Finite element method parametric study on scratch behavior of polymers

Abstract

Parametric studies were performed using finite element analysis (FEA) to learn how material and surface properties of polypropylene (PP) affect scratch behavior. Three-dimensional FEA modeling of scratching on a PP substrate with a spherical-tipped indenter is presented. Three different loading conditions, that is constant scratch depth, constant normal load, and linearly increasing normal load, are adopted for this parametric study. From the FEA findings, it is learned that Poisson's ratio has a negligible effect on scratch performance, whereas raising the coefficient of adhesive friction induces a significantly larger residual scratch depth and tangential force on the scratch tip. Increasing the Young's modulus of a material does not necessarily improve its overall scratch performance. On the other hand, modifying the yield stress of a material has a major impact on scratch resistance as a higher yield stress reduces the residual scratch depth. From this numerical effort, it is concluded that the yield stress and coefficient of adhesive friction are the most critical parameters to influence the scratch performance of a material. Analyses also suggest that the general trend in the parametric effect of the above four parameters on scratch behavior is independent of the applied normal load level.

Tribology behavior on scratch tests: Effects of yield strength

“…Previous FEM results based on the traditional Coulomb friction failed to explain it and even displayed an opposite trend; with a growth of yield strength the scratch friction coefficient reduces (e.g., Ref. [8,10]). The reason was that with the growth of yield strength the deformation of surface material was suppressed, which led to a decrease of the surface deformation friction coefficient (SDFC) and caused a further drop in the SFC [13].…”

Tribology behavior during indentation and scratch of thin films on substrates: effects of plastic friction

“…11(b) than in Fig 11(a). Previous FEM results 12 showed that with a growth of the yield strength, scratch friction coefficient reduced. However, when plastic slippage is considered the scratch friction could increase with an increasing yield strength in our model, which is consistent with experimental observation for most metals.…”

“…When indentation depth is on the micro-scale, coatings with crystalline materials due to GND have a strong size effect and classic elastoplasticity fails to describe it. This topic is out of range of previous finite element models [8][9][10][11][12][13][14][15][16] and this paper. However, if strain-gradient theories are applied to substitute the total system of equations in the Appendix, then compared with current results the extrusion and deformation of the coating will decrease and the area of the plastic slippage region will also reduce due to the additional hardening caused by GND under a micron indentation depth.…”

“…8, 20, and 21, which is effective with a small CFC or a small indentation depth. When the friction stress reaches shear yield strength, previous models in Refs [8][9][10][11][12][13][14][15][16] are invalid and plastic sliding should be employed. With a rising CFC, the SFC µ d grows but friction coefficient change rate α decreases.…”

“…We have not noticed any discussion about the effects of plastic friction during scratch in previous works. Despite great success in describing scratch behaviors and interpreting experimental phenomena in pioneering FEM models, [8][9][10][11][12][13][14][15][16] they are based on traditional Coulomb friction and will fail when plastic sliding occurs. In addition, additive decomposition of the scratch friction coefficient µ s = µ + µ d into traditional µ and surface deformation µ d components is widely accepted (see Refs.…”