Protective Zr-containing SiO2 coatings resistant to thermal cycling in air up to 1400 °C

The article reports on the evolution of the structure, mechanical properties and protection ability of 7 μm thick Zr-containing SiO2 coatings during thermal cycling in air. The films were reactively sputtered from a composed target (a Si plate fixed by a Zr ring with inner diameter ØinZr = 20 mm) using a closed magnetic field dual magnetron system operated in AC pulse mode. Main attention is devoted to the investigation of the effect of the film structure on the thermal stability of the mechanical properties and the protection of the substrate. Only the SiO2 film with a low content of Zr was investigated in detail. It was found that (1) the Si31Zr5O64 film sputtered at the substrate temperature Ts = 500 °C is amorphous, (2) the structure of the Si31Zr5O64 film gradually changes during thermal annealing in air from the amorphous to that containing a crystalline t-ZrO2 phase, (3) the t-ZrO2 phase is stable in wide range of Ta up to 1500 °C and no conversion of the t-ZrO2 phase into the m-ZrO2 phase is observed during subsequent cooling to room temperature (RT), (4) the hardness H and effective Youngʹs modulus E⁎ of the Si31Zr5O64 film are thermally stable during heating at temperatures ranging from the RT up to 1400 °C and (5) the Si31Zr5O64 film interacts with the Al2O3 substrate forming a mixture of t-ZrO2 + m-ZrO2 + SiO2 + Al6Si2O13 on the substrate surface at annealing temperatures Ta ≥ 1500 °C. Main issue of this investigation is the finding that the properties of a protective coating do not change during thermal cycling as far the structure of a coating is unchanged during the increase and decrease of the annealing temperature Ta.