Oxidation/ablation behaviors of hafnium carbide-silicon carbonitride systems at 1500 and 2500 C

The oxide scales of hafnium carbide (HfC) typically exhibit a porous structure after oxidation/ablation due to the release of gas oxidation products, which allows oxygen penetration to promote the rapid oxidation of the HfC matrices. Here, we report that the oxidation/ablation resistance of HfC was enhanced by the incorporation of amorphous silicon carbonitride (SiCN). HfC-SiCN ceramics with 10 vol % SiCN showed a significant improvement in the oxidation/ablation resistance compared with pure HfC. The HfC-10 vol % SiCN ceramic has a higher density with good mechanical properties. After being oxidized at 1500 °C for 2 h, a dense and homogeneous HfO₂-HfSiO₄ layer with low oxygen permeability is formed. The ablation resistance of the HfC-10 vol % SiCN ceramic is improved due to the formation of the triple-layer structure oxide with good thermal stability and mechanical scouring resistance. After ablation under an oxyacetylene flame for 60 s, the mass and linear ablation rates of HfC-10 vol % SiCN ceramic are −0.019 mg cm⁻² s⁻¹ and -0.156 μm s⁻¹, respectively.     

Ablation behavior of a novel HfC-SiC gradient coating fabricated by a facile one-step chemical vapor co-deposition

To protect carbon/carbon composites against ablation at ultra-high temperature, a novel HfC-SiC gradient coating was fabricated by a facile one-step chemical vapor co-deposition. The phase composition, microstructure, bonding strength and ablation behaviour were investigated, and the mechanical properties of ablated coatings were characterized as well. The bonding strength of HfC-SiC gradient coating is 19.6?±?0.5?N (176% higher than that of HfC coating). HfC-SiC gradient coating shows excellent ablation resistance under oxyacetylene flame. The mass and linear ablation rate of HfC-SiC coating were only 0.153?±?0.02?mg·s^?1 and ?0.998?±?0.08?μm·s^?1, respectively. After ablation for 60?s, the hardness and elastic modulus of ablated HfC-SiC gradient coating are higher than those of ablated HfC coating. The excellent ablation resistance of HfC-SiC gradient coating results from its high bonding strength and the adhesion effect of Hf-Si-O sticky glass phase.     

Preparation of carbon/carbon‐ultra high temperature ceramics composites with ultra high temperature ceramics coating

In order to increase the oxidation resistance of carbon/carbon (C/C) composites at long‐term high temperature, C/C‐Ultra High Temperature Ceramics composites (UHTCs) with a dual‐layer UHTCs oxidation coating was successfully designed and fabricated. The microstructure and ablation resistance were investigated and discussed. After ablation in arc‐heated wind tunnel with temperature being 2200°C for 1000s, the mass ablation rate and linear ablation rate were ?1.9 × 10^?2 mg/cm²s and 2.9 × 10^?5 mm/s, respectively. The formation of thermodynamically compatible oxide scale including ZrO₂ skeleton and SiO₂ or Zr–Si–O glass on the surface were mainly contributed to the excellent ablation resistance of the composite.     

Strong and tough HfC-HfB2 solid-solution composites obtained by reactive sintering with a SiB6 additive

HfC-HfB₂ composite ceramics were successfully reactively spark plasma sintered with a unique SiB₆ additive. The incorporation of the SiB₆ not only promotes the densification of HfC (up to ~99%), but also significantly enhances the toughness from 4.3 ± 0.5 MPa m^1/2 to 14.2 ± 1.4 MPa m^1/2. The flexural strength of the HfC-HfB₂ composite ceramics was simultaneously improved to 529 ± 48 MPa, which is about 1.4 times higher than that of HfC. This improvement is attributed to the dense composite microstructure comprising an in-situ formed HfB₂ and a solid solution of Si and O in HfC and HfB₂ grains.     

Preparation,Mechanical Properties and Cyclic Oxidation Behavior of the Nanostructured NiCrCoAlY-TiB2 Coating

TiB₂-Metal composite coatings with excellent oxidation resistance become ideal candidates using at high temperature ranging from 600 to 1000?°C. In order to maintain both the superior mechanical properties and oxidation resistance in severe working conditions, the nanostructured NiCrCoAlY-TiB₂ coating was fabricated by the activated combustion high velocity air-fuel spraying (AC-HVAF) with the composite powders prepared by ball milling and plasma spheroidization. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the phase constituents and microstructure. It was found that the coating with nanostructured TiB₂ particles uniformly distributed in the NiCrCoAlY matrix has the same structure as the composite powders. The coating shows excellent mechanical properties, such as high microhardness (991.43 HV_0.3), good fracture toughness (4.12?MPa?m^?1/2) and large bonding strength (75.43?MPa), and excellent oxidation resistance with low weight gain (1.56?×?10^?6 mg² cm^?4 s^?1). The cyclic oxidation behavior is in accordance with the parabolic law.     

Oxidation and ablation behaviour of multiphase ultra-high-temperature ceramic Ta0·5Zr0·5B2–Si–SiC protective coating for graphite

To provide reliable oxidation protection for carbon materials under harsh high-temperature aerobic environments, a dense monolayer-multiphase ultra-high-temperature ceramic Ta_0·5Zr_0·5B₂–Si–SiC (TZSS) coating was fabricated by a combination of dipping and in-situ reaction. The oxidation resistance of the TZSS coating was investigated at 1923 K in air. The results indicated that the TZSS coating could offer at least 70 h of oxidation protection for the matrix material. The synergistic oxygen-blocking effect of the thick oxide layer formed during the oxidation test and the inner coating, played a key role in the oxidation protection process. These were responsible for the excellent oxidation resistance ability of the TZSS coating. Additionally, the ablation performance of the TZSS coating was also investigated under increased heat flux from 2.4 MW/m² to 4.2 MW/m². The ablation behaviours changed from the oxidation and evaporation of coating materials to mechanical scouring, corresponding to increased mass and linear ablation rates. Interestingly, after ablation for 40 s under a heat flux of 4.2 MW/m², a new microstructure composed of “lath-like” Ta₄Zr₁₁O₃₂ solid solution grains was found in the ablation center. This oxide layer possessed few micropores, which could provide reliable protection for the matrix material under ultra-high-temperature oxygen-containing airflow erosion, thus preventing further damage to the composite.     

Microstructure,thermophysical properties,and ablation resistance of C/HfC-ZrC-SiC composites

Carbon/carbon composites modified by HfC-ZrC-SiC were fabricated by reactive melt infiltration with the aim of improving their ablation resistance for application in aerothermal environments. Their microstructure, thermophysical and ablation properties were investigated. Results show that the thermal diffusivity decreases with increasing temperature for all composites. The thermal conductivity of the C/HfC-ZrC-SiC composites decreasing with increasing HfC molar fraction is related to decreased grain size and increased porosity, which impede phonon interaction and increase the phonon scattering. High HfC content effectively improves the oxidation and ablation resistance of the composites. C/HfC-ZrC-SiC composites containing 8.8?mol.% HfC exhibited the best ablation resistance owing to a compact and continuous HfO₂-ZrO₂ mixed layer that formed on the ablated surface.     

Deposition and ablation resistance of HfC-based coatings prepared on SiC-coated C/C composites by supersonic atmospheric plasma spraying

In order to improve the ablation properties of C/C composites, HfC-based coatings with different mass ratios of SiC were deposited on the surface of SiC-coated carbon/carbon composites by supersonic atmospheric plasma spraying. The morphologies and microstructures of the HfC-based coatings were characterised. The ablation resistance test was carried out by oxyacetylene torch. The results show that the as-prepared coatings are multiphase coatings consisting of HfC, HfO₂, SiC and SiO₂. The structure of different coatings is dense. After ablation for 60?s, the ablation centre region of coating is smooth without obvious microcrack and pinhole, and no interlaminar crack can be observed at the cross-section. An Hf–Si–O compound oxide layer is generated on the surface of coating, which is beneficial for protecting the C/C composites from being ablated. Meanwhile, the further generated HfSiO₄ can play a pinning effect, which can prevent crack extension.     

New anti-ablation candidate for carbon/carbon composites: Preparation,composition and ablation behavior of Y2Hf2O7 coating under an oxyacetylene torch

Y₂Hf₂O₇ possesses low thermal conductivity and high melting point, which make it promising for a new anti-ablation material. For evaluating the thermal stability and the potential applications of Y₂Hf₂O₇ on anti-ablation protection of C/C composites, Y₂Hf₂O₇ ceramic powder was synthesized by solution combustion method and Y₂Hf₂O₇ coating was prepared on the surface of SiC coated C/C composites using SAPS. Results shown that the coating exhibits good ablation resistance under the heat flux of 2.4?MW/m² with the linear and mass ablation rates are 0.16?μm?s^?1 and ?0.028?mg?s^?1, respectively, after ablation for 40?s. With the prolonging of the ablation time, the increasing thermal stress causes the increase of cracks. Moreover, the chemical erosion from SiO₂ and the physical volatilization of low temperature molten products aggravate failure of the Y₂Hf₂O₇ coating.     

Multiphase composite Hf0.8Ti0·2B2–SiC–Si coating providing oxidation and ablation protection for graphite under different high temperature oxygen-containing environments

A novel multiphase composite coating composed of Hf_0.8Ti_0·2B₂ solid solution, SiC and Si was prepared by a joint procedure of slurry method and silicon reactive infiltration (SRI). The oxidation and ablation experiments were conducted to investigate oxidation and ablation resistance of the Hf_0.8Ti_0·2B₂–SiC–Si coated graphite samples, respectively. The results revealed that the coated sample was oxidized at 1823 K for 108 h with a mass gain of 1.49%, which was ascribed to the high viscosity oxide layer improved by HfSiO₄ and TiO₂ in conjunction with dense structure of the coating, thereby presenting excellent high temperature stability. Furthermore, after 90 s ablation at 3273 K under a heat flux of 5.62 MW/m², the composite coating was not peeled off, which had mass ablation rate (MAR) and linear ablation rate (LAR) of 3.1 mg/s and 1.5 μm/s, respectively. The refractory oxide layer comprising oxides of Hf and Ti on the surface acted as an oxygen barrier, which can weaken the mechanical erosion force of oxyacetylene flame, finally protecting the inner coating and graphite matrix from further consumption.     

C/C–SiC composite fabricated via PCS–Si slurry reactive melt infiltration

The slurry reactive melt infiltration (RMI) method was used to overcome the shortcomings of the traditional RMI method utilized in preparing the large-irregular shaped C/C preform. C/C–SiC composite was successfully fabricated via PCS–Si slurry RMI. Results show that it has a favorable physical bonding between the PCS–Si slurry and the C/C preform. After RMI, most of the Si in the PCS–Si slurry coating infiltrated into the C/C preform, the density of the C/C preform increased from 1.24 to 1.52g cm⁻³, and the open porosity decreased from 27.22% to 18.04%. SiC was formed on the surface as well as in the pores of the C/C preform. The as-received C/C–SiC composite showed a pseudo-ductile fracture behavior with a flexural strength of 76.4MPa. The mass ablation rate of the C/C–SiC composite is 0.34 mg s⁻¹, exhibiting much better ablation resistance than the C/C preform with a mass ablation rate of 1.80 mg s⁻¹.     

Ablation resistance of HfC–SiC coating prepared by supersonic atmospheric plasma spraying for SiC-coated C/C composites

In order to improve the ablation properties of carbon/carbon composites, HfC–SiC coating was deposited on the surface of SiC-coated C/C composites by supersonic atmospheric plasma spraying. The morphology and microstructure of HfC–SiC coating were characterized by SEM and XRD. The ablation resistance test was carried out by oxyacetylene torch. The results show that the structure of coating is dense and the as-prepared HfC–SiC coating can protect the C/C composites against ablation. After ablation for 30 s, the linear ablation rate and mass ablation rate of the coating are −0.44 μm/s and 0.18 mg/s, respectively. In the ablation center region, a Hf–Si–O compound oxide layer is generated on the surface of HfC–SiC coating, which is conducive to protecting the C/C composites from ablation. With the ablation time increasing to 60 s, the linear ablation rate and mass ablation rate are changed to −0.38 μm/s and 0.26 mg/s, respectively. Meanwhile, the thickness of the outer Hf–Si–O compound layer also increases.     

A dense and fine-grained SiC/Ti3Si(Al)C2 composite and its high-temperature oxidation behavior

A dense SiC/Ti₃Si(Al)C₂ composite was synthesized by in situ hot pressing powders of Si, TiC and Al as a sintering additive at 1500 °C for 2 h under 30 MPa in Ar atmosphere. This composite has a fine-grained and homogeneous microstructure with grain sizes of 5 μm for Ti₃Si(Al)C₂ and of 1 μm for SiC. The SiC/Ti₃Si(Al)C₂ composite possesses an improved oxidation resistance, with parabolic rate constants of 4.57 × 10^?8 kg²/m⁴/s at 1200 °C and 1.31 × 10^?7 kg²/m⁴/s at 1300 °C. This study provides an experimental evidence to confirm the formation of amorphous phases in the oxide scale of the SiC/Ti₃Si(Al)C₂ composite. Microstructure and phase composition of the SiC/Ti₃Si(Al)C₂ composite and oxide scales were identified by X-ray diffractometry and scanning electron microscopy. The mechanism for the enhanced oxidation resistance has been discussed.     

Ultra-high-temperature ablation behavior of SiC–ZrC–TiC modified carbon/carbon composites fabricated via reactive melt infiltration

To improve the ablation resistance under the ultra-high temperature, the matrix of the carbon/carbon (C/C) composite was modified with a ternary ceramic of SiC–ZrC–TiC via reactive melt infiltration. The obtained ceramic matrix was composed of Zr-rich and Ti-rich solid solution phases of Zr_1−xTi_xC and SiC. This composite exhibited an excellent ablation property at 2500 °C with low mass and linear ablation rates of 0.008 mg s⁻¹ cm⁻² and 0.000 μm s⁻¹, respectively. The ablation mechanism was revealed with various microstructure characterizations and compared with those of C/C–SiC and C/C–TiC composites. Results showed that the degradations of these composites were primarily caused by the loss of the protective oxide scale via volatilization under the ultra-high temperature and flushing by high-speed airflow. The high ablation resistance of the C/C–SiC–ZrC–TiC composite was attributed to the protection of a multiphase oxide scale with high viscosity and low volatility.     

High-performance B4C matrix composites fabricated from the B4C–Si–CeO2 system via reactive hot pressing

Boron carbide (B₄C) matrix composites had the advantages of high hardness, high melting point and low density. However, due to the low relative density and poor fracture toughness of B₄C, its comprehensive properties were limited in engineering applications. In this work, in order to improve the comprehensive properties of B₄C composites, B₄C–SiC–SiB₆–CeB₆ composites were designed and fabricated via reactive hot pressing at 2050 °C and 20 MPa with B₄C matrix and novel additives (Double doping of Si and CeO₂) as raw materials. The effects of additive CeO₂ content on the microstructures and mechanical properties of composite were investigated, and reaction mechanisms of B₄C, Si and CeO₂ at different temperatures were studied in detail. The work showed that liquid phase Si and SiB₆ greatly improved the densification of composites. CeB₆ played an indispensable role in the formation of SiC–SiB₆ agglomerate structure, increasing strength and supplementing toughness. When the content of CeO₂ was 6 wt%, the relative density, hardness, flexural strength and fracture toughness reached to 99.7%, 34.9 GPa, 461.46 MPa and 5.57 MPa m^1/2, respectively. Our strategy benefited from the formation of two liquid phases and SiC–SiB₆ agglomerate structure, showing great potential in promoting sintering and improving fracture toughness.     

Improving the corrosion resistance of a-C:H film by a top a-C:H:Si:O layer

During the deposition of a-C:H film, defects (pinholes or discontinuities) caused by excessive stress will inevitably appear, which will reduce the corrosion resistance of the a-C:H film. In this study, top a-C:H:Si:O layers (thickness of approximately 0.3 μm) on the surface of a-C:H films were deposited on a large scale by PACVD technology using acetylene (C₂H₂) and/or hexamethyldisiloxane (HMDSO) as reactants, to improve the corrosion resistance of a-C:H films while ensuring the appropriate overall hardness of the films. The corrosion behaviors of the films were studied by electrochemical impedance spectroscopy (EIS) and Tafel polarization. We found that the a-C:H/a-C:H:Si:O films possess a lower electrolyte penetration rate due to their stronger capacitance characteristics. In addition, the corrosion current density of the a-C:H/a-C:H:Si:O films (10^?10 A cm^?2) were reduced by 2 orders of magnitude compared to the a-C:H film (10^?8 A cm^?2), and by 3 orders of magnitude compared to 316 stainless steel (10^?7 A cm^?2). The impedance results obtained by EIS were simulated using appropriate equivalent circuits, and the corresponding electrical parameters were used to further verify the electrochemical protection behavior of the top a-C:H:Si:O layer.     

The influence of zirconia fibre on ablative composite materials

ABSTRACT

Zirconia fibres have excellent high temperature ablation resistance and have been widely used in ablative materials. In this paper, zirconia fibre was used for reinforcing the ablative composite materials to study the influence of zirconia fibre had upon the mechanical properties and the high temperature ablation properties of such composites. The results showed that the bending strength of the material was also good and reached a maximum of 13.05?MPa. After sintering at 1400°C, the bending strength was also great which could reach 13.05?MPa. In addition, the corrosion resistance of the composites was excellent and the oxygen-acetylene line ablation rate was 0.03?mm?s^?1 when the fibre content was 30?wt-%.     

Scale characterisation of an oxidised (Hf,Ti)C-SiC ultra-high temperature ceramic matrix composite

A hybrid carbide ultra-high temperature ceramics matrix [(Hf,Ti)C-SiC] reinforced with BN-coated carbon fibres was fabricated and tested for surface oxidation resistance. The UHTC composite showed an average mass ablation rate of 0.0014 g/s after exposure to a high heat flux (～17 MW/cm²) oxyacetylene flame test for 30 s above 2500 °C. The cross-sectional profile of the oxides scale formed was characterised and analysed. The scale was multicomponent; consisting of oxides of Hf, Ti and Si, as well as HfTiO₄ and HfSiO₄, which underwent phase separation and immiscibility. Multiple glassy bubbles formed on the scale surface due to the impediment of escaping gases by the glassy layer on the outer scale. The largest pores in the scale and surface bubbles that resisted rupture were the dominant features of the outermost phase-separated layer. Phase separation in the scale top layer improves the resistance to scale rupture.     

Effect of MoSi2 addition on ablation behavior of ZrC coating fabricated by vacuum plasma spray

To improve the ablation resistance of ZrC coating, MoSi₂ incorporated ZrC composite coatings were fabricated by vacuum plasma spray. The ablation resistance of the composite coatings was evaluated using a plasma jet with a heat flux of 1.94?MW/m². The phase compositions and microstructures of the coatings before and after ablation were investigated, and the ablation mechanisms and effect of MoSi₂ were analyzed based on thermal dynamics and microstructure changes. Results showed that MoSi₂ addition could improve the ablation resistance of ZrC coating by means of decreasing the surface temperature and changing the microstructure of the oxidation layer. Si derived from the decomposition of MoSi₂, which occurred within coating, was beneficial to maintain the thickness and integrity of the SiO₂ layer and reduce the oxygen pressure beneath. The thickness of the SiO₂ layer was related to the formation rate (V_f) and the consumption rate (V_c) of SiO₂. The diffusion of Si was in favor of increasing the value of V_f. MoSi₂ could be one choice to improve the ablation resistance of the ZrC coating.