Measurement Of Adhesion And Its Finite Element Simulation During Scratching For Cu/Si(100) Thin Films Deposited By MPPMS

The film-substrate adhesion of the coated parts is a key feature for evaluating its service performance and reliability.The adhesion between film and substrate is one main method for evaluating the coating adhesion.The film-substrate adhesive force of the soft-film/hard-substrate system in a scratch test is difficult to quantify,owing to the unclear relationship between the scratched morphology and coating adhesion,and the failure mode in a scratch test for soft-film/hard-substrate system is still under developed.In this paper,Cu thin films were deposited on Si(100)substrates by closed-field unbalanced modulated pulsed power magnetron sputtering(MPPMS)technique.The effects of sputtering pressure 0.11 Pa-0.70 Pa and target-substrate distance 75 mm-150 mm on the structure,properties and residual stress of Cu thin films were investigated.The failure behavior of Cu/Si(100)thin films was characterized by using the scratched critical load LC1 of first cracking.LC2 of first edge chipping,LC3 of first delamination within the track and LC4 of massive delamination.The mechanical response feature between the indenter and the film/substrate in the scratch process were simulated by finite element method(FEM).The effects of mechanical properties of the film on the stress distribution,plastic deformation.contact force response and strain energy of the film were studied,involving the revealing of the quantitative relationship between the film stress field and the critical load.Under the conditions of low sputtering pressure or near target-substrate distance,SEM and TEM observations showed that the Cu/Si(100)thin films deposited by MPPMS with high ionization rate and low ion energy characteristics showed a fee structure(111)preferred orientation,with compact structure,nano sized fine grain and high hardness and elastic modulus.As the sputtering pressure and target-substrate distance increase,the film microstructure turned from short to coarse columnar crystal structure,with the(111)preferred orientation enhanced,and the surface cracks increased,with the hardness and elastic modulus gradually decreased.The residual tensile stress measured by the arc length method increases correspondingly,then partially released due to surface cracking.In the scratch test,the film deposited with a sputtering pressure of 0.11 Pa and a target distance of 100 mm showed excellent adhesion,with LC1 of 9.40 N,and LC2 of 10.95 N.LC3 and LC4 exceeded the highest value of the applying load,30 N,for the film did not peel off significantly.As the sputtering pressure increases or the target distance becomes longer,the critical load LC1?LC4 all decrease.The FEM simulate and calculate the scratch behavior of Cu/Si(100)thin films by Abaqus.Owing to the micro-scale,large plastic deformation and frictional contact characteristics of the scratches,the densely-divided mesh is used and modeled using the ALE(Arbitrary Lagrangian Eulerian adaptive meshing)mesh adaptive division technique.The interfacial friction coefficient ?i was calculated according to the friction coefficient of the LC2 position in the friction-load curve of the scratch test,the calculated friction-loading curve coincided well with the experimental friction-loading curve.The Cu/Si(100)thin film tested by the scratch method has three typical stress concentration regions corresponding to the scratch damage characteristics LC1?Lc4,which are the zone A directly under the scratch indenter.the zone B surrounding the indenter adjacent to the zone A.and the zone C oblique front of the indenter on both sides adjacent to the zone B.As the load increases,the generation of LC1 corresponds to the loading position where the maximum main tensile stress of the zone B reaches the maximum value,and the LC2 corresponds to the loading position where the maximum tensile stress of the zone C exceeds the zone B,the LC3 and the LC4 correspond to the loading position of the maximum main tensile stress in the zone C and the maximum principal compressive stress in the zone A reaches their maximum,respectively.The maximum principal stress in zones A,B,and C showed fluctuation changes,then the LC2 corresponds to the increased oscillation amplitude,accompanied by the periodic semicircular traces behind the indenter in the A region,and periodic wave traces on both sides of the indenter appearing in zones B and C,LC3 and LC4 correspond to a gradual decrease in the amplitude of the oscillation until it disappears.The effect of yield stress,elastic modulus and residual stress on the film material explains the variation of different scratch failure behavior and critical load in the scratch test.The FEM was used to simulate the periodic wave traces on both sides of the scratch and the periodic semicircular traces in the center of the scratch during the scratching of the Cu/Si(100)thin film.The periodic wave traces on the edge of the scratch were caused by the periodic accumulation and release of the strain energy of the film material around the scratch indenter by a "stick-climb-slip" process.The periodic semicircular traces were related to the semicircular shape von Mises stress distribution of the film.The variation of the fluctuation state of the plastic deformation in the simulated scratch process is consistent with the variation of the scratch test results.