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 in 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 alpha-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 cylinder on-disk apparatus and a Hertzian
pressure of 180 MPa. Two neodymia containing sialon materials with
different alpha:beta-sialon ratios are compared with commercial alumina
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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 alpha/beta-sialon 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.