We.at200 (1996) 206-214
Abrasi~)n resistance of nanostructured and conventional cemented carbides K. Jia, T.E. Fischer * Departmentof MaterialsScienceand Engineering. StevensInstituteof Technology. Hoboken,NJ 07030, USA
Abstract The abrasionresistance,ofnanostmc;uredWC--Cocomposites,synthesizedby a novelsprayconversionmethod,is determinedandcompared with that of conventionalmaterials.Scratchingby diamondindenterand abrasionby I:.~rd(diamond),soft (zirconia) and intermediate(SIC) abrasives was investigated.The size of the scratch formed by the diamond is siu~plyrelated to the hardness of the composite. Plastic deformation,fractureand fragmentationof the WCgrainsincreasewiththeirsize.Nanescalecompositesshowpurelyductilescratchformation. Nanocompositespossess an abrasion resistanceapproximatelydouble that of the most resistantconventionalmaterial:this is a highergain than the increase in hardnesswhichis at most 23%. This large gain is due to a specificgrain size effect on abrasionresistancein the case of diamondand SiC abrasiveand to a veryrapidincreaseof abrasionresistancewithhardaessin the case of the softer (SiC and 7_aOz)abrasives. The observationof the abradedsurfacesof conventionalcompositesreproducedthe knownmechanisms:plasticdeformationand fractureof WC grains by hard abrasives; removal of binder phase and fall-outof we by soft abrasives. Magneticfields from the ferromagneticCo prevent the observationof abrasion mechanismsin the very fine-structurednanocomposites. Keywords: nanostmcture;WC-Co composite;abrasion resistance
1. Introduction Materials with fine-scale micmstructures have long been recognized to exhibit remarkable and technologically attractive properties. When the grain size decreases to the nanometer scale (tens of nanometers), one obtains a novel class of material, called "nano-structured material", which may possess properties different from those of conventional polycrystalline solids [ 1]. Based on the HalI-Peteh relationship, one expects very high hardness. The large fraction of atoms residing in grain boundaries may also increase the toughness of the material or even realize superplasticity. Accordingly, a nanophase may give a material high hardness and toughness. WC-,Co sintered nanostmcture composites have been developed with the anticipation of superior properties . These materials allow the achievement of small mean free path of dislocations in the ductile binder phase at relatively high cobalt concentrations. It was found that the binder phase in the nanostmcturedWC--Cocomposites is richer in tungsten and carbon than in conventional (larger-grained) materials and that it contains amorphous regions . These features
* Correspondingauthor. 0043-16481961515.00 © 1996ElsevierScience$.A. All rights reserved
result in hardness as large as 23.5 GPa without the attendant decrease in toughness . It is the purpose of the present contribution to quantify the benefits of nanomcter size structure to the abrasion resistance of the WC-Co composites. In order to obtain a good reliability of the study and provide a continuum of grain sizes, these nanocomposites arecompared to conventional WC--Co composites with WC grain sizes ranging from 0.7 to 2.5 gm which are subjected to the same abrasion tests and analytical methods, Abrasion of a material is caused by the penetration by abrasive particles and material removal by fracture and plastic flow; these are reflected by the mechanical properties such as hardness and fracture toughness. The variations of grain size of the hard, brittle WC phase and the volume fraction of the ductile cobalt phase give WC--Co composites a wide range of properties between ductile and brittle. Studies have shown that in abrasive wear, material is removed from the surface of cemented carbides by mechanisms involving gross plastic deformation due to the yielding and extrusion of binder metal, spalling due to crack propagation in the [~inder metal, and carbide fragmentation [5-9]. It has been recognized that the microstructure scale plays a decisive role ia determining the fracture mode and mechanical properties, and therefore the wear resistance of cemented carbides [ 10-15 ].
K, Jia, T,E, Fischer/Wear200 (I996)206--214
The microstructure scale of cemented carbides is characterized by carbide grain size and the volume fraction of cementing metal binder phase. Either lowering the cobalt content or decreasing the WC grain size increases the hardness of the composite, and an increase of hardness is accompanied by a loss of bulk fracture toughness in conventional materials [ 10-15]. Evidently, wear is a local event, and the local resistance to deformation and cracking can play a role whose nature and magnitude may not be readily deduced from bulk properties [ 16,17]. Experiments with single hard particles impacted on WC-Co have shown [ 14] that a crater is formed by the cracking of WC grains when the grain size is large, whereas the crater results mainly from plastic deformation of the binder phase with small WC grains. This means that the fracture behavior of WC grains, which has a significant effect on the abrasion resistance of cemented carbides, is related to their size. The above concepts are significant since they suggest that a finer microstructure may give cemented carbides higher abrasion resistance beyond the effect of increased hardness.
2. Materials Conventional carbides, in which the average WC grain size ranges from 0.7 to 2.5 ixm and the cobalt content from 6 to 20 wt.%, were obtained from the Federal Carbide Company (FC) and the RTW company. The nano.structured carbides with carbide grain size of 0.0"/ixm and cobalt contents from 7 to 15 wt.% were sintered by the RTW Company (RTW) from powders synthesized by Nanodyne through the spray conversion technique . The composition and microstructufa/details of the materials and the relevant mechanical properties  are listed in Table 1. Prior to the tests, the surfaces of the samples were prepared by three sequential
steps of polishing: first with a 15 ttm grade diamond grounder, then with a 6 p,m grade, and finally with a I p,m grade diamond paste.
3. Scratch tests 3.1. Experimentalmethod
Scratch tests were ~erformed on a modified Vickers hardhess tester. The experiment consists of performing an indentation with a Vickers hardness diamond indenter, then dragging the loaded indenter parallel to the specimen surface at a speed of 0.05 mm s- t for a length of about 5 mm. The loads used for the scratch tests were kept constant at l/X), 500 and 1000 g. Experiments were performed with one, two or four passes of the indenter in a given scratch. The scratches were examined in the scanning electron microscope. 3.2. Observation of scratches
Fig. I shows single and multiple travel scratches on sample RTW10, which has an average WC grain size of 2.5 tim and 10 wt.% Co. The tracks are bordered by a piled-up ridge of WC grains and binder material that were extruded from the track [Fig. l(a)]. Some of these displaced WC grains are cracked and some underwent plastic deformation with slip steps shown on their surfaces; inside the track as well, a number of WC grains are deformed or fragmented. As the load increases, the cracking of the carbide grains in the track becomes more severe and more material is displaced from the track to the border, Although extrusion of the binder phase alone was seldom observed in the track and at the border, a smaller amount of binder phase found in the scratches than in the original m~tcrial suggests that binder extrusion did take place even in single travel scratching. It is evident that the extruded material and fragmented WC grains can be easily
Table 1 Nominalcomposition and structural characteristicsof testingsamples Sample
F'C20 FCI5 FC6 F426M RTWi0 RTW6 RTW6M NAI5 NAI3 NAT
1.2 1.0 1.0 0.7 2.5 1.2 0.8 0.07 0.07 0.07
30.5 23.6 10.1 I0.1 16.3 10.1 10.1 23.6 20.8 l 1.7
1.06 0.61 0.39 0.25 1.0 0.43 0.14 0.068 0,068 0,039
! 100 1310 1710 1860 1280 1680 1840 1940 2050 2300
N/A N/A 9 8.2 16,4 10 8.4 8 8,3 8.4
The numberin the sampleidentificationindicatesthe cobaltcontentin weight%.Example:NAt5contains15wt.%and 23.6col,%Co,
K J/a, T.E. Fischer/Wear 200 (1996) 206-214
Fig. I. Scratchesproducedwill)a Vickersindenterg 100g loadonconven. lionalWC--CocompositeRTW10with2.5 ~m WCgrainsizeand 10wt.% Co: (a) single scratch, (b) T~'o travel scratch; (c) four-travelscratch (magnificationscaleson bottomrightcomer).
Fig.2+Scratchesproducedwitha Vicke~in¢lentorontheconventionalWCCo compositeRTW6with 1.2p.m ca~idc grain size and 6 wl+%cobalt content:(a) singlescratch;(b) two travelscratch;(c) four-travelscralch (magnificationscaleson bottomrightcorner).
removed in subsequent scratches. This is depicted by comparing the two-travel [Fig. l(b)] and four-travel scratch [ Fig. 1 (c) ] with the single travel scratch [ Fig. 1(a) ]: more material is displaced from the track to the border, and then removed as wear debris in repeated scratches, and WC grains in the track were more severely fractured and more fragmented carbides were removed from the track. Fig. 2 depicts the scratches on RTW6M with 0.8 ttm WC grain size and 6 wt.% Co. The same scratch appearance as on sample RTW10 is observed here: WC grains and binder metal are displaced together to the border, and carbide grains are fragmented and plastically deformed, However, less material is displaced and fewer WC grains are fragmented in the scratch, The result is less material removal in repeated scratching. In addition, less binder phase is removed by extrusion in finer-grained composites, as seen by comparing the
widths of the binder phases in the scratches with those in the unscratehed region, As expected from their hardness, the nano-stmetured cemented carbides exhibit higher scratch resistance. This is depicted in Fig. 3. The scratches are smaller in accordance with the higher hardness of the material; they are smooth with ductile scratch features. Very little material is extruded [Fig. 3(a)], even in the case of multi-travel scratch [Fig. 3(c)]. No micro-cracking is observed in the track of low-load scratches (100 and 500 g loads). Cracks perpendicular to the sliding direction of the indenter are found at the base of the tracks of 1000 g-load scratch. The dimensions of these cracks are one to two orders larger than the carbide grain size ( ~ I00 nm). They have the shape expected in a homogeneous hard material and are caused by the tensile stresses behind the indenter. It is impossible to observe whether there is fracture in the WC grains because of their
K. Jia, T.E.Fischer/Wear200(1996)206-214
4.I.L Diamond.wheel abrasion test Abrasion by diamond was performed with the geometry of pressing a flat sample against a high-concentration diamond cutting wheel with diameter D = 127 mm and thickness t~-0.4 ram. The concentration of diamond particles was about 16 eel.%. The wheel rotated at 60 rpm to give a surface velocity of 0A m s- t. It was dressed after every experimem, and cleaned .... o.,a c,,,J,~u ^~- a with water during the tests. The specimens were abraded under a load of 2.94 N for a period of 50 turns, ultrasonically cleaned and weighed to 0.05 rag. Five runs were made for each sample. To account for the wear of the abrasive wheel and to give a uniform exposure to each sample, tests were performed on the different materials in the sequence ~,--B-C....C-B-A. The results were reproducible to ± 8%. An average value of five runs for each sample was taken and converted to a volumetric wear rate based on the sample's density. 4.I.2. Pin-on-disk abrasion test with SiC and Zr02 Abrasion tests with silicon carbide and zirconia as abrasives were performed on an ML-100 pin-on-disk tribometer. WC-Co samples were used as pins with 2.5 × 2.5 mm crosssections; they were loaded with 8.85 N and slid against disks with bonded abrasive which rotated at 54 rpm. The pin was displaced in a way to increase the radius of the track on the abrasive disk by I mm per turn. This ensured that fresh abrasive was constantly used during the test. The sliding distance of the pin against the disk is
s= ~(~- d)/L
Fig. 3, Scratcheson the nano-struoturedWC-.CocompositeNAt3 with 0.07 p.mcarbidegrainsizeand 13wt.%cobaltcontent:(a) 100g normal load; (b) I kg normaltoad; (e) five-travelscratchat 100g normalload. Notethe differentmagnifications,bonomrightcorner. very small size and the limits imposed on the microscope's resolution by the magnetic fields surrounding the cobalt.
where L is the radial displacement of the pin in one spin of the disk, rt and rz are the distances from the center of the disk to the pin at the start and at the end of a test run. In this study, rl = 5 ram, rz = 110 ram, L = 1 mm and S = 38 m. Two kinds of abrasive disks made by the LECO Corporation were used in this investigation. One is a silicon carbide plain-back abrasive disk in 120 grit ( 106 ~m average particle size). The hardness of the SiC is Hvo.~ffi2600. The other is a zirconia plain-back abrasive disk of the same grade file hardness of the ZrO2particles is Hvo., = 1400. A new abrasive disk was used for each test run. Five runs were made for each sample with the same abrasive. The reproducibility of the measurements is 5: 5%. The average of the five measurements was taken and converted to a volumetric wear rate.
4.1. Abrasion test methods
4.2. Experimental results
The abrasion of materials depends on the quantity of material removed and the abrasion mechanism on the hardness of the material relative to that of the abrasive. Accordingly, the abrasion of We--Co composites was studied with three abrasives: diamond, which is much harder than the composites, ZrOz which is softer than most We--Co composites, and SiC, the hardness of which is slightly higher than that ofthehardest composite. All tests were of the two-body abrasion type where the abrasive is bonded to a solid substrate.
Abrasion rates are measured in terms of the volume of material removed (in mm3) per unit load (N), and distance of sliding against the abrasive (m). The abrasion resistance is defined as !/abrasion rate; its units are Nm mm -3. Fig. 4 shows the abrasion resistance vs. hardness of all the materials investigated when abraded by diamond, SiC and Z,"O2.The abrasion resistance is plotted on a logarithmic scale in order to display all the values which vary over three orders of magnitude. The abrasion resistance increases with the hard-
K.J/a. 7".£Fischer/Wear200 (!996)206-214
SiC abrasive ZrO= zelxame diamondalx'ashre
1 2 0 0 1 4 0 0 1600 1800 2000
Hams, (l~mm=) Fig. 4. The vmrindonof abrasion~esislance(in Nmmm-~) with hardness of WC-Coabradedby diamond.SiCandZtOzabrasives.The logarithmic scaleisusndloallowapn~enlationofalldalaovertheeeordersofmagnitude, Fullsymbols.*conventionalcomposites.Emptysyml~ls:nanocomposites. hess of the material, hut does so much more rapidly against soft than against hard abrasives. The abrasion resistance of the cermets against SiC is lower than against the har&r diamond wheel. We attribute this to the configuration of the two abrasives. Wear oftbe repeatedly used diamond during the test and smearing by Co reduced the cutting efficiency so that &essing of the wheel was necessary before each test. In addition, the volume fraction of diamond was only 16%, which allowed a sizable amount of the normal load to be carried by the metallic support of the wheel. In the case of SiC, by contrast, the sample was always exposed to fresh abrasive which carried the entire load.
~ ( g4~mZ ) Fig. 5. The variationof t~sislag'ewithhardnessof we-Co compositesto abrasionby diamond.Fullsymbols:nanocomposites.Opensquares:conventionalcermets. k vs WC groin =lz~ 0,055
RTW6MF ~ l F~,lD,.,.. - , ' ' ' ' ' ' ' ' l r
0.040i 0.035 0.030
~ 0.025 0.020 0.015
4.23. Abrorion by diamond
The resistance of the materials to abrasion by diamond is shown in Fig. 5. Since the diamond is much harder than all the composites, the data can be considered to follow the classic law of abrasion:
H where V is the volume of material removed, F. the total no:real force pressing the sample on the abrasive, and S the total distance the sample travels against the abrasive. The abrasion rate defined above is VIF.S= kill. The abrasion behavior of the material can also be characterized by the abrasion coefficient k, which is dimensionless. Examination of Fig. 5 reveals that Eq. ( 1) does not describe the data with precision: another quantity influences the abrasion rate. In Fig. 6, the value ofkcalculatedaccording to Eq. (1) is plotted against the WC grain sizes in rite composites. The samples with hardness greater than i4~,~Lg ram- 2 including the nano¢omposites, fa!! on a line that can be expressed as k = 0.027 + 0.OlTd, where the average WC grain size d is
1 2 3 AveraoeWCgm~size(~n) Fig.6. Dependenceof the abrasioncoeflidentk in Eq, (I) on theaverage WCgrainsizeofWC---Cocompositesabradedbydiamond.Triangles:nanocompositcs.Circles:sampleswith,6 wt.%Co.Squares:softsamples. expressed in micrometers. It is interesting to note that the effect of the grain size reduction on k follows the same trend for the conventional materials with 6 wt.% Co and the nanocomposites. The softer samples, with average WC grain sizes 1, 1.2 and 2.5 Itm, present a separa:: and similar dependence ofk on d. The increase in abrasion resistance provided by the nanocomposites (NAT) over the highest value oftheconventional corrects (FC6M) is a factor of 1.65, and of this the increment in hardness is factor of 1.23 and the decrease in k from Fig. 6 contributes a larger factor of 1.33. Thus the direct effect of the WC grain size reduction is larger than that of the hardness;
K. Jia, T.E. Fischer/Wear200 0996) 206..214
deformation of the composite and by fragmentation of the WC grains. The smaller the WC grain, the more resistant it is to fracture [ 14]. Thereforewe attribute the general increase of abrasion resistance to the hardness of~o material and the dependence of the k value on grain size to the resistance to WC grain fragmentation. Fig. 7(b) shows that the abrasion of the nanocomposites is entirely by plowing. The small size of the microstructure and the limit on the microscope resolution imposed by the magnetic field of the Co prevent us from obtaining evidence on the possible fracture of the 70 nm WC grains. 4.2.2. Abrasion by Zr02
Fig.7. Scanningelectronmicrographsof WC-Cosurfacesabradedby diamond:(a) conventionalWC-CocompositeRTWI0; (b) n~o-slructured WC--CocompositeNAI3 (magnificationscaleson bottomrightcomer). this is also apparent from the jump in abrasion resistance between the FC6M and NAI5 in Fig. 5. Scanning electron micrographs of the abraded surfaces shed light on the abrasion mechanisms. In the conventional composite [Fig. 7(a)], material removal occurs by plastic
As Fig. 4 shows, the resistance to abrasion by the soft ZrO, increases very rapidly with the hardness of the material: as the hardness doubles, the abrasion resistance increases by a factor of 400, in a stark departure from Eq. (1). We recall that the hardness of zirconia is 1400 kg mm-: (or 14 GPa), which is in the middle of the hardness range of the composites. The accepted theory  predicts a transition from "hard" to "soft" abrasive behavior when the ratio of hardness of the abrasive to that of the abraded material I-II/ Hm= 1.2, which corresponds to a material hardness of 1160 kg mm-2. Thus we may consider the abrasion of all the compogites to occur in the soft abrasive mode in which penetration of the material by the abrasive occurs less and less as its hardness increases. The differencebetween abrasion by diamond and by zirconia is apparent in a comparison of the abraded surfaces, Fig. 8(a) and (b) and Fig. 7(a). In the conventional material [Fig. 8(a) and (b)], the soft zirconia
Fig. 8. Scanningelectronmicrog~h:s~f wc-co ~urfaees::~:bi:zi~0hia: (i)~d structuredWC-CocompositeNA!3 (magnificationscaleson bottomrightcomer).
(c} and (d) nano-
K. Jia, T,E.Fischer/Wear200(1996)206-214 Abrasion R e s i m ~ of We.co
uprooting of carbide grains. It seems that this abrasion p r ~ s s is less dependent on the local fracture strength than in abrasion by diamond, and therefore less on the carbide grain size. The abrasion resistance changes so rapidly with hardness that it is not possible to decide ~'rom Fig. 4 whether there is a specific influence of the WC grain size. Fig. 8(c) and (d) depict the surface of the nanoslructured sample NAI3 abraded by Z~2. Its appearance corresponds much more to sliding (adhesive) wear than to abrasion. No surface material displacement can be seen. Flakes of material are observed within the scratches, some of which are still bonded to the sample's surface. The material was removed by the delamination process involving crack development at the subsurface in friction-generated shear. The flake wear debris shown in Fig. 8(d) is further evidence for this wear process.
m u g IK3O ItlWl0
! ,= 1000
mrdnm (Jq~/,.,,,.1) Fig.9. The variationof abrasionresistancewithhardnessof WC-Cocomposites~,bradedby siliconcarbideabrasives(Hv,=2600kg ram-2). Open squmes- hardabrasivebehavior[Eq. ( 1)], fullSCluares-transitionto soft abrasiveb~havior,circles= nan•composites. abrasive is not able to form groves in the material or scratch the harder WC grains. The abrasion by zirconia has removed the cobalt binder [Fig. 8(b)] and exposed the WC grains, some of which have been fractured. This suggests an abrasion mechanism of WE-Co composite described by Larsen-Basse and Tanouye  and Larsen-Basse and Koyanagi : the abrasive wear of cemented carbides proceeds by the removal of the binder material followed by the fragmentation or
Eg. 10.Scanningelectronmict~phs OfWC-C0s ~ (¢) a n d ( d ) napo-Mt'ucRm~ W C - C o ¢ o m p ~ i t e N A I 3 .
4.2.3. Abrosion by SiC Fig. 9 presents the abrasion resistance of the materials against SiC. The hardness of SiC Hv = 2600 kg ram-2 so that the transition from hard to soft abrasive should occur [ 18] at a ratio of H,/Hm=I.2, i.e. at a material hardness of 2150 kg mm -2, namely at sample NA13. For sample hardness lower than 1680 kg m m - ,2 the abrasion can be described by Eq. (1) with a value of k = 0.13 as shown by the straight line obtained by linear regression. For harder materials, the abrasion resistance increases rapidly, as expected [ 18]. As in the case of abrasion by diamond, one can put forward the argument that the increase in abrasion resistance from sample RTW6 to FC6M is due to the direct effect of WC grain size,
by~liconcarbide:( a ) ~ ( b ) ~
K. J ia, I.E. Fischer/Wear 200 (1996) 206-214
as is the jump in resistance from FC6M to NAI5. Since this is also *.hehardness range where the transition from hard to soft abrasive occurs, the effects of hardness and grain size cannot be neatly separated. The rapid increase in abrasion resistance of the nano, o'aposites is due to the transition to soft abrasion. The surface of a hard conventional composite RTW6 abraded by SiC is shown in Fig. 10. The appearance of the track Fig. lO(b) is intermediate between those of Fig. 7(a) and Fig. 8(b). The binder material has been partially removed around the carbide grains, and individual WC grains are standing in relief on the surface. They exhibit cracks and missing fragments at the edges. The selective removal of the binder phase is also proven by the lower cobalt content of the abraded surface examined by EDS. Fig. 10(c) and (d) present the surface of n ano-strucmred sample HAl 3 abraded by SiC. Selective binder removal was not observed, and EDS examination shows no difference in the cobalt content between the abraded and unabraded surfaces. The material has clearly been abraded: the surface is similar to that of Fig. 7(b) and does not show evidence of adhesive wear as against zirconia [Fig. 8(c) and (d)]. This is in agreement with the fact that abrasion of the nanocomposites by SiC is 100 times faster than by zireonia (Fig. 4). The maximum abrasion resistance obtained with the nanocomposites is three times higher than the highest resistance of the conventional material ( FC6M ). In this case, the benefit is mostly due to the increased hardness, approaching that of the abrasive.
5. Discu~ion The abrasion resistance of the WC--Co composites has been found to depend on their hardness and on the size of the WC grains. The hardness of the composite determines the penetration of the abrasive into the material and the WC grain size influences the fracture and material removal mechanisms. The hardness is measured by an indentation which is larger than the grains and represents an average property of the material. Abrasion events occur on a scale that is smaller than the WC grains in a coarser, conventional composite, but larger than the grains in the nanocomposites. Therefore the details of the abrasion processes depend on the microstructure of the material as the SEM micrographs have shown. The hardness of a WC-Co deposit increases with decreasing dislocation mean-free path A = acd in the Co, where c is the concentration of Co, d is the average WC grain size and a is a factor that depends on the shape of the WC grains. a = 1/3 for grains that are cubic or approximately spherical. Thus a decrease in Co content and of grain size increases the hardness and the abrasion resistance. The data indicate that a reduction of the WC gTain is more desirable than a decrease in Co content. Further reductions of the grain size to the nanometer scale, as we have seen, extends the range of hardnesses that are possible, provides a ductile behavior of the
materials in scratching and in abrasion, and further extends the abrasion resistance by reducing WC grain fracture.
Against all three abrasives, the highest abrasion resistance of the nanocomposites is approximately twice that of the best conventional material. This increase is larger than that of the hardness alone, which is about 26%. 2. The resistance of WC--Co composites to abrasion by diamond increases with increasing hardness of the material and decreasing WC grain size. The latter contributes more to the gain in abrasion resistance of nanocomposites than hardness does. 3. Against ZzO2, the abrasion resistance increases rapidly with hardness and a specific dependence on grain size cannot be established. Abrasion by SiC presents an intermediate case where a rapid increase of hardness and the grain size factor both contribute to increased abrasion resistance. 4. The abrasion mechanism of the conventional composites depends on the hardness of the abrasive and on the size of the WC grains. A hard abrasive causes material removal by fragmentation of the larger grains in addition to plastic deformation. Soft abrasives cause material removal predominantly by the removal of binder followed by WCCo fragmentation in the softe,"materials.
Acknowledgements Support for this work by the Office of Naval Research under contract no. N00014.91-J-1661 is gratefully acknowledged. The authors also thank Professor Bernard Kear of Rutgers University and Dr Larry McCandlish of Nanodyne Inc. for providing them with the samples, and Professor Bernard Gallois for valuable discussions.
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K. Jia, ZE. Pbcher /Wear 200(1996) 206-214
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