The desirability of providing carbon bodies having oxidation resistance is well-known. Carbonaceous materials such as monolithic carbon, graphite, and carbon-carbon composites of fibers in carbon have excellent strength-to-weight properties at high temperatures, e.g., 1400.degree. C. and higher, and are generally superior to conventional construction materials such as metals and super alloys at these temperatures. In addition, the mechanical strength of a carbon body increases as the temperature increases, whereas in conventional structural metals, the strength typically decreases with increased temperature.
The use of carbon bodies in high temperature applications has been limited due to the relatively high reactivity of carbon, principally with oxygen, at temperatures above about 400.degree.-500.degree. C. Such reactivity results in erosion of the carbon body due to the reaction between carbon and oxygen, yielding carbon monoxide and carbon dioxide. Accordingly, many attempts have been made to provide oxidation-resistant coatings for carbon bodies in order to permit their use in oxidizing environments and at elevated temperatures.
Major difficulties have been encountered in attempting to provide oxidation-resistant coatings on carbon bodies. One difficulty is that the coefficient of expansion of the carbon body may be very different from that of the oxidation-resistant coating. The stresses that result from different coefficients of expansion between the coating and the underlying carbon body cause cracking or rupture of the coating, particularly when the part is subjected to thermal cycling. When the coating integrity is thus compromised, oxygen penetrates the coating and attacks the underlying carbon body with resulting loss of structural integrity.
Surface porosity in the carbon body, which results from articles which are not fully densified, may cause pinholes to form in the coating during the coating process. This also may allow oxygen to penetrate to the carbon body. It has also been found that mechanical vibration, debris impingement, and the like may cause cracking of brittle protective coatings.
Successful resistance to high temperature oxidation may be achieved by the process disclosed in U.S. Pat. No. 4,515,860, which is incorporated herein by reference. The oxidation-resistant carbon body disclosed in this patent has thermochemically deposited thereon a silicon alloy coating containing one or more alloying elements selected from the group consisting of carbon, oxygen, aluminum, and nitrogen. The amount of silicon in the coating is in excess of the stoichiometric amount and the alloy coating has a non-columnar grain distribution having substantially equiaxial grains of an average diameter of less than one micron. Because of the exceptionally fine grain size and even grain distribution in the coating, any cracks that may occur are extremely fine in width and form a mosaic pattern. The amount of silicon in excess of the stoichiometric amount fills in these fine cracks when the carbon body is heated to above the melting point of silicon, e.g., above 1410.degree. C., and reacts with any oxygen to form a glassy silicon oxide which acts as a filler sealing the cracks. This patent also contemplates, on an optional basis, particularly where lower temperature crack-resistance is desired, providing an intermediate boron layer. Boron reacts with oxygen to form a glassy boron oxide sealant and flows into any cracks that have formed. In commercial practice the carbon body is usually provided with a preliminary treatment in a mixture of chromic acid and sulfuric acid.
The oxidation resistance conferred by the coatings described in U.S. Pat. No. 4,515,860 provides significant superior characteristics as compared to the coatings of the prior art. Under some circumstances, however, particularly where severe temperature cycling occurs, the protection system may be inadequate to properly seal the cracking which occurs in the brittle coating such that the carbon body is subjected to oxidative attack.
In U.S. patent application Ser. No. 873,004, a coated carbon body is described having improved resistance to oxidation over wide temperature ranges including low temperatures of 500.degree.-1000.degree. C. and high temperatures in excess of 1400.degree. C. The carbon bodies produce by the methods described therein also exhibit excellent oxidation resistance even in environments that involve high temperature thermal cycling. In addition, such carbon bodies exhibit excellent resistance to ablation and erosion.
In the aforesaid patent application, a carbon body is heated to an elevated temperature, generally above 1500.degree. C., and is exposed to preferably gaseous boron oxide. The resulting gas-solid reaction causes the surface of the carbon body to become etched and results in the formation of boron carbide which is contained in the converted and etched surface. The etched and converted surface zone is about 2 to 250 microns deep and is characterized by interconnecting interstices. Following formation of the etched surface, the converted carbon body is provided with a glass forming coating, at least a portion of which is within the interconnecting interstices. The glass forming material may be selected from the group consisting of boron, boron oxide, boron carbide, silicon, silicon alloy, silicon dioxide, germania, and mixtures thereof.
In U.S. patent application Ser. No. 131,479, a continuation-in-part application to Ser. No. 873,004 referred to above, silicon nitride and silicon oxynitride are disclosed as additional glass forming materials.
It has been found that, under certain circumstances, the use of a gaseous boron oxide reactant in the foregoing described method may be difficult. For example, on carbonaceous substrates having complex geometric shapes of, it may be difficult to control the uniformity of the surface etching and conversion. This difficulty is attributable to the formation of concentration gradients of boron oxide present in the gas phase treating the carbon material. In addition, velocity gradients in the fluid flow patterns surrounding the substrate may also contribute to non-uniformity. Moreover, more than one high temperature treatment may be required to form the desired conversion layer in the substrate since certain areas which are poorly exposed to the reactant gas may remain unetched or poorly etched. The foregoing difficulties also make it difficult to scale up the process to accommodate large components, since flow conditions and concentration gradients may be affected as a result of the scale up.