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FILE NAME Brakes BRK DATE 1978 DOC BRK068 DOCUMENT DESCRIPTION Journal Article - Dark Electron Microscopy and Chrysotile Asbestos ' ENVIRONMENTAL RESEARCH 16 383 392 1978 On the Utility of Field Electron Microscopy in the | Determination of the Degree of Deformation In Chrysotile Asbestos An Environmental Research seat Application ATE K. SESHAN Department of Materials Science and Engineering Materials and Molecular Research Division Lawrence Berkeley Laboratory University of California Berkeley California 94720 Received June 21 1977 The degree of microcrystalline deformation in fibers of chrysotile asbestos may be distin- guished using high resolution field electron microscopy This is demonstrated by com- paring undeformed chrysotile with Union Internationale Contre le Cancer UICC standard reference samples The UICC samples are shown to be partially deformed as a result of milling in the mixing process Samples of used and unused brake shoe lining dust were examined using this technique it is shown that chrysotile asbestos in various stages of deformation undeformed to heavily deformed in automobile brake drum dust Such dark images can serve to identify the source of asbestos found in environmental pollution samples INTRODUCTION There is a controversy in the literature whether or not fibers of chrysotile z asbestos survive in automobile brake drum dust two studies Rohl et al 1976 PE Alste et al 1976 report that the fibers do survive and earlier work Lynch 1968 Hickish and Knight 1970 reports that they do not The latter claim that the sotile is converted to forsterite under the high temperatures attained in the braking process As high resolution dark electron microscope images are sensitive to the degree of deformation they can be used to distinguish between deformed and undeformed fibers and thus to resolve these two differing sets of results It is shown using this technique that chrysotile asbestos fibers in various states of deformation undamaged to heavily deformed and recrystallized in automobile brake drum dust SAMPLE SELECTION AND PREPARATION Z Four samples were selected A undeformed chrysotile ore samples from ser- pentine outcrop of Calaveras County California B slightly deformed UICC reference standards of Canadian chrysotile milled during preparation to reduce fiber size Timbrell et al 1969 C unused brake lining dust collected during burnishing prior to installation of new brake shoes and D brake drum dust collected from the front and rear brake drums of a State vehicle obtained during brake shoe service Seshan and Smith 1977 Samples were transferred directly to Formvar electron microscope grids 6 and coated with carbon on both sides : eaertsLeang Tha 2 elas8 Calle 1/17/83 46 Calle Calle Calle 0013-9351 1613-0383 1613-0383 Copyright '1978 by Academic Press Inc. All rights of reproduction in any form reserved : 1/17/83 1/17/83 384 K. SHAN EXPERIMENTAL As the high resolution dark field method is described in great detail elsewhere Hirsch et al 1969 only a very brief description is included here An electron beam striking a polycrystalline specimen with grains of different orientation e.g. A and in Fig ) is diffracted into cones causing the typical polycrystalline ring pattern The resolution field method consists oftilting the incident beam so that the part of the diffracted ring passes through the optic axis of the micro- et, scope Fig 1b The tilting is accomplished with the electronic beam tilt device Then an aperture collects intensity only from those crystallites diffracting into this Coe part of the ring e.g. B Fig 1c Various factors involved in the interpretation of the diffraction patterns of chrysotile asbestos fiber bundles are shown in Fig 2. The actual lattice of chrysotile is a defected scrolled crystal with fiber axis along a Yada 1967 The reciprocal lattice of this crystal should be some form of a spiral equispaced along the a axis Zvyagin 1967 and Whittaker 1966 have studied diffraction effects from concentric cylinders How deformation and shear will affect the diffraction patterns has not to the author's knowledge been studied and is under study here The simpler case of an undeformed defect chrysotile fiber where the fiber is idealized as a series of concentric cylinders as first proposed by Whittaker 1969 is shown in Fig 2i The reciprocal lattice then consists of a series of concentric rings shown in Fig 2ii only the two rings in the 2kl layer are drawn The electron diffraction pattern represents the intersection of the reflecting or Ewald sphere ES Fig 2 with these rings Hirsch et al 1969 This ought to result in a series of spots as shown in the -2kl layer line Streaked patterns are tt however obtained from single fibers of chrysotile Yada 1967 Seshan and Smith 1977 othov The explanation for the streaking probably lies in refraction effects and the fiber shape Yada 1969 it could also result from the various faults produced during the FIG 2 Illustration of t idealized as a senes of co 1967 The reciprocal latte " axis as in the 2k Lays produced as shown on the bly from the spiral shape c fiber bundle fibers of diff typical arcuate pattern growth of the crystal their influence on def When bundles of fi and 3b showing a arcuate patterns of dark image obta 3b should yield unife to be expected only i the experimental obs y b c Fig 1 Hustration of the high resolution dark method a The bright image from a selected area of poly crystal illustrated with two grains A and B b The situation after gun tilt only a portion of the diffracted intensity is collected by placing the objective aperture as shown c The resulting resolution dark image with only favorably oriented grains g B showing dif- fracted intensity or lighting up Bright and dark Fig 3. The dark field as explained above an undeformed cryst The striking featur hollow canals and thi be explained on the F DEFORMATION OF CHRYSOTILE ASBESTOS 385 385 385 t a kl Ikl I i i . 1 loo. I t . | | OXI IX -2kl le f IV N _ \ Lf rc \ \ <_ a XN NN og J . NX N - 4+ &A F2 F2 Illustration of the real and reciprocal lattice of a defect chrysotile fiber i and u idealized as a series of concentric cylinders The actual structure is a defected spiral sheet Yada 1967 The reciprocal lattice of the ideal fiber is then a series of equispaced concentric rings along the - AXIS as the 2kl layer when these rings intersect the Ewald sphere ES spots ought to be p died as shown the -2kl layer line However streaks are observed Yada 1967 arising proba- p's from the spiral shape of the fiber defects and strains formed during the scrolling process When fibe bundle fibers of different orientations is imaged the layer lines are smeared out yielding a typical arcuate pattern as Fig 36 growth of the crystal The influence of these faults on the diffraction patterns and their influence on deformation needs further study When bundles of fibers are involved the streaks are replaced by arcs Figs 2iv and 3b showing a strong tendency toward a texture and yielding the typical arcuate patterns observed by several researchers e.g. Rohl et al 1976 The image obtained by imaging any part of the arc as shown in Figs 1 and : 35. should yield uniform intensity if the crystal is homogeneous This is the case to be expected only in the case of the undeformed fibers is consistent with the the experimental experimental observations in Figs 3a and 3b RESULTS Bright and field images of naturally occurring chrysotile A are shown in Fig 3 The dark field is obtained by imaging a portion of the diffracted intensity as explained above The result is a uniform contrast as would be expected from aada) an undeformed crystal The striking feature of the dark images is the great intensity along the per hollow canals and this needs further investigation The intensity difference cannot ' be explained on the basis of differences in absorption alone it appears that some 386 K. SESHAN Ty abe condenser the fib Fig 3a precaution focused and the b A natural chrysotile a and b Dark and diffraction pattern Notice the Fic 3 Sample and undeformed A faint unusually bright and undamaged internal canals the crystal is homogeneous outline of the objective aperture is seen in the diffraction pattern of Fig 3b c Illustrates the blistering as a result of exposure to the beam are operative Notice that the canals are sometimes bright diffraction processes the intensity is uniform and sometimes dark In the crystalline part however and it is clear that these fibers are free of gross deformation fibers like most sheet silicates are sensitive to 100 elec- Chrysotile trons Langer et al 1974 Seshan 1975 Precaution must therefore be exercised because focusing the while obtaining the high resolution field images upon FIG 4 Sir a that illustrate that th polycrystal polycrystal The spa different set st* obtained Pe _ aad DE FORMATION OF CHRYSOTILE ASBESTOS 387 condenser the fibers tend to become blistered In the dark image shown in Fig 3a precaution was taken to prevent any beam damage the condenser was not focused and the beam was tilted in the dark field mode The condenser was then 0) A reste weet shake care sot y Notice Notice the the . faint BlisterinBlistgering FIG 4 Sample B UICC Standard Reference Canadian chrysotile a and b Dark images that illustrate that the ball milling during the mixing step converts the chrysotile into a grained polycrystal The aperture is moved from one part of the ring to another resulting in an entirely different set of grains lighting up c The bright image from which little information can be obtained 388 K. SESHAN focused to obtain the diffraction pattern shown in Fig 3b The resulting beam damage is shown in Fig 3c The use of beam sensitivity to distinguish chrysotile from other sensitive materials e.g. the amphiboles has been discussed by Langer et al 1974 Dark and bright images of deformed UICC standards B are shown in Figs 4c The effects of deformation are clearly seen in the dark images 4a and 4b and in the electron diffraction patterns but not in the bright images 4c The there appear small microdomains of strongly textured changed to those structural nature ( effect of translatir in Figs 4a and 4b or show diffracted To isolate the e shoe burnishing d B is not inform internal canals s FIG 5. Sample C Burnishing dust from an automobile brake drum prior to installation a Dark field b diffraction pattern and c bright field Notice the preserved canal which shows up the dark image in a indicated by the arrow and arrowhead This and the well preserved diffraction pattern b shows that the fibers are not as deformed as the UICC samples conclusion that cannot be inferred from the bright image c FIG 6 Sample D Although the bright fition patterns are rema this can happen for a Ne a Ge : f ae ee re - By rah i FAS ae ee mn Bape DEFORMATION OF CHRYSO ASBESTOS 389 images 4c The clear internal canals of the undeformed sample A are destroyed there appear small submicron 100 ^ areas which light up as if they were grains or microdomains of different orientations consistent with this observation the strongly textured diffraction patterns of the undeformed fibers Fig 3 are changed to those of a polycrystal At the present the crystallographic and microstructural nature of the deformation is not clear and warrants further study The effect of translating the aperture to a different part of the diffracted ring is shown in Figs 4a and 4b The result is that grains in a different orientation light up or show diffracted intensity signifying that this is truly a diffraction effect To isolate the effects of deformation during the braking process unused brake shoe burnishing dust C was examined Figs 5a and 5b The bright image B is not informative whereas the dark image clearly shows some intact internal canals see arrow and arrowhead Fig 5a resembling the undeformed hy ag FIG 6 Sample D Brake drum dust after use in a state vehicle a Bright field and b dark field SORE Although the bright field is not distinctive the resolution dark field and the selected area diffrac- rene tion patterns are remarkably different The crystal is quite inhomogeneous with very large grain sizes this can happen for a variety of reasons see text ee RIT oR 390 K. SHAN a also small deformation domains which clearly re- chrysotile Fig 3 There are therefore deformed less semble the UICC standard B samples These fibers are than the UICC standard samples Fig 4 as some intact internal canals can still be to derive this conclusion from the bright range seen It is impossible fiber found in brake drum dust are Samples of of heavily deformed chrysotile shown in Fig 6 in the bright and dark field Whereas the effects of deformation are the difference difference in the field image is quite not evident in the bright image is reflected in the electron diffraction striking The crystal is inhomogeneous This This 6c The mottled contrast of black pattern now showing a number of spots ( Fig severe surface deforma- and bright areas could arise from one of secvoenravlercsaiuosnes or grain growth under tion leading to uneven crystal thickness or transformation of local areas heat and deformation to large grain polycrystal forsterite Further investigation into carefully into a new crystallineisphraeqsueiree.dg.to decide which it is It is however quite clear found in the deformed chrysotile that these identifying effects are associated only with the samples used brake drum dust also found in the brake drum dust Undeformed and unaltered chrysotile is after use sample D Fig 7 This was confirmed by electron diffraction collected was calibrated using a which were indexed after the camera constant patterns deposited gold standard with the following results Used brake dust Yada 1967 diameter ^ diameter ^ hhl 2.62 2.60 130 2.34 2.30 220 1.49 1.46 005 clear dark evidence that not all the fibers are deformed There is also found in the brake drum Figure 7 shows the dark- and bright images of fibers asbestos B dust the field image resembles that of the UICC chrysotile 4 and those in the unused brake lining C. shown Fig 5 the grain shown in Fig Based upon this observation it is sizes being the same as in the UICC samples concluded that a variety of products ranging from almost undeformed to com- pletely transformed chrysotile products exist in brake drum dust DISCUSSION AND CONCLUSIONS resolution field electron microscopy can dis- It is demonstrated that resolution asbestos fibers In particular it tinguish the degrteheatofUdIeCfCorsmtaatnidoanrdinchrcyhrsyostotiilleecchhrryyssootitlielehas undergone microdeformation then has been shown The dark method may of ball milling in the preparation step as a result trace the origins of asbestos fibers be used by environmental researchers to in the micro- The dark images suggest that there are significant chaanngdesmicrostructural structure of chrysotile upon deformation the cystallographic details of which are complex and are worthy of further study to the beam sensitivity of chrysotile Yada 1967 There are several references effect to distinguish sotile from et al 1974 The utility of using this Lotahnegrersensitive materials could be of value to environmental pollution research 7 FIG Sample shows that relative field images a are indicating little det This prelimi chrysotile asbe deformation e thank G R problem to my att Walter John and by the US Ener AlsteJ aks pop 10 Hatch D 1970 Hickish E. Occup Hy Hirsch P B.H Crystals Langer A M asbestos fr Lynch JR JR I Rohl A. N. L. N. lining main Seshan K and DEFORMATION OF CHRYSOTILE ASBESTOS Fit 7 Sample D Automobile brake drum dust after use a Dark field and b bright field This shows that relatively undeformed fibers survive in the brake drum dust The grain sizes in the darkfield images a are comparable to those in the burnishing dust Fig 5a or the UICC samples Fig 4a indicating little deformation during use This preliminary study also shows the need to study details of the growth of the ^' chrysotile asbestos and the nature of the defects involved if all the diffraction and a 4 4 deformation effects are to be understood d hee ACKNOWLEDGMENTS ase I thank G R Smith of the Air Industrial Hygiene Laboratory AIHL Berkeley for bringing this J 4 wert problem to my attention and preparing preparing the samples I acknowledge useful discussions with him and Dr MaEt Walter John and thank Professor Washburn for encouragement This work was supported financially cepa by the US Energy Research and Development Administration Dow REFERENCES Ser Alste J. Watson D. and Bagg J 1976 Airborne asbestos in the vicinity of a freeway Atmos Environ 10 583 Hatch D. 1970 Possible alternatives to asbestos as a friction material Ann Occup Hyg 13. 25 Hickish D E. and Knight K. L 1970 Exposure to asbestos during brake maintenance Ann Leys Occup Hyg 13 17 Ih Hirsch P B. Howie A. Pashley D W. and Whelan M. J. 1965 Electron Microscopy of Thin ep Crystals Butterworths London Rea Langer A M Mackler A. D. and Pooley D. 1974 Electron microscopical investigation of RR asbestos fibers Environ Health Perspect 9 63 Be Lynch JR JR 1968 Brake lining decomposition products J. Air Pollut Control Assoc 18 824 ye Rohl A N. Langer A. M. Wolff M. S. and Weisman I. 1976 Asbestos exposure during brake hate lining maintenance and repair Environ Res 12 110 a ty r Seshan K. and Smith G. R. 1977 Characterization of chrysotile asbestos in automobile brake drum K. aa 392 K. SFSHAN dust by transmission electron microscopy Proceedings 35th Annual Meeting of EMSA Claitors Baton Rouge La standard reference Timbrell V. 1969 Characteristics of the international union against cancer samples of asbestos Pneumoconiosis In Proceedings International Conference on Pneumoconiosis Johannesburg p 28. In Oxford Univ Press England Whittaker E. J. W. 1966 Diffraction contrast in electron microscopy of Chrysotile Acta Crystal- logr 21 4616 resolution electron microscope Acta Crystal- Yada K 1967 Study of chrysotile asbestos by a high Zvyalgoign 2B.3 B7041967 Diffraction Analysis of Clay Mineral Structures Plenum New York i : oer ED FEY Be MN ENVIRONMENTAL RESEARCH RESEARCH Development Exposure An aerosol exposal the lungs of kamater concentrations et ^> and were sacrificed to ...ghter...ghter xp wir a month rew ot be reversible increa increa the numbe f ashen No other too tet 1 apparent othe. t Asbestos cement ranks first amorata amorata Leineweber P AC sheets and pipes are being exposed a * to investigate the trolled laboratory de outbred LVG 1 chosen because st^/d in terms of lesion s dusts and th chrysotile asbestos a in the exposed air coniosis and cigare and epithelial less e hamster following following number of investiga investiga 1970 Dontenwillet Dontenwillet fiotti 1968a b 1970 Althoff and Auttan Auttan study of the re re extensively ex gens and convenier convenier 1970 and Do