Engineering Ceramics Research Articles

Study on the Plastic Removal Mechanism of Nano-ZrO<sub>2</sub> Ceramics by Ultrasonic Grinding

The brittleness, plasticity, super-plasticity and the removal mechanisms of critical fragmentation of the nano ZrO2 ceramics were investigated. The formula of critical ductile grinding depth of the common engineering ceramics was inapplicable to the nano-ZrO2 ceramics. A new formula of critical ductile grinding depth of the nano ceramics was established. The ultrasonic vibration grinding experiments showed that the critical ductile grinding depth of the ceramics was 15μm by conventional grinding, but the increment of the critical ductile grinding depth was 60 percent by ultrasonic grinding. The critical ductile grinding depth increased to 25μm. Analyzed by means of SEM, it was transgranular cracking during its cracking process. The nano-ZrO2 ceramics have high toughness so the critical ductile grinding depth increased. The shape, length, width and thickness of the grindings differed greatly from which obtained by conventional grinding.

ZrB2-SiC 세라믹스의 미세구조와 기계적 물성에 미치는 B4C 첨가효과

ZrB₂ has a melting point of 3245℃ and a relatively low density of 6.1 g/㎤, which makes this a candidate for application to ultrahigh temperature environments over 2000℃. Beside these properties, ZrB₂ is known to have excellent resistance to thermal shock and oxidation compared with other non-oxide engineering ceramics. In order to enhance such oxidation resistance, SiC was frequently added to ZrB₂?based systems. Due to nonsinterability of ZrB₂?based ceramics, research on the sintering aids such as B₄C or MoSi₂ becomes popular recently. In this study, densification and high-temperature properties of ZrB₂-SiC ceramics especially with B₄C are investigated. ZrB₂-20 vol% SiC system was selected as a basic composition and B₄C or C was added to this system in some extents. Mixed powders were sintered using hot pressing (HP). With sintered bodies, densification behavior and high-temperature (up to 1400℃) properties such as flexural strength, hardness, and so on were examined.

A Mars hopping vehicle propelled by a radioisotope thermal rocket: thermofluid design and materials selection

Rocket-propelled vehicles capable of travelling a kilometre or more in a ballistic ‘hop’ with propellants acquired from the Martian atmosphere offer the potential for increased mobility and planetary science return compared with conventional rovers. In concept, a radioisotope heat source heats a core or ‘thermal capacitor’, which in turn heats propellant exhausted through a rocket nozzle to provide thrust. A systematic study of the thermodynamics, heat transfer and selection of core materials for a Mars hopper was undertaken. The aim was to advance the motor design and assess technical risks and feasibility. Analytical and numerical motor models were developed; the former to generate thermodynamic performance limits, an ideal hop distance and plot a materials selection chart using simple explicit relations. The numerical model assessed the effect of core configuration and geometry. A hop coefficientChopis shown to characterize the effect of core geometry independently of core material and temperature. The target hop distance of 1 km is shown to be robust. A moderate advantage to pebble-bed cores over a core consisting of straight channels was suggested. High-performance engineering ceramics such as boron carbide offer the longest hop providing the core temperature can be increased significantly above 1200 K.

Machining the Zirconium-dioxide engineering ceramics

In place of brittle ceramics used so far have appeared up-to-date so called tough ceramicmaterials resisting better against mechanical effects [2]. Such material is the zirconium-dioxide, too.The important advantage of hard-turning is the applicability of universal tool. Various outlines can beformed by a tool given. Machining ceramics in case of using traditional machining (turning, milling,drilling) requires special technological conditions (tools, machine-tools, technological parameters, etc.)which are developing presently. We would like extending our research work in this course, too.

Experimental Study on Surface Quality of Al<sub>2</sub>O<sub>3</sub> Engineering Ceramics in Ultrasonic Drilling and Grinding

Engineering ceramics have been widely used in recent years because of its excellent performances. However, the high hardness and brittleness make it very difficult to process with high efficiency and good quality, especially in precision and ultraprecision machining. Therefore, a method of ultrasonic drilling and grinding is provided to the hole machining of Al¬2O3 engineering ceramics. In the paper, with different machining parameters, the surface roughness of micro hole in ultrasonic and traditional drilling and grinding is compared and analyzed. Moreover, the surface micro profile and the wear condition of drilling-grinding tool in ultrasonic and traditional drilling and grinding are observed through SEM. Finally, it is concluded that ultrasonic drilling and grinding is a kind of high efficient processing method adapting to micro holes of hard - brittle materials etc.

Study on the Stress Field in Ultrasonic Grinding Engineering Ceramics

The mechanical model of stress field was established in grinding with/without ultrasonic vibration assistance, based on indentation fracture mechanics. Compared with the stress field in conventional grinding, the research has shown that the grinding stress field changes periodically with the vibration process of the moving grains in ultrasonic grinding. In a vibration cycle, the grains interfere with the workpiece within the time of T+Δt and cut off the material from the workpiece surface. Meantime, the large impact force between the grinding wheel and the workpiece results in stress field on the ground surface. In other period of vibration cycle, the grains are detached from the workpiece surface, and there exists no interaction between them. The grinding force disappears, so does the grinding stress field.

Research on Crack Mechanism of Surface/Subsurface Based on Grinding Stress Field in Ultrasonic Grinding Nano-Ceramics

The crack nucleation and propagation process in ultrasonic grinding engineering ceramics are studied based on indentation fracture mechanics. The study shows ultrasonic grinding can be considered as continuous loading and unloading process in view of a single grain. The loading makes cracks form and preliminarily propagate; while the unloading makes them propagate further and makes the direction of crack propagation offset to the workpiece surface, not so far as to extend to the material interior. In common grinding cracks have extended to the material interior, resulting in deep subsurface crack in the subsurface layers. In this paper, the ground surface of small crack damage can be obtained with large grinding depth in ultrasonic grinding compared with common grinding under the same conditions.

Evaluation of fracture toughness of ZrO 2 and Si 3N 4 engineering ceramics following CO 2 and fibre laser surface treatment

The fracture toughness property ( K 1C) of Si 3N 4 and ZrO 2 engineering ceramics was investigated by means of CO 2 and a fibre laser surface treatment. Near surface modifications in the hardness were investigated by employing the Vickers indentation method. Crack lengths and the crack geometry were then measured by using the optical microscopy. A co-ordinate measuring machine was used to investigate the diamond indentations and to measure the lengths of the cracks. Thereafter, computational and analytical methods were employed to determine the K 1C. An increase in the K 1C of both ceramics was found by the CO 2 and the fibre laser surface treatment in comparison to the as-received surfaces. The K 1C of the CO 2 laser radiated surface of the Si 3N 4 was over 3% higher in comparison to that of the fibre laser treated surface. This was by softening of the near surface layer of the Si 3N 4 which comprised of lowering of hardness, which in turn increased the crack resistance. The effects were not similar in ZrO 2 ceramic to that of the Si 3N 4 as the fibre laser radiation in this case had produced an increase of 34% compared to that of the CO 2 laser radiation. This occurred due to propagation of lower crack resulting from the Vickers indentation test during the fibre laser surface treatment which inherently affected the end K 1C through an induced compressive stress layer. The K 1C modification of the two ceramics treated by the CO 2 and the fibre laser was also believed to be influenced by the different laser wavelength and its absorption co-efficient, the beam delivery system as well as the differences in the brightness of the two lasers used.

Experimental Study on the Characteristics of Surface Roughness in Ultrasonic Aided High-Speed Lapping of Engineering Ceramics

The surface roughness of machined surface is one of the important indicators to evaluate the workpiece machining quality. In order to clarify the change law of surface roughness in ultrasonic aided high-speed lapping with solid diamond abrasive, a series of ultrasonic and conventional lapping experiments of different engineering ceramics, such as Al2O3, ZrO2 and ZTA, were carried out on precision lathe with self-made high-speed ultrasonic lapping device. The research results show that the value of surface roughness in ultrasonic aided lapping is lower than that in conventional lapping. With the assistance of ultrasonic vibration, the surface quality can be improved obviously. In ultrasonic aided lapping, there is an optimum lapping pressure from which a better surface roughness value can be obtained. Because of different physical and mechanical properties, material removal mechanisms of the three kinds of engineering ceramics present different characteristics, which lead to the different surface roughness even though under the same lapping conditions. The research of the paper is helpful to optimize lapping technique and improve the surface quality.

Study on Axial Turning of Engineering Ceramics

Axial turning is a method to cut the cylindrical work piece along its axis, using the kinetic energy of high-speed rotation annularity tool, the abrasives in tool tip (or toes) as the main cutting edge to remove materials and the abrasives in the inner surface of tool as a minor cutting edge to sharpen the processed surface. The cutting thickness and feed rate could be more than 3~5mm and 30mm/min respectively in a cutting, and realized high-efficient, low-cost processing of engineering ceramics. Processing mechanism analyses showed that both median/radial cracks and lateral cracks occurred in the part to be removed, and the intensity of the processed part had little destroyed, only needing adjust the axial force to control the length of transverse cracks.

Investigation on Axial Turning-Grinding of Engineering Ceramics

Axial turning-grinding is a processing method to cut the cylindrical workpiece along its axis, using the kinetic energy of high-speed rotation annularity tool, the abrasives in tool tip as the main cutting edge to remove materials and the abrasives in the outside surface of tool as a minor cutting edge to sharpen the processed surface. Comparing with concentric axial turning-grinding, eccentric axial turning-grinding has higher efficiency. The result indicated that cutting thickness and feed rate could be more than 5~10mm and 200mm/min respectively in a cutting, realizing high-efficiency, low-cost processing of engineering ceramics. According to axial force, chips’ shape and damage degree of surface to be processed, obtaining the proper feed rate range. Removal mechanism analyses showed that both median/radial cracks and lateral cracks occurred in the part to be removed, and the intensity of the processed part had little destroyed, only needing adjust axial force to control the length of transverse cracks.

질화규소 세라믹의 레이저 예열선삭에 관한 연구 (I) - 공정변수에 따른 질화규소의 예열특성 및 산화거동 -

Silicon nitride is widely used as an engineering ceramics because it has high strength, abrasion resistance and corrosion resistance even at high temperature. However, machining of silicon nitride is difficult due to its high hardness and brittleness. Laser assisted machining(LAM) allows effective cutting using CBN tool by locally heating the cutting part to the softening temperature of YSiAlON using the laser beam. The effect of preheating depending on process parameters were studied to find out the oxidation mechanism. If silicon nitride is sufficiently preheated, the surface is oxidized and gas is formed and escapes from the material, thereby making the cutting process more advantageous. During laser preheating process before machining, high temperature results in strong oxidation which makes the bloating, silicate layers and micro cracks. Using the results of these experiments, preheating characteristics and oxidation behavior were found out.

Identification of Brittle-Ductile Material Removal Characteristics in Ultrasonic Aided Lapping of Engineering Ceramics

For the precision engineering ceramics parts, semi-ductile regime machining technologies are usually adopted to obtain the final surface. It is very important to research the brittle-ductile material removal characteristics and corresponding monitoring technology in ultrasonic aided lapping of engineering ceramics to obtain better surface quality. A series of lapping tests were carried out in the paper and the influence law of lapping parameters on the ductile percentage of ultrasonic lapped surface was achieved, which enables to off-line identify the material removal characteristics. Though the material removal characteristics can be online judged through monitoring the ratio of lapping component forces qualitatively, the monitoring effect is worse just because of lower response frequency of dynamometer and the relativity of lapping force. In this paper, a novel online monitoring technology of material removal characteristics is put forward based on AE technology. Through the wavelet package analysis of AE signals, the discrimination index and standard of material removal characteristics can be obtained. It is proved that this technology can realize online identification of the brittle-ductile material removal characteristics in ultrasonic aided lapping of engineering ceramics effectively.

High-Speed Video of Micro-Detonation Arc Machining Process for Engineering Ceramics

Micro-detonation arc machining (MDAM) is a newly proposed special machining technology for engineering ceramics. To study its working principle is of important significance. By means of high speed video and current signal acquisition system, the shape of micro-detonation arc is observed and the formation course of micro-detonation arc is analyzed. The generation process of ablation pits for Si3N4 and Al2O3 is observed by high speed video. Experimental results show that the generating process of micro-detonation arc can be divided into two phases: spark discharge and stable arc. The diameter of micro-detonation arc increases as working current growing. Si3N4 ceramics decompose and Al2O3 ceramics melts when it machined with MDAM. The materials are removed by synergistic effect of high temperature and intensive impact pressure of micro-detonation arc.

Research on the Removal Mechanism of Engineering Ceramics Based on Energy Density

Based on the calculation of energy density for several kinds of non-traditional machining process of engineering ceramics, the material removal mechanism for ceramics was studied. The studies revealed that the key reason for these technologies to machine ceramics effectively is high-energy-density. For Laser machining, EDM and plasma arc cutting, the material was removed by the high temperature ablation while the high density energy is applied to material surface. For high pressure abrasive water jet machining, the material was removed by erosion using the abrasive particles with high density energy. The ceramics was machined by micro-detonation arc under the synergy of ablation and erosion.

Fracture Toughness of a Zirconia Engineering Ceramic and the Effects Thereon of Surface Processing with Fibre Laser Radiation

Vickers hardness indentation tests were employed to investigate the near-surface changes in the hardness of a fibre laser-treated and an as-received ZrO2 engineering ceramic. Indents were created using 5, 20, and 30 kg loads to obtain the hardness. Optical microscopy, white-light interferometry, and a coordinate measuring machine were then used to observe the crack lengths and crack geometry. Palmqvist and half-penny median crack profiles were found, which dictated the selection of the group of equations used herein. Computational and analytical approaches were then adapted to determine the K1 c of ZrO2. It was found that the best applicable equation was: K1 c = 0.016 ( E/ H)1/2 ( P/ c3/2), which was confirmed to be 42 per cent accurate in producing K1 c values within the range of 8 to 12 MPa m1/2 for ZrO2. Fibre laser surface treatment reduced the surface hardness and produced smaller crack lengths in comparison with the as-received surface. The surface crack lengths, hardness, and indentation loads were found to be important, particularly the crack length, which significantly influenced the end K1 c value when K1 c = 0.016 ( E/ H)1/2 ( P/ c3/2) was used. This is because, the longer the crack lengths, the lower the ceramic's resistance to indentation. This, in turn, increased the end K1 c value. Also, the hardness influences the K1 c, and a softer surface was produced by the fibre laser treatment; this resulted in higher resistance to crack propagation and enhanced the ceramic's K1 c. Increasing the indentation load also varied the end K1 c value, as higher indentation loads resulted in a bigger diamond footprint, and the ceramic exhibited longer crack lengths.

Thermodynamic Evaluation of the Si-C-Al-Y-O System for LPS-SiC Application

A thermodynamic dataset for the Si-C-Al-Y-O system was developed for calculations of heterogeneous phase equilibria and reactions in SiC-base engineering ceramics. The quinary system is of major interest for the understanding of the liquid phase sintering of silicon carbide ceramics using additives like alumina (Al2O3) and yttria (Y2O3). The thermodynamic dataset was developed by the CALPHAD method of thermodynamic optimization. Analytical descriptions for the Gibbs free energy functions of all phases in the Si-C-Al-Y-O system were assessed. The sublattice model using the compound energy formalism was used for the treatment of the solid phases. The liquid phase was described by using the partially ionic liquid model presented as (Al3+, Si4+, Y3+)P(O2−, SiO4 4−, Va, AlO3/2, SiO2, C)Q, which covers metallic liquid and oxide liquid as a single phase.

Grinding characteristics in high speed grinding of engineering ceramics with brazed diamond wheels

An investigation was undertaken to explore the grinding characteristics and removal mechanisms in high speed grinding of three engineering ceramics—alumina, silicon nitride, and zirconia—by using brazed diamond wheels of two different grit sizes. The grinding forces and surface roughness were measured and the morphological features of ground workpiece surfaces were examined. The results indicate that material removal mechanisms are different for the three ceramics at high grinding speeds. For alumina, the removal is dominated by brittle fracture. For silicon nitride and zirconia, the ductile removal prevails in the grinding. For each of the three ceramics, grinding power per unit width is found to be nearly proportional to the rate of plowed surface area generated per unit time per unit width, indicating that the grinding energy expended is mainly associated with sliding and plowing.

Machining Mechanism of Abrasive Water Jet on Ceramics

Engineering ceramics feature resistance to high temperature, corrosion, wear and hot impact. However, it is difficult to machine this material in conventional machining methods because of its high hardness and brittleness as well as inconductivity, thus restricting its application area. In recent years, more and more importance has been attached to the new machining method of engineering ceramics, i.e. abrasive water-jet. Feature high efficiency and low cost, the method can be used to process the products of complex shape. However, abrasive water-jet machining of advanced ceramics is a very complex process. The effect of machining on brittle materials, and advanced ceramic materials in particular, have not yet been very well understood. The present research investigates the effect of abrasive water-jet machining on ceramics. The study will increase the general understanding of the machining phenomena for more successful application of abrasive water-jet machining on brittle materials.

Specificity of the strength calculation of components made from brittle materials

The deformation process of structural materials that have been developed intensively on the basis of new technologies, including engineering ceramics, is different from the deformation process of common structural materials. Relationships allowing for a proper description of the process of inelastic nonisothermal loading of components made of semibrittle materials by taking into account the physical peculiarities of the deformation process, such as a violation of the single-curve law, different resistance against strain and compression, a variation of the residual volume, etc., are presented.