Papers by Author: Xun Cai

Abstract: With the development of modern science and technology, the elements such as Al, Si, Mo, C, B will be doped into the TiN and CrN binary films to improve their properties. In this work, a series of Ti-X-N and Cr-X-N films were prepared under the various N2 partial pressures，bias voltages and substrate temperatures by reactive magnetron sputtering using the mosaic target and multi-targets systems. The composition, microstructure, mechanical properties and thermal stability of the films were investigated using EDS, XRD, XPS, AFM, nano-indentation, scratch and thermal stability test. The results indicated that the doping element content, microstructure and mechanical properties of the films can be easily regulated through the deposition parameters, such as the N2 partial pressure，bias voltages and so on. The superhard Ti-Si-N and Ti-Al-N films with the nanohardness of more than 40GPa can be achieved, especially when the lower N2 partial pressure is used.

Abstract: The microstructure of Ag/TiO2 and Ag/Ti/TiO2 multilayer system are investigated by AFM and TEM, and their interfaces are analyzed by the means of spectroscopic ellipsometry (SE). The results show that the multilayer has a sharp interface without interdiffusion and interface reaction products. AFM surface roughness analysis indicates a 2-nm titanium transition layer can improve surface quality of silver films, because it is enabled to change silver island dimension and distribution and improve coverage of silver films on Ti/TiO2 substrate. This can be attributed to titanium ׳s strong oxygen affinity and good wettability to titanium dioxide. The optical properties test of TiO2/Ag/TiO2 multilayer demonstrates surface plasma resonance (SPR) absorption shifts towards long wave region with the increasing of the total thickness of dielectrics.

Abstract: Cross-sectional nanoindentation (CSN) is a new method for measuring interface adhesion of thin films. The interfacial energy release rate (G), characterizing interfacial adhesion, is calculated from the material and geometrical parameters relevant to the test. Effects of residual stresses on G and crack tip phase angle Ψ, have been studied by finite element simulation in this study. The results show tensile residual stresses increase G and compressive stresses reduce it, and they have similar effects on the magnitude of Ψ.

Abstract: The mixture of Ni based alloy powder and WC particles were used as a feeding material to modify the surface properties of cast Al-Si alloy using a CO2 continuous transverse flow laser beam with maximum power of 10 kW. Microstructures and chemical components of the laser surface cladding (LSC) layers were studied using SEM, XRD, TEM and EDS. It is shown that the LSC layers were composed of γ-（ Ni, Cr, Fe, W）matrix phase and many enhancing phases, such as Ni2Al3, Ni3Al, WC, W2C, Cr2B, etc.. The microstructure of the LSC layers was greatly affected by the scanning rate b V and the powder of feeding rate p m under the same laser power. With the increasing of b V and p m , the dissolution phenomenon of WC particles was improved; the length, the diameter and the amount of the acicular constituent were markedly reduced. Microhardness and wear resistance tests were also performed: the average microhardness of the LSC layers was around 5.1 to 5.9GPa, which was five times higher than that of the Al-Si substrate. The wear resistance of the layer was about 20 times as big as that of cast Al-Si alloy when P=6kW, b V =13.3mm s-1, p m =100mg s-1, L=500N. The results showed that the mechanical properties of LSC layers on cast Al-Si alloy can be markedly enhanced with proper processing parameters. However, due to the sudden change of physical and mechanical properties between laser modified layer and substrate, some defects, especially crack, actually occur in the surface modified layer and the interface zone. And finally Ni/WC surface gradient layer was obtained on cast Al-Si alloy through thrice laser scanning technique. The microhardness of the laser gradient layer gradually changed from surface to substrate, so that it can reduce stress concentration in the whole laser surface layer, especially in the interface zone.

Abstract: The effect of a AlN gradient interlayer on the surface mechanical properties was investigated by nano-indentation for the TiN film on N+-implanted Al substrate. The AlN interlayer, 80nm thick measured by Auger Electron Spectrometer (AES), was formed by high energy N+-implantation prior to magnetron sputtering the TiN film in our custom designed multifunctional ion implanter. The nanohardness of N+-implanted aluminum was 450HV at extremely small depth, but quickly decreased to a constant value (65HV). The hardness and Young’s modulus of the TiN film on two different substrates, one with and the other without N+-implantation, kept almost constant up to a small depth of 200nm, and then decreased to the values of the Al substrate with increasing indentation depth, but with a lower decreasing rate for the N+-implanted system. It was found from the load-displacement curves that the interfaces cracked when the indentation load was 38mN for the N+-implanted system, while 18mN for the unimplanted system. Therefore the N+- implantation improved the surface mechanical properties significantly.

Abstract: Crack propagation of X-70 pipeline steel in near-neutral pH solution was studied under different modes of cyclic loading. A revised equation of crack growth rate vs. Δ K was obtained. Average crack growth rate increased with cycles under conditions of different R values. Linear shape notch specimen made cracks much easier to initiate and propagate than V-shaped notch specimen did. For different R values, the curves of crack growth rate with cycles were similar, but the platform propagation period and quick propagation period were different obviously. Crack growth rate at both periods increased and thus failure time decreased markedly with decrease of R value. The propagation directions of cracks were different under different cyclic loading conditions. Under mode I (single tensile stress) cyclic loading, cracks were straight and perpendicular to the tensile stress axis, while under mixed-mode I/III (tensile/shear stress) cyclic loading, cracks were sinuous and did not propagate in the direction perpendicular to the main tensile stress axis.