Lai Fei Cheng

Fibre-reinforced multifunctional SiC matrix composite materials

In the last two decades, fibre-reinforced SiC ceramic-matrix composites (CMCs) have attracted extensive interests. Owing to the designable multi-scale microstructure feature and the tailorable processing methods such as chemical vapour infiltration and polymer derived ceramics, SiC matrix composites attain great potential as multifunctional composites. Through designing the fibre, interphase, matrix and coating, the composite exhibits a multitude of functionalities which are desirable for various technological applications. Besides strengthening and toughening design of CMCs, three inspiring issues of multifunctional CMCs are receiving increasing attention, including crack self-healing, friction self-lubrication, and electromagnetic shielding and absorption, which are the key mechanisms to promote the application of CMCs in hot structures of engines and aerospace vehicles, braking pads/discs, various electronic devices, etc. The present review covers the main mechanisms on strengthening and toughening, crack self-healing, friction self-lubrication, and electromagnetic shielding and absorption of CMCs. Key developments and future challenges in this field are summarised.

Preparation of an oxidation protection coating for c/c composites by low pressure chemical vapor deposition

Preparation defects in a SiC coating on C/C substrates are considered to be the controlling factor which decreases the oxidation protection life at the temperature of preparing. To decrease the number of such defects, a low-pressure chemical vapor deposition (LPCVD) method, instead of general CVD was used, and the substrates were suspended, rather than supported. Multi-deposition was employed to seal the preparation defects at the edges of the substrates produced by the suspension. The SiC coating, which was of a high quality, had a smaller crack width and a better interfacial binding. Every layer in the multi-layer coating was uniform and smooth, and the layer thickness could be easily controlled. The oxidation tests indicated that the preparation defects in the coating could not be sealed by multi-deposition as the gaps between the multi-layers acted as channels for oxygen diffusion towards the defects in the inner layer. The weight loss decreased as the multi-deposition times were increased. An analysis showed that no matter how many layers the coating consisted of, failure always began from the preparation defects in the inner layer which were located at the edges of substrates, and a cavity would be formed beneath the inner layer by direct oxidation of the C/C. The oxidation process in C/C substrates with a multi-layer coating was controlled by the rate of oxygen diffusion along the interlayer gaps which were thinner and longer than the defects. The more the multi-deposition times, the larger the length of the gaps, and the smaller the oxygen transport rate.

Environmental Performance Testing System for Thermostructure Materials Applied in Aeroengines

The conventional ultimate performance test by applying a component in its true application (i.e., in an engine) is often very expensive and impractical when dealing with developmental materials. Simpler, less expensive, and more practical test methods must be utilized. The present work aims toward the applications of an innovative methodology for testing environmental performance of advanced Ceramic Matrix Composites (CMCs) in the presence of combined mechanical, thermal, and environmental applied conditions. To obtain a comprehensive understanding of how a composite might perform in certain application environments, a newly developed environmental performance testing system, which is able to provide the fundamental damage information of the composites in simulating service environments including variables such as temperature, mechanical and thermal stresses, flowing oxidizing gases and high gas pressure, is proposed. The system comprises of two subsystems: (1) equivalent experimental simulating subsystem, and (2) wind tunnel experimental simulating subsystem. The evolution mechanisms of the composites properties and microstructures can be achieved by the former, and then be validated and modified by the latter. Various loading (e.g. fatigue, creep), various atmospheres (e.g. argon, oxygen, water vapor, wet oxygen and molten salt vapor) and various temperature conditions (e.g. constant or cyclic temperatures) can be conducted on the system. Some typical experimental results are presented in this paper. Large quantities of tests have demonstrated the extraordinary stability and reliability of the system.

Morphology and growth mechanism of silicon carbide chemical vapor deposited at low temperatures and normal atmosphere

With the method of phenomenology, a supersaturation–condensation–fusion (SCF) mechanism is proposed to describe the growth of chemical vapor deposition silicon carbide under normal atmosphere. The structure has been characterized by scanning electron microscopy and transmission electron microscopy. Morphology characterization of deposited crystallites and silicon carbide aggregates have been explained in terms of SCF mechanism; Raman spectra analysis indicated that the major chemical bonds of deposit were Si–C and –C=C–. Auger spectra analysis revealed that there were Si, C, S, Cl, and O on the surface of the deposit.

Oxidation behaviour of three-dimensional woven C/SiC composites

Abstract The oxidation behaviour of a three-dimensional woven C/SiC composite protected with an SiC seal coating and with an SiC coating combined with an SiO2–B2O3 glassy coating have been respectively investigated through an experimental approach based on mass and flexural strength changes. Three main temperature domains exist for C/SiC composites protected with an SiC seal coating. At low temperatures (<700°C), the mechanisms of reaction between carbon and oxygen control the oxidation kinetics. At an intermediate temperatures (between 700 and 1100°C), the oxidation kinetics are controlled by gas phase diffusion through a network of microcracks in the SiC matrix and coating. At high temperatures (>1100°C), the oxidation kinetics are controlled by oxygen diffusion through the SiO2 scale formed on the SiC coating. Composites of C/SiC with an SiC/(SiO2–B2O3) coating exhibit better oxidation resistance. The filling of the pores and the microcracks and the flow of the glassy coating at higher temperatures result in a global decrease of mass loss in the composites. By researching the relationship between the residual flexural strength and the mass variation in different temperature ranges, it is shown that the change in the residual flexural strength is dominated by the degradation of carbon phase.

Effect of Pre-Oxidation Treatment on the Thermal Shock Resistance of Thermal Barrier Coatings in a Combustion Gas Environment

Thermal barrier coatings (TBCs) were deposited by an Air Plasma Spraying (APS) technique. The TBC coating comprised of 92 wt.% ZrO2 and 8 wt.% Y2O3 (YSZ), CoNiCrAlY bond coat, and MarM247 nickel base super alloy. After APS of YSZ two batches of TBC specimens were tested, one batch of which was pre-oxidised in air for 10h at 1080 oC. Both types of the specimens were directly pushed into a combustion gas at 1150 oC for 25 min and then out to the natural air for quenching. The combustion gas was produced by burning jet fuel with high speed air in a high temperature wind tunnel, which simulates the real service conditions in an aeroengine. Results show that TBCs prepared by the APS had good thermal shock resistance in the combustion gas. The pre-oxidation treatment of the TBC had a significant effect on its thermal shock life. The as-oxidised TBCs always had worse thermal shock resistance than the as-sprayed ones after thermal shock cycles.

Continuous synchronous composite process for fabricating carbon/silicon carbide composites

The continuous synchronous composite (CSC) process is a new technique, the preparation of reinforcement-phase accompanied simultaneously the deposition of SiC matrix, based on CVI principles for fabrication of ceramic matrix composites. In the CSC process, there was a gradient temperature field on the surface of the graphitic substrate, consisting of high (1000–1200°C), intermediate (900–1000°C) and low (700–900°C) temperature regions, by a bottom heating-element. Following the rotating substrate, micro-pores were well infiltrated in the intermediate temperature regions by gas diffusion transport, and macro-pores were rapidly filled with SiC in the high temperature regions by gas flow transport, respectively. In the present paper, 2-dimension carbon cloth reinforced SiC composites was fabricated by CSC process, and the microstructure, deposition rate and conversion efficiency of methyltrichlorosilane (MTS) were investigated. The densification of C/SiC composites was uniform, and the highest deposition rate within macro-pores was 25 μm h−1, and the conversion efficiency of MTS varied from 11% to a maximum of 27%.

Preparation and mechanical properties of self-reinforced in situ Si3N4 composite with La2O3 and Y2O3 additives

Abstract Self-reinforced in situ Si 3 N 4 composite material was prepared with high amount of La 2 O 3 and Y 2 O 3 additives by two-step hot pressing, and the optimum amount of additives was determined. The volume fraction of boundary glass phase was calculated based on the equilibrium of equivalent number in chemical reaction. For material with 15xa0mol% additives, flexural strength and fracture toughness at room temperature were 960xa0MPa and 12.3xa0MPaxa0m 1/2 , respectively. At temperature of 1350°C, flexural strength was maintained to 720xa0MPa and fracture toughness was significantly increased to 23.9xa0MPaxa0m 1/2 because of the high refractory of oxynitride glass containing compositions of La and Y. Self-reinforced mechanism was mainly responsible for crack deflection along the elongated β-Si 3 N 4 grains.

Effect of Yttria Nanopowder on Multi-Layer Coatings of Yttria And CVD SiC/Graphite

Nano-yttria powder can be synthesized by yttrium citrate-urea precursor, combusted at 600°C in air. The CVD SiC coated on graphite (CVD SiC/Graphite) infiltrated by the yttrium citrate-urea precursor, combusted at of 600°C, and then sintered at 1450°C, the thin yttria film can be achieved. The SEM morphology and EDS result of the thin yttria film show a mass of needle-shaped pining into the CVD SiC layer, which improves the combination of CVD SiC layer and wash yttria coating. Therefore, it is an effective transition layer between CVD SiC coating and wash yttria layer.

Matrix modification of laminated SiCw/SiC ceramic composites

Laminated SiCw/SiC ceramic composites were fabricated by chemical vapor infiltration and tape casting, and the effects of matrix modification on microstructure and strengthening/toughening of the composites were investigated by polymer infltration and pyrolysis and liquid silicon infiltration. With increasing cycles of polymer infltration and pyrolysis, density of the composites increased effectively, and the flexural strength and fracture toughness increased obviously. With increasing time of liquid silicon infiltration, density of the composites increased significantly, but the flexural strength and fracture toughness increased firstly and then decreased gradually. After modification by polymer infiltration and pyrolysis, the bonding of interfacial and interlaminar was not changed, and the density increase led to the property increase. After modification by liquid silicon infiltration, the interlaminar bonding was not changed and interfacial bonding of whisker/matrix was strengthened, and the density increase led to the property increase.