JOURNAL OF MATERIALS SCIENCE 19 (1984) 745--752 Hot isostatic pressing of Si3N, with Y203 additions O. YEHESKEL*,Y. GEFEN*, M. TALIANKER Department of Materials Engineering, Ben-Gurion University of the Negev, Beer Sheva, Israel The effect of Y203 additive on the properties of hot isostatically pressed silicon nitride was studied. The influence of small additions of Y203 on the densification of silicon nitr[de was investigated. The density and elastic moduli of the product increase with increasing of the Y203 additions. The hot isostatically pressed pure silicon nitride con- sists of a-Si3N4, ~-Si3N4 and Si2N20; phase content of the hot isostatically pressed silicon nitride with 10wt % Y203 addition consists of ~-Si3N4, yttrium silicate and Y2Si~O3N4. The effect of the outgassing of the specimens prior to hot isostatical pressing on the properties of the final material is discussed. 1. Introduction Silicon nitride is one of the leading ceramic materials with wide applications at elevated tem- peratures, when high strength and resistance to oxidation, creep and thermal shock are needed. The process of sintering of pure Si3N4 is difficult because of the low self-diffusivity of this covalent material [1]. It is well known, however, that the additions of some oxides to powder of pure Si3N4 provide the formation of intergranular liquid phase which aids the densification of the silicon nitride during the sintering process. Another way to den- sify the material is by applying high pressure to the heated silicon nitride power, i.e. hot pressing. Hot pressed silicon nitride with various oxide additives has been extensively studied [2-8]. Gazza [4, 5] was the first to use Y203 additions in the hot pressing of Si3N4. He found that Y203 additive not only results in dense high-strength specimens of Si3N4 but also improves refractory properties of the material. The amount of about 3 wt % of Y203 was essential for achieving high density of commercial silicon nitride [4], however greater amount of Y203 were needed to densify Si3N4 of higher purity [5]. During the last decade a new process for fabri- cation of fully dense silicon nitride has been developed. This process which promises to com- bine the most attractive features of both sintering and isostatic-pressing technologies is called hot iso- static pressing (HIP) [9, 10]. In the HIP process a gas at high pressure acts via a gas impermeable envelope on a powder of the material during its sintering. The gas pressure in HIP-process is typically higher up to an order of magnitude than that used for uniaxial hot pressing in graphite tools. Hot isostatic pressing offers the possibility of high degree densification of material combined with the ability of fabricating intricately shaped parts. Larker [9] used the HIP of silicon nitride in order to manufacture integrated turbine wheels. Yeh and Sikora [10] consolidated pure Si3N4 at 1760 ~ C at a pressure of 276 MPa, thus obtaining 95% of the theoretical density for the material. The present study was undertaken to investi- gate the effect of small amounts of Y203 additive on the properties of the silicon nitride material fabricated by hot isostatic pressing. Actually the studies were performed within a pseudoternary Si3N4--SiO2-Y203 system for which phase relations obtained at temperatures between 1600 and 1750~ [11] are represented in Fig. 1. Solid circles, corresponding to 0.5, 2, 5 and 10wt% of *Also associated with the Nuclear Research Centre - Negev,PO Box 9001, Beer Sheva, Isxael. 0022--2461/84 $03.00 + .12 Chapman and Hall Ltd. 745