FR2873783A1 - Internal combustion engine piston`s skirts coating method, involves coating skirts with paste which has polyaryletherketone polymer, and heating dry lubricating film to temperature such that film assumes amorphous structure - Google Patents

Abstract

The method involves coating skirts (7a, 7b) with a paste which is disposed in an aqueous solvent and comprises of a polyaryletherketone polymer having a low friction coefficient. The paste is then dried so as to obtain a dry lubricating film (3). The dry lubricating film is heated to a temperature for a predetermined time, such that the film assumes an amorphous structure. An independent claim is also included for skirts of a piston of an internal combustion engine coated with a dry lubricating film.

Description

The present invention relates to a method for coating a surface

  convex, including a piston skirt of an internal combustion engine, a dry lubricant film with high mechanical strength and low coefficient of friction.

  The object of the invention is mainly to reduce the friction between the piston skirt and the cylinder.

  It is indeed known that the main function of the piston skirts is to ensure the guiding of the piston in the cylinder, avoiding any risk of seizure, the upper part of the piston being composed of segment holders which ensure the seal between the chamber. combustion and the lower part of the engine. The friction between the skirt and the resulting cylinder are therefore sources of noise, energy loss and damage to the surfaces.

  In order to overcome these disadvantages, several solutions have been envisaged. The introduction of a set of several microns between the skirt and the cylinder accompanied by a lubricant such as oil makes it possible to avoid seizing problems but creates a significant oil consumption.

  Other solutions consisting in coating the skirt with a coating resistant to galling and with a low coefficient of friction seem to have been the preferred track of car manufacturers and equipment manufacturers.

  Thus, the patent application EP0380415 describes a composite coating, based on a resin of the family of sulfonic polymers, which is fixed to the skirt of the piston by screwing, hooping or gluing. This invention has the disadvantage of adding an assembly step between the piston skirt and the very constraining coating in terms of manufacturing and mechanical strength.

  Similarly, the patent FR2808461 proposes a process for depositing pure polyaryletherketone by pure thermal spraying on the piston skirt, but this has the disadvantage of being constraining in terms of equipment, since it requires the use of projection torches.

  Finally, patent EP0614416 proposes a solution which consists in coating the piston skirt with a pad of a paste comprising a moistened dry lubricant and then drying the paste. The dry lubricant is composed of a fluorinated polymer or graphite or molybdenum disulfide mixed with a binder and a highly volatile organic solvent such as acetone. This method has the advantage of being simple to implement, since it is essentially based on the transfer buffer coating step, but has the disadvantage of using a highly volatile organic solvent, which requires the use of heavy pollution control accessories to manage in a production line.

  In addition, if fluoropolymers, graphite and molybdenum disulfide give the coating a good coefficient of friction, they do not give as far as satisfaction in terms of mechanical strength.

  Therefore, the object of the present invention is to provide a method for coating a convex surface, in particular a piston skirt of an internal combustion engine, a dry film of high mechanical strength and low coefficient of friction. friction overcoming all or part of the disadvantages noted in the prior art detailed above.

  This object is achieved by the fact that said method consists in: Coating the convex surface of a paste made in an aqueous solvent and comprising a polymer with a low coefficient of friction as well as non-organic compounds, Drying said paste so as to obtain a dry lubricating film, - heating said film at a temperature T for a time t, so that said film has an amorphous structure, cooling said film so that said film largely retains its amorphous structure, according to d. Other features: The step of coating the convex surface with said paste may be preceded by a step of preparing the convex surface which comprises at least a first step of cleaning said convex surface by means of a solvent or 'A detergent.

  The step of preparing the convex surface may comprise a second step for increasing or decreasing the roughness of said surface. This may be a sanding or grinding operation.

  The temperature T is preferably between 350 ° C. and 450 ° C.

  The duration t is preferably between 5 s and 15 s.

  Said dough can be made from a mixture of water, a binder in the proportions of 2 to 5% by weight, a dispersant in the proportions of 1 to 3% by weight, an agent anti-foaming agent in the proportions of 5 to 30 ppm, to which is added the polymer with a low coefficient of friction in the proportions of 60 to 80% by weight, and one or more non-organic compounds in proportions of less than 10% by weight. mass.

  Said polymer with a low coefficient of friction is preferably a polyaryletherketone.

  Said non-organic compounds are one or more of the following compounds: molybdenum disulphides, carbon nanotubes, carbon, silicon carbides or graphite.

  Another object of the invention is to provide two piston skirts of an internal combustion engine coated with a dry lubricant film according to the method described above.

  The invention will be better understood on reading the description which follows, which is written by way of non-limiting example, and in the single appended figure which represents the upper part of a piston 1 of an internal combustion engine comprising two skirts 7a, 7b coated with a dry lubricant film 3 according to the invention.

  Prior to the deposition of the film 3 on the skirts 7a, 7b, the outer surface of the piston 1 is cleaned by means of a detergent or a solvent in order to remove impurities as well as any protective coating applied to protect the piston 1 during his transport.

  The operation for increasing the roughness of the skirts 7a, 7b may be, for example, a sanding operation which consists in projecting silica grains at high speed onto the skirts 7a, 7b so as to create small impacts which favor the attachment of the film 3. However, this operation is optional insofar as the skirts 7a, 7b admit a base roughness of about 20 microns which allows sufficient attachment of the film 3 on said skirts.

  The preparation of the dough, not shown in FIG. 1, is made from a mixture composed of water, a binder in the proportions of 2 to 5% by weight, and a dispersant in the proportions of 1 at 3% by weight, an anti-foaming agent in the proportions of 5 to 30 ppm, to which is added a polymer with a low coefficient of friction in the proportions of 60 to 80% by weight, and one or more compounds inorganic in proportions less than 10% by weight.

  The polymer with a low coefficient of friction is a polyaryletherketone marketed under the name Poly Ether Ketone Ether (PEEK TM). It is a semi-crystalline polymer whose basic formula is the following: oxy-1,4-phenylene-oxy-1,4-phenylene-carbonyl-1,4-phenylene, shown in FIG. after: This polymer was chosen as well for its good tribological properties as for its great chemical inertia and good mechanical performance especially at high temperatures. For the invention, it is advantageous to use two ultra-fine powders of low viscosity, either with an average particle size of 25 μm, or with an average particle size of 10 μm.

  As regards the binder, it is advantageous to use RHODOVIOL 25/140 TM which is a polyvinyl alcohol obtained by hydrolysis of polyvinyl acetate. It helps to promote the adhesion of the paste on the surface to be coated.

  Concerning the anti-foaming agent, it is advantageous to use BEVALOID 581B TM. It is a mixture of aliphatic hydrocarbons and nonionic surfactants. It makes it possible to avoid the frothing phenomena that occur when the PEEK TM powder is mixed in the water.

  As regards the dispersant, it is advantageous to use SINNOPAL OP11TM. It is a nonanionic surfactant (Octylphenol ethoxyl), which makes it possible to avoid the settling of the PEEK TM powder in solution.

  Finally, concerning non-organic compounds, the invention recommends the use of molybdenum disulfides, carbon nanotubes, carbon, silicon carbides or graphite in proportions of less than 10% by weight. These inorganic compounds are in the form of a powder having an average particle size, G, according to that specified in the table below: non-organic compounds average particle size: G molybdenum disulfides 5 m <G <12 m carbon nanotubes 0 , 05 pm <G <0.2 m silicon carbides 0.2 m <G <0.5 m 1 μm <G <5 m carbon G <1 m graphite 1 m <G <5 m The presence in the dough one or more of these inorganic compounds is essential, since it is they which confer on the film 3 obtained by the deposition of the paste, improved properties in terms of friction and resistance to wear.

  The process of depositing the paste on the skirt 7a, 7b can be carried out by a method of the screen-printing type, which consists in rotating the piston 1 about its axis 2 and bringing the skirt 7a, 7b into contact with a screen, not shown in Figure 1, through which flows said dough. Advantageously, and unlike a thermal spraying method, it is not necessary to mask the region of the grooves 5 of the piston 1 provided for the segments, to prevent the paste from being deposited there.

  Once the skirt 7a, 7b thus coated, it is advantageous to heat the piston 1 at a temperature of about 200 C in order to partly dry the dough so as to obtain the dry lubricant film 3 and secondly to avoid the thermal shocks resulting from the heat treatment which will follow and which is located at said film 3 and not over the entire thickness of the skirt 7a, 7b.

  Indeed, so that said film 3 admits an amorphous structure, it is necessary to sinter it, ie to heat it at a temperature T for a period t. The heater can use YAG laser technology or CO2 lasers with power between 800 and 5000 Watt.

  Advantageously, the temperature T is between 350 ° C. and 450 ° C.

  Advantageously, the duration t is between 5 s and 15 s.

  So that the film 3 largely retains its amorphous structure, it must then proceed to its cooling. By using the YAG laser heater as described above, a simple cooling with ambient air provides a semicrystalline structure.

  According to the process described above, the thickness of the film 3 is from 20 to 100 μm.

  Thus, the addition of molybdenum disulfide or carbon nanotubes in proportions of 10% by weight makes it possible to reduce by approximately 15% the coefficient of friction corresponding to a film obtained with a paste made from a mixture composed of water, a binder in the proportions of 2 to 5% by weight, a dispersant in the proportions of 1 to 3% by weight, an anti-foaming agent in the proportions of 5 to 30 ppm and polyaryletherketone in proportions of 60 to 80% by weight.

  Likewise, the addition of molybdenum disulphides, carbon nanotubes, carbon, silicon carbides or graphite in proportions of 10% by mass makes it possible to increase the resistance to wear corresponding to a film obtained with a paste made from a mixture of water, a binder in the proportions of 2 to 5% by weight, a dispersant in the proportions of 1 to 3% by weight , an anti-foaming agent in the proportions of 5 to 30 ppm, and polyaryletherketone in the proportions of 60 to 80% by weight.

  According to another embodiment, the film can be sintered by means of another heating device than the laser, such as an induction heating device. The temperature T is then between 380 C and 450 C and the duration t is reduced to an interval of 4 to 8 s. The cooling advantageously comprises a first air quenching intended to cool the surface of the skirt 7a, 7b to a temperature of approximately 260 ° C. and then a second quenching with water, intended to return the film 3 to the temperature while retaining an amorphous structure.

Claims (14)

  1. A method for coating with a dry lubricant film (3) a convex surface (7a, 7b) characterized in that it consists in: a) Coating the convex surface (7a, 7b) with a paste made in a solvent aqueous and comprising a low friction polymer as well as inorganic compounds, b) Drying said paste to obtain a dry lubricating film (3), c) heating said film (3) at a temperature T for a period of time t , so that said film (3) has an amorphous structure, d) cooling said film (3) so that said film (3) largely retains its amorphous structure.   2. A method for coating with a dry lubricant film (3) a convex surface (7a, 7b) according to claim 1, characterized in that the step of coating the convex surface with said paste is preceded by a a step of preparing the convex surface (7a, 7b) which comprises at least a first step of cleaning said convex surface (7a, 7b) by means of a solvent or a lye, 3. Process for coating with a dry lubricant film (3) a convex surface (7a, 7b) according to claim 2, characterized in that the step of preparing the convex surface comprises a second step for increasing the roughness of said surface, 4. A process for coating a convex surface (7a, 7b) with a dry lubricating film (3) according to claim 3, characterized in that the second step consists of a sanding operation, 5. A method for coating with a dry lubricant film (3) a convex surface (7a, 7b) according to any one of the preceding claims, characterized in that the temperature T is between 350 C and 450 C, 6. Process for coating with a dry lubricating film (3) a convex surface (7a, 7b) according to any one of the preceding claims, characterized in that the duration t is between 5 s and 15 s, 7. A process for coating with a dry lubricant film (3) a convex surface (7a, 7b) according to any one of the preceding claims, characterized in that said paste is made from a mixture of water , a binder in the proportions of 2 to 5% by weight, a dispersant in the proportions of 1 to 3% by weight, an anti-foaming agent in the proportions of 5 to 30 ppm, which is added the polymer with a low coefficient of friction in the proportions of 60 to 80% by weight, as well as one or more non-organic compounds in proportions of less than 10% by weight.   8. A method for coating with a dry lubricant film (3) a convex surface (7a, 7b) according to any one of the preceding claims, characterized in that the polymer with a low coefficient of friction is a polyaryletherketone.   9. A process for coating a dry lubricant film (3) with a convex surface (7a, 7b) according to any one of the preceding claims, characterized in that the inorganic compounds comprise molybdenum disulfides.   10. A method for coating with a dry lubricant film (3) a convex surface (7a, 7b) according to any one of the preceding claims, characterized in that the inorganic compounds comprise carbon nanotubes.   11. A process for coating a dry lubricant film (3) with a convex surface (7a, 7b) according to any one of the preceding claims, characterized in that the non-organic compounds comprise carbon.   12. Process for coating a dry lubricant film (3) with a convex surface (7a, 7b) according to any one of the preceding claims, characterized in that the non-organic compounds comprise silicon carbides.   13. A process for coating a dry lubricant film (3) with a convex surface (7a, 7b) according to any one of the preceding claims, characterized in that the non-organic compounds comprise graphite.   14. skirt (7a, 7b) piston (1) of an internal combustion engine, characterized in that they are coated with a dry lubricant film (3) according to the method according to one of the preceding claims.