Nanocrystalline high melting point compound-based materials

The different preparation methods of ultrafine powders of high melting point compounds (carbides, borides, nitrides, and oxides with melting temperatures higher than 2000C) are reviewed. Some properties of these powders are discussed and compared. The consolidation behaviour of these compounds in the nanocrystalline (nc) state is described in detail. Compaction by hot pressing, including high pressures and high temperatures, sintering, and high-energy consolidation methods, is analysed. The microstructure, recrystallization, mechanical and physical properties of nc-carbides, nitrides, and oxides are characterized. Special attention is focused on relationships between structure and properties.     

Search for superhard materials: Nanocrystalline composites with hardness exceeding 50 GPa

The search for superhard materials with Vickers hardness H ≥ 40 GPa (about 4000 kg/mm²) concentrates mainly on polycrystalline diamond, cubic c-BN and C₃N₄ or the substoichiometric CN_x. This approach is based on the theoretical strength which is proportional to the bulk modulus. However, the practically achievable strength (and hardness) of engineering materials is two to four orders of magnitude smaller because their failure occurs due to flaws, and it is determined by their microstructure. Therefore, an alternative approach deals with the design of materials with an appropriate microstructure, such as heterostructures.Recently, we have developed new superhard nanocrystalline composites $\text{nc-Me}{}_{\mathrm{n}}\text{N}\text{a-Si}{}_3\text{N}{}_4$ (Me = Ti, W; V,…). These materials consist of ≤ 4 nm small nanocrystals of a hard transition metal nitride embedded into < 1 nm thin matrix of amorphous silicon nitride. Unlike pure nanocrystalline metals and the heterostructures which show softening when the crystallite size or lattice period decreases below 5–6 nm, the hardness of our composites strongly increases with decreasing crystallite size in that range and approaches the hardness of diamond. In this paper we shall briefly summarize the concept for the design of these materials and experimental results achieved so far. New results to be reported concern the surprising structural stability of these composites and a discussion of the possible origin of the superhardness.     

Nanocrystalline materials for high temperature soft magnetic applications: A current prospectus

Conventional physical metallurgy approaches to improve soft ferromagnetic properties involve tailoring chemistry and optimizing microstructure. Alloy design involves consideration of induction and Curie temperatures. Significant in the tailoring of microstructure is the recognition that the coercivity, (H _c) is roughly inversely proportional to the grain size (D _g) for grain sizes exceeding ∼0·1−1 μm (where the grain size exceeds the Bloch wall thickness,δ). In such cases grain boundaries act as impediments to domain wall motion, and thus fine-grained materials are usually harder than large-grained materials. Significant recent development in the understanding of magnetic coercivity mechanisms have led to the realization that for very small grain sizesD _g<∼100 nm,H _c decreases sharply with decreasing grain size. This can be rationalized by the extension of random anisotropy models that were first suggested to explain the magnetic softness of transition-metal-based amorphous alloys. This important concept suggests that nanocrystalline and amorphous alloys have significant potential as soft magnetic materials. In this paper we have discussed routes to produce interesting nanocrystalline magnets. These include plasma (arc) production followed by compaction and primary crystallization of metallic glasses. A new class of nanocrystalline magnetic materials, HITPERM, having high permeabilities at high temperatures have also been discussed.     

Nanocrystalline ZrO2 thin films as electrode materials using in high temperature–pressure chemical sensors

The nature of the ZrO₂ thin films in the Zr/ZrO₂ electrode plays a vital role to build Zr/ZrO₂ high temperature–pressure chemical sensors, when the electrode is utilized to be in situ measuring electro-chemical properties of hydrothermal fluids. Zr metal was oxidized with a NaCO₃ melt to form a thin film of ZrO₂ on the surface of Zr, then an oxidation–reduction electrode was performed. The electro-chemical properties of the Zr/ZrO₂ electrode were tested from room temperature to high temperatures and high pressures. By using SEM, EPMA and HRTEM, analyses of topography, chemical composition and structure of the ZrO₂ thin films revealed that Zr/ZrO₂ interface is divided in to 5 zones from the outmost zone to the center: 1) prismatic and oxygen-rich ZrO₂, 2) ZrO₂; 3) oxygen-rich Zr; 4) oxygen-bearing Zr; 5) Zr metal. Especially, the outmost oxygen-rich ZrO₂ zone of the thin films is composed of nanometer-sized monoclinic crystals with high oxygen content, when Zr/ZrO₂ electrode is conducted to make a good high temperature chemical sensor.     

电沉积法制备纳米晶材料的研究进展

综述了纳米晶的特点，纳米晶材料电沉积制备的原理和方法，介绍了电沉积纳米晶镍及镍基材料的硬度、拉伸性能、应力、耐磨、耐蚀性及热稳定性等性能研究及其应用现状。认为100μm以下的纳米晶电沉积层的高耐蚀耐磨性在汽车发动机、液压活塞等零部件上将会进一步应用，纳米晶镀层的热稳定性还需改善。     

Nanocrystalline Fe-P-Si alloys

Annealing Fe-P-Si amorphous alloys was found to produce nanocrystalline particles and raise the microhardness of the alloys by a factor of 2 to 3. The most significant strengthening was observed in the alloys containing the smallest amounts of Si and P and the largest amount of the α-Fe-based phase. As shown by x-ray diffraction and electron microscopy, the alloy consisting entirely of nanocrystalline phases with a particle size of about 25 nm crystallizes in three steps.