Evaluation of Mechanical Characteristics of Tribofilm Formed on the Surface of Metal Material Due to Friction under Lubrication with Automatic Transmission Fluid
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-11-29 |
| Journal | Tribology online |
| Authors | Takayuki TOKOROYAMA, Takashi Nishimoto, Yasuhiro MurakĂĄmi, Akiyuki HONDA, Hideaki Mitsui |
| Institutions | Nagoya University |
| Citations | 4 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study investigated the mechanical characteristics, specifically the hardness, of tribofilms formed by Automatic Transmission Fluid (ATF) additives (Fluid A and Fluid B) on SKS3 cold work tool steel substrates.
- Hardness Differential: The phosphorous-derived tribofilm (Fluid A) was calculated to be significantly harder (2.64 GPa) than the sulfur-derived tribofilm (Fluid B, 1.89 GPa).
- Elemental Composition: Fluid A formed a P-derived tribofilm, while Fluid B formed an S-derived tribofilm in the wear track, confirmed by Energy-Dispersive Spectroscopy (EDS) mapping.
- Thickness Measurement: AES depth analysis estimated the tribofilm thickness for Fluid A at approximately 150 nm and for Fluid B at approximately 430 nm.
- Methodology Validation: The AFM nano-scratch method was confirmed as effective for accurately measuring the hardness of thin films, overcoming the issue of substrate interference that complicated results from the standard nano-indentation test.
- Indentation Consistency: Maximum indentation depths (31 nm for Fluid A; 36 nm for Fluid B) obtained via nano-indentation were qualitatively consistent with the hardness ranking determined by the AFM nano-scratch method.
- Tribological Relevance: These findings clarify the relationship between additive chemistry (P vs. S) and the resulting tribofilm mechanical properties, which are critical factors affecting the pitching life and wear resistance of friction surfaces in transmissions.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Fluid A Tribofilm Hardness (AFM Scratch) | 2.64 | GPa | Phosphorous-derived film |
| Fluid B Tribofilm Hardness (AFM Scratch) | 1.89 | GPa | Sulfur-derived film |
| Fluid A Estimated Tribofilm Thickness (AES) | 150 | nm | Based on Fe concentration > 90% |
| Fluid B Estimated Tribofilm Thickness (AES) | 430 | nm | Based on Fe concentration > 90% |
| Fluid A Max Nano-indentation Depth | 31 | nm | Indentation Test Result |
| Fluid B Max Nano-indentation Depth | 36 | nm | Indentation Test Result |
| LFW-1 Test Temperature | 110 | °C | Friction Test Condition |
| LFW-1 Test Load | 1112 | N | Block-on-ring method |
| Fluid A Kinematic Viscosity (100°C) | 5.8 | mm2/s | ATF Lubricant Condition |
| Fluid B Kinematic Viscosity (100°C) | 4.2 | mm2/s | ATF Lubricant Condition |
| AFM Tip Material | Polycrystalline diamond | N/A | Nano-scratch probe |
| AFM Tip Diameter | 150 | nm | Nano-scratch probe radius |
| Normal Load (Tribofilm Scratch) | 2.19 | ”N | AFM Nano-scratch Test |
| Hertzian Mean Contact Pressure | 5.92 | GPa | AFM Nano-scratch Test |
| SKS3 Substrate Indentation Hardness (HIT) | 7.0 | GPa | Base Material Reference |
Key Methodologies
Section titled âKey MethodologiesâThe mechanical characteristics of the tribofilms were evaluated using a multi-step, correlative microscopy approach:
-
Friction Film Formation (LFW-1 Test):
- SKS3 cold work tool steel blocks were tested against a ring using the LFW-1 block-on-ring method (ASTM D2714-94).
- Test conditions included an oil temperature of 110 °C and a load of 1112 N, with sliding speeds gradually reduced over 35 minutes (from 1.0 m/s down to 0.025 m/s).
-
Surface Marking and Positioning:
- Permanent indentations were created on the wear track using a Micro Vickers hardness tester at 100 ”m intervals to serve as precise positioning markers for subsequent high-resolution analysis.
-
Elemental Analysis (EDS Mapping):
- EDS was used to map the elemental composition of the wear track, identifying regions of high concentration for phosphorous (Fluid A) and sulfur (Fluid B) derived from the additives.
-
Tribofilm Thickness Measurement (AES Sputtering):
- Auger Electron Spectroscopy (AES) combined with Ar ion-beam sputtering (2 kV acceleration voltage) was used for depth profiling.
- Thickness was determined by the depth at which the iron (Fe) concentration exceeded 90%, indicating the substrate interface.
-
Hardness Measurement (AFM Nano-scratch):
- The AFM diamond indenter tip (150 nm radius) was used as an abrasive tool to plough the tribofilm.
- The test involved repeated scratching (up to 6 cycles) at the high-concentration elemental spots identified by EDS.
- Hardness (H) was calculated based on the increase in scratch depth (hâ) per cycle, assuming abrasive wear and using the relationship: H = 4W / [Ï(2krhâ - k2hâ2)], where W is the normal load (2.19 ”N) and k is the wear coefficient.
-
Hardness Validation (Nano-indentation):
- Nano-indentation tests were performed 10 times on each tribofilm surface (200 ”N maximum load) to compare results with the AFM scratch data and assess the influence of the underlying SKS3 substrate (HIT â 7.0 GPa).
Commercial Applications
Section titled âCommercial ApplicationsâThe findings regarding tribofilm mechanical properties are crucial for optimizing lubrication and component life in high-stress automotive systems:
- Automatic Transmission Fluid (ATF) Formulation: Direct application in designing next-generation ATF oils, particularly those requiring low viscosity (e.g., 4.2 mm2/s) while maintaining high wear resistance and friction stability.
- Driveline Component Durability: Informing material selection and surface treatment strategies for components subject to boundary lubrication, such as gears, clutches, and bearings, where tribofilm hardness and thickness dictate pitching fatigue life.
- Wear and Friction Control: Utilizing specific additives (P-based vs. S-based) to tailor the mechanical properties of the protective tribofilm, balancing the need for high friction coefficient (for torque transfer) and low wear rate.
- Thin Film Characterization: Validation of the AFM nano-scratch method provides a reliable, high-resolution technique for quality control and research on extremely thin protective films (thicknesses < 100 nm) where traditional indentation methods fail due to substrate effects.
View Original Abstract
In this study, nano-scratch tests were conducted using atomic force microscope (AFM) to clarify the hardness of a tribofilm derived from an additive (Fluid A or Fluid B in automatic-transmission fluid) formed on an SKS3 cold work tool steel substrate surface. Comparisons between nano-indentation hardness tests and AFM nano-scratch tests were performed for each specimen. Prior to these tests, the tribofilms on the SKS3 substrate were examined with energy-dispersive spectroscopy (EDS). In order to calculate the hardness of the tribofilm from the nano-scratch results, we assumed that the AFM diamond tip acted as an abrasive to plough the tribofilm. The phosphorous-derived tribofilm formed from Fluid A was harder than the sulfur-derived tribofilm from Fluid B, and it was calculated that the phosphorous-derived tribofilm was approximately 2.64 GPa and the sulfur-derived tribofilm was 1.89 GPa. After 10 nano-indentation hardness tests on each tribofilm, the maximum indentation depth into the tribofilm formed from Fluid A was approximately 31 nm, while it was approximately 36 nm for Fluid B. These results are qualitatively consistent with the hardness results obtained by the AFM nano-scratch test method.