Microstructure and wear mechanisms of textured CVD alumina and titanium carbonitride coatings

Abstract: The aim of this thesis is to find the reasons behind how the wear performance of hard coatings produced by chemical vapor deposition (CVD) is influenced by their texture, as this is today not fully known. Therefore, differently textured coatings were synthesized, and subsequently analyzed both before and after specially designed machining tests. The main research methods used are analytical electron microscopy, transmission Kikuchi diffraction, electron backscattered electron diffraction, X-ray diffraction, atom probe tomography and Schmid factor simulations. The microstructure, texture and facet development of the as-deposited α-Al2O3 coatings were determined, and the effect of catalyzing gas and diffusion of heavy elements from the substrate were described. After machining tests, terrace formation at the edges is attributed to crystallographic dependent etching. More deformation occurs for the textured coatings in the transition zone, with an associated sub-surface dislocation formation coupled to the number of activated slip systems. The surface morphology in the sliding zone is mainly affected by the surface developed in the previous zones. For the (0001)-textured coating, the observed low wear rate is attributed to homogeneous basal-slip dominating plastic deformation, while for the (01-12) and (11-20) textures the main deformation mechanism is heterogeneous plastic deformation, causing micro-rupture and abrasion, leading to higher wear-rates. For titanium carbonitride coatings, the (211)-textured coating exhibits a more significant and non-uniform deformation than (110), which is related to a heterogeneous response of the relevant slip system. In conclusion, the results presented in this thesis reveal the complex relationships between local wear mechanisms and coating texture. This fundamental understanding can facilitate future development of texture-controlled CVD α-alumina and titanium carbonitride coatings, with the potential of further improving the performance of coated cutting tools.