Yttrium Silicate Coatings on Chemical Vapor Deposition-SiC-Precoated C/C–SiC: Thermodynamic Assessment and High-Temperature Investigation

Yttrium silicates are promising materials for improved oxidation and erosion protection for carbon fiber-reinforced composites. A two-layer coating system of low-pressure plasma-sprayed yttrium silicate on chemical vapor deposition-SiC-precoated C/C–SiC was tested under atmospheric re-entry conditions simulated within a plasma wind tunnel test facility. The thermal expansion behavior of Y₂SiO₅ and Y₂Si₂O₇ was investigated. The chemical compatibility with and without increasing oxygen partial pressure at the interface of the two-layer system was calculated by the CALPHAD method. The calculations were compared with experimental results. Furthermore, a thermodynamic explanation is presented to understand and predict the observed coating failure mechanism, identified as blister formation.     

Development of C/C–SiC Brake Pads for High-Performance Elevators

Novel C/C–SiC materials have been developed for their use as brake pads for high-speed elevators. Under dynamic and stationary conditions, these materials exhibit high thermal shock resistance, high coefficients of friction, and extremely low wear rates. In addition, it has been found that the SiC content of the C/C–SiC materials on the friction surface heavily influences the frictional behavior. Low-cost materials based on short fiber reinforcement and on drastically reduced process times showed their high potential for the manufacture of cost-efficient brake pads.     

Particulate-Reinforced Precursor-Derived Si–C–N Ceramics: Optimization of Pyrolysis Atmosphere and Schedules

Surface oxidation occurred during pyrolysis of SiC particulate-reinforced composites (PRCs) with a precursor-derived Si–C–N matrix. In contrast, such an oxidation was not observed in pure Si–C–N ceramics. The present investigation discusses the possible reasons for this, and reports on the influence of such an oxidation on the microstructure and the mechanical and thermal properties of PRCs by the precursor-impregnation and pyrolysis method. The high-temperature mass stability of the PRCs in Ar deteriorated owing to the decomposition of SiO₂ formed by oxidation. The effects of the pyrolysis schedule on the processing and mechanical properties of PRCs are also investigated.     

Mechanical Characterization of Si–C(O) Fiber/SiC (CVI) Matrix Composites witrTa BN–Interphase

The mechanical behavior of three CVT-processed 2D woven SiC/BN/SiC composite materials with different initial BN interphase thicknesses has been investigated by means of tensile and impact tests. The results have established the efficiency of a BN interphase in promoting a nonlinear/non–catastrophic tensile behavior and high impact resistance. The effect of the initial BN interphase thickness on the resulting mechanical behavior has also been demonstrated. Characterization of the fiber/matrix interfacial zones by AES and TEM has revealed the presence of a SiO₂/C double layer at the BN/fiber interface, which might result from a decomposition undergone by the Si–C(O) Nicalon fiber during processing. It has been suggested that the influence of the initial BN interphase thickness on the mechanical properties of the composites results from both changes occurring in the composition and morphology of the interfacial zones and modifications of the interfacial forces due to accommodation of the radial residual clamping stress.     

氟碳涂料氟含量的红外测定方法研究

采用红外光谱通过建立氟含量定量分析标准工作曲线,选择C—F键特征吸收峰所在的1 400～1 000 cm-1光谱区域作为定量分析信息区,对涂料溶剂可溶物氟含量进行定量分析,并用氟离子电极法对结果准确性进行验证。结果表明,当C—F的质量浓度为1.3～13 mg/mL时,线性回归方程为A=1.04[ρ（C—F）/（mg.mL-1）]＋5.15（R2=0.987）;与氟离子选择电极法对比,回收率在可接受范围内。该方法简单可行、准确可靠,可用于涂料溶剂可溶物氟含量。     

Oxidation Protection Coatings for C/SiC based on Yttrium Silicate

The factor which currently precludes the use of carbon fibre reinforced silicon carbide (C/SiC) in high temperature structural applications such as gas turbine engines is the oxidation of carbon fibres at temperatures greater than 400°C. It is, therefore, necessary to develop coatings capable of protecting C/SiC components from oxidation for extended periods at 1600°C. Conventional coatings consist of multilayers of different materials designed to seal cracks by forming glassy phases on exposure to oxygen. The objective of this work was to develop a coating which was inherently crack resistant and would, therefore, not require expensive sealing layers. Yttrium silicate has been shown to possess the required properties for use in oxidation protection coatings. These requirements can be summarised as being low Young’s modulus, low thermal expansion coefficient, good erosion resistance, and low oxygen permeability. The development of protective coatings based on a SiC bonding layer combined with an outer yttrium silicate erosion resistant layer and oxygen barrier is described. Thermodynamic computer calculations and finite element analysis have been used to design the coating. C/SiC samples have been coated using a combination of chemical vapour deposition and slip casting. The behaviour against oxidation of the coating has been evaluated.     

Reactive Coating of Zircon on Mullite and Mullite—Alumina Substrates

A stable zirconia coating of 20–30 pm thickness on a mullite substrate with a different alumina/silica ratio was obtained by reactive coating of zircon. It is shown that the Al₂O₃/ SiO₂ ratio of the mullite substrate drastically affects the morphology of the zirconia coating. The results are explained on the basis of the Al₂O₃–SiO₂–ZrO₂ phase equilibria.     

Formation of Continuous Pore Structures in Si–C–O Fibers by Adjusting the Melt Spinning Condition of a Polycarbosilane–Polysiloxane Polymer Blend

A polymer blend containing polycarbosilane (PCS) and 15 mass% of polyhydromethylsiloxane (H-oil) was prepared and the properties of the polymer melt were investigated in order to clarify the mechanisms of continuous pore formation. Melt viscosity decreased as the H-oil content increased. Moreover, the saturation of the plasticizer effect of H-oil on the viscosity and apparent turbidity of the melting polymer suggested that the compatibility of H-oil to PCS in melting was limited at 15 mass%. Gas chromatography data after heating showed that an amount of evolved hydrogen at 573 K was increased by 15 mass% of H-oil addition. The fibers formed by the melt-spun of the polymer blend at 578 K mainly possessed a single pore at the center of the fibers' cross section. On the other hand, the fibers melt-spun at 543 K usually included multiple pores, and the fibers melt-spun at 538 K included a number of tiny pores. It is proposed that the evolved hydrogen can be dissolved in the polymer melt, and the desaturation process of the dissolved gas during the fiber spinning with a sudden temperature reduction likely determined the size and location of pores in the fibers.     

In vitro corrosion and mineralization of novel Ti–Si–C alloy

The in vitro electrochemical behaviour of a new titanium based α-alloy (Ti–0.5 wt% Si–0.65 wt% C), fabricated via casting and rapid cooling route, was determined using linear, Tafel, potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS), complemented with ex situ SEM-EDS analysis to evaluate the corrosion mechanism. The experimental results revealed that silicon and carbon, in addition to titanium, resulted in the enhancement of mechanical properties. The polarization tests confirmed that Ti–Si–C alloy possessed excellent corrosion resistance (a low corrosion current density of 0.033 μA cm⁻²), comparable to cp Ti and better than Ti6Al4V in phosphate buffered saline (PBS). The mechanism of corrosion was identified as selective dissolution of titanium solid solution matrix. EIS studies indicated the formation of a stable, passive oxide film on the alloy. Further, in vitro bioactivity was evaluated using mineralization tests i.e. by immersing the pre-treated alloy in a concentrated simulated body fluid (10× SBF). Chemical and microstructural characterization of the mineral layer, formed during immersion, revealed the deposition of fine, porous micron-sized globules of a phase rich in calcium-phosphate (Ca-P). In summary, the bulk properties and excellent in vitro electrochemical and mineralization behaviour of the as-cast Ti–Si–C alloy reveal a high potential for its application as load bearing metallic implants.     

Pressure‐less joining of C/SiC and SiC/SiC by a MoSi2/Si composite

A MoSi₂/Si composite obtained in situ by reaction of silicon and molybdenum at 1450°C in Ar flow is proposed as pressure‐less joining material for C/SiC and SiC/SiC composites. A new “Mo‐wrap” technique was developed to form the joining material and to control silicon infiltration in porous composites. MoSi₂/Si composite joining material infiltration inside coated and uncoated C/SiC and SiC/SiC composites, as well as its microstructure and interfacial reactions were studied. Preliminary mechanical strength of joints was tested at room temperature and after aging at service temperatures, resulting in interlaminar failure of the composites in most cases.     

凝胶注模莫来石/碳化硅复合陶瓷的显微结构与力学性能

以SiC粉和莫来石粉为原料,采用凝胶注模成型工艺制备莫来石/碳化硅复合陶瓷,研究了莫来石含量对复合陶瓷显微结构和力学性能的影响,结果表明:随莫来石含量增加,复合陶瓷气孔率先降低后升高,而力学性能呈相反变化趋势。当莫来石含量为20%(体积分数)时,材料气孔率达到最小值9.4%,线收缩率以及弹性模量达到最大值26.5%和148.6 GPa;在莫来石含量为30%时抗弯强度和断裂韧性达到最大值397.7 MPa和4.17MPa.m1/2。适量莫来石的加入明显改善了材料的力学性能,过多则由于莫来石挥发而在材料中产生缺陷,材料性能下降。     

莫来石/SiC复相多孔陶瓷的制备及性能研究

用高岭土、Al2O3粉和SiC粉末为原料,活性碳为造孔剂制备莫来石/SiC多孔陶瓷.测定了试样的显气孔率、气孔孔径分布和抗弯强度,并分别用XRD和SEM分析晶相组成和断面显微结构.结果表明:莫来石的理论设计质量的分数小于10%时,莫来石/SiC多孔陶瓷的显气孔率随其设计量的增多而急剧降低;莫来石理论设计量继续增加时,试样显气孔率缓慢降低并趋于稳定.气孔孔径随莫来石设计量的增加而急剧减小.抗弯强度随莫来石设计量的增加而先增大,在莫来石理论设计量为20%时达到最大值,此后逐渐降低.SEM分析结果表明:与其他试样相比,莫来石设计量为40%的试样中存在较多的多孔"微区".     

Wet-oxygen corrosion resistance and mechanism of bi-layer Mullite/SiC coating for Cf/SiC composites

A bi-layer oxidation-resistant coating consisting of a mullite outer coating, and a SiC inner coating on the surface of C_f/SiC composites was prepared by the chemical vapour deposition and an air spray sol-gel process, and its corrosion behavior was evaluated in a wet-oxygen coupling environment. Results show that the formation of SiO₂ glass layer and its reaction with mullite particles to form aluminosilicate glass layer, leading to an increase in the density of the mullite outer coating, so that the weight loss of bi-layer Mullite/SiC coating coated C/SiC sample was only 1.11 × 10^?3 g·cm^?2 in the first 100 h of oxidation. However, the weight loss of the coated sample reached 26.82 × 10^?3 g·cm^?2 after 200 h of oxidation due to a part of the mullite outer coating was detached. The SiO₂ glass phase reacted with water vapour to generate gaseous Si(OH)_x, which created distinct holes on the surface of the SiO₂ glass layer or inside the molten aluminosilicate glass layer. Eventually, the mullite outer coating was blistered and detached from the surface of the sample due to the combination and growth of holes.     

氮化铝和莫来石陶瓷衬底的SIMS和XRD研究

利用二次离子质谱（ＳＩＭＳ）和Ｘ射线衍射（ＸＲＤ）研究了用于电子封装的以Ｄｙ２Ｏ３,ＣａＯ为添加剂的ＡＬＮ和以堇青石,ＢａＣＯ３为添加剂的莫来石陶瓷衬底的物相组成和表面成分,尤其是表面杂质,并利用ＳＩＭＳ尝试探讨了ＡｌＮ表面热氧化问题。结果表明,ＡｌＮ和莫来石陶瓷表面不同程度地存在Ｌｉ,Ｃ,Ｆ,Ｎａ,Ｋ,Ｃｌ,Ｔｉ,Ｒｂ等杂质元素的污染;ＡｌＮ表面存在富Ｏ层,在空气中经过８５０℃１０分钟退火后,富     

Self-Crack-Healing Behavior of Mullite/SiC Particle/SiC Whisker Multi-Composites and Potential Use for Ceramic Springs

To improve fracture toughness and encourage excellent self-crack-healing ability, mullite/SiC particle/SiC whisker multi-composites and mullite/SiC whisker composites were hot pressed. The crack-healing abilities and mechanical properties of these sintered composites were investigated. Based on the obtained results, the usefulness of the mullite composite as a material for springs was discussed. The part of mullite/15 vol% SiC whisker/SiC 10 vol% particle containing healed cracks retained high reliability over the whole measured temperature range. When the crack-healing ability was endowed by SiC whiskers alone, the parts containing the healed pre-cracks were found to have a heat-resistance limit temperature. Mullite/15 vol% SiC whisker/10 vol% SiC particle multi-composite had the best potential as a material for springs used at high temperatures, because it had an adequate crack-healing ability as well as shear deformation ability almost two times stronger than that of monolithic mullite.     

Mechanical and Thermophysical Properties of Zr–Al–Si–C Ceramics

The mechanical and thermophysical properties of quaternary-layered carbides, Zr₂[Al(Si)]₄C₅ and Zr₃[Al(Si)]₄C₆ have been investigated and compared with those of Zr₂Al₃C₄ and Zr₃Al₃C₅. These four carbides are generally alike in mechanical and thermophysical properties due to their similar crystal structures that consisting of alternatively stacked ZrC layers and Al₃C₂/[Al(Si)]₄C₃ slabs. The layer thickness of zirconium carbide and aluminum carbide has effects on their properties. Thicker layer of zirconium carbide and/or thinner layer of aluminum carbides are in favor of stiffness, hardness, thermal, and electrical conductivities, but go against density, specific stiffness, Debye temperature, and coefficient of thermal expansion.     

CVD Mullite Coatings in High-Temperature, High-Pressure Air–H2O

Crystalline mullite was deposited by chemical vapor deposition (CVD) onto SiC/SiC composites overlaid with CVD SiC. Specimens were exposed to isothermal oxidation tests in high-pressure air + H₂O at 1200°C. Unprotected CVD SiC formed silica scales with a dense amorphous inner layer and a thick, porous, outer layer of cristobalite. Thin coatings (∼2 μm) of dense CVD mullite effectively suppressed the rapid oxidation of CVD SiC. No microstructural evidence of mullite volatility was observed under these temperature, pressure, and low-flow-rate conditions. Results of this preliminary study indicate that dense, crystalline, high-purity CVD mullite is stable and protective in low-velocity, high-pressure, moisture-containing environments.     

Effect of Temperature on Oxidation Behavior of 3‐Dimensional C/SiC Composites Coated by Different Structural SiC and a‐BCx Coatings in Static Wet Oxygen Atmosphere

The effect of temperature on oxidation behavior of 3‐dimensional C/SiC composites coated with SiC and a‐BC_x coatings was investigated. Below 1000°C, C/SiC composites with SiC coating (sample SSS) showed low residual strength, while the composites with a‐BC_x coating (sample BBB) and the composites with a‐BC_x/a‐BC_x/SiC coatings (sample BBS) showed high residual strength. Above 1000°C, samples SSS and BBS showed high residual strength, while sample BBB showed the opposite results. Sample BBS presented better oxidation resistance during 700–1200°C. The oxidation temperature made an effect on self‐sealing mode of sample BBS.     

Effect of Weave Architecture on Tensile Properties and Local Strain Heterogeneity in Thin-Sheet C–SiC Composites

Room-temperature tensile properties were measured for two thin C–SiC composites fabricated from single sheets of carbon fiber fabric with nominally the same weave architecture, but different fiber packing densities. The SiC matrixes were formed by infiltration and pyrolysis of a polymer precursor (allylhydridopolycarbosilane). The tensile properties are related to microstructural characteristics, observed damage mechanisms, and measurements of local strain concentrations by speckle interferometry. Differences are observed between the responses of these thin-sheet composites and conventional CVI-matrix composites of larger thickness. Debonding between transverse and longitudinal fiber tows allows significant strains due to straightening of initial wavy fiber tows and leads to local stress concentrations. The strength and elastic modulus are affected by the waviness of the longitudinal tows.     

红外光谱和拉曼光谱法研究白炭黑的微观结构

用红外光谱和拉曼光谱分析沉淀法白炭黑和气相法白炭黑的微观结构。结果表明：与气相法白炭黑相比,沉淀法白炭黑表面羟基含量相对较高;未经热处理的沉淀法白炭黑表面覆盖结合水分子,随着处理温度的升高,沉淀法白炭黑表面先失去羟基结合水分子,然后发生羟基缩合反应,直至最后失去所有羟基,形成Si—O—Si网络结构;气相法白炭黑未经热处理时即可明显观察到羟基的存在,且处理温度对其结构影响不明显,这可能是沉淀法白炭黑与气相法白炭黑生产工艺不同造成的。