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Ferrites based infrared radiation coatings with high emissivity and high thermal shock resistance and their application on energy-saving kettle
Accepted Manuscript Title: Ferrites based infrared radiation coatings with high emissivity and high thermal shock resistance and their application on energy-saving kettle Author: Jianyi Zhang Xi’an Fan Lei Lu Xiaoming Hu Guangqiang Li PII: DOI: Reference:
S0169-4332(15)00714-X http://dx.doi.org/doi:10.1016/j.apsusc.2015.03.119 APSUSC 29996
To appear in:
APSUSC
Received date: Revised date: Accepted date:
1-11-2014 9-3-2015 18-3-2015
Please cite this article as: J. Zhang, X. Fan, L. Lu, X. Hu, G. Li, Ferrites based infrared radiation coatings with high emissivity and high thermal shock resistance and their application on energy-saving kettle, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.03.119 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights The
ferrites based infrared radiation coating was prepared by HVOF for the first
time;
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The
infrared radiation coatings were applied firstly on the household kettle;
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The
bonding strength between the coating and substrate could reach 30.7 MPa;
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The
coating kept intact when cycle reached 27 by quenching from 1000 C
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using water;
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The
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energy-saving efficiency of the kettle with coating could reach 30.5%.
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Ferrites based infrared radiation coatings with high emissivity
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and high thermal shock resistance and their application on
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energy-saving kettle
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Jianyi Zhang a, b, Xi’an Fan a, b,*, Lei Lu a, b, Xiaoming Hu c, Guangqiang Li a, b
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a
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China
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b
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and Technology, Wuhan 430081, China
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The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081,
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Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science
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Suzhou Sagreon New Materials Co., Ltd, Zhangjiagang 215625, China
ABSTRACT
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Starting from Fe2O3, MnO2, Co2O3 and NiO powders, the ferrites based infrared radiation coatings with high
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emissivity and high thermal shock resistance were successfully prepared on the surface of carbon steel by high
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velocity Oxy-fuel spraying (HVOF). The coating thickness was about 120 ~ 150 m and presented a typical flat
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lamellar structure. The coating surface was rough and some submicron grade grains distributed on it. The
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infrared emissivity of the ferrites based coating by HVOF was over 0.74 in 3 ~ 20 μm waveband at 800 °C,
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which was obviously higher than that of the coating by brushing process in the short waveband. The bonding
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strength was 30.7 MPa between the coating and substrate, which was five times more than that of conventional
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coatings by brushing process. The combined effect of the superior bonding strength, typical lamellar structure,
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pre-existing microcracks and newly generated pores made the cycle times reach 27 when the coating samples
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* Corresponding author. Tel.: +86 27 68862665; fax: +86 27 68862529. E-mail address: [email protected] (Xi’an Fan).
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were quenched from 1000 °C using water. Lastly, the infrared radiation coatings were applied on the underside
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of household kettle, and the energy-saving efficiency could reach 30.5%. The ferrites based infrared radiation
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coatings obtained in this work are good candidates for saving energy in the field of cookware and industrial high
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temperature furnace.
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Key words: Infrared radiation coating; Ferrites; Emissivity; Energy-saving; HVOF
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1. Introduction
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Infrared radiation materials, which have high emissivity or characteristic emissivity in the infrared
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wavelength, are a kind of important functional materials. When they are coated on the surface of metal or
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refractory substrates in the high temperature furnace, the heat radiation efficiency of the furnace can be
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improved effectively, the heat loss is reduced significantly and the temperature distribution in the furnace is
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more uniform. In addition, the infrared radiation coating materials can provide protective action to the substrate,
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reduce maintenance and improve the working life of the furnace [1]. With increasing energy shortage and
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environmental pollution, the infrared radiation materials have a very wide application prospect in high energy
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consumption industries and daily life.
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At present, the infrared radiation materials have been widely used in industrial applications such as energy
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saving for industrial furnaces [2 ~ 5], efficiency improvement of infrared heaters [6, 7], spacecraft thermal
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control, etc [8, 9]. However, the application is limited severely due to the following defects: low emissivity in
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the short waveband, weak bonding strength with substrate, poor thermal shock resistance and short working life.
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These defects mainly result from their preparation methods. Generally, the infrared radiation coatings are
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prepared by brushing using the slurry which consists of the infrared-active powders mixed with a binder, for its
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convenient construction and low cost. However, the binders will lose effect under high temperature because of 3
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the weak physical bonding strength between the coating and substrate. Meanwhile, the coating easily falls off
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due to the large difference of linear expansion coefficient between the coating and metallic substrate. Therefore,
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it is very important to exploit an infrared radiation coating which can be applied on high-temperature metallic
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substrate. Compared with the coatings prepared by traditional brushing process, the coatings prepared by high
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velocity Oxy-fuel spraying (HVOF) have obvious advantages in quality, such as stronger bonding strength,
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higher hardness, lower porosity and higher coverage rate. This technology has been widely used in
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wear-resistant coatings [10 ~ 12], corrosion resistance coatings [13], thermal barrier coatings [14, 15], etc.
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In order to overcome those defects in structure and property of the infrared radiation coatings prepared by
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traditional brushing process, and make up for the application blank in cookware areas, the ferrites based infrared
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radiation coatings were prepared by HVOF method on the bottom of the civilian kettle, and the infrared
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radiation properties and the energy-saving efficiency were investigated in detail. The infrared radiation
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energy-saving coating obtained in this work had a high emissivity in whole infrared waveband, strong bonding
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strength with substrate and good thermal shock resistance.
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2. Experimental
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2.1 Preparation of ferrites based infrared radiation powders
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All chemicals used in this work were of analytical grade purity and were used as received without further
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purification, and only collected by sieving with a screen having grid size of 38 μm. Firstly, the mixtures of
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Fe2O3 (60 wt.%), MnO2 (20 wt.%), Co2O3 (10 wt.%), and NiO (10 wt.%) were planetary ball milled for 30 min
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(QM-4F, Nanjing Nanda Instrument). Stainless steel pot and balls were used and the inner radius of the milling
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pot is 80 mm. The revolution speed is 400 rpm and the speed ratio of revolution to rotation is 1:2. Then the
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mixed powders were pelletized by slurry spray drying in the spray dry tower from well-round slurry which was 4
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acquired by mixing raw materials, organic binder and water in proportion. Lastly, the ferrite based infrared
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radiation powders were synthesized at 1150 °C for 2 h in the muffle furnace in air atmosphere.
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2.2 Preparation of infrared radiation coating
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In order to obtain good bonding strength between the coating and the substrate, the pretreatment process,
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such as cleaning, sandblasting, and preheating, was undertaken on the carbon steel sheet with 30 mm × 2.5 mm
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in dimensions. Prior to the HVOF deposition of the infrared radiation coating, a Ni3Al bond coating with
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thickness no more than 10 μm was first deposited onto the substrate. A homogeneous and dark infrared radiation
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coating, over 120 μm in thickness, was prepared on the pretreated steel sheet through XM-8000 HVOF
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equipment with oil fuel (Shanghai Xiuma Spraying machinery co., Ltd, China). The HVOF parameters are shown in Tab. 1.
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2.3 Characterization
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The high-temperature solid-state reaction process of the quasi-spherical particles sample was analyzed using
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thermal gravimetric (TG) and differential thermal analysis (DTA) (NETZSCH STA-499C) under air atmosphere
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with a 10 °C /min heating rate. The phase structures of the infrared radiation powders and coating were
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examined by X-ray diffraction (XRD) using an X-Pert Philips diffractometer with Cu K α radiation (=1.5418 Å).
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The microstructures of the infrared radiation powders and coatings were observed by scanning electron
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microscopy (SEM) (FESEM, Nova 400 NanoSEM). The spectral emissivity of the coating sample at 800 °C
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was determined in the wavelength of 3 ~ 20 μm by comparing the radiation of coating sample with a blackbody
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under the same condition (FT-IR, Jasco-6100). The spectral emissivity value was defined using the following
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formula: (λ) = L(λ) / Lb(λ), where L(λ) was radiance of coating sample, Lb(λ) was radiance of blackbody. The
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bonding strength of the coating was tested by the direct pull-off method on a universal testing machine
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(WE-30B), the sample geometry consisted of cylinders with dimensions of 25 mm × 2.5 mm as required by the
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GB/T 8642-200-B (Thermal spraying-Determination of tensile adhesive strength, Chinese National Standards),
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and the type of glue employed was epoxy adhesives E-7. In order to ensure the accuracy of experimental results,
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five parallel tests were used for the coating. Thermal shock resistance was obtained by thermal cycling with
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water cooling from 1000 °C to room temperature. The actual energy-saving efficiency of infrared radiation
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coating was tested by a simple comparison method. The actual energy-saving η of infrared radiation coating was
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given from the following equation:
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t t 1 2 t1
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Where t1 was the heating time of the kettle without coating, t2 was the heating time of the kettle with coating.
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The infrared radiation coating was sprayed on the underside of a kettle and a series of water heating tests were
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done, the heating was done by a common electric stove with power of 2000 W and the amount of water was
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2000 ml.
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3. Results and discussion
(1)
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Fig. 1 shows the TG and DTA traces of the quasi-spherical particles by slurry spraying drying from 50 °C to
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1400 °C. It reveals that the weight loss occurs below 1200 °C, due to the volatilization of residual free water
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(87.0 °C) and impurity phase (Full temperature section), the decomposition of residual organic binder (284.4 °C)
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and manganese dioxide (550.9 °C and 921.1 °C). There are two weak broad endothermic peaks below 400 °C in
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the DTA curve, which mainly correspond to the evaporation of residual free water at 87.0 °C and the
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decomposition of residual organic binder at 284.4 °C, respectively [16]. Because the processes of evaporation
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and decomposition are continuous, these endothermic peaks are broad and weak. With increasing temperature,
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the two typical endothermic peaks at 550.9 °C and 921.1 °C should be caused by the decomposition of
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manganese dioxide, which is also consistent with the TG curve results. The reaction equations and G-T
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equations [17] are as follows: MnO2(s) → 1/2 Mn2O3(s) + 1/4 O2(g)
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Mn2O3(s) → 2/3 Mn3O4(s) + 1/6 O2(g)
G=41500 – 54.98T (J/mol) G=35307 – 28.60T (J/mol)
T = 298 ~ 1000 °C T = 298 ~ 1550 °C
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An exothermic peak appearing at 995.1 °C can be ascribed to the ferrites synthesis, Fe2O3 + Mn3O4 + Co2O3
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+ NiO → Fe–Mn–Co–Ni–O ferrites. In addition, there are two evident endothermic peaks at 1227.1 °C and
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1360.3 °C, which are the melting temperature of the mixed ferrite phases.
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Fig. 2 shows the SEM of infrared radiation powders prepared by spray drying process. It can be seen from
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Fig. 2(a) and Fig. 2(c) that the powders mainly consist of quasi-spherical agglomerated particles with a diameter
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of 20 ~ 100 μm before and after high temperature solid-state reaction, indicating that the high temperature
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process cannot affect the morphology and size of the powders. The agglomerated particles are composed of a
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large amount of small irregular particles, and there are lots of holes with a diameter from 100 nm to 1 μm in the
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agglomerated particles, as shown in Fig. 2(b) and (d). Compared with the sample before high temperature
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solid-state reaction (Fig. 2(b)), these small irregular particles became larger (with a diameter from 200 nm to 3
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μm) and angular after the high temperature process, showing a clear evidence of reaction sintering: the original
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raw material particles reacted and were entirely transformed into angular crystals of ferrite [18], which is in
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agreement with the XRD curve results (Fig. 3(a)). The quasi-spherical agglomerated particles exhibit low
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inter-particle friction and excellent flow ability. For the HVOF process in particular, the agglomerated
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quasi-spherical particles can enhance the flow ability of the infrared radiation powders so that they can be
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continuously injected into the flame center, which is beneficial to high deposition efficiency and more uniform
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microstructure for the coating [19]. The X-ray diffraction patterns of the infrared radiation powders after high temperature solid-state reaction (a) and the coating by HVOF (b) are shown in Fig. 3. The XRD pattern of the powders (Fig. 3(a)) shows that there
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are no obvious diffraction peaks of Fe2O3, Mn3O4, Co2O3 and NiO phases in the high temperature solid-state
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reaction products but new diffraction peaks of ferrites, which include Fe24O32 (JCPDF 96-900-5839),
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Ni0.4Fe2.6O4 (JCPDF 01-087-2336), CoFe2O4 (JCPDF 00-022-1086), NiFe1.95Mn0.05O4 (JCPDF 01-074-2082),
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(Ni0.13Mn0.87)(Ni0.87Mn1.13)O4 (JCPDF 01-088-0241) and so on. It indicates that the ferrite phases can be
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synthesized by transition metal oxides (Fe2O3, MnO2, Co2O3 and NiO) in the experiment. By contrast, the X-ray
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diffraction pattern of the coating by HVOF process is shown in Fig. 3(b). It can be seen from Fig. 3(b) that the
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major crystal phases are still mixed ferrites and the process of HVOF does not result in a new phase. Compared
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with the powders, the intensity of the diffraction peaks for the coating debases sharply and the full width at half
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maximum (FWHM) is larger, indicating that the crystallinity of coating materials has decreased and the grain
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size is smaller after HVOF process. When the semi-molten infrared radiation materials are sprayed onto the
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surface of carbon steel substrate, they will be cooled at a super-fast rate because of the high thermal
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conductivity (45 ~ 48 W/mK) of the carbon steel substrate. There is not enough time to grow up for the grain in
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such a short period of time, similar to the formation of infrared radiation materials with amorphous structure by
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plasma spraying [20]. Furthermore, compared with the powders, the diffraction peaks of the coating slightly
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move to lower angle. It indicates that the HVOF process results in a more severe doping effect, increasing
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interplanar distance and lattice distortion of the ferrite phases [21]. The increasing interplanar distance of the
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ferrite phases may be caused by the migration of some ions to interstitial or substitutional positions during the
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HVOF process, leading to the lattice distortion [22]. In addition, it can be seen from Fig. 3(b) that there are
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obvious peaks of Ni3Al phase in the coating sample, it is likely that some Ni/Al particles were projected
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together with the ferrite powder during the spraying process, namely the powder feeder, the feeding line, the
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torch nozzle, or barrel were contaminated by residuals of Ni/Al during the deposition of the ferrite top layer.
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Fig. 4 shows the surface morphologies and cross-sectional microstructure of coatings. The surface of the
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coating is very rough (Fig. 4(a)). Besides a typical laminated structure (Fig. 4(b)), some semi-molten submicron
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grade particles (Fig. 4(c)) with a diameter from 500 nm to 1 μm distribute on the surface partly, and there are
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some microcracks in the coatings. As shown in Fig. 4(d), the thickness of the coating is about 120 ~ 150 μm and
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the laminated structure displays high internal density and low porosity, which means that the coating has high
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cohesive strength. The cracks can’t be observed between substrate and Ni3Al transitional layer, and between
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Ni3Al transitional layer and infrared radiation coating, indicating that they combine closely. Furthermore, Ni3Al
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distributed near the coating surface in the form of discrete particles or splats confirms the contamination by
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residuals of Ni/Al during the deposition of the ferrite top layer, which is also consistent with the XRD curve
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results (Fig. 3(b)).
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The spectral emissivity values of the ferrites based coating by HVOF were measured in wavelength of 3 ~
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20 μm at 800 °C, as shown in Fig. 5. Simultaneously, the emissivity of the carbon steel substrate and the coating
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by brushing methods are also shown in Fig. 5. For the HVOF sample, the spectral emissivity value fluctuates
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around 0.75 with increasing wavelength from 3 μm to 8 μm, increases from 0.75 to 0.83 with increasing
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wavelength from 8 μm to 9.2 μm, and fluctuates around 0.81 with further increasing wavelength from 9.2 μm to
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20 μm. Compared with the coating by brushing process, the emissivity of the coating by HVOF exhibits a
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higher value in short wavelength of 3 ~ 5 μm. It is because the high temperature flame flow promotes the ion
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migration and strengthens the doping effect of metal ions. The formation of lattice defects improves lattice
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distortion. Simultaneously, these defects can be preserved maximally under fast cooling rate because of the high
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thermal conductivity (45 ~ 48 W/mK) of the carbon steel substrate. They can form localized states and their
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energy values are in the forbidden band [23], which provides the condition for the electron transition and
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promotes the infrared absorption in the short waveband. It is important to improve the infrared radiation
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properties of materials [24, 25]. It is well known that the peak value of energy distribution will move to the
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short waveband with increasing temperature. So, the increase of emissivity in the short waveband is beneficial
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to increase the energy–saving efficiencies, because about 80% of energy is concentrated in the short waveband
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when the temperature is over 800 C. In addition, the coating surface is very rough (Fig. 4(a)). The coating has a
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typical laminated structure (Fig. 4(b)) and some semi–molten submicron grade particles distributed on its
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surface (Fig. 4(c)), which substantially increases the coating surface area. It can decrease the reflectance and
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increase the absorption of the infrared radiation wave [26]. Comparatively, the uncoated steel shows a much
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lower infrared emissivity value about 0.3 ~ 0.4 in 3 ~ 20 μm waveband.
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The bonding strength is 30.7 MPa for the coating by HVOF process, but the bonding strength value is only
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5.9 MPa for the coating by brushing process. The bonding strength between the coating and substrate for the
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coating prepared by HVOF is five times more than that of the coating by brushing process. On one hand, the
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large number of supersonic self-melting powders are sprayed and quickly spread on the surface of substrate,
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which results in a typical laminated structure with high internal density and low porosity. It is believed that this
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microstructure can enhance bonding strength through improving the inter-lamellae contact in the coating [27,
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28]. On the other hand, Ni3Al bond coating’s self adhesive effect can also contribute further to enhancing the
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adhesion strength.
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The thermal shock resistance is evaluated by quenching with water at 1000 C. Most of the coating prepared
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by HVOF still adheres well to the substrate after 27 times, but the coating prepared by brushing process is
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destroyed after 1 time. The XRD pattern of the HVOF-sprayed samples after thermal cycling for 27 times is
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presented in Fig. 6(a). It can be found that the major crystal phases are still mixed ferrites which include
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Fe2.933O4 (JCPDF 01-086-1354), Mn0.43Fe2.57O4 (JCPDF 01-089-2807), NiMn2O4 (JCPDF 01-084-0542),
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(Co0.2 Fe0.8)Co0.8Fe1.2O4 (JCPDF 01-077-0426), Ni0.4Fe2.6O4 (JCPDF 01-087-2336) and so on. As shown in Fig.
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6(b), the surface of coating is rougher than the original ones. Some tetragonal and irregular micron-sized grains
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unevenly distribute on the surface. The pre-microcracks on the surface of original coating disappear and larger
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amounts of pores appear in the irregular micron-sized grains. Compared with primary samples, extensive
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re-crystallization and grain growth can be found in the coating materials after thermal shock test, which is
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caused by the small grain size and large lattice distortion in the coating by HVOF process, providing favorable
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conditions for re-crystallization and grain growth. The secondary electron photo and backscattering photo of the
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cross-sections for the HVOF-sprayed samples after the thermal cycle experiment are shown in Fig. 6 (c) and (d),
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respectively. It can be seen that the thickness of the coating decrease from 120 ~ 150 μm to 100 ~ 120 μm after
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thermal cycling, indicating that a part of the top coating is spalled off by the thermal shock damage, and the
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bond coating is thickened and broken into particles due to high-temperature oxidation and diffusion, while no
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visible delimitation or cracks can be found in the remaining coating from the SEM of cross-sections. This can be
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ascribed to the combined effect of the superior bonding strength, typical lamellar structure, pre-existing
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microcracks and newly generated pores. The thermal stress between coating and substrate is large during
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cooling stage for the thermal cycled coating. For the coating by HVOF technology, the superior bonding
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strength can resist the thermal shock and reduce the formation of cracks between the coating and substrate. The
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typical lamellar structure can cause crack deflection and/or branching, which restrains the propagation of cracks
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in the coating [29]. Moreover, the previously mentioned newly generated pores in thermal cycled coating are
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also beneficial to release thermal stress [19, 20]. At the same time, these pre-existing mircocracks also have a
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microcrack toughening effect and can relieve the thermal stress during thermal cycles [30, 31]. Comparatively,
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the infrared radiation coating prepared by traditional brushing process has weak physical bonding to the
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substrate. It easily peels off from the substrate during thermal shock testing process.
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Fig. 7 shows the temperature-time curves of water heating up in the household kettle. Tab. 2 shows the
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energy-saving rate corresponding to each temperature range. As shown in Fig. 7 and Tab. 2, the household kettle
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with coating can greatly reduce the heating time and energy-saving rate can reach up to 30.5%. On the one hand,
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the infrared radiation energy saving coating applied on the bottom of household kettle can effectively increase
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the absorption of heat radiation from the furnace, according to second law of thermodynamics, the more heat
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energy given to the water per unit time, the shorter the time needed for water boiling. On the other hand, the
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rough coating surface can increase the area of infrared absorption and decrease the reflection. The infrared
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radiation coatings obtained in this work can be widely used in the field of cookware and industrial high
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temperature furnace, and it can play an important role in economic benefit and environmental benefit.
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4. Conclusions
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The ferrites based infrared radiation coatings with high emissivity, strong bonding strength, and excellent
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thermal shock resistance were successfully prepared on the surface of carbon steel by HVOF process. The
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obtained infrared radiation coatings were composed of mixed spinel ferrite phases. The coating thickness was
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about 120 ~ 150 m. The surface of the coating was rough, which presented a typical flat lamellar structure and
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some submicron-size grains were partly distributed on it. The infrared emissivity of the ferrite based coating by
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HVOF was over 0.74 in 3 ~ 20 μm waveband at 800 °C, which was obviously higher than that of the coating by 12
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brushing process in the short waveband. The bonding strength between the coating and substrate was five times
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more than that of conventional coatings by brushing process. The combined effect of the superior bonding
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strength, typical lamellar structure, pre-existing microcracks and newly generated pores made the cycle times
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reach 27 by quenching from 1000 C using water. The infrared radiation coatings were applied on the underside
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of household kettle, and the energy-saving efficiency could reach 30.5%. The infrared radiation coatings
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obtained can be widely used in the field of cookware and industrial high temperature furnace, and it can play an
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important role in economic benefit and environmental benefit.
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Acknowledgements
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This work was financially supported by the National Natural Science Foundation of China (11074195),
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Technology Innovation Fund of Jiangsu Province (SBC201310656) and Technology Innovation Fund of
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Zhangjiagang City (ZKC1205).
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[14] L.Y. Ni, C.G. Zhou. Effects of surface modification on thermal cycling lifetime of thermal barrier coatings with HVOF NiCrAlY bond
coat, Progress in Natural Science: Mater. Int. 22 (2012) 237–243.
[15] J.A. Haynes, K.A. Unocic, M.J. Lance, B.A. Pint, Impact of superalloy composition, bond coat roughness and water vapor on TBC
lifetime with HVOF NiCoCrAlYHfSi bond coatings, Surf. Coat. Technol. 237 (2013) 65–70.
[16] X. D. Chen, J. Min, Z.Q. Zhu, W.P. Ye, Preparation of high emissivity NiCr2O4 powders with a spinel structure by spray drying, Int. J.
Miner. Metall. Mater. 19 (2012) 173–178.
[17] I. Barin, O. Knacke, O. Kubaschewski, Thermochemical properties of inorganic substances, Spring–Verlag, Berlin, New York, 1977.
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radiation coating, J. Mater. Sci. Technol. 25 (2009) 695–698.
[20] L. Lu, X.A. Fan, J.Y. Zhang, X.M. Hu, G.Q. Li, Z. Zhan, Evolution of structure and infrared radiation properties for ferrite–based
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[19] W. Chen, W.P. Ye, X.D. Cheng, W. Duan, F. Mao, D.L. Li, Preparation, microstructure and properties of NiO–Cr2O3–TiO2 infrared
amorphous coating, Appl. Surf. Sci. 316 (2014) 82–87.
[21] X.D. Cheng, W. Duan, W. Chen, W.P. Ye, F. Mao, F. Ye, Q. Zhang, Infrared radiation coatings fabricated by plasma spray, J. Therm.
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SrFe12O19 coatings for electromagnetic wave absorption, J. Eur. Ceram. Soc. 31(8) (2011) 1439–1449.
Spray Technol. 18 (2009) 448–450.
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[18] K. Bobzin, G. Bolelli, M. Bruehl, A. Hujanen, P. Lintunen, D. Lisjak, S. Gyergyek, L. Lusvarghi, Characterisation of plasma-sprayed
[22] Y. Zhang, D.J. Wen, Relationship between infrared radiation and crystal structure in Fe–Mn–Co–Cu–O spinels, Acta Metall. Sin. (Engl.
Lett.) 21(2008) 15–20.
M
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[23] C. Kittel, P. McEuen, Introduction to Solid State Physics, Wiley, New York, 1976.
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[24] I. Kanesaka, Y. Hayashi, Y. Morioka, On infrared intensities of spinel, MgAl2O4, Bull. Chem. Soc. J. 78 (2005) 1256–1258
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[25] Charles F. Windisch Jr., Kim F. Ferris, Gregory J. Exarthos, Shiv K. Sharma, Conducting spinel oxide films with infrared transparency,
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Thin Solid Films 420–421 (2002) 89–99.
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[26] H.J. Yu, G.Y. Xu, X.M. Yan, X.X. Yan, C.M. Shao, C. Hu, Effects of size, shape and floatage of Cu particles on the low infrared
emissivity coatings, Prog. Org. Coat. 66 (2009) 161–166.
[27] K.A. Khort, H. Li, P. Cheang, Significance of melt-fraction in HVOF sprayed hydroxyapatite particles, splats and coatings,
Biomaterials, 25 (2004) 1177–1186.
[28] R.S. Lima, B. Marple, H. Li, K.A. Khor, Biocompatible Nanostructured High-Velocity Oxyfuel Sprayed Titania Coating : Deposition,
Characterization, and Mechanical Properties, J. Therm. Spray Technol., 15(4) (2006) 623–627.
[29] H.J. Kim, C.H. Lee, Y.G. Kweon, The effects of sealing on the mechanical properties of the plasma-sprayed alumina-titania coating,
Surf. Coat. Technol., 139(1) (2001) 75–80.
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plasma spraying, Surf. Coat. Technol., 201 (2007) 7746–7754.
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[31] Y. Wang, W. Tian, Y. Yang, Thermal shock behavior of nanostructured and conventional Al2O3/13 wt% TiO2 coatings fabricated by
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spraying, Surf. Coat. Technol., 197 (2005) 185–192.
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[30] B. Liang, C.X. Ding, Thermal shock resistances of nanostructured and conventional zirconia coatings deposited by atmospheric plasma
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Figure captions
Fig. 1 TG and DTA analysis of the quasi-spherical particles by slurry spraying drying
Fig. 3 X-ray diffraction patterns of the infrared radiation powders (a) and coating (b)
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Fig. 4 SEM of surface (a) (b) (c) and cross-section (d) for the infrared radiation coating
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Fig. 2 SEM of the infrared radiation powders before solid-state reaction (a) (b) and after solid-state reaction (c) (d)
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Fig. 5 Emissivity values curve of the infrared radiation coating and uncoated steel
Fig. 6 XRD pattern (a), SEM of surface (b) and cross-section (c and d) for HVOF-sprayed samples after thermal cycled at 1000 C for 27
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Fig. 7 Temperature-time curves of water heating up in the kettle
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Tab. 1 Processing parameters of HVOF Bond coating
Infrared radiation coating
Kerosene pressure/MPa
0.6 ~ 0.7
0.6 ~ 0.7
Oxygen pressure/MPa
0.6 ~ 0.7
0.6 ~ 0.7
Primary N2 flow rate/L·h–1
400 ~ 500
400 ~ 500
Powder feeding voltage/V
6~7
9 ~ 10
Spraying distance/mm
120
Powder feeding direction
radial
Barrel length/mm
100
Coating thickness/μm
≤ 10
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Item
80
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radial 100
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≥ 120
30 ~ 50
Energy-saving rate/%
33.5
50 ~ 70
70 ~ 80
80 ~ 90
30 ~ 90
27.9
29.0
30.5
30.5
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Temperature interval/°C
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Tab. 2 Energy-saving rate corresponding to each temperature range
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Fig. 1
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Fig. 2(a)
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Fig. 2(b)
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Fig. 2(c)
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Fig. 2(d)
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Fig. 3
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Fig. 4(a)
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S0169-4332(15)00714-X http://dx.doi.org/doi:10.1016/j.apsusc.2015.03.119 APSUSC 29996
To appear in:
APSUSC
Received date: Revised date: Accepted date:
1-11-2014 9-3-2015 18-3-2015
Please cite this article as: J. Zhang, X. Fan, L. Lu, X. Hu, G. Li, Ferrites based infrared radiation coatings with high emissivity and high thermal shock resistance and their application on energy-saving kettle, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.03.119 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
2 3
Highlights The
ferrites based infrared radiation coating was prepared by HVOF for the first
time;
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The
infrared radiation coatings were applied firstly on the household kettle;
5
The
bonding strength between the coating and substrate could reach 30.7 MPa;
6
The
coating kept intact when cycle reached 27 by quenching from 1000 C
7
using water;
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The
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energy-saving efficiency of the kettle with coating could reach 30.5%.
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Ferrites based infrared radiation coatings with high emissivity
2
and high thermal shock resistance and their application on
3
energy-saving kettle
4
Jianyi Zhang a, b, Xi’an Fan a, b,*, Lei Lu a, b, Xiaoming Hu c, Guangqiang Li a, b
5
a
6
China
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b
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and Technology, Wuhan 430081, China
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c
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The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081,
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Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science
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Suzhou Sagreon New Materials Co., Ltd, Zhangjiagang 215625, China
ABSTRACT
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Starting from Fe2O3, MnO2, Co2O3 and NiO powders, the ferrites based infrared radiation coatings with high
12
emissivity and high thermal shock resistance were successfully prepared on the surface of carbon steel by high
13
velocity Oxy-fuel spraying (HVOF). The coating thickness was about 120 ~ 150 m and presented a typical flat
14
lamellar structure. The coating surface was rough and some submicron grade grains distributed on it. The
15
infrared emissivity of the ferrites based coating by HVOF was over 0.74 in 3 ~ 20 μm waveband at 800 °C,
16
which was obviously higher than that of the coating by brushing process in the short waveband. The bonding
17
strength was 30.7 MPa between the coating and substrate, which was five times more than that of conventional
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coatings by brushing process. The combined effect of the superior bonding strength, typical lamellar structure,
19
pre-existing microcracks and newly generated pores made the cycle times reach 27 when the coating samples
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* Corresponding author. Tel.: +86 27 68862665; fax: +86 27 68862529. E-mail address: [email protected] (Xi’an Fan).
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Page 2 of 34
were quenched from 1000 °C using water. Lastly, the infrared radiation coatings were applied on the underside
2
of household kettle, and the energy-saving efficiency could reach 30.5%. The ferrites based infrared radiation
3
coatings obtained in this work are good candidates for saving energy in the field of cookware and industrial high
4
temperature furnace.
5
Key words: Infrared radiation coating; Ferrites; Emissivity; Energy-saving; HVOF
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1. Introduction
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Infrared radiation materials, which have high emissivity or characteristic emissivity in the infrared
8
wavelength, are a kind of important functional materials. When they are coated on the surface of metal or
9
refractory substrates in the high temperature furnace, the heat radiation efficiency of the furnace can be
10
improved effectively, the heat loss is reduced significantly and the temperature distribution in the furnace is
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more uniform. In addition, the infrared radiation coating materials can provide protective action to the substrate,
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reduce maintenance and improve the working life of the furnace [1]. With increasing energy shortage and
13
environmental pollution, the infrared radiation materials have a very wide application prospect in high energy
14
consumption industries and daily life.
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At present, the infrared radiation materials have been widely used in industrial applications such as energy
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saving for industrial furnaces [2 ~ 5], efficiency improvement of infrared heaters [6, 7], spacecraft thermal
17
control, etc [8, 9]. However, the application is limited severely due to the following defects: low emissivity in
18
the short waveband, weak bonding strength with substrate, poor thermal shock resistance and short working life.
19
These defects mainly result from their preparation methods. Generally, the infrared radiation coatings are
20
prepared by brushing using the slurry which consists of the infrared-active powders mixed with a binder, for its
21
convenient construction and low cost. However, the binders will lose effect under high temperature because of 3
Page 3 of 34
the weak physical bonding strength between the coating and substrate. Meanwhile, the coating easily falls off
2
due to the large difference of linear expansion coefficient between the coating and metallic substrate. Therefore,
3
it is very important to exploit an infrared radiation coating which can be applied on high-temperature metallic
4
substrate. Compared with the coatings prepared by traditional brushing process, the coatings prepared by high
5
velocity Oxy-fuel spraying (HVOF) have obvious advantages in quality, such as stronger bonding strength,
6
higher hardness, lower porosity and higher coverage rate. This technology has been widely used in
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wear-resistant coatings [10 ~ 12], corrosion resistance coatings [13], thermal barrier coatings [14, 15], etc.
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In order to overcome those defects in structure and property of the infrared radiation coatings prepared by
9
traditional brushing process, and make up for the application blank in cookware areas, the ferrites based infrared
10
radiation coatings were prepared by HVOF method on the bottom of the civilian kettle, and the infrared
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radiation properties and the energy-saving efficiency were investigated in detail. The infrared radiation
12
energy-saving coating obtained in this work had a high emissivity in whole infrared waveband, strong bonding
13
strength with substrate and good thermal shock resistance.
14
2. Experimental
15
2.1 Preparation of ferrites based infrared radiation powders
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All chemicals used in this work were of analytical grade purity and were used as received without further
17
purification, and only collected by sieving with a screen having grid size of 38 μm. Firstly, the mixtures of
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Fe2O3 (60 wt.%), MnO2 (20 wt.%), Co2O3 (10 wt.%), and NiO (10 wt.%) were planetary ball milled for 30 min
19
(QM-4F, Nanjing Nanda Instrument). Stainless steel pot and balls were used and the inner radius of the milling
20
pot is 80 mm. The revolution speed is 400 rpm and the speed ratio of revolution to rotation is 1:2. Then the
21
mixed powders were pelletized by slurry spray drying in the spray dry tower from well-round slurry which was 4
Page 4 of 34
acquired by mixing raw materials, organic binder and water in proportion. Lastly, the ferrite based infrared
2
radiation powders were synthesized at 1150 °C for 2 h in the muffle furnace in air atmosphere.
3
2.2 Preparation of infrared radiation coating
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In order to obtain good bonding strength between the coating and the substrate, the pretreatment process,
5
such as cleaning, sandblasting, and preheating, was undertaken on the carbon steel sheet with 30 mm × 2.5 mm
6
in dimensions. Prior to the HVOF deposition of the infrared radiation coating, a Ni3Al bond coating with
7
thickness no more than 10 μm was first deposited onto the substrate. A homogeneous and dark infrared radiation
8
coating, over 120 μm in thickness, was prepared on the pretreated steel sheet through XM-8000 HVOF
9
equipment with oil fuel (Shanghai Xiuma Spraying machinery co., Ltd, China). The HVOF parameters are shown in Tab. 1.
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2.3 Characterization
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The high-temperature solid-state reaction process of the quasi-spherical particles sample was analyzed using
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thermal gravimetric (TG) and differential thermal analysis (DTA) (NETZSCH STA-499C) under air atmosphere
14
with a 10 °C /min heating rate. The phase structures of the infrared radiation powders and coating were
15
examined by X-ray diffraction (XRD) using an X-Pert Philips diffractometer with Cu K α radiation (=1.5418 Å).
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The microstructures of the infrared radiation powders and coatings were observed by scanning electron
17
microscopy (SEM) (FESEM, Nova 400 NanoSEM). The spectral emissivity of the coating sample at 800 °C
18
was determined in the wavelength of 3 ~ 20 μm by comparing the radiation of coating sample with a blackbody
19
under the same condition (FT-IR, Jasco-6100). The spectral emissivity value was defined using the following
20
formula: (λ) = L(λ) / Lb(λ), where L(λ) was radiance of coating sample, Lb(λ) was radiance of blackbody. The
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bonding strength of the coating was tested by the direct pull-off method on a universal testing machine
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(WE-30B), the sample geometry consisted of cylinders with dimensions of 25 mm × 2.5 mm as required by the
2
GB/T 8642-200-B (Thermal spraying-Determination of tensile adhesive strength, Chinese National Standards),
3
and the type of glue employed was epoxy adhesives E-7. In order to ensure the accuracy of experimental results,
4
five parallel tests were used for the coating. Thermal shock resistance was obtained by thermal cycling with
5
water cooling from 1000 °C to room temperature. The actual energy-saving efficiency of infrared radiation
6
coating was tested by a simple comparison method. The actual energy-saving η of infrared radiation coating was
7
given from the following equation:
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t t 1 2 t1
9
Where t1 was the heating time of the kettle without coating, t2 was the heating time of the kettle with coating.
10
The infrared radiation coating was sprayed on the underside of a kettle and a series of water heating tests were
11
done, the heating was done by a common electric stove with power of 2000 W and the amount of water was
12
2000 ml.
13
3. Results and discussion
(1)
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Fig. 1 shows the TG and DTA traces of the quasi-spherical particles by slurry spraying drying from 50 °C to
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1400 °C. It reveals that the weight loss occurs below 1200 °C, due to the volatilization of residual free water
16
(87.0 °C) and impurity phase (Full temperature section), the decomposition of residual organic binder (284.4 °C)
17
and manganese dioxide (550.9 °C and 921.1 °C). There are two weak broad endothermic peaks below 400 °C in
18
the DTA curve, which mainly correspond to the evaporation of residual free water at 87.0 °C and the
19
decomposition of residual organic binder at 284.4 °C, respectively [16]. Because the processes of evaporation
20
and decomposition are continuous, these endothermic peaks are broad and weak. With increasing temperature,
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the two typical endothermic peaks at 550.9 °C and 921.1 °C should be caused by the decomposition of
2
manganese dioxide, which is also consistent with the TG curve results. The reaction equations and G-T
3
equations [17] are as follows: MnO2(s) → 1/2 Mn2O3(s) + 1/4 O2(g)
5
Mn2O3(s) → 2/3 Mn3O4(s) + 1/6 O2(g)
G=41500 – 54.98T (J/mol) G=35307 – 28.60T (J/mol)
T = 298 ~ 1000 °C T = 298 ~ 1550 °C
(2)
(3)
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An exothermic peak appearing at 995.1 °C can be ascribed to the ferrites synthesis, Fe2O3 + Mn3O4 + Co2O3
7
+ NiO → Fe–Mn–Co–Ni–O ferrites. In addition, there are two evident endothermic peaks at 1227.1 °C and
8
1360.3 °C, which are the melting temperature of the mixed ferrite phases.
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Fig. 2 shows the SEM of infrared radiation powders prepared by spray drying process. It can be seen from
10
Fig. 2(a) and Fig. 2(c) that the powders mainly consist of quasi-spherical agglomerated particles with a diameter
11
of 20 ~ 100 μm before and after high temperature solid-state reaction, indicating that the high temperature
12
process cannot affect the morphology and size of the powders. The agglomerated particles are composed of a
13
large amount of small irregular particles, and there are lots of holes with a diameter from 100 nm to 1 μm in the
14
agglomerated particles, as shown in Fig. 2(b) and (d). Compared with the sample before high temperature
15
solid-state reaction (Fig. 2(b)), these small irregular particles became larger (with a diameter from 200 nm to 3
16
μm) and angular after the high temperature process, showing a clear evidence of reaction sintering: the original
17
raw material particles reacted and were entirely transformed into angular crystals of ferrite [18], which is in
18
agreement with the XRD curve results (Fig. 3(a)). The quasi-spherical agglomerated particles exhibit low
19
inter-particle friction and excellent flow ability. For the HVOF process in particular, the agglomerated
20
quasi-spherical particles can enhance the flow ability of the infrared radiation powders so that they can be
21
continuously injected into the flame center, which is beneficial to high deposition efficiency and more uniform
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1 2
microstructure for the coating [19]. The X-ray diffraction patterns of the infrared radiation powders after high temperature solid-state reaction (a) and the coating by HVOF (b) are shown in Fig. 3. The XRD pattern of the powders (Fig. 3(a)) shows that there
4
are no obvious diffraction peaks of Fe2O3, Mn3O4, Co2O3 and NiO phases in the high temperature solid-state
5
reaction products but new diffraction peaks of ferrites, which include Fe24O32 (JCPDF 96-900-5839),
6
Ni0.4Fe2.6O4 (JCPDF 01-087-2336), CoFe2O4 (JCPDF 00-022-1086), NiFe1.95Mn0.05O4 (JCPDF 01-074-2082),
7
(Ni0.13Mn0.87)(Ni0.87Mn1.13)O4 (JCPDF 01-088-0241) and so on. It indicates that the ferrite phases can be
8
synthesized by transition metal oxides (Fe2O3, MnO2, Co2O3 and NiO) in the experiment. By contrast, the X-ray
9
diffraction pattern of the coating by HVOF process is shown in Fig. 3(b). It can be seen from Fig. 3(b) that the
10
major crystal phases are still mixed ferrites and the process of HVOF does not result in a new phase. Compared
11
with the powders, the intensity of the diffraction peaks for the coating debases sharply and the full width at half
12
maximum (FWHM) is larger, indicating that the crystallinity of coating materials has decreased and the grain
13
size is smaller after HVOF process. When the semi-molten infrared radiation materials are sprayed onto the
14
surface of carbon steel substrate, they will be cooled at a super-fast rate because of the high thermal
15
conductivity (45 ~ 48 W/mK) of the carbon steel substrate. There is not enough time to grow up for the grain in
16
such a short period of time, similar to the formation of infrared radiation materials with amorphous structure by
17
plasma spraying [20]. Furthermore, compared with the powders, the diffraction peaks of the coating slightly
18
move to lower angle. It indicates that the HVOF process results in a more severe doping effect, increasing
19
interplanar distance and lattice distortion of the ferrite phases [21]. The increasing interplanar distance of the
20
ferrite phases may be caused by the migration of some ions to interstitial or substitutional positions during the
21
HVOF process, leading to the lattice distortion [22]. In addition, it can be seen from Fig. 3(b) that there are
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obvious peaks of Ni3Al phase in the coating sample, it is likely that some Ni/Al particles were projected
2
together with the ferrite powder during the spraying process, namely the powder feeder, the feeding line, the
3
torch nozzle, or barrel were contaminated by residuals of Ni/Al during the deposition of the ferrite top layer.
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Fig. 4 shows the surface morphologies and cross-sectional microstructure of coatings. The surface of the
5
coating is very rough (Fig. 4(a)). Besides a typical laminated structure (Fig. 4(b)), some semi-molten submicron
6
grade particles (Fig. 4(c)) with a diameter from 500 nm to 1 μm distribute on the surface partly, and there are
7
some microcracks in the coatings. As shown in Fig. 4(d), the thickness of the coating is about 120 ~ 150 μm and
8
the laminated structure displays high internal density and low porosity, which means that the coating has high
9
cohesive strength. The cracks can’t be observed between substrate and Ni3Al transitional layer, and between
10
Ni3Al transitional layer and infrared radiation coating, indicating that they combine closely. Furthermore, Ni3Al
11
distributed near the coating surface in the form of discrete particles or splats confirms the contamination by
12
residuals of Ni/Al during the deposition of the ferrite top layer, which is also consistent with the XRD curve
13
results (Fig. 3(b)).
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The spectral emissivity values of the ferrites based coating by HVOF were measured in wavelength of 3 ~
15
20 μm at 800 °C, as shown in Fig. 5. Simultaneously, the emissivity of the carbon steel substrate and the coating
16
by brushing methods are also shown in Fig. 5. For the HVOF sample, the spectral emissivity value fluctuates
17
around 0.75 with increasing wavelength from 3 μm to 8 μm, increases from 0.75 to 0.83 with increasing
18
wavelength from 8 μm to 9.2 μm, and fluctuates around 0.81 with further increasing wavelength from 9.2 μm to
19
20 μm. Compared with the coating by brushing process, the emissivity of the coating by HVOF exhibits a
20
higher value in short wavelength of 3 ~ 5 μm. It is because the high temperature flame flow promotes the ion
21
migration and strengthens the doping effect of metal ions. The formation of lattice defects improves lattice
9
Page 9 of 34
distortion. Simultaneously, these defects can be preserved maximally under fast cooling rate because of the high
2
thermal conductivity (45 ~ 48 W/mK) of the carbon steel substrate. They can form localized states and their
3
energy values are in the forbidden band [23], which provides the condition for the electron transition and
4
promotes the infrared absorption in the short waveband. It is important to improve the infrared radiation
5
properties of materials [24, 25]. It is well known that the peak value of energy distribution will move to the
6
short waveband with increasing temperature. So, the increase of emissivity in the short waveband is beneficial
7
to increase the energy–saving efficiencies, because about 80% of energy is concentrated in the short waveband
8
when the temperature is over 800 C. In addition, the coating surface is very rough (Fig. 4(a)). The coating has a
9
typical laminated structure (Fig. 4(b)) and some semi–molten submicron grade particles distributed on its
10
surface (Fig. 4(c)), which substantially increases the coating surface area. It can decrease the reflectance and
11
increase the absorption of the infrared radiation wave [26]. Comparatively, the uncoated steel shows a much
12
lower infrared emissivity value about 0.3 ~ 0.4 in 3 ~ 20 μm waveband.
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The bonding strength is 30.7 MPa for the coating by HVOF process, but the bonding strength value is only
14
5.9 MPa for the coating by brushing process. The bonding strength between the coating and substrate for the
15
coating prepared by HVOF is five times more than that of the coating by brushing process. On one hand, the
16
large number of supersonic self-melting powders are sprayed and quickly spread on the surface of substrate,
17
which results in a typical laminated structure with high internal density and low porosity. It is believed that this
18
microstructure can enhance bonding strength through improving the inter-lamellae contact in the coating [27,
19
28]. On the other hand, Ni3Al bond coating’s self adhesive effect can also contribute further to enhancing the
20
adhesion strength.
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The thermal shock resistance is evaluated by quenching with water at 1000 C. Most of the coating prepared
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by HVOF still adheres well to the substrate after 27 times, but the coating prepared by brushing process is
2
destroyed after 1 time. The XRD pattern of the HVOF-sprayed samples after thermal cycling for 27 times is
3
presented in Fig. 6(a). It can be found that the major crystal phases are still mixed ferrites which include
4
Fe2.933O4 (JCPDF 01-086-1354), Mn0.43Fe2.57O4 (JCPDF 01-089-2807), NiMn2O4 (JCPDF 01-084-0542),
5
(Co0.2 Fe0.8)Co0.8Fe1.2O4 (JCPDF 01-077-0426), Ni0.4Fe2.6O4 (JCPDF 01-087-2336) and so on. As shown in Fig.
6
6(b), the surface of coating is rougher than the original ones. Some tetragonal and irregular micron-sized grains
7
unevenly distribute on the surface. The pre-microcracks on the surface of original coating disappear and larger
8
amounts of pores appear in the irregular micron-sized grains. Compared with primary samples, extensive
9
re-crystallization and grain growth can be found in the coating materials after thermal shock test, which is
10
caused by the small grain size and large lattice distortion in the coating by HVOF process, providing favorable
11
conditions for re-crystallization and grain growth. The secondary electron photo and backscattering photo of the
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cross-sections for the HVOF-sprayed samples after the thermal cycle experiment are shown in Fig. 6 (c) and (d),
13
respectively. It can be seen that the thickness of the coating decrease from 120 ~ 150 μm to 100 ~ 120 μm after
14
thermal cycling, indicating that a part of the top coating is spalled off by the thermal shock damage, and the
15
bond coating is thickened and broken into particles due to high-temperature oxidation and diffusion, while no
16
visible delimitation or cracks can be found in the remaining coating from the SEM of cross-sections. This can be
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ascribed to the combined effect of the superior bonding strength, typical lamellar structure, pre-existing
18
microcracks and newly generated pores. The thermal stress between coating and substrate is large during
19
cooling stage for the thermal cycled coating. For the coating by HVOF technology, the superior bonding
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strength can resist the thermal shock and reduce the formation of cracks between the coating and substrate. The
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typical lamellar structure can cause crack deflection and/or branching, which restrains the propagation of cracks
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in the coating [29]. Moreover, the previously mentioned newly generated pores in thermal cycled coating are
2
also beneficial to release thermal stress [19, 20]. At the same time, these pre-existing mircocracks also have a
3
microcrack toughening effect and can relieve the thermal stress during thermal cycles [30, 31]. Comparatively,
4
the infrared radiation coating prepared by traditional brushing process has weak physical bonding to the
5
substrate. It easily peels off from the substrate during thermal shock testing process.
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Fig. 7 shows the temperature-time curves of water heating up in the household kettle. Tab. 2 shows the
7
energy-saving rate corresponding to each temperature range. As shown in Fig. 7 and Tab. 2, the household kettle
8
with coating can greatly reduce the heating time and energy-saving rate can reach up to 30.5%. On the one hand,
9
the infrared radiation energy saving coating applied on the bottom of household kettle can effectively increase
10
the absorption of heat radiation from the furnace, according to second law of thermodynamics, the more heat
11
energy given to the water per unit time, the shorter the time needed for water boiling. On the other hand, the
12
rough coating surface can increase the area of infrared absorption and decrease the reflection. The infrared
13
radiation coatings obtained in this work can be widely used in the field of cookware and industrial high
14
temperature furnace, and it can play an important role in economic benefit and environmental benefit.
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4. Conclusions
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The ferrites based infrared radiation coatings with high emissivity, strong bonding strength, and excellent
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thermal shock resistance were successfully prepared on the surface of carbon steel by HVOF process. The
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obtained infrared radiation coatings were composed of mixed spinel ferrite phases. The coating thickness was
19
about 120 ~ 150 m. The surface of the coating was rough, which presented a typical flat lamellar structure and
20
some submicron-size grains were partly distributed on it. The infrared emissivity of the ferrite based coating by
21
HVOF was over 0.74 in 3 ~ 20 μm waveband at 800 °C, which was obviously higher than that of the coating by 12
Page 12 of 34
brushing process in the short waveband. The bonding strength between the coating and substrate was five times
2
more than that of conventional coatings by brushing process. The combined effect of the superior bonding
3
strength, typical lamellar structure, pre-existing microcracks and newly generated pores made the cycle times
4
reach 27 by quenching from 1000 C using water. The infrared radiation coatings were applied on the underside
5
of household kettle, and the energy-saving efficiency could reach 30.5%. The infrared radiation coatings
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obtained can be widely used in the field of cookware and industrial high temperature furnace, and it can play an
7
important role in economic benefit and environmental benefit.
8
Acknowledgements
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This work was financially supported by the National Natural Science Foundation of China (11074195),
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Technology Innovation Fund of Jiangsu Province (SBC201310656) and Technology Innovation Fund of
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Zhangjiagang City (ZKC1205).
12
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Figure captions
Fig. 1 TG and DTA analysis of the quasi-spherical particles by slurry spraying drying
Fig. 3 X-ray diffraction patterns of the infrared radiation powders (a) and coating (b)
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Fig. 4 SEM of surface (a) (b) (c) and cross-section (d) for the infrared radiation coating
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Fig. 2 SEM of the infrared radiation powders before solid-state reaction (a) (b) and after solid-state reaction (c) (d)
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Fig. 5 Emissivity values curve of the infrared radiation coating and uncoated steel
Fig. 6 XRD pattern (a), SEM of surface (b) and cross-section (c and d) for HVOF-sprayed samples after thermal cycled at 1000 C for 27
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Fig. 7 Temperature-time curves of water heating up in the kettle
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Tab. 1 Processing parameters of HVOF Bond coating
Infrared radiation coating
Kerosene pressure/MPa
0.6 ~ 0.7
0.6 ~ 0.7
Oxygen pressure/MPa
0.6 ~ 0.7
0.6 ~ 0.7
Primary N2 flow rate/L·h–1
400 ~ 500
400 ~ 500
Powder feeding voltage/V
6~7
9 ~ 10
Spraying distance/mm
120
Powder feeding direction
radial
Barrel length/mm
100
Coating thickness/μm
≤ 10
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≥ 120
30 ~ 50
Energy-saving rate/%
33.5
50 ~ 70
70 ~ 80
80 ~ 90
30 ~ 90
27.9
29.0
30.5
30.5
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Temperature interval/°C
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Tab. 2 Energy-saving rate corresponding to each temperature range
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Fig. 1
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Fig. 2(a)
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Fig. 2(b)
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Fig. 2(c)
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Fig. 2(d)
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Fig. 3
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Fig. 4(a)
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Fig. 4(b)
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Fig. 4(c)
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Fig. 4(d)
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Fig. 5
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Fig. 6(a)
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Fig. 6(b)
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Fig. 6(c)
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Fig. 6(d)
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Fig. 7
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