Ivi Smid

Self-Lubricating Cold-Sprayed Coatings Utilizing Microscale Nickel-Encapsulated Hexagonal Boron Nitride

It is often beneficial to modify surfaces to gain desirable properties such as improved wear and friction resistance. Self-lubricating coatings can improve the performance of contacting surfaces and extend component lifetimes by reducing the coefficient of friction and/or improving resistance to specific wear modes. With these goals in mind, self-lubricating coatings of hexagonal boron nitride (hBN) particles in a deposited nickel matrix were investigated and optimized for friction and wear. These self-lubricating coatings were created via high-velocity particle consolidation or cold spray using micrometer-sized hBN powder encapsulated by nickel and nickel phosphorous alloys. Relatively thick nickel encapsulation via electrolesss Ni plating was required to aid in coating bonding/formation by “tricking” the hBN into acting as monolithic Ni during deposition. Once deposited on aluminum substrates, the coatings were analyzed and found to exhibit enhanced mechanical and tribological properties such as high bond strength and microhardness, a relatively low coefficient of friction, and improved reciprocating wear behavior relative to pure cold-sprayed Ni coatings. Furthermore, the encapsulation process was found to be both scalable and amenable to relatively small hBN particles.

Cold-Sprayed Ni-hBN Self-Lubricating Coatings

Feedstock preparation strategies were explored to produce composite admixed, milled, and precoated (encapsulated) powders of nickel–hexagonal boron nitride (Ni-hBN) for cold-sprayed self-lubricating coatings. The resulting cold-sprayed coatings were then examined for microstructural homogeneity and composition, as well as bond strength, microhardness, and relevant wear behaviors. Though admixed powders were easy to prepare and economical, milled and precoated formulations provided the advantage of aiding contact between Ni and lubricant powders prior to spraying that ultimately improved deposition and properties. The maximum amount of hBN that could be effectively built into the cold-sprayed Ni coatings was approximately 6 wt%. Results of the study also indicated that the composite coatings exhibited slightly higher hardness and reduced adhesive strength relative to a baseline of pure Ni layers. Moreover, some reductions in friction and expected decreases in bond strength and lubricant uniformity were observed when more than 4 wt% of lubricant was retained in the coatings. Given these findings, the most promising path to improve the amount, uniformity, and influence of the lubricant may be to encapsulate smaller particles with thicker levels of Ni to “trick” the composite particle to bond as pure Ni.

Material Processing and Testing of Plasma-Interactive Components for Fusion Energy Systems

In a global effort, the key players have combined their R&D forces to work jointly on the next generation thermonuclear fusion device. The development of structural, heat-sink and armor materials for plasma interactive components is focusing on neutron irradiation damage, and its impact on property retention and component integrity. Consequently, the processing of materials, realistic component testing methods as well as modeling are being optimized to suit the newest design concept(s). Nondestructive inspection of components is now available at an accuracy needed to ensure long service in a harsh, nuclear environment. The most promising materials and best established manufacturing processes are described; the proposed qualification techniques for fusion in-vessel and heat removal systems are reviewed.

Powder Injection Molding of Niobium

Niobium and niobium-based alloys are used in a variety of high temperature applications ranging from light bulbs to rocket engines. Niobium has excellent formability and the lowest specific weight among refractory metals (Nb, Ta, Mo, W, and Re). Powder injection molding of niobium powder was investigated for efficiency of the process. The sintering of injection molded bars was conducted up to 2000°C in vacuum and low oxygen partial pressure atmosphere. This paper investigates the effect of sintering time, temperature and atmosphere on processing of pure niobium.

In-Situ Agglomeration and De-agglomeration by Milling of Nano-Engineered Lubricant Particulate Composites for Cold Spray Deposition

Nano-engineered self-lubricating particles comprised of hexagonal-boron-nitride powder (hBN) encapsulated in nickel have been developed for cold spray coating of aluminum components. The nickel encapsulant consists of several nano-sized layers, which are deposited on the hBN particles by electroless plating. In the cold spray deposition, the nickel becomes the matrix in which hBN acts as the lubricant. The coating demonstrated a very promising performance by reducing the coefficient of friction by almost 50% and increasing the wear resistance more than tenfold. The coatings also exhibited higher bond strength, which was directly related to the hardenability of the particles. During the encapsulation process, the hBN particles agglomerate and form large clusters. De-agglomeration has been studied through low- and high-energy ball milling to create more uniform and consistent particle sizes and to improve the cold spray deposition efficiency. The unmilled and milled particles were characterized with Scanning Electron Microscopy, Energy-Dispersive X-Ray Spectroscopy, BET, and hardness tests. It was found that in low-energy ball milling, the clusters were compacted to a noticeable extent. However, the high-energy ball milling resulted in breakup of agglomerations and destroyed the nickel encapsulant.

Fragmentation of a Steel Ring under Explosive Loading

There is a great deal of interest in the behavior of metallic materials under high strain rate loading. Finite Element Analysis (FEA) could be used to model these materials with a reduction in the amount of experimentation needed for characterization. A finite element model of a metallic ring under high strain rate loading was developed using the Johnson-Cook failure model in Abaqus Computer Aided Engineering (CAE). The ring was modeled both axisymmetrically and in three dimensions. Failure was determined by defining a failure initiation value to start the process of element deletion. It was found that element deletion would occur when the failure strain initiation value was less than 1x10−4. Results of both axisymmetric and 3-D were found to be within 3% of each other with respect to maximum von Mises stress, and failure modes were identical. The effects of model changes and loading conditions are investigated.