1 - Acta Technica Corviniensis

FLOW WEAR BY SHEAR INSTABILITY IN SLIDING1‐2.INSTITUTE OF STRENGTH PHYSICS AND MATERIALS SCIENCES SB RAS, TOMSK, RUSSIA1.Sergei TARASOV, 2. Valery RUBTSOVABSTRACT: Inhomogeneous character of deformation in subsurface layers of metals in sliding resulted in generation of ananocrystalline layer. Specificity of its deformation behavior is a hydrodynamic flow pattern developing due to shearinstability under conditions of thermal softening. Macroscopic analysis of plastic deformation carried out on theassumption that deformation behavior of the nanocrystalline subsurface layer is similar to that of the parallel‐plane viscousNewtonian flow. It was shown that velocity tangential discontinuity surfaces may exist inside the deforming subsurfacelayer. These surfaces are particular cases of Helmholtz instability and may serve as potential sites where turbulences maynucleate.KEYWORDS: shear instability, nanocrystalline layer, sliding, wearINTRODUCTIONNow there is an interest in studying high‐straindeformation behavior of nanocrystalline materials.The well‐known fact is that inhomogeneousdeformation in subsurface layers of metals in highloadsliding results in generation and flow of ananocrystalline layer [1]. Specificity of its deformationbehavior is a hydrodynamic flow pattern developingdue to shear instability under conditions of thermalsoftening. The nature of shear instability here is acrossover from common shear deformation mode tothe grain rotation governed either by grain boundaryslipping mechanism (GBS) or rotationalrecrystallization mechanism [2] or disclinationmechanism [3] under condition of submicron sizegrain structure formation and dynamicrecrystallization. All these proposed deformationmechanisms might be discussed in studyingdeformation in nanocrystalline materials. However,GBS is the most studied and well‐documentedmechanism, which may serve a basis for analyzing theshear instability. Phenomenon of the shear instabilityin sliding is considered as a product of deep structuremodification, which is a common finding in metalssubjected to high strain rate impact test whenadiabatic shear bands are generated [4].Shear instability of a special type described as a Kelvin‐Helmholtz instability is observed when metal (copper,beryllium or aluminum) plates collide each other in aglancing manner at 2 to 8 mm/μs velocity and smallangle [5, 6]. Wave‐like patterns or eddies are oftenfound at the interface between the plates and areinherent in the said instability. Generation of thispattern may be suppressed by depositing eithergalvanic or electron beam coatings on the surface ofsamples. Such an effect of stabilization is explained bysuppression of shear band generation due to refiningsource metal grains [6]. Judging by this explanationwe may suggest that the developments of shearinstability and shear bands are interrelated.It is reported [7] that generation of eddy‐likestructures during high velocity impact might be bystrain localization zones formed at the previousdeformation stages. Once generated these zonesbecome then involved in a vortex‐like flow [7]. In ouropinion these zones are the results of shear instabilitylike those observed in impact welding, i.e. under highvelocityimpact shear deformation.Eddy‐like structures of another nature may be foundon the worn surfaces of soft Al–Sn and Cu–Pb alloysafter testing at low sliding speeds [8]. It wassuggested [8] that they might nucleate and growunder thermodynamic instability conditions with theiraxes being parallel to the sliding direction. However,the most feasible mechanism for formation of thesestructures may be mechanical mixing as follows frompioneering works of D.A. Rigney.Dynamic high‐speed sliding of aluminum/steel pair wasreported [9]. It was shown that both eddy‐like flowand intermixing occurred at the worn surfaces andresulted in formation of a mechanically mixednanocrystalline layer (MML).The objective of this work is to estimate macroscopicconditions for generation of the eddy‐like flowinstability in sliding on the basis of hydrodynamicapproach including previously obtained bothexperimental and numerical simulation results.EXPERIMENTAL CONDITIONSModern literature sources offer models for gradualformation of nanocrystalline layer in sliding test [9].These models are based traditionally either ondeformation rate or wear debris intermixing withinthe contact zone.However, the nanocrystalline layer structure is verymuch alike the structure of adiabatic shear bandsobtained in explosion loading [10]. It is possible with© copyright FACULTY of ENGINEERING ‐ HUNEDOARA, ROMANIA 91