A functionally graded thermal barrier coating (FG-TBC) of CeO 2 -Y 2 O 3 -ZrO 2 /NiCoCrAlY was prepared using a recently developed supersonic plasma spray (S-PS) system. The system had dual powder feed ports through which the metal alloy powders were fed into the lower temperature region of the plasma plume, to prevent over oxidation, and the ceramic powders were fed into the high temperature region, to produce complete melting. Such an approach enabled fine configurations having a continuously graded composition transition to be obtained. The thermal shock testing of the deposited samples with 1mm thick FG-TBC system on Ni-based alloy substrates was performed using an in-house-designed multi-functional rotational thermal shock tester. In this tester, the heating-cooling curves and surface morphology of tested specimens can also be observed by a microscope with a CCD (charge-coupled device) camera and recorded on line by means of accessorial computer system to evaluate the thermal shock resistance of tested samples while they were heated by a high heat flux of oxygen-acetylene flame to 1200° C in a time interval of 15~20s followed by water-quenched to ambient temperature. The temperature fields and relevant thermal stresses distribution through the thickness of disk samples were calculated by means of ANSYS finite element method. The numerical approach shows that the maximal tensile stress occur at the C-YSZ top coating at the center of disc samples at the start of rapid cooling by water-quenched, where small reticulated surface cracks were observed, which then propagated perpendicularly about 350µm deep to near the interface between the pure C-YSZ coating and the FGMs layer transition between the C-YSZ and the NiCoCrAlY coatings. These vertical cracks appeared to be arrested without any delamination after 200 thermal shock cycles.

Plasma-sprayed yttria-stabilized zirconia particles (~40 µm diameter) were photographed during impact (velocity ~200 m/s) on a glass surface that was maintained at either room temperature or 400°C. A droplet that approached the surface was sensed using a photodetector and after a known delay, a light source was triggered to illuminate the particle in order to photograph it with a charge-coupled device (CCD) camera. A rapid two-color pyrometer was used to collect the thermal radiation from the particles to follow the evolution of their temperature and size, in-flight and after impact. The fully molten particles spread into a thin liquid splat after impacting the surfaces. The partially molten particles disintegrated into small satellite fragments immediately upon impact. The surface area, as indicated by the pyrometric signals, of the partially molten particles during spreading were almost an order of magnitude smaller than that of the fully molten particles. The pyrometric signals, characteristic of the impact of partially molten zirconia, provide a novel method of identifying partially molten ceramic particles after impact on a flat surface.

Plasma-sprayed, molten molybdenum particles (~55 µm diameter) were photographed during impact on grit-blasted glass surfaces that were maintained at either room temperature or at 350°C. Droplets approaching the surface were sensed using a photodetector and after a known delay, a fast charge-coupled device (CCD) camera was triggered to capture time-integrated images of the spreading splat from behind the glass. A rapid two-color pyrometer was used to collect the thermal radiation from the spreading droplets to follow the evolution of their temperature and calculate the splat cooling rates. It was found that as the surface roughness increased, the maximum spread diameters of the molten molybdenum droplets decreased, while the splat cooling rates increased. Impact on non-heated and heated roughened glass with similar roughness values produced splats with approximately the same maximum spread diameters, skewed morphologies, and cooling rates. On smooth glass, the splat morphologies were circular, with larger maximum spread diameters and smaller cooling rates on non-heated smooth glass. An established model was used to estimate the splat-substrate thermal contact resistances. On highly roughened glass, the thermal contact resistance decreased as the glass roughness increased, suggesting that splat-substrate contact was improved as the molten metal penetrated the spaces between the large asperities.

Plasma-sprayed, molten nickel particles (60 µm diameter) were photographed during impact on oxidized 304L stainless steel surfaces that were maintained at room temperature or at 350oC. The steel samples were oxidized at different temperatures. Droplets approaching the surface were sensed using a photo detector and after a known delay, a fast charge-coupled device (CCD) camera was triggered to capture time-integrated images of the spreading splat from the substrate front surface. A two-color pyrometer was used to collect the thermal radiation from the particles to follow the evolution of their temperature after impact. Molten nickel particles impacting on oxidized steel at room temperature fragmented significantly, while heating the surfaces produced splats with disk-like morphologies. Impact on steel that was highly oxidized induced the formation of finger-like splash projections at the splat periphery. The splat cooling rate and thermal contact resistance between the splat and non-heated oxidized steel varied significantly as the degree of oxidation increased; heating the oxidized steel greatly reduced the variations. It was suggested that the large variations in splat cooling rates and thermal contact resistances on the non-heated oxidized steel was due primarily to the presence of adsorbates on the steel surface.

The plasma spraying process is controlled by various parameters that have an influence on powder particle velocities, temperatures and trajectories just before impact to the substrate. In order to fully utilize the thermal and kinetic energy of the plasma it is important to obtain information from these powder particle properties. In this work an intensified CCD camera has been used to detect in-flight particles in an atmospheric plasma spraying process. Plasma spraying was performed using fused and crushed AI2O3 powder. The powder carrier gas flow rate was varied during the spraying experiments. All the other deposition parameters were kept constant. Coatings produced using relatively new spraygun electrodes are compared with ones produced later with the same electrodes when they were worn out. The particle concentration is determined on a relative scale by the fraction of the area of a CCD camera frame covered by particle images. Further investigations necessary to clearify the relationship between the measured relative particle concentration and the true particle concentration are identified. The coatings are analyzed for wear resistance, degree of melting, deposition efficiency, hardness and porosity. The dependence of these coating properties on the relative particle concentration and the effect of electrode wear on the relative particle concentration are studied.

The deposition efficiency (DE) of a particular powder for a particular thermal spray process is very important factor when coating economics is being considered. There are many coating applications, however, where it is also important to know how the deposition efficiency changes during a longer coating process. Normally the DE is determined as mass ratio of powder fed into the process and corresponding weight gain of the sample. In this work the deposition efficiency has been determined for aluminum oxide powder in atmospheric plasma spraying using different spray parameters and electrode wear states. The coating process and in-flight particles were monitored using a fast non-intensified CCD-camera. Using digital image analysis the relative hot particle concentrations and velocity distributions were calculated from images. The possibility to use a CCD camera based monitoring system for in-situ measurement of DE is discussed. Additional laser illumination and PTV measurements were performed to verify the cold particle flux unseen by the plain CCD camera.

This paper describes the development of a diagnostic system that monitors in-flight particle diameters, velocities, and temperatures during thermal spraying. The system is based on a low-cost CCD camera and user-developed software. The camera incorporates a 732 x 282 pixel sensor with high sensitivity in the near IR range where the only radiation is that of the particles. User-developed software modules handle signal processing, image analysis, calibration, and data visualization. In video images, particles appear as light tracks of varying length, width, and intensity, corresponding (respectively) to velocity, diameter, and temperature. A test case in which Cr 2 O 3 powder is sprayed in a plasma jet demonstrates the capabilities of the diagnostic system. Paper includes a German-language abstract.

In plasma spray process, ceramic coatings can be sprayed by using either argon-hydrogen or nitrogen-hydrogen plasma gas mixtures. Starting from a given particle size distribution the question is what are the spray parameters allowing achieving similar coatings with these types of plasmas? The problem is made more complex because a torch working with Ar-H 2 is different from that using N 2 -H 2 as plasma forming gas. It is thus necessary to compare the gas mixture properties, the torches working conditions (mean voltage and thermal efficiency for given current), the arc column diameter relatively to the nozzle internal diameter and the spray parameters, the arc root fluctuations, the powder injection, the particles mean temperatures and velocities as well as their fluctuations linked to those of arc root, the splat formation, the coating porosity and deposition efficiency. This comparison has been achieved for ZrO 2 -Y 2 O 3 (7 vol%) powder with a size distribution between 5 and 25 µm and using the same plasma torch for the two plasma gas mixtures: a 3MB torch with a cylindrical anode nozzle and 5.5 mm in internal diameter.

Understanding the impacting phenomena of yttria-stabilized zirconia (YSZ) particles and following coating formation in plasma spraying process is of importance to control and design the microstructure of coatings such as thermal barrier coatings. To this aim, recently, the authors have developed a novel in situ monitoring system for particle impacts under atmospheric dc plasma spraying conditions. This system utilized a high-speed video camera coupled with a long-distance microscope and was capable of capturing the particle-impinging phenomena at one million frames per second. To understand the coating formation mechanism, two approaches were attempted, that is, observation of the single splat formation and the following coating formation as the integration of splats. In the former case, the deformation and cooling processes of YSZ droplets impinging on substrates were captured successfully. In the latter case, multiple-droplet-impacting phenomena were observed as an ensemble treatment. Representing coating process, the tower formation (1- dimensional) and bead formation (2-dimentional) were observed under typical plasma spray conditions for thermal barrier coatings. By using a triggering system coupled with the motion of a robot, impact events were recorded for every pass. The obtained images clearly showed the coating formation resulted by the integration of single splats.

Instabilities in plasma spray jets can result in coatings with inconsistent properties. The arc root fluctuation and shear layer instability due to strong gradients are of foremost concern. The shear layer instabilities result from shear between the high velocity, low density hot core gas, the intermediate density and velocity boundary layer, and the high density quiescent environment. A cold-flow facility with density gradients similar to a plasma torch has been used for implementation of traditional fluid dynamics measurements such as hot-wire anemometry. Methods to control these instabilities are developed and tested using both the plasma torch and the cold flow facility. Through nozzle design modifications the instabilities resulting from arc root fluctuations and high density gradients have been reduced. The effectiveness of the control on the plasma jet is determined using in-flight particle characterization along with high speed imaging and photodiode measurements of the jet.

Thermal Spray Toolkit is a developing toolkit for thermal spray applications based on RobotStudio™ software which is developed by Asea Brown Bovrie Ltd (ABB). This toolkit is composed of several functional modules including PathKit, ProfileKit and MonitorKit. PathKit provides numerous methods to create trajectory on different surfaces including square and rectangular surfaces, round surfaces, curved surfaces and rotating workpieces. ProfileKit permits coating surface analysis by importing a surface profile and it can also guide users to select the right spray parameters according to specific materials. MonitorKit can capture the robot displacement in real-time during the spray process. Thermal Spray Toolkit is a toolkit developed to apply robotics in thermal spray applications whose reliability has been confirmed by the experimental results.

This paper presents what is our actual knowledge in the development of the on-line control of the plasma spray process. First the influence of the different parameters on coating properties is recalled. The dependence of particle parameters at impact on macroscopic parameters such as the gas composition, its enthalpy, the injection conditions… is discussed together with the possible actions to compensate for the voltage drop due to electrodes erosion. The transient aspect of the plasma jet due to the arc root fluctuations and their consequence on particle parameters is also presented. Then the different sensors able to work in booths harsh environment are described. The way they are used, informations they provide, the possibility they give to monitor the spray parameters for a good working area is discussed. At last what would really be necessary for a real on-line control is presented and it is concluded that we are still far to achieve this objective.

Arc instability in dc plasma guns has a detrimental effect on coating quality and electrode life. This paper investigates the impact of a single-cathode plasma column design on process stability, versatility, and component life. The design increases arc length and minimizes restrike, enabling high voltage, low current operation as well as the use of ternary gas to increase enthalpy. Arc behavior is assessed by monitoring fluctuations in electrical signals and correlated with particle temperature and velocity. The takeover-steady mode is shown to be the typical operating mode of the plasma jet for this design, resulting in steady particle characteristics over long periods of maintenance-free operation.

One approach for controlling the twin wire arc spray (TWAS) process is to use optical properties of the particle beam like length or brightness of the beam as input parameters for a process control. The idea is that changes in the process like eroded contact nozzles or variations of current, voltage and/or atomizing gas pressure can be detected through observation of optical properties of the particle beam. It can be assumed that if these properties deviate significantly from those obtained from a beam recorded for an optimal coating process the spray particle and so the coating properties change significantly. Thus, the goal is to detect these optical deviations and compensate occurring errors by adjusting appropriate process parameters for the wire arc spray unit. One cost effective method for monitoring optical properties of the particle beam is to apply the process diagnostic system PFI (Particle Flux Imaging): PFI fits an ellipse to an image of a particle beam thereby defining easy to analyze characteristical parameters by relating optical beam properties to ellipse parameters. Using artificial neural networks (ANN) mathematical relations between ellipse and process parameters can be defined. Thus in the case of a process disturbance through the use of an ANN-based control new process parameters can be computed to compensate particle beam deviations. In this paper, it will be shown that different process parameters can lead to particle beams with the same PFI parameters.

Plasma spraying using liquid precursors makes it possible to produce finely-structured coatings with a broad range of microstructures and properties. Nonetheless, issues with coating reproducibility and control of deposition efficiency continue to be a concern. With conventional dc plasma torches that inject liquid feedstock transversely into the plasma stream, coating quality depends on transient interactions between the liquid and plasma jet. Numerical models may assist in understanding these interactions provided they are able to predict droplet fragmentation, which determines the trajectories of droplets and their behavior in the plasma flow. Although various models for droplet fragmentation have been proposed in the literature, they include parameters and constants that need to be validated for plasma spraying conditions. This study simulates liquid material injection and break-up in the plasma jet using an enhanced Taylor analogy break-up (TAB) model. Model constants are adapted to plasma spray conditions by observation of liquid behavior in the plasma flow, which is accomplished by means of a shadowgraph system using pulsed backlight illumination.

High-Sn bearing alloys have been used for more than a century in many areas of industry. They are typically applied by casting, although thermal spraying is gaining in use, particularly for component repairs. This study evaluates the effects of HVOF spray parameters on the velocity, temperature, size, and trajectory of Babbitt particles and correlates the in-flight characteristics with coating porosity and intermetallic phase distribution. In the experiments, Babbitt layers up to 370 μm thick were deposited on carbon steel substrates while measuring particle properties and deposition efficiency. Coating samples were analyzed by means of optical and electronic microscopy and some were chemically etched to reveal the size and distribution of intermetallic phases. Test results show a significant refinement in intermetallic phase distribution when compared with commercial flame and arc-sprayed coatings.

In this investigation, alumina powders prepared by different methods were sprayed on carbon steel substrates using a conventional plasma torch with radial injection. The spraying process and powder injection parameters were varied and the injection behavior of the powder was studied. Changes in particle acceleration, deceleration, and impact were measured with novel spray diagnostic equipment and are correlated with the structure and properties of the coatings obtained. Dense coatings were achieved with several agglomerated-and-sintered Al 2 O 3 powders, although higher impact velocities and temperatures were recorded for the larger and denser fused-and-crushed particles.

The quality and durability of coatings produced by virtually all thermal spray techniques could be improved by increasing the velocity with which coating particles impact the substrate. Additionally, better control of the chemical and thermal environment seen by the particles during flight is crucial to the quality of the coating. A high velocity thermal spray device is under development through a BMDO SBIR project which provides significantly higher impact velocity for accelerated particles than is currently available with existing thermal spray devices. This device utilizes a pulsed plasma as the accelerative medium for powders introduced into the barrel. Recent experiments using a Control-Vision diagnostic system showed that the device can accelerate stainless steel and WC-Co powders to velocities ranging from 1500 to 2200 m/s. These high velocities are accomplished without the use of combustible gases, and without the need of a vacuum chamber, while maintaining an inert atmosphere for the particles during acceleration. The high velocities corresponded well to modeling predictions, and these same models suggest that velocities as high as 3000 m/s or higher are possible.

In the past ten years, significant progress has been made in the field of advanced sensors for particle and spray plume characterization. However, there are very few commercially available technologies for online characterization of the as-deposited coatings. In particular, coating thickness is one of the most important parameter to monitor and control. Current methods such as destructive tests or direct mechanical measurements can cause significant production downtime. This paper presents a novel approach that enables online, real-time and non-contact measurement of individual spray pass thickness during deposition. Micron-level resolution was achieved on various coatings and substrate materials. The precision has been shown to be independent from surface roughness or thermal expansion. Results obtained on typical HVOF and plasma sprayed coatings are presented. Finally, current fields of application, technical limitations and future developments are discussed.

In plasma spraying, the arc root fluctuations, modifying the length and density of the plasma jet, have an important influence on particle thermal treatment. These voltage fluctuations are strongly linked to the properties of the cold boundary layer, surrounding the arc column, depending on the plasma spray parameters (composition and plasma forming gas flow rate, current, etc.) and the plasma torch design (anode-nozzle internal diameter and shape, etc.). In order to determine the influence of these different spray parameters on the cold boundary layer properties and voltage fluctuations, experiments were performed with two different plasma torches from Sulzer Metco. The first one is a PTF4 torch with a cylindrical anode-nozzle, working with Ar-H 2 plasma gas mixture and the second one is a 3MB torch with both a conical and a cylindrical anode-nozzle, working with a N 2 -H 2 plasma gas mixture. Moreover, the arc voltage fluctuation influence on particle thermal treatment was observed through the measurements of temperature and velocity of particles, using an yttria partially stabilized zirconia powder with a size distribution between 5 to 25 µm.