Wear transitions and tribochemical reactions in ceramics

Abstract

Structu ral ceramics are used in diverse tribological applications due to their unique properties that include resistance to abrasio n and erosion, resistance to corrosive wear, wear resista nce at elevated temperatures, low density and unique electrical, thermal and magnetic properties. Applications include precision instrument bearings, cutting tool inserts, prosthetic articulating joints and engine components. F ollowing a brief overview of the processing methods applied to alumina, silicon nitrid e and silicon carbide, the wear behaviour of these ceramics is reviewed. The wear behaviour changes as the load or the coef®cient of frictio n is increased. This change is associated with a drastic increase in wear at a critical load that depends on the material and test conditions. This transition from mild to severe wear occurs through a microfracture process at the sliding contact. Contact mechanics is used to develop a simp le model to determine whether contact failure occurs by a classical brittle process (i.e. cone cracking) or a quasi-plastic behaviour (i.e. small microcracks distrib uted below the contact surface). The model provides estimates of the transition load within a factor of two of the experimentally obtained values and indicates that the brit tle behaviour dominates the observed transition from mild to severe wear in the three ceramics evaluated in this study. The limita tions of the model and its use in design of tribological components are discussed. The role of lubrication and tribochemical reactions between ceramic surfaces and atmospheric moisture on the wear behaviour is discussed.

Wear mechanisms of silicon carbide-whisker-reinforced alumina (Al2O3–SiCw) cutting tools when high-speed machining aged Alloy 718

“…Such layer is known to form on ceramic tools during machining as a result of chemical reactions with workpiece material or environment [16,19]. Two separate zones of tribolayer (see Fig.…”

“…Most of the reaction products become either plastic or liquefy under the cutting temperature thus forming tribolayer on the tool surfaces. Advantage of the tribolayer is that in liquid state it supresses tool/workpiece adhesion and reduces friction coefficient [19]. Use of SiC whisker reinforcement in ceramics is expected to additionally intensify the formation of tribolayer through oxidation of whiskers.…”

The Rationale for Silicon Nitride Bearings in Orthopaedic Applications

“…27 Published friction and wear data for different types of silicon nitride show a wide scatter due to different test conditions used in various studies. 28 Prototype total hip bearings have been fabricated using sintered Si 3 N 4 composites (Amedica Corporation, Salt Lake City, Utah, USA); these have confirmed improved fracture toughness and strength over medicalgrade alumina, and when tested in a hip simulator, both cobalt-chromium and silicon nitride femoral heads produced low wear rates against silicon nitride acetabular bearings that were comparable to Al 2 O 3 -Al 2 O 3 wear rates, which are the lowest of any orthopaedic bearing. 24 Other work has validated the observation that water-lubricated silicon nitride has very low friction when sliding against itself.…”

“…In laboratory testing with pure silicon nitride, both mechanisms of wear have been confirmed. 28 Accordingly, for stable, long-term steady performance of silicon nitride orthopaedic bearings, a chemically-inert material composition that is impervious to surface oxidation is mandatory. Precise control of the raw powders and processing parameters is critical in manufacturing bearing components with predictable, reliable in vivo behavior that can last for the remaining decades of a patient's lifespan.…”

Ultra Low-Friction Characteristics of Nanostructured Surfaces on Silicon Nitride in Aqueous Medium

“…Yapısal seramiklerin aşınma başlangıcı her malzemenin kendisine ait eşik yük değerinin aşılmasına müteakip küçük (mikro seviyede) kırılmalar/kopmalar ile olmaktadır. Ayrıca pek çok seramikte karşılaşılan sulu/su buharlı ortamda seramik ile su molekülleri arasındaki yüksek reaksiyon potansiyeli sonucu, seramik yüzeyinde oksitler ve hidroksitler oluşması kaçınılmazdır [2]. Bu reaksiyonlar aşınma testleri esnasında hızlandığından dolayı tribo-kimyasal reaksiyonlar adını alır.…”