Papers by Author: Ying Qin

Abstract: Thermal barrier coatings are used as thermal insulation and thermal protection for high-temperature components of aircraft engines. Service failure of the components is often caused by premature breaking of the coatings. The distributions of residual stress in a ZrO₂/NiCoCrAlY thermal barrier coating during thermal cycling was here simulated using finite element method. Four kinds of models involving planar and curved interfaces between the bond and the ceramic top coat layers were established. The simulated results showed that the high residual stress which is about 100-300MPa is present at the interface between bond layer and ceramic layer. The residual stress in curved interface is larger than that in planar one, and concentrates mainly at the troughs. Structure for planar interface exhibits better than curved interface. The residual stress would increase obviously with the presence of thermally grown oxide layer.

Abstract: High current pulsed electron beam is an effective technique for surface sealing of ceramic thermal barrier coatings prepared by electron beam physical vapor deposition. Due to the rapid remelting and solidification, the outer layers of ceramic coatings become smooth and dense, and the protective performance for turbine blades is effectively improved. Because of the complex multi-layered structures in the coatings, a high-current pulsed electron beam treatment requires specific parameter inputs which are related to the temperature field induced by electron energy deposition in the coatings. In this paper, a two-dimensional temperature simulation was performed to demonstrate the melting depth and temperature evolution in ceramic coatings treated by high-current pulsed electron beam. Different energy densities and pulses were studied and discussed for obtaining optimized parameters.

Abstract: High current pulsed electron beam (HCPEB) is a fairly new technique for improving surface properties such as corrosion and wear resistances. One of the negative effects induced by HCPEB is the potential formation of craters on the surface of the HCPEB treated materials. These changes can impair the corrosion-resistance by promoting pitting. The mechanisms of nucleation and growth are detailed and the effect of the number of pulses on crater formation is discussed.

Abstract: Due to the special heating mode of High Current Pulsed Electron Beam (HCPEB) irradiation, intense stresses such as thermoelastic stress, quasi-static stress, and shock wave stress can be generated by a dynamic thermal field. A dynamic thermo-stress is the origins of these stresses. The simulations for non-melting and melting modes are respectively compared with related phenomena such as bending, surface plastic deformation, and residual stress (non-melting mode, quasi-static stress-related), crater formation, depth distribution of microhardness, fragmentation of pearlites (melting mode, shock-stress wave-related).

Abstract: High current pulsed electron beam (HCPEB) is now becoming a promising energetic source for the surface treatment of materials. When the concentrated electron flux transferring its energy into a very thin surface layer within a short pulse time, superfast processes such as heating, melting, evaporation and consequent solidification, as well as dynamic stress field induced by an abrupt thermal distribution in the interactive zone impart surface layer with improved physicochemical and mechanical properties. The present paper reports mainly our experimental research work on this new-style technique. Investigations performed with a variety of constructional materials (aluminum, carbon and mold steel, magnesium alloys) have shown that the most pronounced changes of composition, microstructure and properties occur in the near-surface layers, while the thickness of the modified layer with improved mechanical properties (several hundreds of micrometers) is significantly greater than that of the heat-affected zone due to the propagation of stress wave. The surfaces treated with either simply several pulses of bombardment or complex techniques, such as rapid alloying by HCPEB can exhibit improved mechanical and physicochemical properties to some extent.

Abstract: The simulation of the temperature reveals an ultra high heating/cooling rates in the order of 108~109 K/s and melted layer thickness micrometers in depth. A temperature-induced dynamic thermal stress fields can then generate three principal stress, the quasi-static stress, the thermoelastic stress, and the shock stress, the latter two being stress waves. The thermoelastic stress wave has small amplitudes less than 0.1 MPa. The shock stress wave however is a typical nonlinear wave, several hundreds of MPa in amplitude, much stronger than the thermoelastic stress wave, and has a strong impact on materials structure and properties far beyond the heat-affected zone. The maximum compressive quasi-static stress in the surface layer in aluminum reaches several hundreds of MPa, which easily induces surface deformation in metallic materials.