航空刹车材料应用及发展

本文介绍了航空刹车材料的发展历程,综述了航空刹车使用的粉末冶金材料、C／C复合刹车材料、C／C—SiC复合材料等摩擦制动材料的特点性能以及它们在航空刹车中的应用发展状况。通过分析和比较它们的性能优缺点,指出C／C-SiC复合刹车材料具有较好的适应性,能够满足航空刹车构件的要求,将取代现在使用的粉末冶金刹车材料和C／C复合刹车材料,成为新一代航空刹车材料。     

粉煤灰增强树脂基摩擦材料磨屑形貌及成分分析

通过干法热压工艺制备了粉煤灰增强树脂基摩擦材料,利用D-MS型定速式摩擦试验机测其在不同温度下的摩擦磨损性能,并收集磨屑。借助扫描电子显微镜观察、分析样品摩擦后的表面和磨屑形貌,研究了摩擦材料磨损的内在机制,并借助X射线衍射仪、X射线能谱仪、红外光谱仪进行综合表征。结果表明,添加粉煤灰的树脂基摩擦材料摩擦表面形成连续、稳定的摩擦膜,能够有效改善树脂基摩擦材料的摩擦磨损性能。对磨屑的分析实验证明摩擦膜的产生与粉煤灰中SiO2和Al2O3有关。     

C/C复合材料的摩擦转变和磨损机理

总结了国内外各种解释C/C复合材料摩擦磨损转变的观点,C/C复合材料摩擦磨损转变归因于摩擦膜的变化、水和氧的解吸附、磨屑氧化和应力集中四大原因.归纳了不同刹车条件下C/C复合材料的磨损机理,并指出国内关于C/C复合材料摩擦磨损的研究主要集中在摩擦后磨屑和摩擦膜的"静态"观察上,提出了进一步研究应关注"动态"磨损过程.     

载流摩擦磨损中影响起弧的因素分析

以铜基粉末冶金/铬青铜为摩擦副,在销-盘式摩擦磨损试验机上进行了载流摩擦学特性研究。在多种试验条件下对载流摩擦磨损中影响起弧的因素进行了分析。结果表明,载荷主要是通过影响接触状态和摩擦副表面温度,进而影响材料的电弧性能。载荷有一个最佳值,在此载荷条件下燃弧率、电弧能量、摩擦系数、质量磨损率均最小;电流在两电极间产生电场是形成电弧放电的必要条件,电流主要影响了摩擦副的表面温度,从而使摩擦副表面状态变差。     

高合金铁基粉末冶金材料摩擦磨损性能的研究

为了获得高性能的耐磨材料,采用粉末冶金方法制备了高合金颗粒强化铁基材料.利用光学显微镜、扫描电镜、能谱分析仪和摩擦磨损实验研究了干摩擦状态下材料的摩擦磨损性能.结果表明:材料的磨损机理是由粘着磨损、磨料磨损和接触疲劳磨损共同作用的.本文研究的高合金铁基粉末冶金耐磨材料,在50 s内进入稳定磨损阶段,能在很短的时间里达到最佳使用效果.干摩擦120 min后,材料的比磨损量小,仅为2.096×10-15 kg/(m.N),摩擦系数变化不大,在0.05～0.06保持相对稳定,说明材料的耐磨性能较好.材料组织中M6C碳化物能保证材料很好的耐磨性能,而M2C碳化物则降低材料的耐磨性能.     

三维针刺C/SiC刹车材料的摩擦磨损性能

通过化学气相渗透法(CVI)结合反应熔体浸渗法(RMI)制备了三维针刺,C/SiC刹车材料 , 利用 MM21000型摩擦磨损试验机系统研究了C/SiC刹车材料的摩擦磨损性能,采用光学显微镜和扫描电子显微镜分别对摩擦表面和磨屑形貌进行了观察。结果表明:干态刹车条件下,当初始刹车速度相同时,摩擦系数随着刹车压力的升高而逐渐降低;当刹车压力相同时,摩擦系数随着初始刹车速度的增加先升高后降低。湿态摩擦性能衰减小(衰减约8 %) 、恢复快;静态摩擦系数高(为0. 56～0. 61),摩擦系数随着初始刹车温度的升高而显著降低。当刹车压力相同时,磨损率随着初始刹车速度的增加而增大;当初始刹车速度大于20 m/s时,刹车压力的增大使磨损率显著增加。      

聚醚醚酮摩擦磨损行为和机理的研究

本工作通过摩擦过程中聚醚醚酮(PEEK)结构(包括材料与磨屑结构)的变化研究了PEEK的摩擦磨损机理。结果表明,低负载下,材料的摩擦表面不熔融,但表层的凝聚态结构发生了变化,摩擦状态不变,磨损是由于摩擦对偶的连续作用下产生疲劳而脱落。高负载下,材料表面熔融并发生氧化交联,摩擦状态改变,材料外形结构与内层凝集态结构发生变化,磨损是熔融了的表面层粘附于对偶上经进一步的交联反应后,受剪切力作用而脱落。材料在摩擦磨损过程中的表面状态决定了其摩擦磨损机理,并导致了材料摩擦磨损行为对负载的特殊依赖性。     

载流条件下铜基粉末冶金材料的摩擦磨损行为

载流条件下材料的摩擦学性能对于铁铬弓网系统、电极电刷以及航天航空运用都有重要意义.以铜基粉末冶金材料为对象,在专用的载流摩擦磨损试验机上,系统地研究了载流电流密度、滑动速度与接触压力对摩擦磨损特性的影响规律.结果表明:电流、接触压力与滑动速度对载流摩擦副的摩擦磨损性能存在着强烈的交互作用.电流介入产生的电弧损伤严重,恶化了摩擦副的摩擦磨损性能;一定的接触压力有利于获得比较稳定的载流摩擦表面接触,对于高速、大功率电力机车,一定的接触压力有利于保证良好的载流质量,减少电弧产生.     

60Si_2Mn钢的低周拉扭复合微动疲劳特性

测量60Si_2Mn钢在拉扭复合载荷作用下的低周微动疲劳特性,研究了不同轴向循环拉伸应力幅值对微动疲劳寿命、循环软化特性以及摩擦磨损表面和断口形貌的影响.结果表明,随着循环拉伸应力幅值的提高,60Si_2Mn钢的微动疲劳寿命降低幅度不同,发生循环软化的时期不断提前,完成循环软化的疲劳周期也不断缩短。同时,微动摩擦副产生的氧化物磨屑对微动磨损性能有重要影响,在疲劳前期加剧摩擦磨损,在疲劳后期减轻摩擦磨损。微动疲劳裂纹源形成于试样发生微动摩擦磨损的表面,并出现疲劳台阶。在扭矩产生的切向剪切应力作用下,疲劳裂纹沿着与轴向45°角的方向扩展,最终在断口上留下显著的舌状凸起,拉应力的幅值越大舌状凸起越明显。     

Sialon,Al2O3和SiC自配对干摩擦研究

本文研究了高温结构陶瓷Ｓｉａｌｏｎ、Ａｌ２Ｏ３、ＳｉＣ的自配对滑动干摩擦磨损性能，运用ＳＥＭ、ＴＥＭ、ＸＰＳ手段观察和分析了磨损面的表面形貌、磨屑形状及相组成，目的是揭示陶瓷材料在试验条件下的滑动干摩擦磨损规律和摩擦磨损机理，结果表明：Ｓｉａｌｏｎ$ 微剥离磨损，摩擦过程中产生的氧化产物能改善材料摩擦；Ａｌ２Ｏ的磨损机理是磨屑边界润滑下的晶粒拔出和沿晶断裂；ＳｉＣ表现为犁沟－磨屑－犁沟过程的磨损，并     

现代轨道交通刹车材料的发展与应用

分析了轨道交通车辆用盘形制动装置中闸片/制动盘组成的摩擦副的工作条件及其对材料的要求,介绍了铸铁闸片、树脂基闸片、铁基和铜基粉末冶金闸片的性能特点及适用领域,重点分析了粉末冶金闸片各组元的功能及摩擦磨损性能调控机制.基于制动盘热斑形成机理,阐明了闸片形状与排布对制动盘热源分布的影响规律.阐述了铸铁、铸钢、锻钢、金属基复合材料和C/C复合材料制动盘的研究进展,并指出了现代轨道交通刹车材料设计与制造的研究热点及研究方向.     

Influence of processing parameters on microstructure and mechanical response of a high-pressure die cast aluminum alloy

Motor sealing brackets were fabricated by pressurized die casting (PDC) process under 408 MPa pressure via semisolid thick slurry of Aluminum Alloy 6061 base metal and 20% powder-chip of similar composition (average 500 μm). Initially, 81 microns powder (40% by weight) and micro-size chips (60% by weight) were mixed by stirring near 300°C until the diffusion of powder takes place and then refined by ball milling process. Influence of thick semisolid slurry modified by powder-chip reinforcement was investigated according to unmelted chip distribution inside the formed cavities, reinforcement incorporation factor (RIF), density measurement, blister formation along the surfaces and microhardness. The obtained results reveal that the unmelted particles significantly improved the microhardness value and density as 134.85 HV and 2.71 g/cm³, respectively, also with the reduction in the size of cavities. Refined microstructures were observed at several locations of the components due to the effect of processing parameters.     

W2Cu面对等离子体梯度热沉材料的制备和性能

采用粉末冶金法制备了 W2Cu面对等离子体梯度热沉材料。对其显微组织、 界面以及重要的热学、 力学性能进行了研究。显微组织观察表明 : 截面成分呈梯度分布 , 并通过高温下元素的扩散 , 实现了组织的连续变化 , 层间没有明显界面 ; 烧结后 Cu形成了连续的网络结构 , 分布在 W颗粒周围。W2Cu梯度材料的化学元素分- 1布和热学、 力学性能沿厚度方向呈梯度变化 , 材料整体的热导率达 151. 4 W·(m·K) 。在 800 ℃ 温差条件下 ,对材料分别进行抗热震和耐热疲劳实验。热震实验后 , 界面处未发现裂纹和开裂现象 , 表现出良好的抗热震性能。经过 83次热循环冲击后 , 观察到了裂缝 , 并探讨了裂缝形成机制。     

Ball milling of stainless steel scrap chips to produce nanocrystalline powder

Nanocrystalline stainless steel powder was produced by ball milling of austenitic stainless steel scrap chips. The structural and morphological changes of samples during ball milling and after subsequent heat treatment were investigated by X-ray diffractometery, scanning electron microscopy and microhardness measurements. During ball milling the austenite in as-received chips partially transformed to the martensite phase with nanoscale size grains of ∼15 nm. This structure exhibited high microhardness value of about 850 Hv which is much higher than that for original samples. The deformation-induced martensite partially transformed to austenite after annealing at 700 °C for 1 h reducing the hardness of powder particles.     

钛酸钾含量对汽车摩擦材料性能的影响

钛酸钾是替代石棉用于摩擦材料中的一种新兴增强材料。以一种低金属陶瓷配方为基础,采用粉末冶金法制备钛酸钾增强摩擦材料,研究钛酸钾含量(质量分数,%)对摩擦材料的物理性能、力学性能、摩擦磨损性能及制动噪音的影响。结果表明,随钛酸钾含量增加,摩擦材料的气孔率增加,密度降低,pH值增加;洛氏硬度增加,压缩性及内剪切强度降低;摩擦系数稳定性增强,磨损量先降低后增加;噪音发生频次先降低后增加。钛酸钾含量为12%时,磨损量最低,噪音表现最佳。     

WC颗粒增强Fe基粉末冶金复合材料研究进展

介绍了WC颗粒增强Fe基粉末冶金复合材料,讨论了WC颗粒增强Fe基粉末冶金复合材料的WC含量、组织结构、制备工艺、WC颗粒形貌及强韧化机理,并指出研发一种短流程、低成本的高性能WC颗粒增强Fe基粉末冶金复合材料及其制备工艺是Fe基复合材料未来发展的趋势之一。     

SOME OBSERVATIONS ON SMALL FATIGUE CRACKS IN A SUPERALLOY

Abstract —An investigation into the applicability of linear elastic fracture mechanics to very small fatigue cracks growing in a powder metallurgy nickel base superalloy is described. An unusual specimen was designed to facilitate the study of these small cracks. The stress intensity factor for the specimen was estimated and then calculated from the plastic zone size as determined by interferometry. The crack tip deformation field was also observed in the SEM and by stereoscopic viewing. These observations showed that the macroscopic deformation field was the same for both the long and short cracks, and was controlled by the stress intensity. The fatigue crack tip was found to interact strongly with the material microstructure and the localgrowth rate cannot be correlated with fracture mechanics quantities.     

Study on the reaming process of aluminum alloy 7050-T7451 under different cooling conditions

Aluminum alloy 7050 is widely used in the aeronautical industries. However, owing to their highly ductile property, chips created during high-speed machining cannot be naturally broken, and long continuous chips are unavoidably formed, impacting the machining stability and quality of the parts. Because a smaller cutting allowance is required compared with conventional machining operations, the behavior of the chips during reaming operation may be more complex and different from those determined in previous investigations. Therefore, studying the characteristics of chip formation and hole quality during the reaming process is essential to improve the machinability of aluminum alloy 7050. In this study, three different cooling conditions were applied to reaming aluminum alloy 7050-T7451 with polycrystalline diamond (PCD) reamers. The finite element models (FEMs) were established to simulate the chip formation. The macro- and micro-morphologies of chips under the three cooling conditions were compared to analyze the chip behaviors. The diameter, surface roughness, and micro-morphologies of the reamed holes were also analyzed to evaluate the hole quality. The results showed that the chip morphology was strongly influenced by the cutting parameters and cooling strategies. It was found that the desired chip morphologies, satisfactory geometrical accuracy and surface quality during the reaming of aluminum alloy 7050-T7451 could be achieved using internal cooling at a spindle speed of 8 000 r/min and a feed rate of 0.01 mm/z. This study also demonstrates the feasibility of an internal cooling strategy for breaking chips when reaming aluminum alloy 7050-T7451, which opens new possibilities for improving the chip-snarling that occurs during hole machining.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-021-00364-z     

球磨工艺对太阳能电池正面银浆用玻璃粉的影响

研究了球磨时间、固液比、料球比和磨球尺寸与级配对太阳能电池银浆用玻璃粉粒度及形貌的影响,并将其与一定比例的银粉、有机载体配制成导电银浆,印刷、烧结在硅片上形成晶体硅太阳能电池片,测试了其电学性能。实验结果表明,将玻璃粉用作正面银浆粘接剂时,其最佳的行星球磨工艺参数:球磨时间为4h,固液质量比为1∶0.8,料球质量比为2.5∶1,磨球级配w(大)∶w(中)∶w(小)为3∶2∶1。此时,制备的多晶硅太阳能电池串联电阻为7.15mΩ,光电转换效率可达16.56%。     

On the mechanics of edge chipping from spherical indentation

Edge chipping is a basic failure mode in brittle materials which is dictated by a wealth of material and geometric variables. Here we examine the effect of indenter bluntness on chipping load and chip dimensions. Soda-lime glass and YTZP plates are subject to surface-normal loading near an edge by a W/C ball or a Vickers tool. The ball radius r is varied from 0.2 to 8.7 mm while the indent distance h is varied from several millimeters down to a few microns. Although cone cracks are a common feature under spherical indentation, the chipping event is dominated by median-radial cracks formed under the contact. The fracture behavior is characterized by a “large” indent distance regime where the median cracks progress stably up to chipping and a “small” one where they grow unstably to form a chip once initiated. Closed-form relations for chipping load P _F under spherical indentation is developed with the aid of the test data and non-dimensional arguments. While in the “large” distance regime P _F is proportional to h ^3/2 irrespective of tool bluntness, in the “small” regime P _F is proportional to r ^1/2 h ^3/4. Interestingly, the chip dimensions are virtually independent of ball radius, varying linearly with h. Beyond relevance to structural integrity, the chipping test facilitates a simple means for determining fracture toughness K _C as well as the load needed to initiate median cracks in opaque brittle materials. An attempt is made to extend the static analysis to low-velocity impact. The results show that the damage formed during the fracture process has a major influence on dynamic chipping.