Fatigue wear mechanism of structural ceramics.

Abstract

The role of fatigue as a possible factor in ceramic wear during repeat pass sliding was investigated to improve fundamental understanding of material removal processes during surface loading of structural ceramics. A new experimental technique, consisting of synchronized biaxial repeated loading of a sphere on a flat plate, was developed to simulate the stress conditions in repeat pass sliding. This technique was applied as a bench test for evaluating wear behavior and quantifying contact-induced surface damage accumulation of structural ceramics. A Cyclic Fatigue Indentation Device was designed to meet requirements for this research. This study consisted of repeated indentation tests with uniaxial normal load and with synchronized biaxial (normal and tangential) loads in a partial slip contact condition. Tests were conducted with 99.5% alumina balls and flat plates of three structural ceramics materials: sintered alumina-based, hot-pressed silicon nitride-based, and hot-pressed alumina/titanium carbide. In the cyclic indentation tests, micro-scale surface damage accumulation (debris) was observed on the contact surface. No large scale material removal through macro-scale crack linkage was observed; actual wear occurred via a fatigue mode on a micro-scale by grain pull-out; the removed grains subsequently fragmented in a brittle mode. The micro-scale surface damage became more severe as the number of indentation cycles increased, and it significantly increased when cyclic horizontal tractions were added to normal loads. Macro-scale ring/horse shoe cracks initiated and propagated along grain boundaries near the contact edge, and it was shown that ceramics had threshold values for fatigue macro-crack initiation under repeated indentations. In order to quantify the damage amount, the approaching contact displacement between two bodies was directly measured during cyclic loading. An increase in compliance of material, which was measured as an increment of contact displacement with increasing micro and macro damage, was represented as a change in the apparent elastic constant.