How are some metals shiny and some dull?
For the metal-to-metal coefficient of static friction, it depends a lot on the composition of the different metals, their surface finish, and the amount and composition of any lubricant or surface contaminant. Ditto for leather on metal. As a rule, for clean, smooth metal-to-metal surfaces, dissimilar metals tend to give lower values for static friction than two surfaces of the same metal that would have a tendency to stick to each other (i.e., seize) and refuse to slide at all. This is especially true if one of the metals is very hard and the other is relatively soft, which is one reason why sleeve bearings are made of relatively soft stuff. It also helps if the surfaces are smooth but not too smooth. Minimum values you can expect for the coefficient of static friction would be in the range of 0.1. From a theoretical standpoint, leather on metal is even more complicated than metal on metal because the leather is a chemically and structurally complex mix of substances that can be expected to change with age, usage, and environment. That said, the tables in the reference will give you some idea what to expect.
