Tham khảo tài liệu 'modern tribology handbook 2010 part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | FIGURE Tribologically important properties in different zones of the coated surface. for yield stress based on the layer spacings. Bunshah and co-workers also deposited microlaminate coatings by evaporation techniques both metal metal Bunshah et al. 1984 and metal ceramic Sans et al. 1983 couples being studied. In general an improvement in mechanical properties was observed for decreasing layer thicknesses. In recent years the literature and at a certain level the theoretical understanding of multilayer films has grown in particular with regard to compositionally modulated superlattice thin films Barnett 1993 Barnett and Shinn 1994 . However many of the theories developed are applicable to highly epitaxial or single-crystal layers and while considerable progress has been made in understanding the theoretical aspects of superlattice hardening and strengthening mechanisms their value in practical tribological systems is somewhat limited. There are several reasons for this not the least of which is the difficulty in growing true superlattices . epitaxial layers on real polycrystalline engineering components. A more practical approach is to consider how the requirements of a surface differ at different locations within it that is at the interface with the substrate within the coating itself and at its surface as shown by Figure . Multilayer coatings offer the possibility of designing the surface according to such requirements Holleck 1986 1990 Holleck and Schier 1995 Voevodin et al. 1996b . This is the concept behind the functionally graded coatings discussed in Section . Holleck tended to use multilayers of different ceramic materials . TiC and TiB2 selected on the basis of their dominant bonding mechanism . metallic covalent or ionic . He demonstrated improvements in hardness indentation toughness adhesion and wear performance under optimized layer thickness conditions Holleck et al. 1990 and attributed this in part to the crack deflection .
