Combustion synthesis: mechanically induced nanostructured materials

Combustion synthesis (CS) is a specific approach for fabrication of a variety of advanced materials through use of self-sustaining chemical reactions. Controlling over the microstructure of the material is a key factor in defining the maturity of a technology. In this work, we demonstrate that under specific conditions, morphology and microstructure of the initial reaction media do not change during the CS process. Thus, one may control the microstructure of CS materials by preparing the desired structure of the initial reaction media. Specifically, using examples from several systems, which include intermetallics (NiAl), ceramics (SiC) and refractory carbides (TiC), we demonstrate that combination of short-term high-energy ball milling and CS allows precise control over the morphology and phase composition of product powders.     

Synthesis and characterization of nanostructured silicon carbide

Equiatomic nanostructured silicon carbide was successfully prepared by milling elemental Si and C powders, using a planetary ball mill. The synthesis of this carbide proceeded at milling conditions corresponding to 5.19 W/g shock power. The reaction was gradual and completed after 15 h. After 20 h of alloying duration and towards the end of the process, the SiC diffraction crystallite size (DCS) reached a critical value of 4 nm. At this same alloying duration, SEM characterization revealed that the powders exhibit homogeneous distribution of the particles with 0.3 µm in size.     

Microstructural study of titanium carbide coating on cemented carbide

Titanium carbide coating layers on cemented carbide substrates have been investigated by transmission electron microscopy. Microstructural variations within the typically 5m thick chemical vapour deposited TiC coatings were found to vary with deposit thickness such that a layer structure could be delineated. Close to the interface further microstructural inhomogeneities were obsered, there being a clear dependence of TiC deposition mechanism on the chemical and crystallographic nature of the upper layers of the multiphase substrate.     

A new one-dimensional tungsten carbide nanostructured material

A new medium specific surface area one-dimensional tungsten carbide nanostructure was obtained by the Shape Memory Synthesis method, in which the macrostructural features of a carbonaceous 1D-preformed template were maintained during the carburization of tungsten oxide and determined the resulting carbide morphology.     

利用电石渣制备针叶形碳酸钙的研究

为有效利用电石渣资源，以电石渣为原料合成了针叶形碳酸钙。利用XRD、SEM等研究了电石渣的预处理方式、溶液的pH值对碳酸钙纯度和白度的影响，以及碳化反应温度、添加剂对针叶形碳酸钙形成的影响。结果表明：电石渣在105℃下干燥2h的方式进行预处理，且控制碳化反应溶液的pH＞7时，可获得纯度＞97％、白度＞98的针叶形碳酸钙。碳化反应温度是影响针叶形碳酸钙形成的主要因素，合成针叶形碳酸钙的最佳碳化反应温度为80℃左右，其含量可达94％左右。添加剂MgCl2对针叶形碳酸钙晶体的长径比也有较大影响。利用工业副产物电石渣可以合成高纯度针叶形碳酸钙。     

The magnetic behaviour of nanostructured zinc ferrite

We have investigated a series of nanostructured ZnFe₂O₄ samples produced by mechanical activation (mean particle sizes d ～50-8 nm) by variable temperature neutron diffraction measurements (2-535 K) supported by DC magnetisation measurements (4.2-300 K). The systematic increase in the mean inversion parameter (c ～0.04-0.43) with increasing milling time is accompanied by a gradual decrease in the occurrence of the long range antiferromagnetic ordering observed in the starting ZnFe₂O₄ material, as well as a gradual decrease in the related diffuse short range order peak. The neutron diffraction patterns of particles with d < ～15 nm and c> ～0.2 are consistent with the occurrence of ferrimagnetic order and exchange interactions of the type Fe³⁺A—O^2?—Fe³⁺ [B]. Diagrams summarising the magnetic regions of nanostructured ZnFe₂O₄ are presented. The magnetic behaviour overall agrees well with the enhanced magnetisation and ferromagnetic behaviour reported for nanostructured, ultrafine and thin films of ZnFe₂O₄ by other groups using mainly magnetisation and Mössbauer spectroscopy measurements.     

Synthesis, characterization and annealing of mechanically alloyed nanostructured FeAl powder

Elemental powders of Fe and Al were mechanically alloyed using a high energy rate ball mill. A nanostructure disordered Fe(Al) solid solution was formed at an early stage. After 28 h of milling, it was found that the Fe(Al) solid solution was transformed into an ordered FeAl phase. During the entire ball milling process, the elemental phase co-existed with the alloyed phase. Ball milling was performed under toluene to minimise atmospheric contamination. Ball milled powders were subsequently annealed to induce more ordering. Phase transformation and structural changes during mechanical alloying (MEA) and subsequent annealing were investigated by X-ray diffraction (XRD). Scanning electron microscope (SEM) was employed to examine the morphology of the powders and to measure the powder particle size. Energy dispersive spectroscopy (EDS) was utilised to examine the composition of mechanically alloyed powder particles. XRD and EDS were also employed to examine the atmospheric and milling media contamination. Phase transformation at elevated temperatures was examined by differential scanning calorimeter (DSC). The crystallite size obtained after 28 h of milling time was around 18 nm. Ordering was characterised by small reduction in crystallite size while large reduction was observed during disordering. Micro hardness was influenced by Crystallite size and structural transformation.     

An antiemission coating based on zirconium carbide

The main requirement for the formation of an antiemission coating of intermetallic Pt₃Zr compound is the presence of a buffer layer of stoichiometric zirconium carbide (ZrC) that can be formed with the aid a vacuum-arc plasma source. It is shown that ZrC layer can be obtained through vacuum annealing of a multilayer film comprising nanolayers of zirconium (Zr), nonstoichiometric zirconium carbide (ZrC_{1 ? x} ), and zirconium carbide with excess carbon (ZrC + C) sequentially deposited from vacuum-arc-discharge plasma.     

Tribological evaluation of high-speed steels with a regulated carbide phase

Wear resistance of a commercial steel and titanium–niobium high-speed steels with a regulated carbide phase was evaluated by employing a micro-scale abrasive wear test with alumina particles. The worn volumes and corresponding wear coefficients were the lowest for the new non-ledeburitic grades containing titanium, then the two niobium grades, the conventional (both wrought and by powder metallurgy) steels exhibited the worse wear resistance. Fractography SEM observations together with energy-dispersive X-ray (EDX) chemical analysis revealed the decisive role of the steels' MC particles in the wear process. These carbides influenced the abrasion by stoppage of the wear scars and/or changing their trajectories. Directional and nondirectional abrasion modes in the steels tested using alumina and carborundum abrasives were found and are discussed.     

Tribological behaviour of proposed polymeric bearing materials

The aim of the present work is to introduce new polymeric bearing materials. The proposed polymeric composites are consisting of polyethylene (PE), polypropylene (PP) and polystyrene (PS) and filled by fibres of polytetrafluoroethylene (PTFE) in concentration up to 25 wt.% as well as different types of natural oils such as (corn oil, olive oil, paraffin oil, glycerin oil, castor oil and sunflower oil) in concentration up to 10 wt.%. The frictional behavior of the proposed composites and wear resistance are investigated at different values of applied load. Based on the experimental observations, it was found that for composites free of oil, friction of PS and PE specimens decreased, while friction of PP specimens slightly increased with increasing PTFE content. PP composites filled by corn oil showed slight friction increase. Besides, friction coefficient displayed by PS and PE specimens filled by glycerin oil decreased with increasing oil content, while friction coefficient displayed by PP specimens showed consistent trend. It was noted that, PE filled with 7.5% glycerin oil and 20 wt.% PTFE displayed the minimum value of friction coefficient (0.07). This friction coefficient values recommend those composites to be used as bearing materials. PE filled by glycerin oil displayed relatively lower friction values due its common known good lubricating property. Friction of PE composites filled by paraffin oil drastically decreased with the increasing oil content. PP composites showed the lowest wear values. In addition to that, it was shown that wear displayed by composites filled by glycerin oil was higher than that containing corn oil, while wear of the tested composites filled with olive oil showed lower values than that displayed by corn oil filled composites. Composites containing 5.0–7.5% paraffin oil content showed good wear resistance which recommends them to be used as bearing material. Wear of PP, PS and PE composites filled with sunflower oil and 15 wt.% PTFE drastically decreased with increasing oil content. The minimum wear was displayed by PP and PE composites filled with 10% oil content.     

Analysis of nanostructured coatings synthesized by ballistic impaction of nanoparticles

SiC nanostructured coatings were synthesized by ballistic impaction of nanoparticles using a process called hypersonic plasma particle deposition (HPPD). X-ray diffraction spectra of typical samples showed the presence of crystalline SiC and Si. Grain sizes obtained through transmission electron microscopy showed particles in the sub 10 nm range with primarily crystalline β-SiC and some crystalline Si particles present. These results correlate well with particle size distributions measured using an aerosol sampling probe coupled to a scanning electrical mobility spectrometer. Interestingly, particle size distributions indicated only small changes in the particle size distributions when Si deposition was compared to SiC. Examination of adhesion characteristics highlighted the importance of a chemically bound interlayer during SiC deposition on Mo and steel substrates.     

Structure and properties of bulk nanostructured alloys synthesized by flux-melting

Nanomaterials can easily be prepared as thin films and powders, but are much harder to prepare in bulk form. Nanostructured materials are prepared mainly by consolidation, electrodeposition, and deformation. These processing techniques have problems such as porosity, contamination, high cost, and limitations in refining the grain size. Since most bulk engineering metals are initially prepared by casting, we developed a casting technique, flux-melting and melt-solidification, to prepare bulk nanostructured alloys. The casting technique has such advantages as simplicity, low cost, and full density. In our method, Ag–Cu alloys were melted in B₂O₃ flux, which removed most of the impurities, mainly oxides, in the melts. Upon solidifying the melt at a relatively slow cooling rate on the order of 10¹–10² K/s a large undercooling of ∼0.25 T _m (where T _m is the melting temperature) was achieved. This large undercooling leads to the formation of bulk nanostructured Ag–Cu alloys composed of alternative Ag/Cu lamella and nanocrystals, both ∼50 nm in dimension. Our liquid-processed alloys are fully dense and relatively free from contamination. The nanostructured Ag–Cu alloys have similar yield strength in tension and in compression. The as-quenched alloys have yield strength of 400 MPa, ultimate tensile strength (UTS) of 550 MPa, and plastic elongation of ∼8%. The UTS was further increased to ∼830 MPa after the as-quenched alloy rod was cold drawn to a strain of ∼2. The nanostructured Ag–Cu alloys show a high electrical conductivity (∼80% that of International Annealed Copper Standard), a slight strain hardening (strain-hardening coefficient of 0.10), and a high thermal stability up to a reduced temperature of 2/3 T _m. Some of these behaviors are different than those found in previous bulk nanostructured materials synthesized by solid state methods, and are explained based on the unique nanostructures achieved by our flux-melting and melt-solidification technique.     

Single crystalline silicon carbide nanorods synthesized by hydrothermal method

A new and simple hydrothermal process has been used to synthesize single crystalline silicon carbide nanorods. The synthesized silicon carbide nanorods have a length of about 1 μm and nearly same diameter of about 40 nm. We have observed that the nanorods possess a well-defined single crystalline structure with a thin layer of amorphous silica on the surface. The X-ray diffraction analysis demonstrates that the structure of silicon carbide nanorods is β-SiC. Raman shifts of the silicon carbide nanorods are discussed and the oxide-assisted growth mechanism is proposed to explain the formation and growth of silicon carbide nanorods.     

化学气相反应制备碳化硅纳米线

用硅粉、二氧化硅和石墨粉作原料，在无催化剂的条件下，在1400℃下用高温化学气相反应法制备了碳化硅纳米线，并用高分辨扫描电镜观察了所得碳化硅纳米线的形貌。所得碳化硅纳米线直径为100-500nm，长度可达几百微米。还提出了描述碳化硅纳米线的生长机理。     

环氧基倍半硅氧烷杂化膜的摩擦性能研究

采用溶胶-凝胶法,将不同含量的正硅酸乙酯(TEOS)与r-缩水甘油醚基丙基三甲基硅烷(GPMS)共水解缩合,所得终产物为有机-无机纳米杂化材料,用浸渍法使其在玻璃基体上成膜,并利用扫描探针电镜(SPM)对膜的形貌进行了表征.通过MTS Nano Indenter XP纳米压痕仪研究了TEOS的含量对杂化体系摩擦性能的影响.结果表明,环氧基倍半硅氧烷杂化膜在TEOS含量为5%时表面粗糙度最小、弹性恢复能力最大,TEOS含量为10%时摩擦系数最小.     

Laser in-situ synthesis of mixed carbide coating on steel

A 2.5 KW Nd:YAG laser was employed to modify the surface of a AISI 1010 steel deposited with a precursor powder mixture of Fe, Ti, Cr and C. In-situ formation of TiC and chromium carbides [M₇C₃ (M = Fe, Cr) and Cr₇C₃] was observed as function of laser processing power at constant scan speed. Although TiC was present in all the samples, the chromium carbides were absent in samples processed at certain laser powers. Corresponding to this behavior, variation in mechanical properties of the coating was observed. The hardness and wear properties of the samples without chromium carbides was inferior in comparison to samples with both TiC and chromium carbides.     

Deformation behaviour of single crystals of titanium carbide

Single crystals of titanium carbide were deformed in compression over a wide range of temperature, and the operative slip systems were determined by etch-pitting and electron microscopy. Around the brittle-ductile transition temperature, the slip system undergoes a gradual change from {1 1 0} 1 ¯1 O to {1 1 1} 1 ¯1 0; this is interpreted to be the mechanism governing the brittle-ductile transition in titanium carbide.On leave from Defense Metallurgical Research Laboratories, Hyderabad 500258, India.     

Microstructure evolution of Fe-based nanostructured bainite coating by laser cladding

A Fe-based coating with nano-scale bainitic microstructure was fabricated using laser cladding and subsequent isothermal heat treatment. The microstructure of the coating was observed and analyzed using optical microscope (OM), field-emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). The results showed that nanostructured bainitic ferrite and carbon-enriched retained austenite distributed uniformly in the coating. Blocky retained austenite was confined to the prior austenite grain boundaries resulting from the elements segregation. The bainitic microstructure obtained at 250 °C had a finer scale compared with that obtained at 300 °C. The volume fraction of austenite increased with increasing transformation temperature for the fully transformed bainitic coating. The bainitic transformation was accelerated as a result of the fine prior austenite generated during the laser cladding. The evolution of the carbon contents in bainitic ferrite and retained austenite revealed the diffusionless mechanism of the bainitic transformation.     

Corrosion behaviour of squeeze-cast aluminium alloy-silicon carbide composites

The corrosion behaviour of squeeze-cast Al alloy (LM11) separately dispersed with 10 vol% SiC fibres and SiC particles was investigated in 3% aqueous NaCl solution by general corrosion as well as potentiodynamic polarization techniques. Erosion-corrosion tests were also performed on the specimens in the solution. The base alloy was also subjected to identical tests to examine the influence of the presence of SiC in the matrix. The base alloy showed a lower corrosion rate than the composites. Furthermore, the alloy containing SiC fibres showed a higher corrosion rate than the one with SiC particle dispersion. Erosioncorrosion tests indicated that the rate of material loss followed a trend similar to that in other corrosion tests. The material loss was significantly higher in the case of erosion-corrosion tests. In addition to pitting and attack at the CuAl₂ precipitate-Al interface in the matrix, dispersoid-matrix interfacial attack by the corrosion medium was also observed in the case of composites. On the other hand, erosion-corrosion revealed occasional partial removal of the dispersoid due to the impingement of the electrolyte. The tendency of the dispersoid removal by the impinging electrolyte was predominantly more in the case of the composites dispersed with SiC fibres. Results are explained in terms of the interfacial bonding as well as the shape of the dispersoid.