Materials Science

Stabilized by Stress

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Science  08 Apr 2005:
Vol. 308, Issue 5719, pp. 167
DOI: 10.1126/science.308.5719.167b

Thin films are often used as protective coatings against wear and corrosion; nitride films are also used as decorative layers. Typically, these films have a 1:1 metal:nitrogen composition, as in TiN, ZrN, or CrN, all of which have a NaCl-like structure and are highly conductive. Although it is difficult to make films with higher proportions of nitrogen, there have been reports of films with a M3N4 composition. Under high pressure inside a diamond anvil cell, the orthorhombic structure of these materials transforms into a metastable cubic structure, and the films become transparent and less conductive.

Chhowalla and Unalan have designed an industrially viable, filtered cathodic arc process to create cubic Zr3N4 films. Metal vapor is generated by an arc discharge on a pure Zr cathode and reacted with fully ionized atomic nitrogen. The deposition process produces films with inherent compressive stresses and, in combination with the localized high temperatures where the plasma is deposited, creates conditions that stabilize the cubic phase of Zr3N4. These films were found to be much harder than either ZrN or orthorhombic Zr3N4, and showed excellent wear resistance when used to coat a steel-milling tool. — MSL

Nat. Mater. 10.1038/nmat1338 (2005).

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