Oxidation Behavior of Silicon-Infiltrated Carbon/Carbon Composites in High-Enthalpy Convective Environment

Thermal response and oxidation behavior of commercial metal-silicon-infiltrated carbon/carbon composites (MICMAT^TM; Si-CC) were evaluated in a high-enthalpy convective environment using an arc jet facility (an arc wind tunnel). Composite specimens were put into a supersonic plasma air stream having a gas enthalpy of 12.7–18.8 MJ/kg for 50–600 s. Cold-wall heat fluxes measured by a Gardon-type calorimeter ranged from 1.0 to 1.8 MW/m², and the maximum surface temperature reached 1300°–1660°C. After the arc jet testing, no surface recession was observed in the samples, and the mass loss rate of the composites was far less than that of graphite. The excellent oxidation resistance was caused by formation of a porous SiC layer at the surface of the composite. Oxidation behavior of the composites is discussed based on a simplified airflow blocking model of the porous SiC layer. The composites exhibited excellent oxidation resistance for short-term exposure in high-enthalpy airflow.     

Kinetics and Mechanisms of Oxidation of 2D Woven C/SiC Composites: I, Experimental Approach

The oxidation behavior of a 2D woven C/SiC composite partly protected with a SiC seal coating and heat-treated (stabilized) at 1600°C in inert gas has been investigated through an experimental approach based on thermogravimetric analyses and optical/electron microscopy. Results of the tests, performed under flowing oxygen, have shown that the oxidation behavior of the composite material in terms of oxidation kinetics and morphological evolutions is related to the presence of thermal microcracks in the seal coating as well as in the matrix. Three different temperature domains exist. At low temperatures (<800°C), the mechanisms of reaction between carbon and oxygen control the oxidation kinetics and are associated with a uniform degradation of the carbon reinforcement. At intermediate temperatures, (between 800° and 1100°C), the oxidation kinetics are controlled by the gas-phase diffusion through a network of microcracks in the SiC coating, resulting in a nonuniform degradation of the carbon phases. At high temperatures (>1100°C), such diffusion mechanisms are limited by sealing of the microcracks by silica; therefore, the degradation of the composite remains superficial. The study of the oxidation behavior of (i) the heat-treated composite in a lower oxygen content environment (dry air) and (ii) the as-processed (unstabilized) composite in dry oxygen confirms the different mechanisms proposed to explain the oxidation behavior of the composite material.     

Oxidation Kinetics of Powdered Silicon Nitride

The oxidation kinetics of powdered silicon nitride were studied in dry oxygen and dry air at 1 atm pressure between 1065° and 1340°C. An automatic recording electrobalance was used to measure the weight gain as a function of time. Parabolic oxidation kinetics were observed with an activation energy of 61 kcal/mol in dry oxygen and 68 kcal/mol in dry air. The oxidation rate in dry oxygen was approximately twice that in air. The solid oxidation product was tridymite above 1125°C and amorphous silica at 1067°C.     

Improvement in the Oxidation Resistance of Oxide-Matrix Silicon Carbide-Particulate Composites by Mullite Infiltration

A SiC–∼50 vol% mullite-particulate composite fabricated by melt infiltration was found to exhibit excellent oxidation resistance at temperatures >1500°C in air. Cristobalite was found on the surface of samples after 100 h oxidation at temperatures of 1515°, 1620°, and 1650°C. The oxidation rate constant at 1515°C was almost comparable to hot-pressed bulk SiC, and at least 1 order of magnitude lower than the lowest value for the oxide-matrix SiC-particulate composites made by conventional processes, as reported in the literature and made by melt infiltration in the present study.     

Strength of Silicon Nitride after Thermal Shock

A water-quenching technique was used to evaluate the thermal-shock strength behavior of silicon nitride (Si₃N₄) ceramics in an air atmosphere. When the tensile surface was shielded from air during the heating and soaking process, the quenched specimens showed a gradual decrease in strength at temperatures above 600°C. However, the specimens with the air-exposed surface exhibited a ∼16% and ∼29% increase in strength after quenching from 800° and 1000°C, respectively. This is because of the occurrence of surface oxidation, which may cause the healing of surface cracks and the generation of surface compressive stresses. As a result, some preoxidation of Si₃N₄ components before exposure to a thermal-shock environment is recommended in practical applications.     

Oxidation Effects on the Mechanical Properties of a SiC-Fiber-Reinforced Reaction-Bonded Si3N4 Matrix Composite

The room-temperature mechanical properties of a SiC-fiberreinforced reaction-bonded silicon nitride composite were measured after 100 h treatment in nitrogen and oxygen environments to 1400°C. The composite heat-treated in nitrogen to 1400°C showed no appreciable loss in properties. In contrast, composites heat-treated in oxygen from 600° to 1000°C retained ∼65% and 35% of the matrix fracture and ultimate strength, respectively, of the as-fabricated composites, and those heat-treated from 1200° to 1400°C retained greater than 90% and 65% of the matrix fracture and ultimate strength, respectively, of the as-fabricated composites. For all nitrogen and oxygen treatments, the composite displayed strain capability beyond the matrix fracture strength. Oxidation of the fiber surface coating, which caused degradation of bond between the fiber and matrix and reduction in fiber strength, appears to be the dominant mechanism for property degradation of the composites oxidized from 600° to 1000°C. Formation of a protective silica coating at external surfaces of the composites at and above 1200°C reduced oxidation of the fiber coating and hence degrading effects of oxidation on their properties.     

Effect of Environment on the Stress-Rupture Behavior of a Carbon-Fiber-Reinforced Silicon Carbide Ceramic Matrix Composite

Stress-rupture tests were conducted in air, under vacuum, and in steam-containing environments to identify the failure modes and degradation mechanisms of a carbon-fiber-reinforced silicon carbide (C/SiC) composite at two temperatures, 600° and 1200°C. Stress-rupture lives in air and steam-containing environments (50–80% steam with argon) are similar for a stress of 69 MPa at 1200°C. Lives of specimens tested in a 20% steam/argon environment were about twice as long. For tests conducted at 600°C, composite life in 20% steam/argon was 30 times longer than life in air. Thermogravimetric analysis of the carbon fibers was conducted under conditions similar to the stress-rupture tests. The oxidation rate of the fibers in the various environments correlated with the composite stress-rupture lives. Examination of the failed specimens indicated that oxidation of the carbon fibers was the primary damage mode for specimens tested in air and steam environments at both temperatures.     

Oxidation of BN/Nicalon Fiber Interfaces in Ceramic-Matrix Composites and Its Effect on Fiber Strength

Microstructural changes at the interface were analyzed in two Nicalon-fiber ceramic-matrix composites with a dual BN/SiC coating on the fibers after thermal exposure at different temperatures (in the range 800°-1400°C) and in different environments (air and argon). The outer SiC coating acted as a barrier to oxygen, which penetrated into the composite via pipeline diffusion along the BN/fiber interfaces. Oxygen penetration led to the formation of an SiO₂ layer by oxidation of the fiber surfaces. The in situ fiber strength at different temperatures, as determined from the radius of the mirror region on the fiber fracture surface, indicated that this SiO₂ layer severely degraded the fiber strength. Oxidation was highly dependent on the nature of the BN/fiber interface. The presence of a thin carbon-rich interlayer, which burned out rapidly at high temperature, favored the entry of oxygen and accelerated oxidation of the fibers.     

Thermal Oxidation of Aluminum Nitride Powder

The oxidation kinetics, morphology, and crystallinity of aluminum nitride (AlN) powder thermally oxidized in flowing oxygen were determined from 800° to 1150°C. At 800°C the oxidation became detectable with weight change. AlN powder was almost completely oxidized at 1050°C after only 0.5 h. Amorphous aluminum oxide formed at relatively low temperatures (800°–1000°C), with a linear oxidation rate governed by the oxygen–nitride interfacial reaction. Transmission electron microscopy displayed individual aluminum oxide grains which formed a discontinuous oxide layer at this temperature range. The aluminum oxide was crystalline at higher temperatures (>1000°C), as detected by X-ray diffraction, and the density of oxide grains increased with temperature.     

Kinetics and Mechanisms of Oxidation of 2D Woven C/SiC Composites: II, Theoretical Approach

A model for the oxidation kinetics of SiC-coated 2D C/SiC composites is developed on the basis of mechanisms derived from TGA data (between 700° and 1500°C in dry oxygen or air, P = 100 kPa). Carbon/oxygen reaction, gas phase diffusion through microcracks present in the external SiC coating, and silica growth are the main phenomena taken into account in the modeling. A morphological characterization of the microcracks network based on a compression test and SEM observations has been developed. The differential equations of the model are solved according to an iterative procedure. The mass variations of the composite during an oxidation test, as derived from the model, are in good agreement with the experimental data. The model is then used to predict the oxidation rate variations when (i) the oxygen partial pressure is decreased, and (ii) the state of damage of the external SiC coating is increased by mechanical loading.     

Characterization of Oxidized Polymer-Derived SiBCN Fibers

The oxidation behavior of a developmental amorphous SiBCN fiber was investigated. Fibers were heat-treated in stagnant laboratory air at temperatures of 1300°–1500°C for 1 or 2 h. The oxidized SiBCN fibers contained three distinct concentric layers, each increasing in oxygen concentration from the core to the outer surface. The unreacted fiber core retained its amorphous nature. The first oxidation layer next to the core consisted of a mixture of amorphous SiBCNO and turbostratic BN, which evolved into a more oxygen-rich glass with hexagonal and turbostratic BN grains dispersed throughout nearer the surface. The second layer consisted of essentially pure silica glass with no detectable B, C, or N present. The outermost layer in the fiber oxidized at 1500°C had devitrified to cristobalite. The fiber suffered significant strength degradation after oxidation.     

Preparation and Thermal Ablation Behavior of HfB2–SiC-Based Ultra-High-Temperature Ceramics Under Severe Heat Conditions

HfB₂–SiC-based ultra-high-temperature ceramics with aluminum nitride (AlN) as a sintering aid were hot pressed at 1850°C. The sinterability and mechanical properties were investigated and compared with the composite without a sintering aid. It was shown that the addition of AlN greatly improved the powder sinterability and enabled the production of a nearly full-dense composite. The mechanical properties, especially the flexural strength, were enhanced remarkably through the improvement in the sinterability and microstructure. The oxidation resistance of a composite doped with 10 vol% AlN was evaluated by a plasma arc heater and the ablation mechanism was discussed.     

Carbon Dioxide Laser Cutting of a Carbon-Fiber–Silicon Carbide-Matrix Composite

CO₂ laser scribing and cutting were studied on a carbon-fiber-silicon carbide-matrix (C/SiO) composite nominally containing 45 vol% of carbon fibers. The scribing and cutting were performed in continuous-wave (CW) mode using laser powers between 750 and 1500 W, and specimen translation velocities between 0.5 and 4 cm/s. The laser spot size was 300 μm in diameter. The groove width and depth were measured as functions of power and velocity. The results were compared to theoretically predicted values obtained by solving the quasi-steady-state heat conduction equation in three dimensions for a moving body. Reasonably good agreement between theory and experiment was found. The microstructures of the laser-cut surfaces indicated the formation of redeposit by condensation from the vapor phase. X-ray diffraction and Raman spectroscopy analyses of the redeposit showed the presence of β-SiC and graphitic carbon. The four-point bending strength of the laser-cut composite was found to be approximately 20% lower than the corresponding strength of the diamond-cut composite. The strength was fully recovered after removing 180 ± 10 μm of the material from the lased surface by grinding. The oxidation resistance of the laser-cut and diamond-cut composites was studied with a thermogravimetric balance at 1103°, 1304°, and 1402°C in air. The oxidation behavior at all investigated temperatures for both materials was dominated by a rapid initial mass loss due to the oxidation of carbon and a possible active oxidation of SiC, followed by a slow mass gain due to the passive oxidation of SiC. At 1304°C the rate of passive oxidation of SiC in the laser-cut material was somewhat higher than in the diamond-cut material. At 1402°C, the diamond-cut surface oxidized more rapidly than the taser-cut surface. The differences in oxidation rates were attributed to the differences in microstructure.     

Thermal Cycling Damage Mechanisms of C/SiC Composites in Displacement Constraint and Oxidizing Atmosphere

A constraint stress of 62.5 MPa is created on a three-dimensional C/SiC composite specimen whose both ends are fixed when temperature is cycled between 900° and 1200°C. The cyclic stress results in a maximum damage strain of 0.06% within 50 cycles owing to coating and matrix cracking, fiber debonding, sliding, and breaking in the composite. This constrained specimen elongation also leads to a final compressive stress of 14 MPa on the composite through a decrease in the baseline constraint stress. Wet oxygen atmosphere at a high cyclic temperature, concomitant with stresses, can aggravate the damage situation by alternate oxidation between internal and external fibers in composites.     

Effect of Thermal Exposures on the Strengths of Nextel™ 550 and 720 Filaments

The effects of thermal exposure on the strengths of Nextel™ 550 and 720 tows, bare and coated with carbon, were determined by room-temperature tensile testing of single filaments extracted from tows that had been exposed to different thermal environments (i.e., air or vacuum) at temperatures from 550° to 1400°C. The results help define the allowable composite processing conditions when using these tows. A 28% drop in the strength of Nextel 550 filaments occurred after a thermal exposure at 1100°C for 2 h in air. After an exposure of 1300°C/2 h/air, a strength degradation of ∼47% resulted. Filaments exposed above 1100°C under vacuum showed more severe strength degradation than filaments exposed in air. The observed strength degradation may stem from a combination of phase transformations of the alumina, the onset of mullite crystallization, and/or exaggerated mullite grain growth. Strength after heat treatment under vacuum at 1050° and 1150°C did not deteriorate as rapidly as after heat treatment under vacuum between 950° and 1050°C or between 1150° and 1250°C. This may be a result of the competition between healing of flaws by the amorphous silica and its evaporation (leading to an increase in its viscosity or loss) and/or densification of the filaments. Nextel 720 filaments exhibited about 9% strength loss after an exposure at 1100°C/2 h/air. The filaments maintained 75% of their strength after a 1300°C/2 h/air heat treatment. The observed strength degradation may stem from thermal grooving, grain growth, and/or annealing of the mullite subgrain boundaries. Thermal exposure of >10 h at 1300°C was required to produce measurable grain growth. Strength loss between 1200° and 1300°C (air heat treatment) was not as great as between 1100° and 1200°C or 1300° and 1400°C.     

Thermal Degradation of Fiber Coatings in Mullite-Fiber-Reinforced Mullite Composites

The thermal degradation behavior of single-layer BN and of double-layer BN/SiC chemically vapor-deposited fiber coatings in mullite-fiber-reinforced mullite composites was investigated by means of transmission electron microscopy after processing and heat treatment of the composites at 1000°, 1200°, and 1300°C for 6 h in air. The single-layer BN coatings were ˜0.7 mu m thick and consisted of turbostratic BN with (0001) basal planes lying parallel to the surfaces of the fibers plus nanosized areas that had no preferential orientation. This microstructure remained unchanged up to 1000°C; however, distinct coarsening of the randomly oriented BN crystallites occurred in the temperature range of 1000°-1200°C. The single-layer BN coatings were stable against oxidation, up to 1200°C. At higher temperatures, degradation of the coatings via oxidation occurred. Double-layer BN/SiC coating systems consisted of BN that was 0.08 mu m thick and SiC layers that were 0.16 mu m thick and deposited onto the mullite fibers. The turbostratic BN was highly anisotropic and did not undergo any microstructural change, up to 1300°C. The outer SiC layer of the double-layer coating system improved the oxidation resistance of BN in the 1200°-1300°C temperature range, despite a partial oxidation of SiC to SiO₂.     

Kinetics of Thermal Oxidation of Silicon Nitride Powders

The kinetics of thermal oxidation of H. C. Starck M-11 and Ube SN-E10 Si₃N₄ powders was evaluated in the temperature range 650°-1200°C using isothermal and nonisothermal thermogravimetric analysis, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. Between 700° and 1200°C, the isothermal kinetics was modeled equally well using the Ginstling-Brounshtein and Zuravlev-Lesokhin-Tempeľman equations. Despite their similar particle sizes and surface areas, the two powders exhibited different oxidation kinetics. The activation energies for oxidation of the M-11 and SN-E10 powders were determined to be 400 and 540 kJ/mol, respectively, between 1000° and 1200°C, and 230 and 260 kJ/mol, respectively, between 700° and 1000°C. The parabolic rate constants for oxidation of the two powders were comparable to those reported in the literature for monolithic, chemically-vapor-deposited Si₃N₄ at the higher temperatures. At lower temperatures, the oxidation kinetics of the M-11 powder was nearly linear, whereas the kinetics of the SN-E10 powder remained power-law dependent.     

Effect of Zirconia Content on the Oxidation Behavior of Silicon Carbide/Zirconia/Mullite Composites

The oxidation of hot-pressed SiC-particle (SiC_p)/zirconia (ZrO₂)/mullite composites with various ZrO₂ contents, exposed in air isothermally at 1000° and 1200°C for up to 500 h, was investigated; an emphasis was placed on the effects of the ZrO₂ content on the oxidation behavior. A clear critical volume fraction of ZrO₂ existed for exposures at either 1000° or 1200°C: the oxidation rate increased dramatically at ZrO₂ contents of >20 vol%. The sharp transition in the oxidation rate due to the variation of ZrO₂ content could be explained by the percolation theory, when applied to the oxygen diffusivity in a randomly distributed two-phase medium. Morphologically, the composites with ZrO₂ contents greater than the critical value showed a large oxidation zone, whereas the composites with ZrO₂ contents less than the critical value revealed a much-thinner oxidation zone. The results also indicated that the formation of zircon (ZrSiO₄) at 1200°C, through the reaction between ZrO₂ and the oxide product, could reduce the oxidation rate of the composite.     

Thermal Oxidation of Sputter-Coated Reaction-Bonded Silicon Nitride

Ceramic coatings prepared by sputtering and reactive sputtering were applied to reaction-bonded silicon nitride surfaces to prevent extensive oxidation of the underlying material. The high-density nitride-based coatings retard the oxidation of the substrate by forming an oxygen diffusion barrier which seals the open porosity while maintaining dimensional and thermal stability. The oxidation kinetics of the coated and uncoated reaction-bonded silicon nitride substrates were compared at T = 1000 ° to 1200°C. Oxidation of the underlying material in this temperature range was substantially reduced when suitable coatings were used and the crystalline oxidation product (cristobalite) was essentially eliminated.     

High-Temperature Tensile Behavior of a Boron Nitride-Coated Silicon Carbide-Fiber Glass-Ceramic Composite

Tensile properties of a cross-ply glass-ceramic composite were investigated by conducting fracture, creep, and fatigue experiments at both room temperature and high temperatures in air. The composite consisted of a barium magnesium aluminosilicate (BMAS) glass-ceramic matrix reinforced with SiC fibers with a SiC/BN coating. The material exhibited retention of most tensile properties up to 1200°C. Monotonic tensile fracture tests produced ultimate strengths of 230–300 MPa with failure strains of ∼1%, and no degradation in ultimate strength was observed at 1100° and 1200°C. In creep experiments at 1100°C, nominal steady-state creep rates in the 10⁻⁹ s⁻¹ range were established after a period of transient creep. Tensile stress rupture experiments at 1100° and 1200°C lasted longer than one year at stress levels above the corresponding proportional limit stresses for those temperatures. Tensile fatigue experiments were conducted in which the maximum applied stress was slightly greater than the proportional limit stress of the matrix, and, in these experiments, the composite survived 10⁵ cycles without fracture at temperatures up to 1200°C. Microscopic damage mechanisms were investigated by TEM, and microstructural observations of tested samples were correlated with the mechanical response. The SiC/ BN fiber coatings effectively inhibited diffusion and reaction at the interface during high-temperature testing. The BN layer also provided a weak interfacial bond that resulted in damage-tolerant fracture behavior. However, oxidation of near-surface SiC fibers occurred during prolonged exposure at high temperatures, and limited oxidation at fiber interfaces was observed when samples were dynamically loaded above the proportional limit stress, creating micro-cracks along which oxygen could diffuse into the interior of the composite.