Dongfeng Diao

Fracture Mechanisms of Thin Amorphous Carbon Films in Nanoindentation

Abstract The nanoindentation fracture of amorphous carbon films on silicon substrate was studied. Load-displacement curves were obtained during indentation in conjunction with the scanning electron microscope (SEM) observations of fractured surfaces at different loads. The fracture process was found to progress in three stages: (1) first ring-like through-thickness cracks form around the indenter by high stresses in the contact area; (2) delamination and buckling occur around the contact area at the film/substrate interface by high lateral pressure; and (3) second ring-like through-thickness cracks and spalling are generated by high bending stresses at the edges of the buck led film. The strain energy release in cracking was estimated from a step observed during the loading cycle of the load-displacement curve. An equation for calculation of fracture toughness of thin films is introduced based on the analysis of the energy release rate. The methodology is used to obtain the fracture toughness of thin films. The results show that the calculated values are in good agreement with those measured by conventional methods.

The maximum tensile stres on a hard coating under sliding friction

Abstract In the friction of a hard coating the maximum tensile stress in the sliding direction generated at the friction surface is important for predicting crack propagation in the coating. The finite element method is employed to evaluate the stress field in the hard coating and the substrate under frictional loads, and the ratio between the values of maximum tensile stress and the maximum contact pressure is calculated under various contact conditions. Finally, a simple equation is introduced for the calculation of the maximum tensile stress at the friction surface. This equation is a function of friction coefficient, maximum contact pressure, coating thickness, contact width and elastic moduli of coating and substrate, and gives the value of the maximum tensile stress which is affected by the existence of the substrate.

Graphene sheets embedded carbon film prepared by electron irradiation in electron cyclotron resonance plasma

We used a low energy electron irradiation technique to prepare graphene sheets embedded carbon (GSEC) film based on electron cyclotron resonance plasma. The particular π electronic structure of the GSEC film similar to bilayer graphene was verified by Raman spectra 2D band analyzing. The phase transition from amorphous carbon to GSEC was initiated when electron irradiation energy reached 40 eV, and the growth mechanism of GSEC was interpreted as inelastic scattering of low energy electrons. This finding indicates that the GSEC film obtained by low energy electron irradiation can be excepted for widely applications with outstanding electric properties.

Magnetic behavior of graphene sheets embedded carbon film originated from graphene nanocrystallite

We found paramagnetic behavior at 300 K of graphene sheets embedded carbon (GSEC) film, which is deposited under low energy electron irradiation in electron cyclotron resonance plasma. The origin of the magnetic properties of GSEC film is ascribed to the formation of graphene nanocrystallite. With higher irradiation energy, the size of nanocrystallite barely changed, while the density in GSEC film became higher, leading to a dramatically increase of saturation magnetization and residual magnetism. This finding indicates that GSEC film with higher magnetization can be expected, which has the potential for magnetic and spintronics applications.

The Local Yield Map of Hard Coating under Sliding Contact

The stress distribution in the hard coating and substrate under elastic sliding contact is analyzed with the finite element method. The elliptical distribution of normal contact pressure is supposed and the combinations of coating thickness, friction coefficient and elastic modulus of coating and substrate are changed in wide range. The position of yield initiation is found with the calculated result and the local yield map is introduced in relation to the ratio of the coating yield strength to substrate yield strength and the ratio of the coating thickness to the half-contact width. The local yield map shows that the yield at the coating interface is the most general case in wide range of contact condition and the yield at the coating surface becomes the next general case as the friction coefficient becomes large.

Finite element analysis on local yield map and critical maximum contact pressure for yielding in hard coating with an interlayer under sliding contact

Abstract Micro-crack in the hard coating initiates usually from the local yield position. To prevent the crack to occur, the most important criterion is to satisfy the condition that the von Mises stress is less than the yield strength of material. In this paper, the local yield map showing the yielding position are introduced by using the finite element method and the criterion. The critical maximum contact pressures for yielding at the different positions were calculated by using the coating thickness, friction coefficient and the yield strength, where a critical range of yield strength ratio for the transition of yielding positions was found for 0.4∼0.5. When the yield strength ratio is less than the range, yielding initiates from the interlayer and when the ratio is larger than the range, yielding initiates at the surface for high friction coefficient and in the base for low friction coefficient.

Effect of interlayer on maximum contact stresses of hard coating under sliding contact

Abstract In order to prevent surface damage or spalling of a coating with an interlayer under sliding contact, the understanding of the effect of the interlayer on the maximum contact stresses (maximum tensile stress, maximum shearing stress, maximum von Mises stress) is necessary, Because the damage or the spalling of coating initiates usually from the positions of these maximum stresses. Therefore, in this paper, the sliding contact problem of a hard coating with an interlayer was analyzed using the finite element method. The results show the effect of four typical elastic moduli of the interlayer, i.e., E inter = 2 E topp , ( E top + E base )/2, E base and E inter = E base /2 (here E inter is the elastic modulus of interlayer, E top is the elastic modulus of the hard coating and E base is the elastic modulus of the base) on these maximum stresses. In the case of maximum tensile stress on the surface, E inter = 2E top is most useful for decreasing the stress as compared to other interlayers, while in the case of maximum shearing stress on the interface between the coating and the substrate, E inter = E base /2 is most useful for decreasing the stress. In the case of maximum von Mises stress, E inter = 2 E top shows the largest effect of interlayer on the stress. In addition, the effect of the coating thickness and the friction coefficient on these maximum stresses are also presented in the paper.

Fracture mechanisms of ceramic coating during wear

Abstract A typical buckling phenomenon of the coating on the wear groove caused by the residual compressed stress was analyzed by the interface fracture mechanics and the buckling theory. It has been found that there is a critical thickness of coating on the wear groove for the buckling. The critical thickness can be calculated by t b c d = [ 12(1 − v 2 f )σ R π 2 E f ] 1 2 (here tb is the coating thickness, cd the length of the interfacial crack, vf the Poissons ratio of the coating, σR the residual compressed stress in the coating, and Ef the elastic modulus of the coating).

Magnetism induced by excess electrons trapped at diamagnetic edge-quantum well in multi-layer graphene

In this paper, we clarified a robust mechanism of magnetism generated by excess electrons captured by edge-quantum well of diamagnetic armchair edges. Consistency between density functional theory calculations and electron cyclotron resonance experiments verified that: (1) Multi-layer armchair nanoribbons are stable with proper amounts of excess electrons which can provide net spin; (2) Since under-coordination induces lattice relaxation and potential well modulation, electrons tend to be trapped at edges; and (3) Neither large amount of excess electrons nor positive charges can induce magnetism. This work shed light on the development of graphene devices in its magnetic applications.

Three-Dimensional Local Yield Maps of Hard Coating Under Sliding Contact

The local yield maps for the identification of the yield initiation positions of hard coating on three-dimensional (3D) elastic half space under sliding contact were developed. In this study, the semi-analytical method (SAM), which is based on the conjugate gradient method (CGM) and the discrete convolution and fast Fourier transform (DC-FFT) technique, was employed to analyze the contact problem. By using this method, the von Mises stress distributions for various combinations of coating thicknesses, friction coefficients, and elastic moduli of the coating and substrate were calculated. Then, the positions of yield initiation were found with the calculated results by comparing the critical maximum contact pressure Pmax,c for von Mises yielding at or in the different positions (surface, coating, interface, and substrate), and the 3D-local yield maps were introduced in relation to the yield strength ratio of the coating to the substrate (Yf/Yb) and the ratio of the coating thickness to the Hertzian contact radius (t/a0). Finally, the effect of critical friction coefficient on the transition of yielding positions was discussed. [DOI: 10.1115/1.4005265]