Technical Briefing: Hardness of Electrical Contact Materials
May 4, 2021at4:00 AM
by Keith Hoge
Figure 1 (above): An electrical contact being tested for hardness with a Vickers hardness indenter
Electrical contact materials must be produced with specific and consistent material properties to ensure they perform well in an application. Variation in characteristics such as conductivity, density, or even surface roughness can cause an electrical contact to fail. This can result in a potentially dangerous, even life-threatening situation. As such, it is important that electrical contact manufacturers hold their quality standards to a very high level and perform quality inspections throughout the production program consistently.
One important characteristic that must be monitored closely is hardness. Electrical contacts must be designed to withstand significant contact pressures, especially in medium to high voltage applications. Hardness plays a crucial role in a contact’s ability to withstand these pressures. If a contact is too soft, it can be prone to irreversible plastic deformation which will alter the function of the contact (at best) or cause the contact to fail completely (at worst). If a contact is too hard, it might crack under these loads or it might not have sufficient asperity* when contact is made. There is a middle ground of hardness values where the contact will not exhibit plastic deformation and it will not crack, chip, or have excessive contact resistance due to low asperity.
*Asperity is the amount of surface area that is in contact with the counterface. Since all materials have some degree of surface roughness, not all of the contact’s surface will be touching the counterface when contact is made. When asperity is low, contact resistance can increase which is problematic for many reasons.
We now see that hardness is a critical material property of electrical contact materials. But what is hardness exactly? Of course, we all know that a bowling ball is harder than a tennis ball, but how do we quantify exactly how much harder? The answer isn’t necessarily straightforward. There are many different types of hardness tests and hardness scales, each reserved for different materials. Technically, there is no one scale that can be used for all materials, so it is important to ensure the correct scale is being used for the material that is tested. With all that said, all types of hardness tests and scales are intended to quantify one thing – a material’s ability to resist deformation.
To properly understand exactly what hardness means, we must first look at something called the “Stress-Strain Curve”. This curve shows the relationship between stress and strain, both of which are defined below:
Stress: a measurement of force applied to an object divided by the area over which the force is acting
Strain: the amount of deformation experienced by an object under load in a particular direction divided by the original length of the object in that dimension.
For most materials, when the relationship between stress and strain is shown on a graph, it looks something like the chart shown in Figure 2.