A new possibility of building up thin diamond films in precipitation of carbon from the gaseous phase

A new method for building up thin diamond films is suggested based on the difference in the rates of sublimation of graphite and diamond. It is recommended that this method be used for producing diamond films at temperatures above ∼1000°C without application of a chemical etching agent. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 73, No. 2, pp. 454–457, March–April, 2000.     

A model SiC-based fibre with a low oxygen content prepared from a polycarbosilane precursor

A model SiC-fibre has been prepared from a polycarbosilane precursor by means of an irradiation oxygen-free curing process. The chemical composition remains unchanged after heat treatments under an inert atmosphere for pyrolysis temperatures of 1600°C. At this temperature, the fibre consists of SiC nanocrystals (mean size 6–10 nm) and free carbon. However, a slow grain growth takes place as the temperature is increased. The fibre retains a high strength at room and high temperatures up to temperatures of 1600 °C when the pyrolysis has been performed under nitrogen. The electrical conductivity was studied as a function of the pyrolysis temperature Tp: For 1100≤Tp≤1200 °C, the conductivity increases by several orders of magnitude due to the reorganization of the free carbon phase at the SiC grain boundaries. Oxidation kinetics of the filaments remain parabolic from 1000–1400 °C. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of the La<Subscript>2</Subscript>O<Subscript>3</Subscript> sol–gel coating on the alumina scale adherence on a model Fe–20Cr–5Al alloy at 1100 °C

The effect of lanthanum sol–gel coatings was studied in order to improve the alumina scale adherence during the model Fe–20Cr–5Al alloy oxidation, at 1100 °C, in air. Various sol–gel coating procedures were applied. Argon annealing of the lanthanum sol–gel coating was tested at temperatures ranging between 600 and 1000 °C. The coating crystallographic nature was characterized by X-ray diffraction (XRD) depending on the annealing temperature. The oxidation process has been examined at 1100 °C by in situ XRD on blank Fe–20Cr–5Al, sol–gel coated and argon-annealed specimens. This study shows that the coating argon annealing at 1000 °C leads to the preferential formation of LaAlO₃ instead of La₂O₃. This coating procedure leads to an alumina scale formation showing the best adherence under thermal cycling conditions at 1100 °C.     

Mechanical properties and oxidation behaviors of carbon/carbon composites with C–TaC–C multi-interlayer

To improve the mechanical properties and oxidation-resistance properties, a C–TaC–C multi-interlayer structure was introduced in carbon/carbon (C/C) composites by chemical vapor infiltration. Compared with conventional C/C composites, a higher fracture toughness and strength have been achieved by using the C–TaC–C multi-interlayer. In addition, the composites also exhibit a higher preliminary oxidation temperature and a lower mass loss at high temperatures. The oxidation rate of the composites increases with temperature increasing in the range of 700–1300 °C, reaching a maximum value at 1300 °C, then decreases in 1300–1400 °C. A hexagonal structure of Ta₂O₅ phase is obtained when being oxidized at 700–800 °C, and it transforms to an orthorhombic phase at temperatures above 900 °C. The structures of C–TaC–C multi-interlayer are intact without cracks or porosities after being oxidized at 700–800 °C. In 900–1300 °C, the composites are oxidized uniformly with the formation of pores. At temperatures above 1300 °C, there are oxidation and non-oxidation regions with the oxidation process being controlled by diffusion.     

Oxidation behaviour of the near α-titanium alloy IMI 834

Oxidation behaviour of the near α-titanium alloy IMI 834 was investigated over a range of temperatures, from 600–800°C, in air. Specimens were solution-treated in the α + β and β phase fields for 1 h and 1/2 h, respectively and cooled in air to room temperature. The solution treated samples were subjected to stabilization treatment at 700°C for 2 h, followed by cooling in air. Oxidation behaviour of these samples was studied from 600–800°C in air, for 50 h. The morphology of the scales formed was examined by SEM and the phases present in the scales were characterized by X-ray diffraction. While there was little oxidation at 600°C, the rate of oxidation increased at higher temperatures. In general, the rate of oxidation was found to be more in the α + β treated condition than that in the β treated one. The results are discussed in terms of the characteristics of the oxide film formed under different conditions.     

Correlation between microstructure and mechanical behaviour at high temperatures of a SiC fibre with a low oxygen content (Hi–Nicalon)

An oxygen free Si–C fibre has been studied in terms of the chemical, structural and mechanical properties produced as a function of annealing treatments. In spite of its high thermal stability with regard to a Si–C–O fibre the Si–C fibre was subject to moderate SiC grain growth, organization of the free carbon phase and densification within the temperature range 1200–1400°C. The strength reduction at ambient for temperatures ≤1600°C could possibly be due to SiC coarsening or superficial degradation. Bend stress relaxation (BSR) and tensile creep tests show that the as-received fibre undergoes a viscous flow from 1000°C. The thermal dependance of the creep strain rate strongly increases at temperatures ≥1300°C. This feature might be partly explained by the structural evolution of the fibre occurring above this temperature. Heat treated fibres (1400–1600°C) exhibit a much better creep strength, probably due to their improved structural organization. This revised version was published online in November 2006 with corrections to the Cover Date.     

Microstructures and high temperature oxidation resistance of alloys from Nb–Cr–Si system

Oxidation behavior of Nb–30Si–(10,20)Cr alloys have been evaluated in air from 700 to 1400 °C by heating for 24 h and furnace cooling them. The lower weight gain per unit area has been observed for 20Cr alloy at 1200, 1300, and 1400 °C. Pesting has been observed at lower temperatures (700, 800, 900 °C). Analysis indicates that the powder formation at 900, 100, 1100 °C may be associated with β form of Nb₂O₅ (base centered monoclinic form). However the m-monoclinic form of Nb₂O₅ evolves at temperatures above 900 °C while o-orthorhombic Nb₂O₅ forms at below this temperature. The phases in the alloys have been calculated using the Pandat^TM software program at different temperatures using calculated Nb–Cr–Si phase diagrams.     

Conversion of copper and manganese metallic films to spinel coating

Oxidation of copper + manganese metallic thin films on UNS 430 stainless steel was studied at temperatures of 600–950 °C. The Cu–Mn metallic films were converted to Cu–Mn spinel coating in the temperature range of 600–950 °C as confirmed by X-ray diffraction. Conversion mechanisms of metals to the spinel were investigated. Examination by SEM showed that negligible oxidation of the substrate alloy occurred in the coated samples at temperatures up to 750 °C; a transition layer formed between the substrate and spinel coating at higher temperatures. Cu–Mn spinel coating can provide protection to the metallic substrate at temperatures up to 850 °C. Formation of the spinel coating was due to reactions of CuO, which formed through outward diffusion of copper, with manganese oxides, which formed through inward diffusion of oxygen.     

Dynamic constitutive behavior of Hastelloy X under thermo-mechanical loads

An experimental investigation has been conducted to study the dynamic constitutive behavior of Hastelloy X (AMS 5754) at room and elevated temperatures under varying rates of loading. A split Hopkinson pressure bar (SHPB) apparatus was used in conjunction with an induction coil heating system for applying dynamic loads at elevated temperatures. Experiments were carried out at different temperatures ranging from room temperature (25 °C) to 1,100 °C at an average strain rate of 5000/s. Room temperature experiments were carried out at varying strain rates from 1000 to 4000/s. The results show that as the strain rate increases from quasi-static to 4000/s, the yield strength increases by approximately 50%. Also, under dynamic loading, the yield stress decreases with temperature up to 700 °C, after which it shows a peak at 900 °C before beginning to decrease again as the temperature is further increased. The Johnson–Cook model was used to predict the dynamic plastic response under varying rates of loading and at different temperatures.     

Surface changes during pyrolytic conversion of hybrid materials to oxycarbide glasses

Hybrid materials from TEOS–TBOT–PDMS have been prepared and pyrolyzed between 400 and 1,000 °C. The surface characteristics of this type of materials have been studied by nitrogen adsorption, mercury intrusion porosimetry, and inverse gas chromatography at infinite dilution (IGC). IGC has been used for obtaining the dispersive and acid–base surface energies of the different materials. The specific surface areas and pore volumes of the studied samples have resulted to increase the pyrolysis temperatures ranging between 400 and 600 °C and decrease for higher temperatures. On the other hand, surface energies increase when the materials are pyrolyzed between 400 and 800 °C and then decrease after pyrolysis at 1,000 °C. When the material is pyrolyzed at the highest temperature, the surface energies are close to that of typical glasses. It has been observed that pyrolyzing at 800 °C the material has the highest values of both components of the surface free energy (dispersive and specific). The surface energy–pyrolysis temperature variation does not correspond to the formation of micropores in the material during the pyrolysis process. Therefore, it has been assumed that high energy active sites must be formed on the surface when the materials are pyrolyzed at 800 °C.     

Modeling silicon–germanium interdiffusion by the vacancy exchange and interstitial mechanisms

We present a comprehensive model for the interdiffusion of silicon (Si) and germanium (Ge) in single quantum well (SQW) structures. The model includes the vacancy exchange mechanism as well as interstitial diffusion. We explicitly account for lattice site conservation and the effect of Ge on the various material properties. We use well-established values from the literature without need of any adjustable parameters. The model fits experimental data for up to 25% Ge over the temperature range 800–1,200 °C under inert and oxidizing conditions. At high temperatures, nitriding anneals are also considered. We find that diffusion is dominated at temperatures below ∼1,050 °C by the vacancy exchange mechanism, whereas the silicon interstitial dominates the diffusion at higher temperatures at least up to 15% Ge.     

Deposition of diamond films on metal substrates

In this paper we report on the growth of polycrystalline diamond films on Mo, W, and Ni substrates using oxy-acetylene combustion flame technique. Effect of substrate temperature on the growth of diamond films has been studied in the temperature range 600–1100°C. The deposits and their surface morphology has been characterized by X-ray diffraction and scanning electron microscopy (SEM). A short duration pretreatment of Mo substrates by outer zone of the oxy-acetylene flame at lower substrate temperatures, results in the improvement of quality and adherence of the films. Growth of diamond as well as other intermediate compounds depending on the nature of substrates and interface layers is discussed. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

High-temperature fluorohydroxyapatite-titanium interaction

We have studied chemical interaction between fluorine-substituted hydroxyapatite and metallic titanium in the temperature range 700–1000°C. The results indicate that the phase relations in this system differ markedly from those in the hydroxyapatite-titanium system under similar conditions. The most important result is that the apatite phase persists in the former system, whereas in the hydroxyapatite-titanium system the hydroxyapatite decomposes at 900°C to form calcium titanate. The process common to the two systems is the complete oxidation of titanium and the formation of rutile at temperatures above 800°C.     

Preparation of ceria-based polishing powders from carbonates

The preparation of Ce(IV) oxide-based polishing powders by thermal decomposition of carbonates was studied and the effect of the calcination process conditions on the properties of product was examined. High-grade polishing powders can be produced at calcination temperatures 700–1200°C, the maximum polishing efficiency is achieved by calcining at 1000–1100°C. The observed dependences were interpreted from the standpoint of the present mechanochemical theory of glass polishing.     

Fullerenes catalyze the graphite-diamond phase transition

The effect of fullerenes on the graphite-diamond phase transition was experimentally studied. Adding a catalytic amount of fullerenes to graphite during the diamond synthesis at relatively low pressures (4.5–5.5 GPa) and temperatures (about 1200°C) increases the percentage graphite to diamond conversion by a factor of 1.8 as compared to the case of an initial charge containing only graphite or fullerene. Adding fullerene to graphite allows the pressure and temperature of the synthesis to be decreased.     

Interfacial interaction of solid cobalt with liquid Pb-free Sn–Bi–In–Zn–Sb soldering alloys

Dissolution kinetics of cobalt in liquid 87.5%Sn–7.5%Bi–3%In–1%Zn–1%Sb and 80%Sn–15%Bi–3%In–1%Zn–1%Sb soldering alloys and phase formation at the cobalt–solder interface have been investigated in the temperature range of 250–450 °C. The temperature dependence of the cobalt solubility in soldering alloys was found to obey a relation of the Arrhenius type c _s = 4.06 × 10² exp (−46300/RT) mass% for the former alloy and c _s = 5.46 × 10² exp (−49200/RT) mass% for the latter, where R is in J mol⁻¹ K⁻¹ and T in K. For tin, the appropriate equation is c _s = 4.08 × 10² exp (−45200/RT) mass%. The dissolution rate constants are rather close for these soldering alloys and vary in the range (1–9) × 10⁻⁵ m s⁻¹ at disc rotational speeds of 6.45–82.4 rad s⁻¹. For both alloys, the CoSn₃ intermetallic layer is formed at the interface of cobalt and the saturated or undersaturated solder melt at 250 °C and dipping times up to 1800 s, whereas the CoSn₂ intermetallic layer occurs at higher temperatures of 300–450 °C. Formation of an additional intermetallic layer (around 1.5 μm thick) of the CoSn compound was only observed at 450 °C and a dipping time of 1800 s. A simple mathematical equation is proposed to evaluate the intermetallic-layer thickness in the case of undersaturated melts. The tensile strength of the cobalt-to-solder joints is 95–107 MPa, with the relative elongation being 2.0–2.6%.     

Continuous SiC-based model monofilaments with a low free carbon content: Part II From the pyrolysis of a novel copolymer precursor

Quasi stoichiometric model SiC monofilaments (C/Si atomic ratio ≈ 1.02) with still some free carbon (≈3 mol%) and residual oxygen have been produced from a novel copolymer precursor, itself prepared from methylphenyldichlorosilane and 2,4-dichloro-2,4-disilapentane. The continuous green fibre was melt spun at 230°C, cured by electron-beam irradiation, and pyrolysed under argon at temperatures, T_p, in the range 1000–1600°C. The fibre remained nanocrystalline at high temperature with the SiC grain size growing from 1.5 nm to 7.3 nm when T_p was raised from 1400°C to 1600°C. Its Young's modulus continuously increased as T_p was raised (with E=320 GPa for T_p=1400°C), whereas its tensile strength at room temperature underwent a maximum for T_p=1200°C (σ_R≈1850 MPa for L=10 mm and d≈20 μm). This revised version was published online in November 2006 with corrections to the Cover Date.     

Structural and chemical changes of thermally treated bone apatite

The thermal behaviour of the animal by-product meat and bone meal (MBM) has been investigated in order to assess how it is affected structurally and chemically by incineration. Initially composed of intergrown collagen and hydroxyapatite (HAP), combustion of the organic component is complete by 650 °C, with most mass loss (50–55%) occurring by 500 °C. No original proteins were detected in samples heated at 400 °C or above. Combustion of collagen is accompanied by an increase in HAP mean crystallite size at temperatures greater than 400 °C, from 10 nm to a constant value of 120 nm at 800 °C or more. Newly formed crystalline phases appear beyond 400 °C, and include β-tricalcium phosphate, NaCaPO₄, halite (NaCl) and sylvite (KCl). Crystallite thickness as judged by small angle X-ray scattering (SAXS) increases from 2 nm (25–400 °C) to 8–9 nm very rapidly at 550 °C, and then gradually increases to approximately 10 nm. The original texture of HAP within a collagen matrix is progressively lost, producing a porous HAP dominated solid at 700 °C, and a very low porosity sintered HAP product at 900 °C.     

Polymer–ceramic conversion of a highly branched liquid polycarbosilane for SiC-based ceramics

Liquid polycarbosilane (LPCS) with a highly branched structure was characterized by fourier-transform infrared spectrometry (FT-IR) and ¹H, ¹³C, ²⁹Si nuclear magnetic resonance spectrometry (NMR). The LPCS was then cured and pyrolysized up to 1,600 °C under flowing argon. The structural evolution process was studied by thermogravimetric analysis and differential scanning calorimetry (TG-DSC), FT-IR, and X-ray diffraction (XRD). Hydrosilylation, dehydrocoupling, and polymerization cross-linking reactions between Si–H and C=C groups occurred at low temperatures, which mainly accounted for the high ceramic yield (70%) up to 1,400 °C. The organic groups gradually decomposed and the structure rearranged at high temperatures. The FT-IR analysis revealed that Si–CH₂–Si chains, the backbone of original polymer, can be retained up to 1,200 °C. At temperatures higher than 1,200 °C, the Si–CH₂–Si chains broke down and crystalline SiC began to form. The final crystalline products were β-SiC and a small amount of carbon.     

Properties of polymeric compositions based on epoxy and functional petroleum polymeric resins

We studied cross-linking, water resistance, and impedance characteristics of polymeric compositions based on epoxydiane and maleinized petroleum polymeric resins. It is shown that epoxy and petroleum polymeric components interact at temperatures of 150°C and higher to form cross-linked copolymers. The gel-fraction content of the composition increases with the acid number of the petroleum polymeric component. Small amounts of tertiary amine enhance the efficiency of its cross-linking. The water resistance and protective properties of epoxy-petroleum polymeric compositions improve as the acid number of petroleum polymeric resins increases and the content of the liquid fraction decreases. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 6, pp. 37–42, November–December, 1996.