Microstructure of fracture of a laser–electric spark coating on titanium after abrasive wear

We present results of a study of the microstructure of a ZrB₂-containing combined laser–electric spark coating deposited on a titanium alloy and subjected to wear with a nonrigidly fastened abrasive performed by scanning electron microscopy in combination with an electron-probe microanalysis. It is established that the coating is characterized by a gradient microstructure over the thickness, and its subsurface layer is modified by Zr, O, Si, and C additives. We put forward the assumption that an increase in the hardness of the coating after abrasive treatment is due to its plastic deformation with the fragmentation of the structural elements and alloying with the indicated additives.     

Effect of TiB2 content on the characteristics of spark plasma sintered Ti–TiBw composites

Spark plasma sintering (SPS) was employed to fabricate monolithic titanium and in-situ formed TiB whisker (TiB_w) reinforced titanium matrix composites (TMCs) by adding different amounts of TiB₂ as boron source. The sintering process was completed at 1050 °C for 5 min under 50 MPa. The influences of TiB₂ content (0.6–9.6 wt. %) on microstructural evolution and mechanical properties of TMCs were investigated. Thermodynamics, XRD analysis and microstructural investigations confirmed the in-situ formation of TiB_w in the composite samples. However, some semi-reacted TiB₂ phases, surrounded by TiB coronas, were remained in the microstructure due to the unfinished chemical reaction between the components during a short-time sintering process. The results showed that all samples were appropriately densified by SPS process into the almost dense parts with relative density no less than 97.5%. While bending strength decreased and hardness increased with increasing TiB₂ content, the sample with 4.8 wt. % TiB₂ had the maximum tensile strength. Fractographical assessments showed that the addition of TiB₂ hindered the grain growth of titanium matrix. With increasing TiB₂ content, fracture mode changed from a multiple pattern to a predominantly transgranular and brittle state.     

Fabrication of nano-grained Ti–Nb–Zr biomaterials using spark plasma sintering

Nanostructured near-β Ti–20Nb–13Zr at % alloy with non-toxic elements and enhanced mechanical properties has been synthesized by spark plasma sintering (SPS) of nanocrystalline powders obtained by mechanical alloying. The consolidated bulk product was characterized by density measurements and Vickers hardness (HV), and X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) combined with energy-dispersive spectroscopy (EDX), and transmission electron microscopy (TEM) for structural details. The temperature during spark plasma sintering was varied between 800 and 1200 °C, while the heating rate and holding time of 100°K/min and 10 min were maintained constant in all the experiments. The effect of SPS temperature on the densification, microstructure, and HV was discussed. The results show that a nearly full density structure was obtained after SPS at 1200 °C. The microstructure of the obtained alloy is a duplex structure with the α-Ti (hcp) region having an average size of 70–140 nm, surrounding the β-Ti (bcc) matrix. The obtained alloy was chemically homogenized with a micro hardness value, HV of 660. The developed nanostructured Ti–20Nb–13Zr alloy is suggested for biomedical use as in implant material in dental and orthopedic applications.     

Sol–gel synthesis and spark plasma sintering of Ba0.5Sr0.5TiO3

Ultrafine Ba_0.5Sr_0.5TiO₃ powders were prepared by using barium nitrate, strontium nitrate, tetrabutyl titanate, and ammonia via citrate–nitrate combustion process at low temperature (500 °C), along with the X-ray diffraction (XRD), differential scanning calorimetry (DSC)/thermogravimetry analysis (TGA) and scanning electron microscopy (SEM) analytic reports. Spark plasma sintering was carried out to obtain the ultrafine crystalline BST and to improve the dielectric properity. It was found that the sintered BST showed ultrafine crystalline microstructure. At 25 °C, the dielectric constant and dissipation factor of the sintered sample were 1533 and 0.0063 at 10 kHz.     

Surface preparation of bioactive Ni–Ti alloy using alkali,thermal treatments and spark oxidation

The primary aim of this study was to compare different surface treatments used for bioactivation of pure titanium surfaces––thermal, alkali treatment and spark oxidation, and to assess their suitability as treatments for Ni–Ti alloys. This was considered by examining the surface properties, calcium phosphate precipitation from a physiological solution, and nickel ion release. Additionally, changes in the transformation temperature were measured for thermally treated samples. These studies indicate that the native surface of Ni–Ti alloy is highly bioactive when assessing the precipitation of calcium phosphates from Hank’s solution. Low temperature heat treatments also produced promising surfaces while high temperature treatment resulted in a very low rate of Ca and P precipitation. Alkali treatment and spark oxidation resulted in some bioactivity. Nickel ion release was greatest for alkali treated and sparks oxidized samples, and the rate of its release from these two samples was on the verge of daily safe dose for adolescent human. The other analyzed samples revealed very low rates of nickel ion release. Heat treatment at 400°C resulted in significant increase in the transformation temperatures, and a further increase of the treatment temperature up to 600°C caused a drop of the transformation temperature.     

Characterization of hydroxyapatite– and bioglass–316L fibre composites prepared by spark plasma sintering

To improve the mechanical properties of pure hydroxyapatite (HA) ceramics and pure 45S5 bioglasses, HA–316L fibre composites and bioglass 45S5–316L fibre composites were produced by spark plasma sintering (SPS) at 950 and 850 °C, respectively. While the HA phase in the HA–316L fibre composites did not decompose after the SPS process, microcracks were found around the 316L fibres in the composites. Consequently, the HA–316L fibre composites could not effectively improve the mechanical properties of the pure HA ceramics. In contrast, the bioglass 45S5–316L fibre composites showed no microcracks around the 316L fibres and thus exhibited bending strengths of up to 115 MPa.     

Mechanical properties of carbon nanotube–alumina nanocomposites synthesized by chemical vapor deposition and spark plasma sintering

Carbon nanotubes–alumina (CNT–Al₂O₃) nanocomposites with variable CNT content were directly synthesized by chemical vapor deposition (CVD). The as-grown CNT–Al₂O₃ mixture was densified by spark plasma sintering (SPS) at 1150 and 1450 °C. Vickers hardness of 9.98 GPa and fracture toughness of 4.7 MPam^1/2 were obtained for 7.39 wt.% CNT–Al₂O₃ nanocomposite. The addition of CNTs gives rise to 8.4% increase in hardness and 21.1% increase in toughness over that of the pure Al₂O₃. The optimum amount of CNTs is considered to be able to significantly enhance the mechanical property of ceramics in composites.     

Microstructures and mechanical properties of spark plasma sintered Cu–Cr composites prepared by mechanical milling and alloying

Nanograined Cu–8 at.% Cr composite was produced by a combination of mechanical milling (MM), mechanical alloying (MA) and spark plasma sintering (SPS). Commercial Cu and Cr powders were pre-milled separately by MM. The milled Cu and Cr powders were then mechanically alloyed with as-received Cr and Cu powders respectively. After milling, the powder mixtures were separately subjected to SPS. It was found that pre-milling Cr can efficiently decrease the size of grain and reinforcement, resulting in remarkable strengthening. The grain size of Cu matrix was about 82 nm after SPS. The Vickers hardness, compressive yield strength and compression ratio of the composite were 327 HV, 1049 MPa and 10.4%, respectively. The excellent mechanical properties were primarily attributed to dispersion strengthening of the Cr particles and fine grain strengthening of the Cu matrix. The strong Cu/Cr interface and dissolved Cr atoms can also contribute to strengthening of the composite.     

Can the use of pulsed direct current induce oscillation in the applied pressure during spark plasma sintering?

Abstract

The spark plasma sintering (SPS) process is known for its rapid densification of metals and ceramics. The mechanism behind this rapid densification has been discussed during the last few decades and is yet uncertain. During our SPS experiments we noticed oscillations in the applied pressure, related to a change in electric current. In this study, we investigated the effect of pulsed electrical current on the applied mechanical pressure and related changes in temperature. We eliminated the effect of sample shrinkage in the SPS setup and used a transparent quartz die allowing direct observation of the sample. We found that the use of pulsed direct electric current in our apparatus induces pressure oscillations with the amplitude depending on the current density. While sintering Ti samples we observed temperature oscillations resulting from pressure oscillations, which we attribute to magnetic forces generated within the SPS apparatus. The described current–pressure–temperature relations might increase understanding of the SPS process.     

Densification behavior and mechanical properties of spark plasma-sintered ZrC–TiC and ZrC–TiC–CNT composites

Titanium carbide (TiC) and carbon nanotubes (CNTs) were introduced into zirconium carbide (ZrC) ceramics to improve the fracture toughness. ZrC–TiC and ZrC–TiC–CNT composites containing 0–30 vol.% TiC and 0.25–1 mass% CNT were prepared by spark plasma sintering at temperatures of 1750–1850 °C for 300 s under a pressure of 40 MPa. Densification behavior, microstructure, and mechanical properties of the ZrC-based composites were investigated. Fully dense ZrC–TiC and ZrC–TiC–CNT composites with a relative density of more than 98 % were obtained. Vickers hardness of ZrC-based composites increased with increasing TiC content and the highest hardness was achieved with the addition of 20 vol.% TiC. Addition of CNTs up to 0.5 wt% significantly increased the fracture toughness of ZrC-based composites, whereas the addition of TiC did not have this effect.     

Processing and thermal properties of Al/AlN composites in continuous solid–liquid co-existent state by spark plasma sintering

Aluminum nitride-particle-dispersed aluminum–matrix composites were fabricated in a unique fabrication method, where the powder mixture of AlN, pure Al and Al–5 mass%Si alloy was uniquely designed to form continuous solid–liquid co-existent state during spark plasma sintering (SPS) process. Composites fabricated in such a way can be well consolidated by heating during SPS processing in a temperature range between 798 K and 876 K for a heating duration of 1.56 ks. Microstructures of the composites thus fabricated were examined by scanning electron microscopy and no reaction product was detected at the interface between the AlN particle and the Al matrix. The relative packing density of the Al/AlN composite was almost 100% when volume fraction of AlN is between 40% and 60%. Thermal conductivity of the composite was higher than 180 W/mK at an AlN fraction range between 40 and 65 vol.%, approximately 90% of the theoretical thermal conductivity estimated by Maxwell–Eucken’s model. The coefficient of thermal expansion of the composite falls in the upper line of Kerner’s model, indicating strong bonding between the AlN particle and the Al matrix in the composite.     

Fabrication and characterizations of Zn1?xCoxO bulk ceramics prepared by solid state reaction combined with spark plasma sintering

Zn_1?xCo_xO (x?=?0.01, 0.05 and 0.1) bulk ceramics were prepared through a two-step, solid state reaction method combined with spark plasma sintering technique. The single phase Zn_1?xCo_xO powders were synthesized using ZnO and Co₃O₄ at 935?°C in air for 3?h. The Zn_1?xCo_xO bulks were prepared at sintering temperature from 900 to 1,100?°C for 5?min by SPS. The relative density of Zn_1?xCo_xO bulk ceramics sintered at 1,100?°C is higher than 99% of the theoretical value. The Structure, composition analysis, optical absorption, Raman and XPS measurements revealed that the Co²⁺ substituted Zn²⁺ ions and was incorporated into the lattice of ZnO in both of the single phase Zn_1?xCo_xO powders and bulk ceramics. Room- and low-temperature magnetization measurements reveal a paramagnetic behavior and that the paramagnetic Co amount is smaller than the nominal Co concentration for all of Zn_1?xCo_xO samples at 4?K. The paramagnetic magnetism of bulk ceramics is apparently larger than that of powders with the same composition. The electrical properties measurements reveal that the Co concentration has a slight influence on the electrical properties of Zn_1?xCo_xO bulk ceramics. The carriers concentration is about 1?×?10²⁰?cm^?3 and with the Co concentration increases the resistivity slightly increases from 3.56?×?10^?3 (x?=?0.01) to 5.58?×?10^?3 (x?=?0.1) Ωcm.     

Microstructure,mechanical properties,and in vitro biocompatibility of spark plasma sintered hydroxyapatite–aluminum oxide–carbon nanotube composite

In the present work, HA reinforced with Al₂O₃ and multiwalled carbon nanotubes (CNTs) is processed using spark plasma sintering (SPS). Vickers micro indentation and nanoindentation of the samples revealed contrary mechanical properties (hardness of 4.0, 6.1, and 4.4 GPa of HA, HA–Al₂O₃ and HA–Al₂O₃–CNT samples at bulk scale, while that of 8.0, 9.0, and 7.0 GPa respectively at nanoscale), owing to the difference in the interaction of the indenter with the material at two different length scales. The addition of Al₂O₃ reinforcement has been shown to enhance the indentation fracture toughness of HA matrix from 1.18 MPa m^1/2 to 2.07 MPa m^1/2. Further CNT reinforcement has increased the fracture toughness to 2.3 times (2.72 MPa m^1/2). In vitro biocompatibility of CNT reinforced HA–Al₂O₃ composite has been evaluated using MTT assay on mouse fibroblast L929 cell line. Cell adhesion and proliferation have been characterized using scanning electron microscopy (SEM), and have been quantified using UV spectrophotometer. The combination of cell viability data as well as microscopic observations of cultured surfaces suggests that SPS sintered HA–Al₂O₃–CNT composites exhibit the ability to promote cell adhesion and proliferation on their surface and prove to be promising new biocompatible materials.     

Low-temperature sintering of ZrW2O8–SiO2 by spark plasma sintering

Amorphous ZrW₂O₈ powder and amorphous SiO₂ powder were prepared by a sol–gel process as raw materials, and high-density ZrW₂O₈–SiO₂ were successfully prepared at a much lower temperature of 923 K for a much shorter holding time of 10 min by spark plasma sintering (SPS) method rather than by conventional melt-quenching method. The relative densities of 0.85ZrW₂O₈–0.15SiO₂ and 0.70ZrW₂O₈–0.30SiO₂ were 99.4% and 96.6%, respectively. The combined technique of a sol–gel process and SPS should enable us to prepare the varied types of high-density composites of ZrW₂O₈ without severe thermal cracking caused by melt-quenching. The thermal expansion properties and dielectric properties of ZrW₂O₈–SiO₂ were also investigated.     

Fabrication of transparent mullite ceramic by spark plasma sintering from powders synthesized via sol–gel process combined with pulse current heating

Mullite nanopowders were synthesized by combining the advantages of the sol–gel process with the rapid synthesis provided by pulse current heating. The mullite ceramic with an infrared transmittance of 83–88% in the wavelength range from 2.5 to 4 μm with a fine grain size of 200 nm was obtained by spark plasma sintering at 1350 °C. Due to the high relative density and the small grain size, the hardness and toughness values of the sample reached 17.82 GPa and 3.6 MPa m^1/2, respectively. In contrast, when the mullite powders synthesized in a muffle furnace, an intermediate phase occurred so that the powder synthesis required high crystallization temperatures and resulted in agglomerated particles. Thus, the mullite ceramics required high temperatures for densification. As a result, the optical and mechanical properties of the ceramics were poor due to the low relative density and the elongated grain growth.     

Effects of molybdenum volume fraction and silica/molybdenum particle-size ratio on relative bulk density and electrical conductivity of silica–molybdenum composites fabricated by spark plasma sintering

SiO₂–Mo composite compact bodies were fabricated by mixing SiO₂ and Mo and then subjected to spark plasma sintering (SPS). In this study, the effects of the volume fraction of Mo and the SiO₂/Mo particle-size ratio of the compact bodies on the relative bulk density and electrical conductivity of the obtained sintered bodies are discussed. The relative bulk density tended to decrease with increasing SiO₂/Mo particle-size ratio and increasing Mo volume fraction, whereas the electrical conductivity increased with increasing SiO₂/Mo particle-size ratio. These results suggest that the density of the regions containing SiO₂ particles in the composite matrix significantly affects the degree of contact between the Mo particles (that form the conduction path). This in turn affects the SPS process, because SPS requires continuous conduction paths for the pulsed direct current to increase the temperature inside the materials. Therefore, when SPS is used to fabricate SiO₂–Mo composite sintered bodies, it is important to select raw material powders that allow the SiO₂/Mo particle-size ratio to be decreased or increased to obtain sintered bodies with high-relative bulk densities or high electrical conductivity, respectively.     

A comparison of the emissions of gasoline–ethanol fuel and compressed natural gas fuel used in vehicles with spark ignition engine in Rio de Janeiro: Brazil

Clean Technologies and Environmental Policy - A comparison of the emissions of gasoline–ethanol fuel and compressed natural gas (CNG) fuel used in vehicles with spark ignition engine was...     

A novel rapid route for synthesizing WC–Co bulk by in situ reactions in spark plasma sintering

A novel rapid route for preparing WC–Co cemented carbides bulk, which integrated the synthesis of the composite powder by in situ reactions and subsequent consolidation in the spark plasma sintering (SPS) system, was proposed. The phase configurations both in the synthesized composite powder and in the sintered bulk were analyzed in details. It was shown that the obtained WC–Co bulk has a homogeneous and fine-grained structure and good combined mechanical properties. As compared with the conventional methods, the present preparation route has been remarkably simplified. Particularly, with the greatly reduced temperature and time in the preparation procedures, it is effective to control the grain coarsening during the processes. By taking into account the characteristics of the SPS technology, the formation mechanism of the WC–Co composite by in situ reduction and carbonization reactions was proposed.     

A novel spark plasma sintering route to process high-strength Ti–4Al–2Fe/TiB nano-composite

Ti–4Al–2Fe alloy and Ti–4Al–2Fe/TiB nano-composite were processed by a novel spark plasma sintering route. KBF4 was used as an alternative and inexpensive boron precursor to form TiB reinforcement in situ during sintering. Fe was used as an alternative to vanadium to make the (α?+?β) Ti matrix. The processed Ti–4Al–2Fe alloy exhibited excellent mechanical properties (CS?=?1798 MPa). The TiB whiskers were distributed homogeneously and were fine (widths 130?nm and lengths from 100?nm to 3?µm). No residual TiB2 was found in the composite, in contrast with other methods. The TiB homogenised and refined the microstructure, while the hardness (710?HV), compressive strength (2414?MPa) and elastic modulus (140?GPa) all increased significantly when compared to the unreinforced alloy.     

Investigation of the Dynamics of Evaporation of Condensed Bodies on the Basis of the Law of Spectral‐Radiation Intensity of Particles

The fundamentals of radiation theory and the mechanism of evaporation of condensed bodies are presented. The distribution functions of particles of a body by energies and by the intensity of their transition from one energy level to another in the process of evaporation have been obtained based on the law of spectralradiation intensity of the body particles. The temperature dependence of the resulting vapor flow on the outer surface of a massive condensed body and a thin layer in equilibrium and nonequilibrium states, which, in the limit, transforms to the known Hertz–Knudsen formula, has been found.