Plasma thin films were deposited from gas mixtures of hexamethyldisiloxane (HMDSO) and oxygen (O-2) using a low-temperature cascade arc torch (LTCAT). Various properties of the deposited HMDSO plasma coatings, including refractive index (RI), surface contact angle, and hardness were evaluated. The characterization results using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, ellipsometry, and water contact angle measurements indicated that, with increased O-2 addition, the deposited HMDSO plasma thin films were of inorganic SiOx nature. It was also found that, in the LTCAT plasma system. O-2 addition significantly improves the hardness of the resulting HMDSO plasma coatings. The film hardness of the deposited HMDSO plasma coatings measured by a standard pencil test (ASTM D3363-05) reached 6H with increased O-2 addition in the HMDSO/O-2 gas mixture. Such hard plasma coatings could be potentially used for many important industrial applications, such as anti-scratch coatings on plastic glasses and various plastic lens materials. (C) 2011 Elsevier B.V. All rights reserved.