Quality Control Microscopy of Coatings

We are surrounded by products that, for either decorative or functional purposes, are covered with coatings; from paintings and lacquers to adhesive or protective coatings, optical, catalytic or insulating coatings. Of all these coatings, conversion phosphate coatings play an important role, especially in the automotive industry, as they are used for corrosion resistance and lubricity. Since these coatings are used for critical parts, the coating process must undergo thorough quality checks. These checks consist of the analysis of the morphology of the coating as well as the percentage of coverage. In this blog, we describe and analyze how automated tools combined with SEMs can be helpful in quality checking phosphate coatings.

Coatings are not only used as a decorative feature, such as paint finishes or lacquers; most of the time, they have a functional purpose:

• Serve as an adhesive
• Have optical, electrical or magnetic properties
• Be catalytic or light sensitive, such as those used to make photographic film

One of the biggest categories is that of protective coatings, ranging from insulation to waterproofing and wear-resistant to anti-corrosion. Coatings can be applied through chemical vapor deposition, physical vapor deposition, spraying, or chemical and electrochemical techniques, such as electroplating.

Phosphate conversion coatings

Within this wide range of functionalities, materials, and coating techniques, we focus our attention on phosphate conversion coatings. These are typically used in the automotive industry and serve as a protection layer on steel parts, preventing corrosion and providing lubricity. The main types of coatings are manganese, iron, and zinc. The coating is applied by immersing the part in a bath containing a phosphoric acid, which causes the growth of a crystalline zinc, manganese, or iron phosphate layer.

Because of the critical use of the coated parts, the coating process must undergo thorough quality checks to ensure the performance of the coating. The quality check consists of the analysis of the coating morphology and the percentage of coverage. One way to carry out this analysis is by using a scanning electron microscope (SEM).

Quality control microscopy for the coating process

SEM is an ideal choice for quality checking of conversion coatings and for the analysis of the crystal morphology. Moreover, imaging with the BSE detector is the most suitable technique to analyze the coverage of the coated sample because of the difference in atomic number between the phosphate coating and the steel.

Since steel is an alloy of iron, and therefore has a higher atomic number than the phosphate coating, it will appear brighter in the backscattered image. Figure 1 shows a BSE image overlaid with the colored EDX map, where the yellow areas consist of iron and the light blue refers to zinc. The brighter areas of the BSE images overlay with the yellow, demonstrating the effectiveness of using these images for the measurement of coating coverage.

Moreover, at the same time, BSE images also reveal the crystal structure of the coating, enabling the coating morphology to be analyzed. Image on the right shows an SEM image of the crystal structure of zinc phosphate coatings.

Automated quality control using scanning electron microscopy

In quality control processes, it is ideal to have a system that enables the user to get fast results and avoid long downtime on the production line. In most cases, the analysis of coated samples is done in a laboratory that is located far from the production line, causing unwanted delays in instances of negative feedback. Having a fast and reliable way to check the quality of phosphate on-site is important. Desktop SEMs are the ideal choice in this case, having a small footprint and being easy to use.

However, this may be not sufficient. An automated tool for quality checking that does not require a dedicated and experienced user gives more advantages in terms of time saving and the reliability of the results. The Thermo Scientific Phenom Programming Interface (PPI) is the right platform to implement an automated tool for quality checks.

The image below shows the process flow of the automated tool designed for the quality checking of the coating coverage. It enables the user to scan a large area of the sample by collecting a set of BSE images at low magnification and saving them to a selected folder.

These images are then automatically analyzed by applying a threshold on the gray level, effectively separating the coating (appears darker) and the uncovered steel (appears brighter). The average coverage percentage and the statistics of the measurement are then saved in a report that the user can print out later. It is also possible to load a set of images, select the correct threshold, do the analysis, and generate the report.