Common polishing methods

Common polishing methods

2022-05-24 13:35:37 7

Metallographic specimens need to be polished after grinding, in order to remove the abrasion marks and deformation layer produced by the grinding process on the specimen, so that it becomes a smooth mirror surface. 

The current method of polishing specimens are mechanical polishing, electrolytic polishing, chemical polishing and compound polishing. The most commonly used polishing method is mechanical polishing. 

The following is a brief introduction to the characteristics of mechanical polishing, electrolytic polishing, chemical polishing each.

I. Mechanical polishing

Polishing on a special polishing machine. Mechanical polishing is mainly divided into two steps: coarse polishing and fine polishing.

Coarse polishing: the purpose is to remove the deformation layer of polishing.

Previously coarse polishing commonly used abrasive is the particle size of 10-20μm α-Al2O3, Cr2O3 or Fe2O3, with water into a suspension to use. At present, artificial diamond abrasive has gradually replaced the alumina and other abrasives, because it has the following advantages.

1) compared with alumina and so on, the particle size is much smaller diamond abrasive, polishing rate to be much larger, for example, 4 ~ 8μm diamond abrasive polishing rate and 10 ~ 20μm alumina or silicon carbide polishing rate is similar;.

2) surface deformation layer is shallow.

3) polishing quality is the best.

Fine polishing: also known as final polishing, the purpose is to remove the deformation layer produced by rough polishing, so that polishing damage is reduced to a minimum.

Commonly used fine polishing abrasives for MgO and γ-Al2O3, of which MgO polishing effect is the best, but the polishing efficiency is low, and not easy to grasp; γ-Al2O3 polishing rate is high, and easy to grasp.

Second, electrolytic polishing

 The use of anodic corrosion method to make the surface of the specimen becomes smooth and bright polishing method. Electrolytic polishing with stainless steel as the cathode, the polished specimen as the anode, the container holds the electrolyte, when the current is connected, the metal ions of the specimen in the solution dissolved, under certain electrolytic conditions, the surface of the specimen slightly convex part of the dissolution than the depression comes faster, so that the surface of the specimen is gradually rough to flat and bright.

Advantages and disadvantages.

    1: compared with mechanical polishing, electrolytic polishing is purely electrochemical dissolution process, no mechanical force, does not cause surface deformation of the metal.

    2: for low hardness of the single-phase alloy and general mechanical polishing is difficult to do aluminum alloy, magnesium alloy, copper alloy, titanium alloy, stainless steel, etc. appropriate to use this method.

    3: electrolytic polishing of specimens with low requirements for the degree of polishing (generally with 800 water sandpaper can be smoothed), fast and efficient.

    4: electrolytic polishing for the material chemical composition of the inhomogeneity, microscopic segregation is particularly sensitive to non-metallic inclusions will be violently corroded, so it is not suitable for segregation of serious metal materials and for inclusions test metallographic specimens.

Note: electrolytic polishing must choose the appropriate voltage, control the current density, too low and too high voltage can not achieve the purpose of normal polishing.

Third, chemical polishing.

    The use of chemical dissolution to get a smooth polished surface. Immerse the specimen in the chemical polishing solution, appropriate stirring or rubbing with cotton, after a period of time to get a bright surface. Chemical polishing also has the role of chemical corrosion, can show the metallographic organization, polishing can be directly observed under the microscope.

Chemical polishing operation is simple, low cost, no special instruments and equipment, the original surface finish of the specimen is not high.

The composition of chemical polishing solution varies with the polishing material. Generally for the mixed acid solution. Commonly used acids are: orthophosphoric acid, chromic acid, sulfuric acid, acetic acid, nitric acid and hydrochloric acid; in order to increase the activity of the metal surface to facilitate the chemical polishing, also add a certain amount of hydrogen peroxide. Chemical polishing solution after use, the solution of metal ions, polishing effect is weakened, need to be replaced frequently. 



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