SOLVING PROBLEMS BY MATERIALS ANALYSIS WITH AN ELECTRON MICROPROBE. G.W. Kagel

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1 SOLVING PROBLEMS BY MATERIALS ANALYSIS WITH AN ELECTRON MICROPROBE G.W. Kagel v1 Jan 2014

2 INTRODUCTION The electron microprobe is an electron microbeam instrument similar to an SEM, but different in that it is highly specialized to provide superior accuracy in elemental analysis using the X-ray emissions induced by an electron beam. The electron microprobe can be an essential tool to provide critical insights needed to answer material questions quickly. Solving difficult materials problems or unraveling a complex failure can be an intensive - and sometimes frustrating - process, but elucidating the root cause of an expensive problem is highly rewarding and absolutely necessary for success. The goal is to solve the problem quickly, and at a cost that is very small compared to the cost of additional failures. When problems arise with a material in development, or an existing material, deliberate and methodical steps are needed to resolve the situation quickly. Likewise with new or modified processes, or materials that suddenly fail without any apparent changes. The goal of this paper is to describe examples of how electron microprobe analysis has been used effectively in problem solving, and to provide some points to consider when facing problems with materials or processes. Three case studies are briefly presented to illustrate how the capabilities of the electron microprobe were used to solve the problem quickly and cost effectively. These examples illustrate how methodical approaches to problem solving, microchemical analyses and collaboration in a cross-functional team have led to rapid identifications of root cause, and successful recovery from difficult situations. EXAMPLE 1: BROKEN WINDINGS IN AN ELECTRIC MOTOR While testing motors manufactured by a new process, breakage of the copper wire in the bobbins was discovered. The manufacturer had to determine the root cause of this failure quickly so the new process could be corrected and implemented. Time was especially critical due to the costs involved with any delay. Figure 1. Microscopic image of failure surface on copper wire This manufacturer followed the optimal path to the ultimate solution. First, a leader was selected to be the focal point of contact for the failure analysis. Next, the leader gathered all the relevant information and methodically documented the findings. After utilizing in-house talent and tools, the leader determined that external laboratory services were needed, and a lab was selected that was particularly suited to his project in terms of tools, talent, accreditation, and availability for quick turnaround time. The initial inquiry to the lab indicated that a microchemical analysis may be needed. Fortunately, the client was familiar with scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (SEM/EDS), and the electron microprobe is especially suited for this type of testing. An analysis was conducted using the electron microprobe to identify the elemental composition of deposits discovered on the failure surfaces. As shown in the EDS spectrum in Figure 2, the presence of a chlorine peak indicated that chloride corrosion caused the failure. Further chemical fingerprinting of materials used in the production process was used to isolate the particular contaminant, which was eliminated from the process. 2

3 Figure 2. EDS spectrum of copper wire failure surface, indicating the presence of chloride EXAMPLE 2: DELAMINATION OF A MULTI-LAYER COATING ON AN ENGINE COMPONENT When an engine component manufacturer was qualifying a new lower-cost supplier, it was found that a coating layer lifted off easily, failing an adhesion test. The engine component required a two-part coating, with each layer provided by a different supplier. The supplier that applied the first layer thought the problem was the second layer, and predictably, the supplier producing the second layer thought the problem was the first layer. To resolve the problem quickly, the engine component manufacturer brought the problem to the electron microprobe at an outside laboratory. The initial task was to lift the second layer and examine the underside in the electron microprobe. A backscattered electron image of the underside of the top layer, shown in Figure 3, revealed that pieces of the first layer were still well adhered. This suggested that delamination was occurring between the base material of the engine component and the first coating layer. Figure 3. Backscattered electron image of underside of coating top layer The next step was to examine the area from which the top layer was lifted. This electron image, shown in Figure 4, shows that the first layer is irregular and thin enough to allow the machining lines on the base material to show through. This seemed to answer the question as to which part of the process had the issue. 3

4 EXAMPLE 3: TROUBLESHOOTING A SPECIALIZED BRAZE ALLOY When a client needed to analyze specialized braze alloys to compare formulations for long-term performance in a harsh operating environment, he turned to the electron microprobe to characterize the alloy inter-metallic and precipitate phases. He was able to determine how formulation changes correlated with chemical microstructure changes and better understand why certain formulations failed to provide the needed performance for certain applications. Figure 4. Backscattered electron image of top surface of the first layer of the coating Finally, a part coated by a consistently successful process was examined and showed a very different surface morphology (Figure 5). This provided the necessary information to the new supplier so they could modify their process until the surface resembled the ideal. In this case the electron microprobe was used to monitor surface morphology as well as chemistry to help the client implement the lower cost supplier. There are many instances where simple SEM/EDS cannot answer questions that are easily handled with the electron microprobe. Many elements have X-ray emission lines that cannot be resolved using EDS. As shown in the EDS spectrum of Figure 6, hafnium, tantalum, and tungsten are clustered in one peak in the lower-energy area of the spectrum, and suffer from poor sensitivity and overlaps with nickel X-ray lines in the higher-energy region. Also shown in Figure 6 is a peak that could be either sulfur or molybdenum. Figure 6. EDS spectrum showing overlapping peaks Figure 5. Backscattered electron image of coating surface with good adhesion An electron microprobe equipped with seven wavelength dispersive spectrometers (WDS) was used to easily resolve all of these overlaps, with far greater sensitivity to trace elements. In addition, light elements like boron can be effectively determined by WDS, but not by EDS. A properlyequipped electron microprobe can mix EDS and WDS signals in the X-ray mapping system, which can rapidly produce information-rich element maps as illustrated in Figure 7. 4

5 chemistries on a microscopic scale. This information was used to make critical material and process changes, resulting in better product performance and lower cost of production. The following key questions should be considered when solving problems with a material or process: - Is there a single contact person who methodically collects all the pertinent data and functions as the liaison to all contacts outside the project team? - Is there a need for services outside our own core competencies? Figure 7. Element map produced by WDS showing segregation of hafnium, tungsten, and chromium CONCLUSION This paper has briefly shown a few points to consider when conducting an investigation in support of new process development, failure analysis, or new material development. Three examples were presented where the electron microprobe was used to rapidly determine - If external laboratory services are needed, do we know of a lab that has the appropriate accreditations for our industry? - Is the laboratory responsive, helpful, and equipped for a wide range of testing services to solve the problem quickly? These guidelines will help narrow the focus when faced with difficult materials challenges that might be met with the help of microchemical analysis 5

6 ABOUT LUCIDEON Lucideon is a leading international provider of materials development, testing and assurance. Through its offices and laboratories in the UK, US and the Far East, Lucideon provides materials and assurance expertise to clients in a wide range of sectors, including healthcare, construction, ceramics and power engineering. The company aims to improve the competitive advantage and profitability of its clients by providing them with the expertise, accurate results and objective, innovative thinking that they need to optimise their materials, products, processes, systems and businesses. ABOUT THE AUTHORS GARY KAGEL Gary Kagel is the electron microprobe analyst at Lucideon and has been a professional analytical chemist since earning his M.S. in chemistry from Montana State University in He has worked in laboratories regulated by the FDA, EPA, and ISO throughout his career.

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contamination analysis for compound semiconductors ANALYTICAL SERVICES B u r i e d d e f e c t s, E v a n s A n a l y t i c a l g r o u p h e l p s y o u C O N T R O L C O N T A M I N A T I O N Contamination