Abstract
The authors have studied the relation between wear intensity, average contact temperature and phase composition of the surface layers ofAISI 1020 steel, copper and NiTi alloy in dry sliding against the steel counterbody under electric current of density higher than 100 A/cm2 . These contact characteristics are considered carefully at the beginning of catastrophic wear, when the surface layers transit to the utmost state. It was noted that relaxation of stresses in the surface layers was due to the structural transformation in normal wear regime. It leads to tribolayers formation. The high strength of the copper tribolayer is first of all due to the formation of FeO oxide on the sliding surface, which prevents adhesion in contact. In addition, signs of a liquid phase were observed on the copper contact surface. It promoted the low rate of formation and accumulation of structural defects. Emergence of areas of melt and FeO oxide on the sliding surface provides high contact wear resistance. These factors, combined with the high thermal copper conductivity, have caused the tribolayer transition to the limit state at high current density and low contact temperature. The absence of oxides on the sliding surface of the NiTi alloy has caused strong adhesion in the contact, high rate of formation and accumulation of structural defects. Therefore, the tribolayer quickly deteriorates and high wear intensity and rapid increase in the contact temperature are observed with current density increase. Therefore, the catastrophic wear of the NiTi alloy begins at a temperature about 350 °C and at low current density. The sliding surface of AISI 1020 steel contained FeO oxide, therefore strong adhesion is not manifested. Formation of FCC-Fe in tribolayer of AISI steel 1020 is detected, that promotes its accelerated deterioration. Therefore, the tribolayer of AISI steel 1020 transites to the utmost state at a relatively low current density and at a higher temperature. The presented contact temperatures corresponding to the beginning stages of the utmost state of the tribolayer do not exceed 350 °С. Comparison of these temperatures with the known contact temperatures of other metals made it possible to assert that raising of the contact temperature of any metal higher than 400 °С leads to its utmost state. Therefore the characteristics of metals contact at temperatures of sliding contact higher than 500 °С is not of practical interest.
Highlights
The authors have studied the relation between wear intensity, average contact temperature and phase composition of the surface lay ers of AISI 1020 steel, copper and NiTi alloy in dry sliding against the steel counterbody under electric current of density higher than 100 A/cm2
These contact characteristics are considered carefully at the beginning of catastrophic wear, when the surface layers transit to the utmost state
It was noted that relaxation of stresses in the surface layers was due to the structural transformation in normal wear regime
Summary
Средней температуры контакта и фазового состава поверхностных слоев стали Ст3, меди и сплава NiTi при сухом скольжении по стальному контртелу под воздействием электрического тока плотностью более 100 А/см. Эти факторы в сочетании с высокой теплопроводностью меди обусловили переход трибослоя в предельное состояние при высокой плотности тока и низкой температуре контакта. Поэтому трибослой стали Ст3 переходит в предельное состояние при относительно низкой плотности тока и при более высокой температуре. Что высокая износостойкость материала в скользящем контакте обеспечивается за счет высокой сдвиговой прочности поверхностных слоев контактирующих тел [1, 2] или, в общем случае, за счет удовлетворительной релаксации напряжений в зоне контакта. Целью настоящей работы является экспериментальная оценка температур, возникающих в интерфейсе сухого скользящего контакта металл – сталь под воз действием электрического тока высокой плотности при переходе поверхностного слоя к предельному состоянию
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