Research

Our research interests are inspired by the need for sustainable design and operation through efficient energy harvesting and conversion, reducing energy consumption as well as prolonging components life by centering on surface engineering. This can be mainly achieved through the application of Tribology which is the science and engineering of interacting surfaces in relative motion dealing majorly with Friction, Wear, and Lubrication. It impacts the design and operation of every device that contains moving components and therefore, our work is of interest to a variety of industries, including power generation and energy conversion, oil and gas, energy harvesting, transportation, MEMS, and health sciences.

Our area of specialty is the combination and application of the numerical, analytical and experimental methods in systems involving material science, thermal and fluid science, and mechanical systems. The research activities are interdisciplinary and require extensive expertise in solid mechanics, fluid mechanics, interface science, materials science, computational mechanics and thermodynamics. Taking on the challenge to improve the mechanical components’ life efficiency, we have focused on modeling the materials subjected to contact degradation processes such as delamination, fatigue, wear and fretting. We have also concentrated on understanding contact and friction phenomenon at multi scales under different environmental and operating conditions and for variety of materials. These analyses are applicable to a range of industrially important applications such as solar cells and wind turbine design which not only need to be designed energy-efficient but also as growing sources of renewable energy production need to have a long operating life and overall reliability.

Ongoing Research

Multiscale contact mechanics and friction modeling of high temperature Ni alloys under helium environment for very high temperature gas reactors

Operating at high temperatures (HT) is critical for reactors as it results in substantial thermal efficiency improvement. In view of design demands, materials that can withstand HT and harsh environments are necessary for reliable and effective nuclear reactor operation. Nickel alloys are the principle candidates in high/very high temperature gas cooled reactors with outlet temperatures of 700-950°C. The objectives of this research project is to systematically evaluate the tribological response of high temperature alloys at relevant reactor operating temperatures. Part of the project focuses on a comprehensive multiscale numerical modeling to investigate and compare the friction, surface damage (wear, fretting, self-welding, corrosion/oxidation) and contact response of tribo-pairs in atmospheric conditions and helium environment. Our research concentrates on modeling hot nano-indentation/scratch to estimate mechanical properties or the bulk material and potential thin oxide film. In addition novel modeling of friction considering surface micro features, creep and temperature fluctuations are performed. The constitutive relationships for an asperity are identified and are extended to populationof asperity enabling the complete multiscale modeling of friction from static to kinetic values. 

Energy Absorption Analysis of Additively-Manufactured Lattice Metamaterials

Recent advances in additive manufacturing techniques along with the growing demands for engineered materials with properties not found in nature have attracted lots of attention to the concept of mechanical metamaterials. These materials are fabricated with complex architectures at the nano/micro scales with specific designs and arrangement of lattice structures giving them superior and sometimes strange mechanical properties at the macroscale. In this study three different mechanical metamaterial architectures were fabricated with different materials through two different 3D printing techniques and subsequently investigated for their mechanical responses and strength. The study involves experimental as well as numerical finite element analyses to obtain a deeper understanding of their deformation and failure mechanisms.

Microfabricated Shark Skin Inspired Patterns for Marine Antifouling Applications

In the maritime industry, biofouling concerns the aggregation of aquatic organisms, such as algae, on ships and other maritime vehicles and structures. The use of environmentally-friendly micro patterns, inspired by natural antifouling structures, as an alternative to toxic antifouling paints has been suggested. We hypothesized that a pattern more similar to the real-life counterpart would further decrease bacterial adhesion. Mimicking these patterns was achieved through various microfabrication techniques and micro-casting of PDMSe on silicon molds to create a novel height gradient. Following growth of Escherichia coli (E. coli) and antibacterial drop-testing, these micro patterns decreased bacterial settlement of E. coli by up to 75% where observed as compared to controlled smooth PDMSe surfaces.

Multiscale Mechanical and Tribological Characterizations of Additively Manufactured Polyamide 12 Parts with Different Print Orientations

The use of additive manufacturing for prototyping and part manufacturing purposes in engineering is becoming a crucial topic in both industry and research. Multi Jet Fusion (MJF) printing technology may possess advantages in fabricating engineering-quality parts, but the dependency of their mechanical characteristics on process parameters is not well understood at this time. In this work, multi scale mechanical analysis of MJF manufactured Polyamide 12 dog bone samples was conducted with respect to the effect of printing orientation on mechanical properties. Results showed a noticeable difference in resultant part characteristics based on whether they were printed in the vertical or horizontal orientation. Tensile experiments showed that orientation, more specifically printing layer hatches, resulted in varying strain and stress characteristics, with vertical samples having 25% smaller fracture strains, but slightly higher (5.8%) fracture stress values than horizontal samples. Nanoindentation experiments showed vertical samples with reduced elastic moduli values double that of horizontal, and with hardness values nearly 40 MPa larger, while nanoscratch tests showed that vertical samples had an almost 50% increase in lateral forces. Also, nanoindentation creep experimental and numerical analyses were performed demonstrating higher creep resistance of vertical samples as compared to the horizontal specimens. In addition, macro friction results showed the independency of the horizontal samples friction coefficient on the direction of sliding while vertical samples coefficient of friction showed about 60% changes at 0° as compared to 90° sliding directions. These results point to an underlying dependence on print orientation in the resulting mechanical properties.

Past Projects

Novel nanotextured protective coating with hybrid self-cleaning characteristics for solar cell applications

Solar energy is one of the best renewable energy sources to meet challenges like energy security and the environmental and global climate change. Unfortunately, the sunniest areas are the dustiest of the world. Accumulation of dust and sand on the PV cells as well as the high velocity sand impact are among the main efficiency drags for the solar power plants. Sufficient rain can wash away dust and soiling, and collector performance are usually restored to nearly original capacity, but it is not the case in desert area with very low precipitation. Therefore, any approach which can reduce the adverse effect of dirt and dust on the efficiency and also increase the absorption of the light on the solar cell are of significant attractiveness. One of those approaches, is the utilization of self-cleaning protective coatings. Currently our focus is on the tribology of a new design for the protective coating with not only passive but also active self-cleaning capabilities which improves the light absorption as well. The sought characteristics are mainly achieved through nano-silica spheres on newly developed highly transparent and wear resistant polymeric coating, so-called ATSP coating. One of the biggest challenges for all recent PV smart coatings are their durability and reliability in often harsh and sandy environments. Our current research seeks to investigate the impact and scratch wear resistance of ATSP coating in comparison to commercial new PV protective glasses under sand impact and thermal load both numerically and experimentally. 

Failure/contact analysis of single-layer, multi-layer and functionally graded soft and hard thin-films

Hard thin film coatings play a vital role in many applications involving sliding contact such as magnetic disk drives, microelectromechanical systems as well as biomedical applications. In such systems, hard coatings are employed to enhance sliding wear resistance of interacting solids. Failure of thin hard coatings during sliding condition or scratch is usually triggered by initiation and propagation of cracks on the coating/substrate interface and consequent flaking of the coating material from the substrate. As a result, delamination or spalling can be prevented if the crack initiation at the coating-substrate interface is restrained. Multi-layering as well as functionally graded material (FGM) are smart designs that have revolutionized coating technologies in the past 20 years and are the best solutions to control sharp changes at the interfaces and also reaching desirable properties both at the surface and the interface. In this research, using excessive computational efforts, useful ready-to-use yield zone maps are constructed for single, bilayer and FGM thin films and coatings. The yield zone maps are constructed based on different loading condition, material properties and film thicknesses. Using experimental nanoindentation / nanoscratch on thin films (less than 40nm), results of numerical modeling are validate against experimental observations. Besides, literature clearly lacks analytical/empirical models to quantitatively understand nanoindnetaiton of FGM thin films. Current study also aims at developing models to be used in order to estimate mechanical properties of FGM films at different depths accurately.

Surface reliability of annealed and tempered solar protective glasses: indentation and scratch behavior (in collaboration with Texas A&M University)

Solar glass is exposed to mechanical contact cleaning and sand particle impact during operation in outdoor environments resulting in optical efficiency loss as well as decrease in mechanical integrity, durability and reliability. The current study investigates the mechanical behavior of two different solar surface glasses through a series of low and high load indentation and scratch experiments. Nanoindentation experiments are performed on the glass substrate in order to measure the hardness and elasticity moduli at different depths below the surface. Scratch experiments are also preformed to find the critical load for the extent of plastic zone or the onset of micro-cracking. The influence of heat treatment for photovoltaic glasses on mechanical properties such as elastic modulus and hardness, and surface properties such as friction coefficient and elastic recovery is examined in this study in which heat treatment is found to affect both mechanical and surface properties. 

Novel contact stiffness modeling and measurement for nanometer rough multi-layer thin films

Currently we perform a comparative study on the accuracy of multiscale statistical rough contact models for thin film applications.  Experimental tests involve contact stiffness measurements for various single and multilayer thin films as well as homogenous materials through pressing a flat tip punch onto a rough surface in the order of mN using nanoindentation. Coating materials under investigation are typically used in micro/nanomechanical applications with thickness of the order of few nanometers. The study involve a novel technique to be able to compare experimental findings with analytical modeling and quantify the effect of major parameter on contact behaviors of rough thin films. Mainly, here we measure the contact stiffness from load-displacement curves investigating the effect of nanometer features on the tribological properties of thin films. Through same methodology multiscale contact models are developed to estimate contact stiffness and compared with experimental results. We try to understand the contact mechanics of nanometer rough surface and the interactions between nanometer-high asperities on the contact response. 

Statistical and deterministic models for rough contact analysis with nano/micro features

Real engineering surfaces are rough at nano/micro level and their interactions involve the contact of surface peaks at discrete spots called asperity tips. These interactions play an important role in the tribological performance of tribological components such as gears, hard disk drives, MEMS as well as biomedical transplants. To consider the effect of roughness, in general, two different approaches namely the statistical model, in which exact micro-contact behavior of the surface is averaged over a selected surface element, and the deterministic model which uses the exact surface profile are commonly used. We seek to develop experimentally validated multiscale contact models for variety of thin films with flat or curved configurations having different structures, thickness and surface features.