Effects of additive and impurity species on the oxide morphology of silicon nitride

Abstract

Si3N4 sintered with Lu2O3 was exposed to 1 atm of oxygen and steam environments at 1200–1400 °C. The effects of additive and impurity species on the morphological development of the oxide layer were examined. Oxide layers grown on as-received samples in oxygen generally contained bubbles and cracks and underwent spallation due to the presence of an initial impurity-laden oxide layer. Oxide layers grown on as-received samples in steam exhibited layered morphology: a glassy outer layer and a cristobalite inner layer with a high population density of Lu2Si2O7 particles between. The Lu2Si2O7 particles accumulated at the interface led to extensive spallation of the upper oxide layer. Removal of the initial oxide by polishing resulted in improved oxidation resistance and oxide morphology in oxygen and in steam.

Effects of Thin Mullite Coating on the Environmental Stability of Sintered Si3N4

“…Additional details of morphologic development of TGOs on SN282 can be found in a study that we reported elsewhere. 11 The application of a thin mullite coating contributed to marked improvement in the oxidation behavior of SN282. According to Figs.…”

“…Such TGO morphology on steamoxidized SN282 was quite similar to those we reported in a separate study. 11 Further discussions of the morphologic characteristics and development can be found in the reference. It is known that silica recession, due to Si(OH) 4 formation and volatilization, destroys air plasma spray mullite coating in steam.…”

Degradation of Silicon Nitride Glow Plugs in Various Environments Part 1: dc Electric Field in Ambient Air

“…Si 3 N 4 degradation by oxidation at high temperatures is determined by the grain‐boundary phases, typically based on oxynitride glasses, formed by a combination of the sintering additives with the parent Si 3 N 4 and SiO 2 impurities 1–3 . While oxidizing pure Si 3 N 4 forms a surface film of SiO 2 , the presence of sintering aids (mostly oxides of rare‐earth elements R 2 O 3 ) induces silicate deposits including R 3+ on the surface 1–5 . The intergranular oxynitride glassy phase in Si 3 N 4 provides a continuous transport medium between the surface silicate scale and the Si 3 N 4 bulk, by the outward diffusion of R 3+ cations and inward diffusion of oxygen 1,2 .…”

“…[1][2][3] While oxidizing pure Si 3 N 4 forms a surface film of SiO 2 , the presence of sintering aids (mostly oxides of rare-earth elements R 2 O 3 ) induces silicate deposits including R 31 on the surface. [1][2][3][4][5] The intergranular oxynitride glassy phase in Si 3 N 4 provides a continuous transport medium between the surface silicate scale and the Si 3 N 4 bulk, by the outward diffusion of R 31 cations and inward diffusion of oxygen. 1,2 While oxidation of Si 3 N 4 -based ceramics in dry air and oxygen was extensively studied, [1][2][3][4][5] it is known that silicon nitride oxidizes up to an order of magnitude faster in wet than in dry oxygen and the mechanism changes from parabolic to paralinear.…”

“…[1][2][3][4][5] The intergranular oxynitride glassy phase in Si 3 N 4 provides a continuous transport medium between the surface silicate scale and the Si 3 N 4 bulk, by the outward diffusion of R 31 cations and inward diffusion of oxygen. 1,2 While oxidation of Si 3 N 4 -based ceramics in dry air and oxygen was extensively studied, [1][2][3][4][5] it is known that silicon nitride oxidizes up to an order of magnitude faster in wet than in dry oxygen and the mechanism changes from parabolic to paralinear. [6][7][8][9][10][11][12] Additionally, the swings of oxidizing/reducing conditions in combustion environments can affect the stability of the silicate film on the component's surface; numerous studies have focused on the corrosion of silicon-based ceramics under these complex combustion conditions.…”