When gas turbine designers and engineers run the initial tests on a new power system, they want to know the temperatures at which the hot-section components, especially the turbine blades, had operated. Accurate temperature measurements tell the specialists if the engine is functioning within its design limits and whether it might run at a higher (maximum) temperature and thus, efficiency level.
Interest in these nondestructive thermal measurement techniques has been spurred of late by the need for power plant operators to ensure that their turbines can withstand repeated rapid ramp-ups to supply power to the grid when new solar and wind stations and other intermittent sources of electricity go off-line, said Jörg Feist, a London-based mechanical engineer who works to develop luminescent sensor materials and associated instrumentation for applications in engines.
Larger-scale aerospace and industrial turbine manufacturers often maintain expensive on-line means to nondestructively measure operating temperatures, which is the business of one of the firms for which Feist works as Managing Director, Southside Thermal Sciences (STS). But some companies, he said, traditionally use "sensor" coatings, such as thermographic paints, instead. The color-sensitive pigments in the paints indicate visually the temperatures that the parts experienced as the engines ran.
Unfortunately, “they’re made with toxic heavy metal,” he said, and in Europe at least, new rules will substantially restrict their use because they contain lead chromates and Pigment Red, which are proscribed under the European Union’s REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) regulations.
Using thermal paints themselves can be a laborious and costly process, he noted. A failed test where paint peels off a component or is briefly overexposed can cost several tens of thousands of dollars.
Feist, working with research colleagues at Imperial College London, has developed a thermal-history paint—a ceramic-based coating—that is faster, more robust, and nontoxic. Past temperature exposures can be determined when the coated component has cooled to room temperature. The new coating is being commercialized by a 2002 spin-off of STS, Sensor Coating Systems (SCS) Ltd.
The thermal indicator paint is based on the light-emitting properties of certain oxide ceramics (yttria-stabilized silicon oxide) when it is doped with lanthanide-series elements (often, europium), Feist explained.
These optically active doping elements, which are also found in mobile phone displays, energy-efficient light bulbs, and LEDs, integrate into the ceramic. When the paint is exposed to high temperatures, it undergoes irreversible changes in its structure and chemistry.
“Basically the engine combustion heat-treats the coating, taking it from an amorphous—non-crystalline—state to a crystalline one,” he said.
The phosphor dopants, he continued, act as atomic-level probes by indicating these structural changes when they are excited by a YAG-Nd laser/fiber optic probe by phosphorescing.
“We can calibrate the amount of crystallinity to the temperature because the optical signal falls off in a predictable way,” Feist explained, likening the process to how the brightness of an old-time radium watch face dims with time (after you "charge" it in a bright light). A company feasibility study has shown that the technique can measure between 250 and 1400°C with an accuracy of ±5 to ±10°C.
The water-based ceramic material can be applied to a component using atmospheric air plasma spraying or electron physical vapor deposition as a long-lasting coating, or, for low-temperature regimes, as a less-robust paint, which gives the end-user greater flexibility.
The readout device can be bench-based or handheld, he said, with the latter enabling surface temperature profiling of a component in-situ to be performed like a borescope-type inspection.
SCS has now established a “user club,” an industrial membership consortium to pool private resources to further R&D on the coating technology. Members include Alstom, MAN Diesel & Turbo, and SNECMA.
“The development program is co-financed by the members, and in return they benefit when the technology is introduced into their design processes,” said Feist.
The sensor coating could find use elsewhere in the chemical and processing industry, as well as among machinery manufacturers, OEMs, Formula One teams, airlines, makers of marine and rocket propulsion systems, and nuclear power companies.