Dependence of adhesion and friction on porosity in porous anodic alumina films

We explore, using atomic force microscopy, adhesion and friction characteristics of porous anodic alumina (PAA) films as the porosity varies. Pore formation greatly reduces the adhesive force, by as much as 30 times, but the porosity of PAA films has little influence on its adhesion properties, showing good agreement with the relative contact area estimated by the Hertzian model and indentation theory. Frictional coefficients vary nonlinearly with the porosity of the PAA films due to intrinsic characteristics of the surface morphology of PAA films.