Fractal processing of AFM images of rough ZnO films

Fractal image processing has been applied to characterize the surface roughness of ZnO films as measured by atomic force microscopy. The simple fractal analysis suggests that the fractal dimension D can be used to describe the change of the whole grain morphology along the growth direction. Multifractal analysis shows that the scaling range is close to three orders of magnitude, which is larger than that of a simple fractal and most empirical fractals. The width of the multifractal spectrum can be used to characterize the roughness of the film surface quantitatively and the shape of multifractal spectrum can describe the ratio between the number of the lowest valleys and the highest peaks statistically.     

Influence of the microstructure on the corrosion resistance of plasma‐nitrided austenitic stainless steel 304L and 316L

Austenitic stainless steels are widely used in medical and food industries because of their excellent corrosion resistance. However, they suffer from weak wear resistance due to their low hardness. To improve this, plasma nitriding processes have been successfully applied to austenitic stainless steels, thereby forming a thin and very hard diffusion layer, the so‐called S‐phase. In the present study, the austenitic stainless steels AISI 304L and AISI 316L with different microstructures and surface modifications were used to examine the influence of the steel microstructure on the plasma nitriding behavior and corrosion properties. In a first step, solution annealed steel plates were cold‐rolled with 38% deformation degree. Then, the samples were prepared with three kinds of mechanical surface treatments. The specimens were plasma nitrided for 360 min in a H₂–N₂ atmosphere at 420 °C. X‐ray diffraction measurements confirmed the presence of the S‐phase at the sample surface, austenite and body centered cubic (bcc)‐iron. The specimens were comprehensively characterized by means of optical microscopy, scanning electron microscopy, glow discharge optical emission spectroscopy, X‐ray diffraction, surface roughness and nano‐indentation measurements to provide the formulation of dependencies between microstructure and nitriding behavior. The corrosion behavior was examined by potentio‐dynamic polarization measurements in 0.05 M and 0.5 M sulfuric acid and by salt spray testing.     

Effects of deformation mode on surface roughening of austenitic stainless steels

Abstract

Free surface roughening of polycrystalline austenitic stainless steels during tensile and deep drawing deformation is investigated using scanning white light interferometry. A fractal analysis method is applied to analyse the surface profile data in two directions which are perpendicular to each other. It is shown that the surface profile is fractal within the length and is on the same order of magnitude as the grain size. The Hurst exponent H of the two deformation modes in different directions stabilises at H≈0·85. While the correlation lengths are different, the value in the longitudinal direction is about two times larger than the one in the transverse direction. The relationship between root mean square roughness and strain appears to be linear, and the roughening rate in tensile deformation is larger than the one in deep drawing deformation.     

Self-heating effect induced by ion bombardment on polycrystalline Al surface nanostructures evolution

We studied the self-heating effect during ion bombardment process on polycrystalline Al foils. An anisotropic surface morphology evolution has been observed. The adjacent peaks?? fusion along the direction perpendicular to the ion beam projection smoothen the surface. Fusion along the parallel direction has been suppressed due to Ar⁺ ion bombardment. It attributes to the result of the competition between the isotropic thermal effect, due to the self-heating effect by energy exchange between incident ions and Al surface, and the suppression by continuous ion bombardment with a certain incident angle. Varying the incident ion beam angle with the angular range 32°?<????<?82°, the ripple wave vector, ??, is found to be parallel to the ion beam direction, whereas for ?? > 82°, ?? is perpendicular to the beam direction. The critical angle, ?? _c , is close to 82°, which is different from Bradley and Harper??s prediction and attributes to the self-heating effect.     

Tantalum metallization and properties of silicon MOS structures under stress

Stresses at the surface of a silicon wafer and at the Si-SiO₂ interface were induced by the sputter deposition of tantalum films. The sputtering of tantalum produced deformation of the silicon wafers by bending, and the radius of curvature was a function of the tantalum sputtering voltage and the tantalum film thickness. The magnitude of the stress induced at the silicon surface was determined from automatic Bragg angle control measurements of the radius of curvature. The surface distribution of the strain field and its distribution in the bulk of the silicon wafer were observed by the use of X-ray transmission topography. The electrical properties of the MOS capacitor were studied as a function of the induced stress. It was found that the largest changes with induced stress occur in the values of recombination time and capture cross section, while the surface state charge density Q_ss and the surface state density N_ss are not significantly affected. Auger electron spectroscopy showed that implanted tantalum atoms diffuse through the SiO₂ and reach the silicon surface.     

Surface roughness induced attenuation and changes in the propagation velocity of long Rayleigh-type waves

Summary The propagation of long Rayleigh waves on a rough surface is studied theoretically by Rayleigh method and for the free rough surface of a homogeneous and isotropic elastic half-space. The frequency dependence of the propagation velocity and attenuation is analysed under the assumption that the frequency is real. Moreover, the changes in the propagation velocity induced by the surface roughness are expressed in terms of the square of the ratio of the root-mean-square departure of the surface from flatness to the correlation length of the distance between successive peaks and valleys in the rough-surface profile.     

Application of plasma polymerized siloxane films for the corrosion protection of titanium alloy

Organosilicon film and SiO_x-like film are deposited on titanium alloy (Ti6Al4V) surfaces by atmospheric pressure (~ 10⁵ Pa) dielectric barrier discharge to improve its corrosion resistance in Hanks solution. Hexamethyldisiloxane (HMDSO) is used to be the chemical precursor. The organosilicon film deposited in Ar/HMDSO system has high growth rate (75 nm/min) and low surface roughness (3 nm), while the SiO_x-like film deposited in Ar/O₂/HMDSO system has lower growth rate (35 nm/min) and slightly higher surface roughness (9 nm). The potentiodynamic polarization tests show that both the two siloxane films coated Ti6Al4V samples have more positive corrosion potential and one order of magnitude lower corrosion current density than the substrate, indicating the corrosion resistance of Ti6Al4V can be improved by depositing siloxane film on its surface. In particular, as the surface is more compact and cross-linked, the SiO_x-like film has better corrosion resistance than the organosilicon film.     

Discrepancies between roughness measurements obtained with phase-shifting and white-light interferometry

Discrepancies between phase-shifting and white-light interferometry have been observed in step-height and surface roughness measurements. The discrepancies have a strong relation to the roughness average parameter of the surface. The skewing effect, which mainly occurs in the vicinity of peaks, valleys, and edges of the sample, causes this problem in white-light interferometry of step height. For roughness, two possible sources of the discrepancy are considered.     

Plastic anisotropy of ultra-thin rolled phosphor bronze foils and its thickness strain evolution in micro-deep drawing

Metal foils are highly advantageous for producing microcomponents with high-aspect-ratio three-dimensional shapes by miniaturizing the process dimensions in sheet-metal-forming technologies. To characterize existing rolled metal foils at manufacturing sites and to clarify the impact of its strong anisotropic properties on micro-sheet formability, tensile tests and micro-deep drawing tests were performed on phosphor bronze foils with thicknesses of 20–300 μm. Focusing on the Lankford value (r-value) as a useful parameter for conventional sheet-metal-formability, the relation between the r-value of ultra-thin rolled foil and its applicability in micro-deep drawing is investigated. Ultra-thin rolled foil is characterized with a higher r-value due to the strong texture of {1 1 0} and {1 1 1} textures. Although the in-plane tendency of the r-value showed a strong correlation with the thickness distribution of micro-drawn cups, the obtained higher r-value for thinner foils does not correspond to the lower formability of thinner metal foils. As relevant parameter for indicating the forming limit for thin-rolled metal foils, the nonuniformity in thickness due to surface roughening is introduced. The importance of a geometrical anisotropy, such as orientation of surface topography and defects, for the unstable deformation of ultra-thin rolled metal foils is experimentally demonstrated.     

Paper‐derived stoneware ceramics: Improvement of properties by calendering and evaluation of pyroplastic deformation behaviour

Paper‐derived ceramics are a new approach to produce thin, lightweight ceramic structures. These ceramics are derived from preceramic papers produced in a paper technological process. The amount of inorganic filler (e. g. stoneware) in the paper is increased up to 85 wt‐%. By firing at high temperatures the cellulose fibres are pyrolized and the inorganic content is sintered. One part of the technological process to produce papers is calendering. A calender consists of two rolls, between which the paper is rolled under pressure. By calendering the paper thickness is reduced and the surface of the paper is improved. Different calender parameters (pressure and temperature) are applied to the preceramic paper and ceramic properties like strength, density and surface roughness are investigated. The strength of paper‐derived stoneware ceramics can be improved by 125% to over 185 MPa by optimizing the calender process. Additionally paper‐derived stoneware ceramics were fired at 1180°C, 1200°C and 1220°C for 1 and for 2 hours at different support distances (10–150 mm) in order to determine the dependence of pyroplastic deformation on support distance. The results show a linear increase between maximum deformation and support distance by 1 hour dwell and a polynomic at 2 hours dwell. Furthermore the Pyroplastic Index (PI) was evaluated according to recent literature. The Pyroplastic Index was correlated to the amorphous content of the differently fired samples. With longer dwell and/or higher temperature the content of amorphous phase increases which lowers the bulk viscosity and supports the pyroplastic deformation. These investigations should give a direction for using paper‐derived stoneware ceramics for special design applications. Paper‐derived ceramics can be systematically deformed in many possible ways to achieve specific shapes.     

Compression‐compression fatigue performance of aluminium matrix composite foams reinforced by carbon nanotubes

The compression‐compression fatigue performance of carbon nanotube (CNT) reinforced aluminium matrix composite foams (AMCFs) were investigated. The ε‐N curves of AMCFs are composed of three stages (the elastic, strain hardening, and rapid accumulation stages), while the fatigue strain of AMCFs accumulates very rapidly in stage III compared with Al foams. The fatigue strength of AMCFs with CNT contents of 2.0, 2.5, and 3.0 wt% increases by 6%, 44%, and 102% than Al foams, respectively. Different from Al foams' deformation of layer‐by‐layer, the main failure modes of AMCFs are the brittle fracture and collapse of pores within significant shear deformation bands under fatigue loading. The uniform distribution of CNTs and good interfacial bonding of CNTs and Al matrix is the important factor for the improvement of fatigue properties of AMCFs.     

Untersuchung der Haftmechanismen kaltgasgespritzter Al‐Schichten auf Al2O3

The metallization of ceramics by means of cold gas spraying has been in the focus of numerous publications in the recent past. However, the bonding mechanisms of metallic coatings on non‐ductile substrates are still not fully understood. Former investigations of titanium coatings on corundum revealed that a combination of recrystallisation induced by adiabatic shear processes and heteroepitaxial growth might be responsible for the high adhesions strengths of coatings applied on smooth ceramic surfaces. In the present work, it is intended to examine the interface area of cold gas sprayed aluminum on alumina substrates. Besides a variation of powder fraction and substrate temperature, it is investigated if a downstream heat treatment has an influence on tensile strength and hardness of the coatings. The splat formation of single particles is investigated by means of scanning electron microscopy, while a high resolution transmission electron microscope is used to examine the Al/Al₂O₃ interface. First results suggest that mechanical clamping is the primary bonding mechanism on polycrystalline coatings with a sub‐micrometer‐scaled surface roughness, while heteroepitaxial growth is the main bonding mechanism for Al coatings on single‐crystalline, atomically smooth sapphire (α‐Al₂O₃) substrates. Heteroepitaxy is promoted by deformation‐induced recrystallisation of the cold gas‐sprayed aluminum.     

The fatigue behavior and mechanism of FV520B-I with large surface roughness in a very high cycle regime

The fatigue properties of FV520B-I up to 10⁹ cycles when the surface roughness R_a ≈ 0.6 were tested and compared with two groups of previously obtained test results. The test results showed that the S-N curve continuously moved downward and the transition stress at which the crack origin changed from the surface to the subsurface decreased with an increase of surface roughness, and the conventional fatigue limit finally appeared. The initiation mechanism of subsurface cracks in a very high cycle fatigue regime was independent of surface roughness. The surface fatigue limit and the high cycle fatigue life were predicted by relevant models. The competition mechanism between surface cracking and subsurface cracking was further discussed.     

Effect of rolling strain on microstructure and tensile properties of dual-phase Mg–8Li–3Al–2Zn–0.5Y alloy

This study was conducted to discuss the effect of rolling strain on microstructure and tensile properties of dual-phase Mg–8Li–3Al–2Zn–0.5Y (wt%) alloy, which was prepared by casting, and then homogenized and rolled at 200?°C. The rolling process was conducted with 10% reduction per pass and five different accumulated strains, varying from 10% to 70%. The results indicate that the as-cast and as-rolled Mg–8Li–3Al–2Zn–0.5Y alloys are composed of α-Mg, β-Li, AlLi and Al₂Y phases. After rolling process, anisotropic microstructure was observed. α-Mg phase got elongated in both rolling direction and transverse direction with the addition of rolling strain. Consequently, the strength of the alloy in both directions was notably improved whereas the elongation declined, mainly caused by strain hardening and dispersion strengthening. The tensile properties of the as-rolled alloys in the RD, no matter the YS, UTS or the elongation, are higher than those of the TD due to their larger deformation strain and significant anisotropy in the hcp α-Mg phase. In addition, the fracture and strengthening mechanism of the tested alloys were also investigated systematically.     

Organic light-emitting devices based on solution-processible quinolato-complex supramolecules

This paper discusses a new type of supramolecular material tris{5-N-[3-(9H-carbazol-9-yl)propyl]-N-(4-methylphenyl)aminesulfonyl-8-hydroxyquinolato} aluminum(III), Al(SCarq)₃, which we synthesized using three 5-N-[3-(9H-carbazol-9-yl)propyl]-N-(4-methylphenyl)aminesulfonyl-8-hydroxyquinoline as bidentate ligands. The peak photoluminescence in the solid phase appears at 488 nm. In cyclic voltammetric measurement, two oxidation peaks, which were obtained at ?5.6 and ?5.9 eV, correspond to HOMO sites of carbazoyl and aluminum quinolates, respectively. In the investigation of solid morphological thin film, the flat surface was investigated using an atomic force microscope. The root mean square (rms) and mean roughness (R_a) were respectively measured to be 0.427 and 0.343 nm. For the fabrication of organic light-emitting devices (OLEDs) using spin-coating techniques, the turn-on voltage and maximum luminescence of the optimized electroluminescence device, glass/ITO (20 nm)/PEDOT:PSS (75 nm)/Al(SCarq)₃ (85 nm)/BCP (8 nm)/LiF (1 nm)/Al (200 nm), were respectively 9.6 V and 35.0 cd m^?2. Due to the electroplex formation between the carbazole (electron-donor) and the aluminum quinolates (electron-acceptor) moieties under an applied DC bias, the chromaticity of electroluminescence shifted to green-yellow with 1931 CIE_x,y (0.40, 0.47).     

Strain hardening and orientation hardening/softening in cold rolled AA 5052 aluminum alloy

The tensile properties of cold rolled sheets were measured for the hot band and annealed hot band of AA 5052 aluminum alloy. The variation in yield strength with rolling true strain was used to represent the hardening rate of cold rolled sheets. The Taylor factor (M?) of cold rolled sheets in tension along the rolling direction was calculated based on the measured orientation distribution functions. The strain hardening and orientation hardening/softening produced by cold deformation were analyzed. The results show that the contribution to the hardening rate of cold rolled sheets comes largely from the deformed microstructure and partly from the texture change. For the annealed hot band the orientation softening occurs at strains below 0.5, while the orientation hardening occurs at strains over 0.5. For the hot band the dM?/dε value is always positive, indicating that orientation softening does not occur.     

Variability of Poisson's Ratio and Enhanced Ductility in Amorphous Metal

Ductile bulk metallic glass of composition 53.0Zr–18.7Cu–12.0Ni–16.3Al (at%) is plastically deformed under uniaxial compression and observed in situ by synchrotron high‐energy X‐ray diffraction. The diffraction patterns reveal the induced atomic strain is orientation dependent. At the onset of plastic deformation, the atomic strain in the compression direction saturates to a close‐nearest‐neighbor distance while atoms relax in the transverse direction. The ever increasing transverse atomic strain expresses in an augmentation of the apparent Poisson's ratio up to ν = 0.5, which is consistent with volume conservation. Contradicting phenomena from linear mechanics, such as the non‐vanishing shear modulus at ν = 0.5 can be explained by the non‐affine character of the deformation, giving rise to characteristics of a localized martensitic phase transformation. The findings explain the often‐reported phenomena such as, the high Poisson's ratio values found in metallic glasses, the partially liquid character of the structure, the free volume increase and the Bauschinger effect.     

Residual stress of as-deposited and rolled wire+arc additive manufacturing Ti–6Al–4V components

Wire + arc additive manufacturing components contain significant residual stresses, which manifest in distortion. High-pressure rolling was applied to each layer of a linear Ti–6Al–4V wire + arc additive manufacturing component in between deposition passes. In rolled specimens, out-of-plane distortion was more than halved; a change in the deposits' geometry due to plastic deformation was observed and process repeatability was increased. The Contour method of residual stresses measurements showed that although the specimens still exhibited tensile stresses (up to 500?MPa), their magnitude was reduced by 60%, particularly at the interface between deposit and substrate. The results were validated with neutron diffraction measurements, which were in good agreement away from the baseplate.

This paper is part of a Themed Issue on Measurement, modelling and mitigation of residual stress.     

Mechanical properties and bonding structure of boron carbon nitride films synthesized by dual ion beam sputtering

The BCN films were synthesized on Si (110) wafers by using dual ion beam sputtering deposition from boron carbide target. The influences of ion assist source energy and N₂ relative flow rate on the surface roughness, mechanical properties and chemical bonding structure of BCN films were investigated systematically. The surface roughness was measured using non-contact optical surface profilometer and the mechanical properties of BCN films were evaluated with nano-indenter. The BCN films were characterized by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. The results showed that the BCN films' surface roughness varied in the range of 5–15 nm, and their hardness and reduced elastic modulus fluctuated in the scope of 18–29 GPa and 192–229 GPa, respectively. When the BCN films' surface roughness varied in the range of 8–12 nm, the values of hardness and reduced elastic modulus were fluctuated slightly. The BCN films with the smoothest surface (R_a = 5 nm) and the highest hardness of 28 GPa were obtained at the ion assist source energy of 200 eV and the N₂ relative flow rate of 50%. The BCN films were amorphous and contained several bonding states such as B–N, B–C and C–N with B–C–N hybridization.     

Microstructure and Mechanical Properties of B4C/6061Al Nanocomposites Fabricated by Advanced Powder Metallurgy

In this study, B₄C/6061Al nanocomposites reinforced with various volume fractions of nano‐sized B₄C particles (B₄C/6061Al NCs) are successfully fabricated by a powder metallurgy route consisting of spark plasma sintering (SPS) and hot extrusion and rolling (HER). The microstructure evolution, phase composition, and mechanical properties of B₄C/6061Al NCs are experimentally investigate. The results show that nearly fully dense (maximum ≈99.21%) as‐SPSed NCs can be fabricated, and this can be attributed to joule heating at the particle contacts and tip spark plasma at the gaps. Nanosized B₄C particles mainly distributed in the 6061Al particles boundaries and formed inhomogeneous network materials in as‐SPSed NCs, while B₄C particles distributed relatively homogeneously in the 6061Al matrix after HER. No new phases are found in the B₄C/6061Al NCs over three deformation stages. The pin effect of the nanosized B₄C can suppress dynamic recovery and improve the driving force for dynamic recrystallization. The mechanical properties are further improved after HER, and the maximum ultimate tensile strength and yield strength for as‐rolled NCs are 305 and 168 MPa. The strengthening mechanisms mainly included load transfer strengthening, dislocation strengthening, Orowan strengthening, and fine‐grain strengthening.