Hui Mei

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.

Comparison of oxidation resistance of NiCoCrAlTaY-coated and -uncoated Mar-M247 superalloys in the air at 1150 °C

Oxidation resistance of NiCoCrAlTaY coated and uncoated Mar-M247 superalloys were compared experimentally in the air at 1150xa0°C. It was found that the oxidation behavior of the NiCoCrAlTaY coated and uncoated Mar-M247 superalloys generally followed the parabolic kinetics with the rate of 1.5 and 9.8xa0×xa010−3xa0mg2/cm4xa0h for each one. Microstructural observation and elemental analysis indicated that after 200xa0h the uncoated alloy covered with an oxide scale of 6–10 times thicker than the coated one, beneath which a fairly continuous and relatively thin α-Al2O3 could be found along the interface close to the substrate. Comparatively, the NiCoCrAlTaY coated alloy uniformly produced the continuous, dense and thick α-Al2O3 layer adhesive to the coating to prevent the metal elements against excessive oxidation, and finally formed the steady oxide scales: outer (Ni, Co)(Al, Cr)2O4 spinels, NiO, (Al, Cr)2Ni3; inner protective α-Al2O3 with a little Cr2O3. On the surface of the uncoated alloy, however, it mainly formed the Ni-rich oxides in which a large quantity of cracks propagated to result in the more oxidation.

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.

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.

Effect of shrinkage constraint on damage of C/SiC composites during thermal cycles in oxidising atmosphere

Abstract Thermal cycling response of a three-dimensional C/SiC composite subjected to a unidirectional shrinkage constraint in wet oxygen was investigated. Constraint stress generated on both ends of the composite specimens during thermal cycles is shown to decrease upon heating and increase with cooling. Nevertheless, the peaks of the constraint stress gradually reduced from the initial 41 MPa to the final constant value of 9·8 MPa after 20 cycles. Residual mechanical properties and microstructure characterisation suggest that the cycled composites suffer a little mechanical degradation and the shrinkage constraint is of advantage to oxidation resistance of the materials during thermal cycles.

Oxidation resistance comparison of coating repaired mechanical cracks in carbon fibre reinforced silicon carbide composite

Oxidation resistance of carbon fibre reinforced silicon carbide composites (C/SiCs) with different coating repaired mechanical cracks was investigated. Structural integrity is critically important for the service performance of components for aerospace applications, and the coatings are reported as ideal choices to repair mechanical impact induced surface cracks. In this paper, chemical vapour deposited SiC, polysilazane (PSN) pyrolysed ceramic and slurry transformed glass coatings were introduced to investigate the oxidation resistance of repaired C/SiCs. The repair effects of various coatings were compared by weight loss and residual mechanical properties. Results showed that both SiC coated and glass coated C/SiCs exhibited preferable oxidation resistance compared with the PSN coated and uncoated specimens. Correspondingly, the former exhibited less weight loss and higher residual flexural strengths than the latter. A scanning electron microscope was employed to observe the microstructures of fracture sections, which gave an intuitive explanation for the repair effect of various coatings.

Non-destructive investigation of quasi-static indentation damage to C/SiC composites and its effects on compressive properties

Abstract Abstract Impact damage was introduced into C/SiC composites by a quasi-static indentation method. Damages with different indentation depths were detected by non-destructive testing (NDT) technologies of thermography, X-ray radiography and ultrasonic C scan, and then the detection resolutions were compared. Effects of the impact damages on the retained compressive strengths of the C/SiCs were investigated. Results indicate that when the indentation depth is <0·5 mm, the C/SiCs reveal the linear elastic behaviour with invisible damage for three NDT methods. Once the indentation depth is >0·75 mm, thermal images and ultrasonic C scan are able to detect the obvious damage, while X-ray radiography did not detect any damage up to the indentation depth of 1·5 mm. The retained compressive strengths decreased gradually with increasing indentation depth, and the descent rate of the compressive strength suddenly increased when the indentation depth is >1·5 mm.

Acoustic Emission Research on Thermal Shock Behaviors of Three-Dimensional C/SiC Composites in Different Environments

Three-dimensional (3D) carbon fiber reinforced silicon carbide matrix composites (C/SiC) were prepared by a low-pressure chemical vapor infiltration method. The thermal shock behaviors of the composites in different environments were researched using an advanced acoustic emission (AE) system. Damage initiation and propagation were easily detected and evaluated by AE. The thermal shock damage to C/SiC composites mainly occurred at the process of cooling and was limited at argon but unlimited at wet oxygen atmosphere. Also correlations have been established between the different damage mechanisms and the characteristics of acoustic emission signals obtained during thermal shock tests. In this way, the paper contributes to the development of the acoustic emission technique for monitoring of damage development in ceramic-matrix composites.