The key to preparing metallographic samples lies in the grinding and polishing steps. The purpose of metallographic polishing is to remove damage caused during the grinding process and prepare for microhardness, optical or SEM/EBSD electron microscopy analysis.


Polishing is the process of using mechanical or chemical mechanical polishing methods to preserve the true microstructure and flatness of the sample surface. If the selection of consumables such as metallographic sandpaper, polishing fluid, polishing cloth, polishing paste, etc. for grinding and polishing is improper, or the cleaning treatment is not in place, or improper operation during the replacement of consumables during the grinding process can cause some problems to occur. For example, there are many scratches, embeddings, convexity, plastic deformation, and comet trailing phenomena that affect the observation of metallographic structure. But it can be said for sure that these phenomena were not caused by metallographic sandpaper. The specific handling methods will be explained to you by the metallographic editor of Lianneng.

   1、 Excessive scratches:


When there are still many scratches that cannot be removed during the fine polishing stage, if it is not due to the quality of the metallographic sandpaper, the more likely reason is the presence of cross contamination. That is to say, before entering the next process, it is necessary to thoroughly rinse the sample, fixture, and grinding disc, which helps to remove residual large diamond particles in the previous process and effectively reduce cross contamination.


But if the sample is porous or has cracks, ultrasonic cleaning should be used as required. It should be noted that when using ultrasonic cleaning, the time should not be too long, otherwise there is a risk of damaging the sample.


Another point is to stop adding polishing fluid in the last 30 seconds of final polishing. After polishing, thoroughly clean the surface of the polishing cloth and sample with water.


When there are too many scratches, the problem can be properly solved by dealing with the above three aspects.

    2、 Embedding:


When embedding occurs during the fine polishing stage, it is due to harder grinding particles embedded in cracks or gaps in softer materials. Generally, metallographic sandpaper is not used, but more fixed abrasives, such as diamond polishing paste, are used to effectively prevent the occurrence of embedding phenomenon. Of course, ultrasonic cleaning before entering the next polishing step can also be used to reduce the possibility of embedding.

    3、 Floating protrusion:


The protrusion phenomenon on the surface of the sample is mainly caused by the protrusion of harder phases or components on a relatively soft substrate. A more effective way to address this phenomenon is to reduce polishing time and use a short bristle polishing cloth in conjunction with diamond polishing paste for polishing. Generally, diamond polishing fluid is not used for polishing to effectively prevent the occurrence of protrusions.

    4、 Plastic deformation:
Plastic deformation is a very serious damage phenomenon that occurs on the surface of a sample. The reason is due to the material being soft or poor lubrication during the polishing process, which is like water ripples and undulates, making it impossible to see the microstructure details clearly. Generally, short plush polishing cloth combined with vibration polishing or fine polishing after corrosion can effectively improve plastic deformation problems.

    5、 Comet Trail


The comet tail phenomenon on the surface of the sample is mainly due to the unidirectional fine grooves originating from particles or pores when the hard phase is poorly bonded in the soft matrix. It seriously affects the observation and evaluation of metallographic structure. Generally, hard lint free polishing cloth is used for polishing, while reducing the applied pressure, which can basically avoid the occurrence of comet tail dragging.


However, for porous materials, using resin or wax to seal the pores and polishing can also effectively prevent this phenomenon from occurring.

    In summary, the small problems that often occur during the preparation of metallographic samples, such as scratches, embeddings, protrusions, plastic deformation, comet tails, etc., are really not caused by metallographic sandpaper! The reasons are different, and each phenomenon requires us to carefully observe and analyze the causes, and then take targeted measures to effectively solve the problem.