Cyclic Oxidation And Hot Corrosion Behavior Of A Zr-doped Pt-modified Aluminide Coating

Pt modified NiAl coating was widely used in the hot-section of advanced gas turbine engines due to its satisfactory oxidation and hot corrosion resistance as well as outstanding mechanical properties.However,different coefficient of thermal expansion(CTE)between the coating and oxide scale often induces significant thermal stress in the oxide scale.Meanwhile,the phase transformation(from NiAl to Ni3Al),caused by Al depletion,is always accompanied by a considerable volume reduction which produces extra stress in the oxide scale.When the accumulated stress is higher than the bonding strength of the oxide scale,cracking and spallation of oxides will occur.In order to further enhance the scale adhesion for NiAl series bond coats,reactive elements(RE)like Zr,Hf and Y were often added to produce a number of favorable effects in improving the oxidation performance.In this study,an ingenious method was used to introduce Zr in Pt modified NiAl coating system.The cyclic oxidation as well as the hot corrosion behavior of the Zr-doped(Ni,Pt)Al coating was evaluated in comparison with Pt-modified NiAl and simple NiAl coating.The effect of Pt,Zr and their interaction on the NiAl coating were discussed.The major findings are listed as follows:(1)A Pt-Zr composite layer,in which Zr particles were either nipped at Pt grain boundaries or wrapped inside Pt grains,was successfully fabricated by co-electronplating method.A uniform suspension with Zr particles in size ranging from 0.1 μm to 10 μm was used as the plating solute.Prior to aluminisation,a vacuum annealing treatment was conducted.The acquired Zr-doped(Ni,Pt)Al coating after aluminisation was composed of an outer(Ni,Pt)Al zone with Zr as the solid solution and an inner interdiffusion zone(IDZ).The average Zr content in the outer(Ni,Pt)Al zone was about 1 at.%(2)During the cyclic oxidation at 1100 ℃ in air,simple NiAl exhibited the highest oxide growth rate,and the spinel of NiCr2O4 appeared after 200 cycles.After 350 cycles,the cracking and spallation has occurred.Compared with simple NiAl,a much slower growth rate and improved oxide scale adhesion was achieved on the(Ni,Pt)Al coating.The first oxide spallation showed up at 400 cycles but it was soon healed by the newly formed oxide.After that,a mono-way weight gain,showed up until the final loss at 800 cycles.(3)For the Zr-doped(Ni,Pt)Al coating,the best oxidation was confirmed among the three coatings during the identical cyclic oxidation test.The steady mass gain continued throughout the cyclic oxidation test and no spinel was detected by X-ray diffraction tests.The oxide scale was adherent to the bond coat with no obvious cracking or spallation.A possible explanation for the enhanced oxidation performance can be attributed to that Zr atoms were precipitated at the grain boundary which acted as a blocker to Al diffusion.As a result,the growth rate of the oxide scale slowed down.On the other hand,Zr was precipitated at the oxide/bond coat interface which intensively inhibited the formation of voids and improved the oxide adhesion consequently.The detailed mechanism of Zr will be investigated in future.(4)The hot corrosion resistance of the before mentioned coatings was evaluated at 850℃ in a mixed salt of Na2SO4 and NaCl(75:25,w/w).The simple NiAl coating specimens showed a straightforward weight loss after 80 h and serious inner oxidation and sulfuration were observed after 200 h.The hot corrosion resistance of NiAl coating was notably improved by the Pt modification.It may be related to Pt could promote the formation of a-A12O3during the initial stage of hot corrosion.Besides,Pt atoms were reported possessing capability of trapping S and alleviating the detrimental effect by sulphur.These resulted in an improved corrosion resistance for the Pt modified NiAl coating.(5)The Zr-doped(Ni,Pt)AI showed the best hot corrosion resistance in terms of not only oxide growth rate but also the extent of internal oxidation/sulfuration.The effect of Zr on hot corrosion resistance of NiAl was similar to that on the cyclic oxidation resistance,where Zr atoms acted as the blocker,to prevent the outward diffusion of A1 and Cr cation,resulting in a much slower oxide growth rate.On the other hand,voids formation in the oxide/bond coat interface was further suppressed by Zr addition,which led to a valid improvement in the oxide adhesion.The detailed mechanism of Zr to hot corrosion will be further investigated as well.