The fine-scale microstructure of thin hard TiN and TiC coatings on steels

Abstract X-ray diffraction (XRD) and transmission electron microscopy (TEM) have been used to investigate the fine-scale microstructure of TiN and TiC obtained by chemical vapour deposition (CVD) and a range of proprietary TiN coatings obtained by physical vapour deposition (PVD) (all 2–5 μm thick) deposited on steels by various commercial processes. The major objective has been to examine the variation between, and variability within, apparently similar coatings. TEM involved both plan-view and edge-on specimens. While selected-area diffraction patterns in the transmission electron microscope reflect local variability and some degree of randomness in coating texture, integration over a larger specimen volume by XRD reveals 220∼ and 111∼ preferential orientations in the CVD and PVD samples respectively. XRD further showed that the coatings are sometimes non-stoichiometric while, for some samples, X-ray mapping indicated trans-interfacial diffusion between titanium from the coatings and chromium from the substrate during coating deposition. TEM revealed considerable point-to-point variability in grain structure and grain size within a given sample and from sample to sample with a high degree of in-grain defect (dislocation line and loop) contrast. The results are presented and discussed in terms of the microstructural influence over the subsurface deformation response controlling hardness, friction and wear.