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
S. Wannaparhun et al /Applied Surface Science 185(2002)183-196 3. 2. Mullite For reference, we also analyze mullite. Detailed Al(2p), Si(2p), and O(ls) XPS spectra are shown in part(b)of Figs. 3, 5 and 8. For a pure mullite sample, all signals are supposed to be from the mullite phase Using peakfit software, a single deconvoluted peak is obtained in all XPS spectra. This indicates th sence of a single phase in the as-received mullite. I (b) Fig. 7b, the deconvoluted Al(2p) at a BE of 74.9eV close to that of Al(2p)in a pure mullite(74.8eV [51). In Fig. 6b, the deconvoluted Si(2p) spectrum indicates a Be of 102.8 eV, which is consistent with a pure aluminosilicate (102.8eV [51)). The shape of the Si(2p)spectrum is symmetric and indicates no shoulder on either side of the spectra as compared 11511010510095 to the as-received alumina. The deconvoluted o(ls) spectrum in Fig. 9b, the BE of 531.7 eV indicates the B E(ev oxygen environment in mullite only (531.6eV Fig. 6. Deconvoluted Si(2p)XPS spectrum of the as-received:(a) 51) lumina matrix;(b) mullite(I: aluminum oxide; 2: aluminosili- The surface si/al ratio of the as-received calculated from the XPs survey scan is close theoretical value of 0.33(based on the mullin mical formula of 3Al2O32SiO2). A slight discrepancy survey scan. The presence of Si(2p) line indicates a in the ratio might come from the peak deconvoluting ery small amount of silicon in aluminosilicate form. procedure or the presence of any surface contaminan To confirm the XPS results, a thin specimen of the as- However, the as-received mullite spectra are very received composite was analyzed using TEM(Fig. 7). consistent with the literature It is worth indicated the presence of silicon in the matrix 3.3. Nextel-720 fiber region of the as-received composite. The preser of gallium ( Ga) was from the TEM sample 3.3.1. XPS Al(2p) preparation using FIB. Source of silicon might be In Fig. 10a, the deconvoluted Al(2p) spectrum from the sol-gel matrix binders during processing. revealed two peaks at 73.5 and 74.2 eV, respectively Thus, we are able to identify not only the foreign phase ( Table 1). The main peak at 74.2 eV with 85.0%o area is in the matrix but also its chemistry. consistent with aluminosilicate (74.6eV 51]).A small peak at 73.5 eV with 15.0% area correspond 3.1.1.3. XPS O(Is). XPS O(1s) spectrum(Fig. &a) to Al2O3 (73.9eV [51])only. According to the litera- indicates an asymmetric feature on its left-hand ture, the fiber was composed of around 60% mullite shoulder. The deconvoluted peaks derived from the and 40%0 a-Al2O3 by mole percent. Based on this spectrum(Fig. 9a) show two peaks at 530.26eV information, the Al(2p) signal of 85.0% would be (aluminum oxide, 89.1% area) and 531.66 eV attributed to the mullite phase and around 15.0%o were (aluminosilicate, 10.9% area), respectively (Table 1). that of the a-Al2O3. Please note that the difference Therefore, it can be concluded that the deconvoluted between the compositional information from the lit O(1s)spectrum was consistent with both Al(2p) and erature and the XPS analysis can be attributed to many Si(2p)spectra. This confirms that the alumina matrix factors. One is that the compositional information was composed of a Si-containing phase, which was an obtained from any XPS analysis is only from the surface region only, while the composition of
survey scan. The presence of Si(2p3 ) line indicates a very small amount of silicon in aluminosilicate form. To confirm the XPS results, a thin specimen of the asreceived composite was analyzed using TEM (Fig. 7). It is worthy to note that the EDS analysis also indicated the presence of silicon in the matrix region of the as-received composite. The presence of gallium (Ga) was from the TEM sample preparation using FIB. Source of silicon might be from the sol–gel matrix binders during processing. Thus, we are able to identify not only the foreign phase in the matrix but also its chemistry. 3.1.1.3. XPS O(1s). XPS O(1s) spectrum (Fig. 8a) indicates an asymmetric feature on its left-hand shoulder. The deconvoluted peaks derived from the spectrum (Fig. 9a) show two peaks at 530.26 eV (aluminum oxide, 89.1% area) and 531.66 eV (aluminosilicate, 10.9% area), respectively (Table 1). Therefore, it can be concluded that the deconvoluted O(1s) spectrum was consistent with both Al(2p) and Si(2p3 ) spectra. This confirms that the alumina matrix was composed of a Si-containing phase, which was an aluminosilicate. 3.2. Mullite For reference, we also analyze mullite. Detailed Al(2p), Si(2p3 ), and O(1s) XPS spectra are shown in part (b) of Figs. 3, 5 and 8. For a pure mullite sample, all signals are supposed to be from the mullite phase. Using peakfit software, a single deconvoluted peak is obtained in all XPS spectra. This indicates the presence of a single phase in the as-received mullite. In Fig. 7b, the deconvoluted Al(2p) at a BE of 74.9 eV is close to that of Al(2p) in a pure mullite (74.8 eV [51]). In Fig. 6b, the deconvoluted Si(2p) spectrum indicates a BE of 102.8 eV, which is consistent with a pure aluminosilicate (102.8 eV [51]). The shape of the Si(2p3 ) spectrum is symmetric and indicates no shoulder on either side of the spectra as compared to the as-received alumina. The deconvoluted O(1s) spectrum in Fig. 9b, the BE of 531.7 eV indicates the oxygen environment in mullite only (531.6 eV [51]). The surface Si/Al ratio of the as-received mullite calculated from the XPS survey scan is close to the theoretical value of 0.33 (based on the mullite chemical formula of 3Al2O32SiO2). A slight discrepancy in the ratio might come from the peak deconvoluting procedure or the presence of any surface contaminant. However, the as-received mullite spectra are very consistent with the literature. 3.3. Nextel-720 fiber 3.3.1. XPS Al(2p) In Fig. 10a, the deconvoluted Al(2p) spectrum revealed two peaks at 73.5 and 74.2 eV, respectively (Table 1). The main peak at 74.2 eV with 85.0% area is consistent with aluminosilicate (74.6 eV [51]). A small peak at 73.5 eV with 15.0% area corresponds to Al2O3 (73.9 eV [51]) only. According to the literature, the fiber was composed of around 60% mullite and 40% a-Al2O3 by mole percent. Based on this information, the Al(2p) signal of 85.0% would be attributed to the mullite phase and around 15.0% were that of the a-Al2O3. Please note that the difference between the compositional information from the literature and the XPS analysis can be attributed to many factors. One is that the compositional information obtained from any XPS analysis is only from the surface region only, while the composition of Fig. 6. Deconvoluted Si(2p3 ) XPS spectrum of the as-received: (a) alumina matrix; (b) mullite (1: aluminum oxide; 2: aluminosilicate). 188 S. Wannaparhun et al. / Applied Surface Science 185 (2002) 183–196
Max⊥ ∠(F (G) Fiber Qualit i ve El ement Ident i fi cat i on by Xray anal ysis (JOEL- TEM Peak list s General 5143694256_12752504 particle(2) [4053169414170634281 Fi b D General(2) 9977313D71561943880 96690002059 Fig. 7. EDS-TEM analysis of a thin composite specimen Mullite PAl2O3 540 530 BE(ev) Fig 8. XPS O(Is)spectrum of the as-received: (a)alumina matrix;(b)mullite
Fig. 7. EDS–TEM analysis of a thin composite specimen. Fig. 8. XPS O(1s) spectrum of the as-received: (a) alumina matrix; (b) mullite
S. Wannaparhun et al /Applied Surface Science 185(2002)183-196 60% mullite and 40% a-Al2O3 by mole percent is ulated based on the Al2O3-SiOz phase diagram, h reflects the volumetric com 3.3.2.XPSS(2p3) In Fig. 1la, the presence of Al(2s),'s second satellite along with Si(2p) spectrum makes this feature very useful for monitoring the surface chemical alteration of the fiber The satellite feature is similar to that of the as-received alumina matrix. The Si(2p) signal is mainly from the aluminosilicate in the fiber. Never theless, the Si(2p)signal could be obtained from the alumina matrix in the composite. Herein, a small contribution of the Si(2p)signal from the alumina matrix is negligible. A deconvoluted peak at 101. 8 eV 525 is obtained (Table 1). The discrepancy in the BE of the deconvoluted peak (101.8 ev)from the literature (102.6eV[5l))is noted. Comparing the surface Si/Al Fig9.Deconvoluted O(ls)XPS spectrum of the as-received: (a) ratio of the as-received fiber with that of the as- umina matrix: (b) mullite (1: aluminum oxide; 2: aluminosili- received alumina matrix, the fiber Si/Al ratio(0.32) B E.(ev) Fig 10. XPS Al(2p) spectrum: (a)the calculated spectrum of the composite(F: fiber and M: matrix);(b)the as-received composit
60% mullite and 40% a-Al2O3 by mole percent is calculated based on the Al2O3–SiO2 phase diagram, which reflects the volumetric composition of the fiber. 3.3.2. XPS Si(2p3 ) In Fig. 11a, the presence of Al(2s)’s second satellite along with Si(2p3 ) spectrum makes this feature very useful for monitoring the surface chemical alteration of the fiber. The satellite feature is similar to that of the as-received alumina matrix. The Si(2p3 ) signal is mainly from the aluminosilicate in the fiber. Nevertheless, the Si(2p3 ) signal could be obtained from the alumina matrix in the composite. Herein, a small contribution of the Si(2p3 ) signal from the alumina matrix is negligible. A deconvoluted peak at 101.8 eV is obtained (Table 1). The discrepancy in the BE of the deconvoluted peak (101.8 eV) from the literature (102.6 eV [51]) is noted. Comparing the surface Si/Al ratio of the as-received fiber with that of the asreceived alumina matrix, the fiber Si/Al ratio (0.32) Fig. 9. Deconvoluted O(1s) XPS spectrum of the as-received: (a) alumina matrix; (b) mullite (1: aluminum oxide; 2: aluminosilicate). Fig. 10. XPS Al(2p) spectrum: (a) the calculated spectrum of the composite (F: fiber and M: matrix); (b) the as-received composite. 190 S. Wannaparhun et al. / Applied Surface Science 185 (2002) 183–196
S. Wannaparhun et al /Applied Surface Science 185(2002)183-196 a 115 74N BE.(ev) Fig. 11. XPS Si(2p)spectrum:(a)the calculated spectrum of the composite(F: fiber and M: matrix); (b)the as-received composite is more than that of the matrix(0.03). Hence, the 3.4. The composite position of Si(2p)in the fiber(101.8 ev) is presented at a higher BE compared to the matrix(101.5 eV It is necessary to understand the geometry of the as- This study is similar to studies of various zeolite received CMC for determining an analyzed surface systems studied by Barr [50,52]. Any change in the using XPS. Herein, the geometry of the CMC can be Al ratio in the complex zeolitic oxide lattice will illustrated using a Cartesian coordinate(Fig. 13a) produce a corresponding change in the core level According to the manufacturing of this particular binding energies results in significant chemical shifts CMC, the woven fiber fabrics are laid upon X-Y plane, in the XPS corelines of interest. The change in the Be infiltrated with alumina, and then stacked up in position of the Si(2p) peak is predominantly attrib- Z-direction to form the CMC of a desired thickness uted to changes in the sial ratio The SEM images indicate two possible surfaces to be analyzed: (i)top surface(Fig. 13b)and(ii)cross- 3.3.3. XPS O(ls) sectional surface(Fig. 13c). It is important to select the Both individual and deconvoluted spectra are listed top surface for the XPS analysis in order to maximize in Fig. 12a. The peaks located at 530.8ev(11.7% the probability of detecting the interface between the area)and 531. 1 ev(88.3%o area) indicate a predomi- fiber and the matrix phase. It is noteworthy that the nant peak of aluminosilicate(531.6 eV) and the minor XPS spectra obtained from the top surface of the Cmo peak is assigned to Al2O3(530.9 eV)(51]. The decon- will depend on the distribution of the matrix on the voluted XPS O(Is) spectra were consistent with the surface. If the fiber fabric at the topmost layer is fully Al(2p) spectrum, where aluminosilicate was also covered with the matrix of the thickness more than the found to be the major component in the Nextel-720 depth resolution of the XPs(depends on the incident angle of the X-ray ), the signal will only come from the
is more than that of the matrix (0.03). Hence, the position of Si(2p3 ) in the fiber (101.8 eV) is presented at a higher BE compared to the matrix (101.5 eV). This study is similar to studies of various zeolite systems studied by Barr [50,52]. Any change in the Si/Al ratio in the complex zeolitic oxide lattice will produce a corresponding change in the core level binding energies results in significant chemical shifts in the XPS corelines of interest. The change in the BE position of the Si(2p3 ) peak is predominantly attributed to changes in the Si/Al ratio. 3.3.3. XPS O(1s) Both individual and deconvoluted spectra are listed in Fig. 12a. The peaks located at 530.8 eV (11.7% area) and 531.1 eV (88.3% area) indicate a predominant peak of aluminosilicate (531.6 eV) and the minor peak is assigned to Al2O3 (530.9 eV) [51]. The deconvoluted XPS O(1s) spectra were consistent with the Al(2p) spectrum, where aluminosilicate was also found to be the major component in the Nextel-720 fiber. 3.4. The composite It is necessary to understand the geometry of the asreceived CMC for determining an analyzed surface using XPS. Herein, the geometry of the CMC can be illustrated using a Cartesian coordinate (Fig. 13a). According to the manufacturing of this particular CMC, the woven fiber fabrics are laid upon X–Y plane, infiltrated with alumina, and then stacked up in Z-direction to form the CMC of a desired thickness. The SEM images indicate two possible surfaces to be analyzed: (i) top surface (Fig. 13b) and (ii) crosssectional surface (Fig. 13c). It is important to select the top surface for the XPS analysis in order to maximize the probability of detecting the interface between the fiber and the matrix phase. It is noteworthy that the XPS spectra obtained from the top surface of the CMC will depend on the distribution of the matrix on the surface. If the fiber fabric at the topmost layer is fully covered with the matrix of the thickness more than the depth resolution of the XPS (depends on the incident angle of the X-ray), the signal will only come from the Fig. 11. XPS Si(2p3 ) spectrum: (a) the calculated spectrum of the composite (F: fiber and M: matrix); (b) the as-received composite. S. Wannaparhun et al. / Applied Surface Science 185 (2002) 183–196 191
S. Wannaparhun et al /Applied Surface Science 185(2002)183-196 E(ev) Fig 12. XPS O(Is)spectrum(a)the calculated spectrum of the composite(F: fiber and M: matrix);(b) the as-received composite matrix(alumina). On the other hand, the XPS signal acquired xPS data from alumina, mullite and Nex will be only obtained from the fiber if there is no tel-720 fiber was used to deconvolute the complex matrix on the surface. For both cases above, the XPs XPS spectra acquired from CMC, since this CMC signal will indicate characteristic features of the alu- composed of alumina and Nextel-720 fiber mina and the fiber and detailed XPs deconvolution (alumina mullite) only. procedure has been adopted to differentiate chemistry of the matrix, fiber and the interface based on chemical 3.4.1. The spectra from the as-received materials shifts [50]. Practically, the top surface of the CMC The XPS survey scan of the as-received composite possesses a certain degree of roughness and hence the shown in Fig. 14 reveals a combined feature of the al- XPS signal obtained from the top surface will carry satellites of Al(2s) from the matrix and the Si(2p) the information depending upon the thickness of the signal from the fiber matrix phase at the surface. If the alumina matrix area thickness is less than the depth resolution of 3.4.1.1. XPS Al(2p). The Al(2p) signal from the as- the XPS, then the XPs will contain the signal from received CMC can be divided into two parts: (a)from the region underneath the matrix, 1. e, the fiber an the alumina matrix and(b) from the Nextel-720 fiber. interface In addition, the Al(2p) signal from the fiber can be In order to investigate any chemical alteration in divided into the signal from the aluminosilicate and CMC during fabrication, two sets of XPS spectra were the alumina. a detailed XPS Al(2p) is shown in quired: (a)as-received and(b) calculated spectra Fig. 10b. The deconvoluted spectrum indicates the from the individual composite components. The peaks at 73.6 eV(88.2% area) and 74.2 eV(11.8%
matrix (alumina). On the other hand, the XPS signal will be only obtained from the fiber if there is no matrix on the surface. For both cases above, the XPS signal will indicate characteristic features of the alumina and the fiber and detailed XPS deconvolution procedure has been adopted to differentiate chemistry of the matrix, fiber and the interface based on chemical shifts [50]. Practically, the top surface of the CMC possesses a certain degree of roughness and hence the XPS signal obtained from the top surface will carry the information depending upon the thickness of the matrix phase at the surface. If the alumina matrix area thickness is less than the depth resolution of the XPS, then the XPS will contain the signal from the region underneath the matrix, i.e., the fiber and the interface. In order to investigate any chemical alteration in CMC during fabrication, two sets of XPS spectra were acquired: (a) as-received and (b) calculated spectra from the individual composite components. The acquired XPS data from alumina, mullite and Nextel-720 fiber was used to deconvolute the complex XPS spectra acquired from CMC, since this CMC is composed of alumina and Nextel-720 fiber (alumina þ mullite) only. 3.4.1. The spectra from the as-received materials The XPS survey scan of the as-received composite shown in Fig. 14 reveals a combined feature of the Alsatellites of Al(2s) from the matrix and the Si(2p3 ) signal from the fiber. 3.4.1.1. XPS Al(2p). The Al(2p) signal from the asreceived CMC can be divided into two parts: (a) from the alumina matrix and (b) from the Nextel-720 fiber. In addition, the Al(2p) signal from the fiber can be divided into the signal from the aluminosilicate and the alumina. A detailed XPS Al(2p) is shown in Fig. 10b. The deconvoluted spectrum indicates the peaks at 73.6 eV (88.2% area) and 74.2 eV (11.8% Fig. 12. XPS O(1s) spectrum: (a) the calculated spectrum of the composite (F: fiber and M: matrix); (b) the as-received composite. 192 S. Wannaparhun et al. / Applied Surface Science 185 (2002) 183–196