Analysis of the high-temperature dry sliding behavior of CoCrFeNiTi0.5Alx high-entropy alloys

In this study,CoCrFeNiTi0.5Alx high‐entropy alloys were produced by induction melting and their dry sliding wear behavior was examined at different temperatures.In addition to face‐centered cubic(FCC)phases,low amounts of a tetragonal phase were detected in the microstructures of alloys without Al and microscratches were formed by wear particles on the worn surfaces of the alloy specimens.Two body‐centered cubic(BCC)phases were detected in the alloy with 0.5Al and a fatigue‐related extrusion wear mechanism was detected on the worn surface.The alloy specimen with a high Al content exhibited the best wear characteristics.No wear tracks were formed in single‐phase BCC intermetallic alloys at room temperature and they exhibited a higher wear strength at high temperatures when compared to other samples.

Interplay between microstructural evolution and tribo-chemistry during dry sliding of metals

Understanding the microstructural and tribo-chemical processes during tribological loading is of utmost importance to further improve the tribological behavior of metals. In this study, the friction, wear and tribo-chemical behavior of Ni with different initial microstructures(nanocrystalline, bi-modal, coarse-grained) is investigated under dry sliding conditions. In particular, the interplay be-tween frictional response, microstructural evolution and tribo-oxidation is considered. Friction tests are carried out using ball-on-disk experiments with alumina balls as counter-bodies, varying the load between 1 and 5 N. The microstructural evolution as well as the chemical reactions beneath the samples’ surface is investigated by means of cross-sections. The samples with finer microstructures show a faster run-in and lower maximum values of the coefficient of friction(COF) which can be attributed to higher oxidation kinetics and a higher hardness. It is observed that with increasing sliding cycles, a stable oxide layer is formed. Furthermore, initially coarse-grained samples show grain refinement, whereas initially finer microstructures undergo grain coarsening converging towards the same superficial grain size after 2,000 sliding cycles. Consequently, the experimental evidence supports that, irrespective of the initial microstructure, after a certain deformation almost identical steady-state COF values for all samples are achieved.

Dry sliding wear performance of 7075 Al alloy under different temperatures and load conditions

Dry sliding wear tests were performed for 7075Al alloy under a load of 25–250 N at 25–200℃. The wear behaviors and mechanisms under various testing conditions were explored. A mild-to-severe wear transition is noticed to occur with an increase in the load at 25–200℃. With the temperature increasing, the wear loss decreases constantly under the low load of less than 50 N. It can be suggested that the 7075 Al alloy presents a high wear resistance under a high ambient temperature and low load. Its high wear resistance is found to be attributed to the existence of mechanically mixing layer (MML). The predominant wear mechanism is adhesive and abrasive wear at room temperature. With the ambient temperature and load increasing, oxidative wear and plastic extrusive wear successively prevail due to thermal oxidation and softening of matrix.

Sliding wear behaviors of Nomex fabric/phenolic composite under dry and water-bathed sliding conditions

A Nomex fabric/phenolic composite was prepared,and its tribological properties were evaluated under dry and water‐bathed sliding conditions by a pin‐on‐disk tribometer.The resulting size of the friction coefficient for the Nomex fabric/phenolic composite in the study occurred in the following order:dry sliding condition>distilled water‐bathed sliding condition>sea water‐bathed sliding condition.The fabric composite’s wear rate from high to low was as follows:distilled water‐bathed sliding condition>sea water‐bathed sliding condition>dry sliding condition.Under water‐bathed sliding conditions,penetration of water into the cracks accelerated the composite’s invalidation process,resulting in a higher wear rate.We also found that the extent of corrosion and transfer film formed on the counterpart pin significantly influenced the wear rate of the Nomex fabric composite.Discussion of the Nomex fabric composite’s wear mechanisms under the sliding conditions investigated is provided on the basis of the characterization results.

Self-lubrication of tribologically-induced oxidation during dry reciprocating sliding of aged Ti–Ni51.5 at% alloy

The tribological behaviors of Ti–Ni51.5 at%alloy strengthened by finely dispersed Ni_(4)Ti_(3) particles in reciprocating sliding against GCr15,Al_(2)O_(3),and ZrO_(2) at room temperature were studied.Interestingly,the coefficient of friction(COF)suffered a sheer drop(from 0.9 to 0.2)when the aged alloy slid against GCr15 at a frequency of 20 Hz under a 20 N load without lubrication.However,severe‐mild wear transition disappeared when a solutionized alloy was used.Moreover,the COF stabilized at a relatively high level when Al_(2)O_(3) and ZrO_(2) were used as counterparts,although their wear mechanisms showed signs of oxidation.Scanning electron microscopy(SEM)and X‐ray element mappings of the wear scars of the counterparts clearly indicate that the formation of well‐distributed tribo‐layer and material transfer between the ball and disk are pivotal to the severe‐to‐mild wear transition in the aged Ti–Ni51.5 at%alloy/GCr15 friction pair.The higher microhardness and superelasticity of the aged alloy significantly accelerate the material transfer from GCr15 to the disk,forming a glazed protective tribo‐layer containing Fe‐rich oxides.

A comprehensive research on wear resistance of GH4169 superalloy in longitudinal-torsional ultrasonic vibration side milling with tool wear and surface quality

To improve the service and machining performance of the workpiece, the tool wear mechanisms, surface machining quality, and wear resistance in conventional side milling(CSM)and longitudinal-torsional ultrasonic vibration side milling(LTUVSM) of GH4169 superalloy at different cutting lengths are investigated systematically. Tool wear mechanisms are revealed and the correlation between machined surface quality with tool wear is analyzed correspondingly. Tool wear patterns mainly include adhesive wear, diffusion wear, abrasive wear, and chipping sticking.Better surface quality is achieved in LTUVSM due to a maximum reduction of flank wear bandwidth and wear rate by 71.9% and 71.5%, respectively, compared to CSM. The friction coefficient,initial wear stage time, and wear volume of dry sliding wear were measured to evaluate the workpiece wear resistance. The maximum reductions in friction coefficient and wear volume in LTUVSM are 18.2% and 15.8% compared to CSM. The regular ultrasonic vibration textures suppress the friction and the growth of contact nodes in the contact area, decreasing the degree of surface wear, which is demonstrated by a 38.8% increase in initial wear stage time compared to CSM.In conclusion, the workpiece in LTUVSM exhibits higher wear resistance because of the improvement of tool wear and the guarantee of surface quality.

Effect of sliding speed on elevated-temperature wear behavior of AISI H13 steel

Elevated-temperature wear tests were performed on AISI H13 steel under 50 and 100 r/min at 400–600℃.Through examining the morphology,structure and composition of worn surfaces as well as the microhardness at subsurfaces,the wear mechanisms in various sliding conditions were explored.H13 steel exhibited totally different elevated-temperature wear behavior at two sliding speeds while the high sliding speed would seriously deteriorate its wear resistance.During sliding at two sliding speeds,the wear rate of H13 steel decreased first and then rose with the increase in temperature and the wear rate reached the lowest value(lower than 1×10^(–6)mm^(3)/mm)at 500℃and 50 r/min.The wear rate at 600℃was lower than that at 400℃for 50 r/min,but the wear rate at 600℃was higher than that at 400℃for 100 r/min(except for 50 N).At 50 r/min,the wear rate decreased first and then increased with the increase in load.However,at 100 r/min,the wear rate monotonically increased with increasing load and reached 33×10^(–6)mm^(3)/mm at 600℃and 150 N,where severe wear occurred.In the other sliding conditions,severe wear did not appear with wear rate lower than 5×10^(–6)mm^(3)/mm.Oxidative mild wear merely prevailed at 500℃and 50 r/min and oxidative wear appeared in the other sliding conditions except for 600℃and 150 N,where severe plastic extrusion wear prevailed.The effect of sliding speed on wear behavior was attributed to the changes of tribo-oxide layers.During elevated-temperature sliding,tribo-oxide particles were more readily retained to form protective tribo-oxide layers on worn surfaces at the lower sliding speed than at the higher sliding speed,so as to protect from wear.

Gradient nanostructured tungsten and the thermal shock response

In fusion reactor or spallation neutron source environment,damage of tungsten components begins at the surface.To enhance the surface toughness,strength,and hardness of tungsten,a nano-to-micro sc ale lamellar gradient surface layer(thickness up to~50μm)was produced on as-rolled pure tungsten using a dry-sliding surface treatment.The dry-sliding temperature and time were established by observing the grain-size evolution from room temperature to 750℃and the coefficient of friction on the surface layer after sliding from 10 to 50 min.The grains changed into a slender fibrous structure at 250℃,which is higher than the ductile-brittle transition temperature but lower than the dynamic recrystallization temperature(750℃).The coefficient of friction of the surface layer decreased with sliding time and stabilized at approximately 50 min,indicating near-s aturation of the surface refinement.After repetitive thermal loads of 10,20,and 30 MW·m^(-2)on the gradient modified layer,it exhibited mainly recrystallization and grain growth.The proportion of low-angle grain boundaries(<10°)reached50.6,42.2,and 17.3%,respectively,and the hardness decreased to 602,582,and 488HV.A heat load of 30 MW·m^(-2)was the threshold for the fast grain growth.Compared with unmodified tungsten,the recrystallization and grain growth are relatively slow.Besides,the gradient layer could suppress the formation of pits and protrusions in the as-rolled tungsten sample.