Effect of thermal treatments in high purity Ar on the oxidation behavior of arc-PVD c-Al0.66Ti0.33N coatings
The effect of temperature in high purity Ar (low oxygen partial pressure) on the oxidation and crystal phase evolution of c-Al0.66Ti0.33N arc-PVD coatings was investigated. High temperature tribology in Ar jet and adhesion behavior of the oxidized coating were addressed. The use of Ar protects the coating from the oxidation reactions and allowed to shed light into the details and paths of the nitride oxidation process. The c-Al0.66Ti0.33N nitrides were slightly oxidized in Ar even at high temperature. The surface chemistry evaluation shows very thin Al- and Ti-based oxide layers formation after 700C in Ar. However, the onset formation of the rutile TiO2 oxide was detected only after 900C and was clearly found by low-angle XRD diffraction after 1000C. XPS analysis of the oxidized samples at 800C indicated the formation of extremely thin layers mainly composed of mixed oxides a-Al2O3 g-Al2O3 and r-TiO2. Gradual changes in the chemical composition of the oxidized coatings observed at 900C clearly demonstrate the formation of gradient boundaries between Al-rich and Ti-rich oxy-nitride layers. After 1000C at least four oxides and oxi-nitrides layers and interlayers were found. Additionally, it is suspected that the transition oxide phases such as c-TiO, c-Al0.54Ti2.46O0.28N4.58, alpha-Al2TiO5 and TiOxNy might be precursors leading to the formation of thermodynamically stable rutile and alumina phases. Surface inspection after high temperature tribology in Ar jet showed formation of tensile cracks in the wear tracks and wear processes due to high size particle adhesion-abrasion and low size particle micro-ploughing of hard nitrides, oxi-nitrides and brittle oxide particles. Adhesion tests performed after high temperature tribology in Ar showed critical load decrease related to cohesive and adhesive damages at the contact point of the scratch track.