扭转复合微动的数值模拟

扭转复合微动是扭动和转动模式微动的复合.以实验研究为基础,在有限元软件ABAQus中建立三维球/平面接触弹塑性有限元模型,通过改变球体旋转轴与平面试样法向的倾斜角度α,实现球/平面接触的扭转复合微动.恒定法向载荷作用下,通过对角位移θ分量做量纲-化处理,建立倾斜角α与完全滑移角位移幅值θ0的函数关系,分析倾斜角对接触状态、接触压力、摩擦剪应力及滑移量的影响规律.结果表明:角位移θ0随倾斜角的增大而减小,并符合量纲-扭动转动分量平方和为一的规律;倾斜角α显著影响接触面力学参数的分布,与纯扭动和纯转动相比,损伤呈现明显的非对称性.     

LZ50车轴钢转动微动摩擦学特性研究

在新型转动微动磨损试验机上,进行LZ50车轴钢/GCr15钢在法向载荷为10N、转动角位移幅值为0.125°～0.5°的转动微动磨损试验。在摩擦动力学行为分析的基础上,结合磨痕的微观分析,研究材料的转动微动磨损机理。结果表明,LZ50车轴钢的微动运行区域仅呈现部分滑移区和滑移区,未观察到混合区。滑移区的摩擦因数明显高于部分滑移区;摩擦因数随着转动角位移幅值的增加而增大。车轴钢在部分滑移区损伤轻微,磨痕呈环状;而在滑移区,接触中心呈现材料塑性流动累积造成"隆起"特征,LZ50钢的转动微动磨损机制主要为磨粒磨损、剥层和氧化磨损。     

粘结MoS2固体润滑涂层的转动微动磨损特性

采用粘结法在LZ50钢表面制备MoS2固体润滑涂层,研究MoS2涂层及LZ50钢基体在干态不同角位移幅值下的转动微动磨损行为。在分析转动微动动力学特性的同时,结合光学显微镜、扫描电子显微镜、电子能谱仪以及轮廓仪对磨痕形貌进行微观分析。结果表明:涂层和基体的转动微动运行区域仅呈现部分滑移区(Partial slip regime,PSR)和滑移区(Slip regime,SR),未观察到混合区。涂层改变基体的微动运行区域,使得PSR缩小,SR运行区域向小角位移幅值方向移动。由于MoS2涂层的固体润滑作用,涂层的摩擦因数在整个试验过程都明显低于基体。在PSR,涂层损伤轻微;在SR,涂层的转动微动磨损机制主要表现为剥层和摩擦氧化。研究表明粘结MoS2固体润滑涂层具有明显的防护作用,显著降低LZ50钢的转动微动磨损。     

桨-毂轴承材料扭动微动磨损行为研究

采用面接触扭动微动形式,以动力定位系统可调距螺旋桨桨-毂轴承摩擦副材料(CuNiAl-42CrMo4)为对象,以不同的角位移幅值模拟海水波动影响下的微动磨损行为,并结合扫描电子显微镜和超景深三维显微镜对磨痕形貌进行分析,探究桨-毂轴承摩擦副材料扭动微动磨损规律。结果表明,随着角位移幅值的增加,扭动微动依次运行于部分滑移区、混合区、滑移区,摩擦因数减小,同时磨损量增加,微动损伤中剥层机制所占的比例逐渐增加,且由于疲劳裂纹扩展的不利影响,实际运行过程中要尽量避开混合区。     

有机玻璃扭动微动磨损的实时观测与摩擦振动分析

在新研制的扭动微动磨损实时观测系统上,以法向载荷为100 N和扭动角位移幅值为0.5°～15.0°的参数,对有机玻璃(Polymethylmethacrylate,PMMA)/GCr15摩擦副进行扭动微动磨损试验,同时录制扭动微动过程并同步采集摩擦振动信号。在摩擦动力学行为分析的基础上,结合实时观测和摩擦振动分析,对PMMA的扭动微动磨损行为进行研究。结果表明,随着角位移幅值增加,PMMA扭动微动呈现三个区域(即部分滑移区、混合区和滑移区);在部分滑移状态(θ=0.5°)下,损伤轻微,振动数据与空载时振动数据相似,相对运动主要靠弹性变形协调;在完全滑移状态(θ=15°)下,整个接触区发生严重损伤,大量磨屑被排出接触区,接触区最外侧呈现纺锤状银纹,每循环次数下振动信号的事件发生数先增大后减小,而振动振幅呈现相反的趋势,主要损伤机制为弹塑性变形、剥层和氧化磨损。     

MoS_2涂层扭动微动磨损特性有限元分析

为探讨MoS2固体润滑涂层在抗扭动微动磨损中应用的可行性,对MoS2涂层在扭动微动下摩擦力学性能进行有限元分析,研究扭转角位移幅值、法向载荷、摩擦因数等对MoS2涂层接触表面力学行为的影响,并与基体材料扭动微动力学行为进行比较。分析结果显示:MoS2涂层扭动微动运行区域的改变消除了混合区裂纹萌生与扩展所产生的损伤;MoS2涂层在部分滑移区和滑移区,其表面塑性应变虽比基体大,但明显较小的摩擦剪应力,在部分滑移区不足以启动MoS2涂层晶体的滑移,在滑移区对塑性流动层的剪切作用不够,因此MoS2涂层表面磨损较轻微,即MoS2能有效发挥抗扭动微动磨损作用。     

离子渗氮／ 渗硫层转动微动摩擦学行为研究

利用渗氮/渗硫复合处理在LZ50钢表面制备离子渗氮/渗硫层,在干态及不同角位移幅值下对渗层及其基体材料进行转动微动磨损试验,利用扫描电子显微镜、能谱仪和2D/3D轮廓仪对磨痕进行微观分析。试验结果表明：渗氮/渗硫层改变了基体材料的微动运行工况图,部分滑移区和滑移区边界向部分滑移区移动,滑移区运行范围增大;在部分滑移区,渗层的摩擦因数明显低于基体材料,其损伤十分轻微;在滑移区,次表层剥落的硬质颗粒使得稳定阶段摩擦因数高于基体材料,磨损机制为磨粒磨损、氧化磨损和剥层。     

60Si2Mn钢复合微动磨损行为的研究

采用球／平面接触,研究了60Si2Mn钢在两种倾斜角(45°和60°)、三种载荷(Fmax=200 N、400 N、800 N)和恒定加载速度(6 mm／min)下的复合微动磨损行为。在详细分析该微动接触条件下的动力学特性的基础上,结合光学显微镜和激光共焦扫描显微镜对磨痕的分析,探讨了不同阶段的微动过程和机制。根据F-D曲线的三种形状,60Si2Mn钢的复合微动运行过程划可划分为三个阶段。第1阶段的磨损以滑移为主,第Ⅱ、Ⅲ阶段的颗粒剥落均按剥层机制进行,但损伤程度明显不同。     

690合金材料的微动磨损特性研究

利用微动磨损试验机,在载荷50N以及位移幅值为60μm、100μm、150μm的工况下,研究了690合金材料在常温下的微动磨损行为及其动力学特性,采用激光共焦扫描显微镜(LCSM)和扫描电子显微镜(SEM)观察磨痕微观形貌。结果表明,载荷和位移幅值对微动特征有很大的影响,微动运行完全处于滑移状态。在滑移区,滑移磨损严重、磨痕面积大。690合金材料的磨损机制主要表现为磨粒磨损与剥层的共同作用。     

扭动微动摩擦力数值分析

为揭示扭动微动接触表面摩擦力分布规律及其对表面磨损的影响,进一步丰富和完善扭动微动损伤机理,基于试验中扭动微动摩擦扭矩的变化,利用ABAQUS用户子程序Fric引入动态变化的摩擦因数,对扭动微动表面摩擦力学行为开展数值模拟分析。摩擦扭矩数值模拟和试验结果的对比分析验证了数值模拟方法的正确性。接触表面摩擦力分布及与试验磨损形貌的对比分析表明:摩擦应力的剪切作用对表面材料损失起着至关重要的作用,不同滑移区摩擦应力分布不同,故呈现的磨痕形貌不同。     

On the mechanisms of various fretting wear modes

According to relative motion directions for a ball-on-flat contact, there are four fundamental fretting wear modes, e.g., tangential, radial, torsional and rotational modes. In this paper, the mechanisms of these four fundamental fretting wear modes, particularly for the later three modes, have been reviewed from results obtained by the authors in combination with results from literature. Some general features have been reported. Differences both in running and degradation behavior have been discussed in detail. Results showed that some similar laws for three fretting regimes (partial slip regime, mixed regime and slip regime), fretting maps (running condition fretting map and material response fretting map), wear and cracking mechanisms obtained from the classic mode (i.e. tangential fretting) were also identified and useful to characterize the other modes. Nevertheless, the occurrence of relative slip for the radial fretting, the formation of mixed regime for the torsional fretting, the evolution of surface morphology for the rotational fretting were quite different compared to that of the classical fretting mode.     

An experimental study on radial fretting behaviour

A new test apparatus has been developed for radial fretting test. Main experimental conditions are as follows: the amplitude of normal load from 200 to 800N, the number of cycles from 1 to 3×10⁵ cycles. Three contact pairs (a 52100 ball against 52100, 1045, 1045 steel with TiN coating) were used for the test. Variations of normal load vs indentation depth between two contact surfaces have been analyzed as a function of cycles. Contact degradation was examined through metallographic expertise on the flat specimen and radial fretting behaviour was compared in the paper.     

Radial fretting behaviours of dental ceramics

Radial fretting tests on a Si₃N₄ ceramics ball opposite to the two dental ceramics flats (Vita VMK95 and Cerec Vita Mark II) have been carried out. The test apparatus was developed from a tension-compression hydraulic machine. Maximum normal load (F_max) was varied from 100 to 800 N with a speed of 6 mm/min, and the number of cycles from 1 to 10⁵. The fretting scars were examined by optical microscope and laser confocal scanning microscope (LCSM). The results of kinetics behaviours showed that all loading and unloading curves of load (F)–displacement (D) curves were almost superposed in the whole fretting process for two dental ceramics under a lower normal load (), and all F–D curves opened, correspondingly some micro-cracks initiated and developed in contact area, when the normal load increased to a higher level (). Under lower normal load, the fretting scars displayed the worn zones in shape of annulus. The cracks in shape of homocentric circularity can be observed for Vita VMK95 at and for Cerec Vita Mark II at . Therefore, there is better radial fretting damage resistance for Cerec Vita Mark II. In the microslip zone, the microscopic analysis showed that the particles detached by the mechanism of delamination. It the meantime, the competing mechanisms of fatigue cracks and wear also were discussed in this paper.     

Dual-motion fretting behavior of mandibular cortical bone against pure titanium

Dual-motion fretting tests of flat cortical bone specimens from fresh human mandible against pure titanium (TA2) ball were carried out on a modified test rig with tilt angle of 45°. The imposed maximal loads varied from 100 to 200 N. Dynamic characteristics of dual-motion fretting tests were analyzed in combination with micro-examinations via optical microscopy (OM), laser confocal scanning microscopy (LCSM) and scanning electron microscopy (SEM) together with energy dispersive X-ray spectrum (EDX). Two types of F-D curves (the trapezoid and elliptic mode) were recorded during the tests. The examination showed that the wear scars of the dual-motion fretting were asymmetric, and the tangential component of dual-motion fretting was in the mixed fretting regime. Under the lower imposed load, only some detachment of particles and scratches without cracking were observed even after 5×10⁴ cycles. The main wear mechanisms of the dual-motion fretting damage were the abrasive and adhesive wear. Under higher imposed loads, the cracks initiated and propagated mainly at the high stress side of contact edges. The wear mechanisms of the dual-motion fretting of cortical bone under higher imposed loads were the combination of the adhesive wear, abrasive wear, cracking and lubrication of the human bone tissue debris. And the lubrication of the debris played an important role during the dual-motion fretting processes.     

An experimental study torsional fretting behaviors of LZ50 steel

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.     

Radial fretting fatigue damage of surface coatings

Radial fretting tests with a 52100 steel ball-on-flat contact have been carried out under different normal loads. TiN, MoS₂ and TiN+MoS₂ coatings on a 1045 steel flat were examined. The normal loads amplitude used were 200, 400 and 800 N at speeds of 12 and 1.2 mm/min. Dynamic analysis in combination with microscopic examinations by SEM and EDX have been performed. It was observed that the vertical stiffness increased with the increase of loading speed and number of cycles. The metallographic examinations showed that little damage was observed for the MoS₂ coating, which exhibited excellent radial fretting fatigue resistance. For the TiN coating, micro-cracks appeared at the lower load while delamination occurred at the higher load. For the TiN+MoS₂ composite coating, the vertical stiffness increased but accompanied by some micro-cracks. As a result of the study, the radial fretting test is proposed as one possible new method to evaluate coating life.     

Mechanical and experimental investigation on nuclear fuel fretting

A mechanical approach to the nuclear fuel fretting problem is studied in this paper to find a possible and efficient way of a wear restraint. Two different contours of the spacer grid spring and dimple were developed to increase the contact area. Fretting wear experiments were carried out for the developed springs and tube specimens. Contact forces of 10 and 30 N, and slip displacements of 50–100 μm were applied under the environment of air as well as water at room temperature. Wear scars on the rods were examined to observe the effect of the mechanical approach on the wear. Especially, the influence of a contour deviation which occurred during fabrication and the wear particle accumulation in the clearance region were investigated in detail. It was found that the contact shape influenced the feature and the behavior of the length, width and volumetric shape of the wear. For the model of fuel fretting wear, equivalent depth (D_e) is suggested as a new parameter that can represent the wear severity.