Surface oxidation of Si3N4 green compacts: Effect of sintering aids

It is well known that the surface chemistry of Si3N4 is strongly influenced by both the synthesis route and the subsequent processing steps, such as the mixing/milling vehicle, the forming procedure, etc. All these processing variables can modify not only the rheological properties of the slips, but also the formation of chemical species at the surface which are directly related to the densification and the properties of the sintered compact. In addition, the additives normally used for sintering can also influence the surface chemistry. The aim of the present work is to determine the possible changes in the particle surface of Si3N4 green compacts prepared by different forming procedures (slip casting and isostatic pressing) with and without sintering aids (Al2O3 and Y2O3). The surface oxidation has been determined, in each case trying to separate the contribution to the oxidation induced by the processing parameters and the contribution provided by the presence of the sintering aids. X-ray photoelectron spectroscopy (XPS) is an extremely useful technique for this purpose. In this work, high resolution XPS has been applied to reveal not only the chemical changes brought about by oxidation of Si3N4 green compacts but also to quantify the extent of surface oxidation.