Techniques for investigating the high temperature deformation of ceramics.

This study investigated the premise that ceramics might sometimes profitably be shaped using techniques more traditionally associated with metals and plastics, particularly forging of solids and extrusion of liquids. This firstly required that the high temperature formability of ceramics be known. Existing forming and associated testing techniques were therefore surveyed, establishing short-comings and pointing to areas requiring new techniques. Plastic deformation mechanisms of principal simple ceramics were reviewed to determine the scope of novel techniques required, and to show evidence of suitability for forming. Emphasis was given to the deformation behaviour of alumina, the ceramic chosen for experimental investigation. Five rigs were constructed for an experimental schedule in three phases: (I) two furnaces and one press were used to study the compatibility of alumina with graphite, tantalum, tungsten and molybdenum under forming conditions. Some original microhardness values for carbides were measured. Observations were made of graphite's spectral emissivity. Sensitive temperature control was achieved. ii) a special melting unit was developed for inflating molten alumina to form hollow spheres, resembling blow-moulding of glass and plastics. Thermal endurances of tungsten heaters in vacuo, in argon and in molten alumina were studied. Heat transfer through sintered and powder alumina was measured. The spectral emissivity of sintered alumina was found. The freezing contraction of molten alumina, and the contact angle with tungsten were found accurately using a novel technique with possible wide application. iii) a multipurpose rig with tungsten tooling was constructed for tests of alternating drivedown - stress relaxation, compressive creep, and forging on recrystallised alumina. The experiments gave information on forgeability, ductility above 1150C, crack healing, tertiary creep rate, pore distribution effect on steady state creep rate, and pore closing contribution to deformation. It was concluded that forging and liquid extrusion of alumina is feasible and is sometimes preferable to traditional shaping methods.