According to the present state of the art “in situ, real-time control systems” are absolutely necessary in modern applications within the field of Thermal Spray technology with regard to the quality and reproducibility of thermal spray coatings for a wide range of industrial applications. Compared with the already existing diagnostic systems and their algorithms which are developed on the basis of CCD cameras, an algorithm can be developed for in situ, real-time control systems, which possesses a by far closer physical approach for the determination of the particle characteristics due to the measurement principle. Thus, about 300 particles can be detected per millisecond as well as average particle velocities within the range of 30 - 1200 m/s and average particle surface temperatures between 700 - 3000 °C can be determined. A statistical distribution of approximately 3000 particles is considered for the computation of the online measured average particle velocities and surface temperatures. The accuracy of the measurement determines the use of an online process control during Thermal Spray processes.

As the requirements placed on thermal coatings become increasingly stringent, being able to acquire more accurate data more quickly is becoming increasingly important. To achieve this, process variables must be measured close to or even inside the gun. This paper introduces a new product which measures process variables directly inside a thermal spray gun. It explains the advantages of measuring the process variables inside the gun, but also why it is important to have gun-relevant data stored on the gun itself.

Measurement of the particle temperature and velocity in detonation spraying is significantly complicated by the pulsed character of the process. In the present study, these parameters are measured for powders with strongly different nature and properties such as WC/Co, Inox and Ti. Experiments are performed using an original computer-controlled detonation spraying (CCDS) installation developed by the authors. The system is distinguished by the mode of powder feeding into the gun barrel which is pulsed in time and localized in space. Evolution of the particle-in-flight velocity and size is examined by an original CCD-camera-based diagnostic tool developed by the authors. A significant spatial separation of the particles along the detonation plume is observed during their acceleration: 15 μm fine particles overtake 45 μm coarse particles by more than 10 plume diameters. For this reason, distributed scanning over the plume length is applied in order to obtain adequate results. A previously developed mathematical model of the process is experimentally validated. Calculations are found to be in a qualitative agreement with the experimental results. As far as particle-in-flight velocity is concerned, the agreement is even quantitative.

The development of coating elaboration processes involving electric arcs depends on process stability and the capacity to ensure a constant reproducibility of coatings properties. This is particularly important when considering the plasma treatment of submicron or nanosized particles in Suspension Plasma Spraying (SPS). Submicron particles closely follow plasma instabilities and have non-homogeneous plasma treatment. Recently, it has been shown that arc voltage fluctuations in dc plasma torches, showing dominant fluctuation frequencies between 4 – 6 kHz, are linked to pressure oscillations in cathode cavity in the rear part of the plasma torch. These resonant oscillations are linked to plasma torch geometry. In this paper, first, we will present a method to isolate the different oscillations modes in measured arc voltage and pressure signals by the use of signal processing methods. Second, correlations between the different modes of oscillations are analyzed following the plasma torch operating parameters.

The effects of powder loading on temperature distributions of yttria-stabilized zirconia particles during flight, which can be used as a melting status indicator, were investigated in atmospheric dc plasma spraying. Commercially available diagnostic systems were utilized to measure the state of in-flight particle parameters. As the powder feed rate was increased, the intensity of a peak related to the latent heat increased, suggesting the increase of semi molten particles. Interesting findings are that the deposition efficiency of the coating actually increased in some conditions at higher powder feed rates. This implies that higher molten degrees of particles do not always give higher deposition efficiencies. The loading effects also affected the result of diagnostics, which requires special care when the diagnostic condition is different from the actual spray conditions.

Powder injection parameters like gas flow, injection angle and injector position strongly influence the particle beam and thus coating properties. The interaction of the injection conditions on particle properties based on DPV-2000 measurements using the single-cathode F4 torch is presented. Furthermore, the investigation of the plasma plume by emission computer tomography is described when operating the three-cathode TriplexPro torch. By this imaging technology, the three-dimensional shape of the radiating plasma jet is reproduced based on images achieved from three CCD cameras rotating around the plume axis It is shown how the formation of the plasma jet changes with plasma parameters and how this knowledge can be used to optimize particle injection.

The aim of this study was to point out the role of each electrode in the droplets formation in twin arc spraying. This way, two consumable wires of different properties, namely steel and copper, were sprayed simultaneously. The DPV 2000 diagnostic system was used to determine the size, temperature and velocity of in-flight particles, detected at the same locations than those previously tested on particles collection. Then, a model based on the particle temperature was developed to separate particles from the anode and the cathode wires. Results showed significant modifications in term of size, velocity, temperature and repartition when changing material electrodes. To validate the proposed model, modelling results were first compared to results found on properties of collected particles, i.e. sizes and percentages. Then, important differences of in-flight particles characteristics, velocity and temperature, were pointed out depending on the electrode nature and on radial locations in the spray jet. Finally, some coatings were sprayed at the same locations and analyzed in term of thickness. Results showed that the thickness distribution was largely dependent on the anode nature, which was in close agreements with in-flight particles analysis.

HVOF sprayed cermet coatings are used for wear protection in numerous application fields. Most feedstock powders can be processed successfully with different spraying guns but optimized parameters for high quality coating production cannot be transferred directly. Besides substrate temperature only particle velocity and temperature inflight and at impact on the substrate determine the resultant coating properties. Two typical HVOF powders, WC/Co 88/12 and Cr 3 C 2 /Ni20Cr 75/25, are sprayed using the kerosene fuel system GTV K2 and particle parameters are recorded just prior to impact on the substrate with the passive diagnostic tool Spray Watch. Detailed correlation of particle parameters and the coating properties porosity and micro hardness as well as deposition efficiency is worked out in order to deduce particle parameter ranges providing coatings with defined optimum properties. The transferability to another kerosene fuel HVOF spraying gun, Tafa JP5000, is tested. It is also shown that effects like wear of the nozzle can be detected by use of the Spray Watch. Secondary correlation of particle parameters and machine settings is carried out to build a base for straight forward closed-loop control of the HVOF spraying process applying process diagnostics

In plasma spraying, the individual droplet behavior at impact is the fundamental element to understand the resulting coating microstructure. A new experimental set-up, developed in SPCTS laboratory (Limoges, F) with two fast shutter cameras (exposure time : 100 ns…1ms) allows visualisation at impact of a single particle plasma sprayed with a direct current (d.c) torch. A fast two color pyrometer enables to monitor particle temperature just prior to its impact, its flattening and its thermal history. Working in parallel with a free falling drop experiment, enables to obtain larger (about three orders of magnitude) time and dimension scale (realized in Advanced Joining Process Laboratory, Toyohashi, J). Each technique gives interesting and complementary results thanks to pyrometric signals and images. Results obtained with plasma sprayed particles allow studying the matter ejections generated on impact splashing .while both techniques allow following the flattening splashing. Calculation and comparison of quenching rates for millimetre sized drops on a stainless steel substrate give indications concerning the disk shaped splat formation.

A special HVOF gun is used for aerodynamic research on internal flows of gas and particles in HVOF gun. The gun has rectangular cross-sectional area and has sidewalls of optical glass or transparent acrylic resin. Compressed air is used as process gas instead of combustion gas to visualize internal flow of the gun. The high-speed gas flows including shock waves in the gun are visualized by Schlieren technique. Particle trajectories in the gun are also visualized by high-speed digital video camera. The observation of erosion pattern created by particle collision on the barrel wall helps understand the particle trajectories throughout the barrel.

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.

In current aircraft engines challenging coatings or coating systems with different functions are used. Many of these coatings are applied to different components with thermal spray processes. Thermal spraying is a very sensitive and very complex process, which is influenced by numerous controllable variables like the powder feed rate, the gas flow rates, etc. as well as not controllable variables like the torch wear, varying powder properties, etc. With conventional process control based on linear algorithms it is not possible to enduringly create constant coating properties, because they cannot describe the complexity of all influencing variables. In this work, the possibility of a closed loop was investigated exemplarily for an atmospheric plasma spray process (APS). During serial production a data base was collected, consisting information about torch and plume conditions as well as powder and coating properties. This data base was used to train different neural networks (NN). With regard to the automation of the APS, the NN obtained target values of relevant coating properties and should calculate the needed control variables. The result of this work shows the difficulties in the quantification of relevant influencing variables and the feasibility of the plasma spray process control with neural network.

Substrate temperature is nowadays recognized as a key parameter to optimise the coating quality in the thermal spraying process. Generally parts being processed are in motion and therefore non contact temperature measurement devices are appropriate. In contrast to thermocouples, optical pyrometers have several advantages. First, they are easy to install and second they do not bring any disturbance to the measured system. Meanwhile, several problems may arise with those devices which are not always considered as they should be and in particular the variation of material emissivity temperature, the effect of the reflection of the external radiation or the attenuation of the optical signal due to the variable transmissivity of the optical path. The aim of this work was to develop algorithms for correcting optical pyrometer temperature measurements during thermal spraying by taking into account emissivity variations and radiation reflexion on the components. Emissivity of some materials with respect to the specific spectral band of the pyrometer and the influence of reflected radiations were measured. Results are discussed in order to point out the influence of each parameter on the temperature value.

Cold spray process is a low temperature spraying method (<600°C) based on particle high kinetic energy at impact (velocity in the range 300-1200 m/s). Powder is injected in a specific designed nozzle and, there, reaches high velocity. Even if bounding mechanisms are not perfectly known, it is established that adhesion can’t exist below a definite particle velocity called critical velocity. This specific velocity depends on both powder and substrate characteristics (chemical composition, mechanical properties…). The large number of parameters makes the determination of critical velocity very complex. But it is a fundamental value to determine the correct operating conditions. This work presents an imaging technique that allows a fast measurement of critical velocity. In this paper, the measuring method is first validated by comparing the critical velocity of copper (sprayed on copper substrate) found in the literature, with the measured one. Its accuracy is then tested with other materials and, finally, some improvements of the method are proposed.

The present work deals with numerical simulations based on a computational heat transfer model for spherical composite particles typically used under plasma conditions. Results describe heat transfer in mono and two layers steel/alumina particles immersed in an uniform infinite plasma. Time dependent behaviours interacting with phase change occurrence and taking into account the contact quality between the two layers are considered.

The distributions of spray particle velocity and temperature were determined with two different instruments, the SpectraViz (Stratonics, Laguna Hills, CA) and the DPV2000 (Tecnar Automation, Montreal, Canada). Experiments were performed with three different powders and a Praxair SG 100 plasma spray torch. The instruments differ by the method they use in imaging the jet ultimately allowing for measurements of velocities and temperatures. The average particle temperature values obtained are very close, indicating that different ways of imaging do not influence the measurement value. The values for the average velocities are in good agreement for two of the powders tested (Al 2 O 3 and CrO 2 ), however, for the YSZ powder a consistent difference in velocity values was found. It appears that for obtaining average values e.g. for process control, both instruments are equivalent.

Diagnostics of particles produced from Value-Arc 100 twin-wire-arc spraying system was found using DPV2000 monitoring tool. Effect of different operating parameters on the diagnostics of the particles, namely, temperature, velocity, and size distributions, were experimentally studied and discussed. It was found that the particle characteristics are not symmetric about the center line of the plume. It was also found that the divergence angle in the plane parallel to both wires is less than the divergence angle in the symmetry plane of the wires, which is observed in the actual oval-shape coating structure and is explained by the geometry of the system. Size distribution and volumetric size distribution of particles are also analyzed and compared with log-normal distribution function. It is shown that the size distribution consists of two different peaks believed to be associated with anodic and cathodic particles in the process.

In the present study, waste PET bottle was used as a recycled PET plate substrate for plasma spraying. The plasma spraying process to PET was analyzed by a heat balance model during the plasma spraying for the purpose of determining the optimal spraying condition. It was found that Cu and TiO 2 powders could be coated on the surface of recycled PET plate. The implantation of Cu and TiO 2 particles into the PET substrate was sometimes observed under a certain spray conditions. Both kinds of Cu and TiO 2 particles retained their original spherical shape. Cu coatings were formed when almost copper particles were melted to 1780 K greater than the melting point of Cu. In anatase TiO2 spraying process, heat input to anatase particles adhered to PET was much less than that to adhered rutile particles so that anatase TiO 2 could be coated on PET plates. Heat input during plasma spraying is, therefore, very important for endowing wasted PET materials with a new function by plasma spraying of metal and ceramics such as Cu and TiO 2 . Although the idea about the heat balance is very simple, the analysis is useful for optimizing the spraying process of PET.

The heat and momentum transfer of the plasma to the injected particles is an important issue in coating formation during plasma spraying. In this study, the plasma temperature and velocity of a Triplex-I torch was measured by means of an enthalpy probe system. Additionally, the properties of injected yttria-stabilized-zirconia powder of a fine sized fraction were recorded spatially resolved by using a DPV2000 system. The plasma temperature and velocity are decreasing by increasing the distance from 45 to 60 mm with respect to the torch exit by approximately 50% from initially 6200°C and 400 m/s, respectively. In contrast, the particles gain temperature up to the melting point at 70 mm stand-off distance as well as the velocity rises up to its highest value of 115 m/s at the maximum flow rate of the particles. Both, plasma and particle characteristics were used to obtain a deeper insight on heat- and momentum transfer of the plasma jet to the single particles.

This paper presents selected research results of the DFG founded project group, consisting of four institutes focusing on diagnostic methods in thermal coating processes. The aim of this group is to characterize the Atmospheric Plasma Spraying (APS) 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 diagnostic tools like Particle Shape Imaging, Laser Doppler Anemometry, Schlieren Technique, Particle Image Velocimetry, Enthalpy Probe, DPV 2000 and Thermography were qualified and adjusted to each other. Most of the results presented in this article are limited to the area close to the substrate which is difficult to handle with diagnostic methods. Additionally, new achievements concerning nozzle design and system enhancements are introduced.