含炭纤维湿式铜基摩擦材料的性能

采用粉末冶金方法制备含短炭纤维的湿式铜基摩擦材料,研究炭纤维含量对湿式摩擦材料的摩擦磨损性能和力学性能的影响,以及制动条件对动摩擦因数的影响。结果表明:随着炭纤维含量及材料的孔隙率增加、硬度及密度均降低,摩擦因数呈先增加后减小的变化趋势,磨损量呈先减小后增大的趋势。炭纤维含量为(质量分数)1%时材料的摩擦磨损性能最好,摩擦因数最大且最稳定,磨损量最小。材料摩擦因数随着载荷增大而增大,随炭纤维含量增加磨损率呈先减小后增大的趋势。炭纤维的加入提高了材料的能量许用值。     

钢纤维含量对汽车摩擦材料性能的影响

通过调整摩擦材料基础配方中钢纤维的含量,采用直接混合工艺制备不同组分的摩擦材料,对其进行理化性能、力学性能、摩擦性能及制动噪音测试。结果表明:随钢纤维含量增加,摩擦材料的密度、p H值和洛氏硬度均增大,常温压缩量减小;气孔率先减小,当钢纤维含量超过21%后,气孔率增大;名义摩擦因数(μnom)先略微降低后升高,最低摩擦因数(μmin)、磨损量和噪音发生率均增加,但当钢纤维含量超过21%后,磨损量和噪音发生率明显增加。     

铜锡合金添加量对车辆制动用摩擦材料性能的影响

对车辆制动用铜锡合金摩擦材料进行力学特性、摩擦性能及摩擦表面表征,分析了不同铜锡合金添加量对车辆制动用摩擦材料性能的影响。结果表明:随着铜锡合金添加量的增加,摩擦材料的密度逐渐增加,pH值基本保持不变;硬度先提高后降低,压缩应力先降低后提高,剪切强度变化不大。随着制动压力的增加,所有样品的摩擦因数呈降低趋势。添加15%(质量分数)铜锡合金的样品表面形成了比较完整的摩擦膜,但摩擦过程中出现了一定程度的剥落。     

列车制动用Cu基粉末闸片材料摩擦磨损性能分析

列车的制动性能与闸片材料的摩擦磨损性能关系密切,在MM-1000Ⅱ型摩擦试验机上测试了自制的Cu基粉末列车闸片材料在不同制动速度下的摩擦磨损特性。结果表明:随着制动速度的增大,摩擦表面的微凸起遭到破坏,摩擦因数随之降低,磨损量增加;在材料接触表面产生大量的摩擦热,造成基体软化,减小了基体对材料中SiO_2等硬质颗粒的夹持能力。摩擦因数和稳定系数均随制动速度增加而降低;而摩擦温度和磨损量随制动速度增加而提高,尤其是在制动速度大于8 r/s时,摩擦表面温度上升,造成基体软化,硬质颗粒脱落,加速了材料的摩擦磨损。为列车制动用Cu基粉末闸片材料摩擦磨损性能的研究提供了理论基础。     

钨含量对铜基摩擦材料性能的影响

为了探究钨含量对制动用铜基摩擦材料性能的影响,采用热压烧结工艺制备了不同钨含量的铜基摩擦材料,对其物理性能、力学性能和摩擦磨损性能进行了测试.研究表明,铜基摩擦材料的密度随着钨含量的增加而增大,而剪切强度和硬度先增大后减小.钨提高了材料磨损表面微凸体接触的结点强度,从而提高了材料的摩擦因数.适量的钨可以有效地减少磨损表...     

钢纤维对摩擦材料性能的影响

树脂基复合材料性能优异,被大量用做汽车制动材料.以酚醛树脂为基体,钢纤维为增强纤维,添加填料,采用热压法制成刹车片,并进行摩擦试验.通过试验研究了酚醛树脂基复合材料中钢纤维不同添加量(质量分数)对复合材料冲击强度、摩擦性能的影响.结果表明:该配方类型中,在100～300℃温度范围内,材料冲击强度先增大后减小,钢纤维质量分数为24%时为转折点;材料的摩擦系数先增大后减小,钢纤维质量分数为26%时为转折点;磨损率随钢纤维添加量的增加而增大,钢纤维最佳添加量为24%～26%.     

干湿态下制动载荷对粉末冶金闸片材料摩擦学性能的影响北大核心

采用粉末冶金技术制备了一种高速列车用铜基闸片材料,研究了干湿环境下制动载荷对铜基闸片材料摩擦学性能的影响,探究了闸片材料及其配对材料(铸钢材料)的磨损规律。结果表明:随着制动载荷的增加,干湿态下摩擦稳定系数均先减小后增加,平均摩擦因数不断降低,闸片材料的磨损率先快速增加后小幅减小,铸钢材料的磨损率先快速增加后缓慢增加。湿态环境可减轻低制动载荷下闸片材料的剥落程度及高制动载荷下材料的犁削,降低了摩擦因数及材料的磨损率。     

WC–Co硬质合金摩擦磨损行为的分子动力学模拟

利用分子动力学模拟研究了WC–Co硬质合金在不同条件下的摩擦过程,分析了晶粒尺寸、摩擦载荷和滑动速率等因素对硬质合金摩擦磨损行为的影响,从原子尺度揭示了硬质合金发生摩擦磨损的微观机制。结果表明,随晶粒尺寸增大,相比于晶粒转动,Co相和WC中的位错滑移逐渐在摩擦引起的塑性变形机制中起主导作用。摩擦载荷增大会导致易变形的Co粘结相被挤出表面而首先去除,通过减小晶粒尺寸可以抑制Co相的挤出–磨损机制,进而提高硬质合金的抗滑动磨损性能。滑动速率升高会降低磨损速率,主要原因是在高速滑动过程中,亚表层各相中位错的形核扩展缺乏持续的驱动应力,位错密度较低,WC不易发生断裂,Co相被挤出表面造成的磨损程度明显减轻。     

制动速度对铜基粉末冶金闸片材料摩擦学性能的影响

以Sn+SiO_2+Al_2O_3+CaF_2作为摩擦组元,石墨+Pb作为润滑组元,制备铜基粉末冶金列车闸片材料,在MM-1000Ⅱ型摩擦试验机上进行摩擦试验,测定制动速度在120~200 km/h范围内材料的摩擦因数、磨损量与表面温度,并观察摩擦表面形貌,研究制动速度对该材料摩擦学性能的影响。结果表明:在制动速度180 km/h时,随制动速度增加,闸片材料的摩擦因数在0.41~0.46之间波动,但制动速度达到200 km/h时,摩擦因数显著减小至0.32,摩擦因数稳定性总体较好。材料的磨损量随制动速度增大而增加,但在制动速度达到180 km/h时磨损量趋于稳定,为119 mg。低速制动下材料的磨损机理主要为疲劳磨损,高速制动时主要为磨粒磨损和氧化磨损。     

混料时间对含石墨的铜基摩擦材料性能的影响

本文采用粉末冶金方法制备了含石墨的铜基粉末冶金摩擦材料,通过调整加入石墨后的混料时间,采用布氏硬度计、摩擦磨损试验机和扫描电镜等设备研究了混料时间对含石墨的铜基摩擦材料性能的影响。研究结果表明:随着加入石墨后混料时间的增加,烧结后摩擦材料中的石墨颗粒分布更加均匀,铜基体的连续性变差,材料密度和硬度均下降,材料的摩擦因数先增大后减小,磨损量先减小后增大。加入石墨后,混料8 h的烧结体的摩擦磨损性能最佳(较其他试样摩擦因数增加了10%左右,磨损量降低了30%左右)。     

Molecular Dynamics Simulations of Dislocation Activity in Single-Crystal and Nanocrystalline Copper Doped with Antimony

Recent experimental and simulation results have indicated that high-temperature grain growth in nanocrystalline (NC) materials can be suppressed by introducing dopant atoms at the grain boundaries. However, the influence of grain boundary dopants on the mechanical behavior of stabilized NC materials is less clear. In this work, molecular dynamics (MD) simulations are used to study the impact of very low dopant concentrations (<1.0 at. pct Sb) on plastic deformation in single-crystal and NC Cu. A new interatomic potential for low Sb concentration Cu-Sb solid-solution alloys is used to model dopant/host and dopant/dopant interatomic interactions within the MD framework. In single-crystal models, the strained regions around the Sb atoms act as heterogeneous sources for partial dislocation nucleation; the stress associated with this process decreases with increasing Sb concentration. In NC models, MD simulations indicate that Sb dopants randomly dispersed at the grain boundaries cause an increase in the flow stress in NC Cu, implying that Sb atoms at the grain boundaries retard both grain boundary sliding and dislocation nucleation from grain boundary regions.     

Recrystallization Phenomena During Friction Stir Processing of Hypereutectic Aluminum-Silicon Alloy

Microstructural evolution and related dynamic recrystallization phenomena were investigated in overlapping multipass friction stir processing (FSP) of hypereutectic Al-30 pct Si alloy. FSP resulted in the elimination of porosities along with the refinement of primary silicon particles and alpha aluminum grains. These alpha aluminum grains predominantly exhibit high angle boundaries with various degrees of recovered substructure and dislocation densities. The substructure and grain formation during FSP take place primarily by annihilation and reorganization of dislocations in the grain interior and at low angle grain boundary. During multipass overlap FSP, small second phase particles were observed to form, which are accountable for pinning the grain boundaries and thus restricting their growth. During the multipass overlap FSP, the microstructure undergoes continuous dynamic recrystallization by formation of the subgrain boundary and subgrain growth to the grain structure comprising of mostly high angle grain boundaries.     

制动摩擦材料研究进展

制动摩擦材料利用运动表面相接触时所产生的摩擦阻力达到减速或终止运动目的,是运载机械中安全保障装置的重要组成部分。本文综述了半金属基、金属基及非金属基制动摩擦材料的研究现状及优缺点,介绍了熔铸法、粉末冶金法及三维编织法等制动摩擦材料制备方法,并从摩擦、磨损、热稳定性等方面分析了制动摩擦材料的关键特性。从研究状况可知,摩擦材料正向少纤维、无纤维型方向发展,高性能、环保型摩擦材料具有较大的发展优势。优化制备工艺、降低生产成本、提高性能、扩大应用领域将是未来制动摩擦材料的研究重点。     

Additive strengthening mechanisms in dispersion hardened polycrystals

Tensile data from polycrystalline samples of copper dispersion strengthened by alumina are analysed. The basis of this analysis is to look at the strain range from 0.05 to 0.20 where the stress-strain curves show a parabolic hardening behaviour and are parallel to one another. The means by which the addition of strength components from various elements of the microstructure and substructure might explain this behaviour are investigated. It is shown that a linear combination of a matrix friction stress, an Orowan bowing stress, a matrix mean stress from the particles and a combined dislocation interaction term can explain this data and also the data from some aluminium-alumina materials. The dislocation interaction term, which dominates, is comprised of terms which cover the pure matrix work hardening, the hardening due to particles and due to the grain boundaries. This term is derived by summing the dislocation density contributions from each of these three sources. The type of additivity suggested here not only gives very good agreement with the stress-strain data but it also uses and is in accord with the experimental measurements of dislocation densities made using transmission electron microscopy.     

超低碳贝氏体钢快速加热奥氏体长大过程中移动晶界上硼的反常偏聚

利用径迹显微照相技术研究了超低碳贝氏体钢焊接热影响区在焊接热循环快速加热过程中硼在奥氏体晶界上的偏聚行为。发现以高密度位贝氏体为原始组织的材料进行快速加热时，新形成的奥氏体晶粒边界上在很高温度下仍会出现反常的晶界硼偏聚。用晶界位错驰原制对这种新的非平衡现象进行了讨论。     

Microstructural Features Leading to Enhanced Resistance to Grain Boundary Creep Cracking in ALLVAC 718Plus

This study focuses on the microstructural features that enhance the resistance of ALLVAC 718Plus to grain boundary creep cracking during testing of samples at 704 °C in both dry and moist air. Fully recrystallized structures were found to be susceptible to brittle grain boundary cracking in both environments. Detailed transmission electron microscopy (TEM) microstructural characterization reveals features that are believed to lead to resistance to grain boundary cracking in the resistant microstructures. It is suggested that dislocation substructures found within the grains of resistant structures compete with the high-angle grain boundaries for oxygen, thereby reducing the concentration of oxygen on the grain boundaries and subsequent embrittlement. In addition, electron backscatter diffraction (EBSD) misorientation maps reveal that special boundaries (i.e., Σ3 boundaries) resist cracking. This is in agreement with previous findings on the superalloy INCONEL 718. Furthermore, it is observed that cracks propagate along high-angle boundaries. This study also shows that in this case, the presence of delta phase at the grain boundaries does not by itself produce materials that are resistant to grain boundary cracking.     

铁粉粒度对粉末冶金材料制动摩擦性能的影响

采用粉末冶金工艺制备含4种粒度(20μm、30μm、50μm、70μm)铁粉增强的铜基摩擦材料,研究铁粉粒度对材料力学性能和制动摩擦性能的影响。采用TM-1型惯性试验台测试材料的制动摩擦性能,试验初速度为50~380 km/h。结果表明:铁粉粒度从20μm增加到70μm时,材料硬度从55.67 HRB降低到31.83HRB,剪切强度从12.56 MPa下降到10.27 MPa。这种硬度和强度的下降使大粒度样品表现出反常的摩擦特性:随着制动速度的提高,铁粉粒度为70μm的F70样品的摩擦因数不降低反而升高,当制动速度从120 km/h上升到380 km/h时,摩擦因数从0.338持续升高到0.356,并且从350 km/h后摩擦因数稳定不变。这种高而稳定的摩擦因数是保证列车在高速下紧急制动、平稳停驶所必需的。     

强变形过程中铁镍合金的微观结构演化机制

 采用透射电镜观察了铁镍(Fe 32%Ni)合金在形变温度500 ℃(＜05Tm)、形变速率10-2 s-1的变形条件下多轴锻造变形过程中的微观结构演变。结果表明,低温多轴锻造强变形可明显细化晶粒,细化过程为：首先,位错墙、位错缠绕等结构通过大量位错滑移运动在原始晶粒内形成;其次,不同方向的变形导致不同方向的滑移系开动,从而致使不同方向的位错墙互相交叉,将原始粗晶粒细分成小尺寸的胞块结构,当变形量达到一定程度时,位错墙和位错缠绕结构内的位错开始重新排列,形成小角度晶界,导致亚晶粒形成;由于变形量不断增加强迫大量的位错在亚晶界处积聚、重排,同时不同方向的变形造成亚晶发生转动,位错重新规则排列及亚晶转动使小角度的亚晶界转变为大角度晶界,从而形成细小的新晶粒。     

Misoriented dislocation substructures and the fracture of polycrystalline Cu–Al alloys

The evolution of the dislocation substructure in polycrystalline Cu–Al alloys with various grain sizes is studied during deformation to failure. A relation between the fracture of the alloys and the forming misorientation dislocation substructures is revealed. Microcracks in the alloy are found to form along grain boundaries and the boundaries of misoriented dislocation cells and microtwins.     

粉末冶金摩擦材料的应用现状及对原材料的要求

粉末冶金技术在中国的应用获得快速发展。系统介绍了粉末冶金摩擦材料在飞机刹车、坦克制动、离合器片、风电制动器主轴、高速列车制动闸片等领域的国内外应用现状,并对粉末冶金摩擦材料的原材料提出了具体要求,以期对粉末冶金技术的进一步发展提供参考。