Effects of sintering additive on the mechanical properties of AlN ceramics

Reference	Presenter	Authors (Institution)	Abstract
09-084	Humberto Naoyuki Yoshimura	Yoshimura, H.N.(Universidade Federal do ABC); Molisani, A.L.(Centro Universitário Fundação Santo André); Goldenstein, H.(Universidade de São Paulo);	AlN (aluminum nitride) ceramics have been used as substrates and packages for microelectronic devices due to their high thermal conductivity and dielectric properties. Although they have potential to be applied as structural material, the study on the mechanical behavior of AlN is limited compared to other engineering ceramics (${\mathrm{Al}}_2$${\mathrm{O}}_3$, SiC and ${\mathrm{Si}}_3$${\mathrm{N}}_4$). In this study, the effects of additive type (${\mathrm{Y}}_2$${\mathrm{O}}_3$ and CaO) and content (0.5 to 8 wt%) on the mechanical properties of an AlN were investigated. The raw powders were mixed, pressed and sintered at 1800 $°$C in ${\mathrm{N}}_2$ atmosphere. The liquid-phase sintered samples had yttrium or calcium aluminides as second-phase at the AlN grain boundaries. In the investigated range, sintering additives had no significant effect on Vickers hardness (~11 GPa) and Young’s modulus (~330 GPa, measured by ultrasonic pulse-echo method), but residual pores decreased these properties. The biaxial flexural strength was strongly affected by the additive type and content: the lowest value was 305 MPa and the maximum values were observed for AlN-0.5% CaO (423 MPa) and AlN-2% ${\mathrm{Y}}_2$${\mathrm{O}}_3$ (466 MPa). The strength variation with the additive content could be correlated with the fraction of transgranular fracture mode of AlN grains, which depended on the AlN grain size. For the microcracks generated at the corners of Vickers impression, the addition of CaO caused more crack deflection around AlN grains than the addition of ${\mathrm{Y}}_2$${\mathrm{O}}_3$, resulting in a decrease in crack length (arresting crack propagation). However, this effect was not observed for AlN resistance to macrocrack propagation, since the fracture toughness (measured by indentation strength method) was almost constant (~2.8 MPa.${\mathrm{m}}^{1/2}$) for all AlN samples. The results of strength are discussed based on the fractographic analysis and on the global and local (grain and grain boundary) fracture toughness.
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