SiO (silicon monoxide) is a granular black material before evaporation and is different than SiO2 (silicon dioxide), which is quartz. SiO is tough and it’s used as a protective layer on top of first surface metal mirror coatings. Sometimes it’s used for A/R coatings. It has poor adhesion over most things if they have already grown a native oxide on them. Some ways to get good adhesion can include heating for certain substrates, or if you have just done a fresh layer of aluminum, silver, or gold, it can stick well. For example, most first surface metal mirrors that you might buy from a place like Edmund Scientific have a protective coating or an anti-reflective coating of either SiO or MgF2 (magnesium fluoride).