INFLUENCE OF OXIDATION IN SO2-AIR MIXTURES ON THE ROOM-TEMPERATURE STRENGTH OF SILICON-NITRIDE

The influence of oxidation in SO2-air mixtures on the mechanical prope rties of two hot-pressed and one pressureless sintered Si3N4 ceramics was investigated. Samples were corroded for 25 h in the range 800-1500 -degrees-C in 1 vol.% SO2 in dry air and the residual flexural strengt h was measured at room temperature. In SO2, the dominant reaction proc ess is oxidation, accompanied by reaction of sulphur oxides with the s urface oxide layers, to yield sulphates where these are stable under t he experimental conditions. This sulphate formation appears not to aff ect the mechanical strength of silicon nitride.