Research on BC/TC Interfacial Failure and Modification Mechanism of Metallic Bond Coat for Thermal Barrier Coatings

Author:Yu Chun Tang

Supervisor:niu zuo bao ze bin


Degree Year:2019





Thermal barrier coatings(TBCs)are widely used in advanced turbine engines(aeronautic and land-based)to increase higher inlet temperatures and improve power output efficiency.A typical TBC system is usually consisted of a ceramic top coating(TC,such as yttria-stabilized zirconia,in short YSZ hereafter)and an underlying bond coating(BC,for instance MCrAlY overlay or Pt-modified aluminide coatings).A thermally grown oxide(TGO,primarily a-A1bO3)scale is inevitably formed and connects ceramic top coating with bond coating,its characteristics like pure,continuous,dense and flat microstructure is play an essential role in determining the high-temperature performance and life time of TBCs.Our current work focuses on the BC/TC interfacial failure and modification mechanism of metallic bond coat for TBCs,and the following results have been obtained:Thermal cyclic oxidation test of NiCoCrAlY-8YSZ TBCs with and without Pt modification was evaluated at 1100℃ up to 1000 cycles,aiming to investigate the effect of Pt on formation of TGO and oxidation resistance.Results indicated that a dual layered TGO(consisted of top(Ni,Co)(Cr,Al)2O4 spinel and underlying α-Al2O3)formed at the NiCoCrAlY/8YSZ interface with thickness of 8.4 μm,accompanying with visible cracks in TGO interior.In contrast,a single-layer,continuous and adherentα-Al2O3 scale with thickness of only 5.6 μm was formed at TC/BC interface of Pt-modified NiCoCrAlY-8YSZ TBCs.The Pt-modified NiCoCrAlY favors the exclusive formation of α-Al2O3 and lower TGO growth rate,and thus could effectively improve high-temperature oxidation resistance and extend service life of TBCs.Acidic and basic single-phase β-(Ni,Pt)Al coatings were prepared by the same gaseous phase aluminizing but via the different Pt electroplating(employing acidic and alkaline Pt plating baths,respectively).The acidic(Ni,Pt)Al coating showed inferior oxidation resistance compared to the basic coating in both isothermal and cyclic oxidation tests at 1100℃.The presence of excess S in acidic(Ni,Pt)Al coating resulted in the poor oxidation performance and higher Al2O3 scale spallation tendency,which deteriorated oxide scale adherence by promoting the formation of pores and cracks in Al2O3 scale.Aluminizing of 5 μm-Pt-electroplated specimens without and with Hf particles has been utilized to prepare Hf-free and Hf-doped(Ni,Pt)Al coatings.Cyclic oxidation tests of two type coatings were investigated by exposing 10 min cyclic oxidation at 1100“C for 1000 cycles.Three phases,L10 martensite,B2(p-(Ni,Pt)Al)and L12(γ’-Ni3Al),were identified at room temperature for Hf-free(Ni,Pt)Al coating after 1000 cycles at 1100℃.The degradation of A1 causes the transformation of B2→L12 in Hf-free(Ni,Pt)Al,accompanied with the 36.2%volume shrink,induce the surface undulate.While for the Hf-doped(Ni,Pt)Al coatings,unique L10 martensite with nano-twinned lamellar structure was obtained.The reversible transformation ofl10→ B2,was the principal reaction throughout the Hf-doped(bNi,Pt)Al bond coatings,accompanied with 3.9%volume change during thermal cycling.The effect of micro-element Hf on L10 martensite was sensitive to the creep resistance of(Ni,Pt)Al bond coatings.