Wear

This chapter outlines common mechanisms that contribute to wear, which is broadly defined to be any form of surface damage caused by rubbing one surface against another. Such wear mechanisms include delamination wear, adhesive wear (where adhesion followed by plastic shearing plucks the ends off the softer asperities, typically described by Archard’s law), abrasive wear (where hard particles or asperities gouge a surface and displace material), and oxidative wear (where surfaces react with atmospheric oxygen prior to being worn). Sliding conditions often determine which wear mechanism dominates, with the main factors being temperature, sliding velocity, oxidation, plasticity, loading force, and mechanical stresses. How wear rates respond to changes to these factors can be diagramed on a wear map. The last part of the chapter discusses how transition state theory can describe nanoscale wear by atomic attrition, and how plasticity and fracture occur at the nanoscale.

Effect of Plastic Flattening on the Shearing Response of Metal Asperities: A Dislocation Dynamics Analysis

Discrete dislocation (DD) plasticity simulations are carried out to investigate the effect of flattening and shearing of surface asperities. The asperities are chosen to have a rectangular shape to keep the contact area constant. Plasticity is simulated by nucleation, motion, and annihilation of edge dislocations. The results show that plastic flattening of large asperities facilitates subsequent plastic shearing, since it provides dislocations available to glide at lower shear stress than the nucleation strength. The effect of plastic flattening disappears for small asperities, which are harder to be sheared than the large ones, independently of preloading. An effect of asperity spacing is observed with closely spaced asperities being easier to plastically shear than isolated asperities. This effect fades when asperities are very protruding, and therefore plasticity is confined inside the asperities.

Numerical Simulation of Cropping

Cropping is a cutting process whereby opposing aligned blades create a shearing failure by exerting opposing forces normal to the surfaces of a metal sheet or plate. Building on recent efforts to quantify cropping, this paper formulates a plane strain elastic–plastic model of a plate subject to shearing action by opposing rigid platens. Shear failure at the local level is modeled by a cohesive zone characterized by the peak shear traction and the energy dissipated by shear failure process at the microscopic level. The model reveals the interplay between shear cracking and the extensive plastic shearing accompanying the cutting process. Specifically, it provides insight into the influence of the material’s microscopic shear strength and toughness on the total work of cropping. The computational model does not account for deformation of the cropping tool, friction between sliding surfaces, and material temperature and rate dependence.

The Degradation of Saturated Clay’s Un-Drained Shearing Deformation Modulus under Cyclic Loading

The un-drained shearing deformation modulus of clay will degenerate under cyclic loading. This degeneration is attributed to two reasons, the first one is the reduction of effective pressure due to the increment of super pore pressure; another one is the change of soil’s structure due to the accumulation of plastic shearing strain. Firstly, based on the result of un-drained shearing tests, two formulations were obtained which include initial deformation modulus ratio versus over-consolidated ratio and initial deformation modulus ratio versus initial shearing strain. Then, the quasi-over-consolidated ratio was introduced to consider the effect of super pore pressure, and a fitting formulation was applied to consider the influence of shearing strain. At last, the degenerate formulation of saturated clay’s deformation modulus was deduced, which was proved reasonably by the test results.

Study on Material Removal Mechanism of Silicon Nitride during ELID Ultraprecision Grinding

Based on the analyzing ultraprecision grinding process of hard and brittle materials, taking ELID grinding of silicon nitride ceramic as an example, active control technology of passivating films state was introduced in this paper. ELID ultraprecision grinding process respectively at adaptive dynamic balance mode, discontinuous electrolyzing mode and discontinuous grinding mode had been comparatively studied. By means of AFM used for analyzing surface topography of parts, studies show that material removal method for ELID grinding is always a combination of micro brittle fracture, plastic shearing, lapping and polishing action, and which is the main material removal mode depends on the actual grinding contact state. Finally, finishing surface generating mechanism for silicon nitride in ELID ultraprecision grinding was summed up.

Wear Mechanisms of Multi-Layer Coated Cemented Carbide Cutting Tools

Turning experiments were performed with cemented WC-Co cutting tools coated with two-layer and three-layer overcoats of TiC/Al2O3 and TiC/Al2O3/TiN, respectively. For comparison, uncoated WC-Co tools were also tested under similar cutting conditions. The predominant wear mechanisms of the various ceramic overcoats and cemented WC-Co were investigated using surface profilometry, scanning electron microscopy, and energy dispersive X-ray analysis. Representative results of the tool wear behavior are presented, and the significance of each ceramic layer on the overall tool wear resistance is interpreted in light of the identified dominant wear mechanisms. Delamination wear characterized by the propagation and linkage of surface, subsurface, and interfacial cracks, abrasion, surface plastic shearing, plucking of carbide grains, and dissolution/diffusion are shown to occur depending on the tool material. These wear processes are not mutually exclusive; they may occur simultaneously at different positions on the same tool surface. Based on nose wear data, correlations between wear lives of coated and uncoated tools and feedrate are established.