Friction — Lubricating Powders

Podczeck, F., Miah, Y. The influence of particle size and shape on the angle of internal friction and the flow factor of unlubricated and lubricated powders. Int. J. Pharm., 144, 1996, 187-194. 

Miller, T. A., and York, P. (1985), Frictional assessment of magnesium stearate and pal-mitate lubricant powders, Powder Technol., 44(3), 219-226. 

Tsuya et al ° studied the optimum concentration of several different solid lubricant powders in copper composites. In all cases except calcium fluoride they found that the friction of cold-pressed copper composites reached a minimum value at a lubricant concentration of about 7 - 10%, and remained fairly constant up to a concentration of almost 90%. 

Strijbos S (1977) Powder-wall friction—effects of orientation of wall grooves and wall lubricants. Powder Technol 18 209-214... 

Figure 19.2 Comparison of coefficient of friction of various solid lubricant powders. Reproduced with permission from Ref. [4].

During compression within a die, there are a multitude of physical and mechanical processes occurring. These include powder flow, percolation, friction, lubrication, fracture, elastic, viscous, and plastic deformation. As a result of these transformations, a powder is converted into a tablet in a matter of some few hundred milliseconds. 

Dry lubricants are usually added to the powder in order to decrease the friction effects. The more common lubricants include zinc stearate [557-05-17, lithium stearate [4485-12-5] calcium stearate [1592-23-0] stearic acid [57-11-4] paraffin, graphite, and molybdenum disulfide [1317-33-5]. Lubricants are generally added to the powder in a dry state in amounts of 0.25—1.0 wt % of the metal powder. Some lubricants are added by drying and screening a slurry of powder and lubricant. In some instances, lubricants are appHed in Hquid form to the die wall. 

Lubricants protect die and punch surfaces from wear and bum-out of the compact during sintering without objectionable effects or residues. They must have small particle size, and overcome the main share of friction generated between tool surfaces and powder particles during compaction and ejection. They must mix easily with the powder, and must not excessively impede powder flow (see Lubrication and lubricants). 

About 88% of Hon powder production is used in the manufacture of P/M parts and friction materials. Detailed statistical data are available (16). About 86% of the copper and copper-base powder produced is used in the production of self-lubricating bearings and P/M parts. 

The success of the compaction operation depends pardy on the effective utilization and transmission of appHed forces and pardy on the physical properties and condition of the mixture being compressed. Friction at the die surface opposes the transmission of the appHed pressure in this region, results in unequal distribution of forces within the compact, and hence leads to density and strength maldistribution within the agglomerate (70). Lubricants, both external ones appHed to the mold surfaces and internal ones mixed with the powder, are often used to reduce undesirable friction effects (71). For strong compacts, external lubricants are preferable as they do not interfere with the optimum cohesion of clean particulate surfaces. Binder materials maybe used to improve strength and also to act as lubricants. 

Molybdenum disulhde (M0S2), graphite, hexagonal boron nitride, and boric acid are examples of lamella materials commonly applied as solid lubricants. The self-lubricating nature of the materials results from the lamella crystalline structure that can shear easily to provide low friction. Some of these materials used to be added to oils and greases in powder forms to enhance their lubricity. Attention has been shifted in recent years to the production and use of nanosize particles of M0S2, WS2, and graphite to be dispersed in liquid lubricants, which yields substantial decreases in friction and wear. 

Capsule formulations usually require lubricants just as do tablet formulations. Lubricants ease the ejection of plugs, reduce filming on pistons and adhesion of powder to metal surfaces, and reduce friction between sliding surfaces in contact with powder. The same lubricants are used in both tablet and capsule formulations. 

Udav Lubricity additive applicable for new and old engines alike and compatible with any mineral lubricant. Contains ultrafine diamond powder for special friction. Reduces noise, wear and smoking while improving power and extending engine life. 

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