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The Materials Science of Thin Films Episode 13

Tham khảo tài liệu 'the materials science of thin films episode 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 578 Metallurgical and Protective Coatings Figure 12-13. a Ball-bearing components inner race or ring outer race or ring ball cage or ball retainer b Schematic representation of unlubricated steel ball-to-steel race contact c Schematic representation of lubricated ball-to-race contact d Schematic representation of unlubricated Tic coated ball-to-race contact. Reprinted with permission from Elsevier Sequoia S.A. H. J. Boving and H. E. Hintermann Thin Solid Films 153 253 1987 . raceways and balls strongly influences the effective life of the bearing. When the roughness is very low then the oil film carries the load as desired and minimal metal-to-metal contact occurs. However when the roughness is high the surface asperities occasionally impinge to cause local contact and microwelding. This is shown in Figs. 12-13b and c representations of the steel ball-steel race contact in the absence and presence of oil-grease lubrication. Even though the microwelds rupture almost instantaneously the bearing interface roughens. The more rapid deterioration of the bearing when no lubricant is present is evident. Many more microwelds form and their fracture releases hard abrasive metal particles. Such a situation arises in bearing applications where no lubricant is permitted because of environmental restrictions. For such demands bearings coated with several microns of hard compounds such as Tic and TiN exhibit dramatically improved behavior as illustrated in Figure 12-13d. In this case a TiC-coated ball contacts an uncoated race. Longer bearing life accompanied by lower noise and vibration occurs 12.4 Tribology of Films and Coatings 579 because of several beneficial effects. When the steel impinges on Tic almost no microwelding or adhesion occurs between these dissimilar materials. Upon contact the harder Tic tends to flatten the raceway asperities by plastic deformation. This process accompanied by a smaller temperature increase than during microweld fracture leads to lower wear .