Superior oxidation resistance, mechanical properties and residual stresses of an Al-rich nanolamellar Ti0.05Al0.95N coating prepared by CVD

Abstract

The influence of microstructure on thermal and oxidation properties of Ti1-xAlxN hard coatings is not fully understood. In this work, an Al-rich Ti0.05Al0.95N coating with a unique self-organized microstructure of alternating soft hexagonal (w) AlN and hard cubic (fcc) TiN nanolamellae synthesized by a low-pressure chemical vapor deposition (CVD) process is analyzed in terms of oxidation resistance, microstructure and phase stability, hardness as well as residual stresses. Multiple coating samples on hard metal substrates were oxidized in ambient air for 1h at temperatures in the range of 700–1200°C. Exceptionally good oxidation resistance up to 1050°C was found, whereas above 1100°C a localized surface degradation caused by the substrate–air interaction with very characteristic oxide blisters was observed. A coating hardness of about 29GPa remained in all unaffected surface areas for temperatures up to 1050°C. This is interpreted by the specific lamellae microstructure that retained a nanocomposite character even after lamellae partly decomposed during annealing. Depth-resolved stress and phase characterization performed by cross-sectional X-ray nanodiffraction in the as-deposited Ti0.05Al0.95N revealed a strong compressive residual stress gradient of up to −2GPa at the coating surface which homogenized but remained still compressive after annealing at 1050°C. Finally, it is demonstrated that the unique dense nanocomposite microstructure is responsible for the superior oxidation behavior as well as high compressive stresses in the Ti0.05Al0.95N coating.