Preparation of millimeter-scale hard layer on the surface of titanium alloy via self-propagating high temperature synthesis combined with hot pressing sintering process

A hard and thick surface strengthening layer with high interfacial bonding strength was innovatively prepared via self-propagating high temperature synthesis (SHS) combined with hot pressing sintering (HPS) process. The thickness of the surface layer reached 1.1 mm, far exceeding the strengthening layer prepared by the traditional surface modification methods of titanium alloy. The raw materials of the hard layer were carefully designed by mixing powders of Ti, C, B and Ni, and the raw material of the matrix was Ti–6Al–4V powder. During the HPS process (1100 °C, 40 MPa), the Ti–Ni reaction released a large amount of heat, which promoted the Ti–C reaction and Ti–B reaction. The test results of X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) suggested that a high amount of Ti₆C_3.75 and TiB were finally generated in the hard layer, and the Vickers hardness value was as high as 1800 HV. Furthermore, the energy dispersive spectrometer (EDS) test results indicated that most Ni atoms diffused to the Ti–6Al–4V side, and NiTi₂ phase was mainly distributed at the interface. Obviously, metallurgical bonding was obtained, accounting for the high interfacial bonding strength of 162 MPa. The present work provided significant insights into the design of novel titanium alloy surface modification process.