Carbon/Carbon is a highly desirable material for use at elevated temperatures in structural applications due to its high strength-to-weight ratio and increasing strength with increasing temperatures.

This presentation will survey the general methods used to fabricate and apply oxidation protection systems to these composites. This will be followed by an overview of typical physical and mechanical properties and selected results from oxidation rate studies.

Carbon-carbon composites are an emerging class of composite materials having a unique combination of high-temperature properties and low densities. These properties are attractive for hot structure and thermal protection system applications in future aerospace vehicles. Aerospace service environments of particular interest are cyclic temperature, oxidizing environments to 3000°F. For carbon-carbon composites to serve as practical engineering materials in such challenging environments, their long-term mechanical and chemical stability is essential.

Many aspects of proposed service environments for these composites pose significant challenges to their satisfactory performance. Among these aspects are the oxidizing nature of the environments (including both high and low oxygen partial pressures), high temperatures, moisture, cyclic temperature service, and foreign-object impact. This paper presents results from materials performance evaluations which cover each of these parameters. The focus is on oxidation-resistant carbon-carbon composites intended specifically for multi-use aerospace applications. Results are presented for the carbon-carbon material currently in use on Space Shuttle and for newer, more advanced structural forms of these composites.

Advanced metal matrix composites are emerging as materials of construction for high performance aerospace applications. The thrust is to develop high specific strength and high specific modulus ultra-lightweight composites using Al or Mg as matrix materials for space structure applications, and for higher temperature applications, to develop materials using various aluminides as matrix materials. This paper presents an overview of the methodology used to develop advanced MMC's and discusses the problems and limitations faced in achieving the composite materials development goals.

Fiberglass-epoxy composites have been considered for use as structural members for the mast of the Space Station solar array panel. The low Earth orbital environment in which Space Station is to operate is composed mainly of atomic oxygen, which has been shown to cause erosion of many organic materials and some metals. Ground based testing in a plasma asher was performed in order to determine the extent of degradation of fiberglass-epoxy composites when exposed to a simulated atomic oxygen environment. During exposure, the epoxy at the surface of the composite was oxidized, exposing individual glass fibers which could easily be removed. Several methods of protecting the composite were evaluated in an atomic oxygen environment and with thermal cycling and flexing: The protection techniques evaluated to date include an aluminum braid covering, an indium-tin eutectic and a silicone based paint. The open aluminum braid offered little protection while the CV-1144 coating offered some initial protection against atomic oxygen. The In-Sn eutectic coating provided initial protection against atomic oxygen, but appears to develop cracks which accelerate degradation by atomic oxygen when flexed. Coatings such as the In-Sn eutectic may provide adequate protection by containing the glass fibers even though mass loss still occurs.

The useful lives of many ceramic composites are influenced, if not controlled, by chemical interactions between various constituents and the gas environment. Thermochemical calculations are extremely valuable in evaluating these concerns. Examples of many possible concerns are presented. The usefulness of a thermochemical approach is demonstrated by a somewhat detailed discussion of the oxidation protection of carbon-carbon composites. This includes discussion of the various possible rate limiting steps and the detrimental effects of CO gas formed as a result of carbon oxidation.

Particulate composites of TiB2 in ZrO2, Y2O3 and Al2O3 have been prepared by vacuum hot pressing. Specimens were tested in oxyacetylene flames at 1650 to 2050°C; in vacuum at 1600°C and by induction heating in air at 1400 to 1600°C in order to evaluate the chemical compatability between the diboride/oxide combinations, as well as the effectiveness of the oxides as diffusion barriers. Results indicate that the large vapor ressure of B2O3 formed at the diboride/oxide interfaces presents serious problems under all the conditions tested.

The objective of this work was to determine the mechanisms of oxidative degradation of two non-oxide reinforced oxide composites. The composite systems studied were: (1) SiC whiskers in MgAl2O4 spinel and (2) SiC whiskers in 3Al2O3–2SiO2 (mullite). The components of the composites were chosen for their stability, purity, thermochemical compatibility, and oxidation resistance.

Advanced rocket engine combustion devices operate in a highly dynamic thermochemical environment that challenges the material research community to provide solutions to heretofore unsolved technical problems. The paper outlines a range of needs and corresponding operating environments that typically exceed the melting temperature of most materials, imposes heating and cooling rates measured in thousands of degrees per second, provides cyclic, highly oxidizing, and reducing combustion environments often found side-by-side, including nonequilibrium gas compositions not readily producible in the laboratory.

The stringent design requirements for minimum weight, impeccable quality control, and high reliability most often make the material cost contribution a secondary factor in relation to the total end-item cost. This provides new opportunities for the use of ultra high-tech material/material processing that would ordinarily be rejected for economic reasons.

Selective corrosion resistance of the chromium implanted brass as well as vacancy formation were investigated. Dealloying was observed to create excess vacancies. Chromium and boron and many other potentially beneficial alloying elements like C, N, Si, P, V, Co, Ni, Cu, Ge, As, Y, Nb, Pd, Ag, Sn, Sb, have a repulsive interaction with vacancies and they could increase the dezincification resistance in alpha brass but not in beta brass. Boron implanted brass was found to be more resistant to selective corrosion than the chromium implanted brass.

The stability of metals (or lack thereof) upon exposure to the indoor or outdoor atmosphere places strong constraints upon many of their uses, including uses in advanced technologies. Two recent developments permit increased understanding of the processes that occur: a detailed knowledge of the chemical constitution of indoor and outdoor atmospheres and of precipitation, and extensive analytical studies of the corrosion layers of degraded metals. Among the insights revealed from these perspectives are (1) the participation in materials degradation of the sulfuric and nitric acids in small aerosol particles and in precipitation; (2) the presence of organic acid anions within corrosion films, revealing interactions with atmospheric formic, acetic, and oxalic acids; (3) the participation of dissolved hydrogen peroxide in the passivation or dissolution of the surface layers of metals; and (4) the frequent occurrence of insoluble mixed salts of hydroxide and either sulfate, chloride, or carbonate anions in the corrosion layers of many metals, suggesting that the rates of formation and dissolution of such minerals may be limiting steps in many stabilization or degradation processes.

The vibrational spectra of silicate glasses provide some clues on the structure and stability of the glass-forming network. Wavenumbers of Raman bands are related to the types of linkages within the network and Raman intensities to the “speciation” within the network. Absolute infrared intensities which can be obtained from deconvolution of specular reflectance spectra can be used to obtain dipole derivatives which in turn can be correlated with effective charges on bridging and non-bridging oxygens. IR and Raman spectra measured on a variety of silicate glasses are used to extract network polymerization and bond effective charges which in turn are related to the durability of the glasses.

There is renewed appreciation for understanding glass corrosion mechanisms due in large part to the urgency of nuclear waste management. Although the ultimate goal - to predict specific corrosion performance - has not yet been reached, theories continue to be fine tuned so that this objective now appears attainable.

However, even at our present level of understanding, chemical durability theory is a vital tool in reading the secrets of the past as contained in both ancient and natural glasses, designing products such as optical waveguides for a new technological age and predicting how those products, such as nuclear waste glass, will perform - far into the future.

Studies of the corrosion process of silicate and boro-silicate glasses under a broad range of contact times, surface-to-volume ratios and leachant compositions indicate that dissolution of such glasses in aqueous media can be interpreted in terms of activity of dissolving amorphous silica. This activity is modified by the presence of elements such as Al, Mg and Fe. The effects of these elements, when initially present in the glass or in the leachant, and, in general, the effects of glass composition on the course of the corrosion process can be viewed in terms of the formation of a surface layer on the leached glass. The type, composition and structure of this layer, which are determined by glass composition, control the dissolution behavior of the glass.

A thermodynamic model of glass durability has been applied to natural, ancient, and nuclear waste glasses. The durabilities of over 150 different natural and man-made glasses, including actual ancient Roman and Islamic glasses (Jalame ca. 350 A. D., Nishapur 10-11th century A. D., and Gorgon 9-11th century A.D.), have been compared. Glass durability has been shown to be a function of the thermodynamic hydration free energy, δGhvd, which can be calculated from glass composition and solution pH. Using this approach the durability of the most durable nuclear waste glasses examined was ˜106 years by comparison with the durability of the natural basalts of ˜106 years. The least durable waste glass formulations were comparable in durability to the most durable simulated medieval window glasses of ˜103 years. In this manner, the durability of nuclear waste glasses has been interpolated to be ˜106 years and no less than 103 years.

Hydration thermodynamics have been shown to be applicable to the dissolution of glass in various natural environments. Groundwater-glass interactions relative to geologic disposal of nuclear waste, hydration rind dating of obsidians, and/or other archeological studies can be modeled, e.g. the relative durabilities of six simulated medieval window glasses have been correctly predicted for both laboratory (one month) and burial (5 year) experiments.

The effects of solution pH on glass dissolution has been determined experimentally for the 150 different glasses and can be predicted theoretically by hydration thermodynamics. The effects of solution redox (oxidation potential expressed as Eh) on dissolution of glass matrix elements such as Si and B have been shown to be minimal. The combined effects of solution pH and Eh have been described and unified by construction of thermodynamically calculated Pourbaix (pH-Eh) diagrams for glass dissolution. The Pourbaix diagrams have been quantified to describe glass dissolution as a function of environmental conditions by use of the data derived from hydration thermodynamics.

Eh-pH diagrams are useful for comparing the probable geochemical behavior of many elements including U and the transuranics Np, Pu, Am. Revised Eh-pH diagrams have been constructed for U, Np, Pu and Am at 25 C, 1 bar conditions. These diagrams, based on new and revised thermodynamic data, include aqueous and solid species in the generic system M-C-O-H. The species M(OH)− is considered herein, although it's existence has not been verified.

Aqueous U(VI) species are dominated by below pH 5 and by carbonate complexes at higher pH. U(IV) species include important fields of UO2, and U(OH)4. If Si is present, USiO4 may be important. U3O8, occurs between the U(IV) and U(VI) species.

Np(IV) solid species are problematic. NpO2 covers a wide range of Eh-pH, but its precursors, NpO. (OH)2 and Np(OH)4 cover more restrictive Eh-pH ranges with more important. This increases potential for radionuclide migration. Np(V, VI) carbonate complexes are important only at high Eh.

Pu(IV) species are dominated by PuO2, but Pu(OH)4 does not occupy as much Eh-pH space, being replaced in large part by . With carbonate present, Pu(III) species also replace part of the Pu(OH)4 field, and Pu2 (CO3)3 becomes important. Pu(VI) carbonate complexess occur only at high Eh.

Am(III) species dominate Eh-pH space in the system Am-C-O-H, with Am2 (CO3)3 a major phase. Under carbonate-poor conditions, Am(OH) 3 is important. AmO2 occurs at high Eh, pH, and at high Eh and very high pH.

The importance of the M(OH)4 precursors to MO2 species is the enhanced possibility of radionuclide migration from buried spent fuel rods, even under reducing condtions.

Results are presented from a two-year dynamic leach test of nuclear waste glass under conditions designed to simulate those of the Stripa granite repository. Solution and surface analytical techniques were used to assess the glass leach rate as well as surface composition and morphology. Glass leach rates were observed to decrease by a factor of two during the first six months. This effect is attributed to the formation of a protective surface layer. Analysis of this layer shows it to be rich in silicon and iron and depleted in lithium, sodium and boron. It was also found that the layer is subject to dissolution.

Reactions of glasses with the environment occur directly on the glass surface and inside the glass after diffusion of molecules into it. Water, either vapor or liquid, is the most important reactant, but other gases can react with surface and internal silicon-hydroxyl and silicon-oxygen groups. Substantial amounts of water can be introduced into glasses at high temperatures and pressures; the resultant hydrosilicates have unusual properties.

In multicomponent alkali silicate glasses water preferentially leaches components; the mobile alkali ions are leached over a wide pH range. Analysis of alkali profiles is straightforward, but measurement of hydrogen profiles requires special care because of outgassing of water. Hydration transforms the surface structure of less durable silicate glasses, leading to rapid outgassing and rapid ionic transport in the transformed layer.

The rate of dissolution of the silicon-oxygen network depends on temperature, glass composition, and solution components, especially pH.

Heavy metal fluoride glasses dissolve in water, and glass components precipitate on the glass surface. These glasses are quite durable in humid air, with a slow reaction with water to form HF vapor.

The corrosion of silicate glasses is described in terms of four stages of surface attack which result in six types of surface conditions depending upon the glass composition and various environmental and physical factors. Type I and II surfaces correspond to durable glasses which have a very high surface SiO2 content whereas Type IV and V surfaces are unstable. Type IIIA surfaces include bioactive glasses which chemically bond to bone and Type IIIB surfaces are characteristic of the alkali borosilicate nuclear waste glasses which form multiple layers of precipitated surface films.

Fluoride glasses are being considered for use in long distance, mid-infrared transmitting optical fibers. The multi-phonon edge of these fibers is shifted to longer wavelengths compared to silica fibers. The intrinsic loss is therefore lower (< 0.01 dB/km) and occurs at about 2.5 microns. Applications for these fibers include ultra-low loss communication links, radiation resistant links, remoting of IR focal plane arrays and IR power delivery. In addition, this ultra low loss optical fiber may be used for fiber guided missiles with capability for distances longer than 500 km. Most of the interest however, has been focused on development of ultra long (thousands of kilometers), repeaterless communication links. Such repeaterless links may decrease system cost and cable weight, while increasing overall reliability and ease of repair compared to silicate optical fibers.

This paper presents a review of our current understanding of environmentally induced crack growth in glasses, single crystal ceramics, and polycrystalline ceramics. It is shown that the rate of crack growth is controlled by the chemical activity of the active species in the environment as well as by the crack tip stress. A molecular model of a stress-induced chemical reaction between vitreous silica and water is described, and the implications of this model for predicting the effects of other chemical species on crack growth are discussed. Effects of chemical bonding in the solid as well as in the reacting solutions, on the crack growth mechanism are elucidated. Finally, the complicating effects of multigrain arrays on crack extension in polycrystalline ceramics are pointed out.