Sub-atmospheric pressure chemical vapor deposition (CVD) processes have been developed to produce theoretically dense, highly pure, void-free and large area bulk materials, SiC, Si, ZnSe, ZnS and ZnSxSe1-x. These materials are used for optical elements, such as mirrors, lenses and windows, over a wide spectral range from the vacuum ultraviolet (VUV) to the infrared (IR).

In this paper we discuss the effect of CVD process conditions on the microstructure and properties of these materials, with emphasis on optical performance. In addition, we discuss the effect of chemical composition on the properties of the composite material ZnSxSe1-x.

We first present a general overview of the bulk CVD process and the relationship between process conditions, such as temperature, pressure, reactant gas concentration and growth rate, and the microstructure, morphology and properties of CVD-grown materials. Then we discuss specific results for CVDgrown SiC, Si, ZnSe, ZnS and ZnSxSe1-x.

Preferred conditions for deposition of thick α-Si3 N4 plate from HSiCl3 -NH3-H2 on the vertical surfaces of a lowpressure, hot-wall CVD reactor were identified by means of a designed experiment. The design included the range of temperatures 1300°C-1500°C, pressures 0.5–2.0 Torr, and residence times 0.01–1.0 sec. The vertical deposition surfaces received a viscous, laminar flow of well mixed, thermally equilibrated reactants. Plates 0.05–0.5 mm thick were produced on multiple vertical substrates 350 cm2 in area at deposition rates 5–70 μm/hr. Plates 0.5–4.0 mm thick were produced on horizontal substrates at deposition rates of 60–120 μm/hr. When NH3 flows in stoichiometric excess, deposition rates on vertical surfaces increase approximately linearly with the flow rate of HSiCl3 but depend little on temperature, as would be expected if the reaction proceeds under mass transport control with product depletion. Multiple correlation analyses show that thickness variations in the deposit are reduced by increasing the temperature and decreasing the gas residence time. CVD silicon nitride plate produced under the optimized conditions exhibits theoretical density and is free of pores and cracks. It exhibits a columnar morphology in which the <222> and <101> crystallographic directions are oriented preferentially normal to a surface, which consists of well-defined trigonal facets 10–50 μm across. Crystallite sizes determined by Xray line broadening range from 0.06–1.0 μm. This CVD plate is gray and contains approximately 0.5 w/o C and 0.5 w/o 0 as principal impurities.

Specific compositions of boron-carbon-silicon ceramics exhibit improved abrasive wear and good thermal shock resistance, but require bulk sintering at temperatures in excess of 2100K. The formation of such phases by chemical vapor deposition was investigated in the temperature range of 1073K–1573K. Methyltrichlorosilane (CH3SiCl3), boron trichloride, and methane were chosen as reactant gases, with hydrogen as a carrier gas and diluent. The coatings were deposited in a computer-controlled, hot-wall reactor at a pressure of 33 MPa (200 Torr).

Below 1473K the coatings were amorphous. At higher temperatures non-equilibrium reactions controlled the deposition process. The most common coating consisted of a silicon carbide matrix and a silicon boride, SiB6, dispersed phase. Multiphase coatings of B+B4C+SiB6 and SiC+SiB6+SiB14 were also deposited by controlling the partial pressure of methane and boron trichloride. Non-equilibrium thermodynamic analysis qualitatively predicted the experimentally deposited multiphase coatings.

Non-reflective or high emissivity optical surfaces require materials with given roughness or surface characteristics wherein interaction with incident radiation results in the absorption and dissipation of a specific spectrum of radiation. Coatings have been used to alter optical properties, however, extreme service environments, such as experienced by satellite systems and other spacecraft, necessitate the use of materials with unique combinations of physical, chemical, and mechanical properties. Thus, ceramics such as boron carbide are leading candidates for these applications. Boron carbide was examined as a coating for optical baffle surfaces. Boron carbide coatings were deposited on graphite substrates from BCl3, CH4, and H2 gases employing chemical vapor deposition (CVD) techniques. Parameters including temperature, reactant gas compositions and flows, and pressure were explored. The structures of the coatings were characterized using electron microscopy and compositions were determined using x-ray diffraction. The optical properties of the boron carbide coatings were measured, and relationships between processing conditions, deposit morphology, and optical properties were determined.

The reaction of (100)Si with C2H2 in a hot wall CVD reactor has been studied using a X-ray photolectron spectroscopy, and a scanning electron microscopy. The growth of the SiC films was observed through the behavior of Si2p peaks and their plasmons. Smooth surface morphology with a monolayer of SiC was obtained at 950°C for 7 minutes and defects were observed for longer reaction times at this temperature. For higher reaction temperatures (e.g. 1000°C), defects were observed for reaction times as short as 10 seconds. The formation of defects was correlated to the out-diffusion of Si in the carborization process.

The effect of annealing the SCS-6* SiC fiber for one hour at 2000°C in an argon atmosphere is reported. The SiC grains in the fiber coarsen appreciably and the intergranular carbon films segregate to the grain junctions. It would appear that grain growth in the outer part of the fiber is primarily responsible for the loss in fiber strength and improvement in fiber creep resistance.

Plasma-assisted chemical vapor deposition (PACVD) is used extensively to coat planar (2-dimensional) substrates. In principle, the technique can be used to deposit coatings on 3-dinensional objects. However, extending PACVD to coat 3-dimensional objects uniformly requires careful control of the plasma, substrate temperature, and reactant concentrations over a large volume. A novel low-temperature radio frequency PACVD reactor design was developed to deposit coatings uniformly and reproducibly on 3-dimensional metallic substrates. The design features a temperature-controlled reaction chamber fitted with one or more rf-driven electrodes to generate uniform, large-volume plasma. The reactor was used to develop a series of silicon carbide coatings, which were deposited at or below 500°C. The coatings contain SiC and varying amounts of free silicon and/or amorphous carbon (diamond-like carbon), depending on reagent gas composition and reactor operating parameters. The coatings significantly reduced wear on stainless steel samples in ball-on-disk and abrasive wear tests and provided oxidation protection to molybdenum and titanium alloy.

Thin film properties of LPCVD TiN barriers deposited on Si(100), using TiCl4 and NH3 as reactants, were investigated as a function of deposition temperature between 400 °C and 700 °C. The TiN film chemistry and film composition were studied by AES and RBS techniques, while the microstructural properties (grain size, lattice parameter and texture) were evaluated by XRD. The TiN deposition rates and film resistivities were also determined. Finally the film properties of the TiN barriers as determined by surface analysis were related to the process parameters.

The technology of chemical vapor infiltration (CVI) has progressed dramatically over the past twenty-five years and stands now as the leading process for fabrication of high temperature structures using ceramic matrix composites. Modeling techniques also have advanced from extensions of catalyst theory to full 3-D finite element code with provision for temperature and pressure gradients. These modeling efforts offer insight into critical factors in the CVI process, suggest opportunities for further advances in process technology and provide a tool for integrating the design and manufacture of advanced components.

Early modeling identified the competition between reaction and diffusion in the CVI process and the resulting trade-off between densification rate and uniformity. Modeling of forced flow/thermal gradient CVI showed how the evolution of material transport properties provides a self-optimizing feature to this process variation.

“What-if” exercises with CVI models point toward potential improvements from tailoring of the precursor chemistry and development of special preform architectures.

As a link between component design and manufacture, CVI modeling can accelerate successful application of ceramic composites to advanced aerospace and energy components.

Following the evolving microstructure of composites through all stages of chemical vapor infiltration (CVI) is a key to improved understanding and control of the process. X-ray Tomographic Microscopy (XTM), i.e., very high resolution computed tomography, allows the microstructure of macroscopic volumes of a composite to be imaged nondestructively with resolution approaching one micrometer. Results obtained with XTM on dense SiC/SiC composites and on woven SiC fiber preforms illustrate how details of the densification process can be followed using this technique during interruptions in processing. Ways in which the three-dimensional microstructural information may be used to improve modeling are also indicated.

We prescnt effective diffusion coefficients of gases in porous media whose structure can be represented as an assemblage of cylindrical fibers, such as the media used as substrates in chemical vapor infiltration. Structures consisting of non-, partially, or freely overlapping fibers of various orientation distributions are considered, and effective diffusion coefficients are computed by means of a Monte Carlo simulation scheme. In order to be able to examine the interrelation of ordinary, transition, and Knudsen diffusivities and tortuosities, computations are carried out over the whole diffusion regime, i.e., from bulk to Knudsen. Our simulation results are compared with variational bounds and experimcntal values of tortuosity of fibrous beds reported by other investigators.

A model, incorporating both gas-phase and surface reactions, for simulating thickness profile of SiC, deposited from trichloromethylsilane (TMS), along the longitudinal direction of a single pore is presented in this paper. The transport mechanisms considered include both forced-flow and diffusion. With the nonlinear nature of this model, a finite element model was developed to solve the problem numerically. Simulation results were in good agreement with the reported experimental data by Fedou et al. (1990). Effects of critical parameters, such as deposition temperature, ratio of sticking coefficients of TMS and intermediate species, and forced-flow, on the deposition thickness profile were investigated. Forced-flow effect was found to be small for the chemical vapor infiltration (CVI) processes at high deposition temperatures.

A forced-flow thermal-gradient chemical vapor infiltration process has been developed to fabricate composites of thick-walled tubular geometry common to many components. Fibrous preforms of different fiber architectures (3-dimensionally braided and filament wound) have been investigated to accommodate components with different mechanical property requirements. This paper will discuss the fabrication of tubular, fiber-reinforced SiC matrix composites and their mechanical properties.

Boron nitride has been identified as a good fiber/matrix interface coating for SiC fibers in a Si3N4 matrix. Boron nitride thin films prepared from BCl3 and NH3 at 1000°C and from B2 H6 and NH3 at 550°C were deposited on SiC (Nicalon®) cloth in a low pressure chemical vapor deposition reactor. The coating thickness was varied from 0.1 μm up to 4 μm. Six layers of cloth for each coating thickness were incorporated into a chemical vapor infiltrated matrix of amorphous Si3N4.

Flexural strength data and composite toughness were obtained from three-point bend tests. Scanning electron microscopy and metallographic analysis of fracture surface morphology were used to determine the failure mode.

A microwave assisted process for production of continuous fiber reinforced ceramic matrix composites is described. A simple apparatus combining a chemical vapor infiltration reactor with a conventional 700 W multimode oven is described. Microwave induced inverted thermal gradients are exploited with the ultimate goal of reducing processing times on complex shapes. Thermal gradients in stacks of SiC (Nicalon) cloths have been measured using optical thermometry. Initial results on the “inside out” deposition of SiC via decomposition of methyltrichlorosilane in hydrogen are presented. Several key processing issues are identified and discussed.

Structural fibers are currently being considered as reinforcements for intermetallic and ceramic materials. Some of these fibers, however, are easily degraded in a high temperature oxidative environment. Therefore, coatings are needed to protect the fibers from environmental attack.

Silicon carbide (SiC) was chemically vapor deposited (CVD) on Textron's SCS6 fibers. Fiber temperatures ranging from 1350 to 1500 °C were studied. Silane (SiH4) and propane (C3H8.) were used for the source gases and different concentrations of these source gases were studied. Deposition rates were determined for each group of fibers at different temperatures. Less variation in deposition rates were observed for the dilute source gas experiments than the concentrated source gas experiments. A careful analysis was performed on the stoichiometry of the CVD SiC-coating using electron microprobe. Microstructures for the different conditions were compared. At 1350°C, the microstructures were similar;however, at higher temperatures, the microstructure for the more concentrated source gas group were porous and columnar in comparison to the cross sections taken from the same area for the dilute source gas group.

The TiB2, TiC and TiN coatings were studied as potential protective coatings for SiC fibre reinforced titanium alloys. These coatings were deposited by a CVD technique on resistively heated SiC monofilament fibres at reduced pressure in a cold wall reactor. The deposition conditions that are required to coat each of these materials were determined. The morphology, microstructure, phase composition and nature of the coatings were examined. The comparison and evaluation of the effectiveness and potential of these as protective coatings were carried out by incorporating the coated fibre into Ti-6A1–24V using the diffusion bonding method at 1100°C under 10 MPa for 1 hour.

In many composite systems it has become apparent that coatings on the reinforcements are necessary to achieve high toughness materials. In order to examine materials which may be used as coatings on ceramic monofilaments and remain stable in high temperature, oxidizing environments, the deposition of a number of refractory metals has been attempted. The results of coating experiments using silicon carbide fibers as substrates as well as general observations concerning the prospects of continuously coating long lengths of fibers will be discussed. The materials studied include carbon, cobalt, zirconium, molybdenum, tantalum, tungsten, and iridium. Carbon has been deposited from methane and propylene onto both SiC and sapphire fibers. Deposition of the metals has been achieved by direct chlorination of the metals followed by hydrogen reduction at the fiber. Iridium(III)2,4-pentanedionate has been used to deposit iridium metal. All metals were deposited at low pressure in a hot wall reactor with fibers continuously spooled through the reactor.

The development of new and improved ceramic fibers has spurred the development and application of ceramic composites with improved strength, strength/weight ratio, toughness, and durability at increasingly high temperatures. For many systems, the ceramic fibers can be used without modification because their properties are adequate for the chosen application. However, in order to take maximum advantage of the fiber properties, it is often necessary to coat the ceramic fibers with materials of different composition and properties. Examples include (1) boron nitride coatings on a ceramic fiber, such as Nicalon silicon carbide, to prevent reaction with the ceramic matrix during fabrication and to enhance fiber pullout and increase toughness when the ceramic composite is subjected to stress[l]; (2) boron nitride coatings on ceramic yarns, such as Nicalon for use as thermal insulation panels in an aerodynamic environment, to reduce abrasion of the Nicalon and to inhibit the oxidation of free carbon contained within the Nicalon[2]; and (3) ceramic coatings on carbon yarns and carbon-carbon composites to permit use of these high-strength, high-temperature materials in oxidizing environments at very high temperatures[3,4].

The stability of BN thin film coatings (2–5 nm thick) on MgO and TiO2 substrates was investigated using transmission electron microscopy (TEM). The samples were heated in air for at least 16 hours at temperatures ranging from 773 K - 1273 K. On MgO supports, the BN thin film coating was lost by 1073 K due to a solid state reaction with the substrate leading to formation of Mg2B2O5. No such reaction occurred with the TiO2 substrate and the BN was stable even at 1273 K. However, the coating appeared to ball up and phase segregate into islands of near-graphitic BN and clumps of TiO2 (rutile). The oxidizing treatment appears to promote the transformation from turbostratic BN to graphitic BN.