FR2784438A1 - High temperature joint for space vehicle has metal substrate with two coating layers, upper having conformable surface for joint sealing - Google Patents

Abstract

The high temperature joint for a space vehicle has a metal substrate (1). A first coating is of a high elastic strength metal with a thickness of about fifteen microns and a second coating is of metal with a thickness of between two and six microns and low hardness but an ability to flow to accommodate surface roughness on the joint sealing face.

Description

Field of the invention
The present invention relates to high temperature metallic static seals which must provide a high level of tightness for a sufficient period of time under extreme operating conditions, in particular in temperature ranges which may exceed 1000 K, in particular for applications in the space sector. .

Prior art
Various types of metal static seals of various configurations are already known which are intended to be used in harsh environments, with operations under high pressure and in very high or on the contrary very low temperature ranges.

 Examples of such seals are described in particular in documents EP-A-0241350, EP-A-0711938 and EP-A-0851158.

 Among the known static seals, however, no static seal has the power to maintain the same level of tightness in a very wide temperature range which can range from 73 K to 1100 K. Certain solutions have been proposed to allow temperature resistance of a seal for a temperature value of the order of 1000 K. However, such solutions imply a deterioration in the quality of the seal compared to standard temperature conditions.

 A first solution already proposed is illustrated in Figures 11 to 14.

 According to this first known embodiment, a static seal comprises a metal substrate 40 on which at least in the zones of sealing contact with a part 50 vis-à-vis which sealing must be carried out, a coating is formed. 'silver 60. There is shown a part 50 whose surface condition 51 has irregularities.

 FIG. 11 represents the known joint before it is brought into contact with the part 50. The reference 67 designates the interface between the coating 60 in silver and the metal substrate 40. The reference 66 designates a limited diffusion of the silver in the substrate . The reference 61 designates the regular outer surface of the silver coating 60 which is intended to come into contact with the irregular surface 51 of the part 50.

 The silver coating 60 aims to allow adaptation of the surface of the seal to the irregular surface 51 of the part 50. Thus, as can be seen in FIG. 12, when the seal is pressed against the part 50 by an effort symbolized by the arrow F, the silver coating 60 flows and fills the free spaces defined between the roughness of the surface 51 of the part 50. The free surface 62 of the silver coating 60 then follows the shape of the irregular surface 51 of Exhibit 50.

 FIG. 13 shows the phenomena which occur when the surrounding temperature exceeds 950 K. Silver particles 64 are transferred into the part 50 on the other side of the irregular surface 51 and cause a phenomenon of sticking of the silver coating 60 on Exhibit 50.

 FIG. 14 shows the phenomena which occur after the phenomenon of bonding of the silver coating 60 on the part 50. After a period of operation at high temperature (950 to 1000 K) of the order of 15 minutes or after several thermal cycles , a break occurs in the connection between the seal and the part 50 by cracking of the silver coating 60. The crack 80 formed in the silver coating 60 constitutes a loss of sealing which ensures degradation of the seal, which can no longer be considered as correctly fulfilling its function.

 Another technology will also be described, with reference to FIGS. 15 and 16, proposed for the production of seals which must withstand high temperature.

 According to this other technology, a static seal comprises a metal substrate 40 on which, at least in the zones of sealing contact with a part 50 vis-à-vis which the sealing must be carried out, a coating of nickel 70. FIG. 15 represents the known joint before it is placed against the part 50, the surface condition of which 51 has irregularities.

 Reference 77 designates the interface between the coating 70 in silver and the metal substrate 40. Reference 76 designates a limited diffusion of nickel in the substrate. The reference 71 designates the regular outer surface of the nickel coating 70 which is intended to come into contact with the irregular surface 51 of the part 50.

 The nickel coating 70 constitutes a hard material with high elastic resistance intended to avoid a phenomenon of sticking at high temperature. However, as can be seen in FIG. 16, when the seal is in contact with the part 50 and pressed against the latter by a force symbolized by the arrow F, the surface 72 of the coating 70 made of nickel is deformed without however being able perfectly adapt to the surface 51 of the part 50, due to the high hardness of the nickel. There thus remain free spaces 73 between the surface 51 of the part 50 and the deformed surface 72 of the coating 70 made of nickel, which has the effect of reducing the performance and reliability of the seal.

Subject and brief description of the invention
The invention aims to remedy the aforementioned drawbacks of making it possible to obtain, at high temperatures, a level of tightness which is comparable to those which are obtained with static seals under standard conditions.

 These aims are achieved by means of a metallic static seal at high temperature, characterized in that it comprises at least one sealing contact zone constituted by a metallic substrate on which a first coating of a first hard metallic material is successively deposited. with high elastic resistance whose thickness is at least about 15 μm, and a second coating of a second metallic material whose thickness is between about 2 and 6 μm and whose hardness is low and the capacity of creep allows an adaptation of the geometry of the sealing contact zone to the surface state of a part with which the joint must seal.

 By virtue of such a structure, an adaptation of the surface of the seal in the contact zone can be carried out completely without the risk of cracking occurring.

 For example, the first coating is nickel and the second coating is silver.

 Preferably, the first coating has a thickness of between 20 μm and 40 μm.

 Advantageously, the second coating has a thickness of between 2 zm and 4 zm.

 According to a particular characteristic, the first coating is made of a metallic material whose hardness is between approximately 100 and 150 HV (Vickers hardness), and preferably of the order of 120 HV.

 Advantageously, the second coating is made of a metallic material whose hardness is of the order of 50 HV.

 The sealing contact area of the joint may be essentially flat or have a large radius of curvature.

 However, the seal quality of the seal can be further improved if the seal contact area is convex and has a curvature whose radius is less than or equal to 1 mm.

 The static seal according to the invention is suitable for ensuring a static seal in particular in a temperature range between 950 and 1100 K and can be used in particular for applications in the space field.

Brief description of the drawings
Other characteristics and advantages of the invention will emerge from the following description of particular embodiments, with reference to the appended drawings, in which: FIG. 1 is a three-quarter view (cut) in perspective of an example of static seal to which the invention is applicable, FIG. 2 is a schematic view in axial quarter-section of the example of seal in FIG. 1, FIG. 3 is a view in axial half-section showing an example of 'flange assembly incorporating a seal such as that of Figures 1 and 2, Figure 4 is a sectional view of a portion of the contact area of an example of static seal according to the invention, at t' rest state, FIG. 5 is a sectional view similar to that of FIG. 4, but when the static seal is subjected to a pressure force and is in an environment at ambient temperature of the order of 300 K, FIG 6 is a sectional view similar to that of Figure 5, but lor if the static seal is subjected to a prolonged pressure force and is in a high temperature environment of around 1100 K, FIGS. 7 to 10 show, in section, in cooperation with a part forming a seal seat , different states of the contact zone of a static seal according to the invention, successively in the rest state, when a pressure force is exerted on the seal at room temperature, when the seal is subjected to a temperature greater than 950 K and when the seal is in service at high temperature after a certain operating time, FIGS. 11 to 14 show, in section, in cooperation with a part forming a seal seat, different states of the contact zone d a first static seal of the prior art, successively in the rest state, when a pressure force is exerted on the seal at ambient temperature, when the seal is subjected to a temperature above 950 K and during if the seal is in service at high temperature after a certain operating time, FIGS. 15 and 16 show, in section, in cooperation with a part forming a seal seat, different states of the contact area of a second static seal prior art, respectively in the state of rest and when the seal is in service at high temperature under pressure after a certain operating time, and FIG. 17 is a set of diagrams showing the development of the level of tightness in time, as a function of pressure, at high temperature, on the one hand for a static seal of the prior art and on the other hand for a static seal according to the invention.

Detailed description of particular embodiments
We will first describe, by way of example, with reference to Figures 1 to 3, a possible configuration for a metallic static seal to which the invention is applicable. However, such a configuration is not limiting and the invention can be applied to seals having very different sections or overall shapes from the example shown in FIGS. 1 to 3.

 FIG. 1 shows the general appearance of an example of a static seal 10 which comprises an annular body 11 of axis Z'Z, extended laterally by lips 12, 13 themselves associated with end portions 14, 15 defining the seal contact surfaces 10. The seal 10 has, in axial half-section (FIGS. 2 and 3), the shape of a coated U.

 As can be seen in particular in FIG. 2, in the rest position, the profile of the seal 10 in axial half-section comprises a curved part 11 in the form of C delimited by a curved external surface 21 and free of angles and by a inner surface 16 also curved and free of angles, the curved part 11 constituting the annular body of the seal.

 The curved part 11 is extended laterally by connecting parts 12, 13 defining the lips of the joint, each of these connecting parts 12, 13 comprising a flat outer surface 22, perpendicular to the axis of the joint and tangentially connected to the surface. curved outer surface 21, and a flat inner surface 17 which is tangentially connected to the curved inner surface 16.

 The convex end parts 14, 15 which seal the end of the holes in the joint are connected to the connecting parts 12, 13 in a progressive manner, with no re-entrant angle. The convex end portions 14, 15 can thus each be delimited, on the one hand, by a flat internal surface which extends and is in alignment with the flat internal surface 17, so as to form therewith a continuous flat surface and, on the other hand, by a curved outer surface 24 which constitutes a contact area and protrudes from the planar outer surface 22, being progressively connected to the latter.

 Figure 3 shows an example of possible mounting of the seal 10 in a flange assembly. The seal 10 is arranged on a countersink 122 of a first part 102 formed by a pipe 120 terminated by a flange 121. One of the faces 112 of the housing 140 is constituted by the front face of a flange 111 of a second part 101 also defining a pipe section 110. The flanges 111, 121 are assembled by connecting means such as bolts 130. This mounting example is in no way limiting and is simply intended to illustrate the cooperation between contact areas of the seal 10 and external parts 110, 120 defining a seat or a housing for the joint.

 The particular structure of the static seal according to the invention will now be explained with reference to FIGS. 4 to 6 which represent in section, a seal contact area, such as for example the convex end portions 14, 15 of the U-shaped seal 10 in the figures. 1 to 3.

 The joint contact zone comprises a metal substrate 1 covered with a hard metallic material, which at high temperature does not present any risk of sticking to the interface part cooperating with the joint in the contact zone intended to produce the seal. . The coating 2 is made of a metallic material whose hardness is advantageously between 100 and 150 HV, and preferably is of the order of 120 HV. The coating 2 made of hard material, such as nickel, preferably has a thickness greater than 15 μm, and more particularly a thickness of between 20 μm and 40 m.

 The coating 2 of hard material is itself covered with a thin layer 3 of a metallic material of low hardness, for example of the order of 50 HV, which, by its creep capacity, allows adaptation to the defects of surface of the interface parts cooperating with the seal. The coating 3 can be produced for example in silver.

 The thickness of this silver coating 3 can be between 2 and 6 μm and preferably between 2 and 4 m.

At ambient temperature, of the order of 300K, the coating 3 has, under the effect of a pressure force (exerted according to arrow F in FIG. 5) which can be low, a creep which produces extra thicknesses 3a able to penetrate the surface irregularities of the interface parts and thus capable of ensuring an adaptation of the external surface of the seal to the irregular surface of the interface part. Up to temperatures of the order of 950 K, the silver coating 3 does not present any risk of sticking. When the temperature rises,
The silver simply has a larger creep.

 The outward migration of the extra thicknesses 3a of silver due to creep is favored by the convex profile C of the joint contact zone.

This convex profile allows the silver to escape to either side of the contact area.

 FIG. 6 shows the state of a seal according to the invention at high temperature, of the order of 1000 to 1100 K.

 Gradually, as the temperature tends towards the 1000-1100 K range, the nickel of the coating 2 is exposed in a zone 2b.

The risk of sticking of the silver is then low due to the small thickness of the coating 3 and to the fact that the silver has continued to be spread in extra thicknesses 3b on either side of the main contact zone, in regions where the residual contact force is negligible.

 At high temperature, and if the contact force exerted in the direction of arrow F is sufficient, the nickel of the coating 2 itself exhibits a creep phenomenon with extra thicknesses 2a which are created on either side of the zone of main contact. The contact force is increased when the profile C is convex as shown in FIGS. 4 to 6. The radius of the convex part C of the joint is advantageously less than 1 mm and can be of the order of 0.4 to 0 , 6 mm.

 Note however that the invention is applicable to any joint profile as soon as the material of the substrate 1 is compatible with the deposition of a coating 2 of a material such as nickel.

 The behavior of a static seal in accordance with the invention subjected to temperature variations ranging from ambient temperature or from low temperatures to very high temperatures of the order of 1100 K will be described more explicitly with reference to FIGS. 7 to 10.

 FIG. 7 shows the initial state of a portion of joint according to the invention, which comprises a metal substrate 1 on which is deposited a first coating 2 of nickel defining an interface 177 with the substrate 1. The first coating 2 may have a thickness preferably between 20 and 40 lim. The reference 176 designates a weak diffusion of nickel particles in the substrate 1. A second coating 3 in silver is deposited on the first coating 2 in nickel.

The optimal thickness of the second coating 3 is between 2 and 4 μm. However, good results can still be obtained in a thickness range of between 2 and 6 µm. In the initial state the second coating 3 defines a regular interface 171 with the first coating 2 and has an external free surface 161 also regular, without marked roughness.

 In FIG. 7, the portion of the seal intended to ensure sealing is not yet in contact with the interface part 150 having a surface 151 provided with asperities, with which it must cooperate. FIG. 7 thus shows the surface condition 161 of a seal sealing track before its first contact with the part 150 forming a housing or seal seat.

 FIG. 8 represents the state of a portion of seal according to the invention after it has been brought into contact with the interface part 150 at ambient temperature, with a pressure force which is exerted in the direction of arrow F and can remain relatively modest. Taking into account the good creep characteristics of the silver coating 3, the assembly of the two coatings 2 and 3 is deformed so as to ensure an excellent adaptation of the external surface of the seal to the irregular surface 151 of the external part 150. outer surface 162 of the coating 3 thus perfectly matches the irregular surface 151 of the part 150. The interface 172 between the coatings 2 and 3 in nickel and silver is also deformed without however exactly corresponding to the surfaces 151 and 162, taking into account the hardness of nickel which prevents as great an adaptation as silver. The larger spaces 163 formed between the surface 151 and the interface 172 are however entirely occupied by silver from the coating 3 so that an excellent seal is already achieved at room temperature.

 FIG. 9 corresponds to an increase in temperature to a level of the order of 950 K. It can be seen that the seal between the seal and the part 150 is retained in its entirety, but that silver particles 164 aired in room 150.

 FIG. 10 represents the state of the seal at high temperature, for example at 1100 K, after a certain period of use, for example of the order of 15 to 30 minutes. It is noted that, taking into account the small thickness of the coating 3, it cannot occur in this coating 3 of crack in a zone located in the joint, below the irregularities of the surface 151, as for example in the achievements of Prior art of FIG. 14, and taking into account the presence of the silver coating 3 on the nickel coating 2, there can also be no significant free space in the vicinity of the surface 151, as c this is the case for example with the embodiments of the prior art of FIG. 16.

 In the seal according to the invention, if a slight bonding of the silver coating 3 on the surface 151 of the part 150 sometimes occurs at high temperature after a certain operating time, this slight bonding does not cause anything other than a minute slot 180 which perfectly matches the irregularities of the surface 151 of the part 150 and therefore does not affect the sealing barrier, as long as the silver remains present in the zones 163 where the distance between the interface 172 separating the coatings 2 and 3 and the surface 151 of the part 150 is the largest and since no crack can occur in the coating 2 taking into account the hardness of the material constituting this coating.

 FIG. 17 illustrates the improvement in the level of tightness obtained with a static seal according to the invention comprising on a metal substrate 1 a first coating of relatively thick nickel 2 and a second coating of very thin silver 3 (2 hum ), with respect to a seal of the prior art such as that illustrated in FIGS. 15 and 16, in which only a relatively thick nickel coating 70 is deposited on a metal substrate 40.

 The curve F1 illustrates the evolution of the level of leakage in Pa. M3 I s of the prior art seal as a function of the time expressed in hours, at a temperature of 1000 K for a pressure evolving progressively as a function of time up to 20 M Pa (200 bar) according to curve P1.

 Curve F2 illustrates the evolution of the level of leakage in Pa. M31 s of the seal according to the invention as a function of the time expressed in hours, at a temperature of 1000 K for a pressure which increases progressively as a function of time up to 25 MPa (250 bar) according to curve P2.

 The seal of the prior art and the seal according to the invention differ in the nature of their coating, but are identical in terms of their general shape. In the example taken into account, the seals have a configuration which essentially corresponds to that illustrated in FIGS. 1 to 3.

 It can be seen that the maximum level of leakage obtained with the seal according to the invention remains approximately a million times lower than with a seal of the prior art. Furthermore, with the seal according to the invention (curve F2), stabilization is quickly obtained over time at a satisfactory level of tightness, of the order of 10-8 Pa. M3 / s.

 On the contrary, with the seal according to the prior art, the level of leakage increases sharply with the increase in pressure, and when the latter is stabilized at a value of 20 MPa (200 bar), the level of tightness does not '' improves only very slowly (curve F1), the residual leakage level remaining, after several hours, still on the order of a thousand times higher than the residual leakage level obtained already after half an hour of operation at high temperature with a seal according to the invention (curve F2).

 It can thus be seen that the invention brings major progress in the production of metallic static seals which are capable of maintaining high performance in terms of sealing over a wide range of temperatures from low temperatures, for example of the order of 73 K, up to temperatures of the order of 1100 K passing through standard temperatures of the order of 300K.

 These performances make the seals according to the invention particularly useful in the space field.

 In the foregoing, it has been specified that at least the contact areas of the joint must include a metal substrate 1 covered with a double coating 2,3. Naturally, for reasons of manufacturing convenience and homogeneity of the joint, the entire surface of the static joint, such as the U-shaped joint 10 in FIGS. 1 to 3 may have the double coating, including in areas n 'ensuring no sealing contact.

Claims (8)

 1. High temperature metallic static seal, characterized in that it comprises at least one sealing contact zone constituted by a metallic substrate (1) on which are successively deposited a first coating (2) of a first metallic material hard with high elastic resistance whose thickness is at least about 15 μm, and a second coating (3) of a second metallic material whose thickness is between about 2 and 6 m and whose hardness is low and the creep capacity makes it possible to adapt the geometry of the sealing contact zone to the surface state of a part with which the joint must seal.  2. Static seal according to claim 1, characterized in that the first coating (2) is made of a metallic material whose hardness is between 100 and 150 HV.  3. Static seal according to any one of claims 1 and 2, characterized in that the second coating (3) is made of a metallic material whose hardness is of the order of 50 HV.  4. Static joint according to any one of claims 1 to 3, characterized in that the first coating (2) has a thickness between 20 pm and 40 m.  5. Static seal according to any one of claims to 4, characterized in that the second coating (3) has a thickness between 2 hum and 4 m.  6. Static seal according to any one of claims 1 to 5, characterized in that the first coating (2) is nickel and the second coating (3) is silver.  7. Static seal according to any one of claims 1 to 6, characterized in that the sealing contact area is convex and has a curvature whose radius is less than or equal to 1 mm.  8. Static seal according to any one of claims 1 to 7, characterized in that it is adapted to ensure a static seal in a temperature range between 950 and 1100 K.