GRINDING SUPPLIES FOR SAMPLE PREPARATION
The purpose of the grinding step is to remove damage from cutting, planarize the specimen(s), and to remove material approaching the area of interest.
The most common metallographic abrasive used is Silicon Carbide – SiC. It is an ideal abrasive for grinding because of its hardness and sharp edges. For metallographic preparation, SiC abrasives are used in coated abrasive grinding papers ranging from very coarse 60 grit to very fine 1200 grit sizes. Some of the application procedures are given below.
• Soft non-ferrous metals - Initial grinding is recommended with 320 grit SiC abrasive paper followed by 400, 600, 800 and 1200 grit SiC paper. Because these materials are relatively soft they do not easily break down the SiC paper. Thus initial grinding with 320 grit is generally sufficient for minimizing initial deformation and yet maintain adequate removal rates. For extremely soft materials such as tin, lead, and zinc it is also recommended that the abrasive paper be lightly coated with paraffin wax. The wax reduces the tendency of the SiC abrasive to embed into the soft specimen.
• Soft ferrous metals - are relatively easy to grind with the depth of deformation being a major consideration. 240 grit SiC abrasives provide a good initial start with subsequent use of 320, 400, 600, 800 and 1200 grit SiC.
• Hard ferrous metals - require more aggressive abrasives to achieve adequate material removal. Thus coarse SiC abrasives (120 or 180 grit) are recommended for stock removal requirements. Once planarity and the area of interest are obtained a standard 240, 320, 400 and 600 grit series is recommended.
• Superalloys - are generally of moderate hardness but have extremely stable elevated temperature characteristics and corrosion resistance. The procedures for preparing superalloys are very similar to that for most non-ferrous metals.
• Ceramics - are extremely hard, corrosion-resistant and brittle materials. They fracture producing both surface and subsurface damage. Proper grinding minimizes both of these forms of damage. This requires the application of a semi-fixed abrasive which are held rigidly for grinding but can be dislodged under high stress in order to minimize subsurface damage. The abrasive size is also important because very coarse abrasives will remove material quickly but can seriously damage the specimen. For ceramics, consideration of the damage produced at each preparation step is critical to minimizing the overall preparation sequence.
• Composites - are perhaps the most difficult specimens to prepare because of the wide range of properties for the materials used. For example, a metal matrix composite (MMC) such as silicon carbide ceramic particles in an aluminum metal matrix is a difficult specimen to prepare. This composite contains extremely hard/brittle ceramic particles dispersed in a relatively soft/ductile metal matrix. As a rule of thumb, initial grinding should