Research Status of Ultra-precision Machining of Plastics in Brittle Materials

0 Preface

With the development of modern science and technology, brittle materials play an important role in many fields of modern high-tech industries, especially in aerospace, optics, and electronics. They often have very high processing accuracy and surface quality. Claim. But so far, the processing of brittle materials is still a difficult thing. Because one of their most prominent characteristics is the material's high brittleness, low fracture toughness, the material's elastic limit and strength are very close. When the load on the material exceeds the elastic limit, it breaks and breaks, and cracks and dents have formed on the surface of the female mouth, seriously affecting the surface quality and performance [12,13]. Therefore, the brittle material has extremely poor machinability. In the past, people have been using ancient grinding and polishing processes to finish brittle materials. These processing methods have low production efficiency, and the processing accuracy is not easy to guarantee. Moreover, the processing process is not easy to achieve computer control. Even complex workpieces with curved surfaces cannot even be processed, and thus they are far from meeting the requirements of modern high-tech development and high efficiency.

In recent years, people have done a lot of exploration and experimentation on the processing of germanium materials. Beyond that, with the rapid development of science and technology, the manufacturing technology of diamond tools and ultra-precision machine tools has reached a very high level, making ultra-Precision Machining of brittle materials possible. Simply cutting the surface of brittle materials through diamonds is an emerging technology that has only developed in the past decade. It mainly achieves high-quality surfaces by performing ultra-precision cutting of plastic domains on brittle materials. This method has a wide range of application prospects due to its advantages such as high production efficiency, easy control of the production process, and the ability to machine workpieces with complex curved surface shapes. This paper summarizes the research status of ultra-precision processing of brittle materials and analyzes some of the existing problems. It is hoped that this will provide inspiration for the further study of brittle materials' plastic domain processing.

1 The theory of brittle-plastic deformation of sharp indenter on brittle materials

The possibility of ultra-precision turning of brittle materials with sharp diamond tools is based on the fact that brittle materials can be plastically deformed under sharp diamond indentations. In the past few decades, many scholars have conducted a large number of indentation experiments on various hard and brittle materials, that is, pressing the diamond indenter into a certain depth of the material with a certain vertical force to observe the deformation of the material. In a complete cycle of loading and unloading of an imprinting experiment, the cracking process from failure to expansion is shown in Fig. 1 [1].

From Fig. 1, it can be seen that even a brittle material will produce a certain amount of plastic deformation under the effect of a very small load. As the load increases, the material will change from a plastic deformation mode to a brittle failure, creating brittle cracks on the interior and surface of the material. In this transition process, the vertical load applied when the crack is just generated is called the critical load, and the depth at which the indenter presses is called the critical depth. These two concepts are very important to describe when brittle materials change from plastic deformation to brittle failure under external forces. Critical pressure depth

(a) Initial loading: The contact zone creates a zone of permanent plastic deformation without any crack damage. Deformation zone size increases with increasing load.

(b) Critical zone: When the load increases to a certain value, an intermediate crack (MedianCraclE) is generated at the stress concentration directly below the indenter.

(c) Crack Growth Zones: As the load increases, so does the interstitial crack.

(d) Initial unloading phase: Intermediate cracks begin to close but do not heal.

(e) Lateral crack generation: Further unloading, due to the mismatch of elasto-plastic stress in the contact zone, a tensile stress is generated which is superimposed on the stress field, resulting in a series of lateral cracks extending laterally (Lateral Crack).

(f) Complete Displacement: Lateral cracks continue to expand, forming broken debris if the crack extends to the surface. The calculation formula is shown in (1) [2,3]:

In the formula, dc is the critical pressure depth, E is the elastic modulus of the material, H is the hardness of the material, Kc is the fracture toughness of the material, and ψ is a constant related to the geometry of the indenter.

In the printing process of brittle materials, intermediate cracks are always generated first, and it is perpendicular to the surface of the material and spreads to the inside. The damage to the material is most serious. Therefore, the length and vertical load imposed on the intermediate crack by scholars The relationship between the two was studied in detail and the following relationship was obtained [4]:

In the formula, c represents the crack length, P is the vertical load applied, β is a constant depending on the shape of the indenter, and φ is the half-vertex angle of the indenter.

It was precisely because people realized from the press and scribing experiments that even with brittle materials, plastic deformation occurs when the depth of penetration is small, and the length of the crack is related to the applied load. Cracks do not extend to the ductile region of brittle materials that have been machined.

2 Ultra-precision grinding of brittle materials

Ultra-precision grinding technology is a new method of processing brittle materials that has recently been developed. It uses high-rigidity and ultra-compact grinding machines to grind materials with diamond grinding wheels. Evans and Marshall used a diamond indentor to sculpt the surface of a brittle material such as glass to simulate the cutting process of tiny abrasive grains on a diamond wheel. When the applied load is greater than the critical load, the brittle cracking system under the abrasive grain is shown in Figure 2. [5]. To achieve ultra-precision grinding of brittle materials, the key is to make the material removed by plastic deformation. Under the action of the abrasive particles, the surface of the material is just finely cracked, and the thickness of the abrasive grain is called the critical cutting thickness. Many scholars have studied the brittle transition of brittle materials under grinding conditions [6-8], American scholar T. G. Based on a large number of grinding experiments on brittle materials such as glass and ceramics, bifano obtained the critical thickness formula for ultra-precision grinding[6]:

In the formula, dc is the critical cutting thickness, E is the elastic modulus of the material, H is the material hardness, and K is the fracture toughness of the material.

Japanese scholar Naoya Ikawa et al. used the abrasive grains with different particle sizes to perform indentation experiments on the surfaces of single-crystal silicon and lithium niobate. It was found that the influence of abrasive grains with different particle sizes on the surface of the material was different [9]. Therefore, there is reason to believe that in the ultra-precision grinding process, the size of the diamond wheel abrasive grain used will inevitably affect the critical cutting thickness. However, there is still a lack of research in this area.

In the late 1980s, an ultra-precision grinding machine was successfully developed at the Precision Engineering Center of North Carolina State University. The spindle stiffness of the machine tool was 50 MN/m. On this machine tool, a specially-ground diamond grinding wheel is used to grind a variety of optically brittle materials such as single crystal silicon, single crystal crucible, and amorphous glass, and a smooth surface without cracks can be processed [10]. In addition, the British phase of Japan has also developed its own super-precision grinding machines, of which the UK's Cranfield Precision Engineering Center grinder is the world's largest rigid uranium machine tool, Japan's Toyota Seiki Co., Ltd. developed a ceramic material developed Zero-expansion coefficient spindle super-precision grinding machine. They also grind hard and brittle materials, and the surface quality of the material is superior to that of optical surfaces processed using conventional polishing methods. However, there are the following problems in this processing technology [11,12]: How to ensure that the abrasive grains on the grinding wheel are uniformly distributed and highly uniform: Because the abrasive grains are very small, it is easy to produce grinding debris that blocks the grinding wheel and causes the grinding force to increase. Cracks develop on the surface.

In order to solve the above problems, the Japanese Institute of Physical Chemistry Omori et al. proposed the ELID grinding technology in 1987. The basic principle is to use on-line electrolysis to trim the metal-based grinding wheel, that is, in the grinding process, the grinding grinding fluid is poured between the grinding wheel and the tool electrode and a DC pulse power is applied to make the grinding wheel metal bonding agent as an anode. The anodic dissolution effect gradually removes, and the abrasive grains that are not affected by electrolysis protrude from the surface of the grinding wheel, thus realizing the dressing of the grinding wheel, and the sharpness of the grinding wheel can always be maintained during the processing.

The emergence of ELID grinding technology successfully solved the problem of metal-based superabrasive grinding wheel dressing. At the same time, the micro-trimming effect of on-line electrolysis enables the ultra-fine-grained grinding wheel to maintain its sharpness during the grinding process, in order to achieve stable ultra-precision. Grinding creates favorable conditions. The grain size of the grinding wheel used in the study in Japan has reached 5 nm, and the grinding surface roughness Ra is less than 1 nm.

ELID grinding technology has also been paid attention to and applied in the United States, the United Kingdom, Germany and other countries, and is used to perform ultra-precision machining on the surface of brittle materials. In China, Harbin Institute of Technology started research on EID grinding technology in 1993. At present, mirror grinding is performed on brittle materials such as cemented carbide, ceramics, and optical glass, and the surface roughness values ​​are ground under the same machine conditions. Compared with the ordinary grinding wheel, the surface roughness Ra of some workpieces has reached nano level, and the roughness of the grinding surface of silicon crystallized glass can reach Ra 0.012 μm. This shows that the ELID grinding technology can achieve ultra-precision machining of brittle materials. However, there are still oxide films on the grinding wheel surface or the surface layer of the grinding wheel is not pressed into the surface of the workpiece to form a surface layer glaze and electrolytic grinding fluid. The ratio of the other issues, these issues will seriously affect the quality of the surface processing, pending further study.

3 Brittle materials ductile ultra-precision turning

Since 1987, Blake and Scattergood, a scholar at Carolina State University in the United States, first conducted a series of ultra-precision turning experiments on brittle optical material single crystal crucibles, and successfully realized plastic ultra-precise vehicle-creation of brittle materials. Its surface roughness Ra reaches 8nm [13]. Blake et al.'s cuttings for the first time proposed the relationship between the cutting section geometry and the critical cutting depth when cutting material with a circular-arc diamond turning tool, and proposed a brittle-to-turn cutting model as shown in FIG. 3 . In the figure, f is the feed amount, z is the distance between the center of the tool tip and the brittle transition zone, yc is the length of the crack, and t is the cutting thickness at a point on the circular-arc edge.

Due to the use of a circular-arc tool, the effective cutting thickness increases from zero to maximum from the tool tip to the surface to be machined. When the cutting thickness reaches a certain critical value dc, the material begins to produce fracture failure; when the cutting thickness is less than the critical value, the material removal method is mainly plastic deformation. Assuming that the crack propagation depth at the critical value dc is the crack propagation depth is yc, if the crack propagation depth yc does not affect the machined surface, a smooth surface can be obtained. The part of the fracture that occurs on the arc surface will be cut off during the subsequent cutting process.

From this cutting model, it can be seen that the so-called plastic field turning of brittle materials is not that the material is to be plastically removed during the entire cutting process, but that only the material near the cutting edge is removed by plastic deformation. Most other materials are cut off in a brittle manner, which is an important imperative for the turning of brittle materials. In this process, the effect of the feed rate is very important, the critical cutting thickness dc. The relative position of the arc blade of the turning tool and the value of the feed amount are closely related. When the feedrate is increased, the critical cutting thickness dc will move down the turning edge of the turning tool, and the crack will easily spread to the machined surface. He proposed the formula for critical cutting thickness [13]:

In the formula, f is the amount of feed, and R is the radius of the turning tool.

As shown in Fig. 3, because the depth of crack propagation yc has a reconvergence effect on the formed arc surface, considering the influence of yc, the following formula is improved: [14] :

Blackey conducted a series of cutting experiments on single crystal silicon and single crystal silicon. The experimental results show that the crack propagation depth yc is approximately 3 to 10 times the critical cutting thickness dc.

In order to improve the analysis accuracy, Blackey et al. further studied the chip morphology of diamond-cut single crystal crucibles using a scanning electron microscope. The chips cut near the tool tip are thin and continuous strips, which are plastic deformation. The chips far away from the tip of the tool are relatively thick, showing a discontinuous broken state, mainly brittle deformation. Obvious brittle transformation boundaries can be observed on the chips. Blackey gives the following relation [5]:

In the formula, Wductile is the width of plastic deformation region of the chip. The accuracy of the critical depth of cut dc obtained by the above formula has been further improved. However, the above analysis is based on experimental measurement values ​​and is greatly affected by experimental conditions.

In 1990, Japanese scholar Nakasuji and others also studied brittle transitions in brittle materials turning [16]. Their research methods and results on critical cutting thickness are basically consistent with the results of Backey et al. But at the same time, they also found that the surface roughness of the single-crystal silicon turning surface showed a bright and dark fan-shaped distribution characteristics, which is due to the anisotropic characteristics of the single product material. In the (100) crystal plane of single crystal silicon, the experiments of right-angle cutting along the <110> and <100> crystal orientations also show that the cutting force varies greatly when turning along different directions, which is bound to affect the material. Surface quality affects. Takayuki Shibata supplies explain the cause of this sector shape to the effect on the slip system [20] , but the brittle material will inevitably produce brittle failure during the turning process, so the explanation from the plastic slip alone has a certain degree of one-sidedness.

Because the cutting thickness in the ultra-precision machining of brittle materials is very small, the cutting edge radius and cutting thickness of the cutting tools are basically in the same order of magnitude, so the influence of the cutting edge radius on the cutting process can not be ignored. Although some scholars have studied the influence of cutting edge radius on cutting force, the study on the effect of cutting edge radius on brittle material transition of brittle materials for ultra-precision turning is still a blank and needs further study.

KiovanolaJH found that in the car-creation of the glass, the tool used -30 ° rake angle is easy to form a continuous strip cutting, plastic processing. Later, in the process of ultra-precision turning of materials such as single-crystal silicon and silicon, it has also been found that the use of a large negative rake angle facilitates the plastic zone processing of brittle materials. Japanese scholars have used a method of applying a negative pressure to the surface of a workpiece to carry out turning, and on a precision lathe, a plastic domain processing of a brittle material is realized. However, scholars did not give a good explanation for the plastic domain processing of brittle materials with a large negative rake diamond turning tool.

4 Conclusion

From the research status of the ultra-precision machining of brittle materials in the plastic domain, we can see that the ultra-precision machining of brittle materials has made important progress and has a deep understanding of the cutting process. However, people lack deep research on the mechanism of brittle-to-plastic transformation of brittle materials under ultra-thin cutting conditions. Therefore, the determination of critical depth of cut is based on experimental measurements and lacks theoretical guidance. Compared with the research level of ultra-precision machining of plastic materials, the study of ultra-precision machining of brittle materials in the plastic domain is still in its infancy. Many problems remain to be explored and studied.