However, the traditional methods lubricant formulators and equipment manufacturers have used to evaluate fretting wear and corrosion leave room for improvement.

Amping Up Electrification

There has been a sharp increase in the volume of electrical and electronic components used in commercial, industrial, consumer and military applications over the past decade. In particular, the global trend is moving toward adding even more electronic controls to automotive applications-or complete conversion, as is the case with electric and hybrid electric vehicles.

There can be more than 400 connectors with over 3,000 individual electrical terminals in an automobile, and the number of electronic components in the average car will most likely double with the advancement of electric vehicle technology and the continued integration of convenience and social media into the consumer automobile.

While most connectors and terminals operate reliably for their expected lifetime, each ultimately represents a potential point of failure. The life of these components is directly related to the reliability of electrical contacts. Safety is also of the highest concern for automotive applications. Together, reliability and safety directly impact the perceived quality of an automobile.

Studies show that nearly 30 percent of signal and accessory circuit failures and more than 50 percent of power circuit failures in cars can be attributed to connectors. The same holds true for other markets and applications. Anecdotally, the Apollo space program experienced more connector failures than any other technical problem. A main cause of these failures is fretting corrosion or wear.

Fretting wear is one of the major mechanical forms of deterioration and failure of electrical terminals and contacts. The automotive environment offers severe challenges for even the highest quality connector. In the engine compartment, connectors must survive thermal shock (rapid heating and cooling cycles that cause expansion and contraction), as well as corrosive gasses, fuel, salt water and dirt. Power mirrors, door locks and other external systems must resist water and even detergent from car washes. Inside the passenger compartment, temperatures can soar when the car sits in the sun and can drop well below freezing in cold climates.

While auto designers try to place connectors away from moisture and water sources, they remain susceptible to failure from fretting corrosion. At the heart of these applications are the electrical contacts, which are subjected to oxidation, humidity, vibration and fretting.

Fretting corrosion is the result of micro-movement between the mating surfaces of the electrical terminals, which wears away the surface and allows an oxide layer to form at the contact. This micro-motion is created by vibration from the engine, drivetrain, suspension, fans, small motors and thermal shock.

As fretting occurs in a system, the pressure on the terminal will force through the contact surface to create wear. As time goes on, worn areas oxidize and harden. Because of the small amplitude, this oxidized wear material has difficulty escaping from the contact zone, where it continues to create additional wear and oxides. As this wear is happening and the oxides are being created, the contact resistance in the terminal increases until a point when continuity is lost.

For electrical terminals and other components, this amount of relative movement between mated components leads to several types of surface damage resulting in fretting corrosion or wear. In fretting wear, the top layer of material wears away due to pure mechanical wear, but in fretting corrosion, a passive film is created in the contact. This passive film is a very thin layer of oxidized corrosion that forms on the nascent metal, which leads to an increase in contact resistance and loss of continuity.

Fretting corrosion creates an insulative layer between the contacts that causes an open circuit, increasing the contact resistance across the terminal. This causes the connector to act like a resistor and consumes power (heats up) rather than passing it through to the operating devices. Coating the contacts with an anti-fretting lubricant reduces mechanical wear, provides an oxygen barrier and helps keep oxide debris away from the contact area.

In a lubricated contact, the lubricant will provide a film that separates the moving surfaces and encapsulates the wear particles to help prevent oxidation. To improve the durability and reliability of automotive components under fretting conditions, lubricants are known to help extend the operational life of the contact.

This is accomplished in several ways. First, the lubricant helps reduce friction, which eases the coupling of sliding surfaces through a thin film between the mating surfaces. Lubricants also help seal porous coatings, which is the case in electrical terminals where substrates like copper can bloom up through a porous metal such as gold and cause advanced fretting corrosion. Finally, lubricants protect against oxidation corrosion by insulating the lubricated contacts from the environment, which prevents oxide formation and more aggressive wear.

Improving Research Methods

To study the fretting wear of various automotive components, including electrical terminals and connectors, Nye Lubricants evaluated the current methodologies that have been employed either at the research level or by commercial manufacturers. This research showed that current fretting testing was being done either as a simulation using test specimens (crossed cylinders, cylinder on plates, etc.) or, if electrical terminals were used, it was performed in a single-specimen fashion with either mechanical vibration or heat cycling used to create fretting.

While experimental simulation testing focused primarily on generic test specimens and not actual terminals, there is a gap in the research of fretting corrosion for automotive applications. It should also be noted that in most research, lubricating grease was not investigated as a possible solution to fretting corrosion.

The approach of only using test specimens has two main weaknesses. It either has a very small testing sample set, which makes it difficult to show statistical significance, or the geometry of the specimens will produce results that do not correlate well to commercial mechanisms. This led researchers at Nye to develop the multi-terminal fretter, which allowed an experiment to be designed with ten electrically isolated terminals to be run simultaneously for statistical significance.

The MTF is made up of two electrically isolated testing blocks that can have various components affixed to them. A power source is applied to an isolated testing block, with a four-wire resistance measurement taken across the testing specimen. Once a test specimen is affixed on the testing block, a linear actuator with absolute encoder is used to oscillate the testing stage and create the fretting wear testing profile.

The frequency of fretting is controlled by a voice coil actuator that allows for motion control down to 1 nanometer over the fretting range. This actuator operates based on the Lorentz force principle, which measures the electromagnetic force exerted on a charged particle. As current is applied to the motors coil, a magnetic field is created, which generates a force. The change in this current (flux) can then alter the applied force.

When a test is set up, great care must be taken for the proper alignment of the test components, specifically electrical terminals, as problems can be caused by misalignment. A typical test is performed by first measuring the static resistance of the affixed sample and then continuously reading the four-wire resistance until the failure criteria has been met or the number of test cycles has been exceeded.

As the MTF allows for multiple samples to be run at once, the test will run until all samples have failed. Typically, a 100-500 milliohm change in contact resistance from the start of the test will constitute a failure. This has been accepted as a representative failure level by the automotive industry.

Making the Connection

Using this new testing methodology to study the factors that affect fretting wear, researchers found that durability in electrical terminals is reduced significantly as the frequency increases and the amplitude decreases. Larger amplitude allows for wear particles to be removed from the contact surface while smaller sliding amplitude typically encourages wear particles to stay in the contact area. As most electrical terminal applications see fretting in the low amplitude range, it is understandable that automotive electrical problems occur due to fretting, especially if the terminals are not lubricated.

It is also apparent from the results that lubricants greatly improve the durability of electrical terminals, as research has shown dramatic increases in the life expectancy of lubricated terminals. It should be noted that contact lubricants without a metal passivator do not typically pass the 1 million-cycle screening test. This is because the metal passivator helps to prevent the oxidation of the tinned copper, which causes fretting corrosion. The combination of fretting wear and fretting corrosion is very severe on tinned copper electrical terminals as a hard oxide film forms on contacts made of this soft metal. From here, the soft layer is easily broken through and the wear particles press into the asperities of the surface, causing a dramatic increase in contact resistance and a discontinuity.

With this new fretting test methodology and apparatus for terminal and connector testing, the industry can now determine the durability and reliability of electrical terminals and the lubricants used for them. Through testing of lubricants to their ultimate fretting failure point, formulators can design electrical terminal systems that are optimized with a lubricant to help reduce or eliminate fretting failures that are causing warranty and safety issues in the automotive industry.

Jason Galary is director of research, development and innovation at Nye Lubricants and has over 20 years of lubricants industry experience. He holds a B.A. in electrical engineering, an M.S. in mechanical engineering and a Ph.D. in. applied mechanics and materials. Galary founded the Application Development and Validation Testing Laboratory at Nye, which develops new testing equipment and methods. Contact him at jgalary@nye
lubricants.com.