Three tests: (1) dynamic fatigue; (2) flexure strength, and (3) direct, double-cantilever beam stress intensity-crack velocity measurements carried out in air (22 C) and liquid nitrogen (-196 C) all showed that dense sintered AlN undergoes slow crack growth (SCG) in air at room temperature. These and other results from the literature indicate that SCG in AlN occurs intergranularly due to moisture. The severity of the SCG varies from that of Al2O3 (e.g. n values of 20-60) to N values of several hundred or more. A comparison with data on SiC and Si3N4 indicate that SCG in AlN occurs due to residual oxide additives in grain boundary, but no clear distinction was observed between AlN densified with CaO or Y2O3 additions. Testing of AlN samples which had undergone surface (post fired) or cofired (e.g. via) metallization showed significant strength reductions rather than increases in testing at - 196 C, but metallized samples immersed in liquid nitrogen without testing, removed, and then tested in air at 22 C showed no strength decrease, indicating that the lower strengths in liquid nitrogen were due to added thermal expansion mismatch stresses between the metallization and AlN, not the formation of cracks due to the stresses.
Citation: International Journal of Applied Ceramic Technology
Pub Type: Journals
aluminum nitride, fracture mode, grain boundary phases, slow crack growth