rus
Issue #1/2016
E. Zakharevich, V. Gorokhov, V.Lapshin, M.Shavva
Experimental Definition Of The Boundaries Of Brittle-Ductile Transition When Cutting
Experimental Definition Of The Boundaries Of Brittle-Ductile Transition When Cutting
THE BRITTLE MATERIALS
Treatment of brittle optical materials is the main task of modern industry. In order to enhance the efficiency of brittle material treatment it is necessary to reduce the share of cutting time, which is spent on rough and semifinish operations, and increase the quality of the surfaces obtained after polishing, chemical and mechanical treatment.
Treatment of brittle optical materials is the main task of modern industry. In order to enhance the efficiency of brittle material treatment it is necessary to reduce the share of cutting time, which is spent on rough and semifinish operations, and increase the quality of the surfaces obtained after polishing, chemical and mechanical treatment.
Теги: the quality of the surfaces obtained after polishing treatment of brittle optical materials качества обрабатываемой поверхности. обработка хрупких оптических материалов
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reatment of brittle optical materials is the main task of modern industry. The products and components of glass-ceramics, ceramics, quartz glass and leucosapphire with the roughness and accuracy of surface shape within the nanometer range have widespread application in instrument engineering, optical, electronic and defense industries. In order to enhance the efficiency of brittle material treatment it is necessary to reduce the share of cutting time, which is spent on rough and semifinish operations, and increase the quality of the surfaces obtained after polishing, chemical and mechanical treatment.
The process of interaction of brittle material with the cutting tool surface during the treatment by the edge and abrasive tools usually has several stages. The first stage is characterized by the formation of cracks. The crack enlargement, which continues during the second stage, causes chipping of the material fragment and its detachment from the treated surface. During the crack enlargement the increase of cutting force is observed. At the third stage, the material detached due to chipping leaves the treated surface, and the cutting force decreases to zero [1]. The process of treatment of brittle materials described by these three stages is characterized by the formation of edge and conical cracks. The edge cracks directly cause material chipping. Dimensions of conical cracks determine the depth of defective layer after the treatment.
Thus, the treatment of brittle materials based on the chipping of particles from the treated surface results in the occurrence of defective layer and increase of roughness of treated surface. Many foreign and domestic researchers note that there are some intermediate states between the processes of plastic material deformation observed during the treatment of metals, for instance, when treated brittle material acts as the ductile material. These states stipulate the process of plastic deformation of chips and treated brittle material [2, 3]. The fact that extruded and cut out scratch is observed during scratching of hard alloy, ceramics and glass-ceramics serves as the proof of such state [2].
The treatment of brittle materials by plastic deformation makes it possible to decrease the probability of cracks formation, which results in the decrease of the depth of defective layer and enhancement of the quality of treated surface. The necessity of additional polishing stands no longer. This advantage gives great effect in case of the treatment of the surfaces with irregular shapes.
Conditions for plastic deformation of brittle material upon cutting can be determined on the basis of many factors: properties of treated material, geometrics of cutting tools, speed of material deformation, rate of applied load [2]. All listed factors determine the boundary of brittle-ductile transition of material; however, the critical thickness of cut out layer refers to the most significant parameter, which is also connected with the concept of boundary.
Within the framework of this paper, the attempt of experimental definition of the critical thickness of cut out layer is made for potassium dihydrogen phosphate, single-crystalline silicon and germanium.
The works were performed using high-accuracy test bench [4] equipped with contactless capacity-type sensors, which are necessary for the measurement of elastic releases of cutting tools in the process of treatment.
Determination of the critical thickness of cut out layer was performed by the application of the cuts with various depth on the workpiece surface with diamond single-crystalline tool. As known from the references [5, 6, 7], it is required to use diamond tools with zero or negative front rake and corner radius within the range of 30–50 nm for the edge cutting machining of brittle materials under the conditions of cutting, which provide its plastic deformation [8]. The diamond cutter used in experimental research activities has corner radius of not less than 50 nm; the measurements were performed with the use of scanning electronic microscope Tescan Mira3.
Series of scratches with various cutting depth were made on the samples of potassium dihydrogen phosphate – Fig. 1. Before the scratch formation, the place of its application was treated by the same tool with consecutive decrease of cutting depth and sparking-out. The depth of such preliminary treatment was equal to 15 µm. This action was performed in order to remove the defective layer formed on the workpiece surface after the previous treatment. The cutting speed was equal to 300 mm/min.
Table gives the depth of scratch cutting on the workpieces and value of elastic release of cutting tool.
Figure 2 (a, b, c, d, e) illustrates the appearance of scratch surfaces of potassium dihydrogen phosphate, 1–5 respectively.
If the quality of treated surface of scratch determined on the basis of number of cleavages is taken as the criterion of plasticity of treated material, then it follows from Figure 2 that the highest quality is observed on the surface of the scratch No. 5. Therefore, upon the depth of cutting of 0.08 µm this material is mainly treated by the method of plastic deformation.
Appearance of surface of the scratches applied on single-crystalline silicon is given in Figure 3.
It follows from the figure that upon the depth of cutting of 100 µm, material is in boundary condition. Brittle chipping and plastic deformation take place simultaneously. However, upon the decrease of the depth of cutting to 80 µm on the surface of the scratch No. 3 scratches and chips are practically not observed.
Analogous results were obtained during the treatment of germanium.
When applying the cuts on the samples of brittle materials, formation of flow chips was registered, and this fact proves the material transition into boundary state.
On the basis of results of these activities, we can say that the methods for definition of critical thickness of cut out layer of brittle optical materials have been developed. The critical thickness of cut out layer for potassium dihydrogen phosphate, silicon and germanium has been determined. Its value lies within the range of 80–100 nm. However, during the treatment of listed materials not only this parameter should be taken into account but also the state of cutting tool, equipment rigidity and speed of cutting of treated material.
The article is written within the framework of Agreement No. 14.579.21.0042 dated 25.08.2014 (unique identifier RFMEFI57914X0042) concluded between JSC "VNIIINSTRUMENT" and Ministry of Education and Science of the Russian Federation on the topic "Development of Technology and Equipment for Nano-Scale Treatment of Optical Materials by Diamond Single-Crystalline and Abrasive Tools under Conditions of Quasi-Ductile Cutting".
reatment of brittle optical materials is the main task of modern industry. The products and components of glass-ceramics, ceramics, quartz glass and leucosapphire with the roughness and accuracy of surface shape within the nanometer range have widespread application in instrument engineering, optical, electronic and defense industries. In order to enhance the efficiency of brittle material treatment it is necessary to reduce the share of cutting time, which is spent on rough and semifinish operations, and increase the quality of the surfaces obtained after polishing, chemical and mechanical treatment.
The process of interaction of brittle material with the cutting tool surface during the treatment by the edge and abrasive tools usually has several stages. The first stage is characterized by the formation of cracks. The crack enlargement, which continues during the second stage, causes chipping of the material fragment and its detachment from the treated surface. During the crack enlargement the increase of cutting force is observed. At the third stage, the material detached due to chipping leaves the treated surface, and the cutting force decreases to zero [1]. The process of treatment of brittle materials described by these three stages is characterized by the formation of edge and conical cracks. The edge cracks directly cause material chipping. Dimensions of conical cracks determine the depth of defective layer after the treatment.
Thus, the treatment of brittle materials based on the chipping of particles from the treated surface results in the occurrence of defective layer and increase of roughness of treated surface. Many foreign and domestic researchers note that there are some intermediate states between the processes of plastic material deformation observed during the treatment of metals, for instance, when treated brittle material acts as the ductile material. These states stipulate the process of plastic deformation of chips and treated brittle material [2, 3]. The fact that extruded and cut out scratch is observed during scratching of hard alloy, ceramics and glass-ceramics serves as the proof of such state [2].
The treatment of brittle materials by plastic deformation makes it possible to decrease the probability of cracks formation, which results in the decrease of the depth of defective layer and enhancement of the quality of treated surface. The necessity of additional polishing stands no longer. This advantage gives great effect in case of the treatment of the surfaces with irregular shapes.
Conditions for plastic deformation of brittle material upon cutting can be determined on the basis of many factors: properties of treated material, geometrics of cutting tools, speed of material deformation, rate of applied load [2]. All listed factors determine the boundary of brittle-ductile transition of material; however, the critical thickness of cut out layer refers to the most significant parameter, which is also connected with the concept of boundary.
Within the framework of this paper, the attempt of experimental definition of the critical thickness of cut out layer is made for potassium dihydrogen phosphate, single-crystalline silicon and germanium.
The works were performed using high-accuracy test bench [4] equipped with contactless capacity-type sensors, which are necessary for the measurement of elastic releases of cutting tools in the process of treatment.
Determination of the critical thickness of cut out layer was performed by the application of the cuts with various depth on the workpiece surface with diamond single-crystalline tool. As known from the references [5, 6, 7], it is required to use diamond tools with zero or negative front rake and corner radius within the range of 30–50 nm for the edge cutting machining of brittle materials under the conditions of cutting, which provide its plastic deformation [8]. The diamond cutter used in experimental research activities has corner radius of not less than 50 nm; the measurements were performed with the use of scanning electronic microscope Tescan Mira3.
Series of scratches with various cutting depth were made on the samples of potassium dihydrogen phosphate – Fig. 1. Before the scratch formation, the place of its application was treated by the same tool with consecutive decrease of cutting depth and sparking-out. The depth of such preliminary treatment was equal to 15 µm. This action was performed in order to remove the defective layer formed on the workpiece surface after the previous treatment. The cutting speed was equal to 300 mm/min.
Table gives the depth of scratch cutting on the workpieces and value of elastic release of cutting tool.
Figure 2 (a, b, c, d, e) illustrates the appearance of scratch surfaces of potassium dihydrogen phosphate, 1–5 respectively.
If the quality of treated surface of scratch determined on the basis of number of cleavages is taken as the criterion of plasticity of treated material, then it follows from Figure 2 that the highest quality is observed on the surface of the scratch No. 5. Therefore, upon the depth of cutting of 0.08 µm this material is mainly treated by the method of plastic deformation.
Appearance of surface of the scratches applied on single-crystalline silicon is given in Figure 3.
It follows from the figure that upon the depth of cutting of 100 µm, material is in boundary condition. Brittle chipping and plastic deformation take place simultaneously. However, upon the decrease of the depth of cutting to 80 µm on the surface of the scratch No. 3 scratches and chips are practically not observed.
Analogous results were obtained during the treatment of germanium.
When applying the cuts on the samples of brittle materials, formation of flow chips was registered, and this fact proves the material transition into boundary state.
On the basis of results of these activities, we can say that the methods for definition of critical thickness of cut out layer of brittle optical materials have been developed. The critical thickness of cut out layer for potassium dihydrogen phosphate, silicon and germanium has been determined. Its value lies within the range of 80–100 nm. However, during the treatment of listed materials not only this parameter should be taken into account but also the state of cutting tool, equipment rigidity and speed of cutting of treated material.
The article is written within the framework of Agreement No. 14.579.21.0042 dated 25.08.2014 (unique identifier RFMEFI57914X0042) concluded between JSC "VNIIINSTRUMENT" and Ministry of Education and Science of the Russian Federation on the topic "Development of Technology and Equipment for Nano-Scale Treatment of Optical Materials by Diamond Single-Crystalline and Abrasive Tools under Conditions of Quasi-Ductile Cutting".
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