Single-source-precursor synthesis and air-plasma ablation behavior of(Ti,Zr,Hf)C/SiC ceramic nanocomposites at 2200℃

Dense monolithic(Ti,Zr,Hf)C/SiC ceramic nanocomposites with four different molar ratios of metallic elements in the(Ti,Zr,Hf)C phase(i.e.,Ti:Zr:Hf=1:1:1,2:3:5,2:3:3,and 1:2:1)were prepared upon pyrolysis of novel(Ti,Zr,Hf)-containing single-source precursors(SSPs),followed by spark plasma sintering(SPS).A thorough characterization was conducted to elucidate the synthesis of the SSPs,polymer-to-ceramic transformation,chemical/phase compositions,and microstructure of the SiTiZrHfC-based ceramics.The results revealed the feasibility of synthesizing nanocomposites with high(Ti,Zr,Hf)C contents using the SSP method.These nanocomposites were characterized by a unique microstructure with in situ generated(Ti,Zr,Hf)C@C core-shell nanoparticles homogeneously mixed withβ-SiC.The ablation behavior of the nanocomposites was evaluated on an air-plasma device for 60 s.Impressively,the nanocomposites exhibited excellent ablation resistance,and the lowest linear ablation rate reached−0.58μm/s at 2200°C.Notably,the ablation resistance can be dramatically improved by precisely tailoring the atomic ratios of metal elements within the(Ti,Zr,Hf)C phase via the molecular design of the SSPs.The formation of a multiple-oxide layer with both a high-meltingpoint phase((Ti,Zr,Hf)O_(2))and low-melting-point phases((Zr,Hf)TiO_(4))and glassy SiO_(2),as well as their structure,played a critical role in the enhanced ablation resistance.The uniform distribution of the high-melting-point(Ti,Zr,Hf)O_(2)nano/microparticles throughout the glassy SiO_(2)matrix significantly enhanced the viscosity and stability of the oxide layer by the pinning effect,offering superior protection against the ingress of oxygen atoms and excellent resistance to mechanical erosion.

Effects of air plasma flame on the ZrB_(2)-based UHTC coatings:Microstructure,phase evolution and ablation resistance

As for the air plasma sprayed ZrB_(2)-based coatings,B content change caused by inevitable oxidation is predictable but commonly ignored.Affected by air plasma flame,the B element loss and residual B_(2) O_(3) in the sprayed ZrB_(2) coating were observed.Moreover,how the B content change affects the microstructure,phase evolution,and ablation resistance(2.4 MW/m^(2),60 s)of ZrB_(2)-based coatings with different sec-ondary phases(SiC,MoSi_(2),and TaC)was investigated.The B element loss contributed to the increase in surface temperature and the decline in the sintering degree of the ZrO_(2) layer.The evaporation of residual B_(2)O_(3) caused damage to the coating structure in the form of pores,whose negative effect was enhanced and reduced by MoSi_(2) and TaC secondary phases,respectively.This work will provide some insight into thermally sprayed non-oxide ceramic coatings in the atmosphere.

Sublayer design and ablation resistance of CVD-TaC alternate coatings with different crystallite morphologies for C/C composites

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.

Two birds with one stone:Construction of bifunctional-POSS hybridized boron-silicon ceramicized phenolic composites and its ablation behavior

To further enhance the ablation resistance properties of thermosetting phenolic resin matrix composites,in this work,bifunctional alkalic heptaphenyltrisilanol sodium salt polyhedral oligomeric silsesquioxane was utilized to catalyze the synthesis of boron phenolic resin and provide silicon source to obtain boron-silicon hybrid phenolic resin(BPOSSPR)with excellent ablative resistance.BPOSSPR possesses a low cur-ing activation energy(101.4 kJ/mol)and excellent thermal properties(initial decomposition temperature was 453.0℃and char yield at 1000℃was 72.7%).The mechanical and thermal insulation properties of carbon fiber reinforced BPOSSPR composites(CF/BPOSSPR)and high silica fiber reinforced BPOSSPR composites(HSF/BPOSSPR)are significantly enhanced.The linear ablation rate and mass ablation rate of CF/BPOSSPR are as low as 0.003 mm/s and 0.0354 g/s;those of HSF/BPOSSPR are 0.119 mm/s and 0.0264 g/s.The ablation-resistance mechanism of BPOSSPR composites is mainly due to the formation of ceramic thermal barrier layers under high temperature conditions,such as B_(2)O_(3),SiO_(2),borosilicate glass,SiC,which might play an effective role in protecting against heat flow erosion.As a result of these ex-cellent properties,the innovative heat shielding BPOSSPR composites could offer the ability to tolerate harsher environment in future aerospace applications.

Effect of Ni content on high power laser ablation behavior of coatings sprayed by Ni covering graphite/SiO2 powders

Faced with the challenge of high energy ablation problems, especially for laser ablation, effective energy dissipation protective materials fabricate by efficient preparation method is a feasible solution. The Ni-graphite/Si O2 coatings with different Ni content were prepared by plasma spraying method with optimized plasma spraying parameters. All coatings are pure without oxidation and dense. Their ablation behaviors were investigated by high power continuous wave laser. The results indicate that the Ni-graphite/Si O2 coating with appropriate Ni content could realize the purpose of energy consumption by endothermal reaction of graphite/Si O2 and reflection improvement. High Ni content will block the occurrence of endothermal reaction of graphite/Si O2 and increase the heat diffusion to interior part of coating, which can make the ablation situation of coating more serious.

Ablative Property of C/C–SiC–HfC Composites Prepared via Precursor Infiltration and Pyrolysis under 3,000 °C Oxyacetylene Torch

C/C–SiC–HfC composites were fabricated via precursor infiltration and pyrolysis using a mixture solution of organic hafnium-containing polymer and polycarbosilane as precursor. The microstructures and the phases of the composites were analyzed by scanning electron microscopy and X-ray diffraction. The ablation resistance of the composites was evaluated under 3,000 °C oxyacetylene torch. After ablation for 120 s, the composites exhibit good ablation properties with the linear and mass ablation rates of 9.1 9 10-4mm/s and 1.30 9 10-3g/s, which are far lower than those of the C/C–SiC composites. The excellent ablative property of the C/C–SiC–HfC composites is resulted from the formation of HfO2 molten layer on the surface of the composites, which could play a positive role in reducing heat transfer and preventing oxygen transport to the underlying carbon substrate.