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 situ and 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]
- Size effects (effect of reduced system dimensions) on the crystal structure and properties of nanostructured ceramic thin films and nanolaminates. [NSF funded]
- 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]
- 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]
- Processing, structure, and property interrelationships of metal-ceramic, ceramic-ceramic, and carbon nanotube-metal hybrid composites. [AFRL funded]
- 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]
- Development of hard and lubricious sputtered coatings on metallic alloys and composite biomaterials for orthopedic implants. [not currently 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.
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 .
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 diamondlike carbon thin films during solid and liquid lubricated sliding contacts [10-13] and nickel-multiwalled carbon nanotube (MWCNT) composites , 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  and ultrananocrystalline diamond thin films  and their potential in lubricated contacts.
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  and lubricious oxide nanolaminates [7-9], sputtered titanium doped tungsten disulphide thin films  and MoS2/Sb2O3/Au nanocomposite coatings , hybrid CVD/sputter processed tungsten carbide diamondlike carbon nanocomposite thin films , and polymer composites [5,19].
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].
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.
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 .
- T.W. Scharf, S. V. Prasad, T. M. Mayer, R. S. Goeke, and M. T. Dugger, “Atomic Layer Deposition of Tungsten Disulphide Solid Lubricant Coatings,” Journal of Materials Research, 19 3443-3446 (2004).
- T.M. Mayer, T.W. Scharf, S. V. Prasad, M. T. Dugger, P.G. Kotula, R.K. Grubbs, and R. S. Goeke, and, “Atomic Layer Deposition of Highly Conformal Tribological Coatings,” Report No. SAND2005-5742, Sandia National Laboratories, Albuquerque, NM, (2005).
- T.W. Scharf, S. V. Prasad, M. T. Dugger, P.G. Kotula, R. S. Goeke, and R.K. Grubbs, “Growth, Structure, and Tribological Behavior of Atomic Layer Deposited Tungsten Disulphide Solid Lubricant Coatings with Applications to MEMS,” Acta Materialia, 54, 4731-4743 (2006).
- T.W. Scharf, D. R. Diercks, B. P. Gorman, S. V. Prasad, and M. T. Dugger, “Atomic Layer Deposition of Tungsten Disulphide Solid Lubricant Nanocomposite Coatings on Rolling Element Bearings,” Tribology Transactions, 52,284-292 (2009).
- M.C. Romanes, N.A. D’Souza, D. Coutinho, K.J. Balkus, Jr. , and T.W. Scharf, “Surface and Subsurface Characterization of Epoxy-Mesoporous Silica Composites to Clarify Tribological Properties,” Wear, 265, 88-96 (2008).
- T.W. Scharf, M.C. Romanes, K. Mahdak, J.Y. Hwang R. Banerjee, R.D. Evans and G.L. Doll, “Atomic-scale Structure and Composition of Tungsten Carbide Reinforced Diamond-like Carbon Films,” Applied Physics Letters, 93, 151909 (2008).
G.L. Doll, B.A. Mensah, H. Mohseni and T.W. Scharf, “Chemical Vapor Deposition and Atomic Layer Deposition of Coatings for Mechanical Applications,” Journal of Thermal Spray Technology,19,510-516 (2010).
H. Mohseni and T.W. Scharf, “Tribological Improvement of Carbon/Carbon Composites by Infiltration of ZnO/Al2O3/ZrO2 Solid Lubricant Coatings,” Tribology and Lubrication Technology, 66(8), 20-21 (2010).
F. L. Kuo, M.-T. Lin, B.A. Mensah, T.W. Scharf and N.D. Shepherd, “A Comparative Study of the Photoluminescence and Conduction Mechanisms of Low Temperature Pulsed Laser Deposited and Atomic Layer Deposited Zinc Oxide Thin Films,” Physica Status Solidi A, 207, 2487–2491 (2010).
T.W. Scharf and I.L. Singer, “Third Bodies and Tribochemistry of DLC Coatings,” in Tribology of Diamond-like Carbon Films: Fundamentals and Applications, eds. A. Erdemir and C. Donnet, Springer, pp. 201-236 (2007).
T.W. Scharf and I.L. Singer, “Role of the Transfer Film on the Friction and Wear of Metal Carbide Reinforced Amorphous Carbon Coatings during Run-in," Tribology Letters, 36, 43-53 (2009).
T.W. Scharf, J.A. Ohlhausen, D.R. Tallant, and S.V. Prasad, “Mechanisms of Friction in Diamond-like Nanocomposite Coatings,” Journal of Applied Physics, 101, 063521-1 -063521-11, (2007).
- This article was selected for the October 27, 2008 issue (Volume 18, Issue 17) of Virtual Journal of Nanoscale Science & Technology covering a focused area of frontier research on nanometer-scale structures.
S.V. Prasad, T.W. Scharf, P.G. Kotula, J.R. Michael, and T.R. Christenson, “Application of Diamond-Like Nanocomposite Coatings on LIGA Microsystem Parts,” Journal of Microelectromechanical Systems, 18, 695-704 (2009).
T.W. Scharf, A. Neira, J.Y. Hwang, J. Tiley and R. Banerjee, “Solid Lubrication of Laser Deposited Carbon Nanotube Reinforced Nickel Matrix Nanocomposites, J. Appl. Phys., 106, 013508-1 - 013508-7 (2009).
- This article was selected for the April 9, 2007 issue (Volume 15, Issue 14) of Virtual Journal of Nanoscale Science & Technology covering a focused area of frontier research on nanometer-scale structures.
R. E. Stallcup II, Y. Mo, T.W. Scharf, and J.M. Perez, “Formation of nanometer-size high-density pits on epitaxial diamond (100) films,” Diamond and Related Materials, 16 1727–1731 (2007).
D.S. Grierson, A.V. Sumant, A.R. Konicek, M. Abrecht, J. Birrell, O. Auciello, J.A. Carlisle, T.W. Scharf, M.T. Dugger, P.U.P.A. Gilbert, and R.W. Carpick, “Tribochemistry and Material Transfer for the Ultrananocrystalline Diamond-Silicon Nitride Interface by X-PEEM Spectromicroscopy,” Journal of Vacuum Science and Technology B, 25(5) 1700-1705 (2007).
T.W. Scharf, A. Rajendran, R. Banerjee, and F. Sequeda, “Growth, Structure and Friction Behavior of Titanium Doped Tungsten Disulphide (Ti-WS2) Coatings,” Thin Solid Films, 517, 5666-5675 (2009).
T.W Scharf, P.G. Kotula and S.V. Prasad, “Friction and Wear Mechanisms in MoS2/Sb2O3/Au Nanocomposite Coatings,” Acta Materialia, 58, 4100-4109 (2010).
W. Brostow, W. Chonkaew; K.P. Menard, and T.W. Scharf, “Modification of an Epoxy Resin with a Fluoroepoxy Oligomer for Improved Mechanical and Tribological Properties,” Materials Science and Engineering A, 507, 241-251 (2009).
J.Y. Hwang, A. Neira, T.W. Scharf, J. Tiley, and, R. Banerjee, “Laser-deposited Carbon Nanotube Reinforced Nickel Matrix Composites,” Scripta Materialia, 59, 487-490 (2008).
A.R.P. Singh, J.Y. Hwang, T.W. Scharf, J. Tiley and R. Banerjee, “Bulk Nickel-Carbon Nanotube Nanocomposites by Laser Deposition,” available online Materials Science and Technology, (2011). DOI: 10.1179/174328409X411899.
S. Gopagoni, A. R. P. Singh, J. Y. Hwang, N. Bunce, B.A. Mensah, T. W. Scharf, J. Tiley, and R. Banerjee, “Microstructural evolution in laser deposited nickel-titanium-carbon in situ metal matrix composites,” Journal of Alloys and Compounds, 509, 1255–1260 (2011).
S. Samuel, S. Nag, T.W. Scharf, and R. Banerjee, “Wear Resistance of Laser-Deposited Boride Reinforced Ti-Nb-Zr-Ta Alloy Composites for Orthopedic Implants,” Materials Science and Engineering C: Biomimetic Materials, Sensors and Systems, 28, 414-420 (2008).
- This article was selected for the July 20, 2009 issue (Volume 20, Issue 3) of Virtual Journal of Nanoscale Science & Technology covering a focused area of frontier research on nanometer-scale structures.