A Macro-scale, Tribological Modeling Framework for Simulating Multiple Lubrication Regimes and Engineering Applications
Creators: name: Gagan  Srivastava
Publisher: name: Carnegie Mellon University
Year: 2015
Subjects: subject: Mechanical Engineering
subject: FOS: Mechanical engineering
subject: FOS: Mechanical engineering

description: Tribology is the science of interacting surfaces and the associated study of friction lubrication and wear. High friction and wear cause energy loss and deterioration of interacting surfaces. Lubrication, using hydrodynamic liquids is the primary mechanism to reduce friction and wear. Unfortunately, not all applications can be ideally lubricated to operate in a low friction zone. In the majority of cases, the relative velocities between the moving components is either too low, or the transferred force is too high for them to get perfectly lubricated, with minimal solid to solid contact. In such conditions they operate in the boundary or mixed lubrication regime, where there is significant solid-solid contact. Examples of such conditions are commonplace in our daily lives. From the food in our mouth to a floating hard disk drive read/write heads, or artificial hip joints to a polishing process, all operate in the mixed lubrication regime. In this thesis, a generalized numerical modeling framework has been developed that can be applied to simulate the operation of a large variety tribological applications that operate in any of the three lubrication regimes. The framework called the Particle Augmented Mixed Lubrication - Plus (PAML+), accounts for all the major mechanical interactions encountered in any tribosystem. It involves coupled iii iv ABSTRACT modules for solid mechanics and fluid mechanics. Depending on the application, additional fidelity has been added in the form of modules relevant to the physical interactions unique to the application. For example, modeling of the chemical mechanical polishing process requires treatment of particle dynamics and wear to be able to generate predictions of meaningful quantities such as the material removal rate. Similarly, modeling of artificial hip joints requires additional treatment of mass transport and wear to simulate contamination with debris particles. The fluid mechanics have been modeled through the thin film approximation of Navier Stokes equations, known as the Reynolds Equation. The solid mechanics have been modeled using analytical or semi-analytical techniques. Statistical treatments have been applied to model particle dynamics wherever required to avoid huge computational requirements associated with deterministic methods such as the discrete element method. To demonstrate the strengths and general applicability of the modeling approach, four major tribological applications have been modeled using the new modeling approach in order to broadly impact key industries. The four tribological applications are (i) Pin-on-disk tribosystems (ii) Chemical mechanical polishing (CMP) (iii) Artificial hip joints, and (iv) Mechanical seals. First the model was employed to simulate pin-on-disk interfaces to evaluate different surface texture designs. It also served as a platform to test the model’s ability to capture, and seamlessly traverse through different lubrication regimes. The model predicted that an intermediate texture dimension of 200mm resulted in 80% lesser wear than a larger texture of 200mm, and up to 90% lesser wear than an untextured sample. Second, the framework was employed to study the CMP process. Overall, the model was found to be at least 50% more accurate than the previous generation model. Third, the model v was tailored to study the artificial joints. Wear predictions from the model remained within 5% error upon comparing against the experiments, while studying different “head” sizes. It was discovered that textured joints can reduce the concentration of the wear debris by at least 2:5% per cycle. For an expected lifetime of 12 years, that translates to lifetime enhancement of 3 months. Lastly, the model was employed to study the performance of mechanical seals. Even though the model was much more computationally efficient, it remained within 5% of much more detailed and computationally expensive FEA models. The model also predicted that the seals allow the highest leakage at shaft speeds of about 950 RPM.