Damage-free machining of monocrystalline silicon carbide

Cutting tests of monocrystalline SiC, on the surface of which an amorphous layer was preformed by ion implantation, were performed. Ductile-mode machining was observed at a depth of cut smaller than 60 nm. At a depth of cut larger than 60 nm, cracks were observed on the work surface. However, transmission electron micrographs show that crack propagation was obstructed at the interface between the amorphous and crystalline layers even under brittle-mode machining, and no subsurface damage extended into the crystalline layer. The results suggest that the damage-free machining of monocrystalline SiC is possible by surface modification to an amorphous structure.     

Slurry based multilayer environmental barrier coatings for silicon carbide and silicon nitride ceramics — I. Processing

Multilayer mullite/gadolinium silicate (Gd₂SiO₅) environmental barrier coatings (EBCs) were deposited on α-SiC (Hexaloy) and Si₃N₄ (SN282) substrates through cost-effective slurry based dip-coat processing. Coatings applied by two approaches, alcohol and sol-based slurries, were examined and optimized in terms of their recipes and air sintering temperatures. A significant increase in densification rates was found for the sol-based EBCs applied on both SiC and SN282 substrates due to the fine mullite particles formed during reaction sintering of well-mixed silica and alumina sols. Mechanical alloying of the starting powder mixtures instead of their simple rotary-blending was found to be beneficial in terms of enhanced coat sintering kinetics. Dense thick coatings that were well-bonded to the substrate were obtained.     

Shape adaptive grinding of CVD silicon carbide

Because of the direct relationship between removal rate and surface roughness in conventional grinding, ultra-precision finishing of hard coatings produced by chemical vapour deposition (CVD) usually involves several process steps with fixed and loose abrasives. In this paper, an innovative shape adaptive grinding (SAG) tool is introduced that allows finishing of CVD silicon carbide with roughness below 0.4 nm Ra and high removal rates up-to 100 mm³/min. The SAG tool elastically complies with freeform surfaces, while rigidity at small scales allows grinding to occur. Since material removal is time dependent, this process can improve form error iteratively through feed moderation.     

Slurry based multilayer environmental barrier coatings for silicon carbide and silicon nitride ceramics — II. Oxidation resistance

In part I of this study, the dip-coat processing of mullite/gadolinium silicate (Gd₂SiO₅) environmental barrier coatings (EBCs) applied on α-SiC and SN282™ Si₃N₄ through alcohol based and sol based slurries was presented. Here, the performance of selected EBCs by evaluating their oxidation resistances during thermal cycling in simulated combustion (90% H₂O-balance O₂) environment between 1350 °C and RT for up to 400 cycles is being reported. Oxidation of un-coated α-SiC was severe, leading to aligned and layered porous silica scale formation (~ 17 μm thick) on its surface with frequent scale spallation when exposed to 100 cycles. Mullite/Gd₂SiO₅/B₂O₃ (83.5/11.5/5 wt.%) EBCs remained adherent to α-SiC substrate with an underlying porous silica layer formed at substrate/coating interface, which was ~ 12 μm after 100 cycles, ~ 16 μm after 200 cycles, and ~ 25 μm after 400 cycles. In contrast, α-SiC substrate coated with mullite/Gd₂SiO₅ (88/12 wt.%) EBC had only limited oxidation of ~ 10 μm even after 1350 °C/400 cycles. The sol based mullite/Gd₂SiO₅ (88/12 wt.%) EBC on α-SiC substrate after 400 cycles was adherent, but showed more interfacial damages (~ 20 μm after 400 cycles) though it had increased coating density. However, the mullite/Gd₂SiO₅ (88/12 wt.%) EBC (alcohol based) delaminated from the SN282™ Si₃N₄ substrate after 1350 °C/100 cycles, because of the formation of interconnected interfacial voids and hairline cracks. Parabolic growth kinetics for the underlying silica was observed for both the alcohol and sol based coated samples.     

铸铁用碳化硅质泡沫陶瓷过滤材料的研制与应用

根据铸铁件生产的特点，提出铸铁对泡沫陶瓷过滤材料性能的要求。较详细地介绍了研制铸铁用碳化硅泡沫陶瓷过滤材料方面所做的工作。以较充实的试验结果证明所研制的碳化硅泡沫陶瓷过滤铸铁液效果良好，并对泡沫陶瓷过滤的机理进行了合理的分析。     

Thermal plasma chemical vapor deposition of wear-resistant, hard Si-C-N coatings

Wear-resistant, hard Si-C-N coatings were synthesized in a triple torch plasma reactor using a thermal plasma chemical vapor deposition process. In this reactor, three dc plasma torches were angled so that their jets converge to form a highly chemically reactive region at the substrate. Vaporized hexamethyldisilazane (HMDSN) was injected through a central injection probe, while nitrogen or hydrogen gases were added through the torches to the argon plasma.Various dissociation, recombination and intermediate reactions were considered to determine what major species exist in the gas phase during the deposition of Si-C-N films. Reactant flow rates were varied to evaluate the thermodynamic equilibrium compositions across a linear temperature profile above the substrate and to identify the species that lead to the production of wear-resistant, hard Si-C-N films.A series of experiments were conducted at low HMDSN flows (∼ 1 sccm) and varying hydrogen and nitrogen flows. Films were characterized by micro X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. Indentation tests were conducted on the polished film cross-sections, while wear tests were carried out on the film surfaces. At substrate temperatures below 1000 °C, amorphous Si-C-N films were deposited, while higher temperatures produced crystalline composite films of α- and β-Si₃N₄ and α- and β-SiC. Films produced with hydrogen at low HMDSN flows displayed non-columnar morphology and therefore had higher wear-resistance, indicating the benefit of low reactant-to-plasma gas flow concentrations on film growth. At low HMDSN flows, low nitrogen-to-hydrogen ratios had also shown an increase in film linear density. Small variations in mechanical properties and wear were observed between films grown under low N:H flow ratio conditions (smooth film surfaces). Wear-resistance of films with columnar structures from high N:H conditions was significantly lower, while the hardness was unobtainable. This result indicates the importance of film morphology on mechanical performance.     

Hybrid polishing mechanism of single crystal SiC using mixed abrasive slurry (MAS)

Single crystal SiC is a mechanically hard and chemically inert material used in optical and power devices. This work proposes the development of a hybrid polishing technique using a mixed abrasive slurry (MAS) with colloidal silica and nano-diamond. Hybrid removal mechanism of the MAS on the SiC is investigated by polishing results, chemical analyses and AFM studies. Each role of two abrasives is distinguished by scratching tests with AFM contact mode on the chemically reacted SiC surface. Finally, this paper provides an optimum MAS condition to realize highly efficient material removal rate (MRR) keeping defect-free surface.     

SiC颗粒增强铝基复合材料制造工艺及性能

将２０－４０μｍ的ＳｉＣ颗粒（ＳｉＣｐ）经预处理后在真空中与铝共熔，然后冷至铝合金固液两相区搅拌，可明显改善ＳｉＣｐ／基体间的润湿性和增强相分布的均匀性；加镁也可改善其润湿性，该材料的强度是铝合金的二倍，弹性模量也提高两倍以上。     

YT合金改性研究

对 YT合金 ( YT5、YT1 4、YT1 5)从采用复合型粘结剂、制取非均匀结构、增加添加剂和调整工艺参数等几个方面进行了改性分析研究。结果表明 :改性后的 YT合金其物理和机械性能较改性前均有不同程度的提高 ,显微组织结构明显改善 ,使用性能有了较大幅度提高 ,为进一步增强 YT产品质量的稳定性、可靠性 ,以较好的功能价格比赢得市场奠定了基础。     

Studies on electric-discharge machining of non-contact seal face grooves

In this paper, the electric-discharge machining (EDM) of non-contact seal grooves was studied. Two types of material, namely: tungsten carbide and silicon carbide were tested by EDM due to their hard machining behaviors. The geometry of seal grooves is another reason why grinding or other precision machining processes cannot be applied. Four parameters of EDM processes were studied, namely: electrode material, pulse duration, discharge current, and polarity. According to experimental results, the optimal process parameters were obtained based on the specimen’s qualities, such as surface roughness and depth of cut. An industrial example was also studied in the final stage of the proposed paper.     

Investigation of interfacial bonding between Na2O–B2O3–SiO2 solder and silicon carbide substrate

Abstract

The interfacial characteristics of the SiC/glass solder/SiC joining part were investigated by means of thermodynamic and wetting behaviour calculation, microstructure observation, chemical structure and element analysis. The results of wavelength dispersive spectroscopy linear scan across the joint showed a relatively low increase in sodium content at the interfaces, which indicated that the sodium silicates in glass could react with silicon carbide substrate at the joining temperature, but it is not the reason for the good bonding between SiC and the interlayer. The mechanism of the good interfacial bonding lies in the formation of oxycarbide phase at the interface.     

Processing and mechanical properties of porous silica-bonded silicon carbide ceramics

A simple processing route for manufacturing highly porous, silica-bonded SiC ceramics with spherical pores has been developed. The strategy adopted for making porous silica-bonded SiC ceramics entails the following steps: (i) fabricating a formed body through a combination of SiC and polymer microbeads (employed as sacrificial templates) and (ii) sintering the formed body in air. SiC particles are bonded to each other by oxidation-derived SiO₂ glass. By controlling the microbead content and the sintering temperature, it was possible to adjust the porosity such that it ranged from 19 to 77%. The flexural and compressive strengths of the porous silica-bonded SiC ceramics with ≈40% porosity were ≈65 MPa and ≈200 MPa, respectively. The superior strengths were attributed to the homogeneous distribution of small (≤30 μm), spherical pores with dense struts in the porous silica-bonded SiC ceramics.     

Magnetic properties of vanadium-doped silicon carbide nanowires

This study reports the magnetic properties of vanadium (V) doped single crystalline silicon carbide nanowires. The first principle calculation indicated that the V-doped cubic SiC phase can exhibit half-metallic ferromagnetic properties that are essential for the realization of spintronic devices. Based on this calculation, V-doped SiC nanowires were fabricated in a chemical vapor deposition process. The single crystalline β-SiC nanowires, which are doped with ca. 4 at.% of V, had diameters of < 100 nm and a length of several μm. High-resolution transmission electron microscopy observations revealed vanadium carbide (VC) phases in the nanowires, even at this low concentration of dopants. Magnetic characterization implies that the nanowires are a mixture of the diamagnetic phase of VC and ferro- or paramagnetic phases of V-doped SiC. These results suggest that the doping of transition metal having high solubility to the SiC phase can lead to the realization of dilute magnetic semiconductor behavior at very low temperature.     

A Possible Relationship between Schottky Barrier Heights and Adhesion Energies of Metal／Semiconductor or

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Structural and mechanical properties of amorphous silicon carbonitride films prepared by vapor-transport chemical vapor deposition

The deposition of amorphous silicon carbonitride (a-SiCN:H) films has been successfully achieved through an in-house developed vapor-transport chemical vapor deposition (VT-CVD) technique in a nitrogenated atmosphere. Polydimethylsilane (PDMS) was used as a single-source precursor for both silicon and carbon, while NH₃ was mixed with argon to ensure the in-situ nitrogenation of the films. The chemical bonding and the atomic composition of the a-SiCN:H films were systematically investigated, as a function of their N content, by means of Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). AFM was used to obtain 2-D and 3-D views of the films. The mechanical properties [(hardness (H) and Young's modulus (E)] of the freshly prepared films were investigated by the nanoindentation technique. It is shown that by controlling the NH₃/Ar gas flow ratio in the reactor, a-SiCN:H films with various N contents [(0-27) at.% range] are achieved. On the microstructural level, the increase incorporation of N in the a-SiCN:H films is found not only to lead to C atom substitution by N atoms in the local Si-C-N environment but also to an enhanced incorporation of hydrogen bonded to both Si and N. Furthermore, the increase incorporation of N in the a-SiCN:H films resulted in an increase of the average R_rms surface roughness from 4 to 12 nm. Moreover, the films became porous with pore size and density increase as a result of increasing N at.%. Ultimately, both H and E of the a-SiCN:H films were found to be sensitive to their N content, as they decrease (from ~ 17 GPa and 160 GPa to ~ 13 GPa and 136 GPa, respectively) when the N content is increased from 0 to 27 at.%. The formation of Si-N, Si-H, and N-H bonds at the detriment of the more stiff Si-C bonds is thought to account for the observed lowering of the mechanical properties of the a-SiCN:H films as their N content increased.     

碳化硼薄膜的电子束蒸发制备及表面分析

采用电子束蒸发技术制备碳化硼薄膜,利用X射线衍射(XRD)分析了薄膜的结构,测量了薄膜的X射线光电子能谱(XPS),并利用原子力显微镜(AFM)对薄膜进行表面分析.XRD结果表明:薄膜的结晶性随着衬底温度的升高逐渐转好,在较低的衬底温度下制备出多晶碳化硼薄膜.XPS分析得到了碳化硼薄膜表面的化学成分和结构特性,其主要成分为B_4C.AFM结果表明,薄膜表面光滑平整、均匀致密,随着衬底温度的升高薄膜均方根(RMS)粗糙度逐渐增大.     

SiC粉体中Fe元素的赋存形态及其提纯工艺

为了探究SiC块体中Fe元素的赋存形态及其粉体纯化机制,分别以冶炼后SiC块体以及经气流磨破碎后的粉体为主要原料,以HCl、HNO3、H2SO4和HF作为酸洗剂,对SiC粉体进行酸洗。通过XRD、SEM和ICP等检测手段对SiC块体和酸洗前后粉体的物相组成、微观结构及Fe元素含量进行表征,重点研究微观状态下SiC中Fe元素的赋存形态及其酸洗提纯工艺。结果表明:Fe元素以FexSiy固溶体的形式赋存于SiC块体的内部,并且微观状态下呈现出被游离Si所包裹的形态;当控制液/固比为5:1时,最佳酸洗的工艺参数为:HF浓度0.15 mol/L,浸出温度70℃,浸出时间2 h,在此工艺下Fe元素的去除率可达94.1%。