Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:firstname.lastname@example.org
Physical Characteristics and Testing of
PART II – THICKNESS TESTING
Methods of testing thickness of plated deposits
Beta Back scattering Metallurgical cross section/microscope
X-ray Fluorescence Coulometric
Magnetic Eddy Current
Dropping test Spot test
Jet Test Micrometer
Weight increase Ultrasonic
Each test method has limitations and is subject to errors. Knowledge of these inherent errors is essential to producing reliable results. There is not enough space to discuss all these methods and point out possible errors. I may have missed a few methods. However these are the ones I know something about. I have used most of them. The nice thing about editorials is that I don’t have to follow the rules of good scientific paper writing.
I will start with Beta back scattering, because it has interesting attributes.
The beta back scattering is non-destructive and can be used on many different plated deposits. The way it works is interesting. It has a radioactive source in the instrument that emits beta radiation onto the metal to be measured. The metal scatters the radiation and a Geiger counter reads the amount of returned beta radiation. The amount of returned radiation is proportional the atomic number (and thus the atomic mass) of the atoms in the metal being tested. The “Betascope” and other names for Beta back scattering instruments is widely used because of its relative accuracy, ease of use, is relatively fast and is non-destructive.
To assure accuracy, the density of the deposit must be known within reasonable accuracy. For example, the thickness measurement = Betascope thickness X Gold density-standard
Standards are available from NTIS (National Institute for Technology and Standards) and from suppliers of the instruments. The instrument is calibrated using the pure gold standard, density 19.3 gm/cm3. Hard gold deposits range from 17.4-to18 gm/cm3.
. Using the standards and knowing the density of the metal or alloy to be tested other metals and alloys can be tested for thickness as well as gold.
Metallurgical cross section thickness measurements can be very accurate using the ASTM recommended procedure. This method requires cutting a crossection from the part to be tested. Cutting should be done with a fine grain diamond saw, or equivalent so that a minimum of burrs are left and the cut is exactly perpendicular the surface. The specimen is mounted in a plastic mold and polished to a fine finish. Again the specimen must not be at an angle that would result in a high reading. Using a microscope with a filar eyepiece, measure the thickness directly.
X-ray fluorescence is used to measure thickness of heavy metals. Nickel copper tungsten molybdenum, tin-lead alloys, etc. are the typical metals tested by this means for the best accuracy. Layers of deposits can be measured. For example tin-lead over nickel, Gold over nickel, chromium over nickel over copper, and many more similar layers of coatings.
Coulometric instruments, also known as anodic stripping, use an electrochemical process to etch away a plated or metallic layer at a predetermined rate. The amount of time to remove the plated layer provides an indication of coating thickness. Coulometric measurement is a destructive technique.
Eddy current, penetrating radar and other electromagnetic thickness gauge techniques are used to detect or measure flaws, bond or weld integrity, electrical conductivity, coating thickness, detect the presence metals. The eddy current method is also useful in sorting alloys and verifying heat treatment. Eddy current thickness gages use an electromagnet to induce an eddy current in a conductive sample. The response of the material to the induced current is sensed. Since the probe does not have to contact the work surface, eddy current testing is useful on rough surfaces or surfaces with wet films or coatings.
Laser thickness gauges include methods such as laser shearography, magneto-optical, holographic interferometry or other optical techniques to detect flaws, residual stress or measure thickness.
Mechanical gages physically contact a sample to measure thickness using a gap and/or comparison to a known dimensional standard or master. Micrometers and calipers are common types of mechanical gages used for dimensional gaging.
Ultrasonic instruments use beams of high frequency acoustic energy that are introduced into the material and subsequently retrieved. Thickness or distance calculations are based on the speed of sound through the material being evaluated. The most widely used of all UT techniques is the pulse-echo technique.
Magnetic measurements using the “Magnagage” has been around longer then I have. A small magnet attached to a flexible wire is lowered onto the surface of a plated non-magnetic coating and slowly tension is applied while a gage measures the force required to separate the probe from the surface being tested. The magnetic method is limited to magnetic substrates and non-magnetic coatings. It is a quick rough measure of thickness.
What have I missed?