Microstructure and oxidation resistant property of C/C composites modified by SiC–MoSi₂–CrSi₂ coating

Abstract

To prevent carbon/carbon (C/C) composites from oxidation, a dense and multiphase inlaid SiCMoSi 2 -CrSi 2 coating was prepared by pack cementation. The coatings were characterised by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. The effect of pack composition on the oxidation properties was studied on low density C/C composite, and then optimum composition was finalised. The optimum composition was the one containing 5 wt%Cr and oxidation behaviour of high and low density C/C composites coatings with this composition was studied. Oxidation test shows that, when the Cr content was 5 wt-%, the SiCMoSi 2 -CrSi 2 coating possessed excellent oxidation resistant ability. After application of the SiCMoSi 2 -CrSi 2 multiphase coating, the low density C/C composite showed a mass loss of only 1?36% after oxidation at 1873 K for 43 h, while the mass loss of the coated high density sample was up to 7?45% after oxidation at 1873 K for 8 h.

Thermal fatigue behavior of C/C composites modified by SiC–MoSi2–CrSi2 coating

“…1, the peaks of SiC are rather strong, illustrating that the coating is primarily composed of SiC. In addition, the peaks of Si are also rather strong compared with the one of MoSi 2 and CrSi 2, inferring that a large number of Si phases are existent in the coating, which is advantageous to the compactness of the multiphase coating [15]. Fig.…”

“…Then molten Si congregated to the surface of C/C composites would react with C/C to form SiC coating, which tends to reduce the defects in C/C composites and relax the stress at the tip of cracks and heal up cracks in the coating [6]. On the other hand, holes in the porous structure of the coating were filled with some CrSi 2 and MoSi 2 grains in company with the flow of liquid Si, forming a dense and multiphase inlaid coating [15]. This inlaid structure is advantageous in avoiding the formation of through-coating cracks and improving the oxidation protective ability of the coating.…”

“…The weight loss curve of modified C/C samples after thermal cycling between 1773 K and room temperature in Ar for 20 times was almost a horizontal and straight line with values approaching zero, indicating that the modified C/C samples were not oxidized. While that of modified C/C samples after thermal cycling between 1773 K and room temperature in air for 20 times can be characterized by a sloping and straight line, by reason that the oxidation process of C/C matrix is mainly controlled by the rate of O 2− reaction with C/C matrix [15]. Although modification by SiC-MoSi 2 -CrSi 2 multiphase coating could improve the thermal shock resistance of C/C samples, the existence of edges and corners on the surface of C/C samples would cause the stress concentration at the tip of these edges and corners due to the mismatch of thermal expansion coefficients between the coating (˛M oSi2 = 8.1 × 10 −6 K −1 , CrSi2 = 10.5 × 10 −6 K −1 , ˛S i = 2.6 × 10 −6 K −1 , ˛S iC = 4.5 × 10 −6 K −1 ,) and C/C composites (˛C /C = 1.0 × 10 −6 K −1 ) during thermal cycling [16][17][18].…”

“…The temperature for preparing modified C/C samples was 1973-2173 K. Details of preparing modified C/C samples were reported in Ref. [7].…”

“…In addition, provided that the coating materials could infiltrate into C/C composites deeply through the cracks or holes, the favorable inlaid structure and intensive interfacial bonding would be formed at the C/C-coating, which could further improve the oxidation resistance and mechanical properties of coated C/C composites. Low-density C/C composites have been considered as potential candidates into which could be readily infiltrated by the coating materials through the simple and low-cost techniques [6][7][8].…”