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Silicon carbide based plasma sprayed coatings
Surface & Coatings Technology 207 (2012) 182–189
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Silicon carbide based plasma sprayed coatings Mario Tului a,⁎, Barbara Giambi a, Stefano Lionetti a, Giovanni Pulci b, Fabrizio Sarasini b, Teodoro Valente b a b
Centro Sviluppo Materiali S.p.A., Via di Castel Romano, 100, 00128 Rome, Italy Sapienza Università di Roma, Department of Chemical Engineering Materials Environment, Via Eudossiana 18, 00184 Rome, Italy
a r t i c l e
i n f o
Article history: Received 26 March 2012 Accepted in revised form 19 June 2012 Available online 27 June 2012 Keywords: Plasma spray Silicon carbide Oxidation resistance
a b s t r a c t Coatings containing up to 66 vol.% of silicon carbide were deposited by plasma spray. Potential applications can be found in the protection of CMC (ceramic matrix composite) against wear and high temperature oxidation. It is well known that SiC cannot be deposited by thermal spray because it decomposes before melting. To face this problem, a mixture of SiC and ZrB2 was deposited, since those two compounds form a eutectic phase, at a temperature lower than the one of SiC decomposition. Coating microstructure was characterized by XRD, SEM, and EDS, confirming the presence of SiC in the deposited layer and the formation of the eutectic phase during spraying. Samples of the coatings were exposed in air at high temperature, in the range of 650 °C to 1700 °C. The oxide scale was investigated by means of XRD, SEM, EDS and WDS. It was constituted by a SiO2 layer, which includes islands of ZrO2. Test results showed the good potential of the material investigated to be used as a protection against the high temperature oxidation. © 2012 Elsevier B.V. All rights reserved.
1. Introduction High temperature materials have been widely investigated and developed in order to improve the performances of systems enabling them to withstand extreme thermal, mechanical, and chemical conditions. Typical applications and research fields are thermal protection systems and hot structures of vehicles for hypersonic flights, advanced gas turbine engines and heat exchangers [1,2]. In oxidizing environments, for service temperatures higher than 1200–1300 °C, usually ceramic materials or ceramic matrix composites have to be selected because of their oxidation resistance and good mechanical strength at high temperature; the state of the art of high temperature materials includes the innovative and promising Ultra High Temperature Ceramics (UHTCs), such as ZrB2–SiC or HfB2–SiC, and the more conventional fiber-reinforced advanced ceramic composites such as C/SiC and SiC/SiC [1–3]. Carbon–carbon (C/C) composites exhibit good mechanical properties at very high temperatures (higher than 2000 °C) but are not oxidation-resistant and need to be protected by coatings. C/SiC composites exhibit oxidation resistance up to 1600 °C in dry oxidizing environments but, if stressed by thermal cycling, their service life is modest because of the cracking of matrix leading to direct oxidation of the carbon fiber reinforcement.
⁎ Corresponding author. Tel.: +39 06 5055 742; fax: +39 06 5055 204. E-mail addresses: [email protected] (M. Tului), [email protected] (B. Giambi), [email protected] (S. Lionetti), [email protected] (G. Pulci), [email protected] (F. Sarasini), [email protected] (T. Valente). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.06.062
Also SiC/SiC ceramic matrix composites often have to be protected by means of environment barrier (EBC) or oxidation resistant coatings in wet (steam) oxidizing environments that leads to active oxidation with the formation of volatile Si(OH)n, or if the temperature exceeds 1600 °C [2,3]. Typically, ceramic matrix composites are protected by means of silicon-based coatings (SiC, MoSi2, mullite, barium–strontium–alumina-silicates, scandium silicate, yttrium silicate etc.) obtained by CVD or sol–gel techniques [4–11], as summarized in Table 1. In this paper the possibility of obtaining an UHTC coating with high SiC content (up to 66 vol.%) deposited by plasma spray process was investigated. Thermal spraying of SiC based materials on CMC could be very interesting because it permits (i) the deposition of thick coatings, (ii) the use of UHTC compositions (e.g. SiC–ZrB2) able to increase the maximum service temperature [12], and (iii) the reduction of the deposition time with regard to CVD process. Moreover, SiC based UHTC plasma sprayed coatings could be very interesting because of their chemical (adhesion) and mechanical (thermal expansion) compatibility with C/SiC and SiC/SiC composites. Mechanical properties and oxidation behavior of ZrB2–SiC plasma sprayed coatings were previously investigated [12–16]: results show their excellent oxidation resistance in air up to 1800 °C, due to formation of an adherent, multiphase and protective oxide scale on the exposed surface; nevertheless there are not any available data regarding the oxidation properties of the high SiC content UHTC coatings. The experimental activities reported in this paper are focused on the deposition and characterization of SiC based coatings, in order to verify whether the increase of SiC content leads to a higher silicon carbide decomposition and/or to a reduced oxidation resistance if compared with low SiC content UHTC coatings.
M. Tului et al. / Surface & Coatings Technology 207 (2012) 182–189
183
Table 1 Protective coatings for C/SiC and SiC/SiC composites. Coating materials
Deposition technique
Oxidation test temperature
Substrate
Ref.
MoSi2 Mullite (3 Al2O32 SiO2) Barium strontium aluminosilicate (BSAS)
Liquid reaction Plasma spraying Slurry and sintering Plasma spraying Plasma spraying Slip casting and sintering Plasma spraying Plasma spraying Slurry and sintering Plasma spraying
1400 °C 1400 °C N.A. 1200 °C 1300–1400 °C 1600 °C 1200 °C 1200 °C N.A. 1380 °C
C/SiC SiC/SiC SiC based CMC SiC/SiC C/SiC SiC/SiC C/SiC SiC based CMC SiC/SiC C/SiC C/SiC SiC/SiC
[7] [4] [5] [3] [9] [6] [3] [3] [10] [11]
Mullite + BSAS Yttrium silicate Calcium aluminosilicate Rare earth silicate + BSAS
Table 2 Spraying parameters.
a
Pressure (Ar gas) Spray distance Plasma gas (Ar) Plasma gas (H2) Plasma power input
(mbar) (mm) (slpm) (slpm) (kW)
1200 110 55 15 48
Zr (wt.%)
Si (wt.%)
40.42 41.8 42.7
35.02 34.4 31.5
Table 3 Chemical analysis results.
Starting composition ZrB2–SiC powder ZrB2–SiC coating
b +
+ = ZrB2 ! = SiC
Counts (a.u.)
+
!
+
+! ++
+
Coating
+ !+
Powder
10
20
30
40
2θ
50
60
70
80 Fig. 3. SEM micrographs of the coating cross section.
Fig. 1. XRD patterns of starting powder and coating.
3500
Temperature (°C)
L 3000
B
SiC + L
2500
ZrB2 + L
A
2000
SiC + ZrB2 1500 0
20
40
60
80
100
ZrB2 Molar content (%) Fig. 2. Pseudo-binary phase diagram of the ZrB2–SiC system.
Fig. 4. SEM micrograph of the coating at high magnification.
184
M. Tului et al. / Surface & Coatings Technology 207 (2012) 182–189
0 DTA
16
-20
12
-40 8
-60
TGA
4 0
0
DTA (µV)
Weight gain (%)
20
250
500
750
1000
-80 1250
Temperature (°C) Fig. 5. TGA–DTA curves.
2. Experimental ZrB2 and SiC commercial powders, with an average size of 2 μm and 700–800 nm, respectively, were mixed to obtain the composition of ZrB2 34 vol.%–SiC 66 vol.%. The mixture was agglomerated by employing spray-drying method, obtaining a powder, constituted by spherical grains, with an average grain size of 60 μm, well suited for thermal spray deposition.
Self standing coatings up to 3 mm in thickness, were obtained by deposition of the powder under inert atmosphere on a flat graphite substrate, successively mechanically removed. A CAPS (controlled atmosphere plasma spray) equipment (Sultzer Metco, Switzerland), which allows one to control atmosphere and pressure in the deposition chamber during spraying, was used for deposition. Spraying parameters are reported in Table 2. Chemical analysis was carried out both on the powder and on the coatings to verify their composition. The Optima 3300DV ICP OES (inductively coupled plasma; optical emission spectrometer) (Perkin Elmer, USA) was used. Coatings were characterized by XRD (X-ray diffraction). A Siemens D500/DIFFRAC apparatus was employed. For comparison, a powder sample was also analyzed. Some coating samples were cut, embedded in resin, polished and their cross section was analyzed by a JEOL JSM-6480LV SEM (scanning electron microscope) equipped with EDS (energy dispersion spectroscope) and WDS (wavelength dispersion spectroscope) INCA Oxford system. Density measurements were carried out on coating samples according to ISO EN 623-2, by weighting samples before and after immersing them in water, according to Archimedes' method. Simultaneous TGA–DTA (thermal gravimetric analyses–differential thermal analyses) were carried out in static air, from room temperature
Fig. 6. SEM micrograph of a sample exposed in air at 650 °C for 30 min along with WDS maps for B, O, Si and Zr.
M. Tului et al. / Surface & Coatings Technology 207 (2012) 182–189
up to 1250 °C using a NETZSCH STA 409 Thermobalance, with a heating rate of 3 °C/min. Oxidation tests were conducted on 10 × 10 × 3 mm samples in a high temperature furnace in air at various temperatures and different dwelling times for each temperature, namely: 30 min for the samples exposed at 650 °C, 850 °C, 1100 °C and 1300 °C; 30, 60, 90 and 150 min for the samples at 1500 °C and 1700 °C. Before testing, samples were ultrasonically cleaned in acetone and dried. They were placed on zirconia refractory bricks and they were tilted to ensure minimal contact area. The following thermal cycle was used for the samples exposed at 1500 °C and 1700 °C: (i) heating rate of 10 °C/min from room temperature up to 1350 °C; (ii) heating rate of 5 °C/min up to the maximum temperature (1500 or 1700 °C); (iii) hold time at maximum temperature for 30, 60, 90 and 150 min; (iv) cooling rate of 10 °C/min from maximum temperature up to room temperature. Weights before and after oxidation at 1500 °C and 1700 °C were measured using a balance with 0.1 mg resolution. Microstructural analyses were performed on the surfaces and cross sections using a scanning electron microscope (Philips XL 40) equipped with an energy dispersive spectroscope (EDS, EDAX Falcon). The cross sections of the oxidized specimens were prepared for microstructural analysis by polishing procedures down to a 1 μm finish. Specimens were gold sputtered before SEM observation.
185
X-ray diffraction (Philips X-PERT) was used to identify the phases present after exposure. A step-scan mode was used in the 2θ range from 5° to 80° with a step width of 0.01° and a counting time of 7 s per step. The employed radiation was monochromated CuKα (40 kV–30 mA). 3. Results and discussion 3.1. Coating characterization Apparent solid density equal to 4.12 g/cm 3 was obtained averaging the values measured on three specimens. Chemical analysis and XRD were carried out both on the starting powder and on coating samples. Table 3 reports the chemical analysis results; for the sake of comparison, the composition of the starting mixture is also reported. XRD patterns are reported in Fig. 1. The XRD pattern of the coating can be fully indexed as ZrB2 and SiC, i.e., only the starting phases are present in the coating. Other phases, e.g., SiO2, ZrO2, B2O3, ZrSiO4, were not detected. The absence of oxide phases in the coating is due to the spraying parameters used (deposition under inert atmosphere, H2 as plasma gas). Previous works [12–16] showed that it is possible to obtain ZrB2–SiC coatings when SiC content is lower than ZrB2 content. SiC decomposition does not occur during spraying because of a liquid phase formation
Fig. 7. SEM micrograph of a sample exposed in air at 850 °C for 30 min along with WDS maps for B, O, Si and Zr.
M. Tului et al. / Surface & Coatings Technology 207 (2012) 182–189
at a temperature lower than the SiC decomposition one, as suggested by the ZrB2–SiC pseudo-binary phase diagram, reported in Fig. 2 [17], where a eutectic phase at 2207 °C is present. The compositions investigated in the previous works were located, in the phase diagram, near to the ZrB2 axis, while the composition investigated in the present work is positioned, in the phase diagram, on the opposite side with respect to the eutectic phase, i.e., near the SiC axis, as indicated by the dashed line in Fig. 2. The already mentioned mechanism of liquid phase formation, at a temperature lower than the SiC decomposition temperature, is supposed to be still valid with this coating composition. Fig. 3 shows the cross section of a coating observed by SEM at different magnifications: it appears to be compact and homogeneous (Fig. 3a); no unmelted particles were observed. The observed variety of gray color levels suggests the presence of compounds with different Zr and Si contents. EDS-WDS analyses confirmed that the dark gray areas are composed of SiC particles, white areas are made of ZrB2 particles, and intermediate gray level intensity areas presented different Zr and Si contents. Metallographic analysis seems to confirm the hypothesis of liquid phase formation according to the phase diagram. The coating appears to be formed by splats of different morphologies (Fig. 3b). This can be explained supposing that some particles impacted on the substrate in a partially melted status, i.e., at a temperature within the solidus–liquidus range; other particles were completely melted, i.e., above liquidus temperature. Those two situations correspond, respectively, to points A and B in Fig. 2. This can occur because particles could have: i) different sizes; ii) different compositions (which fluctuate around the nominal one); iii) different thermal histories during spraying.
a
* = ZrO2 ° = ZrSiO4
* * Counts (a.u.)
186
1500°C
* *
* * * * ** * *
* * *
** * * ** ** * *** * * *
* * *
**
°
*
*
*
+
* *
1700°C
10
*
* °
+ = SiO2
* 20
30
40
2θ
50
60
80
Fig. 9. X-ray diffraction patterns of the surface of ZrB2–SiC specimens oxidized for 90 min at 1500 °C (a) and 1700 °C (b).
High cooling rate, typical of plasma spraying, together with low miscibility of the phases ZrB2 and SiC, that reduces crystallization kinetics, causes the splat morphology to be strongly affected by particle condition at impact. EDS analyses carried out on the coating cross section area reported in Fig. 4 show that it is composed of SiC particles (average size b 1 μm) embedded in a Zr enriched matrix. Therefore, this splat could be generated by a particle which impacted in a partially melted status (point A in Fig. 2). 3.2. Oxidation behavior Thermogravimetric analysis and differential thermal analysis were simultaneously carried out on coating specimen from room temperature up to 1250 °C. TGA and DTA curves during heating are shown in Fig. 5.
b
SiO2
SiO2
ZrO2
ZrO2
c
70
d
SiO2
SiO2
ZrO2
ZrO2
Fig. 8. SEM micrograph of the cross sections of samples exposed for 30 min at 1100 °C (a), 1300 °C (b), 1500 °C (c), 1700 °C (d).
M. Tului et al. / Surface & Coatings Technology 207 (2012) 182–189
The following phenomena can be observed: i) weight gain in the range 600–800 °C associated with an exothermic reaction (DTA curve); ii) reduction of this phenomenon between 800 and 1100 °C; iii) further weight increase above 1100 °C during heating. TGA and DTA results don't differ from the ones obtained on lower SiC content samples, suggesting the same oxidation mechanisms, as reported in previous papers [15,16]. At the lower temperatures (points i) and ii) of the above discussion), ZrB2 oxidation is the most relevant phenomena, according to the following reaction: 2ZrB2 þ 5O2 ↔2ZrO2 ðsolidÞ þ 2B2 O3 ðliquidÞ: B2O3 tends to form a protective layer that reduces oxidation kinetics and protects SiC from oxidation till 1100 °C. Above such a temperature, B2O3 evaporates allowing SiC oxidation [18,19]. The SEM observation and EDS analyses carried out on the surfaces of the samples exposed for 30 min at 650 °C and 850 °C, reported in Figs. 6 and 7, respectively, are coherent with the proposed oxidation mechanism. On the surface of the sample exposed in air at 650 °C (Fig. 6), polygonal grains of B2O3 can be observed; they correspond to the formation of the first B2O3 nuclei, in the initial stage of the oxidation. Vice versa, in the sample exposed in air at 850 °C (Fig. 7), the B2O3 film is already formed and partially covers the surface of the sample. Unoxidized SiC particles can be observed in both the samples. Fig. 8 shows the cross sections of samples exposed for 30 min at 1100 °C, 1300 °C, 1500 °C and 1700 °C, respectively. The samples exposed at 1100 °C and 1300 °C show an oxidized layer formed by porous zirconium oxide and silicon oxide islands. In the samples exposed at higher temperatures a compact silicon oxide scale, including ZrO2 islands, is present.
187
Boron was never found in the oxide layers, likely because of the B2O3 formation and its evaporation at temperatures above 1100 °C. This element is present under the oxidized layers as ZrB2 particles. The oxidation reactions and products for the ZrB2–SiC specimens have been analyzed in the temperature range 1500 °C–1700 °C to understand their oxidation mechanisms. The XRD spectra obtained from the oxide scales formed during isothermal exposure at 1500 °C and 1700 °C for 90 min are shown in Fig. 9. The results in this figure indicated that the main phase present in the oxide scale is monoclinic ZrO2 while tetragonal zirconia was detected only for specimens oxidized for 30 min at 1500 °C. It can be supposed that tetragonal zirconia formed during oxidation but transformed to the monoclinic one during cooling [20]. Some traces of zircon (ZrSiO4) were found in specimens oxidized both at 1500 °C and 1700 °C but for the shortest exposure time, i.e. 30 min. Although the formation of stable zircon might be predicted on the basis of ternary isothermal section of the ZrO2–B2O3–SiO2 system at 1500 °C, this compound is often not reported in ZrB2–SiC composites oxidized at high temperatures (> 1500 °C) [20,21]. It is interesting to note that in the XRD pattern of the sample exposed at 1500 °C, a bump due to the presence of amorphous SiO2 can be observed, while at 1700 °C crystalline SiO2 (cristobalite) is present; this is thought to result from a progressive devitrification of the amorphous silica film [12]. The conclusions of the XRD analysis are confirmed by the examination of the surfaces of the oxidized samples. Fig. 10 (a–d) shows the SEM micrographs of surfaces of specimens oxidized for 30 and 90 min at 1500 and 1700 °C. Comparison of these images shows the oxide scale formed at 1500 °C to be rougher and more granular in comparison to that formed at 1700 °C. All these oxide scales appear porous with extensive bubble formation. In both cases (1500 and 1700 °C) the surface is covered by a thick silica glassy layer with aggregated zirconia particles with different sizes and shapes. The surface appearance is consistent with the oxidation mechanisms of ZrB2–SiC bulk composites, as outlined in [22]. In fact, during high temperature oxidation of
Fig. 10. SEM micrographs showing surface morphologies of the scale of specimens after oxidation at 1500 °C for 30 min (a); 1700 °C for 30 min (b); 1500 °C for 90 min (c); 1700 °C for 90 min (d).
M. Tului et al. / Surface & Coatings Technology 207 (2012) 182–189
ZrB2–SiC, a borosilicate oxide (B2O3–SiO2) film forms on the outer surface. Due to the high vapor pressure of boria at these temperatures compared to silica, boria is preferentially evaporated from the borosilicate liquid. The liquid oxide film at the outer surface then becomes a mainly viscous SiO2-rich borosilicate liquid which is protective and fills the pores of the porous oxide scale. The formation of the large bubbles is most likely due to the coalescence of the gaseous products (i.e., CO) inside the external forming glass. The main difference caused by the exposure temperature is the increasing amount of oxide scale with increasing temperature and the simultaneous decrease of zirconia particles appearing on the surface with the presence of cracks and discontinuities. A typical SEM (BSE) image depicting the cross section of the specimen oxidized at 1500 °C for 30 min is shown in Fig. 11. Examination of this figure indicates that the oxide scale is made of a continuous SiO2-rich layer containing various amounts of particulate zirconia, as confirmed by the EDS maps (Fig. 11). The formation of an external silica-based glass layer is very effective in limiting the inward diffusion of oxygen into the inner bulk, thus enhancing the oxidation resistance.
400 1700°C 1500°C
Oxide thickness (µm)
188
300
200
100
0 0
20
40
60
80
100
120
140
160
Time (min.) Fig. 12. Oxide thickness of ZrB2–SiC specimens oxidized at 1500 °C and 1700 °C as a function of oxidation time.
Fig. 11. Backscattering electron microscopy (BSE) image of the cross section of the ZrB2–SiC specimen tested at 1500 °C for 30 min along with the EDS maps for O, Si and Zr.
M. Tului et al. / Surface & Coatings Technology 207 (2012) 182–189
Specific mass gain (mg/cm2)
30
189
confirm the retention of ZrB2 and SiC phases in the deposited coatings and the chemical analysis highlights an amount of silicon consistent with the starting powders. Coatings are compact and homogeneous, with a low porosity content. Oxidation tests were performed up to 1700 °C: they showed the good potentiality of the material investigated to be used as a protection against the high temperature oxidation of CMC hot structures.
25 20 15 10
References
1700°C 1500°C
5 0 0
20
40
60
80
100
120
140
160
Time (min.) Fig. 13. Mass change of ZrB2–SiC specimens after oxidation at 1500 °C and 1700 °C as a function of exposure time.
The thickness of the oxide layer for each specimen tested was measured by averaging 150 values at various locations due to the nonuniformity of the oxide layer, which resulted in large standard deviations. The thickness increases with increasing oxidation time and temperature, as shown in Fig. 12. Fig. 13 shows mass gain per unit surface area as a function of oxidation at 1500 and 1700 °C. With increasing oxidation time, mass gain increased. It is likely that both thickness and mass gain increase rapidly during the first stages of oxidation but then steadily increase with time.
4. Conclusions Reported results demonstrate that it is possible to deposit coatings containing up to 66 vol.% of SiC by plasma spraying: the XRD patterns
[1] M.M. Opeka, I.G. Talmy, J.A. Zaykoski, J. Mater. Sci. 39 (2004) 5887. [2] H.E. Eaton, G.D. Linsey, E.Y. Sun, K.L. More, J.B. Kimmel, J.R. Price, N. Miriyala, In: Proceedings of ASME TURBOEXPO 2001, June 4–7, 2001, New Orleans, Louisiana, USA. [3] H.E. Eaton, G.D. Linsey, J. Eur. Ceram. Soc. 22 (2002) 2741. [4] K.N. Lee, R.A. Miller, J. Am. Ceram. Soc. 79 (1996) 620. [5] F. Bezzi, P. Fabbri, A. Brentari, C. Mingazzini, E. Burresi, L. Beaulardi, S. Sangiorgi, Adv. Sci. Technol. 66 (2010) 100. [6] J.D. Webstera, M.E. Westwooda, F.H. Hayesa, R.J. Daya, R. Taylora, A. Duranb, M. Apariciob, K. Rebstockc, W.D. Vogel, J. Eur. Ceram. Soc. 18 (1998) 2345. [7] Y. Xu, L. Cheng, L. Zhang, H. Ying, W. Zhou, J. Mater. Sci. 34 (1999) 6009. [8] Y. Zhangy, H. Li, X. Qiang, K. Li, J. Mater. Sci. Technol. 26 (2010) 1139. [9] K.N. Lee, D.S. Fox, J.I. Eldridge, D. Zhu, R.C. Robinson, J. Am. Ceram. Soc. 86 (2003) 1299. [10] Z. Hong, L. Cheng, J. Deng, L. Lu, Y. Wang, J. Inorg. Organomet. Polym. 22 (2012) 692. [11] K.N. Lee, D.S. Fox, N.P. Bansal, J. Eur. Ceram. Soc. 25 (2005) 1705. [12] C. Bartuli, T. Valente, M. Tului, Surf. Coat. Technol. 155 (2002) 260. [13] G. Pulci, M. Tului, J. Tirillò, F. Marra, S. Lionetti, T. Valente, J. Therm. Spray Technol. 20 (2011) 139. [14] T. Valente, C. Bartuli, G. Pulci, Adv. Sci. Technol. 45 (2006) 1505. [15] M. Tului, S. Lionetti, G. Pulci, F. Marra, J. Tirillò, T. Valente, Surf. Coat. Technol. 205 (2010) 1065. [16] M. Tului, S. Lionetti, G. Pulci, E. Rocca, T. Valente, G. Marino, Surf. Coat. Technol. 202 (2008) 4394. [17] S.S. Ordan'jan, A.I. Dimitrev, E.S. Moroskina, Izv. Akad. Nauk. SSSR Neorg. Mater. 10 (1989) 1752. [18] A. Rezaie, W. Fahrenholtz, G. Hilmas, J. Eur. Ceram. Soc. 27 (2007) 2495. [19] E. Eakins, D. Jayaseelan, W. Lee, Metall. Mater. Trans. A 42A (2011) 878. [20] S.N. Karlsdottir, J.W. Halloran, J. Am. Ceram. Soc. 95 (2009) 1328. [21] S.N. Karlsdottir, J.W. Halloran, A.N. Grundy, J. Am. Ceram. Soc. 91 (2008) 272. [22] T.A. Parthasarathy, R.A. Rapp, M. Opeka, M.K. Cinibulk, J. Am. Ceram. Soc. 95 (2012) 338.
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Silicon carbide based plasma sprayed coatings Mario Tului a,⁎, Barbara Giambi a, Stefano Lionetti a, Giovanni Pulci b, Fabrizio Sarasini b, Teodoro Valente b a b
Centro Sviluppo Materiali S.p.A., Via di Castel Romano, 100, 00128 Rome, Italy Sapienza Università di Roma, Department of Chemical Engineering Materials Environment, Via Eudossiana 18, 00184 Rome, Italy
a r t i c l e
i n f o
Article history: Received 26 March 2012 Accepted in revised form 19 June 2012 Available online 27 June 2012 Keywords: Plasma spray Silicon carbide Oxidation resistance
a b s t r a c t Coatings containing up to 66 vol.% of silicon carbide were deposited by plasma spray. Potential applications can be found in the protection of CMC (ceramic matrix composite) against wear and high temperature oxidation. It is well known that SiC cannot be deposited by thermal spray because it decomposes before melting. To face this problem, a mixture of SiC and ZrB2 was deposited, since those two compounds form a eutectic phase, at a temperature lower than the one of SiC decomposition. Coating microstructure was characterized by XRD, SEM, and EDS, confirming the presence of SiC in the deposited layer and the formation of the eutectic phase during spraying. Samples of the coatings were exposed in air at high temperature, in the range of 650 °C to 1700 °C. The oxide scale was investigated by means of XRD, SEM, EDS and WDS. It was constituted by a SiO2 layer, which includes islands of ZrO2. Test results showed the good potential of the material investigated to be used as a protection against the high temperature oxidation. © 2012 Elsevier B.V. All rights reserved.
1. Introduction High temperature materials have been widely investigated and developed in order to improve the performances of systems enabling them to withstand extreme thermal, mechanical, and chemical conditions. Typical applications and research fields are thermal protection systems and hot structures of vehicles for hypersonic flights, advanced gas turbine engines and heat exchangers [1,2]. In oxidizing environments, for service temperatures higher than 1200–1300 °C, usually ceramic materials or ceramic matrix composites have to be selected because of their oxidation resistance and good mechanical strength at high temperature; the state of the art of high temperature materials includes the innovative and promising Ultra High Temperature Ceramics (UHTCs), such as ZrB2–SiC or HfB2–SiC, and the more conventional fiber-reinforced advanced ceramic composites such as C/SiC and SiC/SiC [1–3]. Carbon–carbon (C/C) composites exhibit good mechanical properties at very high temperatures (higher than 2000 °C) but are not oxidation-resistant and need to be protected by coatings. C/SiC composites exhibit oxidation resistance up to 1600 °C in dry oxidizing environments but, if stressed by thermal cycling, their service life is modest because of the cracking of matrix leading to direct oxidation of the carbon fiber reinforcement.
⁎ Corresponding author. Tel.: +39 06 5055 742; fax: +39 06 5055 204. E-mail addresses: [email protected] (M. Tului), [email protected] (B. Giambi), [email protected] (S. Lionetti), [email protected] (G. Pulci), [email protected] (F. Sarasini), [email protected] (T. Valente). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.06.062
Also SiC/SiC ceramic matrix composites often have to be protected by means of environment barrier (EBC) or oxidation resistant coatings in wet (steam) oxidizing environments that leads to active oxidation with the formation of volatile Si(OH)n, or if the temperature exceeds 1600 °C [2,3]. Typically, ceramic matrix composites are protected by means of silicon-based coatings (SiC, MoSi2, mullite, barium–strontium–alumina-silicates, scandium silicate, yttrium silicate etc.) obtained by CVD or sol–gel techniques [4–11], as summarized in Table 1. In this paper the possibility of obtaining an UHTC coating with high SiC content (up to 66 vol.%) deposited by plasma spray process was investigated. Thermal spraying of SiC based materials on CMC could be very interesting because it permits (i) the deposition of thick coatings, (ii) the use of UHTC compositions (e.g. SiC–ZrB2) able to increase the maximum service temperature [12], and (iii) the reduction of the deposition time with regard to CVD process. Moreover, SiC based UHTC plasma sprayed coatings could be very interesting because of their chemical (adhesion) and mechanical (thermal expansion) compatibility with C/SiC and SiC/SiC composites. Mechanical properties and oxidation behavior of ZrB2–SiC plasma sprayed coatings were previously investigated [12–16]: results show their excellent oxidation resistance in air up to 1800 °C, due to formation of an adherent, multiphase and protective oxide scale on the exposed surface; nevertheless there are not any available data regarding the oxidation properties of the high SiC content UHTC coatings. The experimental activities reported in this paper are focused on the deposition and characterization of SiC based coatings, in order to verify whether the increase of SiC content leads to a higher silicon carbide decomposition and/or to a reduced oxidation resistance if compared with low SiC content UHTC coatings.
M. Tului et al. / Surface & Coatings Technology 207 (2012) 182–189
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Table 1 Protective coatings for C/SiC and SiC/SiC composites. Coating materials
Deposition technique
Oxidation test temperature
Substrate
Ref.
MoSi2 Mullite (3 Al2O32 SiO2) Barium strontium aluminosilicate (BSAS)
Liquid reaction Plasma spraying Slurry and sintering Plasma spraying Plasma spraying Slip casting and sintering Plasma spraying Plasma spraying Slurry and sintering Plasma spraying
1400 °C 1400 °C N.A. 1200 °C 1300–1400 °C 1600 °C 1200 °C 1200 °C N.A. 1380 °C
C/SiC SiC/SiC SiC based CMC SiC/SiC C/SiC SiC/SiC C/SiC SiC based CMC SiC/SiC C/SiC C/SiC SiC/SiC
[7] [4] [5] [3] [9] [6] [3] [3] [10] [11]
Mullite + BSAS Yttrium silicate Calcium aluminosilicate Rare earth silicate + BSAS
Table 2 Spraying parameters.
a
Pressure (Ar gas) Spray distance Plasma gas (Ar) Plasma gas (H2) Plasma power input
(mbar) (mm) (slpm) (slpm) (kW)
1200 110 55 15 48
Zr (wt.%)
Si (wt.%)
40.42 41.8 42.7
35.02 34.4 31.5
Table 3 Chemical analysis results.
Starting composition ZrB2–SiC powder ZrB2–SiC coating
b +
+ = ZrB2 ! = SiC
Counts (a.u.)
+
!
+
+! ++
+
Coating
+ !+
Powder
10
20
30
40
2θ
50
60
70
80 Fig. 3. SEM micrographs of the coating cross section.
Fig. 1. XRD patterns of starting powder and coating.
3500
Temperature (°C)
L 3000
B
SiC + L
2500
ZrB2 + L
A
2000
SiC + ZrB2 1500 0
20
40
60
80
100
ZrB2 Molar content (%) Fig. 2. Pseudo-binary phase diagram of the ZrB2–SiC system.
Fig. 4. SEM micrograph of the coating at high magnification.
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0 DTA
16
-20
12
-40 8
-60
TGA
4 0
0
DTA (µV)
Weight gain (%)
20
250
500
750
1000
-80 1250
Temperature (°C) Fig. 5. TGA–DTA curves.
2. Experimental ZrB2 and SiC commercial powders, with an average size of 2 μm and 700–800 nm, respectively, were mixed to obtain the composition of ZrB2 34 vol.%–SiC 66 vol.%. The mixture was agglomerated by employing spray-drying method, obtaining a powder, constituted by spherical grains, with an average grain size of 60 μm, well suited for thermal spray deposition.
Self standing coatings up to 3 mm in thickness, were obtained by deposition of the powder under inert atmosphere on a flat graphite substrate, successively mechanically removed. A CAPS (controlled atmosphere plasma spray) equipment (Sultzer Metco, Switzerland), which allows one to control atmosphere and pressure in the deposition chamber during spraying, was used for deposition. Spraying parameters are reported in Table 2. Chemical analysis was carried out both on the powder and on the coatings to verify their composition. The Optima 3300DV ICP OES (inductively coupled plasma; optical emission spectrometer) (Perkin Elmer, USA) was used. Coatings were characterized by XRD (X-ray diffraction). A Siemens D500/DIFFRAC apparatus was employed. For comparison, a powder sample was also analyzed. Some coating samples were cut, embedded in resin, polished and their cross section was analyzed by a JEOL JSM-6480LV SEM (scanning electron microscope) equipped with EDS (energy dispersion spectroscope) and WDS (wavelength dispersion spectroscope) INCA Oxford system. Density measurements were carried out on coating samples according to ISO EN 623-2, by weighting samples before and after immersing them in water, according to Archimedes' method. Simultaneous TGA–DTA (thermal gravimetric analyses–differential thermal analyses) were carried out in static air, from room temperature
Fig. 6. SEM micrograph of a sample exposed in air at 650 °C for 30 min along with WDS maps for B, O, Si and Zr.
M. Tului et al. / Surface & Coatings Technology 207 (2012) 182–189
up to 1250 °C using a NETZSCH STA 409 Thermobalance, with a heating rate of 3 °C/min. Oxidation tests were conducted on 10 × 10 × 3 mm samples in a high temperature furnace in air at various temperatures and different dwelling times for each temperature, namely: 30 min for the samples exposed at 650 °C, 850 °C, 1100 °C and 1300 °C; 30, 60, 90 and 150 min for the samples at 1500 °C and 1700 °C. Before testing, samples were ultrasonically cleaned in acetone and dried. They were placed on zirconia refractory bricks and they were tilted to ensure minimal contact area. The following thermal cycle was used for the samples exposed at 1500 °C and 1700 °C: (i) heating rate of 10 °C/min from room temperature up to 1350 °C; (ii) heating rate of 5 °C/min up to the maximum temperature (1500 or 1700 °C); (iii) hold time at maximum temperature for 30, 60, 90 and 150 min; (iv) cooling rate of 10 °C/min from maximum temperature up to room temperature. Weights before and after oxidation at 1500 °C and 1700 °C were measured using a balance with 0.1 mg resolution. Microstructural analyses were performed on the surfaces and cross sections using a scanning electron microscope (Philips XL 40) equipped with an energy dispersive spectroscope (EDS, EDAX Falcon). The cross sections of the oxidized specimens were prepared for microstructural analysis by polishing procedures down to a 1 μm finish. Specimens were gold sputtered before SEM observation.
185
X-ray diffraction (Philips X-PERT) was used to identify the phases present after exposure. A step-scan mode was used in the 2θ range from 5° to 80° with a step width of 0.01° and a counting time of 7 s per step. The employed radiation was monochromated CuKα (40 kV–30 mA). 3. Results and discussion 3.1. Coating characterization Apparent solid density equal to 4.12 g/cm 3 was obtained averaging the values measured on three specimens. Chemical analysis and XRD were carried out both on the starting powder and on coating samples. Table 3 reports the chemical analysis results; for the sake of comparison, the composition of the starting mixture is also reported. XRD patterns are reported in Fig. 1. The XRD pattern of the coating can be fully indexed as ZrB2 and SiC, i.e., only the starting phases are present in the coating. Other phases, e.g., SiO2, ZrO2, B2O3, ZrSiO4, were not detected. The absence of oxide phases in the coating is due to the spraying parameters used (deposition under inert atmosphere, H2 as plasma gas). Previous works [12–16] showed that it is possible to obtain ZrB2–SiC coatings when SiC content is lower than ZrB2 content. SiC decomposition does not occur during spraying because of a liquid phase formation
Fig. 7. SEM micrograph of a sample exposed in air at 850 °C for 30 min along with WDS maps for B, O, Si and Zr.
M. Tului et al. / Surface & Coatings Technology 207 (2012) 182–189
at a temperature lower than the SiC decomposition one, as suggested by the ZrB2–SiC pseudo-binary phase diagram, reported in Fig. 2 [17], where a eutectic phase at 2207 °C is present. The compositions investigated in the previous works were located, in the phase diagram, near to the ZrB2 axis, while the composition investigated in the present work is positioned, in the phase diagram, on the opposite side with respect to the eutectic phase, i.e., near the SiC axis, as indicated by the dashed line in Fig. 2. The already mentioned mechanism of liquid phase formation, at a temperature lower than the SiC decomposition temperature, is supposed to be still valid with this coating composition. Fig. 3 shows the cross section of a coating observed by SEM at different magnifications: it appears to be compact and homogeneous (Fig. 3a); no unmelted particles were observed. The observed variety of gray color levels suggests the presence of compounds with different Zr and Si contents. EDS-WDS analyses confirmed that the dark gray areas are composed of SiC particles, white areas are made of ZrB2 particles, and intermediate gray level intensity areas presented different Zr and Si contents. Metallographic analysis seems to confirm the hypothesis of liquid phase formation according to the phase diagram. The coating appears to be formed by splats of different morphologies (Fig. 3b). This can be explained supposing that some particles impacted on the substrate in a partially melted status, i.e., at a temperature within the solidus–liquidus range; other particles were completely melted, i.e., above liquidus temperature. Those two situations correspond, respectively, to points A and B in Fig. 2. This can occur because particles could have: i) different sizes; ii) different compositions (which fluctuate around the nominal one); iii) different thermal histories during spraying.
a
* = ZrO2 ° = ZrSiO4
* * Counts (a.u.)
186
1500°C
* *
* * * * ** * *
* * *
** * * ** ** * *** * * *
* * *
**
°
*
*
*
+
* *
1700°C
10
*
* °
+ = SiO2
* 20
30
40
2θ
50
60
80
Fig. 9. X-ray diffraction patterns of the surface of ZrB2–SiC specimens oxidized for 90 min at 1500 °C (a) and 1700 °C (b).
High cooling rate, typical of plasma spraying, together with low miscibility of the phases ZrB2 and SiC, that reduces crystallization kinetics, causes the splat morphology to be strongly affected by particle condition at impact. EDS analyses carried out on the coating cross section area reported in Fig. 4 show that it is composed of SiC particles (average size b 1 μm) embedded in a Zr enriched matrix. Therefore, this splat could be generated by a particle which impacted in a partially melted status (point A in Fig. 2). 3.2. Oxidation behavior Thermogravimetric analysis and differential thermal analysis were simultaneously carried out on coating specimen from room temperature up to 1250 °C. TGA and DTA curves during heating are shown in Fig. 5.
b
SiO2
SiO2
ZrO2
ZrO2
c
70
d
SiO2
SiO2
ZrO2
ZrO2
Fig. 8. SEM micrograph of the cross sections of samples exposed for 30 min at 1100 °C (a), 1300 °C (b), 1500 °C (c), 1700 °C (d).
M. Tului et al. / Surface & Coatings Technology 207 (2012) 182–189
The following phenomena can be observed: i) weight gain in the range 600–800 °C associated with an exothermic reaction (DTA curve); ii) reduction of this phenomenon between 800 and 1100 °C; iii) further weight increase above 1100 °C during heating. TGA and DTA results don't differ from the ones obtained on lower SiC content samples, suggesting the same oxidation mechanisms, as reported in previous papers [15,16]. At the lower temperatures (points i) and ii) of the above discussion), ZrB2 oxidation is the most relevant phenomena, according to the following reaction: 2ZrB2 þ 5O2 ↔2ZrO2 ðsolidÞ þ 2B2 O3 ðliquidÞ: B2O3 tends to form a protective layer that reduces oxidation kinetics and protects SiC from oxidation till 1100 °C. Above such a temperature, B2O3 evaporates allowing SiC oxidation [18,19]. The SEM observation and EDS analyses carried out on the surfaces of the samples exposed for 30 min at 650 °C and 850 °C, reported in Figs. 6 and 7, respectively, are coherent with the proposed oxidation mechanism. On the surface of the sample exposed in air at 650 °C (Fig. 6), polygonal grains of B2O3 can be observed; they correspond to the formation of the first B2O3 nuclei, in the initial stage of the oxidation. Vice versa, in the sample exposed in air at 850 °C (Fig. 7), the B2O3 film is already formed and partially covers the surface of the sample. Unoxidized SiC particles can be observed in both the samples. Fig. 8 shows the cross sections of samples exposed for 30 min at 1100 °C, 1300 °C, 1500 °C and 1700 °C, respectively. The samples exposed at 1100 °C and 1300 °C show an oxidized layer formed by porous zirconium oxide and silicon oxide islands. In the samples exposed at higher temperatures a compact silicon oxide scale, including ZrO2 islands, is present.
187
Boron was never found in the oxide layers, likely because of the B2O3 formation and its evaporation at temperatures above 1100 °C. This element is present under the oxidized layers as ZrB2 particles. The oxidation reactions and products for the ZrB2–SiC specimens have been analyzed in the temperature range 1500 °C–1700 °C to understand their oxidation mechanisms. The XRD spectra obtained from the oxide scales formed during isothermal exposure at 1500 °C and 1700 °C for 90 min are shown in Fig. 9. The results in this figure indicated that the main phase present in the oxide scale is monoclinic ZrO2 while tetragonal zirconia was detected only for specimens oxidized for 30 min at 1500 °C. It can be supposed that tetragonal zirconia formed during oxidation but transformed to the monoclinic one during cooling [20]. Some traces of zircon (ZrSiO4) were found in specimens oxidized both at 1500 °C and 1700 °C but for the shortest exposure time, i.e. 30 min. Although the formation of stable zircon might be predicted on the basis of ternary isothermal section of the ZrO2–B2O3–SiO2 system at 1500 °C, this compound is often not reported in ZrB2–SiC composites oxidized at high temperatures (> 1500 °C) [20,21]. It is interesting to note that in the XRD pattern of the sample exposed at 1500 °C, a bump due to the presence of amorphous SiO2 can be observed, while at 1700 °C crystalline SiO2 (cristobalite) is present; this is thought to result from a progressive devitrification of the amorphous silica film [12]. The conclusions of the XRD analysis are confirmed by the examination of the surfaces of the oxidized samples. Fig. 10 (a–d) shows the SEM micrographs of surfaces of specimens oxidized for 30 and 90 min at 1500 and 1700 °C. Comparison of these images shows the oxide scale formed at 1500 °C to be rougher and more granular in comparison to that formed at 1700 °C. All these oxide scales appear porous with extensive bubble formation. In both cases (1500 and 1700 °C) the surface is covered by a thick silica glassy layer with aggregated zirconia particles with different sizes and shapes. The surface appearance is consistent with the oxidation mechanisms of ZrB2–SiC bulk composites, as outlined in [22]. In fact, during high temperature oxidation of
Fig. 10. SEM micrographs showing surface morphologies of the scale of specimens after oxidation at 1500 °C for 30 min (a); 1700 °C for 30 min (b); 1500 °C for 90 min (c); 1700 °C for 90 min (d).
M. Tului et al. / Surface & Coatings Technology 207 (2012) 182–189
ZrB2–SiC, a borosilicate oxide (B2O3–SiO2) film forms on the outer surface. Due to the high vapor pressure of boria at these temperatures compared to silica, boria is preferentially evaporated from the borosilicate liquid. The liquid oxide film at the outer surface then becomes a mainly viscous SiO2-rich borosilicate liquid which is protective and fills the pores of the porous oxide scale. The formation of the large bubbles is most likely due to the coalescence of the gaseous products (i.e., CO) inside the external forming glass. The main difference caused by the exposure temperature is the increasing amount of oxide scale with increasing temperature and the simultaneous decrease of zirconia particles appearing on the surface with the presence of cracks and discontinuities. A typical SEM (BSE) image depicting the cross section of the specimen oxidized at 1500 °C for 30 min is shown in Fig. 11. Examination of this figure indicates that the oxide scale is made of a continuous SiO2-rich layer containing various amounts of particulate zirconia, as confirmed by the EDS maps (Fig. 11). The formation of an external silica-based glass layer is very effective in limiting the inward diffusion of oxygen into the inner bulk, thus enhancing the oxidation resistance.
400 1700°C 1500°C
Oxide thickness (µm)
188
300
200
100
0 0
20
40
60
80
100
120
140
160
Time (min.) Fig. 12. Oxide thickness of ZrB2–SiC specimens oxidized at 1500 °C and 1700 °C as a function of oxidation time.
Fig. 11. Backscattering electron microscopy (BSE) image of the cross section of the ZrB2–SiC specimen tested at 1500 °C for 30 min along with the EDS maps for O, Si and Zr.
M. Tului et al. / Surface & Coatings Technology 207 (2012) 182–189
Specific mass gain (mg/cm2)
30
189
confirm the retention of ZrB2 and SiC phases in the deposited coatings and the chemical analysis highlights an amount of silicon consistent with the starting powders. Coatings are compact and homogeneous, with a low porosity content. Oxidation tests were performed up to 1700 °C: they showed the good potentiality of the material investigated to be used as a protection against the high temperature oxidation of CMC hot structures.
25 20 15 10
References
1700°C 1500°C
5 0 0
20
40
60
80
100
120
140
160
Time (min.) Fig. 13. Mass change of ZrB2–SiC specimens after oxidation at 1500 °C and 1700 °C as a function of exposure time.
The thickness of the oxide layer for each specimen tested was measured by averaging 150 values at various locations due to the nonuniformity of the oxide layer, which resulted in large standard deviations. The thickness increases with increasing oxidation time and temperature, as shown in Fig. 12. Fig. 13 shows mass gain per unit surface area as a function of oxidation at 1500 and 1700 °C. With increasing oxidation time, mass gain increased. It is likely that both thickness and mass gain increase rapidly during the first stages of oxidation but then steadily increase with time.
4. Conclusions Reported results demonstrate that it is possible to deposit coatings containing up to 66 vol.% of SiC by plasma spraying: the XRD patterns
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