Chemical vapor deposition has been utilized to produce ternary, multiphase coatings of various compositions of silicon carbide (SiC) with Ti, Cr, and Mo. Thermodynamic calculations have been performed for a variety of experimental conditions in each system. Scanning, transmission and analytical electron microscopy, and X-ray diffraction techniques have been used to characterize the microstructures and to determine compositions.

Monolithic SiB4±x and SIB6 plates about 1 nm in thickness were prepared by CVD using SiCl4, B2H6 and H2 gases as source materials. The CVD-SiB4±x, plates have nonstoichiometric compositions between B/Si=3.1 and 5.0. The lattice parameters of the CVD-SiB4±x, plates are a=0.633 nm and c=1.262 to 1.271 nm. The density of the CVD-SiB4±x ranges from 2.39 to 2.45 g/cm3. The CVD-SiB6 plates have a constant composition of B/Si=6.0. The lattice parameters of the CVD-SiB6plates are a=1.444 nm, b=1.828 nm, c=0.9915 nm, and their density Is 2.42 g/cm3. These measured densities are both almost in agreement with theoretical values. The electrical conductivity, Seebeck coefficient, Hall mobility and thermal conductivity of the CVD-SiB4±x and CVD-SiB6 plates were examined in the temperature range from 300 to 1000K. The conduction mechanism Is discussed and their figure of merit values for thermoelectric materials are evaluated.

Tungsten disilicide (WSi2 ) was formed by annealing 185 nm and 750 nm thick LPCVD-W films deposited on <100>-Si substrates. The thickness of the formed WSi2 was observed by Rutherford backscattering measurements (RBS) to increase parabolically with the annealing time. This agrees with the behavior reported in the literature for sputter deposited or evaporated W films. However, a higher stlicide growth rate was found in this work. An activation energy of 2.6 eV/atom was measured, which is smaller than those for sputter deposited or evaporated W films. The crystal structures of the formed WSi2 and the unreacted W films were analyzed using X-ray diffraction (XRD) technique. The thermal history of the samples was found to play an important role for the crystal structure of the unreacted W and formed WSi2. The as-deposited W films became more oriented to the <100> direction with increasing film thickness, while the unreacted W films on top of the formed WSi2, from the samples annealed at temperatures from 700 to 800°C, became more <100> direction dominated with decreasing W film thickness. The preferred orientation of the grown WSi2 films also varied with the silicidation conditions. These observations indicate that the simple method of determining W film thickness by comparing the W diffraction peak heights of the remaining W films after heat treatment to a reference sample will unavoidably introduce large errors. A compensation technique was suggested where both the W and WSi2 diffraction signals were used. A parameter x, a function of reacted W thickness, was introduced to fit the compensation procedure. The conclusion of this study is that attention should be paid to the orientation change of W and WSi2 films during heat treatment if XRD is employed to study the kinetics of WSi2 formation.

The effect of deposition temperature and SiH4/WF6 flow ratio on the resistivity and impurity content of SiH4/WF6 tungsten films has been investigated. It is found that low deposition temperatures and high SiH4/WF6 ratios result in films with a high resistivity up to 150 μΩcm, whereas films deposited at high temperatures have a resistivity of 9 μΩcm. From Electron Probe Micro Analysis and Auger Electron Spectroscopy we conclude that the resistivity increase is due to silicon which is incorporated in the tungsten film during deposition. This results in a resistivity increase of 20 to 40 μΩcm per at% Si.

Thin films of tungsten grown in a CVD reactor by the reduction of hydrogen and silane consisted of a two phase microstructures; a matrix phase of bcc tungsten, and a second phase of W20O58. The second phase is uniformly distributed in the film and does not afeoct 5athe electrical resistivity of hydrogenreduced films (8 μohm-cm). However, dissolved oxygen in the silane-reduced film contributes to the observed higher electrical resistivity (13 μohm-cm) along with smaller grain size. The larger amount of oxide in the hydrogen-reduced film correlates with its slower growth rate when compared to the silane-reduced film which contained a smaller amount of oxide phase.

A novel replication process for silicon carbide surfaces has been developed. A polished polycrystalline chemical vapor deposited (CVD) β-SiC surface was reproduced from a SiC substrate (mandrel) by a two-step pretreatment process followed by CVD of SiC. In this paper we describe the process and present characterization data for both the substrate and the replicated surfaces. Based on the characterization results, we have developed a model which describes the chemical reactions that occur during the replication process.

The electrical resistivity of LPCVD TiN films deposited at temperatures between 450 and 700°C has been found to vary from about 900 to 150 μ Ωcm. The elemental composition (Ti:N), impurity content and the microstructure of the films have been investigated in order to explain the resistivity variation. It appears that this variation is directly related to the chlorine (impurity) content of the films. Films deposited at 700°C have both the lowest chlorine content and the lowest value of resistivity. Additionally the difference between the resistivity of this latter film and the (lower) value for bulk TiN is discussed.

The adhesion as determined by indentation of diamond films deposited by microwave plasma-assisted CVD on various substrates was investigated. The critical loads for crack initiation and the propagation of the lateral crack under different loads were measured. The effects of the coating conditions and surface pretreatment of the substrate on the adhesion were examined.

An increasing problem faced by the aerospace industry is the erosive wear of engine components. Various PVD protection coatings have been used previously to combat this wear problem, but with limited success. This paper presents results obtained with a low-temperature CVD, erosion resistant, multilayer tungsten carbide coating system. The coating can be deposited at temperatures below 500°C on ferrous alloys used in the aerospace industry. The low temperature of deposition eliminates the problem of degradation of mechanical properties that occurs with normal high-temperature chemical vapor deposition processes. High-cycle fatigue strength and other mechanical property data are presented for coated stainless steel, as well as results on erosion and impact resistance at 90° impingement.

Amorphous SiO2 films were deposited by the low pressure pyrolysis of organometallic precursors at temperatures of 350 to 500°C. Films were deposited on both single crystal silicon and fused quartz substrates. Intrinsic stresses in the films were measured by a curvature method as a function of deposition conditions and thermal processing conditions. As–deposited films showed significant tensile stress of the order of 400 MPa. The intrinsic stresses in the films increased as the deposition temperature decreased and deposition pressure increased. Small concentrations of dopants like B2O3 and P2O5 significantly decreased the intrinsic stress of as–deposited films. Stress reduction was also achieved by post deposition annealing. SiO2 film stress is correlated to the Si-O-Si bond angle distribution in the structure of the resultant film.

The microstructure and mechanical properties (adhesion, hardness) of beta-SiC coatings, prepared in a cold wall reactor, were studied by examining different substrates and deposition process conditions. Hard (3500 HK), fine grained beta-SiC coatings were deposited onto graphite at relatively high deposition terperatures (1473–1673 K) using SiC14,C3H8,H2 gas mixtures. The thermal decomposition of methyltrichlorosilane (MTS) in hydrogen and argon allowed the production of beta-SiC coatings onto a variety of substrate materials in the temperature range 1173–1373 K. The growth mechanisms of SiC deposited from MTS/H2/Ar system onto graphite and polycrystalline alfa-SiC substrates were studied. At low (40 kPa) and atmospheric pressure, the growth kinetics is limited by surface reactions. Also, a strong dependence of the microstructure and crystallographic orientation on the deposition parameters was observed. Hard metals (WC/Co) were succesfully coated with beta-SiC using the MTS/H2/Ar system. Problems of chemical compatibility between the substrate and the beta-SiC coating were minimized by deposition of an intermediate layer of TiN.

As part of an ongoing program to develop high performance silicon carbide coatings for graphite, a research study into the effects of deposition conditions on residual stresses was initiated. A series of experiments was conceived and implemented to explore the microstructure and the elastic properties of the coatings and substrates as well as to directly measure the residual stress states. The results of these experiments are analyzed with respect to the effects of deposition temperature on the microstructure and properties of CVD SiC on graphite and how the observed differences affect the residual stresses in the coatings.

SICABO fiber, which is one of the silicon carbide coated fibers, has been found to be more stable than BORSIC fiber in titanium matrix composites because SICABO fiber has excess carbon in the SiC layer which was determined by an AES depth profile study. Tensile strength data also show that the SICABO fiber has a higher tensile strength after heating in titanium during hot pressing and chemical vapor deposition processing. Ti5Si3 and TiSi phases were identified from XRD studies on titanium coated BORSIC, but there was no TiSi phase identified in the titanium coated SICABO fiber. AES depth profile results have shown that the BORSIC-Ti has a broad interdiffusion zone.

Oxidation resistant coatings of iridium on graphite, carbon/ carbon or molybdenum and of aluminum oxynitride on graphite and carbon/carbon have been produced by low pressure CVD from halide starting compounds (IrF6 and AlCl3). The resulting deposits are adherent, non porous and have proved to be impervious to oxygen in subsequent heating to 2000° C in air for more than 30 minutes (no evidence of deterioration).

These coatings materials are promising for effective protection against oxidation in rocket engines, gas turbines and thermal protection tiles space shuttles or against corrosion in the glass industry (iridium).

Silicon carbide whiskers have excellent mechanical and chemical properties, making them very desirable as a reinforcement for structural ceramic and other composite materials. Los Alamos has developed a laboratoryscale batch process for producing very high quality SiC whiskers by the vapor-liquid-solid (VLS) method which is an active candidate for technology transfer involving significant scale-up. The process, however, involves quite complex chemical/physical and parametric relationships and has not as yet lent itself successfully to modeling. An expert computer system was therefore developed to facilitate the transfer of this technology to industry. Optimum conditions were determined by relating the many process parameters to product results to establish a set of rules for running the process. These are incorporated in a two-phase expert system designed to guide inexperienced users. In Phase I, an expert consultant program provides the user with information that enables him to set up the run. This information is incorporated into the rule base that makes up Phase II- the control system. At present, the operator functions as the controller by responding to the decisions of the expert system; automation can be added later.

The early growth of titanium carbide whiskers from mixtures of TiCl4-CH4-H2 was investigated. Fine nickel particles were the catalytic agent in a hot-wall CVD reactor. It was confirmed that whisker growth is initiated by the vapor-liquid-solid mechanism at the growth temperature of 1423 K. The initial growth proceeds by the diffusion of the constituent species through a Ni-Ti liquid droplet and is characterized by a rapid increase in whisker length, the diameter remaining essentially constant. In the later stage, the whiskers grow primarily in the axial direction at a reduced rate with a gradual increase in the diameter. The whisker diameter (D) and its rate of increase are independent of the nickel particle sizes, but increased proportionately to the reactant supersaturations. The whisker length (L) also increased with increasing nickel particle size and reactant supersaturation, resulting in large aspect ratios (L/D = 12.5). The whiskers exhibited extensive branching and kinking and a <100> primary growth direction.

A process for the preparation of fiber-reinforced SiC composites by chemical vapor deposition has been developed at Oak Ridge National Laboratory. Composites are prepared by infiltrating fibrous preforms with reactant gases that decompose at elevated temperatures to deposit silicon carbide between and around the fibers. Because the infiltration process utilizes both temperature and pressure gradients, SiC is deposited under conditions that vary considerably from the hot face to the cool face of the composite. Matrix characterization of composite samples by transmission electron microscopy and Raman spectroscopy are described.

The potential of Chemical Vapor Deposition, as a process for applying erosion and corrosion resistant ceramic and metal coatings to ferrous and non-ferrous alloys has long been recognized. Unfortunately, the high deposition temperatures (900°-1000°C) cause deleterious and generally nonreversible metallurgical changes in the coated parts. Corresponding shape stability and dimensional tolerances are also lost. For more than twelve years, Plasma Enhanced Chemical Vapor Deposition (PECVD) technology has evolved from the laboratory into production operations within the semiconductor industry.

This acceptance is primarily due to the low PECVD deposition temperatures (<450 ° C). The PECVD processes have recently demonstrated an adaptability to coating alloy components and carbon/ceramic fibers with a wide range of carbide, boride, nitride, oxide and metal coatings to render them corrosion and erosion resistant. Typical Knoop hardness control of sili on carbide coatings can be controlled from about 2300 to more than 7000 kgfmn−2 at 10% penetration of the indenter while coating thickness and uniformity can be controlled from 300A ± 5%. Selected coating applications and performance are described.

We have investigated the pyrolytic deposition of TiN thin films on molybdenum and 304 stainless steel substrates by continuous wave (CW) and pulsed CO2 (λ=10.6 μm) laser radiation. A flow rate of NH3: TiCl4 of 5:1 was maintained during the deposition process. The substrate temperature was controlled by varying the intensity of the incident laser irradiation. The TiN films were characterized by Transmission Electron Microscopy (TEM), Auger Electron Spectroscopy (AES) and Rutherford Backscattering Spectrometry (RBS). TEM results showed that the films were polycrystalline of equiaxed nature with diffraction patterns containing the characteristic TiN rings. The films had a lattice parameter of 4.24Å. AES results showed a slight incorporation of oxygen.

An ArF excimer laser (193 nm) aligned parallel to the substrate has been used to initiate photo-activation of the W-CVD process. W was deposited on silicon and SIMOX at a 310 °C substrate temperature using WF6 as a W precursor, together with H2. The samples were analyzed after deposition with SEM and 2 MeV He+ RBS, to obtain information about the reaction between WF6 and Si in laser-induced CVD of W. The data indicate that the reaction between WF6 and Si is suppressed upon laser irradiation. The suppression increases with increasing laser-pulse frequency. An explanation for this phenomenon is given in terms of atomic Hformation after (partial) photodissociation of WF6.