Densification Behaviour and Mechanical Properties of Aluminium Oxide and Cerium Oxide-Doped Yttria Tetragonal Zirconia Polycrystal Ceramics Using Two-Step Sintering

The densification behaviour, mechanical properties, and microstructure of high-purity Al 2 O 3 and CeO 2 -doped Y-TZP with different weight percentage varied from 0.3 to 1 wt% were investigated. The samples were pressed uniaxially at 200MPa into rectangular bars and discs and pressureless-sintered at temperature ranging between 1250C and 1450C for 2 h while the microstructure was characterized with a scanning electron microscope (SEM). Two-step sintering process works well for temperature higher than 1400C and it createdmost tetragonal phase arrangement for stable structure to delay ageing through phase transformation. The mechanical properties in terms of bulk density, Young’s modulus, Vickers hardness, and fracture toughness were also measured. The results indicate that the addition of dopants accelerated the densification parameters and reinforced and toughened the obtained bodies.Themaximum values for the mechanical properties of the Al 2 O 3 and CeO 2 -doped Y-TZP ceramics were 6.01, 220GPa, 13.8 GPa, and 7MPa for density, Young’s modulus, Vickers hardness, and fracture toughness, respectively, which are higher than those of the doped samples.


Introduction
Yttria-stabilised tetragonal zirconia polycrystalline ceramics (Y-TZP) are becoming popular engineering materials due to their excellent mechanical properties, studied and used in many engineering applications, such as engine parts, valves, cutting tools, and moulds, due to their good fracture toughness, high strength, elastic modulus, and wear resistance [1][2][3].
In recent years, yttria-tetragonal zirconia polycrystals (Y-TZP), with their superior combination of mechanical properties and chemical inertness, have been employed in the biomedical field as an implant material [4][5][6][7][8][9].In general, when a restricted number of ZrO 2 particles undergo the transformation during cooling from the sintering temperature, the accompanying volume expansion would cause the development of a fine distribution of microcracks in the ceramic matrix.These microcracks would increase the toughness by interacting with a propagating crack, causing deflection and blunting of the crack.Due to this nature, the engineering application of pure ZrO 2 proves to be nonviable as the sintered body would crumble to pieces upon cooling from the sintering temperature [10,11].
To overcome this undesirable phase transformation, stabilisers such as magnesia (MgO), calcia (CaO), ceria (CeO 2 ), and yttria (Y 2 O 3 ) have been added in various quantities in zirconia.In general, alloying zirconia with these oxides reduces the change of chemical free energy, which in turn lowers the tetragonal to monoclinic transformation temperature to below ambient temperature [12][13][14][15][16]. Garvie et al. [17] highlighted the potential of increasing both the strength and toughness of zirconia through a mechanism involving the phase transformation of metastable tetragonal particles induced by the presence of the stress field ahead of a crack.
Kobayashi et al. [18] first discovered a serious limitation of Y-TZP ceramics for applications near 250 ∘ C in moist environment.Results revealed that the ceramic can suffer a slow, tetragonal to monoclinic phase transformation at the samples surface in a humid atmosphere, followed by microcracking and a serious loss in strength, a phenomenon subsequently known as ageing or low temperature degradation (LTD).Ever since then, many researchers have experimented with Y-TZP with an attempt to understand the basic micro mechanisms of the ageing-induced () to () phase transformation and to suppress this LTD phenomenon [19][20][21][22][23][24].Although the experimental observations of the ageing phenomenon are well understood, the actual mechanism responsible for the degradation has not been unequivocally explained.The effect of copper oxide (CuO) doping Y-TZP has been studied by .It was found that mechanism involving liquid phase enhanced due to the low melting point of CuO in ZrO 2 matrix densification of the ceramics.The effect of TSS on the densification and grain growth of nanocrystalline 3Y-TZP ceramic was investigated by Mazaheri et al. [28].From the results it was found that the grain growth rate of nanocrystalline 3Y-TZP in conventional sintering route is significantly lower than that of other ceramics such as ZnO and Al 2 O 3 ; also full stabilized tetragonal zirconia (3Y-TZP) ceramic can be obtained at grain size <275 nm.
It has been reported that improved Y-TZPs with optimized mechanical properties and ageing resistant could be obtained by the addition of more than one stabiliser to zirconia.For instance, Xu et al. [29].It was found that the addition of CeO 2 to Y-TZP could prevent ageing, while retaining relatively high fracture toughness of 7-9 MPam 1/2 .Similar observation was made by Sato et al. [30,31] who found that the ageing kinetics of 2, 3, and 4 mol% Y-TZP sintered at 1400 ∘ C to 1650 ∘ C decreased with increasing yttria content.Results also showed that increasing yttria content resulted in a reduction in the onset critical ageing temperature for phase transformation.

Materials and Method
The experiment was conducted from a starting Y-TZP powder containing 3 mol% yttria and cerium oxide-aluminium oxide powders of 99.9% purity as dopants manufactured by Kyoritsu Ltd., Japan.Three different compositions of CeO 2 -Al 2 O 3 -doped Y-TZP powder mixtures (0.3, 0.5, and 1 wt%) and undoped Y-TZP sample were prepared.The ratio dopant to Y-TZP was 1 : 99, whereby, regardless of the percentage of dopant in each composition, the addition of both dopants will always be equal to 1 wt%.Each powder was weighed carefully and then underwent an ultrasonification process, whereby the base powder (Y-TZP) is first added with 150 mL of ethanol and left for 6 minutes, followed by the addition of the two dopants (Al 2 O 3 and CeO 2 ), and further ultrasonification of 22 minutes follows.The powder was composed of primary particles with an average size of ∼30 nm (Figure 1(a)) and spherical granules (Figure 1(b)).The powder was uniaxially compact in a standard procedure [32] at about 1 KN using hardened steel mold and die assembly.The powder was processed by cold isostatic pressing at 200 MPa, followed by pressureless sintering at temperature ranging from 1250 ∘ C to 1450 ∘ C in air.Unlike regular pressureless sintering, twostep sintering involves increasing the temperature to about 1000 ∘ C and is maintained for a period of one hour, before further heating to 1250 ∘ C. The sintered samples were ground on one face by Silicon Carbide, SiC papers of 120, 240, 600, 800, and 1200 grades successively, followed by polishing with 6 m and 1 m diamond paste to produce an optical reflective surface.The Vickers indentation test performed using a load of 10 kn on polished surfaces in order to measure the hardness and indentation toughness.Bulk density of the specimens was determined by the Archimedes method using distilled water.The Young's modulus of the sample can be calculated using the standard test method (ASTM E1876-97) [33][34][35].The microstructure of the samples was examined by JSM-6310 scanning electron microscopy (SEM).The results for undoped Y-TZP at low sintering temperature 1300 ∘ C shows that the relative density achieved was below 95%.In the case of the 0.5 wt% Al 2 O 3 and CeO 2 -doped ceramics, the two-step sintering at 1400 ∘ C was found to be beneficial in enhancing the densification of the ceramics.From the sintering regime employed, 1400 ∘ C was found to be the optimum sintering temperature as all Y-TZPs sintered at this temperature recorded densities that were close to 98%.In addition, for the 0.5 wt% Al 2 O 3 and CeO 2doped Y-TZP, maximum density of 6.01 Mgm −3 (i.e., 8% T.D.) was recorded at this temperature and further increase in the sintering temperature resulted in the decline of its bulk density.This phenomenon is probably attributed to a phase transformation from tetragonal to monoclinic zirconia that starts after 1400 ∘ C.During the phase transformation, a volume expansion of zirconia grains (i.e., expansion of the zirconia lattice) occurs and therefore a significant decrease in density is reported.Similar results have been reported by other researchers working on Al 2 O 3 or CeO 2 as a sintering additive in Y-TZP [36,37].Figure 3 shows that an  value of above 200 GPa could be achieved with the additions of ≥0.However, in the present work, bulk density is unlikely to be the governing factor for the decline of hardness observed from 1400 ∘ C to 1450 ∘ C because all Y-TZPs except for the 0.5 wt% Al 2 O 3 and CeO 2 -doped Y-TZP were more than 97% dense at these temperatures as shown in Figure 2. The decline of hardness could possibly be due to grain growth resulting from sintering at higher temperatures.The 1 wt% Al 2 O 3 and CeO 2 -doped Y-TZP display the lowest hardness for sintering above 1400 ∘ C as shown in Figure 4. Two possible explanations can be made for the decline in hardness of this sample when sintered above 1400 ∘ C; firstly, hardness was strongly dependent on bulk density, which decreased considerably during the temperature range as shown in Figure 2 and, secondly, it can be associated with the reduction of tetragonal phase content and a concomitant increased in the cubic phase formation in the zirconia matrix with increasing temperature.The reduction of tetragonal phase content had been cited by Lawson et al. [38] and Liu and Chen [39] to be the reason for the lower hardness of Y-TZP doped with CuO and Sr 2 Nb 2 O 7 , respectively.

Young's
3.4.Fracture Toughness.The effect of Al 2 O 3 and CeO 2 -doped Y-TZP and sintering temperature on the fracture toughness ( Ic ) is shown in Figure 5.In general, additions of up to 0.3 wt% Al 2 O 3 and CeO 2 have negligible effect on the fracture toughness of Y-TZP for the entire sintering temperature investigated.The fracture toughness of these samples was found to fluctuate between 4.5 MPam 1/2 and 5.18 MPam 1/2 .The fact that the  Ic did not change significantly indicates that the additions of Al 2 O 3 and CeO 2 below 0.3 wt% did not affect the tetragonal phase stability of Y-TZP.Higher Al 2 O 3 and CeO 2 additions of 0.5 and 1 wt% also showed similar trend at sintering temperatures below 1300 ∘ C.However, sintering above 1300 ∘ C, the  Ic of both Al 2 O 3 and CeO 2 -doped Y-TZP increased by increasing the temperature.This observation was more pronounced for the 0.5 wt% Al 2 O 3 and CeO 2doped Y-TZP, which exhibited a significant increase in  Ic from 4.6 MPam 1/2 at 1300 ∘ C to 7.0 MPam 1/2 at 1450 ∘ C.

Microstructural and Phase Development.
Scanning electron microscopic images of 0.5 wt% Al 2 O 3 and CeO 2 -doped Y-TZP sintered at 1450 ∘ C are presented in Figure 6(a).The analyses revealed that samples sintered at 1450 ∘ C showed particles that are nonspherical in shape and with little agglomeration and porosity.The surface topographies confirmed the presence of little agglomeration and porosity.For all of them, the zirconia appears agglomerated in light contrast and the aluminium oxide and cerium oxide appear as dark background particles.The average particle size obtained from the particle size distribution analysis using the grain intercept method was 1.13 m.
The SEM results are in good agreement with the density of the 0.5 wt% Al 2 O 3 and CeO 2 -doped Y-TZP sintered at 1450 ∘ C.
On the other hand, relatively rough surfaces were observed on surface sintered at 1250 ∘ C but no grains structures were clearly observed as depicted in Figure 6(b).The likely reason that 0.5 wt% Al 2 O 3 and CeO 2 -doped Y-TZP sintered at 1250 ∘ C did not have grain structure could be due to the presence of a glassy phase that suppressed the crystal growth and sealed the grain boundary.
It was also found that 1 wt% Al 2 O 3 and CeO 2 -doped Y-TZP sintered at 1450 ∘ C resulted in severe phase transformation which was also accompanied by networks of micro-and macrocracks on the surface as typically shown in Figure 7.
High angle XRD analysis performed on the 1450 ∘ C sintered sample revealed that very high percentage of cubic phase (up to 48%) was calculated in the zirconia matrix.This result indicated that sintering of 1 wt% Al 2 O 3 and CeO 2 -doped Y-TZP at 1450 ∘ C resulted in the formation of monoclinic and cubic phases in the zirconia matrix.The spontaneous phase transformation upon cooling from sintering and the development of the cubic phase was not observed for the 0.5 wt% Al 2 O 3 and CeO 2 -doped when sintered at 1450 ∘ C. The variation of Young's modulus with sintering temperature of all composition studied was in good agreement with the variation in bulk density.In general, it was found that Y-TZPs containing above 0.5 wt% Al 2 O 3 and CeO 2 attained  values above 200 GPa when sintered at temperature 1350 ∘ C.

Conclusion
In general, the hardness of all Al 2 O 3 and CeO 2 -doped Y-TZPs was higher than the undoped material when sintered at 1250 ∘ C and 1300 ∘ C. In particular, the addition of 0.3-1 wt% Al 2 O 3 and CeO 2 was most effective in enhancing the hardness of Y-TZP.These doped samples exhibited hardness of >13 GPa as compared to ∼9.7 GPa for the undoped ceramics sintered at 1250 ∘ C.
The additions of up to 0.3 wt% Al 2 O 3 and CeO 2 were found to have negligible effect on the fracture toughness of Y-TZP throughout the sintering regime employed.The KIc of these samples was found to fluctuate between 4.67 MPam 1/2 and 5.18 MPam 1/2 , which implied that the tetragonal phase stability of Y-TZP was not disrupted.However, as the temperature increased above 1400 ∘ C, an increasing KIc trend  was noted for both ceramics.In particular, the 1 wt% Al 2 O 3 and CeO 2 -doped Y-TZP exhibited a significant increase in the fracture toughness, from 4.8 MPam 1/2 at 1400 ∘ C to 7.1 MPam 1/2 at 1450 ∘ C.

3. 1 .
Bulk Density.The variation of bulk density for Y-TZPs with different amounts of Al 2 O 3 and CeO 2 sintered at temperatures 1250 ∘ C-1450 ∘ C is shown in Figure 2. All Y-TZP compositions including the undoped shared a common trend.Addition of Al 2 O 3 and CeO 2 were found to be most beneficial at sintering temperatures of 1250 ∘ C to 1400 ∘ C.
Modulus.The effect of aluminium oxide and cerium oxide doped Y-TZP on the Young's modulus is shown in Figure 3.The addition of Al 2 O 3 and CeO 2 was beneficial in enhancing the elastic modulus of Y-TZP, especially at low sintering temperatures of 1250 ∘ C and 1300 ∘ C when compared to the undoped Y-TZP.

3. 3 .
Vickers Hardness.The effect of sintering temperature and Al 2 O 3 and CeO 2 on the Vickers hardness of Y-TZP is shown in Figure 4.The results obtained in the present work confirmed that the Al 2 O 3 and CeO 2 additions were beneficial in improving the hardness of zirconia when sintered at low sintering temperatures.

2 )Figure 5 :
Figure 5: Variation of sintering temperature and Al 2 O 3 and CeO 2 addition on the fracture toughness of sintered Y-TZP.
Sinterability and mechanical properties of Al 2 O 3 and CeO 2doped Y-TZP ceramics were studied in the present work.The beneficial effect of Al 2 O 3 and CeO 2 in enhancing the densification of Y-TZP has been revealed.The Al 2 O 3 and CeO 2 -doped Y-TZPs achieved almost full density (>97% of theoretical density) at 1300 ∘ C. The study revealed that 1350 ∘ C was the optimum sintering temperature for all Y-TZPs to achieve >98% of theoretical density.

Figure 6 :
Figure 6: (a) SEM image showing lines intercepting on grain boundaries of sample at 1450 ∘ C. (b) SEM image showing lines intercepting on grain boundaries of sample at 1250 ∘ C.