Metallographic Polishing for deformation-free and scratch-free sample surfaces

Metallographic polishing, just like metallographic grinding, is the final stage in the sample preparation process of metals for subsequent analysis. Its purpose is to rectify the deformations caused by previous work steps (during sectioning and cutting). 
The following introduction provides some general considerations as well hints for sample preparation processes.

Metallographic polishing consists of several steps, with each step using a finer abrasive than the previous one. Tyically, a destinction between three operations is made: Pre-, intermediate and final polishing.

The ultimate goal is to produce a deformation-free, scratch-free, and highly reflective sample surface. For accurate results, a selection of appropriate consumables, a polishing machine, and a matching preparation guide with individual instrument settings (optimized for the according material) is required.

QATM is a leading manufacturer and supplier of high-quality consumables as well as a range of innovative metallographic polishing machines, from manual to fully automized models. The QATM application experts combine decades of experience with thousands of processed materialographic samples as well as diverse materials from a wide range of industry sectors and will be happy to assist with your application.

Metallographic Diamond Polishing

In metallography, diamond polishing agents are differentiated according to their shape. There are poly- and monocrystalline diamonds.

Agents containing polycrystalline diamonds have clear advantages over agents containing monocrystalline ones. The polycrystalline diamond has a uniform grain shape and many sharp edges.

The removal rate of diamond agents depends on the number and size of diamond cutting edges that are working simultaneously.

For the right choice of diamond abrasive (i. e. whether mono- or polycrystalline), it is important to determine its requirements beforehand. Basically, a good result can also be achieved with the cheaper monocrystalline agents, even though the way to get there requires more effort.

Diamond agents also differ in the type of their carrier material. Which of the three common carrier materials is chosen depends on the circumstances. 

Diamond paste: High concentration on the cloth, an additional lubricant is always necessary

Diamond suspension: Ensure well and uniform distribution on the cloth, pay attention to segregation in case of a longer stationary period

Diamond spray: Ensure well and uniform distribution on the cloth, an additional lubricant is always necessary, higher consumption due to dosage

Metallographic Oxide Polishing

In metallography, oxide agents are primarily used for the final process step.

A distinction is made between silicon dioxide and aluminium oxide suspensions. Both are available as "ready-to-use" aqueous or alcoholic suspension. The grain sizes are between 0.1 and 0.06 µm.

There are no absolute rules for the application of the suspensions. Alcohol-based suspensions should be used for specimen susceptible to corrosion.

Silicon dioxide: All austenitic steels, aluminium alloys, titanium alloys, precious metal alloys

Aluminium oxide: Brass, low to unalloyed steels, grey cast iron grades

With the use of oxide agents, a mechanical or chemo-mechanical removal can be achieved, which removes the smear and deformation layers that may still be present from previous metallographic polishing; the quality of this process ranges from good to very good.

The 'double' attack also results in short polishing times. Significantly lower material costs is another advantage over diamond agents

Metallographic Polishing Cloths and Discs

Cloths for metallographic polishing are available with different surface structures or materials (e. g. silk, artificial silk, wool fabrics, flocked and foamed plastics). Specially developed discs with integrated diamonds are also available.

Metallographic pre-polishing is usually carried out on less elastic (hard) cloths using lubricant (low impact elasticity is important to maintain edge sharpness). If edges are rounded off by cloths that are too soft, this has a negative effect on the evaluation of thin layers.

During final polishing, switch to more elastic, long-napped cloths and to lubricants with a higher viscosity.

The elasticity of the cloths for intermediate polishing lies between the two extremes just described. 

Tips and tricks for metallographic grinding and polishing

Grinding and polishing is a standard technique in metallography. Expert knowledge in processing is available for almost all materials. However, a number of potential sources of error must be taken into account when searching for the appropriate procedure. The procedure is very time-consuming due to the multi-stage workflows, especially in manual processing. On the other hand, larger investments must be expected if a higher degree of automation is desired.

The following risks must be evaluated in each individual case:

• The use of abrasive materials for metallographic polishing can result in scratched sample surfaces, especially if the material contains phases with different hardness values. This can never be ruled out in case of composite materials.
• Edges and grain boundaries can be rounded.
• Coolants and lubricants must not react with the material as organic materials, such as paper or fibers, would swell. If the cooling performance is not sufficient, the sample materials suffer thermal degradation and/or microstructural changes.

The market for grinding and polishing equipment is very diverse and offers suitable solutions for a large variety of applications. Depending on application and capacity, simple semi-manual to fully automated systems are available. The wide range of applications also speaks for itself. With carefully matched  consumables and suitable preparation parameters, excellent results can be achieved for almost all materials. Mechanical grinding and polishing often is the only preparation technique suitable for ceramics, composite materials and filled/reinforced plastics.

To provide assistance to the user, a few tips and tricks are listed below. 

2. Comet Tails

In metallography, the so-called comet tails usually occur with non-metallic inclusions, pores and hard phases in a soft basic matrix, if the section is only held in one direction during manual preparation. This artefact is typical for large material samples. For optimum metallographic sample preparation, the specimen should be moved in a way that prevents creation of a preferred direction. 

4. Deposition of metallographic grinding and polishing agents

Abrasives and other polishing agents for metallography, such as silicon carbide particles from the grinding paper, diamonds from diamond polishing agents or oxides from the oxidic final polishing agents, can be pressed into the surface during preparation.
This may occur in the following situations:

• In case of dry metallographic grinding of composite materials consisting of very hard and very soft materials
• In case of excessively high pressure when preparing extremely soft materials and/or composites with components of soft and hard phases
• In case of excessively high abrasive concentration in combination with too high pressure
• Incorrect selection of the grain size of the agent
• When the lubricating film breaks down during polishing. If there is too little lubricant is added, the samples can heat up considerably
• When using lubricant with very low viscosity
• If the wrong cloth is chosen (too hard or too soft), pay attention to impact elasticity
• In case of too one-sided sample movement