Ceramic processing

Ceramic cutting fluid is a product used for cutting, grinding, drilling, and other processing of non-metallic materials. Specially used for the processing of non-metallic materials such as ceramic materials, zirconia, gemstones, jade and glass. MB-135, as a water-soluble fully synthetic cutting tool, has excellent machining effects on non-metallic materials at an affordable price and excellent performance.

How to choose cutting fluid

When selecting a metal cutting fluid, the first step is to make a preliminary judgment based on the cutting process conditions and requirements, and choose a pure oil-based metal cutting fluid or a water-soluble metal cutting fluid. Usually, we can choose based on the recommendation of the machine tool supplier; Secondly, it can also be selected based on conventional experience. For example, when using high-speed steel tools for low-speed cutting, pure oil-based metal cutting fluid is usually used. When using hard alloy tools for high-speed cutting, water-soluble metal cutting fluid can usually be used; For situations where it is difficult to supply fluids or the cutting fluid is difficult to reach the cutting area, pure oil-based metal cutting fluids (such as tapping, internal hole broaching, etc.) can be used. In other cases, water-soluble metal cutting fluids can usually be used. In summary, the specific cutting fluid type should be selected based on specific cutting conditions and requirements, the different characteristics of pure oil-based metal cutting fluids and water-soluble metal cutting fluids, and the different actual conditions of each factory, such as ventilation conditions in the workshop, waste liquid treatment capacity, and the use of cutting fluid in the previous and subsequent processes.

Wear and Lubrication of Ceramic Tools in Cutting

1. Ceramic cutting tools are characterized by high hardness, excellent wear resistance and high-temperature mechanical properties, good chemical stability, and are not easy to bond with metal,It can be widely used for cutting difficult to machine materials, ultra high speed cutting, high-speed dry cutting, and hard cutting. The optimum cutting speed of ceramic tools is 3~10 times higher than that of cemented carbide tools, which can greatly improve the cutting productivity. In the past three decades, due to the effective control of raw material purity and grain size in the manufacturing process of ceramic tools, the addition technologies of various carbides, nitrides, boride, oxides, whiskers or a small amount of metals have been developed, and a variety of toughening and reinforcement mechanisms have been adopted, which have greatly improved the strength, toughness, impact resistance, etc. of ceramic tools. But ceramic cutting tools are not omnipotent. Ceramic cutting tools are subject to high temperature and high pressure in the process of cutting, and will inevitably be worn or damaged to varying degrees. The existing research shows that each kind of ceramic tool has its specific processing range, and different ceramic tools (or the same kind of ceramic tools) will have very different wear patterns and tool life when machining different workpiece materials, so there is an optimal matching problem between ceramic tools and cutting objects. There have been some literature reports on this aspect both domestically and internationally, but due to different experimental conditions and research methods, there are also differences in the experimental results and research conclusions of different researchers.   

On the basis of the existing research of the author and referring to the relevant literature reports at home and abroad, this paper makes a comprehensive review of the wear and lubrication of ceramic tools in cutting and the optimal matching of ceramic tools and machining objects, with a view to playing a guiding and reference role in the research and development of new ceramic tool materials, the selection and wear control of ceramic tools in actual machining.   

2. Wear mechanism of ceramic tools during cutting  

In the cutting process of ceramic tools, there are always two friction pairs, that is, the friction pair between the rake face and the chip and the friction pair between the rake face and the workpiece. Among them, the former affects the wear of the front tool surface, while the latter affects the wear of the back tool surface and the quality of the machined surface. The wear of the front and back tool surfaces both affect the tool life. Ceramic tools are mainly used for high-speed cutting. The cutting temperature is often as high as 800~1000 ℃ (even higher), and the cutting pressure is also very high. Therefore, the wear of ceramic tools is the result of the combined action of mechanical wear and chemical wear, and its wear mechanisms mainly include abrasive wear, adhesive wear, chemical reaction, diffusion wear, oxidation wear, etc. The existing research shows that the wear of ceramic tools is closely related to the cutting conditions. When different ceramic tool materials process different workpiece materials under different cutting conditions, the dominant wear mechanism may be different. When cutting at low speeds, the wear mechanism often manifests as abrasive wear due to the lower cutting temperature; In high-speed cutting, adhesive wear, chemical reaction, oxidation wear, and diffusion wear caused by high temperature are the main causes.   

The author's research shows that the wear mechanism of Al2O3 based ceramic tool in continuous cutting steel parts is mainly abrasive wear with micro chipping edge and adhesive wear, while in cutting cast iron it is mainly abrasive wear. Wayne and Brandt et al. have studied the machining of Inconel 718 material with Al2O3/SiCw ceramic tools and concluded that abrasive wear and adhesive wear are the main wear mechanisms of ceramic tools under low speed cutting conditions; Under the condition of high-speed cutting, adhesive wear, chemical reaction wear and diffusion wear are the main wear mechanisms of ceramic tools. Because Inconel 718 material has high high temperature strength, large plastic deformation and severe work hardening, cutting force and cutting temperature are very high. When the cutting temperature is less than 900 ℃, the front face of the tool is mainly subject to adhesive wear; When the temperature reaches 1200 ℃, Ni begins to diffuse towards the center of the tool. Due to the diffusion of Ni, on the one hand, the surface hardness of the tool material decreases and its performance decreases; On the other hand, it increases the affinity between the tool and the workpiece, leading to increased adhesive wear. Therefore, cutting fluid must be used when machining Inconel 718 with Al2O3/SiCw ceramic tools (cutting fluid containing chlorinated paraffin is better).  

  Casto et al. concluded that the wear mechanism of the tool is mainly adhesive wear and abrasive wear by studying the processing of AISI 1040 material with Al2O3/ZrO2 ceramic tools, while there are serious chemical reactions on the tool surface when Si3N4 ceramic tools are used to process AISI 1040 steel. When machining AISI 4337 steel with Al2O3/ZrO2 and Al2O3/TiCN ceramic tools, the wear mechanisms of the front and rear tool surfaces are different. Chemical reactions and plastic deformation are the main reasons for the wear of the front blade surface, while the wear mechanism of the rear blade surface is caused by the fracture between ceramic particles, which leads to the detachment of ceramic particles. Brandt found the plastic deformation of the surface layer when cutting Al2O3 based ceramic tools, and believed that this was due to the reaction of Al2O3 with FeO (steel surface oxidation products) or MgO (ceramic additives) to form a spinel structure, or the interaction of Al2O3 with SiO2 and CaO to form a compound with low melting point and low hardness. The author's research shows that Al2O3/TiB2 ceramic tools have better wear resistance when machining high-strength steel and hardened steel. With the increase of TiB2 content, the wear resistance of the tools increases.  

For whisker toughened ceramic tools, the whiskers are directionally distributed on the plane perpendicular to the hot pressing axis during hot pressing, resulting in different distribution of whiskers on different surfaces. Therefore, the wear resistance of whisker toughened ceramic tools is related to the orientation of whiskers, θ= The wear resistance of the 0 ° surface is the worst, while θ= The 90 ° surface has the best wear resistance. When the wear of the cutting tool surface is the main factor, it should be selected θ= 90 ° surface as the back face of the tool; When the tool is mainly worn on the previous cutting surface, it should be selected θ= The 90 ° surface serves as the front face of the tool. When there is significant wear on both the front and rear cutting surfaces of the tool, it should be selected θ= The 45 ° surface serves as the front (back) face of the tool to improve its wear resistance.   

Si3N4 based ceramics have been used as tool materials since the late 1970s and have been widely used in the cutting of cast iron and nickel based alloys. The Si3N4 based ceramic tool mainly suffers from abrasive wear when cutting cast iron at high speed, and chemical wear when cutting carbon steel at high speed. Chemical wear itself generally accounts for a small proportion of the total wear of ceramic tools, but chemical action can greatly aggravate the degree of mechanical wear. For example, chemical dissolution and diffusion can cause the strength of ceramic surface to weaken, and increase the bond between the tool and the workpiece, leading to severe bond wear and micro fracture wear. When using Si3N4 ceramic tool to cut AISI 1045 steel, its wear rate is two orders of magnitude higher than that when cutting gray cast iron; The mutual diffusion of elements such as Fe and Si between the workpiece and the cutting tool during the cutting of cast iron is much smaller than that during the cutting of steel. When cutting steel, the wear of Si3N4 ceramic tools is mainly related to the chemical interaction between the tool and the workpiece. Due to the chemical dissolution of Si3N4 particles and their continuous removal from the glass phase, Si3N4 ceramic tools show a high wear rate. The high wear rate of Si3N4 ceramic tool when cutting steel is mainly attributed to the following two factors: ① the SiO2 layer formed on the tool surface due to Si3N4 oxidation is constantly removed; ② SiO2 forms a low melting point eutectic mixture with FeO on the surface of the workpiece. The chemical interaction between Sialon ceramic tool and iron base alloy has been specially studied. The results show that: β′- Sialon particles undergo chemical reactions with iron based alloys, and silicon and nitrogen dissolve and diffuse in the iron based alloys. The alloying elements in steel have a certain impact on the reaction activity between Sialon and steel. Elements such as nickel, silicon, carbon, and phosphorus can reduce the reaction activity, while elements such as chromium, molybdenum, titanium, and vanadium can increase the reaction activity.

Although the wear of ceramic tools is closely related to the cutting conditions, the main factors determining the wear characteristics of ceramic tool wear are still the composition and microstructure of ceramic materials. The basic phenomenon of ceramic tool wear is the fracture and transfer of materials, so the formation and propagation of cracks will have an important impact on wear. Ceramic tool materials are mostly multiphase ceramics, and there are glass phase, pores, impurities, etc. at the grain boundary, and there are thermal expansion mismatch and elastic modulus differences between the phases. The existence of grain boundary pores can lead to stress concentration, and as the source of cracks, pores will induce grain boundary cracks. Moreover, since pores mainly occur at grain boundaries, cracks extend to pores and connect with them, thereby accelerating the propagation of cracks. The research of Rice et al. shows that the increase of porosity greatly reduces the wear resistance of ceramic tools, and the excessive residual stress caused by the mismatch between elastic modulus and thermal expansion will lead to the cracking of materials without external load. Due to the fact that the additives added to polycrystalline ceramics mainly exist on the grain boundaries in the form of glass phase during the sintering process, under high temperature conditions generated by high-speed cutting, the viscosity of the glass phase decreases and plastic flow occurs, leading to grain boundary slip and stress concentration at the grain boundary boundary. If stress concentration causes complete plastic deformation of adjacent grains, it will cause stress relaxation. If it cannot adapt to the deformation of adjacent grain boundaries, stress concentration will cause cracks to occur at the grain boundaries. After the nucleation of cracks, as the degree of grain boundary slip continues to increase, cracks will occur. A large number of dislocations in the crystal of ceramic tool materials provide another way for crack nucleation. With the continuous wear process, dislocations continue to proliferate, and more microcracks caused by dislocations will form at the grain boundary. These cracks will form continuous cracks when they are connected, which will lead to the decline of wear resistance of ceramic tools.

3. Lubrication during ceramic cutting  

At present, there is no consensus on whether ceramic tools need lubrication in cutting. Some scholars believe that ceramic tools have the characteristics of high hardness, high melting point, high temperature resistance, poor thermal shock resistance, and are very sensitive to thermal stress. Improper cooling will cause thermal cracks and damage of tools. Therefore, ceramic tools can meet the requirements of use without cooling and lubrication during cutting. However, many researchers believe that when ceramic tools are used to process some difficult to machine materials (such as machining nickel base superalloys with whisker toughened ceramic tools), cutting fluids must be fully used (cutting fluids containing chlorinated paraffin are better). Proper cooling and lubrication are very beneficial to reduce the wear of ceramic tool wear and extend their service life. Tonshoff et al. studied the role of lubricants in turning hardened steel with Al2O3/TiC ceramic tools. Cutting tests were carried out under dry cutting and different lubricant lubrication conditions respectively. The results indicate that tool wear, machined surface quality, and chip formation are all affected by lubricants. Compared with dry cutting, using lubricants prolongs the tool life and significantly improves the surface quality of the workpiece. This is mainly because the extreme pressure additives in lubricants undergo frictional chemical reactions with the surface of the workpiece under cutting conditions, forming a chemical adsorption film. Through the composition analysis of the workpiece surface under the condition of lubrication cutting, it is found that the extreme pressure lubrication film containing FeS and FePO4, etc., reduces the cutting friction, inhibits the occurrence of adhesion, and thus reduces tool wear.

Cheryl's friction and wear tests of Si3N4/TiC ceramic materials at 900 ℃ showed that Si3N4 and TiC undergo oxidation at high temperatures, generating an oxide protective film containing Si and Ti on the friction surface, which can significantly reduce the friction coefficient and improve the wear resistance of the material. The cutting test of nickel base alloy with Si3N4 based and Al2O3 based ceramic tools shows that the main reason for tool failure under dry cutting conditions is the serious wear of the rake face and the adhesion of chips on the tool. The use of cutting lubricants improves the cutting performance of the tool, improves the cutting efficiency and improves the surface quality of the workpiece. Someone has studied the lubrication effect of various lubricants and additives on ceramic metal friction pairs and found that oil-based cutting fluids are more effective than water-based cutting fluids. If lubricating oil containing zinc dialkyl disulfide phosphate (ZDDP) is used for lubrication, the wear rate of Si3N4 ceramic tool when cutting 45 steel can be reduced by two orders of magnitude compared with dry cutting, and the wear rate when cutting stainless steel can be reduced by one order of magnitude compared with dry cutting. Surface analysis revealed that friction chemical reaction products such as ZnO, FeS, and FePO4 were generated on the worn surface of Si3N4 and the workpiece. The author has conducted an experimental study on the dry cutting of hardened steel with Al2O3/TiB2 ceramic tool. The results show that the ceramic tool has a self-lubricating function during high-speed dry cutting. When the cutting speed is low, the cutting temperature is also low, and the wear mechanism of the tool is mainly manifested as abrasive wear and adhesive wear; When the cutting speed is very high, the average cutting temperature of the tool surface is high, and the actual instantaneous maximum temperature is higher than the average temperature. The TiO2 diffraction peak appears in the XRD spectrum of the tool wear area after cutting, which indicates that TiB2 is oxidized under the action of high cutting temperature. The oxide TiO2 of TiB2 can act as a solid lubricant between the chip and the tool rake surface, thereby reducing the cutting force and average friction coefficient of the rake surface μ， And it can reduce the adhesive wear of the tool and improve its wear resistance.  

4. Matching of ceramic tools with machining objects

Each kind of ceramic tool has its specific processing range, and different ceramic tools (or the same kind of ceramic tools) have very different wear patterns and tool life when machining different workpiece materials. Therefore, every kind of ceramic tool has its best machining object, that is, there is the problem of the best matching between the ceramic tool and the machining object.   

Al2O3 based ceramic tools contain aluminum, so Al2O3 based ceramic tools have greater affinity when machining aluminum and aluminum alloys, and the tools will have greater adhesive wear and diffusion wear. Ceramic tools such as Al2O3/TiC and Al2O3 (/W, Ti) C contain aluminum and titanium elements. They also have greater affinity when machining titanium and titanium alloys, aluminum and aluminum alloys, so they are not suitable for machining aluminum, titanium and their alloys. The bonding tendency between pure iron and Al2O3 tools is greater than that between steel and cast iron. Pure Al2O3 ceramic tools begin to bond at about 500 ℃ when cutting pure iron. Compared with other superhard tools (such as diamond and cubic boron nitride tools), the diffusion effect between Al2O3 tools and iron is the smallest.   

Al2O3 tools toughened with SiC particles or SiC whiskers exhibit excellent cutting performance when machining nickel based alloys. However, when machining steel, the tool material undergoes rapid wear due to the easy reaction of Fe with SiC. When machining hardened steel with ceramic tools containing SiC, under the action of high cutting temperature, SiC is easy to react with Fe in the workpiece, and the reaction formula is 

4Fe+SiC→FeSi+Fe3C    

The higher the cutting speed, the higher the cutting temperature, which will further intensify the reaction rate between Fe and SiC. After the reaction of SiC whiskers with Fe, the original hardness and wear resistance of the whiskers are reduced, and the bonding strength between the whiskers and the matrix is weakened. Therefore, the whiskers are prone to detachment under the action of abrasive particles, thereby weakening the toughening effect of the whiskers. In addition, ceramic tools will also produce solution wear at high temperatures. Table 1 shows the solubility of each component of ceramic tool material and Fe at 1323 ℃. From the table, it can be seen that Al2O3 and ZrO2 have the lowest solubility in Fe, and the order of solubility from high to low is: SiC → TiN → TiC → Al2O3 → ZrO2. At high temperatures, the solubility of SiC in Fe is more than two orders of magnitude higher than that of TiC and TiN. Due to the chemical reaction and mutual dissolution of Fe and SiC whiskers, the content of Fe element in the tool material increases, further increasing the adhesion tendency between the tool and the workpiece, which is unfavorable for the wear resistance of the tool. Therefore, ceramic tools containing SiC particles or SiC whiskers are not suitable for machining steel parts.   

Table 1 Solubility of ceramic tool material composition in Fe at 1323 ℃ Material composition solubility (mol%) ZrO2-3.6 × 10-8 Al2O3-5.6 × 10-7 TiC-1.0 × 10-3 TiN-1.9 × 10-3 SiC-6.4 × 10-1

Al2O3 and ZrO2, the material components of Al2O3/ZrO2 ceramic tools, have good chemical stability at high temperatures, and their solubility with Fe is very small, so they are not easy to diffuse and dissolve into the workpiece materials. Therefore, Al2O3/ZrO2 has good wear resistance. Because the toughening effect of Al2O3/ZrO2 ceramic tools will be significantly reduced at high temperatures (above 1170 ℃), Al2O3/ZrO2 ceramic tools are not suitable for high-speed or ultra-high speed cutting at higher temperatures, and are only suitable for machining at lower cutting speeds. Si3N4 based ceramic tools are suitable for high-speed cutting of cast iron, and satisfactory results can be obtained when machining nickel based alloys, but they are severely worn when cutting austenitic stainless steel. Due to the high affinity between Si3N4 and Fe, as well as the mutual diffusion between Si and Fe, the high temperature generated by high-speed cutting greatly intensifies the chemical interaction and element diffusion between Si3N4 and such workpieces, exacerbating the wear of Si3N4 tools. Therefore, Si3N4 tools are not suitable for high-speed cutting of materials such as pure iron and carbon steel. In general, Al2O3 based ceramic tools have good wear resistance and high temperature resistance (both higher than Si3N4 based ceramic tools), and their high temperature chemical stability is very good, and they are not prone to mutual diffusion or chemical reaction with iron. Therefore, Al2O3 based ceramic tools have the widest range of applications, suitable for high-speed cutting of steel, cast iron and their alloys; The fracture toughness and hot cracking resistance of Si3N4 based ceramic tools are higher than those of Al2O3 based ceramic tools, which are suitable for intermittent machining of cast iron and cast iron alloys; ZrO2 toughened ceramic tools have high room temperature toughness and are suitable for intermittent cutting, but are not suitable for high-speed or ultra-high speed cutting at higher temperatures; Ceramic tools with SiC are most suitable for machining nickel base superalloys, pure nickel and high nickel alloys, but not for machining steel and cast iron.  

5. Conclusion  

To sum up, different kinds of ceramic tools (or the same kind of tools) have different wear patterns when machining different workpieces. Cooling and lubrication will have a great impact on the wear and tool life of ceramic tools. Proper cooling and lubrication are very beneficial to reduce the wear and prolong the service life of ceramic tool wear.   

In practical applications, each kind of ceramic tool has its own specific processing range. There is an optimal matching problem between ceramic tools and their processing objects. The appropriate tool material should be selected according to the workpiece material to be processed, and the optimal cutting amount should be determined according to whether the tool material contains components that are prone to diffusion and chemical interaction with the workpiece material under high temperature.