Abstract:
The present invention relates to a plasmatron for a plasma coating apparatus including an anode having an axial bore through which gas is passed around a cathode and an electric arc is established between the anode and cathode. A powder feed line or conduit is connected between the anode and a powder feed source. The feed line has a straight section along a portion of its length terminating at the axial bore of the anode. The straight section of feed line has a ratio of length to internal diameter at least about 4.8, preferably 10 and even more preferably 15.

Description:
This is a continued prosecution application of application Ser. No. 09/506,621, filed Feb. 18, 2000, which claimed priority from provisional application Ser. No. 60/121,976, filed Feb. 27, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to plasma coating of coating materials on a substrate, and more particularly to equipment and processes for injecting powder into a plasma coating machine. 
     2. Description of the Prior Art 
     One of the ongoing problems in the plasma coating industry is “spitting” of prematurely molten powder from the plasmatron or plasma gun. Spitting becomes more of a problem when coating large parts, continuously over a relatively long period. Spitting can occur when applying metal and/or ceramic coatings. When spitting occurs the affected parts have to be stripped and re-coated, and this has a major cost impact. 
     Several factors are believed to contribute to spitting. One of the major factors is the way powdered material is injected into the plasma stream. Most plasma systems have plasma guns with external powder injection, whereby the powdered coating materials are injected into the plasma stream outside of the anode of the plasma gun through use of inert gas and a tubular injection conduit. Although this outside powder injection reduces the risk of spitting, it also reduces the deposit efficiency as well as the coating quality. 
     The most efficient way of conventionally injecting powder into a plasma stream is internally, that is, whereby the powdered coating materials are injected into a space between the anode and cathode of the plasma gun through use of inert gas and a tubular injection conduit. This internal powder injection method produces superior quality coatings, but does so at an increased risk of spitting. 
     Other factors that contribute to spitting are believed to be distance between the arc discharge and the point injection; the temperature of the plasma; and the energy concentration of the plasma. It is known that powder injection close to the arc discharge into a very hot and concentrated plasma stream with high energy density can result in premature melting of the powdered material. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The present invention relates to equipment and processes for positioning powder injection ports in a plasma coating machine to minimize and/or prevent spitting of prematurely molten powder. 
     In accordance with the present invention, it has been discovered that not only are the injection angles relative to the plasma stream, important both in axial and radial direction, but also that the length of the powder injection conduit in relation to its diameter has a significant influence on spitting. It has been discovered that if the length of the powder injection conduit adjacent the entrance to the plasma stream is greater than 4.8 times its diameter, without any substantial interruption or deviation in diameter throughout this length, spitting is reduced by a factor of about 4. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become apparent from the foregoing detailed description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a schematic sectional elevation view of a plasmatron in a plasma coating system, shown depositing a coating onto a substrate using a coating material in powder form; 
     FIG. 2 is a schematic sectional elevation view of a prior art anode in a plasmatron of the type shown in FIG. 1; 
     FIG. 3 is a schematic sectional elevation view of a prior art plasmatron; 
     FIG. 4 is a schematic sectional elevation view of a preferred embodiment of a plasmatron in accordance with the present invention; 
     FIG. 5 is a schematic elevation view of an alternate embodiment of a plasmatron in accordance with the present invention; 
     FIG. 6 is a schematic sectional elevation view of another alternate embodiment of a plasmatron in accordance with the present invention; and 
     FIG. 7 is a sectional elevation view of the most preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings, wherein like reference numerals designate like or corresponding parts, and more particularly to FIG. 1 thereof, an apparatus  30  is shown for plasma deposition of materials onto a substrate. The plasma spray machine  30 , made by Electro-plasma, Inc. in Irvine, Calif., is available now from Sulcer Metco Company in Switzerland. It is used primarily for depositing nickel alloys and other specialized material on various parts, such as turbine blades for protection from the high temperature erosion influences in jet turbine engines. 
     The plasma spray machine  30  includes a main chamber  35  within which a low pressure, inert gas atmosphere can be established. The enclosure includes a transfer chamber  40  through which parts can be passed into and out of the main chamber  35  without contaminating the atmosphere in the main chamber  35  or affecting the gas pressure therein. Gas feed and exhaust lines connect to fittings on the main chamber  35  and the transfer chamber  40  for exhausting and purging to establish the desired atmospheric composition and pressure. A plasmatron  50  is disposed in the main chamber  35 , preferably on a robotic arm  55  by which the plasmatron  50  can be manipulated remotely within the chamber  35  by controls outside the chamber. 
     With reference to FIG. 2, the plasmatron  50  has a nozzle  60  and a conical cavity  65  within which a cathode  70  is suspended centrally, and an annular passage  72 . The cathode  70  and the wall of the conical cavity  65  of the anode  60  are separated by an annular gap of about 0.150″. This gap is commonly referred to as the “anode through diameter”. 
     In operation, the main chamber  35  is evacuated to a pressure of about 50 militorr through one of the gas lines by a vacuum pump  75 , and then backfilled with clean (99.995% pure) nitrogen or argon gas to about 300 torr. The chamber  35  is again evacuated to about 50 militorr and recharged with inert or non-reactive gasses such as argon or a mixture of argon and helium or argon and hydrogen to an operating pressure of about 30 torr. The hydrogen moiety is believed to function as an oxygen getter in the chamber, i.e., to reduce the oxygen content in the powder coating materials to negligible amounts, on the order of 15-30 ppm or less in materials that must have a low oxygen content. 
     One or more parts  80  are transferred into the chamber  35  from the transfer chamber  40 . This transfer chamber  40  had been evacuated during each transfer operation to between 50 to 100 militorr, and backfilled to 100 torr, evacuated again and backfilled to 30 torr before opening the transfer valve. A part  80  previously put into the transfer  40  chamber is then manipulated into position under the plasmatron  50  by use of conventional, remotely operable manipulating equipment  85  in preparation for the coating operation. 
     Two powder sources  90  (only one of which is illustrated in FIG. 2) of known design and commercially available are filled with a powder of the coating material and evacuated to 50 militorr, then backfilled with pure argon to 4 psig. This process is repeated two more times to minimize the oxygen content in the powder feeders and in the powder. The powder is a gas atomized powder having particle diameters in the range of 10-45 micrometers. Powders used in these machines and processes are well known and commercially available from numerous suppliers. 
     During the coating process, a plasma gas comprising a mixture of about 82% argon and 18% hydrogen is flowed through the annular passage  72  at a rate of about 150 schf argon and 34 schf hydrogen. A conventional DC plasma power supply  95  is energized to create an arc in the passage  72 , and 71.5 kW of power is applied at 1300 amp and 55 volts. 
     The plasma gas exits the nozzle  60  in a plasma gas stream at high temperature and velocity, and impinges on a part or substrate  105  positioned about 17 inches below the nozzle  60 . The temperature of the part surface rises quickly to about 400° C. whereupon a negative DC transfer arc power supply  110  is energized to cause electrons to flow in a reverse transfer arc  115  out of the heated substrate surface and to flow countercurrent through the plasma gas stream  100  to the nozzle  60  (or to a separate electrode coupled to the plasma gas stream  100 , not shown). The action of the reverse transfer arc  115  preferentially discharges at the substrate surface where oxides and other contaminants exist, and acts to vaporize and otherwise eliminate the contaminants until the substrate surface is metallurgically clean. 
     The powder feeders  90  are then turned on to feed powder at a rate of about 50 grams/minute with a carrier gas flowing at a rate of about 15 schf. The powder is entrained in the plasma gas stream  100  and ejected from the nozzle  60  at supersonic speeds. It travels with the plasma gas stream in a diverging or conically shaped flow pattern, and impacts against the substrate surface at high speed. The impact of the high energy and partially melted powder particles on the extremely clean substrate surface result in diffusion and flattening of the powder particles when they impact against the substrate surface. The diffusion of the powder particles into the substrate surface results in an intimate bond between the powder coating and the substrate surface. 
     In the standard design of the tubular powder injector, the powder injection conduit includes an external feed tube and a channel in the anode. The external tube is connected to and feeds into the tubular path a hole or channel in the anode, as shown in FIG.  3 . The internal channel in the anode has a bend in it, and the ratio of the distance between this bend and the powder exit orifice at the anode wall is less than 4.8. As such, this powder injection conduit has a longitudinal centerline, or axis having two legs or sections at an acute angle to each other as shown in FIG.  3 . It has been discovered that this bend, which is adjacent to or closest to the plasma stream creates turbulence of the powder flow within the conduit. The bent axis designs are believed to create the following problems: 
     1. High and low pressure zones are created in the powder injection conduit, which causes the finer particles to slow down relative to the larger particles, and this results in premature melting of the finer particles, while the larger particles continue moving along the conduit; 
     2. The bend in the powder injection conduit causes the powder stream to deflect and bounce off the conduit wall just before injection into the plasma stream at angles that are not the proper or ideal angle of injection; and 
     3. With an injection conduit length to diameter ratio of less than 4.8:1, due to the above two phenomena, the variously sized, entrained powder particles move at varying velocities and at different directions at the time of injection, thus causing spitting. 
     It has been discovered that using a powder injection conduit having a tubular shape and a length to diameter ratio of about 15:1, the following improvements were achieved: 
     1. Lowered risk of spitting by a factor of at least 4; 
     2. Increased deposition efficiency by about 3%, due to precise powder injection; 
     3. Improved coating quality due to a relatively lower amount of unmelted particles. 
     In accordance with the present invention it has been found that the risk of spitting with an internal powder injection plasma gun is reduced when the powder injection conduit is a tube, its length-to-diameter ratio is at least 4.8:1 and the tube is essentially straight over this length. Also, it has been discovered that the preferred powder injection tube length-to-diameter ratio is 10:1 or greater. 
     It has also been discovered that in the most preferred embodiment, the ratio of the powder injection tube diameter to anode through diameter is at least 4:1, and the ratio of the powder injection tube, in its straight portion, to its diameter is at least 4.8:1, and preferably 10:1 or greater. 
     Also, although tubular shaped powder injection lines are preferred, other cross sectional shapes, such as square, rectangular, oval, and triangular, are considered to be equivalent so long as they provide substantially the same results as do tubular-shaped powder injection lines. 
     It is understood that the above-described preferred embodiments examples, and figures are illustrative of the general principals of the present invention. Other formulations, arrangements, assemblies and materials may be used by those skilled in this art and embody the principals of the present invention. It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations as they are outlined within the description above and within the claims appended hereto. While the preferred embodiments and application of the invention have been described, it is apparent to those skilled in the art that the objects and features of the present invention are only limited as set forth in the claims appended hereto.