square: glass; filled square: bronze; open circle: graphite; filled circle: MoS ; X: unfilled, half-filled

It may be of interest to note that unfilled Teflon has the property that its coefficient of friction, µ, is not given µ=Fn/Ft, but rather by , where Fn denotes the force normal to the contact surface and Ft denotes the force tangential to the surface [6]. Fillers may modify this property by an amount that depends upon the kind, amount, or orientation of the filler.