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Characterization and testing of layered HfC/SiC protective coatings
Abstract
The harsh environments encountered in aerospace reentry and propulsion applications necessitate protective coatings that combine high temperature capability and corrosion (oxidation) resistance. Hard refractory carbides, including silicon carbide (SiC) and hafnium carbide (HfC), offer many desirable properties but are typically limited by the durability and stability of the protective surface oxide layers that form during use. SiC is thus limited to a peak sustained use temperature of about 1650 °C (3000 °F) due to the rapid degradation or loss of the SiO2 protective layer at higher temperatures. HfC, with a protective HfO2 layer, offers higher temperature capability but has poor thermal cycling resistance due to spalling of the oxide. A microlayered HfC/SiC coating developed at Ultramet has demonstrated outstanding resistance to temperatures up to 2480 °C (4500 °F) and excellent thermal cycling resistance. The HfC/SiC coating forms a complex oxycarbide surface layer that is resistant to temperatures significantly higher than those acceptable for SiO2, and remains adherent during thermal cycling. The coating, applied to various aerospace thermal protection system substrate materials, has been tested and characterized under severe conditions (e.g. torch, arcjet, rain erosion) representative of potential operational applications.
In space propulsion applications, the development of new ceramic matrix composites with improved resistance to oxidation and ablation at high temperature is needed and ultra-high temperature ceramics-based ones appear the most suitable. Combination of both powder impregnation (ZrB2, C) and liquid silicon infiltration enabled manufacturing of UHTC based matrices in Cf/C preforms with less than 10 vol% open porosity and various proportions and homogeneous distribution of C, ZrB2, SiC and Si. Oxidation behaviour was evaluated on composite structures using an oxyacetylene torch at temperatures higher than 2000°C. Chemical analyses and microstructural observations before and after oxidation testing evidenced the protection ability of ZrB2-SiC-Si matrices thanks to the formation of multi oxide scales which resisted even tested durations of 6 minutes and pointed the unharmful presence of residual 12 vol% silicon on the composite for use at high temperature under high gas flows.
Silicon oxycarbide (SiOC) is a promising protective coating material. However, so far there has not been any comprehensive review to systematically evaluate the characteristics and properties of this material as coatings. In this review, SiOC compositions and fillers are examined based on their fundamental functions in the coating materials. From a processing point of view, different coating formation processes, such as plasma-enhanced chemical vapour deposition, dip coating, and spin coating, are discussed. As an intrinsic requirement and one of the most challenging aspects for SiOC to be used as coating materials, the stability of SiOC in air conditions is analysed and the degradation mechanisms of SiOC are discussed. Lastly, for different application needs, the physical and mechanical behaviours, electrical conductivity, and optical property of the coatings are presented. This first-ever systematic review on the SiOC system as coating materials is intended to provide guidance for future SiOC coating development.
SiC coatings have been grown by direct liquid injection of organosilanes in a hot-wall chemical vapor deposition reactor (DLICVD). 1,3-disilabutane (DSB) and polysilaethylene (PSE) were used as single-source precursors. Amorphous and stoichiometric SiC coatings were deposited under low pressure on various substrates in the temperature range of 923–1073 K. Thickness gradients due to the temperature profiles and the precursor depletion were observed along the reactor axis but the thickness uniformity could be improved as a function of the deposition conditions. Growth rates as high as 90 μm·h− 1 were obtained using pure precursors. The injection of PSE solutions in toluene significantly reduces the deposition rate due to the decrease of the PSE mole fraction but allows a better control of the growth rates and the microstructure of coatings. They exhibit a smooth surface morphology and a very dense structure. The films grown using pure precursors exhibit an Si:C atomic ratio equal to 1:1 while those using toluene solutions are slightly C-rich (54 at.% C). The presence of solvent vapor in the CVD reactor becomes a source of carbon contamination at deposition temperatures equal to or higher than 1073 K. The influence of the growth conditions is discussed, in particular the presence of toluene vapor.
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