Particle Characterization - Features of Advanced Quality Control Analyzer in Characterizing Foreign Particles by Aspex Corporation

Topics Covered

Background
Quality Control in Automotive and Industrial Manufacturing
Evaluating Contaminants with Scanning Electron Microscopes (SEM)
Applications of Personal Scanning Electron Microscopes (PSEM)
Fundamental Analysis Using Backscatter Electron Detectors (BSED)
Particle Characterization
Data Analysis
   Traditional SEM Reporting
   Fully Automated Reporting and Validation
Predicting Imminent Failures
   Repeatability for Total Area of Particles
   Understanding Size Distribution Shifts
   Evaluating Material Characterization Shifts with Ternary Diagrams
   Implications of Data
Summary

Background

Since its inception, ASPEX has dedicated itself to becoming the recognized expert in automated microanalysis software and instrumentation. Over recent years, ASPEX has expanded its focus to supply automated microcontamination quality-control systems to customers with particle contamination and manufacturing cleanliness problems, as well as innovative microanalysis solutions for monitoring the dynamics of wear and structural fatigue in engines, motors, and other mechanical systems as part of predictive maintenance programs.

Our mission at ASPEX is to bring our customers A TOTAL SOLUTION to their microanalysis requirements in everyday production environments - solutions that can help them achieve new standards of product quality, reduce operating and warranty costs, and enhance their bottom lines.

Quality Control in Automotive and Industrial Manufacturing

Traditional control practices in the automotive and industrial manufacturing companies typically rely on visual inspections of filter membranes after they have been gravimetrically weighed to determine the amount of debris present. Particles are directly implicated for various failure modes of products and can be linked to increased warranty costs. In addition, optical microscopes and optical scanners can be used to obtain basic sizing information. Over the last decade additional emphasis has been put on identifying the source of the contamination, and more importantly reducing and/or eliminating it from the process. Unfortunately traditional methods of contamination monitoring fall short in the respect - no material speciation can be obtained. As a result, an electron microscope is often employed to manually inspect the filter membrane to characterize only a few key particles of interest. This overall approach to contamination control is effective; however it is labor intensive and is known to be irreproducible and highly variable.

Evaluating Contaminants with Scanning Electron Microscopes (SEM)

The implementation of a more streamlined approach, more specifically, one which requires only a single analysis step, is critical to sustaining a successful particle contamination laboratory. More often than not, manufacturers consider the utilization of a scanning electron microscope (SEM) to conduct contamination evaluations as an excellent tool for imaging and materials characterization. However, one key functionality of an SEM which is overlooked is that it is a very efficient scanner. Taking this into account, an Advanced Quality Control (AQC) system was developed to allow investigators spend more time on determining a corrective action than on analyzing the samples themselves. Particles of interest in an SEM are resolved by their average atomic number. Since debris is collected on filter membrane material, an inherent contrast mechanism exists such that the particles appear bright and the background appears dark. Consequently, the analyzer can determine what part of the sample is a particle and what part of the sample is background. Through the use of a unique set of hardware, computer algorithms, and data management solutions, the AQC maintains transparency of the instrument to the process and its operators by scanning samples fully unattended.

Applications of Personal Scanning Electron Microscopes (PSEM)

In 1992, ASPEX Corporation introduced the original Personal Scanning Electron Microscope® (PSEM) and was the first offering of a microscope designed for multi-user environments and a low cost alternative to microscopes being produced at the time. Over the years, the Personal SEM® has been employed in various applications ranging from forensics to military applications. More recently, an industrialized packaging and reduction of size makes it an ideal tool for manufacturers. One of the core fundamental capabilities of the Personal SEM® is its ability to run unattended and scan for particles. In addition, a data management system has been developed improve report generation and decrease the work load of the operators.

Fundamental Analysis Using Backscatter Electron Detectors (BSED)

The AQC is based on a standard electron microscope configuration (Figure 1). It utilizes a Tungsten emitter source to generate electrons. Electrons are then focused to a point on the membrane surface and the beam is swept left-to-right and top-to-bottom in a coarse stepping pattern. During the interaction of the beam at each pixel, carbon and oxygen molecules of the membrane generate backscatter electrons (BSE). The efficiency of BSE generation from the membrane is fairly inefficient so very few BSE are detected by the backscatter electron detector (BSED) position above the sample.

Figure 1. Analyzer Set-Up

As beam continues to scan through the sample (Figure 2), the BSED signal is continually monitored and once the beam interacts with a feature, the BSED signal will increase. This is due to an observable increase in the average atomic number of the sample, in this case the atomic changes from C-O to an aluminum or stainless steel particle.

Figure 2. Automated Feature Analysis

An increase in the atomic number increases the efficiency of BSE generation (Figure 3), resulting in an increase BSED signal on the analyzer. Once a feature is detected, a measuring algorithm is engaged, whereby the size and shape of the particle is determined. Finally, the beam is positioned on the particle to determine its elemental composition. This process is unique to the PSEM due to its integrated hardware and software package and allows particle detection ranging from 30 nm to 5 mm. Without require image processing like all other SEM equipment requires, analysis times are reduced upwards of 10-fold. Finally, the PSEM provides additional flexibility for particle shape analysis of “complex” irregular shaped features.

Figure 3. Backscatter Electron Contrast

Particle Characterization

To fully understand particle contamination, the first step is understanding what is present in the system currently (Figure 4). The main goal of these projects is to implement a Quality Control program that works. As can be seen by the flow chart of implementation, basic information about the particles must first be understood and use that information to control the generation of particles in the process. This requires the identification of the material and ultimately the removal or minimization of the material of concern. Upon implementation of the sampling protocol, the AQC can provide this advanced characterization information.

Figure 4. Overview of implementation for the Quality Department.

The instrument first determines the location, size and shape of a particle (Figure 5).

Figure 5. Particle Size and Shape. Right Image - indicates the use of a segmentation algorithm to characterize complex and overlapping particles.

In addition the particle is characterized with Energy Dispersive X-ray Spectroscopy (EDS) to determine the elemental composition of the feature detected (Figure 6). The AQC then stores all the 'particle-by-particle' information into a database for further interpretation.

Figure 6. Particle Characterization. Left - Energy Dispersive Spectrometer Spectra. Right - Automated determination of the composition of the particle based on the spectrum acquired.

Data Analysis

Traditional SEM Reporting

Reporting using standard SEM/EDS technology and software can be time consuming and require considerable expertise in the technology (Figure 7). Once prepared, samples are manually or automatically scanned for contamination and a data array can be created and used for spreadsheet based reporting. Further review and historical referencing is required by an analyst to approved data integrity and process control.

Figure 7. Traditional reporting using SEM data.

Fully Automated Reporting and Validation

The need for a routine contamination control program is important in most pharmaceutical, automotive, and industrial manufacturers. In these cases, time is most effectively spent creating appropriate actions for correcting contamination problems. Traditional electron microscopes require a significant amount of time not only to acquire the data for particle analysis but also for labor intensive data reviewing and reporting, as mentioned above.

As a result, ASPEX® has developed the Perception™ eXecutive™ software suite (Figure 8). Perception™ eXecutive™ is an advanced data-management solution which utilizes a unique set of hardware, computer algorithms, and automated reporting software to allow for fully automated reporting of particle analysis data. In addition, Perception™ eXecutive™ can support data management for a multiple instrument environment. These features allow ASPEX® to create a user experience unlike any other automated SEM/EDS system in the industry-transparency of the instrument to the process and its operators.

Figure 8. AQCTM PerceptionTM eXecutiveTM Software Suite overview.

Predicting Imminent Failures

Repeatability for Total Area of Particles

In order to better ascertain quality of products and predicting imminent failures, the reliability of the measurement and several benchmarking measures must be determined. Typically these are also used for trending purposes to determine when the process is 'in control'. As can be seen by Figure 9, the AQC offers a superior reliability and repeatability of measures in comparison to traditional SEM and optical microscope techniques.

Figure 9. Gage R&R results from an analysis of particles on a filter membrane.

Understanding Size Distribution Shifts

Size distribution shifts can be used to indicate additional fatigue on a product in use. From the data below (Figure 10), there are clear differences between a normal baseline sample and Sample A and Sample B. Both A and B are results from different types of failures in the process and during operation. Sample A in particular is the result of an increase in wear debris from M50NiL bearing in a Jet Engine. Using these results, the manufacturer is able to predict imminent failures of the engines and determine the reliability of the aircraft for flight.

Figure 10. Size Distribution Changes for samples from Jet Engine failures

Evaluating Material Characterization Shifts with Ternary Diagrams

Through chemical analysis, unique groups of particles can easily be identified and traced back to their source. Here we demonstrate the ability of ternary diagrams to easily identify shifts in populations and wear characteristics (Figure 11). Each dot indicates a single particle that was analyzed. The color of the dot indicates the surface area of the particle.

Figure 11. Ternary diagrams illustrating material characterization shifts.

Implications of Data

Overall red flags & warning signals can be implemented based on the historical data, otherwise known as trending (Figure 12 & 13). Upon analysis, engineers can then conduct the necessary investigations into the process to improve future products.

Figure 12. Historical References and Trending with built-in RED and GREEN warning signals.

Figure 13. GO-NO-GO Reporting for process control.

Summary

The advent of new manufacturing processes and tight tolerance designs has increased emphasis on control and manufacturing costs. Particle-by-particle counting, sizing, shaping, and identification can enumerate a wealth of information for an individual user. The ASPEX® AQC package was developed to alleviate the need for highly trained SEM experts and data analysis personnel. To accomplish these tasks, a database architecture was used to take large amounts of information and perform automated data analysis to generate an output that is easily interpretable. When combined with ASPEX®'s high performance particle and feature analyzers, Perception™ eXecutive™ seamlessly provides a new data reporting capability that works for you - not against you. The AQC package in addition offers some standard industrial based reporting templates, such ISO 16232 for particle contamination in automotive parts, to streamline implementation timeframes and shorten the return on investment timeframe. Utilizing these new advanced quality control algorithms, engineers now have the ability to easily track foreign particle material and how it effect their processes. By comparison, this new analyzer and software algorithms affords superior usability and reproducibility.

Source: Aspex Corporation

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