the invention concerns a method for treating a surface for the protection and functionalisation of polymers by gas plasma deposit in a confined chamber of one or several silicon alloy layers . the silicon alloy is selected among silicon and its oxides , nitrides , oxynitrides ; the deposit is performed at a temperature less than the degradation temperature of the polymer , and a physico — chemical surface pre - treatment by plasma is performed in the same chamber before the silicon alloy is deposited ; the pre - treatment consisting in a surface treatment comprising etching a surface zone of the polymer and step which consists in depositing a polymeric carbon compound .