Alumina is widely used for conventional structural applications because of its high melting pint, high wear and corrosion resistance, electrical insulation, and good high-temperature strength. However, it has a restriction in advanced structural applications because of its low fracture toughness. In...
Alumina is widely used for conventional structural applications because of its high melting pint, high wear and corrosion resistance, electrical insulation, and good high-temperature strength. However, it has a restriction in advanced structural applications because of its low fracture toughness. In this study, in-situ formation of platelike LaAl_(11)O_(18) (10 and 20 vol%) and anisotropic grain growth of alumina matrix were tried separately or in combination to improve the fracture toughness of alumina by developing an interlocking microstructure of elongated grains. The addition of SiC nanoparticles (0.5~5 vol.%) was also employed to avoid strength degradation by controlling the degree of the grain growth of alumina. The LaAl_(11)O_(18) phase was formed by the reaction between alumina and LaAlO_(3) during sintering at 1600℃. SiO_(2) sol (240 ppm) or CaO-SiO_(2) (1:1 by mole, 0.5 or 1 wt.%) was added to induce the anisotropic grain growth. Using these approaches, three composite systems were prepared: Al_(2)O_(3)-LaAlO_(3)-SiO_(2), Al_(2)O_(3)-CaO-SiO_(2), and Al_(2)O_(3)-LaAlO_(3)-SiC. The microstructure, phases, and mechanical properties of the composites were investigated to determine the effectiveness of the current approaches in attaining improved mechanical properties. The anisotropic grain growth of alumina was not obtained in the Al_(2)O_(3)-LaAlO_(3)-SiO_(2) system contrary to the expectation. The anisotropic grain growth occurred in the Al_(2)O_(3)-CaO-SiO_(2) system, but its degree was not significant as expected. The addition of 2 vol.% SiC nanoparticles was found to be enough to suppress the grain growth of alumina with no difficulty in densification (i.e., 97~99% theoretical density). The platelike LaAl_(11)O_(18) grains were formed at the grain boundaries of alumina with their length limited by the grain size of alumina. The Al_(2)O_(3)-LaAlO_(3)-SiC system showed improved strength and fracture toughness compared with pure alumina, because the LaAl_(11)O_(18) platelets and SiC particles not only retarded the grain growth of alumina effectively but also induced toughening mechanisms such as crack deflection and crack bridging presumably. For instance, in the case of Al_(2)O_(3)/20 vol.% LaAl_(11)O_(18)/2 vol.% SiC, the strength and fracture toughness obtained were 524 MPa and 4.3 MPa·m_(1/2), respectively, which in the case of pure alumina, they were 413 MPa and 3.3 MPa·m_(1/2).
Alumina is widely used for conventional structural applications because of its high melting pint, high wear and corrosion resistance, electrical insulation, and good high-temperature strength. However, it has a restriction in advanced structural applications because of its low fracture toughness. In this study, in-situ formation of platelike LaAl_(11)O_(18) (10 and 20 vol%) and anisotropic grain growth of alumina matrix were tried separately or in combination to improve the fracture toughness of alumina by developing an interlocking microstructure of elongated grains. The addition of SiC nanoparticles (0.5~5 vol.%) was also employed to avoid strength degradation by controlling the degree of the grain growth of alumina. The LaAl_(11)O_(18) phase was formed by the reaction between alumina and LaAlO_(3) during sintering at 1600℃. SiO_(2) sol (240 ppm) or CaO-SiO_(2) (1:1 by mole, 0.5 or 1 wt.%) was added to induce the anisotropic grain growth. Using these approaches, three composite systems were prepared: Al_(2)O_(3)-LaAlO_(3)-SiO_(2), Al_(2)O_(3)-CaO-SiO_(2), and Al_(2)O_(3)-LaAlO_(3)-SiC. The microstructure, phases, and mechanical properties of the composites were investigated to determine the effectiveness of the current approaches in attaining improved mechanical properties. The anisotropic grain growth of alumina was not obtained in the Al_(2)O_(3)-LaAlO_(3)-SiO_(2) system contrary to the expectation. The anisotropic grain growth occurred in the Al_(2)O_(3)-CaO-SiO_(2) system, but its degree was not significant as expected. The addition of 2 vol.% SiC nanoparticles was found to be enough to suppress the grain growth of alumina with no difficulty in densification (i.e., 97~99% theoretical density). The platelike LaAl_(11)O_(18) grains were formed at the grain boundaries of alumina with their length limited by the grain size of alumina. The Al_(2)O_(3)-LaAlO_(3)-SiC system showed improved strength and fracture toughness compared with pure alumina, because the LaAl_(11)O_(18) platelets and SiC particles not only retarded the grain growth of alumina effectively but also induced toughening mechanisms such as crack deflection and crack bridging presumably. For instance, in the case of Al_(2)O_(3)/20 vol.% LaAl_(11)O_(18)/2 vol.% SiC, the strength and fracture toughness obtained were 524 MPa and 4.3 MPa·m_(1/2), respectively, which in the case of pure alumina, they were 413 MPa and 3.3 MPa·m_(1/2).
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