Ceramic ductile particles

The thermal shock behaviour of hot-pressed alumina, hot-pressed and sintered alumina reinforced with iron particles (reprinted from Journal of the European Ceramic Society, 19, Aldridge and Yeomans, The thermal shock behaviour of ductile particle toughened alumina composites, 1769-1775, copyright 1999, with permission from Elsevier). 

Figure 8.12 illustrates a solid particle impinging on a surface. It has been found that the erosive wear rate depends upon the impingement angle, a, the particle velocity, vq, and the size and density of the particle, as well as the properties of the surface material. It has also been found that there is a difference in erosive wear properties of brittle and ductile materials. The maximum erosive wear of ductile materials occurs at a = 20°, whereas the maximum erosive wear for brittle materials occurs near a = 90°. Since the impingement angle is probably lower than 90° for these type of flow situations, we might consider only brittle materials, such as ceramics for this application. Let us examine brittle erosive wear in a little more detail first. 

Some reports have cited bridging across the relatively ductile metal particles as mechanisms which contribute to the increase in fracture strength.54 Other reports demonstrate cracks propagating at the metal-ceramic interfaces,... 

Figure 6.1. Examples of damage sites due to single-particle impacts (a) On mild steel, a ductile metal (b) On polycrystalline alumina, a brittle ceramic (from Hutchings, 1987 reproduced with permission).

Figure 4. Tensile flow curves of two Al-4.5 wt.% Cu reinforced with aluminium matrix composites, reinforced with angular and polygonal alumina particles of roughly the same size. The data show the significant improvements brought by matrix alloying (see Fig. 3) as well as the strong influence of the ceramic particle type on the composite mechanical performance. With the stronger ceramic, the material features high strength and acceptable tensile ductility despite the fact that it is more than 50% ceramic.

Somewhat higher ductility is obtained by combining ceramic particles with a metallic binder phase. Such ceramic-metallic materials (cermets) are excellent under erosive and abrasive conditions. A well-known material is WC Co. Better corrosion resistance is obtained by alloying Co with Cr or by using alloys of Ni, Cr and possibly Mo as the binding phase. 

Concerning the cutting mechanism, there are differences between processing ductile, metallic materials and brittle hard materials such as glass or ceramics. In case of ductile materials, microplastic deformations of surfaces, hardening, and embrittlement as well as breakaways of particles... 

E. Claxton, B. A. Taylor, R. D. Rawlings, Processing and Properties of a Bioactive Glass-ceramic Reinforced with Ductile Silver Particles, J. Mat. Sci. 37, 3725—3732 (2002). 

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