STRUCTURE AND MECHANICAL AND TRIBOLOGICAL PROPERTIES OF MAGNETRON SPUTTER DEPOSITION OF STAINLES-STEEL NITRIDE AND CARBIDE THIN FILMS WITH TRANSITION METAL ADDITIVES

Since the initial discovery of the S-phase in 1985, understanding the structural nature of this phase and the anomalous shift of the (200) diffraction peaks has been a challenging problem. Austenitic stainless steels, ternary Fe–Cr–Ni alloys, like AISI 304, demonstrate excellent corrosion resistance and relatively good levels of toughness and strength. For this reason, they are widely used engineering materials in areas such as aerospace, construction buildings, piping, telecommunications, chemical and petrochemical applications. However, stainless steels have a relatively low hardness, and this leads to a poor wear resistance, resulting in a short lifetime that limits its use in industrial applications. Therefore, surface treatment methods have been developed to improve its mechanical properties without loss of corrosion resistance. Surface hardening of stainless steels can be accomplished using a combination of nitrogen implantation and diffusion to create a hardened surface layer. The incorporation of nitrogen into stainless steels by these techniques results in expansion of the fcc (austenite) lattice; this phase is referred to as “expanded austenite,’’ or the “S-phase’’. A notable feature of the S-phase is the displacement of the (200) reflection from its expected position. The reactive magnetron sputtering process has been used to deposit thin films of nitrogen-supersaturated stainless steels. In addition, new hybrid coatings were studied by combining stainless steel targets with other transition metals, as well as carbon, in the deposition process. A variety of advanced characterization methods were used to examine the structural, compositional, mechanical and tribological properties of these films. These techniques include x-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM-EDX), transmission electron microscopy (TEM), micro-hardness (Knoop indenter), nano-Indentation, and both pin-on-disk and optical microscopy tests for tribological evaluations. In addition, the structural nature of the films was further examined using area-detector based x-ray diffractometry. Using 304 stainless steel sputtering targets, films were deposited in a mixed Ar/N atmosphere using a variety of Ar/N ratios, as well as parametric variations in substrate bias and temperatures and sputter gun power ratios. XPS analysis showed nitrogen supersaturation levels near 40 at.% in these films. X-ray diffraction analysis showed the structures of the films were strongly temperature dependent: above 450 °C, the films were a mixture of CrN, bcc-Fe, and Ni; below 450 °C, the films were nominally fcc-structured. However, the common anomalous deviation in the position of the (200) reflection was observed, indicating the presence of the S-phase. Area-detector based X-ray diffraction studies, which allowed peak position measurements as a function of the inclination of the diffraction vector (angle ψ), showed a200 declined with increasing ψ, but always remained greater than a111, which was relatively constant with ψ. Hardness was measured and also found to be a strong function of substrate temperature, with the highest hardness of 2100 kg/mm2 obtained for films deposited at room temperature. SEM and TEM cross-section samples showed uncommon morphological features which provided insight into the structural nature of the S-phase. Hybrid stainless steel /titanium nitride (SS-Ti-N) films, as well as a hybrid stainless steel/chromium nitride (SS-Cr-N) coatings were investigated and showed superior mechanical properties that may be promising new coatings. The S-phase was also produced in these hybrids coatings. In the SS-Ti-N, titanium concentrations of up to ~14 at.% were obtained, in which case the nitrogen levels were near stoichiometric (50 at.%N). Hardness levels of 18-24 GPa (~1800-2500 Kg/mm2) were obtained for the films that had titanium concentrations between 10-14 at.%. These S-phase films made by co-sputteirng from both stainless steel and titanium targets could increase the hardness by nearly 100% compared to films made with only stainless steel. A tribological analysis of the films was conducted using a pin-on-disk test with an alumina ball, and the optimal results were obtained on a SS-Ti-N film deposited at 150oC/ -140V, where the average friction coefficient was 0.39. It should be noted that the average of regular stainless steel is 0.6 For the SS-Cr-N films, chromium concentrations of up to 54% were obtained and showed a maximum hardness of ~ 4639.8 Kg/mm2 for a film deposited at 250C and -140V. These films tend to have a nitrogen concentration of ~ 40%. The S-phase was formed in these coatings and the (200) peak also shifted from expected positions. The friction coefficient of the SS-Cr-N coated films was examined and showed an improved friction coefficient (0.41) at film deposited at 150C. Further studies of N-supersaturated films deposited stainless steel and stainless steel co-sputtered with titanium were conducted to better understand the structural nature of the S-phase. In order to quantify the peak shift in these films, a term denoted the “R-value” was used, which for an FCC structure is given by: (1) An R-value of 0.75 is expected for normal fcc structures; a value of R\u3e0.75 indicates the presence of the S-phase. The effect of nitrogen and titanium concentrations, substrate temperature and the morphology on R-value was investigated. R-values were generally \u3e 0.75, indicating a deviation from the common fcc structure. The samples with R closest to 0.75 were films with higher titanium levels (10-14 at.% Ti), and these films had stoichiometric nitrogen concentration levels (~50 at.% N). Also, films that have a nitrogen content of 30-43% do not show a consistent relationship to high or low R-values. SEM cross-section of the S-phase films deposited at lower bias showed a layered or ribbed morphology in the coarse columns. TEM images revealed a central spine and branched structure in films deposited at 150C and 250 oC, with fewer branches at 350C. Additionally, increasing the substrate temperature from 150 to 350 oC led to a decrease in the R-value (from 0.802 to 0.779) made the films denser. The effect on the peak shift (2) calculated and the shift was 0.022o, however, this number was far from the value of 2 measured from our XRD data. It was concluded that the observed layered morphology does not explain the measured R-values. Films of stainless steel/carbon were also deposited by co-sputtering. This was done because carbon offers another way to make an alternative version of S-phase using carbon instead of nitrogen. These films maintained S-phase structure when deposited below 450oC. Carbon concentrations near 50% were obtained in several cases, and the hardness of these films reached a maximum value of 2256 Kgf/mm2 at a deposition temperature of 250oC. In comparison to SSN, SSC has an improved hardness

Corporate governance in Gulf Cooperation Council (GCC) countries : ownership structure, executive compensation and firm performance

Gulf Cooperation Council (GCC) countries showed a remarkable economic growth in the past decade due to being a major oil exporter, attracting significant foreign investment, becoming a major global trading partner and integrating further to the global economy. These developments have resulted in the need to institute advanced corporate governance norms for GCC companies. However, corporate governance systems and practices in the GCC countries are of recent origin and, albeit the importance of corporate governance practices in transforming GCC economies, there is a dearth of academic research on corporate governance in GCC countries. Our motivation for this research is the increasing importance of GCC economies within the world economy and the lack of research on corporate governance mechanisms in these countries. We believe we make a significant contribution to the existing research by providing comprehensive empirical insights into different aspects of corporate governance practices and firm performance in the GCC countries. In particular, our focus is on how corporate performance is affected by ownership structure and executive compensation. We test these relationships using a uniquely constructed and hand collected data of 349 listed companies in Kuwait, Saudi Arabia, Oman, Qatar and United Arab Emirates for the years between 2006 and 2011.We start by formulating a theoretical base for the study through a discussion on how major theories of corporate governance evolved over time and reviewing the state of empirical research in corporate governance in general as well as in the context of the GCC region. We find that the corporate governance practices and standards prevailing in the GCC countries are not comparable to those followed in other developed countries because of many challenges identified by previous research. We also find that there is a large scope for the implementation of globally comparable corporate governance practices in the GCC countries.In our first empirical analysis, we investigate the impact of ownership structure on firm performance. The issue as to whether ownership structure matters for the performance of firms has been an important subject of debate in the corporate finance literature. Our objective here is to explore in more detail the factors that motivate particular types of ownership structure and the potential impact of ownership structure and firm performance in the Gulf region. We find that higher insider ownership leads to better performance, especially when insider owner is the Chairman. A larger ownership by institutional investors has a positive impact on firm performance. On the other hand, we find that GCC firms with high family ownership perform does not perform well.In our second empirical analysis, we examine the key determinants of the executive compensation to top five executives in GCC firms. We find that larger firms and firms with potential future growth pay higher total compensation to their executives. Concentrated ownership structure leads to lower compensation levels while firms with high family ownership as well as external ownership tend to pay higher compensation. We also find that managerial as well as institutional ownership of companies tends to lead to higher salaries. Companies that have more growth potential seem to pay higher bonuses. We also find that the choice between behaviour versus outcome oriented compensation is mainly influenced by firm size, the presence of executive members on the board, and managerial ownership. Larger companies prefer to pay bonus as part of executive compensation. Companies that have higher number of executives or owned by managers are also tend to pay more compensation through bonuses.In our third empirical analysis, we examine the relationship between executive compensation and firm performance. We find that higher total executive compensation leads to better firm performance. On the other hand, we do not find any significant relationship between the components of compensation (salary and bonus) and performance. We point out that specific GCC economic environment and features peculiar to this region has more impact on the firm performance rather than the executive compensation.Overall, findings of our study is valuable in assisting key decision-makers, such as the shareholders or policy makers, in enhancing firm performance through diversifying their ownership structures and utilising the right policies in compensation of executives in GCC countries

High-Speed Indoor Optical Wireless Links Employing Fast Angle and Power Adaptive Computer-Generated Holograms With Imaging Receivers

In this paper, we introduce an adaptive optical wireless system that employs a finite vocabulary of stored holograms. We propose a fast adaptation approach based on a divide and conquer methodology resulting in a number of adaptation algorithms: fast angle adaptive holograms (FAA-Holograms), fast power adaptive holograms (FPA-Holograms), and fast angle and power adaptive holograms (FAPA-Holograms) and evaluate these in mobile optical wireless (OW) systems in conjugation with imaging reception. The ultimate goal is to improve the signal-to-noise ratio (SNR), to reduce the effect of intersymbol-interference (ISI), to speed up the adaptation process, and to eliminate the need to calculate the hologram in real-time at each transmitter and receiver location. The system operates at high data rates under the impact of multipath dispersion, background noise and mobility. At a data rate of 2.5 Gb/s and under eye safety regulations, the proposed FAPA-Holograms offers around 20 dB SNR in the presence of background shot noise, receiver noise, multipath dispersion, and mobility. Simulation results show that the proposed system, FAPA-Holograms, can reduce the time required to identify the optimum hologram position from 80 ms in the original beam angle and power adaptive line strip multibeam system (APA-LSMS) to about 13 ms

Hybrid diffuse IR transmitter supporting VLC systems with imaging receivers

Indoor visible light communication (VLC), using white-LED lighting, generally assumes lights are ON during communications. In this paper, we propose a new Hybrid diffuse Infrared transmitter (HDIrT) coupled with an imaging receiver to support VLC system when the light is dimmed or is totally turned off. Infrared (IR) optical communications boasts similar advantages as VLC systems. It can also provide high transmission rates. The ultimate goal of our proposed system is to increase the signal to noise ratio (SNR), mitigate the channel delay spread and the effect of inter-symbol-interference (ISI) when the system operates at a high data transmission rate. The delay spread is reduced from 1.55 ns to about 0.1 ns when a narrow field-of-view (FOV) imaging receiver replaces a wide FOV non-imaging receiver. At a higher data rate of 2.5 Gb/s, the simulation results show that the imaging HDIrT system achieves about 17 dB SNR in the presence of multipath dispersion, receiver noise and mobility

A Simple User Grouping and Pairing Scheme for Non-Orthogonal Multiple Access in VLC System

In this paper, a simple user grouping and pairing scheme is proposed for non-orthogonal multiple access (NOMA) and is applied for the downlink visible light communication (VLC) system. The proposed scheme is a mix of both NOMA and the conventional orthogonal multiple access (OMA) schemes. In the proposed scheme, every two users are paired using NOMA and all pairs are allocated with conventional OMA. The performance of the proposed scheme is compared to the performance of the conventional OMA in terms of the maximum sum rate. Simulation results show that the proposed scheme provides higher sum rate compared with the OMA scheme