Microstructure and composition evolution of a single-crystal superalloy caused by elements interdiffusion with an overlay NiCrAlY coating on oxidation

MCrAlY(M=Ni and/or Co)overlay coating is widely used as a protective coating against high temperature oxidation and corrosion.However,due to its big difference in chemical composition with the underlying superalloy,elements interdiffusion occurs inevitably.One of the direct results is the formation of interdiffusion zone(IDZ)and secondary reaction zone(SRZ)with a high density of fine topological closed-packed phases(TCPs),weakening dramatically the mechanical properties of the alloy substrate.It is by now the main problem of modern high-temperature metallic coatings,but there are still hardly any reports studying the formation,growth and transformation of IDZ and SRZ in deep,as well as the precipitation of TCPs.In this work,a typical NiCrAlY coating is deposited by arc ion plating on a single-crystal superalloy N5.Elements interdiffusion between them and its relationship on microstructure were clarified.Cr rather than Al from the coating diffuses into the alloy at high temperatures and segregates immediately beneath their interface,contributing largely to the formation of IDZ.Simultaneously,diffusion of Ni from the deep alloy to IDZ leads to the formation and continuous expansion of SRZ.

Influence of Sputtered Nanocrystalline Coating on Oxidation and Hot Corrosion of a Nickel-based Superalloy M951

The isothermal and cyclic oxidation behaviors in air and hot corrosion behaviors in Na2SO4 ＋ 25 wt% K2SO4 salt of M951 cast superalloy and a sputtered nanocrystalline coating of the same material were studied. Scanning electron microscopy, energy dispersive X-ray spectroscope, X-ray diffraction, and transmission electron microscopy were employed to examine the morphologies and phase composition of the M951 alloy and nanocrystalline coating before and after oxidation and hot corrosion. The as-sputtered nanocrystalline layer has a homogeneous y phase structure of very fine grain size （30-200 nm） with the preferential growth texture of （111） parallel to the interface. Adherent AI203 rich oxide scale formed on the cast M951 alloy and its sputtered coating after isothermal oxidation at 900 and 1000 ℃. However, when being isothermal oxidized at 1100℃ and cyclic oxidized at 1000 ℃, the oxide scale formed on the cast alloy was a mixture of NiO, NiAl2O4, Al2O3 and Nb205 and spalled seriously, while that formed on the sputtered coating mainly consisted of Al2O3 and was very adherent. Nanocrystallization promoted rapid formation of Al2O3 scale during the early stage of oxidation and enhanced the adhesion of the oxide scale, thus improved the oxidation resistance of the substrate alloy. Serious corrosion occurred for the cast alloy. The sputtered nanocrystalline coating apparently improved the hot corrosion resistance of the cast alloy in the mixed sulfate by the formation of a continuous Al2O3 and Cr2O3 mixed oxide layer on the surface of the coating, and the pre- oxidation treatment of the coating led to an even better effect.

Modification of NiCoCrAlY with Pt: Part Ⅱ. Application in TBC with pure metastable tetragonal(t’) phase YSZ and thermal cycling behavior

A thermal barrier coating system comprising Pt-modified NiCoCrAlY bond coating and nanostructured 4mol.% yttria stabilized zirconia(4YSZ, hereafter) top coat was fabricated on a second generation Ni-base superalloy. Thermal cycling behavior of NiCoCrAlY-4 YSZ thermal barrier coatings(TBCs) with and without Pt modification was evaluated in ambient air at 1100?C up to 1000 cycles, aiming to investigate the effect of Pt on formation of thermally grown oxide(TGO) and oxidation resistance. Results indicated that a dual layered TGO, which consisted of top(Ni,Co)(Cr,Al)_2O_4 spinel and underlying α-Al_2O_3, was formed at the NiCoCrAlY/4 YSZ interface with thickness of 8.4μm, accompanying with visible cracks at the interface. In contrast, a single-layer and adherent α-Al_2O_3 scale with thickness of 5.6μm was formed at the interface of Pt-modified NiCoCrAlY and 4 YSZ top coating. The modification of Pt on NiCoCrAlY favored the exclusive formation of α-Al_2O_3 and the reduction of TGO growth rate, and thus could effectively improve overall oxidation performance and extend service life of TBCs. Oxidation and degradation mechanisms of the TBCs with/without Pt-modification were discussed.

An in-situ formed ceramic/alloy/ceramic sandwich barrier to resist elements interdiffusion between NiCrAlY coating and a Ni-based superalloy

NiCrAlY coatings are widely applied on various alloy components to enhance oxidation and/or corrosion resistance at high temperatures.However,elements interdiffusion occurs between them due to composition difference.Although various diffusion barriers(DBs)are reported,this problem is still far from completely solved as most ceramic barriers suffer from poor adherence,while the metallic barriers play a limited role.In this study,NiCrAlY coating was deposited onto a second-generation single-crystal superalloy by arc ion plating.A novel simple method is provided to address elements interdiffusion.By pre-oxidation at a moderate temperature,a thin scale of Ni(Co)O forms at the alloy surface.It transforms to be an alumina/NiCoCr alloy/alumina sandwich by an in-situ reaction with the overlaying NiCrAlY coating and the alloy substrate at high service temperatures,which offers good barrier ability in conjunction with strong adhesion.In the presence of such an alumina/alloy/alumina DB,the NiCrAlY coating provides high resistance to oxidation and scale spallation for the alloy substrate.

Corrosion Behaviors of Nitride Coatings on Titanium Alloy in NaCl-Induced Hot Corrosion

TiN and TiAlN coatings were deposited by arc ion plating on titanium alloys to study their hot corrosion resistance when they were exposed to NaCl at 600℃.The microstructure and corrosion behaviors of nitride coatings were studied using scanning electron microscope,X-ray diffraction,electro-probe microanalyzer and X-ray photoelectron spectroscopy.The results showed that nitride coatings with the different compositions and the ones with the same composition but different thicknesses presented different hot corrosion resistance.TiN and thin TiAlN coatings showed poor corrosion resistance.Serious internal oxidation attacked the alloy substrate.Their corrosion products were mainly consisted of non-protective TiO2 and sodium salt.By contrast,the thick TiAlN coating presented outstanding corrosion resistance.Besides sodium salt,the corrosion products were composed of protective Al_(2)O_(3).The increasing thickness of TiAlN significantly enhanced the hot corrosion resistance.The corrosion mechanisms of alloy,TiN and TiAlN coatings were discussed in detail.

Oxidation and Thermal Shock Behavior of a Glass-Alumina Composite Coating on K38G Superalloy at 1000℃

The glass-alumina composite coatings were successfully prepared on the K38G superalloy substrates.Their isothermal oxidation and thermal shock behavior at 1000 ℃ were characterized.With a post-annealing process at 850 ℃,the composite coatings possessed an improved protective effect for the alloy substrates from isothermal oxidation and a higher resistance to thermal shock.Crystallization from the glass matrix and interfacial reaction between the matrix and alumina inclusions,which caused the composites more refractory and tough,accounted for this improvement.The micromechanisms for the formation of oxidation results of spinel ZnCr_2O_4 were also discussed.

Phase structure of sputtered Ta coating and its ablation behavior by laser pulse heating(LPH)

Phase structure of sputtered Ta coating in the negative glow space and LPH effect were explored.The whole coating/substrate system is substrate→physically gas-absorbed Fe surface→oxygen-enriched TaOx layer→amorphous Ta→αandβdual phase→singleαphase.After LPH course,micro structure of Ta coating shows intact,only a few cracks emerge after 100 laser pulses,exhibiting thin HAZ but thick Fe/Ta ICZ,without martensitic transformation.For the electrodeposited Cr coating,continuous thermal stresses produce many extra micro-crack,substrate oxidation and martensitic transformation,leading to crack propagations and final bulk delamination,without any ICZ.

Benefits of Zr addition to oxidation resistance of a single-phase (Ni,Pt)Al coating at 1373 K

A single-phase (Ni,Pt)Al coating with lean addition of Zr was prepared by co-electroplating of Pt-Zr com posite plating and subsequent gaseous alum inization treatm ents. Isotherm al and cyclic oxidation behavior of the Zr-doped (Ni,Pt)Al coating sam ples was assessed at 1373K in static air in comparison with plain nickel alum inide (NiAl) and norm al (Ni,Pt)Al coatings. Results indicated th at Zr-doped (Ni,Pt)Al coating dem onstrated a lower oxidation rate constant and reduced tendency of oxide scale spallation as well as surface rumpling, in which the enhanced oxidation perform ance was m ainly attributed to the segregation of Zr at oxide scale grain boundaries and the im proved Young's modulus of the coating. Besides, the addition of Zr effectively delayed oxide phase transform ation of Al2O3 from θ phase to α phase in the early oxidation stage and coating degradation of β-NiAl to γ'-Ni3Al in the stable oxidation stage.

On the rumpling mechanism in nanocrystalline coatings:Improved by reactive magnetron sputtering with oxygen

Surface rumpling is detrimental to high temperature protective coatings as it shortens their lifetime and leads to adhesion losses and unexpected corrosion degradation.The driving force and mass transport mechanism behind of rumpling remains to be clarified.In the present investigation,we subjected two types of nanocrystalline coating systems to avoid the influence of interdiffusion on rumpling study.One group was an ordinary nanocrystalline coating,and the other group was designed and prepared with trace oxygen by reactive magnetron sputtering.Systematic cyclic oxidation test at 1100°C was also car-ried out.Results show the ordinary nanocrystalline coating oxidized rapidly,which leads to the fast consumption of Al and the acceleration of phase transition in the coating.Meanwhile,severe surface rumpling is observed due to the stress release of nanocrystals through plastic deformation.Besides,the reactive doping of oxygen can significantly reduce the consumption process of Al in nanocrystalline coat-ing.The rumpling is controlled due to the improvement of coefficient of thermal expansion and Young’s modulus of the coating.Thereafter,the cyclic oxidation resistance is improved.

Comparative study of mechanical and wear behavior of Cu/WS_2 composites fabricated by spark plasma sintering and hot pressing

The mechanical and wear behavior of copper-tungsten disulfide（Cu/WS_2） composites fabricated by spark plasma sintering（SPS） and hot pressing（HP） was investigated, comparatively. Results indicated that the addition of lubricant WS_2 substantially reduced wear rate of the Cu matrix composites fabricated by SPS,and the optimum content of WS_2 is 20 wt% with regard to the wear behavior. However, it affected a little to the wear rate while dramatically decreased the friction coefficient of the composite fabricated by HP.This difference in friction behavior of the self-lubricating composites fabricated by the two techniques was closely related to their different mechanical properties. Severe interfacial reaction occurred during spark plasma sintering, leading to brittle phase formation at interface.

A general strategy towards improving the strength and thermal shock resistance of glass-ceramics through microstructure regulation

Glass-ceramics are usually obtained through controlled crystallization.In this work,we propose a new strategy to add an appropriate amount of oxide particles to the parent glass to improve the performance of glass-ceramics.Different amounts of Al_(2)O_(3) or/and CeO_(2) particles were added into a SiO_(2)-Al_(2)O_(3)-ZnOCaO-ZrO_(2)-TiO_(2)based glass,and crystallization behavior,fracture strength,and thermal shock behavior were systematically evaluated.The results indicate that with the addition of Al_(2)O_(3) or/and CeO_(2) particles of moderate amount,the unfavorable needle-like ZrSiO_(4),Zn 2 SiO_(4),and CaTiSiO_(5) crystals were largely inhibited when annealed at 900℃ .Accordingly,fracture strength is maintained high after heating at high temperatures.The thermal shock resistance is also enhanced drastically.The additive Al_(2)O_(3) is thermodynamically favorable to react with the glass,forming particulate ZnAl_(2)O_(4) instead of precipitating the needle-like crystals of Zn_(2)SiO_(4) and CaTiSiO_(5);while CeO_(2) will combine with ZrO_(2) to form a solid solution and promote the precipitation of primary crystal CaZrTi_(2)O_(7) that will not transform to ZrSiO_(4) with prolonging thermal exposure.

Enamel coating for protection of the 316 stainless steel against tribo-corrosion in molten zinc alloy at 460℃

Hot-dip galvanizing provides excellent corrosion and wear resistance for steels.However,the equipment itself,such as the steel roller,immerged in corrosive molten zinc suffers serious material loss during steel's production.Its protection has become the main technique problem in modern galvanizing line.In this study,an enamel coating was designed and prepared.Its tribo-corrosion in molten zinc alloy(Zn-0.2 wt%Al)at 460℃was investigated in comparison with the traditional WC-12 Co composite coating and the 316 stainless steel.Results indicate that the steel suffers serious material damage.Various corrosion products of Fe_(2) Al_(5) Znx form at the worn surface and the wear scar has reached 200μm deep after merely 5 h tribo-corrosion.Though the two coatings provide an improved tribo-corrosion resistance,for the WC-12 Co coating,its chemical reaction with the molten zinc increases brittleness and promotes cracking.The synergistic wear and corrosion cause its degradation.The enamel coating performs better during tribo-corrosion.It is chemically stable in molten zinc thus able to provide high corrosion resistance.In addition,the amorphous[Si04]network and the self-lubricating CaF_(2) crystallite help it to build up an intact amorphous glaze layer readily at surface on sliding,leading to a reduced wear loss.During the whole tribo-corrosion process,the enamel coating is completely free of cracking,and the Zn penetration is inhibited.

Microstructure and cavitation erosion behavior of sputtered NiCrAlTi coatings with and without N incorporations

The NiCrAlTi coatings free of N and with N incorporations were deposited on austenitic stainless steel304 L by magnetron sputtering in Ar and in gas mixture of Ar and N2,respectively.The N incorporated in the coatings existed as nitride precipitates(from^3 vol.%to^17 vol.%)after vacuum annealing.All the NiCrAlTi coatings,whatever free of N or with N incorporations,exhibited better resistance against cavitation erosion than ion plating Ti N coating and the substrate 304 L in ultrasonic cavitation tests.The NiCrAlTi coating free of N incorporation presents superior cavitation erosion resistance.However,the nitrogen incorporation within the NiCrAlTi coatings showed negative effects on the resistance against cavitation erosion.

Water vapor effects on the oxidation resistance of modified potassium silicate coating at high temperature

An inorganic potassium silicate coating with pigments of alumina,aluminum phosphate,NiCrAlY and copper chromite black was prepared on CB2 stainless steel.Oxidation behavior in either ambient air or O_(2)+H_(2) O mixture at 630℃ for 2000 h was comparatively studied,and the coating exhibited excellent resistance under both test conditions.The water vapor considerably accelerated the oxidation of the uncoated CB2 steel,as the hydroxide,the main constituent of the coating,had a negligible evaporation rate at test temperature,while it had a limited effect on the coated sample.Meanwhile,the existence of coating may prolong or eliminate the incubation period in the O_(2)+H_(2) O mixture at 630℃.After oxidation,the coating matrix is in an amorphous state and fillers as alumina and copper chromite black are stable in the coating.Leucite(KAlSi_(2) O_(6))formed by Al from NiCrAlY and potassium silicate in the coatings was detected after tests either in O_(2) or O_(2)+H_(2) O mixture.