Friction measurements on muscovite mica and glass have been performed with a friction force microscope using Si3N4 tips. The environment was changed from ambient conditions to N2‐ or Ar‐gas conditions. In ambient conditions, the friction‐versus‐load curves showed a nonlinear behavior closely following the Hertzian contact mechanics of a single‐asperity contact: friction force ∼load2/3. For the same tip under gaseous conditions the friction force increased linearly with the load, indicating multi‐asperity contact. A new model is proposed to explain this change in the nature of the friction behavior. In this composite‐tip model, the tip is formed by the actual Si3N4 tip and ‘‘solidlike’’ contaminants present in enclosed cavities between tip and sample surface. © 1995 American Institute of Physics.