On Thermoelastic Contact And Dynamic Sliding Instability Of Functionally Graded Coated Structures

Functionally graded materials (FGMs) possess properties that vary gradually with location within the materials. Used as coatings and interfacial zones they tend to reduce stress concentration resulting from material property mismatch, increase the bonding strength, improve the surface properties and reduce thermal and residual stresses in an elevated temperature environment. In this thesis the thermo-elastic contact and sliding dynamic instability of FGM coated structures are studied, and possibility of the improvement of the resistance to contact damage and the sliding dynamic stablility by using the FGM coating is discussed. The main contents and conclusions include:1. The two-dimensional (2D) thermo-elastic sliding frictional contact of the FGM coated half-plane under the plane strain deformation is investigated. The thermo-mechanical properties of the FGM coating vary exponentially or arbitraily along the thickness direction. The results show that the distributions of the surface contact stresses and temperature can be altered by adjusting the gradient of the thermo-elastic parameters of the coating. It is indicated that the FGM coatings have potential applications in improving the resistance to thermo-elastic contact damage. The results also show that we can alter the distribution of the contact stresses and surface temperature by adjusting the gradient type of the coating material.2. The2D elastic dynamic instability problem of the FGM coated half-plane sliding against a homogeneous half-plane is investigated by examining the stability of the elastic wave caused by perturbations. The results show that the stability can be improved by adjusting the gradient index of the coating. Compared with the homogeneous coated structure, the stresses at the bonded interface are continuous in the FGM coated structure. By changing the coating thickness, the tensile stress amplitude of the sliding and bonded interfaces can be reduced. In other words, the FGM coated structure is more effective in controlling the interfacial tensile stress, and thus in reducing the possibility of interfacial and contact failure. Meanwhile, the sliding dynamic stability may be improved through adiusting the FGM coating thickness.3. With consideration of the frictional heating and thermal effects, the2D thermoelastic dynamic instability problem of the coated half-plane sliding against a homogeneous half-plane is investigated by examining the stability of the thermo-elastic wave caused by perturbations. Firstly, we consider the case without a coating, i.e. the problem of two elastic half-planes sliding against each other. It is shown that the material properties and uniform pressure have obvious effects on the stability. Sliding is instable for an arbitrary friction coefficient when the material mismatch is small enough. Instable sliding occurs at the finite values of the friction coefficient when the material mismatch is large. Secondly, we consider the case of a homogeneous coating and analyze the effects of the coating materials and thickness on the stability. It is shown that favorable thickness can be chosen for different coating materials to keep stable sliding. At last, we investigate the thermo-elastic dynamic instability of the FGM coated half-plane sliding against a homogeneous half-plane. The results show that the stability, including the critical friction coefficient and critical sliding speed, can be modified by adjusting the gradient of the FGM coating. Furthermore, compared with structure with a homogeneous coating, the tensile stresses at bonded interface are continuous in the FGM coated structure. The amplitude of the tensile stresses at the sliding and bonded interfaces can be reduced significantly through adjusting the coating thickness. So an FGM coated structure is more favorable in controlling the interfacial tensile stress, and thus in reducing the possibility of interfacial and contact failure.The present investigation, which may be a positive contribution to the theory of thermo-elastic contact mechanics and dynamic instability of FGMs, is significant to optimal design and practical applications of FGM coating for the purpose of improving the resistance to thermo-elastic contact damage and dynamic instability.