Atmospheric plasma is an emerging material processing technique that enables depositing coatings and modifying surface properties on large and/or complex geometry substrates, and also substantially decreases the initial capital investment.We explored the application of atmospheric plasma for depositing dense and/or hybrid silica coatings on plastics. The coating deposition rate, molecular network structure, density, elastic modulus and adhesion to plastics exhibited a strong dependence on the precursor delivery temperature and rate, and the functionality and number of silicon atoms in the precursor molecule. A carbon-bridged organosilane precursor at high delievery temperature produced hybrid silica coatings with more than twice higher elastic modulus and adhesion compared to commercial polysiloxane hard coatings on plastics. Atmospheric plasma was also explored for modifying the plastic surface to enhance adhesion with plasma coatings. A short atmospheric plasma treatment increased the adhesion up to four times due to newly formed surface functional groups and increased surface roughness; while prolonged treatment dramatically decreased the adhesion by over-oxidizing the surface and forming a low-molecular-weight weak layer. Interestingly, this low-molecular-weight weak layer can be removed by compressed air cleaning or ethanol rinsing of the surface, resulting in dramatically improved adhesion. The combination of atmospheric plasma and compressed air/solvent treatments opened up a way to form well functionalized and at the same time highly crosslinked polymer surface for adhesion enhancement.