Nanostructured MCrAlY powders were first produced via cryomilling and sprayed using conventional OEM-approved thermal spray processes and parameters.  All yttria partially stabilized zirconia (YPSZ) top coats were sprayed using APS.  Samples with the nanostructured bond coat showed favourable results against static oxidation and thermal cycling.  Delayed formation of mixed oxide thermally grown oxide (TGO) was clearly evident on the nanostructured NiCrAlY bond coat samples.  As well, dramatic improvements in thermal cycling performance were realized for thermal sprayed TBCs with nanostructured NiCrAlY bond coats, as compared to their conventional counterparts [1-4].  However, the high processing costs associated with the need for liquid nitrogen as an integral part of cryomilling were an obstacle to commercial viability.

n-WERKZ Inc. has recently developed a new powder processing approach that does not use liquid nitrogen to form equi-axed nanostructured metal powders (Figure 1).  The avoidance of liquid nitrogen, which has until now been an essential ingredient in processing similar nanostructured metal powders, is crucial, since the high cost of liquid nitrogen is generally cost-prohibitive for most commercial applications.  In addition, the new approach offers the advantage of reduced processing time as compared to cryomilling.

For the preliminary testing of the non-cryogenically milled (NCM) NiCrAlY powder, all bond coats were sprayed using HVOF and all YPSZ top coats using APS.  The coatings did not undergo vacuum heat treatment prior to testing.  Characterization and analysis of the failed thermal cycle coupons were carried out by the United States Naval Academy. The preliminary results from static oxidation and thermal cycling tests look very promising.  As per the coatings derived from cryomilled powder, compared to thermal spray coatings using conventional NiCrAlY bond coats, the nanostructured bond coat samples showed delayed formation of mixed oxide thermally grown oxide (TGO) [5, 6] and the TBC samples showed 50% improvement in thermal cycles to failure (Figure 2).  The rapid formation of a continuous and dense alpha-alumina TGO layer seems to play a major role in the enhanced thermal properties observed in samples with NiCrAlY bond coats.   

Studies have been carried out at McGill University on the processing and testing of nanostructured NiCoCrAlY for petrochemical applications [7, 8]. The results from their study parallel the findings of cryomilled and NCM NiCrAlY.

n-WERKZ has recently signed an agreement with a European coatings company to optimize, evaluate, and apply specified nanostructured MCrAlYs for various industrial gas turbine engine manufacturers.

REFERENCES

1.   G.E. Kim, Proven and Promising Applications of Thermal Sprayed Nanostructured Coatings, proceedings of the International Thermal Spray Conference, Seattle, USA, May 15-18, 2006

2.   J.M. Schoenung, F. Tang, L. Ajdelsztajn, G.E. Kim, V. Provenzano, Processing and Characterization of Thermal Barrier Coatings with Cryomilled Bond Coats, Materials Forum, Vol 29, p 414-419, 2005

3.   L. Ajdelsztajn, J.A. Picas, G.E. Kim, F.L. Bastian, J. Schoenung, V. Provenzano, Oxidation behavior of HVOF sprayed nanocrystalline NiCrAlY powder, Materials Science and Engineering, A338, p 33-43, 2002

4.   G.E. Kim, J.M. Schoenung, V. Provenzano, E.J. Lavernia, L. Ajdelsztajn, Patent No. US 7,361,386 B2 – FUNCTIONAL COATINGS FOR THE REDUCTION OF OXYGEN PERMEATION AND STRESS AND METHOD OF FORMING THE SAME, April 22, 2008

5.   G.E. Kim, T. Addona, P. Richer, B. Jodoin, A. Al-Mathami, M. Brochu, Characterization and Evaluation of Nanostructured Bond Coats from Non-cryogenically Milled Feedstock”, proceedings of the International Thermal Spray Conference, Beijing, China, May 14-16, 2007

6.   G.E. Kim, M. Brochu, A. Moran, T. Addona, Enhanced TBC Performance with Nanostructured Bond Coats, proceedings of the International Thermal Spray Conference, Las Vegas, Nevada, USA, May 4-7, 2009

7.   D. Mercier, G.E. Kim, M. Brochu, ‘Surface Oxide Selectivity of Nanostructured CoNiCrAlY and NiCoCrAlY Materials’, oral presentation at 2009 TMS Annual Meeting & Exhibition, February 15-19, 2009: San Francisco, CA.

8.   C. Kaplin, D. Mercier, M. Brochu, ‘Nanostructured MCrAlY Coatings for High Temperature Oxidation in Petrochemical Applications’, oral presentation at Materials Science & Technology 2009, October 25-29, 2009: Pittsburgh, PA.

George E. Kim, Ph.D.

F.W. Gartner

Perpetual Technologies, Inc.

email: gkim@perpetualtech.ca