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High-temperature friction and wear properties of various sliding materials against aluminum alloy 5052
Tribology International 60 (2013) 45–52
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High-temperature friction and wear properties of various sliding materials against aluminum alloy 5052 T. Murakami n, S. Kajino, S. Nakano National institute of Advanced Science and Technology (AIST), Tsukuba, Ibaraki 305-8564, Japan
a r t i c l e i n f o
abstract
Article history: Received 4 January 2012 Received in revised form 19 October 2012 Accepted 25 October 2012 Available online 2 November 2012
The friction and wear properties of Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3, WC-6mass% Co and BN-50mass% Ni disk specimens sliding against aluminum alloy 5052 were examined at 823 K in air. The AISI H13 steel, AISI 52100 steel and BN– 50mass% Ni disk specimens exhibited relatively stable friction coefficients as low as 0.4–0.5, although the AISI 52100 steel and BN-50mass% Ni disk specimens exhibited the largest volume increase and the highest specific wear rates, respectively. On the other hand, the AISI H13 steel, ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens exhibited much smaller volume changes than AISI 52100 steel, Inconel 600 and BN-50mass% Ni disk specimens. However, the ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens exhibited unstable friction coefficients and the friction coefficients as high as 0.6, respectively. SEM-EDS analyses revealed large amount of Al and oxygen on the worn surfaces of the Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel and ZrO2–3 mol% Y2O3 disk specimens, while much smaller amount of Al and oxygen were observed on the worn surfaces of the Inconel 600, WC-6mass% Co and BN-50mass% Ni disk specimens. In addition, distinct oxygen peaks were observed on the worn surfaces of all the aluminum alloy pin specimens, but such oxygen peaks were hardly observed on the non-worn surfaces of all the pin specimens. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Aluminum alloy High temperature Unlubricated condition
1. Introduction Aluminum alloys are widely used in the automotive and aerospace industries because they are lightweight and recyclable materials. Aluminum alloys such as 5052, 5056, 6061 and 6063 are sometimes fabricated by hot extrusion at high temperatures [1–8] because they are more difficult to fabricate by cold working than steels [9]. The hot extrusion of the aluminum alloys were normally performed by hard material coated-AISI H13 steel [5–7] and WC-Co alloy dies [8]. However, aluminum alloys are known to be high friction materials, and they easily stick to conventional molds at high temperatures. Also, it is well known that martensitic steels such as AISI H13 steel become softer at high temperatures while 700–800 MPa pressure is applied to the dies during the hot extrusion processes. It is expected that the temperatures at the worn surfaces of the dies are much higher than the extrusion temperatures. On the other hand, it is necessary to develop much more finely fabricated aluminum alloy products such as heat sinks to improve their thermal efficiency. Therefore, it is necessary to develop low friction and low wear rate extrusion die materials for aluminum alloys. Pellizzari reported that the
n
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friction coefficients of the AISI H11 steels coated with TiAlN, CrN, and TiCþTiN were around 1.0 when they were slid against aluminum alloy 6082 at around 573 K [5]. Morijiri et al. also reported that TiAlN-coated steel specimens exhibited friction coefficients as high as 0.5–1.0 under an unlubricated condition at 673 K [10]. However, the high-temperature friction and wear properties of various sliding materials have not been investigated sufficiently when paired with aluminum alloys. Si3N4- and ZrO2based ceramics are considered to be possible candidates for such sliding materials because they exhibit high hardness at high temperatures, at which hot extrusion of aluminum alloys is performed. Also, self-lubricating materials containing solid lubricants such as hexagonal BN (h-BN) might be candidates for such sliding materials. It was found in our previous study that the friction coefficients and wear rates of Al2O3 and ZrO2–Y2O3–Al2O3 specimens were fairly reduced by coating with solid lubricants such as BaSO4 and SrSO4 [11–14]. In this study, we prepared Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel with a composition of Fe-(4.80–5.50)mass% Cr-(0.80– 1.20)mass% Si-(0.32–0.42)mass% C-(1.00–1.50)mass% Mo-(0.80– 1.15)mass% V-(0.25–0.50)mass% Mn, AISI 52100 steel with a composition of Fe-(1.30–1.60)mass% Cr-(0.15–0.35)mass% Si-(0.95– 1.10)mass% C, Inconel 600 with a composition of Ni-(14.0– 17.0)mass% Cr-(6.0–10.0)mass% Fe, ZrO2–3 mol% Y2O3, WC-6mass% Co and BN-50mass% Ni disk specimens, and investigated their friction
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Table 1 Microvickers hardness and density of disk and pin specimens. Density (Mg/m3)
Disk specimen
Microvickers hardness at room temperature (GPa)
Microvickers hardness at 823K (GPa)
Si3N4–8mass% Al2O3–6mass% Y2O3 AISI H13 steel AISI 52100 steel Inconel 600 ZrO2–3 mol% Y2O3 WC-6mass% Co BN-50mass% Ni Aluminum alloy 5052
17.2
15.9
3.27
5.4 8.1
5.3 2.0
7.70 7.78
2.8 14.7
2.3 9.2
8.34 6.05
24.9 0.4 0.9
16.2 0.2 0.2
14.85 3.50 2.68
Fig. 1. Scheme of the high-temperature pin-on-disk tribometer used in this study.
and wear properties when sliding against aluminum alloy 5052 pin specimens under unlubricated conditions at 823 K in air. Si3N4– 8mass% Al2O3–6mass% Y2O3, AISI 52100 steel, ZrO2–3 mol% Y2O3, WC-6mass% Co were chosen as disk materials because they are used as sliding materials very often. Also, the Inconel 600 was chosen as disk material because the nickel-based alloys are often used as high temperature structural materials. In order to investigate the effects of the BN on the friction and wear behavior clearly, the Ni composite disk specimens containing a large amount of BN phase were prepared in this study although the BN-50mass% Ni disk specimens were very soft materials as shown in Table 1. In addition, the worn surfaces on the disk and pin specimens were examined using a scanning electron microscope (SEM) with an energy dispersive spectroscopy (EDS) attachment.
2. Experimental procedures Fig. 2. XRD pattern of the disk and pin specimens prepared in this study.
Commercial Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100steel, Inconel 600, ZrO2–3 mol% Y2O3, WC-6mass% Co plates were used as the disk specimens. In addition, aluminum alloy 5052 was used as their paired pin specimens. The microvickers hardness and density of each disk and pin specimen are shown in Table 1. The microvickers hardness was measured at a load of 9.8 N for a loading time of 15 s. In addition, the microvickers hardness at 823 K was measured in an Ar gas atmosphere. Most of the specimens exhibited lower microvickers hardness at 823 K than at room temperature. Especially, the microvickers hardness of the AISI 52100 steel at 823 K was about one quarter of that at room temperature while the microvickers hardness of the AISI H13 steel at 823 K was as high as that at room temperature. Furthermore, BN-50mass% Ni disk specimens were prepared by spark plasma sintering (SPS) of granulated BN-50mass% Ni powder with a particle size of 20–53 mm at 1023 K under a pressure of 40 MPa in an Ar gas atmosphere for 600 s. SPS is a hot pressing process [14–17], and the sample powder is heated by flowing an electric current through a graphite mold containing the sample powder. SPS is reported to produce dense ceramic and metal powder compacts very easily in very short sintering times. Friction and wear tests were performed at 823 K in air at relative humidity ranging 40–50% using a high-temperature pinon-disk tribometer [14] as shown in Fig. 1. The dimensions of the pin specimens were as follows; diameter: 5 mm, length: 20 mm and tip curvature radius: 2.5 mm. Before the friction and wear tests, each disk and pin specimen was polished using 4000 grit emery paper and cleaned in a mixture of 50 vol% acetone and 50 vol% petroleum benzene for 1.2 ks using an ultrasonic cleaner. During the friction and wear test, each specimen was heated
using high-frequency induction heating. The friction and wear test conditions were as follows: load: 4.9 N, frequency: 1 Hz, stroke: 10 mm and testing time: 1.2 ks. In this study, two to three friction and wear tests were performed for each disk material. The friction coefficient was calculated by measuring the friction force at 1 ms intervals, obtaining the average friction force (absolute value) of one cycle of the reciprocation (1 s) and dividing the average friction force by load [14]. After the friction and wear tests, all of the disk specimens and their paired pin specimens were cleaned in a mixture of 50 vol% acetone and 50 vol% petroleum benzene for 1.2 ks using an ultrasonic cleaner. The specific wear rates of the disk specimens and their paired pin specimens were calculated using the following procedures. The wear depth profiles of each disk specimen were examined using a surface profilometer. A set of equally spaced depth profiles (3 profiles for each wear track) covering the whole wear track area was used to evaluate the volumetric wear [18]. The wear volume of aluminum alloy removed from the paired pin specimens was calculated by measuring the diameter of the wear scars on the pin specimens with an optical microscope. The wear scars were assumed to be perfectly circular and flat in order to simplify the calculations [19,20]. The specific wear rates were obtained by dividing the wear volume (mm3) by the load (4.9 N) and the total sliding distance (24 m). In addition, the worn surfaces on each disk and pin specimen were observed using a SEM with an EDS attachment. Before the SEM-EDS analyses, all of the disk and pin specimens were coated with a 10 nm thick Pt film to enable electrical conductivity to the surfaces of the non-electrical conductivity phases.
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3. Results Fig. 2 shows the X-ray diffraction patterns of the disk and pin specimens prepared in this study. The Si3N4–8mass% Al2O3– 6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3, WC-6mass% Co and Al alloy 5052 mainly consisted of Si3N4, Fe, Fe, Ni, ZrO2, WC and Al phases, respectively. Al2O3, Y2O3 and Co peaks were not observed in the XRD patterns of the Si3N4–8mass% Al2O3–6mass% Y2O3, ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens. These results would be because it is generally difficult to detect small amount of phases using the xray diffractometer. In addition, the BN-50mass% Ni disk specimen
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mainly consisted of h-BN and Ni phases as expected from the Ni– B–N phase diagram reported by Smid et al. [21]. Fig. 3 shows the microstructure of the cross sections of the disk and pin specimens. All of the specimens except for the BN50mass% Ni disk specimens had homogeneous microstructure and few pores. A small amount of white spotty Y-, Al- and O-rich phases were observed in the Si3N4–8mass% Al2O3–6mass% Y2O3 disk specimens while the AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3 and aluminum alloy 5052 specimens mostly seemed to be single phase materials. On the other hand, the WC-6mass% Co disk specimens exhibited very fine microstructure, in which the grain size was smaller than 1 mm. The
Fig. 3. SEM photograph showing a cross section of the (a) Si3N4–8mass% Al2O3–6mass% Y2O3, (b) AISI H13 steel, (c) AISI 52100 steel, (d) Inconel 600, (e) ZrO2–3 mol% Y2O3, (f) WC-6mass% Co, (g) BN-50mass% Ni, and (h) Aluminum alloy 5052 specimens prepared in this study.
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BN-50mass% Ni disk specimens consisted of h-BN and Ni phases, and any reaction phases were not observed, although the size of the h-BN phases was uneven. Fig. 4 shows the average friction coefficients of the disk specimens as a function of testing time. The Si3N4–8mass% Al2O3–6mass% Y2O3, Inconel 600, and ZrO2–3 mol% Y2O3 disk specimens exhibited unstable friction coefficients, and their friction coefficients sometimes exceeded 0.6. Also, the testing time exhibiting such high friction coefficients was not periodic and repeatable. Therefore, it is considered that the unstable friction coefficients of the Si3N4–8mass% Al2O3–6mass% Y2O3, Inconel 600, and ZrO2–3 mol% Y2O3 disk specimens were due to the non-repeatable phenomena such as flaking. In addition, the WC6mass% Co disk specimens mostly exhibited friction coefficients as high as 0.5–0.7. On the other hand, the AISI H13 steel, AISI 52100 steel and BN-50mass% Ni disk specimens exhibited relatively stable friction coefficients as low as 0.4–0.5. It is considered that the reason why the AISI H13 steel and AISI 52100 steel disk specimens exhibited low friction coefficients was due to the formation of low friction materials such as iron oxides on the worn surfaces. Umeda reported that Al2O3–Fe3O4 composites exhibited the friction coefficients as low as 0.4–0.5 because Fe3O4 worked as a solid lubricant [22]. On the other hand, the reason why the BN-50mass% Ni disk specimens exhibited low friction coefficients was thought to be that the h-BN phase in the disk specimens worked as a solid lubricant. Fig. 5 shows the specific wear rates of the disk specimens and their paired pin specimens. A negative value for a specific wear rate means an increase in the volume of that disk specimen. It is considered that the volume increase was caused by the transfer from the paired materials and some tribochemical reactions on the worn surfaces. The AISI H13 steel, ZrO2–3 mol% Y2O3 and WC6mass% Co disk specimens exhibited the much smaller volume changes than the AISI 52100 steel, Inconel 600 and BN-50mass% Ni disk specimens. It is considered that the smaller volume
Fig. 4. Average friction coefficients of the disk specimens sliding against aluminum alloy 5052 pin specimens.
Fig. 5. Specific wear rates of the disk specimens and their paired pin specimens. A negative value for a specific wear rate means an increase in the volume of the disk specimen.
changes were due to the higher microvickers hardness of the AISI H13 steel, ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens than the AISI 52100 steel, Inconel 600 and BN-50mass% Ni disk specimens at 823 K as shown in Table 1. On the other hand, the Si3N4–8mass% Al2O3–6mass% Y2O3 disk specimens exhibited a little larger volume changes than the AISI H13 steel, ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens although the microvickers hardness of the Si3N4–8mass% Al2O3–6mass% Y2O3 disk specimens was much higher than the AISI H13 steel and ZrO2–3 mol% Y2O3 disk specimens. Hillert et al. reported that the mixture of Si3N4 and Al partially melted at 873 K [23]. Therefore, it is considered that the tip of the aluminum alloy pin specimens reacted with a small amount of Si3N4, becomes softer conditions and easily stuck to the Si3N4–8mass% Al2O3–6mass% Y2O3 disk specimens when the disk specimens were slid against the aluminum alloy pin specimens at 823 K. On the other hand, the AISI 52100 steel disk specimens exhibited the largest volume increase of all the disk specimens while the Inconel 600 disk specimens exhibited the volume reduction. However, the microvickers hardness of the AISI 52100 steel was similar to that of the Inconel 600 at 823 K as shown in Table 1. According to the Fe–Al [24] and Ni–Al binary phase diagrams [25], brittle NiAl3 intermetallic compound, which has the lowest melting point (1127 K) of all the Ni–Al type intermetallic compounds, has 300 K lower melting point compared to FeAl3, which has the lowest melting point (1433 K) of all the Fe–Al type intermetallic compounds. Therefore, it is considered that the brittle phases such as NiAl3 were easily formed and flaked off on the worn surfaces of the Inconel 600/aluminum alloy tribopairs while such brittle intermetallic compounds were hardly formed on the AISI 52100 steel/ aluminum alloy tribopairs. Moreover, the BN-50mass% Ni disk specimens, which had relatively stable and low friction coefficients as shown in Fig. 4, exhibited the highest specific wear rates of all the disk specimens. It is considered that the highest specific wear rates were due to the low hardness of the BN-50mass% Ni disk specimens as shown in Table 1. Fig. 6 shows the worn surfaces of the Si3N4–8mass% Al2O3– 6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3, WC-6mass% Co and BN-50mass% Ni disk specimens after sliding against the aluminum alloy pin specimens. Except for the BN-50mass% Ni disk specimens, Al-rich phases were observed on all the disk specimens. It is considered that the h-BN phase in the
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Fig. 6. SEM photographs showing the worn surfaces on the (a) Si3N4–8mass% Al2O3–6mass% Y2O3, (b) AISI H13 steel, (c) AISI 52100 steel, (d) Inconel 600, (e) ZrO2–3 mol% Y2O3, (f) WC-6mass% Co and (g) BN-50mass% Ni disk specimens after sliding against the aluminum alloy pin specimens.
BN-50mass% Ni disk specimens prevented the aluminum content from sticking to the worn surfaces of the disk specimens. In addition, the Al-rich phases on the worn surfaces of the Inconel 600 and WC6mass% Co disk specimens were much smaller than those on the Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel and ZrO2–3 mol% Y2O3 disk specimens. It is considered that the smaller amount of Al-rich phase on the Inconel 600 disk specimens was due to the flaking off as described in Fig. 5. Also, it is considered that the smaller amount of Al-rich phase on the WC-6mass% Co was due to slow reaction rate of Al with WC because of extremely high melting point of WC (3049 K) [26]. On the other hand, adhesive wear
was observed on the worn surfaces of the AISI H13 steel, AISI 52100 steel and Inconel 600 disk specimens, indicating that these alloys were easily deformed at 823 K. Fig. 7 shows the EDS spectra of the worn surfaces on the Si3N4– 8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3, WC-6mass% Co and BN-50mass% Ni disk specimens. Strong Al and oxygen peaks were observed on the worn surfaces of the Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel and ZrO2–3 mol% Y2O3 disk specimens. This result indicates that Al content easily get stuck to the Si3N4–8mass% Al2O3– 6mass% Y2O3, AISI H13 steel, AISI 52100 steel and ZrO2–3 mol% Y2O3
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Fig. 7. EDS spectra of the worn surfaces on the Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3, WC-6mass% Co and BN-50mass% Ni disk specimens after sliding against the aluminum alloy pin specimens.
disk specimens as shown in Fig. 6, and that these worn surfaces were mostly covered with oxide films. In particular, iron oxide films seemed to be formed on the worn surfaces of the AISI H13 and AISI 52100 steel disk specimens because strong Fe peaks were observed. It is considered that these iron oxides caused the low friction coefficients of the AISI H13 and AISI 52100 steel disk specimens as shown in Fig. 4. On the other hand, much weaker Al and oxygen peaks were observed on the worn surfaces of the Inconel 600, WC6mass% Co and BN-50mass% Ni disk specimens as described in Fig. 6. Fig. 8 shows the worn surfaces on the aluminum alloy 5052 pin specimens after sliding against the Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3, WC-6mass% Co and BN-50mass% Ni disk specimens. The worn surfaces on the aluminum alloy pin specimens were found to be mostly covered with oxide films after sliding against the Si3N4– 8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens. Most of these oxide films were smooth, although they were partially flaked off. It is considered that these flaking-off phenomena caused the unstable friction coefficients of the ZrO2–3 mol% Y2O3 and WC6mass% Co disk specimens as shown in Fig. 4. These flaking-off phenomena were observed on the worn surfaces of the pin specimens slid against the AISI H13 and AISI 52100 steel disk specimens although these disk specimens exhibited relatively stable friction coefficients as shown in Fig. 4. As described in Fig. 6, the worn surfaces of the AISI H13 and AISI 52100 steel disk specimens were easily deformed during their friction and wear tests. Also, both the AISI H13 and AISI 52100 steel disk specimens exhibited the volume increase as shown in Fig. 5. For these reasons, the wear debris flaked from the pin specimens might have easily penetrated into the worn surfaces of the AISI H13 and AISI 52100 steel disk specimens, by which the stable friction coefficients might have been obtained. We will investigate these mechanisms much more clearly in the near future. On the other hand, the worn surfaces on the aluminum alloy pin specimens slid against the BN-50mass% Ni disk specimens were smoother than the other pin specimens and mostly covered with Band N-rich phase and Ni-rich phase. These results indicate that h-BN phase in the BN-50mass% Ni disk specimens worked efficiently as a solid lubricant. Fig. 9 shows EDS spectra of the worn surfaces on the pin specimens after sliding against the Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol%
Y2O3, WC-6mass% Co and BN-50mass% Ni disk specimens. Distinct oxygen peaks were observed on the worn surfaces of all the aluminum alloy pin specimens, while such oxygen peaks were hardly observed on the non-worn surfaces of all the pin specimens. These results indicate that the oxidation reactions of the aluminum alloy pins were promoted by the friction and wear tests performed in this study. In addition, much weaker oxygen peak and weak Ni peaks were observed on the worn surfaces of the aluminum alloy pin specimens after sliding against the BN50mass% Ni disk specimens. This would be because h-BN phase in the BN-50mass% Ni disk specimens worked as a solid lubricant efficiently as described in Fig. 8. Moreover, weak Fe peaks were observed on the worn surfaces of the aluminum alloy pin specimens after sliding against the AISI H13 and AISI 52100 steel disk specimens while weak Ni and Cr peaks were observed on the worn surfaces of the aluminum alloy pin specimens after sliding against the Inconel 600 disk specimens. It is considered that these Fe, Ni and Cr peaks were caused by adhesive wear as described in Fig. 6. As described above, the AISI H13 steel, AISI 52100 steel and BN-50mass% Ni disk specimens exhibited relatively stable friction coefficients as low as 0.4–0.5 (Fig. 4), although the AISI 52100 steel and BN-50mass% Ni disk specimens exhibited the largest volume increase and the highest specific wear rates (Fig. 5), respectively. On the other hand, the AISI H13 steel, ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens exhibited small volume changes (Fig. 5), although the ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens exhibited unstable and high friction coefficients (Fig. 4). These results indicate that the AISI H13 steel is the most appropriate die material of all the disk specimens. This would be because low friction iron oxides formed on the worn surfaces worked as solid lubricants efficiently. However, adhesive wear was observed on the worn surfaces of the AISI H13 steel disk specimens. In order to avoid this problem, some Fe-based intermetallic compounds will be prepared, and their tribological properties will be investigated in the near future, because a lot of Fe-based intermetallic compounds such as Fe7Mo6 would exhibit higher strength than martensitic steels such as AISI H13 steel at temperatures higher than 823 K. Moreover, it is considered that ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens containing 1–5 mass percents of solid lubricant such as h-BN exhibit both low friction coefficients and low specific wear rates when slid against aluminum alloys. It was found in our previous study that the friction coefficients and wear rates of Al2O3 and ZrO2–Y2O3–Al2O3 specimens were fairly reduced by coating with solid lubricants [11] and containing them [12–14]. In addition, it is considered that ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens containing 1–5 mass percents of solid lubricant have higher strength compared to the martensitic steels at around 823 K. These self-lubricating composites will be prepared, and their tribological properties will be also investigated in the near future.
4. Summary In this study, the friction and wear properties of various materials sliding against aluminum alloy 5052 were examined at 823 K in air. After the friction and wear tests, the worn surfaces were examined using a SEM with an EDS attachment. The conclusions obtained in this study are as follows. 1. The AISI H13 steel, AISI 52100 steel and BN-50mass% Ni disk specimens exhibited relatively stable friction coefficients as low as 0.4–0.5 although the AISI 52100 steel disk specimens
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Fig. 8. SEM photographs showing the worn surfaces on the aluminum alloy 5052 pin specimens after sliding against the (a) Si3N4–8mass% Al2O3–6mass% Y2O3, (b) AISI H13 steel, (c) AISI 52100 steel, (d) Inconel 600, (e) ZrO2–3 mol% Y2O3, (f) WC-6mass% Co and (g) BN-50mass% Ni disk specimens.
exhibited the largest volume increase and the BN-50mass% Ni disk specimens had the highest specific wear rates. 2. AISI H13 steel, ZrO2–3 mol% Y2O3, WC-6mass% Co disk specimens exhibited much smaller volume changes than AISI 52100 steel, Inconel 600 and BN-50mass% Ni disk specimens, although the ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens exhibited unstable friction coefficients and the friction coefficients as high as 0.6, respectively. 3. According to the EDS analyses, large amount of Al and oxygen were observed on the worn surfaces of the Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel and ZrO2–
3 mol% Y2O3 disk specimens. On the other hand, much smaller amount of Al and oxygen were observed on the worn surfaces of the Inconel 600, WC-6mass% Co and BN-50mass% Ni disk specimens. 4. Distinct oxygen peaks were observed on the worn surfaces of all the aluminum alloy pin specimens, while such oxygen peaks were hardly observed on the non-worn surfaces of all the pin specimens. In addition, much weaker oxygen peak was observed on the worn surfaces of the aluminum alloy pin specimens after sliding against the BN-50mass% Ni disk specimens.
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Fig. 9. EDS spectra of the worn surfaces on the pin specimens after sliding against the Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3, WC-6mass% Co and BN-50mass% Ni disk specimens.
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[9] Ikeda T. Tribology of aluminum alloy in cold forming. Tribologist 2010;55:854–9. [10] Morijiri A, Hatsuno K, Watanabe T, Masuda T, Hobo M Effect of tribological properties of aluminum alloy on TiAlN film coated by PVD method, Prof. JAST Tribol. Conf. Tokyo 2011-5, pp. 123–4 (2011). [11] Murakami T, Ouyang JH, Umeda K, Sasaki S. High-temperature friction properties of BaSO4 and SrSO4 powder films formed on Al2O3 and stainless steel substrates. Materials Science Engineering A 2006;432:52–8. [12] Ouyang JH, Murakami T, Sasaki S, Li Yu-Feng, Wang Ya-Ming, Umeda K, et al. High temperature tribology and solid lubrication of advanced ceramics. Key Engineering Materials 2008;368–372:1088–91. [13] Li YF, Ouyang JH, Wang YM, Zhou Y, Murakami T, Sasaki S. Influences of SrSO4 and Ag on high temperature tribological properties of spark-plasma-sintered ZrO2(Y2O3)–Al2O3 composites. Key Engineering Materials 2010;434– 435:138–43. [14] Murakami T, Ouyang JH, Sasaki S, Umeda K, Yoneyama Y. High-temperature friction and wear properties of X-BaSO4 (X: Al2O3, NiAl) composites prepared by spark plasma sintering. Materials Transactions 2005;46:182–5. [15] Tokita M. Industrial applications of advanced spark plasma sintering. American Ceramic Society Bulletin 2006;85:32–4. [16] Murakami T, Kaneda K, Nakano M, Mano H, Korenaga A, Sasaki S. Friction and wear properties of Fe–Mo intermetallic compounds under oil lubrication. Intermetallics 2007;15:1573–81. [17] Murakami T, Ouyang JH, Sasaki S, Umeda K, Yoneyama Y. High-temperature tribological properties of spark-plasma-sintered Al2O3 composites containing barite-type structure sulfates. Tribology International 2007;40:246–53. [18] Huq MZ, Celis JP. Fretting wear of multilayered (Ti,Al)N/TiN coatings in air of different relative humidity. Wear 1999;225–229:53–64. [19] Shuaib M, Davies TJ. Wear behaviour of a REFEL SiC containing fluorides up to 900 1C. Wear 2001;249:20–30. [20] Ruff AW. ASM Handbook Volume 18: Friction, Lubrication and Wear Technology. In: Blau P, editor. Materials Park, OH: ASM International; 1990. p. 363. [21] Smid I, Rogl P., Phase equilibria and structural chemistry in ternary systems: transition metal-boron–nitrogen, Conf. Ser. - Inst. Phys. 1986;75:249–57. [22] Umeda K Basic research in high temperature solid lubrication of ceramics and development of self-lubricating ceramics, Doctoral Dissertation. Toyohashi Institute of Technology, Japan; 1998. [23] Hillert M, Jonsson S. Prediction of the Al–Si–N system. Calphad—Computer Coupling of Phase Diagrams and Thermochemistry 1992;16:199–205. [24] Kattner UR. Al–Fe (Aluminum–Iron). In: Massalski TB, Peterson DE, Bale C, Pelton AD, Itkin VP, Alcock CB, et al., editors. Binary Alloy Phase Diagrams. Materials Park, OH: ASM International; 1996. [25] Singleton MF, Murray JL, Nash P. Al–Ni (Aluminum–Nickel). In: Massalski TB, Peterson DE, Bale C, Pelton AD, Itkin VP, Alcock CB, et al., editors. Binary Alloy Phase Diagrams. Materials Park, OH: ASM International; 1996. [26] Nagender Naidu SV, Sriramamurthy AM, Rama Rao P. C–W (Carbon–Tungsten). In: Massalski TB, Peterson DE, Bale C, Pelton AD, Itkin VP, Alcock CB, et al., editors. Binary Alloy Phase Diagrams. Materials Park, OH: ASM International; 1996.
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Tribology International journal homepage: www.elsevier.com/locate/triboint
High-temperature friction and wear properties of various sliding materials against aluminum alloy 5052 T. Murakami n, S. Kajino, S. Nakano National institute of Advanced Science and Technology (AIST), Tsukuba, Ibaraki 305-8564, Japan
a r t i c l e i n f o
abstract
Article history: Received 4 January 2012 Received in revised form 19 October 2012 Accepted 25 October 2012 Available online 2 November 2012
The friction and wear properties of Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3, WC-6mass% Co and BN-50mass% Ni disk specimens sliding against aluminum alloy 5052 were examined at 823 K in air. The AISI H13 steel, AISI 52100 steel and BN– 50mass% Ni disk specimens exhibited relatively stable friction coefficients as low as 0.4–0.5, although the AISI 52100 steel and BN-50mass% Ni disk specimens exhibited the largest volume increase and the highest specific wear rates, respectively. On the other hand, the AISI H13 steel, ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens exhibited much smaller volume changes than AISI 52100 steel, Inconel 600 and BN-50mass% Ni disk specimens. However, the ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens exhibited unstable friction coefficients and the friction coefficients as high as 0.6, respectively. SEM-EDS analyses revealed large amount of Al and oxygen on the worn surfaces of the Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel and ZrO2–3 mol% Y2O3 disk specimens, while much smaller amount of Al and oxygen were observed on the worn surfaces of the Inconel 600, WC-6mass% Co and BN-50mass% Ni disk specimens. In addition, distinct oxygen peaks were observed on the worn surfaces of all the aluminum alloy pin specimens, but such oxygen peaks were hardly observed on the non-worn surfaces of all the pin specimens. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Aluminum alloy High temperature Unlubricated condition
1. Introduction Aluminum alloys are widely used in the automotive and aerospace industries because they are lightweight and recyclable materials. Aluminum alloys such as 5052, 5056, 6061 and 6063 are sometimes fabricated by hot extrusion at high temperatures [1–8] because they are more difficult to fabricate by cold working than steels [9]. The hot extrusion of the aluminum alloys were normally performed by hard material coated-AISI H13 steel [5–7] and WC-Co alloy dies [8]. However, aluminum alloys are known to be high friction materials, and they easily stick to conventional molds at high temperatures. Also, it is well known that martensitic steels such as AISI H13 steel become softer at high temperatures while 700–800 MPa pressure is applied to the dies during the hot extrusion processes. It is expected that the temperatures at the worn surfaces of the dies are much higher than the extrusion temperatures. On the other hand, it is necessary to develop much more finely fabricated aluminum alloy products such as heat sinks to improve their thermal efficiency. Therefore, it is necessary to develop low friction and low wear rate extrusion die materials for aluminum alloys. Pellizzari reported that the
n
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friction coefficients of the AISI H11 steels coated with TiAlN, CrN, and TiCþTiN were around 1.0 when they were slid against aluminum alloy 6082 at around 573 K [5]. Morijiri et al. also reported that TiAlN-coated steel specimens exhibited friction coefficients as high as 0.5–1.0 under an unlubricated condition at 673 K [10]. However, the high-temperature friction and wear properties of various sliding materials have not been investigated sufficiently when paired with aluminum alloys. Si3N4- and ZrO2based ceramics are considered to be possible candidates for such sliding materials because they exhibit high hardness at high temperatures, at which hot extrusion of aluminum alloys is performed. Also, self-lubricating materials containing solid lubricants such as hexagonal BN (h-BN) might be candidates for such sliding materials. It was found in our previous study that the friction coefficients and wear rates of Al2O3 and ZrO2–Y2O3–Al2O3 specimens were fairly reduced by coating with solid lubricants such as BaSO4 and SrSO4 [11–14]. In this study, we prepared Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel with a composition of Fe-(4.80–5.50)mass% Cr-(0.80– 1.20)mass% Si-(0.32–0.42)mass% C-(1.00–1.50)mass% Mo-(0.80– 1.15)mass% V-(0.25–0.50)mass% Mn, AISI 52100 steel with a composition of Fe-(1.30–1.60)mass% Cr-(0.15–0.35)mass% Si-(0.95– 1.10)mass% C, Inconel 600 with a composition of Ni-(14.0– 17.0)mass% Cr-(6.0–10.0)mass% Fe, ZrO2–3 mol% Y2O3, WC-6mass% Co and BN-50mass% Ni disk specimens, and investigated their friction
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Table 1 Microvickers hardness and density of disk and pin specimens. Density (Mg/m3)
Disk specimen
Microvickers hardness at room temperature (GPa)
Microvickers hardness at 823K (GPa)
Si3N4–8mass% Al2O3–6mass% Y2O3 AISI H13 steel AISI 52100 steel Inconel 600 ZrO2–3 mol% Y2O3 WC-6mass% Co BN-50mass% Ni Aluminum alloy 5052
17.2
15.9
3.27
5.4 8.1
5.3 2.0
7.70 7.78
2.8 14.7
2.3 9.2
8.34 6.05
24.9 0.4 0.9
16.2 0.2 0.2
14.85 3.50 2.68
Fig. 1. Scheme of the high-temperature pin-on-disk tribometer used in this study.
and wear properties when sliding against aluminum alloy 5052 pin specimens under unlubricated conditions at 823 K in air. Si3N4– 8mass% Al2O3–6mass% Y2O3, AISI 52100 steel, ZrO2–3 mol% Y2O3, WC-6mass% Co were chosen as disk materials because they are used as sliding materials very often. Also, the Inconel 600 was chosen as disk material because the nickel-based alloys are often used as high temperature structural materials. In order to investigate the effects of the BN on the friction and wear behavior clearly, the Ni composite disk specimens containing a large amount of BN phase were prepared in this study although the BN-50mass% Ni disk specimens were very soft materials as shown in Table 1. In addition, the worn surfaces on the disk and pin specimens were examined using a scanning electron microscope (SEM) with an energy dispersive spectroscopy (EDS) attachment.
2. Experimental procedures Fig. 2. XRD pattern of the disk and pin specimens prepared in this study.
Commercial Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100steel, Inconel 600, ZrO2–3 mol% Y2O3, WC-6mass% Co plates were used as the disk specimens. In addition, aluminum alloy 5052 was used as their paired pin specimens. The microvickers hardness and density of each disk and pin specimen are shown in Table 1. The microvickers hardness was measured at a load of 9.8 N for a loading time of 15 s. In addition, the microvickers hardness at 823 K was measured in an Ar gas atmosphere. Most of the specimens exhibited lower microvickers hardness at 823 K than at room temperature. Especially, the microvickers hardness of the AISI 52100 steel at 823 K was about one quarter of that at room temperature while the microvickers hardness of the AISI H13 steel at 823 K was as high as that at room temperature. Furthermore, BN-50mass% Ni disk specimens were prepared by spark plasma sintering (SPS) of granulated BN-50mass% Ni powder with a particle size of 20–53 mm at 1023 K under a pressure of 40 MPa in an Ar gas atmosphere for 600 s. SPS is a hot pressing process [14–17], and the sample powder is heated by flowing an electric current through a graphite mold containing the sample powder. SPS is reported to produce dense ceramic and metal powder compacts very easily in very short sintering times. Friction and wear tests were performed at 823 K in air at relative humidity ranging 40–50% using a high-temperature pinon-disk tribometer [14] as shown in Fig. 1. The dimensions of the pin specimens were as follows; diameter: 5 mm, length: 20 mm and tip curvature radius: 2.5 mm. Before the friction and wear tests, each disk and pin specimen was polished using 4000 grit emery paper and cleaned in a mixture of 50 vol% acetone and 50 vol% petroleum benzene for 1.2 ks using an ultrasonic cleaner. During the friction and wear test, each specimen was heated
using high-frequency induction heating. The friction and wear test conditions were as follows: load: 4.9 N, frequency: 1 Hz, stroke: 10 mm and testing time: 1.2 ks. In this study, two to three friction and wear tests were performed for each disk material. The friction coefficient was calculated by measuring the friction force at 1 ms intervals, obtaining the average friction force (absolute value) of one cycle of the reciprocation (1 s) and dividing the average friction force by load [14]. After the friction and wear tests, all of the disk specimens and their paired pin specimens were cleaned in a mixture of 50 vol% acetone and 50 vol% petroleum benzene for 1.2 ks using an ultrasonic cleaner. The specific wear rates of the disk specimens and their paired pin specimens were calculated using the following procedures. The wear depth profiles of each disk specimen were examined using a surface profilometer. A set of equally spaced depth profiles (3 profiles for each wear track) covering the whole wear track area was used to evaluate the volumetric wear [18]. The wear volume of aluminum alloy removed from the paired pin specimens was calculated by measuring the diameter of the wear scars on the pin specimens with an optical microscope. The wear scars were assumed to be perfectly circular and flat in order to simplify the calculations [19,20]. The specific wear rates were obtained by dividing the wear volume (mm3) by the load (4.9 N) and the total sliding distance (24 m). In addition, the worn surfaces on each disk and pin specimen were observed using a SEM with an EDS attachment. Before the SEM-EDS analyses, all of the disk and pin specimens were coated with a 10 nm thick Pt film to enable electrical conductivity to the surfaces of the non-electrical conductivity phases.
T. Murakami et al. / Tribology International 60 (2013) 45–52
3. Results Fig. 2 shows the X-ray diffraction patterns of the disk and pin specimens prepared in this study. The Si3N4–8mass% Al2O3– 6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3, WC-6mass% Co and Al alloy 5052 mainly consisted of Si3N4, Fe, Fe, Ni, ZrO2, WC and Al phases, respectively. Al2O3, Y2O3 and Co peaks were not observed in the XRD patterns of the Si3N4–8mass% Al2O3–6mass% Y2O3, ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens. These results would be because it is generally difficult to detect small amount of phases using the xray diffractometer. In addition, the BN-50mass% Ni disk specimen
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mainly consisted of h-BN and Ni phases as expected from the Ni– B–N phase diagram reported by Smid et al. [21]. Fig. 3 shows the microstructure of the cross sections of the disk and pin specimens. All of the specimens except for the BN50mass% Ni disk specimens had homogeneous microstructure and few pores. A small amount of white spotty Y-, Al- and O-rich phases were observed in the Si3N4–8mass% Al2O3–6mass% Y2O3 disk specimens while the AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3 and aluminum alloy 5052 specimens mostly seemed to be single phase materials. On the other hand, the WC-6mass% Co disk specimens exhibited very fine microstructure, in which the grain size was smaller than 1 mm. The
Fig. 3. SEM photograph showing a cross section of the (a) Si3N4–8mass% Al2O3–6mass% Y2O3, (b) AISI H13 steel, (c) AISI 52100 steel, (d) Inconel 600, (e) ZrO2–3 mol% Y2O3, (f) WC-6mass% Co, (g) BN-50mass% Ni, and (h) Aluminum alloy 5052 specimens prepared in this study.
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BN-50mass% Ni disk specimens consisted of h-BN and Ni phases, and any reaction phases were not observed, although the size of the h-BN phases was uneven. Fig. 4 shows the average friction coefficients of the disk specimens as a function of testing time. The Si3N4–8mass% Al2O3–6mass% Y2O3, Inconel 600, and ZrO2–3 mol% Y2O3 disk specimens exhibited unstable friction coefficients, and their friction coefficients sometimes exceeded 0.6. Also, the testing time exhibiting such high friction coefficients was not periodic and repeatable. Therefore, it is considered that the unstable friction coefficients of the Si3N4–8mass% Al2O3–6mass% Y2O3, Inconel 600, and ZrO2–3 mol% Y2O3 disk specimens were due to the non-repeatable phenomena such as flaking. In addition, the WC6mass% Co disk specimens mostly exhibited friction coefficients as high as 0.5–0.7. On the other hand, the AISI H13 steel, AISI 52100 steel and BN-50mass% Ni disk specimens exhibited relatively stable friction coefficients as low as 0.4–0.5. It is considered that the reason why the AISI H13 steel and AISI 52100 steel disk specimens exhibited low friction coefficients was due to the formation of low friction materials such as iron oxides on the worn surfaces. Umeda reported that Al2O3–Fe3O4 composites exhibited the friction coefficients as low as 0.4–0.5 because Fe3O4 worked as a solid lubricant [22]. On the other hand, the reason why the BN-50mass% Ni disk specimens exhibited low friction coefficients was thought to be that the h-BN phase in the disk specimens worked as a solid lubricant. Fig. 5 shows the specific wear rates of the disk specimens and their paired pin specimens. A negative value for a specific wear rate means an increase in the volume of that disk specimen. It is considered that the volume increase was caused by the transfer from the paired materials and some tribochemical reactions on the worn surfaces. The AISI H13 steel, ZrO2–3 mol% Y2O3 and WC6mass% Co disk specimens exhibited the much smaller volume changes than the AISI 52100 steel, Inconel 600 and BN-50mass% Ni disk specimens. It is considered that the smaller volume
Fig. 4. Average friction coefficients of the disk specimens sliding against aluminum alloy 5052 pin specimens.
Fig. 5. Specific wear rates of the disk specimens and their paired pin specimens. A negative value for a specific wear rate means an increase in the volume of the disk specimen.
changes were due to the higher microvickers hardness of the AISI H13 steel, ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens than the AISI 52100 steel, Inconel 600 and BN-50mass% Ni disk specimens at 823 K as shown in Table 1. On the other hand, the Si3N4–8mass% Al2O3–6mass% Y2O3 disk specimens exhibited a little larger volume changes than the AISI H13 steel, ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens although the microvickers hardness of the Si3N4–8mass% Al2O3–6mass% Y2O3 disk specimens was much higher than the AISI H13 steel and ZrO2–3 mol% Y2O3 disk specimens. Hillert et al. reported that the mixture of Si3N4 and Al partially melted at 873 K [23]. Therefore, it is considered that the tip of the aluminum alloy pin specimens reacted with a small amount of Si3N4, becomes softer conditions and easily stuck to the Si3N4–8mass% Al2O3–6mass% Y2O3 disk specimens when the disk specimens were slid against the aluminum alloy pin specimens at 823 K. On the other hand, the AISI 52100 steel disk specimens exhibited the largest volume increase of all the disk specimens while the Inconel 600 disk specimens exhibited the volume reduction. However, the microvickers hardness of the AISI 52100 steel was similar to that of the Inconel 600 at 823 K as shown in Table 1. According to the Fe–Al [24] and Ni–Al binary phase diagrams [25], brittle NiAl3 intermetallic compound, which has the lowest melting point (1127 K) of all the Ni–Al type intermetallic compounds, has 300 K lower melting point compared to FeAl3, which has the lowest melting point (1433 K) of all the Fe–Al type intermetallic compounds. Therefore, it is considered that the brittle phases such as NiAl3 were easily formed and flaked off on the worn surfaces of the Inconel 600/aluminum alloy tribopairs while such brittle intermetallic compounds were hardly formed on the AISI 52100 steel/ aluminum alloy tribopairs. Moreover, the BN-50mass% Ni disk specimens, which had relatively stable and low friction coefficients as shown in Fig. 4, exhibited the highest specific wear rates of all the disk specimens. It is considered that the highest specific wear rates were due to the low hardness of the BN-50mass% Ni disk specimens as shown in Table 1. Fig. 6 shows the worn surfaces of the Si3N4–8mass% Al2O3– 6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3, WC-6mass% Co and BN-50mass% Ni disk specimens after sliding against the aluminum alloy pin specimens. Except for the BN-50mass% Ni disk specimens, Al-rich phases were observed on all the disk specimens. It is considered that the h-BN phase in the
T. Murakami et al. / Tribology International 60 (2013) 45–52
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Fig. 6. SEM photographs showing the worn surfaces on the (a) Si3N4–8mass% Al2O3–6mass% Y2O3, (b) AISI H13 steel, (c) AISI 52100 steel, (d) Inconel 600, (e) ZrO2–3 mol% Y2O3, (f) WC-6mass% Co and (g) BN-50mass% Ni disk specimens after sliding against the aluminum alloy pin specimens.
BN-50mass% Ni disk specimens prevented the aluminum content from sticking to the worn surfaces of the disk specimens. In addition, the Al-rich phases on the worn surfaces of the Inconel 600 and WC6mass% Co disk specimens were much smaller than those on the Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel and ZrO2–3 mol% Y2O3 disk specimens. It is considered that the smaller amount of Al-rich phase on the Inconel 600 disk specimens was due to the flaking off as described in Fig. 5. Also, it is considered that the smaller amount of Al-rich phase on the WC-6mass% Co was due to slow reaction rate of Al with WC because of extremely high melting point of WC (3049 K) [26]. On the other hand, adhesive wear
was observed on the worn surfaces of the AISI H13 steel, AISI 52100 steel and Inconel 600 disk specimens, indicating that these alloys were easily deformed at 823 K. Fig. 7 shows the EDS spectra of the worn surfaces on the Si3N4– 8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3, WC-6mass% Co and BN-50mass% Ni disk specimens. Strong Al and oxygen peaks were observed on the worn surfaces of the Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel and ZrO2–3 mol% Y2O3 disk specimens. This result indicates that Al content easily get stuck to the Si3N4–8mass% Al2O3– 6mass% Y2O3, AISI H13 steel, AISI 52100 steel and ZrO2–3 mol% Y2O3
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Fig. 7. EDS spectra of the worn surfaces on the Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3, WC-6mass% Co and BN-50mass% Ni disk specimens after sliding against the aluminum alloy pin specimens.
disk specimens as shown in Fig. 6, and that these worn surfaces were mostly covered with oxide films. In particular, iron oxide films seemed to be formed on the worn surfaces of the AISI H13 and AISI 52100 steel disk specimens because strong Fe peaks were observed. It is considered that these iron oxides caused the low friction coefficients of the AISI H13 and AISI 52100 steel disk specimens as shown in Fig. 4. On the other hand, much weaker Al and oxygen peaks were observed on the worn surfaces of the Inconel 600, WC6mass% Co and BN-50mass% Ni disk specimens as described in Fig. 6. Fig. 8 shows the worn surfaces on the aluminum alloy 5052 pin specimens after sliding against the Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3, WC-6mass% Co and BN-50mass% Ni disk specimens. The worn surfaces on the aluminum alloy pin specimens were found to be mostly covered with oxide films after sliding against the Si3N4– 8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens. Most of these oxide films were smooth, although they were partially flaked off. It is considered that these flaking-off phenomena caused the unstable friction coefficients of the ZrO2–3 mol% Y2O3 and WC6mass% Co disk specimens as shown in Fig. 4. These flaking-off phenomena were observed on the worn surfaces of the pin specimens slid against the AISI H13 and AISI 52100 steel disk specimens although these disk specimens exhibited relatively stable friction coefficients as shown in Fig. 4. As described in Fig. 6, the worn surfaces of the AISI H13 and AISI 52100 steel disk specimens were easily deformed during their friction and wear tests. Also, both the AISI H13 and AISI 52100 steel disk specimens exhibited the volume increase as shown in Fig. 5. For these reasons, the wear debris flaked from the pin specimens might have easily penetrated into the worn surfaces of the AISI H13 and AISI 52100 steel disk specimens, by which the stable friction coefficients might have been obtained. We will investigate these mechanisms much more clearly in the near future. On the other hand, the worn surfaces on the aluminum alloy pin specimens slid against the BN-50mass% Ni disk specimens were smoother than the other pin specimens and mostly covered with Band N-rich phase and Ni-rich phase. These results indicate that h-BN phase in the BN-50mass% Ni disk specimens worked efficiently as a solid lubricant. Fig. 9 shows EDS spectra of the worn surfaces on the pin specimens after sliding against the Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol%
Y2O3, WC-6mass% Co and BN-50mass% Ni disk specimens. Distinct oxygen peaks were observed on the worn surfaces of all the aluminum alloy pin specimens, while such oxygen peaks were hardly observed on the non-worn surfaces of all the pin specimens. These results indicate that the oxidation reactions of the aluminum alloy pins were promoted by the friction and wear tests performed in this study. In addition, much weaker oxygen peak and weak Ni peaks were observed on the worn surfaces of the aluminum alloy pin specimens after sliding against the BN50mass% Ni disk specimens. This would be because h-BN phase in the BN-50mass% Ni disk specimens worked as a solid lubricant efficiently as described in Fig. 8. Moreover, weak Fe peaks were observed on the worn surfaces of the aluminum alloy pin specimens after sliding against the AISI H13 and AISI 52100 steel disk specimens while weak Ni and Cr peaks were observed on the worn surfaces of the aluminum alloy pin specimens after sliding against the Inconel 600 disk specimens. It is considered that these Fe, Ni and Cr peaks were caused by adhesive wear as described in Fig. 6. As described above, the AISI H13 steel, AISI 52100 steel and BN-50mass% Ni disk specimens exhibited relatively stable friction coefficients as low as 0.4–0.5 (Fig. 4), although the AISI 52100 steel and BN-50mass% Ni disk specimens exhibited the largest volume increase and the highest specific wear rates (Fig. 5), respectively. On the other hand, the AISI H13 steel, ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens exhibited small volume changes (Fig. 5), although the ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens exhibited unstable and high friction coefficients (Fig. 4). These results indicate that the AISI H13 steel is the most appropriate die material of all the disk specimens. This would be because low friction iron oxides formed on the worn surfaces worked as solid lubricants efficiently. However, adhesive wear was observed on the worn surfaces of the AISI H13 steel disk specimens. In order to avoid this problem, some Fe-based intermetallic compounds will be prepared, and their tribological properties will be investigated in the near future, because a lot of Fe-based intermetallic compounds such as Fe7Mo6 would exhibit higher strength than martensitic steels such as AISI H13 steel at temperatures higher than 823 K. Moreover, it is considered that ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens containing 1–5 mass percents of solid lubricant such as h-BN exhibit both low friction coefficients and low specific wear rates when slid against aluminum alloys. It was found in our previous study that the friction coefficients and wear rates of Al2O3 and ZrO2–Y2O3–Al2O3 specimens were fairly reduced by coating with solid lubricants [11] and containing them [12–14]. In addition, it is considered that ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens containing 1–5 mass percents of solid lubricant have higher strength compared to the martensitic steels at around 823 K. These self-lubricating composites will be prepared, and their tribological properties will be also investigated in the near future.
4. Summary In this study, the friction and wear properties of various materials sliding against aluminum alloy 5052 were examined at 823 K in air. After the friction and wear tests, the worn surfaces were examined using a SEM with an EDS attachment. The conclusions obtained in this study are as follows. 1. The AISI H13 steel, AISI 52100 steel and BN-50mass% Ni disk specimens exhibited relatively stable friction coefficients as low as 0.4–0.5 although the AISI 52100 steel disk specimens
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Fig. 8. SEM photographs showing the worn surfaces on the aluminum alloy 5052 pin specimens after sliding against the (a) Si3N4–8mass% Al2O3–6mass% Y2O3, (b) AISI H13 steel, (c) AISI 52100 steel, (d) Inconel 600, (e) ZrO2–3 mol% Y2O3, (f) WC-6mass% Co and (g) BN-50mass% Ni disk specimens.
exhibited the largest volume increase and the BN-50mass% Ni disk specimens had the highest specific wear rates. 2. AISI H13 steel, ZrO2–3 mol% Y2O3, WC-6mass% Co disk specimens exhibited much smaller volume changes than AISI 52100 steel, Inconel 600 and BN-50mass% Ni disk specimens, although the ZrO2–3 mol% Y2O3 and WC-6mass% Co disk specimens exhibited unstable friction coefficients and the friction coefficients as high as 0.6, respectively. 3. According to the EDS analyses, large amount of Al and oxygen were observed on the worn surfaces of the Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel and ZrO2–
3 mol% Y2O3 disk specimens. On the other hand, much smaller amount of Al and oxygen were observed on the worn surfaces of the Inconel 600, WC-6mass% Co and BN-50mass% Ni disk specimens. 4. Distinct oxygen peaks were observed on the worn surfaces of all the aluminum alloy pin specimens, while such oxygen peaks were hardly observed on the non-worn surfaces of all the pin specimens. In addition, much weaker oxygen peak was observed on the worn surfaces of the aluminum alloy pin specimens after sliding against the BN-50mass% Ni disk specimens.
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Fig. 9. EDS spectra of the worn surfaces on the pin specimens after sliding against the Si3N4–8mass% Al2O3–6mass% Y2O3, AISI H13 steel, AISI 52100 steel, Inconel 600, ZrO2–3 mol% Y2O3, WC-6mass% Co and BN-50mass% Ni disk specimens.
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