Plasma spray coating process

Transcription

1 Plasma spray coating process Afsaneh Ansari Narges Amoultani Material Engineering Department Isfahan University Of technology Dr. Salimi Jazi 90/2/28

2 Table of Contents Introduction.1 Thermal spraying process 1 Detonation spraying...1 Wire arc spraying...1 Flame spraying...2 High velocity oxy-fuel coating spraying (HVOF).2 Warm spraying...2 Cold spraying.3 Plasma spraying.3 General remarks.5 Spray torches...6 Spraying environment.8 Vacuum plasma spraying 9 Application..9 References 10

3 Introduction: Thermal spraying techniques are coating processes in which melted (or heated) materials are sprayed onto a surface. The "feedstock" (coating precursor) is heated by electrical (plasma or arc) or chemical means (combustion flame). Thermal spraying can provide thick coatings (approx. thickness range is 20 micrometers to several mm, depending on the process and feedstock), over a large area at high deposition rate as compared to other coating processes such as electroplating, physical and chemical vapor deposition. Coating materials available for thermal spraying include metals, alloys, ceramics, plastics and composites. They are fed in powder or wire form, heated to a molten or semimolten state and accelerated towards substrates in the form of micrometer-size particles. Combustion or electrical arc discharge is usually used as the source of energy for thermal spraying. Resulting coatings are made by the accumulation of numerous sprayed particles. The surface may not heat up significantly, allowing the coating of flammable substances. Coating quality is usually assessed by measuring its porosity, oxide content, macro and micro-hardness, roughness. Generally, the coating quality increases with increasing particle bond strength and surface velocities. but stilll widely used processes such as flame spraying and wire arc spraying, the particle velocities are generally low (< 150 m/s), and raw materials must be molten to be deposited. Plasma spraying, developed in the 1970s, uses a high-temperature plasma jet generated by arc discharge with typical temperatures >15000 K, which makes it possible to spray refractory materials such as oxides, molybdenum, etc. [1 1] Detonation spraying: The Detonation gun basically consists of a long water cooled barrel with inlet valves for gases and powder. Oxygen and fuel (acetylene most common) is fed into the barrel along with a charge of powder. A spark is used to ignite the gas mixture and the resulting detonation heats and accelerates the powder to supersonic velocity down the barrel. A pulse of nitrogenn is used to purge the barrel after each detonation. This process is repeated many times a second. The highh kinetic energy of the hot powder particles on impact with the substrate result in a build up of a very dense and strong coating. Several variations distinguished: of thermal spraying are Detonation spraying Wire arc spraying Flame spraying High velocity oxy-fuel coating spraying (HVOF) Warm spraying Cold spraying Plasma spraying Wire arc spray: Wire arc spray is a form of thermal spraying where two consumable metal wires are fed independently into the spray gun. These wires are then charged and an arc is generated between them. The heat from this arc melts the incoming wire, which is then entrained in air jet from the

4 gun. This entrained molten feedstock is then deposited onto a substrate. This process is commonly used for metallic, heavy coatings. [1] Flame Spraying : Flame spraying is the oldest of the thermal spraying processes. A wide variety of materials can be sprayed by this process, and the vast majority of components are sprayed manually. Flame spraying uses the heat of combustion of a fuel gas (usually acetylene or propane) and oxygen mixture to melt the coating material, which can be fed into the spraying gun in two forms, either powder or solid wire/rod. The two consumable types give rise to the two process variants: powder flame spraying and wire flame spraying. In the case of the powder flame spraying process, powder is fed directly into the flame by a stream of compressed air or inert gas (argon or nitrogen). Alternatively, in some basic systems, powder is drawn into the flame with air by a venturi effect, which is sustained by the fuel gas flow. It is important that the powder is heated sufficiently as it passes through the flame. The carrier gas feeds powder into the centre of an annular combustion flame where it is heated and propelled towards the substrate. A second outer annular gas nozzle feeds a stream of compressed air around the combustion flame, which accelerates the spray particles towards the substrate and focuses the flame. In the wire flame spraying process, the wire feed rate and flame settings must be balanced to produce continuous melting of the wire and a fine particulate spray. The annular compressed air flow atomises and accelerates the particles towards the substrate. Oxyfuel (HVOF) Spraying: The most recent addition to the thermal spraying family, high velocity oxyfuel spraying has become established as an alternative to the proprietary, detonation (D-GUN) flame spraying and the lower velocity, air plasma spraying processes for depositing wear resistant tungsten carbide-cobalt coatings. HVOF spraying differs from conventional flame spraying in that the combustion process is internal, and the gas flow fates and delivery pressures are much higher than those in the atmospheric burning flame spraying processes. The combination of high fuel gas and oxygen flow rates and high pressure in the combustion chamber leads to the generation of a supersonic flame with characteristic shock diamonds. Flame speeds of 2000ms-1 and particle velocities of ms-1 are claimed by HVOF equipment suppliers. A range of gaseous fuels is currently used, including propylene, propane, hydrogen and acetylene. Warm spraying: Is a novel modification of high velocity oxy-fuel spraying, in which the temperature of combustion gas is lowered by mixing nitrogen with the combustion gas, thus bringing the process closer to the cold spraying. The resulting gas contains much water vapor, unreacted hydrocarbons and oxygen, and thus is dirtier than the cold spraying. However, the coating efficiency is higher. On the other hand, lower temperatures of warm spraying reduce melting and chemical reactions of the feed powder, as compared to HVOF. These advantages are especially important for such coating materials as Ti, plastics, and metallic glasses, which rapidly oxidize or deteriorate at high temperatures. [1]

5 Cold spraying: In cold spraying, particles are accelerated to very high speeds by the carrier gas forced through a converging diverging de Laval type nozzle. Upon impact, solid particles with sufficient kinetic energy deform plastically and bond metallurgically to the substrate to form a coating. The critical velocity needed to form bonding depends on the materials properties, powder size and temperature. Soft metals such as Cu and Al are best suited for cold spraying, but coating of other materials (W, Ta, Ti, MCrAlY, WC Co, etc.) by cold spraying has been reported. [1] The deposition efficiency is typically low for alloy powders, and the window of process parameters and suitable powder sizes is narrow. To accelerate powders to higher velocity, finer powders (<20 micrometers) are used. It is possible to accelerate powder particles to much higher velocity using a processing gas having high speed of sound (helium instead of nitrogen). However, helium is costly and its flow rate, and thus consumption, is higher. To improve acceleration capability, nitrogen gas is heated up to about 900 C. As a result, deposition efficiency and tensile strength of deposits increase. [1] Plasma spraying: Plasma spraying is part of thermal spraying, a group of processes in which finely divided metallic and non-metallic materials are deposited in a molten or semi-molten state on a prepared substrate [2,3]. The thermal plasma heat source (direct current (dc) arc or radio frequency (RF) discharge) with temperatures over 8000K at atmospheric pressure allows the melting of any material. However, to avoid too low a deposition efficiency, the melting temperature Tm must be at least 300K lower than the vaporization or decomposition temperature. Powered materials are injected within the plasma (RF discharges) or the plasma jet (dc arcs) where particles are accelerated and melted, or partially melted, before they flatten and solidify onto the substrate (forming lamellae or splats), the coating being built by the layering of splats (see figure 1(a)). Numerous industries, in recognition of the versatility and cost-efficiency of thermal and plasma sprays have introduced these technologies in the manufacturing environment, with thermal plasmas being generally used to spray high-added value coatings. The base material/coating combination can be tailored to provide resistance to heat, wear, erosion and/or corrosion as well as unique sets of surface characteristics. Coatings are also used to restore worn or poorly machined parts to the original dimensions and specifications, or for their capability in near-net-shape manufacturing of high performance ceramics, composites, refractory metals and functionally graded materials. The first industrial plasma spray torches (dc arcs) appeared in the 1960s, but only about a decade later soft vacuum plasma spraying was introduced in industry. In the 1980s, process robotization was started, together with the development of RF plasma torches for spraying. The 1990s were mainly devoted to the implementation of robust sensors able to work in the harsh environment of spray booths and new torches. Sensors give information on the particle temperature Tp, velocity vp, radial heat flux distributions prior to their impact, and on the substrate and coating temperature evolution before (preheating) during and after (cooling) spraying (see figure 1(a)). This decade has also seen the development of new dc torches (axial injection, Triplex rotating torches to spray in cylinder bores) and RF torches (supersonic). At last, it seems that, at the beginning of the new

6 century, most efforts are directed towards the development of closed-loop systems for an on-line control of the process (with the help of robust sensors) to improve coating reliability and reproducibility, as well as towards the implementation of new processes to achieve finely structured coatings, if not nano-structured ones (see figure 1(a)). It must be emphasized that most of the important strides during the last five decades have been achieved due to the intensive research in the field through both measurements and modelling. They have allowed a much better understanding of interactions between particles and plasma and the development of simplified sensors in booths. However, studies about the formation of splats and their layering, i.e. coating formation, are still in their infancy and much is still to be done. The complexity of the process is summarized in figure 1 for dc plasma spraying, which represents more than 99% of the industrial plasma spray equipment. Figure 1(a) represents the way particles conveyed by a carrier gas and injected within the plasma (for 30μm particles about particles s 1 are injected) with the resulting splat ormation and layering. According to particle velocities, their residence times in the plasma jet core and its plume are between 0.1 and a few milliseconds. In figure 1(a) we also present the recent developments of finely or nano-structured coatings with the spraying of either agglomerated nanoparticles heated to be in a mushy state before impact or suspensions of nano- or micro-metric particles. In the former case, the nano-particles, mixed at impact with molten material, are embedded in a cement made of the solidified molten material, while in the latter case the particles (between 0.05 and a few μm) are fully wide range of times involved in conventional coating formation [3] varying from microseconds to hours (the size of the parts to be coated being between a few centimetres and tens of metres). Of course, the materials deposited represent a hierarchy of microstructures across various length scales: nanoelements, micro-sized grains contained within meso-scale splat structures and a variety of nano-, microand meso-scale defect structures comprising voids, micro-cracks and oriented boundaries. Conventional coating properties depend on three subsystems on which the operator can exercise some control. They are listed below and, after some general introductory remarks, the presentation will follow. The sub-systems are (1) plasma formation and its interaction with its environment, (2) powder and its injection with the resulting particle parameters (temperature, diameter, velocity, number flux) at impact, (3) splat formation, splat layering and coating formation. The fourth and last part of the presentation will be devoted to the recent researches related to plasma sprayed finely structured coatings. a)

7 b) Figure 1. Principle of dc plasma sprayed coating formation. (a) Typical particle and splat sizes in conventional and finely structured coatings. (b) Time ranges in coating formation. 1. General remarks 1.1. In-flight conditions First of all, the choice of the plasma source depends on the particle size to be sprayed. For example, RF torches are mostly used to spray metallic particles larger than 130μm to Manufacture the matrix of a carbon carbon-fibre reinforced part. Besides, as spraying is achieved in a controlled atmosphere chamber, oxidation is drastically limited. The ideal situation in plasma spraying would be when all particles that are injected reach the substrates with a temperature over their melting point and are uniformly heated (but not over heated: no vaporization, for example) with velocities as high as possible but compatible with a fully melted state. High velocities decrease particle residence times and thus their heating. This dream is of course impossible for the following reasons [2 4]. (a) Particles, resulting from milling and atomization processes have a relatively wide size distribution: for example, a very favourable and conventional case is a size between 22 and 45μm in diameter. However, for porous coatings, a situation where only a part of the particles is fully melted, and thus forms a cement of larger particles only partially melted or in a mushy state, is desirable. This is, for example, the case of the ceramic part of thermal barriers where a size distribution between 11 and 125μmis used for plasma sprayed stabilized or partially stabilized zirconia. In any case, the choice of the size distribution is critical but it has to be done among the products available at powder manufacturers, in order to keep powder prices compatible with what customers are ready to pay. b) Particles, at least in dc arc spraying, should be injected with a momentum similar to that of the plasma jet. This is achieved by using a carrier gas and an injector (a simple tube whose internal diameter (id) is between 1.2 and 2 mm). For a given injector, at its exit, particles exhibit velocity vectors that are not all parallel to the injector axis. These depend on their collisions with the injector wall and between themselves and are also linked to their size distribution and carrier gas flow rate. This divergence increases when the mean particle size decreases, especially when it is below 20μm in diameter. The injector, even when water cooled, cannot be positioned too close to the plasma jet, to avoid clogging. Thus, with dc plasma spraying, part of the injected particles will not penetrate into the plasma jet, due to the watering can effect, and thus will not be heated. Besides, depending on their trajectories and masses, all those penetrating the plasma will not necessarily be melted. (c) The particle melting, for a given size range, depends on two parameters: their residence times linked to their velocity within the plasma, whose length is limited (about 3 20 cm depending on the surrounding pressure and means of production), and the plasma gas composition. The latter mainly controls the heat transfer to particles, which is the lowest for pure Ar and the highest for ternary mixtures such as Ar He H2, with

8 intermediate values for Ar He and Ar H2 mixtures. Too high a heat transfer with a low conductivity particle (such as zirconia, for example) will induce a heat propagation phenomenon within the particle which is not uniformly heated and melted. The two parameters controlling particle melting are in fact closely linked to the choice of the spray torch. (d) Actually, the main differences between both spray torches: RF discharge and dc plasma jet is due to the torch id. RF torches can be used to spray large metallic particles (up to μm) or small ceramic ones (d < 40 μm) both with pure argon while dc torches will melt only metallic particles smaller than 40μm with pure argon and will require a secondary gas (H2, He) to melt ceramic particles of the same size. (e) Another important point, which has to be underlined, is the plasma surrounding atmosphere which is entrained by the plasma flow and can react with the molten particles. This is the case when spraying in air, resulting in metal oxidation and carbide decomposition. Compared to dc arcs, RF discharges used for spraying only work in a controlled atmosphere with a pressure of a few tens of kilopascals. (f) If it is easier to melt small particles, the choice of the size remains limited because of the momentum that has to be given to particles for their penetration within the plasma jet. When the particle size decreases, the carrier gas velocity has to be increased drastically At impact conditions Coating adhesion depends strongly on the substrate preparation, preheating temperature, morphology and composition of the oxide scale formed at the surface of a metallic substrate. The preheating of the substrate, generally achieved with the plasma spray jet, is thus a key issue, especially for the oxide layer formation, and it has to be controlled according to the size and thickness of the part to be sprayed. Substrate and coating temperature during spraying is also linked to the coating residual stress distribution and it is very important to control this parameter. It depends on the spray torch and the plasma forming gases, the cooling systems used and the relative displacement of the torch/substrate. This relative displacement (spray pattern and relative velocity of the torch/substrate) not only controls the residence time of the plasma jet at a given location (heating and thickness of layered splats) but also the impact angle of the particles, which has to be as close as possible to 90 (impact orthogonal to the substrate). 2.Spray torches 2.1. RF torches. Compared to dc torches the main difference is in the torch id, resulting in flow velocities below 100ms 1 and the axial injection of particles. As can be seen in figure 2(a), the injector is positioned almost at the middle of the coil. As the coupling between the coil and the plasma occurs in a ring close to the wall, the gas close to the torch axis is only heated by convection conduction, and the water cooled injector can be positioned axially with no coupling to the coil. In the spray torches supplied by TECKNA the conventional quartz tube has been replaced by a ceramic tube (as shown in figure 2(a)) with a higher thermal conductivity. The coil is inserted in the torch body and it allows a perfect alignment and a smaller separation between the coil and the discharge and, thus, a better coupling [5]. The combination of these elements with a careful aerodynamic design of gas injectors and laminated high-velocity water cooling allows reliable functioning with a high power density. Spray torches generally work at 3.6 MHz, with power levels up to 100kW. As the gas velocity is roughly inversely proportional to the

9 b) square of the torch id it means that plasma gas velocity is below 100ms 1, corresponding to particle velocities below 60ms 1 and of course high residence times (in the tens of milliseconds range). It allows the melting of metallic particles up to 200μm with argon in spite of its low thermal conductivity. Argon as the plasma forming gas allows us to achieve an easy coupling at reasonable power levels [5,6] but the sheath gas can be pure oxygen if necessary, allowing us, for example, to spray materials that are very sensitive to oxygen losses such as perovskites [9]. A general trend, derived from results obtained with coatings sprayed by a high velocity oxifuel flame [3], is to improve coating properties, through better interlamella a) Figure 2. RF plasma torches from TECKNA. (a) Conventional PL 50. (b) Supersonic: TECKNA PL35: characteristics sizes of Mach 3 torch: dc = 4.57 mm, do = 7 mm, L = 21.5 contacts, achieved with impact velocities over ms 1. Unfortunately, with dc torches, especially with ceramic particles, it implies increasing the heat transfer to particles (with He and/or H2) drastically in order to compensate for their shorter residence time in the plasma jet. However, it often results in non-uniform melting and, in spite of impact velocities close to ms 1, in poorer interlamellar contacts. That is why, recently [8], a supersonic nozzle has been adapted to the RF torches (see figure 2(b)). Particles are first heated and melted in the coil region where their velocity is low and then, once they are uniformly melted (argon heating), they are accelerated in the coil divergent. Gas velocities between 1500 and 2500ms 1 are achieved [8] in the gas expansion area with resulting particle velocities up to 600ms DC arc plasma torches. Compared to conventional RF plasma torches gas velocities are between 600 and 2300ms 1 (still subsonic velocities according to plasma temperatures) and the particle injection is radial. A conventional plasma torch (more than 90% of industrial torches) with a stick type cathode is shown schematically in figure 3. The cathode is made of thoriated (2 wt%) tungsten and an anode-nozzle of high purity oxygen free copper, often with an insert of sintered tungsten. The arc column (3 in figure 3) develops from the conical cathode (_ in figure 3) tip pumping part of the plasma forming gas (1 in figure 3), the other part flowing along the anode wall (cold boundary layer 2 in figure 3). The arc attachment to the anode wall [2, 11] through the connecting column (4 in figure 3) continuously fluctuates in length and position. This is due to the movements induced by the drag

10 force of the gas flowing in the cold boundary layer (2 in figure 3) and the magneto-hydrodynamic forces, both resulting in upstream and downstream short circuits. The corresponding transient voltage, depending on the cold boundary layer thickness in the arc attachment area [12], exhibits a restrike (saw tooth shape), take-over (regular periodic variation) or mixed mode, and its value can reach ±35% of the time-averaged voltage. The restrike mode is the most probable with plasma forming gases containing diatomic gases, while the take-over mode is mainly obtained with mono-atomic gases. Of course, depending on the relative anodenozzle id and arc column diameter, controlling the cold boundary layer thickness, it is possible to shift from one mode to the other. In the restrike mode (with Ar H2, N2, N2 H2 and Ar He H2 plasma forming gases), depending on the torch working conditions, the voltage fluctuations vary between ±15% and ±35%. It results in plasma jet cores (area where T >8000 K) whose lengths can vary between 15 and 60 mm. With pure Ar or Ar He mixtures, where the take-over mode is the most probable, these fluctuations are limited to ±20% at the maximum. These fluctuations are partly controllable by playing with the gas composition, mass flow rate and arc current for a given nozzle id. However conditions where the lowest fluctuations are obtained are not necessarily those which are the best to spray a given material with a given distribution. As usual, with plasma spraying a compromise has to be found. The arc root fluctuations allow us to keep the anode integrity because, at the arc root heat fluxes can be as high as 1011Wm 2, the residence time of the arc root has to be limited to about 150μs. But, correspondingly, the power dissipated in the arc fluctuates with its voltage (the torch being fed with a current source) resulting in plasma jets continuously fluctuating in length and position (see the bottom part of figure 3) at frequencies ranging between 2000 and 8000 Hz depending on the cold boundary layer thickness [12]. Of course, these fluctuations are not favourable for a uniform heating of particles. The plasma jet (5 in figure 3) exits the torch anode at high velocities: from 600ms 1 with pure Ar up to 2200ms 1 with Ar H2 (still subsonic velocities). This high velocity jet creates vortex rings that coalesce and result in large scale eddies (6 in figure 3) which entrain cold surrounding gas bubbles. The latter are times denser than the plasma jet and they are mixed with it when they are heated enough while the plasma has cooled down, i.e. mostly in the plasma plume. To limit the plasma jet fluctuations, a new torch called TRIPLEX from Sultzer Metco [11], has been recently introduced. Figure 3. Top: schematic of a conventional dc arc spray torch with: _ stick type thoriated tungsten cathode, anode; nozzle 1 the plasma forming gas injection, 2 the cold boundary layer at the anode wall, 3 the arc column, 4 the connecting arc column, 5 the plasma jet exiting the nozzle, 6 the large scale eddies, 7 the surrounding atmosphere bubbles entrained by the engulfment process, 8 the plasma plume. Bottom: plasma jet pictures taken with a shutter time of 10 4 s. Spraying environment: air plasma spraying (APS), performed in the ambient air controlled atmosphere plasma spraying (CAPS), usually performed in a closed chamber, either filled with inert gas or evacuated

11 variations of CAPS: high-pressure plasma spraying (HPPS), lowpressure plasma spraying (LPPS), extreme case of which is vacuum plasma spraying (VPS, see below) underwater plasma spraying Another variation consists of having a liquid feedstock instead of a solid powder for melt, this techniques is known as Solution precursor plasma spray Vacuum plasma spraying: Vacuum plasma spraying (VPS) is a technology for etching and surface modification to create porous layers with high reproducibility and for cleaning and surface engineering of plastics, rubbers and natural fibers as well as for replacing CFCs for cleaning metal components. This surface engineering can improve properties such as frictional behavior, heat resistance, surface electrical conductivity, lubricity, cohesive strength of films, or dielectric constant, or it can make materials hydrophilic or hydrophobic. The process typically operates at C to avoid thermal damage. It can induce non-thermally activated surface reactions, causing surface changes which cannot occur with molecular chemistries at atmospheric pressure. Plasma processing is done in a controlled environment inside a sealed chamber at a medium vacuum, around Pa. The gas or mixture of gases is energized by an electrical field from DC to microwave frequencies, typically W at 50 V. The treated components are usually electrically isolated. The volatile plasma by-products are evacuated from the chamber by the vacuum pump, and if necessary can be neutralized in an exhaust scrubber. In contrast to molecular chemistry, plasmas employ: Molecular, atomic, metastable and free radical species for chemical effects. Positive ions and electrons for kinetic effects. Plasma also generates electromagnetic radiation in the form of vacuum UV photons to penetrate bulk polymers to a depth of about 10 µm. This can cause chain scissions and cross-linking. Plasmas affect materials at an atomic level. Techniques like X-ray photoelectron spectroscopy and scanning electron microscopy are used for surface analysis to identify the processes required and to judge their effects. As a simple indication of surface energy, and hence adhesion or wettability, often a water droplet contact angle test is used. The lower the contact angle, the higher the surface energy and more hydrophilic the material is. Vacuum plasma spraying Application: This technique is mostly used to produce coatings on structural materials. Such coatings provide protection against high temperatures (for example thermal barrier coatings for exhaust heat management), corrosion, erosion, wear; they can also change the appearance, electrical or tribological properties of the surface, replace worn material, etc. When sprayed on substrates of various shapes and removed, free standing parts in the form of plates, tubes, shells, etc. can be produced. It can be also used for powder processing (spheroidization, homogenization, modification of chemistry,

12 etc.). In that case, the substrate for deposition is absent and the particles solidify during flight or in a controlled environment (e.g., water). A polymer dispersion aerosol could be injected into the plasma discharge in order to create a grafting of this polymer at a substrate surface. [3] This application is mainly used to modify the surface chemistry of polymers. For example, Aluminum oxide, AL2O3, more often referred to as alumina, is an exceptionally important ceramic material which has many technological applications. It has several special properties like high hardness, chemical inertness, wear resistance and a high melting point. It is reported that the corrosion resistance of alumina coatings are higher than that of cermet and metallic coatings [15]. Ceramic coatings usually are characterized by a relatively high open porosity which is deleterious when the coatings have to perform in an aggressive environment. The porosity allows a path for electrolytes from the outer surface to the substrate [13,14,15]. There are also adhesion problems between the oxide coating and metallic substrate. A viable solution is to insert a metallic "bond coat" between the substrate and the coating [16] References: [1] ^ a b c d e f g Kuroda, Seiji; Kawakita, Jin; Watanabe, Makoto; Katanoda, Hiroshi (2008). "Warm spraying a novel coating process based on high-velocity impact of solid particles". Sci. Technol. Adv. Mater. 9: doi: / /9/3/ [2] Pawlowski L 1995 The Science and Engineering of Thermal Spray Coatings (New York: Wiley) [3] Fauchais P, Vardelle A and Dussoubs B 2001 J. Thermal Spray Technol [5] Boulos M 1997 J. High Temp. Mater. Process [6] Boulos M 1992 J. Thermal Spray Technol [7] Mailhot K, Gitzhofer F and Boulos M-I 1997 Thermal Spray: A United Forum for Scientific and Technological Advances [8] Leveill e V, Boulos M and Gravelle D 2003 Thermal Spray 2003: Advancing the Science and Applying the Technology vol 2, ed C Moreau and B Marple (Ohio, USA: ASM International Materials Park) pp [9] Gitzhofer F, Boulos M, Heberlein J, Henne R, Ishigaki T and Yoshida T 2000 MRS Bull. July [10] Morischita T 1991 Plasma Technick 2nd Symp. vol 1, ed S Blein-Sandmeier et al (Wohlen, CH: Plasma Technik) pp [11] Zierhut J, Halbeck P, Landes K D, Barbezat G, M uller M and Schutzl M 1998 Thermal Spray: Meeting the Challenges of the 21st Century vol 2, ed C Coddet (Ohio, USA: ASM International Materials Park) pp [12] Heberlein J 2002 J. High Temp. Mater. Process [13] Y.Dianran., H. Jining et al, The corrosion behavior of a plasma spraying AL2O3 ceramic coating in dilute HCL solution, Surface and Coating Technology 89(1997), pp [14] E.Celik, I.Sengil, Effect of some parameters on corrosion behavior of plasmasprayed coatings, Surface and Coating Technology 97(1997), pp [15] E.Celik, I.Ozdemir et al, Corrosion behavior of plasma sprayed coatings, Surface and Coating Technology 193(2005), pp [16]M.Rosso., A.Scrivani et al., Corrosion resistance and properties of pump pistons coated with hard materials", Refractory Metals and Hard Materials19 (2001), pp [4] Vardelle A, Moreau C and Fauchais P 2000 MRS Bull. July 32 7

13