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Abstract
This article highlights the importance of scanning electron microscopy (SEM) in analyzing impurities for technical cleanliness applications. SEM can classify particles that optical microscopy might miss or misclassify.
Introduction
In industries like automotive and aerospace, maintaining clean technical components is crucial for the longevity and performance of equipment and processes. Contaminant particles can settle on machined parts during manufacturing or along the supply chain. Even small amounts of these contaminants can cause severe damage, shorten the lifespan of components, or affect the processes they are involved in.
Technical cleanliness is used to determine the cleanliness level of machined parts. These checks follow internationally recognized standards (such as VDA-19.1 and ISO-16232 for the automotive industry) or customer-specific guidelines, which describe the steps for particle collection, analysis methods, and results documentation to ensure analyses are reproducible and comparable. However, they do not define cleanliness specifications. Customers determine if a part meets their cleanliness standards.
Technical cleanliness inspection
An interdisciplinary team, comprising various departments such as design, production, logistics, and quality assurance, is responsible for determining the cleanliness specifications (i.e., the allowable number and types of particles that a component can tolerate without affecting its functionality) and the inspection strategy for a given system or component.
The inspection strategy involves defining the methods for particle extraction, separation, and analysis. Typically, contaminant particles are removed from the component of interest by washing and are collected on a membrane through filtration. These membranes, usually PET or PA mesh filters with 5 µm pores, are then analyzed using various methods, with light microscopy and scanning electron microscopy being the most common. Finally, the results, including images and quantitative data, are documented. The report specifies whether the cleanliness specification is met.
Figure 1: Overview of the technical cleanliness process. After defining the cleanliness specifications for a given component, the particles are extracted, filtered, and analyzed. Finally, the results are summarized in a report.
Analysis methods: Optical microscopy and scanning electron microscopy
Technical cleanliness is assessed using various methods described in ISO 16232 and VDA 19.1 standards. These include visual inspection, particle analysis, and analytical techniques to quantify and classify contaminants and determine if cleanliness levels are met.
Optical microscopy
Optical microscopes use visible light to characterize particles and a digital camera to capture images, achieving a resolution of approximately 500 nm. Modern optical microscopes are equipped with software for automated particle detection, which operates based on a contrast threshold between the particles and the background.
Optical microscopes are affordable, easy to use, and capable of characterizing hundreds of particles within a few minutes. These qualities make this technique the gold standard for technical cleanliness analysis, especially for particles larger than 50 µm. However, it has two limitations due to the intrinsic principles of light microscopy. First, small particles are hard to detect or may appear as aggregates. Second, particles with low contrast, such as minerals, are difficult to detect.
Scanning electron microscopy
Scanning electron microscopy (SEM) detects particles using backscattered electrons and resolves features smaller than 10 nm. The contrast of the particles depends on the atomic number, making materials composed of heavier elements brighter. Mineral particles, for example, can then be visualized more clearly than with optical microscopy. While optical microscopy can provide hints about the type of material, microanalysis techniques make it possible to more precisely determine elemental composition. SEM coupled with an energy dispersive X-ray spectroscopy (EDS) detector reliably classifies particles by their chemical composition.
| Technique | Size detection | Advantages | Disadvantages |
| Optical | >5 µm |
|
|
| SEM-EDS | >0.5 µm |
|
|
Table 1: Advantages and disadvantages of optical microscopy and scanning electron microscopy for technical cleanliness inspection.
Particle contamination
There is a wide range of particle types that can contaminate a sample. However, not all particles cause the same harm. In the context of technical cleanliness, the hardness of a material is directly linked to the potential damage that it can cause to a technical component or process. Hard materials, such as minerals and some metals, will likely cause more damage than soft, organic materials.
Minerals like corundum (Al2O3), silicon carbide (SiC), and sand (SiO2), which originate from surface treatment processes, are highly abrasive and therefore can damage components in irreversible ways. When observed with optical microscopes, they present very weak contrast that can make them invisible for detection (Figure 2).
Figure 2: Corundum (top green circle) and sand (red bottom circle) present low contrast in optical microscopy, but with SEM-EDS they are easily recognizable. Images courtesy of RJL.
Metallic particles originate from manufacturing processes. Their hardness, and therefore their damage potential, depends on the specific type of metal. Some optical devices can distinguish metal from non-metal by the reflections from their surfaces but cannot unambiguously determine the composition. Moreover, while optical microscopy gives a rough estimate of the number of metallic particles contaminating a sample, it can also be misleading. Oxidized or corroded metallic particles do not have a shiny surface, and therefore they would not be recognized as such by optical microscopes.
The limitations of optical microscopy are overcome by using SEM with EDS, which can detect smaller particles and determine their chemical composition. This information is crucial when investigating the nature, location, and origin of contaminants.
Figure 3: In this sample, zinc (top green circle), steel (middle red circle), and aluminum (bottom yellow circle) do not show a shiny reflection under the optical microscope and would not be classified as metallic particles. SEM-EDS confirms that these particles are indeed metallic and determines their elemental composition. Images courtesy of RJL.
Automated SEM-EDS: Perception Software for technical cleanliness applications
While optical microscopy remains the standard for technical cleanliness analysis due to its cost-effectiveness and speed, it has limitations. It may not detect low-contrast particles like minerals or accurately identify metallic particles.
SEM-EDS addresses these limitations by providing detailed information about the size, shape, and composition of individual particles, which help identify the source of contamination. SEM-EDS can be integrated with advanced image analysis algorithms and automation for sophisticated particle analysis solutions.
Thermo Scientific Perception Software, available with the Thermo Scientific Phenom XL Desktop SEM and the Thermo Scientific Axia ChemiSEM System, is designed for automated, high-throughput particle analysis and classification. With Perception Software, you can easily set up analysis areas for multiple samples and design a recipe that includes all the parameters needed to automatically detect and sort particles according to morphology and chemical composition. The automated analysis can be monitored in real time, providing continuous feedback on the results. Finally, the results are collected and organized into customizable reports. Perception Software offers repeatable and accurate results and is compatible with ISO 16232 and VDA 19.1 standards.
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