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Tribology International

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A micromechanical experimental study of kaolinite-coated sand grains

S.S. Kasyap, K. Senetakis∗

Department of Architecture and Civil Engineering, City University of Hong Kong, Kowloon, Hong Kong SAR

A R T I C L E I N F O

Keywords:Inter-particle frictionMicromechanicsLaboratory testCoated grains

A B S T R A C T

The micromechanical behaviour of sand grains coated with a clay agent is examined in the study. The coatingwas applied using a suspension of sand grains and kaolinite powder with a wide spectrum of coating percentagesand durations of vibration to achieve different classes of coating. Element composition, measurement of surfaceroughness and single grain crushing tests were used for the basic characterisation of the materials. From puresand grains to lightly coated grains, there was observed a significant shift of the inter-particle friction but thedifferences were smaller between lighter and heavier coating. Microscopic images showed the creation of debrisduring shearing which might have contributed to the increase of friction from the uncoated to the coated grains.

1. Introduction

Advancements in the application of the discrete element method(DEM) in geomechanics [1] have necessitated more systematic micro-mechanical studies to be conducted in the laboratory. Such experi-mental studies can provide important information associated with thebehaviour of geological materials at the grain scale, including grainstrength and grain contact response, so that the findings from labora-tory research can be further utilized in DEM analysis as input. Some ofthe important quantities to be explored in the laboratory include thetangential and normal load – displacement relationships as well as theinter-particle coefficient of friction [2]. Particularly for the inter-par-ticle coefficient of friction, previous studies have highlighted its im-portant influence on the macro-scale monotonic and cyclic behaviour ofgranular materials [3–6].

In terms of micromechanical studies investigating the normal ortangential load – displacement behaviour and inter-particle friction ofreal soil grains or reference grains, previous works [7–13] have focused,majorly, on clean and/or uncoated surfaces. This has been deliberatelyperformed by cleaning the surfaces of the grains (for example in thestudies by Refs. [7–9]) to remove any impurities that could influencethe output from the micromechanical tests. Cleaning of the grain sur-faces can help to isolate any unforeseen factors on the contact me-chanics behaviour of soil grains and purely examine the frictional me-chanisms at the grain contacts providing fundamental insights into themicromechanical behaviour of geological materials. In other micro-mechanical studies [11,14–16], it was not explicit whether the surfacesof the tested materials had been cleaned. Nevertheless, it can be con-sidered that most of these works focused on uncoated grain surfaces, so

that effects such as, for example, surface roughness, material type andcomposition or surface morphology could be examined in a straight-forward way.

In nature, granular soils comprise a mixture of coarser and finergrains and it is common that the surfaces of the grains may be coated.This coating can be because of chemical or biological substances,weathering processes or fine grained material covering sand grains. Thelatter may be the case for sand-silt or sand-clay mixtures (which arecalled as gap-graded soils). The coating of the surfaces of geologicalmaterials is applicable not only within the context of soil mechanicsapplications, but it is also of major interest in the rock mechanics dis-cipline. For example, rock joints are commonly filled with a coatingmaterial which may affect markedly the mechanical behaviour ofgouges [17]. Using laboratory element testing or numerical analyses,researchers have studied the influence of the presence of finer geolo-gical materials, i.e. silt and clay fractions, on the frictional behaviourand shear strength of coarse grained soils [18–22]. This kind of studiesalso influence the research on the liquefaction behaviour of granularmaterials due to the presence of fines [23–25]. Rheological studies havealso been carried out varying the concentration of the granular sus-pension, which have enhanced our understanding on problems related,for example, to debris flows [26]. Problems such as liquefaction ordebris flows (i.e. granular flows) are of major interest in the disciplinesof geotechnical and geological engineering. Even though many ad-vancements have taken place recently in the coating techniques andstudies on coating effects on the tribological behaviour of interfaces[27–32], there have been relatively limited studies related with thecoating characteristics of geological materials. For example, Barrowset al. [33] studied the natural formation of coating on the surfaces of

https://doi.org/10.1016/j.triboint.2018.05.021Received 19 February 2018; Received in revised form 21 April 2018; Accepted 10 May 2018

∗ Corresponding author.E-mail addresses: ssarvadev2-c@my.cityu.edu.hk (S.S. Kasyap), ksenetak@cityu.edu.hk (K. Senetakis).

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limestone grains in soil due to wetting and drying cycles. The coatingformed on the grains was observed to be characteristic with the sur-rounding soil and that the combination of effects such as weathering,recrystallization and adsorption were found to be the major reasons forthe formation of coating. Scheidegger et al. [34] worked on coatingsilica sand with goethite (i.e. an iron oxide compound). Torrey et al.[35] described different procedures for coating of quartz crystals withnanoparticles. These works were biased either towards chemical in-teractions with soils or coatings at the nanoscale.

In this research work, an attempt was made to study in the la-boratory the micromechanical behaviour of sand-sized grains coatedwith clay minerals in order to quantify the frictional and normal contactresponse of coated sand grain interfaces. For this purpose, the com-mercially available kaolinite was used as the coating compound, andLeighton Buzzard sand (LBS) was used as the coarse-grained material,which quartz sand has been thoroughly examined in previous works[8,9,13,36]. The three mainstream objectives of this work were: (i) todevelop a new technique in coating natural sand-sized grains with claypowder, (ii) to provide characterisation of the coated grains in terms ofelement composition, surface roughness and grain strength and (iii) toexamine the effect of the coating presence and amount on the tribolo-gical behaviour of natural grains.

2. Material description

Leighton Buzzard sand (LBS) of fraction 2.36–5.00mm was used forboth coating studies and micro-mechanical testing. This is a typicalquartz sand which consists of Silicon (Si) and Oxygen (O) as the pri-mary elements. Traces of other elements such as Aluminium (Al) andIron (Fe) are also found in the particles. Typical energy-dispersive X-rayspectroscopy (EDS) test results of pure LBS are shown in Fig. 1. Averagepercentage weights of elements for fifty pure LBS particles were used inthe current study, which are also shown in the same figure. These re-sults from the different particles showed consistent values with smalldeviations. Wang and Coop [37] described the LBS grains as sub-rounded and translucent white or yellow in colour with a greasy lustre.On the surface of the LBS grains, coating of clay minerals was appliedusing the commercially available kaolinite powder. The chemicalcomposition of the kaolinite is Al2Si2O5(OH)4 as a repeating unit,whereas, quartz has SiO2 as a repeating unit and hence the elementalanalysis of ‘kaolinite coated LBS particles’ should show an increase in Alpercentage. This is a key parameter used in the current study toquantify the amount of coating on the surfaces of the grains. The EDSresults and scanning electron microscope (SEM) images of pure kaoli-nite are also shown in Fig. 1. It can be observed that the kaoliniteparticles have flaky shape and varying sizes. This could be used as areference to observe the adsorption of kaolinite on the LBS grains. Theliquid limit and plastic limit of the kaolinite are 30% and 15%, re-spectively, resulting in a low to medium plasticity material with Plas-ticity Index (PI) of 15%. Kaolinite was made as a solution for coatingand distilled water was used as the solvent. The adopted procedure forcoating is explained in the next section. From Sandeep and Senetakis[38], the average surface roughness (Sq) of pure LBS grains was ob-served to be around 0.223 μm with a standard deviation of± 0.061 μmas measured from the optical surface profiler of the City University ofHong Kong. This value was confirmed from representative tests carriedout by the authors using the same optical profiler.

As an established practice, the LBS grains were cleaned withButanone before testing [8,9] in order to remove any oil or other im-purities from their surfaces that could disorder the actual behaviour.For this purpose, ultrasonic cleaner with distilled water at a tempera-ture of 50–60 °C was used for 30min to remove any kind of impuritiesfrom the material surface before the application of the coating. Hencethe sole effects of the coating material could be studied without theintervention of pre-occupied debris on the surfaces of the grains.

3. Coating procedure

Six different concentrations and three different time intervals werechosen to examine the influence of the coating material on the amountof adsorption on the surfaces of the LBS grains. The LBS grains weremade in contact with 25, 50, 100, 150, 200 and 300mg/ml con-centrations of kaolinite solution for time intervals of 100, 500 and1000 s.

At a required concentration, a uniform solution of kaolinite wasprepared with distilled water as the solvent by shaking the mixture. Thetemperature of the distilled water used in this process was around40–50 °C to achieve uniformity more readily. Shaking of the solutionwas carried out using an orbital shaker which moves its platform in ahorizontal orbital motion for set time and at set RPM. The beakers withmixture are fixed to this platform (Supplementary Fig. S1). To avoid theformation of lumps, kaolinite powder was gradually added in smallamounts to the distilled water which was already in motion. During thisprocess, it was observed that the clay particles were not always com-pletely dissolved in the water, particularly for lower concentrations.Because of this, the mixture should be kept in constant motion withinthe total duration of the coating procedure to maintain a uniform so-lution. Thereafter, the LBS grains were dropped into that uniform so-lution. After a few trials, an RPM of 220–250 was found to be appro-priate for the solution to be uniform and for minimum impact of thegrains. After the time of interaction was set, the grains were separatedfrom the solution and they were oven dried. The effectiveness of thiscoating procedure was evaluated qualitatively with SEM and digitalmicroscope images and quantitatively with energy dispersive spectro-scopy (EDS) graphs.

4. Coating analysis

EDS analysis provides a spectrum of elements as well as their per-centage weights available in the functional scan area of the grain. Withrespect to the size and shape of the grains, the maximum possiblesurface was scanned for the analysis by adjusting the magnification. Foreach combination of concentration and time, five grains were coatedand analysed. As discussed in the previous section, the coated LBSgrains should show an increase in aluminium (Al) content because ofthe chemical composition of the materials. This statement could besupported, primarily, with EDS graphs of coated LBS grains. Typicalspectra obtained at different concentrations are shown in Fig. 2. It isobserved that the peak value of Al increased with increasing con-centrations. Hence, the governing parameter could be either the alu-minium percentage on the coated surface or the change in the alumi-nium percentage after coating. Finally, the change in aluminiumpercentage was arbitrarily chosen for the current study as the trend forboth parameters was similar. Fig. 3 shows the variation of the change inpercentage weight of aluminium with the increase in concentration atthree different exposure times. Their associated standard deviations andpolynomial trendlines for bar graphs are also shown in the same figure.The trendlines were fitted very well to the data with admirable R2

values. There was observed a strict increase in the adsorbed aluminiumcontent with increase in concentration for given time of exposure. Also,there was an increase in the adsorption, though small, with the increasein time of exposure. As shown in Fig. 3, from 500″ to 1000″, the re-sponses were within the standard deviations of each other and theadsorption was observed to reach a saturation level in the range of200–300mg/ml of concentration. This was not the case for 100″ time ofexposure where no hint of saturation was observed even up to 300mg/ml. Based on the results from the coating analysis, it was understoodthat heavier concentrations were required to reach saturation at 100″ ofexposure time. At lower concentrations, all the curves started with asaturation behaviour irrespective of the time of exposure. This explainsthat any concentration below 25mg/ml would show similar values ofthe adsorbed aluminium contents. The said phenomenon could not be

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supported experimentally since the adsorption phenomenon at con-centrations lower than 25mg/ml was very unrepeatable and unreliablefor the given material combination (i.e. LBS with kaolinite). Hence, therange of concentrations chosen for the current study had a lower boundof 25mg/ml.

Supplementary Table 1 gives a summary of the numerical values ofthe bar graphs and their standard deviations as illustrated in Fig. 3. Thestandard deviation values are in considerable range with a maximumof±27% and a minimum of±2.5%. Precisely, very small deviationsare observed for the 300mg/ml condition. The morphology of thegrains could be, perhaps, one of the primary aspects that was accoun-table for non-uniform adsorption of the coating material on the surfaceof the LBS grains. After the grains were shaken in the kaolinite solution,the solution was observed to be stagnant in the micro troughs on thesurface of the grains. After oven drying, deposition of excessive kaoli-nite in troughs than in crests occurred. This phenomenon was observedmore extensively for medium concentrations. At lower and higherconcentrations, the adsorption was comparatively uniform. Typicalmicroscopic images of the grains after coating are shown in Fig. 4 attwo different magnifications and two extreme concentrations. It is

observed that with increasing concentration, the adsorption of theclayey material is very much visible to the naked eye. It is shown thatthe LBS surface was completely covered with flaky kaolinite particlesfor higher concentration and that at lower concentration the LBS sur-face was scarcely covered.

The important role of surface roughness on the frictional behaviourof variable types of interfaces has been well acknowledged in the lit-erature [7,38–40]. The root mean square deviation of the heights on thesurface is a typical parameter used for roughness quantification. Thesurface roughness of the coated LBS grains at selected concentrations(i.e. 25 and 300mg/ml at 500″ exposure time) was evaluated. For thispurpose, a set of five particles for each concentration were analysed andthe surface roughness values for sub-regions of 20 μm×20 μm area atfive distinct locations on a single coated grain were found. The peak of agrain is the prime region for micromechanical sliding tests and so theselection of sub-regions was biased towards peak. The roughness valuesare noted after flattening the curved surface profile of the particle.Fig. 5 shows 3-dimensional images of flattened surface profiles at saidconcentrations. The texture of the kaolinite on the surface of the LBSgrains is clearly visible. The average surface roughness values observed

Fig. 1. Element composition of LBS and Kaolinite: (a) EDS graph of pure LBS - Average percentages weights of elements with standard deviation (b) EDS graph ofkaolinite powder – Percentage weights of elements (Note: Fig. 1(b) includes SEM image of kaolinite powder).

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are provided in Supplementary Table 2. There was a significant increaseof the roughness from pure LBS (Sq= 0.223 ± 0.06 μm) to the coatedgrains prepared in suspension with 25mg/ml powder concentration(Sq= 0.572 ± 0.22 μm) and 300mg/ml concentration(Sq= 1.013 ± 0.14 μm).

With the observations from the SEM and microscopic images, EDSanalysis and roughness values, two extreme concentrations i.e. 25 and300mg/ml were chosen for the conduction of the micromechanicalloading tests. All the samples were prepared at 500″ of exposure time.Hereafter the said two cases of the coated particles are referred as Type– L and Type – H (i.e. 25 and 300mg/ml, respectively), where “L” and“H” denote light and heavy concentrations.

5. Crushing behaviour of coated LBS particles

The study against grain crushing is important to be obtained for acomplementary understanding of the behaviour of geological-granularmaterials. Crushing is part of the inherent process of sedimentation andlithification where soils are compacted at greater pressures, as well aspart of variable phenomena including granular flows or compression-shearing of soils under the application of external loads. This workprovided a preliminary study into grain crushing of the selected ma-terials to understand the role of coating on their compression andsubsequent brittle failure.

The crushing tests were conducted on pure LBS and Type – H

Fig. 2. EDS graphs of coated LBS grain at different concentrations (a) 25mg/ml; (b) 100mg/ml; (c) 150mg/ml; (d) 300mg/ml.

Fig. 3. Variation of percentage increase of Al with different concentrations for 100s, 500s and 1000s of exposure (standard deviations and trendlines included).

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particles to examine and compare extreme cases of uncoated andheavily coated grains. The apparatus and procedure described by Wangand Coop [37] were used for the tests but only digital micro cameraswere used for the monitoring of the tests (i.e. high-speed camera wasnot used in the study). For the crushing tests, the grains were chosen tobe fairly spherical in outline so that the effect of morphology could berelatively diminutive. The expression proposed by Cavarretta andO'Sullivan [41] was used for calculating stresses (Eq. (1)) and also thesurvival probability of the grains was calculated (Eq. (2)) with the ex-pression proposed by McDowell and Bolton [42] based on Weibull [43].From the Weibull survival probability, plots similar to Lim et al. [44],the Weibull modulus (m), which explains the variability of strengthwithin the population, and the characteristic strength (σc), which de-scribes the minimum strength that 37% of the population possess, werecalculated.

=σ 0.9*Ndf 2 (1)

⎜ ⎟= ⎡⎣⎢

−⎛⎝

⎞⎠

⎤⎦⎥

P exp σσs

c

m

(2)

In Eq. (1), N is the peak load or failure load and d is the geometricmean of d2 and d3 (d2 and d3 are the intermediate and smallest di-mensions of the smallest cuboid circumscribing the grain). In Eq. (2), σis the peak load obtained for a test and σc is the characteristic strengthfor the population.

Pure LBS and Type – H particles were tested with 30 samples each ata loading speed of 0.01mm/min. Fig. 6 shows the survival probabilityagainst the peak stress for the LBS and the Type – H particles. Fig. 7shows typical normal load-displacement curves obtained for pure LBSand Type – H particles. Though there was some scatter in the crushingbehaviour of the particles, it was observed that the Type – H particleshad extended failure displacements than that of pure LBS with a moreplastic behaviour in the initial stage of crushing. This plastic de-formation stage in the beginning of the trend has smoothly transformedinto nonlinear behaviour to reach the failure point. There were no

Fig. 4. Typical images of coated LBS grains at different scales (a) microscopic image for 25mg/ml (b) microscopic image for 300mg/ml (c) SEM image for 25mg/ml(d) SEM image for 300mg/ml.

Fig. 5. Typical flattened surface profile of coated LBS grains (a) 25mg/ml (b) 300mg/ml.

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abrupt changes in the trend that can signify any differences in thecrushing behaviour of kaolinite-coated and pure LBS particles. One ofthe reasons for this behaviour could be that the coating material, be-tween the platen and the LBS surface both at top and bottom, is undercontinuous compression with the increase of load. This could be ob-served from the images of a representative test while crushing proceedsas shown in Fig. 8, where chipping of the coating material on the sidesof the particle could be observed before the actual failure was reached.This plastic deformation however did not affect the failure load of thegrains and its variability. This can be observed from the comparison ofthe survival probability of both pure LBS and Type – H particles inFig. 9. It is observed that the Weibull's modulus and the characteristicstrength for both classes of grains are similar. The σc and m valuesobtained in this study are in good comparison with the values obtained

by Ref. [37] for 2.36–5.00mm pure LBS grains. For a complete char-acterisation of the crushing behaviour of coated LBS, additional numberof tests at different concentrations, dealing with all the parametersgoverning the strength and also understanding mechanisms is required,which was out of the scope of the current study.

6. Grain contact testing program and results

6.1. Micromechanical apparatus and testing program

A custom-built micromechanical loading apparatus (SupplementaryFig. S2), present at the City University of Hong Kong, was used in thestudy [36,45]. The apparatus consists of three major systems, one in thevertical direction which is used for the study of the normal load –displacement behaviour and the application of the confinement at graincontacts, one in the horizontal direction for the conduction of shearingtests and one in the out-of-plane horizontal direction. Each systemconsists of micro linear actuators, load cells (100 N of capacity) andeddy current displacement sensors (10−5mm of resolution). The grains,after mixing with coating solution and oven drying, were glued on thebrass mounts, and fixed rigidly into the brass wells. One brass well isplaced on the sled of the apparatus which is supported by ball bearingsbeneath and the second brass well is fixed on a rigid frame. Images ofcoated grains during the setting of a sliding test are shown inSupplementary Fig. S3. The apparatus is housed inside a Perspexchamber and a humidity controller was used to maintain the humidityat about 60% during the experiments.

Two classes of inter-particle shearing tests were conducted (i.e.Type – L and Type – H) varying the concentration of the coating ma-terial on the surface of the LBS grains. Pure LBS test results were ex-tracted from Sandeep and Senetakis [13,38] to compare with the be-haviour of the coated grains. Details of the testing program and asummary of the results are given in Table 1. For each class of grains,monotonic shearing tests were conducted at five different normal loadsbetween 1 N and 10 N, using a new pair of grains for each test. A total offourty-five tests was conducted with the variation of normal load andconcentration of coating as presented in Table 1. Both the normal loadapplication and shearing were carried out in a displacement-controlledmanner with a rate of 0.3 mm/h and 0.1 mm/h, respectively. The testswere carried out for a shearing displacement of around 50 μm where asteady state or micro-slip condition was observed. These tests on coatedgrains were repeated for three different pairs at each given normal load,to assess the repeatability of the normal and tangential load behaviour.With the said tests, the normal contact response and the frictional

Fig. 6. Probability of survival curves from grain crushing tests for pure LBS andType – H.

Fig. 7. Typical normal load-displacement behaviour from grain crushing test showing plastic behaviour of Type – H sample.

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behaviour of coated LBS grains at different concentrations and normalloads were studied and compared with the behaviour of pure LBS. Forselected tests, digital microscope images were taken after the comple-tion of the shearing to observe possible damage to the surfaces.

6.2. Normal load – displacement behaviour

After the grains are fixed in position, the top grain is moveddownwards towards the stationary bottom grain at a required rate. The

movement is stopped after the required load is attained. Fig. 10 gives acomparison of normal load-displacement behaviour of pure LBS, Type –L and Type – H cases for 10 N loading condition. The range of the load-displacement behaviour is shifted towards increasing displacementsfrom the pure LBS to the heavier coated grains and in general, thenormal contact response becomes softer for the heavier coated grains.In the maximum loading case of 10 N, the final displacement is less than10 μm for pure LBS, but for Type – H particles, this displacement rangesfrom about 20 to 35 μm and beyond that. For each type of grains (i.e.

Fig. 8. Type – H particle under crushing test (a) before loading (b) Chipping of coating material while loading (c) particle breakage at peak load.

Fig. 9. Survival probability curves showing Weibull modulus and characteristic strength.

Table 1Summary of micromechanical shearing testing program and results.

No. Kaolinite coating Code Normal load (N) Tangential load (N) Inter-particle coefficient of friction - μ Friction Angle (deg.)

1 Pure LBSa Pure LBS 1 0.25 0.25 10.20

2 2 – –3 5 0.95 0.194 7 1.26 0.185 10 1.7 0.176 25mg/ml Type – L 1 0.21 0.24 0.29 0.21 0.24 0.29 13.90

7 2 0.42 0.5 0.56 0.21 0.25 0.288 5 1.1 1.3 1.35 0.22 0.26 0.279 7 1.89 1.68 1.54 0.27 0.24 0.2610 10 2.2 2.7 2.5 0.22 0.27 0.2511 300mg/ml Type – H 1 0.33 0.3 0.31 0.33 0.3 0.31 14.20

12 2 0.5 0.54 0.48 0.25 0.27 0.2413 5 1.1 1.4 1.3 0.22 0.28 0.2614 7 1.47 1.96 1.75 0.22 0.28 0.2515 10 2.6 2.7 2.4 0.26 0.27 0.24

a Pure LBS values considered after Sandeep and Senetakis [13,38].

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LBS, Type – L, Type – H), there was observed a scatter to the data,which has also been reported for other types of geological materials[13,14]. It is possible that this is influenced by the variabilities of thegrain surface characteristics for each pair of grains which is reasonableto be expected for natural-geological materials. However, the trends ofthe normal load – displacement curves are clear and consistent as theamount of coating increases.

Fig. 11 shows the normal displacements at which the requirednormal loads were reached for Type – L and Type – H and also thevariation in the displacements for the total set of tests at each givenmaximum normal load. A slight variation in the normal displacementwith normal load can be observed and the probable reason, as discussedabove, is because of variabilities of the surface characteristics for thedifferent pairs of grains at a given concentration. It is also observed thatthis variation is greater for Type – H than Type – L because of heavieradsorption on Type – H. It is required to note that measurement of the

thickness of the coating on the surface of LBS particles was not carriedout.

6.3. Tangential load – displacement behaviour

After the required normal load is reached, the lower particle ismoved horizontally at a target rate in a displacement-controlled modeagainst the surface of the upper-stationary grain, maintaining thenormal load in a force-controlled mode. The grains were displaced forabout 40–50 μm where a steady state or a micro-slip condition wasobserved. Fig. 12 shows typical tangential load – displacement curves atdifferent normal loads for Type – L and Type – H. For a given con-centration, it is observed that the steady state (or maximum reachedtangential load) increases with the increase of the normal load, which isthe general case observed as well for uncoated sand grains [13]. Incontrast to the displacements reported by Sandeep and Senetakis [13]

Fig. 10. Typical normal load-displacement behaviour of 10 N loading for three particle cases.

Fig. 11. Final normal displacements for each normal load case and their variation.

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for uncoated LBS, the coated grains reach a steady state at much greaterdisplacements. Ni and Zhu [46] described that the asperities breakageoccurring continuously would cause a micro-slip condition and thatbreakage at the macro level would cause a steady state behaviour,which observations have been confirmed for geological materials[12,13]. Micro-slip condition was observed more severely for few cases,particularly at higher normal loads (Fig. 12, curves at 10 N of normalload). The higher displacements that were needed to reach a steadystate or the micro-slip condition could be attributed, partly, to the softflaky clay particles present in the contact region which particles werecompressed between the top and bottom grains. A significant shift ofthe required displacements to reach a steady state (or micro-slip be-haviour) in comparison to reported data on LBS, was also observed bySandeep et al. [47] examining the micromechanical behaviour ofweathered volcanic granules. These granules have a heavy naturalcoating on their surfaces which consists, majorly, of clay to silt sizedgrains. It is hypothesised that the softer nature of the Type – L or Type –H grains, similar to the weathered volcanic granules tested by Ref. [47],may have contributed to this shift of the displacements. A comparison

of the tangential load – displacement curves of pure LBS and coatedgrains at a normal load of 5 N is given in Figu.13(a). There is observed ashift of the curves towards lower displacements in the initial regime ofbehaviour for the LBS grains. The shape of the curves reaching a steadystate, or the micro-slip condition, also differs for each class. The coatedgrains showed a softer behaviour, whereas the pure LBS behaviour wasobserved to be very stiff reaching, for the example of Fig. 13, a steadystate at very small displacements.

The envelopes of the tangential load – normal load for the Type – Land Type – H grains along with the envelope for pure LBS are given inFig. 13(b). Each data point on the graph represents a shearing test on agiven pair of grains and the slopes of the trendlines correspond to theaverage inter-particle coefficient of friction (μ) for each class. It is ob-served that the coated grains have greater μ values in comparison topure LBS. The increase of μ was of the order of 35% (from 0.181 to0.246) from LBS to Type – L, while this increase was comparativelysmaller between Type – L and Type – H. Ref. [38] reported that theincrease of the surface roughness and/or the decrease of the apparentYoung's modulus of the contacted surfaces may lead to an increase of

Fig. 12. Typical tangential load-displacement behaviour at different normal loads for (a) Type – H and (b) Type – L.

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the inter-particle coefficient of friction. Qualitatively, the results inFig. 13 can be attributed to the softer nature of the coated LBS grains,which in turn increases the area of contact, as well as the greaterroughness for Type – L and Type – H in comparison to pure LBS. Asummary of the results, in terms of inter-particle coefficient of frictionfor each given test as well as the average inter-particle friction valuesfor the different classes are given in Table 1. This increase can be as-certained from the coefficient of inter-particle friction and friction an-gles shown in Table 1.

Representative microscopic images showing the damage on thegrain surface after shearing (Type – H at 2 N and 10 N) are shown inFig. 14. After shearing, the coating material on the surface of the par-ticle was scraped in the direction of shearing and it was accumulated atthe end of the traverse. The image of the surface of the particle showsthe accumulation of the coating material at the end of the shearingpath. It is hypothesised that the material being scraped provided ad-ditional resistance against sliding since debris was created between thecontacted surfaces and thus, increase in tangential load was observed.

Damage analysis of pure LBS was studied by researchers and at highernormal loads, the damage on the surface was severe and was termed asploughing [13], which ploughing behaviour was also noticed by Yanget al. [11]. Since the coating material is not an integral part of theparticle, scrapping phenomenon was observed even for lower normalloads in the present study. The area of contact between the grains isexpected to be greater at higher normal loads, so that the amount ofdamage occurred to the kaolinite coating on the surface of the grains isalso wider, which is confirmed from the images in Fig. 14.

7. Summary and conclusions

In this study, the normal and tangential load-displacement beha-viour of artificially coated LBS particles was examined. Commerciallyavailable clay mineral, kaolinite, was used as the coating agent. Orbitalshaker was used to apply the coating on the LBS particles using differentconcentrations of solutions and various times of exposure. Coating ef-ficiency was analysed using EDS analysis and SEM images as well as

Fig. 13. Effect of coating compound on the frictional response of LBS grains: (a) comparison of tangential load-displacement behaviour at 5 N normal load showingshift in steady state or micro-slip condition (b) tangential load versus normal load variation corresponding to steady state.

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surface roughness measurements of the grains. EDS results showed anincrease in adsorption (change in Al contents on the surface of theparticle) with increasing concentration for a given time of exposure.Increased roughness values were observed for coated particles com-pared to pure LBS. Representative particle crushing tests were carriedout for pure LBS and grains with heavier coating. The peak crushingloads, the characteristic strengths and the Weibull's moduli were foundto be similar between pure LBS and coated grains. However, for thecoated grains, notable plastic deformation was observed which shiftedthe normal-load displacement curves to greater displacements. This isdue to the deformation of soft coating material on the surface of LBS.Two extreme cases of coating (lowest and highest, i.e. 25 and 300mg/ml with 500″ exposure time) were selected to conduct micromechanicalsliding tests and also pure LBS results from the literature were used forcomparison. Similar to particle crushing tests, higher normal displace-ments were required to reach a given normal load. This trend was moreapparent as the amount of coating increased. In the tangential direc-tion, the curves were shown to be softer for heavier coating, and par-ticularly at greater normal loads, micro-slip was observed rather than asteady state sliding for many of the experiments. The tangential dis-placements required to reach steady state sliding or micro-slip in-creased with the increase of coating level. The inter-particle frictionincreased markedly from the uncoated LBS to the grains with lightercoating, but the differences were relatively smaller between lighter andheavier coating. Microscopic images of the surface of grains after theshearing was completed, for representative tests, showed that theclayey material was scrapped during shearing. This phenomenon wasfound to be more dominant in heavily coated particles. This creation ofdebris might have contributed to the increase of the inter-particlefriction in comparison to the uncoated grains.

Acknowledgements

This research work was supported by the grants from the ResearchGrants Council of the Hong Kong Special Administrative Region, China,Project No. T22-603/15 N (CityU 8779012)) and Project No. 9042491(CityU 11206617). The anonymous reviewers are acknowledged fortheir kind suggestions that helped us to improve the quality of themanuscript.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.triboint.2018.05.021.

References

[1] Cundall PA, Strack ODL. A discrete numerical model for granular assemblies.Geotechnique 1979;29:49–65.

[2] O’Sullivan C. Particulate discrete element modelling: a geomechanics perspective,Applied Geotechnics, vol. 4. Spon Press; 2011.

[3] Yang ZX, Yang J, Wang LZ. On the influence of inter-particle friction and dilatancyin granular materials: a numerical analysis. Granul Matter 2012;14(3):433–47.

[4] Huang X, Hanley KJ, O'Sullivan C, Kwok CY. Exploring the influence of interparticlefriction on critical state behaviour using DEM. Int J Numer Anal Meth GeoMech2014;38(12):1276–97.

[5] Dai BB, Yang J, Zhou CY. Observed effects of inter-particle friction and particle sizeon shear behavior of granular materials. International Journal of GeomechanicsASCE 2016;16(1). 04015011. 10.1061/(ASCE)GM.1943-5622.0000520.

[6] Sazzad Md M, Suzuki K. Effect of Inter-particle friction on the cyclic behavior ofgranular materials using 2D DEM. J Geotech Geoenviron Eng 2011;137(5):545–9.

[7] Cavarretta I, Coop MR, O'Sullivan C. The influence of particle characteristics on thebehaviour of coarse grained soils. Geotechnique 2010;60(6):413–23.

[8] Cavarretta I, Rocchi I, Coop MR. A new interparticle friction apparatus for granularmaterials. Can Geotech J 2011;48(12):1829–40.

[9] Senetakis K, Coop MR, Todisco MC. The inter-particle coefficient of friction at thecontacts of Leighton Buzzard sand quartz minerals. Soils Found 2013;53(5):746–55.

[10] Senetakis K, Sandeep CS, Todisco MC. Dynamic inter-particle friction of crushedlimestone surfaces. Tribol Int 2017;111:1–8.

[11] Yang L, Wang D, Guo Y, Liu S. Tribological behaviors of quartz sand particles forhydraulic fracturing. Tribol Int 2016;102:485–96.

[12] Nardelli V, Coop MR, Andrade JE, Paccagnella F. An experimental investigation ofthe micromechanics of Eglin sand. Powder Technol 2017;312:166–74.

[13] Sandeep CS, Senetakis K. Grain-scale mechanics of quartz sand under normal andtangential loading. Tribol Int 2018;117:261–71.

[14] Cole DM, Peters JF. Grain-scale mechanics of geologic materials and lunar simu-lants under normal loading. Granul Matter 2008;10:171–85.

[15] Cole DM, Mathisen LU, Hopkins MA, Knapp BR. Normal and sliding contact ex-periments on gneiss. Granul Matter 2010;12:69–86.

[16] Cole DM, Hopkins MA. The contact properties of naturally occurring geologicmaterials: experimental observations. Granul Matter 2016;18:1–16.

[17] Tembe S, Lockner DA, Wong TF. Effects of clay content and mineralogy in frictionalsliding behaviour of simulated gouges: binary and ternary mixtures of quartz, illite,and montmorillonite. J Geophys Res C Oceans Atmos 2010;115(B3).

[18] Takahashi M, Mizoguchi K, Kitamura K, Masuda K. Effects of clay content on thefrictional strength and fluid transport property of faults. J Geophys Res C OceansAtmos 2007;112(B8).

[19] Yang J, Wei LM. Collapse of loose sand with addition of fines: the role of particleshape. Geotechnique 2012;62(12):1111–25.

[20] Dai B, Yang J, Luo X. A numerical analysis of the shear behavior of granular soilwith fines. Particuology 2015;21:160–72.

[21] Phan VT, Hsiao DH, Nguyen PTL. Effects of fines contents on engineering propertiesof sand-fines mixtures. Procedia Engineering 2016;142:213–20.

[22] Zhang X, Baudet BA, Hu W, Xu Q. Characterisation of the ultimate particle sizedistribution of uniform and gap-graded soils. Soils Found 2017;57(4):603–18.

[23] Lade PV, Yamamuro JA. Effects of non-plastic fines in static liquefaction of sands.Can Geotech J 1997;34(6):918–28.

[24] Wei LM, Yang J. On the role of grain shape in static liquefaction of sand-finesmixtures. Geotechnique 2014;64(9):740–5.

[25] Kwa KA, Airey DW. Effects of fines on liquefaction behaviour in well-graded ma-terials. Can Geotech J 2017;54(10):1460–71.

[26] Sosio R, Crosta GB. Rheology of concentrated granular suspensions and possibleimplications for debris flow modelling. Water Resour Res 2009;45(W03412):16.

[27] Capece M, Dave RN. Solventless polymer coating of microparticles. Powder Technol2014;261:118–32.

[28] van Kampen A, Kohlus R. Systematic process optimization of fluid bed coating.Powder Technol 2017;305:426–32.

[29] Freireich B, Li J. A renewal theory approach to understanding interparticle coatingvariability. Powder Technol 2013;249:330–8.

[30] van Kampen A, Hitzmann B, Kohlus R. Assessment of coating quality by use ofdissolution kinetics. Powder Technol 2015;286:325–31.

[31] Bar-Hen M, Etsion I. Experimental study of the effect of coating thickness and

Fig. 14. Microscopic images of Type – H grains after shearing at (a) 2 N (b) 10 Nnormal loads.

S.S. Kasyap, K. Senetakis Tribology International 126 (2018) 206–217

216

substrate roughness on tool wear during turning. Tribol Int 2017;110:341–7.[32] Tonelli L, Martini C, Ceschini L. Improvement of wear resistance of components for

hydraulic actuators: dry sliding tests for coating selection and bench tests for finalassessment. Tribol Int 2017;115:154–64.

[33] Barrows HL, Simpson EC, Tu HY. Formation of surface coatings on limestone par-ticles in soil. Soil Sci Soc Am Proc 1966;30:318–20.

[34] Scheidegger A, Borkovec M, Sticher H. Coating of silica sand with goethite: pre-paration and analytical identification. Geoderma 1992;58:43–65.

[35] Torrey JD, Krischling TL, Greenlee LF. Processing and characterization of nano-particle coatings for quartz crystal microbalance measurements. J. Research ofNational Institute of Standards and Technology 2015;120.

[36] Nardelli V. An experimental investigation of the micromechanical contact behaviorof soils. Architechture and civil engineering departmen. City university of HongKong; 2017.

[37] Wang W, Coop MR. An investigation of breakage behaviour of single sand particlesusing high-speed microscope camera. Geotechnique 2016;66(12):984–98.

[38] Sandeep CS, Senetakis K. Effects of Young’s modulus and surface roughness on theinter-particle friction of granular material. Materials 2018;31(2).

[39] Joe J, Scaraggi M, Barber JR. Effect of fine-scale roughness on the traction betweencontacting bodies. Tribol Int 2017;111:52–6.

[40] Hanaor DAH, Gan Y, Einav I. Static friction at fractal interfaces. Tribol Int2016;93:229–38.

[41] Cavarretta I, O'Sullivan C. The mechanics of rigid irregular particles subject touniaxial compression. Geotechnique 2012;62(8):681–92.

[42] McDowell GR, Bolton MD. On the micromechanics of crushable aggregates.Geotechnique 1998;48(5):667–79.

[43] Weibull W. A statistical distribution function of wide applicability. J Appl Mech1951;18:293–7.

[44] Lim WL, Mcdowell GR, Collop AC. The application of Weibull statistics to thestrength of railway ballast. Granul Matter 2004;6:229–37.

[45] Senetakis K, Coop MR. The development of a new micro-mechanical inter-particleloading apparatus. Geotech Test J 2014;37(6):1028–39.

[46] Ni J, Zhu Z. Experimental study of tangential micro deflection of interface of ma-chined surfaces. J Manuf Sci Eng 2001;123(2):365–7.

[47] Sandeep CS, Todisco MC, Senetakis K. Tangential contact behaviour of weatheredvolcanic landslide material from Hong Kong. Soils Found 2017;57:1097–103.

S.S. Kasyap, K. Senetakis Tribology International 126 (2018) 206–217

217