On the high temperature stability of γ-Al2O3/Ti0.33Al0.67N coated WC–Co cutting inserts

Abstract

Abstract The high temperature stability of γ-Al2O3 films deposited using filtered cathodic arc and plasma assisted chemical vapor deposition on Ti0.33Al0.67N coated WC–Co cutting inserts is investigated. X-ray diffractometry reveals that filtered cathodic arc deposited films transform partially into the thermodynamically stable α-Al2O3 phase at a temperature of 1000°C. The γ to α-Al2O3 transformation for plasma assisted chemical vapor deposition grown films is observed at 900°C. These results are in qualitative agreement with differential scanning calorimetry measurements. Transmission electron microscopy on filtered cathodic arc and plasma assisted chemical vapor deposition films annealed at 900°C reveals the existence of hexagonal AlN in the Ti0.33Al0.67N interlayer, as well as Al depletion at the Al2O3/Ti0.33Al0.67N interface. After annealing the plasma assisted chemical vapor deposition sample at 900°C, α-Al2O3 grains with a size of ∼100 nm are observed inside the γ-Al2O3 matrix, while for filtered cathodic arc samples only the γ-phase is identified. Transmission electron microscopy analysis on both filtered cathodic arc and plasma assisted chemical vapor deposition samples annealed at 1000°C shows that the original Al2O3/Ti0.33Al0.67N/WC–Co layer architecture is no longer intact. The formation of TiO2 is detected along the growth direction of the Al2O3 films. The present study suggests that not only the morphology and the impurities incorporated into γ-Al2O3 but also stability of the Ti0.33Al0.67N interlayer determine the high temperature stability of γ-Al2O3/Ti0.33Al0.67N coated hardmetal.