tailieunhanh - Metal Machining Episode 5

Tham khảo tài liệu 'metal machining episode 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Tool materials 113 -phase . In any case using a P-type substrate would give added life to a tool used to cut steels once the coating wore off or if it failed. To compensate for loss of toughness on changing from K- to P-type such substrates were typically manufactured with a greater Co for a given grade of duty than if they were uncoated. Thus began the development of special substrate compositions for coated tools. By the early 1980s substrates were being manufactured with surface layers containing from to 3 times the amount of cobalt to that in the bulk and from 10 mm to 30 mm thick on near WC-Co bulk compositions. Toughness is maintained near the surface without reducing the hardness of the bulk. Considering the high thermal expansion coefficient of cobalt Table the surface layer of the substrate is better thermally matched to the coating materials and thermal strains are reduced. CVD-coated tools began to find uses in interrupted turning and light milling operations. Considering the thicknesses of both the coatings and modified substrate surface layers the composition and hence thermal and mechanical properties of CVD-coated tools can vary over depths of up to around 40 mm. This is not insignificant relative to the size of the stressed and heated regions during cutting. Detailed understanding of the interactions between the graded surface compositions and the mechanical and thermal fields generated in machining leading to still further improvements in tool design continues to develop. PVD coatings An alternative process for manufacturing coatings is Physical Vapour Deposition PVD . It is similar to CVD in its productivity in its basic form deposition rates are also around 1 mm hr but requires substrates to be heated only to a few 100 C say 500 C so coatings can be deposited without the need to guard against unfavourable changes to the substrate. In contrast to CVD in which the metallic elements of the coating are obtained from gases at around 10 .