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.

A high-speed video recording system was developed to measure the velocity and temperature of thermal spray particles in flight. The system consists of three image sensors and a cubic beam-split prism that directs light, based on color or wavelength, to three imaging planes. In this study, one of sensors is used to measure particle velocity while the other two measure particle temperature. The prospect of simultaneously measuring the temperature and velocity of individual particles in flight is discussed along with the insight it provides on the formation of spray coatings.

In this paper, we describe a new sensor for monitoring inflight particles in thermal spray processes. The sensor can measure simultaneously and in real-time, the mean velocity and mean temperature of the particle jet for a very broad range of powder feed rates. The thermal radiation emitted by the hot particles is collected by a lens and focused on two optical fibers. Knowing the distance between the optical fibers and the magnification of the optics, the mean particle velocity is computed by measuring the time delay between the signals collected in the two fibers by cross-correlation. The signals are band-pass filtered to prevent spurious reflection, equipment movement and noise from disturbing the measurement. Using the same signals filtered at two specific wavelengths, the mean temperature of the particle jet is obtained by the two-color pyrometry technique. In this technique, the temperature is computed from the ratio of the light intensity detected at two different wavelengths.

In the thermal spray process, particle state at impact is among the key factors influencing the quality, characteristics and properties of the deposit formed. Measuring and eventually controlling in-flight particle jet characteristics can help ensure the repeatability of desired coating properties. Moreover, monitoring particle jet can lead to cost savings by allowing replacement of consumable parts on a need-basis rather than on a preventive-basis. However, successful use of in-flight particle sensors in a production environment requires developing an implementation strategy that minimizes impact on coating cycle time, limits operator intervention and reduces analysis time, while still generating useful data. The usefulness of in-flight particle sensor in a high yield production environment is investigated. A preliminary study was first conducted to define a strategy to implement the data acquisition process using an ensemble in-flight particle monitoring system that allows real-time measurements of average particle temperature and velocity and overall particle jet profile. Data was collected to capture representative torch and particle jet behaviors over the life of several consumable hardware sets. Evolution of the particle jet parameters over time is compared to that of some of the process parameters, as well as to the deposition efficiency and one selected characteristic of the coating deposited. Some trends are identified, and potential benefits and drawbacks of using in-flight particle monitoring in a high yield production environment are highlighted.

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.

The Accuraspray is a new in-flight particle sensor that provides information on the average in-flight particle temperature, using two-color pyrometry, and velocity, using a cross-correlation calculation. Various aspects influencing the reliability of the sensor estimates are studied. First, the sensitivity of the temperature and velocity estimates to the positioning of the sensor with respect to the particle jet, such as the angular orientation of the fibers and the working distance to the spray plume, is evaluated. Then, the influence of the plasma radiation on the temperature measurement is estimated. This influence can be reduced significantly by filtering out the low frequency components of the pyrometric signals, which contain most of the plasma fluctuations. Finally, a lower limit in the signal-to-noise ratio (SNR), for which an acceptable temperature estimate is obtained, is evaluated. A valid velocity estimate can still be obtained with a lower SNR. All these studies were performed under various spraying conditions, including plasma spraying and HVOF, using various feedstock materials (YSZ, Al-Si, cermets).

A two-wavelength particle imaging pyrometer has been developed to measure temperature, velocity and size of individual particles within a field of view and a depth of field that spans the entire particle stream in most thermal spray devices. The pyrometer provides continuous updates to particle condition profiles, histograms and correlations. The software locates particle streaks, determines the intensity ratio and dimensions of each streak, and calculates the particle temperature, velocity and size. Many forms of advanced materials processing technologies, such as thermal spray, spray-forming and atomization processes, have considerable need of process control sensor technology. These measurements provide the basis for application of the pyrometer to many of these processes. Particle temperature measurements of plasma-sprayed ceramic powder were obtained using a spectrometer and the pyrometer. Comparisons of the measurements show that the vision-based pyrometer has excellent accuracy. The standard deviation of the measurements was 40 K or about 1.3 %. Additional pyrometer measurements were used to determine its minimum detectable temperature and velocity change, which were 12.4 K and 2.77 m/s, or 0.4 % and 1.5 %, respectively. The vision-based particle sensor can now be applied to high performance control strategies to provide stable particle temperatures and velocities over long duration plasma spray processes.

This study evaluates the performance characteristics of three different torches. A variety of different sensor systems were used to determine how each system resolves the differences in torch spray conditions. Torch performance characteristics include the long term drift behavior in terms of spray pattern and coating profile, the ability to control the particle state, and each torches nonlinear characteristics such as bifurcation (sudden jumps) and on/off repeatability. Sensors include commercially available particle sensors (Tecnar’s DPV 2000, Oseir’s Spraywatch, and Inflight’s IPM), as well as monitoring of plume intensity, acoustic emission, and high frequency voltage/current. Torches include Praxair’s SG 100 (internal injection), Sulzer Metco’s 9MB, and Progressive Technologies 100 HE. The study focused on spraying YSZ under each torches typical operating conditions. Abstract only; no full-text paper available.

Samaria-doped ceria (SDC) has become a promising material for the fabrication of intermediate-temperature, metal-supported solid oxide fuel cells (SOFCs). While typical SOFC materials, such as yttria-stabilized zirconia (YSZ), require high temperatures (> 700°C) to exhibit suitable ionic conductivity for high cell performance, SDC displays similar ionic conductivities at lower temperatures (600°C – 650°C). The atmospheric plasma spray (APS) process is a promising technique for manufacturing metal-supported SOFCs. In this study, the in-flight characteristics, such as particle velocity and surface temperature, of spray-dried SDC agglomerates were analyzed at various plasma spray conditions using the DPV-2000 in-flight particle sensor manufactured by Tecnar Automation. Coatings of SDC were applied on stainless steel substrates using a range of spray conditions, and their resulting microstructures and deposition efficiencies were analyzed. It was found that particle temperature could be related to the specific plasma energy, and that coating porosity was related closely to the measured average particle temperature.

Thermal spray technology became established in various parts of industry (e.g. aircraft industry, medical industry etc.). Some of the applications are highly sensitive, therefore the coating quality plays an important role. Most of the common quality control methods are based on destructive testing methods which are undesirable for economical reasons. In recent years the interest in non-destructive online measurement methods increased and still is growing. More detailed knowledge of the relationship between process parameters can help to improve the coating quality standards and to understand different phenomena in thermal spraying. This will make online process control possible and improves the acceptance of thermal spray technology. In this paper results of fundamental studies on the atmospheric plasma spraying process (APS) are presented. Al 2 O 3 and NiCr powder were sprayed as model materials. In the experiments modem on-line monitoring systems were used to investigate the entire process from gun parameters to the coating quality. The faces in this paper are set on investigations of the in-flight properties of spray particles with the particle sensor DPV 2000 and their relationship with resulting coating properties. A method will be presented to extract characteristic values (velocity and temperature) out of the spray process with DPV 2000. With this method the APS-process was monitored and process parameters were correlated with main coating properties.

Three actual applications of spray plume sensors for industrial coating operations are presented. Two of these are in the aeronautic industry and one in the automotive industry. Depending on the application, different types of sensors were used ranging from plume shape monitoring (particle trajectories) to full plume characterization including in-flight measurement of individual particle properties. In each case, the process monitoring scheme developed depended on the manufacturer's requirements as well as the nature of the information provided by the sensor. The significant differences in usage and working conditions compared to an R&D environment are examined. Advantages and drawbacks in each of the three situations are detailed. A tentative forecast of coming developments in this field is also given.

This paper investigates the need and requirements for developing an improved sensing scheme for plasma spray, which would aid in compensating for the large variations in deposition rate. Experimental evaluation of a variety of current sensors reveals poor correlation with deposition rate. Important issues that explain these results include reconsidering what particle state characteristics should be measured and determining what sensing characteristics are required to achieve better correlation with deposited mass. A new particle flux sensor is proposed that meets these challenges is evaluated.

Over the last decade there has been an explosion in terms of available tools for sensing the particle spray stream during thermal spray processes. This has led to considerable enhancement in our understanding of process reproducibility and process reliability. However, in spite of these advances, the linkage to coating properties has continued to be an enigma. This is partially due to the complex nature of the build-up process and the associated issues with measuring properties of these complex coatings. In this paper, we present an integrated strategy, one that combines process sensing, with process modeling and extracting coating properties in situ through the development of robust and advanced curvature based techniques. These techniques allow estimation of coating modulus, residual stress and non-linear response of thermal sprayed ceramic coatings all within minutes of the deposition process. Finally, the integrated strategy examines the role of process maps for control of the spray stream as well as design of thermal spray coatings. Examples of such studies for both MCrAlY and YSZ coatings will be presented.

A Sulzer-Metco 6P Powder Flame Spray Torch spraying an alumina-titania ceramic powder RX60 6-axis robotic was characterized using an Accura G3(Tecnar Automation; Quebec, Canada) and a DPV-2000 (Tecnar Automation; Quebec, Canada). The two sensors were mounted side-by-side and a robot was used position the torch in relation to each sensor. Process gas flows were set using laminar flow element mass flow controllers. Accura and DPV measurements of particle temperature (Tp) and particle velocity (Vp) were made in succession at each operating condition without changing torch operating conditions. Data for a single designed experiment was collected with both sensors allowing for comparison of the two sensors across the operating space of a typical powder flame spray process.

Real-time control offers the potential to reduce plasma spray variations that affect yield and coating quality. Important factors for designing such controllers are discussed including dominant nonlinearities, cross-coupling interactions, and sensor issues. The performance of several alternative strategies to achieve better coating thickness control are evaluated.

Innovation and improvements are described which yield a 20 fold increase in the signal-to-noise level of a two fiber, twin wavelength high speed pyrometer used for in-flight particle diagnostics. Examples are given of how these developments extend the application range of the technology to low temperature processes such as flame spraying of low emissivity materials.

Plasma-spray injection conditions have a major effect on particle properties and thus the characteristics of deposited coatings. In this investigation, opposing particle streams were injected into a plasma jet and the effect of different configurations and injection parameters was systematically studied. First- and second-order process maps were constructed from particle diagnostic data and in-situ beam curvature measurements, which served to guide optimization efforts.

This paper discusses the different ways plume sensors can be used in thermal spraying and presents examples highlighting the use of a commercial plume sensor in actual production situations.

The ability to respond timely and cost effectively is bringing rapid manufacturing techniques into the mainstream production/repair arena. The Army can benefit from such technologies to avoid long lead times and high costs of procuring and inventorying replacement parts. But, a technology that offers versatility in terms of product size, cost and the ability to fabricate production components rapidly is yet to be available. This paper describes thermal spray technologies as applied to rapid manufacturing, part refurbishing and reengineering. A hybrid thermal spray system that can deposit a variety of materials with high throughput and density has been developed. Several generic materials that can meet the functional requirements of many parts and at the same time can be easily deposited by a thermal spray technique are discussed. An expert system software assisting the operator from CAD information, reverse engineering to part refurbishing is presented. Examples on the use of multiple schemes to remanufacture a part are also discussed. These schemes offer new avenues to fabricate replacement parts and/or salvage damaged parts and can benefit the Army considerably.

A wide range of manufacturing processes are used to supply yttria stabilized zirconia powders for plasma sprayed TBC applications. From previous studies it is known that the difference in coating properties can potentially result from variations in powder feedstock as a consequence of particle inflight behavior and particle impact. An additional strong contribution to splat variation results from changes in the particle in flight behavior. In order to understand the variation in particle condition as a consequence of different powder morphologies, a detailed diagnostic analysis was carried out for plasma densified (PD), fused and crushed (FC) and agglomerated & sintered (A&S) powders. In this study a “3D multiple sensors” based integrated approach was used to evaluate these differences. Direct feed back sensors were used for optimization and combined with sophisticated diagnostics for in-depth studies. To obtain comparable results, three batches of commercial powders were sized to the same specification. For a given set of spraying parameters the recorded spray stream characteristics such as plume position, particle temperature, size and velocity deviated strongly for the given morphologies. By optimizing injection, the different powders can be made to follow nominally similar trajectories. This study reveals the sensitivity of each powder to process parameters and the variability in particle state that can result from it. Some techniques are suggested to optimally inject the different powders and to achieve similar particle states for these morphologies