ICMCTF2005 Session A4: Clearance Control

The difficulties associated with generating material property data for thermally sprayed coatings are well known. This is especially an issue for abradable coatings, due to their inherently low mechanical strength and often complex thermal behaviour. In addition, a lack of globally recognised testing standards has lead to a range of company specific in-house test methods, which often provide data that is difficult or impossible to cross-reference. These factors have highlighted the need for robust industry standard methods for the testing and analysis of abradable coating materials.

Material property data, which can be linked to abradability performance and thermal behaviour, is an important element in developing a robust fundamental understanding of abradable coatings. This data will not only assist with the coating manufacture and source control of current abradable systems, but also provide an essential component in the development of new abradable coatings.

This paper highlights the key aspects of a two-phase process, aimed at the development of a suite of internationally recognised test standards for abradable coating materials. These will facilitate a greater fundamental understanding abradable materials and support future coating developments.

Phase one of the process involved evaluating the test methodologies that are currently available, and defined where possible, the standardised parameters and process conditions. This work initially focused on the increasing number of abradability test facilities and the testing of abradable materials over a range of parameters and test conditions. The capability of each facility was assessed against specific testing objectives and analytical requirements. Generic abradability parameters, which are independent of facility specific variables, such as disc size and specimen geometer were defined. These will provide the building block of a test standard for future abradability testing.

Phase two highlights strategic areas, where there is a requirement for robust test methods or analysis techniques. These encompass fundamental mechanical test methods as well as the latest non-destructive evaluation (NDE) techniques, which have potentially far reaching applications from source control at manufacture to 'on-wing' inspection.

As part of this study, a process has been developed by Rolls-Royce (patent pending) for the mechanical testing of freestanding thermally sprayed coating materials. This enables abradable material test pieces to a range of geometries, to be tested using standard mechanical testing equipment in a reproducible manor.

NDE techniques could potentially offer significant quality control and cost saving benefits to coating suppliers and engine manufacturers alike. Recent developments in capability and equipment mean that robust techniques, can be carried out on an industrial scale often using potable or even hand held equipment with extremely high precision.

This study covers the aspects of abradable material testing and analysis discussed above and highlights future requirements of abradable testing, which would benefit from standardisation and globally recognised procedures.

The abradable testing facility at Sulzer Innotec consists of a rotor, movable specimen stage and heating device; the infrastructure being that of a large turbine balancing facility with a 2 MW electric rotor drive. A wide range of aero, power and steam turbine OEMs have used the rig over the past ten years, usually in partnership with abradable coating developers.

The special features of the rig are its capacity for abradable evaluation at temperatures up to 1200°C, blade tip speeds from 30 to 410 m/s and blade incursion rates into abradable shrouds from 2 to 2000 µm/s. The broad range of test conditions available cover the entire in-situ conditions experienced by fan, compressor and turbine clearance control systems.

In recent years, the facility has diversified into elevated temperature testing of labyrinth seal systems using a dedicated rotor with removable seal strip capability. Also available are testing of instrumented blades, with measurement and evaluation of blade foil and root vibration modes during incursion at temperature into shroud materials. An overview of the rig features will be provided together with a brief look at the evolution of some general abradable technology trends over the past ten years.