Multifunctional dream ceramic matrix composites are born

Researchers at Osaka University have produced composites consisting of alumina (AI₂O₃) ceramics and titanium (Ti), namely AI₂O₃/Ti composites. They designed a percolation structure for forming a continuous conduction pathway by dispersing fine-sized Ti particles into an AI₂O₃ matrix, optimizing the particle size of metallic Ti powder and sintering processes. They improved fracture toughness and electrical conductivity of AI₂O₃/Ti composites while simultaneously giving them photocatalytic ability through chemical and/or thermal treatment. (Figure 1)

Various types of metal-ceramic composites have been researched and developed, but their combination and fine structures were limited. In particular, the combination of ceramics such as alumina used as matrices and titanium, a biocompatible metal, has a problem in that the structure of composites is not uniform because of the high reactivity of titanium (oxidation happens and chemical compounds are produced) and the large particle size of commercially-available Ti powder (several tens of micrometers). Thus, it was difficult to produce composites that have advantages of both ceramics and metal: that is, composites in which metallic Ti powder is homogeneously dispersed in the matrix and has excellent mechanical properties.

The group prepared ball-milled titanium hydride (TiH₂) fine powder mixed with alumina powder, producing AI₂O₃/Ti composites using a method based on the in situ decomposition of TiH₂ to Ti and simultaneous sintering with Al₂IO₃, which process inhibited AI₂O₃ dissolution into Ti by diffusion through interfacial reaction between AI₂O₃ and Ti during sintering. As a result, they minimized reactivity of Ti and AI₂O₃ to disperse significantly finer and more homogeneous Ti (compared to those produced with conventional methods) in AI₂O₃, realizing composites with a percolation structure by controlling the content of added Ti.

In this way, the group improved fracture toughness of inherently brittle AI₂O₃ through dispersion of fine Ti particles into AI₂O₃ and, due to percolation of metallic Ti particles, contributing electrical conductivity to insulator ceramics AI₂O₃. They also demonstrated that AI₂O₃ ceramics could be machined by electrical discharge machining like metals. (Usually, ceramics are not electrically conductive.) In addition, they formed a nano-porous- or nanorod- structured titania layer on the surface of the composite by selectively oxidizing Ti via NaOH treatment and/or heat treatment. Through this, they demonstrated that the photocatalytic ability to break down organic substances could also simultaneously be given to AI₂O₃/Ti composites.

Group leader Tohru Sekino says, "AI₂O₃/Ti composites will be used as ceramic matrix composites that have excellent mechanical properties and can be machined by electrical discharge machining. They will also be used for industrial products and biomaterials as new multi-functional composites that have an active surface layer with antibacterial properties and a photocatalytic ability to break down pollutants."