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

Abstract

We investigated the effects of a chemically‐vapor‐deposited mullite coating (∼100 nm) on the oxidation resistance of sintered Si3N4 in air and steam environments. The coating was sacrificially incorporated into the thermally grown oxide (TGO) on Si3N4 during isothermal oxidation in air at 1400°C, leading to significantly reduced TGO growth as well as markedly improved TGO morphology. This improvement can be attributed to the refractory and viscous nature of the SiO2‐Al2O3 system, compared with SiO2, when under the influence of alkali and/or alkaline‐earth fluxing elements. However, the mullite coating had little effect on the stability of the ceramic in the steam environment at 1200°C, due likely to high activity of SiO2 in mullite.

Degradation of Silicon Nitride Glow Plugs in Various Environments Part 3: NGDI Engine

“…The estimated 1200 h life in service on the engine is not acceptable for this application (minimum 5000 h and ideally 20,000 h are desired), so means to protect the surface of the ceramic heater operating continuously at 1300°C in NGDI engine must be implemented. A variety of techniques to modify the Si 3 N 4 surface by ion implantation, coatings, hydrothermal surface treatment, have been studied in to improve the oxidation and corrosion resistance . Deposition of a mullite‐mullite coating, followed by in‐situ calcination by powering the glow plug has been evaluated in our labs (Wong et al [2010], unpublished data) and it looks very promising, as tested in air.…”

Slurry based multilayer environmental barrier coatings for silicon carbide and silicon nitride ceramics — I. Processing

“…To date, atmospheric plasma spraying [7,8] and chemical vapor deposition [9,10] are the widely used techniques to apply the EBCs on silicon based materials. However, these techniques are time consuming expensive processes.…”

“…Mullite and mullite based EBCs for SiC and Si 3 N 4 ceramics are very attractive due to their good chemical compatibility and excellent environmental stability [7][8][9]. To date, atmospheric plasma spraying [7,8] and chemical vapor deposition [9,10] are the widely used techniques to apply the EBCs on silicon based materials.…”

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

“…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 13–20 . The need to protect nonoxide structural ceramics in such combustion environments, including gas turbines, triggered an active search for “Environmental Barrier Coatings” that could prevent (or slow down) the degradation of such components in oxidizing/reducing conditions including water vapor 21–26 . The additional damage mode for Si 3 N 4 ‐based ceramic heaters of GP is the degradation of the internal structure of the ceramic heater due to the applied electrical current.…”