Determination of mechanical properties of graded coatings using nanoindentation

Abstract Surfaces can be protected against mechanical damages by coatings that have to be adapted to the substrate and the conditions in the application. To reach the optimum protection, the covering coating should not be a single layer but an optimised layer stack or a coating with graded mechanical properties. Recently, it was shown that B–C–N films can be used as model system for the production of graded coatings and for the investigation of the mechanical properties due to the large variation range of their properties. For the purpose of the following investigation, single layers with mechanical properties on the opposite ends of the variation range of B–C–N were deposited with a thickness of about 0.8 μm on two different silicon samples. Additionally, a 1.55-μm coating with stepwise increasing Young's modulus reaching from about 105 to 255 GPa was produced in 31 deposition steps on a third silicon sample as model for a graded coating. The mechanical properties of all coatings were measured by nanoindentation with a Berkovich indenter and two different spherical indenters of 3 and 12 μm radius. The Young's moduli of the two homogeneous coatings were derived from wholly elastic measurements with the spherical indenters. Elastic force–displacement curves were also measured on the graded coating. A new mathematical model was used to calculate the curve for a graded coating on a Si substrate and to fit it to the measurement data. Using the data from two different indenters, it was possible to derive the correct moduli at the lower and topmost part of the graded coating. The agreement of measured and calculated curve over the whole depth range was better than 2% of the maximum depth. Plastic properties were also studied in detail. Beside the measurement of hardness, two innovative methods for the determination of the yield strength were applied and compared. The gradient in the plastic properties could be proved. The results confirm the usefulness of the mathematical model to predict the mechanical behaviour of graded coatings and to analyse mechanical stresses in such samples.