Nano crystalline α-alumina powder (purity of 99.9%, crystal size 40nm, average particle size 150 nm, Advance materials, USA), 3 mol% yttria stabilized zirconia powder (purity 99.9%, crystal size 30 - 60 nm, Advance materials, USA) and magnesium oxide (99%, RCL Labean Ltd, Bangkok, Thiland) powders were thoroughly mixed to obtain a homogeneous distribution. In doing so, proper amount of Al₂O₃, ZrO₂ and MgO was taken in a HDPE pot. Wet milling was carried out for 24 hours in ethanol media in a motor driven pot mill using yttria stabilized zirconia balls as grinding media. 3Y-ZrO₂ was added into alumina from 0, 2, 5, 10, 15 and 20 vol%. 0.2 wt% of magnesium oxide was also added to the alumina-zirconia mixture. The slurry of the powder mixtures was dried in an vacuum oven (VO400, Memmert, Germany) at 90˚C for 6 hours and 5% PVA (polyvinyl alcohol) solution was added as a binder. The mixture was milled for uniform mixing and again dried and screened. The dried blend was unidirectionally pressed into pallets at 210 MPa using Universal Testing Machine (FS 300 KN, Testometric Co. Ltd. England).

In the single stage sintering schedule (Figure 1) depicted below that there was a holding period of three hours at 600˚C for binder removal. The samples were finally sintered at 1580˚C for two hours in air according to sintering schedule using high temperature furnace (Z18-40, Micropyretics Heaters International, USA). The sintered samples were used for characterization. For all sample same sintering schedule was maintained.

Density of the sintered sample was measured by Archimedis’ law using the following equation:

where = sintered density, = water density, = mass of sintered pellet, = mass of pellets in water.

The percentage of total porosity was calculated from the bulk density and the theoretical density according to the standard formula [22] :

where, = the total porosity of the sample, = density of sintered pellet and = theoretical density for samples.

Microhardness of the composites was measured by Vickers indentation using Micro-hardness Tester (HMV-2T, Shimadzu Corporation, Japan). The applied load was 19.614 N for 6 second over polished surface. The values of microhardness were calculated from the following equation [8] :

where, H_v = Vickers hardness (GPa), P = applied load (N), d = arithmetic mean of the two diagonal length (mm)

The porosity dependence Elastic Modulus can also be measured using Dewey- Mackenzie relation [18] :

where, E is the effective elastic modulus of porous composites. E_o is the elastic modulus of the dense composite and is the porosity of the sample.

Fracture toughness of the composite was measured by Vickers indentation using equation [23] :

where, K_IC, H_v, E, P and C_o stand for fracture toughness(Gpa), Vickers hardness (GPa), elastic modulus(GPa), indentation load (MN) and radial crack length(m), respectively. In this case applied load was 20 kg for 15 second (DVK-2, MATSUZAWA, Japan). All fracture surfaces were observed in an optical microscope (NMM-800TRF, MTI Co., USA). Before indentation the specimen surface was polished successively using 1200 and 4000 grit SiCpaper, respectively. Following this, the samples were polished in 6 µm and 1 µm diamond paste on a texmet cloth.

Phase identification was performed on sintered surface by X-ray diffractometry (XRD) with CuKα radiation (λ = 1.54046 Å) in D8 Advance diffractometer (BRUKER AXS GmbH, Germany) using 40 KV voltage and 40 mA tube current. The scanning speed was maintained 2˚/min, 2θ = 20˚ - 70˚ range. The volume fraction of monoclinic zirconia (V_m) was calculated according to the following equation [24] .

where I is the integral intensity and the subscripts m and t refer to the monoclinic and tetragonal phase, respectively.

Microstructure of the sample was observed by Scanning Electron Microscopy (SEM). The polished samples were sputter coated with platinum for 2 - 3 minute to make the surface conducting. The specimens were observed by SEM (JEOL-6490, Japan) in secondary electron (SE) mode at 20 KV accelerating voltage. The grain size of alumina and zirconia were measured by comparison method.

Density and porosity are important parameters on which mechanical properties depend. Composites with higher density and lower porosity offer better mechanical properties [8] . The presence of porosity can reduce fracture toughness by the reduction of the resistant area by the effect of stress concentration in the pores. So reduction of porosity and production of composite with higher density is an important factor to obtain better mechanical properties. Homogeneous distribution of different phases (alumina, zirconia, magnesia) in the mixture can enhance the densification process. High compaction pressure (210 Mpa) was used to obtain compact green sample.

Density and porosity of Alumina-Zirconia composites (sintered) as a function of vol% of 3Y-ZrO₂ content are presented in Figure 2 and Figure 3, respectively.

The density increases due to addition of dense powder (the density of zirconia is higher than that of alumina) and due to removing the pores between the powder. From SEM analysis it was observed that addition of 3Y-ZrO₂ led to

decrease in grain size of alumina, which is also responsible for increase in density. It is well known that densification of powdered particle during sintering is a thermally assisted phenomenon. Thermal energy helps to diffuse atom and thus bonding is developed between the adjacent particles. With the continual supply of thermal energy more and more contact points are created between the particles and the intervening porosity diffuses out of the contact area. Hence a substantial increase in density resulted and porosity of the composite is reduced. In the mean time the existing grains inside the particle have an opportunity to grow in order to minimize the overall energy of the system. Thus densification and grain growth occurs simultaneously. Density of composite also depends on t-ZrO₂ retention, because transformation of m-ZrO₂ to t-ZrO₂ leads to volume compression. For pure alumina 95.2% of theoretical density was achieved (porosity 4.80%) whereas 99.31% of theoretical density was achieved for composite containing 20 vol% of 3Y-ZrO₂. This density was the maximum density where the porosity was 0.69%. The density value conforms to some previous works those used pressureless sintering technique [25] [26] . It is observed that the porosity is reduced as 3Y-ZrO₂ content increases. The porosity decreases due to densification of the composite and due to pore removal by secondary phase of ZrO₂. This could be explained for the homogenity of the microstructure obtained in the present work as well as the small diameter and the narrow size distribution of the secondary phase.

The Vickers hardness values of sintered Alumina-Zirconia composites as a function of vol% of 3Y-ZrO₂ is represented in Figure 4.

From Figure 4 it is observed that initially microhardness of the composite increased slightly with 3Y-ZrO₂ content up to 5 vol%. After that it decreased significantly. Zirconia has lower hardness than alumina. Hence there is a general decreasing tendency in hardness values with increasing zirconia content. From the SEM analysis it was observed that grain size of alumina decreased significantly with lower 3Y-ZrO₂ content up to 5 vol% and after that the changes in grain size of alumina was not significant. Decrease in grain size of alumina may be a cause of increase in microhardness. So, maximum microhardness was obtained for composite containing 5 vol% 3Y-ZrO₂ due to combined effect of lower hardness of ZrO₂ and decrease in grain size of alumina. In this investigation maximum hardness value (17.46 MPa) was obtained for composite containing 5 vol% of 3Y-ZrO₂ and minimum hardness value (13.02 GPa) was obtained for composites containing 20 vol% 3Y-ZrO₂.

Values of elastic modulus of composites with 3Y-ZrO₂ content are presented in Figure 5. In this study elastic modulus of pure alumina was obtained 347.39 GPa. Maximum elastic modulus (350.48 GPa) was obtained for composite with 2 vol% of 3Y-ZrO₂ and minimum elastic modulus (335.31 GPa) was obtained for composite with 20 vol% of 3Y-ZrO₂.

According to Tan et al. [27] elastic modulus depends on the porosities, microcrack and different phases present in the composite. They found that the elastic modulus of Alumina-Zirconia composite could be increased by the addition of <5 wt% of ZrO₂. They found maximum elastic modulus for containing 3 wt% of ZrO₂. Similar trend was found in this investigation. When 2 vol% 3Y-ZrO₂ was added to alumina the elastic modulus was slightly increased and maximum elastic modulus was obtained at this composition.

The flexural strength and fracture toughness values of the composite as a function of 3Y-ZrO₂ content are presented in Figure 6 and Figure 7, respectively.

Figure 6 and Figure 7 reveal that addition of 3Y-ZrO₂ to alumina leads to increase in flexural strength and fracture toughness significantly. For pure alumina

maximum flexural strength was obtained 425.23 MPa and fracture toughness was obtained 3.95 MPa∙m^0.5. Flexural strength and fracture toughness increased with zirconia content continuously and maximum flexural strength (684.32 MPa) and fracture toughness (10.33 MPa∙m^0.5) were obtained for Alumina- Zirconia composite containing 20 vol % of 3Y-ZrO₂. The increase in flexural strength and fracture toughness maybe obtained due to high density of the composite and high t-ZrO₂ retention at 20 vol% 3Y-ZrO₂.

Improvement of mechanical properties of composites can be explained by t ® m transformation. The martensitic t to m transformation is induced by cooling or by external loading under isothermal conditions. The change increases the volume of the particle by about 3% - 4% and produces compressive stress in the alumina matrix. These stresses in turn close the crack and act as energy barrier to further crack growth. From XRD analysis it is observed that m-ZrO₂ phase increase with increase in 3Y-ZrO₂ content and at the same time t-ZrO₂ retention decrease. Although the microhardness and elastic modulus of the composites slightly reduced with zirconia content the flexural strength and fracture toughness obviously improved. By incorporating proper amount of zirconia to alumina we may obtain composites with high flexural strength and fracture toughness. Similar results were also published earlier [28] .

The microstructure of Alumina-Zirconia composite samples containing different

vol% 3Y- ZrO₂ were examined using Scanning Electron Microscopy (SEM). Figures 14(a)-(c) show the SEM micrographs of Alumina-Zirconia composites. Comparatively dark and large grains are Al₂O₃ whereas bright and small grains are ZrO₂. When 3Y-ZrO₂ was added, the grains become finer compared to that of pure Al₂O₃, as shown in Figure 14(b) and Figure 14(c). Observations show highly homogeneous microstructures without agglomerates, pores or abnormally grown alumina grains. The zirconia grains are well-distributed in the material and the alumina grains are surrounded by them.

There is interplay between densification and grain growth during solid state sintering of polycrystalline ceramics. Densification occurs by the flux of matter from the grain boundaries (the source) to the pores (the sink). Rapid densification requires that the diffusion distance between the source of matter and the sink be kept small, i.e., the grain size must remain small. Rapid grain growth causes a drastic reduction in the densification rate, so prolonged sintering time is needed to achieve the required density, which increases the possibility for abnormal grain growth to occur.

From Figure 14(b) and Figure 14(c), it can also be observed that ZrO₂ particles are uniformly dispersed throughout the alumina matrix. A homogeneous distribution of zirconia throughout the alumina matrix as well as the typical intergranular location of zirconia at the grain boundaries of the alumina was observed. This result is similar to that published elsewhere [32] . As the amount of 3Y-ZrO₂ content increases, the Al₂O₃ grain size decreases significantly. The ZrO₂ phase creates a pinning effect around Al₂O₃ grain and obstructs its growth. When ZrO₂ is added with Al₂O₃, evenly distributed fine ZrO₂ grains act as grain growth inhibitor and led to smaller Al₂O₃ grains. This is possibly due to increase in density as well as enhancement of mechanical properties.

It was observed that the microstructure became highly homogeneous and finer at sintering temperature 1580˚C. It can also be observed that the grain sizes of

ZrO₂ increase and hinders the grain growth of Al₂O₃ at 1580˚C which contributes to the lowering of porosity. A fine grain size and refined microstructure are necessary for improved mechanical properties especially for wear resistance of dental implant.

Figure 15 and Figure 16 show variation of alumina and zirconia grain size in Alumina-Zirconia composites with respect to 3Y-ZrO₂ content.

From Figure 15 it is observed that grain size of Al₂O₃ decreased significantly when a small amount of 3Y-ZrO₂ (5 vol%) is added with alumina. Further addition of 3Y-ZrO₂ led to decrease in alumina grain size slowly and linearly. In the present work maximum average grain size was obtained 6.25 µm for pure alumina. For 5 vol% of 3Y-ZrO₂ addition alumina grain size decreased to 2.58 µm. Minimum grain size of alumina was obtained 1.34 µm for Alumina-Zirconia composite containing 20 vol% of 3Y-ZrO₂. On the other hand, grain size of zirconia in Alumina-Zirconia composite increased linearly with zirconia content (Figure 16). In the present study minimum and maximum grain size of zirconia was obtained 0.35 µm and 0.48 µm for Alumina-Zirconia composite containing 5 vol% and 20 vol% 3Y-ZrO₂ respectively. The present results are harmonious with some previous works [9] [33] . The increase in density with zirconia content of Alumina-Zirconia composite may due to increase in zirconia grain size and decrease in alumina grain size.