Papers by Author: Hasan Mandal

Abstract: - SiAlON ceramics were produced from different starting Si3N4 powders including β-Si3N4 and α-Si3N4 powders and mixtures of these powders. Gas pressure sintering was used for sintering. After sintering, resultant fracture toughness values were correlated with microstructure and starting powders. By optimizing chemistry and process parameters; - SiAlON ceramics with reasonable fracture toughness can be produced from rather coarse β-Si3N4 powder. This could improve the economic viability of SiAlON ceramics since -Si3N4 powders are less costly.

Abstract: In the present study, interactions between AlN and SiAlON laminated couples were investigated after gas pressure (GPS) and spark plasma sintering (SPS) by scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX) with the aim to produce laminated composites. In the laminated couples sintered by GPS, a significant reaction zone (~100-150 μm), containing a high aspect ratio of elongated polytypoid grains, was observed at the interface. However, in the case of laminated couples sintered by SPS, a considerably thin reaction region (~2-3 μm) was observed, elongated polytypoid grain formations were also detected.

Abstract: 25α:75β SiAlON composition was designed with different cations and at different molar ratios. Effect of the type of cations both on the composition and the type of intergranular phase investigated after gas pressure sintering and further post sintering heat treatment.

Abstract: In this study, effect of seed addition on the microstructural development of Y-Sm α-SiAlON system has been investigated. Besides using pre-produced rod-like α-SiAlON seeds, growth in the glassy phase and harvested by chemical treatment, powdered Y-Sm/α-SiAlON was also used as nuclei. Microstructural results of both systems were compared with un-seeded samples. It was found that the powdered α-SiAlON addition has also caused the acicular α-SiAlON grain formation, which indicates the dominant effect of thermodynamic stability on the preferential growth of α-SiAlON grains in the c direction.

Abstract: In this study, textured α-SiAlON ceramic was produced by templated grain growth process. In order to enhance the alignment of the seeds, gated tape casting arrangement was utilized. Y-Ce codoped seeds were synthesized to induce the preferred orientation. For the production of the seeds, Ce3+ was used as a sintering additive due to its well-known effect on the formation of elongated α-SiAlON grains. In addition, Y3+ was used to stabilize Ce3+ in α-SiAlON structure. Gated tape casting arrangement was also used to enhance the alignment of the seeds in the green tape. After the lamination and sintering, preferentially aligned α-SiAlON grains were observed by the microstructural investigation. In addition, XRD results obtained from the planes parallel and perpendicular to the casting direction indicated the alignment of grains in α-SiAlON ceramics. Hardness measurements also confirmed that the alignment of the grains results in 8% increase in hardness value of the plane perpendicular to the casting direction with respect to that of the parallel one.

Abstract: Aqueous dispersions of SiAlON forming powders (Si3N4, AlN and Y2O3) were obtained. Dispersibility of powders was found to be improved by addition of sodium tripolyphosphate. Stable aqueous suspensions of SiAlON forming powders were prepared by using a phosphate ester, sodium tripolyphosphate (STPP, Na5P3O10). Effect of the STPP on rheological properties of the powder mixture was investigated by zeta potential measurements, sedimentation and milling studies. It has been shown that addition of the STPP to the aqueous SiAlON forming powder mixture improves the dispersion behavior significantly. These results suggest that STPP can be utilized as a dispersant in such ceramic systems.

Abstract: CeramTec AG is a well-known manufacturer of cutting tool materials, especially for inserts based on silicon nitride. Some years ago it started the production of alpha/beta-SiAlONs because of their superior properties. Due to strong competition in the cutting tool business there is a constant need for further development of the cutting tool materials. One possibility to increase wear resistance and life time of cutting tools is to increase the hardness and maintaining the good toughness at the same time. This can be done by adding hard particles like e.g. SiC to an alpha/beta-SiAlON. In 2003 an improved grade had been presented: a SiC-reinforced alpha/beta-SiAlON. This composite material shows a significantly reduced wear during machining of cast iron. The effect of SiC-addition on mechanical and thermal properties is discussed.

Abstract: In the present work, multi-cation doped (Sr2+-Mn4+) SiAlON ceramics were investigated. The mixture was sintered at in a gas pressure sintering furnace (GPS). XRD results showed that sintered samples were composed of β and α-SiAlON phases and this result is also confirmed by back scattered SEM images. EDX analysis of the individual α-SiAlON grains showed that Mn can incorporate into the α-SiAlON structure when it is codoped with Sr.