tailieunhanh - Mechanical Engineer's Reference Book 2011 Part 10

Tham khảo tài liệu 'mechanical engineer's reference book 2011 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Wear and surface treatment 9 81 textiles and filled plastics. It can also be applied to shafts gears or plain bearings in equipment handling abrasive slurries. If there is an element of corrosion then the substrate itself needs to be resistant. Alternatively TiN can be applied over the top of another coating for instance electroless nickel . It can be applied to carburized surfaces with only minimal tempering but if it is to be applied to a nitrided or nitrocarburized material the compound layer must be removed first. There are now PVD chambers that can coat components up to 3 m long and processing time and costs are being continuously reduced. PVD is a semi- line-of-sight process and so holes and re-entrant areas will receive less coating. Typically at the equivalent of one diameter deep down a hole the coating thickness will be 30 of that outside the hole. The coatings replicate the underlying surface texture without build-up on edges or corners. Finally PVD is a high-technology process. It is best suited for high-quality components bright clean free from oxide burrs and contaminants. Chemical vapour deposition64-65 Chemical vapour deposition CVD is in many ways a competitor of physical vapour deposition. The process is used to deposit metals and ceramics by the decomposition of a reactive gas at the surface of components placed within the chamber. For instance titanium carbide is produced by a reaction between titanium tetrachloride and methane or titanium nitride can be produced by replacing the methane with nitrogen or by using hydrogen and ammonia. Deposition temperatures are very high usually above 800 C. Thus there are potential problems with softening and distortion of substrates. In fact the CVD process is most commonly used to coat tungsten carbide tooling particularly the indexable inserts used for high-speed turning. The cobait-cemented carbide is an ideal substrate because it has a similar coefficient of thermal expansion to the TiC or TiN coatings. .

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