Engineering ceramics have many excellent properties, such as high hardness and strength, strong corrosion resistance, wear resistance, high temperature resistance and good chemical inertia, so in aerospace, chemical, military, mechanical, electronic and electrical Applications in the field of precision manufacturing are becoming more widespread. At present, developed countries such as Germany, Japan, the United States, and Britain attach great importance to the development and application of engineering ceramics. Since the 1980s, countries have been competing for a large amount of capital and manpower, and have made great progress in engineering ceramic processing theory and technology, product development and application.
Due to the high hardness and high brittleness of ceramic materials, most of the processed ceramic components produce various types of surface or subsurface damage, which leads to a decrease in the strength of the ceramic component, thereby limiting the adoption of large material removal rates. For ceramic high-efficiency grinding, the fundamental goal is to achieve maximum material removal while maintaining material surface integrity and dimensional accuracy. At present, the processing cost of ceramics has reached 80% to 90% of the cost of the entire ceramic component. The high processing cost and the difficulty in measuring and controlling the surface damage layer limit the wider application of ceramic components.
The broad application prospects and complex processing characteristics of ceramic materials require a comprehensive and in-depth understanding of the grinding process of ceramics. Since the 1990s, scholars at home and abroad have conducted a lot of research, new ways of ceramic grinding, material removal mechanism of ceramic grinding, grinding burns, grinding surface integrity and other influencing factors, different grinding conditions. Positive research results have been achieved in many aspects such as the best grinding parameters. This paper mainly summarizes and summarizes the research status and development of ceramic grinding.
1 Development of grinding mechanism of ceramic materials Research on grinding mechanism Due to the random size of the grinding wheel, the randomness of the abrasive grain distribution and the complexity of the grinding motion law, it has brought great difficulties to the research of grinding mechanism. . In ceramic grinding, due to the high hardness and high brittleness of ceramics, most studies have used the "indentation fracture mechanics" model or the "cutting" model to approximate processing. In the early 1980s, Frank and Lawn first established three mechanism analysis models of blunt indenters, sharp indenters and contact sliding, and proposed the stress intensity factor formula K=aE·P/C2/3, according to brittle fracture. The mechanical condition K≥KC, the critical load PBC=Cb·K of brittle fracture is derived. According to the yield condition s≥sY of the material, the critical load PYYC=s3/g3 (or PYYC=H3Y/g3) in plastic deformation mode is derived. ). The research indicates that the removal mechanism of ceramic materials is usually crack propagation and brittle fracture. When the hardness of the material is reduced, the radius of the indentation is small, the friction is severe, and the load is small, plastic deformation occurs. In 1987, I. Inasaki further suggested that the removal of ceramic materials in different ways depends on the size and density of defects on the material, such as the size of cracks, cracks and stress fields. Hirano Hirano also proposed in his monograph that the removal mechanism of materials is affected by high temperature strength. In 1991, Professor Zheng Huanwen and Cai Guangqi from Northeastern University conducted grinding experiments on molybdenum-containing cermets. The granules were explored by measuring the unit grinding force, grinding energy and grinding ratio, and using SEM to observe the ceramic surface and cutting area. Material removal mechanism.
In 1994, Keio University R.Rentsch first used the molecular dynamics method for the grinding mechanism research, and gave the simulation results of the first grinding process to illustrate the phenomenon of grinding debris accumulation during grinding, and pointed out the grinding. The similarities and differences between the simulation of the cutting process and the simulation of the cutting process.
In 1996, S. Malkin of the Massachusetts Institute of Technology reviewed the mechanism of ceramic grinding. It is believed that the research mechanism of grinding is the basis for the realization of low-cost and high-efficiency grinding of ceramic materials. The specific research methods are summarized as indentation fracture mechanics and processing observation. The indentation fracture mechanics model is based on an idealized crack system and the deformation produced by the indenter. This method analyzes the interaction between the abrasive particles and the workpiece, using an ideal indentation in a small range, and analyzes the relationship between stress, deformation, and material removal. The processing observation method includes the measurement of the grinding force, the processing of the surface topography and the microscopic observation of the chips. Both provide important insights into the grinding mechanism of ceramic materials.
In 1999, G Warnecke of the University of Kaiserslautern in Germany pointed out that the grinding process and results are closely related to the material removal mechanism when grinding hard and brittle materials such as new ceramics and hard metals. The material removal mechanism is determined by the interaction of material properties, abrasive geometry, abrasive cut-in motion, and mechanical and thermal loads acting on the workpiece and abrasive particles. In addition, the surface grinding process is also affected by the dynamic characteristics of the contact zone.
For general grinding, extensive and intensive research has been carried out on the grinding mechanism and the grinding process. In the precision and ultra-precision grinding, high-speed and high-efficiency grinding, especially for the grinding mechanism and grinding process of engineering materials such as ceramics and glass with special processing properties, some research has been carried out at home and abroad, but it is still not comprehensive, yet To form a complete theoretical system, it is necessary to conduct more in-depth research to find out its inherent laws.
The basic removal mechanism of materials In the study of ceramic material processing, the most complicated is the material removal mechanism. Studies have shown that in the ceramic grinding process, material removal is mainly based on the following removal mechanisms: grain removal, spalling, brittle fracture, fracture, grain boundary micro-crushing and other brittle removal methods, powder removal and plastic removal methods.
Brittleness removal mechanism of materials In general, in the process of ceramic grinding, material brittle removal is accomplished by forming or stretching, spalling and chipping of voids and cracks. The specific methods are as follows: grain removal, Material spalling, brittle fracture, fine grain boundary, etc. During the grain removal process, the material is removed in such a way that the entire die is detached from the surface of the workpiece. In 1990, K. Subramanian et al. pointed out that the grain removal was accompanied by the removal of the material. The peeling removal method is an important removal method in the grinding of ceramic materials. In 1992, DWRicherson proposed that in the material spalling removal mechanism, the material is partially spalled due to the expansion of transverse and radial cracks generated during the grinding process. The main drawback of this approach is that the crack propagation greatly reduces the mechanical strength of the workpiece. In 1995 and 1996, Xu, HHK, and Jahamir.S successively pointed out that the processing of ceramic materials such as alumina, glass ceramics, silicon nitride, and silicon carbide showed that the grain boundaries were slightly broken and materials during ceramic grinding. Grain dislocations also play a key role in the material removal process. In 1998, V Sinhoff of Achen Production Engineering Research Institute of Germany studied the grinding of optical glass with cup-shaped diamond grinding wheel. The focus was on the study of brittle ductile transition characteristics, and the stress distribution and crack geometry in the material were considered as The grinding evaluation model is established by the functions of abrasive grain geometry, material properties and external load. Then the TGBifano energy conservation law is used to describe the transition process of the material's brittleness removal to ductility removal process.
The powder removal mechanism of the material During the precision grinding process, when the grinding depth is on the submicron level, the fragmentation and the fracture do not occur, and the material powdering phenomenon may be mainly generated at this time. The mechanism of material powder removal means that the abrasive particles cause static compressive stress of the fluid during the grinding process, and the local shear stress field surrounded by the compressive stress causes grain boundary or intergranular micro-crushing, thereby causing material powdering phenomenon. The grain of the ceramic material is broken into finer grains by powder removal and forms a powder domain.
The plastic removal mechanism of the material is similar to the chip forming process in metal grinding, which involves slipping, ploughing and chip forming, and the material is removed by shearing chip formation. The plastic removal mechanism mainly refers to the ductile domain grinding of ceramic grinding. Under certain processing conditions, any brittle material can be removed by plastic flow. The indentation fracture mechanics model preliminarily produces the critical load of transverse crack. When the processing condition is lower than this critical load, the material will be mainly removed by plastic deformation. At present, many experts and scholars at home and abroad are studying the development of ductile grinding and semi-ductive grinding technology for ceramics to reduce micro-cracks and cracks on the surface of the workpiece and improve the performance of the workpiece.
In 1989, TG Bifano clearly proposed a new ductile grinding process for processing brittle materials. It is believed that by using a high-stiffness, high-resolution precision grinding machine, by controlling the feed rate, the hard and brittle material can be removed in a ductile domain mode and given The critical grinding depth expression: DC=0.15 (E/H)(KC)2, and the relationship between feed rate and material properties when describing ductile domain grinding is described according to the law of energy conservation. In 1991, BifanoT, DowTA, and ScattergoodRO systematically studied the ductile grinding of ceramic materials using a special grinding machine equipped with an ultra-precision feed control device. The results show that there is a certain relationship between the grinding feed rate and the material properties (such as fracture toughness, hardness and elastic modulus) of various brittle materials in the corresponding brittle transition. In the case where the grinding depth is sufficiently small, all brittle materials will be removed by plastic flow rather than by brittle fracture.
Although the ductile domain grinding method can obtain a fairly good surface quality, the efficiency is low and the processing cost is high. The use of high grinding wheel grinding speeds increases plastic flow and results in high removal rates. In 1993, Inoue et al. used 120# diamond grinding wheel to grind RESN. The experimental results show that at 170m/s, the proportion of surface cracking of the workpiece is reduced from 48% to 12% of 25m/s. In 1994, KOvch et al. used a ceramic bond diamond grinding wheel to grind ceramics at a speed of 160 m/s to obtain a high grinding ratio of 5100. In 1996, research conducted by Malkin et al. further demonstrated that the reduction in surface fracture and the significant increase in plastic flow in high-speed ultra-high speed grinding may be related to the glass phase formed at higher grinding temperatures.
There are many factors influencing the actual grinding process, such as machine stiffness, grinding depth, wheel speed, abrasive size, shape, geometry and temperature. To achieve ductile grinding, the conditions are quite demanding. At present, most of the semi-ductive grinding is used. At this time, the machined surface is formed by the alternating mixing of the micro-crushed surface and the plastic plane to complete the cutting, which can reduce the surface defects to a minimum and obtain a good surface integrity and improve the surface. Performance of the workpiece such as strength. In the semi-ductive grinding process, the ceramic material is removed by a large amount of micro-crushing and plastic deformation at the abrasive action. When the stress field caused by the cutting edge of the abrasive grain cut into the workpiece is smaller than the defect, the material will be removed by plastic deformation; on the contrary, when the stress field is larger than the defect, the local concentrated brittle failure caused by crack propagation will play a major role. Due to the bluntness and height distribution of the abrasive grains on the grinding wheel, the grinding depth of each abrasive grain is different, so that the material is removed by the joint action of brittle fracture and plastic deformation, thereby achieving semi-ductive grinding.
Ke Hongfa et al. proposed that in the semi-ductive grinding of ceramics, due to the poor thermal conductivity of ceramics, the rapid cooling of the cooling liquid will increase the brittleness of the ceramic, resulting in microcracks on the surface. If a good machined surface is to be obtained, the coolant should not be used in order to remove the ceramic as much as possible in a plastically deformed manner.
2 New development of ceramic grinding methods The research and development of new ceramic materials continue to promote and promote the development of ceramic processing technology. On the other hand, the production of these new grinding methods also provides strong technical support for the application of ceramic materials. Due to the special processing characteristics of ceramic materials, traditional grinding methods are difficult to meet the requirements of practical applications, so people have been exploring new ways of grinding ceramics. In recent years, ultrasonic vibration grinding, ELID (on-line electrolytic dressing diamond grinding wheel), ECD (electrochemical online control dressing), ECDM (electrochemical discharge machining), MEEC (mechanical-electrolytic-electric spark grinding), etc. A representative new type of composite processing. These grinding methods not only solve the processing problems of difficult-to-cut materials, but also improve the processing efficiency, and can improve the processing quality. They are briefly described as follows:
Ultrasonic grinding Ultrasonic machining is the application of ultrasonic vibration on a processing tool or a material to be processed. A liquid abrasive or a paste abrasive is applied between the tool and the workpiece, and the tool is pressed against the workpiece with a small pressure. During processing, due to the ultrasonic vibration between the tool and the workpiece, the abrasive particles suspended in the working fluid are forced to continuously impact and polish the surface to be processed with a large speed and acceleration, plus cavitation and overpressure effects in the processing area. Thereby a material removal effect is produced. It combines with other processing methods to form a variety of ultrasonic composite processing methods. Among them, ultrasonic grinding is more suitable for the processing of ceramic materials, and the processing efficiency increases as the brittleness of the material increases.
Xin Zhijie and others from North China Institute of Technology conducted research on ultrasonic vibration honing and grinding technology and developed ultrasonic vibration grinding devices. This technology has great potential in high-efficiency finishing of hard and brittle materials such as ceramics. Wang Jun et al. pointed out that the material removal mechanism of ultrasonic vibration plastic grinding and ordinary plastic grinding is also different. Ultrasonic vibration plastic grinding not only causes shear failure of materials, but also causes fatigue damage of materials under high frequency vibration, accelerating materials. It is removed more efficiently than ordinary grinding. The conditions for achieving ultrasonic vibration plastic grinding are not only related to the depth of the grinding but also to the amplitude and frequency. Ultrasonic vibration grinding can not only use a larger amount of grinding, but also reduce the dressing time of the grinding wheel, so the processing efficiency is more than double that of ordinary grinding. Tianjin University proposed that ultrasonic grinding can combine the characteristics of ultrasonic machining and high-speed grinding. The processing efficiency is about ten times higher than that of ultrasonic machining, which can improve the surface quality of the workpiece. It is of great value to the microporous processing of ceramic materials.
ELID (Online Electrolytic Dressing Diamond Wheel)
ELID grinding is a novel grinding method that combines electrolytic dressing wheels with conventional mechanical grinding during machining. During the ELID grinding process, the weak electrolysis causes the metal bond on the surface of the grinding wheel to be continuously ionized and dissolved, and the resulting easily ruptured passivation film can prevent the grinding debris from adhering to the grinding wheel, thus ensuring that it is always A certain amount of abrasive particles protruded out. Efficient grinding and mirror grinding can be achieved with a choice of binders. The technique was first proposed by Hitoshiohmori et al. of the Institute of Physical Chemistry of Japan in 1987. They used a fine abrasive cast iron fiber-based diamond grinding wheel to precisely machine the silicon wafer; the grinding machine was used in the grinding process by a common machine tool. Online trimming enables mirror grinding of silicon wafers. In 1995, Omori conducted further research on ELID. ELID grinding of single crystal silicon, optical glass and ceramics was carried out with cast iron-based grinding wheels of several micrometers to submicron diamond abrasive grains. The size and roughness of the abrasive grains were systematically studied. Degree relationship. The surface roughness after processing is up to several angstroms, which can replace grinding and polishing.
MEEC (Mechanical - Electrolytic - EDM Grinding)
Electrolytic and electric spark composite grinding process (MEEC) is a three-composite machining method based on mechanical grinding. It combines mechanical, electrical and chemical functions to achieve high-speed and high-precision machining. The working principle is that during the rotation of the grinding wheel, when the non-conductive part is in contact with the workpiece, the abrasive grains mechanically grind the workpiece, and when the conductive part approaches the workpiece, the electrolysis is caused by the grinding fluid sprayed between the grinding wheel and the workpiece. The effect is to improve the quality of the machined surface. The spark discharge that occurs at the moment when the conductive portion leaves the surface of the workpiece, in addition to removing the workpiece material to some extent, the resulting high temperature also causes the bonding agent around the abrasive grains on the grinding wheel to melt and vaporize to maintain the sharpness of the grinding wheel and Some workpiece materials, such as ceramics, are subject to heat for grinding. This method can process non-conductive materials (ceramics) that cannot be electrically or electrolytically processed. Through research on the MEEC method, Guangdong University has proposed ways to reduce energy consumption, improve processing efficiency, and improve processing accuracy.
ECD (electrochemical online control trimming)
In 1999, D. Kramer et al. proposed the ECD technology, which is different from ELID. The ECD process does not require the formation of oxide and hydroxide films. Instead, it controls the electrochemical trimming by measuring the grain edge and the surface state of the workpiece. Process, the results show that the technology can significantly improve the surface quality of materials in the process of grinding ceramics, PCBN, PCD and cemented carbide. In 2000, D. Kramer et al. further proposed that the use of a controlled electrochemical process to trim metal bond grinding wheels on-line can provide a new way to grind new cutting materials that are extremely difficult to machine with conventional grinding methods.
ECDM (Electrochemical Discharge Machining)
ECDM is a controlled on-line electrochemical process that combines electrochemical machining (ECM) with electrical discharge machining (EDM). M. Schoepf et al. of Zurich, Switzerland, call it the ideal method for sharpening metal bond grinding wheels and cost-effectively grinding ceramic materials.
3 Conclusion In summary, the development of ceramic material processing technology, the application of various new abrasives and adhesives, especially the emergence of new grinding methods, have created conditions for the application of ceramic materials in a wider range of fields; The development of ceramic material grinding mechanism, especially the theoretical research of ceramic material removal mechanism has been greatly developed at home and abroad, and provides a theoretical basis for the further application of ceramic materials. At present, research in the fields of non-stationary grinding and non-invasive grinding has also attracted people's attention. It is foreseeable that in the near future, with the deepening of theoretical research and the emergence of new processing technologies, ceramic grinding technology will certainly be applied and promoted in more fields.
Due to the high hardness and high brittleness of ceramic materials, most of the processed ceramic components produce various types of surface or subsurface damage, which leads to a decrease in the strength of the ceramic component, thereby limiting the adoption of large material removal rates. For ceramic high-efficiency grinding, the fundamental goal is to achieve maximum material removal while maintaining material surface integrity and dimensional accuracy. At present, the processing cost of ceramics has reached 80% to 90% of the cost of the entire ceramic component. The high processing cost and the difficulty in measuring and controlling the surface damage layer limit the wider application of ceramic components.
The broad application prospects and complex processing characteristics of ceramic materials require a comprehensive and in-depth understanding of the grinding process of ceramics. Since the 1990s, scholars at home and abroad have conducted a lot of research, new ways of ceramic grinding, material removal mechanism of ceramic grinding, grinding burns, grinding surface integrity and other influencing factors, different grinding conditions. Positive research results have been achieved in many aspects such as the best grinding parameters. This paper mainly summarizes and summarizes the research status and development of ceramic grinding.
1 Development of grinding mechanism of ceramic materials Research on grinding mechanism Due to the random size of the grinding wheel, the randomness of the abrasive grain distribution and the complexity of the grinding motion law, it has brought great difficulties to the research of grinding mechanism. . In ceramic grinding, due to the high hardness and high brittleness of ceramics, most studies have used the "indentation fracture mechanics" model or the "cutting" model to approximate processing. In the early 1980s, Frank and Lawn first established three mechanism analysis models of blunt indenters, sharp indenters and contact sliding, and proposed the stress intensity factor formula K=aE·P/C2/3, according to brittle fracture. The mechanical condition K≥KC, the critical load PBC=Cb·K of brittle fracture is derived. According to the yield condition s≥sY of the material, the critical load PYYC=s3/g3 (or PYYC=H3Y/g3) in plastic deformation mode is derived. ). The research indicates that the removal mechanism of ceramic materials is usually crack propagation and brittle fracture. When the hardness of the material is reduced, the radius of the indentation is small, the friction is severe, and the load is small, plastic deformation occurs. In 1987, I. Inasaki further suggested that the removal of ceramic materials in different ways depends on the size and density of defects on the material, such as the size of cracks, cracks and stress fields. Hirano Hirano also proposed in his monograph that the removal mechanism of materials is affected by high temperature strength. In 1991, Professor Zheng Huanwen and Cai Guangqi from Northeastern University conducted grinding experiments on molybdenum-containing cermets. The granules were explored by measuring the unit grinding force, grinding energy and grinding ratio, and using SEM to observe the ceramic surface and cutting area. Material removal mechanism.
In 1994, Keio University R.Rentsch first used the molecular dynamics method for the grinding mechanism research, and gave the simulation results of the first grinding process to illustrate the phenomenon of grinding debris accumulation during grinding, and pointed out the grinding. The similarities and differences between the simulation of the cutting process and the simulation of the cutting process.
In 1996, S. Malkin of the Massachusetts Institute of Technology reviewed the mechanism of ceramic grinding. It is believed that the research mechanism of grinding is the basis for the realization of low-cost and high-efficiency grinding of ceramic materials. The specific research methods are summarized as indentation fracture mechanics and processing observation. The indentation fracture mechanics model is based on an idealized crack system and the deformation produced by the indenter. This method analyzes the interaction between the abrasive particles and the workpiece, using an ideal indentation in a small range, and analyzes the relationship between stress, deformation, and material removal. The processing observation method includes the measurement of the grinding force, the processing of the surface topography and the microscopic observation of the chips. Both provide important insights into the grinding mechanism of ceramic materials.
In 1999, G Warnecke of the University of Kaiserslautern in Germany pointed out that the grinding process and results are closely related to the material removal mechanism when grinding hard and brittle materials such as new ceramics and hard metals. The material removal mechanism is determined by the interaction of material properties, abrasive geometry, abrasive cut-in motion, and mechanical and thermal loads acting on the workpiece and abrasive particles. In addition, the surface grinding process is also affected by the dynamic characteristics of the contact zone.
For general grinding, extensive and intensive research has been carried out on the grinding mechanism and the grinding process. In the precision and ultra-precision grinding, high-speed and high-efficiency grinding, especially for the grinding mechanism and grinding process of engineering materials such as ceramics and glass with special processing properties, some research has been carried out at home and abroad, but it is still not comprehensive, yet To form a complete theoretical system, it is necessary to conduct more in-depth research to find out its inherent laws.
The basic removal mechanism of materials In the study of ceramic material processing, the most complicated is the material removal mechanism. Studies have shown that in the ceramic grinding process, material removal is mainly based on the following removal mechanisms: grain removal, spalling, brittle fracture, fracture, grain boundary micro-crushing and other brittle removal methods, powder removal and plastic removal methods.
Brittleness removal mechanism of materials In general, in the process of ceramic grinding, material brittle removal is accomplished by forming or stretching, spalling and chipping of voids and cracks. The specific methods are as follows: grain removal, Material spalling, brittle fracture, fine grain boundary, etc. During the grain removal process, the material is removed in such a way that the entire die is detached from the surface of the workpiece. In 1990, K. Subramanian et al. pointed out that the grain removal was accompanied by the removal of the material. The peeling removal method is an important removal method in the grinding of ceramic materials. In 1992, DWRicherson proposed that in the material spalling removal mechanism, the material is partially spalled due to the expansion of transverse and radial cracks generated during the grinding process. The main drawback of this approach is that the crack propagation greatly reduces the mechanical strength of the workpiece. In 1995 and 1996, Xu, HHK, and Jahamir.S successively pointed out that the processing of ceramic materials such as alumina, glass ceramics, silicon nitride, and silicon carbide showed that the grain boundaries were slightly broken and materials during ceramic grinding. Grain dislocations also play a key role in the material removal process. In 1998, V Sinhoff of Achen Production Engineering Research Institute of Germany studied the grinding of optical glass with cup-shaped diamond grinding wheel. The focus was on the study of brittle ductile transition characteristics, and the stress distribution and crack geometry in the material were considered as The grinding evaluation model is established by the functions of abrasive grain geometry, material properties and external load. Then the TGBifano energy conservation law is used to describe the transition process of the material's brittleness removal to ductility removal process.
The powder removal mechanism of the material During the precision grinding process, when the grinding depth is on the submicron level, the fragmentation and the fracture do not occur, and the material powdering phenomenon may be mainly generated at this time. The mechanism of material powder removal means that the abrasive particles cause static compressive stress of the fluid during the grinding process, and the local shear stress field surrounded by the compressive stress causes grain boundary or intergranular micro-crushing, thereby causing material powdering phenomenon. The grain of the ceramic material is broken into finer grains by powder removal and forms a powder domain.
The plastic removal mechanism of the material is similar to the chip forming process in metal grinding, which involves slipping, ploughing and chip forming, and the material is removed by shearing chip formation. The plastic removal mechanism mainly refers to the ductile domain grinding of ceramic grinding. Under certain processing conditions, any brittle material can be removed by plastic flow. The indentation fracture mechanics model preliminarily produces the critical load of transverse crack. When the processing condition is lower than this critical load, the material will be mainly removed by plastic deformation. At present, many experts and scholars at home and abroad are studying the development of ductile grinding and semi-ductive grinding technology for ceramics to reduce micro-cracks and cracks on the surface of the workpiece and improve the performance of the workpiece.
In 1989, TG Bifano clearly proposed a new ductile grinding process for processing brittle materials. It is believed that by using a high-stiffness, high-resolution precision grinding machine, by controlling the feed rate, the hard and brittle material can be removed in a ductile domain mode and given The critical grinding depth expression: DC=0.15 (E/H)(KC)2, and the relationship between feed rate and material properties when describing ductile domain grinding is described according to the law of energy conservation. In 1991, BifanoT, DowTA, and ScattergoodRO systematically studied the ductile grinding of ceramic materials using a special grinding machine equipped with an ultra-precision feed control device. The results show that there is a certain relationship between the grinding feed rate and the material properties (such as fracture toughness, hardness and elastic modulus) of various brittle materials in the corresponding brittle transition. In the case where the grinding depth is sufficiently small, all brittle materials will be removed by plastic flow rather than by brittle fracture.
Although the ductile domain grinding method can obtain a fairly good surface quality, the efficiency is low and the processing cost is high. The use of high grinding wheel grinding speeds increases plastic flow and results in high removal rates. In 1993, Inoue et al. used 120# diamond grinding wheel to grind RESN. The experimental results show that at 170m/s, the proportion of surface cracking of the workpiece is reduced from 48% to 12% of 25m/s. In 1994, KOvch et al. used a ceramic bond diamond grinding wheel to grind ceramics at a speed of 160 m/s to obtain a high grinding ratio of 5100. In 1996, research conducted by Malkin et al. further demonstrated that the reduction in surface fracture and the significant increase in plastic flow in high-speed ultra-high speed grinding may be related to the glass phase formed at higher grinding temperatures.
There are many factors influencing the actual grinding process, such as machine stiffness, grinding depth, wheel speed, abrasive size, shape, geometry and temperature. To achieve ductile grinding, the conditions are quite demanding. At present, most of the semi-ductive grinding is used. At this time, the machined surface is formed by the alternating mixing of the micro-crushed surface and the plastic plane to complete the cutting, which can reduce the surface defects to a minimum and obtain a good surface integrity and improve the surface. Performance of the workpiece such as strength. In the semi-ductive grinding process, the ceramic material is removed by a large amount of micro-crushing and plastic deformation at the abrasive action. When the stress field caused by the cutting edge of the abrasive grain cut into the workpiece is smaller than the defect, the material will be removed by plastic deformation; on the contrary, when the stress field is larger than the defect, the local concentrated brittle failure caused by crack propagation will play a major role. Due to the bluntness and height distribution of the abrasive grains on the grinding wheel, the grinding depth of each abrasive grain is different, so that the material is removed by the joint action of brittle fracture and plastic deformation, thereby achieving semi-ductive grinding.
Ke Hongfa et al. proposed that in the semi-ductive grinding of ceramics, due to the poor thermal conductivity of ceramics, the rapid cooling of the cooling liquid will increase the brittleness of the ceramic, resulting in microcracks on the surface. If a good machined surface is to be obtained, the coolant should not be used in order to remove the ceramic as much as possible in a plastically deformed manner.
2 New development of ceramic grinding methods The research and development of new ceramic materials continue to promote and promote the development of ceramic processing technology. On the other hand, the production of these new grinding methods also provides strong technical support for the application of ceramic materials. Due to the special processing characteristics of ceramic materials, traditional grinding methods are difficult to meet the requirements of practical applications, so people have been exploring new ways of grinding ceramics. In recent years, ultrasonic vibration grinding, ELID (on-line electrolytic dressing diamond grinding wheel), ECD (electrochemical online control dressing), ECDM (electrochemical discharge machining), MEEC (mechanical-electrolytic-electric spark grinding), etc. A representative new type of composite processing. These grinding methods not only solve the processing problems of difficult-to-cut materials, but also improve the processing efficiency, and can improve the processing quality. They are briefly described as follows:
Ultrasonic grinding Ultrasonic machining is the application of ultrasonic vibration on a processing tool or a material to be processed. A liquid abrasive or a paste abrasive is applied between the tool and the workpiece, and the tool is pressed against the workpiece with a small pressure. During processing, due to the ultrasonic vibration between the tool and the workpiece, the abrasive particles suspended in the working fluid are forced to continuously impact and polish the surface to be processed with a large speed and acceleration, plus cavitation and overpressure effects in the processing area. Thereby a material removal effect is produced. It combines with other processing methods to form a variety of ultrasonic composite processing methods. Among them, ultrasonic grinding is more suitable for the processing of ceramic materials, and the processing efficiency increases as the brittleness of the material increases.
Xin Zhijie and others from North China Institute of Technology conducted research on ultrasonic vibration honing and grinding technology and developed ultrasonic vibration grinding devices. This technology has great potential in high-efficiency finishing of hard and brittle materials such as ceramics. Wang Jun et al. pointed out that the material removal mechanism of ultrasonic vibration plastic grinding and ordinary plastic grinding is also different. Ultrasonic vibration plastic grinding not only causes shear failure of materials, but also causes fatigue damage of materials under high frequency vibration, accelerating materials. It is removed more efficiently than ordinary grinding. The conditions for achieving ultrasonic vibration plastic grinding are not only related to the depth of the grinding but also to the amplitude and frequency. Ultrasonic vibration grinding can not only use a larger amount of grinding, but also reduce the dressing time of the grinding wheel, so the processing efficiency is more than double that of ordinary grinding. Tianjin University proposed that ultrasonic grinding can combine the characteristics of ultrasonic machining and high-speed grinding. The processing efficiency is about ten times higher than that of ultrasonic machining, which can improve the surface quality of the workpiece. It is of great value to the microporous processing of ceramic materials.
ELID (Online Electrolytic Dressing Diamond Wheel)
ELID grinding is a novel grinding method that combines electrolytic dressing wheels with conventional mechanical grinding during machining. During the ELID grinding process, the weak electrolysis causes the metal bond on the surface of the grinding wheel to be continuously ionized and dissolved, and the resulting easily ruptured passivation film can prevent the grinding debris from adhering to the grinding wheel, thus ensuring that it is always A certain amount of abrasive particles protruded out. Efficient grinding and mirror grinding can be achieved with a choice of binders. The technique was first proposed by Hitoshiohmori et al. of the Institute of Physical Chemistry of Japan in 1987. They used a fine abrasive cast iron fiber-based diamond grinding wheel to precisely machine the silicon wafer; the grinding machine was used in the grinding process by a common machine tool. Online trimming enables mirror grinding of silicon wafers. In 1995, Omori conducted further research on ELID. ELID grinding of single crystal silicon, optical glass and ceramics was carried out with cast iron-based grinding wheels of several micrometers to submicron diamond abrasive grains. The size and roughness of the abrasive grains were systematically studied. Degree relationship. The surface roughness after processing is up to several angstroms, which can replace grinding and polishing.
MEEC (Mechanical - Electrolytic - EDM Grinding)
Electrolytic and electric spark composite grinding process (MEEC) is a three-composite machining method based on mechanical grinding. It combines mechanical, electrical and chemical functions to achieve high-speed and high-precision machining. The working principle is that during the rotation of the grinding wheel, when the non-conductive part is in contact with the workpiece, the abrasive grains mechanically grind the workpiece, and when the conductive part approaches the workpiece, the electrolysis is caused by the grinding fluid sprayed between the grinding wheel and the workpiece. The effect is to improve the quality of the machined surface. The spark discharge that occurs at the moment when the conductive portion leaves the surface of the workpiece, in addition to removing the workpiece material to some extent, the resulting high temperature also causes the bonding agent around the abrasive grains on the grinding wheel to melt and vaporize to maintain the sharpness of the grinding wheel and Some workpiece materials, such as ceramics, are subject to heat for grinding. This method can process non-conductive materials (ceramics) that cannot be electrically or electrolytically processed. Through research on the MEEC method, Guangdong University has proposed ways to reduce energy consumption, improve processing efficiency, and improve processing accuracy.
ECD (electrochemical online control trimming)
In 1999, D. Kramer et al. proposed the ECD technology, which is different from ELID. The ECD process does not require the formation of oxide and hydroxide films. Instead, it controls the electrochemical trimming by measuring the grain edge and the surface state of the workpiece. Process, the results show that the technology can significantly improve the surface quality of materials in the process of grinding ceramics, PCBN, PCD and cemented carbide. In 2000, D. Kramer et al. further proposed that the use of a controlled electrochemical process to trim metal bond grinding wheels on-line can provide a new way to grind new cutting materials that are extremely difficult to machine with conventional grinding methods.
ECDM (Electrochemical Discharge Machining)
ECDM is a controlled on-line electrochemical process that combines electrochemical machining (ECM) with electrical discharge machining (EDM). M. Schoepf et al. of Zurich, Switzerland, call it the ideal method for sharpening metal bond grinding wheels and cost-effectively grinding ceramic materials.
3 Conclusion In summary, the development of ceramic material processing technology, the application of various new abrasives and adhesives, especially the emergence of new grinding methods, have created conditions for the application of ceramic materials in a wider range of fields; The development of ceramic material grinding mechanism, especially the theoretical research of ceramic material removal mechanism has been greatly developed at home and abroad, and provides a theoretical basis for the further application of ceramic materials. At present, research in the fields of non-stationary grinding and non-invasive grinding has also attracted people's attention. It is foreseeable that in the near future, with the deepening of theoretical research and the emergence of new processing technologies, ceramic grinding technology will certainly be applied and promoted in more fields.
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