Volume I I, number 5.6,7 MATERIALS LETTERS June 1991 15N MAS NMR in the YSiAlON system D. Kruppa, R. Dupree and M.H. Lewis Received 30 January I99 1; in final form 20 March I99 I “N MAS NMR has been used to characterise an YSiAlON glass and its crystallisation products. In the glass twofold and threefold coordinated nitrogen was identified. In addition in the crystallised material a peak of large chemical shift is ascribed to singly coordinated nitrogen substituted into the Y,.41S0,2 YAG structure. 1. Introduction Silicon nitride ceramics are promising structural materials for applications involving high tempera- tures. Sintering aids are necessary in their manufac- ture which react with the SiOz found on the surface of the Si3N, grains and this leads to glassy inter- granular phases. Unfortunately these amorphous phases soften at high temperatures so that the full potential of the Si3N4 ceramic is not realized. Study of the YSiAlON system is important because of its relation to the intergranular region of Y203 and A1203 fluxed Si3N,. The properties of glasses in the YSiAlON system are significantly affected by the ni- trogen content. The hardness, glass transition tem- perature and relative fracture toughness increase while the thermal expansion coefficient decreases with increasing N content [ 1,2]. The modification in properties is thought to be due to N substituting for bridging oxygens and thereby increasing the cross linking in the glass structure. The products of glass crystallisation are also sensitive to nitrogen content in both the intergranular glass form and the isolated state. The object of this study was to characterize the crystallisation of YSiAlON glasses of high nitrogen content using “N magic angle spinning nuclear mag- netic resonance (MAS NMR). This technique has great potential for adding to the body of knowledge on nitrogen-containing glasses and ceramics. Evi- dence for the inclusion of N in the glass network has previously been obtained from 29Si MAS NMR [ 3 1. 15N NMR provides more direct evidence for this and the coordination of the N atoms. Previous work [ 4,5] on YSiAlON glasses using energy dispersive analysis in an electron microscope (TEM-EDAX) showed that significant levels of Al or Si substitution oc- curred in some of the crystallising phases. It is prob- able that nitrogen concurrently substituted for oxy- gen in these phases but at concentrations which are not detectable even with “windowless” EDAX sys- tems. Using 15N NMR direct evidence for N substi- tution can be obtained. The main difficulty is the need to use 15N enriched compounds because of its low natural abundance. Fig. 1 shows the Janecke prism representation for the YSiAlON system. The glass forming region is en- closed in a dotted line. Also shown are the crystalline phases which formed during the annealing of the zyxwvutsrq Y,SiAQN -2&q 2M : 3X PLANE Fig. I. Janecke prism for the YSiAlON system. The glass forming region is enclosed in a dotted line. The compositions of the glasses in this study are shown as dots along a line of constant Y: Si : Al ratio within the glass forming region. 0167-577x/91 /$ 03.50 0 1991 - Elsevier Science Publishers B.V. ( North-Holland ) 195