Abstract:
A method for coating a heat engine part, particularly a turbo-machine part made of a superalloy and adapted for use in aeronautical applications, comprises electrophoretically depositing a metallic structure of cellular form with uniformly disposed cells of predetermined size. The deposition is performed using an electrophoresis bath containing methanol, aluminum chloride as an electrolyte, and a powder containing Cr, Al, Y, Ta and Ni. The cellular metallic structure is consolidated by a sintering treatment, which may be reactive, or metallization, preferably in the vapor phase, and the coating is completed by applying a ceramic material by plasma spraying.

Description:
This is a division of Ser. No. 07/197,318, filed on May 23, 1988, now U.S. Pat. No. 5,057,379. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to heat engine parts made of an alloy having good mechanical strength and resistance to high temperatures, and comprising a protective coating, and relates especially to turbo-machine parts made of superalloy, particularly if nickel-based, having a protective coating for protection against corrosion and oxidation. The invention also relates to a method of forming the protective coating on such parts. 
     2. Summary of the prior art 
     The search for high performance in the development of turbo-machines, particularly for aeronautical applications, has led to ever increasing operating temperatures, while rationalization of the use of equipment makes it necessary to prolong the life of the machine parts. The result of this is the adoption of numerous solutions relating to protective coatings for providing turbo-machine parts subjected to high temperatures with protection against oxidation and/or corrosion. 
     U.S Pat. No. 4328285 discloses the protection of superalloy gas turbine parts by a metallic undercoat having a composition of the M, Cr, Al, Y type, where M denotes Fe, Ni, Co or a mixture of these metals, applied by plasma spraying, followed by a ceramic-based coating comprising zirconium oxide and at least 15% by weight of cerium oxide, also formed by plasma spraying. 
     U.S. Pat. No. 4248940 discloses another example of a heat barrier coating for superalloy parts, the coating being formed by plasma spraying a mixture of powders comprising a bonding material of the M, Cr, Al, Y type, where M denotes Fe, Ni, Co or a mixture thereof, and a ceramic type material based on zirconium oxide stabilized by another oxide, the coating having an increasing percentage of ceramic in the direction away from the substrate. 
     However, no previously known solution is entirely satisfactory in relation to the particular conditions of use and taking into account the operating requirements and the improvement of the properties of thermal insulation and of resistance to combined oxidation and corrosion agents of various kinds. A particularly noticeable phenomenon has been observed which may be described as the development and the propagation of cracks or fissures under the action of stresses which develop in the coating and which are of thermal origin in particular. 
     Other heat engines, particularly diesel engines, also have parts which have been provided with a protective coating for the improvement of operating performance. 
     It is an object of the invention, therefore, to provide heat engine parts with an improved protective coating structure, and in particular a structure capable of modifying the manner of rupture observed in the coating under critical conditions of operation of the coated parts. 
     SUMMARY OF THE INVENTION 
     According to a first aspect, the invention provides a heat engine part made of an alloy having good mechanical strength and resistance to high temperatures, especially a turbo-machine part made of superalloy which is preferably nickel-based, said part having a protective coating comprising a metallic structure, preferably composed of M, Cr, Al, and Y where M is a metal selected from the group consisting of nickel, cobalt, iron, and mixtures thereof, with the possible addition of tantalum, said metallic structure being obtained by electrophoretic deposition and a consolidation treatment, said metallic structure being of a cellular form wherein the cells are substantially evenly distributed and are of a predetermined size depending on the conditions selected for said electrophoretic deposition, and said metallic structure having a modified composition and being bonded to said alloy part as a result of said consolidation treatment, and a ceramic based material applied to said metallic structure by atmospheric plasma spraying. 
     The consolidation treatment preferably comprises a sintering process, which may be reactive, or a metallization process, particularly a vapour phase process, at a temperature and for a period known per se for the application to said alloy. 
     The protective coating of the alloy part in accordance with the invention provides significant advantages in the way of improved working life and operating performance. An attempt at explaining the observed phenomenon may be begun with the tests which have been carried out. 
     FIGS. 1a, 1b and 1c show diagrammatically sectional views of a substrate 1a coated by a known method with a metal undercoat 1b and a ceramic top coat 1c by means of plasma spraying. From the inception of a critical crack, shown at 2 in FIG. 1b, as a result of the application of thermal shocks representative of the operating conditions of the coated part, FIG. 1c shows how a coating break appears as a result of propagation of the crack 2 upon continuation of the thermal shocks. 
     FIGS. 2a, 2b, 2c show diagrammatic views similar to those of FIGS. 1a, 1b and 1c, but of a substrate 2a coated in accordance with the invention, wherein the metallic structure 2b obtained by electrophoretic deposition has the required cellular form with a controlled size of cells. As a result of the applied thermal shocks a critical crack 2 is also started, as shown in FIG. 2b. However, the similarity stops there, since the invention provides a different fissuring mechanism. As shown in FIG. 2c, a deflection of the crack is observed at 3 and the crack no longer propagates in a direction parallel to the surface of the coating or to the planes of the various metal/ceramic interfaces as in the earlier coating shown in FIG. 1c. After that, propagation of the crack is observed to stop at 4 where it meets an element of the metallic cell structure which is more resistant to fissuring. 
     This attempt at an explanation, however, is only partial, and other advantages of the structure of the coating in accordance with the invention leading to an improvement of the results must be mentioned. The modification of the manner of rupture is also obtained through an improvement of the mechanical adherence at the metal/ceramic interface, the cellular structure facilitating in particular an interpenetration between the two layers. In addition, the structure obtained brings about a modification of the distribution of the stresses at the ceramic/metal interface, the result of which is, not only particular properties of crack propagation as detailed above, but also particular conditions which advantageously bring about a delay in the occurrence or inception of such fissuring or cracking. Depending on the applications of the invention, a structure of the type shown in FIGS. 2a 2b and 2c may be desired or, in some cases, a structure of the type shown in FIG. 2d may be obtained in which the cellular metallic structure 2b is flush with the outer surface of the completed protective coating. 
     According to a further aspect of the invention the protective coating on a heat engine part made of an alloy having good mechanical strength and resistance to high temperatures, particularly a turbomachine part of superalloy, may be formed by a method comprising the steps of: 
     a) depositing on said part a metallic structure, preferably composed of M, Cr, Al, and Y, where M is a metal selected from the group consisting of Ni, Co, Fe, and mixtures thereof, with the possible addition of Ta, by electrophoretic deposition under conditions so as to obtain a structure of cellular form wherein the cells are of a predetermined size and are substantially evenly distributed; 
     b) subjecting said part with said deposited cellular metallic structure to a consolidation treatment, preferably consisting of a sintering process, which may be reactive, or a metallization process, particularly a vapour phase process, under conditions of temperature and time known per se for application to said alloy, so as to consolidate said structure on said part; and 
     c) applying a ceramic-based powder to said consolidated structure on said part by atmospheric plasma spraying to complete said protective coating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1a, 1b and 1c are diagrammatic sectional views of a part with a known form of protective coating, and have been described earlier; 
     FIGS. 2a, 2b and 2c are similar sectional views of a part with a coating in accordance with the invention and FIG. 2d is a sectional view of a part with an alternative form of coating in accordance with the invention; 
     FIGS. 3a and 3b show test pieces used in carrying out protective coating performance tests on a coated superalloy part in accordance with the invention; 
     FIGS. 4, 5 and 6 are graphs showing the variation of the mass of metallic powder deposited according to different electrophoretic deposition parameters in a method in accordance with the invention; 
     FIG. 7 is a diagrammatic view illustrating the cellular form of the metallic structure obtained by electrophoretic deposition in the method of the invention; 
     FIGS. 7a and 7b are sectional diagrams illustrating the structure of the final coating obtained in two embodiments of the invention; 
     FIGS. 8a, 8b, 8c and 8d show scanning electron microscope photographs of different cellular metallic structures obtained according to the parameter values selected for the electrophoretic deposition in a method of the invention; 
     FIGS. 9a and 9b show scanning electron microscope photographs of two cellular metallic structures after consolidation treatment of the electrophoretic deposition; 
     FIGS. 9c and 9d are scanning electron microscope photographs showing details of the bond between the deposited coating and the substrate; 
     FIG. 10 shows a scanning electron microscope photograph of a final coating structure in accordance with the invention, and FIG. 10a shows a detail of FIG. 10 to a larger scale; 
     FIG. 11 is a diagram plotting a heat cycle applied to a test piece coated in accordance with the invention; and, 
     FIG. 12 shows diagrammatically the results of thermal shock behaviour tests carried out on various test pieces following the cycle of FIG. 11. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Test pieces 10 and 11 represented in FIGS. 3a and 3b were used for the production of a protective coating in accordance with the invention. In this example, the basic material of the test pieces 10 and 11 was a nickel-based superalloy of the following composition in percentages by weight: 
     
         ______________________________________C = 0.05-0.15;        Si = 1 maximum;                      Mn = 1 maximum;Cr = 20.5-23.0;        Fe = 17.0-20.0;                      Mo = 8.0-10.0;Co = 0.50-2.50;        W = 0.20-1.0  andNi the balance to 100.______________________________________ 
    
     After preparation by cleaning and polishing in a known manner, a test piece, such as 10 or 11, was mounted in a device, known per se, permitting the production of an electrophoretic deposition, the said test piece being mounted in the cathode position. 
     In this example the bath used had a base of methanol --CH 3  OH, and the electrolyte was aluminium chloride Al 2  Cl 6 . Various concentrations of electrolyte were tested, particularly at 0.5 g/litre, but was kept below 1.5 g/litre. The powder to be deposited was of type M, Cr, Al, Y as defined earlier and in this example had the following composition in percentage by weight: 
     Cr=21; Al=8.47; Y=0.59; Ta=5.7; and Ni the remainder. 
     The powder comprised spherical particles having diameters between 45 μm and 75 μm. 
     Various quantities of powder between 1500 and 2000 g/l were tested, and good results were achieved using 2000 g/l. 
     The electric field applied was kept below a strength of 2500 V.cm -1  and the current density at a value below 100 mA.cm -2 . The temperature of the bath was maintained at a temperature between 15° and 35° C., and good results were obtained at an ambient temperature between 18° and 21° C. In the course of the electrophoretic deposition process the different chemical reactions may be represented as follows: 
     the solution of the aluminium chloride in methanol produces the reactions: 
     a) with the residual water contained in the methanol, 
     
         Al.sub.2 Cl.sub.6 +6H.sub.2 O→2 [Al(OH).sub.3 ]+6H Cl 
    
     b) with the methanol, 
     a first ionization; 
     
         Al.sub.2 Cl.sub.6 +6 CH.sub.3 OH→2 [AlCl.sub.2 OCH.sub.3 ]+4 CH.sub.3 OH+2 H Cl 
    
     a second ionization: 
     
         Al.sub.2 Cl.sub.6 +6 CH.sub.3 OH→2 [AlCl(OCH.sub.3).sub.2 ]+2 CH.sub.3 OH+4 H Cl 
    
     and (possibly a third ionization: 
     
         Al.sub.2 Cl.sub.6 +6 CH.sub.3 OH→2 [Al(OCH.sub.3).sub.3 ]+6 Hcl 
    
      Under these conditions methanol and hydrochloric acid react to give a gaseous release of methyl chloride CH 3  Cl(catalytic effect of Al 2  Cl 3 ); 
     on the introduction of the NM, Cr,Al, Y powder, the aluminium hydroxide and the aluminium alkoxide and chloroalkoxides are adsorbed on the surface of the M, Cr, Al, Y particles to generate a surface charge density; 
     after the application of the electric field, an electrophoresis and an electrolysis take place simultaneously, the voltage between the electrodes corresponding to the voltage supplied by the generator and simultaneously to the deposition of M, Cr, Al, Y powder onto the surface of the cathode constituted by the part or test piece 10 or 11 to be coated, there also being a release of hydrogen at the cathode. 
     Under the established conditions indicated, the deposition obtained exhibits a cellular structure brought about by the said release of hydrogen. An even distribution of the cells is obtained under the conditions indicated and the size of the cells may be adjusted, depending upon the desired structure for the particular application envisaged, by varying certain parameters of the electrophoretic deposition process, particularly the strength of the electric field or the temperature. 
     FIG. 4 illustrates the variation of the mass of powder deposited in mg/cm 2 , plotted as ordinates, according to the time of deposition in seconds, plotted as abscissae, with set conditions of temperature at 23° C., electrolyte concentration at 1 g/l, and initial quantity of M, Cr, Al, Y powder at 2000 g/l, and different electric field strengths as follows: 
     54V.cm -1  for curve 4A 
     108V.cm -1  for curve 4B 
     180V.cm -1  for curve 4C 
     360V.cm -1  for curve 4D 
     710V.cm -1  for curve 4E 
     Similarly, FIG. 5 illustrates the variation of the mass of powder deposited in mg/cm 2 , plotted as ordinates, according to the strength of the electric field applied in V.cm -1 , plotted as abscissae, with the same conditions of temperature, concentration of electrolyte, and quantity of M, Cr, Al, Y powder as in FIG. 4, and different deposition periods as follows: 
     9 s for curve 5A, 
     15 s for curve 5B, 
     30 s for curve 5C, 
     60 s for curve 5D. 
     Similarly, FIG. 6 illustrates the variation of the mass of powder deposited in mg/cm 2 , plotted as ordinates, according to the temperature of the bath in ° C., plotted as abscissae, with the same conditions of electrolyte concentration and quantity of M, Cr, Al, Y powder as in FIGS. 4 and 5, a deposition period of 15 seconds, and different electric field strengths as follows: 
     55 v.cm -1  for curve 6A 
     80 V.cm -1  for curve 6B 
     110 V.cm -1  for curve 6C 
     FIG. 7 shows a diagrammatic representation of an example of the cellular structure of the metal undercoat obtained by electrophoretic deposition under the conditions defined. As shown, an even distribution of cells 12 is obtained. 
     FIGS. 8a, 8b, 8c, 8d depict examples of the cellular structure obtained by varying parameters of the electrophoretic deposition, i.e. the strength of the electric field and/or the temperature, the other conditions and the deposition time (equal to 9 seconds) being fixed. 
     FIG. 8a shows a structure with small cells of size d c  below 100 μm obtained at 8° C. and 100 V.cm -1 . On the other hand, FIG. 8b shows a structure exhibiting large cavities of size d c  of the order of 500 μm and obtained at 31° C. and 130 V.cm -1 . 
     Low cell densities and variations of coating thickness may also be obtained, depending on the strength of the electric field. For example, FIG. 8c shows a structure with a single layer deposition of a thickness of the order of 50 μm obtained at 23° C. and 20 V.cm -1 , whereas FIG. 8d shows a relatively compact deposition structure of the order of 500 μm thickness obtained at 23° C. and 110 V.cm -1 . 
     The bath used comprising methanol with an aluminium chloride electrolyte provides additional advantages in permitting very short deposition times, thus preventing the heating up of the bath, and preventing stray depositions, the presence of aluminium hydroxychloride in particular being below 1 mg/cm -2 . In addition, the drying of the deposition as it comes out of the electrophoretic solution is immediate as a consequence of the low vapour pressure of methanol. 
     The need for adequate mechanical strength, amongst other things, of the electrophoretically deposited M, Cr, Al, Y structure, leads to the provision of the consolidation treatment for the cellular metallic structure coating the superalloy part. This treatment also aims at ensuring satisfactory chemical protection properties for the coating. One method of carrying out this treatment comprises performing a thermo-chemical aluminizing treatment in the vapour phase. The temperature conditions and the duration of this treatment suitable for the superalloy constituting the basic substrate of the part to be coated are well established, and have been described in particular in U.S. Pat. No. 3486927. It will not therefore be necessary to expand on other details of the treatment which are standard knowledge. 
     FIGS. 9a and 9b show scanning electron microscope photographs of two test pieces having undergone this aluminizing treatment in the vapour phase. For the test piece of FIG. 9a the duration was 1 hour at 1155° C. The initial structure is preserved and the sectional view of the test piece shown in FIG. 9c, as well as the detail of the bond between the substrate and the deposit shown in FIG. 9d, shows the absence of detachment and the good bonding with the substrate. In the test piece of FIG. 9b the duration of the process was 3 hours at 1150° C. A good consolidation is also obtained, but the deposit is slightly less porous. 
     The protective coating is completed by the application of a ceramic material forming a thermal barrier. The constituent chosen is zirconium oxide ZrO 2  having its phase stability ensured by another mixed oxide. In the example produced, the powder used comprised 8% Y2O 3  by weight mixed with ZrO 2 , and had a grain size between 45 and 75 μm. An atmospheric type plasma spraying under operating conditions usual for this type of application was carried out to apply the ceramic powder material. 
     After spraying the ceramic, the initial cellular form of the consolidated metallic structure was retained. FIG. 7a shows a diagrammatic representation of a fully coated part, showing at 10 the superalloy substrate, at 12a the metallic structure of cellular form, and at 13 the ceramic material. Depending on the intended usage of the part, a coating structure of the type shown in FIG. 7a may be desired. Alternatively, a structure as shown in FIG. 7b may be obtained, in which parts of the cellular metallic structure 12a are flush with the surface of the coating obtained after application of the ceramic material 13. FIG. 10 shows a scanning electron microscope photograph of an example of a coated part in accordance with the invention showing the filling of the cells of the metallic structure with the ceramic material, and FIG. 10a shows a magnified detail. 
     Various tests of plasma spraying of the ceramic concerned were carried out successfully with varying morphology of the cell structure of the metallic undercoat used, e.g. structures with a cell size d c  which is either below 100 μm, between 100 and 300 μm, or greater than 300 μm. 
     Tests were also carried out to test the stability of. coated superalloy parts in accordance with the invention under conditions representative of the conditions likely to be experienced by the coated parts during use. A particular and significant test relates to thermal shock resistance. It consisted of subjecting the test pieces coated in accordance with the invention to thermal cycles corresponding to the cycle represented in FIG. 11 and decaying in 15 minutes to 110° C. followed by a cooling down in ambient air for 15 minutes. 
     FIG. 12 shows in diagrammatic form the results obtained on six test pieces. Two reference test pieces T1 and T2 were coated solely by plasma spraying with a metal undercoat of M, Cr, Al, Y composition and with an outer ceramic coating, while four test pieces E 1 , E 2 , E 3 , E 4  were given a coating in accordance with the invention. The length of life of each test piece is represented in FIG. 12 by the number of cycles indicated as ordinates corresponding to each test piece. With the reference test pieces T 1  and T 2 , fissuring and detachment of the ceramic coating were observed after the number of cycles indicated. Test piece E 1 , at a duration equal to that of T 2 , exhibited low fissuring but no detachment. Test pieces E 2  and E 3  have a longer life than T 2 , and after 2083 cycles (instead of 780 cycles for T 2 ) E 3  showed fissuring but no detachment. E 4  was subjected to a more severe thermal cycling comprising 8 minutes at 1100° C. and 2 minutes forced cooling in compressed air, but nevertheless its life was greater than 2000 cycles. 
     From these results and the micrographic observations carried out it has been shown that the intended aims of the invention have been attained, particularly the modification of the distribution of the stresses, especially of thermal origin, at the interface between the cellular metallic structure and the outer ceramic coating. As noted earlier with reference to FIGS. 2a, 2b and 2c, the propagation of cracks is opposed or blocked by the presence of cells in the metal undercoat, but it seems also that a lower level of stresses is obtained at the metal/ceramic interface as a result of improved ductility of the metallic structure due to its cellular form. As a result of the cellular structure there is, in particular, an improved accommodation of thermal expansion, and rupture inception points may occur at the metal/ceramic interface in a very dispersed manner, permitting distribution of the stresses at a lower level at each point. Indeed, the level of stresses resulting from differential metal/ceramic expansion is no longer determined by the dimensions of the coated parts but by the size and the distribution of the cells formed in the coating. 
     Other advantages have been noted resulting from the particular structure of the protective coating in accordance with the invention. In particular, the heat insulation provided by the coating is increased as a result of the presence of the cells in the metallic structure which are filled with ceramic material. Moreover, the thermo-chemical aluminizing treatment in the vapour phase applied in the described embodiments of the invention, in addition to the consolidation of the cellular metallic structure also ensures excellent chemical protection from the said treatment. 
     Other test examples have also been made using flat plates of 30×30×5 mm of superalloy and have led to the same good results, which shows that superalloy parts of various shapes can be coated in accordance with the invention.