Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:firstname.lastname@example.org
Physical Characteristics and Testing of Plated Deposits
HARDNESS AND TESTING
Hardness is defined as resistance to penetration. The hardness of various electroplated and electroless plated deposits has been and continues to be the subject of much discussion and controversy. For example, hard chrome is often reported to have 1000 to 1200 Vickers hardness. Actually the hardness of hard chrome can vary significantly if the plating conditions of temperature and current density are not controlled in certain specific ranges. The hardness can be as low as 670 Vickers hardness number or as high as 1200. It is the greatly dependent on plating conditions. Often hard chrome platers do not know what temperature and current density are required for the highest hardness. There are charts in the literature.
In addition, the hardness test results can vary. If the deposit thickness is insufficient to support the indentation, low readings result. If the gram load is not specified in the report of hardness, the result is meaningless. Most of the early tests were done using a Vickers indenter. ASTM-384, (ASTM is the American society for Testing Materials) an older specification calls for a 100-gram load for hard deposits such as hard chrome and electroless nickel. Some people have used lesser or higher loads. The Vickers indenter is a pyramid-shaped diamond. The thickness must be at least 0.001" ( 1 mil). Thinner deposits are likely to result in incorrect readings. Higher gram loads can cause cracking of the deposit resulting in an error. Lower loading for hard deposits can also result in erroneous readings. Vibration of the indenter during tests can cause errors. Poor specimen preparation such as the specimen is mounted on a slight slant, or the polishing is not done properly.
The most recent ASTM specification calls for the use of the Knoop indenter that uses a rhombic-shaped indenter. It is thinner and longer than the Vickers. Both ASTM and NIST (National Institute of Standards and Technology) specify the Knoop hardness test for hard deposits and Vickers is no longer recommended. The Rockwell indenter is spherical and is not recommended for plated deposits. There are charts in the literature that show the approximate relationship between the various hardness tests. These are not accurate because of the differences in results due to the procedures. Knoop hardness is specified for soft metals but with lower gram loads.
Electroless nickel varies in hardness, similarly to chrome, but for different reasons. The lower the phosphorus in the EN deposit, the harder is the deposit until the phosphorus (P) content is somewhat lower then 4%. The highest as plated hardness is about at about 4% phosphorus and is about 700-750 Knoop, 100-gram load (about the same using Vickers).
The heat-treated hardness is from 970 to 1000 Knoop, 100 gm load (or Vickers 980 to 1010) The heat-treat temperature is best at about 385 degrees C.
The as plated hardness of 10-11% P is about 500 Knoop-100 gm load. These deposits can be heat-treated to a hardness of about 890 Knoop, 100 gm load. The Heat-treat temperature for maximum hardness is about 395 degrees C. The literature usually suggests a heat-treat temperature of 400 degree C (750F) for 1 hour. Note if the oven is already at temperature it really only takes about Ĺ hour to reach the maximum hardness. Much longer times than 1 hour can lower the hardness somewhat. And higher temperatures above 400 degrees C can soften the deposit.
For Mid Phosphorus electroless nickel deposits 6-8% P are about 900-950 Knoop, 100- gm load. Be sure to prepare the specimen according the recommended practices of ASTM (American Society for Testing Materials) and/or NIST (National Institute of Standards).
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 donít 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
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 cross-section 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.
CORROSION RESISTANCE TESTING
Well, I guess I have to start with salt spray testing. The oldest and most used accelerated environmental simulated testing method around. ASTM B-117, 5% neutral salt fog tests. How good is at predicting failure in real life in the environment? Consider the following: the U. S. Bureau of Standards prepared and sent to numerous laboratories doing B-117 salt fog testing samples to test and report the results. The bottom line was that there was no correlation between the various (and numerous) salt fog cabinets. That means there were many different answers. A test made comparing salt fog to other corrosion test also showed that "there is usually not a direct relation between salt spray resistance and resistance to corrosion in other media." So, why use it? Well because a large number of less informed people require it.
There are numerous tests now floating around that are reported to be better than salt spray. For example, Chrysler Corp specifies Corrodkote. A paste painted on the surface to be tested, then exposed to humidity. Test conditions are according to ASTM B368
CASS (copper-acetic acid modified salt spray) this is much more corrosive than the ASTM B-117. Now specified by many of the Automakers. It is ASTM B368. It consists of 5% sodium chloride+ copper chloride.2H2O acidified with acetic acid to a pH of 3.2.
Out door exposure sites are scattered about the country for the purpose of evaluating industrial atmosphere exposure, these tests require years of observation.
"Controlled humidity" test: there are fifteen ASTM standards relating different variations of creating and controlling fog and humidity in cabinets for corrosion testing of a broad spectrum of products, from decorative electrodeposited coatings to the evaluation of the corrosivity of solder fluxes for copper tubing systems. The basic humidity test is most commonly used to evaluate the corrosivity of materials or the effects of residual contaminants. Cyclic humidity tests are conducted to simulate exposure to high humidity and heat typical of tropical environments."
A new corrosion test was developed by the Corrosion Task force of the auto/Steel Partnership, a consortium whose members includes the three major U.S. automakers and nearly all of the major steel producers. It is called "Cosmetic Corrosion Lab Test, SAE J2334. It is a rather simple test to perform and takes only 24 hours per test cycle and has three stages. It can be run manually or automatically. The tests were correlated with a series of real life tests. One of the tests consisted of mounting panels on pick up trucks driven in Montreal, Quebec and St. Johnís Newfoundland, for five years; two of the most corrosive environments in North America. Similar tests were run in Michigan, Ohio and Pennsylvania. The tests showed the best correlation to the real world.
The last tests I will mention is anodic stripping and electrochemical anodic polarization that detects unseen pits and corrosion cell on the surface of the specimen. There are special lab instruments for these and are usually run by chemists or electrochemists. I am sure there are more tests that I donít remember or simply donít know.
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