Temperature effect on the material removal mechanism of soft-brittle crystals at nano/micron scale

Highlights

• Decreasing hardness, modulus and increasing toughness can be obtained for KDP at elevated temperature.

• Temperature-dependent visco-elastic behaviour has been found for KDP for the first time.

• Brittle cracks and micro grits are main deformation forms in nano cutting KDP at room temperature.

• Crystal lattice misalignment and nano crystals contribute to plastic deformation at elevated temperature.

• Critical UCT of KDP can be extended 8.6 times from 0.4 μm at 23 °C to 3.61 μm at 170 °C.

Abstract

Soft-brittle crystals, e.g. KH₂PO₄ (KDP), are difficult-to-cut due to their high brittleness which can easily generate crack during the machining process. The conventional method to machine this kind of material is by inducing ductile cutting mechanism at room temperature with ultra-precision machining, which can only remove materials at nanoscale level and hence yields very low material removal rate. While some thermal-assisted processes have been recently attempted to improve the machinability of some difficult-to-cut materials, e.g. ceramics, there is no systematic understanding of the temperature effect on material removal mechanism of soft-brittle KDP crystals yet. In this work, the temperature effect on the material removal mechanism has been investigated for the first time using nano-scratch technique. While a decreased hardness and elastic modulus have been observed with the increase of temperature, an increase of fracture toughness has been revealed with a contradictory tendency, indicating a higher capacity of plastic deformation at elevated temperature. In contrast to the almost totally brittle scratch at room temperature (RT) caused by crack propagation and edge chipping, the scratch at 170 °C can achieve more ductile-regime surfaces with a larger critical undeformed cutting depth (3.61 μm), e.g. a significant increase of 8.60 times compared with that at RT (0.42 μm). Moreover, the TEM analysis on the subsurface microstructures shows that a great number of nano grits was generated in the subsurface at RT as the result of crack propagation and interaction, while at elevated temperature some crystallographic lattice misaligned structures (LMS) and nano crystals have been brought about due to the nucleation and evolution of thermal-activated dislocations, which explains the higher plasticity of KDP at elevated temperature. The results present in this paper are of great significance for understanding the specific temperature effect on the brittle-to-ductile transition of the cutting mechanism for future designing thermal-involved processes to machine soft-brittle materials.

Introduction

Soft-brittle crystals, like potassium dihydrogen phosphate (KH₂PO₄/KDP) and calcium fluoride (CaF₂), are widely applied in the laser-driven Inertial Confinement Fusion (ICF) facilities [1,2]. To achieve the pre-designed optical performance, these soft-brittle crystals must be prepared by ultra-precision machining; nevertheless, while due to their low hardness and low fracture toughness as well as the extreme anisotropy of these mechanical properties on different crystal planes, this type of material is effectively difficult to machine as they could result in insufficient surface integrity. Recently, many advanced machining methods, such as single-point diamond cutting (SPDT) [2] and precision micro-milling [3], have been employed to produce ultra-smooth surfaces (Ra <3 nm) to meet the strict engineering requirements in ICF facilities. These processes bring material removal to nanoscale level through plastic deformation rather than fracture propagation. However, these processes, on one hand, yield very low material removal rate, while on the other hand, micro/nano cracks can be observed on the machined surfaces due to their unique combination of material properties (extremely soft, fragile and anisotropic) [1]; hence, machining of soft-brittle materials is still facing considerable challenges [2].

The ductile cutting mode has been proposed to obtain a crack-free surface in machining of brittle materials, such as YAG crystals [4], in which case, the practical uncut chip thickness (UCT) must be smaller than the critical value of the uncut chip thickness (UCT) for the brittle-to-ductile transition (BDT) [5]. The practical UCT is mainly dependent on the dynamic conditions (e.g. dynamics of machine tools [6]) of machining processes while the critical UCT is always regarded as a theoretical value of BDT which can be calculated by Bifano's model [7]. For KDP crystals, the critical UCT has been revealed as less than 340 nm [8], meaning that only when the practical UCT is below this value can the material be removed in ductile-regime. This nanoscale material removal not only requires a high accuracy of the machine tool but also results, as a side effect, in low machining efficiency which, eventually, results in high manufacturing costs of KDP optics. In the last decades, to improve the machining accuracy and efficiency under the premise of ductile-regime cutting mode, considerable efforts have devoted to achieving higher values of the practical UCT, by investigating its relationship with different machining factors such as the dynamics of machine tools [4], the geometries of cutting tools [2], and the selection of process parameters [3]. However, very limited attention has been paid to the value of critical UCT, which is normally considered as a fixed value to evaluate the BDT. In fact, according to Bifano's model [7], the critical UCT is mainly determined by the mechanical properties (e.g. hardness, elastic modulus, and fracture toughness) of machined materials while these mechanical properties are closely linked with the temperature. For instance, the hardness and elastic modulus of reaction-bonded silicon carbide (RB-SiC) could decrease by 46.1% and 41.8%, respectively, at 1200 °C when compared with the values at room temperature (RT) [9]. Therefore, the temperature is supposed to have an underlying influence on the critical UCT (e.g. BDT of cutting modes) by impacting the mechanical properties. If this is the case, it would induce a significant change in the material removal mechanism for KDP crystals, which will further bring about a fundamental evolution of cutting methods and, eventually, improve the machinability and machining efficiency of this group of soft-brittle materials.

To understand the properties of KDP crystals, the mechanical behaviours under indentation have been studied where the average hardness of the (001) and (100) planes of KDP crystals has been revealed as 2.0 and 1.6 GPa, respectively [10]. The corresponding Mohs hardness of KDP is 2.5, as small as that of aluminum, which leads to the difficulty of grinding this material as the abrasive particles can easily be embedded into the soft workpiece surfaces [11]. Meanwhile, by conducting nano tests, Guin identified two slip systems in KDP crystals, the first of which is on the planes of (110), (101), (112) and (123) with a common Burgers vector of 1/2<111> while the other one is (010) <100>, contributing to the understanding the plastic deformation of brittle KDP crystals [12]. More interesting, an obvious Indentation Size Effect (ISE) for KDP crystals has been reported in Fang's work [13], where the Vickers hardness on (001) planes has decreased nearly 30% when the indentation load increases from 0.029 N (H_v = 2.0 Gpa) to 1.96 N (H_v = 1.41 GPa). This phenomenon clearly indicates that the mechanical behaviour of KDP crystals at nano/micro scale is quite different from that at macro scale. Furthermore, some sudden displacement bursts (e.g. pop-in events) with an average displacement drift of about 8 nm were found in the loading curves when referring to Borc's Berkovich indentations [14], which were attributed to the plastic deformation induced at small indentation loads. Nevertheless, all these aforementioned studies were carried out at room temperature with a conventional indentation technique while temperature effect has not been taken into consideration. To authors best knowledge, up to now the only research that reported the temperature-dependent mechanical responses for single-crystal KDP was focused on the material strength for evaluating its feasibility application in high-temperature environments [15]; nevertheless, the specific material properties, including hardness, elastic modulus and fracture toughness of KDP crystals at elevated temperature and their influence on the cutting mechanism, have not been investigated. Thus, it is of key importance to understand the evolution of aforementioned material properties with the increase of temperature, and to further explore how the temperature can affect the theoretical BDT in nano machining of KDP and the corresponding cutting mechanism.

Furthermore, to achieve cutting in the ductile regime for KDP crystals, scratch tests have been performed [16], which revealed that only under a low scratch load (<2000 μN) can a smooth scratch groove be achieved on (001) plane through plastic material removal, while once the scratch load exceeds this value brittle cracks would take place. It has also been found that the anisotropic crystallographic structure of KDP has a significant influence on the critical UCT of BDT [1], causing that even in the same plane there is a severe variation of UCT along various directions. Moreover, the friction coefficient in scratch processes has also been identified to be closely related to the crystallographic orientation [17]. While previous researches mainly focused on the influence of anisotropy of KDP crystals on the critical UCT with a simple aim to predict whether the material is removed by plastic chips or brittle cracks, very little study reported the material deformation behaviour under the scratch load and how the deformed material is removed in ductile-regime. While the material removal mechanism induced by scratch in some hard-brittle materials like RB-SiC [18], has been revealed recently, the deformation mechanism in soft-brittle material, e.g. KDP crystals, and its resultant on material removal mechanism is still unexplored.

Furthermore, while all the aforementioned scratch tests were performed at RT, no explorations have been carried out to exploit the temperature effect on the mechanical properties to facilitate the material removal as well as the brittle-to-ductile transition when machining soft-brittle materials. Although some thermal-involved processes, such as laser-assisted milling, have been attempted to improve the machinability of some difficult-to-cut metal materials [19,20], these investigations do not explain well the specific function of the temperature effect on the deformation behaviours and the resultant removal mechanism of soft-brittle materials.

In light of this, the present work is aimed at investigating the fundamentals of the temperature effect on the material removal mechanism of soft-brittle KDP crystals at nanoscale level. To achieve this, the evolution of mechanical behaviours (e.g. hardness, elastic modulus, and fracture toughness) with the increase of temperature were firstly investigated by nanoindentation tests. Following this, varied-depth nanoscratch tests were performed at different temperatures to understand the material removal mechanism caused by temperature effect. In particular, the underlying deformation mechanism of KDP crystals induced by temperature effect was revealed for the first time with the transmission electron microscope (TEM), which could provide a valuable understanding about the temperature effect on the BDT of cutting modes for developing appropriate setups/methods for future thermal-assisted machining processes able to improve the machinability and efficiency of soft-brittle materials.