Influence of an O2 Type Oxidizing Environment on SiCf/SiC Composites: Properties/Microstructure Relationship

Ceramic matrix composites have been identified as a potential material of core structure for the fourth generation of fission nuclear reactors. Regarding their excellent mechanical behavior in very harsh conditions (high temperature and high irradiation flux), the CVI–SiC_f/SiC composites with pyrocarbon interlayer are of prime interest for the fuel cladding in the gas-cooled fast reactor. Although the working atmosphere is helium in these advanced reactors, the presence of oxidizing impurities could have a significant role on the mechanical behavior of materials subjected to long-term exposures. Within this framework, this study was intended to investigate the influence of oxidation on the SiC_f/SiC composites mechanical properties. Different pre-damage states were intentionally introduced by mechanical tensile tests on plate specimens before performing an oxidation treatment of 1,000 h at 1,000 °C under helium with 10 ppm of O₂. The degradation of the composite was determined from the mechanical behavior of post-exposure specimens. Results were correlated both with microstructural observations of the damage and with characterizations of the generated oxides at the surface of the composites. The most severe decline of mechanical properties occurs for the higher predamaged loadings. Indeed in this case, the silica formed during the oxidation of SiC is not in sufficient quantities to fill the cracks.     

Glass coating for SiCf/SiC composites for high-temperature application

This work reports on the fabrication of a self-healing, oxidation-resistant glass coating for SiC_f/SiC composites. The glass-coating material is a thermally stable boro-silicate glass with excellent wetting properties and viscosity.Some glass-coated composites were prepared by a simple and low-cost slurry technique, then treated in air for 100 h at 1200°C. Three-point bending tests were performed on the glass-coated composites after the thermal treatment, and the results were compared with the bending strength of the uncoated SiC_f/SiC before and after the same thermal treatment. The mechanical behavior of the glass-coated composite remained almost virtually unaffected after being heated at 1200°C in air for 100 h.     

Effects of dip-coated BN interphase on mechanical properties of SiCf/SiC composites prepared by CVI process

BN interphase was successfully synthesized on SiC fiber fabrics by dip-coating process using boric acid and urea as precursors under N₂ atmosphere. The morphology of BN interphase was observed by SEM, and the structure was characterized by XRD and FT-IR spectra. The SiC_f/SiC composites with dip-coated BN interphase were fabricated by chemical vapor infiltration (CVI) process, and the effects of BN interphase on the mechanical properties of composites were investigated. The results show that the SiC fibers are fully covered by BN interphase with smooth surface and turbostratic structure (t-BN), and the thickness is about 0.4 μm. The flexural strengths of SiC_f/SiC composites with and without BN interphase are about 180 and 95 MPa, respectively. Compared with the as-received SiC_f/SiC composites, the composites with BN interphase exhibit an obvious toughened fracture behavior. From the microstructural analysis, it can be confirmed that the BN interphase plays a key part in protecting the fibers from chemical attack during matrix infiltration and weakening interfacial bonding, which can improve the mechanical properties of SiC_f/SiC composites remarkably.     

High Temperature Oxidation Behavior of Alloy 617 and Haynes 230 in Impurity-Controlled Helium Environments

The high temperature oxidation behavior of alloy 617 and Haynes 230 have been investigated for VHTR intermediate heat exchanger applications. Oxidation tests were carried out for up to 500 h at 900 °C and 1000 °C in impure helium environments containing H₂, H₂O, CO, CO₂, and CH₄. The oxidation kinetics of the alloys followed a parabolic rate law in all cases. In the impure helium environments with very low oxygen, the external oxides of alloy 617 were composed of a Cr₂O₃ layer, TiO₂ ridges on the grain boundaries, and isolated MnCr₂O₄ grains on top of the Cr₂O₃ layer. On the other hand, those of Haynes 230 consisted of a Cr₂O₃ inner layer and a protective MnCr₂O₄ outer layer, which increased the oxidation resistance. The effect of small amounts of CH₄ and H₂ on the oxidation kinetics of the alloys was insignificant. Irregular oxide morphology, such as cellular Cr₂O₃ oxides for alloy 617 and MnCr₂O₄ platelets for Haynes 230, was formed in the impure helium environment at 900 °C. For Haynes 230, along with platelets, whiskers were frequently found at the tip of the MnCr₂O₄ oxide crystals.     

Interfacial reaction studies of B4C-coated and C-coated SiC fiber reinforced Ti–43Al–9V composites

The interfacial reactions of B₄C-coated and C-coated SiC fiber reinforced Ti–43Al–9V composites were investigated by scanning electron microscope and transmission electron microscope. The detailed microstructures as well as the chemical composition throughout the reaction zone were identified. For SiC_f/B₄C/TiAl composite, the reaction zone from B₄C coating to TiAl matrix is composed of 4 layers, namely, a carbon-rich layer, a mixed layer of TiB₂ + amorphous carbon, a TiC layer and a mixed layer of TiB + Ti₂AlC. For SiC_f/C/TiAl composite, the reaction zone from C coating to TiAl matrix is composed of 3 layers, namely, a fine-grained TiC layer, a coarse-grained TiC layer and a thick Ti₂AlC layer. For both kinds of composites, the reaction mechanisms of the interfacial reactions were analyzed, and the corresponding reaction kinetics were calculated. The activation energies of interfacial reaction in SiC_f/B₄C/TiAl composite and SiC_f/B₄C/TiAl composite are 308.1 kJ/mol and 230.7 kJ/mol, respectively.     

Characterization of hot-pressed Ti3SiC2–SiC composites

The high-density Ti₃SiC₂-SiC composites with different SiC volume contents were fabricated by hot pressing technique under 35 MPa in a vacuum atmosphere at 1550 °C for 30 min. Microstructural observation showed that the distribution of SiC particulates in the Ti₃SiC₂ matrix was uniform which improved the hardness of Ti₃SiC₂–20 vol% SiC sample (13.9 GPa), compared to monolithic Ti₃SiC₂ (7.1 GPa). The sample containing 15 vol% SiC showed the highest flexural strength value, compared to the other Ti₃SiC₂-SiC samples and the monolithic Ti₃SiC₂. The fracture toughness of the Ti₃SiC₂-SiC samples was also lower than that of the monolithic Ti₃SiC₂ MAX phase.     

Wetting of molten Sn-3.5Ag-0.5Cu on Ni-P(-SiC) coatings deposited on high volume faction SiC/Al composite

The wetting of molten Sn-3.5Ag-0.5Cu alloy on the Ni-P(-SiC) coated SiC_p/Al substrates was investigated by electroless Ni plating process, and the microstructures of the coating and the interfacial behavior of wetting systems were analyzed. The SiC particles are evenly distributed in the coating and enveloped with Ni. No reaction layer is observed at the coating/SiC_p/Al composite interfaces. The contact angle increases from ～19° with the Ni-P coating to 29°, 43° and 113° with the corresponding Ni-P-3SiC, Ni-P-6SiC and Ni-P-9SiC coatings, respectively. An interaction layer containing Cu, Ni, Sn and P forms at the Sn-Ag-Cu/Ni-P-(0,3,6)SiC coated SiC_p/Al interfaces, and the Cu-Ni-Sn and Ni-Sn-P phases are detected in the interaction layer. Moreover, the molten Sn-Ag-Cu can penetrate into the Ni-P(-SiC) coatings through the Ni-P/SiC interface and dissolve them to contact the SiC_p/Al substrate.     

HfB2/SiC as a protective coating for 2D Cf/SiC composites: Effect of high temperature oxidation on mechanical properties

In the field of thermal shielding for aerospace applications C_f/SiC composites are raising great interest, provided that they are protected from oxidation by suitable coatings. Conversely, ultra high temperature ceramics, and in particular HfB₂, are among the best oxidation resistant materials known. A coating made of a HfB₂/SiC composite (20% weight SiC) was tested as an oxidation protection on a C_f/SiC composite. The composite was produced by Polymer Impregnation Pyrolysis (PIP), which is a simple and low cost method; the coating was applied by painting a slurry on the surface of the composite and by heat treating. The thermal behaviour was studied by thermo-gravimetric analysis, and mechanical tests were conducted before and after oxidation. The HfB₂/SiC composite seems to effectively protect the underlying C_f/SiC composite, with a mechanical strength reduction of only 20% after 30 min at 1600 °C, even if some weight loss due to partial carbon fibre damage is observed. A first analysis of thermal cycling in oxidizing environment suggested that the HfB₂/SiC coating reduces continual damage thanks to the sealing effect of the glassy surface layer.     

Comparison of thermal and ablation behaviors of C/SiC composites and C/ZrB2-SiC composites

A comparison was presented of the thermal and ablation behaviors of two carbon fiber reinforced ceramic-matrix composites (one with a SiC matrix and the other with a ZrB₂-SiC matrix). The C/SiC composite possessed a lower thermal conductivity (TC) and a higher emissivity in comparison to the C/ZrB₂-SiC composite. The two composites exhibited the good ablation-resistive properties with no obvious erosion rate after the arc-heated wind tunnel ablation tests. The surface of the C/SiC composite appeared to be coarse and had many rounded protrusions while a denser and more homogeneous glass oxide scale was formed for the C/ZrB₂-SiC composite. The maximum surface and back side temperatures of the C/ZrB₂-SiC composite were about 50 °C lower than those of the C/SiC composite, respectively, which was mainly attributed to the evaporation of the B₂O₃ as well as its higher TC.     

Microstructures and mechanical properties of Al 2519 matrix composites reinforced with Ti-coated SiC particles

Ti-coated SiC_p particles were developed by vacuum evaporation with Ti to improve the interfacial bonding of SiC_p/Al composites. Ti-coated SiC particles and uncoated SiC particles reinforced Al 2519 matrix composites were prepared by hot pressing, hot extrusion and heat treatment. The influence of Ti coating on microstructure and mechanical properties of the composites was analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the densely deposited Ti coating reacts with SiC particles to form TiC and Ti₅Si₃ phases at the interface. Ti-coated SiC particle reinforced composite exhibits uniformity and compactness compared to the composite reinforced with uncoated SiC particles. The microstructure, relative density and mechanical properties of the composite are significantly improved. When the volume fraction is 15%, the hardness, fracture strain and tensile strength of the SiC_p reinforced Al 2519 composite after Ti plating are optimized, which are HB 138.5, 4.02% and 455 MPa, respectively.     

Process study, microstructure, and matrix cracking of SiC fiber reinforced MoSi2 based composites

SiC fiber reinforced SiAlON-MoSi₂ composites have been manufactured by a concurrent fiber winding and low pressure plasma spraying (LPPS) technique to produce a multilayer, circumferentially fiber reinforced composite ring. The LPPS parameters for SiAlON-MoSi₂ powder were optimized by a two-level experimental design approach followed by further optimization, which provided a smooth sprayed surface, low matrix porosity, and high deposition efficiency. The microstructure of SiAlON-MoSi₂ matrix consisted of a lamellar structure built up of individual splats and a uniform distribution of discontinuous SiAlON splats throughout the MoSi₂ matrix. The spray/wind composites exhibited 2% porosity and well-controlled fiber distribution. High temperature consolidation led to the formation of a thick reaction zone at the fiber-matrix interface by a chemical reaction between C coating and MoSi₂. Matrix cracking occurred in SiC _f (15 vol.%)/MoSi₂ after cooling from 1500 to 25 °C and was attributed to the large tensile residual stresses in the matrix developed on cooling because of coefficient of thermal expansion (CTE) mismatch between matrix and fiber. The addition of 40 vol.% SiAlON into the MoSi₂ effectively eliminated the matrix cracking by reducing the matrix-fiber CTE mismatch. Predictions of matrix cracking stress on the basis of residual stresses in the composites showed that the maximum permissible fiber volume fraction to avoid matrix cracking was 6% for SiC _f /MoSi₂ and 23% for SiC _f /SiAlON(40 vol.%)-MoSi₂.     

Fabrication and mechanical properties of SiCw/MoSi2–SiC composites by liquid Si infiltration of pyrolyzed rice husk preforms with Mo additions

Dense SiC ceramic matrix composites containing SiC whiskers (SiC_w) and MoSi₂ phase (SiC_w/MoSi₂–SiC) are fabricated by a liquid Si infiltration (LSI) method. Pyrolyzed rice husks (RHs) containing SiC whiskers, particles and amorphous carbon are mixed with different amounts of Mo powder to form preforms for the infiltration. Microstructure and mechanical properties of the composites are studied. Fracture mode of the composites is discussed. Results show that the SiC whiskers and fine particles in the pyrolyzed RHs were preserved in the composites after the LSI process. The amorphous carbon and Mo powder in the preforms reacted with molten Si, forming SiC and MoSi₂ in the composites. The presence of MoSi₂ in the composite increases the elastic modulus but lowers the flexure strength. Content of MoSi₂ of ca. 20 wt.% provides an enhanced fracture toughness of 4.1 MPa m^1/2 for the composite. But too large amount of MoSi₂ caused crack formation in the composite. The compressive residual stress introduced by the formation of MoSi₂ and SiC, and the de-bonding of the fine SiC particles and SiC whiskers from the residual Si phase are considered to favor the fracture toughness of the composites.     

Effect of SiC Addition on Oxidation Behavior of ZrB<Subscript>2</Subscript> at 1273 K and 1473 K

The oxidation behavior of ZrB₂–SiC composites with different contents of SiC addition was investigated at 1273 and 1473 K in air for 12 h in this study. The SiC addition contents ranged from 0 to 30 wt%. The results showed that when ZrB₂–SiC composites were oxidized at 1273 K in air, a two-oxide layer-structure forms: a continuous glassy layer and a ZrO₂ layer contained unoxidized SiC. When SiC content is 5 and 10 wt%, the glassy layer is mainly composed by B₂O₃. When SiC content is 20 and 30 wt%, a borosilicate glass could be formed on the top layer, which could improve the oxidation resistance of ZrB₂. When ZrB₂–SiC composites were oxidized at 1473 K in air, the oxide layer was composed of ZrO₂ and SiO₂ and unreacted SiC. Additionally, when SiC addition content was higher than 10 wt%, a continuous borosilicate glass layer could be formed on the top of the oxide layer at 1473 K. With the increase of SiC content in ZrB₂, the oxide layer thickness decreased at both 1273 and 1473 K.     

Joining of Cf/SiC Composite and TC4 Using Ag-Al-Ti Active Brazing Alloy

Carbon fiber reinforced SiC (C_f/SiC) composite was successfully joined to TC4 with Ag-Al-Ti alloy powder by brazing. Microstructures of the brazed joints were investigated by scanning electron microscope, energy dispersive spectrometer, and x-ray diffraction. The mechanical properties of the brazed joints were measured by mechanical testing machine. The results showed that the brazed joint mainly consists of TiC, Ti₃SiC₂, Ti₅Si₃, Ag, TiAl, and Ti₃Al reaction products. TiC + Ti₃SiC₂/Ti₅Si₃ + TiAl reaction layers are formed near C_f/SiC composite while TiAl/Ti₃Al/Ti + Ti₃Al reaction layers are formed near TC4. The thickness of reaction layers of the brazed joint increases with the increased brazing temperature or holding time. The maximum room temperature and 500 °C shear strengths of the joints brazed at brazing temperature 930 °C for holding time 20 min are 84 and 40 MPa, respectively.     

Composite material for instrumental applications based on micro powder Al2O3 with additives nano-powder SiC

The paper provides novel basis for composite material Al₂O₃-SiC as an instrumental material. The advantage of this composite material in comparison with the material having additives made out of SiC fibers is analyzed and justified. The microstructures of composites under various modes of electric sintering (electric consolidation) are considered. Some mechanical properties of the composite materials obtained by electric sintering, their phase compositions are determined, and comparative assessment of the properties is given. Some advantageous mechanical properties of obtained Al₂O₃-SiC composites were pointed out, obtained with less energy consumption than other reported composites.     

C_f/SiC复合材料与钛合金Ag-Cu-Ti-C_f复合钎焊

采用Ag-Cu-Ti-Cf(Cf:碳纤维)复合钎料作中间层,在适当的工艺参数下真空钎焊Cf/SiC复合材料与钛合金,利用SEM,EDS和XRD分析接头微观组织结构,利用剪切试验检测接头力学性能.结果表明,钎焊时复合钎料中的钛与Cf/SiC复合材料反应,在Cf/SiC复合材料与连接层界面形成Ti3SiC2,Ti5Si3和少量TiC化合物的混合反应层.复合钎料中的铜与钛合金中的钛发生互扩散,在连接层与钛合金界面形成不同成分的Cu-Ti化合物过渡层.钎焊后,形成碳纤维强化的致密复合连接层.碳纤维的加入缓解了接头的残余热应力,Cf/SiC/Ag-Cu-Ti-Cf/TC4接头抗剪强度明显高于Cf/SiC/Ag-Cu-Ti/TC4接头.     

SiCf/Ti-60热稳定性研究

本文对采用磁控溅射先驱丝法制备的SiC_f/Ti-60复合材料进行不同温度下长时间热暴露实验,分析了热等静压态和热暴露态复合材料界面区结构稳定性及元素扩散规律。研究结果表明,界面反应层主要产物为TiC,纤维中C、Si元素和基体中Ti及其它合金元素进行互扩散;C元素扩散速率较快,在界面处和基体内形成TiC,基体中的TiC主要集中分布在α相晶界处。SiC_f/Ti-60复合材料反应层长大受扩散控制并遵循抛物线定律,界面反应层长大指数因子为2.27×10_-4 m/s_1/2,界面反应层长大激活能为118 kJ/mol。     

Formation mechanism and oxidation behavior of MoSi2–SiC protective coating prepared by chemical vapor infiltration/reaction

In order to protect C/C composites from oxidation, SiC-MoSi₂ composite coating was synthesized by chemical vapor infiltration /reaction (CVI/CVR) technology. A porous Mo layer was prefabricated on SiC coated C/C composites, and then MoSi₂ and SiC were subsequently prepared in a CVI /CVR process using methyltrichlorosilane (MTS) as precursor. The deposition and reaction mechanism of the MoSi₂-SiC composite coating was investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The oxidation behavior of SiC-MoSi₂ coated specimens was tested. The results show that the porous Mo layer can be densified with SiC phase decomposed from MTS, and transformed into SiC-MoSi₂ by reacting with MTS as well. A dense composite coating was prepared with optimized deposition parameters. The coated specimen exhibits a good oxidation resistance with a little mass loss of 1.25% after oxidation at 1500 °C for 80 h.     

Oxidation of ZC63 Mg alloys reinforced with SiC particles between 390 °C and 500 °C in air

The ZC63 magnesium alloys reinforced with 10 wt.% of SiC particles with an average particle size of 50 μm were cast. The fabricated SiC_p/ZC63 composite consisted of an α-Mg matrix, unreacted α-SiC particles, and an intergranularly formed CuMgZn compound. It was oxidized at 390 °C to 500 °C up to 5 h in air. The oxide scales were thin and compact below 430 °C, but became porous and loose above 450 °C. They consisted primarily of MgO and a small amount of Mg₃N₂. SiC particles were stable over the temperature range explored.     

SiC nanowire-toughened MoSi2-SiC coating to protect carbon/carbon composites against oxidation

To protect carbon/carbon (C/C) composites against oxidation, a SiC nanowire-toughened MoSi₂-SiC coating was prepared on them using a two-step technique of chemical vapor deposition and pack cementation. SiC nanowires obtained by chemical vapor deposition were distributed random-orientedly on C/C substrates and MoSi₂-SiC was filled in the holes of SiC nanowire layer to form a dense coating. After introduction of SiC nanowires, the size of the cracks in MoSi₂-SiC coating decreased from 18 ± 2.3 to 6 ± 1.7 μm, and the weight loss of the coated C/C samples decreased from 4.53% to 1.78% after oxidation in air at 1500 °C for 110 h.