Initial deformation and wear of cemented carbides in rock drilling as examined by a sliding wear test

Initial deformation and wear of cemented carbides in rock drilling as examined by a sliding wear test

Accepted Manuscript Initial deformation and wear of cemented carbides in rock drilling as examined by a sliding wear test Jannica Heinrichs, Mikael O...

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Accepted Manuscript Initial deformation and wear of cemented carbides in rock drilling as examined by a sliding wear test

Jannica Heinrichs, Mikael Olsson, Staffan Jacobson PII: DOI: Reference:

S0263-4368(16)30624-2 doi: 10.1016/j.ijrmhm.2016.12.011 RMHM 4388

To appear in:

International Journal of Refractory Metals and Hard Materials

Received date: Revised date: Accepted date:

14 October 2016 16 December 2016 19 December 2016

Please cite this article as: Jannica Heinrichs, Mikael Olsson, Staffan Jacobson , Initial deformation and wear of cemented carbides in rock drilling as examined by a sliding wear test. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Rmhm(2016), doi: 10.1016/j.ijrmhm.2016.12.011

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ACCEPTED MANUSCRIPT Initial deformation and wear of cemented carbides in rock drilling as examined by a sliding wear test

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Jannica Heinrichsa*, Mikael Olssona,b, Staffan Jacobsona a Ångström Tribomaterials Group, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden b Materials Science, Dalarna University, SE-791 88 Falun, Sweden

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*Corresponding author e-mail address: [email protected]

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ABSTRACT

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Due to a combination of high hardness and toughness, resulting in excellent wear resistance, cemented carbides are commonly used as the rock crushing component in rock drilling. The present paper presents a unique study where the very initial stages of deformation and wear of cemented carbide in sliding contact with rock are followed in small incremental steps. After each step, a pre-determined area within the wear mark is characterized using high resolution SEM and EDS. This facilitates analysis of the gradual deformation, material transfer, degradation and wear. The deterioration mechanisms found in this sliding test are similar to those observed in actual rock drilling. Cemented carbide grades with different microstructures show significant differences, where a higher amount of Co and a larger WC grain size both are associated to more wear.

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1. INTRODUCTION

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Key words; Cemented carbides, sliding, wear, deformation, granite, rock drilling

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Cemented carbides constitute a range of composite materials, consisting of hard carbide particles bonded together by a metallic binder. The basic grades consist of uniformly distributed WC grains in a binder of Co. These composites display a combination of high hardness and toughness, resulting in excellent wear resistance in many demanding engineering applications, such as rock drilling. However, despite their proven superiority in rock drilling applications, the detailed understanding of the prevailing wear mechanisms is far from complete. With a deepened knowledge about the wear, drill bit buttons could possibly be designed to last longer, thereby decreasing the costs for down time, materials and regrinding. Characterization of the surface of a worn drill bit button usually reveals a very smooth worn surface, where the roughness typically is limited to the size of individual WC grains, or even less, indicating that the deformation and wear chiefly operates on a sub-µm scale [1,2]. Beste and Jacobson [1] have estimated the wear depth to be 1 µm per 20 impacts for rotary/percussive drilling in granite. The low wear rate and the observed plastic deformation, cracking, chipping and crushing of individual WC grains indicate that wear is dominated by removal of individual grains or grain fragments, while removal of large clusters of grains is unusual [1,2]. In addition to the fractured WC grains, the worn drill bit button also shows depletion of Co in the top surface and instead rock material intermixed with Co act as a binder between worn WC grains.

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A standard method for testing the abrasive wear resistance of cemented carbides comprises a cemented carbide block that is pressed against a steel wheel rotating in a slurry containing aluminium oxide abrasive particles [3]. The wear rate is evaluated by measuring the weight loss of the cemented carbide specimen, with an accuracy of 0.0001 g. References [4-7] employ this standard test. They all show that the wear resistance generally increases with increasing hardness. However, this is said to be valid only when the wear is dominated by plastic deformation, as in cemented carbides of relatively low hardness. When the hardness increases, micro fracture becomes an important wear process, and thus the wear resistance depends also on WC grain size, Co binder content and composition of the Co binder, all of which are microstructure parameters influencing the response to tribological contact [6-9].

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When it comes to rock drilling, other properties than abrasive wear resistance come into play. While field tests often are complicated and expensive, some laboratory tests better imitating the real rock drilling application can be used. Several authors have performed rock drilling in a laboratory environment. In [10] percussive rock drilling in granite is performed in lab scale. The authors conclude that cemented carbide grades with near-nano sized WC grains wear faster than grades with medium to medium-coarse grains, in grades of the same hardness. These grades were given the same hardness by adjusting the amount of Co. This higher wear rate is due to the reduced fracture toughness of the grades with near-nano sized grains, causing micro-chipping and micro-cracking and thus detachment of large WC-Co agglomerates. In [11] rotary drilling tests is performed in sandstone. The authors showed that an increase in WC grain size and Co mean free path increase wear, although the amount of Co is just slightly changed. They also concluded that the grades that wear the most have high fracture toughness, although the main wear mechanism was observed to be fracture of the WC skeleton. The contradiction in this conclusion suggested that the mechanisms must be studied more thoroughly and earlier in the process.

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Due to the complexity of the rock drilling process, other simplified tests not including actual drilling, have been reported. In [12] a cemented carbide rock drill button is pressed against a rotating cylinder of granite and wear is measured by weight loss of the button. The authors report that water addition increase wear and so do addition of alumina particles. Addition of silica particles on the other hand does not affect the wear. It is concluded that all worn surfaces look similar to those worn in actual rock drilling, except for those were alumina was added. In these the wear was increased and dominated by large scratches, not present on rock drill button surfaces after actual drilling. In the above tests, several WC grain layers are removed from the surface before the test is interrupted and the worn surfaces analysed. Therefore, all traces of the initial degradation and wear initiation have been lost. References [13-17] present single asperity tests using diamond tips, performed to study the very initial response of cemented carbides in high stress sliding contact. The localised contact allows for careful Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD) studies and provides very detailed information about the deformation against a harder, inert and static counter surface. This approach is however not necessarily directly applicable to rock drilling, where the counter surface typically is softer than the cemented carbides or at least softer than its tungsten carbide hard phase. By using styli manufactured from rock material [17,18], also very localised interaction with rock can be studied, and the very initial stages of several degradation and wear mechanisms also present in rock

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The present paper employs a new unique approach facilitating studies of the first successive stages of the degradation process of cemented carbide in contact with rock. The test is based on a lathe set-up where a small polished cemented carbide cylinder is pressed against a larger rotating granite cylinder, in crossed cylinders geometry. To study the initial interaction with the rock material, the sliding is repeatedly interrupted, the distance is limited to a few centimeters and exactly the same pre-determined area within the wear mark is repeatedly analyzed. In this way it is possible to keep track of the gradual deformation, degradation and wear of the microstructure. Four commercial cemented carbide grades, representing grades commonly used in rock drilling, are evaluated. 2. EXPERIMENTAL 2.1. Materials

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A range of commercial cemented carbide grades have been selected for evaluation, see Table 1. The grades selected include one of the most common grades in rock drilling with 6 wt% Co and small (~1 µm) WC grains, one with larger grain size and two grades with higher Co content. The samples were cylindrical in shape (ø 12.8 mm, L 20 mm). To allow microstructure studies, all samples were polished to Ra <40 nm. The composite micro hardness was measured according to Table 1. Nanoindentation was used to measure the hardness of the individual phases, see Table 2.

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Table 1. Composition, WC grain size, micro hardness and Co mean free path of the respective cemented carbide grades. Compositions and grain sizes are given by the manufacturer. The Co mean free path (λ) is calculated using the relation λ= d VCo /(1- VCo), where VCo is the Co volume fraction and d is the WC grain size [19]. Note that the number in the designation represents the wt% of Co and the final letter designates the grain size, fine or coarse.

Composition [wt%] WC 94, Co 6 WC 94, Co 6

11CoF 15CoF

WC 89, Co 11 WC 85, Co 15

Hardness Co mean free path HV1 [kg/mm2] [µm] Fine (1 µm) 1620 ± 35 0.11 Coarse (3 µm) 1310 ± 40 0.34

WC grain size

Fine (1 µm) Fine (1 µm)

1290 ± 30 1150 ± 30

0.22 0.31

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Cemented carbide grade (designation) 6CoF 6CoC

Granite (Bohus granite) was chosen to represent the rock material. It is a composite material consisting mainly of quartz, biotite and feldspar. It was shaped into a cylinder (ø 42 mm, L 148 mm) and to imitate the contact against fractured rock in drilling, the surface was first roughened using 120 grit SiC grinding paper. Table 2. Nanohardness of the respective constituents in the cemented carbides and Bohus granite. The hardness of the cemented carbide phases are measured in 15CoF (Co) and 6CoC (WC). The hardness of Bohus granite is from [18]. All measurements have an indentation depth of 100 nm.

Phase

Nanoindentation hardness [GPa]

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7.7 ± 0.4 27 ± 1.3 13 ± 1.0 10 ± 1.3 3.4 ± 1.2

2.2. Tribological testing

Sketch showing the crossed cylinder geometry and lateral feed in the lathe set-up sliding test. The large cylinder represents the rock material and the small cylinder the cemented carbide.

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Fig.1

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The cemented carbide cylinder was pressed against the larger rotating granite cylinder in crossed cylinders geometry, using a lathe set-up, see Fig. 1. It is a pure sliding contact, and the cemented carbide cylinder was fed in the lateral direction, to always be in contact with fresh rock. The sliding speed was set to 7.3 cm/s. The normal force, F=60 N, was applied by a spring and monitored during the test. The normal load is chosen high enough to cause fracture of the rock material in the contact, in similarity to actual rock drilling. The test was interrupted for scanning electron microscopy (SEM) investigations at short intervals, each involving only a few centimetres sliding, adding up to a total sliding distance of 100 cm.

2.3. Post test surface characterization

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3. RESULTS

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The cemented carbide surfaces were studied post testing using high resolution SEM (Zeiss Merlin) and Energy Dispersive X-ray Spectroscopy (EDS; Oxford Aztec X-max). Focused Ion Beam (FIB; FEI Strata DB235) was used for cross-sectioning of selected samples, preceded by in situ platinum deposition to protect the very surface from ions, during milling and polishing using the ion beam.

3.1. Mechanism study of 6CoF Here, the behaviour of the 6CoF sample, being a commonly used cemented carbide grade for drilling in granite, is presented separately. The behaviours of the other grades are then reported in 3.2. using this more detailed report as a reference. Already after the first 8 cm of sliding, several observations of degradation were made in the wear mark on the cemented carbide, Fig. 2. The WC grains have shifted slightly in position, thereby exposing the grain boundaries. Single grains show some plastic deformation and occasional cracking. Some Co has been replaced by (or covered by) granite, especially where larger volumes were present. Thin layers of granite have also transferred to the surface.

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Continued sliding leads to removal of small WC fragments and more extensive granite transfer, resulting in increased surface roughness and intensified mechanical interaction between the granite and cemented carbide. As a result, the tendency to tilting, cracking and fragmentation of the superficial WC grains increase with increasing sliding distance. Fragment by fragment, whole WC grains are removed and replaced by granite, resulting in an increasing fraction of granite cover. Occasionally, some of the transferred granite becomes removed. Already after less than 100 cm sliding, the original microstructure with WC separated by Co is no longer recognisable.

Fig.2

Exact same area of 6CoF imaged before and during successive sliding, indicated in the images. The dark features are transferred granite. Sliding direction of the granite is from bottom to top. Tilt 45°. (SEM, 3 kV)

After 100 cm sliding, almost all Co is removed from the surface or buried under transferred granite, see Fig. 3. Traces of Co are present in the near-surface area. However, the superficial microstructure is dominated by Si and O, from the granite, and W and C, from the WC grains.

Fig.3

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Elemental mapping of the worn 6CoF cemented carbide surface after sliding 100 cm against the granite cylinder. The O and Si signal represents transferred granite. Tilt 45°. (SEM/EDS, 5 kV)

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A similar area studied in cross-section, Fig. 4, confirms that the surface layer is dominated by fragmented WC grains in a granite binder. However, this short test has mainly affected the top surface while the microstructure is seemingly unaffected only a couple of micrometers into the cemented carbide.

Fig.4

The worn 6CoF in surface and cross-section view, after sliding 87 cm against the granite cylinder. Combination of two images. (SEM, 3 kV)

In higher magnification, Fig. 5, it is indeed observed that the surface layer consists of cracked grains and small WC fragments imbedded in granite.

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3.2. Degradation study of the four cemented carbide grades

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Details of the cross section in Fig.4. Bright WC grains, light grey cobalt binder and dark transferred granite with small, imbedded WC fragments. The light grey layers covering the structure are the platinum film used to protect the surface during the cross sectioning. (SEM, 3 kV)

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Fig.5

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The different grades have very different microstructures, as shown in Fig. 6. To compare the wear initiation of the different grades, the successive sliding test was repeated for all grades. Differences appear already after the first very short sliding against the granite, Fig. 7. The 6CoC and 15CoF grades both have incorporated substantial amounts of granite onto their surfaces and into their microstructures.

Fig.6

Microstructures of the different polished cemented carbide grades before testing. Tilt 45°. (SEM, 3kV)

The exact same areas of the different polished cemented carbide grades as in Fig. 6 after sliding ~8 cm against the granite cylinder. Sliding direction of the granite is from bottom to top. Tilt 45°. (SEM, 3kV)

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Fig.7

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Continued sliding leads to significant transfer also to the 11CoF grade, while the transfer to 6CoC and 15CoF escalates, Fig. 8.

The exact same areas of the different polished cemented carbide grades as in Fig. 7 after sliding ~17 cm against the granite cylinder. Sliding direction of the granite is from bottom to top. Tilt 45°. (SEM, 3kV)

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Fig.8

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Further sliding, Fig. 9, leads to significant transfer of granite to the 6CoC, now not only in between WC grains, but also covering groups of grains. The 15CoF grade, on the other hand, still has the granite mostly filling the space in between WC grains. However, the surface starts to look rough and worn. The surface irregularities now have the dimensions of individual WC grains, implying that whole grains have already been removed, however most probably fragment by fragment.

The exact same areas of the different polished cemented carbide grades as in Fig. 8 after sliding ~40 cm against the granite cylinder. Sliding direction of the granite is from bottom to top. Tilt 45°. (SEM, 3kV)

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Fig.9

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After a total of 70 cm sliding against the granite cylinder, Fig. 10, the original microstructure can still be recognized only for the 6CoF grade. Here, individual grains from Fig. 7 can still be identified. The other grades all have rough and worn appearances, with minimal similarities to the original structure.

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Fig.10 The exact same areas of the different polished cemented carbide grades as in Fig. 9 after sliding ~70 cm against the granite cylinder. Sliding direction of the granite is from bottom to top. Tilt 45°. (SEM, 3kV)

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The deformation and wear are however not affecting the microstructures to great depths, as shown by the cross sections in Figs. 4 and 5. This is also confirmed by the use of different ―imaging depths‖ in the SEM (by varying the electron energy), Fig. 11. This technique reveals that the rock layers are relatively thin. The layers are locally thicker, where filling out irregularities in the worn cemented carbide, but these height variations are typically in the range of individual WC grains.

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Fig.11 The 6CoF after sliding 100 cm against the granite cylinder. The exact same area is imaged using different acceleration voltages, as indicated in the respective images. At the highest acceleration voltage, the transferred granite is almost completely transparent, revealing most of the underlying topography. Sliding direction of the granite is from bottom to top. SEM, tilt 45°.

4. DISCUSSION

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4.1. Wear conditions in the test method compared to actual drilling

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The present test deviates from actual drilling in several respects. The dynamic situation is very different, since the test involves pure sliding, without impacts as typical of e.g. rotary percussive drilling. Neither is water added, as is typically used for cooling in underground drilling. In drilling, the drill buttons meet not only the solid rock but also recently produced rock fragments. The test did not include addition of rock fragments. However, as has previously been shown, neither water nor rock fragments are needed to achieve the same types of wear mechanisms as in rock drilling [12]. Also in the present test, the worn surfaces are very similar to those observed on actual drills. Two of the most important characteristic features of actual drilling are included: the contact pressure is high enough to locally fracture the rock and the tool continuously meets new counter material. The ability of the test to generate the true dominant wear mechanisms is a strong indicator that it is very suitable for detailed studies of the initial degradation and wear of cemented carbides for rock drilling. The test samples have a simple geometry and are easy to fabricate. The test itself is cheap, although the surface investigations require an SEM. The present investigation has focused on mechanisms and wear initiation. For quantitative wear rate comparisons between different grades, the test procedure described in [12] is more suitable.

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The observations of the initial deterioration is summarised in Fig. 12. The deterioration starts with plastic deformation and preferential wear of the binder, against the passing rock material. This allows for granite to penetrate into the Co pockets and also allows the WC grains to tilt. These modifications increase the surface roughness and change the composition of the top layer, thereby further aggravating the interaction between the mating surfaces. The intensified contact stresses cause the WC grains to deform and break. Small fragments of the grains become removed, while no loss of whole WC grains or clusters of WC grains are observed. It is worth noticing that unlike in actual drilling, the samples used here were polished before testing. This enables studies of a smooth and virtually undeformed microstructure in a controlled manner of the very first roughening stages of the deterioration, probably not a separable stage when the surfaces are already rough. The subsequent repetitive process involving transfer of rock material, deformation and fracturing of WC grains and material loss mainly by removal of WC fragments is however believed to be representative also for an originally rough surface.

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While after the first few incremental sliding stages the worn surfaces show similarities to previously published single asperity sliding tests [17,18], the appearances gradually become very similar to those observed on drill bit buttons used in rock drilling [1,2].

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This strong similarity in surface appearance indicates that the described wear process involving fracturing and successive removal of WC fragments (as illustrated by Fig. 12) likely is a dominant wear mechanism also in actual rock drilling. This process results in the typical severely fractured and deteriorated worn appearance with a topography that is smooth compared to the WC grain size.

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Fig.12 Schematic illustration of the observed initial deterioration mechanisms. The rock counter surface is moving from left to right.

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4.3. Influence from the cemented carbide microstructure

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The sliding tests reveal clear differences in appearance between the different cemented carbide grades, after the same sliding distance. However, the wear mechanism observed is similar. The differences rather arise from the events occurring earlier and escalating quicker with the grades with lower wear resistance. The grades with higher amount of Co and larger grain size are known to wear quicker, also from other tests [11]. The Co mean free path increases with Co content and WC grain size [4,6]. Since the degradation starts with plastic deformation of the Co, the grades with a larger Co mean free path are more sensitive to this. The subsequent exchange of cobalt for granite is also more prominent if the preceding Co volumes are larger. Also the tilting of WC grains is easier with less WC stabilising the microstructure. This consequently leads to the transferred granite getting a stronger mechanical interlocking, which also significantly increases the surface roughness. These processes make the wear escalate much quicker for the grades with the higher mean free path, although the course of events is similar. The 6CoC and 11CoF grades have similar hardness, but their initial deterioration rates were found to differ. The wear rate of 11CoF is lower and the original microstructure is preserved for a

ACCEPTED MANUSCRIPT longer time. This is likely explained by the fact that, despite its higher Co content, the 11CoF grade offers better support to its WC grains due to its smaller Co mean free path. 5. CONCLUSIONS

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The presently taken experimental approach has for the first time allowed detailed studies of the very first stages of degradation and wear of cemented carbides in a contact situation resembling rock drilling. By following the gradual microscopic alterations within a small area, new details regarding the strong dynamics of the process, and the short time to establish the typical steady state appearance of worn rock drill buttons, were revealed.

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 Already during the first centimeters of sliding against the granite cylinder, single WC grains become plastically deformed or cracked and Co becomes removed.  During continued sliding, the WC grains are removed fragment by fragment until whole grains have been removed. Granite penetrates and fills the cavities of the cemented carbide surface.  Already after less than one meter sliding, the appearance of field worn rock drill buttons is quite closely imitated. This ability to mimic the dominant surface features of actual worn rock drill buttons implies that this simple test can offer valuable detailed understanding of the wear mechanisms.  The test discriminates between different cemented carbide grades and associates an increasing amount of Co and increasing WC grain size to higher wear rates.

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ACKNOWLEDGEMENT

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The authors would like to thank Dr. Stefan Olovsjö, Atlas Copco Secoroc, for providing the test materials. REFERENCES

U. Beste, S. Jacobson, A new view of the deterioration and wear of WC/Co cemented carbide rock drill buttons, Wear 264 (2008) 1129–1141 [2] S. Olovsjö, R. Johanson, F. Falsafi, U. Bexell, M. Olsson, Surface failure and wear mechanisms of cemented carbide rock drill buttons – The importance of sample preparation and optimized microscopy settings, Wear 302 (2013) 1546-1554 [3] American National Standards Institute. Standard test method for abrasive wear resistance of cemented carbides. Philadelphia: American Society for Testing and Materials (ASTM B 611-85); 1985 [4] S. Luyckx, N. Sacks, A. Love, Increasing the abrasion resistance without decreasing the toughness of WC–Co of a wide range of compositions and grain sizes, Int. J. of Refractory Metals & Hard Materials 25 (2007) 57–61 [5] A. J. Gant, M. G. Gee, B. Roebuck, Rotating wheel abrasion of WC/Co hardmetals, Wear 258 (2005) 178-188 [6] D. G. F. O'Quigley, S. Luyckx, M. N. James, An Empirical Ranking of a Wide Range of WC-Co Grades in Terms of their Abrasion Resistance Measured by the ASTM Standard B 611-85 Test, Int. J. of Refractory Metals & Hard Materials 15 (1997) 73-79 [7] I. Konyashin, B. Ries, Wear damage of cemented carbides with different combinations of WC mean grain size and Co content. Part I: ASTM wear tests, Int. J. of Refractory Metals & Hard Materials 46 (2014) 12–19 [8] B. Roebuck, A. J. Gant, M. G. Gee, Abrasion and toughness property maps for WC/Co hardmetals, Powder Metallurgy 50 (2007) 111-114 [9] M. G. Gee, A. Gant, B. Roebuck, Wear mechanisms in abrasion and erosion of WC/Co and related hardmetals, Wear 263 (2007) 137-148 [10] I. Konyashin, B. Ries, Wear damage of cemented carbides with different combinations of WC mean grain size and Co content. Part II: Laboratory performance tests on rock cutting and drilling, Int. J. of Refractory Metals & Hard Materials 45 (2014) 230–237

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[11] J. Larsen-Basse, C. M. Perrott, P. M. Robinson, Abrasive wear of tungsten carbide-cobalt composites. I. Rotary drilling tests, Materials Science and Engineering 13 (1974) 83-91 [12] J. Angseryd, A. From, J. Wallin, S. Jacobson, S. Norgren, On a wear test for rock drill inserts, Wear 301 (2013) 109–115 [13] K. P. Mingard, M. G. Gee, EBSD examination of worn WC/Co hardmetal surfaces, Wear 263 (2007) 643–652 [14] M. Gee, K. Mingard, B. Roebuck, Application of EBSD to the evaluation of plastic deformation in the mechanical testing of WC-Co hardmetal, Int. J. of Refractory Metals & Hard Materials 27 (2009) 300–312 [15] M. G. Gee, L. Nimishakavi, Model single point abrasion experiments on WC/Co hardmetals, Int. J. of Refractory Metals & Hard Materials 29 (2011) 1–9 [16] A. J. Gant, M. G. Gee, D. D. Gohil, H. G. Jones, L. P. Orkney, Use of FIB/SEM to assess the tribo-corrosion of WC/Co hardmetals in model single point abrasion experiments, Tribology International 68 (2013) 56-66 [17] J. Heinrichs, M. Olsson, S. Jacobson, Surface degradation of cemented carbides in scratching contact with granite and diamond – the roles of microstructure and composition, Wear 342-343 (2015) 210-221 [18] M. Olsson, J. Heinrichs, K. Yvell, S. Jacobson, Initial degradation of cemented carbides for rock drilling — Model studies of the tribological contact against rock, Int. J. of Refractory Metals & Hard Materials 52 (2015) 104-113 [19] E. Underwood, Quantitative Stereology, Addison/Wesley, New York, 1970

ACCEPTED MANUSCRIPT Highlights Cemented carbides studied in sliding test against granite to imitate rock drilling. Plastic deformation and wear of binder enable tilting of WC and transfer of granite. WC grains deform and crack and are subsequently removed fragment by fragment.

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The test successfully discriminates between different cemented carbide grades.

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An increasing amount of Co and increasing WC grain size is associated to higher wear.