Investigation of porous NiTi coatability on the surface of Co-Cr-Mo alloy via SHS method

In this study, porous NiTi was coated on the surface of Co-Cr-Mo alloy by using SHS method and microstructural and mechanical examinations were performed in these produced samples. Metal powders determined as 50.5% Ni and 49.5% Ti in terms of atomic percentage were mixed homogenously for 24 hours by adding ceramic balls in a specially designed mixer. Then the mixtures prepared were cold-pressed under 50-100-150-200 MPa pressure in a specially designed die. Cold pressed samples were joined by being diffused for 30 minutes under 5 MPa pressure at five different temperatures as 950, 1050, 1100, 1150, 1200 ºC under argon gas atmosphere in the induction melting furnace. While microstructure examinations of samples, which were produced after diffusion, following the metallographic process were determined by using OM, SEM, EDS, and EDX, their transformation temperatures and enthalpies were determined by using DSC-DTA. Then the density of porous NiTi produced by using SHS method was calculated according to Archimedes' principle and microstructures of diffused sample were taken and thus their experimental studies were completed. As a result of the experimental studies; it was observed that a more efficient bonding occurred together with the increase in the diffusion temperature and contrarily the density linearly increased based on the increase in the diffusion length, considerable grain coarsening in the Tamz main alloy, micro hardness and cold pressing pressure; whereas the porosity linearly decreased based on the micro cracks and cold pressing pressure. When the interfaces bonded at 950 and 1050 ºC diffusion temperatures were examined as a result of the microstructure examinations; it was decided that such temperatures were insufficient because a poorly bonded interface with significant openings, i.e. micro cracks was obtained. Examining the EDX results received from the top region of NiTi section together with the increasing diffusion temperature; it was determined that when low presence of nickel in the structure in other words, presence of nickel by forming components in inner regions is considered as biomaterial, it is a required result. According to the DSC measurement of NiTi alloy produced via SHS method that was taken by 10 ºC/min. heating and cooling rates; while the Austenite start temperature was (As) 92.1 ºC and Austenite finish temperature was (Af) 122.5 ºC; martensite start temperature (Ms) was 61.6 ºC and martensite finish temperature was 43.2 ºC. Enthalpy transformation during Austenite transformation (ΔH) was determined as 4.58 J/g and enthaply change value during martensite transformation (ΔH) was determined as 4.04 J/g. According to the TG measurements, there was no increase in the alloy mass up to approximately 600 oC. There was a logarithmic increase in the mass after 600 oC. The reason of this increase can be explained as the increase in the mass due to the oxidization of the NiTi alloy. In this study; it was seen that porous NiTi coating with SHS method on the Co-Cr-Mo alloy had the best microstructure and some mechanical properties at the diffusion temperature of 1200 ºC.