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Thermal shock properties and microstructure investigation of LVPS and HVOF-CoNiCrAlYSi coatings on the IN738LC superalloy
Vacuum 88 (2013) 124e129
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Thermal shock properties and microstructure investigation of LVPS and HVOF-CoNiCrAlYSi coatings on the IN738LC superalloy M. Mohammadi a, *, S. Javadpour a, A. Kobayashi b, S.A. Jenabali Jahromi a, K. Shirvani c a
Department of Materials Science and Engineering, Shiraz University, Shiraz, Iran Joining & Welding Res. Inst., Osaka University, Ibaraki, Osaka 567-0047, Japan c Department of Advance Materials and New Energies, Institute of Advance Technologies, IROST, Tehran, Iran b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 November 2011 Received in revised form 2 February 2012 Accepted 3 February 2012
The thermal shock behavior and microstructural investigation of CoNiCrAlYSi coatings applied by Low Vacuum Plasma Spray (LVPS) and High-Velocity Oxy Fuel (HVOF) techniques on the Ni-base superalloy IN738LC were examined. Thermal shock resistance was investigated by forced water quenching of coated samples from high temperature (1100 C) to room temperature. Thermal shock life time of coatings were determined by measuring the weight gain of the specimens at regular intervals for a duration of 65 cycles. Characterizations of the coatings and oxide scales were investigated by SEM equipped with EDS and XRD analyses. Oxidation kinetic obtained from weight measurement and morphological observations indicate better thermal shock resistance of LVPS than HVOF coatings. Larger grain size of protective oxides on the HVOF coatings led to more thermal stresses emanated from severe thermal cycling and spallation of the coatings. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: HVOF LVPS Cyclic thermal shock Spallation Thermal stresses
1. Introduction Materials for high temperature turbine environments face severe operating conditions. These components are subjected to degradation due to high temperature oxidation and corrosive gasses that evolve during combustion and service operation. Nibased superalloys are mainly used in high temperature application. The compositions of these superalloys are such that, they can provide good strength and creep resistance at high temperature but do not provide suitable environmental protection alone; therefore, coatings are used to protect the underlying materials from degradation at harsh working condition [1e3]. Extensive research efforts over decades led to advanced protective coatings for turbine components. Therefore the duplex protective system consisting of overlay MCrAlY bond coat (BC) and ceramic top coat (TC) such as 8 wt.% yttria partially stabilized zirconia (8PYSZ) are used mainly in the turbine component and aero engines against high temperature oxidation and high temperature corrosion. This protective system, called thermal barrier coating (TBC), allows the increase of the gas inlet temperature [4e7]. The service-life of MCrAlY coatings depends greatly on the aluminum and chromium content, which would be consumed as a result of scale growth and spallation of Al2O3/Cr2O3 in thermal * Corresponding author. E-mail address: [email protected] (M. Mohammadi). 0042-207X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2012.02.003
cycles and aggressive environments [8,9]. Formation of adherence, continuous, dense and stable protective oxide, is affected by some factors such as chemical composition of bond coat, environmental condition, working temperature and coating processes [1,2,10]. Effect of coating process on the oxidation and hot corrosion resistance were reported by many researchers, but thermal shock resistance of different coating methods were less discussed and compared. Thermal shock behavior of NiCoCrAlYSiB coatings were investigated by Wang et al. [11,12]. According to this research, accelerated oxidation can be attributed to the accelerating oxide formation and breakdown due to compressive stresses accumulation in the oxides due to oxide growth at high temperature and thermal mismatch during cooling. In this study, cyclic thermal shock properties and microstructural change of CoNiCrAlYSi coatings, prepared by LVPS and HVOF on the IN738LC substrate, have been investigated.
2. Experimental procedure 2.1. Substrate material The chemical composition of the Nickel-based superalloy IN738LC used in this study is shown in Table 1. Specimens with dimensions of 15 mm 10 mm 2 mm were grounded using No. 1000 SiC abrasive paper and grit-blasted with alumina powder in
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Table 1 Nominal compositions of IN738LC and CoNiCrAlYSi coating that used in this research. Material
Chemical composition
IN738LC
61.7%Nie15.5%Cre8.5%Coe3.5%Ale3.5% Tie2.6%W1.8%Mo1.6%TaTrace Coe29%Nie26%Cre8%Ale0.6Sie0.8%Y
CoNiCrAlY powder (Sicoat 2231)
order to increase adherence between the CoNiCrAlYSi coating and the substrate and then ultrasonically cleaned in ethanol. 2.2. Coating process CoNiCrAlYSi (Sicoat 2231) powder, which has its chemical composition as described in Table 1, was used to produce MCrAlY overlay in this study. The bond coats were deposited using a Sulzer Metco low vacuum plasma spray and a HVOF (Metallisation METJET II) spray machines. In each type of coating, the thickness of the coating was around 250 mm. Argon and hydrogen were used as primary and secondary gases in the LVPS process and kerosene (C7H16) and oxygen were used as fuel in the HVOF process. The coating parameters, which were adjusted based on the real coating conditions produced by suppliers of the turbine Blade Company, were listed in the Table 2. Finally coated samples were subjected to solution and aging heat-treating for 4 h at 1100 C and 20 h at 850 C in a vacuum furnace. 2.3. Thermal shock test Thermal shock test was performed in a thermal cycling furnace. Each cycle contains specimens directly inserted into the furnace at the 1100 C, holding for 10 min and then forced water quench by immersing them in water (20e30 C) for 5 s. There were three parallel specimens for each coating. [11e13]. Samples weight gain was measured after every 5 cycles at initial stage and after every 2 cycles at the end of test by an electronic balance with the 0.1 mg accuracy. Cyclic test was continued until the specimen weight change become negative and the intersection of the weight change curve and time axis namely thermal cycling life, was determined [14,15]. Before and after subjecting to hot corrosion, the specimens were investigated by an X-ray diffractometer (XRD) (Rigacu, Ultima IV, Japan) using monochromatic Cu-Ka radiation operated at 40 kV and 40 mA. Surface morphology and cross-section of the hot corrosion tested specimens were examined using a field emission scanning electron microscopy (ERA-8800FE) after the specimens were mounted in an epoxy resin and subjected to a polishing process. 3. Results and discussion
Fig. 1. Cross-section microstructure of as-coated samples after heat treatment, (a) LVPS and (b) HVOF coatings.
no cracks and good adhesion to the substrate. A dual-phase structure consisting of an austenitic solid solution g phase with fcc structure and a intermetallic b-(Co,Ni)Al phase with bcc structure are presented as light and dark areas respectively [2,3]. Moreover, an interdiffusion zone (IZ) can be seen in the coating/substrate interface due to inward and outward diffusion of elements during heat treatment. X-ray diffraction patterns of LVPS and HVOF-CoNiCrAlYSi coatings after the heat treatment are shown in Fig. 2. Same phases with different intensity can be seen for the two types of coatings. According to these results, CoNiCrAlYSi coatings is composed of g/g0 phase, b-(Co,Ni)Al phase and a small amount of a-Al2O3.
3.1. As-coated samples
3.2. Weight changes
Cross-sectional SEM images of the heat-treated LVPS and HVOFCoNiCrAlYSi coatings in Fig. 1 indicates that they were dense, with
During the thermal cycling up to 50 cycles, no macro-damage and delamination were detected in LVPS coatings. For HVOF
Table 2 Plasma spray parameters of two types of coatings. LVPS coating parameter Parameter Current Amount 450 A
Primary gas (Ar) 50 l/min
Secondary gas (H2) 6 l/min
Powder feed rate 35 g/min
Spray distance 15 cm
Chamber pressure 20 mbar
Pass No. 16
HVOF coating parameter Parameter Fuel Amount Kerosene C7H16
Fuel flow rate 200 ml/min
Oxygen flow rate 900 l/min
Powder feed rate 45 g/min
Spray distance 30 cm
Fuel pressure 5.5 (kg/cm2)
Oxygen pressure 7 (kg/cm2)
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Fig. 2. X-ray diffraction patterns of as-sprayed CoNiCrAlYSi coatings after heat treatment.
coatings, some cracks and spallation were detected only after 10 cycles of thermal shock. Fig. 3 shows the weight gains of two types of coatings during the thermal shock. This net mass change is the representative of the mass gain owing to oxidation and mass loss due to spallation and delamination of oxide and coating during thermal shock processes [11e13]. It is seen that severe spallation of oxide scales in the HVOF coatings has occurred only after 10 cycles. LVPS samples exhibited normal weight gains until 35 cycles and after this cycle, the weight loss occurred due to spallation and delamination was overcome by the weight gains due to oxidation. It can be concluded that LVPS coatings have a better thermal shock resistance in contrast to HVOF coatings.
Fig. 5 shows X-ray diffraction patterns of the surface oxides formed on the CoNiCrAlYSi coatings after thermal shock cycling. XRD results show that after 10 cycles of thermal shock surface oxides consist mainly of a-Al2O3 for LVPS samples, and a-Al2O3 and slight amounts of Co3O4 and NiO for HVOF samples. Formation of non-protective oxide such as NiO and Co3O4 in HVOF samples at the primary stage of cyclic thermal shock causes the rapid degradation of these coatings [4]. Investigation of XRD results from Fig. 5 shows that the intensity of some oxide such as NiO and Co3O4 is increased in the HVOF samples and some mixed oxide of Cr and Al created at the oxide scale after 30 cycles of thermal shock from 1100 C. In this condition a-Al2O3 is the main oxide phase on the surface of LVPS samples, and this can be the main reason for good thermal shock resistance and persistence of coating as shown in the Fig. 3. FESEM morphology of CoNiCrAlYSi coatings after 30 cycles of thermal shock is shown in Fig. 6. It is obvious that the surface morphology of HVOF coatings is comprised of some coarse and fine oxide particles and also some cracks are observed at the interface of coarse particles. Existence of these cracks, produced due to poor adherence of some non-protective oxide, is the main cause for easy spallation and degradation of these coatings [12,13]. Morphological investigation of LVPS samples show that fine oxide particles composed mainly of a-Al2O3 cover the entire surface and create a continuous oxide layer on the coating. It is clear that no cracking and spallation is observed between these fine oxide particles.
3.3. Thermal shock behaviors Fig. 4 shows the surface FESEM morphologies of CoNiCrAlYSi coatings a after 10 cycles of thermal shock from 1100 C. It is clear that particle sizes of oxide scale are different in two types of coatings and some microcracks are observed between oxide particles in the HVOF samples. LVPS samples morphology consist of some valley and lumps and no cracks are observed during the thermal shock test. The surface of the LVPS coatings remained almost unaffected except the separation of some particles.
Fig. 3. The weight change of LVPS and HVOF coated specimens in thermal shock cycling.
Fig. 4. Surface FESEM morphology of specimens after 10 thermal shock cycles: (a) LVPS and (b) HVOF coatings.
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Fig. 5. XRD patterns of LVPS and HVOF-CoNiCrAlYSi coatings after various thermal shock cycling.
Fig. 7 represents the surface morphology of the samples after 65 cycles of thermal shock. Although XRD result in Fig. 5 shows that the main surface oxide for LVPS samples after 65 cycles is a-Al2O3, this coating undergoes degradation due to severe thermal stresses on the oxide layer [12]. According to the XRD result, intensity of a-
Fig. 7. Surface morphology of CoNiCrAlYSi coatings after 65 cyclic thermal shock at 1100 C, (a) LVPS and (b)-HVOF coatings.
Fig. 6. Surface morphology of CoNiCrAlYSi coatings after 35 cyclic thermal shock at 1100 C, (a) LVPS and (b) HVOF coatings.
Al2O3 has increased in HVOF coatings after 65 cyclic thermal shocks due to degradation of b depleted zone and formation of alumina on some part of the coating but existence of cracks in the oxide scale prevent protection and adherence. SEM cross sections examination of the two types of coatings after 65 cycles of oxidation in Fig. 8 shows that severe degradation and internal oxidation has happened in the HVOF samples, and this degradation can be seen from the small amount of b phase, internal oxidation and extension of interdiffusion zone. Smaller thickness of b-depletion zone and larger amount of b phase in the LVPS samples could be the main reason for better thermal shock properties of these coatings in contrast to HVOF coatings. EDAX analysis of the black particle in (marked as 1 in Fig. 8b) the b depleted zone of HVOF samples is shown in Fig. 8c, which proves that severe internal oxidation has happened in this sample. It is obvious that rapid cooling and heating of coated samples enhance the oxide scale failure in comparison to isothermal and cyclic oxidation tests. Resistance of CoNiCrAlYSi coating to the thermal shock is widely dependent on adherence, compaction, and kind of protective oxide on the coating surface. It can be concluded that cracks play the most important role in the thermal shock resistance of coatings. Three sources of oxide spallation occur in cyclic thermal shock. First is the spallation due to growth stresses in oxide film, this factor is important when samples remain much time at high temperature. Another factor is the thermal strain produced from thermal gradient in oxide scale, this thermal gradient is produced
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As a result of rapid cooling from high temperature and because of tension stress in the coating/oxide interface, some deformation has occurred at this interface. When this thermal shock continues for more cycles, this deformation can lead to a flow in the coating surface; this flow can reduce the thermal expansion coefficient of the coating to a value more compatible with that of the oxide scale. In high cycles, reduction of CTE continues and it became similar to oxide scale. Eventually, state of stress in the oxide is changed and compressive stresses become tensile in the rapid cooling. Although the transient tensile stress is considerably lesser than the compressive stress, the oxides suffer tensile failure easily for their low tensile strengths [11e13]. In present study, failure of LVPS coating attributed to the compressive stresses developed from thermal expansion mismatches during cooling and transient tensile stresses produced in oxide scales. It is notable that both kinds of stress result in failure of the oxide, and compressive stress can cause some tension in coating-oxide interface and separation of oxide. Some factors affect the thermal shock behavior of coatings, one of them is the grain size of coating. Because of more diffusion path, smaller grains are in favor of the selective oxidation of the Al to form protective alumina layer on the surface. The second factor is the addition of small quantities of reactive elements that have been known to enhance the adherence of alumina scales. Another factor is the oxide particle size formed on the coating, since small particles have more opening space due to high density of grain boundaries, so larger amount of thermal strains can be damped by grain boundaries and less stress being applied on the oxide particles [11,12,16]. 4. Conclusions Thermal shock cycling properties of HVOF and LVPSCoNiCrAlYSi coatings were studied in this research and following conclusions can be drawn: 1. Fine oxide morphologies and small grain size of coatings can improve the thermal shock behavior of CoNiCrAlYSi coatings. 2. Formation of non-protective oxide such as NiO, Co3O4 on the top of HVOF coatings leads to the quick spallation and degradation of this coating in cyclic thermal shock. 3. LVPS coatings showed better thermal shock resistance, due to formation of adherence, stable and dense oxide of a-Al2O3. Acknowledgment The authors would like to thank Dr. M. Bahmani, Mr. Rahimipour and Mr. Younesi in Parto Company (Iran) for their support in developing the coatings and also Dr. B. Subramanian (CSIR e CECRI, Karaikudi, India) for helpful comments and editing English of the paper. References
Fig. 8. Cross-sectional image of CoNiCrAlYSi coatings after 65 cycles of thermal shock, (a) LVPS and (b) HVOF show severe degradation in the HVOF coatings, and (c) EDS analysis of internal oxide in HVOF sample.
at high heating and cooling rate. Difference between coefficient of thermal expansion (CTE) of oxide and coating is the cause of third factor. In rapid cooling, oxide scales experience compressive stresses because of their smaller CTEs as compared with the base metal. At rapid heating, stress state change and oxide scale experience tensile stresses [11e13].
[1] Goward GW. Surface and Coatings Technology 1998;108e109:73e9. [2] Pomeroy MJ. Materials and Design 2005;26:223e31. [3] Sidhu TS, Agrawal RD, Prakash S. Surface and Coatings Technology 2005;198: 441e6. [4] Wang B, Gong J, Wang AY, Sun C, Huang RF, Wen LS. Surface and Coatings Technology 2002;149:70e5. [5] Zhao L, Parco M, Lugscheider E. Surface and Coatings Technology 2004;179: 272e8. [6] Ajdelsztajn L, Picas JA, Kim GE, Bastian FL, Schoenung J, Provenzano V. Materials Science and Engineering A 2002;338:33e43. [7] Gurrappa I, Sambasiva RA. Surface and Coatings Technology 2006;201: 3016e29. [8] Mahesh RA, Jayagathan R, Prakash S. Materials Chemistry and Physics 2010; 119:449e57. [9] Jiang SM, Peng X, Bao ZB, Liu SC, Wang QM, Gong J, et al. Corrosion Science 2008;50:3213e20.
M. Mohammadi et al. / Vacuum 88 (2013) 124e129 [10] Brandla W, Tpma D, Grabkeb HJ. Surface and Coatings Technology 1998; 108e109:10e5. [11] Wang QM, Li H, Guo MH, Ke PL, Gong J, Sun C, et al. Materials Science and Engineering A 2005;406:337e49. [12] Wang QM, Li H, Guo MH, Ke PL, Gong J, Sun C, et al. Materials Science and Engineering A 2005;406:350e7.
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[13] Nicholls JR, Bennett MJ. Material at High Temperature 2000;17(3):413e28. [14] Firouzi A, Shirvani K. Corrosion Science 2010;52:3579e85. [15] Shirvani K, Firouzi S, Rashidghamat A. Corrosion Science 2012;55: 378e84. [16] You XQ, Si TZ, Liu N, Ren PP, Xu YD, Feng JP. Ceramics International 2005;31: 33e8.
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Thermal shock properties and microstructure investigation of LVPS and HVOF-CoNiCrAlYSi coatings on the IN738LC superalloy M. Mohammadi a, *, S. Javadpour a, A. Kobayashi b, S.A. Jenabali Jahromi a, K. Shirvani c a
Department of Materials Science and Engineering, Shiraz University, Shiraz, Iran Joining & Welding Res. Inst., Osaka University, Ibaraki, Osaka 567-0047, Japan c Department of Advance Materials and New Energies, Institute of Advance Technologies, IROST, Tehran, Iran b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 November 2011 Received in revised form 2 February 2012 Accepted 3 February 2012
The thermal shock behavior and microstructural investigation of CoNiCrAlYSi coatings applied by Low Vacuum Plasma Spray (LVPS) and High-Velocity Oxy Fuel (HVOF) techniques on the Ni-base superalloy IN738LC were examined. Thermal shock resistance was investigated by forced water quenching of coated samples from high temperature (1100 C) to room temperature. Thermal shock life time of coatings were determined by measuring the weight gain of the specimens at regular intervals for a duration of 65 cycles. Characterizations of the coatings and oxide scales were investigated by SEM equipped with EDS and XRD analyses. Oxidation kinetic obtained from weight measurement and morphological observations indicate better thermal shock resistance of LVPS than HVOF coatings. Larger grain size of protective oxides on the HVOF coatings led to more thermal stresses emanated from severe thermal cycling and spallation of the coatings. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: HVOF LVPS Cyclic thermal shock Spallation Thermal stresses
1. Introduction Materials for high temperature turbine environments face severe operating conditions. These components are subjected to degradation due to high temperature oxidation and corrosive gasses that evolve during combustion and service operation. Nibased superalloys are mainly used in high temperature application. The compositions of these superalloys are such that, they can provide good strength and creep resistance at high temperature but do not provide suitable environmental protection alone; therefore, coatings are used to protect the underlying materials from degradation at harsh working condition [1e3]. Extensive research efforts over decades led to advanced protective coatings for turbine components. Therefore the duplex protective system consisting of overlay MCrAlY bond coat (BC) and ceramic top coat (TC) such as 8 wt.% yttria partially stabilized zirconia (8PYSZ) are used mainly in the turbine component and aero engines against high temperature oxidation and high temperature corrosion. This protective system, called thermal barrier coating (TBC), allows the increase of the gas inlet temperature [4e7]. The service-life of MCrAlY coatings depends greatly on the aluminum and chromium content, which would be consumed as a result of scale growth and spallation of Al2O3/Cr2O3 in thermal * Corresponding author. E-mail address: [email protected] (M. Mohammadi). 0042-207X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2012.02.003
cycles and aggressive environments [8,9]. Formation of adherence, continuous, dense and stable protective oxide, is affected by some factors such as chemical composition of bond coat, environmental condition, working temperature and coating processes [1,2,10]. Effect of coating process on the oxidation and hot corrosion resistance were reported by many researchers, but thermal shock resistance of different coating methods were less discussed and compared. Thermal shock behavior of NiCoCrAlYSiB coatings were investigated by Wang et al. [11,12]. According to this research, accelerated oxidation can be attributed to the accelerating oxide formation and breakdown due to compressive stresses accumulation in the oxides due to oxide growth at high temperature and thermal mismatch during cooling. In this study, cyclic thermal shock properties and microstructural change of CoNiCrAlYSi coatings, prepared by LVPS and HVOF on the IN738LC substrate, have been investigated.
2. Experimental procedure 2.1. Substrate material The chemical composition of the Nickel-based superalloy IN738LC used in this study is shown in Table 1. Specimens with dimensions of 15 mm 10 mm 2 mm were grounded using No. 1000 SiC abrasive paper and grit-blasted with alumina powder in
M. Mohammadi et al. / Vacuum 88 (2013) 124e129
125
Table 1 Nominal compositions of IN738LC and CoNiCrAlYSi coating that used in this research. Material
Chemical composition
IN738LC
61.7%Nie15.5%Cre8.5%Coe3.5%Ale3.5% Tie2.6%W1.8%Mo1.6%TaTrace Coe29%Nie26%Cre8%Ale0.6Sie0.8%Y
CoNiCrAlY powder (Sicoat 2231)
order to increase adherence between the CoNiCrAlYSi coating and the substrate and then ultrasonically cleaned in ethanol. 2.2. Coating process CoNiCrAlYSi (Sicoat 2231) powder, which has its chemical composition as described in Table 1, was used to produce MCrAlY overlay in this study. The bond coats were deposited using a Sulzer Metco low vacuum plasma spray and a HVOF (Metallisation METJET II) spray machines. In each type of coating, the thickness of the coating was around 250 mm. Argon and hydrogen were used as primary and secondary gases in the LVPS process and kerosene (C7H16) and oxygen were used as fuel in the HVOF process. The coating parameters, which were adjusted based on the real coating conditions produced by suppliers of the turbine Blade Company, were listed in the Table 2. Finally coated samples were subjected to solution and aging heat-treating for 4 h at 1100 C and 20 h at 850 C in a vacuum furnace. 2.3. Thermal shock test Thermal shock test was performed in a thermal cycling furnace. Each cycle contains specimens directly inserted into the furnace at the 1100 C, holding for 10 min and then forced water quench by immersing them in water (20e30 C) for 5 s. There were three parallel specimens for each coating. [11e13]. Samples weight gain was measured after every 5 cycles at initial stage and after every 2 cycles at the end of test by an electronic balance with the 0.1 mg accuracy. Cyclic test was continued until the specimen weight change become negative and the intersection of the weight change curve and time axis namely thermal cycling life, was determined [14,15]. Before and after subjecting to hot corrosion, the specimens were investigated by an X-ray diffractometer (XRD) (Rigacu, Ultima IV, Japan) using monochromatic Cu-Ka radiation operated at 40 kV and 40 mA. Surface morphology and cross-section of the hot corrosion tested specimens were examined using a field emission scanning electron microscopy (ERA-8800FE) after the specimens were mounted in an epoxy resin and subjected to a polishing process. 3. Results and discussion
Fig. 1. Cross-section microstructure of as-coated samples after heat treatment, (a) LVPS and (b) HVOF coatings.
no cracks and good adhesion to the substrate. A dual-phase structure consisting of an austenitic solid solution g phase with fcc structure and a intermetallic b-(Co,Ni)Al phase with bcc structure are presented as light and dark areas respectively [2,3]. Moreover, an interdiffusion zone (IZ) can be seen in the coating/substrate interface due to inward and outward diffusion of elements during heat treatment. X-ray diffraction patterns of LVPS and HVOF-CoNiCrAlYSi coatings after the heat treatment are shown in Fig. 2. Same phases with different intensity can be seen for the two types of coatings. According to these results, CoNiCrAlYSi coatings is composed of g/g0 phase, b-(Co,Ni)Al phase and a small amount of a-Al2O3.
3.1. As-coated samples
3.2. Weight changes
Cross-sectional SEM images of the heat-treated LVPS and HVOFCoNiCrAlYSi coatings in Fig. 1 indicates that they were dense, with
During the thermal cycling up to 50 cycles, no macro-damage and delamination were detected in LVPS coatings. For HVOF
Table 2 Plasma spray parameters of two types of coatings. LVPS coating parameter Parameter Current Amount 450 A
Primary gas (Ar) 50 l/min
Secondary gas (H2) 6 l/min
Powder feed rate 35 g/min
Spray distance 15 cm
Chamber pressure 20 mbar
Pass No. 16
HVOF coating parameter Parameter Fuel Amount Kerosene C7H16
Fuel flow rate 200 ml/min
Oxygen flow rate 900 l/min
Powder feed rate 45 g/min
Spray distance 30 cm
Fuel pressure 5.5 (kg/cm2)
Oxygen pressure 7 (kg/cm2)
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Fig. 2. X-ray diffraction patterns of as-sprayed CoNiCrAlYSi coatings after heat treatment.
coatings, some cracks and spallation were detected only after 10 cycles of thermal shock. Fig. 3 shows the weight gains of two types of coatings during the thermal shock. This net mass change is the representative of the mass gain owing to oxidation and mass loss due to spallation and delamination of oxide and coating during thermal shock processes [11e13]. It is seen that severe spallation of oxide scales in the HVOF coatings has occurred only after 10 cycles. LVPS samples exhibited normal weight gains until 35 cycles and after this cycle, the weight loss occurred due to spallation and delamination was overcome by the weight gains due to oxidation. It can be concluded that LVPS coatings have a better thermal shock resistance in contrast to HVOF coatings.
Fig. 5 shows X-ray diffraction patterns of the surface oxides formed on the CoNiCrAlYSi coatings after thermal shock cycling. XRD results show that after 10 cycles of thermal shock surface oxides consist mainly of a-Al2O3 for LVPS samples, and a-Al2O3 and slight amounts of Co3O4 and NiO for HVOF samples. Formation of non-protective oxide such as NiO and Co3O4 in HVOF samples at the primary stage of cyclic thermal shock causes the rapid degradation of these coatings [4]. Investigation of XRD results from Fig. 5 shows that the intensity of some oxide such as NiO and Co3O4 is increased in the HVOF samples and some mixed oxide of Cr and Al created at the oxide scale after 30 cycles of thermal shock from 1100 C. In this condition a-Al2O3 is the main oxide phase on the surface of LVPS samples, and this can be the main reason for good thermal shock resistance and persistence of coating as shown in the Fig. 3. FESEM morphology of CoNiCrAlYSi coatings after 30 cycles of thermal shock is shown in Fig. 6. It is obvious that the surface morphology of HVOF coatings is comprised of some coarse and fine oxide particles and also some cracks are observed at the interface of coarse particles. Existence of these cracks, produced due to poor adherence of some non-protective oxide, is the main cause for easy spallation and degradation of these coatings [12,13]. Morphological investigation of LVPS samples show that fine oxide particles composed mainly of a-Al2O3 cover the entire surface and create a continuous oxide layer on the coating. It is clear that no cracking and spallation is observed between these fine oxide particles.
3.3. Thermal shock behaviors Fig. 4 shows the surface FESEM morphologies of CoNiCrAlYSi coatings a after 10 cycles of thermal shock from 1100 C. It is clear that particle sizes of oxide scale are different in two types of coatings and some microcracks are observed between oxide particles in the HVOF samples. LVPS samples morphology consist of some valley and lumps and no cracks are observed during the thermal shock test. The surface of the LVPS coatings remained almost unaffected except the separation of some particles.
Fig. 3. The weight change of LVPS and HVOF coated specimens in thermal shock cycling.
Fig. 4. Surface FESEM morphology of specimens after 10 thermal shock cycles: (a) LVPS and (b) HVOF coatings.
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Fig. 5. XRD patterns of LVPS and HVOF-CoNiCrAlYSi coatings after various thermal shock cycling.
Fig. 7 represents the surface morphology of the samples after 65 cycles of thermal shock. Although XRD result in Fig. 5 shows that the main surface oxide for LVPS samples after 65 cycles is a-Al2O3, this coating undergoes degradation due to severe thermal stresses on the oxide layer [12]. According to the XRD result, intensity of a-
Fig. 7. Surface morphology of CoNiCrAlYSi coatings after 65 cyclic thermal shock at 1100 C, (a) LVPS and (b)-HVOF coatings.
Fig. 6. Surface morphology of CoNiCrAlYSi coatings after 35 cyclic thermal shock at 1100 C, (a) LVPS and (b) HVOF coatings.
Al2O3 has increased in HVOF coatings after 65 cyclic thermal shocks due to degradation of b depleted zone and formation of alumina on some part of the coating but existence of cracks in the oxide scale prevent protection and adherence. SEM cross sections examination of the two types of coatings after 65 cycles of oxidation in Fig. 8 shows that severe degradation and internal oxidation has happened in the HVOF samples, and this degradation can be seen from the small amount of b phase, internal oxidation and extension of interdiffusion zone. Smaller thickness of b-depletion zone and larger amount of b phase in the LVPS samples could be the main reason for better thermal shock properties of these coatings in contrast to HVOF coatings. EDAX analysis of the black particle in (marked as 1 in Fig. 8b) the b depleted zone of HVOF samples is shown in Fig. 8c, which proves that severe internal oxidation has happened in this sample. It is obvious that rapid cooling and heating of coated samples enhance the oxide scale failure in comparison to isothermal and cyclic oxidation tests. Resistance of CoNiCrAlYSi coating to the thermal shock is widely dependent on adherence, compaction, and kind of protective oxide on the coating surface. It can be concluded that cracks play the most important role in the thermal shock resistance of coatings. Three sources of oxide spallation occur in cyclic thermal shock. First is the spallation due to growth stresses in oxide film, this factor is important when samples remain much time at high temperature. Another factor is the thermal strain produced from thermal gradient in oxide scale, this thermal gradient is produced
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As a result of rapid cooling from high temperature and because of tension stress in the coating/oxide interface, some deformation has occurred at this interface. When this thermal shock continues for more cycles, this deformation can lead to a flow in the coating surface; this flow can reduce the thermal expansion coefficient of the coating to a value more compatible with that of the oxide scale. In high cycles, reduction of CTE continues and it became similar to oxide scale. Eventually, state of stress in the oxide is changed and compressive stresses become tensile in the rapid cooling. Although the transient tensile stress is considerably lesser than the compressive stress, the oxides suffer tensile failure easily for their low tensile strengths [11e13]. In present study, failure of LVPS coating attributed to the compressive stresses developed from thermal expansion mismatches during cooling and transient tensile stresses produced in oxide scales. It is notable that both kinds of stress result in failure of the oxide, and compressive stress can cause some tension in coating-oxide interface and separation of oxide. Some factors affect the thermal shock behavior of coatings, one of them is the grain size of coating. Because of more diffusion path, smaller grains are in favor of the selective oxidation of the Al to form protective alumina layer on the surface. The second factor is the addition of small quantities of reactive elements that have been known to enhance the adherence of alumina scales. Another factor is the oxide particle size formed on the coating, since small particles have more opening space due to high density of grain boundaries, so larger amount of thermal strains can be damped by grain boundaries and less stress being applied on the oxide particles [11,12,16]. 4. Conclusions Thermal shock cycling properties of HVOF and LVPSCoNiCrAlYSi coatings were studied in this research and following conclusions can be drawn: 1. Fine oxide morphologies and small grain size of coatings can improve the thermal shock behavior of CoNiCrAlYSi coatings. 2. Formation of non-protective oxide such as NiO, Co3O4 on the top of HVOF coatings leads to the quick spallation and degradation of this coating in cyclic thermal shock. 3. LVPS coatings showed better thermal shock resistance, due to formation of adherence, stable and dense oxide of a-Al2O3. Acknowledgment The authors would like to thank Dr. M. Bahmani, Mr. Rahimipour and Mr. Younesi in Parto Company (Iran) for their support in developing the coatings and also Dr. B. Subramanian (CSIR e CECRI, Karaikudi, India) for helpful comments and editing English of the paper. References
Fig. 8. Cross-sectional image of CoNiCrAlYSi coatings after 65 cycles of thermal shock, (a) LVPS and (b) HVOF show severe degradation in the HVOF coatings, and (c) EDS analysis of internal oxide in HVOF sample.
at high heating and cooling rate. Difference between coefficient of thermal expansion (CTE) of oxide and coating is the cause of third factor. In rapid cooling, oxide scales experience compressive stresses because of their smaller CTEs as compared with the base metal. At rapid heating, stress state change and oxide scale experience tensile stresses [11e13].
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