Tribological behavior of various ceramic materials sliding against CF/PTFE/graphite-filled PEEK under seawater lubrication

In order to select appropriate antifriction and wear resistance material combinations for key frictional pairs in seawater hydraulic components, tribological characteristics of SiC, Si3N4, Al2O3, and ZrO2 ceramics sliding against carbon fiber/polytetrafluoroethylene/graphite-filled polyetheretherketone composite lubricated with seawater were comparatively investigated with a ring-on-ring test rig. The results show that the nonoxide ceramics (Si3N4 and SiC), especially the Si3N4 ceramic, exhibited lower friction coefficients and smaller wear rates than those of oxide ceramics (Al2O3 and ZrO2). And the tribological behaviors of polyetheretherketone/Si3N4 tribopair under dry friction, pure water, and seawater lubrications are further comparatively studied to explore the effect of lubricating medium on the tribological characteristics of Si3N4 ceramic. It is found that the lubricating effect of SiO2 and Si(OH)4 films generated by tribo-chemical reaction between Si3N4 and water is the main factor for the relatively low friction coefficient and wear rate of polyetheretherketone/Si3N4 tribopair under aqueous lubrication. Under seawater lubrication, the Mg(OH)2 and CaCO3 deposition layers caused by the chemical reaction of Mg2+ and Ca2+ in seawater could inhibit the generation of SiO2 and Si(OH)4 films and increase the counterface roughness. As a result, the tribological behaviors of polyetheretherketone/Si3N4 tribopair are worse under seawater lubrication than that of pure water lubrication.

Oxidation behavior of pressureless liquid-phase-sintered α-SiC in ambient air at elevated temperatures

The long-duration oxidation behavior of a pressureless liquid-phase-sintered (LPS) α-SiC with 10 vol% Y3Al5O12 additives was studied by furnace oxidation tests in ambient air at 1100 to 1450 °C. The oxidation of this LPS SiC ceramic was found to be passive throughout these temperatures due to the formation of oxide scales, with a change in the oxidation behavior occurring at 1350 °C. It was also found that the oxidation behavior is very complex, exhibiting two distinct stages at all temperatures: (i) initial nonparabolic oxidation, where the rate-limiting mechanism is the outward diffusion of Y3+ and Al3+ cations from the secondary intergranular phase into the oxide scale with the activation energy of the oxidation being 504 ± 32 kJ/mol, followed by (ii) parabolic oxidation below 1350 °C, where the rate-determining mechanism is the inward diffusion of oxygen through the oxide scale with the activation energy being 310 ± 47 kJ/mol, or paralinear oxidation at and above 1350 °C, where oxidation is controlled by some mixed reaction/diffusion process. The existence of two oxidation regimes reflects the progressive crystallization of the oxide scale during the oxidation. Finally, guidelines are provided for the design and fabrication of low-cost, highly oxidation-resistant LPS SiC or other LPS nonoxide ceramics.