Elastic properties of multilayer oxide coatings on float glass

• Nanoindentation can measure the elastic properties of 200 nm oxide coatings on glass.
• Good elastic modulus data requires careful consideration of the properties of the glass substrate.
• A simple model for the contact modulus of a coated sample has been extended to multilayer coatings.
• The ISO14577 Part 4 extrapolation approach underestimates the elastic properties of 200 nm oxide coatings on glass.
• Oxide coatings on glass should be at least 400 nm thick for direct elasticity measurement.

The damage resistance of multilayer oxide coatings on glass used for optical applications often depends on contact damage and the stress generation and relaxation mechanisms during coating deposition and subsequent application. Whereas it is relatively easy to design the optical properties of a multilayer coating to meet a particular specification it is more difficult to design its mechanical response. In part this is due to a lack of reliable mechanical property data for design, chief of which is the elastic modulus of each coating layer in the multilayer stack. Indentation tests can be used to determine the elastic and plastic properties of relatively thick (>1 μm) coatings but, whereas it is possible to measure the plastic response of submicron coatings provided the indenter penetration is less than 10% of the coating thickness this is not the case for elastic properties where a much lower penetration is required. In this study the use of nanoindentation testing in conjunction with a simple modelling approach to determine suitable elastic properties of individual oxide layers in a multilayer coating stack is assessed. It is demonstrated that the ISO14577 extrapolation approach underestimates the contact moduli for 200 nm thick coatings but a multilayer model can be used to determine more realistic properties. Variations in elastic properties of the glass substrate in the near-surface region need to be taken into consideration to get good data from the model.