Oleg L. Khasanov

Spark Plasma Sintering of Commercial Zirconium Carbide Powders: Densification Behavior and Mechanical Properties

Commercial zirconium carbide (ZrC) powder is consolidated by Spark Plasma Sintering (SPS). Processing temperatures range from 1650 to 2100 °C. Specimens with various density levels are obtained when performing single-die SPS at different temperatures. Besides the single-die tooling setup, a double-die tooling setup is employed to largely increase the actual applied pressure to achieve higher densification in a shorter processing time. In order to describe the densification mechanism of ZrC powder under SPS conditions, a power-law creep constitutive equation is utilized, whose coefficients are determined by the inverse regression of the obtained experimental data. The densification of the selected ZrC powder is shown to be likely associated with grain boundary sliding and dislocation glide controlled creep. Transverse rupture strength and microhardness of sintered specimens are measured to be up to 380 MPa and 24 GPa, respectively. Mechanical properties are correlated with specimens’ average grain size and relative density to elucidate the co-factor dependencies.

Ceramic Powder Dry Compaction Under Powerful Ultrasound Action

The uniaxial compaction of dry ceramic nanopowder under powerful ultrasound action is described. The description of nanopowder compaction using a dimensionless equation allows the determination all of the basic pressing parameters to optimize the coefficients of die-wall, interparticle friction, and springback. As a result, a homogeneously dense, unstrained green compact is formed. Introduction Ceramic nanopowders (CNP) are characterized by high elasticity, hard particles with high values of interparticle and die-wall friction owing to their high specific surface area and a lot of interparticle contacts. It is desirable to produce an uniformly dense green compact of a complex shape from dust-like CNP, and to retain the nanostructure of the compact after sintering. It is also important to avoid introducing additional porosity and impurities in the sintered ceramics from the binders used for compaction. This paper describes a method to uniaxially dry press homogeneously dense CNP green compact using powerful ultrasound action (PUA). Theoretical Approach The compaction equation in the dimensionless form is [1,2]: 1 ln + ⋅ = P b ρ , (1) where ρ is a relative density of a green compact; b is constant describing compressability of compacted CNP; P = P/Plim is the relative compaction pressure, that relates compaction pressure P to the critical pressure Plim, at which the theoretical density is reached. Using Eq. 1 it is possible to identify the contributions of the elastic (ρel) and plastic (ρpl) components in a CNP compaction curve, as well as to express a density differential ∆ρ throughout a green compact height: ⋅ + ⋅ = el pl ρ ρ ρ (2) R h f b F F b S S f b ξ ξ ρ 2 0 тр 0 s = ⋅ = ⋅ ⋅ ⋅ = ∆ , (3) where ξ is coefficient of lateral pressure, f =f0 /ξ = coefficient of die-wall friction (f0 is the friction coefficient of a pair «a die material on a powder material»); Ss is the green compact lateral surface; S0 is its hydraulic surface; Ffr, F0 are die-wall friction force and loading force; h/2R = t is an aspect ratio of the green compact (h = the green compact height, R = the hydraulic radius). Eq. 3 allows one to determine physical meaning of the coefficient b in Eq. 1: it is the differential in relative density in the height of a green compact when the loading force is greater than the die-wall friction force. Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 73-76 doi:10.4028/www.scientific.net/KEM.264-268.73

Influence of ultrasonic compaction conditions of the (Ba,Sr)TiO/sub 3/ nanopowder on the pore distribution in sintered ceramics

The pore distribution on the wafer surface of ferroelectric ceramics Ba/sub 0.6/Sr/sub 0.3/Ca/sub 0.1/TiO/sub 3/ was investigated. The samples with diameter of 38-40 mm were sintered from green compacts being compacted under ultrasound action with a power of 1-3 kW and by uniaxial static moulding. The pore distributions were determined inside of wafer surface regions located from central point toward edges using SEM data. The software ImageJ1.16 was used. The distribution regularities of small (less than 5 microns) and large pores depending on sample manufacture conditions were determined.

Specific features of the charge neutralization of silicon carbide in sintering by electron beam in the forevacuum range of pressures

It is shown that a noticeable role in the electron beam charge neutralization in the course of electron-beam sintering of compacted silicon carbide samples is played, as the sample temperature increases, by the electrical conductivity of a sample being sintered, as well as by thermionic emission from its surface. Experimental results obtained for compacted silicon carbide are used to determine its energy gap width and the electron work function.

Microstructure investigation of 13Cr-2Mo ODS steel components obtained by high voltage electric discharge compaction technique

Refractory oxide dispersion strengthened 13Cr-2Mo steel powder was successfully consolidated to near theoretical density using high voltage electric discharge compaction. Cylindrical samples with relative density from 90% to 97% and dimensions of 10 mm in diameter and 10–15 mm in height were obtained. Consolidation conditions such as pressure and voltage were varied in some ranges to determine the optimal compaction regime. Three different concentrations of yttria were used to identify its effect on the properties of the samples. It is shown that the utilized ultra-rapid consolidation process in combination with high transmitted energy allows obtaining high density compacts, retaining the initial structure with minimal grain growth. The experimental results indicate some heterogeneity of the structure which may occur in the external layers of the tested samples due to various thermal and electromagnetic in-processing effects. The choice of the optimal parameters of the consolidation enables obtaining samples of acceptable quality.

Electron Beam Sintering of Zirconia Ceramics

The work demonstrated the sintering of zirconium dioxide ceramics by means of an electron beam produced by a plasma-cathode e-beam source operating at fore-vacuum pressure. The sintered ceramics consist of tetragonal-modified zirconium dioxide with grain size from 0.7 to 10 micrometers, depending on the sintering conditions. At constant sintering temperature, the density of the material and its grain size depend on the integrated energy injected into the sintered material by the electron beam.

Investigation of Structural Hierarchy of Nanoceramics Compacted by Dry Pressing under Powerful Ultrasound Action

AFM techniques were used to investigate the microstructure, internal grain structure and intergrain interfaces of zirconia nanostructured ceramics sintered from green compacts being compacted by the common uniaxial dry compacting of nanopowders and under powerful ultrasound action. It was confirmed an essential influence of nanopowder compacting conditions on microstructure of sintered ceramics. Introduction Structural or functional nanocrystalline ceramics has improved mechanical and functional properties (fracture toughness, strength, hardness, thermal conductivity, specific electronic or magnetic properties) caused by the reduction of grain size to the nanometer range. However, the microstructure of nanoceramics have an impact on these properties [1]. It has been shown that among various techniques of nanoceramics production the method of the uniaxial dry compacting of initial nanopowders under powerful ultrasound (PU) action is effective to sinter homogeneously dense ceramics having nanostructured grains. Using SEM and XRD analysis we observed the structural-scale hierarchy and phase composition of zirconia nanoceramics (ZrO2 5.4wt%Y2O3) fabricated from the plasma chemically synthesized zirconia nanopowder compacted with PU-action and without it. PU-action at compacting leads to the decrease of grain sizes and grain aggregate sizes in sintered ceramics. Nanoscale grains with mean size of 300 nm in such ceramics are layered packs of subgrains (around 20 nm x 200 nm x 200 nm), with the scaly particles of initial nanostructured powder as their nuclei. The ceramics consist of cubic and tetragonal phases of zirconia [2]. The aim of this work is AFM investigation of microstructure of the zirconia ceramics prepared using the above compacting method at varying PU power (W = 0, 1, 2, 3 kW) and compaction pressure (P = 50, 100, 150, 200 MPa). Experimental Methods and Specimens Nanoceramics were sintered from green compacts of ZrO2 –5%wtY2O3 nanopowder, produced by plasma-chemical synthesis [2]. The powder particles had the shape of polycrystalline plates (scaly morphology) with the average linear size of 150 nm; the mean crystallite size in such a plate was 23 nm. The elemental composition of the powder under study was measured by means of the electronprobe microanalysis using the EPMA-1400 (Shimadzu Inc.) installation. According to the measurements the powder contained 74.13% of Zr, 18.16% of O, 4.28% of Y, and 3.43% of C. The uniaxial dry compacting of green compacts 50 mm in diameter and with an aspect ratio of 0.05 was carried out under PU-action (PU-samples) and without it (O-samples) by the techniques described in [3,4]. Then compacts were sintered in air at Тs =1650°C up to density of 88%. The AFM measurements of the fracture surface of ceramics were performed in semi-contact mode using a scanning probe microscope P47-MDT (NT-MTD, Russia). We used the AFM studies in a mode of phase contrast (PC) images constructed by monitoring the phase shift of cantilever oscillation during the scan. The PC images contain information related to micromechanical properties of the sample materials (elastic modulus, viscoelasticity, or damping) and adhesion generated between the probe tip and the sample surface (surface energy, capillary forces). Recent works conducted by several investigators showed that the phase contrast images were highly effective for estimating structural inhomogeneity of materials [5]. The chemical composition of the fracture surfaces was studied by techniques of XPS-analysis described in [6]. Results and Discussion The microstructure of ceramics sintered from non-sonicated compacts has been determined to be different from that of the sonicated samples. By AFM investigation of the O-ceramics (W = 0 kW), the samples compacted at P=50 MPa had aggregates close to 2 μm involving grains of ~500 nm (Fig. 1a). At Р= 100 and 150 MPa scaly subgrain structure is formed with less subgrain sizes of 100-150 nm (Fig. 1b). The scaly structure becomes less pronounced and grains grow in size with the increase of P up to 200 MPa (Fig. 1c). Apparently, particle deformation at compacting results in the fusion of scales. Fig. 1. AFM image for O-ceramics (a P= 50 MPa, b P= 150 MPa, c P= 200 MPa). In support of above the histograms of grain size Dgr distribution in ceramic samples are shown in Fig. 2. They were calculated and plotted from AFM images using ImageJ 1.28 software. Regarding the PU-ceramics, all samples preserve in grains the scaly structure of initial powder (Fig. 3), because of the sonication of green compacts enables a lower rate of plastic deformation of particles during densification. At the same time the particle structure varies due to a mechanochemical activation under PU vibrations. Subgrains of 25-35 nm detected in the AFM-phase contrast mode for PU-samples are likely to correspond to the regions of stress concentration, that emerge at the secondary recrystallization in sintering process of the samples. The size of the detected structure is similar to the grain size of the initial powder. According to the AFM-data on size distribution of grains, the increase of W together with the increase in P sets a grain size limit, after which a reverse process – coagulation – becomes dominant. In this case grains can be as large as 1500 nm, which is comparable with the agglomerate sizes (Fig. 4a). Fig. 2. Ceramics grain size histograms. a b c

Influence of Ultradispersed Fraction of Boron Carbide Powder on Strength Properties of the Ceramics Manufactured by SPS Method

The experimental results for manufacturing ceramics from industrial boron carbide powder with 10 wt% of ultradispersed fraction of a powder of the same composition by spark plasma sintering have been presented. Under optimal process conditions such sintering results in a decrease of the sintering temperature and synthesis time, and the addition of ultradispersed fraction increases microhardness and fracture toughness of the ceramics.

Lower sintering temperature of nanostructured dense ceramics compacted from dry nanopowders using powerful ultrasonic action

Nanostructured high dense zirconia ceramics have been sintered from dry nanopowders compacted by uniaxial pressing with simultaneous powerful ultrasonic action (PUA). Powerful ultrasound with frequency of 21 kHz was supplied from ultrasonic generator to the mold, which was the ultrasonic wave-guide. Previously the mold was filled by non-agglomerated zirconia nanopowder having average particle size of 40 nm. Any binders or plasticizers were excluded at nanopowder processing. Compaction pressure was 240 MPa, power of ultrasonic generator at PUA was 1 kW and 3 kW. The fully dense zirconia ceramics has been sintered at 1345°C and high-dense ceramics with a density of 99.1%, the most grains of which had the sizes Dgr ≤ 200 nm, has been sintered at low sintering temperature (1325°C). Applied approach prevents essential grain growth owing to uniform packing of nanoparticles under vibrating PU-action at pressing, which provides the friction forces control during dry nanopowder compaction without contaminating binders or plasticizers.

Influence of powerful ultrasonic treatment on the structure of dry ceramic nanopowders

Influence of the non-cavitational powerful ultrasonic action (PUA) on the particle size distribution and lattice parameters of crystallites of zirconia and alumina dry nanopowders have been studied. The ultrasonic treatment occurred into the acoustic waveguide having a cavity, filled by dry nanostructured powder (NP). The PUA of different power was used to various dry NP: ZrO<inf>2</inf>-8%Y<inf>2</inf>O<inf>3</inf> (TZ-8Y, TOSOH); ZrO<inf>2</inf>-3%Y<inf>2</inf>O<inf>3</inf> (TZ-3YS, TOSOH); ZrO<inf>2</inf>-3%Y<inf>2</inf>O<inf>3</inf> (PCZY, Siberian Chemical Plant, SCP); Al<inf>2</inf>O<inf>3</inf> (UDPO, SCP). It was found that non-cavitational PUA influences the lattice parameters and average agglomerate sizes of dry zirconia and alumina NP. There are extreme PUA powers at which extreme values of the lattice parameters and average agglomerate sizes are correlated. The crystal structure of the TZ-3Y (synthesis by spray-drying) is more stable against PUA than PCZY (plasma-chemical synthesis).