Ring-on-ring tribological experiments were performed with hybrid mechanical seats with silicon nitride (Si3N4) rings. The K x P V product, where K is the wear coefficient, P the seating pressure, and V the linear speed, is proposed as a novel parameter to characterize the total working range of a mechanical seal system, with the advantage of directly indicating the thickness reduction for a certain time of service. The K x P V criterion is represented in a map form called "functionality diagram." The rotary Si3N4 rings show the better behavior (K x P V = 0.05 mu m/h) against WC-Co mating faces.