18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS COMPARISON OF MICROWAVE AND CONVENTIONAL SINTERING OF Al 2 O 3 -ZrO 2 COMPOSITES T. Thongchai 1 *, S. Larpkiattaworn 2, D. Atong 3 , M. Kitiwan 3 1 Dep. of Industrial Engineering, Faculty of Engineer, Naresuan University, Pisanuloke, Thailand 2 Thailand Institute of Scientific and Technological Research, Pathumthani, Thailand 3 National Metal and Materials Technology Center, Thailand Science Park, Pathumthani, Thailand * Corresponding author (Tanikan_9@hotmail.com) Keywords : Microwave, Alumina, Zirconia, Composite 1. Introduction heating rate, short cycle time, higher toughness and Oxide ceramic materials was used in variety of fine grain size, higher mechanical properties, higher modern technological process due to a unique density, lower firing temperature. However, very combination of physicochemical properties. little literature reported on the microwave heating of Alumina has an excellent property in high Al 2 O 3 -ZrO 2 composite. The aim of this paper is to temperature stability, high strength, high hardness, present a comparative study of the microwave biological resistance, thermal stability, and chemical heating and conventional heating applied to Al 2 O 3 - resistance. It is widely used for various applications ZrO 2 composite in related to their properties of such as spark plug, ball mill and pot mill, electronic density, porosity, and strength. substrate, and etc [1]. Zirconia exists as a monoclinic crystal at room temperature and inverts 2. Experimental Procedure to tetragonal phase above approximately 1200 ° C. It Starting materials used to prepared the samples are has high density, high fracture toughness, high hardness, low thermal conductivity, chemical high purity Al 2 O 3 (98.9%) with average particle sizes of 3.2 μ m and fine ZrSiO 4 powder with inertness, and ware resistance [2]. Al 2 O 3 -ZrO 2 average particle sizes of 0.3 μ m and 9.7 μ m. Two composite consists of an alumina matrix in which there are embedded zirconia particles, either batch compositions were prepared, AZ1: 80%wt unstabilized or stabilized. It is well known that this Al 2 O 3 + 20%wt ZrSiO 4 (0.3 μ m) and AZ2: 80%wt second phase addition results in an enhancement of Al 2 O 3 + 20%wt ZrSiO 4 (9.7 μ m). The mixture of their mechanical properties such as high strength, each batche was wet milled for 5 h, dried and sieved. and high toughness. Due to their excellent The green pellets were formed by uniaxial pressing properties, Al 2 O 3 -ZrO 2 composite remains an at 2 tons. The dimensions of green pellets were 1.23 interesting subject for materials researchers and has cm in diameter and 0.35 cm in thickness and been used in various application in recent year [3]. dimension for the bar shape was 1 x 5.5 x 0.7 cm. Microwave sintering has gained increasing attention The firing profile was 5 ° C/min up to 600 ° C and to scientist because of its advantages over then 10 ° C/min up to 1300, 1400 and 1500 ° C, conventional sintering for ceramic materials. respectively. The bulk density, porosity and water Microwave belong to electromagnetic spectra with absorption were measured by the Archimedes’ wavelengths from 1 mm to 1 m. The commonly used frequency for microwave heating are 2.45 and 0.915 method. The flexural strength were determined from GHz [4]. In conventional heating, energy transferred three-point bending test with a span length of 30 mm to the materials through convection, conduction, and and loading rate of 0.5 mm/min. Phases of samples radiation of heat from surfaces of the material. In after firing were characterized by X-ray diffraction, microwave heating, energy is delivered directly to with Ni filtered Cu K α radiation (XRD:Shimadza, materials through molecular interaction with the Japan). Microstructures were observed from electromagnetic field. This difference of energy Scanning electron microscopy (SEM: JOEL JSM- delivery can result in many advantages of 6340F). microwave heating such as uniform heat, rapid
3. Results and Discussion phase and the tetragonal zirconia (t- ZrO 2 ) was not observed (Fig.1). By increasing sintering Typical X-ray diffraction pattern of samples (AZ1) temperature to 1400°C, higher intensity peaks of after sintering at 1300°C and 1400°C in both m-ZrO 2 and t- ZrO 2 were observed and peaks of conventional and microwave have been presented in ZrSiO 4 were rarely appeared. For samples sintered in Fig. 1 and 2, respectively. It was found that sintering microwave furnace, both m-ZrO 2 and t-ZrO 2 were at 1300°C in conventional furnace, α -Al 2 O 3 and found at temperature 1300°C and 1400°C and ZrSiO 4 were observed as major phases while the intensity of peaks increase with increase monoclinic zirconia (m-ZrO 2 ) showed as minor temperature. However, ZrSiO 4 peaks were not observed for both sintering temperature in microwave furnace. This can be explained that during sintering ZrSiO 4 will transform to m-ZrO 2 and to t-ZrO 2 at higher temperature. By microwave sintering, the volumetric interaction of the electromagnetic fields with a ceramic material will lead to a higher heating efficiency and faster reaction rates when compared with conventional heating at the same temperature [5, 6]. This resulted in more ZrO 2 phase formed in the sample sintered by microwave. The results of bulk density, apparent porosity and water absorption of AZ1 and AZ2 samples sintered at 1300, 1400 and 1500 ° C in conventional and microwave furnaces were shown in Fig. 3, 4 and 5, respectively. AZ2 sample sintered in conventional Fig. 1. XRD pattern of AZ1 sintered at 1300°C furnace at 1300°C presented the lowest density of 2.4 g/cm 3 , highest porosity and water absorption of 42 and 17.5%, respectively. This high porosity is caused from densification retarding of bigger particle size of ZrSiO 4 . However, this phenomenon can be compensated by using microwave sintering. Microwave sintering process can produce higher final density of sample than the conventional sintering process. It can enhance densification, especially at higher sintering temperature presents more evident effect. This is because microwaves absorb the electromagnetic energy volumetrically, and transform it into heat which generate within the materials first and then transfer to the entire volume. This is different from conventional heating, in which heat is transferred between particles by the mechanisms of conduction, convection and radiation, the material’s surface is first heated Fig. 2. XRD pattern of AZ1 sintered at 1400°C followed by the heat moving inward. This means that there is a temperature gradient from the surface to the inside.
3.5 The results of flexural strength and compressive Average Bulk Density (g/cm 3 ) strength from the effect of ZrSiO 4 starting particle 3 sizes, sintering temperature and furnaces were 2.5 shown in Fig. 6 and Table 1. It was found that 2 flexural strength and compressive strength of sintered samples increased significantly with 1.5 AZ1 (Conventional Sintering) increasing sintering temperature under the same AZ1 (Microwave Sintering) 1 particle size and furnace. AZ1 sample performed AZ2 (Conventional Sintering) higher flexural and compressive strength compared 0.5 AZ2 (Microwave Sintering) to AZ2 sample sintered at the same temperature and 0 furnace. It was found that flexural and compressive 1300 1400 1500 Temperature ( ° C) strength increase with decreasing starting particle sizes. Samples prepared from smaller particle size performed higher flexural and compressive strength Fig. 3. Average bulk density of AZ1 and AZ2 than those prepared from larger particle size under the same sintering temperature and furnace. This is 45 due to small particle size creates high density of 40 sintered body which results in high strength of Apparent Porosity (%) 35 particle bonding. The composite samples sintered by 30 the microwave method exhibited higher flexural and compressive strength than that samples sintered by 25 conventional one. This improve of strength can be 20 AZ1 (Conventional Sintering) explained by homogeneous microstructure of sample 15 AZ1 (Microwave Sintering) sintered in microwave furnace. This result revealed 10 AZ2 (Conventional Sintering) superior mechanical properties of microwave 5 AZ2 (Microwave Sintering) sintered sample compared to conventional sintered 0 samples. Furthermore, in Al 2 O 3 -ZrO 2 composite , t- 1300 1400 1500 Temperature ( ° C) ZrO 2 can be formed at all temperature by microwave sintering. Fig. 4. Apparent porosity of AZ1 and AZ2 500 20 Flexural strength (MPa) 450 Water Absorption (%) 15 400 AZ1 (Conventional Sintering) 350 10 AZ1 (Microwave Sintering) AZ1 (Conventional Sintering) 300 AZ2 (Conventional Sintering) AZ1 (Microwave Sintering) 5 AZ2 (Conventional Sintering) AZ2 (Microwave Sintering) 250 AZ2 (Microwave Sintering) 1300 1400 1500 0 Temperature ( ° C) 1300 1400 1500 Temperature ( ° C) Fig. 6. Flexural strength of AZ1 and AZ2 Fig. 5. Water absorption of AZ1 and AZ2
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