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The wear of diamonds in impregnated diamond bit drilling
Wear,
311
141 (1991) 311-320
The wear of diamonds in impregnated diamond bit drilling Duncan Miller and Anthony Department of Materials (South Africa)
Engineering,
Ball University
of Cape Town, Rand.ebosch
7700
(Received December 21, 1989; accepted July 3, 1990)
Abstract The wear of diamonds in impregnated diamond microbits was investigated in the laboratory by drilling a variety of rock types. The wear was studied by optical and scanning electron microscopy after drilling under a range of bit pressures, rates of advance, and rotational velocities using bits of different diamond size and concentration. The nature of the diamond wear modes did not vary with the drilling conditions but the relative proportions of different types of diamond wear changed with drilling performance. For stable drilling in any given rock type a characteristic threshold pressure existed above which desirable microfracture of the exposed diamonds was promoted over undesirable wear flat generation. The flats are produced by a sliding wear mode with the silicate minerals ploughing plastic grooves in the heated surfaces of the diamonds. Microfracture mode is the result of progressive growth and coalescence of cleavage microcracks promoted by the cyclical stresses experienced by the diamonds at pressures sufficient to cause indentation of the rock.
1. Introduction
Diamonds have been used in rock machining operations such as drilling, sawing and grinding for many decades. Relatively little is known about the wear of diamond in such applications [ 11. Despite sustained research even the wear of diamond in general is not well understood [Z] and there is thus a need to document the wear behaviour of diamond in specific applications. It is known that in hard rock drilling with impregnated diamond bits the drilling performance is influenced by the strength of the diamonds, which also affects the predominant mode of diamond wear [3, 41. Attempts have been made to quantify diamond wear in hard rock sawing by optical inspection and classification of the worn diamonds into arbitrary categories [ 5, 61. A similar approach is used here in reporting the wear of diamonds in impregnated diamond microbits drilled in a variety of materials.
2. Machine and materials The tests were conducted in the laboratory using an instrumented drilling rig consisting of a modified pillar drill with computer-monitored transducers 0043-1648/91/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
to record net power consumption, torque, bit pressure, rotational velocity and penetration rate 17-9 1. Tests were conducted under both set thrust and set rate of advance conditions, varying the bit pressure, rate of advance, rotational velocity, diamond mesh size and concentration, and rock type. The drill bits used were impregnated diamond microbits with inner and outer diameters of 12 and 20 mm respectively and six 2 mm wide waterways placed symmetrically. The diamonds, ranging in size from 20-80 U.S. mesh, were De Beers SDA 100 synthetic grit in a sintered bronze matrix at concentrations of 30 and 50. (A concentration of 100 is conventionally defined as 0.88 g of diamond per cubic centimetre of imprecation: approximately 25% by volume.) Eight rock types were drilled. ‘Bushveld norite’ is a dense, moderately hard igneous rock consisting mostly of felspar and pyroxene. ‘Cape granite’ consists predominantly of large alkali felspar crystals with interstitial quartz and is very resistant to drilling. ‘Red granite’ is a readily d.rilIable syenite conta~g mostly large crystals of plagioclase felspar and hornblende. The marble used consists almost entirely of soft, fine-aged, rec~s~~zed calcite. ‘Jaspilite’ is a very fine-grained, very resistant cherty ironstone. The quartzite used is of moderate drilling resistance and is composed of partially recrystallized, moderately angular grains of quartz. The sandstone, selected for its abrasiveness, is weakly cemented with angular quartz grains in a micaceous matrix.
3. Testing procedure New bits and bits suitable for re-use were conditioned by drilling 0.3-0.5 m in the abrasive sandstone to remove a layer of diamonds and to expose fresh, randomly orientated crystals. The tests were about 1.5 m long, with rotational speeds equivalent to l-4 m s-‘, an average load per diamond of up to 5 kgf, and rates of advance up to 0.1 mm rev-‘. Tap water flowing at about 350 1 h- ’ at 200 kPa pressure was used as a coolant and fIushing medium. Diamond wear was evaluated visually with a stereo optical microscope at 40 X magnification and studied in the scanning electron microscope [lo]. The frequency of different wear types was recorded as a percentage of the total number of exposed stones on each bit. Previous work has shown that in impregnated bit drilling the bit pressure has a crucial role in dete~~g the drilling performance [7-lo]. If the bit pressure is too low for a given rock and bit combination then the pene~tion rate decreases steadily (if drilling under set thrust) or the reactive load increases to impede progress (if drilling at set rate of advance). Tests were conducted in both the suboptimal and steady drilling regimes to study the effect of bit pressure on diamond wear. The wear of each exposed diamond was classified at regular intervals after driIIing set incremental distances in norite.
313
4. Results
mica1 examples of diamond wear are illustrated in Fig. 1. Recently exposed, fresh diamonds are angular with sharp points and edges fJ?ig. l(a)).
(a>
(e> Fig. 1. Scanning electron micrographs of typical examples of diamond wear: (a) unworn diamond; (b) wear flat; (c) microfracture; (d) hacldy macrofracture flush with the matrix; (e) pull-out hole. (Arrows indicate direction of travel.)
Wear flats are typically grooved or striated and can develop on originally flat diamond faces or on abraded edges and points (Fig. 1 (b)). Macrofractured diamonds have a surface covered with multiple sharp points (Fig. l(c)) and grade into more severely fractured diamonds which may protrude very little from the matrix (Fig. l(d)). Entire diamonds or residual fragments may be lost through pull-out to form holes in the bit matrix (Fig. l(e)). Diamonds with wear flats (Fig, l(b)) predominated at low bit pressures associated with suboptimal drilling and a declining penetration rate (Fig. 2). At higher bit pressures microfractured diamonds (Fig. l(c)) were more numerous and steady drilling took place above a transitional bit pressure of 5-6 MPa with the particular bits used. Drilling at bit pressures above this transition produced a characteristic sequence of diamond wear development with microf~cture predom~at~g over wear flats (Fig. 3). The transition from suboptimal to stable drilling took place at a calculated mean pressure on individual diamonds of about 400 MPa in norite irrespective of mode of control, rate of advance, rotational velocity, diamond mesh size,
.
wear
-m-
microfracture
.
*
500’ DRILLING
,A.
a
macrofracture
l
pull-out
. I
2000
1500
1000
DISTANCE
flat
*
--_ .
“nwor”
-a-
(mm)
Fig. 2. Plot of diamond wear type percentage for tests drilled in norite at set bit pressure increments, showing increase in microfracture at higher thrust levels.
l
l
l
l *
l
l
4 .
.
l
-
_.--
p--
l.-_.. I 0
V---o__
) A 250
. -;-
. . v _I________-_----., .
.
wear
-w A
microfracture macrofracture
l
flat
pull-out
-
-.--_ .
,.
,
500
750
GRILLING
.
unworn
-*-
-*--_-.
I, 5300
DISTANCE
r(---:---’ 1250
I 1500
(mm)
Fig. 3. Plot of diamond wear type percentage against distance for tests d&led in norite at 6 MPa bit pressure showing the preponderance of microfracture over wear flat development with stable drilling.
315
diamond concentration. This pressure is of the same order of magnitude as the uniaxial compressive strength of the rock, measured as 287 + 28 MPa. The compressive strength of the diamonds measured between corundum anvils was 4.44 GPa [ 71. At the transition the specific energy of drilling was at a minimum (Fig. 4). This specific energy minimum occurred at higher bit pressures in stronger rocks, indicating that the threshold diamond pressure for the predominance of microfracture and steady drilling is dependent on the rock strength. Two different sequences of diamond wear were observed. In suboptimal drilling a freshly exposed diamond tended to develop a large, stable wear flat, as did the few stones that had fractured. With steady drilling at bit pressures above the transition a freshly exposed diamond tended to experience a more complicated sequence. Rounding of any sharp points led to the development of a transitory wear flat which broke up by microfracture, with a possible cyclical repetition of these two stages. Eventually wholesale failure reduced the diamond to splinters with ensuing loss in most cases. The wear flats were striated with grooves of variable depth in the direction of travel (Figs. 5(a) and 5(b)). The finer grooves were narrow and straight. At high magnification it was evident that they had rounded intervening ridges and median cracks extending into the body of the stone (Figs. 5(c) and 5(d)). Usually, parallel steps were developed more or less transverse to the direction of abrasion (as indicated in Fig. 6(a)). These steps contributed to the eventual microfracture of the surface when drilling at pressures above the transition. Crystallographically controlled microfracture produced asurface covered by numerous points 0.01-o. 1 mm in size (Fig. 6(b)) which promoted stable drilling. More extensive hackly fracture led to minimal projection of the diamond from the bit matrix (Fig. 6(c)) or complete loss of the diamond. The nature of these wear features did not vary with rock type or diamond size when drilling the silicate rocks. In the soft marble abrasion of the diamonds was minimal and they tended to remain angular and to fracture without developing wear flats (Fig. 6(d)). or
i
i
2 2000 cl 1500 tz 5 1000 v L = 500 kf m o0
'
\ \ \ \ \ \ \ . .'\. ;-;.A
' 2
'
' 4
/ T/C /' ._F' . ,' l
' 6
BIT PRESSURE
'
1 8
'
10
12
IMPa)
Plot showing the effect of bit pressure on specific energy calculated from power consumption for tests in norite at set thrust.
Fig. 4.
Fig. 5. Scanning electron micrographs of wear flats: (a) developing wear flat on exposed edge; (b) large wear flat; (c) lightly grooved wear flat surface; (d) detail of grooved wear flat surface. (Arrows indicate direction of travel.)
5. Discussion
It is significant that the transition from suboptimal to steady drilling takes place at a mean diamond pressure of the same order of magnitude as the compressive strength of the rock and not of the diamond. At a low diamond pressure few diamonds indent the rock, the energy transmitted is low, and most of the exposed diamonds experience sliding and the consequent development of wear flats. Those diamonds that do indent the rock experience higher stresses and are more liable to fracture. As the pressure is increased the number of diamonds which indent the rock increases with a proportional decrease in sliding, and an increase in torque, rock fracture and diamond fracture. At the point at which the average pressure per diamond exceeds the indentation strength of the rock most of the exposed points indent the rock. A further increase in load causes greater depth of indentation rather than a greater number of indents, and sliding is greatly reduced in favour of rock and diamond fracture. This process is interactive, with the bit pressure and diamond wear ultimately controlhng the drilling performance.
(4
cc>
(4
Fig. 6. Scanning electron micrographs of diamond fracture: (a) developing microfracture; @> crystallographically controlled microfracture; (c) macrofractured diamond; (d) fractured diamond drilled in marble. (Arrows indicate direction of travel.)
The observed diamond wear is similar to the reported wear of diamonds in hard rock sawing [5, 6, 1 l] despite the higher speeds used in sawing. Sharp diamond points blunt rapidly in sliding relatively slowly on hard materials such as quartz over distances as short as 150 mm [ 121. In the tests reported in this paper the exposed diamonds rapidly developed rounded corners or edges when drilling silicate rocks. There was indirect evidence that the diamond surfaces had reached temperatures in excess of 700 “C at which the oxidation of diamond in the presence of air is possible [ 131. Sintered flakes of rock flour were a conspicuous component of the detritus produced by drilling the norite [7, 81, the major constituent of which is plagioclase felspar of labradorite composition. This melts at 1000-l 150 “C at a pressure of 500 MPa in the presence of water [ 141. The diamond wear flats experienced a calculated pressure of this magnitude and the sintering of rock flour indicates local temperatures of about 1000 “C or more. Thermally activated mechanisms play a crucial role in the wear of diamond at high speeds [ 151. As the initial rounding of sharp
diamond asperities showed no evidence of grooving it was concluded that it was caused by oxidation, and perhaps graphitization of the surface layers, and not directly by a mechanical process. The rate of wear of diamond depends on the nature of the material being rubbed, with an incubation time which also depends on the hardness of the rubbed material [ 2, 161. The incubation time for discernible abrasion of diamond in calcite is evidently longer than the test duration of about 10 mm. The absence of wear flats on diamonds drilled in marble also indicated that wear flats were not the result of abrasion by small released diamond particles. On the bits that had drilled marble there were a number of fractured diamonds which would have provided fme particles. The fractures were hack& and were probabiy caused by impact with loose diamonds. There is little evidence for dislocation movement in diamond at room temperature but plastic deformation is possible at 1000 “C [ 171. The potentially plastic layer is thin because of the high thermal conducted of di~ond. A sharp cold asperity traversing the heated diamond surface generates high local pressures and ploughs an essentially plastic, narrow, straight groove a few microns deep. A brittle crack forms at the bottom of the track in the cooler, less plastic body of the diamond. The groove and crack is widened
and deepened by subsequent
abrasion, oxidation, and graphitization.
The steps on the wear flat surfaces have three possible origins. They may be the expression of layers of differing physical strength due to different densities of growth defects on the (111) growth planes [ 181. They may be slip steps produced in accordance with the { 11 l}( 110) slip system of diamond [ 19 ] activated by frictional heating [ 201. They may result from tensile opening of cleavage cracks on { 11 l> planes [ 2 I]. As a result of their nearly ubiquitous presence on wear flats and the role they play in initiating the subsequent microfracture we believe the latter mechanism to be the most probable. The microfracture occurred by the removal of fragments released by the coalescence of cleavage microcracks opened up by sustained ~uctuat~g stresses on the surface as the diamonds indent, fracture and impact the hard silicate mineral grains. The diamond microfracture frequently started from the trailing edge which was more susceptible to failure in tension. Progressive subcritical crack growth has been recognized as generally important in the wear of diamonds where cyclical stressing of the surface is involved [ 2, 15, 16, 19, 20, 221. The microfracture observed here is strong evidence for a process of cumulative microdeformation in diamond.
6. Conclusions Cl) For any effective combination of bit and rock type there is a minimum threshold bit pressure above which microfracture is promoted over wear flat development, and steady drilling can take place as a result. This pressure must be sufficient for the average pressure per exposed diamond to exceed the compressive strength of the rock.
319
(2) Typically grooved wear flats which dominate in the suboptimal drilhng condition are produced by sliding. Cold hard silicate mineral asperities are capable of ploughing grooves through the heated, plastic surface layers of the diamonds. (3) Microfracture of worn diamond surfaces which dominates under steady drilling conditions takes place after sufficient strain build-up in response to cyclical loading which opens up cleavage microcracks to initiate failure. (4) Although the drilling performance in a given rock type is affected by changes in the drilling parameters the general nature of the diamond wear modes in drilling hard silicate rocks with impregnated diamond bits is independent of the drilling conditions or rock type.
Acknowledgments This work formed part of a cooperative research program with Boar-t Research Centre. Financial support from Boart and a research bursar-y from the CSIR is acknowledged with gratitude. The technical staff of the Department of Materials Engineering, University of Cape Town, are thanked for their considerable contribution.
References 1 C. M. Perrot, Tool materials for driiing and mining, Ann. Rev. Muter. Sci., 9 (1979) 23-50. 2 E. M. Wilks and J. Wiiks, The abrasion resistance of natural and synthetic diamond, Wear, 81 (1982) 329-346. 3 C. J. Bullen and M. W. Bailey, SDA 100 in hard rock drilling, Ind. Diamond Rev., 39 (1979) 352355. 4 C. J. Bullen, Hard rock drilling-some recent test results, In& Diamond Rev., 44 (1984) 270-275. 5 M. W. Bailey and C. J. Bullen, Sawing in the stone and civil engineering industries, Ind. Diamond Rev., 39 (1979) 48-52. 6 W. Ertingshausen, Wear processes in sawing hard stone, Ind. Diamond Rev., 4.5 (1985) 254-258. 7 D. E. Miller, Rock drilling with impregnated diamond microbits, Ph.D. Thesis, University
of Cape Town, 1986. 8 D. Miller and A. Bali, Rock driiing with impregnated diamond microbits-an experimental study, Int. J. Rock Mech. Min. Sci., 27 (1990) 363-371. 9 D. Miller and A. Ball, The evaluation of drill bit performance and rock drillability with an ~st~ented laboratory machine, J. S. A& Is&. Min. Me&U., in the press. 10 D. E. Miller, The wear of diamonds in rock drilling with impregnated microbits, Proc. Electron Microsc. Sot. S. Afr., 14 (1984) 167-168. 11 A. Biittner, Coolant additives in the sawing of granite, In&. Diamond Rev., 40 (1980) 332335. 12 G. J. Glover, The brittle and plastic response of quartz, Ph.D. Thesis, University of Cape Town, 1980. 13 T. Evans, Changes produced by high temperature treatment of diamond. In J. E. Field (ed.), The Properties of Deans Academic Press, London, 1979, pp. 403-424.
320
14 W. A. Deer, R. A. Howie and J. Zussman, An I~tro~~~~t~~. to the Rock Fat-ming ~~~(YI~, Longman, London, 1972. 15 D. Tabor, Adhesion and friction. In J. E. Field (ed.) T& Proj?erZ&s of Dionw&, Academic Press, London, 1979, pp. 326-350. 16 I_?.Crompton, W. Hirsc and M. G. W. Howes, The wear of diamond, Proc. R. Sot. ~~, Se?: A, 333 (1973) 435-454. 17 C. A. Brookes, V. R. Howcs and A. R. Parry, Multiple slip in diamond due to a nominal contact pressure of 10 GPa at 1000 “C, Nutwe (lkmdm.., 332 (1988) 139-141. 18 J. E. Field, Strength and fracture properties of diamond. In J. E. Field (ed.), The Properties oj’IXnnz&, Academic Press, London, 1979, pp. 282-324. 19 C. A. Brookes, Inden~tion hardness. In J. E, Field (ed.), The E4yqptrties of ~~~~, Academic Press, London, 1979, pp. 383-402. 20 F. P. Bowden and D. Tabor, Deformation, friction and wear of diamond. In R. Berman (ea.), Ph&cul Pxptrlies ofDiam.md, Clarendon, Oxford, 1965, pp. 184-220. 81 V. R. Howes, Ring cracks of diamond surfaces. In R. Berman (ed.), P~&xJ~ f+C$Oerties qf ~~~Y~~~~~,Clarendon. Oxford, 1965, pp. 174-183. 82 C. A. Brookes and R. M. Booper, Wear and hardness of new tool materials, &fet%fs socie& Book 27~7. Metals Society, London 1982, pp. 32-36.
311
141 (1991) 311-320
The wear of diamonds in impregnated diamond bit drilling Duncan Miller and Anthony Department of Materials (South Africa)
Engineering,
Ball University
of Cape Town, Rand.ebosch
7700
(Received December 21, 1989; accepted July 3, 1990)
Abstract The wear of diamonds in impregnated diamond microbits was investigated in the laboratory by drilling a variety of rock types. The wear was studied by optical and scanning electron microscopy after drilling under a range of bit pressures, rates of advance, and rotational velocities using bits of different diamond size and concentration. The nature of the diamond wear modes did not vary with the drilling conditions but the relative proportions of different types of diamond wear changed with drilling performance. For stable drilling in any given rock type a characteristic threshold pressure existed above which desirable microfracture of the exposed diamonds was promoted over undesirable wear flat generation. The flats are produced by a sliding wear mode with the silicate minerals ploughing plastic grooves in the heated surfaces of the diamonds. Microfracture mode is the result of progressive growth and coalescence of cleavage microcracks promoted by the cyclical stresses experienced by the diamonds at pressures sufficient to cause indentation of the rock.
1. Introduction
Diamonds have been used in rock machining operations such as drilling, sawing and grinding for many decades. Relatively little is known about the wear of diamond in such applications [ 11. Despite sustained research even the wear of diamond in general is not well understood [Z] and there is thus a need to document the wear behaviour of diamond in specific applications. It is known that in hard rock drilling with impregnated diamond bits the drilling performance is influenced by the strength of the diamonds, which also affects the predominant mode of diamond wear [3, 41. Attempts have been made to quantify diamond wear in hard rock sawing by optical inspection and classification of the worn diamonds into arbitrary categories [ 5, 61. A similar approach is used here in reporting the wear of diamonds in impregnated diamond microbits drilled in a variety of materials.
2. Machine and materials The tests were conducted in the laboratory using an instrumented drilling rig consisting of a modified pillar drill with computer-monitored transducers 0043-1648/91/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
to record net power consumption, torque, bit pressure, rotational velocity and penetration rate 17-9 1. Tests were conducted under both set thrust and set rate of advance conditions, varying the bit pressure, rate of advance, rotational velocity, diamond mesh size and concentration, and rock type. The drill bits used were impregnated diamond microbits with inner and outer diameters of 12 and 20 mm respectively and six 2 mm wide waterways placed symmetrically. The diamonds, ranging in size from 20-80 U.S. mesh, were De Beers SDA 100 synthetic grit in a sintered bronze matrix at concentrations of 30 and 50. (A concentration of 100 is conventionally defined as 0.88 g of diamond per cubic centimetre of imprecation: approximately 25% by volume.) Eight rock types were drilled. ‘Bushveld norite’ is a dense, moderately hard igneous rock consisting mostly of felspar and pyroxene. ‘Cape granite’ consists predominantly of large alkali felspar crystals with interstitial quartz and is very resistant to drilling. ‘Red granite’ is a readily d.rilIable syenite conta~g mostly large crystals of plagioclase felspar and hornblende. The marble used consists almost entirely of soft, fine-aged, rec~s~~zed calcite. ‘Jaspilite’ is a very fine-grained, very resistant cherty ironstone. The quartzite used is of moderate drilling resistance and is composed of partially recrystallized, moderately angular grains of quartz. The sandstone, selected for its abrasiveness, is weakly cemented with angular quartz grains in a micaceous matrix.
3. Testing procedure New bits and bits suitable for re-use were conditioned by drilling 0.3-0.5 m in the abrasive sandstone to remove a layer of diamonds and to expose fresh, randomly orientated crystals. The tests were about 1.5 m long, with rotational speeds equivalent to l-4 m s-‘, an average load per diamond of up to 5 kgf, and rates of advance up to 0.1 mm rev-‘. Tap water flowing at about 350 1 h- ’ at 200 kPa pressure was used as a coolant and fIushing medium. Diamond wear was evaluated visually with a stereo optical microscope at 40 X magnification and studied in the scanning electron microscope [lo]. The frequency of different wear types was recorded as a percentage of the total number of exposed stones on each bit. Previous work has shown that in impregnated bit drilling the bit pressure has a crucial role in dete~~g the drilling performance [7-lo]. If the bit pressure is too low for a given rock and bit combination then the pene~tion rate decreases steadily (if drilling under set thrust) or the reactive load increases to impede progress (if drilling at set rate of advance). Tests were conducted in both the suboptimal and steady drilling regimes to study the effect of bit pressure on diamond wear. The wear of each exposed diamond was classified at regular intervals after driIIing set incremental distances in norite.
313
4. Results
mica1 examples of diamond wear are illustrated in Fig. 1. Recently exposed, fresh diamonds are angular with sharp points and edges fJ?ig. l(a)).
(a>
(e> Fig. 1. Scanning electron micrographs of typical examples of diamond wear: (a) unworn diamond; (b) wear flat; (c) microfracture; (d) hacldy macrofracture flush with the matrix; (e) pull-out hole. (Arrows indicate direction of travel.)
Wear flats are typically grooved or striated and can develop on originally flat diamond faces or on abraded edges and points (Fig. 1 (b)). Macrofractured diamonds have a surface covered with multiple sharp points (Fig. l(c)) and grade into more severely fractured diamonds which may protrude very little from the matrix (Fig. l(d)). Entire diamonds or residual fragments may be lost through pull-out to form holes in the bit matrix (Fig. l(e)). Diamonds with wear flats (Fig, l(b)) predominated at low bit pressures associated with suboptimal drilling and a declining penetration rate (Fig. 2). At higher bit pressures microfractured diamonds (Fig. l(c)) were more numerous and steady drilling took place above a transitional bit pressure of 5-6 MPa with the particular bits used. Drilling at bit pressures above this transition produced a characteristic sequence of diamond wear development with microf~cture predom~at~g over wear flats (Fig. 3). The transition from suboptimal to stable drilling took place at a calculated mean pressure on individual diamonds of about 400 MPa in norite irrespective of mode of control, rate of advance, rotational velocity, diamond mesh size,
.
wear
-m-
microfracture
.
*
500’ DRILLING
,A.
a
macrofracture
l
pull-out
. I
2000
1500
1000
DISTANCE
flat
*
--_ .
“nwor”
-a-
(mm)
Fig. 2. Plot of diamond wear type percentage for tests drilled in norite at set bit pressure increments, showing increase in microfracture at higher thrust levels.
l
l
l
l *
l
l
4 .
.
l
-
_.--
p--
l.-_.. I 0
V---o__
) A 250
. -;-
. . v _I________-_----., .
.
wear
-w A
microfracture macrofracture
l
flat
pull-out
-
-.--_ .
,.
,
500
750
GRILLING
.
unworn
-*-
-*--_-.
I, 5300
DISTANCE
r(---:---’ 1250
I 1500
(mm)
Fig. 3. Plot of diamond wear type percentage against distance for tests d&led in norite at 6 MPa bit pressure showing the preponderance of microfracture over wear flat development with stable drilling.
315
diamond concentration. This pressure is of the same order of magnitude as the uniaxial compressive strength of the rock, measured as 287 + 28 MPa. The compressive strength of the diamonds measured between corundum anvils was 4.44 GPa [ 71. At the transition the specific energy of drilling was at a minimum (Fig. 4). This specific energy minimum occurred at higher bit pressures in stronger rocks, indicating that the threshold diamond pressure for the predominance of microfracture and steady drilling is dependent on the rock strength. Two different sequences of diamond wear were observed. In suboptimal drilling a freshly exposed diamond tended to develop a large, stable wear flat, as did the few stones that had fractured. With steady drilling at bit pressures above the transition a freshly exposed diamond tended to experience a more complicated sequence. Rounding of any sharp points led to the development of a transitory wear flat which broke up by microfracture, with a possible cyclical repetition of these two stages. Eventually wholesale failure reduced the diamond to splinters with ensuing loss in most cases. The wear flats were striated with grooves of variable depth in the direction of travel (Figs. 5(a) and 5(b)). The finer grooves were narrow and straight. At high magnification it was evident that they had rounded intervening ridges and median cracks extending into the body of the stone (Figs. 5(c) and 5(d)). Usually, parallel steps were developed more or less transverse to the direction of abrasion (as indicated in Fig. 6(a)). These steps contributed to the eventual microfracture of the surface when drilling at pressures above the transition. Crystallographically controlled microfracture produced asurface covered by numerous points 0.01-o. 1 mm in size (Fig. 6(b)) which promoted stable drilling. More extensive hackly fracture led to minimal projection of the diamond from the bit matrix (Fig. 6(c)) or complete loss of the diamond. The nature of these wear features did not vary with rock type or diamond size when drilling the silicate rocks. In the soft marble abrasion of the diamonds was minimal and they tended to remain angular and to fracture without developing wear flats (Fig. 6(d)). or
i
i
2 2000 cl 1500 tz 5 1000 v L = 500 kf m o0
'
\ \ \ \ \ \ \ . .'\. ;-;.A
' 2
'
' 4
/ T/C /' ._F' . ,' l
' 6
BIT PRESSURE
'
1 8
'
10
12
IMPa)
Plot showing the effect of bit pressure on specific energy calculated from power consumption for tests in norite at set thrust.
Fig. 4.
Fig. 5. Scanning electron micrographs of wear flats: (a) developing wear flat on exposed edge; (b) large wear flat; (c) lightly grooved wear flat surface; (d) detail of grooved wear flat surface. (Arrows indicate direction of travel.)
5. Discussion
It is significant that the transition from suboptimal to steady drilling takes place at a mean diamond pressure of the same order of magnitude as the compressive strength of the rock and not of the diamond. At a low diamond pressure few diamonds indent the rock, the energy transmitted is low, and most of the exposed diamonds experience sliding and the consequent development of wear flats. Those diamonds that do indent the rock experience higher stresses and are more liable to fracture. As the pressure is increased the number of diamonds which indent the rock increases with a proportional decrease in sliding, and an increase in torque, rock fracture and diamond fracture. At the point at which the average pressure per diamond exceeds the indentation strength of the rock most of the exposed points indent the rock. A further increase in load causes greater depth of indentation rather than a greater number of indents, and sliding is greatly reduced in favour of rock and diamond fracture. This process is interactive, with the bit pressure and diamond wear ultimately controlhng the drilling performance.
(4
cc>
(4
Fig. 6. Scanning electron micrographs of diamond fracture: (a) developing microfracture; @> crystallographically controlled microfracture; (c) macrofractured diamond; (d) fractured diamond drilled in marble. (Arrows indicate direction of travel.)
The observed diamond wear is similar to the reported wear of diamonds in hard rock sawing [5, 6, 1 l] despite the higher speeds used in sawing. Sharp diamond points blunt rapidly in sliding relatively slowly on hard materials such as quartz over distances as short as 150 mm [ 121. In the tests reported in this paper the exposed diamonds rapidly developed rounded corners or edges when drilling silicate rocks. There was indirect evidence that the diamond surfaces had reached temperatures in excess of 700 “C at which the oxidation of diamond in the presence of air is possible [ 131. Sintered flakes of rock flour were a conspicuous component of the detritus produced by drilling the norite [7, 81, the major constituent of which is plagioclase felspar of labradorite composition. This melts at 1000-l 150 “C at a pressure of 500 MPa in the presence of water [ 141. The diamond wear flats experienced a calculated pressure of this magnitude and the sintering of rock flour indicates local temperatures of about 1000 “C or more. Thermally activated mechanisms play a crucial role in the wear of diamond at high speeds [ 151. As the initial rounding of sharp
diamond asperities showed no evidence of grooving it was concluded that it was caused by oxidation, and perhaps graphitization of the surface layers, and not directly by a mechanical process. The rate of wear of diamond depends on the nature of the material being rubbed, with an incubation time which also depends on the hardness of the rubbed material [ 2, 161. The incubation time for discernible abrasion of diamond in calcite is evidently longer than the test duration of about 10 mm. The absence of wear flats on diamonds drilled in marble also indicated that wear flats were not the result of abrasion by small released diamond particles. On the bits that had drilled marble there were a number of fractured diamonds which would have provided fme particles. The fractures were hack& and were probabiy caused by impact with loose diamonds. There is little evidence for dislocation movement in diamond at room temperature but plastic deformation is possible at 1000 “C [ 171. The potentially plastic layer is thin because of the high thermal conducted of di~ond. A sharp cold asperity traversing the heated diamond surface generates high local pressures and ploughs an essentially plastic, narrow, straight groove a few microns deep. A brittle crack forms at the bottom of the track in the cooler, less plastic body of the diamond. The groove and crack is widened
and deepened by subsequent
abrasion, oxidation, and graphitization.
The steps on the wear flat surfaces have three possible origins. They may be the expression of layers of differing physical strength due to different densities of growth defects on the (111) growth planes [ 181. They may be slip steps produced in accordance with the { 11 l}( 110) slip system of diamond [ 19 ] activated by frictional heating [ 201. They may result from tensile opening of cleavage cracks on { 11 l> planes [ 2 I]. As a result of their nearly ubiquitous presence on wear flats and the role they play in initiating the subsequent microfracture we believe the latter mechanism to be the most probable. The microfracture occurred by the removal of fragments released by the coalescence of cleavage microcracks opened up by sustained ~uctuat~g stresses on the surface as the diamonds indent, fracture and impact the hard silicate mineral grains. The diamond microfracture frequently started from the trailing edge which was more susceptible to failure in tension. Progressive subcritical crack growth has been recognized as generally important in the wear of diamonds where cyclical stressing of the surface is involved [ 2, 15, 16, 19, 20, 221. The microfracture observed here is strong evidence for a process of cumulative microdeformation in diamond.
6. Conclusions Cl) For any effective combination of bit and rock type there is a minimum threshold bit pressure above which microfracture is promoted over wear flat development, and steady drilling can take place as a result. This pressure must be sufficient for the average pressure per exposed diamond to exceed the compressive strength of the rock.
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(2) Typically grooved wear flats which dominate in the suboptimal drilhng condition are produced by sliding. Cold hard silicate mineral asperities are capable of ploughing grooves through the heated, plastic surface layers of the diamonds. (3) Microfracture of worn diamond surfaces which dominates under steady drilling conditions takes place after sufficient strain build-up in response to cyclical loading which opens up cleavage microcracks to initiate failure. (4) Although the drilling performance in a given rock type is affected by changes in the drilling parameters the general nature of the diamond wear modes in drilling hard silicate rocks with impregnated diamond bits is independent of the drilling conditions or rock type.
Acknowledgments This work formed part of a cooperative research program with Boar-t Research Centre. Financial support from Boart and a research bursar-y from the CSIR is acknowledged with gratitude. The technical staff of the Department of Materials Engineering, University of Cape Town, are thanked for their considerable contribution.
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