Enhancing the hardness of Al/W nanostructured coatings

Two-component multilayer thin films frequently show hardness enhancements at specific repeat periods above that of the constituent layers. This study of hardness enhancements in W/Al nanostructured coatings provides strong new evidence that hardness enhancements in this system arise not only from the presence of a layered structure, but also from the presence of defects introduced by changing the deposition conditions. Samples with well defined layers of W and Al were produced by sputtering to cover a wide range of periods from 10 to 200 nm. No evidence of enhanced hardness in these films was found by nanoindentation. On the other hand, samples deposited from cathodic arc sources showed strong hardness enhancement above that of pure W. However, the samples of highest hardness did not contain Al layers for much of their thickness. The hardening mechanism therefore could not be attributed to the presence of a multilayer structure. Examination of the microstructure showed that the interruptions to the W deposition caused by operation of the Al source introduced defects which acted as pinning sites for dislocations. The nanoindentation hardness data were well described using a modified Hall-Petch relation.