Pressureless sintering curve and sintering activation energy of Fe–Co–Cu pre-alloyed powders

The kinetic characteristics of Fe–Co–Cu pre-alloyed powders in the pressureless sintering process have been investigated. The expansion ratio, linear shrinkage, densification rate and effect of heating rate on the sintering have been analyzed. Based on the classical Arrhenius curve, the sintering activation energy has been calculated. Results show that the samples have a smaller expansion ratio before contracting when the Fe content is higher, and the final linear shrinkage ratio is larger too. The sintering carries out more efficiently and the final linear shrinkage ratio is larger when the samples at a lower heating rate. In the initial and final stage of sintering, the Arrhenius curve is suitable for the Fe–Co–Cu pre-alloyed powders and diffusion is the main transport mechanism. At the initial stage of sintering the sintering activation energy of Fe25%–Co15%–Cu60% powder is 453.11 kJ/mol, Fe45%–Co15%–Cu40% powder is 638.28 kJ/mol and Fe65%–Co15%–Cu20% powder is 504.6 kJ/mol, respectively. At the final stage of sintering the sintering activation energy of Fe25%–Co15%–Cu60% powder is 31.17 kJ/mol, Fe45%–Co15%–Cu40% powder is 20.09 kJ/mol and Fe65%–Co15%–Cu20% powder is 35.13 kJ/mol, respectively. The sintering activation energy in the middle stage is dominated by not only one diffusion mechanism so it is not suitable for the Arrhenius curve.     

Gas pressure sintering of β-silicon nitride

The sintering behaviour of -Si₃N₄ powder was investigated in 980 kPa (10 atm) nitrogen at 1800–2000 °C. It is shown that -Si₃N₄ has a higher sinterability than the finer -Si₃N₄. The solution of small grains and reprecipitation on large grains occurred during sintering at >1600 °C. The rate-determining step in the liquid-phase sintering is believed to be the diffusion of material through the liquid phase at grain boundaries. There was no abnormal grain growth during gas pressure sintering of -Si₃N₄. The microstructures of gas pressure sintered materials from -Si₃N₄ were more uniform than those from -Si₃N₄. The densification mechanism of -Si₃N₄ is discussed in relation to that of -Si₃N₄.     

Pressureless sintering of dense hydroxyapatite–zirconia composites

Hydroxyapatite (HA)–TZP (2.5 mol% Y₂O₃) containing 2, 5, 7.5 and 10 wt% TZP were prepared using calcium nitrate, diammonium hydrogen orthophosphate, zirconium oxychloride and yttrium nitrate. The composite powder was prepared by a reverse strike precipitation method at a pH of 10.5. The precipitates after aging and washing were calcined at 850°C to yield fine crystallites of HA and TZP. TEM study of the calcined powder revealed that while HA particles had both spherical and cuboidal morphology (∼50–100 nm) the TZP particles were only of spherical nature (∼50 nm). X-ray analysis showed that the calcined powder of all the four composition had only HA and t-ZrO₂. Uniaxially compacted samples were sintered in air in the temperature range 1,150–1,250°C. High sintered density (>95% of theoretical) was obtained for composites containing 2 and 5 wt% TZP, while it was 92% for 7.5 wt% and 90% for 10 wt% TZP compositions. X-ray analysis of sintered samples shows that with 2 wt% TZP, the retained phases were only HA and t-ZrO₂. However, for 5, 7.5 and 10 wt% TZP addition both TCP and CaZrO₃ were also observed along with HA and t-ZrO₂. Bending strength was measured by three point bending as well by diametral compression test. While in three point bending, the highest strength was 72 MPa, it was 35.5 MPa for diametral compression. The strength shows a decreasing trend at higher ZrO₂ content. SEM pictures show near uniform distribution of ZrO₂ in HA matrix. The reduction in sintered density at higher ZrO₂ content could be related to difference in the sintering behaviour of HA and ZrO₂.     

Low-temperature microwave sintering of TiN–SiC nanocomposites

Densification kinetics study during microwave sintering of titanium nitride-based nanocomposite has been conducted. A series of TiN–SiC compositions with 1, 3, 5 wt% of silicon carbide were microwave sintered at relatively low sintering temperatures (900–1,300 °C) for 0–30 min. The SiC content influenced on heating uniformity and final density and grain-size achieved. Densification process during microwave sintering obeyed the mechanism of grain-boundary diffusion with activation energy of 235 kJ mol⁻¹. Microwave sintering resulted in fine microstructure (~300 nm) and hence high values of micro hardness (~20 GPa).     

Effect of additives on sintering of α-cristobalite

Sintering is performed for the pure α-cristobalite, with TiO₂ additive, and with CaO additive powder compacts. The sintering temperatures were 1500, 1515, and 1525 °C for different dwell times. The master sintering curve theory is applied to the densification data and the activation energies are obtained. The splitting strength is measured for the sintered specimens. It is found that CaO decreases the activation energy due to the formation of liquid phase at the used sintering temperatures. The TiO₂ increases the activation energy because it exists as solid particles at the used sintering temperatures. The measured splitting strengths of the sintered specimens are shown to be best expressed by a sigmoid function of the work of sintering.     

Gas pressure sintering of silicon nitride — current status

Gas pressure sintering of silicon nitride is now common for obtaining a dense as well as tough product. A review of earlier works has revealed that there are still controversies in prediction of mechanism of the sintering process. Analysis of the kinetics of the process obtained by integration of a dilatometer inside the furnace has been presented. The mechanism of sintering silicon nitride powder compacts in presence of hyperbaric nitrogen atmosphere has been discussed. It has been observed that intermediate stage densification kinetics is very much susceptible to sintering atmosphere and time of pressurisation. The microstructure of the final product has been found to be dependent primarily on the method of incorporation of additive in the starting silicon nitride powders.     

Microwave sintering of nanocrystalline γ-Al2O3

The densification and phase transformation behavior of gas condensation synthesized nanocrystalline γ-A1₂O₃ sintered with microwave radiation has been studied. The polymorphic nucleation and growth phase transformations which occurred as the material was heated through the temperature range of 800–1300°C present significant obstacles in the achievement of specimens which possess high bulk densities. These phase transformations are accompanied by a change in particle morphology, crystallite size, and surface area. Alumina derived from a chemically synthesized boehmite precursor has been shown to exhibit the same nucleation and growth phase transformation behavior when conventionally heated. It is concluded that nanocrystalline γ or δ alumina will not be a viable starting material for the production of dense bodies with grain sizes of less than 100 nm.     

Rapid production of β-FeSi2 by spark-plasma sintering

Thermoelectric materials of Fe_0.91Mn_0.09Si₂ were produced with two methods (i.e., hot pressing (HP) and spark-plasma sintering (SPS)) and their thermoelectric properties were compared. The relative density of the samples sintered at the same temperature for thirty minutes was similar. The relative density of the SPS specimen reached 90% in five minutes at 1173 K, while the HP specimen reached 90% in thirty minutes at 1173 K. The optimum condition of the heat treatment was found to be for 3.6 ks at 943 K for the transformation to the -phase. The peritectoid reaction ( + ) mainly occurred at and above 953 K, and the eutectoid reaction mainly occurred at 943 K. The -phase ratio of the HP specimen was larger than that of the SPS specimen because the peritectoid reaction occurred during longer cooling period from 1173 K to 943 K. Consequently, the Seebeck coefficient of the SPS specimen was slightly smaller than that of the HP specimen. However, the figure of merit was very similar because the thermal conductivity of the SPS specimen was smaller than that of the HP specimen. Thus, the SPS method is superior to the HP method with respect to mass-production because the production time is significantly shorter.     

Compaction and sintering behaviour of glass–alumina composites

The effects of Al₂O₃ additions on the compaction and sintering behaviour of a leadborosilicate glass (LG) have been investigated. LG powder was prepared by melting, fritting and milling a glass of the composition: 77PbO, 10B₂O₃, 10SiO₂, 2Al₂O₃ and 1P₂O₅ (wt.%). The mean particle sizes of the powders were: LG, 6.5 μm and Al₂O₃, 3.3 μm. The compaction behaviour of LG–Al₂O₃ powder mixtures can be represented by a new compaction equation: [(D_g−D₀)/(1−D₀)]=(P/P_f)ⁿ, where D_g is the relative green density, D₀ the relative tap density and n and P_f are material constants. The exponent n decreases from 0.192 to 0.065 as the Al₂O₃ content is increased from 0 to 100 vol.%. The Frenkel equation for isothermal shrinkage has been found to be valid. It is shown that in the glass matrix composites the minimum sintering temperature can be determined by measuring the dilatometric deformation temperature. The presence of Al₂O₃ in excess of 15 vol.% has been found to strongly retard the sintering kinetics. An addition of 45 vol.% Al₂O₃ increases the activation energy for sintering from 67 to 112 kcal mol⁻¹. The presence of Al₂O₃ particles also induced a partial crystallisation in LG matrix.     

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.     

Microwave-assisted rapid discharge sintering of a bioactive glass�Cceramic

Bioactive glass-ceramics have been developed as successful bone graft materials. Although conventional sintering in an electrically-heated furnace is most commonly used, an alternative microwave plasma batch processing technique, known as rapid discharge sintering (RDS), is examined to crystallise the metastable base glass to form one or more ceramic phases. Apatite-mullite glass-ceramics (AMGC) were examined to elucidate the effects of RDS on the crystallization of a bioactive glass-ceramic. By increasing the fluorine content of the glass, the fluorapatite (FAp) and mullite crystallization onset temperatures can be reduced. Samples were sintered in a hydrogen and hydrogen/nitrogen discharge at temperatures of ≈800 and 1000 °C respectively with the higher sintering temperature required to form mullite. Results show that the material can be densified and crystallised using RDS in a considerably shorter time than conventional sintering due to heating and cooling rates of ≈400 °C/min.     

Preparation of β-FeSi2 thermoelectric material by laser sintering

Laser sintering has been applied for preparing β-FeSi₂ based thermoelectric alloy for the first time. Effects of laser sintering time on alloying, phase transformation and microstructure of FeSi₂ were investigated by X-ray diffraction (XRD) analysis and scanning electron microscope (SEM). Effects of annealing temperature and time on phase transformation were also studied through Seebeck coefficients. The results show that for 90 s laser-sintered samples, it takes only 15 h to obtain β phase under Ar atmosphere followed by an annealing at 1073 K. These samples exhibit homogeneous microstructure with average grain size of less than 5 μm. A maximum Seebeck coefficient at room temperature could reach 115 μV/K. It indicates that laser sintering could be an alternate faster preparation method to generate high quality β-FeSi₂ thermoelectric material with little contamination due to its advantages of rapid heating rate, high cooling rate and rapid solidification.     

Selective laser sintering of barium titanate–polymer composite films

A selective laser sintering process has been used to consolidate electro-ceramic thin films on silicon substrates. Methods of forming pre-positioned layers of barium titanate were investigated by spin-coating the feedstock powder mixed with a commercial polymer photo-resist. The ceramic–polymer composite was deposited directly onto a nickel film which was evaporated onto a silicon substrate, pre-oxidised to form an electrically insulating layer. A range of laser processing parameters was identified in which consolidated barium titanate layers could be formed. The laser power was found to be more influential in forming sintered microstructures than laser exposure time. The microstructure of barium titanate films is sensitive to the SLS laser-processing conditions, with the optimum laser powers for the processing of the BaTiO₃–polymer found to be in the range 17–20 W. This article highlights the possibility of using ‘direct write’ techniques to produce piezoelectric materials upon silicon substrates.     

Solid solution formation at the sintering of hydroxyapatite–fluorapatite ceramics

Fluorhydroxyapatite ceramics are increasingly studied for the use as biomaterials due to their good integration ability in the bone tissue and higher resorption resistance compared to the common hydroxyapatite (HA) ceramics. This study is aimed at the X-ray diffraction investigation of the interaction between HA and fluorapatite (FA) particulates in the sintering temperature range up to 1300 °C. The lattice parameters were calculated in dependence of both the FA content in the powder mixtures and the sintering temperature. From those data, the solid solution formation is concluded, at least in the temperature range from 1200 to 1300 °C. Energy-dispersive X-ray microanalysis confirmed the fluorine distribution to be almost uniform in the sintered at 1300 °C ceramics.     

Effect of metal-oxide addition on the sintering of β-calcium orthophosphate

The effect of metal-oxide addition (110 mol %) on the sintering of -calcium orthophosphate (–Ca₃(PO₄)₂) was examined by pressureless sintering at 1070 °C for 5 h. The metal-oxide additives used were as follows: (i) monovalent metal-oxides, Li₂O, Na₂O, and K₂O; (ii) divalent metal-oxides, MgO, CaO, SrO, and BaO; (iii) trivalent metal-oxides, Al₂O₃ and Fe₂O₃; and (iv) tetravalent metal-oxides, SiO₂, TiO₂, and ZrO₂. The relative densities of the sintered –Ca₃(PO₄)₂ compacts were reduced with increasing amounts of monovalent and divalent metal-oxide additions, except for the case of MgO addition when the relative density remained with increasing amount of MgO up to 4 mol %. In the case of trivalent metal-oxide additions, the relative density of sintered –Ca₃(PO₄)₂ compact was reduced with increasing amount of Al₂O₃ addition, whereas it was enhanced with increasing amount of Fe₂O₃ addition and reached 98.7% at 10 mol % Fe₂O₃ addition. In the case of tetravalent metal-oxide additions, relative densities of the sintered –Ca₃(PO₄)₂ compacts were slightly reduced with increasing amounts of SiO₂ and ZrO₂ additions, whereas no appreciable changes in the relative density were observed with increasing amount of TiO₂ addition.     

Pulse electric current sintering of hydroxyapatite/β-tricalcium phosphate composites

Hydroxyapatite (HA)/β-tricalcium phosphate (β-TCP) composites attract attentions as bone implant materials. As one of the fabrication method of HA/β-TCP is mixing of HA and β-TCP powder in advance of sintering. This method enables to control the ratio of content of β-TCP easier. However, it is difficult to obtain dense composites. In this study, we focused on pulse electric current sintering (PECS) to obtain dense HA/β-TCP composites. The sinterability is evaluated with relative density and grain size measurements. Composition of sintered body was also characterized by X-ray diffraction. In comparison with pressureless sintering, PECS increased relative density of the composites without grain growth. In HA/β-TCP sintered by PECS, the phase transformation from β-TCP to α-TCP was promoted. This is due to higher thermal energy by spark discharge during PECS. On the other hand, sintering additives (MgO) inhibited phase transformation. It was suggested that sinterability of HA/β-TCP composites was improved by PECS.     

Pressureless sintering of β-SiC with Al2O3 additions

-SiC was pressureless sintered to 98% theoretical density using Al₂O₃ as a liquid-phase forming additive. The reaction between SiC and Al₂O₃ which results in gaseous products, was inhibited by using a pressurized CO gas or, alternatively, a sealed crucible. The densification behaviour and microstructural development of this material are described. The microstructure consists of fine elongated -SiC grains (maximum length 10 m and width 2–3 m) in a matrix of fine equi-axed grains (2–3 m) and plate-like grains (2–5m). The densification behaviour, composition and phases in the sintered product were studied as a function of the sintering parameters and the initial composition. Typically, 50% of the -phase was transformed to the -phase.     

Microwave sintering of β-SiAlON–ZrO2 composites

Densification, phase transformation, microstructure evolution and hardness of microwave sintered β-SiAlON–ZrO₂ composites were investigated and compared with conventionally sintered samples. Sintering trials were performed by a high vacuum capable 2.45 GHz microwave furnace without decomposition. Microwave sintered samples showed better densification behavior than conventional sintered samples. The higher density observed in the case of microwave sintered samples was attributed to volumetric fast heating. X-ray diffraction results of conventionally sintered samples showed β-SiAlON, tetragonal ZrO₂ and ZrN phases, while, ZrO₂ reacted with nitrogen and completely transformed to ZrN in the case of microwave sintered samples. The aspect ratios of microwave sintered β-SiAlON grains were higher than conventional sintered samples whereas, hardness remained lower.     

Mechanical properties and microstructure of β-SiAION-β-SiC composites by pressureless sintering

-SiAION--SiC composites containing up to 12 wt% -SiC were prepared by pressureless sintering. The strength of composites at room temperature remained relatively unchanged, whereas strength at 1200 °C increased for composites. The fracture toughness (K _IC) for composites was higher than that for -SiAION ceramics. The maximum value was 5.4 MPa m^1/2 for 6 wt% -SiC, and this was an improvement of 15% in comparison with -SiAION ceramics. From SEM observations, an improvement inK _IC values was attributed to crack deflections and branching-off of cracks. Intra-granular fractures were frequently observed in -SiAION. From TEM observations, -SiAION crystals were nanocomposites, within which existed the fine crystals in -SiAION crystal. For composite, -SiAION and -SiC crystals were directly in contact. The mismatching zone was observed in -SiC.