X-ray nanodiffraction investigation of stress gradients in textured CVD alpha-alumina coatings

Purpose and goal

The goal of the project was to use transmission X-ray nanodiffraction in order to investigate the residual stress gradients along the growth direction in state-of-the-art CVD textured TiCN-a-Al2O3 wear-resistant multilayer coatings, and how these residual stress gradients are effected by post-treatment of the coatings. Information that would hard to obtain using other experimental techniques. Coatings with three different textures for the a-Al2O3 layer, and with and without post-treatment, where successfully investigated at P03, Petra III, Hamburg.

Expected results and effects

From the obtained diffraction data, it was possible to extract how the residual stress gradients varied in the coatings with submicrometer spatial resolution. It was also possible to not only extract how the post treatment protocol relaxed the residual stress gradients but how the protocol influences the microstructure in the coatings by introducing defects and how this varies through the depth of the coatings. The latter is something that is of high interest to optimize for improved coating performance and where X-ray nanodiffraction is a unique technique to investigate this.

Planned approach and implementation

Samples for synchrotron nano-diffraction were cut from as-deposited and treated inserts, using a combination of mechanical cutting and focused ion beam. The beamsize and energy at the beamline was 350 nm and 12.98 keV. Scanning the sample in the beam and collecting diffraction patterns at different positions through the coating thickness allowed determination of the residual stresses gradients. By extracting the interplanar spacing of planes as a function of azimuthal angle along the collected Debye-Scherrer rings, standard sin2psi method could be used to calculate the stress.