PROCESO DE CONTRATACIÓN

briefly reviewed, and those applicable to HPSN and HPSiAlON materials

discussed. Only material relevant to the creep of Syalon Ceramics is

considered and for a full appreciation of creep processes the liter­ ature must be considered.

creep behaviour are also considered, but, for the sake of brevity, the derivation of equations has not been included and may be found in the referenced works.

3.2.1 Creep deformation of silicon nitride ceramics at elevated temperature

atlon which occurs, especially at elevated temperatures, upon applic­ ation of loads producing stress very much lower than the fracture stress.

In this section creep mechanisms elucidated for high

The various methods available for characterisation of

A comprehensive review

'-*0-1

Pul

more recent review1-

-1

mechanisms with experimental data g'eaned from the literature. The

reader is directed to these sources for reference.

Most high temperature creep mechanisms predict a steady state creep rate, and are described by the general equation for creep¡-

^ ■ w ■G f • a r

in which D = D o<exp(-Q/RT)

where, A = a dimensionless constant,

D = the appropriate diffusion coefficient, G = the shear modulus,

b = the Burgers vector, d = the grain size,

m = the inverse grain size exponent, n = the stress exponent,

Dq = a frequency factor,

Q = the activation energy for the diffusion process, k, R and T have their usual meanings.

For polycrystalline materials identification of the rate-controlling creep mechanism is made by reference to the values of n, m and Q deriv­

ed experimentally. Creep of silicon-nitride based materials is usually

described by intergranular mechanisms and thus the state and properties of the grain boundaries becomes important:-

(1) Grain boundaries separated by viscous layers

If the viscous layer is thick, adjacent grains and bound­ ary asperities offer no resistance to the shearing process and a

Newtonian-viscous strain rate is recordedj i e . £ ®( tf n where n =

1

If the viscous layer is thin, boundary irregularities may lead to non- viscous behaviour, le. n >

1

(2) Grain boundaries with no second phase

Since elastic distortion of grains is insignificant in the case of Si^N^ ceramics, grain movement is described by diffusion controlled or cavitation accomodated, grain boundary sliding mechanisms.

I

4 6

iWl

A study of HPSiA10NL J ceramics showed that material fabricated with a small amount of residual glass phase crept with a stress exponent n = 1.5-1

.6

In the same study another HPSiAlON ceramic.fabricated with only segregated impurity ions at grain boundaries.exhibited creep

characterised by n =

1

296

U55 KJmol-^ . A pre-oxidised specimen (l350°C/250hrs.) crept with n = 1

and Q = 830 KJmol-^ and it was proposed that oxidation removed the segregated intergranular impurity ions to the oxide scale, changing the creep mechanism from one of grain boundary diffusion (Coble) to one of lattice diffusion (Herring-Nabarro) rate control.

In another study a series of HPSN ceramics fabricated with MgO additive was produced with variable grain boundary glass vol­

umes

'-8

strain recovery upon unloading) were explained in terms of grain bound­ ary sliding accomodated by elastic strain arising at asperities on

/3-Si^N^ grains. Again the effects of the oxidation reaction were

noted, the continual removal of the intergranular glassy phase, to the scale, improving creep resistance. Thus, a truly steady state second­ ary creep stage was not observed, but values of stress exponent, n, were derived in the range n «

0

9

2

0

creep. Compositions furthest from the ternary eutectic and therefore

of lower glass content exhibited n

2

1

The equation:-

cav

was presented, where, £.cav = creeP strain due to cavltational

mechanism,

= volume of grain boundary liquid,

g = applied stress, l^_ = liquid viscosity,

t = time,

from which it can be seen that the relation between cavltational strain

4 7

and applied stress cannot be expressed by a simple power law. Thus for materials dominated by both diffusional and cavitational creep, power law analysis of the apparent steady state condition, as a func­ tion of stress, would result in an apparent stress exponent, n > 1. This is consistent with experimental values which range between 1 and 3•

Also presented, assuming diffusion along viscous bound­ ary phase, was the equation

of the experimental data and it was shown that cavitation creep domin­ ates at high volume fraction of glass, large stresses, lower viscosities and long test periods. Thus a material can be dominated by diffusional creep at low stress levels and by cavitational creep at high stress levels.