Researches For Interfacial Delamination Mechanism Of Coatings/Substrates System Subjected To Adhesive Contact Load

To avoid the adhesion failure and increase the system performance,coatings are widely applied in contact surface of micro/nano electro-mechanical systems to reduce surface adhesion.However,the mismatch of coating and substrate properties and the residual stress caused by preparation process can lead to interfacial delamination.The theoretical study of this issue is urgently intensified,which has significant guidance for the preparation of coating.The dissertation focuses on the issue of interfacial delamination of the coating/substrate system at the micro/nano-scale and aims to construct delamination mechanism maps to quantitatively characterize the interface bonding property.The main contents are as follows.(1)In the absence of surface adhesion,the finite element model of the contact between a rigid sphere and an elastic coating on a semi-infinite elastic-plastic substrate is established.By conducting detailed parametric studies,we constructed delamination mechanism maps which show the critical indentation depth and force required to initiate interface delamination in the absence of adhesion.It is found that increasing the cohesive strength/energy can reduce the probability of interfacial delamination.Additionally,the coating/substrate interface can receive better protection with lower coating elastic modulus.Moreover,increasing the coating thickness will prompt delamination nucleation for lower indentation depths,but an opposite trend occurs for higher indentation depths.A coated system with tensile residual stress may be likely to suffer from interfacial delamination than that with compressive stress.(2)The nonlinear spring elements with a force-displacement relationship derived from the Lennard-Jones potential are used to model the surface adhesion between the indenter and the coating.The issues of adhesion-delamination are investigated.Results show that the unloading response consists of five sequential stages: elastic recovery,interface damage(crack)initiation,damage evolution(delamination),coating elastic bending,and abrupt surface detachment(jump-out).Additionally,in the indentation process,the three contact instabilities are observed: abrupt surface contact(jump-in),crack initiation,and abrupt surface detachment(jump-out).Moreover,the normal stress along the coating surface is found to be tensile around the edge of contact zone at the instant of maximum indentation depth(3)Delamination mechanism maps considering surface adhesion are constructed.It is found that increasing the adhesion work will prompt delamination nucleation.For relatively stronger adhesion,the higher the coating elastic modulus or the coating thickness is,the greater the critical indentation depth becomes.However,for relatively weaker adhesion,the effect of the coating elastic modulus and the coating thickness on the critical indentation depth is similar to that of adhesionless.Additionally,in the presence of surface adhesion,the transition of compressive stress to tensile stress in the coating can reduce the risk of interface delamination for lower indentation depths,whereas the opposite happens for higher indentation depths.