Development and cytotoxicity evaluation of SiAlONs ceramics

SiAlONs are ceramics with high potential as biomaterials due to their chemical stability, associated with suitable mechanical properties, such as high fracture toughness and fracture resistance. The objective of this work was to investigate the mechanical properties and the cytotoxicity of these ceramic materials. Three different compositions were prepared, using silicon nitride, aluminum nitride and a rare earth oxide mixture as starting powders, yielding Si₃N₄–SiAlON composites or pure SiAlON ceramics, after hot-pressing at 1750 °C, for 30 min. The sintered samples were characterized by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). Furthermore, hardness and fracture toughness were determined using the Vicker's indentation method. The biological compatibility was evaluated by in vitro cytotoxicity tests. Ceramic with elevated hardness, ranging between 17 and 21 GPa, and high fracture toughness of 5 to 6 MPa m^1/2 were obtained. Since a nontoxic behavior was observed in the cytotoxicity tests, it may be assumed that SiAlON-based ceramics are viable materials for clinical applications.     

Particulate silicon nitride-based composites

In an attempt to optimize the structure and properties of silicon nitride ceramics, a variety of novel processing techniques and materials compositions have evolved over the last 15 years. Among the most important, was the development of various silicon nitride-based composites. A review of particulate, silicon nitride-based composites other than whisker- or platelets-reinforced, is presented. Materials based on silicon nitride and SiAlONs, with additions of carbides, nitrides and borides of transition metals are described. Special emphasis is placed on TiN- and TiC-containing ceramics. The manufacture of composites by hot pressing, reaction sintering, pressureless and gas-pressure sintering is discussed. The data on properties, including conductivity, density, Young's modulus, strength, fracture toughness, hardness, thermal expansion, wear, creep and oxidation resistance are presented. Analysis of actual and potential uses of the selected composites demonstrates that the particulate composites are very promising as tool, structural and electronic materials.     

Hardness of superhard polycrystalline materials and its temperature dependence

Data on the hardness and fracture toughness of diamond and cubic boron nitride-based polycrystalline superhard materials and the temperature dependence of these materials’ hardness have been considered. It has been found that the temperature dependence of hardness of polycrystals, based on diamond and boron nitride, produced from powders having various dispersions, shows a decrease in hardness of all the materials and difference in the materials thermostability.     

Structure, mechanical and functional properties of aluminum nitride-silicon carbide ceramic material

Two-phase ceramic composites of the dielectric-semiconductor type having different semiconducting phase content (aluminum nitride ceramics with uniformly distributed inclusions of silicon carbide of a certain size) have been produced by pressureless sintering. These composites are characterized by Vickers hardness HV (150 N) 9.5–15.8 GPa, Palmqvist fracture toughness 3.0–4.2 MPa m^0.5, bending strength 132–209 MPa, thermal conductivity 37–82 W/(m K), and by a coefficient of the microwave electromagnetic energy attenuation to 36.3 dB/cm. It has been found that as the size of silicon carbide grains in aluminum nitride-based ceramics increases, the thermal conductivity increases and microwave energy attenuation decreases, which is indicative of the decisive role of grain boundaries in scattering both phonons and microwave radiation.     

On the possibility of silicon nitride as a ceramic for structural orthopaedic implants. Part I: processing, microstructure, mechanical properties, cytotoxicity

Notwithstanding the good combination of mechanical and tribological properties, the suitability of silicon nitride for application as prosthesis in bone reconstruction or in articular joints replacements is still controversial. This study aims to design and produce three different silicon nitride-based ceramics and to test the materials. In this Part I the microstructure and mechanical properties evidence outstanding characteristics and the cytotoxicity studies confirm that all the materials are extremely inert and biocompatible. In Part II, the wear performance and the wettability and chemical stability against different aqueous media and physiological solutions are investigated and discussed.     

The effect of nano-sized sintering aids on toughening behavior of silicon nitride

Processing plays an important role in determining the microstructure of silicon nitrides which in turn influences the mechanical properties, such as hardness and toughness. Sintering aids are an important processing parameter. The influence of the chemistry of sintering aids on properties of silicon nitrides is a well-explored subject. Here the size of sintering aids used and its impact on microstructure and mechanical properties is explored. Specifically the use of nano-scale versus micron-scale sintering aids is examined. Microstructures and mechanical properties for silicon nitrides hot-pressed with nano-sized sintering aids are compared with a reference silicon nitride hot-pressed with micron-sized sintering aids. Hardness and fracture toughness are determined at room temperature using hardness indents. Grain size and aspect ratio distributions are determined for the two silicon nitrides and their impact on mechanical properties are examined. Toughening behavior is studied by experimentally determining R-curves. Both toughness and toughening (R-curve) behavior are shown to improve with the use of nano-scale sintering aids.     

无压烧结氮化硅陶瓷的力学性能和显微结构

以MgO-Al₂O₃-SiO₂为烧结助剂,借助XRD、SEM、TEM、EDS、HRTEM等手段,研究了无压烧结氮化硅陶瓷材料的力学性能和显微结构,着重探讨了材料制备工艺、力学性能和显微结构之间的关系,通过调整制备工艺改善材料微观结构以提高材料的力学性能.强化球磨混合的试样经1780℃无压烧结3h后,抗折强度高达1.06GPa,洛氏硬度92,显微硬度14.2GPa,断裂韧性6.6MPa·m^0.5.材料由长柱状β-Si₃N₄晶粒组成,晶粒具有较大的长径比,长柱晶的近圆晶粒尺寸0.3-0.8μm,长度3-6μm,长径比约7-10,显微结构均匀.     

Micron-sized fracture experiments on amorphous SiOx films and SiOx/SiNx multi-layers

In this study miniaturized monolithic cantilevers of thermally grown silicon oxide and multi-layer cantilevers of plasma enhanced chemical vapor deposited silicon oxide and nitride were mechanically characterized. In order to determine the fracture stress as well as the fracture toughness, un-notched and focused ion beam pre-notched cantilevers were tested. While the thickness of the monolithic cantilevers was varied from 280 nm to 2380 nm, the individual sub-layer thickness of the multi-layer cantilevers was adjusted to 50 nm. Bending experiments reveal a small increase of the fracture stresses with decreasing cantilever thicknesses. For the multi-layer stacks the tensile stress at fracture slightly exceeds the strength values of the corresponding monolithic materials. Furthermore, it is demonstrated that the specimens pre-notched by focused ion beam do not show significant changes in fracture toughness with varying pre-notch size. This makes the applied test a reproducible technique to determine fracture toughness of brittle films.     

水溶性胶态成型工艺制备氮化硅耐磨结构陶瓷

以氮化硅粉末为原料, 采用水溶性胶态成型工艺制备高耐磨氮化硅陶瓷. 采用正交设计的方法来优化制备高品质注浆料, 并研究了掺杂分散剂后Zeta电位的变化. 同时, 还对氮化硅陶瓷烧结体的显微结构、力学性能和耐磨性能进行了研究. 结果表明: 当氮化硅浆料中固相体积分数为45%时, 可制得体积密度较高的精细氮化硅陶瓷材料, 断裂韧性可达7.21MPa·m^1/2, 硬度为9.30GPa. 通过抗耐磨损实验研究表明: 干摩擦条件下, 氮化硅陶瓷发生了晶粒脆性断裂和脱落; 水润滑条件下, 摩擦表面产生了氢氧化硅 反应膜, 降低了磨损, 主要是化学腐蚀磨损.     

R-curve measurement of silicon nitride ceramics using single-edge notched beam specimens

R-curve behaviour of three kinds of silicon nitride-based ceramics has been studied using the single-edge notched beam (SENB) technique. If the notch is deep enough, the specimen shows stable fracture during the bending test, even when the sample is a brittle material. The conditions required to obtain stable fracture in the bending test are clarified by the analysis. The crack length of the specimen was also calculated from the changing load during the fracture test. In this study, coarse-grained silicon nitride shows a large increase of theR-curve. On the other hand, silicon nitride with silicon carbide whiskers shows noR-curve increase. The rise of theR-curve should be related to the microstructure of the ceramics, and especially to the grain size of the specimen, because silicon carbide whiskers are not large compared to the silicon nitride grains, and silicon carbide can reduce the grain growth of silicon nitride during sintering.     

Instrumented and Vickers Indentation for the Characterization of Stiffness,Hardness and Toughness of Zirconia Toughened Al_2O_3 and SiC Armor

Instrumented and Vickers indentation testing and microstructure analysis were used to investigate zirconia toughened alumina(ZTA) and silicon carbide(SiC).Several equations were studied to relate the Vickers indentation hardness,Young's modulus and crack behavior to the fracture toughness.The fracture in SiC is unstable and occurs primarily by cleavage leading to a relatively low toughness of 3 MPa m~(1/2),which may be inappropriate for multi-hit capability.ZTA absorbs energy by plastic deformation,pore collapse,crack deviation and crack bridging and exhibits time dependent creep.With a relatively high toughness around 6.6 MPa m~(1/2),ZTA is promising for multi-hit capability.The higher accuracy of mediar equations in calculating the indentation fracture toughness and the relatively high c/a ratios above 2.5suggest median type cracking for both SiC and ZTA.The Young's modulus of both ceramics was most accurately measured at lower indentation loads of about 0.5 kgf,while more accurate hardness and fracture toughness values were obtained at intermediate and at higher indentation loads beyond 5 kgf respectively.A strong indentation size effect(ISE) was observed in both materials.The load independent hardness of SiC is 2563 HV,putting it far above the standard armor hardness requirement of 1500 HV that is barely met by ZTA.     

R-curve behaviour and microstructure of sintered silicon nitride

R-curves for two in situ reinforced silicon nitrides A and B, of different microstructures, were characterized using indentation-crack growth measurements. Silicon nitride B, with its coarser microstructure and 8 MPa m^1/2 toughness, showed higher resistance to crack growth and more damage tolerance than silicon nitride A, with its finer microstructure and 7 MPa m^1/2 toughness. However, silicon nitride A showed a higher Weibull modulus than that of silicon nitride B due to the relatively narrow critical grain-size distribution. These results suggest that a coarse microstructure with narrow flaw-size distribution is beneficial to toughening, damage tolerance, and reliability.     

Mechanical Properties Measurement of PECVD Silicon Nitride after Rapid Thermal Annealing Using Nanoindentation Technique

Abstract:  This paper presents the results of mechanical characterisation of residual stress, elastic modulus, hardness and fracture toughness of plasma-enhanced chemical vapour deposited (PECVD) silicon nitride films subjected to rapid thermal annealing (RTA), processed between 200 and 800 °C. Additional tensile residual stresses were generated during the RTA period and the stress reached peak values after a 400 °C RTA process. On the other hand, nanoindentation testing revealed that both the modulus and hardness varied significantly with different RTA temperatures. Finally, the fracture toughness of the nitride was estimated to be 1.33 MPa √m based on a series of Vickers micro-indentation tests and it can be enhanced by the RTA process. These results should be useful for microelectromechanical systems (MEMS) or integrated circuit (IC) structure fabrication as regards maintaining the structural integrity and improving fabrication performance.     

Microwave sintering of Si3N4 with LiYO2 and ZrO2 as sintering additives

Phase transformation, microstructure development and mechanical properties of 2.45 GHz microwave-sintered silicon nitride (Si₃N₄) with lithium yttrium oxide (LiYO₂) and zirconia (ZrO₂) sintering additives were investigated. It was found that α to β phase transformation completed at a lower temperature of 1500 °C. Scanning electron microscopy (SEM) micrographs revealed a bimodal microstructure with a large number of elongated β-Si₃N₄ grains in addition to smaller grains. Surface residual porosity was observed in all sintered samples due to selective localized over heating of grain-boundary glassy phase. The high aspect-ratio of β-Si₃N₄ grains exhibited significant crack deflection, debonding and pull-out. It was observed that Vickers hardness and indentation fracture toughness increased with increasing sintering temperature.     

Determination of the fracture mechanical parameters of porous ceramics from microstructure parameters measured by quantitative image analysis

The porosity that takes an important part in the failure process of three different ceramic materials (mullite, silicon carbide and silicon nitride) was characterised by means of Quantitative Image Analysis (QIA). Several parameters such as size, shape and orientation of pores have been evaluated. In parallel, the mechanical properties such as fracture toughness and Weibull modulus were directly measured. In order to appreciate the relevance of the use of QIA, the mechanical parameters have also been deduced from the microstructural features, and a comparison between measured and determined values was carried out. The results show a remarkable concordance.     

Comparison of the Properties of TiC/Fe-AI Composites Produced by Self-Propagating High-Temperature Synthesis and Hot Isostatic Pressing

Self-propagating high-temperature synthesis (SHS) and hot isostatic pressing (HIP) techniques were used to produce TiC-20 wt.% Fe-Al and TiC-40 wt.% Fe-Al composite materials. The microstructure of the materials produced by the SHS technique consisted of spherical carbide particles embedded in an iron aluminide matrix whereas the microstructure of the materials produced by the HIP technique was less regular. A maximum hardness of 1820 HV was measured for the material produced using HIP and a maximum fracture toughness of 16.3 MPa ^1/2 was obtained for the material produced by SHS. Hardness values obtained from samples produced by the HIP technique were higher than those obtained from samples produced by the SHS technique. The SHS samples had better fracture toughness. The results of the oxidation resistance tests showed that TiC/Fe-Al composite materials can be recommended for use in oxidative environments holding temperatures up to 800^°C.     

On the possibility of silicon nitride as a ceramic for structural orthopaedic implants. Part II: chemical stability and wear resistance in body environment

In Part I, the processing, microstructure and mechanical properties of three silicon nitride-based ceramics were examined and their non-toxicity was demonstrated. In this Part II, some features critical to biomedical applications were investigated: (i) the wetting behaviour against aqueous media, including physiological solutions; (ii) the chemical stability in water and in physiological solutions; and (iii) the wear resistance, measured under experimental procedures that simulate the conditions typical of the hip joint prosthesis. The results confirmed that silicon nitride may serve as a biomaterial for bone substitution in load bearing prosthesis.     

Fracture toughness of silicon nitride determined by cyclic-fatigue-cracked specimens

An experimental technique for determining fracture toughness has been developed. In this method, a fatigue precrack is introduced in single-edge notched beam specimens by cyclic fatiguing in four-point bend at an elevated temperature. The resulting fatigue precracks satisfy all conditions required by the ASTM Standard Test for plane-strain fracture toughness of metallic materials. The applicability of this technique to provide reproducible fracture toughness values is demonstrated by experimental results obtained for silicon nitride sintered in two different ways in comparison with those obtained by means of the indentation technique for the same materials.     

Silicon nitride: the engineering material of the future

The purpose of this review is to present the recent developments in silicon nitride (Si₃N₄) ceramics and to examine the achievements regarding our understanding of the relationship between processing conditions, chemical composition, microstructure and mechanical properties of Si₃N₄. Si₃N₄ is one of the most important structural ceramics because it possesses a combination of advanced properties such as good wear and corrosive resistance, high flexural strength, good fracture resistance, good creep resistance and relatively high hardness. These properties are obtained through the processing method involving liquid phase sintering in which a tailored microstructure, with high aspect ratio grains and chemistry of intergranular phase, triggers the toughening and strengthening mechanisms leading to the development of high fracture toughness and fracture strength. However, despite high fracture toughness and strength, Si₃N₄ ceramic materials still break catastrophically, and the fracture behaviour of this ceramic is considered to be the major obstacle for its wider use as a structural material. In addition to the macrostructure–mechanical properties relationship, this paper also reviews new designs involving laminates possessing no plane of weakness and some theoretical developments involving crack opening displacement. Proposals of how to improve the fracture resistance were also discussed.     

High-temperature deformation and fracture processes in sintered reaction-bonded silicon nitride

Studies of the high-temperature deformation behaviour of sintered reaction-bonded silicon nitride (SRBSN) materials were conducted at 1200 °C in air under selected stress levels, which were applied at a single stress or as a sequence of stepwise increasing stresses. The objective was to evaluate the effects of the fabrication methods (conventional versus microwave heating process), microstructure, and precursor silicon powder purity on the deformation and fracture processes during creep loading of SRBSN materials containing a mixture of 3 wt% Al₂O₃ and 9 wt% Y₂O₃ sintering additives. Results indicated that all of the SRBSN materials exhibited a threshold stress above which the dominant process underwent transition from creep to extensive creep-assisted crack growth (CACG) from existing pores. In addition, the microwave SRBSN materials exhibited a better resistance (higher threshold stress) to CACG process, compared with those fabricated by conventional heating with the same metallurgical grade of silicon powder. The higher threshold stress observed in microwave SRBSN is mainly associated with the increased number density of elongated grains and the related higher fracture toughness. However, the minimum creep rates and stress exponents obtained in the creep regime were independent of the heating method. The microwave SRBSN material fabricated with lower purity silicon also exhibited a higher threshold stress for multiple crack formation and growth as compared with that processed with higher purity silicon. Conversely, the creep rate of microwave SRBSN materials was decreased by decreasing the impurity level (i.e. iron) in silicon powder.