Structural and Mechanical Properties of Zirconia Toughened Alumina (ZTA) Composites

Structural and Mechanical Properties of Zirconia Toughened Alumina (ZTA) Composites

1. Moazzam Hossen1*, F.-U.-Z. Chowdhury2, M. A. Gafur3, A. K. M. Abdul Hakim4

   1Department of Computer Science & Engineering, International Islamic University Chittagong, Chittagong, Bangladesh.

   2Department of Physics, Chittagong University of Engineering and Technology, Chittagong, Bangladesh.

   3PP and PDC, BCSIR, Dhaka, Bangladesh.

   4Department of Materials and Metallurgical Engineering, Bangladesh University of Engineering and Technology, Dhaka,

   Bangladesh.

   Abstract – The Zirconia toughened Alumina (ZTA) composites with 0-16 wt % of ZrO2 were prepared by slurry method and sintered at temperature (Ts) 1500°C and 1600°C for 2 hours. The density, porosity, structural properties and mechanical properties of Al2O3-ZrO2 composites with respect to ZrO2 content as well as sintering temperature have been explored in the present work. About 2% higher density of 16 wt % ZTA has been achieved and the microstructures are highly homogeneous and finer with less porosity when compared to pure Al2O3. The surface morphology of the samples was studied by using SEM. The effect of zirconia content on hardness and elastic modulus were investigated. At 1500°C, maximum hardness shows 15.79 GPa and at 1600°C, it is 19.76 GPa, which is observed for pure Alumina. However, at same temperatures, minimum hardness shows 11.24 GPa and 12.96 GPa, respectively, that is observed for 16 wt% ZrO2. Elastic modulus also shows same behaviour as shown for hardness. Flexural strength increases with the increase of both zirconia content and sintering temperature. The approach adopted in the present study may provide an alternative to design Al2O3- ZrO2 composites with improved mechanical properties.

   Keywords: ZTA; X-Ray Diffraction; SEM; Vickers microhardness; Flexural strength.

   1. INTRODUCTION

      The demand for composite materials over metals is increasing. Zirconia-toughened alumina (ZTA) composites have received significant scientific and technological attention during the last two decades for use in cutting tools, dies or prosthesis components due to their excellent room temperature strength, hardness, toughness and wear resistance [1]. More recently, ZTA has become increasingly important as a structural material for biomedical implants, such as hip prosthesis. A key issue for such implants is to increase their lifetime. For these reasons, research efforts currently focus on long-lasting devices based on new materials characterized by superior strength and toughness, optimal tribological properties and long-term biocompatibility [2].

      In this frame, ZTA composites have demonstrated their effectiveness for orthopedic applications. In this system, alumina provides high strength and hardness, whereas tetragonal zirconia exerts a toughening effect, by controlling transformation into the monoclinic phase [3, 4]. It has been recognized that the magnitude of toughening and flexural

      strength is completely dependent on the microstructure of the aluminazirconia composite (i.e., volume fraction, size, shape, location and size distribution of ZrO2) [5, 6]. Generally, ZTA composites are prepared by slurry method, whose main issue is keeping a homogeneous microstructure in the final, sintered materials. In fact, zirconia aggregates can lead to localized aging phenomena [7, 8], whereas alumina ones could behave as preferential sites for

      crack propagation. Zirconia particles can be embedded in a variety of different matrices to form transformation- toughened composites provided the zirconia particles have a sufficiently small size. Requirements are that the matrix will not react with the ZrO2 or vice-versa and that the matrix has sufficiently high elastic modulus to maintain a low MS (martensitic transformation) temperature. Usually, zirconia particles are likely to separate from each other and grow during sintering. However, evidence suggests that zirconia particles are mobile in nature and move within the grain boundary of the matrix. The presence of dispersed tetragonal zirconia particles in the matrix can affect the strength and toughness of the material in two ways: (a) the stress field of a crack can initiate the martensitic transformation and compressive stress on the crack surface is developed due to the expansion of transformed particles against the matrix, and (b) the transformation may take place near the surface of the macroscopic piece due to surface grinding and result in a compressive surface which leads to higher strength values. 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 ZTA without any grain growth during sintering. Therefore, in order to control the properties of the product, it is important to understand these aspects at each and every stages of the process. Excellent control over composition can be achieved, most notably in the preparation of multi-component oxides that are homogenous at a near-atomic scale. On the basis of the above findings and importance, the particular interest of this study was to develop a series of Al2O3-ZrO2 composites through chemical route and characterize mechanical properties.

      The present work deals with the structural and mechanical properties of Al2O3-ZrO2 composites by slurry

      V I(111) I(111) m

      m

      method. Preparation of various Zirconia Toughened Alumina compositions was done. X-ray Diffraction (XRD)

      m I

      (111) m

      I(111)m

      I(111)t

      patterns and microstructures were analyzed. Physical properties such as density and porosity and mechanical properties such as microhardness, elastic modulus and flexural strength were determined.

   2. EXPERIMENTAL

      1. Sample Preparation

         The commercially available powders used in this work

         were the nanocrystalline, – Al2O3 powder and yttria- stabilized ZrO2 (3 mol%) powder from the manufacturing

         Theoretical density o of the sample was calculated from the actual density of the starting powders and their weight percentages using the rule of mixtures [9]:

         o AVA ZVZ

         where A, Z are the density and VA, VZ are the volume fraction of Al2O3 and ZrO2 respectively. Values of A= 3.97 gm/cm3 and Z =6.10 gm/cm3 have been used here to calculate o . The measured bulk density b was determined using the formula:

         company Inframat® Advanced MaterialsTM, USA. Alumina presented crystal size ~40 nm and particle size ~150 nm and zirconia presented average particle size 30-60 nm. To

         b

         m

         r 2 h

         prepare different composition, related powders were weighed by following unitary method, keeping the total weight constant.

         Different compositions had been prepared by varying ZrO2 content from 4 to 16 wt% in the step of 4 wt%. A pure Al2O3 sample was also prepared to compare its properties with those of ZTA composites. To attain the compositions, mechanical milling was performed in distilled water with 65 wt% of solid content for 24 hours in ball milling, with zirconia ball as a grinding media. Ammonium polyacrylate was used as dispersant. The slurry of the power mixtures was dried in an oven for 24 hours at 100°C and the dried lumps were crushed and passed through a plastic sieve. Hand milling was done for several hours to obtain a homogeneous distribution. Powder compacts with dimension of 5×6×50 mm were confirmed by uniaxiallly pressing a 60 MPa. Before compaction, polyvinyl alcohol (PVA) was mixed as a binder with the powder to provide some green strength for subsequent handling. The withdrawal of the binder was carried through in a muffle furnace with heating rate 6 ºC/min at 600 ºC per 180 min. The sintering was carried out in a box furnace at 1500 ºC and 1600 ºC for two hours with a heating rate of 20 ºC/min. The bulk densities of the sintered samples were measured from the ratio weight/volume. The sintered specimens were machined longitudinally with a 325 grit-resin bonded diamond wheel at a depth of 5 µm/pass. The final dimensions of the specimens were 3×4×35 mm.

      2. Measurements

      Phase identification was confirmed by powder X-ray diffractometry (XRD) technique using BRUKER X-Ray Diffractometer of Model D8 Advance with Cu-K radiation (= 1.54 Ã…). For each sample a scan has been performed from 20 to 75 with a step size of 0.02.The volume fraction of monoclinic zirconia (Vm) was calculated according to the following equation:

      Where m is the mass, r the radius and h the height of the

      sample.

      The porosity P of the sample was determined using the relation.

      P (1 b ) 100%

      0

      Microstructures for the samples of different compositions were examined by using Philips XL-30 scanning electron microscopy (SEM). Mean linear intercept

      length l is used to determine the ASTM grain size number, G, by using the linear intercept method:

      G 3.2877 6.6439log10 l

      where l is in millimeters at 1× Magnification.

      The methodology used for the determination of hardness, was in accordance with ASTM C1327-99 [10]. Five Vickers impressions had been carried through in the surfaces of each one of the samples, which already were polishing, using an applied load of 2 kgf (19.6141 N) during six seconds. After the diagonal length measurement, the values of the Vickers hardness (GPa) were calculated, by the following equation [10, 11]:

      H 0.0018544 F

      v d 2

      Where F is the load in N and d is the average length of the two diagonals of the indentations in mm.

      The porosity dependence Elastic Modulus can also be measured using the Dewey-Mackenzie relation [12]:

      E Eo (1 2P)

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

      The flexural strength of the specimens was determined according with ASTM D79010 by three-point bending method (Hounsfield Model H10KS-universal test machine), with a span of 30 mm and crosshead speed of 0.5 mm/min

      t

      m m

      m m-ZrO

      2

      t

      t t-ZrO

      2

      Al O

      2 3

      at ambient room temperature conditions. The flexural strength was calculated from the following equation.

      m t m t

      m t t t

      x=16% x=12%

      flexural

      3WL

      2BD2

      Intensity (arb. units)

      x=8% x=4%

      where W is the load (N), B is the width (mm) and D is the thickness (mm) of the sample.

      20 30 40 50 60 70

      2 Theta (degree)

      x=0%

   3. RESULT AND DISCUSSION

1. XRD analysis

   The XRD patterns of a ZTA sample containing 0, 4, 8, 12 and 16 wt% ZrO2 sintered at 1500°C and 1600°C are shown in figure 1. and figure 2. respectively which confirms the formation of multi-phase ZTA.

   Figure 2. XRD pattern of ZTA composites containing different wt% ZrO2 and sintered at 1600°C for 2 hours.

   ZrO2 retained in the metastable tetragonal phase as well as on the relative distribution of Al2O3 and ZrO2 in the matrix [13]. Finer particle size of both Al2O3 and ZrO2 will not only enhance the chances of a uniform Al2O3 and ZrO2 distribution, it also increases the possibility of ZrO2 being retained as metastable tetragonal phase [14]. With the

   2

   m m-ZrO

   Intensity (arb. units)

   t

   m

   t t

   m

   m t m m

   t t-ZrO Al O

   2 3

   2

   t

   t t

   x=16% x=12% x=8% x=4% x=0%

   increase of ZrO2 content, intensity of some t-ZrO2 peaks are reduced, whereas some m-ZrO2 peaks are appeared and further intensity increases. So it can be inferred that monoclinic phase is indeed retained at the room temperature due to the presence of the hard alumina matrix and this phase is responsible for flexural strength improvement by transformation toughening mechanism. The effect of sintering temperature 1500°C and 1600°C on the XRD pattern of a ZTA composite containing 0, 4, 8, 12 and 16 wt% ZrO2 has been shown in the related figure. From figure

   1. and figure 2. it is observed that with the increase of sintering temperature more peaks for ZrO2 have been found. It also reveals that with the increase of sintering

      20 30 40 50 60 70

      2 Theta (degree)

      Figure 1. XRD pattern of ZTA composites containing different wt% ZrO2 and sintered at 1500°C for 2 hours.

      XRD analysis of the samples indicate that only – Al2O3, t and m-ZrO2 are the crystalline phases present in the pure Al2O3 and in the ZTA composites, respectively. It is observed that at 4 wt% and 8 wt% ZrO2 content, there is no m-ZrO2 phase and all are t-ZrO2 phase, while with the increase of ZrO2 content, m-ZrO2 phase increases and the t- ZrO2 phase decreases. However, t-ZrO2 retention becomes much easier to trigger the transformation to monoclinic. The extent of toughening achieved in the composites depend on the

      particle size of Al2O3 and ZrO2, volume fraction of

      temperature, more m-ZrO2 phases appear and the intensity

      of t-ZrO2 decreases.

      Densification of powdered particle is a thermally assisted phenomenon due to sintering at 1500°C and 1600°C. Figure 3 shows the result of density as a function of zirconia content. Presence of ZrO2 results in a continuous increase in density. The theoretical density of ZrO2 is around 6.10 gm/cc whereas that of Al2O3 is 3.97 gm/cc, therefore the effective density values of the alumina- zirconia rise with increasing ZrO2 content and a compact structure is formed. Sintering in practice is the control of both densification and grain growth. 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 is resulted. 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. It shows that density increases with sintering temperature. It is known that matter transport during the sintering of polycrystalline composites occurs by diffusion, a thermally activated process, which can occur

      along different paths in the solid, giving rise to the different mechanisms of diffusion: lattice, grain boundary and surface diffusion. It is also known that there is an exponential dependence of diffusion coefficient (D) on temperature.

      4.1

      Figure 4. shows the effect of ZrO2 content and sintering temperature on the porosity of ZTA composites, respectively. It is observed that the porosity is decreased with ZrO2 content as well as sintering temperature. It resembles that the composites are in more compact form at the higher temperatures. During the diffusion process of

      Theoretical Density

      4.2 Bulk Density

      Theoretical Density (gm/cc)

      4.1

      4.0

      Bulk Density (gm/cc)

      3.9

      sintering, the intervening porosity diffuses out, resulting in reduction of porosity. It is observed that the porosity is decreased with ZrO2 content as well as sintering

      4.0

      Theoretical Density

      1500C

      3.8

      3.7

      temperature. This porosity usually originates from entrapped pore during green compaction.

      With the increase of temperature, sample becomes/p>

      4.2 Bulk Density

      4.1

      4.0

      1600C

      4.2

      4.1

      4.0

      more homogeneous. During 100°C interval, porosity decreases sharply from low temperature to high temperature. It is also seen that the percentage of porosity is minimum at the sintering temperature 1600°C for all the composites than that of at 1500°C, i.e., it reveals that the

      0 4 8 12 16

      Zirconia Content

      Figure 3. Theoretical and Bulk Density as a function of zirconia content sintered at different temperatures for 2 hours.

      Rising in temperature increases the value of D and hence density increases with sintering temperature. The density is also influenced by phase transformation of monoclinic to tetragonal zirconia above 1170°C during heating. The more the tetragonal phase retains in the structure, the more the density of the composite attains. The density result reveals that the elimination of pore, simultaneous grain growth with grain boundary diffusion and lattice diffusion are the predominant mechanism to compact the material at high temperature. In this work, the theoretical density calculated from the rule of mixture was found around 94.21% for pure Al2O3 and that of 94.51- 96.11% for 16 wt% of ZTA sintered at 1500°C for 2 hours. In addition, about 98.65% of theoretical density was observed for pure Al2O3 and 99.05-99.28% that for same ZTA sintered at 1600°C for 2 hours. About 3-6% density has been changed due to increase of 100°C for both pure Al2O3 and ZTA. This resembles that after increasing sintering temperature particles become more compact and dense. Moreover, increasing temperature and Zirconia content, density increases more linearly than that of lower temperature and content.

      sample is more compact at this temperature.

2. Scanning electron microscopy

   Figure 5. (a)(e) show the SEM micrographs of the ZTA composites containing different wt% ZrO2 at 1600°C. When ZrO2 was added, the grains become finer compared to that of pure Al2O3, as shown in Figure 5. (b)-(e). Observations show highly homogeneous microstructures without agglomerates, pores or abnormally grown alumina grains. The micrograph exhibits the Al2O3 and ZrO2 grains as dark and whitish color respectively, where zirconia grains are embedded around the alumina grain and within the grain. However, small fractions of intragranular zirconia grain are also being noticed.

   From the figure, it can also be observed that ZrO2 particles are uniformly dispersed throughout the alumina matrix. As the amount of 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 refiner and led to smaller Al2O3 grains. This is possibly due to increase in density as well as enhancement of mechanical properties. It can also be observed that the grain sizes of ZrO2 increase and hinders the grain growth of Al2O3 at 1600°C, which contributes to the lowering of porosity. A fine grain size and refined microstructure are necessary for improved mechanical

   96.0

   95.5

   Relative Density (%)

   95.0

   94.5

   94.0

   1500C

   Relative Density

   Porosity

   5.6

   5.2

   4.8

   4.4

   Porosity (%)

   4.0

   1.4

   properties especially for wear resistance of dental implant.

   It is observed from Figure 6 that, grain size of Al2O3 and ZrO2 vary with the amount of Zirconia content added with the pure Al2O3. In the present work, it is seen that when ZrO2 was added to Al2O3 then the grain size decreases slowly.

   99.30

   99.15

   99.00

   98.85

   98.70

   Relative Density

   Porosity

   1.3

   1.2

   1.1

   1.0

   0.9

   0.8

3. Hardness and elastic modulus

   The Vickers hardness of Al2O3-ZrO2

   composites as a

   98.55

   1600C

   0 4 8 12 16

   Zirconia Content

   0.7

   function of zirconia content and at temperature 1500°C and 1600°C, respectively are shown in figure 7. Figure 7. shows the variation in hardness of ZTA composites with the

   Figure 4. Relative Density and Porosity as a function of zirconia content sintered at different temperatures for 2 hours.

   amount of zirconia content. Hardness decreases with the increase in ZrO2 content. Zirconia has lower hardness than

   alumina and it can also obstruct the densification of the alumina phase. Again, amount of zirconia has an adverse affect on the hardness of the composites due to coarsening of the zirconia grains and formation of subsequent porosity. From the figure it is also clear that, for any particular composition, hardness increases with sintering

   1. Al2O3

   2. Al2O3+4 wt% ZrO2

   3. Al2O3+8 wt% ZrO2

   4. Al2O3+12 wt% ZrO2

   5. Al2O3+16 wt% ZrO2

Figure 5. Microstructure of Different Compositions sintered at 1600°C

temperature. As higher sintering temperature offers better densification, hardness value increases as a whole. Similar trend in hardness has been observed by R. P. Rana [15]. Similar trend in hardness has also been observed by Basu et al. [16]. The hardness increases with sintering temperature for all compositions. At 1500°C, maximum hardness shows

15.79 GPa and at 1600°C, it is 19.76 GPa, which is observed for pure Alumina. However, at same temperatures, minimum hardness shows 11.24 GPa and 12.96 GPa, respectively, that is observed for 16 wt% ZrO2. At the same temperatures, maximum and minimum hardness of 4, 8 and 12 wt% ZrO2 are in between these values. In addition, for all the compositions, hardness increases moderately with TS. The above increase in the hardness value of the Al2O3-ZrO2 composite is related to the density increment. It appears that the phases of the sintered composite have least influence on the hardness.

1.9

1500C

3.5 Flexural Strength

1.8 1600C

1.7

Grain Size (m)

1.6

1.5

1.4

1.3

1.2

1.1

1.0

The Flexural strength increases linearly with increasing zirconia content as well as sintering temperature that has shown in figure 8. The flexural strength was found around 401 MPa for pure Al2O3 and that of 656 MPa for 16 wt% of ZTA sintered at 1500°C for 2 hours. In addition, about 432 MPa of flexural strength was observed for pure Al2O3 and 691 MPa for 16 wt% of ZTA sintered at 1600°C.

1500C

700 1600C

650

0 4 8 12 16

Zirconia Content

Figure 6. Variation of grain size with zirconia content at various sintering temperature

600

Flexural Strength (MPa)

550

500

450

16 Hardness

15 Elastic Modulus

14

13

330

327

Elastic Modulus (GPa)

324

400

0 4 8 12 16

Zirconia Content

Hardness (GPa)

12

11

1500C

10

20

18

16

14

1600C

12

Hardness

Elastic Modulus

321

318

360

355

350

345

340

Figure 8. Variation of Flexural strength with zirconia content at different sintering temperature

The martensitic transformation and frontal process zone are predominant mechanisms to increase the flexural strength of Al2O3-ZrO2 nanocomposite. Since the tetragonal zirconia grains do not undergo the stress-induced transformation to the monoclinic form when grain size is

0 4 8 12 16

Zirconia Content

Figure 7. Variation of hardness and elastic modulus with zirconia content at different sintering temperature

Elastic modulus of the composite depends on the volume fraction of the phases present and their individual modulus of elasticity. At 1500°C, densification of the composites is improved by reducing pore volume fraction of the matrix and as a result of which the modulus of elasticity improves. Alumina has a higher modulus of elasticity cmpared to zirconia. So, samples containing 4 wt% zirconia have the highest modulus of elasticity, whereas specimen containing 16 wt% zirconia has lower levels of densification at a given sintering temperature compared to other samples having lower amount of zirconia. Similar phenomena also have been observed for 1600°C. Simultaneously, the grain size of zirconia increases and hinders the grain growth of alumina at 1600°C, which contributes to the lowering of elastic modulus. Nicholson

smaller than a critical size. The particle sizes lower than the critical value does not affect the strength. The reduction of size improves the bulk density but not the transformability of the tetragonal zirconia grains. Usually, the effect of grain size on strength in transformation-toughened composites is a complex phenomenon [18]. An increase in grain size improves the transformability, therefore, increasing strength. Microstructure reveals that the zirconia grain size increases with increasing zirconia content. The m-ZrO2 phase increases with increase in ZrO2 content. The retention of t-ZrO2 varies during fracture and t-ZrO2 fraction decreases with increasing zirconia content. However, t-ZrO2 retention becomes much easier to trigger the transformation to monoclinic. As a result, its contribution to transformation toughening predominates although retention of m-ZrO2 aids to microcrack toughening.

The addition of zirconia is very effective in hindering the grain growth of alumina during sintering, even for the lowest zirconia content. The addition up to 16 wt% ZrO2 prevents grain growth without allowing the formation of zirconia agglomerates, which are very deleterious for the mechanical stability of the composite. However, the premature zirconia agglomeration can be observed in case of composites containing 15 mol% ZrO2 [16].

1. CONCLUSION

   The Zirconia-toughened alumina (ZTA) composite has been successfully prepared by slurry method and they have been sintered at 1500C and 1600C for two hours. The sintering temperature and sintering time plays vital role in changing the properties. The X-ray diffraction studies clearly showed the formation of -Al2O3, t and m-ZrO2 phases both in the pure Al2O3 and in the ZTA composites, respectively. It is seen that the ZrO2 addition promotes composites with higher densities. The density has been found to increase with increasing zirconia content and sintering temperature, while porosity decreases with the same change of zirconia content and sintering temperature. SEM images show the microstructures of prepared ZTA which reveals that the grains are homogeneous and finer at higher TS. It can also be observed that the grain sizes of ZrO2 increase and hinders the grain growth of Al2O3 at 1600°C compared to that of at 1500°C, which contributes to the lowering of porosity. Hardness and elastic modulus was found to be increased with sintering temperature, while the addition of ZrO2 decreases the hardness and elastic modulus of the composite. The Flexural strength increases with increasing zirconia content as well as sintering temperature. The martensitic transformation and frontal process zone are predominant mechanisms to increase the flexural strength of Al2O3-ZrO2 nanocomposite. The zirconia substitution in pure Al2O3 and the change of sintering temperature was found to have a significant

   effect on structural and mechanical properties.

   ACKNOWLEDGMENT

   The authors are thankful to PP and PDC, BCSIR, Dhaka-1000, Bangladesh for extending experimental facilities.

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