Materials Science Forum Vols. 546-549

Abstract: Halide-activated pack cementation and Air plasma spraying methods were utilized to deposit Si-modified MCrAlY coatings. Nb-base alloys were coated with MCrAlY by Air plasma spraying and followed by pack siliconizing process at 1100°C for 8h and 10h. The pack powders were consisted of 5 Wt.℅NH4Cl、35 Wt.℅Si、60 Wt.℅Al2O3. Si-modified MCrAlY coatings have three-layer microstructure. The outer layer was composed of NbSi2 and Nb3Ni2.4Cr1.6Si6, while the middle layer comprised the compound of Nb、Ti、Ni、Cr、Si. The inner layer adjacent to the substrate was silicide. Isothermal oxidation resistance of Si-modified MCrAlY coatings was tested at 1250°C in static air. Si-modified MCrAlY coatings had excellent oxidation resistance, because continuous Cr2O3 and SiO2 scales which serve as obstacle of oxygen diffusion were formed after oxidation.

Abstract: As a kind of white paint, ZnO/silicone thermal control coatings are used commonly in space. In order to implement more effective thermal control in space, nanotitanium dioxide was doped to the coatings. In this paper, different percent of nanotitanium dioxide was appended to the coatings, and thermal control principle was discussed. Optical properties of these coatings were discussed by spectrophotometry and infrared reflectometer. The experimental results show that solar absorptance (αs) of nanotitanium dioxide doped ZnO/silicone thermal control coating is lower in all wavelength region than the coatings without doping, especially in visible region and infrared region. The total solar absorptance of 1% nanotitanium dioxide doped coating is 0.09, which decreases by 36% than the undoped coating..

Abstract: Using Hall ion source assisted MF twin targets unbalanced magnetron sputtering, series of Ti/TiN, Ti/TiN/Ti(C,N) and Ti/TiN/Ti(C,N)/TiC hard anti-wear films are deposited on different materials like stainless and high speed steels by changing atmosphere, bias mode, sputtering and ion beam assisted currents. The color, crystal structure, hardness and binding force of film are tested and analyzed, respectively. Experimental results show that color of film is sensitive to atmosphere, slight change of atmosphere can influence the coating’s color seriously. The preferred orientation of Ti(C,N) films prepared by MF twin targets unbalanced magnetron sputtering has no obvious change compared with the film deposited by other PVD technologies. The substrate material has great influence on film’s hardness, binding energy and surface modification. The application of Hall current can effectively improve the binding force between film and substrate.

Abstract: 10mol% Nd2O3 and Yb2O3 co-doped YSZ thermal barrier coatings were produced by electron beam physical vapor deposition (EB-PVD). Compared to the traditional YSZ coating, the deposited coating has shown tree-like microstructure in each column. Due to this, the co-doped coating is more porous than the YSZ coating. The microstructure evolution of the coating during high temperature exposure at 1373 K was studied. The tree-like microstructure disappeared due to joining of sub-grains during sintering. Thermal growth oxide (TGO) grew quickly at the first few hours and then the growth of TGO became slow in the subsequent high temperature exposure. Cracks generated and propagated in the ceramic top coat and along the interface of the top coat and TGO layer. Finally, the coalescence of such cracks resulted in failure of the TBCs.

Abstract: 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.

Abstract: The friction and wear characteristics of a cathodic arc ion-plated CrSiN coating in sliding against different counter materials have been investigated under both unlubricated and boundary-lubricated conditions. Under boundary-lubricated conditions, the CrSiN coating exhibits a friction coefficient of 0.12 to 0.20 against a SAE 52100 steel ball and of 0.04 to 0.07 against a CrN-coated steel pin, depending on wear parameters used. The effects of load, temperature and counter materials on friction and wear of the CrSiN coating were discussed. The worn surfaces and wear debris generated under different wear conditions were examined by means of scanning electron microscopy and Raman spectroscopy to identify the tribochemical reaction and phase structures generated during the wear process. Under unlubricated conditions, iron-based oxides formed in sliding against a steel ball, while some chromium oxides were identified on worn surfaces of the CrSiN coating in sliding against an alumina ball in air.

Abstract: Ti2AlNb orthorhombic alloy is a promising high temperature structural material for aero-industries due to its advantageous properties. However, insufficient wear-resistance is a major drawback that has restricted the actual uses of this alloy in many circumstances. A treatment of double glow plasma surface chromizing on Ti2AlNb alloy has been carried out as an attempt to resolve this problem. This paper mainly investigated the electrochemical corrosion behaviors of this alloy after chromizing. The microstructure of the chromized layer was analyzed by X-ray diffraction (XRD). The sectional morphology of chromized layer was surveyed through scanning electronic microscopy (SEM).The polarization curves of specimens in three corrosive media, 5% H2SO4, 5% HCl and 3.5% NaCl, were measured. The eroded surface morphologies were also surveyed by SEM. The results indicate that surface chromizing treatment slightly decrease the alloy’s corrosion resistance, but still exhibit good performance.

Abstract: Metal-semiconductor-metal (MSM) planar photodiodes and photoconductors were fabricated on unintentionally doped homoepitaxial diamond thin films deposited on Ib and IIa-type diamond substrated. The MSM photoconductor on the epilayer grown on the Ib-type substrate exhibits the highest discrimination ratio up to 108 between 210 nm and visible light and a photocurrent gain around 33 at 220 nm. The persistent photoconductivity is rather week for such kind of photoconductor.

Abstract: In this work Inconel 600 alloy was coated with two different types of coatings, Crmodified aluminide coating and Y- doped chromium modified aluminide coating . Diffusion coating was carried at 1050 oC for 8 hrs under Ar atmosphere by single step aluminizing- chromizing process and by single step aluminizing- chromizing- yttriumizing process. The cyclic hot corrosion tests of IN 600 and its coated systems deposited with 2 mg / cm2 NaCl / Na2SO4 (100/0, 50/50, and 0/100 wt. %) deposits were conducted at 900 oC in air for 105 hrs at 15 hrs cycle. The hot corrosion kinetic of uncoated Inconel 600 alloy follows parabolic rate law when oxidized with 100% Na2SO4 deposits, whereas it follows a linear rate law when oxidized with 100% NaCl deposits and with (50% NaCl + 50% Na2SO4) deposits. In cyclic hot corrosion tests, the parabolic rate constant (kP) values for Cr- modified aluminide coating when oxidized with NaCl / Na2SO4 concentrations (100/0, 50/50, and 0/100 wt. %) deposits are: 2.67x10-6, 2.73x10-6, and 8.34x10-7 (mg2/cm4)/s. respectively. But for Y- doped chromium modified aluminide coating are: 2.10x10-6, 1.51x10-6, and 6.66x10-7 (mg2/cm4)/s. respectively, under the same test conditions. The kP values for both coated systems oxidized with 100% Na2SO4 deposits are one order of magnitude lower than that for 100% NaCl, and for (50% NaCl + 50% Na2SO4) deposits under the same test conditions.

Abstract: The ceramic coatings with thickness up to 120 μm were prepared on Ti3Al-based alloy in silicate electrolyte by microarc oxidation method. The morphology, microstructure, composition and phase constituent of the coatings were investigated. Microhardness profiles along the cross-section of the coating were determined. The maximum microhardness in the coating was about 3 times higher than that of the substrate. The thickness of the coating increased with the increasing of MAO time. There were two layers in the ceramic coatings: a dense layer and a loose layer. The coating was mainly composed of (Ti0.6Al0.2Nb0.2)O2 phase. Scratching test revealed a good adhesion between the Ti3Al substrate and the coating.