Initial surface failure and wear of cemented carbides in sliding contact with different rock types

Initial surface failure and wear of cemented carbides in sliding contact with different rock types

Wear 408–409 (2018) 43–55 Contents lists available at ScienceDirect Wear journal homepage: Initial surface failure and...

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Wear 408–409 (2018) 43–55

Contents lists available at ScienceDirect

Wear journal homepage:

Initial surface failure and wear of cemented carbides in sliding contact with different rock types


Jannica Heinrichsa, , Mikael Olssona,b, Bjarne Almqvistc, Staffan Jacobsona a

Applied Materials Science, Uppsala University, Sweden Materials Science, Dalarna University, Sweden c Earth Sciences, Uppsala University, Sweden b



Keywords: Cemented carbides Sliding Wear Deformation Rock drilling

The initial wear, deformation and degradation of cemented carbide in contact with different rock types are studied using a crossed cylinder sliding test. The sliding distance is limited to centimetres at a time, interrupted by successive SEM analysis. This allows for careful studies of the gradually changing microstructure of the cemented carbide during the test. Five different rock types are included; granite, metal sulphide ore, mica schist, quartzite and marble. All rock types are very different in microstructure, composition and properties. The cemented carbide grade used for the evaluation contains 6 wt% Co and fine (~ 1 µm) WC grains, a grade commonly used in rock drilling. The results show that the cemented carbide microstructure becomes altered already during the very first contact with rock. The initial wear rate and wear character is highly influenced by the rock type. The initial wear of the cemented carbide is highest against quartzite and lowest against marble.

1. Introduction Cemented carbides are composite materials consisting of hard carbide particles in a more ductile binder phase. The most common combination is tungsten carbide (WC) particles in a cobalt (Co) binder. When used as drill bit buttons in rock drilling these composite materials generally show low wear. The combination of high hardness and toughness minimizes both plastic deformation and brittle fracture in contact with the rock. The cemented carbide microstructure is usually well defined and manufactured following a powder metallurgy process route optimized to achieve the composition and properties sought for. On the other hand, the counter material in rock drilling, the rock, is a natural material generally showing large variations in both microscale, e.g. multimineral rocks, and macroscale, e.g. variation of rock type in a borehole, which will affect the drillability and penetration rate [1–3]. This wide group of rock types also differs quite a lot in both chemical, physical and mechanical properties. Just a few papers have looked into the properties of rock from the fundamental tribological perspective [4–6]. In [4] the friction between cemented carbide and a variety of rock types was measured in a pin-on-disc set up and the authors concluded that the differences in friction did not correlate to the commonly observed large differences in wear of the cemented carbide. In [5] the hardness distribution of different rocks was measured both in small scale

Corresponding author. E-mail address: [email protected] (J. Heinrichs). Received 19 March 2018; Received in revised form 26 April 2018; Accepted 26 April 2018 Available online 30 April 2018 0043-1648/ © 2018 Elsevier B.V. All rights reserved.

(indentation depth 1 µm) and commonly used Vickers measurements with 500 g load. They concluded that hardness needs to be measured in small scale to be representative for the scale in which wear of cemented carbides occur in rock drilling, and to be able to identify hard inclusions/phases that are present also in minerals with relatively low hardness. The corresponding wear of a cemented carbide pin was evaluated in a scratch test set up in [6] and was found to correlate quite well to the measured small scale hardness in [5], but was also influenced by e.g. grain size of the rock, where smaller grains (i.e. cemented carbide pin passing more grain boundaries) were correlated to more wear. A common strategy is to focus on the cemented carbide, which can be designed to achieve certain properties, and use a counter rock material to compare these against each other [7–9]. This approach could be used to optimize the cemented carbide grade used in a certain application. A recent study combines actual drilling and three different lab-scale tests of a set of cemented carbides. It includes both comparisons of wear ranking between the actual drilling and the lab tests and micro graphical studies of the worn drill buttons from the different tests [10]. Another strategy is to study the wear mechanisms of the cemented carbide against a certain rock material, to increase the knowledge about the wear in that application [8–17]. In [8,9,13–15] the focus is on detailed wear mechanisms against granite, references [11,12] deal with the occurrence of reptile skin in drilling of magnetite and also wear

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Fig. 1. The appearance of the included rock types. Width of photograph (top row) 14 mm and light optical micrograph (second row, crossed-polarized light) 5 mm. Colour refers to the photographs and the birefringence of minerals in the micrographs are used to quantify the main minerals (> 1%) in each rock type.

Table 1 Quantity, grain size and nanohardness of the respective minerals (> 1%) in the included rock types. Rock type


Chemical formula

Quantity [%]

Grain sizea

Hardness [GPa]


Quartz Feldspar (alkali) Mica (Biotite) Pyrite Amphibole (Cummingtonite) Quartz Sphalerite Othersc Quartz Mica (Muscovite) Quartz Mica (Muscovite) Quartz Calcite

SiO2 KAlSi3O8 K(Mg,Fe)3AlSi3O10(F,OH)2 FeS2 Fe2Mg5Si8O22(OH)2 SiO2 (Zn,Fe)S – SiO2 KAl2AlSi3O10(F,OH)2 SiO2 KAl2AlSi3O10(F,OH)2 SiO2 CaCO3

25–40 40–50 15–20 35–45 5–10 10–20 15–25 10–15 50 50 95–97 3–5 1–2 > 97

Medium Medium Medium Medium Medium Medium Medium – Small Small Small Small Small Large

12.8 ± 1.0 10.5 ± 1.3 3.4 ± 1.3 20.7 ± 1.1 12.2 ± 0.8 –b 3.2 ± 0.3 – 11.5 ± 0.7 2.8 ± 0.5 12.1 ± 0.8 4.4 ± 0.3 10.5 ± 0.9 2.2 ± 0.2

Metal sulphide ore

Mica schist Quartzite Marble

a b c

Small refers to a mean grain size < 200 µm, medium 500–800 µm and large > 1.4 mm. No quartz was identified in the nanoindentation sample although significant amounts were found in the thin section. Other mafic minerals (e.g. pyroxenes, biotite mica) and feldspar. Less than 1% of each mineral.

the rocks, and to which depth the rock infiltrated the microstructure, were pointed out [23]. However, although the observations from real drilling are very valuable, the cemented carbide grade, drilling depth and drilling parameters are not the same for all materials, since it has been optimized already for the application, i.e. the rock type, based on experience. Hence, the influence on wear from rock type has to our knowledge not yet been systematically investigated. Most of the field tests also include drilling for a long time, meaning that although several similarities are observed in steady state wear [23], differences in wear initiation might be hard to separate. In the present work, the very initial deformation, degradation and wear of cemented carbide in contact with different rock types are studied using a crossed cylinder sliding test, previously shown to result in

against limestone [16] and sandstone [17] has been investigated. In recent years some papers summarizing the wear mechanisms of cemented carbides in contact with different rocks has been published [11,18–21]. After studying the literature both [11,18] give suggestions for choosing the cemented carbide grade based on rock type and which problems that arise, respectively. In [22,23] buttons that have been drilling in quartzite, quartzitic granite, magnetite, chromite, manganese, coal/sandstone and gypsum are compared. Several similarities were observed; partial rock cover on the surface, formation of an intermixed layer with rock/Co/WC and penetration of rock deep into the cemented carbide button. Mechanisms of deterioration and removal of materials was suggested based on the different observations from the buttons [22]. Also differences, like formation of reptile skin for some of


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Fig. 2. All nanoindentation hardness measurements of the respective constituents in the rock types and cemented carbide. Hardness of cemented carbide constituents are exemplified by measurements from [15]. The hardness of the cemented carbide composite (converted from HV1 in [14]) is indicated by a dotted line.

microstructure, composition and properties. The cemented carbide grade used for the evaluation is a commonly used grade in rock drilling including 6 wt% Co and small (1 µm) WC grains. The results are discussed in the light of the rock drilling application. 2. Experimental 2.1. Materials

Fig. 3. a) Sketch showing the lathe test set-up, where the large rotating cylinder represents the rock material and the small cylinder fed in the lateral direction represents the cemented carbide drill bit button. b) Typical appearance of resulting wear scar on the small cylinder. (SEM, 3 kV).

The rock types included in the study were granite, metal sulphide ore, mica schist, quartzite and marble, with the appearance and main mineral composition according to Fig. 1. The samples were drill cores (ø 37–52 mm) collected from different sites in the Boliden Garpenberg mine (Sweden), chosen to represent a wide range of rock types, with different microstructure, mineral composition, chemical and mechanical properties. Thin sections, about 2 cm × 3 cm, were prepared from each rock type and petrographic microscopy was used to identify the minerals. The quantity and grain size of the identified minerals were estimated from the microscope images. The nanohardness of each individual mineral was measured on polished cross sections by

similar wear surfaces as often observed in rock drilling [9]. The sliding distance is limited to centimetres at a time, interrupted by successive studies of a pre-defined area in the wear mark, using high resolution scanning electron microscopy (SEM). This allows for careful studies of the gradually changing microstructure of the cemented carbide during the test. Five different rock types are included; granite, metal sulphide ore, mica schist, quartzite and marble, all with very different

Fig. 4. Average friction coefficient for each run with connecting lines to guide the eye. Each run corresponds to about 0.1 m sliding against the rock cylinder.


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Fig. 5. Each cemented carbide sample imaged before test against respective rock material, indicated in the image. Tilt 41°. (SEM, 3 kV).

compare different cemented carbide grades in contact with rock [9]. The cemented carbide cylinder was fed in the lateral direction, generating a helical contact track on the rock cylinder, to always be in contact with new rock, Fig. 3. The normal force was applied by contraction of a spring and set to 57–70 N, compensating for the differences in rock cylinder diameter (similar hertz contact pressure). The resulting pressure is high enough to cause fracture and wear of the rock cylinders in the sliding contact. The sliding speed was set to 0.1 m/s. The testing was interrupted after every 0.1 m sliding for SEM (Zeiss Merlin) investigations, to be able to study the gradual deformation, wear and degradation. Before imaging, loose wear fragments were carefully removed using a stream of pressurized air. After imaging, the test was continued at the exact same position, adding up to a total sliding distance of 1 m when the test was terminated. To allow precise repositioning, the cemented carbide cylinder is fixed in its specimen holder when taken out of the test rig for SEM investigation. The specimen holder makes sure that the cylinder cannot rotate and that it always returns to the same vertical position when repositioned. No specific effects of the interruptions were noted, neither regarding friction nor degradation mechanisms. It can be noted that the contact in real drilling is very far from continuous, and involves much wider

nanoindentation using a diamond Berkovich indenter and an indentation depth of 100 nm. The mineral characteristics are given in Table 1. The quantity of each mineral is given as a range, showing the substantial variations in mineral composition over the thin section. Some of the minerals display a relatively large standard deviation in nanohardness due to their anisotropic character and thus each individual measurement is reported in Fig. 2. Before testing all cylinders were ground using a 120 grit SiC grinding paper, to achieve a rock surface characterized by fractured mineral grains, similar to the rock in drilling. Cemented carbide cylinders (ø 12.8 mm, L 20 mm) of a commercial grade, containing 6 wt% Co and fine (~ 1 µm) WC grains, were used as model material. All samples were polished to Ra < 40 nm, using 1 µm diamond spray in the last preparation step, to enable careful studies of the microstructure. The composite micro hardness HV1 was 1620 ± 35 kg/mm2 [14] and the nanohardness of the individual phases was 28 ± 3.7 (WC) and 6.0 ± 1.1 (Co) [15]. 2.2. Tribological testing The sliding tests were performed in a lathe, where a cemented carbide cylinder was pressed against a larger rotating rock cylinder in crossed cylinder geometry. This test set-up has been used previously to


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Fig. 6. The exact same areas of the polished cemented carbide as in Fig. 5 after sliding 0.1 m against the different rock types. Sliding direction of the rock is from bottom to top. Tilt 41°. (SEM, 3 kV).

in relatively low friction, between 0.2 and 0.3, while mica schist, quartzite and marble result in significantly higher values, 0.5–0.6.

temperature variation than do this test. 2.3. Post test surface characterisation

3.2. Surface appearance High resolution SEM (Zeiss Merlin) and Energy Dispersive X-ray Spectroscopy (EDS; Oxford Aztec X-max) was used to analyse the worn cemented carbide surfaces after testing. Selected samples were analysed in cross-section prepared by Focused Ion Beam (FIB; FEI Strata DB235) and imaged by SEM (Zeiss Merlin). To protect the very surface of the cross-section samples from the ion beam during sample preparation, platinum was deposited locally on the surface in situ in the vacuum system.

A pre-defined area of all polished cemented carbide samples were imaged before testing, see Fig. 5. During testing, the exact same area of the samples was again imaged, after every 0.1 m sliding against the respective rock material. In Figs. 6–10 selected images of the successive sliding can be observed. The cemented carbide samples are named CC followed by their respective rock counter surface. During the first 0.1 m sliding against the rock cylinder, several changes to the microstructures have already occurred, Figs. 6 and 7. The CC-Granite sample shows several traces from the contact with granite, which has transferred to the surface, visible as dark areas in between WC grains and as thin layers on top of WC grains. Further, the grain boundaries between neighboring WC grains have been enhanced, due to small movements of the individual grains with respect to their neighbours. Some individual WC grains are also plastically deformed, as indicated by visible slip steps in the previously polished WC grain surface, or even cracked. The CC-Metal Sulphide Ore sample is similar

3. Results 3.1. Friction measurements The friction coefficient is measured during the test and the average friction coefficient during each run (engagement and disengagement excluded) is shown in Fig. 4. The rock type clearly influences the friction coefficient. Tests against granite and metal sulphide ore both result


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Fig. 7. Detail images of the polished cemented carbide after sliding 0.1 m against the different rock types. Sliding direction of the rock is from bottom to top. Examples of cracked WC grains are indicated with an arrow. Tilt 41°. (SEM, 3 kV).

geometry. The CC-Quartzite shows more wear than after 0.1 m, and the grains visible in between transferred rock are cracked and fragmented. The CC-Marble is again completely covered by rock. During the following 0.4 m, the wear process continues on all cemented carbide samples, Fig. 9. More rock is gradually transferred to the CC-Granite, while small or previously cracked WC grains are removed as fragments. In the CC-Metal Sulphide Ore some small WC fragments have been replaced by rock, however mainly polishing of the WC grains has continued. On the CC-Mica Schist transfer of rock has become even more extensive, now covering large parts of the surface. Some of the transferred rock is however occasionally removed (cf. left side of Figs. 9 and 8). The more severe wear of CC-Quartzite has continued and the cemented carbide surface has become rough. The rock coverage on CC-Marble has partly flaked off, showing the surface underneath, where a thinner and partly covering rock transfer layer is visible. It may also be noted that the cracked marble transfer layer is several micrometers thick. After completing the test, Fig. 10, the CC-Granite has become quite rough, now consisting of cracked and fragmented WC grains with transferred granite acting as a binder. However, several grains from the

to CC-Granite, but the number of plastically deformed and cracked WC grains is significantly lower. The CC-Mica Schist sample is again similar to CC-Granite, with cracked and deformed WC grains, however the mica schist is rather transferred as particles positioned on top of the cemented carbide, than filling the gaps in between the WC grains. The largest changes to the cemented carbide microstructure are however taking place against quartzite. Already after this short sliding length, the CC-Quartzite shows cracking and fragmentation of WC, wear of the composite and lots of transfer, where the quartzite is present both between WC grains and on top of the composite. The CC-Marble on the other hand is completely covered by transferred marble, concealing the cemented carbide. Continued sliding (Fig. 8) causes removal of small WC fragments, more cracking and fragmentation of WC grains and more transfer of rock to the CC-Granite. The same is valid for CC-Mica Schist, however the transfer of rock is more extensive. The CC-Metal Sulphide Ore shows some wear and removal of WC fragments, but more importantly the surface is being polished. The slip lines in the previously deformed WC grains, that were visible after the first contact (Figs. 6 and 7), are disappearing and the WC grains are starting to get a more rounded


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Fig. 8. The exact same areas of the polished cemented carbide as in Fig. 5 after sliding 0.2 m against the different rock types. Sliding direction of the rock is from bottom to top. Tilt 41°. (SEM, 3 kV).

surface, indicating that no (or minor) wear has occurred underneath the transfer film. Elemental analysis by EDS was performed on the final surfaces, see Fig. 12. It is observed that the rock coverage, represented by O (one of the main elements in quartz, feldspar, mica, amphibole and calcite) in the elemental maps, is substantial on all surfaces, and especially so on CC-Mica Schist, CC-Quartzite and CC-Marble. In the latter two analyses, the transferred rock is also represented by C. Marble includes the carbon rich calcite, but no such phases were identified in the natural quartzite thin section. The occasional small amount of calcite, or other carbon rich minor phases that might be present in the natural quartzite, seems to transfer preferentially. The uncovered cemented carbide is dominated by W and C, from the WC grains. However, also Co is showing in some of the corresponding areas in almost all maps. It is only in the analysis of CC-Metal Sulphide Ore that Co is barely seen. FIB cross-sections were prepared from the final surfaces imaged in Fig. 10 (CC-Metal Sulphide Ore, CC-Mica Schist and CC-Quartzite) and Fig. 11 (CC-Marble). For CC-Granite an area similar to the one in Fig. 10 has been studied previously [9] and is here reproduced. All cross-sections show that although the surface is changed, in some cases radically,

original microstructure are still present in the surface (cf. Fig. 5). The CC-Metal Sulphide Ore experienced more extensive wear in the end, visible as exchange of WC/Co for large patches of transferred rock in Fig. 10. However, studying the whole wear mark this was very rare, and the polished appearance and lack of Co separating the WC grains observed to the left in Fig. 10 was more typical. The CC-Mica Schist surface was typically showing minor wear, with areas covered by transferred rock and, where the cemented carbide was showing, deformed and cracked WC grains together with pits, which were not filled by rock. The CC-Quartzite continued to show wear and is characterized by cracked and deformed WC grains in between transferred quartzite. There are no similarities to the original microstructure imaged in Fig. 5. The CC-Marble again shows complete rock coverage, concealing the cemented carbide surface. However, as previously observed the rock coverage of CC-Marble is occasionally cracked and removed from the surface, and although the area successively studied in SEM is covered, some other areas are exposed. Such an area, previously covered by rock, is imaged in Fig. 11. The previously observed thin and partly covering transfer layer is visible, however at higher magnification, Fig. 11b, traces from the polishing can still be observed on the cemented carbide


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Fig. 9. The exact same areas of the polished cemented carbide as in Fig. 5 after sliding 0.6 m against the different rock types. Sliding direction of the rock is from bottom to top. Tilt 41°. (SEM, 3 kV).

from its original appearance, it is only the top surface layer that is affected, Fig. 13. Below the top two microns, the microstructure appears almost unaffected in all samples. Besides that, CC-Granite shows granite filling the gaps between WC grains, and cracking of some WC grains. CC-Metal Sulphide Ore shows smooth and polished grains at the surface, and no separation of the grains, neither by Co nor by rock. Where wear of the cemented carbide has occurred, rock has been transferred and the surface is thus relatively smooth. CC-Mica Schist shows very shallow wear, although pits were observed in top view. The transferred rock does not fill these pits in the microstructure, but is rather located on top of the WC grains. The CC-Quartzite surface is the roughest, however the wear is still shallow and visually there are no traces from the sliding contact below the upper microns. No wear or roughening of the surface can be observed in CC-Marble. The wear characteristics are summarized in Table 2.

ranges from 0.2 to 0.6. The rock types showing the lowest friction, granite and metal sulphide ore, cause moderate and low wear of the cemented carbide, respectively. Both also show a relatively small tendency to transfer of rock to the cemented carbide surface. The rock types resulting in high friction, mica schist, quartzite and marble, cause quite different tendencies to initial wear of the cemented carbide. Mica schist causes low wear, quartzite causes extensive wear and marble does not cause any observable wear at all. What these three have in common though, is that they all result in extensive transfer of rock to the cemented carbide surface. This implies that the high friction coefficient does not indicate severe wear of the cemented carbide, but rather indicates sliding contact between rock and rock material transferred to the cemented carbide. The relatively large fluctuations in average friction coefficient would then be explained by the occasional partial removal of transferred rock from the wear mark.

4. Discussion

4.2. Initial wear characteristics

4.1. Friction coefficient

Irrespective of rock type immediate changes of the cemented carbide occur, exemplified by tilting, plastic deformation and cracking of WC, but also extensive transfer of rock material or wear of Co binder

The friction coefficient is highly influenced by the rock material and


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Fig. 10. The exact same areas of the polished cemented carbide as in Fig. 5 after sliding 1 m against the different rock types. Sliding direction of the rock is from bottom to top. Tilt 41°. (SEM, 3 kV).

still only affecting the upper WC grain layer, and transferred rock fills the gaps between WC grains, smoothening the top surface. Where no wear, but only transfer, has occurred the transfer film is of even thickness and smooth, as long as no flaking occurs. The grain size of the minerals in all rock types is several orders of magnitude larger than the WC grains in the cemented carbide, even for the fine-grained rocks. The transferred particles found on the cemented carbide are on the other hand in the micrometer range, meaning that the wear and transfer of rock is on a scale much smaller than the mineral grain size. Thus, the mineral grains are efficiently crushed in the contact, either directly in the crossed cylinder contact or as wear particles trapped between the two cylinders.

and/or WC fragments and grains. The phenomenon that rock, containing no mineral harder than the WC in the cemented carbide, deforms cemented carbide to such a degree is interesting and has previously been shown in several papers [7–11,13–15,24]. The more ductile Co allows for individual WC grains to move a bit when exerted to high pressures from the rock asperities or neighboring WC grains, which causes the tilt and small relocations of grains in the Co matrix. When such movement is restricted by neighboring grains, plastic deformation, cracking and fragmentation of WC grains occur [24]. The Co binder is softer than at least the hardest mineral in all tested rock types and can therefore be abrasively worn, or squeezed out from in between WC grains when these move and subsequently become removed from the surface. To what extent these different processes occur and which characteristics that are most typical vary between the tests and are strongly dependent on the type of rock counter material. The observed differences in initial wear of the cemented carbide depend only on rock type, since everything else is kept constant. Although all cemented carbide surfaces show lots of traces from the contact with rock making them rough in microscale, the worn surfaces are actually quite smooth. Where significant wear has occurred, it is

4.3. Influence from rock type on cemented carbide wear Quartzite resulted in the highest initial wear of the cemented carbide. The high wear rate can partly be explained by the small mineral grain size, thus more grain boundaries are passed when sliding a certain distance compared to a coarser grained rock, which is known to result in high wear [6]. However, mica schist is also characterized by minerals


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Fig. 11. The polished cemented carbide after sliding 1 m against marble. In an area previously covered by rock, the cemented carbide has become exposed. b) The area indicated in a) in higher magnification. Bottom row; selected elemental analysis of the area in a). The area imaged in b) is indicated. Sliding direction of the rock is from bottom to top. Tilt 41°. (SEM/EDS, 3 kV).

The mica schist transfer film is thinner than the marble transfer film and not fully covering the cemented carbide surface. However, it seems like this thin and patchy film is capable of protecting the cemented carbide surface quite well, after the initial wear of mainly Co. The CCGranite and CC-Quartzite also have patchy transfer films that potentially could protect the cemented carbide, however the transferred rock is incorporated in the cemented carbide surface and when rock particles are removed parts of the cemented carbide could follow, contributing to the larger wear of these samples. The mica schist transfer particles are also occasionally removed, however since it is not incorporated in the cemented carbide microstructure the resulting wear is still low. The contact against metal sulphide ore shows a very different effect not observed for any of the other rock types, which resembles chemical polishing of the cemented carbide. This is a very interesting effect, however further investigations are needed to understand the chemical reactions taking place during the current contact conditions.

with small grain size and causes significantly lower initial wear of the cemented carbide. Thus, the respective hardness and share of each mineral is probably also of high importance, although the cemented carbide composite is harder than all the minerals (except pyrite). All the studied rock types contain quartz, the second hardest mineral in the investigation (well above the hardness of Co), but to very different amounts. Quartzite is almost pure quartz, with only small additions (3–5%) of mica (and occasional small amounts of e.g. calcite), while mica schist on the other hand contains about 50% each of these minerals. Quartzite results in high wear and mica schist cause significantly lower initial wear. Thus, it is not only the hardness of the hardest mineral that is important. Some minerals, like mica in mica schist, seem more able to distribute the pressure in the contact between rock asperities and the cemented carbide. The more even distribution prevents critical contact pressures with subsequent deformation and cracking of WC grains, thereby lowering the initial wear rate of the cemented carbide. This assumption is strengthened by the results for the hardest mineral in the investigation, pyrite, which is the main constituent in metal sulphide ore. Somewhat unexpectedly, the initial wear rate against the metal sulphide ore is very low, which is probably an effect of the presence of the less hard mineral sphalerite. Marble causes the lowest initial wear, even below the limit of visual detection in the SEM. Since it consists almost solely of calcite, which is softer than Co, it seems reasonable that the marble is heavily worn while the cemented carbide is practically unaffected. Proof of that marble is heavily worn can be seen from the thick transfer film generated on the cemented carbide surface. This transfer film has no resemblance to the original rock, being much thinner than the size of the grains in the rock and is probably generated from small rock fragments sintered together in the contact to generate the smooth film. Small micrometer-sized particles, not yet incorporated in the film, can be observed on top of the film, see e.g. Fig. 6. Occasionally parts of the thick transfer film become detached (Fig. 9) but the film quickly regenerates from new wear fragments. The film efficiently keeps the cemented carbide separated from the rock cylinder, and thereby protects from wear also when occasional harder minerals enter the contact.

4.4. Implications for rock drilling The connection between drilled rock type and wear of the cemented carbide is evident, both from laboratory testing and actual rock drilling experience. The observations made in this investigation contribute knowledge about the initiation of wear and indicate that the cemented carbide displays different weaknesses depending on rock type; some rocks preferentially wear the Co binder, some wear the composite via chemical attack and some wear the composite by crushing the WC grains. This opens up for tests with different types and amounts of binder and WC grains depending on rock type towards an understanding about how to increase the wear resistance by tailoring the cemented carbide to resist the tribological impact from specific rock types. 5. Conclusions This is a first contribution towards a new understanding of “drillability” of rock types, ultimately useful for tailoring cemented carbides.


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Fig. 12. Elemental analysis of the respective surfaces in Fig. 10 (CC-Granite, CC-Metal Sulphide Ore, CC-Mica Schist and CC-Quartzite) and Fig. 11 (CC-Marble). (SEM/EDS, 3 kV).


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Fig. 13. FIB cross sections of the worn cemented carbide surfaces after 1 m sliding against the respective rock cylinders (CC-Granite was originally published in [9] (after 87 cm sliding) and is here reproduced). Each individual image is a combination of a top view and a cross-section image. Top view is tilted 41°. (SEM, top view 3 kV and cross-section 10 kV, except CC-Granite 3 kV). Table 2 Subjective characterisation of the wear of the cemented carbide after sliding against the different rock types, based on numerous micrographs including those presented in the paper. The severity of each observation increases from + to ++++ and – indicates not observed.

Wear of Co Wear of WC Transfer of rock Integration of transferred rock Total wear of cemented carbide Average coefficient of friction


Metal sulphide ore

Mica schist



+++ +++ ++ +++

+++ + + +++

+++ ++ +++ ++

++++ ++++ +++ +++

– – ++++ –










• • • • • •

The most important observations can be summarized as:

• Sliding contact with rock immediately results in changes to the cemented carbide microstructure; rock is transferred, WC grains


become plastically deformed, cracked and worn and Co becomes removed. The initial wear rate and character of this process has been shown to be highly dependent on the type of rock. A high initial wear rate of the cemented carbide results in irregular and uneven wear surfaces on a microscopic scale. Still the wear is shallow, limited to the upper microns. The wear and transfer of rock occurs on a scale much smaller than the mineral grain size. The hardness of the rock does not solely explain the difference in wear rate, although significant amounts of minerals of low hardness decrease the wear rate and lack of low hardness minerals increases it. Transferred rock, which has not become incorporated into the cemented carbide microstructure, successfully provides protection and decreases the wear of the cemented carbide. The friction coefficient is not correlated to the wear of the cemented carbide. Extensive transfer of rock to the cemented carbide surface leads to rock-rock contact and high friction. The observed differences in wear character may enable tailoring of a cemented carbide grade to delay initiation of wear against a specific rock type.

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