Single Point Diamond Turning of Calcium Fluoride Optics

Transcription

1 Key Engineering Materials Vol. 516 (212) pp Online available sine 212/Jun/14 at (212) Trans Teh Publiations, Switzerland doi:1.428/ Single Point Diamond Turning of Calium Fluoride Optis Xihun Luo 1, a, Jining Sun 1, b, Wenlong Chang 1, and James M Rithie 1, d 1 Shool of Engineering and Physial Sienes, Heriot-Watt University, Edinburgh, EH14 4AS, UK a X.Luo@hw.a.uk, b JS173@hw.a.uk, WC129@hw.a.uk, d J.M.Rithie@hw.a.uk Keywords: diamond turning, diamond tool wear, CaF 2 optis, brittle frature Abstrat. This paper aims to develop a ost-effetive diamond turning proess to obtain nano-smooth CaF 2 optis. Diamond tool wear was also arried out through a number of utting trials. Three CaF 2 speimens (diameter of 5 mm and thikness of 5 mm, rystal orientation of (111)) were diamond turned on an ultra preision lathe (Moore Nanoteh 35UPL) by a number of faing uts. In the utting trials feed rate varied from 1 µm/rev to 1 µm/rev. White spirit mist was used as the oolant. Cutting fores were measured by a dynamometer (Kistler BA9256). Surfae roughness of the CaF 2 optis and tool flank wear were measured by a white light interferometer (Zygo Newview 5) and a sanning eletron mirosope (FEI Quanta 3D FEG), respetively. It was found that using a feed rate of 1 µm/rev surfae roughness Ra of 2 nm ould be obtained. When the ratio of the normal utting fore to the tangential utting fore was lower than 1 tool wear would initiate. In diamond turning of alium fluoride abrasive wear was the main tool wear mehanism. Using white spirit mist as theoolant ould avoid generation of thermal type brittle frature on the mahined CaF 2 surfaes. Introdution Calium fluoride (CaF 2 ) is an optial material whih is highly transmissive in both ultraviolet and infrared ranges. It is partiularly well suited for manufaturing lenses, prisms and mirrors for dark field imaging systems and large-sale mirolithography systems and eximer lasers [1]. Single point diamond turning has been applied in the manufaturing of CaF 2 optis to ut rough blanks into semi-finished optis [2, 3, 4]. Diamond turned CaF 2 optis will be polished by magnetorheologial finishing (MRF) or floating polishing in order to obtain the required nano-smooth surfaes and remove subsurfae damage layers [5]. CaF 2 is a typial brittle material. It possesses low frature toughness and low hardness, but an extremely high thermal expansion oeffiient (5.8 times that of silion) and very low thermal ondutivity (1/17 that of silion). For these reasons speial attention needs to be paid to the thermal problem in proess development. Running large numbers of diamond turning trials while using a straight-nosed diamond tool, Yan et al. [6] found that there were two types of brittle frature on the mahined surfae. A-type brittle frature whih was 1 to 2 µm in size appeared in a utting region when a high feed rate was used. On the other hand B-type brittle frature whih was of the order of 1 µm in size was observed in low feed rate utting regions. Further studies have found that A-type brittle frature is due to the size effet and rystallographi effet of the dutile-brittle transition. B-type brittle frature results from thermal effets. Performing dry utting ould eliminate B-type brittle frature, but the tool wear will beome another problem. This paper aims to develop a ost-effetive diamond turning proess to obtain nano-smooth CaF 2 optis. Eonomial mahining feed rate to maintain damage-free (both mehanial and thermal) nano-smooth diamond turned CaF 2 optial surfaes will be investigated through experimental studies. Theoretial maximum feed rate to maintain dutile mahining mode Same as for other brittle materials like Si and Ge, diamond turned CaF 2 surfaes are produed by a ombination of brittle frature and dutile mode removal mehanisms. Above the brittle to dutile transition point, the effetive hip thikness is bigger than the so-alled ritial depth of ut, so All rights reserved. No part of ontents of this paper may be reprodued or transmitted in any form or by any means without the written permission of TTP, (ID: /7/12,12:28:8)

2 Key Engineering Materials Vol miro-frature damage ours and the removal is by brittle frature. Below the transition point, the effetive hip thikness is smaller than the ritial depth of ut, so plasti deformation takes plae and dutile regime removal is ahieved. For CaF 2, its ritial depth of ut (d ) is about.1 µm. It is obtained by [7]: d E K = β [ H H 2 ]. (1) where β is a onstant. E, H and K are elasti modulus, hardness and frature toughness of CaF 2 respetively. The maximum feed rate for dutile mode diamond turning an be alulated by [7]: f max = d 2( d R + y ). (2) where R is the tool nose radius and y is the surfae damage depth. Based on Eq. (2), f max = 6 µm was obtained when R = 2 mm. However, this is only a theoretial estimation. More aurate value will have to be obtained through utting trials. Cutting trials Setup for diamond utting trials. The utting trials were arried out on an ultra preision diamond turning mahine Moore Nanoteh 35 UPL. This mahine utilizes hydrostati linear bearings and linear motors for linear ontrolled motions, and an air bearing spindle with a diretly oupled servo motor drive. High resolution position and angular enoders are used throughout, whih with a high speed eletroni ontroller, ensure a high level of ontrolled motion performane. The experimental setup is shown in Fig.1. The CaF 2 work piee was attahed to the vauum huk. A diamond tool was installed on a tool holder whih was mounted on a kistler dynamometer. Two oolant nozzles were used in the utting trial. CaF 2 work piee Diamond tool Tool holder Dynamometer X Z Fig. 1 Experimental setup

3 41 Proeedings of Preision Engineering and Nanotehnology Mahining onditions. Two CaF 2 work piees (5 mm diameter, 5 mm thik, (111) fae) were diamond turned by using the mahining onditions listed in Table 1. The diamond tool travelled from outside toward the work piee entre in the utting proess. The utting sheme is shown in Fig. 2, in whih the tool feed rate varied from 1 µm/rev to 1µm/rev with a utting length of.5 mm. Surfae roughbess Ra, (nm) mm....5 mm Tool feed diretion Fig. 2 Cutting sheme Table 2 Mahining ondition and tool geometry Spindle speed 1,2 rpm Depth of ut 1 µm Feed rate 1 1 µm/rev Coolant White spirit Tool rake angle -15º Clearane angle 1º Nose radius 2 mm Fig. 3 Surfae roughness of the mahined surfae against feed rate for sample 1 Sample measurement and inspetion. A Zygo Newview 5 white light interferometer was used to measure the mahined surfae roughness. A Form Talysurf 12L was utilized to measure the form auray of the mahined surfae. The mahined work piee surfae and diamond tool before and after utting trials were also inspeted by a FEI Quanta 3D Sanning Eletron Mirosope (SEM) and an atomi fore mirosope (DI Dimension 31). Results and disussions Surfae roughness and form auray. Fig. 3 shows the variation of surfae roughness (Ra) when utting sample 1 under different feed rates. It an be seen that when a feed rate of 1 µm/rev was used surfae roughness (Ra) of 2 nm was ahieved. Surfae roughness Ra was lower than 4 nm when the feed rate varied from 1 µm/rev to 1 µm/rev. The measured surfae topography by SEM onfirmed that dutile mode utting was ahieved in the utting trials. However, as shown in Fig 4. the peak to value of the mahined surfae began to inrease when the feed rate was higher than 5 µm/rev. Therefore, a feed rate of 5 µm/rev is reommended to ut CaF 2 in order to obtain nano-smooth surfae and good form auray. Fig. 5 shows the mahined surfae roughness of sample 2 in whih a sharp rise of surfae roughness was observed. When a feed rate of 1 µm/rev was used the surfae roughness Ra was above 1 nm. It indiated that brittle frature had ourred on the mahined surfaed due to tool wear. Later inspetion of the mahined surfae and the diamond tool by SEM onfirmed this onlusion.

4 Key Engineering Materials Vol PV (nm) Fig. 4 Form auray of mahined CaF 2 surfae under different feed rates for sample 1 Surfae roughness, Ra (nm) Brittle frature ours Fig. 5 Surfae roughness of mahined surfae against feed rates for sample 2 Cutting fores. Cutting fores ontain very useful information about the state of the mahining Cutting fores (N) Fn (sample 1) Ft (sample 1) Fn (sample 2) Ft (Sample 2) Fig. 6 Variation of utting fores against feed rate

5 412 Proeedings of Preision Engineering and Nanotehnology proess. Fig. 6 shows the utting fores for utting sample 1 and sample 2 under different feed rates. It an be seen that the utting fores were all at sub-newton level. The negative rake angle tool ontributed to inreased ompressive stress in front of the tool. Suh stress state is favourable to suppress frature damage. Therefore, it was found that the normal utting fores are bigger than the tangential utting fores under feed rates from 1 µm/rev to 1 µm/rev. However, as shown in Fig. 6 when utting sample 2 the normal utting fore beame smaller than the tangential utting fore. It indiated that the dutile mode utting ondition had hanged to brittle mode. After utting sample 2 the diamond tool was then inspeted. Tool wear. The diamond tool was inspeted by a SEM before and after the utting trials. The SEM images are shown in Fig. 7. Clear tool flank wear Clearane fae was observed after a utting distane of 8 km on the utting edge Flank tool wear whih was in the form of many mirogrooves. This is a strong indiation of the development of abrasive (a) Before utting trials (b) After utting trial (utting distane of 8 km) wear in its early stage. The width of flank tool Fig. 7 SEM images of the utting edge of the diamond tool before and wear (VB) measured by after the utting trials SEM was only 1.36 µm. The tool wear hanged the original brittle-dutile transition ondition and aused brittle fratures on the mahine surfae. Therefore, the surfae roughness of mahined surfae on sample 2 was higher than that on sample 1. Fig. 8 shows mahined surfae topography measured by SEM and a zoomed image. The size of fratures is within 1 to 2 µm. This is the so-alled A-type brittle frature. B-type brittle fratures were not observed on the mahined surfae. This showed that using white spirit an improve thermal onditions. But (a) Overall view of brittle fratures (b) Zoomed brittle fratures further utting trials are Fig. 8 Brittle frature on the mahined surfae (sample 2) needed in the future to onfirm the tool life when using a feed rate less than 5 µm/rev. Another CaF2 work piee (Sample 3) was ut by using a feed rate of 1 µm/rev, a depth of ut of 1 µm and white spirit as the oolant. After a utting distane of 6 km brittle fratures appeared on the mahined surfae. The size of brittle frature was also of the order of 1 to 2 µm. This further onfirmed that white spirit helps to redue thermal brittle frature.

6 Key Engineering Materials Vol Conlusions Through the experimental studies, the following onlusions an be drawn: (1) Nanosmooth diamond turned CaF 2 optial surfaes were obtained when a feed rate from 1 µm/rev to 1 µm/rev, depth of ut of 1 µm, spindle speed of 12 rpm, a -15º rake angle diamond tool and white spirit as the oolant were used. (2) Surfae roughness (Ra) of 2 nm and form auray (PV) of 26 nm on the mahined surfae of CaF 2 optis were obtained when using a feed rate of 1 µm/rev. (3) A feed rate of less than 5 µm/rev is reommended for better form auray and high mahining effiieny. (4) White spirit is a suitable oolant for utting CaF 2 optis as B-type brittle frature, whih is related to thermal effets, did not our in the experiment. Aknowledgement The authors would like to thank EDTC, Mr. Mat Norris from Spanopti, Mr. Andrew Cox from Contour Fine Tooling, Prof. Paul Shore, Dr. Paul Comley and Mr. Alan Heume for providing finanial support, CaF 2 speimens and aess to the Moore Nanoteh 35 UPL for diamond turning trials in this projet. Referenes [1] D.J. Ewing, Eximer lasers at 3 years, Opt. Photonis News 14 (23) [2] D.L. Deker, D.J. Grandjean, J.M. Bennett, Optial and surfae physial harateristis of diamond-mahined infrared windows, NBS Spe. 562 (1979) [3] J. Yan, K. Syoji, J. Tamaki, Crystallographi effet in miro/nanomahining of single-rystal alium fluoride, J. Va. Si. Tehnol. B22 (24) [4] E.R. Marsh, B.P. John, J.A. Couey, J. Wang, R.D. Grejda, R.R. Vallane, Prediting surfae figure in diamond turned alium fluoride using in-proess fore measurement, J. Va. Si. Tehnol. B23 (25) [5] Y. Nambaa, N. Ohnishia, S. Yoshidaa, K. Harada, Y. Yoshida, T. Matsuo, Ultra-preision float polishing of alium fluoride single rystals for deep ultra violet appliations, CIRP Ann.-Manufaturing Tehnology 53 (24) [6] J. Yan, J. Tamaki, K. Syoji, T. Kuriyagawa, Single point diamond turning of CaF 2 for nanometri surfae, Int. J. Adv Manuf Teh. 24 (24) [7] P. Blake, R.O. Sattergood, Dutile-regime mahining of germanium and silion, J, Am. Ceram. So. 73 (199)

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