The selected properties of Al2O3-Mo composites in the strength test aspect
* Marek Barlak, ** Mirosław Kozłowski * Politechnika Warszawska, Instytut Technologii Materiałowej, ul. Narbutta 85, 02-524 Warszawa ** Instytut Technologii Próżniowej, ul. Długa 44/50, 00-241 Warszawa
Annals 2 No. 4, 2002 pages 153-156
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abstract Because of considerable differences of materials properties (coefficient of thermal expansion, elastic modulus), the immediate joining of metals with ceramics is very difficult. The generated stresses in the ceramic-metal joints during the temperature change are difficult problem. One universal method of the joining metals with ceramic not exists. The problem of minimisation of generated stresses can be solved by the using similar materials (similar values of coefficient of thermal expansion, elastic modulus etc.). These materials must characterise suitable mutual bondability. Figure 1 presents the temperature change of coefficient of thermal expansion (a) and elastic modulus (b) of selected metals and Al2O3 ceramic. Al2O3 ceramic and molybdenum have similar values of physical properties and these characterise suitable mutual bondability. Melting point for Al2O3 ceramic is 2054°C and melting point for molybdenum is 2623°C. Three kinds of Al2O3-Mo composite materials were produced: 25%Al2O3-75%Mo, 50%Al2O3-50%Mo and 75%Al2O3- 25%Mo (by volume). These composites were produced in sintering processes of powder mixtures (temperature: 1400°C, time: 30 min, pressure: 25.5 MPa, vacuum: 1.33 · 10-3, grain size of Al2O3 ceramic: about 20 µm, grain size of Mo: about 1 µm). Table 1 shows the selected results of the investigations of Al2O3-Mo composites. It presents values of shear strength, fractures, hardness, theoretical and real density, porosity, Young’s modulus, shear modulus, bulk modulus, Poisson’s ratio, velocity longitudinal and velocity shear. Table 2 presents the macrostructure and the microstructure of Al2O3-Mo composites (longitudinal and transverse cross-section in proportion to force in sintering process). Figure 2 shows the surface morphology of Al2O3-Mo composite and the surface distributions of Mo and Al. On presented photographs we can see non-homogeneity of material structure. The linear analysing of distribution of Mo and Al (Fig. 3) and X-ray diffraction analysis of Al2O3-Mo composite materials show rather diffusion type of interfaces between Al2O3 ceramic and molybdenum grains. The crack of materials (Fig. 3) shows big stresses in the structure. The first results of investigations are not satisfactory (e.g. big porosity), but the investigations will be continued. The increasing of sintering temperature, the introduction of initial powder concentration and the increasing of the homogeneity of powder mixtures should lead to obtain good constructional material.