Rock penetration into cemented carbide drill buttons during rock drilling

Rock penetration into cemented carbide drill buttons during rock drilling

Available online at www.sciencedirect.com Wear 264 (2008) 1142–1151 Rock penetration into cemented carbide drill buttons during rock drilling Ulrik ...

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Available online at www.sciencedirect.com

Wear 264 (2008) 1142–1151

Rock penetration into cemented carbide drill buttons during rock drilling Ulrik Beste a , Staffan Jacobson b,∗ , Sture Hogmark b a

b

Atlas Copco Secoroc AB, Box 521, SE-737 25 Fagersta, Sweden Tribomaterials Group, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden

Received 26 October 2004; received in revised form 13 December 2006; accepted 17 January 2007 Available online 6 March 2007

Abstract In percussive and rotary percussive rock drilling, the rock is crushed into small fragments by the repeated hard impact of the drill bit, and subsequently removed by flushing water or air. To avoid excessive wear, the steel drill bit is equipped with a set of cemented carbide buttons that protrude from the bit to take the actual impact. The severe contact against the rock results in some wear of the button, but also in formation of surface layers of rock material and penetration and impregnation of rock material into the cemented carbide structure. This situation, with serious implications for the wear and fracture of the buttons, have previously not been reported. The present findings represent a significantly new understanding of the wear of the rock button material. The deterioration mechanisms are described in detail, using examples from a range of real drilling applications in different rock types. During operation, material in the surface layer of the drill button shifts from that of the original cemented carbide into an uncontrolled composite. This composite is formed by the WC carbide hard phase and a binder consisting of a mixture of cobalt and rock. This new material should be expected to exhibit properties significantly different from the original cemented carbide. © 2007 Elsevier B.V. All rights reserved. Keywords: Rock drill; Wear; Deterioration; Rock penetration; Cemented carbide; Hard metal

1. Introduction Rock drilling is performed by a number of techniques ranging from rotary/percussive drilling in very hard rock, via rotary/crushing drilling in medium hard rock, down to cutting in soft rock types. Typically, the tools used are equipped with WC/Co cemented carbide buttons that perform the actual fragmentation of the rock. Cemented carbide (CC) has a unique combination of high hardness and high toughness [1] that facilitate the capacity to crush rock while suffering minimum fracture and wear. Typical rock drill grades have rather coarse WC grains, 2–5 ␮m, and 5–10% Co binder, a composition offering a very good balance between wear resistance and fracture resistance against a range of rock types. DOIs of original articles:10.1016/j.wear.2007.01.030, 10.1016/j.wear.2007.01.031 ∗ Corresponding author. Tel.: +46 18 471 30 88; fax: +46 18 471 35 72. E-mail address: [email protected] (S. Jacobson). 0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.01.029

Despite the highly developed material properties, wear of the CC drill buttons is usually the life-limiting factor for the rock drill bit. This wear has been examined numerous times. However, the experimental difficulties are significant. Firstly, direct observation of the drill button inside the deep hole is impossible and it is extremely hard to measure the contact conditions (normal and friction forces, temperature, etc.). Further, the immense variation in properties between rock types from all over the world, but also within a single mine, makes it difficult to compare and evaluate the results. The wear process has often simply been described as “abrasion”. However, it is actually much more complex. The most important wear mechanisms reported in the literature are fragmentation of WC grains, binder phase extrusion and thermal fatigue (forming “reptile skin”/“heat check” patterns and large cracks) [2–4]. The worn surface is typically considered to retain the properties of the CC material. However, in his 1973 rock drill review Larsen-Basse reported a “skin effect” in rock drilling [5]. The skin effect signifies a lower ini-

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Table 1 Drill data and rock penetration of the investigated buttons Button no.

1 2 3 4 5 6 7 8 9 10

Rock type

Quartzite Quartzite Quartzitic granite Quartzitic granite Magnetite Magnetite Chromite Chromite Manganese Gypsum

Drilling depth [m]

18 280 18 500 1 20 17 144 102 3300

Thickness of intermixed layer [␮m]

[# WC grain layers]

10 15 10 20 2 10 10 40 30 30

3 5 4 8 1 2 1 16 12 12

Maximum rock channel depth [␮m]

a Times

30 180 50 80 10 40 130 100 300 800

0 8 0 26 0 0 0 Unknown Unknown 0

reshaped

All penetration data are average values from estimations made on several micrographs from each button. a The drill buttons are reshaped when the risk for large-scale fracturing due to reptile skin or sharp edges is high. Usually, reshaping does not affect the top centre of the drill button, which is the position where the studies are made.

tial wear rate of the button compared to the steady state wear rate. Jonsson reported an increased hardness down to 1–2 mm below the surface of buttons used to drill magnetite [6]. In a simplified lab-investigation of the wear mechanisms, Beste et al. used a CC tip to scratch different rock types [7]. It was concluded that large, hard mineral grains (notably the quartz grains in sandstone) lead to rapid CC wear and a complex mixture of surface damage types, including fragmentation of WC grains and adhered rock. No directionality or signs of two-body abrasion were observed, despite the very “abrasive” character of the sandstone. The present investigation is based on examples selected from a range of field drilling applications in different rock types. An alternative sample preparation technique – targeting microsectioning [8] – is used to expose the character of the surface layer of the CC drill buttons.

3. Observations 3.1. General observations Generally, the worn drill buttons were partly covered by strongly adhered rock material, see Fig. 1. This layer of rock material appears very firmly integrated into the surface of the button.

2. Experimental A set of drill buttons was selected from rotary/percussive drilling operations in a variety of rock types, see Table 1. All buttons (except no. 7) had peripheral positions on the drill bits that ranged in diameters from 50 to 120 mm. Peripheral buttons usually wear more than central ones. Button 7 is a centrally placed button used in rotary drilling. The buttons were removed from the drill bits and investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The specimens were prepared according to the targeting micro-section technique described in Ref. [8]. Using this technique, the drill button specimens are cross-sectioned by firstly making a deep cut from the bottom almost all the way through, leaving only a small bridge of the surface layer. A wedge (e.g. a screwdriver) is then forced into the cut to fracture this bridge. The brittle fracture will always follow the weakest path through the surface layer. Hence, weakened or pre-cracked parts of the structure will automatically be revealed in the cross-section. A few of these samples were further investigated using an instrument that combines focused ion beam milling with a conventional SEM (FIB-SEM).

Fig. 1. Typical examples of targeting micro-sections through used drill buttons. The selected views show the outer surface and the fracture cross-section. A large part of the outer surfaces is covered by strongly adhered rock material (dark). A dark roughly 10 ␮m thick intermixed layer is revealed in (a). In (b) deep rock channels are clearly visible as bright patches. The arrows indicate thickness of the intermixed layer and maximum rock channel depth, respectively.

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Table 2 Characteristics of the investigated rock drill buttons Button no.

Cemented carbide composition

WC [␮m]

Hardness [HV30]

Rock type

Mine locations

1 2a 3 4 5 6 7 8 9 10

94% WC, 6% Co 94% WC, 6% Co 94% WC, 6% Co 94% WC, 6% Co 94% WC, 6% Co 94% WC, 6% Co 90% WC, 10% Co 94% WC, 6% Co 94% WC, 6% Co 94% WC, 6% Co

3 3 2.5 2.5 2.5 5 7 2.5 2.5 2.5

1500/1050/1480 1500/1050/1480 1430 1430 1430 1230 1200 1430 1430 1430

Quartzite Quartzite Quartzitic granite Quartzitic granite Magnetite Magnetite Chromite Chromite Manganese Gypsum

S¨odra Sandby, Sweden S¨odra Sandby, Sweden Karlshamn, Sweden Karlshamn, Sweden Kiruna, Sweden Kiruna, Sweden South Africa South Africa Nchwang, South Africa Knauf Bergwerke, Germany

All values of composition, WC grain size and CC hardness are nominal. a These buttons are manufactured with an approximately 3 mm thick surface layer in which the Co content is reduced, followed by a Co-enriched layer and finally bulk composition. The hardness of each layer is given.

Fig. 2. The three basic types of integration of rock material into the WC/Co structure, found on all rock drill buttons investigated. The WC grain size is typically 2–5 ␮m in rock drill bits. (a) Partial rock cover, (b) intermixed layer, and (c) rock channels.

Fig. 3. Two buttons drilled in quartzite, almost completely covered by rock material and exhibiting distinct intermixed layers. (a) Button 1, revealing a dark intermixed layer. (b) Button 2 exhibiting a similar dark intermixed layer, but also traces of some deeper rock channels (indicated with arrows), down to approximately 180 ␮m.

Fig. 4. Button 3, exhibiting partial rock cover, intermixed layer and deeper penetration. (a) Overview of targeting micro-sectioned sample. (b) Close-up of one of the globules formed by the deeply penetrating granite.

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Fig. 5. Targeting micro-sections of Button 6, that has drilled in magnetite rock. (a) Typical area close to the drill button surface exhibiting large amounts of magnetite (dark) and a large crack originating from a reptile skin valley on the surface. The presence of magnetite was confirmed by EDS analysis. (b) Close-up of area indicated in (a) showing penetrated magnetite adhering to WC grains. (c) Close-up of (b). (d) A section of the button below the penetrated magnetite. Here, the original Co binder between the WC grains shows a ductile fracture behaviour.

Furthermore, rock material was often found to have penetrated into the cemented carbide structure. In this way, the Co binder was frequently replaced by – or intermixed with – rock material, forming an up to 40 ␮m thick layer with a radically modified binder, cp. Fig. 1a. It is denoted the intermixed layer. Even more strikingly, rock material locally penetrates the CC structure to extensive depths. Channels filled with rock material were found down to almost 1 mm below the surface. When these rock channels are revealed by targeting microsectioning they typically appear as white spots in the SEM, cp. Fig. 1b. These three types of rock material integration are schematically illustrated in Fig. 2, and estimated rock channel depths and intermixed layer thicknesses are presented in Table 2. Generally, the cobalt content was substantially reduced in the intermixed layers, compared with the original composition. Some intermixed layers showed no traces at all of Co, indicating a content below 1 at% (below for the sensitivity limit of the EDS technique). The thickness of the intermixed layers typically varies between 1 and 40 ␮m. A tendency towards increasing thickness with increasing drilling depth can be noted. However, the thickness appears surprisingly independent of the rock material drilled, at least after relatively short time use. All buttons used for drilling around 20 m exhibit around 10 ␮m thick layers, irrespective of rock hardness. For these drills (nos. 1, 3, 6 and 7) the minerals range from the very hard (quartzite and quartzitic

granite) to the soft (chromite and magnetite). On button 5, which was used only very shortly, the magnetite rock formed a very thin intermixed layer. On the magnetite button that has drilled 20 times deeper (no. 6), however, the intermixed layer is significantly thicker. The maximum rock channel depths fell in the interval 10–800 ␮m. Also, this depth was found to increase with increasing drilling depth, with the chromite buttons as the only exception. Buttons 7 and 8 were used in soft chromite ore, including some platinum. These ores are known to sometimes be very difficult to drill, with early onset of button damage. Here, they were found to form thick intermixed layers and deep penetration channels. 3.2. Influence of rock material The rock intermixing and channel penetration – as visualised in Fig. 2 – may exhibit different characteristics, as exemplified for a number of rock types below. Rock penetration through channels often reveals itself by isolated nodules of rock found at relatively large depths beneath the surface or as continuous rock filled channels extending to large depths. 3.2.1. Quartzite Drilling in quartzite, one of the hardest rock types, results in the formation of an intermixed layer, and after further drilling, also in the formation of rock channels, see Fig. 3.

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Fig. 6. Button 7 (chromite), examined in the SEM as prepared by targeting micro-sectioning. (a) At the drill button surface. (b) Close-up of a region showing fractured chromite in the intermixed layer. (c) Region at around 60 ␮m below the surface, which is about half of the maximum channel depth. (d) Close-up of (c) revealing penetrated chromite material (dark stains) smeared out on several WC grains. The rock channel transporting this chromite could be followed up to the outer surface.

3.2.2. Quartzitic granite Button 3, that drilled in quartzitic granite shows a complex surface zone, including both an intermixed layer and a special type of deep rock penetration revealed as globules, with a morphology resembling solidified glass, see Fig. 4. 3.2.3. Magnetite Button 6 shown in Fig. 5 was used to drill magnetite, and, as characteristic for magnetite, a “reptile skin” pattern was formed on the surface. Substantial magnetite intermixing and deep cracks were found, as shown in Fig. 5a. The magnetite is also revealed as globules on many WC grain edges, see Fig. 5b and c. No, or very small, traces of Co were found in the fracture surface through the intermixed layer. Further down into the button, the original Co binder dominates, see Fig. 5d. In this fracture surface the Co binder has become plastically deformed, and there is no evidence of trans-granular fracture through the WC grains. 3.2.4. Chromite In chromite drilling, the rather soft chromite penetrates into the structure, following the binder phase layers and adhering to WC grains or forming large flakes, see Figs. 6 and 7. Fig. 6c and d shows a section where a rock channel could be followed from the surface to a depth of 60 ␮m. At the end of the channel, the rock is smeared out on several WC grain surfaces. Chromite

was not found to form globules on WC grain edges, such as observed in the magnetite case. Just as magnetite, chromite is known to often generate reptile skin patterns in the drill button surface. 3.2.5. Manganese Button 9, used for drilling manganese rock, exhibited a very different, shiny appearance. It also showed a shallow “reptile skin”. Targeting micro-sectioning revealed deep and extensive rock penetration, forming both intermixed layers and deep channels, see Fig. 8. Rock material in the globular shape found in the magnetite case (Fig. 5) was also observed. Further, peculiar very thin and long “rock filaments” could be followed in the bulk CC structure. The rock nodules were further investigated using the FIBSEM technique, see Fig. 9. One rock nodule was intersected by the ion beam and then imaged, also using the ion beam. The rock material forming the nodule proved to be highly porous and very fine grained or amorphous, see Fig. 9c and d. 3.2.6. Gypsum Finally, the button used to drill in gypsum, which is a very soft rock, showed “reptile skin” and extensive plastic deformation in a form not observed on any other button, see Fig. 10. The combination of very soft rock, no resharpening, and very large

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Fig. 7. Button 8 (chromite), examined after targeting micro-sectioning (SEM in BSE mode). This button also displayed a pronounced reptile skin. (a) The characteristic distribution of chromite (dark areas). (b) Close-up showing two large nodules of chromite inside the CC material.

drilling depth obviously resulted in very deep and widespread rock penetration in the shape of flakes. 4. Discussion The most significant result in this investigation is that integration and penetration of rock material into the cemented carbide was found in each drill button investigated, as easily detected in the SEM after targeting micro-sectioning. These phenomena takes on many forms. Most striking is perhaps the penetration to depths far below the drill button surface, sometimes down to around 1 mm. The ever-present integration of rock material into the CC structure surprised us completely. No references to these phenomena could be found in the drill wear literature. Why has it not been observed before? Several explanations are plausible. The intermixed layers and the rock channels are difficult to observe in cross-sections prepared according to normal metallographic cutting and polishing techniques. Such samples are dominated by cross-sectioned WC grains. In contrast, the targeting micro-sections primarily reveal the binder phase, and the outer surface of WC grains. Further, the intruded material is very brittle, and likely to fragment and fall out during metallographic polishing. Lost rock material

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Fig. 8. Typical appearance of Button 9 (manganese), when examined by targeting micro-sectioning. (a) Extensively intermixed layer (manganese in darker areas) and a very narrow, thin rock thread (indicated by arrow). (b) The thin rock thread in (a) followed to its end.

results in small regions out of focus in the optical microscope, which results in a porous “bad preparation” appearance. Further, Si peaks in the EDS spectrum are overlapped by W peaks, which make it hard to reveal most minerals in the SEM. The following suggested mechanisms for rock intermixing and penetration are supported by observations on the penetration depth, morphology and structure of intermixed and penetrated rock material. 4.1. Rock intermixing The first stages of button wear in rock drilling are associated with an extensive intermixing of rock material into the surface layer of the button. This is facilitated by the extreme local conditions of temperature and pressure associated to the intense intermittent contact between drill button and rock material. Despite the average temperature of the buttons being kept low by the extensive cooling from water or high-pressurized air, the local intermittent temperatures are certainly high enough to at least cause substantial softening of the rock material. Thus, rock material will fill any crack or cavity that results from wear and fatigue of the button surface. Notably, the Co binder content was observed to be very low in the superficial CC layer of used buttons. In fact, Co has been partly replaced by rock material to form a WC/rock material composite. The thickness of the intermixed layer corresponds

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Fig. 9. A targeting micro-section of Button 9 (manganese), investigated by FIB-SEM. (a) Typical manganese nodules found very deep below the surface (SEM). (b) Close-up of some of these rock nodules as imaged by the ion beam to enhance the contrast between rock material and CC. (c) Cross-section through one of the rock nodules, made by the ion beam and imaged by SEM. (d) The structure of the manganese nodule at higher magnification. Note the porous internal morphology of the nodule and the incomplete adherence to the CC (SEM).

to the depth to which rock material can be directly “hammered” into the CC material by the external forces. Naturally, this thickness increases with the gradual disintegration of the CC material that follows with increased drilling depth. Note that the thickness of the intermixed layer is of the order of 1–15 WC grain diameters, for all rock types, see Table 2. This indicates that it is the character of the cemented carbide rather than that of the rock material that is decisive for the thickness of the intermixed layer. Note also that the layer has grown relatively thick already after a short drilling depth, indicating a decelerating growth rate. Based on these observations, it should be expected that the wear rate of a drill button would primarily be controlled by the properties of the intermixed layer, rather than those of the original CC. A transition should follow after a short incubation corresponding to a few meters of drilling. This is probably the mechanism behind the skin effect reported by Larsen-Basse [5], i.e. an initial low wear rate while “wearing through the skin”. It also implies that the original CC material is more wear resistant than the intermixed layer.

4.2. Rock penetration and Co intrusion The deep rock penetration is the most striking, and also the most unexpected phenomenon. What could be the mechanism for deep penetration into such a solid structure? At some weak local regions, rock material starts to penetrate from the intermixed layer further into the interior of the drill button. The weak regions are probably cracks and regions of Co depletion where the rock material can find a path, thus forming and filling long, narrow channels. Initially, the driving force may be a direct hammering from the impact between rock and drill button. However, the further penetration has to occur by some other mechanism. It is not possible to force long, narrow “spaghetti” of rock material, winding deep into the solid cemented carbide, by pushing or hammering on its end. Instead, a mechanism based on travelling shock waves and thermal softening of the rock is proposed. Each local rock impact results in a compressive shock wave and an associated high-temperature wave, induced by intense friction and plastic deformation. The shock wave travels through the button with the speed of sound (about 5800 m/s for a typical

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4.3. Rock penetration—influence of rock material The resulting rock penetration depth depends on the difference between the penetration rate and the wear rate of the intermixed layer. Naturally, both these factors depend on the rock material. The penetration rate should also slow down with increasing penetration depth. Clearly, the buttons used to drill rock types causing rapid wear (quartzite and granite) show the lowest penetration depths, and drill buttons from the least aggressive rock types (chromite, manganese and gypsum) show the most extreme penetration depths, especially after extensive depths, see Table 2. A further external factor to consider here is depth drilled after the last reshaping operation, which naturally also influences the observed penetration. 4.4. Morphology of penetrated rock material The penetrated rock material was found to form very different morphologies. These included flakes (Figs. 8 and 11), nodules (Fig. 10), threads (cp. rock wool) (Fig. 9) and filled channels (Figs. 1 and 4). The globular and thread-like morphologies may be artefacts, indicating that penetrated rock material undergoes melting during the targeting micro-section fracturing. The energy release associated to the deformation during crack propagation and opening is partly transformed into heat, which possibly could be sufficient to melt some rock material in the crack vicinity. The thread-like morphology may result from straining of soft globules during the cleavage and subsequent adherence to the fracture surface. This clearly needs further investigation to be adequately clarified. The internal porous structure of the nodule shown in Fig. 9c and d may be a more representative picture of the morphology of penetrated rock, offering further clues to understanding the mechanisms of rock penetration. Fig. 10. General appearance of targeting micro-section through Button 10 (gypsum). (a) A large number of rock channels (dark) found deep below the surface (BSE mode). (b) The same types of channels seen in a lateral targeting microsection (i.e. a cross-section perpendicular to the drilling direction). Note also the reptile skin on the external surface. (c) Close-up of (b) revealing a typical rock flake.

rock drill grade). As it passes through the CC structure, any open channel will momentarily be constricted by the state of hydrostatic stresses within the wave, and subsequently relaxed when the wave has passed, see Fig. 11. This causes a pumping effect, squeezing any material inside the channel a step in the direction of wave propagation. (Compare the action of a rolling-pin when rolling out a piece of dough.) This pumping effect would contribute both to intrusion of Co and inwards penetration of rock material. The steady state mechanism of Co-depletion in the superficial CC layer may also be due to this “squeeze and pump” mechanism rather than the commonly proposed outwards extrusion [3–6,9–11].

4.5. Influence on the cemented carbide properties Since cemented carbides are sensitive to tensile stresses, a surface crack filled with rock material may cause severe damage. The rock material will act like a wedge, which can accelerate the crack growth when the button is exposed to impact and varying temperature. A corresponding behaviour is well known in geological sciences, as ice-wedges or sand-wedges act to accelerate the growth of large-scale crack patterns on the ground [12]. Larsen-Basse reported a lower initial wear rate and a higher steady state wear rate after some length of drilling [5]. Based on the findings presented here, it can be proposed that this skin effect is the result of the formation of the intermixed rock layer. The initial surface of unaffected CC is more wear resistant, and after some drilling time, an intermixed layer with a lower wear resistance has developed. Intermixing of rock material is also suggested to explain the higher hardness measured in the surface zone, as reported by Jonsson [6].

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Fig. 11. A sequence of sketches illustrating the proposed mechanism of deep rock penetration. A shock wave is generated by the rock impacting on the drill button. The wave (involving high pressure and high temperature) passes through the structure, squeezing a small amount of material downward the channel. When the wave has passed, the channel is slightly extended. Channel lengths varying from 10 to 800 ␮m have been noted in this work. The porosity of the resulting, uncompressed and cooled rock material reflects the amount of compression of the CC structure and the subsequent thermal expansion of the rock during the pressure wave passage.

Significant integration of rock material in the cemented carbide structure of rock drilling buttons has been revealed. It was observed in drill buttons selected from rotary/percussive drilling of a broad variety of rock types. Rock material integration essentially takes on three forms; partial rock covers, intermixed layers, where the binder is partly replaced by or intermixed with rock material, and deep rock channels, which are the result of local penetration of rock material deep into the cemented carbide structure. The present findings bring about a significantly new view on the action of cemented carbide material during rock drilling. The surface layer transforms from the original WC/Co structure into a new composite. The intermixed layer is probably the modification being most significant for the performance of the drill button. Here, the carbide phase is relatively intact while the binder consists of a mixture of cobalt and rock material. It should be expected that this composite exhibits properties significantly different from the original CC, due to the following alterations:

Also the partial rock covers may have a significant influence on the button wear by frequently offering some protection. The thickness of the intermixed layer is of the order of a few WC grain diameters, and hence the steady state wear is controlled by removal of material from this layer. The intermixed layers and channels grow with increasing drill depth and the resulting growth rate is determined by the competition between penetration rate and wear of the surface layer. Low wear rates result in deeper penetration. Deep rock penetration (in some cases found to extend down to almost 1 mm) is proposed to be a gradual process driven by the combined action of direct hammering on rock material in the surface layer and compressive shock waves squeezing the rock further into the cemented carbide structure. The Co depletion from the surface layer of buttons is proposed to be driven by the same mechanism as rock penetration, rather than Co being extruded out of the surface, as earlier reported in the literature. The rock penetration phenomena discovered in this investigation motivate special considerations in the development of new cemented carbide grades for rock drilling.

• a less ductile binder; • possibly a harder binder; • probably an increased level of compressive stresses in the surface layer; • probably wedge effects operating to widen cracks.

• Wear resistant CC materials should be designed to either obstruct intermixed layer formation or to generate such layers with high wear resistance. • CC materials with improved fracture resistance should be developed to avoid rock material penetration.

5. Conclusive summary

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Acknowledgements Torbj¨orn Hartzell and Stefan Ederyd at Sandvik Hard Materials are gratefully acknowledged for invaluable information and discussions. Fredrik Bj¨orklund is gratefully acknowledged for his systematic work with analysing the rock penetration in a large number of drill buttons and hot rolls. Roger M˚ansson and Lasse Larsson are acknowledged for their practical assistance in rock drilling and open-minded discussions. Finally, the financial support from Sandvik AB is gratefully acknowledged. References [1] H.E. Exner, Physical and chemical nature of cemented carbides, Int. Met. Rev. 4 (1979) 149–173. [2] U. Beste, T. Hartzell, H. Engqvist, N. Ax´en, Surface damage on cemented carbide rock drill buttons, Wear 249 (3–4) (2001) 324–329. [3] J. Larsen-Basse, Binder extrusion in sliding wear of WC–Co alloys, Wear 105 (3) (1985) 247–256.

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[4] K.G. Stjernberg, U. Fisher, N.I. Hugoson, Wear mechanisms due to different rock drilling conditions, Powder Metall. 18 (35) (1975) 89–106. [5] J. Larsen-Basse, Wear of hard-metals in rock drilling: a survey of the literature, Powder Metall. 16 (13) (1973) 1–32. [6] H. Jonsson, Wear of cemented carbide bits during percussive drilling in magnetite-rich ore, Planseeberichte f¨ur Pulvermetallurgie Bd. 24 (1976) 108–134. [7] U. Beste, A. Lundvall, S. Jacobson, Micro-scratch evaluation of rock types—a means to comprehend rock drill wear, Tribol. Int. 37 (2) (2004) 203–210. [8] U. Beste, S. Jacobson, Targeting micro sectioning—a technique to reveal weak zones in worn surfaces, Wear 264 (2008) 1152–1156. [9] J. Larsen-Basse, Effect of composition, microstructure, and service conditions on the wear of cemented carbides, J. Met. 35 (11) (1983) 35– 42. [10] S.G. Bailey, C.M. Perrot, Wear processes exhibited by WC–Co rotary cutters in mining, Wear 29 (1974) 117–128. [11] C.M. Perrot, Tool materials for drilling and mining, Annu. Rev. Mater. 9 (1979) 23–50. [12] P.J. Williams, M.W. Smith, The Frozen Earth, Press Syndicate of the University of Cambridge, New York, 1989, p. 306.