FRICTION BETWEEN A CEMENTED CARBIDE AND
DIFFERENT ROCK TYPES
U. BESTE, S. JACOBSON
Uppsala University, The Ångström Laboratory, Box 534, 751 21 Uppsala, SWEDEN;
e-mail: [email protected]
SUMMARY
WC/Co cemented carbide is the most common material for rock drilling because of its superior combination of
toughness and hardness. The cemented carbide buttons wear is primarily caused by abrasion and accelerated by the
material degradation due to repeated impact and thermal cycling, which induce surface cracks. The heat generated is
linear to the friction coefficient. To elucidate the relationship between the known wear mechanisms and the sliding
friction performance, a friction test series with a 94 wt % WC / 6 wt % Co cemented carbide rock drill button sliding
against polished granite, magnetite, hematite, quartzite, calcite, granulite and sandstone has been performed in a pin-ondisc tribometer. The test was performed in 25 °C air, at 350 °C and also in water. The load was 20 N and the velocity
was 0,26 m/s. Scanning electron microscopy, energy dispersive spectroscopy and light optical microscopy were used to
analyse the surfaces. At room temperature the cemented carbide exhibited the highest stable dry friction against calcite,
and the lowest against granulite. At 350 °C granulite gave the highest friction and hematite the lowest. In water, granite
showed the highest friction and quartzite the lowest. A “remaining water lubricant effect” was noticed where calcite was
able to keep low, water lubricated friction value even when seemingly run dry, while granulite, sandstone and hematite
were not. In this investigation, no direct correlation between drillability, bit life and friction could be seen.
Keywords: cemented carbide, friction, rock, drilling
1
INTRODUCTION
Owing to its unique combination of hardness (from 2100
to 3000 HV50) and toughness (up to 30 MNm −3 2 ),
WC/Co is the dominant material in rock working tools
[1]. However, when drilling or milling in rock the
cemented carbide is exposed to many different rock
types at high loadings, and the extreme conditions lead
to some special types of wear [2]. Usually, abrasion is
the dominant wear mechanism, while sometimes the
haphazard cracks from the reptile skin also become very
important. Reptile skin is as a surface fatigue effect that
leads to large scale cracks that does not form against
rock types with high abrasiveness. However, it is
unknown if the reptile skin is continuously worn away,
or if it does not form at all [3].
A typical rock drill hits the rock at 50 Hz with a
hydraulic impact pressure of 17-20 MPa and a feed
pressure of 9-10 MPa, while rotating 75-200 rpm. It is
flushed with high-pressure water to keep the temperature
low and to blow the crushed stone away [3]. The exact
working temperature on the rock drill buttons is not
known, since so many parameters are involved. Clark
has mentioned that during impact, typical temperatures
are 450 to 500 °C and at equilibrium 150 °C [4]. Clark
also mentioned (referred from older articles) the
temperature 320 °C.
A problem in mining is the varying character of the
rocks. Even within the same ore a drill becomes exposed
to different wear mechanisms and of course, the friction
also varies. As always, the friction depends on the
sliding materials (rock types), their surface roughness as
it forms during drilling, the temperature, the water
presence and the presence of cuttings between the drill
and the solid rock.
This article describes a simple test, intended to give an
idea of the friction behaviour when the drill button
slides against different rock types under mild, but well
controlled conditions. The friction between one rock
drill grade and seven common rock types are measured.
2
MATERIALS AND EXPERIMENT
The tested rock types and their reptile skin forming
tendencies and abrasiveness are listed in table 1.
Rock
Compo- Hard- Reptile Abrasivetype
sition
ness
skin?
ness?
Magnetite Fe3O4
361
Yes
Very small
Hematite Fe2 O3
486
No
Medium
CaCO
754
No
Very small
Calcite
3
Granite
*
810
No
Medium
Granulite *
1028
No
Medium
Sandstone *
1230
No
High
SiO2
Quartz
1417
No
High
The * denotes multi-mineral rock types.
Table 1. The tested rock types compositions and their
measured average hardness. Reptile skin tendencies and
abrasiveness from [3].
The tested rock types can be divided into two groups.
One group with isotropic character, including magnetite,
hematite, calcite and quartz. The second group
containing complex (or multi-mineral) rocks including
granite, granulite and sandstone.
This division is mainly a question of grain sizes and
mineral types in the rock. The granite has large grains of
different minerals, and magnetite small grains of just
magnetite. Granite consists of quartz, felspar, mica and
0,2
Quartzite
Sandstone
Granite
Granulite
Hematite
Magnetite
Calcite
In water
Granite
Sandstone
0,6
0,4
0,2
Quartzite
Granulite
Calcite
0
Figure 2. The stable friction for all rock samples in air
at 25 °C, 350 °C and in water. The friction is
considered stable after 20 minutes of sliding.
1
100
0,8
80
0,6
60
0,4
40
0,2
20
Quartzite
Sandstone
Granulite
Granite
0
Calcite
0
Percent of the friction in air
Friction, run dry after the water test
Percent of the friction in air
The friction coefficient
During the wet tests, the water could be emptied from
the specimen holding container. When run dry like this
an interesting effect could be noted. After seemingly run
dry, the friction coefficient did not climb to the normal
dry friction values for all the materials. This remaining
water lubricant effect is shown in Fig. 3.
At 350 °C
0,8
RESULTS
The dry friction values initially varied between 0,37 and
0,72 and generally rose up to between 0,46 and 0,87
before stabilising, see Fig. 1 and Fig. 2. At the higher
temperature the initial friction was typically similar to
the room temperature values, while they stabilised at
lower values. Granulite stands out from the other rock
types by consistently exhibiting substantially higher
friction at the higher temperature.
In air
Magnetite
3.1
0,4
1
Friction coefficient
3
0,6
Hematite
The tests run in water were also used to give a friction
when run dry. This feature provides a special effect; that
is the remaining lubricant effect from water. When the
samples went dry after a couple of minutes testing, the
friction changed. By filling in new water the friction
could be cycled back to its former values.
0,8
Magnetite
The friction was considered stable after 20 minutes of
sliding. However, at 350 °C, the friction was considered
stable after 4 minutes when high wear rate prevented
longer test times.
In water
Figure 1. Start friction for all rock samples in air at
25°C, 350 °C and in water. The start friction is
measured after 10 s when sliding on polished samples
before a smoother sliding track has formed.
Stable friction coefficient
A start friction is measured after 10 seconds, before any
substantial polishing or wear of the rock could be
observed. However, the friction between a rock drill and
the rock is obtained during extremely short times, and
on surfaces quite different from the polished surfaces in
this test.
At 350 °C
0
A common cemented carbide rock drill button, with 6
wt.% Co, 94 wt.% WC and with 2.5 µm large WCgrains was sliding against these rock specimens in a pinon-disc tribometer.
The tests were performed in normal air, with 20 N load
and 0,26 m/s velocity, in at least 20 minutes. The
velocity corresponds to the true velocity of a rock drill
button. In rock drilling, the load on each button is about
2 kN. In this tests the rocks are not intended to be
crushed and load is set to 20 N.
In air
Hematite
The rock samples were set in a protecting resin (or in
cement for the test performed at 350 °C) and polished
using 3 µm diamond on cloth as a final step. The
hardness values presented in table 1 are the mean values
of six micro hardness indents at 25, 50 and 500 g,
respectively.
1
Start friction coefficient
hornblende of different grain sizes. Granulite is a
mixture of 50-1000 µm-grained minerals mainly
consisting of silicon acid rich rock types. Sandstone is a
sedimentary rock type with 200-2000 µm grain size [5].
Figure 3. Friction coefficient in the seemingly dry
sliding conditions occurring when the wet tests run dry.
The relation to the ordinary dry friction level is also
indicated.
3.1.1
In air at 25 °C
The cemented carbide buttons showed the highest
stable dry friction against calcite, µ=0,87, and the
lowest against granulite, µ=0,46.
Magnetite, hematite and quartzite showed lower
initial friction than after stabilising while the other
rocks showed higher initial friction.
3.1.2
In air at 350 °C
Granulite resulted in the highest friction with µ=0,80
and hematite the lowest, µ=0,42.
Against magnetite and quartzite the initial friction
was lower than the stable level while the other rocks
gave the opposite behaviour.
Hematite and granulite showed higher start friction
at 350 °C than at 25 °C. Only granulite showed
higher stable friction at 350 °C than at 25 °C.
3.1.3
Figure 4. Magnetite debris in sliding track from water
test, located in the magnetite grain borders.
In water
Granite showed the highest friction, µ=0,21 and
quartzite (the hardest rock) the lowest, µ=0,06.
In water, all rock samples showed a polishing effect
after running-in, leading to lower friction.
3-5 µm large magnetite mineral wear particles
adhere to magnetite grain borders in sliding track.
3.2
The sliding track appearance
The wear of the cemented carbide was not studied.
However, a light survey of the rock wear was
performed, based on examination in optical and
scanning electron microscope.
In all room tempered air and water tests, the sliding
tracks on the rocks look similar. No significant
differences in wear rate could be seen. At 350 °C, the
wear differed more between the rock types; the lowest
wear resistance at high temperature was had by calcite
followed by granite, which also cracked. The other rocks
had equal wear resistance, and the quartzite had the
highest resistance.
The magnetite showed an exceptional phenomenon, as
seen in Fig. 4. After sliding in water, the track was
covered with wear particles, typically of two sizes,
0.5 µm and 3-5 µm large. The larger were located at the
grain borders and valleys while the smaller were found
everywhere. EDS-analysis showed that these particles
consisted of magnetite. In dry tests, the sliding track was
covered by a brown tribofilm of rust, not allowing the
particles to allocate to borders.
The hematite sliding track showed nothing remarkable,
but the friction was very sensitive and increased highly
when running dry in water. That means that the hematite
surface in Fig. 5 allows boundary lubrication only when
there is sufficient amount of water around.
The dry sliding track on sandstone is showed in Fig. 6.
Here, the large grains have become abraded to form
polished plateaus. Sandstone had the highest start
friction in air, occurring when the grains are unpolished.
Figure 5. Sliding track formed on the hematite sample
in the wet test. The original surface is seen in the upper
right corner. The friction coefficient raised highly when
run dry.
Figure 6. Sliding track formed on the sandstone sample
in the dry, 25 °C test.
4
4.1
DISCUSSION
The remaining water lubricant effect
The friction of the rock types showed different
behaviour when running dry after running in water.
When sandstone, granulite and hematite went dry, the
friction values returned to the level in the dry tests, see
Fig. 3.
On the contrary, when quartzite, granite and magnetite
went dry in the water tests, the friction rose to about half
of the dry stable friction. Thus, a remaining lubricant
effect from water could be seen. The calcite showed the
largest remaining effect: friction remained at 0,1-0,2
every time the rock sample ran dry, compared to the dry
friction in air that was measured to be 0,87.
wear rate and a long bit life. However, a comparison
between the drillability of a rock, bit endurance and the
present friction data gives no distinct correlations. This
implies that the differences in friction between rocks and
drill bits are of minor importance to the wear
mechanisms.
The significant differences between the tested rock
types, are probably due either to different abilities to
adhere or adsorb water on the surface or within the
structure, or due to different requirements on the amount
of water molecules needed to provide a substantial
friction reduction.
5
4.2
No direct correlation was found between friction
values and the reptile skin forming effect.
No obvious connection between the isotropy of a
rock and its friction could be drawn.
A "remaining water lubricant effect" has been
observed, implying that the friction stays low for a
period although the water seemingly has
disappeared from the surface. The length of this
period differs significantly between the tested rock
types, due either to different abilities to adhere or
adsorb water on the surface or within the structure,
or due to different requirements on the amount of
water molecules needed to provide a substantial
friction reduction.
Assuming that the friction in rock drilling is best
represented by the start friction in water, the
differences in friction are relatively small: from the
granite level, µ=0,27 to hematite µ=0,21.
In this investigation, no direct correlation between
drillability, bit life and friction could be seen.
Comparison between the similar rock types
magnetite and hematite
The differences between hematite and magnetite
include:
Magnetite has higher start friction in air and in water
but lower at 350 °C.
A magnetic effect occurs in water, which leads to the
localisation of wear particles at magnetite grain
borders.
Hematite gives a stronger remaining water lubricant
effect.
Hematite has higher hardness.
From this it is difficult to draw conclusions about
connections between the friction values and wear.
Magnetite is known to produce reptile skin and has one
of the largest dry friction values. However, some rock
types give higher dry friction but do not produce reptile
skin.
Probably, the wear mechanisms depend much more on
the character of the rock than on the friction coefficient.
The fast percussions in rock drilling could lead to
extremely high normal loads, which crushes the rock
and minimizes the sliding.
4.3
Friction, drill rate index and tool endurance
The drill rate index (or drillability) of a rock is a
measure of how easy it is to drill a rock type. Another
common measure is the bit life. These two measures
may correlate, but normally they do not. This means that
a rock that is very difficult to drill may still give a low
CONCLUSIONS
6
REFERENCES
[1] Exner, H.E.: Physical and chemical nature of
cemented carbides. International Metals Review, 4
(1979) , 149-173
[2] Larsen-Basse, J.: Wear of hard-metals in rock
drilling: a survey of the literature. Powder Metallurgy,
16 (1973) No. 13, 1-32
[3] Beste, U., Hartzell, T., Engqvist, H., and Axén, N.:
Surface damage on cemented carbide rock drill buttons.
Wear, 249 (2001) 3-4, 324-329
[4] Clark, G.B.: Principles of rock drilling and bit
wear. Colorado school of mines Quartely, (1982)
[5] Fredén, C.: Sveriges Nationalatlas - Berg och Jord.
2:nd ed. Sveriges Nationalatlas, ed. U. Arnberg. 1998.