Surface Engineering of Zirconium for Tribological Properties Enhancement
Zirconium and its alloys have found increasing applications in several industrial sectors including the nuclear power generation, the chemical processing and the biomedical industries, mainly due to the combination properties of neutron transparency, excellent corrosion resistance and good biocompatibility. However, with a base hardness of about 200 HV, zirconium and its alloys have poor tribological properties and find limited applications in other fields of engineering. Efforts have been made in this work to develop surface engineering techniques to enhance the tribological properties of commercially pure zirconium (CP-Zr) and to characterise the structures and properties of surface engineered CP-Zr. It can be stated that there is limitation in the current research, no sufficient information on thermal oxidation and carburising of Zr has been released in open literature. In this research better wear resistances have been achieved for surface engineered zirconium using Thermal Oxidation (TO) and Pack Carburising (PC) treatments. Two surface engineering techniques have been investigated in this work. One is thermal oxidation (TO) and another is pack carburising (PC). Both processes have been investigated under a wide range of processing conditions, including processing temperature, time, surface roughness and compositions. The structures and compositions of the resultant surface and subsurface layers have been characterised using a variety of analytical and experimental techniques, including metallography, scanning electron microscope, X-ray diffraction, glow discharge spectrometer and ball-cratering. The properties of the surface engineered CP-Zr have been characterised by microhardness testing, scratch testing, and tribological testing under both dry, unlubricated and simulated body fluids (Ringer’s solution) conditions. The results show that TO is a very effective surface engineering technique to enhance the tribological properties of CP-Zr. TO produces a hard ZrO2 oxide layer (OL) of 5 to 12 microns on the surface and an oxygen diffusion zone (ODZ) of a few microns in the subsurface. The OL offers good wear resistance while the ODZ provides load bearing capacity. Thus, the combination of the OL and ODZ offers CP-Zr excellent tribological properties under high contact loads. However, the performance of TO CP-Zr depends on the TO process conditions and the surface roughness of the TO surface. This work investigated the effect of TO temperature, time, initial surface toughness and roughness after TO, on the tribological performance. It has been determined that the optimum TO temperature is 650oC and optimum time is 6 h. Too high a temperature and too long a TO time can lead to the formation of pores and cracks in the OL, leading to deterioration in tribological properties. This happens due to the fact that the created OL using those conditions can be poor, damaged and flakes off easily. It has also been found that a slightly rough surface before and/or after TO is beneficial in delaying crack formation in the OL during sliding and enhancing the load bearing capacity of TO CP-Zr. This happens because there is minimal contact between the alumina ball and surface of the sample during friction and wear testing. A further investigation has been conducted to compare TO Zr with TO Ti. Both Zr and Ti are important biometals used in medical implants. But they show very different TO characteristics in terms of OL growth kinetics and mechanical properties. This investigation has shown that TO produces a much thicker OL on Zr than on Ti and the OL on Zr is very adherent to the substrate. As a result, the TO Zr performs much better during sliding tests under dry conditions and in Ringer’s solution. Another surface engineering technique investigated is pack carburising (PC). Although very few work has been reported on carburising of zirconium, there have been some reports on pack carburising of titanium. It is thus necessary to investigate the feasibility of pack carburising CP-Zr in this work. PC was conducted at various temperatures (825 − 980oC) and for various duration (3-40 h) and with different pack compositions. The results show that CP-Zr can be effectively carburised at temperatures higher than 900oC for sufficiently long duration (more than 10 h). Low temperatures and short duration favour oxidation rather than carburisation. Successfully carburised CP-Zr comprises a ZrC carbide layer of a few microns on the surface, followed by a thick diffusion zone (200 microns) containing oxygen and carbon in the subsurface. The carburised CP-Zr offers enhanced tribological properties, but is not as effective as thermal oxidised CP-Zr.
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