Reinforcement alumina

The SiC whisker-reinforced alumina composite, a model for engineered materials, has opened new vistas for tool material development. Whereas SiC whisker-reinforced alumina is used extensively for the machining of nickel-base superaHoys, SiC whiskers react chemically with steel, causing rapid wear on the rake face. Attempts are underway to replace SiC whiskers with less reactive whiskers such as TiC or TiN. 

SiC whisker-reinforced alumina is a major advance in tool material development, as it provides a means to increase the fracture toughness of the material via the composite material approach. It is entirely possible that in the next century many new whiskers of refractory, hard materials will be made... 

Explain why silicon carbide-reinforced alumina is stronger and tougher than pure alumina. 

Todd, R.I., Morsi, K. and Derby, B. Neutron diffraction measurements of thermal residual microstresses in ceramic particle reinforced alumina , Brit. Ceram. Proc. 57 (1997) 87-101. 

Runyan, J.L. Bennison, S.J. (1991) Fabrication of flaw-tolerant aluminium-titanate-reinforced alumina. J. Eur. Ceram. Soc. 7, 93-99. 

For most metal-reinforced nanocomposites the thermal expansion coefficient of the metal phase will be larger than that of the matrix, reversing the expected stress fields compared to SiC-reinforced alumina. Thus while the tensile radial stresses surrounding occluded particles may induce transgranular cracking, the compressive hoop stresses may inhibit crack propagation if the particles are located at grain boundaries. Macrostresses in sub-micron Ni... 

The initial interest in ceramic matrix nanocomposites arose from reports by Niihara and co-workers indicating enhanced mechanical properties due to the presence of ceramic (SiC) particles.53 With the development of various processing routes to introduce nanometer-sized metal particles in a ceramic matrix, variations in functional (i.e. magnetic) properties are possible. In the following we briefly review the microstructurally dependent properties, with emphasis on the possible mechanisms leading to improved properties and using SiC-reinforced alumina as a point of comparison. 

In metal-reinforced ceramic matrix nanocomposites, matrix grain refinement has also been demonstrated by Niihara and co-workers.12 At the same time, as mentioned in the previous section, significant residual stress fields have been measured for Ni-reinforced alumina nanocomposites,23 although with reversed compression-tension fields compared to SiC-reinforced alumina. Thus for metal particles below the critical size for intrinsic cracking11 and... 

In an opposite trend, the addition of Zr02 particles to Ni-reinforced alumina... 

Lee, W.J., Case, E.D. (1989), Cyclic thermal shock in SiC-whisker-reinforced alumina composite , Mater. Sci. Eng., A119, 113-126. 

J. R. Porter, Observations of Non-Steady State Creep in SiC Whisker Reinforced Alumina, in Whisker- and Fiber-Toughened Ceramics, eds. R. A. Bradley, D. E. Clark, D. C. Larsen, and J. O. Stiegler, ASM International, Metals Park, OH, 1988, pp. 147-152. 

K. Xia and T. G. Langdon, The Mechanical Properties at High Temperatures of SiC Whisker-Reinforced Alumina, in High Temperature/High Performance Composites, Materials Research Society Symposium Proceedings, Vol. 120, eds. F. D. Lemkey, S. G. Fishman, A. G. Evans, and J. R. Strife, Materials Research Society, Pittsburgh, PA, 1988, pp. 265-271. 

P. Lipetzky, Creep Deformation of Whisker-Reinforced Alumina, Ph.D Thesis, Brown University, Providence, RI, 1992. 

It is well established that improvements in the room-temperature fracture toughness of ceramics can be achieved by whisker reinforcement, and various mechanisms have been proposed to explain the phenomenon. Recently, two studies have examined the fracture behavior of SiC whisker-reinforced alumina composites at high temperatures to determine the dependence of fracture toughness on temperature, the effect of test atmosphere on fracture toughness, and mechanisms of fracture.33,34 In both studies, single-edged-notched bars... 

S. R. Nutt and P. Lipetzky, Creep of Whisker-Reinforced Alumina, Mater. Sci. Engr., A166, 199-209 (1993). 

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