Coating processes – Spray coating utilizing flame or plasma heat – Metal or metal alloy coating
Reexamination Certificate
2000-03-23
2001-08-28
Bareford, Katherine A. (Department: 1762)
Coating processes
Spray coating utilizing flame or plasma heat
Metal or metal alloy coating
C427S455000, C427S331000, C427S353000, C427S367000
Reexamination Certificate
active
06280796
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to methods for producing a slide surface on a light metal alloy and also to reciprocating piston engines such as internal combustion engines having cylinder linings with slide surfaces.
German Offenlegungsschrift No. 44 40 713 discloses a method of producing slide surfaces on cast iron parts consisting of several steps in which lubrication pockets are exposed in a slide surface so as to provide hydrodynamic lubrication in operation. These steps include machining of the slide surfaces and then processing the surfaces with a chemically and electrochemically inactive liquid applied under a pressure suitable for flake removal. By a combination of liquid erosion and frictional smoothing the lubrication pockets are exposed in the slide surface, forming in the aggregate a system of pressure microchambers to provide the requisite hydrodynamic lubrication. These pockets are produced by a honing operation which extracts small titanium carbide and nitride particles from the surface and the craters so formed are closed by further treatment. Liquid erosion and frictional smoothing expose these pockets again.
U.S. Pat. No. 5,080,056 discloses a process in which a substantially pore-free aluminum bronze alloy layer is applied by high velocity flame spraying to parts made of a magnesium alloy, the thickness of the applied alloy layer being then reduced by honing from about 1 mm to a final dimension of about 127 microns.
In contradistinction to the formation of a system of pressure microchambers, it is known that a communicating groove system may be obtained, for example on the surfaces of cylinder liners for internal combustion engines by a honing operation. Such honing produces intersecting scores which axe interconnected at the points of intersection and constitute an overall open system. A disadvantage of this system is that the part sliding on the slide surface, for example a piston ring on a piston of an internal combustion engine, will push oil contained in the scores ahead of it, preventing any significant build-up of hydrodynamic oil pressure on the slide surface. Consequently, along the edges of the scores there will be mixed friction between the two materials. The mixed friction lubrication systems which are in widespread use for surfaces of gray iron castings, however, are not applicable to slide surfaces on parts made of a magnesium alloy for reasons of weight.
U.S. Pat. No. 2,588,422 discloses an aluminum engine block having cylinder liners which are formed by thermal spraying. These liners are built up in two layers on the untreated surface of the engine block, the top layer being a hard slide layer such as steel about 1 mm in thickness and the lower layer being a molybdenous interlayer about 50 microns in thickness. The interlayer, containing at least 60% molybdenum, does not constitute a slide layer, but is necessary in order to bind the hard slide layer to the aluminum block. Preferably, the uniting layer is made up of pure molybdenum. The slide layer is a layer of hard metal, as for example carbon steel, bronze or stainless steel, in which the steel may be an alloy containing nickel, chromium, vanadium or molybdenum for example. In principle, this two-layer structure provides a good slide layer, but the cost of the double coating is substantial.
British Published Patent Application No. 2,050,434 discloses various coatings obtained by thermal spraying and having thicknesses from 0.5 to 2 mm. These coatings are formed on steel or cast parts for internal combustion engines, as for example piston rings or cylinder linings. In this case it is found that coatings consisting of equal parts of powdered molybdenum and powdered carbon steel are considerably less abrasion-resistant than coatings containing only 0.5-4.5 wt. % of molybdenum with 20-97 wt. % of metal carbides and possibly iron or ferrous alloys. To bind these coatings to magnesium alloys, the process described above with respect to U.S. Pat. No. 2,588,422 must be used.
British Patent No. 1,478,287 describes a powdered mixture for plasma coating of steel parts or castings such as piston rings, cylinder blocks or cylinder linings to produce layers bearing a thickness of about 762-1270 microns. The powder is a mixture of molybdenum, boron and cast iron containing at least as much cast iron as molybdenum and the boron is usually present is amounts up to 3% of the sum of molybdenum and cast iron. Such coatings, as example 1 of British Published Patent Application No. 2,050,434 A shows, no longer meet present-day performance requirements.
Another coating for cylinder liners is disclosed in U.S. Pat. No. 3,620,137 in which a plasma coating containing nickel and chromium, boron, silicon and possibly also iron in addition to at least 65 wt. % molybdenum is described. This coating is intended for cylinder liners made of gray cast iron and exhibits very small pores of 0.1 to 2 microns and an over-all porosity of 15%, corresponding to the coatings described in British Application No. 2,050,434 and British Patent No. 1,478,287.
As described above, various plasma coatings are known for aluminum alloys. The application of the same coatings to magnesium alloys, however, is not possible in principle, because magnesium alloys have a coefficient of heat expansion about ⅓ greater than that of aluminum alloys. Thus, the ratio of the heat expansion coefficients of such plasma coatings to those of magnesium alloys is about 1:3, so that high stresses can be expected between the plasma sprayed layer and the magnesium alloy.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method for producing a slide surface on a light metal alloy which overcomes disadvantages of the prior art.
Another object of the invention is to provide a method of producing a slide surface on a magnesium or aluminum alloy in which a single coating operation is sufficient.
A further object of the invention is to provide a reciprocating piston engine having an engine block made of a magnesium or aluminum alloy with improved slide surfaces.
These and other objects of the invention are attained by thermal spraying a coating of a mixture containing about 10 to 70 weight percent molybdenum and about 90 to 30 weight percent steel, and preferably about 20 to 60 weight percent molybdenum and about 80 to 40 weight percent steel, onto the surface of a magnesium or aluminum alloy. Thereafter, the coating is machined by honing or the like to form the slide surface. In one embodiment the slide surface is on a cylinder lining in the cylinders of an internal combustion engine in an aluminum or magnesium engine block.
Magnesium alloys on which such slide surfaces are produced usually contain at least 20 wt. % magnesium and preferably magnesium is the chief constituent of the alloy. The situation is much the same for aluminum alloys.
Even though a magnesium alloy has a coefficient of heat expansion of about 27×10
−6
K
−1
and an aluminum alloy has a coefficient of about 21×10
−6
K
−1
while the applied according to the invention has a coefficient of about 10×10
−6
K
−1
, the coatings according to the invention provide good adhesion to such light metal alloys. It has been found that, with such molybdenum coatings, some roughness of the light. metal alloy surface will provide a good bond between the light metal alloy and the applied coating. For especially good bonding, it is preferable to detrine the best depth of roughness for the light metal alloy surface by a sees of tests with the particular light metal alloy and molybdenous coating. Likewise, it is preferable to optimize the spray conditions for good bonding by a series of tests, as is the practice in conventional spray plasma technology. Thus, for example, in the use of powders as spray media, the powder particle size, the ratio of molybdenum to steel, and also the type of steel may be varied, in which case optimal results can be achieved with only a few trials.
The method according
Kloft Manfred
Schlegel Udo
Strich Reinhard
Baker & Botts LLP
Bareford Katherine A.
Volkswagen AG
LandOfFree
Method of producing a slide surface on a light metal alloy does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of producing a slide surface on a light metal alloy, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of producing a slide surface on a light metal alloy will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2502435