Loads carrying capacity map for the surface texture design of SiC thrust bearing sliding in water

Formation of tribochemical layer of ceramics sliding in water and its role for low friction

Silicon nitride ceramics is a generic term for a variety of alloys of Si3N4 with additional compounds necessary for a complete densification of the Si3N4 starting powder. They are heterogeneous, multicomponent materials characterised by the inherent properties of the crystalline modifications α and β of Si3N4 and the significant influence of the densification additives. With a view to ability of the α and β modification to form solid solutions α-Si3N4 (αss) and β-Si3N4 (βss) solid solutions can be distinguished. Each group contains engineered materials with interesting properties for special applications. Phase relations and micro-structures determine the properties decisively. Composition of the phases, the distribution of the grains, their aspect ratio and the grain boundary phase are pronounced microstructural features. The formation of the microstructure strongly depends on the one hand on the quality of the Si3N4 starting powders, which closely is related to the chemistry of the production process, and on the other on the liquid phase sintering as the most important step in the densification route. The interrelation between pure Si3N4, the densification of the powder including the role of sintering additives, microstructural engineering, physicochemical properties of the sintered Si3N4 ceramics (SSN, GPSN, HPSN, HIP-SSN, HIP-SN) are described in more detail and compared to reaction bonded Si3N4 ceramics (RBSN), which are produced by nitridation of silicon powders.

Silicon Nitride Ceramics

Ionic liquids were studied to determine their effectiveness as boundary lubricant additives for water. The chemical and tribochemical reactions that govern their behavior were probed to understand lubrication mechanisms. Under water lubricated conditions, silicon nitride ceramics are characterized by a running-in period of high friction, during which time the surface is modified causing a dramatic decrease in friction and wear. Two mechanisms have been proposed to explain the friction and wear behavior. Si3N4 sliding against itself may result in tribochemical reactions that form a hydrated silicon oxide layer on the surface of the sliding contact. This film has been suggested to mediate friction and wear. Others have suggested that tribo-dissolution of SiO2 results in an ultra smooth surface and after a running-in period of high wear, the lubrication mode becomes hydrodynamic. The goal of this study was to examine the effects that ionic liquids have on the friction and wear properties of Si3N4, in particular their effects on the running-in period. Tribological properties were evaluated using pin-on-disk and reciprocating tribometers. The tribological conditions of the tests were selected to produce mixed/hydrodynamic lubrication. The relative lubrication mode between mixed and hydrodynamic was controlled by the initial surface roughness. Solutions containing 2 wt% ionic liquids were produced for testing purposes. Chemical analysis of the sliding surfaces was accomplished with X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The test specimens were 1 in diameter Si3N4 disks sliding against 1/4 in Si3N4 balls. The addition of ionic liquids to water resulted in dramatically reduced running-in periods for silicon nitride from thousands to the hundreds of cycles. Proposed mechanisms include the formation of BFx and PFx films on the surface and creation of an electric double layer of ionic liquid.

Ionic Liquid Lubrication Effects on Ceramics in a Water Environment

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Junguo Xu

Papers

The transition of wear mode during the running-in process of silicon nitride sliding in water

The effect of water vapor on the agglomeration of wear particles of ceramics

The effect of temperature and humidity on the friction and wear of magnetic tape