Wear mechanism of activated zinc phosphate conversion coatings under dry and lubricated sliding

• Uniformly adsorbed zinc phosphate activators affected conversion coating formation. • Activated coatings showed lower roughness, open pores, and dry-sliding friction. • Activation eliminated porous interlayer and reduced zinc-iron phosphate formation. • Dry sliding led to coating indenting into substrate and substrate grain refinement. • MCT lubrication stabilized low friction and reduced damage to coating and substrate. Zinc phosphate conversion coatings are widely used to improve the functional performance of steel surfaces. A pretreatment step known as activation is commonly applied to enhance nucleation density and deposition efficiency. However, conventional activation methods rely on complex chemistries. In this work, a cost-effective activation strategy based on pure zinc phosphate particles is introduced to control coating formation and improve its performance. The adsorption of zinc phosphate particles on the steel surface prior to phosphating was directly confirmed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and scanning electron microscopy (SEM). These pre-adsorbed particles layer altered the coating formation mechanism. SEM and X-ray diffraction (XRD) analyses revealed a pronounced refinement of crystal size and a reduced contribution of the zinc-iron phosphate phase in activated coatings. Cross-sectional focused ion beam (FIB)-SEM and scanning transmission electron microscopy (STEM) analyses showed that, with activation step, zinc phosphate coating was directly attached to the substrate, while without activation, coating formed on a porous interface layer. Electrochemical impedance spectroscopy (EIS) combined with surface profilometry demonstrated a significant reduction in open porosity and surface roughness. Under dry sliding conditions, activated coatings consistently exhibited lower friction than non-activated coatings. Wear track cross-sections revealed wear initiation through crystal fragmentation and third-body compaction, followed by localized substrate indentation and grain refinement at extended sliding times. Under lubrication with medium-chain triglyceride oil, friction remained stable at low values over more than 5 h of continuous sliding. Cross-sectional analyses confirmed that lubrication preserved the zinc phosphate layer and effectively suppressed substrate damage.