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Characterisation of wear properties of ultrafine-grained hardmetals using a special abrasive wheel test
International Journal of Refractory Metals & Hard Materials 19 (2001) 371±379 www.elsevier.com/locate/ijrmhm
Characterisation of wear properties of ultra®ne-grained hardmetals using a special abrasive wheel test M. Herr a
a,*
, T. Sailer a, H.G. Sockel a, R. Schulte b, H. Feld b, L.J. Prakash
c
Institut f ur Werkstowissenschaften, Universitat Erlangen-Nurnberg, Lehrstuhl I, Martensstrasse 5, D-91058 Erlangen, Germany b Tigra Hartsto GmbH, Gewerbering 2, D-86698 Oberndorf am Lech, Germany c WTP Materials Engineering, Konrad-Adenauer-Str. 27, D-72108 Rottenburg, Germany Received 5 March 2001; accepted 15 August 2001
Abstract Three-body abrasive wear tests were carried out on super ultra®ne-grained hardmetals (WC intercept length about 0.2 lm) with dierent Fe, Ni, Co binder systems, on hardmetals with Co binder and on standard materials with coarser WC grains. Apart from the standard materials, the hardmetals were all produced in laboratory scale from commercial nanocrystalline WC powders from dierent companies. The practical application of these hardmetals is in the ®eld of cutting dry wood. The main purposes of this work were to characterise the wear properties of the produced grades and to derive predictions from laboratory wear tests regarding practical applications. Therefore a new wear test apparatus was built. The wear results showed a logarithmic correlation with hardness and a threshold to the low wear region, which was exceeded by some grades, at binder mean free paths of less than 40 nm. Moreover the laboratory wear results correlate well with results from ®eld testing. A tribo®lm was found on the worn surface and characterised by scanning electron microscope (SEM), AFM and TEM. In an analysis of the wear mechanisms the role of this tribo®lm must be considered. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Hardmetal; Wear; Fe, Ni, Co binder systems; Low wear region; Tribo®lm
1. Introduction Wear is the predominant damage mechanism that limits the lifetime of hardmetal cutting knives used for dry wood cutting. A very important parameter of the tribological system involved in woodcutting are the high cutting speeds (60±100 m/s). At these high speeds mineral particles in the wood and the wood chips themselves act as abrasive grains that cause abrasive wear at the surface of the cutting knife [1]. An attempt was made to transfer this to a corresponding tribological system in laboratory scale. Therefore a new three-body-abrasivewheel-test apparatus that combines the advantages of ASTM G65 and ASTM B611-85 was built to carry out wear experiments at room temperature. The apparatus has to ful®ll the requirements of wear tests in general [2]: 1. Reproducibility of the wear experiments on one material grade. * Corresponding author. Tel.: +49-9131-8527474; fax: +49-91318527504. E-mail address: [email protected] (M. Herr).
2. Dierentiability of wear properties between dierent material grades. 3. Transferability of the wear data to the corresponding tribological system in practical application. Wear experiments were carried out on super ultra®negrained hardmetals with Co binder and complex binder systems produced in laboratory scale. In addition, wear experiments were carried out on standard materials. Microstructural investigations were carried out by scanning electron microscope (SEM), TEM and AFM to link microstructural parameters with wear mechanisms.
2. Investigated materials and microstructural parameters All investigated hardmetals (grade 45±81) (Table 1) were produced in laboratory scale by TIGRA Hartsto GmbH. The nano WC powders used (BET surface (m2 =g) 1.98±3.56) were from four dierent powder companies, an ``a'' after the powder number refers to the undoped version of the WC powder, a number after the
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Table 1 Composition and microstructure parameters of the investigated super ultra®ne-grained hardmetals Hardmetal 1 2 3 4 5
WC powder
Binder (wt%)
Binder composition
Inhibitor
Hardness HV 10
LWC (lm) (measured)
LBi (lm) (measured)
LBi (lm) (calculated)
Powder Powder Powder Powder Powder
1.1 1.2 1.1 1.3 1.2
3 8 10 4 6
Co Co Co Co Co
Cr3 C2 , VC Cr3 C2 Cr3 C2 , VC VC Cr3 C2
2100 1710 1810 1800 1800
0.3 0:54 0:33 0:27 0:13 0:71 0:39 0:53 0:28
0:23 0:15 0:10 0:06 0:17 0:09 0:23 0:18
0.041 0.145 0.092 0.091 0.104
Stand. Stand. Stand. Stand. Stand.
mat. mat. mat. mat. mat.
Grade Grade Grade Grade Grade Grade
45 46 48 80 84 85
Powder Powder Powder Powder Powder Powder
2 3 4 4a 1.1 3
10 10 10 10 2.5 2
Co Co Co Co Co Co
Cr3 C2 , VC Cr3 C2 , VC Cr3 C2 Cr3 C2 VC Cr3 C2 , VC
1860 2040 1860 1920 2370 2470
0:25 0:12 0:18 0:10 0:21 0:14 0:25 0:11 0:36 0:17 0.25
0:09 0:06 0:08 0:05 0:09 0:07 0:14 0:10 0:11 0:07
0.085 0.061 0.072 0.088 0.021 0.019
Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade
55 56 53 54 61 52 81 57 58 59 68 69 70 71 72 66 73 74 75 76 77 78
Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder
4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 3
7.5 4 10 10 10 10 10 12.5 15 17.5 10 10 12.5 15 17.5 30 7.5 10 2 2.35 2.35 2
Fe Co Ni Fe Co Ni Fe Co Ni Fe Co Fe Ni Fe Co Ni Fe Co Ni Fe Co Ni Fe Co Ni Fe Co Ni Fe Ni Co Fe Ni Co Fe Ni Co Fe Ni Co Fe Ni Co Fe Ni Co Fe Ni Co Fe Ni Co Co Ni Co Ni Co Ni Co Ni
VC VC VC VC VC No Inh. VC VC VC VC VC No Inh. VC VC VC VC VC VC Cr3 C2 , VC VC Cr3 C2 , VC Cr3 C2 , VC
2090 2240 1780 2060 1880 1760 1970 1890 1740 1650 1960 1860 1830 1740 1680 1320 1990 1870 2290 2070 2290 2170
0:21 0:12 0:23 0:12 0:27 0:16 0:20 0:11 0:17 0:11 0:30 0:25 0.2 0:17 0:10 0:18 0:11 0:19 0:11 0:19 0:08 0:23 0:13 0:23 0:13 0:21 0:12 0:20 0:11 0:23 0:12 0.21 0:25 0:13 0.32 0:35 0:24 0:32 0:19 0.25
0:07 0:04 0:06 0:04 0:08 0:06 0:07 0:04 0:10 0:07 0:13 0:12
0.059 0.047 0.100 0.075 0.064 0.112 0.074 0.082 0.107 0.135 0.071 0.085 0.115 0.124 0.142 0.331 0.057 0.092 0.017 0.028 0.030 0.015
powder number denotes another WC powder from the same producer (Table 1). Nano WC powder, binder powder(s), a small amount of grain growth inhibitor and carbon powder (free carbon) were milled in an attritor. After a special granulation procedure and drying process the grades were pressed and sintered under a certain pressure. The results are super ultra®ne hardmetals characterised according to [3] by a mean intercept WC size between 0.1 lm and 0.3 lm. The hardmetals investigated have Co binders or complex binder systems consisting of Fe, Co, Ni. The motivation to produce these hardmetals was to combine the advantages of ®ne-grained hardmetals (higher hardness at same toughness and abrasion resistance [3,4]) and the advantages of hardmetals with complex binder systems (price, heat treatment, hardness, abrasive wear resistance [5]). The standard materials investigated (Table 1) were granulated by conventional spray conversion process and sintered by TIGRA Hartsto GmbH.
0:09 0:06 0:11 0:08 0:12 0:09 0:10 0:06 0:10 0:06 0:09 0:07 0:10 0:09 0:11 0:10 0:27 0:30 0:12 0:08 0:08 0:04 0:06 0:06
The basic microstructural parameters with their standard deviation are also given in Table 1. The mean intercept WC length (LWC ) and the binder mean free path (LBi ) are determined by intercept chord length measurements [6] from SEM images. It was not trivial to get images with sucient quality from SEM to make quantitative microstructure analysis. The reason is the small WC grain size, which needs high resolution of the SEM and, moreover, a special sample preparation procedure [7] to allow a high image quality. The LBi is the microstructural parameter with the lowest accuracy of measurement, especially for hardmetals with low binder content and small LWC . For this reason, Table 1 also shows the calculated values of LBi (Eq. (1)), that were calculated from the measured LWC , the theoretical binder volume content (vol% binder) and the theoretical WC volume content (vol% WC) and does not take into account the measured LBi . For hardmetals with <5 wt% of binder the inhibitor content was included in the theoretical binder volume, too. The LWC values on a grey
M. Herr et al. / International Journal of Refractory Metals & Hard Materials 19 (2001) 371±379
background in Table 1 are estimated values. Where hardmetals with low binder contents show bigger WC grain sizes because of another sintering procedure. LBi 2
LWC vol% binder A vol% WC
with A 1:148:
1
A similar equation can be found in [8]. The ®tting parameter A was found on a hardmetal with 1.5 lm of LWC , where the measurement of LBi is quite exact. The 2 in front of Eq. (1) is a geometric factor.
3. Experimental All wear experiments were carried out at room temperature with the apparatus shown in Fig. 1. A specimen
373
(Fig. 3) is pressed against a rotating wheel (100 rpm, 10 mm thick, 195 mm diameter) with a de®ned normal load (FN ) that is applied in horizontal direction by a linear guide bar and a cantilever arm with a relocatable weight. The wheel is wet continuously (55 ml/min) with water. A corrosive medium could also be used. The dry abrasive (SiO2 , Al2 O3 or SiC), with a grain size of about 250 lm, ¯ows in a de®ned rate (45 g/min), realised by a kind of a paddle wheel, from the upper sandbox onto the wet wheel close to the contact area of wheel and specimen (Fig. 1). Thus, there is an abrasive slurry between wheel and specimen with a constant ratio of abrasive and water. With this apparatus it is possible to measure the friction force (FF ) and normal force (FN ) with two weighing cells, the wear depth (D) with an inductive displacement sensor and the ¯ow rate of the medium
Fig. 1. Three-body abrasive wheel test apparatus ± a combination of ASTM G65 and ASTM B611-85.
Fig. 2. Data recorded online from a wear experiment on WC hardmetal with 10 wt% Fe, Ni and Co binders.
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Fig. 3. Specimen geometry: schematic diagram, specimen in initial state, worn state.
with a ¯ow meter over the whole test time (Fig. 2) and to 3 record the results online. The wear resistance (m=mm ), which is constant over the test time, is determined by calculating the volume loss and the sliding distance from the online measured data. The sensitivity of this apparatus allows the determination of wear and friction behaviour for hardmetals with only slight variations in binder content and binder composition. This apparatus not only combines ASTM G65 and ASTM B611-85, but has a further advantage compared to the ASTM standards: the normal load is applied at right angles to the tangent of the wheel over the whole test time. This is achieved using a linear guide bar which only moves in horizontal direction (Fig. 1). The normal load is applied by the cantilever arm in this guide bar. All experiments were carried out at FN 100 N and with alumina No. 60 as abrasive, unless otherwise mentioned.
4. Results and discussion 4.1. Wear experiments Fig. 4 shows the comparison between practical application and laboratory wear tests on hardmetals with
Fig. 4. Comparison between practical application (cutting length) and laboratory wear tests (wear resistance).
binder contents lower than 5 wt% and dierent WC grain sizes. The hardmetal cutting tools were used for cutting a certain length of chipboard. The cutting length is determined by the quality of the cut chipboard. It makes no sense to compare absolute values of cutting length and wear resistance from laboratory experiments because of the dierent tribological systems. But the ranking of qualities in cutting length and laboratory wear tests is the same. So laboratory wear tests in the tribological system investigated can give an idea of the relative ranking of the cutting tools in practical application. For comparison, cutting tests were made with PCD coated hardmetal cutting tool. Concerning the cutting length, Fig. 4 shows that some of the cutting qualities of the hardmetal knives produced under laboratory conditions are comparable to those of PCD coated cutting tools. Correlations between wear resistance and microstructural parameters and physical measurements were found. Fig. 5 shows the correlation between hardness and wear resistance. The grain size of the abrasive (Al2 O3 ) is about 250 lm. So a large amount of WC grains and binder are worn by one abrasive particle. This means that bulk properties like hardness aect the wear resistance strongly (Fig. 5), i.e., the combined properties of binder phase and WC phase and not the
Fig. 5. Correlation between wear resistance and hardness (abrasive: Al2 O3 ; FN 100 N).
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375
properties of the individual phases determine the wear resistance [9]. With linear increasing hardness a logarithmic increase of wear resistance is observed. So the chosen tribological system is optimal for distinguishing hardmetals that dier only slightly in binder content and binder composition with respect to their wear resistance. The wide variation of the wear resistance (over three decades) is due to the abrasive used (Al2 O3 ). According to the literature [10±13], abrasive wear occurs at a low or a high level depending on the ratio of hardness of the abrasive (Ha ) to the hardness of the material (Hm ) being worn. To demonstrate these wear levels, silica sand (SiO2 /975 HV), alumina (Al2 O3 =1900 HV) and silicon carbide (SiC/2350 HV) were used as abrasives (Fig. 6). The mean values for the mineral hardness were taken from [14]. All abrasives had an abrasive grit size of about 250 lm and a similar grit size distribution (measured by sieve analysis). The tested grades had the same binder composition and almost the same WC grain size but dierent binder contents to eliminate the in¯uence of binder composition and WC grain size on the scatter of the wear resistances. The values used for Fig. 6 are circled in Fig. 5. The experiments with SiO2 as abrasive were carried out at FN 140 N to achieve a measurable wear rate. For comparison all, wear rates in Fig. 6 were normalised with respect to FN to obtain the speci®c wear resistance. The wear resistance of Ha =Hm 1 is in the transition area from low to high wear level conditions which is typical for multiphase materials. From Fig. 6 it is obvious that wear experiments using Al2 O3 as abrasive are in the transition area. This fact makes it easy to develop a relative ranking of the wear resistance of the grades used. At a ratio of about 1:2Ha =Hm there is a change in wear mechanisms, because, above this ratio, indentation of abrasive grits into the hardmetal surface is possible [14]. Literature [14] suggests that a ratio higher than 1.2 leads to fracture of WC and plastic deformation of WC
and binder phase. Ratios lower than 1.2 lead to a kind of preferential removal of the binder phase in conjunction with fracture and detachment of WC. Microstructural investigations on a hardmetal having a ratio near 1.2 showed that, in this case, these mechanisms may have to be reviewed. The correlation observed between wear resistance and hardness (Fig. 5) provided the motivation to investigate the correlation between the wear resistance and, following the Hall±Petch relation, the inverse root of the 1=2 binder mean free path (LBi ) (Fig. 7). High/low wear levels and a transition area can be found in Fig. 7, too (the calculated values of LBi (Table 1) were used here). The area of the high wear level is not so obvious, because only Al2 O3 was used as abrasive in order to highlight the transition area as shown in Fig. 6. The area of low level wear is reached at a binder mean free path of about 40 nm. This means hardmetals with a binder mean free path of less than 40 nm show only a slight increase in wear resistance with faster decreasing mean free path in the tribological system considered. So a further decrease in binder mean free path will not result in a signi®cant increase in wear resistance. This result corresponds with [3], where a change in dislocation movement is expected at a critical value of 40 nm. The binder mean free path of a hardmetal is determined by the binder content and the WC grain size. The line in Fig. 7 connects the data of hardmetals with Fe, Ni, Co binders that have almost the same composition. The scatter around this line is caused by dierent binder compositions which are not taken into account in the binder mean free path. Fig. 8 shows the wear resistance of ®ve hardmetals taken from Fig. 7 with almost the 1=2 1=2 same LBi (LBi 3:4 to 3.9) plotted against the Ni and Co content of the complex Fe, Ni, Co binder systems (all ®ve hardmetals have 10 wt% binder content). For visualisation the measured wear resistances are columns for a calculated surface graph (Fig. 8). With increasing amount of Ni in the binder phase the wear resistance
Fig. 6. High/low wear level for hardmetals (circled in Fig. 5) with same binder composition and WC grain size but dierent binder contents.
Fig. 7. Correlation between wear resistance and inverse root of binder 1=2 mean free path (LBi ).
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about 10 wt% (or a WC content of about 80 vol%) which still lie in the low wear region. The consequence would be advantages in strength, toughness and sintering procedure, while retaining almost the same wear properties in the tribological system investigated. 4.2. Microstructural investigations
Fig. 8. Wear resistance of hardmetals with 10 wt% complex binder content versus wt% of Co and Ni in the Fe, Ni and Co binder phases.
decreases, but a further increase of Ni would lead to an increase of the wear resistance as observed in [5]. In agreement with [5], at a Ni content of about 20 wt% in the binder phase, there is a maximum in wear resistance at about 10 wt% of Co (Fig. 8). A decrease in the Ni content and a simultaneous increase in the Co content would lead to higher wear resistances (Fig. 8). 1=2 The correlation between LBi with WC volume content and WC grain size is shown in Fig. 9. Dierent WC grain sizes (0.1, 0.2, 0.5 lm) in Fig. 9 are characterised by calculated lines (Eq. (1)). The scatter line is the threshold to the low wear region for hardmetals with LBi less than 40 nm as shown in Fig. 7. Most of the tested grades fall near the line of 0.2 lm WC grain size (Fig. 9). For these grades, the threshold to the low wear region is exceeded at a binder content lower than 3 wt% (or a WC content of more than about 90 vol%). If the WC grain size were reduced from 0.2 to 0.1 lm, it would be possible to produce hardmetals with a binder content of
Fig. 9. Correlation between inverse root of binder mean free path (LBi1=2 ) with vol% WC and WC grain size (calculated lines for 0.1, 0.2, 0.5 lm WC grain size).
Further microstructural investigations have so far been carried out on grade 59 (Table 1) with 17.5 wt% Fe, Ni, Co binder and a WC intercept length of 0.19 lm. This hardmetal has a wear resistance near the high wear region (Ha =Hm 1:15; Fig. 6), as indicated by the square in Fig. 7. According to the wear mechanisms mentioned in the explanations of Fig. 6, wear on the surface of this hardmetal should cause preferential removal of the binder phase due to displacement of WC grains and extrusion of the binder phase. The removal of binder phase between the WC grains leads to detachment and cracking of the WC grains [14]. For ratios greater than 1:2Ha =Hm , fracture of WC grains and plastic deformation of binder and WC phase should be the dominating wear mechanism [14]. So at a ratio near 1:2Ha =Hm (the investigated hardmetal has a ratio of 1:15Ha =Hm ) there should probably be a mixture of both wear mechanisms for higher and lower values of 1:2Ha =Hm . Microstructural investigations on this hardmetal showed in contrast to the above-mentioned wear mechanisms that the worn surface is covered with a tribo®lm. A top view on the worn surface by SEM only shows a diuse surface with smaller WC grains compared to the polished state, as shown in, Fig. 10. At lower magni®cation in the SEM, macroscopic wear grooves are visible (Fig. 11), due to microploughing of the abrasive particles. So it is assumed that the worn surface (the tribo®lm) suers massive plastic deformation during the wear process. The tribo®lm is obvious in the cross-section view of Fig. 12. The tribo®lm has a thickness of about 2 lm, and it seems that the ®lm consists of some just created large fragments of WC grains (Fig. 12, left) and very small grains that are, perhaps, rounded (Fig. 12, right). The ®lm has a smooth outer surface and a sharp transition to the bulk material under the ®lm. A similar tribo®lm was found in dry sliding with conform contact on hardmetals with large WC grains (7 and 1.5±2 lm) by [9]. Preliminary EDX analysis on the tribo®lm in a cross-section and on bulk material were carried out using SEM. The Fe-based binder phase consists of Fe, Ni and Co. All binder elements were detected with the measurements over the bulk material. In measurements over the ®lm, only Fe was found as binder material. So the ratios between WC and Fe (wt%) in the bulk and in the ®lm could be compared (Table 2). Table 2 indicates that there is a lower Fe content, i.e., binder content, in the ®lm than in
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Fig. 10. Top view on polished (left) and worn surface (right) of grade 59 (SEM).
Table 2 Ratio between wt% W and wt% Fe in bulk material and tribo®lm
Fig. 11. Top view on worn surface of grade 59 with wear grooves caused by Al2 O3 abrasive particles (SEM).
the bulk material. This may be correspond to the mentioned wear mechanisms for preferential removal of the binder phase discussed above. But here the binder phase is removed from the tribo®lm and not between WC grains from the bulk material. Moreover, Table 2 indicates with the higher value of the standard deviation of
Bulk material (W/Fe)
Tribo®lm (W/Fe)
40 8
68 18
the measurements over the ®lm that the composition of the tribo®lm is less homogeneous than that of the bulk material, due to inhomogeneous preferential removal of binder from the tribo®lm. AFM results with non-contact mode on the tribo®lm from the top view are shown in Figs. 13 and 14. The topography contrast image (left) and the phase contrast image (right) indicate a massive change in the WC grain shape and size compared to the unworn state. The grains at the top of the tribo®lm are rounded and have a grain size much lower than in the unworn state. There are still some contours of coarser rounded grains visible (left image), indicating that coarser rounded grains fracture into nanocrystalline grains. Moreover, there is more fracture of WC grains in parallel (from left to right) to the wear groves (only visible in the left image). TEM investigations verify the AFM results. The TEM samples were prepared from the bulk side to have
Fig. 12. Cross-section view on the worn surface of grade 59 (SEM).
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Fig. 13. Top view on worn surface of grade 59 (AFM); topography contrast (left), phase contrast (right).
round structures of the tribo®lm consist of an amorphous phase containing WC, Fe and Ni. Between the round grains, a higher amount of Fe was found, indicating this to be a kind of binder phase for the tribo®lm. 5. Summary
Fig. 14. Top view on polished surface of grade 59 (AFM); topography contrast.
transmission through the tribo®lm from the top view (see Fig. 15). For further TEM investigations, a sample with better transmission is needed to show some diffraction patterns. The results obtained so far are that the
1. Laboratory wear tests can provide the relative ranking of the quality of the cutting tool in practical application. 2. Wear experiments in the tribological system considered were carried out with the above-mentioned parameters in the transition area from low to high wear level. This fact makes it easy to distinguish wear properties of dierent hardmetals that only show slight variations in their composition. 3. The wear resistance of hardmetals with complex binder systems consisting of Fe, Co and Ni is increased with simultaneous decrease of Ni content and increase of Co content in the binder phase. For
Fig. 15. Bulk material (left) and top view on tribo®lm (right) of grade 59 (TEM).
M. Herr et al. / International Journal of Refractory Metals & Hard Materials 19 (2001) 371±379
hardmetals containing 20 wt% Ni in the binder phase a maximum in wear resistance was observed at a Co content of 10 wt%. 4. At a binder mean free path of ca. 40 nm there is a threshold to the low wear region (maximum wear resistance) for the tribological system considered. Therefore, a further decrease in binder mean free path will not result in a signi®cant increase in wear resistance. The threshold was exceeded by hardmetals with a binder content of 2±3 wt% at a WC intercept length of about 0.3 lm. Calculations showed that, if the WC grain size were reduced from 0.3 to 0.1 lm, it would be possible to produce hardmetals with a binder content of about 10 wt% which still exhibit a low wear region. Thus, advantages in strength, toughness and sintering procedure without reduction in wear performance would be expected for the tribological system investigated. 5. A tribo®lm was found on the worn surface. The ®lm consists of nanocrystalline rounded grains with probably a lower binder content compared with the bulk material. First TEM investigation showed that these grains consist of an amorphous phase containing WC, Fe and Ni. Possible wear mechanisms should depend strongly on the formation of the ®lm and on the behaviour of the ®lm during wear. References [1] Feld H. Hardmetal for woodcutting applications. Woodworking Int 1988;4.
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[2] Walter P, Feld H. Bestimmung der Abriebfestigkeit von Hartmetall mit dem Schleifrad. Planseeber Pulvermetall 1976;24: 100±7. [3] Richter V. On hardness and toughness of ultra®ne and nanocrystalline hard materials. Refract Met Hard Mater 1999;17: 141±52. [4] Jia K, editor. Microstructure, mechanical properties and wear resistance of WC/Co nanocomposites. Mat Res Soc Symp 1997;457:303±8. [5] Prakash L. Weiterentwicklung von WC Hartmetallen unter Verwendung von Eisen-Basis-Binderlegierungen. PhD Thesis, Karlsruhe, 1979. [6] Nordgren A. Microstructural characterisation of cemented carbides using SEM based automatic image analysis. Refract Met Hard Mater 1991;10:61±81. [7] Sailer T, Herr M, Sockel H-G, Schulte R, Feld H, Prakash LJ. Microstructure and mechanical properties of ultra®negrained hardmetals. Int J Refract Met Hard Mater 2001; 19:553±9. [8] Underwood EE. Quantitative stereology. Reading, MA: 1970. [9] Engqvist H. Microstructural aspects on wear of cemented carbides. PhD Thesis, Uppsala, 2000. [10] Wahl H, editor. Verschlei probleme im Braunkohlenbergbau, Braunkohle. Warme Energie 1951;5/7:75±87. [11] Wellinger K, Uetz H. Gleitverschlei, Sp ulverschlei Strahlverschlei unter der Wirkung von k ornigen Stoen. VDIForschungsh 1955;21:449B. [12] Uetz H, F ohl J. Gleitverschleiuntersuchungen an metallen und nichtmetallischen hartstoen unter wirkung k orniger stoe. Warme Energie 1969;21:10±8. [13] Feld H, Walter PH. Beitrag zur kenntnis des mineral-hartmetallverschleies. J Mater Technol 1976;7:300±3. [14] Hutchings IM. Tribology: friction and wear of engineering materials. Cambridge: 1992 [chapter 6].
Characterisation of wear properties of ultra®ne-grained hardmetals using a special abrasive wheel test M. Herr a
a,*
, T. Sailer a, H.G. Sockel a, R. Schulte b, H. Feld b, L.J. Prakash
c
Institut f ur Werkstowissenschaften, Universitat Erlangen-Nurnberg, Lehrstuhl I, Martensstrasse 5, D-91058 Erlangen, Germany b Tigra Hartsto GmbH, Gewerbering 2, D-86698 Oberndorf am Lech, Germany c WTP Materials Engineering, Konrad-Adenauer-Str. 27, D-72108 Rottenburg, Germany Received 5 March 2001; accepted 15 August 2001
Abstract Three-body abrasive wear tests were carried out on super ultra®ne-grained hardmetals (WC intercept length about 0.2 lm) with dierent Fe, Ni, Co binder systems, on hardmetals with Co binder and on standard materials with coarser WC grains. Apart from the standard materials, the hardmetals were all produced in laboratory scale from commercial nanocrystalline WC powders from dierent companies. The practical application of these hardmetals is in the ®eld of cutting dry wood. The main purposes of this work were to characterise the wear properties of the produced grades and to derive predictions from laboratory wear tests regarding practical applications. Therefore a new wear test apparatus was built. The wear results showed a logarithmic correlation with hardness and a threshold to the low wear region, which was exceeded by some grades, at binder mean free paths of less than 40 nm. Moreover the laboratory wear results correlate well with results from ®eld testing. A tribo®lm was found on the worn surface and characterised by scanning electron microscope (SEM), AFM and TEM. In an analysis of the wear mechanisms the role of this tribo®lm must be considered. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Hardmetal; Wear; Fe, Ni, Co binder systems; Low wear region; Tribo®lm
1. Introduction Wear is the predominant damage mechanism that limits the lifetime of hardmetal cutting knives used for dry wood cutting. A very important parameter of the tribological system involved in woodcutting are the high cutting speeds (60±100 m/s). At these high speeds mineral particles in the wood and the wood chips themselves act as abrasive grains that cause abrasive wear at the surface of the cutting knife [1]. An attempt was made to transfer this to a corresponding tribological system in laboratory scale. Therefore a new three-body-abrasivewheel-test apparatus that combines the advantages of ASTM G65 and ASTM B611-85 was built to carry out wear experiments at room temperature. The apparatus has to ful®ll the requirements of wear tests in general [2]: 1. Reproducibility of the wear experiments on one material grade. * Corresponding author. Tel.: +49-9131-8527474; fax: +49-91318527504. E-mail address: [email protected] (M. Herr).
2. Dierentiability of wear properties between dierent material grades. 3. Transferability of the wear data to the corresponding tribological system in practical application. Wear experiments were carried out on super ultra®negrained hardmetals with Co binder and complex binder systems produced in laboratory scale. In addition, wear experiments were carried out on standard materials. Microstructural investigations were carried out by scanning electron microscope (SEM), TEM and AFM to link microstructural parameters with wear mechanisms.
2. Investigated materials and microstructural parameters All investigated hardmetals (grade 45±81) (Table 1) were produced in laboratory scale by TIGRA Hartsto GmbH. The nano WC powders used (BET surface (m2 =g) 1.98±3.56) were from four dierent powder companies, an ``a'' after the powder number refers to the undoped version of the WC powder, a number after the
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Table 1 Composition and microstructure parameters of the investigated super ultra®ne-grained hardmetals Hardmetal 1 2 3 4 5
WC powder
Binder (wt%)
Binder composition
Inhibitor
Hardness HV 10
LWC (lm) (measured)
LBi (lm) (measured)
LBi (lm) (calculated)
Powder Powder Powder Powder Powder
1.1 1.2 1.1 1.3 1.2
3 8 10 4 6
Co Co Co Co Co
Cr3 C2 , VC Cr3 C2 Cr3 C2 , VC VC Cr3 C2
2100 1710 1810 1800 1800
0.3 0:54 0:33 0:27 0:13 0:71 0:39 0:53 0:28
0:23 0:15 0:10 0:06 0:17 0:09 0:23 0:18
0.041 0.145 0.092 0.091 0.104
Stand. Stand. Stand. Stand. Stand.
mat. mat. mat. mat. mat.
Grade Grade Grade Grade Grade Grade
45 46 48 80 84 85
Powder Powder Powder Powder Powder Powder
2 3 4 4a 1.1 3
10 10 10 10 2.5 2
Co Co Co Co Co Co
Cr3 C2 , VC Cr3 C2 , VC Cr3 C2 Cr3 C2 VC Cr3 C2 , VC
1860 2040 1860 1920 2370 2470
0:25 0:12 0:18 0:10 0:21 0:14 0:25 0:11 0:36 0:17 0.25
0:09 0:06 0:08 0:05 0:09 0:07 0:14 0:10 0:11 0:07
0.085 0.061 0.072 0.088 0.021 0.019
Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade Grade
55 56 53 54 61 52 81 57 58 59 68 69 70 71 72 66 73 74 75 76 77 78
Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder
4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 3
7.5 4 10 10 10 10 10 12.5 15 17.5 10 10 12.5 15 17.5 30 7.5 10 2 2.35 2.35 2
Fe Co Ni Fe Co Ni Fe Co Ni Fe Co Fe Ni Fe Co Ni Fe Co Ni Fe Co Ni Fe Co Ni Fe Co Ni Fe Ni Co Fe Ni Co Fe Ni Co Fe Ni Co Fe Ni Co Fe Ni Co Fe Ni Co Fe Ni Co Co Ni Co Ni Co Ni Co Ni
VC VC VC VC VC No Inh. VC VC VC VC VC No Inh. VC VC VC VC VC VC Cr3 C2 , VC VC Cr3 C2 , VC Cr3 C2 , VC
2090 2240 1780 2060 1880 1760 1970 1890 1740 1650 1960 1860 1830 1740 1680 1320 1990 1870 2290 2070 2290 2170
0:21 0:12 0:23 0:12 0:27 0:16 0:20 0:11 0:17 0:11 0:30 0:25 0.2 0:17 0:10 0:18 0:11 0:19 0:11 0:19 0:08 0:23 0:13 0:23 0:13 0:21 0:12 0:20 0:11 0:23 0:12 0.21 0:25 0:13 0.32 0:35 0:24 0:32 0:19 0.25
0:07 0:04 0:06 0:04 0:08 0:06 0:07 0:04 0:10 0:07 0:13 0:12
0.059 0.047 0.100 0.075 0.064 0.112 0.074 0.082 0.107 0.135 0.071 0.085 0.115 0.124 0.142 0.331 0.057 0.092 0.017 0.028 0.030 0.015
powder number denotes another WC powder from the same producer (Table 1). Nano WC powder, binder powder(s), a small amount of grain growth inhibitor and carbon powder (free carbon) were milled in an attritor. After a special granulation procedure and drying process the grades were pressed and sintered under a certain pressure. The results are super ultra®ne hardmetals characterised according to [3] by a mean intercept WC size between 0.1 lm and 0.3 lm. The hardmetals investigated have Co binders or complex binder systems consisting of Fe, Co, Ni. The motivation to produce these hardmetals was to combine the advantages of ®ne-grained hardmetals (higher hardness at same toughness and abrasion resistance [3,4]) and the advantages of hardmetals with complex binder systems (price, heat treatment, hardness, abrasive wear resistance [5]). The standard materials investigated (Table 1) were granulated by conventional spray conversion process and sintered by TIGRA Hartsto GmbH.
0:09 0:06 0:11 0:08 0:12 0:09 0:10 0:06 0:10 0:06 0:09 0:07 0:10 0:09 0:11 0:10 0:27 0:30 0:12 0:08 0:08 0:04 0:06 0:06
The basic microstructural parameters with their standard deviation are also given in Table 1. The mean intercept WC length (LWC ) and the binder mean free path (LBi ) are determined by intercept chord length measurements [6] from SEM images. It was not trivial to get images with sucient quality from SEM to make quantitative microstructure analysis. The reason is the small WC grain size, which needs high resolution of the SEM and, moreover, a special sample preparation procedure [7] to allow a high image quality. The LBi is the microstructural parameter with the lowest accuracy of measurement, especially for hardmetals with low binder content and small LWC . For this reason, Table 1 also shows the calculated values of LBi (Eq. (1)), that were calculated from the measured LWC , the theoretical binder volume content (vol% binder) and the theoretical WC volume content (vol% WC) and does not take into account the measured LBi . For hardmetals with <5 wt% of binder the inhibitor content was included in the theoretical binder volume, too. The LWC values on a grey
M. Herr et al. / International Journal of Refractory Metals & Hard Materials 19 (2001) 371±379
background in Table 1 are estimated values. Where hardmetals with low binder contents show bigger WC grain sizes because of another sintering procedure. LBi 2
LWC vol% binder A vol% WC
with A 1:148:
1
A similar equation can be found in [8]. The ®tting parameter A was found on a hardmetal with 1.5 lm of LWC , where the measurement of LBi is quite exact. The 2 in front of Eq. (1) is a geometric factor.
3. Experimental All wear experiments were carried out at room temperature with the apparatus shown in Fig. 1. A specimen
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(Fig. 3) is pressed against a rotating wheel (100 rpm, 10 mm thick, 195 mm diameter) with a de®ned normal load (FN ) that is applied in horizontal direction by a linear guide bar and a cantilever arm with a relocatable weight. The wheel is wet continuously (55 ml/min) with water. A corrosive medium could also be used. The dry abrasive (SiO2 , Al2 O3 or SiC), with a grain size of about 250 lm, ¯ows in a de®ned rate (45 g/min), realised by a kind of a paddle wheel, from the upper sandbox onto the wet wheel close to the contact area of wheel and specimen (Fig. 1). Thus, there is an abrasive slurry between wheel and specimen with a constant ratio of abrasive and water. With this apparatus it is possible to measure the friction force (FF ) and normal force (FN ) with two weighing cells, the wear depth (D) with an inductive displacement sensor and the ¯ow rate of the medium
Fig. 1. Three-body abrasive wheel test apparatus ± a combination of ASTM G65 and ASTM B611-85.
Fig. 2. Data recorded online from a wear experiment on WC hardmetal with 10 wt% Fe, Ni and Co binders.
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Fig. 3. Specimen geometry: schematic diagram, specimen in initial state, worn state.
with a ¯ow meter over the whole test time (Fig. 2) and to 3 record the results online. The wear resistance (m=mm ), which is constant over the test time, is determined by calculating the volume loss and the sliding distance from the online measured data. The sensitivity of this apparatus allows the determination of wear and friction behaviour for hardmetals with only slight variations in binder content and binder composition. This apparatus not only combines ASTM G65 and ASTM B611-85, but has a further advantage compared to the ASTM standards: the normal load is applied at right angles to the tangent of the wheel over the whole test time. This is achieved using a linear guide bar which only moves in horizontal direction (Fig. 1). The normal load is applied by the cantilever arm in this guide bar. All experiments were carried out at FN 100 N and with alumina No. 60 as abrasive, unless otherwise mentioned.
4. Results and discussion 4.1. Wear experiments Fig. 4 shows the comparison between practical application and laboratory wear tests on hardmetals with
Fig. 4. Comparison between practical application (cutting length) and laboratory wear tests (wear resistance).
binder contents lower than 5 wt% and dierent WC grain sizes. The hardmetal cutting tools were used for cutting a certain length of chipboard. The cutting length is determined by the quality of the cut chipboard. It makes no sense to compare absolute values of cutting length and wear resistance from laboratory experiments because of the dierent tribological systems. But the ranking of qualities in cutting length and laboratory wear tests is the same. So laboratory wear tests in the tribological system investigated can give an idea of the relative ranking of the cutting tools in practical application. For comparison, cutting tests were made with PCD coated hardmetal cutting tool. Concerning the cutting length, Fig. 4 shows that some of the cutting qualities of the hardmetal knives produced under laboratory conditions are comparable to those of PCD coated cutting tools. Correlations between wear resistance and microstructural parameters and physical measurements were found. Fig. 5 shows the correlation between hardness and wear resistance. The grain size of the abrasive (Al2 O3 ) is about 250 lm. So a large amount of WC grains and binder are worn by one abrasive particle. This means that bulk properties like hardness aect the wear resistance strongly (Fig. 5), i.e., the combined properties of binder phase and WC phase and not the
Fig. 5. Correlation between wear resistance and hardness (abrasive: Al2 O3 ; FN 100 N).
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properties of the individual phases determine the wear resistance [9]. With linear increasing hardness a logarithmic increase of wear resistance is observed. So the chosen tribological system is optimal for distinguishing hardmetals that dier only slightly in binder content and binder composition with respect to their wear resistance. The wide variation of the wear resistance (over three decades) is due to the abrasive used (Al2 O3 ). According to the literature [10±13], abrasive wear occurs at a low or a high level depending on the ratio of hardness of the abrasive (Ha ) to the hardness of the material (Hm ) being worn. To demonstrate these wear levels, silica sand (SiO2 /975 HV), alumina (Al2 O3 =1900 HV) and silicon carbide (SiC/2350 HV) were used as abrasives (Fig. 6). The mean values for the mineral hardness were taken from [14]. All abrasives had an abrasive grit size of about 250 lm and a similar grit size distribution (measured by sieve analysis). The tested grades had the same binder composition and almost the same WC grain size but dierent binder contents to eliminate the in¯uence of binder composition and WC grain size on the scatter of the wear resistances. The values used for Fig. 6 are circled in Fig. 5. The experiments with SiO2 as abrasive were carried out at FN 140 N to achieve a measurable wear rate. For comparison all, wear rates in Fig. 6 were normalised with respect to FN to obtain the speci®c wear resistance. The wear resistance of Ha =Hm 1 is in the transition area from low to high wear level conditions which is typical for multiphase materials. From Fig. 6 it is obvious that wear experiments using Al2 O3 as abrasive are in the transition area. This fact makes it easy to develop a relative ranking of the wear resistance of the grades used. At a ratio of about 1:2Ha =Hm there is a change in wear mechanisms, because, above this ratio, indentation of abrasive grits into the hardmetal surface is possible [14]. Literature [14] suggests that a ratio higher than 1.2 leads to fracture of WC and plastic deformation of WC
and binder phase. Ratios lower than 1.2 lead to a kind of preferential removal of the binder phase in conjunction with fracture and detachment of WC. Microstructural investigations on a hardmetal having a ratio near 1.2 showed that, in this case, these mechanisms may have to be reviewed. The correlation observed between wear resistance and hardness (Fig. 5) provided the motivation to investigate the correlation between the wear resistance and, following the Hall±Petch relation, the inverse root of the 1=2 binder mean free path (LBi ) (Fig. 7). High/low wear levels and a transition area can be found in Fig. 7, too (the calculated values of LBi (Table 1) were used here). The area of the high wear level is not so obvious, because only Al2 O3 was used as abrasive in order to highlight the transition area as shown in Fig. 6. The area of low level wear is reached at a binder mean free path of about 40 nm. This means hardmetals with a binder mean free path of less than 40 nm show only a slight increase in wear resistance with faster decreasing mean free path in the tribological system considered. So a further decrease in binder mean free path will not result in a signi®cant increase in wear resistance. This result corresponds with [3], where a change in dislocation movement is expected at a critical value of 40 nm. The binder mean free path of a hardmetal is determined by the binder content and the WC grain size. The line in Fig. 7 connects the data of hardmetals with Fe, Ni, Co binders that have almost the same composition. The scatter around this line is caused by dierent binder compositions which are not taken into account in the binder mean free path. Fig. 8 shows the wear resistance of ®ve hardmetals taken from Fig. 7 with almost the 1=2 1=2 same LBi (LBi 3:4 to 3.9) plotted against the Ni and Co content of the complex Fe, Ni, Co binder systems (all ®ve hardmetals have 10 wt% binder content). For visualisation the measured wear resistances are columns for a calculated surface graph (Fig. 8). With increasing amount of Ni in the binder phase the wear resistance
Fig. 6. High/low wear level for hardmetals (circled in Fig. 5) with same binder composition and WC grain size but dierent binder contents.
Fig. 7. Correlation between wear resistance and inverse root of binder 1=2 mean free path (LBi ).
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about 10 wt% (or a WC content of about 80 vol%) which still lie in the low wear region. The consequence would be advantages in strength, toughness and sintering procedure, while retaining almost the same wear properties in the tribological system investigated. 4.2. Microstructural investigations
Fig. 8. Wear resistance of hardmetals with 10 wt% complex binder content versus wt% of Co and Ni in the Fe, Ni and Co binder phases.
decreases, but a further increase of Ni would lead to an increase of the wear resistance as observed in [5]. In agreement with [5], at a Ni content of about 20 wt% in the binder phase, there is a maximum in wear resistance at about 10 wt% of Co (Fig. 8). A decrease in the Ni content and a simultaneous increase in the Co content would lead to higher wear resistances (Fig. 8). 1=2 The correlation between LBi with WC volume content and WC grain size is shown in Fig. 9. Dierent WC grain sizes (0.1, 0.2, 0.5 lm) in Fig. 9 are characterised by calculated lines (Eq. (1)). The scatter line is the threshold to the low wear region for hardmetals with LBi less than 40 nm as shown in Fig. 7. Most of the tested grades fall near the line of 0.2 lm WC grain size (Fig. 9). For these grades, the threshold to the low wear region is exceeded at a binder content lower than 3 wt% (or a WC content of more than about 90 vol%). If the WC grain size were reduced from 0.2 to 0.1 lm, it would be possible to produce hardmetals with a binder content of
Fig. 9. Correlation between inverse root of binder mean free path (LBi1=2 ) with vol% WC and WC grain size (calculated lines for 0.1, 0.2, 0.5 lm WC grain size).
Further microstructural investigations have so far been carried out on grade 59 (Table 1) with 17.5 wt% Fe, Ni, Co binder and a WC intercept length of 0.19 lm. This hardmetal has a wear resistance near the high wear region (Ha =Hm 1:15; Fig. 6), as indicated by the square in Fig. 7. According to the wear mechanisms mentioned in the explanations of Fig. 6, wear on the surface of this hardmetal should cause preferential removal of the binder phase due to displacement of WC grains and extrusion of the binder phase. The removal of binder phase between the WC grains leads to detachment and cracking of the WC grains [14]. For ratios greater than 1:2Ha =Hm , fracture of WC grains and plastic deformation of binder and WC phase should be the dominating wear mechanism [14]. So at a ratio near 1:2Ha =Hm (the investigated hardmetal has a ratio of 1:15Ha =Hm ) there should probably be a mixture of both wear mechanisms for higher and lower values of 1:2Ha =Hm . Microstructural investigations on this hardmetal showed in contrast to the above-mentioned wear mechanisms that the worn surface is covered with a tribo®lm. A top view on the worn surface by SEM only shows a diuse surface with smaller WC grains compared to the polished state, as shown in, Fig. 10. At lower magni®cation in the SEM, macroscopic wear grooves are visible (Fig. 11), due to microploughing of the abrasive particles. So it is assumed that the worn surface (the tribo®lm) suers massive plastic deformation during the wear process. The tribo®lm is obvious in the cross-section view of Fig. 12. The tribo®lm has a thickness of about 2 lm, and it seems that the ®lm consists of some just created large fragments of WC grains (Fig. 12, left) and very small grains that are, perhaps, rounded (Fig. 12, right). The ®lm has a smooth outer surface and a sharp transition to the bulk material under the ®lm. A similar tribo®lm was found in dry sliding with conform contact on hardmetals with large WC grains (7 and 1.5±2 lm) by [9]. Preliminary EDX analysis on the tribo®lm in a cross-section and on bulk material were carried out using SEM. The Fe-based binder phase consists of Fe, Ni and Co. All binder elements were detected with the measurements over the bulk material. In measurements over the ®lm, only Fe was found as binder material. So the ratios between WC and Fe (wt%) in the bulk and in the ®lm could be compared (Table 2). Table 2 indicates that there is a lower Fe content, i.e., binder content, in the ®lm than in
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Fig. 10. Top view on polished (left) and worn surface (right) of grade 59 (SEM).
Table 2 Ratio between wt% W and wt% Fe in bulk material and tribo®lm
Fig. 11. Top view on worn surface of grade 59 with wear grooves caused by Al2 O3 abrasive particles (SEM).
the bulk material. This may be correspond to the mentioned wear mechanisms for preferential removal of the binder phase discussed above. But here the binder phase is removed from the tribo®lm and not between WC grains from the bulk material. Moreover, Table 2 indicates with the higher value of the standard deviation of
Bulk material (W/Fe)
Tribo®lm (W/Fe)
40 8
68 18
the measurements over the ®lm that the composition of the tribo®lm is less homogeneous than that of the bulk material, due to inhomogeneous preferential removal of binder from the tribo®lm. AFM results with non-contact mode on the tribo®lm from the top view are shown in Figs. 13 and 14. The topography contrast image (left) and the phase contrast image (right) indicate a massive change in the WC grain shape and size compared to the unworn state. The grains at the top of the tribo®lm are rounded and have a grain size much lower than in the unworn state. There are still some contours of coarser rounded grains visible (left image), indicating that coarser rounded grains fracture into nanocrystalline grains. Moreover, there is more fracture of WC grains in parallel (from left to right) to the wear groves (only visible in the left image). TEM investigations verify the AFM results. The TEM samples were prepared from the bulk side to have
Fig. 12. Cross-section view on the worn surface of grade 59 (SEM).
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Fig. 13. Top view on worn surface of grade 59 (AFM); topography contrast (left), phase contrast (right).
round structures of the tribo®lm consist of an amorphous phase containing WC, Fe and Ni. Between the round grains, a higher amount of Fe was found, indicating this to be a kind of binder phase for the tribo®lm. 5. Summary
Fig. 14. Top view on polished surface of grade 59 (AFM); topography contrast.
transmission through the tribo®lm from the top view (see Fig. 15). For further TEM investigations, a sample with better transmission is needed to show some diffraction patterns. The results obtained so far are that the
1. Laboratory wear tests can provide the relative ranking of the quality of the cutting tool in practical application. 2. Wear experiments in the tribological system considered were carried out with the above-mentioned parameters in the transition area from low to high wear level. This fact makes it easy to distinguish wear properties of dierent hardmetals that only show slight variations in their composition. 3. The wear resistance of hardmetals with complex binder systems consisting of Fe, Co and Ni is increased with simultaneous decrease of Ni content and increase of Co content in the binder phase. For
Fig. 15. Bulk material (left) and top view on tribo®lm (right) of grade 59 (TEM).
M. Herr et al. / International Journal of Refractory Metals & Hard Materials 19 (2001) 371±379
hardmetals containing 20 wt% Ni in the binder phase a maximum in wear resistance was observed at a Co content of 10 wt%. 4. At a binder mean free path of ca. 40 nm there is a threshold to the low wear region (maximum wear resistance) for the tribological system considered. Therefore, a further decrease in binder mean free path will not result in a signi®cant increase in wear resistance. The threshold was exceeded by hardmetals with a binder content of 2±3 wt% at a WC intercept length of about 0.3 lm. Calculations showed that, if the WC grain size were reduced from 0.3 to 0.1 lm, it would be possible to produce hardmetals with a binder content of about 10 wt% which still exhibit a low wear region. Thus, advantages in strength, toughness and sintering procedure without reduction in wear performance would be expected for the tribological system investigated. 5. A tribo®lm was found on the worn surface. The ®lm consists of nanocrystalline rounded grains with probably a lower binder content compared with the bulk material. First TEM investigation showed that these grains consist of an amorphous phase containing WC, Fe and Ni. Possible wear mechanisms should depend strongly on the formation of the ®lm and on the behaviour of the ®lm during wear. References [1] Feld H. Hardmetal for woodcutting applications. Woodworking Int 1988;4.
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[2] Walter P, Feld H. Bestimmung der Abriebfestigkeit von Hartmetall mit dem Schleifrad. Planseeber Pulvermetall 1976;24: 100±7. [3] Richter V. On hardness and toughness of ultra®ne and nanocrystalline hard materials. Refract Met Hard Mater 1999;17: 141±52. [4] Jia K, editor. Microstructure, mechanical properties and wear resistance of WC/Co nanocomposites. Mat Res Soc Symp 1997;457:303±8. [5] Prakash L. Weiterentwicklung von WC Hartmetallen unter Verwendung von Eisen-Basis-Binderlegierungen. PhD Thesis, Karlsruhe, 1979. [6] Nordgren A. Microstructural characterisation of cemented carbides using SEM based automatic image analysis. Refract Met Hard Mater 1991;10:61±81. [7] Sailer T, Herr M, Sockel H-G, Schulte R, Feld H, Prakash LJ. Microstructure and mechanical properties of ultra®negrained hardmetals. Int J Refract Met Hard Mater 2001; 19:553±9. [8] Underwood EE. Quantitative stereology. Reading, MA: 1970. [9] Engqvist H. Microstructural aspects on wear of cemented carbides. PhD Thesis, Uppsala, 2000. [10] Wahl H, editor. Verschlei probleme im Braunkohlenbergbau, Braunkohle. Warme Energie 1951;5/7:75±87. [11] Wellinger K, Uetz H. Gleitverschlei, Sp ulverschlei Strahlverschlei unter der Wirkung von k ornigen Stoen. VDIForschungsh 1955;21:449B. [12] Uetz H, F ohl J. Gleitverschleiuntersuchungen an metallen und nichtmetallischen hartstoen unter wirkung k orniger stoe. Warme Energie 1969;21:10±8. [13] Feld H, Walter PH. Beitrag zur kenntnis des mineral-hartmetallverschleies. J Mater Technol 1976;7:300±3. [14] Hutchings IM. Tribology: friction and wear of engineering materials. Cambridge: 1992 [chapter 6].