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

• CrZrN coatings with various Cr–N interlayers were synthesized on WC substrate.
• H/E ratios of Cr–N interlayers varied from 0.021 to 0.079 due to phase change.
• Single phase CrN interlayer showed a median value of H/E ratio to be 0.068.
• CrZrN coating with CrN interlayer showed the highest critical load (Lc3, 46 N).
• Structuring with an optimal gradient of H/E ratio improves adhesion strength.

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.