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Ni cemented carbide tool material irradiated by high-intensity pulsed electron beam
Ceramics International 45 (2019) 15327–15333
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Friction and wear behavior of WC/Ni cemented carbide tool material irradiated by high-intensity pulsed electron beam
T
F.G. Zhang School of Materials Science and Engineering, Shaanxi University of Technology, Hanzhong, 723003, China
A R T I C LE I N FO
A B S T R A C T
Keywords: High-intensity pulsed electron beam Pulse frequency WC/Ni cemented carbide Microstructure Friction coefficient
The friction and wear behavior of high-intensity pulsed electron beam (HIPEB)-irradiated YN13 cemented carbide at energy density of 34 J/cm2 and 10 pulses with pulse frequencies of 1 and 5 Hz were investigated in this work by dry sliding wear test against hardened GCr15 bearing steel at 98 N and 0.47 m/s. Surface characterization was conducted by using scanning electron microscope, surface profilometer, X-ray diffraction and Vickers microhardness tester. The dominant wear mechanisms were identified by the worn surface morphologies. The results indicated that an intense surface remelting and ablation/evaporation was induced by HIPEB irradiation with high pulsed frequency, which resulted in the surface roughening, WC phase transformation and improved tribological properties of the YN13 cemented carbide. The specific wear rate and friction coefficient decreased as pulsed frequency increased. When pulsed frequency increased to 5 Hz, they decreased to 53.5% and 70% for non-irradiated samples. The improvement in wear resistance and the significant friction reduction of irradiated samples with the increase in pulsed frequency should be ascribed to the roughening of surface, the refinement microstructure of remelting, and the decrease in irradiated defect in the remelted layer of irradiated YN13 cemented carbide. The main wear mechanism of irradiated YN13 cemented carbide/GCr15 bearing steel pairs was micro-cutting wear in the initial stage of dry sliding wear and abrasive wear with characteristics of Ni preferential removal and fallout of WC similar to the non-irradiated samples in the stable stage.
1. Introduction WC/Ni cemented carbides, also known as hard alloys or hardmetals, have outstanding mechanical properties of high hardness, strength, and heat stability and excellent toughness and abrasive wear resistance in wear applications [1–4], however, the enhancement in the surface tribological properties of WC/Ni cemented carbides is still an urgent problem. The surface modification of cemented carbides has been effectively performed by high-intensity pulsed electron beam (HIPEB) irradiation [5–7]. HIPEB irradiation induces the remelting of carbides and preferential ablation of metal binder phase by efficient high-density energy deposition during a nanosecond pulse. This method significantly changes the morphology, composition and microstructure of surface to improve its surface properties, such as wear and corrosion resistance. At present, pulsed electron beam technology is making a remarkable progress in surface hardening and is improving the wear resistance of cemented carbides [7–11]. However, most studies on the surface modification of cemented carbide involved the adjustment of irradiation parameters of electron energy, energy density (or beam density), and pulse number and width. In these studies, the used pulse frequency is in the range of 0.03–1 Hz [7–11]. Nevertheless, the microstructure
and properties of irradiated WC-based cemented carbides at high frequency are limited, especially for the friction and wear behavior of irradiated WC-based cemented carbide. The purpose of this investigation was to study the friction and wear behavior of HIPEB-irradiated WC/Ni cemented carbides at 34 J/cm2 and 10 pulses with pulsed frequencies of 1 and 5 Hz under dry sliding wear test against hardened GCr15 steel at 98 N and 0.47 m/s. The wear mechanism of HIPEB-irradiated WC/Ni cemented carbides was also explored. 2. Experimental procedures 2.1. HIPEB irradiation process The WC/Ni cemented carbide of mark YN13 was used as experimental material. The chemical composition (wt%) was 13% of Ni and WC in balance. The samples were machined in the size of 6 mm × 6 mm × 8 mm. Prior to HIPEB irradiation, all the samples were polished using the diamond abrasive powders and 200-grit SiC abrasive paper, followed by alcohol rinse and blow drying. The WC/Ni samples were irradiated on a HIPEB setup described elsewhere [12,13]. The parameters of HIPEB irradiation were as follows: electron energy of
E-mail address: [email protected]. https://doi.org/10.1016/j.ceramint.2019.05.025 Received 25 March 2019; Received in revised form 1 May 2019; Accepted 3 May 2019 Available online 04 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 15327–15333
F.G. Zhang
Fig. 1. Schematic diagram of the block-on-ring type wear test machine.
127 kV, energy density of 34 J/cm2, pulse duration of 180 ns, number of pulses of 10, and pulsed frequencies of 1 and 5 Hz. 2.2. Characterization of HIPEB-irradiated samples Several techniques were used to characterize the surface characteristics of HIPEB irradiated samples. Before and after HIPEB irradiation, the surface morphology and roughness of the WC/Ni samples were studied using scanning electron microscope (SEM, JSM-6390LV) and profilometer (Surfcorder ET-4000A), respectively. The phase structure of the WC/Ni samples was examined by X-ray diffraction (XRD, DX-2500) with Cu Kα radiation. The surface microhardness of the WC/Ni samples was measured with Vickers tester (FEM-7000) with a load of 1.96 N for a holding time of 10 s. 2.3. Wear tests Dry sliding wear tests were conducted on a MM200 block-on-ring type wear test machine, as shown in Fig. 1. The WC/Ni block samples were pressed against a hardened GCr15 bearing steel ring (ø 45 mm × 10 mm, HV2 N 7.02 GPa) with a working load of 98 N and a sliding speed of 0.47 m/s for 2100 s. The working force (FN) applied on the upper body (WC/Ni block) and the accompanying frictional force (FT) of rotating the down body (GCr15 bearing steel ring) over the stationary counterface were periodically recorded during the dry sliding wear tests. The ratio of FN/FT was defined as the friction coefficient (f). Before and after the sliding wear test, the mass of the WC/Ni block sample was weighed by using a digital balance with a scale of 0.1 mg to calculate the specific wear rate. The specific wear rate (k) was calculated by Lancaster’s wear formula [14]:
k=
W Fn s
(1)
where W is the wear volume, which was calculated by the mass loss divided by the density of the cemented carbide tested, Fn is the applied working load, and s is the sliding distance. The surface morphologies in worn regions of the WC/Ni samples before and after HIPEB irradiation were also studied to reveal the wear mechanism of dry sliding wear. 3. Results and discussion 3.1. Surface structure and microhardness Fig. 2 shows the surface SEM images and surface profiles of YN13 cemented carbide before and after HIPEB irradiation under varied pulsed frequency. The surface of the non-irradiated sample presented a relatively smooth pattern with some processing defects during preparation, such as grinding marks, debris, and voids/pores (Fig. 2a). Surface roughness was approximately 0.207 μm. After HIPEB irradiation with a pulsed frequency of 1 Hz (Fig. 2b), the processing defects of
Fig. 2. Surface SEM images and surface profiles of YN13 cemented carbide before (a) and after HIPEB irradiation with pulsed frequency of 1 Hz (b) and 5 Hz (c).
the irradiated surface were eliminated entirely and surface roughness increased to approximately 0.540 μm. Simultaneously, some irradiated defect, such as blow holes and microcracks, appeared on the irradiated surface. Only a few white ablations were observed on the irradiated
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Fig. 3 shows the phase composition of YN13 cemented carbide before and after HIPEB irradiation under varied pulsed frequency. In the initial state, only the sharp peaks of hexagonal WC and cubic Ni phases were observed. Treatment by HIPEB irradiation significantly changed the phase composition of the surface of WC/Ni samples. After HIPEB irradiation with a pulsed frequency of 1 Hz, the diffraction peaks from metastable β-WC1-x, α-W2C, and graphite phases were detected. Therefore, a considerable WC phase transformation occurred on the irradiated surfaces, and this phenomenon decreased the diffraction peak of WC phase. The transformation of WC phase should be ascribed to the rapid cooling process with a cooling rate of 108 K/s and high temperature reaction, that is, reaction ablation of carbon in the WC during beam irradiation [8,16]. The diffraction peak of Ni decreased due to the selective ablation from the action of beam and materials. As the pulsed frequency increased to 5 Hz, the diffraction peaks of β-WC1-x phase remarkably intensified, whereas the diffraction peaks of α-WC and Ni phases greatly weakened. The formation of α-W2C and graphite phase should also be detected on the irradiated surface. The β-WC1-x phase in the remelted layer increased with the increase in pulsed frequency. This phenomenon indicated that the enhancement in thermal effect from the energy deposition of HIPEB irradiation promoted the transformation of WC phase. An enlarged section in the 61o–65° region of the XRD patterns clearly displayed the widening of WC1-x (220) peaks, which indicated that HIPEB irradiation led to the grain refinement of surface remelted layer on the irradiated samples. Fig. 4 shows the Vickers microhardness on the surface of YN13 cemented carbide before and after HIPEB irradiation under varied pulsed frequency. The surface microhardness of HIPEB-irradiated samples was lower than that of the non-irradiated sample and decreased slowly with the increase in pulsed frequency. When the pulsed frequency increased to 5 Hz, the surface microhardness decreased to 10.76 GPa, which was nine-tenths of the non-irradiated sample. Analysis of surface morphology and phase structure of these samples indicated that the reduction in microhardness on the irradiated surfaces with the increase in pulsed frequency should be ascribed to the formation of metastable β-WC1-x phase and surface irradiated defects, which were the negative affecting factors of surface microhardness [8,17]. 3.2. Friction coefficient Fig. 5 shows the friction coefficient of YN13 cemented carbide before and after HIPEB irradiation with varied pulsed frequency during
Fig. 3. XRD patterns of YN13 cemented carbide before and after HIPEB irradiation with the varied pulsed frequency: (a) 2 theta range of 20o-100°; (b) 2 theta range of 61o-65°.
surface. As the pulsed frequency increased to 5 Hz (Fig. 1c), the number of white ablation on the irradiated surface increased, but the number of blow holes decreased. Surface roughness increased to 0.767 μm. Some microcracks were also observed on the irradiated surface. The irradiated defects of blow holes and microcracks were found as well on the surface of cemented carbides treated by pulsed particle beam irradiation [8,15–18]. The blow holes were due to the surface ablation of cemented carbide induced by beam irradiation, and the microcracks were attributed to the rapid heating and cooling from beam irradiation. The weight losses of the irradiated WC/Ni sample with 1 and 5 Hz was 0.75 mg/cm2 and 1.20 mg/cm2, respectively. Therefore, an intense surface remelting and ablation/evaporation was induced by HIPEB irradiation with high pulsed frequency, which was attributed to the enhancement in thermal effect from the energy deposition of HIPEB irradiation due to the decrease in pulse interval.
Fig. 4. Surface microhardness of YN13 cemented carbide before and after HIPEB irradiation with varied pulsed frequency.
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Fig. 5. Friction coefficient curves of YN13 cemented carbide before and after HIPEB irradiation with varied pulsed frequency during wear test with 98 N and 0.47 m/s.
Fig. 6. Influence of pulse frequency on the specific wear rate of the HIPEBirradiated YN13 cemented carbide during wear test with 98 N and 0.47 m/s.
dry sliding wear tests with 98 N and 0.47 m/s. The curves of friction coefficient revealed that the friction process of the non-irradiated and irradiated YN13 cemented carbide should be divided into initial running-in and steady state stages. In the initial running-in stage of dry sliding wear, the friction coefficients fluctuated abruptly. This phenomenon should be strongly related to surface roughness or asperity interaction that resulted in the continuous changes in surface contact condition. The irradiated samples required considerably long runningin time to wear down the higher surface roughness or asperities than that of the non-irradiated samples. After the running-in period, the surface of wear track smoothened and the wear process gradually reached a steady state. In this case, the fluctuations in the friction coefficients diminished and gradually reached a steady state stage. The friction coefficient of irradiated samples/GCr15 bearing steel pairs was lower than that of non-irradiated samples/GCr15 bearing steel pairs and significantly decreased with the increase in pulsed frequency of HIPEB irradiation. When the pulsed frequency increased to 5 Hz, the friction coefficient decreased from 0.8 of non-irradiated sample to 0.56. The significant friction reduction of irradiated samples/GCr15 bearing steel pairs could be explained by the adhesive interaction, which depended on the surface WC phase transformation from HIPEB irradiation, that is, graphite precipitation, which had excellent lubricity for sliding contact for the friction of the surface remelted layer.
3.3. Specific wear rate Fig. 6 shows the influence of pulsed frequency on specific wear rate of the HIPEB-irradiated YN13 cemented carbide during the sliding wear test. The specific wear rate of the irradiated samples was in the 10−7 mm3/Nm range and decreased with increase in pulsed frequency of HIPEB irradiation. When the pulsed frequency increased to 5 Hz, the specific wear rate decreased to 6.42 × 10−7 mm3/Nm, which was approximately 53.5% of the non-irradiated samples. The state of cemented carbides wear could be briefly recognized by “mild wear” and “severe wear” from the point of view of specific wear rate [19,20]. The surface wear of the HIPEB-irradiated samples in the present test condition belonged to a “mild wear”. The HIPEB-irradiated WC/Ni samples had higher wear resistance than non-irradiated samples in the present test condition. However, the surface microhardness of the former samples was considerably lower than that of the latter samples. The wear resistance was in the 107 Nm/mm3 range for the HIPEB-irradiated samples and increased with increase in pulsed frequency of HIPEB
Fig. 7. Relationship of wear resistance with surface microhardness (a) and surface roughness (b) of the HIPEB-irradiated YN13 cemented carbide.
irradiation. Fig. 7 shows the relationship of wear resistance with surface microhardness and surface roughness. The wear resistance increased as surface microhardness decreased (Fig. 7a), which was contrary to the
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Fig. 8. SEM images of worn surfaces of YN13 cemented carbide before (a) and after HIPEB irradiation with varied pulsed frequency of 1 Hz (b) and 5 Hz (c) after sliding tests with 98 N and 0.47 m/s.
usual rule that materials with high hardness have high wear resistance [19,21]. In the present study, the wear resistance linearly enhanced with the improvement in surface roughness (Fig. 7b). This condition indicated that the improved wear resistance of irradiated samples/ GCr15 bearing steel pairs depended not only on surface microhardness, but also on surface roughness, remelting refinement microstructure, and WC phase transformation from HIPEB irradiation. In other words, the formation of WC1-x phases enhanced the coordination between hardness and toughness for the sliding wear of the surface remelted layer [22].
3.4. Wear mechanism Fig. 8 shows the SEM morphologies of wear surface of YN13 cemented carbide before and after HIPEB irradiation with varied pulsed frequency after dry sliding wear tests. As shown in Fig. 8a, the wear track of the non-irradiated sample exhibited an abrasive wear with characteristics of Ni preferential removal and fallout of WC, which should be attributed to the strong deformation and microabrasion [19,21,23–26]. It should be noted that an interesting result was that the harder YN13 cemented carbide was worn down by the much softer steel
(HV2 N 7.02 GPa) during the dry sliding wear process in this paper and similar phenomenon was also observed in the sliding wear process of WC-Co/45 carbon steel pairs [26]. Metallic nickel was softer than WC phase and should be removed preferably from between the carbides by plastic deformation and microabrasion [21,23], which rendered the WC grains prone to break out from the surface. The WC grains began to slip when sufficient support of binder phase had been wiped off and further fracture or fall out of the surface [19]. Fig. 8b and c show that the dry sliding wear of the irradiated samples depended on the remelted layer, which presented a micro-cutting wear despite some blow holes in the remelted layer. A similar phenomenon was found on the wear surface of the WC/Ni treated by high-intensity pulsed ion beam (HIPIB) irradiation in our previous work [27,28]. Many fine black spots on the worn zone of surface remelted layer showed the precipitation of nanograined graphite particles produced by HIPEB irradiation. A similar phenomenon was also found on the wear surface of the WC/Co treated by HCPEB irradiation [29]. The size of blow holes decreased, and nanograined graphite particles refined in the remelted layer with the increase in pulsed frequency. Therefore, the improvement in wear resistance of irradiated samples was depended on not only surface microhardness but also surface roughening and transition of wear
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4. Conclusions (1) High-intensity pulsed electron beam irradiation induced an intense surface remelting and ablation/evaporation at 34 J/cm2 and 10 pluses with high pulsed frequency and led to the surface roughening and WC phase transformation of the YN13 cemented carbide. Surface roughness increased with the increase in pulsed frequency of HIPEB irradiation. Surface roughness increased to 0.767 μm when the pulsed frequency was 5 Hz. (2) The specific wear rate and friction coefficient decreased with the increase in pulsed frequency. When the pulsed frequency increased to 5 Hz, they decreased to 53.5% and 70% for non-irradiated samples. The improvement in tribological properties with the increase in pulsed frequency should be ascribed to the roughening of surface, the refinement microstructure of remelting, and the decrease in irradiated defect in the remelted layer of HIPEB-irradiated YN13 cemented carbide. (3) The main wear mechanism of the HIPEB-irradiated YN13 cemented carbide/GCr15 bearing steel pairs was micro-cutting wear in the initial stage of dry sliding wear, and abrasive wear with characteristics of Ni preferential removal and fallout of WC similar to the non-irradiated samples in the stable stage. Acknowledgements
Fig. 9. Schematics of dry sliding wear for the HIPEB-irradiated YN13 cemented carbide against hardened GCr15 steel at sliding wear contact (a), the initial running-in stage (b) and the steady-state stage (c).
mechanism. The improvement in wear resistance and the significant friction reduction of HIPEB irradiated samples with the increase in pulsed frequency should be ascribed to the roughening of surface, the refinement microstructure of remelting (such as refined nanogrianed graphite particles and WC1-x phase), and the decrease in irradiated defect (such as blow holes) in the remelted layer of irradiated WC/Ni samples. The changes of tribological properties and the SEM results indicated that the dry sliding wear characteristic of the irradiated samples in the initial running-in stage was dependent on the special surface structure of surface remelted layer. When the two surfaces contacted, the surface humps endured the loading and prevented suppression and abrasion of the surface of the GCr15 bearing steel (Fig. 9a). The surface of the soft GCr15 bearing steel had been cut by the surface humps of irradiated samples under the loading because of the hardness of surface humps was higher than that of the GCr15 bearing steel. The surface humps of irradiated samples had also been worn down gradually due to the abovementioned mechanism—an abrasive wear with characteristic of micro-cutting (Fig. 9b). This result was confirmed from the wear surface of irradiated samples (Fig. 8b and c). Wear debris from the dry sliding wear should be collected by the surface concaves of irradiated samples to reduce the occurrence of abrasive wear due to surface contact. A similar phenomenon was also found on the wear surface of the WC/Ni treated by HIPIB irradiation in our previous work [27,28]. As the wear further proceeded, the surface humps of irradiated samples were worn out and the wear process reached a steady state stage (Fig. 9c). In this stage, the irradiated samples had a similar wear mechanism to non-irradiated samples. However, the removal rate decreased as observed from the densified microstructure below the remelted layer in Fig. 8b and c, which was caused by the strengthening of the binder phases due to that the stress wave of HIPEB irradiation propagated from the subsurface layer toward the deep layer of the irradiated samples [30].
The authors are very grateful to Professor M.K. Lei and X. P. Zhu from the Dalian University of Technology for their contributory discussion and technical assistance. This work is supported by the Natural Science Basic Research Program of Shaanxi Province under Grant No. 2018JM5021 and the National Natural Science Foundation of China under Grant No. 51701111. References [1] A.M. Human, H.E. Exner, The relationship between electrochemical behavior and in-service corrosion of WC based cemented carbides, Int. J. Refract. Metals Hard Mater. 15 (1997) 65–71. [2] J.Y. Sheikh-Ahmad, J.A. Bailey, The wear characteristics of some cemented tungsten carbides in machining particleboard, Wear 225–229 (1999) 256–266. [3] H. Engqvist, G.A. Botton, S. Ederyd, M. Phaneuf, J. Fondelius, N. Axén, Wear phenomena on WC-based face seal rings, Int. J. Refract. Metals Hard Mater. 18 (2000) 39–46. [4] H. Engqvist, U. Beste, N. Axén, The influence of PH on sliding wear of WC based materials, Int. J. Refract. Metals Hard Mater. 18 (2001) 103–109. [5] D.I. Proskurovsky, V.P. Rotshtein, G.E. Ozur, Yu F. Ivanov, A.B. Markov, Physical foundation for surface modification of materials with lower energy, high current electron beams, Surf. Coating. Technol. 125 (2000) 49–56. [6] Y.F. Ivanov, V.P. Rotshtein, D.I. Proskurovsky, P.V. Orlov, K.N. Polestchenko, G.E. Ozur, I.M. Goncharenko, Pulsed electron-beam treatment of WC-TiC-Co hard alloy cutting tools: wear resistance and microstructural evolution, Surf. Coating. Technol. 125 (2000) 251–256. [7] Y. Xu, Y. Zhang, S.Z. Hao, O. Perroud, M.C. Li, H.H. Wang, T. Grosdidier, C. Dong, Surface microstructure and mechanical property of WC-6%Co hard alloy irradiated by high current pulsed electron beam, Appl. Surf. Sci. 279 (2013) 137–141. [8] F.G. Zhang, X.P. Zhu, M.K. Lei, Surface characterization and tribological properties of WC-Ni cemented carbides irradiated by high intensity pulsed electron beam, Vacuum 137 (2017) 119–124. [9] W. Peng, S. Hao, J. Chen, W. Li, L. Zhao, J. Deng, Surface composite microstructure and improved mechanical property of YG10X cemented carbide induced by high current pulsed electron beam irradiation, Int. J. Refract. Metals Hard Mater. 78 (2019) 233–239. [10] V.V. Uglov, A.K. Kuleshov, E.A. Soldatenko, N.N. Koval, YuF. Ivanov, A.D. Teresov, Structure, phase composition and mechanical properties of hard alloy treated by intense pulsed electron beam, Surf. Coating. Technol. 206 (2010) 2972–2976. [11] C. Lou, Y. Zhang, X. Lyu, J. Gao, Q. Wang, Surface modification of YW2 cemented carbide cutting tool by high current pulsed electron beam, Mater. Sci. 21 (2015) 203–206. [12] J.L. Liu, H.B. Zhang, Y.W. Fan, Z.Q. Hong, J.H. Feng, Study of low impedance intense electron-beam accelerator based on magnetic core Tesla transformer, Laser Part. Beams 30 (2012) 299–305. [13] J.L. Liu, Y. Yin, B. Ge, X.B. Cheng, J.H. Feng, J. D Zhang, X.X. Wang, A compact high power pulsed modulator based on spiral Blumlein line, Rev. Sci. Instrum. 78 (2007) 103302. [14] J.K. Lancaster, The influence of substrate hardness on the formation and endurance of molybdenum disulphide films, Wear 10 (1967) 103–107.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Friction and wear behavior of WC/Ni cemented carbide tool material irradiated by high-intensity pulsed electron beam
T
F.G. Zhang School of Materials Science and Engineering, Shaanxi University of Technology, Hanzhong, 723003, China
A R T I C LE I N FO
A B S T R A C T
Keywords: High-intensity pulsed electron beam Pulse frequency WC/Ni cemented carbide Microstructure Friction coefficient
The friction and wear behavior of high-intensity pulsed electron beam (HIPEB)-irradiated YN13 cemented carbide at energy density of 34 J/cm2 and 10 pulses with pulse frequencies of 1 and 5 Hz were investigated in this work by dry sliding wear test against hardened GCr15 bearing steel at 98 N and 0.47 m/s. Surface characterization was conducted by using scanning electron microscope, surface profilometer, X-ray diffraction and Vickers microhardness tester. The dominant wear mechanisms were identified by the worn surface morphologies. The results indicated that an intense surface remelting and ablation/evaporation was induced by HIPEB irradiation with high pulsed frequency, which resulted in the surface roughening, WC phase transformation and improved tribological properties of the YN13 cemented carbide. The specific wear rate and friction coefficient decreased as pulsed frequency increased. When pulsed frequency increased to 5 Hz, they decreased to 53.5% and 70% for non-irradiated samples. The improvement in wear resistance and the significant friction reduction of irradiated samples with the increase in pulsed frequency should be ascribed to the roughening of surface, the refinement microstructure of remelting, and the decrease in irradiated defect in the remelted layer of irradiated YN13 cemented carbide. The main wear mechanism of irradiated YN13 cemented carbide/GCr15 bearing steel pairs was micro-cutting wear in the initial stage of dry sliding wear and abrasive wear with characteristics of Ni preferential removal and fallout of WC similar to the non-irradiated samples in the stable stage.
1. Introduction WC/Ni cemented carbides, also known as hard alloys or hardmetals, have outstanding mechanical properties of high hardness, strength, and heat stability and excellent toughness and abrasive wear resistance in wear applications [1–4], however, the enhancement in the surface tribological properties of WC/Ni cemented carbides is still an urgent problem. The surface modification of cemented carbides has been effectively performed by high-intensity pulsed electron beam (HIPEB) irradiation [5–7]. HIPEB irradiation induces the remelting of carbides and preferential ablation of metal binder phase by efficient high-density energy deposition during a nanosecond pulse. This method significantly changes the morphology, composition and microstructure of surface to improve its surface properties, such as wear and corrosion resistance. At present, pulsed electron beam technology is making a remarkable progress in surface hardening and is improving the wear resistance of cemented carbides [7–11]. However, most studies on the surface modification of cemented carbide involved the adjustment of irradiation parameters of electron energy, energy density (or beam density), and pulse number and width. In these studies, the used pulse frequency is in the range of 0.03–1 Hz [7–11]. Nevertheless, the microstructure
and properties of irradiated WC-based cemented carbides at high frequency are limited, especially for the friction and wear behavior of irradiated WC-based cemented carbide. The purpose of this investigation was to study the friction and wear behavior of HIPEB-irradiated WC/Ni cemented carbides at 34 J/cm2 and 10 pulses with pulsed frequencies of 1 and 5 Hz under dry sliding wear test against hardened GCr15 steel at 98 N and 0.47 m/s. The wear mechanism of HIPEB-irradiated WC/Ni cemented carbides was also explored. 2. Experimental procedures 2.1. HIPEB irradiation process The WC/Ni cemented carbide of mark YN13 was used as experimental material. The chemical composition (wt%) was 13% of Ni and WC in balance. The samples were machined in the size of 6 mm × 6 mm × 8 mm. Prior to HIPEB irradiation, all the samples were polished using the diamond abrasive powders and 200-grit SiC abrasive paper, followed by alcohol rinse and blow drying. The WC/Ni samples were irradiated on a HIPEB setup described elsewhere [12,13]. The parameters of HIPEB irradiation were as follows: electron energy of
E-mail address: [email protected]. https://doi.org/10.1016/j.ceramint.2019.05.025 Received 25 March 2019; Received in revised form 1 May 2019; Accepted 3 May 2019 Available online 04 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 15327–15333
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Fig. 1. Schematic diagram of the block-on-ring type wear test machine.
127 kV, energy density of 34 J/cm2, pulse duration of 180 ns, number of pulses of 10, and pulsed frequencies of 1 and 5 Hz. 2.2. Characterization of HIPEB-irradiated samples Several techniques were used to characterize the surface characteristics of HIPEB irradiated samples. Before and after HIPEB irradiation, the surface morphology and roughness of the WC/Ni samples were studied using scanning electron microscope (SEM, JSM-6390LV) and profilometer (Surfcorder ET-4000A), respectively. The phase structure of the WC/Ni samples was examined by X-ray diffraction (XRD, DX-2500) with Cu Kα radiation. The surface microhardness of the WC/Ni samples was measured with Vickers tester (FEM-7000) with a load of 1.96 N for a holding time of 10 s. 2.3. Wear tests Dry sliding wear tests were conducted on a MM200 block-on-ring type wear test machine, as shown in Fig. 1. The WC/Ni block samples were pressed against a hardened GCr15 bearing steel ring (ø 45 mm × 10 mm, HV2 N 7.02 GPa) with a working load of 98 N and a sliding speed of 0.47 m/s for 2100 s. The working force (FN) applied on the upper body (WC/Ni block) and the accompanying frictional force (FT) of rotating the down body (GCr15 bearing steel ring) over the stationary counterface were periodically recorded during the dry sliding wear tests. The ratio of FN/FT was defined as the friction coefficient (f). Before and after the sliding wear test, the mass of the WC/Ni block sample was weighed by using a digital balance with a scale of 0.1 mg to calculate the specific wear rate. The specific wear rate (k) was calculated by Lancaster’s wear formula [14]:
k=
W Fn s
(1)
where W is the wear volume, which was calculated by the mass loss divided by the density of the cemented carbide tested, Fn is the applied working load, and s is the sliding distance. The surface morphologies in worn regions of the WC/Ni samples before and after HIPEB irradiation were also studied to reveal the wear mechanism of dry sliding wear. 3. Results and discussion 3.1. Surface structure and microhardness Fig. 2 shows the surface SEM images and surface profiles of YN13 cemented carbide before and after HIPEB irradiation under varied pulsed frequency. The surface of the non-irradiated sample presented a relatively smooth pattern with some processing defects during preparation, such as grinding marks, debris, and voids/pores (Fig. 2a). Surface roughness was approximately 0.207 μm. After HIPEB irradiation with a pulsed frequency of 1 Hz (Fig. 2b), the processing defects of
Fig. 2. Surface SEM images and surface profiles of YN13 cemented carbide before (a) and after HIPEB irradiation with pulsed frequency of 1 Hz (b) and 5 Hz (c).
the irradiated surface were eliminated entirely and surface roughness increased to approximately 0.540 μm. Simultaneously, some irradiated defect, such as blow holes and microcracks, appeared on the irradiated surface. Only a few white ablations were observed on the irradiated
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Fig. 3 shows the phase composition of YN13 cemented carbide before and after HIPEB irradiation under varied pulsed frequency. In the initial state, only the sharp peaks of hexagonal WC and cubic Ni phases were observed. Treatment by HIPEB irradiation significantly changed the phase composition of the surface of WC/Ni samples. After HIPEB irradiation with a pulsed frequency of 1 Hz, the diffraction peaks from metastable β-WC1-x, α-W2C, and graphite phases were detected. Therefore, a considerable WC phase transformation occurred on the irradiated surfaces, and this phenomenon decreased the diffraction peak of WC phase. The transformation of WC phase should be ascribed to the rapid cooling process with a cooling rate of 108 K/s and high temperature reaction, that is, reaction ablation of carbon in the WC during beam irradiation [8,16]. The diffraction peak of Ni decreased due to the selective ablation from the action of beam and materials. As the pulsed frequency increased to 5 Hz, the diffraction peaks of β-WC1-x phase remarkably intensified, whereas the diffraction peaks of α-WC and Ni phases greatly weakened. The formation of α-W2C and graphite phase should also be detected on the irradiated surface. The β-WC1-x phase in the remelted layer increased with the increase in pulsed frequency. This phenomenon indicated that the enhancement in thermal effect from the energy deposition of HIPEB irradiation promoted the transformation of WC phase. An enlarged section in the 61o–65° region of the XRD patterns clearly displayed the widening of WC1-x (220) peaks, which indicated that HIPEB irradiation led to the grain refinement of surface remelted layer on the irradiated samples. Fig. 4 shows the Vickers microhardness on the surface of YN13 cemented carbide before and after HIPEB irradiation under varied pulsed frequency. The surface microhardness of HIPEB-irradiated samples was lower than that of the non-irradiated sample and decreased slowly with the increase in pulsed frequency. When the pulsed frequency increased to 5 Hz, the surface microhardness decreased to 10.76 GPa, which was nine-tenths of the non-irradiated sample. Analysis of surface morphology and phase structure of these samples indicated that the reduction in microhardness on the irradiated surfaces with the increase in pulsed frequency should be ascribed to the formation of metastable β-WC1-x phase and surface irradiated defects, which were the negative affecting factors of surface microhardness [8,17]. 3.2. Friction coefficient Fig. 5 shows the friction coefficient of YN13 cemented carbide before and after HIPEB irradiation with varied pulsed frequency during
Fig. 3. XRD patterns of YN13 cemented carbide before and after HIPEB irradiation with the varied pulsed frequency: (a) 2 theta range of 20o-100°; (b) 2 theta range of 61o-65°.
surface. As the pulsed frequency increased to 5 Hz (Fig. 1c), the number of white ablation on the irradiated surface increased, but the number of blow holes decreased. Surface roughness increased to 0.767 μm. Some microcracks were also observed on the irradiated surface. The irradiated defects of blow holes and microcracks were found as well on the surface of cemented carbides treated by pulsed particle beam irradiation [8,15–18]. The blow holes were due to the surface ablation of cemented carbide induced by beam irradiation, and the microcracks were attributed to the rapid heating and cooling from beam irradiation. The weight losses of the irradiated WC/Ni sample with 1 and 5 Hz was 0.75 mg/cm2 and 1.20 mg/cm2, respectively. Therefore, an intense surface remelting and ablation/evaporation was induced by HIPEB irradiation with high pulsed frequency, which was attributed to the enhancement in thermal effect from the energy deposition of HIPEB irradiation due to the decrease in pulse interval.
Fig. 4. Surface microhardness of YN13 cemented carbide before and after HIPEB irradiation with varied pulsed frequency.
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Fig. 5. Friction coefficient curves of YN13 cemented carbide before and after HIPEB irradiation with varied pulsed frequency during wear test with 98 N and 0.47 m/s.
Fig. 6. Influence of pulse frequency on the specific wear rate of the HIPEBirradiated YN13 cemented carbide during wear test with 98 N and 0.47 m/s.
dry sliding wear tests with 98 N and 0.47 m/s. The curves of friction coefficient revealed that the friction process of the non-irradiated and irradiated YN13 cemented carbide should be divided into initial running-in and steady state stages. In the initial running-in stage of dry sliding wear, the friction coefficients fluctuated abruptly. This phenomenon should be strongly related to surface roughness or asperity interaction that resulted in the continuous changes in surface contact condition. The irradiated samples required considerably long runningin time to wear down the higher surface roughness or asperities than that of the non-irradiated samples. After the running-in period, the surface of wear track smoothened and the wear process gradually reached a steady state. In this case, the fluctuations in the friction coefficients diminished and gradually reached a steady state stage. The friction coefficient of irradiated samples/GCr15 bearing steel pairs was lower than that of non-irradiated samples/GCr15 bearing steel pairs and significantly decreased with the increase in pulsed frequency of HIPEB irradiation. When the pulsed frequency increased to 5 Hz, the friction coefficient decreased from 0.8 of non-irradiated sample to 0.56. The significant friction reduction of irradiated samples/GCr15 bearing steel pairs could be explained by the adhesive interaction, which depended on the surface WC phase transformation from HIPEB irradiation, that is, graphite precipitation, which had excellent lubricity for sliding contact for the friction of the surface remelted layer.
3.3. Specific wear rate Fig. 6 shows the influence of pulsed frequency on specific wear rate of the HIPEB-irradiated YN13 cemented carbide during the sliding wear test. The specific wear rate of the irradiated samples was in the 10−7 mm3/Nm range and decreased with increase in pulsed frequency of HIPEB irradiation. When the pulsed frequency increased to 5 Hz, the specific wear rate decreased to 6.42 × 10−7 mm3/Nm, which was approximately 53.5% of the non-irradiated samples. The state of cemented carbides wear could be briefly recognized by “mild wear” and “severe wear” from the point of view of specific wear rate [19,20]. The surface wear of the HIPEB-irradiated samples in the present test condition belonged to a “mild wear”. The HIPEB-irradiated WC/Ni samples had higher wear resistance than non-irradiated samples in the present test condition. However, the surface microhardness of the former samples was considerably lower than that of the latter samples. The wear resistance was in the 107 Nm/mm3 range for the HIPEB-irradiated samples and increased with increase in pulsed frequency of HIPEB
Fig. 7. Relationship of wear resistance with surface microhardness (a) and surface roughness (b) of the HIPEB-irradiated YN13 cemented carbide.
irradiation. Fig. 7 shows the relationship of wear resistance with surface microhardness and surface roughness. The wear resistance increased as surface microhardness decreased (Fig. 7a), which was contrary to the
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Fig. 8. SEM images of worn surfaces of YN13 cemented carbide before (a) and after HIPEB irradiation with varied pulsed frequency of 1 Hz (b) and 5 Hz (c) after sliding tests with 98 N and 0.47 m/s.
usual rule that materials with high hardness have high wear resistance [19,21]. In the present study, the wear resistance linearly enhanced with the improvement in surface roughness (Fig. 7b). This condition indicated that the improved wear resistance of irradiated samples/ GCr15 bearing steel pairs depended not only on surface microhardness, but also on surface roughness, remelting refinement microstructure, and WC phase transformation from HIPEB irradiation. In other words, the formation of WC1-x phases enhanced the coordination between hardness and toughness for the sliding wear of the surface remelted layer [22].
3.4. Wear mechanism Fig. 8 shows the SEM morphologies of wear surface of YN13 cemented carbide before and after HIPEB irradiation with varied pulsed frequency after dry sliding wear tests. As shown in Fig. 8a, the wear track of the non-irradiated sample exhibited an abrasive wear with characteristics of Ni preferential removal and fallout of WC, which should be attributed to the strong deformation and microabrasion [19,21,23–26]. It should be noted that an interesting result was that the harder YN13 cemented carbide was worn down by the much softer steel
(HV2 N 7.02 GPa) during the dry sliding wear process in this paper and similar phenomenon was also observed in the sliding wear process of WC-Co/45 carbon steel pairs [26]. Metallic nickel was softer than WC phase and should be removed preferably from between the carbides by plastic deformation and microabrasion [21,23], which rendered the WC grains prone to break out from the surface. The WC grains began to slip when sufficient support of binder phase had been wiped off and further fracture or fall out of the surface [19]. Fig. 8b and c show that the dry sliding wear of the irradiated samples depended on the remelted layer, which presented a micro-cutting wear despite some blow holes in the remelted layer. A similar phenomenon was found on the wear surface of the WC/Ni treated by high-intensity pulsed ion beam (HIPIB) irradiation in our previous work [27,28]. Many fine black spots on the worn zone of surface remelted layer showed the precipitation of nanograined graphite particles produced by HIPEB irradiation. A similar phenomenon was also found on the wear surface of the WC/Co treated by HCPEB irradiation [29]. The size of blow holes decreased, and nanograined graphite particles refined in the remelted layer with the increase in pulsed frequency. Therefore, the improvement in wear resistance of irradiated samples was depended on not only surface microhardness but also surface roughening and transition of wear
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4. Conclusions (1) High-intensity pulsed electron beam irradiation induced an intense surface remelting and ablation/evaporation at 34 J/cm2 and 10 pluses with high pulsed frequency and led to the surface roughening and WC phase transformation of the YN13 cemented carbide. Surface roughness increased with the increase in pulsed frequency of HIPEB irradiation. Surface roughness increased to 0.767 μm when the pulsed frequency was 5 Hz. (2) The specific wear rate and friction coefficient decreased with the increase in pulsed frequency. When the pulsed frequency increased to 5 Hz, they decreased to 53.5% and 70% for non-irradiated samples. The improvement in tribological properties with the increase in pulsed frequency should be ascribed to the roughening of surface, the refinement microstructure of remelting, and the decrease in irradiated defect in the remelted layer of HIPEB-irradiated YN13 cemented carbide. (3) The main wear mechanism of the HIPEB-irradiated YN13 cemented carbide/GCr15 bearing steel pairs was micro-cutting wear in the initial stage of dry sliding wear, and abrasive wear with characteristics of Ni preferential removal and fallout of WC similar to the non-irradiated samples in the stable stage. Acknowledgements
Fig. 9. Schematics of dry sliding wear for the HIPEB-irradiated YN13 cemented carbide against hardened GCr15 steel at sliding wear contact (a), the initial running-in stage (b) and the steady-state stage (c).
mechanism. The improvement in wear resistance and the significant friction reduction of HIPEB irradiated samples with the increase in pulsed frequency should be ascribed to the roughening of surface, the refinement microstructure of remelting (such as refined nanogrianed graphite particles and WC1-x phase), and the decrease in irradiated defect (such as blow holes) in the remelted layer of irradiated WC/Ni samples. The changes of tribological properties and the SEM results indicated that the dry sliding wear characteristic of the irradiated samples in the initial running-in stage was dependent on the special surface structure of surface remelted layer. When the two surfaces contacted, the surface humps endured the loading and prevented suppression and abrasion of the surface of the GCr15 bearing steel (Fig. 9a). The surface of the soft GCr15 bearing steel had been cut by the surface humps of irradiated samples under the loading because of the hardness of surface humps was higher than that of the GCr15 bearing steel. The surface humps of irradiated samples had also been worn down gradually due to the abovementioned mechanism—an abrasive wear with characteristic of micro-cutting (Fig. 9b). This result was confirmed from the wear surface of irradiated samples (Fig. 8b and c). Wear debris from the dry sliding wear should be collected by the surface concaves of irradiated samples to reduce the occurrence of abrasive wear due to surface contact. A similar phenomenon was also found on the wear surface of the WC/Ni treated by HIPIB irradiation in our previous work [27,28]. As the wear further proceeded, the surface humps of irradiated samples were worn out and the wear process reached a steady state stage (Fig. 9c). In this stage, the irradiated samples had a similar wear mechanism to non-irradiated samples. However, the removal rate decreased as observed from the densified microstructure below the remelted layer in Fig. 8b and c, which was caused by the strengthening of the binder phases due to that the stress wave of HIPEB irradiation propagated from the subsurface layer toward the deep layer of the irradiated samples [30].
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