Lubrication in mechanical devices, such as engines, includes continuous improvements to the oils employed to reduce emissions, increase durability, and reduce frictional losses while simultaneously improving overall energy efficiency [1–6]. Lubrication at sliding/rolling contacts in the engine is an interfacial phenomenon with three regimes: boundary, mixed, and hydrodynamic lubrication, wherein the latter provides lower friction and wear. Maximum friction and wear losses occur at the boundary and mixed lubrication regimes in engines, such as the top-ring-reversal region of the piston ring-cylinder liner interface, rolling-sliding contact at cam-follower surfaces, and sliding surfaces in the valve train . Anti-wear additives play a crucial role in reducing wear and preventing the failure of boundary or mixed lubricated interfaces experiencing direct surface asperity collisions due to inadequate lubricant load support. In particular, anti-wear additives are of prime importance in the low viscosity oil currently used in modern engines to substantially reduce viscous energy loss in the engine. Developing more effective automotive lubrication, in combination with lower viscosity base stock and higher-performance, represents an effective route to improve engine efficiency and durability.
Anti-wear additives protect surfaces in relative motion from damage by undergoing mechanochemical dissociation via the formation of surface bond tribofilms. The ability of tribofilms to protect sliding contacts depends, in large part, on their intrinsic properties and their adhesion to the substrate. For example, zinc dialkyl dithiophosphate (ZDDP) is the additive used in current commercial automotive lubricants because of its ability to help form effective tribofilms, and thus act as a sacrificial layer to protect the underlying metal surface . ZDDP tribofilms are well known to form on steel-steel sheared contacts as a result of stress-assisted, thermally activated chemical reactions [9–11]. To date, extensive research work has been conducted to elucidate the properties and tribofilm forming mechanism involving ZDDP. For example, several studies have shown that effective tribofilms are formed only at high temperatures while, in contrast, at room temperature only weakly bounded tribofilms, and in some cases, no tribofilms, are observed [12, 13]. ZDDP tribofilms exhibit patchy morphology and lower elastic modulus and hardness than that engine components (like steel), which allows them to sacrifice or distribute applied stresses under mild friction at the sliding interface to prevent wear . However, despite being a cost-effective multipurpose additive, their application in engine oil is increasingly questioned due to their several disadvantages. These disadvantages include poisoning of exhaust by thiophosphate byproducts, inadequate wear protection in ultralow viscous lubricants, and micro-pitting in thin-film lubrication . These disadvantages illustrate the need to eliminate, or partially replace, ZDDP in the oil. In recognition of these undesirable side effects, engine oil specifications, introduced between 1994 to 2020, reduced the permitted maximum concentration of phosphorus and sulfur in the engine oil . As a result, current engine oil specifications allow phosphorus level up to only 0.08 wt.%. However, there is a strong possibility that in the future permissible phosphorus content in an engine oil might be substantially reduced further [15, 16]. Unfortunately, further reduction in ZDDP content below the 0.08 wt.% of phosphorus will adversely affect the tribological performance of currently used low viscosity lubricants (like GF6). Clearly, it is important to identify novel new additives capable of either replacing or at least reducing, ZDDP content in the engine oil given our increasing concern over energy consumption and undesirable climate effects.
To this end, in recent years, various nanomaterials have been explored as additives in lubricant base oil from the perspective of developing more energy-efficient lubrication to impart superior tribological performance and lower emissions than conventional additives like ZDDP [17–25]. The integration of nanomaterials into tribological systems has several benefits over their micron-sized counterparts, for example, their extremely small size and high specific surface area . Nanoscale materials are believed to involve a different mechanism for friction and wear reduction compared to P-based lubricant additives. For example, metal and metal oxide nanoparticles may get physically pressed, or smeared on the rubbing surfaces, at high contact pressure to form protective surface layers or tribo-sintered films, which decrease resistance to shear stress and provides a cushion against direct asperities collision [19, 26, 27]. Hard carbon-based or ceramic particles are reported to act as mini-ball bearings at the sliding interface [28, 29]. On the other hand, soft carbon-based nanomaterials exfoliate to form a protective film upon rubbing [30–32].
Calcium carbonate nanoparticles are of our particular interest given their excellent chemical stability, frictional and wear reduction properties, and potential as a green lubricant additive in commercial oils [33–36]. Practical lubrication applications of metal oxide nanoparticles face major challenges, such as agglomeration and sedimentation in the base oil. To help overcome these effects, several studies have shown the efficacy of using surface modifications to improve the solubility of metal oxide nanoparticles in oils [18, 37–40].
In the present study, we report tribological results from CaCO3 nanoparticles surface modified with two coatings, one to assist in the film formation and the other to resolve the dispersion problem achieved via a plasma-enhanced chemical vapor deposition (PECVD) technique coating process. The 1st film involves a coating that permits nanoparticles to deposit tribologically beneficial chemistries at sliding interfaces to promote the formation of the tribofilms. The second coating, involving the deposition of hydrophobic films, is employed to ensure that the nanoparticles will disperse uniformly in the oil. To this end, CaCO3 nanoparticles were coated with boron-based polymer films, followed by the second film of a polymeric acrylate-based film on top of the boron-coated CaCO3 nanoparticles (CaCO3BM). Subsequently, the plasma functionalized calcium carbonate nanoparticles were characterized using Fourier transform infrared spectroscopy (FTIR) and X-ray absorption near-edge structure (XANES) spectroscopy.
The main objective of this study was to assess the feasibility of these plasma functionalized CaCO3BM nanoparticles to help reduce the amounts of phosphorus currently employed in engine oils. Specifically, we focused on reducing the amount of P to 350 ppm, approximately half of that currently employed commercially with zinc dialkyl dithiophosphate (ZDDP) and ashless dialkyl dithiophosphate (DDP) additives. For this purpose, oil formulations were prepared with CaCO3BM nanoparticles in combination with/without ZDDP and DDP, and tribological properties were assessed through high frequency reciprocating tribometer under the boundary lubrication regime. In addition to friction and wear properties, electrical contact resistance data were acquired in-situ during tribological tests to evaluate the dynamics of tribofilm formation at the sliding interface. Subsequently, surface characterization techniques, including atomic force microscopy (AFM) and XANES, were employed to determine the morphology and chemical make-up of the tribofilms. The results obtained provide interesting fundamental new insights into the anti-wear mechanism and nature of the tribofilm formed, along with encouraging evidence of possible reduction of phosphorus content in the oil.