Hardness characterisation of thin Zr(Hf,N) coatings

In this study, depth-sensing Vickers indentation tests were conducted on Zr(Hf,N) coatings, containing different amounts of Hf (0–21 wt.%). The coatings were deposited on hardened AISI D2 cold-work tool steel with a physical vapour deposition technique. Hardness characteristics of the coatings were analysed under six different indentation loads (0.01–0.1 N) according to load and energy methods. The same Vickers hardness numbers were obtained from the conventional load method under indentation loads that create indents having depths lower than 10% of the coating thickness. An energy method, which was derived from the work of indentation, gave similar Vickers hardness numbers over a wide range of indentation depths (up to 25% of the coating thickness). The hardness numbers of Zr(Hf,N) coatings calculated by these two methods, which are independent of the indentation depth, did not change with increasing Hf content of the coatings. The average Vickers hardness number obtained from the energy method is approximately half that from the load method. After correcting the conventional-load-method HV numbers according to the Oliver and Pharr procedure, the difference between HV numbers calculated by load and energy methods significantly decreased.