Subcritical crack growth in Al2O3 with submicron grain size

Hochdichte Al2O3-Keramiken mit Submikrometer-Gefüge und Reinheit > 99,99 % wurden durch Gelcasting mit höchster Festigkeit und in komplizierten Formen hergestellt. Höchstauflösende Elektronenmikroskopie zeigt, dass alle Korngrenzen frei sind von selbst dünnsten amorphen Interfaces. Mit einer neuen Methode wurden subkritische Risswachstumsraten in Wasser und an Luft bis hin zu sehr kleinen Werten von 10exp-13 m/s erfasst. Die hohe Reinheit der Keramiken bedingt eine Rissausbreitung, die trotz der Feinkörnigkeit des Gefüges gleich oder geringer ist als für herkömmliche Al2O3-Biokeramiken gegenüber dem Verhalten an Luft wurde keine Intensivierung der Rissausbreitung in Wasser beobachtet.

Gelcast sintered alpha -Al2O3 (corundum) ceramics were developed with a sub-grit grain size at densities >99%. Highly perfect samples with a minimum of flaws were prepared by an approach that maintains the high purity of the raw powder > 99.99% Al2O3 throughout processing. As a consequence, all grain boundaries are free of even thinnest amorphous interface films, amorphous triple junctions are 150 nm, and their frequency is low. Subcritical crack growth was investigated by an approach recording growth rates as low as 10exp-13 m/s. The outstanding purity of grain boundaries gives rise to a resistance against subcritical crack growth which is similar or even below that of coarser conventional alumina ceramics. No significant promotion of subcritical crack growth by water was observed for the new gelcast high-purity ceramics with grain sizes <1 mu m, and there is no indication of a threshold K10 below which no crack growth would occur. The results suggest that in sintered alumina ceramics with a given purity of grain boundaries the subcritical crack- growth mechanism of stress corrosion is independent of the grain size. With their high mechanical reliability, these corundum grades are promising candidates for the use in new prostheses for joints with a high load bearing capability and with small calliper sizes.