Hard and brittle materials refer to materials having high hardness and high brittleness, usually non-conductors or semiconductors, such as various stone materials, glass, silicon crystals, quartz crystals, cemented carbides, ceramics, and the like. With the development of science and technology and modern industry, the application fields of hard and brittle materials are expanding, and hard and brittle material processing technology is also developing. Cutting processing plays an important role in various hard and brittle material processing methods. For example, in the processing of architectural decorative panels and precision parts of rock materials, sawing is the first process of machining, and the cost of sawing processing accounts for more than 50% of the total processing cost. At present, the cutting process of hard and brittle materials such as stone mainly uses various diamond cutting tools. Since diamond is the hardest substance known in nature, its excellent performance determines its broad development prospects in the field of cutting and processing of hard and brittle materials such as stone.
The processing methods of using diamond tools for sawing hard and brittle materials are: circular saw blade cutting, diamond band saw cutting, diamond frame saw cutting, diamond bead saw cutting and the like. Although each method has its own different characteristics and application range, its cutting mechanism and diamond wear mechanism are almost the same. Since rock cutting is the most important use of diamond cutting tools, it is of great significance to study the stone sawing mechanism and the wear mechanism of diamond cutting tools for the rational manufacture and correct use of diamond cutting tools. For a long time, experts and scholars at home and abroad have done a lot of experiments and research on the processing mechanism of diamond tool sawing granite, the wear mechanism of diamond tools and the sawing force in the sawing process, and have achieved remarkable results. Processing and research and development of diamond tools have played a positive theoretical guiding role.
Study on the sawing mechanism of 2 diamond cutting stone
Diamond abrasives are typically made into a cutting tool by sintering or electroplating. The cutting process of the diamond tool is similar to the grinding process, but due to the influence of the material, the processing mechanism of the hard and brittle materials such as rock and ceramic is different from the metal working mechanism, and the processing process is more complicated. Since diamond cutting tools were first used in stone cutting, there are many researches on the mechanism of diamond cutting stone. Domestic and foreign scholars have carried out long-term research on the processing mechanism of diamond tool sawing granite: from the early application of rock under the intrusion of the intrusion theory, the single-grain diamond scratch surface morphology observation method has gradually developed to the comprehensive application of polarized light microscope and scanning electron microscope Observe the surface morphology of the rock processing and the law of crack generation and propagation, and evaluate the cutting state of the rock with acoustic emission signals. However, due to the complicated cutting state and cutting process of hard and brittle materials such as rock, the research on the cutting mechanism has not yet formed a unified understanding.
Similar to the grinding process, the mechanism of action between a single diamond particle and a stone during cutting is first studied. Early experimental studies have shown that when single-grain diamond is used to cut granite under different conditions, the failure mode of rock is mainly brittle fracture; meanwhile, plastic deformation is still generated in rock according to different mineral composition.
Figure 1. P.Bienert model of single-grain diamond-cut rock
Figure 2: Single-grain diamond-cut rock model improved by M.Meding
P. Bienert proposed a model for single-grain diamond-cut rock in a doctoral thesis on concrete processing (see Figure 1). The model summarizes the process of sawing the rock as follows: (1) In front of the diamond particles, due to the shearing action caused by the compressive stress, the rock material is broken, forming the main chip, and is collapsed and extruded into the cutting zone; Below, due to the high pressure and possible temperature effects, the rock material is plastically deformed to form secondary chips, forming a smooth surface in a certain thin layer; 3 behind the abrasive grains, due to sudden elastic stress release, resulting in larger chips The formation consists of loose blocky chips and secondary chips. The P.Bienert model describes the chip formation process of diamond abrasive cutting rock in detail, but fails to study the stress distribution in the cutting zone and the law of crack generation and expansion, and does not reflect the front and lower pressure of the blade. The situation of the entity.
M.Meding improved the P.Bienert model (see Figure 2). It is believed that there are three deformation zones in the cutting process: 1 The first deformation zone is located in front of and in the vicinity of the abrasive grain. The compressive stress generated by the negative rake angle causes shear failure of the rock, and the fragmented rock particles fly out from the front of the abrasive grain, and the rock is pressed to both sides of the abrasive grain. 2 The second deformation zone is located below the abrasive grains. For limestone and marble, a plastic deformation zone is formed on the surface in contact with the abrasive grains, the surface of the workpiece is smooth (mainly caused by compressive stress), and the strong plastic deformation is only a few microns thick; the granite will also be produced under the action of high temperature and high pressure in the contact zone. Local plastic deformation. 3 The third deformation zone is located behind the abrasive grain. In the vicinity of the abrasive grains, some tails composed of fine rock particles are formed. It is inferred from the test results that the surface stress of the scratches is changed from compressive stress to tensile stress after the abrasive grains are scratched.
Many scholars in China have also studied the sawing mechanism of granite and other stone materials. Xu Xipeng et al. observed through scanning electron microscopy of the sawing surface of granite that the fracture forms of quartzite are mainly intergranular fracture and transgranular fracture, and the deformation mode is mainly determined by the deformation mode of quartz, the main component of rock; other granites are mainly The structural components are quartz, feldspar and plagioclase, so the deformation characteristics are determined by the three. Among them, the dissociation of mica is the most complete and most easy to remove, followed by feldspar and plagioclase, while quartz hardly undergoes dissociation and fracture, so it is the most difficult to cut. The extrusion action of diamond cutting granite will cause brittle fracture of granite. This is because there are various defects and stress concentrations in the granite stone, which cause cracks to occur and expand under the action of extrusion, leading to brittle failure of granite.
As a non-destructive testing method, acoustic emission measurement has been used for damage and wear monitoring of cutting tools, metal and rock fracture process analysis. Some studies believe that the root mean square value (AErms) of acoustic emission has a good correspondence with the machinability of rock. The hardness of rock is proportional to the value of AErms. Tests have shown that the larger the AErms value, the worse the processability of sawing a rock with a diamond circular saw. Wang Chengyong used DIN50103 to measure the single-grain grinding test on the TypFP3NC milling machine with Rockwell hardness diamond indenter. The relationship between acoustic emission signal and grinding depth, rock type and mineral composition was analyzed. Studies have shown that the acoustic emission signal of single-grain diamond grinding granite is affected by factors such as granite type, mineral composition and grinding depth. When grinding and sawing granite and quartz with good machinability (or large grinding depth), the average value of AErms is large, and there are many signals in the high peak range. The AErms value also reflects the fracture mode during the grinding process. For granite, the average value of AErms is small, and the signal in the low peak range is large, indicating that the micro-crush component is high and the crushing energy consumption is high.
Although people have done a lot of research on the mechanism of stone sawing from different angles, because the rock sawing process is quite complicated, people's understanding of the physical nature of the sawing process needs further study. The rock sawing process is like a black box, and the correspondence between input and output parameters can only be established by a suitable measuring instrument. Therefore, some sawing models currently established reflect the law of the sawing process to a certain extent, but they cannot fully explain the physical nature of the sawing process.
1. Friction between substrate and chip 2. The substrate is abraded by chips and flakes 3. First sheet area 4. Stone and abrasive grain friction 5. Plastic deformation 6. Elastic deformation
Figure 3: Mechanical action between the tool and the workpiece when cutting the stone
Study on the wear mechanism of 3 diamond tools
When diamond tools are used to saw hard and brittle materials such as stone, diamond abrasives and substrates inevitably wear out due to high pressure, severe friction and possible high temperature effects. The wear and tear of the diamond abrasive grains and the wear of the substrate determine the sawing effect and tool life. Balogh pointed out that the main factors affecting the life and efficiency of diamond saw blades include cutting speed, characteristics of the material being cut, saw blade quality and operator skill level. LiaoY.S. studied the wear characteristics of diamond sintered blocks when diamond circular saw blades were used to cut granite. Studies have shown that the failure modes of the agglomerates are mainly erosion corrosion, cavitation corrosion and wear. British scholar LuoS.Y. has conducted fruitful research on the wear of diamond circular saw blades. He conducted an experimental study of diamond saw blade wear as early as 1991. The saw blade used in the experiment has a diameter of 205 mm, a core thickness of 5 mm, a diamond sintered block size of 40×7×10.5 (mm), a saw blade outer circle speed of 30 m/s, a feed speed of 1 m/min, and a cutting depth of 0.2mm; the coolant is water; the workpiece material is Indian red granite; the wear of the diamond abrasive grains is analyzed by SEM, and the wear amount and force of the saw blade are measured. The experimental results show that the etched pit on the diamond surface of the tool block is very small, and the wear of the working surface is mainly caused by microscopic damage and polishing of the diamond particles. At this time, the cutting force is small and the saw blade is relatively wear-resistant. On the contrary, when there are a large number of expandable pits on the surface of the diamond, the wear form is mainly microscopic breakage and particle extraction, at which time the cutting force is large and the saw blade is not wear-resistant. In 1996, LuoS.Y. further studied the wear characteristics of diamond during circular saw cutting. The research shows that the failure of the saw blade is mainly caused by the damage and extraction of the abrasive grains. When more than one third of the abrasive grains are damaged or pulled out, When it comes out, the cutting efficiency is significantly reduced, and even in severe cases, the saw blade fails. HKTönshoff and J.Asche studied the wear of diamond tools when cutting stone and established a model of a single diamond-cut stone. The model describes the mechanical action between the tool and the workpiece when cutting a stone with a diamond circular saw blade (see Figure 3), including the elastic and plastic deformation of the workpiece under the action of cutting forces, the friction between the stone and the diamond, and between the stone and the substrate. Friction, friction between the chip and the substrate, etc. Research suggests that the wear mechanism of diamond can be divided into four types: 1 adhesive wear: diamond adheres to the surface of the stone and is cut off a part; 2 friction and wear: extremely hard particles in the rock scratch the diamond surface; 3 diffusion wear: The chemical reaction between the workpiece and the diamond reduces the strength and hardness of the diamond; 4 abrasive particle breakage: diamond breakage caused by mechanical overload, thermal overload or fatigue.
Many scholars in China have also studied the wear and failure modes of diamond saw blades. Song Yueqing and others from Beijing Nonferrous Metals Research Institute analyzed the relationship between the wear behavior of diamond particles and the wear properties of tool carcass and the tool cutting performance by observing the cutting performance and wear surface morphology of different diamond saw blades. The effect is that the diamond that is smoothed or polished in the tool is not good for the cutting performance of the tool, and the increase of the new blade and the micro-crushed diamond particles is beneficial to improve the cutting performance of the tool. Yang Weiguang and others from the Beijing Institute of Powder Metallurgy studied the wear mechanism of diamond tools. Scanning electron microscopy observations show that the wear of diamond tools includes both minor wear and severe wear. Slight wear includes diamond furrow wear, delamination wear, and pitting wear; severe wear includes diamond fracture wear, carcass and diamond interface extrusion, relief and overall detachment. Studies have shown that the edge height h of the diamond tool decreases with slight wear and increases with severe wear. The severe wear of diamonds increases processing efficiency but affects tool life. Wang Dian of Central South University of Technology will study the wear mechanism of diamond saw blades when cutting hard stone. Studies have shown that diamond saw blades have four wear mechanisms: impact shear, fatigue failure, particle extraction and heat effects. Surface erosion is caused by thermal influences. Impact shearing and fatigue can cause microscopic fracture of diamond particles, and particle extraction increases the cutting force of individual particles. Xu Xipeng of Huaqiao University, through the study of the wear mechanism of diamond tools, believes that diamond is subjected to the direct friction and wear of granite, and is also affected by the impact and wear of granite cutting debris. Therefore, the wear type of diamond can be summarized as abrasive grains. Wear, impact wear and impact wear caused by solid particles in the fluid. The actual wear process of diamond can go through different paths, starting from the complete crystal form, undergoing micro-crushing to macro-crushing, and finally falling off, or it can be detached from the beginning. The specific method of wear depends on the quality of the diamond, the load it is subjected to, and the properties of the binder. In addition, many scholars have studied the friction and wear characteristics of diamond saw blades when cutting stone, and obtained some valuable conclusions.
4 sawing force research
In the process of stone sawing, the sawing force is a very important parameter. The size of the sawing force not only determines the power of the machine tool, but also determines the load on the tool, which determines the sawing ability of the tool. Since the sawing force of the diamond tool is the sum of the sawing forces acting on each diamond particle, it is necessary to study the relationship between the sawing force, the chip and the diamond abrasive grain geometry, and to study the process parameters for a single diamond. The effect of the pellet and the cutting performance of the entire tool.
In an early study of sawing force, Tönshoff obtained the force on a single diamond abrasive grain and the relationship between the size of the abrasive particles, the feed rate and the feed pressure. Tönshoff believes that the ratio of feed pressure to tangential force is 5 to 15, so the feed pressure is the main sawing force component; the larger the abrasive grain, the greater the feed pressure acting on the abrasive grain; As the amount increases, the feed pressure will decrease, which is due to the increase in the chip breaking area, which causes the diamond abrasive grains to break, and the self-sharpening effect makes the abrasive grains sharper.
Chen Xian believes that the sawing force includes the fracture resistance of the rock, the friction between the diamond and the rock, the friction between the sawdust and the diamond and the metal carcass. Obviously, the fracture resistance of rock is related to the physical properties, chemical composition, mineral composition and sawing process parameters of the rock. Although the crushing mechanism of granite is not completely clear, it is generally believed that the formation process of sawdust is brittle and the energy consumed is not large, so the crushing resistance component is very small, accounting for only about 15% of the sawing force component. The power loss caused by friction is about 82% to 87% of the sawing power. Xu Xipeng's research on sawing force also proves this point. The results show that the fracture energy and chip kinetic energy during the sawing process are negligible, and the energy is mainly consumed in friction.
Jerro et al. used the finite element method to analyze the cutting process of diamond saw blades and established a cutting force calculation model for cutting hard and brittle materials. In addition to the processing technology and tool parameters, such as the peripheral speed of the saw blade, the feed rate, the saw blade diameter, the cutting depth, the size of the abrasive grain, the density and distribution of the diamond agglomerate, the model also includes the workpiece material performance parameters, such as Modulus of elasticity, Poisson's ratio, etc. The finite element method can be used to first calculate the cutting of a single abrasive grain, and then calculate the cutting force of a single agglomerate and the entire saw blade.
Due to the complexity and randomness of the sawing process, the research on sawing force is mostly based on experiments and empirical formulas based on experiments, and theoretical research is relatively rare. Zhou Canfeng of Beijing Institute of Petrochemical Technology theoretically studied the sawing force of diamond circular saw blades when sawing stone. By referring to the relevant formula derived by G.Wener, the relevant theoretical formula was obtained. The formula reflects the microstructure distribution of the workpiece material. The effect of uniform and non-linear characteristics on sawing force, but does not consider the particularity of sawing brittle materials.
5 Conclusion
In summary, so far, people have done a lot of research on the sawing mechanism of hard and brittle materials such as diamond tool sawing stone and the wear mechanism of diamond tools, and have achieved many results. However, due to the complexity of the sawing process, most of these studies are in the exploratory stage. There are also many major theoretical issues such as the microscopic mechanism of the sawing process, the theoretical calculation of the sawing force, the microscopic wear mechanism of the diamond abrasive grains, the diamond abrasive grains and sintering. Micro-interface analysis of the substrate or electroplated coating is urgently needed to be further explored and studied.
Figure 1. P.Bienert model of single-grain diamond-cut rock 
Figure 2: Single-grain diamond-cut rock model improved by M.Meding 
Figure 3: Mechanical action between the tool and the workpiece when cutting the stone Study on the wear mechanism of 3 diamond tools When diamond tools are used to saw hard and brittle materials such as stone, diamond abrasives and substrates inevitably wear out due to high pressure, severe friction and possible high temperature effects. The wear and tear of the diamond abrasive grains and the wear of the substrate determine the sawing effect and tool life. Balogh pointed out that the main factors affecting the life and efficiency of diamond saw blades include cutting speed, characteristics of the material being cut, saw blade quality and operator skill level. LiaoY.S. studied the wear characteristics of diamond sintered blocks when diamond circular saw blades were used to cut granite. Studies have shown that the failure modes of the agglomerates are mainly erosion corrosion, cavitation corrosion and wear. British scholar LuoS.Y. has conducted fruitful research on the wear of diamond circular saw blades. He conducted an experimental study of diamond saw blade wear as early as 1991. The saw blade used in the experiment has a diameter of 205 mm, a core thickness of 5 mm, a diamond sintered block size of 40×7×10.5 (mm), a saw blade outer circle speed of 30 m/s, a feed speed of 1 m/min, and a cutting depth of 0.2mm; the coolant is water; the workpiece material is Indian red granite; the wear of the diamond abrasive grains is analyzed by SEM, and the wear amount and force of the saw blade are measured. The experimental results show that the etched pit on the diamond surface of the tool block is very small, and the wear of the working surface is mainly caused by microscopic damage and polishing of the diamond particles. At this time, the cutting force is small and the saw blade is relatively wear-resistant. On the contrary, when there are a large number of expandable pits on the surface of the diamond, the wear form is mainly microscopic breakage and particle extraction, at which time the cutting force is large and the saw blade is not wear-resistant. In 1996, LuoS.Y. further studied the wear characteristics of diamond during circular saw cutting. The research shows that the failure of the saw blade is mainly caused by the damage and extraction of the abrasive grains. When more than one third of the abrasive grains are damaged or pulled out, When it comes out, the cutting efficiency is significantly reduced, and even in severe cases, the saw blade fails. HKTönshoff and J.Asche studied the wear of diamond tools when cutting stone and established a model of a single diamond-cut stone. The model describes the mechanical action between the tool and the workpiece when cutting a stone with a diamond circular saw blade (see Figure 3), including the elastic and plastic deformation of the workpiece under the action of cutting forces, the friction between the stone and the diamond, and between the stone and the substrate. Friction, friction between the chip and the substrate, etc. Research suggests that the wear mechanism of diamond can be divided into four types: 1 adhesive wear: diamond adheres to the surface of the stone and is cut off a part; 2 friction and wear: extremely hard particles in the rock scratch the diamond surface; 3 diffusion wear: The chemical reaction between the workpiece and the diamond reduces the strength and hardness of the diamond; 4 abrasive particle breakage: diamond breakage caused by mechanical overload, thermal overload or fatigue. 
