REACTIONS OCCURRING IN POST HEAT-TREATED ALPHA BETA SIALONS - ON THE THERMAL-STABILITY OF ALPHA-SIALON/

To a powder Mixture of an overall alpha-sialon composition R(0.4)Si(10 .2)Al(1.8)O(0.6)N(15.4), with R = Nd Sin, DJ and Yb, were added extra amounts of powder mixtures, (20%), having Si:Al:R and O:N atonic ratio s of 2:1:1 and 3:1, respectively. Series of alpha-rich mixed alpha/bet a-sialon ceramics containing about 20-30 vol% glassy phase were prepar ed from these powders by pressureless sintering at 1750 degrees C, The as-prepared samples were subsequently heated at 1750 degrees C and th en quenched to loom temperature or to 1150, 1300 and 1450 degrees C (w ith a cooling I ate exceeding 400 degrees/min), and were annealed at t hese temperatures for various rimes. The samples quenched to room temp erature revealed that alpha-sialon coexists with beta-sialon and a liq uid phase at 1750 degrees C. The post heat-treatment at the lowest tem perature involved a devitrification of the glassy phase and resulted i n mixtures of mainly rare earth oxynitrides like the U-phase R(3)Si(3) Al(3)O(12)N(2), wollastonite RSiO(2)N or the B-phase Dy2SiAlO5N. Post heat-treatment at 1450 degrees C induced a reaction between residual l iquid and alpha-phase in the Nh and Sm-systems and yielded a mixture o f an oxygen- and a nitrogen-rich phase in all systems. Thus the melili te phase, R(2)Si(3-x)Al(x)O(3+x)N(4-x), is formed with all I are earth elements except Yb, which yields Yb4Si2O7N2. The oxygen-rich phase in the Nd- and Sm-systems was the aluminate RAlO(3), while in rite Dy- a nd Yb-systems the garner phase, R(3)Al(5)O(12), was formed. Similar re sults were obtained with samples quenched to 1300 degrees C, These fin dings suggest that the stability of alpha-sialon is related to the typ e of sintering aid used The phase assemblage found in the as-prepared samples is discussed in view of the findings obtained in the annealing experiments.