Vickers hardness and deformation of Ni/Cu nano-multilayers electrodeposited on copper substrates

Ni/Cu nano-multilayers were fabricated by an electrodeposition technique. Ratio of the Ni:Cu layer thickness was kept at 1:1. By laminating nickel and copper layers at a very narrow spacing, we obtained highly-densified parallel interfaces which can give rise to high strength. Dependence of Vickers hardness and tensile deformation on individual layer thickness h was investigated on the Ni/Cu multilayers. The Vickers hardness increased with decreasing layer thickness for the multilayers of h≥ 10 nm. This change in the hardness was consistent with the Hall-Petch relation. At the 10 nm layer thickness, the hardness attained more than three times higher than that of the copper substrate. On the other hand, the hardness decreased rapidly with the layer thickness at h < 10 nm. The tensile deformation tests were also carried out at the substrates coated with the multilayer of h = 5, 20 and 100 nm. The SEM observations revealed that the slip lines of the deformed substrates were terminated by the multilayer at the multilayers of h = 20 and 100 nm. On the other hand, a lot of slip lines penetrated into the multilayer of h = 5 nm. These slip observations were compatible with the layer thickness dependence of the hardness.     

Analysis of Co/Cu multilayers by SNMS reverse depth profiling

The overall quality of multilayer thin films prepared by electrodeposition could strongly be influenced by the surface and interface roughness. The roughness, however, may increase with the number of layers. For that very reason the reliable analysis of the first few layers is essential. However, in depth profiling methods based on sputtering techniques the first layer is always found at the bottom of the sputter crater. Since the depth resolution might decrease during sputtering, the analysis of the first few layers is often very difficult. In order to circumvent this problem, we performed SNMS depth profiling in the direction from the substrate. We prepared thin film samples in two ways. First, Co/Cu multilayer stacks were electrodeposited on Si/Cr/Cu substrates and SNMS depth profiling was carried out from the direction of the topmost layer. Secondly, electrodeposited Co/Cu multilayer stacks were coated with a few microns thick Ni layer and detached from the Si substrate in order to study the film structure from the side of the substrate. Using this latter method, which we denote as “reverse depth profiling”, we were able to analyze the first and, probably, the most even layers of the thin film structure with high resolution.     

Oxidation study of Co/Cu multilayers by resonant X-ray reflectivity

The outer oxide layer of magnetron-sputtered Co/Cu multilayers has been studied by means of resonant and classical X-ray reflectometry. The variation of the oxide layer thickness has been monitored versus the upper-Co layer thickness. The surface oxidation seems to stop at the Co/Cu interface and hence no Cu oxide is produced. The contact potential at the Co/Cu interface may be at the basis of this effect. In addition, the dependence of the oxide layer roughness with its thickness has been studied, varying the roughness inversely to the oxide layer thickness, something which results comprehensible on the basis of the low temperature oxidation process.     

Growth types and surface topography of Co, Cu and Co/Cu multilayers studied by AES and STM/SFM

A thin film of cobalt, copper and Co/Cu multilayers deposited on Si(1 0 0) has been studied by an in situ combination of Auger electron spectroscopy and scanning tunnelling/scanning force microscopy. We show that thickness-dependent Auger peak intensity measurements, taken in situ during deposition of constituents of Co/Cu multilayers, combined with microscopy can bring valuable information about growth type of the system components.     

Microstructure and ultrahigh strength of nanoscale Cu/Nb multilayers

The microstructure and mechanical properties of Cu/Nb multilayers were investigated by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and nanoindentation. Ultrahigh strength of 3.27 GPa is achieved at the smallest layer thickness of 2.5 nm, which agrees well with the theoretical prediction based on the deformation mechanism of crossing of dislocations across interfaces. After that, the strength decreases with the increasing layer thickness and the transition of the deformation mechanism to confined layer slip occurs at the layer thickness of 6.5 nm. Additionally, strength of the Cu/Nb multilayers increases with increasing loading strain rate because of enhanced strain hardening.     

Fracture toughness,strength and Vickers hardness of yttria-ceria-doped tetragonal zirconia/alumina composites fabricated by hot isostatic pressing

Yttria-ceria-doped tetragonal zirconia ((Y, Ce)-TZP)/alumina (Al₂O₃) composites were fabricated by hot isostatic pressing (HIP) at 1400–1600 °C and 147 MPa for 30 min in Ar gas using fine powders prepared by hydrolysis of ZrOCl₂ solution. The mechanical properties of these ceramic composites were evaluated. The fracture toughness and bending strength of the composites consisting of 25 wt% Al₂O₃ and tetragonal zirconia with compositions 4 mol% YO_1.5-4 mol% CeO₂-ZrO₂, 2.5 mol% YO_1.5-4 mol% CeO₂-ZrO₂ and 2.5 mol% YO_1.5-5.5 mol% CeO₂-ZrO₂ fabricated by HIP at 1400 °C were 6–7 MPa m^1/2 and 1700–1800 MPa. Fracture toughness, strength and hardness of (Y, Ce)-TZP/Al₂O₃ composites were strongly dependent on HIP temperature. The fracture strength and hardness were increased, and grain growth of zirconia grains and phase transformation from the tetragonal to the monoclinic structure of (Y, Ce)-TZP during HIP in Ar at high temperature (1600 °C) were suppressed by the dispersion of Al₂O₃ into (Y, Ce)-TZP.     

Mechanical characterization of Ti-added MgB2/Cu superconducting wires using Vickers hardness test

The Ti-added (0%, 5%, and 10%) MgB₂/Cu superconducting wires were prepared by the powder-in-tube (PIT) method. Mechanical properties of the samples were then characterized by using a dynamic ultra-microindentation experimental technique. We observed that the sink-in effect is significant in our samples. So, the loading–unloading (P–h) curves were analyzed by the displacement approach to indentation. It was found that hardness (H) and the effective elastic modulus (E) values increased with added Ti. In addition, these values showed peak load dependence (i.e. indentation size effect (ISE)).     

Co/Cu纳米多层膜的制备及巨磁阻性能的研究

采用恒电位双电解槽法在硼酸镀液体系中,以单晶Si(111)为基底电沉积制备Co／Cu多层膜,确定了双槽法制备多层膜的工艺条件,为得到优良的多层膜巨磁阻材料,镀液体系中加入了自制的添加剂。并用扫描电镜(SEM)表征了多层膜的断面形貌,小角度X射线衍射(LXRD)谱图中出现了2个衍射峰,大角度X射线衍射(MXRD)谱图中强衍射峰的两侧出现了卫星峰,表明多层膜具有超晶格结构。用物性测量系统(PPMS)测试了Co／Cu多层膜的巨磁阻(GMR)性能,GMR值达到52．52％。     

Co/Cu多层膜层间耦合作用的调节

利用在Fe（5nm）/[Co/Cu]30多层膜的隔离层（Cu）中掺入磁性元素Co，引入与相邻Co层之间反铁磁的RKKY作用相竞争的直接的铁磁耦合作用，降低了相邻Co层间的交换耦合作用。在Cu层中掺入不同原子数比例的磁性元素时，饱和磁场有所降低，从而提高了多层膜对外磁场变化的灵敏度。     

Magnetoresistance and microstructure evolution upon rapid thermal annealing of giant magnetoresistive Co/Cu/Co/CoNbZr multilayers

The magnetoresistance (MR) variation of Co/Cu/Co/CoNbZr spin valves as a result of rapid thermal annealing has been investigated. MR ratio of 3.8% was obtained in the as-deposited sample and a considerable increase to 6.86% was observed in the 450°C×60 s treated sample. Microstructure studies show that the enhancement of MR ratio is a consequence of the nano-crystallization of amorphous CoNbZr soft layer. The nano-crystallized CoNbZr possess fine and dense microstructure and excellent electrical and soft magnetic properties which leads to the MR enhancement. With increasing annealing temperature or annealing time, interface roughness caused by rapid grain growth decrease the MR ratio rapidly. XRD studies imply that the interfusion of Cu atom into the Co layer is another possible degradation mechanism of Co/Cu/Co/CoNbZr spin valves at annealing temperature beyond 550°C.     

电沉积纳米复合材料的超塑性研究

采用脉冲电沉积技术制备了含不同Al2O3颗粒的Al2O3/Ni-Co纳米复合材料,在应变速率从8.33×10-4s-1到1.67×10-2s-1,温度从723K到823K的范围内,研究了它们的超塑性拉伸变形行为,确定了最佳超塑性条件并获得了最大延伸率.采用SEM和TEM对电沉积和超塑变形前后试件的显微结构进行了表征.应用晶粒长大行为和协调机制对合金和复合材料的超塑性进行了对比研究和讨论.     

缓冲层Ta对退火Co/Cu/Co薄膜微观结构和界面互扩散的影响

用直流磁控溅射法在Si/SiO₂基底上制备了Co/Cu/Co薄膜和加入缓冲层的Ta/Co/Cu/Co薄膜,用扫描电子显微镜、原子力显微镜、X射线衍射和俄歇电子能谱研究了薄膜的微观结构、表面形貌、织构和界面互扩散现象。结果表明:退火后薄膜中均存在{111}和{002}衍射峰,加入缓冲层Ta后,Co/Cu/Co薄膜的衍射峰强度明显增强,并存在较强的{111}纤维织构,薄膜表面孔洞及粗糙度大幅减小。退火后薄膜界面处产生互扩散现象,层状结构被破坏。缓冲层Ta提高了薄膜与基底材料间的润湿性,可有效缓解界面互扩散现象。     

Microstructure control in TiAlN/SiNx multilayers with appropriate thickness ratios for improvement of hardness and anti-corrosion characteristics

TiAlN/SiN_x multilayers were fabricated by a reactive magnetron sputtering system combining r.f. and d.c. power sources. The SiN_x layer thickness (l_SiNx) was 0.4 and 1 nm, while the layer thickness ratios (l_TiAlN/l_SiNx) of TiAlN to SiN_x were adjusted to be 4/0.4 and 4/1, respectively. Characterizations by XRD, TEM, SEM and nano-indentation revealed the dependence of l_SiNx on the preferred orientation, crystalline behavior, microstructure and hardness. The crystalline SiN_x grew epitaxially and formed the coherent interfaces with the TiAlN, exhibiting the maximum hardness of 42 GPa. However, SiN_x evidently transformed from crystalline to amorphous when the l_SiNx increased to 1 nm, while microstructure of films changed from columnar feature to more densified one. The corrosion resistance of coatings in 3.5 wt % NaCl aqueous solution was investigated by potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS). The denser microstructure exhibited the lower corrosion rate and higher polarization impedance. It was revealed that the amorphous SiN_x altered the coherent interfaces and the superlattice structure, leading to the improved anti-corrosion performances.     

Microstructure and magnetic properties of electrodeposited Co/Cu multilayer nanowire arrays

Highly uniform Co/Cu multilayer nanowire arrays had been electrodeposited into the nanochannels of porous anodic aluminum oxide template. X-ray diffraction pattern showed that Co and Cu grow in their HCP and FCC structures, respectively. Each nanowire had the same length with 20 μm and the diameter with 50 nm. The thickness of Co was 50 nm and Cu layer was about 5 nm. Magnetic measurements of the nanowire arrays showed that the magnetic coercivity for the applied field parallel to the nanowires is larger than that perpendicular to the anowires. The magnetic coercivity of Co multilayer nanowire arrays is smaller than that of the Co/Cu nanowire arrays and the crystal direction of Co layers were not obviously affected by Cu layer. The Co/Cu nanowire arrays exhibited excellent Giant Magneto Resistive ratio of about 75%.     

Influence of a Pb buffer layer on structural and magnetotransport properties of Co/Cu multilayers

In this paper changes of structure and magnetotransport properties of Co/Cu multilayers were observed as a function of the Pb buffer layer thickness. Structural analysis indicated that the Pb buffer leads to the decay of superlattice periodicity. Surface topography of the top layer of the Co/Cu multilayers observed by SFM allowed the determination of surface roughness which is relatively large and weakly depends on buffer thickness. This effect is accompanied by the continuous rise of island size that reaches a diameter around 200 nm for Co/Cu multilayers deposited on 40 nm Pb buffer. AES experiments show significant segregation of Pb to the surface. A small magnetoresistance effect ΔR/R measured for Co/Cu multilayers deposited on an Pb buffer is almost independent of the thickness of the buffer layer. This behavior of ΔR/R could be understood by assuming that discontinuous ferromagnetic layers, bridged through the Cu spacer, are formed.     

Microstructures and Vickers hardness of rapidly solidified Al-Cu alloys near the Al-Al2Cu equilibrium eutectic composition

Al-Cu alloys near the Al-Al₂Cu equilibrium eutectic composition were rapidly solidified by the single-roller method. Two-zone structures were observed in transverse sections of Al-24.8, 30.0, 32.7, 33.2, 37.0 and 40.3 mass % Cu alloy ribbons, which were interpreted on the basis of the difference in cooling rates and of the temperature gradients between the chilled and free-surface sides of the ribbons. Non-equilibrium eutectic structures, cellular and irregular lamellar, were seen at the chilled side because the alloy liquids were directionally solidified with coupled eutectic growth. The coupled zone of the Al-Cu alloy expanded over the composition range 24.8–40.3 mass % Cu by rapid solidification. Dendritic structures were formed at the unchilled side of Al-24.8, 30.0, 37.0 and 40.3 mass % Cu alloys. Vickers hardness values at the chilled side differed from those at the unchilled side due to the difference in the microstructure.