Cutting In The Quasiplastic Mode
t the moment, the majority of domestic enterprises engaged in the production of high-precision parts made of brittle materials for optical, electronic, medical and aviation industries, use multiple grinding of machined surfaces and their subsequent lapping and finishing (polishing) (Fig. 1) as a fundamental technology. This technology is very effective and time tested. It allows you to obtain the necessary accuracy and roughness for simple surfaces (plane, sphere) in almost all cases. However, it has often lack time. Processing of one part with a form accuracy of not more than 100 nm and a surface roughness of not more than 10 nm can take tens of hours. The twenty-first century is intolerant to time losses, there is an important task before modern technologies: to reduce the processing time of high-precision parts made of brittle materials, while ensuring the required accuracy and surface roughness. The task is not easy, but challenges are interesting by their nonstandard solutions which allow you to see the problem from a different angle.
Quasiplasticity, ductile cutting, processing in the quasiplastic mode are the modes of brittle material machining, when surface layers of the material are tested in respect of ductile properties . It means that it is possible to set such a speed and a depth of cutting, a cutter feed speed or blank supply when so high stress emerges in the processed material that the mechanism of brittle fracture is replaced by the mechanism of ductile deformation. In other words, under certain cutting modes, machining the details of monocrystalline quartz is equivalent to machining the parts of structural steel. It is only needed to figure out how to set such cutting modes and what equipment enables to carry out them? The answer to this question lies in the history of ductile cutting.
So, ductile cutting was first discovered in 1954. In experimental studies on frictional wear of rock salt crystals, completely by accident, it was noted that the faces of some of the following contact marks were formed not by chipping with typical cracks around the edges, but as if melted. Ductile deformations of the brittle material were obvious. That phenomenon was, if not unique, then at least wonderful. But how to use it and how to create the conditions in the cutting zone, similar to those that had place when frictionizing numerous faces of salt crystals?
Bifano T. and Dow T. tried to give the answer to this question. In the late 80s of the twentieth century, they put numerous experiments on ductile grinding.
During experimental studies, scientists have advanced a theory that the cutting depth, at which a ductile deformation of brittle materials is observed, is directly proportional to the square of stress-intensity factor (fracture toughness) and modulus of material elasticity, and inversely proportional to the cube of hardness . As is often happens, the practice has proven the advanced theory only partially. However, Bifano T. and Dow T. could do the main thing, they found the borders of brittle-ductile transition for quartz, germanium, glass-ceramic, quartz glass, optical glass and other materials (Fig. 2).
Despite the fact that Biff’s and Dow’s data were received more than twenty years ago, many foreign studies are still based on the above modes. Along with the experiments on ductile grinding, the technology of ductile turning for brittle materials develops. Toh and McPherson  have found that plastically deformed chips, when processing ceramic materials, appears when the cutting depth is less than 1 micron. At the same time, Blake and Scattergood  who studied the processing of germanium and silicon, have come to the conclusion that the main parameter that controls the transition from brittle fracture to ductile fracture, is the thickness of the cutting layer, i. e., depth of cutting. Puttic , who performed the diamond turning of glass with a cutting depth of about 100 nm, reached a surface roughness Ra of 0,6 microns. Leung  performed directly the machining of silicon on the turning lathe with a roughness of 2.86 nm, and also came to the conclusion that in order to obtain a high-quality surface, it is necessary to carry out the processing in the ductile mode and the chip thickness (cutting depth) should be less than the critical value set by Biff and Dow.
When ductile turning, chips formed in the corner radius zone of the cutting edge has a form of "comma". Chip sizes vary from zero near the top of the equipment to a maximum value at the chip ends (Figure 3). As already mentioned above, Blake came to the conclusion that the transition from brittle mode to ductile mode of chip removal is determined by the critical chip thickness. Damages, which are expected when ductile processing, are not subject to the processed surface, as they are gradually eliminated by subsequent overtravel due to the consistency of the equipment radius and supply, which in turn determine the chip cross-section.
Thus, ductile cutting: ductile grinding and ductile turning are phenomena observed in a very small, nanometer depth of cutting. Performing the processing in the quasiplastic mode requires:
The use of a super hard ultrahigh-precision machine capable of providing the necessary cutting modes;
Kinematics of cutting, which ensures the minimum thickness of cut chips;
The use of single-crystal diamond with a cutting edge sharpness of 20…50 nm or abrasive tools with micron grit.
OJSC "VNIIINSTRUMENT" together with "Resurs Tochnosti, Ltd." and N. E. Bauman MSTU have been conducting development and research in the field of ultrahigh-precision equipment for more than three years to handle a wide range of materials. In 2014, within the framework of the Federal Target Program "Research and development in priority directions of scientific-technological complex of Russia for 2014–2020", OJSC "VNIIINSTRUMENT" entered into an agreement to perform applied research on the topic "Development of technology and equipment for nanoscale processing of optical materials with single crystal diamond and abrasive tools in the quasiplastic cutting mode".
Within the project, it is planned to create a super hard ultrahigh-precision experimental stand for diamond turning and grinding in the quasiplastic mode (Fig. 4), as well as for processing brittle materials by the ductile cutting method.
At the moment, on the available ultrahigh-precision equipment, preliminary experimental studies are conducted on processing of: dihydrogen phosphate of potassium, sapphire single crystal quartz, quartz glass and glass-ceramic. Material processing was performed by grinding with a diamond single crystal tool and by diamond milling (Fig.5, 6) when implementing a special cutting kinematic scheme that ensures a thickness of the removed chips in the range of 4 to 34 nm.
As a result of experiments in terms of grit diamond grinding 2/3 microns, the processed surface areas were obtained with a roughness of 2 to 34 nm (depending on the material and processing conditions) (Fig. 7). On the surfaces "feeding traces’ of the cutting tool and the absence of chips and cracks are significantly obvious. This ensures that when processing the material was in the quasiplastic condition in the cutting zone.
When diamond milling potassium dihydrogen phosphate, the roughness of the processed surface was 1–3 nm (Fig. 8). There were also traces of cutter feeding. Fig. 7 also shows the boundary of the brittle-ductile transition.
The future plans of OJSC "VNIIINSTRUMENT" is development of manufacturing technology for flat, spherical, aspherical and general surfaces using methods of diamond turning and grinding in the cutting quasiplastic mode. It is also planned to manufacture the components and install a special ultrahigh-precision stand for implementation of cutting quasiplastic modes when processing brittle materials.
Thus, the processing using the ductile cutting method and a special tool (a diamond wheel on the metallic bond, a cutter or a mill) allows to receive the optical surface when processing almost any brittle material, on the surfaces of virtually any shape (lattices, freeform surfaces with nanometer precision and roughness with almost complete absence of the damaged layer. In this case, the operation of polishing is either completely eliminated or takes minimal time.
This article was prepared within the framework of the Federal Program "Research and development in the priority directions of scientific-technological complex of Russia for 2014–2020" under the Agreement No. 14.579.21.0042 of 25.08.2014 "Development of technology and equipment for nanoscale processing of optical materials with monocrystalline diamond and abrasive tools in the cutting quasiplastic mode" between OJSC "VNIIINSTRUMENT" and the Ministry of Education and Science of the Russian Federation.