| 1. |
- Shoja, S., et al.
(författare)
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Enhanced steel machining performance using texture-controlled CVD alpha-alumina coatings : Fundamental degradation mechanisms
- 2024
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Ingår i: International Journal of Machine Tools and Manufacture. - 0890-6955. ; 197
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Tidskriftsartikel (refereegranskat)abstract
- Cemented carbide inserts coated with CVD α-alumina, particularly those exhibiting a (0001) texture, have proven highly effective in steel turning. Despite the established superior performance of (0001) textured alumina coatings, the underlying reasons remain unclear. This study explores the influence of the crystallographic texture of alumina on wear mechanisms in various chip-tool contact zones on the insert rake face. The objective is to establish a fundamental understanding of the active degradation mechanisms and machining performance by relating coating texture to the orientation and deformation of individual Al2O3 grains. Two multilayered coatings, Al2O3 on Ti(C,N), featuring (0001)- and (112‾0)-textured CVD α-alumina, were assessed in dry turning of a bearing steel. The wear rate of the (112‾0) coating was double that of the (0001) coating. Worn coatings exhibit nano-terrace formation at the insert edge, likely due to chemical etching. In the sticking zone, plastic deformation leads to larger facets for grains oriented with the chip flow direction, while rounded surfaces result if this condition is not met. In the transition zone, both (0001) and (112‾0) textured coatings undergo increased plastic deformation accompanied by sub-surface dislocations. (0001) texture deforms more by basal slip creating a wavy coating pattern with steps present at larger misalignments of the lattice planes in neighboring grains while (112‾0) texture deforms by several slip systems creating elongated ridges and ruptured-like areas resulting in rougher surface. This difference in surface morphology is then inherited by the abrasion of submicron coating fragments embedded in the chip (more in (112‾0) texture) in the sliding zone resulting in an even rougher surface. Chemical reaction with the hot chip may also contribute to wear acting as an additional mechanism. This fundamental understanding contributes to the potential enhancement of steel machining using texture-controlled CVD alumina coatings, ultimately improving coated cutting tool performance.
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| 2. |
- Shoja, Siamak, 1980, et al.
(författare)
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Calculated and experimental Schmid factors for chip flow deformation of textured CVD α-alumina coatings
- 2021
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Ingår i: Surface and Coatings Technology. - : Elsevier BV. - 0257-8972. ; 412
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Tidskriftsartikel (refereegranskat)abstract
- A thorough analysis of Schmid factors (m) for three different (basal and two prismatic) slip systems and three different coating textures, (0001), (011¯2) and (112¯0), was done in order to understand the influence of CVD α-alumina coating textures on the ability of the coatings to deform plastically at different locations on the rake face of a cutting tool insert during a metal machining operation. Schmid factor diagrams were constructed using MATLAB/MTEX in order to visualize the angular dependence of an external force relative to the α-Al O crystals (grains) on the Schmid factor. The diagrams were also used to extract m-value frequency distributions for different slip systems and textures. In addition, lateral m-value distribution maps were obtained from experimental textured coatings using electron backscatter diffraction. These maps show the ability for neighboring grains to deform plastically in the coatings. Cutting tool inserts with differently textured α-Al2O3 coatings were subjected to dry machining of a quench-tempered steel. Using scanning electron microscopy, the microstructure and surface topography of the worn alumina layers were investigated and it was found that a flatter surface morphology and higher probability of discrete plastic deformation are connected to less wear. This was observed in the (0001)-textured sample, which also exhibited the highest m-values in the wear zone with highest temperatures and external forces. It was observed that basal slip is most easily activated, followed by prismatic slip systems 1 and 2 in this case. For (011¯2) and (112¯0) textured coatings the differences in m-values for the three slip systems are not that big, and the distributions are relatively wide. It is clear that the Schmid factor analysis forms a basis that is important for understanding crater wear, especially when it is connected to local plastic deformation, of textured CVD α-Al2O3 coatings. The methodology of this work can be expanded to other coating systems and also more generally to applications where it is of interest to analyze the deformation behavior and local plastic anisotropy of textured materials.
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| 3. |
- Shoja, Siamak, 1980, et al.
(författare)
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Microstructure investigation of textured CVD alumina coatings
- 2020
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Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 87
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Tidskriftsartikel (refereegranskat)abstract
- This study concerns the interfacial microstructure and texture development in three textured CVD alpha-Al2O3 coatings using X-ray diffraction, transmission Kikuchi diffraction, scanning transmission electron microscopy and energy dispersive X-ray analysis. It is well known that the performance of these types of coatings relies on the degree and type of texture. The aim of this work is to study the microstructure of three different CVD alpha-Al2O3 layers when deposited onto a Ti(C,N,O) bonding layer. The coatings were deposited onto cemented carbide/cobalt substrates (WC/Co). It was observed that grain boundary diffusion of W and Co occurred through the TiN and Ti(C,N) layers to the bonding layer/alpha-Al2O3 interface. This may disturb the alumina layer nucleation and early growth. Interfacial porosity was observed at the bonding layer/alpha-Al2O3 interface. The number of voids that were detected in the (0001) and (01 (1) over bar0)-textured coatings was similar when H2S was not used during the initial deposition step. When H2S was present during the nucleation step deposition of the alpha-Al2O3 more voids were introduced at the interface for the (0001)-textured samples. The alumina grain morphology developed from small (similar to 100 nm) equiaxed grains at the start of the alumina coating to larger (several microns) columnar grains at the top of the coating. The inner part of the alumina coatings had a more random orientation. The texture changed either: (i) gradually over several grains, or (ii) more abruptly from one grain to another; from more random orientations to the desired texture. The desired texture develops earlier when using H2S at the start for the (0001)-textured coatings, while the (01 (1) over bar0)-texture development benefits from the absence of H2S. Thus, in this study, H2S promotes (0001) texture and interfacial void formation.
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| 4. |
- Shoja, Siamak, 1980, et al.
(författare)
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On the influence of varying the crystallographic texture of alumina CVD coatings on cutting performance in steel turning
- 2022
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Ingår i: International Journal of Machine Tools and Manufacture. - : Elsevier BV. - 0890-6955. ; 176
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the mechanisms at the tool/chip interface during metal cutting is crucial in the production of almost every metallic component used in engineering applications. It is critical to have rapid, durable, and reliable machining processes. This work contributes to the understanding of mechanisms occurring on the tool in the secondary shear zone, and it is focusing on the tool side of the contact. Crystallographic textured Chemical Vapor Deposited (CVD) α-Al2O3 coated cutting tools are dominating the steel turning area, as they show an increased performance compared to coatings with randomly oriented grains. In this study, we investigate the effect of three different CVD α-Al2O3 textures on the initial rake crater behavior. This was done using a turning test designed to generate crater wear only in the alumina layer, which was deposited onto an inner Ti(C,N) layer, which in turn was deposited on a cemented carbide insert. With this approach, the influence of the underlying coating layer and substrate was reduced. Pre- and post-machining characterization of the different contact areas on the surfaces of the three textured CVD α-Al2O3 coatings, (0001)(0001), (011‾2) and (112‾0), was performed using scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and energy dispersive X-ray spectroscopy (XEDS). Plastic deformation, micro-rupture, abrasion and chemical reactions with the workpiece material are all identified as mechanisms involved in crater formation during turning. For the (0001)-textured coating, the observed low wear-rate is attributed to homogeneous basal-slip dominating plastic deformation, while for the (011‾2) and (112‾0) textures the main deformation mechanisms are attributed to heterogeneous plastic deformation, causing micro-rupture and abrasion, leading to higher wear-rates. The effect of a larger coating grain size is mainly seen as the formation of wider ridges and valleys, while the effect on wear rate was limited.
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