Microstructure and mechanical behavior of are-evaporated Cr-N coatings

Cr-N coatings were grown by are evaporation onto high speed steel substrate s. The coatings were each grown using a different negative substrate bias v oltage, V-S, between 20 and 400 V. X-ray diffraction showed a microstructur e containing primarily the NaCl structured CrN phase along with the BCC-Cr and HCP-Cr2N. Auger electron spectroscopy and transmission electron microsc opy indicate a substoichiometric (N/Cr = 0.85+/-0.08) composition and dense columnar microstructure, respectively. At V-S = 400 V, equiaxed grains and microcracks parallel the substrate-coating interface were observed. The fi ber textured coatings are in a compressive residual stress state that incre ases from 2.9 (V-S = 20 V) to 8.8 GPa (V-S = 100 V). At higher bias voltage s, a decrease of the compressive residual stress is seen, which is discusse d in terms of lattice defect annihilation in the collision cascade and latt ice defect diffusion during deposition. Nanoindentation showed a maximum ha rdness at V-S = 100 V of 29 GPa. The critical loads for cohesive failure in a scratch test decreased monotonically with increasing negative substrate bias. The scratch results suggest a transition in deformation mechanism fro m plastic deformation to cracking, which occurs at lower applied loads when V-S is increased. Similar behavior was also seen in a crater grinding wear test where a shift in wear mechanism from plastic deformation to chipping occurred at V-S = 200 V. The influence of the microstructure on the deforma tion transition is discussed in terms of lattice defect density and the pre sence of equiaxed grains. (C) 1999 Elsevier Science S.A. All rights reserve d.