Preparation, oxidation and ablation resistance of IrAl intermetallic coating

Iridium (Ir) has been selected as the protective coating on the rhenium thruster chamber of the liquid rocket engine, due to its high melting point, excellent corrosion resistance and quite low oxygen permeability. However, Ir forms gaseous oxides rather than a protective oxide barrier above 1100°C under oxidizing environments, leading to a limited lifetime at high temperatures. To improve the oxidation resistance, in the present work pure Ir was modified by pack cementation to produce a single phase IrAl intermetallic coating. The bond strength of the coating was examined by coating-pull-off test. The oxidation and ablation resistance was assessed by cyclic oxidation test at 1800°C and high frequency plasma wind tunnel test (heat flux: 2.03MW/m2 and enthalpy: 19MJ/kg), respectively. It was found that the IrAl coating is well boned to the substrate with a bond strength above 30MPa. The oxidation and ablation resistance of the Ir was significantly enhanced after the pack cementation treatment (see Figure 1). The improvement in oxidation and ablation resistance can be ascribed to the excellent comprehensive properties of the in-situ formed Al2O3 barrier and outstanding physical and chemical compatibility among the phases in the multilayer coating system. Please click Additional Files below to see the full abstract

Feasibility research of gaining “refractory high entropy carbides” through in situ carburization of refractory high entropy alloys

Abstract: High entropy alloys (HEAs) refer to solid solution alloys which contain five or more principal elements in equal or near equal atomic percent. Due to their unique structures, HEAs have superior comprehensive properties compared with the conventional alloys based on only one element. The property improvement based on the effect of high entropy may works on the refractory carbides used in ultra-high temperature ceramics. Therefore, a solid carburization method was employed on a refractory high entropy alloy of HfZrTiTa to prepare in situ the possible “Refractory High Entropy Carbides”. The microstructure, micro-hardness and oxidation resistance of the carburized layer obtained at 900 ℃ for 10 h were investigated. It can be concluded that the carburized layer, ~120 μm thick in total, had a double layer structure. The Ti-depleted outer layer had an average hardness of ~1200 HV, while the inner layer, with evenly distributed and equimolar elements, had a maximum hardness of ~1590 HV. Although the final phase identification by TEM is under way, we believe that the carburized layer is composed of uniformly distributed carbides based on the results of hardness and element distribution. The substrate closely adjacent to the carburized layer had a higher hardness of ~725 HV compared with the HfZrTiTa alloy (~500 HV), due to the formation of Zr and Hf-rich Needle-like phases. The cyclic oxidation test showed that the carburization treatment on HfZrTiTa can improve its oxidation resistance to some extent. Please click Additional Files below to see the full abstract

Laser Ablation Applications in Ablation-Resistance Characterization of Materials

Owing to the rapid heating and large power intensity, the laser beams were successfully used to characterize the ablation-resistant performance of materials, which provided us more knowledge about the usability of materials in the ablation environment and developing protection against laser irradiation. In this chapter, we comparatively introduced some experimental methods for ablation-resistance characterization of materials. The fundamentals of laser-material interactions were discussed from the physical and chemical aspects to help understand the laser ablation mechanism. Finally, we presented some practical applications of laser ablation in ablation-resistance characterization of ultra-high-temperature ceramics (UHTCs) and ceramic matrix composites and discussed the laser ablation behavior and mechanism

Novel Ir-X thermal protection coatings designed for extreme aerodynamic heating environment

Due to the rapid evaporation of SiO2 protective layer, most Si-containing oxidation resistant coatings could not withstand a temperature above 1800℃, which is not enough for hypersonic voyage in upper atmosphere. With a higher melting point (2440℃) and lower oxygen permeability（10-20g·m-1·s-1）, iridium is supposed to be a promising coating material for ultra-high temperature applications. However, Iridium has a low emissivity ε(0.017 for 2.5-25μm) and high recombination coefficient γ(0.64 at 1200℃) of atomic oxygen, resulting in a much higher thermal response compared with the ceramic materials under the same aerodynamic environment. To solve this problem, elements such as Al, Cr, Zr etc. were selected to modify pure Ir to form Ir-X (X=Al, Cr or Zr) coating. The modification element X in Ir-X coating forms high emissivity and low recombination coeffcient oxide on Ir, which meanwhile prevents the Ir from atomic oxygen. It was found that Ir-Al, Ir-Cr, Ir-Ti, Ir-Zr, Ir-Ta and Ir-Hf diffusion coating could be prepared via pack cementation. The recombination coefficient and emissivity of as-oxidized Ir-Al were changed to 0.0089 and 0.723, respectively. Please click Additional Files below to see the full abstract

Effect of infiltration time on the microstructure and mechanical properties of C/C-SiC composite prepared by Si‐Zr10 alloyed melt infiltration

Low cost C/C-SiC composites were prepared through reactive melt infiltration with Si-Zr10 alloy infiltrant under different infiltration time. Effect of infiltration time on the microstructure and mechanical properties of the composite were investigated. ZrC tended to be formed in the composite and the amount of carbon phase decreased with an extension in the infiltration time according to the X-ray diffraction results. Phase transformation of the C/C-SiC composite was analyzed based on C-Si-Zr phase diagram. Flexural strength of the composite prepared by preform 0.9 g/cm³ decreased with an increase in the infiltration time while that of the composite prepared by preform 1.38 g/cm³ increased initially and then decreased reversely. The highest flexural strength of the composite was found at about 324 MPa. Flexural strength of the composite is considered to depend on its phase composition and fiber-matrix interface.This work was supported by National Natural Science Foundation of China (51302315), Innovation Foundation for Excellent Postgraduate of National University of Defense Technology and Hunan Provincial Innovation Foundation for Postgraduate. Yonggang Tong also thanks the support from China Scholarship Council

Microstructure，mechanical property and oxidation behavior of HfZrTiTaBx HEAs

The unique structural and thermal features of high-entropy alloys (HEAs) conduce to their excellent stability and mechanical properties. Recent researches have suggested that the high-entropy alloys composed of refractory metals exhibit competitive phase-stability and strength at elevated temperatures, which made them the promising candidate materials for high-temperature structural applications at even higher temperatures compared with the Ni-based superalloys. However, the alloys barely consisting of refractory metal elements are usually oxidized easily in oxidizing environment at high temperatures. This work aims to prepare a refractory HEA with both excellent mechanical properties and outstanding oxidation resistance by alloying of B element. In this study, an equimolar quaternary HfZrTiTa alloy and three kinds of HfZrTiTaBx(x=1.1, 2.3, 4.7) alloys with different amounts of B-addition were produced by vacuum arc melting technique in argon atmosphere. The structures of the prepared alloys were characterized via X-Ray diffraction and TEM. The oxidation behaviors of these alloys were investigated by differential scanning calorimeter (DSC)from 25℃ to 1300℃ in air. Their mechanical properties at room temperature and phase-stability at different annealing temperatures from 800℃ to 1600℃ were also examined. The results show that the HfZrTiTa alloy consists of a fully disordered body-centered cubic (BCC) solid solution phase due to the high mixing entropy, while the alloys with B addition have some nano particles uniformly distributed in the BCC solid solution matrix. The lattice parameters and Vicker hardness of the B-containing alloys increase with increasing B content due to the interstitial solid solution strengthening of B element and nanoprecipitation strengthening. The BCC structure of all alloy samples remains stable up to 1200℃. The quaternary HfZrTiTa alloy has a flexural strength of 2.3GPa with a typical dimple fracture morphology, indicating that the alloy shows ductile to some extent. The oxidation rates of the HfZrTiTaBx (x=1.1, 2.3, 4.7) alloys at 1300℃ were about 0.13~0.15g•mm-2•h-1, obviously lower than that of the HfZrTiTa alloy (0.454g•mm-2•h-1)

Laser ablation resistance and mechanism of Si-Zr alloyed melt infiltrated C/C-SiC composite

Ablation resistance of C/C-SiC composite prepared via Si-Zr alloyed reactive melt infiltration was evaluated using a facile and economical laser ablation method. Linear ablation rates of the composite increased with an increase in laser power densities and decreased with extended ablation time. The C/C-SiC composite prepared via Si-Zr alloyed melt infiltration presented much better ablation resistance compared with the C/SiC composite prepared by polymer infiltration and pyrolysis process. The good ablation resistance of the composite was attributed to the melted ZrC layer formed at the ablation center region. Microstructure and phase composition of different ablated region were investigated by SEM and EDS, and a laser ablation model was finally proposed based on the testing results and microstructure characterization. Laser ablation of the composite experienced three distinct periods. At the very beginning, the laser ablation was dominated by the oxidation process. Then for the second period, the laser ablation was dominated by the evaporation, decomposition and sublimation process. With the further ablation of the composite, chemical stable ZrC was formed on the ablated surface and the laser ablation was synergistically controlled by the scouring away of ZrC melts and evaporation, decomposition and sublimation process.This work is supported by National Natural Science Foundation of China (51641502)

Porous bulk superhydrophobic nanocomposites for extreme environments

Robust superhydrophobic materials providing protections from harsh weather events such as hurricanes, high temperatures, and humid/frigid conditions have proven challenging to achieve. Here, we report a porous bulk nanocomposite comprising carbon nanotube (CNT)-reinforced polytetrafluoroethylene (PTFE). The nanocomposites are prepared using a templated approach by infusing a CNT/PTFE dispersion into a sponge followed by thermal annealing and decomposition of the sponge template. Importantly, an excess accretion of CNT/PFFE particle mixture on the sponge resulted in nanocomposites with unique and hierarchical porous microstructure, featuring nanochannels near the surface connected to microscale pores inside. The superhydrophobic nanocomposite could resist liquid jets impacting at a velocity of �85.4 m s1 (Weber number of �202,588) and exhibits excellent high-temperature resistance as well as mechanochemical robustness. The porous nanocomposites display excellent icephobicity both with and without infusion with polydimethylsiloxane/silicone oil. These properties should facilitate exploitation as stiff/strong structural polymeric foams used in a variety of fields

Research progress in high thermal conductivity of silicon carbide matrix composites reinforced with fibers

As one kind of advanced high temperature structural and functional materials, it is necessary for fiber reinforced silicon carbide matrix composites (SiC CMCs) in the field of thermal management (TM) to combine the efficient heat transfer and high temperature heat resistance. Common fibers reinforced SiC CMCs, such as carbon fibers reinforced SiC CMCs (Cf/SiC or Cf/C-SiC), silicon carbide based fiber reinforced SiC CMCs (SiCf/SiC), etc., have a low degree of graphitization of the reinforcing fiber and are difficult to form an effective heat transport network. The latest research progress on the preparation and properties of fiber reinforced SiC CMCs with highly thermal conductivity was reviewed in this paper. The heat transport ability of fiber reinforced SiC CMCs can be improved by introducing highly thermal conductive phase, optimizing interfacial structure, making silicon carbide crystal coarse-grained, and designing preform structure. Moreover, the development of the fiber reinforced SiC CMCs with highly thermal conductivities was prospected, that is, comprehensively considering the factors that affect the performance of SiC CMCs, flexibly using the structure-activity relationship between the microstructure and properties of the composites, in order to prepare fiber reinforced SiC CMCs with stable size, excellent properties

A Novel Recombined Potato virus Y Isolate in China

This study reports the findings of a distinct Potato virus Y (PVY) isolate found in Northeast China. One hundred and ten samples (leaves and tubers) were collected from potato plants showing mosaic symptoms around the city of Harbin in Heilongjiang province of China. The collected tubers were planted and let to grow in a greenhouse. New potato plants generated from these tubers showed similar symptoms, except for one plant. Subsequent serological analyses revealed PVY as the causing agent of the disease. A novel PVY isolate (referred to as HLJ-C-44 in this study) was isolated from this sample showing unique mild mosaic and crisped leaf margin symptoms. The complete genome of this isolate was analyzed and determined. The results showed that HLJ-C-44 is a typical PVY isolate. Phylogenetic analysis indicated that this isolate belongs to the N-Wi strain group of PVY recombinants (PVYN-Wi) and also shared the highest overall sequence identity (nucleotide and amino acid) with other members of this strain group. However, recombination analysis of isolate HLJ-C-44 revealed a recombination pattern that differed from that of other PVYN-Wi isolates. Moreover, biological assays in four different potato cultivars and in Nicotiana tabacum also revealed a different phenotypic response than that of a typical PVYN-Wi isolate. This data, combined, suggest that HLJ-C-44 is a novel PVY recombinant with distinct biological properties.A Novel Recombined Potato virus Y Isolate in ChinapublishedVersio