Optical Nondestructive Condition Monitoring of Thermal Barrier Coatings

Abstract

This paper describes recent developments of the thermal barrier sensor concept for nondestructive evaluation (NDE) of thermal barrier coatings (TBCs) and online condition monitoring in gas turbines. Increases in turbine inlet temperature in the pursuit of higher efficiency will make it necessary to improve or upgrade current thermal protection systems in gas turbines. As these become critical to safe operation, it will also be necessary to devise techniques for online condition monitoring and NDE. The authors have proposed thermal barrier sensor coatings (TBSCs) as a possible means of achieving NDE for TBCs. TBSCs are made by doping the ceramic material (currently yttria-stabilized zirconia (YSZ)) with a rare-earth activator to provide the coating with luminescence when excited with UV light. This paper describes the physics of the thermoluminescent response of such coatings and shows how this can be used to measure temperature. Calibration data are presented along with the results of comparative thermal cycle testing of TBSCs, produced using a production standard air plasma spray system. The latter show the durability of TBSCs to be similar to that of standard YSZ TBCs and indicate that the addition of the rare-earth dopant is not detrimental to the coating. Also discussed is the manufacture of functionally structured coatings with discreet doped layers. The temperature at the bond coat interface is important with respect to the life of the coating since it influences the growth rate of the thermally grown oxide layer, which in turn destabilizes the coating system as it becomes thicker. Experimental data are presented, indicating that dual-layered TBSCs can be used to detect luminescence from, and thereby the temperature within, subsurface layers covered by as much as 500 μm of standard TBC material. A theoretical analysis of the data has allowed some preliminary calculations of the transmission properties of the overcoat to be made, and these suggest that it might be possible to observe phosphorescence and measure temperature through an overcoat layer of up to approximately 1.56 mm thickness.

Phosphorescent thermal history sensors

“…Reversible changes in thermographic phosphors have been used for many years to measure surface temperatures or heat fluxes under conditions relevant to gas turbines [12][13][14][15][16][17][18]. There are, however, temperature driven processes associated with phosphors that lead to irreversible changes in their emission characteristics.…”

“…Optical (usually UV) excitation can be used and since the relaxation transitions are often forbidden by the selection rules, emission is long lived and classified as phosphorescence. Commonly used dopants are Europium (Eu) [19], Terbium (Tb) [20], Dysprosium (Dy) [17] and Manganese (Mn) [13]. Relaxation from an excited energy level often occurs by a combination of radiative and non-radiative processes.…”

Accelerated thermal profiling of gas turbine components using luminescent thermal history paints

“…Materials for this application are usually comprised of an oxide ceramic doped with an optically active ion such as a transition metal or rare-earth ion (Brübach et al, 2008;Heyes et al, 2008;Rabhiou et al, 2013). For the lifetime decay method, the dopant ions are excited by irradiating the material with short laser pulses.…”

The integrated monitoring system for running parameters of key mining equipment based on condition monitoring technology

“…Condition monitoring technology has evolved rapidly in recent years, with a host of new tools becoming available to assist mine operators in improving the management of equipment. These tools have capabilities such as remote real-time vital signs and performance monitoring, alarm/event handling, trend analysis, and maintenance workflow management (Werner and Lewis, 2006;Heyes et al, 2008). When these monitoring tools are used in the mining industry, the vital signs data can also be integrated with maintenance history, predictive maintenance data, and operational information.…”