CA1073283A - Wear resistance properties of soft metal films - Google Patents

Abstract

WEAR RESISTANCE PROPERTIES
OF SOFT METAL FILMS

Abstract of the Disclosure This invention relates to metal surfaces having thin coatings thereon of a soft metal of limited maximum thickness.
the thin coating offsets the sliding wear effects by delamination that can occur within a metal substrate when the surface of said substrate is in sliding contact with another surface.

Description

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Background o~ the ~nvention 1. ~ield of the Invention This invention relates to eliminating or substantially reducing sliding wear properties utllizing the properties of thin soft metal films, and more particularly, relates to a means ~for improving the wear properties of metal surfaces by applying a thin soft metal coating over said surface.

2. Description of Prior Art Wear of metals where one metal surface is in sliding contact with another is a complex phenomenon. In part, wear properties depend on conditions under which one metal surface slides over another and the properties of -the metals in sliding relation with each other. When examining metal wear, a number of mechanisms have been considered, i.e., abrasive, adhesive, diffusive, corrosive, fatigue, and fretting wear. The existence of abrasive, diffusive and corrosive wear mechanisms has been established.
With regard to sliding wear, one theory is proposed by Archard and disclosed in the Journal of Applied Physics 24 981-88 (1953). This theory postulates that when asperities come into contact and adhere strongly to each other, the subse~uent separation occurs in the bulk of the weaker asperity. This process is assumed to create a particle from the weaker surface which is attached to the other material. When these transferred particles become free, loose wear particles are formed. The -theory assumes that wear particles are hemispherical, that their size is related to asperity contact, and that the deformation is confined to a region near the asperity contact. While attempting to relate wear volume, sliding speed, ; -2-~0'732~.i normal load and hardness with experimental results, ~rchard's theory fails to address itsel~ to the physical metallurgy of metal deformation and provides no insight whatsoever as to the wear of metals under diferent sliding conditions.
- A second theoretical explanation of adhesive wear is based on dislocation theory, plastic deformation and fracture of metals near the surface of the metal. This explanatlon, known as the delaminatlon theory oE wear, was disclosed by N.P. Suh in ~ear 25, 111-124 (1973).
The delamination theory is founded upon the following observations:
(a) During wear, the material at and very near the sur-face does not have a high dislocation density, due to the elimina-tion of dislocations by the image force acting on those disloca-tions nearly parallel to the surface. This image force arises due to the existence of a stress-free surfacé. As a result of ; -this force, the material very near the surface cold-works less than that of the sub-surface layer.
(b) As sliding continues, accumulations of dislocations located at a finite distance from the surface results in void - formation. The formation of these voids will be enhanced if the material contains a hard second phase against which disloca-tions are forced.
(c) With time,-the aforesaid voids coalesce, either by growth or shearing of the metal effecting a crack parallel to the wear surface.
(d) When the crack reaches some critical length (depen-dent on the material), the material will crack and the surface ~, will shear, yielding a sheet-like flake. The formation of such ; 30 flakes :.

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will hereina~ter be referred to as delamination.
With this recent delamination theory of wear as a hind-sight explanation of effectiveness, numerous solutions had been proposed in the prior art to reduce sliding metal wear by preven-ting the Eormation of flakes thereby prolonging the useful life of the metal surface. The most common prior art material employs conventional lubricating oils or solids such as molybdenum di-sulfide, graphite or babbitt metal as described in U.S. Patent No. 3,165,567. Use of lubricating oils is not effective in high temperature and high vacuum environments. Similarly, conventional lubricants adversely affect thermoplastic resins, typically used in molding and die casting. Use of such lubricants is also ineffective in eliminating metal to metal contact in many moving parts due either to design constraints of geometry of frequent start-up and shut-down cycles.
To counter these problems, soft metal films have been used to replace conventional lubricants. The lubricating action of soft metal films, such as ~ead or indium~ on hard steel sub-strates have been demonstrated by sowden and Taylor in "The Friction and Lubrication of Solids", Part 1, p. 111, Clarendon Press, London, 1954. Unfortunately, most soft metals are poor lubricants, low melting and not resistant to oxidation. One exception to this generalization is gold, which is a soft, inert metal and has a high melting point~ Due to these charac-teristics, gold has attracted attention as a lubricant to separate metal surfaces and provide an easily sheared region.
Some illustrations of this use are disclosed by R. Takagi and T. Liu in ASLE Transactions, 10, 115-123 (1967) and ASLE Trans-actions, 11, 68-71 (1968). These 3Z8~

gold ~ilms were Formulated either to thicknesses of 5 to 20 ~m or combined with another layer of dissimilar metal, for example nickel, as an integral part~
For plastic molding, numerous metal coat;ngs have been I
used. These include, for example, aluminum having a coating thickness of 2-7 x 106`~um as disclosed in U.S Patent No.
2,979,773; bronze having a thickness of about 3000 ~m as dis-closed in U.S. Patent No. 2,851,331; and tin having a thickness of about 2500-4000 ~um as disclosed in U.S. Patent No.2,860,947. I
These metal coatings are ùsed to efFect efficient heat conduction while protecting the hard metal mold surface from chemical inter-action with the thermoplastic resins. Other metal coatings, such as amalagamated mercury, as disclosed in U.S. Patent No.
2,705,669 in addition to preventing chemically induced deterior-ation of the metal~surface also decreases the resin's tendencyto adhere to the metal t`ool sùrface. Such amalgamation increases the useful life of the molding tool ten-fold.
Though the aforesaid metal coatings do, in m~ny cases~
increase the useful life of the metal surface, they are not free from difficulty. For example, many are temperature sensitiv~
and melt at temperatures which are relatively low for many molding operations. More importantly, delamination wear occurs within the metallic coating itself so that although the metal surface may not wear, the coating may wear creating the same -problems encountered with the delamination of the metal surface.
Statement of the Invention The present invention is based upon the discovery that t~ e wear resistance of a metal surface in sliding contact with ~¦ anoth metal surface can be substantially ir,creased over the : I , .
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wear resistance obtainable us;ng prior art methods by application of a relatively thin coating of a soft metal to the metal surface The coatings utilized by the invention are distinguished from the use of soft metal coatings as reported in the prior art by the thickness (or more accurately, thinness~ of the coating). In this respect, using cadmium as an example of a coating metal, the meaximum thickness of the coa-ting is about l.0 ~m and preferably, varies between about 0.05 and 0.5 ~um. This is to be contrasted wlth prior art coatings, such as those described above, where the lO tinnest coating reported is about 2.5 ~m for tin. !
Though not wishing to be bound by theory, it is believed I;
that the thin coatings of this invention are more effective than the relatively thicker coatings of the prior art as the coa-~ings of this invention are too thin to undergo delimination wear as will be described in greater detail below.
Description of the Drawings In the drawingsg Fig. 1 graphically represents the wear of two cylinders sliding against each other in milligrams o~
cadmium-pla~ed 1018 steel as a function of the thickness of a cadmium coating in microns under a 5 lb. load in an atmosphere of both argon and air;
Fig. 2 graphically represents coefficient of friction of two cylinders of cadmium-plated 1018 steel sliding against each other as a function of the thickness of a cadmium coating in microns under a 5 lb. load in an atmosphere of both argon and air ;~
Fig. 3 is a photomicrograph of wear tracks of unplated steel in argon gas under a load of 2.25 kg after ~5 m sliding distance;
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Fig. 4 is a photomicro~raph of the wear tracks of 0.1 ~m cadmium plated steel in argon gas under a load of 2.25 kg aFter 45 m sliding distance.
Fig. 5 is a photomicrograph of sub-surface damage and deformation of unplated steel in argon gas under a load of 2.25 kg after 54 m sliding distance. -Fig. 6 is a photomicrograph of sub-surface damage and deformation of O.l,um cadmium plated steel in argon gas under a load of 2.25 kg after 54 m sliding distance.
Description of the Preferred Embodiments As noted abovej substantially increased life of a metal surface in sliding contact with' another surface is attained by provision of a relatively thin coating of a soft metal over said metal surface. The term "soft metal" as used herein, is intended lS to mean those metals having a face centered cubic or hexagonal close packed with C/A ~ 1.633 structure and alloys thereof.
Examples of metals within the ~nventlon inc1ude tin, aluminum, indium, lead, platinum, palladium, ~ and copper. Preferred ! metals are cadmium and gold, cadmium being most preferred.
20 ~ The improved results obtained by using a thin coating in !I contrast to the thicker coating of the prior art are believed to ¦I be explained by reference to the above described delamination ji theory of wear. As noted above, plastic deformation occurs when ! two surfaces are in sliding contact with each other, generating dislocations in the surface, which are eliminated by the image l ~ force due to the stress-free surface. However, sub-surface plasti,c -~ , deformation also occurs, generally to a depth of 40-80~um which ¦
deformations remain and accumulate in the sub-surface layer effecting void and crack nucleation in the deformed sub-surface.
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As this sub-surface deformat;on increaaes, the voids grow and sub-surface cracks)nearly parallel to the wear track, are generated.
These cracks finally shear to the surface and produce thin wear sheets by delamination. The prior art soft metal coatings while of some initial lubricative benefit due to the lower dis-location density in the soft sub-surface layer, soon undergo a delamination process of wear within the soft metal layer itself.
This delamination wear due to void Formation within the film itsel produces wear particles destructive to the required lubricat;on.
An illustration of this intra-film delamination is disclosed by R. Takagi and Ti Liu in ~SLE Transactions, 11 64-71 (1968).
There, thin gold flakes of moderate size were observed and attri-buted to a peeling from the substrate due to fatigue at the plati 9 substrate interface. In fact, it is believed that such flakes were indicative of intra-film delamination.
The thin metal coat;ng, in accordance with this invention is too thin to retain dislocations within its own layer and therefore, delamination wear is eliminated. This is due to the fact that the thin coating continuously deforms without significan ` work-hardening since , when dislocations are generated, they pile up at the interface between the deposited metal and the substrate and as the slide surface moves forward, these dislocations are released through the surface of the wear track. In addition to this image force, acting on dislocations proximate to the free surface, another force acts on dislocations in metals with low shear moduTe;s such that these dislocations are repelled from the interfa e. In he case of high st esses, there will be some :`:

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transfer to and generation of disloeations in the substrate material. However, the dislocation transfer will- be considerably less than would be observed on an uncoated material' because of r the lower tangential force transmitted Thus, the composite sur-face will markedly delay delamination of the substrate material.
In view of the above theory, the thicknéss of the coatinof the invention over the metal surface to be protected must not exceed a given maximum value before delamination occurs within the metal coating itself. The maximum thickness'varies dependent upon the coating metal used and a general range inclusive for all soft metals cannot be given. It can only be stated that the thickness cànnot exceed that thickness whereby delamination occur ~-within the coat;ng. An estimate of thickness (h) of any given layer can be calculated from the following equation:
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'l ~ ' G is the shear module~ ln ~/cm2, f the friction stress in ~ /cm2, '-b the Burgers vector in A, and v Poisson's ratio. :
Applying the formula to copper where G = 2.2 X 106 i ~f = 4 ' ' '' v = 0.33 b = 2 to 5A . ' the preferred thickness (h) is found to be 10 to 20~um.
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¦ Within this criterion, one can determine the appropriate ¦ thickness of the coating for any given soft metal by the appli-¦ cation of routine exper;mentation.
I With regard to minimum thickness ~ the 0ating, the thick-I ness must be sufficient to provide the desired result of reducing ¦wear. This minimum is dependent upon the metal used but typically is at least 0.05 ~m and preferably at least O.lJum.
The substrate over which the metal coating is applied Ican be any metal, non-metal, plastic or carbide composition having 10 I a higher flow stress than the coating. More preferably, the sub-¦strate is quite hard relative to-the coating. This relative hardness can also be effected by employment-of multiple coatings.
Thus, a base of intermediate hardness may be coated with a layer ¦of very hard material, said hard layer being overcoated with a ¦thin layer of soft metal as taught by this invention.
The substrate is typically one used for sliding contact.
¦Thus, substrates coated in accordance with the invention have Imany diverse applications as parts in molds, journal bearings, bal ; ¦bearings, gears and the like. -¦ The method of applying the coating is not critical and an coating method known to the art is applicable. Thus, vacuum deposition, and electrolytic and electroless plating methods are suitable. , :
I In the most preferred embodiment, the metal substrates 1 in slidin3 contact are each coated with dissimilar metals, i.e.9 ¦ cadmium and gold or copper-cadmium. This dua1 c~ating markedly ~decreas s al loying and th~s reduces the friction between the Iayer~.

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The invention will be better understood by reference to the Following examples which were carr;ed out by wear tests on a lathe employing a cylinder on cylinder geometry. The specimens were rotated at a surface speed of l80 cm/min. and stat;onary pins were pushed against the specimens by a normal load of 2.25 kg The normal and frictional forces were measured by a dynamometer and a Sanborne recorder. The material tested was AlSi 1018 steel, commercially electroplated in varying thicknesses with various metals. The specimens were ~ indiamater by 3" long with a 16 RMS ground finish, and annealed at 670C for 10 hoursin 5X10 5 torr before plating. The slider pins were of the same material and electroplated in a similar fashion.
Prior to testing, the specimens were degreased and weigh ~d to an accuracy of 0.05 mg. The wear tests were carried out dry, both i~n air and argon gas at room temperature for 54 meters sliding distance.
Following the wear tests, the specimens were carefully brushed to remove the loose wear particles and weighed. The wear tracks were observed with a scanning electron microscope.
Examples 1 - 4 Electro-deposited cadmium , -8 Example ~ Coatinq (~m) Wear Rate ~q/cmX10 ) . .
1 0.075 280(1) ~
2 0.1 0-5

3 1.0 3.0

4 10 85.o i (1) Also includes a contribution from ab asive wear due to extreme thinness of coating. ~¦

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The criticality of Film thickness can best be seen by l ¦ reference to Figures l and 2 of the drawings, which illustrate I
¦ dependence of wear rate and coef~icient oF friction on substrate i ¦ coating ~hickness. In both cases, the subs~antially improved

5 I wear behavior of substrates plated in accordance with this i ¦ invention can be observed relative to the inferior behavior ¦ obtained by the use of thicker coatings as taught in prior art.
Figures 3 and 4 enable a comparison of the wear tracks l oF unplated samples with those of 0.1 ,um cadmium plated samples.
: ¦ The wear tracks of the unplated samples (Fig. 3) appear to be l I very rough ~ith a cratered appearance indicative that many wear i I ¦ sheets have been separated from.the wear track. The wear track l of the plated sample, on the other hand, is much smoother with :
parallel furrows in the sliding direction. Only a few indicia of delamination are observed.
Figures 5 and 6 compare the sub-surface damage and de-formation of the unplated steel (Figure 5) and the 0.1 ym cadmium plated steel in argon (Fig. 6). The plated sample has I
undergone less deformation and contains few sub-surface voids .
and cracks. The fact that the plated samples have some voids and cracks in the steel substrate indicates that the cadmium layer will èventually be removed when the steel substrate del- . ~
aminates. This hypotheses was tested and indeed the cadmium ~..
. layer was removed by sub-surface delamination after 86 m of I 25 sliding, yet the unplated steel for the same distance of sliding ,:
would ve worn more by at least a factor of 5000.

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, ~ 3 l . .. 11 Examples 5 - 9 : El ectro-deposited copper Example Coatinq (um) Wear Rate (q/cmX10~8) 0.1 302

6 1.0 315 1'

7 5.0 200

8 10.0 10

9 20.0 35 Examples 11 - 12 Example ^ Coatinq (um~ - CoefFicient of Friction 11 cadmium(0.1)- .
copper(10) 0.1 .
: 12 cadmium(0.1)- :
gold (5) 0.08 . .:
The substantial decrease in metal wear rate obtainable : I
. by use of the thin, soft metal films .in accordance with this invention is a significant advance as it increases the useful .
life of a sliding surface by substantially delaying delamination . 20 due to plastic deformation and further inhibits intra-film ;
1~ delamination whi-ch phenomenon substantially limited the use of lubric lVe pricr art coatinqs D scft metals.

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Claims (22)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Substrate surfaces in sliding contact with each other each of said surfaces being coated with a soft metal selected from the group of face centered cubic, hexagonal close packed (c/A > 1.633) and alloys thereof, each coated substrate being of substantially higher flow stress than its coating, and each soft metal coating having a minimum thickness sufficient to reduce wear and a maximum thickness, h, defined in accordance with the following equation:

wherein G is the shear modulus in kg/cm2, .delta.f the friction stress in kg/cm2, b the Burgers vector in A and V the Poisson ratio for said soft metal, whereby delamination cannot occur within the soft metal coating.

2. The surface of claim 1 where the minimum thickness is 0.05µm.

3. The coated surface of claim 1 wherein the substrate is a metal.

4. The surfaces of claim 1 where each surface is coated with a different soft metal.

5. The surfaces of claim 1 wherein the soft metal is cadmium.

6. The surfaces of claim 5 wherein the maximum thick-ness is 5.0µm.

7. The surfaces of claim 5 wherein the maximum thickness is 1.0µm.

8. The surfaces of claim 1 wherein the soft metal is gold.

9. The surfaces of claim 8 wherein the maximum thick-ness is 0.5µm.

10. The surfaces of claim 1 wherein the soft metal is copper.

11. The surface of claim 10 wherein the maximum thick-ness is 10µm.

12. A method for substantially reducing wear between substrate surfaces in dynamic contact with each other, said method comprising coating each of said surfaces with a soft metal coating selected from the group of face centered cubic, hexagonal close packed (c/A > 1.6333 and alloys thereof, said substrates being of substantially higher flow stress than its coating, and each soft metal coating having a minimum thickness sufficient to reduce wear and a maximum thickness, h, defined in accordance with the following equation:

wherein G is the shear modulus in kg/cm2, .delta.f is the friction stress in kg/cm2, b the Burgers vector in .ANG. and V the Poisson ratio for said soft metal coating, whereby delamination cannot occur within the coating.

13. The method of claim 12 where the minimum thickness is 0.05µm.

14. The method of claim 12 wherein the substrate of the coated surface is a metal.

15. The method of claim 12 where each surface is coated with a different soft metal.

16. The method of claim 12 wherein the soft metal is cadmium.

17. The method of claim 16 wherein the maxim thickness is 1.0µm.

18. The method of claim 16 wherein the maximum thickness is 0.5µm.

19. The method of claim 12 wherein the soft metal is gold.

20. The method of claim 19 wherein the maximum thickness is 5µm.

21. The method of claim 12 wherein the soft metal is copper.

22. The method of claim 21 wherein the maximum thickness is 10µm.