published at the ITSC 2005, Basel, CH, May 2 nd - 4 th 2005 Modified Supersonic nozzles for the Vacuum Plasma Spraying

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1 published at the ITSC 2005, Basel, CH, May 2 nd - 4 th 2005 Modified Supersonic nozzles for the Vacuum Plasma Spraying A. Schwenk, S. Mihm, G. Nutsch, Ilmenau / D, A. Wank, Chemnitz / D, H. Gruner, Mägenwil / CH Two new inner contours of de Laval nozzles (V70, V21) for F4 torch, which have already successfully been established for the Atmospheric Plasma Spraying (APS) are investigated for application under Vacuum Plasma Spraying conditions (VPS). Studies have been performed with regard to power efficiency, sound level and arc voltage fluctuations as well as coating quality and deposition efficiency for use of CoNiCrAlY powder as feedstock material. The results are compared to the commonly used standard VPS nozzle. CFD calculations and enthalpy probe measurements of plasma gas velocity and temperature distribution in the centreline of the supersonic plasma jets are carried out in order to get insight in the basic dependencies. 1 Introduction For almost two decades, Vacuum Plasma Spraying (VPS) is a manifold used thermal spray technique in a wide range of coating industry. Especially with the use of reactive feedstock materials, VPS is due to its inert gas environment and the high kinetic energy of the generated plasma jet an appropriate thermal spray process to produce dense and oxide free coatings. As investigations during the last decade have already shown, the nozzle inner contour is one of the most effecting parameters for thermal spray processes especially for VPS [1-6] and based on newer results also for APS [7-11]. The flow characteristic of the plasma jet in terms of flow dynamics is determined by the nozzle inner contour. For low pressure applications like VPS a supersonic plasma jet at the nozzle exit is obtained. Since 1982 the utilization of so called standard nozzles for VPS is state-of-the-art. The inner contour of the standard VPS nozzle is designed convergentcylindrical-divergent with a conical divergent exit. Depending on the chamber pressure a non-adjusted supersonic under- or overexpanded plasma jet at the nozzle exit is obtained in most cases of VPS applications. Under- or overexpanded gas jets are defined by compression and expansion zones, so called shock nodes, which cause disturbances and losses of kinetic energy in flow direction. By comparison it is possible to achieve adjusted plasma jet flow conditions with the application of new designed de Laval nozzles (V70, V21) with a convergent-divergent inner contour. In contrast to the standard VPS nozzle the divergent exit of the new designed de Laval nozzles is not conical but bell-shaped in order to generate a so called wall-bounded plasma flow until the nozzle exit. The avoidance of a flow separation from the nozzle inner wall results in less recirculation effects, which cause turbulences in the boundary layer of the near wall region. With the implementation of a simple 2D-model within commercial FLUENT software, the influence of the inner contour of the de Laval nozzles is studied in comparison to the standard VPS nozzle in terms of gas velocity and Mach number distribution along the nozzle axis. Enthalpy probe and time dependent arc voltage measurements have been performed in order to show the influence of the nozzle inner contour on the VPS process with regard to plasma jet properties as well as the arc movement on the anode wall. A goal of this study is to figure out the advantages and disadvantages with the utilization of de Laval nozzles with bell-shaped divergent exit for VPS concerning deposition efficiency and coating quality (porosity, oxide content) compared to the standard VPS nozzle. 2 Experimental Set-up Two different Vacuum Plasma Spraying systems are used in this study. For enthalpy probe measurements a MC60 Plasma torch from Medicoat AG, Mägenwil, CH, is used, whereas coatings are sprayed with a Sulzer Metco F4 Plasma torch using a VPS system Plasmatechnik A3000S. The two torches have the same configuration concerning their main components such as the finger-tipped tungsten cathode surrounded concentrically by the tungsten-copper anode. A DC arc with A operates between the two water cooled electrodes, which corresponds to a plasma torch power of 40 kw with the use of an Ar- H 2 -plasma gas mixture (Ar 60 l/min, H 2 5 l/min). Internal powder injection is used for the standard VPS and V70 nozzle and an external powder injection for the V21 nozzle. The powder injectors inner diameter is 1.8 mm for all experiments. The carrier gas flow rate is set to 2.5 l/min Ar with a powder feed rate of 15.7 g/min using Co32.5Ni21Cr8Al0.5Y with a grain size of 15 µm < d < 45 µm as feedstock material. The powder is injected with an angle of 105 to the axis of the expanding plasma jet. 3 de Laval nozzle 3.1 Nozzle Design - Theoretical background The design of the divergent exit of the new de Laval nozzles is based on the characteristics method of Prandtl and Busemann [12]. According to the calculations of the transition curve of the nozzle inner contour, the plasma gas flow is accelerated to supersonic velocities at the nozzle exit depending on the throatexit area ratio A * /A (A * - throat area, A - exit area), the pressure ratio of the plasma gas flow at the nozzle exit p 0 /p s (p 0 - stagnation pressure, p s - static pres-

1a and b) is calculated for low pressure conditions using an Ar-H 2 -plasma gas mixture (12 vol.-% H 2 ) and the boundary conditions shown in Tab. 1.

2 sure) and the adiabatic exponent κ of the plasma gas mixture as already described in detail in [13]. With the assumption of an isentropic flow, thermodynamic equilibrium and the absence of a boundary layer, the new design of the de Laval nozzles V70 and V21 (Fig. 1a and b) is calculated for low pressure conditions using an Ar-H 2 -plasma gas mixture (12 vol.-% H 2 ) and the boundary conditions shown in Tab. 1. To prevent evolution of shock nodes in axial flow direction, the plasma gas flow has to be completely expanded at the exit of the de Laval nozzles. This occurs, if the static pressure p s of the emanating plasma jet is equal to the environment pressure p E. Otherwise the plasma jet is over- or underexpanded. Tab. 1. Parameters of de Laval nozzle design. Nozzle V70 V21 D * [mm] 6 7 T Gas,m [K] M ω [ ] D [mm] L [mm] p 0 /p s Ar [slpm] H 2 [slpm] Computional Fluid Dynamics (CFD) With the implementation of a simple axis-symmetric 2D-model within the commercial software Fluent using the finite-volume-method, CFD calculations of the plasma gas flow through the nozzle are performed for given boundary conditions under low pressure conditions (VPS). The one equation Spalart-Allmaras model for wall-bounded flows, which solves a model transport equation for the kinematic eddy (turbulent) viscosity, is chosen in the case of all three types of nozzle. The CFD calculations for the standard VPS, V70 and V21 nozzle are carried out for Mach number and gas velocity distribution along the axis inside the nozzle (Fig. 2) for the pressure conditions shown in Tab. 2. Tab. 2. Pressure boundary conditions for CFD calculations. Nozzle Standard V70 V21 p 0 [kpa] p E [kpa] p 0 /p E Fig. 2a. Axial velocity distribution inside the standard VPS nozzle. Fig. 1a. Scheme of the convergent-divergent V70 nozzle with a prolongation of 7 mm. 12,1 Fig. 1b. Scheme of the convergent-divergent V21 nozzle with a prolongation of 2.1 mm. Fig. 2b. Axial velocity distribution inside the V70 nozzle.

Fig. 2c. Axial velocity distribution inside the V21 nozzle. For a given chamber pressure p c = p E = 40 kpa at the exit of each nozzle, an overexpanded plasma jet is obtained.

for V70 and V21 nozzles. In contrary, the plasma jet is emanating the standard VPS nozzle with a maximum velocity of around 1800 m/s due to a lower inlet pressure p 0.

3 Fig. 2c. Axial velocity distribution inside the V21 nozzle. For a given chamber pressure p c = p E = 40 kpa at the exit of each nozzle, an overexpanded plasma jet is obtained. The results of the CFD calculations for the velocity distribution in axial flow direction show, depending on the stagnation pressure p 0 at the inlet of the nozzle, velocity values of around 2000 m/s for V70 and V21 nozzles. In contrary, the plasma jet is emanating the standard VPS nozzle with a maximum velocity of around 1800 m/s due to a lower inlet pressure p 0. The CFD calculations are performed assuming that stagnation pressure does not change due to to wall friction effects neither for de Laval nozzles nor for the standard VPS nozzle. Considering the plasma gas dynamics in the case of the emanating overexpanded plasma jet, a compression (vertical shock wave) inside each of the three nozzles is obtained. This causes not only a flow separation from the wall but also a deceleration of the expanding plasma gas flow in the divergent exit of the nozzle. The results of the Mach number distribution along the axis inside the three nozzles are shown in Fig. 3. Fig. 3b. Axial Mach number distribution inside the V70 nozzle. Fig. 3c. Axial Mach number distribution inside the V21 nozzle. Taking into account that all three nozzles are not used for their adjusted flow conditions, the greatest turbulences are obtained in the conical divergent exit of the standard VPS nozzle with recirculation velocities up to 150 m/s in the near wall region. Both the de Laval nozzles and the standard VPS nozzle can only reach Mach numbers in a range of M = for p C = 40 kpa. 4 Results and Discussion 4.1 Deposition Efficiency Fig. 3a. Axial Mach number distribution inside the standard VPS nozzle. First investigations are accomplished to determine the influence of the de Laval nozzles inner contour of the on the deposition efficiency compared to the standard VPS nozzle. CoNiCrAlY coatings are sprayed with the utilization of the three nozzles and the constant parameter set of section 2 with regard to the plasma gas mixture (Ar 65 l/min, H 2 8 l/min), carrier gas flow (2.5 l/min Ar), powder feed rate (15.7 g/min) and chamber pressure (p c = 40 kpa) as well as spray time for two varying spray distances z. The mean coating thicknesses d coating are given in Tab. 3.

4 Tab. 3. Coating thicknesses of VPS sprayed coatings using standard VPS, V70 and V21 nozzles. nozzle Standard V70 V21 z [mm] d coating [µm] z [mm] d coating [µm] The determination of deposition efficiency for VPS according to DIN pren ISO/DIS is part of current investigations. First results show with the utilization of the standard VPS nozzle a deposition efficiency up to 84 %. Taking into account the coating thicknesses in Tab. 3, deposition efficiency for use of V21 nozzle can be even higher but at least in the same range compared to the standard VPS nozzle. Regarding the substantially worse results for use of the V70 nozzle, it can be concluded that the spray parameters are not optimized yet. Therefore, further investigations need to be carried out. 4.2 Arc voltage fluctuations Arc voltage fluctuations as a consequence of the hydrodynamic forces of the plasma gas flow inside the nozzle as well as the Lorentz force of the magnetic field caused by the arc itself are still a disadvantage of the DC Plasma Spray process. As can be found in literature the DC Plasma torch can operate in three different modes, i.e. restrike, take-over and steady mode [14]. In most cases, the take-over or the mixed mode (as a superposition of the restrike and take over mode) are obtained with typical frequency peaks of 4-7 khz in the power spectra using gas mixtures of argon with a certain percentage of molecular gases such as hydrogen or nitrogen. The frequency of 4 khz, which is predominantly observed for nozzle inner contours with a cylindrical part is due to the restrike of the arc and causes sound levels of around 120 dba of the DC Plasma torch as already proofed in the case of APS applications [13]. In contrary the axial location of the arc anode attachment can be fixed in the divergent exit part of a de Laval nozzle with avoidance of arc restrike. The rotational component of the arc anode attachment movement prevents the local overheating of the anode wall. Arc voltage fluctuations of the F4 plasma torch are measured with a digital oscilloscope (DL2100A) and the utilization of all three types of nozzles. The results for chamber pressure p c = 40 kpa are shown in Fig. 4. U [V] ,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 t [ms] Standard VPS V70 V21 Fig. 4. Arc voltage fluctuations for use of standard VPS, V70 and V21 nozzle for the VPS. The results for utilization of V70 and V21 nozzles show reduced arc voltage fluctuations of U = ± 3 V due to the fact that the anode attachment is more or less fixed in the divergent exit of the de Laval nozzles. The 4 khz frequency peak is not observed any more in the power spectra, which results in a noticeable lower sound level of the plasma torch compared to the standard VPS nozzle. This is caused by greater arc voltage fluctuations U = ± 6 V due to the cylindrical part of the standard VPS nozzles inner contour, which causes the restrike of the arc as already verified for APS. The U-I-characteristic of the arc shows for use of de Laval nozzle V70 at a constant arc current and a chamber pressure p c = 40 kpa higher arc voltages compared to the standard VPS and V21 nozzle (Fig. 5). This is due to different arc lengths depending on the nozzle inner contour and the plasma gas dynamics inside of the nozzle. U [V] Strom [A] VPS V21 V70 Fig. 5. U-I-characteristic of the plasma torch using the standard VPS, V70 and V21 nozzle. 4.3 Plasma jet properties The gas velocity and temperature distribution obtained from enthalpy probe measurements within the plasma jet using the standard VPS, V70 and V21 nozzle at chamber pressure p c = 40 kpa and electrical input power of 40 kw are shown in Fig. 6. The minimum measure distance from the nozzle exit

commonly used conical exit of the standard VPS nozzle. This is due to the different effects of flow separation from the anode wall and the deceleration of the plasma gas flow inside the nozzle.

5 amounts to 75 mm for the standard VPS nozzle due to the high heat flux of the argon-hydrogen gas mixture to the probe tip. As written above, at low pressure conditions the bell-shaped divergent exit of the V70 and V21 nozzle leads to higher axial velocities and an extended hot core of the plasma jet compared to the commonly used conical exit of the standard VPS nozzle. This is due to the different effects of flow separation from the anode wall and the deceleration of the plasma gas flow inside the nozzle temperature [K] Fig. 7a. Cross section of a CoNiCrAlY coating sprayed with standard VPS nozzle at p c = 40 kpa distance z [mm] VPS - nozzle V21 - nozzle V70 - nozzle Fig. 6a. Axial temperature distribution in the centreline of the plasma jet for standard VPS, V70 and V21 nozzles velocity [m/s] distance z [mm] VPS - nozzle V21 - nozzle V70 - nozzle Fig. 6b. Axial velocity distribution in the centreline of the plasma jet for standard VPS, V70 and V21 nozzles. 4.4 Coating quality To evaluate the different nozzles for VPS, the as sprayed coatings are investigated with regard to the microstructure and the mean porosity. First results of the CoNiCrAlY coatings sprayed with utilization of standard VPS and V21 nozzle are shown in Fig. 7. Coating cross-sections indicate a dense microstructure. Systematic determination of the mean porosity by digital analysis is part of current investigations. Fig. 7b. Cross section of a CoNiCrAlY coating sprayed with V21 nozzle at p c = 40 kpa. 5 Conclusions For adjusted flow conditions the undisturbed supersonic plasma jet generated by V70 and V21 nozzle is much longer compared to the standard VPS nozzle. This is due to an improved expansion inside the new designed bell-shaped exit of the de Laval nozzles compared to the conical exit of standard VPS nozzles. A large number of VPS applications implicate nonadjusted plasma flow conditions, which result in either over- or undexpanded supersonic plasma jets. In this study an overexpanded plasma jet is investigated with regard to velocity and Mach number distribution in the centreline, arc voltage fluctuations and sound level of the DC Plasma torch, deposition efficiency and coating quality. The use of new de Laval nozzles (V70 and V21) at a given chamber pressure of 40 kpa, an electrical input power of 40 kw and an Ar-H 2 plasma gas mixture (65 l/min Ar, 8 l/min H 2 ) results in reduced arc voltage fluctuations of U = ± 3 V as well as a reduced sound

6 level of the DC Plasma torch. Due to the bell-shaped divergent exit of de Laval nozzles, an improved expansion of the emanating plasma gas jet from the nozzle can be obtained, which has been predicted by a simple axis-symmetric 2D-model by the commercial software Fluent. First results for CoNiCrAlY powder feedstock with grain size of 15 µm < d < 45 µm show deposition efficiencies up to 84 % for use of the standard VPS and V21, whereas the parameter set is not optimized for V70 nozzle yet. The as-sprayed coatings show low porosity, which will be quantified in further investigations. To increase the process stability as well as the deposition efficiency and coating quality, de Laval nozzles with a bell-shaped divergent exit are an competitive alternative to standard VPS nozzles. 5 Literature [1] Bolot, R., D. Klein and C. Coddet, Influence of the nozzle design on the structure of a Plasma jet under vacuum conditions, Conference Proceedings of the International Thermal Spray Conference, E. Lugscheider, DVS, Essen, 2002, p [2] Bolot, R., D. Klein and C. Coddet, Design of a nozzle extension for thermal spray under very low pressure conditions, Proceedings of the ITSC, Osaka, Japan, 2004, conference CD [3] Cao, M., F. Gitzhofer, D.V. Gravelle, R. Henne and M. I. Boulos, A torch nozzle design to improve plasma spray techniques, Plasma Sources Science Technology, 6 (1997), p [4] Henne, R., W. Mayr and A. Reusch, Influence of nozzle geometry on particle behaviour and coating quality in high-velocity VPS, Conference Proceedings of the Thermal Spraying Conference, DVS, Aachen, 1993, p [5] Jodoin, B., M. Guidrat, J.-L. Dorier, C. Hollenstein, M. Loch and G. Barbezat, Modelling and diagnostics of a supersonic DC plasma jet expanding at low pressure, Proceedings of the ITSC, Essen, Deutschland, 2002, p [6] Rat, V., E. Boyer and R. H. Henne, DC Plasma Diagnostics for Improvement of Plasma Spraying Process under Soft Vaccum Conditions, Proceedings of the ITSC, Orlando, Florida, 2003, p [7] Schwenk, A., H.Gruner and G.Nutsch, Einfluss der Düsenkontur auf atmosphärisch DCplasmagespritzte Al 2 O 3 -Schichten, Proceedings of the ITSC, Essen, Deutschland, 2002, p (in German). [8] Schwenk, A., Ch. Schneider, J. Sachs, H. Gruner, and G. Nutsch, Lavaldüsen Eine innovative Alternative für das Atmosphärische Plasmaspritzen, Tagungsband Workshop Plasmatechnik, G. Nutsch, ISBN , Ilmenau, 2002, p (in German). [9] Schwenk, A., H. Gruner and G. Nutsch, Modi-fied Nozzle for the Atmospheric Plasma Spraying, Proceedings of the ITSC, Orlando, Florida, 2003, p [10] Schwenk, A., H. Gruner, S. Zimmermann, K. Landes and G.Nutsch, The influence of inner nozzle contour on the plasma torch performance for atmospheric plasma spraying, 16 th International Symposium on Plasma Chemistry, Taormina, Italy, June 22-27, paper 519 [11] Schwenk, A., H. Gruner and G. Nutsch, Eine neue Düsengeneration für das Atmosphärendruck Plasmaspritzen, Tagungsband zur 5. Industriefachtagung Oberflächen- und Wärmebehandlungstechnik, , Chemnitz, Germany, ISBN , p (in German) [12] Prandtl, L., and A. Busemann, Näherungsverfahren zur zeichnerischen Bestimmung von ebenen Strömungen und Überschallgeschwindigkeiten, Stodola Festschrift, Zürich, 1929 (German) [13] Schwenk, A., H. Gruner, S. Zimmermann, K. Landes and G. Nutsch, Improved Nozzle Design for de-laval-type-nozzles fort he Atmospheric Plasma Spraying, Proceedings of the ITSC, Osaka, Japan, 2004, conference CD [14] Heberlein, J., Approaches for control of the dynamic Characteristics of Plasma Spray torches, Tagungsband Worhshop Plasmatechnik, G. Nutsch, ISBN X, Ilmenau, 2001, p. 1-11

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