Surface treatment of Ti-6Al-4V alloy by rf plasma nitriding

The Ti-6Al-4V alloy was treated by inductively coupled rf plasma nitriding. The effects of plasma-processing time in the range of 5-35 min on the microstructure and the mechanical properties of the plasma-nitrided Ti-6Al-4V samples were studied. The plasma power input was adjusted at 450 W and pure N-2 gas was introduced to establish a treatment pressure of 8.0-8.4 x 10(-2) mbar. The characteristics of the nitrided layers have been investigated by microhardness testing, surface roughness measurements, optical microscopy, and x-ray diffraction. The results show that the surface microhardness increases as the plasma- processing time increases to reach 2000 HV0.1 at a plasma-processing time of 35 min. A high nitriding rate of 2.81 mu m(2) s(-1) at a plasma-processing time of 25 min was achieved. The formation of the hard phases TiN, Ti2N, and Ti( N) in the Ti-6Al-4V surfaces are found to be the reason for the increased microhardness. Surface energy, yield strength and Young's modulus for the nitrided Ti-6Al-4V alloy were calculated from the Vickers microhardness data.