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.     

Identification of a new oxidation/ dissolution mechanism for boria-accelerated SiC oxidation

Boria effects on accelerated SiC oxidation kinetics were investigated by conducting thermogravimetric analysis on SiC substrates coated with sol-gel derived borosilicate glass isothermally exposed to dry O₂ and argon at 800°C and 1200°C for 100 hours. Boria concentrations in the glass coatings were 0, 14-38, and 92-94 mol%, balance silica. Accelerated weight gain was observed for SiC exposures in dry O₂ at 800°C when boria concentrations were ≥ 92 mol%, corroborated by oxide thickness ranging from 3.5 to 10 µm. The oxide thickness predicted for pure SiC exposed to these conditions in the absence of boria is 0.15 µm. Microstructural analysis of SiC surfaces after oxide removal revealed that boria etched the underlying SiC substrate. Oxidation exposures at 1200°C in dry O₂ suppressed boria effects on accelerating SiC oxidation kinetics due to rapid boria volatilization coupled with the formation of a protective thermally grown silica scale. Accelerated weight gain or oxide growth did not occur with argon exposures at either temperature. A new mechanism for boria-accelerated SiC surface-reaction kinetics is presented based on evidence for boria etching of SiC.     

Borosilicate Glass‐Induced Fiber Degradation of SiC/BN/SiC Composites Exposed in Combustion Environments

Three SiC/BN/SiC composite specimens reinforced with different SiC fibers (Sylramic, Sylramic‐iBN, and Hi‐Nicalon Type S) were exposed in a combustion environment. Exposures were carried out for 151 h in a fuel‐lean high pressure burner rig at 0.9 MPa total pressure, sample temperatures near 1573 K, and a gas velocity of 15 m/s. Weight loss of all three composites was observed. Extensive oxidation of SiC fibers was observed in cracked locations. A mechanism based on borosilicate enhanced oxidation coupled with volatilization of boria is described. Ramifications of this degradation mechanism are discussed for long‐term applications of SiC/BN/SiC composites in combustion environments.     

Oxidation kinetics strength of Hi‐NicalonTM‐S SiC fiber after oxidation in dry and wet air

Hi‐Nicalon?‐S SiC fiber strengths and Weibull moduli were measured after oxidation for up to 100 hours between 700°C and 1400°C in wet and dry air. SiO₂ scale thickness and crystallization extent were measured by TEM. The effect of furnace environment on trace element levels in the SiO₂ scales was characterized by secondary ion mass spectroscopy. Crystallization kinetics and Deal‐Grove oxidation kinetics for glass and crystalline scale, and the transition between them, were modeled and determined. Crystallization retards oxidation kinetics, and scale that formed in the crystalline state was heavily deformed by the growth stress accompanying SiC oxidation volume expansion. Glass scales formed in dry air slightly increased fiber strength. Glass scales formed in wet air did not increase strength, and in some cases significantly decreased strength. Scales more than 200 nm thick were usually partially or completely crystallized, which degraded fiber strength. Contamination of scales by trace impurities such as Al and Ca during heat treatment inhibited crystallization. The oxidation kinetics and the strengths of oxidized Hi‐Nicalon?‐S fibers are compared with previous studies on SiC fibers, bulk SiC, and single‐crystal SiC. Empirical relationships between oxidation temperature, time, scale thickness, and strength are determined and discussed.     

Model for SiC fiber strength after oxidation in dry and wet air

The strengths of oxidized SiC fibers were modeled from the effects of SiO₂ scale residual stress on fracture. Surface tractions from scale residual stress were determined for SiC surface flaws. The residual stress was the sum of the growth stress from oxidation volume expansion, thermal stress from SiO₂-SiC thermal expansion mismatch, and stress from phase transformations in crystallized scale. The partial relaxation of tensile residual stress from scale cracking was also calculated. Scale thicknesses were determined using Deal-Grove oxidation kinetics for glass and crystalline scales. Kolmogorov-Johnson-Mehl-Avrami (KJMA) kinetics was used to determine scale crystallization rates. Strengths of fibers with glass and with crystalline scales formed by oxidation in dry and wet air between 600° and 1400°C were modeled. The effects of partially crystallized scales were calculated using Weibull statistical methods. Modeled strengths were compared with measurements. Slight strength increases after glass scale formation, large decreases that accompany scale crystallization, and some differences between dry and wet air oxidation were accurately modeled. This suggests that under some conditions the scale residual stress dominates the changes in strength after SiC fiber oxidation. However, modeled strengths were significantly higher than those measured for some fibers oxidized in wet air, which suggests another degradation mechanism is active for these conditions. Modeling assumptions and implications for SiC fiber strength after oxidation for long times 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.     

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.     

Microstructural,strength, and creep characterization of Sylramic™, Sylramic™-iBN and super Sylramic™-iBN SiC fibers

The chemical composition, microstructure, strength, and thermal stability of polymer-derived Sylramic? SiC fibers fabricated by Dow Corning and COI Ceramics, Inc., and nitrogen-treated Sylramic? SiC fibers, referred to as Sylramic?-iBN and Super Sylramic?-iBN SiC fibers, were investigated and compared. The baseline Sylramic? SiC fibers fabricated by both vendors as well as the nitrogen-treated Sylramic? SiC fibers are composed mostly of β-SiC (～97 wt%) with small amounts of TiB₂ (～2 wt%), amorphous carbon (～1 wt%) and trace amounts of B₄C. Most of the amorphous carbon is segregated at the core/interior of the fibers. Both baseline and nitrogen-treated Sylramic? SiC fibers have similar grain size and pore size distribution, except for a thin layer of in-situ grown crystalline BN (30–70 nm) on the surface of Sylramic?-iBN and Super Sylramic?-iBN fibers. Wide variation in strength within a batch as well as between batches is observed in both baseline and nitrogen-treated Sylramic? SiC fibers but both types of fibers are microstructurally stable at temperatures to 1800 °C in argon and nitrogen environments compared to Nicalon?-S and Tyranno®-SA SiC fibers. Under the same creep condition, Super Sylramic?-iBN fibers show better creep resistance compared to Sylramic?, Sylramic?-iBN, Hi-Nicalon?-S, and Tyranno®-SA fibers. Possible reasons for strength variability and the mechanism of in-situ BN formation on Sylramic? SiC fibers are discussed.     

Na2SO4 deposit-induced hot corrosion of SiC fibers relevant for SiC CMCs

Hi Nicalon, Hi Nicalon S, Sylramic, and Sylramic iBN SiC fibers were exposed to ~60 μg/cm² of Na₂SO₄ in a 0.1% SO₂/O₂ gaseous environment for times between 0.75 and 24 h at 1000°C. After exposure, the corrosion products were characterized using SEM, EDS, ICP-OES, TEM, and EFTEM to determine their high-temperature resistance to Na₂SO₄ and key reaction mechanisms. All SiC fiber types tested in this work exhibited little resistance to Na₂SO₄ deposit-induced attack relative to their behavior in dry O₂ environments. It was found that Hi-Nicalon displayed the least resistance to Na₂SO₄ deposit-induced attack due to excess carbon content resulting in the formation of a highly porous crystalline oxide and promotion of basic corrosion conditions. All fiber types formed a crystalline SiO₂ reaction product, either cristobalite or tridymite. Sylramic and Sylramic iBN formed a crystalline SiO₂ reaction layer containing TiO₂ needles due oxidation of TiB₂ particles. Additionally, Na₂SO₄ deposits resulted in pitting of all fiber surfaces.     

Self‐Generating High‐Temperature Oxidation‐Resistant Glass‐Ceramic Coatings for C–C Composites Using UHTCs

Carbon–carbon (C–C) composites are ideal for use as aerospace vehicle structural materials; however, they lack high‐temperature oxidation resistance requiring environmental barrier coatings for application. Ultra high‐temperature ceramics (UHTCs) form oxides that inhibit oxygen diffusion at high temperature are candidate thermal protection system materials at temperatures >1600°C. Oxidation protection for C–C composites can be achieved by duplicating the self‐generating oxide chemistry of bulk UHTCs formed by a “composite effect” upon oxidation of ZrB₂–SiC composite fillers. Dynamic Nonequilibrium Thermogravimetric Analysis (DNE‐TGA) is used to evaluate oxidation in situ mass changes, isothermally at 1600°C. Pure SiC‐based fillers are ineffective at protecting C–C from oxidation, whereas ZrB₂–SiC filled C–C composites retain up to 90% initial mass. B₂O₃ in SiO₂ scale reduces initial viscosity of self‐generating coating, allowing oxide layer to spread across C–C surface, forming a protective oxide layer. Formation of a ZrO₂–SiO₂ glass‐ceramic coating on C–C composite is believed to be responsible for enhanced oxidation protection. The glass‐ceramic coating compares to bulk monolithic ZrB₂–SiC ceramic oxide scale formed during DNE‐TGA where a comparable glass‐ceramic chemistry and surface layer forms, limiting oxygen diffusion.     

Determination of silicon carbide fiber electrical resistivity at elevated temperature

The resistance of Sylramic‐iBN and Hi‐Nicalon SiC fiber tows was measured at elevated temperature in air. Resistivity could not be directly measured, since the fibers passed through a furnace with varying temperature along the length. The resistivity of the isothermal section of the fiber tow was modeled by a series circuit of finite elements. Existing data for Hi‐Nicalon resistivity vs temperature was used to verify the model and then extend it to Sylramic‐iBN, for which there is no literature data readily available. The model matched experimental values with low overall error (<±14%). Fiber resistivity decreased by more than two orders of magnitude when heating from 25°C to 1400°C. Sylramic‐iBN tow resistance was also measured during a 500 hour hold at 1315°C. The resistance increased by more than 140% during heat treatment. The resistance change correlated well with the decrease in SiC fiber diameter that resulted from oxidation.     

Effect of activation on boron nitride coating on carbon fiber

Boron nitride (BN) thin coating has been formed on the surface of chemically activated polyacrylonitrile (PAN) carbon fibers by dip coating method. The chemical activation of PAN fibers was carried out by two different chemicals, i.e. nitric acid (HNO₃) and silver nitrate (AgNO₃) solution. The chemical activation changes the surface properties, e.g. surface area and surface microstructure of the carbon fibers. These surface modifications ultimately influence properties of boron nitride coating on carbon fibers. The boron nitride coating on carbon fibers showed better crystallinity, strength and oxidation resistance when carbon fibers were activated by HNO₃. This improvement in strength and oxidation resistance is attributed to better crystallinity of boron nitride coating on HNO₃ activated PAN fibers.     

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.     

In situ Y2Si2O7 coatings on Hi-Nicalon-S SiC fibers: Phase formation and fiber strength

Y₂Si₂O₇ coatings were formed on Hi-Nicalon-S SiC fibers by reaction of solution-derived YPO₄ coatings with glass SiO₂ scales formed by fiber oxidation. Two oxidation methods were used: pre-oxidation, where fibers were oxidized prior to YPO₄ coating, or post-oxidation, where fibers were first coated with YPO₄ and then oxidized. Fibers with YPO₄/SiO₂ films were heat-treated in argon at 1200°C for 20 hours to react YPO₄ and SiO₂ to Y₂Si₂O₇. The effects of SiO₂ to YPO₄ film thicknesses on fiber strength and on the Y₂Si₂O₇formation kinetics were investigated. An optimized process to obtain single-phase continuous Y₂Si₂O₇ coatings on Hi-Nicalon-S fibers with low loss in fiber strength is suggested.     

Fatigue of three advanced SiC/SiC ceramic matrix composites at 1200°C in air and in steam

High‐temperature mechanical properties and tension‐tension fatigue behavior of three advanced SiC/SiC composites are discussed. The effects of steam on high‐temperature fatigue performance of the ceramic‐matrix composites are evaluated. The three composites consist of a SiC matrix reinforced with laminated, woven SiC (Hi‐Nicalon?) fibers. Composite 1 was processed by chemical vapor infiltration (CVI) of SiC into the Hi‐Nicalon? fiber preforms coated with boron nitride (BN) fiber coating. Composite 2 had an oxidation inhibited matrix consisting of alternating layers of silicon carbide and boron carbide and was also processed by CVI. Fiber preforms had pyrolytic carbon fiber coating with boron carbon overlay applied. Composite 3 had a melt‐infiltrated (MI) matrix consolidated by combining CVI‐SiC with SiC particulate slurry and molten silicon infiltration. Fiber preforms had a CVI BN fiber coating applied. Tensile stress‐strain behavior of the three composites was investigated and the tensile properties measured at 1200°C. Tension‐tension fatigue behavior was studied for fatigue stresses ranging from 80 to 160 MPa in air and from 60 to 140 MPa in steam. Fatigue run‐out was defined as 2 × 10⁵ cycles. Presence of steam significantly degraded the fatigue performance of the CVI SiC/SiC composite 1 and of the MI SiC/SiC composite 3, but had little influence on the fatigue performance of the SiC/SiC composite 2 with the oxidation inhibited matrix. The retained tensile properties of all specimens that achieved fatigue run‐out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated.     

SiC Depletion in ZrB2–30 vol% SiC at Ultrahigh Temperatures

The formation of a porous SiC‐depleted region in ZrB₂–SiC due to active oxidation at ultrahigh temperatures was characterized. The presence/absence of SiC depletion was determined at a series of temperatures (1300°C–1800°C) and times (5 min–100 h). At T < 1627°C, SiC depletion was not observed. Instead, the formation of a ZrO₂ + C/borosilicate oxidation product layer sequence was observed above the ZrB₂–SiC base material. At T ≥ 1627°C, SiC was depleted in the ZrB₂ matrix below the ZrO₂ and borosilicate oxidation products. The SiC depletion was attributed to active oxidation of SiC to form SiO(g). The transition between C formation in ZrO₂ (T < 1627°C) and SiC depletion in ZrB₂ (T ≥ 1627°C) is attributed to variation in the temperature dependence of thermodynamically favored product assemblage influenced by the local microstructural phase distribution. The growth kinetics of the SiC depletion region is consistent with a gas‐phase diffusion‐controlled process.     

Oxidation Behavior of Hot Pressed ZrB2‐ZrC‐SiC Ceramic Composites

Dense ZrB₂‐SiC ceramics containing 40 vol% ZrC particles are fabricated via hot pressing method. Then the sintered ceramics are oxidized in air up to 1500°C, and the oxidation kinetics of the ceramic composites is deduced in combination with the reacted fraction curves. As indicated by the experimental results, the oxidation kinetics changes from reaction‐controlled process to diffusion‐controlled one with increasing of oxidation temperature. In addition, the oxidation kinetics parameters are obtained, which indicates that the oxidation resistance decays at elevated temperatures. Furthermore, the evolution of surface morphology and oxide scale during oxidation process is clarified.     

Surface structures of sodium borosilicate glasses from molecular dynamics simulations

Surface plays an important role in the physical and chemical properties of oxide glasses and controls the interactions of these glasses with the environment, thus dominating properties such as the chemical durability and bioactivity. The surface atomic structures of a series of sodium borosilicate glasses were studied using classical molecular dynamics simulations with recently developed compositional dependent partial charge potentials. The surface structural features and defect speciation were characterized and compared with the bulk glasses with the same composition. Our simulation results show that the borosilicate glass surfaces have significantly different chemical compositions and structures as compared to the bulk. The glass surfaces are found to be sodium enriched and behave like borosilicate glasses with higher R (Na₂O/B₂O₃) values. As a result of this composition and associated structure changes, the amount of fourfold boron decreases at the surface and the network connectivity on the surface decreases. In addition to composition variation and local structure environment change, defects such as two‐membered rings and three‐coordinated silicon were also observed on the surface. These unusual surface composition and structure features are expected to significantly impact the chemical and physical properties and the interactions with the environments of sodium borosilicate glasses.     

Initial Stages of ZrB2–30 vol% SiC Oxidation at 1500°C

The initial oxidation behavior of ZrB₂–30 vol% SiC was analyzed with the goal of understanding any relationship to the variable oxidation performance observed at longer times. A box furnace was used to oxidize samples for times as short as 10 s and up to 100 min at 1500°C in air. The samples were characterized using mass change, scanning electron microscopy, energy dispersive spectroscopy, X‐ray diffraction, and X‐ray photoelectron spectroscopy to explore the oxidation behavior. The presence of borosilicate glass and ZrO₂ was observed on the surface at times as early as 10 s. Bubble formation in the borosilicate glass was observed after 30 s of oxidation and is attributed to uneven distribution of the glass. The impact of surface roughness on oxidation was also explored and found to be negligible for times greater than 30 s.     

Microstructural characterization of ZrB2–SiC based UHTC tested in the MESOX plasma facility

Microstructures were investigated for ZrB₂–SiC and ZrB₂–HfB₂–SiC ultra high temperature ceramics that were subjected to a high temperature plasma environment. Both materials were tested in the MESOX facility to determine the recombination coefficient for atomic oxygen up to 1750 °C in subsonic air plasma flow. Surfaces were analyzed before and after testing to gain a deeper insight of the surface catalytic properties of these materials. Microstructural analyses highlighted oxidation induced surface modification. Oxide layers were composed of silica with trace amounts of boron oxide and zirconia if the maximum temperature was lower than about 1550 °C and zirconia for higher temperatures. The differences in the oxide layer composition may account for the different catalytic behavior. In particular, the presence of a borosilicate glass layer on the surface of ZrB₂–SiC materials guarantees atomic oxygen recombination coefficients that are relatively lower than the coefficients measured when only zirconia is present. The oxidation processes of ZrB₂–HfB₂–SiC materials, associated with catalytic tests carried out up to 1550 °C, lead to the formation of hafnia as well as silica, and zirconia. The higher recombination coefficients measured in the case of ZrB₂–HfB₂–SiC materials can be correlated with the presence of hafnia which is probably characterized by higher catalytic activity compared to zirconia. In any case, the investigated materials demonstrate a low catalytic activity over the inspected temperature range with maximum values of recombination coefficients close to 0.1.