Fraunhofer Institute for Mechanics of Materials IWMModern high-performance machines, ranging from electric motors to wind turbine gearboxes, subject lubricants to extreme conditions: pressures in the gigapascal range, shear rates exceeding one million per second, and lubricant films as thin as a single molecule. In this critical range, classical computational models fail. The project uses molecular dynamics simulations to develop a physically sound continuum description for precisely this critical lubrication regime, enabling an extension of the engineering-friendly Reynolds equation for application in this extreme regime.
Molecular dynamics and continuum description of highly sheared lubricant films in boundary friction
Completed research project
Project description
Market Demand and Societal Challenge
Friction and wear account for an estimated 20–30% of global energy consumption in industry. Lubricants are the key means of minimizing tribological losses. With the industrial trend toward higher power densities, smaller components (downsizing), and more environmentally friendly, low-viscosity lubricants, machine components such as rolling bearings, gear wheels, and cam followers are increasingly operated in the so-called boundary lubrication (BL) regime. In this regime, the lubricant film is only a few nanometers thick—or even just a single molecular layer. Pressures of several gigapascals and shear rates exceeding 10⁶ s⁻¹ drive the lubricant into extreme non-equilibrium states in which classical hydrodynamic models and viscosity laws fail. As a result, designers cannot reliably design lubrication clearances in this regime, leading to conservative oversizing, unplanned failures, or suboptimal lubricant selection.
Project Approach
Project ReCon is developing a physically grounded continuum description for the entire transition range from thin-film lubrication to dry contact and back. The approach combines three methodological levels: (1) Large-scale atomistic molecular dynamics (MD) simulations of tribological model contacts provide reference data; (2) Parametric MD studies of small, representative volume elements provide constitutive laws for lubricant film dynamics in extremely narrow gaps (e.g., viscosity, wall sliding, and 2D transport properties as functions of pressure, temperature, shear rate, and gap size); (3) A classical Reynolds solver for the continuum mechanics of the lubricating film is extended to include these constitutive laws and validated against large-scale simulations. Two research paths are being pursued: the “starvation path” (continuous reduction of the lubrication gap down to a monolayer) and the “relubrication path” (refilling a dry contact from a lubricant reservoir).
Contribution to Addressing the Challenges
The project will culminate in a Reynolds solver capable of quantitatively describing all lubrication regimes—from thick fluid films to partially dry contact—in a manner suitable for practical applications. The results provide the scientific basis for more energy-efficient, low-wear machine components that can be operated with low-viscosity, environmentally friendly lubricants—a direct contribution to industrial sustainability and to reducing global energy consumption due to friction.
Fraunhofer IWM’s Work Packages in the Project:
- Large-scale molecular dynamics simulations of tribological model contacts with converging-diverging channel geometries, surface roughness, and gap widths down to the monolayer range
- Formulation of constitutive laws for the nonlinear lubricant viscosity η(P, T, γ̇, h) and the wall slip velocity v_slip(τ, P, T, h) as functions of pressure, temperature, shear rate, and gap size
- Development of an extended Reynolds lubrication equation solver (RLE code) that integrates nonlinear viscosity laws, wall slippage, compressibility, and monolayer transport—thereby significantly exceeding the scope of classical EHD codes
- Derivation of microscopically grounded parameters for the Eyring-MKT sliding law from molecular trajectories, thereby establishing a solid physical foundation for the previously empirical constitutive relations
Based on the project results, Fraunhofer IWM can offer industrial companies the following research and development services:
1. Simulation-based lubricant qualification under extreme conditions
Fraunhofer IWM characterizes lubricant formulations under high-pressure and high-shear-rate conditions based on validated molecular dynamics simulations—without the use of costly high-pressure rheometers.
- Determination of viscosity, shear thinning behavior, and wall-sliding properties at pressures up to the GPa range
- Analysis of lubricant behavior at shear rates > 10⁶ s⁻¹
- Characterization of PAO oils, synthetic hydrocarbons, and other lubricant classes
2. Design of lubrication gaps in the limit friction range
Using a newly developed extended Reynolds solver, the Fraunhofer IWM simulates tribological contacts under operating conditions where classical EHD codes fail.
- Simulation of bearings, cams, and gear teeth in the boundary and mixed friction regions
- Quantitative prediction of friction force, pressure distribution, and film thickness down to the nanometer range
- Analysis of operating conditions beyond the scope of conventional simulation methods
3. Analysis and Prevention of Lubricant Film Breakdown (Starvation)
The Fraunhofer IWM identifies critical operating conditions under which there is a risk of transition from full lubrication to dry friction—and provides the basis for targeted prevention.
- Simulation and identification of critical operating points with an increased risk of starvation
- Data-driven analysis of the influence of lubricant quantity, viscosity, and operating parameters
- Derivation of specific optimization recommendations to prevent lubricant film breakdown
4. Development and validation of constitutive laws for new lubricant classes
Fraunhofer IWM develops customized viscosity and sliding laws for new and innovative lubricant classes that can be directly integrated into common engineering simulation tools.
- Development of tailored constitutive laws for bio-based oils, ionic liquids, low-viscosity synthetic oils, and other lubricant classes
- Validation of the models using experimental or simulated reference data
- Provision of models prepared for immediate implementation in COMSOL, OpenFOAM, commercial EHD solvers, and others
