Thermal stability of nanocomposite CrC/a-C:H thin films

The thermal stability of low-friction Me-C/a-C:H coatings is important for their potential applications in the tool and automotive industry. Recently we showed that CrC_x/a-C:H coatings prepared by unbalanced magnetron sputtering of a Cr target in Ar + CH₄ glow discharges exhibit a nanocomposite structure where metastable fcc CrC nanocrystals are encapsulated by an a-C:H phase. Here, we present the structural evolution of these nanocomposite CrC/a-C:H coatings during annealing. High-temperature X-ray diffraction in vacuum and differential scanning calorimetry (DSC) combined with thermo-gravimetric analysis in Ar atmosphere indicate decomposition of the formed metastable fcc CrC phase and subsequent formation of Cr₃C₂ and Cr₇C₃ and structural transformation of the a-C:H matrix phase towards higher sp² bonding contents at temperatures above 450 °C. Combined DSC and mass spectrometer analysis as well as elemental profiling after annealing in vacuum by elastic recoil detection analysis relate this transformation to the loss of bonded hydrogen at temperatures above 200 °C.Due to these structural changes the coefficient of friction depends on the annealing temperature of the nanocomposite a-C:H coatings and shows a minimum of ∼ 0.13 for T = 200 °C. The more complex tribochemical reactions, influenced by the hydrogen loss from the coating during in-situ high temperatures ball-on disc tests, result in coefficient of friction values below 0.05 for T < 120 °C.     

Chemical vapor deposition and photoluminescent properties of polycrystalline AlN films

Thick polycrystalline aluminum nitride films have been grown by chemical vapor deposition using metallic aluminum and ammonium chloride. The films have been characterized by scanning electron microscopy and photoluminescence spectroscopy. The results have been used to analyze correlations between the main process parameters.     

Chemical stability of doped ZnO thin films

We present the results of a chemical etching stability study carried out on ZnO thin films doped with several elements deposited by spray pyrolysis. Prior to the etching, a structural study was done by X-ray diffraction and the texture of the samples was obtained by scanning electron microscopy. The samples were etched employing a solution of dilute hydrochloric acid. The etching rates obtained for the different samples depend on the dopant element and our results confirm that films doped with Cr present the highest stability against chemical etching.     

High temperature oxidation behavior of CrN/AlN superlattice films

The oxidation behavior of CrN/AlN superlattice films with different bilayer periods (Λ), Al/(Cr + Al) ratios, and crystal structures of the AlN layer was investigated. The films were deposited using a pulsed dc closed field unbalanced magnetron sputtering system. The oxidation tests were carried out in the ambient air at elevated temperatures from 700 to 1100 °C for 1 h. The changes in the crystal phase, microstructure and hardness of the films after the oxidation tests were characterized using X-ray diffraction, scanning electron microscopy and nanoindentation, respectively. When both CrN and AlN layers were in the NaCl cubic structure, the film with Λ = 3.8 nm and an Al/(Cr + Al) ratio of 0.6 exhibited a superior oxidation resistance than the film with Λ = 12.4 nm and an Al/(Cr + Al) ratio of 0.19. The film with Λ = 3.8 nm maintained the nanolayered structure with an oxidation temperature up to 1000 °C by the protection of a thin and dense X-ray amorphous oxide layer. In contrast, when the AlN layers were in the Wurzite hexagonal structure, the film with Λ = 22.5 nm and an Al/(Cr + Al) ratio of 0.67 exhibited poor oxidation resistance. The film lost the superlattice structure at 800 °C and was completely oxidized at 1000 °C due to the formation of a porous crystalline oxide layer on the surface.     

Fabrication, microstructure and properties of chemical vapour deposited AlN/Mo laminated composites

The synthesis of AlN/Mo multilayer composites was investigated. Laminates of this type are used as ceramic–metal units in electrical and vacuum devices. Appropriate chemical vapour deposition (CVD) technologies have been developed for AlN/Mo laminated composites. The extreme behaviour of the thermodynamic and technological parameters as a function of CVD reactor temperature was studied and several types of AlN coating microstructures were observed. The highest AlN crystallization rate and the best properties were obtained in the "bimodal" type microstructure. The mechanisms for the different types of deposited microstructures are discussed. The temperature dependence of the thermal resistance and the long-term thermal stability in a vacuum and in caesium vapour media were studied for AlN/Mo three-layer laminates. Only the laminates with a "bimodal" type AlN microstructure were found to be satisfactory. Fully dense laminates of this type were characterized by high electrical and corrosion resistance for a minimum of 5000 h. It was concluded that the laminated composite based on molybdenum/aluminium nitride is an excellent alternative to the presently existing niobium/corundum laminates. This revised version was published online in November 2006 with corrections to the Cover Date.     

AlN/EG纳米流体的制备及稳定性研究

通过两步法制备了氮化铝/乙二醇(Al N/EG)纳米流体,研究了超声分散时间、p H值、分散剂种类及添加量3种因素对其稳定性的影响。结果表明,超声分散时间太长或太短都不利于流体的稳定性,实验中取30 min最好;酸溶液的加入使Al N/EG纳米流体稳定性急剧恶化;碱溶液的加入也使Al N/EG纳米流体稳定性恶化,但速度较酸的加入慢。适量分散剂PVP的加入能够改善Al N/EG纳米流体的稳定性。     

Micromagnetics of laminated Permalloy films

The edge curling wall (ECW) is observed in an optical Kerr microscope. Conditions are derived energetically favoring single transverse domains with ECWs over other multiple closure domains or single longitudinal domains that are undesirable because of their low permeability. Computed figures illustrate how the maximum permissible spacer thickness depends on Permalloy sublayer thickness, uniaxial anisotropy, width of the magnetic strip, mismatch of Permalloy thickness, and perpendicular anisotropy. The case with strip width W=100 mu m and vanishing magnetostriction or stress should require only a few sublayers. However, if W is very small, or if stress-induced or some other form of perpendicular anisotropy is excessive, then attainment of single transversely magnetized domains requires many sublayers. Some of these conclusions are supported by microscopic observations.<>     

The thermal stability of AlN

The thermal stability of AlN powders and thin films has been investigated using reflection high-energy electron diffraction (RHEED) and X-ray diffraction. AlN powder was treated thermally and chemically to assess the oxidation resistance of this compound and to identify the phases formed. The results show that AlN is stable up to 1000° C in air and remains stable up to 1400° Cin vacuo. -AIOOH is formed when AlN is treated with water at 100° C but AlN does not react readily with atmospheric moisture at room temperature. The thermal stability of thin films of AlN on GaAs has been evaluated at temperatures between 900 and 1100° C in a nitrogen atomosphere. It was found that AlN did not oxidize under these conditions. Pure AlN is a suitable encapsulant for GaAs at high annealing temperatures in an inert atmosphere.     

AlN/Si衬底上ZnO薄膜的择优取向生长

当ZnO薄膜直接沉积在Si衬底上时,由于ZnO与Si的晶格失配度大,不易于获得高质量的ZnO薄膜.因此,选择合适的衬底材料沉积ZnO薄膜,对提高其质量非常重要.本文采用射频磁控溅射法,通过在Si(100)衬底上预沉积AlN作为ZnO薄膜生长的缓冲层,获得了择优取向的ZnO薄膜.我们还讨论了ZnO薄膜在AlN/Si衬底上的取向生长机理.     

Characteristics of nanocrystalline AlN:Er films co-deposited by using AlN, Er, and SiO2 targets

We report the characteristics of AlN:Er films that were co-deposited by using AlN, Er, and SiO₂ targets. The PL emission spectra show strong green emissions of Er³⁺ ions in AlN:Er films annealed at an optimal temperature of 750 °C, which is attributed to the intra-4f Er³⁺ transitions of ²H_11/2 → ⁴I_15/2 and ⁴F_7/2 → ⁴I_15/2. This optimal temperature can activate Er species as an efficient visible luminescence center. High-resolution transmission electron microscopy (HREM) observations showed that the AlN:Er film annealed at 750 °C exhibits the microstructure of AlN nanocrystallites embedded in the amorphous matrix. The occurrence of strong Er³⁺ emissions in the amorphous-nanocrystalline AlN:Er films by thermal annealing might contribute to an increased number of excitation Er³⁺ centers and the presence of oxygen related to Er³⁺ excitation and recombination processes. A distinct visible bluish green emission is also confirmed from the EL device with an amorphous-nanocrystalline AlN:Er active layer.     

Synthesis of c-axis oriented AlN thin films on different substrates: A review

Highly c-axis oriented AlN thin films have been deposited by reactive sputtering on different substrates. The crystallographic properties of layered film structures consisting of a piezoelectric layer, aluminum nitride (AlN), synthesized on a variety of substrates, have been examined. Aluminum nitride thin films have been deposited by reactive pulsed-DC magnetron sputtering using an aluminum target in an Ar/N₂ gas mixture. The influence of the most critical deposition parameters on the AlN thin film crystallography has been investigated by means of X-ray diffraction (XRD) analysis of the rocking curve Full-Width at Half Maximum (FWHM) of the AlN-(0 0 0 2) peak. The relationship between the substrate, the synthesis parameters and the crystallographic orientation of the AlN thin films is discussed. A guide is provided showing how to optimize these conditions to obtain highly c-axis oriented AlN thin films on substrates of different nature.     

AlN/TiSiN纳米多层膜的微观组织和力学性能研究

采用TiSi复合靶和Al靶,用射频磁控溅射工艺沉积不同TiSiN层厚度的AlN/TiSiN纳米多层膜。采用X射线衍射仪(XRD)、扫描电子显微镜(SEM)、高分辨透射电子显微镜(HRTEM)和纳米压痕仪研究了不同TiSiN层厚度对AlN/TiSiN纳米多层膜的微观组织和力学性能的影响。结果表明,随着TiSiN层厚度的增加,AlN相的结晶程度先增加后降低,涂层的硬度先提高后降低,当TiSiN层厚度为0.5nm时具有最高的硬度和弹性模量。HRTEM观测可知,在TiSiN层厚度为0.5nm时,TiSiN层在AlN层的模板作用下呈密排六方结构,并与AlN层呈共格外延生长,薄膜的强化主要与共格外延生长结构有关。     

Craze yielding and fracture mechanism in PE/PS/PE laminated films

Mechanical behaviour of laminated films of PS and PE, especially the altered mechanical properties of PS in the laminated state, were studied as functions of PE volume fraction. It was observed that crazing in PS can be modified by laminating layers of PE to both sides. Elongation at break and hence fracture energy increased conspicuously when PE volume fraction increased. Craze yielding stress in PS layers also increased with increasing PE volume fraction because craze formations in PS layers were suppressed by the reduction of tensile stress concentration effect at craze tips. Craze initiations were always found at the free side edges of the laminated films, which can be correlated with the transverse interlaminar shear stress concentrations existing at the edges of the laminated films caused by the difference in Poisson's ratios between PS and PE.     

Craze yielding and fracture mechanism in PE/PS/PE laminated films

The change in polystyrene (PS) layer thickness, which has been simultaneously determined during post-yield deformation, shows that crazing is the basic mechanism of toughening in all laminated films, and that shear deformation supplements the contribution of crazing especially for samples with high polyethylene (PE) volume fractions. Crazes formed in PS layers in the laminated films are slender and regular compared with the short and lenticular crazes formed in bulk PS film. When PE volume fraction increased, craze advance speed decreased because of the reduction of the stress concentration effect at craze tips. The life-time of the first mature craze to be formed at a given strain rate increased with PE volume fraction because the PE supporting the mature crazes could effectively inhibit craze rupture and blunt out the propagating crack by absorbing the stored elastic energy in the PS layer that would have been dissipated as fracture surface energy.     

Chemical bonding of a-Ge1−xCx:H films grown by RF reactive sputtering

Amorphous hydrogenated germanium-carbon (a-Ge_1−xC_x:H) films were deposited by RF reactive sputtering pure Ge (1 1 1) target at different flow rate ratios of CH₄/(CH₄+Ar) in a discharge Ar/CH₄, and their composition and chemical bonding were investigated using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR). XPS and FTIR results showed the content of germanium in the films decreased with the increase of the flow rate ratio CH₄/(Ar+CH₄), and the Ge-C, Ge-H, C-H bonds were formed in the films. The fraction of Ge-C, Ge-H, and C-H bonds was strongly dependent on the flow rate ratio. Raman results indicated that the films also contain both Ge-Ge and C-C bonding. Based on the change of the chemical bonding of a-Ge_1−xC_x:H films with the flow rate ratio CH₄/(CH₄+Ar), an optimal experimental condition for the application of infrared windows was obtained.     

Elastic properties in different nano-structured AlN films

A study of the transverse acoustic phonons on nano-structured AlN films has been carried out by using high-resolution micro-Brillouin spectroscopy. Dense films have been deposited by radio frequency (r.f.) magnetron sputtering under ultra high vacuum at room temperature. Films with different morphologies were prepared and investigated by transmission electron microscopy and Brillouin Spectroscopy (equiaxed nano-sized, nano-columnar grains and amorphous phase). Results show a dependence of the transverse modes on the nano-structure. The nano-columnar film exhibits two transversal modes as expected for the AlN würtzite while the equiaxed nano-sized and the amorphous films only exhibit one isotropic transverse mode as expected in amorphous materials. One important result is that the sound propagation velocity in the AlN amorphous phase is higher than the one in the non-textured nano-crystalline phase. This phenomenon has, however, already been observed in ferroelectric ceramics.     

Some remarks on Ag/C:H nanocomposite films

Nanocomposite films Ag/C:H were prepared using DC unbalanced magnetron sputtering operated in Ar and n-hexane mixture. Optical emission spectroscopy was used to monitor deposition process as witnessed by XPS quantitative analysis. UV-vis transmittance of Ag/C:H films as a function of Ag content show well-known anomalous optical absorption. FTIR (SEIRA) spectroscopy and HRTEM images were used to reveal an interface region between plasma polymer C:H and Ag nanocluster surface.     

Non-equilibrium crystallization-aggregation mechanism of a-C:H layer in a-C:H/a-Se complex films

Also representing: CCAST (World Laboratory), Beijing 100080, People's Republic of China and the Structure Research Laboratory, University of Sciences and Technology of China, Hefei 230026, People's Republic of China.