Non-destructive evaluation of porous MgO ceramics using acoustic techniques

Two acoustic techniques, ultrasonics (US) and acousto-ultrasonics (AU), were used to evaluate MgO extrudate in both the green and sintered states. In US, the velocity was measured and in AU ringdown counts, pulse amplitude, pulse width and spectrum were used to quantify the output signal. The acoustic measurements showed good reproducibility, especially velocity. Under some sintering conditions the ultrasonic velocity and the porosity were lower than in the green state: this was attributed to pore growth in the early stages of sintering. It was found that as the temperature and time of sintering increased, the density, compressive strength and ultrasonic velocity increased. However, the response of the AU parameters to changes in sintering conditions was more complex, for example, the peak amplitude increased whereas pulse width and ringdown counts decreased with increasing temperature of sintering. The US and AU data on the sintered MgO extrudate were correlated with the structure and properties. Empirical relationships between the acoustic parameters and porosity and compressive strength were obtained which gave better coefficients of regression for the ultrasonic velocity than for acoustoultrasonic parameters. It was concluded that US, and in some instances AU, are feasible as quality control techniques/non-destructive evaluation for ceramics.