Ultrathin TiB 2 protective films
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TiB2 thin films demonstrate considerable potential for use as protective overcoats in the magnetic recording industry due to their excellent mechanical and tribological properties and good chemical and thermal stability. In the many studies performed on TiB2 films, the relative effectiveness of ultrathin TiB2 films has not been systematically investigated for very thin TiB2 films. In the present investigation, film stress and microstructure in as-sputtered and annealed ultrathin TiB2 films were investigated as a function of thickness. Ultrathin TiB2 films, as thin as 5 nm, were observed to adequately protect an underlying magnetic layer from oxidation up to 400 °C.
I. INTRODUCTION
Titanium diboride (TiB2) based ceramic coatings are receiving increasing consideration as potential protective overcoats for high-density magnetic recording media due to their high intrinsic hardness, favorable electrical and thermal conductivity, and good chemical and thermal stability.1–7 TiB2 films have been deposited using a variety of methods including plasma spraying,8 chemical vapor deposition,3,4,9 electron-beam vaporization,10 ion-beamassisted deposition (IBAD),11,12 ion beam sputtering,13 and various magnetron sputtering techniques.5,6,14–27 Of these deposition methods, sputtering has important advantages such as low deposition temperature, uniform coverage over large areas, and flexibility in controlling stress states and in tailoring the film chemistry, as well as the absence of toxic or explosive precursors. The microstructures and/or tribomechanical properties of sputtered TiB2 films have been the topic of numerous investigations.5,6,12,13,18–27 Deng et al.,21 for example, demonstrated high hardnesses and high residual stresses in relatively thick (i.e., 500 nm) TiB2 films produced via dc magnetron sputtering. They also demonstrated that the mechanical and tribological properties could be further improved with slight nitrogen doping.21 Several other investigators have also reported similar mechanical and/ or tribological properties for sputtered TiB 2 coatings13,22,24–27 as well as excellent chemical inertness and resistance to oxidation.6 For example, Wiedemann and Oettel,6 in a study of TiB2 coatings deposited on steel substrates, observed no change in chemical composition, structure, or hardness after annealing in air up to 400 °C. At 600 °C, however, they observed the formation of a detectable layer of TiOx.6 Several groups have also investigated the thermal stability of TiB2 films under a J. Mater. Res., Vol. 16, No. 4, Apr 2001
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variety of conditions.5,20,28,29 Sade and Pelleg5 observed that thick (approximately 250–500 nm) cosputtered amorphous TiB2 films annealed in vacuum did not begin to crystallize up to 890 °C annealing. This result was later supported by others for as-evaporated amorphous TiB2 films,28 whereas Chen and Barnard20 reported that the grain size of ultrathin (35 nm) TiB2 films increased as much as four times after annealing in air at 400 °C. T
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