Numerous thermal spray coatings are applied onto gas turbine engine components to enhance performance and efficiency, and to extend life.  Depending on the component, specific coatings are applied to protect against wear, oxidation, hot-corrosion, as well as to provide superior seals for enhanced fuel efficiency.  The following are examples of nanostructured coatings that are at various stages of development and may find applications in gas turbine engines.  To date, the developments have been mainly on refinement of the coating structure of existing materials and not on the development of new coating material chemistries.

Previous blogs [1, 2] provide descriptions of the development of nanostructured MCrAlYs and yttria partially stabilized zirconia (YPSZ) coatings for high temperature applications, including thermal barrier and abradable coatings.  The quick formation of a dense α-alumina TGO layer that mitigates formation of mixed oxides observed in nanostructured MCrAlYs may prove to be beneficial in extending the life of hot-section components such as cylinder liners, transition pieces, and exhaust ports.  The same components may benefit from the superior resistance to sintering, lower thermal conductivity, and higher strain tolerance reported for solution precursor plasma spray (SPPS) n-YPSZ top coats.  Thermal spray coatings have been widely used to minimize gaps (to provide seal) between rotating and stationary components in a gas turbine engine.  A new APS-applied n-YPSZ coating, with controlled nanostructure phase and porosity, shows promise for high temperature abradable applications.

Gas turbine engines tend to generate cyclic loading that leads to wear of its components.  Wear resistant coatings such as carbide cermets, CoMoCrSi, Cr2O3-Co, and CuNiIn are thermal sprayed onto turbine engine components such as dovetail interlocks, discs, shafts, casings and nozzle wear pads.  Nanostructured WC-Co coating with high-retention of WC nanoparticles possess very high abrasion resistance [3] and high hardness [4].  Work carried out at UC, Irvine [5] on cryomilling conventional CrC-NiCr powder and spraying with HVOF led to a coating with superior hardness and fracture toughness.  An added benefit with nanostructured carbide coatings relates to their surface finishing.  Zhang et al. [6] have shown that nanostructured WC-12Co coatings require larger grinding force due to their higher wear resistance; however, the surface finish is better than their conventional counterpart coatings.  In addition, the better surface finish of the nanostructured coating was obtained at a reduced depth of cut.  It is important to note that surface finishing can represent a significant cost for the overall coating and finishing process. 

Oxide coatings are also used to protect components such as knife edge tips and face of labyrinth seal fins and seal teeth, as well as blade tips or outer airfoil section of compressor blades.  Although the main role of these coatings is to protect against wear, they can also mitigate overheating of a component.  The superior adhesion strength, wear resistance, and fracture toughness characteristics [7] observed in nanostructured oxide coatings may provide superior performance for these components. 

With the mounting pressure to ban hard chrome electroplating, more and more components of gas turbine engines may be protected/rebuilt using thermal spray and possibly nanostructured coatings.

REFERENCES

1      G.E. Kim, Nanostructured MCrAlYs, available at: http://blog.fwgts.com/blog/2009/11/23/nanostructured-mcralys.html, 2009.

2      G.E. Kim, Applications for Nanostructured Thermal Spray Coatings, available at: http://blog.fwgts.com/blog/2010/1/7/applications-for-thermal-spray-nanostructured-coatings.html, 2010.

3      G.E. Kim, Nanostructured Coatings and Their Benefits for Wear Applications, available at: http://blog.fwgts.com/blog/2010/1/13/nanostructured-coatings-and-their-benefits-for-wear-applicat.html, 2010.

4      B. Zha, H. Wang, X.S. Xian, Nano Structured WC-12Co Coatings Sprayed by HVO/AF, Thermal Spray 2004: Advances in Technology and Applications.

5      J. He, M. Ice, E.J. Lavernia, Synthesis of nanostructured Cr₃C₂-25(Ni20Cr) coatings, Metallurgical and Material Transactions A, Vol. 31, No. 2.

6      B. Zhang, X.B. Liu, An Investigation of Microgrinding of Nanostructured Material Coatings, Presented at UEF Conference on Novel Synthesis and Processing of Nanostructured Coatings for Protection Against Degradation, Davos, Switzerland, 2001.

7      G.E. Kim, Why Thermal Spray Nanostructured Ceramic Coatings are so Tough, available at: http://blog.fwgts.com/blog/2009/9/24/why-thermal-spray-nanostructured-ceramic-coatings-are-so-tou.html, 2009.

George E. Kim, Ph.D.

F.W. Gartner

Perpetual Technologies, Inc.

email: gkim@perpetualtech.ca

Thermal spray nanostructured ceramic coatings have proven to possess characteristics that are favorable for wear resistance applications.  A unique characteristic of improved toughness without compromising on hardness has led to dramatic improvements in abrasion-resistance, as well as low- and high-impingement angle slurry erosion [1-3].  An added advantage of these novel ceramic coatings relates to the reduced surface finishing time required to attain target smoothness (reductions of 40 % have been observed). The time and effort saved in surface finishing translates to notable cost savings.  There are currently thousands of military and industrial components that are benefiting from nanostructured coatings. 

Examples of military applications include wear and corrosion resistant nanostructured ceramic coatings for ship bearing journals, submarine ball valves, and ship reduction gear in air conditioner unit [4].  The United States Navy is saving tens of millions of dollars annually from the use of these coatings.

Thousands of ball valves and numerous stirring blades in high pressure acid leach and pressure oxidation facilities around the world are coated with nanostructured ceramic coating [5].  These components are exposed to sliding, abrasion, and slurry erosion conditions, all within a very corrosive environment.

Initial research on the development of a thermal spray nanostructured WC-base cermet coating was not successful due to the high level of carbide degradation.  However, recent work carried out by FW Gartner and Perpetual Technologies on depositing nanostructured WC-Co have led to a higher retention of WC nanoparticles within the coating.  Samples of these coatings were tested at Syncrude Canada’s research laboratory.  The very promising result is shown in the figure below where the red column represents the weight loss of the latest FWG-PT nanostructured WC-12Co coating against three-body abrasion compared to those of other thermal sprayed WC-12Co coatings in Syncrude Canada’s database.  In fact, the wear loss is low enough to be approaching those found in welded coatings such as plasma transferred arc (PTA) process.

FW Gartner and Perpetual Technologies are continuing their effort towards further enhancing the performance of nanostructured carbide-base cermet coatings.  Several examples of target applications include offshore gate valves, bitumen pump impellers, power valves, and slat and flap tracks.

George E. Kim, Ph.D.

F.W. Gartner

Perpetual Technologies, Inc.

email: gkim@perpetualtech.ca