Mechanics of Materials
Tribological Behaviors of B6O/Si3N4 and B6O/Al2O3 Sliding Pairs in Water
2020 Volume 61 Issue 3 Pages 475-481
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2020 Volume 61 Issue 3 Pages 475-481
In this study, B6O powder compacts were fabricated by spark plasma sintering B6O powder at 1800°C for 600 seconds. We then investigated the microstructures and mechanical properties of the powder compacts and assessed the tribological behaviors of the powder compacts by sliding them against commercially available Si3N4 and Al2O3 bearing balls in water at room temperature. The average friction coefficients of the B6O/Si3N4 sliding pairs were as low as 0.17, while the average friction coefficients of the B6O/Al2O3 sliding pairs gradually increased from 0.10 to 0.18. Also, the B6O/Si3N4 sliding pairs showed slightly more stable friction coefficients with smaller error bars than the B6O/Al2O3 sliding pairs 1200 seconds after starting the friction tests. The specific wear rates of the B6O powder compacts ranged from approximately 2 × 10−6 to 5 × 10−6 mm3/Nm when the Si3N4 and Al2O3 balls were used as the paired materials. We conjectured that the low friction coefficients and low specific wear rates stemmed from the formation of extremely thin H3BO3 films on the wear tracks of the powder compacts.
Fig. 4 Friction behaviors of B6O/Si3N4 and B6O/Al2O3 sliding pairs as a function of testing time. The blue and red filled circles represent the average friction coefficients.
Water lubrication systems have attracted much attention in industries that rely on the use of water pumps, medical devices and food machinery, given the advantages of water as an ecological material.1) Si3N4-, Al2O3-, and SiC-based ceramics have recently been widely used as sliding materials in water lubrication systems by dint of their high hardness, high wear resistance, and high corrosion resistance. In one notable report, Si3N4/Si3N4 and SiC/SiC sliding pairs showed extremely low friction coefficients of less than 0.01 in water after sliding a few thousand meters.2) However, the friction coefficients of the same Si3N4/Si3N4 and SiC/SiC sliding pairs were 0.95 and 0.45 in the beginning of their friction tests, respectively.2) They also exhibited high friction coefficients (≧0.5) at low sliding velocities under boundary lubricating conditions. In another report, Al2O3/Al2O3 sliding pairs exhibited a little comparatively high friction coefficients $( \simeq 0.5)$ in water.3) In an earlier study by our group, AlB12 and SiB6 ceramics showed low friction coefficients and high wear resistance in water when Si3N4, SiC and Al2O3 balls were used as the paired materials.3,4) These borides also have high hardness and corrosion resistance, in addition to favorable friction coefficients, and thus can be potentially applied as sliding parts in water. Dense AlB12- and SiB6-based ceramics, however, are very difficult to obtain by the hot pressing of blended elemental powders, as the Al and Si content in the blended powders oxidizes very easily during sintering, even in an Ar gas atmosphere at high temperature.3) For this reason, the AlB12 and SiB6 powder compacts fabricated by the sintering of blended elemental powders are likely to contain abundant Al2O3 and borosilicate phases with low hardness, respectively.
B6O, on the other hand, is the third hardest material, after diamond (70 to 100 GPa) and cBN (60 GPa).5–12) He et al. have reported that B6O single crystal has a high hardness of 45 GPa13) and a fracture toughness (4.5 $\text{MPa}\sqrt{\text{m}} $)13) that is fairly close to that of diamond single crystal (5 $\text{MPa}\sqrt{\text{m}} $) and higher than that of cBN single crystal (2.8 $\text{MPa}\sqrt{\text{m}} $).14) Moreover, Ogunmuyiwa et al. have reported that the hot-pressed polycrystalline B6O ceramics have the microvickers hardness as high as 30 GPa.15) The relative ease with which B6O ceramics can be obtained by hot pressing has led the way to numerous studies to fabricate B6O ceramics by hot pressing in order to investigate their mechanical and thermoelectric properties.15–17) No earlier studies have been performed, however, to sufficiently investigate the tribological behaviors of B6O ceramics. The high wear resistance and low friction coefficients of these ceramics are expected to prompt such investigations, as the materials exhibit high hardness and icosahedral B12 crystal structures similar to those of AlB12 and SiB6. He et al. have also reported that B6O has high corrosion resistance.13)
For the present study, therefore, we decided to fabricate B6O powder compacts by spark plasma sintering (SPS), and evaluate their tribological behaviors by sliding the B6O powder compacts against commercially available Si3N4 and Al2O3 bearing balls in water at room temperature because both the Si3N4 and Al2O3 balls exhibited low friction coefficients when these balls were slid against the borides such as AlB12 and SiB6.3,4)
B6O powder was obtained using methods similar to those reported by Solodkyi et al.16) First, B2O3 powder was dissolved in water and mixed with B powder to obtain a mole ratio of B2O3:B = 1:14. After drying at 100°C, the mixture was heated at 1300°C for 7200 seconds in a vacuum to obtain B6O powder. Next, B6O powder compacts with a diameter of 30 mm and a thickness of 10 mm were obtained by spark plasma sintering the B6O powder at 1800°C under a pressure of 40 MPa for 600 seconds in argon atmosphere using a Sinter Land LABOX-650F SPS machine. A 50-µm-thick Ta film was inserted between the B6O powder and graphite mold before the sintering to prevent the powder from reacting with the mold.16) After the SPS, the surface of each B6O powder compact was polished using diamond paste with a particle size of 1 µm. The surface roughness of the polished powder compacts was 0.3 µmRa. All of the powder compacts were then ultrasonically cleaned in a 1:1 mixture of acetone and petroleum benzene for 1200 seconds.3,4,18,19) The microvickers hardness of each B6O powder compact was measured five times using a Shimadzu HMV-1 hardness tester at a load of 9.8 N, for a loading time of 15 seconds. The fracture toughness of each powder compact, meanwhile, was investigated by the indentation fracture method.20,21) The x-ray diffraction (XRD) patterns of the B6O powder and B6O powder compacts were examined using a Rigaku Miniflex x-ray diffractometer. The cross sections of each powder compact were also observed using a JEOL JSM-7400F scanning electron microscope (SEM) with an energy dispersive x-ray spectrometry (EDS) attachment. The B6O powder compacts were slid against Si3N4 and Al2O3 balls with a diameter of 9.52 mm a few times using a Rhesca FPR 2001 rotary ball-on-plate tribometer to investigate the tribological behaviors. The powder compact and counterface ball were submerged in 100 ml of water in each test, with no refilling of the water during the test. All of the friction tests were performed at an applied load of 9.8 N, a sliding velocity of 1.57 mm/s, and temperature of 25°C for 3600 seconds.3) The specific wear rates of the powder compacts and counterface balls were calculated using a Tokyo Seimitsu SURFCOM surface profilometer and a Shodensya GR3400J optical microscope. The wear tracks of each powder compact were observed by two methods: first, by SEM using a JEOL JSM-7400F SEM with an EDS attachment; second, by x-ray photoelectron spectroscopy (XPS) using a Thermo VG Theta Probe XPS.
Figure 1 shows the XRD patterns of the B6O powder and B6O powder compacts fabricated in our present study. While the B6O powder was mainly composed of the B6O phase, it also contained a small amount of B phase that appeared to be a reaction residue of the B6O powder. The B6O powder compacts, on the other hand, were mainly composed of the B6O phase but also contained small amounts of TaB2 phase, probably as products from the reaction of the B6O powder with Ta film during the SPS.
XRD patterns of B6O powder and B6O powder compact fabricated by SPS.
Figure 2 shows a cross section of the B6O powder compact. Before the SEM observation, the powder compact was coated with a thin Pt film for the electrical conductivity of the powder compact. As demonstrated in Fig. 1, the powder compacts were mainly composed of the B6O phase and had a small number of pores. In addition, the white spotty phase seen in Fig. 2 was identified as the Pt phase according to the EDS analysis. On the other hand, TaB2 phase, whose weak peaks were detected in the XRD patterns of the B6O powder compact (Fig. 1), was hardly observed in Fig. 2, indicating that the TaB2 concentration was very small in the B6O powder compacts.
Composition image showing a cross section of a B6O powder compact fabricated by SPS (visualized using a JEOL JSM-7400F SEM).
Figure 3 shows the microvickers hardness and fracture toughness of the B6O powder compacts, Si3N4 balls and Al2O3 balls. The microvickers hardness of the B6O powder compacts was approximately 28 GPa, which was close to the value reported by Ogunmuyiwa et al.15) The microvickers hardness of the B6O powder compacts was higher than those of the Si3N4 and Al2O3 balls. In addition, the fracture toughness of the B6O powder compacts was lower than those of the Si3N4 and Al2O3 balls. This lower fracture toughness of the B6O powder compacts is consistent with the results reported by Ogunmuyiwa et al.15) They reported that the hot-pressed B6O powder compacts showed brittle fracture toughness.
Microvickers hardness and fracture toughness of B6O powder compacts, Si3N4 balls and Al2O3 balls.
Figure 4 shows the friction behaviors of the B6O powder compacts sliding against the Si3N4 and Al2O3 balls in water. The average friction coefficients of the B6O/Si3N4 sliding pairs were as low as 0.17, while the average friction coefficients of the B6O/Al2O3 sliding pairs gradually increased from 0.10 to 0.18. Also, the B6O/Si3N4 sliding pairs showed slightly more stable friction coefficients with smaller error bars than the B6O/Al2O3 sliding pairs 1200 seconds after starting the friction tests.
Friction behaviors of B6O/Si3N4 and B6O/Al2O3 sliding pairs as a function of testing time. The blue and red filled circles represent the average friction coefficients.
Figure 5 shows the wear behaviors of the B6O powder compacts and Si3N4 and Al2O3 counterface balls. The specific wear rates of the powder compacts slid against both types of balls ranged from approximately 2 × 10−6 to 5 × 10−6 mm3/Nm. The specific wear rates of the compacts slid against the Si3N4 balls were slightly larger than those of the compacts slid against the Al2O3 balls. This would be due to slightly higher hardness of the Si3N4 counterface balls than the Al2O3 counterface balls (Fig. 3). The specific wear rates of the Si3N4 and Al2O3 counterface balls were smaller than those of the B6O powder compacts, although the powder compacts were much harder than the counterface balls. We expected that a small amount of H3BO3 was formed on the wear tracks of the B6O powder compacts by the reaction of B6O with water during the friction tests, although He et al. reported that B6O has high corrosion resistance.13) On the other hand, the specific wear rates of the Si3N4 counterface balls were slightly smaller than those of the Al2O3 counterface balls.
Wear behaviors of B6O powder compacts and Si3N4 and Al2O3 counterface balls.
Figure 6 shows composition images of the wear tracks of the B6O powder compacts obtained using a JEOL JSM-7400F SEM. Smooth surfaces were observed on the wear tracks of the powder compacts after sliding against the Si3N4 and Al2O3 balls. Tribofilms were scarcely observed on the wear tracks of the powder compacts after sliding against the Si3N4 balls, though some low-friction tribofilms such as H3BO3 films appeared, presumably as a consequence of the low and stable friction coefficients of the B6O/Si3N4 sliding pairs. Very thin, scarcely detectable tribofilms were thus thought to have formed on the wear tracks of the B6O powder compacts after they were slid against the Si3N4 balls. While it is well known that SEM has a detection depth of about 1 µm,22) many tribofilms with thicknesses of less than 100 nm have been reported by many researchers. Another possibility is that some of the tribofilms on the wear tracks of the B6O powder compacts dissolved in water during the friction tests. It is well known that H3BO3 easily dissolves in water.4) On the other hand, a large area of Al- and O-rich white film was observed on the wear tracks of the B6O powder compacts after they were slid against the Al2O3 balls. The Al- and O-rich film was thought to have formed mainly through the transfer of the material from the Al2O3 balls. We also believe that the rise of the friction coefficients and their amplitudes during the friction tests of the B6O/Al2O3 sliding pairs (see Fig. 4) was due to the formation of the Al- and O-rich films, given that we previously observed a little comparatively high friction coefficients $( \simeq 0.5)$ of Al2O3/Al2O3 sliding pairs in water.3)
Composition images showing wear tracks of B6O powder compacts after they slid against (a) Si3N4 and (b) Al2O3 balls.
Figure 7 shows the wear scars of the Si3N4 and Al2O3 counterface balls. Smooth surfaces with small numbers of shallow scratch marks were observed on the wear scars of the Si3N4 and Al2O3 counterface balls.
Optical-microscope photographs showing the wear scars of the Si3N4 and Al2O3 counterface balls.
Figure 8 shows the EDS spectra of the wear tracks of the B6O powder compacts slid against the Si3N4 and Al2O3 balls, alongside the EDS spectrum of the non-wear tracks of the B6O powder compacts. No differences were observed between the wear tracks and non-wear tracks of the powder compacts after the compacts were slid against the Si3N4 balls. The low-friction tribofilms on the wear tracks were probably too thin to detect by SEM. On the other hand, an Al peak was observed on the wear track of the B6O powder compact after it was slid against the Al2O3 ball. We believe that this Al component was derived from the Al- and O-rich white film shown in Fig. 6.
EDS spectra of the wear tracks of the B6O powder compacts after they were slid against (a) Si3N4 and (b) Al2O3 balls. The lowermost plot (c) shows the EDS spectrum of the non-wear tracks of the B6O powder compacts.
Figure 9 shows the XPS spectra of the wear track area of the B6O powder compact after sliding against the Si3N4 ball. The composition of this wear track area was 44.06B–34.39C–2.29N–18.58O–1.29Si (at%). Most of the carbon content was thought to be due to carbon contamination.23) A comparison with the NIST x-ray photoelectron spectroscopy database24) indicates that B6O, Si3N4, H2O and SiO2 peaks appeared on the wear tracks of the B6O powder compacts after the compacts were slid against the Si3N4 balls. In addition, a peak at about 400 eV was also detected in Fig. 9. It is assumed that this peak was assigned to an ammonium salt. Saito et al. have reported that hexagonal BN specimens showed low friction coefficients in water, and that low-friction H3BO3 films with a layered crystal structure were detected by XPS.25,26) On the other hand, the AlB12 and SiB6 powder compacts we fabricated in our previous study showed low friction coefficients in water when Si3N4 balls were used as the paired materials. In that study, however, we observed films of Al2O3, Al(OH)3 and SiO2, and not H3BO3, on the wear tracks of the AlB12 and SiB6 powder compacts.4) Therefore, it was considered that extremely thin low-friction H3BO3 films were formed on the wear tracks of the AlB12 and SiB6 specimens, and that the formation of the films caused the low friction coefficients of the AlB12/Si3N4 and SiB6/Si3N4 sliding pairs.4) We conjectured that most of the H3BO3 films were promptly removed during the friction tests with the AlB12/Si3N4 and SiB6/Si3N4 sliding pairs, because H3BO3 easily dissolves in water.4) Besides, it has been reported that the vapor pressure of H3BO3 is 1.6 × 10−2 Pa at 25°C,27) while the XPS spectra of each powder compact were obtained in a high vacuum of 10−6–10−7 Pa in our present study. Therefore, we also considered that most of the H3BO3 content remaining on the wear tracks of the AlB12 and SiB6 powder compacts volatilized before the XPS analysis. Adachi et al., meanwhile, have described the formation of a low-friction Si(OH)4 gel on SiO2 films on the wear tracks of Si3N4 specimens in water.28,29) It thus appeared, in the present study, that Si(OH)4 gel films and very thin H3BO3 films had been formed on the wear tracks and caused the low friction coefficients of the B6O/Si3N4 sliding pairs in water, as shown in Fig. 4. We also speculate that the ammonium salt films might have reduced the friction coefficients of the B6O/Si3N4 sliding pairs. Sato and Kanno et al. have described the formation of NH3 on the surfaces of the Si3N4 in water through the following equation.30,31)
| \begin{equation*} \text{Si$_{3}$N$_{4}$}+\text{6H$_{2}$O} = \text{3SiO$_{2}$} + \text{4NH$_{3}$} \end{equation*} |
XPS spectra of the wear track area of a B6O powder compact after it was slid against a Si3N4 ball. The dotted lines reveal the fitted peaks and backgrounds.
Figure 10 shows the XPS spectra of the wear track area of the B6O powder compact after sliding against the Al2O3 ball. The composition of this wear track area was 3.33Al–16.45B–64.12C–0.85N–15.23O (at%). Al2O3, B6O, C contamination and H2O peaks were observed on the wear track of the B6O powder compact after it was slid against the Al2O3 ball. Further, the ammonium salt peak observed on this wear track was much smaller than that observed on the wear track of the B6O powder compact slid against the Si3N4 ball. The reason for more abundant ammonium salt detected on the wear tracks of the B6O powder compact slid against the Si3N4 ball remains unclear. We speculated that the low friction coefficients of the B6O/Al2O3 sliding pairs (Fig. 4) stemmed from the formation of a scanty H3BO3 film through the same process observed in the B6O/Si3N4 sliding pairs. The Al2O3 films on the wear track of the B6O powder compact also appeared to increase the friction coefficients and their amplitudes (Fig. 4).
XPS spectra of the wear track area of a B6O powder compact after sliding against an Al2O3 ball.
As described above, the B6O powder compacts fabricated by SPS showed low friction coefficients and low specific wear rates when Si3N4 and Al2O3 balls were used as the paired materials. Therefore, it is thought that the B6O ceramics can become a candidate of the sliding materials in water lubrication systems. We also plan to investigate the tribological behaviors of B6O powder compacts slid against paired materials in air at both room temperature and high temperature in the near future. Because McMillan et al. have reported that the melting point of the B6O ceramics is as high as 2000°C.32)
In this study, B6O powder compacts were fabricated by SPS, and their microstructures, and mechanical properties were examined. Also, the tribological behaviors of the powder compacts were investigated by sliding them against Si3N4 and Al2O3 balls in water. The following results were obtained.
This work was supported by the management expenses grants of National Institute of Advanced Industrial Institute, Japan (grant number AAZ30382K54).
