About Grinding and Polishing

Preparation goals

Regardless of the preparation requirements, the overall goals of the preparation are the same:

 

• All structural elements must be retained.
• The surface must be without scratches or deformation.
• No foreign matter may be introduced on the specimen's surface.
• The specimen must be plane and highly reflective.
• The optimal price per sample should be obtained.
• All preparations must be 100% reproducible.

The basic process of mechanical specimen preparation is material removal, using abrasive particles in successively finer steps to remove material from the surface until the required result is achieved.

There are three mechanisms for removing material: grinding, polishing, and lapping. They differ in the tendency to introduce deformation in the specimen's surface.

How to grind and polish

The goal of materialographic specimen preparation is to reveal the true structure of the specimen, whether it is metal, ceramic, sintered carbide, or any other solid material.

The easiest way to accomplish this is with a systematic preparation method. When the work routinely involves examining the same material, in the same condition, achieving the same result each time is desirable. This means that the preparation result must be reproducible.

Sample preparation follows certain rules which are valid for most materials. Different materials with corresponding properties (hardness and ductility) respond similarly and require the same consumables during preparation. Therefore, all the materials can be displayed in the Metalogram according to their properties, and not because they belong to a certain material group.

Case story: Preparation time reduces from 60 to 11 minutes

How to select a preparation method

The Metalogram displays materials according to specific physical properties: hardness and ductility. 

• Hardness: The easiest attribute to measure but is not sufficient information about a material to find the correct preparation method.
• Ductility: The ability of a material to deform plastically is important for grinding and polishing. This property expresses how the material responds to mechanical abrasion.

The X-axis represents the hardness in Vickers (HV). The values are not shown in a linear progression, because the variety of preparation methods for softer materials is greater than for hard ones. The shape of the Metalogram results from soft materials generally being more ductile and hard materials usually being more brittle.

Selection of a Preparation Method

1. Find the hardness on the X-axis.

2. Move up or down, depending on the ductility of the material. Unlike hardness, ductility is not easily defined in precise numbers.

3. Place the material on the Y-axis according to any previous experience. A prerequisite is to have an idea of how a ductile or brittle material will perform.

METALOGRAM METHODS

The Metalogram is based on ten preparation methods. Seven methods, A - G, cover the complete range of materials. They are designed to produce specimens with the best possible results. In addition, three short methods, X, Y, and Z, are displayed. These methods are for very quick, acceptable results.

Some materials such as composites, coatings, or other materials consisting of various phases or components cannot be easily placed in the Metalogram. In these cases, the following rules can be applied when deciding on the preparation method:

• Predominant component Select a method which is suited for the material's predominant component.
• Artifacts Check the samples after each step and, if preparation artifacts do occur, consult troubleshooting for advice. The most common artifacts are edge rounding, relief, pull-outs, and porosity.

Preparation Parameters

Preparation methods present a balanced set of parameters for a grinding and polishing procedure described by the headlines below.

Lubricant

Depending on the type of material and the preparation stage, different lubricants combine levels of lubricating and cooling levels and liquid characteristics.

This may include thin lubricants with high cooling and low lubrication effect, special lubricants for polishing of soft and ductile materials, alcohol-based or water-based, etc.

Depending on the type of material and the grinding/polishing disk used for preparation, the amounts of lubrication and cooling have to be balanced. Generally, it can be said that soft materials require high amounts of lubricant to avoid damage, but only small amounts of abrasive as there is very little wear on the abrasive. Hard materials require less lubricant but higher amounts of abrasive, due to faster wear. The amount of lubricant has to be adjusted correctly to get the best result.

The polishing cloth should be moist, not wet. Excess lubricant will flush the abrasive from the disk and remain as a thick layer between the specimen and disk, thus reducing material removal to a minimum.

For two-in-one diamond suspensions, lubrication and cooling liquids are included and balanced in the bottle to optimize the relevant preparation method.

Troubleshooting - Grinding and Polishing

There Are a Few Basic Rules Which Should Always Be Followed:

• To improve the preparation of a particular material, make sure that it has been prepared according to a suitable method from the  Metalogram.
• If the material is being prepared for the first time, it is important to examine the specimen after every step with a microscope. This makes it easier to see when preparation artifacts occur.
• Before proceeding to the next step, be sure that all damage from the previous step, such as scratches, pull-outs, or embedded grains, are removed completely. If this is not done, artifacts from an early step might show up on the finished surface, in which case it would be impossible to be sure where they originated. It must be known when artifacts begin to occur to be able to improve the method.
• Keep preparation times as short as possible. Unnecessarily long preparation times waste consumables and may even damage the specimen, by causing edge rounding, comet tails, and relief, for example.
• New polishing cloths or grinding disks may need to be "run in" for a short time, or dressed or cleaned before use, to give the best results.

Case story: How Uddeholm AB learned to troubleshoot halfmoon-shaped scratches after grinding

15 Costly Grinding and Polishing Troubles – How to Avoid Them

Scratches, smearing, staining and deformation are just a few of the troubles you want to avoid when grinding and polishing for materialographic analysis. Don’t miss these important tips to avoid the 15 most common grinding and polishing troubles.

1. TROUBLESHOOTING - Scratches 

• Scratches are grooves in the surface of a sample, produced by the points of abrasive particles.
• Make sure that after PG the surface of all samples in the specimen holder shows the same uniform scratch-pattern over the whole surface.
• Repeat PG if necessary.
• To avoid contamination of the grinding/polishing surface through large abrasive particles from a previous step, clean the samples and sample holder carefully after every step.
•  If there are still scratches left over from the previous step after finishing the current step, increase the preparation time by 25% to 50% as a first measure. If that does not help, use the expert system.

2. TROUBLESHOOTING - Smearing

The plastic deformation of larger sample areas is called smearing. Instead of being cut away or removed, material is pushed across the surface. Smearing occurs because of an incorrect application of abrasive, lubricant, polishing cloth, or a combination of these, which makes the abrasive act as if it was blunt. There are three ways to avoid smearing:

• Lubricant: Check the amount of lubricant and, if necessary, increase it as smearing often occurs when the lubricant level is too low.
• Polishing cloth: Due to high resilience of the cloth, the abrasive can be pressed deep into the cloth and it cannot cut. Change to a cloth with lower resilience.
• Abrasive: The diamond grain size might be too small, which means the particles cannot cut. Use a larger grain size.

3. TROUBLESHOOTING - Staining

• Staining is often seen after cleaning or etching specimens.
• When there is a gap between the sample and resin, water or alcohol or etchant can bleed out.
• Areas on the specimen surface can be discoloured and make the examination difficult or even impossible.
• Clean and dry specimens immediately after each preparation step.
• Avoid the use of compressed air when drying your specimens after final polishing, because compressed air can contain oil or water.
• OP polishing can result in a white film left on the specimen surface if the cleaning is not carried out correctly.

If your polisher is not equipped with automatic water flushing after the oxide polishing step during the last ten seconds of OP polishing, flush the polishing cloth with water to clean both the specimens and the cloth.

• Do not use hot water for cleaning specimens, because hot water is more aggressive than cold water and subsequent etching will be intensified.
• Never leave specimens in normal room conditions because humidity might attack the specimen. Always store specimens in a desiccator if you want to keep them.

 

4. Troubleshooting - Deformation

There are two types of deformation: elastic and plastic. Elastic deformation disappears when the applied load is removed. Plastic deformation, which may also be referred to as cold work, can result in subsurface defects after grinding, lapping, or polishing. Remaining plastic deformation can first be seen after etching.

Only deformation introduced during the preparation is covered here. All other types from previous operations like bending, drawing, and stretching are not considered, because they cannot be changed or improved by changing the preparation method.

• Deformations are artifacts which first show up after etching (chemical, physical, or also optical etching).
•  If a supposed deformation line also is visible in brightfield in unetched condition, please see the scratches section on how to improve the preparation method first.

5. Troubleshooting - edge rounding

Using a polishing surface with high resilience will result in material removal from both the sample surface and the sides. The effect of this is edge rounding and can be seen with mounted specimens if the resin wears at a higher rate than the sample material. Please check your samples after each step to see when the fault occurs so you can determine what changes you will need to make in the preparation.

6. Troubleshooting - relief

Material from different phases is removed at different rates due to varying hardness or wear rate of the individual phases. 

Relief is usually not noted until polishing begins, so it is important to begin the preparation with grinding media that will keep the samples as flat as possible. However, for the best possible starting conditions, MD-Largo should be used for fine grinding of materials with a hardness below 150 HV, and MD-Allegro should be used for fine grinding of materials with a hardness of 150 HV and higher.

• Plane grinding with diamond is the best choice to ensure flat samples from the very beginning of the preparation.
• Fine grinding with either MD-Largo or MD-Allegro will provide the best possible planeness.
• To avoid relief, preparation time and the type of polishing cloth used are the most important parameters.
• The preparation time should be kept as short as possible. When developing a new method, the samples have to be checked at short intervals (one to two minutes).
• The polishing cloths have a strong influence on the planeness of the samples. A polishing cloth with low resilience produces samples with less relief than a cloth with high resilience.
• See Edge Rounding for the correct way to change preparation parameters.
• To avoid relief with layers and coatings, mounting may help to improve the result. Look in the "About Mounting" section for more detailed information.

7. Troubleshooting - pull-outs

Pull-out is a general term used to describe a number of material irregularities such as:

• Loss of structural elements (for example: unsupported particles in spray coatings, longitudinal fibers in composites).
• Cavities or pits that remain after water-sensitive inclusions have been dissolved or eroded.
• Holes created when inclusions such as oxides have been broken out of the matrix material.
• Damage caused by aggressive grinding that has not been removed yet (such as broken grains in brittle ceramics and other hard/brittle materials that do not suffer plastic deformation).

The above-described issues normally occur during the early steps of materials preparation: sectioning, mounting, and plane/coarse grinding.  Avoid these situations by:

• Take care during cutting and mounting not to introduce excessive stress that could damage the specimens.
• Use MD-Largo when possible to avoid pull-outs. as it is less aggressive than MD-Allegro.
• Do not use higher forces or more coarse abrasives than needed for plane grinding or fine grinding.
• The margins between each abrasive grain size should not be too large so that it would prolong the preparation time unnecessarily.
• A napless polishing cloth should be used when possible, as it does not tend to "pluck" particles out of the matrix. Also most of the napless cloths have a lower resilience providing higher removal rates.
•  Every step has to remove the damage from the previous one and has to introduce as little damage as possible of its own.
•  Check the samples after every step to find out when pull-outs occur.

 

8. Troubleshooting - gaps

Gaps are voids between the mounting resin and sample material. When examining samples with a microscope, it is possible to see if there is a gap between the resin and the sample. Gaps can result in a variety of preparation faults: edge rounding, contamination of polishing cloth, problems when etching, and staining.

• Vacuum impregnation using epoxy will provide the best result.
• The samples should always be cleaned and degreased to improve the adhesion of the resin to the sample.
• Hot mounting: choose the correct resin and cool the samples in the press under pressure to avoid gaps.
• Cold mounting: avoid too high curing temperatures. For large mounts, use a stream of cold air for cooling or place the molding cups in a shallow tray of cool water.
• To save a sample with a gap, try to fill the void with epoxy under vacuum. Clean and dry the sample carefully, put it into the vacuum chamber, and use a small amount of epoxy to fill the gap. The preparation has to be started all over again to remove any excess epoxy on the sample surface.

9. Troubleshooting - cracks

Cracks are fractures in brittle materials and materials with different phases. The energy used to machine the sample is greater than can be absorbed. The surplus energy results in the cracks.

Cracks occur in brittle materials and samples with layers. Care has to be taken throughout the complete preparation process.

This section does not deal with cracks in ductile materials, as these are not caused by the preparation but are already present in the sample prior to preparation.

• Cutting: The appropriate cutoff wheel has to be chosen, and a low feed rate should be used.
• When cutting coated samples, the wheel should pass through the layer(s) first, so that the base material can act as support.
• Clamping of the sample should be carried out in a way that no damage can occur. If necessary, use padding between sample and clamp.
• Mounting Avoid hot compression mounting for fragile materials or samples.  Use, instead, cold mounting, preferably with vacuum impregnation. The only exception is ClaroFast, Struers’ thermoplastic resin which can be used in either CitoPress-15/-30 or any mounting press in which the resins can be pre-heated and softened without pressure.

Note: Vacuum impregnation will only fill cracks and cavities connected with the surface. Be careful not to use mounting materials with high shrinkage. They might pull layers away from the base material.

10. Troubleshooting - false porosity

Some materials have natural porosity, for example, cast metals, spray coatings, or ceramics. It is important to get the correct values, and not to provide incorrect readings because of preparation faults.

Depending on the properties of a material, two contrary effects regarding porosity can be seen:

• Soft and ductile materials can be deformed easily. Therefore, pores can be covered by smeared material. An examination might show porosity percentage that is too low.
• The surface of hard, brittle materials is easily fractured during the first mechanical preparation steps, thus exhibiting more porosity than is actually the case.

Contrary to the ductile material, where the initial porosity seems to be low and pores have to be opened, brittle materials seem to have a high porosity. The apparent fracturing of the surface has to be removed.

• Polishing with diamonds is necessary, regardless of the material hardness or ductility. Examine the specimens every two minutes with a microscope, inspecting the same area each time to determine if there is improvement. One way to make sure you are looking at the same area is to mark an area with a hardness indentation (for brittle materials, care has to be taken not to introduce additional stress).
• Once there are no further changes in porosity, proceed to the next polishing step.
• If needed, to remove the last of any smeared metal, the final step should be an oxide polish to remove material slowly, without introducing new deformation.

  

11. Troubleshooting - hard/brittle materials

Hard brittle materials often get fractured at the surface during the first mechanical preparation steps. The surface might show a porosity higher than the real one.

Contrary to the ductile material, where the initial porosity seems to be low and pores have to be opened, brittle materials seem to have a high porosity. The apparent fracturing of the surface has to be removed.

12. Troubleshooting - comet tails

Comet tails occur adjacent to inclusions or pores, when the motion between sample and polishing disk is unidirectional. Their characteristic shape earns the name "comet tails." A key factor in avoiding comet tails is the polishing dynamics.

1. During polishing, use the same rotational speed for the samples and the disk.

2. Decrease the force.

3. Polishing for extended time on a soft cloth is a contributing factor. Ensure that as little deformation as possible must be removed by the next polishing step, especially when a cloth with high resilience is needed.

13. Troubleshooting - contamination

Material from a source other than the sample itself, which is deposited on the sample surface during mechanical grinding or polishing, is called contamination.

• Contamination can occur on all types of materials.
• During polishing, dirt particles or material removed during a previous step can be deposited on the specimen or on the polishing cloth.
• Microscopic examination can show "inclusions" or phases in a structure which are anomalies or deformation.
• Be sure to store polishing disks in a dustproof cabinet to avoid contamination of the disk surface.
• Should there be any doubt if a phase or particle is correct, please clean or change the polishing cloth and repeat the preparation from the fine grinding step.
• Above all, make sure that the specimens are cleaned well between preparation steps

14. Troubleshooting - embedded abrasive

An embedded abrasive is a loose abrasive particle pressed into the surface of a specimen. With soft materials, abrasive particles can become embedded. Embedded abrasives can occur because of a small abrasive particle size, the grinding or polishing cloth used has a low resilience, or a lubricant with a low viscosity is used. Often, a combination of these reasons takes place.

• When plane grinding, abrasive particles can become embedded in soft materials. Continue with a somewhat finer grit surface (i.e. MD/DP-Pan with DiaPro Pan 15 um) as a second plane grinding step and MD-Largo for fine grinding. Embedded particles should be removed after the fine grinding step.
• MD-Molto 220 for Aluminum and Al alloys, or MD-Mezzo for Titanium and Ti alloys should be used for plane grinding those specific nonferrous metals/alloys.
• MD-Allegro should not be used for materials with hardness lower than 150 HV. Instead of being pressed into the disk, the abrasive particles will be pressed into the sample and stay there, firmly embedded. Use the MD-Largo instead of MD-Allegro.
• When polishing soft materials, grain sizes of 3.0 µm and smaller should only be used on cloths with high resilience.
• For the last diamond polishing steps of soft materials, when fine abrasive particles are used:

1. DiaPro NAP R 1.0 um when MD/DP-Nap cloth is used
2. DiaPro Mol R 3.0 um when MD/DP-Mol cloth is used
3. DP-Lubricant, Red, a lubricant with high viscosity, is used with the diamond abrasive.
4. If the material is water sensitive, use DP-Lubricant, Yellow with the diamond abrasive.

15. Troubleshooting - Lapping tracks

Lapping tracks are indentations on the sample surface made by abrasive particles moving freely on a hard surface. These are not scratches, like from a cutting action, but are the distinct tracks of particles tumbling over the surface without removing material.

• If an abrasive particle is not held in a fixed position while the sample is passing over it, it will start rolling. Instead of removing material, the grain is forced into the sample material, creating deep deformation and only chipping small particles out of the sample surface.
• Lapping tracks can be produced during both grinding and polishing.
• The causes are: incorrect disk/cloth surfaces for the actual operation or the wrong force. Also, combinations of these faults can cause lapping tracks.

Polishing

Like grinding, polishing is used to remove the damage remaining from the previous steps. This is achieved with steps of successively finer abrasive particles. Polishing is divided into two different processes:

Thus, the removal rate (the amount of material removed in a certain time period) is very low during lapping, leading to very long process times. With soft materials, the abrasive particles are often pressed into the specimen surface where they become firmly embedded. Both the deep deformations and embedded grains are undesirable in materialographic specimen preparation. This means that lapping is only used for the preparation of very hard, brittle materials, such as ceramics and mineralogical specimens.