AU2011304382B2

AU2011304382B2 - A rock drill bit for percussive drilling and a rock drill bit button

Info

Publication number: AU2011304382B2
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: 2015-01-15
Anticipated expiration: 2031-09-20
Also published as: AU2011304461B2; RU2571783C2; BR112013006623A2; RU2013118269A; BR112013006621A2; EP2619396A1; EP2434086B1; CN103261561A; CN103339339A; AU2011304382A1; WO2012038428A1; US20130180785A1; KR20140009149A; AU2011304461A1; AU2015238819A1; RU2013118265A; US20130180784A1; EP2434086A1; CA2811674A1; ZA201302143B

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

1 A ROCK DRILL BIT FOR PERCUSSIVE DRILLING AND A ROCK DRILL BIT BUTTON 5 TECHNICAL FIELD OF THE INVENTION AND BACKGROUND ART The present invention relates to a rock drill bit for percussive drilling and a rock drill bit button according to the preambles of 10 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 hole hammer drill just as well as a top hammer drill, although 15 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 sence of limitations applies to the intended percussion fre 20 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 rock drill bit in high frequency assemblies operating at a fre 25 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 1 and Fig 2. The drill bit has a bit head 2 configured to be at 30 tached at an end of a drill element, for example in the form of a 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 35 diameter than the bit head. Other ways of connecting the drill bit to the drill element are conceivable and known within the art. 4171442_1 (GHMatters) P93028.AU JENNIFER 2 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 buttons are configured to engage material to be crushed and to 5 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 for engaging material to be crushed. It is also indicated how a 10 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 close to the walls of a hole 6 in which the rock drill bit with said 15 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. 20 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 of known rock drill bits of this type is dependent upon the wear 25 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 penetration speed. These flat surfaces 10 will during the 30 operation of the rock drill bit grow and finally result in a 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 35 defined in the introduction. 4171442_1 (GHMatters) P93028.AU JENNIFER 3 SUMMARY OF THE INVENTION Disclosed is a rock drill bit and rock drill bit button in which at least one button has a shank portion at least partially comprising 5 a bearing portion, the material of which is harder than the binder phase, the bit 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 and rotation of the rock drill bit to rotate about 10 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 penetra tion speed of the rock drill bit longer than would it be fixed in the 15 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 bearing portion is substantially homogenous or stated another 20 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. According to another embodiment of the invention the bearing 25 portion is at least partially coated with a barrier coating, which substantially stops dissolution of binder phase. According to another embodiment of the invention the bearing portion can have a friction coefficient against steel which is less 30 than 0.5 that will substantially avoid wear on the hole wall. According to another embodiment of the invention the bearing portion may have a microhardness (HV 0.05) of at least 3000 to make the bearing portion endure abrasion. 35 4171442_1 (GHMatters) P93028.AU JENNIFER 4 According to another embodiment of the invention the bearing portion comprises anyone of or several of titanium-aluminium nitride (TiAIN), aluminum-chromium nitride (AICrN), titanium carbide (TiC), titanium nitride (TiN), chromium nitride (CrN), 5 zirconium nitride (ZrN) and/or diamond coatings to achieve non abrasive effect on the hole. According to another embodiment of the invention the button comprises button retaining means such that the button may be 10 reliably held in the rock drill bit while being allowed to rotate. According to another embodiment of the invention a base portion of at least one button rests against or contacts a bottom of a button hole to transfer impact forces to the button while 15 allowing the base portion to move thereon when rotating. The invention also relates to a rock drill bit button according to the invention for percussive rock drilling into earth material, such as rock. 20 The invention also relates to a use of a rock drill bit according to the invention for percussive rock drilling into earth material, such as rock. 25 Further advantages as well as advantageous features of the in vention will appear from the following description. BRIEF DESCRIPTION OF THE DRAWINGS 30 With reference to the appended drawings, below follows a spe cific description of embodiments of the invention cited as exam ples. 35 4171442_1 (GHMatters) P93028.AU JENNIFER 5 In the drawings: Fig 1 is a very simplified view of a rock drill bit according to prior art in operation, 5 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 10 drill bit according to the present invention, 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, 15 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 20 cording to a second embodiment of the invention, Fig 7 is an exploded view of the rock drill bit according to Fig 6, 25 Fig 8 is a simplified view corresponding to Fig 4 of a rock drill bit according to a third embodiment of the inven tion, Fig 9 is a simplified view of a button allowed to rotate in a 30 bit head of a rock drill bit according to a fourth em bodiment of the invention, and Fig 10 is a very simplified view of a drilling assembly for per cussive rock drilling according to an embodiment of 35 the present invention in operation. 4171442_1 (GHMatters) P93028.AU JENNIFER 6 Fig. 11 is a graph showing drilled meters versus drill penetration speed, wherein drill bits B and C represent the present invention. 5 DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVEN TION Disclosed is a rock drill bit for percussive drilling comprising a bit head configured to be attached at an end of a drill element of 10 a drilling assembly, said bit head having at a front end as seen in the intended drilling direction a plurality of buttons distributed over said bit head and configured to engage material to be crushed, at least one of said buttons having a shank portion of a substrate material, comprising particles of a first material 15 embedded in a binder phase, said first material being harder than the binder phase, the shank portion at least partially comprising a bearing portion, the material of which is harder than the binder phase, and in that at least one of said buttons is allowed to rotate about its own symmetry axis. 20 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 symmetry axis by being received in holes 22 in a drill bit body in 25 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. Each button may be manufactured from pressed and sintered 30 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 will be discussed more in detail. In certain cases, it may be 35 justified that at least the exposed part of the button comprises 4171442_1 (GHMatters) P93028.AU JENNIFER 7 superhard materials such as polycrystalline diamond or cubic boron nitride. A rock drill bit 30 according to a first embodiment of the present 5 invention will now be described while making reference to Figs. 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 10 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. 15 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 20 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 25 (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 30 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 35 second member. 4171442_1 (GHMatters) P93028.AU JENNIFER 8 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 ing. This means that impact forces are transferred to the gauge 5 buttons from a surface 32 located inside the drill bit. The first 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 10 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 15 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 20 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 25 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 30 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 35 gauge buttons be fixedly arranged in the bit head. 4171442_1 (GHMatters) P93028.AU JENNIFER 9 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 secured to an end 52 of a drill element 53 and having a ring 5 surface 54 forming a support for a base portion 55 of each 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 10 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. 15 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. 20 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 25 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 30 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 35 has also at least one flushing hole 68 (see the arrows F indicating the flow of flushing medium) opening at the first end 4171442_1 (GHMatters) P93028.AU JENNIFER 10 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 inside the hole 58. The function of this embodiment of the 5 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 invention is very schematically shown in Fig 8. This rock drill bit 10 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 largest diameter of the button. A blind hole 82 in the bit head 15 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 button 80 is pushed into the hole 82 the lock ring will first be 20 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. Fig 9 illustrates an alternative way of locking a button 90 to a bit 25 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 pin 92 is used instead of a lock ring, and this lock pin is after 30 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. The base portions 44, 55 and the annular groove 91 are all 35 examples of button retaining means and each said portion may define a largest diameter of the button. 4171442_1 (GHMatters) P93028.AU JENNIFER 11 Fig 10 illustrates very schematically a drilling assembly for per cussive rock drilling according to the present invention having a rock drill bit 70 according to an embodiment of the invention 5 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, which in turn makes said drill element 73 and the rock drill bit to 10 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. The drilling assembly has also means 74, such as a compressed 15 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 operation of the power means 72 so as to adapt the frequency of 20 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 frequencies above 250 Hz, preferably above 350 Hz and most 25 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 than with rock drill bits already known, since the penetration 30 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. The inventors of the present invention found during tests that 35 button hole wear is of major importance. Numerous experiments were made to avoid hole wear including hardening of the steel 4171442_1 (GHMatters) P93028.AU JENNIFER 12 bit body, different flushing solutions for avoiding cuttings to enter into the holes, polishing of the buttons, etc. The results of the tests regarding button hole wear showed that surface hardness of the drill bit body and entrance of rock cuttings into 5 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 wear rate increases rapidly after a certain time of use of 10 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 wear rate in the hole increases rapidly. 15 The aim of the further tests was to maintain the integrity of the envelope surface of the button. One way of achieving that aim is to coat at least the shank 20 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 that also the button retaining means and/or the exposed portions 25 of the rotatable buttons are coated. Two coating materials were used in tests, i.e. one material comprising TiAIN and one material comprising AICrN. 30 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 (TiAIN), and C - a drill bit with coated, rotatable gauge buttons, 35 the coating being BALINIT® ALCRONA PRO, i.e. aluminum chromium nitride (AICrN). The coating thickness was about 3 4171442_1 (GHMatters) P93028.AU JENNIFER 13 micrometers in both cases. All bits had fixed, i.e. pressed-in, front buttons during the drilling tests. Each drilled hole was 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 5 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 the drill bits A with uncoated gauge buttons. While drill bit A 10 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 feed was increased since by then the buttons became blunt and 15 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 has a friction coefficient against steel which is less than 0.5, 20 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 thickness at the bearing portion can be thin such as 1-5 25 micrometers, preferably 2-4 micrometers, most preferably about 3 micrometers. The coatings form diffusion barriers which prevent the interaction between the hole wall and the button substrate 30 material. Other coatings that can be used are titanium carbide (TiC), titanium nitride (TiN), chromium nitride (CrN), zirconium nitride (ZrN) and diamond coatings. A material generally free from particles that are harder than the 35 surrounding material is here called substantially homogenous. 4171442_1 (GHMatters) P93028.AU JENNIFER 14 It is preferable that the base portion of each rotatable button rests against or contacts the bottom of the hole to transfer impact forces to the button and while allowing the base portion to move thereon when rotating. 5 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 without departing from the scope of the invention as defined in 10 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. 15 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. "Substantially" used in the expressions "substantially a frusto 20 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. The disclosures in EP Patent Application No. 10178387.6, from 25 which this application claims priority, are incorporated herein by reference. While the technology has been described in reference to its 30 preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes may be made without departing from its scope as defined by the appended claims. 4171442_1 (GHMatters) P93028.AU JENNIFER 15 It is to be understood that a reference herein to a prior art document does not constitute an admission that the document forms part of the common general knowledge in the art in Australia or in any other country. 5 In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an 10 inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the technology. 4171442_1 (GHMatters) P93028.AU JENNIFER

Claims

1. A rock drill bit for percussive drilling comprising a bit head configured to be attached at an end of a drill element of a 5 drilling assembly, said bit head having at a front end as seen in the intended drilling direction a plurality of buttons distributed over said bit head and configured to engage material to be crushed, at least one of said buttons having a shank portion of a substrate material, comprising particles of 10 a first material embedded in a binder phase, said first material being harder than the binder phase, the shank portion at least partially comprising a bearing portion, the material of which is harder than the binder phase, and in that at least one of said buttons is allowed to rotate about its own 15 symmetry axis.

2. A rock drill bit according to claim 1, the material of the bearing portion being substantially homogenous. 20

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

4. A rock drill bit according to anyone of claims 1 to 3, _the 25 bearing portion being at least partially coated with a barrier coating.

5. A rock drill bit according to anyone of claims 1 to 4,_the bearing portion having a friction coefficient against steel 30 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.

6. A rock drill bit according to anyone of claims 1 to 5,_the bearing portion having a microhardness (HV 0.05) of at least 35 3000, preferably in the range of 3000-3500, most preferably in the range of 3100-3400. 4171442_1 (GHMatters) P93028.AU JENNIFER 17

7. A rock drill bit according to anyone of claims 1 to 6,_the bearing portion comprising anyone of or several of titanium aluminium nitride (TiAIN), aluminum-chromium nitride 5 (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,_the 10 button comprising button retaining means.

9. A rock drill bit according to anyone of claims 1 to 8, a base portion of at least one button resting against or contacting a bottom of a button hole to transfer impact forces to the 15 button while allowing the base portion to move thereon when rotating.

10.A rotatable rock drill bit button, said button having a shank portion of a substrate material, comprising particles of a first 20 material embedded in a binder phase, said first material being harder than the binder phase, the shank portion at least partially comprising a bearing portion, the material of which is harder than the binder phase. 25

11.A rock drill bit button according to claim 10, the material of the bearing portion being substantially homogenous.

12.A rock drill bit button according to claim 10 or 11, the bearing portion comprising a material generally free from particles 30 that are harder than the surrounding material.

13.A rock drill bit button according to anyone of claims 10 to 12, the bearing portion having a friction coefficient against steel which is less than 0.5, preferably in the range of 0.1 - 0.5, 35 most preferably in the range of 0.2-0.4. 4171442_1 (GHMatters) P93028.AU JENNIFER 18

14.A rock drill bit button according to anyone of claims 10 to 13, the bearing portion having a microhardness (HV 0.05) of at least 3000, preferably in the range of 3000-3500, most preferably in the range of 3100-3400. 5

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

16.A rock drill bit button according to anyone of claims 10 to 15, the button comprising button retaining means . 15

17. Use of a rock drill bit according to any of claims 1-9 for per cussive drilling into earth, such as rock. 4171442_1 (GHMatters) P93028.AU JENNIFER