Oxidation Protection Coatings for C/SiC based on Yttrium Silicate

The factor which currently precludes the use of carbon fibre reinforced silicon carbide (C/SiC) in high temperature structural applications such as gas turbine engines is the oxidation of carbon fibres at temperatures greater than 400°C. It is, therefore, necessary to develop coatings capable of protecting C/SiC components from oxidation for extended periods at 1600°C. Conventional coatings consist of multilayers of different materials designed to seal cracks by forming glassy phases on exposure to oxygen. The objective of this work was to develop a coating which was inherently crack resistant and would, therefore, not require expensive sealing layers. Yttrium silicate has been shown to possess the required properties for use in oxidation protection coatings. These requirements can be summarised as being low Young’s modulus, low thermal expansion coefficient, good erosion resistance, and low oxygen permeability. The development of protective coatings based on a SiC bonding layer combined with an outer yttrium silicate erosion resistant layer and oxygen barrier is described. Thermodynamic computer calculations and finite element analysis have been used to design the coating. C/SiC samples have been coated using a combination of chemical vapour deposition and slip casting. The behaviour against oxidation of the coating has been evaluated.     

Mullite Coatings on SiC and C/C–Si–SiC Substrates Characterized by Infrared Spectroscopy

Mullite (3Al₂O₃·2SiO₂) coatings on SiC substrates and SiC precoated carbon/carbon composite (C/C-Si-SiC) substrates were produced by pulsed laser deposition (PLD) using pressed mullite powder targets. The layers can be characterized efficiently by IR reflection spectroscopy in the spectral range between 650 and 5000 cm⁻¹. The deposited coatings turn into mullite upon oxidation in air at temperatures between 1400° and 1600°C. Fabry-Perot interferences indicate a high quality and homogeneity of the mullite coating/SiC substrate interface. Amorphous SiO₂ gradually forms during prolonged heating or at higher temperatures.     

Development of C/C–SiC Brake Pads for High-Performance Elevators

Novel C/C–SiC materials have been developed for their use as brake pads for high-speed elevators. Under dynamic and stationary conditions, these materials exhibit high thermal shock resistance, high coefficients of friction, and extremely low wear rates. In addition, it has been found that the SiC content of the C/C–SiC materials on the friction surface heavily influences the frictional behavior. Low-cost materials based on short fiber reinforcement and on drastically reduced process times showed their high potential for the manufacture of cost-efficient brake pads.     

Chemical Vapor Deposition of Epitaxial Films of Yttrium Iron Garnet and Gallium-Substituted Yttrium Iron Garnet and a Thermodynamic Analysis

Epitaxial single-crystal films of yttrium iron garnet (YIG) and Ga: YIG (Y₃Fe_{5- x} Ga _x O₁₂) were grown by chemical vapor deposition. The garnet is deposited by the reaction of gaseous metal chlorides with O₂ at 1150°C and 5 torr. The chlorides are generated in-line by reacting Cl₂ gas with a Y-Fe(-Ga) alloy. A computer-aided thermodynamic calculation was developed to determine which of several possible solids in a multicomponent CVD system will be deposited for a given set of growth conditions. The calculation determines an intermediate equilibrium among all gaseous species and then identifies the solid phase most favored to be deposited from the supersaturated gas phase. The results of the calculations agreed well with experiment and have been useful in finding the conditions for optimum growth of garnet. This type of calculation can be applied to other multicomponent CVD systems.     

Braking Behavior of C/SiC Composites Prepared by Chemical Vapor Infiltration

Carbon fiber-reinforced silicon carbide matrix composites have the potential to overcome the shortcoming of the currently used carbon/carbon friction materials in aircraft brakes. In this article, the carbon/silicon carbide (C/SiC) composites were prepared by chemical vapor infiltration method, and the brake disks with different densities and component content were finally obtained. The friction coefficient and friction stability can be significantly improved by increasing both material density and carbon content. When the density of C/SiC composite is 2.3 g/cm³, the coefficient of friction measured is 0.23, the coefficient of friction stability remains about 0.43, the liner wear rate is less than 9.3 μm/cycle, and the weight wear rate is less than 9.1 μm/cycle. The rapid increase of friction coefficient approaching the end of braking is mainly related to the increasing of surface temperature in a short time and the enhanced adhesion and abrasion of contact conjunctions and asperities. The C/SiC composites exhibited a good stability of braking against fading versus the braking number and surface temperature. The surfaces of C/SiC brake disks were covered with wear debris including the fragment of carbon fibers after the braking tests. The wear on the surfaces is significantly determined by cyclic mechanical and thermal stresses, which result in the micro-cracks in the SiC matrix, the thin flakes of the surface materials as well as the grooves.     

Fabrication and Microstructure of C/SiC Composites Using a Novel Heaterless Chemical Vapor Infiltration Technique

Fabrication of C/SiC composites by using the heaterless chemical vapor infiltration (HCVI) technique, which is an improved technology based on the conventional chemical vapor infiltration, is reported for the first time in this paper. In the HCVI process, a gradient temperature field formed in the fiber preform overcomes the problems of slow diffusion and restricted permeability of gaseous reactant species to some extent, and the electro-deposition is necessary to accelerate the SiC deposition rates. The highest linear deposition rates of SiC matrix within inter-fiber pores are 0.33 μm/h. Microstructures of the C/SiC composites are uniform, and the inter-fiber and inter-ply pores can be well infiltrated. The longitudinal and circumferential microcracks are found in the composites.     

Numerical Simulation of Effect of Methyltrichlorosilane Flux on Isothermal Chemical Vapor Infiltration Process of C/SiC Composites

A two-dimensional axisymmetrical mathematical model for the isothermal chemical vapor infiltration process of C/SiC composites was developed. Transport phenomena of momentum, energy, and mass in conjunction with infiltration-induced changes of preform structure were taken into account. The integrated model was implemented by the finite-element method to simulate numerically the isothermal chemical vapor infiltration (ICVI) process of C/SiC composites at different methyltrichlorosilane (MTS) fluxes. The influence of MTS flux on concentration distribution and time-dependent densification behaviors of C/SiC composites was studied in detail. Calculation results imply that MTS flux has an obvious influence on infiltration in micro-pores and little influence on infiltration in macro-pores. Increasing flux will lead to an evident acceleration for infiltration in micro-pores. Moderate flux is preferable by a combination of both a relatively high infiltration rate and a relatively low fabrication cost. This model is helpful to understand the fundamentals of the ICVI process for the fabrication of C/SiC composites.     

Microstructure and Mechanical Properties of Three-Dimensional Textile Hi-Nicalon SiC/SiC Composites by Chemical Vapor Infiltration

Three-dimensional textile Hi-Nicalon SiC-fiber-reinforced SiC composites were fabricated using chemical vapor infiltration. The microstructure and mechanical properties of the composite materials were investigated under bending, shear, and impact loading. The density of the composites was 2.5 g·cm⁻³ after the three-dimensional SiC perform was infiltrated for 30 h. The values of flexural strength were 860 MPa at room temperature and 1010 MPa at 1300°C under vacuum. Above the infiltration temperature, the failure behavior of the composites became brittle because of the strong interfacial bonding and the mismatch of thermal expansion coefficients between fiber and matrix. The fracture toughness was 30.2 MPa·m^1/2. The obtained value of shear strength was 67.5 MPa. The composites exhibited excellent impact resistance, and the dynamic fracture toughness of 36.0 kJ·m⁻² was measured using Charpy impact tests.     

Through Thickness Mechanical Properties of Chemical Vapor Infiltration and Nano‐Infiltration and Transient Eutectic‐Phase Processed SiC/SiC Composites

The through thickness (interlaminar) shear strength and trans‐thickness tensile strength of three different nuclear‐grade SiC/SiC composites were evaluated at room temperature by the double‐notched shear and diametral compression tests, respectively. With increasing densification of the interlaminar matrix region, a transition in failure locations from interlayer to intrafiber bundle was observed, along with significant increases in the value of the interlaminar shear strength. Under trans‐thickness tensile loading, cracks were found to propagate easily in the unidirectional composite. The 2D woven composite had a higher trans‐thickness tensile strength (38 MPa) because the failure mode involved debonding, fiber pull‐out and fiber failure.     

Thermodynamic Analysis of the Chemical Vapor Deposition of Composite 〈Si3N4〉-〈BN〉 Coatings

A thermodynamic analysis of the chemical vapor deposition of the composite ceramic (Si₃N₄(BN) ⋆ was performed using a general computer program for the calculation of heterogeneous equilibria. Reactants assumed were (H₂SiCl₂), (BCI₃), and (NH₃). The equilibrium conditions for the deposition of condensed phases in the B-Si-N system wire determined as a function of the variables temperature, total system pressure, and gram-atomic reactant gas fractions B/(Si + B) and N/(CI + N). The phase assemblage (Si₃N₄)-〈BN〉 was found to be stable over a large region. Predominant gaseous species at equilibrium were (HCI), (N₂), (H₂), and the silicon chlorides. Deposition efficiencies at equilibrium for (Si₃N₄) and (BN) were high, particularly in the presence of excess (NH₃) and at temperatures below 1600 K.     

Microwave Heated Chemical Vapor Infiltration: Densification Mechanism of SiCf/SiC Composites

Silicon carbide fiber-reinforced silicon carbide matrix composites (SiC_f/SiC) have been produced using microwave heated chemical vapor infiltration. Preferential densification of the composite from the inside out was clearly observed. Although an average relative density of only 55% was achieved in 24 h, representative of an ∼26% increase over the initial fiber vol%, the center of the preform densified to 73% of the theoretical. The densification mechanisms were investigated using X-ray absorptiometry and scanning electron microscopy. The initial inverse temperature profile obtained, which was found to result in the efficient filling of the intratow porosity, although not the intertow porosity, flattened out after approximately 6 h as the densification front moved outward toward the edges. Although not investigated directly, the evidence suggested that this was caused by changes in both the thermal conductivity and microwave absorption characteristics as the samples densified.     

Combined Spectroscopic and Thermodynamic Investigation of Nextel-720 Fiber/Alumina Ceramic-Matrix Composite in Air and Water Vapor at 1100°C

The high-temperature performance of a Nextel-720 fiber/Al₂O₃ composite in air and water vapor at 1100°C was investigated using X-ray photoelectron spectroscopy (XPS) and thermodynamic calculations. In the presence of water vapor, the formation of volatile Si(OH)₄ was responsible for the loss of mullite phase in the fiber of the ceramic-matrix composite (CMC). XPS analysis revealed the formation of surface aluminosilicates for the water-vapor-exposed CMC for 1000 h.     

Deposition Rate,Texture, and Mechanical Properties of SiC Coatings Produced by Chemical Vapor Deposition at Different Temperatures

Silicon carbide (SiC) coatings were produced on carbon/carbon (C/C) composites substrates using chemical vapor deposition (CVD) at different temperatures (1100°C, 1200°C, and 1300°C). The deposition rate was found to increase with deposition temperature from 1100°C to 1200°C. From 1200°C to 1300°C, the deposition rate decreased. SiC coating produced at 1200°C exhibited a strong (111) texture compared with the coatings produced at other temperatures. Both hardness and Young's modulus were also found to be higher in the coating produced at 1200°C. The variation in mechanical properties with the increase in temperature from 1100°C to 1300°C showed a direct correlation with the change in deposition rate and (111) texture. Microstructure analysis shows that the change in CVD temperature leads to the change in grain size, crystallinity, and density of stacking faults of SiC coatings, which appears to have no significant effect on mechanical properties of SiC compared with the texture observed in SiC coating. For the coating deposited at 1200°C, both the hardness and Young's modulus increased gradually from the substrate/coating interface to the top surface. The nonuniformity of mechanical properties along the cross‐section of the coating is attributed to the nonuniform microstructure.     

Synthesis and Growth Mechanism of SiC/SiO2 Nanochains Heterostructure by Catalyst‐Free Chemical Vapor Deposition

The SiC/SiO₂ nanochains heterostructure with double amorphous layers was successfully synthesized by a catalyst‐free chemical vapor deposition process at 1280°C. The SiC/SiO₂ nanochains experience an apparent regular periodic structure, whose SiO₂ beads with diameters of about 160 nm are positioned on the SiC strings beside one another. Their growth mechanism could be mainly ascribed to Rayleigh instability and vapor‐solid mechanism. The double layers structure of amorphous silica on the surface of the strings results from the different silica deposition stages. SiC nanowires with diameters of 20–50 nm were also found accompanied with the SiC/SiO₂ nanochains and not changed into the chains‐shaped morphology because of their small diameters and much higher additive surface pressure of bending silica melt layer on the nanowires.     

High-Temperature Chemical Stability of Plasma-Sprayed Ca05Sr05Zr4P6O24 Coatings on Nicalon/SiC Ceramic Matrix Composite and Ni-Based Superalloy Substrates

The potential application of Ca05Sr05Zr4P6O24 (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 ∼200 (xm 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 ∼200 (xm thick metallic bond coat layer were used as substrate materials. Both the powder and coating contained ZrP2O7 as an impurity 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/O2 atmospheres at 1000°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.     

High-Temperature Oxidation at 1500° and 1600°C of SiC/Graphite Coated with Sol–Gel-Derived HfO2

Oxidation of SiC compositionally graded (SCGed) graphite coated with HfO₂ derived from HfCl₄ by a sol–gel process was performed at 1500° and 1600°C in a flowing gas mixture of Ar and O₂ (80/20 kPa). SCGed graphite was produced by reaction of graphite with either molten Si or SiO gas at 1450°C. The sol–gel-derived HfO₂ precursor was deposited on SCGed graphite by a dip-coating method. Isothermal and cyclic oxidation of uncoated- and HfO₂-coated SCGed graphite was studied by monitoring overall weight change using an electro-microbalance. Scanning electron microscopy with energy-dispersive X-ray analysis was used to observe the surfaces and cross-sections of the oxidized HfO₂-coated SCGed graphite. The formation of HfSiO₄ was confirmed on the outer layer of the oxidized sample, beneath which a thin silica layer was formed. The improved oxidation resistance of SCGed graphite by coating with HfO₂ is discussed on the basis of the formation of these two layers.     

Formation of β-Si3N4 Coatings by Chemical Vapor Deposition

BetaSi₃N₄ coatings were obtained by chemical vapor deposition in a fused-silica reaction tube by outside heating of the system SiCl₄-NH₃-N₂ at a deposition temperature (reaction tube temperature) of 1300°C, whereas α- and α+β-phase coatings were obtained at depositon temperatures of 1150° and 1250°C, respecively. Formation of β-phase coatings at relatively low temperatures is explained in terms of the effect of a catalytic impurity, SiO vapor from the reaction tube. The X-ray diffraction patterns and sulface morphologies of the coatings were studied.     

Effect of Weave Architecture on Tensile Properties and Local Strain Heterogeneity in Thin-Sheet C–SiC Composites

Room-temperature tensile properties were measured for two thin C–SiC composites fabricated from single sheets of carbon fiber fabric with nominally the same weave architecture, but different fiber packing densities. The SiC matrixes were formed by infiltration and pyrolysis of a polymer precursor (allylhydridopolycarbosilane). The tensile properties are related to microstructural characteristics, observed damage mechanisms, and measurements of local strain concentrations by speckle interferometry. Differences are observed between the responses of these thin-sheet composites and conventional CVI-matrix composites of larger thickness. Debonding between transverse and longitudinal fiber tows allows significant strains due to straightening of initial wavy fiber tows and leads to local stress concentrations. The strength and elastic modulus are affected by the waviness of the longitudinal tows.     

Pressure‐less joining of C/SiC and SiC/SiC by a MoSi2/Si composite

A MoSi₂/Si composite obtained in situ by reaction of silicon and molybdenum at 1450°C in Ar flow is proposed as pressure‐less joining material for C/SiC and SiC/SiC composites. A new “Mo‐wrap” technique was developed to form the joining material and to control silicon infiltration in porous composites. MoSi₂/Si composite joining material infiltration inside coated and uncoated C/SiC and SiC/SiC composites, as well as its microstructure and interfacial reactions were studied. Preliminary mechanical strength of joints was tested at room temperature and after aging at service temperatures, resulting in interlaminar failure of the composites in most cases.     

Oxidation Behavior from Room Temperature to 1500°C of 3D-C/SiC Composites with Different Coatings

A carbon-fiber-reinforced silicon carbide composite (3D-C/SiC) was prepared by chemical vapor infiltration. A SiC and SiC/Si-Zr coating were deposited on the composite to investigate the effect of different coatings on the oxidation behavior of 3D-C/SiC composites. The 3D-C/SiC(SiC/Si-Zr) composite decreased in weight below 1000°C and increased in weight above 1000°C. With an increasing oxidation time, the weight loss increased greatly and the weight gain increased little. The 3D-C/SiC(SiC) composite always decreased in weight over the full temperature range. With an increasing oxidation time, the weight loss increased rapidly below 1000°C and reached its minimum value at 1400°C. The 3D-C/SiC(SiC/Si-Zr) composite had a higher oxidation resistance above 1000°C, and the 3D-C/SiC(SiC) composite had a higher oxidation resistance below 1000°C. The wider the coating cracks, the larger the maximum weight loss and the lower the temperature corresponding to the maximum weight loss. With an increasing oxidation time, the activation energy of the 3D-C/SiC(SiC/Si-Zr) composite increased from 96 to 138 kJ/mol, and the 3D-C/SiC(SiC) composite increased from 130 to 180 kJ/mol.