A IR Methods to Evaluate Surface Texture. TRUFASU Aurelian Ovidius 1,a

Transcription

1 Applied Mechanics and Materials Online: ISSN: , Vol. 332, pp doi: / Trans Tech Publications, Switzerland A IR Methods to Evaluate Surface Texture TRUFASU Aurelian Ovidius 1,a 1 Universitatea Politehnica Bucuresti, strada Splaiul Independentei 313, sector 6, Bucuresti, Romania a a_trufasu@yahoo.com. Keywords: IR, Testing. Abstract. This paper is about IR technology for roughness evaluation of upper layer of mineral glass lenses, prisms or reticules. Based on simple observation that friction is linked with higher temperature, authors evaluated the same optical surfaces by two methods: microscopic and IR evaluation. Facts are extremely encouraging, because results are similar; zones with higher level above average are warmer and those under average are colder. Introduction-generalities Surface roughness is a useful parameter for morphological separation. Real roughness is a shell script that automatics the process, but users must be aware that it uses real surface area to calculate both real and planar area for each grid cell (sub-regions) and that raster resolution plays an important role on area estimations, [1]. Super-finishing optical surfaces of any piece can be done by several methods; all of them are intended to create levelling of surface and at the same time to reduce the temperature of upper layer, because a higher temperature induces local or general deformation. On upper layer take place a movement of dislocated particles even these are due to chemical reaction or mechanical process. The difficulties of usually evaluation consist in total square surface evaluated by microscopically method, which is too small to cover the whole optical surface of a medium lens with 20 mm 2 total surface. During some processes the optical pieces are immersed in liquid to support chemical reaction and to short technological time. In this case, an inspection on optical surface is impossible till the finishing the final stages. Novelty Authors proposed a noncontact method and made experience with it, an IR inspection, images acquired by IR CCD camera (FLIR type) with high resolution, enough to prove the places with higher temperature have some picks over the others. Algorithm Process evolution was monitored all the time: and regularly, in the first 10 minute and aftermath from 30 to 30 minutes, till the final result was achieved or when on one surface was free of any small holes, visible with a 6x magnifier. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-09/04/16,08:55:25)

2 552 OPTIROB 2013 Figure 1 Structure Scheme In the Fig. 1 is shown the structure scheme for the working bench. It was a classical one modified with new devices, added to keep under surveillance all the time optical pieces and fluids involved in the process. Temperature transducer is logged to the same computer as FLIR; another transducer measure in real time the concentration of waste fluid; if the liquid it too concentrated, means that collateral products (used powder and small particles of upper layer) is too high and the power of suspension is lower; in that case there are the premises to rise temperature and deformation of pieces at the same time. The optical piece used for experiments carried out is shown in Fig. 2. Figure 2 Sample design and dimensions

3 Applied Mechanics and Materials Vol Know how The experiments have been carried out on electro-mechanical specialized device, but powder supplying was manually in a certain type of technology. Two different polishing technologies have been used: a) a classical way, known as polishing stage; suspensions was very concentrated and the grinding plate was wet all the time all operations carried out by human resource. b) a combined way, consists in a classical way of polishing, during which, the grinding plate is kept wet with the liquid of polishing suspension, nanopowder being melt in the solution. For both ways an active controller, measuring the waste water temperature, powder concentration and pressure in the centre of the block was in place. A certain values for both parameters were under control, over limit values induce reaction for human supervisor or cut the electric supply. Technological parameters of polishing process Experiments took place on a Karger type machine with 5 pieces displayed on working places. Working parameters have been set up to the following values: Samples block diameter: 30 mm; Samples number in block: 5 pieces ; Diameter polishing device: 100 mm; Rotation speed of lower shaft: 100 rot/min; Number of oscillations of upper shaft: 72 osc/min; Working pressure, located in the centre of block of samples: 0,5 kg/m 2 ; Local temperature for the waste liquid < 65 0 C; Concentration for powder < 15gr/litre. Samples were placed on F120 diameter plate, in a proper manner to cover the whole surface and at the same time to have enough space in between to eliminate unwanted or waste material and to prevent overheating. The cooling channels are evenly distributed all over the place and the cooling liquid flows the plate all the time. The temperature was recorded in different stages of manufacturing, and in Fig. 4 is shown the last picture over one sample. Elongation for each optical raw material depends on temperature and elongation coefficient is greater or lower depending on organic or mineral structure. For the same material is a linear variation of sample thickness, due to modification of local temperature, by the thermal conductivity coefficient λ and linear thermal expansion coefficient α, as its showed in Fig 3. Table 1 N-BK7 Thermal conductivity coefficient λ [kcal*h -1 *m -1 *deg -1 ] C 0 0 C C C Linear thermal expansion coefficient α [α 20/t *10 7 ] C C C C 56 62

4 554 OPTIROB 2013 Figure 3 Elongation distribution (µm) Have to be noticed that local temperature is different from point to point, but reflected and atmospheric temperatures are constant all over the plate, controlled by air conditioning central unit, so they do not disturb in any way local temperature and elongation is proportional with this local temperature. In fact, local heat transfer is constant all over the plate and all the time. Experiments Figure 4 Local temperature records on one optical piece surface in 12 points The profit is taken by higher temperature belonging to the hills helping them to get the same level with the most points of surface. The local points are placed randomly on the plate and samples were labelled with this numbers. In Fig. 4, one optical piece is detached from working place after what is considered finished process. The piece was manufactured by classical way and previous experience was an advantage to contribute to the final nanometric topography of surface. IR image was shooting in 1 minute after detaching from plate and cooling process was not enough to influence temperature distribution over the piece. In fact, immediately after ending process temperature raises a little because liquid cooling effect stopped. In Fig. 5, one optical samples is detached from working place after what is considered finished process. The sample was manufactured by classical way and previous experience was an advantage to contribute to the final nanometric topography of surface. IR image was shooting in 1 minute after detaching from plate and cooling process was not enough to influence temperature distribution over the piece. In fact, immediately after ending process temperature raises a little because liquid cooling effect stopped.

5 Applied Mechanics and Materials Vol Figure 5 IR view of a sample detached from the first experiment type working place FL IR S y s te m s Sp1 1 Sp1 Sp C Sp5 Sp1 2 Sp4 Sp6 Sp8 Sp9 Sp Sp3 Sp2 Figure 6 Temperature distribution, highlighting important values As it can be seen in Fig. 6, the differences between local temperatures are not enough higher to be seen as stains on sample surface, even the clearance is higher. But, comparing with human face temperature, human face seen in background, the differences are noticeable and pretty much higher, because the temperature values spread all over the sample is around 22 0 C and human face has 36 0 C. Even on human face the differences can be seen clearly (nose or left eye) so the camera accuracy is proved to be good Figure 7 Original IR image acquired by FLIR

6 556 OPTIROB 2013 Figure 8 Nanostructure as it can be seen by AFM in 3-D representation On the selected optical piece where marked several areas with major difference of temperature. These selected areas 20x20 µm 2 where controlled by AFM (atomic force microscope) and image of one sample is shown in Fig. 6. On the right side of picture is a scale of thickness (or real image of roughness). Most of surface is red or orange dominant which means 4-6 nm highest. Structure of surface is uniform and some isolated peaks are lower than 16 nm. So, topography of surface can be assumed to uniform levelling and temperature in this area is uniform too and below average. If the surface is enlarged to 40x40 µm 2 the surface topography is much clear even is represented as a 3-D view on a plate appearance. There are two difference surfaces with reddish colours appearance (light red and dark red) and some white peaks. These peaks are stains created by washing liquid. Figure 9 Nanostructure as it can be seen by AFM with lower magnitude Optical pieces were distributed on working plate in a star shape, nothing in the middle and five around. Distribution is key factor to eliminate waste influence on future surface topography. A crowded plate keep the waste in the middle and cooling factor is lower, but less pieces make circulation faster and polishing properties of powder is not used enough. From previous experience a distance between optical pieced of 2 mm between their sides is optimal. This optimal distribution is shown better on IR image presented in Fig. 10. The pressure over the upper plate is changed in accordance with optical piece material, dimensions and level of accuracy. Changing the pressure is up to previous experience of manufacturer, it determines the time of manufacturing and surface quality. The pressure can be changed during the process, if temperature is higher than set limit, because between those two parameters is a linear relation. In Fig. 10 can be seen that sides of pieces have higher temperature then upper face due to cooling effect of liquid. Temperature on sides differs from piece to piece according to their thickness, but there is no influence to the upper structure, as authors make no link. Considering that temperature can rise in different places after removing upper plate, authors founded their theory only on images acquired during the polish process, an example being showed in Fig. 9. As it can be seen on Fig. 9, optical pieces are greenish

7 Applied Mechanics and Materials Vol comparing with plate support and upper polish plate. The temperature range is [28,1 29,3] 0C. Temperature are not equal on surface even surface belong to the same optical piece. So, the only clue is a link between different levels of thickness of piece surface and pressure on them, pressure applied on upper plate to speed up the polish process. The truth of this link is proved with AFM inspection on supervised surfaces. Conclusion Figure 10 Samples distribution on working plate Measuring temperature on local points is a certain way to assess the level of thickness and roughness on large pieces. Due to different local temperature, plate is not plane, is an irregular surface with peaks and holes, the difference between highest and lowest point depending on the difference of temperature. At the same time the peaks are permanently in contact and under pressure with upper plate, so temperature is higher all the time. In the upper structure can be imagine a melting process, hills facing a rush process than valleys. Melting process is initiated chemical way, temperature level or both. The slopes are easy and quality of surface is better on highest points rather than lower. References [1] [2] #roughness_parameters.htm [3] ews_vol4-roughness.pdf. [4] GRIFFITH P., MARLEN A., Optical Fabrica-tion lies on Tried and True Methods, Laser Fo-cus World, Oct [5] MELLOT N.P., BRANTLEY S.L., Evaluation of Surface preparation Methods for Glass, Surf. Interface Analysis no. 31, 2001, pg [6] LANGHORN CH., HOWE A., Optical Mor-phology, Coherent Auburn Group Publication, 2006.

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