Scratch and fragmentation tests were performed on 100 nm thick SiOx coatings (1.1 less than or equal to x less than or equal to 2) deposited on PET substrates with and without prior N-2-plasma treatment. A correlation was found between the critical normal forces measured during scratching and crack densities at fragmentation saturation. Quantification of the works of adhesion, and a comparison of their values for the scratch and fragmentation tests were possible by a comparison of the model developed by Bull, Burnett and Rickerby for the former, and Kelly and Tyson＇s model combined with the approach from Nardin and Schultz for the latter.