Residual stress gradients in α-Al2O3 hard coatings determined by pencil-beam X-ray nanodiffraction: The influence of blasting media

• Stress-depth-profiles for CVD α-Al2O3 coatings determined by X-ray nanodiffraction
• Pencil-shaped X-ray beam enables cross-sectional studies on coarse grained coatings.
• Different stress-depth-profiles introduced by different blasting media highlighted
• Kinetic energy and contact radius of blasting medium determine stress-depth-profile.
• Stresses are widely relaxed after annealing at 900 °C.

Post-deposition blasting treatments are widely used to introduce compressive residual stress into CVD hard coatings, which are typically in a tensile stress state after deposition on cemented carbide substrates. Within this work, α-Al2O3 coatings grown by CVD on TiCN base-layers were dry-blasted using a globular as well as an edged blasting medium and subsequently annealed at 900 °C. The as-deposited, blasted and annealed samples were characterized using cross-sectional synchrotron X-ray nanodiffraction using a pencil X-ray beam of 10 μm × 100 nm in size as well as complementary synchrotron energy dispersive and laboratory monochromatic X-ray diffraction. The results document that the maximum compressive stress of 4 GPa in the samples blasted with the edged medium is significantly higher compared to the samples blasted with the globular medium, which showed maximum compressive stress of 2 GPa. The stress gradient obtained after blasting with the edged medium is steeper, while the zone with compressive stress reaches deeper into the coating for the samples blasted with the globular medium. In the substrate, significantly increased compressive stress of 400 ± 60 MPa compared to 90 ± 30 MPa in the as-deposited state was observed only after blasting with the globular medium and relaxed fully after annealing. In addition, the observed stress gradients were corroborated by the particle impact using a finite element contact mechanics approach.