Effect of TiN nano-coating on the interface microstructure of carbon fibres-az91 alloy composite

Transcription

1 IOP Conference Series: Materials Science and Engineering Effect of TiN nano-coating on the interface microstructure of carbon fibres-az91 alloy composite To cite this article: A Olszówka-Myalska and A Botor-Probierz 2012 IOP Conf. Ser.: Mater. Sci. Eng Related content - Studies of the AZ91 magnesium alloy / SiO 2 -coated carbon fibres composite microstructure A Olszówka-Myalska and A Botor-Probierz - Microstructural analysis of titanium aluminide formed in situ in an aluminium matrix composite A Olszówka-Myalska and W Maziarz - Application of EBSD method for the investigation of microstructure and crystallographic orientation in RE 2 Zr 2 O 7 TBC B Chmiela, M Sozaska and G Moskal View the article online for updates and enhancements. This content was downloaded from IP address on 06/12/2017 at 02:40

2 Effect of TiN nano-coating on the interface microstructure of carbon fibres-az91 alloy composite A Olszówka-Myalska 1 and A Botor-Probierz Silesian University of Technology, Faculty of Materials Engineering and Metallurgy, ul. Krasińskiego 8b, PL Katowice, Poland anita.olszowka-myalska@polsl.pl Abstract. Magnesium matrix composites reinforced with carbon fibres, C f,without surface modification and coated with TiN nanolayer, (C f ) TiN, were investigated. AZ91 magnesium alloy and carbon fibres of T300B (Toray) PAN type were chosen as components. In the experiment infiltration in vacuum as a method of component consolidation and chemical vapour deposition (CVD) for carbon fibres surface modification were applied. Structural investigations were performed in the Hitachi 3200S field-emission scanning electron microscope (FE-SEM) and on a FEI Tecnai G2 FEG high-resolution transmission electron microscope (TEM) equipped with energy-dispersive X-ray spectrometer (EDS) and high angle annular dark field (HAADF) detectors. SEM observations revealed that at the interface of AZ91-C f composite destructive phases were formed. An application of TiN nano-coating as a surface modification ensured proper wettability of carbon fibres by liquid metal and protection against the destructive products formation. The microstructure of AZ91-(C f ) TiN composite interface was multilayered. TEM investigation revealed a continuous layer of mixed alumina and magnesia just at the carbon surface, followed by a layer with a dominant concentration of titanium, enriched with Al, O, Mg and C and subsequently a layer of nano-sized Al 12 Mg 17 crystals in the magnesium matrix. Therefore, an applied TiN nano-coating can be classified as an active barrier in AZ91-C f system. 1. Introduction Light weight metallic materials, such as Mg and its alloys, are getting popular in the designing of magnesium matrix composites, especially reinforced with carbon materials. The greatest advantage of this system is the comparable density of both components ~ 1.8 g/cm 3. The carbon-magnesium system offers unique properties in comparison to conventional metals and alloys. The system is characterized as a material with high specific strength, good thermal conductivity and low density [1-4]. It is necessary to solve some problems for composite processing, such as proper wetting and reactivity of carbon fibres in contact with liquid metal [5-6]. Because of their technological properties magnesium alloys containing aluminium (i.e., AZ91) have been used as composite matrix and described in several papers [7-9]. However, hydrophilic carbides (Al 4 C 3, Al 2 MgC 2 ) form at the fibre-matrix interface. Therefore, modification of magnesium alloy composition and application of technological coatings in order to eliminate the formation of hydrophilic carbides were proposed in some papers [10-12]. 1 To whom any correspondence should be addressed. Published under licence by IOP Publishing Ltd 1

3 For instance, TiN nano-coating was proposed by Popowska et al. [12]. They revealed an improvement of wettability and a decrease of carbides formation intensity but the microstructure of the interface and stability of TiN was not characterized in detail. The latter problem was analyzed for the SiO 2 nano-coating application in the AZ91-C f system [13] where it has been shown that the transformation of the initial SiO 2 into a mixed oxide layer containing silicon and aluminium took place. The aim of present paper is to characterize the interface microstructure in AZ91-(C f ) TiN composite with SEM and TEM methods and to analyze the role of TiN in the interface formation. 2. Experiment 2.1. Composite processing Surface modification of carbon fibres. Carbon fibres of FT300B (Toray) type in roving form were applied in experiments. Chemical vapour deposition (CVD) was used for fibre surface modification. During CVD at the temperature of 980 C the mixture of N 2 and TiCl 4 gases was applied Components consolidation. Carbon fibres uncoated and coated with TiN were applied as a reinforcement of AZ91 alloy (9 % Al and 0.6 % Zn). Composite samples were obtained in the same conditions by infiltration process in Degussa press. The process was performed in vacuum at a temperature of 670 C for 20 minutes followed by cooling under the loading. Composite samples with a 20 mm diameter each were obtained Methods of materials microstructure investigation The microstructure was preliminarily investigated using a scanning electron microscope (FE-SEM Hitachi 4200S, Hitachi Ltd., Tokyo, Japan) at an accelerating voltage of 15 kv, equipped with an energy-dispersive X-ray spectrometer (EDS) (Noran 3500, Thermo Electron Corporation, Waltham, MA, Si-Li detector, 130 ev resolution). The Kα X-ray series (for all elements) were used for elemental mapping and line-scans. X-ray diffraction measurements (XRD) were made with a Jeol JDX-7S diffractometer (equipped with plate graphite monochromator, using Cu Kα radiation at 40 kv and 20 ma). X-ray diffraction patterns of fibres were recorded in the range of diffraction angles 2θ from 10 up to 90 with a step size of More detailed structural investigations of the fibre-matrix interface were performed on a FEI Tecnai G2 field-emission high-resolution transmission electron microscope equipped with a HR CCD Gatan UltraScan camera, high angle annular dark field (HAADF) detector, at an accelerating voltage of 200 kv equipped with EDS (Edax Phoenix Si(Li)-detector with an UTW ultra-thin window for light elements analysis). 3. Results 3.1. Composite processing As a result of fibre surface modification, a thin coating of TiN with a thickness of approximately 100 nm was formed. It covered the fibres very well and was uniform. It was composed of very fine and thickly packed grains and it had the golden colour typical for TiN (figures 1a and 1b). X-ray diffraction measurements (XRD) confirmed the presence of crystalline TiN (figure 2). After components hot-pressing consolidation the composite samples of 20 mm diameter each were obtained for subsequent structural examination. 2

4 a b Figure 1. SEM micrographs of TiN coating deposited on T300B carbon fibres by the CVD method. Figure 2. XRD patterns of carbon fibres roving coated with TiN by the CVD method Composites microstructure investigation SEM observation. The observation was performed on polished and fractured cross-sections (figures 3-6). The reinforcement was properly surrounded with nonporous metal matrix. However, in the case of the composite made with uncoated fibres micro-pores were formed at the interface on the polished cross-sections (figure 3a and 3b) as a result of contact with water, while that effect was not recorded in the AZ91-(C f ) TiN composite (figure 4). Moreover, in the AZ91-C f system very thin needle-like phases oriented from fibre into the matrix were observed (figure 3b). This type of interface suggests the presence of hydrophilic carbide. Additionally, in case of the AZ91-(C f ) TiN 3

5 EMAS 2011: 12th European Workshop on Modern Developments in Microbeam Analysis IOP Publishing IOP Conf. Series: Materials Science and Engineering 32 (2012) doi: / x/32/1/ composite a characteristic thin and regular layer was detected around the fibres (figure 4). On the fractured cross-sections of both composites (figures 3c and 5) the effect of carbon fibres pulling-out was not observed. a b c Figure 3. Microstructure of the AZ91-Cf composite without surface modification of the fibres. a & b) polished cross-section; and c) fractured cross-section (SEM). a b Figure 4. SEM micrograph showing the microstructure of a polished cross-section of a AZ91-(Cf)TiN composite. As a results of X-ray microanalysis (figures 6 and 7) it was found that in the AZ91-(Cf)TiN composite the interface layer was enriched with aluminium, titanium and oxygen TEM observation. The observation of the fibre-matrix interface by TEM (figure 8) indicated the presence of a regular zone between fibre and matrix with irregular plate-shaped crystals located near its vicinity. Phase identification of the continuous zone by selected-area diffraction pattern (SADP) was impossible, but irregular crystals around that zone were characterized as Al12Mg17-phase, visibly alike to the other plate shaped crystals visible in the matrix. The HAADF image with X-ray maps (figure 9) showed that the continuous zone consisted of two layers. One of them located directly at the 4

6 a b Figure 5. SEM micrograph showing the microstructure of fractured cross-section of a AZ91-(C f ) TiN composite. Figure 6. Interface of the AZ91-(C f ) TiN composite. X-ray mapping of: a) Mg; b) Al; c) C; d) Ti; and e) O. carbon surface consisted of oxygen, aluminium and magnesium, while in the next layer titanium was detected as the dominant element and additionally oxygen, carbon, aluminium and magnesium were observed. The presence of nitrogen at the interface was not revealed. 5

7 Figure 7. X-ray line scans along the interface of the AZ91-(C f ) TiN composite (SEM+EDS). Figure 8. Interface of the AZ91-(C f ) TiN composite. Bright field image (BFI) with selected-area electron diffraction pattern (SADP) of the Al 12 Mg 17 -phase. 4. Discussion Results of SEM observations of both composites indicate that the parameters applied in the process of components consolidation are proper from the point of view of good infiltration of the carbon fibres and the close continuous contact of the lamellar-shaped fibres with the metal. The absence of pulling-out effect in the composites indicates that chemical bonding was formed in both composite with and without carbon fibres surface modification (figures 3c and 5). Generally, the results of the performed investigations suggest that fibre surface modification with nano-coating is desirable for a composite with an AZ91 matrix with respect to the TiN-coating protection against formation of hydrophilic carbide, which appeared at the interface in AZ91-C f composite without modification (figure 3) and led to partial degradation of AZ91-C f composite interface. The latter result has been confirmed in the literature data for similar experiments on C-Mg alloy systems [12]. 6

8 Figure 9. Interface of the AZ91- (C f ) TiN composite. HAADF (z-contrast and X-ray elemental mapping). Results of SEM/EDS investigations of AZ91-(C f ) TiN composite revealed only that the interface is enriched with aluminium, titanium and oxygen (figures 5 and 6). Not all light element X-ray lines had sufficient intensity. The nitrogen Kα intensity was not measured because of an overlap with the Ti Lα-line. The oxygen Kα intensity is distinctly higher in the transition region than in the matrix (which can be considered as a background (bremsstrahlung)). Carbon Kα mapping gave only the visible difference in the intensities in the carbon fibre and magnesium alloy matrix but its content in the transition region has to be examined by TEM/EDS for samples prepared by FIB technique. For a more accurate characterisation of the interface microstructure, TEM examination was necessary in order to reveal its multilayered nature (figure 7). EDS measurements show the presence of Ti, Al, Mg, O and C in the zone composed of a uniform double layer and of an additional region of nano-sized plates distributed in the magnesium matrix. A double layer of new phases instead of the initial TiN nano-layer was formed during components consolidation by hot pressing. Their precise phase identification requires further investigations but the X-ray maps suggest the appearance of some unexpected effects. The oxide layer formed exactly on the carbon fibres is the result of interaction between oxygen absorbed by the fibres during their manufacturing and atoms of metals (Al and Mg), which have diffused through the nano-grained TiN technological coating. Moreover, an initial TiN nano-layer has been subjected to transformation into a layer of similar geometry but with different chemical composition. It consisted of titanium as dominant element, with additional oxygen and carbon, and was doped with some amount of aluminium and magnesium. That may suggest the presence of titanium oxide and titanium carbide molecules in the layer. However, nitrogen was not detected by SEM/EDS and TEM /EDS but its presence cannot be excluded. The electron SADP of nano-sized plates in interface region showed Al 12 Mg 17 -phase formation. Generally, it is the phase typical for AZ91 magnesium alloy and its crystallisation around carbon fibres was observed also in other composites [14-16]. 7

9 Investigations of the AZ91-(C f ) TiN interface microstructure showed that the applied TiN nano-layer does not eliminate chemical interaction between carbon reinforcement and AZ91 matrix, therefore, the TiN is only an active barrier, suitable for diffusion process control at the interface. It explains the results of some experiments [12] where the protective role of TiN was insufficient. A very important and a new result of the presented experiment is the detection of an evident oxide layer around the fibres. That effect was observed in composites without any coating and one may expect that the presence of the oxides is the result of oxygen presence in the magnesium alloy. An application of a TiN barrier showed that oxygen absorbed by carbon reinforcement participated in oxide layer formation. This observation allows to explain the process of interface microstructure formation in the case of carbon fibre modification with a SiO 2 nano-layer, where SiO 2 transformation into complex oxide took place [13], which was not evident upon structural investigations of the AZ91-(C f ) SiO2 system. 5. Summary Some conclusions of the performed investigations can be drawn: 1. Proper infiltration of Ti coated carbon fibres by AZ91 magnesium alloy was obtained and fibres pulling-out effect was not observed what suggests that a chemical bonding was formed in the AZ91-(C f ) TiN composite. 2. The TiN nanolayer is however the effective protection in the AZ91-C f system against a formation of hydrophilic phases in applied conditions of components consolidation but the nanolayer microstructure evolves as a result of diffusion processes. Therefore, the role of TiN can be characterized as a reactive diffusion barrier. 3. A region of several nanolayers was formed at the interface of AZ91-(C f ) TiN composite which consisted of continuous complex oxide and complex Ti containing layers, as well as the zone of nano-plates of Al 12 Mg 17 phase surrounded by Mg matrix. 4. Presence of complex oxide nanolayer between fibre and Ti containing nanolayer suggests that oxygen absorbed by carbon reinforcement affects final interface microstructure in C-Mg system independently on applied carbon surface modification. Acknowledgements This work was supported by the Polish Ministry of Science and Higher Education (project no. 4689/B/T02/2007/33). References [1] Hufenbach W, Andrich M, Langkamp A and Czulak A 2006 J. Mater. Proc. Tech [2] Lindroos V K and Talvitie M J 1995 J. Mater. Proc. Tech [3] Ye H Z and Liu X Y 2004 J. Mater. Sci [4] Mordike B L and Ebert T 2001 Mater. Sci. Engng. A [5] Bouix J, Berthet M P, Bosselet F, Fawe R, Peronnet M, Viala J C, Vincent C and Vincent H 1997 J. Phys. IV France [6] Feldhoff A, Pippel E and Woltersdorf J 1997 J. Microsc. Oxford [7] Korner C, Schaff W, Ottmuller M and Singer R F 2000 Adv. Engng. Mater [8] Oakley R, Cochrane R F and Stevens R 1995 Key Engng. Mater [9] Kleine A, Hemptenmacher H, Dudek H J, Kainer K U and Krüger G 1995 J. Mater. Sci. Lett [10] Pei Z L, Li K, Gong J, Shi N L, Elangovan E and Sun C 2009 J. Mater. Sci [11] Feldhoff A, Pippel E and Woltersdorf J 2000 Adv. Engng. Mater [12] Popovska N, Gerhard H, Wum D, Poscher S, Emig G and Singer R F 1997 Mater. Design [13] Olszówka-Myalska A and Botor-Probierz A 2010 IOP Conf. Series: Mat. Sci. Engng

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