The Use of Acoustic Emission to Characterize Fracture Behavior During Vickers Indentation of HVOF Thermally Sprayed WC-Co Coatings

  • N. H. Faisal
  • J. A. Steel
  • R. AhmedEmail author
  • R. L. Reuben
Peer Reviewed


This paper describes how acoustic emission (AE) measurements can be used to supplement the mechanical information available from an indentation test. It examines the extent to which AE data can be used to replace time-consuming surface crack measurement data for the assessment of fracture toughness of brittle materials. AE is known to be sensitive to fracture events and so it was expected that features derived from the AE data may provide information on the processes (microscale and macroscale fracture events and densification) occurring during indentation. AE data were acquired during indentation tests on samples of a WC-12%Co coating of nominal thickness 300 μm at a variety of indentation loads. The raw AE signals were reduced to three stages and three features per stage, giving nine possible indicators per indentation. Each indicator was compared with the crack profile, measured both conventionally and using a profiling method which gives the total surface crack length around the indent. A selection of the indents was also sectioned in order to make some observations on the subsurface damage. It has been found that reproducible AE signals are generated during indentation involving three distinct stages, associated, respectively, with nonradial cracking, commencement of radial cracking, and continued descent of the indenter. It has been shown that AE can give at least as good a measure of cracking processes during indentation as is possible using crack measurement after indentation.


acoustic emission fracture toughness HVOF surface crack length Vickers indentation WC-12%Co coating 



Vickers indentation size for half diagonal 1 and 2


average Vickers indentation half diagonal size


radial crack length along Vickers indentation diagonal 1 and 2


average radial crack length c = l + a


acoustic emission energy


Vickers hardness


fracture toughness empirical constant


acoustic emission-based fracture toughness


fracture toughness


average surface radial crack length


crack path unit length


total surface crack length


indentation load


acoustic emission ring-down count


average surface roughness


linear correlation coefficient




acoustic emission event duration


individual acoustic emission event duration




absolute voltage


serrated crack path unit length


fracture toughness variable



initial value


final value


resolvable crack length


number of cracks



acoustic emission


high velocity oxygen fuel



The authors would like to thank Deloro Stellite Ltd., UK for supplying the as-sprayed HVOF WC-12%Co coated samples for this study. The authors would also like to express their thanks to Metallurgist, Bill Taylor, at Struers Ltd., UK, for his support in the metallurgical preparation of the cross section of the Vickers indent for subsurface damage assessment.


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Copyright information

© ASM International 2009

Authors and Affiliations

  • N. H. Faisal
    • 1
  • J. A. Steel
    • 1
  • R. Ahmed
    • 1
    Email author
  • R. L. Reuben
    • 1
  1. 1.Department Mechanical Engineering, School of Engineering and Physical SciencesHeriot-Watt UniversityEdinburghUK

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