Effect of 3 mol % Yttria Stabilized Zirconia Addition on Structural and Mechanical Properties of Alumina-Zirconia Composites

Alumina-Zirconia (Al2O3-ZrO2) composites especially Zirconia Toughened Alumina (ZTA) shows better mechanical properties over alumina. Al2O3-ZrO2 composites were prepared by powder compaction method varying 3 mol% yttria stabilized zirconia (3Y-ZrO2) content from 0 to 20 vol% using small amount of MgO as sintering aid. The composites were sintered for two hours in air at 1580 ̊C. At this temperature maximum density was achieved 99.31% of theoretical density for composite containing 20 vol% 3Y-ZrO2. Density measurement of sintered composites was carried out using Archimedes’s method. Hardness and fracture toughness measurement was carried out using Vickers indentation. Phase content and t-ZrO2 retention were detected by means of X-ray diffraction (XRD). Microstructure of the composites and grain size of alumina and zirconia was determined by Scanning Electron Microscopic (SEM) analysis. Maximum microhardness (17.46 GPa) was achieved for composite containing 5 vol% ZrO2 and maximum flexural strength (684.32 MPa) and fracture toughness (10.33 MPam) was achieved for composite containing 20 vol% of 3Y-ZrO2. The aim of the present work is to investigate the optimum 3Y-ZrO2 content for obtaining maximum density, microhardness, flexural strength and fracture toughness of Al2O3-ZrO2 composites.


Introduction
Alumina is a suitable candidate for preparing engineering ceramics because of its several inherent properties such as low thermal conductivity, comparatively high thermal expansion coefficient, high hardness and pronounced chemical stability.
Alumina ceramics is used for biomedical applications.It has been developed as an alternative to surgical metal alloys for total hip prosthesis and tooth implants.
As an inert bioceramic Al 2 O 3 is used in load bearing hip prosthesis and dental implants in dense and pure state for its corrosion resistance and high were resistance.In the early 70's bioceramics were employed to perform singular biologically inert roles, such as to provide parts for bone replacement.The realization that cells and tissues in the body perform many other vital regularity and metabolic roles has highlighted the limitations of synthetic materials as tissue substitutes.Demand of bioceramics has changed from maintaining an essentially physical function with eliciting a host response, to provide a more integrated interaction with the host.This has been accompanied by increasing demand for medical devices to improve the quality of life as well as extended durability.It is a problem for common alumina that it shows low mechanical resistance and fracture toughness [1].Alumina-Zirconia composites have been widely studied over last two decades [2] [3] [4] [5].It is well known that the mechanical properties of alumina ceramics can be considerably increased by the incorporation of fine zirconia particles [6] [7] [8] [9].By improving the mechanical behavior alumina may be more widely used for various biomedical applications.Alumina-Zirconia composites are an important class of ceramic which exhibit high strength and toughness over alumina due to stress induced phase transformation (tetragonal to monoclinic zirconia) [10] [11] [12].This phenomenon was first discovered by Clauson in 1976 [6].Pure zirconia has three polymorphic (crystalline) forms: monoclinic (m, at low temperature), tetragonal (t, at intermediate temperature) and cubic (c, at high temperature) [13].The t to m transformation is martensitic in nature.It is used to improve the mechanical properties of ZrO 2 ceramics and ZrO 2 particle-reinforced composites by the mechanism of transformation toughening.The t to m transformation occurs at around 950˚C on cooling.Transformation toughening occurs because of the volume expansion occurs (3% -4%) that accompanies t → m transformation producing a reduction in the stress intensity at the crack tip, leading to a dissipation of energy of the propagating crack.Cracks become self-limiting because an advancing crack must overcome both the energy required for material transformation associated with the volume expansion of transformed material [14] [15] [16].The grain size of zirconia is also important for retaining t-phase in the composite.The size must be lower than a critical size to ensure the t-phase at room temperature [17].
The increased volume fraction of zirconia is also responsible to reduce the stability of the tetragonal phase.Hence, an optimum amount of zirconia addition is expected to retain the favorable amount of tetragonal content which can enhance the thermal and mechanical properties of Alumina-Zirconia composites without any grain growth during sintering [18].When compared with single phase α-Al 2 O 3 , Alumina-Zirconia composites offers high hardness of Al 2 O 3 matrix, coupled with improvements in fracture toughness.Because t-ZrO 2 particles are present within the matrix, overstabilization of tetragonal zirconia phase is also a problem.If the tetragonal phase is overstabilized, the stress required for transformation in the high stress region around a crack tip may be higher than the fracture toughness.As a result the fracture toughness of the composite will be low, similar to that of alumina.It has been observed that a mixture of tetragonal and monoclinic zirconia gives the highest toughness in zirconia toughened alumina [19].In this investigation, a small amount of sintering aid (MgO) was used.The mechanical properties are also dependent on relative distribution of Al 2 O 3 and ZrO 2 matrix [20].For getting homogeneous microstructure of the composite homogeneous distribution of Al 2 O 3 and ZrO 2 is very much essential.
Densification is another important factor for obtaining better mechanical properties.Composite with low density show poor mechanical properties.So, proper processing route should be followed for getting high mechanical properties.Relative density of Alumina-Zirconia composites increases with increasing sintering temperature but at the same time the fraction of retained t-ZrO 2 decreases probably due to the combined effect of low matrix constraint provided by the porous compacts as well as due to an increase in grain size [21].So, optimization of sintering temperature plays a vital role on properties of composite.Sintering temperature was optimized as 1580˚C.
Introduction of a small amount of sintering aid (CaO, MgO) in the ceramic process enhances the mass transport during solid state sintering so that ceramic reached full density at low temperature.Moreover decreasing grain growth a stronger ceramic was obtained [7].To fulfill this purpose 0.2 wt% MgO was used as sintering aid in sample preparation.To obtain homogeneous structure of the composite and high density ceramic body slurry was prepared in ethanol media and powder compaction method was adopted for sample preparation in this investigation which may be an alternative design for preparing Al 2 O 3 -ZrO 2 composites.An alternative sintering schedule has also been followed in the present study.

Sample Preparation
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 Al 2 O 3 , ZrO 2 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-ZrO 2 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 unidirec-tionally 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.

Measurements
Density of the sintered sample was measured by Archimedis' law using the following equation: where s ρ = sintered density, w ρ = water density, s m = mass of sintered pellet, w m = mass of pellets in water.
The percentage of total porosity ϕ was calculated from the bulk density and the theoretical density o ρ according to the standard formula [22]: where, ϕ = the total porosity of the sample, ρ = density of sintered pellet 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.
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].

Density and Porosity
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-ZrO 2 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-ZrO 2 led to

Vickers Hardness (Hv) and Elastic Moduluds (E)
The Vickers hardness values of sintered Alumina-Zirconia composites as a function of vol% of 3Y-ZrO 2 is represented in Figure 4. From Figure 4 it is observed that initially microhardness of the composite increased slightly with 3Y-ZrO 2 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-ZrO 2 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-ZrO 2 due to combined effect of lower hardness of ZrO 2 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-ZrO 2 and minimum hardness value (13.02GPa) was obtained for composites containing 20 vol% 3Y-ZrO 2 .
Values of elastic modulus of composites with 3Y-ZrO 2 content are presented in Figure 5.In this study elastic modulus of pure alumina was obtained 347.39 GPa. Maximum elastic modulus (350.48GPa) was obtained for composite with 2 vol% of 3Y-ZrO 2 and minimum elastic modulus (335.31GPa) was obtained for composite with 20 vol% of 3Y-ZrO 2 .
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 ZrO 2 .They found maximum elastic modulus for containing 3 wt% of ZrO 2 .Similar trend was found in this investigation.When 2 vol% 3Y-ZrO 2 was added to alumina the elastic modulus was slightly increased and maximum elastic modulus was obtained at this composition.

Flexural Strength and Fracture Toughness
The flexural strength and fracture toughness values of the composite as a function of 3Y-ZrO 2 content are presented in Figure 6 and Figure 7, respectively.Similar results were also published earlier [28].

Phase Analysis of Raw Materials
The XRD patterns of major raw materials (alumina and stabilized zirconia) which were used for preparation of Alumina-Zirconia composites are presented in Figure 8 and Figure 9, respectively.The XRD patterns confirm their corresponding phases.Figure 9 shows the XRD pattern of raw 3 mol% yttria stabilized zirconia (3Y-ZrO 2 ) which confirms the presence of tetragonal zirconia by ICDD 072-7115.

Phase Analysis of Sintered Sample
The XRD patterns of sintered pure alumina and Alumina-Zirconia composite of stabilized zirconia is presented in Figure 10 and Figure 11, respectively.
Figure 10 shows the XRD pattern of alumina pellet sintered at 1580˚C.As like as raw alumina the phase was identified by ICDD 089-7717 as α-alumia (corundum).Three characteristic peaks were obtained at 2θ values 35.14˚, 43.36˚ and 57.51˚ which are almost similar to that that of raw alumina.But the peak intensity is significantly higher than the peak intensity of raw alumina.The crystallite size of alumina obtained was 32.48 nm which is also larger than that of raw alumina.

Variation of XRD Pattern with Zirconia Content
The XRD patterns of Alumina-Zirconia composites containing 2, 5, 10, 15 and 20 vol% stabilized ZrO 2 and sintered at 1580˚C for 2 hours are shown in Figure 12.
From Figure 12 it is observed that though t-ZrO 2 (3Y-ZrO 2 ) was used for composite preparation a significant peak of m-ZrO 2 was obtained.Peak of maximum intensity of t-ZrO 2 was obtained for composite containing 20 vol% 3Y-ZrO 2 .But intensity of m-ZrO 2 continuously increased with zirconia content.
Low ZrO 2 content is favorable to retain t-ZrO 2 due to the presence of more alu-

t-ZrO2 Retention
Although 3Y-ZrO 2 (t-ZrO 2 ) was used for preparation of Alumina-Zirconia composites in this study, peaks of considerable intensity of m-ZrO 2 was observed from Figure 12.When the vol% of 3Y-ZrO 2 increased the peak intensity of t-ZrO 2 decreased, simultaneously the peak intensity of m-ZrO 2 increased.However it is confirm that tetragonal phase is indeed retained at room temperature as a metastable phase due to presence of hard alumina matrix around, and this phase is responsible for fracture toughness improvement by transformation toughening mechanism.The calculated volume fraction of t-ZrO 2 as a function of zirconia content is shown in Figure 13.From Figure 13 it is observed that 100% t-ZrO 2 retention was found for composites containing 2 and 5 vol% 3Y-ZrO 2 .For Alumina-Zirconia composites containing 20 vol% 3Y-ZrO 2 content t-ZrO 2 retention was found 87.5%.In earlier works it was observed that, fraction of tetragonal phase increase when the quantity of ZrO 2 decrease (30vol% of tetragonal phase for 5 vol% ZrO 2 , but only 5% of tetragonal phase for 20 vol% of ZrO 2 ) [30] [31].If the tetragonal phase is overstabilized, the stress required for transformation in the high stress region around a crack tip may be higher than the fracture stress.As a result the flexural strength of the composite will be low, similar to that of Al 2 O 3 .Our resultant Alumina-Zirconia composite consist a mixture tetragonal and monoclinic zirconia where tetragonal phase which is not overstabilized.So the composite is suitable for showing higher mechanical properties.

Scanning Electron Microscopic (SEM) Analysis
The microstructure of Alumina-Zirconia composite samples containing different 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 ZrO 2 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-ZrO 2 content increases, the Al 2 O 3 grain size decreases significantly.The ZrO 2 phase creates a pinning effect around Al 2 O 3 grain and obstructs its growth.When ZrO 2 is added with Al 2 O 3 , evenly distributed fine ZrO 2 grains act as grain growth inhibitor and led to smaller Al 2 O 3 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  Figure 16.Effect of 3Y-ZrO 2 content on Zirconia grain size in alumina-zirconia composites.
From Figure 15 it is observed that grain size of Al 2 O 3 decreased significantly when a small amount of 3Y-ZrO 2 (5 vol%) is added with alumina.Further addition of 3Y-ZrO 2 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-ZrO 2 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-ZrO 2 .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-ZrO 2 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.

Conclusion
In this present study, Alumina-Zirconia composites containing 0 -20 vol% 3Y-ZrO 2 have been synthesized successfully using powder compaction method.
α-alumina, m-zirconia and t-zirconia phase were present in sintered composites.
Addition of 3Y-ZrO 2 to alumina resulted composite materials with high density, high flexural strength and high fracture toughness.Microhardness and elastic modulus were found maximum for composite containing 5 vol% and 2 vol% of 3Y-ZrO 2 , respectively.Flexural strength and fracture toughness were found maximum for composite containing 20 vol% of 3Y-ZrO 2 .The characteristics of the composites exhibited better properties when compared with pure alumina.
The ease of preparation of the composites and better properties support the viability of the process for industrial application.
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.

Figure 6 andFigure 6 .
Figure 6 and Figure 7 reveal that addition of 3Y-ZrO 2 to alumina leads to increase in flexural strength and fracture toughness significantly.For pure alumina

Figure 11 shows
Figure 11 shows XRD pattern of Alumina-Zirconia composite containing 20 vol% 3 mol% yttria stabilized zirconia sintered at 1580˚C which confirms the formation of multi-phase composite.Phases of α-alumina (corundum), monoclinic zirconia (m-ZrO 2 ) and tetragonal zirconia (t-ZrO 2 ) were identified by ICDD 089-7717, ICDD 037-1484 and ICDD 072-7115, respectively.Though t-ZrO 2 was used for composite preparation, significant peaks of m-ZrO 2 were observed because m-ZrO 2 particles are present in the Al 2 O 3 matrix.The improvement in strength and toughness of Alumina-Zirconia composite results from the volume expansion and shear strain arising from t → m ZrO 2 transformation.Each tetragonal particle release energy and expand to stable size in monoclinic form if a crack tries to form under stress.

Figure 13 .
Figure 13.Tetragonal ZrO 2 retention as a function of zirconia content.

Figure 15 .
Figure 15.Effect of 3Y-ZrO 2 content on grain size of alumina in Alumina-Zirconia composites.