The mechanical properties of ceramic matrix composites (CMCs) are governed by the relationships between the matrix, the interface material, and the fibers. In non-oxide matrix systems the use of compliant pyrolytic carbon or BN have been demonstrated to be effective interface materials, allowing for absorption of mismatch stresses between fiber and matrix and offering a poorly bonded interface for crack deflection. The resulting materials have demonstrated remarkable strain/damage tolerance together with high strength. Carbon or BN, however, suffer from oxidative loss in many service environments, and thus there is a major search for oxidation resistant alternatives. This paper will review the issues related to developing a stable and effective interface material for non-oxide matrix CMCs.

Chemical vapor deposition is well suited for preparing many advanced products, including single-phase and composite materials, coatings, and films. Significant success in the manufacture of electronic semiconductors, beginning in the early 1970s, accelerated the development of CVD processes for the production of other materials. Hard and wear-resistant materials (TiC, TiN, TiB2, Al2O3, and diamond and diamondlike carbon) have found numerous and growing applications such as coatings for cutting tool inserts, knives, and metallic turbine blades. The high strength and exceptional fracture toughness of fiber-reinforced ceramic matrix composites fabricated by CVI make them applicable for many high temperature structural applications. Multilayer CVD coatings of SiC/glass or SiC/oxide have proved effective for protecting carbon/carbon composites from high temperature oxidation. Application of PACVD has continued in the electronics industry for the manufacture of semiconductor devices, protective coatings for electronic circuits, and solar cells. Laser-assisted CVD is finding increased utility in component preparation, and, should its technological limitations be overcome, could play a major role in producing radiation damage free, high density, complex integrated circuits and other electronic components.

A chemical vapor deposition process was developed for the fabrication of high-temperature particulate filters. Fibrous Nicalon (SiC)felt was used as the filter material. Preliminary evaluation of filters fabricated from Nicalon revealed that cleanablefilters with collection efficiencies of >99.9% could be fabricated. However, weaknesses identified during the initial testing and evaluation spurred the development of several improved filter designs for future investigation.

An analytical model has been developed for the forced-flow/thermal-gradient chemical vapor infiltration process as applied to the Nicalon/SiC system. The model utilizes various substrate properties and deposition kinetics to predict the progress of infiltration. Preforms have been modeled to determine surface area/density relationships, and it has been shown that a 'series resistance' model for thermal conductivity exhibits good fit to experimental data. Model predictions based on these parameters and first-order deposition kinetics have shown excellent agreement with experimental trends. The model predicts an increase in the cool-face temperature and the differential pressure across the substrate that are directly related to the degree of densification. These parameters can be used to monitor and modify the infiltration process. An infiltration system has been equipped with a computer-interfaced process control and data acquisition system. The system has been used to monitor and alter conditions during infiltration, thus improving deposition efficiency and uniformity.

Toughened ceramic composites were produced by the simultaneous chemical vapor deposition of two phases. Fracture toughness values were nearly double the value of SiC when CH/sub 3/SiCl/sub 3/ and TiCl/sub 4/ vapors were used to produce SiC-TiSi/sub 2/ composites. Other systems consisting of SiC and the metal or metal carbides of Ni, Cr, W, and Mo were examined. Equilibrium thermodynamic analyses of these systems were performed as a function of the chemical vapor deposition variables: temperature, total pressure, and reactant concentration. These calculations confirmed that the volatile species used early in this work (organometallic compounds of Ni and Cr and the fluorides of W and Mo) were not suitable for the production of toughened composites. Additional work is under way to produce composites from the oxychlorides or carbonyls of Ni and Cr or the chlorides or oxychlorides of W and Mo. Thermodynamic calculations also indicated that SiC and TiC should be codeposited when CH/sub 3/SiCl/sub 3/, TiCl/sub 4/, and a hydrocarbon are used as the coating gases. Much additional work remains to produce SiC-TiC composites and to control the morphology of the TiSi/sub 2/ dispersed phase so that optimum fracture toughness values can be obtained.

A process has been developed for the fabrication of fiber-reinforced SiC composites by chemical vapor infiltration. Infiltration times for the low-density fibrous structures were reduced significantly from previous processes by simultaneously utilizing a thermal gradient and forced gas flow. Uniform matrix deposition occurred throughout unidirectional-fiber, cloth, or random chopped-fiber preforms to produce composites with densities approaching 90% of theoretical. Fabrication of composites in this fashion produced specimens with high flexural strengths and with a strain at maximum load which significantly exceeded those of conventional SiC bodies.

Oxide fiber-reinforced silicon carbide matrix composites were fabricated employing the forced-flow, thermal gradient chemical vapor infiltration (FCVI) process. NextelTM fibers of varying composition were incorporated into the SiC matrix and the influence of fiber composition on densification was examined. Composites were prepared to investigate the effectiveness of each NextelTM fiber as a reinforcement for the given matrix. A carbon interface coating was used for the baseline materials, however, alternate interlayers with improved oxidation resistance were also explored. Room-temperature flexure strengths of as-fabricated composites and specimens heated in air at 1273 K were measured and compared to results for other SiC-matrix composites.

Forced-flow thermal gradient chemical vapor infiltration (FCVI) has been developed fbr the rapid densification of ceramic matrix composites. For preforms of >3 mm thickness FCVI can produce a near-net-shape part in less than one day as opposed to isothermal, isobaric CVI which requires several weeks to density such a component. Efforts at ORNL and elsewhere have resulted in capability to produce prototypical thick-walled heat exchanger tubes and turbine disk blanks. This paper will review recent modeling and experimental efforts re fated to the FCVI of cylindrical forms.

Crystalline mullite was deposited by chemical vapor deposition (CVD) onto SiC/SiC composites overlaid with CVD SiC. Specimens were exposed to isothermal oxidation tests in high‐pressure air + H2O at 1200°C. Unprotected CVD SiC formed silica scales with a dense amorphous inner layer and a thick, porous, outer layer of cristobalite. Thin coatings (∼2 μm) of dense CVD mullite effectively suppressed the rapid oxidation of CVD SiC. No microstructural evidence of mullite volatility was observed under these temperature, pressure, and low‐flow‐rate conditions. Results of this preliminary study indicate that dense, crystalline, high‐purity CVD mullite is stable and protective in low‐velocity, high‐pressure, moisture‐containing environments.

Corrosion resistant Cs0.6Mg0.4Zr4(PO4)(6) (CMZP) and Ca0.5Sr0.5Zr4(PO4)(6) (CS-50) coatings for fiber-reinforced SiC-matrix composite heat exchanger tubes have been developed. Aqueous slurries of both oxides were prepared with high solids loading. One coating process consisted of dipping the samples in a slip. A tape casting process has also been created that produced relatively thin and dense coatings covering a large area. A processing technique was developed, utilizing a pre-sintering step, which produced coatings with minimal cracking.

Chemical vapor infiltration (CVI) is simply chemical vapor deposition (CVD) on the internal surfaces of a porous preform and has been used to produce a variety of developmental and application materials. The greatest use of CVI is to infiltrate continuous-filament preforms taking advantage of the relatively low-stress CVD process. In CVI, reactants are introduced in the porous preform via either diffusion or forced convection and the CVD precursors deposit the appropriate phase(s). As infiltration proceeds, the deposit on the internal surfaces becomes thicker. Thus, after some length of time, the growing surfaces meet bonding the preform and fill much of the free volume with deposited matrix. The forced-flow/thermal-gradient technique (FCVI) developed at Oak Ridge National Laboratory overcomes the problems of slow diffusion and restricted permeability, and has demonstrated a capability to produce thick-walled, simple-shaped, SiC-matrix components in times of the order of hours. A model has been developed for the process that predicts flow, thermal and density profiles as a function of time. The results have been compared with an initial set of experiments and indicate qualitative agreement. It is expected that improved property relationships, such as permeability and thermal conductivity as a function of density, will allow the model to closely represent the FCVI process and be useful in fabrication and product optimization.

Coatings of Ca0.5Sr0.5Zr4(PO4)6 (CS-50), an ultra-low thermal-expansion material, have been successfully developed to protect SiC-based materials from sulfate salt corrosion. The coatings were applied to fiber-reinforced, SiC-matrix composite heat exchanger tubes and clay-bonded SiC. Aqueous slurries were prepared and applied to the SiC substrates. One coating process for heat exchanger material substrates, consisted of mechanically dipping the samples in a slip. A spin-coating process has also been developed that produced thin, non-obstructing coatings for hot-gas filter substrates. The thermal shock and corrosion resistance of coated parts were evaluated, and preliminary results are encouraging for the use of CS-50 as a protective coating.

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Mullite and aluminum titanate precursor polymeric sols were developed for applying as coatings on Nicalon{trademark} fabrics and tows. A Nicalon{trademark}/SiC composite with a mullite interface was fabricated. The mullite precursor interface coatings were applied by a vacuum infiltration method and the SiC matrix was deposited by a forced flow chemical vapor infiltration process. Thin, uniform mullite interface coatings were obtained. However, the Nicalon{trademark}/SiC composite exhibited brittle fracture. Mullite and alumina-titania coatings were applied on Nicalon{trademark} tows and the effect of heat treatment at 1000{degrees}C in air is discussed.

The interaction between several low-expansion NZP materials and Na{sub 2}SO{sub 4} at 1000{degrees}C in pure O{sub 2} was studied. Ba{sub 1.25}Zr{sub 4}P{sub 5.5}Si{sub 0.5}O{sub 24} experienced extensive cracking and delamination upon reaction with Na{sub 2}SO{sub 4}. On the other hand, Ca{sub 0.5}Sr{sub 0.5}Zr{sub 4}P{sub 6}O{sub 24} remained intact in terms of visual appearance, and had no significant weight loss or gain. However, the ion exchange between Na{sup +} ions and Ca{sup +2} ions was observed to be sufficiently rapid to allow the penetration of the Na{sup +} ions into the test specimens in 100 h. The segregation of Ca to the specimen surface was observed due to the ion exchange. Ca{sub 0.6}Mg{sub 0.4}Zr{sub 4}P{sub 6}O{sub 24} was also tested, but its stability could not properly be assessed because the as-received specimens contained a significant amount of MgZr{sub 4}P{sub 6}O{sub 24} as an impurity phase.

Continuous fiber ceramic matrix composites require fiber/matrix interfaces which allow load transfer from the matrix to the fibers when the composite materials are st-eased. Crack deflection and fiber pullout are also necessary components of the mechanical behavior of composites. Screening interface materials and determining their optimum characteristics is a lengthy and expensive procedure if standard chemical vapor infiltration composite processing is used. A procedure for fabricating minicomposites was developed to address this problem. Minicomposites require very little material and much less labor than is necessary to produce a standard composite. Also, the mechanical property measurements made on minicomposites target the behavior of the interface coatings and their effect on the properties of the composite. Tensile testing of minicomposites was used to optimize the matrix infiltration process and will be utilized in the future to study the mechanical behavior of new materials systems, and specifically, new interface coating materials.

This feature article explores the concept of creating functionally graded metal-ceramic composite microstructures for thermal barrier coatings used in gas-turbine applications, From a thermomechanical perspective, this concept offers the possibility of significantly improving the life and reliability of thermal barrier coatings, However, prior research reveals that progress has been somewhat limited because of the oxidative instability exhibited by some metal-ceramic composite microstructures, The present study addresses some of the materials criteria and research issues associated with preparing chemically stable, yet mechanically durable, graded metal-ceramic microstructures for realistic application environments.

A novel type of hot-gas filter based on a ceramic fiber-reinforced ceramic matrix was developed and extended to full-size, 60-mm OD by 1.5-meter-long, candle filters. A commercially viable process for producing the filters was developed, and the filters are undergoing testing and demonstration throughout the world for applications in pressurized fluidized-bed combustion (PFBC) and integrated gasification combined cycle (IGCC) plants. Development activities at Oak Ridge National Laboratory (ORNL) and at the 3M Company, and testing at the Westinghouse Science and Technology Center (STC) are presented. Demonstration tests at the Tidd PFBC are in progress. Issues identified during the testing and demonstration phases of the development are discussed. Resolution of the issues identified during testing and the status of commercialization of the filters are described.

The potential application of Ca0.5Sr0.5Zr4P6O24 (CS50) as a corrosion-resistant coating material for Si-based ceramics and as a thermal barrier coating material for Ni-based superalloys was explored, A similar to 200 pm thick CS50 coating was prepared by air plasma spray with commercially available powder A Nicalon/SiC ceramic matrix composite and a Ni-based superalloy coated with a similar to 200 pm thick metallic bend coat layer were used as substrate materials. Both the powder and coating contained ZrP2O7 as an im purity phase, and the coating was highly porous as-deposited, The coating deposited on the Nicalon/SiC substrate was chemically stable upon exposure to air and Na2SO4/O-2 atmospheres at 1000 degrees C for 100 h, In contrast, the coating sprayed onto the superalloy substrate significantly reacted with the bond coat surface after similar oxidation in air.