Development Of Graded Self-lubricating Ceramic Cutting Tool Material And Its Friction And Wear Mechanism

Aiming at the problem that existing homogeneous self-lubricating ceramic cuttingtool materials are not available to possess rational combination of antifriction andantiwear properties, a new cutting tool material—graded self-lubricating ceramiccutting tool material was developed in this paper by applying the design concept offunctionally graded materials. The new cutting tool material has primary characteristicsthat the solid lubricant content decreases gradually from the material’s exterior into itsinterior and residual compressive stress exists on its surfaces.Graded self-lubricating ceramic cutting tool materials usually consist of acomposite matrix and solid lubricants, yet so far there is not a compositionaldistributional model used for materials with more than two components in formalpublications. In this paper a compositional distributional model and its function formulticomponent functionally graded materials were initially proposed and then appliedto compositional design of graded self-lubricating ceramic cutting tool materials.Both Al₂O₃/（W,Ti）C/CaF₂and Al₂O₃/TiC/CaF₂graded self-lubricating ceramiccutting tool materials were prepared by technological process of dispersing andblending powders with wet process, laminated padding and then axially hot press invacuum. A handy and practical layering method was explored. There was no reactionamong CaF₂in the prepared materials and components in the ceramic matrix.Reasonable residual compressive stress was formed on the surface layers of the gradedself-lubricating ceramic cutting tool materials.In comparison with the corresponding homogeneous materials, mechanicalproperties of graded self-lubricating ceramic cutting tool materials have significantimprovements. Flexural strength, hardness and fracture toughness of Al₂O₃/（W,Ti）C/CaF₂graded material increase by24.6%,19.2%and5.5%, respectively. ForAl₂O₃/TiC/CaF₂graded material, the rates of increase are21%,16%and5.9%,separately.The components in adjacent graded layers bonded with each other at theinterfaces and their microcosmic boundaries become dim. Microstructures of thefracture surfaces formed in the bending test were analysed. Mixed transgranularfracture and intergranular fracture are fracture modes of the graded self-lubricatingceramic cutting tool materials. The toughening mechanism of the graded self-lubricating ceramic cutting tool materials is the synergistic effect of lamellartoughening and the toughening mechanism inside each graded layer.Friction-wear tests of the graded self-lubricating ceramic cutting tool materialswere carried out and their antifriction and wear resisting mechanisms were studied.Along with increase of the normal load, the friction coefficient showd a downwardtrend whereas the wear rate increased. Both the friction coefficient and the wear ratedecreased along with increase of the revolving speed. Under the same operatingcondition, friction coefficient of the graded self-lubricating ceramic cutting toolmaterials is close to that of the corresponding homogeneous materials and wear rate ofthe former is smaller than the latter. The increase of wear resisting property of thegraded self-lubricating ceramic cutting tool materials is due to the residualcompressive stress existing on their surfaces.