Coherent growth and superhardness effect of heterostructure h-TiB2/c-VC nanomultilayers

TiB₂/VC nanomultilayers with different VC layer thicknesses have been prepared by a multi-target magnetron sputtering system. X-ray diffraction, high-resolution transmission electron microscopy and nanoindentation measurements were employed to investigate the microstructure and mechanical properties of these films. The results revealed that a metastable structure of VC has been formed in epitaxial TiB₂/VC multilayers with VC layer thickness ≤0.8 nm. Meanwhile, the multilayers exhibited coherent interface between layers resulting in a significantly enhanced hardness of the films, with a maximum value of 43.9 GPa. The stable cubic structure of VC was observed for VC layer thickness ≥1.3 nm, which causes a gradual disruption of the coherent interface of the multilayers, resulting in the quick decrease of hardness.     

Crystallization of amorphous AlN and superhardness effect in VC/AlN nanomultilayers

VC/AlN nanomultilayers with various AlN layer thicknesses have been prepared by multi-target magnetron sputtering. Microstructure evolution and mechanical properties of the multilayers have been investigated. Under the “template effect” of cubic VC, as-deposited amorphous AlN has been crystallized to cubic structure with AlN layer thickness < 1 nm, correspondingly the multilayers exhibit coherent growth and obtain significantly enhanced hardness with the maximum of 40.1 GPa. A further increase of AlN layer thickness causes the formation of amorphous AlN, which blocks the coherent growth of the multilayers, resulting in a rapid decline of hardness.     

Microstructure and mechanical properties of as-deposited and annealed TiB2/BN superlattice coatings

TiB₂/BN multilayers with the modulation ratios (t_TiB2:t_BN) ranging from 1:1 to 16:1 and a constant modulation period of 24 nm were prepared by magnetron sputtering. The TiB₂/BN multilayers were subsequently annealed in a vacuum environment at temperatures of 500-700 °C for 30 min, then characterized by extensive measurements. All multilayers exhibited small grain sizes and stable layer structures with polycrystalline with TiB₂(001), TiB₂(101), TiB₂(002) textures or amorphous BN, resulting in higher hardness and elastic modulus than that of individual monolithic TiB₂ or BN coatings. The hardness of as-deposited multilayer can reach as high as 39.34 GPa at t_TiB2:t_BN = 13:1, meanwhile the friction coefficient got to 0.028, which was also the lowest. The hardness and friction were almost unchanged after annealing at 500-700 °C, which was attributed to good thermal stability in the layer structure and the existence of stable TiB_xN_y phases.     

纳米多层膜中的非晶晶化与超硬效应

通过对TiN/SiC、TiN/TiB₂和TiN/SiO₂纳米多层膜微结构和力学性能的研究, 展示了通常溅射沉积态为非晶的SiC、TiB2和SiO₂薄膜, 在立方结构的TiN晶体层模板作用下的晶化现象, 以及多层膜由此产生的生长结构和力学性能的变化. 结果表明: SiC在层厚0.6nm时晶化为立方结构后,可以反过来促进TiN/SiC多层膜中TiN层的晶体完整性; TiB₂在层厚2.9nm时晶化为六方结构, 并与TiN形成{111} _TiN//{0001} _TiB2, <100> _TiN//<11-20> _TiB2 的共格关系; SiO₂在层厚0.9nm 时晶化为立方结构的赝晶. 多层膜中SiC、TiB₂和SiO₂晶化后都与TiN形成共格外延的生长结构, 并相应产生了硬度升高的超硬效应. 随着SiC、TiB₂和SiO₂层厚的增加, 它们又转变为非晶态, 多层膜的共格外延生长受到破坏, 其硬度亦明显降低.     

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.     

Influence of modulation periods and modulation ratios on the structure and mechanical properties of nanoscale TiAlN/TiB2 multilayers prepared by IBAD

Alternate hard TiAlN/TiB₂ multilayers with different modulation periods (Λ) ranging from 0.6 to 27 nm and modulation ratios (t_TiAlN:t_TiB2) ranging from 8:1 to 25:1 were prepared using an ion beam assisted deposition (IBAD) system. The effect of Λ and t_TiAlN:t_TiB2 on the hardness, elastic modulus, residual stress, and fracture resistance were investigated using various characterization techniques. All multilayers with clear interfaces displayed higher hardness than individual TiAlN and TiB₂ layers. The maximum hardness of 35 GPa and critical load of 84 mN were obtained for the multilayer with a Λ of 2.2-8.8 nm and t_TiAlN:t_TiB2 of 8:1. Strong TiAlN (111) crystallographic texture as well as multilayer structure is thought to be be responsible for the increasing hardness of the TiAlN/TiB₂ multilayers.     

Structural and mechanical properties of titanium and titanium diboride monolayers and Ti/TiB2 multilayers

Nano-multilayers represent a new class of engineering materials that are made up of alternating nanometer scale layers of two different components. In the present work a titanium (Ti) monolayer was combined with titanium diboride (TiB₂) to form a Ti/TiB₂ nano-multilayer. Designed experimental parameters enabled an evaluation of the effects of direct current bias voltage (U_b) and bilayer thickness (Λ) during multilayer deposition on the mechanical properties of reactively sputtered Ti/TiB₂ multilayer films. Their nanostructures and mechanical properties were characterized and analyzed using X-ray photoelectron spectroscopy (XPS), low-angle and high-angle X-ray diffraction (XRD), plan-view and cross-sectional high-resolution transmission electron microscopy (HRTEM), and microindentation measurements. Under the optimal bias voltage of U_b = − 60 V, it was found that Λ (varied from 1.1 to 9.8 nm) was the most important factor which dominated the nanostructure and hardness. The hardness values obtained varied from 12 GPa for Ti and 15 GPa for TiB₂ monolayers, up to 33 GPa for the hardest Ti/TiB₂ multilayer at Λ = 1.9 nm. The observed hardness enhancement correlated to the layer thickness, followed a relation similar to the Hall-Petch strengthening dependence, with a generalized power of ∼ 0.6. In addition, the structural barriers between two materials (hcp Ti/amorphous TiB₂) and stress relaxation at interfaces within multilayer films resulted in a reduction of crack propagation and high-hardness.     

Tribological and mechanical behaviors of TiN/CNx multilayer films deposited by magnetron sputtering

TiN/CN_x multilayer films with bilayer periods of 4.5-40.3 nm were deposited by direct-current magnetron sputtering. Layer morphology and structure of the multilayered films were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy. The TiN/CN_x multilayers exhibited coherent epitaxial growth due to the mutual growth-promoting effect at small bilayer period and some crystalline regions going through the interface of TiN/CN_x. Nanoindentation tests showed that the hardness of the multilayers varied from 12.5 to 31 GPa, with the highest hardness being obtained with a bilayer period of 4.5 nm. The tribological properties of the films were investigated using a ball-on-disk tribometer in humid air, and the TiN/CN_x multilayer with a bilayer period of 4.5 nm also exhibited the lowest friction coefficient and the highest wear resistance.     

Influence of modulation ratio on the structure and mechanical properties of TiB2/TiAlN multilayered coatings

TiB₂/TiAlN multilayered coatings with various modulation ratios (t_TiB2:t_TiAlN) were grown using radio-frequency magnetron sputtering at room temperature. Nanoindentation, tester for material surface properties, and XRD were used to investigate the influence of modulation ratio on microstructure and properties of the multilayers. All multilayers showed improved mechanical properties, compared with the average value of the monolithic TiB₂ and TiAlN coatings. The multilayer with modulation ratio of 5:2 displayed the highest hardness (36 GPa) and longest time to crack during wear. A marked layer structure with the strong mixture of TiAlN (111), AlN (111), and TiB₂ (001) textures with smaller grain sizes was responsible for the enhanced hardness.     

Microstructure and mechanical properties of CrAlN/SiNx nanostructure multilayered coatings

Multilayered CrAlN and SiN_x films were deposited periodically by radio frequency reactive magnetron sputtering. In the CrAlN/SiN_x multilayered coatings, the thickness of CrAlN layer was fixed at 4 nm, while that of SiN_x layer was adjusted from 4 nm to 0.3 nm. The dependence of the SiN_x layer thickness on the preferred orientation, crystalline behavior and mechanical properties of multilayered coatings were discussed with the aid of XRD patterns and HRTEM. It was demonstrated that amorphous SiN_x layer transformed to a crystallized one when the thickness decreased from 4 nm to 0.3 nm. The crystalline SiN_x layer grew epitaxially, formed the coherent interface with the CrAlN layer, and the columnar structure was exhibited. The critical layer thickness for the transition from amorphous SiN_x to a crystallized one was found to be around 0.4 nm, and maximum hardness of 33 GPa was revealed.     

SiO2层晶化对TiN/SiO2纳米多层膜结构和性能的影响

采用多靶磁控溅射法制备了一系列具有不同SiO2调制层厚的TiN/SiO2纳米多层膜.利用X射线衍射、X射线能量色散谱、扫描电子显微镜、高分辨电子显微镜和微力学探针表征和研究了多层膜的生长结构和力学性能.结果表明,具有适当厚度(0.45～0.9 nm)的SiO2调制层,在溅射条件下通常为非晶态,在TiN层的模板作用下晶化并与TiN层共格外延生长,形成具有强烈(111)织构的超晶格柱状晶多层膜;与此相应,纳米多层膜产生了硬度和弹性模量异常增高的超硬效应(最高硬度达45 GPa).随着SiO2层厚度的继续增加,SiO2层转变为非晶态,阻断了多层膜的共格外延生长,使纳米多层膜形成非晶SiO2层和纳米晶TiN层的多层结构,多层膜的硬度和弹性模量逐渐下降.     

Structural characteristics and mechanical properties of Ti(Cr) films produced on Si substrate

A series of nanogranular Ti₉₀Cr₁₀ thin films have been fabricated by pulsed-laser deposition on Si substrates at different temperatures. The crystal structure and mechanical properties of these films were investigated. The X-ray diffraction and transmission electron microscope images with selected area diffraction showed that the structure of as-prepared films is dependent on film thickness and deposition temperature. It was found that the Ti₉₀Cr₁₀ films consisted of fine hexagonal close packed microstructure with columnar grains, while body close-packed cubic structure of Cr films are composed of irregular grains, meanwhile, a chromium disilicide (CrSi₂) layer formed in the interface between the substrate and Cr films which deposited at temperature of greater than 600 °C. The crystalline and columnar grains improved with an increase of the thickness of the films and an optimum microstructure is obtained under the present experimental condition of about 50 nm thickness and deposited temperature of 500 °C for Ti₉₀Cr₁₀ films. Deposited at 300 °C, the Ti₉₀Cr₁₀ films have hardness of 12.7 GPa and elastic modulus of 174.6 GPa. Improved to 600 °C the sample shows higher hardness of 13.1 GPa and higher elastic modulus of 183.2 GPa. Using Benjamin-Weaver model, adhesion shearing force can be calculated as 34.9 MPa for 300 °C Ti₉₀Cr₁₀ film while higher value of 44.4 MPa for higher temperature of 600 °C.     

Realization of superhard chromium nitride-based films: A superlattice nanocrystalline-Cr2N/amorphous-WC film

Generally, it is a challenge for superhard nitride films to possess strong enough fracture toughness. In the present study, a superhard nanocrystal- (nc-) Cr₂N/amorphous- (a-) WC film was fabricated by arc ion plating and dc magnetron sputtering. The superlattice structure of nc-Cr₂N with 9 nm thickness and a-WC with 3 nm thickness was alternately grown on amorphous Cr/WC buffer layer. Accordingly, the multilayers nc-Cr₂N/a-WC nanocomposite showed a superhardness effect (~ 40 GPa) which presents an anomalous enhancement of hardness and elastic modulus. The adhesion strength of nc-Cr₂N/a-WC multilayers on the steel substrate exceeded 60 N. The tribological behavior of the nc-Cr₂N/a-WC film was proven that the superlattice nc-Cr₂N/a-WC nanocomposites have significant potential for high-speed dry machining and other wear-resistance precise parts.     

Effect of AlN layer thickness on structure and magnetic properties of FePt/AlN multilayers

FePt (50 nm) and [FePt(xnm)/AlN(1, 2, 3 nm)]₁₀ (x=2, 3 nm) films were prepared by RF magnetron sputtering technique, then were annealed at 550 °C for 30 min. This work investigates the effect of AlN layer thickness on structure and magnetic properties of FePt/AlN multilayers. Superlattice (0 0 1) peaks can be found in the grazing incidence X-ray diffraction of FePt and [FePt (3 nm)/AlN (1, 2, 3 nm)]₁₀ films, which indicate that the FCC phase has been partially transformed into ordered L1₀ phase. Compared with the single layer FePt film, superlattice (0 0 1) peaks of FePt/AlN multilayers are weak and wide, which indicates that the introducing of AlN hinders the growth of FePt particle, and also shows the introducing of AlN is not beneficial to the transformation from FCC phase to L1₀ phase. In addition, the low-angle XRD spectra show the layered structure of FePt/AlN has been broken after annealing. The coercivities, particle size, intergrain exchange interactions of FePt/AlN films are decreased with increasing AlN layer thickness.     

Microstructure and hardening mechanisms in a-Si3N4/nc-TiN nanostructured multilayers

Amorphous/nanocrystalline Si₃N₄/TiN nanostructured multilayer films were fabricated by radio-frequency reactive magnetron sputtering. The microstructure and properties of these films were measured using an X-ray diffractometer, X-ray photoelectron spectroscope, high-resolution transmission electron microscopy and nanoindenter. The superhardness effect was found in Si₃N₄/TiN multilayers. The hardness of Si₃N₄/TiN multilayers is affected not only by modulation periods, but also by layer thickness ratio and deposition temperature. The hardness value is about 40% higher than the value calculated from the rule of mixtures at a deposition temperature of 500 °C and a layer thickness ratio (l_Si3N4/l_TiN) of 3/1. The hardening mechanisms in this system are discussed in the light of our experimental results. Results of calculation of the theoretical stress distribution in the multilayers suggests that alternating stress fields caused by thermal mismatching between Si₃N₄ and TiN is one of the main reasons for the superhardness effect observed in Si₃N₄/TiN multilayers.     

Microstructure and optical properties of Al2O3/ZrO2 nano multilayer thin films prepared by pulsed laser deposition

The aluminium oxide/zirconium oxide (Al₂O₃/ZrO₂) nanolaminate thin films (5/20 nm with 4 bilayers, 5/15 nm with 5 bilayers and 5/10 nm with 7 bilayers) were deposited on Si (100) and quartz substrates at an optimized oxygen partial pressure of 3 × 10⁻² mbar at room temperature using pulsed laser deposition. The multilayer films were characterized using X-ray diffraction, X-ray reflectivity, Atomic force microscopy and UV–Visible spectroscopy. The X-ray diffraction studies showed amorphous nature for 5/20 nm film, whereas 5/15 nm and 5/10 nm multilayers showed only tetragonal zirconia at room temperature. X-ray reflectivity studies showed the Kiessig fringes and Bragg peaks, indicating the well defined formation of individual layers and bilayer periodicity in the multilayer films. The AFM studies showed the RMS roughness values of 0.7 nm, 0.9 nm and 1.1 nm for 5/10 nm, 5/15 nm and 5/20 nm multilayers respectively. The optical performance of the combined Al₂O₃/ZrO₂ nanolaminates showed that the refractive indices of the films increased from 1.75 to 1.99 with the decrease of ZrO₂ layer thickness from 20 to 10 nm.     

Structural and electrical properties of room temperature pulsed laser deposited and post-annealed thin SrRuO3 films

Good quality strontium ruthenate (SrRuO₃) thin continuous films (15 to 125 nm thick) have been synthesized on silicon (100) substrates by room temperature pulsed laser deposition under vacuum followed by a post-deposition annealing, a route unexplored and yet not reported for SrRuO₃ film growth. The presence of an interfacial Sr₂SiO₄ layer has been identified for films annealed at high temperature, and the properties of this interface layer as well as the properties of the SrRuO₃ film have been analyzed and characterized as a function of the annealing temperature. The room temperature resistivity of the SrRuO₃ films deposited by laser ablation at room temperature and post-annealed is 2000 μΩ·cm. A critical thickness of 120 nm has been determined above which the influence of the interface layer on the resistivity becomes negligible.     

Microstructure and Vickers hardness of Co/Cu multilayers fabricated by electrodeposition

In order to investigate thermal stability of Co/Cu multilayers fabricated by electrodeposition, Vickers hardness tests and microstructure observations were conducted on both as-deposited and annealed Co/Cu multilayers having a layer thickness of 100 nm. The multilayers were annealed at temperatures ranging from 473 to 1,273 K for 1 h. It is confirmed that even after the annealing at 1,023 K, the multilayer maintained the high hardness (Hv231) which was comparative to that of the as-deposited Co/Cu multilayer. When the annealing temperature was higher than 1,073 K, the hardness decreased rapidly with increasing temperature. Scanning electron microscopy (SEM) observation revealed that the multilayered structures were still maintained without any layer damages after the annealing at the temperatures less than 873 K. At the cross sections of the Co/Cu multilayers annealed at T > 923 K, several copper layers were fragmented. The layered structure finally disappeared by the annealing at 1,273 K. The rapid decrease in the hardness at T > 1,073 K is simply understood from the annihilation of the Co/Cu interfaces.     

Microstructure and mechanical properties of TiAlN/SiO2 nanomultilayers synthesized by reactive magnetron sputtering

TiAlN/SiO₂ nanomultilayers with different SiO₂ layer thickness were synthesized by reactive magnetron sputtering. The microstructure and mechanical properties were investigated by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and nano-indentation. The results indicated that, under the template effect of B1-NaCl structural TiAlN layers, amorphous SiO₂ was forced to crystallize and grew epitaxially with TiAlN layers when SiO₂ layer thickness was below 0.6 nm, resulting in the enhancement of hardness and elastic modulus. The maximum hardness and elastic modulus could respectively reach 37 GPa and 393 GPa when SiO₂ layer thickness was 0.6 nm. As SiO₂ layer thickness further increased, SiO₂ transformed back into amorphous state and broken the coherent growth of nanomultilayers, leading to the decrease of hardness and elastic modulus.     

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.