The Design Of Blister Equipment And The Characterization Of Mechanical Properties For Film Materials

With the development of the micro/nano science and technology, film materialsare widely used in surface engineering, interface engineering and related practicalapplication field, such as diamond film on cutting tools, anticorrosion coating of plateand thermal barrier coatings on turbine blades, etc. Mechanical loads and heatingloads applied to the film at service state can cause stress mismatch. It may lead tocrack, delamination or completely failure of thin films. The mechanical properties ofthin films are the key to design, improve and enhance the service life and reliability offilm materials. Obviously, the general techniques used to measure these properties inbulk materials cannot be applied to thin films with micro/nano scale. Therefore, it isnecessary to develop new experimental methods and equipment and obtain the keymechanical parameters of films. This thesis aims to develop a high accuracy andfull-field measurement blister equipment, which is used to characterize themechanical properties of thin films. The main research contents are summarized asfollows:1. The key problems of blister method are proposed by the investigation ofpresent methods and equipments. With the deep analysis of present theoretical model,measurement equipment and sample preparation method, their problems andinsufficiency are summarized. This paper concludes that some critical issues shouldbe considered to realize the characterization of mechanical properties for thin filmthrough blister testing. It consists of three aspects:(1) precise measurment ofdisplacement field and the design of corresponding equipment,(2) a characterizationmethod of yield strength at small deflection, and (3) a characterization model ofinterfacial adhension properties for elastic-plastic film materials.2. A novel testing setup is developed by combining blister test and digitalspeckle correlation method. The measuring principle of digital speckle correlationmethod is presented at first. Then telecentric lens is used to overcome the parallaxproblem of traditional lens. This equipment provides high accuracy, reliable andconvenient experimental data with whole field information.3. Yield strength of nickel film is characterized at small defletion, which offsetsthe gap of present model. In this thesis, the relation between pressure and deflection atsmall deflection deformation is studied. The Young’s modulus of nickel can be gained from the slope of curve. The result is in good agreement with the data obtained bytensile test, which confirms the reliability of the equipment. The stress field can beobtained from deflection field of nickel film based on the theory of small deflectiondeformation. Then the yield strength of nickel film is gained from the critical pointdeviates from the fitting line of deflection-pressure curve in the range of smalldeflection deformation.4. Distinguish from the common work on adhesion of circlar film, the interfacialadhesion properties of film/substrate with square observing window is characterized.For some special coating materials, such as films deposited on Si substrate, theobserved window of freestanding film is square or rectangle due to the anisotropicetching of Si substrate. In this thesis, the bonding properties of polypropylenefilm/steel with square freestanding film are studied. The deformation and debondingprocess are observed by the deflection measurement in whole field. The crackappeared on the center of edges of square film and propagated outwards to the corners.The experimental phenomena are proved by the theory of film large deflection.5. Based on plastic flow rule and the bending theory of beams, an elastic-plasticfilms/substrates model is established. The adhesion of nickel/steel is studied in thisthesis. The method of sample preparation is explored for the special nickel film/steelsubstrate materials. The elastic deformation of film under uniform pressure is studied.The adhesion properties of the elastic-plastic debonding are investigated under theconcentrated loads. Interfacial adhesion energy model of elastic-plasticfilms/substrates system is offered, which offsets the deficiency of present model.In a word, the methods and equipments of blister test are studied systematicallyin this thesis. A high accuracy and full-field measurement blister equipment isdeveloped to characterize the key parameters of mechanical properties of thin films,including Young’s modulus, yield strength and interfacial adhesion properties. Theresult of the study will provide valuable information for design optimization,performance improvement and reliability study of thin film.