Surface Carbide Layer Research Articles

Influence of helium plasma on the structural state of the surface carbide layer of tungsten

<abstract> <p>This paper presents the results of the experimental studies of the helium plasma interaction with a surface carbide layer of tungsten. The experiments were carried out on a plasma beam installation (PBI) at a constant energy of incoming ions of 2 keV and at a surface temperature of the tungsten carbide layer of ~905 and ~1750 ℃. The local parameters (T<sub>e</sub>, n<sub>0</sub>) of the helium plasma were evaluated using the probe method and spectrometric analysis of the plasma composition. The helium plasma irradiated two types of the carbide layer on the tungsten surface, WC and W<sub>2</sub>C. The mechanisms of changing the tungsten surface morphology in the result of the plasma irradiation have been described. The study of the surface structure of the tungsten samples with a carbide layer of two types (WC, W<sub>2</sub>C) after the exposure to the helium plasma has revealed two different types of the formation of helium bubbles and changes in the surface morphology. The physical mechanism of the formation of helium bubbles consists in the capture of helium atoms by the thermal vacancies generated at high temperature by the material surface. However, with a significant increase in temperature to 1750 ℃, the formation of the bubbles was no longer observed and the sample surface had a developed coral-like structure with crystallographically oriented grains.</p> </abstract>

Mechanism of Crack Generation in Carbide Surface Layer of Laser-Clad Iron Alloys

The mechanism of crack generation in the surface chromium carbide layer formed by laser cladding of an iron substrate with a mixed powder of raw materials has been investigated for different laser melting procedures. On the vertical section of the specimens subjected to cross laser scanning, vertical cracks across the surface carbide layer were observed on one side or both sides of the laser melted track, and some interfacial cracks were observed at the interface between the carbide layer and the iron substrate. Stress analysis revealed that vertical cracks may be caused by a tensile stress generated just after the solidification of the laser melted region due to the restriction of the unmelted carbide on both sides, while interfacial cracks may be generated due to a stress resulting from a steep temperature gradient beneath the top surface and the difference in the coefficient of thermal expansion between the chromium carbide and the iron substrate.

KINETICS OF CARBONIZATION OF TITANIUM ALLOYS IN A CARBON-OXYGEN-CONTAINING MEDIUM

We investigate the kinetics of saturation of surface layers of VT1-0 and OT4 titanium alloys in the process of carbonization from graphite in an atmosphere of argon at temperatures of 900 and 1000°C for 1 ‐ 15 h. We revealed the formation of a deep hardened zone for long-holding times. The redistribution of oxygen in the case of dissociation of a surface oxide film promotes the formation of complex TiC x O y -type compounds based on titanium carbide, which additionally harden surface layers. Thermochemical treatment, in particular carbonization, is an efficient method for increasing the durability of products made of titanium alloys [1 ‐ 3]. The rate of formation and growth of surface carbide layers, their phase composition, and the structure are determined by the activity of the saturating medium, the temperature, and the time of treatment [4]. Using the results of investigation related to carbonization, one can estimate the intensity of interaction of a metal with the environment and trace the evolution of phase transformations in its surface layers. Saturation of titanium alloys from a powder substance (graphite) in argon [4] is one of the efficient methods for their carbonization. Using this fact, we investigate the kinetics of carbonization of titanium alloys from this medium. Methodical Aspects We investigate specimens 15 × 10 × 1 mm in size made of commercial titanium VT1-0 (commercially pure titanium) and OT4 (Ti ‐ 3.5, Al ‐ 1.5 Mn) alloys. The specimens were polished ( R a = 0.4 µm ) and subjected to the thermochemical treatment at temperatures of 900 and 1000°C for an isothermal holding of 1 ‐ 15 h. The specimens were saturated from graphite in a static atmosphere of argon (100 kPa) under a partial oxygen pressure < 0.2 Pa. We determine the phase composition of surface layers performing X-ray phase analysis with a DRON-3.0 diffractograph in a monochromatic CuK α -irradiation focused according to the Bragg ‐ Bretano scheme. The voltage across the anode of the X-ray tube is equal to 30 kV and the current through it equals 20 µA. The results are processed with a software of the diffractograph. To identify the obtained diffraction spectrum of the specimen presented in the form of an array of the values of d i and T i , we compare them with the data of the JCPDS‐ASTM card file [5]. We estimate hardening on the basis of microhardness (a PMT-3 device). We identify a hardened layer with a zone whose hardness is greater than the hardness of the core by 0.2 GPa [6]. Results and Discussion

Carbide surface coating of Co-Cr-Mo implant alloys by a microwave plasma-assisted reaction

A technique to grow a hard carbide surface coating on Co-Cr-Mo implant alloys used in artificial joints was developed. The carbide surface coating was applied to as-cast and forged Co-Cr-Mo alloys to improve their wear properties. The surface carbide layers were produced by reactions between the alloy surface and a methane-hydrogen mixed gas by a microwave plasma-assisted surface reaction. The new carbide layers showed “brain coral-like” surface morphology and appear to consist of mixed phases including Cr3C2, Cr2C, Cr7 C3, Cr23C6, and Co2C. The Vickers microhardness of thin carbide coatings (∼3 μm thick) was about HV 1100 regardless of the test location. The Vickers microhardness of thick carbide coatings (∼10 μm thick) showed a wide range of hardnesses from HV 1000 to HV 2100. Co-deposition of soot and diamond films occured on a small area of the forged alloy substrates and diamond particles were sparsely dispersed on as-cast alloy substrates. The carbide surface layer has the potential to increase the wear resistance of the Co-Cr-Mo alloy as a wear resistant coating.

GLOW DISCHARGE PLASMA CHROMISING

The layer morphology and structure resulting from the glow discharge plasma chromising of steels containing 0·22, 0·45, 1·0, and 1·2%C are discussed. The growth kinetics of chromium and carbide layers has been investigated; the relationship between layer thickness and chromising time follows a parabolic law. An equation for growth of the surface carbide layer as a function of matrix carbon activity and chromising time has been derived. The calculated and measured values are in good agreement. It is shown that ion bombardment accelerates chromising.

Effect of carbides on the hydriding and oxidation behavior of a Zr-2.5Nb alloy

The surface carbide layer, interior chain-like carbide arrangements and the carbide/matrix interfaces present in an initial Zr-2.5Nb alloy were studied in hydriding and oxidation experiments. Hydriding performed by cathodic charge in 0.1 M H 2SO 4 solution showed that neither the inner carbide chains nor the corresponding interfaces offered short circuit paths for hydrogen diffusion, while the surface carbide layer acted as a protective layer. The oxidation tests performed in autoclaves during 24 h in deaerated steam at 400°C and 105 bar showed weight gains higher than one order of magnitude with respect to the samples free of carbides. The surface carbides promoted the accelerated oxidation of the surrounding matrix and were stripped during the experiments. Evidence was found of the oxidation of the inner carbides, probably due to the transformation strain energy produced by the former oxidation of the surrounding matrix. Moreover, the carbide/ matrix interfaces acted as short circuit paths for oxygen diffusion.

Diffusion Chromising of Steel by CVD Using Chloride Compounds: 4 Influence of Steel Composition

The effects of carbon content and low titanium additions on the structure and thickness of chemical vapour deposited chromium coatings produced at 930°C have been studied. Diffusion of chromium in these steels has been examined by calculation of the average diffusion coefficients and determination of the concentration dependence of the interdiffusion coefficient by the Boltzmann- Matano method. The structure and thickness of the coating obtained depend largely on the amount of soluble carbon in the steel. The steels that contained 0.002–0.004% soluble carbon were most suitablefor chromising since only a very thin surface carbide layer was formed. Those steels which contained &gt;0·05% soluble carbon at 930°C produced coatings containing dispersions of (Cr,Fe)23C6 carbide. Such coatings remain thinner and are no longer pure diffusion coatings.

Stability of bulk and surface carbide layers and their relation to the Fischer-Tropsch hydrocarbon synthesis

Using a combination of the recently developed selfconsistent layer Koringa-Kohn-Rostoker LKKR) scheme and a non-selfconsistent cluster method, the local densities of states for bulk and surface carbides of iron, nickel and copper have been determined. Ia the bulk carbides, a decrease in carbon-metal bonding is observed, due both to decreased splitting of the carbon p orbitals, and an increase in filling of the antibonding p orbital. This exactly mirrors the observed trend in stability of the carbides, decreasing from iron to nickel, with copper carbide unknown. Similar trends are observed from the LDOS of the surface carbides, and it is concluded that the carbidic carbon species is thermodynamically unstable at copper surfaces. The catalytic implication of these results and those of Feibelman for ruthenium and rhodium carbides is discussed, with particular reference to their implications for the mechanism and selectivity of these metals in Fischer-Tropsch synthesis.

ChemInform Abstract: Formation of a Surface Carbide Layer During Sintering of Titanium Diboride.

AbstractA thin TiC surface layer forms during sintering of TiB2 in a graphite furnace by a mechanism which involves surface oxidation by CO2/CO.

Formation of a Surface Carbide Layer During Sintering of Titanium Diboride

Titanium diboride, vacuum sintered in a graphite furnace, can develop a thin TiC surface layer. A mechanism for carbide formation is proposed which takes into account the changing p(CO2)/p(CO) ratio as a function of temperature. At low temperature, TiB2 is oxidized by CO2 to give a surface oxide layer. As the temperature is increased, the reduced p(CO2)/p(CO) ratio allows formation of TiC via a reaction between the oxide layer and CO. The proposed mechanism is supported by thermochemical calculation and microstructural analysis.

Influence of sulfur on the nature and morphology of carbides formed on the surfaces of Fe-Ni-Cr alloys at high temperatures

The influence of sulfur upon surface carbide formation on a range of Fe-Ni-Cr alloys has been investigated at a temperature of 1273 K. The addition of small amounts of H2S, up to 100 ppm (range of $$p_{S_2 } $$ = 2.2×10−12 to 5.5×10 su−11 bar), to a H2-CH4 carburizing atmosphere (ac=0.8) was found to significantly modify the behavior from that normally observed in the sulfur-free environment. The carburization of these alloys in the H2-CH4 atmosphere led to the formation of globular particles of M7C3 on the surface of the alloy, but the addition of H2S affected the type, morphology, and distribution of the surface carbides. Initially, the lower carbon containing M23C6 was formed, which transformed to M7C3 at a rate determined by the concentration of sulfur in the environment. The morphology of the M7C3 was modified by the presence of sulfur, and the carbide exhibited a preferred crystallographic orientation in the [001] direction. Particles of manganese sulfide were formed on the commercial alloys at $$p_{S_2 } $$ > 2.3 × 10−11 bar at 1273 K, and these served as nucleation sites for carbides sothat, in contrast to the behavior in sulfur-free conditions, complete surfacecarbide layers were formed.

Carbide coating process by use of molten borax bath in Japan

Pore-free and smooth surface carbide layers can be formed on metals and carbon immersed in a molten borax bath above 1073 K. The carbide layers, consisting of VC, NbC, TiC, or Cr7C3, are formed by the reaction between the carbon atoms in the substrate and the carbide forming element atoms dissolved into the fused borax from additive powders such as ironvanadium, ironniobium, iron-titanium, ironchromium, and chromium. Core hardening of steel substrates can be simultaneous or by reaustenitizing after carbide coating. Carbide coated steels show excellent resistance to wear, seizure, corrosion and oxidation. The carbide layers are adherent to the substrate and do not exfoliate in severe service as in cold forming. The process effectively improves performance of dies, tools, machine parts, and is applied in various fields of Japanese industries.

Formation of a diffusion layer on sintered ZhGr2 material

1. A study was made of the formation of a diffusion layer on sintered ZhGr2 iron—graphite material during complex diffusion alloying. It is shown that the rate of formation of a surface diffusion layer on a part varies as a function of impregnation time. 2. The chemical composition of the material investigated was found to affect the process of diffusion layer formation. It was established that a surface carbide layer forms through diffusion of “carbide” carbon into the outer kinetic range. Diffusion of impregnating elements toward the carbide/starting-basis interface leads to the formation of an alloyed γ solid solution layer.

Formation of carbides on rubbing metal surfaces and its influence on wear resistance

Data were obtained showing that lubricated friction between alloys containing carbide-forming elements may lead to the formation of etch-resistant surface carbide layers. Metallographie and X-ray diffraction analyses established that the necessary condition for the formation of surface layers of this kind is local seizing of the rubbing metal surfaces. The formation of these layers leads to intense wear of at least one of the friction pair components.

Diffusion phenomena on rubbing surfaces in relation to wear resistance

When steel parts are working under heavy-duty conditions, there is always a possibility of the formation of carbides on their rubbing surfaces as a result of decomposition of lubricants and subsequent reaction of carbon with the metal. This process is inevitably associated with local seizing and mutual material transfer. As a result, the carbide phase formed during friction betwen two steels of a different composition may differ from the adjacent material in respect to its content of carbide-forming elements. When strong carbide-forming elements are present in a steel with a friction-induced surface carbide layer, heating such a steel to relatively low temperatures leads to localized decarburization which, under certain circumstances, may reduce the wear resistance of steel. In a simple case of a binary alloy, the rate of decarburization depends on the affinity of the alloying element to carbon.