Sheng Long Zhu

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.

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.

The Effects of N2 Flow Rates on the Properties of Ti-Al-Si-N Films Deposited by Arc Ion Plating

Ti-Al-Si-N films were deposited on 1Cr11Ni2W2MoV steel at different N2 flow rates from 0 to 600 sccm by AIP using a Ti60Al30Si10 target. The effects of the N2 flow rates on the deposition rate, composition, microstructure, microhardness and adhesion property of the films were investigated by SEM/EDS, XRD, microhardness and scratch tests, respectively. The results show that the deposition rate of the films decreased initially and then began to increase at N2 flow rate higher than 300 sccm. The microhardness of the films increased with the N2 flow rate except a slight drop at 450sccm. The evolution of adhesion property of the films was very similar to that of microhardness. All the Ti-Al-Si-N films had B1 NaCl structure with (220) preferred orientation except that at 600 sccm. The effects of the N2 flow rate on the properties of the Ti-Al-Si-N films were discussed.

Corrosion and Oxidation Protection of Ti6Al4V Alloy by an Inorganic Metal Modified Silicate Composite Coating

An inorganic metal modified silicate composite coating was developed to protect the corrosion and oxidation of Ti6Al4V alloy at 650°C. The properties of the coatings were investigated by oxidation and corrosion experiments, and the microstructure of the coatings was studied by SEM. The isothermal oxidation results indicated that the Ti6Al4V alloy suffered serious oxidation after exposed for 100h at 650°C and the mass of the alloy gained almost lineally, while the coated alloy had almost no mass gain and the coatings keep intact after oxidation. The salt spray test for 20h and oxidation at 650°C for 2h was carried out to investigate the corrosion and oxidation properties of the coatings. The results indicated that the alloy experienced serious corrosion after experiment for 20 cycles, while the coated alloy did not corrode after the experiments. The thermal shock experiments indicated that the coatings could withstand 50 times thermal shock when kept for 5min at 700°C and then immediately put into 3.5%NaCl solution at room temperature.

Solid Reactions between Enamel and O-Phase Ti-Al-Nb Intermetallics at 800 °C

The specimens of O-phase Ti-22Al-25Nb (at%) intermetallics coated with silica-based enamel received mass gains of about 1 mg/cm2, after 300 h of oxidation or hot corrosion at 800 °C. These rates were much faster than the growth rates of silica films at the same temperature. To understand this phenomena, the specimens were analyzed using SEM, XRD, EPMA and TEM. An oxide layer with thickness of several μm was observed at the enamel/substrate interface of the coated specimens after either oxidation or hot corrosion. XRD and TEM analysis revealed the newly formed oxide layer was composed of α-Al2O3, Al2SiO5, Al2TiO5, rutile-TiO2 and NbO2. It was shown by EPMA profiling that an Al-depleted zone was located just beneath the oxides. It was proposed that the solid reactions between the enamel coating and the O-phase Ti-Al-Nb played important roles for the oxidation and hot corrosion behavior of the coated specimens.

Cyclic Oxidation Behavior of Sputtered Nanocrystalline Coating with Yttrium Additions in Air

K38 nanocrystalline coatings with various amounts of yttrium addition were deposited by magnetron sputtering. Cyclic oxidation tests were conducted at 800-1000oC in air in order to reveal the effect of reactive elements on the oxidation behavior of nanocrystalline material. The results indicated that the influence of yttrium was not observable at 800oC. At 900 and 1000oC, addition of 0.1 wt.% Y decreased the growth rate of scale, while 0.5 wt.% Y addition significantly increased the oxidation rate of nanocrystalline coating.

Effect of Al Content on Corrosion Resistance of (Ti1-xAlx)N Coatings under NaCl Deposit in Wet Oxygen at 600°C

(Ti1-xAlx)N(x=0, 0.1, 0.3) coatings were deposited on 1Cr11Ni2W2MoV stainless steel by arc ion plating. The (Ti1-xAlx)N coatings had B1 NaCl structure, however its preferred orientation change from (111) to (220) with the increase of Al content. A number of nodule-shaped spots identified as rutile TiO2 by XRD formed on the surface of TiN coating after corrosion beneath NaCl deposit in wet oxygen at 600 oC. With the addition of Al, a thin scale composed of Al2O3 and TiO2 formed on the surface of the (Ti1-xAlx)N coatings instead of a scale of TiO2 on TiN coating, thereby their corrosion resistance was remarkably enhanced.