Liquid flame spraying

J. Aerosol Sci., VoL 26. Suppl 1, pp. $229-$230, 1995 Elsevier Science Ltd Printed in Great Britain 0021-8502/95 $9.50 + 0,00 Pergamon LIQUID FLAME SPRAYING J. Tikkanen,V. Pitk&nen,J. Larjo, J. Keskinen, G. Graeffe Tampere University of Technology, Department of Physics, P.O.Box 692 FIN 33101 Tampere, Finland K. Gross, Wo Smith, S. Raghu, and C.C. Berndt State University of New York at Stony Brook, Department of Materials Science and Engineering, Stony Brook, NY 11794-2275, U.S.A. M. Rajala University of Industrial Arts, Department of Ceramics and Glass Design, H&meentie 135 C, FIN 00560 Helsinki, Finland. Keywords -- material synthesis, flame spraying, atomization, coatings INTRODUCTION Thermal spraying is a process in which molten particles are deposited onto a substrate. The purpose of spraying is to form a coating, thereby modifying the surface properties. Thermal spraying utilizes a variety of deposition techniques, such as flame spraying (FS), atmospheric/vacuum plasma spraying (APS/VPS), and high velocity oxy-fuel spraying (HVOF). Thermal energy is generated either by an oxy-fuel burner or an ionized plasma, resulting in temperatures ranging from 2500 to 3500 K or 6000 to 14000 K, respectively. Feedstock is commonly powder but may also be as wire or rod. Powder of the size range between 5-70 microns is injected into the flame and travels at velocities between 50 and 900 m/s, depending on the technique used. The number of applications for thermal spraying is increasing through wider acceptance in the industry and through new techniques (Pawlowski, 1995; Smith, 1992). As the powder particle size requirement within thermal spray applications is continuously decreasing, being at the moment around 20 Ilm, particle feed problems increase. Intensive effort is directed to find techniques to overcome this problem. A novel thermal spraying technique has been developed, which replaces solid feedstock with liquid materials (Tikkanen et al., 1994). Liquid Based Flame Spraying (LBFS) atomizes liquid feedstock into a oxy fuel burner, which accelerates and converts it to fine molten droplets prior to impact with the substrate. This study focuses on thermal and flow properties of the flame spray and particularly on the formation and behavior of particles in the new spraying process. EXPERIMENTAL SYSTEM Hydrogen and oxygen were used as burning gases to provide a clean burning flame. Flow rates up to 100 I/min and pressures of 10 bar for hydrogen and 50 I/min, 5 bar for oxygen could be used. Physical properties of the LBFS were analyzed. The atomized droplet size distribution was analyzed with a Laser Diffraction Analyzer (LDA). Flame temperature distribution was measured using Rayleigh Back Scattering (RBS). Control Vision (CV) qualitatively monitored the flame turbulence and the atomized material as it AS26/9suPPl-a $229 $230 J. TIKKANENet al. was transported in the flame. Finally, Laser Doppler Velocimetry (LDV) was employed to determined the flame and molten droplet velocity. RESULTS AND DISCUSSION The LBFS gas atomizer produces fine liquid droplets with a mass median of less than 10 Ilm. Solid particle size can be further changed by varying the solution concentration. Liquid feed rates are typically 400-1000 ml/h, but are limited to the maximum enthalpy of the flame. Feedstock can be any suitable liquid suspension or precursor of the desired coating material. A maximum flame temperature of 2500-3000 K was measured with RBS, occurred 50 mm from the nozzle exit. A real-time image produced by CV illustrates very little spread of the atomized liquid emitted from the torch (fig. 1). A smaller mean droplet size and chemical homogeneity are two attributes of the process. LBFS has led to successful results in coating production and material synthesis. Good coating quality is ensured by reactions occurring on the heated substrate. Further experiments are being carried out using Laser Doppler Velocimetry (LDV) and Electrical Low Pressure Impactor (ELPI) to measure the particle velocity and particle size distribution in the flame. ELPI measurement makes it possible to do chemical analysis on the collected particles as a function of size, which will be useful to gauge the extent of reactions in the flame. Figure 1. Control Vision image (20 x magnification) of LBFS process. REFERENCES Pawlowsk~ L.: The Science and Engineering of Thermal Spray Coatings, John Wiley & Sons, New York (1995), 414 p. Smith, R.W.: Equipment and Theory, A Lesson from Thermal Spray Technology (edited by Willis, B.), Engineering Institute, ASM Intemational, (1992), 66 p. Tikkanen et al., Finnish Patent Application No. 930200 (1994). Materials