Research Status of Wear of Diamond Tools in Processing Hard and Brittle Materials and Its Influencing Factors
Keywords : diamond tool, wear
1. Introduction
With the advancement of science and technology and the development of modern industry, hard and brittle materials (such as laser and infrared optical crystals, ceramics, quartz glass, silicon crystals and stone) are increasingly used. Due to the high hardness and brittleness of hard and brittle materials, their physical and mechanical properties, especially toughness and strength, are quite different from those of metallic materials. Therefore, these materials are difficult or even impossible to process by ordinary processing methods. Diamond is the hardest material known in nature, and its excellent properties make it a promising field in the field of hard and brittle materials processing. At present, the use of diamond tools for cutting and grinding hard and brittle materials is still an effective processing method, such as cutting stones with diamond cutting tools, grinding ceramics with diamond wheels. 
Diamond tools for processing hard and brittle materials mainly include various diamond saws and diamond grinding wheels. Although the application range and processing characteristics of various tools are different, the wear mechanism is almost the same. Because the wear of diamond tools has a great influence on the processing quality and processing of the workpiece, the wear performance of the tool is an important indicator to reflect the tool performance and process parameters. Therefore, the research on the wear mechanism of the diamond tool is reasonable for guiding the diamond tool. Reasonable selection of manufacturing and process parameters is of great importance. For a long time, many scholars at home and abroad have devoted themselves to the research on the wear mechanism of diamond tools, and have achieved gratifying results. 
2. Research on Wear Mechanism of Diamond Tools
When a hard and brittle material is processed with a diamond tool, the tool inevitably wears due to severe friction, high temperature, etc., and wear is a very complicated process. 
(1) Three stages of wear
The wear of the diamond tool consists of three phases: an initial rapid wear phase (also called a transition phase), a steady wear phase with a wear rate of approximately constant, and a subsequent accelerated wear phase. The accelerated wear phase indicates that the tool cannot continue to work and needs to be reworked. 
(2) Abrasive wear form
Abrasive wear forms can be divided into: integral abrasive grains, micro-crushed abrasive grains, macro-crushed abrasive grains, abrasive particles falling off and abrasive grain smoothing. The proportion of these forms of wear depends on the different stages of wear, the tools used and the materials being processed. TW Liao et al. quantitatively studied the wear of diamond grinding wheels when micro-feed grinding of structural ceramics and pointed out that the wear of the grinding wheel was different during the transition phase and the stable wear phase. The wear during the transition phase depends not only on the specifications of the grinding wheel, the material properties and the grinding conditions, but more importantly on the method of preparation of the grinding wheel. In the transitional stage, since the grinding wheel has just been trimmed, the abrasive grains protrude the most, and many abrasive grains do not participate in the cutting, so the proportion of the overall abrasive grains is higher than the stable phase; at the same time, the trimming causes many abrasive grains to protrude too large, and the holding force is insufficient, grinding The proportion of particle shedding is higher than that of the stable phase; in addition, the dressing weakens some of the abrasive particles, making the proportion of micro-crushed abrasive particles higher than the stable phase. Abrasive wear in the steady stage is mainly friction and wear. The lower proportion of slightly broken abrasive grains and the relatively high friction and wear ratio indicate that the grinding wheel does not appear sharp, which is unfavorable for processing. Qiu Zhongjun of Harbin Institute of Technology and others used alumina grinding wheels to grind alumina ceramics. It was pointed out that the wear of the grinding wheel was mainly abrasive wear and abrasive falling off. SY LUO studied the diamond saw blade cutting granite, pointing out that the failure of the saw blade is mainly due to the macroscopic fracture of the abrasive grains and/or the abrasive particles falling off. When the number of broken and falling abrasive grains exceeds 1/3, the tool fails. Yu Yiqing of Huaqiao University, through a large number of follow-up observations and SEM analysis of the diamond tool surface and diamond bond process in the process of cutting stone, concrete and other materials, shows that the longer the overall abrasive grain is kept, the easier the processing is, the tool wear resistance The higher the degree of macro-crushing, the higher the diamond cutting edge height, and even the loss of cutting ability; after a long period of tool cutting, the abrasive grains are naturally blunt, the height of the blade is reduced, and the wear of the bonding agent around the abrasive particles is further The holding power of the diamond is weakened, causing the diamond to fall off. Xu Xipeng et al. studied the wear of diamond segments when sawing granite. It is believed that the actual wear process of diamonds needs to go through different paths: from the complete crystal form, after micro-crushing to macro-crushing, and finally falling off; This depends on factors such as the quality of the diamond, the load it is subjected to, and the binder. The wear process of the metal bond used for the diamond-retaining carrier has more complicated tribological properties. Granite chips and diamond chips erode the surface of the bond under the action of the cutting fluid, forming erosion marks on the surface, and the binder is abraded under fluid-like grinding conditions. The crater of the diamond front end shows that the fluid forms air pockets during the sawing process and causes erosion. In addition, the binder is also scraped and ploughed by hard spots in the granite, which is similar in texture to the texture in metal grinding. 
(3) Diamond tool wear mechanism
The wear mechanism of diamond tools generally has the following types: friction wear, abrasive particle breakage, binder fracture, abrasion wear, surface fatigue and impact, etc. Friction and wear make the abrasive grains blunt and smooth, and the binder breaks off, causing the abrasive particles to fall off. Abrasive wear promotes abrasive detachment due to weakened bond strength. The wear mechanism of the different processing processes is different. For example, when the structural ceramics are ground by the micro-feed of the diamond grinding wheel, the wear of the diamond grinding wheel mainly has three forms: friction wear, abrasive particle breakage and binder fracture; Another form of wear is the wear of the bond. Yiyang Zhou et al. used ceramic bond grinding wheels to grind sapphire. It is believed that the wear of ceramic bond grinding wheel is mainly brittle fracture of the bond, while the wear of metal and resin bond grinding wheel is mainly wear and abrasion. The author of the literature [5] cuts granite by using a diamond saw blade, pointing out that there are four main wear mechanisms of the tool: friction wear, abrasion wear, surface fatigue and impact. Xu Xipeng et al. studied the wear of diamond segments when sawing granite. It is believed that diamond is subjected to the direct friction and wear of granite, and is also impacted and corroded by granite cutting debris. The type of wear can be summarized as: grinding Particle wear, impact wear and erosion caused by solid particles in the fluid. The XP XU and the like quantitatively analyze the load acting on the abrasive grains and the temperature of the cutting zone. It is considered that the abrasive particle crushing is mainly caused by the impact force, but the temperature change is also important because it causes thermal fatigue damage and thermal stress.
3. Factors affecting the wear of diamond tools
(1) Tools
The diamond grade, content, particle size, matching of the bond to the diamond and the shape of the tool are factors that are important to the tool wear. 
Generally, the diamond content is low and the power consumption is low. However, the diamond content is too low, and the macro-crushing will increase sharply, resulting in insufficient blade height and increasing power consumption. When the diamond content is high, the power consumption is increased, which leads to diamond shedding and tool resistance. The grinding is actually reduced. If the diamond grade is higher, at a lower content, the complete crystal form ratio is still higher, the block wear resistance is high, and the power consumption is low, but the diamond grade should be matched with the binder selection. Wu Jian et al analyzed the influence of diamond quality dispersion on the sawing process. It pointed out that the high-quality diamond has good wear resistance, and the binder must also have good wear resistance. Only in this way can the full-quality diamond be fully utilized. For low-quality diamonds, because of their poor resistance to pressure and impact, even when cutting easier mineral components, more obvious wear and tear will occur. When it is particularly difficult to cut, macro-cracking will generally occur. When the cutting ability is lost, the requirement for the wear resistance of the bonding agent should be relatively low at this time to ensure that the diamond has sufficient edge height. The literature [10] also pointed out that the quality dispersion of diamond should be minimized. In [8], by analyzing the spindle deformation during the processing of sapphire by ceramic bond diamond grinding wheel (deformation shows the change of normal load required for grinding), it is found that the deformation changes periodically, indicating that the grinding wheel has self-sharpness, because The wear of the ceramic bond wheel is a brittle fracture of the bond material, so that new abrasive particles can quickly appear. The wear of metal and resin bond grinding wheels is mainly friction wear and abrasion. YC Fu et al. gave an optimized model of grinding wheel grinding. Through this model, the grinding wheel (including the size, concentration, elongation and effective abrasive grain space) can be optimized according to the processing requirements and grinding parameters. Grinding wheels and machining requirements optimize grinding parameters (including grinding depth, grinding wheel speed and workpiece feed rate). In addition, many scholars have conducted research in this area. 
(2) Processing conditions
Since the wear of the diamond tool is closely related to its load state, the processing conditions have a significant effect on wear. Dai Xiangguo et al analyzed the influence of process parameters on the life of the grinding wheel when the diamond grinding wheel cuts the engineering ceramics. It is considered that when other parameters are fixed, there is an optimal grinding wheel speed for each cutting depth; when the cutting depth and the grinding wheel speed are constant, there is one The optimum feed rate; the order of the influence of each process parameter on the radial wear of the grinding wheel is: grinding wheel speed - cutting depth - feed rate. The literature [17] pointed out that the tool wear has a great relationship with the processing parameters and the coordination of the processing parameters, so the process parameters must be optimized. ZJ Pei analyzed the situation when polishing the silicon wafer and concluded that when the grinding wheel speed is 4350 rpm and the chuck rotation speed is 590 rpm, the grinding force gradually increases, and when it reaches a certain peak value, it returns to a lower value. And continue to follow this cycle, indicating that the grinding wheel gradually becomes dull, when the grinding force reaches a certain value, the abrasive grains are broken; when the grinding wheel speed is 2175 rpm and the chuck rotation speed is 40 rpm, the grinding wheel is broken when the grinding force is small. Therefore, the grinding force is substantially constant and the wear rate is high.
(3) Materials to be processed
Different workpiece materials have different fracture toughness and hardness, so the properties of the workpiece material also affect the wear of the diamond tool. In [19], the wear of the cutters when cutting different stone materials with a diamond circular saw was studied. It was pointed out that when cutting hard granite, the wear of the diamond circular saw is mainly the macroscopic fracture and shedding of the abrasive grains, because the holding power is not enough. The cutting conditions are poor; when the soft granite is cut, the micro-crushing of the intact crystal form and the abrasive grains accounts for a large proportion, and the proportion of the abrasive particles falling off is still high, and the wear keeps the saw blade in cutting ability. Xu Xipeng believes that the nature and deformation mechanism of different minerals in granite are the key to determining the way diamond fails. The higher the quartz content, the more intense the diamond wear; if the content of feldspar is significantly higher, the sawing process is relatively difficult; under the same sawing conditions, the coarse-grained granite is finer than the fine-grained granite. It is more difficult to cause cleavage fracture. 
4. Conclusion
Domestic and foreign scholars have done a lot of research on the wear mechanism of diamond tools and its influencing factors, and have made remarkable achievements, which are of great significance for guiding processing practice and further research. At the same time, there are still many problems in the field of diamond tools for processing hard and brittle materials, such as the microscopic mechanism of research tool failure, the theoretical model for establishing the life of diamond tools, etc., and the researchers are making unremitting efforts to solve them.