Cutting Edge Technologies (1984) / Chapter Skim
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High-Technology Ceramics
High-Technology Ceramics
Pages 117-132
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From page 117... ...
Of course, these are not traditional ceramics—those beautiful, usually rather fragile examples of the potter's art made from naturally occurring substances such as clays, talc, and feldspars (Figure 1~.~ Early materials technologists were able to increase the durability of such products somewhat by applying glazes, substances formulated both to improve appearance and to have a coefficient of thermal expansion smaller than that of the substrate ceramic. On cooling, the substrate placed the glaze into compression, making it more difficult for minor scratches to develop into catastrophic cracks.
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From page 118... ...
Because the strength of a ceramic solid is inversely related to the square root of the size (d) of its largest flaw, and since d usually depends directly on the size of its component "grains," the modern ceramist prefers to build up ceramic shapes from chemically homogeneous particles, typically 1 micrometer (lam)
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From page 119... ...
Ideally, comminution, or grinding, is now used only to break up loosely bound clusters of fine particles or to add strain energy to them to accelerate subsequent sintering reactions. Developments leading to the generation of HTCs, or, as the Japanese prefer to call them, "fine ceramics," are occurring in three areas: the production of powders, of shapes, and of the important property of toughness.
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From page 120... ...
The sintering temperature required in this case was some 300°C below that conventionally used for TiO2. Lead zirconium titanate (PZT)
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From page 121... ...
In the latter case researchers at MIT and elsewhere are making fine particles of silicon (Si) , SiC, and Si3N4 from gaseous reactants such as SiH4, NH3, and C2H4, using a CO2 laser.
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From page 122... ...
has been introduced to further density the sintered part and so minimize the number of fracture-initiating voids remaining. This technique can produce complex ceramic shapes of <2 percent porosity and with pore sizes sufficiently small that room-temperature tensile strengths of 50 to 100 kpsi (thousand pounds per square inch)
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From page 123... ...
HIGH-TECHNOLOGY CERAMICS 123 ROCing Process ~ ~ MOLD Cold-compacted "green" piece. Cast "Fluid Die," e.g., a proprietary glass, around _ green piece.
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From page 124... ...
In comparison, a HIPing cycle usually takes several hours. THE DEVELOPMENT OF TOUGH CERAMICS Over the past few years, materials scientists have sought to circumvent the intrinsic fragility of ceramics by reducing the size and concentration of preexisting flaws through the use of ultrafine particles and compacting processes capable of producing components of near-theoretical density, as just discussed, and by introducing into the ceramics a variety of synthetic crack-retarding entities, such as phase-transforming particles, fibers, and distributions of cracks (see Figure 5~.
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From page 125... ...
unstressed, showing coherent tetragonal precipitate particles; (b) stressed by indenter, revealing transformed monoclinic particles near indentation (arrows)
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From page 126... ...
126 ADVANCES IN STRUCTURAL MATERIALS LAS-SIC composite 24 cat ~ 20 us Lo an I Cal To ~ 4 Unidirectional ~ composite ~ _ L~16 I2 8 Cross-pied (0°190°) \` ~ composite , _ ·~ _ {~/ .
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From page 127... ...
. It seems more likely that over the next few years parts of conventional engines will be produced in ceramics, with different ceramics being chosen for different parts to meet specific operating requirements.
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From page 128... ...
and Cummins Engine Company recently teamed up to build an uncooled diesel engine for an army truck (see Figure 9~.22 This truck has operated well so far, providing 9 mpg as compared with the standard 6 to 7 mpg. By mid-1984, the team hopes to demonstrate an oilless version of this engine, with an intrinsic thermal efficiency of ~54 percent as compared with values in the low 30s for conventional gasoline engines.
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From page 129... ...
in the 1960s, the objective always being improved tool life and enhanced rates of metal removal. Recently, however, and as a spin-off from their research on ceramics for gas turbine engines, Ford Motor Company has demonstrated that Si3N4 has excellent cutting characteristics, doubling the productivity of conventional cutting tools in the machining of cast-iron auto parts, e.g., wheel drums and clutch components.
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From page 130... ...
Among the many interesting developments in this area are Kyocera's use of singlecrystal sapphire to produce a range of products from hip prostheses to dental implants (see Figure 10) ,24 and Corning's introduction of potassium-magnesium-silicate ceramic crowns that are cemented directly to the remaining tooth structure and do not require any metal bridgework.3 Of course, before high-technology ceramics can truly be considered
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From page 131... ...
Alumina-based ceramic adhesives also are beginning to appear.3 Many innovative developments are expected to have occurred in the field of high-technology ceramics by the year 2000. By then, multicomponent, self-reinforced ceramic alloys, heat-treated to optimize properties, protected by compressive surface layers that are perhaps applied by ion bombardment or laser glazing (a new approach to a traditional process)
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From page 132... ...
Rice, Naval Research Laboratory, Washington, D.C.; H Kent Bowen, Massachusetts Institute of Technology, Cambridge, Massachusetts; and a number of the authors' colleagues at Martin Marietta Laboratories, Baltimore, Maryland.
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