Abstract:
The invention relates to a plasma torch, comprising: a plasma generator comprising a cathode extending along an axis X and an anode ( 24 ), the cathode and the anode being arranged so as to be capable of generating, in a chamber ( 26 ), an electric arc between the anode and the cathode due to an electrical voltage, the plasma generator also comprising a plasmagen gas injection device ( 30 ) comprising an injection pipe ( 72 ) leading, along an injection axis (I i ), to an injection opening ( 74 ) in the chamber; a means for injecting a material to be discharged into a plasma flow generated by said plasma generator, the plasma torch being characterized in that: the relationship R″ between: the radial distance (y i ) of said injection opening, defined as the minimum distance between the axis X and the center of said injection orifice; the largest transverse size (D C ) of the cathode in the region of the chamber downstream from the position P AC , wherein P AC  denotes the axial position of maximum radial mutual encroachment of the anode and the cathode, is less than 2.5; and the projection of the injection axis (I i ) into a transverse plane passing through the center of the injection orifice of said injection conduit forms an angle β less than 45° with a radius extending into said transverse plane and passing through the axis X and through the center of said injection orifice.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a plasma generator and a plasma torch employing such a plasma generator. 
         [0002]    Plasma spraying is used to form a coating on a substrate. It generally consists in producing an electric arc, in blowing a plasmagen gas through this electric arc so as to generate a very high-temperature, high-speed plasma flux, then in injecting into this plasma flux particles so as to spray them onto the substrate. The particles melt, at least partially, in the plasma and can thus adhere well to one another and to the substrate when they cool. This technique may thus be used to coat the surface of a substrate made of a metal, ceramic, cermet, polymer, organic material or a composite, in particular a composite comprising an organic matrix. This technique is especially used to coat parts having various shapes that have for example planar or axisymmetric geometries, especially cylindrical geometries, or complex geometries, these parts possibly having various sizes—the only limit being access by the jet of particles. The aim may be, for example, to provide a substrate with a surface functionality such as wear resistance, or to modify the friction coefficient, the thermal barrier or the electrical insulation. 
         [0003]    This technique may also be used to manufacture bulk parts, by way of a technique called “plasma forming”. By virtue of this technique it is thus possible to apply a coating a number of millimeters in thickness, even more than 10 mm in thickness. 
         [0004]    Plasma torches, or plasmatrons, are for example described in WO 96/18283, U.S. Pat. No. 5,406,046, U.S. Pat. No. 5,332,885, WO 01/05198 or WO 95/35647 or U.S. Pat. No. 5,420,391. 
         [0005]    The performance parameters of a plasma torch for industrial purposes may be said to be the following:
       high spray productivity, the spray productivity being defined as the amount of material deposited per unit time;   high deposition efficiency, the deposition efficiency being defined as the ratio, in wt %, between the amount of material deposited and the amount of material injected into the plasma flux;   maximum coating quality, and in particular the ability to produce a uniform and reproducible coating, including with a high material flow rate;   minimum energy consumption;   lowest possible maintenance time with the highest possible time interval between two consecutive maintenance operations; and   reduced contamination via loss of the cathode material.       
 
         [0012]    One object of the exemplary embodiments is to provide a plasma torch that at least partially meets these criteria. 
       SUMMARY OF THE INVENTION 
       [0013]    For this purpose, exemplary embodiments include a plasma generator comprising:
       a cathode extending along an axis X and an anode, the cathode and the anode being placed so as to be able to generate, in a chamber, an electric arc between the anode and the cathode under the effect of a voltage; and   a device for injecting a plasmagen gas comprising an injection duct opening via an injection orifice into the chamber.       
 
         [0016]    In a first principal embodiment, the ratio R between:
       the axial distance x between the axial position p AC  of minimum radial distance between the anode and cathode and the axial position p i  of said injection orifice; and   the largest transverse dimension D C  of the cathode in the region of the chamber downstream of the position p AC , called the “arc chamber”,
 
is smaller than 3.2, preferably smaller than 2.5 and/or larger than 0.5.
       
 
         [0019]    In a second principal embodiment, the ratio R′ between:
       the axial distance x′ separating the axial position p C  of the downstream end of the cathode and the axial position p i  of said injection orifice; and   the largest transverse dimension D C  of the cathode in the arc chamber,
 
is smaller than 3.5, preferably smaller than 3.0 and/or larger than 1.2.
       
 
         [0022]    In a third principal embodiment, the ratio R″ between:
       the radial distance y i  of said injection orifice, defined as the minimum distance between the axis X and the center of said injection orifice; and   the largest transverse dimension D C  of the cathode in the arc chamber,
 
is smaller than 2.5 and preferably larger than 1.25.
       
 
         [0025]    Whatever the principal embodiment considered, the inventors have observed that a plasma generator according to exemplary embodiments enables deposition with a very high productivity and efficiency and with a limited amount of electricity consumption and a limited contamination by the cathode. 
         [0026]    In particular, the third principal embodiment provides excellent performance when the plasmagen gas turns around the cathode, forming a vortex. 
         [0027]    Whatever the principal embodiment considered, preferably, a plasma generator according to exemplary embodiments may also comprise one or more features of the other principal embodiments. It may furthermore have one or more of the following optional features:
       among the set of injection orifices of said injection device, said injection orifice is that or one of those having the furthest downstream axial position;   the axial distance x is preferably shorter than 25 mm, preferably shorter than 18 mm and/or preferably longer than 5 mm, a distance x of about 13 mm being particularly well suited;   the axial distance x′ is preferably shorter than 30 mm, preferably shorter than 25 mm and/or preferably longer than 9 mm, even longer than 15 mm, a distance x′ of about 20 mm being particularly well suited;   the radial distance y i  is preferably shorter than 27 mm, preferably shorter than 20 mm, even shorter than 15 mm and/or preferably longer than 6 mm, even longer than 10 mm, a distance y of about 12 mm being particularly well suited;   the axial distance x″ separating the axial position p AC  from the axial position p A  of the furthest downstream point of the anode is preferably shorter than 60 mm, preferably shorter than 50 mm and/or preferably longer than 30 mm, a distance x″ of about 45 mm being particularly well suited;   the ratio R′″ between the minimum radial distance y AC  between the anode and the cathode in the axial position p AC  and the largest transverse dimension D C  of the cathode in the arc chamber is preferably smaller than 1.25, preferably smaller than 0.5 and preferably larger than 0.1, preferably larger than 0.2, a ratio R′″ of about 0.3 being particularly well suited; and   the injection device comprises a plurality of injection orifices, at least one of the conditions, and preferably all the conditions, imposed on the ratios R, R′, and R″, and on the distances x, x′, x″ and y, being true whichever injection orifice is considered.   The injection device is an injection device according to exemplary embodiments, as described below.   The cathode comprises, at its free end, a conical portion, preferably having a pointed or rounded shape. The angle δ at the apex of this conical portion is preferably larger than 30°, preferably larger than 40° and/or smaller than 75°, preferably smaller than 60°. The length, along the axis of the cathode, of the conical portion is preferably longer than 3 mm and/or shorter than 15 mm, preferably shorter than 8 mm. The largest diameter of this conical portion (at its base) is preferably larger than 6 mm, preferably larger than 8 mm and/or smaller than 14 mm, preferably smaller than 10 mm. Preferably, the free end of the conical portion is rounded, the radius of curvature of this end preferably being greater than 1 mm and/or less than 4 min.   The cathode comprises, preferably immediately upstream of the conical portion, a cylindrical portion. The cylindrical portion preferably has a length longer than 5 mm, preferably longer than 8 mm and/or shorter than 50 mm, preferably shorter than 25 mm, more preferably shorter than 20 mm, preferably shorter than 15 mm. The cylindrical portion preferably has a circular cross section and a diameter larger than 4 mm, preferably larger than 6 mm, preferably larger than 8 mm and/or smaller than 20 mm, preferably smaller than 14 mm, more preferably smaller than 10 mm. Preferably, the cylindrical portion has a diameter substantially equal to the largest diameter of the conical portion, so as to extend continuously from the latter.   Preferably, the cathode comprises, preferably immediately upstream of the cylindrical portion, a frustoconical portion. Preferably, the frustoconical portion extends as far as the back (referenced  59  in  FIG. 2 ) of the chamber in which the electric arc is generated. Preferably, the angle at the apex γ of this frustoconical portion is larger than 10°, preferably larger than 30° and/or smaller than 90°, preferably smaller than 45°. The length of the frustoconical portion may be longer than 5 mm and/or shorter than 15 mm. Preferably, the largest diameter of the frustoconical portion is larger than 6 mm, preferably larger than 10 mm and/or smaller than 30 mm, preferably smaller than 20 mm, more preferably smaller than 18 mm and/or the smallest diameter of said frustoconical portion is larger than 4 mm, preferably larger than 6 mm, preferably larger than 8 mm and/or smaller than 20 mm, preferably smaller than 14 mm, more preferably smaller than 10 mm. Preferably, this smallest diameter is equal to the diameter of the cylindrical portion, so that the frustoconical portion prolongs the cylindrical portion.   In one embodiment, the length of the conical portion is shorter than the length of the cylindrical portion. The ratio between the length of the conical portion and the length of the cylindrical portion may in particular be larger than 0.5 and/or smaller than 1.   In one embodiment, the length of the cylindrical portion is substantially identical to the length of the frustoconical portion.   Preferably, the cathode comprises a cylindrical portion, preferably of circular cross section, preferably prolonged coaxially, into the arc chamber, by a conical portion. More preferably, the cathode comprises, coaxially, a frustoconical portion prolonged by a cylindrical portion, preferably of circular cross section, preferably prolonged, into the arc chamber, by a conical portion.   Preferably, the cathode comprises a frustoconical portion and at least one, preferably all the injection orifices are placed in one or more transverse planes cutting said frustoconical portion. In one embodiment, all the injection orifices may be located in the same transverse plane. This transverse plane may be placed, for example, at a distance from the base of the frustoconical portion (corresponding to the largest diameter of the frustoconical portion) lying between 30% and 90%, preferably between 40% and 70% of the length of the frustoconical portion.   The cathode is a blown-arc plasma cathode, preferably a rod-type hot cathode.   In one embodiment, the cathode may be a single part, i.e. made of a single material. In another embodiment, the cathode comprises a rod of tungsten and a copper part, into which the tungsten rod is inserted.   The chamber comprises a cylindrical part upstream and/or an intermediate convergent part (convergent in the downstream direction) and/or a downstream cylindrical part. The intermediate convergent part may especially be frustoconical or comprise a plurality of frustoconical parts, in particular two frustoconical parts, extending coaxially prolonging each other (i.e. without a step at the transition between these frustoconical parts). Preferably, the angle at the apex ψ 1  of a first frustoconical part upstream of a second frustoconical part is larger than the angle at the apex ψ 2  of said second frustoconical part. The angle at the apex ψ 1  may in particular lie between 50 and 70°, The angle at the apex ψ 2  may in particular lie between 10 and 20°.   Preferably, the chamber comprises in succession, and coaxially from upstream to downstream, an upstream cylindrical part, an intermediate convergent part and a downstream cylindrical part. Preferably, the length of the upstream cylindrical part is longer than 5 mm and/or shorter than 40 mm, preferably shorter than 20 mm. Preferably, the length of the intermediate convergent part is longer than 10 mm and/or shorter than 80 mm, preferably shorter than 40 mm and preferably longer than 20 mm and/or shorter than 30 mm. Preferably, the length of the downstream cylindrical part is longer than 10 mm and/or shorter than 80 mm, preferably shorter than 40 mm and preferably longer than 20 mm and/or shorter than 30 mm.   Preferably, the diameter of the upstream cylindrical part is larger than 10 mm, preferably larger than 15 mm and/or smaller than 70 mm, preferably smaller than 40 mm, preferably smaller than 30 mm.   The largest diameter of the intermediate convergent part (base) is larger than 15 mm and/or smaller than 40 mm, preferably smaller than 25 mm. Preferably, the diameter of the upstream cylindrical part is larger than the largest diameter of the intermediate convergent part, so that there is a step between these two parts.   The smallest diameter of the intermediate convergent part is larger than 4 mm, preferably larger than 5 mm and/or smaller than 20 mm, preferably smaller than 12 mm, preferably smaller than 9 mm.   The diameter of the downstream cylindrical part is larger than 4 mm, preferably larger than 5 mm and/or smaller than 20 mm, preferably smaller than 12 mm, more preferably smaller than 9 mm.   More preferably, the smallest diameter of the intermediate convergent part is substantially equal to the diameter of the downstream cylindrical part, so that the downstream cylindrical part may extend continuously the intermediate convergent part.   The length of the upstream cylindrical part is longer than the length of the frustoconical part of the cathode.   More preferably, the sum of the length of the upstream cylindrical part and of the intermediate convergent part is longer than the length of the cathode in the chamber. In one embodiment, the free end of the cathode extends substantially to halfway along the intermediate convergent part of the chamber, In particular, it may extend a distance, from the base of the intermediate convergent part, lying between 30 and 70%, preferably between 40% and 60% of the length of the intermediate convergent part.       
 
         [0054]    Exemplary embodiments also relate to a plasmagen gas injection device arranged so as to create a vortex around the cathode, in particular around the downstream part of the cathode which extends into the arc chamber. 
         [0055]    An injection device according to exemplary embodiments may also comprise one or more of the following optional features:
       the injection device is placed upstream of the part of the cathode extending into the arc chamber. The injection device may in particular be placed at the upstream end of the chamber;   the injection device comprises at least one injection duct. Preferably, the injection device comprises at least four injection ducts, even at least 8 injection ducts;   the diameter of the injection orifice of an injection duct is preferably larger than 0.5 mm and/or smaller than 5 mm, preferably about 2 mm;   an injection duct is placed so that the projection of the injection axis in a radial plane passing through the center of the injection orifice of said injection duct makes an angle α, to the axis X, larger than 10°, larger than 20° and smaller than 70° or smaller than 60°;   an injection duct is placed so that, in an assembled position in which the injection device is integrated into a plasma generator having an axis X, the projection of the injection axis in a transverse plane passing through the center of the injection orifice of said injection duct makes an angle β with a radius lying in said transverse plane and passing through the axis X and through the center of said injection orifice, the angle β being smaller than 45°, preferably smaller than 30° and/or larger than 5°, preferably larger than 10°, even larger than 20°;   a plurality of injection ducts, preferably all the injection ducts, have the same values for x and/or x′ and/or α and/or β;   the injection device has the shape of a ring, preferably extending along a transverse plane, the axis of the ring being the axis X; and   the injection device comprises a plurality of injection orifices equiangularly distributed about the axis X.       
 
         [0064]    Exemplary embodiments also relate to a plasma torch comprising:
       a plasma generator according to exemplary embodiments; and   means for injecting a material to be sprayed into a plasma flux generated by said plasma generator.       
 
         [0067]    The means for injecting the material to be sprayed may open into the interior of the plasma generator, and in particular into the arc chamber, or open onto the exterior of the plasma generator, in particular at the mouth of the arc chamber. 
         [0068]    Said means for injecting the material to be sprayed may be arranged so as to inject said material to be sprayed along an axis extending in a radial plane (passing through the axis X) and forming, with a plane transverse to the axis X, an angle θ, having an absolute value smaller than 40°, smaller than 30°, smaller than 20°, an angle smaller than 15° being well suited. 
         [0069]    The injection duct may be turned inward (negative angle θ, as shown in  FIG. 8 ) relative to the plasma flux, turned outward (positive angle θ), or be perpendicular to the axis X of the plasma generator (θ=0, as shown in  FIG. 1 ). 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0070]    Other features and advantages of the exemplary embodiments will become clearer still on reading the detailed description which follows and with regard to the appended drawings in which: 
           [0071]      FIG. 1  shows, in longitudinal cross section, a plasma torch in an embodiment; 
           [0072]      FIG. 2  shows a detail of  FIG. 1 ; 
           [0073]      FIGS. 3   a  and  3   b  show, in longitudinal cross section and in transverse cross section (along the plane A-A shown in  FIG. 3   a ), a plasmagen gas injection device employed in the plasma torch in  FIG. 1 ; 
           [0074]      FIG. 7   a  shows in longitudinal cross section a plasmagen gas injection device employed in the variant of the plasma torch according to  FIG. 6  and  FIGS. 7   b  and  7   c , showing this device in transverse cross section along the planes A-A and B-B shown in  FIG. 7   a , respectively; 
           [0075]      FIGS. 4 ,  5 ,  6  and  8  show, in longitudinal cross section, variants of plasma torches according to exemplary embodiments; 
           [0076]      FIG. 9  shows a cathode in a preferred embodiment; 
           [0077]      FIG. 10  shows an anode in a preferred embodiment. 
       
    
    
       [0078]    In the various figures, identical references are used to denote identical or analogous elements. 
         [0079]    The detailed description and the drawings are provided for the purposes of nonlimiting illustration. 
       DEFINITIONS 
       [0080]    In the present description, the terms “upstream” and “downstream” are used relative to the flow direction of the flux of plasmagen gas. 
         [0081]    A “transverse plane” is a plane perpendicular to the axis X. 
         [0082]    A “radial plane” is a plane containing the axis X. 
         [0083]    The expression “axial position” is understood to mean a position along the axis X. In other words, the axial position of a point is given by its normal projection on the axis X. 
         [0084]    The axial position p AC  of minimum radial distance between the anode and cathode is defined as the position, on the axis X, of the transverse plane in which the distance between the anode and the cathode is smallest. This radial distance (i.e. measured in a transverse plane) is called the “minimum radial distance” and denoted y AC  as shown in  FIG. 2 . If the distance between the anode and the cathode is a minimum in a plurality of transverse planes, the position p AC  denotes the position of the furthest upstream plane. 
         [0085]    The “chamber” is the volume which extends from the aperture of the outlet through which the plasma exits from said plasma generator towards the interior of the plasma generator. The chamber consists, upstream, of an “expansion chamber” into which the plasmagen gas is injected, and an “arc chamber” in which the electric are is generated. The transverse plane in the position p AC  is considered to mark the boundary between the expansion chamber and the arc chamber. 
         [0086]    The largest transverse dimension D C  of the cathode in the arc chamber is measured taking into account only the part of the cathode which extends into the arc chamber. When, as in the preferred embodiment, the cathode comprises, extending into the arc chamber, a cylindrical portion of circular cross section ending in a conical portion forming a point, this transverse dimension corresponds to the diameter of the cylindrical portion of the cathode. 
         [0087]    The expression “comprising a” is understood to mean “comprising at least one” unless the contrary is indicated. 
       DETAILED DESCRIPTION 
       [0088]    Reference is presently made to  FIG. 1 . 
         [0089]    A plasma torch  10  comprises a plasma generator  20  and means  21  for injecting a material to be sprayed into the plasma flux produced by the plasma generator  20 . 
         [0090]    The plasma generator  20  comprises a cathode  22  extending along an axis X and an anode  24  arranged so as to enable an electric arc E to be generated, in a chamber  26 , under the effect of a voltage produced by means of a power source  28 . The plasma generator  20  also comprises an injection device  30  for injecting a plasmagen gas G into the chamber  26 . 
         [0091]    The plasma generator may also comprise a chamber (not shown) for regulating the pressure and pressure uniformity of the plasmagen gas, upstream of the injection device  30 . 
         [0092]    The plasma generator  20  finally comprises a body  34  for securing the other elements. 
         [0093]    The body  34  houses a cathode holder  36  to which the cathode  22  is fastened, an anode holder  38  to which the anode  24  is fastened, and an electrically isolating body  40  placed between the assembly consisting of the cathode holder  36  and the cathode  22 , on the one hand, and the assembly consisting of the anode holder  38  and the anode  24 , on the other hand, so as to electrically isolate them from each other. 
         [0094]    The body  34  is in general formed from two jackets  34 ′ and  34 ″ which fit closely around the anode and cathode holders and the injection device, as shown in  FIG. 1 . Preferably, the body  34  is a single part. In particular, in one embodiment, the injection device and the anode holder are a single part, as shown for example in  FIG. 8 . Advantageously, a single part makes it possible to improve the central alignment of the parts relative to the axis of the torch and makes it easier to assemble and disassemble the torch. 
         [0095]    The electrically isolating body  40  preferably consists of a material that is able to withstand radiation from the plasma. The nature of the means used for the electrical isolation may also be selected depending on the local temperature. For example, as shown in  FIG. 8 , an isolating part  41  of reduced thermal resistance may be placed in the region which is not directly exposed to the plasma. 
         [0096]    The cathode holder  36  and the anode holder  38  are at the same electrical potential as the cathode  22  and the anode  24 , respectively. However, the cathode  22  and the anode  24  may be consumables made of copper and tungsten whereas the cathode body  36  and anode body  38  may be made of a copper alloy. 
         [0097]    The + and − terminals of the power source  28  are connected directly or indirectly to the anode  24  and cathode  22 , respectively. The power source  28  is able to generate, between the anode and the cathode, a voltage higher than 40 V and/or lower than 120 V. 
         [0098]      FIG. 2  shows that the cathode  22 , in the shape of a rod of axis X, comprises in succession, coaxially, from upstream to downstream, a frustoconical portion  45  of decreasing diameter, a cylindrical portion  46  of circular transverse cross section and a conical portion  48  with a rounded apex. 
         [0099]    In one embodiment, the cylindrical portion has a diameter larger than 5 mm, larger than 6 mm and/or smaller than 11 mm, smaller than 10 mm, a diameter of about 8 mm being well suited. 
         [0100]    The diameter of the cylindrical portion  46 , denoted D C , is called the “diameter of the cathode”, and is preferably about 8 mm. The axial position of the downstream end  50  of the cathode  22  is referenced p C  herein below. 
         [0101]    The cathode  22  may be made of tungsten, optionally doped with a dopant that reduces the work function of the metal of the cathode relative to the work function of tungsten. The tungsten may in particular be doped with thorium oxide and/or lanthanum oxide and/or cerium oxide and/or yttrium oxide. This advantageously makes it possible to increase the current density at the melting point of the metal or reduce the operating temperature by a few hundred degrees Celsius, relative to a pure tungsten cathode. 
         [0102]    The cathode may or may not be made of a single material. For example, in  FIG. 8  the cathode  22  comprises a rod  22 ″ made of tungsten, whether doped or not, and a part made of copper  22 ′ for fastening to the cathode holder. 
         [0103]    The anode  24  takes the form of a sleeve of axis X, the internal surface  54  of which comprises in succession, from upstream to downstream, a frustoconical portion  56  and a cylindrical portion  58  of circular cross section. 
         [0104]    In the same way as the cathode, the anode may or may not be made of a single material. 
         [0105]    In order to reduce erosion of the anode by the arc root of the plasma column, at least part of the internal surface  54  of the anode, and in particular downstream of the arc initiation zone (located on the frustoconical portion  56 ), is made of a refractory conductive metal, preferably of tungsten. 
         [0106]    The internal surface of the cylindrical portion  58  of the anode may also be protected by a coating or a sleeve  57 , for example made of tungsten, as shown in  FIG. 8 . 
         [0107]    The axial position of the anode  24  is such that part of the cylindrical portion  46  and the conical portion  48  of the cathode  22  are placed facing the frustoconical portion  56 , i.e. in the volume of the chamber  26  bounded radially by the frustoconical portion  56 . 
         [0108]    In the embodiment shown in  FIG. 1 , the axial position p AC  is located substantially level with the junction between the cylindrical portion  46  and the conical portion  48  of the cathode  22 . 
         [0109]    The chamber  26  comprises in succession, from upstream to downstream, an expansion chamber  26 ′ extending axially from the back  59  of the chamber  26  as far as the position p AC , then an arc chamber  26 ″ extending axially from the position p AC  as far as the position p A  of an outlet aperture  60  bounded by the downstream end of the anode, and through which the plasma exits from the plasma generator. 
         [0110]    Preferably, the diameter of the outlet aperture  60  is larger than 4 mm, preferably larger than 5 mm and/or smaller than 15 mm, preferably smaller than 9 mm. 
         [0111]    The chamber  26  may open onto the outlet aperture  60  via a nozzle that preferably extends along the axis X and the diameter of which may vary depending on the position of the transverse cross section considered, as shown for example in  FIG. 4 , or be constant, as shown in  FIG. 1 . 
         [0112]    The injection device  30 , shown in greater detail in  FIGS. 3   a  and  3   b , is arranged and located so as to create a gas flux that turns about the cylindrical portion  46 , even about the conical portion  48 , of the cathode  22 . Preferably, the injection device  30  takes the form of a ring of axis X. 
         [0113]    The lateral wall  70  of this ring is pierced with eight substantially rectilinear injection ducts  72 . Each injection duct  72  opens towards the interior of the ring via an injection orifice  74 . The center of an injection orifice  74  defines the axial position p i  and the radial distance y i  of this injection orifice. 
         [0114]    The transverse cross section of an injection duct  72  is substantially cylindrical and has a diameter D lying between 0.5 mm and 5 mm. 
         [0115]    The radial distance y i  between the axis X and the center of any one of the injection orifices is constant. It is preferably longer than 10 mm and/or shorter than 20 mm, a radial distance y i  of about 12 mm being well suited. 
         [0116]    The injection orifices  74  are located in the same transverse plane P (in a cross section A-A). They all have the same diameter D, the same axial position p (=p i ) and the same radial distance y (=y i ). 
         [0117]    An injection duct  72  opens, towards the axis of the ring, along an injection axis I i . In a radial plane passing though the center of the injection orifice  74 , the projection of the injection axis I i  makes, with the axis X, an angle α of 45°, as shown in  FIG. 3   a.    
         [0118]    In a transverse projection plane, passing through the center of the injection orifice  74 , the injection axis I i  makes, with a radius passing through the axis X and the center of said injection orifice  74 , an angle β of 25°, as shown in  FIG. 3   b.    
         [0119]    The injection device  30  is placed in the expansion chamber  26 ′. 
         [0120]    The axial distance between the axial position p AC  of minimum radial distance between the cathode  22  and the anode  24  and the position p of the injection orifices in the furthest downstream plane P is denoted x. The ratio R between x and the diameter D C  of the cylindrical portion  46  of the cathode  22  is denoted R (R=p AC /D C ). In the embodiment of  FIG. 1  or of  FIG. 2 , x is about 15 mm and the ratio R is about 1.88. 
         [0121]    The axial distance separating the axial position p C  of the downstream end  50  of the cathode  22  and the position p is denoted x′. The ratio between x′ and the diameter D C  of the cathode  22  is denoted R′ (R′=x′/D C ). In the embodiment of  FIG. 1  or of  FIG. 2 , x′ is equal to about 20 mm and the ratio R′ is 2.5. 
         [0122]    Finally, the ratio between the radial distance y between the axis X and the injection ducts  72  and the diameter Dc of the cathode  22  is denoted R″ (R″=y/D C ). In the embodiment of  FIG. 1  or  FIG. 2 , y is equal to about 13 mm and the ratio R″ is equal to about 1.63. 
         [0123]    Without being bound to one theory, the inventors have observed that when at least one of the ratios R, R′ and R″ is such as defined in exemplary embodiments, the performance of the plasma torch is particularly good, especially when the plasmagen gas is injected upstream of the cathode, and in particular injected so as to be able to turn about the cathode. The use of an injection device according to exemplary embodiments has been shown to be particularly advantageous for this purpose. According to exemplary embodiments, the plasmagen gas is injected very close to the downstream end of the cathode. The jet of plasmagen gas is little slowed over this short distance and the plasmagen gas is also cooler when it reaches the arc. It therefore preserves a high viscosity making sustaining and lengthening the arc easier and thus making it possible to increase the power of the plasma generator. In addition, the rotation of the gas about the cathode also advantageously enables wear of the electrodes to be limited. 
         [0124]    The plasmagen gas G, the flow of which is shown in  FIG. 2  by the arrow F, is preferably a gas chosen from argon and/or hydrogen and/or helium and/or nitrogen. 
         [0125]    The plasma generator  20  also comprises cooling means able to cool the anode  24  and/or the cathode  22  and/or the cathode holder  36  and/or the anode holder  38 . In particular these cooling means may comprise means for circulating a coolant, for example water, preferably in a turbulent state, the Reynolds number defining the turbulent state of this fluid possibly being preferably higher than 3000, more preferably higher than 10000. 
         [0126]    A cooling chamber  76  of axis X may in particular be housed in the anode holder  38  so as to permit the coolant to circulate near the anode  24 . 
         [0127]    The cooling means may also be common to the body  34 , the anode and the cathode, as shown in  FIG. 8 . 
         [0128]    The plasma torch  10  comprises, in addition to the plasma generator  20 , injection means  21  placed, in the embodiment shown, so as to inject particles to be sprayed near the outlet aperture  60  of the chamber  26 . All the injection means used, internal or external to the arc chamber  26 ″, may be envisioned. Thus the means for injecting particles to be sprayed are not necessarily external to the plasma generator, but may be integrated therein, as shown in  FIG. 5 . 
         [0129]    In the embodiment shown in  FIG. 1 , the injection means  21  are placed so that at least some of the material to be sprayed is injected towards the axis X along an axis making, to a transverse plane P′, an angle θ of about 0°. In  FIG. 8 , the angle θ is about 15°. 
         [0130]      FIG. 9  shows a variant of the cathode  22 . 
         [0131]    The cathode  22  comprises a rod  22 ″ made of tungsten and a copper part  22 ′, in which the rod  22 ″ made of tungsten is inserted. 
         [0132]    An upstream part  22   a  and a downstream part  22   b  of the cathode may be seen, intended to extend out of the chamber  26  and into the chamber  26 , respectively (see for example  FIG. 2 ). In the remainder of the description, only the downstream part  22   b  is described. The free end of the downstream part  22   b  is formed from a conical portion  82  having a rounded point. The radius of curvature of this end is larger than 1 mm and smaller than 4 mm. The angle at the apex δ of this conical portion is about 45°. The length L 82 , along the axis of the cathode, of the conical portion  82  is larger than 3 mm and smaller than 8 mm. The largest diameter D 82  of this conical portion (at its base) is larger than 6 mm and smaller than 10 mm. 
         [0133]    The cathode  22  comprises, immediately upstream of the conical portion  82 , a cylindrical portion  84  of circular cross section, having a diameter equal to D 82 . The cylindrical portion  84  has a length L 84  longer than 5 mm and shorter than 15 mm. 
         [0134]    The cathode also comprises, immediately upstream of the cylindrical portion  84 , a frustoconical portion  86 . The angle at the apex γ of this frustoconical portion  86  is larger than 30° and smaller than 45°. The length L 86  of the frustoconical portion  86  is longer than 5 mm and shorter than 15 mm. The largest diameter D 86  of the frustoconical portion  86  is larger than 6 mm and/or smaller than 18 mm. The smallest diameter of said frustoconical portion  86  is substantially equal to D 82 , so that the frustoconical portion  86  prolongs the cylindrical portion  84 . 
         [0135]    Preferably, the cathode is arranged so that in operation, at least one, preferably all, of the injection orifices are located in a transverse plane Pi cutting said frustoconical portion  86 . In one embodiment, this plane is located a distance “z” from the base of the frustoconical portion  86  lying between 30% and 90% of the length L 86  of the frustoconical portion  86 . 
         [0136]      FIG. 10  shows a variant of the anode  24 . This anode comprises a first part  24   a  made of copper or a copper alloy and a second part  24   b  made of tungsten or a tungsten alloy. The second part  24   b  is inserted in the first part  24   a  so as to define with it a downstream part of the chamber  26 , extending downstream of an upstream cylindrical part  26   a , drawn with dashed lines, and defined by the injection device  30 . 
         [0137]    The second part  24   b  is in particular intended to define the arc chamber. 
         [0138]    The downstream part of the chamber  26  comprises in succession, from upstream to downstream, an intermediate convergent part  26   b  (converging in the downstream direction) and a downstream cylindrical part  26   c.    
         [0139]    The intermediate convergent part  26   b  comprises first and second frustoconical parts  26   b ′ and  26   b ″, extending coaxially and prolonging each other. The angle ψ 1  at the apex of the first frustoconical part  26   b ′ upstream of a second frustoconical part, of between 50 and 70°, is larger than the angle ψ 2  at the apex of said second frustoconical part  26 ″, of between 10 and 20°. 
         [0140]    The length L 26a  of the upstream cylindrical part  26   a  lies between 5 and 20 mm. 
         [0141]    The length L 26b  of the intermediate convergent part  26   b  is about 24 mm. 
         [0142]    The length L 26b′  of the first frustoconical part  26   b ′ lies between 2 and 10 mm, for example about 5 mm. 
         [0143]    The length L 26c  of the downstream cylindrical part  26   c  lies between 20 and 30 mm. 
         [0144]    The diameter D 26a  of the upstream cylindrical part  26   a  is larger than 10 mm and smaller than 30 mm. 
         [0145]    The largest diameter D 26b  of the intermediate convergent part  26   b  (base) is about 18 mm. 
         [0146]    The diameter D 26a  of the upstream cylindrical part is larger than the largest diameter D 26b  of the intermediate convergent part, so that there is a step  80  between these two parts. 
         [0147]    The smallest diameter d 26b  of the intermediate convergent part  26   b  is larger than 4 mm and smaller than 9 mm. 
         [0148]    The diameter of the downstream cylindrical part  26   c  is equal to d 26b . 
         [0149]    Preferably, the length L 26a  of the upstream cylindrical part  26   a  is longer than the length L 86  of the frustoconical portion  86  of the cathode  24 . More preferably, the sum (L 26a +L 26b ) of the length of the upstream cylindrical part  26   a  and of the intermediate convergent part  26   b  is greater than the length L 22b  of the cathode  22  in the chamber  26 . When the cathode  22  is installed in its operating position in the chamber  26  defined by the anode  22 , the free end of the cathode preferably extends substantially to half-way along the intermediate convergent part of the chamber. 
         [0150]    The operation of a plasma torch according to exemplary embodiments is similar to that of related art plasma torches. A voltage is generated by a power supply  28  across the cathode  22  and the anode  24  so as to create an electric arc E. Plasmagen gas G is then injected with a flow rate of typically higher than 30 l/min and lower than 100 l/min, at a temperature higher than 0° C. and lower than 50° C., and at an absolute pressure lower than 10 bars by means of the injection device  30  upstream of the downstream end  50  of the cathode  22 . The flux of plasmagen gas G turns about the cathode  22  as it progresses into the chamber  26  towards the outlet aperture  60 . By passing through the electric arc E, the plasmagen gas G is converted into plasma at a very high temperature, typically at a temperature higher than 8000 K, even higher than 10000 K. The plasma flux exits from the chamber  26 , substantially along the axis X, at a velocity typically higher than 400 m/s and lower than 800 m/s. 
         [0151]    Simultaneously, the material to be sprayed is injected, in the form of particles, into the plasma flux by means of injection means  21 . 
         [0152]    The material to be sprayed may in particular be a mineral, metal and/or ceramic and/or cermet powder, even an organic powder, or optionally a liquid such as a suspension or a solution of the material to be sprayed. 
         [0153]    This material is then carried along by the plasma flux and heated, even melted by the heat of the plasma. When the plasma torch  10  is directed towards a substrate, the material is thus sprayed against this substrate. During cooling the material solidifies and adheres to the substrate. 
       Examples 
       [0154]    The following examples are provided for the purposes of illustration and do not limit the scope of the exemplary embodiments. 
         [0155]    Two plasma torches T 1  and T 2 , similar to that shown in  FIG. 8 , were compared to two related art torches, an “F4” torch and a latest-generation tricathode torch. The operating conditions (electrical parameters, composition of the plasmagen gas, powder injection flow rate, spraying distance) of the two related art torches corresponded to the nominal conditions recommended by the manufacturer or to conditions considered as being even better. The operating conditions of the plasma torches T 1  and T 2  were chosen so as to obtain the best possible performance. 
         [0156]    Table 1 below collates the technical features of the plasma torches tested and the test conditions. The two related art plasma torches had orifices for injecting plasmagen gas which opened onto the back of the chamber. The dimensional parameters defining the injection device for the plasmagen gas according to exemplary embodiments therefore did not apply to these two plasma torches. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Related 
                 Latest- 
               
               
                   
                   
                   
                 art 
                 generation 
               
               
                   
                   
                   
                 “F4” 
                 tricathode 
               
               
                 Plasma Torch 
                 T1 
                 T2 
                 torch 
                 torch 
               
               
                   
               
             
             
               
                 Position of the device for injecting the plasmagen gas 
                 lateral 
                 lateral 
                 from the 
                 from the 
               
               
                 relative to the cathode 
                   
                   
                 back 
                 back 
               
             
          
           
               
                 Device for 
                 Angle α 
                 45° 
                 45° 
                 Not 
                 Not 
               
               
                 injecting the 
                 Angle β 
                 25° 
                 0° 
                 applicable 
                 applicable 
               
               
                 plasmagen gas 
                 x (= p AC  − p i ) 
                 13 mm 
                 13 mm 
               
               
                   
                 R (= x/D C ) 
                 1.6 
                 1.6 
               
               
                   
                 x′ (= p C  − p i ) 
                 20 mm 
                 20 mm 
               
               
                   
                 R′ (=x′/D C ) 
                 2.5 
                 2.5 
               
               
                   
                 y 
                 12.5 mm   
                 12.5 mm   
               
               
                   
                 R″ (= y/D C ) 
                 1.75 
                 1.75 
               
               
                 Arc chamber 
                 Cathode diameter (D C ) 
                  8 mm 
                  8 mm 
               
               
                   
                 R′″ (=y AC /D C ) 
                 0.3 
                 0.3 
               
               
                   
                 x″ (=p A  − p AC ) 
                 43.5 mm   
                 43.5 mm   
               
               
                   
                 Outlet aperture diameter 
                 6.5 mm  
                 6.5 mm  
                   
                   9 mm 
               
               
                   
                 (cylindrical channel) 
               
               
                 Power source 
                 Current (A) 
                 750 
                 700 
                 630 
                 530 
               
               
                   
                 Voltage (V) 
                 72 
                 66 
                 68.5 
                 103 
               
               
                   
                 Power (kW) 
                 54 
                 46.2 
                 43 
                 55 
               
               
                 Plasmagen gas 
                 Argon (l/min) 
                 50 
                 40 
                 38 
                 30 
               
               
                   
                 Hydrogen (l/min) 
                 16 
                 12 
                 13 
                 0 
               
               
                   
                 Helium (l/min) 
                 0 
                 0 
                 0 
                 35 
               
               
                 Powder 
                 Carrier gas 
                 Ar 
                 Ar 
                 Ar 
                 Ar 
               
               
                 spraying 
                 Carrier gas flow rate (l/min) 
                 3 × 4 ± 1 
                 1 × 4.5 ± 1 
                 3.2 
                 3 × 3.5 
               
               
                   
                 Powder injection flow rate (g/min) 
                 120 
                 45 
                 40 
                 100 
               
               
                   
                 Spraying distance (outlet aperture- 
                 140 
                 120 
                 110 
                 90 
               
               
                   
                 substrate distance) (mm) 
               
               
                   
                 Orifice diameter for injection of the 
                  2 mm 
                  2 mm 
                 1.5 mm 
                 1.8 mm 
               
               
                   
                 powder to be sprayed 
               
               
                   
                 Distance between the means for 
                  9 mm 
                  9 mm 
                   6 mm 
                 6.5 mm 
               
               
                   
                 injecting the powder and the axis of 
               
               
                   
                 the torch 
               
               
                   
                 Injection angle relative to the axis 
                 90° 
                 90° 
                 90° 
                 90° 
               
               
                   
                 of the torch 
               
             
          
           
               
                   
                 Powder composition sprayed 
                 Chromium oxide 
                 Chromium oxide 
               
               
                   
                 Particle size of the powder sprayed 
                 17-45 μm 
                 17-45 μm 
               
             
          
           
               
                 Results 
                 Deposition efficiency (%) 
                 52 
                 45 
                 40 
                 50 
               
               
                   
                 Productivity (g/min) 
                 62.4 
                 20 
                 16 
                 50 
               
               
                   
                 Amount of energy consumed per kg 
                 14.4 
                 38.5 
                 44.8 
                 18.3 
               
               
                   
                 deposited (kWh) 
               
               
                   
               
             
          
         
       
     
         [0157]    As is clearly shown, a plasma torch according to exemplary embodiments makes it possible to achieve a particularly high efficiency and productivity with reduced energy consumption. 
         [0158]    Comparing the performance of the plasma torches T 1  and T 2  shows that the plasma torch T 1  makes it possible to obtain, for a deposition efficiency that is similar (52%) or even higher (deposition efficiency of T 2 : 45%), a productivity (higher than 62%) that is more than three times greater than that of the plasma torch T 2  (about 20%) for which the angle β is zero. 
         [0159]    Wear measurements have shown that, at equivalent powers, the wear of the electrodes of one plasma torch according to exemplary embodiments, in particular with the angles α and β such as described above, is lower than that of the related art torches, and in particular that of the electrodes of the F4 plasma torch. Advantageously, contamination with copper and/or tungsten of the deposited layer is thereby reduced. 
         [0160]    Of course, the invention is not limited to the embodiments described and shown. In particular, a plasma torch according to exemplary embodiments may be of any known type, in particular of the “blown-arc plasma” or “hot cathode” type, especially a “rod-type hot cathode”. 
         [0161]    The number and the shape of the anodes and cathodes are not limited to those described and shown. 
         [0162]    In another embodiment, the plasma generator comprises a plurality of anodes and/or a plurality of cathodes, and in particular at least three cathodes. Preferably however, the plasma generator comprises a single cathode and/or a single anode. 
         [0163]    Advantageously, the plasma generator is easier to control. 
         [0164]    The shape of the chamber is also nonlimiting. 
         [0165]    The injection device may also be different to that shown in  FIG. 1 . 
         [0166]    For example, it may comprise a single ring or a plurality of rings. 
         [0167]    The number of injection ducts is nonlimiting. Their cross section is not necessarily circular, and could be, for example, oblong or polygonal, in particular rectangular. 
         [0168]    The arrangement of the injection ducts could also be different to that shown in  FIG. 1 . The injection ducts could for example be arranged in a helix pattern or, more generally, placed so that the injection orifices are not all in the same transverse plane. They could especially lie in two (as shown in  FIG. 6 ), three, four or more transverse planes. In the injection device shown in  FIG. 6  and detailed in  FIGS. 7   a ,  7   b  and  7   c , twenty injection orifices  74  are distributed in the first and second transverse planes P 1  and P 2 . Eight injection orifices  74   1 , equiangularly distributed about the axis X, lie in the first transverse plane P 1 . They all have the same diameter D 1  and the same radial distance y 1 . The projection of an injection axis I 1  of an injection orifice  74   1  in a transverse plane makes an angle β 1  with a radius extending in said transverse plane and passing through the axis X and through the center of said injection orifice. 
         [0169]    The twelve other equiangularly distributed injection orifices  74   2  lie in the second transverse plane P 2  downstream of P 1 , and have the same diameter D 2 , larger than D 1 , and the same radial distance y 2 , equal to y 1 . The projection of an injection axis I 2  of an injection orifice  74   2  in a transverse plane makes an angle β 2  with a radius extending in said transverse plane and passing through the axis X and through the center of said injection orifice. The angle β 2  is smaller than the angle β 1 . 
         [0170]    Preferably, the ratio of the cumulated cross section S 1  of the orifices  74   1  and the cumulated cross section S 2  of the orifices  74   2  (=S 1 /S 2 ) lies between 0.25 and 4.0. The expression “cumulated cross section” is understood to mean the sum of areas of all the cross sections of a set of orifices. 
         [0171]    In another embodiment y 1  could be different to y 2 . The orifices belonging to a given transverse plane could also have radial distances y i  that differ one from the other. 
         [0172]    The injection orifices could also be grouped in groups of two, three or more. Thus, in one embodiment, the injection device may comprise four pairs of holes, said pairs preferably being equiangularly distributed. 
         [0173]    When the injection orifices are placed in a plurality of transverse planes, the injection orifices of a first plane may be aligned along the direction of the axis X or offset with those of a second plane, for example angularly offset by a constant angle.