Erosion and rolling contact wear mechanisms in silicon nitride hybrid bearings.

Abstract

One remarkable advantage of hybrid bearings over all steel bearings is the elimination of separate oil lubricant system in applications, such as compressors and pumps in refrigeration and air conditioning units. High speed test runs restricted increase in speed due to material wear, which eventually affected the life of bearings. Being low saturation temperature fluids, change of phase is very common in refrigerants and cryogenic liquids, which lead to cavitation. Silicon nitride rolling elements with different sintering additives, properties and microstructure were experimentally studied to understand the nature of cavitation erosion. Advanced surface analysis technique was used study the erosive wear correlation to microstructure of test materials. Cavitation erosion wear initiated by multiple intergranular and transgranular fracture, leading to erosion pit formation. Grain size and grain boundary composition have shown to be the dominant factors for providing resistance to cavitation. Effect of surface defects and lubricant viscosity on cavitation erosion was investigated and is detailed in this thesis. A rotary specimen method was designed to study the effect of cavitation on rolling bodies. Computational modelling of acoustically generated cavitation was attempted and is also reported in this work. A novel test methodology was designed and manufactured by modifying a rotary tribometer to allow controlled experimental testing of two different phenomena rolling contact fatigue and cavitation erosion. This testing made it possible to study rolling and erosive wear mechanisms of rolling elements. Cavitation created far away in this new system is shown to be transported to the rolling contacts. The mechanism of material damage was by surface weakening due to mechanical impact of bubbles, which enhance fluid entrance and hydrodynamic pressure leading to wear initiation. Micro erosion pits formed in the rolling contact, which accelerated the damage by dislodging grains and bunch of grains. This testing method is suitable for a qualitative assessment of cavitation-RCF damage for different fluids with varying viscosities, and operating conditions.