Investigation on abrasion-corrosion properties of WC-based composite with fractal theory

Journal Pre-proof Investigation on abrasion-corrosion properties of WC-based composite with fractal theory

Liang Peng, Bingsuo Pan, Zhijiang Liu, Ziyu Liu, Songcheng Tan PII:

S0263-4368(19)30704-8

DOI:

https://doi.org/10.1016/j.ijrmhm.2019.105142

Reference:

RMHM 105142

To appear in:

International Journal of Refractory Metals and Hard Materials

Received date:

30 August 2019

Revised date:

24 October 2019

Accepted date:

28 October 2019

Please cite this article as: L. Peng, B. Pan, Z. Liu, et al., Investigation on abrasioncorrosion properties of WC-based composite with fractal theory, International Journal of Refractory Metals and Hard Materials(2018), https://doi.org/10.1016/ j.ijrmhm.2019.105142

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© 2018 Published by Elsevier.

Journal Pre-proof

Investigation on abrasion-corrosion properties of WC-based composite with fractal theory Liang Penga, Bingsuo Pan a, b,1 , Zhijiang Liua, Ziyu Liua, Songcheng Tana a

Faculty of Engineering, China University of Geosciences, Wuhan 430074, China

b

International Joint Research Center for Deep Earth Drilling and Resource Development, Wuhan 430074, China

Abstract: Morphology analys is of a worn surface is one of the significant methods to study wear mechanism. To surmount the weakness of conventional surface characterization system and investigate the abrasion-corrosion properties of WC-based matrix, the fractal geometry combined with three-dimension cloud point has been adopted

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in this study. Tests were conducted in various saline slurry with a modified abrasion test rig capable of monitoring the in-situ electrochemical behavior. The microstructures and compositions of the worn surface were characterized by scanning electron microscopy (SEM) and Raman spectrometer respectively. Results indicate that the increased sodium chloride in the slurry can enhance the variation of the worn surface morphology, leading to a higher value of fractal dimension (FD). Specifically, a raise from 2.444 to 2.515 can be witnessed with the saline gradually

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increasing to saturation. Furthermore, the corrosion products on the worn surface were identified as a mixture of oxides and hydroxychloride, and the multi-oxide films were much more common. Interpretation for the change of FD combined with abrasion-corrosion mechanism has also been detailed in this paper.

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Keyword: fractal; surface; abrasion-corrosion; point cloud; matrix 1. Introduction

Due to the surging demand for oil, gas and various minerals, there is a trend for geological exploration to

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shift from shallow to deep or ultra-deep drilling, and to extend to frozen lands, oceans and other complex areas. Faced with some technical problems in drilling such deep and complex reservoir e.g. wellbore stability [1-3],

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researchers have designed a variety of saline drilling fluids. Thanks to the high conductivity and chloride ion concentration of those drilling fluid [4, 5], the rotary rock-breaking tools downhole, e.g. impregnated diamond bit,

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have to bear severe corrosion attack besides conventional abrasion damage, implying the wear mechanism changes from mechanical wear to the combination of abrasion, corrosion and their synergy[6, 7], which is a totally different damage mechanism from abrasion in ambient air [8]. In a drilling process, the abrasion between the rock and the bit, accompanied by chemical corrosion, plays a crucial role in the drilling efficiency, which will further influence the total cost of a project. Actually, the adverse corrosive effect induced by acid or alkaline environment on the degradation mechanism of similar WC-based hardmetals has already drew some researcher’s attention [9-11]. However, there has been relatively little coverage on the response of impregnated diamond bit to the corrosive environment such as a saline slurry. It becomes apparent that revealing the abrasion-corrosion mechanism is of great significance to tinker with the bit design and manufacture and thus prolong its service life. In essence, diamond bit consists of cutting elements, i.e. diamond grains, and bonding elements, i.e. metal matrix. Therefore, investigation on the abrasion-corrosion properties of bit matrix is a fundamental step in understanding the interaction between the bit and the rock. It is known that surface topography is one of the key factors that indicate the wear mechanism

[12-14]

.

Therefore, surface characterization cannot be neglected when shedding light on the wear process. Generally, statistical parameters, such as the arithmetic mean deviation of profile Ra, are employed to characterize a worn surface [15]. However, the random nature of an engineering surface prevents an exact quantification of the surface 1

Corres ponding author. E-ma il a ddress: [email protected] (B. Pan).

Journal Pre-proof [16]

profile features by these Euclidean geometry methods . There are two main reasons for this. One difficulty in classifying tribological surfaces is the repeatability of the statistical parameters, which means some parameters are correlated with each other. Thus, it is hard to determine which parameters to adopt to distinguish the difference between two surfaces properly. Another concern is the limitation of the stationary random process hypothesis postulated in traditional statistical approaches. Actually, researches have proved that the surface topography is a non-stationary random process [17-19], meaning that those statistical parameters are scale-dependent. In other words, the accuracy of those characteristic parameters is influenced by the sampling length and resolution of the measuring instrument. Alternatively, fractal theory, as a tool to characterize engineering surface topography, has gained much wider attention in recent years since it can not only reflect the natural and intrinsic properties of random phenomena, such as complexity, irregularity, but also overcome the disadvantages of conventional statistical analysis [20-22].

on the basis of previous studies

[23-26]

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Based on the above analys is, this paper aims to illustrate the abrasion-corrosion properties of WC-based impregnated diamond bit matrix. Experiments have been conducted on a modified abrasion test rig to understand the tribological behaviors of the composite and the wear mechanisms have been particularly discussed. Moreover, a modified fractal dimension (FD) calculation method based on three dimensional point cloud has been proposed . Although the objective of this paper is to demonstrate the tribocorrosion

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characteristics of the WC-based matrix, the attempt to combine the point cloud with fractal geometry to characterize worn surfaces will assist in the understanding of wear mechanism in other analogous researches. 2. Experimental

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2.1 Materials

Samples used in this investigation were manufactured by hot sintering technique. As shown in Table 1, the powder mixtures based on the matrix formula were mixed in a ball mill for 8h before the sintering process, which

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Content (wt%)

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was conducted on an SM-100A automatic sintering machine (Hubei Changjiang Jinggong Material Technology Co., Ltd.). The three key sintering parameters and specimen dimensions are given in Table 2. Table 1 M atrix formula

WC

Co

CuSn10*

30

16

54

* A pre-alloyed metal powder with 90% (wt) Cu and 10% (wt) Sn Table 2 Sintering parameters and specimen dimensions

sintering temperature

sintering pressure

retention

Specimen

(℃)

(MPa)

time (min)

dimension (mm)

860

13

2.5

56×24×4

2.2 abrasion-corrosion test As shown in Fig.1, a modified LS-225 abrasion test rig (Shanghai Textile Testing Technology Co. Ltd) was used to evaluate the abrasion-corrosion properties of WC-based composites in a saline environment. The specimen was secured in the holder and loaded against the rotated steel wheel of 150mm diameter, whose circumference was bonded with a 10mm thick rubber ring. Moreover, six metallic blades were welded to the flank of the steel wheel to agitate the slurry during the rotation. Prior to the abrasion experiment, the specimens were polished and electrically insulated from the surrounding environment except for the target surface for research. Load applied to the specimen was transferred through the L-shaped arm, which could rotate around the pivot but whose movement in the horizontal direction was limited. The abrasive used in this experiment were 40 mesh and 100 mesh silica sand with a mass mixed ratio of 1:1 to simulate a field condition. And the prepared abrasive

Journal Pre-proof slurry was a blend of 510 g dry abrasive and 1700g saline solution. Five different sodium chloride concentrations varying from 0% to saturation were adopted for this research. In the process of wear, air was constantly pumped into the slurry to prevent the abrasive particles from flocculating, To make the in situ open-circuit potential (OCP) measurements with respect to time during the wear process, a three-electrode electrochemical cell connected to an electrochemical workstation (Wuhan CorrTest, CS310H) was embedded in the sample holder, in which the specimen served as a working electrode and a graphite sheet and a saturated calomel electrode (SCE) were selected as the counter electrode and reference electrode respectively. In addition, the in situ electrochemical polarization curve was obtained by scanning from 200 mV nobler than OCP to 100mV more negative than the OCP at a rate of 2 mV/s with reference to SCE.

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Pivot

WE

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RE CE

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Rubber rim

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Metal blade

Load

Sample

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Slurry

Fig.1 Schematic of the modified abrasion-corrosion test rig (WE: Working Electrode, RE; Reference Electrode, CE: Counter

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Electrode).

Following the wear process, the micro topography and elements distribution of the worn specimen surface

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were analyzed by scanning electron microscopy (SEM, G2 PRO, Phenom) equipped with an energy-dispersive X-ray spectroscope (EDS, EM-30AX PLUS +, Coxem). What’s more, the corrosion products on the worn surface

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were detected by a Roman spectrometer (RS, RM-1000). Finally, weight change of the specimen before and after the abrasion test was measured by an electronic balance with an accuracy of 0.1 mg. The detail test conditions were given in Table3. Table 3 Abrasion-corrosion test conditions

Load (N)

100

Speed of rotation (m/min)

260

Time of rotation (min)

30

Rotated wheel diameter (cm)

178

Sodium chloride concentrations

0% ;3.5%;10% ;20% ;saturated(26.5%)

Mass fraction of abrasive in slurry

0.3

2.3 Fractal dimension calculation The FD calculation method used in this research was optimized according to the principle of box-counting algorithm owing to its wide acceptance [27-30]. In contrast to the previous methods, the three dimensional point cloud of the surface, rather than the binary image, was introduced into the proposed method to get higher calculation accuracy. Therefore, no specific image size, e.g. a M×M matrix, is required in this method. Before the worn sample was sent to the operation platform of a confocal laser scanning microscope (VK-X100K, manufactured by KEYENCE), an ultrasonic bath and drying process were essential pretreatment steps.

Journal Pre-proof For consistency, the laser completes the scanning of a stripe-shaped region with 0.3mm in width and 14mm in length along the sliding direction in the center of wear scar for all specimens with a magnification of 200 times. Subsequently, the preprocessing of point clouds including filtering, smoothing, hole filling, and point cloud simplification was conducted on CloudCompare software, which is fundamental for FD calculation. Based on the results of point clouds processing, the surface matrix is set as 𝑋 × 𝑌 × 𝑍, where X and Y denotes the 2-D position with the third coordinate (Z) denoting the surface amplitude. The surface projection on the XOY plane is partitioned into grids of size 𝑎 × 𝑏 and the surface matrix is covered by the box of size 𝑋 𝑌 𝑍 𝑎 × 𝑏 × 𝑐 , where (𝑎 ) = (𝑏) = (𝑐 ) = 𝛿 is satisfied. On each grid (i,j), the maximum and the minimum

amplitude drops into the 𝑚𝑡ℎ and 𝑛𝑡ℎ box respectively. Therefore, the number of boxes in the certain grid (i,j) can be express as 𝑛(𝑖, 𝑗) = 𝑚 − 𝑛 + 1 when the scale factor is 𝛿. Taking the contributions of all the grids, the total number of boxes that cover the whole surface is 𝑁𝛿 = ∑𝑖𝑗 𝑛(𝑖, 𝑗) − 𝑁𝑛𝑢𝑙𝑙 where 𝑁𝑛𝑢𝑙𝑙 is the number of

log(𝑁𝛿 ) ⁄log(𝛿)

(1)

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𝐷=

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boxes containing no points. Since N is counted for different sizes of box, i.e. different values of 𝛿, the FD can be demonstrated by Eq. (1).

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Referring to all those analyses exhibited above, a program based on Matlab had been des igned for the FD calculation.

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3. Results and discussion

3.1 wear behavior Results on the influence of salinity on the FD were present in Fig.2, together with the weight loss. It’s

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obvious to notice that both the FD curve and the weight loss curve show a growing tendency as the salinity of the slurry increases. The value of FD of the worn surface rises form 2.444 in pure water to 2.515 in saturated salinity

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slurry, which indicates a more detailed worn surface topography since the value of FD is the indictor of the surface complexity. Moreover, it is in good agreement with the change of three-dimensional profile of the worn

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surface as compared in Fig.3, in which the worn surface peaks is relatively finer and denser in slurry with a higher concentration of NaCl. Besides, the scratches on the surface get deeper, meaning a much more severe surface degradation. Similar to the evolution of the FD, the weight loss value also sees a dramatic increase overall, with 1.58mg in clean water and 3.5mg for a saturated salinity slurry.

Fig.2 Influence of slurry salinity on fractal dimension and weight loss of the test sample

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Fig.3 Three dimensional surface morphologies of the worn tracks under different salinity slurry : (a) 0%; (b) 10%; (c) saturated

3.2 Electrochemical characteristic

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The variations of in situ OCP over time with different salinity slurry are shown in Fig. 4.

Fig.4 In situ OCP vs. time for the samples in different salinity

Clearly, every curve can be divided into three regions. In the first region, 0-60s, the OCP shifted gradually toward the negative direction, showing active behavior of the test sample. This is because the surface exposed to the corrosive slurry has been polished in advance and corrosion could occur owing to the high content of active chemical component, such as Cu and Co, on the surface. In the second region, from 60s to 1860s, the curve shows a dramatic rise immediately after the rotation begins, and one important thing to notice is that the marked shift of OCP is toward the positive direction, rather than the negative direction as shown in previous research

[31-33]

. This

counterintuitive result is not surprising considering the experimental rig (especially for the blade of wheel) employed in this study. When the wheel rotates initially, the slurry will splash and abundant oxygen will dissolve in. Simultaneous ly, these agitated slurries can act as a consecutive electrolyte film on the whole surface of the

Journal Pre-proof sample since it is not fully immersed. As a result, the fresh surface passivates quickly, leading to a more noble value of OCP. Since then the OCP maintains a relatively more positive value and fluctuates slightly, which is likely due to the competing depassivation and repassivation related to the inconsistent abrasion within the wear scar, and the downward sloping curve demonstrates that the effect of former outweighs that of the latter. This has been confirmed by the in situ measurements of potentiodynamic polarization curves as shown in Fig.5. Compared with the result measured in static condition, the free corrosion potential (𝐸𝑐𝑜𝑟𝑟 ) becomes the noblest with rotation in absence of applied load. Once the load is applied, there is a shift in 𝐸𝑐

in active direction, implying the

effect of mechanical abrasion. However, the 𝐸𝑐𝑜𝑟𝑟 obtained is still nobler than that collected under static condition owing to the limited abrasive damage to the passive film, or the finite area of wear track compared with the unworn area. As for the final region, after the rotation stopped and the liquid level returns to calm at 1860s, there is an instantaneous recovery in OCP towards negative direction for the discontinuity of passive film, which provides

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the penetration path for the corrosive slurry to the underlying substrate. This further corroborates the explanation for the tilted curve in second region.

Fig.5 Polarization curves of sample under different operating condition in 10% NaCl (1: immersion with no applied load; 2: rotation

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with no applied load; 3: rotation with applied load)

Besides, the OCP curve, as a whole, witnesses an overall migration to the active direction as the salinity in

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slurry increased despite the fact that the extent of such increase is restrained at higher salinity. Reasons for these two could be interpreted as the higher concentration of chloride ions and limited area of active materials respectively. 3.3 surface characteristic

b

c

Fig.6 (a)SEM image of the worn surface; (b)(c)corresponding EDS measurement of point A and B in (a) respectively.

As seen from Fig.7, the SEM images of the worn surface formed in 10% and saturated NaCl slurry were

Journal Pre-proof selected and comparatively analyzed with that shaped under clean water of their micro surface features, before which an SEM inspection of the worn surface with the corresponding EDS measurements has been present in Fig.6 to confirm the composition. It can be definitely identified that the soft phase (mainly copper and cobalt) corresponding to dark regions distribute along the WC particles corresponding to the bright ones. Compared with the worn surface formed in clean water with no noticeable corrosive defects, the corrosive attack toward the periphery of the WC particles together with some pits has come into sight in Fig.7(c). As the salinity in the s lurry increases to saturated, the gap marked by red rectangle becomes increasingly obvious, representing a more active corrosion activity. Furthermore, the same phenomenon with grooving mode of abrasive flow is easy to be observed in all the images, which could be related to the plastic-dominated failure mode for fine silica sand

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particles.

Fig.7 SEM images of the worn surface of sample as the salinity varied: (a) 0%; (c) 10%; (e) saturated and (b) (c) (d) are the enlarged SEM images of the red rectangle area respectively

Journal Pre-proof To examine the passive film constitution mentioned above in detail, the surface worn in saturated NaCl slurry was sent to a Raman spectrometer and several typical points on the surface were singled out and analyzed to represent the entire corrosive products. Fig.8 depicts the Raman spectroscopy results with the corresponding optical micrographs indicating the approximate locations where the Raman spectra were collected. For the selected point located at relatively blue areas (Fig.8(b)), it can be identified that the main phase of the corrosion products was Cu2 O for the Raman bands at 218/525/623 cm −1[34-36]. The Cu2 O phase has also been detected in the deep yellow areas in Fig.8(c) for the similar peaks at 220/530 cm−1, and the blunt peak around the 610 cm −1 may [37, 38]

relate to the presence of CuO . As for the relative dark regions in Fig.8(d), the Cu2 Cl(OH)3 and CoO films −1 exhibit their characteristic Raman bands around 114/513 cm (according to the data form RRUFF Project) and −1 [39]

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467/673 cm respectively. Basically, the passive film formed during the abrasion process is a mixture of oxides and hydroxychloride, and the formation of multi-oxide films is much more prevailed.

Fig.8 Raman spectra of worn surface and its corresponding location: spectrum line 1,2,3 is related to the position in (b) (c) (d), respectively.

3.4 Discussion The potential problem of conventional methods employed to characterize rough surface is that the parameters used to describe the rough surface depend on a portion of the specific scales, such as sampling length and instrument resolution, which leads to the limitation to distinguish the surface properties. One solution is to use scale-independent parameter, such as FD, to characterize a rough surface. Since worn surface morphology is firmly related to the wear mechanism, it’s therefore an effective way to provide insight into the wear mechanism by analyzing the fractal dimension of the worn surface. FD relates to the complexity of the worn surface morphology and a higher value of FD indicates more irregular abrasive surface topographies [40]. As the data shown in Fig.2, the high salinity slurry associated with

Journal Pre-proof higher value of FD denotes the increased number of asperities on the surface, leading to the larger contact area. Due to the presence of corrosive aqueous media, such increased contact area, to some extent, means the increased damage as a result of abrasion–corrosion interaction, which has been further corroborated by the change of weight loss. Over the wear process, material removal occurs simultaneous ly by mechanical wear and electrochemical corrosion. The multiple indentations and grooves in Fig.3 indicate the prominent abrasive wear, which will damage the passive film resulting in the exposure of the bare matrix to aggressive environment. At the same time, the corrosion attack of the matrix phase at the WC/matrix interface loosens the carbides and finally leads to the pullout of carbides as can be inferred form the presence of cavities in the wear scar. The removal of hard particles means that the soft matrix loses its protection, causing lower wear resistance and more severe worn surface. As a result, larger number of scratches and pits are produced on the surface, the number of detailed structures is

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growing, which will enhance the variation degree of the surface morphology and thus increase the fractal dimension. Examination of electrochemical behavior reveals more supportive details of the abrasion-corrosion mechanism that has occurred. During the abrasion process, the dissolved oxygen is adsorbed onto the surface and reacts with copper and cobalt to form oxide films though they are weak in terms of mechanical property [41] and

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can be easily removed by abrasion. The abrasive particles and wear debris entrained into the contact surface with the active-passive sites on the worn surface will both cause the micro-galvanic coupling, preventing the potential from moving further into positive and getting fluctuating OCP value. Also, the high affinity of copper and cobalt for chlorine ions complicates as well as enriches the surface chemistry

[42]

. All of those factors will have a

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detrimental impact on the composite properties, making it more vulnerable to abrasion damage. With increasing NaCl content, the anodic behavior becomes more active and as a consequence degradation related to corrosion

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4. Conclusions

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becomes more intensive. Hence, severe abrasive wear occurs and the variation of the worn surface topography gets higher, which finally leads to the increased FD value.

Based on fractal theory, this paper reveals the relationship between the worn surface topography and the

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abrasion-corrosion mechanism in saline environment. According to the discussions and results above, several conclusions can be drawn as follows: 1. An attempt has been made to combine the calculation of FD with three dimensional point cloud, in which the limitation of the image size required conventionally has been broken down. With the increase in NaCl concentration, the wear resistance of the specimen gets worse due to the loss of skeletal carbides. Besides, the corrosion effects make it more susceptible to abrasion damage. Consequently, the more severe degeneration occurs on the surface, leading to the enhanced variation in surface morphology associated with a higher FD value. More precisely, The value of FD of the worn surface rises form 2.444 in clean water to the peak of 2.515 in saturated salinity slurry. 3.

SEM micrographs of the worn surface reveal that the preferential corrosion attacks the WC/matrix interface which subsequently leads to the pullout of carbides and severe damaged surface. The dominant corrosive products have been detected as a mixture of oxides and hydroxychloride.

Acknowledgements This work was supported by the National Natural Science Foundation of China [Grant number 41872187].

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[38] Xu J F, Ji W, Shen Z X, et al. Raman spectra of CuO nanocrystals[J]. Journal o f Raman spectroscopy, 1999,30(5):413-415. [39] Tang C, Wang C, Chien S. Characterizat ion of cobalt o xides studied by FT-IR, Raman, TPR and TG-MS[J]. Thermochimica Acta, 2008,473(1-2):68-73. [40] Papanikolaou M, Salonitis K. Fractal roughness effects on nanoscale grinding[J]. Applied Surface Science, 2019,467-468:309-319. [41] Wood R J K, Herd S, Thakare M R. A crit ical review of the tribocorrosion of cemented and thermal sprayed tungsten carbide[J]. Tribology International, 2018,119:491-509. [42] Chan H Y H, Takoudis C G, Weaver M J. Oxide Film Format ion and Oxygen Adsorption on Copper in Aqueous Media As Probed by Surface-Enhanced Raman Spectroscopy[J]. The Journal of Physical Chemistry B, 1999,103(2):357-365.

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Journal Pre-proof The abrasion-corrosion properties of WC-based composite were interpreted.

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A modified FD calculation method based on 3D point cloud was proposed.

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The presence of sodium chloride could increase the FD value of worn surface.

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The corrosion products on the worn surface were multi-oxides and hydroxychloride.

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Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be

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considered as potential competing interests: