Understanding phase images
To understand Phase images, you need to know that in tapping mode the tip is forced to oscillations with a frequency near its resonant frequency. While scanning across the regions of different mechanical properties (hardness, adhesive, stiffness, friction etc.) the interaction of the tip and the surface will cause the oscillation to lag (delay) or in other words giving a phase shift. This phase shift is between the driving signal and the feedback signal from the tip.
Differences in phase images are often attributed to some kind of boundary like material boundary, grain boundary, edges of particles just to mention a few of them.
Phase contrast images can give certain information that might be lost in just a topographic image.
Because there are a lot of different factors affecting the phase, the corresponding information from phase images is often more qualitative rather than quantitative.