Microstructure Of Deposits Research Articles

Quasi-amorphous, nanostructural CoCrMoC/a-C:H coatings deposited by reactive magnetron sputtering

CoCrMo-C coatings have been deposited on CoCrMo ISO 5832-12 alloy substrates by reactive magnetron sputtering from targets of the same alloy in the C2H2/Ar atmosphere. Their structure evolution with increasing carbon content as well as chemical and phase composition were investigated by X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy. Hardness, Young's modulus, and tribological properties in ambient air were studied. Results show that microstructure of deposits evolves from biphasic, containing γ (fcc) and ε (hcp) metallic phases for low carbon content, through nanocrystalline and amorphous for carbon content between 10 and 20 at.%, to quasi-amorphous nanostructural for even higher content of carbon. Segregation of amorphous carbon leads to the formation of self-organized tubular nanostructure for coatings containing more than 30 at.% of carbon. Introduction of carbon into CoCrMo alloy results in a significant increase of hardness up to 14 GPa and improvement of load-bearing capacity which reached 120 MPa. Compared to uncoated CoCrMo substrate, deposited coatings with high carbon content exhibited superior tribological properties with low friction coefficient around 0.2 and wear rate about 2 × 10−15 m3/N/m.

Deposition of wet microparticles on a fiber: Effects of impact velocity and initial spin

Inertial deposition of wet microparticles on a fiber is numerically investigated by means of discrete element method, taking the effects of impact velocity and initial spin into consideration. Results indicate that the overall, normal and tangential coefficients of restitution remain zero as the impact velocity is lower than a critical value, beyond which these restitution coefficients will increase nonlinearly with increasing the impact velocity. As the impact angle increases, the normal critical impact velocity will show a parabolic decrease and the tangential critical impact velocity will increase linearly, different from dry collisions where the critical impact velocities almost keep constant. As the impact velocity increases, the capture efficiency of wet microparticles by the fiber decreases exponentially, meanwhile the deposit microstructures tend to be denser. It becomes much more complex if clockwise or anti-clockwise initial spins are encountered. The rotational coefficient of restitution will first decrease with increasing the rotational rate until reaching a minimum value, beyond which the rotational restitution coefficient will increase as the rotational rate increases further. However, the overall restitution coefficient changes slightly if the rotational rate is less than about 105 rad/s. The effect of clockwise spin on capture efficiency is found to be distinctly different from that of anti-clockwise spin, and it can be reasonably interpreted by their effects on the overall coefficient of restitution.

Bubble-assisted densification of cobalt deposit during electrowinning in CoCl2 solution

Preparation of densified cobalt deposit during electrowinning can enhance the quality of cobalt products effectively for more application. Here we proposed a bubble-assisted electrowinning process in aqueous solution of CoCl2 to achieve the production of high-density cobalt deposit. Electrowinning processes with and without the assistance of a flow of gaseous bubbles was compared by characterizing the microstructure and phase composition of the cobalt deposits. Moreover, electrochemical behavior of cobalt ions was investigated. Upward flowing bubbles can eliminate unsatisfactory influence of dendritic growth and hydrogen evolution on the product, forming compact deposit consisting of fine cobalt crystals. The flowing bubbles are available for preferred growth of cobalt deposit. The current density used in this work and the concentration of CoCl2 can be elevated to an industrial level without the growth of dendritic product. This bubble-assisted method can be employed for the densification of various metals during electrowinning in aqueous solution.

Modification of the Mechanical Properties of Vitreous Enameled Aluminium Substrate Adding Graphene Flakes

Vitreous enamel is a very interesting materials thanks to its glassy nature, which permits to obtain high protection against corrosion and fire. In addition, the smooth surface with an elevate hardness favors good abrasion resistance. However, a negative aspect of this kind of coating is the high brittleness, which limits the application in case of impulsive or high loads. The nucleation and propagation of cracks leads to the destruction of layer integrity with loss of properties. In recent years graphene has received increasing attention as toughening addictive for organic and metal coatings as well as composite materials. This work is a first attempt to evaluate if the addition of graphene in an enamel deposit can increase its mechanical properties. The graphene was mixed with the dry frit. A milling process was then carried out before producing the torbida. In this way the formation of graphene clusters and accumulations is prevented. The addition of 1% graphene has shown to be effective in the increase of mechanical properties without negative influence on the deposit microstructure.

In Situ NMR Analysis of Lithium Metal Surface Deposits from Electrolytes with Varying Salt Concentrations

Lithium metal constitutes a promising anode material, based on its high theoretical specific capacity (3 860 mAh g−1), and low electrochemical potential (−3.04 V vs. SHE). Non-homogeneous lithium deposition, however, may lead to the formation of reactive high surface area lithium (HSAL) upon cycling, eventually yielding losses of active material, safety risks, and insufficient cycling efficiency. HSAL with e.g. needle-like structure does not only cause higher capacity losses based on side reactions and solid electrolyte interface (SEI) formation, but also could result in internal short circuits by penetration of separator layers. In contrast, low surface area lithium (LSAL) such as nodule-like deposits[1] may reduce unwanted side reactions as well as safety risks. The actual microstructure of lithium deposits is governed by extrinsic factors including cell pressure, temperature, current densities, as well as surface properties of lithium metal electrodes but also the explicit choice of electrolyte constituents, and is therefore crucial for the overall cell performance.[2] In this work, highly concentrated electrolytes are exploited to achieve improved cycling stability and reduced HSAL formation, where the formation of a highly robust SEI, more uniform lithium deposits, fewer side reactions on the anode surface, as well as higher viscosity of the electrolytes were identified as key factors.[1, 3] Electrochemical in situ investigations, scanning electron microscopy (SEM) [4] and in situ 7Li nuclear magnetic resonance (NMR) spectroscopy [5, 6] are applied to in detail elucidate lithium deposition phenomena in symmetrical Li/Li cells. In addition to SEM data, in situ 7Li NMR constitutes an alternative, highly viable spectroscopic method that affords (semi-) quantitative and qualitative monitoring of lithium microstructure growth upon cycling, thereby unraveling critical processes during electrodeposition while delivering counterstrategies for unrestrained HSAL growth and options for achieving significantly improved electrochemical performance and life time of the considered cells. [1] J. Qian, et al., Nature Communications, 6 (2015) 6362. [2] X.-B. Cheng, et al., Chemical Reviews, 117 (2017) 10403-10473. [3] L. Suo, et al., Nature Communications, 4 (2013) 1481. [4] G. Bieker, et al., Physical Chemistry Chemical Physics, 17 (2015) 8670-8679. [5] H.J. Chang, et al., The Journal of Physical Chemistry C, 119 (2015) 16443-16451. [6] R. Bhattacharyya, et al., Nature Materials, 9 (2010) 504-510.

Microstructure and Mechanical Properties of Medium Carbon Steel Deposits Obtained via Wire and Arc Additive Manufacturing Using Metal-Cored Wire

Wire and arc additive manufacturing (WAAM) is a 3D metal printing technique based on the arc welding process. WAAM is considered to be suitable to produce large-scale metallic components by combining high deposition rate and low cost. WAAM uses conventional welding consumable wires as feedstock. In some applications of steel components, one-off spare parts need to be made on demand from steel grades that do not exist as commercial welding wire. In this research, a specifically produced medium carbon steel (Grade XC-45), metal-cored wire, equivalent to a composition of XC-45 forged material, was deposited with WAAM to produce a thin wall. The specific composition was chosen because it is of particular interest for the on-demand production of heavily loaded aerospace components. The microstructure, hardness, and tensile strength of the deposited part were studied. Fractography studies were conducted on the tested specimens. Due to the multiple thermal cycles during the building process, local variations in microstructural features were evident. Nevertheless, the hardness of the part was relatively uniform from the top to the bottom of the construct. The mean yield/ultimate tensile strength was 620 MPa/817 MPa in the horizontal (deposition) direction and 580 MPa/615 MPa in the vertical (build) direction, respectively. The elongation in both directions showed a significant difference, i.e., 6.4% in the horizontal direction and 11% in the vertical direction. Finally, from the dimple-like structures observed in the fractography study, a ductile fracture mode was determined. Furthermore, a comparison of mechanical properties between WAAM and traditionally processed XC-45, such as casting, forging, and cold rolling was conducted. The results show a more uniform hardness distribution and higher tensile strength of the WAAM deposit using the designed metal-cored wires.

Flow Kinetics of Molten Silicates through Thermal Barrier Coating: A Numerical Study

Infiltration of molten calcium–magnesium–alumina–silicates (CMAS) through thermal barrier coatings (TBCs) causes structural degradation of TBC layers. The infiltration kinetics can be altered by careful tailoring of the electron beam physical vapor deposition (EB-PVD) microstructure such as feather arm lengths and inter-columnar gaps, etc. Morphology of the feathery columns and their inherent porosities directly influences the infiltration kinetics of molten CMAS. To understand the influence of columnar morphology on the kinetics of the CAMS flow, a finite element based parametric model was developed for describing a variety of EB-PVD top coat microstructures. A detailed numerical study was performed considering fluid-solid interactions (FSI) between the CMAS and TBC top coat (TC). The CMAS flow characteristics through these microstructures were assessed quantitatively and qualitatively. Finally, correlations between the morphological parameters and CMAS flow kinetics were established. It was shown that the rate of longitudinal and lateral infiltration could be minimized by reducing the gap between columns and increasing the length of the feather arms. The results also show that the microstructures with long feather arms having a lower lateral inclination decrease the CMAS infiltration rate, therefore, reduce the CMAS infiltration depth. The analyses allow the identification of key morphological features that are important for mitigating the CMAS infiltration.

Microstructure and mechanical property of high growth rate SiC via continuous hot‐wire CVD

AbstractAn average growth rate of SiC at 3.4‐28.5 μm/min can be achieved via continuous hot‐wire CVD method with input powers of 300‐380 W. Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS) analysis, and X‐ray diffraction (XRD) method, combined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were applied to investigate the microstructure of deposits. At 300 W, amorphous SiC and Si were main products in deposit, which were rapidly replaced by crystallite SiC containing high density stacking faults at 340 W and above. Moreover, with the grain size of SiC increasing from ~25 to ~420 nm, the stacking faults probability decreasing from 0.179 to 0.125, as well as the surface morphology changed from loose‐packed granules to a well‐defined faceted structure with strong (111) texture. Structural changes led to the increase of deposit's Young's modulus from 266.3 to 341.5 GPa, and the mean tensile strength of SiC filament from 1.57 to 3.03 GPa. The successive growth of W/SiC interfacial layer above 360 W resulted in the reduction in mean tensile strength and Weibull moduls of SiC monofilaments, which agrees with the prediction from critical interfacial layer thickness theory.

Quartz grain surface microtextures of dam-break flood deposits from a landslide-dammed lake: A case study

A landslide-dam break flood can cause great damage to the downstream area in a mountainous watershed. The analysis of quartz sand surface textures is a valid microscale method by which to study the genetic mechanisms of flood deposits. However, previous studies have not considered the microstructures of dam-break outburst deposits. In this research, samples from 24 sites along the Diexi dam-break flood deposits in southwestern China were examined and analyzed using scanning electron microscopy (SEM) photomicrographs. In addition, the frequency statistical results of quartz sand microtextures were also compared in different sedimentary environments. On one hand, the upstream end of dam-break flood deposits contained quartz grains with breakage blocks and inherited microtextural characteristics, while the downstream quartz sands are characterized by v-shaped and small conchoidal fractures. The different combinations of quartz sand surface textures indicate the transformation process changing from high energy collisions to low energy collisions over a short distance. On the other hand, the results exhibit the difference between dam-break flood deposits and other sediments such as debris-flow deposits or normal flood deposits, which reveals the potential of this microscopy method for recognition of dam-break flood deposits. The features of quartz sand microtextures also indicate that channel topography, sediment concentration and other factors may play important roles in routing of dam-break flood events, thus we suggest that the quartz sand microtextures can be used as a complementary method rather than the only identification criterion of dam-break flood deposits.

Influence of butynediol and tetrabutylammonium bromide on the morphology and structure of electrodeposited cobalt in the presence of saccharin

Abstract To develop cobalt coating for use in diamond tools, cobalt electrodeposition was conducted in Watts-type baths. The effects of tetrabutylammonium bromide and 2-butyne-1,4-diol on the surface morphology, crystallographic structure, surface roughness and microhardness of thick cobalt deposits were investigated in the presence of saccharin. The addition of tetrabutylammonium bromide had a modest effect on the morphology, crystallographic structure and microhardness of cobalt deposits, while it reduced the surface roughness noticeably. On the other hand, the addition of 2-butyne-1,4-diol favored the formation of a much smoother deposit, but unexpectedly, it dramatically decreased the microhardness as a result of increased grain size, accompanied by the changes both in the morphology from fine-grained structure to lenticular structure and in the crystal structure from (0002) preferred orientation to (10 1 ¯ 0) preferred orientation. With the addition of a mixture of saccharin, tetrabutylammonium bromide and 2-butyne-1,4-diol, a harder and smoother thick cobalt deposit was obtained. To understand the role of these additives, linear sweep voltammetry and transmission electron microscopy were also used to investigate the reduction behavior of Co ions and the microstructure of cobalt deposits, respectively, which were compared with the morphological, structural and mechanical properties of cobalt deposits. Possible mechanisms for the alteration in the properties of cobalt deposits were discussed.

Effect of laser remelting on deposition quality, residual stress, microstructure, and mechanical property of selective laser melting processed Ti-5Al-2.5Sn alloy

Laser remelting (LR) is often used during selective laser melting (SLM) processes to improve the densification degree and top surface quality of the products. However, researches regarding its effects on side surface quality, residual stress, microstructure, and mechanical property are still quite lacking. The influence of the repeated usage of LR on a solidified layer is also not very clear. To address these issues, LR treatments with the cyclic numbers of 1–3 on every solidified layer have been employed during the SLM processes of a Ti-5Al-2.5Sn alloy in this study and their influences on deposition quality, residual stress, microstructure, and mechanical property were researched. The results first indicated that although the improvement in top surface quality and densification degree would be more and more notable with the increase of LR cycles, the side surface quality could not be improved through all the LR treatments. Then, it was shown by the hole-drilling tests on the surface centers of several 15 × 15 × 3 mm3 thin-plate specimens that when 1 cycle of LR was conducted on every solidified layer, the principal residual stress at the hole bottom of the SLM sample could be far beyond that of the non-treated one. In contrast, when 2 or 3 cycles of LR were conducted, the measured principal residual stresses could be reduced to lower than that of the non-treated ones. It was also proved that unlike the non-treated sample which presented a martensite-dominated microstructure with a very weak texture, all the LR-treated samples exhibited two kinds of preferential orientations. Finally, tensile properties of the stress-relieved samples were tested and it was seen that under the combined influence of the densification and the microstructure factors, the elongations of the LR-treated samples became slightly higher than that of the non-treated one whereas the tensile strengths were nearly identical. In summary, this paper demonstrates that there is still limitation in using LR as an auxiliary process of the SLM production of titanium products. The possible increase in residual stress and the formation of strong textures should be particularly considered.

Measurement of temperature-dependent thermal conductivity for PVD Ti0.55Al0.45N ceramic coating by time domain thermo-reflectance method

Accurate measuring temperature-dependent thermal conductivities of (Ti,Al)N ceramic coating is important to quantitatively analyze the thermal effects of (Ti,Al)N ceramic coating on the heat generation and heat transfer during cutting process by PVD (Ti,Al)N coated cemented tools. In this paper, the temperature-dependent thermal conductivities of PVD Ti0.55Al0.45N ceramic coating with coating thickness of 2 µm in the temperature ranges of 25–500 °C are measured by time domain thermo-reflectance method. Firstly, the detailed deposition process and microstructure characterization of PVD Ti0.55Al0.45N ceramic coating are presented. Secondly, the measuring principle and measuring procedure of PVD ceramic coating thermal conductivity with the time domain thermo-reflectance are proposed. Then, the measured thermal properties of substrate are used in the calculation algorithm to obtain the thermal conductivity of Ti0.55Al0.45N ceramic coating at the specific temperature, which can eliminate the variation influences of substrate thermal properties with temperature. Lastly, the obtained thermal conductivities of PVD Ti0.55Al0.45N ceramic coating are verified to be increased with the increase of temperature in the temperature ranges of 25–500 °C. The relationship between the thermal conductivity of PVD Ti0.55Al0.45N coating and temperature can be represented by quadratic polynominal equation. The applied time domain thermo-reflectance method is useful for measuring the temperature-dependent thermal conductivities of similar ceramic coatings in the temperature ranges of 25–500 °C.

Influence of hot isostatic pressing on structure and properties of titanium cold-spray deposits

In this paper, an experimental study of the influence of capsule-free hot isostatic pressing (HIP) on the microstructure and tensile strength of cold-spray pure titanium deposits is considered. It is shown that hot isostatic pressing at 110 MPa and 1173 K significantly changes the microstructure of titanium cold-spray deposit. Total porosity was decreased from 4.3 to 2.2% due to elimination of small-scale porosity (pores with size less than 5 μm), whereas larger pores were not completely closed. Measured ultimate tensile strength (UTS) of as-sprayed and HIP-treated samples was 110 and 480 MPa, correspondingly. Increase of UTS is explained by material diffusion and microstructure changes during the HIP cycle. At the same time, ductility of the samples after HIP was only ~ 8% which is significantly lower than that for the bulk pure titanium.

Effect of pulse current on wear behavior of Ni matrix micro-and nano-SiC composite coatings at room and elevated temperature

Abstract This work presents the effect of the pulse-current on the wear behavior of Ni/nSiC and Ni/μSiC electrodeposits. The deposits microstructure and SiC content were investigated by means of microstructural, crystallographic e chemical analysis. The mechanical properties were evaluated through microhardness measurements. Ball-on-disc tests were performed at both room temperature and 300 °C. Wear rate and mechanisms were determined by morphological and microscopical characterization. The composite-coatings produced under pulse current present enhanced hardness and wear resistance, at both temperatures, linked to microstructural refinement and SiC content. The main wear mechanism observed was triboxidation, accompanied by abrasive wear mainly on the micro-composite coatings.

The microstructure evolution and strengthening mechanism of a γ′-strengthening superalloy prepared by induction-assisted laser solid forming

The microstructure evolution and strengthening mechanism of a γ′-strengthening superalloy prepared by induction-assisted laser solid forming (LSF) were clarified by an investigation of the LSFed IN-738LC alloy with different solution treatments (at 1070 °C, 1120 °C and 1160 °C). The results show that both the as-deposited and heat-treated deposits were dominated by the columnar grains with a width of 100–270 μm. The equiaxed grains mainly originated from the columnar-to-equiaxed transition (CET) during LSF while a small portion of the equiaxed grains originated from recrystallization during the heat treatment, due to the relatively low temperature gradient in the induction-assisted LSF. Fine grain strengthening was still working in the columnar-grains dominated LSFed IN-738LC deposits. However, γ′ strengthening was found to play a major role in the strengthening mechanism by comparing the microstructure and properties of different heat-treated deposits. The γ′ dissolution and coarsening kinetics were calculated and the interaction mechanism between the dislocation and γ′ phase in LSFed IN-738LC was clarified. The deformation of the coarse (d > 200 nm) near cubic γ′ was found to be beneficial to the increase of ultimate tensile strength (UTS) and elongation (EL), while a brittle continuous phase formed at the grain boundary (GB) which was rich in Ni, Cr, Al and Ti was found to be detrimental to the deposit's plasticity. The γ′ phase deformation and desquamating, carbide breaking and GB phase breaking are the predominant fracture failure mechanisms. On the basis of relatively high strength, the IN-738LC deposits with a lower yield strength (YS)/UTS ratio than the cast and SLMed ones were obtained when solution-treated at 1070 °C.

Effective inhibition of zinc dendrites during electrodeposition using thiourea derivatives as additives

The electrodeposition of zinc with thiourea derivatives as additives was investigated. The dynamic deposition and dissolution processes of zinc deposits were monitored in situ with synchrotron radiation X-ray imaging to reveal the inhibition role of additives on zinc dendrites. The real-time images show a large amount of zinc dendrites grow directly on the substrate surface in blank electrolyte. The microstructure of zinc deposits is sensitive to the molecular structure of additives. After adding thiourea derivatives, both the nucleation overpotential and the degree of polarization increase in the order of additive-free < thiourea (TU) < allylthiourea (ATU) < methylthiourea (MTU). TU can suppress partly the formation of zinc dendrites but the zinc deposits present a very loose structure. Both ATU and MTU can effectively inhibit the growth of zinc dendrites and get smooth deposits. Moreover, the anodic stripping of deposited zinc in the presence of MTU can proceed with a homogenous dissolution. These results will potentially benefit for the zinc-based rechargeable batteries.

Validation of parameters selection of welding with micro-jet cooling by using method of fundamental solutions

The aim of the paper was analyzing the main welding process with parameters of micro-jet cooling just after the welding process and subjecting this process to verification based on numerical methods. To accomplish this purpose, it was decided to get a varied amount of acicular ferrite (AF) in WMD (weld metal deposit). The high amount of acicular ferrite influences positively impact toughness of weld. During research with different micro-jet parameters the chemical analysis, micrograph tests and Charpy V impact test of the metal weld deposit on pendulum machine were carried out. Independently, it was decided to check the heat distribution in the weld metal deposit using numerical method. The methods allow checking the heat distribution during the use of a new welding process (with micro-jet cooling). A varied amount of acicular ferrite in weld metal deposit (in the range 55–68%) was obtained only because of the possibility of using micro-jet technology. Main micro-jet cooling parameters depend on the number of the jets, micro-jet gas pressure and diameter of cooling jets. This high amount of acicular ferrite is unheard in weld metal deposit in another way or other methods of welding such as MAG or TIG. Micro-jet cooling is the way to steer the microstructure of weld metal deposit. It is important because consequently it could be used to the steering of weld joint structure and mechanical properties (for example impact toughness). The most effective cooling gas is helium. Welding with micro-jet cooling could be treated as a very promising process with a high industrial application. Also, using the Method of Fundamental Solutions, the influence of the heat flow process in WMD after welding with micro-jet cooling was checked. Controlled influence of the heat flow process in the welded joint will allow for proper selection of micro-jet cooling parameters. The first time the possibilities of an innovative and still developing welding process were verified by the Method of Fundamental Solutions.

High rate reactive sputtering of Al2O3 coatings by HiPIMS

High rate reactive sputtering of insulating metal oxide coatings, e.g. alumina (Al2O3), by high power impulse magnetron sputtering (HiPIMS) is of strong interests, as the increased ionization of target species in a HiPIMS process can be used to improve the structure and properties of the coatings. The challenges for reactive sputtering of Al2O3 using HiPIMS include arc suppression on the target surface and low deposition rates due to target poisoning. In this study, AlxOy coatings were reactive sputtered by deep oscillation magnetron sputtering (DOMS), one version of HiPIMS, which generates large oscillatory micro-pulses within long modulation pulses. The process stability was evaluated by measuring micro arc counts on the target at different combinations of on and off times of the oscillatory micro-pulses. Virtually arc-free depositions for Al2O3 were observed by utilizing combinations of short oscillation micro-pulse on time (<4 μs) and longer oscillation micro-pulse off time (>40 μs) at a peak target current of 130 A. The mechanisms of generating arc-free discharge for insulating coating reactive sputtering using deep oscillatory micro-pulses are discussed. The hysteresis behavior with open-loop for reactive sputtering of AlxOy using DOMS was studied. The closed-loop control for reactive sputtering was achieved by controlling the oxygen partial pressure (PO2) using the feedback signal from a remote penning plasma emission monitoring sensor, which generates an ionized sample plasma away from the deposition zone. The deposition rate, microstructure, and mechanical and optical properties of the AlxOy coatings deposited at different PO2 in the transition region were investigated. Up to 56% of the metallic deposition rate has been achieved for obtaining stoichiometric Al2O3 coatings by controlling the PO2 around 0.0143 Pa. The DOMS Al2O3 coatings show improved mechanical properties and optical transmission as compared to those obtained by pulsed dc magnetron sputtering reported in the literature.

Investigation into the Ash Deposits in a Coal-Fired Traveling Grate Boiler with Ammonia Present in the Flue Gas

A study was undertaken of the ash deposits on different positions in a 140 MW coal-fired traveling-grate boiler with ammonia present in the flue gas. Several ash samples were collected from the slag tubes and three-stage heat exchanger of this boiler. The ash samples were examined using the combination of X-ray fluorescence and X-ray diffraction analysis to obtain their chemistry and mineralogy. Scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis was conducted to determine the microstructure and compositions of the ash deposits. The results showed that the fouling of the slag tubes was mainly attributed to Fe2O3. At the three-stage heat exchanger, complicated sulfates played an important role in the deposition formation. The deposition mechanism of sulfates gradually changed with the gas temperature at different stages of the heat exchanger. NH4Al(SO4)2, Na3Fe(SO4)3, and KAl(SO4)2 were more prone to deposit on the surface of the heat exchanger than ammonium salts [e.g., (NH4)3H(SO4)2] at higher temperatures (>340 °C). (NH4)3H(SO4)2 was found in the ash deposits at the outlet of the second and third stages of the heat exchanger, where the flue gas temperature was below 240 °C.

Microstructure control of cold-sprayed pure iron coatings formed using mechanically milled powder

Nanocrystalline metallic materials with grain sizes of less than 100 nm exhibit high strength and unique characteristics, such as high levels of ductility and corrosion resistance. Mechanical milling techniques can be used to produce large amounts of nanocrystalline metallic powder. However, it is essential to perform a sintering treatment, which induces coarsening of the crystal grains, to form a bulk material from the mechanically milled powder. The cold spray technique is expected to effectively form a nanocrystalline metallic coating using the mechanically milled powder. This study focuses on pure iron coating, which is formed by a cold spray technique using mechanically milled pure iron powder. Pure iron powder, with crystal grains of ~100 nm, was obtained by a mechanical milling process, and was deposited onto a low-carbon steel substrate by cold spraying. It was confirmed that a dense, nanocrystalline coating with a hardness of over 300 HV can be achieved; however, the deposition efficiency of this coating was half of that of the coating formed using the as-received powder. The improvement of the deposition efficiency and properties of such coatings was attempted using powder mixtures comprising both milled and as-received powders. The results indicate that the deposition efficiency and microstructure, directly linked to the mechanical properties of the coating, can be controlled by varying the content of the as-received powder of the mixture.