PDF of figure
PDF of text
New Multilayer Interlayer Creates Al2O3 Joints
Stable to 1100°C
Andreas Glaeser
A new method of joining ceramics for high-temperature applications
has been developed by a research team directed by Andreas M. Glaeser
of the Ceramic Science Program in the Center for Advanced Materials.
The method can be used to make aluminum oxide ceramic joints that
retain useful levels of strength up to 1100°C, far beyond
the 600°C limit of standard commercial techniques.
Ceramic materials have a number of desirable properties-strength
at elevated temperature, corrosion resistance, high strength to
weight ratio, good wear resistance-that make them an attractive
choice for high-temperature, high-stress applications such as
high-temperature energy conversion and power generation systems.
However, the difficulty of shaping ceramics into complex parts
has limited their use in these applications. Assembling complex
parts by joining simpler components would appear to be a solution
to this problem. However, while techniques for joining ceramics
to themselves and to metals have been developed, many are only
applicable to relatively low temperature applications. Indeed,
there is a general lack of brazes for application temperatures
above 600°C.
Research into ceramic joining techniques in the Ceramic Science
Program has focused on developing methods based on multilayer
metallic interlayers designed to lower the required process temperature
and bonding pressure (to avoid metal/ceramic reactions that might
degrade the microstructure) while maximizing the service temperature.
Recently, the MSD team used an interlayer consisting of a thick
(125 mm) core layer of niobium surrounded on both sides by a thin
(3 m_) layer of copper to join alumina ceramics by brazing at
temperatures between 1150°C and 1400°C. By adjusting the
processing temperature and bonding pressure, joints with highly
reproducible, increased strength could be produced. In these joints,
room temperature failure at stresses of 240 MPa ± 20 MPa
occurs in the alumina not the joint (see figure). Bend tests performed
at up to 1100°C on these optimized joints indicate that significant
strength is retained at high temperature with failures, when they
occur, again located in the ceramic, not the joint.
Joints were formed with transparent sapphire to allow optical
characterization of the interface, and to examine the mechanism
of joining. Grain boundary ridges and asperities on the bonding
surfaces provide initial points of Al203-Nb contact. It was shown
that during processing, copper forms a discrete phase of "islands"
through interfacial dewetting that appears to result in direct
bonding between the Al203 and niobium over most of the interface.
Thus, at temperatures approaching the melting temperature of copper
(1083°C), it is the higher melting temperature niobium (m.p.
= 2468°C) that supports the applied load and is responsible
for the strength at elevated temperature. Although the mechanism
by which the use of copper leads to strengthening is not completely
understood, the simplest explanation is that without the copper,
which serves as a high diffusivity path for niobium, the development
of comparable levels/degrees of Nb-Al203 contact (which creates
the bond) would require substantially longer processing times.
There is no theory and insufficient experimental data to quantify
exactly how much longer more processing would be required, but
it is estimated that it would be at least a factor of ten, and
possibly a factor of 100 to 1000 greater.
Work with higher purity alumina has suggested that further improvement
in joint strength and temperature capability are possible. Samples
were tested at temperatures up to 1300°C, and again instances
of failure/deformation that occurs in the ceramic are observed.
In addition, the team is developing metal combinations to enable
extension of the technique to the preparation of strong, high-temperature
joints for other material combinations, for example, joining other
ceramics to each other or to metals.
Andreas Glaeser (510)486-7262, Materials Sciences Division
(510)486-4755, E. O. Lawrence Berkeley National Laboratory
R. A. Marks, D. R. Chapman, D. T. Danielson, and A. M. Glaeser,
"Joining of alumina via copper/niobium/copper interlayers,"
Acta. Mater. 48, 18-19, 4425
Materials Sciences Division
Ernest Orlando Lawrence Berkeley
National Laboratory
One Cyclotron Road, Mail Stop 66, Berkeley, California 94720 USA