Tribo-Oxidative Protection and Transfer Mechanisms of Molybdenum-Based PVD Coatings

Solid lubricant coatings deposited by physical vapor deposition (PVD) play a crucial role in protecting components against wear under dry sliding conditions. In this study, the tribological behavior of molybdenum (Mo) PVD coatings on rolling bearing steel is investigated, with a particular focus on tribofilm formation. Tribological tests are performed using a ball-on-plate washer configuration, which enables defined stressing of coated specimens under controlled conditions.

To gain deeper insights, a lamella is extracted from the tribofilms using focused ion beam (FIB) milling. Structural and chemical characterization is carried out by optical microscopy, scanning electron microscopy (SEM), laser scanning microscopy (LSM), and energy-dispersive X-ray spectroscopy (EDX). Complementary cross-section analyses provides detailed information on the chemical composition of near-surface reaction products as well as the internal microstructure and layer interfaces at the nanometer scale.

The results reveal that a stable tribo-oxidative film with a thickness of up to 200 nm is formed. The tribological system actively restructures the entire contact zone through bidirectional material transfer, leading to the formation of a stable 𝑀⁢𝑜⁢𝑂𝑥-on-𝑀⁢𝑜⁢𝑂𝑥 pairing and a self-lubricating condition. In this way, the tribofilm effectively prevents abrasive wear in the sliding contact and protects both surfaces. The PVD coating acts as a sacrificial reservoir within a self-regenerating mechanism, where in-situ film formation planarizes the counter-body surface while preserving the Mo-based reservoir.

The findings provide direct microstructural evidence for a self-healing tribological system based on molybdenum oxides, highlighting their potential for applications requiring reliable dry lubrication.