This work investigates the effect of atmospheric plasma spraying (APS) parameters on in-flight particle properties, splat morphology, and coating microstructure for conventional and nano-size YSZ powders. Particle temperature and velocity were measured using a dual-slit velocimeter and individual splats and coating microstructures were examined in a scanning electron microscope. The results show that total porosity increases with decreasing arc current and increasing stand-off distance and that conventional powder coatings have higher total porosity at higher arc currents than coatings made from nanopowder. The effect of substrate temperature on splat formation was also assessed. Splat flattening and circularity increase with increasing substrate temperature, particularly for nanopowders.

In order to identify means to improve plasma spray consistency, various modifications to the design of a commercial plasma torch nozzle have been investigated. The modifications consist of preparing anode inserts with grooves in the axial direction (spline insert), and introducing a fraction of the plasma gas through a ring of micro-nozzles surrounding the anode nozzle (micro-jet ring). Different designs for each modification have been investigated, and these modifications have also been paired with a modified upstream gas injector. For each of the modified designs, a wide range of characteristics have been measured for the arc, the plasma jet, the in-flight particles and the coating. The results show that most nozzle modifications lead to higher particle velocities and temperatures. The plasma jet is significantly elongated by using some of the modified nozzles, and the cold gas entrainment somewhat reduced. The arc voltage and the luminosity fluctuations are little affected by the nozzle modifications, however, the modification of the gas injector does change the frequencies of the fluctuations. Each of the nozzle modifications can be easily implemented offering an economical way to enhance process reliability.

Build up of strain within thermal barrier coatings has been identified as one of the main reasons for coating failure. The large volume fraction of grain boundaries in nanostructured materials has been predicted to partially relieve the strain in a coating structure. In this study, the difference in morphological characteristics of regular PSZ (r-PSZ) particles and nano-agglomerate PSZ (n-PSZ) particles have been investigated to improve our understanding of the stresses and strains within a coating. The Triple Torch Plasma Reactor, a reduced pressure plasma spray system along with a pair of tungsten and molybdenum apertures was used to isolate spray particles from the plume to deposit splat sample. Cross sections of the splat samples were prepared. Image analysis techniques were used on scanning electron microscopy (SEM) images to characterize both the surface and the cross sectional features of the splat samples. Semi-molten structures were found to be the defining characteristic of the n-PSZ samples. Peak height distribution (PHD) was defined to quantify the distribution of the height of semi-molten structures. PHD was estimated for different samples and was found to track the changes in the morphological characteristics as a function of the deposition conditions.

A reactor and process are described that allow the variation of the plasma deposition conditions such that a wide range of coating properties can be obtained. The reactor consists of three plasma torches mounted such that the deposition precursors can be injected centrally into the merging jets. This reactor has been used to deposit the different layers of a solid oxide fuel cell, including the very dense (99.5% density) zirconia electrolyte layer. Investigation of the zirconia layer showed a microstructure which has characteristics of both plasma sprayed coatings and vapor deposited coatings. The region of particle heating and acceleration has been characterized with enthalpy probes resulting in velocity and temperature fields. Calculations have been performed to describe the particle heating histories. The results show that a significant fraction of the particles evaporate and condense at the surface thus contributing to the formation of the dense layer. This hybrid process combines the possibility of obtaining high density coatings as with a PVD process with the rapid deposition rate of a plasma spray process.

Instabilities in plasma spray jets can result in coatings with inconsistent properties. The arc root fluctuation and shear layer instability due to strong gradients are of foremost concern. The shear layer instabilities result from shear between the high velocity, low density hot core gas, the intermediate density and velocity boundary layer, and the high density quiescent environment. A cold-flow facility with density gradients similar to a plasma torch has been used for implementation of traditional fluid dynamics measurements such as hot-wire anemometry. Methods to control these instabilities are developed and tested using both the plasma torch and the cold flow facility. Through nozzle design modifications the instabilities resulting from arc root fluctuations and high density gradients have been reduced. The effectiveness of the control on the plasma jet is determined using in-flight particle characterization along with high speed imaging and photodiode measurements of the jet.

Multilayer thermal barrier coatings are being investigated for high temperature applications by employing a strain accommodating interlayer. Plasma sprayed coatings of nanostructured feedstock have shown promise in this direction. Layers of nanostructured yttria stabilized zirconia (nano–YSZ) and conventional YSZ were deposited on mullite substrates using the triple torch plasma reactor (TTPR), and on NiCrAlY coated steel substrates using the Praxair SG-100 plasma torch. The coatings were heat treated and the microstructure evaluated. Heat treating the samples lead to the formation of larger pores with a significant proportion of partially molten particles. The porosity evolved from the partial sintering of the nano-agglomerates. Porosity change during the sintering process was measured and the microstructure observed using electron microscopy. The nanostructured coatings were compared to conventional YSZ coatings.

Instabilities of plasma spray jets have been a source of inconsistencies in coating properties. These instabilities can be minimized through the use of central injection torches or torches with fixed anode attachment. However, any low density ( < ~0.7) jet is globally unstable to small disturbances. Globally unstable jets are characterized by a short potential core, rapid spreading, and high entrainment, all of which are present in a plasma jet. Plasma jets have ratios of jet density to density of the surrounding gas on the order of 0.01, as well as rather low Reynolds numbers and thick boundary layers. In the present work, the instabilities are investigated through analysis of the disturbance growth in the shear layer between the plasma and the cold surrounding gas. These investigations are using two types of experiments, one consisting of a SG 100 spray torch with several optical diagnostic methods being applied to the shear layer analysis. The other experiment simulates the plasma jet at low temperatures by using a helium core jet exhausting into a sulfur hexafluoride (SF6) environment. The simulated plasma jet (SPJ) has a density ratio of 0.03. The simulated plasma jet (SPJ) allows controlled variation of the boundary layer through different fluid dynamic arrangements. It further allows use of diagnostics such as hot wire anemometry and PIV to clearly characterize the shear layer. Some of the characteristics of the shear layer are presented and comparisons between the plasma jet and the simulated plasma jet, and initial results on controlling the jet instability, are discussed. Abstract only; no full-text paper available.

Applying an environmental barrier coating (EBC) and a thermal barrier coating (TBC) on the next generation gas turbine structural materials such as silicon carbide matrix composites will lead to large stresses due to thermal expansion mismatch; thereby limiting the coating's effectiveness and lifetime. Nanostructured materials possess a large volume fraction of grain boundaries and are conjectured to partially relieve the strain in the coating structure. A Triple Torch Plasma Reactor (TTPR) was used to spray multi-layered TBCs consisting of a mullite EBC deposited either on a silicon carbide or a mullite substrate, a nano-phase partially stabilized zirconia coating (n- PSZ), and a yttria stabilized zirconia coating (YSZ) as the TBC. The nanostructure of the n-PSZ could be maintained during the deposition process. The coatings were heat treated at 1300°C and the change in microstructure and mechanical properties were analyzed using scanning electron microscopy (SEM), micro-indentation and scratch testing applied to the coating cross section. While a change in the microstructure was observed, in particular grain growth, the hardness and elastic modulus appeared to be little affected by the heat treatment giving a preliminary validation of the multilayer concept.

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.

Voltage fluctuations are investigated for a wide range of operating conditions using the Praxair SG-100 plasma torch operated with Ar/H 2 as plasma gas. The voltage signals are characterized using statistically averaged values and power spectra. The process of anode erosion and its effect on the characteristics of the signal are described. It has been found that erosion has only little effect on the average voltage, but two characteristics of the voltage fluctuations obtained from image analysis of the voltage trace are found to be excellent indicators of anode erosion. An estimate of arc root speeds based on the voltage traces yields values of 50-100 m/s. It is estimated that the range of arc root movement and arc root speed is significantly reduced to less than one half of the original value due to anode erosion. Investigation of the effect of operating conditions and anode erosion on power spectra has been investigated and it has been observed that high frequency peaks are predominantly a characteristic of the geometry of the anode.

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.

Anode erosion in plasma spray torches results in coating deterioration. The usable life of a torch anode is strongly dependent on the fluid dynamic behavior of the plasma inside the torch, which in turn depends on the geometric design of the anode and the operating parameters. In order to study the relative importance of these effects, cold flow investigations have been performed with a torch having a glass anode with the same geometric dimensions as a Praxair-TAFA SG 100 torch. The density differences between the arc and the cold gas were simulated by injecting heated helium from the tip of the cathode into the cold argon gas flow from the regular gas injector. Flow visualization was achieved by seeding the flow with micron-sized particles. A finite element CFD code was used to simulate the cold flow structure. The results were compared with erosion patterns observed with an actual plasma torch. The results indicate that recirculation eddies inside the torch will force a preferred anode attachment which is different for different gas injectors. Minimization of such recirculation regions by appropriate fluid dynamic design will result in more random attachment of the arc and prolonged anode life.

In this paper, atmospheric plasma spraying is used to produce metal-insulator-metal structures that emit ultraviolet light via electrical discharge when sufficient voltage is applied. The metal layers are sprayed with molybdenum alloy powders and the insulator, a 50 to 100 μm dielectric layer, is sprayed with zirconia powder. The authors describe the fabrication process, assess the characteristics of the layers, and present test results that quantify the radiant power of the emitted UV light. Output power and efficiency of the plasma-sprayed structures are compared with that of commercial excimer lamps and the effects of coating erosion due to electrical discharge are discussed. Paper includes a German-language abstract.

Operating parameters which affect the droplet generation characteristics for a single wire arc spray, SWAS, torch have been studied. The droplet size distribution, velocity, axial divergence, and deposition efficiency can be controlled by the selection of wire size, wire feed rate, arc voltage, and gas jet back pressure. Coatings have been formed under a variety of operating conditions with structures from 3 mm to 4.5 mm in width. The average deposition efficiency is greater than 80 % and the average axial divergence angle is 2.4°. The SWAS torch has been shown to operate for extended periods of time with minimal erosion of non-consumable electrode.

The wire arc spraying process, one of several thermal spray processes, gained a sizable part of the thermal spray market, however, more control is needed for this process to be used for high precision coatings. This study is aimed at investigating the liquid metal droplet formation process in order to identify methods for droplet trajectory control. A high speed Kodak imaging system has been used to observe the droplet formation for different operating conditions. Decreasing the upstream pressure and the current levels lead to the reduction in the asymmetric melting of both anode and cathode. By decreasing the interactions of the large eddy structures with the formed metal agglomerates one can achieve better control of the particle trajectories and jet divergence. Thus, coatings can be obtained with higher definition and improved reliability.

The control over coating quality in plasma spraying is partly dependent on the arc and jet instabilities of the plasma torch. Different forms of instabilities have been observed with different effects on the coating quality. We report on an investigation of these instabilities based on high-speed end-on observation of the arc. The framing rate of 40,500 frames per second has allowed the visualization of the anode attachment movement and the determination of the thickness of the cold gas boundary layer surrounding the arc. The images have been synchronized with voltage traces. Data have been obtained for a range of arc currents, mass flow rates, for different gas injectors and for anodes displaying different amounts of wear. The analysis of the data has led to quantitative correlations between the cold gas boundary layer thickness and the instability mode for the range of operating parameters. The arc instabilities can be seen to enhance the plasma jet instabilities and the cold gas entrainment. These results are particular useful for guiding plasma torch design and operation to minimize the influence of plasma jet instabilities on coating properties.

This paper is devoted to the investigation of the dynamics of the plasma jet, which is produced by a subsonic DC spray torch operated with an argon-helium mixture as the plasma gas. It focuses on studying the effect of some parameters which influence the arc attachment inside the anode nozzle. For this purpose, the paper uses different means of gas injection, that is straight flow injection and vortex flow injection, and anodes which have experienced different degree of wear. A heavily eroded anode is characterized by large voltage fluctuations at relatively low frequencies, while straight gas injection at high current levels led to a low average voltage with small fluctuations and to low burner performance. The results are interpreted by assuming changes in the thickness of the cold interface between the arc and the anode, and conclusions are drawn as to the voltage characteristics indicative of good torch operation. Paper includes a German-language abstract.

This paper aims to achieve improved control over the spray pattern in a wire arc torch by means of constructive measures to influence the flow dynamics. It focuses on evaluating different fluid dynamic designs of the spray torches with respect to the droplet trajectories and velocity distributions and the deposit distribution. First, the paper describes the approaches for novel nozzle designs including the addition of shrouds and the torch evaluation diagnostics, and then presents the results for four different nozzle arrangements. It is observed that the nozzle design and the placement of the wire tips within the nozzle have the greatest effect on the divergence of the spray pattern. Paper includes a German-language abstract.

There is widespread industrial interest in wire arc spraying as it has proven to be a relatively inexpensive alternative providing high quality coatings in many applications. Modeling and experimental studies have been conducted with different nozzle geometries to study the influence of fluid dynamics on the behavior of the particles which form the coating. Schlieren optical diagnostics and image analysis are used to study flow patterns obtained with different nozzle geometries. Supplementary measurements are made with static and total pressure probes. A particle breakup model is used to predict the trajectory and breakup of molten particles in the jet. This investigation indicates that nozzle designs that minimize or eliminate shock formation before the atomizing gas reaches the wire tips result in lower coating porosity and a more uniform particle size distribution.

Despite the fact that plasma spraying has been a widely used technology over the past three decades, industries using this technology still need higher quality products. Presently, only a small degree of process control is used in most plasma spraying systems. Improved process control should lead to more consistent results and higher quality products. We discuss a relatively simple control scheme consisting of a microphone as a primary sensor and a fuzzy logic look-up model indicating the condition of the anode. Selected frequency peaks in the power spectrum of the microphone signal are analyzed online, and the results are correlated with an average jet length obtained from a series of high speed images. The jet length, in turn, is correlated with coating characteristics. A simple feedback control system is proposed which will counteract the negative effects of an eroded anode on coating quality.