Papers by Keyword: Nitride

Abstract: TiB2 coatings were plasma sprayed in air and were studied by XPS. There are five elements in the top surface of the studied coatings, namely, B, C, N, O and Ti. Oxygen pick-up in the coatings results in formation of oxides of boron and titanium. Nitrogen was shown to exist in N-Ti and O-N-Ti in the coating. Depending on the spraying parameters, mono-boride as well as di-boride was also detected in the studied coatings. With careful peak fitting it was shown that oxygen may exist in the coating as dissolved atomic O in addition to as lattice oxygen in the oxide of boron and titanium.

Abstract: The cubic γ-Si3N4 phase was synthesized by the shock technique from the hexagonal β-Si3N4 phase. The thermal stability of the γ-Si3N4 was investigated during heating in vacuum up to 1773 K. An exothermal heat effect was found at 1690 K and structural investigations revealed complete transformation of γ-Si3N4 to β-Si3N4. Corresponding heat effect value was estimated as 51.3±7.7 KJ/mol. The high-pressure-high-temperature treatment (P=13 GPa, T=1300-2300 K) was applied to γ-Si3N4 to make bulk polycrystalline non-porous samples. It was found that temperatures below 1623 K do not change content of the cubic γ-Si3N4 while temperatures above 2273 K decrease it substantially. Mechanical properties of these bulk samples were measured by acoustic wave and nanoindentation techniques. The maximum values belong to cubic γ-Si3N4: hardness 39-44 GPa, Young’s modulus 475 Gpa and bulk modulus 263 GPa. Equilibrium γ-β line position in P-T phase diagram was estimated by using data obtained in this work. The equilibrium pressure at T=300 K was estimated as P300=7.0±2.0 GPa.

Abstract: A new nitriding method has been devised which requires only a simple vacuum furnace and enables direct nitridation of solid aluminium without any prior surface treatment. It can be used to produce thick aluminium nitride surface layers on aluminium, under nitrogen at atmospheric pressure. A critical element of the process is the use of a magnesium vapour source that reduces/disrupts the natural, protective oxide film on the aluminium surface and facilitates nitriding. The nitride surface layers form through two distinct modes, one growing outward from the aluminium plate surface and the other growing into the aluminium. Studies of the nitride layers utilizing optical microscopy, TEM, SEM, XRD and XPS have been conducted. Details of the composition, structure and growth as well as possible mechanisms for the nitride formation are presented. Understanding of the reaction may have important implications for the production of wear resistant coatings on bulk Al as well as for the production of Al/AlN composites.

Abstract: Self-propagating high temperature (combustion) synthesis (SHS) is being used to develop several synthesis and processing routes for the next generation of ceramic nuclear fuels. These fuels are based on an actinide nitride within an inert matrix. The application of SHS is particularly important in the synthesis of americium (Am) based ceramics; since the rapid heating and cooling cycles used in this process will help to minimize vaporization loss of Am, which is a major problem in synthesizing Am-based ceramics. Manganese, praseodymium, and dysprosium are being used as physical and chemical surrogates for various actinides. Actinide nitride powders produced using auto-ignition combustion synthesis (AICS) are subsequently reacted with zirconium powder using SHS to produce a final fuel pellet. This paper will discuss the research to date on the synthesis of Am-N powders as well as the production of dense Zr-Am-N pellets as a model ceramic fuel system.

Abstract: In this study, α-Si3N4 powder was produced by carbothermal reduction and nitridation (CRN) of quartz from Can-Canakkale. Carbon with a specific surface area of 110 m2g−1 and quartz powders were mixed then the powder mix was placed in an alumina tube furnace and reacted in between 1300-1500°C for 4 hours under nitrogen flow. The quartz powder was carbothermally reduced and nitrided to form silicon nitride powders. XRD results showed that the reaction product was mainly α-Si3N4 and contained some β-Si3N4 and residual quartz. In order to reduce amount of unreacted quartz, the raw materials mixture was grinded either with carbon black or with no carbon. After CRN reactions of separate grinded quartz powders with carbon, residual quartz was disappeared, reaction temperature was decreased and α-Si3N4 rate was increased. Hence, a better mixing of carbon and fine silica enhanced the α phase formation. SEM images and XRD pattern showed that sub micron particles (0.6–0.87m), high α-phase content Si3N4 powders can be produced at 1450°C for 4 h in flowing nitrogen gas during the CRN process.

Abstract: A frequent criticism of nitride materials during the last 30 years, and especially those designed for structural applications has been that the cost is too high by a factor of (say) 10. In the competition with cheaper materials (albeit with poorer properties and shorter lifetimes), users have generally preferred to go for the cheaper option, rather than the more expensive nitrides. Despite many attempts to address this issue, the cost of nitride processing has remained high – due to the high price of starting materials, the high temperatures needed for firing, and also the finishing costs (often involving diamond machining), and this has been a major factor limiting the market share enjoyed by these materials. A number of studies have been reported recently using the technique of mechanochemical synthesis, in which nitrogen is incorporated (usually via ammonia) into the starting powders during a high-energy milling process (at room temperature). In the subsequent firing, considerably lower temperatures are needed to produce the resulting final nitride product(s). In this presentation, the technique of mechanochemical synthesis is discussed, the range of materials that have been produced are reviewed, and the potential of this technique for reducing the cost of bulk nitride production is reviewed.

Abstract: This paper was designed to assess the adhesive properties of hard coatings made by physical vapor depositions on various substrates (AISI D2, AISI H-13 and M2) with and without an intermediate nitrided layer. An estimation of adhesion was carried out using the scratch test, where adhesion is measured by the critical load (Lc). This value was determined as the normal force affecting the indenter and causing the coating detachment as well as the acoustic emission signal containing the information on the extent of coating damage. The scratch track after the scratch test was also examined with an optical microscope to observe the failure modes of each coating. Hard coatings TiN, CrN and TiAlN were chosen for this study. Results of the test showed that harder substrates and coatings give higher values of critical loads.

Abstract: Composite nanoparticles of Ni-TiC and Ni-TiN were prepared by an active plasma-metal reaction method. The structure and morphology were evaluated by X-ray diffraction and transmission electron microscopy observations. The morphology of the composite particles is dice-like or dumbbell-like, where the outer sides are metallic and the inner part of the rod (or dice)-like structure is TiC or TiN. The formation mechanism of the composite particles is considered by analogy to the VSL mechanism. The thermal stability of the nanocomposite particles is vastly superior to that of the metal particle. The excellent catalytic property of the Ni-TiN composite particle was confirmed when compared to the well-known Raney Ni particle and mixed particles of Ni and TiC.

Abstract: The nickel-free austenitic stainless steel produced by solution nitriding (Fe-25%Cr-1%N alloy) was subjected to isothermal heat treatment, and then the microstructure formed through the decomposition of austenite was investigated in terms of the morphology of eutectoid structure and the size of eutectoid block. On the isothermal heat treatment at 873K~1223K for the solution-nitrided steel, the austenite decomposed to eutectoid structure composed of ferrite and Cr2N nitride. This transformation could be completely finished after long time heat treatment in the above temperature range. The nose temperature of T.T.T. curve was around 1173K, and the time to start the eutectoid transformation was only 100~200s. The eutectoid structure was formed mainly along austenite grain boundaries and then grew into the untransformed austenite region. Finally, the austenite was completely decomposed into ferrite and Cr2N nitride. As a result of OIM observation for the specimen after isothermal heat treatment, the eutectoid structure was found to be divided into small-sized ferrite blocks, in which lamellar Cr2N plates were finely distributed. The block size and the mean ferrite path of eutectoid structure were decreased with lowering the heat treatment temperature. In the 873K heat-treated material, these values were estimated at 20 microns and 0.1 microns, respectively.