A process control tool has been developed for air plasma spraying of a NiAl (bond coat) and Al 2 O 3 (top coat) coating systems. The process is employed at Volvo Aero Corporation for abrasive purposes, such as knife-edge applications on compressor parts. In-flight particle temperatures, velocities and diameters were measured by the DPV2000 system. Several samples were sprayed and the coating microstructures were evaluated using Image Analysis techniques on optical and scanning electron microscope images. Top and bond coat thickness, oxides, porosity, grit blast residues, delaminations, surface roughness (on top, bond and substrate) and tensile strength were evaluated. Statistical regression analysis was then used to establish relationships between process parameters (i.e. current and primary gas flow), particle in-flight characteristics (i.e. velocity and temperature), microstructure properties, and mechanical properties. The equations derived were finally used for development of a tool, which can be used by the operator for on-line monitoring and control of the coating characteristics based on information of the current particle inflight characteristics. The tool makes it possible to continuously adjust the process set points, ensuring a high reproducibility and stability of the process.

Revealing the true properties is of utmost importance for the optimized use of thermal sprayed coatings and for the realization of the ultimate spray booth. The necessity of the true properties for a successful correlation as well as development of process windows for on-line control cannot is emphasized. Only the true properties can be expected to correlate to the spray parameters or the particle properties in the flame. What is actually meant when stating the following? - The coating has 5% porosity - Tensile bond is 50MPa - Hardness is 1000HV - Coating thickness is 100 nm Different loopholes during the steps of metallographic preparation, hardness and tensile bond strength testing are highlighted and discussed. The microstructure of a thermally sprayed coating typically consists of a multiphase matrix (often a mix between hard, soft and amorphous), pores, oxides, delaminations, cracks, grit residues and unmelted particles. Due to this complexity in the structure there are a number of possible errors that often can be made in metallographic laboratories. Several examples and explanations are given. It is shown that improper handling in the materials laboratory may lead to smearing and pullouts etc. This will ruin the results and effectively hide the true microstructure. Furthermore the use of different epoxies and curing methods effectively varies the tensile bond strength test results by more than 300% for various coatings. By using different preparation routines the micro hardness can be varied by more than HV 0.3 100.

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.).

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).

Quality management (QM) standards, used as a support to safeguard product quality, show the way to implement a quality management system and/or to adapt and revise an already existing QM system. This article describes the quality requirements for thermal spraying companies and qualification requirements for spraying personnel. In addition, it provides the guidelines for the education and examination of spray personnel.

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.

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.