Summary of Research
Since joining UNT in August 2005, my principal research activities have been primarily
in synthesis and processing (thin films and laser processed bulk composites) and characterization
(structure and tribological properties) interrelationships of ceramic, metallic and
polymeric materials and their composites. To this end, I have established a thin films
tribology laboratory to measure friction, wear and lubrication processes by in situand ex situ methods, and a thin films apparatus to synthesize novel atomic layer deposited thin
films. Specifically, I am involved in seven principal research thrust areas [with current funding noted]:
- Viscous flow atomic layer deposition (ALD) to synthesize nanoscale solid lubricant
and wear resistant coatings for many applications including moving mechanical assemblies,
such as miniature rolling element bearings, and to study fundamentals of rolling contact
fatigue. [NSF funded]
The first research thrust area was originally initiated while I was a staff member
at Sandia National Laboratories [1-4]. The principle of ALD is based on sequential pulsing of chemical precursor vapors,
both of which form about one atomic layer each pulse, thereby generating pinhole free
coatings that are extremely uniform in thickness, even deep inside pores, trenches
and cavities. This was the motivation to continue the work at UNT since nanoscale
processing and manufacturing is a currently relevant research area with numerous potential
funding opportunities. Within a year at UNT, my students and I independently setup
the ALD reactor and grew novel nanolaminate and nanocomposite thin films on moving
mechanical assemblies (MMA), such as miniature rolling element bearings, micro electro
mechanical systems (MEMS) and other precision components. My graduate students have
presented results at international conferences, and have published several publications
[5-9] in this thrust area.
- Size effects (effect of reduced system dimensions) on the crystal structure and properties
of nanostructured ceramic thin films and nanolaminates. [NSF funded]
The second thrust area has focused on advancements in MMA to better understand their
structure-property relationships, since the reduction in the dimensions of these devices
increases the influence of dynamic tribological surface interactions on performance
and reliability. Drs. Ryan Evans and Gary Doll from Timken Co., a worldwide commercial
leader in bearings and other fine precision components that involve sliding and/or
rolling contacts, and my group have studied fundamental structure and properties of
nanocomposite tribological films deposited with plasma-discharge vapor deposition
systems, including nanocrystalline films containing nanoscale metal carbides in mixed
sp3/sp2 carbon matrices. The research has successfully addressed the composition/structure
dependence of tribological thin films on performance and mechanisms of film modification
by tribo-chemical reactions in MMA applications. We have correlated film initial nanostructure
and properties to macro-scale friction and wear performance to further increase scientific
understanding and support the development of predictive models. The results have been
presented at numerous conferences and an Applied Physics Letters paper was published
in 2008 [6].
- In situ and ex situ Raman tribology studies of solid lubricants and lubricant additives to diamond and
diamondlike carbon and self-lubricating materials. [ACS and AFRL funded]
The third area of my research activities was originally initiated while I was a Postdoctoral
Fellow at the Naval Research Laboratories. I have been successful in transferring
that knowledge to UNT by identifying Raman active tribochemical processes that control
friction and wear and adhesion of diamond like carbon thin films during solid and
liquid lubricated sliding contacts [10-13] and nickel-multiwalled carbon nanotube (MWCNT) composites [14], which will play a crucial role in implementing condition-based maintenance to government
and private sectors. In addition, peer-reviewed journal papers have been published
on Raman spectroscopy of diamond [15] and ultrananocrystalline diamond thin films [16] and their potential in lubricated contacts.
- Advanced analytical nanostructural and compositional techniques, such as 3D atom probe
tomography and transmission electron microscopy, to determine what controls properties
at sliding and rolling interfaces. [NSF funded]
The aforementioned second and third research areas have been partially supported by
the fourth thrust area which has incorporated state-of-the-art diagnostic characterization
tools and methods at UNT to study interfacial composition, periodicity, and morphological
changes occurring at the surface and in the subsurface regions with focused ion beam
cross-sectional SEM, analytical high resolution TEM, local electrode 3D atom probe
tomography (local electrode atom probe, LEAP), and Raman spectroscopy spatial mapping.
LEAP has been and is currently being used to characterize elemental partitioning,
morphology of nanoscale precipitates in 3D and sliding/rolling interfaces. The LEAP
is used in conjunction with TEM to determine the most accurate information at the
nanometer to Ångstrom size scales. These tools have been instrumental in fundamental
and applied studies of ALD nanocomposites [4] and lubricious oxide nanolaminates [7-9], sputtered titanium doped tungsten disulphide thin films [17] and MoS2/Sb2O3/Au nanocomposite coatings [18], hybrid CVD/sputter processed tungsten carbide diamondlike carbon nanocomposite
thin films [6], and polymer composites [5,19].
- Processing, structure, and property interrelationships of metal-ceramic, ceramic-ceramic,
and carbon nanotube-metal hybrid composites. [AFRL funded]
The fifth area of my research activities is processing, structure, and property interrelationships
of ceramic-ceramic and metal-carbon nanotube hybrid composites, which is sponsored
by Air Force Research Laboratory (AFRL) for the UNT program “Institute for Science
and Engineering Simulation (ISES). Specifically, hybrid nanocomposite coatings chemically
infiltrated into carbon-carbon composites and graphite foams are being studied for
thermal protection systems. Hybrid nanocomposite protective coatings require minimal
processing and potentially undergo in situ modifications during use to optimize their properties and thus enhance the operating
temperatures of carbon-based composites and foams for hypersonic aircraft. The protective
coatings being deposited are ALD ZrO2 and yttria-stabilized ZrO2 (YSZ) infiltrated
into carbon-carbon composites and graphitic foams, thus forming a thermally stable
oxidation barrier. As these coatings are employed at elevated temperatures, the ZrO2
and the carbonaceous matrix can react to form a ZrC interfacial layer. This in situ reaction is thermodynamically favored in air above ~1657°C. Lubricious, thermodynamically
stable ZnO will also be deposited on ZrO2 to mitigate fretting wear in bushings that
are used in jet engine blade integrated disks. The research aims to understand the
thermochemical mechanisms of their formation and their oxidative, thermal and high
temperature tribological properties. In addition, nickel-multiwalled carbon nanotube
(MWCNT) composites have been developed in collaboration with MSE Associate Professor
Raj Banerjee and his group. Processing-structure-tribological interrelationships have
been studied for this composite and numerous publications have resulted from this
work [14, 20-22].
- Inductively-coupled plasma mass spectrometer to understand solid lubricant tribochemical
reactions/mechanisms and study thermal reduction and interfacial chemical reactions
in ceramic-ceramic hybrid matrix composites. [AFOSR funded]
The sixth research area will make use of a recent AFOSR-funded Defense University
Research Instrumentation Program (DURIP) acquisition of an inductively-coupled plasma
mass spectrometer with laser ablation source to help understand the mechanism(s) for
thermal reduction by characterizing the aforementioned ZrO2/ZrC/C interfacial chemical
reactions. Their thermodynamics and kinetics will also be critical if these composites
are to be used in thermal protection system applications. The tool will also help
in understanding solid lubricant tribochemical reactions/mechanisms after room and
high temperature fretting and sliding wear tests.
- Development of hard and lubricious sputtered coatings on metallic alloys and composite
biomaterials for orthopedic implants. [not currently funded]
The seventh and final area of my research activities involves functionally-graded
hard and lubricious sputtered coatings on metallic alloys and composite biomaterials
for orthopedic implants in collaboration with MSE Associate Professor Raj Banerjee
and his group. While not currently funded, we are in the process of writing proposals
to NSF and NIH. In order to submit successful proposals to these federal funding agencies,
initial experiments have been conducted and a peer-reviewed paper has been recently
published in a leading biomaterials journal [23].