Alumina/Epoxy Interpenetrating Phase Composite Coatings: I, Processing and Microstructural Development

Interpenetrating phase composite (IPC) coatings consisting of continuously connected Al₂O₃ and epoxy phases were fabricated. The ceramic phase was prepared by depositing an aqueous dispersion of Al₂O₃ (0.3 μm) containing orthophosphoric acid, H₃PO₄, (1–9.6 wt%, solid basis) and heating to create phosphate bonds between particles. The resulting ceramic coating was porous, which allowed the infiltration and curing of a second-phase epoxy resin. The effect of dispersion composition and thermal processing conditions on the phosphate bonding and ceramic microstructure was investigated. Reaction between Al₂O₃ and H₃PO₄ generated an aluminum phosphate layer on particle surfaces and between particles; this bonding phase was initially amorphous, but partially crystallized upon heating to 500°C. Flexural modulus measurements verified the formation of bonds between particles. The coating porosity (and hence epoxy content in the final IPC coating) decreased from ∼50% to 30% with increased H₃PO₄ loading. The addition of aluminum chloride, AlCl₃, enhanced bonding at low temperatures but did not change the porosity. Diffuse reflectance FTIR showed that a combination of UV and thermal curing steps was necessary for complete curing of the infiltrated epoxy phase. Al₂O₃/epoxy IPC coatings prepared by this method can range in thickness from 1 to 100 μm and have potential applications in wear resistance.     

Effects of Temperature and Reagent Concentration on the Morphology of Chemically Vapor Deposited β-Ta2O5

Crystalline β-Ta₂O₅ coatings were deposited on hot-isostatically-pressed Si₃N₄ by reacting TaCl₅ with H₂ and CO₂ in the temperature range of 1000°–1300°C and at a pressure of 660 Pa. The Ta₂O₅ coatings generally consisted of wellcoalesced 2–3 μm grains, resulting in the formation of a nonporous coating morphology. However, the presence of microcracks on the as-deposited surface was consistently observed. The surface morphology, texture, and growth rate of the coatings were examined as a function of deposition parameters.     

Thermodynamic and Experimental Study of High-Purity Aluminum Nitride Formation from Aluminum Chloride by Chemical Vapor Deposition

Thermodynamic calculations in the systems Al-Cl, Al-Cl-N, and H-Al-Cl-N were used to assess the capabilities of AlCl₃ or mixtures of AlCl₃ with Al to produce AlN by chemical vapor deposition (CVD) techniques. Direct nitridation (N₂ as reaction agent) is possible only at high temperatures (≥1500 K), using AlCl₃–Al mixtures. Reaction with NH₃ at equilibrium gives low yields but the suppression of NH₃ dissociation yields near 100%, which makes the method suitable for powder production, coating, and single-crystal growth. AlN with less than 1 wt% oxygen was obtained from technical grade AlCl₃ by this process. The formation of both amorphous AlN powder and crystalline AlN coatings was observed. It is assumed that the formation of AlCl₃· x NH₃ adducts by mixing of Al-Cl vapor and NH₃ at temperatures ≤1273 K prevents NH₃ dissociation and favors the production of amorphous AlN.     

Deposition Mechanism for Chemical Vapor Deposition of Zirconium Carbide Coatings

Zirconium carbide (ZrC) coatings were fabricated by chemical vapor deposition (CVD) using ZrCl₄, CH₄/C₃H₆, and H₂ as precursors. Both thermodynamic calculation results and the film compositions at different temperatures indicated that zirconium and carbon deposited separately during the CVD process. The ZrC deposition rates were measured for CH₄ or C₃H₆ as carbon sources at different temperatures based on coating thickness. The activation energies for ZrC deposition demonstrated that the CVD ZrC process is controlled by the carbon deposition. This is also proven by the morphologies of ZrC coatings.     

CVD Mullite Coatings in High-Temperature, High-Pressure Air–H2O

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.     

Preparation of Magnesium Nitride Powder by Low-Pressure Chemical Vapor Deposition

Submcrometer-scale magnesium nitride (Mg₃N₂) powder was prepared by low-pressure chemical vapor deposition, i.e., the reaction of Mg vapor with mixed NH₃–N₂ gases at 800°C under a pressure of ∼1 kPa. The mixing ratios of NH₃–N₂ gases were 0% NH₃–100% N₂ (pure N₂), 20% NH₃–80% N₂, 40% NH₃–60% N₂, 60% NH₃–40% N₂, 80% NH₃–20% N₂, and 100% NH₃–0% N₂ (pure NH₃). The reactions between Mg vapor and NH₃–N₂ gases produced platy Mg₃N₂ particles <0.2 μm and acicular particles with long-axis length of ∼0.2 μm, whereas the reaction of Mg vapor with pure NH₃ gas produced spherical Mg₃N₂ particles with diameters of ∼0.1 μm.     

Process-Structure-Reflectance Correlations for TiB2 Films Prepared by Chemical Vapor Deposition

The process-structure-reflectance interrelationships for TiB₂ films prepared by CVD were determined using statistically designed experiments. A hot wall CVD reactor employing graphite substrates and the TiCl₄+ BCl₃+ H₂ reagent system were used at pressures of 2.7 and 6.7 kPa. Single-phase polycrystalline TiB₂ films were obtained. An increasing percentage of the grains were oriented with their (001) planes parallel to the substrate as the deposition temperature was increased and as the BCl₃:TiCl₄ ratio decreased. Grain size increased from ∼0.5 to 3 µm as the deposition temperature was increased from 900° to 1100°C and as the coating rate was decreased from 0.6 to 0.1 µm/min. Fine-grained, smooth, highly reflective films were obtained at low deposition temperatures and high BCl₃:TiCl₄ ratios.     

Fabrication of Silicon Carbide Whisker/Alumina Composite by Thermal-Gradient Chemical Vapor Infiltration

SiC( w )/Al₂O₃ composites were made from an AlCl₃-H₂-CO₂ mixture by a thermal-gradient chemical vapor infiltration (CVI) method. Al₂O₃ was deposited from the reaction of AlCl₃ and H₂O, which was produced from the oxidation of H₂ by CO₂. The densification rate was measured at various reactant compositions and total pressures. When the reaction rate or total pressure increased, the rate-controling step shifted from H₂O production to AlCl₃ diffusion, which led to premature pore closing. To obtain dense composites in a short infiltration time, the diffusion rate of AlCl₃ had to be increased by decreasing the total pressure.     

Kinetic and Thermodynamic Analyses of Chemical Vapor Deposition of Aluminum Nitride

AIN coatings were prepared by chemical vapor deposition from the AlCl₃—NH₃—Ar reagent system using an impinging jet reactor in the temperature range of 700° to 1100°C. A mass transfer model and thermodynamic calculations were used to analyze the deposition data. The AIN-CVD process could be approximated by calculating mass transfer—thermodynamic limits at low AlCl₃ concentrations. The AIN deposition rate decreased drastically with increasing temperature above 1000°C in agreement with thermodynamic predictions. At high AlCl₃ concentrations, a surface kinetic mechanism involving AlCl₃ adsorbed on the deposition surface appeared to be the rate-limiting step. The AIN deposition rate decreased on increasing the AlCl₃ concentration or total pressure. The crystalline structure of AIN was strongly influenced by the processing parameters. The AIN coatings became highly crystalline and preferentially oriented with an increase in the AlCl₃ concentration or pressure.     

Dy:YAG Phosphor Coating Using the Solution Precursor Plasma Spray Process

Dy:YAG phosphor coatings were deposited using the solution precursor plasma spray process. The phase composition, microstructure, and photoluminescent properties of the as-deposited coatings were investigated. X-ray diffraction analysis confirmed that the coating is mainly composed of the YAG phase with a small amount of an intermediate YAP phase. Scanning electron microscopy micrograph revealed that the as-sprayed coating has columnar structures with a thickness of ∼60 μm. The measurement of photoluminescent properties indicated that the phosphor coating exhibits two predominant emission regions: a blue region (470–500 nm) and a yellow region (560–600 nm), which are assigned to ⁴F_9/2–⁶H_15/2 and ⁴F_9/2–⁶H_13/2 transitions, respectively.     

Dense Alumina–Zirconia Coatings Using the Solution Precursor Plasma Spray Process

For the first time, dense coatings have been made by the solution precursor plasma spray (SPPS) process. The conditions are described for the deposition of dense Al₂O₃–40 wt% 7YSZ (yttria-stabilized zirconia) coatings; the coatings are characterized and their thermal stability is evaluated. X-ray diffraction analysis shows that the as-sprayed coating is composed of α-Al₂O₃ and tetragonal ZrO₂ phases with grain sizes of 72 and 56 nm, respectively. The as-sprayed coating has a 95.6% density and consists of ultrafine splats (1–5 μm) and unmelted spherical particles (<0.5 μm). The lamellar structure, typical of conventional plasma-sprayed coatings, is absent at the same scale in the SPPS coating. The formation of a dense Al₂O₃–40 wt% 7YSZ coating is favored by the lower melting point of the eutectic composition, and resultant superheating of the molten particles. Phase and microstructural thermal stabilities were investigated by heat treatment of the as-sprayed coating at temperatures of 1000°–1500°C. No phase transformation occurs, and the grain size is still in the nanometer range after the 1500°C exposure for 2 h. The coating hardness increases from 11.8 GPa in the as-coated condition to 15.8 GPa following 1500°C exposure due to a decrease in coating porosity.     

Infiltration-Inhibiting Reaction of Gadolinium Zirconate Thermal Barrier Coatings with CMAS Melts

The thermochemical interaction between a Gd₂Zr₂O₇ thermal barrier coating synthesized by electron-beam physical vapor deposition and a model 33CaO–9MgO–13AlO_3/2–45SiO₂ (CMAS) melt with a melting point of ∼1240°C was investigated. A dense, fine-grained, ∼6-μm thick reaction layer formed after 4 h of isothermal exposure to 1300°C. It consisted primarily of an apatite phase based on Gd₈Ca₂(SiO₄)₆O₂ and fluorite ZrO₂ with Gd and Ca in a solid solution. Remarkably, melt infiltration into the intercolumnar gaps was largely suppressed, with penetration rarely exceeding ∼30 μm below the original surface. The microstructural evidence suggests a mechanism in which CMAS infiltration is arrested by rapid filling of the gaps with crystalline reaction products, followed by slow attack of the column tips.     

Oxidation of CVD Si3N4 at 1550° to 1650°C

A thermo gravimetric study of the oxidation behavior of chemically vapor-deposited amorphous and crystalline Si₃N₄ (CVD Si₃N₄) was made in dry oxygen (0.1 MPa) at 1550° to 1650°C. The specimens were prepared under various deposition conditions using a mixture of SiCl₄, NH₃, and H₂ gases. The crystalline CVD Si₃N₄ indicated a parabolic oxidation kinetics over the whole temperature range, whereas the amorphous CVD Si₃N₄ changed from a parabolic to a linear law with increased temperature. The oxidation mechanism is discussed in terms of the activation energy for the oxidation and the microstructure of the formed oxide films.     

Characterization of Si3N4 Coated with Chemically-Vapor-Deposited Mullite after Na2SO4-lnduced Corrosion

Si₃N₄ substrates coated with chemically-vapor-deposited, crystalline mullite (3Al₂O₃.2SiO₂) were subjected to a corrosive environment containing Na₂SO₄ and O₂ at 1000°C for 100 h. The composition and microstructure of the as-deposited and corroded specimens were examined and compared. The coating appeared to be effective in preserving and therefore protecting the surface microstructure of the underlying Si₃N₃ substrates. However, a small degree of Na penetration through mullite grain boundaries was observed to a coating depth of ∼1 μm.     

Effects of Oxygen Partial Pressure on the Nucleation Behavior and Morphology of Chemically-Vapor-Deposited Zirconia on Hi-Nicalon Fiber and Si

A ZrO₂ coating was prepared on Hi-Nicalon fiber and single-crystal Si by chemical vapor deposition (CVD) using ZrCl₄, CO₂, and H₂ as precursors at 1050°C. The effects of oxygen partial pressure on the nucleation behavior of the CVD-ZrO₂ coating were systematically studied by intentionally varying the controlled amount of O₂ into the CVD chamber. Characterization results suggested that the number density of tetragonal ZrO₂ nuclei apparently decreased with increasing the oxygen partial pressure from 4 × 10⁻³ to 1.6 Pa. Also, the coating layer became more columnar and contained larger monoclinic ZrO₂ grains. The observed relationships between the oxygen partial pressure and the nucleation and morphologic characteristics of the ZrO₂ coating were attributed to the grain size and oxygen deficiency effects, which have been previously reported to cause the stabilization of the tetragonal ZrO₂ phase in bulk ZrO₂ specimens.     

Preparation of Ultrafine Silicon Nitride, and Silicon Nitride and Silicon Carbide Mixed Powders in a Hybrid Plasma

Ultrafine Si₃N₄ and Si₃N₄+ SiC mixed powders were synthesized through thermal plasma chemical vapor deposition (CVD) using a hybrid plasma which was characterized by the superposition of a radio-frequency plasma and an arc jet. The reactant, SiCl₄, was injected into an arc jet and completely decomposed in a hybrid plasma, and the second reactant, CH₄ and/or NH₃, was injected into the tail flame through multistage ring slits. In the case of ultrafine Si₃N₄ powder synthesis, reaction effieciency increased significantly by multistage injection compared to single-stage injection. The most striking result is that amorphous Si₃N₄ with a nitrogen content of about 37 wt% and a particle size of 10 to 30 nm could be prepared successfully even at the theoretical NH₃/SiCl₄ molar ratio of ∼ 1.33, although the crystallinity depended on the NH₃/SiCl₄ molar ratio and the injection method. For the preparation of Si₃N₄+ SiC mixed powders, the N/C composition ratio and particle size could be controlled not only by regulating the flow rate of the NH₃ and CH₄ reactant gases and the H₂ quenching gas, but also by adjusting the reaction space. The results of this study provide sufficient evidence to suggest that multistage injection is very effective for regulating the condensation process of fine particles in a plasma tail flame.     

Wet-Chemical Routes Leading to Scandia Nanopowders

Two wet-chemical routes have been used to synthesize Sc₂O₃ nanopowders from nitrate solutions employing ammonia water (AW) and ammonium hydrogen carbonate (AHC) as the precipitants. The precursors and the resultant oxides are characterized by elemental analysis, X-ray diffractometry, differential thermal analysis/thermogravimetry, high-resolution scanning electron microscopy, and Brunauer-Emmett-Teller analysis. Crystalline γ-ScOOH· n H₂O ( n ≈ 0.5) is the only phase obtained by the AW method. This phase dehydrates to Sc₂O₃ at ∼400°C, yielding hard aggregated nanocrystalline Sc₂O₃ powders. Three types of precursors have been synthesized by the AHC method, depending on the AHC/Sc³⁺ molar ratio ( R ): amorphous basic carbonate [Sc(OH)CO₃·H₂O] at R ≤ 3, crystalline double carbonate [(NH₄)Sc(CO₃)₂·H₂O] at R ≥ 4, and a mixture of the two phases at 3 < R < 4. Among these precursors, only the basic carbonate shows spherical particle morphology, ultrafine particle size (∼50 nm), and weak agglomeration. Sc₂O₃ nanopowders (∼28 nm) with high surface area (∼49 m²/g) have been prepared by calcining the basic carbonate at 700°C for 2 h.     

Thermodynamic Calculations for the Chemical Vapor Deposition of Silicon Nitride

The importance of silicon nitride (Si₃N₄) prepared by the chemical vapor deposition (C VD) technique is discussed briefly. A computer code was used to calculate CVD phase diagrams useful for the preparation of this material. Theoretical deposition efficiencies and gaseous phase compositions are also reported. The reactants include NH₃/SiH₄, NH₃/SiH₂Cl₂, NH₃/SiCl₄, and NH₃/SiF₄. The effect of N₂, H₂, and Ar additions, as well as the system pressure, are evaluated. The results are compared with experimental data from the literature.     

Analytical Modeling of Chemical Vapor Infiltration in Fabrication of Ceramic Composites

A model for chemical vapor infiltration is applied to the study of the growth of alumina from the chemical reaction among AlCl₃, H₂, and CO₂ within a SiC-fiber bundle which is situated in an isothermal hot-wall reactor. The pore space between the fibers is simulated by cylindrical capillary tubes. The model considers binary diffusion of CO₂ and H₂, chemical reaction on the inner surface of the tube, and deposition film growth. Furthermore, diffusion-controiled and chemical-reaction-controlled processes are taken into account to determine the dominating process in chemical vapor infiltration. Both molecular diffusion and Knudsen diffusion are considered sequentially in this model during the infiltration process. Based upon this model, the optimum processing conditions required for chemical vapor infiltration to form a SiC/Al₂O₃ composite can be predicted for different fiber preform systems.     

Effects of Melt Chemistry on the Spectral Absorption Coefficient of Molten Aluminum Oxide

Values of the spectral absorption coefficient (α) of liquid aluminum oxide were determined by transmission of a pulsed dye laser beam incident on continuous-wave (CW) CO₂-laser-melted pendant drops attached to sapphire filaments. Measurements were made on molten drops of Verneuil sapphire at wavelengths of 0.450 and 0.633 μm, at ambient oxygen partial pressures from 10^-10 to 1 bar in eight pure gases (Ar, CO, CO₂, H₂, H₂O, HCI, N₂ and O₂), in CO/CO₂ mixtures, and in H₂/H₂O mixtures, and at a temperature of ca. 2400 K. Specimens contaminated with iron, magnesium, silicon, and tungsten were also investigated in an oxygen atmosphere. At a wavelength of 0.633 μm, the value of α was greater than 50 cm^-1 under reducing or inert gas conditions. It decreased to a minimum at intermediate oxygen partial pressures of 5 × 10^-5 bar in CO/CO₂ mixtures and 5 × 10^-3 bar in H₂/H₂O mixtures, and increased at larger oxygen partial pressures. The specimens were opaque (α > 55 cm^-1) in hydrogen, in HCI at pressures above 0.04 bar. Specimens contaminated with 5000-10000 ppm of Fe, Mg, Si, or W were also opaque. At a wavelength of 0.45 μm the liquid aluminum oxide specimens were opaque in Ar and oxygen, and gave α= 46 cm^-1 in CO₂. The dynamic response when the ambient gas was changed from CO₂ to argon showed that the transmission maximum for = 0.45 μm was at p (O₂) < 0.1 bar.