Influence of fibre content on the strength of carbon fibre reinforced HfC/SiC composites up to 2100 °C

The influence of carbon fibre content on the mechanical behaviour of HfC/SiC composites was investigated up to 2100 °C for specimens containing 40 or 55 vol% fibres. Silicon carbide was added as a sintering aid during hot pressing. Increasing the fibre content made infiltration more difficult, which resulted in higher porosity in the specimen with 55 vol% fibres. The room temperature flexural strength ranged from 340 to 380 MPa, and it increased to more than 400 MPa at 1800 °C due to stress relaxation. Increasing temperature was accompanied by a decrease in the slope of the load-displacement curve, indicating a decrease in elastic modulus, but plastic deformation was not observed below 2100 °C. At 2100 °C, the specimen containing a higher fibre content underwent significant deformation due to low interfacial strength between the fibre plies, retaining a strength at the proportional limit of 290 MPa and an ultimate strength of 520 MPa.     

Oxidation and cracking/spallation resistance of ZrB2–SiC–TaSi2–Si coating on siliconized graphite at 1500 °C in air

A ZrB₂–SiC–TaSi₂–Si coating on siliconized graphite substrate was prepared by a combination process of slurry brushing and vapor silicon infiltration. The high-temperature oxidation behavior and cracking/spallation resistance of the as-prepared coating were investigated in detail. It was revealed that the oxidation kinetics at 1500 °C in static air followed a parabolic law with a relatively low oxidation rate constant down to 0.27 mg/(cm²·h^0.5). The crack area ratio of the as-prepared coating was determined as 3.8 × 10⁻³ after severe thermal cycling from 1500 °C to room temperature for 20 times. Apart from the formation of ZrO₂ as skeleton phase with SiO₂ as infilling species, the good oxidation and cracking/spallation resistance of the coating also could be attributed to its unique duplex-layered structure, i.e., a dense ZrB₂–SiC–TaSi₂ major layer filled with Si and an outermost Si cladding top layer. Meanwhile, the strong adhesion strength of the SiC transition layer with the graphite substrate and the outer ZrB₂–SiC–TaSi₂–Si layer was a vital factor as well.     

The effect of a graphite addition on oxidation of ZrB2–SiC in air at 1500 °C

Dense ZrB₂ containing 15 vol.% SiC and 15 vol.% graphite was exposed to flowing air at 1500 °C. A layered scale structure developed that consisted of (1) a uniform SiO₂-rich layer on the surface, (2) a layer of ZrO₂ and SiO₂, (3) a layer of ZrO₂ (4) a partially oxidized layer composed of porous ZrB₂, ZrO₂, and graphite, and (5) unaffected ZrB₂–SiC–C. A thermodynamic model based on volatility diagrams and consistent with the experimental observations was constructed to explain the development of the layered structure. Oxidation behavior was consistent with passive oxidation and formation of a protective surface layer. Analysis indicated that it may not be possible to form a protective surface layer without actively oxidizing SiC and producing a porous partially oxidized layer between the outer protective layer and the underlying unoxidized material.     

Mechanical behaviour of carbon fibre reinforced TaC/SiC and ZrC/SiC composites up to 2100°C

The microstructure and elevated temperature mechanical properties of continuous carbon fibre reinforced ZrC and TaC composites were investigated. Silicon carbide was added to both compositions to aid sintering during hot pressing. Fibres were homogeneously distributed and no fibre degradation was observed at the interface with the ceramic matrix even after testing at 2100 °C. The flexural strength increased from 260 to 300 MPa at room temperature to ∼450 MPa at 1500 °C, which was attributed to stress relaxation. At 1800 °C, the strength decreased to ∼410 MPa for both samples. At 2100 °C plastic deformation resulted in lower strength at the proportional limit (210–320 MPa), but relatively high ultimate strength (370–440 MPa). The sample containing ZrC had a lower ultimate strength, but higher failure strain at 2100 °C due to the weak fibre/matrix interface that resulted in fibre-dominated composite behaviour.     

Critical oxidation behavior of Ta-containing ZrB2 composites in the 1500–1650 °C temperature range

The microstructure evolution of ZrB₂ hot pressed with 15 vol% TaSi₂ was studied in the as-sintered state and after oxidation for 15 min at 1500 and 1650 °C in stagnant air. In the pristine material, the original ZrB₂ nuclei are surrounded by a mixed (Zr,Ta)B₂ solid solution. Refractory Ta-compounds are located at triple junctions and wetted grain boundaries are distinctive of this ceramic. After oxidation, the solid solution evolves into ZrO₂ grains encasing intragranular nano-structured TaB₂ particles. Here we show that the operating limit temperature of this composite is related to the critical oxidation of TaB₂ to Ta₂O₅ above 1600 °C, accompanied by large volume expansion and local liquid formation, which ruptures the ZrO₂ grains and structure.     

In situ studies of oxidation of ZrB2 and ZrB2–SiC composites at high temperatures

High temperature oxidation of ZrB₂ and the effect of SiC on controlling the oxidation of ZrB₂ in ZrB₂–SiC composites were studied in situ, in air, using X-ray diffraction. Oxidation was studied by quantitatively analyzing the crystalline phase changes in the samples, both non-isothermally, as a function of temperature, up to ～1650 °C, as well as isothermally, as a function of time, at ～1300 °C. During the non-isothermal studies, the formation and transformation of intermediate crystalline phases of ZrO₂ were also observed. The change in SiC content, during isothermal oxidation studies of ZrB₂–SiC composites, was similar in the examined temperature range, regardless of sample microstructure and composition. Higher SiC content, however, markedly retarded the oxidation rate of the ZrB₂ phase in the composites. A novel approach to quantify the extent of oxidation by estimating the thickness of the oxidation layer formed during oxidation of ZrB₂ and ZrB₂–SiC composites, based on fractional conversion of ZrB₂ to ZrO₂ in situ, is presented.     

Long-term oxidation and ablation behavior of Cf/ZrB2–SiC composites at 1500, 2000, and 2200°C

Although C_f/ZrB₂–SiC composites prepared via direct ink writing combined with low-temperature hot-pressing were shown to exhibit high relative density, high preparation efficiency, and excellent flexural strength and fracture toughness in our previous work, their oxidation and ablation resistance at high and ultrahigh temperatures had not been investigated. In this work, the oxidation and ablation resistance of C_f/ZrB₂–SiC composites were evaluated via static oxidation at high temperature (1500°C) and oxyacetylene ablation at ultrahigh temperatures (2080 and 2270°C), respectively. The thickness of the oxide layer of the C_f/ZrB₂–SiC composites is <40 μm after oxidizing at 1500°C for 1 h. The C_f/ZrB₂–SiC composites exhibit non-ablative properties after oxyacetylene ablation at 2080 and 2270°C for >600 s, with mass ablation rates of 3.77 × 10⁻³ and 5.53 × 10⁻³ mg/(cm² s), and linear ablation rates of −4.5 × 10⁻⁴ and −5.8 × 10⁻⁴ mm/s, respectively. Upon an increase in the ablation temperature from 2080 to 2270°C, the thickness of the total oxide layer increases from 360 to 570 μm, and the carbon fibers remain intact in the unaffected region. Moreover, the oxidation and ablation process of C_f/ZrB₂–SiC at various temperatures was analyzed and discussed.     

A ZrB2–SiC/SiC oxidation protective dual-layer coating for carbon/carbon composites

In order to improve the oxidation resistance of C/C composites, a ZrB₂–SiC/SiC oxidation protective dual-layer coating was prepared by a pack cementation combined with the slurry paste method. The phase and microstructure of the coating were characterised by X-ray diffraction, scanning electron microscope and energy-dispersive spectrometer analyses. The anti-oxidation and thermal shock resistance of the coating were also investigated. It was found that the ZrB₂–SiC/SiC coating could effectively improve the oxidation resistance of the C/C composites. The weight loss of the coated samples was only 1.8% after oxidation at 1773?K for 18?h in air. The coating endured 20 thermal shock cycles between 1773?K and room temperature with only 4.6% weight loss.     

Creep behavior and failure mechanisms of CVI and PIP SiC/SiC composites at temperatures to 1650 °C in air

Creep properties of 2D woven CVI and PIP SiC/SiC composites with Sylramic™-iBN SiC fibers were measured at temperatures to 1650 °C in air and the data was compared with the literature. Batch-to-batch variations in the tensile and creep properties, and thermal treatment effects on creep, creep parameters, damage mechanisms, and failure modes for these composites were studied. Under the test conditions, the CVI SiC/SiC composites exhibited both matrix and fiber-dominated creep depending on stress, whereas the PIP SiC/SiC composites displayed only fiber-dominated creep. Creep durability in both composite systems is controlled by the most creep resistant phase as well as oxidation of the fibers via cracking matrix. Specimen-to- specimen variations in porosity and stress raisers caused significant differences in creep behavior and durability. The Larson-Miller parameter and Monkman-Grant relationship were used wherever applicable for analyzing and predicting creep durability.     

The effect of YB4 addition in ZrB2-SiC composites on the mechanical properties and oxidation performance tested up to 2000 °C

The influence of YB₄ and Y₂O₃ on densification, mechanical properties and oxidation performance of ZrB₂-SiC (ZS) composite was studied. The oxidation tests were performed in static air up to temperature of 1650 °C for 1 h as well as under dynamic conditions of oxyacetylene torch at 2000 °C. Static oxidation of ZS led to the formation of protective silica-based glass on the surface. However, ablation tests showed absence of silica in ablation centre. Only dense zirconia layer was left on the top of ZS. Composites with Y-containing additives exhibited significantly inferior oxidation performance in static conditions, since severe spallation and deeper degradation of the material were observed. On the contrary, the depth of material degradation after ablation was comparable with ZS. Samples were covered by solid solution of zirconia and yttria. Due to very low vapor pressure, yttria-based oxidation products are of interest considering even higher application temperatures exceeding 2000 °C.     

Influence of SiAlON addition on the microstructure development of hot-pressed ZrB2–SiC composites

The impact of SiAlON on densification behavior and microstructure of the ZrB₂-SiC composite was investigated. ZrB₂, SiC, and SiAlON were used as the initial materials to produce ZrB₂-SiC composite by hot pressing at 1900 °C. A fully dense composite was obtained having ~99.9% relative density. High-resolution X-ray diffraction (HRXRD) assessment verified the in-situ formation of ZrC, and the presence of residual carbon, SiAlON, and ZrB₂ and SiC phases in the as-sintered ceramic. Furthermore, the thermodynamic calculations confirmed the results attained by HRXRD. In addition, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized for the microstructural investigation. SEM fractographs indicated the impact of SiAlON on the hindering of grain growth and the formation of flaky phases (graphitized carbon or solidified liquid phase) at the grain boundaries. TEM studies revealed the presence of a transparent glassy phase at the particle interfaces. A significant impact of liquid phase sintering was also affirmed in the clean interfaces.     

Kinetics and evolution of oxide scale during various stages of isothermal oxidation at 1300 °C in spark plasma sintered ZrB2 – SiC – LaB6 composites

The kinetics and oxide scale evolution during isothermal exposure of spark-plasma sintered ZrB₂-20 SiC–LaB₆ (7, 10 or 14 vol%) composites at 1300 °C for 1, 8, or 24 h have been examined. Random mass change observed during the first hour stems from a non-protective scale. The variation of mass gain with time is expressed by near-parabolic rate law during 0−8 h time period, and by relations indicating slower kinetics during 8−24 h, with parabolic rate constants (k_p) decreasing sharply. Microstructural examination has shown a continuously evolving layered oxide scale comprising La₂Si₂O₇, borosilicate glass (BSG), ZrSiO₄, and ZrO₂, where the BSG layer growth rate scales linearly with LaB₆ content during 0−8 h, but shows an opposite trend on further exposure. During both time periods (0−8 and 8−24 h), k_p decreased following a linear relationship with increasing BSG layer thickness, indicating its key role as diffusion barrier for oxygen.     

On the oxidation behavior of ZrB2–SiC–VC composites

In this study, near-fully dense ZrB₂–SiC–VC (75-20-5 vol%) composite was manufactured through hot pressing at 1850°C under the pressure of 40 MPa for 60 min. Then the oxidation examination of the composite was carried out under different durations and temperatures. The microstructure and phase evolution after hot pressing and oxidation processes were examined by scanning electron microscopy, and X-ray diffractometry. The VC addition led to the formation of ZrC and VSi₂ phases, which assisted the densification of the composite by removing ZrO₂ from the particles’ surface. The oxides of ZrO₂, SiO₂, ZrSiO₄, V₂O₅, and VO₂ formed distinct layers on the sample during the oxidation at 1700°C for 4 h with a parabolic regimen and activation energy of 177.5 kJ/mol.     

Ablation behavior of ZrB2–SiC protective coating for carbon/carbon composites

Ablation behavior of ZrB₂–SiC protective coating for carbon/carbon composites during oxyacetylene flame test at 2500 °C was investigated by analyzing the microstructure differentiation caused by the increasing intensity of ablation from the border to the center of the surface. After ablation, a continuous SiO₂ scale, a porous SiO₂ layer inlaid with fine ZrO₂ nuclei, and a continuous ZrO₂ scale respectively emerged in the border region, the transitional region, and the center region. In order to investigate the ablation microstructure in the initial stage, the sub-layer microstructure was characterized and found to be mainly formed by coral-like structures of ZrO₂, which showed huge difference with the continuous structure of ZrO₂ on the surface layer. A kinetic model concerning the thickness change induced by volatilization and oxidation during ablation was built to explain the different growth mechanisms of the continuous ZrO₂ scale and the coral-like ZrO₂ structure.     

Tensile properties of two-dimensional carbon fiber reinforced silicon carbide composites at temperatures up to 2300 °C

Carbon fiber reinforced silicon carbide (C/SiC) composites are of the few most promising materials for ultra-high-temperature structural applications. However, the existing studies are mainly conducted at room and moderate temperatures. In this work, the tensile properties of a two-dimensional plain-weave C/SiC composite are studied up to 2300 °C in inert atmosphere for the first time. The study shows that C/SiC composite firstly shows linear deformation behavior and then strong nonlinear characteristics at room temperature. The nonlinear deformation behavior rapidly reduces with temperature. The Young’s modulus increases up to 1000 °C and then decreases as temperature increases. The tensile strength increases up to 1000 °C firstly, followed by reduction to 1400 °C, then increases again to 1800 °C, and lastly decreases with increasing temperature. The failure mechanisms being responsible for the mechanical behavior are gained through macro and micro analysis. The results are useful for the applications of C/SiC composites in the thermal structure engineering.     

Isothermal oxidation behavior of YAl3C3 at 900–1300 °C in air

The isothermal oxidation behavior of YAl₃C₃ in air has been investigated at 900−1300 °C for 20 h. At 900 and 1000 °C, the oxidation kinetic curves of YAl₃C₃ obey parabolic rate law. The oxide scales have a bilayer structure. The outer layer is composed of YAG and amorphous Al₂O₃, and the inner layer is a carbon-rich layer. The oxidation kinetic curves of YAl₃C₃ obey linear law above 1100 °C. The oxide scales have a monolayer structure composed of Al₂O₃ and YAG. The crystallization transformation of amorphous Al₂O₃ causes the oxidation kinetic curve changing from a parabolic to a straight line.     

A SiC–Si–ZrB2 multiphase oxidation protective ceramic coating for SiC-coated carbon/carbon composites

In order to improve the oxidation protective ability of SiC-coated carbon/carbon (C/C) composites, a SiC–Si–ZrB₂ multiphase ceramic coating was prepared on the surface of SiC-coated C/C composite by the process of pack cementation. The microstructures of the coating were characterized using X-ray diffraction and scanning electron microscopy. The coating was found to be composed of SiC, Si and ZrB₂. The oxidation resistance of the coated specimens was investigated at 1773 K. The results show that the SiC–Si–ZrB₂ can protect C/C against oxidation at 1773 K for more than 386 h. The excellent oxidation protective performance is attributed to the integrity and stability of SiO₂ glass improved by the formation of ZrSiO₄ phase during oxidation. The coated specimens were given thermal shocks between 1773 K and room temperature for 20 times. After thermal shocks, the residual flexural strength of the coated C/C composites was decreased by 16.3%.     

Microstructure of friction surface developed on carbon fibre reinforced carbon–silicon carbide (Cf/C–SiC)

We have used TEM to study the microstructure of friction surface of carbon fibre/carbon–silicon carbide composites brake discs after multi braking stop by using organic pads. A friction surface layer was developed consistently on the top of Si regions of the composites, but inconsistently on that of SiC and C. Inside the layer, amorphous silicon/silicon oxides appeared extensively with various non-metallic and metallic crystallites dispersed inside with sizes ranging from a few nanometers to several microns. A coherent interface between the friction layer and the composite surface was established under the braking conditions, whilst its sustainability varied notably in SiC and C regions. Microcracking near the friction surface appeared in SiC and C_f/C regions largely due to the extensive ductile deformation of SiC and weak interfaces between C and C_f. Material joining mechanisms were discussed to enlighten the friction transfer layer development on the surface of the composite discs.     

Effect of oxidation treatment on KD–II SiC fiber–reinforced SiC composites

Continuous silicon carbide fiber–reinforced silicon carbide matrix (SiC_f/SiC) composites have developed into a promising candidate for structural materials for high–temperature applications in aerospace engine systems. This is due to their advantageous properties, such as low density, high hardness and strength, and excellent high temperature and oxidation resistance. In this study, SiC_f/SiC composites were fabricated via polymer infiltration and pyrolysis (PIP) with the lower–oxygen–content KD–II SiC fiber as the reinforcement; a mixture of 2,4,6,8–tetravinyl–2,4,6,8–tetramethylcyclotetrasiloxane (V4) and liquid polycarbosilane (LPCS), known as LPVCS, was used as the precursor; while pyrolytic carbon (PyC) was used as the interface. The effects of oxidation treatment at different temperatures on morphology, structure, composition, and mechanical properties of the KD–II SiC fibers, SiC matrix from LPVCS precursor conversion, and SiC_f/SiC composites were comprehensively investigated. The results revealed that the oxidation treatment greatly impacted the mechanical properties of the SiC fiber, thereby significantly influencing the mechanical properties of the SiC_f/SiC composite. After oxidation at 1300 °C for 1 h, the strength retention rates of the fiber and composite were 41% and 49%, respectively. In terms of the phase structure, oxidation treatment had little effect on the SiC fiber, while greatly influencing the SiC matrix. A weak peak corresponding to silica (SiO₂) appeared after high–temperature treatment of the fiber; however, oxidation treatment of the matrix led to the appearance of a very strong diffraction peak that corresponds to SiO₂. The analysis of the morphology and composition indicated cracking of the fiber surface after oxidation treatment, which was increasingly obvious with the increase in the oxidation treatment temperature. The elemental composition of the fiber surface changed significantly, with drastically decreased carbon element content and sharply increased oxygen element content.     

Effect of oxidation on the bending fatigue behavior of an advanced SiC/SiC CMC component at 1000 °C in air

The present study elucidates the effect of oxidation during static and fatigue loading in SiC/SiC CMC structured component, which shows damage in the stress-concentrated region. It is made of Tyranno SA3 fiber, BN (Boron nitride) interphase, and CVI (chemical vapor infiltration) + PIP (polymer impregnation and pyrolysis) hybrid matrix. The comparison based on strength and fracture morphology was made. After annealing, the as-received sample showed minute oxidation and slightly enhanced strength. The fatigued sample without annealing under low stress showed higher retained strength than the as-received sample due to smooth debonding. The fatigued sample with annealing under high stress showed a loss in strength than the as-received sample owing to the formation of a significant amount of borosilicates glasses, which further promoted SiO₂ formation between fiber and matrix and caused the brittle failure. However, simultaneous filling borosilicate glasses into the pores oppositely aided in maintaining the retained strength.