Influence of substrate hardness on the response of W-C-Co-coated samples to depth-sensing indentation

Depth-sensing indentation tests were used to determine the hardness of amorphous W-C-Co coatings deposited on different steel and copper substrates. The hardness of the film, Hf was chosen to be always greater than the hardness of the substrate Hs and within the range Hf/Hs = 2 to 18.5, The influence of the ratio Hf/Hs on the ratio (t/h(D))(C) between the film thickness t and the critical value of the indentation depth (h(D))(C), for which the substrate starts to deform plastically, was studied. Two independent methods were used to determine (h(D))(C) values, One utilized the differential analysis of the loading part of the indentation curve, and the other was based on the plot of (Hc - Hs)/(Hf - Hs) versus t/(h(D)), Hc being the measured hardness of the film/substrate composite at a given indentation depth (h(D)). A good correlation between both methods was found.