Wear performances and mechanisms of ultrahard polycrystalline diamond composite material grinded against granite

Int. Journal of Refractory Metals and Hard Materials 54 (2015) 46–53

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Wear performances and mechanisms of ultrahard polycrystalline diamond composite material grinded against granite Gang Yan a, Wen Yue a,b,⁎, Dezhong Meng a, Fang Lin a, Zongyi Wu c, Chengbiao Wang a,b a b c

School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, China Key Laboratory on Deep Geo-drilling Technology of the Ministry of Land and Resources, China University of Geosciences (Beijing), Beijing 100083, PR China Beijing Huayou Guanchang Environment and Energy Science & Technology Development Co., Ltd., Beijing 100020, China

a r t i c l e

i n f o

Article history: Received 4 May 2015 Received in revised form 22 June 2015 Accepted 8 July 2015 Available online 16 July 2015 Keywords: Ultrahard polycrystalline diamond CVD diamond Grind Granite Wear performance Wear mechanism

a b s t r a c t The wear characteristics of a novel ultrahard polycrystalline diamond (UHPCD) were verified comparing with polycrystalline diamond (PCD) through a lathe used here by grinding against granite. The wear ratios of UHPCD and PCD against granite in grinding were calculated according to the sample weight loss. The hardness and the wear morphologies of UHPCD and PCD were characterized by a micro hardness tester, a confocal laser scanning microscope (CLSM) and a scanning electron microscope (SEM). Additionally, both UHPCD and PCD were used in the manufacture of the core drilling bits. The field application tests of the bits were carried out. The results showed that the wear resistances of UHPCD under different rotate speeds are about two times than that of PCD. A cambered face which was fitted to the granite column surface formed at the end face of PCD after the wear process. Differently, a convex structure with the cutting edge of CVD diamond formed at the end face of UHPCD. The hardness of the core of UHPCD (105–115 GPa) which is higher than that of PCD (53–57 GPa) led to the different wear topographies. Higher hardness with a strong support contributes to a superior wear resistance performance. The results of field application tests reveal that the lifespan of UHPCD bits (up to 132.33 m) is superior to that of PCD bits (up to 83.76 m). These findings provide a basis for further geological application of UHPCD. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Superhard materials with excellent wearing resistance and high efficiency [1,2], are urgently required for the deep or ultra-deep hole drilling. The polycrystalline diamond (PCD) has been used in rock drilling bits for nearly 40 years due to its high hardness and wear resistance [3,4]. Chemical vapor deposition (CVD) diamond is one of the ideal superhard materials for its unique chemical and physical characteristics, such as high hardness and wear resistance, good thermal conductivity, low friction, thermal expansion coefficient, and so on. Although CVD diamond is much harder than ordinary PCD, its application is limited by its weak fracture toughness [5,6]. The wear mechanisms of PCD and CVD diamond in grinding have been reported in many references [7–13], as well as the comparative studies of the wear properties of these two materials [14,15]. Deng et al. [7] researched the tribological behaviors of PCD milling against Al2O3 ceramic ball at high temperatures. The extrusion of Co phase resulted in the surface damage at 600 °C, and the graphitization occurred when the temperature exceeded 700 °C. Uhlmann et al. [11] tested thin and thick CVD diamond coating in grinding AlSi17Cu4Mg. They found ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (W. Yue).

http://dx.doi.org/10.1016/j.ijrmhm.2015.07.014 0263-4368/© 2015 Elsevier Ltd. All rights reserved.

that the main wear mechanism of thin, thick CVD diamond was surface adhesion. The long-term use of CVD diamond was affected by the increasing of roughness and rounding of cutting edge during wear. Arumugam et al. [14] researched the machining properties of PCD and CVD diamond in dry machining Al–Si alloy by presenting the correlation among diamond tool morphology, machining parameters, nonferrous workpiece properties, and particulate emission. The primary wear mechanism of the PCD inserts was abrasive wear, and for CVD diamond inserts was massive delamination of the coating. During tests different kinds of chips were generated, and PCD exhibited better cutting continuity than CVD diamond. CVD diamond induced more brittle fracture in the chips due to its higher thermal conductivity. PCD and CVD diamond presented extremely different wear mechanisms due to the compositions and structures. It is supposed that a superhard material which possesses a higher hardness as well as a higher toughness would exhibit excellent antiwear properties. A novel superhard material which was composited of PCD and CVD diamond, noted as the ultrahard polycrystalline diamond (UHPCD), was proposed by Shulzhenko et al. [16]. The UHPCD was sintered by two-anvil high-pressure and high-temperature (HP/HT) press facility with CVD diamond inside the diamond powder. This material had a good wear resistance reaching 0.6 mg/km, which was much lower than that of the PCD material produced in the same synthesis process.

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20 µm

Fig. 1. SEM image of diamond powder. Fig. 4. Schematic diagram of cubic press.

A split sleeve for granite

Nucleation side

Granite

Growth side A special workholder for sample

Flank side

Sample

800 µm

Fig. 2. Optical image of CVD diamond. Fig. 5. Photo of the test bench.

Meng et al. [17] had studied the thermal stability of UHPCD made by the two-anvil press facility. They found that the hardness of CVD diamond in UHPCD decreased from 133 ± 7 GPa to 109 ± 3 GPa after thermal treatment at 1200 °C in argon for 10 min, but it was still higher than 94 GPa the hardness of the CVD diamond. However, the wear mechanism of UHPCD in grinding against rock has not been reported. The wear properties of UHPCD and PCD were tested by grinding granite on lathe, which corresponds to the working condition simulation of the drilling bit. The wear ratio, hardness, Raman spectra as well as the wear morphologies were applied to identify the wear characteristics and wear mechanisms. Moreover, the effects of the field application of UHPCD and PCD on the drilling bits were assessed. It aims to obtain the further understanding of the wear properties of UHPCD in geological application.

2. Experimental details 2.1. Materials synthesis Diamond micron powders mixed with Si micron powder and CVD diamond were used as the raw materials. The Si with purity of 99.99% was adopted here as binder. During the high-temperature and highpressure synthesis process the Si–C bonds form between the melting Si and diamond, which ensures the bonding of diamond particles. The synthesis technology of PCD with SiC bonding phase was proposed early and has been applied commercially [18,19]. The size of diamond powder ranged from 20 to 30 μm as shown in Fig. 1. The weight proportion of Si in mixed synthetic powder was 5 wt.%. The mixed powder was

Relative motion direction

Sample

Fig. 3. Physical image of cubic press.

Granite

Fig. 6. Schematic of relative position between sample and granite.

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G. Yan et al. / Int. Journal of Refractory Metals and Hard Materials 54 (2015) 46–53

(a)

(b)

(c)

20 mm

20 mm

20 mm

Fig. 7. Images of bits: (a) bit without insert, (b) bit with PCD, (c) bit with UHPCD.

(a)

(b) CVD diamond

Point 1

500 µm

Point 2

Point 3

500 µm

Fig. 8. Optical images of PCD and UHPCD: (a) PCD; (b) UHPCD.

treated in high temperature vacuum condition by a vacuum furnace for 24 h in order to remove the adsorption of oxygen and water vapor. The temperature in the vacuum furnace was up to 650 °C and the vacuum degree was about 3 × 10−3 Pa. CVD diamond was produced by a deposition in a hot wire CVD apparatus in methane–hydrogen mixtures. As shown in Fig. 2, the CVD diamond was cut into 0.8 × 0.8 × 4 mm slender column by laser. Fig. 2 shows from top to bottom the nucleation side, growth side and the flank side of CVD diamond. The diamond grain sizes of the diamond nucleation side are smaller than that of growth side due to the nanocrystalline character. The difference between UHPCD and PCD in assembling was that CVD diamond columns were inserted into the assembly blocks for UHPCD. The UHPCD was synthesized by a CS-4 type hinge cubic press facility as shown in Fig. 3, which was widely used in China in fabricating hard materials. The working principle of cubic press is illustrated in Fig. 4. With the hydraulic pressure control, six hydraulic cylinders transmit pressure synchronously to the synthesis center in order to achieve the purpose of high pressure. Meanwhile, the high temperature in synthesis center is implemented by connecting any two opposite hydraulic cylinders with electric circuit. The synthesis process included two steps. At the first stage the assembly was subjected to a treatment at the pressure of 5.5 GPa and temperature of 1170 K for 50 s, and then the temperature increased to 1570 K (silicon melting point) at the given pressure for

90 s. The gained samples were acid pickled to remove the impurities like graphite residues. 2.2. Characterization method The wear tests were carried out on a CS6140A screw-cutting lathe in turning granite as shown in Fig. 5. A split cylinder and a split sleeve were used for fixing the rock column, as well as a special workholder shaped like a drill chuck for sample to fit to the lathe. The rock cylinder was ground to be cut longitudinally at a given penetration. The granite used from Miyun District, Beijing in the tests was made into a cylindrical size of Φ75 × 400 mm. Its compressive strength and drillability grade is 129.6 MPa and IX level respectively. The movement direction of the

Table 1 Hardness of the original CVD diamond, UHPCD and PCD. Position

Original CVD CVD diamond in Edge of UHPCD Center of PCD diamond UHPCD (point 2) (point 3) (point 1)

Hardness (GPa)

90–98

105–115

53–59

53–57 Fig. 9. Raman spectra of the center of UHPCD and PCD.

G. Yan et al. / Int. Journal of Refractory Metals and Hard Materials 54 (2015) 46–53

Fig. 10. Average wear resistance of PCD and UHPCD under different rotate speeds.

(a1)

(b1)

200 µm

(a2)

200 µm

200 µm

(e1)

200 µm

200 µm

200 µm

(h1)

200 µm

(g2)

200 µm

200 µm

(d2)

(g1)

(f2)

200 µm

200 µm

200 µm

200 µm

(d1)

(c2)

(f1)

(e2)

workpiece relative to the granite is shown in Fig. 6. Taken into consideration of the work environment of PCD drilling bits, a selection of water lubrication condition was given, the cutting depth ap = 0.25 mm, the tool feed f = 0.026 mm/r. The parameter of tool feed was chosen in order to maintain a relatively stable work environment. Four levels of spindle speed (200, 280, 400, and 560 rpm) are used in the experiments. The different rotating speeds are related to the fact that the lifespan of PCD bits can be affected by the condition of speed in field drilling application [20–22]. The rotate speed of 200 and 560 rpm would result in maximum cutting speeds of 47.1 and 132.0 m/min, respectively. For comparison with concrete cutting, Moseley et al. [9] studied wear mechanisms of PCD cutters arranged on bits ranging from 52 to 152 mm in diameter and rotational speeds of 90–500 rpm resulting in maximum surface speeds of over 200 m/min. Considering the possibility of rupture of samples during tests, the test was set to be a single stroke and the value of the tool path length l was 200 mm. The wear resistance, I, was defined as the ratio between the

(c1)

(b2)

49

200 µm

(h2)

200 µm

200 µm

Fig. 11. Confocal microscopy images of wear surface morphology of PCD and UHPCD after wear testing under different rotate speeds, PCD samples: (a1) 200 rpm, (b1) 280 rpm, (c1) 400 rpm, (d1) 560 rpm; UHPCD samples: (e1) 200 rpm, (f1) 280 rpm, (g1) 400 rpm, (h1) 560 rpm; and (a2), (b2), (c2), (d2), (e2), (f2), (g2), (h2) show the surface height distribution of the corresponding images, respectively.

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mass wear of the rock destruction element Δm and the drifting for a tool element L, i.e. I ¼ Δm=L

ð1Þ

Δm ¼ min –m f

ð2Þ

where min and mf were the sample masses before and after testing, respectively. It could be calculated that L½mg=km ¼ πDn  t=60 ¼ πDn  ðl= f nÞ=60 ¼ πDl=ð1000  f Þ

ð3Þ

where D was the diameter of rock cylinder, l was the tool path length, n and f were rotating speed and feeding amount, respectively. Referred to the superhard material abrasion ratio measurement in Russia (rotating frequency — 355 rpm, the penetration depth — 1.0 mm) [14], and according to the chosen feeding amount f = 0.026 mm/r, the wear length L here was 1500–1800 m. Each test in the experimental conditions of given speed were repeated three times. A PTM-3 micro hardness tester which adopted a Vickers diamond pyramid as an indenter was used to determine the hardness of the CVD diamond and the different locations of UHPCD and PCD. The value was found by the generally accepted formula HV = 1.8544 P/d2, where P is the indentation load, d is the arithmetical mean of two diagonals of the indent. The indentation load of 4.9 and 9.8 N were chosen to be considered as the optimal one since in using both loads practically the same values were obtained [23]. The indent diagonals were measured using a Neophot optical microscope. In addition, during the hardness measurements no fracture of the tested material or indenter was observed under the indentation loads used. The Raman spectra were measured by a LabRAM HR Evolution spectrometer. A discrete emission line of an Ar–Kr laser (λexc = 514.5 nm) was used as the source of the optical excitation and the diameter of a laser spot on the sample surface was 0.2–0.5 μm. The space spectral mapping of the sample was conducted by displacing table at a step of 0.1 μm and the frequency of the spectral line was accurate to 0.15 cm−1. A BX51M optical microscope and a VK-X100 confocal microscopy were used to observe the appearance of both materials and the wear surface morphologies of these two materials, respectively. The microscopic morphologies of the wear surface were measured by a SSX-550 scanning electron microscope (SEM).

high) due to the shape of mold primarily. Both UHPCD and PCD are dense, and no fracture can be observed in the surface. The hardness of original CVD diamond, UHPCD and PCD are given in Table 1, and the points can be found in Fig. 8. The hardness of original CVD diamond is 90–98 GPa, and it is 105–115 GPa measured on the CVD diamond in the UHPCD. A promotion of hardness occurred in the CVD diamond after high-temperature and high-pressure synthesis process. The hardness' promotion can be understood as the result of the formation of a combined structurally stressed state at high pressure. The state was generated by the plastic deformation of diamond grains in the course of the formation of a rigid skeleton around CVD diamond during high-temperature and high-pressure process [24]. Meanwhile, the hardness of PCD in the UHPCD (53–59 GPa) is close to that of PCD (53–57 GPa). The only difference is that UHPCD has a CVD diamond column. It is supposed that UHPCD can exhibit better antiwear properties for that the wear resistance of material is directly related to hardness [25]. Raman spectroscopy can be used to distinguish different forms of carbon [26,27]. The Raman spectra of the central position of UHPCD and PCD are shown in Fig. 9. The spectral line of CVD diamond in the center of UHPCD only has one pronounced peak at 1330 cm−1 responding to the diamond peak. A weak G peak can be observed at the dash line near 1560 cm−1. Whereas, two peaks can be observed on PCD, a diamond peak at 1331 cm−1 and the other peak at 540 cm−1 which signifies the existence of Si [28]. No obvious composition change occurred in the CVD diamond. 3.2. Wear behaviors The wear results of UHPCD and PCD after testing are shown in Fig. 10. The wear ratios of both materials at all conditions were less

2.3. Field application of drilling bits In order to test the effects of field application of UHPCD and PCD on the drilling bits, the coring bits fabricated were 76 mm in diameter. The bit was designed for core drilling with 8 segments, and the ceramic carbide was impregnated with the type AC160T diamond whose granularity was 40/50 mesh. Fig. 7(a), (b), (c) images show bit without inserts, bit inserted with PCD and bit inserted with UHPCD, respectively. The bits were used in Qianchen mining area in Zhaoyuan, Shandong. The rocks are mostly hard whose grades are 9–11. A XY-6B type drilling rig and a NBB260/7 type slime pump were used during the drilling. By adjusting the rotating speed and weight on bits, the rate of penetration (ROP) was maintained in a certain level 3–3.3 m/h during the field experiment. Working conditions of the bits were recorded such as the rotate speed, rate of penetration, drilling depth. 3. Results and discussion 3.1. Characterizations The optical images of UHPCD and PCD are shown in Fig. 8. The square shaped objects observed clearly in Fig. 8(b) is CVD diamond. The UHPCD and PCD samples are in similar shape (4 mm in diameter and 4.5 mm

Fig. 12. Raman spectrum of these two materials after wear testing under different rotate speed: (a) the worn area of CVD diamond in UHPCD, (b) the worn area of PCD.

G. Yan et al. / Int. Journal of Refractory Metals and Hard Materials 54 (2015) 46–53

than 0.5 mg/km. This result is close to the reference [16] in which the wear ratio of ultrahard material with CVD diamond reaches 0.6 mg/km. The wear ratio of UHPCD reaches the peak value of 0.25 mg/km and the wear ratio of PCD reaches the peak value of 0.46 mg/km under 400 rpm. The wear ratio values of UHPCD are about half of that of PCD under different rotate speeds. It demonstrates that the wear properties of UHPCD are superior to that of PCD. The wear ratios of UHPCD and PCD are independent on the varied rotate speeds. It means that the rotate speed is not the key factor influencing the wear properties of UHPCD and PCD in this test. The trends in wear ratios of the two materials are similar, but it shows no obvious regular pattern compared to the reference [13]. It can be attributed to the irregular of the granite. Granite is a collection of kinds of minerals which are in extreme inhomogeneity and irregular. The minerals include the quartz, mica, potash feldspar, soda feldspar and so on. It is obviously shown that the CVD diamond make the UHPCD have much better wear resistance than that of PCD. Fig. 11 shows the confocal microscopy images of wear surface topography of PCD and UHPCD after testing, as well as the corresponding image of surface height distribution. Due to the distinction of the composition and structure, wear morphologies of the two materials in grinding against granite are quite different. However, there is no observable difference among the wear surface morphologies of PCD or UHPCD under four different rotate speed conditions. It can be visibly confirmed that there is no direct relationship between the rotate speed and the abrasion performance as stated in preceding paragraph. Combined with Fig. 8 and the surface height distribution images in Fig. 11(a2), (b2), (c2), (d2), it can be found that the end face of PCD was slightly changed in the testing. The wear surface of PCD gradually fitted to the surface of the granite column due to the grain loss of PCD. It means that the cutting edge of PCD was corroded [29]. Although UHPCD and PCD were tested in almost identical conditions, the wear surfaces of UHPCD are significantly different from

(a)

51

those of PCDs. A convex structure was formed on the wear surface of UHPCD, in which the CVD diamond acted as a boss top and the polycrystalline material surrounded acted as support. It can be found by both the surface morphologies and the surface height distribution images as shown in Fig. 11(e2), (f2), (g2), (h2). The reason is that the remarkable different hardness of CVD diamond and PCD. CVD diamond is much harder than the polycrystalline diamond surrounded, as a result slower damage of the CVD diamond. That is why that the boss shaped cutting edge of UHPCD can be maintained as shown in Fig. 11. It can be pointed that UHPCD has a better self-sharpening ability [30,31] relative to PCD. The ability of self-sharpening means UHCPD can remain an acuate shape and reduce the diameter of the penetration tunnel in drilling. Fig. 12 shows the Raman spectrum of the worn area in PCD and CVD diamond of UHPCD after grinding under different rotate speed conditions. It can be observed that no obvious difference is in the spectra of the wear regions of UHPCD and PCD, respectively. It also manifests that the change of rotate speed has no significant effects on the process of UHPCD and PCD grinding against granite. An obvious diamond peak at 1331 cm−1 and a weak peak at the dash line position at about 1580 cm−1 G peak can be found in Fig. 12(a). A similar phenomenon is observed at the dash line position in Fig. 12(b), but higher graphite peak intensity is shown in the PCD materials. It is implied that a certain graphitization occurred in both UHPCD and PCD during the wear for the G peak at 1580 cm−1. The Si binder contained in PCD can promote the graphitization. It can be induced that the UHPCD can maintain a better wear performance during wear testing. Compared with Figs. 9 and 12(b), the relative intensity of Si peak at 540 cm−1 increased after the wear testing. The increase of the Si peak intensity can be considered as that the diamond grains failed off and the Si particles exposed during the testing [32]. Based on above results, the rotate speeds have no obvious effect on the wear process of UHPCD and PCD. The wear morphologies of

(b)

50 µm

10 µm

(c)

A

4 µm Fig. 13. SEM images at different magnification of worn PCD under 200 rpm: (b), (c) is the enlargement of the block area in (a), (b) in order.

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G. Yan et al. / Int. Journal of Refractory Metals and Hard Materials 54 (2015) 46–53

(a)

(b)

10 µm

50 µm

(c)

(d)

C A B

4 µm

1 µm

Fig. 14. SEM images at different magnifications of worn UHPCD under 200 rpm: (b), (c), (d) is the enlargement of the block area in (a), (b), (c) respectively.

UHPCD and PCD under 280, 400, 560 rpm are similar to that under 200 rpm. Therefore, the followed micro wear morphologies under 200 rpm would be only discussed. The microscopic wear morphologies of PCD are shown in Fig. 13. The polycrystalline characteristics are shown in Fig. 13(a) and the signs of pits also appear. Evident scratches on the grain surface as well as some pits caused by the shedding off of the grains can be seen in Fig. 13(b). The scratches and the pits become more signal in the ellipse A in Fig. 13(c). It can be drawn that the main wear mechanism of PCD in grinding against granite is abrasive wear. The abrasive wear is also one of the main mechanisms in turning concrete for PCD tools [9]. The abrasive particles can be the hard debris of the granite or the particles dropped from PCD. It declares that the breakage of PCD material is mainly caused by the drop of diamond particles. Si was adopted as the binder in synthesis of PCD. The PCDs contain not only D–D bonds, but also the bonds of C–Si and elemental Si [33]. The falling off of particles can be attributed to the low intergranular bonding strength and the shock of the testing. The microscopic wear morphologies of UHPCD are shown in Fig. 14. The boundary between the CVD diamond and the polycrystalline diamond surrounded can be clearly observed in the lower right corner in Fig. 14(a). CVD diamond is flat and dense. Its appearance is different from the surrounded polycrystalline diamond. An extension of fracture traces in the lower left corner and a delamination phenomenon in the middle can be seen obviously in Fig. 14(b). In Fig. 14(c) the phenomenon of delamination can be discovered clearly. The wear area is divided into three layers. A row of scratches can be found from the top down in the area C in Fig. 14(d). It indicates that the main wear mechanism of UHPCD in grinding granite is abrasive wear. The abrasive particles can be the hard debris of the rock and the diamond particles dropped from UHPCD. A markedly track is shown in the area A in Fig. 14(d). In the area B the origin of the crack can be found in the interior of the diamond

grain instead of along the grain boundary. Certain angle (not the right angle) appears between the cracks and the scratches as shown in Fig. 14(d). Due to the growth mode in preparation stage, the bonding among the internal grains of CVD diamond is strong. That is the reason why no pits of particles shed off is observed here. Whereas, the continuous shocks caused by the lathe and the variation of granite components led to the surface fatigue. The surface fatigue sparked the cracks. With the continuous shocks, the cracks extended and the furcation took place in some certain depth of the CVD diamond. It can be declared that the breakage of CVD diamond material is mainly caused by bed separation resulted from fatigue [12]. The wear surface stratification phenomenon in Fig. 14(b), (c) also confirms this point. 3.3. Field application The results of three kinds of bits in drilling tests are summarized in Table 2. It can be found that all the bits were used normally. The

Table 2 Working conditions of the bits with or without inserts in drilling test. Inserts

Hole no.

Drilling depth (m)

Rotate speed (rpm)

Rate of penetration (m/h)

Drilling footage (m)

None None None PCD PCD PCD UHPCD UHPCD UHPCD

6762K6 240/2k813 240/2k813 4082k2 4082k2 240/2k813 6762K6 4082k2 4082k2

1154.70–1176.76 991.11–1068.06 1273.30–1296.53 834.22–917.98 917.98–986.78 1131.20–1155.25 1176.76–1309.09 674.38–749.49 749.49–834.22

490 475 475 475 475 475 490 490 490

3.3 3 3 3 3 3 3.3 3.3 3.3

22.06 76.95 23.23 83.76 68.80 24.05 132.33 75.11 84.73

G. Yan et al. / Int. Journal of Refractory Metals and Hard Materials 54 (2015) 46–53

experimental results show that inserting of UHPCD material or PCD material in the bit could significantly prolong the bit service life. With a view to the polytrope and uncertainty of the strata [34], it can be suggested that the bits with inserts of UHPCD show a better performance in service life than those of the other two kinds of bits. The UHPCD material has a superior performance in prolonging the service life of bit than PCD material. The advantages of UHPCD to PCD are of relatively higher wear resistance as well as self-sharpening. Compared to the PCD material, UHPCD can significantly extend the service life of bits when applied to drilling bits. Hence, it can be speculated that the UHPCD can improve the work efficiency of drilling bits (i.e. increasing the drilling rate). In addition, the application fields in drilling of CVD diamond is broadened by the material of UHPCD. 4. Conclusions The following conclusions can be drawn based on this study: 1) Both UHPCD and PCD materials show good performance in grinding against granite. The wear resistance of UHPCD is about two times than that of PCD in four different test conditions. At 400 rpm the wear ratio of UHPCD reaches the maximumvalue of 0.25 mg/km and that of PCD reaches 0.46 mg/km. 2) The wear surface of PCD gradually jointed the column surface of granite during grinding, while a convex structure as the cutting edge is formed in UHPCD. The UHPCD possesses a better selfsharpening ability related to PCD. 3) The higher wear resistance and the convex structure of UHPCD can be attributed to the high hardness of CVD diamond. 4) The UHPCD material has been successfully applied in the geological core drilling. Moreover, UHPCD material has a significant effect on prolonging the lifespan of drilling bits. Acknowledgment Financial support from the National Natural Science Foundation of China (51375466), International Science and Technology Cooperation Project of China (2011DFR50060) and Fundamental Research Funds for the Central Universities (2652015074, 2652015072) are gratefully acknowledged. The authors are grateful to Prof. Alexandr Shulzhenko, Dr. Alexandr Sokolov, Prof. Anatoliy Zakora and Vladislav Gargin from Synthesis and Sintering of Superhard Materials Department, Bakul Institute for Superhard Materials, National Academy of Sciences of Ukraine, for their help in synthesis and characterization experiments. The authors would also express their gratitude to Mr. Zhiyong Ouyang, from the Institute of Prospecting and Engineering, Beijing, for manufacturing the drilling bits, and Mr. Ruixiang Yu, from the 6th Geology & Mineral Resources Survey Institute, Shandong, for field testing of the drilling bits. References [1] Q. Huang, D. Yu, B. Xu, Nanotwinned diamond with unprecedented hardness and stability, Nature 510 (7504) (2014) 250–253. [2] J.B. Liu, Z.F. Shi, X.J. Li, Development and application of super-hard cutter material, Key Eng. Mater. 480 (2011) 676–680. [3] H.T. Hall, Sintered diamond: a synthetic carbonado, Science 169 (1970) 868–869. [4] R.H. Wentorf, R.C. DeVries, F.P. Bundy, Sintered superhard materials, Science 208 (1980) 873–880. [5] A. Inspektor, E.J. Oles, C.E. Bauer, Theory and practice in diamond coated metalcutting tools, Int. J. Refract. Met. Hard Mater. 15 (1997) 49–56.

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