Ablation Behavior of SiC/ZrB 2 Multilayer Coating Prepared by Plasma Spray Method
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TODAY, new materials for hypersonic flight vehicles, re-entry vehicles, nozzles, etc., are required. These materials must be able to tolerate oxidizing and corrosive atmospheres at temperatures > 2270 K for long durations.[1–3] Carbon materials, including graphite and C/C composites, which are known as a class of advanced materials, are the best candidates for these applications. Carbon materials possess a unique combination of desirable properties including low density, high toughness and high specific strength (strength to weight ratio), a very low coefficient of thermal expansion and mechanical property retention at high temperatures. However, the poor oxidation resistance of carbon materials above 773 K potentially limits their high-temperature applications.[4,5] Therefore, carbon materials must be protected against oxidation. One of the most effective methods to improve the oxidation resistance of carbon materials is to use ceramic coatings. Among various ceramics, silicon-based ceramics such as SiC coatings show great
SHAHLA TORABI, ZIA VALEFI, and NASER EHSANI are with the Faculty of Materials and Manufacturing Processes, Malek Ashtar University of Technology, Tehran, Iran. Contact e-mail: valefi@mut.ac.ir Manuscript submitted April 9, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
potential for the protection of carbon materials from oxidation at high temperatures because of the beneficial formation of a SiO2 protective film that can efficiently prevent the diffusion of oxygen into the substrate.[6–9] Unfortunately, the SiC coatings have two main disadvantages: (1) Due to thermal expansion mismatches between the carbon substrate and SiC (aSiC = 3 9 106 – 5 9 106/C, aC = 2.2 9 106 – 3 9 106/C), poor adhesion is generally obtained along with a high crack density in the coating.[10,11] One of the approaches employed to solve this problem is the formation of the SiC coating with the transition interface. A functional gradient layer due to the formation of a material concentration gradient at the substrate/coating interface minimizes the influence of the CTE mismatch.[12,13] In fact, a gradual change in the concentration from 100 pct silicon carbide on the surface to 100 pct carbon at the substrate and lack of a clear interface between the substrate and coating result in a gradual change of CTE from the surface toward the substrate. As a result, crack formation at the coating/substrate interface due to thermal shocks is prevented.[10,14,15] (2) The maximum working temperature of the SiC coating is limited to about 1873 K because of the active oxidation of SiC and volatilization of SiO2 at higher temperatures.[16] To protect carbon materials at higher temperatures, the idea of using another ceramic material with high thermal resistance on the SiC has been raised. Due to the unique properties of ultra-high temperature ceramics
(UHTC) in an oxidizing atmosphere > 2273 K, recent studies have been carried out on the application of these ceramics as coatings in thermal protection systems (TPS).[17,18] Among the fam
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