So far in this series we have concentrated primarily on the detection of oil and other hydrophobic films on surfaces. However, not all contaminating films are oil and all are not hydrophobic. What about contaminants that are not hydrophobic and may even by hydrophilic (residual soaps and surfactants, for example) as well as things like oxides? These are just as potentially problematical yet won’t be detected by many of the tests described so far. So let’s talk about some tests that can potentially detect these other surface contaminants.
Many materials that may contaminate cleaned surfaces fluoresce under ultraviolet light. Others that are not naturally fluorescent may be made to fluoresce by the addition of a fluorescent dye for detection purposes. The test is simple, a source of ultraviolet light is directed at the surface in a darkened room. Sources of ultraviolet light are readily available as “black lights” in the form of fluorescent tubes. I should caution that some “ultraviolet” bulbs used for purposes of tanning as well as incandescent bulbs with a “black light” coating are not suitable for performing this test as they emit too much light in the visible range. Fluorescent tubes used for this test should be of the “BLB” variety and should appear dark in color. If the contaminant is present, it will be seen as a bright (usually green or white) area on the part under ultraviolet light. Although this is usually detected visually, a sensitive camera and/or computer setup may be used to enhance the sensitivity of the test and quantify the result. This test is very effective on irregular surfaces which do not lend themselves to testing by other means. Residues in the roots of threads and in undercuts are extremely evident. Although we are not talking about particles yet, some particles are also fluorescent and may also be detected using this test.
Electron Emission Testing
Many surfaces will emit electrons when stimulated by an intense source of photons (light) as well as other energy sources. The test involves detecting the quantity of electrons emitted (current) as a measure of contamination on the surface. This test, unlike many others, will detect oxides as well as other foreign materials on a surface. The applicability of the test may be limited on surfaces that are not flat and uniformly reflective. The equipment required for this type testing according to suppliers is relatively inexpensive and easy to operate. The test is totally non-invasive having no effect on the surface being tested. Electron emission testing is popular in many industries including the manufacture of laser enclosures and other critical components. It is recommended that readers interested in more detailed information on this type of test search the internet. A good (although older) article on the subject was published in Precision Cleaning Magazine and can be found at http://www.p2pays.org/ref/02/01816.htm.
One of the very most sensitive tests available utilizes radioactive tracers introduced either into the contaminant or into a solvent that is evaporated from the surface. Although very sensitive and should be noted, it is out of the range of most users. Again, an internet search is suggested for those with further interest.
Although not all-inclusive, the tests described in this and previous blogs represent a good cross section of the types of tests that are used to detect surface films to measure cleanliness. Next we will explore methods of detecting particles contaminating surfaces.
– FJF –