It has been recently demonstrated that a physically thick (about 50 nm) interfacial region ('interphase') is formed between a plasma deposited film and the underlying plasma treated or untreated polymer substrate. This can have an important effect on the performance of optical coatings, protective coatings, barrier films for packaging, and others. In the present work we study the effect of N-2 and He plasma treatment of polycarbonate on the surface and near-surface (composite) mechanical properties. Multi-technique mechanical testing (scratching and depth-sensing indentation) has been performed using a broad range of applied loads (1 mN-10 N). We demonstrate that low-load scratching provides a more surface-specific information on the hardness and scratch resistance than do other methods. We also point out the advantage of constant-load, multipass scratching compared to the progressive-load approach.