三种机械密封材料的摩擦磨损性能研究

研究了3种核主泵用机械密封陶瓷材料(氮化硅、氧化铝和碳化硅)在室温干摩擦条件下及水润滑条件下分别与氮化硅陶瓷球对磨的摩擦磨损性能。研究结果表明,在与氮化硅球干摩擦的3种材料中氧化铝陶瓷具有最大的摩擦系数和最小的磨损质量,氮化硅具有最小的摩擦系数。在氮化硅陶瓷自配对摩擦副摩擦磨损试验中,水润滑条件下氮化硅摩擦系数及摩擦质量损失都有很大程度的减小,且摩擦系数随转速增加而减小。综合考虑力学性能和摩擦磨损性能,选择氮化硅陶瓷作为核主泵机械密封材料更合适。     

绝缘陶瓷材料电火花加工技术的研究进展

陶瓷材料特性鲜明，有着广阔的应用前景。辅助电极法电火花加工绝缘陶瓷是一种新兴的加工工艺，介绍了陶瓷材料电火花加工技术的基本原理及研究进展。     

Artifact level produced by different femoral head prostheses in CT imaging: diamond coated silicon nitride as total hip replacement material

Commercial femoral head prostheses (cobalt–chromium alloy, yttria partially stabilized zirconia (Y-PSZ) and alumina) and new silicon nitride ceramic ones (nanocrystalline diamond coated and uncoated) were compared in terms of artifact level production by computed tomography (CT). Pelvis examination by CT allows the correct diagnosis of some pathologies (e.g. prostate and colon cancer) and the evaluation of the prosthesis-bone interface in post-operative joint surgery. Artifact quantification is rarely seen in literature despite having a great potential to grade biomaterials according to their imaging properties. Materials’ characteristics (density and effective atomic number), size and geometry of the prostheses can cause more or less artifact. A quantification procedure based on the calculation of four statistical parameters for the Hounsfield pixel values (mean, standard deviation, mean squared error and worst case error) is presented. CT sequential and helical scanning modes were performed. Results prove the artifact reproducibility and indicate that the cobalt–chromium and Y-PSZ are the most artifact-inducing materials, while alumina and silicon nitride (diamond coated and uncoated) ceramic ones present a low level of artifact. Considering the excellent biocompatibility and biotribological behaviour reported in earlier works, combined with the high medical imaging quality here assessed, diamond coated silicon nitride ceramics are arising as new materials for joint replacement.     

The characterization of plasma-sprayed/rapid omnidirectional compaction — processed tantalum coatings as applied to densified ceramic substrates

The application of rapid omnidirectional compaction (ROC) processing to plasma-sprayed tantalum coatings in order to produce dense wear-resistant tantalum carbide coatings on a variety of predensified ceramic substrates is reported. Substrate materials included aluminium nitride, silicon carbide, silicon nitride, and WC-6Co. In particular, the phase chemistry which evolves during ROC processing, the surface morphology of the coating as-deposited and after ROC processing, and the erosive wear resistance of the coatings, are discussed.     

Excimer laser-induced microstructural changes of alumina and silicon carbide

Surface treatments with a KrF excimer laser were applied on alumina and silicon carbide ceramic materials. Results on the surface modifications induced by laser were related to the processing parameters: laser fluence (1.8 and 7.5 J/cm²), number of laser pulses (1 to 500), frequency (1 to 120 Hz), pulse duration (25 ns), sample speed under the laser beam and working atmosphere. It was ascertained that alumina can be laser treated under air, while silicon carbide needs an inert atmosphere to avoid surface oxidation. Microstructural analyses of surface and cross section of the laser processed samples evidenced that at low fluence (1.8 J/cm²) the surface of both ceramics is covered by a scale due to melting/resolidification. At high fluence (7.5 J/cm²) there are no continuous scales on the surfaces; material is removed by decomposition/vaporisation and the depth of material removal is linearly dependent on the number of pulses. On alumina surface, a network of microcracks formed, while on silicon carbide different morphologies (flat and rugged areas, deposits of debris and discontinuous thin remelted scales) were detected. The evolution of surface morphology and roughness is discussed with reference to composition, microstructure and physical and optical properties of the two tested ceramics and to laser processing parameters.     

Regression analysis of temperature-dependent mechanical and thermal properties of dielectric technical ceramics

Regression analysis is performed on a data set of temperature-dependent material properties of several ceramic materials. The materials considered are alumina, aluminium nitride, beryllia, fused quartz, sialon, and silicon nitride. The properties considered are density, Young’s, bulk, and shear moduli, Poisson’s ratio, tensile, flexural and compressive strength, thermal conductivity, specific heat capacity, and thermal expansion coefficient. The data set, previously reported by de Faoite et al. (J Mater Sci 47(10):4211, 2012), was compiled to facilitate the materials selection and design of a ceramic component for the Variable Specific-Impulse Magnetoplasma Rocket (VASIMR^®). Temperature-dependent material property data are required for accurate thermo-structural modelling of such ceramic components which operate over a wide temperature range. The goal of this paper is to calculate a set of regression coefficients to reduce this data set to a tractable format for use in the materials selection and design of such components. Regression analysis could not be performed for all material properties for all of these materials, due to a lack of data in the literature, and these gaps in the available data are highlighted.     

Ceramic materials and phosphors based on silicon nitride and sialon

This review covers some of the main trends in the field of synthesis and the applications of materials based on silicon nitride presented in publications over the past 10–15 years. Attention is paid to the technique for synthesizing nitride and oxynitride compounds and for the production of ceramic construction materials and phosphors in silicon nitride systems.     

Entwicklung und Verschleißverhalten von Si3N4-Schneidkeramik

Development and wear behaviour of silicon nitride ceramic Cutting tools based on silicon nitride ceramic have been employed successfully in industrial ranges of application especially in cutting of grey cast iron. Nevertheless their availability is still limited due to various reasons, depending on the specific material properties. In order to improve the wear behaviour and the reliability of these tools a new silicon nitride ceramic matrix, bonded with grains of carbides, was developed. The paper presents results obtained during investigations on material specific wear mechanisms in face milling and turning. The responsible wear mechanism in turning are the high tool temperatures. They leads to a softening of the grain boundary phase so that the Si₃N₄-grains can be removed by the chip flow. The wear in milling is due to the high mechanical load accompanied with the frequency in period of cutting     

PROCESSING ASPECTS OF SHAPING ADVANCED MATERIALS BY ELECTRICAL DISCHARGE MACHINING

The principles of applying electrical discharge in die-sinking and wire-cutting machines are reviewed. This technique causes material to be eroded by switching electrical current on and off between the tool and the work-piece through a dielectric fluid. For the case of metals and many ceramics and ceramic composites, the mechanism for this erosion is melting or perhaps evaporation/condensation. For refractory materials, a new mechanism for erosion, thermal spalling, was observed. The role of the dielectric fluid, its purity, and the proper nitration system are discussed. The effect of various operating conditions, including the current, pulse time duration, and wire feed rate, on the material removal rate and the surface quality is also reviewed. It was seen that EDM can be applied to machining advanced materials, including single phases and composites of ceramic/ceramic and ceramic/metal, if a minimum electrical conductivity is met.     

Effects of an Electrode Material on a Novel Compound Machining of Inconel718

Compound machining (CM) compounded by arc machining and electrical discharge machining (EDM) milling is a new and fast processing method used to machine so-called “difficult-to-machine” materials. This method has an exciting maximum material removal rate that reaches 11,218 mm³/min with the relative electrode wear rate (REWR) of 1.54% when machining Inconel718. The electrode material is an important factor that affects the processing cost and quality of CM. Traditionally used electrode materials in arc machining and EDM, including pure tungsten (W), cerium tungsten (WCe20), copper (Cu), tungsten copper alloy (W80), and graphite (C), were used as electrode materials to process Inconel718. Experimental results show that tubular C is the best electrode material for CM. When tubular C is unavailable, WCe20 is the suitable electrode material for rough machining and W is a better choice in finish machining. Cu electrode is unsuitable for CM because of its low melting point. Results of this work provide guidance for selecting electrode materials for the industry application of efficient CM.     

Comparative study of high temperature composites

Two classes of composite made using either ceramic matrix with high temperature fibers or carbon/carbon have been used for various applications that require high temperature resistance, over three decades. However, their use has been limited to special applications because of the high costs associated with fabrication. Typically the composites are cured at more than 1000°C, and in most instances the heating has also to be carried out in controlled environments. In addition, because of the high processing temperature, only certain type of expensive fibers can be used with the ceramic matrices. A recently developed inorganic matrix, called polysialate can be cured at temperatures less than 150°C, making it possible to use carbon and glass fibers. Composites made using carbon, glass and combinations of carbon and glass fibers have been tested in bending and tension. This paper presents the comparison of processing requirements and mechanical properties of carbon/carbon composites, ceramic matrix composites made with silicon carbide, silicon nitride and alumina fibers and carbon/polysialate composites. The results indicate that carbon/polysialate composite has mechanical properties comparable to both carbon/carbon and ceramic matrix composites at room and high temperatures. Since the polysialate composites are much less expensive, the authors believe that it has excellent potential for more applications in aerospace, automobile and naval structures.     

Machining damage of monocrystalline silicon by specific crystallographic plane cutting of wire electrical discharge machining

The machining damage on the certain crystal face of single crystal silicon, the manufacture of which are under diverse parameters of wire electrical discharge machining (WEDM), is tested by means of the micro-observation and X-ray diffraction rocking curve method. In the process of monocrystalline silicon machined by WEDM, when the pulse width is small, the basic methods of material removal are melting and gasification, which are also called normal removal methods. When the pulse width increases to a certain degree beyond the normal removal, thermal spalling removal also occurs, which is considered a compound removal method. The depth of machining damage is difficult to control. The structure of machining damage under two conditions is divided into normal removal and compound removal in this study. To make the depth of machining damage easy to control, compound removal should be avoided when processing the single crystal silicon by certain crystal face cutting of WEDM. Such an approach can provide the premise and guarantee for the subsequent processing of single crystal silicon with certain crystal face in the future.     

Improved mechanical long-term reliability of hip resurfacing prostheses by using silicon nitride

Although ceramic prostheses have been successfully used in conventional total hip arthroplasty (THA) for many decades, ceramic materials have not yet been applied for hip resurfacing (HR) surgeries. The objective of this study is to investigate the mechanical reliability of silicon nitride as a new ceramic material in HR prostheses. A finite element analysis (FEA) was performed to study the effects of two different designs of prostheses on the stress distribution in the femur–neck area. A metallic (cobalt–chromium-alloy) Birmingham hip resurfacing (BHR) prosthesis and our newly designed ceramic (silicon nitride) HR prosthesis were hereby compared. The stresses induced by physiologically loading the femur bone with an implant were calculated and compared with the corresponding stresses for the healthy, intact femur bone. Here, we found stress distributions in the femur bone with the implanted silicon nitride HR prosthesis which were similar to those of healthy, intact femur bone. The lifetime predictions showed that silicon nitride is indeed mechanically reliable and, thus, is ideal for HR prostheses. Moreover, we conclude that the FEA and corresponded post-processing can help us to evaluate a new ceramic material and a specific new implant design with respect to the mechanical reliability before clinical application.     

Friction Welding of Aluminum to Ceramics

This paper addresses the feasibility of friction welding a:s an approach to join aluminum to ceramics. Potential advantages include the localised heating below the melting point of the parent materials, and the short time required to produce the bond. Metal / ceramic combinations examined include aluminum / silicon nitride, aluminum / silicon carbide, aluminum / zirconia and aluminum / aluminum nitride.     

A comparative study of the effect of γ-radiation on electrical parameters of oxide dielectrics

The effect of ionizing radiation on electrical properties of oxide dielectrics is studied. A comparative analysis of the temperature dependence of electrical conduction is performed for non-irradiated and irradiated samples of three types of oxide dielectrics—aluminum oxide, silicon dioxide, and alumina ceramics—in the temperature range between 200 and 700 K. The data obtained are correlated with special features of thermoradiation modification of dielectric parameters of oxides. The behavior of the dielectric loss tangent and permittivity is determined in a wide range of temperatures and γ-radiation doses. The peak conductivity parameters of ceramic samples are calculated. An assumption is made on the nature of the observed features of temperature and dose dependences of electrical properties of oxide dielectrics as well as on a possible admittance mechanism in these materials.     

The microstructures of silicon nitride/alumina ceramics

The microstructures of materials formed by sintering or hot-pressing mixtures of silicon nitride and alumina have been studied by transmission electron microscopy. The probable mechanism of transformation of the reactants to form β′-silicon aluminium oxynitride (β′-sialon) via a liquid phase sintering process, which is analogous to a similar transformation in hot-pressed silicon nitride containing a magnesia additive, is proposed. The origin and crystal symmetry of an unknown second phase is discussed. The residual quantity of this phase, known as the X-phase, is controlled mainly by the silica impurity content of the initial silicon nitride powder.     

氮化硅陶瓷直接凝固注模成型的凝固动力学

直接凝固注模成型是一种新的近净尺寸陶瓷成型工艺,它是通过可控反应速率的酶催化反应来实现浆料原位固化的,本文半悬浮体流变学与陶瓷浆料的成型工艺学有机结合,通过陶瓷浆料的动态驰豫实验,研究了Ｓｉ３Ｎ４浆料ＤＣＣ成型的凝固动力学。结果表明：“凝固时间”可以成功地描述瓷浆料的凝固过程,动态驰豫实验投资亿浆料的凝固动力学提供了一条简单可行的研究方法,在Ｓｉ３Ｎ４浆料ＤＣＣ成型工艺条件优化的基础上,制备出了气     

精细雾化抛光氮化硅陶瓷的抛光液配制参数优化

采用超声精细雾化施液抛光对氮化硅陶瓷基体进行抛光,研究了不同的pH值、磨料浓度以及氧化剂含量对氮化硅陶瓷基体抛光的材料去除率的影响,优化了pH值、磨料浓度及氧化剂含量,并与传统的化学机械抛光进行了对比。结果表明:当二氧化硅磨粒质量分数为5wt%,氧化剂含量为1wt%,pH值为8时,材料去除率MRR为108.24nm/min且表面粗糙度Ra为3.39nm。在相同的抛光参数下,传统化学机械抛光的材料去除率MRR为125nm/min,表面粗糙度Ra为2.13nm;精细雾化抛光的材料去除率及表面粗糙度与传统抛光接近,但精细雾化抛光所用抛光液用量仅为传统抛光所用抛光液用量的1/9。     

Employing reactive synthesis for metal to ceramic joining for high temperature applications

Joining of dissimilar materials allows the properties of both materials to be exploited in a device or structure. The main reasons for the incorporation of dissimilar materials are to achieve function, improve efficiency and to reduce cost.Silicon nitride is an engineering ceramic that has outstanding properties but has yet to find its full commercial potential. Silicon nitride is suitable for high temperature applications, however, its incorporation into devices or structures tends to be restricted due to a lack of suitable joining techniques.This paper presents the results of joining between the high temperature and corrosion resistant iron-chromium-aluminium alloy (Fecralloy) with silicon nitride by a nickel aluminide (NiAl) interlayer. The formation of NiAl from its constituent elements (Ni-Al compact was used) by reactive synthesis is highly exothermic and this was utilised to cause partial melting of the Fecralloy interface and reactive wetting of the silicon nitride interface.Joints with average shear strength of 94.3 MPa were fabricated under optimum processing conditions (900°C, 15 min, 45 MPa). Thermal cycling at 850°C in air showed that the joints could be used at this temperature.The primary focus of this work was on the effects of process conditions upon the microstructure and mechanical properties of the joint. The reactive synthesis of NiAl was studied using differential thermal analysis (DTA), where the effects of varied heating rate were investigated.