Much of the excitement surrounding the advent of nanotechnology is due to the elucidation of the behavior of surfaces in relative motion at the topography-scale and below. Understanding the tribology study of friction, lubrication, and wear of the rubbing surfaces from the macro-scale to the nanoscale is of great scientific and industrial relevance. Additionally, when real surfaces under load rub together, wear debris or foreign particles often exist in the fluid (i.e., air or liquid) filled interface. This problem known as "particle-Augmented Mixed Lubrication" (PAML) is very dynamic and hence difficult to predict; yet, it is of great technical importance to the advancement of nanotechnology when the particles are nano-sized or sub-micron. With the explosive growth in computational power, complex particulate-based tribosystems can soon be completely modeled with multi-physics approaches by simulating dynamic processes that evolve over varying length and time scales without simplifying the scope of the problem such as lubrication approximations and globally applied wear rate relations. Consequently, this effort proposes to build a research and education program to study PAML-based tribosystems using a multiphysics modeling approach, with experimental validation, research-based education, and outreach.
The intellectual merit of this research program is that a generalized, multi-physics numerical particle augmented mixed lubrication model will be developed in conjunction with a educational program that gives students experience in applying fundamental tribology concepts to a cutting-edge problems in nanotechnology. Because existing models of PAML tribosystems are typically based on single or even dual mathematical physics descriptions, they are unable to capture the relevant PAML phenomena that occur over several length and time scales. For example, chemical mechanical polishing (CMP) is a PAML-based process where a rotating wafer, deposited with thin films, is polished as it is pressed against a rotating pad; the pad is flooded with a chemically reactive slurry with nanoparticles in it.
This complex PAML-based problem is one of the most critical steps in the fabrication of nano-enabled devices. As with most PAML processes, attempts to predict its behavior have been inadequate in that they were unable to capture all the relevant physics phenomena occurring over several space scales. Therefore, the proposed research offers a breakthrough modeling approach for PAML systems that will be validated by using chemical mechanical polishing as the experimental test-bed. The proposed research approach consists of three overlapping and related phases: (i) modeling, (ii) validation experiments and characterization, and (iii) research-based education and outreach. In phase I, a numerical multi-physics particle augmented mixed lubrication model will be developed. In phase II, nano-characterization and CMP experiments will be conducted to validate the particle augmented mixed lubrication model. In phase III, a particleassociated tribology simulation tool that employs the PAML modeling approach, will be developed to train students to understand modeling of complex tribological systems.
The broader impacts of the proposed work are that the PAML modeling framework will yield fundamental breakthroughs in understanding wear associated particle-based tribology. This will enable advancements in a wide range of technologies such as (i) integrated circuit (IC) and data storage nanotechnology, (ii) total joint replacement, (iii) nanoparticulate/fluid lubrication, (iv) coal flow energy systems, (v) dental tribology, and (vi) other technologies that encounters particles in fluidic environments. The proposed research-based education plan will also broadly impact the tribology community by teaching students to use fundamental tribology models as components to a larger more complex multiphysics tribology problem. Students will have the opportunity to participate in an enhanced tribology course with strategically coordinated assignments designed to teach (1) fundamental tribology problemsolving skills, related to (2) an educational multi-physics tribology simulation tool, where models can be (3) validated by laboratory experiments. Finally, the CAREER research results will be used as materials in pre-college workshops which aim to increase the number of minority students pursuing careers in science, technology, engineering, and mathematics.
" led to significant advancements in the field of Tribology (the study of interacting surfaces and their associated friction, lubrication, and wear), in general engineering research, and in engineering education. The first two of five outcomes below relate to the intellectual merit of this work. The first outcome is that a computational modeling approach was developed to predict the behavior of sliding surfaces undergoing particle augmented mixed lubrication (PAML). A difficult tribology problem to predict, PAML exists when surfaces in sliding contact have thin fluid films and small particles in between them. When the particles are harder than surfaces, the surfaces typically wear as is seen in applications such as automobile bearings, artificial hip and knee joints, teeth, and a semiconductor polishing processes needed to fabricate the world’s computer chips. High-fidelity and lower fidelity PAML computer models were developed for macro-scale and nano-/micro-scale tribology problems. They enabled macro-scale predictions of engineering applications, such as chemical mechanical polishing (CMP), which is used to polish semiconductor wafers when fabricating computer chips with nano-scale features. The second outcome is that a proprietary, in-house, computational software platform called the Particle-Surface Tribology Analysis Code (PSTAC) was developed to predict the tribological behavior of systems involving sliding surfaces. The software includes a module to solve PAML problems and many other tribology problems. The final three outcomes of this work relate to the broader impact of the proposed effort. The third outcome of this work is related to engineering education. A new graduate course in Computational Fluid Dynamics (CFD) was created at Carnegie Mellon as a direct result of this NSF research. While the course is listed in the mechanical engineering department curriculum, it was designed to be a multidisciplinary course. For example, it progressively teaches students without graduate level fluid mechanics backgrounds to understand how to solve thermal and fluid mechanics problems on the computer, including advanced the data visualization of the results. Additionally, a ‘research ethics’ paper was published and presented at an international Tribology conference to inspire the next generation of students to pursue high-quality, ethical research. The fourth outcome involves the development of a vast PhD recruitment and retention framework for students in engineering and Tribology. Online videos were created to motivate talented students in America and beyond to pursue doctoral degrees in engineering. Numerous minorities received research and training experiences in the PI’s lab and those of his colleagues. Efforts such as this led to the number of minorities entering the field of tribology to more than triple since the inception of this award. The fifth and final outcome was related to outreach. The PI believes that becoming one "the best and the brightest" is largely a function of one’s exposure and experiences. Therefore, pre-college engineering seminars and lab tours for minority, at-risk Pittsburgh public school students from Lincoln Technology Academy were conducted.
