Relationships between hardness, Youngʹs modulus and elastic recovery in hard nanocomposite coatings

The paper is devoted to an assessment of the mechanical behavior of hard and superhard nanocomposite coatings from loading/unloading curves measured by a computer-controlled Fischerscope H 100 microhardness tester and a maximum depth dmax of the diamond indenter impression into the coating at a given load L. It is shown that: (1) the area between the loading/unloading curve and the value of dmax decreases with increasing (i) hardness H, (ii) effective Youngʹs modulus E*=E/(1−ν2) and (iii) universal hardness HU, where E and ν are the Youngʹs modulus and the Poisson ratio, respectively; and (2) there is no simple relation between the mechanical response of the coating and H or E* alone; however, this response is strongly dependent on the ratio H/E*. The last fact gives a possibility of tailoring the mechanical properties of a coating for a given application, e.g. to prepare coatings with high hardness H, high resistance to plastic deformation (∼H3/E*2), high elastic recovery We, but with low E* and high dmax. Special attention is also given to the analysis of problems in accurately measuring the hardness of superhard (≥60 GPa) coatings. It is shown that a high elastic recovery We≥80% of superhard films with H≥60 GPa (1) strongly decreases the gradient dH/dL and (2) shifts the region L, where H(L)≈constant and the hardness H is correctly measured, to higher values of L. This means that the lowest load L used in the hardness measurement must be higher than L used in measurements of coatings with H<60 GPa to prevent the value of H measured from being significantly higher than the real hardness of the coating.