Novel Materials for Transient Liquid Phase Ceramics and Metal Joining

Joining is an enabling technology for many ceramics applications. Often ceramics are only useful in a system of components, requiring that they be bonded in some fashion to other ceramic components of the same composition or dissimilar materials such as metals or other ceramics. This is particularly true in the practical applications of fuel cells, gas separation membranes, and sensors, where a wide variety of ceramic-ceramic and ceramic-metal joints are required. The objective of this project is to bond a metal to a ceramic by the formation of a liquid-phase ceramic that wets to the surface of the metal and the ceramic. This liquid phase diffuses into the metal and bulk ceramic, and as the chemical composition of the liquid changes, it becomes a solid. This process is known as transient liquid phase (TLP) sintering. Ideally, this solid will be the bond between a metal and a ceramic. There are a variety of existing methods for joining ceramics to themselves or other materials. These joining methods have the disadvantage of leaving behind an interfacial phase with thermal and physical properties inferior to that of the materials being joined and may degrade the environmental stability of the parent material. Consequently, industry and academia have sought for many years to develop joining methods which leave behind effectively no interfacial phase, or a compatible, refractory phase with virtually the same thermal expansion coefficient as the joined parts. Although this work has not yet yielded a successfully novel metal to ceramic joint, a new transient liquid phase ceramic, SrMoO4, has been synthesized. This ceramic was sintered at 830°C and possesses a melting temperature of over 1200°C. Additionally a baseline study of the environmental stability of metal-to-metal brazes was conducted. This work is in collaboration with the Central Metallurgical Research Institute of Egypt with the goal of extending the operating range of high temperature materials and increasing their operational life

Lunar Rover Traction Concepts in Reduced Gravity

Future exploration of the moon and Mars will require systems capable of transporting both humans and cargo in reduced gravity environments. The combination of lunar regolith and reduced gravity creates issues for vehicular traction. NASA is interested in testing transportation systems in lunar environments to establish the relationship between the weight of a vehicle and its traction abilities on the lunar surface. The greater the mass of a vehicle, the better traction it will have in reduced gravity. However, a heavier vehicle requires more fuel to deliver the payload from the Earth to lunar orbit to the lunar surface and results in a costlier mission. NASA is especially interested in investigating ways to lower the weight of lunar exploration vehicles without compromising their traction. This experiment will observe how different wheel geometries interact with lunar regolith simulant under variable loads in lunar gravity. The project includes building a test apparatus with digital data acquisition, designing a test matrix, performing the experiment in lunar gravity, and sharing the experience with the community. Key components include a DC motor providing enough torque to overcome the static friction of the lunar regolith, software to control the motor, and an enclosure that houses the regolith and the wheel test apparatus. This project is a preliminary step for future research and experiments

Microgalvanic Corrosion Behavior of Cu-Ag Active Braze Alloys Investigated with SKPFM

The nature of microgalvanic couple driven corrosion of brazed joints was investigated. 316L stainless steel samples were joined using Cu-Ag-Ti and Cu-Ag-In-Ti braze alloys. Phase and elemental composition across each braze and parent metal interface was characterized and scanning Kelvin probe force microscopy (SKPFM) was used to map the Volta potential differences. Co-localization of SKPFM with Energy Dispersive Spectroscopy (EDS) measurements enabled spatially resolved correlation of potential differences with composition and subsequent galvanic corrosion behavior. Following exposure to the aggressive solution, corrosion damage morphology was characterized to determine the mode of attack and likely initiation areas. When exposed to 0.6 M NaCl, corrosion occurred at the braze-316L interface preceded by preferential dissolution of the Cu-rich phase within the braze alloy. Braze corrosion was driven by galvanic couples between the braze alloys and stainless steel as well as between different phases within the braze microstructure. Microgalvanic corrosion between phases of the braze alloys was investigated via SKPFM to determine how corrosion of the brazed joints developed

Corrosion and High Temperature Oxidation Behavior of 316L Stainless Steel Joined with Cu-Ag Based Braze Alloys

316L samples of various geometries were joined under Argon atmosphere using Cu-Ag based braze alloys. Following joining, elemental and phase composition across the braze and parent metal interface was characterized with optical microscopy and SEM with EDS. Baseline electrochemical testing was performed on each of the braze alloys in the fired and unfired condition. Additionally, metal-to-metal braze specimens were prepared in order to expose the braze interface to 0.6 M NaCl electrolyte where the free corrosion potential was monitored. Following exposure to the aggressive solution, the corrosion damage morphology was characterized to determine the mode of attack and likely initiation areas. The critical potential for localized corrosion initiation was also investigated for the braze alloys when connected galvanically to 316L samples to determine the impact of brazing on localized corrosion. Initial results indicate dissimilar metal driven corrosion attack at the braze metal interface into the parent 316L as well as preferential dissolution of the Cu rich phase within the braze alloy when exposed to 0.6 M NaCl