Molybdenum-sulphur coatings

Details Molybdenum-sulphur coatings Molybdenum-sulphur coatings

1998-10-02

2002-07-23

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

Composite

Of metal

C428S472000, C428S697000, C428S698000, C428S699000, C051S297000, C420S429000, C075S231000

Reexamination Certificate

active

06423419

ABSTRACT:

TECHNICAL FIELD
This invention relates to improvements in techniques for depositing metal-sulphur coatings, for example MoS
2
, relates to metal-sulphur coatings with improved properties, and relates to articles coated with the new coating.
BACKGROUND ART
MoS
2
coatings have been used as solid lubricants in a variety of applications, but mostly as coatings on bearings in aerospace applications. They are renowned for being soft and for deteriorating in water-containing atmospheres.
More recently, MoS
2
coatings have been used to improve the efficiency of cutting tools. (German Patents 982616, or DE 2 345 355, and 4414051 C1. U.S. patent application Ser. No. 07 946 642, and Swiss Patent Application 2893/91).
MoS
2
coatings can be deposited by various methods but the preferred method is sputtering. In the past MoS
2
coatings deposited by sputtering have been poorly adhered, of low density, and with columnar structure.
MoS
2
is conventionally very soft and powdery. It is most definitely not considered a hard coating. It can be scratched with a fingernail. Indeed, it is so soft that it is difficult to measure on the Vickers Hardness scale—it is right at the bottom end of the scale, and almost off the bottom end of the scale. It has a coefficient of friction that is very dependent upon humidity. In a dry atmosphere or in a vacuum it has a coefficient of friction of about 0.02. In humid atmospheres it has a higher coefficient of friction, and at atmospheric humidity it fails quickly because of the moisture in the air. This has restricted its use, for example to space-going applications where there is no water vapour present.
MoS
2
is so soft that it cannot be used as a coating where the load is too high (above, say, 20N a pin on disc test with a 5 mm ball as the pin).
Recent developments at Teer Coatings using Closed Field Unbalanced Magnetron Sputter Ion Plating (CFUMBSIP) have produced MoS
2
coatings of improved quality. In particular, the coatings are highly adherent, have a dense structure, a good wear rate, and are surprisingly hard. These coatings have the properties required for good tribological performance.
Our earlier patent application EP 0 521 045 discloses a closed field unbalanced Magnetron Sputter ion plating system suitable for performing the present invention, and its contents are hereby incorporated by reference. The reader is directed to read EP 0 521 045 A1 now to appreciate the disclosure incorporated. (“Closed Field” Magnetron Sputter Ion plating is the use of flux linking between adjacent magnetrons to provide a closed magnetic field between them, reducing the number of ionising electrons that escape from the system).
It has been found in practice that when using the sputtering method the results can be variable and that good coatings can be produced in one deposition sequence and better coatings in another deposition sequence which is intended to be identical.
We have linked some problems with variability in MoS
2
coatings with the condition of the coating system and the condition of the MoS
2
sputtering target, and in particular with the presence of contaminants in the deposition system. Two contaminants causing problems are water vapour from the atmosphere and Sulphur which is present in the chamber from previous MoS
2
depositions. The source of water contamination is the humidity in the atmosphere. This can be absorbed on all surfaces within the coating chamber but in particular is readily absorbed by the MoS
2
sputtering target.
Sulphur may also be present in the sputtering chamber following previous coating operations and can form compounds such as H
2
S which also contribute to contamination. The water vapour and/or the sulphur in the sputtering chamber can cause acidic contamination on the substrate surface prior to deposition leading to coatings with poor adhesion. Water vapour and/or sulphur compounds can also cause contamination during coating. The contaminants can lead to coatings with less desirable properties. For instance brittle coatings may be deposited due to the presence of contaminants.
The aim of the present invention is to produce a better molybdenum-sulphur coating.
SUMMARY OF THE INVENTION
According to a first aspect of the invention we provide a molybdenum-sulphur low friction coating having a thickness of at least 200 nm and a substantially pore-free homogenous non-columnar structure.
This has hitherto been entirely unachievable. Molybdenum-sulphur coatings have traditionally been thought of as very, very, soft. 500VHN is harder than stainless steel. We believe that our coating may be so much harder than has previously been achieved because it does not have pores, or voids.
One of the leading works on MoS
2
films is by T Spalvins (see “A Review of Recent Advances in Solid Film Lubrication” by T Spalvins published in J Vac Sci Technol A5(2) March/April 1987). As described by Spalvins an MoS
2
coating has an equiaxed structure overlapping a ridge-type structure. If the thickness of the equiaxed region is greater than about 200 nm it transforms above 200 nm into a columnar fibre structure with growth structure defined by voided boundaries, or pores. The equiaxed structure extending for perhaps up to 200 nm is pore free and is a densely packed structure, but above that fibres of a columnar structure grow, extending perpendicular to the substrate and are separated by open voided boundaries A few tens of nanometres wide. Such coatings are notoriously soft and break at the fibrous columnar region.
FIG. 9
illustrates this hole-filled columnar structure.
The above is discussed in U.S. Pat. No. 5,268,216 of Keem et al who propose having multiple thin (less than 200 nm) layers of MoS
2
to build up striated coatings with each layer being too thin for fibrous columnar growth to start.
We have found that by using our closed ring flux magnetron sputter ion plating system having a relatively high ion current density at the article to be coated we can create remarkable coatings, contrary to the teaching of Spalvins and Keem.
Preferably the coefficient of friction is 0.1 or less, and maybe 0.05 or less, and under some circumstances 0.005 or less.
MoS
2
is also notoriously bad in even slightly humid atmospheres. Our MoS
2
coating can operate successfully in normal air atmosphere. Our coating may have a coefficient of friction of about 0.02 when exposed to an atmosphere having 20% specific humidity, or 40% specific humidity.
In addition to producing pure, or substantially pure MoS
2
coatings we have found that by incorporating another metal, or metal from a second target, we get even better coatings. The metal in the molybdenum sulphur material may be selected from the group;
Titanium; zirconium; hafnium
tungsten; niobium; platinum
vanadium; tantalum; chromium; gold; molybdenum.
We have been able to achieve coatings which incorporate Titanium with a Vickers hardness of at least 1000, and with a Vickers hardness of at least 1500, and with a Vickers hardness of at least 2000.
The ratio of molybdenum plus other metal to sulphur of maybe about 1:2. The other metal may be dissolved in the MoS
2
crystal structure so as to have substantially no areas of elemental said other metal. The amount of said other metal may not be more than about 18% by weight (this is the figure for Titanium), but other metals may have different solubility in the MoS
2
structure and have other upper limits.
The coating is preferably substantially homogeneous. It is preferably substantially amorphous. The coating may be such that substantially no crystal grains can be seen using a transmission electron microscope at atomic resolution.
The coating may have Titanium also present and have a composition Mo
x
Ti
y
S
z
where x+y≈1 and Z≈2, and where X is at least 4 times Y, and where the coating is a homogeneous amorphous coating showing substantially no crystal grain boundaries when viewed with a transmission electron microscope at atomic resolution, and which has a coefficient of friction of no more than 0.1, and which shows substantially no discrete element

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