Alwin Nagel

Rheology of aqueous silicon nitride suspensions

Abstract Highly concentrated silicon nitride slips with solid-volume concentrations φ of up to 0·475 were prepared from preoxidized, submicron powders in aqueous solutions at pH>8. Viscosity measurements reveal a pseudo-plastic flow behaviour at φ >0·35, which could not be described by existing flow equations. On the basis of the elastic-floc model for highly concentrated suspensions, a modified flow equation was derived, which is only determined by the initial floc-volume ratio and the particle-packing density. The calculated viscosity fits very well with the experimental values from low to high particle-volume concentrations φ from 0·2 to 0·475.

Processing, Microstructure and Mechanical and Tribological Properties of Rare Earth Containing Sialons

Sialon ceramics with two types of additive elements and varying amounts of sintering aid have been studied with regard to their microstructural behaviour and their tribological performance. In this investigation, neodymium and ytterbium have been chosen as stabilising elements because of their different ionic radii. According to the general sialon formula MxSi12-m-nAlm+nOnN16-n (x = m / 3) the starting compositions have m values of 0.5 and 1.0, respectively, and n values of 1.0. With gas pressure sintering the sinterability strongly depends on the fraction of liquid phase and thus the amount of additive in the starting composition. Here, excess additive means a larger portion of rare earth oxide than calculated from the sialon formula. Excess additive contents change the microstructure in pure α-sialon ceramics from equiaxed to needle-like and increase the indentation fracture toughness up to 6.3 MPam 1/2 . Reciprocating sliding tests were performed with a cylinderon-disk apparatus and a Hertzian pressure of 180 MPa. Two neodymia containing sialon materials with different α:β-sialon ratios are compared with commercial alumina and silicon nitride materials. Isooctane is used as lubricant for simulating a fuel injection pump and shows the lowest friction coefficients of 0.3 for the sialon parts. With water as lubricating liquid the neodymium based α/βsialon reveals a friction coefficient of 0.08 compared to 0.4 for alumina and 0.95 for silicon nitride after a running-in distance of 250 m.