The protective effect of Yb2SiO5/Si and Yb2Si2O7/Si coatings on CVD-SiC coated C/C composites in high temperature wet oxygen environments was investigated by first-principle calculation and experiment, respectively. The calculation results reveal that the Yttrium silicate (Yb2SiO5, YbMS) phase exhibits stronger bonding energy compared to the yttrium disilicate (Yb2Si2O7, YbDS) phase. Moreover, the Si-O bond within the YbMS structure is more stable under similar water vapor corrosion conditions. Consequently, the YbMS structure contributes to the formation of a thinner thermally grown oxide (TGO) and diffusion reaction layer (DRL) at the interface between the Si bond coat and YbMS topcoat under wet oxygen conditions at 1773 K. However, the higher coefficient of thermal expansion (CTE) of the YbMS coating resulted in the formation of larger penetration cracks in YbMS-coated C/C samples during cooling. After 20 h of oxidation in a water vapor environment at 1773 K, the weight change of the YbMS-coated sample was -4.374%, while the YbDS coated sample was -0.610%.

The effect of transition metal on oxidation process of ZrB2 was studied by Density Functional Theory (DFT) and experimental techniques. The energy barriers of O atom diffusion on Hf-doped surface and migration inward are 0.60 eV and 7.67 eV, which are all higher than other surfaces. This is due to electronic structure's regulation of ZrB2. After oxidation for 8 h, the mass gain of (Zr, Hf) B2 is 11.34 %, which is lower than other ceramics. It indicates that (Zr, Hf) B2 has excellent oxidation resistance. The thickness of oxidation layers of (Zr, Hf) B2 is lower than that of ZrB2.

To compare the influence of crystallite morphology on the synergistic effect between different sublayers under oxyacetylene flame (O2/CH4 flux ratio: ~1.33), (TaC/SiC)3 alternate coatings were prepared on C/C composites by chemical vapor deposition via triple cycles of TaC/SiC sequences, whose TaC sublayers were composed of acicular (AS-6) and columnar crystals (CS-6) respectively. After the cyclic ablation tests with a heat flux of 2.48 MW/m2, the CS-6 sample exhibited favorable thermal shock resistance through layer-by-layer protection than coating exfoliation of the AS-6 specimen. Their SiC layers also performed different ablative behaviors, which transformed from acute evaporation of SiO2 glass for the SiC single layer to the eroded surface with pits distributed of the AS-6 specimen, and further the integrated scale with glass covered of the CS-6 sample. The stress relaxation resulting from the formation of porous middle layer and the enhanced reinforcement of Ta2O5 particles for SiO2 glass to improve evaporation suppression may be the main reasons for the superior synergistic effect of the CS-6 sample.

In this study, the oxidation processes of SiC coatings with different crystal structures were systematically investigated by experimental and density functional theory (DFT) calculations. The SiC coatings were prepared on carbon/carbon (C/C) composites through pack cementation (PC) and chemical vapor deposition (CVD), respectively. The SiC coating prepared by PC method has superior oxidation resistance to that prepared by CVD. It was ascribed to a strong adhesion of PC-coating and substrate. And, the addition of Al, which improves the strength of Si-O bonding, inhibiting the diffusion of O in the SiO2. Additionally, it is found that α-SiC, prepared by PC, has better intrinsic oxidation resistance than β-SiC. The reason is that α-SiC has high adsorption energy of the O atom on the C-terminated surface and low adsorption energy of the O atom on the Si-terminated surface. Moreover, the diffusion of O on the surface of α-SiC at 1773 K was more difficult due to the more stable Si-O bonding. These findings demonstrate that the oxidation resistance of α-SiC is superior to β-SiC and can help guide the structural design of the high-temperature coatings.

In this paper, we have successfully addressed the challenges of cracking and spalling in HfB2 coatings during ablation through the novel synthesis of (Hf, Zr)B2 solid solution coatings using the chemical vapor deposition (CVD) method. The ablation resistance of the coating exhibited an initially improving trend followed by a subsequently deterioration as the Zr content increased. When the Hf/Zr molar ratio in the (Hf, Zr)B2 solid solution coating was 1:1, a thicker Hf-Zr-O amorphous oxide layer was formed during ablation, which acted as a barrier, preventing the inward diffusion of oxygen. As a result, the (Hf0.5Zr0.5)B2 solid solution coating exhibited superior ablation resistance with a mass ablation rate of only -0.216 mg/s and a linear rate of -0.089 mu m/s. These findings underscore the enormous potential of boride solid solution UHTC as exceptional candidates for ultra -high temperature ablation resistant applications.

Tri -layer environmental barrier coatings (EBCs) were developed to provide complete encapsulation for carbon/ carbon (C/C) composites, consisting of SiC inner layer, Si bond layer, and mixed Yb2Si2O7-Yb2SiO5 topcoats with varying mass ratios. The protective properties of the EBCs were assessed by exposing the coated samples to a corrosive gas mixture of 50 % O2 and 50 % H2O at 1500 degrees C, the mass change, phase composition, and microstructure evolution were investigated. The results showed that a small amount of Yb2SiO5 doping could improve water vapor corrosion resistance, but the excessive content of Yb2SiO5 led to a mismatch of the coefficient of thermal expansion (CTE) with inner coating, resulting in coating cracking. The mixed topcoat with 60 wt% Yb2Si2O7 and 40 wt% Yb2SiO5 showed superior water vapor corrosion resistance. After corrosion for 20 h at 1500 degrees C, no penetrating cracks of large size appeared, the thickness of thermal growth oxide (TGO, 4.6 +/- 0.4 mu m) was relatively lower, and the C/C substrate was effectively protected from damage.

In this work, TaC/SiC alternate coatings with different sublayer numbers were fabricated on C/C composites by chemical vapor deposition, whose ablation resistance and defect tolerance under oxyacetylene torch can be improved by structural optimization. The two-layer sample with less layer number presented coating spallation owing to the incomplete relaxation of thermal stress, and the six-layer specimen exhibited mechanical denu-dation of TaC outer layer induced by its thinner thickness, which both indicate their inferior ablative perfor-mances. However, the toughness elevation of alternate structure, the synergistic effect of SiO2-Ta2O5 compound oxide and the sublayer reliability of TaC for anti-scouring as well as SiC for oxygen-barrier all result in the better ablation resistance of the four-layer specimen. These results can provide reference values for the structural design of high-temperature applications.

In order to make carbon/carbon composites suitable for application in gas turbine engine, it is necessary to develop environmental barrier coatings (EBCs) to protect them from reacting with water vapor. In our previous work, a novel high-entropy rare-earth disilicate (Lu0.2Yb0.2Er0.2Tm0.2Sc0.2)2Si2O7 ((5RE0.2)2Si2O7) has been developed and verified as a promising candidate for EBCs. In this work, the (5RE0.2)2Si2O7 coating was syn-thesized on the surface of SiC coated C/C composites by supersonic atmospheric plasma spraying method. The protective performance and mechanism of this coating under high temperature water vapor environment was explored in detail. Results showed that the weight change of the sample coated with (5RE0.2)2Si2O7 was only 0.2% after corrosion for 100 h at 1500 oC, which proved that (5RE0.2)2Si2O7 coating could significantly improve the resistance of C/C composites against water vapor corrosion. This work may provide theoretical basis for the design and application of high-entropy rare-earth silicates as EBCs.

Tantalum carbon(TaC)alternate coatings with sublayers comprised of different crystallite morphologies were prepared on carbon/carbon composites by chemical vapor deposition.Their ablative behaviors and defending mechanisms were both investigated.The specimen with the sublayer composed of columnar crystals exhibited a better ablation resistance due to the toughness enhancement induced by the lami-nated structure.However,the mechanical denudation of the sample only containing acicular crystals and the coating spallation caused by superfluous gaseous products of the sample with the sublayer composed of nanocrystals both indicate their inferior anti-ablation properties.It is believed that the results will be helpful for the structural design and practical application of chemical vapor deposition(CVD)alternate coatings.

A thermodynamic calculation on the HfB2 coating prepared by chemical vapor deposition (CVD) through HfCl4-BCl3-H-2-Ar system was performed, together with the relevant verification experiments. The calculation results indicated that HfB2 coating could be obtained above 900 degrees C with the ratios of BCl3/HfCl4 and H-2/HfCl4 higher than 1 and 12, respectively. The experimental results demonstrated that the deposition temperature, H-2 and BCl3 flow rates had significant effects on the grain size, growth rate and phase composition of HfB2 coatings. A dense and uniform HfB2 coating was prepared at 1150 degrees C with a BCl3/HfCl4 ratio of 3 and a H-2/HfCl4 ratio of 20, whose mass and linear ablation rates were 15.61 mg/s and 15.58 mu m/s under oxyacetylene flame.

Three types of TaC coatings with different crystallite morphologies were successfully fabricated on carbon/ carbon composites by chemical vapor deposition, and their ablation performance was investigated. The TaC coating with acicular crystal exhibited a more favorable ablation resistance as certified by the compact surface morphology and lower ablation rates due to the formation of a stable oxide layer. As for others, the powdering of TaC coating with nano crystal induced by gas evaporation of acute oxidation, and the exfoliation of TaC coating with columnar crystal caused by grain overgrowth all resulted in their weak protection for C/C substrates during ablation.

To improve fracture toughness and oxidation resistance of SiC coating, we designed and prepared a SiC/PyC laminated coating with biomimetic laminated structure on carbon/carbon (C/C) composites by chemical vapor deposition (CVD) method. The microstructure, mechanical properties and oxidation resistance of the coating were investigated. The results showed that the laminated structure could effectively improve the toughness of SiC coating and avoid the formation of penetrating cracks, thus improved the oxidation resistance of coating. The weight loss of laminated SiC/PyC coated sample decreased 75.2 % compared with that of single SiC coated sample after oxidation at 1773 K. The good oxidation resistance and mechanical properties of SiC/PyC laminated coating were attributed to the toughening effect of laminated structure including crack deflection and interface debonding. Our work could bring beneficial enlightenment to improve the toughness of ceramic coating.

TaC coatings with four kinds of crystal structures were prepared on carbon/carbon composites by chemical vapor deposition. The surface morphologies, microstructure transformation, and ablative behaviors of TaC coated C/C specimens were investigated. Results show that the coatings with acicular crystal structure exhibit better ablation resistance with lower ablation rates, especially the needle-piled sample. The bistratal structure composed of dense glass layer and compact TaC layer is beneficial for maintaining the integrity of acicular samples after ablation. But the formation of lamella grains and porous middle layer is the main factor for the damage and even exfoliation of the columnar specimens.

SiC ceramic coating, for prevention of C/C composites against oxidation, was prepared by pressure-less reactive sintering to investigate the oxidation behaviour in an oxidising environment containing water vapour at 1773 K. The experimental results demonstrated that the oxidation behaviour of porous SiC ceramics could be divided into two stages, following the parabolic model, which was attributed to the variation in the contact area involved in the oxidation reactions. During the entire oxidation process, water vapour could accelerate the oxidation of the SiC ceramics, according to the weight change. By first-principle calculations, the accelerated oxidation rate of the SiC ceramics was attributed to weakened Si?O and Al?O bonds in the formed glassy scale, which were caused by hydroxide radicals from the water. Atomic thermal motions at high temperature could lead to the breakage of the network structure, promoting the diffusion and solution of oxidising gases. When the as-prepared SiC ceramics were applied as anti-oxidative coatings for the C/C composites, the SiC ceramic coating and C/C matrix could be sealed and protected faster per unit time, because water vapour was beneficial to the formation of a glassy layer. The weight loss of the C/C matrix could be attributed to unsealed microcracks inside the SiC coating in the initial stage.

Transition metal carbide, especially one-dimensional (1D) hafnium carbide (HfC), have attracted increasing attention from researchers of ultra-high temperature field. Herein, we used chemical vapor deposition method to fabricate 1D HfC including HfC nanowries (HfCNWs) and whiskers (HfCWs) in carbon/carbon (C/C) composites to construct three dimensional HfCNWs-C/C and HfCWs-C/C networks composites. Results showed that the HfCNWs and HfCWs were radially and evenly distributed on the surface of carbon fibers. The in situ grown HfCNWs and HfCWs decreased cracks in pyrocarbon matrix. Benefiting from the unique chemical and physical properties of 1D HfC, the HfCNWs-C/C composites exhibit improved thermal conductivity and ablation-resistance.

SiC ceramics and SiC ceramic coatings on carbon/carbon (C/C) composites were prepared by chemical vapor deposition (CVD) method to investigate their oxidation behaviors in wet oxygen at 1773 K. The experimental results demonstrated that water vapor could increase the oxidation rate of SiC, and promote the crystallization of SiO2. By the Raman and infrared spectroscopic analyses, the existence of Si-OH vibration modes in the oxide scales indicated that the reaction between water vapor and silica could occur at high temperatures. Besides, first-principle molecular dynamics (FPMD) simulations and the static calculations based on density functional theory (DFT) also confirmed the occurrence of the reaction and illustrated that the reaction was caused by the co-effect of O and H atoms from the water, resulting in the breakage of the Si-O bonds in silica. After oxidation for 30 h, the weight loss of CVD-SiC coated C/C samples in wet oxygen reduced to 3.47%, which was ascribed to that the formation of abundant oxide scale acted as an effective oxidation barrier by healing cracks on SiC coatings. However, the crystallization and structural variation of the oxide scale might be harmful to the long-term oxidation in the condition of high-speed gaseous scour.

First-principle calculations were employed to investigate the initial oxidation behaviors of CVD-SiC in wet oxygen. The H2O/O-2 co-adsorption behaviors with different H2O/O-2 molecular ratios and coverages on (3 x 3) 3C-SiC (111) surface were studied by geometry optimization and property calculation. The results showed that the O-O bond in Si-O=O structure had higher bond energy than that in Si-O-O-H structure, indicating the weaker bond energy of O-O bond in the formed hydroperoxyl. First-principle molecular dynamic (FPMD) simulations confirmed that the formation of hydroperoxyl as an intermediate was feasible during the dissociation of molecular oxygen, which played an important role in O-2 activation. The static calculation results indicated that the formation of hydroperoxyl needed to experience a lower activation energy barrier (0.35 eV), which demonstrated more likely occurrence than the direct dissociation of molecular oxygen. In brief, the introduction of molecular water could enhance the oxidation rate of 3C-SiC in initial stage by the formation of hydroperoxyl. This work revealed the initial oxidation mechanism of 3C-SiC (111) in wet oxygen and the role of water in initial oxidation, which could also provide a new view to explain the accelerated oxidation rate of SiC in wet oxygen.

The oxidation behaviors of carbon/carbon composites prepared by chemical vapor infiltration method in wet oxygen were investigated by experiments and first-principle calculations. The results showed that the graphite components in mixed structure were easily destroyed and transformed into amorphous carbon in wet oxygen, accelerating the weight loss of C/C composites. The reason was ascribed to that hydroxyl radicals promoted the surface and inward diffusion of O atoms by uneven distribution of charges and formation of an exothermic reaction. Besides, the H transfer as a feasible way could result in the gather of epoxy radicals, increasing the risk of cracking.

Hafnium carbide nanowires (HfCnws) were in-situ grown in carbon/carbon (C/C) composites, and subsquently the preforms were densified by isothermal chemical vapor infiltration to obtain HfCnws modified carbon/carbon (HfCnws-C/C) composites. Morphology and microstructure of HfCnws were examined, and the effect of HfCnws on the mechanical property and ablation resistance of C/C composites were also investigated. Results show that introducing HfCnws refined the grain size of pyrolytic carbon (PyC). The out-of-plane compression, interlaminar shear and flexual strength of HfCnws-C/C composites increased by 120.80%, 45.60% and 94.65%, respectively compared with pure C/C, and the HfCnws-C/C shows good ablation resistance under oxy-acetylene flame ablation.

To improve the ablation resistance of carbon/carbon (C/C) composites at temperature about 2200 K, a ZrB2-SiC-TiSi2 ultra-high temperature ceramic coating was prepared by supersonic atmosphere plasma spraying on SiC coated C/C composites. The ZrB2-SiC-TiSi2 coating could protect C/C composites for more than 240 s under heat flux of 2400 kW/m(2). The maximum surface temperature of the sample can reach to 2230 K. In addition, the mass and linear ablation rates of the coated samples after ablation for 240 s are only (0.314 +/- 0.065) x 10(-3) g/s and (0.221 +/- 0.026) x 10(-3) mm/s, respectively. The ZrB2-SiC-TiSi2 coating was converted into three layers after ablation: an outer layer, a particle-stacking layer, and a molten filled layer. The production of ZrTiO4, acting as sintering additive, could promote the sintering of ZrO2 and fill the pores, which effectively improved the ablation resistance of ZrB2-SiC-TiSi2 coating. (C) 2020 Elsevier B.V. All rights reserved.