An AFM microscope affords the measurement of interatomic forces exerting between a probe associated to a cantilever and a biological or chemical sample. A force spectrum f(z) shows the force evolution as a function of the probe-sample distance z. A reproduction of this analysis in conjunction with the scan of the sample surface yields a force-volume image f(x,y,z). Today, the analysis of a force-volume image remains mainly descriptive. The signal processing challenges include the decomposition of a force spectrum into elementary patterns and the factorization of a force-volume image, in order to provide a precise characterization of each pixel (x,y) of the sample surface. In this communication, we introduce a mixture model for force-volume images and we discuss its validity by means of experimental data.