Diamond Grinding and Fine Grinding Discs

Diamond Grinding Disc Instructions

Things to check before you start

Magnetic support disc

It is important to check the flatness of the magnetic support disc. If it is not level, you can level it by grinding off the high spots using a 1"x1"x5" dressing stick and use a straight edge for measuring flatness. Get a straight edge that as a minimum covers the distance from the centre to the edge of your magnetic disc, preferably the whole disc diameter.

Run the disc at 150 rpm with an ample supply of water. Use the dressing stick to remove the excess magnetic rubber and check with the straight edge that the disc is level. The dressing stick will get dark from the debris, but this will disappear next time you dress a diamond disc. Because of ease of use, the magnetic support disc is widely used. However many people do not realize that a magnetic disc needs maintenance to function properly. The discs magnetic surface is made from a soft polymer and the continued force acting on the center area will make the material flow outwards. Hence, over time it becomes hollow in the center.

A second issue is that the edge of disc is continuously exposed to lubricant and diamond suspension and will over time swell and make the disc higher at the outside. Both of these issues will cause scratches at the edges of your samples, prolonging each preparation step unnecessarily. Therefore it is recommended that you check the level of your disc regularly.

Force setting

This is possibly where the most variance between the different machines occur. For various reasons you cannot expect the force setting to be accurate. Metallographic polishers use different principles to generate force, and the suppliers do not agree on a standard way of measuring the force. Further, they all measure the force while the machine is at a complete stop, not while it is running. For this reason, 20 N on one machine brand may not be equal to 20 N on another brand. Indications on too high forces are premature wear of grinding discs and polishing cloths and samples that are hot after polishing and have heavy scratch patterns. Too low forces will generally result in very long grinding or polishing times.

The Method

A preparation method consist of 3 - 5 steps. The purpose of each step is to remove the deformation created by the previous step and to improve the surface finish. When testing a new method it is therefore of extreme importance to check the surface of your samples after each step to make sure the finish is as required and the sample is ready for the next preparation step. Omitting this can cause a non satisfactory final finish with artifacts that are difficult to identify the origin of.

Plane-grinding

When new, the grinding disc needs to be dressed before using it for the first time. Place the diamond grinding disc on the magnetic support disc, let the machine rotate at 150 rpm and turn on the water. Use the dressing stick to remove the uneven top layer. This is an extremely important step as an undressed diamond grinding disc performs very badly with very little removal and deep scratches.

After grinding your samples for the recommended time they should have an even finish over the entire surface, and be perfectly flat. The finish you can check visually and the flatness by using the straight edge. Possible errors on single samples are coarser scratches along the edges from a non level magnetic disc. It can also be contributed by too high samples. The height should be no more than 2⁄3 of the diameter. In a sample holder you can expect prolonged grinding times if your samples were cut uneven, or if you did not level all samples to the same plane in the levelling device. Either relevel your samples or add a coarse diamond grinding disc grit 80 plane grinding step.

Fine-grinding

Fine grinding discs need diamond abrasive added during use. Because of their effectiveness in bringing diamonds in contact with the sample, they do neither require large amounts of diamonds nor any run-in time. With diamond suspensions you should dose about 0.5 ml/min on a 250 mm disc. Less on a 200 mm disc and more on a 300 mm disc. After the fine grinding step your samples should have a nice even semi glossy finish. If visible scratches from the grinding step are present you will need to increase the fine grinding time. If the scratches are at the edge you might still have a level problem with your magnetic disc.

Dosing of diamond suspensions

We recommend a dosing level of 0.5 - 1.5 ml/min of diamond suspension depending on the size of the disc and number of samples. To ensure these values, it is recommended to flush your automatic dosing system with diamond suspension and then do a dry run where you collect the suspension dosed by your system and adjust your settings accordingly. Most trigger sprays/manual dosing bottles dose approximately 0.5 ml per trigger activation. The suspension should be dosed about 1/3 off the centre of the disc. In this area, the samples pass most often and therefore everything dosed here will spread quickly to the rest of the preparation surface. The dosing level must be adjusted to keep the preparation disc moist but not wet. Excessive dosing will not result in faster preparation, on the contrary the “aqua-planing” effect will extend the preparation time. When using diamond suspension, the formation of foam on the polishing cloth is a clear indication of overdosing.

Polishing

Because of their absorbent nature all polishing cloths will require a certain preparation, before the removal rate is optimised. There are 2 ways of doing so:

1. The traditional way of pre-dosing with diamond suspension of 1 ml/min for 2-3 minutes and to run one or two dummy samples that are not needed for examination.
2. A faster and better way is to impregnate the polishing cloth before starting the preparation with a diamond stick in the same grain size as the suspension to be used. Thus the polishing cloth is ready for immediate use. Rotate the polishing cloth at 150 rpm and move the Diamond Stick from the centre of the disc to the outside to create a spiral pattern. Then start the preparation and apply diamond suspension as described above.

When running a polishing step, the recommended time should be followed quite closely. Extending the polishing time 2 or 3 times is not recommended, as this will most likely result in samples that are over polished, showing rounded edges due to the long polishing time. However, if the samples still display scratches from the previous step, you need to increase the polishing time.

Final Polishing with Colloidal Silica

Colloidal Silicas do contain anti drying agents but they will eventually dry out and ruin the polishing cloth. A handy way of avoiding this is to flush the cloth with water during the last 10-20 s of the preparation. This way both cloth and samples are clean, and the CMP reaction is neutralised.