Structural-phase and morphological features of the surface layers of the SHS TiNi

Porous biocompatible TiNi-based alloys obtained by the method of self-propagating high-temperature synthesis (SHS) are used as an implant material. Surface properties are important for the biocompatibility of porous nickelide titanium implants. However, the causes of high corrosion resistance have not yet been studied in detail and require further study. The difficulty of directly studying the surface of a porous alloy explains this situation. The article presents a comprehensive study of the structural-phase and morphological features of the TiNi surface layers, obtained by the SHS method in the layer-by-layer burning regime. By means of XRD, SEM and optical microscopy, it was shown that the SHS process produces thin but dense surface layers of Ti₄Ni₂(O,N,C) intermetallic oxycarbonitrides that cover the entire surface. It is established that the surface layer consistently consists of three layers: the foam layer, the upper and lower dense layer. A nanocrystalline intermetallic foam, which is carried by the reaction gases, forms the first non-uniform layer. This layer is most massive in large open macropores and absent in closed pores. The second layer is the upper part of a gradient dense layer with a thickness of 20–100 nm. The lower part of the third gradient dense layer is variable and amounts to 750–1500 nm. The EDS method revealed a gradient of concentration composition in the upper and lower dense layers of the alloy. It is assumed that the cause of the concentration gradient is the diffusion of nitrogen into the surface of the peritectic melt.