Patent Application
20080163751
1. Technical Field
This invention is generally related to pistons and methods of their manufacture. More particularly, it is related to pistons having coatings to improve scuff and wear resistance, friction, and noise and metal surface treatment to improve the adhesion of the coating(s) and methods of their manufacture.
2. Related Art
Al alloys, particularly Al—Si alloys are commonly used in the manufacture of pistons for application in internal combustion engines for automotive and other applications. While Al—Si alloy pistons have generally acceptable performance in a wide variety of internal combustion engine applications, there remains a need to improve the performance of pistons manufactured from these alloys with respect to critical sliding contact regions of these pistons.
One such sliding contact region during engine operation relates to the portion of the piston surface which functions as the interface between the piston and the cylinder sidewall, particularly the piston skirt. In internal combustion engines which use reciprocating pistons, the pistons incorporate a piston skirt. The piston skirt generally comprises the lowermost sidewall of the piston and is generally located beneath and extends downwardly from the lowermost ring groove.
The piston skirt is a portion of the piston during engine operation, and frequently incorporates a coating which is intended to improve its friction and wear properties during operation, such one of a number of different engineering thermoset and thermoplastic polymer coatings. These polymer coatings are generally applied as one of the final steps in the manufacture of Al alloy pistons. They are generally applied after the final machining of the piston surface, including the piston skirt, and ring grooves to obtain the desired profile and surface finish. However, such polymer coatings may not have sufficient adhesion strength when bonded directly to the finished surface Al alloy pistons. Therefore, pretreatment of the surface to be coated with an adhesion promoting process is frequently employed. One such pretreatment method and coating involves phosphate conversion of the surface of the piston. Phosphating is typically carried out with commercially available chemical processes. While generally effective at improving the adhesion of polymer coatings over those applied to untreated Al alloy surfaces control of the process can be difficult and costly and the phosphated surface can be detrimental on some un-coated parts of the piston. In addition to the limitation of process control of phosphated surfaces, the phosphate treatment process requires significant treatment of waste water and other effluents associated with the process.
In view of ever increasing demands for performance improvement in internal combustion engines, it is desirable to develop improved primer or adhesion promoting coatings for use on Al alloy pistons to improve the reliability of polymer coatings on piston wear surfaces, such as piston skirts. It is also desirable to develop improved coating processes which require reduced post-process treatment of residual process materials
One aspect of the invention is directed to an Al alloy piston, preferably an Al—Si alloy piston, having an improved coating on a predetermined portion of its outer surface. The predetermined portion of the outer surface is the piston skirt. The coating includes a primer coating of a silane and a polymer based coating, such as an epoxy or polyamide-imide/graphite/molybdenum disulphide coating. The silane of the primer coating and the polymer coating are selected so as to promote strong chemical bonding, including polymerization reactions, between them.
Pistons having such coatings represent an improvement over related art pistons and piston manufacturing methods, such as those which utilize phosphate primers, by utilization of a method which includes: fabricating a piston having an outer surface; applying a primer coating comprising a silane to at least a predetermined portion of the outer surface of the piston; drying the primer coating onto the outer surface of the piston; and applying a polymer coating to a predetermined portion of the outer surface of the piston.
According to another aspect, a method is provided for making a piston assembly, including preparing a piston, a connecting rod and a wrist pin. The piston is joined to the connecting rod by the wrist pin to establish an articulated piston joint with multiple contact surfaces. At least one of the contact surfaces is coated with a coating containing silane to reduce friction. The contact surfaces are preferably free of sliding bearings and/or bushings, with the silane material serving as a bearing layer material at the at least one contact surface.
These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein like figures have like designations, and wherein:
Referring to
Referring again to
A silane is the principal constituent of primer coating 32. A “silane” is any silicon analog of an alkane hydrocarbon. Silanes consist generally of a chain of silicon atoms covalently bound to hydrogen atoms. The general formula of a silane is SinH2n+2. This use includes cyclosilanes, or silanes in a ring configuration, analogous to cycloalkanes or alkanes in a ring configuration. It also includes various forms of branched silanes, again analogous to branched alkanes. As used herein, silane also refers generally to silanes in any of these forms having any number of associated organic or inorganic functional groups attached. The silane of primer coating 32 may be any silane that promotes or enhances the adhesion strength of polymer coating 34 to the Al alloy 19 which comprises outer surface 18 of piston 10. Preferably, a silane will be selected which contains functional groups which are adapted to provide a strong covalent or other chemical bond to one or more constituents of polymer coating 34. Therefore, the silane of primer coating 32 and polymer coating 34 will be selected to optimize the bond strength between them, such as by selecting them to promote cross-linking between the functional groups of the silane and the constituents of polymer coating 34. For example, if polymer coating 34 comprises an epoxy-based polymer, preferred silanes include gamma-aminopropyltrialkoxysilane, vinyltrialkoxysilane and various other epoxy silanes, such as those which include; in the uncured form, one or more reactive epoxide or oxirane groups Primer coating 32 will preferably be selected from a group consisting of: Primer coating 32 is preferably applied in a very thin layer, generally having a thickness of 1 μm or less, which is dried onto the predetermined portion of 30 of outer surface 18.
Polymer coating 34 may be any polymer coating which will provide wear resistance to piston 10 in a given application environment. Polymer coating 34 will also preferably include a plurality of particles 36 of a solid lubricant dispersed in polymer coating 34 as a matrix. Polymer coatings may include, for example, epoxy-based, polyamideimide-based and phenolic-based coatings and the like. The plurality of particles 36 of solid lubricant may include, for example, particles of graphite, molybdenum disulphide, polytetrafluoroethylene, hexagonal boron nitride (HBN) and the like. Polymer coating 34 will preferably have a thickness in a range of between about 2-40 μm, and more preferably in the range of about 8-20 μm.
According to another aspect of the invention, and referring to
The step of fabricating 110 a piston 10 having an outer surface 18 may be performed using conventional materials and processes. Piston will preferably be formed from an Al-alloy 19, and more preferably from an Al—Si alloy. Piston 10 may be fabricated by casting the general form of the piston followed by machining and the like of the features described above and shown in
Following the step of fabricating 100 piston 10, method 100 optionally, but preferably, includes a step of cleaning 120 the outer surface 18 of the piston 10. Cleaning 110 may be performed using any of a number of known cleaning techniques to remove oils and other residual materials from outer surface 18. This may include dry processes, such as plasma etching or the like, or various wet processes, such as applying liquid cleaning solutions to outer surface 18. Generally, wet processes utilizing alkaline cleaning solutions, such as those comprising KOH or NaOH are preferred. Cleaning solution concentrations, times and temperatures for use with Al alloys, including Al—Si alloys, are well-known.
When alkaline cleaning solutions are employed, method 100 also optionally, but preferably, includes a step of rinsing 130 the cleaning solution from the outer surface 18 of piston 10. When alkaline cleaning solutions, such as KOH or NaOH are utilized, rinsing 130 preferably employ the use of a rinse comprising purified water, such as water purified by deionization or by reverse osmosis filtration or the like. Rinsing times and temperatures for removing residual cleaning solutions of the types described herein are also well-known.
Method 100 also includes a step of applying 140 a primer coating 32 comprising a silane to at least a predetermined portion 30 of the outer surface 18 of the piston. Silanes are generally in the form of a liquid at room temperature and may be applied by any of a number of known methods for applying a liquid to a surface having the geometry of outer surface 18, including dipping piston 10 in a solution comprising the silane, spraying a solution comprising the silane onto outer surface 18, or other methods of application including screen printing, painting, pad printing or the like. When dipping is employed, it is preferable to immerse piston 10 in the silane solution, thereby providing primer coating over all of outer surface 18; however, partial immersion of the predetermined portion 30 of outer surface 18 also may be employed. When spraying, pad printing, painting or other application techniques are employed, primer coating may be applied to all of outer surface 18, or it may be limited just to predetermined portion 30, or variations between these bounds. Applying 140 of primer coating 32 by any of the techniques generally described herein may be performed using any of a number of well-known equipment and application methods. For many types of silanes and application methods, such as spraying or dipping, it is possible to collect and reuse the excess material, such as sprayed material which either does not strike, or strikes but does not remain on (i.e., drips off) the outer surface 18, or such as dipped material which runs off from outer surface 18. As such, this is principally an additive process which enables recycling of excess materials. This represents a significant improvement and simplification over methods used to apply related art primer coatings, such as phosphate coatings, where processes are often complex and the reactants used to prime the piston surface have limited recyclability, and where the reaction byproducts frequently require extensive and costly waste treatment.
Following the step of applying 140 primer coating 32, method 100 optionally, but preferably, includes a step of drying 150 the primer coating onto the portion of outer surface to which it has been applied, particularly if the solution comprising silane incorporates any volatile constituents. Drying 150 may include room temperature drying, or drying at an elevated temperature.
While it is possible to provide pistons 10 having only primer coating 32 applied as a product, whereupon, polymer coating 34 may be applied subsequently or not, it is preferred that method 100 also include a step of applying 160 polymer coating 34 of the types described herein to predetermined portion 30 of outer surface 18. The method of application will preferably be selected based upon the nature and properties of the polymer coating 34 to be applied. For application of polymer coatings 34, or unpolymerized or partially polymerized precursors to polymer coatings, as a liquid, the methods described above with regard to primer coating 32 are also applicable. For application of polymer coating 34 as a solid, other well-known application techniques may be employed. For example, if polymer coating 34 comprises an epoxy, the uncured epoxy precursor materials may be applied as a liquid, such as by pad printing, screen printing or painting.
Following the step of applying 160 polymer coating 34 to predetermined portion 30, it may be desirable to cure unpolymerized or partially polymerized constituents using any of a number of well-known polymerization techniques, including application of heat, light or polymerization initiators.
According to another aspect of the invention, the above described silane materials may be applied to one or more of the contact surfaces of the piston joint to decrease friction and wear. As such, the silane-containing material may be applied to the pin bore contact surfaces of the piston, or the contact surface(s) of the wrist pin, or the pin bore of the connecting rod, or combinations thereof. In such case, the piston, connecting rod and wrist pin may be made of various materials not limited to those discussed above.
Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. The invention is defined by the claims.
This application claims priority to U.S. Provisional Application Ser. No. 60/884,114, filed Jan. 9, 2007 and is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60884114 | Jan 2007 | US |
