Ravikumar Dumpala

Extremely high wear resistance and ultra-low friction behaviour of oxygen-plasma-treated nanocrystalline diamond films

The diamond nanowire (DNW) film was deposited by N2-enriched microwave plasma-enhanced chemical vapour deposition (MPECVD) process. As-deposited DNW film was treated in O2 plasma which resulted in chemical and microstructural modification. Sheath of the DNW film is chemically constituted by amorphous carbon (a-C)- and graphite (sp2C=C)-like bonding. However, nanowires transformed into ultra-small spherical grains after the O2-plasma treatments. In this condition, a-C and sp2C=C bonding significantly reduced due to plasma etching caused by oxygen atoms. After the O2-plasma treatment, formation of functional groups such as C=O, C?O?C, O?H, O?CH3 and H2O was observed on the surface and inside the wear track as evident from the micro FTIR analysis. H2O is hydrogen bonded to oxygen-containing groups such as ?OH and ?H. The O2-plasma-exposed DNW film exhibits surface charging and causes formation of dangling bonds and electron trapping centres. This results in significant decrease in contact angle, hence superhydrophilic behaviour. The friction coefficient of O2-plasma-treated film showed super low value ?0.002 with high wear resistance 2???10?12?mm3?N?1?m?1. In the reciprocating ball-on-disc tribology test, only ?80?nm wear loss was observed after the 1?km of sliding distance at 10?N loads. Such an advance in tribological properties is explained by passivation of covalent carbon bonding and transformation of sliding surfaces by weak van der Waals and hydrogen bondings. High surface energy and the consequent superhydrophilic behaviour of film is attributed to the formation of the above-mentioned functional groups on the surface. This protects against deformation of the wear track leading to extremely high wear resistance.

Machining characteristics of fine grained AZ91 Mg alloy processed by friction stir processing

Abstract AZ91 Mg alloy was considered and friction stir processing (FSP) was adopted to achieve grain refinement to investigate the effect of grain size and secondary phase on machining characteristics during drilling at various speeds and feeds. Super saturated AZ91 Mg alloy was obtained after FSP and the grain refinement was achieved from (166.5±8.7) µm to (21.7±13.5) µm. Surprisingly, hardness reduced for FSP AZ91 Mg alloy (88.95±6.1) compared with AZ91 alloy (108.2±15.6), which was attributed to the reduced secondary phase. However, the mean cutting force for FSP-treated (FSPed) AZ91 Mg alloy was marginally increased. The edge damage of the drilled holes was lower for FSPed AZ91 Mg alloy compared with unprocessed AZ91 Mg alloy. Hence, it can be understood that the grain refinement may slightly increase the cutting forces during drilling but better edge finishing can be achieved in machining of AZ91 Mg alloy.

Teaching of mechanical engineering concepts through three-dimensional geometric modeling

In the present pedagogical work, the importance of teaching mechanical engineering concepts using three-dimensional spatial visualization was emphasized and explained using specific examples. Elementary concepts of the mechanical engineering such as vortex flow, geometric and dimensional tolerances, orthographic projections, machine components, and hydraulic machinery were depicted in three dimensions, using a computer-aided geometric modeling software. The conventional schematic methods used to describe the mechanical engineering concepts were mapped into spatial three-dimensional geometric models. These three-dimensional geometric models bring clarity in understanding the complex engineering concepts in an accurate and precise way because of the ease in visualization. An improvement in students’ level of understanding is recorded through percentage marks obtained in the students’ performance in the classroom environment.