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The Tribological Properties Of Anhydrous Calcium Grease With α-ZrP, MoS2 And Graphite
Abstract
The tribological properties of α-zirconium phosphate particles as an additive in anhydrous calcium grease were studied by using an Optimol SRV-V oscillating reciprocating tester and a four-ball tester.
Fortunately, α-Zr(HPO4)2.H2O (α-ZrP)grease exhibits excellent properties in anti-friction and wear-resistant, load-carrying capacity, and extreme pressure properties. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) and 3D analysis show that α-ZrP particles appear to form a protective film allowing increased load capacity and operating frequency of the rubbed pairs.
The tribological properties of α-zirconium phosphate particles as an additive in anhydrous calcium grease were studied by using an Optimol SRV-V oscillating reciprocating tester and a four-ball tester.
Fortunately, α-Zr(HPO4)2.H2O (α-ZrP)grease exhibits excellent properties in anti-friction and wear-resistant, load-carrying capacity, and extreme pressure properties. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) and 3D analysis show that α-ZrP particles appear to form a protective film allowing increased load capacity and operating frequency of the rubbed pairs.
Meanwhile, α-ZrP particles can provide low friction coefficient and wear loss during a long-term test.
Introduction
α-Zr(HPO4)2.H2O (α-ZrP) had good performance in anti-wear, friction-reduction, and load-carrying capacity as a solid lubricating additive in lithium grease .
Whether under rolling mode or reciprocating mode, α-ZrP has better tribological property than the conventional solid lubricant molybdenum disulfide. Similar to molybdenum disulfide, α-ZrP is a layered cation exchange material of Zr(HPO4)2.H2O, whose crystal structure consists of Zr4+ions positioned alternately slightly above and below the ab plane, bonded with the oxygen atoms of six different tetrahedral phosphate groups, producing a two-dimensional cross-linked covalent network plane.
The weak bond strengths between the planes allow for easy shearing of the crystal, resulting in a lamellar mechanism of lubrication. α-ZrP particles can form a protective film to prevent the two friction pairs direct contact.
These properties make α-ZrP promising candidates for lubricant additives.
In this study, the tribological properties of α-ZrP as an additive in anhydrous calcium grease were investigated by using an Optimol SRV-V tester and a four-ball tester.
These properties make α-ZrP promising candidates for lubricant additives.
In this study, the tribological properties of α-ZrP as an additive in anhydrous calcium grease were investigated by using an Optimol SRV-V tester and a four-ball tester.
The findings of this study will further expand the application scope of α-ZrP as a solid lubricant.
The anhydrous calcium grease is outstanding in water resistance, colloid stability, corrosion resistance, and salt spray corrosion resistance.
In order to better understand the tribological properties of α-ZrP, we selected MoS2 and graphite as the reference in this research.
The anhydrous calcium grease is outstanding in water resistance, colloid stability, corrosion resistance, and salt spray corrosion resistance.
In order to better understand the tribological properties of α-ZrP, we selected MoS2 and graphite as the reference in this research.
Because MoS2 and graphite are the conventional solidlubricant, which are usually used as additives in calcium grease.
The tribological performance of α-ZrP, MoS2, and graphite anhydrous calcium grease was evaluated at different applied loads, frequencies, temperatures, and durations. A 3D optical profiling system, scanning electron microscopy(SEM), and energy dispersive X-ray spectroscopy (EDS) were employed to study traces of the friction surface to obtain an adequate.
understanding of the lubrication mechanism.
Experiment
The laboratory anhydrous calcium-based grease was obtained as follows.
The mixture of weighed 12-hydroxystearic acid and PAO8 was first put into the grease kettle and heated to 85℃ until it became homogeneous.
The mixture of weighed 12-hydroxystearic acid and PAO8 was first put into the grease kettle and heated to 85℃ until it became homogeneous.
Second, a water slurry of calcium hydroxide was added slowly to the above mixture.
The mixture was heated to 120 ℃for 1.5 h, then the temperature increased to 140℃ for 10 min, the remaining PAO8 was added to the grease kettle, stirred until the temperature cooled to room temperature, and the base anhydrous calcium-based grease was obtained.
The mixture was heated to 120 ℃for 1.5 h, then the temperature increased to 140℃ for 10 min, the remaining PAO8 was added to the grease kettle, stirred until the temperature cooled to room temperature, and the base anhydrous calcium-based grease was obtained.
Varying concentrations(1.0–7.0 wt %) of α-ZrP, MoS2, and graphite were added to the calcium grease to investigate the effect of these additives on the tribological behavior of grease.
Each calcium grease sample was mixed via mechanical stirring and ground three times in a triple-roller mill.
Results and Discussion
Determination of Suitable Concentration of Additives
The Effect of Different Loads on the Performance of Grease
Results and Discussion
Determination of Suitable Concentration of Additives
The suitable concentration of the three additives in anhydrous calcium grease depended on the evaluation of wear volume losses of the steel discs.
The concentration of additives ranged from 1.0 to 7.0 wt %.
The results for each additive differed depending on additive concentration. From Figure 1, it can be seen that the anti-wear capacities of base grease were improved by the addition of α-ZrP, MoS2, and graphite, respectively. The addition of α-ZrP led to the lowest wear volume of the steel discs in the test range. For MoS2, the wear volume had a significant increase when the concentration reached 5.0 wt %.
The concentration of additives ranged from 1.0 to 7.0 wt %.
The results for each additive differed depending on additive concentration. From Figure 1, it can be seen that the anti-wear capacities of base grease were improved by the addition of α-ZrP, MoS2, and graphite, respectively. The addition of α-ZrP led to the lowest wear volume of the steel discs in the test range. For MoS2, the wear volume had a significant increase when the concentration reached 5.0 wt %.
For graphite, the wear volume declined gradually with the increase of the concentration.
According to the results of the above experiment, the optimized concentration is based on having effective lubrication at the lower concentration. Considering that the friction test should be under the same condition, the feasible concentration for the three additives in anhydrous calcium grease was chosen to be 3.0 wt %. Typical properties are listed in Table 1.
The Effect of Different Loads on the Performance of Grease
The Effect of Different Frequencies on the Performance of Grease
The Effect of Different Temperatures on the Performance of Grease
The Effect of Long Durations on the Performance of Grease
Based on actual operating condition of grease, we performed a prolonged test to study the quality of grease with α-ZrP, MoS2, and graphite in test conditions of 600 N–40 Hz and 600 N–20 Hz.
The test time lasted 360 min. Under the test conditions of 600 N–20 Hz–360 min, the wear volumes (10-4mm3) for α-ZrP, MoS2, and graphite grease were 16.10 ±2.02, 44.31 ±5.11, and 37.15 ±6.09, respectively (Figure 7).
When the test conditions changed to 600 N–40 Hz–360 min, the wear volumes (10-4mm3) for α-ZrP, MoS2, and graphite grease were 18.86 ±2.14, 77.40 ±5.03, and 41.26 ± 6.21, respectively.
Obviously, α-ZrP grease worked steadily and had the lowest wear volume among the three additives after a long-duration experiment. Graphite grease had better anti-wear performance than that of MoS2 grease.
The average friction coefficient for α-ZrP, MoS2, and graphite exhibited analogous trends.
To better understand the tribological behavior of the long duration experiments, Figure 8 presents the 3D surface profiles of the corresponding wear tracks on discs after 360 min friction tests.
It is obvious that the wear tracks lubricated by α-ZrP grease are small and narrow at the running frequency of 20 Hz or 40 Hz. The wear tracks with MoS2and graphite grease are relatively wider and deeper.
SEM micrographs and EDS spectra of the lower disks are shown in Figure 9. It can be found that the worn surface lubricated by α-ZrP grease was smoothest and no sharp furrows were present on the wear track, which demonstrates that α-ZrP shows the best friction-reducing and antiwear properties, corresponding to the results of tribological test.
Meanwhile, the worn surface lubricated with MoS2 and graphite grease displayed grooves and scuffings. The furrows of graphite grease were shallower than that of MoS2, revealing that graphite had the better antiwear properties, as discussed earlier.
Meanwhile, the worn surface lubricated with MoS2 and graphite grease displayed grooves and scuffings. The furrows of graphite grease were shallower than that of MoS2, revealing that graphite had the better antiwear properties, as discussed earlier.
From the data of the EDS spectra, the feature elements (Zr, P) of α-ZrP were discovered on the wear surface.
Additionally, the phenomenon was also found on the disc surface of MoS2 grease.
Additionally, the phenomenon was also found on the disc surface of MoS2 grease.
Although the EDS results showed the appearance of α-ZrP and MoS2 particles on the rubbing surface, α-ZrP can be more effective to alleviate abrasive wear than MoS2.
Conclusions
In this study, the tribological properties of anhydrous calcium grease with α-ZrP, MoS2,and graphite particles have been researched via reciprocating and rotational motion. Based on the results obtained above, the following conclusions are listed:
1. α-ZrP is considered to be a promising candidate for lubricant additives because of its outstanding lubricating properties.
The maximum applied load and operating frequency for α-ZrP grease can reach up to 900 N, 70 Hz, respectively. α-ZrP grease can run steadily throughout the test and hold the lowest wear volume regardless of severe contact condition.
2. SEM-EDS and 3D analyses indicate that the outstanding performance of α-ZrP grease comes from a stable and compact tribofilm with low shearing strength, which seems stronger and more durable than MoS2and graphite.
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