Continuous Protective Layer Research Articles

Preparation of microencapsulated carbon microspheres coated by magnesium hydroxide/polyethylene terephthalate flame–retardant functional fibers and its flame retardant properties

In order to improve the compatibility between the flame retardants of carbon microspheres coated by magnesium hydroxide (MH@CMSs) and the PET matrix and improve the spinnability of the masterbatch, MH@CMSs have been microencapsulated by PET to obtain microencapsulated carbon microspheres coated by magnesium hydroxide flame retardants – MMH@CMSs.Morphologies and structures of MMH@CMSs have been studied by scanning electron microscope (SEM), transmission electron microscopy (TEM), and FTIR, which showed that an organic shell layer of PET as capsule wall was coated on the surface of MH@CMSs. A series of MMH@CMSs/PET fibers with different MMH@CMSs contents were successfully prepared through the melt-spinning method. The morphology and structure of MMH@CMSs/PET fibers were characterized by SEM and FTIR. The flame retardancy of MMH@CMSs/PET fibers was determined via limiting oxygen index (LOI) test and cone calorimeter. Results showed that the MMH@CMSs/PET fibers possessed optimum flame retardancy when the MMH@CMSs content is 0.6 wt.%, at which the LOI reached a maximum of 25.8, and the pk-HRR, total heat release, and total smoke release were reduced by 27.4, 20, and 13.6%, compared with pure PET fibers, respectively. Moreover, the flame-retardant mechanism was studied by thermogravimetric analysis, thermogravimetric analysis-infrared spectrometry, and the SEM of the residue char, which disclosed that MMH@CMSs enhanced the thermal stability of PET fibers, and promoted PET fibers to form a dense and continuous protective char layer that effectively blocked heat transfer and combustible gas release.

Surface oxidation of heating resistors made from Kanthal AF: Increasing the lifetime of glow plugs

Glow plugs in the automotive industry are based on heating resistors made from FeCrAl alloys, often referred to as Kanthal. Metals and alloys exposed to high temperatures tend to react with the surrounding atmosphere, often resulting in high-temperature corrosion. Some high-temperature alloys use aluminium to form an Al2O3 protective layer in order to improve the oxidation resistance. In the temperature range above 1200 °C, SiO2 and Al2O3 are the two oxides capable of forming an efficient, protective barrier against further oxidation. The alloy nominally requires more than 4 wt % Al to form a continuous protective layer. In this paper we have investigated the Al2O3 formation and growth on the surface of iron aluminides prepared using different heat-treatment parameters to achieve a continuous protective layer on the surface of the heating resistors. SEM/EDX mapping analyses were performed to confirm the elemental distribution on the cross-section of the resistors heat treated in air or pure oxygen, or in a humid H2/Ar atmosphere, for different times and temperatures to achieve a continuous coverage of the oxide layer. The results of different heat treatments and their influences on the oxide growth are compared.

Preparation of carbon microspheres coated magnesium hydroxide and its application in polyethylene terephthalate as flame retardant

A novel flame retardant, carbon microspheres coated magnesium hydroxide (Mg(OH)2@CMSs) was synthesized. The morphology and structure of Mg(OH)2@CMSs was characterized by scanning electron microscope (SEM) and fourier transform infrared spectroscopy (FTIR). Then, Mg(OH)2@CMSs was added into polyethylene terephthalate (PET) to improve the flame retardancy of PET. The prepared composite (Mg(OH)2@CMSs/PET) with only 1 wt% Mg(OH)2@CMSs obtained a LOI value of 27.5% and reached UL94 V-0 rating. The flame retardant mechanism was studied by cone calorimeter measurement, thermogravimetric analysis (TGA), SEM and FTIR. The results showed that Mg(OH)2 and CMSs had synergistic flame retardant effect on PET. And there were two effects for Mg(OH)2@CMSs to improve the flame retardancy of PET. On one hand, CMSs and MgO as the bridge supporting points were dispersed into the basic framework of char layer in order to further strengthen the three-dimensional network structure of char layer. Thus a continuous and compact protective layer was formed, resulting in the decrease of pk-HRR and pk-MLR. On the other hand, the FTIR spectra of Mg(OH)2@CMSs/PET char residues showed that Mg(OH)2@CMSs induced PET degrading into small molecules with CC, which prevented PET from further decomposition, thus increased the amount of residue and decreased the THR value.

Si添加鋼のFe-Zn合金化反応に及ぼすMn添加量の影響

The influence of the Mn content of Si-added steel sheets on the Fe-Zn galvannealing reaction was investigated. Three steel sheets, 1.5 mass%Si-1.4, 1.9 and 2.7 mass%Mn, were annealed in a 10vol%H2-90vol%N2 atmosphere. Si and Mn oxides were analyzed by reflectance Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive X-ray spectrometry. SiO2 and Mn2SiO4 formed as selective oxides at the steel surface after recrystallization annealing. The ratio of the oxide species changed depending on the Mn content in the steel. When the Mn content was lower, formation of SiO2 was promoted and that of Mn2SiO4 was suppressed. In the selective oxide layer which formed on the surface of the 1.5 mass%Si-1.4 mass%Mn steel sheets, Mn2SiO4 formed at the outer side, and SiO2 formed at the inner side. This can be explained by consideration of the thermodynamic oxygen potential gradient. Furthermore, areas where SiO2 mainly formed and those where Mn2SiO4 mainly formed were distributed on the surface of the 1.5 mass%Si-1.4 mass%Mn steel sheets. In this case, the Fe-Zn intermetallic compound (IMC) formed preferentially on the Mn2SiO4 between the zinc coating and the substrate steel after galvanizing, and the Fe-Zn galvannealing reaction was suppressed on the SiO2 layer. It is considered that a dense and continuous protective SiO2 layer acted as a barrier to the Fe-Zn galvannealing reaction.

A study on nanostructured in-situ oxide dispersed NiAl coating and its high temperature oxidation behavior

Improving the efficiency and lifetime of high temperature coatings in a cost-effective way has always been receiving more attention. In this study, it was focused to investigate the high temperature oxidation behavior of thermally sprayed Ni-5wt.% Al coating as a foundation of protective Ni-based alloys under the influence of in-situ formation and dispersion of oxide nanoparticles and grain size refinement. Therefore, high-energy ball-milling process and High Velocity Oxy-fuel (HVOF) system were employed to produce nanocrystalline powders and coatings. Isothermal oxidation evaluation of free-standing coatings was carried out at 950°C. Also the microhardness of the nanostructured coatings was examined and compared with conventional Ni–5Al coatings. The oxidation rate constant (kP) of the modified coating was considerably lower than that of the conventional coating. This phenomenon happened because the continuous protective alumina layer was formed as a thermally grown oxide (TGO) on the nanostructured dilute NiAl from the beginning of the oxidation. The presence of the in-situ alumina nanodispersoids, formed during the milling process, as nucleation sites and numerous grain boundaries as easy diffusion paths facilitates the α-Al2O3 protective layer. The microhardness test results also confirmed the higher hardness of nanostructured coating.

Microstructure, mechanical and oxidation characteristics of detonation gun and HVOF sprayed MCrAlYX coatings

Microstructure, mechanical property and oxidation resistance of MCrAlYX coatings prepared by detonation gun (D-gun) and HVOF spraying were investigated. Lamellar microstructure and uniform microstructure formed in D-gun sprayed MCrAlYX coating and HVOF sprayed coating, respectively. Element redistribution and formation of new phase took place during the detonation process. Besides, the porosity of D-gun sprayed coating was much lower than that of HVOF sprayed coating. On the mechanical property, the micro-hardnesses of the two coatings were in the same level (∼HV 910). However, D-gun sprayed MCrAlYX coating exhibited larger standard deviation of microhardness due to its lamellar microstructure, and exhibited better bend bonding strength owing to the existence of residual compressive stress between the layers and particles. Meanwhile, due to the much more compact microstructure, D-gun sprayed MCrAlYX coating showed superior oxidation resistance to the HVOF sprayed coating. The continuous dense protective layer can form earlier in D-gun sprayed coating and thus suppress further oxidation and control the oxide thickness at a relatively low level.

Sintering in a graphite powder bed of alumina-toughened zirconia/carbon nanotube composites: a novel way to delay hydrothermal degradation

Zirconia-based ceramics have been introduced in biomedical applications, for example, in hip implants. Certain zirconia composites are prone to spontaneously transform from the tetragonal phase to the monoclinic phase during long-term storage in the presence of moisture at low temperatures. This phenomenon is time-dependent and can be accelerated by water or water vapour. Herein, we report strong experimental evidence of a delayed t→m ZrO2 phase transformation in alumina-toughened zirconia (ATZ) and ATZ/multi-walled carbon nanotube (MWCNT) composites when pressureless sintered in air in a graphite powder bed. The m-ZrO2 phase in ATZ and ATZ/MWCNT composites sintered in a graphite powder bed after hydrothermal ageing for 20h at 134°C decreased by 81% and 87%, respectively, compared to an ATZ sample sintered in an alumina powder bed. The enhanced hydrothermal stability could be attributed to the formation of a thin continuous alumina protective layer covering the surface of the ceramic composites.

Carburization behavior of high velocity oxy fuel sprayed metallic coatings at 900°C

Purpose– The purpose of this investigation was to evaluate the performance of high velocity oxy fuel (HVOF) coated SS-310 samples in a carburizing environment.Design/methodology/approach– The carburization behavior of metallic coatings with three different compositions was studied under isothermal carburizing exposure conditions at 900°C for 125 hours. The coatings were deposited on SS 310 substrates using the HVOF technique. The ASTM Standard method was used to evaluate coating adhesion. Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, X-ray diffraction and weight gain were used to evaluate the surface morphology, microchemical composition, phase constitution and degree of environmental protection imparted by the coatings.Findings– The experimental results indicate that Ni-rich coating offered better protection to SS 310 alloy compared to Co-rich coatings in carburizing environments. This was thought to be due to the formation of a continuous protective layer of Cr2O3on the Ni-rich coating surface.Originality/value– The study has direct practical relevance to the petrochemical industry, particularly for refinery applications. In refinery service, SS310 is used in header damper plates. The useful service life of such header plates can be extended by the use of high temperature corrosion resistant metallic coatings. The present investigation highlighted the protection offered by Ni-based HVOF coated SS-310 samples in carburizing environment.

Abrasion resistance of magnesium alloys with surface films generated from phosphonate imidazolium ionic liquids

Surface films formed by treatment with the ionic liquids (ILs) 1,3-dimethylimidazolium methylphosphonate (LMP101), 1-ethyl-3-methylimidazolium methylphosphonate (LMP102) and 1-ethyl-3-methylimidazolium ethylphosphonate (LEP102) on magnesium alloys have been studied. The abrasion resistance of the coated alloys was studied by microscratching under progressively increasing load, and compared with that of the uncoated materials. Abrasion-protective phosphorus-containing films are generated on AZ31B, with an order of abrasion resistance as a function of the IL of LEP102>LMP101>LMP102, with a reduction in penetration depth of a 67% for the sample treated with LEP102 with respect to the uncovered alloy. This is attributed to the formation of a continuous adhered phosphorus-containing film. In contrast, the abrasion resistance of EZ33A alloy is not improved due to the presence of the less reactive Zn-rich phase at the grain boundaries, which prevents the formation of a continuous protective layer. The results are discussed from contact angles, SEM–EDX and XPS analysis.

Flexible and flame resistant poly(lactic acid)/organomontmorillonite nanocomposites

ABSTRACTThe PLA/OMMT nanocomposites were produced using a melt compounding technique with isopropylated triaryl phosphate ester flame retardant (FR; 10–30 parts per 100 resin). The flammability of the PLA/OMMT composites was evaluated with an Underwriter Laboratory (UL‐94) vertical burning test, and their char morphology was studied using scanning electron microscopy (SEM). The thermal properties of the PLA/OMMT were characterized with a thermogravimetric analyzer (TGA) and a differential scanning calorimeter (DSC). The thermal analyses showed that adding FR reduced the decomposition onset temperature (To) of PLA/OMMT. Both PLA/OMMT/FR20 and PLA/OMMT/FR30 showed excellent flame retardant abilities, earning a V‐0 rating during the UL‐94 vertical burning test. A compact, coherent and continuous protective char layer was formed in the PLA/OMMT/FR nanocomposites. Additionally, the DSC results indicated that the flexibility of the PLA/OMMT composites increased after adding FR due to the FR‐induced plasticization. The impact strength of PLA/OMMT was greatly increased by the addition of FR. Flexible PLA nanocomposites with high flame resistance were successfully produced. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41253.

Atomic oxygen resistance of polyimide/silicon hybrid thin films with different compositions and architectures

Silicon (Si)-containing polyimides (PIs) with superior atomic oxygen (AO) resistance are promising materials for space applications. Here, in this study, we present the synthesis and characterization of eight Si-containing PI thin films and evaluate their AO durability. The resulting PI films exhibited high thermal stability and preferable AO resistance but showed slightly reduced mechanical performance relative to pristine PI. The highest optical transparency at 550 nm was observed for PI/octaaminopropylsilsesquioxane, while the lowest value was observed for PI/silica (SiO2) hybrids. X-Ray photoelectron spectroscopic study suggested that the topmost surface of PI was degraded at the early stage and an SiO2 inert protective layer was finally formed on the surface of hybrid films after AO exposure. It is found that Si-containing units of higher oxidation states and with higher Si/O molar ratio are favorable to improve the AO resistance. Dispersion of Si at molecular level contributes to improving anti-AO property as well as optical transparency of the prepared films. The characterization of scanning electron microscopy indicated a continuous SiO2 protective layer was crucial to prevent AO from eroding the bulk matrix.

Nanocrystalline ZnCO3—A novel sorbent for low-temperature removal of H2S

The reactivity of a nanocrystalline ZnCO3 toward H2S (0.2vol% in N2/H2 mixture) at 140–180°C was characterized by thermal gravimetric analysis and by breakthrough curves measurements. We have found that under used conditions transformation of ZnCO3 into ZnS is complete and the rate determining step of the sulfidation is the surface reaction. Such behavior is in strike contrast with that of ZnO whose sulfidation is severely limited by diffusion. The higher reactivity of ZnCO3 in comparison with ZnO is attributed to the different microstructure of ZnS layer formed in these materials after a partial sulfidation. As in ZnO–ZnS transformation the molar volume increases (from 14.5 to 23.8cm3/mol), a continuous protective ZnS layer is formed hampering the access of H2S to the non reacted ZnO core. By contrast, in ZnCO3–ZnS transformation the molar volume decreases (from 27.9 to 23.8cm3/mol), which produces a discontinuous non-protective ZnS layer enabling a complete transformation of ZnCO3 even at 140°C. The higher reactivity of ZnCO3 results in a considerable increase of the breakthrough sulfur capacity of the carbonate in comparison with oxide. The material has therefore a good potential for being used as a disposable sorbent for H2S capture at low temperature.

Preparation of Nickel-Based Nanolubricants via a Facile In Situ One-Step Route and Investigation of Their Tribological Properties

Nickel-based nanolubricants containing size-tunable monodispersed nickel nanoparticles were in situ synthesized in poly-alpha-olefin (denoted as PAO6) via a simple one-step thermal decomposition method with Ni(HCOO)2·2H2O as the Ni source, PAO6 as base oil, as well as oleylamine and oleic acid as the surface-capping agents. The size-dependent tribological properties of as-synthesized Ni-based nanolubricants were evaluated with a four-ball friction and wear tester. The morphology of wear scar and chemical states of some typical elements on worn steel surfaces were analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy. The morphology and structure of Ni nanoparticles separated from as-synthesized nanolubricants were characterized by means of transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectrometry. It has been found that, when the volume of PAO6 in the reaction system is adjusted as 80, 40 and 20 mL, respectively, the size (average diameter) of Ni nanoparticles can be tuned from 7.5 to 13.5 and 28.5 nm. Ni nanoparticles separated from as-synthesized nanolubricants consist of face-centered cubic nanocrystal and have good dispersibility in a variety of organic solvents and good thermal stability as well. Moreover, as-synthesized Nickel-based nanolubricants exhibit good antiwear behavior even at a low Ni concentration of 0.05 % (mass fraction). This is because surface-capped Ni nanoparticles in as-prepared nanolubricants are able to release highly active Ni nanocores and O- and N-containing organic modifying agents that can readily form boundary lubricating film on sliding steel surfaces. In the meantime, Ni nanoparticles with a smaller size are of high surface activity and can be readily deposited on sliding steel surfaces to form a stable and continuous protective layer thereon. As a result, both the boundary lubricating film and the chemically adsorbed and deposited Ni layer contribute to greatly improve the antiwear behavior and load-carrying capacity of PAO6 base stock.

Effect of CrTiAlN Coatings on High-Temperature Oxidation Behavior of H13 Steel

CrTiAlN hard coatings have been deposited on H13 steel for improving the oxidation resistance by Closed Field Unbalanced Magnetron Sputter Ion Plating (CFUMSIP). The phase components, surface and section morphology of H13 steel before and after oxidation were analyzed by XRD and SEM. The results showed that the columnar crystal film possesses three major parts, i.e., the internal Cr bonding layer, the intermediate CrN transition layer, the external CrTiAlN gradient coating. The selective oxidation of Cr element was found on the surface of uncoated H13 steel while oxidation, but a continuous protective layer was not formed because of low Cr content, finally a lot of alpha Fe2O3 particles took place on the surface of H13 steel owing to the oxidation of the matrix element. O element can diffuse much more easily through the coating and oxide scales at 800 °C, and spallation and cracks were not found during the oxidation. These results indicated that the CrTiAlN coating have excellent high-temperature oxidation resistance at 800 °C.

Atomic Oxygen Erosion Resistance of Polysiloxane/POSS Hybrid Coatings on Kapton

Abstract Polysiloxane/POSS hybrid coatings were deposited on Kapton substrate by copolymerizing epoxy-modified polysiloxane resin with trisilanolphenyl-silsesquioxane (POSS) and their atomic oxygen (AO) erosion resistance was tested in a ground-based AO simulator. The surface morphologies and structures of the exposed and unexposed hybird coatings were analyzed by scanning electronic microscopy and X-ray photoelectron spectroscopy. The results indicate that the polysiloxane/POSS hybrid films are capable of providing protection against AO attack. Unlike the polysiloxane coating, no crack was observed in the hybird coatings during AO exposure. The results of XPS analysis indicated that a continuous high-quality protective SiO2 surface layer was formed on on the polysiloxane/POSS hybrid coatings after the AO exposure. It can prevent further degradation of the underlying polymer with increased exposure to the AO flux and provide high-quality erosion protection for these materials. The erosion yield of the polysiloxane/POSS hybrid coatings decreased by a factor of more than two orders of magnitude compared with that of the polyimide film, reduced to 0.4% of Kapton film.

The effects of zirconium diboride particles on the ablation performance of carbon–phenolic composites under an oxyacetylene flame

ZrB2 modified carbon–phenolic (C–Ph) composites are fabricated by the impregnation method, and the ablation performance of the composites is tested using an oxyacetylene flame. In addition, the microstructure of the ablation zone is observed with a scanning electron microscope (SEM). The results indicate that the linear ablation rate decreases with increasing ZrB2 content, and the ZrB2–C–Ph composites have their lowest linear ablation rate when the ZrB2 to phenolic resin weight ratio is 0.09 : 1. The main ablation mechanisms of the composites with low loading of ZrB2 are oxidation and char spallation. When more ZrB2 particles are introduced, a continuous protective ZrO2 layer is properly formed on the ablated surface of the specimen, and the stripping of the protective ZrO2 layer and the oxidation of carbon are the main ablation mechanisms. Furthermore, the forming–stripping process of a protective ZrO2 layer occurred continuously and was repeated during the ablation process.

Surface modification of carbon sorbents with polycarbosilane

Modified carbon sorbents containing silicon oxycarbide, which did not form a continuous protective layer on the surface, were prepared by the liquid-phase supporting of polycarbosilane onto the surface of carbon sorbents followed by thermal treatment. The silicon-containing sorbents were characterized by increased stability to oxidation by atmospheric oxygen at high temperatures (as compared with that of the initial sorbents) with the retention of sufficiently high sorption activity.

Effect of Atomic Oxygen Exposure on Si Implanted Siloxane Coatings in Low Earth Orbit Environment

To improve the AO erosion resistance of siloxane coating, the Si ion implantation with nominal dosage of 7×1016 ions/cm2 in siloxane coatings on the polyimide substrate was carried out by using the plasma immersion ion implanter. The Si implanted siloxane coatings before and after AO exposure in the ground-based AO simulation facility were investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS). The results of mass measurement shown that, in initial stage of AO exposure the implanted sample had a small mass change, and then was stabilized. The erosion yield of the implanted siloxane coating decreased by a factor of more than two orders of magnitude compared with that of the polyimide film, and decreased by a factor of more than an order of magnitude than that of unimplanted siloxane coatings. The results through SEM and XPS indicated that a continuous high-quality protective SiO2 surface layer was formed on the implanted siloxane coatings after the AO exposure. It can prevent further degradation of the underlying polymer with increased exposure to the AO flux and provide high-quality erosion protection for these materials. The Si implanted materials have a markedly increased erosion resistance than unimplanted siloxane coatings in AO environment.

High-temperature oxidation of sprayed NiAl–Re coatings subjected to magnetic treatment

The effect of structural changes that occur in heat-resistant detonation-sprayed NiAl–Re coatings treated in the magnetic field on their oxidation is examined. It is shown that the surface of coatings subjected to magnetic treatment is rich in nickel and layers of nickel oxides form more intensively and to a greater depth (as compared with those without magnetic treatment) until a continuous layer of alumina forms in the scale. The removal of the nickel-rich layer uncovers aluminum-rich surface where a continuous protective layer of Al2O3 scale can be formed in high-temperature oxidation.

Growth of the E. Coli Outer Membrane

The outer membrane (OM) of E. coli is composed of four elements: lipopolysaccharide (LPS), phospholipids, OM proteins, and lipoproteins. Together these elements form a continuous protective layer, defending the bacterium against harsh environments and toxic chemicals. Maintenance of an intact OM requires that synthesis and insertion of new OM components keep pace with bacterial growth. At present, little is known about where new OM is incorporated, or how its growth is regulated. We use video microscopy to examine the behavior of fluorescently labeled LPS and specific OM proteins on the surfaces of growing bacteria. Initially, labeled LPS and OM proteins in an individual cell exhibit a uniform peripheral distribution. As the bacterium elongates, fluorescent spots emerge, subsequently drift apart from one another, and occasionally bifurcate. Arresting bacterial growth with Rifampin halts the motion of the fluorescent spots, resulting in a fluorescence pattern which remains stable over a period of hours. We hypothesize that the appearance and divergence of these fluorescent spots of labeled OM is due to insertion of newly synthesized, unlabeled OM components. We track the motion of these spots on the surfaces of E. coli, and measure the convergence and divergence of adjacent tracks on the periphery of the cell. Our data suggest that new OM is incorporated in patches and distributed non-uniformly, with the bulk of the new material inserted along the lateral walls of the cell and lower rates of insertion in the polar regions of the cell.