Friction and wear of carbonate rocks under high velocity sliding BONEH1, 3, Yuval, SAGY2, Amir, and RECHES3, Ze'ev 1 Earth and Planetary Sciences, Washington University, One Brooking Drive, MO 63130. 2 Geological Survey of Israel, 30 Melkhe Israel St., Jerusalem, Israel. 3 School of Geology & Geophysics, University of Oklahoma, Norman, OK 73019.

Motivation: Friction and wear relations

Rotary Shear Apparatus

Wear-rate calculations

Experimental faults

Observations: Fault slip surfaces

Dover limestone Kasota dolomite

Thermal Correction - The displacement normal to the

slip direction (FND) is first corrected for the thermal

expansion of the sample due to frictional heating.

Fitting and differentiate – The FND signal is fitted

and differentiate to determined the wear-rate in units of:

[μm/m] - (micron of fault wear/ meter of fault slip)

We analyze here the wear-rate and friction

coefficient during the steady-state state.

Steady-state stage Transient

stage

Non-linear Coulomb friction

Slip along faults is always associated with frictional resistance and

surface wear. The dominant factors that control friction and wear are:

rock composition (lithology, grain size, mineral hardness),

environmental conditions (fluids chemistry/pressure, humidity,

temperature), fault-zone conditions (roughness, hardness, gouge

thickness, gouge grain size), and mechanical loading (normal stress,

slip velocity, pore pressure).

Observations: Wear–rates and Friction coefficients

The two graphs on the right display wear-rates

as function of slip velocity. They indicate:

Under Low velocity :

Wear-rates depend on normal stress

Under High velocity:

1. Wear-rates are low

2. Wear-rates do NOT depend on normal

stress (in contradiction with Archard

equation, below)

G – wear volume; K – material constant

H – hardness; x – slip distance ; σn – normal stress

Archard’s wear equation - Archard (1953) wear equation states that

total wear is linearly related to slip distance and normal stress.

Rotary shear apparatus at Oklahoma University

(Reches & Lockner (2010).

The experiment configuration (left) includes: an

Upper block (UB) with a ring-shaped contact

sliding on a flat, lower block (LB). Two metal

disks, which are fixed on each block, host eddy

currents sensors (in green) that measure the fault

normal displacement (FND). Thermocouples

(TC) are embedded in the upper block ~ 3 mm

from the slip surface. We continuously monitored:

shear and normal stress, velocity and FND

Friction coefficient and Power - density The friction coefficient depends on BOTH slip velocity normal stress (fig. below, left).

It correlates best with the power-density (Fig. below, right).

Power Density = Shear stress * Velocity [Work per unit square], namely

Three friction regimes are apparent:

1. High friction coefficient (μ ~ 0.9) under low power-density of < 0.05 MW/m2.

2. Low friction coefficient (μ ~ 0.3) under high power-density > 0.4 MW/m2.

3. Transition zone for intermediate power density of 0.05 – 0.4 MW/m2.

𝜏 ∗ 𝑉 = 𝑀𝑃𝑎 ∗𝑚

𝑠=

𝑀𝑊

𝑚2

Experimental settings

We experimentally analyze the wear and friction of experimental faults

made of three carbonate rocks: Kasota dolomite (KD), Dover limestone

(DL), and dolomite-quartzite pair (KDBQ), and tested a wide range of

normal stress (up to 7 MPa) and up to seismic slip-velocity (1 m/s).

The main objectives:

A. Determine the effects of normal stress and slip velocity on wear-rate;

B. Investigate the relations between wear-rate and frictional strength.

UB

LB

TC

Lower blocks of Kasota dolomite and Dover limestone samples after low-velocity

experiments. The ring areas that are covered with light colored, gouge powder are the

sliding fault slip surfaces. They are surrounded by inner and outer rings of ejected

gouge.

We found above that the wear-rate and friction depends on both slip velocity and normal stress. An effective way to present these

relations is by Wear map and Friction map (below) that are frequently used in the engineering literature to depict frictional

resistance and wear-rate behavior of materials under a wide range of loading conditions. The wear and friction maps for carbonate

rocks (below) can be used for predicting fault behavior.

Wear and Friction Maps

Wear map Friction map

Predicted dynamic friction for various velocities

Synthesis: Fault surface evolution

Thermally decomposed carbonate slip surfaces:

A below displays a transition from gouge powder (bright

yellow, P) to patches of decomposed crust (light brown,

DC) on the slip surface. Dark gray area (R) are exposed

rock surfaces. Dolomite on quartzite sample under

V = 0.14 m/s.

B below displays continuous coverage by the decomposed

shining surfaces in limestone, formed under V = 0.36 m/s.

In an ideal Coulomb material, the shear stress

is a linear function of the normal stress.

We observed however (left), that

where a and b are constants, and where b is a

function of slip velocity (right).

These relations indicate more ductile behavior

at high velocities.

= 𝒂𝒏 𝒃

Under low slip–velocity (namely: high friction and low

temperature), the slip surfaces are covered with powder (A below).

Under high slip–velocity (namely: low friction and high

temperature), the slip surfaces are covered with hard, dark and

shinny crust (Siman-Tov et al., in pre). These surfaces were

probably created by thermal decomposition (Han et al, 2007).

A B

Queener et al. 1965

V = total wear

β, n = constants

x = slip distance

K = steady-state

1.5 mm

B

A

P

DC

R

Brittle regime:

Low velocity and high

normal stress =

High wear-rate, high

friction, and gouge

covered surface. Brittle Ductile

Transition

Combined: Wear and Friction maps

Ductile regime:

High velocity and

high normal stress =

Low wear-rate, low

friction, and shiny,

hard surface.

- Arachard, J. F., 1953. Contact rubbing of flat surfaces. Journal of Applied Physics, v. 24, 8, p. 981-988.

- Boneh, Y., 2012. Wear and gouge along faults: Experimental and mechanical analysis. MS Thesis, U of Oklahoma.

- Han, R. et al., 2007, Ultralow friction of carbonate faults caused by thermal decomposition: Science, v. 316.

- Queener, C.A., et al., 1965, Transient wear of machine parts: Wear, v. 8, p. 391-400.

- Siman-Tov et al., in prep.

Wea

r-ra

te [m

m/m

]

Wea

r-ra

te [m

m/m

]

• Two regimes of wear-friction-fault morphology in experimental carbonate faults:

• Brittle regime

• Conditions: Low velocity and high normal stress

• Fault properties: High wear-rate, high friction (m > 0.7) , gouge covered surface

• Deformation mechanism: asperity fracture, three-body shear

• Ductile regime

• Conditions: High velocity (seismic) and high normal stress

• Fault properties: Low wear-rate, low friction ( m < 0.5), shining-hard surface

• Deformation mechanism: plastic flow, thermal decomposition

Conclusions