@article{wei_sankar_sharma_yamagata_narayan_2001, title={Effect of chamber pressure and atmosphere on the microstructure and nanomechanical properties of amorphous carbon films prepared by pulsed laser deposition}, volume={19}, ISSN={["0734-2101"]}, DOI={10.1116/1.1322641}, abstractNote={We have investigated the effect of chamber pressure and atmosphere on the microstructure and nanomechanical properties of amorphous carbon thin films prepared by pulsed laser deposition. The amorphous carbon films were deposited in various atmospheres such as nitrogen and argon at different chamber pressures. We used Raman spectroscopy to study the bonding characteristics of the deposited amorphous carbon films. Atomic force microscopy and optical microscopy were utilized to observe the surface conditions and the microstructures of the deposited films. Nanoindentation measurements were carried out on various samples prepared under different conditions to study the effect of chamber pressure and atmosphere on the elastic modulus and nanohardness of the films. It was found that reduced vacuum leads to formation of amorphous carbon films with reduced elastic modulus and nanohardness. Amorphous carbon films prepared under higher chamber pressures exhibit an increased density of particulates and significantly roughened surface. The results were understood in combination with the optical emission and electrostatic measurements of the laser plasma plume. It was found that the presence of atmosphere decreases the leading edge ionic energies of the species in the laser plasma plume and increases the thermalization of the laser plasma due to an increased possibility of collision.}, number={1}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A-VACUUM SURFACES AND FILMS}, author={Wei, Q and Sankar, J and Sharma, AK and Yamagata, Y and Narayan, J}, year={2001}, pages={311–316} }
 @article{wang_sharma_kvit_wei_zhang_koch_narayan_2001, title={Mechanical properties of nanocrystalline and epitaxial TiN films on (100) silicon}, volume={16}, ISSN={["2044-5326"]}, DOI={10.1557/JMR.2001.0373}, abstractNote={We investigated mechanical properties of TiN as a function of microstructure varying from nanocrystalline to single crystal TiN films deposited on (100) silicon substrates. By varying the substrate temperature from 25 to 700 °C during pulsed laser deposition, the microstructure of TiN films changed from nanocrystalline (having a uniform grain size of 8 nm) to a single crystal epitaxial film on the silicon (100) substrate. The microstructure and epitaxial nature of these films were investigated using x-ray diffraction and high-resolution transmission electron microscopy. Hardness measurements were made using nanoindentation techniques. The nanocrystalline TiN contained numerous triple junctions without any presence of amorphous regions. The width of the grain boundary remained constant at less than 1 nm as a function of boundary angle. Similarly the grain boundary structure did not change with grain size. The hardness of TiN films decreased with decreasing grain size. This behavior was modeled recently involving grain boundary sliding, which is particularly relevant in the case of hard materials such as TiN.}, number={9}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Wang, H and Sharma, A and Kvit, A and Wei, Q and Zhang, X and Koch, CC and Narayan, J}, year={2001}, month={Sep}, pages={2733–2738} }
 @article{wei_sankar_narayan_2001, title={Microstructural changes due to heat-treatment of annealing and their effect on the creep behavior of self-reinforced silicon nitride ceramics}, volume={299}, ISSN={["0921-5093"]}, DOI={10.1016/s0921-5093(00)01403-9}, abstractNote={In order to understand the improvement of creep resistance by furnace and microwave annealing, we have investigated the effect of heat-treatment on the microstructural characteristics of the crept self-reinforced silicon nitride (Si3N4) ceramic GS44. X-ray diffraction was performed on the as-sintered and heat-treated samples to study the phase changes due to annealing treatment. Optical microscopy and scanning electron microscopy (SEM) were used to study the fracture surface and to identify the creep mechanism. High resolution and analytical transmission electron microscopy (TEM) were employed to analyze the microstructures of the crept samples with as-received and heat-treated conditions. It has been reported that both conventional furnace and microwave annealing enhance the creep resistance of the material, and microwave annealing had the most significant effect. Fractography showed that the microwave annealed samples exhibits least creep damage. Furnace annealing also reduces the creep damage, but the effect is far less as compared to microwave annealing. Scanning electron microscopy (SEM) analysis showed significant amount of multiple-junction cavitation in the creep-tested samples. TEM observations showed significant devitrification of the amorphous phases in the microwave annealed specimens, as verified by micro-diffraction studies of the junction phases. This is also confirmed by X-ray diffraction and high-resolution lattice image of the triple junction phases. The microstructural observations were combined with a recent model of the effect of amorphous residues in ceramics on the creep behavior to explain the improvement in the creep resistance due to annealing.}, number={1-2}, journal={MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING}, author={Wei, Q and Sankar, J and Narayan, J}, year={2001}, month={Feb}, pages={141–151} }
 @article{wei_sankar_sharma_oktyabrsky_narayan_narayan_2000, title={Atomic structure, electrical properties, and infrared range optical properties of diamondlike carbon films containing foreign atoms prepared by pulsed laser deposition}, volume={15}, ISSN={["0884-2914"]}, DOI={10.1557/jmr.2000.0094}, abstractNote={We investigated the atomic structure, electrical, and infrared range optical properties of diamondlike carbon (DLC) films containing alloy atoms (Cu, Ti, or Si) prepared by pulsed laser deposition. Radial distribution function (RDF) analysis of these films showed that they are largely sp3 bonded. Both pure DLC and DLC + Cu films form a Schottky barrier with the measuring probe, whereas DLC + Ti films behave like a linear resistor. Pure DLC films and those containing Cu exhibit p-type conduction, and those containing Ti and Si have n-type conduction. Photon-induced conduction is observed for pure DLC, and the mechanism is discussed in terms of low-density gap states of highly tetrahedral DLC. Our results are consistent with relative absence of gap states in pure DLC, in accordance with theoretical prediction by Drabold et al.37 Temperature dependence of conductivity of DLC + Cu shows a behavior σ ∞ exp(−B/T1/2), instead of the T−1/4 law (Mott–Davis law). Contributions from band-to-band transitions, free carriers, and phonons to the emissivity spectrum are clearly identified in pure DLC films. The amorphous state introduces a large contribution from localized states. Incorporation of a small amount of Si in the DLC does not change the general feature of emissivity spectrum but enhances the contribution from the localized states. Cu and Ti both enhance the free carrier and the localized state contributions and make the films a black body.}, number={3}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Wei, Q and Sankar, J and Sharma, AK and Oktyabrsky, S and Narayan, J and Narayan, RJ}, year={2000}, month={Mar}, pages={633–641} }
 @misc{wei_narayan_2000, title={Superhard diamondlike carbon: preparation, theory, and properties}, volume={45}, ISSN={["0950-6608"]}, DOI={10.1179/095066000101528340}, abstractNote={Abstract One of the many forms of carbon, diamondlike carbon (DLC) or tetrahedral amorphous carbon (ta-C) consists mainly of sp3 bonded carbon atoms. If properly prepared, DLC can have properties that rival those of crystalline diamond. The beneficial properties of DLC stem from the continuous rigid random networks of sp3 carbon atoms, and the properties can essentially be tailored by controlling the sp3/sp2 ratio. Techniques that have been successfully used to prepare high quality DLC coatings or thin films include pulsed laser ablation, filtered cathodic vacuum arc deposition, and mass selected ion beam deposition. Diamondlike carbon coatings that possess properties close to diamond in terms of hardness, atomic smoothness, infrared transparency, and chemical inertness can be processed easily with these techniques. In the past decade, tremendous progress has been made in experimental and theoretical investigations of hydrogen free DLC. Experimental and commercial applications in areas including microelectronics, microtribology, and biomedical technologies have been demonstrated. Potential applications include sensors, flat panel displays (field emitters), and photodiodes. Past and recent developments in synthesis and processing, properties, and modelling of hydrogen free superhard amorphous DLC are comprehensively reviewed. The techniques of fabrication, theoretical modelling, physical and mechanical characterisation, properties, and present and potential applications of DLC are discussed.}, number={4}, journal={INTERNATIONAL MATERIALS REVIEWS}, author={Wei, Q and Narayan, J}, year={2000}, pages={133–164} }
 @article{wei_sharma_sankar_narayan_1999, title={Mechanical properties of diamond-like carbon composite thin films prepared by pulsed laser deposition}, volume={30}, ISSN={["1879-1069"]}, DOI={10.1016/S1359-8368(99)00035-9}, abstractNote={We have investigated the mechanical properties of diamond-like carbon (DLC) thin films that contain foreign atoms. The DLC films were prepared by pulsed laser deposition. A novel target design was adopted to incorporate foreign atoms into the DLC films during film deposition. Copper, titanium and silicon are chosen as the dopants. The chemical composition of the doped films was determined using Rutherford backscattering spectrometry, X-ray photoelectron spectroscopy and calibrated extrapolation. Experimental results of both visible and UV Raman are presented and discussed in terms of peak shape and position. The effect of dopants on the Raman spectrum is also analyzed. Optical microscopy of the pure DLC of a certain thickness showed severe buckling. A brief review of the theoretical background of adhesion is given and the possible mechanisms of adhesion that may work in DLC coatings are discussed. Qualitative scratch tests on the specimens show that pure DLC has quite poor adhesion due to the large compressive stress, while the doped DLC films exhibit much improved adhesion. Wear tests show improved wear resistance in the doped DLC coatings. Nanoindentation results give an average hardness above 40 GPa and effective Young's modulus above 200 GPa for pure DLC. The copper doped DLC films showed slightly decreased hardness and Young's modulus as compared to pure DLC films. Ti and Si can reduce the hardness and Young's modulus more than Cu. All these can be understood by analyzing the internal stress reduction as derived from Raman G-peak shift to lower wavenumbers. A preliminary model of the stress reduction mechanism is discussed.}, number={7}, journal={COMPOSITES PART B-ENGINEERING}, author={Wei, Q and Sharma, AK and Sankar, J and Narayan, J}, year={1999}, pages={675–684} }
 @article{wei_sankar_kelkar_narayan_1999, title={Microstructure evolution accompanying high temperature; uniaxial tensile creep of self-reinforced silicon nitride ceramics}, volume={272}, ISSN={["0921-5093"]}, DOI={10.1016/s0921-5093(99)00497-9}, abstractNote={Extensive transmission electron microscopy (TEM) has been performed to study the microstructure evolution of a self-reinforced silicon nitride associated with high temperature creep. A large population of strain whorls is observed in samples crept at relatively high temperatures and the strain whorls are not necessarily asymmetrical with respect to the grain boundary normal. Large angle convergent beam electron diffraction (LACBED) at the grain boundaries where strain whorl contrast is visible reveals severely curved Bragg lines, implying large residual strains. This indicates that grain boundary interlocking might be effective to enhance the creep resistance at high temperatures. Dislocation pile-ups, arrays and tangles are present in certain silicon nitride grains. However, a simple analysis rules out dislocations as the major creep mechanism. Most dislocations started from grain boundaries. The role of dislocations is to relieve the stress concentrations at the strain whorls. This adds to the diffusion mechanism of stress relaxation at the strain whorls and facilitates other creep mechanisms such as grain boundary sliding. A large density of multiple-junction cavities is observed in the samples crept at relatively high temperatures. It is proposed that grain boundary sliding and cavity formation, in addition to stress relaxation through nucleation of dislocations at the strain whorls act together to produce a much shorter life to failure at high temperatures. While at lower temperatures, the creep is more diffusion controlled which gives a stress exponent of unity.}, number={2}, journal={MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING}, author={Wei, Q and Sankar, J and Kelkar, AD and Narayan, J}, year={1999}, month={Nov}, pages={380–388} }
 @article{wei_narayan_sharma_sankar_narayan_1999, title={Preparation and mechanical properties of composite diamond-like carbon thin films}, volume={17}, ISSN={["0734-2101"]}, DOI={10.1116/1.582074}, abstractNote={We have investigated mechanical properties of diamond-like carbon (DLC) thin films, particularly the internal compressive stress and ways to alleviate it. Foreign atoms such as copper, titanium, and silicon were incorporated into the DLC films during pulsed laser deposition. The chemical composition of the doped films was determined using Rutherford backscattering spectrometry (RBS) and x-ray photoelectron spectroscopy (XPS). Optical microscopy of the doped films showed that DLC films containing Cu exhibit much less particulate density as compared to the films containing Ti and Si. Visible Raman spectroscopy was used to characterize the films. The effect of dopants on the Raman spectrum was analyzed in terms of peak shape and position. Optical microscopy of the pure DLC of a certain thickness showed severe buckling. The mechanisms of adhesion associated with DLC coatings were discussed. Qualitative scratch tests on the specimens showed that pure DLC films have relatively poor adhesion due to a large compressive stress, while the doped DLC films exhibit much improved adhesion. Wear tests show improved wear resistance in the doped DLC coatings. Nanoindentation results suggest that pure DLC has an average hardness above 40 GPa and effective Young’s modulus above 200 GPa. The doped DLC films showed slightly decreased hardness and Young’s modulus as compared to pure DLC films. These results can be rationalized by analyzing the internal stress reduction as derived from Raman G-peak shift to lower wavenumbers. A preliminary interpretation of the stress reduction mechanism is discussed.}, number={6}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A-VACUUM SURFACES AND FILMS}, author={Wei, Q and Narayan, RJ and Sharma, AK and Sankar, J and Narayan, J}, year={1999}, pages={3406–3414} }
 @article{wei_narayan_narayan_sankar_sharma_1998, title={Improvement of wear resistance of pulsed laser deposited diamond-like carbon films through incorporation of metals}, volume={53}, ISSN={["0921-5107"]}, DOI={10.1016/s0921-5107(98)00150-0}, abstractNote={We have investigated the characteristics of diamond-like carbon (DLC), DLC doped with Cu, and DLC doped with Ti deposited by a sequential pulsed laser ablation of two targets. The composition of these films was determined by Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy and transmission electron microscopy studies showed typical features of DLC with a high fraction of sp3 bonded carbon in the doped films as well as in the undoped films. Wear resistance measurements made on the samples by means of the `crater grinding method' showed that DLC+2.75% Ti has the highest wear resistance, while that of pure DLC has the lowest amongst the samples. Careful analysis of the Raman data indicates a significant shift to shorter wavelength with the addition of metal, which means that the compressive stress in the DLC films has been reduced. We envisaged that the reduction in the compressive stress promotes the wear resistance of the coatings. The XPS studies showed evidence for the formation of Ti–C bonding in the Ti doped sample. Thus metal-doped DLC coatings are expected to improve the tribological properties and enhance the performance of components coated with metal-doped DLC.}, number={3}, journal={MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY}, author={Wei, Q and Narayan, RJ and Narayan, J and Sankar, J and Sharma, AK}, year={1998}, month={May}, pages={262–266} }
 @inproceedings{wei_sharma_narayan_ravindra_oktyabrsky_sankar_muth_kolbas_narayan_1998, title={Microstructure and IR range optical properties of pure DLC and DLC containing dopants prepared by pulsed laser deposition}, booktitle={Advances in laser ablation of materials: Symposium held April 13-16, 1998, San Francisco, California, U.S.A.  (Materials Research Society symposia proceedings ; v. 526).}, publisher={Warrendale, Pa.: Materials Research Society}, author={Wei, Q. and Sharma, A. K. and Narayan, R. J. and Ravindra, N. M. and Oktyabrsky, S. and Sankar, J. and Muth, J. F. and Kolbas, R. M. and Narayan, J.}, year={1998}, pages={331} }
 @article{jagannadham_sharma_wei_kalyanraman_narayan_1998, title={Structural characteristics of AIN films deposited by pulsed laser deposition and reactive magnetron sputtering: A comparative study}, volume={16}, ISSN={["0734-2101"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032370406&partnerID=MN8TOARS}, DOI={10.1116/1.581425}, abstractNote={Aluminum nitride films have been deposited on Si(111) substrates at different substrate temperatures using two techniques; pulsed laser deposition or reactive magnetron sputtering. The films deposited by either of the techniques have been characterized by x-ray diffraction and transmission electron microscopy to determine the crystalline quality, grain size, and epitaxial growth relation with respect to the substrate. The bonding characteristics and the residual stresses present in the films have been evaluated using Raman and Fourier transform infrared spectroscopy. Secondary ion mass spectrometry has been performed to determine the nitrogen stoichiometry and the presence of impurities such as oxygen and silicon. The adhesion strength of the AlN films to the silicon substrate and the wear resistance have been determined by scratch test and a specially designed microscopic wear test. A comparison of the different characteristic features associated with the AlN films deposited by pulsed laser deposition or magnetron sputtering is presented with particular emphasis to electronic and tribological applications.}, number={5}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A-VACUUM SURFACES AND FILMS}, author={Jagannadham, K and Sharma, AK and Wei, Q and Kalyanraman, R and Narayan, J}, year={1998}, pages={2804–2815} }