Nanoscratch testing of atomic layer deposition and magnetron sputtered TiO2 and Al2O3 coatings on polymeric substrates

Abstract

Titania and alumina thin films are commercially important materials, finding a wide variety of applications. These materials are commonly deposited onto glass substrates by reactive magnetron sputtering techniques, or more recently by atomic layer deposition (ALD). Such films have been well-characterized in terms of their structures and properties, including their nanomechnical and nanotribological properties. There is increasing demand, though, to deposit thin (<100 nm) functional films onto polymeric substrates, which can provide cost and weight savings and opens up the potential of deposition in large throughput roll-to-roll coating systems. However, polymeric substrates impose severe limitations due to both their thermal and mechanical properties. In the former case, production is restricted to low temperature processes and in the latter case, the substrate offers negligible load support for the thin ceramic coating. While nanoindentation and nanotribological testing of thin ceramic coatings on rigid substrates have become established techniques, the same cannot be said of testing similar “hard” coatings on “soft” flexible polymeric substrates. It is recognized that the coating/substrate system is very different in nature and the mechanisms of failure may also be very different. Sub-100 nm titania and alumina coatings have, therefore, been deposited onto a range of polymeric substrate materials by low temperature ALD and, for comparison purposes, magnetron sputtering techniques, including mid-frequency pulsed dc and high power impulse magnetron sputtering. Nanoscratch testing has been carried out on these coatings usingspherical indenters. Indenter radius and loading rate were varied and their influence on the elastoplastic properties of the film and the substrate are considered and comparisons are made withthesame coating materials deposited onto “rigid” substrates.