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Research Papers: Gas Turbines: Turbomachinery

Phenomena of Foreign Object Damage by Spherical Projectiles in EB-PVD Thermal Barrier Coatings of Turbine Airfoils

[+] Author and Article Information
Sung R. Choi

Naval Air Systems Command,
Patuxent River, MD 20670
e-mail: sung.choi1@navy.mil

Jennifer M. Wright, D. Calvin Faucett, Matthew Ayre

Naval Air Systems Command,
Patuxent River, MD 20670

Note an extensive number of the data points was generated for the HPT blades, which amounted to a total of more than 140.

The steel ball projectiles that were retrieved after the FOD experiments revealed little plastic deformation, even at the highest impact velocity of 300 m/s.

There might be tensile stress(es) occurring at the topcoat-and-bondcoat interface when traveling compressive stress wave reflects off at the interface.

1Corresponding author.

Contributed by the Turbomachinery Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received February 17, 2014; final manuscript received March 14, 2014; published online May 9, 2014. Editor: David Wisler.

This material is declared a work of the US Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.

J. Eng. Gas Turbines Power 136(10), 102603 (May 09, 2014) (9 pages) Paper No: GTP-14-1107; doi: 10.1115/1.4027362 History: Received February 17, 2014; Revised March 14, 2014

Thermal barrier coatings (TBCs), attributed to their inherent brittleness, are vulnerable to damage by impacting foreign objects when kinetic energy of the objects surpasses certain limits. The damage is termed foreign object damage (FOD) and results in various issues to coatings as well as to substrates, from plastic impression to delamination to spallation to cracking, depending on the severity of impact. The FOD experiments were conducted utilizing a ballistic impact gun for vane airfoil components coated with 220 μm-thick, 7% yttria–stabilized zirconia (7YSZ) by electron beam physical vapor deposit (EB-PVD). The testing was performed with impact velocities ranging from 150 m/s to Mach 1 using 1.6-mm hardened chrome-steel ball projectiles. The resulting FOD was in the forms of impact impressions, cone cracking, and delamination of the coatings/substrates. Prediction of delamination crack size as a function of impact velocity was made based on an energy-balance approach through a quasistatic, first-order approximation. The prediction was in reasonable agreement with experimental data considering a presumable compaction of the TBCs upon impact.

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Figures

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Fig. 1

Examples of FOD in EB-PVD TBCs/substrates of hot-section airfoils in aeroengines (U.S. Navy): (a) micro-FOD in TBCs by a miniature particle, probably by a molten metallic droplet; (b) intermediate FOD probably by a pointed particle causing deformation and compaction of TBCs/substrate; and (c) significant FOD by big metallic particles (>5 mm in size)

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Fig. 2

A schematic indicating the locations of impact in hot-section vane airfoils employed in FOD testing

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Fig. 3

Typical appearances of external impact damages occurring in EV-PVD TBCs of hot-section vane airfoils by 1.6-mm steel ball projectiles impacted at: (a) 200 m/s and (b) 300 m/s. The inset in (b) represents details of region “A”.

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Fig. 4

Impact impression damage size (diameter and depth) as a function of impact velocity in EV-PVD TBCs of hot-section vane airfoils impacted by 1.6-mm steel ball projectiles. The lines are arbitrary for clarity of data. The error bars represent ±1.0 standard deviation.

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Fig. 5

Impact damage size as a function of impact velocity in EV-PVD TBCs of HPT blades impacted by 1.6-mm steel ball projectiles obtained from a previous study [11,12]. The damage at 300 m/s was in a form of spallation of TBCs originating from impact sites. The “EH” represents so-called “engine hours,” a logistical term.

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Fig. 6

Weibull distribution of impact damages in 220 -μm-thick EV-PVD TBCs of hot-section vane airfoils by 1.6-mm steel ball projectiles impacted at 150–300 m/s, showing a unimodal distribution. The previous obtained data on 110 -μm-thick EV-PVD HPT blades [11,12] are included for comparison, which show a bimodal distribution characterized with the lower and higher probability regions.

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Fig. 7

A typical cross-sectional view of impact damage in EV-PVD TBCs of hot-section vane airfoil by 1.6-mm steel ball projectile impacted at 150 m/s. (a) Overall view showing delamination of TBCs; (b) details of region A showing cone cracking or “shear band”; and (c) top- and bondcoat interfaces showing delamination crack and TGO layer. “TC”: topcoat; “BC”: bondcoat.

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Fig. 8

Examples of fracture surfaces showing occurrence of cone cracking when impacted by 1.6-mm steel ball projectiles at 300–400 m/s for: (a) SiC/SiC ceramic matrix composite (CMC) [14]; and (b) silicon nitride [24]

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Fig. 9

Delamination size (a) as a function of impact velocity (V) determined from 220 -μm-thick EV-PVD TBCs of hot-section vane airfoils impacted by 1.6-mm steel ball projectiles. The line is arbitrary for clarity of data.

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Fig. 10

Schematics of an impact event and its mechanical responses used in the quasi-static, first-order prediction of delamination crack of EB-PVD TBCs in response to impact by 1.6-mm steel ball projectiles. (a) Overall impact event; (b) responses of projectile and coatings; and (c) unibiaxial state of stresses in the coatings.

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Fig. 13

Results of prediction of delamination crack size as a function of impact velocity for EV-PVD TBCs of hot-section vane airfoils impacted by 1.6-mm steel ball projectiles. The lines represent predictions at three different levels of compaction factors of ξ = 1.0, 0.8, and 0.6. The experimental data (Fig. 9) are included for comparison.

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Fig. 12

Indent area (Ain) as a function of indent force (Pin) in EV-PVD TBCs of a hot-section vane airfoil determined via static indentation with 1.6-mm steel ball projectiles. The error bars represent ±1.0 standard deviation. The line indicates the best functional fit. The “contact yield stress” of TBCs, py, corresponds to an inverse of the slope of the best-fit line.

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Fig. 11

Determinations of curve fittings from the data of impression damage size as a function of impact velocity. The best-fit line for the damage diameter (dp) was determined by a polynomial regression analysis, while the line for the damage depth (zp) was calculated based on Eq. (3) together with the best-fit line for dp (Eq. (14)).

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