surface science ELSEVIER Applied Surface Science 185(2002)183-196 www.elsevier.com/locate/apsusc Surface chemistry of Nextel-720 alumina and Nextel-720/ alumina ceramic matrix composite(CMC) using XPS-A tool for nano-spectroscopy S. Wannaparhun', S Seal, V. desai Advanced Materials Processing and Analysis Center(AMPAC) and Mechanical, Materials and Aerospace Engineering(MMAE). University of Central Florida, Eng. 381, 4000 University Blvd., Orlando, FL 32816, USA Received 5 July 2001: accepted 11 September 2001 Abstract Oxide-based ceramic matrix composites( CMCs)are prime candidates for high temperature turbine applications. Increasing demand of CMCs necessitates the development of quality monitoring procedures. Sol-gel derived Nextel-720 fiber/alumina matrix CMC is one of the potential candidate material for land-based gas turbine applications. X-ray photoelectron spectroscopy(XPS) and transmission electron microscopy (TEM) were utilized to investigate any surface/interface chemical alteration of the Nextel-720 fiber reinforcement and the alumina matrix during fabrication. The calculated XPS spectra of the composite were obtained by simply adding the spectra of the as-received Nextel-720 fiber and the alumina matrix. The calculated XPS spectra and the acquired XPS Al(2p), Si(2p), and o(ls)spectra from the as-received materials were compared using a superimposition method to investigate any chemical alteration during composite fabrication for quality control measures. This paper is aimed to serve as a reference for future XPS studies of CMCs exposed to aggressive turbine environments. C 2002 Elsevier Science B V. All rights reserved PACS: 87. 64 Lg Keywords: Ceramic matrix composites; Nextel-720 fiber; XPS or ESCA; TEM; Aluminosilicate; Gas turbines 1. Introduction candidate for high temperature land-based gas turbine component because it can reduce NOx and CO emis Ceramic matrix composites(CMC) are widely used sion for no cooling medium systems [8-10 as high temperature materials for power generation An interfacial property of a CMC plays an and aerospace applications for structural advantages tant role on their ductile-brittle transition, which ver their metallic counterparts [1-7. Nextel-720/ depends on various crack-propagating modes [11 alumina CMC, an oxide-based CMC, is a potential 18]. Crack deflection within the bulk region of the composite is aimed to obtain high ductility as well as Corresponding author. Tel :+1-407-823-5277: high strength from the fiber reinforcement. Sol-gel is E-mail address: seal mail ncf. edu(S. Seal). the primary process for manufacturing a continuous Currently: Ph. D. student, Materials Science and Engineering fiber-reinforced ceramic composite(CFCC)[19, 20] University of Florida, Gainsville. During fabrication, a surface and interfacial chemical 0-4332/02/- see front matter C 2002 Elsevier Science B V. All rights reserved. S0169-4332(01)00594-3
Surface chemistry of Nextel-720, alumina and Nextel-720/ alumina ceramic matrix composite (CMC) using XPS–A tool for nano-spectroscopy S. Wannaparhun1 , S. Seal* , V. Desai Advanced Materials Processing and Analysis Center (AMPAC) and Mechanical, Materials and Aerospace Engineering (MMAE), University of Central Florida, Eng. 381, 4000 University Blvd., Orlando, FL 32816, USA Received 5 July 2001; accepted 11 September 2001 Abstract Oxide-based ceramic matrix composites (CMCs) are prime candidates for high temperature turbine applications. Increasing demand of CMCs necessitates the development of quality monitoring procedures. Sol–gel derived Nextel-720 fiber/alumina matrix CMC is one of the potential candidate material for land-based gas turbine applications. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) were utilized to investigate any surface/interface chemical alteration of the Nextel-720 fiber reinforcement and the alumina matrix during fabrication. The calculated XPS spectra of the composite were obtained by simply adding the spectra of the as-received Nextel-720 fiber and the alumina matrix. The calculated XPS spectra and the acquired XPS Al(2p), Si(2p3 ), and O(1s) spectra from the as-received materials were compared using a superimposition method to investigate any chemical alteration during composite fabrication for quality control measures. This paper is aimed to serve as a reference for future XPS studies of CMCs exposed to aggressive turbine environments. # 2002 Elsevier Science B.V. All rights reserved. PACS: 87.64 Lg Keywords: Ceramic matrix composites; Nextel-720 fiber; XPS or ESCA; TEM; Aluminosilicate; Gas turbines 1. Introduction Ceramic matrix composites (CMC) are widely used as high temperature materials for power generation and aerospace applications for structural advantages over their metallic counterparts [1–7]. Nextel-720/ alumina CMC, an oxide-based CMC, is a potential candidate for high temperature land-based gas turbine component because it can reduce NOx and CO emission for no cooling medium systems [8–10]. An interfacial property of a CMC plays an important role on their ductile–brittle transition, which depends on various crack-propagating modes [11– 18]. Crack deflection within the bulk region of the composite is aimed to obtain high ductility as well as high strength from the fiber reinforcement. Sol–gel is the primary process for manufacturing a continuous fiber-reinforced ceramic composite (CFCC) [19,20]. During fabrication, a surface and interfacial chemical Applied Surface Science 185 (2002) 183–196 *Corresponding author. Tel.: þ1-407-823-5277; fax: þ1-407-823-0208. E-mail address: sseal@mail.ncf.edu (S. Seal). 1Currently: Ph.D. student, Materials Science and Engineering, University of Florida, Gainsville. 0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0169-4332(01)00594-3
184 S. Wannaparhun et al /Applied Surface Science 185(2002)183-196 reaction can lead to alteration in the physiochemical deposited on a preform of a woven Nextel-720 fiber properties of the composite [21-30]. Therefore, it is fabric during composite fabrication process developed important to establish the correlation between chemi University of California at Santa Barbara. The woven cal and mechanical properties(physiochemical prop- fabric was cut and laid up in layers according to the erties)of the as-manufactured composite for future desired thickness and volume fraction reinforcement. mass-production of all oxide CMCs. Both quantitative The as-received composite has the following properties and qualitative measurements of physiochemical as reported by the manufacturer: density, 2.60 g/cm; properties are important for monitoring the process volume fraction, 43. 6% fiber and porosity, w28.2 X-ray photoelectron spectroscopy (XPS)is selected Further information regarding processing of the com- as a surface analytical tool to provide key chemical state posite was reported elsewhere [15, 16 information of the CMCs. This is because XPs is pable of providing chemical information of a 2. 2. X-ray photoelectron spectroscopy large-thinregion. This particular advantage will accom- modate us to extract the information from the interface A PHI 5400 ESCA was used in the present study as a formed between the fiber and the matrix phase in CMCs major surface analytical tool. Mg Ko of energy a few detailed investigations have been documented 1253.6 eV was used as the X-ray source. After acquir 31-33 for the use of XPS valence band to investigate ing the XPS spectra, the binding energy (BE)values any chemical interaction between carbon fiber and were shifted due to the differences in the polarizability phenolic matrix. Besides, XPS has been successfully and chemical potential of the compounds. Any char a key component of the Nextel-720 fiber[34-38. 53). at 284.6ev 14448). Using a peakfit software In this study, XPS was used in detail in conjunction with TEM to monitor any chemical alteration in the using Gaussian/Lorentzian peak shape, which include CMCduring fabrication XPS analyses of the individual X-radiation satellites in the fitting routine composite components(e. g, Nextel-720 fiberandalum- ina matrix)are compared to the as-manufactured CMCs 23. Process monitoring concept The present study was intended to investigate the via bility of XPS as a tool for quality control of oxide CMC Fig. I shows an analytical route for monitoring any chemical interaction between the fiber and the matrix 2. Experimental and o(ls) spectra of the as-received Nextel-720 fiber 2.. A Nextel-720/alumina CMC (Al(2p)Fiber, Si(2p)Fiber, and o(ls)Fiber), the alumina matrix(Al(2p)Matrix, Si(2p)Matrix, and O(ls)Matrix) A Nextel-720M/alumina CMC was manufactured and the composite(Al(2p)As-received, Si(2p )As-received, by Composites Optics Ceramics Company(COI-C) and O(Is)As-received) were acquired. The calculated underan Air Force SBIR contract using sol-gelprocess. Spectra(Al(2p)cal, Si(2p)cal, and O(s)ca)were nens were supplied by Sie obtained by adding the XPS spectrum of the as- of Nextel-720 fibers infiltrated in the alumina matrix. BE scale. The calculated XPS spectra were obtained The Nextel-720 fiber was manufactured by 3M Corporation and supplied as eight-harness satin fabric. Al(2p)cal= Al(2p)Fiber+ Al(2p)Matix in the fabric roximately 400 Si(2p)cal= Si(2pE filaments, 10-12 um in diameter, and 0/90 orienta tions. The chemical composition of the fiber is approx o(ls)Cal=o(ls)Fiber +o(1s)Mat mately 85%A1 0, and 15% SiO, by weight. In terms of Ideally, there would be no chemical interaction at the phase composition, the CMC consists of 60% mullite fiber/matrix interface, if and only if the following and 40% a-A1 0, by weight [39-42) according to a conditions are satisfied binary Al2O3-SiO2 phase diagram [43]. Alumina was Al(2p)Cal= Al(2p)As-received (4)
reaction can lead to alteration in the physiochemical properties of the composite [21–30]. Therefore, it is important to establish the correlation between chemical and mechanical properties (physiochemical properties) of the as-manufactured composite for future mass-production of all oxide CMCs. Both quantitative and qualitative measurements of physiochemical properties are important for monitoring the process. X-ray photoelectron spectroscopy (XPS) is selected as a surface analytical tool to provide keychemical state information of the CMCs. This is because XPS is capable of providing chemical information of a large-thinregion.Thisparticularadvantagewillaccommodate us to extract the information from the interface formed between thefiberandthematrixphaseinCMCs. A few detailed investigations have been documented [31–33] for the use of XPS valence band to investigate any chemical interaction between carbon fiber and phenolic matrix. Besides, XPS has been successfully utilized in studying various aluminosilicates chemistry, a key component of the Nextel-720 fiber [34–38,53]. In this study, XPS was used in detail in conjunction with TEM to monitor any chemical alteration in the CMC during fabrication. XPS analyses of the individual compositecomponents(e.g.,Nextel-720fiberandaluminamatrix)arecomparedtotheas-manufacturedCMCs. The present study was intended to investigate the viability of XPS as a tool for quality control of oxide CMC. 2. Experimental 2.1. A Nextel-720/alumina CMC A Nextel-720TM/alumina CMC was manufactured by Composites Optics Ceramics Company (COI-C) underanAirForce SBIRcontract usingsol–gelprocess. Composite specimens were supplied by Siemens Westinghouse Corporation. The composite consisted of Nextel-720 fibers infiltrated in the alumina matrix. The Nextel-720 fiber was manufactured by 3M Corporation and supplied as eight-harness satin fabric. The tows in the fabric contain approximately 400 filaments, 10–12 mm in diameter, and 0/908 orientations. The chemical composition of the fiber is approximately 85% Al2O3 and 15% SiO2 by weight. In terms of phase composition, the CMC consists of 60% mullite and 40% a-Al2O3 by weight [39–42] according to a binary Al2O3–SiO2 phase diagram [43]. Alumina was deposited on a preform of a woven Nextel-720 fiber fabric during composite fabrication process developed byUniversityofCaliforniaatSantaBarbara.Thewoven fabric was cut and laid up in layers according to the desired thickness and volume fraction reinforcement. The as-received composite has the following properties as reported by the manufacturer: density, 2.60 g/cm3 ; volume fraction, 43.6% fiber and porosity, 28.2%. Further information regarding processing of the composite was reported elsewhere [15,16]. 2.2. X-ray photoelectron spectroscopy A PHI 5400 ESCAwas used in the present study as a major surface analytical tool. Mg Ka of energy 1253.6 eV was used as the X-ray source. After acquiring the XPS spectra, the binding energy (BE) values were shifted due to the differences in the polarizability and chemical potential of the compounds. Any charging shifts were removed by fixing the C(1s) BE at 284.6 eV [44–48]. Using a peakfit softwareTM, non-linear least square curve fitting was performed using Gaussian/Lorentzian peak shape, which include X-radiation satellites in the fitting routine. 2.3. Process monitoring concept Fig. 1 shows an analytical route for monitoring any chemical interaction between the fiber and the matrix during composite fabrication. XPS Al(2p), Si(2p3 ), and O(1s) spectra of the as-received Nextel-720 fiber (Al(2p)Fiber, Si(2p3 )Fiber, and O(1s)Fiber), the alumina matrix (Al(2p)Matrix, Si(2p3 )Matrix, and O(1s)Matrix), and the composite (Al(2p)As-received, Si(2p3 )As-received, and O(1s)As-received) were acquired. The calculated spectra (Al(2p)Cal, Si(2p3 )Cal, and O(1s)Cal) were obtained by adding the XPS spectrum of the asreceived fiber and matrix with respect to a constant BE scale. The calculated XPS spectra were obtained using the following equations: Alð2pÞCal ¼ Alð2pÞFiber þ Alð2pÞMatrix (1) Sið2p3 ÞCal ¼ Sið2p3 ÞFiber þ Sið2p3 ÞMatrix (2) Oð1sÞCal ¼ Oð1sÞFiber þ Oð1sÞMatrix (3) Ideally, there would be no chemical interaction at the fiber/matrix interface, if and only if the following conditions are satisfied: Alð2pÞCal ¼ Alð2pÞAs-received (4) 184 S. Wannaparhun et al. / Applied Surface Science 185 (2002) 183–196
S. Wannaparhun et al /Applied Surface Science 185(2002)183-196 185 Nextel 720 fiber As-received Sol-gel process As-received trixs spectr umina matrix (100% matrix from sol-gel process) The as-received composite Fiber Spect (F)+(M) Composite (F)+(△M)+() F)+(M) Any during the processing? Fig. 1. Use of XPS to evaluate the interfacial chemistry of the composite material through spectral interpretation. Photoelectron from: F, fiber; As-received (5) 3.. Alumina o(Iscal=o(1s) XPS survey scan of the as-received alumina shown To compare the calculated and the as-received spectra, in Fig. 2 revealed two predominant peaks: (i)the main a graphical superimposition method was used. For Al(2p)coreline between 60 and 80 eV, and (ii) Al(2s) TEM analysis, the authors have followed a specimen and its satellites between 80 and 120 e V. The Be of preparation technique using a double layer model Al(2s) was around 119 eV and its first satellite was at (DLM) and focused ion beam(FlB)for an effective 9.4 eV downfield from the main Al(2s) peak. The surface/interface analysis of this insulating material second satellite of Al(2s) was located around 90- 49] 110 eV, close to Si(2p) lines. Similar satellites were observed for 1(2p) but with varying The surface Si/Al ratio calculated from XPS spectra 3. Results and discussion vas≈0.03 A surface Si/Al ratio is a good measurement to Primarily, both mullite and alumina were analyzed indicate the presence of silicon in the matrix, if any as reference materials to understand the fundamental Any change in Si/Al ratio will affect the position surface chemistry of this composite intensity of all the peaks in the XPS spectrum. A
Sið2p3 ÞCal ¼ Sið2p3 ÞAs-received (5) Oð1sÞCal ¼ Oð1sÞAs-received (6) To compare the calculated and the as-received spectra, a graphical superimposition method was used. For TEM analysis, the authors have followed a specimen preparation technique using a double layer model (DLM) and focused ion beam (FIB) for an effective surface/interface analysis of this insulating material [49]. 3. Results and discussion Primarily, both mullite and alumina were analyzed as reference materials to understand the fundamental surface chemistry of this composite. 3.1. Alumina XPS survey scan of the as-received alumina shown in Fig. 2 revealed two predominant peaks: (i) the main Al(2p) coreline between 60 and 80 eV, and (ii) Al(2s) and its satellites between 80 and 120 eV. The BE of Al(2s) was around 119 eV and its first satellite was at 9.4 eV downfield from the main Al(2s) peak. The second satellite of Al(2s) was located around 90– 110 eV, close to Si(2p3 ) lines. Similar satellites were also observed for Al(2p) but with varying intensity. The surface Si/Al ratio calculated from XPS spectra was 0.03. A surface Si/Al ratio is a good measurement to indicate the presence of silicon in the matrix, if any. Any change in Si/Al ratio will affect the position and intensity of all the peaks in the XPS spectrum. A Fig. 1. Use of XPS to evaluate the interfacial chemistry of the composite material through spectral interpretation. Photoelectron from: F, fiber; M, matrix; I, interface. S. Wannaparhun et al. / Applied Surface Science 185 (2002) 183–196 185
S. Wannaparhun et al /Applied Surface Science 185(2002)183-196 A1(2s) d B E.ev) Fig. 2. XPS survey spectrum of the as-received alumina matrix: (a)first satellite of Al(2s); (b)Si(2p)region; (c)second satellite of Al(2s) (d) first satellite of Alo decrease in Si/Al ratio will shift all the peaks to lower indicates the presence of Al2O3(73.7eV [51]. The Be due to ionicity/covalency (IC) effect [50]. This peak at 74.6 eV with 6.3% area was aluminosilicate particular feature was also observed in this study. Thus (74.6eV [51, 52]). The latter is consistent with the the presence of silicon was evident in the as-received Si/Al ratio(0.03)mentioned previously for the as- alumina matrix received alumina with silicon as a foreign phase L.. Detailed ch 3.1.1.1. XPS Al(2p). Al(2p)spectra for mullite and alumina are illustrated in Fig 3. As shown in Fig. 4a. the deconvoluted Al(2p) spectrum was composed of two peaks located at 73.6 and 74.6eV (Table 1), respectively. The peak at 73.6eV with 93. 7%area Mullite 70 80 BE.(ev) Fig 4. Deconvoluted Al(2p) XPS spectrum of the as-received:(a) Fig. 3. XPS Al(2p) spectrum of the as-received: (a) alumina alumina matrix;(b)mullite(1: aluminum oxide; 2: aluminosili- matrix;(b)mullite
decrease in Si/Al ratio will shift all the peaks to lower BE due to ionicity/covalency (IC) effect [50]. This particular feature was also observed in this study. Thus the presence of silicon was evident in the as-received alumina matrix. 3.1.1. Detailed chemistry 3.1.1.1. XPS Al(2p). Al(2p) spectra for mullite and alumina are illustrated in Fig. 3. As shown in Fig. 4a, the deconvoluted Al(2p) spectrum was composed of two peaks located at 73.6 and 74.6 eV (Table 1), respectively. The peak at 73.6 eV with 93.7% area indicates the presence of Al2O3 (73.7 eV [51]). The peak at 74.6 eV with 6.3% area was aluminosilicate (74.6 eV [51,52]). The latter is consistent with the Si/Al ratio (0.03) mentioned previously for the asreceived alumina with silicon as a foreign phase. Fig. 2. XPS survey spectrum of the as-received alumina matrix: (a) first satellite of Al(2s); (b) Si(2p3 ) region; (c) second satellite of Al(2s); (d) first satellite of Al(2p). Fig. 3. XPS Al(2p) spectrum of the as-received: (a) alumina matrix; (b) mullite. Fig. 4. Deconvoluted Al(2p) XPS spectrum of the as-received: (a) alumina matrix; (b) mullite (1: aluminum oxide; 2: aluminosilicate). 186 S. Wannaparhun et al. / Applied Surface Science 185 (2002) 183–196
S. Wannaparhun et al /Applied Surface Science 185(2002)183-196 187 Table 1 analysis of the as-received mullite, Nextel-720 fiber, alumina matrix, and composite Material SU/Al Spectrum Peak fitted data BE(ev) FWHM (e Percentage of area Phase [51] mullite Aluminosilicate Si(2p) 1028 Aluminosilicate Aluminosilicate Nextel-720 fiber 0.3208 73.5 Aluminum oxide Si(2p) 101.8 117 Aluminum oxide 31.1 88.3 Aluminosilicate Alumina matrix 0.0302 A(2p) 937 Aluminum oxide 1.66 Aluminosilicate S 1017 142 00 Aluminosilicate 530.3 2.0 Aluminum oxide 317 10.9 Aluminosilicate omposite A(2p) 73.6 88.2 118 S 101.5 530.1 2.05 71.9 5312 3.1.1.2. XPS Si(2p). Fig 5a indicates a left shoulder of the deconvoluted peak(101.7 eV, Table 1)and the of Al(2s)peak around 115 eV. The first and the second literature(102.6eV [51) can be explained based on satellite of Al(2s)are located at 110 and 95-105 e V, the IC effect [50] mentioned earlier. At very low Si/Al respectively. In Fig 6a, the deconvoluted Si(2p)peak ratio, the Si(2p)peak should be located at a low BE was found embedding in the range of the Al(2s)'s position. In this case, the Si/Al ratio was very low such second satellite. The discrepancy between the position that the Si(2p)peak could not be observed in the Mullite 115 Fig.5. XPS Si(2p,) spectrum of the as-received: (a)alumina matrix:(b)mullite(* denotes first satellite of Al(2s)
3.1.1.2. XPS Si(2p3 ). Fig. 5a indicates a left shoulder of Al(2s) peak around 115 eV. The first and the second satellite of Al(2s) are located at 110 and 95–105 eV, respectively. In Fig. 6a, the deconvoluted Si(2p3 ) peak was found embedding in the range of the Al(2s)’s second satellite. The discrepancy between the position of the deconvoluted peak (101.7 eV, Table 1) and the literature (102.6 eV [51]) can be explained based on the IC effect [50] mentioned earlier. At very low Si/Al ratio, the Si(2p3 ) peak should be located at a low BE position. In this case, the Si/Al ratio was very low such that the Si(2p3 ) peak could not be observed in the Table 1 XPS analysis of the as-received mullite, Nextel-720 fiber, alumina matrix, and composite Material Si/Al Spectrum Peak fitted data BE (eV) FWHM (eV) Percentage of area Phase [51] Mullite 0.5158 Al(2p) 74.9 1.91 100 Aluminosilicate Si(2p3 ) 102.8 1.96 100 Aluminosilicate O(1s) 531.7 2.58 100 Aluminosilicate Nextel-720 fiber 0.3208 Al(2p) 73.5 2.09 15 Aluminum oxide 74.2 2.09 85 Aluminosilicate Si(2p3 ) 101.8 1.84 100 Aluminosilicate O(1s) 530.8 2.54 11.7 Aluminum oxide 531.1 2.54 88.3 Aluminosilicate Alumina matrix 0.0302 Al(2p) 73.6 1.66 93.7 Aluminum oxide 74.6 1.66 6.3 Aluminosilicate Si(2p3 ) 101.7 1.42 100 Aluminosilicate O(1s) 530.3 2.07 89.1 Aluminum oxide 531.7 2.07 10.9 Aluminosilicate Composite 0.0622 Al(2p) 73.6 1.64 88.2 Aluminum oxide 74.2 1.64 11.8 Aluminosilicate Si(2p3 ) 101.5 1.98 100 Aluminosilicate O(1s) 530.1 2.05 71.9 Aluminum oxide 531.2 2.05 28.13 Aluminosilicate Fig. 5. XPS Si(2p3 ) spectrum of the as-received: (a) alumina matrix; (b) mullite ( denotes first satellite of Al(2s)). S. Wannaparhun et al. / Applied Surface Science 185 (2002) 183–196 187