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Friction between a cemented carbide rock drill button and different rock types
Wear 253 (2002) 1219–1221
Short communication
Friction between a cemented carbide rock drill button and different rock types U. Beste∗ , S. Jacobson Uppsala University, The Ångström Laboratory, Department of Materials Physics, Tribomaterials Group, Box 534, 751 21 Uppsala, Sweden Received 22 January 2002; received in revised form 18 July 2002; accepted 15 August 2002
Abstract WC/Co cemented carbide is the most common material for rock drilling due to its superior combination of toughness and hardness. To elucidate the relationship between the known wear behaviour against different rock types and the corresponding sliding friction performance, a friction test series has been performed. A cemented carbide rock drill button with 94-wt.% WC and 6-wt.% Co was sliding against polished granite, magnetite, hematite, quartzite, mica schist, leptite and sandstone in a pin-on-disc tribometer. The test was performed in 25 ◦ C and 350 ◦ C air and also in 25 ◦ C water. The load was chosen to 20 N (which is considerably lower than required for crushing the rock) and the sliding velocity was 0.26 m/s. At 25 ◦ C, the cemented carbide exhibited the highest stable dry friction against mica schist, and the lowest against leptite. At 350 ◦ C, leptite gave the highest friction and hematite the lowest. In water, granite showed the highest friction and quartzite the lowest. A “remaining water lubricant effect” was noticed where mica schist was able to keep low, water lubricated friction value even when seemingly run dry, while leptite, sandstone and hematite were not. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Friction; Cemented carbide; Rock; Drilling
1. Introduction
2. Experiment
Owing to its unique combination of hardness and toughness, WC/Co cemented carbide is the dominant material in rock working tools [1]. The wear mechanisms of the WC/Co surface are very complex, and not very well understood. It has been speculated that varying friction against different rock types has an influence on the wear by both influencing the contact temperature and the local shear stresses. However, little data has been published on the friction in this system. To get a better platform for understanding the differences in wear rates and wear mechanisms, a simplified friction measurement has been performed between a common WC/Co rock drill grade and seven common rock types. The tests were performed in room tempered air and water and at 350 ◦ C. The load applied is considerably lower than the actual drill load, which crushes the rock. The lower load allows ordinary sliding friction to be measured. Differences in this friction are discussed in relation to known rock drill wear.
A common cemented carbide rock drill button, with 6 wt.% Co, 94 wt.% WC and with 2.5 m large WC-grains was made sliding in a pin-on-disc tribometer against the rock specimens listed in Table 1. The button had 6 mm top radius and a hardness of 1270 HV. The tests were performed in normal air, with 20 N load and 0.26 m/s velocity, in at least 20 min. The velocity corresponds to the actual velocity of a rock drill button. In rock drilling, the load on each button is about 2 kN when the rock is crushed, and much lower between the impacts. In these tests, the intention is not to crush the rocks and the load is therefore set to 20 N. The composition of the tested rock types were analysed by X-ray diffraction and their Vickers hardness measured as presented in Table 1. The rock types can be divided into two groups, the first group with isotropic character, including magnetite, hematite, quartz and sandstone. The second contains complex rock types including mica schist, granite, and leptite. However, the character of the rock types is also correlated to the grain sizes, and the approximate values are presented in Table 1.
∗ Corresponding author. Tel.: +46-18-471-31-14; fax: +46-18-471-35-72. E-mail address: [email protected] (U. Beste).
0043-1648/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 ( 0 2 ) 0 0 2 3 9 - 9
1220
U. Beste, S. Jacobson / Wear 253 (2002) 1219–1221
Table 1 The composition of the rock types, their measured average hardness and approximate grain size Rock type
Composition
Magnetite Hematite Mica schist
Fe3 O3 Fe3 O3 Quartz, and the feldspars albite and microcline Quartz, microcline, hornblende, some biotite and albite Quartz, albite Large grains of quartz SiO2
Granite
Leptite Sandstone Quartz
Hardness
Grain size (m)
360 490 750
200 50 300
810
700
1030 1230 1420
20 400 Single crystal
The rock samples were set in a protecting resin (or in cement for the test performed at 350 ◦ C) and polished using 3 m diamond on cloth as a final step. The hardness values presented in Table 1 are the mean values of six micro hardness indents at 25, 50 and 500 g, respectively. An initial friction was measured after 10 s, before any substantial polishing or wear of the rock could be observed. The friction level typically became stable after some minutes of sliding, and the steady-state friction value was read after 20 min of sliding. However, at 350 ◦ C, the friction was measured after 4 min, when high rock wear rate prevented longer test times. The tests run in water were also used to determine another value, namely the friction when run dry. This elucidates a special effect; that is the remaining lubricating effect from the water, also when the surface seemingly has become dry.
Fig. 1. Initial friction for all rock samples at 25 and 350 ◦ C air and in 25 ◦ C water. The initial friction is measured after 10 s sliding on polished samples, before a smoother sliding track has formed.
3.2. In air at 350 ◦ C • Leptite resulted in the highest friction with µ = 0.80 and hematite the lowest, µ = 0.42. • Against magnetite and quartzite, the initial friction was lower than the stable level while the other rocks gave the opposite behaviour. • Hematite and leptite showed higher initial friction at 350 ◦ C than at 25 ◦ C. Only leptite showed higher steady-state friction at 350 ◦ C than at 25 ◦ C.
3. Results The dry, room tempered friction values initially varied between 0.37 and 0.72 and generally rose up to between 0.46 and 0.87 before stabilising (see Figs. 1 and 2). At the higher temperature, the initial friction was typically similar to the room temperature values, while they stabilised at lower values. Leptite stands out from the other rock types by consistently exhibiting substantially higher friction at the higher temperature. When the samples went dry after a couple of minutes testing, the friction changed. By filling in new water, the friction could be cycled back to its former values. 3.1. In air at 25 ◦ C • The cemented carbide buttons showed the highest steady-state dry friction against mica schist, µ = 0.87, and the lowest against leptite, µ = 0.46. • Magnetite, hematite and quartzite showed lower initial friction than after stabilising while the other rocks showed higher initial friction.
Fig. 2. The steady-state friction for all rock samples at 25 and 350 ◦ C air and in 25 ◦ C water. The steady-state friction was read after 20 min of sliding.
U. Beste, S. Jacobson / Wear 253 (2002) 1219–1221
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Fig. 3. Friction coefficient in the seemingly dry sliding conditions occurring when the wet tests run dry. The relation to the ordinary dry friction level is also indicated.
3.3. In 25 ◦ C water • Granite showed the highest friction, µ = 0.21 and quartzite (the hardest rock) the lowest, µ = 0.06. • In water, all rock samples showed a polishing effect after running-in, leading to lower friction. • Three to five micrometer large magnetite wear particles adhere to magnetite grain boundaries in the sliding track, probably due to their magnetic properties. During the wet tests, the water could be emptied from the specimen holding container. When run dry like this an interesting effect could be noted. After seemingly run dry, the friction coefficient did not climb to the normal dry friction values for all the materials. If water was again added, the friction consistently returned to the level typical of sliding in water. This remaining water lubricant effect is shown in Fig. 3.
4. Conclusions • Assuming that the friction between the cemented carbide drill tip and the rock in rock drilling is best represented by the initial friction in water, the differences are relatively small: ranging from the granite level, µ = 0.27 to hematite µ = 0.21.
• These differences in friction are probably too small to explain the large differences in wear rate and mechanisms in rock drilling. • Water in the contact zones reduces the initial friction, typically by a factor of 2. • A “remaining water lubricant effect” has been observed, implying that the friction stays low for a period although the water seemingly has disappeared from the surface. The length of this period differs significantly between the tested rock types, due either to different abilities to adhere or adsorb water, or due to different requirements on the amount of water molecules needed to provide a substantial friction reduction.
Acknowledgements The financial support from Sandvik AB is gratefully acknowledged. Reference [1] U. Beste, T. Hartzell, H. Engqvist, N. Axén, Surface damage on cemented carbide rock drill buttons, Wear 249 (2001) 324–329.
Short communication
Friction between a cemented carbide rock drill button and different rock types U. Beste∗ , S. Jacobson Uppsala University, The Ångström Laboratory, Department of Materials Physics, Tribomaterials Group, Box 534, 751 21 Uppsala, Sweden Received 22 January 2002; received in revised form 18 July 2002; accepted 15 August 2002
Abstract WC/Co cemented carbide is the most common material for rock drilling due to its superior combination of toughness and hardness. To elucidate the relationship between the known wear behaviour against different rock types and the corresponding sliding friction performance, a friction test series has been performed. A cemented carbide rock drill button with 94-wt.% WC and 6-wt.% Co was sliding against polished granite, magnetite, hematite, quartzite, mica schist, leptite and sandstone in a pin-on-disc tribometer. The test was performed in 25 ◦ C and 350 ◦ C air and also in 25 ◦ C water. The load was chosen to 20 N (which is considerably lower than required for crushing the rock) and the sliding velocity was 0.26 m/s. At 25 ◦ C, the cemented carbide exhibited the highest stable dry friction against mica schist, and the lowest against leptite. At 350 ◦ C, leptite gave the highest friction and hematite the lowest. In water, granite showed the highest friction and quartzite the lowest. A “remaining water lubricant effect” was noticed where mica schist was able to keep low, water lubricated friction value even when seemingly run dry, while leptite, sandstone and hematite were not. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Friction; Cemented carbide; Rock; Drilling
1. Introduction
2. Experiment
Owing to its unique combination of hardness and toughness, WC/Co cemented carbide is the dominant material in rock working tools [1]. The wear mechanisms of the WC/Co surface are very complex, and not very well understood. It has been speculated that varying friction against different rock types has an influence on the wear by both influencing the contact temperature and the local shear stresses. However, little data has been published on the friction in this system. To get a better platform for understanding the differences in wear rates and wear mechanisms, a simplified friction measurement has been performed between a common WC/Co rock drill grade and seven common rock types. The tests were performed in room tempered air and water and at 350 ◦ C. The load applied is considerably lower than the actual drill load, which crushes the rock. The lower load allows ordinary sliding friction to be measured. Differences in this friction are discussed in relation to known rock drill wear.
A common cemented carbide rock drill button, with 6 wt.% Co, 94 wt.% WC and with 2.5 m large WC-grains was made sliding in a pin-on-disc tribometer against the rock specimens listed in Table 1. The button had 6 mm top radius and a hardness of 1270 HV. The tests were performed in normal air, with 20 N load and 0.26 m/s velocity, in at least 20 min. The velocity corresponds to the actual velocity of a rock drill button. In rock drilling, the load on each button is about 2 kN when the rock is crushed, and much lower between the impacts. In these tests, the intention is not to crush the rocks and the load is therefore set to 20 N. The composition of the tested rock types were analysed by X-ray diffraction and their Vickers hardness measured as presented in Table 1. The rock types can be divided into two groups, the first group with isotropic character, including magnetite, hematite, quartz and sandstone. The second contains complex rock types including mica schist, granite, and leptite. However, the character of the rock types is also correlated to the grain sizes, and the approximate values are presented in Table 1.
∗ Corresponding author. Tel.: +46-18-471-31-14; fax: +46-18-471-35-72. E-mail address: [email protected] (U. Beste).
0043-1648/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 ( 0 2 ) 0 0 2 3 9 - 9
1220
U. Beste, S. Jacobson / Wear 253 (2002) 1219–1221
Table 1 The composition of the rock types, their measured average hardness and approximate grain size Rock type
Composition
Magnetite Hematite Mica schist
Fe3 O3 Fe3 O3 Quartz, and the feldspars albite and microcline Quartz, microcline, hornblende, some biotite and albite Quartz, albite Large grains of quartz SiO2
Granite
Leptite Sandstone Quartz
Hardness
Grain size (m)
360 490 750
200 50 300
810
700
1030 1230 1420
20 400 Single crystal
The rock samples were set in a protecting resin (or in cement for the test performed at 350 ◦ C) and polished using 3 m diamond on cloth as a final step. The hardness values presented in Table 1 are the mean values of six micro hardness indents at 25, 50 and 500 g, respectively. An initial friction was measured after 10 s, before any substantial polishing or wear of the rock could be observed. The friction level typically became stable after some minutes of sliding, and the steady-state friction value was read after 20 min of sliding. However, at 350 ◦ C, the friction was measured after 4 min, when high rock wear rate prevented longer test times. The tests run in water were also used to determine another value, namely the friction when run dry. This elucidates a special effect; that is the remaining lubricating effect from the water, also when the surface seemingly has become dry.
Fig. 1. Initial friction for all rock samples at 25 and 350 ◦ C air and in 25 ◦ C water. The initial friction is measured after 10 s sliding on polished samples, before a smoother sliding track has formed.
3.2. In air at 350 ◦ C • Leptite resulted in the highest friction with µ = 0.80 and hematite the lowest, µ = 0.42. • Against magnetite and quartzite, the initial friction was lower than the stable level while the other rocks gave the opposite behaviour. • Hematite and leptite showed higher initial friction at 350 ◦ C than at 25 ◦ C. Only leptite showed higher steady-state friction at 350 ◦ C than at 25 ◦ C.
3. Results The dry, room tempered friction values initially varied between 0.37 and 0.72 and generally rose up to between 0.46 and 0.87 before stabilising (see Figs. 1 and 2). At the higher temperature, the initial friction was typically similar to the room temperature values, while they stabilised at lower values. Leptite stands out from the other rock types by consistently exhibiting substantially higher friction at the higher temperature. When the samples went dry after a couple of minutes testing, the friction changed. By filling in new water, the friction could be cycled back to its former values. 3.1. In air at 25 ◦ C • The cemented carbide buttons showed the highest steady-state dry friction against mica schist, µ = 0.87, and the lowest against leptite, µ = 0.46. • Magnetite, hematite and quartzite showed lower initial friction than after stabilising while the other rocks showed higher initial friction.
Fig. 2. The steady-state friction for all rock samples at 25 and 350 ◦ C air and in 25 ◦ C water. The steady-state friction was read after 20 min of sliding.
U. Beste, S. Jacobson / Wear 253 (2002) 1219–1221
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Fig. 3. Friction coefficient in the seemingly dry sliding conditions occurring when the wet tests run dry. The relation to the ordinary dry friction level is also indicated.
3.3. In 25 ◦ C water • Granite showed the highest friction, µ = 0.21 and quartzite (the hardest rock) the lowest, µ = 0.06. • In water, all rock samples showed a polishing effect after running-in, leading to lower friction. • Three to five micrometer large magnetite wear particles adhere to magnetite grain boundaries in the sliding track, probably due to their magnetic properties. During the wet tests, the water could be emptied from the specimen holding container. When run dry like this an interesting effect could be noted. After seemingly run dry, the friction coefficient did not climb to the normal dry friction values for all the materials. If water was again added, the friction consistently returned to the level typical of sliding in water. This remaining water lubricant effect is shown in Fig. 3.
4. Conclusions • Assuming that the friction between the cemented carbide drill tip and the rock in rock drilling is best represented by the initial friction in water, the differences are relatively small: ranging from the granite level, µ = 0.27 to hematite µ = 0.21.
• These differences in friction are probably too small to explain the large differences in wear rate and mechanisms in rock drilling. • Water in the contact zones reduces the initial friction, typically by a factor of 2. • A “remaining water lubricant effect” has been observed, implying that the friction stays low for a period although the water seemingly has disappeared from the surface. The length of this period differs significantly between the tested rock types, due either to different abilities to adhere or adsorb water, or due to different requirements on the amount of water molecules needed to provide a substantial friction reduction.
Acknowledgements The financial support from Sandvik AB is gratefully acknowledged. Reference [1] U. Beste, T. Hartzell, H. Engqvist, N. Axén, Surface damage on cemented carbide rock drill buttons, Wear 249 (2001) 324–329.