Grinding is an essential operation in the preparation of metallographic specimens. It involves abrading the surface of the specimen through a series of operations using progressively finer abrasive grit. The objective of grinding is to minimize mechanical surface damage that must be removed by subsequent polishing operations. In this article, we will discuss the importance of grinding, the types of abrasives used, and the proper techniques for effective grinding.
Importance of Grinding
Grinding allows the operator to minimize mechanical surface damage that must be removed by subsequent polishing operations. Even if sectioning is done in a careless manner resulting in severe surface damage, the damage can be eliminated by prolonged grinding. However, prolonged polishing will do little toward eliminating severe surface damage introduced by grinding. Thus, it is important to follow a proper grinding sequence to achieve the desired results.
Types of Abrasives Used
The abrasive grit used for grinding is usually categorized by its size. Grit sizes from 40 mesh through 150 mesh are considered coarse abrasives, while grit sizes from 180 mesh through 600 mesh are fine abrasives. Silicon carbide (SiC), aluminum oxide (Al2O3), emery (Al2O3 -Fe3O4), diamond particles are some of the commonly used abrasives in specimen preparation.
Silicon carbide is preferred over other abrasives for metallographic grinding of almost all types of metal. It has a hardness of 9.5 on the Mohs scale, near that of diamond, and has very sharp edges and corners, making it very effective for grinding.
Aluminum oxide, a synthetic corundum with a hardness of 9.1 on the Mohs scale, is also a commonly used abrasive. Emery, an impure, fine-grained variety of natural corundum, is another abrasive used in specimen preparation.
Proper Grinding Techniques
Grinding should commence with a 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 wheel. Rough surfaces such as those produced by hacksawing, bandsawing, or other methods usually require abrasive grit sizes in the range of 80 to 150 mesh.
The abrasive used for each succeeding grinding operation should be one or two grit sizes smaller than that used in the preceding operation. A satisfactory grinding sequence might involve grit sizes of 180, 240, 400, and 600 meshes. 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.
The operator should apply medium to moderately heavy pressure firmly to achieve the best results. Most grinding of metallographic specimens 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.
To ensure the complete elimination of previous grinding scratches, the direction of grinding must be changed 45 to 90 degrees between successive grit sizes. Additionally, microscopic examination of the various ground surfaces during the grinding sequence may be worthwhile in evaluating the effect of grinding.
Understanding Abrasive Materials Used for Grinding and Polishing
Grinding and polishing are essential processes for preparing samples in various industries, including metallurgy, electronics, and biology. Abrasive materials are used to grind and polish these samples to obtain a smooth and flat surface. In this article, we will discuss the different abrasive materials used for grinding and polishing, including boron carbide and diamond.
Boron Carbide
Boron carbide is a commonly used abrasive material for grinding ceramics and other extremely hard materials. It has a hardness of almost 10 on the Mohs scale, making it an excellent material for grinding hard samples. However, it is used less frequently than other abrasive materials due to its high cost.
Diamond Abrasives
Diamond abrasives have become increasingly popular in recent years due to their high hardness and sharp cutting edges. Carefully sized diamond abrasive particles are available in various sizes, ranging from 280 microns to 0.25 microns. Coarser 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 are available as suspensions in oil-soluble and water-soluble paste vehicles known as diamond compounds. The extreme hardness and sharp cutting edges of diamond particles make them ideal for grinding harder alloys and refractory materials.
Advantages of Diamond Abrasives
One of the significant advantages of diamond abrasives is that they offer a high cutting rate, which means that samples can be ground and polished quickly. They also produce a consistent surface finish and have a long lifespan compared to other abrasive materials. Diamond abrasives are suitable for grinding and polishing samples of different sizes and shapes, making them a versatile choice for various industries.
Grinding is a crucial step in the preparation of metallographic specimens. It minimizes mechanical surface damage that must be removed by subsequent polishing operations. The proper selection of abrasives and the application of appropriate grinding techniques are critical to achieving the desired results. By following a proper grinding sequence, the operator can produce specimens with clean-cut, uniform scratches and minimal surface damage.
Hand Grinding
Metallography, the study of the microstructure of metals and alloys, requires accurate specimen preparation for examination under a microscope. The preparation process involves several steps, including cutting, mounting, grinding, polishing, and etching. In this article, we will focus on the manual and planar grinding steps of the process.
Manual Grinding
Manual grinding is the first step after mounting the specimen. Its purpose is to remove any previous damage created during cutting and to prepare the surface for further grinding and polishing. To achieve this, the specimen should be rotated 90 or 45 degrees and ground continuously until all scratches from the previous grinding direction are removed. If necessary, the abrasive paper can be replaced with a newer one to increase cutting rates.
A simple setup for manual grinding can be provided by a piece of plate glass or other hard, flat surface, on which an abrasive sheet rests. The operator holds the specimen by hand against the abrasive sheet and moves it in a rhythmic 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
Planar 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, especially for very brittle materials such as silicon.
The machine parameters that affect the preparation of metallographic specimens include 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 that 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.
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.
Automated Grinding: A Guide to Efficient Preparation of Materials
Grinding is an essential step in the preparation of materials for analysis, testing, or other applications. However, the manual process can be time-consuming and inconsistent. Automated grinding is an efficient way to achieve uniformity and precision in the grinding process. In this article, we’ll explore the different types of automatic grinding devices and techniques for successful preparation.
Types of Automatic Grinding Devices
Automatic grinding devices use lap surfaces where paper-backed disks are placed or abrasive powder is charged. The lap can be either a rotating or vibrating disk. Vibratory grinding is a common method used in automated preparation. The key to successful automated preparation is to clean the specimens thoroughly between each abrasive grit size used.
Coarse Grinding
Grinding can be achieved in a variety of ways, using a variety of abrasives. Fixed abrasive surfaces are available using diamond or cubic boron nitride (CBN) abrasives. The method used to bind the abrasives to the wheel affects the grinding characteristics. The harder or more rigid the bonding medium, the more aggressive the grinding action of the surface. Therefore, metal bonded fixed abrasive wheels are the most aggressive grinding surfaces, whereas resin bonded wheels are less aggressive.
Coarse grit Silicon Carbide (SiC) or Alumina abrasives may be used, but the durability or characteristics of such materials may be inappropriate for certain materials. Generally, grinding papers need frequent changing to maintain sharp abrasive particles. It is important to follow the manufacturer’s recommendations and advice.
Fine Grinding
SiC paper is the traditional method used for fine grinding and is adequate when used properly. However, SiC paper blunts quickly and should be discarded after a short period of grinding to maintain efficient ‘stock’ removal. Grinding on a surface that has blunt abrasives causes a great deal of surface damage by smearing, ‘burnishing,’ and local heating.
It is important to ensure that sharp abrasives are used and to follow the manufacturer’s instructions with regard to grinding speeds, direction, force, times, and lubricants used. Damage injected during grinding may be invisible in the polished surface. Remember that different materials have different abrasion characteristics. The selection of grinding material and conditions can, therefore, be specific to a given sample.
Final Inspection
After every grinding stage, it is advisable to inspect the ground surface using a light microscope to ensure that all damage from the previous stage, whether that be a cutting or grinding stage, is completely removed. Advance in this manner to the finest abrasive size required, ready for polishing. Care at this stage will greatly reduce the amount of polishing required to achieve a good surface.
Conclusion
Automated grinding is an efficient way to achieve uniformity and precision in the preparation of materials. Cleaning specimens between each abrasive grit size used is key to successful preparation. Following the manufacturer’s recommendations and advice with regard to grinding material, conditions, and lubricants used is crucial for optimal results. Careful inspection of the ground surface after every grinding stage will greatly reduce the amount of polishing required to achieve a good surface.