Papers by Keyword: CrAlN

Abstract: Chromium aluminum nitride (CrAlN) has been extensively studied because of high hardness, high oxidation and corrosion resistance, and good wear resistance. However, utilizing substrate treatments such as heating and voltage biasing during film deposition usually leads to relatively high surface roughness that affects wear rates. It has been found that sputter deposition at low substrate temperatures can produce nano-grain coatings with enhanced structure and mechanical properties. For this reason, the CrAlN in this study was prepared by a reactive co-sputtering technique without the substrate treatments. Effects of Al content on structure and mechanical properties were investigated by X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray spectrometry, atomic force microscopy, X-ray photoelectron spectroscopy, and nanoindentation. The results suggest that these CrAlN films formed as solid solutions by substitution of Al for Cr in the CrN crystalline structure. The deposition with increasing Al but fixed N leads to N deficiency, therefore at high Al content these films form under 1:1 stoichiometric nitride. This lowers film crystallinity and hence refined film morphology. Surface roughness and hardness of the films decreased from 5.737 to 1.135 nm and from 31.69 to 26.56 GPa, respectively. However, the solid solution strengthening arising from the further increase of the Al content causes these values to rebound to 2.466 nm and to 30.16 GPa.

Abstract: For better selection of coated cutting tools, TiAlN (Ti₅₀Al₅₀N) and CrAlN (Cr₅₀Al₅₀N) coatings were deposited onto ball-nose and square end mills using arc evaporation, and their cutting performances were evaluated using steel workpieces of various hardnesses. In particular, cutting tests were performed on three types of workpieces, made from S50C, SKD61, and SKD11 steels, having Brinell hardness numbers of HB220, HRC40, and HRC60, respectively. The results of the cutting experiments were elucidated and discussed in terms of the mechanical properties and anti-oxidation resistances of the different coatings. The results revealed that TiAlN-coated square end mills at high cutting speeds (V = 200 m/min ) had superior performance when used on steels with high hardness (SKD11), whereas CrAlN-coated ball-nose end mills were superior when used on low hardness steel (S50C). Therefore, CrAlN-coated ball-nose end mills are concluded to be suitable for the machining of low hardness steels, whereas TiAlN-coated square end mills are preferable for the machining of high hardness steels (SKD11).

Abstract: Sheet-bulk metal forming is an innovative process with a high potential to generate load-adapted parts with high precision. Bulk forming processes of sheet metals especially require high process forces, resulting in an intense contact pressure and, thus, in a very high abrasive and adhesive wear. As a method to reduce or avoid these common wear phenomena, even hardened or coated tool surfaces are not sufficient. The objective of this paper is to show an improvement of the tool resistance during an incremental forming process by an adapted tool design and the application of structured tool surfaces combined with coatings. For the tool surface the structure of the scarabaeus beetle serves as the basis for a bionic structure. This structure was manufactured by micromilling. Despite the high hardness of the tool material and the complex geometry of the forming tools, very precise patterns were machined successfully using ball-end milling cutters. The combination of bionic structures with coating techniques like physical vapor deposition (PVD) on plasma nitrided tool surfaces is very promising. In this work, the influence of process parameters (workpiece material, lubrication, tool design, stepwise infeed) on the tool resistance during the forming operation was analyzed experimentally. The results of the optimized forming tools were compared to conventional, unstructured, uncoated, and only plasma nitrided forming tools. The different tools were applied to 2 mm thick metal sheets made of aluminum (AlMg3) and steel (non-alloy quality steel DC04). As a result, the process forces could be reduced by a modified shape and surface of the tools. Thus, the lifetime of the tools can be enhanced.

Abstract: Sheet-bulk metal forming is a process used to manufacture load-adapted parts with high precision. However, bulk forming of sheet metals requires high forces, and thus tools applied for the operational demand have to withstand very high contact pressures, which lead to high wear and abrasion. The usage of conventional techniques like hardening and coating in order to reinforce the surface resistance are not sufficient enough in this case. In this paper, the tool resistance is improved by applying filigree bionic structures, especially structures adapted from the Scarabaeus beetle to the tool’s surface. The structures are realized by micromilling. Despite the high hardness of the tool material, very precise patterns are machined successfully using commercially available ball-end milling cutters. The nature-adapted surface patterns are combined with techniques like plasma nitriding and PVD coating, leading to a multilayer coating system. The effect of process parameters on the resistance of the tools is analyzed experimentally and compared to a conventional, unstructured, uncoated, only plasma nitrided forming tool. Therefore, the tools are used for an incremental bulk forming process on 2 mm thick metal sheets made of aluminum. The results show that the developed methodology is feasible to reduce the process forces and to improve the durability of the tools.

Abstract: The chromium nitride thin layers have became more and more popular in the last decade because of their good physical, chemical and mechanical properties. The present study deals the morphological characterization of hard Cr-N and CrAlN thin layers deposited on silicon (100) substrates by physical vapour deposition (PVD). The effect of the annealing temperature on the adherence and the thermal stability of CrN layers are considered. SEM observations and EDS microanalyses were performed. Cr-N films, 1μm thick, were annealed for 1 hour at 600 to 1000°C under flux of nitrogen. These layers presented a good thermal stability at low temperatures. Moreover SEM observations showed that the CrN films had a low adhesion at high temperatures. In addition, after annealing at 700°C for 4 hours, CrAlN coating of 1μm thickness presents a better thermal stability than CrN coating but with lower adherence. The results given by SEM-EDS and XRD are compared.

Abstract: CrN and CrAlN coating tools were heated and heat-preserved in electric stove to make oxidation experiment. The results were obtained through SEM, EDX and XRD. The results are as follows: The oxidation of CrN and CrAlN coating is induced by the micro-holes which were formed by the diffusion of nitrogen atoms and chromium ions towards the coating surface under sufficient thermal energy. With the incorporation of Al, CrAlN coating showed stronger oxidation resistance as compared to CrN. The dense mixture of Cr₂O₃ and Al₂O₃ makes oxygen diffusion through the film difficult and then oxidation and decomposing are alleviated effectively, therefore CrAlN coating can remain high oxidation resistance and high hardness after high temperature which make this coating tool be competent for high speed machining of the hardworking materials. In addition, the failure of the coating is primary induced by oxidation and the different expansion coefficients between coating and substrate.

Abstract: Recently, Cr-Al-N coatings have received more and more attention of researchers owing to its standing-out mechanical performances and superior chemical stability. In this present work, CrAlN ternary coatings were deposited by a bipolar pulsed dual magnetron sputtering (BPDMS) method and the effects of substrate negative bias on deposition rate, structure, hardness and adhesion of the coatings were investigated. The application of appropriate negative substrate bias voltage leads to dense structure, high hardness and excellent adhesion for CrAlN coatings.

Abstract: It is very important to minimize wear and friction in forming processes in order to avoid adhesion between work piece and tool. For the realization of these requirements, the PVD-coating system CrAlN was deposited and tested on substrates made from high speed steel 1.3343 by means of a reactive sputter process. The coatings were deposited as single- and multilayers with a metallic Cr-interlayer. Prior to realizing the appropriate coating design, the substrates were pre-structured. For this purpose natural surfaces were used as a pattern and tested in this research work. The skin of an insect serves as a model and its fine structures were reproduced on the substrate surface by milling. The generated specimens were analyzed with a scanning electron microscope, nanoindenter and ball on disc tester to compare the performance of the coating systems.