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Embedding grains of a hard, high yield strength phase into a ductile matrix has been widely explored in macrocomposites made of ceramics and metals, which are known as cermets. However, if grain sizes and matrix separations are reduced to a nanometer level, dislocation activity as a source of ductility is eliminated and new mechanisms are required to provide toughness enhancement. Uniquely, these nanocomposites containa high volume of grain boundaries with a crystalline/amorphous transition across grain–matrix interfaces, limiting initial crack sizes and helping to deflect and terminate growing cracks. These mechanisms helped to explain the fracture resistance of novel super-hard composites, where both amorphous ceramic and metal matrices were used to encapsulate nanocrystalline ceramic grains. More on the super-hard nanocomposite coating design and discussion of the active mechanisms can be found in available review articles. Grain boundary diffusion and grain boundary sliding were suggested as mechanisms for improving ductility and providing super-plasticity of single phase ceramic nanocrystalline systems. The recent research indicates also that high ductility can be more easily achieved in multiphase structures and that grain boundary sliding can be a primary mechanism of super-plasticity. It was also found that equiaxial grain shapes, high angle grain boundaries, low surface energy, and the presence of an amorphous boundary phase facilitate grain boundary sliding. The ongoing verification of possible deformation mechanisms also indicates the importance of nanovoid opening and nanocrack branching around nanocrystalline grain inclusions for crack energy dissipation. These findings were used to develop tough tribological nanocomposite coatings, whose design is schematically shown in Fig. 1. The design includes the following main concepts: (1) a graded interface layer is applied between the substrate and crystalline/amorphous composite coating to enhance adhesion strength and relieve interface stresses (combination of functional gradient and nanocomposite designs). (2) encapsulation of 3–10 nm sized hard crystalline grains in an amorphous matrix restricts dislocation activity, diverts and arrests macrocrack development, and maintains a high level of hardness similar to super-hard coating designs; (3) a large volume fraction of grain boundaries providesductility through grain boundary sliding and nanocracking along grain/matrix interfaces. These design concepts provide toughness enhancement through stress minimization, crack deflection, and ductility. They are close but still differ from the design concepts of super-hard nanocomposite coatings. The primary differences are viewed in the selection of a matrix phase with a lower elastic modulus, relaxation of the requirement for strong binding between the matrix and grains, and selection from a greater range of acceptable grain sizes of the nanocrystalline phase that is embedded in an amorphous matrix. The conceptual design in Fig. 1 provides both high cohesive toughness and high interface (adhesive) toughness in a single coating. Several examples of tough wear resistant composite coatings are provided. Two of them combined nanocrystalline carbides with an amorphous DLC matrix designated as TiC/DLC and WC/DLC composites. In another example, nanocrystalline YSZ grains were encapsulated in a mixed YSZ–Au amorphous matrix as shown in Fig. 2. In all cases, the large fraction of grain boundary phase provided ductility by activating grain boundary slip and crack termination by nanocrack splitting. This provided a combination of high hardness and toughness in these coatings. Fig. 3, compares Vickers indentations made at the highest load of the machine (1 kg). There are no observable cracks in these coatings, even after significant substrate compliance (indentation marks are 9 lm deep into 1 lm thick coatings). The coating hardness was quite high ranging from 18 to 30 GPa, and for most other typical hard coatings at these loads, cracks in the corners of the indentations are expected. Thus, novel nanocomposite designs of tough tribological coatings are very promising. They explore fundamentally different concepts of toughness improvement, when compared to macrocomposite materials such as cermets. Furthermore, their design can be taken to the next level by realizing that both matrix and nanograins can serve as reservoirs of solid lubricants. These lubricants will be then released in a friction contact during the course of sliding. The active tribological role of matrix and encapsulated grains is a basis for the tough and low friction ‘‘chameleon’’ coatings, which are discussed in the next section. |
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