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Microstructural characterization of plasma sprayed conventional and nanostructured coatings with nitrogen as primary plasma gas
Surface & Coatings Technology 235 (2013) 424–432
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Microstructural characterization of plasma sprayed conventional and nanostructured coatings with nitrogen as primary plasma gas V. Bolleddu, V. Racherla, P.P. Bandyopadhyay ⁎ Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur 721302, India
a r t i c l e
i n f o
Article history: Received 16 May 2013 Accepted in revised form 31 July 2013 Available online 8 August 2013 Keywords: Plasma spraying CPSP Coating Nanostructured coatings Bimodal microstructures
a b s t r a c t Air plasma sprayed conventional and nanostructured Al2O3–13 wt.%TiO2, WC–12–17 wt.%Co, and ZrO2–7– 8 wt.%Y2O3 coatings were deposited using nitrogen as the primary plasma gas. In nanostructured coatings, as critical plasma spray parameter (CPSP) – which is defined as the ratio of gun or arc power to the primary gas flow rate – controls the volume fraction of unmelted/partially melted regions, which in turn control the properties of coatings, coatings for all systems considered in this work were obtained as a function of CPSP. Coating properties, e.g. porosity, microhardness and percentage of partially melted/unmelted regions, for nanostructured coatings, were seen to be monotonic functions of CPSP. However, the effect of CPSP on percentage of partially melted/unmelted regions was significantly smaller for nitrogen as compared to argon as the primary plasma gas, particularly for nanostructured Al2O3–13 wt.%TiO2 coatings. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Thermal spray coatings from nanostructured agglomerated ceramic powders have received considerable attention in the recent past owing to their improved performance over coatings from corresponding conventional powders [1–3]. By choosing process conditions such that particle temperatures before they hit the substrate are just around the melting point, it can be ensured that coatings from nanostructured agglomerated powders have a nano character to them [4]. When complete melting of a fraction of agglomerated particles is prevented, the unmelted and partially melted particles appear as inclusions within the fully melted matrix resulting in a bimodal coating microstructure. In addition to uniform mixing of constituents, the enhanced performance of nanostructured coatings has also been attributed to the above described bimodal nature of microstructures, particularly for alumina–13 wt.% titania coatings [5–7]. Research on nanostructured coatings is relatively new, having started in late 1990s, and has been limited to alumina–13 wt.% titania (Al2O3–13 wt.%TiO2), titania (TiO2), tungsten carbide cobalt (WC–8–25 wt.%Co), yttria stabilized zirconia (ZrO2–7–8 wt.%Y2O3), and hydroxyapatite (HA) coating systems. Further, among these, majority of earlier works compare the performance of alumina–titania, tungsten carbide cobalt and yttria stabilized zirconia coatings obtained using conventional and nanostructured powders. It may be noted that the formation of bimodal microstructures is aided by the fact that, in several coating systems, the constituents in agglomerated powders have different melting points. For example, in alumina– ⁎ Corresponding author. Tel.: +91 3222 282950; fax: +91 3222 282278. E-mail addresses: [email protected], [email protected] (P.P. Bandyopadhyay). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.07.069
titania powders, while titania has a melting point of 1855 °C [8] alumina has a melting point of 2072 °C. One important parameter for nanostructured coatings is CPSP, i.e., the ratio of arc power and primary gas flow rate [7,9–12]. The effect of CPSP on coating properties like hardness, porosity, degree of bimodality, has not been studied systematically for materials other than alumina–titania. In addition, essentially all earlier works on air plasma spraying of nanostructured coatings use argon as the primary plasma gas. In this work nitrogen has been used since this gas is used as the primary plasma gas in air plasma spraying in many industries, particularly in developing countries, as it is cheaper and more widely available than argon. In this context, this work aims to systematically study the effect of CPSP on coating properties for three most widely used nanostructured coatings: Al2O3–13 wt.%TiO2, WC–17 wt.%Co, and ZrO2–7 wt.% Y2O3, deposited using nitrogen as the primary plasma gas. 2. Experimental procedure Air plasma sprayed coatings were deposited on AISI 1020 steel substrates of 65 × 46 × 5 mm size using a Sulzer Metco 3MB plasma gun. Nitrogen and hydrogen were used as primary and secondary plasma gases. The process parameters used for deposition of coatings are listed in Tables 1 and 2. The conventional powders have been sprayed using parameters corresponding to the highest CPSP used for its nanostructured counterpart (Table 2). To prepare the specimens for deposition of coatings, an Alex NH 500 surface grinder was used to grind the top and bottom faces of substrates to remove the oxide layers and create flat surfaces. The substrates were then grit blasted using alumina grits of grit size 24 at 100 psi air pressure and cleaned ultrasonically for 20 min in acetone/iso-propanal bath. Next, they were preheated to
V. Bolleddu et al. / Surface & Coatings Technology 235 (2013) 424–432 Table 1 Common process parameters used for deposition of coatings by atmospheric plasma spraying (APS). Serial number
Process parameter
Value
1 2 3 6 7 8 9 10 11
Spraying distance (mm) Primary gas Secondary gas Gas flow ratio in SCFH (N2/H2) Substrate preheating temperature (°C) Hydrogen pressure (psi) Nitrogen pressure (psi) Powder injection angle (°) Powder feed rate (g/min)
125 Nitrogen (N2) Hydrogen (H2) 3.3 200 80 80 90 25
around 200 °C using the plasma gun, with nitrogen as the only plasma gas, before depositing a 100–120 μm thick Ni–20 wt.%Cr (Metco 43CNS) bond coat. A 250–350 μm thick top coats were then deposited using three conventional and nanostructured agglomerated feed stock powders: Al2O3–13 wt.%TiO2 (Metco 130 SF, Nanox S2613S), WC–12–17 wt.%Co (Metco 72F-NS, Infralloy S7417-15) and ZrO2–7– 8 wt.%Y2O3 (Metco 204B-NS, Nanox S4007). The tap densities for nanostructured Al2O3–13 wt.%TiO2, WC–17 wt.%Co and ZrO2–8 wt.%Y2O3 are 1.4 g/cm3, ~5 g/cm3, and 1.55 ± 0.15 g/cm3, respectively. While, nanostructured agglomerated powders were sourced from Inframat Inc., CT, USA, conventional powders were obtained from Sulzer Metco, Westbury, NY, USA. For further analyses, specimens of 10 × 5 × 5 mm size were cut from coated samples, using a low speed diamond saw and cross sectional specimens for metallographic examination have been prepared from the sliced coupons. Optical micrographs are obtained using an Olympus optical microscope (Model: GX41, DP 12). Coating porosity was measured from optical micrographs taken at six randomly selected locations, and analyzed using “ImageJ” software at a fixed grey scale threshold value of 188. Mean and standard deviation of porosity are estimated based on area fraction of pores obtained from six images. Coating microstructures were observed using a Zeiss Evo 60 scanning electron microscope (SEM) and a Zeiss Supra 40 field emission scanning electron microscope (FE-SEM). Image analyses of SEM and FESEM micrographs, using ten different images for each coating, were used for estimating the volume percentage of partially melted/unmelted regions in nanostructured coatings. More specifically, at a given CPSP, for each coating, ten SEM/FE-SEM images were taken at a fixed magnification of 1000×. Each image was printed on an A4 sheet and via visual inspection the partially melted/unmelted regions were identified and cut out. The initial weight of the sheet and the cumulative weight of partially melted/unmelted regions were then used to calculate the area/volume fraction of partially melted/unmelted regions in the image. Mean and Table 2 Currents and gas flow rates used to obtain variation in CPSP. Coating system
Current (A)
Voltage (V)
N2 gas flow rate (SCFH)
H2 gas flow rate (SCFH)
CPSP
Nanostructured Al2O3–13 wt.%TiO2
400 400 450 450 500 500 400 400 450 450 400 400 450 450 500 500
84 88 90 91 90 92 82 83 85 85 83 86 85 88 90 94
50 65 50 65 50 65 50 65 50 65 50 65 50 65 50 65
15 20 15 20 15 20 15 20 15 20 15 20 15 20 15 20
672 542 810 630 900* 707 656 511 765* 588 664 530 765 610 900* 723
Nanostructured WC–17 wt.%Co
Nanostructured 7 wt.% Yttria stabilized zirconia (YSZ)
⁎Conventional coatings were deposited at these CPSPs for the three coating systems.
425
standard deviation of volume fraction of partially/unmelted regions in the coating at a given CPSP were then calculated based on readings obtained from all images obtained at that CPSP. Hardness measurements were made using a LECO LM 700 microhardness tester having Vickers indenter at 100 g load and 15 s dwell time. Ten readings were taken for each coating and the average has been reported. X-ray diffraction studies were carried out using a PW1070 instrument with CuKα radiation with tension and intensity of generator at 40 kV and 30 mA, respectively, scanning step of 0.016711° and step time of 0.13 s. 3. Results and discussion Powder morphologies for conventional and nanostructured agglomerated Al2O3–13 wt.%TiO2 feedstock powders are shown in Fig. 1a, b. It is clear that conventional alumina–titania powders prepared using the fusing and crushing technique are blocky, dense, and angular, whereas the corresponding nanostructured agglomerated powders prepared using the spray drying and sintering technique are porous and nearly spherical. The spherical morphology of agglomerated powders results in better powder flow characteristics. Further, the porous nature of agglomerated powders results in lower thermal diffusivities for these powders and is expected to lead to partial melting of some of the powder particles. SEM micrographs for conventional and nanostructured agglomerated WC–12–17 wt.%Co powders are shown in Fig. 1c, d. It can be seen that the conventional powders prepared via the sintering and crushing method are porous, irregular and blocky. The corresponding agglomerated powders prepared using the spray drying and sintering techniques are spherical and porous. Once again, the spherical morphology of agglomerated powders results in better flow characteristics. Morphologies of conventional and nanostructured agglomerated ZrO2–7–8 wt.% Y2O3 powders are shown in Fig. 1e, f. It is known that the conventional yttria stabilized zirconia powders prepared using the hallow oven spherical particle (HOSP) process are spherical and hollow with a dense outer shell. These powders have a tap density of 2.3 ± 0.2 g/cm3. The corresponding nanostructured agglomerated powders are also spherical but have a lower tap density of 1.55 ± 0.15 g/cm3. Cross-sectional SEM micrographs of conventional and nanostructured Al2O3–13 wt.%TiO2 coatings are shown in Fig. 2. The following are evident from the Fig. 2a–d: (i) Four distinct features can be seen in coatings deposited from nanostructured alumina–titania powders: Partially melted (PM) regions composed of nano alumina particles in titania rich matrix embedded in fully melted alumina–titania matrix, titania rich spots that appear as white patches in micrographs observed using a backscattered electron detector, uniform distribution of micro-cracks nearly aligned with the coating direction, and preferential distribution of nearly spherical pores in PM regions, (ii) PM regions in nanostructured coatings tend to arrest micro-cracks as seen from insert in Fig. 2b, (iii) segregation of titania in fully melted regions decreases with increasing CPSP as confirmed by minimal presence of white titania rich splats in Fig. 2d, (iv) porosity of nanostructured coatings is higher than that for conventional ones, particularly at lower CPSPs, (v) inter splat bonding for conventional as well nanostructured coatings, particularly those at higher CPSPs, is “good”, and (vi) nearly uniform distribution of micro-cracks, in the coating direction, occurs in conventional as well as nanostructured coatings. It may be noted that white titania rich regions in conventional and nanostructured coatings, PM regions in nanostructured coatings, and nearly spherical pores appearing in nanostructured coatings act as crack arrestors. It may also be noted from Fig. 2b–d that the percentage of PM region obtained with nitrogen as primary plasma gas, even at lowest CPSP considered here, is far smaller than those typically obtained for corresponding coatings deposited using argon as the primary plasma gas [7,10,11]. This is attributed to the fact that nitrogen in the ionised state has a much higher specific enthalpy at a given temperature as compared to argon [13]. In addition, presence of hydrogen increases the thermal conductivity of the gas followed by heat transfer to the particles [14–16]. Hence, the
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a
b
3µm
10µm
10µm
c
d
10µm
3µm
e
f
10µm
10µm
Fig. 1. Powder morphologies for conventional and nanostructured (a, b) alumina–titania, (c, d) tungsten carbide–cobalt and (e, f) yttria stabilized zirconia, powders.
b
a Pores
CPSP = 542
PM region
Fully melted region Titania rich splats
20µm
c
CPSP = 707
10µm
10µm
d
CPSP = 900
10µm
Fig. 2. Coating microstructures for (a) conventional alumina–13 wt.% titania and (b–d) nanostructured alumina–13 wt.% titania coatings deposited at CPSPs 542, 707 and 900, respectively. The insert in b shows an enlarged view of a partially melted region (PM region) at 542 CPSP. In this figure and the following ones, CPSP value is mentioned only for nanostructured coatings and that conventional coatings are deposited at process parameters corresponding to the highest CPSP for that coating system.
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nitrogen–hydrogen combination brings about a more intensive particle melting. As stated earlier, titania has a lower melting point (1855 °C) than that for alumina (2072 °C) and hence at a lower CPSP, i.e., at lower particle temperatures, melting and segregation of titania is promoted while alumina is still in solid state [17]. Cross-sectional SEM and FE-SEM micrographs for conventional and nanostructured tungsten carbide–cobalt coatings, respectively, are shown in Fig. 3. The following observations can be made from the Fig. 3a–d: (i) Partially melted regions occur in conventional as well as nanostructured coatings. In fact, volume percentage of PM regions in conventional coatings deposited at process parameters corresponding to the highest CPSP (i.e. 765 CPSP) appears to be higher than that of corresponding nanostructured coatings deposited at any of the CPSPs, (ii) inter splat bonding appears to be weaker for conventional coatings and for nanostructured coatings deposited at lower CPSPs, (iii) as expected, percentage of unmelted, sharp edged, sub-micron, tungsten– carbide particles in partially melted regions is highest at lowest CPSP, (iv) sub-micron cracks along the coating direction and high levels of porosity in PM regions present in alumina–titania coatings, are absent here, (v) shape, orientation, and degree of bonding of partially melted regions with surrounding matrix, depends on the fraction of unmelted particles within the PM region. For powder particles undergoing less degree of melting, a large number of unmelted WC particles make the corresponding droplet highly viscous and less amenable to spreading, leading to lower aspect ratio splats with possibly high surface roughness and weak bonding with surrounding matrix. This indicates possible weaker inter splat bonding for nanostructured coatings deposited at lower CPSPs. Cross-sectional SEM micrographs of conventional and nanostructured yttria stabilized zirconia coatings are shown in Fig. 4. The following qualitative observations can be made from the Figure: (i) Crack density in conventional coatings (Fig. 4a) is much higher than that in nanostructured coatings (Fig. 4b–d). Further, unlike cracks in alumina– titania coatings which were nearly aligned with the coating direction, cracks in conventional yttria stabilized zirconia coatings do not have a preferred orientation, (ii) among all the coatings, nanostructured coatings deposited at highest CPSP have the lowest porosity (conventional coatings also are deposited at the highest CPSP) and the “best”
a
inter splat bonding, (iii) conventional as well as nanostructured yttria stabilized zirconia coatings do not exhibit well defined splat structures, (iv) the unmelted/partially melted regions in nano-structured coatings are irregular, highly porous and weakly bonded to the fully melted matrix. Therefore, they are not expected to improve the fracture toughness or wear resistance of the coatings. More specifically, the nanostructured coatings deposited at the highest CPSP are expected to have the best wear resistance and cohesive fracture toughness, and (v) interconnected crack density in yttria stabilized zirconia coatings (except for nanostructured coatings at highest CPSP) is much higher than for alumina–titania or tungsten carbide–cobalt coatings. This is expected to result in weaker wear resistance of the coatings, especially at higher loads and higher sliding velocities, when fatigue damage – which is sensitive to crack density – controls the wear rate and mass loss. It may be noted that the cross-sectional SEM micrographs for conventional and nanostructured yttria stabilized zirconia coatings obtained here are aligned with the observations reported for corresponding coatings deposited using argon as primary plasma gas [18–20]. Porosity for conventional coatings of these three systems sprayed at process parameters corresponding to the respective highest CPSPs are 6, 4.2 and 7.6%, respectively. Porosity in nanostructured Al2O3– 13 wt.%TiO2, WC–17 wt.%Co, and ZrO2–7 wt.%Y2O3 coatings as a function of CPSP are shown in Fig. 5. The following can be noted from the Fig. 5a–c: (i) Porosity decreases monotonically with CPSP in all coating systems. Further, the decrease with CPSP in all systems is nearly linear, (ii) the observed porosities are in line with observations reported in literature for corresponding coatings deposited with argon as primary plasma gas [28,29]. For example, air plasma sprayed nanostructured coatings deposited at standard process parameters using Argon as primary plasma gas were observed to have porosities of around 8–10.5% and around 8% for Al2O3–13 wt.%TiO2 [11] and ZrO2–7 wt.%Y2O3 [21] coatings, respectively, (iii) porosities of Al2O3–13 wt.%TiO2 and ZrO2– 7 wt.%Y2O3 are far higher than that for WC–17 wt.%Co coatings, (iv) effect of CPSP on coating porosity is strongest in yttria stabilized zirconia coatings, and (v) porosity of conventional and nanostructured tungsten carbide–cobalt coatings is of the same order. However, while nanostructured Al2O3–13 wt.%TiO2 coatings are more porous as compared
b
Fully melted region
CPSP = 511
Fully melted region PM region
PM region
10µm
10µm
c
427
CPSP = 656
10µm
d
CPSP= 765
10µm
Fig. 3. Coating microstructures for (a) conventional tungsten carbide–12 wt.% cobalt, (b–d) nanostructured tungsten carbide–17 wt.% cobalt coatings deposited at CPSPs 511, 656 and 765, respectively.
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a
b
CPSP = 530
10µm
10µm
c
d
CPSP = 664
CPSP = 900
10µm
10µm
Fig. 4. Coating microstructures for (a) conventional 8 wt.% yttria stabilized zirconia, (b–d) nanostructured 7 wt.% yttria stabilized zirconia coatings deposited at CPSPs 530, 664 and 900, respectively.
12
a
8 6 4
4 3 2
2 0 500
b
5
Porosity (%)
Porosity (%)
10
WC-17wt% Co coating
6
Al2O3-13wt% TiO2 coating
1
550
600
650
700
750
800
850
900
950
0 450
500
550
600
CPSP
700
750
800
ZrO2-7wt% Y2O3 coating
12
c
10
Porosity (%)
650
CPSP
8 6 4 2 0 500
550
600
650
700
750
800
850
900
950
CPSP Fig. 5. Porosity variation in nanostructured (a) alumina–13 wt.% titania, (b) tungsten carbide–17 wt.% cobalt and (c) 7 wt.% yttria stabilized zirconia coatings as a function of CPSP. Porosity for conventional coatings of these three systems sprayed at process parameters corresponding to the respective highest CPSPs are 6, 4.2, and 7.6% respectively.
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to their conventional counterparts, nanostructured yttria stabilized zirconia coatings are less porous as compared to their conventional counterpart. It may also be noted that alumina–titania coatings deposited using nitrogen as the primary plasma gas are slightly more porous than corresponding coatings deposited using argon as the primary plasma gas [28,29]. Hardness for the conventional coatings of these three systems sprayed at process parameters corresponding to the respective highest CPSPs are 9.91, 15.6, and 6.74 GPa, respectively. Microhardness of nanostructured coatings as a function of CPSP is shown in Fig. 6. The following can be seen from the Fig. 6a–c: (i) In all coating systems, microhardness increases nearly linearly with CPSP. This is on expected lines as porosity in all coating systems decreases with increasing CPSP, (ii) effect of CPSP on microhardness of coatings, is strongest in ZrO2–7 wt.%Y2O3 nanostructured coatings, and (iii) the micro hardness values obtained here, for nanostructured coatings deposited using nitrogen as primary plasma gas, are similar to that for corresponding coatings deposited using argon as the primary plasma gas. For example, while microhardness values of around 8.5–11, 8, and 8.4 GPa were obtained for air plasma sprayed nanostructured Al2O3–13 wt.%TiO2 [9,28,29], WC–17 wt.%Co [21], and ZrO2– 7 wt.%Y2O3 [22] coatings, respectively, deposited using argon as the primary plasma gas. The values obtained in this work for the above three coating systems are around 8–12, 10–14, and 5–8 GPa, respectively. Percentage of partially melted regions as a function of CPSP, for each of the coating systems, along with some comparative results from literature are shown in Fig. 7. It is clear from Fig. 7a, b that, the percentage of partially melted regions in nanostructured Al2O3–13 wt.% TiO2 coatings are much more sensitive to CPSP when the coatings are deposited using argon as compared to nitrogen as the primary plasma gas [7,28].
Further, while highest amount of partially melted regions obtained for nanostructured Al2O3–13 wt.%TiO2 coatings deposited using nitrogen as the primary plasma gas is around 17% that for corresponding coatings deposited using argon as primary plasma gas is around 45% [7]. Interestingly, as seen from Fig. 7a, c, d, the amount of partially melted regions in nanostructured coatings, is of the order of 10–17%, for all the three coating systems considered in this work. Thus, it is evident from Figs. 5–7 that optimal performance of nanostructured coatings deposited using nitrogen as the primary plasma gas can be expected at higher CPSPs where the coating porosity is lower, hardness is higher, and the fraction of PM regions is moderate and not very different from that obtained at lower CPSPs. X-ray diffraction patterns for nanostructured Al2O3–13 wt.%TiO2, WC–17 wt.%Co, and ZrO2–7 wt.%Y2O3 coatings, deposited at a low and a high CPSP, are shown in Figs. 8–10. It is evident from Fig. 8 that ‘γ-Al2O3’ is the dominant phase in the coatings. More specifically, by comparing the integrated peak area intensities of the phases as per the procedure cited in [31] the volume fraction of ‘α-alumina’ phase in coatings deposited at 542 and 900 CPSPs was found to be around 2.63% and 3.67%, respectively. Note that the ‘α-alumina’ content in the coatings is much smaller as compared the content in coatings deposited using argon [12] where ‘α-alumina’ volume fractions of about 5–30% were observed. Note that, the volume fraction of α-Al2O3 is low at higher as well as lower CPSPs in coatings deposited using nitrogen as the primary plasma gas. This is on expected lines as higher specific enthalpy of nitrogen, at a specific plasma gas temperature, would result in greater melting of nanostructured agglomerated particles, which in turn results in lower volume fractions of PM regions and lower volume fraction of the ‘α-alumina’ phase.
16
14
WC-17wt% Co coating
Al2O3-13wt% TiO2 coating
14
a Hardness HV100 (GPa)
12
Hardness HV100 (GPa)
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10 8 6 4 2 0 500
b
12 10 8 6 4 2
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c
Hardness HV100 (GPa)
8
6
4
2
0
500
550
600
650
700
750
800
850
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CPSP Fig. 6. Microhardness of nanostructured (a) alumina–13 wt.% titania, (b) tungsten carbide–17 wt.% cobalt and (c) 7 wt.% yttria stabilized zirconia coatings as a function of CPSP.
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Al2O3-13wt% TiO2 coating
16
Al2O3-13wt% TiO2 coating
b
a
Amount of PM region (%)
Amount of PM region (%)
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14 12 10 8 6 4
Results from Luo et al., 2003 for argon as primary plasma gas
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CPSP
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CPSP
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Fig. 7. Variation of amount of partially melted region (PM region) in nanostructured coatings with CPSP for (a, b) alumina–13 wt.% titania (c) tungsten carbide–17 wt.% cobalt and (d) 7 wt.% yttria stabilized zirconia coatings.
It may be noted from Fig. 9 that there is significant decarburization in nanostructured tungsten carbide–cobalt coatings leading to the formation of tungsten (W) and brittle W2C phases. In fact, the decarburization in nanostructured tungsten carbide–cobalt coatings deposited at 511 CPSP is so high that tungsten rather than tungsten carbide is the dominant phase in coatings. Though, not shown here, the decarburization is much smaller in conventional coatings, where WC is the dominant phase and the relative intensities of the decarburised species are on similar levels as reported in literature for conventional coatings deposited
1000
using argon as the primary plasma gas [23–25]. The high temperature in a plasma jet (around 12,000 K) brings about decomposition in a WC–CO coating and hence, the preferred mode of deposition of such coating is high velocity oxy-fuel (HVOF) spray, a process that involves much lower temperature (around 3000 K) [26]. Note from Fig. 10, that nanostructured yttria stabilized zirconia coatings predominantly contain the metastable, non-transformable, t′-tetragonal zirconia phase, which occurs from rapid cooling of coatings during air plasma spraying. These coatings also contain a small volume fraction of cubic zirconia phase as
Al2O3-13wt% TiO2 coating
a
1400
Al2O3-13wt% TiO2 coating
b
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-Alumina
: -Alumina 400 : Rutile Titania
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: Alumina
400 200 O
O
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O
0
0 60
(degree)
80
100
-200
20
40
60
80
(degree)
Fig. 8. XRD patterns for nanostructured alumina–13 wt.% titania coatings deposited at CPSPs (a) 542 and (b) 900, respectively.
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3000 2500
511 CPSP
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Fig. 9. XRD patterns for nanostructured tungsten carbide–17 wt.% cobalt coatings deposited at CPSPs (a) 511 and (b) 765, respectively.
7000 6000
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Intensity (a.u)
Intensity (a.u)
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b
80
100
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(degree)
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60
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(degree)
Fig. 10. XRD patterns for nanostructured 7 wt.% yttria stabilized zirconia coatings deposited at CPSPs (a) 530 and (b) 900, respectively.
indicated by the peak at 73.238° in Fig. 10. This result is in line with the literature [21,22,27,30] on YSZ coatings deposited using argon as the primary plasma gas. 4. Conclusions Air plasma spraying was used to deposit nanostructured Al2O3– 13 wt.%TiO2, WC–17 wt.%Co, and ZrO2–7 wt.%Y2O3 coatings along with relevant conventional coatings using nitrogen as the primary plasma gas. The microstructure and mechanical properties of the obtained coatings were analyzed using several characterization techniques. The following conclusions were drawn based on this work: • The volume fraction of partially melted regions in coatings deposited using a wide range of process parameters for all three coating systems considered in this work were in the range of around 10–17%. In this case, nitrogen has been used as the primary plasma gas. This volume fraction range is considered to be low for similar coatings deposited using argon as the primary plasma gas. This is attributed to a higher specific enthalpy of nitrogen at a specified gas temperature, bringing about a more intensive melting action. • Porosity and hardness of nanostructured coatings are on similar lines as that of corresponding coatings deposited using argon as the primary plasma gas. • Based on relative intensities of α and γ-Al2O3 peaks in X-ray diffraction patterns, it is noted that percentage of α-Al2O3 in nanostructured
alumina–titania coatings is much smaller than percentages in corresponding coatings deposited using argon. Further, critical plasma spray parameter does not seem to influence the volume fraction of α-Al2O3 phase significantly. • Significant decarburization occurs in nanostructured WC–17 wt.%Co coatings. In fact, at a lower critical plasma spray parameter, tungsten (W) was found to be the dominant phase rather than tungsten carbide (WC). • Based on porosity, microhardness, and volume fraction of partially melted regions of nanostructured coatings deposited using nitrogen as the primary plasma gas, for three coating systems considered in this work, coatings deposited at the highest critical plasma spray parameter are expected to offer the best performance. References [1] X.Q. Zhao, H.D. Zhou, J.M. Chen, Mater. Sci. Eng. A 431 (2006) 290–297. [2] C. Zhou, N. Wang, H. Xu, Mater. Sci. Eng. A 452–453 (2007) 569–574. [3] E. Sanchez, E. Bannier, V. Cantavella, M.D. Salvador, E. Klyatskina, J. Morgiel, J. Grzonka, A.R. Boccaccini, J. Therm. Spray Technol. 17 (2008) 329–337. [4] R.S. Lima, B.R. Marple, J. Therm. Spray Technol. 16 (2007) 40–62. [5] Y. Wang, S. Jiang, M. Wang, S. Wang, T.D. Xiao, P.R. Strutt, Wear 237 (2000) 176–185. [6] E.H. Jordan, M. Gell, Y.H. Sohn, D. Goberman, L. Shaw, S. Jiang, M. Wang, T.D. Xiao, Y. Wang, P. Strutt, Mater. Sci. Eng. A 301 (2001) 80–89. [7] H. Luo, D. Goberman, L. Shaw, M. Gell, Mater. Sci. Eng. A 346 (2003) 237–245. [8] R.S. Lima, B.R. Marple, Mater. Des. 29 (2008) 1845–1855. [9] L.L. Shaw, D. Goberman, R. Ren, M. Gell, S. Jiang, Y. Wang, T.D. Xiao, P.R. Strutt, Surf. Coat. Technol. 130 (2000) 1–8.
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[10] M. Gell, E.H. Jordan, Y.H. Sohn, D. Goberman, L. Shaw, T.D. Xiao, Surf. Coat. Technol. 146–147 (2001) 48–54. [11] S.T. Aruna, N. Balaji, S. Jyothi, V.K. William Grips, Surf. Coat. Technol. 208 (2012) 92–100. [12] D. Goberman, H. Sohn, L. Shaw, E. Jordan, M. Gell, Acta Mater. 50 (2002) 1141–1152. [13] H.S. Ingham, A.J. Fabel, Weld J. 1 (1975) 101. [14] R.B. Heiman, VCH, Weinheim, Germany, First edition (1996). [15] C. Escure, M. Vardelle, P. Fauchais, Plasma Chem. Plasma Process. 23 (2003) 185–221. [16] C. Tekmen, Y. Tsunekawa, M. Okumiya, Surf. Coat. Technol. 203 (2009) 1649–1655. [17] D. Wang, Z. Tian, L. Shen, Z. Liu, Y. Huang, Surf. Coat. Technol. 203 (2009) 1298–1303. [18] F.A. Guo, N. Trannoy, D. Gerday, J. Eur. Ceram. Soc. 25 (2005) 1159–1166. [19] W.B. Gong, C.K. Sha, D.Q. Sun, W.Q. Wang, Surf. Coat. Technol. 201 (2006) 3109–3115.
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Microstructural characterization of plasma sprayed conventional and nanostructured coatings with nitrogen as primary plasma gas V. Bolleddu, V. Racherla, P.P. Bandyopadhyay ⁎ Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur 721302, India
a r t i c l e
i n f o
Article history: Received 16 May 2013 Accepted in revised form 31 July 2013 Available online 8 August 2013 Keywords: Plasma spraying CPSP Coating Nanostructured coatings Bimodal microstructures
a b s t r a c t Air plasma sprayed conventional and nanostructured Al2O3–13 wt.%TiO2, WC–12–17 wt.%Co, and ZrO2–7– 8 wt.%Y2O3 coatings were deposited using nitrogen as the primary plasma gas. In nanostructured coatings, as critical plasma spray parameter (CPSP) – which is defined as the ratio of gun or arc power to the primary gas flow rate – controls the volume fraction of unmelted/partially melted regions, which in turn control the properties of coatings, coatings for all systems considered in this work were obtained as a function of CPSP. Coating properties, e.g. porosity, microhardness and percentage of partially melted/unmelted regions, for nanostructured coatings, were seen to be monotonic functions of CPSP. However, the effect of CPSP on percentage of partially melted/unmelted regions was significantly smaller for nitrogen as compared to argon as the primary plasma gas, particularly for nanostructured Al2O3–13 wt.%TiO2 coatings. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Thermal spray coatings from nanostructured agglomerated ceramic powders have received considerable attention in the recent past owing to their improved performance over coatings from corresponding conventional powders [1–3]. By choosing process conditions such that particle temperatures before they hit the substrate are just around the melting point, it can be ensured that coatings from nanostructured agglomerated powders have a nano character to them [4]. When complete melting of a fraction of agglomerated particles is prevented, the unmelted and partially melted particles appear as inclusions within the fully melted matrix resulting in a bimodal coating microstructure. In addition to uniform mixing of constituents, the enhanced performance of nanostructured coatings has also been attributed to the above described bimodal nature of microstructures, particularly for alumina–13 wt.% titania coatings [5–7]. Research on nanostructured coatings is relatively new, having started in late 1990s, and has been limited to alumina–13 wt.% titania (Al2O3–13 wt.%TiO2), titania (TiO2), tungsten carbide cobalt (WC–8–25 wt.%Co), yttria stabilized zirconia (ZrO2–7–8 wt.%Y2O3), and hydroxyapatite (HA) coating systems. Further, among these, majority of earlier works compare the performance of alumina–titania, tungsten carbide cobalt and yttria stabilized zirconia coatings obtained using conventional and nanostructured powders. It may be noted that the formation of bimodal microstructures is aided by the fact that, in several coating systems, the constituents in agglomerated powders have different melting points. For example, in alumina– ⁎ Corresponding author. Tel.: +91 3222 282950; fax: +91 3222 282278. E-mail addresses: [email protected], [email protected] (P.P. Bandyopadhyay). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.07.069
titania powders, while titania has a melting point of 1855 °C [8] alumina has a melting point of 2072 °C. One important parameter for nanostructured coatings is CPSP, i.e., the ratio of arc power and primary gas flow rate [7,9–12]. The effect of CPSP on coating properties like hardness, porosity, degree of bimodality, has not been studied systematically for materials other than alumina–titania. In addition, essentially all earlier works on air plasma spraying of nanostructured coatings use argon as the primary plasma gas. In this work nitrogen has been used since this gas is used as the primary plasma gas in air plasma spraying in many industries, particularly in developing countries, as it is cheaper and more widely available than argon. In this context, this work aims to systematically study the effect of CPSP on coating properties for three most widely used nanostructured coatings: Al2O3–13 wt.%TiO2, WC–17 wt.%Co, and ZrO2–7 wt.% Y2O3, deposited using nitrogen as the primary plasma gas. 2. Experimental procedure Air plasma sprayed coatings were deposited on AISI 1020 steel substrates of 65 × 46 × 5 mm size using a Sulzer Metco 3MB plasma gun. Nitrogen and hydrogen were used as primary and secondary plasma gases. The process parameters used for deposition of coatings are listed in Tables 1 and 2. The conventional powders have been sprayed using parameters corresponding to the highest CPSP used for its nanostructured counterpart (Table 2). To prepare the specimens for deposition of coatings, an Alex NH 500 surface grinder was used to grind the top and bottom faces of substrates to remove the oxide layers and create flat surfaces. The substrates were then grit blasted using alumina grits of grit size 24 at 100 psi air pressure and cleaned ultrasonically for 20 min in acetone/iso-propanal bath. Next, they were preheated to
V. Bolleddu et al. / Surface & Coatings Technology 235 (2013) 424–432 Table 1 Common process parameters used for deposition of coatings by atmospheric plasma spraying (APS). Serial number
Process parameter
Value
1 2 3 6 7 8 9 10 11
Spraying distance (mm) Primary gas Secondary gas Gas flow ratio in SCFH (N2/H2) Substrate preheating temperature (°C) Hydrogen pressure (psi) Nitrogen pressure (psi) Powder injection angle (°) Powder feed rate (g/min)
125 Nitrogen (N2) Hydrogen (H2) 3.3 200 80 80 90 25
around 200 °C using the plasma gun, with nitrogen as the only plasma gas, before depositing a 100–120 μm thick Ni–20 wt.%Cr (Metco 43CNS) bond coat. A 250–350 μm thick top coats were then deposited using three conventional and nanostructured agglomerated feed stock powders: Al2O3–13 wt.%TiO2 (Metco 130 SF, Nanox S2613S), WC–12–17 wt.%Co (Metco 72F-NS, Infralloy S7417-15) and ZrO2–7– 8 wt.%Y2O3 (Metco 204B-NS, Nanox S4007). The tap densities for nanostructured Al2O3–13 wt.%TiO2, WC–17 wt.%Co and ZrO2–8 wt.%Y2O3 are 1.4 g/cm3, ~5 g/cm3, and 1.55 ± 0.15 g/cm3, respectively. While, nanostructured agglomerated powders were sourced from Inframat Inc., CT, USA, conventional powders were obtained from Sulzer Metco, Westbury, NY, USA. For further analyses, specimens of 10 × 5 × 5 mm size were cut from coated samples, using a low speed diamond saw and cross sectional specimens for metallographic examination have been prepared from the sliced coupons. Optical micrographs are obtained using an Olympus optical microscope (Model: GX41, DP 12). Coating porosity was measured from optical micrographs taken at six randomly selected locations, and analyzed using “ImageJ” software at a fixed grey scale threshold value of 188. Mean and standard deviation of porosity are estimated based on area fraction of pores obtained from six images. Coating microstructures were observed using a Zeiss Evo 60 scanning electron microscope (SEM) and a Zeiss Supra 40 field emission scanning electron microscope (FE-SEM). Image analyses of SEM and FESEM micrographs, using ten different images for each coating, were used for estimating the volume percentage of partially melted/unmelted regions in nanostructured coatings. More specifically, at a given CPSP, for each coating, ten SEM/FE-SEM images were taken at a fixed magnification of 1000×. Each image was printed on an A4 sheet and via visual inspection the partially melted/unmelted regions were identified and cut out. The initial weight of the sheet and the cumulative weight of partially melted/unmelted regions were then used to calculate the area/volume fraction of partially melted/unmelted regions in the image. Mean and Table 2 Currents and gas flow rates used to obtain variation in CPSP. Coating system
Current (A)
Voltage (V)
N2 gas flow rate (SCFH)
H2 gas flow rate (SCFH)
CPSP
Nanostructured Al2O3–13 wt.%TiO2
400 400 450 450 500 500 400 400 450 450 400 400 450 450 500 500
84 88 90 91 90 92 82 83 85 85 83 86 85 88 90 94
50 65 50 65 50 65 50 65 50 65 50 65 50 65 50 65
15 20 15 20 15 20 15 20 15 20 15 20 15 20 15 20
672 542 810 630 900* 707 656 511 765* 588 664 530 765 610 900* 723
Nanostructured WC–17 wt.%Co
Nanostructured 7 wt.% Yttria stabilized zirconia (YSZ)
⁎Conventional coatings were deposited at these CPSPs for the three coating systems.
425
standard deviation of volume fraction of partially/unmelted regions in the coating at a given CPSP were then calculated based on readings obtained from all images obtained at that CPSP. Hardness measurements were made using a LECO LM 700 microhardness tester having Vickers indenter at 100 g load and 15 s dwell time. Ten readings were taken for each coating and the average has been reported. X-ray diffraction studies were carried out using a PW1070 instrument with CuKα radiation with tension and intensity of generator at 40 kV and 30 mA, respectively, scanning step of 0.016711° and step time of 0.13 s. 3. Results and discussion Powder morphologies for conventional and nanostructured agglomerated Al2O3–13 wt.%TiO2 feedstock powders are shown in Fig. 1a, b. It is clear that conventional alumina–titania powders prepared using the fusing and crushing technique are blocky, dense, and angular, whereas the corresponding nanostructured agglomerated powders prepared using the spray drying and sintering technique are porous and nearly spherical. The spherical morphology of agglomerated powders results in better powder flow characteristics. Further, the porous nature of agglomerated powders results in lower thermal diffusivities for these powders and is expected to lead to partial melting of some of the powder particles. SEM micrographs for conventional and nanostructured agglomerated WC–12–17 wt.%Co powders are shown in Fig. 1c, d. It can be seen that the conventional powders prepared via the sintering and crushing method are porous, irregular and blocky. The corresponding agglomerated powders prepared using the spray drying and sintering techniques are spherical and porous. Once again, the spherical morphology of agglomerated powders results in better flow characteristics. Morphologies of conventional and nanostructured agglomerated ZrO2–7–8 wt.% Y2O3 powders are shown in Fig. 1e, f. It is known that the conventional yttria stabilized zirconia powders prepared using the hallow oven spherical particle (HOSP) process are spherical and hollow with a dense outer shell. These powders have a tap density of 2.3 ± 0.2 g/cm3. The corresponding nanostructured agglomerated powders are also spherical but have a lower tap density of 1.55 ± 0.15 g/cm3. Cross-sectional SEM micrographs of conventional and nanostructured Al2O3–13 wt.%TiO2 coatings are shown in Fig. 2. The following are evident from the Fig. 2a–d: (i) Four distinct features can be seen in coatings deposited from nanostructured alumina–titania powders: Partially melted (PM) regions composed of nano alumina particles in titania rich matrix embedded in fully melted alumina–titania matrix, titania rich spots that appear as white patches in micrographs observed using a backscattered electron detector, uniform distribution of micro-cracks nearly aligned with the coating direction, and preferential distribution of nearly spherical pores in PM regions, (ii) PM regions in nanostructured coatings tend to arrest micro-cracks as seen from insert in Fig. 2b, (iii) segregation of titania in fully melted regions decreases with increasing CPSP as confirmed by minimal presence of white titania rich splats in Fig. 2d, (iv) porosity of nanostructured coatings is higher than that for conventional ones, particularly at lower CPSPs, (v) inter splat bonding for conventional as well nanostructured coatings, particularly those at higher CPSPs, is “good”, and (vi) nearly uniform distribution of micro-cracks, in the coating direction, occurs in conventional as well as nanostructured coatings. It may be noted that white titania rich regions in conventional and nanostructured coatings, PM regions in nanostructured coatings, and nearly spherical pores appearing in nanostructured coatings act as crack arrestors. It may also be noted from Fig. 2b–d that the percentage of PM region obtained with nitrogen as primary plasma gas, even at lowest CPSP considered here, is far smaller than those typically obtained for corresponding coatings deposited using argon as the primary plasma gas [7,10,11]. This is attributed to the fact that nitrogen in the ionised state has a much higher specific enthalpy at a given temperature as compared to argon [13]. In addition, presence of hydrogen increases the thermal conductivity of the gas followed by heat transfer to the particles [14–16]. Hence, the
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a
b
3µm
10µm
10µm
c
d
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3µm
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Fig. 1. Powder morphologies for conventional and nanostructured (a, b) alumina–titania, (c, d) tungsten carbide–cobalt and (e, f) yttria stabilized zirconia, powders.
b
a Pores
CPSP = 542
PM region
Fully melted region Titania rich splats
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c
CPSP = 707
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Fig. 2. Coating microstructures for (a) conventional alumina–13 wt.% titania and (b–d) nanostructured alumina–13 wt.% titania coatings deposited at CPSPs 542, 707 and 900, respectively. The insert in b shows an enlarged view of a partially melted region (PM region) at 542 CPSP. In this figure and the following ones, CPSP value is mentioned only for nanostructured coatings and that conventional coatings are deposited at process parameters corresponding to the highest CPSP for that coating system.
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nitrogen–hydrogen combination brings about a more intensive particle melting. As stated earlier, titania has a lower melting point (1855 °C) than that for alumina (2072 °C) and hence at a lower CPSP, i.e., at lower particle temperatures, melting and segregation of titania is promoted while alumina is still in solid state [17]. Cross-sectional SEM and FE-SEM micrographs for conventional and nanostructured tungsten carbide–cobalt coatings, respectively, are shown in Fig. 3. The following observations can be made from the Fig. 3a–d: (i) Partially melted regions occur in conventional as well as nanostructured coatings. In fact, volume percentage of PM regions in conventional coatings deposited at process parameters corresponding to the highest CPSP (i.e. 765 CPSP) appears to be higher than that of corresponding nanostructured coatings deposited at any of the CPSPs, (ii) inter splat bonding appears to be weaker for conventional coatings and for nanostructured coatings deposited at lower CPSPs, (iii) as expected, percentage of unmelted, sharp edged, sub-micron, tungsten– carbide particles in partially melted regions is highest at lowest CPSP, (iv) sub-micron cracks along the coating direction and high levels of porosity in PM regions present in alumina–titania coatings, are absent here, (v) shape, orientation, and degree of bonding of partially melted regions with surrounding matrix, depends on the fraction of unmelted particles within the PM region. For powder particles undergoing less degree of melting, a large number of unmelted WC particles make the corresponding droplet highly viscous and less amenable to spreading, leading to lower aspect ratio splats with possibly high surface roughness and weak bonding with surrounding matrix. This indicates possible weaker inter splat bonding for nanostructured coatings deposited at lower CPSPs. Cross-sectional SEM micrographs of conventional and nanostructured yttria stabilized zirconia coatings are shown in Fig. 4. The following qualitative observations can be made from the Figure: (i) Crack density in conventional coatings (Fig. 4a) is much higher than that in nanostructured coatings (Fig. 4b–d). Further, unlike cracks in alumina– titania coatings which were nearly aligned with the coating direction, cracks in conventional yttria stabilized zirconia coatings do not have a preferred orientation, (ii) among all the coatings, nanostructured coatings deposited at highest CPSP have the lowest porosity (conventional coatings also are deposited at the highest CPSP) and the “best”
a
inter splat bonding, (iii) conventional as well as nanostructured yttria stabilized zirconia coatings do not exhibit well defined splat structures, (iv) the unmelted/partially melted regions in nano-structured coatings are irregular, highly porous and weakly bonded to the fully melted matrix. Therefore, they are not expected to improve the fracture toughness or wear resistance of the coatings. More specifically, the nanostructured coatings deposited at the highest CPSP are expected to have the best wear resistance and cohesive fracture toughness, and (v) interconnected crack density in yttria stabilized zirconia coatings (except for nanostructured coatings at highest CPSP) is much higher than for alumina–titania or tungsten carbide–cobalt coatings. This is expected to result in weaker wear resistance of the coatings, especially at higher loads and higher sliding velocities, when fatigue damage – which is sensitive to crack density – controls the wear rate and mass loss. It may be noted that the cross-sectional SEM micrographs for conventional and nanostructured yttria stabilized zirconia coatings obtained here are aligned with the observations reported for corresponding coatings deposited using argon as primary plasma gas [18–20]. Porosity for conventional coatings of these three systems sprayed at process parameters corresponding to the respective highest CPSPs are 6, 4.2 and 7.6%, respectively. Porosity in nanostructured Al2O3– 13 wt.%TiO2, WC–17 wt.%Co, and ZrO2–7 wt.%Y2O3 coatings as a function of CPSP are shown in Fig. 5. The following can be noted from the Fig. 5a–c: (i) Porosity decreases monotonically with CPSP in all coating systems. Further, the decrease with CPSP in all systems is nearly linear, (ii) the observed porosities are in line with observations reported in literature for corresponding coatings deposited with argon as primary plasma gas [28,29]. For example, air plasma sprayed nanostructured coatings deposited at standard process parameters using Argon as primary plasma gas were observed to have porosities of around 8–10.5% and around 8% for Al2O3–13 wt.%TiO2 [11] and ZrO2–7 wt.%Y2O3 [21] coatings, respectively, (iii) porosities of Al2O3–13 wt.%TiO2 and ZrO2– 7 wt.%Y2O3 are far higher than that for WC–17 wt.%Co coatings, (iv) effect of CPSP on coating porosity is strongest in yttria stabilized zirconia coatings, and (v) porosity of conventional and nanostructured tungsten carbide–cobalt coatings is of the same order. However, while nanostructured Al2O3–13 wt.%TiO2 coatings are more porous as compared
b
Fully melted region
CPSP = 511
Fully melted region PM region
PM region
10µm
10µm
c
427
CPSP = 656
10µm
d
CPSP= 765
10µm
Fig. 3. Coating microstructures for (a) conventional tungsten carbide–12 wt.% cobalt, (b–d) nanostructured tungsten carbide–17 wt.% cobalt coatings deposited at CPSPs 511, 656 and 765, respectively.
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a
b
CPSP = 530
10µm
10µm
c
d
CPSP = 664
CPSP = 900
10µm
10µm
Fig. 4. Coating microstructures for (a) conventional 8 wt.% yttria stabilized zirconia, (b–d) nanostructured 7 wt.% yttria stabilized zirconia coatings deposited at CPSPs 530, 664 and 900, respectively.
12
a
8 6 4
4 3 2
2 0 500
b
5
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Al2O3-13wt% TiO2 coating
1
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ZrO2-7wt% Y2O3 coating
12
c
10
Porosity (%)
650
CPSP
8 6 4 2 0 500
550
600
650
700
750
800
850
900
950
CPSP Fig. 5. Porosity variation in nanostructured (a) alumina–13 wt.% titania, (b) tungsten carbide–17 wt.% cobalt and (c) 7 wt.% yttria stabilized zirconia coatings as a function of CPSP. Porosity for conventional coatings of these three systems sprayed at process parameters corresponding to the respective highest CPSPs are 6, 4.2, and 7.6% respectively.
V. Bolleddu et al. / Surface & Coatings Technology 235 (2013) 424–432
to their conventional counterparts, nanostructured yttria stabilized zirconia coatings are less porous as compared to their conventional counterpart. It may also be noted that alumina–titania coatings deposited using nitrogen as the primary plasma gas are slightly more porous than corresponding coatings deposited using argon as the primary plasma gas [28,29]. Hardness for the conventional coatings of these three systems sprayed at process parameters corresponding to the respective highest CPSPs are 9.91, 15.6, and 6.74 GPa, respectively. Microhardness of nanostructured coatings as a function of CPSP is shown in Fig. 6. The following can be seen from the Fig. 6a–c: (i) In all coating systems, microhardness increases nearly linearly with CPSP. This is on expected lines as porosity in all coating systems decreases with increasing CPSP, (ii) effect of CPSP on microhardness of coatings, is strongest in ZrO2–7 wt.%Y2O3 nanostructured coatings, and (iii) the micro hardness values obtained here, for nanostructured coatings deposited using nitrogen as primary plasma gas, are similar to that for corresponding coatings deposited using argon as the primary plasma gas. For example, while microhardness values of around 8.5–11, 8, and 8.4 GPa were obtained for air plasma sprayed nanostructured Al2O3–13 wt.%TiO2 [9,28,29], WC–17 wt.%Co [21], and ZrO2– 7 wt.%Y2O3 [22] coatings, respectively, deposited using argon as the primary plasma gas. The values obtained in this work for the above three coating systems are around 8–12, 10–14, and 5–8 GPa, respectively. Percentage of partially melted regions as a function of CPSP, for each of the coating systems, along with some comparative results from literature are shown in Fig. 7. It is clear from Fig. 7a, b that, the percentage of partially melted regions in nanostructured Al2O3–13 wt.% TiO2 coatings are much more sensitive to CPSP when the coatings are deposited using argon as compared to nitrogen as the primary plasma gas [7,28].
Further, while highest amount of partially melted regions obtained for nanostructured Al2O3–13 wt.%TiO2 coatings deposited using nitrogen as the primary plasma gas is around 17% that for corresponding coatings deposited using argon as primary plasma gas is around 45% [7]. Interestingly, as seen from Fig. 7a, c, d, the amount of partially melted regions in nanostructured coatings, is of the order of 10–17%, for all the three coating systems considered in this work. Thus, it is evident from Figs. 5–7 that optimal performance of nanostructured coatings deposited using nitrogen as the primary plasma gas can be expected at higher CPSPs where the coating porosity is lower, hardness is higher, and the fraction of PM regions is moderate and not very different from that obtained at lower CPSPs. X-ray diffraction patterns for nanostructured Al2O3–13 wt.%TiO2, WC–17 wt.%Co, and ZrO2–7 wt.%Y2O3 coatings, deposited at a low and a high CPSP, are shown in Figs. 8–10. It is evident from Fig. 8 that ‘γ-Al2O3’ is the dominant phase in the coatings. More specifically, by comparing the integrated peak area intensities of the phases as per the procedure cited in [31] the volume fraction of ‘α-alumina’ phase in coatings deposited at 542 and 900 CPSPs was found to be around 2.63% and 3.67%, respectively. Note that the ‘α-alumina’ content in the coatings is much smaller as compared the content in coatings deposited using argon [12] where ‘α-alumina’ volume fractions of about 5–30% were observed. Note that, the volume fraction of α-Al2O3 is low at higher as well as lower CPSPs in coatings deposited using nitrogen as the primary plasma gas. This is on expected lines as higher specific enthalpy of nitrogen, at a specific plasma gas temperature, would result in greater melting of nanostructured agglomerated particles, which in turn results in lower volume fractions of PM regions and lower volume fraction of the ‘α-alumina’ phase.
16
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WC-17wt% Co coating
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a Hardness HV100 (GPa)
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CPSP Fig. 6. Microhardness of nanostructured (a) alumina–13 wt.% titania, (b) tungsten carbide–17 wt.% cobalt and (c) 7 wt.% yttria stabilized zirconia coatings as a function of CPSP.
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Al2O3-13wt% TiO2 coating
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Al2O3-13wt% TiO2 coating
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Results from Luo et al., 2003 for argon as primary plasma gas
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Fig. 7. Variation of amount of partially melted region (PM region) in nanostructured coatings with CPSP for (a, b) alumina–13 wt.% titania (c) tungsten carbide–17 wt.% cobalt and (d) 7 wt.% yttria stabilized zirconia coatings.
It may be noted from Fig. 9 that there is significant decarburization in nanostructured tungsten carbide–cobalt coatings leading to the formation of tungsten (W) and brittle W2C phases. In fact, the decarburization in nanostructured tungsten carbide–cobalt coatings deposited at 511 CPSP is so high that tungsten rather than tungsten carbide is the dominant phase in coatings. Though, not shown here, the decarburization is much smaller in conventional coatings, where WC is the dominant phase and the relative intensities of the decarburised species are on similar levels as reported in literature for conventional coatings deposited
1000
using argon as the primary plasma gas [23–25]. The high temperature in a plasma jet (around 12,000 K) brings about decomposition in a WC–CO coating and hence, the preferred mode of deposition of such coating is high velocity oxy-fuel (HVOF) spray, a process that involves much lower temperature (around 3000 K) [26]. Note from Fig. 10, that nanostructured yttria stabilized zirconia coatings predominantly contain the metastable, non-transformable, t′-tetragonal zirconia phase, which occurs from rapid cooling of coatings during air plasma spraying. These coatings also contain a small volume fraction of cubic zirconia phase as
Al2O3-13wt% TiO2 coating
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Al2O3-13wt% TiO2 coating
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Fig. 8. XRD patterns for nanostructured alumina–13 wt.% titania coatings deposited at CPSPs (a) 542 and (b) 900, respectively.
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Fig. 9. XRD patterns for nanostructured tungsten carbide–17 wt.% cobalt coatings deposited at CPSPs (a) 511 and (b) 765, respectively.
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Fig. 10. XRD patterns for nanostructured 7 wt.% yttria stabilized zirconia coatings deposited at CPSPs (a) 530 and (b) 900, respectively.
indicated by the peak at 73.238° in Fig. 10. This result is in line with the literature [21,22,27,30] on YSZ coatings deposited using argon as the primary plasma gas. 4. Conclusions Air plasma spraying was used to deposit nanostructured Al2O3– 13 wt.%TiO2, WC–17 wt.%Co, and ZrO2–7 wt.%Y2O3 coatings along with relevant conventional coatings using nitrogen as the primary plasma gas. The microstructure and mechanical properties of the obtained coatings were analyzed using several characterization techniques. The following conclusions were drawn based on this work: • The volume fraction of partially melted regions in coatings deposited using a wide range of process parameters for all three coating systems considered in this work were in the range of around 10–17%. In this case, nitrogen has been used as the primary plasma gas. This volume fraction range is considered to be low for similar coatings deposited using argon as the primary plasma gas. This is attributed to a higher specific enthalpy of nitrogen at a specified gas temperature, bringing about a more intensive melting action. • Porosity and hardness of nanostructured coatings are on similar lines as that of corresponding coatings deposited using argon as the primary plasma gas. • Based on relative intensities of α and γ-Al2O3 peaks in X-ray diffraction patterns, it is noted that percentage of α-Al2O3 in nanostructured
alumina–titania coatings is much smaller than percentages in corresponding coatings deposited using argon. Further, critical plasma spray parameter does not seem to influence the volume fraction of α-Al2O3 phase significantly. • Significant decarburization occurs in nanostructured WC–17 wt.%Co coatings. In fact, at a lower critical plasma spray parameter, tungsten (W) was found to be the dominant phase rather than tungsten carbide (WC). • Based on porosity, microhardness, and volume fraction of partially melted regions of nanostructured coatings deposited using nitrogen as the primary plasma gas, for three coating systems considered in this work, coatings deposited at the highest critical plasma spray parameter are expected to offer the best performance. References [1] X.Q. Zhao, H.D. Zhou, J.M. Chen, Mater. Sci. Eng. A 431 (2006) 290–297. [2] C. Zhou, N. Wang, H. Xu, Mater. Sci. Eng. A 452–453 (2007) 569–574. [3] E. Sanchez, E. Bannier, V. Cantavella, M.D. Salvador, E. Klyatskina, J. Morgiel, J. Grzonka, A.R. Boccaccini, J. Therm. Spray Technol. 17 (2008) 329–337. [4] R.S. Lima, B.R. Marple, J. Therm. Spray Technol. 16 (2007) 40–62. [5] Y. Wang, S. Jiang, M. Wang, S. Wang, T.D. Xiao, P.R. Strutt, Wear 237 (2000) 176–185. [6] E.H. Jordan, M. Gell, Y.H. Sohn, D. Goberman, L. Shaw, S. Jiang, M. Wang, T.D. Xiao, Y. Wang, P. Strutt, Mater. Sci. Eng. A 301 (2001) 80–89. [7] H. Luo, D. Goberman, L. Shaw, M. Gell, Mater. Sci. Eng. A 346 (2003) 237–245. [8] R.S. Lima, B.R. Marple, Mater. Des. 29 (2008) 1845–1855. [9] L.L. Shaw, D. Goberman, R. Ren, M. Gell, S. Jiang, Y. Wang, T.D. Xiao, P.R. Strutt, Surf. Coat. Technol. 130 (2000) 1–8.
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