Fu Hui Wang

On the Development and the Oxidation of Novel Ni-Cr and Ni-Al Nanocoatings by Composite Electrodeposition

Novel Ni-based nanostructured coatings against high temperature oxidation were fabricated by composite eleoctrodeposition. The electrodeposited nanocoatings (ENCs) were prepared, separately from a nickel sulphate electrolyte bath with addition of Cr nanoparticles in an average size of 39 nm, or with Al nanoparticles in an average size of 75 nm. The Cr or Al nanoparticles were dispersed among the nanocrystalline Ni grains of ENCs. The ENC Ni-10.9Cr at 900 o C and the ENC Ni-28.0Al at 1050 o C exhibited a superior oxidation resistance as a result of the formation of continuous chromia or alumiuna scale, respectively. It is proposed that the dispersion of the Cr or Al nanoparticles among the nanocrystalline Ni grains greatly increased the surface density of the nucleation sites for Cr2O3 or Al2O3; simultaneously, the nanostructured coatings favored rapid healing of the narrowed Cr2O3 or Al2O3 nuclei during a very short transient period, which consequently led to the suppression of NiO growth.

Oxidation and Hot-Corrosion Behavior of Sputtered Ti-48Al-8Cr-2Ag Nanocrsytalline Coating

Magnetron-sputter deposition was used to produce a Ti-48Al-8Cr-2Ag (at.%) coating on a cast alloy substrate with the same composition. The oxidation behavior of the cast Ti-48Al-8Cr2Ag alloy and its sputtered coating was investigated in air at 800°C―1000°C. Hot corrosion was carried out in molten 75% Na2SO4 + 25% K2SO4 at 900 0 C. The results indicated that the oxidation rate of sputtered nanocrystalline coating was lower than that of the cast alloy at 900°C. The former formed a scale of merely Al2O3 and the latter formed a scale of Al2O3 + TiO2 mixture. For the same reason, the sputtered nanocrystalline coating exhibited much better hot corrosion resistance than the cast alloy in molten 75% Na2SO4 + 25% K2SO4 at 900°C. However, the nanocrystalline showed a little bit higher oxidation rate than the cast alloy at 800°C and 1000°C, because spherical surface and boundaries or free surface between columnar of the coating gave a larger actual oxidation area than the original alloy.

Long-Term Oxidation Resistance of (Ti,Al,Y)N Coatings at 800 °C

Composite metastable Ti0.5Al0.5N, Ti0.49Al0.49Y0.02N and Ti0.48Al0.48Y0.04N coatings were deposited on a wrought martensite steel 1Cr11Ni2W2MoV for aero-engine compressor blades by arc ion plating respectively all coatings possessed a dense surface and the same B1NaCl phase structure and the lattice parameter increased with increase of the incorporated element Y. Oxidation-resistance of (Ti,Al,Y)N coatings at 800 for up to 500 hr was investigated and the results showed that the introduction of yttrium into the coatings dramastically improved the oxidation-resistance of the coatings in air. The presence of Y in the nitride coatings suppresses the outer diffusion of Fe through the nitride coating. And the oxide of yttrium may segregate to the grain boundaries of oxides, suppressing the outward diffusion of metallic elements in coating and the inward diffusion of oxygen. Besides, the existence of adequate Y in the coatings is in favor of suppression of the growth of oxide crystals, formation layered oxide scale and decrease of compressive stress of oxidation.

High Temperature Oxidation of (Ti,Al)N and (Ti,Al,Y)N Coatings on a Steel Prepared by Arc Ion Plating (AIP)

Abstract: Based on the characteristics of arc ion plating (AIP) technique, the microstructure and composition of synthesized compound coatings may be tailored by controlling the relevant processing parameters such as composition of target materials, substrate bias voltage, and nitrogen partial pressure and so on. In this study, the effect of substrate pulse bias and nitrogen partial pressure on droplet formation, chemical composition and phase constituent of (Ti,Al)N and (Ti,Al,Y)N coatings was studied. Furthermore, (Ti,Al)N and (Ti,Al,Y)N coatings were deposited on a wrought martensite steel 1Cr11Ni2W2MoV under optimum parameters, and thereafter oxidation resistance of the coatings was evaluated.

Hot Corrosion Behavior of a Novel Enamel-Based Thermal Barrier Coating

A novel thermal barrier coating comprised two layers: sputtered bond coat and enamel-zirconia composite top layer. The hot corrosion tests were carried out at 900 °C for 100 h. The salt used in the tests was 25wt% NaCl + 75wt% Na2SO4. Mass loss and spalltion of oxide scales of uncoated K444 specimens was obvious. For the coated specimens, no oxide scale spallation and no weight loss were observed. The TGO formed on the coated specimens was thin layer of mixture oxides of Al, Ti and Cr, while thick multi-layered oxide scales formed on the uncoated specimens. Besides, deep internal oxidation zone was observed on the uncoated specimens. The coatings after hot corrosion tests contained t-ZrO2 and NaAlSi3O8, while the oxide scales formed on the uncoated K444 were TiO2, Cr2O3, NiCr2O4 and Na2Cr2Ti6O16.

The Investigation of the Interdiffusion Barrier in TiAl/MCrAlY System

TiAl based alloys are promising candidates for structural applications at high temperature. However, the poor oxidation resistance above 800oC obviously restrains their applications. Although NiCrAlY overlay coatings can remarkably improve the high temperature oxidation resistance of TiAl, serious inward diffusion of Ni from the coating to the substrate occurs which could reduce the lifetime of the coating/substrate system. Apparently, the development of interdiffusion barrier could overcome the disadvantage of the NiCrAlY/TiAl system. In this work, Ta, TiN and Cr2O3 interlayers were deposited between NiCrAlY coating and γ-TiAl substrate as diffusion barrier (DB). The interdiffusion behavior of the TiAl/DB/NiCrAlY system was investigated at 1000°C. The results showed that the metallic and nitride interlayers cannot retard the interdiffusion of Ni effectively. As an active diffusion barrier, the oxide interlayer obviously suppressed the inward diffusion of Ni from the coating to the substrate by the formation of alumina-rich layers at both the TiAl/DB and DB/NiCrAlY interfaces.

Influence of Micro-Structure on Oxidation Behavior of a Ni-Based Superalloy at 1000°C

The isothermal and cyclic oxidation behaviors of a Ni-based superalloy with singlecrystalline (SC), polycrystalline and nanocrystalline (NC) structures were studied at 1000°C. Results indicated that a uniform oxides scale consisted of external Cr2O3 with little TiO2 and internal continuous Al2O3 formed on SC alloy. A non-uniform external oxide of which some locations were nodule-like scale was formed on surface of cast alloy. The nodule-like parts consisted of TiO2, Cr2O3 and serious internal oxidation of Al, and rest flat surface was a Cr2O3 and Al2O3 layer. A continuous Al2O3 layer formed on the sputtered NC coating. The micro-structure influenced the oxidation mechanism and resulted in different oxide scale formed on three materials, which greatly influenced materials’ oxidation and cyclic-oxidation resistance.

Depth Distribution of Residual Stress in Graded (Ti, Al) N Coating and Fatigue Properties of Coated Stainless Steel

The depth distribution of the residual stress in graded (Ti, Al) N coating deposited on steel by arc ion plating was measured by the Stripping Layer Substrate Curvature Technique, and the effect of graded (Ti, Al) N and mono-layered (Ti, Al)N coating on the fatigue properties of 1Cr11Ni2W2MoV stainless steel were investigated. The depth distribution of the residual stress in mono-layered Ti70Al30N and TiN were also measured for comparison. The results show that the residual stresses in the coatings are compressive, which increase gradually from the coating/substrate interface and reach a maximum value at the middle region, then decrease until the surface. Compared with TiN and Ti70Al30N, the stress maximum value in the graded coating is nearer to the coating surface. It is also shown that the fatigue strength of the graded (Ti, Al)N and Ti70Al30N coated samples are superior to that of the uncoated substrate. The improvement of the fatigue properties for the coated samples is thought to be attributed to the hard coatings with high compressive stress.

Influence of NiCoCrAlY and Diffusion Aluminide Coating on Oxidation and Hot Corrosion of a Ni-Based Superalloy

The most common metallic coatings used in today’s gas turbine engines are MCrAlX (where M is Ni and/or Co and X is one or more reactive elements such as Y, Hf, etc.) type overlay coatings. However, overlay coating techniques (plasma and flame spraying or physical vapor deposition) are line-of-site processes, and so, it is possible not to deposit coating on some surface of the complex turbine components. The diffusion aluminide coatings can solve this problem. A NiCoCrAlY and diffusion aluminide coating were prepared on K38G cast alloy by multi-arc ion plating and low pressure chemical vapor deposition (LP-CVD) techniques, respectively. The isothermal oxidation behavior of K38G and the coatings was studied in air at 900 and 1000 oC. Their hot corrosion behaviors in the presence of 75 wt.% Na2SO4+K2SO4 and 75wt.%Na2SO4+NaCl film at 900oC were studied. The results showed that the two kinds coatings exhibited low oxidation rate at 900 and 1000oC and the presence of salt accelerated the oxidation rate. The NiCoCrAlY coating showed the better hot corrosion resistance than the aluminide coating.

Influences of MCrAlY Coatings and TBCs on Oxidation Behavior of a Ni-Based Single Crystal Superalloy

DD98M alloy is a second generation nickel-based single crystal superalloy without Re addition, which is developed by Institute of Metal Research, Chinese Academy of Sciences. It is a combination of attractively strong points including high strength and low cost. In this work, Ni-based single crystal superalloy DD98M was adopted as the substrate material. Two types MCrAlY (M denote metal) coatings were deposited on DD98M specimen by arc ion plating and YSZ topcoat (TC) were deposited on the substrate by electron beam physical vapor deposition (EB-PVD) on the NiCrAlY bond coat (BC). Experimental results showed that the application of the NiCrAlY and NiCoCrAlYHfSi coatings improved the oxidation resistance of DD98M obviously at 1000 °C. The adhesion of oxide scale of NiCoCrAlYHfSi coating was much better than that of NiCrAlY coating. TBCs application greatly enhanced the operating temperature and significantly improved the durability of the substrate. The thermal growth oxidation (TGO) between the bond coat and topcoat play an important role in adhesion of the whole coating system.