STRESS-RUPTURE STRENGTH OF POLYCRYSTALLINE
OXIDE CERAMIC UP TO 1600ºC
V. S. Bakunov,
E. S. Lukin,
and É. P. Sysoev
Translated from Novye Ogneupory, No. 7, pp. 32 – 40, July 2015.
Original article submitted December 22, 2014.
Experimental data are given for stress-rupture strength (time and deformation to failure) determination for
densely-sintered oxide ceramic with four-point bending in the range 1400 – 1550°C (somewhat above the
brittle-ductile transition temperature) and with loads up to 60 MPa (close to the ultimate strength). It is shown
that elementary mechanical deformation over the whole range of specimen ductile behavior is diffusion-vis
cous flow with different forms, i.e., dislocation and crystal boundary diffusion movement. Data obtained
agree with the suggestion that for transition from ceramic brittle to ductile failure occurrence of some critical
vacancy concentration within it is necessary. Independent of ceramic crystal size and test conditions the sec
ondary creep rate and time to failure are constant, and deformation to failure is also constant.
Keywords: densely-sintered ceramic, periclase, aluminium-magnesium spinel, corundum, stress-rupture
strength, creep, time to failure, deformation before failure.
By stress-rupture strength we normally understand the
that a material withstands to failure under action of a
constant mechanical load during creep tests . An impor
tant characteristic of the process is also deformation before
, and from the point of some authors deformation is
a signal pointing to imminent failure. It is assumed that pro
cesses responsible for material deformation and failure dur
ing creep are interconnected. In the opposite case it would be
impossible to explain formation of nucleating cracks and
their propagation under stress much less than the material
theoretical strength. It has been established by experiment
for metals and alloys is inversely proportional to their
secondary creep rate
, i.e., t
= const .
A considerable number of empirical and theoretical
equations have been proposed for stress-rupture strength,
connecting the main criteria, i.e., time to failure with an in
crease in applied stress s and with temperature T . How-
ever, they describe results of measurements only within lim-
ited measurement ranges of variables; over wide ranges
dependences are normally obtained with inflexions. In prac-
tice for approximating experimental data there is most often
use of power functions of stress and exponential functions of
= Bexp(bT), where A, m, B, b
are empirical coefficients. Apart from these, so-called para
metric dependences have also been proposed: t
= f(s, T).
With s = const they are transformed into temperature
dependences, and with T = const into force dependences.
However, their application is also limited.
Polycrystalline ceramic with action of a mechanical load
in the range up to 1200 – 1300°C behaves as a brittle mate
rial and breaks after a little elastic deformation. At higher test
temperature before failure ceramic shows some, although
low in value compared with metals and alloys, plastic defor
mation (deformation flow). As is well known, elastic defor
mation is connected with small displacements of atoms
(ions) within their own stationary areas, and plastic deforma
tion is connected with their movement. This reaction of ce
ramic to external mechanical action is due to specific fea
tures of the chemical bond of an ion-covalent type in the cor
responding compounds. A high value and directionality of
bonds between anions and cations causes low mobility of
Refractories and Industrial Ceramics Vol. 56, No. 4, November, 2015
1083-4877/15/05604-0375 © 2015 Springer Science+Business Media New York
FGBOU VPO Joint Institute of High Temperatures of the Russian
Academy of Sciences, Moscow, Russia.
FGBOU VPO D. I. Mendeleev Russian Chemical Technology
University, Moscow, Russia.
FGBOU VPO A. G. and N. G. Stoletov Vladimir State University,