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Grinding

Grinding is a most important operation in specimen preparation. During grinding the operator has the opportunity of minimizing mechanical surface damage that must be removed by subsequent polishing operations. Even if sectioning is done in a careless manner, resulting is severe surface damage, the damage can be elimenated by prolonged grinding. However, prolonged polishing will do little toward eliminating severe surface damage introduced by grinding.

Grinding is accomplished by abrading the specimen surface through a sequence of operations using progressively finer abrasive grit. Grit sizes from 40 mesh through 150 mesh are usually regarded as coarse abrasives and grit sizes from 180 mesh through 600 mesh as fine abrasives.

Grinding should commence with coarse grit size that will establish an initial flat surface and remove the effects of sectioning within a few minutes. An abrasive grit size 150 or 180 mesh is coarse enough to use on specimen surfaces sectioned by an abrasive cutoff wheels. Hacksawed, band sawed or other rough surfaces usually require abrasive git sizes in the range 80 to 150 mesh. The abrasive used for each succeeding grinding operation should be one or two grit size smaller than that used in the preceeding operation. A satisfactory grinding sequence might involve grit sizes of 180, 240, 400 and 600 mesh.

As in abrasive-wheel sectioning, all grinding should be done wet, provided water has no adverse effects on any constituents of the microstructure. Wet grinding minimizes loading of the abrasive with metal removed from the specimen being prepared. Water flushes away most of the surface removal products before they become embedded between adjacent abrasive particles. Thus the sharp edges of the abrasive particle remain exposed to the surface of the specimen throughout the operation. If the sharp edges are unexposed the result is smearing of the abraded surface rather then removal of surface metal. The operator must determine, by examining the specimen throughout the sequence of grinding steps, that the abrasive is actually cutting and not merely smearing or burnishing. Burnishing results primarily from using an abrasive beyond its effective limit. Use of worn-out abrasives and dulled cutting edges is determental to good preparation.

Another advantage of the wet grinding is the cooling effect of the water. Considerable frictional heat can develop at the surface of a specimen during grinding and can cuse alterations of the true microstructure - for example, tempering of martensite in steel - that cannot be removed during polishing. Wet grinding provides effective control of overheating. The abraded surface of a specimen may become embedded with loose abrasive particles during grinding. These particles may persist in the surface and appear to be nonmetallic inclusions in the polished specimen. The flushing action of the water removes many of loose particles that might otherwise become embedded. Some laboratories have found that dressing the abrasive material with a solid wax lubricant recommended for grinding and other machining operations can minimize the embedding of abrasive particles.

The purpose of grinding is to lessen the depth of deformed metal to the point where the last vestiges of damage can be removed by series of polishing steps. The scracth depth and the depth of cold worked metal underneath the scratches decrease with decreasing particle size of abrasive. However the depth of cold worked metal is roughly inversely proportional to the hardness of the specimen and may be 10 to 50 times the depth of penetration of the abrasive particle. It is imperative that each grinding steps completely remove the deformed metal produced by the preivious step. The operator usually can assume this is accomplished if he or she grinds more than twice as long as the time required to remove the scratches incurred by the previous step. To ensure the complete elimination of the previous grinding scratches found by visual inspection, the direction of grinding must be changed 45 to 90 degrees between succesive grit sizes. In addition, microscopic examination of the various ground surfaces during the grinding squence may be worthwhile in evaluating the effect of grinding. Each ground surface should have scratches that are clean-cut and uniform in size, with no evidence of previous grinding scratches.

Success in grinding depends in part on the pressure applied to the specimen. A very light pressure removes insufficient metal. Somewhat heavier pressure produce polishing, while still heavier pressure brings about the desired grinding action. Very heavy pressure results in nonuniform scratch size, deep gouges, and embedded abrasive particles. Generally, a medium to moderately heavy pressure applied firmly gives the best results.

Most grinding of metallographic specimen is performed by manually holding the specimen with its surface against a grinding material. To establish and maintain a flat surface over the entire area being ground, the operator must apply equal pressure on both sides of the specimen and avoid any rocking motion that will produce a convex surface. If grinding operation is interrupted - the operator must re-establish contact with grinding material carefully in order to resume grinding in the plane already established.

Specimens should be cleaned after each grinding steps to avoid any carryover of abrasive particles to the next step. Water solutions containing detergents are excellent cleaners and ultrasonic cleaning is an effective technique. Cleanness of the operator's hands is as important as cleanness of specimen. Contamination of the grinding equipment by flying abrasive particles must be avoided.

Grinding Mediums

The grinding abrasives commonly used in the preparation of specimens are silicon carbide (SiC), aluminium oxide (Al2O3), emery (Al2O3 - Fe3O4), diamond particles, etc. Usually are generally bonded to paper or cloth backing material of various weights in the form of sheets, disks and belts of various sizes. Limited use is made of grinding wheels consisting of abrasives embedded in a bonding material. The abrasive may be used also in powder form by charging the grinding surfaces with loose abrasive particles.

Silicon carbide has a hardness of 9.5 on the Mohs scale, which is near the hardness of diamond. Silicon carbide abrasive particles are angular and jagged in shape and have very sharp edges and corners. Because of these characteristics, silicon carbide is very effective grinding abrasive and is preferred to other abrasives for metallographic grinding of almost all types of metal.

Aluminium oxide abrasive material has a trigonal crystal structure and a hardness of 9.1 on the Mohs scale and is a synthetic corrundom.

Emery is an impure, fine-grained variety of natural corundum containing 25 to 45 admixed iron oxide. The hardness of emery is Mohs 8.0. Emery abrasive particles have much smoother surfcaes than either silicon carbide or aluminium oxide abrasive paerticles. For this reason, emery particles do not feel as coarse as silicon carbide or aluminium oxide particles of equivalent grit size and the cutting rate of emery is inferior to that of either of the two other abrasives.

Another abrasive material used occasionally for grinding specimens is boron carbide, which has a hardness of nearly 10 on Mohs scale. Boron carbide is used primarily for grinding ceramic and other extremely hard materials.

Increasing use is being made of diamond as grinding madium as well as polishing medium. Carefully sized diamond abrasive particles are available from 280 microns (about 60 mesh) to 0.25 microns in size. The coaser grades of diamond are used in the form of resin-bonded cloth-backed disks, metal bonded lapping surfaces, and loose particles for charging of grinding surfaces. Diamond abrasives of all sizes are available as suspensions in oil-soluble and water-soluble paste vechicles known as diamond compounds. The extreme hardness (Mosh 10) and sharp cutting edges of diamond particles impart at high cutting rate to diamond abrasives. Diamond abrasives are particularly suitable for grinding the harder alloys and refractory materials.

Hand Grinding

Manual Preparation - In order to insure that the previous rough grinding damage is removed when grinding by hand, the specimen should be rotated 90 degrees and continually ground until all the scratches from the previous grinding direction are removed. If necessary the abrasive paper can be replace with a newer paper to increase cutting rates.

A simple setup for hand grinding can be provided by a piece of plate glass, or other material with hard, flat surface, on which an abrasive sheet rests. The specimen is held by hand against the abrasive sheet as the operator moves specimen in rhytmic style away from and toward him in a straight line. Heavier pressure should be applied on the forward stroke than on the return stroke. The grinding can be done wet by sloping the plate glass surface toward the operator and providing a copious flow of water over the abrasive sheet.

Planar Grinding - or course grinding is required to planarize the specimen and to reduce the damage created by sectioning. The planar grinding step is accomplished by decreasing the abrasive grit/ particle size sequentially to obtain surface finishes that are ready for polishing. Care must be taken to avoid being too abrasive in this step, and actually creating greater specimen damage than produced during cutting (this is especially true for very brittle materials such as silicon).

The machine parameters which effect the preparation of metallographic specimens includes: grinding/polishing pressure, relative velocity distribution, and the direction of grinding/polishing.

Grinding Pressure - Grinding/polishing pressure is dependent upon the applied force (pounds or Newtons) and the area of the specimen and mounting material. Pressure is defined as the Force/Area (psi, N/m2 or Pa). For specimens significantly harder than the mounting compound, pressure is better defined as the force divided by the specimen surface area. Thus, for larger hard specimens higher grinding/polishing pressures increase stock removal rates, however higher pressure also increases the amount of surface and subsurface damage. Note for SiC grinding papers, as the abrasive grains dull and cut rates decrease, increasing grinding pressures can extend the life of the SiC paper.

Higher grinding/polishing pressures can also generate additional frictional heat which may actually be beneficial for the chemical mechanical polishing (CMP) of ceramics, minerals and composites. Likewise for extremely friable specimens such as nodular cast iron, higher pressures and lower relative velocity distributions can aid in retaining inclusions and secondary phases.

Relative Velocity - Current grinding/polishing machines are designed with the specimens mounted in a disk holder and machined on a disk platen surface. This disk on disk rotation allows for a variable velocity distribution depending upon the head speed relative to the base speed.

For high stock removal, a slower head speed relative to a higher base speed produces the most aggressive grinding/ polishing operation. The drawback to high velocity distributions is that the abrasive (especially SiC papers) may not breakdown uniformly, this can result in non-uniform removal across the specimen surface. Another disadvantage is that the high velocity distributions can create substantially more specimen damage, especially in brittle phases. In all cases, it is not recommended to have the head rotating contra direction to the base because of the non-uniform removal and abrasive break-down which occurs.

Minimal relative velocity distributions can be obtained by rotating the head specimen disk at the same rpm and same direction as the base platen. This condition is best for retaining inclusions and brittle phases as well as for obtaining a uniform finish across the entire specimen. The disadvantage to low relative velocity distributions is that stock removal rates can be quite low.

In practice, a combination of a high velocity distribution (150 rpm head speed/300 - 600 rpm base speed) for the initial planarization or stock removal step, followed by a moderate speed and low velocity distribution (120-150 rpm head speed/150 rpm base speed) step is recommended for producing relatively flat specimens. For final polishing under chemical mechanical polishing (CMP) conditions where frictional heat can enhance the chemical process, high speeds and high relative velocity distributions can be useful as long as brittle phases are not present (e.g. monolithic ceramics such as silicon nitride and alumina).

Grinding Direction - The orientation of the specimen can have a significant impact on the preparation results, especially for specimens with coatings. In general, when grinding and polishing materials with coatings the brittle component should be kept in compression. In other words, for brittle coatings the direction of the abrasive should be through the coating and into the substrate. Conversely, for brittle substrates with ductile coatings, the direction of the abrasive should be through the brittle substrate into the ductile coating.

Belt, Disk and Surface Grinders

The most common types of motor-driven grinding equipment are the belt grinder and the disk grinder. In using either, the metallographic specimen is held by hand against a moving, fixed-abrasive grinding material supported by a platen. Belt grinders and disk grinders may be used in either a horizontal or vertical position. Abrasive belts are generally cloth-backed for strength, and the popular belt sizes are 4 by 36 in. and 4 by 54 in. Although cloth-backed disks are available, paper-backed disk are preferred for disk grinding of metallographic specimens, because paper-backed disks rest flat against the platen whereas cloth-backed disks usually curl in form the edge. Most metallographic grinding disks are 8 or 10 in. in diameter. Specimens can be belt or disk ground successfully through all grinding sequences.

Lapping

Is a grinding technique similar to disk grinding. The grinding surface (lap) is a rotating disk whose working surface is charged with a small amount of a hard abrasive material. Laps are made of wood, lead, nylon, paraffin, paper, leather, cast iron and laminated plastics. The abrasive charge may be pressed into lap material by means of a steel block, or the lap may be charged directly with a mixture of abrasive and destilled water during lapping. A groove in the form of a spiral is a direction counter to the lap rotation is often cut in the surface of laps, particularly of lead and paraffin laps. The spiral groove aids retention of cooling water and abrasive.

Automatic grinding

As the name implies, is done without hand assistance. All automatic grinding devices use lap surfaces on which paper-backed disks are placed or abrasive pawder is charged. The lap is either a rotating or a vibarating disk. Use of a latter is described as vibratory grinding. The technique and equipment for automatic grinding are analogous to those described uder Automatic Polishing.

The key to successful automated preparation is to thoroughly clean the specimens between each abrasive grit size used. The tracking of the specimens should also uniformly break down the SiC paper, otherwise non-uniform grinding will occur (especially for hard specimens in soft mounts). In other words, the specimen should track across the entire diameter of the SiC paper.