Can some explain about the chemistry of the coating formation inside the plasma chamber that is taking place with oxygen and HMDSO gas at low pressure.
The thermal stability depends on what kind of coating you have (silicone-like, silicon oxide-like). For more details see e.g. "Degradation and Stability of Plasma Polymers" (doi: 10.1142/9781860945380_0007)
By the way, you have an expert very close at Università Milano Bicocca. Ask for Prof. Claudia Riccardi.