Environmentally Resistant Mo-Si-B-Based Coatings
- PDF / 1,805,641 Bytes
- 12 Pages / 595.276 x 790.866 pts Page_size
- 69 Downloads / 187 Views
PEER REVIEWED
Environmentally Resistant Mo-Si-B-Based Coatings J. H. Perepezko1 • T. A. Sossaman1 • M. Taylor1
Submitted: 7 February 2017 / in revised form: 12 April 2017 / Published online: 19 May 2017 Ó ASM International 2017
Abstract High-temperature applications have demonstrated aluminide-coated nickel-base superalloys to be remarkably effective, but are reaching their service limit. Alternate materials such as refractory (e.g., W, Mo) silicide alloys and SiC composites are being considered to extend high temperature capability, but the silica surfaces on these materials require coatings for enhanced environmental resistance. This can be accomplished with a Mo-Si-Bbased coating that is deposited by a spray deposition of Mo followed by a chemical vapor deposition of Si and B by pack cementation to develop an aluminoborosilica surface. Oxidation of the as-deposited (Si ? B)-pack coatings proceeds with partial consumption of the initial MoSi2 forming amorphous silica. This Si depletion leads to formation of a B-saturated Mo5Si3 (T1) phase. Reactions between the Mo and the B rich phases develop an underlying Mo5SiB2 (T2) layer. The T1 phase saturated with B has robust oxidation resistance, and the Si depletion is prevented by the underlying diffusion barrier (T2). Further, due to the natural phase transformation characteristics of the Mo-Si-B system, cracks or scratches to the outer silica and T1 layers can be repaired from the Si and B reservoirs of T2 ? MoB layer to yield a self-healing characteristic. Mo-Si-B-based coatings demonstrate robust performance up to at least 1700 °C not only to the rigors of elevated temperature oxidation, but also to CMAS attack, hot corrosion attack, water vapor and thermal cycling.
& J. H. Perepezko [email protected] 1
Department of Materials Science and Engineering, University of Wisconsin-Madison, 1509 University Ave., Madison, WI 53706, USA
Keywords environmental resistance high-temperature coatings oxidation
Introduction The continuing drive to attain increased gas turbine alloys and SiC composites engine performance in terms of output power along with reduced emissions and improved efficiency requires the incorporation of new high-temperature materials with capabilities beyond the limitations of current Ni-base superalloys (Ref 1). Both multiphase Mo- and Nbbase refractory metal alloys and SiC composites are receiving increased attention since they offer high temperature strength and creep resistance (Ref 2). While the Mo-Si-B alloys have some intrinsic oxidation resistance, the level is insufficient for operational conditions (Ref 3). In order to provide the supplemental environmental resistance, there is a clear trend to rely on coatings that must exhibit a robust and versatile performance so that they can endure a range of aggressive environments without degradation (Ref 4, 5). For example, in a high temperature combustion environment about 10% is water vapor that attacks pure chromia and silica to form volatile products (Ref 6). Similarly, salt ingestion and
Data Loading...
