can we use graphite lubricant in aluminium powder grinding in ball mill

ball milling - an overview | sciencedirect topics

Ball milling is often used not only for grinding powders but also for oxides or nanocomposite synthesis and/or structure/phase composition optimization [14,41]. Mechanical activation by ball milling is known to increase the material reactivity and uniformity of spatial distribution of elements [63]. Thus, postsynthesis processing of the materials by ball milling can help with the problem of minor admixture forming during cooling under air after high-temperature sintering due to phase instability.

Ball milling technique, using mechanical alloying and mechanical milling approaches were proposed to the word wide in the 8th decade of the last century for preparing a wide spectrum of powder materials and their alloys. In fact, ball milling process is not new and dates back to more than 150 years. It has been used in size comminutions of ore, mineral dressing, preparing talc powders and many other applications. It might be interesting for us to have a look at the history and development of ball milling and the corresponding products. The photo shows the STEM-BF image of a Cu-based alloy nanoparticle prepared by mechanical alloying (After El-Eskandarany, unpublished work, 2014).

Ball milling, a shear-force dominant process where the particle size goes on reducing by impact and attrition mainly consists of metallic balls (generally Zirconia (ZrO2) or steel balls), acting as grinding media and rotating shell to create centrifugal force. In this process, graphite (precursor) was breakdown by randomly striking with grinding media in the rotating shell to create shear and compression force which helps to overcome the weak Vander Waal's interaction between the graphite layers and results in their splintering. Fig. 4A schematic illustrates ball milling process for graphene preparation. Initially, because of large size of graphite, compressive force dominates and as the graphite gets fragmented, shear force cleaves graphite to produce graphene. However, excessive compression force may damage the crystalline properties of graphene and hence needs to be minimized by controlling the milling parameters e.g. milling duration, milling revolution per minute (rpm), ball-to-graphite/powder ratio (B/P), initial graphite weight, ball diameter. High quality graphene can be achieved under low milling speed; though it will increase the processing time which is highly undesirable for large scale production.

Fig. 4. (A) Schematic illustration of graphene preparation via ball milling. SEM images of bulk graphite (B), GSs/E-H (C) GSs/K (D); (E) and (F) are the respective TEM images; (G) Raman spectra of bulk graphite versus GSs exfoliated via wet milling in E-H and K.

Milling of graphite layers can be instigated in two states: (i) dry ball milling (DBM) and (ii) wet ball milling (WBM). WBM process requires surfactant/solvent such as N,N Dimethylformamide (DMF) [22], N-methylpyrrolidone (NMP) [26], deionized (DI) water [27], potassium acetate [28], 2-ethylhexanol (E-H) [29] and kerosene (K) [29] etc. and is comparatively simpler as compared with DBM. Fig. 4BD show the scanning electron microscopy (SEM) images of bulk graphite, graphene sheets (GSs) prepared in E-H (GSs/E-H) and K (GSs/K), respectively; the corresponding transmission electron microscopy (TEM) images and the Raman spectra are shown in Fig. 4EG, respectively [29].

Compared to this, DBM requires several milling agents e.g. sodium chloride (NaCl) [30], Melamine (Na2SO4) [31,32] etc., along with the metal balls to reduce the stress induced in graphite microstructures, and hence require additional purification for exfoliant's removal. Na2SO4 can be easily washed away by hot water [19] while ammonia-borane (NH3BH3), another exfoliant used to weaken the Vander Waal's bonding between graphite layers can be using ethanol [33]. Table 1 list few ball milling processes carried out using various milling agent (in case of DBM) and solvents (WBM) under different milling conditions.

Ball milling of graphite with appropriate stabilizers is another mode of exfoliation in liquid phase.21 Graphite is ground under high sheer rates with millimeter-sized metal balls causing exfoliation to graphene (Fig. 2.5), under wet or dry conditions. For instance, this method can be employed to produce nearly 50g of graphene in the absence of any oxidant.22 Graphite (50g) was ground in the ball mill with oxalic acid (20g) in this method for 20 hours, but, the separation of unexfoliated fraction was not discussed.22 Similarly, solvent-free graphite exfoliations were carried out under dry milling conditions using KOH,23 ammonia borane,24 and so on. The list of graphite exfoliations performed using ball milling is given in Table 2.2. However, the metallic impurities from the machinery used for ball milling are a major disadvantage of this method for certain applications.25

Reactive ball-milling (RBM) technique has been considered as a powerful tool for fabrication of metallic nitrides and hydrides via room temperature ball milling. The flowchart shows the mechanism of gas-solid reaction through RBM that was proposed by El-Eskandarany. In his model, the starting metallic powders are subjected to dramatic shear and impact forces that are generated by the ball-milling media. The powders are, therefore, disintegrated into smaller particles, and very clean or fresh oxygen-free active surfaces of the powders are created. The reactive milling atmosphere (nitrogen or hydrogen gases) was gettered and absorbed completely by the first atomically clean surfaces of the metallic ball-milled powders to react in a same manner as a gas-solid reaction owing to the mechanically induced reactive milling.

Ball milling is a grinding method that grinds nanotubes into extremely fine powders. During the ball milling process, the collision between the tiny rigid balls in a concealed container will generate localized high pressure. Usually, ceramic, flint pebbles and stainless steel are used.25 In order to further improve the quality of dispersion and introduce functional groups onto the nanotube surface, selected chemicals can be included in the container during the process. The factors that affect the quality of dispersion include the milling time, rotational speed, size of balls and balls/ nanotube amount ratio. Under certain processing conditions, the particles can be ground to as small as 100nm. This process has been employed to transform carbon nanotubes into smaller nanoparticles, to generate highly curved or closed shell carbon nanostructures from graphite, to enhance the saturation of lithium composition in SWCNTs, to modify the morphologies of cup-stacked carbon nanotubes and to generate different carbon nanoparticles from graphitic carbon for hydrogen storage application.25 Even though ball milling is easy to operate and suitable for powder polymers or monomers, process-induced damage on the nanotubes can occur.

Ball milling is a way to exfoliate graphite using lateral force, as opposed to the Scotch Tape or sonication that mainly use normal force. Ball mills, like the three roll machine, are a common occurrence in industry, for the production of fine particles. During the ball milling process, there are two factors that contribute to the exfoliation. The main factor contributing is the shear force applied by the balls. Using only shear force, one can produce large graphene flakes. The secondary factor is the collisions that occur during milling. Harsh collisions can break these large flakes and can potentially disrupt the crystal structure resulting in a more amorphous mass. So in order to create good-quality, high-area graphene, the collisions have to be minimized.

The ball-milling process is common in grinding machines as well as in reactors where various functional materials can be created by mechanochemical synthesis. A simple milling process reduces both CO2 generation and energy consumption during materials production. Herein a novel mechanochemical approach 1-3) to produce sophisticated carbon nanomaterials is reported. It is demonstrated that unique carbon nanostructures including carbon nanotubes and carbon onions are synthesized by high-speed ball-milling of steel balls. It is considered that the gas-phase reaction takes place around the surface of steel balls under local high temperatures induced by the collision-friction energy in ball-milling process, which results in phase separated unique carbon nanomaterials.

Conventional ball milling is a traditional powder-processing technique, which is mainly used for reducing particle sizes and for the mixing of different materials. The technique is widely used in mineral, pharmaceutical, and ceramic industries, as well as scientific laboratories. The HEBM technique discussed in this chapter is a new technique developed initially for producing new metastable materials, which cannot be produced using thermal equilibrium processes, and thus is very different from conventional ball milling technique. HEBM was first reported by Benjamin [38] in the 1960s. So far, a large range of new materials has been synthesized using HEBM. For example, oxide-dispersion-strengthened alloys are synthesized using a powerful high-energy ball mill (attritor) because conventional ball mills could not provide sufficient grinding energy [38]. Intensive research in the synthesis of new metastable materials by HEBM was stimulated by the pioneering work in the amorphization of the Ni-Nb alloys conducted by Kock et al. in 1983 [39]. Since then, a wide spectrum of metastable materials has been produced, including nanocrystalline [40], nanocomposite [41], nanoporous phases [42], supersaturated solid solutions [43], and amorphous alloys [44]. These new phase transformations induced by HEBM are generally referred as mechanical alloying (MA). At the same time, it was found that at room temperature, HEBM can activate chemical reactions which are normally only possible at high temperatures [45]. This is called reactive milling or mechano-chemistry. Reactive ball milling has produced a large range of nanosized oxides [46], nitrides [47], hydrides [48], and carbide [49] particles.

The major differences between conventional ball milling and the HEBM are listed in the Table 1. The impact energy of HEBM is typically 1000 times higher than the conventional ball milling energy. The dominant events in the conventional ball milling are particle fracturing and size reductions, which correspond to, actually, only the first stage of the HEBM. A longer milling time is therefore generally required for HEBM. In addition to milling energy, the controls of milling atmosphere and temperature are crucial in order to create the desired structural changes or chemical reactions. This table shows that HEBM can cover most work normally performed by conventional ball milling, however, conventional ball milling equipment cannot be used to conduct any HEBM work.

Different types of high-energy ball mills have been developed, including the Spex vibrating mill, planetary ball mill, high-energy rotating mill, and attritors [50]. In the nanotube synthesis, two types of HEBM mills have been used: a vibrating ball mill and a rotating ball mill. The vibrating-frame grinder (Pulverisette O, Fritsch) is shown in Fig. 1a. This mill uses only one large ball (diameter of 50 mm) and the media of the ball and vial can be stainless steel or ceramic tungsten carbide (WC). The milling chamber, as illustrated in Fig. 1b, is sealed with an O-ring so that the atmosphere can be changed via a valve. The pressure is monitored with an attached gauge during milling.

where Mb is the mass of the milling ball, Vmax the maximum velocity of the vial,/the impact frequency, and Mp the mass of powder. The milling intensity is a very important parameter to MA and reactive ball milling. For example, a full amorphization of a crystalline NiZr alloy can only be achieved with a milling intensity above an intensity threshold of 510 ms2 [52]. The amorphization process during ball milling can be seen from the images of transmission electron microscopy (TEM) in Fig. 2a, which were taken from samples milled for different lengths of time. The TEM images show that the size and number of NiZr crystals decrease with increasing milling time, and a full amorphization is achieved after milling for 165 h. The corresponding diffraction patterns in Fig. 2b confirm this gradual amorphization process. However, when milling below the intensity threshold, a mixture of nanocrystalline and amorphous phases is produced. This intensity threshold depends on milling temperature and alloy composition [52].

Figure 2. (a) Dark-field TEM image of Ni10Zr7 alloy milled for 0.5, 23, 73, and 165 h in the vibrating ball mill with a milling intensity of 940 ms2. (b) Corresponding electron diffraction patterns [52].

Fig. 3 shows a rotating steel mill and a schematic representation of milling action inside the milling chamber. The mill has a rotating horizontal cell loaded with several hardened steel balls. As the cell rotates, the balls drop onto the powder that is being ground. An external magnet is placed close to the cell to increase milling energy [53]. Different milling actions and intensities can be realized by adjusting the cell rotation rate and magnet position.

The atmosphere inside the chamber can be controlled, and adequate gas has to be selected for different milling experiments. For example, during the ball milling of pure Zr powder in the atmosphere of ammonia (NH3), a series of chemical reactions occur between Zr and NH3 [54,55]. The X-ray diffraction (XRD) patterns in Fig. 4 show the following reaction sequence as a function of milling time:

The mechanism of a HEBM process is quite complicated. During the HEBM, material particles are repeatedly flattened, fractured, and welded. Every time two steel balls collide or one ball hits the chamber wall, they trap some particles between their surfaces. Such high-energy impacts severely deform the particles and create atomically fresh, new surfaces, as well as a high density of dislocations and other structural defects [44]. A high defect density induced by HEBM can accelerate the diffusion process [56]. Alternatively, the deformation and fracturing of particles causes continuous size reduction and can lead to reduction in diffusion distances. This can at least reduce the reaction temperatures significantly, even if the reactions do not occur at room temperature [57,58]. Since newly created surfaces are most often very reactive and readily oxidize in air, the HEBM has to be conducted in an inert atmosphere. It is now recognized that the HEBM, along with other non-equilibrium techniques such as rapid quenching, irradiation/ion-implantation, plasma processing, and gas deposition, can produce a series of metastable and nanostructured materials, which are usually difficult to prepare using melting or conventional powder metallurgy methods [59,60]. In the next section, detailed structural and morphological changes of graphite during HEBM will be presented.

Ball milling and ultrasonication were used to reduce the particle size and distribution. During ball milling the weight (grams) ratio of balls-to-clay particles was 100:2.5 and the milling operation was run for 24 hours. The effect of different types of balls on particle size reduction and narrowing particle size distribution was studied. The milled particles were dispersed in xylene to disaggregate the clumps. Again, ultrasonication was done on milled samples in xylene. An investigation on the amplitude (80% and 90%), pulsation rate (5 s on and 5 s off, 8 s on and 4 s off) and time (15 min, 1 h and 4 h) of the ultrasonication process was done with respect to particle size distribution and the optimum conditions in our laboratory were determined. A particle size analyzer was used to characterize the nanoparticles based on the principles of laser diffraction and morphological studies.

high energy ball mills instead of planetary ball mills

Grinding materials to a nano-scale is an important aspect of quality control and R&D. AZoM speaks to Dr. Tanja Butt from RETSCH GmbH about the benefits of using High Energy Ball Mills instead of Planetary Ball Mills.

In laboratories worldwide, Planetary Ball Mills are frequently used, for both, quality control and R&D. They have a reputation to be the best mills to grind semi-hard to brittle samples to very fine particles in the nanometer range < 100 m, always depending on the samples properties. However, such grinding processes can easily take several hours, again, depending on the sample material. With increasing speed, which means also increasing energy input, the chance to obtain actual nano-sized particles increases, but also the risk of warming and rising pressure inside the grinding jars. A practical solution to avoid that problem especially for temperature-sensitive materials - is to interrupt the grinding process and work with cool down breaks. This, however, increases the total process time usually by factor 2-4. Furthermore, the energy input of Planetary Ball Mills is limited, again due to the warming effects and forces inside the machines. Consequently, not all materials can be ground to nanometer size. To offer a solution, RETSCH developed the High Energy Ball Mill Emax which is able to grind many different substances to particle sizes < 100 m and usually in a much shorter time than required by other ball mills. Ideally, the grinding breaks can be skipped altogether, which results in enormous time savings.

The Emax is an entirely new type of ball mill for high energy milling. The unique combination of high friction and impact results in extremely fine particles within a very short process time. The high energy input is a result of the extreme speed of 2000 min-1 and the optimized jar design. Thanks to the revolutionary cooling system with water, the high energy input is effectively used for the grinding process without overheating the sample. Due to the special grinding jar geometry in combination with the circular movement of the jar with fixed orientation - the sample is thoroughly mixed which results in a narrow particle size distribution. The grinding jar supports are mounted on two discs respectively which turn in the same direction. As a result, the jars move on a circular course without changing their orientation. The interplay of jar geometry and movement causes strong friction between grinding balls, sample material and jar walls as well as a rapid acceleration which lets the balls impact with great force on the sample at the rounded ends of the jars. This significantly improves the mixing of the particles resulting in smaller grind sizes and a narrower particle size distribution than has been possible to achieve in ball mills so far. The time required to obtain a specific particle size is often less than in a Planetary Ball Mill, even in processes where no interruption in the grinding times are required in Planetary Ball Mills, thanks to the more efficient grinding mechanism.

The greatest challenge when developing a high energy ball mill is keeping the temperature under control as the enormous size reduction energy leads to considerable heat built-up inside the grinding jar. RETSCH solved this problem with an innovative integrated water cooling system. Hence, the Emax usually doesnt require cooling breaks which are typical for long-term processes in conventional ball mills, even at low speed. This dramatically reduces the grinding time. The cooling system cools the grinding jars via the jar brackets. This is very effective because heat is more easily discharged into water than into air. The user can choose between 3 cooling modes: in addition to the internal cooling, the mill can be connected to a chiller or the tap to further reduce the temperature. For temperature-sensitive samples, the Emax has a great benefit over Planetary Ball Mills. The software allows the user to carry out the grinding process within a defined temperature range, i. e. it is possible to define a minimum and a maximum temperature. On reaching the maximum temperature, the mill automatically interrupts the grinding process and resumes it when the jar has cooled down to the minimum temperature.

When using conventional ball mills the adequate cycles of grinding and cooling need to be ascertained by empirical trials. This may lead to degeneration of the sample or to unnecessarily long processing times. The Emax, in contrast, allows for variable cycles of grinding and cooling within the defined temperature limits. Thus, the entire size reduction process remains reproducible and is carried out in the shortest possible time.

The final size of a certain material always depends on the samples chemical and physical properties. Hard and brittle samples like quartz or some types of pigments can truly be ground to a nanometer size which means D90 < 100 nm! For example, barium titanate or titanium dioxide can be pulverized to D90 < 90 nm. Other, more difficult materials, like the lubricant graphite, can be ground to smaller sizes (D90 = 2.8 m) than in a Planetary Ball Mill. In principle, the Emax reaches final sizes similar to or better than those which are obtained in Planetary Ball Mills.

Special focus was placed on operating convenience and safety when developing the Emax. The grinding jar lids with integrated safety closure, which are simply screwed onto the jars, ensure absolute tightness for wet grinding processes or in cases of pressure increase inside the jar. The grinding jars are quickly and easily placed in the mill and are safely clamped with the ergonomic hand wheel. A sensor monitors the correct position of the jars before starting the machine. Possible imbalances are permanently monitored; if they become too strong the mill stops automatically and the remaining grinding time is displayed. Grinding parameters such as speed, time, interval operation or temperature control are quickly and conveniently set via the color touchscreen. The temperature is displayed during the entire grinding process. The user can store up to 10 grinding programs for routine operations.

Yes, definitely! A huge application field for the Emax is mechanical alloying mainly for R&D. For materials which cannot be alloyed by fusion, mechanical alloying is carried out in ball mills which provide high energy input through impact and friction. Trials have shown that the alloying process in the Emax takes considerably less time than in, for example, a Planetary Ball Mill. Further advantages include a better transformation rate as well as less amorphous particles and less caking of sample material in the jar.

A wide selection of accessories makes the High Energy Ball Mill a versatile instrument.There are grinding jars of three different materials stainless steel, zirconium oxide and tungsten carbide - ensuring contamination-free sample preparation. The first two materials are available in 50 ml jars and in 125 ml jars, tungsten carbide is available 50 ml jar. RETSCH offers a special aeration cover for the grinding jars designed for applications where a special atmosphere is to be maintained in the jar. The grinding balls are available in stainless steel, tungsten carbide and zirconium oxide. Sizes range from 0.1 mm to 15 mm, depending on the material. By selecting the adequate ball numbers and sizes, a wealth of applications can be covered. By continuously measuring pressure and temperature the processes and reactions which take place inside the grinding jar during grinding can be monitored and recorded.

The Emax is not a Planetary Ball Mill it is much more. Higher speed, less warming, finer particles, faster grinding procedures. The high energy input in combination with the unique cooling system provides perfect conditions for effective mechanical alloying or grinding down to the nanometer range. To sum it up, the Emax is faster - finer -cooler!

With more than 100 years of experience RETSCH is the leading solution provider for size reduction and particle sizing technology worldwide. Our philosophy is based on customer orientation and leading-edge technology. This is reflected in instruments whose high-quality components are designed for perfect interaction. Our products not only guarantee representative and reproducible results for size reduction and particle analysis but also allow for easy and comfortable operation. With RETSCH you get: First class product quality thanks to advanced manufacturing methods; Comprehensive application support including free test grindings and product trainings; Excellent sales and service network throughout the world.

The RETSCH website www.retsch.com provides all the details, including a product video of the Emax and an application database. We can also be found at many different trade shows all around the world or people may visit one of our end-user workshops or seminars. To keep our customers up to date with the latest dates and news, we send out a newsletter on a regular basis.

Dr. Tanja Butt studied Biology at the University of Duesseldorf, Germany, and graduated with a Diploma in Biology. After that she earned her PhD in biotechnology at the Forschungszentrum Juelich, Germany. Later, she worked in R&D and project management for different institutions. She joined Retsch in 2013 as Product Manager for the whole milling and sieving product range.

Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of AZoM.com Limited (T/A) AZoNetwork, the owner and operator of this website. This disclaimer forms part of the Terms and Conditions of use of this website.

RETSCH GmbH. (2019, October 30). Nano-Range Grinding with High Energy Ball Mills Instead of Planetary Ball Mills. AZoM. Retrieved on July 10, 2021 from https://www.azom.com/article.aspx?ArticleID=15254.

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practical lubrication of open gearing

Generally, open gear drives are the most economical type of gear drive alternative for use in applications where high load-carrying capacity and long service life under severe shock load conditions are required.

It is these characteristics in addition to flexibility in the machines design that have made open gear drives the most common type of drive used for ball mills and grinding mills, kilns, dryers, draglines and shovels. There are two types of open gear drives; Type 1 utilizes a rack whereas Type 2 utilizes a gear or series of gears.

Type 1 open gear drives consist of an actuator and a rack system used to transmit power. This type of open gearing is primarily used on the cable hoist drums, swing motion drives, mechanical boom lifts, shuttle transfer units and in the hoist and drag drives of mining shovels, draglines and excavators.

These open gears can be bidirectional in motion, and in most applications the tooth geometry, surface finish, pitch line and intermittent loads cause them to operate in thin lubrication film or boundary lubrication condition. Type 1 gears are typically spur-type gears.

If the open gear lubricant is able to provide these characteristics and contaminants and wear debris do not exceed moderate levels, the open gear lubricant can significantly increase the life of the open gearing.

Type 2 open gear drives typically consist of an actuator, pinion and a gear or a series of connecting gears used to transmit continuous loads. They are normally used to power stationary or semi-stationary equipment such as kilns, grinding mills, rotary furnaces, dryers, debarkers, rubber mills, paper mills and finishing mills.

Type 2 open gear drives typically operate at or near their design limits and are often exposed to abrasive contaminants and wear debris. They usually consist of single helical and double helical-type gears.

Open gearing applications, particularly those associated with ball mill and finishing mill applications, are considered some of the most difficult applications a lubricant can encounter. Generally, these types of open gears operate at low pitch line velocities and/or under heavy loads.

However, more recent designs on equipment require open gearing to transmit increasingly higher loads. Because of these considerations, an open gear lubricant must possess the following characteristics and properties:

Several industry-wide specifications and standards have been developed for open gear lubricants by the American Gear Manufacturers Association (AGMA) and different original equipment manufacturers (OEM). The most widely used is AGMA 251.02 (which has been incorporated into AGMA 9005-D94 and AGMA 9005-E02) and U.S. Steel 226 and 236. The requirements are detailed in Table 1.1

The AGMA 9005-D94 and AGMA 9005-E02 stipulate the use of residual compounds 14R and 15R for open gearing when the open gear lubricant is applied by intermittent application methods, where the pitchline velocity does not exceed 7.5 meters per second (1,500 feet/minute). The stated viscosities for 14R and 15R are 428.5 to 857.0 and 857.0 to 1,714 cSt at 100C respectively.

In addition to the AGMA specifications, different OEMs of mining machinery and open gearing units have their own benchmark tests and specifications. Falk specifies that an open gear lubricant must have a minimum viscosity of 857 cSt at 100C, while Sevedala specifies the open gear lubricants have a minimum viscosity of 150 cSt at 100C. The remaining OEM requirements for mining machinery are outlined in Table 2.1

Asphaltic-type open gear lubricants are known as residual compounds or black oils. This type of open gear lubricant has been used for years and is considered a reliable among operators. They are formulated from high-viscosity mineral oils or residual compounds that contain a high level of asphalt or bitumen and a volatile solvent diluent, which is used in the application of the product. Typically the residual black oils used in the formulation have viscosities of 643 cSt or higher at 100C.

Asphaltic open gear lubricants operate on the principle of an oil film separating the mating surfaces of the pinion and the gear. They are generally applied through an automatic spray system. Once applied to the gears, the meshing action causes the solvent diluent to flash or it evaporates, leaving behind a viscous coating of lubricant.

In the past, chlorinated solvents such as 1,1,1-trichloroethane, perchloroethane, perchloroethylene and trichloroethylene were used as the volatile solvent diluent due to their nonflammability, rapid evaporation rates, ability to improve pumpability in automatic lubrication systems, to improve efficiency of spraying and their relatively low cost.

Although the chlorinated solvents provided excellent performance, concerns from environmental and health standpoints arose due to their potential as an ozone depletory and the possibility of being carcinogenic, which lead to the ban of their use.

These concerns resulted in lubricant manufacturers of asphaltic-type open gear lubricants to either use hydrocarbon-type solvents or fluorinated hydrocarbon solvents. The use of these solvents raised concerns over the potential for onboard fires on mining machinery and equipment due to their lower flash points, toxicity issues with fluorinated hydrocarbons, volatile organic compound (VOC) emissions reporting and their slow rate of evaporation.

Asphaltic-type open gear lubricants are used due to the high viscosity imparted by these products that inhibit the meshing gears from contact. The high viscosity also enhances the products adhesiveness. However, the high viscosity of asphaltic lubricants is not necessary for the proper lubrication and the minimization of wear of open gears. Asphaltic-type open gear lubricants are suitable for use in the lubrication of open gearing provided the gear speeds and operating or ambient temperatures remain steady.

At high operating and ambient temperatures, they become less viscous and adhesive resulting in housekeeping problems and higher lubricant consumption rates. High ambient and operating temperatures can also cause the asphaltic-type open gear lubricants to readily oxidize, harden and buildup in the root zone. At low temperatures they tend to provide less-than-satisfactory performance due to their difficulty to dispense and tendency to stiffen, crack and peel-off leaving the gears unlubricated. They also have a tendency to build up in the root zone in low temperatures.

Due to the adhesive nature of the asphalt and bitumen containing oils, asphaltic-type open gear lubricants attract dust, dirt and other contaminants, which can become abrasive wear particles during the meshing action of the gears.

Semifluid greases and semifluid solvent cutbacks were first introduced in Europe approximately 50 years ago. These types of open gear lubricants are widely used and specified in Europe for primary mill open gears and on European manufactured equipment. This type of lubricant is applied before the gear mesh to the loaded side of the tooth in small quantities.

These types of open gear lubricants typically contain a medium- to high-viscosity petroleum base oil, which may contain some asphalt or bitumen. They may also contain a synthetic oil, gelling agent or thickener system such as aluminum complex or lithium complex, solid lubricants such as molybdenum disulfide and graphite, rust inhibitors and extreme pressure (EP) agents.

Because these types of open gear lubricants contain a thickener system, they are commonly referred to as paste-type open gear lubes. Their consistency ranges from an NLGI grade 0 to 1. The cutback versions contain a volatile solvent to enhance the products ability to be applied with spraying and automatic lubrication systems, especially when low temperatures are encountered.

Paste-type open gear lubricants allow the gear and pinion surfaces to operate under boundary lubrication conditions. The paste types EP agents and solid lubricants such as molybdenum disulfide, graphite, acetylene black and other ingredients work together to achieve the desired result of minimizing gear wear under boundary lubrication conditions. Each ingredient has a role to play. First, the thickener system holds the base oils in place.

Secondly, the EP additives protect the base metal of the gears. In short, they prevent welding of the gears. Thirdly, the products base oil does not provide a full-film thickness that is needed to separate the gears even with smooth surfaces. The solid lubricants complement the lubricant by maintaining a solid film that separates the contacting gear surfaces.

Paste-type open gear lubricants, when applied to open gearing, produce a thick, tenacious, semi-dry working film that separates the gears during mesh. This semi-dry film does not buildup in the roots of the gears unless it is overapplied or encounters high operating temperature conditions. The formation of this semi-dry film allows the use of paste-type open gear lubricants in moderately dirty and dusty environments without forming abrasive compounds.

Paste-type open gear lubricants can be formulated to be either thixotropic in nature (shear thinning) or dilatent (shear thickening), as required for the application through a change in composition. These properties can be achieved through the amount of thickener used in the formulation of the lubricant or through the use of volatile solvents. When solvents are utilized it evaporates after application, leaving behind a thick layer of grease that clings to the gear tooth surfaces.

Use as a multiservice lubricant in open gears, bearings and bushings, resulting in simplified lubrication, a reduction of inventories and elimination of lubricant misapplications, which can result in downtime.

Lubricant consumption rates are typically two to three times higher than other open gear lubricants due to the paste types tendency for the base oils to separate from the thickener system when high shock loading conditions or high operating temperatures are encountered.

Gel/polymer thickened-type open gear lubricants were first introduced into the market in the 1980s. These types of lubricants are similar to paste-type open gear lubricants because they contain medium- to high-viscosity petroleum or synthetic base oils that are thickened with a polymeric thickener, EP agents and solid lubricants to enhance their thin film and boundary film performance.

Some products that fall in this classification contain volatile solvents for ease of application. These types of open gear lubricants are typically NLGI 00 to 0 in consistency or are semifluid in consistency.

Gel/polymer-thickened open gear lubricants are thixotropic in nature. The thixotropic property allows friction to be reduced to a minimum, and aids in their ability to be applied by standard intermittent spray systems, or be used in splash or idler immersion systems.

Once applied to the surface of the open gearing, gel/polymer-thickened open gear lubricants form a thick, tenacious, opaque, semitransparent film coating on the gears that can be observed during inspections when using a strobe light while the equipment is operating.

Gel/polymer-thickened open gear lubricants will not build up in the root zones of the gearing. The lubricating film will not rupture, especially during high shock loading conditions. Typically, the stability of the of gel/polymer-thickened open gear lubricants is good while their long-term stability is excellent, because the base oils shows little or no tendency to separate from the gel.

Intermittent spray system maintenance may be reduced and system reliability increased with the use of these open gear lubricants. Due to their good-to-excellent pumpability characteristics, the spray systems pumps, metering devices and conduits are subjected to less stress.

It is less likely that a spray nozzle will foul due to the lubricant drying out, which often occurs when using asphaltic-type open gear lubricants or a product that contains solvents. When the intermittent spray is functioning properly, gel/polymer-thickened open gear lubricants can provide reduced consumption rates below the recommended AGMA guidelines. Even at a reduced application rate, these types of open gear lubricants can reduce pinion temperatures by as much as 30F.

Gel/polymer thickened-type open gear lubricants drain from gear guards and readily flush and remove contaminants from the wearing surfaces of the gears. They do not adhere to dust or dirt contamination and do not present any housekeeping problems other than those associated with over application of the product to the gearing.

High-viscosity synthetic oils appeared in the marketplace in the early 1990s. These are formulated from medium- to very high-synthetic base fluids, such as polyalphaolefin (PAO) and polyol esters, or a combination of both, and contain EP agents and rust and corrosion additive systems.

Some products may also contain solid lubricants such as molybdenum disulfide that is dispersed into the lubricant in a colloidal suspension, or contain viscosity index improvers to enhance the viscometric properties of the synthetic base fluids. Also, the use of viscosity index improvers has traditionally been considered unacceptable by gear manufacturers.

Although these products exhibit high viscosities, concerns exist when under conditions of high loads and shear, the products viscosity index improvers can be sheared. This results in a permanent loss of viscosity that is contributed to the viscosity index improvers.

Various types of synthetic base fluids such as PAOs have inherently low traction properties, which result in low fluid friction in the load zone of the mating gears. This reduced fluid friction produces lower pinion and gear operating temperature and improved gear efficiency.

These types of open gear lubricants conform to the AGMA 14R and 15R viscosity requirements. Like asphaltic-type open gear compounds, they operate on the principle of an oil film separating the surfaces of the gear and pinion and are applied in the same manner as asphaltic-type open gear lubricants.

In some cases, modifications including increasing the operation pressure of intermittent spray systems may be necessary to properly apply the lubricant due to its high viscosity. These types of open gear lubricants, when applied by intermittent systems, need to be utilized according to the guidelines found in ANSI/AGMA standards 9005-D95 and AGMA 9005-E02. If application rates below the AGMA guidelines are used, increased pinion operating temperatures and gear scuffing can occur.

High-viscosity synthetic lubricants are used because of the high viscosity imparted by these products. This viscosity also enhances the products adhesiveness. However, the high viscosity of these gear lubricants is not necessary for the proper lubrication and minimization of wear for open gears. These lubricants are acceptable for use in the lubrication of open gearing provided the gear speeds and operating or ambient temperatures are at optimum levels.

They are primarily used where high operating temperatures are encountered due to their inherent oxidative and thermal stability. At low operating temperatures, heat tracing and drums heaters may be necessary to obtain a proper spray pattern. Though they contain synthetic base fluids, the high-viscosity synthetic base fluids have pour points ranging from -5F to 15F.

High-viscosity synthetic lubricants have a substantially higher initial purchase cost than traditional open gear lubricants. Their economic justification is claimed through reduced lubricant consumption in total loss systems, extended service life in splash, transfer pinion and circulating systems, and reduced lubricant disposal costs.

wet ball milling vs dry ball milling | orbis machinery, llc

Everything we make use of in our day to day activities passes through a milling process. Cement used in building, the cereals we eat, toiletries, paints used in making our house presentable, and the tiles that beautifies the house we live in, all went through a milling process. A ball mill is a grinder which is used to grind, blend and mix materials like chemicals, ores, pyrotechnics, paints, mineral dressing process, paint and ceramic raw materials. Its working principle is impact and attrition. Ball milling have proved to be effective in increasing solid-state chemical reactivity and production of amorphous materials. Milling operations are carried out either wet or dry.

Power The difference between the result gotten from using wet and dry milling are most of the time very large. This difference is attributed to the power. The power to drive a wet ball mill is said to be 30% lesser than that of a similar dry ball mill.

Nature Of Materials In the production of some products both wet ball and dry ball milling processes are required. The grinding of the raw mix in a cement plant, can be carried out either wet or dry but because of the nature of the cement can, grinding it has to be carried out dry.

Quality The quality expected will be the determinant of which ball milling process to be used. For example, if pyrotechnic materials is grounded dry, it gives a product superior characteristics compared to the one which was grounded wet. The grinding of aluminium for the preparation of paint is most of the time carried out using a wet milling process since the method introduces stearic acid, or other antiflocculent

Environment The advantages Wet ball milling has over dry milling are higher energy efficiency, lower magnitude of excess enthalpy, better heat dissipation and absence of dust formation because of the aqueous environment it is being performed.

Introduction Of Active Surface Media Wet ball milling allows easy introduction of surface active media having to do with the reduction of the required energy for the inhibition of aggregation of fine particles. Due to wide adoption, it is only theoretically possible to introduce such material or substance in gaseous or vapour form into dry ball milling. The only practicable method of introducing substance in gaseous form is wet ball milling.

Cost In the production of ethanol, wet ball milling is the process used, because of its versatile process. It produces more products than dry ball milling, but in terms of efficiency, capital, and operating cost, most ethanol plants in the USA prefer to use dry ball milling process. In other words, dry ball milling is cost efficient in ethanol production than wet ball milling. With the above, you should be able to weigh which of the ball milling process is appropriate and cost efficient for your production needs.

nanocarbon in the structure of a hypereutectic aluminum-matrix composite | springerlink

The peculiarities of the fine structure of AlSiNi aluminum-matrix composite with a low thermal coefficient of linear expansion, mechanically activated with the addition of nanoscale reduced graphene oxide (RGO), is investigated. The structure is studied by X-ray diffraction analysis, scanning, transmission and high-resolution transmission electron microscopy. The presence of quasi-graphene layers on the surface of aluminum and silicon particles is detected and it is shown that this shell protects them from clumping upon mechanical alloying, which significantly increases the manufacturability of the process of mechanical activation and subsequent compaction. Thus, it is possible to obtain composite materials with a homogeneous structure and higher physical properties (the use of RGO instead of electrode graphite reduces the thermal coefficient of linear expansion (TCLE) of the composite by 10%).

V. V. Vasenev, V. N. Mironenko, and V. N. Butrim, in Intern. Conf. Powder Metallurgy and Particulate Materials sponsored by the Metal Powder Industries Federation, Chicago, IL, June 2427,2013 (Chicago, 2013), No. 7, p. 156.

Aronin, A.S., Aristova, I.M., Abrosimova, G.E. et al. Nanocarbon in the Structure of a Hypereutectic Aluminum-Matrix Composite. J. Synch. Investig. 14, 668672 (2020). https://doi.org/10.1134/S1027451020040023

graphite grinding machine, graphite ball mill, graphite processing plant

Graphite powder has good chemical stability. Specially processed graphite powder, which has the characteristics of corrosion resistance, good thermal conductivity and low permeability, is widely used in the manufacture of heat exchangers, reaction tanks, condensers, combustion towers, absorption towers, coolers, heaters, filtration , pump equipment. Widely used in petrochemical, hydrometallurgy, acid and alkali production, synthetic fiber, paper and other industrial sectors, can save a lot of metal materials.

The natural flake graphite and the raw coke are fed into the crushing chamber by the compressed air through the feeding ejector, and are distributed in the nozzle around the pulverizing chamber, and the ultra-high-speed airflow is sprayed to the pulverizing chamber, so that the coke powder collides with each other at high speed and rubs into a fine powder, with a fine powder. The gas stream is discharged into the classification chamber by the outlet pipe, the coarser particles are separated, and the powder is again pulverized by the return pipe, and the remaining fine powder is collected by a cyclone separator and a bag collector. Ultrafine graphite powder was produced. Ultra-fine graphite powder is widely used in conductive materials, wear-resistant lubricant batteries, atomic energy industry and defense industry, color picture tube and other fields due to its stable nature. Ultrafine graphite powder can be composited with metal to form metal-based graphite composite conductive material for civilian use, superfine graphite powder is used as lubricant in machinery industry, ultrafine graphite powder is used in color picture tube field for color picture tube made of ultra-fine graphite Graphite emulsion, used as a neutron moderator and protective material in atomic reactors in the atomic energy industry and the defense industry.

exfoliation of graphite by dry ball milling with cellulose | springerlink

Dry ball milling of graphite with cellulose and related polysaccharides was found effective for exfoliation-dispersion of graphene-like carbon. The exfoliation behavior was found to depend strongly on the polymer species; namely, polysaccharides are much more effective than thermoplastic polymers. The compression-molded slabs from co-milled powder with cellulose and carboxymethylcellulose showed an electrical percolation threshold of 1.0% (w/w) or lower. The carbon fraction isolated from milling with carboxymethylcellulose was water-dispersible, containing single- to few-layer graphenes. This method can provide facile and solventless graphene exfoliation and mechanical alloying with polymers.

Bourlinos AB, Georgakilas V, Zboril R, Steriotis TA, Stubos AK, Trapalis C (2009b) Aqueous-phase exfoliation of graphite in the presence of polyvinylpyrrolidone for the production of water-soluble graphenes. Solid State Commun 149:21722176

Hernandez Y, Nicolosi V, Lotya M, Blighe FM, Sun Z, De S, IMcGovern IT, Holland B, Byrne M, GunKo YK, Boland JJ, Niraj P, Duesberg G, Krishnamurthy S, Goodhue R, Hutchison J, Scardaci V, Ferrari AC (2008) High-yield production of graphene by liquid-phase exfoliation of graphite. Nat Nanotechnol 3:563568

Jiang X, Drzal LT (2012) Reduction in percolation threshold of injection molded high-density polyethylene/exfoliated graphene nanoplatelets composites by solid state ball milling and solid state shear pulverization. J Appl Polym Sci 12:525535

Laaksonen P, Kainlauri M, Laaksonen T, Shchepetov A, Jiang H, Ahopelto J, Linder MB (2010) Interfacial engineering by proteins: exfoliation and functionalization of graphene by hydrophobins. Angew Chem Int Ed 49:49464949

Laaksonen P, Walther A, Malho JM, Kainlauri M, Ikkala O, Linder MB (2011) Genetic engineering of biomimetic nanocomposites: diblock proteins, graphene, and nanofibrillated cellulose. Angew Chem Int Ed 50:86888691

Malho JM, Laaksonen P, Walther A, Ikkala O, Linder MB (2012) Facile method for stiff, tough, and strong nanocomposites by direct exfoliation of multilayered graphene into native nanocellulose matrix. Biomacromolecules 13:10931099

May P, Khan U, Hughes JM, Coleman JN (2012) Role of solubility parameters in understanding the steric stabilization of exfoliated two-dimensional nanosheets by adsorbed polymers. J Phys Chem C 116:1139311400

Minami M, Kim Y, Miyashita K, Kazaoui S, Nalini B (2006) Cellulose derivatives as excellent dispersants for single-wall carbon nanotubes as demonstrated by absorption and photoluminescence spectroscopy. Appl Phys Lett 88:093123

Montone A, Grbovic J, Bassetti A, Mirenghi L, Rotolo P, Bonetti E, Pasquini L, Antisari MV (2006) Microstructure, surface properties and hydrating behaviour of MgC composites prepared by ball milling with benzene. Int J Hydrogen Energy 31:20882096

Oyer AJ, Carrillo JM, Hire CC, Schniepp HC, Asandei AD, Dobrynin AV, Adamson DH (2012) Stabilization of graphene sheets by a structured benzene/hexafluorobenzene mixed solvent. J Am Chem Soc 134:50185021

Schniepp HC, Li JL, McAllister MJ, Sai H, Herrera-Alonso M, Adamson DH, Prudhomme RK, Car R, Saville DA, Aksay IA (2006) Functionalized single graphene sheets derived from splitting graphite oxide. J Phys Chem B 110:85358539

Smith CI, Miyaoka H, Ichikawa T, Jones MO, Harmer J, Ishida W, Edwards PP, Kojima Y, Fuji H (2009) Electron spin resonance investigation of hydrogen absorption in ball-milled graphite. J Phys Chem C 113:54095416

Tang J, Zhao W, Li L, Simmons WB, Zhou WL, Ikuhara Y, Zhang JH (1996) Amorphization of graphite induced by mechanical milling and subsequent crystallization of the amorphous carbon upon heat treating. J Mater Res 11:733738

The work was supported by the National Program on Key Basic Research Project (973 Program, no. 2011CB933700), the National Natural Science Foundation of China (51172247, 50773086), and the Chinese Academy of Sciences Visiting Professorships. We thank H.X. Yao (Beijing University of Chemical Technology) for assistance in TEM and J.Y. Fang (Technical Institute of Physics and Chemistry, Chinese Academy of Sciences) for assistance in AFM.

metallographic grinding and polishing insight

Mechanical preparation is the most common method of preparing materialographic specimens for microscopic examination. The specific requirement of the prepared surface is determined by the particular type of analysis or examination. Specimens can be prepared to the perfect finish, the true structure, or the preparation can be stopped when the surface is acceptable for a specific examination.

Mechanical preparation is the most common method of preparing materialographic specimens for microscopic examination. The specific requirement of the prepared surface is determined by the particular type of analysis or examination. Specimens can be prepared to the perfect finish, the true structure, or the preparation can be stopped when the surface is acceptable for a specific examination.

The basic process of mechanical specimen preparation is material removal, using abrasive particles in successively finer steps to remove material from the surface until the required result is achieved. There are three mechanisms for removing material: grinding, polishing, and lapping. They differ in the tendency to introduce deformation in the specimen's surface.

Proper grinding removes damaged or deformed surface material, while limiting the amount of additional surface deformation. The goal is a plane surface with minimal damage that can easily be removed during polishing in the shortest possible time. Grinding removes material using fixed abrasive particles that produce chips of the specimen material (see below). The process of making chips with a sharp abrasive grain produces the lowest amount of deformation in the specimen, while providing the highest removal rate.

The grain is entering the specimen surface. The grain is totally fixed in the X-direction; movement (resilience) in the Y-direction can take place. The chip is started when the grain enters into the specimen material.

This is normally the first step in the grinding process. Plane grinding ensures that the surfaces of all specimens are similar, despite their initial condition and their previous treatment. In addition, when processing several specimens in a holder, care must be taken to make sure they are all at the same level, or "plane," before progressing to the next step, fine grinding. To obtain a high, consistent material removal rate, short grinding times and maximum flatness, totally fixed grains with a relatively large grain size are preferred for plane grinding. Suitable PG surfaces will provide perfectly plane specimens, thus reducing the preparation time on the following fine grinding step. In addition, some surfaces can provide good edge retention. During wear, new abrasive grains are revealed, thus ensuring a consistent material removal.

Fine grinding produces a surface with little deformation that can easily be removed during polishing. Because of the drawbacks with grinding papers, alternative fine grinding composite surfaces are available, in order to improve and facilitate fine grinding, A high material removal rate is obtained by using grain sizes of 15, 9.0 and 6.0 m. This is done on hard composite disks (rigid disks) with a surface of a special composite material. Thus, the diamond grains, which are continuously supplied, are allowed to embed the surface and provide a fine grinding action. With these disks, a very plane specimen surface is obtained. The use of a diamond abrasive on the fine grinding disks guarantees a uniform removal of material from hard, as well as soft, phases. There is no smearing of soft phases or chipping of brittle phases, and the specimens will maintain a perfect planeness. Subsequent polishing steps can be carried out in a very short time.

Diamonds are used as an abrasive to accomplish the fastest material removal and the best possible planeness. No other available abrasive can produce similar results. Because of its hardness, diamonds cut extremely well through all materials and phases. During polishing, a smaller chip size is desirable to ultimately achieve a specimen surface without scratches and deformation. More resilient cloths are used, along with smaller grain sizes, such as 3.0 or 1.0 m, to obtain a chip size approaching zero. A lower force on the specimens will also reduce the chip size during polishing.

Certain materials, especially those that are soft and ductile, require a final polish, using oxide polishing to obtain the best quality. Colloidal silica, with a grain size of approximately 0.04 m and a pH of about 9.8, has shown remarkable results. The combination of chemical activity and fine, gentle abrasion produces scratch-free and deformation-free specimens.

In lapping, the abrasive is applied in a suspension onto a hard surface. The particles cannot be pressed into the surface and secured there. They roll and move freely in all directions, hammering small particles out of the specimen surface and introducing deep deformations. The reason is that the free moving abrasive particle is not able to produce a real "chip" of the specimen surface.

The X-axis represents the hardness in Vickers (HV). The values are not shown in a linear progression, because the variety of preparation methods for softer materials is greater than for hard ones. The shape of the Metalogram results from soft materials generally being more ductile and hard materials usually being more brittle.

The Metalogram is based on ten preparation methods. Seven methods, A - G, cover the complete range of materials. They are designed to produce specimens with the best possible results. In addition, three short methods, X, Y, and Z, are displayed. These methods are for very quick, acceptable results.

Some materials such as composites, coatings, or other materials consisting of various phases or components cannot be easily placed in the Metalogram. In these cases, the following rules can be applied when deciding on the preparation method:

Surfaces are carefully selected according to relevant equipment in use, sample material, and requirements for preparation. Within each group of surfaces: grinding stones, grinding or polishing paper, disks or cloth, the difference in characteristics include type of abrasive bond, abrasive type, hardness, resilience, surface pattern, and projections of fibers.

The preparation is always started with the smallest possible grain size to avoid excessive damage to the specimens. During the subsequent preparation steps, the largest possible intervals from one grain size to the next are chosen in order to minimize preparation time.

The removal rate in grinding and polishing is closely related to the abrasives used. Diamonds are one of the hardest known materials, as they have a hardness of approximately 8,000 HV. That means it can easily cut through all materials and phases. Different types of diamonds are available. Tests have shown that the high material removal, together with a shallow scratch depth, is obtained because of the many small cutting edges of polycrystalline diamonds. Silicon carbide, SIC, with a hardness of about 2,500 HV, is a widely used abrasive for grinding papers for mainly non-ferrous metals. Aluminium oxide, with a hardness of about 2,000 HV, is primarily used as an abrasive in grinding stones. It is mainly used for the preparation of ferrous metals. It was also extensively used as a polishing medium, but since the introduction of diamond products for this purpose, it has largely lost its usefulness in this application. Colloidal silica is used to produce a scratch-free finish in oxide polishing steps In general, the abrasive must have a hardness of 2.5 to 3.0 times the hardness of the material to be prepared. Never change to softer abrasives - this might lead to preparation artifacts. The amount of abrasive applied depends on the grinding/polishing surface and the hardness of the specimen. The combination of cloths with low resilience and hard specimens requires a larger amount of abrasive than cloths with high resilience and softer specimens, because the abrasive particles wear faster.

Depending on the type of material and the grinding/polishing disk used for preparation, the amounts of lubrication and cooling have to be balanced. Generally, it can be said that soft materials require high amounts of lubricant to avoid damage, but only small amounts of abrasive as there is very little wear on the abrasive. Hard materials require less lubricant but higher amounts of abrasive, due to faster wear. The amount of lubricant has to be adjusted correctly to get the best result.

The polishing cloth should be moist, not wet. Excess lubricant will flush the abrasive from the disk and remain as a thick layer between the specimen and disk, thus reducing material removal to a minimum.

For PG, a high disk speed is used to get a fast material removal. For FG, DP, and OP, speeds of 150 rpm are used for both grinding/polishing disks and specimen holders. They are also both turning in the same direction. When working with loose abrasives, high speeds would throw the suspension from the disk, thus requiring higher amounts of both abrasive and lubricant.

The force is expressed in Newton. The figures stated in the preparation methods are typically standardized for six specimens of 30 mm diameter, clamped in a specimen holder. The specimens are mounted, and the specimen area should be approximately 50% of the mount. If the specimens are smaller, or there are fewer specimens in a holder, the force has to be reduced to avoid damage, such as deformations. For larger specimens, the force only needs to be slightly increased. Instead, the preparation time shall be extended. Higher forces increase the temperature because of higher friction, so thermal damage may occur.

Preparation time is the time during which the specimen holder is rotating and pressed against the grinding/polishing disk. The preparation time is stated in minutes. It should be kept as short as possible to avoid artifacts such as relief or edge rounding. Depending on the specimen size, the time may have to be adjusted. For larger specimens, the time shall be extended. With specimens smaller than the standard, the time is kept constant and the force reduced.

The plastic deformation of larger sample areas is called smearing. Instead of being cut away or removed, material is pushed across the surface. Smearing occurs because of an incorrect application of abrasive, lubricant, polishing cloth, or a combination of these, which makes the abrasive act as if it was blunt. There are three ways to avoid smearing:

If your polisher is not equipped with automatic water flushing after the oxide polishing step during the last ten seconds of OP polishing, flush the polishing cloth with water to clean both the specimens and the cloth.

There are two types of deformation:elasticandplastic. Elastic deformation disappears when the applied load is removed. Plastic deformation, which may also be referred to as cold work, can result in subsurface defects after grinding, lapping, or polishing. Remaining plastic deformation can first be seen after etching.

Only deformation introduced during the preparation is covered here. All other types from previous operations like bending, drawing, and stretching are not considered, because they cannot be changed or improved by changing the preparation method.

Using a polishing surface with high resilience will result in material removal from both the sample surface and the sides. The effect of this is edge rounding and can be seen with mounted specimens if the resin wears at a higher rate than the sample material.Please check your samples after each step to see when the fault occurs so you can determine what changes you will need to make in the preparation.

Relief is usually not noted until polishing begins, so it is important to begin the preparation with grinding media that will keep the samples as flat as possible. However, for the best possible starting conditions, MD-Largo should be used for fine grinding of materials with a hardness below 150 HV, and MD-Allegro should be used for fine grinding of materials with a hardness of 150 HV and higher.

Gaps are voids between the mounting resin and sample material. When examining samples with a microscope, it is possible to see if there is a gap between the resin and the sample. Gaps can result in a variety of preparation faults: edge rounding, contamination of polishing cloth, problems when etching, and staining.

Note:Vacuum impregnation will only fill cracks and cavities connected with the surface. Be careful not to use mounting materials with high shrinkage. They might pull layers away from the base material.

Some materials have natural porosity, for example, cast metals, spray coatings, or ceramics. It is important to get the correct values, and not to provide incorrect readings because of preparation faults.

Contrary to the ductile material, where the initial porosity seems to be low and pores have to be opened, brittle materials seem to have a high porosity. The apparent fracturing of the surface has to be removed.

Contrary to the ductile material, where the initial porosity seems to be low and pores have to be opened, brittle materials seem to have a high porosity. The apparent fracturing of the surface has to be removed.

Comet tails occur adjacent to inclusions or pores, when the motion between sample and polishing disk is unidirectional. Their characteristic shape earns the name "comet tails." A key factor in avoiding comet tails is the polishing dynamics.

3. Polishing for extended time on a soft cloth is a contributing factor. Ensure that as little deformation as possible must be removed by the next polishing step, especially when a cloth with high resilience is needed.

An embedded abrasive is a loose abrasive particle pressed into the surface of a specimen. With soft materials, abrasive particles can become embedded. Embedded abrasives can occur because of a small abrasive particle size, the grinding or polishing cloth used has a low resilience, or a lubricant with a low viscosity is used. Often, a combination of these reasons takes place.

Lapping tracks are indentations on the sample surface made by abrasive particles moving freely on a hard surface. These are not scratches, like from a cutting action, but are the distinct tracks of particles tumbling over the surface without removing material.

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laboratory ball mills for ultrafine grinding

Nanotechnology, which revolutionizes industries such as semiconductor technology, materials science, pigments, food, and pharmaceutics, is one of the most innovative developments today. It deals with particles ranging from 1 to 100 nm. These particles have special properties because of their size, as their surface is very much enlarged in relation to their volume, the so-called size-induced functionalities. For instance, ultrafine particles are harder and more break-resistant compared to larger particles. Nanotechnology brings effects that occur in nature to a commercial level, such as, for instance, the lotus effect: nanocoated fabrics or paints are dirt- and water-repellent just like the lotus flower.

How are nanoparticles generated? The Bottom-Up technique synthesizes particles from molecules or atoms. The Top-Down technique involves reducing the size of larger particles to nanoscale, for instance, with laboratory mills. Small particles are attracted to each other by their electrostatic charges because of their significantly expanded surface in relation to the volume. Nanoparticles are generated by colloidal grinding which involves dispersion of the particles in liquid in order to neutralize the surface charges. Depending on the sample material, both alcohol and water can be used as dispersion medium. In some cases, the neutralization of surface charges can be achieved through the addition of a buffer such as sodium phosphate or molecules with long, uncharged tails such as diaminopimelic acid (steric or electrostatic stabilization).

Several factors, including size reduction principle and energy input make ball mills the best option for the production of nanoparticles. The most important factors for choosing a mill and suitable accessories are:

Nanoparticles are produced with the Top-Down method by colloidal grinding employing a suitable dispersant in order to keep the particles from agglomerating. A high energy input is needed to reduce small particles with mechanical force to smaller sizes. The selection of the correct grinding jar filling and suitable grinding tools are other factors that need to be considered.

An initial size reduction step can be useful depending on the size of the original sample material and desired final fineness. A dry grinding method with grinding balls of >3 mm is generally performed by filling one third of the jar with sample material and one third with grinding balls. The homogenized sample is then employed for the actual colloidal process.

With its planetary ball mills and the novel high-energy ball mill Emax, RETSCH provides two types of ball mills that offer the energy input necessary for colloidal grinding down to the nanometer range. Grinding balls and jars made up of an abrasion-resistant material such as zirconium oxide are well-suited for this kind of application. Here, 60% of the grinding jar level is filled with grinding balls ranging from 0.5 to 3 mm , offering many frictional points. The actual sample fills about one third of the jar volume. The consistency of the sample should become pasty by adding an appropriate dispersant (e. g. buffer, isopropanol, or water), providing perfect conditions for colloidal grinding. If an extremely high final fineness is needed, it is advisable to continue with a second colloidal grinding step with grinding balls ranging from 0.1 to 0.5 mm , particularly if 2 to 3 mm balls were employed in the initial process (the balls need to be 3 x bigger compared to the particle size of the initial material). In order to separate the sample from the grinding balls, both are placed on a sieve (with aperture sizes 20-50% smaller than the balls) with a collecting pan. 60% of the jar is filled with small beads for the subsequent colloidal grinding. The suspension from the earlier grinding is carefully mixed with the grinding beads until a pasty consistency is achieved.

During grinding, some materials tend to become very pasty, preventing the grinding balls from moving around in the suspension and thus making additional size reduction almost impossible. So, during the grinding process, it is recommended to check the consistency of unknown sample materials. If required, the sample/ball mixture can be diluted further by adding more dispersant. If a sample swells easily, the sample/dispersant ratio must be adapted accordingly. The addition of surfactant to stabilize the consistency is another option.

The grinding jar should be removed carefully as it may have a temperature of up to 150 C, owing to the heat produced during the grinding process. Furthermore, pressure increases inside the grinding jar. Hence, it is recommended to use the optional safety closure for the comfort grinding jars used with RETSCHs planetary ball mills to ensure safe handling. After the grinding process, the jar must cool down for a while. The Emax jar is equipped with an integrated safety closure. Furthermore, the mills effective cooling system prevents the jars from heating too much. Both jar types are provided with optional aeration covers for working under inert atmosphere.

Each grinding jar in the planetary ball mill represents a planet. This planet is placed on a circular platform, the so-called sun wheel. When the sun wheel rotates, every grinding jar turns around its own axis, but in the opposite direction. As a result, centrifugal and Coriolis forces are activated, causing rapid acceleration of the grinding balls (Figure 4). The outcome is very high pulverization energy that allows for the generation of very fine particles. The massive acceleration of the grinding balls from one wall of the jar to the other creates a strong impact effect on the sample material and causes additional grinding effects through friction. The ratio between the speed of the sun wheel and the speed of the grinding jar is 1:-2 for colloidal grinding and many other applications. This means that when the sun wheel rotates once, the grinding jars rotate twice in the opposite direction.

The result of grinding of alumina (Al2O3) at 650 min-1 in the PM 100 is shown in Figure 5. The mean value of the particle size distribution is 200 nm after 1 hour of size reduction in water with 1 mm grinding balls; it is 100 nm after 4 hours. In a further test, the material was initially ground for 1 hour with 1 mm grinding balls and after that for 3 hours with 0.1 mm grinding balls (Figure 6). In this case, an average value of 76 nm was obtained. As demonstrated with these grinding results, the planetary ball mills can generate particle sizes in the nanometer range.

The Emax is a new kind of ball mill designed specifically for high energy milling. The impressive speed of 2,000 min-1, thus far unrivaled in a ball mill, together with the special grinding jar design produces a huge amount of size reduction energy. The unique combination of friction, impact, and circulating grinding jar movement produces ultrafine particle sizes in a very short amount of time. Thanks to the novel liquid cooling system, excess thermal energy is rapidly discharged, thus preventing the sample from overheating, even after long grinding times.

The new size reduction mechanism of the Emax is characterized by intensive friction and high-frequency impact, resulting in unequaled grinding performance. This unique combination is produced by the oval shape and the movement of the grinding jars which do not turn around their own axis as seen in the case of planetary ball mills. The interaction of jar geometry and movement produces strong friction between jar walls, sample material, grinding balls, and rapid acceleration which allows the balls to impact with large force on the sample at the rounded corners of the jars. This considerably enhances the mixing of particles, resulting in a narrower particle size distribution and smaller grind sizes than obtained in traditional ball mills.

The grinding energy ensuing from the friction of a large amount of small grinding balls is enhanced even more in the Emax by the high speed of 2,000 min-1. As the unique liquid cooling system rapidly discharges the frictional heat, the high energy input is fully exploited for the grinding process. Both sample and mill would overheat without effective cooling. Based on the grinding mode and the sample characteristics, cooling breaks of approximately 60% of the total grinding time are suggested for traditional planetary ball mills to prevent overheating. On the other hand, the Emax is suitable for continuous grinding without breaks, thanks to its effective liquid cooling system.

In a comparative test, the pigment titanium dioxide was pulverized in the Emax (50 ml grinding jar of zircomium oxide, 110 g matching grinding balls 0.1 mm , 10 g sample, 15 ml 1 % sodium phosphate) and in the most powerful planetary ball mill. The d90 value of the Emax sample was 87 nm after 30 minutes. The planetary ball mill accomplished a grind size of only 476 nm after this time (excluding cooling breaks). As a result, the Emax provided a 5 times higher final fineness compared to the planetary ball mill (Figure 9).

Figure 10 displays the outcome of grinding graphite in the Emax at 2,000 min-1 (50 ml grinding jar of zirconium oxide, 110 g matching grinding balls 0.1 mm , 5 g sample, 13 ml isopropanol) and in the most powerful planetary ball mill. Since graphite is a lubricant, it needs a particularly high energy input for size reduction. 90% of the Emax sample showed a fineness of 13 microns after just 1 hour of grinding, whereas the planetary ball mill achieved this grind size only after 8 hours of grinding (excluding cooling breaks). Regarding the final fineness obtained in the Emax after 8 hours of grinding, its excellent performance again becomes obvious: With a d90 value of 1.7 m, the grind size is 7 times finer than the one obtained in the planetary ball mill (12.6 m).

An integrated water cooling system cools the grinding jars of the Emax. The mill can be connected to the tap or a heat exchanger to further reduce the temperature. The cooling circuit of the Emax is shown in Figure 11. The grinding jars are cooled through the jar. The cooling system is extremely effective because heat is discharged more easily into water than into air. The Emax software enables users to perform the grinding process within a defined temperature range, that is, they can fix a minimum and a maximum temperature. The mill automatically stops when the maximum temperature is exceeded, and it starts again after reaching the minimum temperature.

For many years, nanoparticles (i. e., particles with a diameter of < 100 nm) have been the object of scientific research. There are different techniques to generate nanoparticles. The Top-Down technique involves size reduction of very large particles to the nanometer range. The best results are obtained using ball mills which provide the necessary energy input. Besides the planetary ball mills, including PM 100, PM 200 and PM 400, the new high energy ball mill Emax is particularly suitable for colloidal grindings down to the nanometer range due to the high energy input and innovative water cooling system.

AZoM speaks to David Moulton, UK Managing Director at Camfil, about the company's air filtration solutions and how they help to provide a safer working environment for those in theconstruction industry.