Diamond blades exceptionally wear resistant and extremely sharp P. Gluche 1, S. Strobel 1, H.-J. Fecht 2 1 GFD Gesellschaft für Diamantprodukte mbh, Lise-Meitner-Str. 13, 89081 Ulm, Germany 2 University of Ulm, Institute of Micro and Nanomaterials, Albert-Einstein-Allee 47, 89081 Ulm, Germany Abstract: Slitting is mainly seen as an insignificant part of web handling and changing blades permanently is a necessary evil. Blades with a coated finish are often regarded as being not sharp enough. A new approach to overcome this dilemma is the use of diamond coated, plasma sharpened blades (PSDblades). These blades typically surpass the lifetime of uncoated steel (ceramic/carbide) blade by a factor 800-1000 (20-40) times. This enables for the first time a very high and constant cutting quality and very low maintenance effort. The technical base as well as best practice examples will be presented. Introduction In almost all industrial applications cutting is an important process in the production chain. For the converting industry, e.g. sheeting or sizing of plastic foils, paper, fibre, cloth, the cutting procedure is realized by slitting machines using circular and slotted blades. Since there is a strong tendency toward tougher and more robust materials, the cutting performance and lifetime of standard steel blades often reach their limit. The most important parameters determining the performance of a cutting blade are: 1) The sharpness; better described as cutting ability This property includes many parameters, like the radius of curvature r c at the cutting edge, the friction, the blade angle, the surface roughness etc. In this paper we concentrate on the most important parameter, the radius of curvature r c, that determines the separation of the material at the cutting edge at a microscopic scale. 2) The durability or lifetime; better described as edge-holding property This property includes also many parameters, like the hardness of the blade material or the micro geometry. But it is also but also influenced by the cutting process itself like speed, friction, temperature. A blade fails, if the cutting quality failed the requirements, which corresponds to a cutting ability failure. Standard wear is the fundamental mechanism that dulls steel blades by increasing the
radius of curvature. The blade has to be changed if a certain r cmax is reached. The harder the blade material is, the lower is the wear propagation and has consequently higheredge-holding properies. The hardness of a blade can be increased by applying a thin hard-coating (e.g. by PVD) like TiN or CrN on the blade s surface of the finished steel blade. However, this approach reduces automatically the cutting ability since the radius of curvature is increased by the thickness of the hardcoating. A maximum lifetime increment by a factor of 5 is expected in contrast to a standard steel blade. Another approach is the substitution of the blade material e.g. by cemented carbide (also called solid carbide, tungsten carbide, WC) or ceramics (typically ZrO). The production of these blades require diamond grinding tools and the fabrication process is therefore expensive. These types of blades can exhibit the lifetime of a standard steel blade by a factor of 20-40. Here we report on the utilization of the hardest possible coating - diamond. In contrast to other thin film hard-coatings the diamond film is applied as a thick film on a cemented carbide substrate blade. The film thickness is in comparison to PVD hard-coatings rather thick (12-15µm) and needs to be re-sharpened after coating. The cutting ability (small r c ) is typically achieved by mechanical grinding and polishing. Even standard razor blades are fabricated this way. Another sophisticated approach is to utilize wet chemical etching in order to form a pointed cutting edge applied for instance for the fabrication of scalpel blades. This procedure is however not easy to control. In this paper we will report on a non-mechanical procedure to re-sharpen blades based on the utilization of plasma sharpening. Experimental A) Diamond deposition For the hard-coating we are using a nanocrystalline diamond film, which is deposited by hot filament CVD (Chemical Vapor Deposition) [1]. The substrate blades are cleaned, nucleated and placed in the reactor vacuum chamber close to the filaments. Methane (CH 4 ) serves as carbon source. By heating the filaments between 1900-2200 C the methane is dissociated and CHx radicals are generated. At the substrate surface the carbon condenses as strong bound carbon (sp 3, diamond) or weak bound carbon (sp 2, graphite). The unwanted sp 2 -bonded carbon has to be removed. This is done simultaneously during the deposition process by adding a huge amount of hydrogen to the gas phase (typically 1-5% CH 4 in 99-95% H 2 ). The hydrogen is dissociated by passing the filaments and forming atomic hydrogen, which etches the sp 2 -bonded carbon on the substrate s surface. Thus, if the dynamic equilibrium of etching and deposition of sp 2 - and sp 3 -hybridized carbon is adjusted correctly, a pure diamond film can be grown. In contrast to PVD (sputtering), no ion bombardment is involved and the film shows a very homogeneous morphology. In this report we concentrate on nanocrystalline diamond films with a thickness of 12-18 µm and an average grain size of approx. 10 nm. These films show a very low surface roughness (approx. 10-20 nm) and a very high fracture toughness of 5 GPa. Figure 1(a) shows a cross-sectional scanning electron microscope (SEM) picture of the asdeposited diamond film on cemented carbide. Figure 1(b) shows the surface roughness (16 nm) of a nanocrystalline diamond film deposited on atomically smooth silicon.
nano crystalline diamond film surface roughness: approx. 1µm average crystal size: 30-50nm 10µm (a) (b) Fig 1. Cross-section of a nanocrystalline diamond film on a carbide blade (SEM picture) (a) and roughness analysis (atomic force microscopy) of a nanocrystalline diamond film deposited on a very smooth surface (b). Since the deposition process requires high temperatures, steal is not a suited substrate. Well suited substrates for example are cemented carbides with low Co content of typically less than 12 %. B) Plasma sharpening In contrast to thin hard-coatings we used thick, diamond film (12-18 µm). In consequence the cutting ability is fully lost. Fig. 2 shows SEM photographs of a cemented carbide blade before and after diamond coating. (a) (b) Fig. 2 SEM micrograph of the cutting edge of a cemented carbide substrate. (a): as grinded; (b): diamond coated. The increase of the radius of curvature r c is clearly visible. The high coating thickness fulfils the following requirements. It mechanically stabilizes the substrate-diamond interface and offers a high residual wear volume after plasma sharpening.
In order to reduce the radius of curvature, we apply the plasma sharpening diamond process (PSD). In contrast to the diamond deposition, this process uses directed ion bombardment. Activated ions and radicals are accelerated under a small angle towards the cutting edge. This results in an anisotropic removal of the diamond film. The angle of ion bombardment can be adjusted in such a way that approx. 75 % of the initial diamond thickness remains untouched. This process can be considered being similar to sandblasting. However having reactive components and having the smallest possible blasting powder with atomic-sized grains. The Ion energy can be adjusted very precisely allowing a controlled impact of ions and resulting in a highly reproducible removal rate. Fig 3. SEM cross-section analysis: (left) uncoated carbide blade; (middle) diamond coated carbide blade;(right) diamond coated and plasma sharpened carbide blade. The cross-sectional SEM photographs of blades (Fig. 3) clearly show the sharpening effect resulting in a visible reduction of the radius of curvature. Fig. 4 shows a top view of the cutting edge before and after plasma sharpening. Fig. 4 SEM top view of the cutting edge of as-coated (a) and plasma sharpened diamond blade (b).
The intensity of the plasma polishing process is well controllable by the exposed time in the plasma, meaning that the radius of curvature r c can be adjusted to customer s needs. Figure 5 shows the extreme case of a plasma sharpened diamond razor blade for shaving purposes. The radius of curvature could be successfully reduced to a level below 50 nm. Fig. 5 SEM cross-section SEM photograph of diamond coated and plasma sharpened razor blade. Middle and right: high resolution SEM. The radius of curvature r c could be reduced below 50nm. In order to characterize the cutting ability a string cutting test has been established. There the cutting edge is driven perpendicular against a biased (F y ) polymer string. The cutting force F x is measured during the displacement process resulting in a force vs. dislocation curve F x =f(s x ). The maximum force is used to characterize the cutting ability (indirect characterization of r c ) of the blades. Fig. 6 shows the experiential setup and Fig. 7 schematically shows the measurement process.
Fig 6. Photo of the experimental set up of the string cutting test. Fig. 7 Schematic setup of string cutting test and experimental measurement result Results and discussion In order to prove the influence of the process time on the cutting ability, a blade with 15 cutting angle was sharpened successively. The cutting ability of the uncoated blade was in the range of 100-800 mn. The strong variation of force readings is originated from the inhomogeneous grinding process of the cutting edge (see also Fig. 2a). The diamond coating smoothens out the initial chipping of the cutting edge, which in turn yields in a lower variance of the cutting forces after the sharpening.
The sharpening process was interrupted after certain process steps and the blade was characterized utilizing the string cutting test. Fig. 8 shows the result of the plasma sharpening process. This graph demonstrates that the sharpness can be adjusted to the customer s needs. Fig. 8 Influence of process time on the cutting ability. The sharpness increases with increasing process time. In order to compare industrial available blades, the string cutting test was applied to different commercially available blades (see Fig. 9).
Fig. 9 Maximum force readings of commercially available cutting blades Finally different blade geometries were fabricated and tested in industrial environments. In order to compare these results, tungsten carbide and ceramics blades were also introduced into the test. In order to receive statistically relevant data, a total of 150 blades has been tested. As a testing material, a 0,2 mm thick plastic foil with TiO filling material has been used. These test results are presentd in Fig. 10. The tungsten carbide and ceramic blades showed nearly the same low lifetime of approx. 1.5 to 2 days. The diamond blades however showed a significant increased lifetime of approx. 36 days. In comparison to carbide blades, this corresponds to a lifetime increase of 24 times.
Fig. 10. Comparison of the lifetime of tungsten carbide, ceramic and plasma sharpened diamond cutting blades. Summary In order to maximise the lifetime of cutting blades, we applied the hardest available material, diamond, as a thick coating on cemented carbide blades. In contrast to conventional thin hardcoatings like TiN, a thickness in the range of 12-18 µm has been chosen in order to enable a high wear volume at the cutting edge after sharpening and to protect the diamond/carbide interface. A plasma sharpening process enables a controllable and adjustable cutting ability. The final blade offers a high residual wear volume (approx. 75 % of the initial diamond thickness) and additionally reduces the radius of curvature even below the value of the initial blade. From the string cutting characterization we showed, that this technique can be utilized in a wide span of applications from industrial blades to shaving blades. Cutting performance tests in industrial environments showed a lifetime increase of up to 24 times in contrast to conventional cemented carbide blades. This technology allows to generate custom sharpened blades, optimized for a maximum lifetime in industrial applications. References [1] M. Wiora, K. Brühne, A. Flöter, P. Gluche, T.M. Willey, S.O. Kucheyev, A.P. van Buuren, A.V. Hamza, J. Biener and H. J. Fecht, Grain size dependent mechanical properties of nanocrystalline diamond films grown by hot filament CVD. Diamond and Related Materials 18, pp. 927 930 (2009). About the Authors Dr.-Ing. P. Gluche is the managing director of GFD Gesellschaft für Diamantprodukte mbh in Ulm, Germany, founded in 1999. He received his PhD in engineering at the University of Ulm. Stefan Strobel is a senior engineer at GFD Gesellschaft für Diamantprodukte mbh and head of the diamond blade development and production. He received his diploma degree in physics from the University of Ulm. Prof. Dr. H.-J. Fecht is director of the Institute of Micro and Nanomaterials, which he founded in 1997 at the University of Ulm, Germany. As an expert in the field of materials science, materials engineering, and nanotechnology, he received the prestigious G. F. Leibniz award in 1998 and has co-authored more than 400 technical papers and co-organized several international conferences and workshops.