Reciprocating Wear Test of AISI 52100 Bearing Steel in h-BN-based Aqueous Lubricants

Abstract

In this study, reciprocating wear tests are performed on AISI 52100 bearing steel to investigate its tribological behavior in a hexagonal boron nitride (h-BN) water solution. The h-BN-based aqueous lubricant is prepared using an atoxic procedure called ultrasonic sonication in pure water. Ball-on-flat reciprocating sliding experiments are conducted, where the ball is slewed on a fixed flat at 50-㎛ displacement. The lubricating behavior of h-BN is compared with that of deionized (DI) water. Results show that the friction coefficient is higher in h-BN testing than that in DI tests, but the results are equalized as the friction coefficient reaches a stable level. Scanning electron microscopic images reveal significant material loss in the center and mild abrasion on the edge of the wear scar in h-BN tests. However, these effects are minor in DI water situations. The results of energy-dispersive X-ray spectroscopy show that considerable oxidation occurs in the central zone of the wear scar in h-BN cases with strong adhesion and material removal. These findings reveal the importance of determining ideal circumstances that can tolerate material friction and wear.

1. Introduction

Bearing steel is utilized to combat the extreme loading conditions that exist at the bearing portion of the machineries/mechanisms. Workplace hazards include extreme temperatures, large loads, and high velocity. High surface hardness, strength, toughness, and fatigue performance are also desired. To withstand these conditions steel and their alloys are widely used in most of the industries. Out of the wide variety of steels used, AISI 52100 bearing steel is one of the common candidate to withstand the aforementioned situations due to its high hardness (~58 HRC) and tensile strength (~1748 MPa) [1,2].

The AISI 52100 bearing steel has a wide range of mechanical applications, including anti-friction bearings, crankshafts, gearboxes, camshafts, and nuclear reactors [3,4]. Because bearings are used in almost every mechanical system, both the research community and industry are facing challenges in designing, material selection, and surface modifications to improve the performance of industrial tribological systems [5,6,7].

To lubricate the aforementioned systems, numerous lubricants are available; however, due to the adaptability of nanotechnology, nanomaterials have entered the arena.

Hexagonal boron nitride (h-BN) is one of the promising materials where they are used in versatile fields such as production industries, chemicals, water filtration, agriculture fertilizers, biomedical, pesticides and etc. [8]. H-BN is a 2-d nanomaterial which has a lamellar structure similar to the other 2-d nanomaterials that are widely used as lubricants such as MoS₂, Graphene, Graphite etc. The h-BN has good thermal resistance and stability, chemical inertness and the ability to control the coefficient of friction(COF). That is the reason h-BN is not only used as solid lubricants but also as additives, infusions and a part of nanocomposite coatings for lubricant applications. Apart from the other properties of h-BN, it is an eco-friendly material and often referred as green material. This is an added advantage that it doesn’t harm our environment even if they are used in a commercial scale. Previously, researchers have reported the deposition of boride and nitride coatings on steel for the assessment of tribological and mechanical performance due to its hydrophobic and dielectric nature [9]. Because of its superior thermal, electrical, and optical properties, the hexagonal form of BN offers a wide range of applications in 2D-sheet form with/without functionalization [10]. AISI 52100 bearing steel and h-BN are employed in this work due to their versatile applications. H-BN can be easily mixed/blended in water for producing water based lubricants without the use of any chemicals or surfactants. As h-BN is a green lubricant, when mixed with a natural fluid like water makes it a 100% green lubricant compared to the other 2-d nanomaterials which require complicated procedures to make aqueous based lubricant solutions. This water based h-BN lubricant may replace the oil/petroleum products based conventional lubricants

Recently, it was reported that no chemicals or surfactants are required to disperse h-BN lubricant in pure water [11,12]. These findings motivate us to utilize the h-BN solution as green lubricant. In this study, AISI 52100 balls and flats are evaluated under the lubrication of DI water and h-BN based aqueous solution to investigate the effect of h-BN lubricant on wear behavior during sliding.

2. Research Materials and Techniques

2-1. Lubricant and test specimen preparation

The h-BN solution was prepared using a bath sonication process, and deionized (DI) water was used as obtained. A concentration of 0.1wt% of h-BN nanoparticles were mixed with DI water using sonicated assisted hydrolysis method to produce h-BN lubricant solution.

Many pilot tests were conducted to determine the best h-BN concentration for the studies. Based on the findings of these pilot experiments, h-BN solution of 0.1% concentration was selected because, the h-BN nanoparticle were evenly dispersed having lesser agglomerations and a proper zeta potential and particle dimensions. It was also seen that the friction inhibition ability was good at this concentration. This led to the selection of 0.1wt% h-BN lubricant solution for this work.

To synthesize the h-BN solution, the h-BN powder was mixed and stirred with DI water for 10 minutes and then the solution was sonicated for 2 hours at a normal room temperature. To ensure that the h-BN nanoparticles were evenly distributed throughout the DI water, the sonicated mixture was centrifuged for 6 minutes at 700 RCF. Before inserting the centrifuged h-BN solution to the contact zone of the test site, sonication was done to the h-BN solution for about 6 minutes to avoid aggregation and held at room temperature for approximately 10 minutes to stabilize the solution temperature.

A 0.1wt% h-BN aqueous solution was distributed over Si wafers and dried using a hot plate to determine the physical dimensions of the h-BN particles. After that, the topography and dimensions of the dried lubricant particles were then measured by employing AFM in an intermittent contact method. Figure 1(a) depicts an atomic force microscopy (AFM, Hitachi 5100N) scan of an aqueous h-BN lubricant. As shown in Fig. 1(b), the outer dimensions of the representative h-BN lubricants are 2500 nm and 210 nm, respectively. In case of the analysis conducted using nanoparticle analyzer the size of h-BN lubricants was about 2600 nm and the zeta potential was about −50 mV as given in Fig. 2. Furthermore, the zeta potential of -50 mV represents that the h-BN is stable and the lubricants are dispersed perfectly with negligible aggregations in the synthesized solution.

Fig. 1. Topography analysis of h-BN flakes using AFM (a) topography, (b) height profile.

Fig. 2. Analysis of h-BN solution using Nanoparticle analyzer (a) Size and (b) Zeta potential.

AISI 52100 steel balls and flats were used for the sliding friction tests. The flats with a dimension 69 × 27 × 4 mm³ and balls having a radius of 10 mm were used for experiments. To maintain a mirror finish on the surfaces of the specimens and to avoid any blemishes, all balls and flats underwent a lapping process. The polished specimens were obtained from R&B company, Republic of Korea where the average roughness of the as-prepared AISI 52100 flat specimens were approximately R_a =0.026µm. To perform the reciprocating sliding experiments, a good mirror polish is always maintained after the lapping procedure to avoid uneven surface roughness.

2-1-1. Wear tests

The sliding friction tester procured from R&B is used in this work. The schematic of the sliding friction experiment setup is depicted in Fig. 3. The sliding tester has operational parameters such as 10 and 90 µm of displacement amplitude, a 30 Hz of maximum operating frequency, and an allowable normal load of 300 N. The AISI 52100 flats and balls were cleaned with DI water for 10 minutes in a bath sonicator prior to each experiment. The specimens were cleaned for a period of 10 minutes using an ultrasonic sonicator by utilizing solvents such as acetone and ethanol at 99.5% and 70% purity. The flat specimen was placed inside the lubricant bath, and the ball was mounted to the piezoelectric actuator. Then the ball was made to contact with the flat specimen and the lubricant, then the ball was given a reciprocating push on the stationary flat counterpart. After attaching the ball to the actuator on the flat, a normal load of 6 kgf was applied. The normal load is applied using weight flats on top of the actuator using an arrangement installed to hold the weight plates. Finally, the h-BN-based aqueous lubricants were introduced into the lubricant bath containing mating surfaces. A sufficient lubricant level was maintained to cover the test specimens. To better represent the test system a schematic of the sliding friction test is shown in the Fig. 3.

Fig. 3. Schematic of the sliding friction test.

The test parameters were measured using a computer controlled system which acquired all data for each 1 ms. The wear tests were carried out for 84 minutes at a reciprocating velocity of 200µm/s with a displacement amplitude of 50 µm. The friction coefficient was calculated by taking the mean absolute value of the friction data from each reciprocating cycle. All of the sliding friction tests were performed at 23℃.

3. Results and Discussion

After each wear tests, ex-situ characterizations were done to analyze the tested flat surface. At first the surface has been visualized using an optical microscope to roughly evaluate wear mechanisms. The worn regions of AISI 52100 flats tested with DI water and h-BN solution are depicted in Fig. 4. The DI water test shows much lesser abrasion marks than h-BN solution. Also, the center has more material removed in the hBN test as represented in Fig. 4(b).

Fig. 4. Optical morphologies of AISI 304 studied under (a) DI water and (b) h-BN water solutions at a reciprocating sliding stroke(δ) of 50 µm and a normal load of 58.8 N.

The evolution of friction coefficient was higher initially with h-BN tests than DI water cases as shown in Fig. 5. This is due to the metal-to-metal contact of the mating surfaces in the h-BN cases before the nanoparticles of h-BN reaches the interfacial region to reduce the friction by its sliding lamellar structure. The difference in friction coefficient is much minimal after 5000 cycles but its slightly lower in case of h-BN tests compared to the DI water tests. In the present test condition h-BN lubricant has insignificant effect on the friction. As illustrated in Fig. 6, the wear scar surface profiles obtained from the investigations in h-BN solution and pure DI water, however, varied significantly. It is known that the AISI 52100 steels are prone to react with oxygen in water environment, showing oxidative wear. Therefore, the wear behavior needs to be discussed in the perspective oxidative wear.

Fig. 5. Histogram of friction coefficient for AISI 52100 platen studied under DI water and h-BN solution at a reciprocating sliding stroke of 50 µm and a normal load of 6 kgf.

Fig. 6. Wear damage profiles of AISI 52100 platens measured using stylus profilometer examined with (a) DI water and (b) h-BN water solutions at a reciprocating sliding stroke of 50 µm and a normal load of 6 kgf.

According to the majority of the research, lubricated reciprocating wear behavior is primarily determined by the lubricant's and oxygen's capacity to enter the contact [13,14]. Additionally, scientists found that the concentration of oxygen in air was nearly six times higher than that of lubricant, leading them to believe that the rate at which oxygen diffuses through a lubricant is directly related to the inverse of its viscosity [15].

The basic rule is that the lubricant's effect on oxygen diffusion into the contact zone and its penetration into the interfacial region define the wear rates and processes of lubricated fretting contacts. Since, oxidation on the surface and diffusion of oxygen into the lubricant into the contact area plays an important role in controlling the friction and wear, this is the reason for employing the detection of oxygen through energy-dispersive X-ray spectroscopy (EDS) analysis was done as shown in Fig. 7 and 8.

Fig. 7. AISI 52100 platen tested with h-BN at a reciprocating sliding stroke of 50 µm with a normal load of 6 kgf (a) SEM (b) EDS of the marked region of Fig. 7(a).

Fig. 8. SEM and EDS micrographs showing oxygen signals of AISI 52100 (a) as-procured and (b) examined with DI water, (c) h-BN at a reciprocating sliding stroke (δ) of 50 µm with a normal load of 6 kgf.

As shown in scanning electron microscope (SEM) image (Fig. 7(a)), there are pits at the middle of wear scar where the surface material has been removed due to continued abrasion and wear debris acceleration. The wear debris along with the h-BN lubricants have moved from the center to the outskirts of wear scar. In the material removed pit oxidation is significant and high in concentration due to high oxygen diffusion allowed by the h-BN solution. This high oxygen concentration is evident from the EDS plot given in Fig. 8(b). This higher oxygen diffusion behavior of hBN solution is unexpected and instead of protecting the surface the h-BN lubricants in combination with wear debris created a 3-body interaction scenario. During the 3-body interaction the wear debris, h-BN nano-particles and the mating surfaces interact together to produce more friction at the interfacial contact zone, if the lamellar structure of h-BN disintegrates due to the reciprocating motion of the ball on the flat surface Apart from the un-oxidized wear debris and h-BN particles, the oxidized wear debris also involves in abrasion. The oxidized wear debris mixed with h-BN particles are evident from Fig. 7 where the EDS accounting for oxygen is represented.

In DI water tests there is slight abrasion around the outskirts of the central (Fig. 8(b)). The abrasion scenario is high in case of h-BN cases but at the center, adhesion along with material removal is evident from Fig. 8(c).

In case of h-BN test oxygen diffusion is higher in all regions due to the higher oxygen content at all the three regions (stick, slip and outer) as shown in the EDS map of Fig. 8(c). However, there is comparatively negligible oxygen content in the DI water case in the unworn regions. This evidently informs that at this sliding stroke DI water proves to be a better lubricant.

4. Conclusion

The lubricating effectiveness of h-BN dispersed solution and DI water was tested on AISI 52100 ball and flat tribopairs. Wear damage was higher with h-BN tests than DI water cases. This was due to the disintegration of h-BN lubricants during sliding motion and mixing of accelerating wear debris with the h-BN lubricants.

The h-BN lubricants take their own time period to stabilize the friction levels during the test. This is evident from the initial spike of friction coefficient in h-BN case which later equalized after some time period due to the time taken for the lubricants to be distributed all over the sliding area. At this sliding stroke oxidation diffusion was higher with h-BN cases which made way for greater material removal at the central zone and significant abrasion at the surroundings of the central zone.

To conclude, a proper concentration of h-BN solution should be used to reduce the disintegration of h-BN lubricants in order to protect the material, If the lubricants of h-BN are dispersed stably and inhibit the diffusion of oxygen during the sliding oscillations h-BN will be a better lubricant of the future.