Fracture Toughness of Thermal Spray Ceramics: Measurement Techniques and Processing Dependence
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Fracture Toughness of Thermal Spray Ceramics: Measurement Techniques and Processing Dependence Gregory M. Smith1
•
Adam Smith1 • Sanjay Sampath1
Submitted: 12 April 2018 / in revised form: 27 August 2018 ASM International 2018
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 orientationdependent toughness properties of TS coatings. Keywords cracking fracture mechanical properties microstructure tensile bond strength
& Gregory M. Smith [email protected] 1
Center for Thermal Spray Research, Stony Brook University, Stony Brook, NY 11794, USA
Introduction Ceramics, glasses, and ceramic composite materials have been used across various industries and have seen numerous advances with regard to materials versatility, processing, and applications for structural and functional uses (Ref 1). Ceramic materials are notable for their potential high stiffness and strength, but also their lack of plasticity and propensity for brittle failure linked to internal flaws (Ref 2, 3). This can be overcome to some extent through creative material design. There are a range of bio-materials and ceramic composites which use advantageous engineering, orientation/layering, and hierarchical structures to achieve increased damage tolerance (Ref 4, 5). Damage tolerance in this context can be described as the ability to limit or arrest crack propagation despite the presence of internal defects or flaws within the system, often originating from the material’s formation or processing. These defects can manifest as internal crack origination sites or as internal stress concentration points. Measurement of a crack and characterization of its propagation can be inherently difficult. A variety of techniques have been developed for mea
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