Micromechanical manufacturing of abrasive surfaces for fundamental studies on wear and grinding

WEAR ELSEVIER

Wear217 (1998) 231-2-';6

Micromechanical manufacturing of abrasive surfaces for fundamental studies on wear and grinding Rickard GAhlin *, Staffan Jacobson Upp.rula University, ,~ngstriJmLaboratory, Department ~f Materials Selene(', Box 534. S-751 21 Uppsala. Sweden

Received6 August 1997:accepted 13 February 1998

Abstract An etching procedure has been developed with the objective to produce silicon abrasive surfaces in a controlled and reproducible way. These surfaces are intended for studies of fundamental aspects of abm,;ion. The procedure is based on techniques commonly used for manufacturing of silicon micro mechanical structures. The resulting surface is controlled with respect to abrasive tip shape, tip size and distance between tips. Further. it is possible to control the lip radius by mechanical after-treatment. The resulting tip shape and quality was evaluated in a scanning electron microscope (SEM) and the abrasive properties in a pin-on-di.~ test. Moreover, tip failures were studied m yield information about the tip strength. The process developed proved successful in manufacturing well defined abrasive surfaces in silicon. showing nearly constant tip shape and size over large areas. Further. the tips proved tough and wear resistant enough to abrade tin with very limited wear. Hence. the manufactured abrasive surfaces are well suited for fundamental stndies on wear and grinding. © 1998 El~vier Science S.A. All rights reserved. Keywords: Abrasivewear:Grinding:Fundamentalstudies:Abeasivesurface:Micromechanical:Silicon

I. Introduction Abrasive wear and grinding are related processes characterised by hard particles removing materials from a .softer countersurface by some scratching or cutting action. In grinding the material removal is intentional to bring a workpiece into a well defined shape and surface finish. The work material is cut by sharp grits of rather irregular shape and orientation, often attached to paper or cloth or cemented together to form a grinding wheel. In abrasive wear the material removal is unwanted. The abrasive particles or asperities causing the wear are normally less aggressive than the grinding grits, but in most respects the material removal processes are the same. It involves microculting, briRle microcracking along the .scratches. plus ridge formation and other forms of plastic deformation even.. tually resulting in local fatigue fractures and detachment of wear debris. The wear rate ( material removal rate) depends on a large number of parameters. These include the worn material prop* Correspondingauthor. U~psalaUniversity.Deparlmcntof Technology. Materials Science Division. box 534. S-751 21. Uppsala. Sweden. Tel.: +46-18-471-3036: lax: +46-18-4.71-3572: e-mail: rlga@mercur. teknikum.uu.se 0043-1648/98/$19.00 © 1998El~vier ScienceS.A. All rights reserved. PII S0043-1648 ( 9 8 ) O 0 1 7 2 - 0

erties, the load and sliding speed of the particular situation and the shape, size distribution, orientation, lateral di,qribulion and material propertiesof the abrading tips.Further, the differentfactorsm a y influence each other in differentways. This large number of parameters, ~ of which also m a y he hard to control, oftgn make it very hard to perform fun-

Fig. I.Commerciallyavailablesd~r~ivesm'facemamaf~tmed witha replicati~mtechnique(SEM ).

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damemal research regarding the abrasive wear process or to op|imise grinding tools for maximum removal or surface finish. This paper presents one way of improving the situation by using micro mechanical etching techniques, which offer the possibility to produce relatively large surfaces covered by abrasive tips of extremely well defined shape, size, orientation and lateral distribution. The first step in this work was to choose a suitable tip shape. The shape should constitute a good representation of actual grinding grits and wearing particles: further, it should be possible to produce them in a controlled fashion and modify them to cover a range of sharpness, etc. In addition, it is important that the tips are capable of abrading the counter-

material while self suffering very limited damage. The choice fell on the pyramid shape, for which micro mechanical etching techniques have proven suitable [ I-4]. It should be mentioned that other techniques are possible, among them replication which has been used by 3M to produce controlled abrasive surfaces (see Fig. I ). In the present paper the technique developed to manufacture controlled abrasive surfaces is briefly described and some resulting surfaces are shown before and after use. Further, the potential of the technique in studying fundamental aspects of abrasion and the possibilities to develop novel grinding and polishing tools are discussed.

2. The manufacturing process Oxidation

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Patternof

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It Ill W Fig. 2. Majo¢~cpsof the micromechanicaletchingtechnique.

To produce the desired surfaces. ( 100)-silicon wafers 7.6 cm (3 in.) in diameter were treated by a micro mechanical etching technique consisting of several steps ( see Fig. 2). The manufacturing process involves the following major steps: Firstly. the wafers are wet oxidised (02) to obtain a

Fig. 3. ExampIc~. of a.,,-pr~vJu,,:cdc.ilicon tip. (SEM). { a ) Shmp tip. (h) Truncated tip produced by interrupling the etching. (c) Rounded tip pr~luccd by mc¢~hanicalpoli~.hing of sharptip..

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R. G~hlin.$. Jacob.winI Wear217 (1908)231-236

SiO., layer of approximately I/zm. Secondly. a uniform pattern of oxide islands (50/zm in radius) is produced by standard photolithographic techniques. This SiO, spot pattern resists etching and thus determines the position of the individual tips and (depending on the etch used) also the tip shape. The tip size and distance between tips are chosen by adjusting the applied photolithographic pattern. Larger spots in the lithographic mask produce larger tips and longer dis-

tances between the spots results in surfaces with a mote spar~ tip distribution. Finally, the unprotected silicon is ani~tropically etched, gradually undercutting the oxide discs which eventually fall off. leaving a uniform structure of pyramidal silicon tips. To he able to produce pyramidal abrasives with small variations in shape and size over large areas, a new etch formula hascd on the HNA system (hydrofluoric acid HF. nitric acid HNO3 and acetic acid CH?OOH) had to be developed. The HNA system is a polishing etch producing smooth surfaces. Parts of the intricate HNA system are well characterised and some standard ~lutions have been developed for orientation independent etching at room temperature (22°C) 15,61. These solutions contain low HF and high HNO3 concentrations and yield isotropic etching. However, for high HF and law HNO3 concentrations they etch anisotropicafiy but tend to be difficult to initiate and the etch rate is strongly time dependent. After an initial optimisation procedure, the composition HF, HNO~ and CH3OOH to the proportions 3: 1:8 was chosen. The etching process was triggered by distributing a small amount of HNO.~ over a "dummy" wafer and etching it for about I0 rain before commencing the actual manufacturing etch. The time to undercut the oxide discs was approximately 4 min. The developed procedure resulted in almost ideally sharp tips, see Fig. 3a and Fig. 6. Further, it proved possible to change the pyramid angle simply by altering the etching time. If the etching is interrupted immediately after the oxide discs have become undercut, a pyramid angle of about 90° results. To increase the tip angle the etching must continue after the undercut, e.g., 2 rain extra etching is required to produce a tip angle of about 130°. However, due to the time dependent etch rate. it is hard to produce wafers with a pre specified pyramid angle (see Fig. 4). The sharp shape can readily he modified by minor changes of the etching procedure or by mechanical after-treatment. By interrupting the etching procedure before the silicon oxide discs are fully undercut and removing the remaining oxide with buffered HF. a truncated pyramidal tip shape is produced ( see Fig. 3b ). As obvious from Fig. 2 (stage !11). the size of the truncation is directly related to the tip angle. Blunt tips are easily obtained by polishing the sharply etched tips with 0.25 /.t,m diamond particles on soft cloth discs (see Fig. 3c). In this way tips with appro,~imately 3 and 9/,tin tip radii were produced by altering the polishing time (see Fig. 5). It should he noted that the polishing procedure affects the shape of the tips, which becomes more cone-like

25

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15

20

25

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35

40

Etch that [ m q Fig. 4. Schematicdiagramof the elch rate.

Fig. 5. Differen¢tip radii p ¢ o ~

by p o l i n g ate ~ d i a n w m l tips with

0.25 p.m d i a ~ particleson .,;oftclothdi~s. (a) Approxima~radius3 /zm and (b) approximalcradius0/xm. with the longer the polishing time. The effect is significant when producing tips with a radius larger than about 6 ,urn.

3. Examinalicm olr ~

albcasive surfaces

3. I. Shape and quali~ control in the SEM

An examination of the manufactured structures in the .~anning electron micro~ope (SEM) displayed pyramidal

R. Gdhfin. $. Jacohson I Wear 217 ¢1998) 231-236

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235

250

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2

3

4

5

6

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(b)

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3

4

5

6

Sliding distance [m]

Fig. 9. The specimen shortening vs. the sliding distance for an abrasive silicon structure (a packing density of 24 tips / ram-" and a cone angle of approximately I00') sliding against a) brass and b) tin specimen.

and from 0.004 (4000 mesh) to 0 4 m m ' / N m (80 mesh) for tin. The size a n d distribution of the produced abrasive tips compares best to a paper of 320 mesh with a mean pa~t~,:le size of approximately 45 .u.m {.see Fig. 10). For this paper the wear rates were 0.04 mmVN m against brass and 0.3 mm~/N m against tin. It was found that the wear rate was strongly dependent on the tip angle. In Fig. I I. the wear rates of live different tip angles are shown.

. ~

:

4. Discussion

Fig, 10. An ordinary SiC abrasive I~t~¢r of 3.20 mesh ( mean particle size of

about 45 # m ). from the measured specimen shortening vs. sliding distance diagram (see Fig. 9), were about 0.04 mm3/N m for brass and about 0.1 mma/N m for tin. The corresponding wear rates against ordinary SiC abrasive papers ranged from 0.001 (4000 mesh. corresponding to a mean particle size of approximately 5 ttm) to 0.05 mm3/N m (80 mesh. corresponding to a mean particle size of approximately 200/~m ) for brass

In the process of developing the etch ~lution. stand;~rd compositions of the potassium hydroxide (KOH) and HNA systems 15-71 were investigated. The ethylenediamine pyrocatechol ( E D P ) system 15-71 constitutes a third promising candidate but was not tested within this project. The problem found with the KOH system was that fo¢ ~ m e compositions, it produced relatively coarse surfaces and for others, locally varying shapes. In the HNA system the main work was on optimising the composition for the ants•tropic etch needed to produce t~,c desired shapes, since the system initially is developed for i,c, tropic etching. The ability to produce well controlled abrasive surfaces offers several possibilities. It facili)ates studies of fundamen-

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Pyramid angle [ o 1 Fig. I I. Wear rate ,,'s. pyramid angle lot shalp tips with a packing density of 24 tips/ram-'.

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R. Gdhlin, £ Jm'obson I Wea r 217 ( 19981 231-2J6

tal aspects of abrasion and the development o f novel grinding and polishing ',ools. An investigation o f the so-called size effect 181 in abrasive wear has been the first application in fundamental wear research 191. In this case. previous research has been restricted to abrasive surfaces with rather large distributions in particle shape and size ( i.e., commercially available abrasive papers), e.g.. Refs. I I0,1 1 I, or restricted to single controlled shape tip action 1121. These limitations were overcome by the present technique. Next, the influence of the tip angle 1131 on the abrasion rate will be further investigated. In grinding, the present technique could be used to develop o p t i m i ~ d surfaces for material removal rate, best surface finish or minimum subsurface damage. These optimisations would certainly lead to reduced costs and an improved product quality. Other possibilities include development o f grinding tool yielding high removal rates combined with good surface finish which could result in fewer process steps and hence, significantly reduced production costs. Another possibility is to produce surfaces with specified roughness values or textures.

References I II R.N. Thomas, R.A. Wickstrom. D.K. Schroder, H.C. Nathanson. Solid-StateElectron. 17 (1974) 155. 121 A. Hariz. H.G. Kim, M.R. Haskard. IJ. Chung. J. Micromech. Mieroeng.5 (1995) 282, 131 V.G. Litovehenko, A.A. Evtukh. R.I. Marchenko. N.I. Klyui. V.A. Semenovieh.J. Micromeeh.Microeng.7 (1997) I. 141 A. Boi~n, O. Han~n. S. Bouwstra, J. Mierorneeh. Microeng. 6 (1996) 58. 151 K.E, Bean. IEEE Trans. Electron Devices ED-25 I0 (1978) 1185. [6 ] K.E. Pctersen. Prec. |EEE 70 (5) ( 1992) 420. 171 H. Seidel. L. Csepegi. A. Heuberger.H. Baumgartel.J. Electrechem. Soc. 137 ( I I 1 ( 199013612. [81 H.C.Sin, N. Saka. N.P. Sub. Wear5 (1979) 163. 191 R. G~hlin. S. Jaeobson, The size effect in abrasion studied by controlled abrasive surfaces. Wear. submitted. 1101 T. Hisakado. it. Suda. T. Tsukui, Wear 155 (1992) 297, I 111 J. Gtxldard, H. Wilman. Wear5 (1962) 114. I 121 H.R.Shetty, T.H. Kos¢l,N.F. Fiore. Wear 80 ( 1982) M7. 1131 T.O. Mulhearn.L.E. Samuels. Wear 5 (1962) 478.

Biographies 5. Conclusions • The present investigation has demonstrated one successful method of manufacturing well defined abrasive tips. showing practically constant shape and size over a silicon wafer. 7.6 cm ( 3 in. ) in diameter. • The silicon tips are capable of abrading soft metals such as tin. while self suffering very limited damage. Against harder metals such as brass, the unprotected silicon cannot sustain the large stresses generated, but the tips rapidly li-'acturc. • The technique of manufacturing controlled abrasive surfaces in silicon has proven to be well suited for fundamental research on abrasion, with restriction that soft metals must he used to aw~id damage of the abrasive surface. Acknowledgements The authors gratefully acknowledge the linancial support from the Swedish Research Council for Engineering Sciences ( TFR ).

Rickard G~thlin: received his MSc degree in Material Physics at Uppsala University in 1993. Since January 1994, he has been working as a PhD student at the Materials Science Division, Angstrom Laboratory. Uppsala University. His work has concentrated on development, evaluation and application of a micro abrasive wear test. Today. his field of r c ~ a r e h is focused on fundamental studie~ on wear and grinding at a micro scale. Staffan Jacobson: obtained his DSc in Materials Science in 1988 at Uppsala University. where he currently holds a position as Assistant Professor. His primary field o f research is tribology focused on abrasive and fretting wear mechanisms and surface coatings. He has developed models and a methodology for evaluation of abrasive wear resistance o f bulk materials, composites and surface coatings. Fields o f application are found within wear parts, metal cutting tools, electrical contacts, automotive brakes, etc.