Silicon carbide fiber oxidation behavior in the presence of boron nitride

The oxidation behavior of Sylramic SiC fibers without a boron nitride surface layer was compared to Sylramic iBN SiC fibers with a boron nitride surface layer by conducting thermogravimetric analysis in dry O₂ at temperatures ranging from 800 to 1300°C for times up to 100 hours. Sylramic fibers followed the Deal and Grove oxidation kinetic model. A transient period of accelerated oxidation kinetics was observed with Sylramic iBN fibers. Raman spectroscopic analysis of oxidized fibers provided evidence for a borosilicate glass structure. The boron concentrations in the oxides, quantified by inductively coupled plasma‐optical emission spectrometry, were correlated with the weight change behavior, oxide thickness, and fiber recession of the oxidized fibers. Oxides formed from Sylramic iBN fibers were typically higher in boron concentration, which led to initial rapid oxidation rates that were 3‐10 times faster than observed for pure SiC. Slower oxidation rates followed as the oxide surface became increasingly enriched with SiO₂ due to boria volatilization, thus limiting boria effects on SiC fiber oxidation kinetics. The accelerated high‐temperature oxidation of SiC fibers due to the presence of BN are discussed in terms of the borosilicate glass structure and composition.     

Effect of moisture on the oxidation behavior of ZrB2

Oxidation studies of ZrB₂ were performed under wet air and dry air conditions at 1200°C, 1400°C, and 1500°C for 1, 4, and 10 h. Compared to dry air, the presence of water vapor was found to enhance the oxidation kinetics by a factor of 7 to 30, depending on the temperature. Thermodynamic calculations suggested that water vapor promoted the formation of additional volatile species such as boric acid (HBO₂), in addition to boria (B₂O₃) produced in dry air, which increased the evaporation rate of B₂O₃. Compared to dry air, the presence of water vapor leads to more rapid evaporation of boria and the transition from parabolic oxidation kinetic behavior (ie, rate controlled by diffusion through boria) to linear (ie, underlying ZrB₂ is directly exposed to the oxidizing environment) at shorter times and lower temperatures.     

Investigations into the thermal stability of sol-gel-derived glasses as models for thermally grown oxides

The thermal stability of sol-gel-derived silica and borosilicate glasses exposed to dry O₂ at 800 and 1200°C for 100 hours was characterized by weight change, thermal transitions, morphology, structure, and composition to investigate suitability as models for thermally grown oxides. Rapid weight loss was observed in the first few hours of isothermal exposure for borosilicate glasses, followed by constant weight loss at a low rate for the balance of the exposure. Weight loss resulted from loss of residual hydroxyl species retained from the sol-gel synthesis, and from oxidation of carbon retained from thermal decomposition of the organic precursors by pyrolysis. Characterization of the sol-gel-derived glasses showed structural similarities to silica and binary borosilicate glasses synthesized by melt or vapor deposition methods, and to thermally grown oxides. Oxygen transport mechanisms through the sol-gel-derived glasses is not thought to be affected by the retained carbon. However, a silica-enriched glass surface resulting from boria volatility, observed from a borosilicate glass exposed dry O₂ at 1200°C, will slow O₂ transport rates. The results show that sol-gel-derived silica and borosilicate glasses can be used as models for thermally grown oxides.     

Fabrication and oxidation behavior of Al4SiC4 powders

Al₄SiC₄ powders with high purity were synthesized by heating the powder mixture of aluminum (Al), silicon (Si), and carbon (C) at 1800°C in argon. The microstructure is characterized as platelike single grain. Both the nonisothermal and isothermal oxidation behavior of Al₄SiC₄ was investigated at 800°C‐1500°C in air by means of thermogravimetry method. It is demonstrated that Al₄SiC₄ powder possesses good oxidation resistance up to 1200°C and is almost completely oxidized at 1400°C. At 800°C‐1100°C, the oxide scales consist of an Al₂O₃ outer layer and a transition layer. Al₄SiC₄ remains the main phase. At 1200°C, some spallation resulting from the increment of Al₂O₃ and the mismatch of thermal expansion coefficient between different product layers can be observed. Above 1300°C, the oxide layer is composed of two part, i.e., large‐scale Al₂O₃ crystals (outer layer) and mullite with less amount of SiO₂ (inner layer). The oxidation behavior changes due to the different oxide products. For the reaction kinetics, a new kind of real physical picture model is adopted and obtains a good agreement with the experimental data. The apparent activation energy is calculated to be 176.9 kJ/mol (800°C‐1100°C) and 267.1 kJ/mol (1300°C‐1400°C).     

Steam pressure and velocity effects on high temperature silicon carbide oxidation

The effects of steam pressure, velocity, and composition on SiC oxidation kinetics were studied. Pressure effects were tested at 1200°C from 0.1 to 1.4 MPa at a steam velocity of 0.25 cm/s. Velocity effects were tested in two furnaces at 0.45 MPa, 1200°C and 0.1 MPa, 1600°C with velocities ranging from 0.25 to 137 cm/s. Steam composition was altered by changing the reaction vessel material. Oxide morphology and composition were determined using optical and electron microscopy, and X-ray diffraction. Porous oxides were observed whenever structural SiC from the reaction vessel saturated the steam with volatilized silica, H₂, and CO. Oxidation kinetics were calculated by the change in SiC thickness. The steam velocity/recession rate followed a power-law relationship of ~ 0.35 while the steam pressure/recession rate followed a power-law relationship of ~ 1.78.     

Short-term oxidation of a ternary composite in the system AlN–SiC–ZrB2

The present study investigates the oxidation behaviour in air of a structural ceramic composite with the following volumetric composition: 55% AlN–15% SiC–30% ZrB₂. This kind of ternary composite is electroconductive (3 × 10⁻⁴ Ω cm) and has significant strength (∼700 MPa) and toughness (4 MPa m^1/2) up to 1000 °C. Oxidation tests were carried out in a TG equipment from 700 to 1300 °C with exposition time of 30 h. Significant weight gain is observed at T > 1000 °C. In the range 700–900 °C, the process is dominated by the oxidation of ZrB₂ into zirconia and boria and the kinetic is nearly parabolic. At temperatures in the range 1000–1100 °C, boria reacts with alumina forming aluminium borate and borosilicate glass and the kinetic largely deviates from parabolic behaviour. In the samples oxidized at temperatures in the range 1200–1300 °C, aluminium borate and mullite crystallize on the surface. The kinetics is para-linear in this temperature range but at 1300 °C, the rupture of the outer layer, results in accelerated damage of the sample. The composite is recommended for applications up to 1100 °C.     

Insights into internal oxidation of SiC/BN/SiC composites

The article presents new observations of the physical manifestations of internal oxidation and volatilization in SiC/BN/SiC composites. The observations are made on both unbroken and broken minicomposite specimens before and after 12 h exposures at 1000°C in dry air with 10 ppm water vapor. The observations are enabled by a sample preparation method involving ion-mill sectioning and polishing. Complementary analyses of volatilization and closure of resulting gaps are also presented. The observations show that BN is generally consumed in two stages: (i) through reaction with oxygen along the interfaces with both the fiber and the matrix, producing two concentric annular pockets of borosilicate glass and an intervening annulus of progressively thinning BN; and (ii) subsequent volatilization, through the reaction of boria with trace amounts of water vapor in the environment to form borohydroxide gases. The spatial extent to which these processes proceed is governed by a competition between the outward diffusion of reaction gases through both matrix cracks and interface gaps produced by boria volatilization, and the formation of oxides on the newly exposed surfaces of fibers, matrix, and coating.     

High-temperature oxidation of BN-coated sylramic SiC fibers in dry and wet atmosphere

The oxidation behavior of SiC Sylramic fibers coated with chemically vapor deposited Si-doped boron nitride (BN) was investigated at temperatures between 800 and 1200°C in dry and wet O₂ atmospheres. Thermogravimetric analysis was used to study the oxidation kinetics of the fiber and the influence of the BN layer and the environment. The morphology and composition of the thermally grown oxide scale was determined posttest by scanning electron microscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma optical emission spectrometry. This study gives new insights into the synergistic effects of BN and water vapor on the oxidation behavior SiC Sylramic fibers. The vulnerability of the BN fiber interphase and the behavior of the fiber under conditions relevant to high-temperature turbine applications are discussed.     

Interfacial reactions between B2O3 and spark plasma sintered Yb2Si2O7

Reactions between boria (B₂O₃) and Yb₂Si₂O₇ were studied via a series of idealized interfacial “well” tests. Boria oxidizes out of SiC/SiC ceramic matrix composites (CMCs) where BN is used as a fiber/matrix interphase and boron-rich inclusions often serve as aids in the melt infiltration process. Borate phases are highly reactive and can react with the rare earth silicates currently being utilized as environmental barrier coatings (EBCs) for these CMC systems. Ytterbium disilicate substrates for these well tests are prepared via spark plasma sintering. The well is then drilled into the substrates and filled with a boria glass plug. Exposures in a stagnant-air box furnace show that the boria is reacting with the disilicate via a substitution reaction leaving YbBO₃ and amorphous silica glass as the product phases. This phase was characterized with scanning electron microscopy and elemental dispersive spectroscopy (SEM/EDS), micro-focus X-ray diffraction, and selected-area electron diffraction (SAED). Inductively coupled plasma optical emission spectroscopy (ICP-OES) was also used to analyze water-soluble glassy phases left on the surface of the substrates post-exposure, indicating that the boron content of the glass was decreasing with both increasing exposure times and temperatures. There are few data on the borate product phase properties, however the results of this study suggest that the boria formed via oxidation from the SiC/BN/SiC composites could be detrimental to the performance of Yb₂Si₂O₇ environmental barrier coatings via formation of the borate phase and silica.     

SiAlC ceramics from a new polyaluminocarbosilane: Synthesis,structure and oxidation behaviors

SiAlC ceramics were prepared with a new polyaluminocarbosilane (PACS-N01) which was synthesized using methylaluminoxane (MAO) and liquid hyperbranched polycarbosilane (LPCS), and had a quite high ceramic yield (around 82.7%) at 900 °C in argon after curing. The obtained SiAlC ceramics consisted of β-SiC as major building units, small amounts of α-SiC and Al-containing phases with Al–O and Al–C bonds, and stayed partially amorphous till 1600 °C. Oxidation behaviors of SiAlC ceramics and SiC ceramics prepared through the same procedure were studied in the air at 1200 °C for different times. Dense oxide scales free of pores were formed on the surface of both samples. Measurements of oxide scale thickness revealed that the oxidation followed parabolic reaction kinetics and that the oxidation rate constant was apparently smaller for SiAlC than that for SiC. Besides, –Al–O–Si– network structure could be formed in SiAlC's oxide scale, which was supposed to block the pathway of oxygen diffusion and thus lowered the oxidation rates.     

Oxidation of SiC/BN/SiC ceramic matrix composites in dry and wet oxygen at intermediate temperatures

The oxidation behavior of SiC/BN/SiC ceramic matrix composites (CMCs) was evaluated from 400° to 800 °C in 100% O₂ and 50% H₂O/50% O₂ gas mixtures. Thermogravimetric analysis (TGA) was utilized to measure weight change during controlled environment exposures at elevated temperatures for 1 and 50 hours. Oxidized CMCs and their oxides were studied post-exposure with scanning electron microscopy and energy dispersive spectroscopy. The oxidation onset and composition transition temperatures were evaluated. Key observations include oxide composition, oxide wetting, oxygen solubility in Hi-Nicalon SiC fibers and BN fiber coating oxidation and volatility behavior as a function of temperature. Degradation in wet environments at 600 °C was most extensive due to the formation of a non-wetting, non-protective surface oxide, allowing oxidant access to the BN fiber coatings followed by oxidation and volatilization. Implications of the CMC oxidation behavior are discussed for CMCs in service.     

Crystallization kinetics for SiO2 formed during SiC fiber oxidation in steam

The crystallization kinetics for SiO₂ formed by oxidation of Hi-Nicalon^™-S SiC fiber between 800 and 1600°C in Si(OH)₄(g) saturated steam were determined. Glass SiO₂ scale always formed first. Glass scale eventually crystallized to cristobalite, and during further oxidation the scale formed directly as cristobalite. Growth stress relaxed by viscous flow in SiO₂ that formed as glass. Cristobalite formed by crystallization of this glass was relatively undeformed. In SiO₂ that formed directly as cristobalite, growth stress relaxed by intense plastic deformation accompanied by dynamic recrystallization. There were therefore two layers in cristobalite scale: a heavily deformed inner layer and an undeformed outer layer. These layers were distinguished by TEM. SiO₂ crystallization times were determined from the thicknesses of undeformed cristobalite and the SiC oxidation kinetics for glass scale formation. SiO₂ crystallization kinetics were determined from the crystallization time distributions at different SiC oxidation temperatures in steam. For all temperatures the crystallization time growth exponent (n) was 1. There was a large decrease in crystallization rate between 1000 and 1100°C. Between 800 and 1000°C the activation energy (Q) for crystallization was 65 kJ/mol, between 1100 and 1500°C it was 110 kJ/mol, and at 1600°C it was ~500 kJ/mol. Analysis methods and results are discussed.     

Study of oxide layer formation on Inconel 718 during isothermal oxidation between 800 °C to 1200 °C in hot air

The present study is to investigate the isothermal oxidation behavior of Inconel 718 (a Cr/Ni/Mo/Si-based alloy) because of their excellent thermo-mechanical properties, which are important in aerospace applications. X-ray diffraction (XRD) study confirmed the formation of spinel element of NiCr₂O₄ along with Cr₂Ni₃ at 1000 °C. X-ray photoelectron spectroscopy (XPS) is used to define the elemental composition of oxides and Raman spectroscopy to ascertain the state of oxides. Different morphologies such as tetragonal biaxial pyramids, flakes, and polygonal plates were observed on heated samples. Cross-sectional and EDS studies clearly revealed the drastic increase of oxide layer from 800 °C to 1200 °C. Finally, electrochemical impedance spectroscopy (EIS) analysis was carried out at 800 °C, 1000 °C and 1200 °C with a tenant period and ramping rate of 24 h and 10 min respectively. The EIS results divulged that charge transfer resistance values drastically increased with increasing temperature due to oxide layer.     

Oxygen diffusion mechanisms during high‐temperature oxidation of ZrB2‐SiC

Oxygen diffusion mechanisms during oxidation of ZrB₂‐30 vol% SiC were explored at temperatures of 1500°C and 1650°C using an ¹⁸O tracer technique. Double oxidation experiments in ¹⁶O₂ and ¹⁸O₂ were performed using a modified resistive heating system. A combination of scanning electron microscopy, energy‐dispersive spectroscopy, and time‐of‐flight secondary ion mass spectrometry was used to characterize the borosilicate and ZrO₂ oxidation products. Oxygen exchange with the borosilicate network was observed to occur quickly at the oxygen‐borosilicate surface at both 1500°C and 1650°C, while evidence of oxygen permeation was only observed at 1650°C for short time (<1 min) exposures. At longer times, >5‐9 min, complete oxygen exchange throughout both the borosilicate glass and ZrO₂ was observed at both temperatures preventing identification of the oxygen transport mechanisms, but demonstrating that oxygen transport is rapid in both oxide phases.     

Steam oxidation of ytterbium disilicate environmental barrier coatings with and without a silicon bond coat

The current generation of multilayer Si/Yb₂Si₂O₇ environmental barrier coatings (EBCs) are temperature limited by the melting point of Si, 1414°C. To investigate higher temperature EBCs, the cyclic steam oxidation of EBCs comprised of a single layer of ytterbium disilicate (YbDS) was compared to multilayered Si/YbDS EBCs, both deposited on SiC substrates using atmospheric plasma spray. After 500 1-h cycles at 1300°C in 90 vol%H₂O-10 vol%air with a gas velocity of 1.5 cm/s, both multilayer Si/YbDS and single layer YbDS grew thinner silica scales than bare SiC, with the single layer YbDS forming the thinnest scale. Both coatings remained fully adherent and showed no signs of delamination. Silica scales formed on the single layer coating were significantly more homogeneous and possessed a markedly lower degree of cracking compared to the multilayered EBC. The single layer EBC also was exposed at 1425°C in steam with a gas velocity of 14 cm/s in an alumina reaction tube. The EBC reduced specimen mass loss compared to bare SiC but formed an extensive 2nd phase aluminosilicate reaction product. A similar reaction product was observed to form on some regions of the bare SiC specimen and appeared to partially inhibit silica volatilization. The 1425°C steam exposures were repeated with a SiC reaction tube and no 2nd phase reaction product was observed to form on the single layer EBC or bare SiC.     

Kinetics of Cristobalite Formation in Sintered Silica

The kinetics of the cristobalite transformation are reported for sintered silica glass from 1200°C to 1650°C and plotted as a time–temperature–transformation diagram. The 1200°C–1350°C transformation data were fit to the Johnson–Mehl–Avrami–Kolmogorov expression with an time exponent of 3.0 ± 0.6 and an apparent activation energy of 555 ± 24 kJ/mol for the kinetic constant. The temperature of maximum transformation rate was found to fall between 1500°C and 1600°C. Seeding amorphous silica powder with cristobalite resulted in accelerated transformation kinetics. Silica glass powder containing residual quartz had faster transformation kinetics than fully amorphous powder seeded with cristobalite.     

Determination of Retained B2O3 Content in ZrB2‐30 vol% SiC Oxide Scales

The composition of the borosilicate glass layer formed during oxidation of ZrB₂‐30 vol% SiC was determined to elucidate the extent of B₂O₃ retention in the oxide during high‐temperature oxidation. Oxidation was conducted in stagnant air at 1300°C, 1400°C, and 1500°C for times between 100 and 221 min. Specimens were characterized using mass change and scanning electron microscopy. After oxidation, the borosilicate glass layer was dissolved from the specimens sequentially with deionized H₂O and HF acid, to analyze the glass composition using inductively coupled plasma optical emission spectrometry. It was found that the average B₂O₃ content in the glass scale ranged from 23 to 47 mol%. Retained B₂O₃ content in the bulk of the glass decreased with increasing temperature, confirming increased volatility with temperature. Boron depth profiles were also obtained in the near surface region using X‐ray photoelectron spectroscopy and energy dispersive spectroscopy. The measured B concentrations were used to estimate the B₂O₃ concentration profile and B diffusion coefficients in the borosilicate glass. Implications for the ZrB₂‐SiC oxidation process are discussed.     

Oxidation treatments for SiC particles and its compatibility with glass

Different thermal treatments were performed to produce a protective coating on the surface of SiC particles in order to allow their incorporation in a glass matrix. These oxidation treatments were carried out in air at different temperatures ranging from 800 °C to 1500 °C and different times at 1200 °C (10 min–48 h). The oxidation kinetics followed the Deal–Grove model and the thickness of the protective coating increased with temperature and SiC particle size. Protected SiC particles with different particle sizes were incorporated in a borosilicate glass. With small particles sizes foam glasses were obtained, whereas particles with higher grain size, i.e., higher coating thickness, were stable in the glass matrix and a smooth glass was obtained.     

Surface oxidation behavior in air and O2-H2O-Ar atmospheres of continuous freestanding SiC films derived from polycarbosilane

In this contribution, thermodynamic computational calculations firstly carried out on Ar-Si-C-O/Ar-Si-C-O-H database demonstrate that passive oxidation is main reaction of continuous freestanding SiC films in both air and 14%H₂O/8%O₂/78%Ar atmospheres. SiC films were subsequently annealed at 1300?°C, 1400?°C and 1500?°C for 1?h in air and O₂-H₂O-Ar atmospheres. Results suggest that modulus, hardness and resistivity decrease whereas crystallite size of β-SiC and α-cristobalite increase with elevated annealing temperature. In particular, hardness of wet oxidized samples is lower than that of air oxidized ones. Additionally, their oxidation kinetics models were also established and verified by annealing at 1200?°C in air and wet oxygen for different time from 1?h to 100?h. Oxidation of continuous freestanding SiC films is identified to follow parabolic oxidation kinetics, and water could effectively enhance the oxidation rates. It is revealed that SiO₂ layer can protect SiC films from further oxidation, and their thickness increases with prolonged annealing time. In this study, a dense and uniform SiO₂ layer with a thickness of 1.1–1.6?µm was produced for sacrificial and passivation layer based on suitable thermal oxidation process (annealing at 1000?°C for 5?h in O₂-H₂O-Ar environment). Interestingly, fast diffusion paths in this oxide layer could effectively accelerate oxidation process of SiC films. These obtained achievements would promote further applications of SiC films on microelectromechanical systems (MEMS) devices in harsh environments.     

Evolution of structure during the oxidation of zirconium diboride–silicon carbide in air up to 1500 °C

The structures that developed as dense ZrB₂–SiC ceramics were heated to 1500 °C in air were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction. The oxidation behavior was also studied using thermal gravimetric analysis (TGA). Below 1200 °C, a protective B₂O₃-rich scale was observed on the surface. At 1200 °C and above, the B₂O₃ evaporated and the SiO₂-rich scale that formed was stable up to at least 1500 °C. Beneath the surface, layers that were rich in zirconium oxide, and from which the silicon carbide had been partially depleted, were observed. The observations were consistent with the oxidation sequence recorded by thermal gravimetric analysis.