Damage and loading rate effects on the microfailure behaviour of Al2O3-ceramics studied by SEM

摘要

A scanning electron microscopy (SEM)-X-ray combined experimental approach to qualitative and semi-quantitative characterization of microfailure behaviour of an Al2O3-ceramic material in terms of induced damage is presented. The qualitative approach was based on representative fractographical images and data obtained by SEM whereas the semi-quantitative approach was based on a new technique of the X-ray electron probe microanalysis (X-ray EPMA) capable to detect the localized subsurface damage. The damage was induced by a simple rotary notch-cutting procedure where the associated damage parameters can be controlled by the cutting rate. By correlating certain characteristic macro- and microfractographic features/patterns with well-known microfailure mechanisms it was possible to make qualitatively, in an indirect way, evident the existence of induced damage which was assumed to be ideal brittle having only microcracking component. In the same correlating way the stimulating effect of internal pores on the damage development was deduced. Observed loading rate effects on the fractographic behaviour expressed by changes in fracture roughness and micromorphology were attributed to pore-assisted microcracking linkage. Low porosity under high loading rates tends to lower the fractographic roughness, a fact which can be related with a reduction in the ability of energy dissipation. On the other hand, high porosity under increasing loading rates leads to non observable changes in the fractographic roughness, a fact which is indicative of corresponding no appreciable changes in the dissipative character of the material. The measured local damage distribution ahead of the notch-tip shows a monotonic increasing of damaging effects toward the tip. By means of this distribution the total or average damage degree and process zone length ahead of the notch-tip were evaluated. The increase of these two damage parameters is not proportional to the increase in the cutting speed i.e. the rate of damage development. Finally, by assuming brittle damage an experimental approaching procedure for the estimation of the induced energy required for the microcracking damage was proposed.