Direct Multimodal Nanoscale Visualization of Early Phosphorus-Based Antiwear Tribofilm Formation

Abstract

Understanding the mechanism of antiwear (AW) tribofilm formation and how to tune surface chemistry to control functionality is essential for the development of the next generation of oil lubricants. In particular, understanding and optimizing early AW tribofilm formation can increase the energy efficiency of mechanical systems. However, the mechanism for how these films form is not well understood. The majority of prior work has focused on analyzing only end-of-test surfaces long after the film has formed. In this work, we develop an in situ multimodal chemical imaging methodology to directly visualize the early formation of AW films on steel surfaces. We investigate an oil formulation containing a phosphorus-based additive commonly used to protect surfaces from wear and fatigue processes in machine elements, such as gears, bearings, and sliding contacts. Using nanoscale multimodal chemical imaging on combined platforms of atomic force microscopy (AFM) coupled directly with in situ nano-infrared (nano-IR) spectroscopy, and further combined ex situ with time-of-flight secondary ion mass spectrometry (ToF-SIMS), we demonstrate a direct correlation between changes in friction and local surface chemistry. In these experiments, the AFM probe acts as a single asperity contact to generate the tribofilm as well as a tool to analyze it in situ as it is forming. To verify our in situ measurements, we compare these results to the ex situ ToF-SIMS of macroscale block-on-ring tribometer-formed samples. The understanding gained here on how AW films form and how film properties can be modified by tuning the chemistry of the additives will facilitate developing transmission fluids to meet increasing demands for vehicle performance and efficiency.

Nanocrystalline Iron Pyrophosphate-Regulated Amorphous Phosphate Overlayer for Enhancing Solar Water Oxidation

“…This indicates that different decomposition of the phosphate species in FePy@FePi and FePi overlayer, thereby they are composed of different kinds of phosphate species [ 50 ]. Noticeably, the absence of Fe − fragments indicated the absence of iron phosphide species in these samples [ 51 ].

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Recent Advancements in Understanding of Growth and Properties of Antiwear Tribofilms Derived from Zinc Dialkyl Dithiophosphate Additives under Nanoscale Sliding Contacts

“…The sliding tests are performed on the sample immersed within the lubricant, subjected to elevated temperatures (∼100 °C) using a heating stage. Otherwise, an alternative method involves the direct application of a thin coating of oil onto a pristine steel surface, where the AFM probe establishes an oil meniscus between the surface and the probe . This methodology facilitates the nanoscale development of the tribofilm within an oil environment, as illustrated in Figure (b).…”

“…Interestingly, the absence of displaced Fe within the tribofilm showed the inconsistency of the HSAB reaction model. In a recent study, a similar AFM-based in situ and ex situ nanoscale multimodal imaging approach was adopted integrating an AFM with nano-IR and ToF-SIMS . This approach had some uniqueness because, given the limitations of AFM in providing direct insights into the chemical composition of the tribofilm during its formation, the study introduced a developed workflow that effectively enabled monitoring the dynamic chemical evolution of the tribofilm during its growth using nano-IR, as shown in the schematic in Figure (a).…”

“…Otherwise, an alternative method involves the direct application of a thin coating of oil onto a pristine steel surface, where the AFM probe establishes an oil meniscus between the surface and the probe. 33 This methodology facilitates the nanoscale development of the tribofilm within an oil environment, as illustrated in Figure 3(b). Commercially available sharp probes made of Si or diamond-like carbon (DLC) or diamond-coated Si at the cantilever's free end are used to observe the topography with subnanometer spatial resolution, while microsphere probes are used to measure the friction force in multiasperity contact across the sliding region, as shown in Figure 3(c).…”

Quantification Characterization of Hierarchical Structure of Polyurethane by Advanced AFM and X-ray Techniques

“…To achieve quantitative characterization, AFM nanomechanical mapping (AFM-NM) technology captures force curves at each scanning position, enabling the analysis of the material’s elastic modulus and adhesive force. , AFM-NM visualizes the distribution of mechanical properties of the components, providing observation of the material’s microstructure and mechanical characteristics of the components. − Moreover, AFM-NM can measure materials with Young’s modulus ranging from 0.7 MPa to 70 GPa and has been widely adopted for phase identification of various polymer microstructures. , Despite these technical advances, current characterization techniques and equipment still have limitations when it comes to correlating the PU phase structure with its chemical composition. To address this limitation, AFM-IR combines AFM with infrared spectroscopic analysis, offering chemical imaging at the nanometer and microregion scale. − In summary, AFM-NM and AFM-IR provide valuable tools for visualizing and quantitatively studying PU’s phase structure, microstructure, and chemical composition. These techniques offer insights into the microphase separation and the characteristics of the interface between the HS and SS, enabling a more comprehensive understanding of PU’s properties.…”