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).

The influence of arc root fluctuations in DC plasma spraying on the physical state of the particle jet is investigated by correlating individual in-flight particle temperature and velocity measurements with the instantaneous voltage difference between the electrodes. In-flight diagnostics with the DPV-2000 sensing device involves two-color pyrometry and time-of-flight technique for the determination of temperature and velocity. Synchronization of particle diagnostics with the torch voltage fluctuations is performed using an electronic circuit that generates a pulse when the voltage reaches some specific level; this pulse, that can be shifted by an arbitrary period of time, is used to trigger the acquisition of the pyrometric signals. Unlike what has been predicted by numerical modeling, time-dependent particle temperature and velocity due to power fluctuations induced by the arc movement can be very important. Periodic variations of the mean particle temperature and velocity, reaching ΔT = 600°C and Δv = 200m/s, are recorded during a voltage cycle. Moreover, very few particles are detected during some part of the cycle. The existence of quiet periods suggests that particles that are injected at some specific moments in the plasma are neither heated nor accelerated efficiently. To our knowledge, this is the first time large time-dependent effects of the arc root fluctuations on the particle state (temperature and velocity) are experimentally demonstrated with quantitative measurements.

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.

On-line monitoring of two thermal spray processes by means of an imaging diagnostic technique capable of measuring several spray particle properties, such as individual and average particle temperatures, velocities and number of particles with spatial distributions, was studied by using the Spray-Watch thermal spray monitor. Aim of the work was to demonstrate the capabilities of this novel monitor in quick optimisation of certain spray gun parameters to obtain desired particle characteristics in-flight, and hence desired coating structure and properties with high deposition efficiency. Examples are given in plasma spraying of Al 2 O 3 and HVOF spraying of NiCoCrAlY.

The effect of chamber pressure on the temperature and the velocity of the plasma sprayed particles was measured quantitatively through the chamber window. As the chamber pressure increases, the plasma flame shortens. At the same time, the particle slows down and the particle temperature increases. The multivariate analysis among the spray parameters and the measured values reveal that the particle temperature and the particle velocity can be controlled individually as a function of the chamber pressure and the spray distance. The zirconia coatings were fabricated by controlling the particle temperature and velocity at the impinging upon the substrate individually. Increase of the particle temperature increased the deposition efficiency, the density and the hardness of the coating. On the other hand, increase of the particle velocity up to a certain value also increased them, but the excessive velocity decreased them. Because the shortened dwelling time prevented the powder core from melting, even if the surface was sufficiently heated.

During last 10 years, it has been pointed out that the reproducibility and reliability of air plasma sprayed (APS) coatings depend, among other parameters, on the particle velocity and temperature distributions prior to their impacts to the target surface. On-line control systems have been designed to follow these parameters in the harsh environment of booths. However, in spite of significant strides, works have yet to be carried out to establish relationships between deposit properties and in-flight particle parameters and/or surface target temperature, roughness and oxidation stage. The SPCTS laboratory at the University of Limoges has developed the SDC (Spray and Deposit Control) system in collaboration with SNECMA Services. It controls the stability of the spray jet and the mean particle trajectory together with the target surface temperature. It has been used with a device allowing to measure the deflection of a rectangular beam during APS of WC-Co17wt% or ZrO 2 +8wt%Y 2 O 3 powder on Hastelloy X (Ni base alloy) substrate. The aim of this study was to determine which spray parameters influence the residual stresses, in order to achieve a mean compressive residual stress in the WC-Co17wt% coating on Hastelloy X and to control it with the deposit 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.

This paper describes an experimental investigation of plasma jet properties of a DC torch operated at low pressure (below 10 mbar). A modified enthalpy probe system is described, which allows gas sampling from the plasma jet at pressures down to the mbar range. Measurements of the specific enthalpy, temperature and velocity throughout the jet for different pressures are presented and discussed. In the pressure range investigated, the jet flow is supersonic and compressible theory is used to infer the velocity from the dynamic pressure measured at the probe tip. In addition, optical emission spectroscopy of the plasma jet is used to evidence the differences of these low-pressure plasmas with respect to common, atmospheric pressure thermal jets. These preliminary measurements are the starting points towards a better understanding of plasma jets at low operating pressures in view of new process development and optimisation.

The coating characteristics of plasma sprayed layers are strongly related to the spray parameters and the gun design which also determine the state of the sprayed powder particles upon impact. On-line optical measurements of particle velocity and temperature can be used for the optimization of the spray parameters as they provide information of both, characteristic of the coating as well as process stability and performance of the plasma process. With the DPV-2000 diagnostic system the correlation between particle and coating characteristics have been investigated, especially for the new Triplex technology. Differences observed in the distribution of the particle properties between the Triplex and F4 gun are due to different standard spray parameters and correlated to the inherent difference of the operation principle of both plasma guns. The typical three fold symmetry of the Triplex plasma and powder injection is observed within the particle distribution pattern which also depends on the type of powder and spray parameter used. With the help of DPV-2000 measurements it is also possible to demonstrate the excellent process stability of the Triplex gun and to define and adjust parameter windows for different applications in order to obtain the desired range of coating properties. The advantages of the DPV-2000 diagnostic should also contribute to an enhanced basic understanding of new spray processes such as the LPPS Thin Film technology. Experiments have been performed to prove the feasibility of particle diagnostics under these special spray conditions. First results are presented.

The effects of the plasma spray process parameters on the microstructure and properties of coatings have been recognized for a long time. It is now clearly proved that the quality and the properties of the deposits are strongly dependent on the in-flight particle characteristics as well as on the splat morphology. The aim of this study was to examine in detail: firstly the interaction between an Al 2 O 3 /13%TiO 2 powder and the spraying process, secondly the splat formation in order to understand the morphology of the deposits. A good way to reach a better understanding of the particle/ plasma interaction is to determine the particle parameters at impact. The particles parameters: velocity, temperature and diameter, prior to their impact on the substrate were measured by the DPV-2000 diagnostic system. The effect of particle velocity, temperature and size on the splat morphology and deposits properties for different plasma conditions were examined. The splat morphology was characterized using the Sommerfeld dimensionless number, K; the value of which is directly related to the impact mode. These analyses were correlated to microhardness, roughness and to the tribological properties of the coatings.

Many thermal spray coating applications require an optimum performance regarding the thermal and mechanical stability of the layer composite. The maximum loads that a composite can sustain, are not only dependent on the intrinsic material properties of the coating, but are also subject to the quality of deposition. The quality of the coating is predominantly influenced by the temperature distribution during the deposition process thereby influencing the residual stress development. Therefore failure of a thermally sprayed coating under mechanical and/or thermal load often could be avoided by an adequate deposition process with well controlled heat and mass transfer, i. e. by avoiding hot-spots on the surface that result in high residual stresses in the composite. With the help of Infrared (IR) thermography an imaging of the lateral and spatial temperature field of a workpiece surface and its evolution in time can be monitored and visualised. In the presented work the atmospheric plasma spraying process serves as an example to demonstrate the suitability of thermographic imaging as a quality control and process optimisation technique for online process monitoring and control in thermal spraying. The results indicate that IR-thermography can be used as a flexible tool for on-line process control of coating manufacturing via thermal spraying, it offers a powerful way to optimise the deposition process.

The innovative diagnostic system, PFI, records the radiation of the plasma jet as well as of the luminous particle flux. A desktop computer immediately converts these brightness distributions to a set of ellipses. The simple set-up of the system and the fast algorithm enables the utilization of the system in serial production. A statistical design of experiments (DOE) was applied for various process parameters to correlate the determined characteristic ellipses to the process parameters. Measurements of the process by DPV2000 and properties of the resultant coating were used to validate the established correlations. The results demonstrate the suitability of the diagnostic method PFI for quality control and quality assurance in serial production.

The HVOF process with reduced heat effect on the substrate and therefore minimal degradation of fatigue properties is now finding wide application in fatigue critical applications. The critical parameters for process control are residual stress in the deposit and maximum substrate temperature. Quality control tools for these parameters are deflection of Almen Strips (similar to shot peening) for simulating residual stress and the use of infared pyrometry for temperature measurement. Both of these methods are technically sensitive particularly in spraying of coupons to evaluate the effect of coating on material properties. Lessons learned will be presented and recommendations made for applications of these tools in controlling the HVOF process.