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Metal matrix nanocomposites (MMNCs) are particulate metal matrix composites with nano size reinforcement. Reinforcement enhances the mechanical behaviour of matrix materials. These materials possess excellent strength-to-weight ratio. These are now replacing MMCs, conventional alloys and materials in new and existing designs.
Tin Babbitt is an idle material for sliding contact bearings (M.V.S. Babu, 2015). It is designer’s first choice when it comes to the selection of journal bearing material. It possesses all desirable properties to be called as an idle bearing material. However, its load carrying capacity is only 5.5 – 10.5 MPa (Goswami, 2004). It is very small when compared with other commercially available bearing materials like Al alloys and bronzes (Sturk & Whitney, 2013). In this paper, we have adopted a strategy of fabricating a metal matrix nanocomposite with Tin Babbitt as matrix and reinforcing with nanoparticles.
Tin Babbitt is a soft material with a hardness of about 24.2 BHN (ASTM, 2014). If Babbitt is reinforced with hard ceramic reinforcements like SiC and Al2O3, then these may scratch the smooth surface of the steel shaft supported by these Babbitt bearings. Rough surface resulting from the scratching will increase the friction force leading to rise in bearing temperature. So, a ceramic reinforcement with a medium hardness of about 5 - 6 MHN was chosen to reinforce the Tin Babbitt. Ilmenite or Iron Titanium Oxide (FeTiO3) is mineral (“Ilmenite,” 2015) abundantly available (Rao, Ramana, Venugopal, & Rao, 2005) along the sea coast of India. It is an important ore of Titanium. It is a weakly magnetic, appears in black or steel-gray solid, brittle in nature with hardness varying from 5 – 6 MHN. For a systematic study of the change in the tensile behaviour of the matrix, the central composite design was used. Effect of three process parameters stirring time, ultrasonic treatment time and wt.% of nano-Ilmenite on the ultimate tensile strength (UTS) was studied.
Metal Matrix Nano Composites (MMNCs) are produced by powder metallurgy route (Abdizadeh, Ashuri, Moghadam, Nouribahadory, & Baharvandi, 2011; Hassan & Gupta, 2005), high energy ball milling (Lu, Thong, & Gupta, 2003), sputtering (Musil, 2000) and stir casting (Ezatpour, Sajjadi, Sabzevar, & Huang, 2014; Nie et al., 2011; Valibeygloo, Azari Khosroshahi, & Taherzadeh Mousavian, 2013), etc. Among all of these methods stir casting shown in Figure 1(a) is regarded as the most productive and economical. Uniform distribution of nanoparticles and wettability of nanoparticles in metal matrix is a challenge. If the stirring time is increased to achieve uniform distribution, it results in too much gas and oxidation to matrix melt. So it is necessary to reduce the stirring time to fabricate high-quality composites. Many investigators have developed alternate ways to achieve uniform particle distribution and good wettability.
Figure 1. Working principles of stir casting and ultrasonic assisted casting processes: (a) Schematic of the stir casting process; (b) Schematic of ultrasonic assisted casting.
Ultrasonic treatment of metals shown in Figure 1(b) is one of such efforts to improve distribution and wettability of nanoparticles. Ultrasonic treatment is used to disperse nanoparticles by ultrasonic cavitation effect (Nie et al., 2011). The cavitation affect would produce much ultrasonic cavitation in the melting metal, which would collapse with high pressure (above 1000 atm), temperature (about 5000 0C) and heating and cooling rates (above 1010 K/s) (Suslick & Price, 1999). It would disperse the existed agglomeration consisted of nano-reinforcement and clean the surface of reinforcement particles. Also, ultrasonic treatment can effectively degas melt.