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TECHNICAL PAPERS

Residual Strain Measurements in Thermal Spray Cermet Coatings via Neutron Diffraction

[+] Author and Article Information
R. Ahmed1

School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UKr.ahmed@hw.ac.uk

H. Yu, S. Stewart

School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK

L. Edwards

Department of Materials Engineering, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK

J. R. Santisteban

Rutherford Appleton Laboratory, ISIS, Didcot, OX11 0QX, UK

1

Corresponding author.

J. Tribol 129(2), 411-418 (Jan 09, 2007) (8 pages) doi:10.1115/1.2647503 History: Received March 15, 2006; Revised January 09, 2007

The impact and fatigue resistance of overlay coatings is significantly influenced by the residual strain (or stress) field induced during coating deposition, post-treatment, and in-service loading. Optimization of the residual strain field is therefore critical to the life and performance of components. Nondestructive measurement of these strain fields in relatively thin (300400μm) thermal spray coatings, however, poses a challenge because conventional techniques, such as deep hole drilling, x-ray diffraction, synchrotron diffraction, and changes in beam curvature either make these techniques destructive and/or provides only a very near-surface strain measurement. This particularly complicates the strain analysis in cermet coatings, e.g., WC-Co deposited by the thermal spraying process, where the low penetration depth of x-ray and synchrotron-diffraction ray can only provide a through thickness measurement of stress or strain profile via the destructive layer removal technique. Recent investigations have therefore concentrated on the use of neutron diffraction technique for such analysis, and this paper reports some of the early findings of the comparison of through thickness strain measurements in relatively thin (400μm) as-sprayed and post-treated WC-12wt.%Co coatings via the neutron diffraction technique. Since neutrons are not charged, they do not interact with the electron cloud surrounding the atom (unlike x-ray); hence, diffraction results from the interaction with the atomic nucleus. Neutrons therefore have greater penetration depth in most engineering materials, and therefore provide a nondestructive through thickness strain measurement. Results of strain measurement are discussed with the structure property relationships and contact fatigue performance, and indicate that post-treatment of these coatings results in harmonization of the strain field within the coating, and at the coating substrate interface. This significantly influences the contact fatigue performance by improving both the cohesive and adhesive strength of these coatings.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Factors influencing the generation of residual stress in thermal spray coatings

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Figure 2

Horizontal scan of the coated sample relative to the fixed gage volume in reflection and transmission mode to measure strain. Cross indicates the center of gravity of gage volume.

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Figure 3

Vertical (z) scan used for neutron diffraction measurements for in-plane strain. 2θ is fixed at 90deg for both detectors. Strain is averaged from the two detectors. Cross indicates the center of gravity of gauge volume

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Figure 4

Schematic illustration of the cup assembly for rolling contact fatigue tests

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Figure 5

Microstructure of 400μm thick coatings: (a) as-sprayed coating, (b) HIPed coating, and (c) coating/substrate interface after HIPing

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Figure 6

XRD pattern of the coatings: (a) as-sprayed coating and (b) HIPed coating

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Figure 7

Neutron diffraction pattern of the 20μm diffusion layer formed at the coating/substrate interface after the HIPing post-treatment

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Figure 8

Hardness and indentation modulus measurement results for the as-sprayed and HIPed coatings

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Figure 9

Vertical scan results comparing microstrain and residual stress in 0.4mm thick WC-Co coatings in the as-sprayed and HIPed conditions

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Figure 10

RCF test results for the as-sprayed and HIPed coatings in steel and ceramic ball configurations

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Figure 11

Surface observations of 400μm thick coatings subjected to 2.7GPa contact stress using conventional steel ball contact configuration: (a) spalling in the as sprayed coating and (b) macropitting in the HIPed coating

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Figure 12

Surface observations of 50μm thick coatings subjected to 2.7GPa contact stress using conventional steel ball contact configuration: (a) delamination in the as sprayed coating and (b) surface pitting in the HIPed coating

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