# Ultra-Precision Cutting Mechanism of KDP Crystal in Microplastic Region via Heating Assistance
## 1 Introduction  
KH\({}_{2}\)PO\({}_{4}\) (KDP) crystals, which have interesting photophysical properties, such as piezoelectricity, dielectricity, and electro-optical properties, are optical materials with excellent performance. KDP crystals, which are required in inertial confinement nuclear fusion [1; 2; 3; 4; 5], can be used under a wide transmission band, large nonlinear optical coefficient, and high damage threshold (greater than 15 J/cm\({}^{2}\), 1 ns). In addition, they can grow large-diameter crystals (\(\Phi\)500-600 mm) [6; 7; 8]. In the 1950s, KDP, as a high-performance piezoelectric crystal material, was mainly used in the manufacturing of sonar and civilian piezoelectric transducers. In the 1960s, with the emergence of laser technology, KDP crystals were developed with a large nonlinear optical coefficient and a high laser damage threshold, and the crystals had high transmittance from the near-infrared to ultraviolet bands and a high birefringence coefficient. Thus, they were usually used in the second-, third-, and fourth-harmonic-generation devices of Nd: YAG lasers (at room temperature). At the same time, KDP is a kind of crystal material with a high electro-optic coefficient; thus, it is widely used in electro-optic modulators, Q switching, shutters for high-speed photography, and other components [9; 10]. This crystal shows application prospects in major technologies such as in controlled thermonuclear reactions of high-power laser systems, nuclear explosion simulations, etc. Therefore, research on ultra-large, high-quality KDP optical crystals has attracted much attention from researchers worldwide [11; 12; 13]. Applying KDP crystals in high-power solid-state lasers requires higher levels of surface quality. The surface processing quality of KDP crystals affects the beam quality in laser targeting and pointing accuracy. Recently, many scholars have studied this problem considering the surface quality properties of KDP crystals. Most KDP crystal surfacesare currently processed using ultraprecision single-point diamond fly cutting (UPSDFC) technology [14; 15; 16]. To improve the quality of KDP crystal surfaces, [PERSON] et al. [17] studied the effect of the ultraprecision single-point diamond cutting process of KDP crystals on a PNEUMO precision machine tool. The surface quality was improved to produce a smooth surface, achieving an accuracy of 3.6 \(\upmu\)m by changing the machining parameters and the cutting tool edge's radius. [PERSON] et al. analyzed the influence of processing parameters on the surfaces of KDP crystals through a large number of single-point diamond-turning experiments. Based on the influence of the mechanical properties of the KDP crystals on surface quality properties, the ultrasonic-assisted cutting of KDP crystals was carried out, which improved the surface processing quality properties of the KDP crystals [18]. [PERSON] et al. found that KDP crystals were difficult to machine due to their low fracture resistance, and fracture fringes were easily generated during processing. Based on material performance tests under different temperature fields, the hardness and elastic modulus properties of the KDP crystals decreased by 21.4% and 32.5% at high temperatures (160 \({}^{\circ}\)C), respectively [19]. [PERSON] et al. [20] reported improvements in crystal surface quality when a particular orientation was used to process the KDP crystals. The experimental results showed that the cutting state of the material's surface in the ultraprecision cutting of KDP crystals changed with changes in the factors, such as the back-cutting tool amount, the tool cutting radius, and the crystal direction, which affected the processing quality of the KDP crystal's surface. [PERSON] et al. [21] established a model to simulate surface morphology, which included surface performance indicators, including waviness, roughness, and 3D surface morphology.