THE FINE-SCALE MICROSTRUCTURE OF THIN HARD TIN AND TIC COATINGS ON STEELS

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-mu-m 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.