Surface formation mechanism in ultraprecision diamond turning of coarse-grained polycrystalline ZnSe

Highlights

• Two kinds of surface defects occur due to the plowing and tearing effects of a tool.

• A large nose radius tool with a zero rake angle is effective for complete ductile machining.

• Grain boundary step forms when a tool passes cross a twin boundary at a large angle.

• Cutting chips undergo a zinc-blende to cinnabar phase transformation and metallization.

• A smooth surface of 1.5 nm Sa without detectable defect is achieved.

Abstract

Zinc selenide is an excellent infrared optical material. In this study, ultraprecision diamond turning experiments were carried out on coarse-grained polycrystalline ZnSe (p-ZnSe) under various conditions and the corresponding surface formation mechanisms were investigated by examining the surface topography, chip morphology, material microstructural change, and cutting forces. Two kinds of surface defects were observed (i.e. plowing-induced micron-scale cleavage craters and tearing-induced submicron-scale tearing pits). It was determined that plowing induced micron-scale cleavage craters could be suppressed by reducing undeformed chip thickness. Furthermore, it was ascertained that tearing-induced submicron-scale tearing pits could be restrained by using a cutting tool with a zero rake angle. The minimal surface roughness was dominated by grain boundary steps formed when the cutting tool crossed twin boundaries at a large angle. A model was proposed for correlating surface defects and boundary steps with crystal orientations and cutting directions. By using a large-nose diamond tool for cutting, a smooth surface of 1.5 nm Sa was obtained. In addition, a zinc blende to cinnabar phase transformation was observed in the cutting chips, and metallization of cutting chips occurred at an extremely small undeformed chip thickness (~20 nm). Moreover, it was discovered that phase transformation inside the workpiece depends on the radius of the tool nose. The findings of this study provide an important reference for ultraprecision machining of brittle polycrystalline materials.

Introduction

Zinc selenide (ZnSe) is a striking infrared optical material because of its high transmissibility at infrared wavelengths [1,2], hence, it is often used in windows of high power CO₂ lasers [3,4] and lenses for night vision systems of cars with autopilot, among others. ZnSe has a relatively low hardness quality [5], but it has a high brittleness quality [6], making it very difficult to be machined. The conventional method for creating optical surfaces with ZnSe is chemo-mechanical polishing [7,8]. However, the polishing efficiency thereof is low. Besides, it is also difficult to polish complex surfaces with high accuracy. As an alternative, ultraprecision diamond turning has been proposed as a promising solution for crafting complex shapes with high surface integrity [9].

While the existing research on single-crystalline materials has been extensive, those on polycrystalline materials are relatively few. In diamond turning of reaction-bonded silicon carbide [10], a super hard polycrystalline composite material, grain dislodgement became a major reason for surface roughness. In diamond turning of polycrystalline titanium [11], a low elastic modulus metal material with low thermal conductivity, it was found that the cracks could spread along the boundaries of large grains. In diamond turning of polycrystalline copper [12], a soft and ductile metal material, it was determined that surface crack patterns were less likely to occur. However, polycrystalline ZnSe, a soft and brittle material with different mechanical properties from the aforementioned polycrystalline materials, may result in a distinct difference in terms of the cutting mechanism. Zong et al. [9] studied oblique diamond turning of polycrystalline zinc sulfide (ZnS), whose mechanical property is, to some extent, similar to polycrystalline ZnSe, albeit less brittle than the latter, potentially causing different machinability, whether it be microscale or nanoscale.

For ultraprecision diamond turning of ZnSe, several studies have been conducted. Fang et al. [13] conducted diamond turning experiments on ZnSe (crystal structure unknown) and achieved nanometric surface finishes by using a zero rake angle tool. They mentioned that fractures would occur when a highly negative rake tool is used, however, they failed to clarify their reason for such. Shojaee et al. [14] investigated the effects of cutting parameters on residual stress as well as the crystal quality of a machined surface in diamond turning of polycrystalline ZnSe (p-ZnSe). They found that certain grains or twins exhibit signs of fracture, but the relationship between the fracture and the crystallographic orientation was not characterised. Li et al. [15] investigated the ultraprecision diamond turning capability of physical vapour deposited p-ZnSe on a smooth surface which met the requirement for the infrared optical image systems. They found that the intended result was achieved by setting low feed rates and depths of cuts. However, even the fine finished surface still had some defects, with the formation mechanism of which kept unrevealed. Xiao et al. [16] conducted plunge-cutting experiments of p-ZnSe whose grain size was at the submicron level. They reported that grain dislodgement dominated the surface formation and that a larger brittle-ductile transition depth could be realised by using a tool with a highly negative rake angle or a large nose radius. It should be noted that the effect of the rake angle reported by Xiao et al. [16] is exactly contrary to that reported by Fang et al. [13]. The reason might be that the workpieces used in those studies had different crystal structures, leading to different material removal mechanisms. However, the relationship between ultraprecision machinability and workpiece crystal structure has not been well-understood. Moreover, it is inferred that the machining-induced changes of the crystal structure (i.e. phase transformation) might have significant effects on the material machinability. However, up to date, there has not been any systematic study on the phase transformation of ZnSe in diamond turning.

Currently, ZnSe windows used for infrared systems are mostly p-ZnSe due to its low production cost, the grain size of which being a few tens or hundreds of microns [15,17], far larger than that used in the study of Xiao et al. [16]. For coarse-grained polycrystalline materials, grain dislodgment may be suppressed, but grain boundary step (i.e. a height difference between two adjoining crystal grains appearing at their common border due to the crystallographic effect in the material property) is a critical factor of surface roughness [18,19]. Nevertheless, the relationship between the formation of grain boundary step with cutting conditions is still unclear. The grain boundary step may result from the different elastic recovery of grains following tool pass [20,21] or anisotropic plasticity dominated by dislocation slip and grain boundary accommodation, as shown by finite element simulations [22,23]. Liu et al. [24], through a molecular dynamics simulation, concluded that sub-grains with transitional crystal orientations could be formed at the grain boundary through the plowing of the cutting edge and crystal rotation. The misalignment in the slip directions between sub-grains and original grains resulted in the grain boundary step. However, these arguments have not yet been verified by experimental studies.

In this study, ultraprecision diamond turning tests were performed on coarse-grained p-ZnSe. The mechanisms of surface defects generation, phase transformation, and grain boundary step formation, for the tool nose radius, cutting conditions, and crystallographic orientation, were systematically investigated. The phase transformation behaviours in both the machined surface and the cutting chips were identified and compared. Finally, a smooth surface without detectable defects was realised by optimising the cutting conditions. The findings of this study provide insights into the fundamental physics of machining brittle polycrystalline material.