SYNCHROTRON ANALYSIS OF THERMAL SPRAY NANOSTRUCTURED COATINGS

I wish to thank Professors Thomas Tsakalakos and E. Koray Akdogan, at Rutgers University, for providing much of the material used for this blog.  The group at Rutgers has been using the synchrotron at the National Synchrotron Light Source (NSLS) located in Brookhaven National Laboratory (BNL) to characterize nanostructured materials in both bulk and coating form.  Through the energy dispersive X-ray diffraction (EDXRD) method and the synchrotron light source, measurement of phase composition and strain can be deduced from its 3D-mapping capabilities. [i]  Conventional X-ray and neutron scattering have been used to provide strain profiling of materials as a function of depth only; however, due to the high intensity, its high parallelism, and its simultaneous collection of many Bragg lines, the synchrotron emerges as a promising means of determining residual stress in 3D.  A schematic illustrating the comparison between conventional and Rutgers transmission-mode EDXRD techniques is presented in Fig. 1. [ii]

An interesting general finding from synchrotron EDXRD on as-received and grit blasted titanium substrates is presented in Fig. 2. [iii] 

 It can be noted that grit blasting instills a compressive residual stress due to the peening effect of the impinging grit particles.  This compressive stress introduces a bending moment in the other underlying bulk material which is clearly visible by the sloping data between 0<x3<1.5.

One of the materials considered by Tsakalakos and his group include the characterization of thermal spray nanostructured coatings such as titanium oxide.  A 3-D phase map of a nanostructured TiO2 coating by EDXRD as a function of depth used for quantitative phase analysis and strain measurement as a function of depth is presented in Fig. 3. ⁱⁱ

Some of the key results are as follows:

• ·      the coating consisted of rutile and anatase phases (FWG’s n-TiO2 was predominantly rutile)
• ·      the phase composition of the coating varied from the coating surface to coating/substrate interface, with higher rutile phase closer to the interface
• ·      both starting nanostructured feedstock powder and coating stoichiometries werere in a slightly reducing state, i.e., Ti_2-δ. [iv]

Very revealing data on the advantages of nanostructured coatings was attained from studying the response of conventional and nanostructured Al2O3-TiO2 coatings to tensile and compressive stresses. [v] The nanostructured coating had higher Young’s modulus in tension (~ 300 GPa vs. ~ 260 GPa) and higher crack initiation load (40 lbs vs. > 0 lbs in tension and 450 lbs vs. 300-400 lbs in compression).  These results are in line with prior observations of thermal sprayed nanostructured oxide coatings.

Prof. Akdogan summarizes the results as, “Nanostructured coatings have nanograin which causes crack deflection and also contributed to stoppage or slowing down of crack growth which manifests itself as an enhancement in mechanical properties. Also, they appear to be more uniform as compared to micro-structured coatings - in other words, the flaw size in such coatings is inherently smaller as compared to micro-coatings”.  

REFERENCES

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[i] N.I. Ahmedi, Nondestructive Evaluation of Residual Stresses in Nanostructured Coatings by Synchrotron Radiation, Master of Science Thesis – Rutgers University, UMI 1474656, ProQuest, January 2010

 [ii] E.K. Akdogan, L. Balarinni, Z. Zhong, A. Ignatov, M. Croft, T. Tsakalakos, 3D Quantitative Phase Analysis and Strain Mapping of Nanostructured Coatings by Synchrotron Energy X-ray Diffraction, Proceedings of the Twentieth (2010) International Offshore and Polar Engineering Conference, Beijing, China, June 20-25, 2010

 [iii] T. Tsakalakos, M. Croft, Z. Zhong, C-C Kao, N. Jisrawi, J. Skaritka, T. Fast, R. Sadangi, V. Shukla, Strain Mapping in Nanostructured Coatings by Synchrotron Probe, Oral Presentation at International Offshore and Polar Engineering Conference 2008, Vancouver, Canada

 [iv] T. Tsakalakos, M. Croft, Z. Zhong, C-C Kao, N. Jisrawi, J. Skaritka, T. Fast, R. Sadangi, V. Shukla, Strain Mapping in Nanostructured Coatings by Synchrotron Probe, Oral Presentation at International Thermal Spray Conference 2009, Las Vegas, USA

 [v] E.K. Akdogan, L. Balarinni, Z. Zhong, A. Ignatov, M. Croft, T. Tsakalakos, 3D Quantitative Phase Analysis and Strain Mapping of Nanostructured Coatings by Synchrotron Energy X-ray Diffraction, Oral presentation at the Twentieth (2010) International Offshore and Polar Engineering Conference, Beijing, China, June 20-25, 2010

George E. Kim, Ph.D.

F.W. Gartner

Perpetual Technologies, Inc.

email: gkim@perpetualtech.ca