英文翻译原文.pdf

1、Scratch testing of WC/Co hardmetals J.C.P. Zun ega a,n, M.G. Geeb, R.J.K. Wooda, J. Walkera aNational Centre for Advanced Tribology at Southampton (nCATS), University of Southampton, United Kingdom bNational Physical Laboratory, United Kingdom a r t i c l e i n f o Article history: Received 7 Januar

2、y 2011 Received in revised form 16 February 2012 Accepted 18 February 2012 Available online 18 March 2012 Keywords: WC/Co hardmetals Tribo-layer Wear Coeffi cient of friction a b s t r a c t Two grades of WC/Co hardmetals, a fi ne-grained sample and coarse-grained sample, were tested using single an

3、d multiple scratch tests with a spherical diamond stylus. The variation in damage mechanism with increasing number of scratch repeats was analysed using SEM, FIB and 3D confocal microscopy. An interesting feature is the formation of a thin tribo-layer on the surface of the scratched region, which be

4、comes more evident at increased scratch repeats. Variations in the coeffi cient of friction were attributed to changes in the topography of the stylus and formation of a tribo-layer on the scratched surface. Crown Copyright it was then cleaned by very lightly rubbing a very fi ne (4000 grit) SiC pap

5、er into the stylus tip whilst constantly checking under the SEM if the debris had been removed. SEM imaging then commences the fi nal time before 20m 1m zone of plastic deformation 20m1m 20m 1m Fig. 3. Wear tracks on the fi ne grained sample (a) after 1 pass, (b) higher magnifi cation image after 1

6、pass showing plastic slip of WC grains located near the pile up within the zone of plastic deformation, arrows indicate intragranular fracture of WC grains and circle shows compaction of the adjacent WC grains, (c) after 5 passes, (d) higher magnifi cation showing scratched and adjacent unscratched

7、surface after 5 passes, (e) after 10 passes, and (f) higher magnifi cation showing scratched and adjacent unscratched surface after 10 passes. J.C.P. Zun ega et al. / Tribology International 54 (2012) 778679 putting it back for the next scratch test. This imaging and cleaning procedures for the styl

8、us tip was thus repeated after 1, 2, 5 and 10 passes. The samples subjected to the scratch tests were examined using a confocal microscope for topology and SEM imaging to give information on the wear mechanisms involved whereas EDX and FIB-SIMS analysis were conducted to probe more information on th

9、e tribo-layer generated on the surface of the scratch. 3. Results The results of this study are discussed in four sub-sections covering discussions on the wear damage mechanisms seen on the scratched region; analysis of the tribo-layer formed on the surface of the scratched region; wear measurements

10、; and friction behaviour of the samples tested. 3.1. Wear damage mechanisms Examination of the fi rst scratch obtained on the coarse-grained sample shows that damage on the WC grains is very evident (Fig. 2(a). A higher magnifi cation image (Fig. 2(b) shows that fracture of the WC grains (with arrow

11、s) is the dominant mechan- ism and localized fragmentation is also observed in regions where the WC grains are in contact with other WC grains (encircled). In previous studies 13,14 conducted on the computation of stres- ses created by a spherical sliding contact, the stresses were found to be maxim

12、um on the circumference of the circle of contact, hence it is expected that maximum damage be on the scratched 5m A B C D E F A B C D E F 5m 20m 20m Fig. 4. Wear tracks on coarse grained sample (a) 10 kV accelerating voltage 1 pass, (b) 0.5 kV accelerating voltage after 1 pass, (c) 10 kV acceleratin

13、g voltage after 5 passes, and (d) 0.5 kV accelerating voltage after 10 passes. 1m 1m Fig. 5. Wear tracks on fi ne grained sample (a) 10 kV accelerating voltage 1 pass and (b) using 0.5 V accelerating voltage after 1 pass. J.C.P. Zun ega et al. / Tribology International 54 (2012) 778680 region. Plast

14、ic slip bands were evident on the WC grains adjacent to the scratched region. There was also grooving running along the scratch. This is likely to be caused through movement of the roughened indenter over the surface of the sample and is likely to involve the tribo- layer formed during scratching 4,

15、5 which will be discussed in more detail in the next sub-section. On the second pass, more fragmentation of WC grain fragments took place. As the number of passes increased to 5 and 10 passes (Fig. 2(c) and (e), the WC grains on the scratched region are no longer visible and instead, it is seen that

16、 the surface is covered with a smooth layer of the re- embeddedWCgrainfragmentsdispersedinaComatrix (Fig. 2(d) and (f). The size of the re-embedded WC grains becomes fi ner (Fig. 2(d) versus Fig. 2(f) as the number of repeats increases approaching a nano-sized particle. With the fi ne-grained sample

17、, the images in Fig. 3(a) and (b) showed minimal WC grain fragmentation and less WC grain fracture compared to the observed damage on the coarse grained sample. Hardness is found to strongly infl uence the abrasive wear of WC/Co hardmetals, and the higher hardness of the fi ne grained sample could e

18、xplain the lower level of damage observed on the scratched surface. With 5 passes (Fig. 3(c), the tribo-layer has completely covered the underlying structure. The high magnifi ca- tion image after 5 passes (Fig. 3(d) shows the presence of nano- sized WC grain fragments, which becomes less discernibl

19、e after 10 passes (Fig. 3(e) and (f). The debris accumulated near and on the pile up region was found to contain WC grain fragments dispersed into a re-depos- ited Co matrix (Figs. 2(d), (f) and 3(d), (f). 3.2. Formation of the tribo-layer The tribo-layer which on both of the coarse and fi ne graine

20、d samples has been studied in detail. Using the technique presented by a previous study 4,5 which uses a lower accelerating voltage of 500 V in the SEM, the tribo-layer is visible even on the fi rst pass for the coarse grained samples. Fig. 4(a) and (b) compares the different features that were obse

21、rved. The tribo-layer during the fi rst pass in Fig. 4(b) shows the unevenness of the scratched surface which is manifested by a discontinuity seen on this smeared layer. For example, the features marked A, B, C, D, E, F in Fig. 4(a) were related to the edges of grains A, B, C, D, E, F in Fig. 4(b).

22、 As the number of passes increased, more re-embedment of the WC grains and Co matrix became apparent (Fig. 4(c) and (d). The tribo-layer was also present in the fi rst pass of the fi ne grained sample (Fig. 5(a) and (b) but with less discernible WC grain features present. The fi ner grained sample h

23、as a higher Co content and this observation can be attributed to the lower contiguity of this sample. As the number of passes increases to 5 and 10 passes, the tribo-layer has covered the surface of the scratched surfacea similar observation with the coarse grained sample. This tribo-layer was furth

24、er analysed using FIB-SIMS and was found to be rich in Co. A comparison of Co content on the scratched region and unscratched region (Fig. 6) showed a marked increase in Co content at the scratched region. It is also noted that it was more diffi cult to mill (e.g. more time was needed to mill the sa

25、me depth) the surface of the scratched layer compared to the surface of the unscratched region which could indicate a compacted dense tribo-layer. The FIB image in Fig. 7 shows the cross-section of the tribo-layer after 10 passes which reveals a tribo-layer that is composed of fragmented nano-sized

26、WC grains embedded in a Co matrix which rests on top of the bulk material. Furthermore, the damage is seen to be restricted to 1 WC grain deep and manifesting intergranular fracture and delamination between the Co matrix and WC grains. Severe intragranular fracture was also observed on WC grains tha

27、t are adjacent to the tribo-layer and on WC grains that are adjacent to another WC grain. 3.3. Wear measurements The 3D images obtained on the scratched surfaces enable measurement of the volume removed of material, scratch depth and scratch width. Fig. 8 shows typical images obtained for the fi ne-

28、grained sample and coarse-grained sample. The coarse grained sample has accumulated more pile up compared to the fi ne-grained sample. Comparison of a line profi le taken across the Count per Second (cps) Depth (nm) Scratched surface Unscratched surface 100m A B A Unscratched surface B Scratched sur

29、face Fig. 6. FIB-SIMS of the coarse grained sample after 10 passes (area analysed was 100mm?100m m) comparing the depth profi le of Co on the unscratched surface, A, and scratched surface, B. 2m 1m Fig. 7. FIB image showing depth profi le of the scratched region after 10 passes (a) at high magnifi c

30、ation and (b) at low magnifi cation. J.C.P. Zun ega et al. / Tribology International 54 (2012) 778681 scratch mark (Fig. 9) shows that the depth and width of the scratch are larger for the coarse grained sample than the fi ne- grained sample. The depth of scratch on the fi rst pass for the coarse gr

31、ained sample is about one third of the WC grain size, gradually increasing to 1 and 2 WC grain size after 5 and 10 passes respectively. Whereas, for the fi ne-grained sample, the scratches were measured to be 1, 3, and 5 WC grain size deep on the 1, 5 and 10 scratches respectively. This also shows t

32、hat fragmentation of WC grains is more prevalent on the coarse grained sample. The real volume removed (V) from the sample in the scratch test was directly calculated from measurements of the cross- sectional area of the scratches by subtracting the area of the built up areas, A2and A3, from the are

33、a of the scratched region, A1 (Fig. 10) and expressed mathematically as V V1?A2A3?L1 This ensures that the deformed materials in the pile up region is accounted for and is comparable to the mass loss technique of determining volume removed 910. Fig. 11 shows a summary of the net volume removed versu

34、s number of passes for both coarse- grained sample and fi ne-grained sample. It is seen that the net volume removed on the coarse grained sample is higher than the fi ne-grained sample. Expressing these values as wear rate 17 in terms of the following equation: Wear Rate V=Ps2 where V is the net vol

35、ume removed, P is the applied load and s is the sliding distance. As shown in Fig. 12, we fi nd that the material with lower hardness corresponds to the material with a higher wear rate. These results appear to be higher than those obtained by a previous study but using a lower load of 9.8 N from a

36、pin on disk tribometer 17. It was also demonstrated from previous tests that the use of a higher load 10 as well as the use of a harder abrasive material 10,16 increases the wear rate of WC/Co hardmetals. 0 51 102 154 205 256 48 96 144 192 4 2 0 51 102 154 205 256 48 96 144 192 4 2 0 51 102 154 205

37、256 48 96 144 192 10 5 0 51 102 154 205 256 48 96 144 192 10 5 m m m m m m m m Fig. 8. Sample images obtained using 3D imaging (a) intensity profi le of fi ne-grained sample, (b) height profi le of fi ne grained sample, (c) intensity profi le of coarse grained sample, and (d) height profi le of coar

38、se grained sample. 10 15 20 0 m m Coarse grained sample Fine grained sample 50100150200250 Fig. 9. Sample line profi le cutting across the scratch. Area 1 (A1) A2 A3 Fig. 10. Line profi le showing different areas measured. J.C.P. Zun ega et al. / Tribology International 54 (2012) 778682 3.4. Frictio

39、n measurements A comparison of the coeffi cient of friction of each individual passes of the two samples tested in Fig. 13 shows that there are two main observations in the coeffi cient of friction obtained. Firstly, there is a decrease in the value of the friction coeffi cient of the fi rst pass be

40、tween 1 pass and 5 passes (points encircled). Secondly, there is a gradual increase in the coeffi cient of friction for the 5 passes and 10 passes as the number of repeats are increased. Looking at the stylus tip topology in Fig. 14, the roughness of the stylus tip increased after 2 scratch passes b

41、ut did not change further after the succeeding scratch passes. The grooves within the stylus tip have decreased the actual contact area which could possibly account for the marked decrease in the coeffi cient of friction after 2 passes. The 3D images and surface profi les obtained on the indenter (F

42、ig. 15) also showed that the bulk profi le of the stylus tip remained intact but incurred a rougher surface profi le. After repeated scratching, the debris which was composed of fragmented nano-sized WC grains and Co matrix was transferred into the tip of the stylus fi lling in the grooves (Fig. 16)

43、. The scratch process then occurred between the grooved diamond (with embedded WC/Co debris fi lling in the grooves) scratching on the surface of the WC/Co hardmetal. The gradual increase in the coeffi cient of friction for both samples can be attributed to two factors: an increase in the actual con

44、tact area and therefore adhesive forces and the formation of the tribo-layer as the number of passes progresses. A plot of the average coeffi cient of friction at the end of each scratch pass in Fig. 17 shows a value of 0.46 for the coarse grained sample and 0.42 for the fi ne-grained sample. These

45、values are comparable to multiple scratch experiments of WC/Co using a Berkovich indenter wherein the average coeffi cient of friction obtained was 0.4 11 but is higher compared to a previous study using the same type of polycrystalline diamond indenter with a low load of 4N which had a coeffi cient

46、 of friction of 0.3 6. 4. Discussion of results The results obtained in the scratch test of WC/Co using a diamond indenter reveals a complex interaction of several fac- tors; namely, the bulk material, the formation of the tribo-layer on the surface of the bulk material which is composed of fragment

47、ed nano-sized WC embedded into the Co matrix, and abrasion of the stylus tip used during scratch and its interaction with the tribo-layer. During the fi rst pass, it is assumed that contact is mainly due to the stylus and the WC/Co surface and this perhaps presents an ideal contact scenario. However

48、, as the stylus ploughs through the surface, it carries with it the debris from the WC/Co, which was then deposited in the crevices of the stylus tip. In the same way, this debris was also deposited on the worn or scratched surface of the WC/Co material. In an actual application, this is probably th

49、e more realistic phenomenon to which the coeffi cient of friction obtained shows a stable value after several passes. 0 Volume Removed (m3) Distance (m) Coarse Sample Fine Sample 0 2E-12 4E-12 6E-12 0.010.020.030.040.050.06 Fig. 11. Volume removed versus number of passes comparing coarse-grained sam

50、ple and fi ne grained sample. 0 Wear Rate (m3/Nm) Distance (m) Coarse Sample Fine Sample 0 1E-12 2E-12 3E-12 4E-12 0.010.020.030.040.050.06 Fig. 12. Wear rate of coarse-grained sample and fi ne grained sample. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0 Friction Coefficient Number of Repeats Coarse Grained Sample

51、 0 0.1 0.2 0.3 0.4 0.5 Friction Coefficient Fine Grained Sample 1 Pass 2 Passes 5 Passes 10 Passes 2468100 Number of Repeats 246810 Fig. 13. Coeffi cient of friction measurements. J.C.P. Zun ega et al. / Tribology International 54 (2012) 778683 The wear scars on the fi rst pass shows severe plastic

52、deforma- tion showing cracking and crushing of the WC grains. Slip lines on the WC grains adjacent to the scratch indicate the onset of plastic deformation. The Co phase however is not readily observable but appears to have been ploughed off from the surface of the scratched region and easily smeare

53、d on the surface of the scratched region manifesting the start of the tribo-layer forma- tion. Thus in the fi rst two passes, the tribo-layers formed are predominantly composed of Co with embedded fragments of crushed WC grains. As the number of passes increased, the fragmented WC debris is further

54、ground into fi ner sizes which are also transferred into the stylus tip (Fig. 16). At the same time, the diamond indenter tip experiences abrasion with the forma- tion of pits with successive number of repeats. The spherical surface profi le of the indenter tip after the scratch tests suggests that

55、it was not fl attened or mechanically abraded during the test. Rather, pitting has been observed on the surface of the tip where contact with the hardmetal samples occurred. Polycrystalline diamond is typically produced by sinter- ing diamond grit with a catalyst, which is usually Co 15. Thus, it is

56、 possible that conditions at the surface of contact during scratch testing and the accumulation of Co on the scratched surface after the fi rst two passes may have induced a surface reaction between the Co in the hardmetal samples and the surface of the indenter tip. This could also account for the

57、more pronounced pitting observed in the coarse grained sample which has a higher Co content of 11 wt%. These shallow grooves provided space for the debris to be transferred into the stylus tip during scratching whichleadstoacomplex3bodytribologicalsystemto occurwith the debris acting as the third bo

58、dy. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 1050 Coefficient of Friction Number of Passes Coarse Grained Sample Fine Grained Sample Initial (unused) After 1 pass on coarse grained sampleAfter 2 passes on coarse grained sampleAfter 5 passes on coarse grained sample After 1 pass on fine grained sampleAfter 1m1m 1

59、m 1m 1m1m1m 2 passes on fine grained sampleAfter5 passes on fine grained sample Fig. 14. Correlation of the coeffi cient of friction with images of the cleaned stylus tip before proceeding to the next scratch test. after 10 passes on coarse grained sample after 10 passes on fine grained sample initial profile prior scratch test 0 5 10 15 20 25 m 020406080100120140160 m Fig. 15. Profi les of the stylus tip prior scratch test after cleaning (typical profi le), after 10 passes on coarse grained sample and after 10 passes on fi ne grained sample. J.C.P. Zun ega et al. / Tribol