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NITRIDING OF PURE TITANIUM BY HIGH DENSITY PLASMA USING H2-N2 GAS MIXTURE AT LOW TEMPERATURE

Most plasma nitriding of titanium was successfully carried out at a temperature higher than 500 oC. A lower temperature process is needed to save energy. The experiment was carried out by varying the substrate temperature from 350 to 450 oC. The plasma was generated in a vacuum chamber by a 2 MHz RF power supply. A gas mixture of nitrogen and hydrogen was introduced into the chamber resulted in a significantly effective nitriding of the titanium. The plasma was intensified to a higher density by utilizing a combination of DC bias technique and a hollow cathode. At the optimum mixture of 80% N2 and 20% H2, by the pressure of 35 Pa, a four-hour nitriding resulted in a 20 μm nitrided layer. The hardness of the treated surface was varied between 550 and 1990 HV depending on the process temperature. XRD measurements showed that the nitriding leads to the formation of Ti2N and TiN precipitates. Expansion of the original hcpstructured lattices was also noticed. The hardening was a result of a lattice straining induced by the precipitates and also a possible small percentage of nitrogen solid-solution.