Whisker alignment by slip extrusion

Abstract

Si 3N 4 or Al 2O 3 ceramic slips containing 20–30 vol.% SiC whiskers and small quantities of a dispersant and gelling agent were prepared by ultrasonic dispersion and ball milling. The viscosity was minimized by adjusting the pH of the slip and the amount of organic additives. The slip was extruded into a gelling solution through a hypodermic syringe fitted with a 0.5 mm internal diameter stainless steel needle. The conversion of the extruded slip into a flexible thread was accomplished by an ion exchange reaction in the gelling solution. The orientation of the whiskers in the extruded thread was determined by infiltrating an epoxy resin into the dried thread and measuring the distribution of the angle between the whiskers visible on a polished cross-section and the extrusion direction. The degree of whisker alignment in the thread was defined in terms of a statistical orientation factor 0 which varied from 0 to 1 as the whisker distribution changed from randomly oriented to complete alignment in the extrusion direction. Optimum whisker alignment was achieved at large needle length-to-diameter ratios, low slip viscosities and high extrusion velocities.