Ablation behaviour and mechanical performance of ZrB_(2)-ZrC-SiC modified carbon/carbon composites prepared by vacuum infiltration combined with reactive melt infiltration

The development of advanced aircraft relies on high performance thermal-structural materials,and carbon/carbon com-posites(C/C)composited with ultrahigh-temperature ceramics are ideal candidates.However,the traditional routes of compositing are either inefficient and expensive or lead to a non-uniform distribution of ceramics in the matrix.Compared with the traditional C/C-ZrC-SiC composites prepared by the reactive melt infiltration of ZrSi_(2),C/C-ZrB_(2)-ZrC-SiC composites prepared by the vacuum infiltration of ZrB_(2) combined with reactive melt infiltration have the higher content and more uniform distribution of the introduced ceramic phases.The mass and linear ablation rates of the C/C-ZrB_(2)-ZrC-SiC composites were respectively 68.9%and 29.7%lower than those of C/C-ZrC-SiC composites prepared by reactive melt infiltration.The ablation performance was improved because the volatilization of B_(2)O_(3),removes some of the heat,and the more uniformly distributed ZrO_(2),that helps produce a ZrO2-SiO2 continu-ous protective layer,hinders oxygen infiltration and decreases ablation.

Anti-oxidation properties of ZrB_2 modified silicon-based multilayer coating for carbon/carbon composites at high temperatures

To improve the anti-oxidation ability of silicon-based coating for carbon/carbon （C/C） composites at high temperatures, a ZrB2 modified silicon-based multilayer oxidation protective coating was prepared by pack cementation. The phase composition, microstructure and oxidation resistance at 1773, 1873 and 1953 K in air were investigated. The prepared coating exhibits dense structure and good oxidation protective ability. Due to the formation of stable ZrSiO4-SiO2 compound, the coating can effectively protect C/C composites from oxidation at 1773 K for more than 550 h. The anti-oxidation performance decreases with the increase of oxidation temperature. The mass loss of coated sample is 2.44% after oxidation at 1953 K for 50 h, which is attributed to the decomposition of ZrSiO4 and the volatilization of SiO2 protection layer.

Preparation and oxidation property of ZrB_2-MoSi_2/SiC coating on carbon/carbon composites

To improve the oxidation resistance of carbon/carbon composites,ZrB2-MoSi2/SiC coating on the carbon/carbon substrate was prepared.The inner coating of SiC was prepared by pack cementation and the outer coating of ZrB2-MoSi2 was prepared by slurry painting.The phase compositions and microstructures of the coating were characterized by XRD and SEM,respectively.The preparation and the high temperature oxidation property of the coated composites were investigated.The results show that the outer coating of carbon/carbon composites is composed of ZrB2,MoSi2 and SiC phases.The mass losses of the ZrB2-MoSi2/SiC coated samples with SiC nano-whiskers after 30 h and 10 h of oxidation at 1 273 K and 1 773 K were,respectively,5.3% and 3.0%.The ZrB2-MoSi2/SiC coated samples exhibit self-sealing performance and good oxidation resistance at high temperature.

Preparation and properties of C/C-ZrB2-SiC composites by high-solid-loading slurry impregnation and polymer infiltration and pyrolysis（PIP）

Ultrahigh-temperature ceramics were added to C/C composites to meet their application requirement in a high-temperature oxidizing environment. C/C-ZrB2-SiC composites were fabricated by high-solid-loading slurry impregnation with polymer infiltration and pyrolysis. The dispersion and rheological behavior of ZrB2 slurry and the microstructural, mechanical, and ablation properties of the C/C-ZrB2-SiC composites were investigated. Results indicated that a well-dispersed and low-viscosity ZrB2 slurry was obtained using 0.40 wt.% polyethyleneimine as a dispersant at pH 5. Ceramics were uniformly distributed in the short-cut fiber layer and needle-punched area. The flexural strength of the C/C-ZrB2-SiC composites was 309.30 MPa. The composites exhibited satisfactory ablation resistance under the oxyacetylene flame of 2500℃, and the mass and linear ablation rates were 0.40 mg/s and 0.91 μm/s, respectively. A continuous and compact Zr O2 layer, which could effectively reduce the diffusion rate of oxygen and protect the composites from being ablated, was formed.

C/SiC/MoSi_2-SiC-Si multilayer coating for oxidation protection of carbon/carbon composites

C/SiC/MoSi2-SiC-Si oxidation protective multilayer coating for carbon/carbon （C/C） composites was prepared by pack cementation and slurry method. The microstructure, element distribution and phase composition of the as-received coating were analyzed by scanning electron microscopy （SEM）, energy dispersive X-ray spectroscopy （EDS） and X-ray diffraction （XRD）. The results show that the multilayer coating was composed of MoSi2, SiC and Si. It could effectively protect C/C composites against oxidation for 200 h with the mass loss of 3.25% at 1873 K in static air. The mass loss of the coated C/C composites results from the volatilization of SiO2 and the formation of cracks and bubble holes in the coating.

Joining and machining of(ZrB2-SiC)and(Cf-SiC)based composites

Filler materials of(ZrB_2-SiC-B_4C-YAG) composite were developed for gas tungsten arc welding(GTAW) of the ZrB_2-SiC and Cf-SiC based composites to themselves and to each other. Reaction with filler material,porosity and cracks were not observed at weld interfaces of all the joints. Penetration of filler material in to voids and pores existing in the Cf-SiC composites was observed. Average shear strength of 25.7 MPa was achieved for joints of Cf-SiC composites. By incorporation of Cf-SiC(CVD) ground short fibre reinforcement the(ZrB_2-SiC-B_4C-YAG) composite was machinable with tungsten carbide tool. The joint and machined composites were resistance to oxidation and thermal shock when exposed to the oxy-propane flame at 2300℃ for 300s. The combination of(ZrB_2-SiC-B_4C-YAG) and Cf-SiC based composites can be used for making parts like thermal protection system or nozzles for high temperature applications.

Microstructures and Dry Sliding Wear Properties of ZrB_2/A356 Composites Synthesized by Magneto-chemistry in situ Reaction

In situ A356-x%PVF （particle volume fraction） ZrB2 （ x=1, 3, 5） composites were prepared via magneto-chemistry in situ reaction and the dry sliding wear properties of the composites were investigated. The experimental results show that ZrB2 reinforcement particle is obtained and its morphology mainly present in spherical and regular hexagon. Wear test results show that the values of wear weight loss of the composites decrease with the increase of value under a given sliding time and a certain load of 60 N. Especially, when x=5, the weight loss of the as-prepared composite is 43.1 mg, which is only 36.4% to that ofA356 alloy, 118 rag. The wear mechanism is changed from adhesion wear to adhesion wear and abrasive wear and then to abrasive wear with the increase of x value.

Oxidation behavior of C/C composites with SiC/ZrSiO_4-SiO_2 coating

A SiC/ZrSiO4？SiO2 （SZS） coating was successfully fabricated on the carbon/carbon （C/C） composites by pack cementation, slurry painting and sintering to improve the anti-oxidation property and thermal shock resistance. The anti-oxidation properties under different oxygen partial pressures （OPP） and thermal shock resistance of the SZS coating were investigated. The results show that the SZS coated sample under low OPP, corresponding to the ambient air, during isothermal oxidation was 0.54% in mass gain after 111 h oxidation at 1500 ° C and less than 0.03% in mass loss after 50 h oxidation in high OPP, corresponding to the air flow rate of 36 L/h. Additionally, the residual compressive strengths （RCS） of the SZS coated samples after oxidation for 50 h in high OPP and 80 h in low OPP remain about 70% and 72.5% of those of original C/C samples, respectively. Moreover, the mass loss of SZS coated samples subjected to the thermal cycle from 1500 ° C in high OPP to boiling water for 30 times was merely 1.61%.

Fabrication of ZrC/ZrB2/C Composites Having High Mechanical Properties at Elevated Temperatures

Ultra-high temperature ceramics (UHTCs) are most recently getting much attention for structural parts of hypersonic missiles with their cruising speed of more than Mach 5. Most of the UHTCs are poor sinterability carbides, nitrides, and borides. Therefore, they have been studied and developed for a long time. However, there are still many problems to solve. In this paper, based on the solid-state reaction presented as an equation of (x + y)·ZrC + 2·y·B → x·ZrC + y·ZrB2 + y·C, three-phase ZrC/ZrB2/C composites have been fabricated from ZrC and amorphous B powders using pulsed electric-current pressure sintering at 1373 to 2173 K for 6.0 × 102 s under 50 MPa in a vacuum. ZrC/ZrB2/C = 30/70/C~70/30/C vol% composites with the relative densities Dr of 96.6 to 98.7% were obtained at 2073 K. The 60/40/C vol% composite revealed high bending strength σb (554 MPa), Vickers hardness Hv (19.2 GPa) and moderate fracture toughness KIC (5.25 MPa·m1/2) at room temperature. Furthermore, all composites showed elastic deformation up to 1873 K and revealed σb more than 600 MPa at this temperature, in addition, some composites showed higher σb than 900 MPa at the same temperature. These high mechanical behaviors are discussed with those of the simple binary ZrC/ZrB2 composites which were fabricated under the same conditions except for their starting materials. The best mechanical properties of binary composites were σb (474 MPa), Hv (18.5 GPa), and KIC (4.45 MPa·m1/2) at room temperature, and σb of 400 - 700 MPa at 1873 K. Overall, three-phase composites, nevertheless including soft carbon, have higher mechanical properties than the binary composites.

Dry sliding wear behavior of AA6061/ZrB_2 in-situ composite

The dry sliding wear behavior of AA6061/ZrB2 in-situ composite prepared by the reaction of inorganic salts K2ZrF6 and KBF4 with molten aluminum was investigated.An attempt was made to develop a mathematical model to predict the wear rate of AA6061/（0-10%） ZrB2 in-situ composites.Four-factor,five-level central composite rotatable design was used to minimize the number of experiments.The factors considered are sliding velocity,sliding distance,normal load and mass fraction of ZrB2 particles.The effect of these factors on the wear rate of the fabricated composite was analyzed and the predicted trends were discussed by observing the wear surface morphologies.The in-situ formed ZrB2 particles enhance the wear performance of the composite.The wear rate of the composite bears a proportional relationship with the sliding velocity,sliding distance and normal load.

Non‑Magnetic Bimetallic MOF‑Derived Porous Carbon‑Wrapped TiO2/ ZrTiO4 Composites for Efficient Electromagnetic Wave Absorption

Modern communication technologies put forward higher requirements for electromagnetic wave(EMW)absorption materials.Metal-organic framework(MOF)derivatives have been widely concerned with its diverse advantages.To break the mindset of magneticderivative design,and make up the shortage of monometallic non-magnetic derivatives,we first try non-magnetic bimetallic MOFs derivatives to achieve efficient EMW absorption.The porous carbon-wrapped TiO2/ZrTiO4 composites derived from PCN-415(TiZr-MOFs)are qualified with a minimum reflection loss of−67.8 dB(2.16 mm,13.0 GHz),and a maximum effective absorption bandwidth of 5.9 GHz(2.70 mm).Through in-depth discussions,the synergy of enhanced interfacial polarization and other attenuation mechanisms in the composites is revealed.Therefore,this work confirms the huge potentials of nonmagnetic bimetallic MOFs derivatives in EMW absorption applications.

Effects of in-situ ZrB_2 particle on grain refinement of ZrB_2/AZ91D magnesium matrix composite

In-situ ZrB2/AZ91D magnesium matrix composite was successfully synthesized with AI/K2ZrF6＋NH4BF4 by means of Direct Melt Reaction. The fabricated ZrB2/AZ91D magnesium matrix composite through direct melt mixing method was investigated. Results from X-ray diffraction （XRD） and energy dispersive X-ray spectroscopy （EDS） confirmed the existence of ZrB2 particles in the AZ91D alloy, and most ZrB2 particles were in the size range of just several microns, some even to 100 nm. The cast specimens were studied through corrosion testing and heat treatment. The average grain size of AZ91D decreased markedly from about 250 pm to 50 IJm. In addition, the shape and size of the ,β-MglTAI12 phase as well as the morphologies of primary a-Mg in the magnesium matrix composite were greatly changed. The network structure of the β-MglTAI12 phase was broken into small blocks and the size of a-Mg decreased significantly.

Preparation of ZrB_2-SiC Powders via Carbothermal Reduction of Zircon and Prediction of Product Composition by Back-Propagation Artificial Neural Network

Phase pure ZrB2-SiC composite powders were prepared after 1 450℃/3 h via carbothermal reduction route,by using ZrSiO4,B2O3 and carbon as the raw materials.The influences of firing temperature as well as the type and amount of additive on the phase composition of final products were detailedly investigated.The results indicated that the onset formation temperature of ZrB2-SiC was reduced to 1 400℃by the present conditions,and oxide additive（including CoSO4·7H2O,Y2O3 and TiO2）was effective in enhancing the decomposition of raw ZrSiO4,therefore accelerating the synthesis of ZrB2-SiC.Moreover,microstructural observation showed that the as-prepared ZrB2 and SiC respectively had well-defined hexagonal columnar and fibrous morphology.Furthermore,the methodology of back-propagation artificial neural networks（BP-ANNs）was adopted to establish a model for predicting the reaction extent（e g,the content of ZrB2-SiC in final product）in terms of various processing conditions.The results predicted by the as-established BP-ANNs model matched well with that of testing experiment（with a mean square error in 10^（-3） degree）,verifying good effectiveness of the proposed strategy.

The influence of ablation products on the ablation resistance of C/C-SiC composites and the growth mechanism of SiO_2 nanowires

Ablation under oxyacetylene torch with heat flux of 4186.8（10%kW/m2 for 20 s was performed to evaluate the ablation resistance of C/C-SiC composites fabricated by chemical vapor infiltration（CVI） combined with liquid silicon infiltration（LSI） process.The results indicated that C/C-SiC composites present a better ablation resistance than C/C composites without doped SiC.The doped SiC and the ablation products SiO2 derived from it play key roles in ablation process.Bulk quantities of SiO2 nanowires with diameter of 80 nm-150 nm and length of tens microns were observed on the surface of specimens after ablation.The growth mechanism of the SiO_2 nanowires was interpreted with a developed vapor-liquid-solid（VLS） driven by the temperature gradient.

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TiB_2-B_4C CERAMIC COMPOSITES

The microstructure and mechanical properties of TiB_2 /B_4C composites have been investi- gated.It was found that both the strength and hardness for TiB_2 greatly increase with the ad- dition of 20 to 30 wt-% B_4C,and the fracture toughness K_(IC) value remaines on the original high level.The flexure strength,Vicker's hardness and fracture toughness are 782 MPa,26.2 GPa and 7.2 MPam^(1/2),respectively,for the TiB_2-30 wt-% B_4C composite,compared to 450 MPa,21 GPa and 7.0 MPam^(1/2) for monolithic TiB_2.The toughening and strengthening mechanisms,have also been discussed.

Microstructures and mechanical properties of MgAl_2O_4 particle-reinforced AC4C aluminum composites

MgAl2O4 particle-reinforced AC4C based alloy composites were fabricated by the stirring-casting method. The effects of the average sizes and the size distributions of MgAl2O4 particles on the dispersibility were investigated, and the microstructures, strength, and fatigue properties of MgAl2O4 particle-reinforced AC4C based alloy composites were evaluated. Tensile strength in the MgAl2O4 particle-reinforced AC4C based alloy composite was increased by using the classified particles. The fatigue limit at 107 cycles in the MgA1204 particle-reinforced AC4C-Cu composite increased by 27% compared to the unreinforced alloy at 250 ~C. Dislocations were observed in the matrix around the MgAl204 particle which resulted from the mismatch of thermal expansion coefficients between MgAl2O4 and Al, and resisted failure and caused fatigue cracks to propagate around the MgAl2O4 particles, resulting in extensive crack deflection and crack bowing which contributed to the improvement of fatigue strength.

C/C复合材料ZrB_2-SiC基陶瓷涂层制备及烧蚀性能研究

为提高C/C复合材料的抗烧蚀性能,采用两步刷涂-烧结法制备了ZrB2-SiC基陶瓷涂层。首先利用反应烧结制备ZrB2-SiC-ZrC过渡层,并在此基础上制备了ZrB2-20%SiC-5%Si3N4、ZrB2-15%SiC-20%MoSi2、ZrB2-15%SiC-20%TaC 3种外涂层。利用XRD和扫描电镜研究了涂层的相组成和显微形貌,并采用氧乙炔焰烧蚀仪测试了涂层在2 500℃、60 s的抗烧蚀性能,探讨了涂层的高温烧蚀机理。结果表明:利用反应烧结制备的过渡层与基体结合紧密,且与外涂层无明显分层现象,起到了良好的过渡作用;由于Si3N4及MoSi2起到了烧结助剂作用,使ZrB2-20%SiC-5%Si3N4、ZrB2-15%SiC-20%MoSi2外涂层结构较为致密;ZrB2-20%SiC-5%Si3N4、ZrB2-15%SiC-20%MoSi2涂层表现出了较好的抗烧蚀性能,其中ZrB2-20%SiC-5%Si3N4涂层线烧蚀率及质量烧蚀率分别为0.075 mm/s、0.008 1 g/s,ZrB2-15%SiC-20%MoSi2涂层线烧蚀率及质量烧蚀率分别为0.018 mm/s、0.0064 g/s,而ZrB2-15%SiC-20%TaC涂层由于结构较为松散,未能起到有效的氧化防护,导致涂层被烧穿。

C/C-SiC复合材料表面ZrB_2基陶瓷涂层的制备及高温烧结机理

采用刷涂-烧结法,分别在C/C-SiC复合材料和C/C复合材料表面制备了ZrB2基陶瓷复合涂层。利用EDS,SEM分析陶瓷涂层的成分及微观形貌,通过对比C/C-SiC基体和C/C基体的表面涂层,对C/C-SiC基体表面涂层的高温烧结机理进行了探究。结果表明:高温下C/C-SiC基体中的硅组元会溢出,造成样品质量损失;同时,溢出的硅组元能渗入到陶瓷涂层中,形成了以硅为主要黏结相,ZrB2等陶瓷相弥散分布的陶瓷涂层;与C/C基体相比,硅组元的溢出能有效促进涂层与基体之间的界面结合。在对基体进行预处理的基础上,采用低温真空脱胶,高温常压烧结,能够制备出结构致密、无裂纹并与基体结合牢固的ZrB2基陶瓷涂层。

等离子喷涂制备ZrB_2-SiC涂层及其抗氧-丙烷焰流烧蚀性能

为提高C/C复合材料的抗高温烧蚀性能,利用大气等离子喷涂技术在C/C复合材料表面制备ZrB_2-SiC复合涂层,并对其进行抗高温氧-丙烷焰流烧蚀试验。采用X射线衍射仪(XRD)、扫描电子显微镜(SEM)及能谱分析仪(EDS)对涂层的物相成分、微观形貌等进行检测分析。结果表明:大气等离子喷涂能在C/C复合材料表面制备出均匀致密的ZrB_2-SiC复合涂层,涂覆有ZrB_2-SiC涂层的C/C复合材料分别承受1 600、1 700和1 800℃的氧-丙烷焰流烧蚀300 s后依次增重0.63%、0.76%和0.71%,而烧蚀600s后试样质量烧蚀率分别为9.42×10^(-5)、2.04×10^(-4)和1.04×10^(-3)g/s。ZrB_2-SiC涂层显著提高了C/C复合材料的抗烧蚀性能,涂层氧化生成的玻璃态SiO_2能有效填充孔隙。直到SiO_2耗尽,涂层烧蚀后的孔洞成为环境中的氧进入基体的通道,导致基体烧蚀。

ZrB_2-SiC/C层状复合陶瓷的高温氧化行为

利用流延法成膜和热压烧结工艺制备出了ZrB2-SiC层和石墨层交替排列、层厚均匀、界面清晰的ZrB2-SiC/C层状复合陶瓷。采用循环氧化法对ZrB2-SiC和ZrB2-SiC/C层状复合陶瓷在1000℃及1300℃空气中的氧化动力学曲线进行了研究。结果表明:在1000℃氧化增重时,ZrB2-SiC/C层状复合陶瓷在氧化反应初期表现为氧化增重,随着时间的增加,表现为氧化减重。在1300℃时,ZrB2-SiC/C层状复合陶瓷由于基体层ZrB2-SiC和弱夹层石墨相的氧化规律的相互叠加,使得其氧化增重曲线表现为抛物线规律。由XRD分析及扫描电镜观察发现,1300℃氧化15 h后,试样中不存在弱夹层石墨相,由于石墨相的挥发,材料残留孔隙。