Distribution of carbon contamination in oxide ceramics occurring during spark-plasma-sintering (SPS) processing: II - Effect of SPS and loading temperatures

Carbon contamination during the SPS processing was investigated in the spinel, alumina and zirconia. The carbon contamination changes with the SPS conditions and the target materials. At the high heating rate of 100?°C/min, the contamination occurred over the entire area in the spinel, but only around the surface areas in the alumina and zirconia. For the spinel, the contamination is sensitive to the SPS parameters, such as the heating rate and loading conditions, but less sensitive to the sintering temperature. This suggests that the carbon contamination was caused by evaporation of CO gas from the carbon paper/dies. At the high heating rates, the carbon evaporation is enhanced due to the rapid heating, and then, the evaporated CO gases are encapsulated into the closed pores during the heating process and remain in the matrix. The carbon contamination can be suppressed by a high temperature loading even at the high heating rate.     

Distribution of carbon contamination in MgAl2O4 spinel occurring during spark-plasma-sintering (SPS) processing: I – Effect of heating rate and post-annealing

Carbon contamination from the carbon paper/dies during spark-plasma-sintering (SPS) processing was examined in the MgAl₂O₄ spinel. The carbon contamination sensitively changes with the heating rate during the SPS processing. At the high heating rate of 100 °C/min, the carbon contamination having organized structures occurred over almost the entire area from the surface to deep inside the SPSed spinel disk. In contrast, at the slow heating rate of 10 °C/min, the carbon contamination having disordered structures occurred only around the surface area. The carbon phases transform into high pressure CO/CO₂ gases by post-annealing in air and lead to pore formation along the grain junctions. The pore formation significantly occurs at the high heating rate due to the large amount of the contaminant carbon phases. This suggests that if once the carbon contamination was formed in the materials, it is very difficult to remove the carbon phases from the materials.     

Fine-grained transparent MgAl2O4 spinel obtained by spark plasma sintering of commercially available nanopowders

A high transmittance/small grain size combination for pure spinel ceramics from commercially available nanopowders without sintering aids can be obtained by SPS sintering. By using a low heating rate ≤10 °C/min and a sintering temperature ≤1300 °C, a transparent polycrystalline MgAl₂O₄ spinel was fabricated by SPS with an in-line transmission of 74% and 84% for 550 nm (visible) and 2000 nm (NIR) wavelengths respectively. A small average grain size of about 250 nm was obtained and the pores located at the multiple grain junctions have a mean size of about 20 nm. The high in-line transmission is linked not only to the low residual porosity but particularly to the very small size of pores.     

Polycrystalline transparent magnesium aluminate spinel processed by a combination of spark plasma sintering (SPS) and hot isostatic pressing (HIP)

A two-stage processing approach combining spark plasma sintering (SPS) and hot isostatic pressing (HIP) was employed for the fabrication of relatively large (30?mm diameter) and thick (up to 8?mm) samples of transparent polycrystalline magnesium aluminate. The effects of sample thickness, heating rate during SPS, and the temperature and duration of HIP treatments were investigated. It was established that the heating rate during SPS had a major influence on discoloration due to carbon contamination, which increased with sample thickness. HIP treatment allowed for the elimination of cloudiness due to samples porosity, although carbon contamination present after the SPS step could not be reduced by HIP treatment, regardless of the temperature and duration applied. Highly transparent specimens with thicknesses of 4 and 8?mm exhibiting an in-line transmittance of 85.2 and 83.2% at 600?nm, respectively, were fabricated.     

Influence of Spark Plasma Sintering (SPS) Conditions on Transmission of MgAl2O4 Spinel

The effects of the spark plasma sintering (SPS) parameters, such as heating rate α, temperature T and soak time t_s, and impurities on in‐line transmission T_in were examined in MgAl₂O₄ spinel. The SPS processing at T = 1300°C for t_s = 20 min with a low heating rate of α = 10°C/min is a preferable condition for attaining higher T_in. For the higher T or the longer t_s, grain coarsening enhanced the coalescence of residual pores and second phase precipitation, resulting to the limited T_in even at the slow α. Although the lower T and longer t_s attained fine and dense microstructure simultaneously, the maximum T_in was limited to about 50%. The limited transmission, particularly in the visible range, can be ascribed to the discoloration caused by the carbon contamination. The carbon contamination arose from the preexisting trace CO₃ impurities, irrespective of the SPS conditions. For the higher α, which is the primary advantage of the SPS technique, the additional carbon contamination occurred from the paper/die and remained as glassy carbon in the matrix. To attain higher T_in by the SPS technique, the lower α and T, and the shorter t_s should be utilized after removing the impurities.     

Diffusion bonding of transparent ceramics by spark plasma sintering (SPS) complemented by hot isostatic pressing (HIP)

Diffusion bonding is a viable technique for fabricating larger parts or composite transparent ceramics. Spark plasma sintering (SPS) and hot isostatic pressing (HIP) allow for simultaneous application of high temperature and pressure, making them suitable for bonding various materials, including transparent ceramics. In this study, we demonstrated the combined use of SPS followed by HIP for successful diffusion bonding of transparent MgAl₂O₄ (spinel) and Y₃Al₅O₁₂ (YAG) ceramics. The YAG samples had superior surface quality and were adequately bonded using SPS alone, while rougher spinel samples showed inadequate bonding initially due to insufficient surface contact. However, subsequent HIP treatment effectively closed the voids at the interface and promoted grain growth across the joint, resulting in a strong seamless bond. The combination of SPS and HIP, as a two-step process, holds promise for diffusion bonding of ceramics and offers a potential solution for compensating surface quality issues in the bonded parts.     

A Comprehensive Study of the Carbon Contamination in Tellurite Glasses and Glass‐Ceramics Sintered by Spark Plasma Sintering (SPS)

Highly transparent tellurite glasses and glass‐ceramics based on the 85TeO₂–15WO₃ composition (mol%) were produced by spark plasma sintering (SPS) from powders previously prepared by the conventional melt‐quenching technique and then grinded. We report a study based on the understanding of carbon contamination that is commonly observed by this nonconventional sintering technique and which constitutes a drawback for optical applications. First, the influence of the particle size of the initial amorphous powders as well as an additional pressureless sintering step prior to SPS experiments and the use of a physical carbon diffusion barrier have been investigated to reduce the carbon contamination in glass bulks. Second, once reducing the carbon contamination, glass‐ceramics were obtained by varying the SPS conditions. The noncentrosymmetric γ‐TeO₂ phase crystallized within the bulk volume while maintaining good optical transparency and led to the generation of second harmonic. This approach paves the way to further applications in the domain of linear and nonlinear optics.     

Spark-plasma sintering of boron carbide–silicon carbide composites at 1400 °C from B4C+Si: Densification and sintering/reaction mechanisms

The feasibility of fabricating novel boron carbide–silicon carbide composites by spark-plasma sintering (SPS) of B₄C+Si powder mixtures at only 1400 °C was investigated. First, it is shown that B₄C can be fully densified at 1400 °C if ~20 vol% Si aids are used, leading to bi-particulate composites constituted by boron carbide (major phase) and SiC (minor phase). The formation of these composites is due to the fact that Si acts as a reactive sintering additive during SPS. Lower and higher proportions of Si aids are not optimal, the former leading to porous bi-particulate composites and the latter to dense triplex-particulate composites with some residual free Si. Importantly, it is also shown that these novel boron carbide–SiC composites are fine-grained, nearly-ultrahard, moderately tough, and more affordable to fabricate, a combination that makes them very appealing for many engineering applications. Second, it is demonstrated that during the heating ramp of the SPS cycles a eutectic melt is formed that promotes full low-temperature densification by transient liquid-phase sintering if sufficient Si aids are used. Otherwise, a subsequent stage of solid-state sintering is required at higher temperatures once the eutectic liquid has been consumed in the in-situ formation of SiC. And third, it is demonstrated that during SPS the original B₄C undergoes a gradual isostructural crystallographic transition towards a Si-doped carbon-deficient boron carbide that is more relevant with increasing proportion of Si aids, and it is identified that the carbon source for the formation of SiC is almost exclusively the carbon exsoluted from the B₄C crystals themselves during their isostructural transition. Finally, implications of interest for the ceramic and hard-material communities are discussed.     

Spark plasma sintering of combustion-synthesized β-SiAlON powders

In this paper, the sintering behavior of β-Si_6−zAl_zO_zN_8−z (z=1) powder prepared by combustion synthesis (CS) was studied using spark plasma sintering (SPS). The CSed powder was ball milled for various durations from 0.5 to 20 h and was then sintered at different temperatures with heating rates varying from 30 °C/min to 200 °C/min. The effects of ball milling, sintering temperature, and heating rate on sinterability, final microstructure, and mechanical property were investigated. A long period of ball milling reduced the particle size and subsequently accelerated the sintering process. However, the fine powder was easily agglomerated to form secondary particles, which accordingly decreased the densification of the SPS product. The high sintering temperature accelerated the densification process, whereas the high heating rate reduced the grain growth and increased the relative density of the sintered product.     

Spark-Plasma-Sintering Condition Optimization for Producing Transparent MgAl2O4 Spinel Polycrystal

By controlling the heating rate at <10°C/min during spark-plasma-sintering (SPS) processing, transparent polycrystalline spinel with an in-line transmission of 50% and 70% in the visible- and infrared-wavelengths, respectively, can be successfully fabricated for only a 20-min soak at 1300°C. The high transmission can be attained by reducing the residual porosity and pore size, which was achieved by the low-heating rate. At high heating rates, many closed pores are formed due to the high densification rate during the heating process and remain as large pores around grain junctions. At temperatures >1300°C, the coalescence of the residual pores and the precipitation of second phases, which are caused by rapid grain growth, degrade the transparency. The present study demonstrates that although the high heating rates have been regarded as a primary advantage for the SPS processing, the low heating rate is highly effective in attaining a high transparency in the spinel even at low temperatures and for short sintering times.     

Bending strength and microstructure of Al2O3 ceramics densified by spark plasma sintering

Al₂O₃ ceramics were superfast densified using spark plasma sintering (SPS) by heating to a sintering temperature between 1350 and 1700°C at a heating rate of 600°C/min, without holding time, and then fast cooling to 600°C within 3 min. High-density Al₂O₃ ceramics could be achieved at lower sintering temperatures by SPS, as compared with that by conventional pressureless sintering (PLS). The bending strength of Al₂O₃ superfast densified by SPS in the range of sintering temperature between 1400 and 1550°C reached values as high as 800 MPa, almost twice that obtained by the PLS. SEM observations indicated that intragranular fracture was the preponderant fracture mode in these samples, resulting in these excellent bending strength values.     

Flash sintering of a three‐phase alumina,spinel, and yttria‐stabilized zirconia composite

Three‐phase ceramic composites constituted from equal volume fractions of α‐Al₂O₃, MgAl₂O₄ spinel, and cubic 8 mol% Y₂O₃‐stabilized ZrO₂ (8YSZ) were flash‐sintered under the influence of DC electric fields. The temperature for the onset of rapid densification (flash sintering) was measured using a constant heating rate at fields of 50‐500 V/cm. The experiments were carried out by heating the furnace at a constant rate. Flash sintering occurred at a furnace temperature of 1350°C at a field of 100 V/cm, which dropped to 1150°C at a field of 500 V/cm. The sintered densities ranged from 90% to 96%. Higher electric fields inhibited grain growth due to the lowering of the flash temperature and an accelerated sintering rate. During flash sintering, alumina reacted with the spinel phase to form a high‐alumina spinel solid solution, identified by electron dispersive spectroscopy and from a decrease in the spinel lattice parameter as measured by X‐ray diffraction. It is proposed that the solid solution reaction was promoted by a combination of electrical field and Joule heating.     

Transmittance enhancement of spark plasma sintered CaF2 ceramics by preheating commercial powder

Spark plasma sintering (SPS) is a convenient approach for preparing transparent CaF₂ ceramics. However, carbon contamination is a key issue that should be addressed to achieve high transparency. In this study, a commercially available CaF₂ powder was preheated under vacuum before performing SPS to mitigate carbon contamination. During the preheating of the CaF₂ powder, impurities adsorbed on the particle surface, such as H₂O, CO₂, and O₂, are desorbed. Moreover, the interdiffusion of carbon contaminants is suppressed due to the pre-sintering of the raw powder. The in-line transmittance of the CaF₂ ceramic prepared from the preheated powder increased to 85 % at the wavelength of 1100 nm, which is 38 % higher than that of the ceramic prepared without preheating. In addition, the in-line transmittance increased with increasing grain size of the ceramic, possibly because of the decrease in the number of scattering sources with the reduction in the grain boundary fraction.     

Effect of heating rate on properties of transparent aluminum oxynitride sintered by spark plasma sintering

High heating rates ranging from 50 to 250°C/min are selected to rapidly sinter transparent aluminum oxynitride (AlON) ceramics by spark plasma sintering (SPS) at 1600°C under 60 MPa using a bimodal AlON powder synthesized by the carbothermal reduction and nitridation method. With 1 minute holding time before cooling, all the specimens show high density and high transparence. The maximum transmittance is up to 74.5%-80.6%, where the maximum transmittance is positively correlated with the heating rate. Further analysis reveals that faster heating rates enable the decrease in the amount of the AlON phase decomposed into the α-Al₂O₃ and AlN phases during heating. These α-Al₂O₃ and AlN phases have to be converted back to AlON at the final stage of sintering, which indicates that a decrease in the amount of the α-Al₂O₃ and AlN phases via the boosted heating leads to the higher transmittance of the AlON ceramics. The high heating rates and short holding duration of the SPS utilized in this study result in the fine grain size of the obtained ceramics (1-6 μm) compared to that of the AlON ceramics fabricated by the conventional sintering method. This effect of high heating rates is confirmed by the coupled densification-grain growth modeling. In turn, the obtained AlON specimens exhibit a Vickers hardness of 15.87-16.62 GPa.     

Effect of loading schedule on densification of MgAl2O4 spinel during spark plasma sintering (SPS) processing

An increase in the loading temperature during SPS processing can reduce the residual porosity in a spinel and thus attain a high transmission even at the high heating rate of 100 °C/min. This suggests that load controlling is an important factor as well as the heating rate and sintering temperature. Although the transmission is lower than the maximum value attained at the low heating rates of <10 °C/min, the loading schedule optimization enables utilization of the high heating rate processing that is a primary advantage of the SPS technique.     

The effect of pre-annealing and post-annealing on the transparency of MgAl2O4, prepared by slip casting and spark plasma sintering (SPS)

The aim of this paper is to investigate the effect of slip casting process and the annealing before and after sintering to achieve a transparent MgAl₂O₄. To remove contaminants such as carbon from the structure of shaped spinel bodies, at first, the samples were annealed at temperature of 800?°C, 900?°C and 1000?°C for 2?h and then sintered at 1400?°C. By annealing the sample before sintering at 900?°C, the transmission increased (15% at IR region and 10% at visible region). Although by annealing the samples, the amount of carbon contamination reduced. Annealing the samples after sintering also had some desirable results. The samples annealed at temperature of 1200?°C for a time of 3, 5 and 10?h. The darkness of samples reduced due to the removal of carbon impurities and the sample was annealed at 1200?°C for 5?h had the most transparency in the visible and infrared regions.     

Rapid pressure-less and spark plasma sintering of (Ba0.85Ca0.15Zr0.1T0.9)O3 lead-free piezoelectric ceramics

This work shows for the first time the possibility to sinter BCZT powder compacts by rapid heating rates within one hour of sintering, while achieving good piezoelectric properties. The sintering was performed by rapid (heating rates 100 and 200 °C/min) pressure-less sintering (PLS) at 1550 °C/5-60 min and by SPS sintering (100 °C/min, 1450 °C/5?60 min and 1500 °C/15?45 min). The rapid PLS samples reached a relative density up to 94 % and grain sizes of 17–36 μm acquiring d₃₃ up to 414 pC/N. Although the SPS samples reached full density at 1450 °C, their piezoelectric properties worsened due to smaller grains (10?15 μm) as well as formation of cracks at dwell times > 30 min. At elevated SPS temperature of 1500 °C/30 min, the d₃₃ increased to 360 pC/N sustaining full density. Even higher increase in d₃₃ (424 pC/N) of SPS samples was achieved by post-rapid PLS at 1550 °C/60 min resulting from further expansion in grain size.     

High-pressure spark plasma sintering (SPS) of transparent polycrystalline magnesium aluminate spinel (PMAS)

A high pressure SPS (spark plasma sintering) process was applied for consolidation of un-doped polycrystalline magnesium aluminate spinel. This approach allows fabricating a fully dense transparent ceramic with submicron grain size and high hardness values at a relatively low temperature (1200 °C). The light transmittance of the specimens increases with increasing applied pressure, while the hardness gradually decreases. The optimal combination of properties was achieved after sintering at 1200 °C at a heating rate of 5°/min, a holding time of 15 min and an applied pressure of 350–400 MPa. The specimens display the level of transmittance in the visible wavelengths and hardness values comparable with the best results reported in the literature for the two-stage fabrication process (pressureless sintering and hot isostatic pressing).     

Pressure-assisted microwave sintering: A rapid process to sinter submicron sized grained MgAl2O4 transparent ceramics

This work focuses on the development of an original process based on a 2.45 GHz single-mode microwave cavity equipped with a uniaxial press, to sinter transparent spinel MgAl₂O₄ ceramic in air. The samples were conventionally pre-sintered to a density of 90% TD before microwave sintering to the final stage of densification. The influence of thermomechanical cycle on the material properties was investigated. Transmittance, grain size distribution, hardness and fracture toughness of the samples were measured and correlated to the microstructure. This new sintering process has allowed obtaining transparent samples with sub micrometric grain size and high mechanical properties, with relatively short times and low temperature. These first results can be compared to some obtained by SPS or HIP. The technical input of this method is that all the process is here conducted in air atmosphere.     

Spark plasma sintering of UO2 nanopowders: Pressure,heating rate and current effects

We analysed with different methods the densification of UO₂ nanopowders in SPS under constant heating rate (CHR) and isothermal sintering conditions. The apparent activation energy of densification in SPS (75 kJ/mol with CHR method) is significantly smaller than in conventional sintering. It is shown that this is likely not an effect of the applied current. We also observed a threshold stress at 64 MPa for the transition from pressure-insensitive sintering (stress exponent n≈0) to pressure-assisted sintering, suggesting that the contribution of the capillary stresses in such nanopowders is comparable with the typical stress applied in SPS.