Showing papers on "Carbide published in 1989"

Boron carbide structure by Raman spectroscopy

Abstract: We have obtained and analyzed Raman spectra of single-crystal, hot-pressed, and chemical-vapor-deposited boron carbide materials over their single-phase region (from {similar to}9 to {similar to}20 at. % carbon). These spectra provide insight into the substitutional disorder that characterizes these structurally ordered solids. In particular, although icosahedra and chain structures occupy regular lattice positions, there is local substitutional disorder resulting from the occupancy of certain sites within the icosahedra and chains by either boron or carbon atoms. Comparison of boron carbide Raman spectra with the Raman spectra of {alpha}-rhombohedral boron, boron phosphide, and boron arsenide has confirmed the following structural model derived from theoretical considerations and electrical and thermal transport data. The boron carbide composition with nearly 20 at. % carbon is composed of B{sub 11}C icosahedra linked by carbon-boron-carbon chains. As the carbon content is reduced toward approximately 13 at. %, carbon-boron-carbon chains are progressively replaced by carbon-boron-boron chains. Further reduction in the carbon content results in the replacement of B{sub 11}C icosahedra with B{sub 12} icosahedra.

Processing of boron carbide-aluminum composites

Abstract: The processing problems associated with boron carbide and the limitations of its mechanical properties can be significantly reduced when a metal phase (e.g., aluminum) is added. Lower densification temperatures and higher fracture toughness will result. Based on fundamental capillarity thermodynamics, reaction thermodynamics, and densification kinetics, we have established reliable criteria for fabricating B4C-AI particulate composites. Because chemical reactions cannot be eliminated, it is necessary to process B4C-AI by rapidly heating to near 1200°C (to ensure wetting) and subsequently heat-treating below 1200°C (for microstructural development). [Key words: composites, boron carbide, aluminum, processing, cermets.]

Microstructural Coarsening During Sintering of Boron Carbide

Abstract: This paper reports how the sintering behavior of boron carbide was investigated with particular attention given to microstructure development at various stages in the sintering process. Hot-pressing and pressureless sintering techniques were employed and the effects of heating rate, firing atmosphere, and composition were used to characterize the sintering behavior. Pressureless sintering at temperatures up to 2300{degrees} C produces only limited densification. Microstructural coarsening is responsible for this since it leads to conditions where densification is slow. Hot-pressing and carbon additions suppressed coarsening and permitted densification to {gt} 95% of theoretical density.

Evidence for a silicon oxycarbide phase in the Nicalon silicon carbide fibre

Abstract: The Nicalon silicon carbide fibre has been studied by X-ray photoelectron spectroscopy. Elements entering the fibre are carbon, silicon and oxygen. In addition to previously reported chemical entities (silicon carbide, silica and graphitic carbon) evidence is found of the presence of a new supplementary phase which is attributed to an intermediate silicon oxycarbide phase. As this phase is found to participate in very appreciable proportions to the composition of the fibre, some influence on the properties of this fibre can be anticipated.

Hardmetals and cermets

Abstract: Hardmetals or cemented carbides (sometimes less accurately called sin­ tered carbides) are composite materials with one or more hard but rela­ tively brittle components and a ductile binder metal that imparts a certain degree of toughness to the composite material without impairing the high hardness of the carbide components. This unusual combination of the useful properties of otherwise quite different components led to nearly instantaneous technical acceptance of these materials immediately after their discovery (1). The first patents were issued around 1923 to the German company "Osram Studiengesellschaft"; today there are innumerable patents cover­ ing inventions related to hardmetals. The first hardmetals were marketed by F. Krupp in 1927 under the name "Widia" (from "wie Diamant" = like diamond) and were used primarily for wire drawing dies and wear resistant materials. For metal cutting applications the early hardmetals did not perform much better than conventional high-speed steels. Only after the introduction of titanium carbide (TiC) as a further additive to the basic tungsten carbide-cobalt (WC-Co) composition could a breakthrough in metal cutting technology be achieved (2). Today more than 80% of the total production of hardmetals is used for metal cutting purposes. It is worth mentioning that even today, after sixty-five years of research and development, the majority of hardmetals still consists of tungsten carbide plus various other carbide additions with cobalt as the binder, similar to those compositions claimed and protected by the very first patents. However, the physical reasons why the basic ingredients-WC and Co­ do outperform all other combinations of transition metal carbides with binder metals or alloys are still not entirely clear. Only recently has a class of hardmetals without tungsten carbide as a

Method for producing polycrystalline compact tool blanks with flat carbide support/diamond or CBN interfaces

Abstract: The present invention relates to a method for making a supported PCD or CBN compact comprising placing in an enclosure a cup assembly having a mass of diamond or CBN particles having a surface and the mass of cemented metal carbide having a surface, and optionally a catalyst for diamond (or optionally, CBN) recrystallization, said surfaces being in adjacency to form an interface. The enclosure then is subjected to a high pressure/high temperature process which results in diamond or CBN compacts preferably characterized by diamond-to-diamond or CBN-to-CBN bonding joined to a cemented carbide support at their respective surfaces. The supported compacts are recovered from the enclosures and cup assemblies and finished. The finished supported compacts in the enclosure exhibit non-planar bonded interface resulting in PCD or CBN compacts of substantially non-uniform thickness. The improvement in process of the present invention comprises said carbide mass surface being the mirror image of the finished PCD or CBN non-planar interface for making a finished supported compact of substantially uniform diamond or CBN compact thickness. Preferably, at least two compacts are produced in the process and the catalyst for diamond recyrstallization is provided from the cemented metal carbide mass,

Microstructural Studies of the Interfacial Zone of a SiC-Fiber-Reinforced Lithium Aluminum Silicate Glass-Ceramic

Abstract: Transmission electron microscopy studies have been conducted on interfaces in a lithium aluminum silicate/SiC-fiberreinforced composite. In the as-processed state, interphases of amorphous C and carbides of Nb have been confirmed, with circumferential thermal debonds evident in the C layer. After heat treatment in air at 800°C, the C is found to be replaced by amorphous SiO2, and the carbides of Nb replaced by oxides. The SiO2 thickens with exposure time and typically contains circumferential separations. Some Mg and Al diffusion also accompanies the heat-treatment process and eventually leads to the formation of MgO and Mg silicates in the interfacial zone.

Carbide precipitation during stage I tempering of Fe-Ni-C martensites

Abstract: Microstructural changes which accompany the first stage of tempering have been studied by transmission electron microscopy (TEM) and electrical resistometry in two Fe-Ni-C alloys that form platelike martensite. The e-carbide transition phase in these alloys adopts a platelike shape with a habit plane near {012=α. Electron diffraction data indicate that the carbide may be partially ordered, resulting in orthorhombic symmetry, and therefore, this phase is designated as e′- carbide. The carbide particles contain a fine internal substructure which appears to represent an internal accommodation deformation (faulting) on the carbide basal plane. Detailed analysis of the kinematics of carbide precipitation suggests that the observed habit plane and accommodation deformation permit an invariant-plane strain transformation which minimizes elastic strain energy. The apparent selection of only a limited number of possible orientation variants is explained in terms of the symmetry of the parent martensitic phase, which is known to undergo spinodal decomposition prior to the nucleation of the transition carbide. The martensitic substructure is not found to exert any significant influence on this overall precipitation behavior.

Crystallographic and magnetic structure of ternary carbides of the type Nd2Fe17Cx

Abstract: Neutron diffraction measurements were made at 300 K and 4.2 K on a polycrystalline sample of the composition Nd2Fe17C0.5. The crystal structure of this ternary carbide is very similar to that of the parent compound Nd2Fe17(Th2Zn17-type, R3m), the carbon atoms partially filling up the empty 9(e) site. At 4.2 K, the compound Nd2Fe17C0.5 adopts a magnetic structure in which the easy magnetization direction is perpendicular to the c-axis.

Wetting improvement of carbon or silicon carbide by aluminium alloys based on a K2ZrF6 surface treatment: application to composite material casting

Abstract: A surface treatment with aqueous solutions of K2ZrF6 has been carried out to improve, in dramatic manner, the wetting of carbon (or SiC)-base ceramics by liquid light alloys at low temperatures (ie within the 700 to 900°C range) The mechanism which is thought to be responsible for the wetting improvement involves two steps: (i) K2ZrF6 reacts with aluminium with the formation of K3AlF6, other complex fluoride species and intermetallics, (ii) K3AlF6 dissolves the alumina thin layer, coating the liquid light alloy and enables the wetting of the ceramics The mechanism has been worked out from sessile drop experiments, solid state chemistry experiments and composite casting The K2ZrF6 surface treatment appears to be particularly suitable for processing composite materials made of carbon (or SiC) fibrous preforms and aluminium-base matrices according to techniques directly derived from the light alloy foundry

Multiple metal coated superabrasive grit and methods for their manufacture

Abstract: Multiple metal coated diamond grit for improved retention in a tool matrix comprises a first layer coating of a metal carbide of a strong carbide former, preferably chromium, titanium or zirconium, chemically bonded to the diamond and a second metal coating of an oxidation resistant carbide former, preferably tungsten or tantalum, chemically bonded to the first metal layer. A third metal layer coating of an alloying metal, preferably nickel, can also be added. In accordance with the method of the present invention, the first layer metal can be applied by metal vapor deposition. The second layer metal can be applied by chemical vapor deposition. The third layer of an alloying metal can be applied by electroless or electrolytic plating.

Method of brazing of diamond to substrate

Abstract: A method of making a diamond cutting and abrading tool. The method includes the following steps: (A) Mixing molybdenum with a braze which alloys with the carbide forming substance and a temporary binder to provide a coating material; (B) Applying said coating material to a tool substrate; (C) Applying at least a monolayer of diamond particles thereover; and (D) Heating the product of step (C) at a temperature sufficient to initially form a metal carbide coating on the diamond and thereafter to braze the carbide coated diamond to the tool substrate.

High‐Temperature Behavior of the Aluminum Oxycarbide Al2OC in the System Al2O3–Al4C3 and with Additions of Aluminum Nitride

Abstract: A large number of high-temperature experiments involving binary mixtures of alumina and aluminum carbide (Al4C3) were performed to clarify the behavior of the aluminum oxycarbide Al2OC. It is first shown from heat treatments on the molar composition 55Al2O3·45 Al4 C3 that, in conditions of stable equilibrium, Al2OC decomposes to Al4O4 C and Al4 C3 at 1715°C according to the reaction 4Al2 OC → Al4 O4 C+Al4 C3. However, Al2 OC can be obtained at room temperature from compositions of lower carbide contents (<20 mol%) and by rapid cooling. In that case Al2 OC is solidified in a metastable state, and, when subsequently annealed to temperatures above 1200°C, its lattice reorganizes and progressively transforms into the lattice of Al4 O4 C according to the reaction Al2 O3+xAl2 OC → (1—x)Al2 O3+xAl4 O4 C. This transformation is described in terms of TTT curves and is accompanied by a decrease in hardness and wear resistance. Additions of AlN to Al2 O3 and Al4 C3 have been found to create a solid-solution Al2 OC—AlN in an unexpected region of the ternary system Al2 O3—Al4 C3—AlN. As a result the quasi-binary section Al2 OC—AIN of the ternary diagram Al2 O3—Al4 C3—AlN was extensively investigated. We report the solubility limit of AlN in Al2OC, the improved high-temperature stability of the Al2OC—AlN solid solution compared with pure Al2OC, and eventually the existence of a possible intermediate ternary compound of formula Al10O3C3N4.

The surface composition of silicon carbide powders and whiskers: An XPS study

Abstract: The surface composition and bonding of a wide variety of silicon carbide powders and whiskers have been characterized by x-ray photoelectron spectroscopy (XPS). Ultrafine SiC powders, grown by a radio frequency plasma process, have been shown to exhibit graphitic carbon and a thin suboxide coating. Whiskers of SiC, grown in a vapor-liquid-solid or proprietary commercial process, were generally covered by heavier oxides than the powders and to a variable degree showed silica-like bonding. Most of the materials were subject to sample charging. Procedures were developed to estimate these charging effects and interpret the complete catalog of XPS spectra from these materials with respect to Fermi-level assignments. Charge independent quantities, such as oxygen Auger parameter and O(1s)–Si(2p) peak position difference, were found to agree with accepted values in the literature while exhibiting trends consistent with suboxide and silica bonding assignments. The data give a broad basis for understanding the feedstock surface chemistry which is involved during fabrication of monolithic or composite silicon carbide materials.

The Performance of High-Temperature Reactor Fuel Particles at Extreme Temperatures

Abstract: Coated particles embedded in graphitic elements are the fuel for the High-Temperature Reactor (HTR) Experimental investigations of the performance of particles at extremely high temperatures have been conducted to achieve an understanding of coating failure mechanisms and to establish the data base for safety and risk analyses of hypothetical accidents in large- and medium-sized HTRs The primary mechanism for coating failure and fission product release in the 1900 to 2500/sup 0/C temperature range is thermal decomposition of silicon carbide (SiC) Heating tests have provided the activation energy of this process and the correlation of SiC decomposition with coating failure and subsequent fission product release

Effect of iron and boron carbide on the densification and mechanical properties of titanium diboride ceramics

Abstract: The effect of Fe and B4C on the sintering behavior and mechanical properties of TiB2 ceramics have been studied. Sintering was performed in an Ar atmosphere at 2000° using attrition-milled TiB2 powder (mean particle size = 0.8 μm). When a small amount of Fe (0.5 wt%) was added, abnormal grain growth occurred and the sintered density was low. In the case of B4C added along with 0.5 wt% Fe, however, abnormal grain growth was remarkably suppressed, and the sintered density was increased up to 95% of theoretical. But with excess Fe addition (5 wt%), B4C grains did not act as a grain growth inhibitor, and B4C grains were frequently trapped in large TiB2 grains. The best mechanical properties were obtained for the TiB2–10 wt% B4C–0.5 wt% Fe ceramics, which exhibited a three-point bending strength of 400 MPa and a fracture toughness of 5.5 MPa · m1/2.

Amorphous and Crystalline Silicon Carbide and Related Materials

Abstract: Although silicon carbide has been used for more than half a century, its potential as a high-temperature, corrosion-resistant semiconductor has only recently begun to be exploited. Both crystalline and amorphous forms of SiC offer several advantages over Si, GaAs, and InP for high-frequency, high-power, and high-speed circuits. This volume contains reports on high-temperature SiC MOSFETs and MESFETs, secondary harmonic generation in SiC, a-SiC emitter heterojunction bipolar transistors, and bulk crystal growth of 6H-SiC. For newcomers to the field it provides an up-to-date review of technological developments in SiC and related materials, while specialists will find here recent references and new insights into materials for high-temperature, high-power, and high-speed circuit applications.

Shock compression and release in high-strength ceramics

Abstract: Shock compression and release particle velocity data have been obtained for silicon carbide, titanium diboride, boron carbide, and zirconium dioxide with a laser velocity interferometer (VISAR). Peak impact stresses in these experiments range between 20 and 50 GPa. Iterative numerical methods were used to obtain dynamic compression and release stress-strain behavior of the ceramics. 16 refs., 4 figs., 3 tabs.

Elevated-Temperature Fracture Resistance of a Sintered α-Silicon Carbide

Abstract: The fracture toughness of a dense, sintered commercial α-silicon carbide was determined for temperatures from 20° to 1400°C using both straight- and chevron-notched test specimens and also controlled-surface-microflaw specimens, all in three-point bending. The flexural strengths were also measured for the same range of temperatures and the trend is compared with that of the toughness. Measurements from this study are discussed and also compared with other results in the literature. Analysis reveals the importance of contrasting sharp crack and blunt crack techniques and also the need for addressing the microhardness indentation method separately. It is concluded that the fracture toughness of this silicon carbide is about 3 MPa · m½ and that the crack growth resistance is characterized by a flat R-curve behavior, both of which are independent of temperature from 20° to 1400°C.

Implantation and electrical activation of dopants into monocrystalline silicon carbide

Abstract: The invention is a method of ion implantation of dopant ions into a substrate of silicon carbide. In the method, the implantation takes place at elevated temperatures, following which the substrate may be oxidized or annealed.

Laser processing of a SiC/Al‐alloy metal matrix composite

Abstract: Preliminary studies were conducted on the laser processing of SiC/A356‐Al alloy metal matrix composite (MMC) for such applications as welding/joining and cutting. The SiC/A356‐Al MMC was processed using several different laser specific energies. Microstructural observations after laser processing revealed that the extent of reinforced material (SiC)‐matrix (A356‐Al) reaction is directly proportional to the laser energy input. As energy input increased, SiC particle dissolution became greater and aluminum carbide formation increased in both size and quantity. It appears possible to control substantial change (physical and chemical) in SiC particles during processing by controlling the amount and mode of energy input.

Small area MXPS- and TEM-measurements on temper-embrittled 12% Cr steel

Abstract: Martensitic stainless steel is widely used as corrosion resistant material exhibiting occasionally temper-embrittlement failures after a thermal treatment in the temperature range of 450 ~ 550 ~ The origin of these failures is predominantly the grain boundary segregation of trace elements which causes a reduction of the cohesive forces and thus an intercrystalline embrittlement of the alloys [1]. This process depends on the composition, the microstructure and the thermal and mechanical treatment of the steel. Specifically, the precipitation or dissolution of carbide phases at the grain boundaries triggers the segregation process of phosphorus which is the most critical element for temper embrittlement of chromium steels [2]. This short communication deals with the reversible temper embrittlement of a X-10-Cr-I 2 martensitic steel with 220 ppm phosphorus. Previous investigation of intercrystalline fracture surfaces by Scanning Auger Electron Spectroscopy have shown that the P concentration at the grain boundaries increases with a growing chromium carbide coverage at the grain boundaries [3]. The objective is to identify the carbide phase that is linked to the segregation process. From TEM investigation it is known that Cr23C6 carbides are located at the prior austenite grain boundaries which so far were considered to be the crack planes. Recent studies revealed smaller Cr7C 3 carbide precipitates at subboundaries between the martensite plates very close to the prior austenite boundaries. These Cr7C3 carbides were surrounded by a stress field which is a typical sink for P atoms. After a deembrittling annealing (15 min 625 ~ the very small Cr7C3 carbides were completely dissolved and the phosphorus enrichment of the fracture surfaces significantly reduced [4]. For a better understanding of these reversible temperembrittlement phenomena it is therefore of particular interest to identify the carbides that are present at the fracture surface of temper-embrittled steel. For an unambiguous identification of the type of carbides at the fracture surface small area monochromated photo electron spectroscopy (MXPS) seems at present to be the most appropriate technique. The pronounced chemical shift of the C Is peak clearly distinguishes between different metal carbides and

A mathematical model describing carburization in multielement alloy systems

Abstract: Previously, a finite difference model was set up describing the diffusion of carbon in high-temperature model alloys and its chemical reaction with one of the alloy components. The model described the formation of up to three different chromium carbides, two of which could coexist. This paper describes the further development of the model for application to commercial alloys. The model was extended to enable treatment of an arbitrary number of (a) metallic carbide-forming components in the alloys, (b) carbides which may form, (c) components out of which each carbide may be composed, and (d) carbides which may coexist. With this model, carbon concentration profiles and distribution profiles of precipitated carbides occurring during carburization of binary, ternary, and quaternary Ni-based alloys were calculated. Kinetic and thermodynamic data needed for the calculations were obtained by combining literacture data with experimental results and by fitting measured with calculated concentration profiles. The resulting calculated carbon profiles and carbide distributions were in good agreement with the experimental results.