Micromechanical estimation of composite hardness using nanoindentation technique for thin-film coated system

The hardness characteristics of thin-film coated composite were analyzed using the plastic-zone volume-law of mixtures theory. In calculating the plastic-zone volumes, the elastic/plastic indentation stress field was analyzed; the substrate was assumed to undergo radial deformation, while the cylindrical deformation mode was applied to the film. In addition, the effect of interface was incorporated. The deformation of the softer material was assumed to be constrained by the harder one under the condition of strain matching at the interface. In experiments, the hardness (H) and elastic modulus (E) of Al2O3 film, glass, and 304 stainless steel were evaluated by analyzing the load–depth curve measured by nanoindentation technique. And, the yield strength (Y) was calculated using the relationships between H, E, and Y. Next, we measured the composite hardness values of Al2O3/glass and Al2O3/304 stainless steel systems with increasing load up to 100 mN, and compared with those predicted. For Al2O3/glass system, the measured trend of composite hardness correlated well with the predicted one. On the other hand, there were some deviations for Al2O3/304 stainless steel system, which was expected to result from the film cracking and/or the change of load distribution for hard coating on very soft substrate.