Fracture Toughness of Thermal Spray Ceramics: Measurement Techniques and Processing Dependence

Abstract

Fracture toughness measurements are critical for materials design and characterization but can be difficult to perform on overlay coatings due to a range of geometric factors and substrate constraints. Thermal spray (TS) coatings bring additional complications to measurement interpretation due to their defected, anisotropic structures. Toughness of free-standing coatings has been studied in the past, and literature results indicate promise in measurement with a range of methods. One of these, single-edge, notched beam (SENB) method offers a straight forward approach for measuring fracture toughness and lends itself well for use with TS coatings. In this work, SENB method is used with deliberate modifications to specific parameters of the test specimens, namely free-standing thickness, notch depth, notch sharpness, and heat treatment state, to ascertain the impact of these modifications on the measurement results for air plasma spray Al2O3. Additionally, two methods adapted from the literature, a modified adhesion method and a tensile elongation method, are evaluated for use with three different Al2O3 coatings, including by air plasma spray, flame spray, and high velocity oxy-fuel processes. Results indicate good correlation between SENB and the modified methods for all three coating variants and give insight into the orientation-dependent toughness properties of TS coatings.