Ceramic bearings have been used for femoral heads or acetabular liners of hip prosthesis due to their low wear rate arising from their high hardness and elastic modulus, as well as their excellent long-term biocompatibility as a result of their high chemical stability and resistance to corrosion. Alumina (Al2O3) and zirconia (ZrO2) are two main ceramics which have been used widely in the hip joint replacements. In order to take benefit of advantages of both hardness of alumina and toughness of zirconia and eliminate or limit the disadvantages of them, ZTA (Zirconia Toughened Alumina) composites have been used widely as the hip joint prostheses in clinical surgery consisting of fine ZrO2 particles dispersed in a dense, fine-grained Al2O3 matrix. However, there is still a need to improve their mechanical properties. One of the main possible improvements can be through the addition of small amounts of additives which are expected to give a measurable change in the characteristics of the composites and make a new generation of materials for orthopaedic applications. Titanium oxide is one of the additives which has been widely used in ceramics mainly as a sintering aid, and is found to improve the fracture toughness and wear resistance of alumina ceramics, as well as enhance the density and augment tetragonal phase stability of zirconia ceramics. Moreover, it has been shown that the mechanical properties of conventional ceramics, such as strength, hardness and wear resistance, could be enhanced by reducing the grain size, particularly to the nanoscale dimension. On the other hand, mechanical and tribological properties such as wear, friction and lubrication are the most important indicators to be considered in design and material selection for bearing surfaces. Although the effect of titania on mechanical and wear properties of alumina has been widely studied, there is not much research on its effect on ZTA composites, particularly in nano-sized ceramics. Thus, the current study focused on developing a new ceramic composite for hip joint replacements having higher mechanical properties and better tribological properties by both reducing the grain size and making a titania addition. The samples were prepared by mixing the starting powders, consolidation, sintering, and polishing process. The volume percent of Y-TZP used in this study was kept to 15 vol.% for all the samples, below the percolation limit of 16 vol.%, and the different composite containing no titania (pure ZTA), 0.1 mole.%, 0.5 mole.%, 2 mole.% and 5 mole.% of TiO2 were produced. The starting powders were mixed using an attrition miller for 3.5 hours in the aqueous environment of zirconia and alumina powders and distilled water and the pH value of the slurry brought to 4.5 by adding drops of dilute nitric acid. The slurry was then freeze-dried for 48 hours and crushed using an agate mortar and pestle and passed through a 180, 90, and 45 µm sieves using sieve shaker. The samples with 20mm diameter and 3gr mass were prepared in a graphite die and processed using spark plasma sintering. A uniaxial pressure of 16 KN was applied during the sintering process from 700°C and 3, 5, 7 and 10 minutes dwell time and 1400°C, 1450°C, 1500°C, and 1550°C temperatures were selected for sintering the samples. Then, the samples were ground and polished and etched thermally in the clean furnace for 20 minutes and at the temperature 100°C below their sintering temperature. After the samples were prepared, density measurements, XRD analysis, SEM characterisation, and mechanical testing were carried out on the samples. Due to a very difficult and time-consuming preparation process for wear samples, few samples of each series of ZTA, ZTA + 0.1 mole.% TiO2, and ZTA + 0.5 mole.% TiO2 showing high density and reasonable mechanical properties were selected for reciprocating wear tests. Thus, ZTA samples with sintering time of 7 minuets and temperature of 1450°C, 1500°C, 1550°C; ZTA+ 0.1 mole.% TiO2 samples with sintering time of 5 mintes and temperature of 1450°C, 1500°C, 1550°C; and ZTA+ 0.5 mole.%TiO2 samples with sintering time of 5 minutes and temperature of 1400°C, 1450°C, and 1500°C were prepared. The reciprocation wear test procedures were designed for 24 hours with normal load of 1N, 4N and 8N, and 8 hours with normal load of 16N. The selected reciprocating speed was 600 rpm (10 Hz) and the stroke length was set as 10 mm for all the test procedures. Fresh lubricant of 25 vol.% bovine serum was introduced every 8 hours during the test to avoid degradation. 3D optical microscopy, wear rate calculation, Raman spectroscopy, AFM, LFM and SEM microscopy were then carried out on the worn surfaces. The results showed that all the samples were reasonably dense having relative density more than 97.5% of the theoretical density with greater density at higher sintering temperature. Small amounts of 0.1 and 0.5 mole.% titania addition improved sinterability and densification of the samples; however, the relative density decreased for all sintering temperatures and times for higher content of titania to 2 and 5 mole.%. Similarly, titania addition increased the grain growth with its higher rate for alumina than zirconia. XRD patterns and SEM studies showed the formation of ZrTiO4 occurred adjacent to monoclinic zirconia grains. No traces of aluminium titanate peaks or any particular anisotropic alumina grains or in-situ fibres in SEM images were identified in this research. The opposite effect of the amount of titania on hardness and toughness was found in which titania addition enhanced toughness significantly although it affected hardness adversely, mainly due to its improved crack deflection mechanism. The specific wear rate found to be in mild region with the specific wear rate less than 10-6 mm3/Nm, with higher amount at higher loads and coarser microstructure. Samples containing 0.5 mole.% titania showed tangibly higher wear resistance and less damaged surface, particularly at high loads. The presence of a tribolayer with a thickness around 100 nm was confirmed in this study. Wear mechanisms for tests at low loads and materials with fine microstructures were found to be polishing, differential wear, grain pull-out, 3rd body abrasion and grooves, while large microcracking and removing craters of fractured grains were found to be dominant wear mechanisms in high loads and coarser microstructures. Thus, the significant outcome of this research was that reducing the grain size using SPS technique could significantly improve mechanical and wear properties of ZTA composites and titania addition was found to be beneficially effective in improving these characteristics.
