In this work, computational fluid dynamics (CFD) results confirm earlier calculations indicating that significant evaporation occurs in plasma torch nozzles. In addition, experimental work is performed, investigating the nature of ceramic deposits produced by plasma spray-physical vapor deposition (PS-PVD), particularly coatings composed of nanosized clusters. It was found that as the hot plasma jet comes close to the relatively cool substrate, a boundary layer is formed due to the rapid drop in temperature and velocity. In summary, coatings produced by PS-PVD are a mixture of nanocluster and vapor deposition.

Plasma spray-physical vapor deposition (PS-PVD) has been developed over the past years to produce columnar structured deposits for thermal barrier applications. The non line-of-sight process makes it possible to deposit coatings on complex components in a single production step. The fundamentals of PS-PVD have been well studied and the knowledge obtained is being used to scale up the process. This paper presents initial results on concepts that have been implemented to increase production rates as well as process efficiency.

Recent developments in hybrid low pressure thermal spray technologies such as Plasma Spray-Thin Film (PS-TF), PS-PVD, PS-CVD are being increasingly used to develop functional inorganic coatings and films for emerging high end energy applications. The requirements of such coatings and films are more highly specified than those of conventional plasma spray coatings. Successful film deposition therefore requires not only the development and application of novel operating parameters, but also goes hand-in-hand with tailored feedstock materials development. Targeted development by Sulzer Metco has allowed applications to evolve into fields where conventionally competitive manufacturing technologies would be applied; potentially enabling entirely new fields of plasma spray manufacturing to emerge. Such applications include corrosion protection and electrolytic films in SOFC, gas tight mixed electron and ion conducting membranes for gas separation and thin, transparent functional layers in photo-voltaic applications. This paper provides a brief overview of the status of developments of several high end emerging energy applications which are being developed using such hybrid low pressure plasma spray technologies.

Plasma spray – physical vapor deposition (PS-PVD) is a low pressure plasma spray technology to deposit coatings out of the vapor phase. PS-PVD is part of the family of new hybrid processes recently developed by Sulzer Metco AG (Switzerland) on the basis of the well established low pressure plasma spraying (LPPS) technology. Included in this new process family are plasma spray - chemical vapor deposition (PS-CVD) and plasma spray - thin film (PS-TF) processes. In comparison to conventional vacuum plasma spraying (VPS) and low pressure plasma spraying (LPPS), these new processes use a high energy plasma gun operated at a work pressure below 2 mbar. This leads to unconventional plasma jet characteristics which can be used to obtain specific and unique coatings. An important new feature of PS-PVD is the possibility to deposit a coating not only by melting the feed stock material which builds up a layer from liquid splats but also by vaporizing the injected material. Therefore, the PS-PVD process fills the gap between the conventional physical vapor deposition (PVD) technologies and standard thermal spray processes. The possibility to vaporize feedstock material and to produce layers out of the vapor phase results in new and unique coating microstructures. The properties of such coatings are superior to those of thermal spray and EB-PVD coatings. In contrast to EB-PVD, PS-PVD incorporates the vaporized coating material into a supersonic plasma plume. Due to the forced gas stream of the plasma jet, complex shaped parts like multi-airfoil turbine vanes can be coated homogeneously with columnar thermal barrier coatings using PS-PVD. This paper reports on the progress made by Sulzer Metco to develop a thermal spray process to produce coatings out of the vapor phase.

Low-pressure plasma spray thin film (LPPS-TF) is a recently developed vacuum plasma spray technology that makes it possible to deposit coatings not only by melting feedstock material, but also by vaporizing injected particles. This capability fills a gap between conventional vapor deposition and thermal spray processes. The vaporizing of coating material and formation of layers out of the vapor phase result in unique coating microstructures with superior properties. This paper reports on the progress made in the development of functional coatings built up from the vapor phase of metal and oxide ceramics.

This work shows that low-pressure plasma spraying equipment can be used to deposit layers of varying thickness from liquid or gaseous precursors. In particular, HMDSO and oxygen are used to deposit SiO x thin films over large areas at deposition rates exceeding 35 nm/s and conversion efficiency better than 50%. The coatings are analyzed ex-situ by FTIR absorption spectroscopy and the microstructure and morphology of layer cross-sections are examined by SEM. The effects of various process parameters are investigated as well.

Reliable and economically efficient processes are necessary for the production of high quality coatings for solid oxide fuel cells (SOFC) applications in an industrial scale. In that perspective, Sulzer Metco developed several coating solutions through different processes adapted for each specific applications, in particular on metal supported cells (MSC). Diffusion barrier layers (DBL) using perovskite material, such as Lanthanum Strontium Manganite (LSM), is produced “state-of-the-art” as coating service by Sulzer Metco on metallic interconnects (IC) using the Triplex technology. The newly developed TriplexPro-200 having a long lifetime performance and specific features, like cascaded arc and 3-cathode torch, is the best candidate for producing high quality and reliable coatings in a mass production of SOFC functional layers. LPPS-Thin Film, on the other hand is the technology of choice to deposit very dense, thin and homogeneous layers on various substrates. Yttria stabilized Zirconia (YSZ) layers of 20-40 µm thickness have been deposited on thin metallic substrates (0.7 mm, 140 cm 2 ) without producing any strong deformation of the substrate. Considering the dimension of the metallic substrate the coated cells present very good gas leak tightness performances between 2 and 8 Pa·m/s which is homogeneous on the substrate area. Moreover, LPPS-TF can also be used to produce very dense and thin LSM coatings on interconnects. In this case, LPPS-TF not only produces denser and thinner coatings but also becomes again competitive when considering the manufacturing of DBL for metallic ICs on a high production scale. This paper presents the current developments of these technologies in the domain of SOFC applications.

With the new development of the LPPS Thin-Film technology (LPPS-TF), which is a special modification of conventional LPPS using reduced chamber pressures below 10 mbar, a new window has been opened to deposit uniform and dense thin layers onto large areas in short coating times. This spray process will allow the access to new market areas and will be able to bridge the gap between conventional thin film (< 1 - 10 µm) deposition using PVD/CVD processes and thick (> 50 - 200 µm) thermally sprayed layers. This paper presents the status of the LPPS-Thin Film technology as a hybrid coating process between thermal spray and vapor deposition and gives an overview of potential applications for functional thin coatings and large area coverage.

Low pressure plasma spraying (LPPS) and LPPS-Thin Film (LPPS-TF) processes cover a broad operational pressure range from typically 200 mbar down to a few millibars, filling the gap between conventional thermal spray processes, where coatings are made from the liquid phase, and conventional thin film technologies such as PVD or CVD, where coatings are produced from precursors species in the vapor phase. Using some specific parameters of the LPPS-TF process, the injected material can be partially or even completely in gaseous phase, disqualifying diagnostics based on the detection of solid or liquid particles such as the DPV-2000 (Tecnar, St-Bruno, QC, CA). In this case, other optical diagnostic tools have to be used, such as optical emission spectroscopy (OES) to characterize the LPPS-TF process. In this paper, a qualitative study of the properties of the injected material in the plasma jet using DPV-2000 and optical emission spectroscopy is presented by varying specific plasma parameters. Moreover, in some particular cases, it is shown that the combination of DPV measurements and OES can help to monitor the coating process and to improve the basic understanding of the LPPSTF technology.

This paper describes the investigation of low pressure supersonic plasma jets as found in LPPS processes. The main objective is to develop and validate a two-dimensional axisymmetric mathematical model representing such flows. Due to the supersonic nature of the jet, insertion of a measurement probe leads to the appearance of a detached shock in front of the probe. Consequently plasma values are measured behind the probe-induced shock, namely the stagnation enthalpy ( h o ), the stagnation pressure ( p o ) and the static pressure ( p ). The first two values are taken from enthalpy probe measurements while the third value comes from a new technique. Combining these measurements, a new interpretation method enables the calculation of the free-stream supersonic plasma jet properties. The mathematical model is validated using the enthalpy probe measurements and the free-stream properties from the new interpretation method. Results show that the model does not predict a static pressure as large as the new interpretation method. The principal cause for this discrepancy is attributed to the LTE assumption which is questionable for a 40 mbar plasma jet. The modelling effort reported here confirms the need to develop more detailed mathematical models for low pressure supersonic plasma jets in the future.

Low Pressure Plasma Spraying (LPPS) is nowadays a well-established thermal spray process with a broad variety of important applications for functional surface coatings. The operating pressure for LPPS processes can vary in a wide range from typically 200 mbar down to only a few mbar. This leads to unconventional properties of the plasma jet, in terms of supersonic flow with strong shock structure at moderate pressure, towards rarefaction and frozen flow at very low pressure. In order to optimize and control the spray processes for specific applications, it is necessary to understand the underlying physical mechanisms. However, so far only limited knowledge has been established on the plasma jet properties and its interaction with the spray particles in LPPS conditions. We present several experimental investigations to characterize plasma spray processes under various pressure conditions. Measured plasma jet properties using a dedicated enthalpy probe system and imaging are combined with IR-pyrometry and velocimetry on the particles (DPV2000) to further improve the understanding of the plasma particle interactions. These results, along with spray deposit characterization, can be used to optimize the coating properties and explore further potential applications.

This paper investigates the effect of chamber pressure on plasma jet expansion characteristics. It presents images of the plasma jet corresponding to different chamber pressures and torch parameters and correlates them with enthalpy probe and pressure measurements recorded in different areas of the torch nozzle. A transition from an over-expanded to an under-expanded flow regime, as evidenced by a change in jet topology, is shown to be a function of chamber pressure. This transition pressure strongly depends on torch parameters and is characterized by an estimation of a rarefaction parameter based on nozzle exit and chamber pressure. At low chamber pressures, a progressive change from a continuum to a transition flow regime is shown by the thickening of the shock structures. Paper includes a German-language abstract.

A numerical model of an argon jet exiting a LPPS torch has been developed and validated against enthalpy probe measurements for a slightly overexpanded jet at a chamber pressure of 100 mbar. Visualization of the jet using a CCD camera shows the presence of a small Mach reflection in the first compression-expansion cell with only oblique shock waves in the second cell. This jet topology is also observed in the model results. The images of the enthalpy probe on the axis of the plasma jet reveal that the shock layer, or shock-probe distance, varies according to the axial location of the probe. Shock-probe distance can be as large as 3 mm and should be considered when mapping plasma jets. Paper includes a German-language abstract.

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.