Fretting Wear Characteristics of Nuclear Fuel Cladding in High-Temperature Pressurized Water

In pressurized water reactor(PWR),fretting wear is one of the main causes of fuel assembly failure.Moreover,the operation condition of cladding is complex and harsh.A unique fretting damage test equipment was developed and tested to simulate the fretting damage evolution process of cladding in the PWR environment.It can simulate the fretting wear experiment of PWR under different temperatures(maximum temperature is 350℃),displacement amplitude,vibration frequency,and normal force.The fretting wear behavior of Zr-4 alloy under different temperature environments was tested.In addition,the evolution of wear scar morphology,profile,and wear volume was studied using an optical microscope(OM),scanning electron microscopy(SEM),and a 3D white light interferometer.Results show that higher water temperature evidently decreased the cladding wear volume,the wear mechanism of Zr-4 cladding changed from abrasive wear to adhesive wear and the formation of an oxide layer on the wear scar reduced the wear volume and maximum wear depth.

Wear Characteristics of the Nuclear Control Rod Drive Mechanism(CRDM)Movable Latch Serviced in High Temperature Water

The current research of nuclear control rod drive mechanism(CRDM)movable latch only makes a simple measurement of wear mass.The wear volume and difference in various claw surfaces are ignored and the degradation mechanism of each claw surface is not clear.In this paper,a detailed degradation analysis was carried out on each claw surface of movable latch combined with wear result and worn morphology.Results indicate that the boundary of carbide is preferred for corrosion because carbide presents a nobler Volta potential compared to the metal matrix or boundary region.Due to the oscillation of drive shaft between the claw surfaces of movable latch,the dominant wear mechanism on the upper surface of claw(USC)and lower surface of claw(LSC)is plastic deformation caused by impact wear.Mechanical impact wear will cause the fragmentation of carbides because of the high hardness and low ductility of carbides.Corrosion promotes the broken carbides to fall off from the metal matrix.The generated fine carbides(abrasive particles)cause extra abrasive wear on USC when the movable brings the drive shaft upward or downward.As a result,USC has a higher wear volume than LSC.This research proposes a method to evaluate the wear on the whole movable latches using a 3D full-size scanner.

Anti-loosening performance of coatings on fasteners subjected to dynamic shear load

This paper investigates the self-loosening of threaded fasteners subjected to dynamic shear load. Three kinds of typical coatings, PTFE, MoS_2, and TiN applied to bolts and nuts, are tested in this investigation. The study experimentally examines the loosening mechanisms of fasteners and assesses the anti-loosening performance of the three tested coatings based on their tightening characteristics, loosening curves, and the damage of thread surface. Additionally, the anti-loosening performance of the three coatings is compared under different load forms. The results indicate that the PTFE and MoS_2 coatings have significant anti-loosening effect, whereas the anti-loosening performance of Ti N coating is not satisfactory. It is also found that an appropriate increase of the initial tightening torque can significantly improve the anti-loosening effect. In addition, the microscopic analyses of PTFE and MoS2 coating reveal that a reduced initial tightening torque leads to fretting wear on the thread contact surfaces of fasteners, thereby aggravating the damage.

Investigations on femtosecond laser-induced surface modification and periodic micropatterning with anti-friction properties on Ti6Al4V titanium alloy

Titanium alloys have a wide application in aerospace industries as it has greater strength and low density, but it has poor tribological properties. To improve its friction and wear performance, in present work, a femtosecond laser is used to directly irradiate the Ti6Al4V titanium alloy surface in air conditioning, which results in localized ablation and the formation of periodic microstructures but also a strong pressure wave, propagating the material inside. Through the optimization of processing parameters, surface modification and periodic micropatterning with effective anti-friction properties were successfully induced on the surface. After a treatment of femtosecond laser-induced surface modification(FsLSM), the surface microhardness was improved by 16.6% and compressive residual stress reached-746 MPa. Besides, laser-induced periodic surface structures(LIPSS) with a titanium oxide outer coating were fabricated uniformly on the titanium alloy surface. Rotary ball-on-disk wear experiments revealed that the average coefficient of friction(COF) and wear mass loss of the specimen with Fs LSM treatment were largely reduced by 68.9% and 90% as compared to that of untreated specimens, respectively. It was analyzed that the reason for the remarkable wear resistance was attributed to the comprehensive action of the generation of LIPSS, the titanium oxide outer coating, high amplitude compressive residual stress and gradient grain size distribution on the subsurface during the laser surface treatment. Since the findings here are broadly applicable to a wide spectrum of engineering metals and alloys, the present results offer unique pathways to enhancing the tribological performance of materials.

Development of a novel cycling impact–sliding wear rig to investigate the complex friction motion

In many industrial devices, impact-sliding wear is caused by a variety of complex vibrations between the contacted interfaces. Under actual conditions, impact and sliding motions do not occur in only one direction, and different complex impact-sliding motions exist on the tribology surfaces. In this study, an impact-sliding wear test rig is developed to investigate the wear effect of different complex motions. Using this rig, multi-type impact-sliding wear effects are realized and measured, such as those derived from unidirectional, reciprocating, and multi-mode combination motions. These three types of impact–sliding wear running behavior are tested and the wear damage mechanism is discussed.

Optimization of several surface treatment processes for alleviating fretting damage of a locking pin

The operational safety and reliability of a variable gauge train are affected by the anti-fretting wear performance of the locking mechanism.The main purpose of this study is to optimize the surface treatment process for a locking pin material under actual service conditions to alleviate fretting damage.Based on the two basic principles of surface strengthening and friction reduction,a substrate(AISI 4135 steel)surface was treated by laser quenching(LQ),plasma nitriding(PN),and bonded MoS2 coating.Systematic fretting wear tests were conducted,and the wear behavior and damage mechanism of various treated surfaces were comprehensively investigated.The results indicate that the wear resistances of the LQ-and PN-treated surfaces were significantly improved,and their main wear mechanisms were abrasive wear,delamination,and oxidation wear.The MoS_(2) coating exhibits the lowest friction coefficient and energy dissipation due to its self-lubricating property,but it incurs the highest wear rate and failure in the form of plastic deformation.Furthermore,the rough compound layer with a high hardness on the PN-treated surface is conducive to the formation and maintenance of the third-body contact at the fretting interface,consequently resulting in a significant reduction in wear.An optimal surface treatment process for alleviating fretting damage of the locking pin is recommended via comprehensive evaluation,which provides a reference for the anti-fretting protection of related mechanical components.

Impact-sliding wear response of 2.25Cr1Mo steel tubes:Experimental and semi-analytical method

The impact-sliding wear behavior of steam generator tubes in nuclear power plants is complex owing to the dynamic nature of the mechanical response and self-induced tribological changes.In this study,the effects of impact and sliding velocity on the impact-sliding wear behavior of a 2.25Cr1Mo steel tube are investigated experimentally and numerically.In the experimental study,a wear test rig that can measure changes in the impact and friction forces as well as the compressive displacement over different wear cycles,both in real time,is designed.A semi-analytical model based on the Archard wear law and Hertz contact theory is used to predict wear.The results indicate that the impact dynamic effect by the impact velocity is more significant than that of the sliding velocity,and that both velocities affect the friction force and wear degree.The experimental results for the wear depth evolution agree well with the corresponding simulation predictions.