Papers by Author: Vesselin Stoilov

Abstract: Al/Si alloys are considered to be one of the most promising light weight alloys that can be used extensively in aerospace and automotive industry except for the poor tribological behaviour. However, with advancement and precision of the surface coating depositing techniques, new coating design which significantly enhances the tribological properties of the light weight alloys becomes attainable. In this paper, an innovative coating design is presented and thoroughly analyzed using finite elements method. The proposed model consists of Al/Si 319 as a matrix within which the geometrically defined hard Si particles are dispersed on the surface, and a hard coating layer then deposited in between the Si particles so that the lateral movement of the Si particles is constrained. ABAQUS is utilized to model and address the effects of different parameters, such as coating material, the hard coating thickness, and geometrical shape of the Si particles on the fracture and deboning of the entire structure. Two Si particles shapes are studied: circular and elliptical. Three coating materials are investigated: DLC, CrN and Al2O3. Besides, four coating thicknesses of 4 µm , 8µm, 15µm and 20µm are tested. It is found out that there is no single significant parameter which affects the fracture and deboning of Si particles, yet it is the combination of different parameters. The Si particle geometry plays a major role in determining the critical fracture stress with a circular shape outperforms the elliptical shape. The combination the circular Si particles and the CrN as coating material gives the highest critical fracture stress. Finally, DLC does not perform well with the circular Si Particle and it show the highest possible fracture stress with elliptical Si particle

Abstract: One of the most challenging areas in engineering of composite materials is the fabrication of high quality microstructures. This issue is complicated for tungsten composites due to its high melting temperature and this leads to limitations in terms of possible processing techniques. This research investigates the fabrication of W-12%wtCu composites based on powder metallurgy techniques. Due to the very large difference between the melting point of tungsten and copper, there is no common sintering temperature range for them. In this work, 0.5%Wt nickel was added as an activator to decrease the sintering temperature of tungsten using an activated sintering effect. The effects of pressure, sintering time and temperature in solid state and liquid phase conditions were also investigated. While solid state hot pressing did not result in appropriate microstructures, the liquid phase hot pressing provided high quality samples with a relative density of 98.0%.

Abstract: In the present study molecular dynamics simulations were carried out to investigate the deformation of pure FCC aluminum and diamond cubic silicon interfaces under shear stress. A second nearest-neighbor modified embedded atom method was used to describe the interactions between Al-Al, Si-Si and Al-Si atoms. The critical shear stress (CSS) was determined for various Al/Si and Al/Al interfaces with different alignments and orientations. Structural analyses show that the deformation is localized at approximately 10 Å thickness of the interface in Al. The critical shear stress of Al/Si interface was found to be significantly lower than the critical tensile stress due to the partial stick-slip in sliding. In addition, it has been proven that there is no explicit relationship between shear and tensile critical stresses, which is fundamentally different from isotropic materials, where the shear stress is about half of the tensile stress. The misorientation has a dramatic effect in reducing shear stress at Al/Al interfaces, but has no effect on CSS in Al/Si. As a result, it was shown that introducing Si improves the strength of the interface (and the composite material in general) for different grain orientations.