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Basics of Thermal Spray Technology

I Processes

Andreas Wank GTV Verschleiss-Schutz GmbH, Luckenbach, Germany

1. Introduction

The importance of coating technologies and among them thermal spraying is constantly rising, because coatings permit to exploit cost saving potentials by combination of base materials that fulfill the structural demands (stiffness and strength) with coating materials that fulfill the demands on the components surfaces (corrosion resistance, wear resistance, electrical isolation or conductivity, thermal isolation, catalytic function, decorative effect, etc.). E.g. for components that are subject to severe stress conditions such material compounds can be provided at significantly lower cost than bulk material solutions that cover both structural and surface demands. Additionally repair of damaged or worn surfaces of costly components is an important application field for thermal spray processes.

The standard DIN EN 657 comprises the different processes that belong to the group of thermal spray processes. Therein, thermal spraying is defined as a process, in which the feedstock is fed inside or outside a spraying gun and heated up to molten state or at least to a state of high plastic deformability. The feedstock is propelled unto a prepared surface that is not molten during the spraying process. The most important distinction refers to the applied energy source:

• thermal spraying by atomization of a melt, i.e. melt bath spraying (MBS) • thermal spraying by gaseous or liquid fuels, i.e. wire flame spraying (WFS),

high velocity wire flame spraying (HVWFS), powder flame spraying (PFS), high velocity oxy fuel spraying (HVOF) and detonation gun spraying (DGS)

• thermal spraying by expansion of compressed gases without combustion, i.e. cold gas spraying (CGS)

• thermal spraying by electrical arc or gas discharge, i.e. arc spraying (AS), shrouded arc spraying (SAS), atmospheric plasma spraying (APS), shrouded plasma spraying (SPS), vacuum plasma spraying (VPS), high pressure plasma spraying (HPPS), liquid stabilized plasma spraying (LSPS), inductively coupled plasma spraying (ICPS)

• thermal spraying by high energy density beams, i.e. laser spraying (LS)

Typically, thermal spray coatings are produced with a thickness between 30 µm and 2 mm. However, in some special applications coating thickness can be up to 10 mm.

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Thermal Spray processes feature inherent advantages: • All materials with liquid phase or sufficient ductility below their decomposition

temperature can be sprayed. • Thermal stress of the substrate can be kept low (coating of polymers or wood is

possible). • Coatings can be deposited both on large or locally very limited areas. • Adapted thermal spray equipment can be used on-site, e.g. within power plant

boilers, at bridges, marine landing stages or other steadily mounted constructions.

However, in general there is a comparatively weak bonding between substrate and coating and the coatings contain pores. Post treatment, e.g. by remelting, hot isostatic pressing (HIP) or shot peening, can improve bond strength and / or decrease coating porosity. Usually the exact coating thickness and desired surface roughness is achieved by machining.

2. Melt bath spraying (MBS)

In the MBS process the feedstock material is molten inside a crucible and then atomized through a nozzle (figure 1). The atomizing gas, usually compressed air, is most commonly applied in pre-heated state. The mass flow of the melt can be controlled by adjusting the pressure inside the crucible. The size of the atomized droplets depends on the temperature dependent viscosity and surface energy of the molten feedstock material, the atomizing gas type and flow rate and the nozzle geometry. The particles impacting on the substrate form the coating.

Figure 1: Principle of the melt bath spraying process [1]

These days the process principle is used for production of bulk material (known as spray forming or Osprey process) more often than for production of coatings. The MBS process features outstanding deposition rates, but the heat transfer to the substrate is usually very strong. Thereby metallurgical bonding between coating and substrate can be achieved, but temperature sensitive substrates are easily overheated. Details concerning commonly applied materials and applications are presented in [2].

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3. Flame spraying

3.1 [High velocity] wire flame spraying ([HV]WFS)

For wire flame spraying the feedstock material is applied in wire, rod or flexible cord shape. The diameter of the feedstock can vary in the range between less than 1 mm and 8 mm. Most commonly the feedstock is fed axially into a flame (figure 2), but there are also guns with radial feed of one or more wires. At the wire tip the feedstock is melted and then atomized by means of an atomizing gas. Usually acetylene, propane or hydrogen are applied as combustion gases and compressed air as atomizing gas.

Figure 2: Principle of the wire flame spraying process [1]

WFS is used e.g. for deposition of aluminum or zinc coatings for cathodic corrosion protection of steel structures or for repair of worn component surfaces using similar coating materials like the substrate material. Despite the strong increase in material price in the recent decades molybdenum is still widely applied for production of wear protective coatings, e.g. on automotive piston or gear synchronizing rings, because the coatings feature excellent resistance to galling and fretting. These coatings density and hardness strongly depends on the process parameters. In HVWFS, i.e. WFS at increased process gas pressures and flow rates, there is an extremely fine atomization of the molybdenum melt from the wire tip. Due to the large specific surface area the droplets react strongly with the oxygen of the spray jet atmosphere, which results in coating hardness of up to 1,000 HV0.3. Fine droplets are accelerated very effectively and therefore impinge at high velocity on the substrate, which results in high density coatings. Molybdenum is also used as bond coat material. In that case process parameters are adapted in order to achieve large droplets, because bond strength to both steel and aluminum substrates increases with heat content of the impinging particles as a result of metallurgical interaction. WFS is also used for processing of noble materials like platinum. According coatings are used for protection of components in glass fabrication due to their outstanding durability [3].

Rods and flexible cords are mainly applied for production of ceramic coatings. Al2O3 based coatings are used for wear protection, partially in combination with corrosion protection and ZrO2 based coatings provide excellent thermal insulation properties at high thermal shock resistance. Production of rods by sintering is relatively expensive.

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Therefore flexible cords filled with the according oxide powder permit significant cost savings. Also, cored wires expand the spectrum of applicable coating materials. However, the structural and metallurgical design of these wires need to fit the process characteristics [4-5].

3.2 Powder flame spraying (PFS)

The energy source for melting and acceleration of powder feedstock in PFS is a gaseous fuel - oxygen flame. Inside the combustion gas jet the powder particles are heated up and accelerated (figure 3). Depending on the required heat transfer to the applied feedstock different gun designs with and without injection of additional gases that provide stronger acceleration of powder particles and / or lead to a more focused spray jet are applied.

Figure 3: Principle of the powder flame spraying process [1]

The most important material group for PFS are so called self-fluxing alloys based on the system Ni-Cr-B-Si-C. These alloys feature relatively low melting temperature with a wide range between solidus and liquidus temperature and high melt viscosity. Therefore the relatively high porosity of PFS coatings in the as sprayed state can be removed by a subsequent fusion process for this type of coatings. Diffusion processes during coating fusion also result in metallurgical bonding and accordingly high bond strength. The fusion can be carried out in furnaces, by inductive heating or by use of the spraying guns flame without injection of powders, which is especially suitable for on-site applications. Depending on the chemical composition coating hardness between 20 HRC and 62 HRC can be achieved. Additionally reinforcement with fused tungsten carbide (approximately eutectic mixture of W2C and WC) or WC/Co is applied to respond to demands of extreme wear like in decanters and separators as well as in mining applications. Besides wear protection also corrosion protection capability of these coatings is excellent for a lot of environments. However, heat affection can cause strong distortion of the coated components, which affords subsequent straightening.

Besides metals also ceramics can be processed by PFS. However, deposition rate is significantly lower. Even the manufacturing of thermoplastic polymer coatings is possible by PFS for use of guns with adapted design that permit to prevent

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overheating of the feedstock. Thereby the axially fed granules are shrouded by a coaxial gas, which is heated by an external coaxial flame.

3.3 Detonation gun spraying (DGS)

DGS was developed in the 1950´s in the United States and is the progenitor of the HVOF processes. It is based on a discontinuous combustion process that is characterized by a sequence of charging, in which the combustion gases - usually acetylene and oxygen - and the powder feedstock with the nitrogen carrier gas are injected into the tube, the ignition of the combustible and a flushing of the tube with nitrogen (figure 4). The ignition frequency can vary from 4 - 8 Hz for early developed systems up to about 100 Hz for newly developed spraying guns, which work with fluid dynamical control of the gas injection (HFPD - High Frequency Pulse Detonation).

Figure 4: Principle of the detonation gun spraying process [1]

Detonation gun spraying is characterized by relatively high process gas tempera-tures, which can be up to 4.000 °C, and high particle velocities, i.e. up to 900 m/s. Oxidation of metallic spray particles is limited due to the short interaction time between the hot combustion gases with the powder particles. The high particle velocities result in high coating density and high bond strength.

DGS is mainly applied for production of wear protective cermet coatings based on WC or Cr3C2, but metallic or ceramic coatings can also be manufactured. In comparison to HVOF DGS shows economical advantages due to significantly lower gas consumption. But noise levels of more than 140 dB(A) even exceed the high noise level of HVOF. Especially for guns operating at very low frequency the spraying needs to be carried out in special sound and explosion proof rooms. Additionally the comparatively high weight makes the handling of DGS guns more difficult compared to HVOF guns. Therefore mainly simple geometries that do not require extensive movement of the gun are coated.

3.4 High velocity oxy fuel spraying (HVOF)

The HVOF process has been developed as an advancement of the conventional PFS process, when it was found out that increased particle velocities - like in DGS - permit improved coating density, cohesion and adhesion to the substrates. Therefore gas

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flows were increased to provide high combustion gas velocities. Thereby particle velocities that are typically in the range of 50 m/s for PFS could be increased to about 450 m/s for use of the first HVOF guns in the early 1980´s. These guns showed a convergent-cylindrical nozzle design. Therefore inside the gun the combustion gases could reach a maximum of sound velocity only. Modern HVOF guns that feature a de-Laval, i.e. convergent-divergent, contour (figure 5) permit mean particle velocities up to 850 m/s and development is not yet finished.

Figure 5: Principle of the HVOF spraying process for advanced gun concepts [1]

Besides gaseous fuels like hydrogen, methane, acetylene, ethylene, propylene or propane also liquid fuel can be applied. The choice of the combustible determines the maximum achievable flame temperature (figure 6). By adjusting the ratio between combustible and oxygen flow rate the actual flame temperature can be influenced additionally. For cooling of the flame injection of water or additional gases like nitrogen is possible. Use of compressed air instead of pure oxygen is equivalent to additional injection of nitrogen and so results in decreased flame temperature (figure 6).

The heat transfer to the powder feedstock does not only depend on the flame temperature of the applied mixture, but also on the location of injection and the injection boundary conditions like injection angle, injector internal diameter, carrier gas flow rate and powder feed rate. For axial injection into the combustion chamber there is strong heat transfer, while radial injection in the divergent part of de-Laval nozzles results in significantly lower heat transfer, because the combustion gases have cooled down significantly at this location and the overall time of interaction with the hot environment is strongly reduced. Finally the length and shape of the expansion nozzle influence the heat transfer to the powder particles. As the heat transfer inside the expansion nozzle is stronger compared to the free expanding jet long nozzles result in strong heat transfer. Divergent expansion nozzles result in faster combustion gas jets compared to cylindrical nozzles, which causes reduced dwelling time of particles inside the spray jet and therefore reduced heat transfer to them.

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Figure 6: Flame temperature depending on fuel and combustion stoichiometry [6]

The choice of the combustible must be taken under consideration of the required supply pressure that must be higher than the combustion chamber pressure. The combustion chamber pressure in modern HVOF guns can exceed 1 MPa. Liquefaction prohibits the use of propane under these conditions. Due to safety reasons (exothermic dissociation reaction) acetylene can only be provided at maximum pressures as low as 0.15 MPa. Therefore use of methane, hydrogen and ethylene as gaseous fuels for advanced HVOF guns is advised.

Like for DGS the main application field of HVOF processes is the manufacturing of wear protective cermet coatings based on WC or Cr3C2. The relatively low particle temperatures and the short dwelling time of particles inside the hot combustion gas jet permit to avoid melting of the composite powder particles and the formation of brittle mixed carbides can be limited to low degrees. In optimized HVOF sprayed WC/Co coatings formation of mixed carbide can be kept below the detection limit of X-ray diffraction (XRD). Additionally HVOF replaces VPS sprayed hot gas corrosion protective MCrAlY coatings for turbine blades more and more. Also iron, nickel or cobalt based materials are sprayed for corrosion protection, sometimes also in combination with wear protection.

Due to flame powers up to 250 kW there is a strong heat transfer to the substrates. This necessitates effective cooling of the components that have to be coated.

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HVOF spraying is accompanied by high noise levels of up to 140 dB(A) and the expansion of the combustion gases results in a strong rebound. Therefore and because of the strong heat and dust evolution HVOF spraying is mainly carried out inside sound insulating cabinets with robot handling systems. On-site application with manual handling, e.g. for deposition of corrosion protective coatings in power plant or waste incinerator boilers, is generally possible, but significant effort for occupational safety and health is required.

Coating formation by high velocity particle impact of spray particles on the substrate in solid state also results in compressive residual stress state, which is beneficial concerning life time of coated components under dynamical load. Both for low alloyed steel and also for aluminum alloy substrate materials improvement of fatigue strength and limit by deposition of HVOF coatings has been proved. The existence of a fatigue limit for HVOF coated aluminum parts in contrast to uncoated parts eases their design substantially. So, for some base materials there is high potential to exploit weight savings by light weight concepts considering HVOF coatings. However, for high strength steel substrates fatigue properties are not affected only for very careful pre-treatment by grit blasting and cleaning.

4. Cold gas spraying

Cold gas spraying is a relatively new spraying process that consequently follows the trend to increased particle velocity and decreased particle temperature in order to prevent in-flight oxidation of metallic feedstock materials and to provide compressive residual stress state in the formed coatings. Powders are heated up and accelerated by a process gas that expands through a de-Laval type nozzle with inlet pressures up to 4 MPa at a maximum inlet temperature of 800 °C (figure 7). Typically nitrogen is used as process gas, but helium or compressed air are applied alternatively, also. While the use of compressed air permits cost savings at increased risk of feedstock oxidation helium provides the highest velocities of the expanding process gas due to its high sound velocity and therefore permits higher particle velocities compared to nitrogen. Electrical resistance heating in helical tubes is common. As the powder is fed in the convergent part of the nozzle for standard gun concepts the powder feeder needs to operate at pressures exceeding the process gas inlet pressure.

Figure 7: Principle of the cold gas spraying process [1]

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In contrast to other thermal spray processes in cold gas spraying spreading of spray particles at impact on the substrate due to molten or at least strongly plastified state is not possible. Particle velocity must exceed a critical material dependent value in order to permit sticking instead of rebounding. Typically particle velocity significantly exceeds 500 m/s. For spraying of ductile materials like pure copper or aluminum with adequate size fraction, i.e. 5 µm < d < 25 µm (Cu) and 10 µm < d < 45 µm (Al) respectively, transformation of kinetic to thermal energy upon impact on the substrate permits formation of well adherent coatings with nearly theoretical density by cold welding mechanisms. However, low ductility materials like ferritic stainless steels or hardmetals cannot be sprayed economically and spraying of ceramics is not possible at all [7-8].

The main advantage of cold gas spraying is the avoidance of oxidation both of the spray material and the substrate. The electrical conductivity of cold gas sprayed copper coatings can be as high as 90% of cast material. Coating thickness can be several millimeters and even on polished glass surfaces well adhering conductive layers can be deposited. The small divergence of the particle trajectories permits the manufacturing of free standing complex structures. Presently the most important application of cold gas spraying is the deposition of thin copper coatings on extruded aluminum parts in order to provide low thermal resistance coatings that can be soldered to copper plates. Thereby heat sinks with advanced efficiency for cooling of computer processors are produced [9].

5. Arc spraying (AS)

In arc spraying processes wire shaped feedstock is melted by means of an electrical high current arc. The formed melt at the wire tip is propelled by means of an atomizing gas, most commonly compressed air, unto the surface of the substrate. Due to the process principle only electrically conductive materials that are available in wire shape can be applied. There are single wire arc spraying guns with the arc burning between a constantly fed wire and a non-consuming electrode that also acts as nozzle for the atomizing gas [10]. However, in industries only twin wire arc spraying has found broad acceptance. In according guns two wires, which act as electrodes, are continuously fed towards each other under a defined angle and consumed by melting at their tips (figure 8). The potential difference between the two wires is usually in the range between 15 - 50 V. In order to keep oxidation of the sprayed material as low as possible the lowest voltage providing steady processing is applied, because superheating of the feedstock melt increases with voltage.

Superheating and oxidation of the melt at the wire tip determine the melts viscosity and surface energy. The size of atomized droplets depends on these parameters as well as on the atomizing gas type and flow rate and the nozzle design. In-flight oxidation of spray particles mainly depends on the droplet size dependent specific surface and the oxygen content in the spray jet. Even for use of inert atomizing gases

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like nitrogen or argon entrainment of atmospheric gases into the free expanding spray jet results in considerable oxidation.

Figure 8: Principle of the (shrouded) arc spraying process [1]

For use of inert atomizing gases in combination with inert shrouds (figure 8, green), i.e. a shielding gas jet surrounding the spray jet, oxidation of droplets can be kept low. The sheath gas can also be used to minimize the spray jet divergence, which is particularly important for minimized overspray during coating of small components. High quality coatings of metals with strong affinity to oxygen like titanium alloys are only achieved, if the arc spraying process is carried out inside a closed chamber with controlled inert atmosphere.

With the aim to produce coatings with maximum bond strength atomizing gas flow is kept low in order to produce large droplets that do not oxidize as much as small droplets and provide high heat content for localized metallurgical interactions with the substrate surface forming microscopic diffusion zones. However, porosity and roughness is relatively high for spraying of coarse droplets. Fine structured high density coatings are achieved for high atomizing gas flows and accordingly small atomized droplets.

Among the thermal spray processes arc spraying features outstanding energetic efficiencies and deposition rates. Therefore this process is particularly interesting from the economical point of view, if the application dependent requirements on the coating quality can be met. Additionally robust and mobile equipment that permits on-site application is available. Arc spraying is applied for large area coating of steel and concrete structures with corrosion protective aluminum or zinc based coatings and for (on-site) repair of damaged components. Due to economical advantages compared to alternative spraying processes and high bond strength also bond coats like Ni5Al are often arc sprayed. Zinc sputter targets are produced by shrouded arc spraying with argon as atomizing gas.

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Due to the necessity to provide the coating material in electrically conductive wire shape the spectrum of coating materials is rather small, but cored wires permit a significant extension of the producible material spectrum, e.g. by addition of hard phases as filler material for production of wear resistant coatings [11].

6. Plasma spraying

6.1 Atmospheric [DC] plasma spraying (APS)

In DC plasma spraying a high current arc is used to generate a thermal plasma jet. The arc is started by high frequency high voltage ignition and maintained by DC electrical power supply. Inert gases passing the gap between the electrodes are heated up by the arc. Thereby monoatomic gases like argon or helium are partially ionized and molecular gases like hydrogen or nitrogen are dissociated prior to ionization. During recombination of ions and electrons and re-formation of molecules energy is set free and results in temperatures of the plasma jet up to 10,000 K in the core at the gun exit. Due to the extreme increase in temperature there is strong acceleration of plasma gases inside the plasma torch.

These days most DC plasma torches contain a pin cathode and an anode with axially symmetrical shaped inner contour that acts as nozzle (figure 9). Most commonly both electrodes consist of tungsten doped with thoria or rare earth oxides for improved electron emission properties. Both electrodes are actively cooled to prevent thermal destruction. Mixtures of argon, helium, hydrogen and nitrogen are applied as plasma gas. The powder feedstock is fed into the thermal plasma jet radially inside the nozzle or directly behind the nozzle exit.

Figure 9: Principle of the (shrouded) DC plasma spraying process [1]

The heat transfer to the cathode is significantly lower than that to the anode. Thermal overload of the anode is only avoided because of arc root movement. For conventional inner nozzle contour, i.e. convergent-cylindrical, there is axial as well as circumferential movement of the arc root. However, axial movement causes undesirable deviations of arc length and therefore also of voltage and power. Newly developed nozzles with convergent-divergent inner contour permit axial fixation of the arc without thermal overload because circumferential movement is maintained.

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Besides minimized plasma power fluctuations significant reduction of noise level is achieved. Plasma gas velocity is also strongly reduced which results in increased dwell time of powder particles inside the plasma jet and increased heat transfer. Therefore refractory oxide coatings can be sprayed at increased deposition efficiency [12]. However, for spraying of metals careful adjustment of process parameters and feedstock powders is necessary to avoid excessive oxidation and / or evaporation.

Due to the extremely high temperature of the thermal plasma jet any desired material with liquid phase can be melted, if it is fed into the plasma jet with adequate powder size. Therefore the most important application field of atmospheric plasma spraying is the production of refractory oxide coatings. Al2O3/TiO2 or Cr2O3 coatings are used for wear and corrosion protection. Al2O3 coatings provide good electrical and ZrO2 based coatings (additives: Y2O3, CeO2, MgO, CaO) good thermal insulation properties. But high quality metallic and cermet coatings can also be produced. So, plasma spraying is the most versatile spraying process in terms of coating material spectrum.

Like for arc spraying the use of gas shrouds (figure 9, green) has proven to be effective to prevent excessive oxidation during spraying of reactive materials. Near the torch exit a fast flowing, coaxial laminar stream of inert gases is utilized to prevent entrainment of environmental gases into the plasma jet. The sheath gas also effectively cools the substrate.

Single cathode DC plasma spraying guns with axial powder feeding have not found industrial acceptance due to poor reliability. These days the only commercially available plasma spraying gun with axial feeding of the powder feedstock is the Axial III. It contains three separate plasma jets generated by separate electrode pairs that unite in front of an axial powder injector inside the gun. This gun can be operated economically for large area coating production due to outstanding high deposition rate and efficiency as a result of the small divergence of the particle trajectories and therefore the improved homogeneity of heat transfer in comparison to conventional radial injection in single cathode systems. The major disadvantage of this torch is the high energy and gas consumption due to the operation of three separate plasma jets, each with a comparable power and gas flow like a conventional single cathode torch.

Triple cathode torches permit to overcome the drawbacks of high energy and gas consumption. In such guns the electrical power is divided by three arc columns. So, thermal load of the electrodes is reduced and therefore lifetime increased. Each arc has its fixed anode root attachment. This results in a stable process state, which permits homogeneous heat transfer to the sprayed particles despite of a radial injection in front of the three anode roots. The improved heat transfer conditions permit increased deposition rates with simultaneous increase of powder feed rate. However, coating properties (porosity, hardness, bond strength) remain comparable to use of single cathode APS guns. Also, triple cathode torches show the drawback that the circumferential position of the arc roots on the anode ring vary with process

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parameters (plasma current, plasma gas composition and flow rate). For each set the injection locations need to be adapted according to the actual arc root positions.

The most recent development in the field of APS torch design is a triple anode torch [13]. This type of gun features a single pin cathode and three separate anodic ring segments. Due to the relatively low thermal load on the cathode over-heating is securely avoided even for application of indirect contact cooling. In addition to the advantage of timely fixed arc roots like in triple cathode guns the triple anode gun features constant arc root positions corresponding to the anode ring attachments that do not change their position depending on the process parameters. Therefore, triple anode torches permit the highest possible process stability among DC plasma torches these days. Like for triple cathode torches increase of deposition rate with simultaneous increase of the powder feed rate is possible. Further research is required to decide, if the improved process stability will make improved coating properties accessible, also.

Spraying of wire feedstock with DC plasma torches has been a research topic since the 1980´s, but has not reached industrial acceptance, yet.

6.2 Vacuum [DC] plasma spraying (VPS)

The plasma spraying process has been advanced for use in controlled environments. One example is the vacuum plasma spraying (VPS) process that is also known as low pressure plasma spraying (LPPS) process. Typically chamber pressures are higher than 2 kPa during the coating operation. However, in order to achieve well inert environment the chamber is typically evacuated to a pressure of roughly 1 Pa before it is flooded with an inert gas to the desired working pressure. The process is traditionally used for the production of hot gas corrosion protective MCrAlY coatings, which feature high density and a very homogeneous, oxide free microstructure, on gas turbine blades. Low oxygen content is decisive in order to achieve optimal oxidation resistance. These coatings also act as bond coats for subsequently applied ZrO2 based thermal insulation coatings.

VPS is also important in the field of medical applications, e.g. for production of porous and rough titanium coatings on various types of implants - from hip to tooth implants. Due to the extremely high affinity of titanium to oxygen and nitrogen, it cannot be sprayed under atmospheric conditions. Also hydroxyapatite, i.e. bone material, is processed by VPS in order to achieve implant surfaces that provide optimal preconditions for long-term stable connection between implant and original bone material [14]. There are also some special applications of VPS, e.g. for deposition of tantalum corrosion protective coatings, for production of high quality sputter targets that are used for deposition of thin films from vapor phase or for production of refractory metal free standing bodies [15].

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6.3 High pressure [DC] plasma spraying (HPPS)

Besides VPS also plasma spraying in high pressure environments, i.e. high pressure plasma spraying (HPPS), also known as controlled atmosphere plasma spraying (CAPS) has been developed. Typically chamber pressure is below 0.3 MPa and various gas (mixtures) can be employed with the aim to exploit reactions of spray particles with the environmental gases, e.g. nitriding of titanium particles in order to form TiN. Also high chamber pressure results in low plasma gas velocity and high energy density of the thermal plasma jet, which eases heat transfer to spray particles and is therefore particularly well suited for spraying of high melting point materials. However, up to now HPPS has not found significant industrial acceptance.

6.4 Liquid stabilized plasma spraying (LSPS)

In liquid stabilized plasma spraying torches the plasma gas is produced from a process liquid. Most commonly water is applied, but use of e.g. methanol or ethanol is also possible. Typically the arc is burning between a graphite anode and a rotating, water cooled copper anode behind the nozzle exit (figure 10). The arc is confined and stabilized by the interaction with the vortex liquid flow inside the torch. Due to constant feeding of the liquid the liquid jacket is maintained and acts both as coolant and as a thermal and electrical isolation to the chamber. The thermal plasma is generated as a consequence of partial vaporization, dissociation and ionization of the stabilizing liquid. Plasma temperatures and enthalpies achieved in this type of torches are substantially higher than in common gas stabilized torches, but plasma density is lower [16].

Figure 10: Principle of the liquid stabilized plasma spraying process [1]

Like in most gas stabilized plasma torches the powder feedstock is fed radially into the plasma jet at the torch exit. Due to the strongly oxidizing environment inside the high enthalpy plasma jet liquid stabilized plasma spraying is particularly suitable for high rate spraying of oxide ceramic coatings. For these materials relatively long exposure to oxidizing environments at very high temperature does not result in undesired reactions like oxidation of metallic materials. Despite the robust design of

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respective torches and the relatively low costs for process media - especially for use of water - the rather limited spectrum of suitable coating materials has led to rather seldom use of LSPS in industries.

Hybrid plasma torches with combined gas and liquid stabilization of the arc that result in intermediate processing conditions are still in the stage of laboratory research [16].

6.5 Inductively coupled plasma spraying (ICPS)

Inductively coupled plasma spraying (ICPS), also known as high frequency plasma spraying, utilizes a plasma jet that is generated by inductive coupling of electrical energy into a gas stream (figure 11). The main advantage of this technique is the generation of the thermal plasma jet without contact of the generating components with the plasma gases. Therefore even highly reactive gases, including oxygen, and gas mixtures can be applied and incorporation of eroded electrode material in the coatings is ruled out. Additionally the large plasma volume permits high deposition rates and the particles undergo homogeneous heat treatment due to the axial injection inside the torch through water cooled probes. These days systems with plate powers typically between 25 and 200 kW are available.

Figure 11: Principle of the inductively coupled plasma spraying process [1]

The temperature inside the plasma jet usually does not exceed 10,000 K. In contrast to thermal plasma jets in DC torches the highest temperature is not achieved in the plasma jet core. With increase of the applied frequency the temperature maximum shifts to the jet fringes as a consequence of the skin effect. For spraying under

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atmospheric conditions gas and therefore also particle velocities are relatively low (< 50 m/s). Gravity is commonly exploited to achieve the highest possible velocity. For vacuum plasma spraying with inductively coupled plasma torches that are equipped with adequate nozzles even supersonic flow conditions are possible. However, thereby the plasma jet diameter is reduced accordingly and the reduced chamber pressure also results in significantly decreased energy density.

The main disadvantages of ICPS torches are the poor flexibility concerning handling of the torch due to the dependence of the particle trajectories on the angle between torch and gravity axis for conventional torches, the stiff power supply design and the strong electromagnetic fields, that prevent use of electronically controlled substrate handling systems. Therefore the most important application field of inductively coupled thermal plasma jets are the spheroidization of powders [17-18] and the analysis of materials by emission spectroscopy after complete evaporation in the thermal plasma jet [19].

7. Laser spraying (LS)

The laser spraying process proceeds by injection of a feedstock in powder shape through a suitable powder injector into a laser beam. The laser beam melts the powder particles that approach the substrate surface under the influence of the carrier gas flow and gravity (figure 12). Typically the spray jet is covered by an inert sheath gas jet.

Figure 12: Principle of the laser spraying process [1]

In contrast to laser cladding during laser spraying the substrate surface is not melted or only melted to a negligible degree. Therefore bond strength is comparatively low.

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Due to the rather small volume of the free high energy density laser beam only low deposition rates can be achieved. So, laser spraying is only suitable for well defined, small area coating of component surfaces that must not be melted and the use in industry is not very common. However, the process attracts some interest in research topics that require well defined heat transfer to the material to be processed [20-21].

8. Related processes

8.1 Thermal Plasmajet CVD (TPCVD)

Although conventional plasma spray equipment is used, Thermal Plasmajet CVD does not belong to the thermal spray processes, because coating formation does not proceed by impact of particles on the component to be coated but via gas phase reactions on the components surface. If the precursor for coating formation is not injected into the thermal plasma jet in gaseous state already, it is fully vaporized before the substrate surface is reached. All kinds of plasma spray torches can be applied. As the gas phase composition is decisive for coating formation the process needs to be carried out in controlled atmosphere. Usually relatively low chamber pressure is applied in order to achieve improved homogeneity of properties within the plasma jet and to achieve a small boundary layer thickness on top of the surface.

TPCVD is particularly suitable for high rate deposition of coating materials that cannot be processed by conventional thermal spray methods, because they show neither a liquid phase nor sufficient ductility below their decomposition temperature, i.e. diamond, c-BN, SiC or Si3N4. The simplest and best studied TPCVD coating process is diamond synthesis with methane as precursor [22] (figure 13). In contrast to alternative processes for pure diamond coating synthesis deposition rates exceeding 100 µm/h can be achieved. For SiC and Si3N4 coating formation even 1,500 µm/h is possible [23].

The major drawback of this process is related to the strong gradients in the properties of the plasma jet acting on the substrate surface. Therefore phase composition and thickness of the produced coatings varies strongly between plasma jet axis and the jets fringes. Also heat flux to the substrate is very strong and for diamond coating synthesis substrate temperature is typically > 800 °C.

Figure 13: TPCVD of a diamond coating at the Institute of Materials Technology, University of Dortmund

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8.2 Hypersonic Plasma Particle Deposition (HPPD)

Hypersonic Plasma Particle Deposition (HPPD) is in between TPCVD and conventional thermal spraying, because it proceeds via gas phase on the one hand, but on the other hand coatings are formed by impact of particles on a component to be coated. For this process nearly usual VPS technology is applied. The major difference concerns the chamber pressure which is typically in the range of 100 Pa. Low chamber pressure results in hypersonic velocities of the plasma gases leaving the torch and accordingly fast quenching. Gaseous precursors are added inside a specially designed nozzle and strong heat transfer by the thermal plasma jet results in full dissociation of the injected molecules. During quenching the vapor becomes supersaturated and nanosized particles are precipiated. Due to their small size particles are accelerated effectively. Depending on their size inertia is sufficient to cross the boundary layer on top of the substrate to be deposited and to form a coating by superposition of individual impacting particles. Too fine particles are sucked off by the vacuum pumps together with the plasma gas.

Due to the fact that supersaturated vapor is required precursor feed rate and resultant deposition rate can be significantly higher compared to TPCVD. For silicon deposition more than 3,000 µm/h is possible [24]. The process is still at the stage of laboratory research, but it has already been shown that HPPD is not only capable to produce different nanostructured metallic and ceramic coatings at considerable deposition rate [25]. Also, use of aerodynamic lenses permits production of micro-patterns like micro-towers or micro-walls [26]. So, the process has also high potential to be used for micropart production.

References

[1] DIN EN 657, June 2005, Beuth Verlag GmbH, Berlin, pp. 1-23

[2] Leatham, A.: Spray Forming: Alloys, Products, and Markets, Metal Powder Report, vol. 54, no. 5, 1999, pp. 28-37 (JOM-e: http://www.tms.org/pubs/journals/JOM/9904/Leatham/Leatham-9904.html)

[3] Williams, P.: ACT™ Power Coatings™, Platinum Metals Review, Vol. 46, No. 4, 1 October 2002, pp. 181-187

[4] Wilden, J., A. Wank, F. Schreiber: Wires for arc- and high velocity flame spraying - wire design, materials and coating properties. Proc. Int. Thermal Spray Conf. 2000, Montreal, QC, CAN, Ed.: C.C. Berndt, ASM International, pp. 609-617

[5] Wielage, B., J. Wilden, T. Schnick, A. Wank, P. Fronteddu: Analysis of the Wire Melting Behavior Depending on Wire Design and Process Characteristics. Proc. Int. Thermal Spray Conf. 2002, Eds.: E. Lugscheider, C. C. Berndt, DVS Verlag, Düsseldorf, pp. 446-449, ISBN 3-87155-783-8

[6] Kreye, H., F. Gärtner, A. Kirsten, R. Schwetzke: High Velocity Oxy-Fuel Flame Spraying. State of the art, Prospects and Alternatives. 5th Colloquium "High Velocity Oxy Fuel Spraying”, Erding, D, GTS e.V., 2000, pp. 5-18

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[7] Voyer, J., T. Stoltenhoff, T. Schmidt, H. Kreye: Method and Potential of the Cold Spray Process. 6th Colloquium "High Velocity Oxy Fuel Spraying”, Erding, D, GTS e.V., 2003, pp. 39-47

[8] Krömmer, W.: Cold Spray Process in Practice - Case Studies for Industrial Applications. 6th Colloquium "High Velocity Oxy Fuel Spraying”, Erding, D, GTS e.V., 2003, pp. 115-116

[9] Grasme, D.: First serial application of cold spraying for coating heat sinks. 6th Collo-quium "High Velocity Oxy Fuel Spraying”, Erding, D, GTS e.V., 2003, pp. 119-122

[10] Heberlein, J., M. Kelkar, N. Hussary, R. Carlson: New developments in wire arc spraying, Werkstoffe und Werkstofftechnische Anwendungen, Vol. 4, Proceedings 3rd Werkstofftechnisches Kolloquium, Chemnitz, D, Verlag Mainz, Aachen, 2000, pp. 10-20, ISBN 3-89653-531-5

[11] Wilden, J., A. Wank, F. Schreiber: Wires for arc- and high velocity flame spraying - wire design, materials and coating properties. Proceedings of the International Thermal Spray Conference 2000, Montreal, QC, CA, ASM International, Materials Park, Ohio, 2000, pp. 609-617

[12] Schwenk, A., H. Gruner, G. Nutsch: Modified Nozzle for the Atmospheric Plasma Spraying. Proceedings of the International Thermal Spray Conference 2003, Orlando, FL, USA, ASM International, Materials Park, Ohio, 2003, pp. 573-579

[13] Dzulko, M., G. Forster, K.-D. Landes, J. Zierhut, K. Nassenstein: Plasma Torch Developments. Proceedings of the International Thermal Spray Conference 2005, Basel, CH, DVS-Verlag, Düsseldorf, 2005

[14] Aebli, N., J. Krebs, H. Stich, P. Schawalder, M. Walton, H. Gruner, B. Gasser, J.-C. Theis: In vivo comparison of the osseointegration of vacuum plasma sprayed titanium- and hydroxyapatite-coated implants. Journal of biomedical materials research, Vol. 66A, 2003, No. 2, pp. 356-363

[15] McKechnie, T., Y.K. Liaw, P. Krotz, R. Poorman, F. Zimmerman, R. Holmes: VPS Forming of Refractory Metals and Ceramics for Space Furnace Containment Cartridges. Thermal Spray Coatings: Research, Design and Applications, ASM International, Materials Park, Ohio, 1993, pp. 297-302

[16] Hrabovsky, M.: Generation of thermal plasmas in liquid stabilized and hybrid dc-arc torches. Pure Applied Chemistry, Vol. 74, 2002, No. 3, pp. 429-433

[17] Li, Y.-L., T. Ishigaki: Spheroidization of Titanium Carbide Powders by Induction Thermal Plasma Processing. Journal of the American Ceramic Society, Vol. 84, September 2001, No. 9, pp. 1929-1936

[18] Linke, P., K.-H. Weiss, G. Nutsch: New manufacturing technologies of two phase tungsten carbide. Materialwissenschaft und Werkstofftechnik, Vol. 34, July 1st 2003, No. 7, pp. 613-617

[19] Noelte, J.: ICP Emission Spectrometry - A Practical Guide. 1st edition, December 2002, Wiley-VCH, Weinheim, ISBN-10: 3-527-30672-2

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[20] Luo, Q., Z. Liu, L. Li, S. Xie, J. Kong, D. Zhao: Creating highly ordered metal, alloy, and semiconductor macrostructures by electrodeposition, ion spraying, and laser spraying. Advanced Materials, Vol. 13, February 19th 2001, No. 4, pp. 286-289

[21] Tsukamoto, K., F. Uchiyama, F. Kaga, Y. Ohno, T. Yanagisawa, A. Monma, Y. Takahagi, M.J. Lain, T. Nakajima: New ceramics coating technique for SOFC using the laser spraying process. Solid State Ionics, Vol. 40-41, August 1990, No. 2, pp. 1003-1006

[22] Heberlein, J.V.R., N. Ohtake: Plasma torch diamond deposition. Diamond Films Handbook, Chapter 6, eds.: J. Asmussen, D.K. Reinhard, Marcel Dekker, New York, 2002, pp. 141-210

[23] Wilden, J., A. Wank, M. Asmann, J.V.R. Heberlein, M.I. Boulos, F. Gitzhofer: Synthesis of Si-C-N coatings by thermal plasma jet CVD applying liquid precursors. Applied Organometallic Chemistry, Vol. 15, 2001, No. 10, pp. 841-857

[24] Rao, N.P., N. Tymiak, J. Blum, A. Neuman, H.J. Lee, S.L. Girshick, P.H. McMurry, J. Heberlein: Hypersonic plasma particle deposition of nanostructured silicon and silicon carbide. Journal of Aerosol Science, Vol. 29, June 1st 1998, No. 5-6, pp. 707-720

[25] Hafiz, J., X. Wang, R. Mukherjee, W. Mook, C. Perrey, J. Deneen, J. Heberlein, P.H. McMurry, W. Gerberich, C.B. Carter, S.L. Girshick: Hypersonic Plasma Particle Deposition of Si-Ti-N Nanostructured Coatings. Surface and Coatings Technology, Vol. 188-189, 2004, pp. 364-370

[26] Perrey, C.R., R. Thompson, C.B. Carter, A. Gidwani, R. Mukherjee, T. Renault, P.H. McMurry, J.V.R. Heberlein, S.L. Girshick: Characterization of nanoparticle films and structures produced by hypersonic plasma particle deposition. Proceedings of the Materials Research Society Symposium 2002, Vol. 740, 2002, pp. 133-138