Ultrasonic velocity imaging of slip-cast silicon nitride ceramics

Slip casting is a well established method of manufacturing components from engineering ceramics such as silicon nitride. The technique requires the formation of a stable aqueous suspension of the ceramic powder (the slip), which is then poured into an absorbent mould, into which the liquid from the slip is drawn by suction pressure. This leaves behind a green state ceramic cake consisting of ceramic particles held together as a soft, highly porous and easily damaged material which may then be dried and sintered to produce a hard wearing ceramic with near theoretical density. Two methods for testing Si₃N₄ ceramics have been investigated. The first used a pad of sticky silicone based gel as a substitute for water in an immersion C-scanning system, thus allowing conventional probes and couplant to be used. The second technique was entirely non-contact and used a pulsed laser to generate the ultrasound, and an air-coupled piezoelectric/epoxy component transducer was used as a detector