Method for locally repairing thermal barriers

Abstract

The invention relates to a method for repairing a thermal barrier of a component comprising a substrate coated with such a thermal barrier, said substrate being made of a high-performance alloy, said thermal barrier being adhered to the alloy and having lower thermal conductivity than the alloy, the thermal barrier including at least one ceramic, one region of the thermal barrier being a region to be repaired, wherein said method includes the following steps: a) defining the region to be repaired, using a mask which protects the other regions of the thermal barrier; b) injecting a carrier gas loaded with droplets of ceramic precursor into a plasma discharge inside a plasma chamber of a plasma reactor containing the component to be repaired, while making the concentration of ceramic precursor in the carrier gas dependent on at least one parameter of the reactor selected from among: the pressure of the plasma chamber, the power of the plasma generator and the diameter of the precursor droplets, in order to control the stateliquid, gel or solidof the ceramic precursor having an effect on the region to be repaired; c) injecting a gas not loaded with ceramic precursor into a plasma discharge within the plasma chamber, steps b) and c) being repeated.

Claims

1. A method for repairing the thermal barrier of a component comprising a substrate coated with such a thermal barrier, said substrate being made of an alloy, said thermal barrier adhering to the alloy and being of lower thermal conductivity than the alloy, the thermal barrier comprising at least one ceramic, it being necessary to repair a region of the thermal barrier, comprising the following steps: a) delimiting the region to be repaired, with a mask protecting the other regions of the thermal barrier; b) injecting a carrier gas loaded with droplets of ceramic precursor into a plasma discharge within a plasma chamber of a plasma reactor containing the component to be repaired while controlling the concentration of ceramic precursor in the carrier gas and at least one parameter of the reactor among: pressure of the plasma chamber, power of the plasma generator, diameter of precursor droplets, to control the state of the ceramic precursor impacting the region to be repaired, c) injecting a gas not loaded with ceramic precursor into a plasma discharge within the plasma chamber, steps b) and c) being repeated, and wherein the concentration increases at each iteration of step b) and the parameter decreases at each iteration of step b) to infiltrate delamination cracks of the thermal barrier and then fill the spalled surface zones of said region.

2. The method as claimed in claim 1, comprising a step of cleaning said region, carried out in the plasma chamber prior to step a), comprising injection of a reducing gas into a plasma discharge.

3. The method as claimed in claim 1, comprising a step of preparing said region, carried out in the plasma chamber prior to step a), comprising injection of an oxidizing gas into a plasma discharge.

4. The method as claimed in claim 1, in which a cleaning step takes place before a preparation step.

5. The method as claimed in claim 1, in which step b) comprises pulsed injections of said carrier gas, with a ratio of injection time to resting time between 1/5 and 1/30.

6. The method as claimed in claim 1, in which steps b) and c) are carried out 6 to 30 times.

7. The method as claimed in claim 1, in which the ceramic precursor comprises at least one among hydrated zirconium (IV) oxynitrate, yttrium nitrates, gadolinium nitrates and europium nitrates, with a concentration between 0.05 and 0.5 mole per liter, with pH between 1.2 and 2 and with electrical conductivity between 0.02 and 0.2 Siemens cm.sup.1.

8. The method as claimed in claim 1, in which the power of the plasma discharge is between 60 and 20000 W.

9. The method as claimed in claim 1, in which the pressure in the plasma chamber of the plasma reactor is between 1 and 20000 Pa during steps b) and c).

10. The method as claimed in claim 1, in which injection is carried out via a capillary with a diameter between 50 and 900 m.

11. The method as claimed in claim 1, in which the alloy comprises a superalloy with a base selected from at least one of nickel and cobalt.

12. The method as claimed in claim 1, in which the thickness of the thermal barrier is between 50 and 300 m.

13. The method as claimed in claim 1, in which the thermal barrier is permeable.

14. The method as claimed in claim 1, in which the component to be repaired is permeable with open pores, the open pores being supplied with a fluid with a pressure above the pressure in the plasma chamber, said fluid then occupying the open pores, blocking deposition of the precursors in the open pores and leaving the open pores free after repair.

15. The method as claimed in claim 1, in which the thermal barrier comprises at least one among zirconia, preferably yttria-stabilized or doped with neodymium oxide, gadolinium zirconate, neodymium zirconate and europium zirconate.

16. The method as claimed in claim 1, wherein the ceramic precursor is in a liquid, gel, or solid state.

Description

(1) Other features and advantages of the invention will become clear on examining the following detailed description, and the appended drawings, in which:

(2) FIG. 1 is a sectional view of a component equipped with a thermal barrier in good condition;

(3) FIG. 2 is a sectional view, enlarged compared to FIG. 1, of a component whose thermal barrier is damaged; and

(4) FIG. 3 is a schematic view of equipment for implementing the method.

(5) FIG. 4 is a sectional view of a repaired component; and

(6) FIG. 5 is a detail view of FIG. 4.

(7) The applicant noticed that a technique derived from fuel cells was interesting. The reader is invited to refer to FR 2729400, which describes a method for depositing a thin layer of metal oxide for a fuel cell. A substrate is placed in a vacuum chamber communicating with a plasma chamber that has a convergent nozzle opening into the vacuum chamber via an outlet orifice arranged opposite the substrate and with a diameter between 2 and 5 mm. The pressure in the vacuum chamber is between 10 and 2000 Pa. A slow flow of a gas comprising at least the element oxygen is injected continuously into the plasma chamber and a plasma is generated in the plasma chamber by electromagnetic excitation of the gas. An aqueous solution containing at least the metallic element is nebulized, thus generating an aerosol in a carrier gas that has a pressure above the pressure prevailing in the vacuum chamber and predetermined amounts of the carrier gas loaded with aerosol are caused to enter the plasma chamber sequentially, by aspiration. Starting from the reactor of this method as the basis, the applicant developed a method for repairing locally damaged thermal barriers.

(8) As illustrated in FIG. 1, a component comprises a substrate 1 made of nickel-based superalloy, coated on its outer face with a metallic underlayer 2 of alloy. The metallic underlayer 2 may comprise essentially the elements M, Cr, Al and Y with M denoting iron, cobalt and/or nickel. As a variant, the underlayer comprises nickel aluminide, simple, or modified with a platinum metal, notably Pt, Pd, Ru, Rh, Os, Jr and Re or doped with a reactive element, for example Zr, Hf or Y. The metallic underlayer may alternatively comprise a platinum deposit formed by diffusion. The metallic underlayer 2 comprises an outer face coated with an oxide layer 3. The oxide layer 3 has an outer face coated with the thermal barrier 4.

(9) The thermal barrier may have a thickness between 100 and 150 m. The thermal barrier 4 may be produced by EB-PVD. The thermal barrier 4 may comprise a zirconium-based oxide partially stabilized with yttria. The oxide layer 3 has a thickness between 0.1 and 1 m and preferably between 0.3 and 6 m. The underlayer 2 has a thickness between 10 and 100 m, preferably between 20 and 50 m. The underlayer 2 may comprise platinum-modified or zirconium-doped nickel aluminide, cf. U.S. Pat. No. 7,608,301, or a - alloy, cf. U.S. Pat. No. 7,273,662 and may itself comprise a diffusion barrier according to U.S. Pat. No. 7,482,039. The underlayer 2 is based on the nickel-based single-crystalline superalloy 1. Alternatively, the oxide layer 3 is based directly on the superalloy as indicated in U.S. Pat. No. 5,538,796.

(10) In operation, the component may suffer degradation connected with thermal shocks, spalling of the outer layer, oxidation of the assembly or interactions with the environment. We then observe, see FIG. 2, deposits of CMAS 5 on the surface, surface zones 7 with severe spalling, in which the underlayer is oxidized or corroded, as well as longitudinal cracks 6 from delamination between the superalloy and the thermal barrier, and within the thermal barrier.

(11) Typically, the component has spalling of some mm.sup.2 to some cm.sup.2 with numerous longitudinal delamination cracks between the thermal barrier 4 and the substrate 1. The oxide layer 3 has a thickness between 0.3 and 6 m. The thermal barrier 4 has a columnar structure in this case.

(12) As can be seen in FIG. 3, the system comprises a reactor 11 equipped with a plasma chamber 12 in which a substrate holder 13 is arranged. The component 30 may be mounted on the substrate holder 13. A radiofrequency generator 14 supplies the induction coil 15 arranged around the plasma chamber 12. The electrical frame of the radiofrequency generator is connected to the substrate holder 13. A vacuum pump 16 is connected by a pipe 17 to the interior of the plasma chamber 12. Filters 18 are provided on pipe 17, allowing capture of the acids and dusts, and at least one controlled valve 19.

(13) At least one container 20 is provided for containing the precursors. Probes 21 of a pH-meter 22 and of a conductivity meter 23 are arranged in the container 20. The container 20 is connected to the plasma chamber 12 via a device for feed of the gases and for injection of the precursors 24, comprising a valve. The feed device 24 is also connected fluidically to a gas distributor 25 equipped with at least a mass flowmeter. The feed device 24 comprises a capillary of fixed or adjustable diameter depending on the requirements or a nebulizer optionally coupled to a valve. A cabinet for control, acquisition and monitoring 26 is connected to outputs of the pH-meter 22 and conductivity meter 23 and to the pressure sensor 29 upstream of the pumping valve 19. The control cabinet 26 comprises control outputs connected to the vacuum pump 16, to the radiofrequency generator 14, to the feed device 24 and to the gas distributor 25. Optionally a pipeline 27 is provided between the gas distributor 25 and the substrate holder 13 and a pressure gauge 31 for supplying gas for keeping open the perforations of the component to be repaired. The cabinet 26 is also in communication with a computer 28 equipped with an acquisition card and stores the data from implementation of control, notably Tables 1 to 3.

(14) After sandblasting of the component directed on the regions to be repaired and masking the sound regions of the thermal barrier, the component is mounted in a low-pressure plasma reactor on a substrate holder.

(15) Negative pressure is established in the plasma chamber. The plasma is generated by plasma discharge in an inter-electrode zone. A reducing plasma is then created in order to perform cleaning of the regions to be repaired. The oxide layer that promotes adhesion is then formed with an oxidizing plasma. Then, keeping the plasma oxidizing, the pressure of the plasma chamber and the power of the plasma generator are increased and a solution of hydrated zirconium (IV) oxynitrate ZrO(NO.sub.3).sub.2 6H.sub.2O, and optionally of yttrium nitrate Y(NO.sub.3).sub.2, is sprayed via a capillary. The hydrated zirconium (IV) oxynitrate and optionally the yttrium nitrate undergo oxidation. The nitrates reach the substrate in the liquid state and infiltrate the longitudinal delamination cracks of the thermal barrier.

(16) This is followed by a posttreatment step, keeping the plasma gases and the plasma discharge lit. Thus, the hydrated zirconium (IV) oxynitrate, and optionally the yttrium nitrate Y, deposited in and on the substrate, undergo oxidation again.

(17) The injection and posttreatment cycles are repeated from 6 to 30 times so as to infiltrate the longitudinal cracks between the damaged thermal barrier and the substrate, and the open cracks, whether they are within the thermal barrier or are beneath the thermal barrier, and to cover large spalled surface zones.

(18) The invention is in addition illustrated by the following examples.

EXAMPLE 1

(19) The thermal barrier 4 is produced by EB-PVD and comprises a partially stabilized zirconia-based oxide YpSZ. The thickness of the thermal barrier is between 100 and 150 m. The thermal barrier is on an oxide film about 0.5 m thick. The oxide film was created in situ during deposition of the thermal barrier.

(20) The underlayer may be of the platinum-modified or zirconium-doped nickel aluminide type. The thickness of the underlayer is between 20 and 50 m. The underlayer is based on nickel-based single-crystalline superalloy.

(21) Light sandblasting of the component, for example dry sandblasting with corundum, is then carried out in order to descale the zones covered with CMAS, these zones being easy to remove owing to the delamination cracks created by said CMAS deposits, and the nonadherent thermal barrier zones.

(22) The component is then put in a low-pressure plasma reactor and connected to the substrate holder, which may be cooled or heating, fixed or rotating (depending on the complexity of the component to be treated). The sound parts of the component that do not need repair are protected by a mask, for example aluminum foil.

(23) The plasma is generated for example by an induction coil through which a radiofrequency current passes. The frequency of the current may be of the order of 40 MHz. The generator used for supplying this current may be a TOCCO-STEL generator, transferring power between 60 and 600 W to the gas. The generator comprises two parts, one provided with a compartment that creates a high-voltage direct current starting from the three-phase mains current, the other for producing a high-frequency current. For this purpose, the generator is equipped with a triode and comprises an oscillating circuit based on inductances and capacitances. This compartment supplies a high-frequency current to the terminals of the solenoid, which comprises between 5 and 6 turns. The plasma discharge may be capacitive or inductive.

(24) Starting the process comprises switching on the pumping system in order to create negative pressure in the plasma chamber of the reactor. The pressure is monitored by a gauge, for example of the MKSA type. The pumping system makes it possible to create negative pressure in the reactor for controlling the low-power plasma discharge. The generator is then switched on so as to initiate the plasma discharge. A plasma of hydrogen or of argon-hydrogen or of ammonia is then created in order to effect cleaning of the component in the following conditions: power: 200 W, pressure: 560 to 650 Pa, argon flow rate 1.8 liters per minute STP (in standard conditions of temperature and pressure), nitrogen flow rate 0.215 liter per minute STP, hydrogen flow rate 0.1 liter per minute STP, duration 25 minutes. The impurities and compounds with little adherence then having been removed from the region to be repaired, the oxide layer that promotes adhesion is formed. For this purpose, a plasma of argon/nitrogen/oxygen/steam is initiated in the following conditions: power 260 W, pressure 780 to 1000 Pa, argon flow rate 2.1 liters per minute STP, nitrogen flow rate 0.275 liter per minute STP, oxygen flow rate 0.22 liter per minute STP, water flow rate 0.0015 liter per minute STP by 2-minute pulsed injections according to a cycle of opening of the valve of the feed device for 0.2 second and closure of the valve for 2 seconds, the whole for a duration of 90 minutes, recreating the oxide film in the places where it is absent.

(25) Then, maintaining the argon/nitrogen/oxygen plasma in the aforementioned conditions, the pressure of the plasma chamber is increased to 5300 Pa, the power of the plasma generator is increased to 400 W and a solution of hydrated zirconium (IV) oxynitrate and of yttrium nitrate (ZrO(NO.sub.3).sub.2 6H.sub.2O and Y(NO.sub.3).sub.2) of molar ratio 8.5/1 is sprayed via a capillary with a diameter of 500 m, at a flow rate of 1.5 cm.sup.3 per minute, by 2-minute pulsed injections according to a cycle of opening of the valve of the feed device for 0.2 second and closure of the valve for 2 seconds in the plasma discharge. The electrical conductivity and the pH of the injected solution are measured continuously.

(26) During this step, the hydrated zirconium(IV) oxynitrate and the yttrium nitrate are subjected to the action of the oxidizing species O and OH both in flight and on the substrate. The conditions of pressure in the plasma chamber and of power generated by the reactor combined with the size of the droplets produced by the injector mean that the nitrates reach the substrate in the liquid state and are able to infiltrate the longitudinal cracks and the delamination cracks between the substrate and the thermal barrier.

(27) Next is a posttreatment step with a duration of the order of 8 minutes consisting of maintaining the flow rates of plasma gases and maintaining the plasma discharge. Thus, the hydrated zirconium (IV) oxynitrate and the yttrium nitrate that are deposited in and on the substrate are again subjected to the oxidizing chemistry of the discharge, mainly the action of the species O, since the solution is no longer fed in during this step. Moreover, the reactor may also be supplied with water so as to produce OH species during the posttreatment. The temperature at the level of the substrate holder remains below 400 C.

(28) Taking into account the rate of deposition, which is found to be between 15 and 25 m/h, the injection and posttreatment cycles are repeated 10 times for maximum possible infiltration of the longitudinal cracks, which are notably present between the damaged thermal barrier and the substrate. Then gradually, owing to the control provided between the pumping unit, generator, injector, and the pH measurement or the value of the electrical conductivity of the solution to be injected or both, the pressure in the plasma chamber is decreased, the power of the plasma generator is decreased, the size of the droplets of ceramic precursors is decreased and the concentration of nitrates in the solution is increased. Thus, the precursor droplets pass from the liquid state to a viscous gel state during the intermediate cycles, then solid, allowing covering of large spalled surface zones during the last injection/posttreatment cycles.

(29) At the end of the first 10 cycles of injection/posttreatment, the parameters of pressure in the plasma chamber, power of the plasma generator power, and of the system for injection of the solution of ceramic precursors are controlled, see FIG. 3, as a function of the measurement of electrical conductivity and of pH of the solution of nitrates. The parameters are changed at the start of each injection cycle according to Table 1 given below. This control is provided by adjusting a pumping valve, controlling the generator and for the capillary injector used here, by the diameter of the capillary.

(30) TABLE-US-00001 TABLE 1 No. of the injection/post- Concentration of Diameter treatment nitrates in the Pressure (Pa) of the cycle solution : conductivity Power Controlled at the capillary (2 min/8 min) mol/L pH (S cm.sup.1) (W) start of the cycle (m) 1-10 0.06 1.73 0.033 400 5300 500 11 0.09 1.68 0.039 380 5100 500 12 0.12 1.62 0.045 360 4900 500 13 0.15 1.57 0.057 340 4700 500 14 0.18 1.55 0.062 320 4500 200 15 0.21 1.5 0.073 300 4300 200 16 0.24 1.47 0.081 280 4100 200 17 0.27 1.45 0.088 260 3900 100 18 0.29 1.43 0.096 240 3700 100 19 0.32 1.42 0.103 220 3500 100 20 0.35 1.40 0.110 200 3000 100

(31) The thermal barrier obtained after the injection/posttreatment cycles has, at reactor outlet, a crystalline structure visible on the diffraction pattern, while the temperature in the plasma discharge remains moderate.

(32) The thermal barrier may finally, optionally, undergo annealing at a temperature between 300 and 1400 C. Annealing may be carried out in the reactor, which is then equipped with a heating substrate holder, or outside, under air or under an atmosphere controlled for gas and pressure. Annealing makes it possible to remove residual surface traces of nitrates and water. Annealing also gives rise to a process of germination and growth of the grains constituting the layer.

(33) The method makes it possible to obtain, see FIGS. 4 and 5, good local repair of the thermal barrier by recreating an oxide film 9 on the underlayer 2, in the places where the initial thermal barrier 4 and the initial oxide film 3 have become detached, and by infiltrating a new layer of ceramic 8, thus allowing adherence of the initial thermal barrier 4. The new ceramic deposited 8 comprises micropores and nanopores 10. The new layer of ceramic 8 has very low thermal conductivity and is found to have good adhesion to the initial thermal barrier 4 and to the substrate 1. The open longitudinal cracks initially present in the thermal barrier are also filled. Any surplus of ceramic 8 deposited on top of the initial thermal barrier 4 can be removed by light dry sandblasting or polishing.

(34) During the aforementioned steps, the component to be repaired may be connected to a source of external pressure at increased pressure relative to the pressure of the plasma chamber. Thus, a flow of gas, for example of air, comes from the exterior through the perforated component and leaves via the cooling channels, and will thus prevent deposition of the precursors in the holes. A gas other than air may also be used. An operation of repiercing the holes, which is both expensive and relatively risky for component integrity, is thus avoided.

EXAMPLE 2

(35) The procedure as in example 1 is followed, except that no control is provided for the system for injection of the droplets of ceramic precursors. The diameter of the capillaries remains fixed at 500 m. The control is presented in Table 2.

(36) TABLE-US-00002 TABLE 2 No. of the injection/post- Concentration of Diameter treatment nitrates in the Pressure (Pa) of the cycle solution : conductivity Power Controlled at the capillary (2 min/8 min) mol/L pH (S cm.sup.1) (W) start of the cycle (m) 1-10 0.06 1.73 0.033 400 5300 500 11 0.09 1.68 0.039 380 5100 500 12 0.12 1.62 0.045 360 4900 500 13 0.15 1.57 0.057 340 4700 500 14 0.18 1.55 0.062 320 4500 500 15 0.21 1.5 0.073 300 4300 500 16 0.24 1.47 0.081 280 4100 500 17 0.27 1.45 0.088 260 3900 500 18 0.29 1.43 0.096 240 3700 500 19 0.32 1.42 0.103 220 3500 500 20 0.35 1.40 0.110 200 3000 500

(37) At the level of the repaired zone, we obtain results very similar to those obtained in example 1, namely good infiltration of the deposited ceramic and good adhesion between the initial thermal barrier 4 and the newly deposited ceramic 8. The total thickness of the ceramic 8 deposited is slightly less.

(38) With control on two parameters, pressure in the plasma chamber and power of the plasma generator, instead of three, pressure in the plasma chamber, power of the plasma generator and injection system, control is coarser and the last cycles in which good efficiency is obtained with a solid state of the ceramic precursors are less optimized.

EXAMPLE 3

(39) The procedure in example 1 is followed except that the component is covered with a plasma-sprayed thermal barrier, for example a multiperforated combustion chamber. Conventionally, such a combustion chamber has large spalled zones several cm.sup.2 in area with delamination cracks. The component to be repaired may be connected to the external pressure, either with underpressure or with overpressure relative to the external pressure while being at overpressure relative to the pressure of the plasma chamber.

(40) At the level of the repaired zone, we obtain results very similar to those obtained in example 1, notably good infiltration of the deposited ceramic 8 and good adhesion between the initial thermal barrier 4 and the newly deposited ceramic 8. After repair, the multiperforation holes of the combustion chamber remain open and no operation of repiercing is necessary.

(41) For this example, the table for control of the composition of the solution to be injected, of the pressure in the plasma chamber, of the power of the plasma generator and of the injection system is as follows:

(42) TABLE-US-00003 TABLE 3 No. of the injection/post- Concentration of Pressure (Pa) treatment nitrates in the Controlled at Diameter of cycle solution : conductivity Power the start of the the capillary (2 min/8 min) mol/L pH (S cm.sup.1) (W) cycle (m) 1-10 0.06 1.73 0.033 400 5300 500 11 0.09 1.68 0.039 380 4800 500 12 0.12 1.62 0.045 360 4300 500 13 0.15 1.57 0.057 340 3800 500 14 0.18 1.55 0.062 320 3300 200 15 0.21 1.5 0.073 300 2800 200 16 0.24 1.47 0.081 280 2300 200 17 0.27 1.45 0.088 260 1800 100 18 0.29 1.43 0.096 240 1300 100 19 0.32 1.42 0.103 220 1000 100 20 0.35 1.40 0.110 200 700 100

EXAMPLE 4

(43) The procedure in example 1 or 3 is followed, but adding, after the sequences of injection/posttreatment of a solution of hydrated zirconium (IV) oxynitrate and of yttrium nitrate for repairing the thermal barrier, sequences of injection of ceramic precursors of composition called anti-CMAS such as gadolinium zirconate Gd.sub.2Zr.sub.2O.sub.7 or neodymium zirconate or a zirconia doped with neodymium oxide in order to create an anti-CMAS layer.

(44) The precursors are for example ZrO(NO.sub.3).sub.2.6H.sub.2O and Gd(NO.sub.3).sub.3.6H.sub.2O to form gadolinium zirconate. Results are obtained roughly identical to those in example 1, namely good repair with good infiltration and on the surface, a Gd.sub.2Zr.sub.2O.sub.7 ceramic that is microporous and nanoporous yet has good adhesion.

EXAMPLE 5

(45) The procedure as in examples 1 or 3 is followed, but the sequences of injection/posttreatment of a solution of hydrated zirconium (IV) oxynitrate and of yttrium nitrate are replaced with sequences of injection/post-treatments of other thermal barrier precursors. It is thus possible to inject ceramic precursors of so-called anti-CMAS composition such as ZrO(NO.sub.3).sub.2.6H.sub.2O and Gd(NO.sub.3).sub.3.6H.sub.2O precursors of Gd.sub.2Zr.sub.2O.sub.7 or precursors of zirconia doped with neodymium oxide. Local repair of the initial thermal barrier is then performed with a ceramic of anti-CMAS composition comprising micropores and nanopores. This anti-CMAS composition gives very low thermal conductivity and is therefore very suitable for thermal barrier repair. The anti-CMAS composition has good adhesion to the substrate and to the initial thermal barrier 4 consisting of zirconia partially stabilized with yttria.

EXAMPLE 6

(46) The procedure as in examples 1 or 3 is followed, but injecting a solution with a composition that varies during the cycles of injection/post-treatment. For example, a solution of (ZrO(NO.sub.3).sub.2.6H.sub.2O and Y(NO.sub.3).sub.2), precursors of yttria-doped zirconia, is injected in the first ten cycles. The concentration of these precursors is gradually adjusted, adding Gd(NO.sub.3).sub.3.6H.sub.2O precursor of gadolinium zirconate Gd.sub.2Zr.sub.2O.sub.7, while maintaining a total concentration that is compatible with control. During the last injection/posttreatment cycles, the concentration of yttrium nitrate is gradually brought back to zero, and the other two precursors remain.

(47) Good repair of the damaged thermal barrier 4 is obtained with a ceramic that has a composition gradient: from the composition Zr.sub.2O.sub.3, Y.sub.2O.sub.3 in the vicinity of the substrate 1 of nickel-based superalloy, to the composition Gd.sub.2Zr.sub.2O.sub.7 on the external surface. The microporous and nanoporous structure of the deposited ceramic 8 is barely affected by the composition or by the composition gradient.

EXAMPLE 7

(48) The procedure as in examples 1 or 3 is followed, but injecting a solution of ceramic precursors of a particular composition during the first ten cycles of injection/post-treatment. The purpose of said solution of ceramic precursors is to introduce, for example, compounds possessing particular properties at the interface between the oxide film 9 and the thermal barrier deposited 8. It may for example be a solution of Eu(NO.sub.3).sub.3.6H.sub.2O and ZrO(NO.sub.3).sub.2.6H.sub.2O precursors of europium zirconate Eu.sub.2Zr.sub.2O.sub.7 or of yttria-stabilized zirconia doped with europium or a solution of Eu(NO.sub.3).sub.3.6H.sub.2O, ZrO(NO.sub.3).sub.2.6H.sub.2O and Gd(NO.sub.3).sub.3.6H.sub.2O precursors of gadolinium zirconate doped with europium. These compounds possessing ions with a particular optical property may be used for verifying the stress state of the repaired zone in the context of nondestructive testing. We thus obtain, besides good repair of the damaged barrier with a ceramic, ease of control owing to the sensors near the substrate 1 of the component. The microporous and nanoporous structure of the ceramic deposited is barely affected by the composition of this special layer.

(49) Depending on the requirements, this special solution of ceramic precursors may be injected during cycles of injection/post-treatment other than the first ten. They may also be ceramic precursors mixed with suspensions, colloids or metal alkoxides in order to obtain specific properties locally.

EXAMPLE 8

(50) The procedure as in the preceding examples is followed, but with another device for creating the plasma, which comprises a device for generating and transporting the microwave energy and a device for coupling with the flow, for example an atomizer.

(51) The microwave energy created by two microwave generators in pulsed mode at 2.45 GHz, of the SAIREM GMP 20 KE/D type with power adjustable from 200 to 10000 W each stabilized at 0.1%, positioned opposite. The microwave energy is injected along a diameter of the quartz tube by two waveguides. The device also comprises two impedance adapters, interfaces for computer control and devices for measuring reflected power.

(52) Coupling with the gas is effected in a cylindrical exciter made of stainless steel cooled by circulation of water in a double jacket and placed around a flanged quartz tube in which the plasma is created, cooled for example with air. Flanges are provided, connected to the subassembly allowing injection of the precursors and to the device heating the substrate. The substrate holder may be cooled or heating and used in static or rotating configuration.

(53) The working pressure in the plasma chamber is from 1 to 20000 Pa, with a maximum flow rate of the gases of about 12 liters per minute STP, including 5 liters per minute STP of oxygen and 7 liters per minute STP of argon.

(54) With control between the concentration of the solution of ceramic precursors to be injected, the pressure in the plasma chamber, the power of the microwave generators and the injection system, satisfactory results are obtained.