Abstract Boundary layers on surfaces will change from laminar to turbulent flow after a critical length. Due to the differing heat transfer coefficients of laminar and turbulent flow, the point of transition can be detected by heating the surface and measuring surface temperature by thermographic imaging. Locating the transition point is crucial for the aerodynamic optimization of components. In this study, fiber reinforced polymer composites (FRPCs) were chosen as the test substrate. Experiments were conducted using the flame spray process and NiCrAlY coatings. Multilayered coatings consisting of an aluminum bond coat, a layer of alumina as electrical insulation, and a heating layer of titania were fabricated by atmospheric plasma spraying. Free-flight tests were conducted with a functionalized winglet in order to assess the ability of thermally-sprayed heating elements to detect the location of transition of the flow regime. The results showed that the thermally-sprayed elements heat surfaces uniformly, with sufficient radiation losses for thermographic imaging. It was also shown that the change in temperature at the point of transition was readily observable using thermography.

Abstract This paper presents selected research results of the DFG founded project group, consisting of four institutes focusing on diagnostic methods in thermal coating processes. This group is now working for three years on this topic. Utilizing newest or self invented diagnostic devices, it became possible to describe plasma energy fields, particle feeding and plasma/particle interactions. This goal could be achieved by using the enthalpy probe and streak technique for describing the plasma state combined with results of the particle diagnostics, using the DPV2000 system, a newly invented Particle-Shape-Imaging device (PSI) and Particle Image Velocimetry (PIV). Moreover substrate diagnostics is achieved by using pyrometry and thermography techniques. Paper text in German.

Abstract The combination of the internal combustion engine for drive purposes with an auxiliary power unit (APU) for the electrical vehicle on-board power supply is seen as a particularly promising future automotive concept. Although a first demonstration vehicle has already been realized, the market launch of APUs based on solid electrolyte fuel cells (SOFC) depends very much on the use of an economical manufacturing process suitable for series production. To obtain 2-dimensional images of the heat distribution on SOFC surfaces during plasma spraying, a vacuum-sealed IR-camera was implemented in the low pressure vacuum plasma spraying chamber. The thermographic data obtained should be used as boundary conditions for a finite element model in order to calculate the stresses in the ceramic fuel cells resulting from the plasma spraying process. Paper includes a German-language abstract.

Abstract IR thermography offers efficient methods for optimizing thermally sprayed layer composites. The quality of these layer composites depends crucially on the temperature control during the coating process. With the help of thermography, the effect on the temperature distribution when changing any of these process parameters can be precisely observed. In this paper, these methods for the optimization of plasma sprayed planar layer composites of light metal substrates and ceramics, steel substrates, and ceramics, as well as glass substrates and ceramics are applied and discussed. First, the temperature distribution on the component is recorded over the entire coating process. In a further step, a correlation is established between temperature control during the coating process and the resulting internal stress distribution in the layer composite. It is observed that the burner kinematics alone has a significant influence on the resulting residual stress distributions of the layer composite. Paper includes a German-language abstract.

Abstract This paper presents basic studies on atmospheric plasma spraying using particle pyrometry and thermography. Using the example of aluminum oxide and zirconium dioxide, the influences of the particle properties and the substrate temperature on the resulting layer properties are examined. A method is presented with which the essential particle properties can be identified and the individual processes can be classified. The investigations show that the influence of the substrate temperature must not be neglected if layer properties are to be predicted as part of a quality control. Paper includes a German-language abstract.

Abstract The thermal shock resistance of thermal barrier coating depends strongly on the shear stress generated by the thermal expansion mismatch between the ceramic and bond coat layer. Applying a functionally graded structure composing of NiCoCrAlY and YSZ along the coating can mitigate this effect. The paper studied the improvement of thermal shock properties with different number of intermediate layers (2 - 4) added over the temperature cooling range 900 - 30 °C. Acoustic emission (AE) technique was utilised to determine the moment of occurrence of damage within the coatings, and thus help to identify the corresponding failure mechanisms. Cross section analysis of the coatings after thermal shock tests revealed that the coatings generally failed by two mechanisms: edge delamination and segmentation of zirconia topcoat. Failures in the coatings with 2 and 3 intermediate layers (total of 3 and 4 layers respectively in the overall coating) were dominated by edge delamination while the coating with 4 intermediate layers exhibited only segmentation of the top zirconia layer. This points to the fact that interfacial stresses were not critically affecting the integrity of the 5-layer coating (4 intermediate layers plus the ceramic top layer). The cumulative and rate energy results showed that the energy released by the coatings during the thermal shock tests were in the order of 3-layer coating > 4-layer coating > 5-layer coating. The 5-layer coating had demonstrated the best thermal shock resistance among the four coatings.

Abstract Many thermal spray coating applications require an optimum performance regarding the thermal and mechanical stability of the layer composite. The maximum loads that a composite can sustain, are not only dependent on the intrinsic material properties of the coating, but are also subject to the quality of deposition. The quality of the coating is predominantly influenced by the temperature distribution during the deposition process thereby influencing the residual stress development. Therefore failure of a thermally sprayed coating under mechanical and/or thermal load often could be avoided by an adequate deposition process with well controlled heat and mass transfer, i. e. by avoiding hot-spots on the surface that result in high residual stresses in the composite. With the help of Infrared (IR) thermography an imaging of the lateral and spatial temperature field of a workpiece surface and its evolution in time can be monitored and visualised. In the presented work the atmospheric plasma spraying process serves as an example to demonstrate the suitability of thermographic imaging as a quality control and process optimisation technique for online process monitoring and control in thermal spraying. The results indicate that IR-thermography can be used as a flexible tool for on-line process control of coating manufacturing via thermal spraying, it offers a powerful way to optimise the deposition process.

Abstract In this study, a new laser based technique was evaluated for the characterization of plasma-sprayed oxide coatings. It uses the contactless laser generation and detection of ultrasonic waves in the bi-layered systems. For this purpose, a nanosecond pulsed Nd:YAG laser (λ : 1064 nm, τ =14 ns) was used for irradiating the ceramic coating, whilst the longitudinal displacements of the rear surface of the metallic substrate were detected at the epicenter using a laser heterodyne interferometer. The acoustic signal recorded at the rear surface of the substrate was found to be characteristic of the different events taking place within the irradiated system. In this way, the longitudinal wave velocity, the porosity, as well as the Young's modulus of the coatings can be easily determined, whilst the coating/ substrate adhesion strength can be calculated, taking into account both the thermal, as well as the acoustic effects of the laser radiation. The proposed technique was applied to alumina coatings deposited onto stainless steel coupons by Atmospheric Plasma Spraying and the results were found to be in accordance with those obtained by the techniques commonly used for testing thermal spray coatings (interfacial indentation test, porosity measurement, etc.).

Abstract The control of the coating quality becomes more and more important. Infrared methods are well introduced in industry for several types of inspections. The lock-in thermography is based on modulated heat flux and is proved for composite materials and electronic components. In combination with an IR-camera seems to meet the requirements of a modem quality control system. The off-line measurement takes about 3 up to 5 minutes depending on the geometry of the surface. The paper gives an overview on the principle, the technical details of the measurement and the correlation of the detected signals and the coating properties will be discussed. The accuracy concerning the thickness is determined for different coatings (e.g. WC-Co).

Abstract A thermal cycling test rig and procedure was designed in order to predict the life expectancy of Thermal Barrier Coatings (TBC) under thermal cycling conditions similar to those meet in combustion chambers. Two 2kW-halogen lamps highly focused on the TBC were used to expose the surface of the coating to an intense heat flux. A 25x100 mm TBC is Air Plasma Sprayed (APS) centered onto a substrate 25x370 mm. The thermal cycling can be done either under inert or oxidizing atmosphere in order to separate oxidation-induced acoustic emissions from that resulting from the mismatch of the Coefficient of Thermal Expansion (CTE) of the coating compared to that of the substrate. Two transducers located at each end of the substrate monitor the Acoustic Emission (AE) signals emitted by crack initiation and/or propagation, were recorded and analyzed in order to deduce available information about TBC behavior under thermal load. The use of two transducers with a time of flight approach provides a valuable means of identifying both the crack formation and its location. This thermal cycling test is adequate for the study of various samples, like welded substrates coated with TBC or TBC coated around holes. The presence of cracks is observed using metallography preparation and microscopic observation.

Abstract An industrial and cost-effective online quality control method for thermally sprayed coatings will be presented. A new concept in pulse-thermography allows online, during the spraying process, the non-destructive evaluation of coated surfaces. This technique employs a heat source that produces a heat impulse. The impulse is directed toward the examined surface and the from the surface reflected and/or emitted signal is collected by an infrared-camera and subsequently treated in a computer. It will be demonstrated that the spraying process itself can be used as a heat source. In principle, the fading behavior of the signal captured by a high speed infrared camera is observed, or else the progression of the induced heat wave within the coating. Differences in coating thickness, coating and adhesion defects, microstructural changes, an aggregation of pores as well as oxide or metallic inclusions provoke a significant change in the signal intensity and are therefore detected. Pulse-thermography enables the non-destructive assessment of the quality of thermally sprayed coatings. A coated part can be examined to check if the desired coating structure has been successfully attained or if and where there are any areas with critical deviations in respect to coating thickness or coating microstructure. The simple set-up allows the integration of the technique in the production line.

Abstract A thermal cycling test was designed in order to predict thermal barrier coatings (TBC) life of combustion chambers. The thermal cycle is produced by two 2kW halogen lamps highly focused on the TBC. A 25X100 mm TBC is plasma sprayed centered onto a substrate 25X300 mm. The thermal cycling can be done either under argon atmosphere or air in order to be able to discriminate the oxidation induced acoustic emissions from the expansion mismatch. Two transducers located at each end of the substrate monitor the acoustic signals emitted by crack initiation and/or propagation. The advantage of using two transducers is that with a time of flight approach cracking phenomena can be located along the TBC. This process allows the study of welded substrates coated with TBC, TBC coated around holes and to check for the presence of cracks by using metallography preparation. The challenge of the test is to use the early cycles emission signatures in order to predict the long term durability of the TBC.

Abstract Laser infrared photothermal radiometry (PTR) can be used to determine the thermophysical properties (thermal diffusivity and conductivity) and interfacial defects (i.e. disbonding) of various thermal sprayed coatings on carbon steel substrates. PTR experimental results are compared with a one-dimensional photothermal model that can take into account roughness affects and interfacial defects by considering a roughness equivalent-layer and an equivalent-thermal resistance, respectively. The foregoing thermophysical parameters of the thermal sprayed coatings are obtained when a multi-parameter optimization algorithm is used to fit the PTR experimental results. The potential of the PTR technique for in-situ monitoring of the coating process and the characterization of the thermal sprayed coatings will be discussed in this paper.

Abstract Lock-in thermography is a nondestructive inspection method based on modulated heat transport in combination with infrared imaging. Measurement times are relatively short, about 3 or 4 minutes, and the method is amenable to inspecting a wide range of large and small parts. This paper describes the basic principle behind the method and its application to different thermal sprayed coatings (e.g., Cr-steel, Al2O3). It explains how the test samples were prepared with artificial defects simulating delaminations, inclusions, and other types of imperfections and how the method performed in each case.

Abstract The aim of this work is to study the adhesion of WC-Co coatings using acoustic emission testing. The coatings were produced by HVOF spraying then subjected to four-point bending while in situ acoustic emission measurements were made. The test specimens were then examined under a microscope, revealing regularly spaced transverse cracks on the coating surface as well as interfacial cracking. An analysis of the acoustic emission data revealed two types of acoustic events based on absolute energy and amplitude. This appears to be linked to the cracks that were observed forming the basis for an interfacial cracking model.

Abstract Thermal spray coatings play an important role in the field of surface protection. Threats and opportunities in the thermal spray industry include TS process developments, impact of health and safety, standardization, pre/post treatments, non destructive evaluation of coating characteristics and environment. In this paper, the European thermal spray industry size is estimated. Examples of customer requirements from typical Original Equipment Manufacturers to industry end users are also discussed. The paper provides some views on the ability of a particular spray shop to compete and grow given the market conditions. In addition, it discusses internal benchmarks used by Terolab Services. Paper includes a German-language abstract.

Abstract In the automotive industry, efforts to lower the cost of manufacturing and reduce the weight of an automobile are an ongoing process. Plasma spraying of cylinder bores on engine blocks made of light cast alloys has been developed over the five years. This paper provides an overview of the actual situation of the technology for coating cylinder bores, from cast material to non-destructive testing. The aspects of both finishing using honing technology and testing are to be seen as an important part of plasma spray technology for cylinder bores. The paper discusses the results of laboratory and field tests in Europe, Japan, and the United States. This shows that the plasma spraying process is a proven and cost-effective solution for coating cylinder bores. The launch aspect is also discussed. Paper includes a German-language abstract.

Abstract This paper describes the development of an online process control system for spraying thick thermal barrier coatings as part of a Brite Euram research project. In addition to the evaluation of the substrate temperature, a system for acoustic emission analysis for particle impact and for controlling crack propagation is developed. The procedure, the problems overcome, and the results obtained are described. Paper includes a German-language abstract.

Abstract Thermal spraying is a widespread coating process with various applications in many industry branches. To be able to spray successfully on the sensitive materials an in-situ temperature control is most beneficial. The objective of this paper is to find out the correlation between the mechanical properties of some coating-substrate variations and temperature history of the coating made by HVOF. The paper describes the processes involved in implementation of the thermographic measurement systems into the HVOF spray system to measure the temperature of the coating on aluminum and steel during spraying. It was observed that when spraying on aluminum, the temperature of the coating has a marked influence on the adhesion. Tests with a steel substrate show how the hardness of the WC-CoCr depends on the quenching speed of the coating during spraying. Paper includes a German-language abstract.

Abstract During the coating of components like rolls the major parameters are easily to control. But this is not sufficient to guarantee and stable a high coating quality. Additionally non-controllable parameters, e. g. nozzle wear, influence the process mainly. For this purpose a new testing method is necessary. On base of the acoustic emission analysis a testing equipment was developed in order to control the spraying conditions. In this investigation acoustic emission is applied to anlyse the signals generated by the impact of the molten particels on the component surface. The analysis of the signals is carried out in combination with a new classification method. The results of controlling APS(Atmosperical Plasma Spraying)-process during the disposition of a Cr2O3- coating will be presented. Paper text in German.