Effect of friction in indentation hardness testing: a finite element study

Abstract

The indentation hardness of materials, defined as the resistance to penetration, gives a conveniently rapid indication of their deformation behaviour. There is a contact and friction problem during the indentation process, because the indentor faces slide against the test specimen and drill through the surface of the specimen continuously. According to Tabor [1], the friction coefficient for a polished diamond indentor sliding on most unlubricated metals is 0.1-0.15. If the unlubricated metal surface is polished, as is the case in microindentation hardness testing, the friction coefficient is about 0.2. To study the effect of friction, experiments were done in which both unlubricated and lubricated diamond pyramid indentors (136 °) were applied, and the surface topography of the annealed 70/30 brass specimens was different, i.e. a part of the specimens only went through the large mill, some were polished and others were finely etched. Almost the same hardness values (around 80 Hv) for annealed 70/30 brass were obtained under various experimental conditions [2]. This means that the surface roughness and lubrication have little effect on the size of the indentation produced, if the indentation itself is large compared with the dimensions of the asperities. This result is good agreement with the results of Tabor [1] and Samuels [3]. Mulhearn, over 30 years ago, proposed that indentation by blunt indentors occurs by means of a compression mechanism [4]. His model indicates that there is little or no movement between the surfaces of the indentor and the specimen. It can therefore be concluded that friction between the two does not play a significant role in hardness testing. Although much is now known about contact problems and indentation hardness, there has been little work done on the theoretical analysis of the effect of friction in indentation testing. In order to simulate hardness testing procedures and explain many of the phenomena observed in indentation experiments, using the finite-element method, a detailed quantitative analysis of indentations made by various special indentors, investigating elastoplastic materials, has been carried out [2, 5]. Because the mathematical problem of an indentation hardness testing procedure is a highly non-linear contact problem with varying geometrical boundary conditions, two-dimensional calculations can be performed much more efficiently than three-dimensional ones, due to the lower complexity of the mesh. Meaningful results about the effect of friction