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.

In order to improve gas turbine performance it is possible to decrease back flow gases in the high temperature combustion region of the turbo machine reducing shroud/rotor gap. Thick and porous TBC systems and composite CoNiCrAlY/Al 2 O 3 coatings made by Air Plasma Spray (APS) and composite NiCrAlY/graphite coatings made by Laser Cladding were studied as possible high temperature abradable seal on shroud. Oxidation and thermal fatigue resistance of the coatings were assessed by means of isothermal and cyclic oxidation tests. Tested CoNiCrAlY/Al 2 O 3 and NiCrAlY/graphite coatings after 1000 hours at 1100°C do not show noticeable microstructural modification. The oxidation resistance of new composite coatings satisfied Original Equipment Manufacturer (OEM) specification. Thick and porous TBC systems passed the thermal fatigue test according to the considered OEM procedures. According to the OEM specification for abradable coatings the hardness evaluation suggests that these kinds of coatings must be used with abrasive tipped blades. Thick and porous TBC coating has shown good abradability using tipped blades.

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.

Abradable seals have been used in jet engines since the late 1960's. Today abradable seals are seeing applications not only in low pressure and high pressure sections of the compressor but also in the high pressure turbine module of jet engines and are gaining ever more attention in industrial gas turbines. Thermal spraying is a relative simple and cost effective means to apply abradable seals. These work by minimizing gaps between rotating and stationary components by allowing the rotating parts to cut into the stationary ones. The types of coatings employed are zirconia based abradable material systems with polymer and, in some cases, hexagonal boron nitride additions. The coatings are designed to work at service temperatures of up to 1200 °C. The objective of this paper is to review various types of commercial and experimental ceramic abradable systems and to assess their performance profiles. The paper will review yttria stabilized zirconia based systems with modified polymer additions and with variable particle sizes of the ceramic phase. Alternative stabilizers and their influence on key coating properties such as thermal shock resistance and abradability will be studied. The paper will also review the influence of plasma spray parameters on coating properties and study the general influence of coating porosity on coating erosion properties and abradability.

Low pressure plasma spraying (LPPS) is well established as coating process for the deposition of TBC systems for industrial gas turbines as well as aero applications under industrial standards. With the extension to LPPS Thin Film process, the operating conditions can vary in a wide range from only a few mbar up to typically 200 mbar which imposes different characteristics of the corresponding spray conditions and resulting coating properties. The flexibility of this process allows for subsequent coating of different layers for bond and top coat of a complex TBC system by using only a single TS technology. This simplifies the operational needs for industrial production and offers the potential of increased technological and economical benefits. Depending on the process conditions a broad variety of different TBC layers, multi-layer systems including bond coats & diffusion barriers, dense, porous or graded YSZ layers as well as EB-PVD like layers with columnar microstructure, can be realized using the LPPS-TF technology. Proper adjustment of spraying conditions and suited material selection allows controlling the grade of “columnarity” of the layer. Thermal cycling as well as GE erosion tests have been used to characterize and benchmark the coating quality. Transfer of such coatings onto real turbine blade has demonstrated that all substrate areas could be coated with a columnar TBC structure up to a thickness of about 250 microns. This offers the possibility either to develop coatings with optimized performance or of comparable quality at reduced production costs when compared to established vapour deposition techniques like EB-PVD. This paper presents an overview on the development of various TBC systems such as multi-layers or columnar structures by using the LPPS Thin Film technology.

In the thermal spray industry there remains a constant demand for more stable, controllable and economical processing devices. Most thermal spray applicators apply a wide range of materials especially those who are allied with the turbine markets. Each material coating specification requires a relatively specific range of velocity and temperature transferred to the powder particle to achieve the required material properties on the part. The plasma gun has become a dominant process tool in the turbine industry due to the wide range of parameters that are achievable with the basic tool. In addition, there are today a large number of individual plasma guns that are each known to provide excellent results with some, but not all coating requirements. In many cases the optimum gun running condition can never be achieved due to the interdependence of arc behavior and gas conditions. The turbine industry is currently largely populated with guns that are based on the technology as it was developed in the 1960’s. These guns are characterized by poor voltage stability with a large quasi periodic oscillation in the 3-5 kHz range as described by Bisson [1], with poor performance and life under more extreme operating conditions. A key element of any plasma gun is the nozzle geometry. The cascaded gun types as typified by the Triplex offer a unique opportunity to study and document a wide range of operational parameters, especially at extreme operating conditions.

Plasma spraying at low pressure conditions (LPPS) is a well established thermal spray process with a broad variety of important applications in different industrial segments. The operating conditions for LPPS processes can vary in a wide range from only a few mbar up to typically 200 mbar which imposes different characteristics of the corresponding spray conditions and resulting coating properties. Thermal spray processes have been approved being suitable for integrated fabrication of various layers for SOFC components. Depending on the process conditions, different layers used as functional coatings of SOFC components such as dense electrolyte layers as well as porous electrodes can be realized using the LPPS technology. Due to the flexibility of these processes, an optimized performance of the application on different target materials and geometries is possible with an increased technological and economical benefit compared to conventional thermal spray techniques. This paper presents an overview on the general potential and spray conditions of the LPPS process and its application for the deposition of various functional layers such as electrolyte and electrodes for SOFC components.

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 provides an overview of the tools and techniques used in industry to monitor and control plasma spraying processes. It describes how the tools work and what they can reveal about spraying processes and coating properties. It also presents examples showing how the tools are used to optimize spray parameters for special applications and discusses the possibility of implementing closed loop control for plasma spraying processes. Paper includes a German-language abstract.

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.

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.