Surface engineering of metal, graphite, and metal-graphite composites for improved strength and various material properties
Restricted (Penn State Only)
- Author:
- Foster, Linsea
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 30, 2024
- Committee Members:
- John Mauro, Program Head/Chair
Michael Lanagan, Major Field Member
Randy Vander Wal, Outside Unit & Field Member
Clive Randall, Major Field Member
Ramakrishnan Rajagopalan, Chair & Dissertation Advisor
Daudi Waryoba, Special Member
Daudi Waryoba, Special Member - Keywords:
- Cold sintering
Surface engineering
Powder metal
Powder carbon
Green machining
Soft magnetic composites - Abstract:
- Powder processing through “press and sinter” technology is an immensely useful method for producing complex net shaped three-dimensional products. This methodology allows for processing of multitude of materials that include ceramics, metals, carbons, as well as complex alloys and composites. One of the greatest challenges in powder processing technology is achieving adequate green strength without high-temperature processing. In the powdered metal industry, green strength and green density have a huge impact on determining the final mechanical properties of the sintered components. Also, higher compacted strength achieved at low temperature allows for exploring new energy and cost saving processes such as green machining and design and manufacture of soft magnetic composites. Similarly, low temperature densification and strengthening of graphite provides economic and environmentally friendly avenues to manufacture products for various applications that include electrical contacts, electrodes for nuclear, energy storage and metallurgical industry. Surface engineering that employs warm compaction of modified powders provides a suitable method for increasing the strength of as-compacted powder materials. Surface modification includes the formation of specialized coatings on the individual particle surfaces, achieved through methods such as acid treatment, co-precipitation, and electroless deposition. These carefully crafted coatings facilitate various densification and/or strengthening mechanisms under application of moderate heat and pressure. The work presented here investigates the versatility in adapting cold sintering methodology, a low temperature densification to various powder systems, namely, metals, graphite, and their composites. We demonstrate the use of acid treatment to introduce a hydrated coating on the surface of iron particles, which undergoes dissolution-precipitation during warm compaction. This accelerates particle rearrangement and bonding, improving both density and strength. The concept has also been extended for the development of a sinter hardened iron-based alloy. Using a similar approach, application of a hydrated Cu-Fe-oxalate coating on iron was used to introduce insulating boundary through heat treatment of iron compacts to produce high strength soft magnetic composites. Finally, surface modification of graphite powders using metal nanoparticles resulted in a low temperature densification and strengthening process to form graphite and metal/graphite composites., which has potential application in electrical contact applications. Mechanistic details for improvement in each system were studied in detail. Strength was determined through 3-point bending tests for each material system. Densification evolution was monitored via an in-situ dilatometer with both compaction and heating capabilities. Microstructural study that provided valuable insights on particle bonding, grain boundary composition, porosity and their shape was done using scanning electron microscopy, transmission electron microscopy, and electron dispersive x-ray spectroscopy, respectively. Composition, phase and chemical structure of modified powders were determined for pertinent processing temperatures through Diffuse reflectance infrared Fourier transform spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. Furthermore, system specific characterizations techniques were also performed, including green-phase machinability of iron systems, AC and DC magnetic properties for soft magnetic composites, and compactibility of different graphite powders.