Journal of Thermal Spray Technology

, Volume 20, Issue 5, pp 1071–1084 | Cite as

AE Monitoring and Analysis of HVOF Thermal Spraying Process

  • N. H. Faisal
  • R. AhmedEmail author
  • R. L. Reuben
  • B. Allcock
Peer Reviewed


This work presents an in situ monitoring of HVOF thermal spraying process through an acoustic emission (AE) technique in an industrial coating chamber. Single layer thermal spraying on substrate was carried out through slits. Continuous multilayer thermal spraying onto the sample without slit was also conducted. The AE was measured using a broadband piezoelectric AE sensor positioned on the back of the substrate. A mathematical model has been developed to determine the total kinetic energy of particles impacting the substrate through slits. Results of this work demonstrate that AE associated with particle impacts can be used for in situ monitoring of coating process. Results also show that the amplitude and AE energy is related to the spray gun transverse speed and the oxy-fuel pressure. The measured AE energy was found to vary with the number of particles impacting the substrate, determined using the mathematical model.


acoustic emission HVOF monitoring multilayer spraying non-destructive testing particle impact thermal spray 



Spray spot area

As (t)

Trigonometric spray spot area function of time


Acoustic emission energy

\( \dot{E}(t) \)

Kinetic energy of powder particles as a function of time


Deposit stiffness


Mass of one powder particle

\( \dot{m} \)

Mass flow rate of powder particle impinging on the target

\( \dot{m}_{\text{powder}} \)

Powder particle mass flow rate

\( \dot{N} \)

Number of particles approaching the slit per second

\( \dot{N}_{\text{S}} (t) \)

Number of particles approaching the slit as a function of time


Powder particle radius


Spray spot radius


Ring-down count




Slit passing (scanning) time


Event duration


Time (for angle of arc subtended at the slit Edge 1)


Lamella melting temperature


Substrate temperature


Average powder particle speed


Lateral speed of HVOF spraying gun


Absolute voltage


Threshold voltage


Slit width


Center of slit

Greek Symbols


Angle of arc subtended at the slit Edge 1 as a function of time


Angle of arc subtended at the slit Edge 2 as a function of time


Density of powder particle


Quenching stress


Deposit coefficient of thermal expansion





Coating deposit


Ring-down count




HVOF gun


Lamella melting


Powder particle






Coating substrate





Analog-to-digital converter


Acoustic emission


Computer numerical control


Data acquisition card


High-velocity oxygen fuel


Infinite impulse response


Physical Acoustics Corporation


Lead zirconate titanate


Root mean square


Signal conditioning unit


Scanning electron microscopy


Signal-to-noise ratio


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Copyright information

© ASM International 2011

Authors and Affiliations

  • N. H. Faisal
    • 1
    • 2
  • R. Ahmed
    • 1
    • 2
    Email author
  • R. L. Reuben
    • 1
  • B. Allcock
    • 3
  1. 1.Department of Mechanical Engineering, School of Engineering and Physical SciencesHeriot-Watt UniversityEdinburghUK
  2. 2.College of EngineeringAlfaisal UniversityRiyadhKingdom of Saudi Arabia
  3. 3.Monitor Coatings Ltd, Monitor HouseTyne & WearUK

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