Coating for refractory alloy part

Abstract

A process for coating a part by chemical vapor diffusion is provided and includes placing a powder mixture in a chamber, immersing the part partially in the powder mixture, and applying a heat treatment to the part. The powder mixture includes a first component and a second component forming a gaseous compound during the heat treatment so as to allow deposition of the second component on the part. The part includes a metal refractory allow and the second component forms a solid diffusion alloy by solid diffusion with a metal species of the refractory metal alloy to generate a coating. The solid diffusion allow generates a passivating oxide layer when subjected to oxidizing conditions.

Claims

1. A process comprising: coating a part by chemical vapor diffusion; placing a powder mixture in a chamber, the powder mixture comprising at least one first component, at least one second component and an inert component; the powder mixture including a mass proportion comprised between 6 and 8% of the first component and between 8 and 12% of the second component; the first component comprising at least one halide species selected from NH4Cl, NH4F, AlCl3, CrCl2; the second component comprising at least one species selected from silicon, aluminum, iron, copper, cobalt, nickel, lanthanum, germanium, zirconium, chromium, titanium, hafnium, rhenium and a mixture thereof; immersing the part at least partially in the powder mixture, the part comprising a metal refractory alloy, the metal refractory alloy comprising molybdenum; and heat treating the part, the first component and the second component forming a gaseous compound during the heat treating step so as to allow deposition of the second component on the part; the second component forming a solid diffusion alloy by solid diffusion with at least one metal species of the metal refractory alloy so as to generate a coating; the solid diffusion alloy generating a passivating oxide layer when subjected to oxidizing conditions; wherein the process produces a coating on the part, the coating comprising a plurality of superimposed layers; the superimposed layers comprising: a first layer having a thickness comprised between 0.1 μm and 30 μm, a molar fraction of the second component in the first layer being comprised between 0 and 50%, a second layer covering the first layer and having a thickness comprised between 1 μm and 50 μm, the second layer having a molar fraction of the second component less than 2%; a third layer covering the second layer and having a thickness comprised between 0.5 μm and 50 μm, the third layer having a molar fraction of the second component comprised between 60 and 70%.

2. The process as claimed in claim 1, wherein the powder mixture includes a mass proportion comprised between 1 and 20% of the second component and a mass proportion comprised between 1 and 10% of the first component based on the total mass of the powder mixture.

3. The process as claimed in claim 1, wherein: the second component is a mixture of a silicon powder and an aluminum powder, and the first component is ammonium chloride.

4. The process as claimed in claim 1, wherein the powder mixture further includes a mass proportion comprised between 80 and 86% of an inert component.

5. The process as claimed in claim 4, wherein the powder mixture comprises between 82 and 84% by mass of inert component.

6. The process as claimed in claim 4 wherein the inert component is an alumina powder.

7. The process as claimed in claim 1, wherein the powder mixture comprises: a mass proportion comprised between 6 and 8% of the first component, a mass proportion comprised between 8 and 12% of the second component, the second component being a mixture of aluminum and silicon powder, and a mass proportion of inert component comprised between 80% and 86%, the inert component comprising one refractory mineral oxide powder or a combination of refractory mineral oxide powders.

Description

PRESENTATION OF THE FIGURES

(1) Other features and advantages of the invention will further emerge from the following description, which is purely illustrative and non-limiting, and should be read in conjunction with the appended figures wherein:

(2) FIG. 1 is a schematic representation of a device for implementing a coating process according to the invention;

(3) FIG. 2 is a schematic representation of the structure of a coating according to the invention;

(4) FIG. 3a and FIG. 3b are electron microscope section views representing the coating having first, second and third layers; more precisely, FIG. 3a and FIG. 3b represent cases where the layer comprises molybdenum-aluminum-silicon alloy inclusions.

DESCRIPTION OF ONE OR MORE METHODS OF IMPLEMENTATION AND EMBODIMENTS

(5) Process:

(6) The invention relates to a process for coating a refractory alloy part 1 by chemical vapor diffusion, wherein the part 1 is at least partially immersed in a chamber 3 comprising a mixture of carburizing powders 2 before preforming a heat treatment, the powder mixture comprising at least one activator and at least one doping component, the activator being configured to form a gaseous compound with the doping component during the heat treatment so as to promote deposition of the doping component on the part 1, the doping component being configured to form an alloy by solid diffusion with a species of the refractory alloy so as to generate a coating 4 on the surface of the part 1.

(7) The alloy formed by the doping component and the part 1 is configured to generate a passivating oxide layer when said alloy is subjected to oxidizing conditions.

(8) The powder mixture 2 may comprise a doping component, preferentially in the form of a divided solid (powder), the component being configured to form by solid diffusion an alloy with the refractory alloy support (which is most commonly molybdenum or a molybdenum alloy) to generate the coating 4.

(9) The coating 4 thus formed on the refractory alloy substrate 5 of the part 1 must form a protective layer under oxidizing conditions by generating a passivating oxide layer.

(10) The doping component may comprise: a silicon powder, an aluminum powder, an iron powder, a copper powder, a cobalt powder, a nickel powder, a lanthanum powder, a germanium powder, a zirconium powder, a chromium powder, a titanium powder, a hafnium powder, a rhenium powder.

(11) Advantageously, the doping component may comprise a powder comprising at least one metal salt.

(12) Advantageously, the powder mixture 2 comprises one or more halide-type activator components, and optionally an inert filler to limit the speed of temperature variation during the heat treatment.

(13) The activator component(s) may comprise one compound or a combination of compounds among the following compounds: NH.sub.4Cl, NH.sub.4F, AlCl.sub.3, CrCl.sub.2.

(14) The inert filler is configured to remain in solid form during the heat treatment and not form an alloy with the part 1 nor other species present in the powder mixture or formed during the heat treatment.

(15) It thus increases the volume of the powder mixture 2 so as to immerse the part to be coated 1, but also increases the energy to be supplied to the chamber 3 to increase its temperature, without increasing the mass of reagents, thus controlling the temperature in the chamber 3 and thus the thickness of the coating 4 formed.

(16) The inert filler may optionally comprise alumina Al.sub.2O.sub.3 or magnesium oxide MgO powder, silicates, silica, zirconia, cerium oxide, cristobalite, calcium carbonate.

(17) The heat treatment is then performed so as to form an alloy coating 4 on the surface of the refractory alloy substrate 5.

(18) The alloy comprises one or more molybdenum alloys of form Mo.sub.aX.sub.b and/or one or more solid solutions (i.e., molybdenum dissolved in a species X or a species X dissolved in molybdenum). The composition of the alloy coating 4 between X and Mo are the thermodynamically stable compounds defined in the phase diagrams corresponding to the binary alloy Mo—X and to the relative content of Mo and X in the coating.

(19) At least one of the compounds capable of forming the passivating layer in an oxidizing environment must be in the form of a continuous layer, i.e., a layer extending over the entire surface of the part 1 in contact with the oxidizing environment.

(20) An area not covered with a passivating layer would cause the degradation of the part 1 at this location, degradation which can then spread to the entire part 1 whose oxidation is not passivating and therefore uninterrupted.

(21) The obtaining of a continuous layer is conditioned by the formation of sufficient volatile metal chloride during heat treatment to deposit sufficient metal species on the surface. The obtaining of a continuous layer also suggests that the level of stress in this layer is lower than the stress at rupture which, when exceeded, would cause cracking. The level of stress is a function of the thickness of the coating (chosen <100 μm by adapting the content of doping component, the quantity of activator, the temperature and the time of the heat treatment), the difference in the coefficient of expansion between the coating and the support (chosen lower than 6.10.sup.−6 K.sup.−1).

(22) For example, the silicon in the powder mixture 2 leads to the formation of a molybdenum-silicon alloy phase of form Mo.sub.aSi.sub.b. The MoSi.sub.2 phase forms a passivating silica layer, under oxidizing conditions.

(23) In another example, the aluminum in the powder mixture 2 leads to the formation of a molybdenum-aluminum alloy phase of form Mo.sub.aAl.sub.b, which will form a passivating alumina layer, under oxidizing conditions.

(24) In another example, the nickel in the powder mixture 2 leads to the formation of a molybdenum-nickel alloy phase of form Mo.sub.aNi.sub.b, which will form a passivating nickel oxide layer, under oxidizing conditions.

(25) In another example, the chromium in the powder mixture 2 leads to the formation of a molybdenum-chromium alloy phase of form Mo.sub.aCr.sub.b, which will form a passivating chrome (III) oxide layer, under oxidizing conditions.

(26) Optionally, the powder mixture 2 may comprise several doping components.

(27) These metal elements added to the powder mixture 2 can be combined to form an alloy coating, for example ternary of form Mo.sub.aX.sub.bY.sub.c, or in another example quaternary of form Mo.sub.aX.sub.bY.sub.c,Z.sub.d.

(28) The doping component is configured to generate a volatile chloride which is then reduced on the surface of the part 1 and deposits the metal element of the chloride.

(29) The doping component can also be configured so that it generates a passivating oxide layer under oxidizing conditions.

(30) Optionally, the process comprises a step of oxidation of the part 1, during which the part 1 and the coating 4 are heated in the presence of air at a temperature of 1150° C., so as to generate a passivating oxide layer on the surface of the coating 4.

(31) Advantageously, to limit the reactivity of the part 1 during superalloy casting, the doping component comprises one of the components, or a combination of components, among aluminum, titanium, chromium, zirconium, hafnium whose respective oxides (Al.sub.2O.sub.3, TiO.sub.2, Cr.sub.2O.sub.3, ZrO.sub.2, HfO.sub.2) are oxides with high chemical stability in contact with the molten superalloys.

(32) In this way, the passivating oxide layer formed during the oxidation step is inert in contact with a liquid superalloy at 1550° C. under vacuum, for example during foundry molding of a superalloy gas turbine blade. Inert is understood to mean that the passivating oxide layer prevents the diffusion of species between the part 1 and the superalloy.

(33) Process Example:

(34) The powder mixture 2 is ground for 10 minutes to obtain a homogeneous pack.

(35) It is then advantageously vibrated to increase the bulk density of the powder mixture 2 and allow better contact between the powder and the part 1 to be coated.

(36) The molybdenum or molybdenum-based alloy parts to be coated are polished to remove sharp edges and minimize roughness. The arithmetic mean roughness of the parts after polishing is advantageously less than 0.6 microns.

(37) The parts are cleaned and degreased and then immersed in the powder, preferably totally immersed, and then the powder mixture 2 is advantageously re-vibrated.

(38) The chamber 3 is steamed to remove moisture from the powder and parts at 100° C. for 12 hours. The chamber 3 is then sealed under inert atmosphere by means of ceramic adhesive.

(39) The heat treatment is then applied in inert atmosphere with a first temperature step between 100 and 350° C. to allow firing of the ceramic adhesive and sealing of the crucible, followed by a second temperature step between 1000 and 1100° C. for a period of 10 to 30 hours to achieve the deposit.

(40) Structure of the Coating:

(41) The process is configured to produce a coating 4 on the surface of the part 1, comprising a plurality of superimposed layers.

(42) The coating 4 may comprise, directly in contact with the substrate 5, a first layer 6.

(43) The first layer 6 may comprise a solid solution of aluminum and silicon in molybdenum denoted Mo (Si,Al).

(44) The aluminum molar fraction in the first layer 6 can be comprised between 0% and 50%, preferably between 0% and 35%. The silicon molar fraction in the first layer 6 can be comprised between 0% and 40%, preferably between 0% and 25%.

(45) The thickness of the first layer 6 can be comprised between 0.1 μm and 30 μm and preferably between 0.1 μm and 10 μm.

(46) A second layer 7, covering the first layer 6 in contact therewith, comprises an aluminum-molybdenum alloy. The second layer 7 may comprise a mass proportion greater than 75% of aluminum-molybdenum alloy. The aluminum-molybdenum alloy comprises for example Al.sub.8Mo.sub.3.

(47) The second layer 7 may further comprise a secondary phase having molybdenum-aluminum-silicon alloy inclusions. The second layer 7 advantageously comprises a mass proportion of less than 25% molybdenum-aluminum-silicon alloy inclusions. The composition of these inclusions can be a solid solution of aluminum and silicon in molybdenum denoted Mo(Si,Al) or Mo.sub.5(Si,Al).sub.3 phase.

(48) The thickness of the second layer 7 can be comprised between 1 and 50 μm.

(49) The coating 4 also comprises a third layer 8 covering the second layer 7 in contact therewith.

(50) The third layer 8 comprises a molybdenum-aluminum-silicon Mo(Al,Si).sub.2 alloy capable of forming a passivating oxide layer under oxidizing conditions. The combined molar fraction of silicon and aluminum in the layer 8 is comprised between 60% and 70%, preferably between 65% and 68%.

(51) The third layer 8 can have a thickness comprised between 0.5 μm and 50 μm, preferably between 1 μm and 15 μm.

(52) In this way, the first layer 6 provides cohesion between the coating 4 and the substrate 5. Its intermediate aluminum and silicon content in relation to the upper layers gives it an intermediate coefficient of thermal expansion between that of the substrate 5 and that of the second layer 7.

(53) The coating 4 thus offers a better temperature resistance, in particular by virtue of the reduction of stresses due to differential expansion phenomena between the substrate 5 and the coating.

(54) The second aluminum-rich layer 7 gives the coating 4 a high ductility and thus improves the mechanical strength of the entire coating 4. It limits the problems of crack propagation during thermal cycling, cracks which occur in the layer 8 following the formation of the passivating layer (Kirkendall effect described above).

(55) The layer 8 plays the role of protection against oxidation under oxidizing conditions.

(56) Examples of Embodiments:

(57) Referring to FIG. 3a and FIG. 3b, the coating 4 consists of four layers such as: A first layer 6 of solid solution of aluminum and silicon in molybdenum, in contact with the substrate 5; A second layer 7 comprising an aluminum-molybdenum alloy Al.sub.8Mo.sub.3, the silicon molar fraction of the second layer being less than 2%, the second layer 7 covering the first layer 6 in contact therewith; A third layer 8 comprising a molybdenum-aluminum-silicon alloy Mo(Al,Si).sub.2 on the surface of the coating.

(58) Such a coating 4 can be obtained by incorporating in the chamber 3 a powder mixture 2 comprising: a mass proportion comprised between 1 and 10% of a precursor of a first doping component, here a silicon powder, with a purity of at least 99%, and with a grain diameter of less than 44 μm; a mass proportion comprised between 1 and 10% of a precursor of a second doping component, here an aluminum powder with a purity of at least 99.5%, with a 50% grain diameter of less than 11 μm; a mass proportion comprised between 1 and 10% of an activator component, here ammonium chloride NH.sub.4Cl powder with a purity of at least 99.5%, with a grain diameter comprised between 1 and 3 mm; a mass proportion comprised between 70 and 95% of an inert component, here an alumina powder with a 50% grain diameter of less than 1 μm.

(59) The composition of the powder mixture 2 optionally but advantageously comprises, to within 2% by mass fraction: 7% precursor of the first doping component, 3% precursor of the second doping component, 7% activator component and 83% inert component.

(60) In an embodiment, the powder mixture 2 includes a mass proportion comprised between 6 and 8% of activator component and between 8 and 12% by mass of a mixture of aluminum and silicon powder, the remainder of inert diluent to reach 100%, selected from refractory mineral oxide powders, the mixture of aluminum and silicon powder having a silicon/aluminum mass ratio comprised between 2 and 5.

(61) Such a mixture improves the creation of the protective layer.

(62) Such a mixture makes it possible to manufacture a part 1 made of a refractory metal alloy comprising molybdenum at least partially covered by a coating 4 comprising a plurality of superimposed layers, of which at least: a first layer 6 comprising a solid solution of aluminum and silicon in molybdenum denoted Mo (Si,Al), has a thickness comprised between 0.1 μm and 1 μm, and has an aluminum molar fraction comprised between 0.1% and 35% and a silicon molar fraction comprised between 0.1% and 25%, a second layer 7 comprising a mass proportion of more than 75% of an aluminum molybdenum alloy Al.sub.8Mo.sub.3 and has a thickness comprised between 1 and 30 μm, a third layer 8 having a thickness comprised between 1 μm and 25 μm and comprises a aluminum silicon-molybdenum alloy Mo(Si,Al).sub.2, the third layer having a combined atomic fraction of silicon and aluminum comprised between 65% and 68% and configured to generate a passive oxide layer when exposed to oxidizing conditions.

(63) Carburization can be carried out in a chamber 3 with a volume of 50 cm.sup.3, wherein the powder mixture 2 incorporated contains 1.75 g of silicon powder, 0.75 g of aluminum powder, 1.75 g of ammonium chloride and 20.75 g of alumina powder.

(64) Carburization can be carried out in a chamber 3 of different volume by proportionally adapting the mass amount of powder mixture 2 in the chamber 3 to the volume of the chamber 3.

(65) The AlCl.sub.3 vapor phase concentration obtained in the chamber 3 with such a composition of powder mixture 2 produces a higher aluminum content in the coating 4.

(66) This leads in particular to the formation of an aluminum-molybdenum Al.sub.8Mo.sub.3 phase in the coating in addition to the molybdenum-silicon-aluminum Mo(Si,Al).sub.2 alloy phase.

(67) The ductility of the aluminum-rich coating is superior to that of a coating comprising chiefly a molybdenum-silicon-aluminum Mo(Si,Al).sub.2 alloy.

(68) The presence of the third layer 8, comprising the molybdenum-silicon-aluminum Mo(Si,Al).sub.2 alloy in the coating 4 guarantees the oxidation resistance by allowing the formation of a passivating alumina layer at temperature in an oxidizing atmosphere.

(69) In the case where the part 1 coated with coating 4 is used as a refractory core in an investment casting process for the manufacture of a superalloy turbine blade, the passivating alumina layer on the coating 4 is formed during the steps of firing the ceramic core and firing the shell, which are carried out in the presence of oxygen (usually in air). The alumina layer prevents oxidation at the core of the part 1 during these firing steps. In addition, the low wettability of nickel-based superalloys with alumina makes this layer also protective of any chemical reactivity between the part 1 and the molten superalloy during casting.

(70) In order to control the thickness of the oxide layer on the coating 4, it is possible to subject the part 1 coated with the coating 4 (in this case called the refractory core) to preliminary oxidation of the coating before the steps of firing the ceramic core and the shell. The refractory core coated with the coating 4 can thus be subjected for example to a heat pretreatment in air at a temperature comprised between 1000 and 1500° C.