On a cycle – thermal barrier coatings

Materials World magazine
3 Sep 2010
A coating system consisting of a superalloy substrate, a MCraly bondcoat and the doped thermal barrier is tested under extreme temperature gradient conditions in a laboratory. Visible is the (bluish) flame of the burner impinging on the coating. The surface temperature is about 1,250ºC. The substrate is kept at much lower temperatures. Typical gradients of 200K to 300K are achieved over a coating thickness of 300μm.

Trials of a novel phosphor-doped thermal barrier coating (TBC) have been completed at RWE npower’s power station in Didcot, UK.

Tested for 4,000 hours in a Siemens V94.3 (A) gas turbine, the doped yttria-stabilised zirconia (YSZ) coating is said to have remained in good condition, while the conventional YSZ TBC delaminated. This could enable longer life TBCs, reducing the frequency and costs of maintenance, and increasing efficiency.

The new coating (its exact composition remains confidential) is manufactured using conventional atmospheric plasma spraying equipment, as applied in the power generation sector. It is therefore ready for market entry, claim inventors at UK firm, Southside Thermal Sciences (STS) Ltd, based in London.

Managing Director Joerg Feist says, ‘The mechanism is not completely clear, but some micrographs show that the standard coating ‘crumbles’ at the surface due to phase changes inside the material and [experiences] cracking. The new material shows self-healing where the dopant fuses the cracks’.

Furthermore, STS reports that its product survives temperatures exceeding 1,300ºC in tests at the firm’s laboratory, while standard TBCs show degradation beyond 1,200ºC. This could enable gas turbine engines to operate at higher temperatures for increased efficiencies.

The invention originates from STS’s vision for a smart ‘sensor coating’ for gas turbine engines, where the phosphors’ luminescence under excitation light is remotely monitored to measure the surface and subsurface temperatures (first covered in Materials World, June 2006, p12).

Approximately, 100 doped compositions were formulated and investigated for this purpose using isothermal cycling, harsh thermal gradient cycling (beyond 1,350ºC) and burner test rigs. In doing so, one of the formulations was found to exceed the durability of standard TBCs at elevated temperatures.

Lighting the way

Progress has been made on the sensor solution through a programme funded by the UK’s Technology Strategy Board.

‘The driver is accurate temperature detection,’ explains Feist. ‘If you combine surface and subsurface measurements, you get a heat flux gauge of the component. That would be extraordinary, [as] a 50ºC difference in temperature measurement could reduce the life of your coating by a factor of three. Our technology would predict the temperature plus or minus five degrees’, giving a clearer indication of the TBC’s remaining life.

Furthermore, he says, a more accurate reading of the component’s temperature, will enable operators to run engines as close to maximum temperatures as possible, rather than leaving a bigger margin for fear of part failure.

Research into the remote monitoring system is ongoing, but Feist says a key ‘milestone’ is the development of a ‘unique’ optical probe, which follows a moving light spot on a rotating turbine blade.

He outlines that, although there are thermographic paints in existence for such use, ‘they [unlike the new TBC] come off after several hours of running’, and ‘the image of the spot moves across the optic, distorting the signal’.

He says, ‘We now have a [static] optical probe that allows us to follow a moving light spot with a high acceptance angle of 12 degrees without any deterioration in the signal from some distance. It probably has applications in other areas, such as concentrated photovoltaics’.

The aim is for the light to be collected by the probe, and sent down a fibre optic cable to a remote electronic detector.

Feist adds, ‘The value of the system is relative to the price of the gas you are firing’. The technology could result in a one per cent fuel saving in a market that was worth US$600bln in 2005-6 worldwide, he says.

The team is now looking for potential customers for the coatings, as well as funding to enable testing of its sensing solution on a live operating engine.

The firm also has derivative technologies to offer – a coating that can continuously detect and store information about its thermal history and a product that can identify wear in metals.

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