Mechanism Of Surface Roughening Research Articles

Effect of pre-treatment temperatures on the oxidation behaviors and surface roughening mechanisms of NiCoCrAlYHf coating

The NiCoCrAlYHf coating has been widely used as a bond layer in thermal barrier coating system, of which the oxidation behaviors and surface roughening mechanisms in service are greatly influenced by pre-treatment temperature. To clarify the effects of pre-treatment temperatures, the characteristics of the pre-treated coatings and isothermally exposed coatings were characterized and analyzed, including microstructure, weight change, TGO thickness and surface roughness. When the pre-treatment temperature was relatively low, the coating surface did not form a TGO with integrity. When the pre-treatment temperature was relatively high, mixed oxides were generated on the coating surface during the pre-treatment process, which damaged the integrity of TGO. The overall performance of the coatings pretreated at too high or low temperatures was insufficient. Moreover, the phase transformation of coating is strongly correlated with diffusion of Al element. The pre-treatment process could change the as-deposited coating from a single-phase to a duplex-phase structure due to the diffusion of Al element. In the oxidation process, the duplex-phase structure changed to a single-phase structure due to the consumption of Al element. In addition, it was found that there was a competitive relationship between grain sliding caused by high-temperature stress and Al2O3 growth in the coating depression, which was one of the main surface roughening mechanisms of NiCoCrAlYHf coating.

Li Morphology Evolution during Initial Cycling in a Gel Composite Electrolyte

Li metal anodes are the potential solution for high-energy batteries. One of the challenges of applying such a high-energy anode is Li dendrite growth, which results in short-circuit and thermal runaway. Current battery research focuses on developing solid electrolytes to serve as a physical barrier to prevent dendrite growth. However, the Li morphology change during plating and stripping, and the mechanisms of how Li dendrite grows and propagates into a complex composite solid electrolyte are poorly understood. Understanding and controlling Li morphology evolution, dendrite formation, and growth during cycling are crucial to developing dendrite suppression strategies for solid electrolytes and enabling high-energy lithium metal batteries.In this work, Li morphology evolution during initial cycling in a crosslinked PEO-based gel composite electrolyte full cell with NMC 811 cathode is monitored via post-mortem SEM. The results show that severe surface pitting occurs as early as the second stripping cycle. Pit formation and continuous dissolution is the main cause of Li surface roughening and dendrite growth mechanism in the model gel composite electrolyte. Comparing Li dendrite growth mechanisms in liquid, polymer, and solid electrolytes, the observed dendrite growth mechanism resembles that of the liquid electrolyte the most. This study suggests that strategies to improve the electrochemical reversibility of electrodeposited Li reported in liquid electrolytes to control Li morphology and prevent dendrite growth may be transferrable in a gel electrolyte.This work is sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory (ORNL), managed by UT-Battelle, LLC for the U.S. Department of Energy under Contract No. DE-AC05-00OR22725. Part of the measurements was performed at the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences.

Multi-dimensional study of the effect of early slip activity on fatigue crack initiation in a near-α titanium alloy

During service of gas turbine engines, high cycle fatigue of titanium is a leading cause of component failure highlighting the need for better understanding of the crack initiation mechanism to predict initiation sites. In this study, the relationship between plastic slip activity and fatigue crack initiation was investigated in a near-α titanium alloy using cyclic four-point bending at up to 90% of the proof stress. The finding from surface characterization show that plasticity at such low stress levels was dominated by the basal slip and two types of cracking were seen parallel to basal slip traces. Detailed 3D analysis of both crack types highlighted out-of-plane Burgers vector activity for the observed basal slip associated with crack initiation, consistent with the classic surface roughening mechanism. The transgranular crack initiation was accompanied by the formation of crystallographic facet which was identified to be 6° away from the basal plane due to additional prismatic slip activation during multi-step crack formation. The intergranular crack facet along the boundary between primary α grain pairs, which have their c-axes aligned nearly parallel to each other but with mis-aligned prismatic planes, was formed by an easy cleavage in one step along the basal plane. Statistical evaluation demonstrated that grains combining a moderately high Schmid factor for basal slip, high resolved tensile stress along the c-axis and the Burgers vector being oriented strongly out-of-surface plane favoured transgranular crack initiation. Based on those observations a new parameter involving these three geometrical factors was developed to predict transgranular surface crack initiation sites.

Effect of Phase Transformation on Surface Roughening Behavior in Austenitic Thin Metal Foils

Stainless steels have wide application in the field of micro manufacturing industry. The size effect occur in thin metal foils because of low number of grain. Martensitic phase transformation (MPT) occur after plastic deformation subjected to stainless steel thin metal foils. Beside that, free surface roughening occur in thin metal foils after plastic deformation. The surface roughening mechanism in stainless steel thin metal foils after plastic deformation such as uniaxial tensile test not yet clarified well. MPT and grain misorientation (GMO) have huge effect to surface roughening behavior in stainless steel thin metal foils. The effect of GMO and MPT to surface roughening in SUS 316 and 304 thin metal foils were studied through uniaxial tensile stress state, repeated five times in 6% strain level for one time strain and 30% strain for the total of strain level. After that, an Scanning Electron Microcope-Electron Backscatter Diffraction (SEM-EBSD) analysis applied to 304 and 316 thin metal foils. The result showed that in stainless steel thin metal foil, surface roughening increase proportional both in fine gain (grain size 1,5 μm) and in coarse grain (grain size 9,0μm). The surface roughening in coarse grain, increased higher than in fine grain. The grain strength in SUS 304 is more inhomogeneous compared to SUS 316 that shown by SEM-EBSD results and as a result, increasing ratio of the surface roughness (Ra) is higher in fine grain and coarse grain of SUS 304 compared to SUS 316. The inhomogeneity of the grain strength in SUS 304 thin metal foil is higher than SUS 316 thin metal foil as shown by SEM-EBSD result. Furthermore, the increased surface roughness in stainless steel 304 is higher than stainless steel 316 thin metal foil both in fine grain and coarse grain. Key words : Surface roughening, Martensitic phase transformation (MPT), Grain Misorientation.

Investigations of ion irradiation induced surface damage features in zirconium alloys by atomic force microscopy

Continuous Kr ion irradiation at 400 °C with damage level up to 40 dpa was performed in Zircaloy-4 to investigate the surface damage features of α-Zr matrix. Atomic force microscopy confirms that the irradiation induced surface damage is primarily in the form of damaged pits. The increasing Ra value indicates the gradual surface roughening process with continuous irradiation. The irradiation-induced surface roughening mechanism is suggested as three stages: stage I, the formation and growth along depth direction of damaged pits at low doses; stage II, the extension of damaged region within the surface; stage III, layer by layer removal of surface atoms once the undamaged matrix is depleted.

Multi-Dimensional Study of the Effect of Early Slip Activity on Fatigue Crack Initiation in a Near-α Titanium Alloy

During service of gas turbine engines, high cycle fatigue of titanium is a leading cause of component failure highlighting the need for better understanding of the crack initiation mechanism to predict initiation sites. In this study, the relationship between plastic slip activity and fatigue crack initiation was investigated in a near-α titanium alloy using cyclic four-point bending at up to 90% of the proof stress, with the finding from surface characterization that plasticity at such low stress levels was dominated by the basal slip and two types of cracking were seen parallel to basal slip traces. Detailed 3D analysis of both crack types highlighted out-of-plane Burgers vector activity for the observed basal slip associated with crack initiation, consistent with the classic surface roughening mechanism. The transgranular crack initiation was accompanied by the formation of crystallographic facet which was identified to be 6° away from the basal plane due to additional prismatic slip activation during multi-step crack formation. The intergranular crack facet along the boundary between primary α grain pairs, which have their c-axes aligned nearly parallel to each other but with mis-aligned prismatic planes, was formed by an easy cleavage in one step along the basal plane. Statistical evaluation demonstrated that grains combining a moderately high Schmid factor for basal slip, high resolved tensile stress along the c-axis and the Burgers vector being orientated strongly out-of-surface plane favoured crack initiation. Based on those observations a new parameter involving these three geometrical factors was developed to predict surface crack initiation sites.

Multi-Dimensional Study of the Early Slip Activity on Crack Initiation in a Near α Titanium Alloy

During service of gas turbine engines, high cycle fatigue of titanium is a leading cause of component failure highlighting the need for better understanding of the crack initiation mechanism to predict initiation sites. In this study, the relationship between plastic slip activity and fatigue crack initiation was investigated in a near-a titanium alloy using cyclic four-point bending at up to 90% of the proof stress. Detailed surface characterization demonstrates that plasticity at such low stress levels is dominated by basal slip. Two types of crack mode were seen among four short cracks. Transgranular cracking parallel to basal slip traces was observed within three primary a grains and one case of intergranular cracking was seen along the boundary between primary a grain pairs, which have their c-axes aligned nearly parallel to each other but with mis-aligned prismatic planes. Detailed 3D analysis highlights out-of-plane Burgers vector activity for the observed basal slip associated with crack initiation, consistent with the classic surface roughening mechanism. The plane of the short crack was identified to be a basal facet and lattice rotation around the c-axis close to the crack plane suggests additional prismatic slip activation during multi-step crack formation. Statistical evaluation highlighted that grains combining a moderately high Schmid factor for basal slip, high resolved tensile stress along the c-axis and the Burgers vector being orientated strongly out-of-surface plane favour crack initiation. Based on those observations a new parameter involving these three geometrical factors has been developed that predicts surface crack initiation sites.

Effect of Martensitic Transformation and Grain Size on the Surface Roughening Behavior in SUS 304 and SUS 316 Thin Metal Foils

The surface roughening and martensitic phase transformation (MPT) of SUS 304 and SUS 316 were studied through two experiments: a uniaxial tensile stress test, repeated for five cycles, and an Scanning Electron Microscope–Electron Backscaterr Diffraction (SEM-EBSD) investigation. The MPT and martensitic volume fraction (Mf) were evaluated following the tensile test. The correlation between MPT, Mf, and surface roughening behavior was investigated. The experiment showed that an increase in the strain level from 0.4% to 1% increased the MPT and Mf, which transformed from a metastable austenitic phase in SUS 304. The increased strain level increased the surface roughening for various grain sizes (Dg), from fine grain (Dg < 3 μm) to coarse grain (Dg ≥ 3 μm). SUS 304 and SUS 316 are used so that the surface roughening mechanism between SUS 304 and SUS 316, with different phase conditions and at a similar Dg, can be determined. The results showed that the surface roughening increased for both fine and coarse grain at strain levels of 0.4% and 1%; however, a larger increase of surface roughening was obtained for coarse grain. In coarse grain, surface roughening increased significantly not only with a low MPT, but also with a low grain deformation. In coarse grain, the surface roughening increased proportionally to the strain level (εp) because of the low MPT and weak grain. In fine grain, the surface roughening did not increase proportionally to the εp because of the high MPT, which increased the grain strength in SUS 304. In the fine grain of SUS 304, the increase of surface roughening was nearly the same both at strain levels of 0.4% and 1%, because the MPT and Mf were nearly the same. The surface roughening with the same εp and almost the same Dg in SUS 304 and SUS 316 fine grain was nearly the same, because the grain deformation almost produced the same relative inclination between neighboring grains in the initial state direction to the surface. The inter-grain movement changed the grain orientation. Based on kernel average misorientation (KAM) mapping, the local grain misorientation in SUS 304 was higher than that in SUS 316. This indicated that the fine-grain SUS 304 is harder than the fine-grain SUS 316. There is no MPT in SUS 316 because of the higher austenitic phase, which is affected by the austenitic former, such as Ni.

Toughness evaluation of poly(butylene terephthalate) nanocomposites

This research investigates the toughness of poly(butylene terephthalate) (PBT) nanocomposites, based on mechanical characterizations and subsequent microstructural observations. Montmorillonite (MMT) and nano-precipitated calcium carbonate (NPCC) were selected as nano-reinforcing phases. Notched Izod impact test, tensile test as a quasi-static loading, and mode-I fracture toughness test, were conducted to evaluate the toughness. The corresponding fracture surfaces were analyzed using scanning electron microscopy (SEM). Tensile test results were linked with the morphologies of corresponding tensile-fractured surfaces. The results indicated that massive fibrillation in PBT/MMT nanocomposites dissipated lower energy relative to the large plastic deformation that was induced in the fracture surface of pure PBT. For PBT/NPCC samples, however, surface roughening mechanism was observed to increase the toughness in the tensile test, as compared to plain PBT. The fracture morphology of compact tension (CT) specimens exhibited a low extent of plastic deformation for pure PBT. However, the crack initiation zone of both nanocomposites showed a fibrillated morphology, leading to the increase of stress intensity factor up to 57% and 45% with the application of MMT and NPCC, respectively, as compared to pure PBT. The crack propagation region was, however, associated with shallow fibrils for MMT and local polymer yielding for NPCC.

Characterization of the microscale forming limit for metal foils considering free surface roughening and failure mechanism transformation

The failure behavior of metal foils is greatly influenced by size effect and surface roughness. To explore and characterize the effects induced by the two factors on the formability and failure mechanism in microscale plastic deformation, the micro-scaled forming limit of metal foils (μ-FLC) was investigated by physical experiment and theoretical modeling. A sectionalized failure criterion was proposed according to the modified Considère and Parmar-Mellor-Chakrabarty (PMC) models, which considers the coupled effects of free surface roughening and failure mechanism transformation at microscale. In detail, the modified Considère criterion coupled with the free surface roughening was proposed to predict the right-hand-side μ-FLC, while the PMC model by adjusting the surface roughness parameter was developed to construct the left-hand-side μ-FLC. The physical experiment suggests that the magnitude of surface roughness caused by free roughening can be up to 10˜20% of foil thickness until the fracture of the sample. The failure mechanism of metal foils in microscale deformation changes from localized necking to diffuse instability with the transformation of stress state from uniaxial to equi-biaxial tension. Furthermore, the increasing grain size or the decreasing foil thickness promotes the transformation of failure pattern from the localized necking to diffuse instability. The experimentally determined μ-FLC descend with the increasing grain size and the decreasing foil thickness. The original Considère criterion and Marciniak-Kuczynski (M-K) model are inappropriate for the determination of μ-FLC as they do not consider the significant effects of free surface roughening and failure mode transformation on the micro-formability of metal foils. The developed criterion was validated and corroborated by comparing the theoretically determined μ-FLC with the experimental one. All of these findings advance the insight into the ductile fracture and formability of metal foils influenced by size effect and surface roughness and help to improve the microformed product quality and facilitate the applications of microforming technologies.

Rearrangement of Surface Structure of 4<sup>o</sup> Off-Axis 4H-SiC (0001) Epitaxial Wafer by High Temperature Annealing in Si/Ar Ambient

Mechanism of surface roughening caused by the polishing induced subsurface damage on 4o off-cut 4H-SiC (0001) substrate during thermal etching, CVD epitaxial growth, and the subsequent high temperature annealing was investigated in the wide temperature range of 1000-1800°C. Different from the previous study based on a macroscopic characterization by optical microscopy, microscopic characterization based on a scanning electron microscopy (SEM) was employed in this study. By utilizing the SEM operated under various conditions, disordered step arrangements as well as stacking faults and dislocations were imaged. The obtained results revealed that the SFs cause the fluctuation in the step kinetics, resulting in the step bunching formation during the thermal process.

Physical mechanism of surface roughening on the radial core-shell nanowire heterostructure with alloy shell

We proposed a quantitative thermodynamic theory to address the physical process of surface roughening during the epitaxial growth of core-shell NW with alloy layer. The surface roughening origins from the transformation of the Frank-van der Merwe (FM) mode to the Stranski-Krastanow (SK) mode. In addition to the radius of NW core, the composition and thickness of alloy shell could determine the growth behaviors due to their modulation to the strain. The established theoretical model not only explains the surface roughening caused by the alloy shell layer, but also provides a new way to control the growth of core-shell NW.

Study on Crack Branching Condition for Brittle Crack Propagation in Steels

In brittle material like glass, it is observed that crack branches when the crack speed gets more than the material critical velocity. When the crack branches, the driving force at running crack tip gets lower, so it is possible to prevent the structure from serious failure. In this paper, using the devised under-matched welded joint to implement the higher crack velocity, the mechanism of crack branching and surface roughening is investigated by experiments and the dynamic Finite Element Method (FEM) analysis.In the result, crack surface roughening happens when the crack velocity is around 800m/s, contrary to the findings of the elastic dynamic fracture mechanics. FEM analysis was conducted to explain the phenomenon and it is possible that the high stress triaxiality can be the critical condition for crack branching.

Kinetic surface roughening and wafer bow control in heteroepitaxial growth of 3C-SiC on Si(111) substrates.

A thin, chemically inert 3C-SiC layer between GaN and Si helps not only to avoid the “melt-back” effect, but also to inhibit the crack generation in the grown GaN layers. The quality of GaN layer is heavily dependent on the unique properties of the available 3C-SiC/Si templates. In this paper, the parameters influencing the roughness, crystalline quality, and wafer bow are investigated and engineered to obtain high quality, low roughness 3C-SiC/Si templates suitable for subsequent GaN growth and device processing. Kinetic surface roughening and SiC growth mechanisms, which depend on both deposition temperature and off-cut angle, are reported for heteroepitaxial growth of 3C-SiC on Si substrates. The narrower terrace width on 4° off-axis Si enhances the step-flow growth at 1200 °C, with the roughness of 3C-SiC remaining constant with increasing thickness, corresponding to a scaling exponent of zero. Crack-free 3C-SiC grown on 150-mm Si substrate with a wafer bow of less than 20 μm was achieved. Both concave and convex wafer bow can be obtained by in situ tuning of the deposited SiC layer thicknesses. The 3C-SiC grown on off-axis Si, compared to that grown on on-axis Si, has lower surface roughness, better crystallinity, and smaller bow magnitude.

Effects of surface treatment with coupling agents of PVDF-HFP fibers on the improvement of the adhesion characteristics on PDMS

Surface treatment of polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) fibers was conducted with coupling agents such as epoxy silane, amino silane, and titanate to improve the adhesion characteristics of PVDF-HFP fibers and polydimethylsiloxane (PDMS). The adhesion strength was largest when 4wt% amino silane was used for surface treatment, showing a 250% improvement compared to the untreated case. Surface roughening and shrinking of the PVDF-HFP fibers were observed after surface treatment, but no chemical bonding occurred between the PVDF-HFP fibers and the coupling agents. It was thus concluded that the improvement of the adhesion characteristics of the PVDF-HFP fibers and PDMS was caused by the physical bonding between them due to the surface treatment with coupling agents. In addition, for the surface roughening mechanism, amino silane infiltration into the PVDF-HFP fibers during the surface treatment, followed by extraction during the drying process, was suggested.

Effects of Extreme Temperature Swings ( <inline-formula> <tex-math notation="TeX">$-\hbox{55}\ ^{\circ}\hbox{C}$</tex-math></inline-formula> to 250 <inline-formula> <tex-math notation="TeX">$^{\circ}\hbox{C}$</tex-math></inline-formula>) on Silicon Nitride Active Metal Brazing Substrates

Reliability of power electronic substrates has been one of the main issues of high-temperature packaging technologies. Widely used direct bonded copper (DBC) substrates, if subjected to wide temperature swings, suffer from copper layers peeling off from a ceramic because of the large thermal stresses resulting from the difference in coefficient of thermal expansion (CTE) between the copper and the ceramic (e.g., $\hbox{Al}_{2}\hbox{O}_{3}$ or AlN). Recently, silicon nitride active metal brazing ( $\hbox{Si}_{3}\hbox{N}_{4}$ -AMB) substrate has been developed with the aim to utilize power electronic substrates for extreme environmental conditions. $\hbox{Si}_{3}\hbox{N}_{4}$ -AMB has superior reliability because of the high mechanical strength of $\hbox{Si}_{3}\hbox{N}_{4}$ . In this paper, the effects of extreme temperature swings ( $-\hbox{55} ^{\circ}\hbox{C}$ to 250 $^{\circ}\hbox{C}$ ) on $\hbox{Si}_{3}\hbox{N}_{4}$ -AMB substrates were investigated to evaluate their applicability to harsh environment. Their high resistance to the peeling off of copper layers was confirmed. However, the surface-roughening phenomenon was observed on the surface of copper layers after the temperature cycling test. The mechanism of surface roughening was analyzed. It was found that the out-of-plane displacement of single grains, which is called the orange-peel phenomenon, occurred on the surface of the copper layers.

Roughening of Free Surface During Sheet Metal Forming

This investigation concerns the effect of the process parameters on surface roughening during plastic deformation. The modeling is based on the assumption that surface roughness is proportional to the maximum shear stress on the surface layer. Therefore, an equation is developed in order to describe the relationship between the surface roughness and such process parameters as the initial roughness, grain size, effective strain, and maximum shear strain ratio on the surface layer. In a tensile test, the surface roughness increases nonlinearly with a normal anisotropic value and linearly with the effective strain or grain size. The surface roughness also increases nonlinearly with the effective stress and the strain hardening exponent under some fixed conditions. The normal anisotropic value must be considered in order to evaluate surface roughening during deformation. The experimental results support the proposed model. The proposed model improves our understanding of the mechanism of surface roughening in sheet metal forming.

Physical Mechanism of Surface Roughening of the Radial Ge-Core/Si-Shell Nanowire Heterostructure and Thermodynamic Prediction of Surface Stability of the InAs-Core/GaAs-Shell Nanowire Structure

As a promising and typical semiconductor heterostructure at the nanoscale, the radial Ge/Si NW heterostructure, that is, the Ge-core/Si-shell NW structure, has been widely investigated and used in various nanodevices such as solar cells, lasers, and sensors because of the strong changes in the band structure and increased charge carrier mobility. Therefore, to attain high quality radial semiconductor NW heterostructures, controllable and stable epitaxial growth of core-shell NW structures has become a major challenge for both experimental and theoretical evaluation. Surface roughening is usually undesirable for the epitaxial growth of high quality radial semiconductor NW heterostructures, because it would destroy the core-shell NW structures. For example, the surface of the Ge-core/Si-shell NWs always exhibits a periodic modulation with island-like morphologies, that is, surface roughening, during epitaxial growth. Therefore, the physical understanding of the surface roughening behavior during the epitaxial growth of core-shell NW structures is essential and urgent for theoretical design and experimentally controlling the growth of high quality radial semiconductor NW heterostructures. Here, we proposed a quantitative thermodynamic theory to address the physical process of epitaxial growth of core-shell NW structures and surface roughening. We showed that the transformation from the Frank-van der Merwe mode to the Stranski-Krastanow mode during the epitaxial growth of radial semiconductor NW heterostructures is the physical origin of surface roughening. We deduced the thermodynamic criterion for the formation of the surface roughening and the phase diagram of growth and showed that the radius of the NWs and the thickness of the shell layer can not only determine the formation of the surface roughening in a core-shell NW structure, but also control the periodicity and amplitude of the surface roughness. The agreement between the theoretical results and the experimental data of the Ge-core/Si-shell NW structure implied that the established approach could be applicable to the understanding and design of various semiconductor core-shell NW structures. Consequentially, we used the established theoretical model to study the epitaxial growth of the InAs-core/GaAs-shell NW structure and predict the surface roughening formation, as well as the periodicity and amplitude of the surface roughness, which provided useful information to theoretically design and experimentally control the epitaxial growth of the radial InAs-core/GaAs-shell NW structure.

Surface roughening evolution in pentacene thin film growth

We report investigation on the surface roughening evolution during the pentacene thin film growth, and it turns out to be the overall hopping-up mass transport from the under layers to the upper layers responsible for the surface roughening. A simple interlayer mass transport model utilizing a roughness evolution ratio has been developed, based on which the extent of the interlayer mass transport is found to be reduced with the film thickness. The surface roughening mechanism is considered to be associated with the gradual phase change with the film thickness due to the pentacene polymorphism.

Plasma impact on 193 nm photoresist linewidth roughness: Role of plasma vacuum ultraviolet light

193 nm photoresist patterns printed by optical lithography are known to present significant linewidth roughness (LWR) after the lithographic step that is partially transferred into the underlayers during plasma etching processes. In this study, we identify the factors that impact the photoresist pattern sidewalls roughness during plasma exposure. Among them, plasma vacuum ultraviolet light (110–210 nm) is shown to be the main contributor to the LWR decrease induced by plasma etching processes. In this paper, we also demonstrate the strong correlation between LWR obtained after plasma exposure and the surface roughening mechanisms taking place on top of the resist patterns.