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Precision and micro CVD diamond-coated grinding tools
ARTICLE IN PRESS International Journal of Machine Tools & Manufacture 50 (2010) 420–424
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Precision and micro CVD diamond-coated grinding tools n ¨ Jan Gabler , Sven Pleger Fraunhofer Institute for Surface Engineering and Thin Films IST Bienroder Weg 54 E, 38108 Braunschweig, Germany
a r t i c l e in f o
a b s t r a c t
Article history: Received 17 March 2009 Received in revised form 18 June 2009 Accepted 20 October 2009 Available online 4 November 2009
Both for ultra-precision and for micro-machining diamond is used very often as tool material. The reason is the very high dimensional stability of diamond due to its extreme hardness. Diamond is used for two kinds of machining processes: for cutting, like turning, drilling or milling, as well as for abrasive processes, like grinding. Diamond cutting tools can be made with massive diamond (monocrystal, CVD diamond, PCD) or with diamond coatings. Standard diamond abrasive tools are made by bonding diamond monocrystals onto a base body. A new grinding layer technology is presented: chemical vapour-deposited microcrystalline diamond layers have crystallite tips with very sharp edges that can act for grinding processes. Base body materials and coating technology is presented. Application results of grinding experiments show that very high workpiece quality can be reached, e.g. a roughness Ra of 5 nm with glass workpieces. Truing and recoating techniques are discussed for reuse of worn CVD diamond grinding wheels. Micro grinding tools (abrasive pencils, burrs) can be manufactured with the same coating technology. Very small tools with diameters of 50 mm have been made and successfully tested. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Diamond CVD Tools Grinding Abrasive machining Micro-machining
1. Diamond tools for precision and micro-machining Diamond is used very often as tool material for ultra-precision and micro-machining. The reason for this is the very high dimensional stability of diamond due to its extreme hardness. In ultra-precision machining as well as in micro-machining very small depths of cut (ae) are used, which affords very sharp and stable cutting edges. Diamond is very often the material of the first choice not only for hard, abrasive or difficult-to-machine materials like nickel, glass or cemented carbide but also for ductile and soft materials, like aluminium, copper or plastics. Diamond is used for the two kinds of machining processes: for cutting, like turning, drilling or milling, as well as for abrasive processes, like grinding. 1.1. Diamond tools for cutting Diamond cutting tools can be separated into tools with massive diamond and tools with diamond coatings. 1.1.1. Cutting tools with massive diamond Three different types of massive diamonds are used for tools: monocrystalline diamond, polycrystalline diamond and sintered n
Corresponding author. Tel.: + 49 531 2155 625. ¨ E-mail addresses: [email protected] (J. Gabler), [email protected] (S. Pleger). 0890-6955/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2009.10.008
diamond. The monocrystals can be from natural source or from high-temperature high-pressure (HTHP) synthesis. For ultraprecision cutting mostly natural monocrystals are used [1]. HTHP diamond is used in particular when large widths of cut are required, since these monocrystals are available in larger dimensions (up to 18 mm) than natural monocrystals. Monocrystal ultra-precision cutting tools are mainly used with a corner radius re (see Fig. 1) for turning or milling operations. The two main quality criteria for diamond cutting tools are the waviness of the corner radius re and the cutting edge radius rb. The waviness, which is the amount of deviation from a true circle, can be down to 50 nm for highest quality tools. The cutting edge radius rb of new tools is between 15 and 40 nm. After a runningin, tools can have a cutting edge radius of 100 nm. Metal-matrix sintered diamonds are called ‘‘polycrystalline diamond’’ (PCD) despite they have no polycrystalline nature. Their structure can be called multi-crystalline, since they consist of small micron size diamond crystals that are bonded in a cobalt binder matrix. This two-phase system has a lower hardness than mono- or polycrystalline diamond. The cutting edge radius as well as the form accuracy is lower compared to monocrystal diamond due to the matrix component material structure. Nevertheless PCD can be used for hard precision turning [2]. Chemical vapour deposition cannot only be used to coat tools with a thin diamond layer (see Sections 1.1.2 and 2). It can also be used to fabricate thick (up to 0.5 mm) diamond plates that can be mounted on a tool holder, ground and thus be used similar to sintered PCD diamond plates. These CVD diamond inserts are of
ARTICLE IN PRESS J. G¨ abler, S. Pleger / International Journal of Machine Tools & Manufacture 50 (2010) 420–424
polycrystalline nature and can therefore today not be prepared with the same small cutting edge radius like monocrystal tools. But they have the same hardness as monocrystals. Their
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application range lies therefore between the monocrystals and the sintered PCD diamond. Massive diamond tools can also be used to fabricate microstructures, e.g. V-shaped tools (see Fig. 2). They mostly use monocrystals [4,5], but also PCD [6]. The shape of these micro tools is restricted.
1.1.2. Diamond-coated cutting tools Diamond coated tools can be manufactured with more complex shapes, smaller angles and dimensions. The base body of these tools – mostly cemented carbide – can be shaped easily to very small and tiny geometries, which is not possible with massive diamond. End mills as well as drills with diameters down to 100 mm have been manufactured and successfully tested in ductile workpiece materials (see Fig. 3 right) [7,8]. With very small feed rates they can even perform a ductile machining of hard and brittle materials like silicon (see Fig. 3 right). The accuracy and surface quality of the machined workpiece is restricted due to the roundness of the coated cutting edges. The radius of a coated cutting edge is at least as large as the film thickness, which is in the range 3–8 mm for diamond-coated tools. If smaller cutting edge radii are required, a plasma ion sharpening is possible, which enables cutting edge radii of down to 0.5 mm [9].
1.2. Diamond tools for abrasive machining Fig. 1. SEM picture of a monocrystal diamond cutting edge [3].
Fig. 2. Micro cutting tool with monocrystal diamond [4].
The diamond grinding layers that are used today consist of diamond grains that are bonded onto a tool base body (see Fig. 4). The layer design differs mainly concerning the grain size and the bonding. The diamond is from natural or synthetic source. The grains can be monocrystals (natural or HPHT synthesis) or polycrystalline (from detonation synthesis). The shape differs from compact to blocky or it is complex shaped when polycrystalline diamond grains are used. The bonding may be polymer (resin), ceramic (vitrified) or metal [10]. The different grain sizes are achieved by crushing larger grains. Therefore, the edges of the crushed grains are uneven and mostly dull. Sharpening effects can be achieved, when a truing or crushing is performed with the bonded diamond tool and sharp breakage edges are generated on the diamond grains. The chemical vapour deposition that is used to coat cutting tools with diamond films (see Sections 1.1.2) can also be used to fabricate diamond grinding layers. This approach is the main focus of this contribution and detailed described in the following chapter 2.
Fig. 3. CVD diamond-coated spiral drill, diameter 150 mm (left) and drilled hole diameter 100 mm in silicon (right, [11]).
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2. Technology of CVD diamond-coated grinding tools 2.1. Coating process and material properties CVD diamond grinding layers use the sharp-edged roughness of polycrystalline diamond coatings. In the chemical vapour deposition process, which occurs at low pressure, massive diamond coatings are grown directly onto the substrate. These films have no pores and no binder. If a microcrystalline growth is produced the film surface shows crystallite tips with tetrahedral or pyramidal shape (see Fig. 5). The CVD process uses methane (CH4) as carbon source for the diamond synthesis. The gas phase also contains hydrogen and has to be activated by temperatures higher than 2000 1C. For tool coatings this is done advantageously by hot filaments that have to be arranged near the substrate surface (see Fig. 6) [12].
2.2. Features and advantages compared to other diamond tools The specific advantages of CVD diamond grinding layers compared to bonded diamond grinding layers are: the possibility to manufacture very fine ‘‘grain’’ sizes, a very homogeneous
coating thickness and the possibility to coat nearly every tool geometry including micro abrasive pencils (mandrels, points). The edges of the crystallite tips are extremely sharp. Edge roundnesses of less than 20 nm have been measured. These crystallite edges act as micro cutting edges in the grinding process [13–15]. The CVD diamond coating is characterised by the film thickness ds, the crystallite width bk and the crystallite height hk (see Fig. 5). The height of the crystallite tips can be regarded as the corresponding parameter to the grain protrusion, which is a characteristic value for bonded wheels. The crystallite width corresponds to the grain size. Widths of the crystallite tips below 1 mm up to 20 mm and more can be manufactured. The corresponding height of the crystallite tips is between 0.2 and 10 mm. A prolongation of the deposition process increases the film thickness as well as the size of the crystallite tips. In principle there is no upper limitation. However, film thickness of 30 mm requires coating process times of several days. The unique advantages of CVD diamond grinding layers are therefore the ability to fabricate very ‘‘fine-grained’’ grinding layers, especially for precision and micro-grinding applications. 2.3. Base bodies Coatable base body materials are ceramics (Si3N4 or SiC), which are mainly used for grinding wheels, and cemented carbides that are used for micro abrasive pencils with small diameters due to its higher strength against breakage. Steel and aluminium cannot be coated well with CVD diamond. Parts of the base body, which shall not be coated like flanges or bores, can be covered during the coating process. Tool sizes up to 175 mm diameter have been coated successfully. The available coating reactors allow substrate sizes up to 400 mm.
3. Precision machining with CVD diamond grinding wheels
Fig. 4. Grinding layer with bonded diamond grains D 15.
If the right design of hot-filament array is chosen together with appropriate process parameters (gas flow, methane content, pressure, temperature) very homogenous diamond layers can be deposited. For the grinding wheels of Fig. 6 the mean film thickness was 8.3 mm with a standard deviation of 70.12 mm. The base bodies were manufactured with roundness and cylinder form accuracies of 0.8 mm. The running of the grinding wheel is adjusted after mounting onto the grinding machine. A total running accuracy of 1–2 mm could be achieved as the addition of base body form accuracy, film thickness accuracy and mounting accuracy. These CVD diamond grinding wheels had a crystallite tip width of 8 mm and a crystallite tip height of 2 mm. An external grinding process with glass workpieces was performed at a machine tool
Fig. 5. CVD diamond grinding layer, schematic cross-section, prepared breakage section and inclined view onto the top surface. Geometrical parameters: ds film thickness, bk width of the crystallite tips, hk height of the crystallite tips.
ARTICLE IN PRESS J. G¨ abler, S. Pleger / International Journal of Machine Tools & Manufacture 50 (2010) 420–424
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Fig. 6. Hot-filament activated CVD process to coat two peripheral grinding wheels diameter 80 mm with diamond (left), grinding wheel after coating (right).
Fig. 8. CVD diamond grinding after grinding process showing flattenings of the crystallite tips. Fig. 7. CVD diamond grinding layer with crystallite tip widths of 0.5 mm.
company. A workpiece roughness of Rz= 1.9 mm and Ra =170 nm was achieved. With a much finer grinding wheel – crystallite tip width 0.5 mm and crystallite tip height 0.2 mm (see Fig. 7) – a face grinding of glass was performed at a manufacturer of glass products. The workpieces had a roughness of Rz =33 nm and Ra =5 nm, which can be regarded as optical quality. The wear of CVD diamond grinding layers occurs mainly in the form of flattenings of the crystallite tips (see Fig. 8). A few breakages of some crystallite tips have been observed after grinding of cermet. Film delaminations did never occur. 3.1. Technologies for truing and recoating When CVD diamond grinding layers are worn out their roughness cannot be recreated by sharpening on the grinding machine. So they correspond to bonded single-layer diamond tools. When using CVD diamond grinding tools an intensive consideration of the running errors is therefore inevitable to achieve good workpiece qualities (see above) since they cannot be trued. To avoid this disadvantage new approaches are under investigation, where the CVD diamond grinding layer is ablated by forceless ablation processes. This can be done by electrodischarge machining (EDM) [16] or by a short-pulsed laser, preferably a femto-second laser. The EDM requires electrically conductive diamond films, which can be produced by boron doping during the deposition. The process route foresees firstly a
Fig. 9. CVD diamond grinding layer after EDM.
profiling of the mounted wheel to achieve the required running accuracy and secondly a truing of the diamond layer to receive a specific roughness that enables a grinding with the machined diamond layer. Preliminary results showed, that an electro-discharge machining of a conductive CVD diamond film can produce a sharp-edged, ‘‘fine-grained’’ roughness (see Fig. 9). The roughness height was about 1 mm. With Raman spectroscopy it was proved that the diamond was not damaged in a way that it loses its hardness.
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Fig. 10. CVD diamond abrasive pencils, diameter 100 mm (left) and diameter 50 mm (right), hair as size comparison.
4. Micro-machining with CVD diamond abrasive pencils Another advantage of the CVD diamond grinding layers is the ability to coat very tiny tools. Thus micro abrasive pencils have been manufactured and tested. The tool diameters ranged from 2 mm down to 50 mm (see Fig. 10). With a CVD diamond abrasive pencil diameter of 2 mm a tool life test was performed at the Braunschweig Technical University. Hardened steel was ground and a specific material removal V’w of 410 mm3/mm was achieved. The machining of steel with a diamond tool is not possible under normal conditions, since a massive wear arises during contact of diamond with steel at temperatures above app. 400 1C. This did not happen in this application, since the material removal rate was very low during the micro-machining. As a consequence the cutting temperatures kept low so that no chemical wear occurred. In an industrial application test an internal grinding of steel parts with laser machined nozzle holes of 400 mm diameter have been successfully performed using a cylindrically CVD diamond micro abrasive pencil with a diameter of 125 mm. 5. Conclusion Diamond is the first choice for many precision and micromanufacturing tasks, both for cutting as for abrasive machining processes. A broad range of diamond types are used today: monocrystals, polycrystals, sintered diamond (PCD) and CVD diamond coatings. The latter ones can be used advantageously for grinding wheels as well as for abrasive pencils. Machining tests have shown the performance to manufacture workpiece surfaces with optical quality as well as micro-structures down to 50 mm. References ¨ ¨ [1] E. Brinksmeier, R. Glabe, B. Lunemann, Diamond machining of diffractive optical patterns by using a nanometer-stroke Fast Tool Servo. in: Laser Metrology and Machine Performance VIII, Proceedings of the Eight International Conference and Exhibition on Laser Metrology, Machine Tool, CMM & Robotic Performance, Lamdamap, 2007, pp. 232–241.
[2] F. Klocke, C. Bertalan, High-precision hard turning of cemented carbides. in: M. Weck, European Society for Precision Engineering and Nanotechnology, ¨ Produktionstechnologie, Aachen, Physikalisch-TechFraunhofer-Institut fur nische Bundesanstalt, Braunschweig: EUSPEN International Topical Conference on Precision Engineering, Micro Technology, Measurement Techniques and Equipment (2003, Aachen, Germany), Proceedings, vol. 1, Voerde: Riehm, 2003, pp. 329–332. ¨ Werkzeuganwendungen. in: Otti e.V. [3] P. Feuchter: Massiver CVD-Diamant fur ¨ (Organizer): Otti-Profiforum CVD-Diamant (Wurzburg, Germany, 29.6.2006). Proceedings. Regensburg: Otti e.V. ¨ [4] C. Flucke, R. Glabe, E. Brinksmeier, Manufacturing of moulds for the replication of prismatic microstructures by a novel diamond cutting process, Industrial Diamond Review 1 (2007) 25–30. [5] J. Yan, T. Oowada, T Zhou, T. Kuriyagawa, Precision machining of microstructures on electroless-plated NiP surface for molding glass components. Journal of Materials Processing Technology 209 (10) (2009) 4802–4808. [6] H. Suzuki, T. Moriwaki, Y. Yamamoto, Y. Goto, Precision cutting of aspherical ceramic molds with micro PCD milling tool, CIRP Annals—Manufacturing Technology 56 (1) (2007) 131–134. [7] P. Heaney, A. Sumant, C. Torres, R. Carpick, F. Pfefferkorn, Diamond coatings for micro end mills: enabling the dry machining of aluminum at the microscale, Diamond and Related Materials 17 (3) (2008) 223–233. [8] H. Sein, W. Ahmed, I.U. Hassan, N. Ali, J.J. Gracio, M.J. Jackson, Chemical vapour deposition of microdrill cutting edges for micro- and nanotechnology applications, Journal of Materials Science 37 (23) (2002) 5057–5063. ¨ [9] A. Floter, P. Gluche, K. Bruehne, H. Fecht, Diamond coat hones the cutting edge, Metal Powder Report 62 (2) (2007) 16–20. [10] J. Webster, M. Tricard, Innovations in abrasive products for precision grinding, CIRP Annals—Manufacturing Technology 3 (2) (2004) 597–617. ¨ [11] H.-W. Hoffmeister, J. Gabler, A. Wenda, Production of micromechanical ¨ systems with microcutting. in: F. Waldele, G. Wilkening, J. Corbett, P. ¨ McKeown, M. Weck, J. Hummler (eds.), Progress in Precision Engineering and Nanotechnology, Ninth International Precision Engineering Seminar, Fourth International Conference on Utraprecision in Manufacturing Engineering Braunschweig, vol. 2, pp. 524–526. [12] C.-P. Klages, L. Sch¨afer, Hot-filament deposition of diamond, in: B. Dischler, C. Wild (Eds.), Low-Pressure Synthetic Diamond, Springer, Berlin, 1998, pp. 85–101. [13] C. Borges, P. Magne, E. Pfender, J. Heberlein, Dental diamond burs made with a new technology, The Journal of Prosthetic Dentistry 82 (1) (1999) 73–79. [14] H. Sein, W. Ahmed, C. Rego, Application of diamond coatings onto small dental tools, Diamond and Related Materials 11 (3–6) (2002) 731–735. [15] A. Fernandes, E. Salgueiredo, F. Oliveira, R. Silva, F. Costa, Directly MPCVD diamond-coated Si3N4 disks for dental applications, Diamond and Related Materials 14 (3–7) (2005) 626–630. [16] K. Suzuki, M. Iwai, T. Uematsu, A. Sharma, Development of a grinding wheel with electrically conductive diamond cutting edges, Key Engineering Materials 257–258 (2004) 239–244.
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Precision and micro CVD diamond-coated grinding tools n ¨ Jan Gabler , Sven Pleger Fraunhofer Institute for Surface Engineering and Thin Films IST Bienroder Weg 54 E, 38108 Braunschweig, Germany
a r t i c l e in f o
a b s t r a c t
Article history: Received 17 March 2009 Received in revised form 18 June 2009 Accepted 20 October 2009 Available online 4 November 2009
Both for ultra-precision and for micro-machining diamond is used very often as tool material. The reason is the very high dimensional stability of diamond due to its extreme hardness. Diamond is used for two kinds of machining processes: for cutting, like turning, drilling or milling, as well as for abrasive processes, like grinding. Diamond cutting tools can be made with massive diamond (monocrystal, CVD diamond, PCD) or with diamond coatings. Standard diamond abrasive tools are made by bonding diamond monocrystals onto a base body. A new grinding layer technology is presented: chemical vapour-deposited microcrystalline diamond layers have crystallite tips with very sharp edges that can act for grinding processes. Base body materials and coating technology is presented. Application results of grinding experiments show that very high workpiece quality can be reached, e.g. a roughness Ra of 5 nm with glass workpieces. Truing and recoating techniques are discussed for reuse of worn CVD diamond grinding wheels. Micro grinding tools (abrasive pencils, burrs) can be manufactured with the same coating technology. Very small tools with diameters of 50 mm have been made and successfully tested. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Diamond CVD Tools Grinding Abrasive machining Micro-machining
1. Diamond tools for precision and micro-machining Diamond is used very often as tool material for ultra-precision and micro-machining. The reason for this is the very high dimensional stability of diamond due to its extreme hardness. In ultra-precision machining as well as in micro-machining very small depths of cut (ae) are used, which affords very sharp and stable cutting edges. Diamond is very often the material of the first choice not only for hard, abrasive or difficult-to-machine materials like nickel, glass or cemented carbide but also for ductile and soft materials, like aluminium, copper or plastics. Diamond is used for the two kinds of machining processes: for cutting, like turning, drilling or milling, as well as for abrasive processes, like grinding. 1.1. Diamond tools for cutting Diamond cutting tools can be separated into tools with massive diamond and tools with diamond coatings. 1.1.1. Cutting tools with massive diamond Three different types of massive diamonds are used for tools: monocrystalline diamond, polycrystalline diamond and sintered n
Corresponding author. Tel.: + 49 531 2155 625. ¨ E-mail addresses: [email protected] (J. Gabler), [email protected] (S. Pleger). 0890-6955/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2009.10.008
diamond. The monocrystals can be from natural source or from high-temperature high-pressure (HTHP) synthesis. For ultraprecision cutting mostly natural monocrystals are used [1]. HTHP diamond is used in particular when large widths of cut are required, since these monocrystals are available in larger dimensions (up to 18 mm) than natural monocrystals. Monocrystal ultra-precision cutting tools are mainly used with a corner radius re (see Fig. 1) for turning or milling operations. The two main quality criteria for diamond cutting tools are the waviness of the corner radius re and the cutting edge radius rb. The waviness, which is the amount of deviation from a true circle, can be down to 50 nm for highest quality tools. The cutting edge radius rb of new tools is between 15 and 40 nm. After a runningin, tools can have a cutting edge radius of 100 nm. Metal-matrix sintered diamonds are called ‘‘polycrystalline diamond’’ (PCD) despite they have no polycrystalline nature. Their structure can be called multi-crystalline, since they consist of small micron size diamond crystals that are bonded in a cobalt binder matrix. This two-phase system has a lower hardness than mono- or polycrystalline diamond. The cutting edge radius as well as the form accuracy is lower compared to monocrystal diamond due to the matrix component material structure. Nevertheless PCD can be used for hard precision turning [2]. Chemical vapour deposition cannot only be used to coat tools with a thin diamond layer (see Sections 1.1.2 and 2). It can also be used to fabricate thick (up to 0.5 mm) diamond plates that can be mounted on a tool holder, ground and thus be used similar to sintered PCD diamond plates. These CVD diamond inserts are of
ARTICLE IN PRESS J. G¨ abler, S. Pleger / International Journal of Machine Tools & Manufacture 50 (2010) 420–424
polycrystalline nature and can therefore today not be prepared with the same small cutting edge radius like monocrystal tools. But they have the same hardness as monocrystals. Their
421
application range lies therefore between the monocrystals and the sintered PCD diamond. Massive diamond tools can also be used to fabricate microstructures, e.g. V-shaped tools (see Fig. 2). They mostly use monocrystals [4,5], but also PCD [6]. The shape of these micro tools is restricted.
1.1.2. Diamond-coated cutting tools Diamond coated tools can be manufactured with more complex shapes, smaller angles and dimensions. The base body of these tools – mostly cemented carbide – can be shaped easily to very small and tiny geometries, which is not possible with massive diamond. End mills as well as drills with diameters down to 100 mm have been manufactured and successfully tested in ductile workpiece materials (see Fig. 3 right) [7,8]. With very small feed rates they can even perform a ductile machining of hard and brittle materials like silicon (see Fig. 3 right). The accuracy and surface quality of the machined workpiece is restricted due to the roundness of the coated cutting edges. The radius of a coated cutting edge is at least as large as the film thickness, which is in the range 3–8 mm for diamond-coated tools. If smaller cutting edge radii are required, a plasma ion sharpening is possible, which enables cutting edge radii of down to 0.5 mm [9].
1.2. Diamond tools for abrasive machining Fig. 1. SEM picture of a monocrystal diamond cutting edge [3].
Fig. 2. Micro cutting tool with monocrystal diamond [4].
The diamond grinding layers that are used today consist of diamond grains that are bonded onto a tool base body (see Fig. 4). The layer design differs mainly concerning the grain size and the bonding. The diamond is from natural or synthetic source. The grains can be monocrystals (natural or HPHT synthesis) or polycrystalline (from detonation synthesis). The shape differs from compact to blocky or it is complex shaped when polycrystalline diamond grains are used. The bonding may be polymer (resin), ceramic (vitrified) or metal [10]. The different grain sizes are achieved by crushing larger grains. Therefore, the edges of the crushed grains are uneven and mostly dull. Sharpening effects can be achieved, when a truing or crushing is performed with the bonded diamond tool and sharp breakage edges are generated on the diamond grains. The chemical vapour deposition that is used to coat cutting tools with diamond films (see Sections 1.1.2) can also be used to fabricate diamond grinding layers. This approach is the main focus of this contribution and detailed described in the following chapter 2.
Fig. 3. CVD diamond-coated spiral drill, diameter 150 mm (left) and drilled hole diameter 100 mm in silicon (right, [11]).
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2. Technology of CVD diamond-coated grinding tools 2.1. Coating process and material properties CVD diamond grinding layers use the sharp-edged roughness of polycrystalline diamond coatings. In the chemical vapour deposition process, which occurs at low pressure, massive diamond coatings are grown directly onto the substrate. These films have no pores and no binder. If a microcrystalline growth is produced the film surface shows crystallite tips with tetrahedral or pyramidal shape (see Fig. 5). The CVD process uses methane (CH4) as carbon source for the diamond synthesis. The gas phase also contains hydrogen and has to be activated by temperatures higher than 2000 1C. For tool coatings this is done advantageously by hot filaments that have to be arranged near the substrate surface (see Fig. 6) [12].
2.2. Features and advantages compared to other diamond tools The specific advantages of CVD diamond grinding layers compared to bonded diamond grinding layers are: the possibility to manufacture very fine ‘‘grain’’ sizes, a very homogeneous
coating thickness and the possibility to coat nearly every tool geometry including micro abrasive pencils (mandrels, points). The edges of the crystallite tips are extremely sharp. Edge roundnesses of less than 20 nm have been measured. These crystallite edges act as micro cutting edges in the grinding process [13–15]. The CVD diamond coating is characterised by the film thickness ds, the crystallite width bk and the crystallite height hk (see Fig. 5). The height of the crystallite tips can be regarded as the corresponding parameter to the grain protrusion, which is a characteristic value for bonded wheels. The crystallite width corresponds to the grain size. Widths of the crystallite tips below 1 mm up to 20 mm and more can be manufactured. The corresponding height of the crystallite tips is between 0.2 and 10 mm. A prolongation of the deposition process increases the film thickness as well as the size of the crystallite tips. In principle there is no upper limitation. However, film thickness of 30 mm requires coating process times of several days. The unique advantages of CVD diamond grinding layers are therefore the ability to fabricate very ‘‘fine-grained’’ grinding layers, especially for precision and micro-grinding applications. 2.3. Base bodies Coatable base body materials are ceramics (Si3N4 or SiC), which are mainly used for grinding wheels, and cemented carbides that are used for micro abrasive pencils with small diameters due to its higher strength against breakage. Steel and aluminium cannot be coated well with CVD diamond. Parts of the base body, which shall not be coated like flanges or bores, can be covered during the coating process. Tool sizes up to 175 mm diameter have been coated successfully. The available coating reactors allow substrate sizes up to 400 mm.
3. Precision machining with CVD diamond grinding wheels
Fig. 4. Grinding layer with bonded diamond grains D 15.
If the right design of hot-filament array is chosen together with appropriate process parameters (gas flow, methane content, pressure, temperature) very homogenous diamond layers can be deposited. For the grinding wheels of Fig. 6 the mean film thickness was 8.3 mm with a standard deviation of 70.12 mm. The base bodies were manufactured with roundness and cylinder form accuracies of 0.8 mm. The running of the grinding wheel is adjusted after mounting onto the grinding machine. A total running accuracy of 1–2 mm could be achieved as the addition of base body form accuracy, film thickness accuracy and mounting accuracy. These CVD diamond grinding wheels had a crystallite tip width of 8 mm and a crystallite tip height of 2 mm. An external grinding process with glass workpieces was performed at a machine tool
Fig. 5. CVD diamond grinding layer, schematic cross-section, prepared breakage section and inclined view onto the top surface. Geometrical parameters: ds film thickness, bk width of the crystallite tips, hk height of the crystallite tips.
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Fig. 6. Hot-filament activated CVD process to coat two peripheral grinding wheels diameter 80 mm with diamond (left), grinding wheel after coating (right).
Fig. 8. CVD diamond grinding after grinding process showing flattenings of the crystallite tips. Fig. 7. CVD diamond grinding layer with crystallite tip widths of 0.5 mm.
company. A workpiece roughness of Rz= 1.9 mm and Ra =170 nm was achieved. With a much finer grinding wheel – crystallite tip width 0.5 mm and crystallite tip height 0.2 mm (see Fig. 7) – a face grinding of glass was performed at a manufacturer of glass products. The workpieces had a roughness of Rz =33 nm and Ra =5 nm, which can be regarded as optical quality. The wear of CVD diamond grinding layers occurs mainly in the form of flattenings of the crystallite tips (see Fig. 8). A few breakages of some crystallite tips have been observed after grinding of cermet. Film delaminations did never occur. 3.1. Technologies for truing and recoating When CVD diamond grinding layers are worn out their roughness cannot be recreated by sharpening on the grinding machine. So they correspond to bonded single-layer diamond tools. When using CVD diamond grinding tools an intensive consideration of the running errors is therefore inevitable to achieve good workpiece qualities (see above) since they cannot be trued. To avoid this disadvantage new approaches are under investigation, where the CVD diamond grinding layer is ablated by forceless ablation processes. This can be done by electrodischarge machining (EDM) [16] or by a short-pulsed laser, preferably a femto-second laser. The EDM requires electrically conductive diamond films, which can be produced by boron doping during the deposition. The process route foresees firstly a
Fig. 9. CVD diamond grinding layer after EDM.
profiling of the mounted wheel to achieve the required running accuracy and secondly a truing of the diamond layer to receive a specific roughness that enables a grinding with the machined diamond layer. Preliminary results showed, that an electro-discharge machining of a conductive CVD diamond film can produce a sharp-edged, ‘‘fine-grained’’ roughness (see Fig. 9). The roughness height was about 1 mm. With Raman spectroscopy it was proved that the diamond was not damaged in a way that it loses its hardness.
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Fig. 10. CVD diamond abrasive pencils, diameter 100 mm (left) and diameter 50 mm (right), hair as size comparison.
4. Micro-machining with CVD diamond abrasive pencils Another advantage of the CVD diamond grinding layers is the ability to coat very tiny tools. Thus micro abrasive pencils have been manufactured and tested. The tool diameters ranged from 2 mm down to 50 mm (see Fig. 10). With a CVD diamond abrasive pencil diameter of 2 mm a tool life test was performed at the Braunschweig Technical University. Hardened steel was ground and a specific material removal V’w of 410 mm3/mm was achieved. The machining of steel with a diamond tool is not possible under normal conditions, since a massive wear arises during contact of diamond with steel at temperatures above app. 400 1C. This did not happen in this application, since the material removal rate was very low during the micro-machining. As a consequence the cutting temperatures kept low so that no chemical wear occurred. In an industrial application test an internal grinding of steel parts with laser machined nozzle holes of 400 mm diameter have been successfully performed using a cylindrically CVD diamond micro abrasive pencil with a diameter of 125 mm. 5. Conclusion Diamond is the first choice for many precision and micromanufacturing tasks, both for cutting as for abrasive machining processes. A broad range of diamond types are used today: monocrystals, polycrystals, sintered diamond (PCD) and CVD diamond coatings. The latter ones can be used advantageously for grinding wheels as well as for abrasive pencils. Machining tests have shown the performance to manufacture workpiece surfaces with optical quality as well as micro-structures down to 50 mm. References ¨ ¨ [1] E. Brinksmeier, R. Glabe, B. Lunemann, Diamond machining of diffractive optical patterns by using a nanometer-stroke Fast Tool Servo. in: Laser Metrology and Machine Performance VIII, Proceedings of the Eight International Conference and Exhibition on Laser Metrology, Machine Tool, CMM & Robotic Performance, Lamdamap, 2007, pp. 232–241.
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