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

Thermally sprayed abradable coatings are essential for improving the performance of gas turbine engines. They act as a protective barrier between the stationary casing and rotating blades. Though a lot of research has been done on abradable coatings, little attention has been paid to comprehending wear mechanisms in the abradable-blade tip interaction. The goal of this project is to create a cost-effective test rig that can evaluate different thermally sprayed abradable coatings and understand how they interact with titanium blade tips under application-relevant conditions. Blade tip velocity, incursion rates, incursion depths, reaction forces, and interfacial temperatures are some of the inputs and outputs that the testing rig can provide. Aiming to validate the rig, this study examined the wear behavior of aluminum, thermally sprayed polyester, and AlSi-40Polyester abradable coating. The reaction forces for aluminum and polyester were overall higher when compared to AlSi-40Polyester. However, thermally sprayed polyester showed the highest interfacial temperatures of all materials tested. The difference in the reaction forces and interfacial temperature correlates well with the different wear mechanisms and thermal conductivities. Overall, the equipment showed to be a promising pre-screening methodology to evaluate and develop novel thermal spray abradable coatings.

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.

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.

The abradable coatings had significantly enhanced turbomachinery performance by acting as a sacrificial seal between rotating blades and stationary casing. Further improvement in seal design to meet the higher energy demand and increase the service time has been the key challenges to solve in the gas turbine industry. Honeycomb seals have become the industry standard clearance seal technique due to their unique design and high structural strength with minimum weight. The present study proposes a concept to form a thermal shock resistance structure to achieve higher temperature capability and improve the reliability of abradable seal structures. A cavity layer of honeycomb seal structure made of SS 321 alloy was coated with advanced high-temperature ZrO 2 +7.5%Y 2 O 3 +4% polyester seal material using TriplexPro-210 plasma spray system. The integrity of a seal structure was assessed by a cross-sectional analysis and evaluation of the coating microstructure. Additionally, the microhardness test was performed to estimate coating fracture toughness, and Object-Oriented Finite Element analysis was used to assess its thermo-mechanical performance. The concept proposed in this study should be further validated to develop the most capable innovative technology for advanced gas turbine abradable seal structures.

For the engines used in small turboprop aircrafts, the introduction of abradable coatings represents a feasible way to reach higher levels of overall engine efficiency, specifically by improving the fuel consumption and increasing the inter turbine temperature margin. Abradable coatings on seals also contribute to improved hot restarts capability of an engine and lead to substantial extension of service life of the rotating counter bodies. In our contribution, we concentrate on flame sprayed nickel graphite abradable coating that can be used in turboprop engines both for seals and clearance control. The focus is the impact of spraying parameters on the physical and function properties of the abradable coating.

Chemical composition differences between the feedstock powder and the final coating of a series of composite abradable coatings were investigated. Graphite filler material mass distributions in the coating, overspray powder and burned power is calculated. A preliminary mechanism of graphite loss during the deposition process is established. It is found that the graphite content in the coating is significantly lower than that in the feedstock powder. Over 70% graphite in the feedstock powder is lost during the deposition process. Melting and shrinkage of the nickel shell of the nickel cladded graphite particle as flying through the flame, which resulted in the exposure of the graphite core to the flame and substrate, is the main reason for graphite loss and chemical composition change between the feedstock powder and the final coating. A random manner of particle structure transformation in the flame and its reactions with the spray environment is concluded as an important reason for the poor process repeatability of abradable coatings.

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.

A commercial abradable coating AlSi-hBN was fabricated by atmospheric plasma spraying and was thermal aged at 450 °C in the atmospheric environment for 1000h. Thermal aging effect on coating abradability, hardness, bond strength and microstructure were evaluated. It was found that coating abradability increased obviously after 2h thermal aging and slightly decreased for extended thermal aging time. A sharp decrease in both coating hardness and bond strength were found after 2h thermal aging and a slow increase occurred with the extended aging time. Relationships between coating properties and microstructure were studied. Decomposition of organic binder and sintering of AlSi matrix metal generated by thermal aging were found to be reasons for coating properties changes.

The Ti 2 AlNb blade is used in high pressure compressor aero-engines to provide high thrust force at relatively light weight. A series of abradability tests was carried out on CuAlNi-graphite, NiCrAl-graphite, NiCrAl-bentonite, and NiCrFeAl-hBN abradable coatings rubbed against Ti 2 AlNb dummy blades with the maximum blade-tip velocity of 300 m/s at 500 °C. In consideration of the effects of an engine’s working conditions, some tests were conducted with incursion rate as the single variable. The scratched surfaces of the samples were observed by the stereoscopic optical camera, and a ratio of the blade wear to shroud incursion depth (IDR) was evaluated to characterize the abradability of coatings. The results show that NiCrAl-graphite and NiCrFeAl-hBN abradable coatings perform very well rubbed against the Ti 2 AlNb blade, and the blade-tip wear is not obvious after abradability tests.

Abradable seals are used in aircraft engine compressor and turbine to decrease fuel consumption. Their role is to minimize the clearance between the rotating blades of an engine rotor and the casing to reduce air leakages (compressor) or air-fuel combustion product leakages (turbine). Operating temperatures in turbines (up to 1800°C) can induce a thermal expansion of the blades and give rise to contacts providing damages to the blades or casing. Thus, in case of contact, the blade should remove the abradable seals applied to the casing without being damaged. Besides, the seal must be resistant to the turbine environment. Direct relationships have been observed between plasma spray operating parameters and abradable coating performances. The aim of this study is to determine those relationships for YSZ-Polyester abradable composite coatings. This study is conducted within the frame work of the 7FP European project E-BREAK to reach the environmental objectives of the European Advisory Council for Aviation Research and innovation.

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 series of abradability tests were conducted on AlSi-hBN coatings, which are commonly used in the compressor section of aeroengines for clearance control. The coatings were sprayed on test plates to a thickness of 1.9-2.0 mm and ground to a finish of 10 μm with 400 grit paper. The tests were carried out in an automated test rig with adjustable temperature, blade tip velocity, and incursion rate. The rig is configured such that the coatings are exposed to rotating blades, making contact with the tips as they pass. In this study, investigators monitored the number of contacts, removing and examining abraded coating samples at a given count total ranging from 200 to 4000. It was found that wear characteristics change with each contact between the coating and blade tip, indicating that pass number is a factor that must be considered when testing abradable coatings.

The processing conditions, microstructural and tribological characterizations of plasma sprayed CoNiCrAlY-BN high temperature abradable coatings are reported in this manuscript. Plasma spray torch parameters were varied to produce a set of abradable coatings exhibiting a broad range of porosity levels (34-62%) and superficial Rockwell hardness values (0-78 HR15Y). Abradability tests have been performed using an abradable-seal test rig capable of simulating operational wear at different rotor speeds and seal incursion rates. These tests allowed determining the rubbing forces and quantifying the blade and seal wear characteristics for slow and fast seal incursion rates. Erosion wear performance and ASTM C633 coating adhesion strength test results are also reported. For optimal abradability performance, it is shown that coating hardness needs to be lower than 70 and 50 HR15Y for slow and fast blade incursion rate conditions, respectively. It is shown that the erosion wear performance, as well as, the coating cohesive strength is a function of the coating hardness. The current results allow defining the coating specifications in terms of hardness and porosity for targeted applications.

Thermal spray processes are generally employed to deposit dense coatings. The porosity in a thermal spray coating is limited up to about 20% down to less than 1%. The porous abradable coatings can be deposited by using composite powders containing pore-forms such as polymer. Recently, an effective method to deposit porous coating are being developed by directly utilizing semi-melted spray particles through controlling coating surface temperature. In this article, the recent investigations on the deposition of porous materials and ceramic abradable coatings by surface-melted spray particles are reviewed. The bonding formation between particles by controlling deposit surface temperature is essential to form porous deposits. By using flame spraying, different metallic porous deposits up to tens of millimeter thick from refractory molybdenum (Mo) to stainless steel are fabricated with a porosity level up to 70%. Porous alumina (Al 2 O 3 ) and yttria-stabilized zirconia (YSZ) with a porosity of over 60% are deposited for high temperature abradable coating applications directly by semi-molten ceramic particles. The deposition of convex-shaped YSZ particles is employed to construct the high performance structured cathode for solid oxide fuel cell application. Moreover, the deposited convex-shape particles are also utilized to fabricate effective super-hydrophobic surface. The recent progress on the deposition of surface-melted spray particles will enable many new applications for thermal spraying.

High temperature gas turbines require ceramic abradable coatings for sealing to ensure high efficiency. In this study, a novel method is proposed to deposit porous ceramic coatings through deposition of ceramic spray powder particles in the semi-molten state. The commercially available alumina (Al 2 O 3 ) powders were spheroidized and screened to a particle size range from 40 to 50 µm for spray deposition. Flame spraying was employed for coating deposition. During deposition, the substrate surface was kept at 500°C. The effect of melting degree of spray particles on coating microstructure was investigated by changing the flame power and spray distance. The porosity of flame-sprayed Al 2 O 3 coatings was estimated by image analysis on coating cross-sectional microstructures. The results showed that porous Al 2 O 3 coatings were successfully prepared with a porosity range up to 59% by flame spray. Moreover, spray parameters such as acetylene flow rate and spray distance have significant influences on the particle melting state, thus the microstructure and the porosity of the coating. With the decrease of acetylene flow rate and spray distance, the porosity of coatings increased due to the decrease of the melting degree of the sprayed particles. At a spray distance of 20 mm, when the acetylene flow rate was reduced from 400 to 200 L/h, the porosity increased from 37% to 59%. The results clearly demonstrated the feasibility to prepare porous abradable coatings of high porosity through surface-melted spray particles.

Compressor abradables coming into operational contact with bare, un-tipped titanium alloy rotor blades over a wide range of incursion conditions require excellent cuttability in order to avoid blade tip damage by wear and over-heating. This is more easily achieved for low temperature systems that can make use of low shear strength aluminum matrices than for compressor abradables operating closer to the maximum allowable temperature of advanced titanium alloy blade materials. In this case the rotor path linings will have to incorporate higher temperature resistant Ni and Co alloy matrices. To that end the availability of abradable coatings capable of operating at up to 550°C while showing little thermal ageing effects and excellent abradability over their entire service life can influence the compressor blade material selection and therefore compressor weight and performance characteristics. This paper provides an overview of titanium blade friendly compressor abradable concepts. Particular emphasis will be placed on the abradability of in-service and next-generation coatings designed for use up to the temperature capability of Ti blade rotor materials and beyond. Candidate coatings are also screened for other performance criteria such as thermal cyclic resistance and ageing behaviour.

Gas turbine efficiency is of paramount importance in the modern carbon conscious global economy and the industry is always looking for ways to improve the efficiency of gas turbine engines. Gas bypass between the rotating turbine blade tip and the engine casing affects both the efficiency and the power output of an engine. An increase in this clearance of 125µm can result in an increase of 0.5% in specific fuel consumption. Abradable coatings have been designed to allow the turbine blade abrasive tip to cut a path into shroud abradable coating to improve the seal between the blade tip and the casing. A holistic approach to improving the abradable system – the abradable coating and the blade tip – is necessary. Better blade tips can result in use of denser, more erosion resistant abradables improving performance of the whole system. Current blade tips are limited as the matrix oxidizes at high temperature losing its ability to hold as well as protect the CBN particles. Improvement in blade tips – both in the cutting particles and the matrix which hold these particles – will therefore improve the abradable system performance, as well as allow the use of denser, more erosion resistant abradable materials. This paper represents efforts to improve the matrix oxidation resistance which holds CBN particles. The matrix is a low-aluminum MCrAlHf which is further aluminized to improve the oxidation resistance. New coatings being tested are enhanced aluminide coatings, platinum aluminide coatings and platinum chromide aluminide coatings. The results will be discussed in terms of matrix composition and microstructure as deposited and after static oxidation. The effect of matrix and its impact on the blade tip performance will also be reviewed.

Over the year’s polymers have been used in thermal spray coatings. The most known usage is as a polymer used to seal porosity in thermal spray materials. The sealers mostly used do not really work because they do not penetrate very far into the coating. Once the coating is machined or ground the sealer is gone and we have a porosity coating due to fail. One major sealer resin system used, is Phenolic as the resin, that resin is older than most of the audience and loaded with flammable acetone. It also is not a high temperature material nor does it last long since it is a rigid material and does not expand or contract. The old sealers mostly had solvent systems in them and were replaced with a water born polymer. However, our industry has not kept up with the technology of resin based systems. Water systems sound good environmentally but it does not penetrate and really does not work well for this application. Later polyester was used with a ceramic filler to produce an abradable coating for the seal area for turbine applications. More recently higher temperature polymers were used to create a high temperature abradable coating. I want to show you some new polymeric materials that have done some new applications and also recommend additional work for the thermal spray industry for improved applications using the technology of tomorrow.

Plasma-sprayed ceramic coatings are often used as thermal barrier or abradable coatings in high-pressure stages of gas turbines. They are exposed to high thermo-mechanical loadings, due to the harsh operating conditions. Today, a material typically used in engines as thermal barrier coating material is yttria-stabilised zirconia (YSZ). This material has a low conductivity and a high thermal expansion coefficient, but a limited temperature capability of about 1200°C in long-term applications. For the use as abradable coatings, thicker coatings with a thickness above one millimetre are necessary. However with increasing coating thickness and limited cooling efficiency there is a risk of premature failure. As a result new ceramic materials have been developed. For the lifetime analysis they were tested by thermal gradient cycling tests. In the present work an APS ceramic double-layer topcoat composed of 7YSZ and a top layer of non-stoichiometric magnesia alumina spinel (Mg-Al-Spinel) was used. The layer was sprayed on disc-shaped IN738 superalloy substrates which were coated with a VPS bondcoat. Under specific thermal cycling conditions with temperatures above 1400°C, these samples showed a typical failure mechanism with exfoliation of thin coating lamellae, starting from the coating surface. This failure mechanism was analysed in detail, e.g. by scanning electron microscopy (SEM), X-ray diffraction, and chemical analysis. From these findings, a description of the failure mechanism was developed.