Influence of sintering additives with differing proportions of Y{sub}2O{sub}3/Al{sub}2O{sub}3 on the sintering and material properties of Si{sub}3N{sub}4 ceramics

摘要

To further improve the mechanical strength of Si{sub}3N{sub}4 ceramics, samples were produced using sintering additives containing either only Y{sub}2O{sub}3 or Al{sub}2 O{sub}3, or mixtures of Y{sub}2O{sub}3 and Al{sub}2O{sub}3 in varyingproportions ranging from 0,2 to 4,2. The influence of the additives with respect to the oxygen content in the Si{sub}3N{sub}4 powder (UBE SNP E 10) originating from SiO{sub}2 on the sintering behaviour (dilatometry and gas pressure sintering) as well ason the material properties is examined. Corresponding to their positions in the phase diagram of the system SiO{sub}2 - Y{sub}2O{sub}3 - Al{sub}2O{sub}3, sintering begins in the samples containing Y{sub}2O{sub}3 and Al{sub}2O{sub}3 with the formation ofthe eutecticmelt in the range 1 350... 1 380℃. The onset of shrinkage is influenced by the high heating rates in the dilatometer (30 K per mm) additionally from the amount of liquid phase formed. Therefore, the materials with compositions that are further away fromthe eutectic composition have a somewhat higher shrinkage onset temperature. The samples with the above mentioned additive mixtures havevery similar sintering behaviours, despite the various Y{sub}2O{sub}3/Al{sub}2O{sub}3 proportions.In materials that do not contain Y{sub}2O{sub}3, shrinkage begins at 1420℃ and is initially inhibited by the temporary formation of Si{sub}2N{sub}2O and the mullite-like so-called X phase; at higher temperatures, however, the shrinkage intensifies withhigher sintering rates. In materials containing no Al{sub}2O{sub}3, an SiO{sub}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 1 200 MPa for a broad range of additive compositions, but with Y{sub}O{sub}3/Al{sub}2O{sub}3 proportions in the range of 0,6... 1,1 it results in a marked maximum of the fracturestrength at a testing temperature of1000℃. The maximum mean 4-point bending strength at 1000℃ was calculated to be 1 060 MPa, which indicates the suitability of this optimised material for use in automotive engines.