Mechanical properties of multilayered composites Al2O3/Mo
Hubert Matysiak*, Andrzej Olszyna*, Jan Strzeszewski** *Politechnika Warszawska, Wydział Inżynierii Materiałowej, ul. Wołoska 141, 02-505 Warszawa **Politechnika Warszawska, Wydział Fizyki, ul. Koszykowa 141, 00-662 Warszawa
Annals 3 No. 7, 2003 pages 265-270
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abstract The study was concerned with the multi-layered composites of the .../Al2O3/Al2O3+xvol.%Mo/Al2O3/... type produced by the type casting method. The volumetric contents of molybdenum in the individual layers of the composites were: (4, 9, 14, 21, 28) vol. %. The parameters examined included: density and porosity (by the Archimedes method), Young modulus (by the ultrasonic method), bending strength (three-points bending) and fracture toughness (three-points bending of notched beams). The phases identified in the multi-layered composites by the X-ray analysis were: α-Al2O3, Mo and Mo2C. The sintered composites have a relatively high relative density (the porosity does not exceed 10%) (Table 1, Fig. 3). As the volumetric share of Mo increases, their weight densities increase, whereas the Young moduli E decrease (Table 1, Figs. 3 and 4). The bending strength, the stress intensity factor KIc and the cracking energy are at a maximum in the composite with 14 vol.% of Mo in the layer. The strength and the fracture toughness of this composite are more than twice as great as that of Al2O3, and the fracture energy more than 6 times as great (Table 1, Figs. 4 and 5). The magnitudes of the residual stresses measured during the experiments show that (Table 2 and Fig. 6). The measured magnitude of the stresses depends on two effects. One is associated with the crystallographic anisotropy of the coefficient in corundum, and the other - with the interaction between the components of the composite. Taking these two effects into account, we can state that, with the Mo content between 9 and 21 vol.%, the Al2O3 grains in the Al2O3 layers of the composite are under slight tension, whereas in the composites that contain 4 and 28 vol.% Mo, they are compressed. The Al2O3 grains in the composite Al2O3+xvol%Mo layers are under compression in all the composites. The analysis of the residual stress magnitude in the composites examined is in addition complicated by the appearance of the third phase Mo2C. Its thermal and mechanical properties differ from those of Al2O3 and Mo, and it was probably this phase which is responsible for the disagreement between the expected and measured stress distributions in the composite layers. Moreover, in the composites containing above 14 vol.% of Mo, cracks appear in the Al2O3 layers (Figs. 1 and 2). They result from the tensile strength of the layer material being exceeded. The presence of the cracks also affects the magnitudes and distribution of stresses in the layers (relaxation processes) and in addition reduces the strength and fracture toughness of the composites. Key words: multilayered composites, fracture toughness, residual stresses