Dry-Blasting of α- and κ-Al₂O₃ CVD Hard Coatings: Friction Behaviour and Thermal Stress Relaxation

Post-deposition blasting treatments of cemented carbide cutting tools coated by chemical vapour deposition have gained increasing interest. Within the present work, α- and κ-Al₂O₃ coatings dry-blasted using different pressures as well as globular and edged blasting materials were investigated. A higher pressure resulted in an increase in the compressive residual stresses, as determined by X-ray diffraction. Both blasting materials had a minor influence on the roughness of α-Al₂O₃, while the edged material caused a significant roughness increase for κ-Al₂O₃. For α-Al₂O₃ dry-blasted using the globular material, complete stress relaxation could be observed after annealing at 500 °C, whereas for α-Al₂O₃ dry-blasted using the edged material as well as for the κ-Al₂O₃, an annealing temperature of 900 °C was necessary for stress relaxation. The friction coefficient of the dry-blasted samples first increases with increasing temperature, due to increasing plastic deformation of the blasting material transferred onto the coating surface, until it decreases again above 700 °C, due to softening of the transferred material.