Berechnung der Blitztemperaturen der Mikrokontakte in künstlichen Hüftgelenken unter Benutzung des Kontaktmodells von Greenwood – Williamson

Calculation of Flash Temperatures of Micro‐Contacts in Artificial Hip Joints by Using the Contact Model of Greenwood – Williamson In hip replacement the wear rate is reduced when using femoral heads and acetabular cups made of alumina ceramics. The use of zirconia ceramics (Y‐TZP) is discussed controversy. The process on the articulating surfaces head / cup are not totally understood, especially the temperature rise due to frictional heating. Alumina is a stable material, no problems are expected. When using Y‐TZP, friction temperature can be higher then the coagulate temperature of synovia or the phase transition is induced. The model proposed by Kuhlmann – Wilsdorf was used to calculate the flash temperature under in vivo conditions of micro‐contacts at the different articulating surfaces. The real contact conditions were calculated with the analytical contact model of Greenwood – Williamson. The surface temperature of the wear couple Y‐TZP/Y‐TZP was higher than the one for alumina/alumina.     

Improving Bearing Surfaces of Artificial Joints

The requirements for materials to be used for bearing surfaces in joint replacement are corrosion resistance in the body environment, reliability, hardness, and stiffness. Most important is coverage of the bearing surface with a stable oxide layer, so that the articulating surfaces can be lubricated by the synovial fluid. The synovial fluid, being a protein, can degenerate during frictional heating. This can be avoided if the materials used for wear couples have very good thermal conductivity. Bioceramics for joint replacement have been used since the 1970s. Alumina ceramics reduce the wear rate and solve the problem of implant loosening (osteolysis). Although the in vivo fracture rate for alumina parts is very small, further improved reliability is demanded. Alternative materials may be non‐oxide ceramics, zirconia ceramics, or hard coatings on metals. Advanced non‐oxide ceramics, such as SiC and Si₃N₄, are not suitable for bearing surfaces in knee‐ and hip‐joint replacement because the surface oxide formed is SiO₂, which chips off. Y‐TZP zirconia does not have adequate phase stability. All hard coatings tried hitherto (TiN, DLC) have not been good enough. Alumina matrix composite (AMC) is a new type of bioceramics. AMC offers excellent tribological properties, no frictional heating, improved mechanical strength and fracture toughness, thus more in vivo reliability. So far test results have been very promising.     

TZP陶瓷的高温润滑研究

研究了TZP陶瓷在固体润滑下,室温（25℃）至600℃制备范围内的摩擦学特性．结果表明,使用石墨和MoS₂润滑剂,可在室温至600℃范围内降低TZP／Si₃N₄摩擦副的摩擦系数和磨损率,但当环境温度过高时,摩擦系数和磨损量有所增加;使用CeO₂和Cu对TZP／Si₃N₄摩擦副的摩擦系数影响不大,但可以降低TZP陶瓷的磨损量;CeF₃在高温时,由于结晶化趋势完善及沿（002）面的滑移取向,故可对TZP陶瓷起到良好的润滑作用．     

Novel Zirconia–Alumina Nanocomposites Combining High Strength and Toughness

Alumina toughened zirconia (ATZ) composites, combining a matrix of tetragonal zirconia polycrystals (TZP) and an alumina dispersion, are known for their high strength, toughness, and abrasion resistance. Depending on the type of stabilizing oxide in TZP the materials can be tailored to be either extremely strong (Y–TZP) or extremely tough (Ce–TZP) with a considerable cutback in the complementary property. Novel nanocomposite ATZ materials consisting of TZP co‐stabilized by neodymium and yttrium oxide (Nd–Y–TZP) combined with a reinforcement of 0–40 vol% alumina were produced to achieve high bending strength of 1200–1500 MPa at a toughness of 8–12 MPa·√m. TZP was produced by coating monoclinic nanopowder with 1.5 mol% yttria and 1.5 mol% neodymia. The powder was then blended with submicron size alumina and consolidated by hot pressing. Mechanical and microstructural properties were investigated.     

Ein Sterilisationsprozess kann die Eigenschaften von Biokeramik verändern

Sterilization can change properties of bioceramics Bioceramics made of bioinert alumina or zirconia and bioactive hydroxyapatite are well established implant materials. Implants have to be cleaned and sterilized. When sterilized some bioceramics change their color. This may effect their properties. No decrease of mechanical strength is observed when sterilizing alumina and the novel ceramic biocomposite AMC (Alumina Matrix Composite) with steam or Co⁶⁰ Gamma irradiation. When sterilizing Y‐TZP zirconia with steam a decrease of strength is observed.     

导热相复合陶瓷微波烧结传热模式数值分析

根据微波加热的特点,采用二维稳态导热数值分析法,建立了导热相复合陶瓷微波烧结的传热模式. 研究了导热相的浓度、导热相弥散分布状况、基质类型以及保温方式对复合陶瓷内部二维稳态温度场分布的影响,并设计了导热相SiC晶须复合TZP陶瓷的微波烧结致密化实验,对建立的复合陶瓷的微波烧结传热模式进行了验证. 结果表明：导热相复合陶瓷设计时宜选取具有高导热系数的基体,保持导热相有较高的浓度,使导热相均匀分布于基体中,且微波烧结传热宜采用合适的保温设施.     

Step Sintering of Microwave Heating and Microwave Plasma Heating for Alumina Ceramics

1.IntroductionNumerousstudieshaveshownthatmicrowavesin-teringofceramicshassomeadvantagesoverconven-tionalsinteringprocess,suchassavingsinenergyandprocessingtime,volumetricandrapidheating,lowerdensificationtemperature,uniformmicrostruc-tureofsinteredceramics,improvedmaterialsperfor-mance,productionofnewmaterialsandmicrostruc-tureetc..Basedontheformofmicrowaveenergyused,microwavesinteringcanbebasicallydividedintomicrowaveheatingsintering,microwaveplasmaheatingsinteringandstepsioteringwhichcombin…     

Thermal shock behaviour of paper‐derived alumina ceramics

Micro‐scaled alumina filled preceramic papers are used to produce paper‐derived alumina ceramics. The thin (0.6–0.7mm) and porous ceramics fabricated by this process can be, besides other possible applications, potentially utilized as setter plates on cordierite kiln furniture to avoid contact between the cordierite and powder metal substrate during sintering. The SiO₂ of the cordierite causes objectionable reactions with the powder metal. For this application it is important to investigate the thermal shock behaviour of the paper‐derived alumina ceramics especially focusing the residual strength, because fast cooling is a common technique in the metal sintering industry to avoid cost and save production time. Two differently processed types of paper were investigated: calendered (additionally roll‐pressed) and uncalendered paper. Their remaining strength has been measured by the B3B‐ test after thermal shocks of a temperature difference ΔT = 100 K up to ΔT = 1000 K to evaluate the critical temperature difference ΔT_C and the type of crack growth according to the method of Hasselman. In order to determine the cyclic thermal shock behaviour, at a temperature difference ΔT of 400 K and 600 K the ceramics have been quenched up to 5 times. The results of these investigations have been compared to the properties of tape casted alumina ceramics, a material already commercially used as setter plates. The initial strength of calendered paper‐derived ceramics was 240 MPa. After quenching at a temperature difference ΔT = 600–700 K, 50% strength decrease was observed. Temperature differences of more than 800 K caused >90% strength reduction. Uncalendered paper‐derived ceramics have 185 MPa strength. Between thermo shocks of temperature differences ΔT = 700–800 K, 50% of the strength reduction was measured. With thermo shocks of a temperature difference ΔT = 800 K only 10% of the initial strength remains. Generally the uncalendered ceramics showed a more stable crack growth than the calendered samples. Cyclic shocking at 400 K causes strength losses for both kinds of paper. But they differ in magnitude. While uncalendered paper only lost 7% strength, calendered paper strength was reduced by 20%, compared to their initial strength. This observation gets even more significant by shocking about 600 K. Calendered ceramics start with a higher initial strength and end up with a lower residual strength than the uncalendered ceramics. Only 30 MPa remain after five cycles of shocking, while the uncalendered paper‐derived ceramics have 90 MPa remaining strength. Uncalendered ceramics show a better thermal stress resistance, which can be correlated to their higher porosity and therefore their increased crack deflection in the microstructure. Tape‐casted alumina ceramics show a lower initial strength and lesser thermal shock resistance against simple and cyclic shocking. SEM pictures of the tape‐casted alumina ceramics show large amounts of small, spherical shape pores, while paper‐derived alumina microstructures show long cylindrical pores left by pyrolized cellulose fibres. Considering these results, paper‐derived ceramics are a serious alternative to tape casted ceramics for an application as setter plates.     

Surface characteristics and electrical breakdown of alumina materials

The electrical breakdown of alumina materials is one of the difficulties in higher electric field applications. This breakdown takes place due to localized excess heating, which is induced by surface defects, multipactor, and surface discharge. These factors were evaluated for several commercial alumina ceramics using a scanning electron microscope and comparing these observations with high-power rf transmission tests. The results indicate that alumina ceramics without F-center defects and lower surface charging are feasible for higher electric field operation.     

Sliding friction of ceramics: Mechanical action of the wear debris

The wear behaviour of four commercially available structural ceramics (silicon carbide, SiAION, alumina and partially stabilized zirconia) was investigated at room temperature, under various conditions of sliding friction using closed dry and open lubricated systems. Primary emphasis was placed on the mechanical action of wear debris. It was thus demonstrated that the circulation of wear debris in the sliding interface simultaneously controls the wear rate and the friction response. The parameters which control the accumulation or the elimination of wear debris were determined. The wear behaviour of the examined ceramics was found to be highly dependent upon the quantity of entrapped wear debris.     

Variational approach to the analysis of steady‐state thermo‐elastic wear regimes

A transient wear process on frictional interface of two thermo‐elastic bodies in a relative steady sliding motion induces shape evolution of contact interface and tends to a steady state for which the wear process occurs at fixed contact stress and strain distribution. The temperature field generated by frictional and wear dissipation on the contact surface is assumed to reach a steady state. This state is assumed to correspond to minimum of the wear dissipation power and the temperature field corresponds to maximum of the heat entropy production. The stationarity conditions of the response functionals provide the contact pressure distribution and the corresponding temperature field. The present approach extends the authors previous analyses of optimal or steady‐state contact shapes by accounting for coupled wear and thermal distortion effects. The wear rule is assumed as a non‐linear relation of wear rate to shear stress and relative sliding velocity. The analysis of disk and drum brakes is presented with account for thermal distortion effect. It is shown that the contact shape in a steady thermo‐elastic state essentially differs from that specified for mechanical loading with neglect of thermal effects. The thermal instability regimes are not considered in the paper. Copyright © 2009 John Wiley & Sons, Ltd.     

MECHANISMS OF CYCLIC FATIGUE-CRACK PROPAGATION IN A FINE-GRAINED ALUMINA CERAMIC: THE ROLE OF CRACK CLOSURE

Abstract— Cyclic fatigue-crack growth and resistance-curve behavior have been studied in a fine-grained (∼ 1 μm), high-purity alumina. Specific emphasis is given to the mechanisms associated with crack growth that are controlled by the maximum ( K _max) and the alternating (Δ K ), stress intensities and to the role of crack-face interference (crack closure), which is known to be an important crack-tip shielding mechanism in metal fatigue. Significant levels of subcritical crack growth were detected above a threshold stress intensity of ∼60% of the fracture toughness ( K _c) in the alumina, with growth rates displaying a far larger dependence on K _max compared to Δ K. The role of crack closure was examined using constant- K _max experiments, where the minimum stress intensity ( K _min) was maintained either above or below the stress intensity for crack closure ( K _cl). Where K _min< K _cl, growth rates were found to exhibit a lower dependence on Δ K , which was rationalized in terms of the frictional wear model for crack growth in grain-bridging ceramics. It is concluded that crack closure, as conventionally defined, has little relevance as a crack-tip shielding mechanism during fatigue-crack growth in grain-bridging ceramics, due to the low dependence of growth rates on Δ K compared to K _max.     

Tribologische Kennwerte für Reibwerkstoffe

Tribology parameters for friction materials The tribology parameters of the friction materials are currently the coefficient of friction and the wear coefficient. They are determined depending on the material, the surface pressure and the frictional speed. These two parameters are not sufficient for an evaluation of the friction materials on the basis of a wear theory. Their validity is therefore limited. It is proposed to evaluate the friction materials on the basis of the energetic wear theory. The parameters are then the wear energy density, the shear stress of friction and the linear wear intensity. These values can be shown in a diagram and allow for the direct comparison of widely varying friction materials. The reference coefficient of friction is being introduced as a new wear parameter. The determination of the proposed parameters is made on a flywheel test rig, combined with the laser‐optical online measurement of the wear particles. It was possible to demonstrate that the parameters correctly describe the practice‐relevant requirements using two proven friction materials.     

高速干滑动下材料摩擦学性能的研究进展

综述了近10年来国内外对高速干滑动摩擦磨损行为的研究成果,总结了不同条件下材料的摩擦磨损机理,对影响高速摩擦磨损性能的因素进行了分析,并提出了高速摩擦学研究中存在的问题及研究方向．     

粗颗粒AI_2O_3陶瓷在无润滑条件下磨损特性的探讨

对粗颗粒Al_2O_3陶瓷在无润滑条件下的磨损特性进行了试验研究,绘制了该材料的磨损曲线:磨损速率在初期很低,以后逐渐上升,及至2h以后基本稳定.利用扫描电镜探讨了该材料的磨损机理,认为其磨损以脆性剥落和磨粒磨损为主.     

Microstructure and tribological properties of alumina ceramic with laser-dispersed tungsten additions

The surface of slightly porous, commercially available alumina ceramic was laser-modified with tungsten and/or zirconia additions. The thickness of the resulting multiphase surface layers ranged from 300–800 m depending on the chemical composition and the parameters of the laser process used. Microstructure and worn surfaces were analysed by scanning electron microscopy and energy- dispersive X-ray spectroscopy. Mechanical properties were characterized by using Vickers hardness and nanoindentor testing. Tribological tests were carried out on the surface-modified ceramics using a ball-on-block tribometer. All tests were conducted in unlubricated oscillating sliding contact against balls of alumina in laboratory air at room temperature, relative humidities varying between 3% and 80%, and in distilled water. The multiphase surface layers showed a total volume fraction of second phases up to 40 vol% embedded in the alumina matrix, whereas the average size of the alumina grains was substantially reduced compared with the substrate ceramic. Tungsten dispersoids were distributed homogenously in the ceramic matrix and eutectic Al2O3–ZrO2 phase occurred along the boundaries of the alumina crystallites. Mechanical and tribological properties varied as a function of the microstructure of the laser-modified ceramics, i.e. type and volume fraction of the second phases, and both friction and wear were substantially reduced compared with the commercially available monolithic alumina ceramic used for reference. Friction coefficient and amount of linear wear of the ceramics decreased with increasing relative humidity of the surrounding air. © 1998 Kluwer Academic Publishers     

Wear and corrosion studies of graphite‐aluminum composite reinforced with micro/nano‐TiB2 via spark plasma sintering

The demand for lightweight materials in the automobile and aerospace industries has led to various researches on graphite and graphite‐aluminum composites. The aim of this study was to investigate the effect of the addition of micron/nano TiB₂ particles on the properties of graphite‐aluminum composite particularly the wear resistance. The powders were sintered at 550 °C and 50 MPa with more attention on the effect of the sintering temperature on densification, microhardness, coefficient of thermal expansion, wear and frictional force. The results show that the addition of nano TiB₂ reduces the densification while improving the hardness of Gr?Al composite with the lowest value being 96.0 % of relative density and the highest microhardness of 43.58 HV 0.1. The coefficient of thermal expansion and frictional force of the composite materials increases with increasing TiB₂ content and heating rate (100 °C/min–150 °C/min). TiB₂ particles enhance the wear resistance of graphite‐aluminum composite. The addition of micro/nanoparticles of TiB₂ to graphite‐aluminum composite increases its corrosion rate with improved passivation behavior in 3.5 wt.% NaCl solution. Nevertheless, 5 wt.% nano (100 °C/min) TiB₂ additions do not affect the overall corrosion rate. This work has shown that we can take advantage of some of the properties of TiB₂ to improve the performance of graphite‐aluminum composite.     

Cyclic fatigue of ceramics

The fatigue behaviour of alumina, zirconia-toughened alumina (ZTA) and tetragonal zirconia (TZP) have been investigated using three different techniques. Direct push-pull testing has been used to generate both static and cyclic fatigue data. The results clearly show that all the materials studied are susceptible to both static and cyclic fatigue, and that the times to failure under cyclic loading are considerably shorter than under static loads. The fatigue failure origins have been identified and the influence of surface condition on fatigue life has been assessed. The slow propagation of cracks subject to cyclic tensile and compressive loads has been studied using compact tension specimens and tapered double cantilever beam specimens. These investigations have confirmed the existence of cyclic fatigue effects in coarse-grained alumina and have shown the crack increment per cycle (da/dN) to have a power-law dependence on the peak stress intensity factor. A technique, based on repeated indentation, has been used to investigate the propagation of sub-surface cracks subjected to cyclic loading in both fine-grained alumina and ZTA. The results of the investigation suggest that compressive or closure loads on the crack faces are factors which affect the cyclic fatigue crack growth in ceramics. Based on those observations, an explanation is proposed for the mechanical cyclic fatigue effects in the ceramics investigated.     

Secondary electron emission of TiN-coated alumina ceramics

TiN film is often coated on alumina rf windows to suppress multipactor due to high secondary electron emission (SEE) coefficients. In this paper, SEE coefficients of alumina ceramics and sapphire coated with TiN films of various thicknesses are investigated. The SEE coefficients were measured using a scanning electron microscope with a single-pulse electron beam (100 pA, 1 ms). The SEE coefficients of TiN-coated alumina ceramics were lower than those of uncoated ones and nearly unity for TiN thickness of more than 1 nm, even with incident energy of 1 keV. To emulate multipactor-induced surface heating, the SEE coefficients were also measured at high temperature. The results showed a decrease in the SEE coefficients with temperature for TiN thickness of more than 0.5 nm. TiN coating on an rf window should be as thin as possible, and 0.5-1 nm may be the optimum thickness for suppressing multipactor.     

Alumina versus Zirconia Comparative Survey of Thermic Influence During Dental Ceramic Core Manufacturing Process

For dental ceramic restorations, the manufacturing process presumes a high temperature sample heating treatment process, more than 1,000°C for applied aesthetic veneer and glazer. The present study presents and analyzes a comparative survey for two dental ceramics regarding the link between the heating treatment and reported cases of fractures and failure (delamination) of ceramic core. Two different dental ceramics samples were investigated: alumina based ceramic, respectively zirconia based ceramic materials. For zirconia based ceramic, changes concerning monoclinic (M) and tetragonal (T) crystalline phases occur and peak intensity variation from x-ray diffraction patterns are observed during heating process. For alumina based ceramic, the crystalline structure is less affected by the heating process but a different grain size and orientation were noticed during/after heating treatment. Both of them are affecting the strength and fracture toughness of the dental ceramic core.