Abstract:
A method of operation of a plasma torch. A cold high pressure carrier gas containing a powder material is injected into a cold main high pressure gas flow and then this combined flow is directed coaxially around a plasma exiting from an operating plasma generator and converging into the hot plasma effluent, mixing with the effluent to form a gas stream with a net temperature, based on the enthalpy of the plasma stream and the temperature and volume of the cold high pressure converging gas, such that the powdered material will not melt. The combined flow with entrained is directed through a supersonic nozzle accelerating the flow to supersonic velocites sufficient that the particles striking the workpiece achieve kinetic energy transformation into elastic deformation of the particles as they impact the workpiece forming a cohesive coating.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This invention claims priority to Provisional Application Ser. No. 60/346,540 filed. Jan. 8, 2002 titled “PLASMA SPRAY METHOD AND APPARATUS FOR APPLYING A COATING UTILIZING PARTICLE KINETICS”, by Keith Kowalsky and Daniel Marantz. 
    
    
     FIELD OF INVENTION 
     The present invention is directed to a method and device for low temperature, high velocity particle deposition onto a workpiece surface from an internal plasma generator, and more particularly to a thermal spray method and device in which the in-transit temperature of the powder particles is below their melting point and wherein a cohesive coating is formed by conversion of kinetic energy of the high velocity particles to elastic deformation of the particles upon impact against the workpiece surface. 
     BACKGROUND OF THE INVENTION 
     Until Recently, in thermal spraying, it has been the practice to use the highest temperature heat sources to spray metal and refractory powders to form a coating on a workpiece surface. The highest temperature processes currently in use are plasma spray devices, both using an open arc as well as a constricted arc. These extremely high temperature devices operate at 12,000° F. to 16,000° F. to spray materials, which melt at typically under 3,000° F. Overheating is common with adverse alloying and/or excess oxidation occurring. These problems also occur to a lesser or greater degree during the use of the more recently developed HVOF (high velocity oxy-fuel) processes as well as HVAF (high velocity air-fuel) processes. Both of these are combustion type processes utilizing pure oxygen or air containing oxygen as the-oxidizer in the combustion process. 
     Another prior art method of applying a coating is described in U.S. Pat. No. 5,302,414 Alkhimov et al, issues Apr. 12, 1994, which describes a cold gas-dynamic spraying method for applying a coating of particles to a workpiece surface, the coating being formed of a cohesive layering of particles in solid state on the surface of the workpiece. This is accomplished by mixing powder particles having a defined size of from 1 to 50. microns entrained in a cold high pressure carrier gas into a preheated high pressure gas flow, followed by accelerating the gas and particles into a supersonic jet to velocities of 300 to 1000 meters per second, while maintaining the gas temperature sufficiently below the melt temperature so as to prevent the melting of the particles. In the operation of this cold gas-dynamic spraying method there are a set of critically defined parameters of operation (particle size and particle velocity for any given material) which makes the process very sensitive to control while maintaining consistent coating quality as well as maintaining useful deposit efficiencies. In addition, the cold gas dynamic spray method as described by Alkhimov et al, is limited to the use of 1-50 micron size powder particles. 
     Another prior art method of coating is described in U.S. Pat. No. 6,139,913, Van Steenkiste et al, which describes a kinetic spray coating method and apparatus to coat a surface by impingement of air or gas with entrained powder particle in a range of up to at least 106 microns and accelerated to supersonic velocity in a spray nozzle and preferably utilizing particles exceeding 50 microns. The use of powder particles greater than 50 microns overcomes the limitation disclosed by Alkhimov et al. Van Steenkiste et al, while utilizing the same general configuration of the prior art in which the cold high pressure carrier gas with entrained powder material is injected downstream of the heating source of the main high pressure gas into the heated main high pressure gas overcomes the limitations of Alkhimov et al by controlling the ratio of the area of the powder injection tube to 1/80 relative to the area of the main gas passage. By controlling this ratio, it limits the relative volume of cold carrier gas flowing into the heated main gas flow, thereby causing a reduced degree of temperature reduction of the heated main high pressure gas. The net temperature of the main high pressure gas when mixed with the carrier/powder gas flow is critical to determining the velocity of the gas exiting the supersonic nozzle and thereby to the acceleration of the powder particles. As indicated by Alkhimov et al, a critical range of particle velocity is required in order that a cohesive coating is formed. The particle size, the net temperature of the gas and the volume of the gas determine the gas dynamics required to produce a particle velocity falling into the critical particle velocity range. 
     The cold gas dynamic spray method of Alkhimov et al is limited to the use of a particle size range of 1-50 micron. This limitation has been found by Van Steenkiste et al to be due to the heated main high pressure-gas being cooled by injecting into it the cold high pressure carrier gas/powder. Because of the reduction in gas temperature, the maximum gas velocity that can be achieved is too low to accelerate powder particles larger than 50 microns to the critical velocity required to achieve the formation of a cohesive coating buildup. Van Steenkiste el al improves on this by limiting the amount of cold high pressure carrier gas being injected into the heated high pressure main gas by defining the ratio of the cross sectional area of the bore of the powder injection tube to the area of mixing chamber. This limited the proportion of cold carrier gas mixed into the heated main gas thereby reducing the degree of temperature reduction of the heated high pressure main gas, which then allows for higher gas velocities to be achieved. This provides the ability to accelerate larger particles of a size range greater than 50 microns to a velocity above the critical velocity required to form a cohesively bonded coating buildup. However, the kinetic spray coating method and apparatus of Van Steenkiste et al state an upper limit of the particle size range 106 microns, based on experimental results. 
     In addition in Alkimov et. al. the main gas is heated upstream of the nozzle, then just upstream of the throat of the nozzle, they introduce the particles and cold carrier gas which lowers the final temperature of the combined main gas/carrier gas/particles. This causes the velocity of the particles to be slower than if the temperature of the main gas was not reduced. Accordingly, in Alkimov a much higher main gas temperature must be used to accommodate the cooling effect of the introduction of the cold carrier gas and particles. With standard electric heaters, the main gas temperature can only be increased to 1300 to 1400 degrees Fahrenheit. This limits the velocity of the particles and hence the size of the particles that produce cohesively formed coatings. Although the pressures of the gases can be increased to increase the velocity of the particles this also increases the complexity and the expense of the system. Accordingly Alkimov is limited to particle sizes of 1 to 50 microns. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus by which particles of metals, alloys, polymers and mechanical mixtures of the forgoing and with ceramics and semiconductors having a broad range of particle sizes, may be applied to substrates using a novel plasma spray coating method which provides for first feeding the cold high pressure carrier gas with entrained powder particle material into the cold high pressure main gas prior to heating the combined gases and powder and then converging the cold combined gas/powder mixture coaxially into a plasma flame thereby controllably heating the gas as well as the powder particles. The plasma flame can heat the combined gas and particles in excess of 2500 degrees Fahrenheit. 
     The present invention utilizes a high-pressure plasma generator operating at plasma gas pressures of about 200 psig to 600 psig to produce a very high temperature (about 8,000° F. to about 12,000° F.) plasma flame. A mixture of cold high-pressure gas at a pressure of about 200 psig to about 600 psig, such as air or an inert gas such as argon or helium or a non-reactive gas such as nitrogen, with powder particles entrained in the cold high pressure gas flow is directed to converge coaxially into the high temperature plasma flame and mixing therewith, which causes the powder particles to be heated by the high temperature plasma flame as well as raising the temperature of cold converging high pressure gas. The heated particles in a gas stream consisting of the high temperature plasma gas along with the converged high pressure gas is caused to flow through an extended nozzle to accelerate the gas/powder mixture to a high velocity in the sonic to supersonic velocity range. The centerline of the plasma flame, the converging flow of the cold gas/powder mixture and the centerline of the extended straight bore nozzle are all coaxially aligned. The temperature of the powder particles is elevated to a point below that necessary to cause their thermal softening or melting so that a change in their metallurgical characteristics does not occur. The factors that provide controllability of the temperature of the main high pressure gas mixed with the high pressure carrier/powder gas as well as the particle temperature are the enthalpy of the plasma as well as the volume of high-pressure main/carrier gas mixture. It should be understood that a de Laval nozzle could be substituted for the extended straight bore nozzle in order to achieve higher velocities of the plasma/main gas/carrier gas/powder mixture. A sonic or supersonic flow of the hot gas mixture of plasma/main gas/carrier gas/powder is produced from the extended straight bore or de Laval nozzle and directed as a sonic or supersonic jet of hot gases and particles toward a workpiece surface to be coated. The improvement lies in feeding the cold high pressure carrier gas with entrained powder particle material into the cold high pressure main gas prior to heating the combined gases and powder and then converging the combined gas/powder mixture coaxially into a plasma flame thereby controllably heating the gas as well as the powder particles. The powder particles are controllably heated to the point of less than that required to heat soften the particles, maintaining the in-transit temperature of the particles below the melting point and providing sufficient velocity to the particles to achieve an impact energy upon impact with the workpiece surface capable of transforming the particle kinetic energy to cause elastic deformation to the particles causing them to adhere to the workpiece surface and cohesively build-up a coating thereby forming a dense coating. The improvement over the prior art lays in the fact that, regarding Alkhimov et al, the cold gas dynamic spray method is limited to the use only a particle size range of 1-50 micron. This limitation has been found by Van Steenkiste et al to be due to the heated main high pressure gas being cooled by injecting into it the cold high pressure carrier gas/powder. Because of the reduction in gas temperature, the maximum gas velocity that can be achieved is too low to accelerate powder particles lager than 50 microns to the critical velocity required to achieve the formation of a cohesive coating buildup. Van Steenkiste el al improves on this by limiting the amount of cold high pressure carrier gas being injected into the heated high pressure main gas by defining the ratio of the cross sectional area of the bore of the powder injection tube to the area of mixing chamber. This limited the proportion of cold carrier gas mixed into the heated main gas thereby reducing the degree of temperature reduction of the heated high pressure main gas, which then allows for higher gas velocities to be achieved. This provides the ability to accelerate larger particles of a size range greater than 50 microns to a velocity above the critical velocity required to form a cohesively bonded coating buildup. However, the kinetic spray coating method and apparatus of Van Steenkiste et al state an upper limit of the particle size range 106 microns, based on experimental results. The present invention is novel above the prior art because the cold high pressure carrier gas/powder is injected into the cold high pressure main gas before it is heated. After the step of mixing the carrier and main gas, the combined gas/powder mixture is then heated by mixing it with a very high temperature plasma flame thereby providing the ability to fully control the temperature of the gas mixture prior to acceleration as well as providing a controlled heating of the powder particles. This results in being able to produce higher gas velocities thereby controllably being able to accelerate a very broad range of particle sizes, exceeding 150 microns. 
     Another object of the invention is to use the cold carrier gas and main gas to cool the nozzle instead of water cooling the nozzle. Typically in a water-cooled non-transferred plasma arc spray system approximately 35% of the energy of the plasma ends up heating the water, which is used to cool the nozzle. By using the cold carrier gas and main gas to cool the nozzle, the plasma is then used to heat the carrier gas and main gas and ends up being a very efficient system. 
     Another embodiment of this invention provides for the method and apparatus for depositing a coating onto the internal surface of a bore or cylinder or a concave surface. A plasma device as previously described as pail of this invention is radially disposed with respect to the axis of the bore and supported on a member capable of rotating this plasma device around the axis of the bore. The axis of the plasma device is maintained at ali times during the rotation at a perpendicular position relative to the axis of the bore. Rotating fittings are provided to carry the necessary gases, powder feedstock and electrical power to the rotating plasma device. The plasma device functions in the same manner as the plasma devices previously described as part of this invention. The powder feed stock can be pre-mixed with the main cold gas at a point prior to entering the rotating plasma apparatus or it may be injected or mixed into the main cold gas flow within the plasma device at the point where it enters the plasma torch assembly. A non-transferred high-pressure plasma is established between the cathode electrode and the anode nozzle within the plasma torch forming a plasma flame, into which a high-pressure flow of a mixture of gas and powder particles is caused to converge coaxially into the plasma flame. The high-pressure gas flow can be air or it can be an inert gas such as argon or helium or a non-reactive gas such as nitrogen. The powder particle temperature is elevated to a level below its thermal softening point. The heated particles in the gas stream consisting of the high temperature plasma gas along with the converged high pressure gas flow is caused to flow through an accelerating nozzle such as an extended straight nozzle or a de Laval nozzle to accelerate the gas powder mixture to a high velocity. A sonic or supersonic jet of the hot gas mixture of plasma/gas/powder is produced from the accelerating nozzle and directed as a sonic or supersonic jet of hot gases and particles towards a workpiece surface to be coated. The centerline of the plasma generator and the accelerating nozzle are coaxially aligned. However the axis of rotation of the plasma generator and accelerating nozzle is perpendicular to the axis of rotation of the assembly. As the assembly is rotated and the assembly is traversed axially along the internal surface of the bore is coated. The improvement lies in rotating the plasma generator and accelerating nozzle perpendicular to the axis of rotation, about the axis of rotation, and in the feeding of powder particle material typically with a particle size range greater than 50 microns entrained in a high pressure, high volume carrier gas (typically compressed air) coaxially converging into the plasma flame of the high pressure plasma generator and flowing the plasma/gas/powder mixture into and through an accelerating nozzle such as a straight bore nozzle or a de Laval nozzle, thereby controllably heating the powder particles to a point lower than their thermal softening point and maintaining the in-transit temperature of the particle below the melting point and providing sufficient velocity to the particles to achieve an impact energy upon impact with the workpiece surface capable of transforming the kinetic energy of the particles to cause elastic deformation to the particles causing them to adhere to the workpiece surface and cohesively build-up a coating thereby forming a dense coating while rotating the plasma apparatus perpendicularly about an axis of rotation. 
     Accordingly, it is an object of the invention to provide an improved high pressure plasma spray apparatus for applying a coating utilizing particle kinetics. 
     A further object of the invention is to provide a high pressure plasma apparatus and process in which a sonic or supersonic gas jet is created to cause heating of powder particles typically greater than 50 microns, to a temperature below their melting point and accelerating them to a velocity such that when they impact with the coating surface, their kinetic energy is transformed into plastic deformation of the particles causing them to adhere to the workpiece surface and build-up a coating thereby forming a dense coating. 
     Yet another object of the invention is to provide a high-pressure plasma apparatus and process suitable for coating the internal surfaces of a bore, cylinder or concave surface in which a sonic or supersonic gas jet is created to cause heating of powder particles typically greater than 50 microns, to a temperature below their melting point and accelerating them to a velocity such that when they impact with the coating surface, their kinetic energy is transformed into plastic deformation of the particles causing them to adhere to the workpiece surface and build-up a coating by providing a means of rotation to the high-pressure plasma apparatus such that the plasma assembly is perpendicularly oriented with respect to the axis of rotation. 
     A further object of the invention is to provide a method and apparatus for producing high performance well bonded coatings, which are substantially uniform in composition and have very high density with very low oxides content formed within the coating. 
     Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification. 
     The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to of the others, and the apparatus embodying features of construction, combination of elements, and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the invention, reference is made to the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a high-pressure plasma spray apparatus (HPPS) constructed in accordance with an embodiment of the invention. 
         FIG. 2  is a cross-sectional view of a HPPS apparatus constructed in accordance with an embodiment of the invention, which includes the use of an extended straight bore nozzle. 
         FIG. 3  is a cross-sectional view of a HPPS apparatus constructed in accordance with an embodiment of the invention, which includes the use of an extended de Laval nozzle. 
         FIG. 4  is a cross-sectional view of a HPPS apparatus constructed in accordance with an embodiment of the invention, which includes the use of an extended straight bore nozzle and illustrates an alternative means of injecting powder particles upstream of the converging point of the plasma flame and the cold high-pressure gas flow. 
         FIG. 5  is a cross-sectional diagram of a HPPS apparatus constructed in accordance with an embodiment of the invention, which includes means for rotating the HPPA perpendicularly about an axis of rotation in order to deposit a coating on the internal surface of a bore, cylinder or concave surface. 
       FIG. ( 6 ) is an end view diagram of a HPPS apparatus constructed in accordance with an embodiment of the invention, which includes means for rotating the HPPA apparatus perpendicularly about an axis of rotation. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference is first made to  FIG. 1  in which a high-velocity plasma spray apparatus constructed in accordance with the invention includes a high pressure plasma spray (HPPS) assembly  10 , a high pressure powder feeder assembly  20 , a plasma power supply  30 , a system control console  40  and a gas module  50 . A high pressure plasma gas  11  which typically could be argon, nitrogen or a mixture of argon/hydrogen and having a pressure of between 200 psig and 600 psig, is fed to the gas module  50  through hose  12  and them fed from the gas module  50  through hose  13  to the HPPS torch assembly  10 . Electrical power is supplied to the HPPS  10  from the plasma power supply  30  by means of cables  31  and  32 . High-pressure compressed gas  14 , which can be air, nitrogen, helium or any mixture of these gases and having a pressure of between 200 psig and 600 psig, is supplied to the gas module  50  by means of hose  15  and then fed to the HPPS torch assembly through hose  16 . The high pressure carrier gas  17  having a pressure of between 200 psig and 600 psig is supplied to the gas module  50  through hose  18  and then fed from the gas module  50  to the high-pressure powder feeder  20  by means of hose  19 . From the high pressure powder feeder  20  high pressure carrier gas  17  with powder feed stock entrained in it by the high pressure powder feeder  20  is fed to the HPPS  10  by means of hose  21 . A system control assembly  40  controls the plasma power supply  30  as well as the gas module  50  and the high pressure powder feeder  20 . 
     Reference is now made to  FIG. 2  in which an enlarged cross-sectional view of a HPPS torch assembly  10  is shown. The HPPS torch assembly includes a housing  101 . A gas inlet block  102  is disposed within the housing  101  coaxially with a cathode support  103 . A cathode assembly  104  is attached to the cathode support block  103  and coaxial therewith. A cup-shaped plasma nozzle  105  is disposed about cathode  104  and the cathode support block  103  and the cathode assembly  104  are coaxially aligned within the plasma nozzle support block  106  and electrically insulated from the plasma nozzle by means of insulating sleeve  107  also coaxially aligned with the cathode support block  103  and the cathode assembly  104 . 
     Gas inlet block  102  is formed with a plasma gas inlet port which receives plasma gas and provides its passage through cathode support  103  exiting through tangentially oriented ports  109 , formed within the cathode support. Ports  109  communicate at a right angle with a chamber  110  formed between the cathode electrode  104  and the inner surface of the cup shape plasma nozzle  105 . As the plasma gas exits the tangential ports  109  into chamber  110 , which is formed between the cathode assembly  104  and the plasma nozzle  105 , the plasma gas is formed into a strong vortex flow around the cathode  104  and exits the plasma nozzle constricting orifice.  111  formed within the plasma nozzle  105 . 
     A cup shaped main gas nozzle  112  is disposed about plasma nozzle  105 . A high pressure main gas is fed into a main gas inlet port  113  located in the gas inlet block  102 . The main high pressure gas flows through the gas inlet block  102  to a manifold  114  within the gas inlet block  102  which the passes through a series of ports  115  within the cathode support  103 . The main gas is then caused to flow in an evenly distributed manner into and through ports  116  in thee electrical insulator  107 . A carrier gas and powder inlet tube  117  is located so that it can direct the carrier gas and powder into the main gas flow at a point  118  which is located such that this carrier gas and powder mixes with and evenly distributes itself into the main gas flow within the electrical insulator  107 . It should be understood that the carrier gas and powder can also be mixed into the main gas flow prior to the main gas entering the HPPS torch at the main gas inlet port  113 , thereby eliminating the need for a separate carrier gas and powder inlet tube  117 . The combined main gas and carrier gas with the powder particles evenly distributed within, flows into a manifold formed between the plasma nozzle  105  and the cup shaped gas nozzle  112  and then flows through the conically shaped space  120  formed between the cup shaped gas nozzle  112  and the outer surface of the plasma nozzle causing the combined gas flow to coaxially converge at a point  121  downstream of the plasma nozzle  105 . The negative output of the power supply  30  is connected through lead  32  to the central cathode electrode  104  of the HPPS torch assembly  10 . The positive output of the power supply  30  is connected to the plasma nozzle through electrical power lead  31  so that the plasma nozzle is an anode. 
     Downstream from the plasma nozzle  105  and coaxially aligned with the plasma nozzle  105  and the cup shaped main gas nozzle  112  is a extended straight bore nozzle  122  which is attached and is a part of the HPPS torch assembly  10 . This extended straight bore nozzle  122  is constructed such that its length is at least six (6) times longer than the diameter of its bore. The purpose of the extended bore nozzle  122  to provide a means of causing the total gas flow from the plasma torch  10  with powder particle entrained in the gas to be accelerated to sonic or supersonic speeds, thereby providing the kinetic energy to the powder particles  125  necessary to form a cohesively bonded coating  124  upon impact with the work surface  123 . 
     In operation of the system, a high pressure plasma gas  11  is caused to flow through hose  12  to the gas module  50  and then through hose  13  to the HPPS torch assembly  10 . Additionally high pressure main gas  14  is caused to flow through hose  15  to the gas module  50  and then through hose  16  to the HPPS torch assembly. After an initial period of time, typically two seconds, DC power supply  30  is electrically energized as well as the high frequency generator  33  which is internal to the power supply  30  causing a pilot plasma to be momentarily established. This pilot plasma causes the formation of a high-energy DC plasma formed by an arc current established between the cathode  104  and the plasma nozzle  105 . Instantly with the establishment of the high energy DC plasma, the high frequency generator  33  is de-energized. The DC high energy plasma causes a stream of high pressure hot, ionized gas to flow out of the plasma nozzle  105  mixing with the converging cold high pressure main gas thereby causing the cold main gas to be heated to a controllably set temperature. Once the plasma has been established in a stable manner, high pressure carrier gas  17  is caused to flow through hose  18  to the gas module  50  and then through hose  19  to the high pressure powder feeder  20 . Powder particles of feed stock material are entrained in the carrier gas  17  as it flows through the powder feeder  20  and are caused to flow through hose  21  to the HPPS torch assembly  10  where the high pressure carrier gas  17  and powder enters the torch assembly  10  through tube  17  and is mixed into the cold high pressure main gas  14  at a point  18  so that the carrier gas  17  and powder particles can be distributed within the main gas flow before the gases enter and flow through the conically shaped passage  120  formed between the outer surface of the plasma nozzle and the inner surface of the cup shaped main gas nozzle  112 . As the cold main gas  14  mixed with the cold carrier gas  17  with the powder particle entrained exits the conically shaped passage  120  it converges and mixes with the axial flow of the hot, ionized plasma gas which is exiting the plasma nozzle  105 . The mixing of the hot and cold gases results in a gas temperature which is controllable and is based on the volume, temperature and enthalpy of the plasma gas and the volume and temperature of the main gas mixture and is desirably adjusted to a temperature which is as high as possible while not exceeding the melting or softening point of the powder material. 
     Reference is now made to  FIG. 3  in which a preferred embodiment of the invention is shown. Like numbers are utilized to indicate like parts, the difference between the embodiment of FIG.  2  and that of  FIG. 3  being the use of a de Laval nozzle  126  instead of the straight bore nozzle  122 . The de Laval nozzle consists of three sections, the convergent section  127  and the divergent section  128  and the critical orifice  129 . The employment of a de Laval nozzle  126  provides for improved fluid dynamic flow resulting in producing higher velocities of the exiting gas thereby accelerating the powder feedstock entrained within the gas to higher velocities. This higher velocity of the powder feedstock is required to produce improved coating efficiencies as well as higher coating quality. 
     In reference to  FIG. 4 , this cross-sectional drawing of the HPPS torch is the same as the previously described HPPS torch assembly of this invention as shown in  FIG. 2  with the exception that an alternative point  130  is illustrated for the injection of the carrier gas and powder as compared to the injection point  118  of FIG.  2 . Like numbers are utilized to indicate like parts. As is shown, the point  130  is located within the conically shaped space  120  formed between the cup shaped gas nozzle  112  and the outer surface of the plasma nozzle  105 . Injecting the carrier gas and powder into the main gas flow at this point  130  provides the same advantage as injecting it at a point upstream in the main gas flow such as at point  118  of  FIG. 2  or even to pre-mix the carrier gas and powder with the main gas before the main gas flow enters the HPPS torch assembly at main gas inlet port  113 . 
     Reference is now made to FIGS. ( 5 ) and ( 6 ) in which a cross-section and end view diagram of a HPPS assembly  10  to be employed in a manner suitable for depositing a uniform coating  140  on the concave surface such as a bore  141  is shown. This embodiment includes a HPPS torch assembly  10  similar to HPPS torch assembly  10  described in FIG. ( 2 ), the difference being that HPPS torch assembly  10  is mounted on a rotating member  142  to allow rotation concentrically with respect to bore  141  by means of a motor drive, not shown. 
     The HPPS rotating spray assembly consists of a HPPS torch assembly  10  and a rotating union assembly  11 , which typically can be a commercial two-port rotating union such as a Model No. 1590 manufactured by the Deublin Company. The rotating union  11  consists of a stationary gas block  142  and a rotating member  143 . Contained on the gas inlet block  142  are a main gas inlet port  144  and a plasma gas inlet port  146 . Contained within the rotating union  11  are a passageway  145 , which is a central duct through which the main gas with powder feedstock particle entrained therein flows through, and a passageway  147  through which the plasma gas flows. Attached to the rotating member  143  of the rotary union  11  is a HPPS torch assembly  10 . HPPS torch assembly  10  is mounted at an end of rotating member  142  opposite that of stationary block  143  on the radius of rotating member  142  so that the central axis of the HPPS torch assembly  10  is perpendicular axis of rotation. The HPPS torch assembly  10  is mounted onto the rotating member  143  of the rotary union in such a manner so that the gas passageway  143  of the rotary union  11  is aligned with passageway  148  in the HPPS torch assembly  10  and passageway  147  of the rotary union  11  is aligned with passageway  149  of the HPPS torch assembly  10 , thereby providing means for the main gas with powder feedstock particle entrained therein as well as the plasma gas to flow into and through passageways  148  and  149  respectively in the HPPS torch assembly  10 . Electrical power is brought to the HPPS torch assembly from the plasma power supply  30  of FIG. ( 1 ). The negative connection is brought from the power supply  30  through lead  32  to the stationary block  142  and then is conducted through the body of rotary union  11  to the cathode block  150  of the HPPS torch assembly. Surrounding the cathode block  150  is an insulating sleeve  151  providing electrical insulation between the cathode body  150  and thee plasma anode nozzle  105 . Additionally, electrical insulation is provided between the cathode block  150  and the anode plasma nozzle  105  by means of insulating sleeve  153 . The positive connection from the plasma power supply  30  to the HPPS torch assembly  10  is made through lead  31  which is connected to a brush assembly  154  which commutates the electrical power to an outer jacket  155  which is electrically connected to the plasma anode nozzle  105 . Insulating sleeve  153  additionally serves to manifold the main gas and powder flow in order to uniformly distribute this flow through the passageway  120  which is formed between the outer surface of the plasma anode nozzle  105  and the inner surface of the cup shaped nozzle  112 . The functioning of the HPPS torch assembly  10  of this HPPS rotating assembly is similar to the function and operation of the HPPS torch assembly  10  of FIG. ( 2 ) whereby the cold main gas with powder particles entrained therein is caused to flow into a high temperature plasma which is emanating from the plasma anode nozzle  105 . As the two gas streams mix, the temperature of the cold main gas is raise to a high temperature limited to be below the melting or softening point of the powder material. The velocity of the now heated gas and powder stream is accelerated to sonic or supersonic velocity as the gas stream flows through the de Laval nozzle  126 . As the high velocity powder particles exit the de Laval nozzle  126  they deposit themselves onto the inner surface of the bore  141 . As the coating process proceeds, the HPPS torch assembly is caused to rotate about the centerline of the bore  141  while simultaneously being laterally traversed through the bore  141  thus forming a dense coating buildup  140  uniformly over the desired area of the inner surface of the bore  141 . 
     In the prior art, it has been commonly known that if it is desired to apply a thermal spray coating to an internal surface, prior art cold gas dynamic spray and kinetic spray devices as well as most thermal spray apparatuses, equipped with a deflector head, deflecting the spray pattern 90° is employed and the part to be coated is independently rotated while the spray apparatus is reciprocated up and back along the axis of the concave surface. However, it is not always practical or possible to rotate the part to be coated, such as an automobile engine block, when it is desired to apply a coating to the cylinder bores contained within the engine block. By providing a HPPS torch assembly which is rotatably mounted and rotated about the centerline of a bore while being radially positioned relative to the bore axis a practical process for applying a coating to the inner surface of a concave structure such as a bore is provided. 
     It will thus be seen that the objects set forth above, among those made apparent from the preceding descriptions, are efficiently attained and, since certain changes may be made in carrying out the above method and in the constructions set forth without departing from the spirit and the scope of the invention, it is intended that all matter contained in the above descriptions and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
     It is also to be understood that the following claims are intended to cover all the generic and specific features of the invention herein described and all statements of the scope of the invention, which, as a matter language, might be said to fall there between.