| Single-grit rotating scratch tests have been conducted with a conical diamond tool on pure titanium. The force profiles during the scratch event were captured using high frequency force transducers. The mechanisms of material removal have been characterized by optical and scanning electron microscopes. It was observed that the adhesion between the tool and the deformed material and the hardening properties of the material play active roles in the scratching process. Adiabatic shear band formation followed by cracking was observed to be responsible for the material removal during scratching. The overall frictional coefficient was found to oscillate strongly at the beginning and at the end of the scratch, but increased steadily in the middle of the scratch. The size dependence of overall specific energy was observed and was mainly attributed to the competition between hardening and softening during the scratching process. Instantaneous specific energy and instantaneous scratch hardness have been introduced to characterize the process. These parameters were found to be sensitive to the depth of cut, thus validating the close correlation to the size effect of overall specific energy.; The mechanics of material removal during a single-grit rotating scratch has been investigated analytically. The models for cutting, plowing and mixed modes of material removal are established based on the pressure and the friction on contact faces. The mixed-mode model takes account of the contribution of built-up edge (BITE) ahead of the tool. The single-grit rotating scratch experiments were employed to verify the model result. It is shown that the mixed-mode model captures the salient features of material removal during the rotating scratch. The size effect of overall specific energy may be attributed to the size dependence of the yield pressure.; In order to further explore the intrinsic mechanism of material removal, an upper bound model has been proposed for the single-grit rotating scratch with a conical tool on a rigid perfectly-plastic material. The current model considers the effect of the BUE and makes use of the modification of the BUE to the tool configuration so that the discretization of the plastic region in the vicinity of the tool becomes feasible. The features of the side-ridge, the front-ridge, the BUE and the sublayer plastic zone have been found to be evolving in different ways during the rotating scratch. Three material removal mechanisms were uncovered to be involved with the rotating scratch process, namely, plastic (shear) deformation, contact friction and ductile fracture. In the first half of the scratch, the first two dominate the scratch process, while in the second half, the ductile fracture plays an active role as evidenced by the extensive tearings along both banks of the scratch. |
