Friction in a coated surface deformed by a sliding sphere

Stress and strain modelling and stress field computer simulations are today an important tool for systematic approach and optimisation of tribologically stressed coated contacts. Modelling illustrates and quantifies the dominating parameters resulting in crack initiation, crack growth and failure of coated surfaces. Friction and its components, adhesive and ploughing friction, are necessary input parameters in stress modelling. In Finite Element Method (FEM) modelling the ploughing component is integrated in the model while the adhesive component needs to be determined as input value for stress simulations. This paper presents how adhesive friction is determined for the TiN (μa = 0.066) and DLC (μa = 0.047) coatings from experimental friction measurements. The experimental value is used as an input value in the three dimensional finite element micro-model that simulates the spherical tip sliding on a DLC coated flat substrate with increasing load similar to the conventional scratch test contact. Based on the numerical contact analysis (FEM) similar friction evolution compared to the experimental friction in scratch testing was depicted. However, the analytical approach resulted in a diverse solution.