Faculty Publications
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Item Severity of quenching and kinetics of wetting of nanofluids and vegetable oils(2009) Jagannath, V.; Prabhu, K.N.In the present work, the suitability of vegetable oil blends with mineral oil and alumina based nanofluids as quench media for industrial heat treatment was investigated. Sunflower oil, palm oil, and mineral oil were used for preparing the blends. Alumina based nanofluids of varying concentrations ranging from 0.01-4 % were used. The size of alumina particles was about 50 nm. The severity of quenching and heat transfer coefficients were estimated during quenching of copper probes. Heat transfer coefficients were estimated using a lumped heat capacitance model. The static contact angle was measured on copper substrates having a surface texture similar to the probes used for estimation of heat transfer coefficients. A dynamic contact angle analyzer was used for this purpose. The measured contact angles of nanofluids on copper were high compared to oils, indicating poor wetting by quench media that are polar in nature. Wetting characteristics had a significant effect on heat transfer coefficients estimated during quenching. Copyright © 2009 by ASTM International.Item Wetting behaviour and evolution of microstructure of Sn-Ag-Zn solders on copper substrates with different surface textures(2010) Satyanarayan, S.; Prabhu, K.N.The effect of surface roughness on wetting behaviour and evolution of microstructure of two lead-free solders (Sn-2.625Ag-2.25Zn and Sn-1.75Ag-4.5Zn) on copper substrate was investigated. Both solders exhibited good wettability on copper substrates having rough surface and lower wettabilty on smooth surfaces. The contact angles of solders decreased linearly with increase in surface roughness of the substrate. The exponential power law, ?=exp(-K ?n), was used to model the relaxation behaviour of solders. A high intermetallic growth was observed at the interface particularly on copper substrates with rough surface texture. A thin continuous interface showing scallop intermetallic compounds (IMC) was obtained on smooth surfaces. With an increase in surface roughness, the IMC morphology changed from scallop shaped to needle type at the Sn-2.625Ag-2.25Zn solder/substrate interface and nodular to plate like IMCs for Sn-1.75Ag-4.5Zn solder matrix. Copyright © 2010 by ASTM International.Item Wetting behaviour and evolution of microstructure of Sn-Ag-Zn solders on copper substrates with different surface textures(ASTM International, 2011) Satyanarayan, S.; Prabhu, K.N.The effect of surface roughness on wetting behaviour and evolution of microstructure of two lead-free solders (Sn-2.625Ag-2.25Zn and Sn-1.75Ag-4.5Zn) on copper substrate was investigated. Both solders exhibited good wettability on copper substrates having rough surface and lower wettabilty on smooth surfaces. The contact angles of solders decreased linearly with increase in surface roughness of the substrate. The exponential power law, ?=exp(-KT -1), was used to model the relaxation behaviour of solders. A high intermetallic growth was observed at the interface particularly on copper substrates with rough surface texture. A thin continuous interface showing scallop intermetallic compounds (IMC) was obtained on smooth surfaces. With an increase in surface roughness, the IMC morphology changed from scallop shaped to needle type at the Sn-2.625Ag-2.25Zn solder/ substrate interface and nodular to plate like IMCs for Sn-1.75Ag-4.5Zn solder matrix. Copyright © 2010 by ASTM International.Item Characterisation of water base copper nanoquenchants by standard cooling curve analysis(2011) Ramesh, G.; Prabhu, K.N.Water base copper nanofluids having concentrations varying from 0?001 to 0?1 vol.-% were prepared and used as quench media for immersion quenching. Cooling curve analyses were carried out by using a standard ISO/DIS 9950 quench probe. An inverse heat conduction model is employed to estimate the metal/nanoquenchant interfacial heat flux transients from the measured temperature field and thermophysical properties of the quench probe material. The addition of copper nanoparticles had a significant effect on the occurrence of the vapour blanket stage and nucleate boiling stage. Furthermore, all six cooling curve parameters were found to be altered by adding nanoparticles to water. The contact angle of water decreased from 67 to 39° by adding 0?1 vol.-% of copper nanoparticles indicating the improved wettability of nanofluids. The heat flux curve shows a maximum initially then drops rapidly during quenching. The peak cooling rate and heat flux of water increased by adding copper nanoparticles up to 0?01 vol.-%. Both parameters decreased with further increase in concentration of nanoparticles. The results suggest that the quench severity of water could be altered by adding copper nanoparticles. © 2011 IHTSE Partnership.Item Heat transfer at the casting/chill interface during solidification of commercially pure Zn and Zn base alloy (ZA8)(2012) Ramesh, G.; Prabhu, K.N.Casting/chill interfacial heat transfer during solidification of commercially pure zinc and ZA8 alloy against copper, hot die steel, stainless steel and aluminiuminstrumented chills was investigated. The peak heat flux strongly depends on the thermophysical properties of chill, chill surface condition and superheat of the castingmaterial. Contact angles of alumina coatingmeasured on various substrates suggested that the adhesion of the coating material on copper chill was significantly better as compared to other chill materials. The heat flux curve in the case of coated chills is characterised by a double peak indicating remelting of the solidified casting shell. The second peak in the HTC curve is lower for high conductivity and higher for low conductivity chills as compared to the first peak. It is possible that solid shell formation and remelting occurred in the case of high thermal conductivity chills, whereas shell remelting did not happen in lower thermal conductivity chills. © 2012 W. S. Maney & Son Ltd.Item Effect of temperature and substrate surface texture on wettability and morphology of IMCs between Sn-0.7Cu solder alloy and copper substrate(2012) Satyanarayan, S.; Prabhu, K.N.In the present work, the effect of soldering temperature (270 and 298 °C) and substrate surface texture (0.02 and 1.12 ?m) on wetting characteristics and morphology of intermetallic compounds (IMCs) between Sn-0.7Cu lead-free solder on copper substrates was investigated. It was found that increase in temperature and substrate surface roughness improved the wettability of solder alloy. However, the effect of surface roughness on wettability was significant as compared to that of temperature. The spreading of solder alloy was uniform on smooth substrate, whereas spreading of the alloy on rough substrate resulted in an oval shape. The morphology of IMCs transformed from long needle shaped to short and thick protrusions of IMCs with increase in surface roughness of the substrate. Needle shaped and thick protruded intermetallics formed at the solder/Cu interface were identified as Cu 6Sn 5 compounds. The formation of Cu 3Sn IMC was observed only for the spreading of solder alloy at 298 °C which contributed to improvement in the wettability of solder alloy on both smooth and rough substrate surfaces. © Springer Science+Business Media, LLC 2012.Item The Effect of Wetting Gravity Regime on Shear Strength of SAC and Sn-Pb Solder Lap Joints(Springer New York LLC barbara.b.bertram@gsk.com, 2017) Sona, M.; Prabhu, K.N.The failure of solder joints due to imposed stresses in an electronic assembly is governed by shear bond strength. In the present study, the effect of wetting gravity regime on single-lap shear strength of Sn-0.3Ag-0.7Cu and Sn-2.5Ag-0.5Cu solder alloys reflowed between bare copper substrates as well as Ni-coated Cu substrates was investigated. Samples were reflowed for 10 s, Tgz (time corresponding to the end of gravity regime) and 100 s individually and tested for single-lap shear strength. The single-lap shear test was also carried out on eutectic Sn-Pb/Cu- and Sn-Pb/Ni-coated Cu specimens to compare the shear strength values obtained with those of lead-free alloys. The eutectic Sn-Pb showed significantly higher ultimate shear strength on bare Cu substrates when compared to Sn-Ag-Cu alloys. However, SAC alloys reflowed on nickel-coated copper substrate exhibited higher shear strength when compared to eutectic Sn-Pb/Ni-coated Cu specimens. All the substrate/solder/substrate lap joint specimens that were reflowed for the time corresponding to the end of gravity regime exhibited maximum ultimate shear strength. © 2017, ASM International.Item Thermal conformance parameters for assessment of heat transfer between similar and dissimilar metal contacts(John Wiley and Sons Inc, 2024) Pathumudy, R.D.; Samuel, A.; Prabhu, K.N.A novel approach to assess the thermal conformance between two metallic materials under transient conditions was proposed in the present investigation. Thermal conformance parameters (ƞ, ϴ, tg) were defined to quantify the contact condition between a metal–metal interface. To assess the effect of load and thermophysical properties of the sink and source materials on the degree of thermal conformance, a thermal conformance assessment parameter (TCAP) was proposed. Heat flux transients at the thermal interface was estimated by using an inverse heat conduction approach for various similar and dissimilar metallic surfaces in contact such as Cu─Cu, Al─Al, Al─Cu, and Cu─Al under both load and no load conditions. Commercially available silicone grease (SG) and thermal grease (CTG) were used as thermal interface materials (TIMs). The thermal conformance parameters increased with the increase in load for all the combinations of interfaces with and without TIMs. It was observed that, except for the copper–copper combination, thermal conformance parameters showed a linear relation with the TCAP. The enhancement in the heat transfer due to the application of load and TIM was validated by determining the maximum temperature difference (∆Tmax) across the interface. The experimental study revealed that the ∆Tmax decreases with the application of load and application of TIM leading to enhanced heat transfer. For the copper–copper combination, the thermal conformance depended solely on the load applied. Due to the lower thermal resistance offered by copper source/sink materials, the interfacial resistance between them becomes a dominant factor. The effect of TIM on heat absorbed by the sink was significant for the Cu/Cu interface. © 2024 Wiley Periodicals LLC.