Anti-friction coating composition containing solid lubricants
Reference Number: N 01-02
Inventors: Rao, V. Durga N.; Kabat, Daniel M.; Lizotte, Brian W.
The invention, in a first aspect, is an anti-friction coating composition containing
solid lubricant particles, comprising: (a) a mixture of (i) solid lubricant crystals
that can provide a coefficient of friction of 0.06 or less, at least two of which
are selected from the group of graphite, MoS.sub.2 and BN; (ii) a thermoset resin
and (iii) catalyst for setting the resin (sometimes a crosslinking agent for polymerization
of the resin), and (b) an evaporative medium for carrying said mixture during deposition.
The medium may be a solvent such as butyl acetate, or the medium may be a suspension
carrier such as water. The resin may be bisphenol A epoxy and the ratio of medium
(solvent or suspending carrier) to resin is desirably in the range of 1:5 to 2:1.
The solid lubricants may further comprise 5-20% by weight of at least one selected
from the group of LiF, CaF.sub.2, WS.sub.2, a eutectic of LiF/CaF.sub.2 or LiF/NaF.sub.2.
The ratio of resin to solid lubricants is desirably about 3:2.
The invention, in a second aspect, is a solid film lubricant system useful in protecting a metal wear interface subject to high temperatures. The system comprises: (a) an oil-attracting solid lubricant mixture comprising at least two elements selected from the group consisting of graphite, MoS.sub.2 and BN; (b) means effective to support said mixture under loads of at least 10 psi at temperatures of 600.degree.-800.degree. F. while being stable at such temperatures; and (c) thermoset polymer matrix adhering said mixture to the support or the wear surface, said polymer being stable at 600.degree.-800.degree. F. and having inherent hydrocarbon chemical attraction to form a tenacious oil film of the lubricating oil on the wearing surface.
The means for supporting the mixture may comprise (a) lands in the interface that extend above the polymer, or (b) a transition zone material or substrate between the polymer and interface that provides an elastic modulus exceeding 5 million psi, the material or substrate being comprised of ingredients selected from the group of: (i) nickel, copper, cobalt, iron, or manganese; (ii) intermetallic compounds derived from nickel, chromium, aluminum, vanadium, iron, tungsten, manganese, and moiybdenum; and (iii) cast iron or steel. The system's solid lubricants preferably should have an average 2-25 micron particle size. The proportions for the selected solid lubricants preferably comprise 29-58% by weight graphite, 29-58% by weight molybdenum disulfide, and 7-16% by weight boron nitride. The thermoset polymer is preferably comprised of a thermoset epoxy resin, solvent or water-based carrier (the resin is either soluble or compatible with the carrier), a catalyst curing agent that cross-links the epoxy, and a dispersing or emulsifying agent.
Another aspect of this invention is a method of making anti-friction coated surfaces subject to sliding loads. The method comprises: (a) providing a light metal based cylinder surface (i.e., metal base or alloys of aluminum, titanium, iron or magnesium); (b) exposing nonoxidized metal of the surface; (c) applying a high elastic modulus load-supporting metal layer onto at least portions of the light metal cylinder surface; and (d) simultaneously distributing a solvent-based solid film lubricant mixture and thermoset polymer onto at least portions of the layer at about room temperature (the part may be heated to a temperature appropriate to facilitate rapid removal of the solvent to form an adherent coating) to form a coating of desired thickness. The solid lubricant mixture comprises at least two elements selected from the group consisting of graphite, MoS.sub.2, and BN, and the polymer adheres the mixture to the layer upon flowing thereagainst.
The distribution step may be carried out by different species or modes, including: (i) spraying, roller transferring, or silk screening an acetate, ketone, or mineral spirit solution/emulsion containing the mixture and polymer followed by curing to provide a stable coating; (ii) spraying or roller-transferring, or brush painting or imprinting a water-based emulsion containing the mixture and the polymer, the emulsion being cured to provide a stable coating; or (iii) adhering a tape carrying the mixture and polymer which is subsequently cured. The distribution for the above method should be controlled to coat thinly, usually in the thickness range of 5-20 microns (however, the thickness can be up to 100 microns); the coating is honed, after curing, to a coating thickness of 0.001" or less, as specified. However, higher thicknesses up to 0,003" can be used when exceptional noise control is desired, or to any specified thickness under 0.010" (250 microns) in non-piston system applications and some special piston application as well. The steps (b)-(d) may be required only for that portion of the cylinder wall traversed by the sliding load or piston during a 60.degree. crank angle of movement from top dead center as this is the range where the piston ring/cylinder bore contact pressure is highest. However, for practical purposes the entire surface may be coated.