Shien-Uang Jen

Shien-Uang Jen

Institute of physics, academia sinica, ROC

Title: Hardness of the Fe81Ga19/Si(100) film measured by the nano-identation method


Shien-Uang Jen has completed his Ph.D. at the age of 29 from the Physics Department of Carnegie-Mellon University. He is now a senior research fellow of the Institute of Physics, Academia Sinica. He has published more than 123 papers in reputed journals.


Fe81Ga19 alloy is an auxetic material, which means that its Poisson’s ratio along the [110] direction, V[110], is negative. When this alloy is deposited on a single-crystal Si(100) wafer, the film is highly (110) textured. Thus, if we indent the film plane with a Berkovich indenter vertically, the apparent hardness obtained with the parameter Veff=V[110]=-0.55 < 0 should be larger than that with Veff=V[110]=+0.30. This is reasonable, because if Veff is negative, the deformed material under the pressing indenter tends to push the tip slightly upward (or backward). As a result, the measured depth of circle of contact, hp, of this auxetic film is smaller than it should be under a fixed force, F, and, moreover, since the hardness H≡F/[24.5(hp)2], the apparent (measured) hardness must be larger. We have made a series of (Fe100-xCox)81Ga19/Si(100) films, and measured their hardness. The results indicate that: [1] when 0≤x<7 at %Co, the films (or alloys) are auxetic (Veff<0); [2] when 7≤x≤11 at %Co, they are critical (Veff=0); and [3] when 11 0). Each point on the loading or un-loading curve was analyzed by the finite element analysis to show the side-view of the shear-stress (τ) distributions in the film, film/substrate, and substrate regions, respectively. We can study (or outline) the plastic-flow zone beneath the indenter by adopting the Tresca criterion: τ ≥ H/6.

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