Influence of sintering additives with differing proportions of Y2O3/Al2O3 on the sintering and material properties of Si3N4 ceramics

To further improve the mechanical strength of Si 3 N 4 ceramics, samples were produced using sintering additives containing either only Y 2 O 3 or Al 2 O 3 , or mixtures of Y 2 O 3 and Al 2 O 3 in varying proportions ranging from 0,2 to 4,2. The influence of the additives with respect to the oxygen content in the Si 3 N 4 powder (UBE SNP E 10) originating from SiO 2 on the sintering behaviour (dilatometry and gas pressure sintering) as well as on the material properties is examined. Corresponding to their positions in the phase diagram of the system SiO 2 - Y 2 O 3 - Al 2 O 3 , sintering begins in the samples containing Y 2 O 3 and Al 2 O 3 with the formation of the eutectic melt in the range 1350...1380°C. The onset of shrinkage is influenced by the high heating rates in the dilatometer (30 K per min) additionally from the amount of liquid phase formed. Therefore, the materials with compositions that are further away from the eutectic composition have a somewhat higher shrinkage onset temperature. The samples with the above mentioned additive mixtures have very similar sintering behaviours, despite the various Y 2 O 3 /Al 2 O 3 proportions. In materials that do not contain Y 2 O 3 , shrinkage begins at 1420°C and is initially inhibited by the temporary formation of Si 2 N 2 O and the mullite-like so-called X phase; at higher temperatures, however, the shrinkage intensifies with higher sintering rates. In materials containing no Al 2 O 3 , an SiO 2 -rich melt phase is formed, densification is delayed and less densification is achieved. Gas pressure sintering leads to room temperature strengths on the order of 1200 MPa for a broad range of additive compositions, but with Y 2 O 3 /Al 2 O 3 proportions in the range of 0,6...1,1 it results in a marked maximum of the fracture strength at a testing temperature of 1000°C. The maximum mean 4-point bending strength at 1000°C was calculated to be 1060 MPa, which indicates the suitability of this optimised material for use in automotive engines.