The effects of the H/E ratio of various Cr–N interlayers on the adhesion strength of CrZrN coatings on tungsten carbide substrates

Abstract

Abstract In this work, various Cr–N interlayers were synthesized to improve the adhesion strength of CrZrN coatings on a WC-6 wt.% Co substrate. The hardness to elastic modulus ratio (H/E ratio) of the Cr–N interlayers was controlled to obtain the optimum combination of Cr–N interlayer, CrZrN coating, and WC-6 wt.% Co substrate. The crystalline phases of the Cr–N interlayers were synthesized using various N2 partial pressures from 0 to 0.28 Pa, and the hardness and elastic modulus of the synthesized Cr–N interlayers were measured to be in the ranges of 7 to 28 GPa and 335 to 357 GPa, respectively. This resulted in H/E ratio variations from 0.021 to 0.079. The results from a scratch test showed that a CrZrN coating with a single phase CrN interlayer exhibited the highest adhesion strength (46 N), while the same coating with a Cr interlayer showed the lowest adhesion strength (9 N). The improved adhesion strength could be attributed to the H/E ratio of the Cr–N interlayer in that there existed a gradual decrease in the H/E ratio from the CrZrN coating, to the single phase CrN interlayer, and to the WC substrate (0.090, 0.068, and 0.045, respectively). The coating structure with a gradual decrease in H/E ratios induces a smooth transition of stress in the coating under a loading condition. The adhesion strength of the CrZrN coatings could be improved significantly by structuring the coating with an optimal H/E ratio gradient.