Exploration of the plate bending model for predicting the hardness response of coated systems

In a previous paper, we described a new approach to understanding the hardness enhancements provided to ductile substrates by the application of thin hard coatings. The model, based on plate theory, examined the constraining effect of the coating (after cracking) on the ''up-welling'' movement of plastically displaced material (''pile-up'') in the substrate. The key concepts are that, this upthrust is resisted and constrained by elastic flexure of the coating around the periphery of the contact damage site (e.g. a hardness indentation) and this impedance to deformation appears as an increase in the system hardness. The model led to a plot which can also be used to predict the mechanical properties of coatings necessary to withstand any given scale of contact damage. In this paper the detailed appearance of the plot is further examined with respect to the effects of cracks of differing morphologies. Stronger experimental evidence is also presented to confirm this ''master curve'' or ''design curve'' which can be used in coating selection over a wide range of coated systems and contact scales. Our new data also shows how deviations from the master curve can be used to identify regimes in which differing mechanisms are playing some significant role. Thus our current refinements serve to map out the dominant mechanisms in the deformation response of hard coatings and shed light on exactly how the detailed indentation response of coatings can add significant resistance to contact damage of components, even after cracking of the coating has occurred. (C) 1997 Elsevier Science S.A.