Damage evolution and crystalline orientation effects in ultrafast laser micro/nano processing of single-crystal diamond

Single-crystal diamond is a typical difficult-to-machine material because of its extreme hardness and high brittleness. Compared with traditional machining techniques, ultrafast laser is believed to enable materials processing with high efficiency, high quality and high spatial resolution. To understand the formation mechanism and crystalline orientation effects of damage in ultrafast laser processing diamond, single-path scans were performed in different crystal directions, and groove shape, surface morphology, subsurface damage, phase transformation, and laser-induced periodic surface structure (LIPSS) were systematically analyzed. The experimental results illustrate that the damage evolution can be divided into three different stages according to the pulse number, named weak ablation stage, severe cracking stage, and severe phase transformation stage. Crystal orientation significantly affects the groove shapes, cracks, and phase transformation, and these differences are related to the cleavage energy and atomic rearrangement energy of the crystal plane deposited by the laser energy during processing. At low pulse numbers, the surface graphitization is initiated easier along the <1 1 0> direction, which makes the absorption rate of laser energy locally enhanced, further driving the anisotropy of the processing damage at moderate and high pulse numbers. This work provides a new perspective on ultrafast laser processing of single-crystal diamond, which is crucial for ultra-precision and low-damage fabrication of diamond-based functional micro/nano devices.