Low friction metal-ceramic composite coatings for electrical contacts

An electrical terminal is disclosed where the terminal is formed of an electrically-conductive metal substrate such as copper alloys, aluminum alloys or stainless steel with the substrate having a codeposited composite coating of titanium nitride and gold or silver on the surface. The coating provides wear resistance in high temperature and vibration environments while retaining and demonstrating low friction and low contact resistance properties.

TECHNICAL FIELD 
This invention pertains to improved coatings for use on the contact 
surfaces of metallic electrical terminals. More specifically, this 
invention pertains to certain metal-ceramic composite coatings for such 
terminals operating in high temperature and high vibration environments. 
BACKGROUND OF THE INVENTION 
Modern automotive vehicles have an increasing content of electrical and 
electronic devices that require wires with electrical terminals at the 
ends for their use in vehicles. This increased electrical content in the 
vehicle occurs at the same time that there is also an increase in the 
number of components attached or fixed around the engine in the underhood 
engine compartment of the vehicle. This congestion in the engine 
compartment reduces ambient cooling of the compartment area and leads to 
higher temperatures in the compartment. Because the operating engine 
causes or experiences some vibration, underhood electrical connections on 
passenger cars and trucks can be exposed to temperatures above 150.degree. 
C., combined with severe vibration. Some electrical terminal contact 
materials, such as, for example, gold, have high softening temperatures 
and can withstand operating temperatures above 150.degree. C. However, 
high vibration can quickly degrade their contact properties. Vibration can 
cause microscopic relative movement at electrical contact interfaces that 
quickly wears away existing contact materials. The result is unstable or 
intermittent connection resistance, which can adversely affect vehicle 
performance. 
The vibration resistance of a connector can be improved by minimizing the 
relative movement at contact interfaces. This normally requires improved 
connector locks, seals and wiring harness strain relief features, all of 
which add to the cost of the wiring of the vehicle. An alternative to 
minimizing terminal movement is to develop electrical contact materials 
with improved wear and temperature resistance. It is an object of this 
invention to provide a new terminal coating material with outstanding wear 
resistance and good high temperature electrical performance. 
SUMMARY OF THE INVENTION 
This invention provides a metal-ceramic coating for electrical contact 
applications with low friction, good wear resistance and low contact 
resistance. In a preferred embodiment of the invention, the coating is a 
codeposited layer of titanium nitride and silver on a suitable 
electrically-conductive metal terminal substrate. Examples of suitable 
terminal materials are aluminum alloys, copper alloys and stainless steel. 
The electrically-conductive ceramic, titanium nitride, and silver metal are 
codeposited by physical vapor deposition on a clean metal terminal 
surface. The vapor codeposition of titanium nitride and silver provides a 
hard, relatively low internal stress layer of titanium nitride with 
discrete particles of silver on the outer surface of the ceramic layer. 
Silver preferably constitutes 1 to 10 atomic percent of the composite 
coating and is found largely at the surface of the dense crystalline 
titanium nitride layer. The presence of the silver reduces the friction 
between the coated terminal and a complementary terminal body for making 
easy connections. The combination of the titanium nitride and silver 
provides low contact resistance, even after repeated engagement and 
disengagement of the terminals and after exposure to temperatures of 
150.degree. C. and higher in air. 
The invention has been demonstrated using titanium nitride and silver as 
the codeposited materials. Other suitable metals such as gold can be 
employed. 
Other objects and advantages of the invention will become more apparent 
from a detailed description of the invention which follows. Reference will 
be had to the drawings, which are described below.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1A illustrates a representative male electrical terminal 10 and female 
electrical terminal 12 that might be employed, for example, in an 
automotive application. These terminals may be used singly or a group of 
male terminals and a group of female terminals may be fixed in 
complementary plastic terminal housings so that several connections with 
several wires may be made at the same time. When such connectors are 
employed close to the engine in an underhood automotive application, they 
are subject to high temperatures (as high as 150.degree. C.) and severe 
vibration. The vibration causes the terminals to rub against each other 
and to abrade coatings which are applied in order to prevent oxidation and 
resistance change in the electrically contacting surfaces of the 
terminals. Oxidation and wear of such conductive surfaces increases the 
contact resistance of the connection and impedes the effectiveness of the 
electrical circuit of which they are a part. 
In many applications where a particular circuit is carrying an appreciable 
electrical current, the terminal body preferably is made of a low 
electrical resistance material such as a suitable copper or aluminum 
alloy. Where the particular terminal is in a sensor circuit or other low 
current flow circuit, the terminal body may, for example, be made of a 
higher resistance material such as 301 stainless steel or the like. As 
illustrated in FIG. 1B and in accordance with this invention, the surface 
of the electrically-conductive metal substrate 14 of the terminal 10 is 
provided with a codeposited composite coating that includes an 
electrically-conductive layer 16 of a ceramic such as titanium nitride 
with particles 18 of a highly conductive and preferably relatively low 
friction metal such as silver or gold. As shown in FIG. 1B, titanium 
nitride is a substantially continuous thin layer 16 and the metal is 
present as particles 18 on the surface of the titanium nitride layer 16. 
In accordance with the invention, the codeposition of the ceramic material 
and metallic material is preferably accomplished by physical vapor 
deposition. For example, the deposition of various combinations of 
titanium nitride and silver on stainless steel substrates and titanium 
substrates was carried out in an ultra high vacuum, d.c. magnetron sputter 
deposition system with two sputter sources. The two sources, one a 
titanium target (99.995% titanium), the other a silver target (99.99% 
silver), were d.c. sputtered in a mixture of argon (99.999% pure) and 
nitrogen N.sub.2 (99.999% pure) gases. The argon and nitrogen were 
supplied at a constant flow ratio of 8 standard cubic centimeters per 
minute (sccm) argon and 4 seem nitrogen. In the physical vapor deposition 
apparatus, an anode was added in proximity to the magnetron sources. A 301 
stainless steel substrate (for example) was degreased in a 50/50 mixture 
of Buckeye Shop Master LpH Degreaser and deionized water using an 
ultrasound bath for 15 minutes. The substrate was then rinsed thoroughly 
with deionized water. It was then cleaned in an ultrasound bath with 
acetone and then in a bath of methanol, each for 15 minutes, before 
loading into the load lock of the physical vapor deposition system. 
The base pressure of the deposition chamber was in the low 10.sup.-9 Torr 
range. When the load lock pressure reached 7.times.10.sup.-7 Torr or 
lower, the valve between the load lock and the deposition chamber was 
opened. 
The substrate surface was plasma cleaned for 30 minutes by applying a fixed 
bias of -550V to the substrate in pure argon. For this cleaning operation, 
the argon was supplied at a constant flow of 90 sccm at a pressure of 
about 140 mTorr with the substrate current in the range of 6 to 10 mA. 
The substrate was not intentionally heated or cooled. It was initially at 
room temperature, and its temperature increased to about 95.degree. C. 
during the vapor depositions. During the titanium nitride-silver 
deposition, the total pressure in the deposition chamber was from 5.7 to 6 
mTorr. The titanium sputter source was operated at a fixed current of 
0.475 amperes. The silver sputter source current was fixed at 0.18 amperes 
to generate the TiN.sub.97 Ag.sub.3 nominal composition (Sample 1A) and at 
0.16 amperes to generate the TiN.sub.99 Ag.sub.1 nominal composition 
(Sample 1B). A third sample was prepared with just a titanium nitride 
(i.e., 0% silver) coating (Sample 1C). As is known, titanium atoms and 
silver atoms were ejected from the targets into a plasma containing 
positive nitrogen ions and some positive argon ions. The titanium atoms 
reacted with the nitrogen ions, and TiN was deposited as a layer on the 
substrate. Unreacted silver atoms were codeposited. 
This apparatus could be employed to deposit titanium nitride-silver 
composite coatings on other substrates such as copper, copper alloys, 
aluminum and aluminum alloys and the like. For purposes of comparison, 
some titanium nitride-silver coatings had also been deposited on titanium 
substrates using the above-described apparatus in combination with an 
inductively coupled plasma-assisted vapor deposition process. 
Characterization and Physical Properties of the Codeposited Metal-Ceramic 
Coatings 
The codeposited coating film (16, 18) structure was studied by x-ray 
diffraction (XRD) using a Siemens D500 .theta.--.theta. diffractometer 
with CuK.sub..alpha. radiation. The surface morphology was examined using 
a Hitachi S4000 scanning electron microscope (SEM). The wear spots and 
tracks produced during wear testing were examined with an ISI DS-130 SEM 
fitted with a Kevex Energy Dispersive x-ray Spectroscopy System (EDS). The 
mass thickness and compositions of the films were determined 
quantitatively by electron probe microanalysis (EPMA). The impurity 
content, composition uniformity and interface were studied by x-ray 
photoelectron spectroscopy (XPS) together with sputter depth profiling 
using 4 keV Ar.sup.+ ions. 
SEM observations of Sample 1A, 1B and 1C coatings showed that the 
silver-containing samples have silver nodules on the surface, as indicated 
schematically in FIG. 1B. EPMA of Samples 1A and 1B showed that Sample 1A 
had approximately 3.2 atomic percent silver, and 1B had 1.4 atomic percent 
silver (of the total Ag+TiN content). XPS depth profiling data shows that 
Ag is found at the surface of the film, with less than 1% in the TiN film 
(FIG. 3). Therefore, it is believed that the nodules 18 on the TiN surface 
is Ag. The XRD data (FIG. 2) shows a TiN(111) and Ag(111) peak. This 
indicates the material is a composite. The relative intensity of the 
Ag(111) peak of Sample 1A is larger than that of Sample 1B. The above data 
suggests that the 1A sample, with the Ag source current fixed at 0.18 Amp, 
contains more Ag than the 1B sample, with the Ag source current fixed at 
0.16 Amp. 
The friction coefficient was measured by a pin-on disk method using an 
Implant Sciences Corporation ISC-200 Tribometer. Measurements were made on 
the TiN and TiN--Ag specimens from both sample groups. In each case, the 
pins and disks are of the same material, where the pin is a piece of the 
specimen dimpled with a 3 mm radius mounted on a holder. All measurements 
were carried out at room temperature (21.degree. C.) with 1 N force at 25 
rpm for radii of 3.5 to 6.0 mm, and sliding distance was at least 2.0 m. 
The results were plotted as coefficient of friction versus sliding 
distance. 
The coefficient of friction as a function of sliding distance for TiN on Ti 
and TiN.sub.98 --Ag.sub.2 on Ti samples obtained from pin-on-disk 
experiments is summarized in FIG. 4. For comparison, the coefficient of 
friction of a specimen with 200 nm Ag on TiN--Ag/301ss is also shown in 
FIG. 4. The coefficient of friction of TiN was 0.2 for the initial 0.3 m 
of sliding, then increases to 0.5 after 1.0 m of sliding and remains 
constant. The TiN--Ag films coefficient of friction is 0.2 at the 
beginning of test and increases at different rates as the atomic percent 
of silver is varied. 
The TiN--Ag specimens that have a silver atomic percent less than 10 are 
more dense, while those above 10 atomic percent become less dense and 
porous with cluster. The compositions that are dense have a coefficient of 
friction that remains low for the entire sliding distance. The 
compositions that are less dense have a significant increase in the 
coefficient of friction over the sliding distance. A comparison is made 
with the coefficient of friction for TiN. These results show that by 
adding less than 10 atomic percent of Ag to TiN, a significant reduction 
in the coefficient of friction can be achieved. 
The contact resistance of the coated specimens was measured with a probe as 
per ASTM B667. Measurements were made before and after heat aging at 
150.degree. C., 200.degree. C. and 340.degree. C. The specimens measured 
included TiN on 301 stainless steel and TiN--Ag of different compositions 
on Ti and 301 stainless steel (see the following Table). A solid gold rod 
with a 1.6 mm hemispherical radius was used for the probe tip. The 
measurements of contact resistance as a function of load were carried out 
at 0.5, 1, 2 and 5 N contact force. The contact resistance was averaged 
over five measurements for each contact force. The contact resistance 
measurements were made under dry-circuit conditions to prevent breakdown 
of insulating films. A low contact electrical resistance of a few m.OMEGA. 
can indeed be achieved. 
TABLE 1 
__________________________________________________________________________ 
Contact Resistance vs. Load 
LOAD (N) 
0.5 1 2 5 
Material 
R.sub.m .+-. .sigma. (m.OMEGA.) 
R.sub.m .+-. .sigma. (m.OMEGA.) 
R.sub.m .+-. .sigma. (m.OMEGA.) 
R.sub.m .+-. .sigma. (m.OMEGA.) 
__________________________________________________________________________ 
TiN/Ti 11860 .+-. 5120 
6020 .+-. 1890 
2830 .+-. 690 
850 .+-. 180 
TiN/301ss 
138.25 .+-. 21.36 
76.50 .+-. 10.79 
TiN.sub.97 --Ag.sub.3 
13.93 .+-. 5.96 
7.57 .+-. 0.97 
5.69 .+-. 0.89 
4.35 .+-. 0.71 
TiN.sub.99 --Ag.sub.1 
8.71 .+-. 1.52 
6.42 .+-. 1.12 
5.36 .+-. 1.31 
4.35 .+-. 1.03 
TiN.sub.98 --Ag.sub.2 
5.95 .+-. 1.04 
4.21 .+-. 0.36 
3.13 .+-. 0.15 
2.25 .+-. 0.11 
TiN.sub.87 --Ag.sub.13 
3.85 .+-. 0.53 
2.86 .+-. 0.20 
2.35 .+-. 0.11 
1.91 .+-. 0.09 
TiN.sub.75 --Ag.sub.25 
4.08 .+-. 0.45 
3.33 .+-. 0.1O 
2.89 .+-. 0.05 
2.46 .+-. 0.04 
__________________________________________________________________________ 
In addition to the above friction tests and contact resistance tests, the 
subject titanium nitride-silver coatings were also subjected to severe 
fretting wear tests and heated wiping wear tests and reciprocating wear 
tests. In each situation, the underlying titanium nitride layer with the 
silver particles demonstrated that it had good resistance to such wear 
situations. 
Accordingly, in summary, it is seen that the titanium nitride-silver 
codeposited materials are strong, durable, wear-resistant, 
vibration-resistant coatings that also offer low friction and low contact 
resistance over prolonged usage and exposure. While the specific example 
of titanium nitride and silver has been demonstrated, it is also 
understood that titanium nitride may be used with gold. 
While this invention has been described in terms of some specific 
embodiments, it will be appreciated that other forms can readily be 
adapted by one skilled in the art. Accordingly, the scope of this 
invention is to be considered limited only by the following claims.