David J. Green
Professor of Ceramic Science
and Engineering
B.Sc. (Liverpool), M.S.,
Ph.D. (McMaster)
* DETAILED BIOGRAPHIC INFORMATION
MATSE 414 Mechanical Properties of Ceramics. Detailed Information
MATSE 436
Mechanical Properties
of Materials Detailed Information
MATSE 466 Ceramic Laboratory I. Detailed information
MATSE 545
Mechanical Properties of Brittle Materials
MATSE 509
Composites
MATSE 563 (E MCH 534)
Failure Mechanisms in Materials
Detailed information
MATSE 564 (E MCH 535)
Deformation Mechanisms in Materials
Detailed information
FRACTURE OF BRITTLE MATERIALS
Optical micrograph showing cracks arrested in a glass surface as it is being stressed
Ceramic materials are usually brittle, breaking in a catastrophic manner. Indeed, this behavior often limits the use of ceramics in both structural and non-structural applications. The low amount of energy involved in breaking these materials is a result of their low toughness. In recent years, however, various mechanisms have been identified for increasing the fracture resistance (toughness) of these brittle materials. For example, improvements in fracture resistance have involved the addition of particles, platelets, whiskers, or fibers to a material. Manipulation of residual stresses and structure at various scale levels has also proved to be a useful approach.
This methodology has been very successful and, in some material systems, increases in toughness by an order of magnitude have been obtained. In all these developments, the use of a materials science methodology has been critical philosophy in making the advances. This approach emphasizes the relationships between processing, structure, and mechanical behavior as a way to understand a material, and to identify processing techniques for improving properties. The research in my group utilizes this philosophy and it is aimed at improving the reliability of ceramics and glasses for technological applications. For example, recent research has also led to a process that strengthens glass and stops cracks as the glass is being bent. This leads to a strong glass with a very narrow distribution of strengths.
SEE A MOVIE
THAT SHOWS CRACKS ARRESTING IN THE SURFACE OF GLASS
REQUIRES QuickTime™ Player
STRESS IN THE DENSIFICATION OF POWDER COMPACTS
Photograph showing the disortion of a bi-layer during firing
There is a renewed interest in the ability to co-fire
layered and integrated structures as a result of new applications or unusual
properties for such structures. For example, in the area of structural ceramics,
it has been shown that delamination in ceramic hybrid laminates could be
used to increase the work of fracture. It has also been suggested that laminates
could be tailored to induce stable crack growth, crack arrest and threshold
strengths by using differences in the elastic, fracture toughness and thermal
expansion properties between the layers. Based on economic considerations,
co-firing is an attractive approach for producing these structural laminates.
Layered structures are also of great importance in
electronic applications of ceramics and co-firing is often used to produce
these devices. Multilayer capacitors are a well-established example of this
technology. In this industrial arena, there is a continuous drive for miniaturization
and improved reliability, especially with the move to high frequency applications.
Other applications that use co-fired layered structures include gas sensors,
solid oxide fuel cells and electronic packages. In this last example, there
is a strong interest in the use of low-temperatures co-fired ceramics (LTCC),
especially in the push for wireless applications. LTCC are commercially available
as green tapes that are used to fabricate the electronic package. The tapes
are co-fired with the imprinted metallic conductors. A complex three-dimensional
circuit can be designed by stacking the LTCC layers and allowing the circuitry
to feed through and between the layers. The intent in this technology is
also to integrate other electronic components into the final package. LTCC-based
packages are clearly more complex material systems than laminates with continuous
layers but many of the scientific issues in the co-firing process are similar.
In the co-firing of layered structures, compatibility stresses arise as
a result of differences in the shrinkage behavior between the layers. These
stresses may arise because of differences in the densification rate during
sintering (constrained sintering) and differences in thermal contraction
during cooling after densification. The strain incompatibility between the
layers leads to internal stresses and distortion. The aim of our current
research is to establish the scientific and experimental methodology that
is used to quantify these processes, such that material systems can be designed
that avoid damage and distortion.
SEE A
MOVIE THAT SHOWS A BI-LAYER SPECIMEN UNDERGOING DISTORTION DURING SINTERING
REQUIRES QuickTime™ Player
SEE A
MOVIE THAT SHOWS CERAMIC SPECIMENS UNDERGOING CREEP AT HIGH TEMPERATURE
REQUIRES QuickTime™ Player
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