After experiment comparison, in processing zirconia ceramic, vitrified bond diamond grinding wheel for ceramic is applied with high processing efficiency, the good surface quality of the workpiece and low cost.
With excellent features of low density, high strength, high hardness, low expansion coefficient, wear resistance, corrosion resistance, heat insulation, and chemical stability, zirconia ceramic material is widely used in the fields of energy, aerospace, computer, mechanical processing, etc. It is a kind of ceramic material commonly used for engineering materials and functional materials.
Zirconia ceramic has many excellent mechanical properties, and the orthorhombic, tetragonal, and cubic phases are reversible transitions. This is the basis for the development and application of zirconia as a kind of new material. With toughening and excellent mechanical properties, tetragonal zirconia is the best material for structural ceramic.
It is mainly used in engine and internal combustion engine parts, pumps, bearings, seals, tubes, fiber optic plugs, bioceramic hip joints, dental ceramic, engine parts, textile screw guides, covers and caps of the press, blades, scissors, cutting tools and bulletproof armor in the military. In the adiabatic internal combustion engines, ductile zirconia can also be used for cylinder liners, piston crowns, valve guides, intake and outtake valve seats, bearings, cams, piston rings, and etc.
Cubic zirconia features oxygen ion conduction and is the best material among the 4 oxygen ion conductors. It is mainly used as solid oxide fuel dielectric, oxygen sensor, WHD electrode, filter, converter, resonator and other buzzers, the catalyst for industrial and automotive exhaust.
However, zirconia ceramic materials feature high brittleness, low fracture toughness, and material elastic limit closed to strength, and it is not good for the processing of ceramic materials. It is difficult to effectively grind for the current processing methods. Developing the processing tools with high efficiency, high quality and low cost have become a hot spot in the ceramic grinding field in recent years.
2 Zirconia ceramic processing method
Zirconia ceramics have the features of both engineering ceramics and functional ceramics. As shown below Table 1
Table 1 Main Performance of Zirconia Ceramic
|Fracture toughness/(MPa/m1/2 )||7.5-15||16-9.5||6||4.6|
|Coefficient of thermal expansion/(10-6℃)||9.2||9.2||10.2||—|
|Coefficient of thermal conductivity/(w·m-1·K-1)||0.5-2||0.5-2||0.5-2||0.5-2|
Grinding is the main method for ceramic processing, the methods as follows:
2.1 Ductile grinding technology
Shen JY put forwarded the ductile grinding technology and established the critical cutting thickness model in the field of brittle material grinding by the fracture propagation criterion and the microhardness indentation method, and it controls the surface cracks of ceramic grinding better. This technology mainly applied to high-rigidity and the high-resolution grinding machine makes the brittle material in a ductile mode remove by controlling the grinding depth. That is, the grinding mechanism of the brittle material changes from the original brittle fracture to the plastic flow. Choose the suitable grinding parameters and features of grinding wheel to get the better-machined surface, a kind of nano-scale grinding technology.
For example, Dr. Du’s grinding test of zirconia ceramic with W0.5 grain size diamond grinding wheel shows that the surface roughness Ra=3nmof ultra-smooth mirror can be obtained at the grinding wheel linear velocity Vs=11.8m/s and the feed speed V1=40mm/min. Zirconia ceramic workpiece with thinner thickness causes cracks due to small vibrations when conventional grinding. While ductile grinding is a way of non-destructive processing for ceramic materials. In the subsequent processing of ceramic materials, it is a trend for applying the combination of ultrasonic and ductile grinding.
2.2 Laser preheating assisted grinding
Purdue University scholars put forwarded laser preheating assisted grinding. This method focuses a laser beam with sufficient energy and then irradiates it to the suitable part of ceramic material, preheat it in a short time to make the ceramic surface vaporized or melted. It causes the metallographic structure to change and produces the large thermal stress, and improves the fracture toughness to makes the transition from brittle damage to plastic deformation, thereby realizing the removal of the surface material of the workpiece.
For example, Dr. Luo pointed out that the fixing of the mold is not required in the process of ceramic material. Non-contact grinding is applied. It can process the three-dimensional complex shape materials by controlling the focus position of the laser beam on the surface of the ceramic material.
In the grinding test of zirconia ceramic, when the grinding speed is Vs=25m/s, the feed rate is V=2mm/s, and non-laser preheating, Ra=0.318μm. While laser preheating, roughness is Ra=0.208μm. In the laser preheating, zirconia tends to change from brittle to plastic. And it can reduce the microcrack expansion due to brittleness, improving the roughness of hard and brittleness material.
2.3 ELID grinding technology
It is a kind of machining method for precision ceramic. It mainly applies cast iron fiber-based diamond grinding wheel with fine abrasive grain. It makes the metal bond on the surface of the grinding wheel continuously ionize and dissolve by the weak metallization. The produced passivation film can make the grinding debris not stick to the grinding wheel, and it can increase the self-sharpness of the abrasives.
Choosing a suitable bond can effectively achieve mirror grinding. Later, some scholars improved their technology and used ELID grinding of ceramic materials with several micrometers or even submicron diamond grains, obtaining a mirror with high precision and low surface roughness, which can replace grinding and polishing within a certain range.
2.4 Ultrasonic grinding
Ultrasonic vibration tools are used for ultrasonic grinding, driving the abrasive suspension between the workpiece and the tool, impacting and polishing the workpiece, making the materials in the local part etched away to the powder, thereby achieving the purpose of cutting, grinding and polishing through the hole. There is no microstructure change, residual stress, and burns on the surface of the ceramic workpiece after the technology processing. It is suitable for precision machining of various shapes, sizes and complex requirements, non-conductive hard and brittle materials due to the relatively small macroscopic force during machining.
Dr. Chai did the study of zirconia ceramic grinding by rotary ultrasonic technology from abrasive grain size, abrasive speed, feed rate, and grinding thickness. The study shows that the method features high removal rate of ceramic materials, high processing precision, and high economic profits. When the abrasive grain size is 240 mesh, abrasive speed is 1000mm/min, feed speed is 3mm/s, grinding thickness is 0.1mm, the surface roughness of ceramic R a is 3.2μm.
Compared with electric spark, electrolysis, and laser processing technology, ultrasonic grinding is not dependent on the conductivity of materials and thermophysical. It is especially suitable for micro-machining of hard and brittle materials such as zirconia.
3 Diamond grinding wheel for ceramic (zirconia)
Zirconia ceramic is high hard and hard-to-machine material. Diamond grinding wheels are used for grinding zirconia ceramic. At present, there are mainly resin bond, vitrified bond, metal bind and electroplated bond for the diamond grinding wheel at home and abroad. The corresponding bond diamond grinding wheel is used for different ceramic materials and processing requirements.
3.1 Electroplated diamond grinding wheel is usually used for special purpose grinding.
It is mainly used for the production of special-shaped abrasive with small size and irregular shape. And the coating is extremely thin and life is relatively short. In addition, the range of the roughness of electroplated diamond abrasive tools is large, and the precision is relatively low. Dr. Meng using a ring-shaped electroplated diamond grinding wheel with length 2.25mm, diameter 0.5mm, and abrasive grain size 0.64μm for grinding LT55 ceramic under certain process parameters, the surface morphology was complete, and the roughness value could reach 0.172μm.
Dr. Tong’s research shows that the surface roughness value is larger when the new electroplated grinding wheel grinding. After 160 hours, the surface roughness value can reach 1.6μm, and after 200 hours, the surface roughness value can reach 0.4μm. Therefore, the grinding wheel can be used for rough grinding from the new wheel to 300h and later can be sued for finish grinding.
3.2 Vitrified bond diamond grinding wheel features good heat and wear resistance, precision control easily, easy dressing, suitable for processing difficult-to-machined materials. With the improvement of technology, the emergence of high speed, high rigidity grinding machine and special grinding fluid, vitrified bond diamond abrasive tools are used more and more widely. But the vitrified bond has brittle and poor impact resistance, it can damage the workpiece due to improper control when processing. In the processing, double side vitrified bond diamond grinding wheel is used for zirconia ceramic materials, the average grain size of the diamond is 9μm, concentration is 100%, and the result shows that the roughness value reaches 0.164μm.
Some scholars have used 120# diamond grinding wheel for grinding RBSN. The test shows that the proportion of surface crack of the workpiece is reduced from 48% of 25/s to 12% at 170m/s. Kovach used a vitrified bond diamond grinding wheel for grinding ceramic at speed of 160m/s, achieving the high grinding ratio of 5100 times.
Dr. Tong’s research shows that in the initial stage of grinding, Al2O3 removal rate is highest and ZrO2 removal rate is lowest due to the sharp edge of the diamond grinding wheel under the grinding condition of constant pressure. Owing to the small fracture toughness value of Al2O3, it produces crack and fracture type debris when grinding, forms grain boundary damage, so the grinding removal rate is high. But the fracture toughness of ZrO2 is big, and the ability to resist crack progress is large, and it produces the damage of the plastic flow and the fracture-crush type, so the grinding removal rate is low.
Dr. Chen’s research shows that the average abrasive grain size of a diamond grinding wheel is an important factor affecting the surface roughness of ceramic. If diamond grinding wheel with an average abrasive grain size of less than 25μm is used for grinding, making the ceramic in a plastic mode for grinding to eliminate the surface crack defects.
3.3 There is a large amount of grinding heat when grinding zirconia ceramic. It will have resin carbonization black edge for resin bond diamond grinding wheel, and the service life is short, which increases the cost. It will block for metal bond diamond grinding wheel, and the abrasive grains cannot fall off in time after the blunt grinding, and the self-sharpness is worse.
In the processing of other engineering ceramic, research shows that diamond grinding wheel is used for grinding Si3N4 ceramic at speed of 160m/s, the grinding efficiency is doubled compared with that of 80m/s, and the life of the grinding wheel is 5 times when it is at 80m/s, and 7 times at 30m/s, and obtain good surface quality. The metal bond and resin bond diamond grinding wheel have low grinding speed, usually from 12 to 50m/s, and the grain size is 80-W40. Vitrified bond diamond grinding wheel is more suitable for high-speed grinding, up to 160m/s.
Resin bond, concentration is 50-100%, metal bond, concentration is 75-150%. High concentration is used for rough grain size, form grinding and high efficiency grinding, and the grain size of the vitrified bond is widely chosen.
In recent years, engineering ceramic and functional ceramic have become one of the key points of the development. Some scholars at home and abroad have made many types of research and discussions on the zirconia ceramic grinding, and have made the great process. But in the grinding process, processing efficiency needs to be further improved due to its features.
The vitrified bond diamond grinding wheel for ceramic features high strength, high hardness, and high precision, and it is used on high speed and high precision grinding machines. It will be the first choice for zirconia ceramic grinding in the future. In addition, ultra-high speed vitrified bond diamond abrasive tools are used for automated fine grinding zirconia ceramic materials on high-precision grinding machines, improving the precision of zirconia ceramic surface, reducing micro-cracks, and greatly improve the processing efficiency of zirconia ceramic and save costs.