Non-Contact Coating Thickness Measurement With Advanced Optical Technology

As DB Kimya San. ve Tic. A.Ş., we have been closely following developments in coating technologies worldwide since 2016. As a result of this monitoring, we brought Coatmaster to Turkey in mid-2017. This patented technology was very new and had only been commercialized in 2015. We quickly began trials and demonstrations with leading manufacturers in the sector. In our article you will find some examples of how this system is integrated particularly into automotive and powder coating application lines. In the article we will discuss three different Coatmaster applications with examples. Thermal spray coating thickness measurement in automotive, rubber-metal adhesive thickness measurement in torsional vibration dampers, and powder coating application.

Process Control in Thermal Spray Coatings in Automotive:

Coatmaster provides rapid and non-destructive coating thickness measurement in thermal spray coatings. Process deviations can be detected and corrected early.

Figure 1. For coating thickness measurement, the measuring device is positioned in the cylinder bore (upper bore opening). In this way, coating thickness can be measured at any location around the circumference and at the depth of the cylinder bore.

Thermal spray is classified as a process in which coating material is melted by a thermal source, such as a plasma flame, and applied to a component with a gas flow. Liquid particles rapidly solidify when they contact the surface and produce a layered coating. With powder-based atmospheric thermal spray coatings, virtually all materials can be coated. Coating types range widely from thermoplastics, metals and carbides to ceramics. Iron-based coatings provide the required tribological properties and can be used as wire or powder. Wire arc spray coating, plasma transferred wire arc (PTWA) and rotating single wire (RSW) coatings are examples of wire coating processes. Powder-based atmospheric plasma spray coating also allows the application of ceramic materials in addition to metals. Low-alloy carbon steels are predominantly used for coating the working surfaces of both gasoline and diesel engines. Coatings are heterogeneous and consist not only of melted and solidified particles. Within coatings, depending on process parameters, oxides and carbides form, as well as pores at frequencies between 1 and 4 percent. After honing, these pores serve as a lubricant reservoir and hold the lubricant in place. The friction forces between the piston ring, piston and cylinder walls are thereby minimized. Fuel consumption decreases and the engine's service life is increased. Coating thickness measurement in less than one second: The thickness of unprocessed thermal spray coating must follow narrow tolerance bands. Any deviation from specified tolerances can cause damage to honing tools in the subsequent processing stage and lead to rework. When early coating thickness measurement is used, the layer thickness is kept very small, protecting subsequent processes in the value chain. Traditional coating thickness measurement devices have low repeat accuracy due to rough coatings. For this reason, they are not suitable from a quality assurance perspective. Random inspections via photomicrographs are very time-consuming and do not allow continuous and non-destructive inspection of the coating process. In contrast, the new device from Winterthur Instruments, CoatMaster, measures average coating thickness with a measurement area of 1 to 3 mm in diameter. As a result, even at high surface roughness, high repeat accuracy of 1 to 2 percent is achieved. A measurement takes less than one second and it is therefore possible in series production to test several measurement points for each bore. Coatmaster measures the coating using thermal test principle. The device's light source heats the coating surface by a few degrees Celsius over a duration of several milliseconds. Optical elements and infrared sensors record changes in surface temperature. The light source used is the same as in photographic flash and poses no hazard to humans or the environment. More than 100,000 temperature readings are analyzed for each measurement operation, which then determines the coating thickness. Measurement can be performed from a distance of up to one meter and with an adjustable measurement area having a diameter of 2 to 50 mm. The error of a single measurement is typically below 1 percent. Coating thicknesses can be recorded at a frequency of 2 Hz. By means of an optical measuring probe attached to the device (Figure 1), measurement positions are automatically recorded at fixed intervals across the entire cylinder working surface. CoatMaster makes it possible to measure the thickness of thermally sprayed coating directly, accurately and quickly in a non-destructive manner. Process deviations affecting coating thickness can be detected and corrected quickly (Figure 2).

Figure 2. Coating thickness measurement on the cylinder working surface, at opposite points from bottom to top (a, b). Measurements 1-15 Crankcase 1, Measurements 16-30 Crankcase 2.

Modern Quality Assurance Method for Adhesive Layers

In the automotive industry, measurement of adhesion promoter and adhesive coating thicknesses during the production of torsional vibration dampers meets the high standards of automotive. This section of the article addresses this topic. Torsional vibration dampers are used in internal combustion engine vehicles to reduce radial vibrations. These vibrations are caused by intermittent power transmission from the piston to the crankshaft. These vibrations produce short-term torque oscillations that cause noise and wear in the transmission. In addition, by exerting pressure on the crankshaft, they can lead to buckling fractures. The function of torsional vibration dampers is to suppress these vibrations. Dampers consist of a vibration damping ring and an intermediate rubber element with a hub. During production, the surfaces of the vibration damping ring and hub that contact the rubber are specially pre-treated. These surfaces are then coated with an adhesive. The subsequent vulcanization (curing) process creates a permanent bond between the vibration damping ring, rubber coating and hub.Adhesive layer thickness as a quality characteristic: Adhesive layer thickness is typically measured in micrometers and has a tolerance window. If layer thickness is below the tolerance window, it can lead to adhesion problems between rubber and metal and subsequently cause loss of functionality of the torsional vibration damper. When layer thickness is too high, cracks can form in the adhesive layer under mechanical loading of the component. Therefore, coating thickness measurement during production is a fundamental quality criterion for ensuring the functionality of torsional vibration dampers. Test equipment used for functionally critical quality parameters must be carefully evaluated. For this purpose, quality capability in the automotive industry is defined as Cg. The Cg value is calculated according to the Bosch Handbook 10 using the following formula:

T: tolerance band (upper tolerance limit - lower tolerance limit), Sg: Standard deviation (corresponds to measurement errors for each individual measurement).

Search for an appropriate measuring device: According to this standard, only test devices with a Cg value higher than 1.33 can be used in quality assurance. For example, with a 10 micrometer tolerance window and a single measurement error of 0.9 micrometers, a measuring device would achieve a Cg value of 0.37 and could not be used in quality assurance. In the past, induction current or magneto-inductive measuring devices were used to test layer thickness of adhesion promoters. These typically had standard deviations of several micrometers on pre-treated and coated surfaces. This results in Cg values below 1.33 with 10 or 20 micrometer tolerance windows. These measuring devices were therefore not approved for quality assurance. Currently, power transmission component manufacturers are using coating thickness measuring devices that operate on this thermal principle. An integrated light source heats the surface of the adhesive by a few degrees Celsius for several milliseconds. Optical elements and an infrared sensor determine and record surface temperature. The technical specifications of the light source used pose no hazard to humans or the environment and are comparable to photographic flash tubes. Use of questionable sources such as lasers, beta or X-ray sources has been deliberately avoided. On average, 100,000 temperature readings are analyzed for each measurement operation and layer thickness is determined from this data. Measurement is performed from a distance of up to 1 meter with an adjustable measurement area between 2 and 50 millimeters in diameter. The error of each individual measurement can fall below 1 percent and measured values can be recorded at a frequency of 1 hertz. High-precision contactless measurement: CoatMaster enables contactless layer thickness measurement of the adhesive layer with a measurement error of 70 nanometers (= 0.07 micrometers). This corresponds to a Cg value of 4.5 and thus meets the requirements of the automotive industry. In summary, it can be said that when the measuring device is used by manufacturers and used with very high accuracy, it determines an important parameter.

Savings Potential of Powder Coatings with Inline Coating Thickness Measurement in Field Testing

More than 25 percent powder coating savings and visually noticeable improvement in surface quality are achieved. Powder coating products are becoming increasingly important from ecological and economic perspectives, which means long-term stronger growth in favor of powder coatings. Considering today's raw material supply disruptions and increasing bureaucratic constraints in the paint manufacturing industry, this situation drives us to use resources much more efficiently and brings sustainable technologies to the fore. At this point, the objective in coating applications can be interpreted as working with the lowest possible coating thickness. On the other hand, the technically excellent mechanical properties of powder coatings, at increasing application thicknesses, particularly appear to us as loss of flexibility, which also drives applicators to work at optimum application thicknesses. For precise application of powder coatings at micrometer levels, coating thickness measurement must be performed immediately after the coating process. Traditional measurement methods that measure thickness after baking are therefore excluded from our scope. For the coating thickness measurement process in production areas, the device must meet prerequisites such as having an appropriate measuring range, high angular swing and gap tolerance. The measuring device should be simple to use and reliable and pose no hazard to workers. At Ernst Schweizer AG, tests were conducted over the course of a year with Winterthur Instruments' contactless coating thickness measuring device Coatmaster. Trial results at the modernized facility with Coatmaster showed particularly visible improvement in surface quality, prevention of scrap and 28 percent paint savings. Training hours for new employees also decreased significantly. Following extensive tests with ultrasonic and laser-based measuring devices at the company, the CoatMaster measuring system from Winterthur Instruments was chosen. This measuring system was mounted at the cabinet exit, immediately after powder coating application. Over a distance of 2 meters along a vertical axis, the paint thickness distribution of parts being transported was mapped using the height of the product carrier. The results of coating thickness measurement were displayed graphically on a monitor next to the main control system. The facility operator received quantitative and real-time feedback on changes in process parameters and was able to reduce paint thickness preferentially to the lower tolerance threshold without any risk.   Çağrı Doğançay General Manager DB Kimya San. ve Tic. A.Ş.

Figure 5. Coatmaster: Winterthur Instruments' contactless inline coating thickness measuring device.