Continuous In-Line Chromium Coating Thickness Measurement Methodologies: An Investigation of Current and Potential Technology

Abstract

Coatings or films are applied to a substrate for several applications, such as waterproofing, corrosion resistance, adhesion performance, cosmetic effects, and optical coatings. When applying a coating to a substrate, it is vital to monitor the coating thickness during the coating process to achieve a product to the desired specification via real time production control. There are several different coating thickness measurement methods that can be used, either in-line or off-line, which can determine the coating thickness relative to the material of the coating and the substrate. In-line coating thickness measurement methods are often very difficult to design and implement due to the nature of the harsh environmental conditions of typical production processes and the speed at which the process is run. This paper addresses the current and novel coating thickness methodologies for application to chromium coatings on a ferro-magnetic steel substrate with their advantages and limitations regarding in-line measurement. The most common in-line coating thickness measurement method utilized within the steel packaging industry is the X-ray Fluorescence (XRF) method, but these systems can become costly when implemented for a wide packaging product and pose health and safety concerns due to its ionizing radiation. As technology advances, nanometer-scale coatings are becoming more common, and here three methods are highlighted, which have been used extensively in other industries (with several variants in their design) which can potentially measure coatings of nanometer thickness in a production line, precisely, safely, and do so in a non-contact and non-destructive manner. These methods are optical reflectometry, ellipsometry and interferometry.

Mapping and Imaging of Thin Films on Large Surfaces

“…Methods of large‐area thin‐film characterization have been summarized. According to other studies, [ 2 ] only optical methods are capable of a high‐speed, high‐sensitivity, and nondestructive characterization of thin films over large surfaces (although some potential future candidates have also been mentioned, such as the thermoelectric method with a magnetic readout approach, THz‐TDS, CCM, infrared thermography, as well as XRR or XRF). In case of optical methods, we discussed techniques including spectroscopic reflectometry, transmission, scattering, speckle imaging, and SE in all cases when the areas under study were larger than the wavelength of light.…”

“…Three main groups of the approaches have been identified: 1) depth profiling by sputtering (e.g., secondary ion mass spectrometry [SIMS], secondary neutral mass spectrometry [SNMS], X‐ray photoelectron spectrometry [XPS], Auger electron spectrometry [AES], glow‐discharge optical emission spectrometry [GD‐OES], glow discharge mass spectrometry [GD‐MS], Raman depth profiling), 2) nondestructive techniques (e.g., spectroscopic ellipsometry [SE], Rutherford backscattering spectrometry [RBS], elastic recoil detection analysis [ERDA], X‐ray diffraction [XRD], angle‐dependent X‐ray emission spectroscopy [AXES]), and 3) cross‐section micro(nano)mapping (e.g., scanning transmission electron microscopy [TEM], its combination with energy‐dispersive X‐ray analysis [EDX], scanning Auger electron microscopy, Raman mapping, time‐of‐flight SIMS [TOF‐SIMS]). The tools that are capable of the determination of coating thickness are reviewed by Jones et al [ 2 ] sorting the methods in traditional (coulometry, beta‐particle backscattering, eddy current, magnetic induction, X‐ray fluorescence [XRF], X‐ray reflectometry [XRR], ultrasonic detection), offline (XPS, scanning electron microscopy [SEM], atomic force microscopy [AFM], GD‐OES) and in‐line (thermoelectric method, terahertz time‐domain spectroscopy [THz‐TDS], reflectometry, interferometry, ellipsometry, stimulated Brillouin scattering [SBS], self‐mixing interferometry [SMI], chromatic confocal microscopy [CCM], infrared thermography) categories with the conclusion of reflectometry, interferometry, and ellipsometry being the most suitable methods for in‐line characterization.…”

“…Methods of in‐depth thin‐film characterization [ 1 ] based on either depth profiling by sputtering, complex modeling based on nondestructively measured whole‐depth data, or preparing cross sections destructively and time‐consumingly are all mature to an extent that only requires incremental adaptations and improvements for new tasks such as large‐area mapping. According to Jones et al, [ 2 ] further methods for the determination of coating thickness are reviewed with in‐line and offline capabilities. There are intrinsic features of those methods that make only a few of them capable of collecting the amount of data needed for large‐area measurements within an acceptable time frame.…”

“…Methods of thin‐film and interface characterization can be classified by numerous different aspects such as the way depth profiling is achieved, [ 1 ] according to illumination and detection, [ 3 ] or based on in‐line capabilities. [ 2 ] In Section 2, we briefly summarize the many opportunities and in the rest of the paper focus on those potentially applicable to large‐area mapping. At the end of this article, we discuss some applications and conclude with a short summary of the major characteristics of the methods.…”

Optical Sensors Technology and Applications

“…Ref. [ 12 ] is a careful review of the conventional and innovative coating thickness methodologies for application to chromium coatings on a ferromagnetic steel substrate and determines their advantages and limitations regarding in-line measurements. The X-ray Fluorescence (XRF) method is the most common in-line coating thickness measurement method employed in the steel packaging industry, but it is expensive and is characterized by health and safety concerns due to its ionizing radiation.…”