Effects of electron-beam and sulfur crosslinking of epoxidized natural rubber on the friction performance of semimetallic friction materials

Semimetallic friction composites (SMFCs) consisting of epoxidized natural rubber (50 mol% epoxidation, ENR 50), alumina nanoparticles, steel wool, graphite, and benzoxazine were prepared via melt mixing using a Haake internal mixer at 90 °C and 60 rpm rotor speed. The composites were vulcanized using sulfur and electron-beam (EB) crosslinking systems. The SMFC samples were then subjected to friction, hardness, porosity, and density tests to determine their friction and wear properties. The morphological changes in the samples were also observed under a scanning electron microscope. The friction and wear properties of SMFCs crosslinked via the EB irradiation and sulfur vulcanization systems were compared. The friction coefficients in normal and hot conditions, as well as the hardness and density of the irradiated SMFC, were higher than those of the sulfur-vulcanized samples at all applied doses. The porosity of the irradiated SMFC at 50, 100, and 150 kGy was higher than that of the sulfur-vulcanized samples; however, the irradiated SMFC exhibited a descending trend at 200 kGy. On the other hand, the specific wear rates of the irradiated samples were lower than those of the sulfur-vulcanized samples at all applied doses. The sample crosslinked via EB irradiation at 150 kGy exhibited the greater tribological property compared with the sulfur-vulcanized SMFC, as indicated by the higher friction coefficient (approximately 0.461) and lower wear rate achieved at 150 kGy irradiation.