The Preparation Of Titanium Cermets And Biotribological Properties

Titanium cermets were successfully synthesized and a large quantity of titanium carbide was formed with a gradient distribution on the surface of Ti6Al4V alloy by using a novel sequential carburization under high temperature. At the same time, the titanium cermet femoral head was also produced by this technique.Thermodynamics and kinetics of the carburization were analyzed to understand the sequential diffusion mechanism. Then the surface composition, topography, hardness, fracture toughness, contact angle, corrosion current, frictional coefficient, wear rate and wear particle distribution were systematically investigated to estimate the microstructure, mechanical behaviours, corrosion resistance, wettability and biotribological properties of the titanium cermets.The surface component results indicated that medical grade titanium alloys could be strengthened by sequential carburization, and titanium cermets were synthesized with a thickness of 250μm, including titanium carbide layer and diffusion layer. The element composition profile with depth showed a gradient transition across the coating-substrate, and it showed that there was a good metallurgic bond between the coating and the substrate. All of these resulted in an excellent adhesion and prevented the coating delamination from the substrate. The X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) revealed that uniform and compact titanium carbide with a face-centred cubic structure was formed on the surface of titanium alloy, and it was possible to avoid hydrogen embrittlement as there was no evidence of hydride existence. The XPS and Glow discharge spectrometer (GDS) results indicated that carbon atom concentration showed a gradient distribution in the coating. Furthermore, the surface topography confirmed that a complex micro porous structure was formed due to the stacking of titanium carbide as the disorder degree of crystalline grain orientation increased in the process of carburization.The surface hardness of titanium cermets was significantly improved due to the formation of titanium carbide and complex micro porous structure. The Vickers hardness of titanium cermets reached 778 HV, 2.3 times as high as that of titanium alloy, while the nanohardness value was 12 GPa, 2.6 times as high as that of titanium alloy. In addition, the fracture toughness of titanium cermets was 21 .5×10⁶ Pa·m^1/2,much higher than that of other ceramics, which indicates a combination properties of metal and ceramics. The friction coefficients of both titanium alloy and titanium cermets showed a remarkable load dependence, but had a totally different trend. The friction coefficient of titanium alloy decreased with load while the friction coefficient of titanium cermet increased with load. The formation of titanium carbide and micro porous structure effectively strengthened the surface of titanium alloy and improved the shear resistance. Therefore, the titanium cermets coupled with zirconia showed a bit lower the friction coefficient, much better wear resistance and quite lower debris concentration compared to titanium alloy.On the other hand, there was a remarkable decrease of electrochemical corrosion resistance for titanium cermets; however, it could satisfy the demands for use. It is also found that surface energy is the main mechanism governing the modification of wettability characteristics. The surface energy of Titanium increased from 26 .3×10^-3 J/m² 31 .7×10^-3 J/m² to after the carburization, 20.5% higher than that of titanium alloy; while the contact angle of titanium alloy decreased from 83°to 71.5°after carburization, indicating a significant improvement of wettability, much closer to the optimal water contact angle for cell adhesion of 70°.At last, artificial hip joint experimental results showed that titanium cermets could not only improve the wear resistance of femoral head, but also decrease the wear of UHMWPE joint cup. In addition, titanium cermets could effectively control the UHMWPE debris distribution, and increased the size of UHMWPE debris, thus decreased the cell toxicity of debris.All of the evidence indicated that titanium cermets had an outstanding mechanical properties, excellent wettability, acceptable corrosion resistance and extraordinary biotribological properties, which made it a potential material biomedical materials especially in artificial joint.