Journal of Thermal Spray Technology

, Volume 21, Issue 1, pp 23–40 | Cite as

Residual Strain and Fracture Response of Al2O3 Coatings Deposited via APS and HVOF Techniques

  • R. AhmedEmail author
  • N. H. Faisal
  • A. M. Paradowska
  • M. E. Fitzpatrick
Peer Reviewed


The aim of this investigation was to nondestructively evaluate the residual stress profile in two commercially available alumina/substrate coating systems and relate residual stress changes with the fracture response. Neutron diffraction, due to its high penetration depth, was used to measure residual strain in conventional air plasma-sprayed (APS) and finer powder high velocity oxy-fuel (HVOF (θ-gun))-sprayed Al2O3 coating/substrate systems. The purpose of this comparison was to ascertain if finer powder Al2O3 coatings deposited via θ-gun can provide improved residual stress and fracture response in comparison to conventional APS coatings. To obtain a through thickness residual strain profile with high resolution, a partially submerged beam was used for measurements near the coating surface, and a beam submerged in the coating and substrate materials near the coating-substrate interface. By using the fast vertical scanning method, with careful leveling of the specimen using theodolites, the coating surface and the coating/substrate interface were located with an accuracy of about 50 μm. The results show that the through thickness residual strain in the APS coating was mainly tensile, whereas the HVOF coating had both compressive and tensile residual strains. Further analysis interlinking Vickers indentation fracture behavior using acoustic emission (AE) was conducted. The microstructural differences along with the nature and magnitude of the residual strain fields had a direct effect on the fracture response of the two coatings during the indentation process.


acoustic emission alumina fracture toughness indentation neutron diffraction residual stress thermal spray coating 



Average Vickers indentation half-diagonal size


Ratio of signal gain


Average radial crack length c = l a + a

\( d_{hkl} \)

Lattice interplanar spacing

\( d_{hkl}^{0} \)

Strain-free lattice interplanar spacing


Acoustic emission energy


Elastic modulus of specimen


Elastic modulus of coating and substrate


Reduced elastic modulus


Fracture toughness


AE-based empirical constant


Total surface crack length


Average surface-radial crack length


Indentation load


Center of specimen


Distance from center of specimen


Ring-down count


Surface roughness


Final penetration depth


Maximum penetration depth


Original length


Event duration


Temperature change




Output signal amplitude


Input signal amplitude

\( V_{\text{abs}} \)

Absolute voltage

\( V_{\text{t}} \)

Threshold voltage


Change in length

Greek Symbols


Phase of material composition


Coefficient of thermal expansion


Phase of material composition


Elastic strain




Poisson’s ratio









Lattice planes












Specimen or surface



Acoustic emission


Air plasma spray


Through thickness coating section


Coating surface


Coefficient of thermal expansion




High-velocity oxygen fuel


Substrate section near surface


Environmental scanning electron microscopy


X-ray diffraction



The authors acknowledge the provision of beam time at the STFC ISIS Facility, (experiment number RB810413) for the neutron diffraction measurements. Thanks are due to J. Kitamura and S. Osawa, Thermal Spray Materials Department, Fujimi Incorporated, Japan for thermal spraying the specimens. MEF is supported by a grant through The Open University from The Lloyd’s Register Educational Trust, an independent charity working to achieve advances in transportation, science, engineering and technology education, training and research worldwide for the benefit of all.


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Copyright information

© ASM International 2011

Authors and Affiliations

  • R. Ahmed
    • 1
    • 2
    Email author
  • N. H. Faisal
    • 1
    • 2
  • A. M. Paradowska
    • 3
  • M. E. Fitzpatrick
    • 4
  1. 1.School of EPSHeriot-Watt UniversityEdinburghUK
  2. 2.College of EngineeringAlfaisal UniversityRiyadhKingdom of Saudi Arabia
  3. 3.Rutherford Appleton LaboratoryISISDidcotUK
  4. 4.Materials EngineeringThe Open UniversityMilton KeynesUK

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