Friction works at the 90 degree scanning angle that twists the cantilever with friction force perpendicular to its axis. It also works better with soft tip with longer cantilever at contact mode and maybe faster scanning rate.

Hi,

By chance, I had to calibrate our ca 15 years old Agilent 5500 (only the usual xyz) today. In short, friction works, there is a signal, and it differs for "trace" and "retrace" (the two blue-yellow images, please ignore the labelling), while they are in topo (at left, in the usual WsXM colour, http://www.wsxm.eu/) almost identical, so I show only the trace. I used a rather stiff lever (3N/m), so no chance to get real friction contrast, but principally things do work. In Europe, we have continuing support from www.scientec.es, highly recommendable. I will mail to send you the specifications of our tool.