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| Ballóková B., Hvizdoš P., Lofaj F. |
| MICROSTRUCTURE AND FRACTURE MECHANICAL PROPERTIES |
Abstract
The work studies the influence of microstructure on room and high temperature fracture mechanical properties of SiC based structural ceramics with 30, 40 and 50% MoSi2, respectively, prepared by reactive hot pressing from element powders. The chemical and phase analyses of the materials were carried out by using x-ray diffractography which confirmed presence of MoSi2, SiC, and possibly small amount of MoC, Mo5Si3, Si.
Ceramographic samples were ground on diamond grinding wheels, then lapped and polished down to 1mm. Microstructure was studied using light and scanning electron microscopes. Microstructural features as the size, shape, and distribution of the grains and pores were studied by an image analyzer DIPS 5.0. Materials containing 40 and 50 % of MoSi2 were well sintered to a high density with grain sizes of 3.4?m and 2.3?m, respectively. In both cases equiaxed grains were evenly distributed. In the material with 30% MoSi2, the MoSi2 and SiC grains tended to form clusters that prevent perfect sintering, thus creating a highly porous core of the hot pressed pellet. Therefore, the material contained 0.78% of pores with mean size 5.5?m. Average size of MoSi2 grains was 4.6?m.
High temperature properties were measured in bending at 1400°C at stress 75MPa. The best creep properties were exhibited by the material with 40 vol.% MoSi2. Creep strain rates after 24 hour test were 1.2x10-3 s-1 for SiC+30 % MoSi2 and 2.1x10-4 s-1 for SiC + 40 % MoSi2. Under the same conditions the third material failed after 1 hour.
The material containing 40 % MoSi2 exhibited the highest values of bending strength. The lowest strength value was found in the case of the material with 30% MoSi2 due to its higher porosity. Vickers hardness was measured using the indentation loads of 50, 100, 150 and 200 N. The hardness decreases with increasing MoSi2 content. Fracture toughness was measured using the indentation method and fracture resistance as a function of the crack growth was evaluated. In all three cases the toughening effect was confirmed and the fracture toughness was higher comparing with the monolithic MoSi2 and SiC. In the material with 40 vol.% MoSi2 also a strong R-curve behaviour was found.
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| No 4 (1999), p. 251-256 |
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