Gear joints function mainly in rolling contact as a fundamental operating motion. The gears have their teeth surfaces usually treated e.g. coating, PVD. If the elastic modulus of the two contacting materials are slightly different, this will create a different amount of stretching in the contact area leading to a slip. Reynolds has described this in 1876 and many simulation can be shown the contact area (Hertz) in rolling is actually divided into sub-surfaces of slip and adhesion. This is usually referred to as micro-slipping. So, I think sliding contact does not exist as such in gear joints.