Overview
The target was to develop more reliable, less expensive Thermal Barrier Coatings (TBCs) with increased lifetimes and improved temperature capability. The top coat of TBC systems consists of a low thermal conductivity material such as yttria stabilised zirconia (YSZ). This can be thermally sprayed by means of air plasma spraying (APS) or deposited by (more expensive) electron beam physical vapour deposition (EB-PVD). The PVD process leads to a columnar microstructure giving the coating a high strain tolerance, and a greater lifetime and reliability than plasma-sprayed TBC systems.
The major objective of the project is the development of improved TBC systems using advanced bonding concepts in combination with additional protective functional coatings. The first specific objective will be to use these developments to provide a significant improvement to State-of-the-Art APS coatings and hence provide a cost-effective alternative to EB-PVD.
The second objective will be to combine these new concepts with new coating technologies to provide new, advance material for thermal barrier systems with a capability exceeding the performance of EB-PVD coatings.
In both cases, a major impact on TBC performance, manufacturing and maintenance costs and hence competitiveness of European aviation gas turbine manufactures is expected.
- Work package 1
To have a large database, the consortium decided to prepare 21 different TBC systems consisting of different bondcoats and topcoats. To compare the new specimen with state of the art TBCs, also an APS reference and an EB-PVD reference were produced. Beside the standard materials like spray dried YSZ, advanced materials like nano sized YSZ suspension and YSZ precursor in CVD processes were used. For all coating materials, detailed specifications were defined. Furthermore, the substrate material for the different TBC was defined, and samples from raw material like CMSX4 and IN738 were produced. For special geometries, hollow cylinders and massive pins were manufactured via electrical discharge machining. - Work package 2
The main objective was the production and procurement of powders and precursors. A large batch of fused & crushed YSZ powder was supplied by TIAG, and SM provide a large batch of spray dried YSZ powder. Besides these conventional materials, a YSZ powder with a small amount of Rare Earth was produced in order to use this material as sensor coating material. Further, a YSZ suspension with agglomerated, nano sized particles was developed and optimised by TIAG in collaboration with FZJ. Three batches of fused and crushed magnesia alumina spinel with slightly different contents of Al2O3 were produced also by TIAG. ONERA tested different precursor as starting material for PE-CVD coatings. - Work package 3
As mentioned in the project description, one route to achieve more strain tolerant coatings was the application of 3D interfaces on substrate surfaces. This surface modification was realised by using laser cladding technique. With this method it was possible to produce thin bars on the surface with a height of about 500µm. - Work package 4
This was the largest work package in the project. The main objective was the development of strain tolerant coatings with different approaches to achieve this aim. The performed work can be divided into the following categories:- Development of TBC with segmentation crack densities of about 10 cracks mm-1 using advanced conventional plasma spraying technique;
- Development of TBC systems using new materials like nano sized powders;
- Development of TBCs with columnar microstructure using new techniques like LPPS-TF, PE-CVD.
Altogether, at the end of WP 4 more than 280 test samples with 4 differen
Funding
Results
The four project years revealed several promising results with potential to a commercial use.
- In the field of spraying materials, two promising candidates were detected: the YSZ suspension and the fused & crushed YSZ powder, both produced by TIAG. Coatings produced by these two types of coating material showed good performance in comparison to other project TBC systems.
- The activities in the research subject '3D interfaces' gave a deeper understanding in manufacturing 3D surfaces compatible with the plasma spraying process. These results can help to develop advanced coating systems for different application in gas turbines.
- The development of more strain tolerant coatings with at least a performance equal to EB-PVD coatings in work package 4 was also successful and brought out some results that may be the base for further investigations and qualifications. Highly segmented coatings produced via advanced APS processes show some outstanding test results, while only the thermal conductivity in the same range as that of EB-PVD coatings is a disadvantage. The second innovative TBC system is the LPPS-TF coating, which is also a potential coating system to replace the expensive EB-PVD process.
- Another remarkable achievement of the current project is the further development of APS Sensor Coatings. Beside the already known use of the Sensor TBC as a temperature indicator, an ageing detector or an early delamination sensor, functionality has been proven throughout the TOPPCOAT project: thickness measurement capability. Further, to this the Sensor Coatings has shown remarkable durability during the cyclic oxidation tests which is comparable with the best coating morphologies tested during the project.
- The performance of suspension plasma sprayed coatings was convincing especially in the long term furnace cycling test, and some partner decided to perform further research activities beside the project.
Evaluation of different new TBC systems on real parts could not be finalised within the project, but further qualification tests will show whether highly segmented coatings or LPPS-TF coatings are applicable in modern gas turbines. LPPS-TF TBCs as repair technology for EB-PVD coatings offers a promising solution for lower repair costs of turbine parts. Further, the engine test of a new developed bondcoat showed that costs can be reduced by using a HVOF bondcoat instead of state-of-the-art VPS bondcoats.
Technical Implications
Segmented or columnar structures within the TBC in combination with excellent bonding are expected to improve the durability and reliability of TBC systems.
Preliminary investigations on the development of 3D interfaces by some of the consortium partners had already shown that improvements to bonding can be obtained. This study was continued but the 3D profiling was also investigated as a method to control the microstructure of the TBC layer. A precise control of the microstructure was possible as the location of the segmentation cracks was directly correlated to micro-structural features on the substrate and this was used to initiate segmentation cracks in the coating. By this technique it was expected to gain significantly higher segmentation densities than by using only thermal spray technique on conventional rough substrates.
In order to take full advantage of this, the use of advanced spraying techniques both to initiate the segmentation cracks and to control the coating process in order to obtain desired reproducibility was essential. This included on-line particle diagnostic systems, precise substrate temperature control as high substrate temperatures and the use of advanced APS gun technology. All were essential for control segmentation crack formation.
Within the project the developed segmented APS coatings were compared with standard EB-PVD coatings. It is anticipated that this route will lead to TBC systems with performance close to that of EB-PVD but at a fraction of the cost having impact in both new component and repair businesses.
The project was aimed towards the next generation of TBC systems. In addition to the techniques developed above, further innovate steps were investigated. There are new, emerging technologies which showed the potential to produce highly strain-tolerant coatings in a cost-effective way. The processes that were identified as having the most potential are; thin film - low pressure plasma spraying (TF-LPPS), plasma enhanced chemical vapour deposition (PE-CVD), nano-phase suspension plasma spraying and hollow cathode gas sputtering PVD (GS-PVD). Investigations determined the single most promising process for the deposition of TBC systems. Also new, advanced TBC materials such as aluminates and modified spinels were included.