The document summarizes research on using acoustic emission (AE) monitoring to analyze the high velocity oxygen fuel (HVOF) thermal spraying process. Key points: - AE monitoring can provide insights into particle impact behavior and phase changes during spraying and cooling, important for coating strength. - A kinematic model was developed relating AE energy levels to particle kinetic energy based on spray parameters and slit geometry variables. - AE signals were found to increase with coating buildup over multiple layers due to spray plume widening beyond the theoretical spot size.

1. Application of acoustic emission for monitoring
the HVOF thermal spraying process
N. H. Faisal, J. A. Steel, R. Ahmed, R. L. Reuben, G. Heaton
Dept. of Mechanical Engineering, Heriot-Watt University
Edinburgh, UK, Email: N.H.Faisal@gmail.com
B. Allcock
Monitor Coatings Ltd.
Tyne & Wear, UK
27th European Working Group on Acoustic Emission (EWGAE)
21st September 2006, Cardiff, UK

2. Presentation Structure
• Introduction Aerospace/Turbine
– What is HVOF Thermal Spray Coatings / Applications?
– Why AE Monitoring during Thermal Spraying?
– Quality Control issues in Thermal Spray Coatings
www.ntu.sg
• Experimental Systems and Techniques
Bio-medical/Knee, Hip, Elbow joints
Bio-medical/Knee, Hip, Elbow
• Results
www.bekaert.com
– AE Signal Characteristics
– Development of Kinematic Model of Particle Impact
– Influence of Continuous Multi-layer Thermal Spraying
Multi-
Electronics/Heat sink
• Summary, Conclusions and Future Work
HVOF Spray gun Plasma Spray gun
www.monitorcoatings.co.uk
www.monitorcoatings.co.uk
www.twi.co.uk
Thermal Spray: Industries / Applications

3. HVOF Thermal Spray (TS) Coatings
What is HVOF (High Velocity Oxygen-Fuel) Thermal Spraying process
Coating Flame tempe: Powders velocity: HVOF Thermal
substrate 3000-5000˚C 600-800m/s Spraying system
Spray spot
~ 9-12mm Gas velocity:
AE monitoring system Splat cooling rate: Particle Temp: HVOF nozzle/gun
1500-2000m/s
100-600K/µs 1500-2000˚C
HVOF Coating chamber TAFA JP-5000
Monitor Coatings Ltd. UK
Sources of AE:
• Particle impact
(Kinetic energy / Strain energy released)
• Thermal mismatch, Cracks in layers Noise level in coating chamber ~
• Coating chamber reverberation (noise) 123dB ~ Jet-Engine take-off noise
Why AE monitoring during TS?
WC-10Co-4Cr Powders
• It addresses core technological issues:
Quantifies the partially melted powder particle
landing behaviour & phase changes during
spraying & cooling process – Fundamental to
coatings strength determination
• Monitors quality during the coating formation
• Added advantage over existing quality control
Single WC-10Co-4Cr particle splat After spraying/Partially melted techniques

4. Quality Control issues in TS Coatings
Existing Off-line Techniques (post-spraying process)
Off- post-
e.g., Mechanical Testing procedures: It Requires
‘TEST COUPONS’ (e.g. for Bending, Indentation, Tensile, Bending test
Adhesion, Fatigue, Wear, Thermal Cycling tests)
Existing On-line Techniques (post-spraying process)
On- post-
e.g., Non-Destructive Testing procedures: Ultrasonic,
Radiographic, Electromagnetic, Liquid penetrant, Magnetic
particle, 4-point bending test + AE, Indentation test + AE
There is no ‘on-line’ coatings quality monitoring (i.e., DURING
SPRAYING) available which can quantify the splat landing
behaviour and which can measure the cohesive & adhesive
strengths on ACTUAL COMPONENTS

5. Thermal Spraying / AE measurement
PAC, Micro-80D: Broadband PZT
sensor (0.1-1.0 MHz), Rf = 332kHz
(A) HVOF
System Pre-amplifier: PAC-1220A
Using Slit/mask
TAFA Pre-amp at 40/60 dB,
slit HVOF JP-5000 Gain at SCU = 0 dB
nozzle/gun
12 bit NI, PCI-6115 DAQ, and AE
4-channel system; Sampling rate
2.5MHz/2 sec
CPU, Computer & AE system
(B)
Without Substrate
slit Holder
Coating chamber
Masking-sheet/Substrate Masking-sheet-substrate-
set-up sample holder set-up Masking-sheet (slits) Coating substrate Coating chamber HVOF System,

6. Experimental matrix & Masking-sheet (slit)
Masking-
HVOF Pressure level-P1 Pressure level-P2
Thermal O/F ratio = 1.16 (P1); Oxygen flow rate, 1950 O/F ratio = 1.21 (P2); Oxygen flow rate, 1950
Spray scfh; Powder feed gas, Nitrogen (21 scfh); Fuel scfh; Powder feed gas, Nitrogen (21 scfh);
Parameters flow rate, 6 gallon/min (kerosene); Powder feed Fuel flow rate, 4.5 gallon/min (kerosene);
rate, 80 g/min Powder feed rate, 80 g/min
Slit no. and A B C D A B C D
width (3mm) (2mm) (1mm) (0.5mm) (3mm) (2mm) (1mm) (0.5mm)
Gun
scanning
speed
(mm/sec.)
250 1 1 1 1 1 1 1 1
500 1 1 1 1 1 1 1 1
750 1 1 1 1 1 1 1 1
1000 1 -- -- -- 1 -- -- --
Gun motion Coating Substrate (Mild-Steel)
27 mm Masking-sheet (Mild-Steel)
10mm
A: 3 mm

7. AE Signal Characteristics (Background noise)
I. AE signal during flame spraying only beside slit-substrate without powder
no p owderbesidesample010000.bin -8 Frequency domain:
Coating substrate 0.2 x 10 no powder beside sample010000.bin
8
5, 50, 100, 140kHz
Power spectral density
0.1 6
Ampliture (V)
Coating chamber
0 4 Noise level
-0.1 2
Masking-sheet (slits) -0.2 0
0 0.5 1 1.5 2 0 5 10 15
Time (s) Frequency (Hz) 5
x 10
II. AE signal during flame and powder spraying beside slit-substrate
powderbesidesample010000.bin -8 Frequency domain:
Coating substrate x 10
0.2 4 powder beside sample010000.bin
5, 50, 100, 140kHz
Power spectral density
0.1 3
Ampliture (V)
0 2 Coating chamber
Noise level
-0.1 1
Masking-sheet (slits) -0.2 0
0 0.5 1 1.5 2 0 5 10 15
Time (s) Frequency (Hz) 5
x 10
III. AE signal during flame spraying only on substrate without powder
flamep ass010000.bin -8 Frequency domain:
x 10 flame pass010000.bin
Coating substrate 0.2 4
5, 50, 100, 140kHz
Power spectral density
0.1 3
Ampliture (V)
0 2 Coating chamber
Noise level
-0.1 1
-0.2 0
0 0.5 1 1.5 2 0 5 10 15
Time (s) Frequency (Hz) 5
x 10

8. AE Signal Characteristics
IV. AE signal during full spraying (both flame and powder) at standard spraying pressure P-1
Gun speed: 500mm/sec
Masking-sheet (slit A),
3mm width, 10mm height No. of Slits: 14
Substrate:
Coatings through slit-A
500mm
-7 Frequency domain: hvof12010000.bin
x 10
1.5
5-100kHz (BNG)
SNR ~ 3 to 4 100-200kHz
Power spectral density
300-400kHz
550-650kHz
1 750-850kHz
0.5
BGN
0
0 5 10 15
Frequency (Hz) 5
x 10

9. Development of Kinematic Model (particle impact)
Effective Spraying Area = (1/2) . [R2 {2 (θ-δ) - (Sin 2θ-Sin 2δ)}] = Curve3
Gun Speed, Vg
Phase difference & Area distribution
(3mm slit, 250mm/sec)
Edge-
Edge-1 Edge-2
Edge-
70
Curve1 (Edge-1) area(theta)
60 D D’
area(delta)
50 Slit width, y
Area [mm^2]
effective area
40 Curve3 = [Curve1-Curve2]
D 30
x1 D’ x2
20 Curve2 (Edge-2)
10
Slit centre
0
0 0.01 0.02 0.03 0.04 0.05 0.06
T ime [sec]
Are a distribution (3m m slit, 250m m /se c gun
spe e d)
50
area
40
Curve3 = effective
30
Area
spraying area
20
x1 x2
10
0
0 0.01 0.02 0.03 0.04 0.05 0.06
x1 time [sec] x2

10. Kinematic Model of Particle Impact
K.E of powder particles making impact through effective area is, E
= [(1/2).M.V.V];
hvof11010000.bin E = (1/2) . [N. mp].(R.R/2). [2 (θ-δ) – (Sin 2θ-Sin 2δ)] .V.V
(a)
1 (b) (c) where V is the velocity of sprayed particle
SNR ~ 3 to 4
0.5 Experimental Area Theoretical Area
Theoretical energy [kg.m^2/s^2] per slit
Experimental AE energy [V.s] per slit
Time length = 0.052sec
6000 450
Ampliture (V)
Slit-A: 3mm 400
5000 Slit-B: 2mm
Slit-A: 3mm
Slit-C: 1mm 350 Slit-B: 2mm
Slit-D: 0.5mm
0 4000 300 Slit-C: 1mm
Slit-D: 0.5mm
250
3000
200
2000 150
-0.5 1000
100
50
0 0
0 250 500 750 1000 0 250 500 750 1000
3mm slit width, 250mm/sec gun speed HVOF gun transverse speed [mm/sec] HVOF gun transverse speed [mm/sec]
-1
0 0.5 1 1.5 2
Time (s)
0.6
Kinetic Energy distribution [Kg.m2.s-2]
KE (Slit width: 3mm; Gun Speed: 250mm/sec )
20000
0.4
15000
Ampliture (V)
0.2
Theoretical
10000
0
5000
-0.2
-0.4 0
0.00 0.01 0.02 0.03 0.04 0.05 0.06
Smoothed signal Time of spray gun transversing the slit [seconds]
0.05 0.1 0.15
Time (s)
Experimental Theoretical

11. AE Monitoring During Multilayer Thermal
Transverse Gun Speed
Spraying (without masking slit) Coating builds-up
(cross-section)
Theoretical
spray-spot
~ 9-12mm
Centre-line
Actual
spray-spot
~ 18mm
Actual spray-spot is greater
www.thermalspray.ws than theoretical spray-spot
due to fanning of spray
AE system

12. Influence of Continuous Multilayer
Thermal Spraying
0.00016 AE energy (a.u.)
AE sensor location
AE energy (a.u.) per unit time
0.00014
0.00012
AE sensor location (centre-backside)
0.00010
0.00008
Gun Speed
0.00006
200mm/sec
0.00004
0.00002 1 2 3 4 5 Layers
0.00000
0 50 100 150 200 250 300
No. of data files
-7 Frequency domain: A1010000.sgl
1
A1010000.sgl x 10 [time for 1 data file = 0.004 sec]
5
A1010025.sgl -7 Frequency domain: A1010025.sgl
x 10
1 6
Power spectral density
0.5 4
SNR ~ 4
Ampliture (V)
Power spectral density
3 0.5 Frequency
4 340, 590 & 800kHz
Ampliture (V)
0
2
0
-0.5 Chamber Noise ~ 0.15V 1 2
-0.5
-1 0
0 1 2 3 4 0 1 2 3
Time (s) -3 Frequency (Hz)
-1 6 0
x 10 0 1 x 10 2 3 4 0 1 2 3
Time (s) -3 Frequency (Hz) 6
x 10 x 10

13. Summary
SprAE Monitoring Block Diagram
(Open loop system)
Thermal Spray particle impact
Multilayer Spraying Spraying through slits
AE Signal Thermal Spraying system AE Signal
SprAE monitoring system
Amplitude, Frequency,
Thermal Spray Event (N), Energy (E),
AE Energy Event duration (T)
Process Parameter
distribution Event rate (N/T),
Monitoring Energy rate (E/T)

14. Conclusions
• AE monitoring system developed; High Signal-to-Noise Ratio (3-4) has
Signal-to- (3-
been measured during HVOF
• As gun speed increases values of AE parameters fall through slits
• AE energy increases as the number of layers increase
• Kinematic model has been developed
• Other techniques: Plasma, Detonation and Cold spray coating / AE
Plasma,
Future work • Future experimentation (DOE / Taguchi’s technique) / AE
• Development of post spraying AE tests / Identifying coating
Thermal Spraying system strengths
• Development of Control process during Thermal Spraying / AE
Close loop system • Computational fluid dynamics (CFD) analysis / AE signal
distribution
? • Thermal Spray Nozzle wear rate monitoring using AE
Nozzle
SprAE monitoring SprAE control system
system

15. Thanks!
Tyne & Wear Castle Edinburgh Castle
Cardiff Castle
Any Questions Please?