Carbolite equipment for thermal coatings research

A specially designed thermal cycling rig (TCR) and seven chamber furnaces have been supplied by Carbolite to the Siemens Industrial Turbomachinery factory at Lincoln to expand facilities for research and development work on thermal barrier coatings.

The Lincoln factory manufactures small industrial gas turbines for power generation and mechanical drive applications with outputs from 3mW to 13mW. The Carbolite equipment is being used to evaluate coating materials such as yttria-stabilised zirconia (YSZ) bonded to cast and wrought nickel-based super-alloys used in the production of hot gas path components.

While the chamber furnaces provide isothermal exposure for this work, enabling samples to be heated to a temperature, held and then cooled, the TCR allows samples to be subjected to long cycles of heating and cooling that accurately simulate thermal conditions within a turbine. The equipment has a maximum operating temperature of 1500°C, but the maximum temperatures used by Siemens are generally between 1000°C and 1300°C. In future the company may also use the equipment for thermal fatigue testing.

The TCR consists of a heating zone above a loading and quench zone, with an actuator that raises and lowers samples between the two. In a typical long-term test a sample is lifted into the heating zone, heated to a pre-set temperature, held at that temperature and then lowered into the quench area where it is air-cooled before being raised again into the heating zone for another cycle.

Time and temperature are controlled through a cascade system, with over-temperature protection provided for the heating chamber. A probe thermocouple in the centre of the lifting rod provides temperature readings for the control system and is also linked to a six-channel digital-display chart recorder. The heating elements used have been designed to increase resistance to oxidisation and to provide easy replacement when required.