Nitrides and Oxynitrides III

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Authors: Bernd Bitterlich, Kilian Friederich
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 resistanceand 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 properties is discussed.
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Authors: Sedigheh Salehi, Omer Vander Biest, Jef Vleugels
Abstract: 1.75 mol % Y2O3-stabilized ZrO2-based composites with 35-95 vol % TiN were fully densified by hot pressing for 1 hour at 1550°C under a load of 28 MPa. The TiN grain size was found to increase with increasing TiN content, resulting in a decreasing hardness and strength. The best mechanical properties, i.e., an indentation toughness of 5.9 MPa.m1/2 in combination with a Vickers hardness of 14.7 GPa and an excellent bending strength of 1674 MPa were obtained for the composites with 40 vol % TiN. The active toughening mechanisms were identified and their contribution to the overall composite toughness is discussed. Transformation toughening was found to be the primary toughening mechanism in all investigated composites.
135
Authors: Adem Demir, Derek P. Thompson
Abstract: Nicalon SiC fibre tows have excellent properties for ceramic matrix reinforcement but residual oxygen within the fibres degrades fibre properties when these are incorporated into ceramic matrices at elevated temperatures. β-SiAlON ceramics also have excellent mechanical and physical properties, especially fracture toughness. However, sintering of β-SiAlON is generally carried out at 1650-1750°C, considerably higher than the temperatures above which fibre degradation occurs (>1200°C). In the present study, the refractoriness and strength of Nicalon fibres were improved by high pressure CO heat treatment, and densification temperatures of β-SiAlON were lowered by using different kinds of sintering additives. Heat-treatment of the fibres under 45 bar CO pressure at 1500-1650°C led to an increase in fibre strength and to the formation of a thin carbon layer on the surface of the fibres. These improvements in the Nicalon SiC fibres allowed them to be incorporated successfully into β-SiAlON matrices. The as-received and heat-treated fibres were infiltrated with β-SiAlON starting powder mixes and hot-pressed with low temperature sintering additives at 1600-1700°C for 30 min. Bending strength and fracture toughness measurements showed that samples containing heat-treated fibres provided a significant strength and fracture toughness increase compared with similar samples prepared using as-received fibres, and massive pull-out was observed because of the weak interface resulting from the surface carbon coating on the fibres.
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Authors: Pavol Šajgalík, J. Křest'an, Zoltán Lenčéš
Abstract: The corrosion resistance of sialons made from commercial powders (AlN, Al2O3 and Si3N4) and from powder precursor produced by carbothermal reduction and nitridation of raw aluminosilicate (pyrophyllite) in molten steel were investigated. The corroded zone in sialon made from raw pyrophyllite (P1) is more then two times deeper compared to the corroded zone of sialon made from commercial powders (C1). The corrosion zone of sample P1 is on the average 610 μm deep, while in sample C1 it is only 260 μm. The main corrosion products are γ-Al2O3 and iron silicides. The phase compositions were estimated by neutron Rietveld refinement.
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Authors: N. Karakus, A.O. Kurt, O. Toplan
Abstract: Structural ceramic parts such as those made from Si3N4 or SiAlONs are promising for considering them in replacing metals in the areas where harsh environmental conditions like abrasive corrosion under high stresses along with high temperature effects are dominant. A widespread use of these ceramics depends on the ease of finding them with low cost. This paper highlights some important points on the low cost production technique of Si3N4 or SiAlON ceramics in powdered form via carbothermal synthesis technique using minerals in the light of the current and previous works. The technique gives flexibility of controlling production parameters that affect on the yield and quality of powders produced. A special account is given to the effects of reactants on the production of such ceramic powders. After synthesising ceramics in powdered form, some comparisons were made on the characterisation of the products of homemade ones with that of commercially available ones. After successful experimental works, a raw material of orthoclase clay mineral exhibited a high transformation potential to a mixture of Si3N4 and SiAlON type ceramic powders.
151
Authors: Ahmet Atasoy
Abstract: The behaviour of silica during the carbothermal reduction nitriding process at temperatures between 1300-1500°C was studied by means of X-ray diffraction and scanning electron microscope analysis. The experimental runs were allowed to proceed up to 1 h in presence of nitrogen flow. The following mechanism of reduction nitriding of silica which was based on the experimental observation was proposed. Initially the impurity of the starting material is reduced before 1300°C. SiO2 was reduced into SiO gas phase by active carbon and it was vaporised out of the mixture. The nucleation of α-Si3N4 was formed vapour-gas reaction took place and deposited on the surface of the mixture as well as around the reaction crucible. In the third stage, α-Si3N4 transforms to one dimensional direction which was β-Si3N4 particle. This was followed by the formation of SiC at temperature above 1450°C.
157
Authors: Adem Demir, Zafer Tatli, F. Caliskan, A.O. Kurt
Abstract: In this study, α-Si3N4 powder was produced by carbothermal reduction and nitridation (CRN) of quartz from Can-Canakkale. Carbon with a specific surface area of 110 m2g−1 and quartz powders were mixed then the powder mix was placed in an alumina tube furnace and reacted in between 1300-1500°C for 4 hours under nitrogen flow. The quartz powder was carbothermally reduced and nitrided to form silicon nitride powders. XRD results showed that the reaction product was mainly α-Si3N4 and contained some β-Si3N4 and residual quartz. In order to reduce amount of unreacted quartz, the raw materials mixture was grinded either with carbon black or with no carbon. After CRN reactions of separate grinded quartz powders with carbon, residual quartz was disappeared, reaction temperature was decreased and α-Si3N4 rate was increased. Hence, a better mixing of carbon and fine silica enhanced the α phase formation. SEM images and XRD pattern showed that sub micron particles (0.6–0.87m), high α-phase content Si3N4 powders can be produced at 1450°C for 4 h in flowing nitrogen gas during the CRN process.
163
Authors: Zafer Tatli, Adem Demir, F. Caliskan
Abstract: SiAlON ceramics were successfully produced in the form of powders from high purity kaolin, a hydrated aluminium silicate, Al2Si2O5(OH)4 type of clay mineral (comprises 83.85% kaolinite, 13.59% quartz, 0.88% feldspar, 1.37% others) of Canakkale-Can origin. Factors affecting SiAlON powder production were temperature, holding time, gas-flow rate and preparation methods. System optimisation was achieved following the results succeeded from numerous testing and characterisation (with XRD, SEM, EDS, BET, etc.) of each test. Changing in gas flow rate, temperature and holding time at plateau temperature had influences on the final powder yield, their morphologies and phase formation. The best conversion of kaolin clay mineral to SiAlON ceramic powder was the test run at 1475oC for 4 hour under 1 lt/min N2-flow. Product after the process was mainly of β'- Si3Al3O3N5 (z=3) powder along with small amounts of Al2O3, mullite and AlN phases. Some powder product exhibits furry type of wiskers morphology, which may be useful for using as a reinforcing material in particulate composite bodies.
169
Authors: R. Yilmaz, A.O. Kurt
Abstract: In this study, the Clay – 220 mineral of Turkish origin from Kalemaden A.S., having a composition of the quarts, kaolinite, feldspar and the others with the weight percent of 36.20, 12.66, 48.03 and 3.11, respectively, was used as a raw material to investigate its transferability potential to a technological ceramic. The mineral was in the form of conglomerate rocks, which were prepared in the consecutive mineral processing procedures before taken it to the stage where carbothermal reduction - nitridation (CRN) was performed in an atmosphere controlled environment. Various process parameters, i.e., temperature, time and N2 – flow rate were tested in order to optimise the CRN process. SiAlON was formed at temperatures as low as 1300oC and its content increased with reaction temperature up to 1475oC. Above 1425oC, SiC becomes more predominant phase in the powdered product after CRN. Fine powders formed at temperature of 1350oC, which is the lowest value reported among the current literature, contain single crystalline phase of β-SiAlON. The results indicated that the best transformations and yield to the SiAlON ceramic were occurred on the previously prepared Clay – 220 samples in powder form that were processed at 1350oC for 6 hours with nitrogen flow rate of 1 lt/min.
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Authors: Katsutoshi Komeya, Junichi Tatami
Abstract: Liquid-phase sintering of aluminum nitride (AlN) with additives was reviewed. The most important innovation was the discovery of critical sintering aids for AlN densification, specifically rare-earth compounds and alkali-earth compounds. These additives are extremely valuable for increasing thermal conductivity by trapping and removing oxygen in the AlN lattice during firing. Consequently, thermal conductivities in AlN ceramics of 100 to 260W/mK were developed. We also studied the effects of parameters such as raw powder, additives, composition, and firing condition in liquid-phase sintering with AlN-sintering aids, focusing on oxygen impurities in the system. The sintering behavior of powder compacts was investigated by evaluating the densification, the lattice constant c for AlN, and the dihedral angle of the interface between the AlN grains and the grain boundary liquid-phase. In our results, the change in densification was closely related to changes in the lattice constant c and the dihedral angle. That is, the sintered density increased with an increase in the oxygen dissolved in the AlN grains and with the improvement in wettability between the solid and liquid phase.
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