Experimental study of substrate thermal conditions at APS and HVOF

Transcription

1 Experimental study of substrate thermal conditions at APS and HVOF A. Zagorski, F. Szuecs Alstom Power Customer Service, Baden, Switzerland V. Belashchenko TSD, Concord, NH, USA A. Ivanov Praxair Surface Technologies, Indianapolis, IN, USA S. Siegmann, N. Margadant Swiss Federal Laboratories for Materials Testing and Research, EMPA Thun, Switzerland Abstract Systematic measurements of heat fluxes into the substrate under the conditions of APS and HVOF have been performed with the use of a specially designed calorimeter. Results of measurements are presented in terms of the heat absorbed by a mm diameter disk related to the electrical input power (EIP) and/or to the plasma/gas enthalpy. Depending on the process conditions heat flux into the substrate could reach as much as per cent of the torch input power. Influences of gas composition, torch power and standoff distance on the heat transfer efficiency as well as the radial distributions of heat fluxes have been studied in detail and compared for three types of guns. Data obtained from the measurements of the radial distribution of heat fluxes in the plasma plume allowed estimating effective plasma jet diameter and the heat flux density, respectively. Rates of jet expansion differ significantly between the F and PlazJet plasma torches. Direct measurements of the substrate temperatures with the use of thermal paints have been performed and temperature patterns were compared with the results of D heat transfer modeling. Experimental values of heat fluxes were used in the calculations. Comparison showed that the experimental data on heat transfer efficiency could be used for accurate predictions of the substrate thermal conditions during thermal spraying. Introduction There have been numerous papers and patents regarding influence of substrate and particles temperature conditions on coating formation and coating properties [- among others]. It has been shown that the thermal conditions are the critical factor, which to the great extent determine particle interactions with a substrate as well as the microstructure and thermo-mechanical properties of the coatings. One of the pioneering studies in this area [] showed the very strong correlation between substrate temperature, splat formation and the resulting bond strength of a coating. In that study separate molten particles of different materials were accelerated up to the speed of - m/sec and deposited onto polished substrates under controlled temperature ranged within - o C. From the load, which was applied to detach the splat, and the splat diameter, the bond strength was calculated. Those studies were among the first to show that the bond strength is strongly related to the surface temperature. It was found that the critical substrate temperature exists for coating adhesion. Increase of temperature above the critical value causes significant growth of the bond strength and appearance of a pronounced area with the metallurgical or covalent bonding in the central area underneath the splat as it can be seen in Fig.. m Figure : Substrate microstructure underneath the removed splat; a central spot and concentric areas where the splat was "welded" to the substrate are visible; substrate and powder material - silver (Courtesy of Prof. V.V. Kudinov). Size of welded islands and the fraction of bonded surface atoms at the substrate and between splats to the great extent determine properties of as-sprayed coatings. Rate of the particle cooling down at the substrate, which also critically depends on the surface temperature, is one of the parameters determining the residual stresses in the entire coating [7]. Thus, knowing the substrate surface temperature in the spray spot is a necessity for the appropriate selection of spray parameters in order to control the coating properties.

2 With the use of up-to-date computational methods one can quite easily model a thermal regime of sprayed components providing that the temporal and spatial heat flux distributions are known. One should consider two sources of heat fluxes. The first one is related to the heat brought to the surface by the gas or plasma jets and depends on the heat transfer efficiency at the surface and on the one in the spray gun. The second source is related to the heat transported with the sprayed particles and could be estimated if the particle enthalpy, spray pattern and the deposition rate are known. The latter source could be calculated quite accurately with the use of advanced computational models [8-], whereas the comprehensive simulation of the former one is hardly possible due to the extreme complexity of energy transfer processes inside the gun and between the plasma and surrounding atmosphere. Measurements of heat fluxes into the substrate could allow a researcher to validate theoretical models. Moreover, after parameterization measured data could be directly used in the thermal modeling. The present paper is an attempt to realize such an approach. Direct calorimetric measurements have been carried out for three types of spray guns and variety of gas compositions and process parameters. Measured heat flux data were implemented into the D heat conductivity simulation program and the calculated surface temperature distribution compared with the results from direct temperature measurements. Experimental Technique Heat flux measurements were carried out with the use of the calorimeter, which is shown in Fig.. The device consists of -mm diameter water-cooled copper plate. Two thermocouples measure the water temperature at the inlet and outlet. Also the water flow rate is measured by the volumetric flow meter. In experiments the flow rate varied from to lpm depending on the gun power in order to maintain the outgoing water temperature within the range of to o C. The amount of heat absorbed by the calorimeter can be calculated directly from the water temperature difference and mass flow with the known water heat capacity. The measuring error is determined by the accuracy of thermocouples and of the flow meter and estimated as to % of measured value depending on the regime. Atmospheric plasma spray equipment type M with a modified kw F torch (Medicoat), mm nozzle diameter and Argon-Hydrogen plasma and kw Nitrogen- Hydrogen PlazJet (Praxair/TAFA ) torch with the nozzle diameter of mm were used in experiments. Hydrogen - Oxygen - Kerosene CJS gun (Thermico) with the mm nozzle and operational pressure of 8- bar has been used in HVOF experiments. During the measurements all major gun parameters as well as the flow rate and incoming and outgoing cooling water temperatures were recorded. Figure : General view of the calorimeter installed in a spray booth; -copper plate, -.water hoses, - thermocouples, - heat exchanger. Also direct substrate temperature measurements have been performed. Peak surface temperature distribution at the steel plate was recorded at one pass of the torch using a set of thermal paints (type "Thermochrom" by Faber Castell, Germany) with the color transformation threshold ranged from to 7 o C. Results of Measurements Heat Flux Measurements under APS Conditions Heat flux measurements with different plasma gas compositions, torch electrical power (EIP) and varying nozzle axial and radial distances from the center of calorimeter have been carried out. As it could be expected the amount of heat absorbed changed almost linear with the gun power if the other parameters stayed unchanged. In Fig. typical variations of the total heat flux when the gun moved from outside along the radius to the axis of calorimeter at different standoff positions for two guns are presented. The process parameters of F and PlazJet were Ar/8H - kw and N /9H - 8kW respectively. Using this information a jet diameter can be estimated as follows: Q dq dr D jet, () where Q is the total heat flux. It should be noted that restoring more detail information of the heat flux distribution in the spot would require usage of the smaller calorimeter and special arrangements to minimize edge gas dynamic effects, which are the reasons for non-monotonous behavior of curves in Fig.. Variations of jet diameter with the axial distance assessed according () for the two guns are presented in Fig.. Jet expansion angles are about and degrees for F and PlazJet guns respectively. It is worth noting that the typical expansion angle constitutes - degrees for the highly turbulent gas jets (See, for

3 instance, a Schlieren photo of SG gun in [] or experimental references in []). Due to the lower expansion rate one can expect the fraction of energy delivered to the substrate from the PlazJet torch being higher. Maximum amounts of absorbed heat (the gun was centered against the calorimeter) related to EIP (let us call it the "torch heat efficiency") and to the net plasma enthalpy ("plasma heat efficiency") respectively for two torches are presented in Fig.. The latter characteristic was calculated by subtracting the amount of heat removed with the gun cooling water from the torch net power. In all cases heat transfer efficiency rapidly decreases with the stand off distance as approximately inverse squared distance. It is seen that for the F torch with Argon-Hydrogen mixture the gas composition has a little effect on the plasma heat efficiency, whereas a fraction of electrical power actually transferred to the plasma by the gun increases with the Hydrogen content in plasma. A similar trend was observed for the PlazJet torch. Unfortunately, in this case it appeared technically difficult to separate effects of processes inside the torch nozzle and at the heated substrate since the temperature of outgoing gun cooling water was affected by the thermal conditions in the spray booth. Effect of the hydrogen content on torch efficiency is presented in Fig.. Absorbed Power kw 78 mm 9 mm 79 mm 8 8 Radial Torch Position mm 8 Ab 7 so rb ed Po we r k W a) mm 7 mm mm mm mm Radial Torch Position mm b) Figure : Variations of heat flux absorbed by the surface when the gun moves from the side to the axis of calorimeter a - PlazJet (EIP=8kW), b - F (EIP=kW) Jet Diameter mm 8 F PlazJet Figure : Evolution of plasma jets with the axial distance from the gun nozzle. Torch Efficiency Plasma Efficiency a) b) Figure : Efficiency of heat transfer into the substrate at APS process for F (lines -) and PlazJet (line ) guns with different plasma compositions and torch power: Ar, H 8, kw, Ar, H, kw, Ar, H, kw, Ar, H, 7kW, PlazJet, N, H 9, 8kW; gas flow rates are in slpm; a - absorbed heat related to the gun electric power(torch heat efficiency), b - related to the plasma enthalpy(plasma heat efficiency). Heat efficiency of the PlazJet gun (the upper lines in Figs a, b) is as much as % higher than the one of F torch that correlates to the smaller jet diameter and lower rate of energy dissipation in the ambient atmosphere respectively. Lower level of the turbulence in the PlazJet flow is the most probable explanation of this effect. A principle of "low current-high voltage", which was implemented in the PlazJet concept, results in the higher frequency of arc root pulsations and, correspondingly, in the reduced initial scale

4 Torch Efficiency PJ F.... H content Heat Flux kw 8 7 Figure : Effect of the Hydrogen content in the mixture on the heat energy absorbed by the substrate. Stand off distance 78 mm of turbulence. Also the longer anode channel, which leaves more space for the decay of initial turbulence and produces more uniform velocity profile at the exit, contributes to the reduction of turbulent mixing outside the nozzle. Higher heat efficiency of the laminar or quasi-laminar jets has already found some practical applications. In [] using an advanced laminar plasma gun it appeared possible to optimize a process of spraying self-fluxing powders by depositing the coating at the turbulent regime with the subsequent heat treatment by the laminar jet. Heat Flux Measurements under HVOF Spraying Conditions During experiments a composition of combustion mixture was varied by alternating amounts of all three components. It has been found that the oxygen content in the mixture has a little influence on the amount of heat absorbed by the calorimeter, whereas amounts of hydrogen and kerosene affect it significantly. In Fig. 7 measured values of the total heat flux into the substrate are presented for six conditions. At all regime the oxygen-rich mixtures were used. One can see a clear separation of the curves corresponding to the different amounts of Hydrogen. In Fig. 8 the heat fluxes were normalized to the enthalpy of combustion products in the jet. The latter was calculated as a heat value of the fuel with the deduction of energy removed by the gun cooling water. One can see that the regime with the lowered kerosene content stays above the others. This effect can be explained by the lower heat conductivity of kerosene combustion products compared against ones of Hydrogen. Comparing Figs and 8 one can see that the efficiency of heat transfer from the HVOF jet is much higher than the one of F gun and comparable to the PlazJet plasma spray gun at the same standoff distance. This also should be attributed to the reduced level of turbulence of the supersonic HVOF jet and the lower rate of jet energy dissipation in the atmosphere. Figure 7: Heat fluxes into the substrate at HVOF for different gas compositions: Hydrogen., Kerosene 8., Oxygen, - H., K, O, H8, K, O8, H8, K, O, H., K, O, H, K, O (flow rates in m /h for Hydrogen and Oxygen and l/h for kerosene). Jet Efficiency Figure 8: Heat transfer efficiency of HVOF jet. Fuel compositions as in Fig. 7. Implementation of the Measured Heat Flux Data in the Thermal Simulation of Substrate Experimental data have been used in the D heat conductivity calculations as boundary conditions. A Gaussian distribution of the heat flux has been assumed: In this case q D / 8 r ) / ( r qe. defined by () and, where D is the jet diameter Q q. In order to verify the model temperature field measurements have been performed at the xmm steel plate of -mm thickness at one pass of the F torch. The plate has been marked with thermal paint stripes that change their colors after exceeding the threshold of the specific temperature. The torch EIP was 7kW, standoff distance mm, robot speed mm/s. From the discoloration of the stripes due to the plasma heating as it is presented in Fig. 9a one can restore some information about the imum surface temperature during the process. In Fig. 9b the calculated temperature field is presented. Comparing Figs 9a and 9b one can see that the

5 major features of the temperature distribution are well reproduced by the calculations both qualitatively and quantitatively. Implementation of parameterized experimental data on the heat flux distribution in numerical modeling of substrate thermal conditions allows one for accurate predictions of the surface temperature fields. Acknowledgements Authors are grateful to Prof. O. Solonenko (ITAM, Russia), Dr. D. Wang, Mr. V. Sedov (Praxair Surface Technologies, USA) for the fruitful discussions and experimental support. References: b) Figure 9: Surface temperature distributions from thermal paint measurements (a) and from D calculations (b); temperature iso-lines and threshold paint discoloration temperatures are shown; an arrow indicates the position of the spray gun and direction of its movement. Temperature - in degrees C, dimensions in mm. Summary Direct calorimetric measurements of heat fluxes from APS and HVOF guns into the substrate have been carried out for wide range of process parameters and compositions of working gases. Measured data have been interpreted in terms of heat absorbed by the surface related to EIP and to the plasma enthalpy. It has been shown that the heat transfer efficiency of the HVOF gun and PlazJet torch with Hydrogen - Nitrogen plasma is about per cent higher than the one of the Argon-Hydrogen F torch. The lower level of flow turbulence is the most probable reason for that. Thus, it could be expected that spraying guns with the laminar or quasi laminar flows would have certain advantages when the high heat transfer efficiency to the substrate and/or to the particles required. Increase of hydrogen content in Argon/Hydrogen mixture increases the fraction of electrical energy transferred into the plasma, whereas it has little influence on the efficiency of heat transfer from the plasma into the substrate. a). J. Wigren, L. Rejryd. Thermal barrier coatings-why, how, where and where to, in Thermal Spray: Meeting the challenges of the st century, Proc. th Int. Thermal Spray Conference, Nice, France, 998, pp. -.. V. E. Belashchenko and Yu. B. Chernyak, Stochastic approach to the modeling and optimization of thermal spray coating formation, Journal of Thermal Spray Technology, (), 99, 9.. T. Taylor, Thermal barrier coating for substrates and process for producing it. US Patent,7,. Dec.7, 99.. D. M. Gray, Y.-C. Lau et al. Thermal barrier coatings having an improved columnar microstructure. US Patent,8,8. Nov., P. Nylen, J. Wigren, L. Pejryd, M.-O. Hansson. Modelling of coating thickness, heat transfer and fluid flow and it's correlation with TBC microstructure for a plasma sprayed gas turbine application, in Thermal Spray: Meeting the challenges of the st century, Proc. th Int. Thermal Spray Conference, Nice, France, 998, pp. -.. V.V. Kudinov. Plasma Coatings, Moscow, Nauka, 977 (Russian), p P. Bengtsson, C. Persson. Modelled and measured residual stresses in plasma sprayed thermal barrier coatings. Surface and Coating technology, 9, 997, p M. Vardelle, A. Vardelle, P. Fauchais and M.I. Boulos, Plasma-particle momentum and heat transfer: modelling and measurements, AIChe Journ., v. 9, 98, No, p R. Wetstoff, G. Trapaga and J. Szekely, Plasmaparticle interactions in plasma spraying systems, Metall. Trans. B, V. B, Dec. 99, p R. L. Williamson, J. R. Finke and C.H. Chang, A computational examination of the sources of statistical variance in particle parameters during thermal plasma spraying, Plasma Chem and Plasma Proc., v., No,, p E. Pfender, Fundamental studies associated with the plasma spray process, Surface and Coating Technology., V., 988, p. -.. A. Zagorski, F. Stadelmaier, Full-scale simulation of a plasma spray process. Surface and Coating Technology, V. -7,, p..

6 . S. P. Mates, D. Basak, F.S. Biancaniello, S.D. Ridder and J. Geist, Journ. Calibration of a two-color imaging pyrometer and its use for particle measurements in controlled air plasma spray experiments. Journ. Therm. Spray Technology. V. (),, p. 9.. G. N. Abramovich. The Theory of Turbulent Jets. M.I.T. Press, 9.. O.P. Solonenko. Thje state-of-the-art of thermophysical fundamentals of plasma spraying in O.P. Solonenko and M. F. Zhukov (ed.) Thermal Plasma and New Materials Technologies. Vol.. Cambridge Intersc. Publ., 99, p