The precipitation behavior of Mn-containing dispersoids in Al-Mg-Si 6082 alloys with different Mn contents(0,0.5 and 1.0 wt%)during various heat treatments(300–500℃)was investigated.The effects of dispersoids on ele...The precipitation behavior of Mn-containing dispersoids in Al-Mg-Si 6082 alloys with different Mn contents(0,0.5 and 1.0 wt%)during various heat treatments(300–500℃)was investigated.The effects of dispersoids on elevated-temperature strength and recrystallization resistance during hot-rolling and post-rolling annealing were evaluated.The results showed that the dispersoids in the Mn-containing alloys(0.5 and 1.0%)began to precipitate at 350℃and reached the optimum conditions after 2–4 h at 400℃.However,the dispersoids coarsened with increasing holding time at temperatures above450℃.After the peak precipitation treatment at 400℃for 2 h,the yield strength at 300℃increased from 28 MPa(base alloy free of Mn)to 55 MPa(alloy with 0.5%Mn)and 70 MPa(alloy with 1%Mn),respectively,demonstrating a significant dispersoid strengthening effect at elevated temperature.In addition,the dispersoids were thermally stable at 300℃for up to 1000 h holding owing to its relative high precipitation temperature(350–400℃),leading to the superior constant mechanical performance at elevated temperature during the long service life.During hot rolling and post-rolling annealing,the presence of a large amount of dispersoids results in the higher Zener drag PZcompared with base alloy and then significantly improved the recrystallization resistance.The alloy containing 0.5%Mn exhibited the highest recrystallization resistance among three experimental alloys studied during the post-rolling process,likely resulted from the lower coarsening rate of dispersoids and the lower dispersoids free zone.展开更多
Dense oxide dispersion strengthened(ODS) 316 L steels with different amount of Y2O3 additions were succe s s fully fabricated by selective laser melting(SLM) even though part of the added Y2O3 got lost during the proc...Dense oxide dispersion strengthened(ODS) 316 L steels with different amount of Y2O3 additions were succe s s fully fabricated by selective laser melting(SLM) even though part of the added Y2O3 got lost during the process.The microstructure was characterized in details and the mechanical properties were tested at room temperature,250℃ and 400℃,respectively.The effect of the scanning speed on agglomeration of nanoparticles during SLM process was discussed.Superior properties,e.g.,yield strength of 574 MPa and elongation of 91%,were achieved at room temperature in SLM ODS 316 L with additional 1% of Y2 O3.At elevated temperature s,the strength kept high but the elongations dropped dra matically.It was observed that nano-voids nucleated throughout the whole gauge section at the sites where nanoinclusions located.The growth and coalescence of these voids were suppre s sed by the formation of an element segregation network before necking,which relieved local stress concentration and thus delayed necking.This unusual necking behavior was studied and compared to the previous theory.It appeared that the strong convection presented in the melt pool can evenly redistribute the short-time milled coarse Y2O3 precursor powder during SLM process.These findings can not only solve the problems encountered during the fabrication of ODS components but also replenish the strengthening mechanism of SLM 316 L thus pave a way for further improving of mechanical properties.展开更多
The hardness, tensile strength and impact toughness of one quenched andtempered steel with nominal composition of Fe-0.25C-3.0Cr-3.0Mo-0.6Ni-0.1Nb (mass fraction) both atroom temperature and at elevated temperatures w...The hardness, tensile strength and impact toughness of one quenched andtempered steel with nominal composition of Fe-0.25C-3.0Cr-3.0Mo-0.6Ni-0.1Nb (mass fraction) both atroom temperature and at elevated temperatures were investigated in order to develop high-strengthsteel for long-life gun barrel use. It is found that the steel has lower decrease rate of tensilestrength at elevated temperature in comparison with the commonly used G4335V high-strength gunsteel, which contains higher Ni and lower Cr and Mo contents. The high elevated-temperature strengthof the steel is attributed to the strong secondary hardening effect and high tempering softeningresistance caused by the tempering precipitation of fine Mo-rich M_2C carbides in the α-Fe matrix.The experimental steel is not susceptible to secondary hardening embrittlement, meanwhile, itsroom-temperature impact energy is much higher than the normal requirement of impact toughness forhigh strength gun steels. Therefore, the steel is suitable for production of long-life high-strengthgun barrels with the combination of superior elevated-temperature strength and good impacttoughness.展开更多
The wear behavior and mild−severe(M−S)wear transition of Mg−10Gd−1.5Y−0.4Zr alloy were investigated within a temperature range of 20−200℃.The morphologies and compositions of worn surfaces were examined to identify t...The wear behavior and mild−severe(M−S)wear transition of Mg−10Gd−1.5Y−0.4Zr alloy were investigated within a temperature range of 20−200℃.The morphologies and compositions of worn surfaces were examined to identify the wear mechanisms using scanning electron microscope and energy dispersive X-ray spectrometer.The microstructure and hardness in the subsurfaces were analyzed to reveal the M−S wear transition mechanism.Under a constant loads of 20,35 and 40 N,each wear rate−test temperature curve presented a turning point which corresponded to the M−S wear transition.In mild wear,the surface material was plastically deformed and hence was strainhardened,whereas in severe wear,the surface material was dynamically recrystallized and consequently was softened.It has been found that the critical temperature for M−S wear transition decreases with increasing the normal load,and the normal load exhibits an almost linear relationship with critical temperature for M−S wear transition.This work reveals that the M−S wear transition of the studied alloy conforms to the surface DRX temperature criterion.展开更多
Elevated-temperature pressure swing adsorption is a promising technique for producing high purity hydrogen and controlling greenhouse gas emissions. Thermodynamic analysis indicated that the CO in H-rich gas could be ...Elevated-temperature pressure swing adsorption is a promising technique for producing high purity hydrogen and controlling greenhouse gas emissions. Thermodynamic analysis indicated that the CO in H-rich gas could be controlled to trace levels of below 10 ppm by in situ reduction of the COconcentration to less than 100 ppm via the aforementioned process. The COadsorption capacity of potassiumpromoted hydrotalcite at elevated temperatures under different adsorption（mole fraction, working pressure） and desorption（flow rate, desorption time, steam effects） conditions was systematically investigated using a fixed bed reactor. It was found that the COresidual concentration before the breakthrough of COmainly depended on the total amount of purge gas and the COmole fraction in the inlet syngas.The residual COconcentration and uptake achieved for the inlet gas comprising CO（9.7 mL/min） and He（277.6 mL/min） at a working pressure of 3 MPa after 1 h of Ar purging at 300 mL/min were 12.3 ppm and0.341 mmol/g, respectively. Steam purge could greatly improve the cyclic adsorption working capacity, but had no obvious benefit for the recovery of the residual COconcentration compared to purging with an inert gas. The residual COconcentration obtained with the adsorbent could be reduced to 3.2 ppm after 12 h of temperature swing at 450 °C. A new concept based on an adsorption/desorption process, comprising adsorption, steam rinse, depressurization, steam purge, pressurization, and high-temperature steam purge, was proposed for reducing the steam consumption during CO/COpurification.展开更多
Al-Ti alloy containing rare earth elements can produce fine, uniform dispersion intermetallic phase through rapid solidification (RS) technology. RS Al-Ti-RE alloy can be designed for applications at elevated-temperat...Al-Ti alloy containing rare earth elements can produce fine, uniform dispersion intermetallic phase through rapid solidification (RS) technology. RS Al-Ti-RE alloy can be designed for applications at elevated-temperature since the intermetallic compound has good thermal stability. A transmission electron microscopy investigation shows the intermetallic phase has a diamond cubic structure (a=1.47736 nm), with space group Fd3m. The chemical stoichiometry is Al_(20)Ti_2La. The particle is formed from the melting directly, prior to other phases, and the nucleus is formed from icosahedrons composed with twenty tetrahedrons. Twin crystal structure plays an important role in the nucleation stage.展开更多
Recent structural collapses caused by fire have focused attention on research concerning fire safety in building design. Steel connections are an important component of any structural steel building as they provide li...Recent structural collapses caused by fire have focused attention on research concerning fire safety in building design. Steel connections are an important component of any structural steel building as they provide links between the principal structural members. Considering the importance of this matter this paper describes a spring-stiffness model developed to predict the behavior of bolted angle connections bare-steel joints at elevated temperature. The joint components are considered as springs with predefined mechanical properties i.e. stiffness and strength. The elevated temperature joint’s response can be predicted by assem-bling the stiffness of the components which are assumed to degrade with increasing temperature based on the recommendations presented in the design parameters code. Comparison of the results from the model with existing experimental data showed good agreement. The proposed model can be easily modified to describe the elevated temperature behavior of other types of joint as well as joints under large rotations.展开更多
Zr modification is an effective method for improving hot-cracking resistance and elevated-temperature mechanical properties during laser powder bed fusion(L-PBF)of traditional medium and high strength wrought aluminum...Zr modification is an effective method for improving hot-cracking resistance and elevated-temperature mechanical properties during laser powder bed fusion(L-PBF)of traditional medium and high strength wrought aluminum alloys.This study investigated the l-PBF processability and elevated-temperature mechanical properties of a Zr-modified 2024Al alloy.It was found that the hot-cracking susceptibility increased with the increased scanning speed,which was in reasonable agreement with the modified Rappaz-Drezet-Gremaud criterion.Furthermore,the primary L1_(2)-Al_(3)Zr precipitates,which acted as ef-ficient nucleation sites,precipitated at the fusion boundary of the melt pool,leading to the formation of a heterogeneous grain structure.The yield strength(YS)of the as-fabricated samples at 150,250,and 350℃was 363,210,and 48 MPa,respectively.Despite the slight decrease to 360 MPa of the YS when tested at 150℃,owing to the additional precipitate strengthening from the L1_(2)-Al_(3)Zr precipitates,the YS achieved yield strengths of 253 and 69 MPa,an increase of 20.5%and 30.4%,when tested at 250 and 350℃,respectively.The yield strengths in both the as-fabricated and T6-treated conditions tested at 150 and 250℃were comparable to those of casting Al-Cu-Mg-Ag alloys and superior to those of traditionally heat-resistant 2219-T6 and 2618-T6 of Al-Cu alloys.展开更多
Eutectic high-entropy alloys(EHEAs)that have superior formability are attractive for direct laser deposition technology.In this study,a regular-shaped bulk Ni_(32)Co_(30)Cr_(10)Fe_(10)Al_(18)EHEA without apparent pore...Eutectic high-entropy alloys(EHEAs)that have superior formability are attractive for direct laser deposition technology.In this study,a regular-shaped bulk Ni_(32)Co_(30)Cr_(10)Fe_(10)Al_(18)EHEA without apparent pores and micro-cracks was successfully fabricated by direct laser deposition.The as-deposited alloy showed a high tensile strength of 1.3 GPa with a ductility of 35%at ambient temperature and a tensile strength of 320 MPa at 760℃.The deformation mechanisms of the as-deposited alloy at ambient and elevated temperatures were investigated by coupling the in-situ tensile test with a scanning electron microscope.It is revealed that the excellent combination of strength and ductility originated from the synergic effects of the FCC and B2 phases in eutectic lamellae.And the generation of cracks along phase boundaries restricted its high-temperature strength above 760℃.展开更多
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 subsurf...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.展开更多
基金the financial support of the Natural Sciences and Engineering Research Council of Canada(NSERC)Rio Tinto Aluminum through the NSERC Industry Research Chair in the Metallurgy of Aluminum Transformation at the University of Quebec at Chicoutimi.
文摘The precipitation behavior of Mn-containing dispersoids in Al-Mg-Si 6082 alloys with different Mn contents(0,0.5 and 1.0 wt%)during various heat treatments(300–500℃)was investigated.The effects of dispersoids on elevated-temperature strength and recrystallization resistance during hot-rolling and post-rolling annealing were evaluated.The results showed that the dispersoids in the Mn-containing alloys(0.5 and 1.0%)began to precipitate at 350℃and reached the optimum conditions after 2–4 h at 400℃.However,the dispersoids coarsened with increasing holding time at temperatures above450℃.After the peak precipitation treatment at 400℃for 2 h,the yield strength at 300℃increased from 28 MPa(base alloy free of Mn)to 55 MPa(alloy with 0.5%Mn)and 70 MPa(alloy with 1%Mn),respectively,demonstrating a significant dispersoid strengthening effect at elevated temperature.In addition,the dispersoids were thermally stable at 300℃for up to 1000 h holding owing to its relative high precipitation temperature(350–400℃),leading to the superior constant mechanical performance at elevated temperature during the long service life.During hot rolling and post-rolling annealing,the presence of a large amount of dispersoids results in the higher Zener drag PZcompared with base alloy and then significantly improved the recrystallization resistance.The alloy containing 0.5%Mn exhibited the highest recrystallization resistance among three experimental alloys studied during the post-rolling process,likely resulted from the lower coarsening rate of dispersoids and the lower dispersoids free zone.
文摘Dense oxide dispersion strengthened(ODS) 316 L steels with different amount of Y2O3 additions were succe s s fully fabricated by selective laser melting(SLM) even though part of the added Y2O3 got lost during the process.The microstructure was characterized in details and the mechanical properties were tested at room temperature,250℃ and 400℃,respectively.The effect of the scanning speed on agglomeration of nanoparticles during SLM process was discussed.Superior properties,e.g.,yield strength of 574 MPa and elongation of 91%,were achieved at room temperature in SLM ODS 316 L with additional 1% of Y2 O3.At elevated temperature s,the strength kept high but the elongations dropped dra matically.It was observed that nano-voids nucleated throughout the whole gauge section at the sites where nanoinclusions located.The growth and coalescence of these voids were suppre s sed by the formation of an element segregation network before necking,which relieved local stress concentration and thus delayed necking.This unusual necking behavior was studied and compared to the previous theory.It appeared that the strong convection presented in the melt pool can evenly redistribute the short-time milled coarse Y2O3 precursor powder during SLM process.These findings can not only solve the problems encountered during the fabrication of ODS components but also replenish the strengthening mechanism of SLM 316 L thus pave a way for further improving of mechanical properties.
文摘The hardness, tensile strength and impact toughness of one quenched andtempered steel with nominal composition of Fe-0.25C-3.0Cr-3.0Mo-0.6Ni-0.1Nb (mass fraction) both atroom temperature and at elevated temperatures were investigated in order to develop high-strengthsteel for long-life gun barrel use. It is found that the steel has lower decrease rate of tensilestrength at elevated temperature in comparison with the commonly used G4335V high-strength gunsteel, which contains higher Ni and lower Cr and Mo contents. The high elevated-temperature strengthof the steel is attributed to the strong secondary hardening effect and high tempering softeningresistance caused by the tempering precipitation of fine Mo-rich M_2C carbides in the α-Fe matrix.The experimental steel is not susceptible to secondary hardening embrittlement, meanwhile, itsroom-temperature impact energy is much higher than the normal requirement of impact toughness forhigh strength gun steels. Therefore, the steel is suitable for production of long-life high-strengthgun barrels with the combination of superior elevated-temperature strength and good impacttoughness.
基金financial support from the National Natural Science Foundation of China (No.51775226)。
文摘The wear behavior and mild−severe(M−S)wear transition of Mg−10Gd−1.5Y−0.4Zr alloy were investigated within a temperature range of 20−200℃.The morphologies and compositions of worn surfaces were examined to identify the wear mechanisms using scanning electron microscope and energy dispersive X-ray spectrometer.The microstructure and hardness in the subsurfaces were analyzed to reveal the M−S wear transition mechanism.Under a constant loads of 20,35 and 40 N,each wear rate−test temperature curve presented a turning point which corresponded to the M−S wear transition.In mild wear,the surface material was plastically deformed and hence was strainhardened,whereas in severe wear,the surface material was dynamically recrystallized and consequently was softened.It has been found that the critical temperature for M−S wear transition decreases with increasing the normal load,and the normal load exhibits an almost linear relationship with critical temperature for M−S wear transition.This work reveals that the M−S wear transition of the studied alloy conforms to the surface DRX temperature criterion.
基金financed by Shanxi Province Science and Technology Major Projects of MH2015-06
文摘Elevated-temperature pressure swing adsorption is a promising technique for producing high purity hydrogen and controlling greenhouse gas emissions. Thermodynamic analysis indicated that the CO in H-rich gas could be controlled to trace levels of below 10 ppm by in situ reduction of the COconcentration to less than 100 ppm via the aforementioned process. The COadsorption capacity of potassiumpromoted hydrotalcite at elevated temperatures under different adsorption（mole fraction, working pressure） and desorption（flow rate, desorption time, steam effects） conditions was systematically investigated using a fixed bed reactor. It was found that the COresidual concentration before the breakthrough of COmainly depended on the total amount of purge gas and the COmole fraction in the inlet syngas.The residual COconcentration and uptake achieved for the inlet gas comprising CO（9.7 mL/min） and He（277.6 mL/min） at a working pressure of 3 MPa after 1 h of Ar purging at 300 mL/min were 12.3 ppm and0.341 mmol/g, respectively. Steam purge could greatly improve the cyclic adsorption working capacity, but had no obvious benefit for the recovery of the residual COconcentration compared to purging with an inert gas. The residual COconcentration obtained with the adsorbent could be reduced to 3.2 ppm after 12 h of temperature swing at 450 °C. A new concept based on an adsorption/desorption process, comprising adsorption, steam rinse, depressurization, steam purge, pressurization, and high-temperature steam purge, was proposed for reducing the steam consumption during CO/COpurification.
文摘Al-Ti alloy containing rare earth elements can produce fine, uniform dispersion intermetallic phase through rapid solidification (RS) technology. RS Al-Ti-RE alloy can be designed for applications at elevated-temperature since the intermetallic compound has good thermal stability. A transmission electron microscopy investigation shows the intermetallic phase has a diamond cubic structure (a=1.47736 nm), with space group Fd3m. The chemical stoichiometry is Al_(20)Ti_2La. The particle is formed from the melting directly, prior to other phases, and the nucleus is formed from icosahedrons composed with twenty tetrahedrons. Twin crystal structure plays an important role in the nucleation stage.
文摘Recent structural collapses caused by fire have focused attention on research concerning fire safety in building design. Steel connections are an important component of any structural steel building as they provide links between the principal structural members. Considering the importance of this matter this paper describes a spring-stiffness model developed to predict the behavior of bolted angle connections bare-steel joints at elevated temperature. The joint components are considered as springs with predefined mechanical properties i.e. stiffness and strength. The elevated temperature joint’s response can be predicted by assem-bling the stiffness of the components which are assumed to degrade with increasing temperature based on the recommendations presented in the design parameters code. Comparison of the results from the model with existing experimental data showed good agreement. The proposed model can be easily modified to describe the elevated temperature behavior of other types of joint as well as joints under large rotations.
基金The work was financially supported by the National Key R&D Program of China(No.2016YFB1100100)the Research Fund of the State Key Laboratory of Solidification Processing(NPU),China(No.2020-TZ-02)+3 种基金the Advance Research Projects in the Field of Manned Spaceflight(No.040302)the Shanghai Aerospace Science and Technology Innovation Fund Project(No.SAST2018-066)This work was also supported by the“Fundamental Research Funds for the Central Universities”(No.G2021KY05104)the“Natural Science Basis Research Plan in Shaanxi Province of China”(No.2022JQ-479).We would like to thank Editage(www.editage.com)for En-glish language editing.
文摘Zr modification is an effective method for improving hot-cracking resistance and elevated-temperature mechanical properties during laser powder bed fusion(L-PBF)of traditional medium and high strength wrought aluminum alloys.This study investigated the l-PBF processability and elevated-temperature mechanical properties of a Zr-modified 2024Al alloy.It was found that the hot-cracking susceptibility increased with the increased scanning speed,which was in reasonable agreement with the modified Rappaz-Drezet-Gremaud criterion.Furthermore,the primary L1_(2)-Al_(3)Zr precipitates,which acted as ef-ficient nucleation sites,precipitated at the fusion boundary of the melt pool,leading to the formation of a heterogeneous grain structure.The yield strength(YS)of the as-fabricated samples at 150,250,and 350℃was 363,210,and 48 MPa,respectively.Despite the slight decrease to 360 MPa of the YS when tested at 150℃,owing to the additional precipitate strengthening from the L1_(2)-Al_(3)Zr precipitates,the YS achieved yield strengths of 253 and 69 MPa,an increase of 20.5%and 30.4%,when tested at 250 and 350℃,respectively.The yield strengths in both the as-fabricated and T6-treated conditions tested at 150 and 250℃were comparable to those of casting Al-Cu-Mg-Ag alloys and superior to those of traditionally heat-resistant 2219-T6 and 2618-T6 of Al-Cu alloys.
基金financially supported by the Research Fund of the State Key Laboratory of Solidification Processing(NPU),China(Grant Nos.2020-TS-06,2021-TS-02).
文摘Eutectic high-entropy alloys(EHEAs)that have superior formability are attractive for direct laser deposition technology.In this study,a regular-shaped bulk Ni_(32)Co_(30)Cr_(10)Fe_(10)Al_(18)EHEA without apparent pores and micro-cracks was successfully fabricated by direct laser deposition.The as-deposited alloy showed a high tensile strength of 1.3 GPa with a ductility of 35%at ambient temperature and a tensile strength of 320 MPa at 760℃.The deformation mechanisms of the as-deposited alloy at ambient and elevated temperatures were investigated by coupling the in-situ tensile test with a scanning electron microscope.It is revealed that the excellent combination of strength and ductility originated from the synergic effects of the FCC and B2 phases in eutectic lamellae.And the generation of cracks along phase boundaries restricted its high-temperature strength above 760℃.
基金Natural Science Foundation of Jiangsu Province(No.BK20201231)Jiangsu Colleges and Universities“Cyanine Engineering”and Research Startup Fund Project for High-level Talents of Taizhou University(No.TZXY2017QDJJ013).
文摘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.
