Contact mechanics of FGM coatings

The concept of grading the thermo-mechanical properties of materials provides an important tool to design new materials for certain specific functions. To take full advantage of this new tool research is needed not only for developing efficient processing methods and material characterization techniques but also for carrying out basic mechanics studies relating to the safety and durability of the FGM components. These studies may help to protect certain components against abrasion and wear at the surface where the maximum stress often occurs.; Graded materials, also known as functionally graded materials (FGMs), are generally two-phase composites with continuously varying volume fractions. Used as coatings and interfacial zones they can reduce thermally and mechanically induced stresses resulting from material property mismatch, increase the bonding strength and provide protection against adverse environments.; In this study the contact problem for the FGM coatings is considered. The objective of the study is to obtain a series of analytical benchmark solutions for examining the influence of such factors as material inhomogeneity constants, the coefficient of friction, curvatures and various length parameters on the critical stresses that may have a bearing on the fatigue and fracture of the components with FGM coatings.; In the first part of this study an FGM layer bonded to a homogeneous substrate loaded by a rigid stamp is considered. It is assumed that the coating is inhomogeneous, isotropic and linearly elastic, and the problem is one of plane strain. The shear modulus of the FGM coating is represented by an exponential function. The Poisson's ratio is assumed to be constant. With the applications to load transfer components and abradable seals in mind, it is also assumed that the contacting surfaces are in relative motion with constant coefficient of friction. The related contact problem is solved for various different stamp profiles including flat, triangular, semicircular and cylindrical.; In the second part of this study the contact problem for two elastic load transfer components with FGM coating have been solved. The load transfer components are typically gears, cams, bearings and machine tools, where two elastic solids of given curvatures move relative to each other. The curvatures may be positive or negative. In these components ceramic coatings may be used to increase the wear resistance, whereas the material property grading would improve the toughness, reduce the stress levels and again increase the bonding strength.; After solving the half space problem the Green's functions necessary for the solution of the problem in load transfer components are obtained. A parametric study has been done to find the effects of the parameters such as coefficient of friction, material property grading and surface shear modulus of elasticity of upper/lower or inner/outer cylinders. The contact stresses at the surface of the FGM coating are given and discussed.