| A multicomponent thermosetting system employing existing models of case-core systems used in steel gears is proposed for scientific design of new-generation plastic gears. The system consists of a tough, rubber-modified epoxy core and a hard, unmodified epoxy case. Gradients in hardness, toughness, and residual stress are achieved exploiting density differences between rubber particles and the epoxy matrix. Spinning the system during cure moves lighter particles to the center of the part, leaving a hard rubber-depleted case. Residual stresses in the case are formed by quenching the part after curing.; Because of its simple chemistry and low processing-temperature and viscosity, a model system of diglycidyl ether of bisphenol A epoxy, carboxyl-terminated acrylonitrile-butadiene rubber, and piperidine curing agent was used.; Mechanical testing of epoxies and rubber-toughened systems shows increasing fatigue strength and toughness but decreasing strength and hardness with increased crosslinking. A cure schedule of 16-24 hours at 120{dollar}spcirc{dollar}C results in a material optimized for {dollar}Ksb{lcub}Ic{rcub}{dollar}, fatigue strength and thermal stability. Rubber composition to maximize critical flaw size at yield with a particle size of 1.5 {dollar}mu{dollar}m is approximately 10%.; Thermodynamic and kinetic models consistent with glass transition and cloud point measurements were developed and validated for the prediction of phase separation, cure behavior, and case depths attainable in this system. These models predict that preformed rubber particles are required for the toughening dispersion to achieve desired case depths.; The demonstrated property range gives a core critical flaw size 15 times larger than in the case, and fatigue strength in the case approximately twice that of the core. Further enhancements are expected in the case with controlled residual stress. By using a stronger epoxy system, fatigue strengths greater than those seen in a current Delrin gear material should be achievable.; The feasibility of a functionally graded material formed by centrifugal processing has been clearly demonstrated. |
