High Temperature Reactions in Laser-Assisted Ceramic Additive Manufacturing

Ceramics are the most temperature- and oxidation-stable materials known. Usually only strong acids or bases are capable of inducing ceramics to react. When dealing with extreme temperatures over 3000 °C, however, oxide ceramics can be induced to lose an oxygen and become reactive. This can be used in laser-assisted additive manufacturing (AM) of ceramics. Lasers are used here to sinter or melt the ceramics layer-by-layer. Additionally, each layer and even each spot can be irradiated with almost arbitrary high power and thus extreme heating, which lead to extraordinary reaction kinetics.<br/>As an example, irradiating SiO₂ with a CO₂-laser (λ= 10.6 µm), it can be evaporated with ease, while more stable ceramics like ZrO₂ can be reduced to ZrO, losing one oxygen in the process. This state can be stabilized in inert gas atmosphere and quenched to room temperature, while in oxygen-rich atmosphere the recapture of oxygen takes place upon cooling. When in its reduced state (, i.e., ZrO) can react with other substances to form intermediaries that are usually hard to produce. For instance, by varying the gas atmosphere to forming gas (5% hydrogen, 95% nitrogen), the dark colored ZrO can be converted to the golden ZrN. The reaction zone is clearly molten, so that it can be concluded that the temperature for the reaction is above the melting point of both ZrO₂(2680 °C) and ZrN (2980 °C). Thus, the gas phase can be employed as a reaction medium.<br/>Additionally, the solid phase can trigger certain reactions as well. The addition of Ti powder and a forming gas lead to the formation of an interpenetrating phase composite made from TiN and ZrO. The commonly weak interface between these two very different ceramics is stabilized through mechanical interlocking of the two phases, observed by cross-section elemental mapping in TEM. These TiN films are only a few 100 nm thick, yet present the excellent electrical conductivity that is common for this material. Since ZrO₂ is itself the solid electrolyte in solid oxide electrolyte fuel cells, such a conversion can be used to produce stable contacts for current collection.<br/>In additive manufacturing, the conversion technique can be employed to produce and partially convert ceramics to equip them with special functionalities. For example, Zn can be directly oxidized to the semiconductor ZnO in situ and thus produce functional nanostructures in the manufacturing process.<br/>In this contribution, I will show how high-power CO₂-laser treatments and additive manufacturing can be employed together to create in situ high temperature reaction and ultimately convert and stabilize these products.