Reactive high-power impulse magnetron sputtering of ZrO2 films with gradient ZrOx interlayers on pretreated steel substrates

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High-power impulse magnetron sputtering with a pulsed O2 flow control was used for reactive deposition of densified stoichiometric ZrO2 films with gradient ZrOx interlayers onto floating Si and steel substrates at low substrate temperatures (less than 150 °C). The depositions were performed using a strongly unbalanced magnetron with a planar Zr target of 100 mm diameter in Ar + O2 gas mixtures at the total pressure close to 2 Pa. Two kinds of gradient ZrOx interlayers with different depth profiles of the stoichiometric coefficient, x, from x ≅ 0 to 2, were deposited using the feedback pulsed O2 flow control. Prior to deposition, a modification of the substrate surfaces was performed by high-power impulse magnetron sputtering of the Zr target in Ar gas at the same pressure of 2 Pa and a direct current substrate bias from −965 to −620 V in a target pulse and low substrate temperatures (less than 150 °C) for 10 min. It was shown that the pretreatment of the steel substrates is a necessary condition for good adhesion of the zirconium oxide (both pure ZrO2 and ZrO2 + ZrOx interlayer) films and that the adhesion of the ZrO2 films is substantially higher when the gradient ZrOx interlayers are used. The densified stoichiometric ZrO2 films (refractive index of 2.21 and extinction coefficient of 4 × 10−4 at the wavelength of 550 nm) deposited onto the gradient ZrOx interlayers exhibited a high hardness (15–16 GPa) and an enhanced resistance to cracking.