Kumar Sridharan

Ultralow nanoscale wear through atom-by-atom attrition in silicon-containing diamond-like carbon

Understanding friction and wear at the nanoscale is important for many applications that involve nanoscale components sliding on a surface, such as nanolithography, nanometrology and nanomanufacturing. Defects, cracks and other phenomena that influence material strength and wear at macroscopic scales are less important at the nanoscale, which is why nanowires can, for example, show higher strengths than bulk samples. The contact area between the materials must also be described differently at the nanoscale. Diamond-like carbon is routinely used as a surface coating in applications that require low friction and wear because it is resistant to wear at the macroscale, but there has been considerable debate about the wear mechanisms of diamond-like carbon at the nanoscale because it is difficult to fabricate diamond-like carbon structures with nanoscale fidelity. Here, we demonstrate the batch fabrication of ultrasharp diamond-like carbon tips that contain significant amounts of silicon on silicon microcantilevers for use in atomic force microscopy. This material is known to possess low friction in humid conditions, and we find that, at the nanoscale, it is three orders of magnitude more wear-resistant than silicon under ambient conditions. A wear rate of one atom per micrometre of sliding on SiO(2) is demonstrated. We find that the classical wear law of Archard does not hold at the nanoscale; instead, atom-by-atom attrition dominates the wear mechanisms at these length scales. We estimate that the effective energy barrier for the removal of a single atom is approximately 1 eV, with an effective activation volume of approximately 1 x 10(-28) m.

Materials Challenges for Generation IV Nuclear Energy Systems

Abstract The U.S. Department of Energy is sponsoring the Generation IV Initiative in the United States for the purposes of developing future-generation nuclear energy systems. Six systems have been selected for Generation IV consideration: gas-cooled fast reactor, lead-cooled fast reactor, molten salt-cooled reactor, sodium-cooled fast reactor, supercritical water-cooled reactor, and very high temperature reactor. Critical to the development of Generation IV concepts is successful development and deployment of materials that operate successfully in the aggressive operating environments envisioned in the Generation IV concepts. This paper summarizes the Generation IV operating environments and describes materials challenges and potential solutions, including crosscutting solutions applicable to multiple Generation IV concepts.

Corrosion Behavior of Alloys 625 and 718 in Supercritical Water

Abstract The corrosion behavior of two commercial, nickel-based Inconel alloys, 625 (UNS N06625) and 718 (UNS N07718), has been evaluated after exposure to supercritical water (SCW) at 500°C and 600°C for periods of up to 1,026 h. The 500°C tests were performed in deaerated and 2 ppm dissolved oxygen SCW, while the 600°C tests were performed only in deaerated SCW. Evaluation of corrosion has been performed using weight-change measurements, scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), x-ray diffraction (XRD), Auger electron spectroscopy (AES), and a surface profilometer. Surface oxidation and pitting were the principal corrosion mechanisms for both alloys at 500°C; however, intergranular corrosion was also observed in the Alloy 625 sample exposed to deaerated oxygen SCW at 600°C for prolonged durations. Oxidation was more dominant compared to pitting for the samples exposed to higher oxygen content or higher temperature SCW. The oxide thickness was lower for the higher ch...

An evaluation of metallic coatings for erosive wear resistance in die casting applications

An accelerated testing procedure with a multi-pin die has been established for the study of the erosive wear of die materials and surface treatments in die casting applications, under production conditions. This test procedure utilizes the wear of core pins as a surrogate measure of die erosive wear, and is used to evaluate the efficacy of coatings and die surface treatments in reducing erosive wear in dies under actual production conditions. Metallic coatings of W, Mo, and Pt deposited on the test pins using the plasma source ion implantation technique (PSII) technology in the ion assisted deposition mode, were then evaluated for erosion resistance using this developed test procedure. The results of this evaluation on the effectiveness of these metallic coatings are presented here along with possible reasons for such coating behavior.

Sheath dynamics and dose analysis for planar targets in plasma source ion implantation

A comparison of the experimental measurements and numerical calculations of temporal and spatial sheath evolution for planar targets is presented. The propagating sheath edge initially emanates in an ellipsoidal shape elongated along the plane of the target, then transforms to a spherical shape at a distance of about one diameter from the target, and ultimately the sheath becomes stationary at a distance that depends on the plasma parameters and target dimensions. To complement sheath expansion measurements with dose uniformity in planar targets, silicon wafers were implanted with nitrogen. Surface profilometry and scanning Auger microprobe measurements showed greater sputtering and shallower implantation depths at the edge of the wafer, relative to the centre, in qualitative agreement with the sheath expansion measurements.

Current Status of Knowledge of the Fluoride Salt (FLiNaK) Heat Transfer

Abstract A compilation and reevaluation of data from the 1950s and 1970s from three forced convective heat transfer experiments using the ternary fluoride salt FLiNaK (46.5 LiF–11.5 NaF–42 KF mol%) using presently known thermophysical properties of this salt has been performed. The previous experiments each analyzed their data using different values for the properties of the liquid salt, thus leading to differences in the reported heat transfer coefficients. For turbulent flow in experiments conducted in chambers constructed of Inconel® alloys (as used in these three previous experiments), it was determined that FLiNaK salt behaves as a “normal” fluid and can be modeled using the Dittus-Boelter (DB) correlation within ±15% accuracy. The DB correlation can thus be used for preliminary calculations of salt heat transfer. Despite the success of the DB correlation for tests conducted in Inconel® chambers, forced convective data on heat transfer in nickel and Type 316 stainless steel produced different results. The physical effects contributing to the difference in the data measured in different container materials are not understood. The concentration of Cr (the main corrosion product added to the salt during a test) is similar between Inconel® alloys and Type 316 stainless steel and should affect each test similarly. Nickel is a relatively inert container material to fluoride salts and should not affect the heat transfer. To reconcile the experiments, a simplified approach was undertaken to determine if the radiant heat transfer to the FLiNaK salt could account for the differences. It was found that under the experimental conditions used by previous investigators, the radiant heat transfer from container to salt was <2%. However, the amount of energy transferred by radiation can be significant in applications involving high temperatures (T = 1123 K) and laminar flow conditions (Re < 500) in pipes with a diameter of 1 cm or greater.

Intergranular corrosion of high temperature alloys in molten fluoride salts

Abstract Six Ni and Fe–Ni based austenitic alloys were exposed to molten LiF–NaF–KF: 46.5–11.5–42mol% salt, commonly referred to as FLiNaK, at 850°C for 500 h. Corrosion was noted to occur predominantly from dealloying of Cr resulting in void formation, an effect that was particularly pronounced at the grain boundaries. Alloy weight-loss due to molten fluoride salt corrosion correlated with the initial Cr-content of the alloys up to a Cr content of about 20wt%, and was consistent with the Cr-concentrations measured in the salts after corrosion tests. However, in the nominally 20 wt% to 23 wt% Cr containing alloys, this weight-loss correlated linearly with the carbon content of the alloys, due to the formation of chromium carbide phases at the grain boundaries and its subsequent dissolution into the molten salt along the grain boundaries. The Cr-lean intermetallic precipitates found in the alloys were resistant to corrosion in the molten FLiNaK salt.

Elevated temperature nitrogen ion implantation of incoloy alloys 908 and 909 using the plasma source ion implantation process

Abstract The plasma source ion implantation process has been applied to two FeNi-based superalloys, Incoloy ∗ alloys 908 and 909. Nitrogen was implanted at 53 keV to a dose of 3 x 10 18 atoms cm −2 and the sample temperature was intentionally raised to approximately 550 °C to promote thermal diffusion. Auger analysis for nitrogen concentration indicated a subsurface maximum, then a somewhat lower but constant value up to about 5 μm, followed by a decrease that was more characteristic of thermal diffusion. The matrix lattice parameter at the surface increased owing to nitrogen implantation, and both X-ray diffraction and transmission electron microscopy indicated the formation of nitrides at the surface. Cyclic potertiodynamic corrposion tests in 0.01N NaCl solution showed that the resistance to localized corrosion, after implantation, increased for alloy 908, but for alloy 909 only somne aspects of corrosion showed improvement. Significant improvements in hardness and wear resistance were achieved at the surface as a result of the implantation process. Possible strengthening mechanisms are discussed.

A machine for fretting wear testing of plasma surface modified materials

Abstract Microscopic damage caused by fretting wear is of significant concern in many engineering applications. We report the design and performance of a versatile fretting wear testing machine that uses an electromagnetic actuator under closed-loop control to provide a constant slip amplitude over a wide frequency range. The machine is equipped with a mechanism that allows for gradual sample contact and precise relocation of the stylus. An innovative method for monitoring frictional forces during the course of a test is incorporated. The machine has been successfully applied to materials treated by three types of plasma surface modification: nitrogen ion implantation, thin film alloy deposition and diamond-like carbon film deposition.

Wear-Corrosion Comparisons of Passivating vs Nonpassivating Alloys in Aerated 3.5% Aqueous Solutions of Sodium Chloride

Abstract Metals and alloys that have the capability of forming passive films provide good corrosion resistance in many environments. Under localized wear-corrosion conditions, however, they undergo...