Investigations of r-curves in tool ceramics-alumina ceramics and oxide-carbide-nitride composite
Magdalena Szutkowska*, Marek Boniecki** *Instytut Obróbki Skrawaniem, Zakład Inżynierii Materiałowej, ul. Wrocławska 37a, 30-011 Kraków **Instytut Technologii Materiałów Elektronicznych, Zakład Unikalnych Metod Pomiarowych, ul. Wólczyńska 133, 01-919 Warszawa
Annals 2 No. 5, 2002 pages 378-383
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abstract In spite of many attractive properties of alumina ceramics the main drawbacks are brittleness, week resistance of thermal shock and subcritical crack extension. This fact can also be expressed in low resistance against crack extension, which is characterized by the fracture toughness. Fracture toughness is the commonly parameter used to measure the resistance to brittle crack propagation, but many tool ceramic materials exhibit R-curve behaviour in which the critical stress intensity factor increases with crack length. The fracture toughness and relations of Kr versus crack length c (R-curve) for alumina ceramics and oxide-carbide-nitride composite with addition of 2% wt. zirconia were tested. Behaviour of the fracture toughness with crack extension was determined by means of the experimental method on the basis of the three-point bending (3PB). These specimens with wide polished side surface and vacuum evaporated thin aluminium layer (about 150 nm thick) were notched. An initial 0.9 mm deep notch was produced by diamond saw (0.20 mm thick) and then the notch tip was sharpened manually a razor blade (0.025 mm thick) up to deep 1.1 mm. The controlled crack growth of specimens with double notch was carried with using Zwick testing machine with speed of 1 μm/min. The loading of the specimens has been broken at the moment of crack propagation. The procedure has been repeated many times up to obtaining definite increase of crack length. Individual stages of crack growth at given load has been observed on the computer screen. When the crack length (including length of double notch) has achieved value 2.5 mm the test has been interrupted. The observation of the controlled crack growth reveals the existence of increasing R-curve in the pure alumina ceramics. Addition of TiC-TiN with 2% wt. zirconia limits the occurrence of R-curve. It suggests disappearing of bridging mechanism responsible for appearance of R-curve. The investigations of subcritical crack growth have indicated two times greater fracture energy and slightly greater resistance to crack initiation for the oxide-carbide-nitride composite.