In this study, a wide range of suspension plasma spraying conditions are used to produce YSZ coatings for intended use in liquid gas engines. To meet specifications, the coatings must exhibit a homogeneous microstructure with no vertical cracks or columns, low surface roughness, and low thermal conductivity. The properties of the plasma jet (velocity, enthalpy, stability), droplets (trajectory, number, size), and particles (velocity) were measured during spray trials and are correlated with coating microstructure. Suspension plasma spraying conditions necessary for depositing disk-shaped splats and achieving finely structured coatings with no stacking defects are described along with substrate cooling requirements.

This study evaluates laser-based diagnostic methods that can be used to control the injection of liquid feedstocks into a plasma jet and to monitor the size and velocity of particles and droplets in different zones. It demonstrates the capabilities of shadowgraphy and particle image velocimetry and investigates the spraying characteristics of different liquid feedstocks and solvents. Suspension plasma spraying examples are presented in which variations in droplet-particle velocity and diameter are measured in different areas of the jet.

The residual stress level in coatings is a main issue in controlling in-service deformation, spallation or cracking. Residual stress generation has been widely studied for plasma and HVOF sprayed coatings, but only scare data are available for cold sprayed coatings. This paper describes the measurement and analysis of residual stresses in tantalum cold sprayed coatings. Residual stress measurements were performed by the hole-drilling and curvature methods. The former provided a through-thickness residual stress profile in the coating while the latter was used to investigate the in-situ residual stress evolution during the deposition process. The results from both methods were consistent and showed compressive stress of 350 MPa for a tantalum coating deposited on a 3 mm thick copper substrate at 80°C.

This work deals with the flattening of alumina molten particles, called droplets, on stainless steel substrates either smooth or blasted and preheated at different temperatures. The blasted surface roughness has been limited to Ra= 1.4 µm to image the flattening droplet. Besides flattening and splat cooling, the wettability of melted millimeter-sized alumina drops on the same substrates was measured. The transition temperature, Tt, has been shown to be different between smooth and rough surfaces. For a smooth surface, Tt, is 170°C, and at 200°C 100% of disk shaped splats are obtained. For the rough surface, Tt is close to 300°C with porous splats, becoming almost dense at 450°C. Close to alumina melting temperature, wettability does not vary with the substrate preoxidation, which may not be the case when the temperature is much over the melting one as in plasma spray conditions.

Coatings, a few-millimeter thick, are widely used to protect new mechanical parts against abrasion and erosion or rebuild worn parts. The plasma transferred arc process is a commonly used process to deposit such coatings. It makes it possible to bring about a metal bath inside which melted powders are introduced to form an alloyed coating between the feedstock material and substrate material with metallurgical adhesion. The main parameters of the process are the arc current intensity, plasma and shrouding gas flow rates, distance between the cathode tip and piece, velocity of plasma torch displacement; they all have a notable effect on the produced coating. This study investigates the plasma behavior and properties of the clad by using a design of experiments. The properties of the coating are the dilution level, porosity, and efficiency of material deposition, heat flux transferred to a water-cooled calorimeter, and the hardness in the clad and the substrate to estimate the thermally affected area.

Plasma spraying allows melting totally or partially micrometer sized particles, which flatten in about one is onto the substrate to build the coating by layering resulting solidified splats. The coating adhesion is essential and depends mainly on the behaviour of first lamellae in contact with the substrate. But in the plasma spray process about 108 particles/sec impact onto the substrate, and thus it is difficult to understand the role of the different spray parameters onto the coating quality. In order to get a better understanding of phenomena involved, it is necessary to study a single lamella formation. The experimental set-up is composed of a fast (50ns) two-colour pyrometer and an imaging system, comprising two fast (1 to 10 µs) CCD cameras triggered by the velocity signal of the particle in flight prior to its impact. This work is focused on alumina particles flattening onto stainless steel (304L) substrates preheated at different temperatures during different times.

This study examines the fundamental reactions that, in the solution plasma spraying process, lead to the conversion of the precursor salts to solid material that is deposited onto the substrate. The study specifically focused on the phenomena occurring in-flight and the effect of plasma jet treatment on the mechanical and thermal treatment of the solution injected in the form of a liquid jet. The evolution of precursor droplets in the plasma flow was investigated “in situ” using a shadowgraphy technique. The morphology and structure of material deposited onto smooth stainless steel substrates during single scan experiments were characterized by SEM, GI-XRD and micro-Raman spectroscopy and were correlated to the in-flight observations, in order to evaluate the effect of the plasma-forming gas and solution solvent.

An experimental set-up has been developed, at the SPCTS Laboratory, to produce fully melted, millimeter-sized, ceramic or metallic drops with impact velocities up to 10 m/s. Such impact velocities allow reaching impact Weber numbers, close to those of the plasma spray process (We = 2300). A fast camera (4000 image/s) combined to a fast pyrometer (4000 Hz), allows following the drop flattening. For studding the flattening at the micrometer scale, a DC plasma torch is used to melt micrometer sized alumina particles (around 45 μm). The experimental set-up is composed of a fast (50 ns) two-color pyrometer and two fast CCD cameras (one orthogonal and other tangential to the substrate). The flattening of millimeter and micrometer sized particles is compared. First are studied impacts of alumina drops (millimeter sized) with impact velocities up to 10 m/s. Then are considered micrometer sized alumina particles (about 45 μm in diameter) sprayed with a DC plasma torch. A correlation has been found between both flattening scales and, in spite of the lower impact velocity at the millimeter scale, ejections are also found at the micrometer scales. This work shows that to compare phenomena at the two different scales it is mandatory to have Weber numbers as close as possible in both cases.

The Oxy-Fuel Ionization system (OFI) is a new thermal spray process which consists basically on a high velocity combustion process enhanced by a low energy plasma source. The system is characterized by its stability over a relatively large range of fuel/oxidant conditions, the possibility to use poor fuels like natural one (with low gas consumption) and the high deposition rates that can be achieved in comparison to conventional HVOF guns. The OFI gun has been designed following a modular concept, which in combination with the high flexibility of the system is expected to allow the deposition of coating materials with the most different physical and chemical natures. This work deals with the experimental analysis of the process using methane as fuel gas and its correlation with the deposition of WC-base materials. Two in-flight particle diagnostic systems were used: the Spray Watch diagnostic system (from OSEIR) and the Spray and Deposit Control (SDC) system (developed by the SPCTS laboratory of the University of Limoges). Results are presented for the most representative properties of the optimized coatings (micro hardness distributions on the coating cross section and crystallographic analysis).

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.

The aim of this work is to investigate the effect of substrate surface chemistry (e.g., oxidation and atom diffusion) on the flattening of a single millimeter-sized alumina drop. To that end, a new technique to produce such drops with different impact velocities has been developed. It consists of a rotating crucible heated by a transferred plasma arc and a piston that controls substrate velocity and, as a result, the impact velocity of the drop. A fast camera working in concert with a fast pyrometer precisely records drop flattening and cooling. This system makes it possible to study interface phenomena, such as desorption and wettability, as well as the effects, at impact, of the kinetic energy or Weber number of the flattening drop.

In a DC plasma spray torch, the dynamic behaviour of the arc attachment at the anode nozzle results in fluctuations of arc voltage and the resulting plasma jet instabilities affect the treatment of the particles injected in the plasma flow, and thus, the coating quality. However it is not clear if the experimentally-observed fluctuations of particle temperatures are a major phenomena and if their frequencies are always in unison with those of voltage. In this study, two on-line techniques are used to investigate respectively the time-variation of particle temperatures and their correlations with voltage variations; the first technique makes it possible to analyse plasma voltage instabilities and the second one to investigate the instabilities of particle temperatures. Both allow determining the frequencies and amplitude variations of voltage and particle temperatures. The experiments are carried out with two plasma torches (F4-type and 3MB-type) using respectively argon-hydrogen or nitrogen–hydrogen mixtures as plasma-forming gases. A good correlation between arc voltage and particle temperature fluctuations is observed when the plasma torch is operating with argon-hydrogen while that's not the case when the torch is operating with nitrogen-hydrogen.

The aim of this study is to develop for thermal barrier applications a new process in which coatings exhibit properties between those of APS and EBPVD. This process includes two conventional D.C. plasma torches working in a chamber whose pressure can vary between 30 and 100 kPa. Micro-sized yttria stabilized zirconia powders are injected in both plasma jets to vaporize them, at least partially, and produce finely-structured coatings from vapor and micro-droplets deposition. The torch arrangement allows separating the vapor and the very small particles (less than 1 µm) from the partially vaporized bigger ones. The diagnostics are based on optical emission spectroscopy, pyrometry, imaging of particles trajectories and coating microstructural characterization.

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.

Parametric drifts and fluctuations occur during plasma spraying. These drifts and fluctuations originate primarily from electrode wear and intrinsic plasma jet instabilities. One challenge is to control the manufacturing process by identifying the parameter interdependencies, correlations and individual effects on the in-flight particle characteristics. Such control is needed through methods that (i) consider the interdependencies that influence process variability and that also (ii) quantify the processing parameter-process response relationships. Artificial intelligence is proposed for thermal spray applications. The specific case of predicting plasma power parameters to manufacture grey alumina (Al 2 O 3 -TiO 2 , 13% by wt.) coatings was considered and the influence of the plasma spray process on the in-flight particle characteristics was investigated.

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.

This study deals with a plasma technique that combines two plasma spray torches to produce finely-structured zirconia coatings. Ideally, the deposition process path involves the vaporization of most of the particles injected in the plasma jet and the transport of the vapor to the substrate where it re-condenses. The arrangement of the plasma torches makes it possible to limit the deposition of non-completely evaporated particles onto the substrate. The experimental design of the vapor deposition process has been assisted by experimental characterization of the plasma temperature field and numerical simulations of the two plasma flow interactions and powder vaporization.

Many properties (thermal, electrical, mechanical) of thermal sprayed coatings are strongly linked to the real contacts between the “piled-up” splats. The quality of this contact depends on droplet parameters at impact (size, temperature, velocity) and substrate parameters (temperature, topography). Two different techniques have been developed in order to study the plasma sprayed particle behaviour at impact. The first one allows direct observation under direct current (dc) plasma spray conditions, while the latter one, based on the millimetre sized free falling drop, enables the visualization of flattening phenomena, but at larger scale. These two techniques bring complementary approaches and results. The latter show that flattening time and cooling rate of the lamellae (metallic and ceramic) are improved with the stainless steel substrate surface modification at the nanoscale when corresponding to a positive skewness parameter obtained by preheating it over the transition temperature. Experiments of wettability show that the presence of nanopeaks increases the contact angle of the liquid on the substrates and reduces thermal contact resistance at interface. It has also been shown that, when adsorbates and condensates are not eliminated from the surface, even with a positive skewness, the thermal contact resistance is increased.

In Cold Gas Dynamic Spraying the nozzle is a key part that must be optimized to maximize the injected particle acceleration and improve the coating quality. In this study an axi-symmetric two-dimensional mathematical model is presented and used to predict the flow inside a commercial cold spray nozzle and the particle velocity at the nozzle exit. Comparisons between the model results and the measurements made show that the model allows predicting accurately the particle velocity in the cold spray jet even in the presence of shock waves. The study shows that the particle exit velocity depends on the type of propellant gas used and the stagnation temperature and pressure. Following this work, the design of nozzles for specific applications using this mathematical model can be considered.

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.