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Effect of cooling rate on residual stress and mechanical properties of laser remelted ceramic coating
Accepted Manuscript Title: Effect of cooling rate on residual stress and mechanical properties of laser remelted ceramic coating Authors: Biswajit Das, Muvvala Gopinath, Ashish Kumar Nath, P.P. Bandyopadhyay PII: DOI: Reference:
S0955-2219(18)30215-2 https://doi.org/10.1016/j.jeurceramsoc.2018.04.020 JECS 11830
To appear in:
Journal of the European Ceramic Society
Received date: Revised date: Accepted date:
17-8-2017 27-3-2018 7-4-2018
Please cite this article as: Das B, Gopinath M, Nath AK, Bandyopadhyay PP, Effect of cooling rate on residual stress and mechanical properties of laser remelted ceramic coating, Journal of the European Ceramic Society (2010), https://doi.org/10.1016/j.jeurceramsoc.2018.04.020 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.
Effect of cooling rate on residual stress and mechanical properties of laser remelted ceramic coating Biswajit Das, Muvvala Gopinath, Ashish Kumar Nath, P. P. Bandyopadhyay*
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Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur – 721302, India *
Corresponding author: email: [email protected], phone: + 91 3222 282950, fax: + 91 3222 282278
Abstract: Mild steel substrates were coated with commercially available alumina and chromia powders using the powder flame spraying process. The top layers of the flame sprayed coatings were remelted using a 2kW fiber laser. Thermo-cycles of the laser remelting process were
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monitored on-line using an infrared pyrometer. Cooling rates were varied using different laser
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scanning speeds. Surface morphology, microstructure and phases of the laser treated and as-
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sprayed coatings were investigated using optical microscopy, scanning electron microscopy, Xray diffraction and X-ray tomography. Surface residual stress of the as-sprayed and laser treated
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coatings was measured using X-ray diffraction. The inherent defects like porosity and interlamellar boundary diminish to a great extent upon laser remelting. Surface residual stress of the
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remelted coatings tends to increase with increase in cooling rate. Surface crack density of the
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laser treated coating was reduced appreciably when coatings were preheated prior to laser remelting.
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Keywords: Ceramic oxides, Powder flame spray, Laser remelting, Columnar microstructure,
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Residual stress
1. Introduction
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Thermally sprayed alumina coating is suitable for wear resistance applications owing to its high hardness and chemical stability [1]. This type of coating is used in automotive and electronic industry [2, 3]. On the other hand, chromia coating is widely accepted for corrosion resistant application. These types of coating are used in printing rolls, piston rings of IC engines and pumps [4, 5].
Thermal spraying is a well-known surface modification technique utilized to deposit thick coatings on bulk substrates to modify their surface properties [6, 7]. Flame spraying was the first thermal spray process developed [8]. In the powder flame spraying process, the powder feedstock is carried by a gas flow into the oxy-fuel flame, which heats and propels it towards the substrate. An oxy-acetylene gas mixture is set ablaze to create a flame that acts as a heat source
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to melt the powder. Flame spraying is capable of spraying a wide range of materials ranging from ceramics to polymers and refractory materials [9]. It is possible to spray high melting point oxides like alumina, using this process, since the oxy-fuel flame can generate a temperature high enough to melt these oxides [10]. However, this process has certain differences as compared to plasma spraying, as far as spraying of ceramics is concerned. The flame sprayed coating contains a larger fraction of partially melted regions [11]. Habib et al. [10] observed that it produces
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coatings with higher porosity and bigger grain size.
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These structural aspects can be modified by means of different post processing techniques such
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as laser, plasma and flame heating [12]. A laser post treatment is likely to eliminate these
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deficiencies [13]. Laser remelting is a non-additive process that utilizes a laser beam to impart energy to melt the top layer of the coating. The main advantage of laser is the precise control of
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heat input. This in turn, permits controlling the thickness of the remelted layer while the unmelted part of the coating retains the same properties as before. Microhardness of the laser
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remelted pore free layer is found to be higher as compared to as-sprayed coating owing to densification of the top layer and elimination of porosity [14]. However, cracks may appear
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owing to rapid quenching of the coating material during laser remelting. These cracks can be of two types; longitudinal and transverse. Transverse cracks are beneficial for the high temperature
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application [15]. Surface residual stress of thermally sprayed coating is an important feature to consider since it determines the longevity of the coating during application [16]. Laser remelting
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may change the state of stress of a coated surface, i.e., tensile to compressive or vice versa [17]. Simultaneous or post processing of ceramic deposits using various heat sources was investigated by several authors. Guo et al. [12] studied the effects of different post processing techniques like flame heating, laser remelting, and vacuum furnace heating on the erosive properties of flame sprayed coatings. As-sprayed deposits failed primarily owing to micro-brittle fracture during erosion test. In the post-heated deposits, however, microcutting, micro-ploughing and plastic
deformation played a dominant role. Li et al. [18] used a hybrid technique combining flame spraying with a CO2 laser to deposit an alumina coating on an alumina based refractory substrate. With this technique, a stable α-alumina phase was found to grow in the coating instead of metastable phases (γ, θ, δ, β). Moreover, an increase in the laser power, and decrease in the work-piece traverse velocity, promoted growth of the stable α-alumina phase. They also
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measured the crack density in the hybrid coatings using optical stereometry. Crack density was found to decrease with a reduction in both powder feed rate and work-piece traverse velocity. Process parameters of this hybrid coating procedure were optimized by the same group to minimize the crack density using regression analysis [19]. The results were in agreement with the experimental results reported in ref. [18]. Rico et al. [20] studied the effects of laser remelting on the properties of flame sprayed ZrO2-5CaO coating. Laser remelting produced a
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pore free remelted layer with enhanced Young’s modulus, hardness, fracture toughness and wear
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resistance. Múnez et al. [21] investigated on the thermal barrier characteristics of the laser remelted ZrO2-8Y2O3 coating. It was found that a remelted layer can effectively retard the
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growth of the thermally grown oxide (TGO) layer underneath the topcoat. To the knowledge of the authors no report is available in the literature on the effect of cooling
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rate on the residual stress and mechanical properties of laser remelted flame sprayed ceramic oxide coating. The present work that way is a novel attempt to investigate the mechanical
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properties and surface residual stress of laser remelted flame sprayed ceramic coating. The properties of the as-sprayed coating have also been reported for comparison purpose. It may be
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stressed at this point that flame spraying process is most economical, easy to handle and portable amongst all the thermal spraying processes.
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2. Experimental procedure
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2.1 Materials
Test coupons of C-20 steel of dimension 50 mm×75 mm×5 mm (thickness) were used as the substrate material. Commercially available alumina (Al2O3) and chromia (Cr2O3) powders (Oerlikon Metco, U.S.A.) were utilized as feedstock. The size ranges of alumina and chromia powders were -45+15 µm and -125+11 µm, respectively. A bond coat of Ni-5wt% Al (Oerlikon
Metco, U.S.A.) of size range -90+45 µm was provided between the ceramic top coat and the substrate. 2.2 Flame spraying procedure Prior to flame spraying, C-20 steel coupons were grit blasted inside a grit blasting cabinet
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(Sandstorm, Bangalore, India) using alumina grits of mesh size 24 at 100 psi (0.7 MPa) blasting pressure with a standoff distance of 125 mm. The roughness (Ra) of the grit blasted surface was 6.5 ± 0.4 μm. The grit blasted samples were ultrasonically cleaned for 15 min in 2-propanol solution. Immediately after cleaning the samples, ceramic powders were sprayed using an oxyacetylene neutral flame. The flame spraying parameters used are listed in Table 1.
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Table 1
Powder
Spraying
feed rate
distance (mm)
Oxygen
flow rate
flow rate
(slpm)
(slpm)
125
75
75
Neutral
75
75
Neutral
75
75
Neutral
1
Alumina
25
2.
Chromia
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(g/min)
Acetylene
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Powder
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Sl. no.
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Powders for flame spraying
75
3.
Ni-Al
25
150
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15
Flame type
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2.2 Laser remelting procedure Laser treatment was carried out using Yb-fibre laser (IPG photonics, Model no. YLR 2000)
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operating at 1.07 µm wavelength. The maximum output of the laser is 2 kW. It can be operated in continuous mode as well as in pulsed mode. The laser has multimodal beam intensity profile with higher intensity at center and two annular rings of relatively low intensity. The system is attached with a 5-axis CNC machine capable of moving at a speed up to 20 m/min. Different energy densities were obtained for remelting using various combinations of laser power and scanning speed. The laser beam was kept normal to the coating surface and the beam diameter
on the coating surface was set at 5 mm in every trial. The parameter ranges used to remelt both type of coatings are as follows: Laser power range 250-550 W, scanning speed 200-2000 mm/min, laser beam diameter 5 mm. A single spot monochromatic pyrometer (Micro Epsilon, model: CTLM-2HCF3-C3H, U.S.A.) was used to record the thermo-cycles during the laser remelting process. The pyrometer has the following operational features; operating wavelength
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1.6 µm, vision zone 700 µm, acquisition time 1ms, working temperature range 385 °C-1600 °C. It was also equipped with a pair of guide laser beams to focus at the required point. Thermocycles were recorded keeping the pyrometer static. In order to eliminate the effect of reflected laser radiation, a notch filter of 1064 nm±25 nm spectral range with optical density of 3 was used to block the 1070 nm laser radiation. However, the notch filter blocked a part of the 1.6 µm radiation. Hence, the pyrometer was calibrated with and without the notch filter at the melting
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temperature of different metals, e.g. steel, copper and aluminium. With the notch filter the
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temperature measurement range is 785–3260 °C. For some samples, a preheating technique was employed to minimize the surface crack network in the remelted layers. As-sprayed coatings
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were put into a groove made on a refractory brick and preheated using a defocused laser beam to
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a temperature of either around 550°C or 750°C. The surface temperatures of these samples were monitored online using the same optical pyrometer utilized to monitor the thermo-cycles. The
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remelted surfaces were observed for surface cracking using a scanning electron microscope
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(SEM) (EVO 15, Zeiss, Jena, Germany). 2.3 Characterization of the samples
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For microstructural study, samples of 10×10×5 mm dimensions were cut from the as-sprayed sample using a low speed diamond saw (150 low speed diamond cutter, MTI Corp.). The
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samples were mounted in the resin and they were polished in their cross sections in a semiautomatic polisher using SiC papers and diamond pastes. Laser treated tracks were sliced,
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mounted and polished using same procedure noted above. The polished cross sections were observed under a SEM (EVO 15, Zeiss, Jena, Germany). Phase and surface residual stress investigation was undertaken using a high resolution X-ray diffractometer (Empyrean Cu LFF HR (9430 033 7310x) DK411025, Netherlands) generating Cu Kα radiation. The operating voltage and current were 45kV and 40mA, respectively. For measurement of residual stress, sin2Ψ method was implemented with 9 variations of χ angle, i.e., stage tilt angle (0º, ± 22.79º,
± 33.21º, ± 42.13º, ± 50.77º). The residual stress was measured perpendicular to the laser scanning direction. Stage rotation (angle ϕ) was kept unchanged for uniaxial stress measurement. On the incidence side of the diffractometer, point source and parallel beam X-Ray lens were used along with an X-Ray mask of 4 mm in series with a divergence slit of 7 mm for as sprayed
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coatings. For laser remelted tracks X-Ray mask of 2 mm with a divergence slit of 4 mm was used. A Ni filter of 0.020 mm thickness was fitted in the incident beam optics assembly. The scan was performed with step size 0.1º for each coating. The total time taken for stress measurement was 3 hours 21 minutes for the each coating. A PIXcel1D-Medipix3 point detector was used in 0D mode to monitor the intensity of the diffracted X–Ray radiation. Stress analysis was undertaken by Stress software (Panalytical B.V., Almelo, Netherlands).
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The traditional sin2Ψ method was employed for data analysis. For the as-sprayed alumina coating γ-alumina was the predominant phase [22]. Hence, the peak corresponding to (844) plane
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of γ-alumina (142.30º˂2θ˂148.20º) (PDF card 01-074-2206, Fd-3 m structure, a=7.9060 Å, S1=-
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1.22 TPa-1, 1/2S2=6.54 TPa-1) was employed for stress measurement. Phase transformation
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occurred in alumina on laser treatment and α-alumina (PDF card 01-082-1467, R-3c structure, a=4.7589 Å, c=12.9919 Å, S1=-1.42 TPa-1, 1/2S2=7.59 TPa-1) was the predominant phase in
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remelted coating [23]. Hence, the peak corresponding to (146) plane of α-alumina (134.20º˂2θ˂138.15º) was used for the stress measurement of laser treated coating. For chromia
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coating no phase change was observed before and after laser processing. The peak corresponding to (416) plane (122.45º˂2θ˂128.35º) (eskolaite, PDF card 00-038-1479, R-3c structure,
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a=4.9588 Å, c=13.5942 Å, S1=-1.24 TPa-1, 1/2S2=6.39 TPa-1) was selected for stress measurement [24]. Surface roughness of the coatings was measured using a contact type surface
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roughness tester TAYLOR HOBSON FORM TALYSURF 50 (INTRA 2) 3D profilometer (TAYLOR HOBSON, UK). The traverse length was 4 mm and 3 mm for as-sprayed and laser
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remelted coatings, respectively. Surface roughness was measured in the transverse direction of the laser remelted track. The melting depth of the both laser treated and non-treated coatings were measured using an optical microscope (Zeiss Axio. Vert.A1, Germany). Coating hardness was measured under a load of 3 N (=300 gf) using a microhardness tester (MVH-S AUTO, Omnitech, India). An average of ten hardness readings was considered for microhardness measurement for each coating. To estimate indentation fracture toughness, indentations were
made on the cross section of the sample using a micro hardness tester (MVH-S -AUTO, Omnitech, Pune, India) under a normal load of 300 g, and then the crack lengths were measured using Image J software. Indentation fracture toughness (KIC) values were calculated using the following expression proposed by Evans and Charles [25] c −3/2 H√a
a
(1)
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K IC = 0.15k ( )
ϕ
Where k=correction factor 3.2 for ceramics, ϕ=constraint factor≈3, H = Vickers hardness (MPa), a = half of Vickers diagonal length measured in the direction of crack (m), c = sum of length of the crack and half of the Vickers diagonal in the direction of the crack (m). This method can be used very effectively for the comparison of toughness values in brittle ceramic coatings [26].
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Young modulus of as-sprayed and laser treated coatings were carried out using an instrumented hardness tester (Micro Combi Tester (MCT), CSM instruments, Switzerland) equipped with a
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Vickers indenter. The measurements were performed under a maximum load of 3000 mN,
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loading and unloading rate 6000 mN and dwell time 15 s. Moduli of elasticity of the coating
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were then obtained from the slope of unloading curve. In this case also, an average of ten readings was reported. X-ray tomography instrument (phoenix vǀtomeǀx, GE) was used for
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porosity measurement of the coatings. Tomography parameters were as follows: voxel size 4.99 µm, voltage 100 kV, current 70 µA, detector type dxr-250rt. For imaging subsurface of the as-
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sprayed and the laser remelted coating, a FEI Strata TM DB235 dual beam machine, combining a Field Emission Scanning Electron Microscope (FESEM) (Zeiss Supra 40, Germany) column
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and a high-resolution Focused Ion Beam (FIB) column equipped with a Ga Liquid Metal Ion Source (LMIS) was used. A gold thin film was first applied to protect the area of interest from
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surface damage during ion beam machining. A larger ion beam current (2 nA) was used to mill a pocket in the protected area. The sidewall of the pocket, i.e., the area of interest, was subsequently polished using a lower beam current (100 pA). The samples thus prepared have
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been observed in situ using the FESEM, under an angle of 52º. 3. Results and discussion 3.1 Monitoring of thermo-cycles during laser processing
Fig. 1 and 2 show the time-temperature profile in the zone of interest while laser power and scanning speed was varied in the ranges of 250- 550W and 200 - 1000 mm/min, respectively. It may be noted from figure 2 that the temperature does not rise appreciably with the variation of scanning speed. However, there is a significant increase in the cooling rate with an increase in the scanning speed. At a lower scanning speed, the interaction time of the laser beam with the
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coating is higher. This allows enough time for the accumulated heat to escape to the substrate by conduction. As an effect, the substrate temperature rises and the thermal gradient between the remelted zone and substrate decreases, finally resulting in a decrease in the cooling rate. On the other hand, an increase in laser power beyond 400 W does not show any significant effect on the peak temperature and cooling rate (Fig. 2). It is observed from Fig. 2 that a change in laser power at a fixed scanning speed does not have an appreciable effect on interaction time and hence,
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cooling rate. A similar trend has been observed in the case of chromia coatings (plots not
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included).
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2000
1500
200 mm/min 300 mm/min 400 mm/min 500 mm/min 600 mm/min 1000 mm/min
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Temperature (ºC)
2500
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1000
1000
2000
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0
3000 4000 Time (ms)
5000
6000
7000
Fig. 1. Thermo-cycles obtained during laser remelting of alumina coating using different scanning speeds (Laser Power=400W).
550W 500W 450W 400W 350W 250W
2000
1500
1000 0
1000
2000
3000
4000 Time (ms)
5000
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Temperature (ºC)
2500
6000
7000
8000
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Fig. 2. Thermo-cycles obtained during laser remelting of chromia coating using different laser
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powers (Scanning speed=200 mm/min).
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The approximate cooling rates were calculated for both types of coating from the slopes of the
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cooling curves. Fig. 3 (a) and (b) shows the variation of cooling rate with the variation in laser power and scanning speed. It is observed from Fig. 3 (a) and (b) that the effect of laser power on
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sample cooling rate is limited as compared to scanning speed. Hence, subsequent experiments were carried out at a fixed power of 400 W. The variation in energy density was obtained using
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the following scanning speeds; 200, 300, 400 and 500 mm/min. The corresponding energy
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a
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densities were 96, 12, 16 and 24 J/mm2, respectively. b
Fig. 3. Variation of cooling rate during laser remelting of the coatings using different (a) power and (b) scanning speed. 3.2 Surface morphology
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3.2.1 Surface morphology of as-sprayed coating Typical surface morphology of as-sprayed alumina and chromia coatings is shown in Fig. 4 (a) and (b), respectively. Both types of coatings were formed by accumulation of large numbers of flattened splats. The surface roughness (Ra) of the as-sprayed alumina and chromia coating were
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7.96 ± 0.48 µm and 4.78±0.21 µm, respectively.
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Fig. 4. Secondary electron images of as-sprayed (a) alumina and (b) chromia coating.
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3.2.2 Surface morphology of laser remelted coating Surface roughness of the as-sprayed coating decreases after laser treatment owing to further
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melting and flattening of splats and densification of the remelted layer. Surface roughness values of laser treated coatings processed using different parameters are shown in Table 2. Surface
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roughness was restricted to around 3 µm while scanning speed was varied in the range of 200 500 mm/min. An increase in scanning speed beyond that produces poor surface quality owing to surface rippling effect. During laser processing, a temperature gradient is generated from the center of the laser beam (that coincides with the center line of the remelted track) to the border of the track. Temperature of the remelted layer is expected to be the highest at the center and it is reasonable to assume that the surface tension of the molten layer is lowest at the same point.
Away from the center, the temperature of the molten pool decreases and the surface tension of the melt increases. A higher surface tension results in a localized depression in the surface of the liquid in the central region of track. Away from the center the height of liquid level is higher and a pressure head develops owing to this height difference. This pressure difference sets up a liquid flow towards the center of the molten pool form the periphery. It may be noted that immediately
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after laser remelting the molten pool cools rapidly. Hence, solidification intervenes before height equilibrium is achieved in the melt pool and the quickly frozen liquid surface shows the presence of ripples [27]. Fig. 5 (a) and (b) shows the rippled surfaces of alumina and chromia coatings, respectively.
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Table 2
As-sprayed
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Alumina
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Type of coatings
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Surface roughness of as-sprayed and laser remelted coatings
1.78±0.35
remelted
300mm/min
1.65±0.56
coating
400mm/min
2.78±0.76
500mm/min
3.23±0.34
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200mm/min
As-sprayed
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Chromia
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7.96 ± 0.48
Laser
at 400W
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Surface roughness (µm)
4.78±0.21
Laser
200mm/min
1.21±0.66
remelted
300mm/min
1.93±0.10
coating
400mm/min
2.76±0.75
at 400W
500mm/min
2.97±0.32
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Fig. 5. Surface rippling in laser remelted (a) alumina and (b) chromia coatings (laser power=400W, scanning speed=1000mm/min).
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Fig. 6 (a) and (b) shows the remelted tracks of the alumina and chromia coatings, respectively. The power and scanning speed utilized for these tracks were 500 W and 2000 mm/min,
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respectively. A large number of surface pores appear in both coatings. This effect is attributed to
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(1) bubble formation during the escape of entrapped gases in the coating [28], (2) freezing of the
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displaced liquid before it can fill up the surface pores during laser remelting. It may be noted that a high scanning speed was employed in this case. This in turn induces a high cooling rate and followed by effects that promote the formation of surface pores. It may be noted that no surface
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pore appeared on the remelted layer while the power and speed range were restricted to 400 W
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and 200-500 mm/min, respectively (Section 3.1). b
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a
Fig. 6. Formation of surface pores on laser remelting under higher scanning speed in (a) alumina and (b) chromia coatings (laser power=500W, scanning speed=2000mm/min).
Fig. 7 (a) and (c) shows the higher magnification image of the alumina and chromia coating, respectively. A network of surface cracks was observed on both coatings. The formation of these cracks is attributed to the quenching stress generated during freezing of the remelted layer. A
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higher magnification image of the remelted alumina layer (Fig 7 (b)) reveals a dendritic structure [29]. Fig. 7 (d) shows the higher magnification secondary electron image of the surface of the laser remelted chromia coating. The microstructure appears to be dendritic. Single dendrites arranged themselves together to form a hexagonal structure. Similar results for laser remelted
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zirconia were reported by Morks et al. [30].
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b
d
Fig. 7. (a) Surface crack network in laser remelted alumina, (b) Dendritic structure on the surface of laser remelted alumina, (c) Surface crack network in laser remelted chromia, (d) Dendritic structure on the surface of laser remelted chromia (laser power=400W, scanning
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speed=200mm/min).
3.3 Microstructure 3.3.1 Microstructure of as-sprayed coating
Fig. 8 (a) and 9 (a) show the optical image of the cross section of the alumina and chromia coatings, respectively. The top coat, bond coat, and substrate of the coating are clearly visible.
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The top coat contains pores. However, the coating shows good interfacial integrity. Fig. 8 (b) and
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9 (b) shows the fractured surfaces of the same coatings. From the micrograph, a lamellar structure ubiquitous to thermally sprayed coatings is identified. Growth of columnar grains,
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normal to the lamellar plane is also observed [31]. The inherent lamellar structure of flame
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sprayed coatings is heterogeneous with presence of voids, inter-splat boundary and intra-splat cracks. This heterogeneous structure has a strong influence on the mechanical properties of the
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coating. Fig. 9 (a) and (b) show the optical image of the cross-section and fractured surface of
b
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the as-sprayed chromia coating, respectively.
Fig. 8. (a) Optical image of the cross-section and (b) fractograph of as-sprayed alumina coating.
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b
a
Fig. 9. (a) Optical image of the cross-section (b) fractograph of as-sprayed chromia coating.
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3.3.2 Microstructure of laser remelted coating
Fig. 10 (a) and (b) shows the optical images of laser remelted alumina and chromia coatings,
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respectively. The contrast between laser remelted zone and as-sprayed zone of the coating is
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clearly visible in both coatings. The remelted layer is almost free of pores. A few cracks ran
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through the entire thickness of the remelted layer. These cracks segmented the coatings in a
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number of blocks.
Fig.10. Optical image of (a) alumina coating processed at 400 W and 200 mm/min (Energy density=24 J/mm2) (b) chromia coating processed at 400 W and 200 mm/min (Energy density=24 J/mm2).
The fractographs of laser remelted layers of alumina and chromia are shown in Fig. 11 (a) and (b), respectively. It is observed that upon remelting the inter-lamellar boundaries are no more there. The entire remelted layer appears to be monolithic with columnar grain growing in a
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direction perpendicular to the plane of the coating.
Fig. 11. Fractographs of laser remelted (a) alumina coating processed at 400 W and 200 mm/min density=24 J/mm2).
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3.4 Phases present in the coating
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(Energy density=24 J/mm2) (b) chromia coating processed at 400 W and 200 mm/min (Energy
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Fig. 12 (a) and (b) shows the X-ray diffraction patterns of alumina and chromia coatings, respectively. In case of alumina powder the primary phase is α-alumina. Thermal spraying involves rapid quenching and metastable γ-alumina nucleates from the melt in this circumstance.
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Some particles do not melt entirely and in those cases the α-phase grows from the α-alumina nucleus [22]. After laser remelting the metastable phase of the as-sprayed coating again
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transforms into the stable α-alumina phase. Hence, the peaks obtained from the X-ray diffraction pattern of the remelted layer (Fig 12 (a)) belong to the α-alumina phase. In the X-ray diffraction
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of the chromia powder, the chromia coating and the laser remelted layer show peaks of only one phase, i.e., Eskolaite, a rhombohedral phase. It indicates that chromia did not undergo any phase transformation either during coating formation and laser remelting. Schutz et al., observed existence of other phases like Cr3O4, CrO, or CrO2 in small quantity [32]. In the present investigation phases other than eskolaite were not detected possibly owing to their presence in small amount.
a
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b
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Fig. 12. X-ray diffraction of (a) alumina (b) chromia coatings.
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A preferential growth was observed in the X-ray diffraction pattern of the remelted coatings.
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This is attributed to a directional heat flow during cooling. Preferred orientation can be indexed
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using the texture coefficient values of a set of crystallographic planes under consideration.
1 𝐼 (ℎ 𝑘 𝑙) [∑ ] 𝑁 𝑁𝐼𝑜 (ℎ 𝑘 𝑙)
(2)
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𝑇𝑐 (ℎ 𝑘 𝑙) =
𝐼(ℎ 𝑘 𝑙) 𝐼𝑜(ℎ 𝑘 𝑙)
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Texture coefficient (Tc) for a given (h kl) plane can be calculated using the following equation
Where, I (h k l) is the measured intensity, Io (h k l) is the relative intensity of the corresponding plane
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given in PDF-2 data, and N is the number of reflections.
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The calculated texture coefficients are listed in Table 3. Consistently high values of texture coefficient were found for the (104) and (006) plane for alumina and chromia coating, respectively. This indicates that these two planes grow in preference to others while the remelted
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layers undergo solidification.
Table 3 Texture coefficients of laser remelted coatings Scanning
Texture coefficient (Tc)
power
speed
(W)
(mm/min)
Tc(012)
Tc(104)
Tc(110)
Tc(113)
Tc(012)
Tc(104)
Tc(104)
Tc(006)
400
200
0.02
0.89
3.05
0.0378
0.29
0.21
0.29
2.56
300
0.07
1.16
1. 49
0.595
1.13
0.94
0.49
1. 45
400
0.39
2.68
0.265
0.657
0.79
1.53
0.58
1.09
500
0. 45
2.59
0.53
1.73
0.15
1.86
0.66
1.62
Laser remelted chromia
N
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Laser remelted alumina
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Laser
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3.4.1 Melting depth
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Laser remelting was carried out in continuous mode. In continuous mode, the interaction time is estimated by the ratio of laser beam diameter to laser scanning speed. The laser beam used in this
D
case offers a Gaussian type power distribution with maximum power density in the center and two annular rings in the periphery [33]. Fig. 13 shows the variation of remelting depth (thickness
𝐸=
𝑃 𝑑𝑣
TE
of the remelted layer) for the coatings. The energy density was calculated from the expression, (J/mm2) where P=Laser power (W), d=diameter of the laser beam (mm), v=scanning
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speed (mm/min). The thickness of the remelted layer was measured form the cross sectional micrographs. The melting depth increases with an increase in the energy density for both
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coatings. However, under the same energy density the alumina coating melts to a greater depth. This is attributed to lower melting point of alumina (2033ºC) as compared to chromia (2600ºC)
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[16].
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Fig. 13. Variation of melt depth with laser energy density in the laser remelted coatings.
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3.5 X-Ray Tomography
Fig. 15 (a) and (b) shows the X-ray CT scan of the as-sprayed alumina and chromia coatings,
A
respectively. Presence of pores is considered as defect in thermally sprayed coating [34]. The
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adverse effect of porosity on the coating properties is proportional to the size of the pores [35, 36]. It is apparent from Fig. 15 that flame sprayed chromia coating is more porous as compared
A
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a
EP
TE
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to alumina coating. The porosity values are also listed in Table 4.
b
Fig. 14. CT-scan of flame sprayed (a) alumina and (b) chromia coatings. Fig. 16 (a) and (b) shows the CT scan of laser remelted alumina and chromia coating, respectively, processed using energy density (24 J/mm2) and scanning speed (200 mm/min). Laser treatment of the as-sprayed coating reduces the number of pores and their volumes. The
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porosity data listed in Table 4 indicates that the volumetric pore density decreases with a decrease in the scanning speed, i.e., increase in laser energy density.
b
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A
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a
Fig. 15. CT-scan of laser remelted (a) alumina and (b) chromia coatings processed using 400 W
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laser power and 200 mm/min scanning speed (Energy density=24 J/mm2). 3.6 Mechanical properties of the laser remelted coatings
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Fig. 14 (a) and (b) shows the cross sectional hardness profile of alumina and chromia coating, respectively, plotted against the distance from the edge of the coating. A significant increase in
CC
micro-hardness was observed in both coatings after laser treatment. Hazra et al. [37] made a similar observation in the case of laser remelted mullite coatings. The laser treated coatings have shown a gradual reduction in hardness from the top edge of the remelted layer to the inside of the
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coating. This is attributed to laser assisted densification. The layer immediately below the remelted one is the so-called heat affected zone. The heat affected zone also undergoes some densification resulting in an increase in the hardness as compared to the as-sprayed coating. Below this layer lies the as-sprayed layer unaffected by laser post processing. As expected, it has micro-hardness equal to the as-sprayed coating. Another possible explanation of this improvement, as observed by Morks et al., [30] is grain refinement in the remelted region.
a
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b
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Fig. 16. Depth profile of microhardness in laser remelted (a) alumina and (b) chromia coatings.
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Table 4 indicates that the indentation fracture toughness increases with a decrease in coating porosity for the remelted coatings. Inter-splat boundaries of as-sprayed coating were obliterated
A
upon laser remelting. The cohesion of the monolithic, homogenized laser remelted layer is
M
expected to be higher than that of a porous, lamellar as-spayed flame sprayed coating. This in turn, results in an increase in the indentation fracture toughness of the remelted layer. A
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reduction in porosity also brings about an increase in the Young’s modulus in the remelted
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Table 4
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coatings as compared to their as-sprayed counterpart [38].
Young’s modulus and indentation fracture toughness of the coatings
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Types of coatings
Alumina
as-sprayed
Porosity (%)
Young modulus
Fracture toughness
(GPa)
(MPa m-1/2)
6.49
85.6±11
-
Laser
200mm/min
0.50
162.07±8
2.2±0.2
remelted
300mm/min
0.80
138.45±3
2.4±0.1
coating
400mm/min
1.42
154.96±7
1.97±0.4
at 400W Chromia
500mm/min
as-sprayed
1.78
149.58±9
1.79±0.2
9.17
91.6±8
-
200mm/min
0.37
194.54±6
3.1±0.1
remelted
300mm/min
0.55
178.84±10
2.8±0.1
coating
400mm/min
1.86
186.23±7
1.9±0.1
at 400W
500mm/min
2.09
172.65±3
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Laser
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3.7 Residual stress
1.7±0.1
Generation of residual stress is an important consideration in the case of thermally sprayed
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coatings. Various coating performances such as adhesion strength, resistance to thermal shock,
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etc., depend upon the residual stress developed in the coating [39]. The primary sources of
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residual stress in thermally sprayed coatings are of two types. The first one is the quenching stress that develops during rapid cooling of individual splats after deposition on the substrate
D
surface. The second one is attributed to the differential thermal contraction between the substrate and the coating [40]. Quenching stress is always tensile in nature whereas the differential thermal
TE
contraction stress is either tensile or compressive depending upon the difference in thermal expansion coefficient values of the substrate and the coating material. Moreover, volume
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changes associated with phase transformation during the deposition process may also contribute to the residual stress in the coating.
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Fig. 17 shows the variation of residual stress with cooling rate for alumina and chromia coatings. The as-sprayed alumina coating harbors mild tensile residual stress [24]. This is
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attributed to differential thermal expansion between the bond coat and top coat. It may be noted that thermal expansion coefficient of the alumina top coat and Ni-5wt%Al bond coat is 7×10-6 (1/K) and 12×10-6 (1/K), respectively [9]. The tensile residual stress increases with an increase in the cooling rate, i.e., increase in scanning speed. This increase is attributed to a higher quenching rate obtained at a higher scanning speed [41, 42]. Hence, the magnitude of tensile stress increases with an increase in the cooling rate. For as-sprayed chromia coating, the residual stress was
found to be mildly compressive. Upon laser remelting, a mild compressive residual stress was recorded up to scanning speed 400 mm/min. However, a stress reversal from mild compressive
EP
TE
D
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A
N
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to mild tensile occurs between 500 and 600 mm/min scanning speed.
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Fig.17. Variation in surface residual stress of the remelted coatings with scanning speed.
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3.8 Effect of preheating Fig. 18 (a) shows a FIB milled pocket on the as-sprayed alumina coating. Subsurface cracks are
visible from this image. The surface cracks propagate below the surface to form a network of cracks. This crack network develops during coating deposition. A network of surface cracks also
forms in the laser remelted layer owing to quenching. These cracks also extend below the surface
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of the coating. Fig. 18 (b) shows such a crack in the laser remelted alumina coating.
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Fig. 18. Secondary electron images of the FIB milled pockets of (a) as-sprayed alumina (b) laser
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remelted alumina (processed at laser power=400 W and scanning speed=200 mm/min).
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Fig. 19 (a), (b) and (c) show the top views of the surfaces of a three laser remelted coatings. The
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sample corresponding to Fig. 19 (a) was produced by remelting a coating kept initially at the ambient temperature. Fig. 19 (b) and (c) correspond to the samples that were preheated to 450 ºC
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and 850 ºC, respectively, prior to remelting. A dense network of surface cracks was observed in Fig. 19 (a), i.e., on the sample that was not preheated. The crack network is less dense in Fig. 19
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(b) and almost imperceptible in Fig. 19 (c). This indicates that preheating before remelting reduces the surface crack density significantly. A preheated coating undergoes less quenching
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and hence, less shrinkage during cooling of the remelted layer. The extent of cracking also depends on the preheating temperature. Surface residual stresses of the coatings preheated before
CC
remelting are tabulated in Table 5. The residual stresses are tensile in nature. However, the magnitudes of the residual stresses are significantly lower than those in the remelted layers
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produced without preheating. The sample preheated to 850 ºC was found to be almost stress free. These results again corroborate to the fact that the quenching stress is reduced significantly upon preheating, prior to laser remelting.
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Fig. 19. Surface of laser remelted alumina (a) without preheating (b) preheated at 450 ºC (c) preheated at 850 ºC. Table 5 Residual stress of the preheated samples.
parameter
temperature (ºC)
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400 W power and
Fracture
(MPa)
toughness (MPa m-1/2)
450 ºC
95.6±11.4
-
850 ºC
20.4±6.8
1.67±0.3
No preheating
228.2±54.9
1.79±0.2
TE
D
500 mm/min
Residual stress
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Preheating
A
Alumina
Laser remelting
M
Coating type
The sample preheated at 850 ºC prior to remelting was appeared to be almost crack free. Therefore, it was selected for further investigation. Fig. 20 (a) and (b) show the optical images of
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the cross-section and fractograph of this preheated sample, respectively. The remelted layer was found to be pore free. However, a few transverse cracks were found in the remelted layer. A
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columnar structure perpendicular to the coating surface was found to grow in the remelted layer. These microstructural features of the preheated sample were similar to those of the non-
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preheated sample. However, the melting depth in this case was found to be 165.7±20 µm. Melting to this depth could not be achieved on remelting without preheating (Fig. 14). Upon preheating, absorptivity of the sample surface increases. This is demonstrated by Wang et al. [39]. An increase in laser energy absorption produces an increase in the depth of the remelted layer
a
Columnar structure
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b
Fig. 20. (a) Optical cross-sectional micrograph of the laser remelted alumina coating preheated to 850 ºC (b) Fractured surface of laser remelted alumina coating preheated to 850 ºC (laser power=400 W, scanning speed=500 mm/min).
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Fig. 21. (a) and (b), respectively show the microhardness depth profiles and the phases of the
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alumina coating remelted with and without preheating. Fig 21 (a) shows, preheating does not
A
have a significant effect on the hardness profile. Phases evolved on remelting are same for both the coatings as well (Fig 21 (b)). Indentation fracture toughness of these coatings is listed in table
M
4. A comparison with Table 3 shows, preheating does not have an appreciable effect on the
A
CC
EP
TE
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indentation fracture toughness of the remelted coatings.
Fig. 21. (a) Depth profile of hardness with and without preheating (b) Phases of laser remelted alumina with and without preheating (laser power=400 W, scanning speed=500 mm/min). 4. Conclusions
The work deals with the laser remelting of powder flame sprayed alumina and chromia coatings. It has been found from in-situ temperature history of the molten layer that the cooling rate depends to a greater extent on laser scanning speed as compared to laser power. Both type of remelted coatings present dendritic structure on their surfaces. The lamellar boundaries usually present in the thermally sprayed coating are obliterated upon laser remelting. In the remelted
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layer, columnar grains were found to grow in a plane perpendicular to the coating surface. The laser remelted layer was homogenous and almost pore free. Upon laser treatment, the hardness of the remelted layer was found to increase by up to 31% as compared to the as-sprayed coating. Immediately after laser treatment, a network of surface cracks develops owing to rapid quenching of the remelted layer. Flame sprayed alumina and chromia coatings were found to harbor tensile and compressive residual stress on their surfaces, respectively. Laser remelted
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alumina coating harbors tensile residual stress. The magnitude of the surface residual stress
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increases with an increase in cooling rate. Laser remelted chromia coating preserves its compressive stress state even after laser treatment. In the case of alumina coating, an increase in
A
melting depth was found on preheating the coating to 850ºC prior to remelting. Preheating also
M
produced a decrease in the surface residual stress of the remelted alumina layer.
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S0955-2219(18)30215-2 https://doi.org/10.1016/j.jeurceramsoc.2018.04.020 JECS 11830
To appear in:
Journal of the European Ceramic Society
Received date: Revised date: Accepted date:
17-8-2017 27-3-2018 7-4-2018
Please cite this article as: Das B, Gopinath M, Nath AK, Bandyopadhyay PP, Effect of cooling rate on residual stress and mechanical properties of laser remelted ceramic coating, Journal of the European Ceramic Society (2010), https://doi.org/10.1016/j.jeurceramsoc.2018.04.020 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.
Effect of cooling rate on residual stress and mechanical properties of laser remelted ceramic coating Biswajit Das, Muvvala Gopinath, Ashish Kumar Nath, P. P. Bandyopadhyay*
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Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur – 721302, India *
Corresponding author: email: [email protected], phone: + 91 3222 282950, fax: + 91 3222 282278
Abstract: Mild steel substrates were coated with commercially available alumina and chromia powders using the powder flame spraying process. The top layers of the flame sprayed coatings were remelted using a 2kW fiber laser. Thermo-cycles of the laser remelting process were
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monitored on-line using an infrared pyrometer. Cooling rates were varied using different laser
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scanning speeds. Surface morphology, microstructure and phases of the laser treated and as-
A
sprayed coatings were investigated using optical microscopy, scanning electron microscopy, Xray diffraction and X-ray tomography. Surface residual stress of the as-sprayed and laser treated
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coatings was measured using X-ray diffraction. The inherent defects like porosity and interlamellar boundary diminish to a great extent upon laser remelting. Surface residual stress of the
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remelted coatings tends to increase with increase in cooling rate. Surface crack density of the
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laser treated coating was reduced appreciably when coatings were preheated prior to laser remelting.
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Keywords: Ceramic oxides, Powder flame spray, Laser remelting, Columnar microstructure,
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Residual stress
1. Introduction
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Thermally sprayed alumina coating is suitable for wear resistance applications owing to its high hardness and chemical stability [1]. This type of coating is used in automotive and electronic industry [2, 3]. On the other hand, chromia coating is widely accepted for corrosion resistant application. These types of coating are used in printing rolls, piston rings of IC engines and pumps [4, 5].
Thermal spraying is a well-known surface modification technique utilized to deposit thick coatings on bulk substrates to modify their surface properties [6, 7]. Flame spraying was the first thermal spray process developed [8]. In the powder flame spraying process, the powder feedstock is carried by a gas flow into the oxy-fuel flame, which heats and propels it towards the substrate. An oxy-acetylene gas mixture is set ablaze to create a flame that acts as a heat source
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to melt the powder. Flame spraying is capable of spraying a wide range of materials ranging from ceramics to polymers and refractory materials [9]. It is possible to spray high melting point oxides like alumina, using this process, since the oxy-fuel flame can generate a temperature high enough to melt these oxides [10]. However, this process has certain differences as compared to plasma spraying, as far as spraying of ceramics is concerned. The flame sprayed coating contains a larger fraction of partially melted regions [11]. Habib et al. [10] observed that it produces
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coatings with higher porosity and bigger grain size.
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These structural aspects can be modified by means of different post processing techniques such
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as laser, plasma and flame heating [12]. A laser post treatment is likely to eliminate these
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deficiencies [13]. Laser remelting is a non-additive process that utilizes a laser beam to impart energy to melt the top layer of the coating. The main advantage of laser is the precise control of
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heat input. This in turn, permits controlling the thickness of the remelted layer while the unmelted part of the coating retains the same properties as before. Microhardness of the laser
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remelted pore free layer is found to be higher as compared to as-sprayed coating owing to densification of the top layer and elimination of porosity [14]. However, cracks may appear
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owing to rapid quenching of the coating material during laser remelting. These cracks can be of two types; longitudinal and transverse. Transverse cracks are beneficial for the high temperature
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application [15]. Surface residual stress of thermally sprayed coating is an important feature to consider since it determines the longevity of the coating during application [16]. Laser remelting
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may change the state of stress of a coated surface, i.e., tensile to compressive or vice versa [17]. Simultaneous or post processing of ceramic deposits using various heat sources was investigated by several authors. Guo et al. [12] studied the effects of different post processing techniques like flame heating, laser remelting, and vacuum furnace heating on the erosive properties of flame sprayed coatings. As-sprayed deposits failed primarily owing to micro-brittle fracture during erosion test. In the post-heated deposits, however, microcutting, micro-ploughing and plastic
deformation played a dominant role. Li et al. [18] used a hybrid technique combining flame spraying with a CO2 laser to deposit an alumina coating on an alumina based refractory substrate. With this technique, a stable α-alumina phase was found to grow in the coating instead of metastable phases (γ, θ, δ, β). Moreover, an increase in the laser power, and decrease in the work-piece traverse velocity, promoted growth of the stable α-alumina phase. They also
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measured the crack density in the hybrid coatings using optical stereometry. Crack density was found to decrease with a reduction in both powder feed rate and work-piece traverse velocity. Process parameters of this hybrid coating procedure were optimized by the same group to minimize the crack density using regression analysis [19]. The results were in agreement with the experimental results reported in ref. [18]. Rico et al. [20] studied the effects of laser remelting on the properties of flame sprayed ZrO2-5CaO coating. Laser remelting produced a
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pore free remelted layer with enhanced Young’s modulus, hardness, fracture toughness and wear
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resistance. Múnez et al. [21] investigated on the thermal barrier characteristics of the laser remelted ZrO2-8Y2O3 coating. It was found that a remelted layer can effectively retard the
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growth of the thermally grown oxide (TGO) layer underneath the topcoat. To the knowledge of the authors no report is available in the literature on the effect of cooling
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rate on the residual stress and mechanical properties of laser remelted flame sprayed ceramic oxide coating. The present work that way is a novel attempt to investigate the mechanical
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properties and surface residual stress of laser remelted flame sprayed ceramic coating. The properties of the as-sprayed coating have also been reported for comparison purpose. It may be
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stressed at this point that flame spraying process is most economical, easy to handle and portable amongst all the thermal spraying processes.
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2. Experimental procedure
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2.1 Materials
Test coupons of C-20 steel of dimension 50 mm×75 mm×5 mm (thickness) were used as the substrate material. Commercially available alumina (Al2O3) and chromia (Cr2O3) powders (Oerlikon Metco, U.S.A.) were utilized as feedstock. The size ranges of alumina and chromia powders were -45+15 µm and -125+11 µm, respectively. A bond coat of Ni-5wt% Al (Oerlikon
Metco, U.S.A.) of size range -90+45 µm was provided between the ceramic top coat and the substrate. 2.2 Flame spraying procedure Prior to flame spraying, C-20 steel coupons were grit blasted inside a grit blasting cabinet
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(Sandstorm, Bangalore, India) using alumina grits of mesh size 24 at 100 psi (0.7 MPa) blasting pressure with a standoff distance of 125 mm. The roughness (Ra) of the grit blasted surface was 6.5 ± 0.4 μm. The grit blasted samples were ultrasonically cleaned for 15 min in 2-propanol solution. Immediately after cleaning the samples, ceramic powders were sprayed using an oxyacetylene neutral flame. The flame spraying parameters used are listed in Table 1.
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Table 1
Powder
Spraying
feed rate
distance (mm)
Oxygen
flow rate
flow rate
(slpm)
(slpm)
125
75
75
Neutral
75
75
Neutral
75
75
Neutral
1
Alumina
25
2.
Chromia
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(g/min)
Acetylene
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Powder
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Sl. no.
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Powders for flame spraying
75
3.
Ni-Al
25
150
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15
Flame type
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2.2 Laser remelting procedure Laser treatment was carried out using Yb-fibre laser (IPG photonics, Model no. YLR 2000)
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operating at 1.07 µm wavelength. The maximum output of the laser is 2 kW. It can be operated in continuous mode as well as in pulsed mode. The laser has multimodal beam intensity profile with higher intensity at center and two annular rings of relatively low intensity. The system is attached with a 5-axis CNC machine capable of moving at a speed up to 20 m/min. Different energy densities were obtained for remelting using various combinations of laser power and scanning speed. The laser beam was kept normal to the coating surface and the beam diameter
on the coating surface was set at 5 mm in every trial. The parameter ranges used to remelt both type of coatings are as follows: Laser power range 250-550 W, scanning speed 200-2000 mm/min, laser beam diameter 5 mm. A single spot monochromatic pyrometer (Micro Epsilon, model: CTLM-2HCF3-C3H, U.S.A.) was used to record the thermo-cycles during the laser remelting process. The pyrometer has the following operational features; operating wavelength
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1.6 µm, vision zone 700 µm, acquisition time 1ms, working temperature range 385 °C-1600 °C. It was also equipped with a pair of guide laser beams to focus at the required point. Thermocycles were recorded keeping the pyrometer static. In order to eliminate the effect of reflected laser radiation, a notch filter of 1064 nm±25 nm spectral range with optical density of 3 was used to block the 1070 nm laser radiation. However, the notch filter blocked a part of the 1.6 µm radiation. Hence, the pyrometer was calibrated with and without the notch filter at the melting
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temperature of different metals, e.g. steel, copper and aluminium. With the notch filter the
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temperature measurement range is 785–3260 °C. For some samples, a preheating technique was employed to minimize the surface crack network in the remelted layers. As-sprayed coatings
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were put into a groove made on a refractory brick and preheated using a defocused laser beam to
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a temperature of either around 550°C or 750°C. The surface temperatures of these samples were monitored online using the same optical pyrometer utilized to monitor the thermo-cycles. The
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remelted surfaces were observed for surface cracking using a scanning electron microscope
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(SEM) (EVO 15, Zeiss, Jena, Germany). 2.3 Characterization of the samples
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For microstructural study, samples of 10×10×5 mm dimensions were cut from the as-sprayed sample using a low speed diamond saw (150 low speed diamond cutter, MTI Corp.). The
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samples were mounted in the resin and they were polished in their cross sections in a semiautomatic polisher using SiC papers and diamond pastes. Laser treated tracks were sliced,
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mounted and polished using same procedure noted above. The polished cross sections were observed under a SEM (EVO 15, Zeiss, Jena, Germany). Phase and surface residual stress investigation was undertaken using a high resolution X-ray diffractometer (Empyrean Cu LFF HR (9430 033 7310x) DK411025, Netherlands) generating Cu Kα radiation. The operating voltage and current were 45kV and 40mA, respectively. For measurement of residual stress, sin2Ψ method was implemented with 9 variations of χ angle, i.e., stage tilt angle (0º, ± 22.79º,
± 33.21º, ± 42.13º, ± 50.77º). The residual stress was measured perpendicular to the laser scanning direction. Stage rotation (angle ϕ) was kept unchanged for uniaxial stress measurement. On the incidence side of the diffractometer, point source and parallel beam X-Ray lens were used along with an X-Ray mask of 4 mm in series with a divergence slit of 7 mm for as sprayed
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coatings. For laser remelted tracks X-Ray mask of 2 mm with a divergence slit of 4 mm was used. A Ni filter of 0.020 mm thickness was fitted in the incident beam optics assembly. The scan was performed with step size 0.1º for each coating. The total time taken for stress measurement was 3 hours 21 minutes for the each coating. A PIXcel1D-Medipix3 point detector was used in 0D mode to monitor the intensity of the diffracted X–Ray radiation. Stress analysis was undertaken by Stress software (Panalytical B.V., Almelo, Netherlands).
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The traditional sin2Ψ method was employed for data analysis. For the as-sprayed alumina coating γ-alumina was the predominant phase [22]. Hence, the peak corresponding to (844) plane
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of γ-alumina (142.30º˂2θ˂148.20º) (PDF card 01-074-2206, Fd-3 m structure, a=7.9060 Å, S1=-
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1.22 TPa-1, 1/2S2=6.54 TPa-1) was employed for stress measurement. Phase transformation
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occurred in alumina on laser treatment and α-alumina (PDF card 01-082-1467, R-3c structure, a=4.7589 Å, c=12.9919 Å, S1=-1.42 TPa-1, 1/2S2=7.59 TPa-1) was the predominant phase in
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remelted coating [23]. Hence, the peak corresponding to (146) plane of α-alumina (134.20º˂2θ˂138.15º) was used for the stress measurement of laser treated coating. For chromia
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coating no phase change was observed before and after laser processing. The peak corresponding to (416) plane (122.45º˂2θ˂128.35º) (eskolaite, PDF card 00-038-1479, R-3c structure,
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a=4.9588 Å, c=13.5942 Å, S1=-1.24 TPa-1, 1/2S2=6.39 TPa-1) was selected for stress measurement [24]. Surface roughness of the coatings was measured using a contact type surface
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roughness tester TAYLOR HOBSON FORM TALYSURF 50 (INTRA 2) 3D profilometer (TAYLOR HOBSON, UK). The traverse length was 4 mm and 3 mm for as-sprayed and laser
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remelted coatings, respectively. Surface roughness was measured in the transverse direction of the laser remelted track. The melting depth of the both laser treated and non-treated coatings were measured using an optical microscope (Zeiss Axio. Vert.A1, Germany). Coating hardness was measured under a load of 3 N (=300 gf) using a microhardness tester (MVH-S AUTO, Omnitech, India). An average of ten hardness readings was considered for microhardness measurement for each coating. To estimate indentation fracture toughness, indentations were
made on the cross section of the sample using a micro hardness tester (MVH-S -AUTO, Omnitech, Pune, India) under a normal load of 300 g, and then the crack lengths were measured using Image J software. Indentation fracture toughness (KIC) values were calculated using the following expression proposed by Evans and Charles [25] c −3/2 H√a
a
(1)
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K IC = 0.15k ( )
ϕ
Where k=correction factor 3.2 for ceramics, ϕ=constraint factor≈3, H = Vickers hardness (MPa), a = half of Vickers diagonal length measured in the direction of crack (m), c = sum of length of the crack and half of the Vickers diagonal in the direction of the crack (m). This method can be used very effectively for the comparison of toughness values in brittle ceramic coatings [26].
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Young modulus of as-sprayed and laser treated coatings were carried out using an instrumented hardness tester (Micro Combi Tester (MCT), CSM instruments, Switzerland) equipped with a
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Vickers indenter. The measurements were performed under a maximum load of 3000 mN,
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loading and unloading rate 6000 mN and dwell time 15 s. Moduli of elasticity of the coating
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were then obtained from the slope of unloading curve. In this case also, an average of ten readings was reported. X-ray tomography instrument (phoenix vǀtomeǀx, GE) was used for
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porosity measurement of the coatings. Tomography parameters were as follows: voxel size 4.99 µm, voltage 100 kV, current 70 µA, detector type dxr-250rt. For imaging subsurface of the as-
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sprayed and the laser remelted coating, a FEI Strata TM DB235 dual beam machine, combining a Field Emission Scanning Electron Microscope (FESEM) (Zeiss Supra 40, Germany) column
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and a high-resolution Focused Ion Beam (FIB) column equipped with a Ga Liquid Metal Ion Source (LMIS) was used. A gold thin film was first applied to protect the area of interest from
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surface damage during ion beam machining. A larger ion beam current (2 nA) was used to mill a pocket in the protected area. The sidewall of the pocket, i.e., the area of interest, was subsequently polished using a lower beam current (100 pA). The samples thus prepared have
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been observed in situ using the FESEM, under an angle of 52º. 3. Results and discussion 3.1 Monitoring of thermo-cycles during laser processing
Fig. 1 and 2 show the time-temperature profile in the zone of interest while laser power and scanning speed was varied in the ranges of 250- 550W and 200 - 1000 mm/min, respectively. It may be noted from figure 2 that the temperature does not rise appreciably with the variation of scanning speed. However, there is a significant increase in the cooling rate with an increase in the scanning speed. At a lower scanning speed, the interaction time of the laser beam with the
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coating is higher. This allows enough time for the accumulated heat to escape to the substrate by conduction. As an effect, the substrate temperature rises and the thermal gradient between the remelted zone and substrate decreases, finally resulting in a decrease in the cooling rate. On the other hand, an increase in laser power beyond 400 W does not show any significant effect on the peak temperature and cooling rate (Fig. 2). It is observed from Fig. 2 that a change in laser power at a fixed scanning speed does not have an appreciable effect on interaction time and hence,
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cooling rate. A similar trend has been observed in the case of chromia coatings (plots not
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included).
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2000
1500
200 mm/min 300 mm/min 400 mm/min 500 mm/min 600 mm/min 1000 mm/min
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Temperature (ºC)
2500
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1000
1000
2000
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0
3000 4000 Time (ms)
5000
6000
7000
Fig. 1. Thermo-cycles obtained during laser remelting of alumina coating using different scanning speeds (Laser Power=400W).
550W 500W 450W 400W 350W 250W
2000
1500
1000 0
1000
2000
3000
4000 Time (ms)
5000
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Temperature (ºC)
2500
6000
7000
8000
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Fig. 2. Thermo-cycles obtained during laser remelting of chromia coating using different laser
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powers (Scanning speed=200 mm/min).
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The approximate cooling rates were calculated for both types of coating from the slopes of the
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cooling curves. Fig. 3 (a) and (b) shows the variation of cooling rate with the variation in laser power and scanning speed. It is observed from Fig. 3 (a) and (b) that the effect of laser power on
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sample cooling rate is limited as compared to scanning speed. Hence, subsequent experiments were carried out at a fixed power of 400 W. The variation in energy density was obtained using
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the following scanning speeds; 200, 300, 400 and 500 mm/min. The corresponding energy
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a
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densities were 96, 12, 16 and 24 J/mm2, respectively. b
Fig. 3. Variation of cooling rate during laser remelting of the coatings using different (a) power and (b) scanning speed. 3.2 Surface morphology
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3.2.1 Surface morphology of as-sprayed coating Typical surface morphology of as-sprayed alumina and chromia coatings is shown in Fig. 4 (a) and (b), respectively. Both types of coatings were formed by accumulation of large numbers of flattened splats. The surface roughness (Ra) of the as-sprayed alumina and chromia coating were
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7.96 ± 0.48 µm and 4.78±0.21 µm, respectively.
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Fig. 4. Secondary electron images of as-sprayed (a) alumina and (b) chromia coating.
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3.2.2 Surface morphology of laser remelted coating Surface roughness of the as-sprayed coating decreases after laser treatment owing to further
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melting and flattening of splats and densification of the remelted layer. Surface roughness values of laser treated coatings processed using different parameters are shown in Table 2. Surface
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roughness was restricted to around 3 µm while scanning speed was varied in the range of 200 500 mm/min. An increase in scanning speed beyond that produces poor surface quality owing to surface rippling effect. During laser processing, a temperature gradient is generated from the center of the laser beam (that coincides with the center line of the remelted track) to the border of the track. Temperature of the remelted layer is expected to be the highest at the center and it is reasonable to assume that the surface tension of the molten layer is lowest at the same point.
Away from the center, the temperature of the molten pool decreases and the surface tension of the melt increases. A higher surface tension results in a localized depression in the surface of the liquid in the central region of track. Away from the center the height of liquid level is higher and a pressure head develops owing to this height difference. This pressure difference sets up a liquid flow towards the center of the molten pool form the periphery. It may be noted that immediately
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after laser remelting the molten pool cools rapidly. Hence, solidification intervenes before height equilibrium is achieved in the melt pool and the quickly frozen liquid surface shows the presence of ripples [27]. Fig. 5 (a) and (b) shows the rippled surfaces of alumina and chromia coatings, respectively.
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Table 2
As-sprayed
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Alumina
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Type of coatings
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Surface roughness of as-sprayed and laser remelted coatings
1.78±0.35
remelted
300mm/min
1.65±0.56
coating
400mm/min
2.78±0.76
500mm/min
3.23±0.34
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200mm/min
As-sprayed
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Chromia
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7.96 ± 0.48
Laser
at 400W
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Surface roughness (µm)
4.78±0.21
Laser
200mm/min
1.21±0.66
remelted
300mm/min
1.93±0.10
coating
400mm/min
2.76±0.75
at 400W
500mm/min
2.97±0.32
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Fig. 5. Surface rippling in laser remelted (a) alumina and (b) chromia coatings (laser power=400W, scanning speed=1000mm/min).
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Fig. 6 (a) and (b) shows the remelted tracks of the alumina and chromia coatings, respectively. The power and scanning speed utilized for these tracks were 500 W and 2000 mm/min,
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respectively. A large number of surface pores appear in both coatings. This effect is attributed to
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(1) bubble formation during the escape of entrapped gases in the coating [28], (2) freezing of the
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displaced liquid before it can fill up the surface pores during laser remelting. It may be noted that a high scanning speed was employed in this case. This in turn induces a high cooling rate and followed by effects that promote the formation of surface pores. It may be noted that no surface
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pore appeared on the remelted layer while the power and speed range were restricted to 400 W
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and 200-500 mm/min, respectively (Section 3.1). b
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a
Fig. 6. Formation of surface pores on laser remelting under higher scanning speed in (a) alumina and (b) chromia coatings (laser power=500W, scanning speed=2000mm/min).
Fig. 7 (a) and (c) shows the higher magnification image of the alumina and chromia coating, respectively. A network of surface cracks was observed on both coatings. The formation of these cracks is attributed to the quenching stress generated during freezing of the remelted layer. A
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higher magnification image of the remelted alumina layer (Fig 7 (b)) reveals a dendritic structure [29]. Fig. 7 (d) shows the higher magnification secondary electron image of the surface of the laser remelted chromia coating. The microstructure appears to be dendritic. Single dendrites arranged themselves together to form a hexagonal structure. Similar results for laser remelted
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zirconia were reported by Morks et al. [30].
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A
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b
d
Fig. 7. (a) Surface crack network in laser remelted alumina, (b) Dendritic structure on the surface of laser remelted alumina, (c) Surface crack network in laser remelted chromia, (d) Dendritic structure on the surface of laser remelted chromia (laser power=400W, scanning
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speed=200mm/min).
3.3 Microstructure 3.3.1 Microstructure of as-sprayed coating
Fig. 8 (a) and 9 (a) show the optical image of the cross section of the alumina and chromia coatings, respectively. The top coat, bond coat, and substrate of the coating are clearly visible.
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The top coat contains pores. However, the coating shows good interfacial integrity. Fig. 8 (b) and
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9 (b) shows the fractured surfaces of the same coatings. From the micrograph, a lamellar structure ubiquitous to thermally sprayed coatings is identified. Growth of columnar grains,
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normal to the lamellar plane is also observed [31]. The inherent lamellar structure of flame
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sprayed coatings is heterogeneous with presence of voids, inter-splat boundary and intra-splat cracks. This heterogeneous structure has a strong influence on the mechanical properties of the
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coating. Fig. 9 (a) and (b) show the optical image of the cross-section and fractured surface of
b
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the as-sprayed chromia coating, respectively.
Fig. 8. (a) Optical image of the cross-section and (b) fractograph of as-sprayed alumina coating.
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b
a
Fig. 9. (a) Optical image of the cross-section (b) fractograph of as-sprayed chromia coating.
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3.3.2 Microstructure of laser remelted coating
Fig. 10 (a) and (b) shows the optical images of laser remelted alumina and chromia coatings,
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respectively. The contrast between laser remelted zone and as-sprayed zone of the coating is
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clearly visible in both coatings. The remelted layer is almost free of pores. A few cracks ran
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through the entire thickness of the remelted layer. These cracks segmented the coatings in a
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number of blocks.
Fig.10. Optical image of (a) alumina coating processed at 400 W and 200 mm/min (Energy density=24 J/mm2) (b) chromia coating processed at 400 W and 200 mm/min (Energy density=24 J/mm2).
The fractographs of laser remelted layers of alumina and chromia are shown in Fig. 11 (a) and (b), respectively. It is observed that upon remelting the inter-lamellar boundaries are no more there. The entire remelted layer appears to be monolithic with columnar grain growing in a
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direction perpendicular to the plane of the coating.
Fig. 11. Fractographs of laser remelted (a) alumina coating processed at 400 W and 200 mm/min density=24 J/mm2).
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3.4 Phases present in the coating
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(Energy density=24 J/mm2) (b) chromia coating processed at 400 W and 200 mm/min (Energy
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Fig. 12 (a) and (b) shows the X-ray diffraction patterns of alumina and chromia coatings, respectively. In case of alumina powder the primary phase is α-alumina. Thermal spraying involves rapid quenching and metastable γ-alumina nucleates from the melt in this circumstance.
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Some particles do not melt entirely and in those cases the α-phase grows from the α-alumina nucleus [22]. After laser remelting the metastable phase of the as-sprayed coating again
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transforms into the stable α-alumina phase. Hence, the peaks obtained from the X-ray diffraction pattern of the remelted layer (Fig 12 (a)) belong to the α-alumina phase. In the X-ray diffraction
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of the chromia powder, the chromia coating and the laser remelted layer show peaks of only one phase, i.e., Eskolaite, a rhombohedral phase. It indicates that chromia did not undergo any phase transformation either during coating formation and laser remelting. Schutz et al., observed existence of other phases like Cr3O4, CrO, or CrO2 in small quantity [32]. In the present investigation phases other than eskolaite were not detected possibly owing to their presence in small amount.
a
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b
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Fig. 12. X-ray diffraction of (a) alumina (b) chromia coatings.
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A preferential growth was observed in the X-ray diffraction pattern of the remelted coatings.
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This is attributed to a directional heat flow during cooling. Preferred orientation can be indexed
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using the texture coefficient values of a set of crystallographic planes under consideration.
1 𝐼 (ℎ 𝑘 𝑙) [∑ ] 𝑁 𝑁𝐼𝑜 (ℎ 𝑘 𝑙)
(2)
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𝑇𝑐 (ℎ 𝑘 𝑙) =
𝐼(ℎ 𝑘 𝑙) 𝐼𝑜(ℎ 𝑘 𝑙)
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Texture coefficient (Tc) for a given (h kl) plane can be calculated using the following equation
Where, I (h k l) is the measured intensity, Io (h k l) is the relative intensity of the corresponding plane
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given in PDF-2 data, and N is the number of reflections.
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The calculated texture coefficients are listed in Table 3. Consistently high values of texture coefficient were found for the (104) and (006) plane for alumina and chromia coating, respectively. This indicates that these two planes grow in preference to others while the remelted
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layers undergo solidification.
Table 3 Texture coefficients of laser remelted coatings Scanning
Texture coefficient (Tc)
power
speed
(W)
(mm/min)
Tc(012)
Tc(104)
Tc(110)
Tc(113)
Tc(012)
Tc(104)
Tc(104)
Tc(006)
400
200
0.02
0.89
3.05
0.0378
0.29
0.21
0.29
2.56
300
0.07
1.16
1. 49
0.595
1.13
0.94
0.49
1. 45
400
0.39
2.68
0.265
0.657
0.79
1.53
0.58
1.09
500
0. 45
2.59
0.53
1.73
0.15
1.86
0.66
1.62
Laser remelted chromia
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Laser remelted alumina
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Laser
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3.4.1 Melting depth
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Laser remelting was carried out in continuous mode. In continuous mode, the interaction time is estimated by the ratio of laser beam diameter to laser scanning speed. The laser beam used in this
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case offers a Gaussian type power distribution with maximum power density in the center and two annular rings in the periphery [33]. Fig. 13 shows the variation of remelting depth (thickness
𝐸=
𝑃 𝑑𝑣
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of the remelted layer) for the coatings. The energy density was calculated from the expression, (J/mm2) where P=Laser power (W), d=diameter of the laser beam (mm), v=scanning
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speed (mm/min). The thickness of the remelted layer was measured form the cross sectional micrographs. The melting depth increases with an increase in the energy density for both
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coatings. However, under the same energy density the alumina coating melts to a greater depth. This is attributed to lower melting point of alumina (2033ºC) as compared to chromia (2600ºC)
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[16].
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Fig. 13. Variation of melt depth with laser energy density in the laser remelted coatings.
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3.5 X-Ray Tomography
Fig. 15 (a) and (b) shows the X-ray CT scan of the as-sprayed alumina and chromia coatings,
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respectively. Presence of pores is considered as defect in thermally sprayed coating [34]. The
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adverse effect of porosity on the coating properties is proportional to the size of the pores [35, 36]. It is apparent from Fig. 15 that flame sprayed chromia coating is more porous as compared
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a
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to alumina coating. The porosity values are also listed in Table 4.
b
Fig. 14. CT-scan of flame sprayed (a) alumina and (b) chromia coatings. Fig. 16 (a) and (b) shows the CT scan of laser remelted alumina and chromia coating, respectively, processed using energy density (24 J/mm2) and scanning speed (200 mm/min). Laser treatment of the as-sprayed coating reduces the number of pores and their volumes. The
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porosity data listed in Table 4 indicates that the volumetric pore density decreases with a decrease in the scanning speed, i.e., increase in laser energy density.
b
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a
Fig. 15. CT-scan of laser remelted (a) alumina and (b) chromia coatings processed using 400 W
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laser power and 200 mm/min scanning speed (Energy density=24 J/mm2). 3.6 Mechanical properties of the laser remelted coatings
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Fig. 14 (a) and (b) shows the cross sectional hardness profile of alumina and chromia coating, respectively, plotted against the distance from the edge of the coating. A significant increase in
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micro-hardness was observed in both coatings after laser treatment. Hazra et al. [37] made a similar observation in the case of laser remelted mullite coatings. The laser treated coatings have shown a gradual reduction in hardness from the top edge of the remelted layer to the inside of the
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coating. This is attributed to laser assisted densification. The layer immediately below the remelted one is the so-called heat affected zone. The heat affected zone also undergoes some densification resulting in an increase in the hardness as compared to the as-sprayed coating. Below this layer lies the as-sprayed layer unaffected by laser post processing. As expected, it has micro-hardness equal to the as-sprayed coating. Another possible explanation of this improvement, as observed by Morks et al., [30] is grain refinement in the remelted region.
a
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b
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Fig. 16. Depth profile of microhardness in laser remelted (a) alumina and (b) chromia coatings.
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Table 4 indicates that the indentation fracture toughness increases with a decrease in coating porosity for the remelted coatings. Inter-splat boundaries of as-sprayed coating were obliterated
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upon laser remelting. The cohesion of the monolithic, homogenized laser remelted layer is
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expected to be higher than that of a porous, lamellar as-spayed flame sprayed coating. This in turn, results in an increase in the indentation fracture toughness of the remelted layer. A
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reduction in porosity also brings about an increase in the Young’s modulus in the remelted
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Table 4
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coatings as compared to their as-sprayed counterpart [38].
Young’s modulus and indentation fracture toughness of the coatings
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Types of coatings
Alumina
as-sprayed
Porosity (%)
Young modulus
Fracture toughness
(GPa)
(MPa m-1/2)
6.49
85.6±11
-
Laser
200mm/min
0.50
162.07±8
2.2±0.2
remelted
300mm/min
0.80
138.45±3
2.4±0.1
coating
400mm/min
1.42
154.96±7
1.97±0.4
at 400W Chromia
500mm/min
as-sprayed
1.78
149.58±9
1.79±0.2
9.17
91.6±8
-
200mm/min
0.37
194.54±6
3.1±0.1
remelted
300mm/min
0.55
178.84±10
2.8±0.1
coating
400mm/min
1.86
186.23±7
1.9±0.1
at 400W
500mm/min
2.09
172.65±3
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Laser
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3.7 Residual stress
1.7±0.1
Generation of residual stress is an important consideration in the case of thermally sprayed
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coatings. Various coating performances such as adhesion strength, resistance to thermal shock,
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etc., depend upon the residual stress developed in the coating [39]. The primary sources of
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residual stress in thermally sprayed coatings are of two types. The first one is the quenching stress that develops during rapid cooling of individual splats after deposition on the substrate
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surface. The second one is attributed to the differential thermal contraction between the substrate and the coating [40]. Quenching stress is always tensile in nature whereas the differential thermal
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contraction stress is either tensile or compressive depending upon the difference in thermal expansion coefficient values of the substrate and the coating material. Moreover, volume
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changes associated with phase transformation during the deposition process may also contribute to the residual stress in the coating.
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Fig. 17 shows the variation of residual stress with cooling rate for alumina and chromia coatings. The as-sprayed alumina coating harbors mild tensile residual stress [24]. This is
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attributed to differential thermal expansion between the bond coat and top coat. It may be noted that thermal expansion coefficient of the alumina top coat and Ni-5wt%Al bond coat is 7×10-6 (1/K) and 12×10-6 (1/K), respectively [9]. The tensile residual stress increases with an increase in the cooling rate, i.e., increase in scanning speed. This increase is attributed to a higher quenching rate obtained at a higher scanning speed [41, 42]. Hence, the magnitude of tensile stress increases with an increase in the cooling rate. For as-sprayed chromia coating, the residual stress was
found to be mildly compressive. Upon laser remelting, a mild compressive residual stress was recorded up to scanning speed 400 mm/min. However, a stress reversal from mild compressive
EP
TE
D
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A
N
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to mild tensile occurs between 500 and 600 mm/min scanning speed.
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Fig.17. Variation in surface residual stress of the remelted coatings with scanning speed.
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3.8 Effect of preheating Fig. 18 (a) shows a FIB milled pocket on the as-sprayed alumina coating. Subsurface cracks are
visible from this image. The surface cracks propagate below the surface to form a network of cracks. This crack network develops during coating deposition. A network of surface cracks also
forms in the laser remelted layer owing to quenching. These cracks also extend below the surface
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of the coating. Fig. 18 (b) shows such a crack in the laser remelted alumina coating.
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Fig. 18. Secondary electron images of the FIB milled pockets of (a) as-sprayed alumina (b) laser
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remelted alumina (processed at laser power=400 W and scanning speed=200 mm/min).
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Fig. 19 (a), (b) and (c) show the top views of the surfaces of a three laser remelted coatings. The
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sample corresponding to Fig. 19 (a) was produced by remelting a coating kept initially at the ambient temperature. Fig. 19 (b) and (c) correspond to the samples that were preheated to 450 ºC
D
and 850 ºC, respectively, prior to remelting. A dense network of surface cracks was observed in Fig. 19 (a), i.e., on the sample that was not preheated. The crack network is less dense in Fig. 19
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(b) and almost imperceptible in Fig. 19 (c). This indicates that preheating before remelting reduces the surface crack density significantly. A preheated coating undergoes less quenching
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and hence, less shrinkage during cooling of the remelted layer. The extent of cracking also depends on the preheating temperature. Surface residual stresses of the coatings preheated before
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remelting are tabulated in Table 5. The residual stresses are tensile in nature. However, the magnitudes of the residual stresses are significantly lower than those in the remelted layers
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produced without preheating. The sample preheated to 850 ºC was found to be almost stress free. These results again corroborate to the fact that the quenching stress is reduced significantly upon preheating, prior to laser remelting.
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Fig. 19. Surface of laser remelted alumina (a) without preheating (b) preheated at 450 ºC (c) preheated at 850 ºC. Table 5 Residual stress of the preheated samples.
parameter
temperature (ºC)
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400 W power and
Fracture
(MPa)
toughness (MPa m-1/2)
450 ºC
95.6±11.4
-
850 ºC
20.4±6.8
1.67±0.3
No preheating
228.2±54.9
1.79±0.2
TE
D
500 mm/min
Residual stress
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Preheating
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Alumina
Laser remelting
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Coating type
The sample preheated at 850 ºC prior to remelting was appeared to be almost crack free. Therefore, it was selected for further investigation. Fig. 20 (a) and (b) show the optical images of
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the cross-section and fractograph of this preheated sample, respectively. The remelted layer was found to be pore free. However, a few transverse cracks were found in the remelted layer. A
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columnar structure perpendicular to the coating surface was found to grow in the remelted layer. These microstructural features of the preheated sample were similar to those of the non-
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preheated sample. However, the melting depth in this case was found to be 165.7±20 µm. Melting to this depth could not be achieved on remelting without preheating (Fig. 14). Upon preheating, absorptivity of the sample surface increases. This is demonstrated by Wang et al. [39]. An increase in laser energy absorption produces an increase in the depth of the remelted layer
a
Columnar structure
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b
Fig. 20. (a) Optical cross-sectional micrograph of the laser remelted alumina coating preheated to 850 ºC (b) Fractured surface of laser remelted alumina coating preheated to 850 ºC (laser power=400 W, scanning speed=500 mm/min).
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Fig. 21. (a) and (b), respectively show the microhardness depth profiles and the phases of the
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alumina coating remelted with and without preheating. Fig 21 (a) shows, preheating does not
A
have a significant effect on the hardness profile. Phases evolved on remelting are same for both the coatings as well (Fig 21 (b)). Indentation fracture toughness of these coatings is listed in table
M
4. A comparison with Table 3 shows, preheating does not have an appreciable effect on the
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indentation fracture toughness of the remelted coatings.
Fig. 21. (a) Depth profile of hardness with and without preheating (b) Phases of laser remelted alumina with and without preheating (laser power=400 W, scanning speed=500 mm/min). 4. Conclusions
The work deals with the laser remelting of powder flame sprayed alumina and chromia coatings. It has been found from in-situ temperature history of the molten layer that the cooling rate depends to a greater extent on laser scanning speed as compared to laser power. Both type of remelted coatings present dendritic structure on their surfaces. The lamellar boundaries usually present in the thermally sprayed coating are obliterated upon laser remelting. In the remelted
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layer, columnar grains were found to grow in a plane perpendicular to the coating surface. The laser remelted layer was homogenous and almost pore free. Upon laser treatment, the hardness of the remelted layer was found to increase by up to 31% as compared to the as-sprayed coating. Immediately after laser treatment, a network of surface cracks develops owing to rapid quenching of the remelted layer. Flame sprayed alumina and chromia coatings were found to harbor tensile and compressive residual stress on their surfaces, respectively. Laser remelted
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alumina coating harbors tensile residual stress. The magnitude of the surface residual stress
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increases with an increase in cooling rate. Laser remelted chromia coating preserves its compressive stress state even after laser treatment. In the case of alumina coating, an increase in
A
melting depth was found on preheating the coating to 850ºC prior to remelting. Preheating also
M
produced a decrease in the surface residual stress of the remelted alumina layer.
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