Microgel Additives for Aqueous Lubrication: Tailoring Friction and Wear via Composition and Thermal Responsiveness

The tribological behavior of thermo‐responsive poly( N ‐isopropylacrylamide) (PNIPAAm)‐based microgels is investigated for use as water‐dispersible lubricant additives. Two types of microgels are synthesized using a surfactant‐free emulsion polymerization method: MG0, consisting of pure PNIPAAm with a volume phase transition temperature (VPTT) of ≈33 °C, and MG16, consisting of PNIPAAm copolymerized with hydrophobic tert ‐butyl acrylamide, exhibiting a lower VPTT of around 23 °C. Swelling and lubrication performance are evaluated at 20 and 40 °C. Both microgels significantly reduce friction and wear compared to water alone. At 20 °C, MG0 remains fully swollen and provides effective wear protection through hydrated microgel lubrication. MG16, being near its VPTT, exhibits partial collapse and slightly higher wear. At 40 °C, MG16 demonstrates improved wear resistance, attributed to enhanced film compaction in the collapsed state. Raman spectroscopy and scanning electron microscopy–energy‐dispersive X‐ray spectroscopy confirm that carbon‐rich tribofilms are formed via tribochemical reactions. MG0 produces more graphitic films, while MG16 generates amorphous carbon structures. These findings highlight the tunability of microgel composition for designing adaptive, water‐based lubricants for temperature‐sensitive applications.