We studied the structural change of perhydropolysilazane to silicon nitride during the process of heat treatment by means of in situ X-ray absorption near-edge structure (XANES) and X-ray photoelectron spectroscopy (XPS), together with Fourier transformed infra-red (FT-IR) and temperature programmed desorption (TPD) spectroscopies. It was revealed that nucleophilic attack on silicon atoms in the polymer began to proceed above 200° C, by nitrogen atoms in the polymer itself and also in atmospheric ammonia. From 400° C to 600° C, dehydrogenation also occurred and produced dangling bonds which were attacked by ammonia molecules to form the network of SiN4 tetrahedra. Although oxidation by residual moisture was observed to some extent at 600° C, further heat treatment in ammonia formed Si–N bonds via replacement of oxygen atoms by nitrogen atoms to complete the formation of Si3N4-like structure.
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Yokoyama et al. (1994) studied this question.