Thermal-gradient testing of thermal barrier coatings under simultaneous attack by molten glassy deposits and its mitigation

Degradation of thermal barrier coatings (TBCs) in gas-turbine engines by molten calcium-magnesium-aluminosilicate (CMAS) glassy deposits is becoming a pressing issue, as engines are required to operate under increasingly harsh conditions. While new approaches for mitigating CMAS attack of TBCs are starting to emerge, there is a need for appropriate tests for evaluating the efficacy of those approaches. To that end, we present here a new thermal-cycling test for the evaluation of TBC performance, where a thermal gradient is applied across the TBC. with simultaneous injection of CMAS. The conditions simulated in this new test are closer to actual conditions in an engine, as compared to the conventional furnace test without thermal gradient. This new test is applied to a TBC with a new composition designed for mitigating CMAS attack, where the mitigation mechanisms are observed to be similar to those found in the conventional furnace test. However, based on a previously established mechanics model it is shown here that thermal-gradient cyclic testing is essential for evaluating thermomechanical performance of TBCs under attack by CMAS, and that the use of just the conventional furnace test can lead to the underestimation of the TBC performance. (C) 2010 Elsevier B.V. All rights reserved

Suspension and Solution Plasma or HVOF Spraying

International audienceNanostructured materials offer significant improvements in engineering properties because their grain sizes are smaller than those of conventionally processed materials by a factor of almost 2 orders of magnitude (Ref 1). Since the mid- 1990s, research has been conducted using thermal spray technology for the deposition of finely structured or nanostructured coatings (Ref 2, 3). To produce finely structured coatings by thermal spray techniques, four routes have been suggeste

Liquid phase sintering of silicon carbide with AlN/Y2O3, Al2O3/Y2O3 and SiO2/Y2O3 additions

In this work, the influence of the additive system on the liquid phase sintering of silicon carbide has been investigated. The additives employed were mixtures of AlN/Y2O3, Al2O3/Y2O3 and SiO2/Y2O3. The total additive content was fixed at 20 vol.-%, maintaining the Y2O3 content in each additive system at 35 vol.-%. Cold isostatically pressed samples were sintered at 1900, 2000 and 2100 °C under Ar atmosphere during 30 min. The most promising results have been obtained by samples with AlN/Y2O3 additions sintered at 2000 °C, exhibiting the smallest weight loss of about 6% and the highest flexural strengths of about 433 MPa. Samples with Al2O3/Y2O3 and SiO2/Y2O3 additions exhibited high weight loss, because of reactions of Al2O3 and SiO2 with the SiC matrix, forming gaseous species such as Al2O, SiO and CO, resulting in depletion of the liquid phase, and, consequently, in inferior final densities and mechanical properties. Concerning the SiO2/Y2O3 additive system, the reactions seem to be completed already at temperatures below 1900 °C, turning this additive mixture unsuitable. The microstructural analysis indicated only the presence of the <FONT FACE="Symbol">b</FONT>-SiC phase for all samples; no phase transformation of the <FONT FACE="Symbol">b</FONT>-SiC into <FONT FACE="Symbol">a</FONT>-SiC has been observed