High pressure reactive plasma spray synthesis of titanium nitride based coatings

The term thermal spray covers a wide range of fabrication methods for surface engineering applications. A very promising thermal spray,process involves the production of protective coatings or freestanding components by reactive plasma spray (RPS) techniques. Using reactive gases such as nitrogen or methane it is possible to synthesise hard nitride and carbide phases in reactive metals such as Ti, Cr, and Al. In this investigation RPS was used to produce composite titanium nitride/titanium coatings. Commercially pure titanium powders were used as raw materials and sprayed in a nitrogen atmosphere at up to 300 kPa. The process was carried out by means of a controlled atmosphere plasma spray system, allowing spraying in the pressure range 10-400 kPa. The effects of chamber pressure, plasma gas composition, and spraying distance on the scope of the expected reactions were evaluated. Phase analysis, measurements of nitrogen content and Vickers microhardness, and metallographic investigation of the as sprayed coatings revealed that titanium hard phases were synthesised during spraying and became embedded in the titanium matrix. High pressure significantly enhanced nitride formation (Ti2N, TiN, TiN1-x) in the titanium matrix. Microhardness values up to 1433 HV0.3 were obtained, with maximum nitrogen content of about 11.1 wt-%. Studies on sprayed particles showed that TiN was the dominant phase formed during flight, suggesting that diffusion processes play an important role in the substrate. Compared with pure titanium deposits, coatings produced by RPS are much harder and have better tribological properties.