Processes of formation and destruction of submicrostructure under friction loading are being discussed from the point of view of dislocation representations. Semi-uniform distribution of dislocation clusters of nano- and submicroscopic sizes in surface layers of nickel has been determined. Synergetic aspects of this phenomenon are being discussed.
Modern technologies of intensive plastic deformation make it possible to obtain materials of submicroscopic and nanocrystalline structures with high strength properties. Formation of those structures under friction loading is possible due to localized processes of plastic deformation in surface volumes and accumulation of elastic energy will happen under conditions of rapid deformation.
Specific character of friction loading and temperature gradients facilitate kinetics of surface layers structural transformation. Under contact of surfaces and due to different height and diversity of geometrical shapes of rough edges (extrusions and intrusions), alternate stress functions in surface layer, which along with shear force actions leads to complicated stressed state of crystal lattice. Alternating loading of frictionally interacting extrusions and intrusions transforms into pulsating deformation of surface volumes causing fatigue behaviour. Temperature fluctuations of friction contact have considerable influence and together with initiation of return processes and dynamic recrystallization may result in thermal cycling, viscosity and creepage of surface layer material. These conditions determine formation of peculiar microstructure. Development of microstructure under friction interaction of surfaces of contacting objects should be regarded and discussed in accordance with oscillating character of its kinetics [1-3].
Results of electron-microscopic researches of surface layer microstructure under sliding friction, in accordance with study of dislocation structure by means of ferromagnetic resonance (FMR), are given in this paper [
Transformation of dislocation structure in the near-surface layer of nickel under friction loading has been studied by means of electron microscopy and ferromagnetic resonance. Polycrystalline nickel with purity of 99.9% has been examined. Samples in the form of thin disks (5 × 0.1 mm2) were electrolytically polished and annealed in vacuum (0.133 MPa under 973 K). Friction test was carried out by means of a machine of type АЕ-5 with precision positioning of contact surface. Sliding friction was performed in pair Ni—Mo in air and lubricant CIATIM—201 under load of 82.3 kPa and linear velocity of 0.5 m/s. Number of working operating strokes was in the range of 1 − 36 × 103. Average volume temperature of the sample didn’t exceed 40˚C. Range of ferromagnetic resonance was registered by means of method described in paper [
Under friction loading dislocation structure develops in accordance with oscillation character. This was confirmed by electronic and microscopic researches, as well as by results of ferromagnetic resonance [1-4]. Periodicity of microstructure is associated with development of plastic and plastically destructed stages of deformation. Analysis of structural changes showed that two opposite processes with various degree of mutual exclusion take place in near-surface layer—hardening, which happens due to generation, motion and interaction between dislocations themselves and other defects of structure, and softening, which is associated with relaxation phenomena— annihilation of dislocations by partial phenomena of annihilation and formation of polygonal structures, development of microcracks and drain of dislocations onto the surface. But these relaxation mechanisms are of secondary importance. Researches showed [
Significant grain refinement (up to ≈ 0.2 micrometers) and appearance of fine-grained polycrystalline phase with dimensions of 5 - 10 nm are characteristic for microstructure transformations of surface layer under friction. This close-meshed microstructure consisting of areas of high and low density of dislocations, which are quasiuniformly distributed in the volume of surface layer, is represented at the
Fine-dispersed structure and newly formed large-angle boundaries in the processes of plastic deformation and hardening are extremely important. Firstly, increase of boundary length causes increase of surface layer material plasticity under friction at the expense of grain-boundary slippage. Increased temperature of friction contact stimulates it. Secondly, microscopic elasticity limit increases significantly due to decrease of grain size [
gregation of grain-boundary dislocations along the long plateau of grain boundary, which results in slight cracking along the boundaries of large-grain materials, which can be observed experimentally. Small rectilinear plateau holding small aggregates of grain-boundary dislocations, which quantity is not sufficient for formation and development of cracks, occur in small-grain structures due to multiple bends of boundaries. This phenomenon, to some extent, excludes grain-boundary cracking of small-grain material according to this mechanism. Uniform distribution of dislocation assemblies eliminates formation of local fracture nuclei. Thereby formation of fine-dispersed microstructure is one of the synergetic phenomena in surface layer under friction and that’s why metal surface extends its durability by forming those structures at a certain stage of friction loading. Refinement of coarsegrained structure and development of submicroscopic and microscopic sizes of grains are the results of intensive plastic deformation and decrease of active slip systems [
Germinative microcracks develop during further friction loading and under sufficient dimensions and quantities they consolidate into major crack by force of ductile fracture of material dividing them (
destruction of surface layer of metal under friction. Analysis of micrographs (
Occurrence of multiple micropores inside blocks and
along its borders (
Elements of intercrystalline destruction were detected during tests (
At the stage of intensive destruction of material we may observe multiple micropores inside blocks (
Shear along grain boundaries leads to intensification of local deformation in pores, which results in dissipation of plastic fluidity energy into fracture-like shear cracks. This can explain occurrence of wedge-shaped microcracks at the bottom of pores near boundaries directed with their thin point to the nearest micro hollows (
Function of micro hollows in a grain, which are located in close proximity to its boundary, still remains not enough clarified. It is said that pores, which are located in close proximity to incoherent boundaries, tend to vanish fast, as far as these boundaries act like sources and drains of vacant positions [15,16]. In accordance with our research pores co-exist with boundaries in close proximity to the latter (distance ≈ 0.1 micrometer), and moreover they initiate microscopic chips on these boundaries (
Oscillation behaviour of strength properties was discovered during research of kinetics of dislocation structure and processes of surface layer destruction (nickel) under friction loading. Each change cycle of dislocation density has its corresponding cycle of layer microdestruction. Strong grain refinement to nanocrystalline dimensions occurs under this loading together with quasiuniform distribution of grains along the volume of upper layer. During friction loading, this microstructure transforms into texture elements in the form of thin bundles, upon which it transforms into slide bands and peculiar lamellar struc-
Picture 6. intercrystalline failure of nickel (t = 113 ks).
ture. Microcracks develop along boundaries of these bands, upon which these microcracks consolidate into main cracks and form fragments of material destruction. Multiple micropores inside grains and along boundaries of these grains, as well as coagulation of pores, develop areas of transcrystalline and intercrystalline fracture. Friction loading leads to progressive loosening of upper layer of metal, associated with increase of quantity of fracture areas. At the stage of maximal dispersion this loosening in combination with developed brittleness conditions sharp increase of peeling mass of material in the form of whole layer. This process obtains avalanche-like nature and results in selective discharges of fracture products out of the area of friction contact.