Received 25 Jan 2015; received in revised form 13 Feb 2015; accepted 12 Mac 2015. To cite this article: Rajabi et al. (2015). Effect of second phase morphology on wear resistance of Fe-TiC composites. Jurnal Tribologi 4, pp.1-9. Effect of second phase morphology on wear resistance of Fe-TiC composites A. Rajabi a, M.J. Ghazali a,*, A.R. Daud b a Department of Mechanical and Material Engineering, Faculty of Engineering & Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi, UKM, Malaysia. b School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, UKM, Malaysia. * Corresponding author: mariyam@eng.ukm.my HIGHLIGHTS Fe-TiC composites were successfully produced through in situ casting. SEM observations showed distributed TiC particles in iron matrix. In this study, the best wear resistance was due to a good uniform distribution of uniform shaped TiC by using a re-melt method. ABSTRACT The aim of this paper is to investigate the influence of the second phase morphology (TiC) on wear behavior in Fe-TiC composites. Fe-TiC composites were successfully produced by an in-situ melting method under an inert argon gas in which raw materials, namely cast iron (Fe-3.5wt%C) and 10wt%ferrotitanium (Fe-72wt%Ti) were used. The obtained melting was centrifugally casted in mold ceramic to produce the samples required. XRD instrument was also used to identify the acquired phases. The size of TiC and hardness of composites were examined by an image analyser and Vickers hardness respectively. Morphological investigation of the microstructure was performed using scanning electron microscopy (SEM). Weight lost was measured under the condition of using a pin-on-wheel configuration with normal load 20N, sliding distance up to 2000m and speed 0.8m/s. The results indicated the wear resistance in re-melt composites improved due to TiC good uniform distribution with its regular form. Keywords: Composite Fe-TiC In-Situ Wear resistance 2015 Malaysian Tribology Society (MYTRIBOS). All rights reserved.
1.0 INTRODUCTION Metal matrix particulate composites (MMPC) are considered new materials are made of ceramic hard particles embedded in metal matrix (Kaczmar et al. 2000; Li et al. 2014). Based on previous investigations, incorporation of ceramic particles i.e. Al2O3, SiC, TiB2, WC and TiC in an adequate metal matrix give great improvement on the mechanical properties of the alloys (Prasad & Asthana 2004; Zhou et al. 2008; Razavi et al. 2013). Among these ceramic particles, TiC had drawn much attention as reinforcements in iron matrix due to its high hardness (33% greater than the commercial WC), high chemical stability, high melting point (3100 C), thermal stability, high thermal shock resistance and very low Gibbs formation free energy (Woo et al. 2010; Rajabi et al. 2014; Rajabi et al. 2015). Fe-TiC composites has been highly utilized for more than two decades in various industries due to their unique hardness, wear resistance and toughness. These composites can be produced by several methods including powder metallurgy (PM), self-propagation high temperature synthesis (SHS) and in situ casting (Parashivamurthy et al. 2001; Liu et al. 2010). The last technique has been utilized by several researchers for specific applications due to its advantages e.g. in situ formation of a second phase within the matrix, lack of contamination (oxidation) between two phases and high wettability of TiC within the iron matrix, resulting a strong bond between the matrix and the second phase (Fe and TiC). Moreover, in comparison to conventional processes (PM), lower cost of the aforementioned method (in situ casting) and the possibility of direct productions for nearnett shape are the additional benefits of this method (Feng et al. 2005; Fengjun & Yisan 2007). Since 1950, no sufficient information is available on the second phase morphology and its effect on wear resistance particularly on TiC-ferrous matrix composites. Thus, the present work focuses on the influence of TiC morphology on wear resistant behavior. 2.0 MATERIALS AND METHODS Based on the stoichiometry of Ti and C in the TiC, 1mol of Ti (49 g) and 1mol of C (12 g) can be used to produce 1TiC. It has been established that the titanium carbide is formed through a diffusion of titanium into a carbon site where it precipitates as carbides. TiC is known as the most thermodynamically stable phase during solidifications (ΔG =- 186606+13.22T J/mol) (Rahimpour & Rajabi 2003; Wang et al. 2007). The synthesis of composites was carried out in a 0.05 kg, high frequency induction furnace under argon gas. The melt was centrifugally casted into a ceramic mould. The raw materials composition used in this work are given in Table 1. 2
Table 1: Chemical composition of primary materials (wt%). Alloy composition C Si Mn Ti V Al S P Fe Ferro-titanium - - - 72.00 0.21 0.35 - - Bal Gray cast iron 3.50 0.51 0.52 - - - 0.18 0.15 Bal The samples were polished with 240, 400, 600 and 1000 grit SiC abrasive papers. The final polish was performed using 1 µm alumina slurry. Then, they were etched with 2% nital. Hardness value was determined by a standard Vickers of HV30, with 5 measurements on each sample. The sample was analysed by an x-ray diffraction (XRD) using Cu radiation (λ=1.542a ) and observed by using a scanning electron microscope (SEM) Stereoscan 360 Cambridge. The sliding wear test was carried out under a dry condition using a pin-on-wheel configuration in a 1 atmosphere pressure at 300 K. The pins (samples) were 8 mm in diameter, making a line contact with the periphery of the wheel (40 mm diameter, HRC=35). A schematic diagram of the apparatus is shown in Figure 1. Prior to testing; the pins were ground by sliding against 1000 grit silicon carbide (SiC) abrasive papers. Then, both pin and disc were cleaned using acetone in an ultrasonic bath. The tests were done by rotating the wheel at a peripheral speed of 0.8m/s, with normal load of 20N over distance up to 2000m. Wear resistance was then studied by weight lost. Figure 1: Schematic diagram of the wear test apparatus. 3.0 RESULTS AND DISCUSSION 3.1 Solidification Figure 2 shows an iron-rich corner of a Fe-Ti-C phase diagram. Reactions between carbon and titanium atoms resulted in the formation of TiC primary particles within the melt as the temperature reaches 1600 C. During cooling process, the TiC crystal grows while liquid composition moves toward point A, where the α-ferrite nucleates. At the ternary peritectic B (1320 C) the melt reacts with ferrite dendrites which partially or totally transform into austenite 3
dendrites ( L TiC ). This transformation continues in a solid state while more austenite and TiC co-precipitate down the eutectic valley to ternary eutectic (TE) at 1140 C, in which solidification is completed ( L Fe3C TiC ). The austenite regions are finally transformed into pearlite or martensitic depending on the cooling rate. Here, as expected, retained austenite has transformed into perlite phase due to the low cooling rate. This can be related to the heat conductivity of ceramic moulds used in this study. In this regard, an XRD test also reveals that the phases of perlite (cementite-ferrite) had derived from the γ-austenite phase decomposition (Figure 3). Figure 2: Iron-rich corner of the Fe-Ti-C phase diagram (Wang et al. 2006). Figure 3: X-ray diffraction pattern of Fe-TiC composite 4
In order to investigate the role of TiC morphology on wear behaviour, the Fe-TiC (as-cast) samples were re-melted and unreinforced matrix (gray cast iron) was produced in accordance to the Table 1. Figure 4 shows the scanning electron micrograph of Fe-TiC composites etched by 2% nital. In comparison (Figure 4a and Figure 4b), TiC particles were homogenously and uniformly distributed through a re-melt method. The results of hardness and image analyser are given in Table 2. It was noted that the TiC size had increased a bit due to their inter-collision during the re-melt process. (a) (b) Figure 4: SEM micrograph of Fe-TiC microstructure (a) as-cast, (b) re-melt. Table 2: Hardness and grain size for Fe-TiC composite Samples Hardness (HV.30) Grain size of TiC (µm²) As-cast 567 ± 10 10.27 Re-melt 560 ± 5 10.38 Unreinforced matrix 390 ± 3-5
3.2 Wear behaviour The influence of TiC and its morphology on wear resistance of composites of Fe- TiC is illustrated in Figure 5. It can be observed that, in comparison with the unreinforced matrix, wear resistance in as-cast composites had improved about 25% due to the TiC hard particles within the iron matrix. Past literatures (Du et al. 2011; Rajabi et al. 2014) showed that the wear rate of metal matrix particulate composites had decreased when the second phase emerged in the counterface and acted as load bearing elements. In fact, the smallest weight lost was followed by those subjected to a regular form and good uniform distribution of TiC in the matrix at all sliding distance. At 2000 m of sliding distance, the wear resistance of the re-melt composites had enhanced about 21% and 37% than the ascast and unreinforced samples respectively. This might be due to the fact that good distributions of TiC particles with regular form structure could decrease the weight loss due to the shear stress distribution in the counterface during the wear process. Therefore, the morphology of TiC is a strong key factor on the wear resistance behavior. Figure 6 indicates the worn surface of re-melt samples from the secondary electron image (SEI) and back-scattered electron image (BEI). The SEI displays the surface topography while the BEI shows the difference of contrast on the worn surface, due to the atomic number difference. A corresponding EDAX analysis indicated that dark gray regions corresponded to the transfer of Cr from the disc (counterface) to the sample. In this condition, a dominant wear mechanism can be considered as an adhesive. Figure 5: Weight loss vs sliding distance 6
Figure 7 shows a cross-section of worn surface of as cast method. It can be observed that the surface was damaged due to the cavities created by the pull-out of the TiC particles from the matrix and the particles were trapped between the two solid surfaces but were free to roll as well as slide during the wear test. This is consistent with previous established studies (He & Zhu 2011; Rajkumar & Aravindan 2011) Figure 6: SEM micrograph of (a) SEI and (b) BEI and the corresponding EDAX analysis of worn surface of re-melt sample for a distance of 2000m Figure 7: Scanning electron micrograph, longitudinal cross-section of as-cast sample after sliding against steel disk for a distance of 2000 m. Arrow indicates the TiC pull-out 7
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