Metallographic Polishing and Grinding
Grinding of Specimens
The surface of the specimen is first made flat by an abrasive belt or rotary disc machine. These machines are frequently equipped with coolant attachments to ensure cool cutting and to help wash abraded particles from the belt or disc. Care should be taken to prevent overheating of the specimen during the rough grinding stage.
Abrasive belts or discs for wet grinding usually employ Silicon Carbide as the cutting media. Kemet Met Discs are plain backed and Bramet are Self Adhesive. Abrasive grit size is usually in the 120 - 320 range for rough grinding. Rough grinding produces a flat, plane surface and removes harmful effects resulting from cutting-off operations. It is advisable during the rough grinding operation to bevel the sharp edges of the specimen, or specimen mount to prevent tearing of grinding papers and polishing cloths during subsequent operations. Excessive pressure during rough grinding will form deep scratches, and will increase the depth of disturbed metal on the surface of the specimen. This disturbed layer may extend from ten to fifty times the depth of the scratch produced by an abrasive grain.
The initially deformed layer is reduced in depth by the application of a finer abrasive. However, the finer abrasive also produces another deformed layer, this time of lesser depth. The process is continued with finer and finer grinding and polishing abrasives. The final layer is removed chemically by etching with an acid or some suitable agent.
During fine grinding the specimen is held so that the new finer scratches are introduced at approximately right angles, to those resulting from its previous rough grinding operations. The purpose for this step is to make it easy to recognise the point at which the coarse scratches have been replaced by a series of new finer ones. Fine grinding is usually done on abrasive belts with grit sizes between 320 and 600. Following each grinding stage, the specimen should be cleaned with soap and running water to prevent carrying coarse abrasive particles to the next specimen. One large abrasive grain can ruin the surface of a finely ground and polished specimen.
Cleanliness in metallurgical specimen preparation is essential and cannot be over emphasised. Fine grinding is often done using a water coolant and Silicon Carbide Abrasive Paper mounted on a rotating disc.
Kemet have a wide range of special Diamond and Kemet Composite Wheels, which can often replace the need for Silicon Carbide Discs.
Polishing of Specimens
Of all stages required in quality sample preparation, perhaps the most critical are those concerned with the actual polishing of the specimen.
Smoothing is best carried out by using Kemet Diamond Compound Abrasives or Suspensions, in the 3-9 micron size ranges. For smoothing, a nap-free cloth such as PSU-M should be used on a rotating disc. The Kemet Diamond Compound is generally used in conjunction with Kemet Lubricating Fluids, to uniformly distribute the diamond particles over the surface of the polishing cloth. Diamond Abrasives, which may be used with heavy hand or loading pressure, are cool cutting, and produce a minimum of disturbed metal. During rough polishing the specimen is moved in a clockwise direction around the polishing wheel to ensure equal metal removal from the entire surface, but not allowing prolonged polishing in any one direction.
This is done by mechanical means and follows a similar procedure to that used for smoothing. Kemet Diamond Compound, 0.25 - 1 Micron, can be used for a number of less critical materials.
Certain materials, especially those which are soft and ductile, require polishing with Aluminium Oxide. Aluminium Oxide is often referred to as Alumina and is a popular final polishing abrasive for ferrous and copper based materials. Magnesium Oxide is frequently used for polishing Aluminium and Magnesium Alloys. The abrasive particles used in final polishing are generally carried on a napped or short pile cloth such as Kemet MRE and Kemet NHM. Most Kemet Polishing Cloths can be supplied cut to size and coated with an adhesive backing. The adhesive back eliminates the need for mechanical clamping.
Other common final polishing materials are Colloid Alumina (Acidic) and Colloidal Silica (Alkaline), both used as a chemical-mechanical action. The combination of chemical activity and fine, gentle abrasion produces, scratch and deformation-free specimens. Suited for soft and ductile materials, aluminium, copper/brass, lead-based alloys and also ceramic type materials. They can be used with common laps and pads, but Polyurethane and short nap cloths are recommended.
In addition to conventional mechanical polishing, two common techniques are available - Vibratory Polishing and Electro-Polishing.
Pressure Polishing Systems
The procedure outlined above is a manual method of grinding/polishing. Grinders and Polishers can now be converted to carry out semi-automatic specimen preparation, by means of a Microprocessor Controlled Power Head.
The main advantages are, increase productivity with increased consistency. They can be programmed to control the following functions:
- Specimen polishing time.
- Head direction
- Wheel direction
- Fluid dispensing
The parameters for a large number of specimens can be stored in the memory.
How to polish Metallographic specimens using Lubricating fluid and Diamond Products?
In general there is no definite method for polishing metallographic samples. It is best to practice or have some basic training. Each sample may require a different approach depending on the material, size, finish required and machinery available. In general, a metallographic sample is initially ground on a Wet / Dry Met Papers or Bramet to 600 Grit or finer, then cleaned.
Kemet 6-CM Diamond Compound is applied to a suitable cloth such as PSU-M. Apply Kemet GW2 lubrication to make the cloth damp. During polishing, apply small amounts of GW2 if the cloth starts to dry out. Also apply small quantity of 6-CM Diamond Compound to replenish the cutting action. On large plates / cloths or machines with automated dispensing equipment, use Kemet 6-WP Diamond Suspension. Because this is already a fluid, less GW2 lubrication is required. The Metallographic sample is then cleaned.
After cleaning, apply Kemet 1-CM Diamond Compound or Kemet 1-WP Diamond Suspension to a new cloth such as PSU-M or NHM cloth. During polishing, apply small amounts of GW2 if the cloth starts to dry out. Experiment to determine the best cloth for the sample. The sample should now be ready for examination or etching. If a better finish is required, finer grades of Kemet Diamond Compound and Diamond Suspension are available.