Refractory Oxide Coatings on SiC Ceramics
- PDF / 2,439,606 Bytes
- 4 Pages / 576 x 777.6 pts Page_size
- 97 Downloads / 276 Views
MRS BULLETIN/OCTOBER 1994
all the requirements and thus appears most promising. Solar Turbines, Inc.7 and Oak Ridge National Laboratory8 have done pioneering work on applying refractory oxide coatings on SiC by atmospheric pressure plasma spraying and slurry coating, respectively. A range of coatings were applied, including mullite, alumina, yttria, yttria-stabilized zirconia, hafnia (HfO2), cordierite (2MgO • 2A12O3 • 5SiO2), and multilayer coatings consisting of these oxides. In general, of the refractory oxide coatings tested, mullite and mullite-based coatings such as mullite/ alumina and mullite/yttria coatings were found to adhere and protect best. This is due to the close CTE match between mullite and SiC. Even the mullite or mullitebased coatings, however, tend to crack and debond on thermal cycling. Molten salts penetrate through the cracks and attack the substrate, making the coatings unsuitable for long-term applications. Before applying a coating, the substrate is roughened by etching or grit blasting to provide good bonding between the coating and substrate.7 u On a monolithic ceramic, this process leads to a strength degradation. However, as-plasma-sprayed coatings exhibited a strength distribution equivalent or superior to that of as-received SiC.12 The retained, or improved, strength of the as-plasma-sprayed SiC was attributed to the good mechanical bond between the coating and SiC, presumably providing some load transfer to the coating from failure-initiating defects.12 The cracking and spallation of the early coatings were somewhat surprising given the close CTE between mullite and SiC. Conventionally plasma-sprayed mullite coatings contain a large amount of amorphous phase mullite due to the rapid cooling of molten mullite on a substrate.9"11
The amorphous phase in the coating then crystallizes at 950-1000°C during subsequent exposure in service.9"11"13 Crystallization of amorphous mullite accompanies a volumetric contraction.14 The stress associated with the volume change causes severe cracking. These findings led us to develop a new generation of mullite coatings. In these coatings, the formation of an amorphous mullite phase was eliminated by heating the substrate above the crystallization temperature during the plasma spraying.9"11 The new fully crystalline coatings showed dramatically improved adherence, crack-resistance, and chemical stability under thermal cycling conditions.15 Coating properties can be further tailored to accommodate specific environmental requirements for various applications. One approach to modifying coating properties is to apply an overlay coating by using the mullite as a bond coat. This article describes the environmental durability of mullite coatings in air, oxidizing/reducing atmospheres, and molten salts. Possible overlay coatings and key issues related to the overlay coatings will also be briefly discussed. Oxidation Behavior in Air Oxidation Under Infrequent Cycling When cycled between room temperature and 1300°C with a 20 h cycle, new mullitecoated SiC (Hexo
Data Loading...
