Abstract
Frictional sliding instability, commonly manifested as stick-slip, is of broad interest in seismology, physics, chemistry, and engineering. Although surface roughness is recognized as a key factor influencing the transition from steady sliding to stick-slip, its precise role in controlling frictional stability remains unresolved. Here, we investigate this transition using complementary friction experiments and a minimal Maxwell-slip ensemble model. PMMA plates with systematically varied roughness were subjected to normal and subsequent shear loading. The model represents the interface as independent mass-spring units governed by Coulomb friction and driven at constant velocity, with roughness incorporated probabilistically through a beta distribution of initial block positions, whose standard deviation serves as a quantitative roughness measure. Experiments and simulations show consistent trends: smoother surfaces exhibit stick-slip behavior, whereas rougher surfaces promote steady sliding. In the model, steady sliding arises from asynchronous block slip, while stick-slip corresponds to synchronous collective motion across the interface. A statistically defined critical parameter governs a sharp transition between these regimes and maps directly onto the experimentally observed roughness threshold. These results establish a mechanistic link between microscale interface disorder and macroscopic sliding stability, demonstrating that surface roughness can act as a governing parameter of frictional instability.