Time-Dependent Contact Behaviour of ZDDP-Derived Tribofilms: A Viscoelastic Layered Model Approach

Area of Science:

• Materials Science
• Tribology
• Mechanical Engineering

Background:

• Zinc dialkyldithiophosphate (ZDDP)-derived tribofilms are known to be viscoelastic.
• Previous studies modeled tribofilm creep compliance using a Burgers material model.
• Understanding the contact mechanics of these thin films is crucial for predicting their performance.

Purpose of the Study:

• To develop a contact model for layered materials, extending existing viscoelastic half-space models.
• To investigate the viscoelastic behavior of ZDDP-derived tribofilms using analytical frequency response functions.
• To analyze the influence of operating conditions on the time-dependent characteristics of ZDDP tribofilms.

Main Methods:

• Extended a viscoelastic half-space contact model to account for layered materials.
• Converted analytical frequency response functions into influence coefficients.
• Simulated contact behavior during indentation and sliding against a carbon steel ball.

Main Results:

• When modeled as a half-space, ZDDP tribofilms exhibit a fluid-like response.
• As thin films on a substrate, they show time-dependent behavior (creep, stress relaxation) under low loads and slow speeds.
• Time-dependent effects become negligible at moderate sliding speeds (mm/s to m/s), irrespective of load.

Conclusions:

• ZDDP-derived tribofilms possess significant viscoelasticity.
• Their practical behavior often mimics a soft elastic layer due to typical operating speeds.
• The developed contact model provides insights into the complex mechanical response of ZDDP tribofilms.