key: cord-0259406-kuru5mdz
authors: Navare, Jayesh; Kang, Di; Zaytsev, Dmytro; Bodlapati, Charan; Ravindra, Deepak; Shahinian, Hossein
title: Experimental investigation on the effect of crystal orientation of diamond tooling on micro laser assisted diamond turning of zinc sulfide
date: 2020-12-31
journal: Procedia Manufacturing
DOI: 10.1016/j.promfg.2020.05.088
sha: bc687a007fe1fb56c94103794653ba3d6a2c413f
doc_id: 259406
cord_uid: kuru5mdz

Abstract The present article reports on an experimental study of the effect of crystallographic orientation of single crystal diamond on ductile material removal of ZnS. A multispectral ZnS workpiece was machined with single crystal diamond tools having two different configurations of cutting plane and cutting direction. An x-ray diffractometer (XRD) was used to evaluate the cutting plane and cutting direction of each tool. It was observed that the tools having a {100} cutting plane and <110> cutting direction resulted in considerably lower tool wear and surface roughness values, to that of tools with similar cutting plane orientation and <100> cutting direction.

In this article the effect of crystallographic orientation of diamond tool on ductile material removal (DMR) of ZnS using micro-laser assisted single point diamond turning (SPDT) was investigated. With the advent of ultra-precision diamond turning machines, manufacturing of infrared optical components such as Si and Ge, ZnS and ZnSe lenses and windows, has become very cost effective [1] . The fidelity of the surfaces generated through this process is influenced by several factors such as geometrical accuracy of the cutting edge of the tool, machining dynamical parameters, and the machine tool's nanometric positioning capability and repeatability. Optimal selection and working of these three factors are crucial for machining brittle materials in the ductile regime. Typically, selection of parameters not optimal for DMR, leads to the removal of material by fracture [2] . In case of single crystal materials like Si, Ge, CaF2, etc., if material removal occurs by fracture then an anisotropy in surface roughness is usually observed in distinct zones. Brittle fracture while SPDT of polycrystalline materials like ZnS and ZnSe leads to either pitting or a cloudy appearance on the machined surface.

Several studies have been done to understand the cutting mechanics of brittle materials. The ductile behavior of ZnS and ZnSe during diamond turning was studied by [3] . Blake and Scattergood presented a theoretical model for critical depth of cut that governs the brittle to ductile regime transition during SPDT of Si and Ge [4] .

Oomen and Eisses studied the effect of crystallographic orientation of the diamond tool on surface generation of nonferrous materials and observed that the wear pattern was mainly determined by the workpiece material [5] . Pramanik et al. reported that the diamond tools having their rake face aligned with {110} plane experienced lower wear and gave better finish on electroless nickel compared to those aligned in {100} plane [6] . The cutting force measured for tools with {110} crystallographic plane was more stable and didn't change significantly with the cutting distance whereas the cutting force measured for tools with {100} crystallographic plane varied randomly and had an increasing trend with increase in cutting distance. Similar observations were made

In this article the effect of crystallographic orientation of diamond tool on ductile material removal (DMR) of ZnS using micro-laser assisted single point diamond turning (SPDT) was investigated. With the advent of ultra-precision diamond turning machines, manufacturing of infrared optical components such as Si and Ge, ZnS and ZnSe lenses and windows, has become very cost effective [1] . The fidelity of the surfaces generated through this process is influenced by several factors such as geometrical accuracy of the cutting edge of the tool, machining dynamical parameters, and the machine tool's nanometric positioning capability and repeatability. Optimal selection and working of these three factors are crucial for machining brittle materials in the ductile regime. Typically, selection of parameters not optimal for DMR, leads to the removal of material by fracture [2] . In case of single crystal materials like Si, Ge, CaF2, etc., if material removal occurs by fracture then an anisotropy in surface roughness is usually observed in distinct zones. Brittle fracture while SPDT of polycrystalline materials like ZnS and ZnSe leads to either pitting or a cloudy appearance on the machined surface.

Several studies have been done to understand the cutting mechanics of brittle materials. The ductile behavior of ZnS and ZnSe during diamond turning was studied by [3] . Blake and Scattergood presented a theoretical model for critical depth of cut that governs the brittle to ductile regime transition during SPDT of Si and Ge [4] .

Oomen and Eisses studied the effect of crystallographic orientation of the diamond tool on surface generation of nonferrous materials and observed that the wear pattern was mainly determined by the workpiece material [5] . Pramanik et al. reported that the diamond tools having their rake face aligned with {110} plane experienced lower wear and gave better finish on electroless nickel compared to those aligned in {100} plane [6] . The cutting force measured for tools with {110} crystallographic plane was more stable and didn't change significantly with the cutting distance whereas the cutting force measured for tools with {100} crystallographic plane varied randomly and had an increasing trend with increase in cutting distance. Similar observations were made 

In this article the effect of crystallographic orientation of diamond tool on ductile material removal (DMR) of ZnS using micro-laser assisted single point diamond turning (SPDT) was investigated. With the advent of ultra-precision diamond turning machines, manufacturing of infrared optical components such as Si and Ge, ZnS and ZnSe lenses and windows, has become very cost effective [1] . The fidelity of the surfaces generated through this process is influenced by several factors such as geometrical accuracy of the cutting edge of the tool, machining dynamical parameters, and the machine tool's nanometric positioning capability and repeatability. Optimal selection and working of these three factors are crucial for machining brittle materials in the ductile regime. Typically, selection of parameters not optimal for DMR, leads to the removal of material by fracture [2] . In case of single crystal materials like Si, Ge, CaF2, etc., if material removal occurs by fracture then an anisotropy in surface roughness is usually observed in distinct zones. Brittle fracture while SPDT of polycrystalline materials like ZnS and ZnSe leads to either pitting or a cloudy appearance on the machined surface.

Several studies have been done to understand the cutting mechanics of brittle materials. The ductile behavior of ZnS and ZnSe during diamond turning was studied by [3] . Blake and Scattergood presented a theoretical model for critical depth of cut that governs the brittle to ductile regime transition during SPDT of Si and Ge [4] .

Oomen and Eisses studied the effect of crystallographic orientation of the diamond tool on surface generation of nonferrous materials and observed that the wear pattern was mainly determined by the workpiece material [5] . Pramanik et al. reported that the diamond tools having their rake face aligned with {110} plane experienced lower wear and gave better finish on electroless nickel compared to those aligned in {100} plane [6] . The cutting force measured for tools with {110} crystallographic plane was more stable and didn't change significantly with the cutting distance whereas the cutting force measured for tools with {100} crystallographic plane varied randomly and had an increasing trend with increase in cutting distance. Similar observations were made 

In this article the effect of crystallographic orientation of diamond tool on ductile material removal (DMR) of ZnS using micro-laser assisted single point diamond turning (SPDT) was investigated. With the advent of ultra-precision diamond turning machines, manufacturing of infrared optical components such as Si and Ge, ZnS and ZnSe lenses and windows, has become very cost effective [1] . The fidelity of the surfaces generated through this process is influenced by several factors such as geometrical accuracy of the cutting edge of the tool, machining dynamical parameters, and the machine tool's nanometric positioning capability and repeatability. Optimal selection and working of these three factors are crucial for machining brittle materials in the ductile regime. Typically, selection of parameters not optimal for DMR, leads to the removal of material by fracture [2] . In case of single crystal materials like Si, Ge, CaF2, etc., if material removal occurs by fracture then an anisotropy in surface roughness is usually observed in distinct zones. Brittle fracture while SPDT of polycrystalline materials like ZnS and ZnSe leads to either pitting or a cloudy appearance on the machined surface.

Several studies have been done to understand the cutting mechanics of brittle materials. The ductile behavior of ZnS and ZnSe during diamond turning was studied by [3] . Blake and Scattergood presented a theoretical model for critical depth of cut that governs the brittle to ductile regime transition during SPDT of Si and Ge [4] .

Oomen and Eisses studied the effect of crystallographic orientation of the diamond tool on surface generation of nonferrous materials and observed that the wear pattern was mainly determined by the workpiece material [5] . Pramanik et al. reported that the diamond tools having their rake face aligned with {110} plane experienced lower wear and gave better finish on electroless nickel compared to those aligned in {100} plane [6] . The cutting force measured for tools with {110} crystallographic plane was more stable and didn't change significantly with the cutting distance whereas the cutting force measured for tools with {100} crystallographic plane varied randomly and had an increasing trend with increase in cutting distance. Similar observations were made 48th SME North American Manufacturing Research Conference, NAMRC 48 (Cancelled due to COVID-19) by M. Sharif Uddin et al. while cutting Si, where it was seen that the diamond tools with their rake face aligned with {110} plane gave longer tool life and greater wear resistance [7] . The thrust force measured for tools with {110} orientation was lower than tools with {100} orientation. This behavior was speculated to be the reason for lower wear rate in tools with {110} orientation.

Present article addresses the influence of crystal orientations of diamond tool on DMR of ZnS. The objective of this study is to answer two questions; (1) Could certain crystal orientation of the diamond tool produce surfaces with lower roughness values on ZnS (2) If yes, is tool wear influenced as a result of having the tools in different crystal ordinations?

Micro-laser assisted diamond turning process, hereafter referred to as -LAM, incorporates an emission of a laser beam through the diamond tool which is delivered precisely at the cutting interface between the tool and the workpiece. The delivery of the laser light is made possible by a series of optics integrated into the machining tool post. The laser facilitates a more ductile regime cutting mechanism by locally heating the workpiece and thereby thermally softening it [8] [9] [10] [11] .

The underlying principle of operation of the -LAM process is depicted in Fig. 1 . The heating of the workpiece occurs in a very localized area typically less than 300 m 2 and this heat is dissipated in the form of chips. This localized heating is a crucial aspect of the -LAM process, to preclude introduction of undesired form error, typical in bulk heating processes [12, 13] . 

A multispectral ZnS workpiece was used for the testing purposes of this study. The tests were carried out on a Precitech Nanoform 250, a 2-axis ultra-precision diamond turning machine. -LAM system comprising of a laser delivery and control module, an OptimusT+1 machining tool post was retrofitted to the SPDT machine. The laser used was a continuous wave (CW) Nd:YAG laser, with a 1064 nm wavelength.

Diamond tools with two different crystal orientations were tested. One with cutting plane {100} and cutting direction <100> while the other with cutting plane {100} and cutting direction <110>. Details about the crystal orientation measurement are explained in section 4. The diamond tools used for the tests were manufactured by Edge Technologies and had a -35° rake angle, 10° cylindrical clearance, and 100 µm nose radius. Three tests, ZnS-1, ZnS-2, and ZnS-3, details of which are listed in Table 1 , were done. In each test, four machining cuts were taken and the evolution of the surface topography of the ZnS workpiece was quantified at the end of the fourth cut by measuring the surface roughness. Each machining cut was taken using parameters listed in Table 2 and the laser power exiting the diamond tool was measured before starting each of the three tests using an Ophir power meter, model 30(150)A-BB-18 ROHS.

The workpiece geometry was that of a meniscus lens with a 155 mm convex spherical radius and 415 mm concave spherical radius. The diameter of the workpiece was 144 mm. The part was held using a custom vacuum chuck fixture while machining, see Fig. 2 . The concave side of the workpiece was machined for the three tests and the cut was performed across the entire diameter of 144 mm in a facing configuration. For each test, 4 successive cuts were taken using the prescribed concave spherical path and the length of each cut was approximately 43 km.

To quantify the surface topography of the sample at the end of each test, areal surface measurements were taken on the workpiece using a Zygo ZeGage Pro HR. Average statistical parameters were used for quantifying the roughness values. More details on the surface metrology can be found in Table 3 . Table 3 . Metrology equipment and processing information.

Metrology details

Zygo ZeGage Pro HR Measurement method: Scanning white light interferometry (SWLI).

Objective: 20X Mirau objective, FOV 434 µm  434 µm.

Fitting operation: 4 th order polynomial removed.

Qualifying metric: RMS roughness, Sq (nm); skewness, Ssk; kurtosis, Sku

Evaluation of the crystal orientation of the diamond tools was done using XRD and prior to manufacturing of the tool. Crystal orientation can be defined by specifying a crystallographic plane and a direction with respect to a global coordinate system. In the context of SPDT, the XYZ coordinates of the lathe can be used as a reference to describe the crystal orientation of tools. Fig. 3 illustrates the orientation of the three tools. Tool-1 has {100} <100> orientation, where a {100} crystallographic plane of the diamond is parallel to the XY plane of the lathe (corresponds to the surface of a flat part), and a <100> direction of the diamond is parallel to the Y axis of the lathe (cutting direction). Tool-2 and Tool-3 have {100} <110> orientation, where a {100} plane is parallel to the XY plane, and a <110> direction is parallel to the Y axis.

Siemens D5000 XRD with Cu K radiation at 20 kV and 5 mA was used. A diamond tool was affixed to an aluminum fixture that was mounted onto the stage inside the XRD. The {100} and {110} diffraction peaks were found by orienting the tool such that the intensity of those peaks was maximized on the detector. Figure 3 Schematic of the crystal orientation of the tools (a) Tool-1: a {100} crystallographic plane of the diamond is parallel to the XY plane, and a <100> direction is parallel to the Y axis; (b) Tool-2 and Tool-3: a {100} plane of the diamond is parallel to the XY plane, and a <110> direction is parallel to the Y axis.

The surface roughness interferograms of the machined workpiece after the three tests are depicted in Fig. 4 . For ZnS-1, it is seen that the surface has some extent of brittle fracture. Pitting is observed across the surface as shown in Fig. 4(a) . For ZnS-2 and ZnS-3, the surfaces generated didn't show any detectable signs of pitting, as seen in Fig. 4(b) , 4(c).

Average Sq, Ssk, and Sku evaluated over four measurements for each of the three tests are listed in Table 4 . These statistical parameters point to a higher degree of ductile material removal in ZnS-2 and ZnS-3. For ZnS-1 test, see Fig. 5 (a), VB max was measured to be 30 m while for ZnS-2 and ZnS-3, VB max was measured as 17 m and 5 m respectively as seen in Fig. 5(b) and Fig. 5(c) .

The experimental investigation described here indicates that the rate of wear in diamond tools was influenced by their crystal orientation which subsequently had an impact on the quality of the surface generated. It should be noted that each of the tests were done three times, to ensure the repeatability of the results.

For ZnS-1, the measured surface had distinct zones of brittle fracture. Ssk and Sku of -1.4 and 10.59 respectively indicates the presence of pits and data outliers on the workpiece surface. In case of ZnS-2 and ZnS-3, the measured surfaces were free of any defects and fracture which can be validated by their low Ssk values which were one third of that obtained from ZnS-1. This further confirms the lower degree of ductile material removal with Tool-1 over Tool-2 and Tool-3.

According to [5] , the <110> direction has a higher hardness compared to the <100> direction. While cutting a polycrystalline material like ZnS, where the mechanical properties vary on a very localized scale, it was speculated that Tool-2 and Tool-3 which have a stronger cutting edge compared to Tool-1 were less affected by these local variations thus giving less wear and a better surface finish. Although not reported in this article, it was observed that for shorter cutting distances, Tool-1 produced similar surface finishes to that produced by Tool-2 and Tool-3 which further backs up the hypothesis. The effect of inherent chemistry of diamond tools (synthetic or natural) on the rate of wear was not investigated further as it was beyond the scope of this article. That said, it is believed that the lower wear on the ZnS-3 tool is most likely due to the different chemistry of the tool.

In this article, the influence of crystal orientation of diamond on tool wear and subsequent surface finish of the machined workpiece was experimentally investigated. It was seen that the diamond tools having the tip of their cutting edge normal to <110> direction in {100} plane had a superior finish and better wear resistance compared to the tool having the tip of its cutting edge normal to <100> direction in {100} plane. It was speculated that the higher hardness combined with the possibility of varying elastic recoveries in the vicinity of the cutting interface due to the polycrystalline nature of ZnS, tools where the tip of the cutting edge is normal to <110> direction in {100} plane can survive instantaneous microchipping thus giving better wear resistance.

As part of future work, studies on orientation maps of polycrystal ZnS using electron backscatter diffraction techniques will be used to better examine the mechanical property gradients on the surface of the material. Such studies can provide a more quantitative explanation for the difference in wear of the diamond tools with different crystal orientations.

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