Lapping and Polishing

Lapping and polishing are used to produce smooth or ultrasmooth surfaces (i.e., surfaces with an R of a few nanometers).

They are carried out by placing a slurry of abrasive particles in a liquid vehicle between the specimen and a hard metallic block (lapping) or a soft, flexible pad (polishing). While the block is loaded against the workpiece (specimen) either hydraulically or mechanically, the abrasive particles roll or slide across the specimen so that the wear process is one of three-body abrasion (Ref 20). As in grinding, the abrasive particles have a wide range of sizes, with an average value typically between of 0.05 to 70 ,i,!m. The mean sliding velocity between the abrasive and the specimen in lapping or polishing is typically no greater than 0.5 m/s, which is two orders of magnitude less than that in grinding.

At present there are no reliable calculations of lapping and polishing temperatures. The maximum temperature rise at the abrasive-work interface is thought to be small due to the relatively low sliding velocity between the contacting surfaces, and several arguments support this hypothesis. Recent calculations of the distribution of forces on diamond abrasive particles during the lapping and polishing of ceramics show that the average load applied to a particle is no greater than 1 N under typical conditions (Ref 20). If this load is assumed to act on a particle sliding at a velocity less than 0.5 m/s, a heat partition and heat-transfer analysis carried out along the lines of the grinding calculations described earlier shows that the maximum temperature rise at the work surface is no greater than 100 °C (212 °F) for a friction coefficient of /£= 0.1 between the abrasive and work surface. Even though such an analysis might not be completely rigorous for lapping and polishing, the error in the calculation of the temperature rise is not expected to be significant. Further evidence to support the low temperature rises comes from observations pertaining to residual stresses and microcracking. Lapped or polished surfaces of metals and ceramics are generally found to contain compressive residual stresses due to localized plastic deformation resulting from an indentation/microcutting action of the abrasive particles (Ref 6, 8). Thermally induced residual stresses, in contrast, are tensile in nature; see the previous discussion pertaining to grinding. Metallurgical studies of the structure of lapped or polished steel surfaces show little evidence of structural changes commonly associated with localized high temperatures (e.g., no indication of phase transformations or burn in lapped surfaces of hardened steels). Also, there is very little microcracking to be seen on lapped or polished surfaces of ceramics (Ref 6, 8). Such microcracking, if present, is usually a strong indicator of the occurrence of steep temperature gradients near the surface that cause high, transient stresses.

0 0

Post a comment