Additive Effects on Si₃N₄ Oxidation/Volatilization in Water Vapor

Abstract

Two commercially available additive-containing silicon nitride materials were exposed in four environments which range in severity from dry oxygen at 1 atm pressure, and low gas velocity to an actual turbine engine. Oxidation and volatilization kinetics were monitored at temperatures ranging from 1066 to 1400°C. The main purpose of this paper is to examine the surface oxide morphology resulting from the exposures. It was found that the material surface was enriched in rare earth silicate phases in combustion environments when compared to the oxides formed on materials exposed in dry oxygen. However, the in situ formation of rare earth disilicate phases offered little additional protection from the volatilization of silica observed in combustion environments .It was concluded that externally applied environmental barrier coatings are needed to protect additive-containing silicon nitride materials from volatilization reactions in combustion environments.

Liquid phase sintered silicon carbide (LPS-SiC) ceramics having remarkably high oxidation resistance in wet air

“…However, the rate of volatilisation was substantially decreased in comparison to more conventional Si 3 N 4 materials. 21,22 Oxide layers on SN 282 exhibited a morphology of spherical Lu 2 Si 2 O 7 particles dispersed in a silica matrix, 22,23 and even though Lu 2 Si 2 O 7 was chemically stable in comparison to silica, 23 the silicate grains were partially spalled off during corrosive attack on the film. In the light of these results, the reason for the favourable wet oxidation behaviour of Ho-Lu may be that its higher content of rare earth oxide additive (24 mass%) enables the formation of a more closed cover layer of lutetium and holmium silicates.…”

“…11,18,19 With a water partial pressure of 0.01 MPa at 1400 • C, the wet oxidation conditions used in the present work are less severe than the high gas velocity/high pressure environments encountered in gas turbine engines. 13,18,[20][21][22][23] On the other hand, they are well controlled, very reproducible and permit comparison with a wide range of relevant literature data. 7,8,24 …”

Corrosion and recession of mullite in water vapor environment

“…Mullite bulk was prepared using Al 2 SiO 2 = 3:2 powder was used and homogeneously mixed in an agate mortar using ethanol. The mixed powder was then packed in a graphite die and hot-pressed at 1600 • C for 3 h under an applied pressure of 20 MPa in argon atmosphere.…”

“…[1][2][3] The advanced silicon nitride ceramics developed contain a rare earth silicates phase on the grain boundary aiming to improve the strength as well as creep and oxidation resistance at elevated temperatures. 4,5 Also, these gas turbine components manufactured from Si-based ceramics must exhibit superior resistance to the corrosion and recession process, and thus maintain the long-term dimensional stability under gas turbine conditions.…”

EBC development for hot-pressed Y2O3/Al2O3 doped silicon nitride ceramics

“…But the volume frac stresses generated from coefficients of thermal expansion (CTE) tion of the voids in thermal cycled samples is much too large and mismatch between Si 3 N 4 and coating constituents as shown in must be associated with the moisture induced oxidation, and for- Table 1. But the more serious problem is the formation of large, mation of volatile Si(OH) 4 and nitrogen oxide. Since the coating • C for 3 h, where the coating delaminated from substrate.…”