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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

Abstract

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.

Keywords

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

Nomenclature

a1,2

Vickers indentation size for half diagonal 1 and 2

a

average Vickers indentation half diagonal size

c1,2

radial crack length along Vickers indentation diagonal 1 and 2

c

average radial crack length c = l + a

E

acoustic emission energy

HV

Vickers hardness

k1

fracture toughness empirical constant

KAE

acoustic emission-based fracture toughness

K1c

fracture toughness

la

average surface radial crack length

lyn

crack path unit length

L

total surface crack length

P

indentation load

R

acoustic emission ring-down count

Ra

average surface roughness

r

linear correlation coefficient

t

time

T

acoustic emission event duration

Ti

individual acoustic emission event duration

V

voltage

Vabs

absolute voltage

xn

serrated crack path unit length

X

fracture toughness variable

Subscript

i

initial value

N

final value

n

resolvable crack length

y

number of cracks

Abbreviations

AE

acoustic emission

HVOF

high velocity oxygen fuel

Notes

Acknowledgments

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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