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

Abstract

When friction stress on a contact surface reaches material yield strength in shear, contact slippage can occur even if the slipping condition for Coulomb friction is not satisfied. In this paper, a three-dimensional (3-D) scratch model is proposed, which considers combined Coulomb and plastic friction. Influences of plastic friction are discussed for two continuous displacement loading steps: indentation and scratch. For indentation, initially the sliding on the contact surface can not take place and the complete cohesion condition should be employed; then as the indenter is further compressed down to the coating surface, plastic friction instead of Coulomb friction prevails in most of the contact region. For scratch, the previous complete cohesion at the initial indentation is substituted by plastic or Coulomb slipping, and the slippage becomes plastic-sliding governed for a slightly large indentation depth. The effects of the indentation depth and the Coulomb friction coefficient on the scratch friction coefficient are discussed in detail. Several experimental phenomena are interpreted, which include that with an increase of the normal loading, the scratch friction coefficient reduces for the soft coating but grows for the hard coating; and with the growth of hardness after heat treatment, the scratch friction coefficient increases due to weak plastic slippage. Obtained results help to elucidate tribological behaviors during scratch and are helpful for the interpretation of experimental phenomena and the improvement of numerical simulations for the scratch process.

Tribology behavior on scratch tests: Effects of yield strength

“…[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]. There was a brief attempt to explain this phenomenon in Ref.…”

“…One possible reason in Ref. [13] was given for a special case with soft coatings on a very hard substrate, however, it may not be true in other circumstances. As mentioned in Ref.…”

“…In addition, the yield strengths in both substrate and coating play an important role in tribological performances of the material surface. Another goal is to investigate the effects of the combined yield strengths of the substrate and coating, which will give completely distinct viewpoints from previous FEM results [5][6][7][8][9][10]13].…”

“…There was a brief attempt to explain this phenomenon in Ref. [13] which combined the Coulomb and plastic friction. One possible reason in Ref.…”

“…As mentioned in Ref. [13], the effects of yield strength need to be reexamined and investigated in a separate paper. Consequently, one of the goals of this paper is to study the effects of yield strength in the surface coating and further interpret the experimental phenomenon in detail.…”