Non-destructive eddy current evaluation is a practical and well-established tool in aerospace and other industries, used to find or identify material defects not otherwise detectable. It can also be employed to measure the thickness of various coatings, although it is not yet fully optimized for multi-layer thermal spray systems, such as thermal barrier coatings (TBCs). The first part of this paper aims to look at the underlying mechanisms of the eddy current thickness measurement technique and uses a Design of Experiment (DoE) study to identify key characteristics related to thickness measurement of thermal barrier coatings. The second part of the paper is a case study on the application of the findings into general production, showing the achieved improvements in accuracy and repeatability of thickness measurements.

One of the most popular test methods for the evaluation of thermally sprayed coatings and substrate pre-treatment is the bond strength test in accordance to the standard EN 582 (in Europe) or ASTM C633-79 (in the U.S.). An interlaboratory test, carried out in 2003, has shown that bond strength tests of samples that were joined by different institutions in accordance to DIN EN 582 lead to bond strength values that varied by the factor 2, even if the same adhesive was used. The test not only made clear that the adhesives used to join the loading block to the substrate block but also many other parameters have a tremendous influence on the results of the bond strength tests. It was also clarified that the DIN standard needs to be supplemented in order to improve the comparability and the reproducibility of the bond strength test results. The first aim of this work was to identify the influencing parameters by carrying out Failure Mode and Effect Analyses (FMEA) together with namable institutions. Based on these results, a work instruction based on the DIN standard will be composed in order to avoid its weak spots. Later, a second interlaboratory test will be carried out to evaluate the quality of the work instruction.

Controlling particle state is important to not only achieve the required microstructure and properties in coatings but also to clearly isolate and understand the role of other clusters of variables (such as the various substrate and deposition conditions) on the aforementioned attributes. This is important to design coatings for high performance applications and in the ongoing efforts towards achieving prime reliance. This study examines the variabilities in particle state and explores a few strategies to control them for improved reproducibility with the aid of in-flight particle and plume sensors. The particle state can be controlled by controlling the torch parameters or by directly controlling the particle state itself via feedback from particle and plume sensors such as DPV2000 & TDS. There exist at least a few control protocols to control the particle state (predominantly temperature and velocity) with judicious choice of critical parameters. In the present case the particle state has been controlled by varying the critical torch parameters in a narrow range using 8% YSZ of angular morphology (fused and crushed) with 10-75 microns size distributions in conjunction with a N 2 -H 2 laminar (non-swirl) plasma. Two important results emerge. (1) The particle state resulting from averaged individual particle measurements (DPV 2000) is surprisingly stable with variabilities in T < 1% and variability in V of < 4%. Ensemble approaches yield a somewhat higher variability (5%). In spite of this the variability in basic coating attributes such as a thickness and weight is surprisingly large. (2) Applying a much simpler control strategy to only control the particle injection and hence the particle trajectory results in reduced variabilities in coating attributes.

This paper reviews the new DIN EN ISO personnel qualification and quality management standards and EWF training guidelines for European thermal sprayers. EWF education and training programs run for a period of three weeks, filling critical gaps in knowledge and experience. The primary aim of the ISO standards is to facilitate a reproducible process for spraying coatings of a consistent quality. Paper text in German.

The damaging of the electrodes during spraying can affect the reproducibility of the plasma spray process. Indeed, this may influence the plasma characteristics and the energy transfer to the sprayed particles resulting in significant changes in the coating attributes. In this paper, results from a detailed investigation on the stability of plasma spraying are presented. Specifically designed diagnosis tools were used to study the evolution of key parameters of a plasma spray process during a long-term experiment. A comprehensive analysis is carried out on the collected set of data, with an emphasis on the correlation that may exist among them. Results show significant variations in the particle state and gun characteristics with the spraying time. These variations are reflected in the microstructure of the sprayed coatings. The investigation also gives some indication on how the spray process could be controlled.

With a view to the development of thermal barrier coatings on turbine blades, such coatings were developed on the basis of Y2O3-stabilized ZrO2 and optimized with respect to their reproducibility. Particular attention was given to the spray efficiency and, in particular, the coating porosity. The porosity was measured by Hg porosimetry, which proved to be a reliable method of characterization. The plasma gas flows play an important role for a high reproducibility of the coating properties. High reproducibility of the porosity and spray efficiency was achieved for low argon flows, for which the influences of all the other parameters, such as electric arc current, meandering and sample cooling during plasma spraying or the cathode operating time of the burner, were of minor relevance for the coating properties. The spray efficiency is clearly influenced by the given powder rate and the carrier gas flow for the type of burner used. The change of the grain fraction in the plasma jet was examined for two different powders. Finally, thermal cycling tests were carried out and evaluated for pre-optimized thermal barrier coating samples.

The quality of plasma sprayed coatings depends strongly on substrate surface preparation, especially roughness, grit residue, and oxidation stage; particle spray jet position and size relative to the plasma jet; impacting particle distribution; particle velocity, temperature, and size prior to impact; substrate temperature; and pass thickness. A simple and low-cost spray and deposit control system developed in our laboratory allows to monitor on-line the position, shape, and centroid of the hot particle spray jet. Such a tool has proved to be very sensitive to any drift in powder injection conditions and torch input parameters. Although it gives no direct information on particle velocity and temperature, this system can be easily implemented in an industrial environment and help to maintain constant the particle parameters during spraying. A CCD camera is used in conjunction with a pyrometer making it possible to measure simultaneously substrate temperature. The system can monitor coating parameters such as deposition efficiency and residual stresses. This paper describes how the system can be used to set the tolerance range of process input parameters to obtain coating parameters within given specifications.

The new Triplex-burner was used to develop 7 wt% Y 2 O 3 stabilised ZrO 2 thermal barrier coatings and to optimise the process parameters with respect to reproducibility. The investigations were focused on spray efficiency and coating porosity. Porosity levels and porosity distribution were measured with mercury intrusion, which turned out to be a very reliable characterisation technique. High reproducibility could be achieved in particular for low argon fluxes: the deviation of the porosity levels was below about 4%. Paper text in German.

Reproducibility is a current challenge for the thermal spray industry. Reproducibility associated problems represent a large cost every year not only in terms of rejections and rework, but also in costs for destructive testing and decreased production flow. Thermal spray coatings are moving in the direction of being considered only as a "band aid" to becoming a design element. One of the prerequisites for such a development is an increase in reproducibility for thermal spray coatings. The purpose of this paper is to outline a vision aiming in the direction of a future "ultimate spray booth", where thermal spraying is as reproducible and reliable as machining, grinding or other production processes. A way to increase reproducibility and reliability in the future spray shop involves utilising major parts of IT - technology. This also includes active co-operation design-production in the pre-spray process. This paper will deal with areas such as: operation drawings and lists through multimedia techniques, education programs for operators and designers through multimedia techniques, CAD/CAM, Off-line programming and simulation, On-line diagnostics of flame (particle diagnostics) and coating (temperature & Acoustic emission measurements), on-line Statistical Process Control and Knowledge Based System techniques.

Two sets of plasma spray processing conditions were utilized in the investigation of graded layers, consisting of NiCrAlY and PSZ. Following the optimization of the plasma spray parameters and particle characteristics, the deposition efficiencies (DEs) of various powder species was examined. Selection of the best suited powders for coating production were selected based on the DE results. The base DEs were corrected by conducting the analyses using pre-deposited substrates as targets. Individual mixed coating layers were prepared and their compositions confirmed by image analysis. The effects of standoff distance and substrate temperature were also seen to have an effect on the DE and thus the coating formation. It is suggested that two-feeder, single injector plasma processing may not be the optimal method for the formation of FGMs.

Although very frequently used, traditional (low velocity) flame spraying is a much forgotten process. No major research has been performed during the last decade. This paper focuses on the problem of reproducibility of a typical flame gun used with modern automated process equipment. An on-line diagnostic process control tool measuring the temperature, velocity, size and position of the powder in the flame was applied during spraying of abradable coatings of NiCrAl/Bentonite, coatings which are commonly applied to fan and compressor housings of gas turbines. An automated closed loop flame spray unit with mass flowmeters for the oxygen and acetylene gases was used. Influence of different process parameters on the sprayed particles, such as nozzle design, gas flows, and powder feed rate is discussed. Coating properties, such as erosivity, porosity, microstructure and tensile strength, are evaluated. It is demonstrated that even fairly small process changes influence the flame sprayed particles as well as the properties of the resultant coating.