Expanding Thermal Plasma (ETP) has been demonstrated as a high-rate, large-area process for depositing abrasion resistant coatings onto polycarbonate glazing materials using organosiloxane and O2 reagents with arc plasma sources that exhibit manufacturing robustness. Thorough characterization of the parameter space of the dual reagent plasma system presented challenges, mainly due to the O2 addition Although O2 is incorporated into the depositing coating and influences coating properties, it is not an independent film-forming species. Hence, the normalized energy input parameter that is applied to single reagent deposition systems (power divided by reagent flow rate) does not suitably characterize a system with O2 addition Treating the input parameters as a three-component mixture (Ar+ ion flow rate, reagent flow rate, and O2 flow rate) with refractive index as the response is shown to be a useful representation of the parameter space of the plasma system. The refractive index is shown to correlate to the coating composition, as measured by XPS, thus linking the parameter space of the plasma process to the coating properties. The empirical relationships developed are used for designing experiments to determine the influences of operational parameters on product performance, to optimize these parameters, and to define the appropriate parameter limits.