BACKGROUND OF THE INVENTION Thermal spray coating refers to a variety of processes characterized in that a fusible material such as plastic, metal, ceramic or the like in a particulate form is soften by heat applied thereon and then accelerated towards a target to be coated.

The soften particles hit the target where they are quenched to form a solidified layer of the coating material.

Three major types of processes are known in the art for this kind of operation, namely: plasma spray coating, high velocity flame spray coating and detonation coating.

The plasma spray coating device consists, in general, of a confined passage between an anode and a cathode. A plasma forming inert gas, typically argon or nitrogen, is introduced into the passage and the plasma is usually initiated by a high frequency electric pulse which causes a high current arc discharge between the cathode and anode. The arc heats and ionizes the plasma gases to a high temperature of about 12000°K or even higher, and then, the heated

and expanded plasma is exhausted at high speed through a nozzle orifice in the cathode. In the coating process the feedstock, which is usually used in a powder form, is suspended in a carrier gas. The powder particles are injected into the plasma jet and the carrier gas provides the particles with a sufficient momentum to reach the plane ofthe plasma discharge.

In order to achieve a uniform, high quality coating, several requirements must be met: the feedstock powder must have a narrow particle size distribution, the heat and momentum transfer rates to the particles must be controlled to provide the particles with a narrow range of velocities, and, a sharp surface temperature distribution must be maintained as the particles impinge on the target.

In the plasma spray processes, the velocity of the injected particles is typically limited to low values of 100-300 m/s in accordance with the speed of sound in the propelling media, which consists of both the gases and the dispersed particles. Higher velocities could be attained by increasing one or more of the following parameters: temperature, density and/or carrier gas flow speed.

However, the possibility of exceeding the typical values of these parameters is somewhat limited because of the inherent characteristic resistivity of the ionized plasma. Thus increasing these values would lead to a substantial increase in the electric power required for the arc channel. If the power supply exceeds the optimal working range of 20 - 100 kW, the overall process efficiency decreases due to excessive cathode erosion, magnetic stresses, and also expensive hardware and thermal management under severe conditions. Therefore, a plasma spray coating technique is considered in the art applicable for particles' speed of about 100-300 m/s, whereas the powder application rate, which is also limited by the plasma parameters, would typically be in the range of 2-6 kg per hour.

Several improvements were suggested in the past to overcome the above-described disadvantages associated with plasma spray coating. US 4,982,067 for example, discloses the use of an apparatus comprising four pilot plasma guns located about a common axis, whereby an improved heat transfer

and uniform heating of the feedstock are achieved, together with the improvement of the overall process efficiency.

The second type of thermal spray coating process is high velocity flame spray coating which utilizes the hot gases produced in a combustion process for heating or melting a particulate material. The heated particles are then accelerated to high velocities (300-1000 m/s) towards a target to be coated by a gas jet being at approximately 3000°K. The particles' thermal and kinetic energies thus acquired are converted so as to form a dense adherent coating on the surface, whereas the heat input required for the process is obtained by fuel combustion (where the fuel is in a gas or liquid state) with air or oxygen. A typical configuration of a high velocity thermal spraying device comprises a central orifice for injecting the feedstock powder material and means to feed the fuel and the oxidizer into the reaction zone. The combustion process occurs within the reaction zone and the hot combustion gases evolving therefrom expand to create a jet that accelerates the particles. The powder particles, absorbing heat in the reaction and the expansion zones, are impinged onto the target in a molten or soften state. As a result of the impact they are flattened while generating discrete coated zones on the target. Each of the discrete coated zones tends to adhere to the adjacent ones as well as to the target itself to form the required coating of the target. Devices where such a technology is applied are disclosed for example in US 4,358,053 and US 4,370,538.

The major disadvantages associated with high velocity flame spray coating technology are, the relatively low flame temperature which is below 3000"K, (as compared to temperatures of more than 12,000"K that may be attained in the plasma spray coating process described above). Another disadvantage is the continuous high consumption of fuel and oxygen which sets practical limits on frequent changes in the coating conditions due to the large gas losses.

Furthermore, the specific energy content of the combustion gases is relatively low which sets a limit to the possible rate of powder feeding.

A third technology in which sprayed material at high temperature and high velocity is used for coating, is the "detonation" coating. An apparatus for use in such a technology is disclosed in GB Application 2,104,268 which describes an apparatus comprising a detonation chamber, made in a form of a barrel closed at one end, a gas mixture supply system and a feedstock powder supply system wherein a carrier gas is used for transferring the powder particles. Both, the coating powder and the gas mixture, which typically consists of a fuel gas such as acetylene, an oxidizing gas such as oxygen and an inert powder carrying gas such as nitrogen, are introduced batch-wise into the detonation chamber forming a solid-gas mixture. When the detonation chamber is filled, the solid-gas mixture thus obtained is ignited by an ignition device such as a spark plug, initiating a detonation in the chamber.

The detonation products, traveling at a high velocity to the exit of the barrel, heat and accelerate the powder particles of the coating material, carrying them away from the barrel and impinge them upon the target to be coated, while forming a coating layer thereon. The coating layer consists of a large number of flattened spots of material which are adhered to each other. Such a process has been disclosed for example in US 2,714,563 and 2,774,625.

The major disadvantages in the detonation coating are the inherent complexity in the gas and powder feed control as well as in regulating and controlling the conditions in the detonation chamber. Since the mixture of the gases has to reach the maximum pressure value permitted in the barrel within a very short time interval, the applicable amount of gases and powder per batch, is highly limited.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a coating applicator element that can be used in thermal spray coating processes.

It is another object of the present invention to provide an apparatus and system for carrying out thermal spray coating.

Other objects of the invention will be described along with the description of the invention.

In accordance with the present invention is provided a coating applicator element for use in a thermal spray coating process that comprises a coating powder and a non-gaseous propellant. The coating powder may either be mixed with the non-gaseous propellant or be separated therefrom.

The propellant ofthe present invention may either be an energetic material e.g. a combustible material such as a solid propellant, or a non-energetic material e.g. water, liquid ammonia and the like. In the first case the energy required to gasify the propellant may be derived its combustion, whereas in the latter case, an external source such as plasma discharge is used to deliver the energy required to gasify the propellant. In both cases, the propellant is gasified, and the gaseous product(s) formed is used to motivate the powder particles.

The energetic non-gaseous propellant of the coating applicator element according to the invention may be ignited in any way known per se in the art.

However, a preferred way according to the present invention to initiate the propellant of the applicator element, irrespective of the type of the propellant used, is by using plasma discharge, e.g. by using plasma generating assembly which may be either external to the coating applicator element or located within the applicator element itself. The energy obtained from the plasma generated is used to initiate the propellant, and the gases formed from its initiation, are used to motivate the powder particles.

In the case that the propellant is of the energetic type, only a small portion of the energy required for motivating the powder particles should be derived from the plasma generated, as the substantial part of this energy may be derived from the combustion of the propellant itself. Therefore, the energy obtained from the plasma generated is used mainly for the ignition of the propellant and possibly to provide a minor addition to the energy derived from the propellant combustion.

According to a preferred embodiment of the invention, the coating applicator element further comprises a device for plasma generation. Typically,

such a coating applicator element has a front and rear sections, wherein the non-gaseous propellant and coating powder are located in the front section and the device for plasma generation is located in the rear section.

In operation, the coating applicator element according to the invention is used with an application apparatus which initiates the propellant and the resulting mixture of a high pressure and high density hot gas and coating powder is ejected onto the target to be coated. In a preferred mode of carrying out the invention, the ignition pulse causes plasma generation and the plasma in turn ignites the propellant.

In the case of an energetic propellant accompanied by a plasma pulse, the plasma ignition of the propellant leads to its combustion, a process that in many aspects resembles the combustion of a propellant in a regular firing device. In the case of non-energetic propellant, the plasma formed is mixed with the non-gaseous propellant, causing the gasification of the latter. A high gasification rate can be obtained for example when using a liquid propellant, due to a substantial increase of the propellant surface area exposed to the plasma as a consequence of hydrodynamic instabilities. As a result of the gasification of the propellant (irrespective of the type used), a high pressure and high density hot gas is formed. This gas flows at high velocity towards the target, dragging the coating powder along. The drag force, FD that accelerates the powder particles may be described according to the following expression: FD = SF(Re)(s dp / 8) P(Vg - Vp)2 wherein SF is a coefficient which depends on the Reynolds number, Re, that characterizes the flow around the particles; dp is the particles' mean diameter; p is the gas density; and Vg and Vp are the gas and the particle corresponding velocities.

The mechanism for accelerating the coating powder particles with a propellant that is initiated by plasma generated, is essentially similar to that characterizing the acceleration of a projectile in an electrothermal (ET), electroth- ermal-chemical (ETC) or solid-propellant electrothermal-chemical (SPETC) guns. Some of such methods are described in "Trends in Gun Propulsion for Tactical Army Application" by W.F. Morrision et al. presented at the International Symposium on Ballistics, Stockholm, Sweden, pp. 23-45, 1-3 June 1992, which is hereby incorporated by reference. However, the specific nature of the coating applicator element, which is different from regular cartridges and projectiles, requires some additional features to enable the acceleration of the powder contained in a coating applicator element of the invention, when used in the apparatus of the invention.

Due to the fact that the consumption rate of the propellant is relatively gradual as compared, for example, with that in a detonation process, the process parameters such as gas production rate as well as the pressure and temperature profiles may be predetermined and controlled. This capability of designing and controlling the characteristics of the discharge of the particles' carrying gas in each coating applicator element, presents an important advantage of the present invention, as it allows designing a coating of various characteristics of a target, or alternatively, obtain a highly uniform coating ofthe target.

According to another aspect of the invention there is provided an apparatus for thermal spray coating a target by means of coating applicator elements comprising a non-gaseous propellant and a coating powder, which apparatus comprises a barrel, an initiation assembly, storage and feeding means for the coating applicator elements, and optionally means for connection to a power supply.

According to a preferred embodiment of the invention, the feeding means of the apparatus of the invention is a dual feed mechanism.

The coating applicator elements of the present invention may be applied either in a single mode operation, or preferably, in an automatic mode operation.

The single mode operation refers to the thermal spraying of a single coating applicator element onto a target. Thereafter the coating apparatus may be re-loaded with the next coating applicator element that may be identical with or different from the former coating applicator element used.

In accordance with the automatic mode operation, a plurality of coating applicator elements are arranged so that they can be fed into the coating apparatus continuously, preferably automatically, (e.g. arranged in a chain-like formation which resembles the loading in a machine gun). Typically, the rate of feeding may be in the range of 5-20 coating applicator elements per second, but of course the invention is not limited to any such rate. As may be appreciated, this rate depends primarily on the coating apparatus characteristics, the powder quantity in each coating applicator element, the power source available, etc.

The coating applicator elements, when applied in the automatic mode, may either be similar to each other or different in any one or more of their components, namely the propellant, the coating powder and/or even the medium for plasma generation. Such variations may be for example in types, amounts, particles' size, geometry and the like. Therefore, in accordance with the present invention, a pre-programmed, functionally graded, multi-layered coating of a target may be achieved in a single process as the target being coated by a series of coating applicator elements, containing each different coating powder. Using the apparatus and the coating applicator elements of the invention, also enables a coating of a certain part of the target (or all of the target, if required) with several different coating layers, and also to obtain a non-uniform coating layer by changing the powder quantity in the coating applicator elements used.

According to yet another aspect ofthe invention there is provided a system for thermal spray coating of a target, comprising a spray coating apparatus as described above, an enclosure comprising a target support and optionally cooling and control systems. Preferably, in the system according to the present invention the target support is provided with means for the translatory and

angular displacement of the target, and means to coordinate this displacement with the feeding and ignition of the coating applicator elements.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be illustrated with reference to the drawings. It should be understood that the invention is not limited to the specific embodiments shown herein but also encompasses all other configurations which function similarly to the embodiments described.

In the drawings: Fig. 1 presents schematically a thermal spray coating system of the present invention; Fig. 2 presents a cross sectional view of the apparatus for thermal spray coating; Fig. 3 presents a schematic cross sectional view of the coating applicator element ofthe present invention; Fig. 4 presents another embodiment of the coating applicator element of the invention; and Fig. 5 presents a mechanism for an externally driven apparatus for automatic operation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 presents a system in accordance with the present invention, comprising a spray coating apparatus 2, an enclosure 3 comprising a target support 5 and auxiliary sub-systems such as a control system, a cooling system (which may be used for both the coating apparatus and the target), not shown in this figure. The target to be coated 4, is placed on a support 5 of enclosure 3, where support 5 is connected to motors 8 so as to provide a possibility of displacing target 4 in different directions and/or rotate it about its horizontal or vertical axis. A control system may further be connected to apparatus 2 to monitor several important features of the process, such as the displacement of the

target, the coating rate, identification of malfunctions in the operation of the apparatus, control of the cooling system, etc.

Apparatus 2 comprises the following components: a barrel 10, a plasma generating assembly 11, a connection to power supply unit 12, and a mechanism 13 for feeding the coating applicator elements in a continuous operation mode.

The apparatus further comprises feeding means 14 and storage means 15 for storing the coating applicator elements to be fed. One way of attaching apparatus 2 to enclosure 3, shown in Fig. 1, is by using flexible bellows 9.

Fig 2. shows a cross section of a spray coating apparatus in accordance with the single operation mode embodiment of the invention. The coating apparatus 2' shown here, comprises a barrel 20 with an axial bore 21 extending therethrough. The rear part of the bore has a broader shape so as to form a breech 22, adapted to receive the coating applicator element 23. A plasma ignition device 30 aligned coaxially with the coating applicator element 23 is attached to the inlet opening of barrel 20. The plasma ignition device 30 typically comprises metallic detachable enclosures 31 and 32 designed to withstand very high pressures, e.g.

up to 600 MPa. Typically, the plasma ignition device 30 further comprises a first electrode 41, usually a cathode, being on one side in contact with the rear part of the coating applicator element 23 and on the other side, electrode 41 is placed relatively to a second electrode 42, usually an anode, so that together they are capable of providing an electric ignition pulse within a narrow bore 43 inside electrically shielding member 45. Bore 43 in turn is highly pressurized by the gases evolving during the combustion process and the plasma formed. In accordance with the invention, such a bore may be a capillary which walls are made of a dielectric material e.g. polyethylene. Both electrodes 41 and 42 are located so as to allow the generation as required. The connection of the plasma ignition device 30 to a power supply is not shown in the figure. Member 47 is incorporated to provide a mechanical support, preferably while electrically shielding anode 42 from the metallic detachable enclosure 32. Members 45 and 47 are typically made of composite materials, but also plastic materials such as

high-density polyethylene were found to be satisfactory for the above-described purpose. A detailed description of such a plasma generator is given in A. Loeb and Z. Kaplan, IEEE Transaction on Magnetics, Vol. 25, No. 1, 342 (1989), which is hereby incorporated by reference.

Fig. 3 shows a coating applicator element 70 of the invention. Where used in the apparatus of the present invention, the coating applicator element is adapted to fit within the breech at the inlet of the barrel. The rear part of element 70 is provided with a cavity 72 designed to receive a hollow cathode so that the plasma generated in the medium for plasma generation 74, will penetrate and ignite propellant 76. The propellant may be, for example, a conventional solid propellant having the following burning rate Z: Z=S aP where S is the surface area of the propellant, P is the pressure developing, and a and are constants depending upon the propellant chemistry. Proper choice of a propellant and its geometry, allows pre-designing the gas production rate, and consequently the velocity provided to the coating powder. A bulk of the coating powder 78 may be placed either in the front section of the coating applicator element (as shown in the figure) or be mixed with the propellant. The various components described, are contained according to this embodiment in a cartridge envelope 89. Typically, a coating applicator element comprises a powder which weight is in the range of 20-200% of the propellant weight. An appropriate choice of a plasma and propellant combination, e.g. their geometry and chemical compositions, allows obtaining a sufficiently high combustion rate to ensure high enough pressure in the coating applicator element. Such a phenomenon can also occur when the powder is of a fine grain or arranged in a special geometry, so that the propellant itself may be directly ignited, eliminating the need for plasma source. Using such a coating applicator element although may produce the desired coating, still, the coating applicator elements are likely to be highly sensitive to occasional undesired spontaneous burning. However, utilizing plasma ignition ensures a prompt and homogenous ignition, in a better way than any

other ignition method, due to its rapid flame spreading. This in turn helps achieving a complete bumout/gasfication of the propellant and a better particle velocity distribution. The combination of the fast burning/gasification of the propellant, the subsequent very high breech pressure in a range of 30-600 MPa and the high pressure gradient which is formed along the barrel, generate a fast flow of the gaseous products which carry powder particles together with some propellant's residues, if exist, at very high velocities towards the target.

Preferably, and when properly designed, the propellant will be completely consumed still in the barrel and the particles will gain a velocity sufficient to properly adhere to the target and form high quality coating thereon.

In accordance with the embodiment presented in FIG. 3, the ignition assembly, (not shown in the figure) is located externally to the coating applicator element, for example at the rear part of the breech of the apparatus shown in FIG.

2. According to another embodiment of the invention, plasma generating electrodes are integrated in the coating applicator element, to eliminate the need for external electrodes and a plasma passage means, which may all be subjected to extensive erosion when used repeatedly. The ignition is also simpler in the latter case as a single medium voltage power supply can be used for both the arc ignition and the generation of the plasma. The ignition itself may be carried out by using a metallic fuse as knownper se in the art.

FIG. 4 presents another embodiment of the invention for a coating applicator element that further comprises plasma generation electrodes. An anode 120, and a hollow cathode 121 preferably having a thick ring shape, both of which are preferably made of low cost metal such as iron, may be used as plasma generating electrodes for coating applicator element 122. According to this embodiment of the invention, anode 120 is firmly pressed into the center of a cylinder 123, which is used as an electrical insulator and provides within the required dielectric plasma channel 124 extending between the anode and the cathode. Cylinder 123 may be made of a plastic material such as high-density polyethylene. A socket 125, located in the rear part of anode 120 is adapted to

receive a firing pin that constitutes a part of the breech assembly (not shown in the figure). This pin provides the electrical connection between the power supply and the anode. A thin fuse that may be made for example of aluminum foil 128 is inserted between both electrodes to provide the discharge starter. A separation between the plasma channel 124 and the propellant 132, located adjacent to each other is achieved by separator 130, e.g. a thin mylar foil. Envelope 134 of coating applicator element 122 may be made of various materials, e.g. brass, polymer or any other suitable material. Powder 136 as shown in FIG. 4 is contained in a bulk form in front of the propellant. Alternatively, the powder depending on various requirements that stem from the desired characteristics of the coating, may be placed at different sites in respect ofthe propellant, or may be mixed therewith.

In the automatic operation mode, the operation highly resembles that of an automatic or semi-automatic rifle or a machine-gun, i.e. which fire an uninterrupted series of rounds by a single generation of a continuous firing trigger, where all requisite operational processes follow automatically. The energy required for feeding the ammunition, loading the weapon and ejecting the empty cartridge cases is either derived from the firing energy or sometimes provided externally. Semi-automatic weapons perform similarly to the automatic weapon with the exception of their triggering that must be actuated for each round.

According to another embodiment, the present invention provides an apparatus adapted to operate in an automatic mode. This apparatus functions similarly to an automatic or semi-automatic weapon as explained above. The apparatus may be characterized as being one of the following types, namely, gas-operated, drum-type or as having an external drive.

FIG. 5 presents one of the options provided by the invention for a mechanism to operate the apparatus provided. The mechanism presented is a dual feed mechanism used for an externally driven apparatus that operates automatically. One of the feed sprockets 142 transports a belt 144 containing a plurality of coating applicator elements 148, past a belt link extractor 146. The

coating applicator elements 148 are combed out and the belt links 150 are disengaged. As the coating applicator elements pass on, they are deflected by a deflector 152, the position of which depends on the specific choice of the coating applicator elements content, from sprocket 142 into the stationary indexing wheel 154. Once bolt 156 reaches its rearmost position and comes to a halt, the indexing wheel 154 turns anti-clockwise through about one third of a revolution. Drive may be provided by a gear-unit (not shown in this figure). The rotary motion of the indexing wheel 154 brings a fresh coating applicator element into the feed position 158, whereas the empty case is brought to the discharge position 160.

Subsequent forward movement of the breech bolt in the coating applicator element being fed into chamber 162 and the spent case being discharged by an ejector 164. At the end ofthe forward motion, locking and ignition ofthe coating applicator element fed is effected by a rotating bolt and the spent case is withdrawn again into the indexing wheel. A recoil of the barrel releases interlock; which in turn allows the chain to move on. If ignition does not occur within a given interval of time, a control system would determine a failure in following the operating cycle, and the process may automatically be interrupted.

In the automatic mode operation, the igniting assembly which provides the spark required to initiate the plasma, must be adapted to operate at the rate in which the coating applicator elements are to be applied. Such a device is different from a device for a single mode operation, as it should operate at a relatively high voltage to allow an immediate breakdown between the anode and the cathode of the igniting device. Typical values for breakdown voltage required are about 40-60 kV for 10 mm gap between both electrodes. However, filling the gap between both electrodes with a noble gas such as argon, may reduce this value to about 2-4 kV.

Typical energies provided by the plasma to the propellant, in the case of energetic propellant, are 500-5000 Joules, per unit of a coating applicator element. In the case of less energetic or non-energetic propellant, the plasma should provide the required energy for the gasification of the propellant (in the

range of 30-50 kJ per pulse) Typical value for the pulse duration is about 0. l - 2 ms (where the power is in the range of 10-100 MW). Such set of operating conditions would allow about 30 minutes of continuous operation when using endurable discharge chamber and electrodes, made of materials such as tungsten or tungsten copper alloys.

EXAMPLE A system operating at a single mode as described above, was used for thermal spray coating of a flat stainless-steel target with tungsten carbide powder.

The coating applicator element comprised 6g of tungsten carbide powder, and 6g of solid M8M type propellant, augmented by lkJ plasma jet. The calculated velocity achieved was over 1500m/s. 4g, out of the initial 6g of the powder content in the coating applicator element, reached the 5cm diameter said target located 50 cm away from the apparatus to form a uniform coating thereon of up to 300 microns thickness. No pretreatment of the target was required. Measured hardness of the coating was found to be 1800 kg/mm2 and supreme adhesion of the coated layer to the target was observed.