The Microstructure and Properties of Transition Metal Nitride Nanocrystalline Coatings

The composition, microstructure, mechanical properties and biocompatibility of a number of tantalum nitride coatings, deposited on Ti-Al-V substrates by the double cathode glow discharge technique, were investigated. With increasing nitrogen partial pressure, the composition of the tantalum nitride coatings changed from hexagonal Ta¬2N to fcc TaN. Both coatings exhibited a nanoporous structure comprising fine (~ 10 nm) equiaxed grains together with a homogenous distribution of (~5-10 nm) nanopores. The relatively high hardness and low elastic modulus of these coatings led to improved damage resistance and wear resistance for these tantalum nitride coatings. Further, Ta2N-based coatings also showed good compatibility with hydroxyapatite. However, the introduction of oxygen during the deposition process led to significant degradation of the coating hardness, wear resistance and damage resistance of these coatings. This was due to the presence of Ta2O5 and an amorphous tantalum oxynitride phase arising from the higher oxygen pressure during deposition. In addition to these tantalum nitride coatings, the composition, microstructure, and mechanical properties of zirconium nitride coatings again deposited by double cathode glow discharge technique were also investigated. Zirconium nitride coatings were deposited on both stainless steel and Ti-Al-V substrates. The zirconium nitride coatings on the stainless steel substrates exhibited a bimodal microstructure with both fine grains and more elongated coarser grains. In contrast, the coatings on the Ti-Al-V substrates showed a uniform microstructure comprising fine equiaxed grains and together with a number of nanopores. All these coatings showed comparable hardness values, but lower elastic modulus values, compared with zirconium nitride coatings deposited by other deposition techniques. Therefore, a relative high wear resistance and damage resistance may be expected for the zirconium nitride coatings studied in this thesis.By exploring the influence of nanopores on the mechanical properties of nanocrystalline Ta2N coatings, it was found that the presence of nanoporosity may increase wear resistance and damage resistance of these coatings by significantly reducing elastic modulus without greatly decreasing hardness. This suggests a promising new strategy for improving the mechanical properties of nanocrystalline coatings