Inspecting thermal barrier coatings by IR thermography (original) (raw)
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Detection of Defects in Thermal Barrier Coatings by Thermography Analyses
MATERIALS TRANSACTIONS, 2001
Thermographic imaging under steady-state heat flow was used to nondestructively detect the defects in thermal barrier coatings (TBCs). The finite element method (FEM) analyses and the experimental thermography observations were performed using artificially grooved ZrO 2 plates and the indentation-tested TBC-metal specimens. The FEM results show that: (1) Defects (nonuniformity or internal cracks) of the TBC can be effectively detected by the thermographic imaging method; (2) The apparent thermal images would be far greater than the real sizes of the defects, but the half-height sizes of the temperature profile were found to give good estimates for the latter; and (3) The higher value of heat flow would contribute to the detections. Besides these results, the influence of the defect situations (morphology, size and position) on the thermal images was also predicted by the FEM analysis. Although, due to some difficulties in preparing the test specimens, the quantitative comparison between each FEM result and that of the actual measurement was not performed, however, the experimental results of the grooved ZrO 2 plates were found to be in good agreement with the FEM predictions. For the thermographic experiments of the indentation-tested specimens, both the internal-cracks and the thickness-nonuniformity of the TBCs were successfully observed, and the smallest cracks detectable had a diameter far less than 1 mm.
Defect assessment in gas turbine blade coatings using non-contact thermography
Materials Today: Proceedings, 2020
Thermal Barrier Coatings (TBCs) are inevitably applied on combustion chamber liners, gas turbine buckets, gas turbine vanes, etc. for lowering the metal temperature. They play a major role in enhancing the life of components exposed to higher temperature during operation of gas turbines. Performance of TBCs is dependent on the coating procedure, type of coating material and bond strength between bond coat layer and TBCs. Amongst various techniques which are available for quantitative and quality assessment of the TBCs, non-contact thermography is one of the most promising techniques. The present work deals with simulating thermal cycling on coatings using Finite Element Methods (FEM) for understanding possible locations of residual stress development within the coatings due to thermal cycling. Subsequently, experimental thermal cycling of TBCs and their sequential study of defect formation using non-contact thermography are reported. Comparison of experimental studies and FEM results showed good correlation. Further, microstructural characterization of samples exposed to thermal cycling confirmed that the defects observed during thermography are due to delamination. These studies will be useful to predict defect formation in TBCs during operation of the gas turbine components.
Proceedings of the 2014 International Conference on Quantitative InfraRed Thermography
Spallation of Thermal Barrier Coatings (TBCs) in gas turbines is the result of the insidious initiation and propagation of microcracks at the interface with the substrate. This temperature driven mechanism is difficult to detect in its early stage and the difficulty of determining accurately the interface temperature hinders the development of reliable lifetime prediction models. In this work two non destructive diagnostic methods based on IR thermography and photoluminescence are compared for early damaging assessment of TBCs prepared by a sol-gel route containing precalibrated delaminated areas and integrating a Eu 3+ doped photoluminescent layer at the interface with the substrate. The potential of the later method for through thickness temperature sensing is also discussed.
2005
The infrared thermography (thermal imaging) is considered a flexible, non-contact, and non-destructive tool used to evaluate wide variety of coating attributes (e.g. appearance and protective attributes). This paper addresses the application of thermographic systems in evaluating the protective-coating coverage over steel substrates in two different painting industries. First case is the inspection of the ballast ship tanks coating for "Holidays" or missed coated spots. The second describes the development of an automated thermographic system, designed to inspect for: thin coating, coat cracking, and coat chipping of an anti-corrosive paint applied on automotive steel containers.
Transient thermography testing of unpainted thermal barrier coating (TBC) systems
NDT & E International, 2013
Test piece surfaces are sometimes coated with a black, energy absorbing paint before transient thermography is applied. This practice is not acceptable to some thermal barrier coating (TBC) manufacturers and servicers of these systems since thermal barrier coatings are porous so the paint contaminates the coating and it is very difficult and costly to remove. Unfortunately, unpainted TBC surfaces have low emissivity, and after service their colour is usually uneven. The low emissivity gives low signal levels and also problems with reflections of the incident heat pulse, while the variation in emissivity over the surface gives strong variation in the contrast obtained even in the absence of defects. Additionally, the TBC is translucent to mid-wavelength IR radiation which negatively affects the location of disbonds based on the thermal responses. This paper investigates the effects of uneven discolouration of the surface and of IR translucency on the thermal responses. It has been shown that unpainted TBC systems can be inspected reliably by using higher power flash heating equipment assembled with an IR glass filter and a long wavelength IR camera. The paper also shows that the problem with uneven surface emissivity can be overcome by applying 2nd time derivative processing of the log-log surface cooling curves.
Active IR Thermography Evaluation of Coating Thickness by Determining Apparent Thermal Effusivity
Materials, 2020
Pulsed thermography is a common technique for nondestructive testing (NDT) of materials. This study presents the apparent effusivity method for the quantitative evaluation of coating thickness in a one-sided thermal NDT procedure. The proposed algorithm is based on determining a threshold value of apparent effusivity, which can be found for particular coating-on-substrate structures. It has been found that the square root of the time at which the apparent effusivity curve reaches this threshold is proportional to the coating thickness. The efficiency of the proposed approach is demonstrated by analytical modeling and experimentation performed on thermally-sprayed coatings.
Recent progresses in the inspection of aerospace components by infrared thermography
2008
The use NonDestructive Testing and Evaluation (NDT&E) techniques for the inspection of aerospace materials has progressively increase in the last few decades as commercial and military aircrafts exceed their initial design life. Aerospace components are typically made of aluminium, honeycomb structures, composites or fibre metal laminates (FML), which are affected by different types of anomalies, namely: delaminations, disbonds, water ingress, node failure and core crushing. Possible causes are either material contamination introduced during manufacture, e.g. dirt, solvents, moisture, oils, etc.; or by damages caused during in-flight operation, e.g. after impact. Infrared thermography techniques have proven to be an effective way to detect and, in many cases, to quantify the degree of surface and subsurface damage on such components. Several configurations have been proposed to achieve an active thermography inspection ranging from passive inspection, i.e. without applying any external stimulation, to active approaches, in which an external source of energy is used to produce a thermal contrast between the non-defective and the defective material. The thermal stimulation can be delivered to the specimen surface by optical, mechanical, electromagnetic or other means. The physics and the practical considerations behind these approaches are different in all cases, and the selection of one or another will depend on the specific application and the availability of the equipment. In this paper, a review of recent developments on the thermographic inspection of aerospace materials is proposed. Some guidelines, supported with examples on real and academic specimens, are provided to illustrate the most suitable technique to be considered for some conventional applications.
Influence on Thermal Barrier Coating Delamination Behaviour of Edge Geometry
Fracture of Nano and Engineering Materials and Structures
Ceramic thermal barrier coatings are commonly used in gas turbine hot components (e.g., combustor liners/buckets and guide vane platforms). In components that are only partially coated or have cooling-air outlets, coating-end stress singularities may lead to the spallation of the coating. Depending on the geometry of the transition from coated to uncoated material, the severity of the stress singularity will vary. One way of decreasing the severity of the stress singularity is by introducing a chamfer angle φ < 90° at the coating end. In the present study, a thin thermal barrier coating system has been studied. Bondand top coats have been sprayed to a thickness of 150µm and 350µm, respectively. Vacuum-plasma-spraying technology was used, and the test specimens were rectangular (30x50x5mm) coupons of a nickel-based superalloy, Haynes 230. A NiCrAlSiY bond coat and an YB 2 B OB 3 B partially stabilised ZrOB 2 B top coat were used. In order to achieve well-defined chamfers, sprayed coupons were ground on the edges with SiC grinding paper to desired geometry. By inspections of cross-sections that had not undergone thermal fatigue cycling, it was ensured that no damage was introduced into the system. Mechanical testing was done in a thermal cyclic test rig where specimens are heated in a furnace and cooled with compressed air. FE modelling of the system has been done, aiming to support the findings from thermal fatigue tests. A parametric study including variation of the chamfer angle φ has been made and the stress state near the chamfer evaluated. Evaluation of fatigue damage can be done visually for observation of coating failure (macroscopic observation on coating surface). 20% area with complete spallation was considered as thermal barrier coating failure. For evaluation of damage development, additional light microscopy investigations of cross-sections have been carried out. Results show that the fatigue life benefits from introduction of a chamfer angle at the coating end during thermal fatigue cycling.
Laser Scanning Thermography for Coating Thickness Inspection
Engineering Proceedings
The paper deals with a new approach to laser thermography for the inspection of coating thickness. The approach is based on scanning the specimen surface point by point, using a low-power laser, and recording the temperature responses with an IR camera. A recorded sequence is then transformed into a sequence similar to a flash pulse thermography sequence. Fast Fourier transform was used as a processing technique. The results are compared with a flash pulse thermography measurement. It was shown that the laser thermography measurement provides a higher sensitivity to thickness changes than flash pulse thermography measurement.