The effect of polymer additives on the critical thicknesses of mullite thin films obtained from the monophasic sol–gel precursors

The critical thickness, a maximum thickness of a non-repeating film deposition, above which cracking occurs, is an important characteristic of sol–gel-coating process. In this paper, we used mullite films prepared from the monophasic sol–gel precursors as an example system to study the effects of polymer additives on the coating’s critical thickness. The mullite and mullite–polymer hybrid gels demonstrated monophasic characteristics, enabling low-temperature processing of nanocrystalline, phase-pure mullite films at 1000 °C. The cracking in the films was only observed at the temperatures below 400 °C. Increasing film thickness caused a decrease in the cracking onset temperature. Adding polymers, such as PVP, PEO, and PVA, in the precursor increased the critical thickness. We demonstrated that the reasons of the crack prevention caused by the polymer additives were because they not only slowed down the sol–gel polycondensation process, but also relaxed the stresses during heat treatment. The polymer decomposing in a wider temperature range had a greater critical thickness. The polymer additives also showed optimal concentrations in improving the critical thicknesses. Dense, crack-free, and phase-pure mullite films with thickness up to 450 nm were achieved after firing at 1000 °C. Graphical Abstract