Browsing by Author "Jayaram, V."

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Characterization of Thermal Stability and High-Temperature Tribological Behavior of Electroless Ni-B Coating
(2018) Pal, S.; Sarkar, R.; Jayaram, V.
A preliminary study has been conducted using sequences of isothermal heat treatments and unidirectional high-temperature wear test following ball-on-flat geometry against an alumina counterface, to assess thermal stability and high-temperature tribological properties of the crystalline electroless Ni-B coating, a potential candidate for high-temperature solid lubricant coating. Isothermal heat treatment of 450 C/15 h causes a significant amount of B diffusion into the Fe substrate without altering the coating s through-thickness hardness and nanostructure. At room temperature, a very low wear rate is observed, which increases up to two orders of magnitude above a testing temperature of 100 C. Room-temperature wear behavior is mostly governed by oxidative wear, where friction-induced heating produces a thick oxide scale on the wear track, which subsequently decreases the wear rate by preventing direct contact between the coating and counterface. In the case of wear tests above 100 C, removal of the same oxide layer occurs through local plastic deformation, essentially plastic ratcheting at the contacting region by flow softening of the contacting surface layer due to a local rise in temperature. Worn track morphology shows similarity with the severe wear seen in steel steel contacts. Experimental observations have been explained and validated using the concept of contact point flash temperature. A quantitative assessment of contact point flash temperature has been carried out adopting the methodology, proposed by Ashby et al. The effects of applied normal load, test geometry, choice of counterface material, and testing temperatures on the transition of wear mechanism are critically discussed. 2018, The Minerals, Metals & Materials Society and ASM International.
Characterization of Thermal Stability and High-Temperature Tribological Behavior of Electroless Ni-B Coating
(Springer Boston, 2018) Pal, S.; Sarkar, R.; Jayaram, V.
A preliminary study has been conducted using sequences of isothermal heat treatments and unidirectional high-temperature wear test following ball-on-flat geometry against an alumina counterface, to assess thermal stability and high-temperature tribological properties of the crystalline electroless Ni-B coating, a potential candidate for high-temperature solid lubricant coating. Isothermal heat treatment of 450 °C/15 h causes a significant amount of B diffusion into the Fe substrate without altering the coating’s through-thickness hardness and nanostructure. At room temperature, a very low wear rate is observed, which increases up to two orders of magnitude above a testing temperature of 100 °C. Room-temperature wear behavior is mostly governed by oxidative wear, where friction-induced heating produces a thick oxide scale on the wear track, which subsequently decreases the wear rate by preventing direct contact between the coating and counterface. In the case of wear tests above 100 °C, removal of the same oxide layer occurs through local plastic deformation, essentially plastic ratcheting at the contacting region by flow softening of the contacting surface layer due to a local rise in temperature. Worn track morphology shows similarity with the severe wear seen in steel–steel contacts. Experimental observations have been explained and validated using the concept of contact point flash temperature. A quantitative assessment of contact point flash temperature has been carried out adopting the methodology, proposed by Ashby et al. The effects of applied normal load, test geometry, choice of counterface material, and testing temperatures on the transition of wear mechanism are critically discussed. © 2018, The Minerals, Metals & Materials Society and ASM International.
Effect of Humidity and Temperature on PVD TiAlN-Coating Wear
(Springer, 2025) Cadambi, S.; Dasari, B.P.; Jayaram, V.
Gas turbine blades and disks undergo wear at high temperatures at dovetail joints where tolerances are very small. Thin hard coatings are known to enhance the wear resistance of the superalloy components minimally influencing the tolerance levels. However, fundamental understanding of the coating’s wear mechanisms operating in these harsh conditions is not well understood. In this study, wear tests are performed to understand the wear mechanisms that operate in the temperature range from RT up to 800 °C for thin hard TiAlN coating using simple wear geometry eliminating any influence of wear debris. It is challenging to measure wear of thin hard coatings especially at elevated temperatures but important nevertheless. A coated ball on disk geometry with rough alumina as counterface is used for wear studies to understand exclusively the influence of humidity and temperature coating wear behavior. Cathodic arc evaporation, a physical vapor deposition technique is used to deposit TiAlN coatings on heat-treated IN718 substrates and characterized with, XRD, EPMA, TEM, SEM, nanoindentation, and FIB. The wear at room temperature shows scatter which has been ascribed to seasonal fluctuations in relative humidity. Further, wear results are shown to correlate with Young’s equation for capillary condensation. Wear below 50 pct RH is essentially dry and constant up to 600 °C above which wear increases marginally upto 800 °C. The coefficient of friction shows a maximum at 400 °C, below which friction reduces due to increased adsorption of water vapor, while above 400 °C, TiO2 forms on the surface to reduce the friction. The wear rate at 3 N load in the range of 50–800 °C is ~ 1 × 10?6 mm3/m/N. For 5 N load, the wear rate is same as for 3 N load upto 600 °C but doubles above 700 °C. The average contact pressure through the test is ~ 550 and 650 MPa which is almost twice the design contact pressure. The wear debris gets richer in Ti with increase in temperatures. The Al-rich TiAlN coatings deposited by cathodic arc evaporation (CAE) technique show a low and constant wear behavior over a wide range of temperatures and are ideally suited for the protecting the dovetail joints in gas turbines. © The Minerals, Metals & Materials Society and ASM International 2024.
Effect of Humidity on Wear of TiN Coatings: Role of Capillary Condensation
(2018) Cadambi, S.G.; Jayaram, V.
Coated ball-on-disk wear configuration was used to study the effect of relative humidity, water vapor pressure, and water on wear of TiN coatings in the temperature range of room temperature to 100 C. Two kinds of experiments were designed: one at constant temperature with varying humidity and the second at constant water vapor pressure with varying temperature. Temperature variation experiments were also conducted in water. The trends in wear volume after a fixed sliding distance were analyzed. At constant temperature, the wear volumes increased with humidity/water vapor pressure. However, at a constant vapor pressure, wear volumes remained roughly invariant with temperature until a critical temperature below which they rose sharply. In contrast, the wear rate increased with temperature for tests in liquid water. Considering calculations based on Kelvin s equations and further characterization of the wear surfaces using profilometry, XPS and FIB techniques, it has been strongly suggested that the anomalous temperature dependence of wear in humid air might be due to capillary condensation occurring at the contacting asperities. The wear of TiN showed two regimes in the influence of humidity. Where the humidity was below a threshold value of ~ 50 pct relative humidity (RH), there was negligible condensation and wear was low. Above 50 pct RH, the wear rate increased due to the availability of liquid water at asperities. The reason for the difference in the wear rate in vapor and liquid was hypothesized, based on XPS data, to depend on the formation of soft hydroxides in the presence of water, in contrast to the harder barrier oxide formed in the presence of oxygen, the dominant gas species in the vapor phase. 2018, The Minerals, Metals & Materials Society and ASM International.
Effect of Humidity on Wear of TiN Coatings: Role of Capillary Condensation
(Springer Boston, 2018) Cadambi, S.G.; Jayaram, V.
Coated ball-on-disk wear configuration was used to study the effect of relative humidity, water vapor pressure, and water on wear of TiN coatings in the temperature range of room temperature to 100 °C. Two kinds of experiments were designed: one at constant temperature with varying humidity and the second at constant water vapor pressure with varying temperature. Temperature variation experiments were also conducted in water. The trends in wear volume after a fixed sliding distance were analyzed. At constant temperature, the wear volumes increased with humidity/water vapor pressure. However, at a constant vapor pressure, wear volumes remained roughly invariant with temperature until a critical temperature below which they rose sharply. In contrast, the wear rate increased with temperature for tests in liquid water. Considering calculations based on Kelvin’s equations and further characterization of the wear surfaces using profilometry, XPS and FIB techniques, it has been strongly suggested that the anomalous temperature dependence of wear in humid air might be due to capillary condensation occurring at the contacting asperities. The wear of TiN showed two regimes in the influence of humidity. Where the humidity was below a threshold value of ~ 50 pct relative humidity (RH), there was negligible condensation and wear was low. Above 50 pct RH, the wear rate increased due to the availability of liquid water at asperities. The reason for the difference in the wear rate in vapor and liquid was hypothesized, based on XPS data, to depend on the formation of soft hydroxides in the presence of water, in contrast to the harder barrier oxide formed in the presence of oxygen, the dominant gas species in the vapor phase. © 2018, The Minerals, Metals & Materials Society and ASM International.