A functionally graded material (FGM) is a two-component composite characterised by a compositional gradient from one component to the other. In contrast, traditional composites are homogeneous mixtures, and they therefore involve a compromise between the desirable properties of the component materials. Since significant proportions of an FGM contain the pure form of each component, the need for compromise is eliminated. The properties of both components can be fully utilised. For example, the toughness of a metal can be mated with the refractoriness of a ceramic, without any compromise in the toughness of the metal side or the refractoriness of the ceramic side.
Potential Applications of FGMs
FGMs offer great promise in applications where the operating conditions are severe. For example, wear-resistant linings for handling large heavy abrasive ore particles, rocket heat shields, heat exchanger tubes, thermoelectric generators, heat-engine components, plasma facings for fusion reactors, and electrically insulating metal/ceramic joints. They are also ideal for minimising thermomechanical mismatch in metal-ceramic bonding.
The Origin of FGMs
The FGM concept originated in Japan in 1984 during the spaceplane project, in the form of a proposed thermal barrier material capable of withstanding a surface temperature of 2000 K and a temperature gradient of 1000 K across a cross section <10 mm. Since 1984, FGM thin films have been comprehensively researched, and are almost a commercial reality.
However, most of the “extreme environment” applications for FGMs require bulk FGMs, i.e., FGMs with gradient breadth in the order of millimeters to centimeters, and with continuous gradient profiles. Bulk FGMs remain merely a hypothesis. No commercially viable process has yet been developed to make such a material. While the scientific literature abounds in papers on the modeling of the hypothetical properties of bulk FGMs, the few proposed fabrication methods are labour-intensive specialised laboratory techniques, not low-cost commercial processes.
Wear Resistant FGMs
A low-cost ceramic-metal functionally graded material would be ideal for wear-resistant linings in the mineral processing industry. Such a material would comprise a hard ceramic face on the exposed side, a tough metal face on the rear side that can be bolted or welded to a support frame, and a graded composition from metal to ceramic in between. The gradation would enhance the toughness of the ceramic face and also prevent ceramic-metal debonding. Such a material would uniquely combine the following attributes:
• High abrasion resistance (ceramic face)
• High impact resistance
• Convenience: weldable/boltable to metal supports
There would be little point in developing such a material unless it could compete commercially with the wear-resistant ceramics currently on the market, which range from tens to hundreds of dollars per kilogram, with alumina at the bottom end and imported tungsten carbide at the top end of this range. Unless this can be achieved, the potential benefits of bulk wear-resistant tiles of metal-ceramic FGMs will remain an unrealised possibility.
The Impeller Dry Blending (IDB) Process
To this end, a controlled-blending process has been developed at Sydney University for the fabrication of functionally graded materials of broad regular gradient. This is known as the impeller dry blending process (IDB). The IDB process is characterised by the following features:
• Low-cost process.
• Rapid throughput rates (hundreds of grams per minute).
• Broad gradients millimetres to centimetres in breadth.
• Regular and continuous (not layered) gradients are achievable with a high level of control over gradient profile.
What is Unique About IDB?
A number of laboratory reports have been published on fabrication processes for bulk FGMs. These approaches have mostly involved some sort of controlled segregation approach, i.e., separating a mixture of metal and ceramic powders into a graded profile on the basis of density. In controlled segregation, the driving force for gradation is the action of gravity on the difference in true density of the component powders. Segregation is a slow process with poor gradient control because segregation rates depend strongly on the particle size and morphology of the specific raw materials used. To date, most published papers on bulk FGMs have involved a segregation approach, for example, sedimentation forming, slip casting, centrifugal casting, and thixotropic casting.
In controlled blending, the two FGM components are blended during forming and the ratio is continuously varied from 100% component 1 through to 100% component 2 (or variation thereof). This approach potentially offers the unique advantage of being able to produce precisely controllable regular functional gradients independent of the system-inherent issues of powder density and gravitational settling mechanisms. Also, unlike segregation, controlled blending enables very rapid processing rates.
Use of Controlled Blending
To date, controlled-blending has mostly been used for making FGM thin films. For example, FGM thin films by thermal spraying (blended powder feed), vapour deposition (CVD/PVD blended gas feed), electrophoretic deposition (blended slurries), filter pressing (blended slurries), and blended spray drying.
Note: A list of references can be obtained by contacting the authors.