Volume 8, Issue 7 (July 2011)
Impact Wear Performance of Thin Hard Coatings on TiC Cermets
During metal forming processes like deep drawing of steel, tools are subjected to various loads. Besides adhesion, abrasion, and tribo-oxidation, successive impacts during forming induce Herzian contact pressures and cause fatigue failure. The present study focuses on TiC-based composites with Ni/Mo binders and thin coatings (TiN, (Ti,Al)N, TiCN, nc-(Al,Ti)N/α-Si3N4) produced by the physical vapor deposition technique. The adhesion of the coatings was characterized by Rockwell test and nanoscratch tests in single- and multi-scratch mode. The impact wear tests were performed using the rotating hammer-type surface fatigue wear tester for single spot testing. The effect of substrate surface roughness on the overall substrate/coating system was investigated. Selected surface contact damage sites in all materials were subjected to close examination by high power optical microscopy. Nanocomposite nc-(Al,Ti)N/α-Si3N4 coatings showed the highest hardness value. In addition, these coatings also have lower modulus values, which make them the best candidate for demanding wear applications. TiCN coatings have the second highest hardness value, but due to poor adhesion qualities, especially at higher roughness values, delamination was found to be present even directly after the coating process without any load. This is assumed to be related to the higher inherent residual stresses of TiCN coatings with TiC–Ni/Mo cermets. In this system the gradient nature of the top layer and presence of more ductile additional layer provide better performance in impact wear conditions.