The plasma-spray process features complex plasma-particle interactions that can result in process variations that limit process repeatability and coating performance. This paper reports our work on the development of real-time diagnostics and control for the plasma spray process. The strategy is to directly monitor and control those degrees of freedom of the process that are observable, controllable, and affect resulting coating properties. This includes monitoring of particle velocity and temperature as well as the shape and trajectory of the spray pattern. Diagnostics that have been developed specifically for this purpose and integrated with a closed loop process controller are described.

The introduction of sensor techniques has lead to a major change in the possibility to on-line control the plasma spraying process. By the use of such techniques reproducibility of the process can be enhanced by a more precise process control of flame and particle in-flight characteristics. In this paper a comparative study of four different on-line diagnostic systems and their sensitivity is presented. The systems evaluated are: the Particle Flux Imaging (PFI) from Zierhut Messtechnik GmbH, the PlumeSpector from Tecnar Automation Ltee., the Spray and Deposit Control (SDC) from the University of Limoges/Snecma Inc. and the DifRef M from Flumesys GmbH. Tests were performed using plasma spraying of a Nickel- Aluminium alloy. A statistical design of experiments (DOE) was defined using a ± 5% variation of the five process parameters current, argon flow rate, hydrogen flow rate, powder feed rate and carrier gas flow rate. System principles, sensitivity results are given and the systems suitability for use in industrial production is discussed.

A two-wavelength particle imaging sensor has been developed to measure temperature and velocity of individual particles in most thermal spray devices. The sensor provides continuous updates to particle condition profiles, histograms and correlation’s. The software locates particle streaks, determines the intensity ratio and dimensions of each streak, and calculates the particle temperature and velocity. Many forms of advanced materials processing technologies, such as thermal spray, spray-forming and atomization have considerable need of process control sensor technology. These measurements provide the basis for application of the sensor to many of these processes. Particle temperature and velocity measurements of plasma-sprayed ceramic powder were obtained using the sensor. The average temperature varied from 2800 K to 3000 K as the current to the plasma was increased from 700 amps to 900 amps. The average velocity varied from 85 m/s to 99 m/s over the same range. These results compare favorably with similar measurements, reported in the literature. With its full-stream field of view, the vision-based particle sensor can be applied to control strategies for the purpose of providing stable particle temperatures and velocities over long duration plasma spray processes.

Imaging techniques have gained popularity in thermal spray diagnostics over the last decade. They provide large amounts of data for research, development and process monitoring. The most important measured spray parameters are particle temperature and velocity. The biggest advantage of imaging techniques compared to optical point measurements is that they provide instantaneous distribution measurements. We present the application of high-power diode laser illumination to detect weakly emitting particles in thermal spaying. This extends the feasibility of imaging diagnostics to several demanding environments, including cold spray, powder mixing region of a plasma spraying torch and low-pressure plasma spray.

A simple, low-cost technique for measuring the velocity of particles for spray coating methods is presented. The accuracy of measurement is determined by the precision of the only quantity of geometrical size of the deposited layer. Self-selection of the particles with parameters suitable for deposition occurs. For typical deposition conditions, a velocity of 650 m/s was detected. This technique allows in situ velocity monitoring during process.

This paper investigates the advantages that an advanced control system can provide for plasma spray deposition in terms of variation reduction and greater ability to engineer coating structure. We report our work on implementing a feedback control system that automatically adjusts system inputs to maintain the desired particle states in spite of process variations. The limitations and performance capabilities of both feed forward and real-time control are evaluated. Important system characteristics needed to develop such controllers are discussed including dominant nonlinearalities, dynamics, cross-coupling, distributions, and sensor issues. Performance is evaluated in terms of engineering coating structure and production objectives.

This paper first presents some typical results that illustrate the need for closed-loop stabilization of plasma spraying processes. A strategy to identify the most efficient control actuators is then explained, followed by a description of a robust operator closed-loop feedback method. This method can be directly implemented on current commercial sensors and applied to older spraying equipment than cannot easily be interfaced to modern computers. The method is also capable of implementing tight computer-controlled closed-loop feedback on more modern equipment. A discussion of minimum sensor requirements is also given.

The reproducibility of the coating properties is the result of an efficient control of the thermal spray process. Actually, it is not possible for the sprayer to guarantee the temporal reproducibility of the coating properties without online control. However, adjusting the system fluctuations with regards to the temperature, velocity and size of particle could alter some properties that depend or not on the in-flight particle characteristics. This study aims at discussing the existing correlations between the processing parameters and the coating properties established with a Neural Network methodology. It demonstrates the segmentation of the correlations by comparing the result of the merging procedure of two sub-network structures and the direct correlation from the processing parameters to the coating properties. The sub-structures are built considering respectively the in-flight particle characteristics relationship with the operating conditions and the in-flight particle characteristics relationship with the coating properties.

The resulting thermal and mechanical properties of atmospheric plasma sprayed coatings, are strongly dependent on the particle in-flight characteristics, which in turn depend on the spray gun variables. In industrial production the spray gun variables are set to constant values and closed loop controlled. However, calibrations of the variable levels are regularly performed and variations within specified tolerance limits allowed, which cause variations in the particle in-flight characteristics. The objective of this work was to investigate how these calibration variations affect the particle in-flight characteristics and the final coating properties. The investigation was performed using three-dimensional computational fluid dynamics simulations. The process model correspond to the SM-F-100 Connex gun, spraying ZrO 2 for thermal barrier coating applications. Particle in-flight characteristics were calculated using a stochastic discrete particle model. Validation of the model was performed using the optical DPV2000 system, and fair agreement was found. Voltage, arc current, primary, secondary and carrier gas flow rates were systematically varied one factor at a time and their separate effects on the particle in-flight characteristics evaluated. The most important variables influencing the particle characteristics were current and voltage. Final simulations considering extreme cases enabled determination of the particle characteristics limiting conditions due to tolerance variations. Coating microstructure evaluations of two of these extreme cases revealed that the total porosity could vary up to 4% due to tolerance variations.

Industrial gas product supply has a dramatic influence on the quality, consistency, and performance of thermal spray coatings and their application equipment. The various thermal spray processes test the limits of common gas supply modes in the areas of safety, pressure, purity, storage, and recovery. Selection criteria for the gas and supply mode, using the latest methods, are discussed for plasma, HVOF and cold spray techniques.

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.

While twin wire arc spraying, strong dependences are usually noticed between elaboration process parameters and coating properties. Therefore, control is needed for increasing coating performances. The aim of this paper is to investigate the coating properties in relation with in-flight particle characteristics and with the impact modes. Measurements were achieved on iron particles when changing either the current intensity or the air flow rates. In-flight particle characteristics were determined by using the DPV diagnostic system 200 mm from the gun exit and for three different radial locations. The same locations were kept for the impact analysis. Then, the morphology of the splats was evaluated by its flattening and shape factors when the substrate is preheated or not. Finally, the coating properties were characterized in terms of porosity, oxide contents, microhardness and thickness. Through the determination of related interactions, a better knowledge of coating elaboration conditions will lead to improve reliability and properties of the deposits.

A two-color imaging pyrometer system is employed to examine twin wire arc spray gun used in Spray Tooling Process. Key aspects analyzed in this study include particle temperature, particle velocity, and particle distribution. The influence of process parameters such as gas flow rate, type of gas (N 2 , and air), voltage and current, as well as, spray cap design has been studied. Influence of a super atomizer called the arc jet has also been investigated. The measurements were taken at 80 mm, 160 mm and 200 mm from the gun tip. Finally, the influence of the material on the spray characteristics has been examined. The material list includes 0.8% carbon steel, 0.8% Carbon steel with B, stainless steel, Molybdenum, Ni, Ni-Al and Copper. The type of gas used for atomization significantly influences the particle temperature and velocity. Lowest temperature is observed with nitrogen gas and the highest temperature with the arc jet. Arc jet also produced particles with significantly higher velocity. The flow rate did influence the particle velocity where as temperature wasn’t affected noticeably. The wire material has considerable impact on the particle temperature and velocity. Lower melting point alloys showed higher temperature whereas the velocity has complex dependence on the density and viscosity of the material. Cap type affects the temperature and velocity of the particle. As cap opening increases the temperature and velocity decrease.

Modern thermal spray processes require wide use of diagnostics to gather an extensive process understanding. Today's diagnostic results provide the basis for future designs and advancements, particularly and increasingly on basis of computational approaches. Due to the measuring area in the square centimeter range and its quick and accurate results, the Particle Image Velocimetry (PIV) represents an enriching for thermal spray process diagnostics. Our experimental results obtained from PIV are in accordance to present theoretical and empirical derivations of some kinematic parameters of thermal spray processes. Beneath verification of well-known dominant parameters (for example powder fraction or carrier gas mass flow), this procedure enables the detection and characterization of ancillary influences on the process due to its high accuracy. By statistical analysis of our experiments, using multiple parameter variations per experiment according to the technique of "Design of Experiments" (DoE), we possibly found some hints on interactions between ancillary parameters which shall be analysed in further works carefully. In combination with detailed simulations on plasma - particle interactions and powder injection it should be possible to develop methods for thermal spray processes to minimize the particle flow expansion for an optimization of deposition rate and energy efficiency in the future.

During plasma spraying in-flight particle characteristics are influenced by the many operating parameters associated with the deposition process. The distinctive temperature and velocity signals given by particles as they exit the plasma torch can be used to develop processing maps for defining the optimal operating envelope. Knowing the temperature and velocity history of the particles, the evolution of the microstructure, the amount of porosity and the phase composition can potentially be predicted. In this paper, the relationship of system parameters (stand-off distance, torch power, plasma gas composition and process gas flow) was correlated to in-flight particle characteristics of yttria stabilized zirconia and compared to the resulting coating features such as thickness, microstructure, porosity and phase composition. The appearance of the coating (i.e., color) was also compared after the deposition process. Yttria stabilized zirconia was deposited on grit blasted samples using an F4 (Sulzer Metco) plasma torch. Before depositing each sample on the substrate, the particle properties were measured at the desired stand off distance perpendicular to the particle jet covering an area of 18x18 mm 2 using the Tecnar DPV2000 inflight particle analyzer. The coatings were cross-sectioned for microstructure analysis, thickness measurements and deposition efficiency. Free standing films were used for mercury intrusion porosimetry. Grey levels of the coatings were obtained by optical microscopy and subsequent digital image recording. X-ray-diffraction analysis was also used to obtain the phase composition. Results showed that different particle temperature and velocity conditions lead to specific porosity and varying colors of the deposit. The color of the deposit was correlated directly to the amount of monoclinic phase in the as-deposit material.

Thermal and kinetic history of particles inside the jet plume of atmospheric plasma spray (APS) provide the particles with a certain velocity and temperature prior to the impact onto the substrate. Adjusting these velocity and temperature allows to control final coating's properties. The properties of the obtained coatings mostly depend on particle's conditions prior to the impingement. A set of experiments were done in order to optimize an atmospheric plasma process by means of an online monitoring system. The study is done by changing some spraying parameters that certainly affect the velocity and the temperature of the in-flight particles. Plasma gas flow rates and spraying distance were the parameters changed in this work, whereas all the other spraying conditions were kept constant. On-line monitoring system Spray-Watch (Oseir Ltd.) was used to achieve the parameters optimization by the measurement of velocity, temperature and spatial distribution of the Ni20Cr inflight particles. The study of the coated samples includes microstructure characterization by means of Optical Microscopy (OM), Scanning Electron Microscopy (SEMEDS) and X-ray diffraction to study the effect of the spraying parameters in phase composition. Roughness measurements and mechanical properties are also included.

The objective of this study is to increase the deposition rate in the plasma-spray manufacturing of thermal barrier coatings without altering the quality of the coatings. The experimental part involves the measurement of in-flight particle characteristics and analysis of coatings properties when varying the hydrogen content of the plasma-forming gas, the torch nozzle diameter and the powder feed rate. The experimental results of particle measurements are discussed in the light of the gas flow fields projected by a 3-D model of the plasma spray process.

The aim of the project group consisting of four research centers and founded by the DFG (German Research Society) is to characterize the plasma spraying process by means of diagnostic methods so that, based on the requirement profile of the coating, appropriate adjusting of the process parameters can be realized. For this purpose, different, partly newly-developed diagnostic tools, like Particle Shape Imaging, Laser Doppler Anemometry, Streak Technique, Particle Image Velocimetry, Enthalpy Probe, DPV 2000 and Thermography were qualified and adjusted to each other. The new results presented in this article are limited to the areas of particle injection and substrate which are difficult to handle with diagnostic methods.

The influence of secondary hydrogen and current on the deposition efficiency (DE) and microstructure properties of yttria-stabilized zirconia (YSZ) coatings was evaluated. In order to better understand the influence of the spray process on coating consistency, an YSZ powder, -125+44 µm, was sprayed with nitrogen/hydrogen parameters and a 9MB gun. DE and coating porosity produced using two different spray gun conditions yielding the same input power were compared. Amperage was allowed to vary between 500 A and 560 A and hydrogen was adjusted in order to maintain constant power, while nitrogen flow was kept at a fixed level. Several power conditions, ranging from 32 to 39 kW, were tested. Different injection geometries, i.e., radial with and without a backward component, were also compared. The latter was found to produce higher in-flight temperatures due to a longer residence time of the powder particles in the hotter portion of the plasma. Porosity was based on cross-sectional photomicrographs. In-flight particle temperature and velocity measurements were also carried out with the DPV-2000 for each condition. Test results showed that DE and coating density could vary significantly when a different hydrogen flow rate was used in order to maintain constant input power. On the other hand, DE was found to correlate very well with the temperature of the in-flight particles. Therefore, to obtain more consistent and reproducible DE and microstructures, it is preferable to maintain constant the in-flight particle temperature instead of keeping the input power constant by adjusting the secondary hydrogen flow rate for obtaining more consistent and reproducible DE and microstructures.

In order to ensure reproducibility of thermal sprayed coating characteristics, process control by a sensor system is essential. Volvo Aero has more than 5 years experience of using different sensor systems in plasma and flame spraying. This article presents two examples where such systems have been used for quality control in production. In the first example the DPV2000 system was used in plasma spraying to control the tensile strength of an abrasive application. The study showed that the particle in-flight properties and the tensile strength were very sensitive to the carrier gas flow rate. In the second example the PlumeSpector system was used to control erosion and tensile strength in flame spraying of a composite powder composed of a core of Bentonite clay coated with a shell of NiCrAl alloy. The results show that the capability could be increased by a factor of two using on-line diagnostic control.