FR2467243A1 - CARBON-TYPE MCRALY-TYPE COATINGS, COATED ARTICLES AND METHODS THEREFOR - Google Patents

Abstract

The present invention relates to coatings of the MCrAlY type comprising carbon, articles coated with these coatings and a method of applying these coatings. The protective coating is an MCrAlY type alloy comprising carbon in an amount of 0.6-11% and is characterized in a preferred embodiment in that the matrix comprising carbon contains transition metal carbides having dimensions of 1. of the order of 1 to 2 microns and chromium carbides having dimensions less than 12 microns. The coating is obtained by plasma arc spraying a powder mixture of MCrAlY and Cr3C2 to form a coating comprising both fine carbides and larger carbides of the Cr3C2 type in an MCrAlY matrix. Heat treatment at 1080 degrees C bonds the substrate and the coating and forms more fine carbides. (CF DRAWING IN BOPI)

Description

The present invention relates to protective coatings and

  coated articles and, more specifically, coatings having high resistance to high temperatures against oxidation, corrosion and wear, which can be applied to superalloy articles and a process for applying such coatings to a substrate

superalloy.

  In modern gas turbine engines, certain engine components such as super turbine turbine blades and vanes must be provided with high resistance to oxidation and high temperature wear. These properties are especially important with respect to the region of the notch Z on the sealing ring disposed around the top of the fins which rubs against the notches Z of the adjacent turbine blades and which is subjected to oxidation and severe wear. In the past the Z-notch has been protected by various materials including welded hardening coatings, a typical example of which is a cobalt-based coating having the nominal composition by weight 28% Cr, 5% Ni, 19.5% W, 1% V, the balance being cobalt. Although it is possible to provide protection to the area of the Z-notch of the sealing ring disposed around the top of the fins during engine operation, such curing coatings are expensive.

  and in some! their service life was less than satisfactory

  health. Other more economical techniques for applying these plasma arc plasma cure coatings are unsatisfactory because of insufficient adhesion of the coating during commissioning. Another type of material that has been used as a high temperature protective coating, resistant to heat to wear and corrosion is where hard particles are embedded in a softer matrix including tungsten carbide in a matrix. of

  cobalt is a familiar example for lower temperatures than

  up to 5400C. According to US Pat. No. 3,023,130 the refractory carbide particles are incorporated into heat-resistant iron-based alloys. Carbide particles have often been preferred

  chromium, usually in amounts of up to 90% by weight.

  For example, in US Pat. No. 3,150,938, particles having a size of 40 microns and less have been incorporated into a nickel-chromium alloy (80% -20%). in US Pat. No. 3,556,747, particles have been incorporated into a molybdenum matrix with minimal amounts of nickel-chromium and; in US Pat. No. 3,230,097 they were incorporated into a chromium brazing alloy and

  nickel with lower melting point. These coatings mention

  previously produced are applied by various methods including welding, but plasma arc spraying

  or flame is the most promising process.

  There are two properties of alloys and coatings

  which are to be noted. First, the dies do not have sufficient oxidation-corrosion resistance for applications with Z-notches of gas turbine engines. Second, the chromium carbide particles themselves are incorporated into the coating as used. That is, the exact chromium carbide particles in the applied mixture are the particles that were expected to be present in the alloy of the coating. The function of the alloy of the matrix is simply a binder. As a result, chromium carbide particle and metal matrix coatings known to date are likely to break due to wear, erosion and corrosion and oxidation of the matrix. Therefore the performance of coatings

  composites containing particles is limited by the matrix.

  There is therefore a need to achieve a strong coating

  improved, which tends to be more homogeneous and monolithic

and have a better return.

  It is well known that the family of protective clothing generally referred to as MCrAlY, where M is selected from among nickel, cobalt and iron and their mixture, can give resistance to corrosion oxidation in a high temperature engine environment as compared to other types of coatings and matrix materials of the carbide-containing coatings previously mentioned. See, for example, US Patent Nos. 3 676 085, 3 754 903, 3 928 026 and 3 542 530. However, in the past these alloys of MCrAlY type coatings have been applied to the aerodynamic part and to the root of the superalloy blade.

  o there is no friction or other similar conditions

  favoring wear in a manner almost as severe as that at which the notch Z of the sealing ring

  arranged around the top of the fins is subject.

  Until now, MCrAlY coatings voluntarily did not involve substantial amounts of carbon as its presence was not considered beneficial. In fact, the carbon migration of some

  superaliages has been observed as causing the formation of

  undesirable chromium carbide at the interface

  It is a relatively esoteric case, on the contrary, when certain special alloys contain a relatively high carbon content, for example carbon monoxide. Ni-Ta-C eutectic alloys. Here, as it is described in

  U.S. Patent No. 4,117,179, a coating of MCrAlY containing

  a little carbon (about 0.1% by weight) is used

  to avoid visible carbon migration of the alloy.

  But the carbon content in the coating is minimized to prevent the formation of carbide and there is no suggestion or tendency to obtain resistance

improved wear.

  There is another US patent relevant to the present invention. US Pat. No. 4,124,137 discloses a CoCr alloy coating containing tantalum carbide to withstand high temperature wear. In its broadest form, this coating consists essentially of 17-35 Cr, 5-20 Ta, 0.5-3.5 C, complement Co. Other embodiments contain rare earth metals Al, Si and various metal oxides. Of course, as is well known and indicated in US Pat. No. 4,124,137, Ta is a hardener of

  solid solution in high temperature alloys.

  Although it is preferred to W and Mo for resistance to oxy-

  As a refractory metal Ta, corrosion at best does not improve the oxidation-corrosion resistance in the CoCrAlY-type alloy and more likely other elements in the CoCrAlY alloy.

  degrades it, if it only replaces the system.

  Of course, as is well described in the literature, gas turbine engines operate

  the extreme conditions of the durability of the material.

  A material whose properties have been brought maxiAium for an application, for example oxidation at 1100 OC, can

  behave rather badly in another situation.

  for example hot corrosion at 9000C and vice versa. As a result, there are often necessary trade-offs. The addition of the need for wear resistance adds an additional variable that must be taken into account. So, there are several necessities and room for refinement of coatings alloys to achieve maximum efficiency in a turbine engine

gas.

  It is an object of the present invention to provide a coating alloy resistant to wear, oxidation and corrosion and a superalloy article coated with this coating usable at temperatures up to 10000C.

or more.

  According to the invention the coating perfection-

  born comprises chromium, aluminum, yttrium and carbon, the complement being selected from the group consisting of

  nickel, cobalt, iron or mixtures thereof. The inventors

  This results in a coating consisting essentially of a carbon-rich MCrAlY matrix containing fine carbides having a size of the order of 1 to 2 microns.

  and larger chromium carbides of the order of 12 microns.

  One embodiment comprises a coating composition consisting of 18-80% chromium, 1.2-29% aluminum, up to 4.8% yttrium, 0.611% carbon, the balance being selected from group consisting of nickellcobalt and iron and mixtures thereof. Advantageously, the coating consists of 23-68% of chromium, 4-22% of aluminum, up to 4.4% of yttrium, 1.5-7.8% of carbon, the complement being -5- chosen in the group consisting of nickel, cobalt, and iron and mixtures thereof. In a preferred embodiment, the coating composition consists essentially of 36% chromium, 10% aluminum, 2.6% carbon, 0.52% yttrium and the balance is cobalt. The improved coating consists of complex compounds of the deposited elements compared to the chromium carbide particles simply trapped in a metal matrix as is known in the art. It is admitted that the coating comprises in its structure complex compounds of MCrAlY with a substantial carbon content together with non-stoochiometric transition metal carbides as well as

Cr3C2.

  According to another object of the invention, an improved coating of the MCrAlY type containing chromium carbides is applied by plasma arc spraying of a powder. According to one embodiment, the powder comprises a mixture of

  MCrAlY and Cr3C2 and the deposited coating consists essentially of

  by weight in 18-80 Cr, 1.2-29A1, up to 4.8 Y, 0.6-

  11C, the complement being selected from the group consisting of Ni, Co, and Fe or mixtures thereof. Advantageously, the coating essentially consists of 23-68Cr, 4-22A1,

  up to4.4 Y, 1.5-7.8C, the remainder being claimed above

  below. A particularly preferred composition is 36% Cr, 10% Al, 2.6% C, 0.5% Y, the balance being C. The further improved coatings are characterized by the MCrAlY matrix containing fine carbides of the order of 1 to 2 microns and larger chromium carbides of the order of 12 microns, the carbides being formed during the spraying process when the powder mixture is used. Even better, the substrate on which plasma arc spraying is subjected to a heat treatment at 10800C to obtain the diffusion bonding of the coating and to further form fine transition metal carbides in the matrix which is saturated with carbon. The coating application method finds special application as a protective coating for sealing rings arranged around turbine blade tips made from nickel-based superalloys or

  cobalt and iron to achieve a significant shelf life

  increased in gas turbine engine environments.

  The special morphology of the coating, the combination

  its fine carbides and larger carbides, gives a particularly durable and hard matrix including larger chromium carbide complexes resistant to

wear and well welded.

  So that the invention can be better understood, reference is made to the following figures: FIGS. 1 and 2 are transverse sections through a coating according to the invention which has undergone

  heat treatment, microphotography with magnification

  250x and 500x, respectively after stripping

  electrolytic with a 5% chromic acid solution.

  Figure 3 is a scanning electron micrograph of a cross-section through a coating of the present invention with

magnification 1000 x.

  Figure 4 is similar to Figure 3 but

  represents the coating after heat treatment.

  Superalloys are generally those alloys characterized by alloys of nickel, cobalt or iron which have high mechanical strengths at high temperatures. There are a number of superalloys that are used in gas turbine engines. Among these, the greatest physical properties are generally required of these alloys used for the vanes and vanes of these engines since the vanes and blades experience the highest stresses at the highest temperatures. With regard to the blades, the most severe service with regard to oxidation and wear is experienced by the zone of the notch Z of the sealing rings arranged around the tops of the fins, which zones rub

  against each other during engine operation.

  FIGS. 1, 2, 3 and 4 show a coating having a thickness of 0.023 mm according to the invention; the composition consists of weight of 36% Cr, 10% Al, 2.6% C and 0, 52% Y and the balance is cobalt1 which has been applied to an Inconel 718 superalloy substrate (commercial designation). As can be seen in the figures, there is a multiphase structure which, at the microanalysis, seems to comprise complex carbide particles dispersed more or less randomly through the matrix which comprises carbon. The larger complex carbides and very fine dimensions with an average diameter of about 10 microns and are generally smaller than

-20 microns.

  In order to ensure a high coating density and the desired complex structure, the coating is applied to the substrate by the plasma arc spraying method and the apparatus is described in US Pat. 974 666 filed Nov. 3, 1978. In this advanced technique, the powders needed to form the coating are injected into a stream of cooled plasma gas and then sprayed onto the substrate. This advanced technique was used to form the coating shown in the figures. A physical mixture of two powders having a grain size of less than 44 microns, one a MCrAlY type alloy powder comprising by weight 63% cobalt, 23% chromium, 13% aluminum and 0.65% yttrium and the other a chromium carbide powder (Cr3C2) comprising 87% chromium and 13% carbon, were injected into the plasma gas stream. About 50% of the mixture was CoCrAlY powder. After inert plasma arc spraying by this technique, the article coated with the structure shown in FIG.

  at 10800C for 4 hours to form a diffusion weld

  between the coating and the substrate and a somewhat different structure shown in Figures 1, 2 and 4 is obtained. Other combinations of temperatures and times are applicable to obtain the same result.

as described above.

  The above-described coating and others of a similar nature have been examined by various metallurgical techniques, chemical analyzes, light microscopy, X-ray diffraction and scanning electron microscope to identify the constituents and their morphologies. The coating as depicted in FIG. 3 was determined using electron microsor analysis using an Etec Auto Probe apparatus with Kevex 5100-type X-ray energy analyzer detecting a number of different locations for Co, Cr and Al and calculating Y and C. It was found that the chemical weight distribution was nominally 51% cobalt, 36% chromium, 10% aluminum, 2.6% yttrium. This shows that the powder passed well through the plasma arc spraying device by depositing a composition which results in the ratio of 80% MCrAlY and 20% Cr3C2. Of course, small variations in percentages are normally expected in the composition of the MCrAlY coating powder compositions as well as variations in the analysis.

  of the composition by means of the microsor & electronics.

  Therefore, it must be understood that the conclusions here are subject to these precision limits. It is well known to those skilled in the art of coatings that not all of the powder passing through the plasma arc sputter will deposit on the substrate and that different powders will have different deposition rates. deposit yields

  different for the same spraying conditions.

  Therefore care must be taken to distinguish

  between the material that is sprayed and that which is deposited.

  Such a distinction is not always present in the

technical.

  Many compounds were present that could not be characterized by reference to standard models of the

  X-ray diffraction or preliminary examination of the coatings

  MCrAlY. Therefore, it has been estimated that the coating comprises very fine complex metal carbides (1 to 2 microns), nonstoechiometric carbides, and compounds being metastable. Identifiable phases such as Cr3C2 carbides were present in the coating as deposited as shown in Fig. 3 but in dimensions (rarer than 12 microns) considerably smaller than the average particle size of 15 microns of the carbide particles. which has been incorporated into the mixture passing through the spraying devices. In addition, the microprobe analysis showed that only 5 to 10% of the coating as deposited by weight consisted of crystallographic compound Cr3C2. The remaining chromium and carbon must therefore have been alloyed with or precipitated in the compound ends of the CoCrAlY matrix. This is an unexpected result taking into account what is known in the art that does not seem to describe

  coatings or interactions occur.

  It is likely that the regions identified as Cr3C2 may be partially diluted by matrix metals, at least at their periphery and hence by reference to a particle, the term chromium carbide as used herein should also be

  include more complex and diluted Cr3C2 compounds.

  Examination of the coating after treatment. thermal and showed that the composition has not changed but / can be

  see Figure 4, the morphology was significant

  differently. The particles which may be fine carbides CoCr of the order of 1 micron in diameter can be seen in the matrix; because of their finesse,

  the exact composition or structure was not determinable.

  However, they have been characterized as transition metal carbides as long as only transition metals are present and capable of forming

  carbides in coatings (with the exception of

  improbable and non-substantial combinations of Al and Y). Fine carbides in the untreated coating will be similarly characterized. It has been found that previously observed areas as Cr 3C 2 have substantially changed appearance and are less clearly defiribable since they have been substantially smaller than 10 microns or less. It is assumed that these

  results are due to the spread and formation of alliances

  The X-ray fluorescence of the coating as deposited and after heat treatment indicates that the carbon is dispersed through the coating rather than being

- 10 * -

  fully concentrated in the chromium carbide particles. The amount of carbon and chromium added to the base MCrAlY alloys to produce the new wear-resistant alloys may vary to suit the particular environment to be encountered. For the sake of simplicity, it is indicated that chromium and carbon are added to the MCrAlY base alloy in terms of Cr3C2 whose additions are representative, even though it is

  explained above, the elements are not all chemical-

  combined as Cr3C2 in the coating.

  It has been found that the usable coatings are those comprising 5 to 85% by weight of Cr3C2. This area, when combined with the MCrAlY composition used in the preferred embodiment, gives a coating with the total chromium weight of between 26 and 78%, and carbon

  between 0.65 and 11%. For low temperatures, for example

  At temperatures below 7500 ° C, where applications are more severe, the chromium and carbon content is found in the high-grade portion of the intervale since the carbide phases give resistance to wear. The upper limit is determined by the need for a sufficient matrix to bind the carbons together and to the substrate. Beyond the upper limit, the coating will degenerate due to the physical loss of carbides. At high temperatures, in the 9500C range,

  conditions of lesser wear or requiring higher ductility

  the lower part of the composition range is suitable. The lower limit is determined by the need to obtain improved wear resistance

  compared to the usual alloys of the MCrAlY type.

  Sufficient carbon must be present to obtain the presence of detectable carbide that communicates the wear resistance. Yttrium is incorporated in coatings

  MCrAlY to increase performance with regard to

  do oxidation-corrosion at the highest use temperatures especially above 950 OC. The function of yttrium in MCrAlY alloys has been well defined in the art and the yttrium content of the inventions will be

- he -

  therefore determined by the same criterion. Since yttrium significantly increases the properties at elevated temperatures, it is recognized that at least some yttrium must be present. For applications at lower temperatures, it is possible to omit

  yttrium without having a detrimental effect on performance

  these in the MCrAlY type coatings comprising

carbon according to the invention.

  The hardness of the coating is measured by various tests, for example by means of a diamond penetration hardness measuring device (DPH) using a load of 300 g, giving a printing width of 0.025 mm or further giving a nominal hardness for the matrix. The average hardness of the coating according to the invention can be between 600 DPH up to more than 1000 DPH by varying the carbon-chromium content. The hardness of the matrix obtained by the invention is especially desirable for wear resistance. Even penetrations of the harder chromium carbide regions are thus avoided. The measured apparent hardness of the matrix is attributable to the very fine carbides dispersed therein and obtained by the invention. The most suitable thicknesses for the coating of the invention are determined by the particular application envisaged and the dimension specified is normally that of a

  coating which is in its finished state after machining.

  The preferred coating thickness can range from 0.013 to 0.09 cm and typically lies in the range

  0.020 - 0.038 cm, although of course for

  cations other than for Z-notches, thinner coatings of 0.0025 cm or less may

to be used.

  In order to obtain optimum oxidation and wear resistance as well as optimum adhesion of the coating to the substrate, the density of the coating must be

  high, for example at least 95% of the theoretical density.

  The coating shown has a density of 98%.

  The high hardness in combination with the resistance to oxidation and corrosion of the alloy of the type

- 12 -

  CoCrAlY gives a coating with a structure and a combination

  unique properties that can be used in a wide range of

  no applications in destructive mines. Such properties include a much better combination

  adhesion resistant to oxidation, corrosion and

  at high temperatures that hard surfacing alloys and composite or cermet coatings such as those including scattered chromium carbide particles

  in a nickel-chromium binder or other similar alloys

  res. In addition, the coating of the invention can be economically deposited onto the substrates by the plasma arc sputtering technique described above.

as well as by other techniques.

  It is hoped that the improved wear-resistant garment having a new morphology results from the addition and chromium of the carbon at the MCrAlY type coating intervals described in the art. These ranges have been previously described in connection with various US patents cited

  in the introductory part of this description and

  the compositions of these patents are incorporated herein by reference. (It is also anticipated that such improvements or refinements in the composition of the MCrAlY type coating made in the future will be usable with the present invention.) When the compositions mentioned above, in particular those of US Patent No. 3,676,085 are used with 5 to 85% chromium carbide (Cr3C2),

  the compositional intervals indicated above are obtained

  naked. Whereas chromium and carbon are advantageously added in the form of Cr 3 Cl 2 grains, where chromium and carbon are added in an amount of 87% of chromium and 13% of carbon, they may be added in the form of other compounds such as complex carbides, subcarbons, or

  carbon-rich alloys since it is not nec-

  that carbon-containing particles retain fully intact their Cr3C2 particle identity

  in the coating to implement the invention. Equal-

  In addition, the coatings of the invention can be prepared by making a parent alloy having the desired composition and converting it into a powder and spraying it into the plasma arc. Powders having particle sizes in the range of 5 to 40 microns can

  used depending on the spray installation

  used. Of course other ways to get the

  composition of the desired coating on the article of super-

  alloy can be implemented by these specialists

in the art.

  It may be noted here that compared to other coatings, the coating according to the present invention

  unusual effects. First, there is an interaction of the MCrAlY matrix with the carbide particles

  of chromium to form deposited complex structures.

  With the least complex alloys known in the past

  such an effect has never been observed or considered desirable.

  Second, the composition of the coating alloy

  differs substantially from that of US Patent No. 4,124,137.

  In the present invention, the chromium transition metal is used in addition to the tantalum refractory metal; tantalum is

  a hardener element while chromium is not.

  Conversely, chromium increases the resistance to corrosion whereas tantalum does not. In addition, the chromium carbides of the coating according to the invention are present while in the coating of US Pat. No. 4,124,173 tantalum carbides are present and these carbides have

different properties.

  To better illustrate the present invention, the examples

  The following purely illustrative purposes are offered. EXAMPLE 1 A mixture of two powders of particles of less than 44 micron dimensions, one of nickel-chromium alloy comprising 80% by weight of nickel and 20% by weight of chromium and the other a chromium carbide (Cr3C2), where the nickel-chromium alloy constituted 12% of the mixture was applied by the plasma arc spraying method on a nickel superalloy substrate. The deposited coating consisted of 25% nickelchrome and 75% Cr3C2. X-ray diffraction examination of the coating showed that the nickel-chromium and Cr3C2 components were present in the

- 14 -

  deposited coating. Cr3C2 particles were essentially

  tially present with the particle size of the starting mixture. Since it is well known to those skilled in the art that nickel-chromium has less favorable oxidation and corrosion resistance properties in a gas turbine engine medium than the MCrAlY coating and because that the particles of chromium carbide are present

  in usual quantities in a cermet, one could expect

  that the matrix has the limited properties of nickel-

  chromium is that the particles were likely to be

  pulled out of the matrix. It has been measured that the coating

  had a hardness of 400-700 DPH. Examination of the

  After heat treatment at 10800C for 4 hours did not show any substantial changes in the morphology of the coating compared to that as deposited. Nevertheless, after having subjected the coating to tests, the coating subjected to heat treatment

  was less than the untreated coating

  temperature, had a loss of adhesion to the

  substrate, was spreading and showed a general

  rale. This serves to show the advantage of the invention compared to a material known in the art with respect to the results obtained by the technical treatment.

Example 2

  A powder mixture of particles smaller than 44 microns, one a MCrAlY alloy comprising 63% cobalt, 23% chromium, 13% aluminum and 0.65% yttrium by weight, and other a chromium carbide powder (Cr3C2) where the MCrAlY alloy was% of the mixture, was applied to a nickel alloy substrate IN-718 using the plasma arc spraying method. The coating was heat treated for 4 hours at 10800C. The composition of the coating was found to be nominally 51% Co, 36% Cr, 10% Al, 2.6% C and 0.52% Y. Density was measured and found to be 98% maximum by count. metallographic pores and by calculation. The examination of the

- 15 -

  scanning electron microscope coating and electron probe showed complex carbides

  unidentifiable with diffuse grain boundaries, indi-

  as an interaction of the MCrAlY matrix with the carbides, which was not expected in the state known in

  the technique of metal matrix coatings -carbures.

  The smaller carbides having a diameter of 1 to 2 microns were dispersed in the matrix but could not be identified. The presence of carbide in the metal matrix is unexpected as is the presence of carbides in the MCrAlY coating. Hardness was

measured between 600-700 DPH.

Example 3

  Eleven blades of a third stage of a high-performance gas turbine engine were coated at

  the location of the notch Z on the sealing ring

  The parts were installed in an engine where they were exposed to temperatures of about 92.degree. C. and were wound around the top of the blades by a coating of 0.20 to 0.25 mm in thickness. After more than 500 hours of engine operation, the coatings showed no evidence of degradation

or break.

  Example 4: A coating having the composition 64% chromium, 22.8% cobalt, 5.2% aluminum, 7.8% carbon and 0.2% yttrium was applied to the blades of the turbines

  and subjected to tests similar to those of Example 3.

  A favorable performance was also observed

Example 5

  A coating having a composition of 56.8% cobalt, 29.6% chromium, 11.7% aluminum, 1.4% carbon and 0.6% yttrium was applied to the turbine blades and subjected to similar to those described in Example 3. A favorable performance also

been observed.

  Of course, various modifications have been made by those skilled in the art to the coatings which have just been described solely as examples.

- 16 -

  non-limiting without departing from the scope of the invention.

-17 -

Claims (14)

claims:   1. A protective wear resistant to oxidation and corrosion coating for a superalloy substrate characterized in that it consists essentially of a carbon-saturated MCrAlY type matrix comprising transition metal fine carbides having dimensions of the order of 1 to 2 microns and carbides   of larger chromium of the order of 12 microns.   2. The coating according to claim 1, characterized in that it consists essentially, expressed in percent by weight, of 18-80% of chromium, 1.2 29% of aluminum, up to 4.8% of yttrium, 0.6-11% of carbon, the balance being selected from the group consisting of nickel, cobalt and iron and mixtures thereof, the alloy being constituted by a hard MCrAlY type matrix enriched in carbon and containing at least both fine transition metal carbides and larger chromium carbides. 3. The coating of claim 1, characterized in that the percents by weight of the elements are 23-68%. chromium, 4-22% aluminum, up to 4.4% yttrium, 1.5-7.8% carbon, the balance being selected from the group consisting of nickel, cobalt and iron and mixtures thereof.   4. The coating according to claim 1 for substrate in   superalloy based on nickel and cobalt-based   characterized in that the weight percent of the elements are 36% chromium, 10% aluminum, 2.6% carbon, 0.5%   of yttrium, the complement being selected from the   taking nickel, cobalt, iron or mixtures thereof. 5. Coated article consisting of a metal substrate and characterized by a coating layer of an alloy   according to any one of claims 1, 2 or 3.   6. A coated article according to claim 5, obtained by plasma arc spraying, characterized in that a mixture of powder of MCrAlY and Cr3C2 is sprayed onto a substrate and then the substrate is heated to form a weld by diffusion between the coating and the substrate, the coating comprising a saturated carbon MCrAlY matrix with transition metal fine carbides having dimensions of the order of 1 to 2 microns, and larger carbides of Cr3C2 having dimensions of the order of 12 microns.   7. Article in coated superalloy according to claim 6, characterized in that the coating consists, expressed as a percentage by weight of 18-80% Cr, 1.2-29% Al, up to 4.8% Y, 0.6-11% C, the balance being selected from the group consisting of Ni, Co, and Fe or mixtures thereof. 8. The coating according to claim 1, characterized   in that the matrix has a DPH hardness greater than 725.   9. Article according to claim 6, characterized   the matrix has a DPH hardness greater than 725.   10. Method of applying the coating according to one   any of claims 1 to 4, having a resistance   to corrosion and oxidation wear in a gas turbine engine medium, on a superalloy substrate characterized by: a) mixing MCrAlY powders with Cr3C2 powders and b) pulverizing plasma arc the powders so that they strike or adhere to the substrate, so that the resulting coating consists essentially of a MCrAlY matrix with fine carbides having dimensions of the order   1-2 microns and larger carbides with   10 microns, the coating consists essentially of   a carbon-rich 14CrAlY matrix containing fine carbides with dimensions of the order of 1 to 2 microns and larger chromium carbides having   dimensions of the order of 12 microns.   11. The method of claim 10, characterized in that the coating further consists essentially expressed in percent by weight, 18-80% Cr, 1.2-29% Al, up to 4.8% Y, 0, 6 to 11% C, the complement being chosen from   the group comprising Ni, Co, Fe or mixtures thereof.   12. The method of claim 10, characterized in that it comprises the additional step of heating - 19 -   the plasma arc sprayed substrate to form a diffusion bond between the coating and the substrate and in that the MCrAlY matrix is carbon saturated and additional fine carbides are present in the matrix.   13. Process according to any one of the claims   or 11, characterized in that the mixture of MCrAlY powder with Cr3C2 is replaced by a parent alloy in granular form of MCrAlY and C.   14. Process according to any one of the claims   , 11 and 12 characterized in that the matrix has a hardness DPH greater than 725.