Is the coefficient of friction always less than unity? Is the coefficient of friction always less than unity? What are the cases in which coefficient of friction is greater than unity?
 A: Coefficient of friction may be more than 1. It only implies that frictional force is greater than normal force. It does not violate any well established principle. This is in case of silicone rubber.
You can have a look at this at https://simple.m.wikipedia.org/wiki/Coefficient_of_friction 
A: The coefficient of friction is not a fundamental property of a system. It is an emergent property that results from adhesive forces and surface deformation during the sliding process. I describe this in detail in my answer to How is frictional force dependent on normal reaction?.
The friction coefficient ultimately describes the rate of energy loss as surfaces slide across each other. There are many cases where the sliding causes large scale deformation of the material and this can dissipate large amounts of energy and result in a large value of the friction coefficient. An example of this is in car tyres, and especially in racing car tyres, where the friction coefficient can be well above unity. The energy is dissipated in the deformation (and ultimately destruction) of the soft rubber in the tyres.
