Fracture Of Surface Layers Research Articles

Fracture of the Surface Layers of Alumina Ceramics under the Action of a High-Power Ion Beam of Nanosecond Duration

The fracture of surface layers of alumina ceramic (polycor) under the action of a high-power ion beam of nanosecond duration is investigated. The characteristic features of such fracture are determined. An estimation of the thickness of breakaway fragments of the surface layer of the ceramics gives values in the range from 4 to 12 μm at single high-power ion beam irradiation with a current density of 150 A/cm2. Oxygen depletion in the surface layer of the ceramics is established. The effect of irradiation on the structural-phase state of the ceramics is considered. The possible mechanisms of the observed fracture are discussed.

Relationship of microstructural criteria of fracture and evolution of metal surface and physicochemical properties of medium at friction

In this work, microstructural criteria of fracture of surface layers of metal (nickel) at friction loading of surfaces in lubricating media are studied. Physicochemical criteria of oxidation of surface and film formation are determined. The effect of film on the strength properties of surface layers of metals is determined. Mechanisms of interaction of films with the dislocations of the surface layer are considered. Aspects of internal oxidation and hydrogen attack due to oxygen and hydrogen transfer along the channels of surface dislocations during mechanical and thermal fracture of the lubricant are considered.

Degradation of tungsten under the action of a plasma jet

The degradation of the surface and structure of single-crystal tungsten and sintered powder tungsten during the action of a pulsed plasma jet is studied. It is shown that the degradation of a tungsten target during the action of a plasma jet with an energy flux density of 0.25–1 MJ/m2 is accompanied by surface evaporation and melting and the fracture of surface layers on scales of 150–250 μm. The results of a numerical simulation of the thermomechanical processes that occur in a tungsten target during the action of a plasma jet are presented. The degradation of tungsten during the action of a plasma jet is shown to proceed almost continuously from the action (evaporation, melting) to the times that are more than three orders of magnitude longer than the action time, which is caused by the thermomechanical processes occurring in the tungsten target. Moreover, the action of thermal stresses leads to structural and morphological changes throughout the sample volume, and these changes are accompanied by recrystallization in adiabatic shear bands.

Fatigue wear of materials under dynamic contact loading

Regularities of the fatigue fracture of surface layers of metals, as well as polymer and inorganic materials, under friction are discussed. The kinetics of the growth of fatigue microcracks and mechanisms of the formation of wear particles under various loading conditions are described. It is shown that, at the initial stage, the volume fatigue fracture of friction members that experience dynamic loading results from their fatigue wear.

Effect of PVD coatings on the strain and low-cycle fatigue resistance of stainless steel and titanium alloys

Introduction. The fracture of structural components is known to be often initiated by the local damage of surface layers of the material. The mechanical properties of the surface layer are built up in manufacturing the structural components and affected in service. The major contributors to the fracture of surface layers are their roughness, stress concentrations, and nonuniform distribution of mechanical properties over the surface and in depth of the material [1]. Experimental investigations, e.g., [2], demonstrated that in a homogeneous stressed state, the yield limit of the surface layer of low-carbon steel was 25% lower than that of the base material. As evidenced by some other findings [3], the yield limit of the thinnest specimens is only 20% of that of the thick ones. Therefore, plastic flow on the surface sets in earlier than in the middle layers. Engineering practice employs different treatments, including the application of metallic and nonmetallic films and coatings, to enhance the strength and protective properties of item surfaces. As early as 1926, Rosko [4] began studying the effect of surface oxide films on the mechanical properties of metals in evaluation of the microhardness of cadmium monocrystals. It was revealed that the oxide film less than 20 atoms thick increased the critical stress of cadmium monocrystals by about 50%. It also interferes with plastic strains in the crystal, which contributes to an increase in the hardness of the surface layer. Investigations [5–9] showed that not only oxide films but also PVD coatings (physical vapor deposition) enhanced the tensile and cyclic load resistance, mainly due to the restriction on plastic strains of the base by a hard surface layer with higher mechanical strength, large residual compression stresses, and good adhesion of the coating material to the base [7–9]. An increase in the tensile and cyclic load resistance is associated with the thickness and hardness of PVD coatings and their nitrogen contents [7, 8, 10].

Stribeck Curve Under Friction of Copper Samples in the Steady Friction State

The friction and wear of a pure copper block (99.98 wt% Cu) against a hardened steel disc were studied. The effect of sliding velocity and load on the friction coefficient and wear rate of Cu samples during steady tests was studied. Elasto-hydrodynamic (EHL), mixed (ML) and boundary lubrication (BL) regions were analyzed using the Stribeck curve. The lubrication number of Schipper, Z, was used in the analysis of the Stribeck curve. The transitions from one lubrication region to another are discussed. The mixed EHL region is characterized by stable low values of the friction coefficient, wear rate and temperature. Straight asperity contact is the dominant mechanism under friction of Cu-steel pair in the BL region. High-friction coefficients and wear rates, thin lubricant films and large wear grooves indicate straight asperity contact between rubbed surfaces in the BL region. Although the dominant mechanisms in the mixed EHL and BL regions are different in principle, a steady friction state is preserved in both cases. It is expected that the steady friction state in the BL and mixed EHL regions is associated with deformation and fracture of surface layers but these process occur at different scale levels. It was shown that under friction of Cu-steel pair, two types of ML regions are observed. The first is the stable steady friction of mixed EHL with low values of the friction coefficient and wear rate. The second type of the ML region is the region of unstable friction and wear when a decrease of lubricant film leads to a change of external (roughness, temperature, friction and wear) and internal (strain and stress) parameters. It was found out that a transition to the unstable ML region occurs within a narrow range of Z parameter under definite values of the load and sliding velocity.

Modeling of wear of solids at different scales

The paper puts forward an approach to the analysis of stress state and fracture of surface layers in deformed solids. The approach is based on the consideration of contact interaction processes at several scale levels determined by characteristic structural parameters of the surface and subsurface layers of contacting bodies (micro- and mesoscales) and by their macrogeometry. The approach can be used to estimate the influence of geometrical, mechanical and physical characteristics of materials of contacting bodies and the intermediate medium on the performance of friction pairs. Methods of increasing the wear resistance and service life of the pairs and reducing energy loss owing to the control of wear and friction processes in thin surface layers can be developed.