AU2011304382A1

AU2011304382A1 - A rock drill bit for percussive drilling and a rock drill bit button

Info

Publication number: AU2011304382A1
Application number: AU2011304382A
Authority: AU
Inventors: Mauri Esko; Markku Keskiniva; Pauli Lemmetty; Juha Piispanen
Original assignee: Sandvik Intellectual Property AB
Current assignee: Sandvik Intellectual Property AB
Priority date: 2010-09-22
Filing date: 2011-09-20
Publication date: 2013-04-11
Anticipated expiration: 2031-09-20
Also published as: EP2619396A1; RU2013118269A; AU2011304461A1; CN103339339A; ZA201302143B; WO2012038428A1; US20130180784A1; EP2434086B1; CA2811674A1; KR20140009149A; RU2571783C2; WO2012038342A1; BR112013006623A2; AU2015238819A1; KR20140005880A; RU2013118265A; EP2434086A1; BR112013006621A2; AU2011304461B2; US20130180785A1

Abstract

A button and a rock drill bit (30, 50) for percussive drilling comprising a bit head (39) configured to be attached at an end of a drill element (33, 53) of a drilling assembly, said bit head having at a front end (38, 60) as seen in the intended drilling direction a plurality of buttons (41, 42, 56, 59, 80, 90) distributed over said bit head and configured to engage material to be crushed, at least one of said buttons having a shank portion (41', 56', 80', 90') of a substrate material, comprising particles of a first material embedded in a binder phase, said first material being harder than the binder phase. The shank portion (41', 56', 80', 90') comprises at least partially a bearing portion, the material of which is harder than the binder phase. At least one of said buttons (41, 42, 56, 80, 90) is allowed to rotate about its own symmetry axis.

Description

WO 2012/038428 PCT/EP2011/066323 1 A ROCK DRILL BIT FOR PERCUSSIVE DRILLING AND A 5 ROCK DRILL BIT BUTTON 10 TECHNICAL FIELD OF THE INVENTION AND BACKGROUND ART The present invention relates to a rock drill bit for percussive 15 drilling and a rock drill bit button according to the preambles of the independent claims. The invention is not restricted to any type of drilling assembly for use of a said rock drill bit, but the former may be a down-the 20 hole hammer drill just as well as a top hammer drill, although the rock drill bit shown is especially intended for the latter type. Furthermore, said rock drill bit may have any conceivable size and has normally a diameter of 30 mm - 300 mm. The same ab 25 sence of limitations applies to the intended percussion fre quency and rotational speed of the rock drill bit in operation, although it may be mentioned that these are typically within the ranges 20 Hz - 100 Hz and 20 - 500 revolutions per minute, respectively, but the invention does not exclude the use of the 30 rock drill bit in high frequency assemblies operating at a fre quency above 250 Hz and which may reach more than 1 kHz. A known so-called standard rock drill bit 1 of the type defined in the introduction will now be described while referring to both Fig 35 1 and Fig 2. The drill bit has a bit head 2 configured to be at tached at an end of a drill element, for example in the form of a WO 2012/038428 PCT/EP2011/066323 2 drill tube or drill rod, of a drilling assembly and having a diame ter larger than that of a said drill element. This drill element is not shown in these figures but may be intended to be received in a so-called skirt 3 integral with a bit head and having a smaller 5 diameter than the bit head. Other ways of connecting the drill bit to the drill element are conceivable and known within the art. The bit head has at a front end 4 as seen in the intended drilling direction a plurality of pressed-in gauge buttons 5 distributed along the circumference of the bit button head 2. The gauge 10 buttons are configured to engage material to be crushed and to determine the diameter of a hole 6 (see Fig 1) to be drilled by the rock drill bit. These gauge buttons are made of hard material, such as cemented carbide or tungsten carbide. Front buttons 7 also of hard material are pressed into a front surface 8 15 for engaging material to be crushed. It is also indicated how a flush channel opens at the front by a flushing hole 9 in the front surface. In operation the gauge buttons 5 will engage and break rock 20 close to the walls of a hole 6 in which the rock drill bit with said rod is located and the front buttons 7 will break rock closer to the centre of such a hole by impacts carried out by the rock drill bit in the direction of the arrow A. The drill bit will rotate some what, typically about 50, between each such impact. 25 The operation efficiency of a rock drill bit of this type is of course an important feature and this may be expressed as the penetration speed of the rock drill bit defined as the length of a hole drilled per time unit (meter/minute). The penetration speed 30 of known rock drill bits of this type is dependent upon the wear of said buttons, especially the gauge buttons. It is indicated in Fig 2 that during the operation of such a rock drill bit material is abraded at the periphery of the gauge buttons resulting in a flat surface 10 there, which makes them less sharp and reduces the 35 penetration speed. These flat surfaces 10 will during the operation of the rock drill bit grow and finally result in a WO 2012/038428 PCT/EP2011/066323 3 diameter of a hole drilled determined by said gauge buttons being so much reduced that the rock drill bit has to be replaced. It is of course an on-going attempt to increase the penetration speed and prolong the life time of a rock drill bit of the type 5 defined in the introduction. SUMMARY OF THE INVENTION The object of the present invention is to provide a rock drill bit 10 of the type defined in the introduction being improved in at least some aspect with respect to such rock drill bits already known. This object is according to the invention obtained by providing such a rock drill bit in which at least one of said buttons having 15 a shank portion at least partially comprising a bearing portion, the material of which is harder than the binder phase, and allowing the button to rotate about its own symmetry axis. By rotatably fitting at least one said button in the bit head this button will while drilling be influenced by the impacts thereof 20 and rotation of the rock drill bit to rotate about its own symmetry axis, so that the parts of said button engaging rock will vary and the button will be evenly worn and by that self-sharpened. This means that this button will thanks to the self-sharpening effect maintain its contribution to the penetration speed of the rock 25 drill bit longer than would it be fixed in the bit head. The provision of a bearing portion on the button will substantially avoid any grinding action on the hole wall. According to an embodiment of the invention the material of the 30 bearing portion is substantially homogenous or stated another way it comprises a material generally free from particles that are harder than the surrounding material so as to avoid exposure of abrasive particles towards the hole wall.

WO 2012/038428 PCT/EP2011/066323 4 According to another embodiment of the invention the bearing portion is at least partially coated with a barrier coating, which substantially stops dissolution of binder phase. 5 According to another embodiment of the invention the bearing portion can have a friction coefficient against steel which is less than 0.5 that will substantially avoid wear on the hole wall. According to another embodiment of the invention the bearing 10 portion may have a microhardness (HV 0.05) of at least 3000 to make the bearing portion endure abrasion. According to another embodiment of the invention the bearing portion comprises anyone of or several of titanium-aluminium 15 nitride (TiAIN), aluminum-chromium nitride (AICrN), titanium carbide (TiC), titanium nitride (TiN), chromium nitride (CrN), zirconium nitride (ZrN) and/or diamond coatings to achieve non abrasive effect on the hole. 20 According to another embodiment of the invention the button comprises button retaining means such that the button may be reliably held in the rock drill bit while being allowed to rotate. According to another embodiment of the invention a base 25 portion of at least one button rests against or contacts a bottom of a button hole to transfer impact forces to the button while allowing the base portion to move thereon when rotating. The invention also relates to a rock drill bit button according to 30 the invention for percussive rock drilling into earth material, such as rock. The invention also relates to a use of a rock drill bit according to the invention for percussive rock drilling into earth material, 35 such as rock.

WO 2012/038428 PCT/EP2011/066323 5 Further advantages as well as advantageous features of the in vention will appear from the following description. BRIEF DESCRIPTION OF THE DRAWINGS 5 With reference to the appended drawings, below follows a spe cific description of embodiments of the invention cited as exam ples. 10 In the drawings: Fig 1 is a very simplified view of a rock drill bit according to prior art in operation, 15 Fig 2 is a perspective view of a rock drill bit according to prior art after some time of operation, Fig 3 is a perspective view illustrating the principle of a rock drill bit according to the present invention, 20 Fig 4 shows a longitudinal section through a part of a rock drill bit according to a first embodiment of the inven tion in operation, 25 Fig 5 is an exploded view of the rock drill bit according to Fig 4, Fig 6 is a view corresponding to Fig 4 of a rock drill bit ac cording to a second embodiment of the invention, 30 Fig 7 is an exploded view of the rock drill bit according to Fig 6, Fig 8 is a simplified view corresponding to Fig 4 of a rock 35 drill bit according to a third embodiment of the inven tion, WO 2012/038428 PCT/EP2011/066323 6 Fig 9 is a simplified view of a button allowed to rotate in a bit head of a rock drill bit according to a fourth em bodiment of the invention, and 5 Fig 10 is a very simplified view of a drilling assembly for per cussive rock drilling according to an embodiment of the present invention in operation. 10 Fig. 11 is a graph showing drilled meters versus drill penetration speed, wherein drill bits B and C represent the present invention. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVEN 15 TION Fig 3 shows very schematically the principle of a rock drill bit according to the present invention, in which all gauge buttons 20 and all front buttons 21 are allowed to rotate about their own 20 symmetry axis by being received in holes 22 in a drill bit body in a substantially circumferential ring surface 23 defining a substantially frusto-conical shape as seen in the intended drilling direction and in a front surface 24, respectively. 25 Each button may be manufactured from pressed and sintered cemented carbide. By the term "cemented carbide" is here meant WC, TiC, TaC, NbC, etc., in sintered combination with a binder metal such as, for instance, Co or Ni. The button is preferably at least partially coated with a barrier coating which 30 will be discussed more in detail. In certain cases, it may be justified that at least the exposed part of the button comprises superhard materials such as polycrystalline diamond or cubic boron nitride. 35 A rock drill bit 30 according to a first embodiment of the present invention will now be described while making reference to Figs.

WO 2012/038428 PCT/EP2011/066323 7 4 and 5. The rock drill bit comprises a first member 31 having a substantially circumferential ring surface 32 defining a substan tially frusto-conical shape as seen in the intended drilling direc tion. This first member 31 is provided with means configured to 5 secure this member to a drill element 33, in which this securing means is formed by a sleeve-like portion 34 of the first member 31 provided with engagement means in the form of an internal thread 35 configured to engage engagement means in the form of an external thread 36 on the drill element. 10 The rock drill bit further comprises a second member 37 defining a front end 38 of a bit head 39 of the rock drill bit. This second member is provided with a plurality of through holes 40 receiving the gauge buttons 41 and front buttons 42 while allowing these 15 to rotate about their own symmetry axis. Each gauge button 41 comprises a shank portion 41' preferably integral with a tip portion. Preferably, the shank portion 41' defines a larger diameter than any chosen diameter of the tip portion. The through-holes 40 each have a diameter slightly exceeding 20 (suitably by a diameter difference in the order of 30-80 pm) the diameter of the respective shank portion received therein for allowing the button to move with respect to walls 43 in the second member 37 defining said hole when rotating. However, this difference in diameter has been exaggerated in this figure 25 and also in the embodiment shown in Fig 6 and described below for better illustrating this feature. The gauge buttons as well as the front buttons are provided with a base portion 44 with larger cross-section than the rest of the button and also than the respective hole 40 so as to maintain the button received in the 30 second member. A gauge button 41 rests by the base portion 44 thereof on said ring surface 32 configured to transfer impact forces to the gauge button and allow the base portion to move thereon when rotat 35 ing. This means that impact forces are transferred to the gauge buttons from a surface 32 located inside the drill bit. The first WO 2012/038428 PCT/EP2011/066323 8 member has also surfaces 45 directed in an intended drilling di rection for supporting base portions of front buttons and transferring impact forces thereto while allowing these base portions to move on these surfaces 45 when rotating. Further 5 more, the bit head 39 will through a shoulder 47 on the first member 31 provide a clearance C with respect to this member 31, so that the button 41 may rotate freely without jamming. Particular measures are taken for flushing the surfaces and spaces surrounding the button, which will be explained more in 10 detail below. The rock drill bit comprises means 46 configured to secure the second member 37 to the first member 31. The securing means is preferably configured to releasably secure these members to 15 each other, for instance by mutually securing them by engage ment of threads. This would then mean that it would be possible to remove said second member with buttons for replacement while keeping the first member after the buttons have been that much worn that they have to be replaced. Welding or press 20 fitting are other possible alternatives of said securing means 46 easier to accomplish. When carrying out percussive drilling with the rock drill bit shown in Fig 4 and 5 as illustrated in Fig 4 the buttons thereof 25 will be allowed to rotate about their own axes, which means that the gauge buttons 41 will be worn evenly and maintain their sharpness, so that a high penetration speed may be maintained over a long period of time and the diameter of the hole defined by the gauge buttons will be reduced more slowly than would the 30 gauge buttons be fixedly arranged in the bit head. Figs. 6 and 7 illustrate a rock drill bit 50 according to a second embodiment of the invention. This rock drill bit has a first mem ber 51 in the form of a ring configured to be supported on and/or 35 secured to an end 52 of a drill element 53 and having a ring surface 54 forming a support for a base portion 55 of each WO 2012/038428 PCT/EP2011/066323 9 gauge button 56 in the same way as the corresponding surface 32 in the embodiment shown in Figs. 4 and 5. Each gauge button 56 comprises a shank portion 56' preferably integral with a tip portion. Preferably, the shank portion 56' defines a larger 5 diameter than any chosen diameter of the tip portion. Impact forces will be transferred by the ring surface 54 to the gauge buttons while the base portions thereof are allowed to move thereon when rotating. 10 A second member 57 of the rock drill bit has through holes 58 receiving said gauge buttons and allowing them to move with respect to walls of these holes when rotating. The front buttons 59 are, as an example, in this embodiment fixedly secured to a front end 60 of the second member 57. 15 The second member 57 is in this embodiment provided with means for securing this member to a drill element 53 by having a sleeve-like portion 61 designed to receive a drill element and having engagement means in the form of an internal thread 62 20 for engaging with engagement means in the form of an external thread 63 on the drill element for releasably securing said sec ond member to the drill element and by that also keeping said ring 51, a so-called pusher ring, in place. The first member 51 is provided with a collar 64, so that the first 51 and second 57 25 members are fixed with respect to each other while leaving a clearance 66 therebetween for the button to freely rotate. Proper flushing of a button allowed to rotate is also important. It is indicated in Fig 6 that the rock drill bit has a conventional flush channel 67 extending through the bit head. The flush channel 30 has also at least one flushing hole 68 (see the arrows F indicating the flow of flushing medium) opening at the first end 60 and passing the clearance 66 and the circumference of the button 56 allowed to rotate. This will keep said clearance 66 clear and eliminates wear problems while the button rotates 35 inside the hole 58. The function of this embodiment of the WO 2012/038428 PCT/EP2011/066323 10 invention in operation appears clearly from the above discussion of inter alia the first embodiment of the present invention. A part of a rock drill bit according to a third embodiment of the 5 invention is very schematically shown in Fig 8. This rock drill bit is provided with alternative means to lock a button 80 to a drill bit head 81 while allowing the button to rotate. Each gauge button 80 comprises a shank portion 80' preferably integral with a tip portion. Preferably, the shank portion 80' defines the 10 largest diameter of the button. A blind hole 82 in the bit head designed to receive the button 80 is provided with an annular groove 83, and the shank portion 80' is provided with a cor responding annular groove 84 receiving an elastic lock ring 85, for example a ring, such as a C-ring, made from steel. When the 15 button 80 is pushed into the hole 82 the lock ring will first be compressed until reaching the groove 83 in the bit head. It will then expand outwards into that groove and lock the button to the bit head 81 while allowing the button to rotate. 20 Fig 9 illustrates an alternative way of locking a button 90 to a bit head not shown in a rock drill bit according to a fourth embodi ment of the invention while allowing the button to rotate. This is achieved by providing a shank portion 90' with an annular groove 91 as in the embodiment shown in Fig 8. However, a lock 25 pin 92 is used instead of a lock ring, and this lock pin is after pushing the button 90 into a corresponding hole in the bit head pushed into the groove 91 while locking the button in place and still allowing it to rotate about its own symmetry axis. 30 The base portions 44, 55 and the annular groove 91 are all examples of button retaining means and each said portion may define a largest diameter of the button. Fig 10 illustrates very schematically a drilling assembly for per 35 cussive rock drilling according to the present invention having a rock drill bit 70 according to an embodiment of the invention WO 2012/038428 PCT/EP2011/066323 11 provided with gauge buttons 71. This drilling assembly is a so called top hammer drill acting upon the rock drill bit from a loca tion above the ground and has power means 72, such as diesel engine and hydraulic pump, configured to drive the rock drill 76, 5 which in turn makes said drill element 73 and the rock drill bit to rotate and carry out percussions and by that crush the rock. A design of the drilling assembly as a down-the-hole hammer equipment is also within the scope of the present invention. 10 The drilling assembly has also means 74, such as a compressed air generator, configured to flush cuttings resulted from en gagement of the gauge buttons and the front buttons of the drill bit away from the region occupied by the drill bit. The drilling as sembly has a control arrangement 75 configured to control the 15 operation of the power means 72 so as to adapt the frequency of impacts and the rotational speed of the drill bit. It has turned out that drill bits according to the present invention with buttons al lowed to rotate about their own symmetry axis are particularly well suited to be used in drilling assemblies controlled to have 20 frequencies above 250 Hz, preferably above 350 Hz and most preferred in the range of 350 Hz - 1000 Hz. Drilling with a drilling assembly according to Fig 8 with a rock drill bit according to the present invention will be more efficient 25 than with rock drill bits already known, since the penetration speed may be kept at a high level longer and the stops needed for replacing the rock drill bit or parts thereof will be less frequently occurring. 30 The inventors of the present invention found during tests that button hole wear is of major importance. Numerous experiments were made to avoid hole wear including hardening of the steel bit body, different flushing solutions for avoiding cuttings to enter into the holes, polishing of the buttons, etc. The results of 35 the tests regarding button hole wear showed that surface hardness of the drill bit body and entrance of rock cuttings into WO 2012/038428 PCT/EP2011/066323 12 the hole clearance have no significant effect on wear rate. The inventors surprisingly found that tungsten carbide grains are responsible for the steel wear in the button holes. Surface quality of the button has tremendous effect on wear rate but the 5 wear rate increases rapidly after a certain time of use of polished buttons. It is believed that after a certain period of drilling time the cobalt binder of the cemented carbide dissolves from the button surface thereby exposing abrasive wolfram carbide grains and the button surface quality is lost so that the 10 wear rate in the hole increases rapidly. The aim of the further tests was to maintain the integrity of the envelope surface of the button. 15 One way of achieving that aim is to coat at least the shank portion 41', 56', 80', 90' of the button with a barrier such as a barrier coating to substantially eliminate cobalt dissolution. The button will then substantially maintain the surface quality and the button hole wear is substantially eliminated. It is preferable 20 that also the button retaining means and/or the exposed portions of the rotatable buttons are coated. Two coating materials were used in tests, i.e. one material comprising TiAIN and one material comprising AICrN. 25 Fig. 11 is a graph showing drilled meters versus drill penetration speed of A - a drill bit with fixed, uncoated gauge buttons, B - a drill bit with coated, rotatable gauge buttons, the coating being BALINIT@ FUTURA NANO, i.e. titanium-aluminium nitride 30 (TiAIN), and C - a drill bit with coated, rotatable gauge buttons, the coating being BALINIT@ ALCRONA PRO, i.e. aluminum chromium nitride (AICrN). The coating thickness was about 3 micrometers in both cases. All bits had fixed, i.e. pressed-in, front buttons during the drilling tests. Each drilled hole was 35 about 4.1 m deep. The drill bits all had conical button tips and were made for hole diameter of 48 mm. They all had five 10 mm WO 2012/038428 PCT/EP2011/066323 13 buttons on gauge and three uncoated 9 mm buttons fixed to the front surface. Both drill bits B and C with coated gauge buttons outperformed 5 the drill bits A with uncoated gauge buttons. While drill bit A only could drill about 40 m, drill bit B managed about 80 m and drill bit C about 170 m. Thus with a suitable barrier against binder phase dissolution the life of a drill bit can be extended up to at least 400 %. After drilling about 143 m with drill bit C the 10 feed was increased since by then the buttons became blunt and a further 20 m could be drilled. The latter action is depicted in Fig. 11 as "Higher power". Properties for a suitable coating can be that the bearing portion 15 has a friction coefficient against steel which is less than 0.5, preferably in the range of 0.1 - 0.5, most preferably in the range of 0.2-0.4. The bearing portion may have a microhardness (HV 0.05) of at least 3000, preferably in the range of 3000-3500, most preferably in the range of 3100-3400. The coating 20 thickness at the bearing portion can be thin such as 1-5 micrometers, preferably 2-4 micrometers, most preferably about 3 micrometers. The coatings form diffusion barriers which prevent the 25 interaction between the hole wall and the button substrate material. Other coatings that can be used are titanium carbide (TiC), titanium nitride (TiN), chromium nitride (CrN), zirconium nitride (ZrN) and diamond coatings. 30 A material generally free from particles that are harder than the surrounding material is here called substantially homogenous. It is preferable that the base portion of each rotatable button rests against or contacts the bottom of the hole to transfer 35 impact forces to the button and while allowing the base portion to move thereon when rotating.

WO 2012/038428 PCT/EP2011/066323 14 The invention is of course not in any way restricted to the em bodiments described above, but many possibilities to modifica tions thereof would be apparent to a person with skill in the art 5 without departing from the scope of the invention as defined in the appended claims. For example, the rotatable button can be provided with bearing portion in the shape of a sleeve secured to its shank portion instead of a coating such that the substrate does not reach the hole wall in the drill bit. 10 The number and positions of the buttons of the rock drill bit may differ a lot with respect to the embodiments shown in the fig ures. 15 "Substantially" used in the expressions "substantially a frusto conical shape" and "substantially circumferential ring" also cover the case when cutting recesses or grooves and/or gauge buttons intersect the ring, as shown in the figures. 20 The disclosures in EP Patent Application No. 10178387.6, from which this application claims priority, are incorporated herein by reference. 25

Claims

1. A rock drill bit (30, 50) for percussive drilling comprising a bit head (39) configured to be attached at an end of a drill 5 element (33, 53) of a drilling assembly, said bit head having at a front end (38, 60) as seen in the intended drilling direction a plurality of buttons (41, 42, 56, 59, 80, 90) distributed over said bit head and configured to engage material to be crushed, at least one of said buttons having a 10 shank portion (41', 56', 80', 90') of a substrate material, comprising particles of a first material embedded in a binder phase, said first material being harder than the binder phase, characterized in that the shank portion (41', 56', 80', 90') at least partially comprises a bearing portion, the material of 15 which is harder than the binder phase, and in that at least one of said buttons (41, 42, 56, 80, 90) is allowed to rotate about its own symmetry axis.

2. A rock drill bit according to claim 1, characterized in that the 20 material of the bearing portion is substantially homogenous.

3. A rock drill bit according to claim 1 or 2, characterized in that the bearing portion comprises a material generally free from particles that are harder than the surrounding material. 25

4. A rock drill bit according to anyone of claims 1 to 3, characterized in that the bearing portion at is at least partially coated with a barrier coating. 30

5. A rock drill bit according to anyone of claims 1 to 4, characterized in that the bearing portion has a friction coefficient against steel which is less than 0.5, preferably in the range of 0.1 - 0.49, most preferably in the range of 0.2 0.4. 35 WO 2012/038428 PCT/EP2011/066323 16

6. A rock drill bit according to anyone of claims 1 to 5, characterized in that the bearing portion has a microhardness (HV 0.05) of at least 3000, preferably in the range of 3000-3500, most preferably in the range of 3100 5 3400.

7. A rock drill bit according to anyone of claims 1 to 6, characterized in that the bearing portion comprises anyone of or several of titanium-aluminium nitride (TiAIN), aluminum 10 chromium nitride (AICrN), titanium carbide (TiC), titanium nitride (TiN), chromium nitride (CrN), zirconium nitride (ZrN) and/or diamond coatings.

8. A rock drill bit according to anyone of claims 1 to 7, 15 characterized in that the button comprises button retaining means (44, 55, 91).

9. A rock drill bit according to anyone of claims 1 to 8, characterized in that a base portion (44, 55) of at least one 20 button (41, 42, 56, 80, 90) rests against or contacts a bottom of a button hole to transfer impact forces to the button while allowing the base portion to move thereon when rotating.

10.A rotatable rock drill bit button, said button (41, 42, 56, 59, 25 80, 90) having a shank portion (41', 56', 80', 90') of a substrate material, comprising particles of a first material embedded in a binder phase, said first material being harder than the binder phase, characterized in that the shank portion (41', 56', 80', 90') at least partially comprises a 30 bearing portion, the material of which is harder than the binder phase.

11.A rock drill bit button according to claim 10, characterized in that the material of the bearing portion is substantially 35 homogenous. WO 2012/038428 PCT/EP2011/066323 17

12.A rock drill bit button according to claim 10 or 11, characterized in that the bearing portion comprises a material generally free from particles that are harder than the surroundina material. 5

13.A rock drill bit button according to anyone of claims 10 to 12, characterized in that the bearing portion has a friction coefficient against steel which is less than 0.5, preferably in the range of 0.1 - 0.5, most preferably in the range of 0.2 10 0.4.

14.A rock drill bit button according to anyone of claims 10 to 13, characterized in that the bearing portion has a microhardness (HV 0.05) of at least 3000, preferably in the 15 range of 3000-3500, most preferably in the range of 3100

3400.

15.A rock drill bit button according to anyone of claims 10 to 14, characterized in that the bearing portion comprises anyone 20 of or several of titanium-aluminium nitride (TiAIN), aluminum chromium nitride (AICrN), titanium carbide (TiC), titanium nitride (TiN), chromium nitride (CrN), zirconium nitride (ZrN) and/or diamond coatings. 25

16.A rock drill bit button according to anyone of claims 10 to 15, characterized in that the button comprises button retaining means (44, 55, 91).

17. Use of a rock drill bit (30, 50) according to any of claims 1-9 30 for percussive drilling into earth, such as rock.