- Email: [email protected]
Plasma spray deposition of Al-Al2O3 coatings doped with metal oxides: catalytic applications
Surface and Coatings Technology 123 (2000) 122–128 www.elsevier.nl/locate/surfcoat
Plasma spray deposition of Al-Al O coatings doped with 2 3 metal oxides: catalytic applications L. Pranevicius a,b, L.L. Pranevicius a,b, *, P. Valatkevicius a, V. Valincius a a Lithuanian Energy Institute, 3 Breslaujos St., KaunasLT-3035, Lithuania b Vytautas Magnus University, 28 Daukanto St., KaunasLT-3000, Lithuania Received 22 January 1999; accepted in revised form 19 September 1999
Abstract Al-Al O (70% c-phase) coatings 30–50 mm thick well-adhered to the steel sheets and with a highly developed surface area 2 3 (100–120 m2 g−1) were formed employing plasma-spray technology at atmospheric pressure in air. The plasma-gun with two sequential powder feeders was developed offering the ability to control particle trajectories through the plasma flame, and thus their thermal history. The Al powder is mainly melted and oxidized. Al(OH ) powder passes through the plasma torch with 3 partial dissociation and is incorporated in the matrix of growing film with subsequent decomposition during thermal annealing at 560°C for 90 min. The good adhesion results are explained by the surface pre-treatment effects taking place on the periphery of the plasma torch moving along the surface of steel sheets. The plasma sprayed Al-Al O coatings doped with CuO and Cr O 2 3 2 3 oxides showed characteristic catalytic combustion behaviors. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Coatings; Catalytic behavior; Plasma spraying
1. Introduction During recent years, significant progress has been made in developing atmospheric plasma technology. It combines the universality of the low pressure plasma technology and economic approaches of chemical technologies. Keeping in the mind the fact that chemical technologies are, in many cases, environment unfriendly, plasma technology emerges as the technology of the near future [1,2]. The plasmas generally can be produced by the application of a sufficient high level of energy, e.g. in the form of arcs, sparks, glow discharges, flames or shock waves. It is the activated state of the medium where individual types of particles, e.g. ions, electrons and neutral particles (in the form of non-excited and excited atoms, molecules and radicals) can be grouped in accordance with different temperatures. The field of application of plasma physicochemical treatments embraces: (1) the cleaning of surfaces, beginning with removing grease and oils from sheet metal up to careful photo resist striping; (2) oxidation of surfaces, beginning with * Corresponding author. Tel: +370-7-778602. E-mail address: [email protected] (L.L. Pranevicius)
surface functionalization and ending in artificial aging; and (3) the deposition of thin and thick coatings. The heterogeneous processes initiated by plasma depend on the temperature (energy) and on the concentration of particles reaching the surface from plasma. Usually it is high at atmospheric pressure [3]. In plasma spraying at atmospheric pressure in air the particles injected into the plasma jet may react with the atomic oxygen with the formation of oxides. The goal of this study was to obtain well-adhered Al O coatings with a highly developed surface area on 2 3 steel sheets for catalytic applications employing plasma spraying technology. The basic ideas behind the research were: (1) to realize reactive aluminum powder deposition in plasma torch enriched by Al(OH ) powder at atmospheric 3 pressure in air; (2) to obtain highly adherent coatings on steel sheets; and (3) to identify combustion effects in the ignition and combustion of propane/air mixtures over Al O coatings doped with metal oxides. 2 3 2. Formation of the plasma torch The schematic presentation of the plasma spray gun is shown in Fig. 1. The gun consisted of a hot Hf
0257-8972/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 52 0 - 4
L. Pranevicius et al. / Surface and Coatings Technology 123 (2000) 122–128
Fig. 1. Scheme of the spraying process.
cathode and a water-cooled copper anode. The electrically neutral sections were located between electrodes to increase the voltage drop across the arc and to increase the resistivity of the short electrical connection. The power supply of the arc-gun included a current stabilizer with voltage and current regulation. The arc was initiated between the cathode and anode at atmospheric pressure in air. The geometry of the copper anode was optimized to promote expansion of the gas carrier passing through plasma and to push the plasma content from the plasma-gun forming a ‘flame’ consisting of high velocity, high temperature (energy) electrons, ions and neutrals. It is the so-called equilibrium plasma which exists in arcs, or in plasma torches at higher (atmospheric) pressures, due to the great frequency of collisions between different particles. Reactive plasma spraying is a metal powder spraying process in a reactive plasma. Reactive species, generated by the dissociation of gas precursors, react with heated metal particles and the resulting material splats to form the chemical compounds at the particle or splat surfaces [4]. For example, aluminum powder injected into an atmospheric plasma may react with atomic oxygen. Reactions may occur: (1) in flight with the solid and then with liquid aluminum (the flight time is about 1 ms); (2) at the splat surface as a liquid (t<20 ms) and then as a solid (t<50 ms); and (3) at the beads and then pass surface (t~0.1–30 s) [5]. The plasma torches formed by plasma-guns are nonhomogeneous in temperature and concentration of activated and reactive species in dependence on the coordinates along the torch axis and the radius in the plane of cross-section perpendicular to the axis. The situation becomes more complicated if external gas flows are used to cool the torch sides and substrate. It is well known [6 ] that powder particles will be accelerated immediately by the plasma fluid as soon as they are fed into the arc space by the feeding gas. The main forces of the fluids acting on the powder particles are the fluid viscosity force and the thermophoresis force
123
[7]. As the metal powder is fed into the plasma column, it will consequently be preheated, melted and (or) evaporated. The problem of how to control these processes is not completely solved. Our experimental studies concerning the thermal behavior of Al powders with 50 mm diameter have shown that they are melted in the 100 A central plasma and evaporated in the 200 A central plasma. The arc-plasma spray gun was constructed so as to allow injection of the powders into the torch of plasma internally and externally. The powders used were dried and passed through size filters. The maximum size of Al powder did not exceed 75 mm. The powder of Al(OH ) and metal oxides CuO and Cr O did not 3 2 3 exceed 50 mm. Special mixers provided a continuously vibrating, controlled dosage of powder. The gas carrier (air) and powder were injected into the plasma through a 9.5 mm diameter cylindrical tube with a flow rate of gas carrier of 0.4 g s−1. Al(OH ) powder was mixed 3 with CuO and Cr O powder, injected internally in the 2 3 direction parallel to the axis of the plasma torch passing through the boundary layer between reactor surface and plasma. To decrease the temperature of the boundary layer the additional flux of air was introduced at a flow rate equal to 1.2 g s−1. The Al powder was injected externally into the volume of the plasma channel. The total flow rate of the gas through plasma gun was 2.5–3 g s−1. An additional air flow was directed to the substrate area contacting the plasma torch. The flow rate of the additional air flux was equal to 1–3 g s−1. The mean temperature of the working gas along the axis of the burning arc was 5000–6000 K, while the flow velocity was equal to 400–500 m s−1. The temperature of the working gas leaving the plasma gun was 3000– 3500 K and the mean velocity about 500–650 m s−1. The energy flux density was in the range 106–108 W m−2, and the flux of atomic and particles about 1025 m−2 s−1. A power supply of 30–40 kW was used and the working arc current was 150–200 A [8]. The experimental studies of the parameters of plasma torch have shown that plasma torch parameters sharply depend on the radial distance from the axis and is a function of the maximum temperature of the working gas leaving the gun. If the temperature of the working gas leaving the gun is relatively small (1500–2500 K ) the plasma torch has well defined cylindrical form with length equal to 5–8 diameters of the exit hole of plasma gun. In the range of high temperatures of the working gas leaving the gun (>3000 K ) the radial distribution of temperature and velocity is a step-like function with a sharp decrease in temperature and velocity at a radial distance equal to the radius of the aperture of plasma gun and as the length of plasma torch decreases. It is the result of the turbulence phenomena and gas mixing by the convection process [9]. It is shown that with the increase in gas velocity, the friction forces increase and
124
L. Pranevicius et al. / Surface and Coatings Technology 123 (2000) 122–128
exceed the inertia forces so that the Reynolds number decreases. The distance between the plasma gun and substrate was equal to 100 mm. The steel sheets were rotated during deposition with linear velocity equal to 5 m s–1. The plasma gun moved horizontally in parallel to the treated substrate with a velocity equal to 5×10−3 m s−1.
3. Simultaneous pre-cleaning of moving steel sheets The properties of the coating produced by the particles arriving at the substrate from the central part of the torch depend on the chemical state of steel surface. The effect of a moving torch along the treated surface results in a time and space dependent boundary, in which the fluxes of plasma components differ in composition and velocity according to the radial distance from the axis of the torch. It means that a pre-treatment of the substrate surface takes place before the process of deposition starts. In particular, the bonding between the coating and substrate is affected by any residual compounds which remain from the industrial preparation processes and subsequent surface cleaning. In the present work, low carbon steel sheets with a thickness 0.5 mm were used after cold-rolling with a rolling oil which remains on the steel sheets after the rolling step. Typically the residual oil contaminant is about 300–400 mg m−2. During the annealing in a non-oxidizing atmosphere, which is needed to restore the metallurgical structure which is disturbed during rolling, the various lubricant compounds tend to be eliminated by degradation processes. Nevertheless, residual carbonaceous compounds remain on the surface. In order to improve the adhesion of melted particles arriving from plasma torch it is necessary to remove these residual pollutants as much as possible. Presently, cleaning of the surface of steel sheets before coating deposition is mainly realized in acid or alkaline chemical baths which lead to environmental problems (i.e. the need for treatment of the effluents produced). The alternative to chemical surface cleaning is plasma cleaning. The application of plasmas in the cleaning treatment of metallic surfaces is widely used [10]. The aim of the present study is to combine the plasma cleaning treatment and subsequent deposition processes in one plasma spray torch as it continuously moves along the substrate surface. It is based on the fact that the power density which is adsorbed by a solid surface is non-homogeneous and a function of the radial distance from the axis of the plasma torch. In the central part of the plasma-spray torch the power densities exceed 109 W cm−2 and the heat transfer into the substrate consists of the effect of the hot fluid as an impact heating flow. In the periphery of the plasma torch the
power density decreases, and thus the interior layer of the substrate is weakly affected by plasma-matter interaction, and the main plasma effect occurs at the outermost surface of the substrate. The predominant process at the periphery of the plasma torch consists of bond breaking between the adsorbate and the surface, which provides a chance for the impurities to leave the surface. Indeed, most of the organic compounds show an intense adsorption in the UV range. The surface contaminants may be excited by UV photons to a repulsive antibonding state. A possible relaxation mechanism of this excitation is the departure of the adsorbate from the surface. By employing a plasma torch at the atmospheric pressure in air, one may facilitate rapid breaking of the bonds at the periphery of plasma torch. The deposition of melted material from the central part of the torch then takes place on the plasma cleaned surfaces as the plasma torch is shifted along the surface. The adhesion properties of a coating depend on the interface properties, and it is important therefore that the first contact layer would be deposited on a plasmacleaned and activated substrate surface. In the present work, the construction of the plasma gun and the system of powder injectors forms a plasma torch in such a way that the periphery of the plasma torch performs the functions of surface pre-treatment. As the steel sheets are in continuous motion relative to the plasma torch, the process of surface cleaning takes place in front of the plasma torch where atomic oxygen actively reacts with the plasma-activated and fragmented radicals of organic macromolecules. These components are generally volatile and removed by the external gas flow directed on to the substrate. This means that a clean surface is obtained which is free of oil and fat contamination before deposition starts. The central part of torch is enriched by highly overheated metal aluminum powder which intensively evaporates and reacts with atomic oxygen. The extent of oxidation of the metallic aluminum depends on the plasma parameters and on the time of flight through the plasma torch. The two step process, surface cleaning and deposition, is realized by producing the plasma torch in such a way that the process of surface cleaning prevails on the periphery, and the deposition of oxidized aluminum prevails in the central part of the torch. In all cases the adherence of the coatings so formed was excellent without any chemical preliminary treatment of steel sheets. For catalytic applications it is important to have substrates with a highly developed effective surface area. One possible way of increasing the effective surface area is to form microclusters of material within the growing film which are decomposed and removed by the subsequent thermal treatment. The material incorporated into the matrix of a growing film, in the form of microscopic inclusions, after removal leaves a highly expanded sur-
L. Pranevicius et al. / Surface and Coatings Technology 123 (2000) 122–128
face with numerous pores. In the present research the Al(OH ) powder fulfilled the role of such a material if 3 it was introduced properly into the plasma torch. The construction of the plasma-gun and the powder dosage using the air as gas carrier was done in such a way that Al(OH ) powder of the correct size passed through the 3 plasma without dissociation and was incorporated in the matrix of the growing film. In summary, to obtain highly adherent Al-Al O 2 3 thick coatings (30–50 mm) on the surface of steel sheets, the plasma gun was constructed to make a plasma torch which performed the processes of both surface cleaning and deposition, and also produced a highly expanded surface as Al(OH ) powder was incorporated in the 3 growing film, with dissociation of the powder following during thermal treatment.
125
(a)
4. Coatings properties Two types of coatings have been investigated: firstly, coatings deposited by plasma spray of Al and Al(OH ) powders, and secondly, coatings deposited by 3 plasma spray of Al and Al(OH ) powders with additions 3 of metal oxides powder of CuO and Cr O . 2 3 The Al-Al O (c and a phase) coatings were formed 2 3 in the following way: the first layer of 10–12 mm thickness was sprayed using only Al powder, and the following three layers of 12–15 mm were each formed using Al and Al(OH ) powder in equal portions. After 3 deposition the coated substrates were heated at a temperature of 560°C for 90 min. The doping with oxides for the second type of coatings was done by mixing metal oxide powder with Al(OH ) powder in the same manner as in the case of 3 spray deposition of Al and Al(OH ) powders. The 3 coatings doped with metal oxides were thermally heated at 560°C for 90 min. The adhesion of coatings to the substrate was measured using a bending technique. All coated samples without any preliminary chemical pre-treatment passed 90° bending with a radius of curvature of 2 mm without cracking and peeling — as checked in optical and scanning electron microscopes. The steel sheets after deposition passed mechanical treatments of cutting and folding. The surface structure view of the Al-Al O coating 2 3 in a scanning microscope is presented in Fig. 2(a) and (b) which shows the same area at different magnifications. The surface of the coatings is very rough and porous. A typical view of the cross-section of the same coatings is presented in Fig. 3. It can be seen that at the interface, the layer of metallic aluminum is homogeneously adhered to the substrate. Spherical drops of oxidized aluminum can also be seen. The top layer of the coating consists mainly of oxidized aluminum. X-ray
(b)
Fig. 2. The surface structure view of Al-Al O coatings in scanning 2 3 electron microscope.
analysis results of coatings are presented in Fig. 4 for undoped (Fig. 4(a)) and for doped with 10% CuO and 5% Cr O powder (in mass units) (Fig. 4(b)). Undoped 2 3 Al-Al O coatings after thermal annealing at 560°C for 2 3 90 min include Al O mainly c-phase (about 70%). 2 3 The deposition of sprayed Al and Al(OH ) powder 3 takes place on steel sheets at substrate temperatures of 290–320°C. The substrate temperature has a substantial influence on the ratio of phase fractions of c-Al O and 2 3 a-Al O . With an increase in substrate temperature 2 3 above 290°C a phase transition takes place from c-Al O into a-Al O . At a substrate temperature of 2 3 2 3 770°C practically pure a-Al O is obtained. The addi2 3 tional air cooling prevented the substrate heating above 290°C. The small fraction of a-Al O indicates that only 2 3 small part of oxidized Al was heated above 600–650°C in the plasma, where the intensive formation of a-Al O starts. 2 3 To have highly developed effective surface it is important that the Al(OH ) should not be heated in the 3
126
L. Pranevicius et al. / Surface and Coatings Technology 123 (2000) 122–128
(a) (a)
(b) Fig. 4. The X-ray analysis results of (a) Al-Al O coatings, and (b) 2 3 Al–Al O coatings doped with metal oxides of CuO and Cr O . 2 3 2 3 (b)
Fig. 3. The view of the cross-section of steel sheet covered by an Al-Al O coating. 2 3
plasma to above the temperature of intensive dissociation of Al(OH ) . The plasma-gun construction allowed 3 the Al(OH ) powder to be pushed through the cold 3 boundary layer along the periphery of the plasma torch. This permitted the Al(OH ) powder to be incorporated 3 in the matrix of the coating. During thermal annealing, Al(OH ) incorporated in 3 the matrix of coating dissociates with the formation of c-Al O , and volatile products are desorbed leaving a 2 3 highly developed effective surface. The effective surface area, measured by the thermal desorption technique, was equal to 100–120 m2 g−1 in the samples tested for catalytic applications.
5. Catalytic combustion of propane For experimental studies, a vertically mounted tube furnace (Fig. 5) was used. The pre-mixed propane/air gas stream was introduced through a stainless steel tube at the bottom of the furnace and passed through a
Fig. 5. Scheme of the catalytic combustion test furnace.
12 cm long active element made from two-side coated Al O steel sheets rolled to a cylinder with diameter 2 3 equal to 15 cm. To control the flow of the working gas through the active element, the coated sheets of steel were folded to produce square channels for the flow of propane/air having a characteristic size equal to 7 mm. The temperature of the exiting gas was monitored by a chromel-alumel thermocouple.
L. Pranevicius et al. / Surface and Coatings Technology 123 (2000) 122–128
127
atures and the overall fuel and air conditions and heat flow balance within the test system. Fig. 6 illustrates the catalytic behavior of plasma sprayed Al O undoped (Fig. 6(a)) and doped with 10% 2 3 CuO and 5% Cr O ( Fig. 6(b)). The curves labeled 1 in 2 3 both figures illustrate the time dependence of the temperature installed in the furnace without a flow of propane, and the curves labeled 2 in the case when the equivalence ratio of the propane to air is equal to 1.8. It is seen that the ignition of catalytic combustion of plasma sprayed undoped Al O starts at about 710°C 2 3 followed by homogeneous gas-phase combustion at about 810°C (Fig. 6(a)). The catalytic combustion of Al O coatings doped with metal oxides starts at about 2 3 350°C ( Fig. 6(b)). (a)
6. Conclusions
(b) Fig. 6. Catalytic combustion behavior of (a) Al–Al O coatings, and 2 3 (b) Al–Al O coatings doped with metal oxides of CuO and Cr O . 2 3 2 3
In the experimental procedure, the flow of a premixed propane/air mixture through the furnace was equilibrated for 3 min, and then the heating element of the furnace was switched on. A comparison of the temperature increase behavior when the gas stream contains a propane/air mixture, as opposed to when it contains only air (with all other factors being kept constant) allows one to identify any catalytic combustion effects which may be occurring. A similar experimental procedure was used in Refs. [11,12]. The catalytic combustion of propane in air employing steel sheets covered by an Al O coating doped with 2 3 10% CuO and 5% Cr O were investigated to identify 2 3 catalytic combustion effects. The conducted research concentrated on catalytic combustion of propane which starts at temperatures above the catalytic ignition temperature (T ) with the release of heat above the catalytic ig surface. At T&T , the heat generated at the catalytic ig surface aids in initiating homogeneous combustion above the surface. The homogeneous combustion may then be sustained, or not, depending primarily upon the ability of the catalyst to remain active at high temper-
The Al and Al(OH ) powder mixture doped with the 3 metal oxides of CuO and Cr O was used to form 2 3 catalytic coatings on the surface of steel sheets employing reactive plasma spraying technology at atmospheric pressure in air. Well-adhering coatings consisting of 70% c-Al O and having a highly developed effective 2 3 surface area (100–120 m2 g−1) were formed. It was shown that, by using external and internal injections of the gas carrier/powder and additional cooling of the plasma torch in the gap between plasma-gun and substrate with the flow of air, one can realize the technical conditions necessary for: (1) the surface pre-treatment before deposition, which guarantees good adhesion of arriving deposit material; and (2) incorporation of Al(OH ) powder in growing film without dissociation, 3 which gives a highly developed surface area after thermal treatment at 560°C for 90 min. The plasma sprayed Al-Al O coatings doped with 2 3 metal oxides showed characteristic catalytic combustion behaviors. However, there is lack of fundamental knowledge of the phenomena involved and this is needed to explain the observed experimental results. The plasma flow is complex. Actual turbulence models are not adapted and not completely understood.
References [1] R.W. Smith, Z.Z. Matasim, J. Thermal Spray Technol 1 (1992) 57. [2] M. Bouls, Pure Appl. Chem. 68 (1996) 1007. [3] L. Pranevicius, in: Coating Technology: Ion Beam Deposition, Satas and Associates, Warwick, 1993, p. 453. [4] P. Fauchais, A. Vardelle, A. Denoirjean, Surf. Coat. Technol. 97 (1997) 66.
128
L. Pranevicius et al. / Surface and Coatings Technology 123 (2000) 122–128
[5] P. Fauchais, J.F. Coudert, M. Vardelle, A. Vardelle, A. Denoirjean, J. Thermal Spray Technol. 1 (1992) 117. [6 ] W. Xibao, L. Hua, Surf. Coat. Technol. 106 (1998) 156. [7] J.F. Lancaster, in: Physics of Welding, Pergamon, 1986, p. 413. [8] V. Valincius, P. Valatkevicius, L.L. Pranevicius, D. Milcius, Lithuanian Mater. Sci. 1 (6) (1998) 12.
[9] J.D. Anderson, in: Hypersonic and High Temperature Gas Dynamics, McGraw-Hill, 1989, p. 363. [10] W. Petasch, B. Kegel, H. Schmid, K. Lendenmann, H.U. Keller, Surf. Coat. Technol. 97 (1997) 176. [11] R.L. Jones, Surf. Coat. Technol. 86/87 (1996) 127. [12] R.L. Jones, Surf. Coat. Technol. 94/95 (1997) 95–118.
Plasma spray deposition of Al-Al O coatings doped with 2 3 metal oxides: catalytic applications L. Pranevicius a,b, L.L. Pranevicius a,b, *, P. Valatkevicius a, V. Valincius a a Lithuanian Energy Institute, 3 Breslaujos St., KaunasLT-3035, Lithuania b Vytautas Magnus University, 28 Daukanto St., KaunasLT-3000, Lithuania Received 22 January 1999; accepted in revised form 19 September 1999
Abstract Al-Al O (70% c-phase) coatings 30–50 mm thick well-adhered to the steel sheets and with a highly developed surface area 2 3 (100–120 m2 g−1) were formed employing plasma-spray technology at atmospheric pressure in air. The plasma-gun with two sequential powder feeders was developed offering the ability to control particle trajectories through the plasma flame, and thus their thermal history. The Al powder is mainly melted and oxidized. Al(OH ) powder passes through the plasma torch with 3 partial dissociation and is incorporated in the matrix of growing film with subsequent decomposition during thermal annealing at 560°C for 90 min. The good adhesion results are explained by the surface pre-treatment effects taking place on the periphery of the plasma torch moving along the surface of steel sheets. The plasma sprayed Al-Al O coatings doped with CuO and Cr O 2 3 2 3 oxides showed characteristic catalytic combustion behaviors. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Coatings; Catalytic behavior; Plasma spraying
1. Introduction During recent years, significant progress has been made in developing atmospheric plasma technology. It combines the universality of the low pressure plasma technology and economic approaches of chemical technologies. Keeping in the mind the fact that chemical technologies are, in many cases, environment unfriendly, plasma technology emerges as the technology of the near future [1,2]. The plasmas generally can be produced by the application of a sufficient high level of energy, e.g. in the form of arcs, sparks, glow discharges, flames or shock waves. It is the activated state of the medium where individual types of particles, e.g. ions, electrons and neutral particles (in the form of non-excited and excited atoms, molecules and radicals) can be grouped in accordance with different temperatures. The field of application of plasma physicochemical treatments embraces: (1) the cleaning of surfaces, beginning with removing grease and oils from sheet metal up to careful photo resist striping; (2) oxidation of surfaces, beginning with * Corresponding author. Tel: +370-7-778602. E-mail address: [email protected] (L.L. Pranevicius)
surface functionalization and ending in artificial aging; and (3) the deposition of thin and thick coatings. The heterogeneous processes initiated by plasma depend on the temperature (energy) and on the concentration of particles reaching the surface from plasma. Usually it is high at atmospheric pressure [3]. In plasma spraying at atmospheric pressure in air the particles injected into the plasma jet may react with the atomic oxygen with the formation of oxides. The goal of this study was to obtain well-adhered Al O coatings with a highly developed surface area on 2 3 steel sheets for catalytic applications employing plasma spraying technology. The basic ideas behind the research were: (1) to realize reactive aluminum powder deposition in plasma torch enriched by Al(OH ) powder at atmospheric 3 pressure in air; (2) to obtain highly adherent coatings on steel sheets; and (3) to identify combustion effects in the ignition and combustion of propane/air mixtures over Al O coatings doped with metal oxides. 2 3 2. Formation of the plasma torch The schematic presentation of the plasma spray gun is shown in Fig. 1. The gun consisted of a hot Hf
0257-8972/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 52 0 - 4
L. Pranevicius et al. / Surface and Coatings Technology 123 (2000) 122–128
Fig. 1. Scheme of the spraying process.
cathode and a water-cooled copper anode. The electrically neutral sections were located between electrodes to increase the voltage drop across the arc and to increase the resistivity of the short electrical connection. The power supply of the arc-gun included a current stabilizer with voltage and current regulation. The arc was initiated between the cathode and anode at atmospheric pressure in air. The geometry of the copper anode was optimized to promote expansion of the gas carrier passing through plasma and to push the plasma content from the plasma-gun forming a ‘flame’ consisting of high velocity, high temperature (energy) electrons, ions and neutrals. It is the so-called equilibrium plasma which exists in arcs, or in plasma torches at higher (atmospheric) pressures, due to the great frequency of collisions between different particles. Reactive plasma spraying is a metal powder spraying process in a reactive plasma. Reactive species, generated by the dissociation of gas precursors, react with heated metal particles and the resulting material splats to form the chemical compounds at the particle or splat surfaces [4]. For example, aluminum powder injected into an atmospheric plasma may react with atomic oxygen. Reactions may occur: (1) in flight with the solid and then with liquid aluminum (the flight time is about 1 ms); (2) at the splat surface as a liquid (t<20 ms) and then as a solid (t<50 ms); and (3) at the beads and then pass surface (t~0.1–30 s) [5]. The plasma torches formed by plasma-guns are nonhomogeneous in temperature and concentration of activated and reactive species in dependence on the coordinates along the torch axis and the radius in the plane of cross-section perpendicular to the axis. The situation becomes more complicated if external gas flows are used to cool the torch sides and substrate. It is well known [6 ] that powder particles will be accelerated immediately by the plasma fluid as soon as they are fed into the arc space by the feeding gas. The main forces of the fluids acting on the powder particles are the fluid viscosity force and the thermophoresis force
123
[7]. As the metal powder is fed into the plasma column, it will consequently be preheated, melted and (or) evaporated. The problem of how to control these processes is not completely solved. Our experimental studies concerning the thermal behavior of Al powders with 50 mm diameter have shown that they are melted in the 100 A central plasma and evaporated in the 200 A central plasma. The arc-plasma spray gun was constructed so as to allow injection of the powders into the torch of plasma internally and externally. The powders used were dried and passed through size filters. The maximum size of Al powder did not exceed 75 mm. The powder of Al(OH ) and metal oxides CuO and Cr O did not 3 2 3 exceed 50 mm. Special mixers provided a continuously vibrating, controlled dosage of powder. The gas carrier (air) and powder were injected into the plasma through a 9.5 mm diameter cylindrical tube with a flow rate of gas carrier of 0.4 g s−1. Al(OH ) powder was mixed 3 with CuO and Cr O powder, injected internally in the 2 3 direction parallel to the axis of the plasma torch passing through the boundary layer between reactor surface and plasma. To decrease the temperature of the boundary layer the additional flux of air was introduced at a flow rate equal to 1.2 g s−1. The Al powder was injected externally into the volume of the plasma channel. The total flow rate of the gas through plasma gun was 2.5–3 g s−1. An additional air flow was directed to the substrate area contacting the plasma torch. The flow rate of the additional air flux was equal to 1–3 g s−1. The mean temperature of the working gas along the axis of the burning arc was 5000–6000 K, while the flow velocity was equal to 400–500 m s−1. The temperature of the working gas leaving the plasma gun was 3000– 3500 K and the mean velocity about 500–650 m s−1. The energy flux density was in the range 106–108 W m−2, and the flux of atomic and particles about 1025 m−2 s−1. A power supply of 30–40 kW was used and the working arc current was 150–200 A [8]. The experimental studies of the parameters of plasma torch have shown that plasma torch parameters sharply depend on the radial distance from the axis and is a function of the maximum temperature of the working gas leaving the gun. If the temperature of the working gas leaving the gun is relatively small (1500–2500 K ) the plasma torch has well defined cylindrical form with length equal to 5–8 diameters of the exit hole of plasma gun. In the range of high temperatures of the working gas leaving the gun (>3000 K ) the radial distribution of temperature and velocity is a step-like function with a sharp decrease in temperature and velocity at a radial distance equal to the radius of the aperture of plasma gun and as the length of plasma torch decreases. It is the result of the turbulence phenomena and gas mixing by the convection process [9]. It is shown that with the increase in gas velocity, the friction forces increase and
124
L. Pranevicius et al. / Surface and Coatings Technology 123 (2000) 122–128
exceed the inertia forces so that the Reynolds number decreases. The distance between the plasma gun and substrate was equal to 100 mm. The steel sheets were rotated during deposition with linear velocity equal to 5 m s–1. The plasma gun moved horizontally in parallel to the treated substrate with a velocity equal to 5×10−3 m s−1.
3. Simultaneous pre-cleaning of moving steel sheets The properties of the coating produced by the particles arriving at the substrate from the central part of the torch depend on the chemical state of steel surface. The effect of a moving torch along the treated surface results in a time and space dependent boundary, in which the fluxes of plasma components differ in composition and velocity according to the radial distance from the axis of the torch. It means that a pre-treatment of the substrate surface takes place before the process of deposition starts. In particular, the bonding between the coating and substrate is affected by any residual compounds which remain from the industrial preparation processes and subsequent surface cleaning. In the present work, low carbon steel sheets with a thickness 0.5 mm were used after cold-rolling with a rolling oil which remains on the steel sheets after the rolling step. Typically the residual oil contaminant is about 300–400 mg m−2. During the annealing in a non-oxidizing atmosphere, which is needed to restore the metallurgical structure which is disturbed during rolling, the various lubricant compounds tend to be eliminated by degradation processes. Nevertheless, residual carbonaceous compounds remain on the surface. In order to improve the adhesion of melted particles arriving from plasma torch it is necessary to remove these residual pollutants as much as possible. Presently, cleaning of the surface of steel sheets before coating deposition is mainly realized in acid or alkaline chemical baths which lead to environmental problems (i.e. the need for treatment of the effluents produced). The alternative to chemical surface cleaning is plasma cleaning. The application of plasmas in the cleaning treatment of metallic surfaces is widely used [10]. The aim of the present study is to combine the plasma cleaning treatment and subsequent deposition processes in one plasma spray torch as it continuously moves along the substrate surface. It is based on the fact that the power density which is adsorbed by a solid surface is non-homogeneous and a function of the radial distance from the axis of the plasma torch. In the central part of the plasma-spray torch the power densities exceed 109 W cm−2 and the heat transfer into the substrate consists of the effect of the hot fluid as an impact heating flow. In the periphery of the plasma torch the
power density decreases, and thus the interior layer of the substrate is weakly affected by plasma-matter interaction, and the main plasma effect occurs at the outermost surface of the substrate. The predominant process at the periphery of the plasma torch consists of bond breaking between the adsorbate and the surface, which provides a chance for the impurities to leave the surface. Indeed, most of the organic compounds show an intense adsorption in the UV range. The surface contaminants may be excited by UV photons to a repulsive antibonding state. A possible relaxation mechanism of this excitation is the departure of the adsorbate from the surface. By employing a plasma torch at the atmospheric pressure in air, one may facilitate rapid breaking of the bonds at the periphery of plasma torch. The deposition of melted material from the central part of the torch then takes place on the plasma cleaned surfaces as the plasma torch is shifted along the surface. The adhesion properties of a coating depend on the interface properties, and it is important therefore that the first contact layer would be deposited on a plasmacleaned and activated substrate surface. In the present work, the construction of the plasma gun and the system of powder injectors forms a plasma torch in such a way that the periphery of the plasma torch performs the functions of surface pre-treatment. As the steel sheets are in continuous motion relative to the plasma torch, the process of surface cleaning takes place in front of the plasma torch where atomic oxygen actively reacts with the plasma-activated and fragmented radicals of organic macromolecules. These components are generally volatile and removed by the external gas flow directed on to the substrate. This means that a clean surface is obtained which is free of oil and fat contamination before deposition starts. The central part of torch is enriched by highly overheated metal aluminum powder which intensively evaporates and reacts with atomic oxygen. The extent of oxidation of the metallic aluminum depends on the plasma parameters and on the time of flight through the plasma torch. The two step process, surface cleaning and deposition, is realized by producing the plasma torch in such a way that the process of surface cleaning prevails on the periphery, and the deposition of oxidized aluminum prevails in the central part of the torch. In all cases the adherence of the coatings so formed was excellent without any chemical preliminary treatment of steel sheets. For catalytic applications it is important to have substrates with a highly developed effective surface area. One possible way of increasing the effective surface area is to form microclusters of material within the growing film which are decomposed and removed by the subsequent thermal treatment. The material incorporated into the matrix of a growing film, in the form of microscopic inclusions, after removal leaves a highly expanded sur-
L. Pranevicius et al. / Surface and Coatings Technology 123 (2000) 122–128
face with numerous pores. In the present research the Al(OH ) powder fulfilled the role of such a material if 3 it was introduced properly into the plasma torch. The construction of the plasma-gun and the powder dosage using the air as gas carrier was done in such a way that Al(OH ) powder of the correct size passed through the 3 plasma without dissociation and was incorporated in the matrix of the growing film. In summary, to obtain highly adherent Al-Al O 2 3 thick coatings (30–50 mm) on the surface of steel sheets, the plasma gun was constructed to make a plasma torch which performed the processes of both surface cleaning and deposition, and also produced a highly expanded surface as Al(OH ) powder was incorporated in the 3 growing film, with dissociation of the powder following during thermal treatment.
125
(a)
4. Coatings properties Two types of coatings have been investigated: firstly, coatings deposited by plasma spray of Al and Al(OH ) powders, and secondly, coatings deposited by 3 plasma spray of Al and Al(OH ) powders with additions 3 of metal oxides powder of CuO and Cr O . 2 3 The Al-Al O (c and a phase) coatings were formed 2 3 in the following way: the first layer of 10–12 mm thickness was sprayed using only Al powder, and the following three layers of 12–15 mm were each formed using Al and Al(OH ) powder in equal portions. After 3 deposition the coated substrates were heated at a temperature of 560°C for 90 min. The doping with oxides for the second type of coatings was done by mixing metal oxide powder with Al(OH ) powder in the same manner as in the case of 3 spray deposition of Al and Al(OH ) powders. The 3 coatings doped with metal oxides were thermally heated at 560°C for 90 min. The adhesion of coatings to the substrate was measured using a bending technique. All coated samples without any preliminary chemical pre-treatment passed 90° bending with a radius of curvature of 2 mm without cracking and peeling — as checked in optical and scanning electron microscopes. The steel sheets after deposition passed mechanical treatments of cutting and folding. The surface structure view of the Al-Al O coating 2 3 in a scanning microscope is presented in Fig. 2(a) and (b) which shows the same area at different magnifications. The surface of the coatings is very rough and porous. A typical view of the cross-section of the same coatings is presented in Fig. 3. It can be seen that at the interface, the layer of metallic aluminum is homogeneously adhered to the substrate. Spherical drops of oxidized aluminum can also be seen. The top layer of the coating consists mainly of oxidized aluminum. X-ray
(b)
Fig. 2. The surface structure view of Al-Al O coatings in scanning 2 3 electron microscope.
analysis results of coatings are presented in Fig. 4 for undoped (Fig. 4(a)) and for doped with 10% CuO and 5% Cr O powder (in mass units) (Fig. 4(b)). Undoped 2 3 Al-Al O coatings after thermal annealing at 560°C for 2 3 90 min include Al O mainly c-phase (about 70%). 2 3 The deposition of sprayed Al and Al(OH ) powder 3 takes place on steel sheets at substrate temperatures of 290–320°C. The substrate temperature has a substantial influence on the ratio of phase fractions of c-Al O and 2 3 a-Al O . With an increase in substrate temperature 2 3 above 290°C a phase transition takes place from c-Al O into a-Al O . At a substrate temperature of 2 3 2 3 770°C practically pure a-Al O is obtained. The addi2 3 tional air cooling prevented the substrate heating above 290°C. The small fraction of a-Al O indicates that only 2 3 small part of oxidized Al was heated above 600–650°C in the plasma, where the intensive formation of a-Al O starts. 2 3 To have highly developed effective surface it is important that the Al(OH ) should not be heated in the 3
126
L. Pranevicius et al. / Surface and Coatings Technology 123 (2000) 122–128
(a) (a)
(b) Fig. 4. The X-ray analysis results of (a) Al-Al O coatings, and (b) 2 3 Al–Al O coatings doped with metal oxides of CuO and Cr O . 2 3 2 3 (b)
Fig. 3. The view of the cross-section of steel sheet covered by an Al-Al O coating. 2 3
plasma to above the temperature of intensive dissociation of Al(OH ) . The plasma-gun construction allowed 3 the Al(OH ) powder to be pushed through the cold 3 boundary layer along the periphery of the plasma torch. This permitted the Al(OH ) powder to be incorporated 3 in the matrix of the coating. During thermal annealing, Al(OH ) incorporated in 3 the matrix of coating dissociates with the formation of c-Al O , and volatile products are desorbed leaving a 2 3 highly developed effective surface. The effective surface area, measured by the thermal desorption technique, was equal to 100–120 m2 g−1 in the samples tested for catalytic applications.
5. Catalytic combustion of propane For experimental studies, a vertically mounted tube furnace (Fig. 5) was used. The pre-mixed propane/air gas stream was introduced through a stainless steel tube at the bottom of the furnace and passed through a
Fig. 5. Scheme of the catalytic combustion test furnace.
12 cm long active element made from two-side coated Al O steel sheets rolled to a cylinder with diameter 2 3 equal to 15 cm. To control the flow of the working gas through the active element, the coated sheets of steel were folded to produce square channels for the flow of propane/air having a characteristic size equal to 7 mm. The temperature of the exiting gas was monitored by a chromel-alumel thermocouple.
L. Pranevicius et al. / Surface and Coatings Technology 123 (2000) 122–128
127
atures and the overall fuel and air conditions and heat flow balance within the test system. Fig. 6 illustrates the catalytic behavior of plasma sprayed Al O undoped (Fig. 6(a)) and doped with 10% 2 3 CuO and 5% Cr O ( Fig. 6(b)). The curves labeled 1 in 2 3 both figures illustrate the time dependence of the temperature installed in the furnace without a flow of propane, and the curves labeled 2 in the case when the equivalence ratio of the propane to air is equal to 1.8. It is seen that the ignition of catalytic combustion of plasma sprayed undoped Al O starts at about 710°C 2 3 followed by homogeneous gas-phase combustion at about 810°C (Fig. 6(a)). The catalytic combustion of Al O coatings doped with metal oxides starts at about 2 3 350°C ( Fig. 6(b)). (a)
6. Conclusions
(b) Fig. 6. Catalytic combustion behavior of (a) Al–Al O coatings, and 2 3 (b) Al–Al O coatings doped with metal oxides of CuO and Cr O . 2 3 2 3
In the experimental procedure, the flow of a premixed propane/air mixture through the furnace was equilibrated for 3 min, and then the heating element of the furnace was switched on. A comparison of the temperature increase behavior when the gas stream contains a propane/air mixture, as opposed to when it contains only air (with all other factors being kept constant) allows one to identify any catalytic combustion effects which may be occurring. A similar experimental procedure was used in Refs. [11,12]. The catalytic combustion of propane in air employing steel sheets covered by an Al O coating doped with 2 3 10% CuO and 5% Cr O were investigated to identify 2 3 catalytic combustion effects. The conducted research concentrated on catalytic combustion of propane which starts at temperatures above the catalytic ignition temperature (T ) with the release of heat above the catalytic ig surface. At T&T , the heat generated at the catalytic ig surface aids in initiating homogeneous combustion above the surface. The homogeneous combustion may then be sustained, or not, depending primarily upon the ability of the catalyst to remain active at high temper-
The Al and Al(OH ) powder mixture doped with the 3 metal oxides of CuO and Cr O was used to form 2 3 catalytic coatings on the surface of steel sheets employing reactive plasma spraying technology at atmospheric pressure in air. Well-adhering coatings consisting of 70% c-Al O and having a highly developed effective 2 3 surface area (100–120 m2 g−1) were formed. It was shown that, by using external and internal injections of the gas carrier/powder and additional cooling of the plasma torch in the gap between plasma-gun and substrate with the flow of air, one can realize the technical conditions necessary for: (1) the surface pre-treatment before deposition, which guarantees good adhesion of arriving deposit material; and (2) incorporation of Al(OH ) powder in growing film without dissociation, 3 which gives a highly developed surface area after thermal treatment at 560°C for 90 min. The plasma sprayed Al-Al O coatings doped with 2 3 metal oxides showed characteristic catalytic combustion behaviors. However, there is lack of fundamental knowledge of the phenomena involved and this is needed to explain the observed experimental results. The plasma flow is complex. Actual turbulence models are not adapted and not completely understood.
References [1] R.W. Smith, Z.Z. Matasim, J. Thermal Spray Technol 1 (1992) 57. [2] M. Bouls, Pure Appl. Chem. 68 (1996) 1007. [3] L. Pranevicius, in: Coating Technology: Ion Beam Deposition, Satas and Associates, Warwick, 1993, p. 453. [4] P. Fauchais, A. Vardelle, A. Denoirjean, Surf. Coat. Technol. 97 (1997) 66.
128
L. Pranevicius et al. / Surface and Coatings Technology 123 (2000) 122–128
[5] P. Fauchais, J.F. Coudert, M. Vardelle, A. Vardelle, A. Denoirjean, J. Thermal Spray Technol. 1 (1992) 117. [6 ] W. Xibao, L. Hua, Surf. Coat. Technol. 106 (1998) 156. [7] J.F. Lancaster, in: Physics of Welding, Pergamon, 1986, p. 413. [8] V. Valincius, P. Valatkevicius, L.L. Pranevicius, D. Milcius, Lithuanian Mater. Sci. 1 (6) (1998) 12.
[9] J.D. Anderson, in: Hypersonic and High Temperature Gas Dynamics, McGraw-Hill, 1989, p. 363. [10] W. Petasch, B. Kegel, H. Schmid, K. Lendenmann, H.U. Keller, Surf. Coat. Technol. 97 (1997) 176. [11] R.L. Jones, Surf. Coat. Technol. 86/87 (1996) 127. [12] R.L. Jones, Surf. Coat. Technol. 94/95 (1997) 95–118.