Hard and brittle materials (monocrystalline silicon, sapphire, and optical glass etc.) are widely used in aerospace, semiconductors, optoelectronics, microelectromechanical systems owing to their excellent physicochemical properties such as resistance to abrasion, corrosion, and high temperature [1, 2]. Mechanical drilling micropore in hard and brittle materials is a great challenge. For example, micro holes (Diameter: 0.4–0.8 mm, Depth: 10 mm) were drilled in a monocrystalline silicon electrode plates used in plasma reaction chambers [3]. The machinability of these materials is severely constrained by their inherent high hardness and brittleness characteristics. Most of the tools used for drilling processes are made of tungsten carbide or high-speed steel, which are obviously not hard enough to process hard brittle materials [4]. Moreover, cracking and chipping are easy to occur during the cutting process [5]. Nowadays, more and more tools are made of polycrystalline diamond (PCD) materials.
For micro drilling hard and brittle materials, PCD micro drill is a favored tool with high hardness, high thermal conductivity, high impact, and wear resistance properties [6]. Upon considering the high hardness of the Nd: YAG material, Zeng et al. [7] has used a PCD micro drill with a diameter of 0.5 mm for drilling. The main types of micro drills are twist drill [8], single-lip drill [9], spade drill [10] and D-shape drill [11]. There are difficulties in manufacturing PCD microdrills using conventional techniques such as grinding and electro discharge machining (EDM). The grinding efficiency of PCD is extremely low, and the grinding ratio is about 0.005–0.033 due to the high hardness and brittleness [12]. The ratio of the amount of material removed from the workpiece to the amount of wear on the grinding wheel is defined as the grinding ratio. The grinding ratio of PCD material is only one thousandth to one hundredth of that of cemented carbide. Grinding process could cause severe cracks on the cutting edge and even breakage of the tool. Oliaei [13], Lee [14], Morgan [15], Ohnishi [16] et al. have successfully manufactured PCD drills with simple geometric features by EDM with a brass wire. However, the processing efficiency of EDM is low, too. Both machining methods are difficult to obtain micro twist drill with complex features. To reduce the technical difficulty of manufacturing PCD micro drills and improve the processing efficiency, a method using ultrashort pulsed laser roughing and grinding finishing is presented.
Currently, ultrashort pulse laser (ULPL) providing pulse durations in the femtoseconds (1 fs = 10− 15 s) and picoseconds (1 ps = 10− 12 s) range are commercially available and are gaining more and more attention in the area of precise micro structuring [17]. Ultrashort pulse lasers are now widely used in fields such as metrology, communications, spectroscopy, and materials processing [18]. ULPL can effectively predict laser ablation by suppressing the heat affected zone [19]. Eberle et al. [20] proved that using picosecond pulse laser processing for PCD processing has no significant thermal impact and can achieve higher ablation rates and lower surface roughness. A picosecond pulsed laser with average power 35 W was utilized for the machining of PCD twist drills with a diameter of 2 mm by Warhanek et al. [21]. The drill obtained by laser machining had the longest tool life and the lowest wear compared to CVD-coated carbide drill and drill obtained by EDM methods. Increasingly precise industrial products are driving the trend toward smaller diameters for microdrills. Eberle et al. [22] used a picosecond pulsed laser to machine a PCD twist drill with a diameter of 0.8 mm. The researchers of PCD micro drills are mainly specialized micro ultrahard tool manufacturers such as Karnasch Professional Tools, 6C Tools and Shenzhen Muji Technology Co., Ltd. Smaller diameter PCD micro drills manufacturing is a challenging work because it requires higher positioning accuracy and machining stability performance of machine tools. Also, few people pay attention to the generation of surface integrity of PCD micro drills.
In this paper, an approach of picosecond pulsed laser roughing and grinding finishing for PCD micro drilling was proposed. An industrial picosecond laser source with wavelength of 1064nm and pulse duration less than 10 picoseconds was used. A graphical user interface (GUI) and a computer-aided manufacturing (CAM) modules have been developed. A double helical fluted twist PCD micro drill with a diameter of 0.75 mm was successfully fabricated. The ablation removal mechanism and surface integrity generation mechanism of PCD materials processed by picosecond pulsed laser were analyzed. The feasibility of the approach as an alternative to conventional machining methods was demonstrated. Significant reductions in manufacturing time were achieved. In the future, PCD micro drills with smaller diameters will be manufactured using the approach and laser finishing will be realized.