Lube-Tech
generally have temperature limits and will lose effectiveness in friction prevention once said limits are reached. In this case, the mode of failure would be reaching such extreme conditions, such as high temperatures, where the engine would most likely fail as well. Also, recent research has indicated that OFMs have a pressure limit, where monolayers of OFMs are so tightly packed that structural collapse occurs, rendering the material useless [3,4]. Examples of common OFMs include glycerol mono-oleate (GMO) and oleyl amide. Specifically, glycerol mono-oleate functions as a friction modifier via hydrolysed into oleic acid and adsorption onto metal surfaces for lubrication [5]. It is widely used throughout the lubricant industry, with current studies observing its interactions with other friction modifiers and observing any synergies.
O3
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.155 page 2
alternatives [6]. Providing thicker films decreases the chance of lubricant failure and mechanical wear. For contact potentials, molybdenum creates high contact potentials, which indicates a better formation of a film surface while under stress [6].
Other common elements used within iOFMs include Boron, Carbon (in the form of graphene/graphite), and Fluorine. Boron in iOFMs can come in the form of boric acid (H3
BO3
) as a solid lubricant, and notably
borate derivatives in inorganic compounds [9]. While excelling in antiwear and temperature loading properties (able to withstand temperatures over 500°C), mechanisms for adhesion and lubrication are poorly understood by researchers [9,10]. With carbon- based additives, graphene as carbon nanotubes (CNTs) have shown promise. For graphene, interactions with Fe2
-based multi-walled CNTs showed excellent antiwear and friction reduction depending on the blend. As such, mass production capabilities are readily available and easy given current infrastructure, but accumulations in engines present a challenge for researchers [11]. Finally, fluorinated additives can present as powders, and PTFE shows promise due to reductions in friction. However, results from studies have indicated mixed results while in use.
Figure 1: Typical OFM amphiphilic structure found throughout films [4]
Inorganic friction modifiers (iOFMs) As for inorganic friction modifiers (iOFMs), they serve as the second of three classes of friction modifiers found throughout the engine lubricant industry. Like OFMs, they serve to reduce friction within engines, but with structure breakdown that leads to the formation of films that reduce shear stress [6]. Common elements included in iOFMs include molybdenum, sulphur, and phosphorus; this includes their respective benefits to lubricants discussed in later sections. Molybdenum, for example, shows promise as an alternative to OFM-enriched oils. Thicker protective films and higher contact potentials have indicated iOFMs based in molybdenum as promising
28 LUBE MAGAZINE NO.184 DECEMBER 2024
Functionalised polymers as friction modifiers The final class of friction modifiers falls under functionalised polymers (FPs). FPs are a class of OFMs used as an alternative to traditional friction modifiers. Within engines, functionalised polymers reduce friction primarily through bonding to polar surfaces, forming thicker films than traditional surfactants [13]. The polymeric structure also modifies the temperature-viscosity properties of the lubricant used for the engine, allowing for further friction reduction. Block copolymers, which include more than one monomeric species within a chain in a structured manner, further enhance adsorption on metal surfaces. Recent studies have examined the anchoring chemistry of functionalised polymers and their role in
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