Key Engineering Materials Vols. 353-358

Abstract: Magnesium alloys exhibit great application potential in industries such as automotive, electronics, aerospace and others. But poor wear resistance and corrosion resistance have restricted their actual uses. In this study, a CrN film had been prepared on magnesium alloy AZ91 by arc-glow plasma depositing. The composition and microstructure of the coating had been analyzed by X-ray diffraction (XRD) and glow discharge spectrum (GDS). The morphology of CrN film has been surveyed through scanning electronic microscopy (SEM) and atom force microscopy (AFM). The wear behavior was investigated through ball-on-disc test. The corrosion behavior in 5 wt% NaCl solution was also studied. The results indicate that the specific wear ratio decreases greatly and the corrosion potential also increases relatively comparing to AZ91 matrix.

Abstract: Ti2AlNb orthorhombic alloy is an attractive high temperature structural material for aero-industries, but insufficient wear-resistance is a major drawback which restricts the actual uses of this alloy in many circumstances. A double glow plasma surface molybdenizing on Ti2AlNb alloy had been carried out to resolve this problem. This paper investigated the electrochemical corrosion behaviors of the alloy after molybdenizing. The polarization curves of specimens in three corrosive media, 5% H2SO4, 5% HCl and 3.5% NaCl, were measured. The eroded surface morphologies were surveyed by SEM. The results indicate that surface molybdenizing decreased the alloy’s corrosion resistance slightly, but still exhibit good performance.

Abstract: This research tries to improve the plasma surface W, Mo alloying process by adding inlet methane aside from the original inert gas argon. The carbon and hydrogen particles are incorporated into the ion sputtering, ionization, surface activation and diffusion processes. The W, Mo atoms sputtered from the target diffuse into the surface of the substrate at the same time with the diffusion of carbon atoms. So the synergism of the alloying process and the carburizing process is established in this way. The hydrogen atoms participate the reduction and activation process on the surface of the target and the substrate. The surface HSS combines with substrate via metallurgical bonding and the carbides are all secondary carbides formed at lower temperature during solid state diffusion. These carbides are very soluble to the austenite. This makes the alloy elements fully functional. The carbides with granular shape and distributes homogeneously on the matrix are very fine. No coarse ledeburite eutectic carbide exists. After the co-alloying process of W, Mo, C, direct quench or quench at lower temperature can be applied.

Abstract: Elastic modulus of air plasma sprayed (APSed) YSZ (; ZrO2 stabilized by 8 wt. pct. Y2O3) top coat specimen, which is frequently used for thermal barrier coating (TBC) system for advanced gas turbines, was measured by employing the macro-, micro-, and nano-indentation methods. The elastic modulus was measured, following the Oliver-Pharr method. It was shown that the elastic modulus of the YSZ, as well as the microstructure, was significantly influenced by the spraying conditions employed. Especially the size of ceramic powders used was found to have the most pronounced effect. It was also shown that the elastic modulus revealed significant size effect: that is, there were significant differences in elastic modulus measured by the instruments on the macro-, micro-, and nano-levels. This size effect was discussed, correlating with some relating phenomena: crackings, sink-in, pile-up and spalling; as well as with the characteristic microstructures of the sprayed top coat.

Abstract: The subcommittee on superalloys and coatings, The Society for Materials and Science, Japan (JSMS) was established in 1997. As second stage of the committee, we investigated a fundamental study on thermal plasma sprayed thermal barrier coatings (TBCs). Especially, relationships between spray conditions and coating properties are discussed. It was used 4 spraying conditions such as 1) standard condition, 2) larger particle used and lower velocity, 3) normal particle used and lower velocity, and 4) standard condition and heating up the substrate to 473K. As a round robin test, porosity ratio, residual stress, and bonding strength were evaluated by several techniques for the specimens, which were sprayed by above 4 conditions. As a result, Young’s modulus, and tensile strength increased with decreasing porosity ratio. In case of tests for residual stress evaluation, as-sprayed specimens have small tensile residual stresses. After thermal exposure, residual stresses shifted to compressive.

Abstract: The mechanical properties of the 316L stainless steel subjected to surface mechanical attrition treatment (SMAT) have been studied, these properties are hardness, tensile properties and wear resistance. The research shows that the thickness of the hardened layer increases with the increasing of the treating time. The refined microstructure in the treated layer led to increasing in hardness, strength, and wear resistance. It is obvious that the surface layer hardness and bulk yield strength are increasing when the SMAT time reaches 5 minutes. The increase of surface layer wear resistance is obvious when the SMAT time is 15 minutes. The SEM observation of the wear scars shows that the nanocrystalline layer might reduce the effect of adhesive wear of 316L stainless steel. Therefore, the wear mechanism changes from adhesive abrasion to grinding particle abrasion after SMAT.

Abstract: The effect of silica content on plane-stress fracture toughness of Polyimide/silica (PI/SiO2) composite films was studied using the specific essential work of fracture (EWF) method. The specific essential and non-essential work of fracture for PI/SiO2 hybrid thin films with different silica doping levels was tested by experiment. The shape factor of the plastic region and specific non-essential work of fracture per unit volume were received by finite element (FE) calculation. It demonstrated that the plastic zone for specimens studied here was much different from the traditional result given through optical microscopic observation.

Abstract: A niobium modified layer on Ti-6Al-4V surface was obtained by means of the plasma surface alloying technique. The oxidation behavior of the modified layer was investigated and compared with Ti-6Al-4V at 700°C~900°C for 100h. Composition and microstructure of Ti-6Al-4V and the modified layer after oxidation at 900°C for 100h were analyzed using XRD and SEM respectively. The experimental results showed the oxidation behavior of Ti-6Al-4V at 900°C for 100h was obviously improved after the niobium alloying process.

Abstract: This paper dealt with the processing of anti-rust boronizing layer on steel surface and phosphating as post-treatment to improve the corrosion resistance of the boronized layer. A smooth protective layer with black brightness on the surface of the boronizing layer was obtained. The appearance of the boronized work pieces was greatly improved. The technical parameters of the black phosphating solution were optimized by the experiments and the microstructure and the inoxidizability of the black phosphated coating was also analyzed.

Abstract: Polycrystal samples of La1-xSrxMnO3 (x=0.2, 0.3) have been synthesized using a sol-gel method. The effect of sintering process on the sample microstructure and Sr doping on the magnetoresistance for La1-xSrxMnO3 have been investigated. The results show that the increase of sintering temperature and time can promote the crystallization of samples and reduce the content of impurity. The additive of Sr makes Mn ion in materials relatively compact, and this causes the distorted deformation of the materials structure. The magnetoresistance (MR) changes with the Sr doping. When x=0.2, the maximum MR% of sample is about 43% and keep about 40% between 10~150K. At room temperature, the MR% will decrease to about 6.5%. When x=0.3, the order magnetoresistance (OMR) effect occures above the room temperature.