Dry sliding friction and casing wear behavior of PCD reinforced WC matrix composites

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Dry sliding friction and casing wear behavior of PCD reinforced WC matrix composites Kai Zhang, Zhenquan Wang, Deguo Wang, Yanbao Guo, Bo Zhao

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S0301-679X(15)00179-6 http://dx.doi.org/10.1016/j.triboint.2015.04.028 JTRI3655

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Received date: 28 January 2015 Revised date: 13 April 2015 Accepted date: 16 April 2015 Cite this article as: Kai Zhang, Zhenquan Wang, Deguo Wang, Yanbao Guo, Bo Zhao, Dry sliding friction and casing wear behavior of PCD reinforced WC matrix composites, Tribology International, http://dx.doi.org/10.1016/j.triboint.2015.04.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Dry sliding friction and casing wear behavior of PCD reinforced WC matrix composites Kai Zhang1), Zhenquan Wang2) *, Deguo Wang1), Yanbao Guo1), Bo Zhao1) 1) College of Mechanical and Transportation Engineering, China University of Petroleum-Beijing, 18 Fuxue Road, Changping, Beijing 102249, China 2) College of Petroleum Engineering, China University of Petroleum-Beijing, 18 Fuxue Road, Changping, Beijing 102249, China *Corresponding author: [email protected] Abstract:

The friction and casing wear properties of PCD reinforced WC matrix composites were investigated using a cylinder-on-ring wear-testing machine against N80 casing steel counterface under dry sliding conditions. The results indicate that the friction and casing wear rate of PCD reinforced WC matrix composites are the lowest among the materials. As the applied load and sliding speed steadily increase, the friction coefficients of PCD reinforced WC matrix composites decrease. In addition, the casing wear rates increase with increasing load, but decline with sliding velocity. The dominant wear mechanism of the PCD composite is the micro-cutting wear, accompanied by adhesive wear. Keywords:

Polycrystalline diamond (PCD); composite; sliding wear; wear mechanism 1 Introduction

With the growth in demand for energy, particularly oil, and the development of petroleum exploration and exploitation, new drilling technologies have been developing rapidly, which could bring more serious drill pipe wear and casing damage comparing with traditional technologies [1-5]. Since the 1970s a lot of research works focused on the protection of casing and drill pipe have been carried out. Based on these researching results, a great deal of preventive measures have been developed, such as hardbanding for drill pipe tool joints, non rotation drill pipe protector (NRDPP), drill-stem sub, drill pipe stabilizer, and etc. Among them, the hardbanding is the most widely used method [6-13]. The hardbanding is emerging as a protective layer being used on a drill pipe to protect 1

the drill string. Along with the new drilling technologies used to drill the highly deviated wells, such as horizontal well, extended-reach well or multi-directional well, the wear problems on the drill pipe and casing become severe. To improve the performance of hardbandings, several types of wear resistant alloy hardbandings have been developed [11-14]. Although the performance has been significantly improved, drill pipes and casings still suffer from friction and wear issues. Therefore, new technologies for well drilling applications present a challenge in the choice of hardbanding materials due to the harsh operating environment where they must perform [4-5]. As is known to all, the main wear mechanism of drill pipes is particle wear and the primary wear mechanism of casings is micro-cutting wear. Hence, the new hardbanding materials should have strong resistance to abrasive wear, and at the same time avoid the casing from being micro-cut [14-15]. The polycrystalline diamond (PCD) is known for its homogeneous hardness and good toughness [16-20]. And furthermore, PCD has a high wear resistance and low friction coefficient. Even in a high temperature environment, it can also maintain its excellent performance [21-27]. These advantages contribute to the wide acceptance of PCD in oil and mining explorations. Thus PCD is the optimum choice of the hardbanding material which could be the combination producing the least friction and wear for the tribosystem. However, it’s hard to apply PCD blocks on drill pipes as hardbanding material through conventional means because PCD is a nonmetal material. In previous work [28], we apply PCD blocks on a drill pipe as hardbanding material effectively prepared via special means. First of all, PCD is made into a composite with other materials in a similar way to the infiltration of PDC bits. This kind of PCD composite material can easily be applied on drill pipes in some special way with little damage to the drill pipe. This new hardbanding material, called PCD reinforced WC matrix composite (PCD composite for short), is a kind of composite material with the fine property [29]. Extensive development efforts have been undertaken on hardbandings for drill pipes [1-14]. According to the results of these studies, an ideal drilling material has been requested to have fine fiction and wear characteristics such as low coefficient of friction and high wear resistance, which can not only greatly improve the wear resistance of drill pipe but also reduce the casing wear. High wear resistance is obviously desirable to increase drill pipe and casing life and reduce maintenance while low coefficient of friction is desirable to reduce the energy needed for drilling [5]. Nowadays, the wear

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resistance of almost all common hardbandings is quite well. What is more, any evaluation of the hardbanding materials must include an evaluation of the response of the counterface, i.e., the well casing. As a consequence, apart from their own wear resistance considerations, the optimum choice of hardbanding materials would be the combination producing the least friction and casing wear for this special tribosystem. In addition, few studies have investigated the PCD reinforced WC matrix composites with application on the drill pipe joint. Therefore, the friction and casing wear behaviors of PCD reinforced WC matrix composites remain unclear, and deeper understanding on the casing wear mechanism of these composites is needed. In this work, the friction and casing wear behaviors of PCD reinforced WC matrix composites under dry sliding were investigated by using an experimental system aiming at the drilling operations. 2 Experimental

2.1 Test rig and materials A cylinder-on-ring test rig used to investigate the friction and wear characteristics of hardbanding materials is shown schematically in Figure 1. The lower sample of the hardbanding ring was fixed onto the lower shaft, which was kept stationary during tests because the arm installed on the side of the lower shaft was pressed against a load cell and prevented the shaft from rotation. The upper steel cylinder was fixed into a holder, which was driven by a motor. As indicated in Figure 1(a), the steel cylinder rotating at a selected speed slid against the lower ring sample of the hardbanding materials. The rotation speed was controlled by an AC motor, and the axial load was applied by means of dead weights on a lever as described in the earlier paper [30-31]. As shown in Figure 1(b), the lower specimen is a special kind of ring made of three different hardbanding materials including self-developed PCD composite, FeCrMnNb alloy and FeNb alloy. The samples of PCD composite materials were made into a special ring with an outer diameter of 58 mm and inner diameter of 38 mm by pressureless infiltration. Before the infiltration, the PCD blocks were first fixed on the inside surface of steel cylinder which was put into the special mold cavity made according to the specified requirements after fixing. The PCD blocks used here were made into irregular cuboids with the dimensions 3mm ×3 mm×10 mm. There was a semi-circular structure with a radius of 1.5 mm at each end of the PCD block, as indicated in Figure 1(b). And then, the casting 3

tungsten carbide particles were poured into the mold cavity and jolt-rammed by the vibration machine. The binder metal used in the present study was manganese copper-nickel alloy, which was put on the top of the guide hole of the mold. During the infiltration process, the manganese copper-nickel alloy (38 wt.%) and casting tungsten carbide particles (62 wt.%) were heated to 1135 ℃ for one hour in an sintering furnace to let the matrix alloy infiltrate the PCD blocks without pressure. After being infiltrated, the samples were removed from the furnace at 900℃ and the PCD composite material was obtained after surface treatment [32]. The chemical compositions of them are shown in Table 1. The other two test materials commonly used in a realistic down-hole string were made by overlaying welding on the surface of a 1045 steel ring [33-34]. The hardness and roughness of these three kinds of hardbanding materials are shown in Table 2. A self-developed steel cylinder with a diameter of 4 mm and thickness of 5 mm was used instead of the full-size casing pipe in the realistic down-hole string. In this test, the special cylinder was made of the N80 casing steel which was the most commonly used material in the realizing casing service environment. The surface profile of the test material was observed by a 3D white-light interfering profilometer. The root-mean-square roughness (Sa) value of the steel cylinder used in this study was about 338 nm. 2.2 Experimental procedure In the current study, the tests were conducted using the cylinder-on-ring test rig in dry sliding conditions. The experiments were carried out at a sliding velocity of 0.2-0.8 m/s at room temperature. The test loads were calculated as below [35-36]:

W

16P 2 RL  E*

(1)

where W is the vertical load (N), P is the average contact stress(MPa), R is the radius of the cylinder, L is the length of the cylinder axis and E* is the equivalent elastic modulus. E* can be calculated from:

1 1  12 1  2 2   E* E1 E2

(2)

where E1, E2 are the elasticity modulus of the two contact surfaces. 1 ,  2 are the poisson's ratios of 4

the two contact surfaces. The contact stress between the drill string and the borehole wall is influenced by many kinds of factors. The real contact stress can be commonly selected from zero to dozens of MPa [37]. So, in consideration of the downhole conditions and the limited experimental conditions, the value of the vertical load W ranges from 10-70 N from Eqs. (1). Three repeat tests were conducted for each set of frictional pairs, and the average of the three repeat tests is reported in this paper. Before and after the tests, the mass of the samples was obtained on SartoriusA10S electronic balance with up to 0.1 mg of accuracy. All the specimens were ultrasonically cleaned with petroleum ether and dried in a vacuum drying oven for 30 min before and after each test. The casing wear rate was calculated from the mass loss measurement. The sliding time and friction coefficients were recorded automatically during the tests. The casing wear rate was calculated from the mass loss measurement. After the wear tests, a Quanta200 scanning electron microscopy (SEM) was used to observe the worn surface and debris of the specimens. And the surface compositions of samples were analyzed by X-ray energy dispersive spectrometer (EDS) [38-39]. 3 Results and discussion

3.1 Friction and casing wear properties of different hardbanding materials Figures 2(a) shows the friction coefficient curves of the three materials at an applied load of 50 N and a sliding velocity of 0.2 m/s. It can be seen that the friction processes of the hardbanding materials consist of two stages. The first stage is the running-in period, at which the friction coefficient sharply increases. The second is the stable stage, which indicates the relatively smooth fluctuation of the friction coefficient. With PCD composites, for example, the 3D surface micrographs at different stages can be seen in Figure 2(c)-(f) clearly. In the initial stage of the friction and wear process, the roughness of the sample surface is higher. The contact area between the cylinder and ring is smaller, and the adherent point is relatively rugged. The local stress of the adherent point is much larger, so plastic deformation can easily occur on the surfaces of the steel cylinder, which resists sliding of the steel cylinder over the ring sample. Therefore, the friction coefficient increases sharply at the running-in period. As the friction and wear proceeded, the roughness of the surface is reduced and the convex peak gradually wears off. Thus, the friction coefficient is relatively constant at the stable stage [40-43]. Compared with the other two kinds of materials, the fluctuation of the PCD composite material’s 5

friction coefficient is minimal and its friction coefficient is also minimal (Figure 2(a)), whereas the friction coefficients of the other two materials are slightly different. Because of good wettability between the WC matrix and the PCD blocks, a uniform and compact composite is made [32]. The structures of PCD composites reduce the friction coefficients of composites and effectively improve their stability of friction. The running-in period of the PCD composite material is longest, primarily caused by the high wear resistance of PCD and WC matrix[32, 34]. It is hard for the convex peak on the PCD composite material to wear out, so the stable stage is relatively difficult to reach. Figure 2(b) shows the N80 casing wear rates of three materials under the same conditions. It can be seen that the casing wear rates of PCD composite material, FeCrMnNb alloy and FeNb alloy successively increase, and the casing wear rate of PCD composite material is almost half that of the FeNb alloy. The friction and casing wear properties of PCD composite materials are superior. There are a large number of PCD blocks on the rings. During the friction process, these blocks absorb most of the load and the friction, resulting in less contact between the matrix and cylinders thereby protecting the matrix and reducing the wear rates of N80 casing. These blocks reduce the wear rates of N80 casing materials and effectively improve the wear resistance of composites [34]. 3.2 Effect of varied applied loads on casing wear behaviors of hardbanding materials Figure 3(a)-(c) shows that the friction coefficients of hardbanding materials are gradually increased and continue to remain stable with increasing sliding time. For PCD composite materials, the friction coefficients become smaller with the increase in the applied loads, whereas the friction coefficients of the other two materials are distinctly different. The friction coefficients of FeNb alloy increase with wear load up. It is mainly because of the gradually higher roughness caused by the heavy wear on the worn surface. In the experiments, a number of hard convex peaks and grooves caused by abrasive wear are formed on the worn surface as the experiments proceed. These peaks and valleys cause the ups and downs of the worn surface which results in a significant fluctuation of the friction coefficient [40-43]. Figure 3(d) can show that FeCrMnNb alloy is a “casing friendly” material which can create less casing wear. The friction coefficient of it remains relatively stable with the increase of loads [44]. At the higher applied loads, the stability of PCD composite’s friction coefficient is much higher 6

than that of other two materials because of the increase of the real contact area between the PCD part and N80 casing. As can be seen in Figure 3(d) clearly, the N80 casing wear rates of hardbanding materials are all gradually increased as function of applied load increases. In all experiments, the casing wear rate of the PCD composite is almost always the lowest with the increase in the applied loads. It shows that the PCD composite has fine friction and casing wear characteristics. PCD is a non-metallic material with high hardness, resulting in less adhesive wear with the increase in the applied loads. And furthermore, the elastic-plastic deformation occurs easily in the surface of N80 casings with the increase of the load. In addition, the temperature can rise with increasing applied load, resulting in the softening of the worn surface of N80 casings. All the above factors can reduce the friction coefficients and N80 casing wear rates of composites [44-46]. 3.3 Effect of varied sliding velocities on casing wear behaviors of hardbanding materials Figure 4 shows the wear coefficients of three materials as a function of sliding velocities at 50 N. It can be seen that the friction coefficient of FeNb alloy decreases regardless of sliding velocities. For the other two materials, the friction coefficients obviously decline with increasing velocities. Just like the other two materials, the friction coefficient of the PCD composite increases slightly when the sliding velocity reaches 0.6m/s. However, when the sliding velocity is above 0.6m/s, the friction coefficient of the PCD composite is obviously lower compared with other sliding velocities. This behavior can be attributed to the increased temperatures with increasing sliding velocity. When the sliding velocity reaches 0.6m/s, the temperatures caused by increasing sliding velocity increase significantly, which result in a significant fluctuation of friction coefficient. When the sliding velocity is above 0.6m/s, the increasing temperatures result in the softening of the worn surface which can reduce the friction coefficients of the composites. Figure 4(d) shows that the N80 casing wear rates all decrease as function of sliding velocity increases. The difference of casing wear rates among three materials gradually decreases with the increase of the sliding velocity. It shows that when the sliding velocity increases, the effect of PCD blocks on casing wear rates becomes weak, and the casing wear rates among three materials show little variation.

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3.4 Wear morphology Figures 5 and 6 show SEM images and EDS analysis of the hardbanding materials’ worn surface at the load of 50 N and sliding velocities of 0.2 m/s respectively. Figures 5(a1)-(a2) show that the worn surfaces of composites’ PCD parts only show slight cracks and a few small grooves. A layer of iron oxide is found on the worn surface by EDS analysis (Figures 6(a1)-(a2)), which makes the friction coefficient lower. Figures 5(b1)-(b2) show that the worn surfaces of composites’ WC matrix parts have a large number of deep and long ploughs, and small cavities are found on the part of binder metal. The wear mechanism of the composite is mainly the micro-cuts wearing mechanism and adhesive flaking off from the WC matrix parts. The WC matrix parts of composites were worn down faster than PCD, which made PCD blocks exposed. The large number of PCD blocks in composites result in lower friction coefficient during the friction process of composites. These PCD blocks assume the most load and friction resulting in less contact between the WC matrix and cylinders. Figures 5(c1)-(d2) show that the worn surfaces of FeNb alloy and FeCrMnNb alloy have few small grooves. On the worn surface of the FeCrMnNb alloy, there is a large amount of iron oxide (Figure 6(d2)). These transport metals make rings have less contact with cylinders. Figure 7 shows SEM images of the worn surface of N80 casings sliding against three materials at the applied load of 50 N and a sliding velocity of 0.2 m/s, respectively. Figure 7(b1) shows that the worn surface appears smooth with mild wear characteristics and a shallow plough. Figure 7(b2) shows that the element in the WC matrix part appears on the worn surface of N80 casings. There is a large number of iron oxide, showing that there exists oxidation wear and three body abrasion because of the hard iron oxide [47]. Figures 7 (c1) and (c2) show that the intense plastic deformation is founded on the worn surface, where there also exists oxidation wear and adhesive flaking off. Figures 7 (d1) and (d2) show that the worn surfaces of alloys appear to have oxidation wear and partially exfoliated structure. Figure 8(a) shows the worn debris morphology of PCD composite at the load of 50 N and a sliding velocity of 0.2 m/s. The worn debris of the PCD composite consists of small granules, and the worn scars are thick and big, which indicates serious micro-cutting wear of the WC matrix on the N80 casing cylinder [48]. It was observed from Figures 8(b1)-(c2) that the worn debris consists of small strips and flakes. Compared with PCD composite, the chip size is clearly decreased. The material

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affected by the mutual friction is WC matrix alloy at the prophase of wear, and the friction and casing wear rate are increased. PCD blocks are gradually exposed on the sample surface with the friction process. The large number of debris not ground into powder result from the formation of exposed PCD blocks. During the friction process of composite materials, the debris is attached to the WC matrix alloy not contacting N80 casing. At this point, the dominant wear mechanism of PCD composite is the micro-cutting wear, accompanied by adhesive wear. For the present work, a schematic diagram (not to scale) shown in Figure 9 was put forward to explain the wear mechanism of PCD reinforced WC matrix composites in dry sliding conditions. In the early stage, real contact regions of different parts are as schematically shown in Figure 9(a). When the composite is squeezed against casing surface into a gap of height, its conformational behaviors depend on the asperities shape of casing and the WC matrix part. The contact area between the casing and composite is smaller, and the adherent point is relatively rugged in the initial stage of the friction and wear process. The local stress of adherent points is much larger, so plastic deformation easily occurs on the surfaces of casing. Therefore, the friction coefficient increases sharply. Meanwhile the materials wear off fast, which mainly occurs on the surface of casing and the WC matrix part. In this stage, the wear mechanism of composites is micro-cut wearing and two body abrasion. As the friction and wear proceed, the roughness of the casing surface is reduced and the convex peak is gradually worn off. In the meantime, the wear of the WC matrix part becomes serious because the binder metal is one of the softer metals. Referring to Table 2, the hardness (Hv) of WC matrix part is 136.87nm - similar to binder metal. As the wear amount of binder metal increases, WC particles gradually are exposed or flake off. The exposed or free WC particles make the WC matrix part wear fast. In consequence, the height difference (h in Figure 9) between the PCD part and the WC matrix part gradually increases. Eventually PCD blocks are exposed on the sample surface with the friction process, making the WC matrix part no longer in contact with casing. So at the final stage, the friction coefficient is relatively constant and the wear amount of the composite can maintain a relatively lower level (Figure 2). And it can be seen from Figure 5 to Figure 8, there is large worn debris, comparative smooth surface of casing and badly worn WC matrix parts. All of the above can illustrate it. What is more, as shown in Figure 9(b), the sliding surface temperature and damage increased with

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the friction and wear process. The oxide layer gradually forms as temperature rises (Figures 6 and 7). Meanwhile the free worn debris is also gradually oxidized, which makes the hardness become higher (Figure 8). In this stage, the wear mechanism of composite is micro-cuts wearing and two or three body abrasion. As shown in Figure 9(b), a large area of oxide layer has been forming, which can effectively cut an abrading of friction pairs. The oxide layer on each friction pair appear as adhesive flaking off under the continuous shear force. The oxide layer can be broken into pieces in this case. And as the friction and wear proceed, the pieces of the oxide layer can adhere tightly to the surface of materials to reform oxide layer. The above process will be repeated. In this stage, the friction coefficient is relatively constant and the wear mechanism of the composite is the micro-cutting wear, accompanied by adhesive wear. 4 Conclusions

In order to better understand the friction and casing wear behaviors under dry sliding conditions, PCD reinforced WC matrix composites have been investigated by using an experimental system aiming at the drilling operations. The tribological test results under dry conditions show that the friction and casing wear properties of PCD composites appear to be superior under similar conditions, mainly due to the large number of exposed PCD blocks in the composites. Experimental results also indicate that the friction coefficients of PCD composites tend to be smaller with increasing load or sliding velocity under a dry friction condition. The exposed PCD blocks in the composites are confirmed to play an important role on hardbandings for drill pipes. The present study might provide some complementary insights into the protection of drilling string and the worn behavior of hardbandings in engineering components. Acknowledgments

This work was supported by International S&T Cooperation Program of China (Grant No. 2012DFR70160).

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Figure 1 Schematic description of the test rig(a) and photographs of samples(b).

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Figure 2 Friction and wear behaviors of three materials: (a) Friction coefficient of three material; (b) Wear rate of three materials; (c) 3D surface micrograph of the cylinder at the Stage 1; (d) 3D surface micrograph of the ring at the Stage 1; (e) 3D surface micrograph of the cylinder at the Stage 2; (f)3D surface micrograph of the ring at the Stage 2. (sliding velocity: 0.2 m/s; applied load: 50 N)

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Figure 3 Friction coefficient (a-c) and wear rate under varied applied loads of three materials: (a) PCD composite material; (b) FeNb alloy; (c) FeCrMnNb alloy; (d) Comparison of friction coefficients and N80 casing wear rates of three materials. (Sliding velocity: 0.2 m/s)

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Figure 4 Friction coefficient (a-c) and wear rate at varied sliding speeds of three materials: (a) PCD composite material; (b) FeNb alloy; (c) FeCrMnNb alloy; (d) Comparison of friction coefficients and casing wear rates of three materials. (Applied load: 50 N)

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Figure 5 SEM images of worn surface of three materials:(a1)PCD part (original surface); (a2) PCD part(worn surface); (b1) WC matrix part (original surface); (b2) WC matrix part (worn surface); (c1) FeNb alloy (original surface); (c2) FeNb alloy (worn surface); (d1) FeCrMnNb alloy (original surface); (d2) FeCrMnNb alloy (worn surface). (sliding velocity: 0.2 m/s; applied load: 50 N)

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Figure 6 EDS analysis of worn surface of three materials:(a1) PCD part (original surface); (a2) PCD part (worn surface); (b1) WC matrix part (original surface); (b2) WC matrix part (worn surface); (c1) FeNb alloy (original surface); (c2) FeNb alloy (worn surface); (d1) FeCrMnNb alloy (original surface); (d2) FeCrMnNb alloy (worn surface). (sliding velocity: 0.2 m/s; applied load: 50 N)

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Figure 7 SEM images and EDS analysis of N80 casing worn surface sliding against three materials:(a1), (a2) original surface; (b1),(b2) worn surface against PCD composite material; (c1) ,(c2) worn surface against FeNb alloy; (d1),(d2) worn surface against FeCrMnNb alloy. ( sliding velocity: 0.2 m/s; applied load: 50 N)

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Figure 8 SEM images and EDS analysis showing worn debris morphologies and elements of three materials: (a1), (a2) PCD composite material; (b1),(b2) FeNb alloy; (c1) ,(c2) FeCrMnNb alloy. (sliding velocity: 0.2 m/s; applied load: 50 N)

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Figure 9 Schematic description of PCD reinforced WC matrix composites in dry sliding conditions. The arrows in each picture indicate the normal squeezing load, w.

Table 1 Composition of PCD composite materials (mass fraction, %)

C

O

Si

Ti

7.74

0.97

PCD part 87.11

WC matrix part

4.19

C

O

Al

Sn

Mn

Fe

Ni

Cu

W

5.08

3.25

0.66

2.59

1.39

2.73

3.49

22.79

58.03

Table 2 Mechanical properties of samples a)

No.

Material

Hv

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Sa(nm)

A1

PCD part

7063.42

8.01

WC matrix part

136.87

262

PCD composite material

A2

FeCrMnNb alloy

760

44.5

A3

FeNb alloy

532

85.9

a) Hv, Shore a hardness; Sa, roughness average.

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Highlights  A kind of composite material is developed as a new hardbanding material.  Probing the friction and casing wear behavior of PCD composites under dry sliding condition.  Experimental result is shown under different applied load and sliding speed.  The wear mechanism of PCD composites in dry sliding conditions was modeled.

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