High-Performance Ceramics III

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Authors: He Zhuo Miao, Zhi Jian Peng, Wen Jie Si, Long Hao Qi, Jiang Hong Gong, Wei Pan
Abstract: There are too many methods to enhance the performance of ceramic cutting tools. All the methods can be sorted into two types: inner modification and surface modification. One of the main method to the inner modification of ceramic cutting tools is dispersion strengthening. Usually, in order to enhance the performance of ceramic cutting tools, some dispersed phases of TiN, TiC or TiCN, Al2O3, and/or ZrO2, and so on, and/or some whiskers, or fibers were added into the ceramic matrixes. And the new types of cutting tools, which possessed much more excellent performance than the original ones, were called composite ceramic cutting tools. For the composite Si3N4-based ceramic, Al2O3-based ceramic, and TiCN-based cermet, the cutting efficiency could be enhanced to 3~10 times, compared with cemented carbide tools. And they can be used for rough and finish machining of various cast iron workpieces and hardened steels, respectively, including milling and planning. Ion implantation is a surface modification for ceramic cutting tools. With certain doses of metals, for example, titanium, zirconium and chromium, and so on, implanted into the ceramics, the hardness, Young’s modulus, fractural toughness, and bending strength, etc., can be enhanced. For Al2O3 and Si3N4 ceramics, the hardness, Young’s modulus, and bending strength increased with a maximum factor of 50%, and the flank wear decreased with a factor of 2~12, compared with the unimplanted ceramic cutting tools. However, the main shortcoming of ion implantation to modify ceramics is the thickness of modified layers. They are, usually, too thin for cutting tools. The so-called PHEDP, pulsed high energy density plasma, is another surface modification method for ceramic cutting tools proposed recently. With such method, much thicker coatings of TiN, TiCN and (Ti,Al)N, etc, were deposited onto Si3N4 and WC ceramic cutting tools.The main merits involved in high hardness and Young’s modulus of the coatings, low residual stresses, and good adhesive strength between the coatings and substrates. And the flank wear of the as-depositedtools decreased with a factor of 5~10.
Authors: Sheng Zhu, Bin Shi Xu
Abstract: A novel supersonic plasma spraying system was developed with a maximum power of 80 kW and a maximum working gas flow of 6 m3/h, at which gas and particle velocities of 2400 and 600 m/s can be achieved respectively. This paper deals with novel supersonic plasma spraying system design, the structure of novel supersonic plasma gun includes a special Laval nozzle as the single anode and inner powder supply, and the mechanisms of supersonic plasma jet as well as the effects on the sprayed particles. The spraying process parameters of several ceramic powders such as Al2O3, Cr2O3, ZrO2, Cr3C2 and Co-WC were optimized. The properties and microstructure of the sprayed ceramic coatings were investigated. Nano Al2O3-TiO2 ceramic coating sprayed by using novel supersonic plasma spraying was also studied. Novel supersonic plasma spraying improves greatly ceramic coatings quality compared with conventional air plasma spraying (Metco 9M), as well as it has lower energy and gas exhaustion compared with high power supersonic plasma spraying (Plazjet), which can spray high-performance ceramic coatings at low cost.
Authors: He Zhuo Miao, Zhi Jian Peng, Long Hao Qi, Feng Shi, Wen Jie Si
Abstract: With a newly-developed technique, pulsed high energy density plasma (PHEDP), TiN, TiCN, and (Ti,Al)N coatings were deposited onto silicon nitride and cemented carbide cutting tools. The structures of these coatings were systematically investigated in this paper. The average surface roughness (Ra) of the coated tools were ranged in 20~150 nm. The smooth surface of coated tools means that the coatings are promising candidate for cutting tools of high precision and it is in favor of reducing the fiction coefficients and flank wear of tools. The coating thickness varied, in the range of 3~20 µm, with the deposition conditions of the shot number of pulsed plasma, and the voltages between the inner and outer electrodes of the coaxial gun. The coating has a densified structure compared to the substrate structure and almost no pores and cracks exist in the coating surface. The grain sizes of the coating were small (<100nm), much finer than those of the substrate (>2 µm). Except for TiN-Si3N4 system, no apparent columnar grain structure as presented predominantly in typical vapor deposited coatings was observed. In fact, an equiaxed structure was presented, due to the pulsed mode of plasma bombardment and solid solution strengthening of C or Al into TiN lattices, resulting in disruption, through renucleation, of epitaxy on individual columns. A continuous and densified interface was observed. All these characteristics in structures promised an excellent performance of the coated tools.
Authors: Feng Shi, Zhi Jian Peng, He Zhuo Miao, Wen Jie Si, Long Hao Qi, Wen Zhi Li
Abstract: ZTA (alumina toughened by 20 wt.% zirconia), hot-pressed silicon nitride (with totally 10 wt.% Y2O3 and Al2O3 as additives) and TZP (pressureless-sintered yttria stabilized zirconia) ceramics were implanted by various doses (5 × 1016 ions/cm2 ~ 1 × 1018 ions/cm2) of Ti, Zr, and Cr ions with a MEVVA (metal vapor vacuum arc) source implanter. The bending strength of these ceramics was investigated. It was discovered that, for different ceramics, different behaviors were presented with the same doses of implantation ions. For alumina and zirconia ceramics, the bending strength increased with increasing implantation doses of Ti and Zr ions, but decreased with high dose of Cr ions. For silicon nitride ceramics, however, the bending strength originally increased with smaller doses of metals implanted, and decreased with higher doses of metals of Ti, Zr, and Cr ions. The different behaviors are correlated to the different variations in compositions and microstructures of ceramics after ion implantation.
Authors: Naoki Kondo
Abstract: Grain alignment control to make anisotropic microstructure is one of the most promising techniques to achieve superior mechanical properties in specific directions. Anisotropic silicon nitrides, which were fabricated by a forging technique, can show superior mechanical properties at room temperature as well as at elevated temperatures. A sinter-forged silicon nitride with yttria and alumina additives exhibited very high strength of 2.1GPa at room temperature, meanwhile that with lutetia additive showed high strength of 700MPa at 1500oC. Anisotropic silicon nitrides are also advantageous to achieve higher fracture energy. Such silicon nitrides can show 3~5 times higher fracture energy than isotropic ones. Sinter-forging technique is also applicable to fabricate porous anisotropic silicon nitrides. In this paper, fabrication and mechanical properties of anisotropic silicon nitrides are briefly described.
Authors: Yu Ping Zeng, Naoki Kondo, Kiyoshi Hirao, Tatsuki Ohji, Shuzo Kanzaki
Abstract: Silicon nitride ceramics with and without β-Si3N4 seed addition were prepared via tape casting nonaqueous ceramic slurries, laminating the green ceramic tapes, and gas pressure sintering the green bodies in nitrogen atmosphere. Lu2O3 and SiO2 were used as sintering additives. The results showed that the rod-like β-Si3N4 seed was helpful to enhance β-Si3N4 grain growth unidirectionally. The elongated grains grown from seeds were preferentially oriented parallel to the casting direction, resulting in anisotropic microstructure. When a stress was applied with along the grain alignment, the bending strength of the tape-cast Si3N4 with 3 wt% β-Si3N4 seed addition measured at 1500oC was 738 MPa, which was almost the same as the room temperature bending strength 739 MPa, and the fracture energy can be improved from 301 J/m2 at room temperature to 781 J/m2 at 1500oC. The large fracture energy and bending strength at 1500oC were attributable primarily to the unidirectional alignment fibrous grains and a high melting point grain boundary phase.
Authors: Jun Yang, Bo Wang, Ji Qiang Gao, Jian Feng Yang, Guan Jun Qiao, Hong Jie Wang, Zhi Hao Jin
Abstract: The purpose of this paper is to demonstrate a method of preparation of rod-like β-Si3N4 seeds for self-reinforced silicon nitride ceramics. β-Si3N4 seeds with different size and aspect ratios have been well developed by means of heating the mixture of α-Si3N4 containing various contents of β-Si3N4 powder with certain additives at 1800°С for holding different time under 0.5MPa nitrogen gas. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were performed to investigate the morphology, phase transformation and secondary phase. The influence of the proportion of β-Si3N4 in the initial α-Si3N4 powder on seeds size and morphology was investigated. The uniform β-Si3N4 seeds with controllable size and aspect ratios obtained through the β-Si3N4 addition can be explained by an increased heterogeneous nucleation, and MgO was more effective for enhancing the grain growth by the decreased viscosity.
Authors: Wenli Guo, Ting Xian Xu, Qiang Zou, Chang Chun Ge
Abstract: The production of silicon nitride whiskers in solid mixtures under nitrogen-containing atmospheres was investigated. Green bodies made of micron and nano silicon nitride powders with 2MgO×2Al2O3×5SiO2 as sintering additive were sintered at 1500~1560oC in flowing nitrogen-containing atmospheres. Resultantly whisker-like products were formed on the cold side of the graphite boat. X-ray diffraction analysis indicated that the formed whisker were α–Si3N4. The products were analyzed by transmission electron microscope. The morphologies of the resulting whisker were varied with sintering temperatures and atmospheres. Dislocation structures were observed on the edge of some whiskers. This suggested that the whisker growth proceeded through a vapor-solid (VS) mechanism. The mechanism of whiskers growth at different temperatures and atmospheres was discussed with a thermodynamic calculation based on a database software.
Authors: Jian Feng Yang, Ji Qiang Gao, Guo Jun Zhang, Ichiro Hayashi, Tatsuki Ohji
Abstract: Porous Si3N4 ceramics with different pore morphology have been fabricated, utilizing either organic whiskers or starch as the fugitive agents, through slip-casting and die-pressing technique, respectively. The obtained porous ceramics have rod shaped or equiaxial pore morphology, originated from there two kinds of pore forming agents. The mechanical properties were investigated. The strength decreased considerably when small amount of whiskers were added, however, further increase in the whisker content only cause a moderate decrease of the strength. Gas permeability were measured for the samples with high whisker content of 60 vol% (corresponding to porosity of about 45% in the sintered bodies), and was compared with the counterpart contained the same porosity in which pores were equiaxial. The flexural strength of the samples with these two types of fugitive particles was almost the same, but the permeability of samples with rod-shaped pores were much higher than that with equiaxial pores, which can be understood in terms of a short pass model.
Authors: G.H. Liu, Ke Xin Chen, He Ping Zhou, Xiao Shan Ning, José Maria F. Ferreira
Abstract: In this paper, Yb a-sialon powders with elongated grains have been prepared by combustion synthesis. The effects of diluents, nitrogen pressure, adding a-sialon seeds and NH4F on the phase transformation and microstructure development of the samples are discussed systematically. The experimental results show that the addition of diluents, proper nitrogen pressure and additive NH4F can facilitate the nitridation of Si. It was also found that the added a-sialon seeds and NH4F are beneficial to the nucleation and growth of elongated grains.

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