Synthesis of amorphous carbon nitride using reactive ion beam sputtering deposition with grazing bombardment

Amorphous carbon nitride (a-C:N) thin films were synthesised on steel substrates using reactive ion beam sputtering deposition (RIBSD). A single ion beam is arranged to sputter the graphite target at 75° incidence and concurrently bombard the growing film at grazing incidence angles of the ion beam. Nanoindentation, Raman spectroscopy, FTIR, FT-Raman and XPS were employed to characterise the mechanical and structural properties of the films. It was found that grazing incident bombardment has a significant effect on film structure through an increase in nitrogen content and formation of nitrogen doped structure.     

A novel high entropy CoFeCrNiCu alloy filler to braze SiC ceramics

In order to reduce intermetallic compound formations in brazed joints, a CoFeCrNiCu high entropy alloy was invented and employed to braze SiC ceramics. Results show that SiC ceramics were tightly and strongly brazed with the CoFeCrNiCu filler. Microstructure, phase and shear strength were systematically studied for joints brazed at different temperature. Main compositions were identified as high-entropy FCC, Cu(s, s), Si(s, s), and Cr₂₃C₆ phases, regardless the brazing temperature differences. After being brazed at 1453 K, the joint reached a maximum shear strength of 60 MPa, much higher than those brazed with conventional AgCuTi filler. Thanks to high entropy effect of CoFeCrNiCu filler, random solid solution turned out in the seam and benefitted joint quality. The successful use of CoFeCrNiCu high entropy alloy as fillers can expand the application range of high entropy alloys and provide a new filler system to braze ceramics.     

Tailoring magnetic anisotropy gradients by ion bombardment for domain wall positioning in magnetic multilayers with perpendicular anisotropy

Graded anisotropy magnetic materials possess a coercive field changing laterally with position. A simple fabrication procedure to produce such an anisotropy gradient in a polycrystalline Au/Co layer system without lateral thickness variation and with perpendicular magnetic anisotropy, prototypical for a large variety of thin film systems, is shown. The procedure uses light-ion bombardment without the use of a mask. Magnetization reversal in this polycrystalline layer system takes place by unidirectional movement of a single domain wall only in regions with larger anisotropies and anisotropy gradients. In this anisotropy/anisotropy gradient regime, the domain wall is oriented perpendicular to the coercive field gradient, and it can be positioned along the gradient by an appropriate magnetic field pulse. For smaller anisotropies/anisotropy gradients, the natural anisotropy fluctuations of the polycrystalline layer system induce magnetization reversal dominated by domain nucleation.

PACS

75.30.Gw; 75.70.Cn; 75.60.Ch     

Raman spectroscopy studies of the high-temperature evolution of the free carbon phase in polycarbosilane derived SiC ceramics

The Raman spectra of a number of SiC ceramics synthesized from polycarbosilane at 1200 °C and annealed at 1400, 1600, 1800 and 2000 °C have been recorded using laser excitation wavelength of 532 nm. The peak positions, their intensities (I_D/I_G) and full width at half maximum (FWHM) were used to obtain information about the degree of disorder in the free carbon phases. The increasing ordering with annealing temperature was confirmed by lower FWHM values and G-peak positions obtained from the SiC ceramics annealed at higher temperature. However, the I_D/I_G has shown to be the highest point at 1600 °C, which illustrates that the temperature is one critical point of the microstructure evolution of the free carbon phase changing amorphous to turbostratic with increasing temperatures. Obviously, the oxidation behaviors of the SiC ceramics are significantly affected by the microstructures of the free carbon phases. In the SiC ceramics with above 1600 °C annealing, the oxidation temperatures of the SiC phases are postponed more than 100 °C, because they are surrounded by the free carbon phases.     

钎料合金对钎焊金刚石界面性能的影响

在钎焊金刚石工具制造过程中,金刚石/钎料/基体之间的界面反应产物和微观结构决定了钎料钎焊金刚石与基体之间的结合强度.因此,对界面反应产物及组织微观结构进行研究,进而探讨钎焊工艺的优化十分重要.文章对钎料与基体界面显微组织形貌,高频感应钎焊界面微观,钎焊层的微观组织,钎焊金刚石微结构,钎焊接头界面组织,激光钎焊界面微结构...     

Effect of Zr addition on the interfacial reaction of the SiC joint brazed by Inconel 625 powder filler

Ni-based alloys are believed to be the most suitable brazing fillers for SiC ceramic application in a nuclear environment. However, graphite, which severely deteriorates the mechanical property of the joint, is inevitable when Ni reacts with SiC. In this paper, Different amounts of Zr powders are mixed with Inconel 625 powders to braze SiC at 1400 °C. When Zr addition reaches 40 wt%, the brazed seam confirms the absence of graphite. This research proves that Zr can avoid the graphite’s formation by suppressing Ni’s activity. The room-temperature shear strength of the joint with graphite’s absence is tested to be 81.97 MPa, which is almost three times higher than that of the joint with graphite. The interfacial reaction process and mechanism of the SiC joint are investigated and explained in this paper using thermodynamic calculations.     

Tribological behaviour of transparent ceramics: A review

Owing to superior properties, i.e. high hardness, high wear resistance, and weight reduction of transparent ceramics (TCs) over glasses, TCs have shown promising tribological potential for applications such as face shields, explosive ordnance visors, windows for aircraft, spacecraft and, re-entry vehicles, electromagnetic windows, laser igniter windows, screens for smartphones and more. Researchers globally have been attracted to explore more about TCs, considering the tremendously increasing demand over different other transparent materials. The optical quality of TCs is mostly characterized by the in-line transmittance, and the effect of various processing parameters on transmittance has already been studied by various researchers. In this review, the current research progress regarding tribological performance of TCs is compiled. TCs with potential in tribological applications include MgAl₂O_4, Al₂O₃, AlON, Lu₂O₃, c-BN, Y₂O₃, Si₃N₄, and SiAlON. The relevant strategies to improve the tribological properties, including microstructures and mechanical properties are comprehensively discussed. In addition, the mechanisms of material removal of different transparent ceramics are also presented. It is well observed that surface fracture comprising three stages is found as one of the dominant wear mechanisms during wear. This review aims to provide some meaningful guidelines for development of transparent ceramics with enhanced wear resistance, while identifying the wear mechanisms in particular wear conditions.     

Brazing SiC ceramics and Zr with CoCrFeNiCuSn high entropy alloy

CoCrFeNiCuSn high-entropy alloy and Cu foam composite interlayer was used as a filler for the brazing of SiC ceramics and Zr. The microstructure and mechanical properties of the brazed joint at room temperature and high temperatures as well as the brazing mechanism were systematically investigated. The microstructure is adjusted by controlling the brazing temperature. The main phases in the joint were identified at different brazing temperatures to be a high-entropy alloy phase, α-Zr (s, s), Zr₂Cu, (Zr, Sn) and Zr(Cr, Fe)₂ Laves phase. The joint brazed at 1040 °C for 20 min exhibited a maximum shear strength of 221 MPa at room temperature and an average shear strength of 207 MPa at 600 °C. The room temperature and high temperature-strength obtained here are much higher than those obtained for joints brazed using a conventional filler. Owing to the high-entropy effect, the joint matrix is mainly composed of solid solution phase, which improves the strength and thermal stability of the joint. The existence of the hard Zr(Fe, Cr)₂ Laves phase and the soft α-Zr (s, s) phase in the joint significantly improves its strength and plasticity.     

Fabrication of modified ultra high-temperature ceramic hybrid powders using in situ grown SiC nanowires

Ultra-high temperature ceramic (UHTC) hybrid powders modified using in situ grown SiC nanowires (SiCNWs) were successfully prepared via a simple catalytic method. The self-produce carbon and silicon source promoted the growth of SiCNWs during the pyrolysis process of ZrB₂ polymer precursors coated ZrB₂-SiC powders. The results showed that the growth of SiCNWs could be explained by a tip-growth model and vapor–liquid–solid (VLS) growth mechanism. The SiCNWs with diameter of 200 nm were single crystalline, and the content could be controlled by changing the catalyst content.     

B-site modification of (Ba0.6Sr0.4)TiO3 ceramics with enhanced diffuse phase transition behavior

(Ba_0.6Sr_0.4)TiO₃ (BST) ceramics modified with both Zr and Mn were synthesized via a traditional solid state reaction route. Pure perovskite structure and dense morphology were obtained for all specimens. An enhanced diffuse phase transition (DPT) behavior induced by Mn substitution was detected. We suggest that such enhanced DPT behavior was beneficial for BST practical applications due to the improved dielectric thermal stability while maintaining good dielectric performance. The impact of Mn concentration on the microstructure, dielectric properties, and DPT diffuseness was investigated.     

粉体超微化对碳化硅陶瓷显微结构的影响

研究了粉体超微化后碳化硅陶瓷材料显微结构的变化特征 ,分析了超微化处理对碳化硅陶瓷烧结过程及显微结构的影响机理。经超微化处理后 ,粉体颗粒尺寸减小 ,分布范围变窄 ,硬团聚解聚 ,颗粒形状均匀分布 ;素坯均匀性提高 ,坯体烧结温度降低 ,促进了坯体烧结过程 ;陶瓷材料表面显微结构缺陷明显减少 ,获得了理想的显微结构和良好的力学性能     

Microstructural and mechanical evaluation of a Cu-based active braze alloy to join silicon nitride ceramics

Self-joining of St. Gobain Si₃N₄ (NT-154) using a ductile Cu-Al-Si-Ti active braze (Cu-ABA) was demonstrated. A reaction zone (∼2.5-3.5 μm thick) developed at the interface after 30 min brazing at 1317 K. The interface was enriched in Ti and Si. The room temperature compressive shear strengths of Si₃N₄/Si₃N₄ and Inconel/Inconel joints (the latter created to access baseline data for use with the proposed Si₃N₄/Inconel joints) were 140 ± 49 MPa and 207 ± 12 MPa, respectively. High-temperature shear tests were performed at 1023 K and 1073 K, and the strength of the Si₃N₄/Si₃N₄ and Inconel/Inconel joints were determined. The joints were metallurgically well-bonded for temperatures above 2/3 of the braze solidus. Scanning and transmission electron microscopy studies revealed a fine grain microstructure in the reaction layer, and large grains in the inner part of the joint with interfaces being crack-free. The observed formation of Ti₅Si₃ and AlN at the joint interface during brazing is discussed.     

Preparation,microstructure and ion-irradiation damage behavior of Al4SiC4-added SiC ceramics

Single-phase Al₄SiC₄ powder with a low neutron absorption cross section was synthesized and mixed with SiC powder to fabricate highly densified SiC ceramics by hot pressing. The densification of SiC ceramics was greatly improved by the decomposition of Al₄SiC₄ and the formation of aluminosilicate liquid phase during the sintering process. The resulting SiC ceramics were composed of fine equiaxed grains with an average grain size of 2.0 μm and exhibited excellent mechanical properties in terms of a high flexure strength of 593 ± 55 MPa and a fracture toughness of 6.9 ± 0.2 MPa m^1/2. Furthermore, the ion-irradiation damage in SiC ceramics was investigated by irradiating with 1.2 MeV Si⁵⁺ ions at 650 °C using a fluence of 1.1 × 10¹⁶ ions/cm², which corresponds to 6.3 displacements per atom (dpa). The evolution of the microstructure was investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The breaking of Si–C bonds and the segregation of C elements on the irradiated surface was revealed by XPS, whereas the formation of Si–Si and C–C homonuclear bonds within the Si–C network of SiC grains was detected by Raman spectroscopy.     

Energy dependent structure changes in ion beam deposited a-C:H

The present study investigates the effect of ion energy in the form of acceleration potential on the structure of ion beam deposited a-C:H films using Raman spectroscopy, FTIR spectroscopy and profilometry. The results indicate that for low acceleration potentials (100–200 V), the sp² fraction of the film becomes more ordered as the acceleration potential increases. However, FTIR indicates that the hydrogen bonding in the film is unaffected. For mid-range acceleration potentials (200–800 V) the film structure remains stable, then, as the acceleration potential is increased above 800 V there is a further increase in the order of the sp² fraction in the films and a change in the hydrogen bonding in the film. These structure changes suggest that two separate energy dependent processes effect the structure of IBD a-C:H; a densification/relaxation process at low acceleration potentials and ion damage/sputtering processes at higher acceleration potentials.     

Microstructure and shear strength of ZrB2SiC/Ti6Al4V joint by TiCuZrNi with Cu foam

In this paper, brazing behaviors between ZrB₂SiC and Ti6Al4V by Cu foam interlayer were studied. The microstructure, formation mechanism, mechanical property and fracture surface of the joints were systematically studied. The results showed that the phases in the joints were α+β-Ti, TiCu, Ti₂Cu, Cu(s, s), TiC, TiB₂ and Ti₃SiC₂. An optimum shear strength reached up to 435??MPa?at a brazing temperature of 910?°C and holding time of 20?min. Such a shear strength was 90?MPa higher than the one without the Cu foam. The obtained high shear strength of joint was discussed from microstructure and residual stress. With the increase of brazing time, Cu(s,s) gradually disappeared and the content of Ti₂Cu intermetallic compound increased, which was harmful for the joint. Furthermore, the residual stress of joint with Cu foam was calculated to be 324?MPa, lower than the one without Cu foam interlayer.     

Effect of brazing parameters on microstructure and mechanical properties of Cf/SiC and Nb-1Zr joints brazed with Ti-Co-Nb filler alloy

Reliable brazing of carbon fiber reinforced SiC (C_f/SiC) composite to Nb-1Zr alloy was achieved by adopting a novel Ti45Co45Nb10 (at.%) filler alloy. The effects of brazing temperature (1270–1320 °C) and holding time (5–30 min) on the microstructure and mechanical properties of the joints were investigated. The results show that a continuous reaction layer (Ti,Nb)C was formed at the C_f/SiC/braze interface. A TiCo and Nb(s,s) eutectic structure was observed in the brazing seam, in which some CoNb₄Si phases were distributed. By increasing the brazing temperature or extending the holding time, the reaction layer became thicker and the amount of the CoNb₄Si increased. The optimized average shear strength of 242 MPa was obtained when the joints were brazed at 1280 °C for 10 min. The high temperature shear strength of the joints reached 202 MPa and 135 MPa at 800 °C and 1000 °C, respectively.     

Dielectric and ferroelectric properties of Ta-modified Bi3.25La0.75Ti3O12 ceramics

B-site modified Bi_3.25La_0.75Ti_3-xTa_xO₁₂ ceramics were prepared by the conventional solid-state reaction method. The influence of Ta₂O₅ on microstructure and electric properties of the ceramics was investigated. The results demonstrated that Ta⁵⁺ ions were dissolved into the perovskite lattice and homogeneously distributed in the matrix without forming any minority phase. The conduction mechanism and dielectric response behavior were transformed with Ta substation, which is triggered by varied structural distortion characteristics and defect diploes. The Curie temperature decreased gradually with increasing Ta content and a relaxor-like behavior was observed for x = 0.09 sample. The internal bias field is decreased with Ta doping, because the substitution of Ta⁵⁺ at B-site contributes to release the involved oxygen vacancies in defect diploes. Moreover, further increasing Ta content causes a reduction in the oxygen vacancies located at lattice misfits, resulting in a decrease of coercive fields. An improved ferroelectric properties were obtained for x = 0.09 sample with a relatively lower coercive field and a larger spontaneous polarization.     

Pressureless sintering of silicon carbide ceramics containing zirconium diboride

SiC-5 wt.% ZrB₂ composite ceramics with 10 wt.% Al₂O₃ and Y₂O₃ as sintering aids were prepared by presureless liquid-phase sintering at temperature ranging from 1850 to 1950 °C. The effect of sintering temperature on phase composition, sintering behavior, microstructure and mechanical properties of SiC/ZrB₂ ceramic was investigated. Main phases of SiC/ZrB₂ composite ceramics are all 6H-SiC, 4H-SiC, ZrB₂ and YAG. The grain size, densification and mechanical properties of the composite ceramic all increase with the increase of sintering temperatures. The values of flexural strength, hardness and fracture toughness were 565.70 MPa, 19.94 GPa and 6.68 MPa m^1/2 at 1950 °C, respectively. The addition of ZrB₂ proves to enhance the properties of SiC ceramic by crack deflection and bridging.     

Wetting of SiC by Al-Ti alloys and joining by in-situ formation of interfacial Ti3Si(Al)C2

Two pressureless and reliable procedures for brazing SiC-based materials have been designed. The joining was obtained by the in-situ formation of a Ti₃Si(Al)C₂ MAX phase using simple Al-Ti interlayers. Wettability studies were conducted using several Al-Ti alloys in contact with SiC at 1500?°C. The interfacial microstructures and thermodynamic analysis demonstrated that liquid Al₃Ti in contact with SiC formed a well-bonded Ti₃Si(Al)C₂ interfacial layer. These findings guided the design of two joining methods: one consisted of the simple infiltration of Al₃Ti into the brazing seam, while an Al₃Ti paste/Ti/Al₃Ti paste interlayer assembly was designed for the second process. Sound interfaces without cracks were obtained in both processes. The average shear strength was very high, 296?MPa, for the infiltration method; the drawback was the presence of residual Al. Joining through Al₃Ti/Ti/Al₃Ti interlayers avoided the presence of low-temperature melting phases, with lower shear strength: 85 or 89?MPa depending on the testing method.