Sample preparation

Care and Upkeep of Your Standards

Shawn Wallace, Applications Engineer, EDAX

As I prepared for some analytical work yesterday, I had to repolish a standard block. This made me think about how important these little blocks are and how often they are not cared for properly. With that in mind, I thought it might be useful to pass on some little nuggets of information I have gathered over the years from many sources.

The most important thing about caring for a block is knowing what is in it. Standard blocks can be purchased as a whole or personally made. No matter what, you need to know what you have! To do so, you should keep several copies of the following for every standard you have:

  • Optical light images of the whole block
  • SEM Montage image of the whole block (BSE and SE)
  • Individual image of each standard material
  • Composition of each standard material with sources
  • Notes on each standard
Each of our standard blocks has a name and a duplicate document. This packet has optical, BSE, and SE images of the standard. This allows us to quickly find the standard we want while having all the information easily accessible in hand.

Figure 1. Each of our standard blocks has a name and a duplicate document. This packet has optical, BSE, and SE images of the standard. This allows us to quickly find the standard we want while having all the information easily accessible in hand.

Each of these above items is important. You want to keep both a visual record of your standards, a record of what it is and the condition that it is in, to allow you to track any issues that may pop up (Figure 1). Therefore, having a note section is important. You may find that one of the areas of your standard gives anomalous values and should be avoided. You want to make sure this information is easily accessible to everyone that uses the standard. I suggest scanning and keeping electronic copies in a shared folder on your desktop.

Besides the documentation aspect of care, physical care is just as critical. Most commercial standard blocks come pre-polished and carbon-coated. Over time, both of those will degrade and need to be redone. Usually, the carbon coating damages first, but you also need to check for burn marks and other beam damage done to the standard material itself. When repolishing and recoating, I usually do a solid 10 minute repolish with diamond paste. This removes enough material to eliminate the carbon coating and get new clean, undamaged surfaces while not change the physical appearance all that much. I try my best to avoid using an Al-based polishing material, as they tend to stick around too much and can interfere with my analysis on elements I use. With carbon-based polishing material, it is much easier to see the effects of the carbon. In the end, I do not tend to do quant work on carbon that much, while I often try to quantify aluminum. Whatever you do, document what was done. It can help you both head off and understand issues that may present.

While physically handling your sample, it shouldn’t need to be said, but you should never be touching your sample with ungloved hands. Your oils are bad for both the SEM cleanliness and the sample cleanliness. Avoid any sort of colloidal products with standards, as they do tend to flake with age. When not in use, samples should be held in a desiccator with good desiccant (Figure 3).

A good desiccator should have a rubber molding to help it hold a seal at a minimum. You should try to keep it under vacuum for the best results. While taking this picture, I noticed I should dry my desiccant or replace it. I have seen some users keep a small plastic bag of fresh desiccant in the desiccator as a quick visual reference.

Figure 3. A good desiccator should have a rubber molding to help it hold a seal at a minimum. You should try to keep it under vacuum for the best results. While taking this picture, I noticed I should dry my desiccant or replace it. I have seen some users keep a small plastic bag of fresh desiccant in the desiccator as a quick visual reference.

There are many other tips I can think of sharing, but to wrap it up, standards are valuable in our industry. A good, well cared for standard will last multiple careers while giving consistent results time after time. Take the time to keep your standards in the best condition, and they will repay your time spent on them tenfold.

Want a Free Set of Microanalysis Standards?

Dr. Shangshang Mu, Applications Engineer, EDAX

Modern EDS systems are capable of quantitative analysis with or without standards. Unlike standard-less analysis, the k-ratio is either calculated in the software or based on internal standards. For analysis with standards, it is measured from a reference sample with known composition under the same conditions as the unknown sample. As an applications engineer, sometimes users ask me where to order these standards. Usually, I point them to the vendors that manufacture and distribute reference standards where you can order either off-the-shelf or customized standard blocks. In addition to these commercial mounts, I always tell them that they can request a set of mineral, glass, and rare earth element phosphate standards from the National Museum of Natural History free of charge! These are very useful standards that I’ve seen widely used in not only the geoscience world but also in various manufacturing industries. These free standards are also great for those graduate students with limited budgets and ideal for practicing sample preparation (yes, I was one of them).

This set of standards is officially called the Smithsonian Microbeam Standards and includes 29 minerals, 12 types of glass, and 16 REE phosphates. You can find out more information about these standards and submit a request form by clicking on the link below:
https://naturalhistory.si.edu/research/mineral-sciences/collections-overview/reference-materials/smithsonian-microbeam-standards

I mentioned sample preparation earlier. Yes, you read that right. These standards come in pill capsules containing from many tiny grains to a few larger ones and you need to mount them on your own (Figure 1).

Grains in a pill capsule.

Figure 1. Grains in a pill capsule.

Since you can get the information such as the composition, locality, and references for each standard from the website, what I want to discuss in this blog post is how to prepare them properly for X-ray analysis. The first tricky thing is to get them out of the capsules. The grains in Figure 1 are almost the largest in this set and you won’t get too many of this size. Some of the grains are even too tiny to be seen at first glance. For the majority that are really tiny, you need to tap the capsule a couple of times to release the grains that get stick to the capsule wall, then you can open the capsule very carefully and let the grains slide out with a little tapping.

For mounting, the easiest way is to mount the standards in epoxy using a mounting cup and let it cure. I did this in a fancy way to make it look like a commercial mount (Figure 2). I ordered a 30 mm diameter circular retainer with 37 holes used by commercial mount manufacturers (Figure 3) and filled the holes with standards on my own. I must admit that the retainer is not cheap, but you can machine the mount by yourself or have a machine shop do it for you. In addition to looking pretty, the retainer ensures a good layout so you can quickly locate the standards you need during microanalysis, and you can mount the same type of standards on one block and get rid of the hassle of frequently venting and pumping the SEM chamber to switch standard blocks.

Examples of commercial mounts.

Figure 2. Examples of commercial mounts.

 

30 mm diameter circular retainer with 37 holes.

Figure 3. 30 mm diameter circular retainer with 37 holes.

To prevent the tiny grains from moving and floating up when pouring the epoxy mix, I placed the retainer upside down and pressed it onto a piece of sticky tape (Figure 4a) and positioned the grains on the sticky surface of the tape within the holes. When tapping the capsule to let the grains slide out and fall into the hole, the other holes were covered to prevent contamination (Figure 4b). These holes are small in diameter and pouring the epoxy mix directly will trap air bubbles in the hole to separate the grains from the epoxy mix. To overcome this problem, I filled up the hole by letting the epoxy mix drip down very slowly along the inner surface of the hole.

Positioning grains within the holes of the retainer.

Figure 4. Positioning grains within the holes of the retainer.

For general grinding, I start with wet 240 grit SiC sandpaper with subsequent use of 320, 400, 600, 800, and 1,200 grit wet SiC sandpapers. But coarser grits can grind off tiny grains in this case, so I would recommend starting with a relatively fine grit based on the sizes of the grains you receive and always use a light microscope or magnifier to check the grinding. For polishing abrasive, I used 1 micron and 0.3 micron alumina suspensions on a polishing cloth. For the grains used as standards or quantification in general, the surface needs to be perfectly flat. However, the napped polishing cloth tends to abrade epoxy and the grains at different rates, creating surface relief and edge rounding, especially on tiny grains. To mitigate this effect, the polishing should be checked under a light microscope constantly and stopped as soon as the scratches are removed. A vibratory final polishing with colloidal silica is optional. Followed by ultrasonic cleaning and carbon coating, the standard mount is ready to use.

Note that commercial mount manufacturers may prepare standards individually (especially for metal standards) and insert them into the holes from the back of the retainer and fasten them with retaining rings (Figure 5a). A benefit of this approach is that the standards on the mount are changeable, so you can load all the standards you need on one mount before microanalysis. I used to make several individual mounted standards that can fit into the retainer (Figure 5b) but this process is very time consuming and much trickier to keep the small surface flat during grinding and polishing.

a) The back of a commercial metal standard mount. b) A tiny cylindrical mount that can fit into the retainer holes.

Figure 5. a) The back of a commercial metal standard mount. b) A tiny cylindrical mount that can fit into the retainer holes.

This is definitely a good set of standards to keep in your lab. With EDAX EDS software, in addition to quantification with these standards, you can also use them to create a library and explore the Spectrum Matching feature. The next time you want to quickly determine the specific type of a mineral, you can simply collect a quick spectrum and click the “Match” button, and the software will compare the unknowns to the library you just created.

How to Get a Good Answer in a Timely Manner

Shawn Wallace, Applications Engineer, EDAX

One of the joys of my job is troubleshooting issues and ensuring you acquire the best results to advance your research. Sometimes, it requires additional education to help users understand a concept. Other times, it requires an exchange of numerous emails. At the end of the day, our goal is not just to help you, but to ensure you get the right information in a timely manner.

For any sort of EDS related question, we almost always want to look at a spectrum file. Why? There is so much information hidden in the spectrum that we can quickly point out any possible issues. With a single spectrum, we can quickly see if something was charging, tilted, or shadowed (Figure 1). We can even see weird things like beam deceleration caused by a certain imaging mode (Figure 2). With most of these kinds of issues, it is common to run into major quant related problems. Any quant problems should always start with a spectrum.

Figure 1. The teal spectrum shows a strange background versus what a normal spectrum (red) should look like for a material.

Figure 1. The teal spectrum shows a strange background versus what a normal spectrum (red) should look like for a material.

This background information tells us that the sample was most likely shadowed and that rotating the sample to face towards the detector may give better results.

Figure 2. Many microscopes can decelerate the beam to help with imaging. This deceleration is great for imaging but can cause EDS quant issues. Therefore, we recommend reviewing the spectrum up front to reduce the number of emails to troubleshoot this issue.

Figure 2. Many microscopes can decelerate the beam to help with imaging. This deceleration is great for imaging but can cause EDS quant issues. Therefore, we recommend reviewing the spectrum up front to reduce the number of emails to troubleshoot this issue.

To save the spectrum, right-click in the spectrum window, then click on Save (Figure 3). From there, save the file with a descriptive name, and send it off to the applications group. These spectrum files also include other metadata, such as amp time, working distance, and parameters that give us so many clues to get to the bottom of possible issues.

Figure 3. Saving a spectrum in APEX™ is intuitive. Right-click in the area and a pop-up menu will allow you to save the spectrum wherever you want quickly.

Figure 3. Saving a spectrum in APEX™ is intuitive. Right-click in the area and a pop-up menu will allow you to save the spectrum wherever you want quickly.

For information on EDS backgrounds and the information they hold, I suggest watching Dr. Jens Rafaelsen’s Background Modeling and Non-Ideal Sample Analysis webinar.

The actual image file can also help us confirm most of the above.

Troubleshooting EBSD can be tricky since the issue could be from sample prep, indexing, or other issues. To begin, it’s important to rule out any variances associated with sample preparation. Useful information to share includes a description of the sample, as well as the step-by-step instructions used to prepare the sample. This includes things like the length of time, pressure, cloth material, polishing compound material, and even the direction of travel. The more details, the better!

Now, how do I know it is a sample prep problem? If the pattern quality is low at long exposure times (Figure 4) or the sample looks very rough, it is probably related to sample preparation (Figure 4). That being said, there could be non-sample prep related issues too.

Figure 4. This pattern is probably not indexable on its own. Better preparation of the sample surface is necessary to index and map this sample correctly.

Figure 4. This pattern is probably not indexable on its own. Better preparation of the sample surface is necessary to index and map this sample correctly.

For general sample prep guidelines, I would highly suggest Matt Nowell’s Learn How I Prepare Samples for EBSD Analysis webinar.

Indexing problems can be challenging to troubleshoot without a full data set. How do I know my main issues could be related to indexing? If indexing is the source, a map often appears to be very speckled or just black due to no indexing results. For this kind of issue, full data sets are the way to go. By full, I mean patterns and OSC files. These files can be exported out of TEAM™/APEX™. They are often quite large, but there are ways available to move the data quickly.

For the basics of indexing knowledge, I suggest checking out my latest webinar, Understanding and Troubleshooting the EDAX Indexing Routine and the Hough Parameters. During this webinar, we highlight attributes that indicate there is an issue with the data set, then dive into the best practices for troubleshooting them.

As for camera set up, this is a dance between the microscope settings, operator’s requirements, and the camera settings. In general, more electrons (higher current) allow the experiment to go faster and cover more area. With older CCD based cameras, understanding this interaction was key to good results. With the newer Velocity™ cameras based on CMOS technology, the dance is much simpler. If you are having difficulty while trying to optimize an older camera, the Understanding and Optimizing EBSD Camera Settings webinar can help.

So how do you get your questions answered fast? Bury us with information. More information lets us dive deeper into the data to find the root cause in the first email, and avoids a lengthy back and forth exchange of emails. If possible, educate yourself using the resources we have made available, be it webinars or training courses. And always, feel free to reach out to my colleagues and me at edax.applications@ametek.com!

Seeing is Believing?

Dr. René de Kloe, Applications Specialist, EDAX

A few weeks ago, I participated in a joint SEM – in-situ analysis workshop in Fuveau, France with Tescan electron microscopes and Newtec (supplier of the heating-tensile stage). One of the activities during this workshop was to perform a live in-situ tensile experiment with simultaneous EBSD data collection to illustrate the capabilities of all the systems involved. In-situ measurements are a great way to track material changes during the course of an experiment, but of course in order to be able to show what happens during such an example deformation experiment you need a suitable sample. For the workshop we decided to use a “simple” 304L austenitic stainless-steel material (figure 1) that would nicely show the effects of the stretching.

Figure 1. Laser cut 304L stainless steel tensile test specimen provided by Newtec.

I received several samples a few weeks before the meeting in order to verify the surface quality for the EBSD measurements. And that is where the trouble started …

I was hoping to get a recrystallized microstructure with large grains and clear twin lamellae such that any deformation structures that would develop would be clearly visible. What I got was a sample that appeared heavily deformed even after careful polishing (figure 2).

Figure 2. BSE image after initial mechanical polishing.

This was worrying as the existing deformation structures could obscure the results from the in-situ stretching. Also, I was not entirely sure that this structure was really showing the true microstructure of the austenitic sample as it showed a clear vertical alignment that extended over grain boundaries.
And this is where I contacted long-time EDAX EBSD user Katja Angenendt at the MPIE in Düsseldorf for advice. Katja works in the Department of Microstructure Physics and Alloy Design and has extensive experience in preparing many different metals and alloys for EBSD analysis. From the images that I sent, Katja agreed that the visible structure was most likely introduced by the grinding and polishing that I did and she made some suggestions to remove this damaged layer. Armed with that knowledge and new hope I started fresh and polished the samples once more. And I had some success! Now there were grains visible without internal deformation and some nice clean twin lamellae (figure 3). But not everywhere. I still had lots of areas with a deformed structure and whatever I tried I could not get rid of those.

Figure 3. BSE image after optimized mechanical polishing.

Back to Katja. When I discussed my remaining polishing problems she helpfully proposed to give it a try herself using a combination of mechanical polishing and chemical etching. But even after several polishing attempts starting from scratch and deliberately introducing scratches to verify that enough material was removed we could not completely get rid of the deformed areas. Now we slowly started to accept that this deformation was perhaps a true part of the microstructure. But how could that be if this is supposed to be a recrystallised austenitic 304L stainless steel?

Table 1. 304/304L stainless steel composition.

Let’s take a look at the composition. In table 1 a typical composition of 304 stainless steel is given. The spectrum below (figure 4) shows the composition of my samples.

Figure 4. EDS spectrum with quantification results collected with an Octane Elite Plus detector.

All elements are in the expected range except for Ni which is a bit low and that could bring the composition right at the edge of the austenite stability field. So perhaps the deformed areas are not austenite, but ferrite or martensite? This is quickly verified with an EBSD map and indeed the phase map below confirms the presence of a bcc phase (figure 5).

Figure 5. EBSD map results of the sample before the tensile test, IQ, IPF, and phase maps.

Having this composition right at the edge of the austenite stability field actually added some interesting additional information to the tensile tests during the workshop. Because if the internal deformation in the austenite grains got high enough, we might just trigger a phase transformation to ferrite (or martensite) with ongoing deformation.

Figure 6. Phase maps (upper row) and Grain Reference Orientation Deviation (GROD) maps (lower row) for a sequence of maps collected during the tensile test.

And that is exactly what we have observed (figure 6). At the start of the experiments the ferrite fraction in the analysis field is 7.8% and with increasing deformation the ferrite fraction goes up to 11.9% at 14% strain.

So, after a tough start the 304L stainless steel samples made the measurements collected during the workshop even more interesting by adding a phase transformation to the deformation. If you are regularly working with these alloys this is probably not unexpected behavior. But if you are working with many different materials you have to be aware that different types of specimen treatment, either during preparation or during experimentation, may have a large influence on your characterization results. Always be careful that you do not only see what you believe, but ensure that you can believe what you see.

Finally I want to thank the people of Tescan and Newtec for their assistance in the data collection during the workshop in Fuveau and especially a big thank you to Katja Angenendt at the Max Planck Institute for Iron Research in Düsseldorf for helpful discussions and help in preparing the sample.

Avoid a Distorted View

Dr. Stuart Wright, Senior Scientist EBSD, EDAX

In the world of “fake news” and “alternative facts”, it is important that we dig a little deeper than the headlines to understand the world around us and to avoid a distorted view those in power often want to give us. Ironically, the same is true at the microscale. I recently ran into some work concerning the effects of sample prep on x-ray measurements. It made me reflect on some early work we did to explore the effects of sample prep on EBSD results.

In order to prepare EBSD samples properly it is important to understand that surface finish is not the whole story. It is important that the layer of material sampled by EBSD be distortion free. Charts shown in many metals preparation handbooks clearly show that there can be significant deformation imparted into the sub-surface of a material during preparation. Consider the following chart adapted from a figure in a classic EBSD sample preparation paper: D. Katrakova & F. Mücklich (2001) “Specimen preparation for electron Backscatter Diffraction. Part I: Metals” Praktische Metallographie. 8:547-65. This plot clearly shows why sample prep for EBSD needs to be meticulous.

My longtime colleague, Matt Nowell, did a nice study comparing by grinding two samples, one ground to 240 grit and one to 1200 grit. He then cross-sectioned these samples and carefully prepared the cross-sectioned surfaces. Matt then did OIM scans on the two surfaces. Using a Kernel Average Misorientation (KAM) map, the degree of deformation in the 240 grit sample is clearly more pronounced that in the 1200 grit sample. Matt and I have always wanted to repeat this measurement for more grits and materials but have never found the time to pursue it again.

Many times, students who have asked me “which grinding and/or polishing steps can I skip?” Or, “how many times can I really use a grinding paper?” (I remember as a student we got one paper for each grit for the semester and we would hang them from a wire with clothes pins in the sample prep lab!). Or, “can’t I just do the final grinding step for a longer time and skip the coarser grinding steps?” One thing we’ve learned on our own and in conversations with the sample prep vendors is that the recipes developed with several steps for what intuitively may feel like short times really are the steps that lead to the best results -basically confirming the plot shown above.

The improvement in cameras, image processing and particularly NPAR™ should not be used as an excuse to take shortcuts in sample prep. While it may be possible to get patterns and reasonable maps, are you really looking at the representative microstructure of interest or a distorted version resulting from deformation induced by sample prep?

I believe EBSD has had a positive impact on the metallography community. EBSD has forced us to be more careful in sample preparation over that typically done for light microscopy or even scanning electron microscopy. Hopefully that extra care has resulted in more representative microstructural characterization.