Plasma Jet-Substrate Interaction in Low Pressure Plasma Spray-CVD Processes

Conventional equipment for plasma spraying can be adapted for operation at low pressure so that PECVD-like processing can be performed. The plasma jet generated by the torch is characterized by a high convective velocity and a high gas temperature. The influence of these properties on a deposition process are investigated in the framework of simple theoretical considerations and illustrated by various experimental results obtained with SiOx deposition. A conclusion of this study is that the deposition process is dominated by diffusion effects on the substrate surface: the deposition profiles and the deposition rates are determined by the precursor density and by the gas temperature on the substrate surface. The high velocity of the jet does not play a direct role in the deposition mechanism. On the other hand it strongly increases the precursor density available for the deposition since it efficiently transports the precursor up to the substrate.

Plasma Spray-CVD: A New Thermal Spray Process to Produce Thin Films from Liquid or Gaseous Precursors

Malko Gindrat, Philippe Frédérique Bruno Guittienne, Christoph Hollenstein

New dedicated coating processes which are based on the well-known LPPS (TM) technology but operating at lower work pressure (100 Pa) are being actively developed. These hybrid technologies contribute to improve the efficiencies in the turbine industry such as aero-engines and land-based gas turbines. They also have a great potential in the domain of new energy concepts in applications like Solid Oxide Fuel Cells, membranes, and photovoltaic with the adoption of new ways of producing coatings by thermal spray. Such processes include Plasma Spray-Thin Film (PS-TF) which gives the possibility to coat thin and dense layers from splats through a classical thermal spray approach but at high velocities (400-800 m/s) and enthalpy (8000-15000 kJ/kg). Plasma Spray-PVD (PS-PVD) which allows producing thick columnar-structured Thermal Barrier Coatings (100-300 mu m) from the vapor phase with the employment of the high enthalpy gun and specific powder feedstock material. On the other hand, the Plasma Spray-CVD (PS-CVD) process uses modified conventional thermal spray components operated below 100 Pa which allows producing CVD-like coatings (< 1-10 mu m) at higher deposition rates using liquid or gaseous precursors as feedstock material. The advantages of such thermal spray-enhanced CVD processes are the high ionization degree and high throughput for the deposition of thin layers. In this article, we present an overview of the possibilities and limitations encountered while producing thin film coatings using liquid and gaseous precursors with this new type of low pressure plasma spray equipment and point out the challenges faced to obtain efficient injection and mixing of the precursors in the plasma jet. In particular, SiO (x) thin films from Hexamethyldisiloxane (HMDSO or C6H18OSi2) can be deposited on wafers at deposition rates of up to 35 nm/s at an efficiency of about 50%. The process was also used for producing metal oxide coatings (Al2O3, ZnO, and SnO2) by evaporating different metals in combination with an oxygen gas flow. The effect of process parameters on the deposition rate, coating build up, uniformity, and quality of the coatings are discussed. An overview of different potential applications of this new technology will be also presented.

2011

Characterization of supersonic low pressure plasma jets

Malko Gindrat

Low Pressure Plasma Spraying (LPPS) processes use a DC plasma jet expanding at low pressure for fast deposition of dense coatings in a controlled atmosphere. The LPPS technology is widely used industrially in particular in the aeronautics and medical industries among others. Unlike atmospheric pressure plasma jets, which have been extensively studied experimentally and theoretically, the interest in low pressure DC plasma jets only occurred recently. However, the process development has been mainly based on empirical methods and the basics of the physical mechanisms that govern them still remain to be investigated. Further improvement of the processes requires, in particular, the knowledge of physical properties of the plasma jet such as the temperature, flow velocity and plasma density. Low pressure plasma jets present unconventional properties such as low collisionality, large dimensions and supersonic flow. Therefore specific diagnostics have to be adapted to these conditions. In this study, argon plasma jets at pressures between 2 and 100 mbar are investigated. Imaging has been used to allow a qualitative description of the plasma jet topology for different pressures and torch parameters. Low pressure plasma jets are most of the time supersonic, compressible and in an aerodynamic non-equilibrium, which results in visible successive compression and expansion zones corresponding to a variation of the local pressure, temperature and density. Imaging, combined with pressure measurements inside the plasma torch, has evidenced three different types of flow regimes with respect to the chamber pressure. For chamber pressures below 45 mbar, the flow is under-expanded and is characterized by an exit pressure higher than the chamber pressure. For pressures above 45 mbar, the plasma jet is over-expanded, in this case the exit pressure is lower than the chamber pressure. When the exit pressure is equal to the chamber pressure, the plasma jet is in the so-called design pressure regime. A diagnostic tool, extensively applied on atmospheric plasma jets, the enthalpy probe system, has been modified in order to allow gas sampling from the plasma jet at low pressures. A shock wave appears in front of the probe when it is immersed in a supersonic plasma jet, making the interpretation of enthalpy measurements more difficult.The free-stream properties, like the Mach number, temperature and free-stream enthalpy have to be inferred from stagnation measurements. Two interpretations of enthalpy probe measurements are described in this study. The first method uses the energy conservation equation and LTE assumptions with calorically perfect gas and neglecting the aerodynamic non-equilibrium, whereas the second method, uses a complementary measurement of the static pressure just after the shock using a specially developed tool: the Post Shock Static Pressure Probe (PSSPP). This allows the use of the conservation equations to determine the free-stream properties of the plasma jet without the assumption of calorically perfect gas and aerodynamic non-equilibrium. Determination of the free-stream enthalpy, Mach number and temperature were possible on over-expanded jets for pressures higher than 40 mbar. At 100 mbar with torch parameters of 400 A and 40 SLPM argon flow, the temperature of the plasma jet reaches 10000 K and the velocity is about 3000 m/s on axis. Measurement of plasma jet properties such as the Mach number, electron density and temperature, were performed using double Langmuir probes and Mach probes. In particular, under-expanded jets are studied in detail by performing complete mappings of plasma jet properties at 10 and 2 mbar chamber pressure. These results show that the measured physical properties are consistent with the jet flow phenomenology such as the presence of periodic expansion and compression zones, the effect of the pressure and the location of the shocks. It is shown in particular that for highly under-expanded jets at 2 mbar, the Mach number reaches 2.8 in the first expansion zone followed by a strong drop to subsonic flow revealing the presence of a Mach reflection. The flow is accelerated further and a periodic structure of compression/expansion cells is observed until the local static pressure is in equilibrium with the surrounding pressure. Another diagnostic often used in plasma spraying is optical emission spectroscopy (OES) which is non-intrusive and gives information about the plasma excited species.However, the determination of the excitation temperature, obtained by the Boltzmann plot method, relies on the assumption of local thermodynamic equilibrium (LTE), which is no longer satisfied at low working pressures. The result of the deviation from LTE is that the heavy particle, electron and excitation temperatures are different. In this study, Boltzmann plots have been used to evaluate the deviation from LTE as a function of the working pressure and the location in the plasma jet. It has been shown that the plasma jet is closer to LTE in the compression zones and close to the axis. Measurements of spectral line broadening due to the Stark effect allowed to determine the electron density for under-expanded jets and give results similar than with electrostatic probe measurements.On the other hand, excitation temperatures are systematically lower than the electron temperature for the same plasma conditions. For a 10 mbar plasma jet, the excitation temperature of argon is between 0.73 and 0.78 eV whereas the electron temperature is between 0.7 and 1.2 eV. This shows that at low pressure the plasma jets are not in LTE.These results contribute to the understanding of the supersonic plasma jet behavior at low pressure and can be used to quantify the deviation from local thermodynamic equilibrium (LTE). The extensive mapping of the measured physical properties of the jet will also serve as input for modeling.

EPFL2004

Inclusion of aerodynamic non-equilibrium effects in supersonic plasma jet enthalpy probe measurements

Malko Gindrat, Christoph Hollenstein

Low pressure plasma spraying (LPPS) is a thermal spraying technique that has found a niche for low oxidation products. It uses a low pressure environment (i.e:, chamber pressure between 2 and 90 kPa) and yields supersonic plasma jets. The enthalpy probe technique is a common measurement method in plasmas. However LPPS jets are difficult to diagnose as their supersonic nature forces the apparition of a shock wave in front of any measuring device inserted in the jet. Incomplete or erroneous assumptions are usually invoked to overcome the difficulties associated with this shock wave and carry out the LPPS jet diagnosis from enthalpy probe measurements. In this work, a new device is designed to gain access to an additional physical quantity, which is needed to assess the aerodynamic non-equilibrium state of the jet. It is combined with enthalpy probe measurements, and the resulting set of experimental data is used with a numerical procedure based on gas dynamics theory, yielding free-stream supersonic plasma jet values from the measurements behind the induced shock wave. The results agree well with the phenomenology of supersonic jets in aerodynamic nonequilibrium. However this new method is restricted by the local thermodynamic equilibrium assumption, which is directly linked with the pressure and temperature conditions of the plasma jet.