TECHNICAL FIELD OF THE INVENTION

The present invention refers to a process and an apparatus for depositing coatings on items, and the item obtained from said process.

More in particular, the present invention refers to a process for depositing coatings of high roughness and porosity on endosseous prostheses.

PRIOR ART

Endosseous prostheses, for example the hip prostheses of the non-cemented type on the bone, comprise metal parts on which coatings intended to be directly coupled with the bone tissue are deposited. More in detail, the surface of such coatings is typically characterized by a given roughness, so that the bone cells may proliferate within such roughness so as to mechanically anchor the bone itself to the prosthesis.

One of the technologies most applied for obtaining the deposit of a layer of coating on the metal parts of the endosseous prostheses is that of the so-called ^NN plasma spray", which provides for the use of a plasma torch, of the non-transferred electric arc type, which is used as a means, of melting and propulsion of the coating metal material, typically supplied in form of powder and for example made up of titanium.

for the sake of brief clarification, it should be pointed out that a non-transferred electric arc plasma torch comprises a nozzle, which serves as an anode, housed within which is a cathode: a mixture of gas supplied with suitable pressure, comprising for example argon, hydrogen, helium, nitrogen flows through such nozzle. A high frequency electric pulse generates the first ionisation of the gas. The passage of direct electric current between the anode and cathode then maintains the plasma, which, due to the high forces in question, reaches a very high temperature. Before the coating step, a second current generator placed in connection between the plasma torch and the piece to be coated, may generate an electric discharge, called transferred arc, which has the purpose of heating or cleaning the surface of the piece before the coating. Upon obtaining the desired temperature or degree of cleanliness, the transferred arc generator is disabled and the actual coating step may start.

The particles of the metal coating material, generated by the melting of supplied powder granules, are drawn at high velocity by the mixture of ionized gases, and thus projected on the surface of the metal piece, where they are deposited. More in detail, the spray plasma technology has currently reached good levels, of porosity, in the order of 30% - 60%, wherein porosity stands for the ratio between volumes of the pores and the overall volume of the material: however the surface extension of such pores, which is typically lower than

100 microns, makes them unsuitable for receiving the colonization of bone cells. Could the size of the pores of the plasma spray coatings be increased, the growth of the bone tissue which could be obtained within the porosity of the coating could further improve the quality of the prosthesis/bone interface, thus reducing the problems related to ^stress shielding" associated to a non-gradual interface. The attempt to produce by means of conventional plasma spray technology a coating with larger porosity, in the order of 200-1000 microns

(μm) , generally leads to coatings with poor mechanical properties and with an unacceptable loss of particles.

The transferred arc may also be used during the coating step, so as to increase the energy transferred from the plasma to the powder. This increase of energy transferred to the powder would allow an effect of sintering the powder itself. In the patent application

US 2010/0004753 Al "Open-pore biocompatible surface layer for an implant, methods of production and use", said transferred arc is used in combination with a mixture of powder Ti/Si, whose purpose is that of lowering the titanium melting temperature. The mechanical properties of the coating obtained according to that procedure are however poor. The poor results of this method are clearly due to the inefficiency of the use of Si particles, which are not capable, not even in combination with the transferred arc, to guarantee sufficient sintering of the larger titanium particles. The production of a mixture of powders with different grain size and density with respect to each other is actually problematic: it is very difficult to obtain a homogeneous mixture, while it is also possible that a homogeneous mixture initially tends to lose the homogeneity thereof due, for example, to the mixer in the powders dispenser. Even in cases where it is possible to spray a homogeneous mixture, the in flame injection of two powders having different grain size and density could lead the two powders to penetrate differently into the flame itself, thus not guaranteeing an actual mixing of the powders during the required melting.

Another common technology consists in depositing, on the surface of the metal part of the endosseous prosthesis, sintered beads, for example of titanium or any other material. This technology allows having a porosity of several hundreds of microns, thus allowing the growth of the bone tissue in the coating.

Furthermore, the beads, being sintered, generally guarantee sufficient mechanical stability.

According to the current state of the art, it has been observed that these and other prior art technologies for depositing layers of coating provide results unsuitable to meet the prosthesis application needs: as a matter of fact, the surfaces obtained are porous, but not sufficiently rough, thus not guaranteeing sufficient primary stability. Furthermore, from a production point of view, the techniques of sintering the layers of beads reveal various disadvantages: the high temperature vacuum thermal cycle, required for sintering, is expensive, it deteriorates the mechanical properties of the substrate, particularly reducing the fatigue strength of the prosthesis. Due to said thermal cycle, a mechanical re-machining of the prosthesis is required due to the loss of general tolerances. This mechanical re-machining makes such process even less industrially attractive: alongside implying additional costs, such re-machining is complicated by the poor raachinability of the material after the thermal treatment. Given that it requires lubricants, such mechanical machining may cause hazardous contaminations of the sintered porous layer, such contaminations not always being easily eliminable and being seriously in contrast with the biomedical application in question.

OBJECTS OF THE INVENTION

An object of the present invention is improving the prior art .

Another object of the present invention is that of providing a process for depositing coatings on metal or non-metal pieces, in particular on endosseous prostheses, capable of allowing providing pieces with high surface roughness and with high porosity, as well as with extension and depth of the pores suitable to allow the growth and colonisation of the bone cells in which the prosthesis is implanted.

A further object of the present invention is that of providing metal and non-metal pieces, in particular endosseous prostheses, efficiently couplable with the bone in question and suitable to reduce the known phenomena of "stress shielding" on the bone of the patient, i.e. the accumulation of bone cells in the areas with the highest mechanical load.

Still another object of the present invention is that of providing a process for depositing coatings on metal pieces, in particular on endosseous prostheses, performable in a simpler, more efficient and less expensive manner with respect to -that of sintering layers of beads.

Another object of the present invention is that of providing a process for depositing coatings on metal pieces, in particular on endosseous prostheses, capable of allowing obtaining coatings that are more stable mechanically and more resistant than those obtainable through the known and conventional technologies, with simpler processing steps and with lower production costs.

According to an aspect of the invention, a process for depositing coatings on metal pieces according to the independent claim 1 is provided for.

According to another aspect of the invention a metal piece according to claim 16 is provided for.

The dependent claims refer to preferred and advantageous embodiments of the invention

BRIEF DESCRIPTION OF THE DRAWINGS.

Other characteristics and advantages of the invention shall be more apparent from the description of embodiments of the process for depositing coatings on metal or non-metal pieces, and of the piece obtained through such process, illustrated by way of example in the attached drawings wherein:

figure 1 is a schematic view of a detail of the system for implementing the process according to the invention;

figure 2 is a perspective view of an endosseous prosthesis obtained through the process according to the invention;

figure 3 is a schematic view of an alternative embodiment of the endosseous prosthesis obtained through the process according to the invention;

figure 4 is a perspective magnification, under a microscope, of the surface coating of the endosseous prosthesis of figure 2;

figure 5 is a metallographic section, under a microscope, of a surface coating of an endosseous prosthesis provided according to the so-called ^NS air plasma spray" prior art technique, i.e. a plasma spray in the presence of air;

figure 6 is a metallographic section, under a microscope, of a surface coating of an endosseous prosthesis provided according to the so-called "vacuum plasma spray" prior art technique, i.e. a plasma spray under vacuum;

figure 7 is a metallographic section, under a microscope, of a surface coating of an endosseous prosthesis obtained through the process according to the invention; figure 8 is a perspective view of a further embodiment of the endosseous prosthesis obtained through the process according to the invention;

figure 9 is a sectional schematic view of another embodiment of the system for implementing the process according to the invention;

figure 10 is a sectional schematic view of another embodiment of the system for implementing the process according to the invention;

figure 11 is a sectional schematic view of another embodiment of the system for implementing the process according to the invention;

figure 12 is a sectional schematic view of another embodiment of the system for implementing the process according to the invention.

EMBODIMENTS OF THE INVENTION.

With particular reference to figure 1, and with the aim of better understanding the process according to the present invention, a detail of the system for implementing ^• the process of .depositing a surface coating on metal pieces according to the invention is indicated in its entirety with number 1.

The system 1 comprises at least one plasma torch, indicated in its entirety with 2, associated to actuation members and to control and supply means not represented in the figure, but substantially of the conventional type. The actuation members are used to move the plasma torch, while the control and supply- means manage the operation thereof according to the preset programs and. provide the required power and of gas mixture supply, as better described hereinafter. The plasma torch 2 is positioned facing a piece 3, onto which a surface coating 4 is to be deposited; the piece 3 is mounted on a respective support, not represented in the figures.

The piece 3 may for example be made of metallic material, ceramic material or polymer-based material. By way of non-limiting example, the pieces 3 may be made of metal materials such as the Ti6Al4V, M30NW ISO 5832-9, CoCrMo, CoCr, ZrNb alloys and other titanium alloys; or they may be made of ceramic materials such as Alumina toughened Zirconia, or Zirconia toughened Alumina, silicon nitrides, silicon nitride-titanium nitride composites; or even more, they may be made of polymer materials such as the PEEK (polyetheretherketone) .

In the specific embodiment subject of the present invention, and by way of non-limiting example, the piece 3 on which the surface coating is to be deposited is constituted by an endosseous prosthesis, in particular one of the components of a hip prosthesis. Such component may for example be made to form a hemispherical cap shape, and for example made of titanium metal alloy. The component defines a concave internal surface 5 for coupling either with a frustoconical insert (also called wear insert) or directly with the head of the prosthesis of the femur, or directly with the head of the femur itself; on the other hand, provided for on the external convex surface thereof - or substrate - is the deposit of a layer of surface coating 4, of suitable roughness and porosity suitable to be interfaced with the bone tissue of the acetabulum of the joint, in which implantation is to be performed permanently.

The plasma torch 2 comprises, in a know manner, a metal nozzle 6 which internally defines a chamber 7, ending up with an orifice 8 facing the metal piece 3: an electrode 9 is accommodated within the chamber 7.

The chamber 7 is supplied with a gas mixture, comprising for example argon, hydrogen, helium and nitrogen at suitable percentages, introduced with suitable pressure. The nozzle β also defines one or more pipes 10, supplied along which is the metal material 11, for example in form of powder with grains of suitable size, for providing the coating layer 4. Such metal material 11 may for example be constituted by titanium, but also by other material having suitable characteristics. In an embodiment of the process, such material may for example be constituted by TiβAl4V, i.e. a titanium, aluminium and vanadium alloy.

A first electric generator 12 connects, by means of first connections 13, the electrode 9 with the nozzle β, so that between them, upon the passage of the gas mixture, an electric arc may be established, as better described hereinafter.

A second electric generator 14 connects, by means of second connections 15, the electrode 9 with the metal piece 3 onto which the surface coating 4 is to be deposited, so that a second electric arc may be established between them, as better described hereinafter.

In another embodiment, provided for may be an electric circuit capable of allowing supplying current respectively between the electrode 9 and the nozzle 6 and between the electrode 9 and the metal piece 3, even at different times, by means of a single generator.

According to the invention, the process for the deposit of a surface coating 4 on a metal piece 3, for example an endosseous prosthesis, comprises a step wherein the abovementioned piece is preliminarily prearranged and prepared for machining: in particular, the metal piece 3 is mounted on a special support. Several metal or non-metal pieces 3, identical or even different may be mounted on the same support.

The process also comprises a step of providing at least one plasma torch 2; as schematically illustrated in figure 1, in a non-limiting manner, the plasma torch 2 is substantially positioned facing the metal piece 3, at a suitably defined distance.

The process also provides for a step of creating the vacuum in the environment in which the torch 2 and the metal piece 3 are provided.

Then, there follows a step of supplying the plasma torch 2 with a suitable flow rate of the abovementioned gas mixture. Such gas mixture is supplied through the chamber 7, so that the mixture is forced to exit from the orifice 8.

The process also comprises a step of electrically supplying the plasma torch 2 through the first generator 12, with suitable voltage, so that, upon the passage of the gas mixture, there may be established a first electric arc between the electrode 9 and the nozzle β of high amperage: this causes the heating of the gas mixture up to the ionisation temperature, so as to generate a plasma, indicated schematically with 16- At least one preparatory step of cleaning the substrate on which the surface coating 4 is to be applied, by means of establishing an electric arc between the plasma torch 2 and the metal piece 3 may be provided for, where required by the application.

Furthermore a preparatory step of heating the abovementioned substrate, to facilitate the adhesion of the metal particles which shall then be deposited may also be provided for.

Then, the process provides for a step of supplying the two coating powders 11 at the nozzle 6 of the plasma torch 2, along the pipes 10. The material 11, reaching contact with the plasma 16 melts, at least partly, and it is carried on the metal piece 3 by the plasma itself, being deposited on the surface of the piece. Furthermore, the process comprises a step of electrically supplying the plasma torch 2, through the second generator 14, with suitable voltage, so that, upon the passage of the gas mixture, there may be established a second electric arc between the electrode 9 and the metal piece 3, of high amperage: this allows transferring a further amount of energy from the plasma to the coating, during the formation of the coating itself. Actually, electric microdischarges are generated between the torch 2 and the piece 3, in the order of about 100 amperes, which allow obtaining a higher concentration of energy on the coating and the sintering of the powder particles on the piece 3 itself, thus providing a surface coating 4 with innovative characteristics which shall be described in detail hereinafter.

In particular, said sintering phenomenon occurs under conditions extremely different from those of thermodynamic balance, contrary to the case of the thermal cycles in the oven. This allows obtaining extremely high energy densities per fractions of a second, which allow localized sintering phenomena without heating the entire prosthesis to the sintering temperature itself.

Lastly, at least one step of finishing the surface coating 4 by means of sand-blasting of titanium is provided for.

Figures 2, 3 each illustrate, by way of non-limiting example, a metal piece 3 with a surface coating 4 obtained through the process according to the invention. Such metal piece is particularly constituted by an endosseous prosthesis, having a surface suitable to be implanted into the bone.

Figure 8 instead illustrates, still by way of non- limiting example, a further alternative embodiment of an endosseous prosthesis obtained through the process according to the invention. Figure 8 represents, in particular, a metal piece 3 constituted by a femur prosthesis for the knee provided with a surface coating 4, made through the process according to the invention, at the surface suitable to be interfaced with the bone tissue, i.e. with the tissue of the femur.

The metal piece 3, obtained through the process, comprises a surface coating 4 above all distinguished by a thickness considerably higher with respect to those obtainable through the conventional technologies, i.e. generally comprised between 500 urn and 2000 μm. A further important characteristic of the surface coating 4 attainable through the process according to the present invention consists in the fact that such coating is mechanically more stable and resistant with respect to those obtainable through the conventional technologies, in particular due to the effect of the application of an electric arc during the step of depositing the coating.

Furthermore the surface coating 4 is characterized by high roughness and porosity: the latter in particular is comprised between 30% and 70%, as observable in the magnification of figure 4.

With particular reference to the metallographic section of figure 7, the pores obtained in the surface coating 4, indicated with reference number 17, have a depth generally comprised between 100 um and 1000 μm, and more in particular comprised between 200 μm and 800 μm. Such depth is much higher than that obtainable with the conventional deposit technologies, to which, for example, the metallographic sections of figures 5 and 6 refer to. In particular, the metallographic section under a microscope of figure 5 refers to a surface coating of an endosseous prosthesis made according to the so-called "air plasma spray" of the prior art, i.e. a plasma spray in the presence of air. The metallographic section under a microscope of figure ^β instead refers to a surface coating of an endosseous prosthesis made according to the so-called "vacuvun plasma spray" of the prior art, i.e. a plasma spray under vacuum.

Furthermore, still as observable in figures 4 and 7, the abovementioned pores 17 of the surface coating 4 have a surface extension generally comprised between 100 μm and 500 μm, definitely much higher than that obtainable through the conventional technologies.

Morphologic characteristics of the pores 17 thus made allow, with the endosseous prosthesis implanted into the patient, a much higher and extended colonisation of the bone cells therein, thus leading to the condition wherein the mechanical coupling between the bone and the prosthesis has qualitative characteristics, of ^' resistance and duration considerably higher with respect to those regarding the conventional prosthesis, specifically due to the fact that it is extended on a much greater porous surface.

The process subject of the present invention may be applied, without any limitation, to metal and non-metal pieces of any size and intended for any use, and in particular in the applications that require providing on the piece a surface coating of high stability and mechanical resistance, as well as of extremely high roughness and porosity, and which require extremely large dimensions of the pores - depth and surface extension.

Another embodiment of the process according to the invention is provided through the system represented in detail in figure 9.

It is pointed out that the detailed description that follows, just like in figure 9, the parts corresponding to those of the figures of the previous embodiment are indicated with the same reference number, increased by 100 units.

The system 101 comprises at least one plasma torch, indicated in its entirety with 102, associated to actuation members and to control and supply means not represented in the figure, having the same functions described in the previous embodiment.

The plasma torch 102 is positioned facing a piece 103, on which a surface coating 104 is to be deposited; the piece 103 is mounted on a respective support, not represented in the figures.

The piece 103 may for example be made of metal material, ceramic material or polymer-based material, as described referring to the previous embodiment.

By way of non-limiting example, even in this embodiment the piece 103 on which the surface coating is to be deposited is constituted by an endosseous prosthesis, in particular one of the components of a hip prosthesis. Such component may for example be made to form a heiαi-spherical cap shape, and for example made of titanium metal alloy. The component defines a concave internal surface 105 for coupling either with a frustoconical insert (also referred to as wear insert) or directly with the head of the prosthesis of the femur, or directly with the head of the femur itself; on the other hand, provided for on the external convex surface thereof - or substrate - is the deposit of a layer of surface coating 104, of suitable roughness and porosity suitable to be interfaced with the bone tissue of the acetabulum of the joint, in which implantation is to be performed permanently.

The plasma torch 102 comprises a metal nozzle- 106 which internally defines a chamber 107, ending up with an orifice 108 facing the metal piece 103: an electrode

109 is accommodated within the chamber 107.

The chamber 107 is supplied with a gas mixture, comprising for example argon, hydrogen, helium and nitrogen at suitable percentages, introduced with suitable pressure. The nozzle 106 also defines two pipes 110,118 supplied along which is the metal material for providing the coating layer 104.

More in detail, the nozzle 106 defines a first pipe 110 and a second pipe 118, for example at opposite positions or positions suitable for the injection in the nozzle 106-

A coating powder material of fine grain size, indicated with 111 in figure 9, contained in a first tank 119 is supplied through the first pipe 110. The expression fine grain size is used to indicate a grain size smaller or equivalent to 100 microns, preferably comprised between 5 and 70 micron.

A coating powder material with greater grain size, indicated with 120 in figure 9, contained in a second tank 120, is supplied through the second pipe 118; this implies a titanium powder having a grain size comprised between 50 and 500 microns,, preferably between 100 and 400 microns.

The coating materials 111, 120 may be constituted for example by titanium powder, or Ti6A14V alloy powder, i.e. a titanium, aluminium and vanadium alloy, or tantalum powder, or still zirconium and niobium alloys. A first electric generator 112 connects, by means of first connections 113, the electrode 109 with the nozzle 106, so that between them, upon the passage of the gas mixture, an electric arc may be established, as better described hereinafter.

The process for depositing the surface coating 104 on un metal piece 103, for example an endosseous prosthesis, implemented through the system described above, comprises a step wherein the abovementioned piece is preliminarily provided and prepared for machining: in particular, the metal piece 103 is mounted on a special support. Several metal pieces 103, identical or even different, may be mounted on the same support.

The process also comprises a step of providing at least one plasma torch 102, substantially positioned facing the metal piece 103, at a suitably defined distance. IB2010/053115

The process also provides for a step of creating the vacuum in the environment in which the torch 102 and the metal piece 103 are provided.

Then, there follows a step of supplying the plasma torch 102 with a suitable flow rate of the abovementioned gas mixture. Such gas mixture is supplied through the chamber 107, so that the mixture is forced to exit from the orifice 108.

The process also comprises a step of electrically supplying the plasma torch 102 through the first generator 112, with suitable voltage, so that, upon the passage of the gas mixture, there may be established a first electric arc between the electrode 109 and the nozzle 106 of high amperage: this causes the heating of the gas mixture up to the ionisation temperature, so as to generate a plasma, indicated schematically with 116.

At least one preparatory step of cleaning the substrate on which the surface coating 104 is to be applied, by means of establishing an electric arc between the plasma torch 102 and the metal piece 103 may be provided for, where required by the application.

Furthermore, a preparatory step of heating the abovementioned substrate, to facilitate the adhesion of the metal particles which shall then be deposited may also be provided for. Then, the process provides for the deposit of a first coating layer, called anchoring, with a thickness of 10-100 microns. The first coating layer is made using the powder made of material of fine grain size 111, which exits from the first pipe 110.

After providing the first anchoring coating layer, the process provides for obtaining the second coating layer or the actual coating, of the required roughness and porosity. The second coating layer is provided by means of co-spraying the powder of fine grain size 111 - used for the anchoring layer - and the powder of greater grain size 120. The two powders may penetrate into the flame in an optimised manner, in that each, exiting from a dedicated pipe, may be pushed into the flame at the speed suitable for the grain size and density thereof.

The purpose of maintaining the spraying of the powder of fine grain size 111 during the spraying of the powder of greater grain size 120 is that of supplying the plasma with a powder sufficiently fine to melt, thus providing the material required for firmly anchoring, in a sort of sintering process, the coarse particles, without having to melt them. As a matter of fact, it is quite difficult to melt coarse particles measuring up to 400-500 microns through the VPS technology.

'An important characteristic of the surface coating 104 attainable through the process according to the present embodiment of the invention consists in the fact that ^, such coating 104 is mechanically more resistant and stable with respect to those obtainable through the conventional technologies in particular due to the presence of extremely fine particles of titanium or titanium alloy, applied separately through a dedicated pipe 110.

EXAMPLE

For example, some characteristic values observed on five samples of product with substrate made of titanium, aluminium and vanadium alloy (Ti6A14V alloy) and surface coating made of titanium are provided. The reference directives according to which the tests were performed are indicated between brackets.

Mean volume in % of 40.0-71.0 57.5 7.7 the pores - porosity

in %

(ASTM F1854-09)

Shear fatigue > 107 cycles

strength at 1-10 MPa

(ASTM F1160-05)

Static shear strength 35. 1-39 .5 37 .8 1. 9

[MPa]

(ASTM F1044-05)

Static tensile 35. 7-48 .4 41 .5 6. 0 strength [MPa] (ASTM

F1147-Q5)

Abrasion strength 52. 1-60 .8 55 .8 3. 4

Taber test [mg]

(ASTM F1978-OO(2OO7) )

The table above shows very good results both regarding the porosity and distribution thereof as well as regarding the mechanical characteristics of static and fatigue strength.

Using the same system of figure 9, a powder of fine grain size 111 constituted by a silicon powder may be used alternatively to the titanium powder, or to the powder made of titanium, aluminium and vanadium alloy such as the T16A14V alloy, tantalum powder, or even zirconium-niobium alloys.

In particular, such silicon powder represents with the titanium of greater grain size 120 a eutectic with a low melting temperature: due to the low melting temperature, the powder particles of greater grain size

120 shall be bound together more firmly.

Another embodiment of the process according to the invention is provided through the system represented in detail in figure 10. For practical and clarity reasons, the same reference numbers used in the previous embodiment are used in this embodiment to indicate the same parts of the system.

The system of figure 10 is entirely identical to that of figure 9; further present is a second electric generator 114 which connects, by means of second connections 115, the electrode 109 with the metal piece

103 on which the surface coating 104 is to be deposited, so that a second electric arc may be established between them.

Thus, alongside the steps described regarding the previous embodiment the process also provides for a step of establishing an electric arc which facilitates the melting of the powder particles 111,120.

More in detail, the process comprises a step of electrically supplying the plasma torch 102 through the second generator 114, w.ith suitable voltage, so that, upon the passage of the gas mixture, there may be established a second electric arc between the electrode 109 and the metal piece 103, of high amperage: this allows transferring a further amount of energy from the plasma to the coating, during the formation of the latter. Electric micro-discharges are in fact generated between the torch 102 and the piece 103, in the order of about 100 amperes, which allow obtaining a higher concentration of energy on the coating and the sintering of the powder particles on the piece 103 itself, thus providing a surface coating 104 with innovative characteristics as described in the first embodiment .

Even in this embodiment, the powder made of material of fine grain size 111 may be constituted by titanium powder, or by a powder of titanium, aluminium and vanadium alloy such as the Ti6Al4V alloy, by tantalum powder, or still by zirconium-niobium alloys, or silicon powder, using the same system of figure 9.

Another embodiment of the process according to the present invention is provided through the system illustrated in figure 11. In such figure the same reference numbers of figures 9,10 are used to indicate the same parts.

In this embodiment , the system 101 comprises a third tank 122 containing silicon powder of fine grain size. Furthermore, in this embodiment titanium powder of fine grain size is present in the first tank 119, while titanium powder of greater grain size is present in the second tank 121.

The silicon powder is supplied frσm ' the third tank 122 to the nozzle 106 through a third pipe 123, and it is indicated with 124 in figure 11. The silicon powder 124, alongside the powders of titanium 111,120 of fine grain size and of greater grain size facilitates the creating a low melting point eutectic, so as to provide solid bonds between the particles of greater grain size 120 which constitute the actual coating.

Thus, in this manner, the advantage of using titanium powder of fine grain size Hl alongside that of greater grain size 120, and using silicon powder 124 alongside titanium powder, cooperate synergically obtaining a eutectic which melts at a much lower temperature and has the particles of greater grain size 120, which constitute the actual coating 104, bound firmly and homogeneously.

The embodiment of figure 12 is entirely identical to that of figure 11, but it has the second generator 114 for establishing a transferred arc when supplying the powder material for providing the coating, which facilitates the further transfer of energy and facilitates the melting of the particles 111,120,124, exactly as described regarding the embodiment of figure 10.

The present invention has been described according to preferred embodiments, but equivalent variants may be conceived without departing from the scope of protection provided by the claims.