Fatigue Crack Growth in Bodies with Thermally Sprayed Coating (original) (raw)
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Fatigue Behavior of Bodies with Thermally Sprayed Metallic and Ceramic Deposits
Journal of Thermal Spray Technology, 2008
This paper summarizes the basic results of fatigue testing of bodies with both metallic and ceramic 10 thermally sprayed coatings. Three kinds of ceramic coatings (Al 2 O 3 ,C r 2 O 3 , and olivine) sprayed with DC plasma under identical conditions were investigated together with metallic Ni-5wt.%Al coatings 12 sprayed by wire arc, DC plasma, and HVOF. The elastic modulus of the deposited coatings was 13 investigated using four point bending and resonance method. Bending fatigue tests at resonance 14 frequency were performed with cantilever beam specimens. The processes taking place during the fatigue 15 test are identified and discussed. The morphology of the fracture surfaces was investigated together with 16 microstructure and porosity of the coatings. 17 Keywords elastic modulus, fatigue, fractography, residual 18 stress, thermally sprayed coatings 19 20 21 1. Introduction 22 The most sensitive part of any component from the 23 point of view of fatigue failure is its surface. The surface 24 sensitivity to crack initiation is further enhanced by other 25 forms of surface degradation (caused by loading and/or 26 environmental action). Therefore, the surface properties 27 considerably restrict the usage of classical engineering 28 materials for high-performance applications. Thermally 29 sprayed coatings and other surface modifications are often 30 used to increase component lifetime by preventing surface 31 degradation processes (Ref 1). The thermally sprayed 32 coatings are built of individual splats of different flattening 33 degrees, unmelted powder particles, in-flight resolidified 34 feedstock particles, small ÔdebrisÕ particles resulting from 35 splashing of individual splats at the point of impact, and 36 other features, all of that separated by a porosity network. 37 This complicated nature of thermally sprayed coatings 38 leads to significant differences in mechanical, thermal and 39 fatigue properties compared to bulk material of the same 40 chemical composition (Ref 2-5). Also, the mechanical 41 properties of the substrate material are often significantly 42 changed before and during the deposition process as a 43 result of sample preparation before spraying and/or the 44 thermal and mechanical loading of the substrate during 45 coating deposition. 46 The deposit can influence the fatigue properties of 47 coated part in two principal ways. Hard deposits often 48 show lower than bulk properties due to microcracking of 49 brittle splats. Thus, during fatigue loading the network of 50 small cracks may form or the branching of main fatigue 51 crack may appear in the coating (Ref 6). In this way, the 52 deposit can accommodate the deformation without 53 allowing a major crack to enter the coating/substrate 54 interface. The major fatigue crack then initiates on the 55 substrate surface close to large pores, abrasive particles 56 entrapped by grit-blasting, cracks formed during deposi-57 tion, etc. The crack initiation can be further inhibited by 58 constraining the local plastic deformation by hard deposit 59 as described in (Ref 7). Such mechanism requires dense 60 deposit at least neat the interface and good adherence 61 between the coating and substrate (Ref 7). 62 For softer deposits, near-bulk properties can often be 63 achieved, and considerable residual stresses may develop 64 as a result of particle quenching and deposit cooling. The 65 crack initiation sites can be either in the substrate or in the 66 deposit, depending on the fracture toughness, presence 67 and effectivity of stress concentration centers, and residual 68 stress. Compressive stress can inhibit crack initiation and 69 propagation and prolong fatigue life (Ref 7-10). On the 70 other hand, tensile stress will shorten the fatigue life. The 71 first layer of splats deposited on the substrate makes things 72 more complicated; its residual stress can be different from 73 the rest of the deposit (Ref 11) and may well control the 74 crack initiation in the substrate or act as a crack barrier for 75 cracks propagating from the deposit. Thus, the fatigue 76 resistance of bodies with deposits can be correlated to 77 multiple factors such as residuals stresses at different parts 78 of the deposit, deposit elastic modulus, microstructure, 79 fracture toughness. The properties and characteristics of 80 certain deposits can be found in the literature (Ref 4, 5, 81 12). Some of these deposits were chosen for the present 82 investigation. Alumina, chromia, and olivine sprayed with 83 water-stabilized DC plasma represent ceramics. The alloy 84
Size effect in transient thermal fatigue testing and thermo-mechanical screening of coatings
International Journal of Microstructure and Materials Properties, 2008
In an attempt to bring new ideas to thermal fatigue analysis of tools, we propose the principle of the two driving forces leading to damage by Thermal Fatigue (TF). After a definition of the two driving forces based on temperature and thermo-mechanical loadings, a procedure for evaluating them for in-service tools is proposed. In order to compare thermo-mechanical
Investigating Effect of Industrial Coatings on Fatigue Damage
2011
Investigating Fatigue is one the most important factors in designing most mechanical structure. The reason is that, in many cases, the specimens of the structure break down without any warning or signal. Nano and micron's Coatings are finding more and more applications in industry such as aerospace, automotive, and naval industries. The present article has a purpose. Firstly, it intends to explore the influence of four industrial coatings, namely, hardened chromium, embellished chromium, hardened nickel, and warm galvanizing, all of which have the thickness value at micron levels on fatigue specimens. Moreover, it aims to find the most convenient coating. To achieve these purposes, the abovementioned coatings with the thickness of 13 & 19 were coated on standard specimens who were made of CK45 steel under the same conditions. Then, the S-N curve of each sample was attained empirically according to the standard fatigue testing. Specimens are simulated in the finite element analysis according to experimental conditions and then S-N curve of each sample was attained. Finally, comparing the S-N curves, the most appropriate coating is introduced for the delineated conditions and based fatigue results may be predicted damage of coating.
Fatigue Life of Layered Metallic and Ceramic Plasma Sprayed Coatings
Procedia Materials Science, 2014
The application of thermally sprayed coatings can significantly enhance properties of coated parts such as thermal and wear resistance or biocompatibility. For example coatings prepared by HVOF are used for airplane landing gear parts and plasma sprayed coatings form heat shield on surface of turbine blades and vanes, several types of coatings are used in bone implants. In these application fields the fatigue behavior of coated components is of a paramount importance. The intrinsic properties of the deposited coating (modulus, microstructure, porosity etc.) play an important role. At the same time, the coating process can influence fatigue live through defects and residual stresses introduced to the substrate during spraying and associated preparation steps such as grit-blasting. The influence of substrate surface preconditioning and effect of individual layers of composite coatings on fatigue life were characterized. Plain substrates, grit-blasted substrates, and plasma sprayed specimens with one to three layers of coating were studies. The layered coatings were composed of alternating sequence of ceramic (Cr 2 O 3 ) and metallic (Ni10wt%Al) layers. Deflection controlled resonance bending (R = -1) fatigue test of flat specimens was performed. The deformation amplitude was 1.3 milistrain in crack initiation site and loading frequency was around 80Hz. The significance of the effect of surface treatment on fatigue properties was examined statistically using Wilcoxon test. From the obtained data the effect of individual layers was deduced. In order to explain the observed fatigue behavior, the fractographic analysis and other means of coating and substrate characterization were performed. Strain hardening in substrate was characterized by micro/nanohardness measurement and EBSD analysis. Residual stress in substrate was measured using neutron diffraction.
WIT transactions on engineering sciences, 2007
Impact testing is an efficient experimental procedure that enables the determination of the fatigue resistance of mono-and multilayer coatings deposited on various substrates, which is not possible with the common testing methods previously available. In this paper an advanced impact tester, capable of assessing the fatigue failure resistance of coatings working under cyclic loading conditions, is presented. The fatigue failure of the tested coating was determined by means of scanning electron and optical microscopy. The test results were recorded in diagrams containing the impact load versus the number of successive impacts that the tested coating can withstand.
Thermal fatigue failure induced by delamination in thermal barrier coating
International Journal of Fatigue, 2002
The paper presents the experimental and theoretical investigation on the thermal fatigue failure induced by delamination in thermal barrier coating system. Laser heating method was used to simulate the operating state of TBC (thermal barrier coating) system. The non-destructive evaluation such as acoustic emission (AE) detect was used to study the evolution of TBC system damage. Micro-observation and AE detect both revealed that fatigue crack was in two forms: surface crack and interface delamination. It was found that interface delamination took place in the period of cooling or heating. Heating or cooling rate and temperature gradient had an important effect on interface delamination cracking propagation. A theoretical model on interface delamination cracking in TBC system at operating state is proposed. In the model, a membrane stress P and a bending moment M are designated the thermal loads of the thermal stress and temperature gradient in TBC system. In this case, the coupled effect of plastic deformation, creep of ceramic coating as well as thermal growth oxidation (TGO) and temperature gradient in TBC system was considered in the model. The thermal stress intensity factors (TSIFs) in non-FGM (functional gradient material) thermal barrier coating system is analytical obtained. The numerical results of TSIFs reveal some same results as obtained in experimental test. The model is based on fracture mechanics theory about heterogeneous materials and it gives a rigorous explanation of delaminations in TBC system loaded by thermal fatigue. Both theoretical analysis and experimental observation reveal an important fact: delaminations are fatigue cracks which grow during thermal shocks due to compressive stresses in the loading, this loads the delaminations cracks in mixed I and II mode.
Journal of Alloys and Compounds, 2016
8 wt% yttria stabilized zirconia (8YSZ) is the standard ceramic top coat material used in thermal barrier coatings (TBCs) due to its excellent thermo-physical and thermo-mechanical properties. However, above 1200°C, YSZ has issues such as susceptibility to CMAS (Calcium Magnesium Alumino Silicates) attack and enhanced sintering which could lead to catastrophic failure of the TBC. Pyrochlores of rare earth zirconate composition such as gadolinium zirconate have shown to be resistant to CMAS attack and at the same time possess several other attractive properties. However, poor thermal cycling life of single layer gadolinium zirconate (GZ) TBC compared to single layer YSZ has been reported. Therefore, a double layered GZ/YSZ TBC with YSZ as the intermediate coating and GZ as the top coat and a single layer 8YSZ were deposited by the axial suspension plasma spray process. Additionally, a triple layer TBC (GZdense/GZ/YSZ) comprising of denser GZ coating on top of GZ/YSZ TBC was deposited. SEM analysis revealed a columnar microstructure in the single, double and triple layer TBCs. XRD analysis confirmed the presence of tetragonal prime and defect fluorite phases in the top surface of YSZ and GZ based as sprayed TBCs respectively. The single layer YSZ and GZ/YSZ multi-layered TBCs were subjected to thermal cyclic fatigue (TCF) testing at 1100°C and 1200°C. The triple layer TBC showed a higher thermal cyclic life at both the temperatures compared to the single and double layer TBCs. The failed TBCs at 1100°C were analyzed by SEM/EDS and image analysis. It was found that the failure modes in single layer YSZ and GZ based TBCs were different.
Phase-angle effects on damage mechanisms of thermal barrier coatings under thermomechanical fatigue
Scripta Materialia, 2001
The damage mechanisms of thermal barrier coatings are investigated using in-phase (IP) and outof-phase (OOP) thermomechanical fatigue (TMF) tests. During IP testing, TMF damage is governed by the susceptibility of the coating to buckling. In OOP tests, cracking of the substrate controls life. Cracks initiate either at the unprotected surface of the substrate or within the bond coat, nucleated at defects formed during thermal exposure. Ó
Rolling Contact Fatigue Behavior of Thermal-Sprayed Coating: A Review
Critical Reviews in Solid State and Materials Sciences, 2019
Rolling contact fatigue (RCF) of sprayed coating is complicated process relating to materials, structures, loading conditions, etc. Herein, the research works over the past decades in RCF of sprayed coatings are summarized here to provide a guidance to further explore the new research interests. Firstly, comprehensive summaries including the common RCF-resisted coating materials, the regular RCF setups, typical failure modes of sprayed coatings, and statistical characterization methods for RCF lifetime are illustrated. Secondly, the influence of surface integrity on RCF behavior of sprayed coating is discussed. The according mechanisms are also discussed by fracture analysis, numerical calculation, and finite element method (FEM). Thirdly, the influence of working condition on RCF behavior of sprayed coating is discussed. The shear stress is recognized as the major contribution of the RCF fracture, and the sliding during rolling contact will deteriorate the lifetime of sprayed coating. Additionally, some new techniques and methodologies have been reported in the investigations of RCF behaviors of sprayed coatings. Nondestructive testing (NDT) and signal processing methods are critical in the detection of micro-fractures within the sprayed coatings. Finally, we have proposed some suggests in future investigation of RCF of sprayed coating based on the previous research achievements.
Impact Testing a Capable Method to Investigate the Fatigue Resistance
Fracture of Nano and Engineering Materials and Structures
The impact testing is an efficient experimental procedure that enables the assessment of the fatigue strength of mono-and multilayer coatings, which was not possible with the common testing methods previously available. This paper presents a novel impact testing procedure capable to assess the fatigue failure resistance of HVOF thermal spray coatings working under cyclic loading conditions. From the experimental results it was concluded that the WC-CoCr coating deposited on P91 steel substrate showed superior fatigue strength in comparison to that of CrC-NiCr and Ni20Cr coatings deposited on the same substrate.