Sintering and characterization of YAG dispersed ferritic stainless steels

The present study investigates the effect of yttrium aluminium garnet (YAG) addition on the densification, mechanical, tribological and corrosion behaviour of ferritic (434L) stainless steels. The composites were sintered at both solid-state (1200 °C) and supersolidus (1400 °C) sintering conditions. Supersolidus sintering results in superior densification, hardness and corrosion resistance of both straight 434L stainless steel as well as YAG reinforced 434L stainless steels. The addition of YAG to 434L stainless steels at supersolidus sintered conditions improves the strength and wear resistance of 434L stainless steels without significantly degrading the corrosion performance.     

Electrochemical behavior of sintered YAG dispersed 316L stainless steel composites

The present work investigates the effect of second phase dispersoid addition and sintering temperature on the corrosion behavior of austenitic (316L) stainless steels. Yttrium aluminum garnet (YAG) was added as second phase to the austenitic stainless steels in varying amounts (1, 2.5 and 7.5 wt.%), and the compacts were sintered at 1200 and 1400 °C corresponding to solid-state and supersolidus sintering, respectively. The sintered samples were characterized for their corrosion resistance in 0.1N H₂SO₄ using potentiodynamic polarization. It is shown that YAG addition does not appreciably increase corrosion rate of 316L compacts. However, as compared to solid-state sintering, supersolidus sintering resulted in superior corrosion resistance. The electrochemical behavior of the 316L–YAG composites with sintering temperature is correlated to the densification response and microstructure.     

Mechanical,corrosion, and sliding wear behavior of intermetallics reinforced austenitic stainless steel composites

The present work investigates the effect of supersolidus sintering and intermetallics (Ni₃Al, Fe₃Al) additions on the densification, mechanical, tribological, and electrochemical behavior of sintered austenitic (316L) stainless steels. The performances of the supersolidus liquid phase sintered (SLPS) compacts are compared with the conventional solid-state sintered (SSS) compacts of similar compositions. Correspondingly, the sintering was carried out at two different temperatures 1200 °C (SSS) and 1400 °C (SLPS). Supersolidus sintering results in significant improvement in densification, wear resistance, corrosion resistance, strength, and ductility in both straight as well as aluminide added composites.     

Sintered 434L ferritic stainless steel-Al2O3 particulate composites containing phosphorus

Sintering of 434£ ferritic stainless steel powder compacts containing 0-8 volume %

cx-AI20 a (1 J.1m size) and 0-4 weight % phosphorus in the form of either FesP or Fe2P was carried out in the temperature range 1150°C-1300°C, and the resultant densification parameters, ultimate compressive strengths and hardnesses were measured. The results reveal that temperatures of 1150°C and 1200°C produced liquid-phase sintering when phosphorus was added in the form of FeaP. With Fe2P additions, a higher sintering temperature of 1300°C densified the composites, and an increase in the phosphorus content to 3% uniformly enhanced the sintered properties: The presence of Al20 a particles at an optimum level only improved the properties of the liquid-phase sintered composites.     

Effect of powder reactivity on microwave sintering of alumina

Effect of the reactivity of starting alumina powder of varying crystallinity on the sintering behavior in microwave process was studied. From X-ray amorphous to highly crystalline alumina, powders were obtained by conventional heating of compacts made of the precursor amorphous powder by heating it at different temperatures from 800 to 1500 °C. These samples were then sintered in a multimode microwave field of 2.45 GHz for 10 min at 1500 °C. The microwave effect on densification of the various alumina powders was evaluated by comparing the microwave and conventional sintering data. The results show significant microwave enhancement in the densification of the samples without any pretreatment. This enhancement became less significant as the temperature of the pretreatment increased and finally diminished. Since the pretreatment at elevated temperatures made the powder more stable thermodynamically, this study indicates that the sintering enhancement of a ceramic material in microwave is a metastability-related phenomenon.     

Mechanical properties of hydroxyapatite–zirconia compacts sintered by two different sintering methods

Microwave sintering is traditionally employed to reduce the sintering temperature required to densify powder compacts. The effect of microwave heating on hydroxyapatite (HA)–zirconia (ZrO₂) green bodies has been investigated in order to understand how microwave energy may affect the physical and mechanical properties of the resultant densified composites. Laboratory synthesised nano-sized HA and a commercial nano-sized ZrO₂ powder have been ball milled to create mixtures containing 0–5 wt% ZrO₂ loadings. Compacts were microwave sintered at either 700, 1000 or 1200°C with a 1 h hold time. Comparative firings were also performed in a resistive element furnace using the same heating profile in order to assess the differences between conventional and microwave heating on the physical, mechanical and microstructural properties of the composites. Samples sintered at 700°C show little sign of densification with open porosities of approximately 50%. Composites conventionally sintered at 1000°C were between 65 and 75% dense, whereas the samples microwave sintered at this temperature were between 55 and 65% dense. Samples sintered at 1200°C showed the greatest degree of densification (>80%) with a corresponding reduction in open porosities. TCP generation occurred as a consequence of sintering at 1200°C, even with 0 wt% ZrO₂, and increased degradation of the HA phase to form significant amounts of TCP occurred with increasing additions of ZrO₂, along with increasing open porosity. Nanosized ZrO₂ prevents the densification of the HA matrix by effectively pinning grain boundaries and this effect is more pronounced in the MS materials. Similar strengths are achieved between the microwave and conventionally sintered samples. Greater amount of open porosity and pore interconnectivity are seen in the MS samples, which are considered to be useful for biomedical applications as they can promote osteo-integration.     

316L不锈钢粉末注射成形件的烧结致密化行为

为了控制粉末注射成形零件的最终尺寸精度和力学性能,对316L不锈钢粉末注射成形件的烧结致密化行为进行了试验研究,分析了烧结温度和升温速率对试件致密化行为以及烧结件力学性能的影响.试验结果表明,致密化过程始于1080℃左右,主要在1200～1300℃的升温过程中快速进行,致密化速率随着升温速率的升高而升高.烧结件的抗拉强度、抗弯强度以及延伸率,不但取决于致密化程度,而且与微观结构有关.分析表明,将基于扩散控制和强度控制的烧结理论结合,可以有效地解释316L不锈钢粉末的致密化行为,需在现有的烧结模型中考虑强度影响因素,才能更真实地模拟烧结过程.     

Preparation of aluminium-fly ash particulate composite by powder metallurgy technique

Aluminium-fly ash mixtures containing different weight percentages of fly ash were prepared and compacted at pressures from 138–414 MPa. The compacts prepared at 414 MPa were sintered in nitrogen atmosphere at 600, 625 and 645°C, respectively. The time of sintering ranged from 0.5–6 h. The densification parameter and the green densities of the compacts were determined as a function of compacting pressure and fly ash weight per cent. Density, hardness and strength of the sintered compacts were determined as a function of weight per cent of fly ash particles. Volume changes during sintering of green compacts were also evaluated as a function of increasing fly ash weight per cent. Microscopic studies of green and sintered compacts were done to study the effectiveness of sintering. Green and sintered density of the compacts were found to decrease with increasing weight per cents of fly ash. Sintering results in slight decrease in density and increase in volume of green compacts within the range investigated. Strength of the sintered compacts decreased with increasing weight per cent of fly ash under the present experimental conditions; however, the hardness was found to increase slightly up to 10 wt% fly ash, beyond which it decreased. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of composition on the enhanced microwave sintering of alumina-based ceramic composites

A series of comparative experiments were performed in which a number of alumina-zirconia compositions were sintered in both microwave and conventional furnaces, using identical heating profiles. Measurement of sample end-point densities showed an enhancement of the sintering process associated with the use of microwave heating for all compositions studied. The associated microstructures examined using scanning electron microscopy showed slightly larger grain sizes for the microwave-sintered compacts, as would be expected from their higher densities. The design of a high-temperature sintering dilatometer has allowed continuousin situ monitoring of the densification process in both the microwave and conventional environment. Data obtained in this way have shown that there is an effect of composition on microwave densification. This appears to be related to the increased lossiness of the composite, (increased zirconia content), rather than the effect of zirconia as a sintering aid. In addition the dilatometer results suggest that the microwave enhancement of the sintering process may be due to a reduction in the activation energy for grain-boundary diffusion.Electricity Association Technology Ltd. Registered Office: 30 Millbank, London SW1P 4RD.     

Development of an instrumented and automated single mode cavity for ceramic microwave sintering: Application to an alpha pure alumina powder

This paper describes the development of an instrumented and automated single mode microwave cavity for sintering ceramic powders. This setup includes an optical dilatometer and a motorized plunger to control heating cycles in a wide range of heating rates (from 5 °C ∙ min^− 1 to 200 °C ∙ min^− 1) up to 1600 °C and to allow reliable comparison with conventional sintering. The cavity and the sintering cells for both hybrid and direct microwave sintering were designed using finite element simulation. For accurate temperature measurement, an optical pyrometer calibrated with a specific protocol has been used. Microwave sintering of fine grained (< 100 nm) alpha alumina compacts was thus investigated and compared to conventional sintering. This pure alumina powder has been sintered by direct microwave heating, without any susceptor nor doping element to initiate heating as often achieved in the literature. The comparison of the densification kinetics along an identical thermal cycle evidenced a significant enhancement of sintering under microwaves during the first and intermediate stages.     

Microstructure and abrasive behaviors of TiC-316L composites prepared by warm compaction and microwave sintering

316L stainless steel composites with various weight fractions of TiC particles were prepared using warm compaction and microwave sintering. Abrasion resistance measurements were used to study the abrasive behaviors of TiC-316L stainless steel composites. The effects of TiC content and preparation methods on the microstructure and mechanical properties of 316L stainless steel composites have been investigated. The results showed that the sample prepared by warm compaction and microwave sintering exhibited significantly superior densification, higher hardness, and better abrasion resistance when compared with conventionally processed counterpart. TiC particles reinforcement improved the abrasion resistance of 316L stainless steel, and the abrasion resistance of the composites was considerably better than that of the 316L stainless steel. The volume loss initially decreases with increasing TiC content up to 5 wt.%, it then slightly increases as increase the TiC particles content to 10 and 15 wt.%. In this present abrasion tests, the composites using 5 wt.% TiC addition offers a high abrasion resistance.     

Production of injection moulded 316L stainless steels reinforced with TiC(N) particles

Abstract

Metal matrix composites, based on 316L stainless steel and reinforced with TiC and TiCN particles, were manufactured following a powder injection moulding route: mixing, preparation of feedstock, moulding, debinding and sintering. The 316L stainless steel and carbide powders were dry mixed and moulded with wax based binder. The critical powder loading for injection moulding was 62·5 vol.-% for all samples. Binder debinding was performed by solvent and thermal method. After debinding, the samples were sintered at 1250 and 1385°C for 1 h in pure H₂. Metallographic studies were conducted to extend densification and the corresponding microstructural changes. The sintered samples were characterised by measuring tensile strength, hardness and wear behaviour. Wear loss was determined for all samples after wear tests. All powder, fracture surfaces of moulded and sintered samples, and worn surfaces of all the samples, were examined using scanning electron microscope. The sintered density of injection moulded 316L stainless steel samples, reinforced and unreinforced, increases with increasing sintering temperature. The addition of TiC and TiCN improves the hardness and wear resistance with increasing sintering temperature.     

Sintered duplex stainless steels from premixes of 316L and 434L powders

Duplex austenite–ferrite stainless steels were prepared from the premixes of 316L and 434L stainless steel atomized powders. Pronounced densification was observed after 1350°C sintering in hydrogen. 316L-60w/o 434L steel composition exhibited maximum transverse rupture strength, while 40 and 60w/o 434L containing compositions showed total immunity in 1N H₂SO₄ even after a exposure time of 360 h. Anodic polarization curves also suggest high-corrosion resistance of those two compositions. Magnetic coercivity decreased with increase in sintering temperature while magnetic saturation follows the reverse trend. Wear resistance of the duplex stainless steels under sliding condition was in between the straight steels.     

Preparation and characterization of ZrB 2 -SiC ultra-high temperature ceramics by microwave sintering

ZrB₂-SiC ultra-high temperature ceramic composites reinforced by nano-SiC whiskers and SiC particles were prepared by microwave sintering at 1850°C. XRD and SEM techniques were used to characterize the sintered samples. It was found that microwave sintering can promote the densification of the composites at lower temperatures. The addition of SiC also improved the densification of ZrB₂-SiC composites and almost fully dense ZrB₂-SiC composites were obtained when the amount of SiC increased up to 30vol.%. Flexural strength and fracture toughness of the ZrB₂-SiC composites were also enhanced; the maximum strength and toughness reached 625 MPa and 7.18 MPa·m^1/2, respectively.     

Sintering of MgO and MgO-TiC ceramics by plasma,microwave and conventional heating

The densification of MgO and MgO-TiC ceramics by hollow cathode plasma and microwave sintering techniques was studied. The resultant fractional densities were compared to those of conventionally sintered samples. It was demonstrated that indirect heating and sintering of MgO by microwave radiation was possible. Higher fractional densities were obtained in MgO by microwave sintering in comparison to conventional sintering under the same conditions. In the case of MgO-TiC composites, chemical reaction led to Mg₂TiO₄ formation in all samples. Plasma sintering suppressed this reaction and exhibited reduced sinterability upon TiC addition. On the other hand, small amounts of TiC additions aided the conventional sintering of MgO, resulting in better mechanical properties. Furthermore, commercial grade MgO was sintered to higher densities than relatively pure research grade MgO. Hardness and fracture toughness values are reported.     

Microwave synthesis and sintering characteristics of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript>

CaCu₃Ti₄O₁₂ (CCTO) was synthesized and sintered by microwave processing at 2·45 GHz, 1·1 kW. The optimum calcination temperature using microwave heating was determined to be 950°C for 20 min to obtain cubic CCTO powders. The microwave processed powders were sintered to 94% density at 1000°C/60 min. The microstructural studies carried out on these ceramics revealed the grain size to be in the range 1–7 μm. The dielectric constants for the microwave sintered (1000°C/60 min) ceramics were found to vary from 11000–7700 in the 100 Hz–00 kHz frequency range. Interestingly the dielectric loss had lower values than those sintered by conventional sintering routes and decreases with increase in frequency.     

Nanometric WC-12 wt% AISI 304 powders obtained by high energy ball milling

WC cemented carbides with a greener alternative binder to Co, AISI 304 stainless steel (SS), were processed through high energy ball milling (HEBM). The milling parameters, such as rotation speed, ball-to-powder ratio and milling time were investigated. Selected milling conditions were applied to obtain a nanosized powder of WC-12?wt% SS with a highly uniform distribution of the ductile phase. For comparison, a conventionally wet milled powder was also prepared. Both powders were thermally characterized by dilatometry, up to 1450?°C, using vacuum atmosphere, and structural and microstructural analysis were performed in the sintered samples. The nanometric size of the HEBM powder particles markedly affected its densification and thermal reactivity; when compared with the micrometric powder obtained from conventional milling, early starting densification, with a greater contribution of solid state sintering, and increased reactivity, with formation of a larger amount of (M,W)₆C phase, was noticed during sintering of HEBM powder compacts.     

Studies on sintering behaviour of Al2O3–ZrO2 oxide composites processed by extended arc thermal plasma and conventional heating

Al₂O₃–ZrO₂ composites were sintered using low cost extended arc thermal plasma reactor and conventional heating. Composites prepared in a wide range of composition were studied in terms of their density, shrinkage, hardness, structure, microstructure and dielectric response. Experimental parameter such as sintering time, sintering temperature and plasma power were optimized to achieve higher sintered end product. Highly dense sintered products were obtained by plasma heating route within short sintering time compared with conventional sintered method. Interesting development pertaining to structure and phase evolution, structure and dielectric response are analyzed. It is found that compositional variation in this composite produces structural phase separation at different sintering conditions, which is more in plasma heating product than conventional heated product. Plasma sintered product always shows less dielectric constant as compared to conventional sintered sample.     

Fabrication of hydrothermal ageing resistant of 3 mol % yttria-stabilised tetragonal zirconia polycrystalline via microwave sintering

The influence of microwave sintering on the densification, mechanical performances, microstructure evolution and hydrothermal ageing behaviour of pure 3 mol % yttria-stabilised tetragonal zirconia polycrystalline (3Y-TZP) ceramics was compared with conventional sintered samples. Green bodies were sintered via conventional pressure-less and microwave sintering method between 1200 °C to 1400 °C with dwelling time and firing rate at 120 min, 10 °C/min and 1 min, 20 °C/min. Result showed that reduced processing temperature and holding time is possible with microwave sintering technique for fabricating good resistant zirconia sample with bulk density, Young's modulus, and Vicker's hardness that are comparable to samples sintered with conventional method. However, the microwave sintered samples suffered from hydrothermal ageing where their average grain size is above critical size. The enhancement of hydrothermal ageing resistance of the sintered samples is associated with the decreasing grain size of the sintered samples instead of sintering method.     

Enhancing densification of zirconia-containing ceramic-matrix composites by microwave processing

Various ceramic-matrix composites containing zirconia were sintered using a 2.45 GHz microwave field. The effects of the addition of zirconia and the processing parameters on the sintering and microstructure development were investigated. The results showed that microwave processing enhanced the densification of these composites considerably. The enhancement in sintered density was up to 46% over conventional sintering, depending on the systems. This revised version was published online in November 2006 with corrections to the Cover Date.