Li Tong Zhang

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.

Microstructure and mechanical properties of barium aluminosilicate glass-ceramic matrix composites reinforced with SiC whiskers

BAS glass-ceramic composites reinforced with different volume fractions (0, 10, 20, 30, 40 vol%) of SiC whiskers were successfully fabricated by a hot-pressing method. The microstructure, whisker/matrix interface structure, phase constitution and mechanical properties of the composites have been systematically studied by means of SEM, TEM, XRD techniques as well as three-point bending tests. It was demonstrated that the incorporation of SiC whiskers could significantly increase the flexural strength and fracture toughness of BAS glass-ceramic matrixes. The celsian seeds can effectively promote the hexacelsian-to-celsian transformation in BaAl2Si2O8. The active Al2O3 added to the BAS matrix obviously reduced the amount of SiO2 in the matrix and formed needle-like mullite. The high temperature strengths of the composites were also investigated.

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.

Effect of whisker surface treatment on the mechanical properties of 20vol%SiCw/BAS glass-ceramic composites

Abstract20vol%SiCw/BAS (barium aluminosilicate) composites with as-received, acid-leached and heat-treated whiskers were fabricated by a hot-pressing technique and their mechanical properties were evaluated. The whiskers used in these composites were characterized using Auger electron spectrometry (AES) and scanning electron microscopy (SEM), and the observations were correlated with the mechanical properties. The results show that the oxygen content of the whisker surfaces was reduced by the hydrogen fluoride (HF) acid-leaching treatment and the surface geometry of whiskers was smoothed by the heat-treatment in an inert gas atmosphere. The corresponding composites show an obvious increase (18% and 31%, respectively) in toughness compared with the composites reinforced with as-received whiskers. Fracture surfaces revealed evidence of toughening by the mechanisms of crack deflection, crack bridging and whisker pullout for the composites with surface treated whiskers.

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.

Effects of Interphase Thickness on Damping Behavior of 2D C/SiC Composites

Internal friction of 2D C/SiC composites fabricated by chemical vapor infiltration (CVI) method was measured by dynamical mechanical analysis (DMA) at different frequencies from room temperature (RT) to 400°C in air atmosphere. Internal friction of 2D C/SiC composites increased gradually with increasing temperature and then decreased after damping peak appeared in the temperature range of 250°C to 300°C. Damping capacity and peak value decreased gradually with increasing frequency, accompanied with a shift of damping peak towards lower temperatures. Moreover, the effect of interphase thickness on damping behavior of 2D C/SiC composites was investigated. The results showed that damping peak of the composites increased gradually and the temperature of the peak shifted to the lower temperature with increasing PyC interphase thickness, when the interphase thickness is in the range of 90~296nm. The influence of interphase thickness on interfacial bonding strength, sliding resistance and the microstructure of SiC matrix was discussed, which was considered to be responsible for the results.

Microstructural Characterization of LaB6-ZrB2 Eutectic Composites

Detail microstructure of LaB6-ZrB2 composites has been characterized by TEM and HRTEM. The directionally solidified ZrB2 fibers in LaB6 matrix near LaB6-ZrB2 eutectics present at least three growing relationship systems. In addition to previous report of [001]LaB6 / [0001]ZrB2 relationship, [0 1 1]LaB6 / [0001]ZrB2 and [1 2 0]LaB6 / [0001]ZrB2. were identified. Different with [001]LaB6 / [0001]ZrB2 system, the interfaces of [0 1 1]LaB6 / [0001]ZrB2 and [1 2 0]LaB6 / [0001]ZrB2. show non-coherent and clean interfaces. There is neither glassy phase nor reaction products found at the interfaces

Reaction Kinetic in the CVD Process of Pyrocarbon Deposition from Propylene Pyrolysis

The deposition rate(r) of pyrocarbon deposited from propylene pyrolysis has been explored as a function of residence time (tr=0.2-5s) and deposition temperature (T=1173-1333K) at a constant pressure of 6 kPa. The main feature of r vs. curves was that the deposition rate, firstly increase and then decrease with the residence time. A qualitative chemical model was developed and the kinetic domains were defined for the formation of pyrocarbon. The model could explain the change of deposition rates and kinetic transition due to the occurrence of two different families of ultimate carbon precursor.

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.