Steady-state Friction Coefficient Research Articles

Multiwall Carbon Nanotubes for Solid Lubrication of Highly Loaded Contacts

When lubrication of rolling bearings with oil or grease is not possible, for example because the lubricant evaporates in vacuum, solid lubrication by multiwall carbon nanotubes (MWCNT) is a viable alternative. To understand the mechanisms underlying MWCNT lubrication of highly loaded contacts, we combine an experimental approach with large-scale molecular dynamics (MD) simulations. Tribometry is performed on ground iron plates coated with two different types of MWCNTs by electrophoretic deposition. Although structural differences in the MWCNT materials result in slightly different running-in behavior, most of the tests converge to a steady-state coefficient of friction of 0.18. The resulting wear tracks and tribolayers are subjected to structural and chemical characterization and suggest a tribo-induced phase transformation resulting in tribolayers that consist of MWCNT fragments, iron oxide, and iron carbide nanoparticles embedded in an amorphous carbon matrix. Covalent bonding of the tribolayer to the iron surface and low carbon transfer to the alumina counter body indicate sliding at the tribolayer/ball interface as the dominant mechanism underlying MWCNT solid lubrication. MD simulations of nascent a-C tribofilms lubricated by MWCNT bundles and stacks of crossed MWCNTs reveal two different sliding regimes: a low-load regime that leaves the MWCNTs intact and a high-load regime with partial collapse of the tube structure and formation of a-C regions. The critical load for this transition increases with the filling ratio of the MWCNT and the packing density of the stacks. The former determines the stability of the MWCNT, while the latter controls the local stresses at the MWCNT crossings. For both MWCNT materials, the high-load regime is predicted for the experimental loads. This is confirmed by a remarkable agreement between transmission electron microscopy (TEM) and atomistic simulation images. Based on the findings of this work, a multistep lubrication mechanism is formulated for MWCNT coatings rubbing against alumina on an iron substrate.

Tribological behavior of tribo-pairs in water hydraulic radial piston pumps

Friction and wear tests were performed on five Cu-based materials and three polyether ether ketone (PEEK)-based composites sliding against 17-4PH and 42CrMo4 coated with tungsten carbide-reinforced Ni-based coating. Two types of pin-on-disk specimens were used to represent the slipper/eccentric drive and piston/sleeve gap seal tribo-pairs in water hydraulic radial piston pumps. The experimental results showed that the QSn8-0.3 slipper material exhibited the highest steady-state friction coefficients for sliding against both 17-4PH (0.156) and coated-42CrMo4 (0.894). Glass fiber-reinforced PEEK exhibited the highest steady-state friction coefficients for sliding against the two specimens (0.540 and 0.420). The ZCuPb15Sn8/17-4PH showed the highest wear rate for the representative slipper/eccentric drive tribo-pair (26.86 × 10−6 mm/Nm); glass fiber-reinforced PEEK/17-4PH showed the highest wear rate for the representative piston/sleeve gap seal tribo-pair (46.91 × 10−6mm/Nm). The wear mechanisms of the two types of tribo-pairs were characterized through scanning electron microscopy, profilometry, and energy-dispersive spectroscopy analyses. Furthermore, the effects of hardness, pv values, coating, and reinforcement on the tribological behavior of the two types of tribo-pairs were analyzed.

Frictional behavior and micro-damage characteristics of rough granite fractures

Natural faults or discontinuities in general are usually rough at different scales, and the roughness of fault surfaces plays a dominant role in determining the mode of sliding (stable or unstable). Understanding the frictional behavior of rough fractures is important for investigating the mechanics and nucleation of earthquakes, and other dynamic geo-hazards such as induced seismicity and fault slip rockburst. We perform direct shear tests on rough granite fractures to investigate the shear behavior, microscopic asperity damage and surface wear characteristics under different normal stresses (10 to 40 MPa). We then compare our test results with those obtained using ground saw-cut fault surfaces. Results indicate that stick-slip occurs on all the rough fractures during the sliding stage, and the amount of stress drops and fault gouges tends to increase with normal stress. The peak friction coefficient is much higher than that of ground fault surfaces, while the steady-state friction coefficient is comparable with that of the ground surfaces. We identify three different scales of brittle fractures, which are considered to be closely related to the stick-slip sliding, namely shear-off of asperities, fracturing of the survived asperities and off-fault tensile fractures propagating into the host rock. The micro-damage of the post-shear surface occurs in the form of powder-sized gouge, micro-cracks and grain edge wear; and the micro-crack number, maximum length and aperture tend to increase with increasing normal stress. Influence of different fault types (rough fractures, ground saw-cut surfaces and gouges) on the stress drop is not very discernable, which is partly because that stick-slip is highly susceptible to different loading conditions. Friction drops for different fault types are found to be clustered around 0.1 but lower than 0.3 below 50 MPa normal stress, which also depend on the loading conditons (e.g., stiffness). Our study contributes to a better understanding of the mechanics underlying the dynamic geohazards (earthquakes, induced seismicity, fault slip rockburst) which are associated with the unstable shear failure on rough faults or fracture surfaces.

Visualization of stick-slip shear failure process of granite by 3D reconstruction technique and DEM

Understanding the mechanism of stick-slip instability of faults is of great significance for earthquake forecast and seismic fortification. In this paper, combined with the 3D reconstruction technology and discrete element method (DEM), the stick-slip shear failure process is visualized and the evolution mechanism of each stage of stick-slip process is studied. First, the 3D morphology scanning tests are carried out to define the 3D topography of the sample interface, which is imported into PFC3D to generate particle assembly, and micro-parameters of the particle assembly are calibrated by the experimental results. Second, the loading conditions are applied to the particle assembly, the stick-slip instability of the simulated fault occurs spontaneously, and the evolution of fault displacement and velocity distribution during the whole stick-slip period are studied. Third, the effects of normal stress, shear loading rate and interface roughness on stick-slip instability are investigated. Finally, nine parameters related to the steady-state friction coefficient, namely normal stress, natural density, Young's modulus, Poisson's ratio, maximum static friction coefficient, reference slip velocity, reference friction coefficient, a-b and slip velocity, are selected to establish the steady-state friction coefficient forecast model based on back propagation neural network (BPNN), and the relative importance analysis of relevant parameters is also implemented. Our research is helpful to reveal the seismogenic mechanism.

Utilization of Metallurgical Slags in Cu-free Friction Material Formulations

The aim of our research was to indicate the suitability of metallurgical slags (two blast furnace slags and one steel furnace slag) as the components of Cu-free friction materials. The base mixture consisted of nine components including phenolic resin, graphite, tin sulphide, steel and aramid fibers, iron powder, a mixture of barite with calcite, and vermiculite. To this base mixture, the slags with a particle size below 0.1 mm were added individually in the amount of 20 wt.%. A base friction mixture with alumina in the amount of 20 wt.% represented the reference. Samples for the friction-wear tests were produced in the form of pins by hot press molding. The prepared pins were tested using a pin-on-disc tester in a drag mode at the pressure of 1 MPa and a constant sliding speed of 1.51 m/s for 90 min. The samples with slags exhibited slightly lower values of steady-state friction coefficient compared to the reference composite with alumina, and at the same time produced lower wear particle emissions. The particle concentration was reduced for the samples with slowly cooled blast furnace and steel furnace slag. The results obtained indicated steel furnace slag as a promising component of Cu-free friction composites.

Achieving Superlubricity of Ricinoleic Acid in the Steel/Si3N4 Contact Under Boundary Lubrication

To improve fuel efficiency and minimize environmental pollution, the interest in organic friction modifiers has resurged, especially fatty acids. Efforts have been made to enhance fatty acids’ performance by tuning their saturation state and adding OH lateral groups in the alkyl chain. Here, we compared lubrication properties in the boundary regime for oleic acid (OA), ricinoleic acid (RA), and linoleic acid (LA) under the same viscosity for a steel/Si3N4 tribo-pair. The presence of the OH group in the OA chain reduces not only friction coefficient (CoF) by ~ 70% but also prevents scratches from generating on both surfaces. In the meantime, it determines local friction distribution by controlling top surface terminations. Even at 100 °C, the steady-state friction coefficient of steel/Si3N4 lubricated by RA is only 0.014. A temperature decrease to 80 °C leads to a superlubricity regime (CoF < 0.01) resulting from the cooperation of nanometer-thick liquid film (possibly containing water) and low friction of contact asperities terminated by OH hydroxyl groups provided by RA. In summary, steel/Si3N4 contact lubricated by ricinoleic acid gives unsurpassed tribological performances reaching superlubricity under a wide range of operating conditions.

Effect of tribologically-induced changes in surface termination of silicon-containing diamond-like carbon coatings on the resistance to biomolecule adsorption

Silicon-containing diamond-like carbon (DLC) is a class of thin-film materials with excellent mechanical properties, high thermal stability, and good tribological performance over a wide range of environmental conditions. While non-alloyed/non-doped DLCs also exhibit good biocompatibility and bioinertness, our understanding of the effect of silicon in DLCs on biomolecules/DLC interactions is still elusive. Here, we evaluated the structural, mechanical, and tribological properties of Si-containing DLC coatings with silicon content fraction of 11% and 16%. Tribological tests, performed by sliding a stainless steel pin on the coatings in water, indicated a low friction response (steady-state coefficient of friction <0.11), while quartz crystal microbalance experiments indicated no adsorption of a model biomolecule, namely adenosine triphosphate (ATP), on Si-containing DLCs. Near-edge X-ray absorption fine structure spectromicroscopy analyses performed after tribological experiments provided evidence for an increase in the fraction of silanol surface terminal groups formed in the worn region upon sliding in water without any significant sp3-to-sp2 rehybridization of carbon atoms. The fraction of surface hydroxyl groups in the worn region increases with the silicon content in Si-containing DLC, which leads to a decrease in friction. This tribologically-induced change in surface termination did not lead to the adsorption of ATP upon incubation of tribotested samples in ATP solutions for several hours. These findings open the path for the use of Si-containing DLC in applications requiring good tribological properties in aqueous solution and an excellent resistance to biomolecule surface adsorption that is maintained even after tribologically-induced variations in surface termination.

Sliding wear performance of TiAl-based nitride coatings deposited on ADI by cathodic arc deposition and plasma based ion implantation and deposition

ABSTRACT This work studied the sliding wear performance of TiAl-based nitride coatings synthesized on austempered ductile iron (ADI) by cathodic arc deposition (CAD) and plasma-based ion implantation and deposition (PBIID). Monolayer CAD TiAlN films and bilayer CAD and PBIID TiAl/TiAlN films deposited on an experimental device were analyzed and benchmarked against a commercial bilayer CAD film. Sliding wear was evaluated in a pin–on–disc tribometer. Two test conditions were employed, one intended to prevent damage (low load, short distance) and another intended to promote damage (higher load, longer distance). Regarding low load tests, all coated samples showed friction coefficients of 0.40–0.45 and negligible wear. Regarding high load tests, all coated variants exhibited lower or equal disc and pin wear rates with respect to uncoated ADI. In addition, coated samples displayed steady-state friction coefficients between 0.2 and 0.6 while uncoated ADI steady-state coefficients between 0.6 and 0.85.

Frictional stability and hydromechanical coupling of serpentinite-bearing fault gouge

SUMMARY Observations of slow earthquakes and tremor have raised fundamental questions about the physics of quasi-dynamic rupture and the underlying fault zone processes. The presence of serpentinite at P-T conditions characteristic of deep tremor and slow earthquakes suggests that it plays an important role in controlling complex fault slip behaviour. Here, we report on experiments designed to investigate the frictional behaviour of serpentinite sampled from outcrop exposures (SO1 and SO2) of altered ultramafic rocks present at depth, and recovered from the SAFOD borehole (G27). XRD analyses reveal the presence of chrisotyle, lizardite, kaolinite, talc in SO1; lizardite, clinochlore and magnetite in SO2; and lizardite, quartz and calcite in G27. We sheared fault gouge in a double-direct shear configuration using a true triaxial deformation apparatus. The effective normal stress was varied from 2 to 40 MPa. We conducted velocity stepping tests and slide-hold-slide (SHS) tests in each experiment to characterize frictional stability and healing. At the end of each experiment, post-shear permeability was measured and the samples were recovered for microstructural analysis. The steady-state friction coefficient was μ = 0.17 for SO1, μ = 0.33 for SO2 and μ = 0.53 for G27. Overall, the gouges exhibit velocity strengthening behaviour, and become nearly velocity neutral at 40 MPa effective normal stress. SHS tests show positive healing rates for SO2 and G27, whereas SO1 exhibits zero or negative healing rates. Permeability decreases with increasing σn’, with SO1 (k = 10–20 m2) showing the lowest values. Microstructural observations reveal a well-developed R-Y-P fabric in SO1, which is not observed in SO2 and G27. We posit that the development of shear fabric controlled by mineralogy governs frictional and hydrological properties. In this context, when serpentinite is associated with other weak phyllosilicate minerals, frictional stability and hydrological properties can vary greatly, with a potential control on the mode of fault failure.

Effect of the Anodizing Temperature on Microstructure and Tribological Properties of 6061 Aluminum Alloy Anodic Oxide Films

Anodizing a simple and effective method to improve the wear resistance of 6061 aluminum alloy. In this study, the microstructure of 6061 aluminum alloy oxide films (AAO) was adjusted by changing the electrolyte temperature (5, 15, and 25 °C) using 160 g/L sulfuric acid as the electrolyte. The phase composition, microstructure, and morphological characteristics of the specimens were detected using X-ray diffractometer and field emission scanning electron microscopy (SEM). The hardness, elastic modulus, and tribological properties of the films were examined using hardness testers and a rotary friction tester. The results showed that as the temperature of the electrolyte increases, the surface of the oxide films changes from uniformly distributed small-sized pores to a coral-like loose porous structure, and the thickness of the films increases. The electrolyte temperature has a significant effect on the friction performance of the AAO films. When the solution temperature decreases from 25 to 5 °C, the steady-state friction coefficient decreases from 0.46 to 0.39. According to the morphology of the wear tracks, it can be determined that the main wear mechanism of AAO films gradually changes from delamination wear to abrasive and adhesive wear, and the wear rate drops by ~69%.

High-temperature tribological behavior of Ti containing diamond-like carbon coatings with emphasis on running-in coefficient of friction

Diamond-like carbon (DLC) coatings show a low coefficient of friction (COF) and high wear resistance against aluminum at elevated temperatures, yet they exhibit a high running-in COF (μ R ) prior to reaching a low and stable steady-state COF (μ s ). This study shows that incorporating titanium (Ti) atoms into the DLC structure would reduce the μ R . During pin-on-disk tests conducted on Ti incorporating hydrogenated-DLC (Ti-H-DLC) with 6.2 at.% Ti subjected to dry sliding against 319 Al (Al-6.5% Si), μ s values decreased from 0.27 at 25 °C to 0.11 at 200 °C. The specific wear rate of Ti-H-DLC decreased from 2.44 × 10 −5 mm 3 /Nm at 25 °C to 0.71 × 10 −5 m 3 /Nm at 200 °C. A typical DLC with 40 at.% H (H-DLC) tested at 200 °C showed a low μ s of 0.08 and a wear rate of 1.11 × 10 −5 mm 3 /Nm. However, at 200 °C, Ti-H-DLC showed a lower μ R of 0.16 compared to μ R = 0.78 of H-DLC, and the duration of the running-in period for Ti-H-DLC, t R = 3 revolutions, was shorter than H-DLC with t R of 200 revolutions. Comparisons made with other DLCs, including, NH-DLC, W-DLC, ta-C, and Si-O-H-DLC, in addition to H-DLC, all tested using the same method, revealed that, in the temperature range of 100–250 °C, Ti-H-DLC showed a better running-in behavior making Ti-H-DLC a suitable tool coating for manufacturing processes where high-temperature running-in sliding friction is important, including warm forming and (single-point) turning of aluminum alloys . • High-temperature tribological behavior of Ti-H-DLC against 319 Al was studied. • Low running-in COF and short running-in duration were recorded at 100 and 200 °C. • Compared to other DLC coatings, the lowest μ R and t R were obtained using Ti-H-DLC. • Ti-H-DLC tool coatings are useful for Al warm forming, machining operations ~200 °C.

Fabrication and electrical sliding wear of graphitic Cu–Cr–Zr matrix composites

Abstract The present work was undertaken to prepare graphitic Cu–Cr –Zr matrix composites in order to develop new materials for application under sliding contact in an electrical field. The electrical sliding wear behavior of the graphite (Gr)/CuCrZr composite was investigated from a microstructural viewpoint, compared with the matrix alloy. The results indicate that by using the water atomized Cu–Cr–Zr powder and copper-coated graphite particles as starting materials, compact and homogeneous microstructure can be obtained in the composites. Fine and coherent particulate precipitations are formed in the Cu–Cr–Zr matrix during the aging treatment process, which help to improve the mechanical properties. Under the electrical sliding wear condition, incorporation of graphite in the Cu–Cr–Zr matrix not only decrease the wear loss of the sliding pair but also maintain steady-state friction coefficient. The main reason is that the graphite film that smears on the contacting surfaces can alleviate materials loss induced by electric arc discharge and adhesion wear. Proper aging treatment helps to make full use of the solid lubricant effect of the graphite particles.

Study of the friction coefficient during the steady-state process of continuous hot rolling of steel strips in conditions of the 1950 industrial mill

This paper describes a method for determining the value of the friction coefficient in a steady-state hot rolling under industrial conditions of the casting and rolling complex of &ldquo;Vyksa Steel Works&rdquo; located in Vyksa. The algorithm implies the forward slip method for determining the friction coefficient using the Ekelund formula. Measurements of hot rolling technological parameters from sensors are used as input parameters. Mathematical formulas are implemented in the ibaAnalyzer software, with the help of calculations are made for several thousand strips and data on the friction coefficient are obtained under various conditions. The data processing results demonstrate a normal distribution of the friction coefficient. The highest value of the friction coefficient is typical for the first finisher stands F1-F2, and the lowest values are found for the thinner gauge products rolled at high speed. The influence of the working layer of the work rolls on the friction value is demonstrated. High-chromium cast iron rolls have lower values of the friction coefficients, in contrast to indefinite cast iron, which is used for the working layer of the last finisher stands of mill 1950. A detailed comparison of the results with the classical methods for determining the friction coefficient in a steady-state process has been carried out. The dependence of the ratio of the friction coefficient during initial entry to the steady-state friction coefficient was refined.

Stress- and Time-Dependent Formation of Self-Lubricating In Situ Carbon (SLIC) Films on Catalytically-Active Noble Alloys

Low shear strength (30 MPa) organic films were grown in situ on Pt0.9Au0.1 surfaces via cyclic sliding contact in dry N2 with trace concentrations of ambient hydrocarbons. We present a systematic investigation of the stress- and time-dependent film formation. Steady-state friction coefficients were found to be as low as µ ~ 0.015 and inversely proportional to contact pressure, revealing non-Amontonian behavior. Above a Hertzian contact pressure of ~500 MPa, shear strength dropped, indicating an activated process. Raman spectroscopy identified non-uniformity in areal coverage and relative order with contact pressure. Regions of steady-state low-friction behavior exhibited spectra similar to DLC coatings. Atomic force microscopy was used to study the formation and growth of films at the nanoscale. Stress- and time-dependent measurements suggested a sublinear increase of film volume with time, and a transition from growth to wear at a Hertzian contact pressure of ~1.2 GPa.

Effect of organic-modified nickel phyllosilicates on the tribological, mechanical and thermal properties of epoxy composites

The present study aims at investigating the effect of organic-modified nickel phyllosilicate (NiPS) particles on the structure–property relationships of epoxy (EP) composites. The organic-modified NiPS particles were synthesized using a facile sol–gel method and subsequently mixed into EP matrix by physical blending to improve its tribological, mechanical and thermal properties. Based on a series of tests and analyses, it was found that organic-modified NiPS particles playing a great role in improving the tribological properties under dry sliding. The steady-state friction coefficient was just decreased moderately, whereas the wear rate was dropped significantly to 0.36 × 10 −5 mm 3 /N·m, showing a reduction up to ca. 95% in comparison with pure EP. According to the analysis of worn surfaces, the main mechanism of wear resistance was discussed. The elastic modulus for the EP composites was increased remarkably while the elongation at break decreased. However, the added organic-modified NiPS particles show a negligible influence on the tensile strength. Moreover, the incorporated NiPS particles could enhance the thermal stability of EP composites, showing the notably suppressed mass loss rate and added residual chars. This study provided a promising way for preparing the high-performance EP composites with organic-modified lamellar particles.

Lubricating performances of oil-miscible trialkylanmmonium carboxylate ionic liquids as additives in PAO at room and low temperatures

Ionic liquids (ILs) have recently been demonstrated as a promising lubricating material. However, data on ILs with splendid solubility in non-polar oil and their low-temperature lubricating performances are rarely reported. In this paper, we synthesized four novel oil-miscible protic ionic liquids (PILs) containing different fatty acid anions. They possess low pour point, good low-temperature fluidity, non-corrosiveness and higher thermostability. These PILs were selected as additives in PAO base oil, respectively, and their lubricating performances were tested using a steel ball reciprocating sliding against steel stationary disc at room and low temperatures. At room temperature (~30 ℃), [N888H]C6 with shorter alkyl chains exhibited good lubricating behaviors, both in terms of steady-state friction coefficient and wear volume. However, the lubricating performances of [N888H]OA with longer alkyl chains were relatively poor because the lubricating film formed by it is disordered. At low temperature (-20 ℃), these PIL additives produced excellent antiwear performances, while their friction-reducing properties were unobvious, attributing to the restriction of high kinematic viscosity of PAO base oil.

Influence of the running-in process on the working ability of contact surfaces in lubricated sliding conditions

The influence of the running-in process operating parameters on tribological properties of the block-on-disc samples in lubricated sliding conditions is analyzed and discussed in detail. Different running-in regimes are achieved by varying the normal load and sliding speed. After the running-in period, during which the operating parameters are varied, all samples are placed in a working regime under the same set of operating conditions. At the end of the running-in period, as well as at the end of the working period, an analysis of the changes in the surface roughness, microhardness, wear rate, and coefficient of friction is performed. Less desirable properties in terms of wear rate and steady-state coefficient of friction are noticed for the samples that were run-in with the operating conditions which were the same as the working regime operating conditions. In the defined test conditions, it is shown that the intensity of normal load applied during the running-in process has a dominant influence on the amount of wear and coefficient of friction value. It was also shown that the running-in process can significantly improve the roughness of the initially rough contact surfaces. The results of experimental testing indicate that the variation of the operating parameters during the running-in process can be used to improve the working ability of the sliding contact surfaces under the mixed lubrication regime.

Tribological response of laser-textured Ti6Al4V alloy under dry conditions and lubricated with Hank’s solution

A nanosecond laser was used to define different morphologies of a Ti6Al4V surface: lines, crosshatching and dimples. We analysed the influence of the surface morphology, texture density and orientation (parallel, perpendicular and at 45°) on the friction/wear under dry and lubricated (Hank’s solution) sliding conditions to evaluate the specific laser texturing of Ti alloys for biomedical applications. Under dry conditions, the lowest steady-state coefficient of friction was measured for dimples, whereas in Hank’s solution the lines and crosshatching showed better frictional characteristics after running-in and the removal of the bulges around the laser textures. In terms of wear resistance, the low-density lines pattern and particularly the dimples were the most promising in contact situations likely to occur during starved/dry-lubrication conditions.

Friction characterization and application to warm forming of a high strength 7000-series aluminum sheet

Tribological conditions are challenging when forming high strength aluminum alloys at elevated temperatures due to the sticking behavior of aluminum and potential material transfer to the die in the form of die pick-up. The current study examines the elevated temperature performance of several lubricants as well as a Physical Vapor Deposition (PVD) die coating, in conjunction with a 7000-series aluminum alloy sheet under conditions corresponding to warm forming of automotive structural components. Lubricant performance is characterized from the coupon-to-component level at temperatures in the range of 25–230°C. The classical coupon-level Twist Compression Test (TCT) is modified to perform elevated temperature experiments and used to rank the lubricants in terms of their steady-state coefficient of friction and resistance to lubricant breakdown as a function of temperature. It is found that both the friction coefficient and lubricant breakdown distance need to be considered for lubricant assessment. The studied PVD die coating is found to successfully mitigate lubricant breakdown for the FORGE EASE AL 278 at 230°C in the TCT. In parallel, warm forming experiments considering deep drawn cylindrical cups and U-channel rail sections (representative of a structural side member) are used to evaluate lubricant performance under real stamping conditions. The trends in forming performance (draw-in length, final blank perimeter, visual scoring, and forming force) confirm the lubricant ranking obtained from the TCT coupon tests, with the Teflon® PTFE film as the most effective followed by the FORGE EASE AL 278, OKS 536, and the LPS® Dry Film PTFE Spray Lubricant. For the highest test temperature, the LPS® Dry Film PTFE Spray Lubricant exhibits breakdown in the TCT at short sliding distances and results in fracture during deep drawing.

Influence of dissolution on long-term frictional properties of carbonate fault gouge

Velocity stepping experiments have been performed on a simulated calcite gouge using an annular shear apparatus to investigate the effect of dissolution on the frictional properties of a carbonate fault. The tested material was put in contact with hydrochloric acid at different concentration in order to provoke grains dissolution prior to the experiments. Particle size analysis shows that the small grains tend to disappear due to the chemical reaction, whereas the distribution of large grains is not much affected. The dissolution process induces a decrease of the fractal number of the grain size distribution and an increase of the roughness of the particles. The study of the rate-and-state parameter of the materials, which is commonly used to characterize the ability of the fault to generate earthquakes, shows a decrease towards more negative values with dissolution for dry specimens and an increase for wet samples. Moreover, a decrease of the steady state friction coefficient is observed with dissolution for wet samples. This decrease of the friction would promote the triggering of slip along an existing fault after the injection of an acid fluid like in the cases of CO2 storage or acid gas disposal projects in a carbonate reservoir, but the increase of the rate and state parameter (a–b) of saturated specimens implies that the seismogenic potential of faults could potentially be reduced.