Structure, mechanical and tribological properties of thick CrNx coatings deposited by HiPIMS

Abstract

To address the high desirability of enhanced load capacity and durability of thick protective coatings in harsh working conditions, Cr/CrNx multilayer coatings with various thicknesses of 12–78 μm were deposited using high power impulse magnetron sputtering (HiPIMS). The microstructure, mechanical and tribological properties of Cr/CrNx coatings with different thicknesses were investigated by XRD, XPS, AFM, SEM, nano indentation, Vickers indentation and micro-tribometer, respectively. The coatings were deposited at a higher deposition rate up to 16.8 μm/h. With an increase in coating thickness, the coatings with the mixture CrNx phase containing Cr2N and CrN promote grain refinement and intrinsic stress. The increased thickness led to the improved mechanical property and the tribological properties. 78-μm-thick coating exhibits the highest value of the hardness of 26.6 GPa, H/E* of 0.075, H3/E*2 of 0.15, Rockwell C adhesion (above HF1 level) and fracture toughness of 6.3 MPa/m2, respectively. During dry wear test at a normal load of 10 N for the sliding time of 3 h, adhesion failure on worn surface were detected for 12 μm-thick coating with high friction coefficient (COF) of 0.75 and specific wear rate of 3.27 × 10−5 mm3N−1m−1. However, 78 μm coatings show smooth worn surface with a few abraded and blurry grooves with low COF of 0.55 and specific wear rate of 6.9 × 10−7 mm3N−1m−1 owing to the enhanced hardness, H/E*, H3/E*2, adhesion and fracture toughness. The problem of the balance between the influence of residual stress and mechanical/tribological properties is a key point in deposition of super-thick CrNx coating and that can be realize by the proportion control of Cr2N to CrN phases.