ICMCTF2014 Session A2-1: Thermal and Environmental Barrier Coatings

Thermal barrier coatings (TBCs) are applied to increase lifetime and efficiency of highly loaded turbine components. New topcoat chemistries have been developed for TBCs that offer low thermal conductivity, improved sinter resistance, and higher phase stability. Degradation of those coatings by volcanic ash and calcium-magnesium alumino-silicate (CMAS) deposits is now recognized as an increasingly important fundamental degradation mechanism.

The presentation provides results on several TBCs deposited on top of NiCoCrAlY coated superalloys. Gadolinium zirconate (GdZ), lanthanum zirconate (LaZ), and dyprosia stabilized zirconia (DySZ) were investigated as single and double layers in comparison to standard 7YSZ coatings. In the double-layered TBC systems, a thin layer (~25µm) of 7YSZ was used between the new topcoat material and the bond coat. All coatings were deposited by electron-beam physical vapor deposition. The interaction of the coatings with artificial volcanic ash (AVA) and with CMAS was investigated at various temperatures. While the zirconia-based TBCs provide no mitigation potential due to complete infiltration by AVA and CMAS, the pyrochlore TBCs form crystalline phases rapidly which provides a potential for damage mitigation. A detailed microstructural analysis with special emphasis on the phases formed revealed that besides chemistry the TBC microstructure, in particular the width of the inter-columnar gaps, seems to play a decisive role for the interaction between TBC and deposits.

With the advancement of modern state-of-the-art thermal barrier coatings (TBC), gas turbines can operate at temperatures much higher than the melting point of the substrate. Now, the engine firing temperature is limited to the melting point of external particles with high concentration of calcium–magnesium alumino-silicate (CMAS) entering the gas turbine, which melt at about 1200-1400°C. The molten CMAS forms a glassy phase that reacts with the TBC’s topcoat at high temperatures and causes spallation of the topcoat upon cool down, making it impossible to study the properties of CMAS infiltrated TBCs. In the present work, an easy method for CMAS infiltration involving CMAS delivery to the topcoat pores by epoxy capillary effects and flash melting made it possible to prepare TBC samples with CMAS infiltration up to 20-30% of topcoat thickness uniformly without delamination. For the CMAS infiltrated portion of the topcoat layer, the modulus of elasticity and thermal conductivity were measured respectively by four point bend rig and Laser Flash Analyzer (LFA), while the coefficient of thermal expansion (CTE) was obtained with a recently developed method utilizing the hot stage in Scanning Electron Microscopy (SEM) and quantified with image processing. In addition, the CMAS powder was formed into different sample geometries with a highly dense structure by hot pressing to measure its CTE, thermal conductivity and modulus by Thermo-Mechanical Analyzer (TMA), LFA and Nano-indentation. It was found that the CTE of infiltrated layer decreased by half and the conductivity and modulus increased by factors of 2.8 and 1.6 respectively.

At temperatures above 1200°C, ingested sand and debris particles melt on the TBC surfaces of aero- engine combustion chamber parts and form calcium-magnesium-alumino silicate (CMAS) deposits. The composition and melting point of CMAS vary according to the source and location of the sand and debris. The molten CMAS penetrates the 7 YSZ TBCs, changes the microstructure and reduces their strain tolerance which ultimately leads to failure. A damage assessment of TBC coated combustion chamber parts of aero-engines after service in heavily CMAS loaded areas was performed in this study. Parts from different engines were investigated by SEM and the extent of damage of the 7 YSZ plasma-sprayed TBC due to CMAS deposition and infiltration is estimated. With the help of XRD and EDX, CMAS chemical composition was analyzed and two different typical chemical compositions of CMAS were derived. These model CMAS compositions were synthesized in laboratory and their melting behaviour was determined by means of DSC. Both CMAS variants were subsequently deposited on air plasma sprayed yttria stabilized zirconia (APS-7YSZ) TBC specimens. Samples were subjected to isothermal heat treatments in air at temperatures ranging from 1200°C to 1250°C for times between 10h and 100h. The phase formations and microstructural changes were examined by SEM and XRD. Results from the CMAS/TBC interaction experiments are compared with the damage patterns on the real combustion chamber parts.

Furnace cycle tests (FCT) were conducted at 1135°C on TBC coated buttons with and without CMAS. The extent of TBC damage strongly correlates to the CMAS composition. The presence of Ca-sulphates in the CMAS plays a large role in damaging the 7YSZ and infiltration depth of CMAS. With raising temperature, the depth of infiltration increases rapidly and the CMAS has penetrated completely throughout the TBC thickness at 1250°C. In addition, the time to failure of the TBCs strongly depended on the type of CMAS deposited. The life time of the samples with Ca-sulphate containing CMAS was found to be the least compared to the samples without CMAS and with the sulphate-free CMAS variant.

In the present work it is shown that the lifetime of the APS TBC systems with HVOF bondcoats can be substantially extended by application of a thin APS “flashcoat” layer onto the base HVOF bondcoat. The latter approach allows improvement of the bondcoat roughness profile to the extent typically obtained by LPPS. It is important to mention that the oxidation resistance of the flashcoat strongly depends on the used spray parameters. Deviation from the set of the optimized spray parameters was found to result in the catastrophic oxidation of the flashcoat and corresponding very rapid TBC-failure.

In order to explain the TBC lifetime variations between the various bondcoat morphologies, the roughness profiles of HVOF, LPPS and APS-flashcoat bondcoats were evaluated using fractal analysis. It is suggested that such an approach provides better correlation between the bondcoat morphology and TBC-lifetime than the calculation of the arithmetic mean roughness (Ra) of the bondcoat frequently used in practical application.

Gas turbines are playing an increasingly important role in modern life via, for example, power generation from industrial gas turbines and air transport using jet engines. During their development there has been a steady increase in combusted gas temperatures, with components in this hot, combusted gas stream moving from being uncooled to cooled (using a range of technologies) and to now needing thermal barrier coatings (TBCs) on top of this air cooling. For some gas turbines, the combusted gas temperatures are above the melting points of the component materials. Surface degradation of gas turbine components can occur through simple oxidation, especially at the higher operating temperatures. However, contaminants (such as sulphur, chlorine and alkali species) can enter the gas turbine through the fuel and/or combustion air and cause the hot corrosion of metallic component surfaces (e.g., via type I or type II hot corrosion reactions). Other species (such as calcium, magnesium, aluminium and silicon compounds (i.e., CMAS), possibly mixed with iron and other compounds) can also enter through the combustion air and cause surface damage, especially to thermal barrier coatings. A wide range of coatings for environmental protection has been developed to combat specific forms of surface degradation.

Despite the existence of protective coatings for many current conditions, with the continuing need to increase the efficiencies and reliabilities of gas turbines used in different applications, and to enable fuel and operational flexibility whenever possible, their requirements are changing. Thus coatings for thermal and/or environmental protection form critical parts of the materials systems that are now used in gas turbines. This paper will review state of the art coating systems, and highlight the development of advanced multi-functional coating systems. Examples of these coating systems include: (a) the incorporation of sensing layers into thermal barrier coatings; (b) multi-layered TBCs to resist CMAS/Fe attack; and, (c) multi-layered metallic coatings for oxidation and hot corrosion resistance.