纳米结构纯铜摩擦磨损性能及亚表层结构研究

IMR OpenIR

	纳米结构纯铜摩擦磨损性能及亚表层结构研究
	姚斌
学位类型	博士
导师	卢柯 ; 韩忠
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	纳米结构金属动态塑性变形摩擦磨损机制耐磨性磨痕亚表层结构结构演化动态再结晶 Nanostructured Metals Dynamic Plastic Deformation Wear Mechanism Wear Resistance Worn Subsurface Structure Structure Evolution Dynamic Recrystallization
摘要	"纳米结构材料因其结构独特、性能优异而倍受关注。近十几年来，由于潜在的工业应用价值，纳米结构材料的摩擦磨损性能开始引起研究者的高度重视。但是目前关于纳米结构材料摩擦磨损性能及机理研究缺乏统一认识和深入理解。传统摩擦磨损理论难以解释纳米结构材料独特的摩擦磨损性能。尝试从磨痕亚表层结构演化来理解材料摩擦磨损性能的研究，主要集中在粗晶态样品亚表层微观结构及其演化过程的观察，变形导致纳米晶的形成等。纳米结构金属材料磨痕亚表层结构演化及其与耐磨性之间的关系有待进一步研究。因此，深入系统地研究磨痕亚表层结构及其对摩擦磨损行为的影响对理解纳米结构金属特殊的摩擦磨损机制具有十分重要意义。本研究利用动态塑性变形方法(Dynamic Plastic Deformation，DPD)制备块体纳米结构Cu，开展其摩擦磨损性能的研究。因为选用纯金属Cu，可以排除固溶强化、弥散强化等因素影响，有利于理解晶粒细化对材料摩擦磨损性能的影响。同时，材料的摩擦磨损性能与工况条件及环境密切相关。选择普通变形方式—准静态压缩(Quasi-Static Compression, QSC)和冷轧(Cold Rolling, CR)制备的超细晶结构Cu作为对比材料，本论文深入系统地研究了纳米结构/超细晶结构Cu及其退火态样品在不同工况条件（滑动/微动，干摩擦/油润滑，不同配副，不同温度等）下的摩擦磨损性能及磨痕亚表层微观结构和演化，并探讨了两者之间的关系。主要结果如下： 1. 干摩擦滑动条件下，DPD Cu样品由于强度高而具有优异的耐磨性。具有相同硬度的QSC Cu和CR Cu，其耐磨性相差较大：与CG Cu相比，QSC Cu具有较高耐磨性，而CR Cu耐磨性几乎没有提高。退火态纳米结构DPD Cu耐磨性在硬度为1.38 GPa出现极大值，最佳耐磨性与最高硬度之间不存在对应关系。在QSC Cu和CR Cu退火态样品中均出现相似结果。对不同实验载荷和频率下DPD Cu，CR Cu和CG Cu耐磨性研究表明，随着载荷、频率增大，耐磨性逐渐降低。在相同实验参数下，DPD Cu耐磨性优于CR Cu和CG Cu样品。 2. 磨痕亚表层结构观察发现，几类Cu样品均形成稳态的磨痕亚表层结构，即由严重变形的纳米结构机械混合层(Nanostructured Mixing Layer, NML)和超细晶组成的动态再结晶层(Dynamic Recrystallization layer，DRX layer)组成。CG Cu亚表层结构演化过程中，滑动初始阶段塑性变形导致的晶粒细化起主导作用，变形累积到一定程度后，发生动态再结晶。DPD Cu亚表层结构演化过程包括变形结构的动态再结晶和后续再结晶晶粒的塑性变形。普通变形方式制备的QSC Cu和CR Cu亚表层结构演化过程相似。亚表层中NML和DRX layer结构差异，最终决定了材料耐磨性的高低。 3. 亚表层中NML是磨痕最表层局域高应变区由纳米晶和少量超细晶组成的非均匀结构，其中含有O、W、C等元素。所有Cu样品中形成的NML结构和成分没有明显差异，NML是摩擦磨损过程中磨屑的主要来源。DRX layer主要有再结晶晶粒和变形晶粒两部分组成，二者在该层中均匀混合。CG Cu 中以变形晶粒为主，其它Cu样品中以再结晶晶粒为主。 4. 滑动过程中形成的NML尺寸更小、与DRX layer结合力越强，材料的耐磨性越好。材料耐磨性的优劣直接体现在摩擦磨损过程中抵抗NML形成及剥落能力的强弱上。磨痕亚表层中DRX结构向最表层NML转变，对材料耐磨性起着决定性作用。 5. 干摩擦滑动条件下，无论是纳米结构/超细晶结构Cu，还是CG Cu，磨损量均随着DRX layer中晶粒尺寸增大或显微硬度降低而增大，即材料耐磨性与DRX晶粒尺寸之间存在单调对应关系。由于纯度的降低抑制了亚表层DRX晶粒长大过程而有效地改善了Cu的耐磨性。 6. 油润滑滑动情况下，Cu耐磨性随硬度变化呈单调增加趋势。油润滑滑动磨损量随着DRX晶粒尺寸增大而增大。油润滑/干摩擦微动条件下，DPD Cu耐磨性均明显优于CG Cu。其亚表层结构演化过程与干摩擦滑动情况相似，均为塑性变形导致晶粒细化过程以及累积摩擦热和塑性变形作用下晶粒长大过程的动态平衡。无论是CG Cu还是DPD Cu，其磨损量随DRX晶粒尺寸变增大而增大。 7. 在干摩擦滑动条件下，采用三种不同对磨球，无论是DPD Cu还是CG Cu，按对磨球从WC-Co到AISI 52100及Cu顺序变化，磨损率快速增大。同样对磨球变化顺序，亚表层变形的减弱和摩擦热的快速耗散对晶粒长大过程的抑制作用增强，最终导致亚表层中DRX晶粒尺寸逐渐细化，相应的磨损率快速增加。对比液氮温度(Liquid Nitrogen Temperature, LNT)和室温(Room Temperature, RT)下实验结果，发现LNT下亚表层中再结晶过程因温度降低而被有效抑制，DRX晶粒非常细小，此晶粒承受塑性变形能力减弱，耐磨性变差。配副为Cu 球和LNT两种条件下，控制磨损率的关键过程是亚表层NML中裂纹向DRX结构快速扩展和NML中材料快速剥落。"
其他摘要	"Increasing interests have been focused on nanostructured (NS) materials for their unique properties. During the past decades, their tribological properties have also attracted attention from scientists due to potential industrial applications. However, tribological properties and related wear mechanisms of NS materials are still not fully understood yet. The traditional friction and wear theories could not explain the unique tribological properties of NS materials. However, attempts made on investigations of microstructure evolution beneath the worn surface focus on the observations of the worn subsurface structure and formation of nanocrystalline grains induced by plastic deformation for the coarse-grained (CG) materials. The worn subsurface structure of NS materials and their relation with tribological properties are rarely explored. Therefore, a systematic study on the worn subsurface structure effect on their wear resistance is essential to understand the unique wear mechanisms of NS metals. In this work, bulk NS Cu samples with nano-scale twin bundles (about 30% in volume) embedded in nano-sized grains were synthesized by using dynamic plastic deformation (DPD) technique, and the tribological properties of NS Cu sample have been investigated. Investigations on pure metals can eliminate some extrinsic effects such as solid solution hardening and precipitation hardening, facilitating understanding the grain refinement effect on the tribological properties of NS materials. In addition, the tribological properties of materials depend on the test parameters and environments. Conventional deformation methods, such as Quasi-Static Compression (QSC) and Cold Rolling (CR) were selected for comparison. Friction and wear behaviors for nanostructured/ultra-fined grained Cu and their corresponding annealed samples under different conditions (sliding/fretting, dry/oil lubrication, different conterface and temperature) were investigated as well as the worn subsurface structure and its evolution process, and the relationship between them are also discussed. The main results of this work are: 1. Under dry sliding condition, DPD Cu exhibits an enhanced wear resistance due to its high strength. QSC and CR Cu with almost same hardness show quite different wear resistances. The wear rate of QSC Cu is lower than that of CG Cu, while CR Cu exhibits almost same wear resistance compared to CG Cu. Under dry sliding condition, the highest wear resistance can be found in the annealed DPD Cu samples with a hardness of 1.38 GPa, not corresponding to the highest hardness. Similar results were found in the annealed QSC and CR Cu samples. Under different sliding parameters, the wear resistance of DPD, CR and CG Cu samples decreases with an increasing load and frequency. DPD Cu shows an enhanced wear resistance compared to CR and CG Cu at certain sliding conditions. 2. A steady worn subsurface structure was formed for all Cu samples, which is composed by heavily deformed nanostructured mixing layer (NML) and ultra-fine grained dynamic recrystallization (DRX) layer. For the worn subsurface structure evolution in the CG Cu, grain refinement induced by plastic deformation plays a critical role at the initial of sliding and the DRX occurs after enough plastic deformation accumulation. While for the DPD Cu sample, the worn subsurface structural evolution involves DRX of nanostructured/ultra-fine grains and subsequent plastic deformation of the recrystallized grains. The difference in the microstructure of NML and DRX layer determines the tribological properties of Cu samples. 3. The NML induced by severe plastic deformation is constituted by nano-grains and some ultra-fine grains with foreign elements (oxygen, carbon, tungsten) mixing. The structure and composition of NMLs are similar for all Cu samples, the materials removing within NMLs is the main source of wear debris. Recrystallized grains and deformed grains homogenously mixed within DRX layer. More deformed grains exist in DRX layer for CG Cu, more recrystallized grains for other Cu samples. 4. NMLs developed with smaller sizes and better adherence with the DRX layer correspond to a better wear resistance of Cu samples. The formation and peeling off rate of NMLs directly reflect the tribological properties of Cu samples. The transformation from the DRX structure in the worn subsurface into the top NML is controlling the wear rate of materials. 5. Under dry sliding conditions, a pronounced correlation is identified that the wear rate increases significantly with an increasing grain size or a decreasing micro hardness of DRX layer adjacent to the NML, that is to say, wear resistance increases monotonically with a decreasing DRX grain size. The increasing of impurities of the Cu samples could effectively decrease wear rate. The impurity effect on the wear resistance behaves as a key factor to control the grain growth in the DRX layer. 6. Under oil sliding conditions, wear resistance varies monotonically with hardness, the harder the Cu sample is, the better wear resistance should be. The wear volume increases with an increasing DRX grain size. Under oil/dry fretting conditions, wear resistance of DPD Cu is better than that of CG Cu. The worn subsurface structural evolution process is similar to that in dry sliding conditions, which corresponds to a dynamic balance between grain refinement by plastic deformation and grain growth induced by friction heat and plastic deformation accumulation. For both DPD Cu and CG Cu, the wear volumes increase with an increasing DRX grain size. 7. For DPD Cu and CG Cu, when sliding with a counterface varying from WC-Co to AISI 52100 till Cu ball, the relative wear rate increases quickly under dry conditions. The grain growth in the worn subsurface is suppressed obviously by deformation weakening and friction heat quick dissipation, when counterface is varied from WC-Co to Cu ball. Hence, much more refine grains in DRX layer are developed when sliding with a Cu ball. Under LNT sliding, the grain size in DRX layer is decreased significantly as a result of DRX depression in comparison with RT sliding. In both above conditions, wear rates increase with an obviously decreasing grain size in DRX layer. The wear rate controlling process is transformed correspondingly to NMLs peeling off, which results in higher wear rate. At such a situation, the quick spread of cracks from NMLs into DRX layer and flaking rate of NMLs is controlling the wear rate of materials."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64473
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	姚斌. 纳米结构纯铜摩擦磨损性能及亚表层结构研究[D]. 北京. 中国科学院金属研究所,2012.