A novel pillar indentation splitting test for measuring fracture toughness of thin ceramic coatings

Abstract

The fracture toughness of thin ceramic films is an important material property that plays a role in determining the in-service mechanical performance and adhesion of this important class of engineering materials. Unfortunately, measurement of thin film fracture toughness is affected by influences from the substrate and the large residual stresses that can exist in the films. In this paper, we explore a promising new technique that potentially overcomes these issues based on nanoindentation testing of micro-pillars produced by focused ion beam milling of the films. By making the pillar diameter approximately equal to its length, the residual stress in the upper portion of the pillar is almost fully relaxed, and when indented with a sharp Berkovich indenter, the pillars fracture by splitting at reproducible loads that are readily quantified by a sudden displacement excursion in the load displacement behaviour. Cohesive finite element simulations are used for analysis and development of a simple relationship between the critical load at failure, pillar radius and fracture toughness for a given material. The main novel aspect of this work is that neither crack geometries nor crack sizes need to be measured post test. In addition, the residual stress can be measured at the same time with toughness, by comparison of the indentation results obtained on the stress-free pillars and the as-deposited film. The method is tested on three different hard coatings created by physical vapour deposition, namely titanium nitride, chromium nitride and a CrAlN/Si₃N₄ nanocomposite. Results compare well to independently measured values of fracture toughness for the three brittle films. The technique offers several benefits over existing methods.

Keywords:

• fracture toughness
• nanoindentation
• pillar
• residual stress
• micron scale

Acknowledgements

The authors acknowledge Shiyu Liu, William Clegg (Univ. of Cambridge) and Johann Michler (EMPA) for providing the CrN and CrAlN/Si₃N₄ samples.

Funding

The cohesive zone finite element simulations were performed under the support of NSF grant number CMMI 0926798 and the pillar cracking experiments were performed under support of the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. FIB sample preparation was performed at the interdepartmental laboratory of electron microscopy (LIME lab) of University of Rome “Roma TRE”, with the assistance of Daniele de Felicis.