Further development and precision of an existing FE-model of nanoscratch test for investigation on deformation behavior of PVD nitride hard coatings
Başlık çevirisi mevcut değil.
- Tez No: 644480
- Danışmanlar: PROF. DR. GERHARD HİRT, PROF. DR. KİRSTEN BOBZİN
- Tez Türü: Yüksek Lisans
- Konular: Metalurji Mühendisliği, Metallurgical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2017
- Dil: İngilizce
- Üniversite: Rheinisch-Westfälische Technische Hochschule Aachen
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 96
Özet
Özet yok.
Özet (Çeviri)
The aim of this work is the investigation of deformation and damage behavior of chromium based nitride hard thin PVD coatings, which are deposited by a hybrid process combining dcMS and HPPMS techniques. The type of coatings investigated in this work are widely preferred for the tools used in plastics processing industry due to their high wear and corrosion resistance. Besides, the hybrid deposition technique enables the production of dense, smooth coatings with high deposition rates and also combines the advantages of two different deposition techniques. While there are several mechanical testing techniques such as tensile, compression or impact testing for bulk materials, the mechanical properties of such thin coatings can only be investigated with some special methods like nanoindentation or nanoscratch testing. In such mechanical testing methods, very small loads and indenter tip sizes are used so that the investigated material undergoes deformation in nano- and micro-scale. Nanoindentation is conducted only with the application of a normal load and it allows the determination of indentation modulus EIT and universal hardness HU of such thin coatings. On the other hand, during a nanoscratch test a normal loading is applied together with a tangential motion, which means two different forces are applied on the coating surface in perpendicular directions. After the indentation step, the indenter slides over the coating and deforms it with the chosen force regime. In this way, the resistance of coating and coating/substrate compound against plastic deformation and against cohesive or adhesive damage types can be investigated. Furthermore, in nanoscratch testing a plastic deformation of the coating can be achieved even at very small contact depths. During this work the investigated coating material (Cr,Al)N over AISI 420 substrate was subjected to nanoscratch test with three different constant normal forces F = 400 mN, F = 500 mN and F = 600 mN. Afterwards, the nanoscratch tracks over the surface of (Cr,Al)N coating were analyzed using SEM and CLSM imaging techniques. SEM images of coating surface and cross-section fracture of the coated specimen allowed the examination of plastic deformations and crack formations. With the help of CLSM images, the residual scratch depth profiles over the nanoscratch tracks were generated for each normal force. In each case, the maximum residual scratch depths hmax were found to be more than 10% thickness of the initial (Cr,Al)N coating thickness,meaning a deformation of the substrate also occurred. The SEM images and depth profiles enabled to observe the extent of plastic deformation in (Cr,Al)N coating and compare them for different forces after the nanoscratch tests. For each normal force, the plastic deformation of the AISI 420 steel substrate was also observed along with the coating deformation in SEM images. Along with the experimental investigation methods, the finite element method (FEM) was employed during the work. The goal was to develop and improve an existing FEM model for the simulation of nanoscratch tests using Abaqus FEA software. Furthermore, in contrast to the experiments, FEM simulation of these nanoscratch tests could make the examination and observation of stress and strain distributions at any location of the specimen model possible. This could allow to see where stresses and strains are concentrated during a nanoscratch test, which is quite important to be able to understand the deformation behavior and the failure modes of the coating/substrate system. Furthermore, to be able to simulate the formation of cracks during such nanoscratch tests, XFEM module of Abaqus was employed and coating part of the model was defined as an enriched region. Cohesive segments method in XFEM was chosen to model the crack formation. The compressive residual stresses that are generated during the coating deposition were also incorporated in the model with two different stress magnitudes, in order to examine their effect on the deformation and damage behavior of the coating. The parameters for coating and substrate model regarding their elastic-plastic properties and damage behavior were obtained from the previous works at Surface Engineering Institute (IOT) and from the literature. Before starting nanoscratch simulations, mesh convergence tests were carried out to find out the optimum mesh density, which give the most accurate solution. After running nanoscratch simulations for each constant normal force F = 400 mN, F = 500 mN and F = 600 mN, residual scratch depth profiles similar to experimental ones were created from the deformed specimen model. The experimental and simulation depth profiles were compared for each force. While the shapes of indentations looked the same, some differences in maximum residual scratch depth hmax values were found. It was also observed that this difference decreased with the reduction of normal force. These differences were in fact not surprising, due to the assumptions mentioned in section 5.3. Furthermore, stress and strain distributions throughout the model after the simulation of nanoscratch were presented. Stresses were shown as maximum principal stresses to explain the various possible cohesive or adhesive failure modes of the coating. The regions, where these tensile or compressive stresses were high and concentrated were shown for each different normal force cases and it was explained that these regions indicated possible failure sites. This was also proved by the formation of XFEM cracks in the coating model at the locations where the maximum principal stresses were found to be highest. Furthermore, the distributions of residual plastic strain magnitudes were presented for different normal forces and this allowed to find the locations with highest plastic strains, which means finding the most deformed parts of the coating-substrate system. On the other hand, the effect of the magnitude of the compressive residual stresses of the coating on the deformation and damage behavior of the model was also presented. It was shown that increasing compressive residual stress magnitude in the coating can have improving effects on its deformation and cohesive damage resistance. Finally, the results of experimental investigation and FEM simulations were compared. The possible reasons for differences in the results due to the considered assumptions were subsequently discussed.
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