Foreign Object Damage (FOD) of Ceramic Thermal Barrier Coatings (TBCs) in Gas Turbine Airfoils

Abstract

Ceramic thermal barrier coatings (TBCs), attributed to their inherent brittle nature, are highly susceptible to damage by impacting foreign particles when the impacting kinetic energy exceeds certain limits. The damage is termed foreign object damage (FOD) in related turbine components and results in various issues/problems to coatings as well as to substrates from delamination to spallation to cracking to catastrophic failure depending on the severity of impact. The FOD testing was performed using a ballistic impact gun for turbine airfoil components coated with 7% yittria stabilized zirconia (7YSZ) by electron beam physical vapor deposit (EB-PVD). A range of impact velocities up to Mach 1 was applied with three different projectile materials of steel, silicon nitride, and glass balls. The damage was assessed and characterized in terms of impact velocity, projectile material, and remaining life of turbine components. An energy-balance approach was made to develop a model to predict delamination of the TBCs upon impact.

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

“…The energy balance in an impact event in TBCs may be expressed as follows [13]: where m is the mass of a projectile, V is the impact velocity, F is the impact force, and Vbc is the bouncing-back velocity of a pro jectile right after impact. The impact impression diameter dp in TBCs is defined in Fig.…”

“…Because of limited availability of the airfoil components, only two vanes were used in the experiments. Rela tively large flat areas of TBCs in the hardware, compared to the HPT blades used in the previous work [11][12][13], enabled to cover all the test matrices intended. Impact Morphologies.…”

“…A series of FOD work has been conducted previously using a number of actual airfoils (high-pressure turbine (HPT) blades) coated with 110 mn-thick EB-PVD TBCs (7YSZ) [11][12][13], The airfoil components were used as targets and were subject to ballis tic impact in a range of velocities up to Mach 1 by 1.6-mm spheri cal ball projectiles of hardened chrome steel, silicon nitride, and borosilicate glass. The impact damage was assessed in terms of impact velocity, projectile material, and remaining life of turbine components.…”

“…The vane airfoils were impacted using similar test condi tions that were applied to the HPT blades [11][12][13] but employing only one projectile material, hardened chrome steel. Assessments of the thick TBCs in response to FOD were made from the impact sites as well as from the cross sections, and their behavior was compared with that of the thin (110 /<m) EB-PVD HPT blades.…”

“…A ballistic impact gun, as described elsewhere [11][12][13][14][15], was used to carry out FOD testing for the EB-PVD TBC-coated vane airfoils. The projectiles used in the testing were commercial, 1.6-mm-diameter spheres of hardened chrome bearing steel (grade 5), and their basic physical and mechanical properties are presented in Table 1.…”