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Micro-arc oxidation (MAO) coating with different oxidation times was prepared on aluminum alloy to improve the impact wear resistance of aluminum alloy. Surface morphology and impact wear behavior of the coating with different oxidation times were investigated. Results showed that the MAO coating formed on aluminum alloy had a porous and island structure comprising α-Al 2 O 3 and γ-Al 2 O 3. Coating thickness increased with increasing oxidation time. The coating also had a lower impact force and higher energy absorption ratio than the substrate but had a lower wear volume, which decreased with increasing oxidation time. These results indicated that MAO coating can improve the impact wear resistance of aluminum alloy. Further study revealed that the wear mechanism of aluminum alloy was oxidation wear and contact fatigue spalling, whereas that of MAO coating was contact fatigue wear. • The characterization and impact wear behavior of the MAO coating with different oxidation times was investigated. • The wear mechanism of aluminum alloy and MAO coating was discussed. [ABSTRACT FROM AUTHOR]

Tribological tests were conducted on an AISI A2 steel plate against an AISI 51200 steel ball lubricated by SAE 0W30 and PAO 4 cSt base oils containing no additive package. Friction and wear behaviors were evaluated at room temperature (RT, 23 °C) and a series of elevated temperatures (75, 100, 125 and 175 °C). The steady-state friction coefficient appeared to be proportional to the oil temperature, probably because reduced oil viscosity at a higher temperature caused more surface asperity collisions. In contrast, wear results did not follow the trend: the wear rate surprisingly decreased when the oil temperature increased from RT to 75−100 °C, and then turned around to increase along with the temperature at above 100 °C. Evidentially, there are other significant factors than just the oil viscosity that influence the wear process upon the temperature change. Wear scar morphology examination and surface chemical analysis revealed an oxide-containing surface film on the wear scars and higher oxide content and larger film coverage seemed to reduce the wear rate. Therefore, the wear mechanism is proposed as a combined effect of mechanical material removal and protective surface film formation: the former largely depending on oil viscosity that is inversely proportional to the temperature and the latter involving surface and wear debris oxidation that is promoted by temperature elevation as well as the water content (up to 100 °C) in the oil. [ABSTRACT FROM AUTHOR]

Tribological evaluations were conducted on lubricating base oils of different viscosity grades with and without an anti-wear (AW) additive in lubricating steel-steel and ceramic-steel contacts. Two AW additives were applied: a conventional secondary zinc dialkyldithiophosphate (ZDDP) and an oil-miscible phosphonium-phosphate ionic liquid (IL). Tests were carried out using reciprocating ball-on-flat sliding at room temperature. The flat material was AISI A2 tool steel, and the ball material was either AISI 52100 bearing steel or silicon nitride. Four lubricants were tested: Chevron SAE 15W40 and 0W30 base oils, and the 0W30 base oil plus 1 wt% ZDDP or IL. For the steel–steel contact, the lower-viscosity 0W30 base oil resulted in a higher wear rate than did the 15W40 base oil, as expected. Both the ZDDP and the IL substantially reduced wear, and the IL-additized 0W30 base oil was the best performer. For the ceramic–steel contact, the ZDDP provided moderate wear protection for both ball and flat. In contrast, the IL reduced the steel flat wear more effectively but increased the ceramic ball wear rate. Cross-sectional transmission electron microscopy (TEM) examination and x-ray photoelectron spectroscopy (XPS) chemical analysis were used to reveal the thickness, nanostructure, and composition of the tribofilms formed by ZDDP and IL on the steel and silicon nitride surfaces. [ABSTRACT FROM AUTHOR]

The fretting corrosion behavior of zirconium (Zr) alloy under different pH conditions was investigated. The morphology, microstructure, and wear mechanism of Zr alloy under different pH condition was analyzed. The results indicate that the wear scar Zr alloy in acid condition presents a typical feature of wear debris, furrow, and delamination, indicating that the wear mechanism is oxidation wear and abrasive wear. The microstructure beneath the wear scar is composed of a thick wear debris layer (WDL), deformation layer (DL), and thin tribologically transformed structure (TTS). The WDL is a nano-sized grainy structure and the micro-crack is mainly formed on the interface of WDL and TTS. The largest wear volume belongs to the wear scar in pH of 1 condition. Because the corrosion and corrosion induced by wear would destroy the surface protective film and form a thick WDL, which is easy to separate to increase the material loss volume. The Zr alloy in pH of 1 condition shows the largest volume ratio of wear induced by corrosion and corrosion induced by wear. • The material loss volume by tribocorrosion is lowest at intermediate pH and highest at low pH. • The thick WDL at low pH is generated by the quick oxidation of TTS/DL and the accumulation of oxide particles. • The wear mechanism of Zr alloy at intermediate pH is abrasive wear and at low pH is oxidation wear and abrasive wear. [ABSTRACT FROM AUTHOR]

Abstract: Detailed fretting wear phenomena are usually hidden between the two compact contact interfaces. Direct or in situ observation of fretting wear is supposed to obtain time-dependent damage information. In this paper, the torsional fretting wear tests of plymethyl methacrylate (PMMA) flat against AISI 52100 steel spheres had been investigated on an improved test rig, which provides the real-time in situ observation, friction vibration and noise measurements. The running behaviours (frictional kinetics characteristics), wear mechanisms, oxidative behaviours and debris behaviours of the torsional fretting contact interfaces in combination with the friction vibration analyses were analyzed in detail. The results of observation suggested that a slight damage could be found in the partial slip regime. And the spindle-like detachment occurred in the mixed fretting regime, which presented a radial pattern distribution in the wear scar. The friction lateral vibration decreased accompanied with the accumulation of the debris, and the appearance of third-bodies helped to reduce the friction vibration. In the mixed fretting regime and slip regime, the wear mechanisms mainly were fatigue wear and oxidative wear. [Copyright &y& Elsevier]

Abstract: Four simple fretting modes are defined according to relative motion: tangential, radial, rotational, and torsional fretting. This paper presents a new test rig that was developed from a low-speed reciprocating rotary system to show torsional fretting wear under ball-on-flat contact. Torsional fretting behavior was investigated for LZ50 steel flats against AISI52100 steel balls under various angular displacement amplitudes and normal loads. The friction torques and dissipation energy were analyzed in detail. Two types of T–θ curves in the shape of quasi-parallelograms and ellipticals were found that correspond to gross and partial slips, respectively. The experimental results showed that the dynamic behavior and damage processes depend strongly on the normal loads, angular displacement amplitudes, and cycles. In this paper, the debris and oxidation behaviors and detachment of particles in partial and gross slip regimes are also discussed. Debris and oxidation are shown to have important roles during the torsional fretting processes. The wear mechanism of torsional fretting was a combination of abrasive and oxidative wear and delamination before third-body bed formation. The mechanism was then transformed into third-body wear after a great amount of debris formed. [Copyright &y& Elsevier]

In the heat exchangers of sodium-cooled fast reactors, sodium flow can cause the tubes to vibrate, resulting in fretting wear damage due to the contact between the tubes (2.25Cr-1Mo steel) and their support plate (Gr5C12 alloy). In this work, the effects of temperature on the fretting wear behavior of a 2.25Cr-1Mo heat transfer tube on a Gr5C12 alloy rod were studied. The results showed that the coefficient of friction (COF) and wear volume increased first and then decreased with the increase in temperature. Moreover, 2.25Cr-1Mo showed great wear performance at high temperatures than at room temperature and 80 °C, because of the antifriction nature of the oxidative layer and the high hardness of the tribological transformed structure layer. As the temperature increased, material transfer and plastic deformation became increasingly obvious, but average wear depth decreased. This provides data support for the practical engineering application of 2.25Cr-1Mo steel at elevated temperatures. Wear mechanisms were found to depend modestly on temperature and largely on normal load. As temperature increases, the wear mechanism gradually changes from abrasive wear to adhesive wear. [ABSTRACT FROM AUTHOR]

Various impact-sliding wear behaviors commonly occur among steam generator tubes and their supporters. The main purpose of this study is to investigate the effect of ultrasonic surface rolling process (USRP) on the impact-sliding wear behavior of Inconel 690 alloy tubes. A self-made two-dimensional cycling impact-sliding wear test rig was developed to realize multiple wear patterns by changing motion parameters in both impact and sliding directions. Tests were carried out under different impact and sliding velocities. Results indicated that USRP reduced the impact kinetic energy absorption rate, friction coefficient and increased the wear resistance of Inconel 690 alloy. These can be attributed to the USRP which decreased the grain size of 690 alloy, resulting in the increase of micro-hardness and compressive residual stress. Image 1 • USRP is capable to refine microstructure, improve micro-hardness and compressive residual stresses of 690 alloy tubes. • USRP treated samples showed better wear resistance and lower friction coefficient under impact-wear sliding condition. [ABSTRACT FROM AUTHOR]

Grid-to-rod fretting (GTRF) is the main factor leading to the failures of fuel rods in reactor. The oxide layer formed on pre-wear stage has a significant impact on the wear resistance of zirconium (Zr) alloys fuel cladding. In this research, Zr alloy was immersed in high-temperature water conditions for 30 days,110 days,145 days, and 195 days. The characterization and fretting wear behavior of oxide layer on Zr alloy were investigated. Results show that the oxide layer consists of t-ZrO 2 , m-ZrO 2 , and metal Zr. The thickness and hardness of oxide layer increases with the corrosion time increase. The fretting running region and F-D curve are dependent on the oxide layer. Compared with Zr alloy, the wear volume of oxide layer on Zr alloy is significantly reduced because the structure of oxide layer is dense with a high hardness, leading to that the oxide layer is not easy to be destroyed during the fretting. The wear mechanism of Zr alloy is adhesive wear, and the wear mechanism of oxide layer is abrasive wear. • The fretting wear behavior of pre-oxidized Zr alloy cladding was investigated. • The wear volume of oxide layer on Zr alloy decreases with the corrosion time increase. • The fretting running region and F-D curve are significantly dependent on the oxide layer. [ABSTRACT FROM AUTHOR]

Abstract CoMoCrSi superalloy coatings were prepared on Mar-M247 Ni-based superalloy by high velocity oxygen fuel (HVOF) under various spraying parameters. Dense coatings with porosity < 0.8% were prepared, and the fretting wear properties of the coatings was investigated. The coatings showed good adaptability to the spraying parameters. As the temperature increased from R.T. to 400 °C, the friction coefficient of the coating decreased from 0.8 to 0.6, and the wear morphology gradually changed from pits to bulges. A large amount of material transfer occurred, and the main wear mechanisms at R.T. were abrasive wear and delamination; adhesive wear appeared slowly at elevated temperatures. The coating exhibited better wear resistance than the substrate during the experiments. Highlights • How the spraying parameters effect the fretting wear behavior of high velocity oxygen fuel (HVOF) coating was concerned. • The effect of temperature on the friction and wear properties of coatings has been studied. The friction coefficient decreases with increasing temperature, but the materials loss had the similar rule • The wear mechanism was abrasive wear and delamination in RT, and adhesive wear appeared slowly at elevated temperature. [ABSTRACT FROM AUTHOR]

• The effects of fretting parameter, environmental conditions, irradiation, fretting wear mode, and spacer grid on GTRF are reviewed, and the key influence factor of wear rate is discussed. • The accident tolerant fuel coating and its tribological behavior are investigated. • The highlighted points and research directions of GTRF are prospected. Fuel rods are the most important safety barrier in pressurized water reactors (PWR) and operates at high temperature, high pressure, high speed, and strong irradiation water condition. The grid-to-rod fretting (GTRF) induced by flow-induced vibration (FIV) is the main factor that causes failures in fuel rods. The effects of fretting parameter, environmental conditions, irradiation, fretting wear mode, and spacer grid on the GTRF are systematically reviewed, and the key influencing factor on GTRF is analyzed. The accident tolerant fuel (ATF) coating and its tribological behavior are investigated. Finally, key research guidance and popular topics consideration GTRF are proposed. [ABSTRACT FROM AUTHOR]