Microstructural characterization of diamond/CBN grains steel braze joint interface using Cu–Sn–Ti active filler alloy

In order to develop the new generation superhard abrasive tools of diamond and cubic boron nitride (CBN), the brazing joint experiments of diamond/CBN crystals and AISI 1045 steel matrix using Cu–Sn–Ti active filler powder alloy were investigated in vacuum furnace. The brazing temperature was 930 °C and the dwelling time was 20 min. Interfacial characteristics of the brazing joint among the diamond/CBN grains, the active filler layer and the steel substrate were analyzed using scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction techniques. The results indicated that Ti element in the Cu–Sn–Ti alloys diffused preferentially to the surface of diamond/CBN grits to form a Ti-rich reaction layer in the brazed joints by microanalyses. Moreover, the TiC, TiN and TiB₂ phases in diamond/CBN interface and Cu–Ti phase in steel interface were confirmed by X-ray diffraction phase analysis. The wetting and bonding reactions on diamond/CBN by melting Cu–Sn–Ti alloy were realized through the interfacial reaction products like TiC, TiN and TiB₂ compounds during the brazing process. The adhesive strength experiments of the joint interfaces revealed that the grains were not pulled out from the bond interface. The reliable bonding strength of brazed diamond/CBN grains to the steel substrate can meet the application requirements of high efficiency machining in the industrial field.     

Microstructure instability in TiC-316L stainless steel cermets

Titanium carbide (TiC) based cermets are commonly used in wear and corrosion resistance applications. The microstructural evolution, and related compositional instability, of TiC-based cermets prepared with a 316-L stainless steel binder is described in the present work. Samples were fabricated using a simple vacuum melt-infiltration procedure, with 5 to 30 vol.% binder. Infiltration temperatures ranged from 1475 °C to 1550 °C, held for up to 240 min, typically resulting in sintered samples with densities in excess of 99% of theoretical. It is demonstrated that irregularly shaped grains (concave/hollow) can arise after sintering, especially at 1475 °C, which is discussed in terms of the ‘instability of the solid-liquid interface’ theory. It is demonstrated that a complex, multi-layer core-rim structure arose for the cermets, with accommodation of selected steel constituents into the rim of the TiC grains. In particular, it is shown that the Mo in the original 316-L stainless steel is essentially fully depleted from the metallic binder phase, forming a Mo-rich inner-rim layer on the TiC grain cores.     

Vacuum brazing of TZM alloy to ZrC particle reinforced W composite using Ti-28Ni eutectic brazing alloy

Reliable brazing of TZM alloy and ZrC particle reinforced (ZrC_p) W composite was achieved in this study by using Ti-28Ni eutectic brazing alloy. The typical interfacial microstructure of TZM/Ti-28Ni/ZrC_p-W brazed joint consisted of a Ti solid solution (Ti(s, s)) layer, a continuous Ti₂Ni layer and a diffusion layer mainly composed of W particles and (Ti, Zr)C particles. With an increase of brazing temperature, more ZrC particles and W particles entered the molten brazing alloy, which broadened the brazing seam and diminished the Ti₂Ni layer, resulting in the disappearance of the Ti₂Ni layer eventually. Meanwhile, more Ti(s, s) stripes were observed on the TZM side. The presence of continuous Ti₂Ni intermetallic phase and Ti(s, s) stripes structure in joints deteriorated the joining properties, which resulted in the formation of brittle fracture under shear test. In addition, the fracture path was related to the brazing temperature, and cracks initiate and propagate in the continuous Ti₂Ni layer at lower temperatures. However, the fracture path tended to be located at the TZM substrate close to the interface between TZM and the brazing seam when the brazing temperature exceeded 1040 °C. The optimal room temperature shear strength reached 120.5 MPa when brazed at 1040 °C for 10 min and the fracture surface exhibited cleavage fracture characteristics, and the shear strength at high temperature of 800 °C for the specimens with highest shear strength at room temperature reached 77.5 MPa.     

Effect of vacuum annealing temperature on tribological behaviors of sintered polycrystalline diamond compact

The sintered polycrystalline diamond compacts (PDCs) were annealed at 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, and 800 °C under vacuum environment. The friction and wear behaviors of the annealed PDCs sliding against Si₃N₄ balls were evaluated by a ball-on-disc tribometer in ambient atmosphere. The compositions, microstructures and surface morphologies of PDC discs and wear scars on Si₃N₄ balls were characterized by energy dispersive spectroscopy (EDS), Raman spectroscopy, and scanning electron microscopy (SEM), respectively. The experimental results demonstrated that the steady friction coefficient decreased at the annealing temperature of 200 °C and increased with annealing temperature increasing. While, the wear rate of PDCs and Si₃N₄ balls increased at 200 °C, and sharply decreased from 300 to 800 °C. The surface morphologies and Raman spectra revealed that the variation law of friction coefficient curves at different annealing temperatures was attributed to carbonaceous transfer films formed on Si₃N₄ balls. The residual stress on PDC surface was reduced after the annealing treatment, thus fine diamond grains were easily extracted from PDC surface onto the contact area during the tribotest which led to the wear of PDC and abrasive wear for both counter parts. These results revealed that the friction and wear behaviors of PDC were significantly affected by the vacuum annealing temperature.     

Controlling conditions for synthesis of iron-titanium carbide using two different titanium precursors

Preparation of Fe–TiC composite from mixtures of carbon black and two different titanium bearing minerals (black sand ilmenite and natural rutile) was studied. Milled (mechanically activated) and unmilled carbon containing mixtures were prepared and then heated at temperatures 1200 °C and 1300 °C for 3 h under an inert atmosphere. The reaction progress, as well as reaction products, was evaluated using thermogravimetric analysis (TGA-DTA), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and energy dispersive X-ray spectroscopy (EDX). The feasibility of producing Fe–TiC composites from titanium bearing materials mixed with carbon black was proved. Fe–TiC could be produced by carbothermic reduction of mechanically activated black sand ilmenite containing mixtures milled for 50 h and heated up to 1200 °C. On the other hand, 60 h milling followed by heating up to 1300 °C was needed in case of natural rutile containing mixture. The morphology of the Fe–TiC produced from black sand ilmenite showed a homogeneous distribution of Fe and TiC enriched areas, while the Fe–TiC produced from natural rutile showed intense distribution of TiC phase with traces of iron and lower titanium oxide.     

Investigation on the brazing mechanism and machining performance of diamond wire saw based on Cu-Sn-Ti alloy

Brazing connection between diamond particles and KSC82 carbon steel wire was established by the Cu-Sn-Ti alloy, and a diamond wire saw of 500 m in length and about 0.75 mm in diameter was fabricated. The brazing morphology of the diamond particles was observed using scanning electron microscopy (SEM), and the products and elemental distribution characteristics at the diamond brazed interface were analyzed by the energy disperse spectroscopy (EDS) and X-ray diffraction (XRD). The tensile mechanical properties of the brazed diamond wire saw was obtained through tensile tests, and the morphology of the fracture was observed using the SEM to analyze the tensile fracture mechanism. Further, the diamond wire saw was used for slice processing test of G663 granite, and the failure mode of the wire saw was analyzed. The results showed that there was Ti segregation at the diamond brazing interface, and that Ti₂C new phase was detected at the interface, where brazing connection of diamond particles was achieved through by reactive wetting. The tensile and yield strengths of the brazing diamond wire saw were 1289.08 and 923.18 MPa respectively, its plasticity was twice that of original KSC82 steel wire, and the tensile failure mode of the wire saw was ductile fracture. The stable cutting efficiency of the brazing diamond wire saw cutting the G663 granite with cross-sectional dimensions of 480 mm × 260 mm could reach 15 mm/min. There were three abrasive wear modes for the diamond particles of the wire saw working layer, including normal wear, shear fracture and separation, of which separation accounted for 14.3%. The reason for the separation of diamond was attributed to the oxidation of Ti element in Cu-Sn-Ti alloy and the fatigue crack initiation and growth at the diamond brazing interface.     

The time behaviour of surface applied fluorine inducing the formation of an alumina scale on gamma-TiAl during oxidation at 900 °C in air

Recently the target temperature of components manufactured from gamma-TiAl alloys like turbine blades, turbocharger rotors or automotive valves has been increased to 900 °C. However, there is an insufficient oxidation resistance above 750 °C. One method used to improve the gamma-TiAl oxidation behaviour is the so-called fluorine microalloying effect. After application of fluorine to the TiAl surface by ion implantation or treatment with diluted HF and oxidation at 900 °C in air a dense alumina layer is formed. The aim of this work was firstly to study the short time development of the fluorine concentration during heating up to 400–1000 °C (1 h/air) in steps of 100 °C. Using ion beam analysis the depth profiles of F, Al, Ti and O were obtained simultaneously and non-destructive. A distinct loss of fluorine was found between 400 °C and 500 °C. At temperatures above 800 °C an alumina layer was formed with fluorine maximum located at the metal/oxide interface. Secondly the long time behaviour during oxidation of up to 500 h/900 °C/air was investigated showing a slow fluorine decrease. The alumina layer acts as a diffusion barrier for fluorine, whereas fluorine diffuses into the metal. The diffusion coefficient was calculated. The results fit into the theoretical model assuming a selective transport of gaseous aluminium fluorides at the metal/oxide interface.     

Microstructural evolution during reactive brazing of alumina to Inconel 600 using Ag-based alloy

A metal–ceramic bonding process was developed to produce vacuum tight alumina–Inconel 600 joints using an Ag-based active metal brazing alloy that can withstand continuous operating temperature up to 560 °C. The microstructure and microchemistry of the braze zone was examined using extensive microanalysis of the constituent phases and a mechanism for the interfacial reactions responsible for the bonding is proposed. Prolonged heat treatment at 400 and 560 °C under simulated in-service conditions revealed that the microstructure of braze zone of the joints was stable and maintained leak-tightness and strength. The bond strength of the interface was high enough to cause failure in the alumina side of the joints. Failure of the joints was caused by initiation of crack on the surface of alumina as a result of high tensile residual stress adjacent to the metal–ceramic interface.     

Comparison of the wear behavior of WC/(FeAl-B) and WC-Co composites at high temperatures

In this research, the sliding wear behavior of the hot pressed WC/40 vol%(FeAl-B) composites was investigated at temperatures ranging from the ambient one to those as high as 600 °C. The composites were then compared with hot pressed WC-40 vol%Co and commercial WC-16 vol%Co (H10F) in terms of their mechanical properties and high temperature wear behavior. It was found that the WC/(FeAl-B) composite recorded its maximum wear resistance at all the experimental temperatures, which was higher than that of WC-40 vol%Co at these same temperatures due to the higher hardness of the FeAl-B than that of the Co matrix. Also, WC/(FeAl-B) exhibited a higher wear resistance at lower temperatures and a more proper behavior at higher temperatures than did the commercial WC-16 vol%Co; this was attributed to the higher strength of the FeAl-B matrix at high temperatures. Examination of the wear surfaces revealed that abrasion was the wear mechanism in the commercial WC-16 vol%Co and WC/(FeAl-B) composites at both ambient temperature and 300 °C. At 400 °C, however, the wear mechanism was more of an adhesive one, while binder oxidation was observed at 600 °C.     

Mechanical properties and microstructure of TiB2–TiC composite ceramic cutting tool material

TiB₂–TiC composite ceramic cutting tool material was prepared by sintering during hot-pressing in vacuum. The effects of nano-scale Ni and Mo additives and sintering heating rate on mechanical properties and grain characteristics were investigated. TiB₂ and TiC grains exhibited prismatic and equiaxed shapes respectively. The diameter and aspect ratio of prismatic TiB₂ grains were influenced by nano-scale Ni/Mo additives. A higher heating rate could cause a higher aspect ratio of prismatic TiB₂ grains. The good mechanical properties of TN1((TiB₂–TiC)/Ni composite ceramic sintered at a heating rate of 50 °C/min) were ascribed to a relatively fine and homogenous microstructure. And a brittle B₄MoTi solid solution phase and wider distribution of grain size induced the lower flexural strength of TNM2((TiB₂–TiC)/(Ni,Mo) composite ceramic sintered at heating rate of 100 °C/min), but the higher aspect ratio of TiB₂ grains could prevent cracks from propagating and ameliorated the fracture toughness. The optimum resultant mechanical properties were obtained by (TiB₂–TiC)/Ni composite ceramic sintered at a heating rate of 50 °C/min.     

Anomalous yielding in the complex metallic alloy Al13Co4

The single crystal deformation behaviour of orthorhombic Al₁₃Co₄ has been studied below the brittle-ductile transition temperature observed in bulk material from room temperature to 600 °C, using indentation, microcompression and transmission electron microscopy. At room temperature, slip occurred most easily by dislocation motion on the (0 0 1)[0 1 0] slip system, as observed in the ductile regime at high temperatures. However, as the temperature was increased towards 600 °C, the slip pattern changed to one consisting of linear defects running perpendicular to the loading axis. Serrated flow was observed at all temperatures, although at 600 °C the magnitude of the serrations decreased. Anomalous yielding behaviour was also observed above 226 °C, where both the yield and the 2% flow stress increased with temperature, almost doubling between 226 and 600 °C. It has been suggested that this might arise due to the increasing stability of orthorhombic Al₁₃Co₄ with respect to the monoclinic form with increasing temperature. This is shown to be consistent with the theoretical predictions that exist.     

High temperature oxidation behavior of NiTiNb intermetallic alloys

An intensive experimental study was made of the cyclic oxidation behaviors of NiTiNb alloys (0–7 at.% Nb) with single B2 phase at elevated temperatures of 600 °C and 800 °C in air. The micro-structural evolution of the multi-layers oxide on the alloys was investigated and the effect of Nb on the oxidation characteristics of NiTiNb alloys was evaluated. It is shown that the addition of Nb can remarkably improve the oxidation resistance of NiTiNb alloys at high temperature. Further, the corresponding mechanism for the improvement of the oxidation resistance by Nb addition has been probed. Nb-rich oxide layer formed can effectively impede interdiffusion of oxygen and metallic cations during oxidation process, leading to a remarkable improvement of oxidation resistance of NiTiNb alloys at high temperature.     

Kinetics of Ag3Sn growth in Ag–Sn–Ag system during transient liquid phase soldering process

The kinetics of the interfacial reaction of a thin layer of Sn sandwiched between two pieces of Ag foil has been investigated at temperatures of 260 °C, 300 °C and 340 °C. A time dependence of the form t^1/n with n = 3 was obtained for the kinetics of both the consumption of the Sn remaining and the thickening growth of the Ag₃Sn scallops formed between Sn and Ag. Such a result can be explained well using the model of grain boundary/molten channel-controlled growth of intermetallic compounds. In this case, the diffusion of Ag atoms through the molten channels existing between the previously formed Ag₃Sn scallops is the controlling mechanism for the kinetics. We also report here the derived kinetic constants including reaction constants and the associated activation energy for guiding the practical transient liquid phase soldering of the Ag–Sn–Ag system.     

The carbonization of granular polyaniline to produce nitrogen-containing carbon

Polyaniline (PANI) was prepared by the oxidative polymerization of aniline. The deprotonated product, a PANI base, was carbonized in an inert atmosphere at temperatures up to 800 °C for various times. The mass decreased to 40–50 wt.% at temperatures above 600 °C. The progress of molecular structure during carbonization was followed by infrared and Raman spectroscopies. The carbonization at 650 °C for 1 h is suggested for the optimum conversion of PANI to carbon. The product retained the original globular structure of PANI. The conductivity of the carbonized material was low for carbonizations below 600 °C, <10^?10 S cm^?1, and increased to 10^?4 S cm^?1 after treatment at 800 °C. The content of nitrogen, ～10 wt.%, was not affected appreciably by the carbonization.     

Unlubricated friction and wear behaviors of Al2O3/TiC ceramic cutting tool materials from high temperature tribological tests

The unlubricated friction and wear behaviors of Al₂O₃/TiC ceramic tool materials were evaluated in ambient air at temperature up to 800 °C by high temperature tribological tests. The friction coefficient and wear rates were measured. The microstructural changes and the wear surface features of the ceramics were examined by scanning electron microscopy. Results showed that the temperature had an important effect on the friction and wear behaviors of this Al₂O₃ based ceramic. The friction coefficient decreased with the increase of temperature, and the Al₂O₃/TiC ceramics exhibited the lowest friction coefficient in the case of 800 °C sliding operation. The wear rates increased with the increase of temperature. During sliding at temperature above 600 °C, oxidation of the TiC is to be expected, and the formation of lubricious oxide film on the wear track is beneficial to the reduction of friction coefficient. The wear mechanism of the composites at temperature less than 400 °C was primary abrasive wear, and the mechanisms of oxidative wear dominated in the case of 800 °C sliding operation.     

Oxidation behavior of micro-sized Al2O3–TiC–Co composites prepared from cobalt-coated powders

The oxidation behavior of hot-pressed Al₂O₃–TiC–Co composites prepared from cobalt-coated powders has been studied in air in the temperature range from 200 °C to 1000 °C for 25 h. The oxidation resistance of Al₂O₃–TiC–Co composites increases with the increase of sintering temperature at 800 °C and 1000 °C. The oxidation surfaces were studied by XRD and SEM. The oxidation kinetics of Al₂O₃–TiC–Co composites follows a rate that is faster than the parabolic-rate law at 800 °C and 1000 °C. The mechanism of oxidation has been analyzed using thermodynamic and kinetic considerations.     

Phase and morphology evolution of TiC in the Ti–Si–C system

TiC was synthesized by reactive pyrolysis of a mixture of poly(dimethylsilaacetylide), [-MeSi(H)C≡C-]_n, Ti, and TiSi₂ particles under Ar atmosphere between 1100 °C and 1400 °C. The pyrolysis products were studied by means of X-ray diffraction (XRD), scanning electron microsphere (SEM), and Energy-Disperse Spectrometer (EDS). The results showed that TiC crystals appeared at 1200 °C accompanied by Ti₅Si₃. At 1400 °C, single crystalline phase TiC was obtained. By changing the ratio of raw materials, TiC crystals with different morphologies, including octahedron, truncated-octahedron, and polyhedral were prepared.     

Microstructures and thermal damage mechanisms of sintered polycrystalline diamond compact annealing under ambient air and vacuum conditions

The microstructures and thermal damage mechanisms of sintered polycrystalline diamond compact (PDC) were studied in ambient air and vacuum at the temperature up to 1000 °C. The microstructures and compositions of the annealed PDC were characterized by white light interferometer, X-ray diffractometry (XRD), Raman spectroscopy and scanning electron microscopy (SEM). The results showed that no visible change in the morphologies of surface of PCD layers (PDC surfaces) was observed at 200 °C both in ambient air and vacuum. After annealing at 500 °C, numbers of spalling pits appeared on the PDC surface, and the stress-induced spall mechanism was the dominant thermal damage mechanism in ambient air and vacuum. With the temperature up to 800 °C, the annealed PDC surface in ambient air was seriously damaged with a mixed thermal damage mechanism such as graphitization, oxidation and stress-induced micro-cracks. Whereas, the thermal damage mechanism in vacuum was nearly the same as that at 500 °C. At 900 °C, only a dendritic phase of Co₃O₄ was contained on the annealed PDC surface due to extensive graphitization and oxidation in ambient air. When it comes to vacuum environment, many cracks were observed on the PDC surface and some fine diamond grains near the cracks spalled, which demonstrated that the thermal damage mechanisms consisted of stress-induced crack and spall mechanisms caused by the different thermal expansion coefficients between the diamond and Co phase. Compared with that at 900 °C, the degree of thermal damage reduced at 1000 °C in vacuum because of the diffusion of unevenly distributed Co.     

Direct evidence for stress-induced texture evolution and grain growth of silver thin films upon thermal treatment and self-ion bombardment

Common failure mechanisms in microelectronics, such as electromigration, creep and fatigue, can be positively influenced by microstructure optimization. In this paper a combination of post-deposition heat treatment and self-ion bombardment is proposed as a valid candidate to gain control over the microstructure of (1 1 1) fiber textured thin silver films. Irradiation can induce a strong in-plane texture and hence lead to biaxially textured films through a process of selective grain growth. Moreover, we report microstructural stability of the irradiated regions over a wide range of temperatures (up to 600 °C), in contrast to non-irradiated portions of the film, which underwent abnormal growth of the (1 0 0) out-of-plane oriented grains, and a consequent texture change, at temperatures as low as 195 °C. The thermal stress induced in the film upon heat treatment was quantified in situ and its role in texture change elucidated.     

Investigation of electrical properties of nanostructured carbon films derived from block copolymers

Electrical properties of nanostructured carbon (ns-C) films fabricated by pyrolysis of PAN–b–PBA copolymers were investigated. Films having cylindrical morphology and pyrolyzed at 400, 500 and 600 °C were investigated. Both carbide forming (Zr, Ti) and non-carbide forming (Cu, Pt) metals spanning a wide range of electron work functions (4.1–5.5 eV) formed ohmic contacts to the ns-C films in the as-deposited state. The conductivity of the ns-C films increased roughly three orders of magnitude for every 100 °C increase in the pyrolysis temperature. Hall-effect measurements showed that the films pyrolyzed at 600 °C were n-type with a majority carrier concentration and mobility of 5.8 × 10¹⁸ cm^?3 and 0.97 cm²/V s, respectively. Current–voltage measurements as a function of temperature (I–V–T) were performed on films pyrolyzed at 600 °C, whereas films pyrolyzed at 400 and 500 °C were too resistive for reliable resistivity–temperature and Hall-effect measurements. The resistivity as a function of temperature was analyzed by using the reduced activation energy method and was determined to follow variable-range hopping (VRH) mechanisms at and below room temperature. The data indicates a crossover from Efros–Shklovskii VRH [J. Phys. C 8, (1975) L49] to Mott VRH [J. Non-Cryst. Solids 1, (1968) 1] at temperatures above 100 K.