Quality 101: Understand Coating Thickness Measurement Test Methods
Improving quality while reducing costs is a never-ending goal shared by small one-person companies, as well as by large organizations. Since virtually everything that we are surrounded by has a coating, it stands to reason that tightening tolerances by using coating thickness measurement technology will help the bottom line. This is true for in-house plating and painting applications, companies that perform incoming inspection of goods from suppliers, and third-party inspection firms and consultants. A reduction in costs also can take place by streamlining the time and effort it takes to evaluate the measurements and make adjustments to the process. Less administrative time equates to more time fine tuning.
In order to improve quality and reduce costs, it is important to have a working knowledge of the test methods related to coating thickness measurement and the instruments available for both common and complex applications. Most importantly, the operator should note that just because a test method is capable of measuring a coating, it might not necessarily be the best test method available.
We will discuss available test methods and point out some of the key factors an operator should consider before selecting the proper instrument. Recognizable applications, such as household fixtures, automobile parts and fasteners will be used as examples. We also will discuss a means to streamline the measurement results and help guarantee the integrity of the data.
Test MethodsCommon and complex plating and other coating thickness applications are typically measured using one of the following test methods: X-ray fluorescence, coulometric, beta backscatter, magnetic induction, amplitude eddy current and phase-sensitive eddy current.
Energy dispersive X-ray fluorescence analysis (ED-XRFA) is a method for measuring the thickness of coatings and for analyzing materials. It can be used for the qualitative and quantitative determination of the elemental composition of a material sample, as well as for measuring coatings and coating systems. It works nondestructively and without contact. Measurements can be completed quickly and usually without extensive sample preparation.
With ED-XRFA, it is possible to measure both thickness and chemical composition of homogeneous materials and coatings. Even traces of harmful substances can be detected in the widest variety of samples. Moreover, X-ray fluorescence analysis is a very clean method because no chemicals are used.
Chrome/Nickel/Copper Coatings Over PlasticOne of literally thousands of examples where X-ray fluorescence would be the test method of choice is the task of measuring chrome, nickel and copper coatings applied to a shower head. In most cases the substrate is plastic, thus eliminating most other test methods from being able to perform the measurement. Furthermore, X-ray fluorescence is capable of measuring each individual layer of the decorative chrome in the range of 0-5 microns or less, the nickel coating of 5-10 microns, and the copper layer which might be 30 microns or greater.
As previously mentioned, X-ray fluorescence is nondestructive, providing a major advantage. Care should be taken with regard to the alignment of the part to the detector and correctly focusing the video on the measuring point. The estimated or target thickness of the copper also should be considered in determining if X-ray fluorescence is the most appropriate test method. All things considered, X-ray fluorescence is ideally suited for this application, but it is not the only test method capable of measuring the chrome/nickel/copper coatings over plastic.
The coulometric method measures nearly all metallic coatings including multiple coatings on any substrate material, such as plastic. It is, however, a destructive method because the coatings are successively removed at the measuring area and determined by the time taken for removal.
X-ray fluorescence might still be the preferred choice because it is quick and nondestructive. However, the operator should consider the size of the part and if it will fit inside the chamber of the X-ray system-X-ray chambers can be modified for large parts-and if there might be other alignment or focusing issues that might occur.
The thickness of the copper might also determine which method is best. If, for instance, the copper thickness is up to 50 microns, then the coulometric method could be an important complement to the XRF method.
The beta backscatter method allows for the thickness measurement of organic and metallic coatings on various substrate materials. Even soft coatings, such as well as oil and lubricating films, can be determined with the appropriate probe configuration. Those that have thickness measurement applications that include organic and metallic coatings might choose beta backscatter over other methods because of the wide assortment of coating/substrate combinations that this method is capable of measuring. In addition, beta backscatter is nondestructive.
Magnetic induction and eddy current test methods represent some of the most recognizable coating thickness instruments available. Magnetic induction instruments measure the thickness of nonferromagnetic metal coatings such as chrome, copper, zinc, paint and powder coating over steel and iron. The amplitude eddy current method measures paint, powder coating, as well as anodize coatings over aluminum, stainless steel, copper and brass.
Depending on the capabilities of the particular instrument, there are dual-purpose magnetic induction and eddy current gages and probes. An example when these capabilities would be beneficial is in the case of an automobile where the doors might be steel and the hood might be aluminum. The ability for the device to automatically detect the substrate material and apply the appropriate test method is crucial. It reduces time, reduces the likelihood of erroneous measurements being taken and thus reduces time and costs.
StandardsASTM D7091 Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metals is a resource for those working with many of the aforementioned applications. Additional test methods have their own ASTM and ISO standards.
It is necessary for users to understand that not all gages and probes are ideally suited for all applications. Operators should review specifications such as trueness which is determined using calibration standards of known thicknesses. Trueness can be stated as an absolute value or as a percentage. Another criterion of a particular system is the precision or repeatability standard deviation of X number of single readings.
Other factors include the performance of the gage or probe when measuring concave or convex diameters, the minimum measurement area required, the performance of the gage or probe in relation to the distance of the probe tip to the edge of the part being measured, and the performance of the gage and probe as it relates to the substrate thickness.
Additional criteria to be considered in the selection process is the shape of the part that needs be measured, the type or types of substrates the coating is being applied to, and whether the user requires just a spot checker or an instrument that provides memory and download capabilities.
Zinc Plating Over SteelAnother example where multiple test methods could be used for the same task is the measurement of zinc plating over steel.
Magnetic induction will measure zinc over steel; however, the spot size and measurement area might be too small for truly meaningful results. Likewise, X-ray fluorescence will measure these parts but cost and the portability of a handheld instrument could be a factor in the decision. In this case, the phase-sensitive eddy current measurement test method should be utilized (ISO 21968).
The phase sensitive eddy current method allows for the measurement of metallic coatings on any substrate. This method also is advantageous for this example because it is capable of measuring small objects, such as nuts, bolts and screws.
In other words, the shape of the part has very little influence on the results of the reading. Likewise, measurements of zinc or copper on steel over rough surfaces can be measured. Therefore, truer and more repeatable results can be expected with this technique than with other traditional test methods for this example.
Inspection Plans and Report TemplatesStreamlining the measurement and documentation process is often cumbersome. Making sure the correct number of readings is taken and in the right sequence is extremely important. If done correctly, a selective analysis of different parts or areas on each part can be evaluated. For example, the spray patters on each automobile door, hood and trunk could be looked at individually. Adjustments can then be made to the spray patterns. Quick meaningful evaluations and adjustments to a process have a direct effect on the bottom line.
Many times hard copy documentation of inspection reports is required. Importing, copying and pasting measurement results into inspection forms are an administrative burden and can lead to mistakes. A 5.30 mil value could mistakenly be typed as 3.50 mil, resulting in unintentionally skewing of the statistical results of the data. The direct population of measurement results from the gage to virtually any report template will help reduce the likelihood of errors.
There are multiple test methods available for coating thickness measurement requirements. Utilizing the appropriate test method for the specific application is necessary. Therefore, performance capabilities such as the trueness of the gage and probe should be considered, particularly in regards to factors that influence the readings.
In addition to the capabilities of the gages and test methods, the operator is encouraged to take advantage of technological developments that have greatly enhanced efficiency through hardware and software capabilities. These developments reduce the likelihood of errors. It also provides a selective analysis of parts or areas on each part to be evaluated.
Quicker and more meaningful data through paperless inspection and report templates allow for adjustments to the process. These adjustments will result in the improvement of quality and reduction of costs.