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Gas Turpines: The Black Art of Abradable Coatings

Abradable coatings can result in up to one per cent improvement in the efficiency of gas turbines but the understanding of their properties is relatively slight. Rolls-Royce, in conjunction with a British university, has developed a unique testing process designed to bring forward the understanding of abradable coatings into the 21st century.

Noel Hopkins, Surface Engineering Group, Rolls-Royce, UK and Swansea University, UK

Suck, squeeze, bang, blow! The four-stage process of a gas turbine engine relies on the ability of the engine to maintain high-pressure ratios, and minimise gas leakage. Abradable coatings minimise the clearance gap between compressor blades and rotor path shrouds by wearing preferentially to the rotating compressor blades during engine running, thus minimising ‘over-tip’ gas leakage.

The properties of abradable coatings must be uniquely balanced; providing the strength to withstand the harsh gas turbine environment, and the wear characteristics to minimise compressor blade wear. As thermally sprayed coatings, quantitative property data is relatively sparse and inherently difficult to generate in an accurate and representative manor. As a result the understanding of abradable materials remains at an empirical level despite over 30 years of service experience.

The Freestanding Coating Test Process has been developed by the Surface Engineering Department at Rolls-Royce as a method for the accurate generation of representative thermally sprayed coating property data. It is hoped that this material property data will provide a fundamental understanding of these unique materials and offer some much needed science, to what has been traditionally regarded as a black art technology.

In order to generate representative coating property data in an accurate and reliable manner it is necessary to distinguish between the properties of the coating and the substrate material. This is particularly vital for abradable coatings, which have extremely low strength and stiffness properties compared to the substrate materials.

Thermal spraying replication

The Freestanding Process relies on water soluble moulds manufactured from a polymer-composite material. The mould material is able to withstand temperatures of up to 200à‚ºC, and critically breaks down into a non-toxic powder when placed in water. By arranging the moulds on a rotating stage it is possible to replicate the thermal spraying process used in the manufacture of a compressor rotor path shroud. Abradable coating material is sprayed onto the mould surface until the mould cavity is filled. Following spraying the mould material is dissolved in water leaving a freestanding coating material of near net-shape geometry. Minor machining and finishing operations are then carried out in order to improve the surface finish of the test piece, which can be tested using standard mechanical test equipment as shown in Figure 3.

Figure 3. The Freestanding Coating Process
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Initial validation studies and subsequent mechanical testing focusing on load controlled tensile testing has been carried out at Swansea University in the United Kingdom. Swansea is a key Rolls-Royce University Technology Centre focussing on the development of advanced materials. Figure 1 shows a Freestanding Coating tensile test piece with strain gauge following failure and some typical stress versus strain curves for an Al-Si+Hexagonal Boron-Nitride (hBN) abradable material.

Figure 1. Stress/Strain Curves for Al-Si+hBN abradable and tensile test piece following testing
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A number of notable characteristics can be observed from the results shown in Figure 1. The coating material has a low strength (~18MPa) and a low strain to failure (<0.3 per cent), but the deformation modes during loading are also interesting. From initial loading until failure there is no apparent yield stress at the transition from elastic to plastic deformation. Instead permanent damage is apparent following very low loads. This suggests that the coating fails in a progressive manor as the various phases and interfaces within its structure crack and de-bond.

In order to better understand the failure mechanism associated with an abradable coating under a tensile load it is useful to consider the microstructure of these materials. Within the structure of the abradable testing in Figure 2, there is an Al-Si matrix phase, with randomly distributed particles of hBN. hBN is an extremely low strength and highly lubricious platelet material with similar properties to graphite. It is designed to enhance the abradability of the material. It is therefore reasonable to assume that it is the hBN phases within the material, which plastically deform first.

Figure 2. Microstructure of Al-Si+hBN abradable coating
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The interfaces between hBN particles and Al-Si matrix as well as adjacent Al-Si, which would inevitably contain oxide layers and micro-porosity, form a matrix of weakening planes within the material structure. It is therefore reasonable to assume that these interfaces fail ahead of any intergrannular or particle shear within the Al-Si matrix.

From the results of the initial tensile testing of Freestanding Coating materials, it is clear that abradable materials fail at low strains and at low stresses. It can also be shown that deformation is a progressive plastic mechanism, with the materials displaying no pure elastic deformation or specific yield point.

A crack in the coating material most probably initiates within the weaker hBN particles and at interparticle interfaces. It is therefore reasonable to assume that the coating structure has a strong influence on the thermal fatigue resistance of the material.

Coatings prone to damage

Optimisation and process control are clearly key to producing robust coating systems; however, it is also necessary to understand the key performance drivers necessary for specific failure modes. The Freestanding Coating testing has highlighted the susceptibility of abradable coatings to plastic damage at extremely low tensile stresses.

Aquapour mould containing test piece cavity
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The Freestanding Coating Process allows a range of test pieces to be created, providing the opportunity to generate a fundamental understanding of abradable coatings and other thermally sprayed materials with accurate and representative material property data. The technique is now being evaluated within Rolls-Royce for mechanical analysis of thermal barrier coatings and other thermally sprayed materials.

This article first appeared in Materials World magazine.