Asymmetric Contact Synergy of Unequal-Sized Soft and Hard Clusters in Highly Concentrated ZnCl2 for Heterogeneous Superlubricants

Low ionic concentrations and the chemical stability of the ions involved to water are considered necessary for hydrated ionic solution lubricants. Herein, an ultra-high concentration of chemically active ZnCl2 aqueous solution (up to 20 mol L− 1) is first reported to be used as a liquid superlubricant, with trace amounts of Zn2+ hydrolyzed to generate Zn(OH)2 (i.e., hard clusters) in addition to Zn2+ hydrated with water (i.e., soft clusters), resulting in the formation of heterogeneous phases with a mixture of soft and hard clusters. The coefficient of friction (COF) inversely correlates with the molar concentration of ZnCl2, where the COF of the optimized samples can be as low as 0.006 with a stable macroscopic superlubricated state; the particle size distribution of clusters and their corresponding Spans, however, are positively correlated with the molar concentration, suggesting that asymmetric contact between these unequal-sized soft and hard clusters contributes greatly to the reduction of interfacial shear resistances. This ultra-high-concentration aqueous solution superlubricant not only breaks the convention but also deepens the mechanism of liquid superlubricity from the perspectives of cluster size distribution and interactions between clusters, providing a new insight into the design of advanced water-based ionic solution superlubricants.