Study on lapping performance of agglomerated diamond abrasive

[OBJECTIVES] Grinding is one of the ultraprecision machining methods for efficiently thinning and flattening hard and brittle materials such as sapphire. However, traditional grinding processes cannot meet the requirements for high material removal rates and high surface quality simultaneously. Exploring the use of aggregated diamond abrasives and their corresponding processing methods is beneficial for achieving efficient and stable high-quality grinding of hard and brittle materials.

[METHODS] A novel process was proposed, using ceramic binders and fine diamond abrasives (grain size 3 μm) sintered to form aggregated diamond abrasives (average grain size 30 μm) for grinding purposes. Comparative grinding experiments were conducted on sapphire substrates using the prepared aggregated diamond abrasives and single-crystal diamond abrasives with grain sizes of 3 μm and 30 μm. The grinding performance of the aggregated diamond abrasives was systematically investigated, and a material removal model was established to further reveal the material removal mechanism during processing with these abrasives.

[RESULTS] (1) Aggregated diamond abrasives exhibited a higher material removal rate. Under the same conditions, grinding with aggregated diamond abrasives for 15 minutes achieved a material removal rate of 1.127 μm/min, an 89.1% increase compared to using 3 μm single-crystal diamond abrasives. (2) Aggregated diamond abrasives demonstrated superior processing stability. Over a 120-minute processing cycle, their material removal rate showed the least reduction, with a rate of 0.483 μm/min after 120 minutes, marking a 57.14% decrease from the rate at 15 minutes. In comparison, the material removal rates of 3 μm and 30 μm single-crystal diamond abrasives decreased by 78.02% and 71.2%, respectively.  (3) Aggregated diamond abrasives resulted in better surface quality. The lowest surface roughness Ra after grinding with aggregated diamond abrasives and 3 μm single-crystal diamond abrasives were 9.45 nm and 8.75 nm, respectively, while using 30 μm single-crystal diamond abrasives yielded a lowest Ra of 246 nm. (4) The wear and removal patterns of aggregated abrasives during processing differed from those of single-crystal diamond abrasives. The latter’s wear was through abrasion dulling, characterized by chipping, flattening, and abrasive wear of the cutting edges. In contrast, the aggregated diamond abrasives underwent micro-fracturing, characterized by the shedding of micro-fine single-crystals from the abrasive surface and the binder network disintegration. Statistical analysis of the particle size distribution of the grinding fluid during the grinding process revealed  that after 30 minutes of grinding, the particle size distribution curve of aggregated diamond abrasives shifted to the left, with a 51.5% decrease in peak volume fraction. The change in the abrasive particle size distribution curve was relatively small. At the same time, a trapezoidal peak composed of abrasion debris and detached micro-fine single-crystal abrasives formed on the left side of the curve. In contrast, a significant leftward shift was observed in the curve for 30 μm single-crystal diamond abrasives, with the peak volume fraction decreasing by 82.8%. The change in the abrasive particle size distribution curve was relatively large, forming a low peak wave on the left side of the curve. Aggregated diamond abrasives employed a multi-edge cutting method, primarily relying on multiple micro-fine single-crystal diamond grains on the surface to remove material from the workpiece. In contrast, single-crystal diamond abrasives engaged in single-edge cutting, mainly relying on the edges and corners of the abrasive for material removal.

[CONCLUSION] During the grinding process, aggregated diamond abrasives remove material from the workpiece surface through the combined action of multiple micro-fine single-crystal diamond grains on the surface layer. This ensures consistency in cutting depth and enhances surface quality. Under the impact and compression between the workpiece and the grinding disc, the aggregated diamond grains undergo abrasive wear and micro-fracturing, exposing the micro-fine diamond grains encapsulated within the binder and achievi ng cutting edge renewal and self-sharpening. This leads to higher efficiency and more stable processing capability. Therefore, the multi-edge cutting and micro-fracturing characteristics of aggregated diamond abrasives enable the efficient, stable, and high-quality grinding of sapphire substrates.