N. L. Chernenko

Effect of severe plastic deformation on the microstructure and tribological properties of a babbit B83

The effect of severe plastic deformation carried out at room temperature by the methods of equal-channel angular (ECA) pressing and surface friction treatment (SFT) on the microstructure, rate of wear, and friction coefficient of a babbit B83 (11.5% Sb, 5.5% Cu, Sn for balance) has been investigated. It has been shown that severe plastic deformation that leads to a drop in the grain size of the babbit to 100–300 nm and to a strong refinement of particles of intermetallic phases (SnSb, Cu3Sn) causes a considerable (twofold-fourfold) reduction in the rate of wear and a decrease in the friction coefficient of a steel-babbit pair under test conditions with lubrication at small (0.07 m/s) and enhanced (4.5 m/s) sliding velocities. As was shown by structural investigations performed with the use of scanning electron microscopy, this positive influence of severe plastic deformation on the tribological properties of the babbit is connected with the formation on the deformed-babbit surface of a developed porosity, which improves conditions for lubrication of the babbit-steel friction pair due to the action of the self-lubrication effect and thereby favors the retention of a stable regime of boundary friction of this pair. The formation of porosity is a result of the accelerated spalling of hard brittle intermetallic particles of SnSb and Cu3Sn from the friction surface of the deformed babbit, which is caused by weakening and loss of the bonding of these particles with the plastic matrix (α solid solution based on tin) in the course of severe plastic deformation of the babbit. At the same time, under the conditions of dry sliding friction of the babbit-steel 45 pair, when a fatigue mechanism of wear of the alloy under consideration predominantly develops, this plastic deformation yields an approximately 1.6-fold increase in the rate of wear of the babbit. This increase is mainly due to numerous defects (microcracks) that are introduced into the babbit structure upon its severe plastic deformation and reduce the resistance of the surface layer of this material to the fatigue mechanism of wear.

Influence of the stressed state of the zone of friction contact on the formation of the structure of a surface layer and tribological properties of steels and alloys

An investigation has been performed of the influence of contact stresses, which appear in the zone of friction of iron-manganese alloys with 16.9–40.5 wt % Mn, wear-resistant high-manganese austenitic steels, and carbon steel U13, on the phase composition, structure, and strengthening of surface layers of these materials under conditions of dry friction at small (0.03 and 0.07 m/s) sliding velocities, when the frictional heating of the surface layer of the samples is virtually absent. A quantitative estimation of the arising compressive contact stresses (pressures) has been carried out. It is shown that the value of the compressive contact stresses approximately corresponds to the values of microhardness measured on the friction surface of the materials investigated. These compressive contact stresses initiate the occurrence a γ-ɛ martensitic transformation in the iron-manganese alloys with 16–40% Mn, which is characterized by a negative volume effect, but impedes the development of the ɛ-α and γ-α martensite transformations (in austenitic steels) which occur with an increase in the specific volume. The stresses in question favor the formation of nanocrystalline structures in a thin (≤10 μm thick) surface layer of friction steels and alloys. The contact tensile stresses following the contact compressive stresses initiate the formation and propagation of microscopic cracks in the surface layer of the friction materials, and activate the development of martensitic ɛ-α and γ-α transformations in this layer. It is shown that the structural and phase transformations initiated by contact stresses exert a substantial effect on the microhardness, friction coefficient, and resistance to adhesive wear of the steels and alloys under study.

Structural transformations, strengthening, and wear resistance of titanium nickelide upon abrasive and adhesive wear

Wear resistance and structural transformations upon abrasive and adhesive wear of titanium nickelide Ti49.4Ni50.6 in microcrystalline (MC) and submicrocrystalline (SMC) states have been investigated. It has been shown that the abrasive wear resistance of this alloy exceeds that of the steel 12Kh18N9 by a factor of about 2, that of the steel 110G13 (Hadfield steel), by a factor of 1.3, and is close to that of the steel 95Kh18. Upon adhesive wear in a testing-temperature range from −50 to +300°C, the Ti49.4Ni50.6 alloy, as compared to the steel 12Kh18N9, is characterized by the wear rate that is tens of times smaller and by a reduced (1.5–2.0 times) friction coefficient. The enhanced wear resistance of the Ti49.4Ni50.6 alloy is due to the development of intense strain hardening in it and to a high fracture toughness, which is a consequence of effective relaxation of high contact stresses arising in the surface layer of the alloy. The SMC state produced in the alloy with the help of equal-channel angular pressing (ECAP) has no effect on the abrasive wear resistance of the alloy. The favorable effect of ECAP on the wear resistance of the Ti49.4Ni50.6 alloy takes place under conditions of its adhesive wear at temperatures from −25 to +70°C. The electron-microscopic investigation showed that under conditions of wear at negative and room temperatures in the surface layer (1–5 μm thick) of titanium nickelide there arises a mixed structure consisting of an amorphous phase and nanocrystals of supposedly austenite and martensite. Upon friction at 200–300°C, a nanocrystalline structure of the B2 phase arises near the alloy surface, which, as is the case with the amorphous-nanocrystalline structure, is characterized by significant effective strength and wear resistance.

Effect of frictional heating on the surface-layer structure and tribological properties of titanium nickelide

The effect of frictional heating (whose intensity was varied at the expense of changes in the sliding velocity from 0.35 to 9.00 m/s) on the rate of wear, friction coefficient, friction thermopower, structure, and microhardness of the Ti49.4Ni50.6 alloy in a microcrystalline (MC) state with grains 20–30 μm in size and in a submicrocrystalline (SMC) state with grains 300 nm in size has been investigated. The tribological tests were conducted under the conditions of dry sliding friction in air using the finger-disk (made of steel Kh12M, hardness HRC = 63) scheme at a normal load of 98 N. Due to the frictional heating, the temperature in the surface layer 0.5 mm thick of the samples changed from 150–200 (at a sliding velocity of 0.35 m/s) to 1100°C (at a velocity of 9 m/s). The alloy structure has been studied with the help of metallographic and electronmicroscopic (scanning and transmission microscopy) methods. It has been shown that the rate of wear of the titanium nickelide in the MC and SMC structural states is more than an order of magnitude lower than in the 12Kh18N9 steel and several times less than in the 40Kh13 steel. The fracture of the friction surface of the titanium nickelide occurs predominantly by the fatigue or oxidation-fatigue mechanisms, which are characterized by a relatively low wear rate, whereas the 40Kh13 and 12Kh18N9 steels show a tendency to intense thermal adhesive wear (seizure) at velocities higher than 0.35 m/s. It has been shown by the electron-microscopic investigation that nanocrystalline structures consisting of crystals of the B2 phase, oxides of the TiO2 type, and some amount of martensite B19′ are formed in the process of friction in the surface layer of the titanium nickelide. It has been concluded that an enhanced wear resistance of the titanium nickelide is caused by the high heat resistance (strength) and high fracture toughness of the nanocrystalline B2 phase and by the presence of high-strength thermostable oxides of the TiO2 type formed upon friction.

Formation of a wear-resistant nanocrystalline layer strengthened by TiO2 (Rutile) particles on the surface of titanium

The effect of a thermomechanical treatment including severe plastic deformation under dry sliding friction conditions and subsequent heating in air to 350–650°C with further holding for 1 h on the structure and wear resistance of commercial titanium of grade VT1-0 has been studied. It has been shown that the deformation by friction leads to the formation of a nanocrystalline structure with α crystals 20–100 nm in size in a surface layer of titanium of about 10 μm thick. The heating of titanium deformed by friction at temperatures of 450–650°C for 1 h in air leads to the formation in the surface layer of this material ∼10 μm thick of nanocrystalline particles of the titanium oxide TiO2 (rutile), the volume fraction of which reaches tens of percents, while the dimensions are ∼10 nm. The presence in the surface layer of titanium of a nanocrystalline two-phase (α-Ti + rutile) structure leads to a significant increase in the wear resistance of the VT1-0 titanium in pair with steel 40Kh13. This is explained by the enhanced strength of the arising nanocrystalline layer and its positive influence (as of a transition layer) on the reduction of the level of internal stresses that exist at the interface between the titanium oxide TiO2 and the host metal.

Influence of prolonged heating on thermal softening, chemical composition, and evolution of the nanocrystalline structure formed in quenched high-carbon steel upon friction treatment

Methods of transmission and scanning electron microscopy, X-ray diffraction analysis, micro-hardness, and wavelength dispersive X-ray microanalysis have been used to investigate the changes (in the process of vacuum treatment for 10–1200 min at temperatures of 350–550°C) in the structure, microhardness, and chemical composition of the surface layers of steel U8 (0.83 wt % C) in the initial quenched state and after friction treatment in the argon medium under the conditions of sliding friction using a like spherical indenter-flat sample pair. It has been shown that the layer nanostuctured by friction treatment possesses an enhanced resistance to thermal softening compared with the undeformed quenched steel, not only upon relatively short heating (1–2 h), but also prolonged heating (to 20 h) at temperatures of 350, 450, and 550°C. This is due to the retention of predominantly nanocrystalline structure in the deformed layer upon the prolonged heating to a temperature of 350°C by retarding the processes of the formation of carbide particles and recovery in the α phase and by decelerating the development of recrystallization, including the absence of anomalous growth of separate recrystallized grains upon prolonged high-temperature holdings. After vacuum annealing for 10–1200 min at a temperature of 350°C, an increase was revealed in the carbon concentration by 0.2–0.4 wt % in a layer up to 1 μm thick on the surface of quenched eutectoid steel deformed by friction action.

Modification of the titanium nickelide surface using frictional treatment and subsequent heating in air

The effect of a combined treatment including severe plastic deformation under the conditions of dry sliding friction and heating in air to temperatures of 300–480°C (holding for 1 h) on the structure and wear resistance of the surface layer of the Ti49.4Ni50.6 alloy has been investigated. It has been shown that this frictional treatment results in an amorphous-nanocrystalline structure in the surface layer (of thickness to 10 μm) of the Ti49.4Ni50.6 alloy. Heating to 300°C brings about the complete crystallization of the amorphous phase; as a result, the structure of the deformed surface layer of the alloy becomes single-phase, consisting of nanocrystals of the B2 phase. At 400°C, in this deformed surface layer there arises a nanocrystalline oxide (TiO2) phase whose amount reaches tens of volume percent. The sizes of crystals of the B2 phase and oxide TiO2 are in the range of 1–50 nm. The arising two-phase (B2 + TiO2) nanocrystalline structure is located just below the oxide TiO2 film, which is less than 1 μm thick. With an increase in the heating temperature to 480°C, the deformed surface layer under consideration retains the nanocrystalline two-phase (B2 + TiO2) structure, but an increase in the amount of the oxide phase and a decrease in the microhardness of this structure are observed. In some cases (heating at temperatures of 430 and 450°C), the presence of the two-phase (B2 + TiO2) nanocrystalline surface layer leads to a noticeable (to ∼25%) enhancement in the adhesive wear resistance of the Ti49.4Ni50.6 alloy upon sliding friction in pair with steel 40Kh13.

Effect of the severe plastic deformation and aging temperature on the strengthening, structure, and wear resistance of a beryllium bronze

The effect of the aging temperature, severe plastic deformation by equal-channel angular (ECA) pressing at 400°C, and subsequent aging on the rate of wear, friction coefficient, and microhardness of a commercial beryllium bronze Br.B2 quenched from 800°C has been investigated. The bronze structure before and after tribological tests has been studied with the help of metallographic and electron-microscopic methods of analysis. It has been shown that tempering (aging) of the bronze at 250–400°C for 3 h leading to a sharp growth of the hardness substantially increases the wear rate of the bronze in pair with the steel 45 under the conditions of both dry and boundary sliding friction. The cause of this is a decrease in the toughness of the surface layer of the bronze due to the precipitation of particles of the strengthening γ′ phase (CuBe) from the α solid solution. Severe plastic deformation (ECA pressing) markedly enhances the wear resistance of the bronze subjected to subsequent aging at 250–400°C. This is because ECA pressing favors the formation of an extremely dispersed nanocrystalline structure with α-phase crystals 40–50 nm in size at the wear surface of the bronze; whereas in the friction zone of the quenched bronze or the quenched and aged bronze, there arises a significantly less dispersed structure with matrix crystals to 300 nm in size. It has been assumed that under conditions of the rotational mechanism of plastic deformation operating upon sliding friction in the near-surface layer of the bronze, the increase in dispersity of its structure attained by using ECA pressing enhances the toughness and, correspondingly, wear resistance of the aged bronze.

Structural transformations and wear resistance of abrasive-affected amorphous Fe- and Co-based alloys

The abrasive wear resistance of the Fe64Co30Si3B3, Fe82.6Nb5Cu3Si8B1.4, Co86.5Cr4Si7B2.5, and Fe81Si4B13C2 amorphous alloys (ribbon 30 μm thick) has been investigated upon sliding over fixed abrasives (corundum and silicon carbide). The character of fracture of the surface and structural transformations initiated in these materials by the abrasive action have been studied by the metallographic, X-ray diffraction, and electron-microscopic methods. It has been shown that the abrasive wear resistance of the amorphous alloys is smaller by a factor of 1.6–2.9 than that of the Kh12M and U8 tool steels possessing approximately the same level of hardness. A pronounced softening of the surface layer of the amorphous alloys in the process of wear, which is characterized by a decrease in their microhardness reaching 12.5%, has been found. It has been shown that in the surface layer of these amorphous alloys upon wear there arises a small amount (on the order of several volume percent) of the nanocrystalline structure, which does not exert a marked effect on the microhardness and wear resistance of the alloys. In the alloys under study, the main factor that is responsible for their comparatively low abrasive wear resistance is their local softening in the process of wear caused by specific features of deformation processes occurring heterogeneously under the action of high shear contact stresses.

Friction-induced structural transformations of the carbide phase in Hadfield steel

Structural transformations of the carbide phase in Hadfield steel (110G13) that occur upon plastic deformation by dry sliding friction have been studied by methods of optical metallography, X-ray diffraction, and transmission electron microscopy. Deformation is shown to lead to the refinement of the particles of the carbide phase (Fe, Mn)3C to a nanosized level. The effect of the deformation-induced dissolution of (Fe, Mn)3C carbides in austenite of 110G13 (Hadfield) steel has been revealed, which manifests in the appearance of new lines belonging to austenite with an unusually large lattice parameter (a = 0.3660–0.3680 nm) in the X-ray diffraction patterns of steel tempered to obtain a fine-lamellar carbide phase after deformation. This austenite is the result of the deformation-induced dissolution of disperse (Fe, Mn)3C particles, which leads to the local enrichment of austenite with carbon and manganese. The tempering that leads to the formation of carbide particles in 110G13 steel exerts a negative influence on the strain hardening of the steel, despite the increase in the hardness of steel upon tempering and the development of the processes of the deformation-induced dissolution of the carbide phase, which leads to the strengthening of the γ solid solution.