As mentioned in previous blogs, thermal spray nanostructured coatings have shown unique traits that, in many cases, enhance functional properties such as wear resistance and thermal protection. There are however, additional traits that show promise in enhancing the performance of coatings for other applications.

The National Research Council of Canada (NRC) has developed and evaluated the application of nanostructured titanium oxide (n-TiO2) coating for biocompatible implants ^[[i]].  NRC’s n-TiO2 coating possessed higher hardness, greater bond strength, equivalent or higher osteoblast cell proliferation, and higher contact surface between bone and coating, as compared to current commercial coating - hydroxyapatite (HA).  With the increase in average age of the population as well as in the average life-expectancy in North America, any extension in the life and enhancement in the performance of implant coatings will be beneficial to the recipients’ quality of life and budget.

In addition to the developments in nanostructured MCrAlY bond coats, there have been developments on nanostructured yttria partially stabilized zirconia (YPSZ) top coats.  Inframat and UCONN have been working on a Solution Precursor Plasma Spray (SPPS) approach to the deposition of n-YPSZ coatings ^{[[ii], [iii], [iv]]}.  By using the SPPS processing route, thick TBC top coats with varying degrees and sizes of porosity incorporated into its structure have been produced.  By tailoring the microstructure, this group has produced TBC top coats that are more resistant to thermal cycling, possess greater resistance to sintering, have lower thermal conductivity, and are higher strain tolerance, compared to TBC top coats derived from conventional plasma spray and EB-PVD process.

The NRC has studied the use of APS applied n-YPSZ coating, using agglomerated nanoparticles, for high temperature abradable applications in gas turbine engines ^[[v]].  By adjusting the spray parameters, coatings with differing amounts of nanostructured phase and porosities were attained.  Control of these features provided a certain degree of plasticity and reduced mechanical integrity, favorable for abradable coatings.  The n-YPSZ coatings performed well in abradability tests as compared to the commonly used CoNiCrAlY+BN+polyester metal-based abradable coating. In addition, the resulting wear scars showed no signs of chipping or shattering.  The advantage of this approach rests in the ability to attain good abradable characteristics within a more chemically stable coating material.

 Based on reports from numerous researchers that nanocrystalline metals show greater resistance to localized corrosion, i.e., pitting ^{[[vi], [vii]]}, Perpetual Technologies and the US Navy initiated a study on the cold spray application of nanostructured aluminum coatings for cathodic protection of military assets, including vehicles and aircrafts ^[[viii]].  The objective is to mitigate localized corrosion - a major maintenance issue for the Navy.  In recent months, Army Research Laboratory has joined this effort and has successfully cold sprayed dense nanostructured aluminum alloy deposits with no visible oxidation using an economically viable nanostructured powder.  Corrosion tests will be carried out in 2010 at the US Naval Academy.

 Novel characteristics of nanostructured materials/coatings are continually being realized.  As researchers learn more about these characteristics, their contribution to the enhancement of commercial applications will continue to grow.

[i] R. S. Lima, B. R. Marple, H. Li and K. A. Khor, “Biocompatible nanostructured high-velocity oxyfuel sprayed titania coating: Deposition, characterization, and mechanical properties”, Journal of Thermal Spray Technology, Volume 15(4) December 2006, 623-627

[ii]http://www.growthconsulting.frost.com/web/images.nsf/0/2C054DCA6843B1D36525722900253D0D/$File/TI%20Alert.htm

[iii] M. Gell, L. Xie, X. Ma, E.H. Jordan, N.P. Padture, “Highly durable thermal barrier coatings made by the solution precursor plasma spray process”, Surface and Coatings Technology 177 –178 (2004) 97–102

[iv] M. Gell, E.H. Jordan, M. Teicholz, B.M. Cetegen, N.P. Padture, L. Xie, D. Chen, X. Ma, and J. Roth, Journal of Thermal Spray Technology, Volume 17(1) March 2008, 124-13 

[v] R.S. Lima, B.R. Marple, A. Dadouche, W. Dmochowski, B. Liko, “Nanostructured Abradable Coatings for High Temperature Applications”, Proceedings of the 2006 International Thermal Spray Conference, May 15-18, 2006, Seattle, Washington, USA

[vi] M.R. Zamanzad-Ghavidel, K. Raeissi, A. Saatchi, “ The effect of surface morphology on pitting corrosion resistance of Ni nanocrystalline coatings”, Materials Letters 63 (2009) 1807–1809

[vii] M.M. Sharma and C.W. Ziemian, “Pitting and Stress Corrosion Cracking Susceptibility of Nanostructrued Al-Mg Alloys in Natural and Artificial Environments”, Journal of Materials Engineering and Performance, Volume 17(6) December 2008

[viii] http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA509150&Location=U2&doc=GetTRDoc.pdf

George E. Kim, Ph.D.

F.W. Gartner

Perpetual Technologies, Inc.

email: gkim@perpetualtech.ca