Effect of matrix on the dry friction coefficient of unidirectional fiber-reinforced composite

The coefficients of dry rest (μ₀) and sliding (μ _s ) friction on a polished disk made of quenched steel have been measured for various polymer matrices and fibers and for a composite unidirectionally reinforced with poly(amidobenzimidazole) (PABI) fibers. It is established that μ₀ > μ _s for matrices and fibers with glass transition temperatures T _g below room temperature T _R , otherwise μ₀ ≈ μ _s . This effect is explained by a sharp growth in the plasticity of polymers at T _g , which leads to an increase in the polymer-steel contact area. For a composite with T _g > T _R , the dry friction coefficients obey the relation μ₀ ≈ μ _s ≈ μ _e /C, where μ _e is the coefficients of sliding friction of PABI fibers and C is their concentration. For T _g < T _R (plastic matrix), the friction coefficients of the composite and matrix are close because the latter cannot hold the fiber ends during friction. As a result, they are bent and aligned along the matrix surface or embedded in the surface layer.