Wear Behaviors of Three Typical Bulk Metallic Glasses in Bearing Applications

Abstract

In bearing applications, the development of new materials has become a focus of scientific research in order to make bearing systems smaller and rotate more accurately. Bulk metallic glass (BMG), which has high strength, stiffness and resistance to corrosion, is becoming a promising candidate for bearing and shaft materials. When used as shafts, the friction feature of BMG needs to be evaluated comprehensively. In this work, the friction and wear properties of Ni-based, Zr-based, and Cu-based BMGs sliding against brass lubricated with lithium grease were investigated, using traditional bearing materials (GCr15 steel) as comparison. The results showed that the wear mechanism of the BMGs was primarily abrasive, supplemented by an adhesive wear behavior when sliding against brass plates, just like GCr15 steel. The wear loss of the friction pair (brass plates) increases when the applied normal load increases and the sliding speed decreases. Compared with GCr15 steel, BMGs exhibit better friction performance at low sliding speed, and Ni-based BMG always exhibits a smaller wear loss, especially under large load and low sliding speed. The wear loss of brass plates against Ni-based BMG pin is 24.3% lower than that against GCr15 steel under an applied load of 10 kg, which indicates that Ni-based BMG is an attractive bearing and shaft material for industrial application.

1. Introduction

Bearings and shafts, especially in harsh or critical environments, severely restrict the service life of mechanical systems. Excellent bearing materials can ensure that the mechanical components run with higher precision and longer life. With the ever-growing demands of miniaturization and higher precision for mechanical devices, most traditional bearing materials, such as GCr15 steel, can hardly meet the requirements. In bearing applications, higher strength can reduce volume considerably, while higher stiffness is favorable of realizing higher precision for micro-mechanical components. Moreover, bearings with higher corrosion resistance can improve the environmental adaptability of micro-mechanical components. Therefore, owing to the above-mentioned reasons, much attention is being paid to the development of new bearing materials with enhanced performance. As revealed by much previous research, bulk metallic glasses (BMGs), which possess high strength, high elastic limit, high hardness, and excellent corrosion resistance, are one of the most suitable candidates for bearing or shaft materials [1,2,3,4]. Compared with traditional commercial materials, BMGs such as Ni-based BMG [5], Zr-based BMG [6,7], and Cu-based [8] BMG have many unique performance advantages, such as high strength, high stiffness, satisfactory corrosion resistance, etc., However, the poor glass forming ability (GFA) of BMGs was a big obstacle in the fabrication of components of sufficient size to meet the bearing requirements. In the past few years, this obstacle has been well overcome by many newly discovered BMG components that are free of toxic and precious metals while simultaneously having high GFA, making them suitable for wider applications in industry. Meanwhile, the appearance of new preparation technologies, such as the precision die-casting technique, tilt casting technique, cap-cast technique, net-shaped formability, etc., also make the formation of BMG-based micro-mechanical components realizable [9,10]. For example, Inoue [9] fabricated a micro-gear motor part using Zr-based BMG via a net-shape casting technique. With the assistance of this small gear, the world’s smallest geared-motor, only 1.5 mm in diameter and 9.9 mm in length, was successfully produced. Mamoru Ishida [10] manufactured a micro-gear made of Ni-based BMG via a precision die-casting technique and found that the durability of the micro-gear was 313 times better than that of conventional SK-steel under rotating test. Ma et al. [11] found that the wear resistance of Zr-based BMG rollers was almost twice that of the GCr15 steel rollers under large-load and high-speed rotation tests, and no crystallization took place after the rotation test, which shows the feasibility of Zr-based BMG for utilization as rollers. In a word, the significance and feasibility of BMG for bearing applications have been proved both experimentally and theoretically.

BMGs possess different friction and wear points due to the fact that they have completely different atomic arrangements from traditional alloys. With the same hardness, the behavior of BMGs conforms to a brittle wear mechanism; however, hardened steel exhibits adhesive wear behavior [10]. The wear resistance of BMGs results from the synergistic effects of strength factors, hardness, Young’s modulus, yield strength, glass transition temperature and toughness factors. The formation of naturally and artificially grown surface oxides remarkably improves the wear resistance of BMGs [12]. The influence of annealing also has a very important impact on the tribological properties, and it has been found that the anti-friction properties of BMGs decrease with an increase in annealing degree, in contrast with their anti-wear properties, which increase instead [13]. Environmental factors also affect the tribological properties of BMGs, including sliding velocity, counter pairs, sample size, surface roughness, etc. [14,15,16,17]. The study of friction and wear mechanisms shows that frictional heating plays a central role in controlling tribological properties [18]. As a potential bearing or shaft material, it is very important to investigate its friction and wear properties under the current bearing operating conditions, and it will be helpful to study the tribological properties and further reveal the wear mechanism and failure mode to seek more suitable working conditions and counter pairs. However, few works in the literature have reported the friction and wear properties of BMGs under bearing working conditions. In this work, the wear and friction behaviors of different BMGs against traditional brass under the working environment of a shaft and bearing bush with lithium grease lubrication were investigated in detail, using the traditional bearing material GCr15 steel for comparison. This work may provide some guidance on obtaining the best and most compatible conditions between BMG pins and brass plates.

2. Materials and Methods

Three different typical BMG alloys, Ni₅₃Nb₂₀Ti₁₀Zr₈Co₆Cu₃, Zr₅₅Cu₃₀Al₁₀Ni₅ and Cu₆₀Zr₃₀Ti₁₀, were selected for investigation in this work. All of the BMGs were prepared by arc melting of a mixture of pure raw materials with a purity of 99.99% under a Ti-gettered argon atmosphere and melted five times to ensure the ingots’ homogeneity. Afterwards, a vacuum casting facility was used to cast the melting masters into rods 2 mm in diameter under an argon atmosphere. Pins 6 mm in length were cut from the as-cast 2 mm rods for the wear experiment. The structure of the as-prepared BMG pins was determined by X-ray diffraction (XRD, D8-advance) (Bruker, Bremen, Germany) with a Cu-Kα source. The commercial GCr15 steel pins were also cut to the same size. Vickers hardness was measured by a Vickers microhardness tester (Shanghai Taiming Optical Instruments Co., Ltd., Shanghai, China) under a load of 200 grams and a pressure holding time of 15 s. The HVs of GCr15 steel, Ni-based, Zr-based, Cu-based and brass plate were HV827, HV821, HV531, HV575, HV144, respectively.

The contact sliding experiments were performed on a linear reciprocation UMT-3 pin-on-plate (Bruker, Billerica, MA, USA) tribometer under lubrication conditions with lithium grease, using BMG or GCr15 steel pins against normal brass plates at room temperature. The applied normal loads were set at 1 kg, 3 kg, 5 kg, 10 kg, with a sliding speed of 5 mm/s and a max sliding time of 100 min. The applied sliding speed was set at 1 mm/s, 5 mm/s, and 10 mm/s, under a normal load of 3 kg for a slide distance of 60 m. The roughness of the pins and brass plates were polished to 0.4 µm and 0.1 µm, respectively, prior to the sliding experiments. The weights of the pins and brass plates were measured on a high-precision digital microbalance with a precision of 0.1 mg before and after the sliding tests.

The surface morphologies and elemental distributions were investigated by a scanning electron microscope (SEM, SU-70) (HITACHI, Tokyo, Japan) equipped with an energy dispersive spectroscope (EDS) (Oxford Instruments, Abingdon, UK) after the sliding experiments and after ultrasonic oscillation treatments in acetone for 20 min in order to clean the residual lithium grease away. The cross-sectional morphology was observed by Dektak Stylus Profilers (Bruker, Billerica, MA, USA) for the cleaned samples.

3. Experimental Results and Discussion

3.1. Structural Characterization

Figure 1 shows the XRD patterns of the as-prepared Ni-based, Zr-based, Cu-based BMG pins. As can be seen, only a broad and diffuse peak without any evident crystalline phase was observed, indicating the amorphous nature of all BMG pins.

3.2. Relationship between Wear Rate of Brass Plates and Normal Loads

Figure 2 shows the variation of the wear loss of the brass plates as a function of normal load at a sliding speed of 5 mm/s and a sliding time of 100 min for GCr15 steel, Ni-based, Zr-based, and Cu-based BMGs, respectively. It can be seen that the wear rate increases with an increase in normal load. Under low loads, the wear behavior is mainly controlled by abrasive wear, and the wear loss is smaller due to better lubrication effect of the contact surface. As the load increases, the content of grease on the contact surface decreases, deteriorating the lubrication effect and further enhancing the abrasive wear, which is accompanied by slight adhesive wear. With further increase of the load, the adhesive wear gets more severe, accelerating the wear process. When the normal load is smaller than 3 kg, the wear rates of the brass plates are hardly related to the counter-pair materials, and the wear rates of three BMG pins are relatively lower than that of GCr15 steel, which means that BMGs have better suitability with the brass plates than GCr15 steel. The wear loss of the brass plates against GCr15 steel is 0.054 mg/m under a normal load of 3 kg; however, the average wear loss of the brass plates against BMGs is 0.049 mg/m. With the increase in normal load, the wear rates increase sharply and become closely related to the counter-pair materials. The wear rates of the brass plates against Zr-based and Cu-based BMG pins are higher than that against GCr15 steel under normal loads of 5 kg and 10 kg. However, the wear rate of the brass plates against Ni-based BMG pin is always lower than that against GCr15 steel, regardless of the normal load. Under the condition of 10 kg load, the wear rates of the brass plates against Zr-based and Cu-based BMG pins are 43.1% and 8.1% higher than that against GCr15 steel, respectively. However, the wear rate of the brass plates against the Ni-based BMG pin is 24.3% lower than that against GCr15 steel. It is reasonable to conjecture that the Zr-based and Cu-based BMGs have much lower hardness than GCr15 steel, and consequently result in much higher wear rates. However, Ni-based BMG (HV821) has considerable hardness compared with GCr15 steel (HV827), and its wear rate is much lower than that of GCr15 steel.

3.3. Relationship between Wear Rate of Brass Plates and Sliding Speeds

Figure 3 shows the variation in the wear loss of the brass plates as a function of sliding speed under a normal load of 3 kg after a sliding distance of 60 m for GCr15 steel, Ni-based, Zr-based, and Cu-based BMGs, respectively. The wear rates of the brass plates decrease dramatically with increasing sliding speed. At a sliding speed of 1 mm/s, the wear rate of the brass plates against BMG pins is only 68.8–56.2% of that against GCr15 steel, indicating the greater suitability of the BMG pins with the brass plate at slow sliding speeds. At sliding speeds of 5 mm/s and 10 mm/s, the wear rates of the brass plates are almost the same for all pins. The average wear rates of the brass plates for all pins are in the range of 0.053 and 0.047 mg/m at sliding speeds of 5 mm/s and 10 mm/s, respectively; far below the value at 1 mm/s. It can be concluded that the BMG pins can ensure the counter brass plates a good wear rate at low sliding speeds and a considerable wear rate at high sliding speeds in comparison to GCr15 steel pins.

3.4. Worn Surfaces of Brass Plates under Different Loads

Figure 4 shows the SEM images of the surfaces of the as-worn brass plates with Ni-based and GCr15 steel pins under different normal loads at a sliding speed of 5 mm/s and a sliding time of 100 min, with the inset demonstrating the SEM images for the cleaned worn surfaces of corresponding samples. The surfaces of all brass plates exhibited long continuous parallel grooves, and the as-worn surfaces retain a large amount of abrasive particles of different sizes, which is a typical topography feature of abrasive wear-controlled mechanisms. As the load increases, the shear force increases accordingly. The hard particles on the friction pairs penetrate the surface of the brass plate more intensively, and the spalling debris particles further accelerate the wear. According to the cross-sectional morphology of the wear tracks, as shown in Figure 4f, it can also be confirmed that there are numerous parallel grooves on the worn brass surface. The average wear depths of the brass surfaces with the Ni-based BMG pins were 4.04 µm, 8.51 µm, 8.70 µm, 17.86 µm under the loads of 1 kg, 3 kg, 5 kg, and 10 kg, respectively, which indicates that abrasive wear gets worse with increasing normal load. At a load of 10 kg, the average wear depth of the brass plate against the GCr15 pin was 23.29 µm, 30% higher than the value of 17.86 µm for Ni-based BMG pin. The Ni-based BMG with an equivalent hardness will ensure the friction pairs a better wear resistance than that of GCr15 steel, which further confirms the results of Figure 2.

In the experiment, more residual lithium grease could be found on the brass plates surfaces (as shown in Figure 4a); however, it was almost pushed away under a large normal load (see Figure 4d). It is well known that lithium grease functions as a lubricant and avoids the direct contact between the pins and the brass plates, which reduces the friction and wear rate of the brass plates. Under high normal loads, obvious adhesive wear was found on the brass plates, and it got worse with increasing normal load. Due to the lack of lubrication under high loads, tiny joints formed between the brass surface and the pin. At the local stress concentration, delamination and peeling-off occurred. As a result, the soft brass breaks under the applied pressure of high load, with holes of different sizes left on the brass surface (see Figure 4d,e). Increased adhesive wear is detrimental to the shaft and bearing bush, thus it should be avoided. However, the Ni-based BMG with an equivalent hardness presented a low adhesive wear, while the Cu-based and Zr-based BMG with a low hardness presented a severe adhesive wear compared with GCr15 steel. As the Cu content in Cu-based and Zr-based BMGs is higher than that in Ni-based BMG, the higher inter-solubility between the brass and BMGs is also the cause of severe adhesion wear. It is evident that proper lubrication can effectively reduce adhesion wear. It should also be noted that our experiments were conducted under open circumstances, where the lithium grease could be pushed away, but the actual working environments for bearings are generally closed systems, and the grease may be retained on the contact surface. The BMGs will exhibit mild adhesion wear in actual applications.

3.5. Worn Surfaces of Pins under Different Loads

Figure 5 shows the SEM images of the surfaces of as-worn Ni-based BMG pins under different normal loads at a sliding speed of 5 mm/s and a sliding time of 100 min. The insets show the EDS mapping results of the transfer layers on the pins. It is clear that the transfer layers can be found on the surfaces of all pins, and they are larger and more continuous with increasing normal load. EDS results indicate that the main elements in the transfer layers are Cu and Zn elements, which originate from the counter pairs (brass plates). The transfer layer in Cu-based BMG functions as a solid lubricant during sliding, thus reducing wear. The weight losses of all pins are almost zero, or a small weight gain, regardless of the normal loads (results not shown here), which also confirms the existence of transfer layers. As the BMGs and GCr15 steel are much harder than the brass plates, it is reasonable that only brass is worn away. It is also a reasonable consideration to protect the shafts at the expense of the bearing bushes, based on the mechanical design principle. It is worth mentioning that the pins in this work all exhibit similar behaviors and the results are omitted for simplicity.

3.6. Worn Surfaces of Brass Plates under Different Sliding Speeds

Figure 6a–d shows the SEM images of the surfaces of as-worn brass plates under a load of 3 kg after a sliding distance of 60 m, with the insets illustrating the SEM images of the cleaned surfaces of corresponding samples, and Figure 6e shows the cross-section wear tracks of the cleaned worn surfaces of the brass plates at different sliding speeds. The as-worn surfaces have the same friction feature as described in Figure 4, which indicates that friction at different sliding speeds from 1 mm/s to 10 mm/s also manifests an abrasive wear mechanism, supplemented by adhesive wear. It can be found that more adhesion, delamination and peeling-off occurs at slower sliding speeds, indicating a more serious adhesion wear at slower speeds (see insets of Figure 6a–c). From the wear tracks as shown in Figure 6e, the average wear depths of the brass surfaces against the Ni-based BMG pins were measured as 22.68 µm, 17.13 µm, 16.08 µm at speeds of 1 mm/s, 5 mm/s, and 10 mm/s, respectively, which is indicative of a reverse relation with sliding speed. The BMGs exhibit a non-equilibrium structure and a high energy state, they are easy to oxidize at the increased temperature during wear [19]. When the sliding speed increases, the friction heat and temperature increment on the sliding surface become more apparent, leading to the formation of more metal oxides. Generally, the metal-oxide layer functions as lubrication, further reducing adhesive wear, so the BMG pins have a lower adhesion wear tendency, and consequently a much lower wear loss at high sliding speeds. As can be seen from the as-worn brass surfaces against GCr15 steel and BMG pins at a sliding speed of 1 mm/s (Figure 6a,d), adhesive wear got more serious, and as a result, more and larger peeling holes were found on the friction pair surface of the steel. This may be ascribed to the poor lubrication on the surfaces of steel and brass plate. Under high sliding speed conditions, the lubrication is improved and adhesive wear is apparently reduced; hence, a mild adhesive wear and a resultant low wear loss can be ensured in all pins.

4. Conclusions

In summary, the friction and wear features of different BMGs against traditional brass plates under pin-on-plate and lithium grease lubrication conditions were evaluated experimentally. GCr15 steel is used as a traditional bearing material for the purpose of comparison. It can be concluded from this work that:

(1) Just like the traditional GCr15 steel, Ni-based, Zr-based and Cu-based BMGs all exhibited an abrasive wear mechanism, supplemented by an adhesive wear when sliding against brass plates. There was no mass loss for all the pins under the experimental conditions of this work. The wear loss of the brass plates increases as the applied normal load increases and the sliding speed decreases.

(2) All BMGs exhibited better friction performance at low sliding speed compared with GCr15 steel. The average wear loss of the brass plates against BMG pins was only 68.8–56.2% of that against GCr15 steel at a sliding speed of 1 mm/s. At high sliding speed, the BMGs have a considerable wear loss compared with GCr15 steel.

(3) Compared with GCr15 steel, Ni-based BMG always exhibited a smaller wear loss, especially under large loads and at low sliding speeds. The wear loss of the brass plates against Ni-based BMG pin was 24.3% lower than that against GCr15 steel under an applied load of 10 kg, which indicates that Ni-based BMG is an attractive bearing and shaft material for industrial application.