Structural and Mechanical Properties of Alumina-Zirconia (ZTA) Composites with Unstabilized Zirconia Modulation

Zirconia toughened alumina (ZTA) ceramics are very promising materials for structural and biomedical applications due to their high hardness, fracture toughness, strength, corrosion and abrasion resistance and excellent biocompa-tibility. The effect of unstabilized ZrO 2 on the density, fracture toughness, microhardness, flexural strength and microstructure of some Zirconia-toughened alumina (ZTA) samples was investigated in this work. The volume percentage of unstabilized ZrO 2 was varied from 0% - 20% whereas sintering time and sintering temperature were kept constant at 2 hours and 1580˚C. The samples were fabricated from nanometer-sized (α-Al 2 O 3 : 150 nm, monoclinic ZrO 2 : 30 - 60 nm) powder raw materials by the conventional mechanical mixing process. Using a small amount of sintering aid (0.2 wt% MgO) almost 99.2% of theoretical density, 8.54 MPam ½ fracture toughness, 17.35 GPa Vickers microhardness and 495.67 MPa flexural strength were found. It was observed that the maximum flexural strength and fracture toughness was obtained for 10 vol% monoclinic ZrO 2 but maximum Vickers microhardness was achieved for 5 vol% ZrO 2 although the maximum density was found for 20 vol% ZrO 2 . It is assumed that this was happened due to addition of denser component, phase transformation of monoclinic ZrO 2 and the changes of grain size of α-Al 2 O 3 and ZrO 2


Introduction
All materials can be broadly classified into three categories: Metal, Ceramic and Polymer. For the better high temperature strength, light weight, low wear rate and high hardness properties make ceramic the material of choice for a wide range of applications [1]. Alumina is one of the most extensively used and cost-effective materials in engineering ceramic family. It is one of the hardest materials and its hardness is just only next to diamond (9 out of 10 in Mohs scale). It has strong ionic interatomic bonding that provides its reputed material features. Several inherent properties such as low thermal conductivity, comparatively high thermal expansion coefficient and pronounced chemical stability make it appropriate to choose for ceramic materials. In the early 70s' bio-ceramics were employed to perform singular biologically inert roles, such as to provide parts for bone replacement.
Alumina (Al 2 O 3 ) as an inert bio-ceramic has also biomedical application as an alternative to metal alloys for the replacement of hip prostheses and dental implants.
However, the low fracture toughness property limits its load-bearing capacity which is the major drawback of it [2]. Like alumina, zirconia is also one of the most versatile of refractory ceramic oxide. An excellent corrosion resistance, strength, toughness and chemical inertness properties make it superior for the ceramic composites at temperature well above the melting temperature of alumina. It needs to be stabilized for avoid cracking under stress conditions. It has three polymorphs: monoclinic (m, up to 1170˚C), tetragonal (t, up to 2370˚C) and cubic (c, up to 2680˚C) [3]. With increasing heating temperature, phases transform from monoclinic to tetragonal zirconia associated with almost 5% volume change [4] [5] [6]. During cooling to room temperature, tetragonal phase of zirconia transforms into monoclinic phase with expansion of its volume of nearly 3% -5%. If any stress is applied, the stabilized tetragonal phases of zirconia at room temperature change their phases into monoclinic with expanding their volume by absorbing the energy applied on them and stop the crack to propagate further which is referred to as stress-induced phase transformation of zirconia [7]. Through applying this phase transformation mechanism, it increases the fracture toughness of the composite materials that are generally defined as the toughening mechanism. To retain in tetragonal phase at room temperature, the grain size of zirconia must be smaller than the critical transformation size, below which no phase transformation occurs [8]. Relative distribution of ZrO 2 and volume fraction of ZrO 2 retained in the metastable tetragonal phase are also responsible for the increases of fracture toughness of the composites.
Finer particle size enhances the uniform distribution of both Al 2 O 3 and ZrO 2 particles and tetragonal phase retention possibilities of ZrO 2 particle [9]. On the other hand, in the case of over-stabilization, the stress required for the phase transformation may be higher than the fracture toughness. That's why the fracture toughness will be reduced. So, no stabilizer is used in this experimental procedure. Side by side increases the volume percentages of ZrO 2 also decreases the percentages of tetragonal ZrO 2 retention. So, an optimum amount ZrO 2 ad- The previous studies emphasized the improvement of mechanical properties of ZTA by using stabilized zirconia as reinforcement [17] [18]. The analysis also reported that stabilized zirconia has a great effect (impart toughening mechanism) on the mechanical properties. But the effect of unstabilized zirconia on the properties of ZTA is not extensively studied particularly using the powder compaction method. In this investigation, optimization of the composition will be done to get maximum fracture toughness, microhardness, flexural strength and density by varying the volume percentage of unstabilized zirconia. To get the proper output conventional powder compaction method is applied for the sample preparation. Absolute ethanol is used as a liquid media for uniform mixing of the raw powders during pot milling. A sintering schedule will be followed for all composition of the sample by keeping sintering time and temperature at constant level. That will be an alternative and convenient approach to produce Alumina-Zirconia (Al 2 O 3 -ZrO 2 ) ceramic composites which might be used in replace of other traditional ceramic composites rather than cost effectively.

Materials
Commercially available ceramic powders such as nanocrystalline alpha-alumina powder (purity 99.9%, average particle size ~150 nm, Advance materials, USA), monoclinic zirconia powder (purity 99.9%, particle size 30 -60 nm, Advanced materials, USA) and magnesiumoxide (purity > 97%, particle size < 50 nm, RCL Labean Ltd., Thailand) were taken for this work. Here high purity alumina was used as a base material due to avoid the formation of glassy phases on the grain

Methodology
Initially right amount of powder raw materials is taken into a polypropylene pot.
Absolute ethanol (Merck, Germany) is used as a solvent media for wet milling.
For the homogeneous mixing of the powders 24 hours pot milling is continued by motor driven mill with yttria stabilized zirconia balls as a grinding media.
Then the mixture was dried in a vacuum oven (UE600, Memmert, Germany) at for the complete removal of binder from the sample. The cooling rate of the sample from the sintering temperature was comparatively higher (10˚C/min).
Single stage sintering cycle of this work is shown in Figure 1.
The density of the sintered body was measured by immersion in distilled water using Archimedes' principle. At least six samples were considered for density determination of any type of sintered sample and the variation in the density where, s ρ is the density of the sintered sample, w ρ is the density of the water, s m is the mass of the sintered disc, w m is the mass of the disc in water.
The percentage of total porosity ϕ was calculated from the sintered density and the theoretical density o ρ according to the standard formula [19]: where, ϕ is the total porosity of the sample, ρ is density of sintered pellet and o ρ is the theoretical density for samples.
The microhardness was determined on the polished surface of sintered sample by using microhardness tester (HMV-2 Series, Shimadzu Corporation, Japan) following Vickers indentation approach. This value was measured from the ratio of the applied load to the area of the contact of four faces of the undeformed indenter. Following formula was applied to compute Vickers hardness: where, H v is the Vickers hardness (GPa), P is the applied load (N), d is the average length of two diagonals of the indentations (mm).
The porosity dependence Elastic Modulus can also be measured using Dewey-Mackenzie relation [20]: where, E is the effective elastic modulus of porous composites. E o is the elastic modulus of the dense composite and ϕ is the porosity of the sample.
Fracture toughness of the composite was measured on the polished surface of the sintered composite by Vickers indentation fracture methodusing the follo-wingAnstis equation [21]:

Density and Porosity
The following Figure 2 and Figure 3 indicate the variation of the density and porosity of sintered composites as a function of the vol% of zirconia content.
The reported values are the average of data found from six pellets. It has been seen that the density of the sintered body increases with increasing zirconia contentand the porosity decreases with increasing zirconia. This is due to the addition of denser zirconia (5.68 g/cm 3 ) into the alumina (3.97 g/cm 3 ) matrix.
For the addition of 0 vol% of ZrO 2 to Al 2 O 3 matrix, the density of the sample was

Vickers Microhardness (Hv) and Elastic Modulus (E)
Vickers microhardness of sintered studied materials as a function of vol% of zirconia is shown in Figure 4. This property may be described on the basis of porosity and grain size.  Elastic modulus of the sintered ZTA samples at different concentration of zirconia is represented in Figure 5. Elastic modulus of the composite depends on the volume fraction of the components present in it and their individual modulus of elasticities [26]. As alumina has a higher modulus of elasticity (347.58 GPa) than zirconia (199 ± 2 GPa) [27]. So, the sample containing 2 vol%

Flexural Strength and Fracture Toughness
The flexural strength and fracture toughness of the ZTA composite at different zirconia content are presented in Figure 6 and Figure 7 respectively. It has been shown from Figure 6 and Figure 7 that, the flexural strength as well as fracture toughness of composite increases linearly with increase of zirconia addition up to 10 vol%, then decreased. This is due to the grain size of zirconia increases linearly with increasing zirconia content. An increase in grain size improves the possibilities of phase transformation (tetragonal to monoclinic), as a result increasing strength and toughness. For pure alumina maximum flexural strength was obtained 425.23 MPa and maximum fracture toughness was ob-

Phase Analysis of Raw Materials
The XRD patterns of the raw materials (alumina and unstabilised zirconia) for Alumina-Zirconia composites are presented in Figure 8 and Figure 9 respectively. These patterns confirm their corresponding phases' availability.

Phase Analysis of Sintered Sample
The XRD patterns of sintered pure alumina and ZTA composite containing 10 vol% of unstabilised zirconia sintered at 1580˚C for 2 hours are shown in Figure   10 and Figure 11 respectively which exhibits the content of different phases. Figure 10 shows the XRD pattern of alumina disc sintered at 1580˚C. As like as raw alumina the phase was identified by ICDD 00-089-7717 as α-alumina (corundum). Three major characteristic peaks were obtained at 2θ values 35.14˚, 43.36˚ and 57.51˚ which are almost similar to that of the raw alumina. But the peak intensity is significantly higher than the raw alumina's peak intensity. 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 ZTA ceramics containing 2, 5, 10, 15 and 20 vol% unstabilised ZrO 2 sintered at 1580˚C for 2 hours are shown in Figure 12, which demonstrates the content of the different phases.  Phases of α-alumina (ICDD 00-089-7717)), monoclinic zirconia (00-037-1484) and tetragonal zirconia (ICDD 01-072-7115) were identified for all composites by XRD analysis. It is observed that though m-ZrO 2 was used for composite preparation a significant content of t-ZrO 2 peaks were found. The intensity of t-ZrO 2 initially increased with unstabilised zirconia content. Maximum peak intensity of t-ZrO 2 was obtained for composite containing 10 vol% zirconia. Above 10 vol% zirconia peak intensity of t-ZrO 2 decreased and minimum intensity was obtained from composites containing 20 vol% zirconia. But intensity of m-ZrO 2 continuously increased with zirconia content. Low content of ZrO 2 and high percentage of Al 2 O 3 is favorable to retain t-ZrO 2 due to the presence of more alumina matrix. As sufficient alumina was available to retain the ZrO 2 in tetragonal form, the intensity of t-ZrO 2 increased up to the 10 vol% zirconia addition. Above 10 vol% zirconia addition, sufficient amount of alumina was not available to retain the ZrO 2 in tetragonal form. As a result, intensity of t-ZrO 2 decreased with above 10 vol% zirconia addition. But the intensity of m-ZrO 2 increased continuously. Mixture of both tetragonal and monoclinic ZrO 2 phases imparts to increase strength and toughness of the ZTA ceramics. The extent of toughening achieved in the composites depend on the particle size of Al 2 O 3 and ZrO 2 , volume fraction of ZrO 2 retained in the metastable tetragonal phase as well as on the relative distribution of Al 2 O 3 and ZrO 2 in the matrix [28]. Finer particle size of both Al 2 O 3 and ZrO 2 will not only enhance the uniform distribution of Al 2 O 3 and ZrO 2 particles, but also increases the possibility of ZrO 2 being retained as metastable tetragonal phase [29].

t-ZrO2 Retention
In the present study, ZTA ceramic composite was fabricated using unstabilized-     The present study also shows that, ZrO 2 grains are located at the grain boun-  Figure 15 and Figure 16 show the variation of grain size of alumina and zirconia in sintered ZTA composites with respect to zirconia content.

Grain Size Analysis
From Figure 15 it is observed that grain size of Al   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 ZTA composite containing 5 vol% and 20 vol% ZrO 2 respectively. The present results are harmonious with some previous research [33]. The increase in density of Alumina-Zirconia composite may due to the enhanced compaction with zirconia addition as well as the role of zirconia as a grain refiner to reducing the grain size of alumina.