Department of Metallurgical and Materials Engineering, Istanbul Technical University, Istanbul 34469, Turkey
Received date: 14 January 2010; Accepted date: 03 February 2010
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spark plasma; sintering; alumina; zirconia; titania
Monolithic alumina and zirconia ceramics are widely used for orthopedic implants such as total hip and knee replacement prostheses due to their excellent mechanical properties, high wear resistance, and biocompatibility. It is well known that yttria stabilized tetragonal zirconia (3Y-TZP) has higher strength and fracture toughness than alumina, because of transformation toughening mechanism. A new generation ceramic for joint prostheses, zirconia toughened alumina composites, possesses higher fracture strength and toughness compared to monolithic alumina [2,4,5].
The effects of additives for alumina ceramics have been aimed to decrease the sintering temperature, improve microstructure and properties. The effect of TiO2 addition has been reported to promote the sintering behavior and grain growth phenomena in several studies . TiO2 addition enhanced diffusivity due to the Al3+ vacancies, as generated by Ti4+ substituting for Al3+ [1,3,5].
The purpose of this study is to investigate the sintering behavior, densification and hardness of the Al2O3, Al2O3/3Y-TZP (90–10 vol%) and Al2O3/3Y-TZP (90– 10 vol%) with 5wt% TiO2 composites prepared by spark plasma sintering (SPS) method. The phase analysis and microstructures of the specimens were also investigated.
Al2O3 (Baikowski Grade SM8, France, an average particle size of 0.6 μm), 3Y-TZP (Tosoh Grade TZ-3Y, Japan, an average particle size of 0.05–0.1 μm) and TiO2 (Anatase, Merck, product code 1.00808, Germany) powders were used as starting materials (Table 1). The raw materials were weighed in appropriate quantities, ball milled in ethanol for 24 h and then dried. A graphite die 50mm in inner diameter was filled with the mixture, followed by sintering using an SPS apparatus (SPS-7.40 MK-VII, SPS Syntex Inc.). Pure Al2O3 was sintered at 1350 °C, Al2O3/3Y-TZP composites with and without TiO2 were sintered at 1300 and 1460 °C with a heating rate of 1.7 °C/s in vacuum. A uniaxial pressure of 40MPa and pulsed direct current (12ms/on, 2ms/off) were applied during the entire process. The current was controlled manually during monitoring the displacement behavior of the samples. The crystalline phases were identified by X-ray diffractometry (XRD; Rigaku MiniFlex) in the 2θ range of 10–80◦ with Cu Kα radiation. The densities of the specimen were determined by Archimedes’ method and converted to relative density using theoretical densities of Al2O3, yttria stabilized ZrO2 and TiO2. The fracture surface of the samples were coated with a thin film platinum and subjected to microscopic investigation by a field emission scanning electron microscope (FE-SEM; JEOL JSM-7000F). Microhardness tests were applied to the polished samples under a constant load of 9.8N with 12 s indentation time.
|Material grade||Supplier||Y2O3 (mol%)||Al2O3 (wt%)||Grain sizea (nm)|
a According to the supplier datasheets.
Table 1: ZrO2 starting powders.
The densification of the specimens during SPS process was evaluated by the displacement of punch rods due to the shrinkage of the samples. Figure 1 shows the displacements of Al2O3 and Al2O3/3YTZP composites without TiO2 and with 5wt% TiO2, and isothermal shrinkage at sintering temperatures.
The shrinkage of Al2O3 started at 950 °C and completed at 1310 °C. The starting temperature of shrinkage (1060 °C) for the Al2O3/3Y-TZP composites containing 5wt% TiO2 was significantly lower than that of Al2O3/3Y-TZP (1200 °C). Thus, the presence of 5wt% TiO2 promoted the densification of Al2O3/3Y-TZP and decreased the sintering temperature of Al2O3/3Y-TZP composites from 1460 to 1300 °C.
The XRD patterns of the Al2O3 and Al2O3/3Y-TZP composites sintered at different temperatures for 300 s are shown in Figure 2. X-ray analysis indicated that α-Al2O3 and tetragonal ZrO2 became fully crystalline and monoclinic ZrO2 peaks did not appear at the end of the sintering process.
Figure 3 shows the Vickers hardness of Al2O3 and Al2O3/3Y-TZP composites at load of 9.8N. Al2O3/3YTZP (90–10 vol%) composite had higher hardness than the monolithic Al2O3. The hardness of Al2O3 slightly increased with the addition of 10 vol% yttria stabilized (3 mol%) ZrO2 (3Y-TZP) from 19.8 to 20.2GPa. The addition of 5 wt% TiO2 decreased the Vickers hardness of Al2O3/3Y-TZP composite from 20.2 to 17.3GPa. This relatively low hardness could be mostly due to the 2 wt% porosity.
Microstructures of fracture surfaces of Al2O3 and Al2O3/3Y-TZP composites are shown in Figure 4. In Figure 4(a), Al2O3 consisted of both large and small equiaxed grains 0.5–3 μm in size and straight grain boundaries. Figure 4(b) demonstrates the SEM image of fracture surface of Al2O3-3Y-TZP. In this figure, alumina grains (grey) in size of 0.6–1.5 μm and 3Y-TZP grains (white) in size of 0.05–0.1 μm are shown. Figure 4(c) is the SEM micrograph of Al2O3/3Y-TZP composites containing 5wt%TiO2. The grain size of alumina was not significantly changed by the addition of TiO2.
Al2O3 and Al2O3/3Y-TZP composites were prepared by SPS at temperatures of 1300–1460 °C for 300 s under 40MPa. The addition of 5 wt% TiO2 improved densification of Al2O3/3Y-TZP and decreased the sintering temperature of Al2O3/3Y-TZP composites from 1460 to 1300 °C. The presence of 10 vol% 3Y/TZP slightly increased the hardness of Al2O3 and suppressed the grain growth of alumina.