Suppression of through-the-thickness cracks in SiC coating on C/C composites

Abstract

Two concepts are proposed to minimize coating cracks on the surface of C/ C composites. The first idea of a multi-layered coating is based on the phenomenon that there exists a critical coating thickness below which no coating cracks appear. This phenomenon is experimentally and theoretically confirmed for a SiC coating. Thus, by alternately laminating thin SiC and carbon layers, coatingcracking is expected to be suppressed for a thick and dense coating.The second concept is the use of a fiber-reinforced coating. By the fiber reinforcement, the toughness of a ceramic coating should be improved. In this paper, both concepts are shown to be promising to reduce through-the-thickness coating cracks. In addition, problems especially related to oxidation behavior are discussed in relation with the two-coating systems.

Increase in the refractoriness of carbon composite materials with use of heat-resistant ceramic coatings

“…Considerable hindering of these processes is achieved by volumetric and surface siliciding of carbon material due to a reduction in the transport pore diameter [3]. However, in layers of a coating, due to a difference in thermophysical properties of the coating and carbon material, cracks form that in turn "pass" the oxidizing agent towards the inner surface of the pore volume of carbon material, and the overall oxidation rate is not markedly reduced [4]. Here the wider the cracks in a coating, the greater is the maximum carbon material weight loss and the lower the temperature for the maximum oxidation rate [5].…”

“…A mechanical method for increasing the operating efficiency of protective coatings may be an attempt to reduce the thickness of an SiC-coating to a level with which it has the least internal stresses, and also creation of coatings with alternating thin SiC coatings within which cracks do not form, and thin carbon layers [4]. Another example is reinforcement of an SiC-coating on the surface of a CCCM with SiC-fibers [8].…”