Microstructure and mechanical behavior of arc-evaporated Cr–N coatings

Abstract

Abstract Cr–N coatings were grown by arc evaporation onto high speed steel substrates. The coatings were each grown using a different negative substrate bias voltage, VS, between 20 and 400 V. X-ray diffraction showed a microstructure containing primarily the NaCl structured CrN phase along with the BCC-Cr and HCP-Cr2N. Auger electron spectroscopy and transmission electron microscopy indicate a substoichiometric (N/Cr=0.85±0.08) composition and dense columnar microstructure, respectively. At VS=400 V, equiaxed grains and microcracks parallel the substrate–coating interface were observed. The fiber textured coatings are in a compressive residual stress state that increases from 2.9 (VS=20 V) to 8.8 GPa (VS=100 V). At higher bias voltages, a decrease of the compressive residual stress is seen, which is discussed in terms of lattice defect annihilation in the collision cascade and lattice defect diffusion during deposition. Nanoindentation showed a maximum hardness at VS=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 from plastic deformation to cracking, which occurs at lower applied loads when VS 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 VS=200 V. The influence of the microstructure on the deformation transition is discussed in terms of lattice defect density and the presence of equiaxed grains.