Papers by Author: Guang Nan Chen

Abstract: During the process of Laser beam long-range energy transmission based on the photovoltaic principle, the temperature elevation of the photovoltaic cell under Laser irradiation would greatly affect its photoelectric conversion efficiency. In this work, the heat generation mechanism of thin-film photovoltaic cell under Laser irradiation was analyzed based on the energy conversion relationship between light, electricity and heat. The output voltage characteristic of a thin film GaInP/GaAs/Ge triple-junction photovoltaic cell under irradiation of dual-wavelength Laser beam was investigated, and the results show the dependence of the conversion efficiency on the temperature and the valve effect, which was caused by the long wavelength Laser and would sharply reduce the photovoltaic conversion efficiency of short wavelength Laser. The temperature field of the multi-layer structure subjected to Laser irradiation was analyzed by finite element method, and the results indicate that the temperature elevation pattern depends on the spatial distribution of the light absorption and can be adjusted via optimizing the photovoltaic cell structure.

Abstract: In this paper, the thermal shock induced cracking behavior of a segmented coating on the outer surface of a hollow cylinder has been investigated. The driving force for the propagation of multiple segmentation crack, represented by the Thermal Stress Intensity Factor (TSIF), was determined by combination of the principle of superposition and the finite element method. The maximum TSIF has been shown to occur neither at the beginning nor at the steady state of thermal transients, but at an intermediate instant. As the spacing between multiple segmentation cracks decreases, the magnitude of TSIF first plateaus, and then decreases sharply. This quantitative mechanistic result rationalizes the experimental observations that a segmented coating can exhibit much higher thermal shock resistance than an intact counterpart, if only the segmentation crack spacing is narrow enough. Some other parameters affecting TSIF, such as segmentation crack depth and convection severity, were also discussed.

Abstract: The initial micro-crack density of a chromium coating/steel substrate with and without laser pre-quenching treatment was investigated in this work. It was found the micro-crack density of Cr coating on the pre-quenched substrate surface was much lower than that of the Cr coating on the original substrate surface. Based on the results presented in this work and in the literature, the find presented in this work indicates the lower crack density of the chromium coating on the pre-quenched substrate may improve the mechanical properties of this material system, and it may be an important mechanism responsible for the prolonged lives of workpieces in practice.

Abstract: Adhesion of a coating on its substrate is a crucial parameter determining performance and reliability of coating-substrate system. In this work, the uniaxial tensile and cross-sectional indentation tests are used to investigate the adhesion between the brittle Cr coating and the ductile steel substrate. The uniaxial tensile test results show the maximum shear flow stress level of the ductile steel substrate can only serve as a lower bound estimate on the ultimate shear strength of the interface. The cross-sectional indentation test results show the interfacial decohesion does not occur prior to the cracking of the brittle Cr coating.

Abstract: The evaluation of the interfacial adhesion of coating system has always been a rough task. In this paper, a special testing method of cross-sectional indentation is applied on a model coating system, i.e. electroplated chromium on a steel substrate which is generally regarded as an example of materials pair with strong adhesion. Based on fractography analysis with SEM and interfacial stress simulation with FEM, it is found that interfacial shear stress may induce coating spalling. More interestingly, spalling location is sensitive to substrate pretreatment process. This shows the feasibility of cross-sectional indentation to distinguish interfacial strength at a high level.

Abstract: novel duplex coating is presented on steel substrate by laser discrete quenching prior to chromium coating deposition. Due to the periodical changes in the mesostructures and properties of underlying substrate, the cracking and spalling behaviors of the duplex coating are expected to be different from that of conventional coating. A classic erosion experiment is selected to characterize the positive response of the duplex coating subjected to thermal loads. It is found that the surface cracking is partly restrained and the interface cracking is partly arrested. FEM modeling results show that a unique stress state is induced in the duplex coating with the tempering of the discrete pre-quenched regions due to thermal loading, which is believed to be the mechanism of controlling the damage modes.