Scratches are linear grooves on the surface of the sample, caused by abrasive particles.
After emery polishing, very deep vertical scratches remain. Magnification: 200x
1. Make sure that after rough grinding, the surface of all samples on the sample holder is evenly covered with the same pattern of wear marks;
2. Re-grind if necessary;
3. After each step, the sample and sample holder should be carefully cleaned to remove the interference of the large abrasive particles in the previous process on the grinding/polishing equipment;
4. If there are still grinding marks left by the previous process after the current polishing process, please increase the sample preparation time by 25~50%.
The plastic deformation that occurs in larger areas of the sample is called wrinkling, and when abrasives, lubricants, or polishing cloths are used improperly, or their combination is not suitable, the abrasive will act like a dull knife on the surface of the work, pushing surface, resulting in wrinkles.
Wrinkles on ductile mild steel. Magnification: 15x, DIC
Lubricant: Check the amount of lubricant used. Pushing often occurs when the amount of lubricant is too small, and the amount of lubricant should be increased if necessary.
Polishing cloth: Due to the high recovery of the polishing cloth, the abrasive will be deeply pressed into the bottom of the polishing cloth and cannot perform the grinding function.
Abrasives: The diamond particle size may be too small to be pressed into the sample for grinding. Please use large abrasives. diamond grinding wheels.
3. False color
False coloring is the abnormal coloring of the sample surface, mainly due to contact with foreign substances.
Specimen staining due to gaps between resin and sample. Magnification: 20x
1. Avoid leaving a gap between the sample and the resin when mounting the sample
2. Wash and dry the samples immediately after each sample preparation process.
3. In the last 10 seconds of oxide polishing, rinse the polishing cloth with cold water to clean the sample and the polishing cloth at the same time. Avoid using compressed air to dry the sample after final polishing, because the compressed air contains oil or water.
4. When storing samples, do not place the samples in the air, because moisture may corrode the samples. Samples should be stored in a dry dish.
Plastic deformation (also known as cold working) can lead to subsurface defects after grinding, lapping or polishing. Residual plastic deformation can first be seen after etching.
Short deformation lines, limited to individual particles. Magnification: 100x DIC
1. Deformation is an artifact that appears immediately after etching (chemical, physical, or photoetching).
2. If you can still see the suspected deformation line when observing the unetched sample under bright field, please refer to the “Scratch” section first to see how to improve the sample preparation method.
5. Edge rounding
When using highly resilient polishing cloths, the surface and sides of the sample are sometimes abraded simultaneously, an effect known as edge rounding. This phenomenon occurs if the wear rate of the resin is greater than that of the sample.
The edges will be chamfered due to the gap between the resin and the sample. Stainless steel. Magnification: 500x
Good edge protection, stainless steel. Magnification: 500x
During the grinding process, the edge to be inspected should be well protected, and the edge of the sample should not be chamfered by excessive grinding due to the edge of the inspection sample.
2. When polishing, the side of the sample that needs to be protected faces the back, and the side that does not need to be protected faces the front. Polish in the direction of the rotation of the polishing disc. When polishing, it should be as close to the center of the disc as possible, and the polishing time should not be too long.
The relief is created by the different materials peeling off at different rates due to the different wear rates and hardness of the different phases.
B4C fibers in AlSi, undulations between fiber and substrate. Magnification: 200x
Same as picture above, but without the ups and downs. Magnification: 200x
1. Relief mainly occurs in the polishing stage, and the quality of the sample after grinding should be high to provide a good foundation for polishing.
2. The polishing cloth has a significant impact on the flatness of the sample, and the relief effect caused by the low-recovery polishing cloth is lighter than that caused by the high-recovery polishing cloth.
3. The polishing cloth should maintain a certain humidity during polishing, and control the sample preparation time to avoid excessive sample preparation time. If embossing occurs, the sample must be re-prepared.
During the grinding process, the particles or grains on the surface of the sample are pulled off and the holes left are called exfoliation. Due to the inability of the hard and brittle material to deform plastically, tiny areas of the sample surface shatter and fall off or are dragged off by the polishing cloth.
Inclusions are dragged out. Scratches caused by raised inclusions can be seen. Magnification: 500x, DIC
1. During cutting and mounting, do not apply excessive stress to avoid damage to the sample.
2. During rough grinding or fine grinding, excessive pressure and coarse grinding particles cannot be used.
3. A lint-free polishing cloth should be used that will not “drag” particles from the substrate.
3. Each process must remove the damage caused by the previous process and minimize the damage caused by this process.
4. Check the samples after each process to find out when shedding occurs. Once shedding occurs, it must be re-grinded.
Fractures that occur in brittle and heterogeneous samples are called cracking. When processing a sample with more energy than the sample can absorb, the excess energy promotes cracking.
Cracks between plasma coating and substrate. Cracks originate from cutting. Magnification: 500x
Samples mounted with epoxy resin under vacuum. The crack is filled with a fluorescent dye, proving that the crack was present in the material prior to mounting. Magnification: 500x
Cutting: Proper cut-off wheels must be selected and lower feed speeds should be used, wire cutting techniques should be used if necessary.
Mounting: Avoid hot-compression mounting of brittle materials or samples, and use cold mounting in preference.
Grinding: Avoid using high pressure when coarse grinding.
9. False porosity
Some samples are inherently porous, such as cast metals, sprayed coatings, or ceramics. Therefore, it is important how to obtain accurate data and avoid data errors due to sample preparation errors. Results differ for soft and hard materials.
Superalloys, 3 µm polished for 5 minutes. Magnification: 500x
1 µm additional polishing for 1 minute on top of the above image
On the basis of the above figure, 1 µm was polished for 2 minutes, and the result was correct
Cr2O3 plasma coating after lapping
6 µm after 3 min polishing
After 1 µm additional polishing. correct result
Extensible soft materials can be easily deformed. As a result, holes may be covered with smudged material. Inspection can show that the porosity percentage is too low.
The surface of hard, brittle materials is prone to fracture during the first mechanical preparation steps and therefore exhibits a higher porosity than in reality.
The specimen was inspected with a microscope every two minutes, inspecting the same area each time, to ensure that there was no improvement.
10. Trailing tail
When the sample moves in the same direction as the polishing disc, tailing often occurs around precipitated phases or pores. Its typical shape has earned it the name “trailing tail”.
Trailing. Magnification: 200x, DIC
1. During polishing, the sample and the polishing disc use the same rotational speed.
2. Reduce the polishing force.
3. In order to avoid smearing defects, keep the polishing cloth moist during sample preparation, and the sample should be moved continuously to avoid long-term polishing.
Impurities originating from other parts than the sample itself, and deposited on the surface of the sample during mechanical grinding or polishing, this phenomenon is called contamination.
Due to the slight undulations between the B4C particles and the Al matrix, the surface of the Cu-deposited sample from the previous step. Magnification: 200x
1. This kind of sample can be removed by light polishing again. If you check the polished sample, use alcohol cotton to gently scrub the surface of the sample when blowing it with alcohol.
2. In order to avoid contamination, the samples should be cleaned and dried immediately after each sample preparation process, especially after the last process.
3. When you suspect that a certain phase or particle may not belong to the real tissue, please be sure to clean or replace the polishing cloth, and re-sample from fine grinding.
12. Abrasive pressing
The phenomenon in which free abrasive particles are pressed into the surface of the sample. Since the morphology of embedded sand grains and non-metallic inclusions in steel cannot be distinguished under a metallographic microscope, it will cause misjudgment of defect analysis.
Aluminum, ground with 3 µm diamond, with a low elastic polishing cloth. Various types of corundum were embedded into the samples. Magnification: 500x
1. For samples with cracks and holes, control the intensity of sample preparation, and rinse the samples after each process.
2. If cracks or inclusions in the form of single grains, smaller grains, and separated from the matrix are found in the pores, the energy spectrum of the scanning electron microscope should be used for analysis to determine whether they are inclusions in the steel or brought in during sample preparation.
13. Grinding track
That is, the imprint left on the surface of the sample by the random movement of abrasive particles on the hard surface. Although there were no scratches on the sample, clear marks left by the random movement of particles on the surface were visible. Using the wrong grinding/polishing disc or polishing cloth, or applying inaccurate pressure can combine to cause scratch marks.
Grinding tracks on zirconium alloys: due to rotation or rolling of abrasive particles. Magnification: 200x
1. High elastic polishing cloth.
2. Appropriately increase the intensity of grinding/polishing
Attachment: METALOGRAM sample preparation
Metallographic sample map Metalogram
Introduction to the METALOGRAM method
Metalogram is based on ten metallographic preparation methods. Seven methods, A – G, cover all material. These methods are designed to generate samples with good results. In addition, three rapid sample preparation methods, X, Y, and Z, are indicated, which are suitable for obtaining qualified results quickly.
find the hardness along the x-axis,
Check downward or upward according to the toughness of the material. Different from the hardness, the toughness is more difficult to determine its exact value. Generally, after setting it according to personal experience, find the position of the material on the Y-axis.