RESEARCH OF MACHINING SURFACE QUALITY OF ENGINEERING CERAMICS

Surface quality is an important sublect in the study of machining technology. Because thesurface quality of the ceramic parts has direct effect on their life and reliability, to improve the reliability of structural ceramic parts is an urgent problem which needs to be solved for their furtherwidespread. The authors have done a systematic investigation in the macrostate, geometrical to-pography, surface roughness, deteriorative layer, residual stress and phase change of the machiningsurface of engineering ceramic zirconia and sialon ceramics. Some findings and conclusions of im-portant referential value have been obtained.

MECHANOCHEMICAL POLISHING OF ENGINEERING CERAMICS

Polishing ceramics with a bonded diamond abrasive and fine-grain soft abrasives wascarried out. The different material removeal mechanism of the two kinds of abrasives was pointedout through teh measurement of the material removal rate vs. polishing time and polishing speedrespectively. XPS examination proved that a mechanochemical reaction took place when polishingSialon, a kind of non-oxide ceramics ,with the CrObonded abrasives disc.

Study on Ceramic Cutting by Plasma Arc

Engineering ceramics are typical difficult-to-machine materials because of high hardness and brittleness. PAC (Plasma Arc Cutting) is a very important thermal cutting process and has been successfully used in cutting stainless steel and other difficult-to-machine alloys. PAC’s application in cutting ceramics, however, is still limited because the most ceramics are not good electronic conducts, and transferred plasma arc cannot be produced between cathode and work-piece. So we presented a method of plasma arc cutting engineering ceramics with additional anode. To reduce the kerf width and to improve the kerf quality, the hydro-magnetically confined plasma arc is used to cut engineering ceramic plates. By experiments and analyses the mechanism and characteristics of hydro-magnetic confined plasma are discussed and the effects of secondary confinement on cutting quality, arc properties, and optimal process parameters are presented. When the nozzle diameter is 3 mm, the kerf width of the Al 2O 3 ceramic plate with 6 mm thickness is less than 4.6 mm, and the cutting speed reaches to 0.9～1.2 m/min. Both theoretical analysis and experimental results have proved the feasibility and validity of the newly advanced hydro-magnetic plasma arc cutting, and the following conclusion can be drawn: (1)Synthesizing the advantages of both the water-constriction and magnetic constriction, the hydro-magnetic constriction of plasma arc forms a three-dimensional constriction with improved shape and uniformity of arc column, narrower kerfs, minimal beveling of cuts and higher dross-free cutting speeds than those under either water-constriction or magnetic constriction alone.(2)Hydro-magnetic constriction is capable of improving arc stability, which is reflected in the higher arc voltage at which arc-extinction occurs, than that under any single constriction. (3)For a given diameter of nozzle, quality cut can be produced by using a lower arc current than usually required in conventional plasma arc cutting, but ensuring fine arc shape and capability of cutting simply by employing hydro-magnetic constriction. Meanwhile, the heat load on nozzle could be reduced and service life of nozzle raised.

STUDY ON THE THRESHOLD VALUE OF CERAMIC MICROCRACK PROPAGATION

The fat;igue, behavior of alumina and silicon car-bide has been investigated in this paper. Once visiable macro crack on ceramic materials appears,the specimen ruptures due to the high speed of crack propagation and small ultimate strain. It is indicated that the bending fatigue limit is equal to the threshod value of surface micro crack propagation and is deter-mined by the original defects and grain boundary force of mate-rials. For ceramic materials, the crack grmuth is controlled by the applied stress. By measuring the macroscopic residual strength,we can determine whether the micro-crack is expand-ing and calculate the growth rate of micro-crack.

Interfacial structure and mechanical property of Al2O3 and Invar brazed joint

This paper introduces a brazing process between Al2O3 ceramic and Invar alloy.Al2O3 can be brazed with Invar effectively.The interfacial structure of Al2O3/Invar joint can be expressed as：Invar/Ag（s,s）＋Cu（s,s）＋Fe2Ti（zone Ⅰ）/Ag（s,s）＋Cu（s,s）＋Fe2Ti＋NiTi＋Cu3Ti（zone Ⅱ）/Ag（s,s）＋Cu（s,s）＋Cu2Ti＋Al（s,s）＋TiC＋TiO（zone Ⅲ）/Al2O3.The maximum shear strength of 139 MPa was measured for as-brazed Al2O3/Invar joint brazed at 850℃ for 25 min or 900℃ for 15 min.

Optimization and design of pigments for heat-insulating coatings

This paper reports that heat insulating property of infrared reflective coatings is obtained through the use of pigments which diffuse near-infrared thermal radiation. Suitable structure and size distribution of pigments would attain maximum diffuse infrared radiation and reduce the pigment volume concentration required. The optimum structure and size range of pigments for reflective infrared coatings are studied by using Kubelka-Munk theory, Mie model and independent scattering approximation. Taking titania particle as the pigment embedded in an inorganic coating, the computational results show that core-shell particles present excellent scattering ability, more so than solid and hollow spherical particles. The optimum radius range of core-shell particles is around 0.3 - 1.6 μm. ~rthermore, the influence of shell thickness on optical parameters of the coating is also obvious and the optimal thickness of shell is 100-300 nm.

Unsteady-state Grinding Technology (III) Studies on the Surface Quality

In conventional grinding theory, the chief removed mode of ceramic coprocessor by diamond tools was brittle removal. In order to perform the plastic removal or ductility processing of engineering ceramics the high degree of accuracy and high rigidity grinner must be using micro grain size diamond grinding wheel to direct the processing of micron below rank depth to prevent form the occurring of the brittle processing zone. This will resulted in the high expense of grinding. The expense of grinding could even reach the 80% of the total manufacture cost of the ceramic part. Ultra-precision grinding for advanced ceramics has been achieved by the unsteady-state grinding technique. In this paper, we mainly deals with observing and analyzing the surface quality of the silicon nitride ground by pink fused alumina wheel in different grinding parameters. To optimize the grinding parameters in the process of the unsteady-state grinding, the experiments of X-ray diffraction, energy spectrum analysis, SEM observation and roughness measurement were performed. The results show that: 1. In the process of unsteady state grinding, high line speed (rotational speed) of the grind wheel can improve the roughness of the silicon nitride apparently. It was also evident that the larger the grain mesh size, the better the surface quality. 2. There exists an optimum combination of grinding conditions such as grinding wheel speed, rotational speed of the workpiece, feed rate between the grinding wheel and the workpiece, grinding times and cutting coolant. The surface quality of the silicon nitride can be up to the standard of mirror finishing. 3. By analyzing the finished surface with X-ray diffraction and energy spectrum, the existence of some new phases including titanium and alumina was proved. 4. By utilizing the unsteady state grinding technique, the surface roughness of Ra ≤ 0.030 μm can be achieved by grinding the silicon nitride with the pink fused alumina wheel in low cost. Based on the unsteady-state grinding technique, this paper put forward a new processing method which by utilizing aluminum oxide grinding wheel to perform burnishing process and impudent the low surface roughness processing of engineering ceramic, the Ra is about 0.01 μm. Furthermore, the working efficiency of this method is high, and the process cost is low, so it is a prospective processing method.

Effects of precursor solution concentration on dielectric properties of (Pb,La)(Zr,Ti)O_3 antiferroelectric thick films by sol-gel processing

Pb0.97La0.02Zr0.95Ti0.05O3(PLZT)antiferroelectric thick films derived from different precursor solution concentrations are prepared on platinized silicon substrates by sol-gel processing.The films present polycrystalline perovskite structure with a(100)preferred orientation by X-ray diffraction(XRD)analysis.The antiferroelectricity of the films is confirmed by the double hysteresis behaviors of polarization and double-bufferfly response of dielectric constant under the applied electrical field.Antiferroelectric properties and dielectric constant are improved while the polarization characteristic values are reduced with the increase of precursor solution concentration.The films at higher precursor solution concentration exhibit excellent dielectric properties.