troubleshooting, testing, & analysis

THICKNESS TESTING

BY NORBERT SAJDERA

KOCOUR CO., CHICAGO; WWW.KOCOUR.NET

Coatings are applied to base materials to provide properties not inherent in thebase. These include, but are not limited to, corrosion protection, wear resistance,conductivity, color, reflectivity, and solderability.The amount of coating applied is critical to the final product’s utility andcost. The determination of the amount of coating is, therefore, important inappraising its utility and assessing its cost.Thickness is the most commonly used word to describe the amount ofcoating. A few of the methods used measure the linear depth of the coatingdirectly. These include the micrometer, with variations using styluses attachedto sensitive mechanical and electronic amplifiers, and the microscope, withvarious methods to expose the coating layers for measurement.More commonly, gauges estimating the weight per measured area are used. Thethickness is then calculated using the following equation:

T = m 10/A d, (1)

where T = thickness (μm), m = mass of coating (mg), A = area tested (cm2),and d = density (g/cm3).The instruments using the weight per unit area as the basis for their measurementsare beta backscatter, coulometric, and X-ray.The magnetic and eddy-current methods compare the magnetic and electricalproperties of the base and coating materials to calibrated standards withsimilar properties. The drop test is based on the rate of attack of certain chemicalsolutions. With such a diversity of methods, it is useful to use the summaryin Table I to help choose a measuring system for a particular requirement. Thegravimetric, microscopic, and X-ray are not included in Table I, because theyapply to almost all of the coating combinations listed. A convenient conversiontable for different systems of units is given in Table II.

BETA BACKSCATTER

If a stream of beta particles is directed at matter, it will collide with the atoms inthe matter. This results in a reduction of speed and a change in direction of theparticles. Those particles that leave the matter through the same surface fromwhich they entered are said to be backscattered. The number of particles backscatteredis proportional to the number of atoms per unit area and, therefore,to the atomic weight. The penetration depth of the beta rays is dependent onthe energy level of the radioisotope used as their source.The backscatter can be measured with a Geiger-Müller counter placed in itspath. In a measuring system, a radioisotope is placed between the Geiger-Müllercounter and the coating to be measured. A stream of particles is directed fromthe isotope through an aperture and then to the coating. The backscatter radiatesback through this stream, passes the radioisotope, and then is measuredwith the Geiger-Müller counter.The isotope is chosen on the basis of its maximum energy and half-life. As the thickness of the deposit increases, the energy needed to penetrate the depositis increased. Metals with larger atomic numbers require higher energies for thesame thickness.As particles are emitted by the source, the number of active particles remainingis reduced. The half-life of an isotope is the time necessary to reduce theactivity of the source by one half. As the activity decreases, the instrumentrequires recalibration for the new activity level.Accurate placement of the part is essential for precise measurement. Anaperture is provided to control the area exposed to the beta rays. The apertureis mounted in a probe together with the source to control the distance of thesource to the sample.The Geiger-Müller tube is part of a counting system that records the backscatterand then computes the thickness. A full line of accessories to storedata and provide statistical information is available. Many systems providecomputer prompting for both measuring and calibration procedures.The beta backscatter method applies to coatings and substrates whose atomicnumbers differ by at least 5. ASTM Standard Method B 567 details the considerationnecessary for accurate measurement with this method.

COULOMETRIC METHOD

The coulometric method is based on Faraday’s law. The law states that onegram-equivalent weight of metal will be stripped or deposited for every 96,500coulombs (ampere-seconds) of electricity passed through the electrolyte. Thislaw is so basic that it has been used to define the international ampere.The international ampere is defined as the unvarying electric current that, whenpassed through a solution of silver nitrate, will remove 0.000118 gram of silver persecond from the anode. This figure (0.000118 gram of silver per second) is calledthe electrochemical equivalent of silver.The following equation defines the weight of metal deposited according toFaraday’s law:

M = eit, (2)

where M = mass (g), e = electrochemical equivalent (g/ A-sec), i = current(A), and t = time (sec). To apply the coulometric method tothickness testing, four parameters must be controlled, namely, area, amperage, time, and anodeefficiency. At 100% anode efficiency, by substituting the mass obtained fromFaraday’s law into the thickness formula, the thickness becomes

T = eit 10/A d. (3)

The area to be measured is determined by a flexible rubber gasket. This areacan range from 0.13 to 0.32 cm in diameter. The gasket is an integral part ofthe deplating cell, that holds the solution during the test. The gasket must beflexible so that it will prevent leakage of the solution, yet sufficiently rigid forprecise maintenance of the area.A constant pressure device is included to aid incontrolling the gasket pressure. Since this measurement yields weight per area,accurate control of this diameter is essential.On most instruments, the current source and timer are included in a currentsupply package. This package provides a means for producing a specific constantamperage for each coating to be tested.When the coating is penetrated, there is a voltage change. The rate of changeof this voltage is monitored and used to terminate the test. An electronic timeris used to record the time elapsed. A computer processes the time and amperage,then displays the thickness. Also included in the package are electronic controlsto modify the current and termination sensitivity. These controls are providedto compensate for minute changes in anode efficiency and area.It is necessary to use a specific electrolyte for each combination of coatingand substrate. The electrolyte must satisfy three conditions:

1. The solution must not chemically attack the coating.

2. Anodic dissolution of the coating should be at constant efficiency, ideally 100%.

3. The voltage change on penetration of the coating should be significant.This method is capable of consistently measuring the thickness of a variety ofmetallic coatings to ±10% of their true value. For certain coating and substratecombinations, the accuracy can be higher. The most accurate measurements are inthe range of 40 to 2,000 microinches; however, chromium can be measured in thicknessesas low as 3 microinches. The accuracy of measurement in a specific thicknessrange may be increased by calibrating the instrument with standards in that range.Coatings on wire are measured by means of an auxiliary cell. Tests are performedon sample lengths from 0.5 to 4.0 in. in length.One advantage of this method is the ability to measure combination coatingssuch as copper/nickel/chromium and copper/tin. The instrument manufacturer’sinstructions should be followed precisely for accurate results. Additionalguidelines for achieving accurate measurements are contained in the followingASTM methods:

1. ASTM B 504, standard method for coulometric thickness.

2. ASTM B 298, for silver coatings on copper wire.

3. ASTM B 355, for nickel coatings on copper wire.

The coulometric instrument has found application in measuring other qualitiesof metallic coatings. ASTM B 764 describes a procedure for simultaneousthickness and electrochemical potential (STEP) determination for the layers ofmultilayer nickel deposit.

DROP TESTS

The drop test for measuring plating thickness is based on the rate of attackof specially prepared corrosive solutions on a metal coating. For consistentresults, the drop size, drop rate, temperature, time, and solution compositionmust be controlled.The test is performed by allowing the solution to drop on a particular spotat a rate of 100 drops per minute. The operator then observes the time at whichthe coating is penetrated. For the most common thicknesses, the elapsed timeis less than one minute.Reproducibility of the test is dependent on the skill of the operator. Theoperator must detect the point at which the base metal is exposed and recordthe time. An experienced operator can reproduce his readings within ±5%. Forbest accuracy, the operator should standardize his technique with a standardof known thickness.The accuracy of the system is generally considered to be ±15%, because theoperator cannot control some of the factors that affect the test. These factorsinclude drainage of the solution, alloying at the coating/substrate interface, andcomposition of the coating.Low cost and the ability to measure thickness quickly on irregular shapes arethe chief advantages of the drop test. The greatest disadvantages are destructionof the coating and objections to the use of corrosive solutions in some areas. Thelargest application is in the fastener industry.Standard guidelines for the use of this test are contained in ASTM B 555 Some typical procedures are provided below.

Reagents

1. Cadmium deposits:Ammonium nitrate, 110 g/LHydrochloric acid, 10 ml/L

2. Zinc deposits:Ammonium nitrate, 100 g/LNitric acid, 55 ml/L

3. Zinc and cadmium deposits:Chromic acid, 200 g/LSulfuric acid, 50 g/L

4. Tin deposits:Trichloroacetic acid, 100 g/L

5. Copper deposits:Ferric chloride (FeCl3

.6H2O), 450 g/LAntimony trioxide, 20 g/LHydrochloric acid, 200 ml/LAcetic acid (CP, glacial), 250 ml/L

Operating Conditions

Drop rate: 90 to 110 drops per minute (100 preferred).Temperature: 20 to 30OC (70 to 90OF).Penetration rate: For zinc and cadmium (using separate reagents listed above),each second = 0.00001 in. For copper deposits, two seconds = 0.00001 in. Figure

1 shows the penetration factor as a function of temperature for testing zinc andcadmium deposits with the chromic acid/sulfuric acid reagent.Lacquer or other films are removed from the area to be tested, which is thencleaned with a suspension of magnesium oxide in water. The specimen is heldat an angle of 45O below the dropping tip. To ensure that the reagent impingeson the same spot throughout the test, it is preferable to clamp the specimen inplace rather than to hold it by hand.The apparatus may consist of a separatory or dropping funnel, which is fittedwith a short length of tubing terminating in a drawn-out tip. Special funnels areavailable with two stopcocks. One of these is fully opened and the other used toset the desired drop rate. An automatic drop-test apparatus is available in whichthe drop rate is automatically maintained at 100 drops per minute by means ofa synchronous motor-driven mechanism.

EDDY CURRENT

Eddy current thickness gauges are electromagnetic instruments designed tomeasure the apparent change in impedance of the coil that induces the eddycurrent into the base metal. They are calibrated by comparing the apparentchange in impedance to known thickness of coatings on selected base materials.It is the difference between the conductivity of the base material and thecoating that influences the change in impedance; therefore, the instrument hasits greatest accuracy when testing nonconductive coatings on conductive basematerials and vice versa. The test can be applied to poor electrical conductorsover good electrical conductors with some loss in accuracy.The thickness test is performed with the aid of a specially designed probe.Measurements are made by holding the probe perpendicular to the surface andwith the probe point in contact with the area to be measured. The measurementsare rapid and nondestructive; therefore, some problems with accuracymay be resolved by statistical evaluation of many readings. Thickness gaugesare available with digital display, memory, hard-copy printout, and computerprompting of the calibration procedure.In the range of 5 to 50 μm, the thickness can be determined to within 10%or 1 μm, whichever is greater, of the true thickness. This test is sensitive tomarked differences in the surface contour of the test specimen. Particularattention should be paid to the distance from an edge, surface roughness, andcurvature at a test point.Erroneous results may be avoided by calibrating with standards that approximatethe surface condition and curvature of the specimen to be tested. Thetype of electroplating solution used can influence the electrical conductivityof the deposit and, therefore, the thickness measured.Zinc plated in cyanide, chloride, or alkaline baths is the most prominentexample of this problem. Calibrating the instrument with standards from thesame or similar solutions can greatly reduce error.Eddy current thickness testing is widely applied to anodic coatings onaluminum, nonmetallic coatings on nonmagnetic base metals, and to a lesserextent, to metallic coating/substrate combinations that have different electricalconductivities. The instrument manufacturers’ instructions should befollowed precisely for best results. A standard method for the application andperformance of this test is available in ASTM B 244.

MAGNETIC METHOD

The magnetic method uses the magnetic influence of the coating and substrateon a probe as the basis of a measuring system. Two types of probe systems arein common use. The first to be developed makes use of a mechanical system tomeasure the influence of the coating thickness on the attractive force betweena magnet and the base material.An electromagnetic probe was later developed that measures the influenceof the coating thickness on the reluctance of a magnetic flux path throughthe coating and base material.Three types of coatings can be measured with this system:

1. Nonmagnetic coatings on ferromagnetic base metals.

2. Nickel coatings on ferromagnetic base metals.

3. Nickel coatings on nonmagnetic base materials.The test is performed with the aid of specially designed probes. With thepermanent magnet type, measurements are made by placing the probe perpendicularto the surface to be measured and observing the force necessaryto remove the probe.The electromagnetic-type probe requires placing the probe perpendicularto the surface to be measured and observing the reluctance measurement.The measurements are rapid and nondestructive; therefore, some problemswith accuracy may be resolved by the statistical evaluation of many readings.Commercial instruments are available with analog and/or digital thicknessdisplay, memory, hard copy printout, and computer prompting of calibrationprocedures.The effective thickness range is dependent on the choice of probe system(magnet or reluctance) and the coating/substrate combination. The rangesfor the magnet type are:

1. Nonmagnetic coating on magnetic base, 5 to 25 μm.

2. Nickel coating on magnetic base, 5 to 50 μm.

3. Nickel coating on nonmagnetic base, 5 to 25 μm.The ranges for the reluctance type are from 5 μm to 1 μm for all three coating/

substrate combinations. Both types of instruments are sensitive to markeddifferences in the surface contour of the test specimen.Particular attention should be given to the distance from an edge, surfaceroughness, and curvature at the testing point. Erroneous results may beavoided by calibrating with standards that approximate the surface conditionand geometry of the specimen to be tested. When properly calibrated, themagnetic system can determine the actual thickness within 10%. The instrumentmanufacturer’s instructions should be carefully followed for the mostaccurate results. Two standard methods have been developed for additionalguidance to more reliable readings. They are ASTM B 499 and ASTM B 530.

GRAVIMETRIC

This method requires the measurement of the area to be tested and the determinationof the mass of the coating in that area. The area may be determinedby standard measuring techniques. The coating mass may be determined byone of the following procedures:

1. Weigh coating directly after dissolving the base material withoutattacking the coating.

2. Determine coating mass by analyzing the solution used to dissolve thecoating and all or a portion of the base material.

3. Determine coating mass as the difference between the weight beforeand after dissolving the coating without attacking the base material.

4. Determine coating mass as the difference between the weight beforeand after coating.Usually this method is assigned to a laboratory equipped to handle the corrosivesolutions and to measure the mass and area with sufficient accuracy.After the area and mass have been determined, the thickness may by determinedby using Equation (1).Procedures 1, 2, and 3 are destructive; procedure 4 is nondestructive. Thismethod has the capacity to yield extremely accurate results and is frequentlyused to determine the exact mass of metal used for cost purposes, particularlywith the more precious metals. This method (procedure 4) is used as a techniquefor making thickness standards.ASTM has developed procedures for several specific coatings. They areMethod A 90 for zinc, Method A 309 for terneplate, Method A 630 for tin plate,and Method B 137 for anodized aluminum. ASTM B 767 serves as a guidefor use of this procedure for a variety of plating and substrate combinations.

MICROMETRIC

A micrometer may be used to check the thickness of coatings over 0.001 in. Ifthe micrometer is equipped with a dial indicator, thicknesses of 0.0005 in. maybe measured on uniformly coated cylindrical parts. It is necessary to measurethe same spot before and after plating. Measurements may be obtained by maskingthe deposit and dissolving the unmasked coating, then measuring the stepproduced by this procedure.

MICROSCOPIC

The microscope can be used as a length measuring instrument when it isequipped with a filar eyepiece. The specimen must be carefully polished toprepare a smooth reflective surface and then etched to reveal the various metalsexposed. This generally requires the services of an experienced metallographer.The measurements are generally made on a transverse section of the depositso that the various layers of plating are exposed for viewing. Measurements arealso made on tapered sections to increase the length to be measured. The thicknessis then calculated by correcting the observed length for the taper angle.This method is destructive and time consuming. The thickness results have aprecision of ±2%; however, the accuracy has a constant uncertainty of about 0.8μm (30 microinches). Therefore, despite the precision of the method, it shouldnot be considered as a reference to resolve questions about thicknesses less than8 μm (300 microinches).The filar eyepiece is calibrated by comparing the divisions on the filarmicrometer to the known distance between divisions on a stage micrometer.This method is a true measure of length and does not require a plated standardfor calibration.Due to the high cost of this technique, its use has been reserved for thoseoccasions that require more information than may be obtained from otherthickness-gauging methods. Information concerning porosity, surface roughness,grain structure, and adhesion may be gathered from the specimen preparedfor a thickness test.ASTM B 487 is a standard method outlining the conditions for accurateresults.The wavelength of light limits the resolution of the light microscope to about10 microinches. The scanning electron microscope utilizes the shorter wavelengthof electron waves to measure metallurgical specimens with a 4 microinchresolution. ASTM B 748 is the standard test method for this procedure.Attachments are available for the metallurgical microscope that allow it to beused as an interferometer. The method is mentioned, because its accuracy withthin coatings can be within ±5% of the true thickness. Conditions necessary forthis procedure are contained in ASTM Standard Method B 588.

THE SPOT TEST

This test was developed as a rapid and inexpensive thickness test for chromiumcoatings on nickel and stainless steel. The test has an accuracy of ±20% for coatingsup to 1.2 μm thick. A wax ring is outlined on the part to be tested. A dropof hydrochloric acid is placed in the ring, and the time between the onset andcessation of gassing is recorded. ASTM B 556 provides a standard guide for theperformance of this test.

X-RAY FLUORESCENCE

This method is similar to beta backscatter in that the area to be tested is the targetof radiation, and the energy emitted from that surface is measured. In this method,the radiation used is X-rays produced by an X-ray tube. The radiation measured issecondary emissions from the interaction of the X-rays with the coating and substrate.Unlike beta backscatter, the emissions measured are specific for each metal.Among the unique characteristics of this method are the following:

1. No physical contact with the measured surface is required, thereby protectingthat surface.

2. Extremely small areas may be measured.

3. Since the emissions are specific for each metal, alloy compositions maybe determined.

4. With proper calibration, intermediate coatings may be measured in amultilayer system.Thickness may be measured in the range 0.25 to 10 μm, depending on themetal being measured. With proper calibration, the thickness may be determinedto within 10% of its true value. ASTM B 568 outlines a standard method for thismeasurement system. Due to the noncontacting characteristic and the rapid testtime, this procedure is used to continuously monitor thickness on continuouscoilplating machines and automated plating machines.

STANDARDS

Thickness standards are required for calibrating thickness gauges. In most cases,the standards should be plated from a similar plating solution and on the samebase metal as the product to be tested.After receipt, it is important to have a system to ensure the standard’s reliabilityafter use. One system makes use of primary, secondary, and working standards.The working standards are used for calibrating the thickness gauge daily.Once a week, the working standards are calibrated against the secondary standards.The secondary standards are then calibrated against the primary standardonce a month. The time between calibrations can be varied based on experiencewith the expected life of the standard. When a new standard is purchased, itbecomes the primary standard, and the remaining standards are lowered in rank.

SUMMARY

To measure the thickness of a coating, many properties have been utilized.Measurements have been described that use the atomic configuration, electrochemicalequivalent, corrosion rate, electrical conductivity, magnetic properties,density, and actual linear measure of the coating.In addition to these methods, tests have been successfully performed by usingthe electrical resistance and transparency of the coating. Each of these methodshas its proper application.It is obvious that one system is not capable of satisfying the needs of everyplater. Certainly, a plater coating 10,000 fasteners per barrel load at a rate of 50barrel loads per shift has a problem that is substantially different from a platerthat hard chromium plates printing rolls. Experience and judgment are required,and the responsibility for choosing the most economical thickness-measuringsystem adequate for a particular problem is not a simple task.