The influence of diamond chemical vapour deposition coating parameters on the microstructure and properties of titanium substrates

In contrast to their excellent mechanical properties, titanium alloys possess poor wear characteristics. Diamond coatings appear to be a promising solution for the wear problem. Using standard deposition parameters for silicon (diamond scratched surface; microwave chemical vapour deposition, 500 W, 800 degrees C for 4 h, 50 mbar with an atmosphere of 1% CH4 + 99% H-2) good coatings on pure titanium were obtained. The mechanical properties of the titanium are strongly influenced by the processing temperature and the gas atmosphere. As a result of the hydrogen adsorption, grain coarsening and formation of titanium hydride occur and subsequently the low cycle fatigue strength decreases by an order of magnitude. However, by annealing in vacuum (800 degrees C for 2 h, furnace cooled) the hydrogen can be removed again and the initial mechanical properties can almost completely be restored. Based on scanning electron microscopy micrographs, Auger analysis, X-ray diffraction and microhardness measurements a basic understanding of the microstructural changes was developed. The surface structure may be visualized as consisting of five zones, created during the deposition process (diamond layer/nucleation zone/reaction layer/gradient layer/affected substrate).