Ultra-high performance of DLC-coated Si3N4 rings for mechanical seals

Silicon nitride (Si3N4)-based ceramics are useful materials for tribological purposes due to their superior mechanical properties. One of their applications is as mechanical face seals. However, Si3N4 rings failed as self-mated systems, leading to fluid leakage and catastrophic failure under high-sealing loads. In order to improve the tribological behaviour and to provide efficient full-sealing conditions, the coating of the Si3N4 surfaces with a solid lubricating diamond-like carbon (DLC) film is here proposed. Plasma-enhanced chemical vapour deposition (PECVD) DLC coatings with approx. thickness of 1.5 μm were deposited on Si3N4 rings by conventional RF glow discharge taking only pure methane or a 95:5 methane/silane mixture, under an applied self-bias voltage of −800 V. After 419 h of continuous work (4300 km), the self-mated pure DLC ring-on-ring tribosystem proved its outstanding performance in sealing of 2 bar of internal pressure of water, at 4 m s−1, in the range 0.48-1 kN of applied normal load, without any signs of surface damage and a super low-friction coefficient value less than 0.001. The Si-DLC coatings have a slightly worse performance, with a running-in period and a steady-state friction coefficient of about 0.02.