RESISTANCE OF CERAMICS IN THE Si
– BN SYSTEM
TO LONG-TERM OXIDATION, THERMAL CYCLING,
AND HIGH-TEMPERATURE GAS STREAM
N. I. Ershova
and I. Yu. Kelina
Translated from Novye Ogneupory, No. 12, pp. 25 – 27, December, 2003.
Composite ceramics in the Si
– BN system (BN=0–50%) are tested for resistance to long-term oxida
tion, thermal cycling, and high-temperature gas stream. The results of high-temperature tests suggest a poten
tial of the developed material for application as the rotary seal in gas-turbine engines at 1000 – 1300°C. The
composite material owing to its hardness and resistance to oxidation at lower temperatures can be used for
fabrication of bearing cages.
The mechanical and thermophysical properties of ce-
ramic materials in the Si
– BN system have been studied
in sufficient detail by numerous authors both at home and
abroad. An optimum combination of these properties makes
it possible to use them for fabricating bearing cages and ro-
tary seals in friction assemblies operating under high-tem-
perature conditions. Exposed to heavy-duty conditions, the
ceramic materials are prone to failure because of oxidation,
and thus the serviceability of operating parts is determined
by the resistance of their material to high-temperature oxida
tion. Literature data on oxidative resistance of the
– BN system are desultory. Furthermore, oxidation of
this composite material, which is a multiphase system, like
wise is a complex process.
In this work, we were concerned with the study of a com
posite ceramic in the Si
– BN system. The test specimens
were prepared by hot pressing technology. To prepare the
(MgO), ultradisperse powder composi
tions were used, BN hexagonal particles were use as the
filler. The mass fraction of boron nitride varied within
10 – 60%.
The microstructure of specimens was studied using tech
niques of x-ray and macrostructure analyses, ultrasonic in
spection, and scanning electron microscopy.
The resistance to oxidation of hot-pressed specimens of
varying composition was examined at temperatures up to
1300°C in air for 10 and 50 h, and at 750 – 900°C for 250 h.
These conditions were selected to simulate the service re-
gimes in which two types of operating components — rotary
seals and bearing cages — are employed, exposed to a corro-
sive medium. In regime 1, specimens with the BN content of
0 to 50 wt.% (varying in steps of 10 wt.%) were studied; in
regime 2, it was hot-pressed specimens of composition 20
and 30 wt.% BN.
The extent of oxidation was quantified in terms of the
Dm/m ratio, where Dm is the mass change. The serviceability
of multilayer specimens composed of the system’s materials
was checked using a thermal cycling technique (ten
heat-and-cool cycles) in a hot gas stream with a power of
× K) at 1530°C.
RESULTS AND DISCUSSION
Two concurrent processes are involved in the oxidation
– BN composite ceramics. Oxidation of the matrix
material in the Si
system is accompanied by an
increase in mass owing to the formation of a layer of
a-cristobalite and yttrium silicates; oxidation of boron
nitride causes a decrease in mass owing to the formation of
volatile boron oxide.
The material tested at 700 – 950°C showed a high resis
tance to oxidation: The relative change in mass at 950°C and
holding time of 250 h was – 0.207 and – 0.326% for the ma
terial with 20 and 30% BN, respectively; at 700°C (250 h),
it was – 0.026 and – 0.013%.
Refractories and Industrial Ceramics Vol. 45, No. 2, 2004
1083-4877/04/4502-0091 © 2004 Plenum Publishing Corporation
Tekhnologiya Research and Production Enterprise, Obninsk,
Kaluga Region, Russia.