Toughening in zirconia-toughened alumina composites with non-transforming zirconia

Abstract

Recently, microcrack toughening has been observed in some two-component ceramic systems having substantial thermal expansion mismatch (and consequently residual thermal stresses), in which neither component undergoes a phase transformation. In such systems toughening would arise from stressinduced microcracking in the vicinity of a propagating crack. Zdaniewski [1, 2] described the residual stress-induced microcracking as a principal toughening mechanism in TiB2-A1N composites, where the difference in the coefficients of thermal expansion (CTEs) of the components varied from (1.5 to 5.0) x 10 -6 °C -1. Faber et al. [3] observed a fourfold toughness increase in glass-ceramic systems, following the increase of volume fraction of the dispersed A1203 particles from 0 to 30%. Cai et al. [4] examined the SiC/TiB2 (15 vol % TiB2) systems in which toughening exceeded about 100% over the monolithic SiC matrix material. Magley et al. [5] determined experimentally the residual stresses within grain boundaries between phases in the Sic/TiB 2 composite as being of the order of up to 400MPa with the difference of CTE of 2 . 3 x 1 0 -6°C -~. Cai et al. [4] were the first to provide direct microscopic evidence (by transmission electron microscopy) of the presence of very fine microcracks at the SiCffI'iB 2 grain boundaries. In the present work an increase in fracture toughness of zirconia-toughened aluminium (ZTA) composites containing non-transforming zirconia was observed. It is well known that in such a system the tetragonal-to-monoclinic phase transformation in ZrO2 can be suppressed by the appropriate choice of grain size and/or amount of stabilizer added to the ZrO2 dispersed phase [6, 7]. Therefore, an increase in fracture toughness of ZTA materials with nontransforming ZrO2 cannot be attributed to stressinduced phase transformation, or to the presence of microcracks produced by spontaneous transformation occurring upon cooling from sintering temperatures [8]. Stress-induced microcracking is then probably the dominant toughening mechanism. ZTA compositions, with ZrO2 content varying from 0 to 30 vol %, were wet-processed from dense aqueous suspensions using commercial powders. Alumina (Reynolds, RCHP-DBM) had a reported average particle size of 0.48/xm. ZrO2 powder (3Y-TZ from Tosoh, formerly Toyo Soda), containing 3 mol % Y203 as a stabilizer, was attrition milled