effect of magnesia on zinc oxide stabilized nano … · 724 t. alam et al. / procedia engineering...

Procedia Engineering 68 ( 2013 ) 723 – 729

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1877-7058 © 2013 The Authors. Published by Elsevier Ltd.Selection and peer-review under responsibility of The Malaysian Tribology Society (MYTRIBOS), Department of Mechanical Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysiadoi: 10.1016/j.proeng.2013.12.245

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The Malaysian International Tribology Conference 2013, MITC2013

Effect of Magnesia on Zinc Oxide Stabilized Nano Alumina Ceramic powder in sintering process and its impact on mechanical

properties

T. Alamb, K.H.A. Al Mahmuda, M.F. Hasanb, S.A. Shahira, H.H. Masjukia, M.A. Kalama, A. Imrana, H.M. Mobaraka

aDepartment of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

bDepartment of Materials and Metallurgical Engineering, BUET, Dhaka-1000, Bangladesh

Abstract

In Present work, the properties of Al2O3 powder was studied through incorporation of MgO Nano powder along with ZnO Nano powder and followed by sintering at various temperatures. Previously ZnO and MgO were introduced with Al2O3 separately to improve physical and mechanical properties like densification, micro hardness, fracture toughness etc. In current project, 0.25wt% and 0.50wt% MgO was added with - Al2O3 matrix along with constant 0.1wt% ZnO and sintered those samples at 14500 oC, 15000 oC, 15500 oC respectively. Properties like densification, micro hardness, and wear rate of all these sintered products were measured and discussed where the best composition found was 0.1% ZnO-0.25%MgO-99.65% Al2O3 sintered at 15000oC. Change in properties was observed due to faster densification rate and pinning effect provided by the doping particles. © 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of The Malaysian Tribology Society (MYTRIBOS), Department of Mechanical Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysia. Keywords: Densification rate; Micro hardness; Doping.

1. Introduction

Larger Surface area, which is one of the key characteristics of nano powder makes it a promising candidate for many application [1-3]. Since it is possible to get more interaction area for catalysts, hence better system efficiency can be achieved. Krell reported, improved hardness [4], Wear resistance [5] and Strength [6] are possible to attain by using sub micrometer Alumina. Though, according to S. Yip, some mechanical properties are disintegrated at very small particle size since larger amount of grain boundaries are moving towards deformation [7]. Also alternative sintering behavior was observed in Krells work, which was explained by Large, where he reported that agglomeration of powder acts as an obstacle for locally homogeneous deformation. With all these characteristics, Alumina itself is a brittle ceramic material with relatively strengthened by incorporation of foreign ceramic particles or fibers or whisker at Al2O3 matrix [8, 9]. For instance Toughness and Strength of Alumina is enhanced by addition of Zirconia, to prepare Al2O3 - ZrO2 composite sintered at 1600°C, which is revealed by Tuans work [10], as well as, mechanical properties of Al2O3 is improved by the addition of SiC was reported by Shi [11]. The particle size is also a key factor since fine

© 2013 The Authors. Published by Elsevier Ltd.Selection and peer-review under responsibility of The Malaysian Tribology Society (MYTRIBOS), Department of Mechanical Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysia

724 T. Alam et al. / Procedia Engineering 68 ( 2013 ) 723 – 729

particle size of the additives ensures better dispersion through within the matrix, as well as, higher surface area to volume ratio which will provide enhanced driving force for sintering [12]. When ZnO is incorporated into Al2O3 matrix, ZnO reacts with Al2O3 to form Zinc Aluminate (ZnAlO4), before densification of powder compaction is followed by larger expansion. Negi reported that incorporation of ZnO as an additive in Al2O3 will improve bulk density, apparent porosity and shrinkage value when the sintering temperature is higher than 1650°C and amount of additive does not play an important role on density. A lot of researchers work on Al2O3 with different doping agent to find enhanced properties. Tabrizi and Nassai added CeO2 into Al2O3 matrix and find improved fracture toughness and flexure strength. CeO2 hindered the grain growth as well as densification process [13]. Zirconia toughened alumina provides better wear performance and improved vicker hardness and fracture toughness [14]. Yttria and Ceria revealed toughened alumina which is also very effective to increase hardness and toughness [15]. Yang and Wang reported that flexure Strength and fracture toughness of bulk ceramic will be improved if TiO2 is added in - Al2O3.

Many research works were conducted on magnesia doped Alumina after the report of COBLE, where he

reported, approximate theoretical density is attainable by addition of small amounts of MgO into - Al2O3. According to Radonjic and Srdic, MgO decreases fast and excessive grain growth inhibits grain boundary mobility as well as improves pore attachment, though densification rate is improved by addition of MgO [16]. Another report revealed that doping of MgO in to Al2O3 alters the phase transformation temperature of transitional Al2O3 [17]. Berry and Harner [18] reported that some controversy has invoked by the work of different researchers about the role of MgO on grain growth of Al2O3. In this work, MgO was incorporated into ZnO stabilized - Al2O3 to investigate various properties like theoretical density, micro hardness, and wear performance. 2. Experimental Procedure

In this Project Alumina (Al2O3) nano powder, Magnesia (MgO) nano powder and Zinc Oxide (ZnO) nano powder were used as raw materials for manufacturing test sample. Table 1. Specification of the ceramic powder used in the experiment

Ceramic Powder

Chemical Formula SpecificationPurify (%) Particle size (nm) Specific Surface

Area (m2/g) Density (gm/cc)

Aluminium

Oxide

Al203 99.87 ~40 ~10 3.97

Magnesium Oxide

MgO 99.9 ~30 >50 3.58

Zinc Oxide ZnO 99.7 ~30 ~35 5.606 Four type of sample were produced in current project where compositions are as follows: 1) Pure Al2O3, 2) 99.9 % Al2O3 doped with 0.1% ZnO, 3) 99.65% Al2O3 doped with 0.1% ZnO and 0.25% MgO 4) 99.4% Al2O3 doped with 0.1% ZnO and 0.5% MgO.

Two steps were followed to set final sintered products which were processing steps and sintering. The property of final products depends on efficient processing prior to sintering. Several steps like, mixing by ball milling, cold pressing and drying. At first, all the ingredients were carefully weighed out by using an electric balance followed by pouring in a clean pot containing Zirconia ball. Sufficient amount of acetone which acts as a milling media, was introduced into the pot in such a way that the height of pot should be at least twice or more than the height of powdered raw material. After that, pot was subjected to milling action, where milling was carried out in a locally made milling machine consisting of single motor and two shafts, at 150 rpm speed for 18 to 20


hours. After completion of milling, slurry was extracted from the pot by means of sieve followed by drying in a dryer at 110-120°C for 24 hours. Then few drops of polyvinyl alcohol were added as a binder agent while sampling and again drying was conducted for 20 minute at same temperature. After drying, weighed samples were subjected to lowland cold pressing in a 13 mm diameter disc. Applied pressure was 2.0-2.20 ton for each samples for 2 minute and then it is removed from the disc. Processed Samples were then dried in a dryer at 100-110°C for 24 hours followed by sintering process. Dried samples were then sintered at different sintering temperature in an electrically heated furnace where single stage sintering was performed. At the very beginning samples were slowly heated at the rate of 3°C/min up to 500°C following by holding at that temperature for one hour to remove binder (polyvinyl alcohol) and other volatile matters. The heat rate was carefully handled to extinguish entire entrapped gasses from bulk materials, so that, no cracks (both external and internal) can be formed in it. The samples are then heated at the rate of 15°C/min up to required temperatures, hold there for 2 hours and then slowly cooled to room temperature. The final temperatures used for sintering were as follows: 1450°C, 1500°C and 1550°C with 2 hours soaking period for each case.

Figure. 1. Sintering temperature cycle

In this current investigation several properties like Theoretical density, micro hardness, and wear rate were

determined for every sample. Percentage theoretical density was calculated by the following equation:

% theoretical density =

Micro hardness was calculated by the following formula, HVP = 1.8544 * P/ d2, where P = force (load) in kilograms, d = diagonal length of the impression in mm (milli meters)

Wear rate was calculated by pin on disk testing method, high carbon steel was used for disk material and sintered alumina tablet used as pin. 3. Results and Discussions 3.1 Theoretical Density:

Several factors like average particle size, size distribution, shape, degree of agglomeration, surface charges etc.

play important roles on densification of powders. Generally finer particles tend to form agglomerates, results lower green density, which can be broken by applying optimum pressure for sufficient time during milling, and hence results improved the green density of compacts [6]. By the addition of nano sized MgO as dopants to ZnO stabilized Al2O3% pores are reduced and grains start to grow by movement of grain boundaries. With increasing temperature, diffusion rate and grain boundary migration rate increases. So, there is a race between the diffusion rate and grain boundary migration rate goes on, which increases during sintering. If grain boundary migration rate is high compared to diffusion rate then, there is a strong chance for pore entrapment into the grain. Pore entrapment into the grain decreases the density of sintered alumina. From Fig.2 it is quite clear that for pure Alumina, theoretical density has increased while temperature was raised from


1450°C to 1500°C, but slightly decreased when sintering temperature rose to 1550°C. Thus, it can be said that,

up to 1500°C diffusion rate to grain boundary migration rate trajectory is high and further incrimination of temperature leads to abnormal grain growth. In case of 0.1% ZnO doped 99.9% Al2O3 sample, maximum density 92.41% was obtained at 1500°C, which is higher in a comparison with pure - Al2O3 sintered at the same sintering temperature. Because of doping, grain boundary migration is impeded by the second phase formed at the grain boundary. Therefore, probability of pore entrapment into the grain is hindered. For first two compositions as shown in Fig.2 higher density was found at1500°C. But when MgO of 0.25wt% and 0.5wt% is added to the ZnO stabilized alumina the trend of densification altered. Maximum density has found for the compositions sintered at 1450°C. By doping with MgO surface energy has been changed. MgO has a higher surface energy compared to ZnO; therefore, MgO reduces the sintering temperature for the required density. Theoretical density is the highest for the last two compositions sintered at 1450°C. Therefore, it can be said that diffusion rate to grain boundary migration trajectory is highest for last two compositions sintered at 1450°C. From the micro structure as shown in Fig.6 it has been shown that grain size decreases as a result of adding doping agents. Therefore, from the microstructure it is clear that grain boundary migration is hindered due to pinning effect of second phase formed by doping agent.

Fig. 2. Variation of theoretical density of differently doped alumina ceramic sintered at different sintering temperature. 3.2 Hardness

Hardness of the sintered alumina of different compositions depends on various factors. Mainly thermal stress, densification, grain size and solid solution strengthening is effective here. In same temperature, with the increment of dopant concentration hardness is increasing. As a result of dopant addition, grain size becomes finer comparing to pure alumina. Small grain size means higher density of grain boundary and higher amount of atomic mismatch. Therefore, dislocation will face much resistance for the movement and as a result hardness increases.

Due to addition of dopant, hardness increases in solid solution strengthening process. Solid solution is effective here in high temperature. Same trend is observed for all compositions shown in Fig.3. While sintering at higher temperature, hardness increases for all compositions due to generation of thermal stress. In higher temperature, solid solution strengthening has notable effect to increase the hardness. In the present work, when sintered at 1550°C, diffusion rate is high enough that dopant starts to migrate from grain boundary to interstitial position in the parent crystalline structure of alumina grain. Therefore, in higher temperature two factors are dominant, one is thermal tress and another is solid solution strengthening. Solid solution strengthening is applicable only for doped alumina. The maximum vicars’ Hardness is obtained with 0.1% ZnO-0.25% MgO-99.65% Al2O3 ceramic sintered at 1550°C which is 21.33.


Fig.3 variation of hardness of differently doped alumina ceramic sintered at different sintering temperature.

3.3 Wear Test

Wear rate decreases with an increase in density and hardness. From the following Bar chart, it is evident that, wear rate is gradually decreased up to 1500°C, while, slightly increased at 1550°C. Minimum wear rate was found with sample which has 99.4% A l 2 O 3 stabilized with 0.1% ZnO and 0.5% MgO i n every sintered condition. Grain size has effect on the wear r a t e ; from Fig.5 it has been shown the grain size in 1450°C in different composition. When dopant is added, grain size decreases.This leads to decrement of wear rate. In each composition as shown in Fig.4 wear rate does not vary accordingly with temperature. This happens due to the densification process. At higher temperature density is also high, therefore wears rate decreases. Highest density depends on the trajectory of d i f f u s i o n rate to grain boundary migration. At 1500°C this trajectory is highest, thus, better densification is achieved and, finally wear rate decreases.

Fig.4. variation of wear rate of differently doped alumina ceramic sintered at different sintering temperature.

3.4 Grain Size

Grain size is an important factor that affects the mechanical property of sintered alumina ceramic as described earlier. In Fig.5 compared the microstructure of four different composition sintered at 1450°C. It can be seen from Fig. that by the addition of dopant g r a i n size decreases. Dopant actually hinders the abnormal grain growth in the matrix. In the second composition as found from the SEM micrograph there are some abnormal grain growths. However, by the addition of MgO abnormal grain growth is hindered significantly compared to first two compositions. Grain size is lowest for the fourth composition sintered at 1450°C. It can be said that MgO addition lowers the sintering temperature. ZnO addition of small amount in the second composition is not sufficient to hinder the grain growth. Thus, some abnormal grain growth is observed. In both of the dopant addition second phase is formed. For ZnO addition second phase is zinc aluminate (ZnAl2O4) and for MgO addition second phase is magnesium aluminate (MgAl2O4). Both the second phase hinders the grain growth and this is called pinning effect [19, 20]. However, MgO addition is more benificial than ZnO addition. Because magnesium is more reactive than zinc. Therefore, probability of second phase formation is more pronounced for MgO doping condition.


Fig.5. Grain size of sample A. pure Al2O3, B. doped with 0.1%ZnO, C. doped with 0.1%ZnO and 0.25%Al2O3, D. doped with 0.1%ZnO andaa0.25%Al2O3, sintered at 1450°C (Magnification 1500X).

4. Conclusion

Alumina ceramic is useful especially for its robust mechanical properties. Its hardness, toughness, wear rateproperties draw the attention of scientist to use it as cutting tool, biomedical application, milling media etc. As aresult of adding dopant during sintering process, its mechanical strength increases and also its sinteringtemperature becomes lower. Lowering the sintering temperature increases the commercial value of aluminaceramic and makes it cost effective too. From the above discussion, following conclusions are drawn:

Addition of MgO as a dopant is more beneficial rather than ZnO addition. This is because, due to adding MgO, thegrain size decreased significantly. Smaller grain size is always beneficial, because if the grain size is small hardnessaaand toughness increases but wear rate decreases. If the ceramic is applied for manufacturing cutting tool than wear rateshould get more attention. Therefore, fourth composition sintered at 1500°C is the best composition, because wear rateis lowest in this condition.Third composition showed best hardness property (sintered at 1550°C). Therefore, this composition can be appropriatetto use as milling media and shaping tool since harder substances will cause lower deformation.aa

Almost in every application, best density is always given priority. Therefore, dense product close to theoreticaldensity is appropriate for every mechanical application.

Acknowledgements

The authors would like to thank all the members of the CFES team for helpful discussions. Running project issupported by University of Malaya Research Grant (UMRG), project no: UMRG RP016-2012A. The authors arealso grateful to University of Malaya for the support.

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