How do you measure and state surface resistances of materials? 1

[Tunalover]

Folks-
Does anyone know how to measure and state surface resistivity of materials? We have two different classes of chemical conversion coatings used for aluminum and we need to express and measure surface resistivity for performance comparisons. The spec is MIL-C-5541 and the classes are:
-Class 1A for the best coating for corrosion resistance but the highest surface resistivity, and
-Class 3 for the worst coating for corrosion resistance but the lowest surface resistivity.

Is there a device and/or standard for carrying out the surface resisitivity measurements?

Thanks,
Tunalover

P.S. This was also posted under ASTM issues.

 

[IRstuff]

According to the Quality Assurance section of MIL-C-5541, MIL-C-81706 contains the test method for surface resistivity. It's no longer available from ASSIST, so you'll either need to find a copy or find a corresponding ASTM test method.

Surface resistivity is normally measured with a 4-point Kelvin probe. The probes are arranged in-line, with current flowing in the outer 2 probes. The inner two probes are used to measure voltage drop. There is an equation that converts the distance and probe size into the surface resistivity in micro-ohm*cm. A meter designed for this measurement should have the conversion built-in.

MIL-C-5541 also lists a couple of caveats regarding sufrace roughness and contact flatness effects on the measurement. MIL-C-5541 6.1.2.1 discusses the nominal probe pressure and the expected surface resistance for Class 3.

TTFN

 

[kenvlach]

Several points of clarification.

First, the question is misleading. The contact resistance, not surface resistivity, is measured for MIL-C-5541, Type 3 coatings. See further explanation below.

Second, both MIL-C-81706 and its replacement are available from ASSIST:

http://assist2.daps.dla.mil/eAccess/index.cfm?ident_number=32592

Third, there is no mention of a 4-point Kelvin probe or ‘spreading resistance’ or even ‘surface resistance’ in the specifications.
Contact resistance is measured through the test coupon and coating (top & bottom). It is measured by simply pressing a flat sample between 2 flat, polished copper (or silver-plated copper) contacts at 200 psi. To keep things simple, the top electrical contact is 1x1 inch. The equipment is shown in Figs 1 and 2 of the specification. The following paragraph and Figures in the specs. are identical in both the 1970 and 2002 versions:
“4.5.5 Contact electrical resistance. Panels prepared as specified in 4.4 (before and after the salt spray exposure [see 4.5.1]) shall have the contact electrical resistance of the coating determined for conformance to 3.7. Test equipment and circuitry as shown on figures 1 and 2 shall be used for measuring the electrical resistance of the class 3 material specimens. The applied load shall be within one percent of the calculated 200 psi applied pressure. The contacting electrodes shall be copper or silver-plated copper with a finish not rougher than that obtained by the use of 000 metallographic abrasive paper. The electrodes shall be flat enough so that when the load is applied without a specimen between them, light will not be visible through the contacting surface. The area of the upper electrode shall be one square inch (25 square mm) and the area of the lower electrodes shall be larger. Ten measurements shall be made on each panel in the areas shown on figure 3.”

Fourth, the contact resistance is expressed in ohms per square inch:
“3.7 Contact electrical resistance properties (class 3 only). When tested in accordance with 4.5.5, the contact electrical resistance of aluminum alloy panels treated with class 3 materials under an applied electrode pressure of 200 pounds per square inch (psi) shall be not greater than 5,000 microhms per square inch as applied and 10,000 microhms per square inch after salt spray exposure specified in 4.5.1. Individual readings not greater than 20 percent in excess of the specified maximums shall be acceptable, provided that the average of all readings does not exceed the specified maximum resistance.”

Hope this helps,
Ken

 

[IRstuff]

Sorry for the confusion. I was mislead by the terminology of the question. The question indeed relates to what's called contact resistance and not surface resistivity.

The 4-point probe measurement for surface resistivity uses Van der Pauw's equation to calculate surface resistivity from the spreading resistance directly measured using the the 4-point probe. In fact, semiconductor test devices usually have a specially designed equivalent of the 4-point probe.

TTFN

 

[Tunalover]

Great answers guys! I am grateful for you expertise.