This study presents a general overview of ceramic abradable coatings used in the high-pressure section of turbines that are sprayed over superalloys.

This article describes advanced techniques used in the repair and refurbishment of a platform combustion system (PCS) for an SGT5-8000H turbine. The first part outlines the refurbishment process of the part basket—a key PCS component—covering inspection, repair, recoating, and final assembly steps. The second part highlights the integration of advanced repair technologies, including laser-based cutting and welding, as well as patch repairs using 3D-printed parts via laser powder bed fusion.

The H-class turbine, introduced nearly a decade ago, has reached a significant milestone with its 100th global sale. With 108 units sold and 91 in operation across four continents, accumulating over 3.2 million fired hours, the SGT5-8000H has established itself as a market leader, setting industry benchmarks for performance. Since its launch, the SGT5-8000H's output has increased from 375 MW to 450 MW, and combined cycle efficiency has surpassed 62%. To maintain optimal performance, the platform combustion system (PCS) of the SGT5-8000H has undergone refurbishment in Berlin since 2017. Beginning with a PCS from Samsun, Turkey, the process involves a detailed inspection, repair, recoating, and final assembly. Advanced technologies, such as blue light scanning, enhance efficiency and enable lifecycle assessments. Innovative repair methods, including 3D printed patch repairs using laser powder bed fusion (LPBF), reduce costs. Laser-based cutting and welding automation further minimizes heat input and distortion, ensuring the PCS's reliability and longevity. These technological advancements contribute to the SGT5-8000H's stable and dependable operation.

Hexavalent chromium has been widely used in the coating industry and more specifically in gas turbine hot end component protection. UK REACH (registration, evaluation, authorization, and restriction of chemicals) have given an end date of September 2024 for the use of hexavalent chromium and as such, the industry must adapt to these regulations. Indestructible Paint LTD have developed a new aluminium diffused slurry coating, CFIPAL, that does not contain hexavalent chromium like its predecessor, IP1041. Both CFIPAL and IP1041 were deposited onto Nimonic 75 alloy and underwent metallurgical and chemical analysis which included scanning electron microscopy, energy dispersive spectroscopy, electron backscatter diffraction, hardness testing, contact angle testing, surface roughness testing and finally, salt spray corrosion testing. The results indicated that CFIPAL is a suitable alternative for hexavalent chromium-containing coatings, such as IP1041.

Environmental barrier coatings (EBCs) are required to protect SiC based composites in high temperature, steam containing combustion environments found in the latest generation of high efficiency gas turbine aeroengines. Ytterbium disilicate has shown promise as an environmental barrier coating, showing excellent phase stability at high temperature and a coefficient of thermal expansion close to that of SiC; however, its performance is dependent on the conditions under which the coating was deposited. In this work, a parametric study was undertaken to demonstrate how processing parameters using a widely used Praxair SG-100 atmospheric plasma spraying torch affect the phase composition, microstructure and mechanical properties of ytterbium disilicate environmental barrier coatings. Ytterbium disilicate coatings were deposited using 5 sets of spray parameters, varying arc current and secondary gas flow. The phases present in these coatings were quantified using X-ray diffraction with Rietveld refinement, and the level of porosity was measured. Using this data, the relationship between processing parameters and phase composition and microstructure was examined. Abradable coatings are used throughout gas turbine engines to increase efficiency in the compression and combustion phases of the turbine. Abradable coatings are soft enough to be worn away by turbine blade tips (without damaging the tip itself), allowing for tighter clearances to be used, limiting leakages and increasing efficiency. Using the optimum process parameter window determined in this work, a low density abradable Yb 2 Si 2 O 7 layer will be deposited in future research.

Hot section components of stationary gas turbines such as turbine blades are coated with thermal barrier coatings (TBCs) to increase the high thermal strain tolerance thereby the improvement of the performance for the gas turbines. TBCs represent high-performance ceramics and are mostly composed of yttria-stabilized zirconia (YSZ) in order to fulfil the function of thermal insulation. The microstructure of conventional TBCs should be porous to decrease heat conduction. Besides porous TBCs, the recently developed vertically segmented thermal barrier coatings (s-TBCs) feature outstanding thermal durability. In this work, process parameter development for atmospheric plasma spraying (APS) of s-TBCs is presented. Within the experiments, relevant process parameters such as powder feed rate, surface speed and pathway strategy have been optimized. The aim of this work is to achieve a combination of low internal residual stress and high adhesive tensile strength for s-TBCs. For the formation of vertical cracks, the heat input into the powder feedstock material and the substrate must be controlled precisely.

Abradable seal coatings are widely employed in the gas turbine of aero-engine, which not only strength enough to resist the impact of external particles and airflow, but also excellent wear resistance. In the current study, we concentrate on APS sprayed Aluminum Bronze Polyester abradable coating that can be used in turbo engines both for seals and clearance control. A composite thermal spray powder, substantially in the form of clad particles each of which has coarse polyester powders and sub-particles of Cu-Al alloy powders, was prepared using mechanically clad process. Abradable seal coating was prepared by atmospheric plasma spraying. The microstructure, hardness, bonding strength, thermal shock resistance and corrosion resistance of coatings were researched. Properties of the coating were able to meet the application requirements. The coating microstructures and phase compositions were evaluated via SEM. The corrosion mechanisms of the coating were compared by analyzing the cross-sectional and top surface microstructures of the as-sprayed and eroded coatings.

Increasing operating temperature plays a critical role in improving the thermal efficiency of gas turbines. This paper assesses the capability of advanced thermal barrier coatings being developed for use in 1700 °C class gas turbines. Parts sprayed with these coatings were evaluated and found to have excellent durability and long-term reliability.

Fabrication of Thermal Barrier Coatings (TBCs) with higher lifetime and relatively cheaper processes is of particular interest for gas turbine applications. Suspension Plasma Spray (SPS) is capable of producing coatings with porous columnar structure, and it is also a much cheaper process compared to the conventionally used Electron Beam Physical Vapor Deposition (EB-PVD). Although TBCs fabricated using SPS have lower thermal conductivity as compared to other commonly used processes, they are still not commercialized due to their poor lifetime expectancy. Lifetime of TBCs is highly influenced by the top coat microstructure. The objective of this work was to study the TBCs produced using axial SPS with different process parameters. The bond coat was deposited using High Velocity Air Fuel (HVAF) spray. Influence of the microstructure on lifetime of the coatings was of particular interest and it was determined by thermal cyclic fatigue testing. Thermal conductivity of the coatings was determined by laser flash analysis. The results show that axial SPS could be a promising method of producing TBCs for high temperature gas turbine applications.

Abradable coatings are typically applied on the compressor section of gas turbines to reduce air leakage and increase compressor performance. In pursuit of engine efficiency, the service temperatures of the components are higher than before. The use of nickel-graphite coating in compressor applications in higher temperature environments diminishes the abradable property of the coating. In the current study, a series of abradable coatings were prepared with combustion and plasma spray methods and tested at gas turbine conditions. Coating microstructure, hardness, abradability, and erosion resistance was investigated and compared against conventional nickel-graphite coating. In addition, coatings were aged to mimic the aging cycle in industrial gas turbines and compared to as-sprayed coating properties.

MCrAlY(M=Ni, Co, or Ni-Co)coatings with good high temperature oxidation resistance have attracted great interest. They are widely used in gas turbines as protecting layers, such as thermal barrier coatings and seal coatings. Among many methods developed for preparing MCrAlY coatings, electroplating has drawn great attention due to its perfect bond strength, precise controllability, good coating ability for complex shape and so on. In this paper, the MCrAlY coatings have been prepared by a composite plating way. During this process, the CrAlY particles are wrapped with Ni clad layer. The thickness of the composite coatings is controlled at 150- 200 μm. The plating tests results indicate that the density of the clad layers mainly depend on the electroplating time. After that, these coatings are heat treated under the vacuum condition to make elements diffuse, forming homogeneous M(Ni)CrAlY component. The high-temperature oxidation resistance tests of the prepared coatings show good antioxidant ability at 1000 °C under air condition.

Suspension plasma spraying (SPS) process has attracted extensive effort and interest as a method to produce fine-structured and functional coatings. In particular, thermal barrier coating (TBC) applied by SPS process has gained increasing interest due to its potential for producing coatings that provide superior thermal protection of gas turbine hot-section components as compared to conventional APS-TBC and even EB-PVD TBC. The unique columnar architecture and nano- and submicron sized grains in a SPS-TBC coatings demonstrate some advantages in thermal shock durability, low thermal conductivity, and high-temperature sintering resistance. This work addresses some practical aspects of using the SPS process for TBC applications before it becomes a reliable industry method. The spray capability and applicability of SPS to achieve uniform thickness and microstructure on curved substrates was evaluated in designed spray trials to simulate industrial parts with complex configurations. The performance of SPS-TBCs in erosion, free falling ballistic impact, and indentation loading tests was evaluated to simulate SPS-TBC performance in turbine service conditions. The behaviors of SPS-TBCs in those tests were correlated to key test factors including grit incident angles, impact object sizes, indentation head shapes, and coating surface curvatures. Finally, a turbine blade was coated and sectioned to verify SPS sprayability in multiple critical sections. The SPS trials and test results demonstrate that SPS is promising for innovative TBCs, but some challenges need to be addressed before it becomes an economical and reliable industrial process, especially for gas turbine components.

Cyclic oxidation failure of Atmospheric Plasma Sprayed Thermal Barrier Coatings systems (APS TBCs), commonly used to insulate hot sections in gas turbines, usually results from the spallation of the ceramic top coat. Consequently, in order to predict such spalling phenomena, understanding the mechanisms for cracks initiation and propagation in thermal barrier coatings is of utmost concern for engine-makers. Failure of the TBC is strongly related to the thermal and mechanical properties of each component of the multi-materials system (substrate, bond coat and ceramic) but also to the response of the TBC as a whole. The purpose of the work is to assess the mechanical behaviour of thick TBC using experimental approach for TBC standard lamellar, porous and microcracked microstructure (classically obtained through APS coatings). The experimental characterisation of the mechanical behaviour of the ceramic top coat of the TBC is addressed on specifically designed and prepared free-standing specimens using three points bending (3PB) tests and Small Punch Testing (SPT). The tests are performed on free-standing top coats made of YSZ in the as deposited states and for specimens that undergone isothermal aging at 1100°C for various durations (1h, 10h and 100h). The results of test performed at room temperature using both mechanical testing techniques are compared. This allows to show the evolution of mechanical properties after thermal aging. Tests performed at 850°C in the SPT ring show that the evolution of properties resulting from this aging may be different at room temperature as compare to 850°C.

Improvement in the performance of thermal barrier coating systems (TBCs) is one of the key objectives for further development of gas turbine applications. The material most commonly used as TBC topcoat is yttria stabilised zirconia (YSZ). However, the usage of YSZ is limited by the operating temperature range which in turn restricts the engine efficiency. Materials such as pyrochlores, perovskites, rare earth garnets, etc. are suitable candidates which could replace YSZ as they exhibit lower thermal conductivity and higher phase stability at elevated temperatures. The objective of this work was to investigate different multi-layered TBCs consisting of advanced topcoat materials fabricated by Suspension Plasma Spraying (SPS). The investigated topcoat materials were YSZ, dysprosia stabilised zirconia, gadolinium zirconiate, cerium doped YSZ and yttria fully stabilised zirconia. All topcoats were deposited with TriplexPro-210 plasma spray gun and radial injection of suspension. Lifetime of these samples was examined by thermal cyclic fatigue and thermal shock testing. Microstructure analysis of as-sprayed and failed specimens was performed with scanning electron microscope. The failure mechanisms in each case have been discussed in this article. The results show that SPS could be a promising route to produce multilayered TBCs for high temperature applications.

Water droplet erosion (WDE) is a well-known phenomenon. This type of erosion is due to the impingement of water droplets of several hundred microns to a few millimeters size at velocities of hundreds of meters per second on the edges and surfaces of components. The solution to this problem is in high demand especially for the moving blades of gas turbines’ compressors and those operating at the low-pressure (LP) end of steam turbines. Thermal sprayed tungsten carbide based coatings have been the focus of many studies and are industrially accepted for a multitude of wear and erosion resistance applications. The present work studies the microstructural, phase analysis and mechanical properties and their effects on water droplet erosion resistance of such coatings deposited with high velocity oxygen fuel (HVOF) and high velocity air fuel (HVAF) processes. The feed nano-agglomerated tungsten carbide-cobalt powders are in either sintered or non-sintered conditions. The WDE tests were performed using 0.4 mm water droplets at 300 m/s impact velocity. The study shows promising results for this cermet (better than the Ti6Al4V bulk material) as WDE resistant coatings when deposited using HVOF or HVAF processes under optimum conditions.

This paper analyses the influence of specific coating parameters such as robot velocity, spray distance and part cooling on the risk of crack formation within Chromium- Carbide / Nickel-Chromium coatings. To understand the effect in more detail, metallographic investigations were conducted. These also provide sufficient data to examine other important coating characteristics such as porosity, mechanical stresses and homogeneity. As an additional analytical method Element Mapping is utilised to show the level of oxidation and its impact on the coating microstructure. The methods X-ray diffraction (XRD) and In-situ coating property (ICP)-Sensor are used to investigate the development of stresses in different coatings. With the information from all these examinations a concept was derived to achieve thick, crack-free wear protective coatings.

High efficiency gas turbine needs high temperature sealing by abradable porous ceramic coatings. In this study, porous Al 2 O 3 coatings were deposited by flame spraying through controlling melting of spray powder particle in a semi-molten state. The effect of melting degree of spray particles changed via spray conditions on coating microstructure and porosity was investigated. The melting degree of spray particles was characterized by using 3D confocal laser microscopy. The porosity of the coating was estimated by image analysis. The results showed that the melting degree of alumina particles can be changed from 80 down to 30% and thus the coating porosity can be increased from 30% up to about 60%. The standard hardness test yielded no effective data for the porous coatings deposited by spray particles of a melting degree less than 60%, and hardness of 32-75 HR15Y for Al 2 O 3 coatings deposited by spray particles with a melting degree higher than 60%. The pin-on-disk abrasion test of Inconel 738 nickel-based superalloy spherical pin of 5 mm in diameter at room temperature against porous alumina coating was conducted to evaluate abradability of porous Al 2 O 3 coatings. It was found that for the coatings of hardness less than 32HR15Y and porosity over 40% the wear weight loss of the IN738 pin became negligible despite high wear rate of the coating. It is evident that the flame-sprayed porous alumina coatings of high porosity by the present approach are promising abradable coatings applicable to gas turbine operating at high temperature.

A new method for fabricating microsensors which can provide accurate real-time temperature monitoring of thermal barrier coatings on gas turbine engines was developed. A high temperature K-type thermocouple sensor for hostile environments was deposited using a coaxial pulsed laser cladding process with optimized process parameters giving minimal intrusive features to the substrate and afterwards embedded in typical ceramic layers. The dimensions of the cladded thermocouple were about one hundred microns in thickness and width. The thermal and electrical response of the cladded thermocouple was tested before and after embedding over temperatures ranging from ambient up to approximately 500 °C in a furnace with flowing argon as protective gas. The results were compared to that of a commercial standard K-type thermocouple, which indicate that laser cladding is a promising technology for manufacturing microsensors for in-situ monitoring in harsh operation environments.

With today’s continuously increasing demand for flexibility in heavy duty gas turbines operation, power plant owners are forced to change their operation regime from base load to cyclic operation. The required coating properties for the two regimes are contradicting and cannot be optimized with current MCrAlY systems. Furthermore, for each turbine component, as well as in individual part locations, the loading boundary conditions are differently weighted. For an overall optimized component protection it is therefore of interest to produce coatings with flexible and individually tailored properties. In this context, ALSTOM invested into the development of an Advanced Modular Coating Technology (AMCOTEC), which is based on several powder constituents and a new application method, allowing in-situ compositional changes. With this approach, coating properties, such as oxidation, corrosion, erosion resistance, cyclic lifetime etc. can be modularly adjusted for individual component types and areas. This also includes production of functionally graded coatings, without changing the chemistry of powder fractions.

Degradation of hot section components in the turbine section of gas turbines used in the power generation industry can cause significant problems including financial penalties associated with down time and a decrease in operating efficiency. Thermal barrier coatings (TBCs) have been used effectively for the past two decades to mitigate such losses. The advent of dense and vertically macro-cracked TBCs has seen increasing use, although there is little published data as to its efficacy in performance vis-à-vis standard TBCs. This paper will present some mechanical and high temperature test data of such coatings. Effects of spray process and powders will be examined and discussed. The purpose of this paper is to share some of the experiences of a service provider in discussing various coatings options available to the user of the gas turbine.