Production of Titanium-Nickel alloy metarial and investigation of their wear behavior

Shape memory alloys (SMA) are successfully used for their potential use as functional material in many engineering and medical applications. The technological importance of shape memory alloys is coming from their shape memory effect (SME) and superelastic (SE) capability. SME and SE capability are due to martensitic phase transformation, that is occurred by changing temperature and applied stress, so these alloys are called as intermetallic materials. This study is consists of three phases. In the first stage, classical pin-on-disc type of a standard experimental setup using a load cell units has been made a computer-aided testing machine in order to measure the friction force for testing of any type of materails. In the second stage, pure Ti and NiTi alloy materials were produced by powder metallurgy (PM) method. Microstructures were examined by optical microscope. Hardness, density and porosity measurement were made. In the last stage, dry wear and abrasive wear behavior of these intermetallic materials were investigated under different experimental conditions.In the first stage, to measure the friction coefficient of the wear behavior of the produced alloys, pin-on-disc wear test rig has been revised using a load cell unite and assembled with table type of computer. Thus, friction force would be able to be measured continuously during the experiments. In order to calculate the coefficient of friction correctly, wear tests were carried out on known coefficient friction of PTFE and MS70 materials. The second phase; Ti100, Ni52Ti48 and Ni52Ti48 SMAs were produced by PM method using commercialpurity nickel and titanium powders. The mixture of pre-alloyed and alloyed powders were uniaxially cold compacted using a cylindrical die of 6.5 mm diameter and 20 mm height at a pressure of 650 MPa using a single action press.The compacts were then sintered in a rapid heating pipe type of furnace under a control of Ar gas. By adjusting the auto-controlled power of the induction heat system, the mixed powders were sintered at 1100 oC for 5 hours. The heating rate was about 100 oC/h. Hardness, density, porosity and microstructure by means of optical microscopy were investigated. At the last stage, wear behavior and friction coefficients were measured through the modified pin-on-disc type wear testing machine, which is coupled with table computer. In addition, abrasive wear behavior of the pure Ti and SMAs were studied at various testing conditions.The experimental results showed that the friction force measurements were able to be made correctly due to load cell unite and coupled with computer on the wear experimental set up when wear tests were carried out at any conditions. For example, friction coefficient of pure Ti was measured about 0.37 while riction coefficient was approximately 0.40 for the NiTi alloys, respectively. Optical microscope examination revealed that porosity distribution seemed to be more in pure Ti than that of NiTi alloy, but porosity size was slighly larger for NiTi alloy. It was found that hardness increased with adding Ni content to pure Ti matrix. Hardness of pure Ti and NiTi alloys were measured as 59 HRB and 83 HRB,respectively. The density also increased with Ni content for the base material. Moreover, the experimental results showed that volumetric wear rate of pure Ti and NiTi alloys increased with increasing load, but there was big differences between the matrix and SMAs because of microstructure and hardness values under dry conditions. As for the case of abrasive wear when tested against SiC abrasive paper, wear behaviour of NiTi alloys was found to be much better than pure Ti in spite of the fact that hardness of Ni48Ti52 and Ni52Ti48 alloys was very close to each other. Furthermore, the volumetric wear rate increased with increasing the load, abrasive grain size and time. Among the parametres,abrasive size exerted a great effect on the wear, followed by the load, but the least effect was indicated by the abrasive time.Key Words : SMA, wear, friction coefficient, Ni-Ti, powder metalurgy, .. cold pressing