Superplastic forming of an α-phase rich silicon nitride

The deformation behavior of fine-grained, α-phase rich silicon nitride materials has been studied between 1823 and 1923 K, both in compression and in tension. It is first shown that the higher the α-phase content, the better the superplastic forming ability. A large tension-compression flow asymmetry was evidenced. For instance, shear-thickening flow shows up in compression whereas shear-thinning is observed in tension. Furthermore, much higher flow stresses and hardening rates are reported in compression than in tension. Elongations of more than 80% were achieved for strain rates between 2.5 and 5 × 10⁻⁵ s⁻¹. In the light of our results and of the abundant literature dealing with the high temperature deformation in silicon nitride, a sketch of the different deformation stages is proposed, which emphasizes the tension-compression asymmetry. Starting from the promising results obtained at the laboratory scale, the feasibility for net-shaping of a real part was demonstrated by hot-forging of a parabolic shell.