Coated RCC has very modest mechanical properties and a relatively high CTE in the plane of the fibers. A tensile strength of 62.5Mpa, a tensile elastic modulus of 14.5Gpa, and a CTE of 2.6*10-6C-1 are average RCC in-plane properties. In contrast, the projected structural applications required tensile strengths in the 207Mpa to 345Mpa range. This dictated the use of high-performance carbon fibers that produce CC composites with elastic modulus in the 90Gpa to 117Gpa range and CTE values of approximately 1.8*10-6C-1.
Early systematic oxidation testing of the first structural oxidation-protected CC composite was conducted by two U.S. air force contractors in 1981. The CC composite was a two-dimensional laminate made from high-performance PAN-precursor fibers and was designated advanced carbon carbon (ACC). The best results were obtained with the RCC coating system in which the SiC conversion layer was modified with boron. Burner rig testing that involved rapid heating and cooling between 150C and 1371C with half-hour holds at the maximum temperature resulted in several early failures; this testing also resulted in a number of specimens that performed for periods of 25h to 100h with no weight loss. However, consistently poor performance was shown in static oxidation tests below 760C because of obviously inadequate sealing by the glaze overlay coating and the boronated SiC conversion.
The initial tests of the ACC structural CC with the boron-modified RCC coating were encouraging for limited-life applications in which the material experiences rapid heating and is required to perform for several hours at temperatures in the 1000C to 1400C range. On the other hand, extended-life applications require hundreds of hours of exposure from 649C to 1371C. Oxidation at lower temperatures was a problem with the modified RCC coating system, and a major development effort was clearly needed. Furthermore, it was recognized that the basic RCC coating approach had features that posed serious problems for many of the new structural applications. First, the outer glaze coating was susceptible to flow and particulate erosion. The glaze was also susceptible to alkali loss at high temperatures and acted as an adhesive that prevented movement of mating parts without damage to the coating. A second problem was that the proposed structural parts were often 2.5mm to 5mm thick and the airfoil trailing edges could be as thin as 1mm. Very often the coating approached 1 mm in thickness and the mechanically poor SiC conversion layer constituted a large portion of the part.
The outer glaze and coating thickness problems associated with the RCC type coating have led to the current concept of using a hard, dense outer coating for both limited-life and extended-life applications. The CVD process is used to produce the outer coating that act as the primary barriers to oxygen ingress. The materials SiC and Si3N4 have received the most attention for outer coatings because of their relatively low CTE values and excellent oxidation resistance up to at least 1700C. Recent work for limited-life applications has shown that under conditions of rapid heating to above the coating temperature, in which thermal expansion mismatch cracks in the coating are closed, CVD outer layers can provide excellent CC protection for several hours up to 1750C. cfccarbon.com
Extended-life applications and even limited-life use at temperatures below that in which the coating cracks are open require a glass sealant to fill the cracks. Elemental boron, boron carbide, and several configurations of mixed boron compounds with and without SiC and elemental silicon are now under evaluation as inner layers. Inner layers are being made by particulate slurry coating, CVD, and carbide conversion of the CC surface. The main purpose of the inner layer is to form a borate glass by oxidation through the cracks in the outer coating. Oxidation of the portion of the inner layer beneath the cracks to form a glass produces a 200-percent to 250-percent volume increase that forces the glass into the cracks. In addition to the outer coating and inner glass-forming layer, a base SiC conversion layer that improves bonding to the CC surface will be used often if the glass-forming layer is not a conversion layer.
In addition to acting as the primary oxygen barrier, outer SiC and Si3N4 coating provide hard erosion-resistant bearing surfaces that cover the inner layers and inhibit vaporization of the borate glass sealants. The inner layers provide the sealant glasses but they also must establish and maintain strong bonding with the outer coating and CC to inhibit coating spallation. Conversion layers that are integral to the CC surface and have been densified by some pore-filling technique may be optimum for bonding.
The use of borate and phosphate glasses to protect carbon bodies from oxidation has a long history. Boric oxide is particularly attractive because it melts at approximately 450C and has viscosity, wetting, and thermal stability characteristics that make it an effective sealant over a wide range of temperatures.
Boric oxide has relatively lower vapor pressures at high temperatures and is thermodynamically stable in contact with many materials. For example, B2O3 has a vapor pressure of approximately 10-7 Mpa at 1000C and 10-4 Mpa at 1400C under dry conditions, and it is stable in contact with carbon up to 1575C. Moisture-induced volatility and low viscosity limit the utility of borate glasses to about 1000C unless the glasses are protected by overlay coating. Experience has shown that borate glasses can seal cracks in outer coating during hundreds of hours of thermal cycling in which maximum temperatures of 1400C are maintained for significant fractions of the time. This is due to the outer coating that protects the glass and the presence of inner layers that oxidize to renew glass that is lost by flow or evaporation.