Effect of Yttria on the Synthesis, Microstructure and Mechanical Properties of Partially Stabilized Zirconia in Α-Al2O3Matrix

The effect of Y2O3 addition on the phase transition, microstructure and mechanical properties of ultrafine partially stabilized zirconia (Y-PSZ) in αAl2O3 matrix prepared by co precipitation process has been studied. In the present work, the degree of phase stabilization of different mol% Y2O3 doped zirconia (Y-PSZ) was studied. 10 vol % Y-PSZ (2-3 mol% Y2O3) in α-Al2O3 matrix was prepared with the simultaneous co-precipitation process. The stabilization of crystallographic and thermal phases was characterized by XRD, TG-DTA. The samples were calcined in the range of 700-800°C. The fabricated samples were sintered at a temperature of 1600°C for 2-4 hrs. The sintered samples were characterized for their physical (such as density) and mechanical properties (flexural strength, hardness and fracture toughness). The morphology and microstructure have been observed by FESEM. It has been observed that the significant improvement in fracture toughness with retention of high hardness in the order of~1700 HV The fracture toughness was in the order of 12.0 MPa.m1/2 with the 10 vol% of Y-PSZ (2.5 mol% Y2O3) in αAl2O3. Zirconia based ceramics is one of the most promising structural material because of its high hardness, high corrosion and erosion resistance and high temperature strength. The toughness of this ceramics can be increased using the stress induced phase transformation [1-4]. Zirconia based ceramics are also well known for their excellent electronic, thermal, optical & mechanical properties [1-3]. Pure ZrO2 exists in three crystalline forms namely monoclinic (m), tetragonal (t) and cubic (c). It also exhibit phase transformation: Monoclinic (m) 11700c. Tetragonal (t)22700 Cubic (c) [4] where (m) and (c) are stable phases and (t) is a metastable phase. The transformation of pure zirconia from the ‘m’ phase to‘t’ phase is accompanied by a volume increase of about 4-5% causing cracking and structural failure [4.5]. Due to this volume increase of pure Zirconia makes it unsuitable for any application [6]. For stabilizing the high temperature t-phase at room temperature, different additives/ dopant can be mixed like Y2O3, CeO2, MgO, CaO [7-10]. Yttria was found to reduce t→ m transformation from 1100°C to approximately 550°C [11, 12]. In yttria stabilized zirconia toughened alumina, the ideal microstructure is the single tetragonal phase as it increases the strength, hardness and fracture toughness of the sintered zirconia materials [12, 13]. The high toughness is due to a phase transformation induced by a stress relief due to crack propagating in the metastable t phase. The increase in volume accompanying transformation causes compressive stresses. These stresses effectively close the crack and block further crack growth, which can improve the strength, fracture toughness, and hardness of the ZTA [5]. In particular zirconia stabilized with 2-4 mol% Y2O3 has been used as a structural ceramics, with high hardness, strength [14]and high fracture toughness derived from stress induced phase transformation while 10-15 mol% of CeO2 (Ce-PSZ) plays a vital role on hydrothermal stability and mechanical properties [15]. It has been reported that ceria doped zirconia polycrystals (Ce-PSZ) more than 10 mol% have high fracture toughness and high thermal stability compared to yttria doped zirconia polycrystals (Y–PSZ) ceramics [16]. Bravo et.al reported that the trend towards increasing phase stability with decreasing particle-size of tZrO2 could overcome by adjusting the yttria dopant concentration [17] whereas 2-3 mol% Y2O3 was found to optimize fracture toughness in submicron t-ZrO2 dopant


A B S T R A C T
The effect of Y2O3 addition on the phase transition, microstructure and mechanical properties of ultrafine partially stabilized zirconia (Y-PSZ) in α-Al2O3 matrix prepared by co precipitation process has been studied.In the present work, the degree of phase stabilization of different mol% Y2O3 doped zirconia (Y-PSZ) was studied.10 vol % Y-PSZ (2-3 mol% Y2O3) in α-Al2O3 matrix was prepared with the simultaneous co-precipitation process.The stabilization of crystallographic and thermal phases was characterized by XRD, TG-DTA.The samples were calcined in the range of 700-800°C.The fabricated samples were sintered at a temperature of 1600°C for 2-4 hrs.The sintered samples were characterized for their physical (such as density) and mechanical properties (flexural strength, hardness and fracture toughness).The morphology and microstructure have been observed by FESEM.It has been observed that the significant improvement in fracture toughness with retention of high hardness in the order of~1700 HV The fracture toughness was in the order of 12.0 MPa.m 1/2 with the 10 vol% of Y-PSZ (2.5 mol% Y2O3) in α-Al2O3.
Zirconia based ceramics is one of the most promising structural material because of its high hardness, high corrosion and erosion resistance and high temperature strength.The toughness of this ceramics can be increased using the stress induced phase transformation [1][2][3][4].Zirconia based ceramics are also well known for their excellent electronic, thermal, optical & mechanical properties [1][2][3].Pure ZrO2 exists in three crystalline forms namely monoclinic (m), tetragonal (t) and cubic (c).It also exhibit phase transformation: Monoclinic (m) 1170 0 c.Tetragonal (t)2270 0 Cubic (c) [4] where (m) and (c) are stable phases and (t) is a metastable phase.The transformation of pure zirconia from the 'm' phase to't' phase is accompanied by a volume increase of about 4-5% causing cracking and structural failure [4.5].Due to this volume increase of pure Zirconia makes it unsuitable for any application [6].For stabilizing the high temperature t-phase at room temperature, different additives/ dopant can be mixed like Y2O3, CeO2, MgO, CaO [7][8][9][10].Yttria was found to reduce t→ m transformation from 1100°C to approximately 550°C [11,12].In yttria stabilized zirconia toughened alumina, the ideal microstructure is the single tetragonal phase as it increases the strength, hardness and fracture toughness of the sintered zirconia materials [12,13].The high toughness is due to a phase transformation induced by a stress relief due to crack propagating in the metastable t phase.The increase in volume accompanying transformation causes compressive stresses.These stresses effectively close the crack and block further crack growth, which can improve the strength, fracture toughness, and hardness of the ZTA [5].In particular zirconia stabilized with 2-4 mol% Y2O3 has been used as a structural ceramics, with high hardness, strength [14]and high fracture toughness derived from stress induced phase transformation while 10-15 mol% of CeO2 (Ce-PSZ) plays a vital role on hydrothermal stability and mechanical properties [15].It has been reported that ceria doped zirconia polycrystals (Ce-PSZ) more than 10 mol% have high fracture toughness and high thermal stability compared to yttria doped zirconia polycrystals (Y-PSZ) ceramics [16].Bravo et.al reported that the trend towards increasing phase stability with decreasing particle-size of t-ZrO2 could overcome by adjusting the yttria dopant concentration [17] whereas 2-3 mol% Y2O3 was found to optimize fracture toughness in submicron t-ZrO2 dopant concentrations.To meet all these requirements, several powder processing, forming and sintering processes were investigated.For powder synthesis, methods including attrition milling, CVD synthesis, spray ICP, colloidal processing, hydrothermal reaction, evaporative decomposition, hydrolysis coating, plasma synthesis, sol gel processing and co predicated process were used [18][19][20][21]However, the phase transition and mechanical property evaluation 10 vol% Y-PSZ with different mol % Y2O3 in α-Al2O3 in has not been explored significantly.The main purpose of the present investigation is to examine the effect of different mole % of Y2O3 addition in ZrO2 on the phase transition and growth of Y-PSZ crystal and to find out the effect of different mole % of Y2O3 addition in 10 vol.% of Y-PSZ in α-Al2O3 matrix for better physical and mechanical properties (flexural strength, hardness and fracture toughness) with respect to wear resistant characteristics as cutting tool materials.

Experimental SynthesisofY-PSZ (0-3 mol% Y2O3 doped) and 10 vol% Y-PSZ (2-3 mol % Y2O3 doped) in α-Al2O3Powder
Partially / fully stabilized zirconia by Y2O3 powders varying from 1.0 to 3.0 mol % was synthesized using wet chemical co-precipitation process of requisite amount of ZrOCl2 .8H2Oand Y(NO3)3.5H2Osolution for study of different degree of stabilization.The particular concentration of Y-PSZ with 2-3 mol% Y2O3 doped having more pseudo-tetragonal phase was chosen for 10 vol % reinforcement with α-Al2O3 matrix for making advanced composites by co-precipitation method.These composites were made to correlate the phases with mechanical strength of the material i.e. to study the flexural strength, hardness and fracture toughness.The simultaneous coprecipitation process was done using requisite amount of zirconyloxychloride ZrOCl2.8H2O(>99%, BDH, India), Y (NO3)3.5H2O(Aldrich, USA) and Al (NO3)3•9H2O (LobaChemie,India).The homogenized mixed solution were prepared in a 5 lit glass beaker with a magnetic stirrer system, considering the desired stochiometry of the metal oxides in the final composition of yttria doped zirconia toughened alumina (Y-ZTA) ceramics.The simultaneous co-precipitation was done with dilute NH4OH at pH ≈9 in all the cases of 10 vol % yttria doped zirconia in α-alumina matrix.After complete precipitation, the stirring was continued for few hours to maintain the homogeneity of co-precipitated gel.The hydrated gelatinous precipitate was thoroughly washed with hot distilled water until complete elimination of anions in the solution phase.Then the precipitate was dried in air oven at 80 0 C for several hours.The samples of 10 vol% Y-PSZ (2, 2.5 and 3 mol% Y2O3) in α-Al2O3 were prepared.The dried mass was calcined at 700-800°C.The calcined mass of Y-ZTA was ball milled in alcohol media for 24 hrs using high alumina balls in a 500 ml high alumina jar contained planetary mill (Fritsch, Germany).The dried milled powders were uniaxially and hydraulically compacted at the pressure of 200-300 MPa in a suitable cylindrical (with 10 mm in diameter) die and the compacts were sintered at 1600 0 C for 2-4 hours.The flow chart for the preparation of Y-ZTA is shown in Fig. 1.

Sample Characterization Study
Fig (1).Flow Chart XRD for different mol% of Y2O3 in PSZ was carried out over very short range from 27-33° with a scan rate of 3°/min to study the degree of phase stabilization.After visualizing more stabilized tetragonal phase within 2-3 mol% of Y2O3 in Y-PSZ system, the 10 vol% Y-PSZ (2.5 mol% Y2O3) in α-Al2O3 matrix was studied by XRD (CuKα, Ni filter) using a powder diffractometer (SHIMADZU Model.XRD 6000, Japan) to see whether the t-ZrO2 phase exists or not in presence of α-Al2O3 matrix.The XRD patterns were recorded over the angular range of 20-75° (2θ) with a scan rate of 2.8°/min.Thermal analysis of all the samples was performed in a simultaneous thermal analyzer (Netzsch, STA-449, Jupiter, Germany).Sample of about 30-40 mg were heated from 30 to 1550°C at a heating rate of 10°C/min and

Mechanical Characterization Study
Weighing the samples and determining their volumes by Archimedes method bulk densities and porosities of sintered specimen were evaluated.The hardness and fracture toughness of the sintered polished specimens were measured by Vickers hardness testing machine (Matsuzwa, MXT-70).Fracture toughness was determined by the Vickers indentation technique.The fracture toughness KIC was estimated using the equation KIC = 0.055Ha 0.5 (l/a) -0.5 Where H is the Vickers hardness calculated from H = 0.464 P/a 2 , a is the half diagonal length of the indentation and l is radial crack length from the corner of the indentation.To find out the flexure strength of the specimen, the samples were prepared in the dimension of 2.5 X 3.5 X 30 mm 2 .Then 3 pt bend test with a span of 20 mm at a cross head speed of 0.5 mm/min was conducted in Universal Testing Machine (Tinius Olsen, H50 KS).

Phase Characterization Study Phase Characterization Study of Y-PSZ (0-3 mol% Y2O3 doped) powder
The XRD pattern of sintered samples at 1600°C of Y-PSZ (0-3 mol% Y2O3 doped zirconia) was shown in Fig. 2(a).The stabilization of the phases was studied at low angle (2θ between 27° and 35°) especially to distinguish between monoclinic and cubic/tetragonal phases and to determine the crystallite size of the samples.The phase transition, density, crystallite size and grain size is listed in Table 1.It has revealed that in case of 3 mol% Y2O3 additions in zirconia, the formation of stabilized zirconia specially tetragonal zirconia (t-ZrO2) is predominant whereas in 2.0 mol% Y2O3 addition, the retention of t-ZrO2 is moderate and metastable.It can be concluded from this study that Y2O3 had dissolved in ZrO2 with the formation of stabilized ZrO2 (t-ZrO2) in case of 3 mol% Y-TZP and relatively small amount of monoclinic ZrO2 (m-ZrO2) presents in the same sample.The t-ZrO2 retention also increases with Y2O3 addition with the trend similar to that observed for density variation.The maximum t-ZrO2 retention is 92% at 3 mol% of Y2O3 addition.The t-ZrO2 retention is very low in the composites prepared with 1.0 mol% Y2O3.The density also increases with increase in mol% of Y2O3 addition.To The ratio of the monoclinic to tetragonal phase is calculated with the integral intensities of the monoclinic peaks ( 111

Phase Characterization Study of 10 vol% Y-PSZ (2.5 mol% Y2O3) in α-Al2O3 powder
The X-ray diffraction (XRD) of 10 vol.% Y-PSZ in α-Al2O3 matrix sintered at 1600°C was studied for determining the degree of stabilization of phases as shown in Fig. 2b.The stabilization of phases specially to distinguish between monoclinic and cubic/ tetragonal phases were identified at low angle (2θ between 27 and 35 0 ) and also at higher angle (48-52 0 ).However, in case of 2 and 3 mol % Y203 (Y-PSZ) with same volume % in α-Al2O3 matrix were also carried out but not shown in the figure.The ZrO2 particles are mainly of tetragonal phase embedded in α-Al2O3 matrix, which shows the presence of t-ZrO2 and α-Al2O3 as major phases along with small amount of monoclinic zirconia in case of Y-PSZ system.The main peaks of t-ZrO2 are sharp and high.The tetragonal/ phase in α-Al2O3 matrices play a vital role for enhancing the fracture toughness and hardness of the material.The resulting dried mass (heated at 70°C) of coprecipitated powders of 10 vol.%Y-PSZ (with 2.0, 2.5, 3.0 mol% Y2O3 ) in alumina precursors were studied through thermal analysis (TA) as shown in Fig. 3.In all the cases the endothermic peak range at 180-240 °C corresponds to the decomposition and transformation of γ-AlOOH to γ-alumina in presence of 10 vol % Y-PSZ in alumina precursor.The sharp exothermic peak centered at around 460-472°C.The exothermic peak shifts towards high temperature with increase in Y2O3 content.These exothermic peaks are attributed to the transformation of pseudo tetragonal phase of zirconia.The broad endothermic peak ranging 600-800°C indicates the formation of various transition phases of alumina before conversion to α-alumina at ~1150°C.The phase transition of α-alumina starts at 900°c, fully transform to αalumina at ~1200°C.No exothermic peak was observed in the higher temperature range around 1200 °C, which clearly indicates the absence of monoclinic to tetragonal phase transition.2. The fracture toughness of alumina-zirconia composites depends on the morphology and distribution of zirconia particles, their size, shape, location in alumina matrix and size distribution, which are critically influenced by the processing methodology and Y2O3 content.At 2.5 mol% Y2O3, not only the retention of t-ZrO2 was higher but also sintered density also high as well as the grain sizes also finer.When the Y2O3 content increases from 2.5 to 3.0 mol %, then most of the Y2O3 doped ZrO2 grains transformed to m-ZrO2 during cooling and retention of t-ZrO2 is less and reduces the hardness as well as fracture toughness of the sample.The flexural strength is also increased from 2 mol% to 2.5 mol% of Y2O3 and after that it also reduces.The sintered density and t-ZrO2 retention also affects the flexure strength of the composite which is highest at 2.5 mol% Y2O3 added 10 vol% Y-PSZ in α-Al2O3.

Conclusions
The effect of different mole % of Y2O3 addition in ZrO2 on the phase transition and growth of Y-PSZ crystal has been investigated by XRD using a powder diffractometer.After visualizing the tetragonality factor of ZrO2 crystal, 2-3 mol% Y2O3 doped ZrO2 has been selected for doping in 90 vol% α-Al2O3 to correlate the phases with flexural strength, hardness and fracture toughness of the developed composites.The following conclusions can be drawn from this study  The retention of metastable tetragonal is predominant in case of 2-3 mol% Y2O3 doped ZrO2. The developed sintered Y-ZTA ceramics showed a very uniform dispersion and microstructure of the zirconia grains within the alumina grain matrix. As the Y2O3 content increases, the exothermic peak in the DTA curves of Y-ZTA shifts to high temperature.Significant improvement in fracture toughness, hardness & flexural strength has been observed in the order of ~ 12.20 MPa m 1/2 , ~1700 HV and 510 MPa respectively for 10 vol% of Y-PSZ (2.5 mol% Y2O3) in α-Al2O3 which can be suitable with respect to wear resistant characteristics as cutting tool materials.

Fig 3
Fig 3 DTA curve of 10 vol % Y-PSZ (2-3 mol% Y2O3) in α-Al2O3 matrix Morphology of Y-ZTA Powder Fig. 4(a) and Fig. 4(b) showed the FESEM image of sintered surface and sintered fracture surface of 10 vol % Y-PSZ (2.5 mol% Y2O3) in α-Al2O3 matrix respectively.Homogeneous distribution of finer particles in the range of 20-40 nm was observed in the sintered powder.This sintered compact at 1600°C shows significant grain growth.A close-up view of the sintered Y-ZTA ceramics showed a very uniform dispersion of the zirconia grains within the alumina grain matrix.The average grain size of α-Al2O3 is ~ 1µm and a very fine grain of Y-TZP in the range of 0.2-0.3µm.

Table 1 .
Phase Composition, Density, Crystallite Size and Grain Size of different mol% of Y2O3 in Y-PSZ