Papers by Author: Xin Li Tian

Abstract: Fracture strength is one of the key mechanics performances for engineering ceramics products, greatly influenced by the microscopic topography and residual stress field of ground surface. In this work, several testing equipments, such as the metallurgical microscope, surface profiler and X ray residual stress tester were introduced to investigate the relationships between microscopic topography, surface roughness, residual stress and fracture strength of ground ceramics, after the surface grinding and mechanical polishing. The experimental results show that a smoother machined surface with low roughness and residual stress is obtained through polishing with absolute alcohol for 20 minutes; the fracture strength of Si₃N₄SiC and Al₂O₃ are increased by 6.64%8.18% and 6.58% respectively, comparing to the ceramics without polishing; the surface stress concentration and residual tensile stress of polished ceramics are both reduced after an appropriate time of polishing process, which causes a certain improvement of ground fracture strength.

Abstract: Grinding is the mostly leading machining technology for engineering ceramics. The quality of ground surface can be evaluated by various roughness parameters. And a textural analysis method based on Gray Level Co-occurrence Matrix was employed in researching the roughness of ground ceramics. The relationship between texture features and roughness was investigated through a series of surface images of engineering ceramics collected by a digital microscope. The sampling offset and total gray levels of surface images were determined firstly. Then 4 GLCMs were built up to calculate the average of texture features. And 6 parameters were fixed as main texture parameters. Furthermore, the multiple nonlinear regression theory was used to assess the relation between the texture features and roughness Ra. By statistic test and comparison, the deviation of calculated Ra and actual Ra is less than 0.25. It is shown that this relation is much satisfactory and the method may be suitable for quickly measuring the roughness of ground ceramics.

Abstract: Surface roughness is one of the key factors to evaluate the grinding quality for engineering ceramics. This study introduces a new method based on grayscale information of surface images, to assess and predict the roughness of ground ceramics rapidly and effectively. The investigation sets the functional parameters of digital acquisition device as Brightness 140, Contrast 42, Saturation 24 and Acutance 9. Afterwards, it selects the mean value and the mean square deviation to describe surface roughness, and some image processing techniques are adopted to reduce noises and enhance the images. Lastly, it gives the relation curves on R_a，R_z，R_y versus grayscale information, and concludes a direct proportion law between the grayscale information and ground surface roughness.

Abstract: An edge chipping experiment under static load is adopted to study the Kaiser Effect during the failure process for engineering ceramics. The mechanisms inducing Kaiser Effect are discussed. Some AE signals during the course of Kaiser Effect are analyzed. The results show that there’s obvious Kaiser Effect during the failure process of edge chipping; count, energy and amplitude can be well used to describe the Kaiser Effect. The surface metamorphic layer remained on the ceramic surface during the course of machining is the main factor for Kaiser Effect. Edge distance and mechanics characteristic are the important factors for Kaiser Effect. The study on Kaiser Effect of edge chipping has provided some guidance to prevent edge chipping of engineering ceramics.

Abstract: The surface roughness and surface texture feature both are the key factors to evaluate the ground ceramics surface. The Gray Level Co-occurrence Matrix (GLCM) is introduced to extract and analyze the texture features of ground surface, in order to discover the relationship between texture features and roughness, and predict the roughness of ground ceramics according to the defined texture parameters. Four matrices about different sampling orientations are built up after the sampling offset and total gray levels are determined. By analyzing the correlation about every two features, Contrast, SS and ASM are selected to characterize the texture information of machined surface. Finally, the paper reveals the linear variation laws between R_a, R_y, S, t_p and above features. The grounding quality of ceramic products also can be roughly estimated on the ground of that.

Abstract: Axial turn-grinding 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 as the main cutting edge to remove materials and the abrasives in the inside or outside surface of tool as a minor cutting edge to sharpen the processed surface. Through analysis the simplified model, a slightly stress concentration was found at the junction surface of processed surface and machined surface, which had little damage to the workpiece because of happening in the part to be removed or precision machining allowance, and both cracks propagation caused by material rupture were effective removal after material removal mechanism analysis because the extension of the median/radial crack and lateral crack were both happening in the part to be removed.

Abstract: 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.

Abstract: Coating quality and electroplating efficiency can be improved by application of ultrasonic agitation. The nickel coatings and electroplated diamond tools were fabricated. The effects of ultrasonic agitation on tool fabrication processes were analyzed. Cathode polarization curves and coating hardness were measured. Diamond grains on tool substrate were observed with microscope. Alumina grinding experiments were carried out and grinding ratios were calculated. Experimental results show that cathode polarization degree reduced during ultrasonic electroplating processes. Before diamond grain tack-on operation, ultrasonic agitation can make grain dense and guarantee nice contact between substrate and grains. High grain density of diamond tool can be obtained in tack-on operation after above ultrasonic agitation operation. Ultrasonic agitation is not suitable for the tack-on operation of diamond grains. During buildup processes, some buildup time is needed to obtain enough thickness of nickel coating before ultrasonic agitation is applied. The diamond tools fabricated with ultrasonic agitation present better cutting ability. During tool fabricating processes, the reasonable application of ultrasonic agitation is necessary.

Abstract: With reference to the principle that the strong detonation wave generates the transient dynamic high pressure, the small power pulse power supply and the electrode micro-detonation wave generator are developed. The engineering ceramic material machining with low cost is realized successfully with the devices. It is shown from the study results that there is a degenerative layer about 0.25~0.45mm thick, which is composed of amorphous phases and crystalloid Si and can be machined and removed with the ceramic tool after micro-detonation machining. There are obvious transverse cracks near the interface of the degenerative layer and the matrix, but the cracks along the depth direction are not found. The performance of the degenerative layer is roundly analyzed and its removing mechanism is primitively discussed.

Abstract: Based on the comprehensive summary of latest research achievements of Laser and EDM technologies in home and abroad, the other advanced machining technologies is summarized briefly, such as Ultrasonic Wave, Microwave machining techniques and their composite machining technologies, as well as High-speed (Super High-speed) Grinding, ELID Grinding and Interface Thermal Chemistry Reaction Aided Machining technologies developed on the basis of traditional grinding technology. At last, it is forecasted that the necessary direction of advanced technologies used for ceramics is combinatorial machining technologies of two or more kinds of advanced technologies.