The Role of Boric Acid Additive Size on Green Lubricants Performance

Based on the success of using micron-scale boric acid particles as an additive, efforts were made to determine the relative performance of sub-micron and nano scale boric acid particles in canola oil for metal forming applications. Despite several patents [66, 67] on the subject, there are no studies in the archival literature that quantitatively investigate the performance of boric acid particle additives in the colloidal size range. Recently, Lovell et al. [68] studied the tribological performance of nano-scale (20 nm), sub-micron (average size 600 nm), micron (average size 4 im), and combined sub-micron and micron boric acid additives in base canola oil. As a basis for comparison, 5 i size MoS2 powder additives mixed with canola oil was also studied. Figure 10.9 shows the scanning electro micrographs of micron, sub-micron and nano sized boric acid particles. Once prepared, friction and wear experiments were carried out on the lubricants using the pin-on-disk apparatus at ambient conditions. The pin-on-disk investigation utilized spherical Cu pins (6.5 mm diameter) and Al disks (66 mm diameter). In the experiments, a 100 g normal load was applied and a constant sliding velocity of 120 mm/s was utilized. The friction force was continuously recorded for a sliding distance of 7,500 m (65,000 disk revolutions). Although this sliding distance adequately captured the variation and performance of the lubricants over time, it does not represent the maximum distance for which the lubricants should ultimately be investigated; automotive and industrial applications often operate for several 100 h and over extremely long sliding distances. Figure 10.10 shows the variation of coefficient of friction with sliding distance for the five lubricants studied. Although the exact sliding distance location of the minimum friction varies for each lubricant tested, Fig. 10.10 shows that—with the exception of 20 nm boric acid—each of the lubricants followed a similar trend where the coefficient of friction significantly decreases to a minimum value before slightly increasing over the remainder of the sliding distance. Such a trend suggests that the powder additives in the canola oil form a thin 'protective layer' between the pin and disk surfaces as the micron and sub-micron scale particles are squeezed into the contact interface. This powder layer, which helps carry the load and provides

Fig. 10.9 Scanning electron micrograph of a 250 i, b 4 i, and c nano (20 nm) boric acid particles [68]

Fig. 10.9 Scanning electron micrograph of a 250 i, b 4 i, and c nano (20 nm) boric acid particles [68]

Nano Boric Acid

low shear resistance, increases over time as more particles are forced between the contacting asperities which naturally round-off over time. The fact that the friction increases after a minimum value is attained demonstrates that the boric acid and MoS2 particles 'run-out' and are no longer available to enter the contact interface;

0.02

0 1000 2000 3000 4000 5000 6000 7000 8000

Sliding Distance (m)

Fig. 10.10 Variation of coefficient of friction with sliding distance for various lubricants [68]

Boric Acid 4 |jm Boric Acid 600 nm ^^^ Boric Acid Mix (4 |jm and 600 nm) ^^^ M0S2 5 |om Canola Oil

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0 1000 2000 3000 4000 5000 6000 7000 8000

Sliding Distance (m)

Fig. 10.10 Variation of coefficient of friction with sliding distance for various lubricants [68]

this causes the protective layer to wear away over time which leads to the increase in friction at the latter stages of the experiments. For the case of the 20 nm boric acid particles, the shape of the curve in Fig. 10.10 is unique because the friction continuously decreases to an asymptotic value at the end of the sliding experiments. It is possible that if the experiments had been run to a greater sliding distance, the 20 nm particles would have exhibited a behavior similar to the other additives. Unlike the other additive cases, however, the 20 nm particles go to colloidal suspension within the canola oil; this allows the lubricant particulates to easily enter between the asperities of the pin and disk throughout the sliding experiments. Thus, the 20 nm solution is able to prevent asperity interaction over the entire sliding distance so that no degradation occurs over time. The 20 nm particle lubricant also opens the possibility of additional friction reduction mechanisms that are unique to nano-particles [69].

In terms of the magnitude, the coefficient of friction substantially varied with the type of additives used with the canola oil. As shown in Fig. 10.10, the best frictional performance was exhibited by the 20 nm boric acid particles, followed closely by the MoS2 powder additives (0.5-10 im) and a combination of submicron (0.6 im) and micron-scale (4 im) boric acid additives. The largest measured coefficient of friction over the sliding distance tested was found for the micron (4 im) boric acid particles and the sub-micron (0.6 im) boric acid powder. As described previously, the 20 nm boric acid lubricant combination exhibits the lowest friction because it consists of a colloidal solution that is able to enter into the pin/disk contact interface and provide optimum lubrication without degrading over time. Considering the mixed sub-micron and micron-size powder additives, the reason for the stark differences in the frictional performances of the additives was explained by examining the physical phenomena at play between the different

Fig. 10.11 Schematic diagram of contacting interface in the presence of oil with a micro-scale, b submicro, and c mixed sub-micro and micro-scale powder particles [68]

Fig. 10.11 Schematic diagram of contacting interface in the presence of oil with a micro-scale, b submicro, and c mixed sub-micro and micro-scale powder particles [68]

Conservation Schematic Diagram

sized particles. Figure 10.11a-c shows the schematic diagram of contacting interface in the presence of oil with micro-scale, sub-micro, and mixed sub-micro and micro-scale powder particles, respectively. Boric acid additives that are larger than the asperities size decrease friction by carrying some of the load between contacting asperities (see Fig. 10.11a). The smaller sub-micron-scale particles, in contrast, will 'fill' the asperity valleys and provide a thin, smooth, solid lamellar film between the contacting surfaces (see Fig. 10.11b). Since the protective sub-micron-particulate additives are a better lubricant than individual micron-scale particles, it exhibited a lower coefficient of friction. As shown in Fig. 10.11c, the combined sub-micron and micron-scale powder additives exhibited strong fric-tional performance because both physical phenomena were at play; the small particles formed a protective boundary film while the larger particles helped support the load while being sheared with their well-known low interlayer friction. It is important to note that an identical trend was observed by Hu [36] while studying the frictional behavior of 1.5 im, 30 nm, and mixed 1.5 im and 30 nm MoS2 additives in paraffin oil.

Figure 10.12 shows the wear volume results for the different lubricant mixtures. As shown in Fig. 10.12, the 20 nm boric acid particle lubricant clearly demonstrated the best wear resistance. In fact, the wear rate of the 20 nm lubricant was at

1.20E-010 1.10E-010 1.00E-010 9.00E-011 8.00E-011 7.00E-011 6.00E-011 5.00E-011 4.00E-011 3.00E-011 2.00E-011 1.00E-011

0.00E+000

BA = Boric Acid

BA = Boric Acid

0.00E+000

20 nm BA

600 nm BA

4 ^m BA Lubricants

MoS2

Fig. 10.12 Variation of wear rate of the Cu pins slid against Al disks in the presence of various additives in canola oil-based lubricants at the interface [68]

20 nm BA

600 nm BA

4 ^m BA Lubricants

MoS2

Fig. 10.12 Variation of wear rate of the Cu pins slid against Al disks in the presence of various additives in canola oil-based lubricants at the interface [68]

least an order of magnitude lower than the other lubricants. This again highlights the benefits of placing the nano-meter scale particles into a colloidal solution that is able to readily enter the pin-disk contact interface throughout the experiments. In Fig. 10.12, it is interesting to note that unlike the coefficient of friction, the lubricant mixtures with the combined sub-micron and micron-size powder additives exhibited higher wear than the individual micron-boric acid and sub-micron size boric acid powder. The sub-micron size boric acid powder exhibited the lowest wear rate. While this trend might not seem intuitive, similar phenomena have been observed in reports on dry (i.e. no carrier fluid or oil) powder lubrication. For example, a tribo-system consisting of two surfaces separated by dry third-body powder has been known to exhibit multiple tribological regimes as a function of the particle size (relative to the surface roughness) [22, 70]. If the third-body powder particles are smaller than a critical size Pd1, the surfaces may begin to experience adhesive wear, and if they are greater than the higher critical size Pd2, they will likely experience abrasive wear. This is because below Pdi, the particles are smaller than the asperities and can compact and form solid 'rigid-like' bodies that can promote adhesive wear. Above Pd2, in contrast, the particles are greater than or equal to the surface roughness which leads to more abrasive wear. When the particle size range is Pd1 < Pd < Pd2, a regime known as ''quasi-hydrodynamic'' powder lubrication [70] exists, where the particles undergo shearing to accommodate surface velocity differences similar to that of hydrody-namic fluids and oils. Thus, the combination of sub-micron and micron mixtures in our experiments will promote wear due to the larger micron-particles being squeezed between the pin and disk while sitting on top of the smaller sub-micron-particles (see Fig. 10.11c). The fact that the micron-boric acid powder exhibits lower wear than the combined sub-micron and micron boric acid is likely due to the contact between the micron-boric acid powder additive and the disk surface being more disposed to induce the layered-lattice shearing behavior of boric acid powder. It is noteworthy that the MoS2 additives, which consisted of particle sizes between from 0.5 to 10 i, exhibited significantly greater wear than any of the boric acid additive results. In principal, the MoS2-based lubricants followed a similar trend and can be explained by the combined sub-micron and micron sized boric acid additive discussion above. The wear rate for MoS2, however, increases to a much greater level than the combined boric acid particles and will be the focus of future research by the authors.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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