Damage evolution in coated cemented carbides under compressive contact loads

This work deals principally with two important issues and their interrelation: the evolution of damage in coated cemented carbide tools used for cold forming, and the assessment of mechanical behavior of cemented carbides under compressive contact loads. Damage evolution in ironing tools is qualitatively investigated by planar and cross-sectional microscopical observations. Several mechanisms of damage are identified by taking into account the circumferential distribution along the surface of the ironing tools: coating wear and/ fracture, plastic deformation and fracture of the substrate. In the context of damage location in the substrate, the emphasis is put on the study of damage following all processing steps in substrate and coating preparation. This analysis aims at determining whether the substrate is weakened or not by the mechanical and chemical pre-treatment prior to coating. Spherical indentation testing is used as an experimental procedure to reproduce the compression stress state during cold forming. Large indenter tips (300 microns) are employed for testing cemented carbides in an effort to minimize the effects of material structural inhomogeneities. The behavior of cemented carbides under compressive loads is governed by the reversibility of deformation induced by indentation. A unique feature revealed by analysis of experimental load-displacement indentation curves is the positive energy balance, in that a gain of energy is evidenced after a loading-unloading cycle. Two hypotheses are developed for interpreting such a behavior: the strain induced martensitic transformation of the cobalt ductile phase and the thermal residual stresses in the composite. Substrate weakening subsequent to mechanical and chemical pretreatment prior to coating is expected to have a particular role on the indentation behavior of cemented carbides. The impact of substrate damage on the indentation parameters is identified by a parametric study using finite element simulations in which the damage is considered uniform. In reality, the damage distribution over the surface is rather irregular. In this context, a statistical approach is used for assessment of the material indentation parameters. The failure of coating/substrate systems under compressive contact loads is investigated by finite element simulations. The first failure events are identified by considering that system failure occurs either by substrate plastic deformation or coating fracture. The response of coated systems to spherical indentation is synthesized in damage diagrams which can predict whether the coating or the substrate fails first. The possibility for using such diagrams for assessing the evolution of damage in coated cemented carbide tools is highlighted.


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