# Presentation: 390770

## Aluminide Coatings for Power Generation Applications

**Y. Zhang, Y. Q. Wang and J. P. Stacy**
***Tennessee Technological University***
**B. A. Pint and I. G. Wright**
***Oak Ridge National Laboratory***

**The 20th Annual Conference on Fossil Energy Materials**
**Knoxville, TN**
**June 12-14, 2006**

## Investigate the critical issues associated with aluminide coatings on Fe-base alloys & Ni-based superalloys to develop a comprehensive coating lifetime evaluation approach and to improve long-term coating durability
Task I: Assess long-term interdiffusion between coatings and Fe-base alloys to explore potential benefits of iron aluminide coatings in terms of lifetime and applicable environments
Task II: Develop a type of Pt-enriched +' two-phase coating on Ni-based superalloys and evaluate its cyclic oxidation performance

**Task I:**** Assess long-term interdiffusion between coatings and ****Fe-base alloys**** to explore potential benefits of iron aluminide coatings in terms of lifetime and applicable environments**
**Task II:**** Develop a type of Pt-enriched ********+********' two-phase coating on ****Ni-based superalloys**** and evaluate its cyclic oxidation performance**

**Project Objectives**

## Task I: Why Iron-Aluminide Coatings?

**Improvement of coal-fired power plant efficiency requires increase in steam temperature and pressure**
**Advanced 9%Cr ferritic/martensitic alloys may be creep resistant up to 650°C, but they suffer extensive steam-side oxidation**

**oxide scale < 2 ********m thick**** **

**Aluminide coatings can significantly reduce the oxidation rate of ferritic and austenitic steels in exhaust/steam environment**

**Coated P91, Air + 10%H****2****O, 700********C**

## Research Focus of Task I

**Development of a lifetime model *****(Zhang, Liu, and Pint, submitted to Acta Mater. 2006)***
**Diffusion experiments:**** ****2,000-10,000h in the temperature range of 500-800°C in laboratory air**
**Diffusion modeling**
**COSIM (J. A. Nesbitt, 2000)**
**Heckel’s model (R. W. Heckel, 1977)**
**Pack Cementation Process (above-the-pack)**
**Synthesis of commercial type aluminide coatings with comparable oxidation resistance to CVD model coatings**
**Incorporation of reactive elements such as Hf in the coating**

**Key issues in applications of aluminide coatings on iron-base alloys**
**Base loading use: Al backdiffusion (Al loss to substrate)**
**Peak loading use: mismatch in coefficient of thermal expansion (CTE) between coating and substrate *****(Zhang, Pint, et al. Surf. Coat. Technol. 2004)***

## Very Reproducible, High-Purity CVD Model Coatings for Initial Diffusion & Oxidation Studies

**Al at the coating surface: 26 at.%**
**Cr in the coating: 6 at.%**

**CVD Aluminide Coatings on Fe-9Cr-1Mo**

## Negligible interdiffusion occurred at T ≤ 600°C, while more diffusion was observed at T ≥ 700°C

**The surface Al content of the coating on Fe-9Cr-1Mo decreased from as-coated 30 to 18 at.%**
**No significant change in Cr profile**

**Coatings on Fe-9Cr-1Mo**

## Loss of Al in the iron aluminide coatings was calculated using modified COSIM

**In contrast to overlay coatings with a constant Al profile, diffusion aluminide coatings exhibit a composition gradient**
**Very limited data is available for the concentration-dependent ternary (Fe-Cr-Al) diffusion coefficients of the coating system**

**Approaches in Simulating Al Profiles**

**COSIM (**_***C***_***oating ***_***O***_***xidation & ***_***S***_***ubstrate ***_***I***_***nterdiffusion ***_***M***_***odel*****) was developed to predict ternary concentration profiles associated with oxidation and interdiffusion of NiCrAlY overlay coatings**** *****(J.A. Nesbitt, NASA/TM, 2000-209271)***

## Reasonable Agreement Between the Simulated 

**and Experimentally-Measured Al Profiles **

**The Al profiles calculated using concentration-dependent diffusivities of the binary Fe-Al showed the best overall fit**
**Concentration-independent D (at 24%Al) overestimated interdiffusion**

**Coating on Fe-9Cr-1Mo**
**Diffusion at 700°C**

**Notes:**
As I pointed out already, the interdiffusion coefficient in Fe-Al is a strong function of Al.  As Al decreases the diffusion coefficient decreases.  This explains why when concentration-independent diffusivities (the diffusivities at 25at.%Al) were used, it over-estimated the interdiffusion.

## The model is based on conservation of Al (the area underneath the as-deposited Al concentration profile, M )
If a parabolic behavior applies for coating growth:

**If a parabolic behavior applies for coating growth:**

**The surface Al after diffusion is:**

***	******X******0****** *****and *****X***** are the coating thicknesses before & after diffusion**
***	******K***** is the parabolic constant for the coating growth **
***	******t *****is the diffusion time**

**Heckel’s model does not take into account the Al that is consumed via oxidation (but oxide is minimal at 650-700°C)**

**Complementary modeling work was ****conducted by using a simple mathematical model**

**Notes:**
This model was successfully applied to aluminide coatings on pure iron. In that coating system, the Al has a linear composition profile, as indicated on this schematic.

## Lifetime Prediction at 700°C for Coated Fe-9Cr-1Mo Based on a Critical Surface Al Content 

|  | COSIM |  |  | Heckel’s Model |
| --- | --- | --- | --- | --- |
| Critical Al at Surface
(at%) | Binary Fe-Al |  | Fe-Al-Cr |  |
|  | Conc.-dependent D | Conc.-independent D | Conc.-independent D |  |
| 20 | 6.8 kh | 5.0 kh | 5.6 kh | 7.5 kh |
| 16 | 23.7 | 11.9 | 13.6 | 17.9 |
| 12 | 67.6 | 25.0 | 28.9 | 40.3 |
| 8 | 187.1 kh | 57.2 kh | 66.4 kh | 104.4 kh |

**40,000 h minimal lifetime for power-generation components**** **
**Acceptable sulfidation resistance requires ~20 at.% Al in cast alloys and coatings****  ****(*****Pint et al., Mater. High Temp., 2001 *****)**
**With 8-9% Cr in the coating, the critical Al content is < 13% for oxidation protection in water vapor**** *****(Pint & Wright, Mater. Sci. Forum, 2004)***
**Increase of Al content in the coating can increase its sulfidation resistance, but would worsen the CTE mismatch**

**Notes:**
When a higher critical Al content was used, the difference of predicted lifetimes by different methods was relatively small.  However, when we reduced the critical Al level, and when the coating lifetime increased greatly, the difference became more significant.  I’ll discuss more about the coating lifetime when 12%Al was used as the critical Al content in the next viewgraph.

## Al content and coating thickness can betailored with selection of pack composition

**6%AlCl****3****-25%(Cr-Al)-69%Al****2****O****3**** (wt.%), 1050°C, 6h**** **

**Surface Al: ~ 26 ****at.% Al**

**Surface Al: ~ 32 ****at.% Al**

**Masteralloys with higher Al activities lead to thicker aluminide coatings with higher Al content**

**Cr-25 wt.% Al Masteralloy**

**Cr-15 wt.% Al Masteralloy**

## Thermodynamic calculations indicate that Hf can be incorporated into aluminide coatings by various sources

***Pint and Wright, Mater. Sci. Forum, 2004***

**Partial pressures of Hf & Al halide species are within 2 orders of magnitude at 1050****°****C**

**6AlCl****3****-25(Cr-25Al)-****3Hf/HfO****2****/HfCl****4****-**
**66Al****2****O****3**** (wt.%)**

**Beneficial Effect of**
**Hf on Cast Fe-Al Alloys**

**Calculation Results Using HSC 5.0**

## Pack aluminide coatings on Fe-9Cr-1Mo showed comparable oxidation behavior to CVD model coatings

**Porous Alumina Foam Box**
**(Porosity: 85%)**

**Pack aluminide coatings on Fe-9Cr-1Mo showed ****comparable oxidation behavior to CVD model coatings**

**Clean pack aluminide coatings were synthesized in a non-contact arrangement**
**Pack coatings with Hf registered very low mass gain after 2500h at 700°C**

**Air + 10 Vol.% H****2****O,**
**700°C**

## Aluminide coating was applied to P92 tensile specimens for creep test 

**Pack conditions: 1050°C, 6h**
**25% (Cr-25Al)-6%AlCl****3****-69% Al****2****O****3 ****(wt.%)**

**25 mm long with **
**gauge section of 2 x 2 (mm****2****)**

**	****Specimens are being creep tested at ORNL at 650********C **

***Dryepondt, Zhang, and Pint, Surf. Coat. Technol., 2006, accepted***

## Task II: Why Pt-Enriched + Bond Coat?

**The constant demand for increased operating temperatures in gas turbine engines has been the driving force for development of more reliable thermal barrier coating (TBC) systems**

**Pt-enriched ********+******** two-phase coatings may offer several advantages over ********-(Ni,Pt)Al bond coat **
**Reduced Al interdiffusion with substrates **
**Higher creep strength (less “rumpling”)**
**Improved metallurgical stability**
**Lower manufacturing cost**

**Hayashi et al., *****Acta Mater.*****, 2005**

**Notes:**
A more rigorous control in terms of coating composition, purity, and microstructure.

## ORNL’s Cyclic Oxidation Results of Pt-containing + and -NiAl Cast Alloys at 1100°C 

**With Pt incorporation, 22 at.% Al was sufficient to form a protective Al****2****O****3**** scale at 1100°C**
**Hf addition of ≤ 0.05 at.% appeared to be more beneficial**

## Research Focus of Task II

_**Simple**_** Pt-enriched ********+******** ****coatings:**** ****electroplating the superalloy substrate with ~7********m of Pt, followed by a diffusion treatment in vacuum for 2h at 1150-1175°C**
***	******(Nagaraj et al., US Patent, 5,427,866, 1995; Rickerby et al., US Patent, 5,667,663*****, *****1997)***
_**Modified**_** Pt-enriched ********+******** ****coatings:**** ****to increase the Al content to ~ 22 at.% by a secondary process — short-term pack aluminization**
**Substrate alloys (at.%)**
**DS René 142: Ni-13Al-8Cr-12Co-2Ta-1.5W-0.9Mo-0.9Re**
**142-C: 0.45 Hf, 38 ppma S**
**142-D: 0.52 Hf, 11 ppma S (0.04 Ti)**
**SX Y-free René N5: Ni-13.5Al-8Cr-7.5Co-2Ta-1.5W-0.9Mo-1.0Re-0.06Hf**
**LS-N5 (1 ppma S) & HS-N5 (7 ppma S)**
**Cyclic oxidation: 1h cycles in air at 1100°C**

## Composition of the Simple + Coating: Ni-16Al-18Pt-7Cr-9Co, in at.% (Ni-5Al-43Pt-4Cr-6Co, wt.%)

**Light phase: Pt-enriched ******** phase; dark phase: Pt-enriched ********phase **
**Al in coating: 16-18 at.%; Al in substrate: ~ 13 at.%**
**Pt was substantially higher than in the ******** coating (5 at.%)**
**Incorporation of refractory metals: 1.7 Ta-1.0W-0.7Mo-0.5 Re (at.%)**

**Zhang et al., *****Surf. Coat. Technol.*****, 2005**** **

## Oxidation Behavior of Simple + Coatings on René 142 and N5 Superalloys

**SX N5 alloys with ********+******** coatings experienced lower mass gains (similar to single-phase ********coatings) than ********+********-coated DS 142 alloys**
**For the same type of superalloy, the scale adhesion was insensitive to the S content in substrate**

**Zhang et al., *****Surf. Coat. Technol.*****, 2006, accepted**** **

## The sulfur introduced during Pt plating might overshadow the S in substrate alloys

**The sulfur introduced during Pt plating ****might overshadow the S in substrate alloys**

**Electroplated Pt on LS ****N5**

**GDMS Depth Profile of S**

**Sulfur introduced in the laboratory Pt plating at TTU was significantly lower than that in the industrial plating process**

## Modified Pt-enriched + Coatings

**Modified Pt-enriched ********+******** ****Coatings**

**Pack cementation: 1050°C**
**2NH****4****Cl-10(Cr-15Al)-88Al****2****O****3 ****(wt.%)**
**2h post-plating diffusion in vacuum at 1175°C**
**0 h aluminization at 1050°C**

**The coating consisted of 4 ********m ******** layer + 25 ********m ********+******** sublayer with ~18 at.% Al**

## Al activity in the pack process needs to be reduced to avoid  formation

**The preliminary results suggest that the Al activity in the pack process was too high**
**To reduce the Al activity in pack aluminization:**
**Binary masteralloys with lower Al**
**	*****Bianco and Rapp, J. Electrochem. Soc., 1993***
**More stable activators, such as NaCl or NaBr**
**	*****Levine and Caves, J. Electrochem. Soc., 1974***
**Increasing the distance between the specimen and powder mixture**

**2NaCl-10(Cr-15Al)-88Al****2****O****3**** (in wt.%), 30 min @ 1050****°****C**
**with 2mm thick porous alumina disks on each side of specimen**

## Al at the coating surface was decreased to ~ 22 at.% using NaCl activator

**XRD: ********+******** two phases**
**SEM/EDS at the coating surface: **
**	****Ni-****22Al-23Pt****-2Cr-6Co, in at.%**

**2NaCl-10(Cr-15Al)-88Al****2****O****3**** **
**30 min at 1050****°****C, on René 142-C**

## Summary

**A lifetime model was developed focusing on interdiffusion between aluminide coatings and ferritic & austenitic substrates at 500-800°C **
**For steam/wet air environment with the requirement of 8 at.% Al, a goal of 40,000h lifetime could be easily met; the coating lifetime at 700°C for sulfidizing condition (20% Al) is inadequate**
**Commercially viable pack aluminide coatings were synthesized at TTU with more processing flexibility **
**Pack coatings showed comparable oxidation performance as CVD coatings in water-vapor environment**
**Hf-modified pack coatings exhibited reduced scale growth rate**

**Simple Pt-enriched ********+******** coatings with 16-18 at.% Al showed better oxidation performance at 1100°C on SX N5 than on DS 142 alloys**
**A modified Pt-enriched ********+******** coating with ~ 22 at.% Al was synthesized by using a condensed activator**

## Task I: Aluminide coatings on Fe-based alloys 
Extend the lifetime model to more commercially viable pack aluminide coatings
Apply the pack aluminide coatings to large-sized specimens for power plant testing
Task II: Aluminide coatings on Ni-based superalloys
Optimize the composition of the modified Pt-enriched + two-phase coatings (incorporation of reactive elements) 
Evaluate coating oxidation performance and compare with commercial single-phase -(Ni,Pt)Al coatings

**Extend the lifetime model to more commercially viable pack aluminide coatings**
**Apply the pack aluminide coatings to large-sized specimens for power plant testing**
**Task II: Aluminide coatings on Ni-based superalloys**
**Optimize the composition of the modified Pt-enriched ********+******** two-phase coatings (incorporation of reactive elements) **
**Evaluate coating oxidation performance and compare with commercial single-phase ********-(Ni,Pt)Al coatings**

**Future Work**

## Acknowledgments

**J. A. Nesbitt, NASA Glenn Research Center **
**K. M. Cooley, J. L. Moser, S. Dryepondt, G. W. Garner, and L. R. Walker, ORNL**
**W. E. Hawkins and S. J. Geng, TTU**
**U.S. Department of Energy, Fossil Energy Advanced Research and Technology Development Materials Program, under contract DE-AC05-00OR22725 with UT-Battelle, LLC and subcontract 4000032193 with Tennessee Technological University**
**National Science Foundation-GOALI Program under Grant No. 0504566, Co-PI Dr. Ben Nagaraj (GEAE)**