FEA On Contact Problems Of MEMS Coating

Coatings are often used on the contact surface of micro-electromechanicalsystem(MEMS) to reduce friction, wear and surface adhesion, thus improving contactperformances and service life of the micro device. However, there exists some aspectswhich may induce crack in the interface between the coating and the substrate, such as themismatching of material properties of the coating and substrate, the insufficient interfacialcohesive strength and the loading and unloading cycles. Once a crack is formed, it willextend and result in interface delamination and ultimate failure of the contact system. Theobjective of this dissertation is to analyze the effects of coatings on contact performances,including two categories. One is the contact performances of microball contacts with thecoated race in the silicon microball bearing, the other one is the problems of interfacedelamination and contact performances in MEMS coatings. The contents and conclusionsare as follows.First, a finite element model of the contact between the ball and the silicon racewith/without the diamond like carbon coating(DLC) is developed for a new kind of siliconmicroball bearing. Effects of different sizes, materials of the microball and the design of theDLC on contact performances are investageted. The results show that the density of themicroball can affect the contact stresses obviousely. The proper sizes, the lower materialdensity and elastic modulus of the microball, and the proper elastic modulus of the DLC arebenefical to contact performances of the microball bearing.Second, considering the interface delamination, the finite element model of a rigidsphere contact with an elastic coating and a semi-infinite elastic-plastic substrate isestablished, and the cohesive interface of the coating and substrate is constructed based onthe cohesive zone model. Effects of the maximum loading depth, the cohesive strength, thework of cohesive, the elastic modulus ratio and the multi cyclic loading and unloading onthe delamination are investigated. The results show that crack size increses with the loadingdepth and the elastic modulus ratio. Crack size will decrease as the cohesive strengthincreases, and the work of cohesive can prevent the crack from extending. Multi loadingand unloading cycles will aggravate the damage.Next, considering the surface adhesion, the finite element model of a rigid sphere inclose proximity with an elastic coating and a semi-infinite elastic-plastic substrate is developed, and the surface adhesion exists between the sphere and the coating is modeledbased on the well-known LJ potential. Effects of the maximum loading depth, the elasticmodulus ratio, the thickness of coating, the yield strength of substrate, the cohesive strength,the work of cohesive and multi loading and unloading cycles on the system are investigated.Under the condition of perfect combination of the coating and substrate, the results showthat the pull-off force increases with the loading depth, the elastic modulus ratio and thethickness of coating. Multi loading and unloading cycles and surface adhesion can easilyinduce massive plasticity in the interface zone. Under the consideration of insufficientinterface strength, the cohesive zone model is introduced to represent the interfacedelamination. The results show that there will be the pull-off force at the instant of crackinitiate. The higher the cohesive strength and work of cohesive, the smaller the crack size.Crack size increases at first and then decreases with the increases of yield strength ofsubstrate, and also it will increase with the elastic modulus ratio. Material at the crack tipmay suffer further yield during the unloading half-cycles due to surface adhesion, thusaggravating the damage.