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Now showing items 1 - 16 of 25

  • Brittle coating effects on fatigue cracks behavior in Ti alloys

    Bai, Yanyun   Xi, Yeting   Gao, Kewei   Yang, Huisheng   Pang, Xiaolu   Yang, Xusheng   Volinsky, Alex A.  

    In order to study the coating effects on fatigue crack initiation mechanism in Ti-alloys, two types of brittle coatings, CrAlN and TiN, were deposited on the surface of TC4 titanium alloy by physical vapor deposition. The tension-tension fatigue tests and the in-situ observations of fatigue crack morphology were performed to study the coating effects on the fatigue crack initiation and propagation in the Ti-6Al-4V alloy. It was found that the 510-530 MPa TC4 fatigue limit is reduced to 315-330 MPa due to the CrAlN coating. The brittle coatings impeded the deformation of the TC4 samples at the beginning stage of fatigue tests, while coating cracking promoted the elongation of the tested samples. Fatigue crack was found to be initiated in the brittle coatings and propagated to the coating-substrate interface, inducing micro-damage of the substrate surface. The fracture surface of coated and uncoated samples was quite different, and the formation of non-propagating fatigue cracks was also observed. The coating cracking-induced low cyclic stress substrate damage model was proposed. This study should be of significance for the coating improvement and provides a theoretical basis for improving fatigue properties of coating materials.
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  • Residual stress control in CrAlN coatings deposited on Ti alloys

    Bai, Yanyun   Xi, Yeting   Gao, Kewei   Yang, Huisheng   Pang, Xiaolu   Volinsky, Alex A.  

    In order to control residual stress of CrAlN coatings on Ti substrates, similar to 70 nm CrAl interlayer was deposited at different temperatures. Residual stress and coatings' structure were characterized by X-ray diffraction. Residual stress in the coatings was compressive and increased with CrAl interlayer deposition temperature. Residual stress in 1.5 mu m, 2 mu m and 2.6 mu m thick CrAlN films on TC21 with the interlayer deposited at 100 degrees C (-47.43 MPa, -25.57 MPa and -855.77 MPa, respectively) was smaller than with the interlayer deposited at 300 degrees C (-1.39 GPa, -1.95 GPa and -1.62 GPa, respectively). The coatings on the TC4 substrate showed the same trend (-1.02 GPa, -389.91 MPa and -1.03 GPa for the interlayer deposited at 100 degrees C, respectively, and -921.42 MPa, -2.31 GPa and -1.80 GPa for the interlayer deposited at 100 degrees C, respectively). Changing the interlayer deposition temperature can influence the coatings' residual stress and crystal structure, and improve mechanical properties of the coatings. CrAlN deposition is a convenient and efficient way to improve mechanical properties of Ti alloys.
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  • Brittle coating effects on fatigue cracks behavior in Ti alloys

    Bai, Yanyun   Xi, Yeting   Gao, Kewei   Yang, Huisheng   Pang, Xiaolu   Yang, Xusheng   Volinsky, Alex A.  

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  • Residual Stress and Surface Energy of Sputtered TiN Films

    Pang, Xiaolu   Zhang, Liqiang   Yang, Huisheng   Gao, Kewei   Volinsky, Alex A.  

    Morphology, structure, residual stress, and surface energy of magnetron-sputtered titanium nitride (TiN) thin films, deposited at 300 degrees C with a thickness in the 0.5-1.7 mu m range, were characterized. Film microstructure, the origin of residual stress, and its effect on the surface energy were analyzed. The grain size increased with the film thickness. X-ray diffraction showed (200) to (111) preferred orientation transitions with the increasing film thickness. The stress in the TiN films changed from compressive -0.3 GPa to tensile with the film thickness reaching 0.3 GPa. Larger grain size, initial porosity, and sub-grains generation are the reasons for significant changes in the residual stress. Surface energy was investigated by contact angle of water and glycerol droplets, which both show a significant change with the different stress state and crystal preferred orientation. The TiN films form a contact angle larger than 100 degrees with water as a test liquid, demonstrating their hydrophobicity. While the residual stress changes from compressive to tensile, the contact angle reaches 118 degrees, and the corresponding surface energy changes from 38.8 to 24.2 mJ/m(2). One can expect to achieve a certain desired surface state of TiN films for potential applications.
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  • Interaction between Cu and Cr coadsorption on MnS inclusions in low alloy steels

    Lv, Wenting   Jin, Weichao   Yan, Luchun   Pang, Xiaolu   Yang, Huisheng   Gao, Kewei  

    Interaction between alloy elements Cu and Cr on MnS inclusions was studied by Scanning Transmission Electron Microscope (STEM)/X-ray energy-dispersive spectrometer (EDS) and the first principles calculation method. Assuming that the enrichments of Cu and Cr play a key role on pitting corrosion of low alloy steel, STEM/EDS were used to observe the enrichments of Cu and Cr on MnS inclusions in this study. The first-principles calculation, which is based on density functional theory was used to investigate the segregation behavior of Cu and Cr in the Fe matrix and the adsorption behavior of Cu and Cr on MnS surface. The atomic structures, electronic and energetic properties of clean and Cu or Cr adsorbed MnS surface were calculated. The results showed that both Cu and Cr tend to segregate on the Fe (1 1 0) surface rather than solid dissolved in the Fe bulk. MnS (1 0 0) is the most stable surface amongst the low-index surfaces, and H (hollow) is the most stable adsorption site for both Cu and Cr. Coadsorption of Cu and Cr on MnS (1 0 0) surface was also studied. The results showed that Cu and Cr promote the adsorption of each other on MnS (1 0 0) surface. However, this interaction is effective only within a certain distance between them. Overall, these findings will be helpful to understand the mechanism of pitting corrosion induced by MnS.
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  • Microstructure, residual stress, and fracture of sputtered TiN films

    Zhang, Liqiang   Yang, Huisheng   Pang, Xiaolu   Gao, Kewei   Volinsky, Alex A.  

    Morphology, structure, residual stress, hardness, and fracture toughness of magnetron sputtered titanium nitride (TiN) thin films, deposited at 300 degrees C with a thickness in the 0.3- to 2-mu m range, were characterized. Film microstructure, the origin of residual stress, and its effect on the fracture toughness and hardness were analyzed. The grain size increased with the film thickness, with 1- to 2-mu m-thick films having high pore density. For the 2-mu m film, subgrains appeared at grain boundaries. X-ray diffraction showed (200) to (111) preferred orientation transition. The stress in the TiN films changed from highly compressive (-1.1 GPa) to tensile with the film thickness, reaching 0.68 GPa. Larger grain size, initial porosity, and subgrain generation are reasons for significant changes in the residual stress. Average hardness measured by nanoindentation is 23.2 +/- 0.6 GPa. The hardness of the films in compression is higher than in tension. Hardness variation with the film thickness is mainly due to the grain size and microstructure effects. The fracture toughness decreases with the film thickness, depending on the stress state and value. Compressive stress can significantly improve TIN film fracture toughness, while tensile stress seriously degrades it. (C) 2013 Elsevier B.V. All rights reserved.
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  • Interaction between Cu and Cr coadsorption on MnS inclusions in low alloy steels

    Lv, Wenting   Jin, Weichao   Yan, Luchun   Pang, Xiaolu   Yang, Huisheng   Gao, Kewei  

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  • Residual stress and warpage of AMB ceramic substrate studied by finite element simulations

    Zhang, Shanshan   Yang, Huisheng   Gao, Kewei   Yan, Luchun   Pang, Xiaolu   Volinsky, Alex A.  

    Ceramic substrates with high heat dissipation performance are utilized in high power electronic devices. This study investigates the warpage deformation and residual stress originating during manufacturing of the active material brazing (AMB) ceramic substrate to provide important parameters for the substrate design and ensure good reliability. Finite elements were used to analyze the effects of ceramic, metal and solder thickness, ceramic substrate size and pressure on residual stress distribution and warpage deformation. Calculation results of thermal elastic and thermal elastic-plastic finite elements are compared. Plastic deformation during the welding process greatly affects calculation results accuracy. It is found that the maximum axial stress is concentrated on the ceramic side and axial residual stress is the main factor causing cracking of the ceramic substrate. The thickness of ceramic, metal and solder, along with the substrate size have significant effects on residual stress and warpage deformation, which both can be reduced by applying external pressure.
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  • Comparative study of Ti and Cr adhesion to the AlN ceramic:Experiments and calculations

    Zhang, Shanshan   Jin, Weichao   Yang, Huisheng   Gao, Kewei   Pang, Xiaolu   Yan, Luchun   Volinsky, Alex A.  

    In this manuscript, ceramic substrates with different adhesion layers were prepared by the direct-plated-copper (DPC) method. Microstructure and mechanical properties were investigated by scanning electron microscopy (SEM) and adhesion strength tests. SEM results showed that the AlN/Ti/Cu interface had fewer defects than AlN/Cr/Cu and AlN/Cu. The adhesion strength of the Ti layer samples was about 16.5 MPa, which is much higher compared with Cr and no adhesion layer samples. The adhesion strength of the AlN/Ti/Cu sample was the highest, followed by AlN/Cr/Cu, and AlN/Cu adhesion was the lowest. The AlN (0 0 1)/Ti (0 0 1) and AlN (0 0 1)/Cr (1 1 0) interfaces were studied by the first principles calculations based on the density functional theory (DFT). It appears that the Ti-N bonds were formed by the Ti-spd and N-p orbital hybridization. Moreover, the electron transition from Ti atom to N atom was higher than that from Cr to N. The bond length of Ti-N bond is shorter than Cr-N bond length, and its population has proven that Ti-N has stronger covalent character, which may be the reason why the AlN/Ti/Cu interface has higher interfacial stability than AlN/Cr/Cu.
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  • Fracture Toughness and Adhesion of Transparent Al:ZnO Films Deposited on Glass Substrates

    Pang, Xiaolu   Ma, Hongjian   Gao, Kewei   Yang, Huisheng   Wu, Xiaolei   Volinsky, Alex A.  

    Al-doped zinc oxide films (AZO) of different thicknesses were deposited by reactive magnetron sputtering on glass substrates. Fracture toughness and adhesion of transparent AZO films were measured by indentation in air and water. Fracture toughness of glass is about 0.63 MPa center dot m(1/2). Under the same normal load, radial crack length shortened with the increase of AZO film thickness. When indented under deionized water, cracks in both glass and AZO films got longer, signifying corresponding decrease in the fracture toughness.
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  • Interfacial microstructure of chromium oxide coatings RID A-7974-2009

    Pang, Xiaolu   Gao, Kewei   Yang, Huisheng   Qiao, Lijie   Wang, Yanbin   Volinsky, A. A.  

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  • Thickness effect on the band gap of magnetron sputtered Pb45Se45O10 thin films on Si

    Sun, Xigui   Gao, Kewei   Pang, Xiaolu   Yang, Huisheng   Volinsky, Alex A.  

    Oxygen doped PbSe thin films with different thickness were grown on the Si (100) substrates by magnetron sputtering, and characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and physical properties measurement system. As the film thickness increased, the intensity of the (200) PbSe prominent diffraction peak increased, while the (220) peak almost vanished, indicating the primary growth direction. The change rate between the light and dark resistance increased with the film thickness, and the maximum of 64.76% was obtained. According to the density functional theory calculations and the experimental results, the band gap of the PbSe thin films decreased from 0.278 eV to 0.21 eV when doped with oxygen. Doping with oxygen during the deposition process is a viable way to prepare PbSe thin films with a tunable band gap. The band gap increased almost linearly with the lattice constant, confirmed by the calculated and experimental results. (C) 2014 Elsevier B.V. All rights reserved.
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  • AlTiN layer effect on mechanical properties of Ti-doped diamond-like carbon composite coatings

    Pang, Xiaolu   Yang, Huisheng   Gao, Kewei   Wang, Yanbin   Volinsky, Alex A.  

    Ti/Ti-doped diamond-like carbon (DLC) and Ti/AlTiN/Ti-DLC composite coatings were deposited by magnetron sputtering on W18Cr4V high speed steel substrates. The effect of the AlTiN support layer on the properties of these composite coatings was investigated through microstructure and mechanical properties characterization, including hardness, elastic modulus, coefficient of friction and wear properties measured by scanning electron microscopy, Raman spectroscopy, scratch and ball-on-disk friction tests. Ti and AlTiN interlayers have a columnar structure with 50-80 nm grains. The hardness and elastic modulus of Ti/Ti-DLC and Ti/AlTiN/Ti-DLC coatings is 25.9 +/- 0.4, 222.2 +/- 6.3 GPa and 19.3 +/- 1, 205.6 +/- 6.7 GPa, respectively. Adhesion of Ti-DLC, Ti/AlTiN/Ti-DLC and AlTiN/Ti-DLC coatings expressed as the critical lateral force is 26.5 N, 38.2 N, and 47.8 N, respectively. Substrate coefficient of friction without coatings is 0.44, and it is 0.1 for Ti/Ti-DLC and Ti/AlTiN/Ti-DLC coatings. Wear resistance of Ti/AlTiN/Ti-DLC composite coatings is much higher than Ti/Ti-DLC coatings based on the wear track width of 169.8 and 73.2 mu m, respectively, for the same experimental conditions. (C) 2011 Elsevier B.V. All rights reserved.
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  • Microstructures and properties of Ag-Cu-Ti-In composite fillers for electronic packaging applications

    Zhang, Shanshan   Yan, Luchun   Gao, Kewei   Yang, Huisheng   Yang, Lihang   Wang, Yanbin   Wan, Xiaoling  

    With excellent thermal cycling performance, active metal brazing (AMB) ceramic substrates have been highly competitive materials for high power packaging. In related researches, composite filler is an effective way to improve the comprehensive performance of ceramic substrates. The effects of particle hardness on microstructure and properties (i.e., coefficient of thermal expansion (CTE), elastic modulus and compression properties) of Ag-Cu-Ti-In composite fillers were studied in this paper. The results showed that the microstructures of composite fillers were refined with the increase of graphite and TiC content. Meanwhile, the reduction of CTE of composite fillers added with TiC particles is better than that of graphite particles. The CTE of G10 sample is 7.4% lower than that of the sample without particle, and T10 sample is 8.9% lower. Furthermore, TiC particles increased both elastic modulus and yield strength of the composite fillers while graphite particles mainly had an opposite effect. The bulk modulus of the composite fillers drops sharply from 95.96 to 12.64GPa when the content of graphite particles was 10wt% compared with that of the filler metal without particles. The yield strength of composite filler with graphite content of 1wt% reaches 380MPa, which is caused by the in situ reaction between graphite and Ti. However, the elastic modulus and the yield strength of the composite fillers increases with the increase of TiC content.
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  • Role of deposition parameters on microstructure and mechanical properties of chromium oxide coatings

    Luo, Fei   Pang, Xiaolu   Gao, Kewei   Yang, Huisheng   Wang, Yanbin  

    Chromium oxide coatings were deposited by reactive magnetron sputtering on high speed steel (HSS) substrate under various oxygen flow rates and radio frequency (RF) powers. The effect of deposition conditions on the microstructure, hardness and critical load of chromium oxide coating failure was studied. The results indicated that a crystalline chromium oxide coating formed at a high oxygen flow rate and a low RF power exhibited a higher hardness and a lower critical load as compared to a chromium oxide coating with an amorphous microstructure. (C) 2007 Elsevier B.V. All rights reserved.
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  • Annealing effects on microstructure and mechanical properties of chromium oxide coatings RID A-7974-2009

    Pang, Xiaolu   Gao, Kewei   Luo, Fei   Yang, Huisheng   Qiao, Lijie   Wang, Yanbin   Volinsky, Alex A.  

    Reactive radio frequency magnetron sputter-deposited chromium oxide coatings were annealed at different temperatures and times. The influence of annealing temperature on the microstructure, surface morphology and mechanical properties was examined by X-ray diffraction, nanoindentation, pin-on-disc wear and scratch tests, respectively. X-ray results show that the chromium oxide sputtered at room temperature in low oxygen flux is primarily amorphous. Annealing below 400 degrees C did not cause much change, while annealing at higher temperature of 500 degrees C caused a significant change in microstructure and mechanical properties. Hardness increased from 12.3 GPa to 26 GPa, and the wearability improved with higher annealing temperature due to the formation of crystalline Cr2O3 phase, which occurs at 470 degrees C. Annealing time had little effect on mechanical properties and microstructure, although coating surface roughness increased with a longer annealing time. Coating adhesion was improved by annealing, due to residual stress relief and possible interfacial interdiffusion. (C) 2007 Elsevier B.V. All rights reserved.
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