The present communication proposes a new model for the computation of the composite hardness of coated systems as a function of the relative indentation depth, the hardness of both coating and ...substrate and two material constants that characterize the performance of the film during the indentation test. The model is developed from several important considerations which can be summarized as follows: (1) The substrate starts to contribute to the composite hardness at penetration depths of the order of 0.07–0.2 times the coating thickness, as suggested in the literature. (2) Above such a boundary the composite hardness depends mainly on the intrinsic hardness of the coating, whereas below it such a hardness is determined essentially by the hardness of the mixture that encompasses the remaining part of the film and the plastically deformed substrate material. (3) The hardness of such a mixture is assumed to be constant, except for the possible indentation size effect that could be displayed by the substrate. (4) The composite hardness is given by a linear law of mixtures in terms of the hardness of the coating and such a mixture, and the volume fraction of both materials under the indenter, at any given depth of the latter. It is shown that the model proposed describes very well the hardness data obtained in different systems including: Ti and TiC formed on a chromium steel of a high carbon content; TiN; TiCN and CrN deposited on M2 steel; and TiN
0.55, TiN
0.65, TiN
0.75, ZrN
0.50, ZrN
0.60 and ZrN
0.70 deposited on 316L stainless steel substrate. The results are also compared with those derived from the models earlier advanced by Jönsson and Hogmark, Burnett and Rickerby, Chicot and Lesage and Tuck et al., without taking into consideration the indentation size effect of the film. It is shown that the modified version of the earlier model put forward by Korsunsky et al., published recently by Tuck et al., constitutes a particular form of the model here proposed.
The elastic modulus of thin films can be directly determined by instrumented indentation when the indenter penetration does not exceed a fraction of the film thickness, depending on the mechanical ...properties of both film and substrate. When it is not possible, application of models for separating the contribution of the substrate is necessary. In this work, the robustness of several models is analyzed in the case of the elastic modulus determination of a porous aluminium oxide film produced by anodization of an aluminium alloy. Instrumented indentation tests employing a Berkovich indenter were performed at a nanometric scale, which allowed a direct determination of the film elastic modulus, whose value was found to be approximately 11GPa. However, at a micrometric scale the elastic modulus tends toward the value corresponding to the substrate, of approximately 73GPa. The objective of the present work is to apply different models for testing their consistency over the complete set of indentation data obtained from both classical tests in microindentation and the continuous stiffness measurement mode in nanoindentation. This approach shows the continuity between the two scales of measurement thus allowing a better representation of the elastic modulus variation between two limits corresponding to the substrate and film elastic moduli. Gao's function proved to be the best to represent the elastic modulus variation.
The present study has been conducted in order to investigate the coupled effects of substrate microstructure and the sulphuric acid anodizing process, on the fatigue life of a 2017A aluminum alloy, ...by means of plane bending fatigue tests (R=−1). The effect of two different tempers, namely artificially aging (T6) and recovery annealing, prior to anodizing, was studied. The microstructural analysis of the substrate and anodic films was carried out by means of different techniques. It has been determined that the anodic film gives rise to a decrease in the fatigue properties of the heat treated alloy at all stress levels. However, at low stresses, the reduction in fatigue life is more pronounced for the annealed material. The effect of substrate microstructure on the morphology of the anodic layer and on the reduction of the fatigue life of the alloy is also discussed.
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A novel differential form of the integrated constitutive description earlier advanced by Jonas et al., for modeling the flow stress of austenite, is developed. The temperature and strain rate ...dependencies are introduced in the formalism through the temperature-dependent shear modulus of the material, the yield, saturation and steady-state stresses, as well as the time to achieve 50% dynamic recrystallization. The correlation between each of the above stresses and the Zener-Hollomon parameter is carried out by means of the Sellars-Tegart-Garofalo model employing the activation energy for the self-diffusion of Fe in austenite (Q=284kJmol−1). The proposed formalism involves the determination of the flow stress of the material by means of the numerical integration of three differential equations. In this way, it is possible to compute the flow stress both during the work-hardening and dynamic recovery stage, as well as from the onset of dynamic recrystallization up to the work softening transient and final achievement of the steady-state stress. Therefore, it is possible to predict the flow stress curves of austenite under both sharp and ramped transient loading conditions leading to the occurrence of dynamic recrystallization, a novel feature that cannot be accomplished by means of the early advanced model.
The determination of the elastic modulus of thin films by indentation at nano or micrometric scale is possible by applying different models allowing the separation of the substrate influence from the ...indentation measurement. However, for an accurate determination of the elastic modulus, the indentation depth and/or the contact area must be corrected by taking into consideration the frame compliance of the instrument and the blunted tip effect of the indenter. In nanoindentation, these two corrections are performed prior to the indentation experiments. However, in microindentation, some works showed that the compliance term does not have a constant value. As a result, a more consistent modeling must necessarily consider this compliance term as an unfixed parameter, at least for this range of loading. In addition, a discussion about the calculation of the contact area according to the methodologies of Oliver and Pharr (1992) 13 and of Hochstetter et al. (1999) 14 is also proposed to consider the sinking-in or piling-up effects, respectively, which can take place during the indentation process, depending on the mechanical behavior of the material. Finally, different weight functions and the model of Tricoteaux et al. (2010) 12 are applied to the raw and corrected data. This methodology is applied in microindentation and validated by nanoindentation for the determination of the elastic modulus of a TiHfCN thin film. A convergence of the results is observed only if the two corrections are taken into account. Accordingly, the elastic modulus of the TiHfCN film is found to be close to ~500GPa.
► The elastic modulus of TiHfCN thin film has been determined by indentation. ► Models are applied to separate the contribution of the substrate. ► The tip defect of the indenter and frame compliance are considered. ► The methodologies of Oliver and Pharr and of Hochstetter et al. are discussed. ► The elastic modulus of the film is found to be close of 500GPa.
A methodology for determining the thin film hardness from a microindentation loading curve is proposed. The loading curve is modelled to compute the dynamic Martens hardness using the indentation ...depth reached during the test. Moreover, the indentation size effect is taken into account by applying the strain gradient plasticity theory. Then, the dynamic Martens hardness and the hardness length-scale factor are used to express the applied load as a function of the indentation depth. The proposed model involves three parameters: (
i) the dynamic Martens macro-hardness, equivalent to the hardness obtained for an infinite applied load, (
ii) the hardness length-scale factor, which represents the material resistance to plastic deformation under indentation and (
iii) a corrective load, considering the rounded tip effect of the indenter and the zero shift. The model is validated on a 316L stainless steel which subsequently is used as a substrate material for two different Diamond Like-Carbon thin films. The coated systems involved both a hydrogen-free mostly amorphous carbon–chromium (a-C) film of ∼
2.6
μm in thickness and a hydrogenated, amorphous carbon (a-C:H) solid lubricant of ∼
2
μm.
The wear resistance of a PVD (Ti
0.7Al
0.3)N coating deposited on an as-received and gas nitrided AISI H13 has been examined by using ball-on-disc tests at room temperature and at 600
°C. In order to ...determine the influence of a previous heat treatment on this type of steel on the wear resistance of the (Ti
0.7Al
0.3)N coating, two commercial heat treatments were employed which gave rise to the same substrate hardness. Surface microhardness measurements have been carried out to determine the load-carrying capacity of the coated systems. In general, the wear behavior was found to be independent of the nature of the heat treatment applied to the substrate prior to the nitriding process but strongly dependent on the testing temperature. At room temperature, there were small variations between the different systems tested, whereas at high temperatures, clear differences were found between them. At 600
°C, a typical temperature that could be achieved during the aluminum extrusion processes, the nitrided H13 steel/(Ti
0.7Al
0.3)N PVD duplex coating shows a satisfactory wear resistance compared to both the nitrided steel and the steel substrate only coated with (Ti
0.6Al
0.4)N, which exhibited the worst performance. The satisfactory wear resistance observed for the duplex coating system at high temperature is mainly a consequence of two different aspects. Firstly, its higher load-carrying capacity due to the existence of a hard nitrided layer, as well as its high H/E ratio. Both parameters allow the presence of higher elastic strains without the failure of the ceramic layer, which would normally occur in the case of TiAlN PVD coatings deposited directly on the AISI H13 steel. Secondly, the intrinsic characteristics of the coating, i.e. its chemical constitution, which allows the formation of a dense oxide mixture inside the wear track that impedes both its further oxidation and the deterioration of the mechanical properties as consequence of nitrogen diffusion.
The present investigation has been carried out with the aim of determining the tribological behavior of a VPS chromium carbide coating both in the as-deposited and heat-treated conditions. A ...commercial powder of Cr2C3–25% NiCr was sprayed employing a VPS system (Medicoat AG, Switzerland) onto plain low carbon steel coupons. The samples were subsequently annealed for 2h at 600°C, 800°C and 900°C in Ar. The microstructural characterization was carried out by using SEM and XRD before and after the heat treatment of the samples. SEM observations were employed for determining the degradation mechanisms that took place during the wear tests. When the coated systems rubbed against alumina under a 5N normal load in air, a progressive change in the mechanism, from a mixed adhesive and abrasive, to a predominant abrasive was observed, as the heat treatment temperature increased. The wear constants were found to be of the order of approximately 10−6mm3/N.m, which indicates a wear resistance of nearly 4 times higher in comparison with the wear results reported in the literature for similar coating systems obtained by employing HVOF deposition. However, the heat treatment carried out at 900°C brought about only 20% increase in the sliding wear resistance of the coated system.
► We have investigated the sliding wear behavior of a Cr2C3-25%NiCr VPS thermal spray coating. ► Heat treatments for 2 hours have not an important consequence on the coating sliding wear resistance. ► The as-deposited coating exhibited a good sliding wear performance. ► This coating could compete with a Cr2C3-25%NiCr coating deposited by HVOF.
The present investigation has been conducted in order to study three commercially available hard AlCrN-based coated substrates by means of nano- and microindentation, aimed at determining the ...intrinsic hardness of the films. The results obtained are subsequently discussed taking into consideration the residual stresses and structure (grain size and texture) determined by X-ray diffraction and in the light of published data available in the literature. The study is first carried out with monolithic CrN and AlCrN coatings and it is further extended to the analysis of multilayer AlCrN/TiSiN coatings. Hardness obtained from nanoindentation at low depth and modelling the microindentation data match with values obtained at 22, 30 and 31 GPa for CrN, AlCrN and AlCrN/TiSiN coatings respectively. Residual stresses change from slightly tensile for CrN coating to highly compressive for AlCrN and AlCrN/TiSiN ones while the average grain sizes neglecting microstrains are similar for the three coatings (10–15 nm). AlCrN and AlCrN/TiSiN response similarities result from the low influence played by the stratification as compared to the nanostructure of the polycrystalline sublayers (nanometric grain size).
Wear experiments in the range of 25–600°C have been conducted on samples of D2 tool steel in different conditions involving unnitrided, nitrided and nitrided and coated with Balinit® A (TiN) and ...Balinit® Futura (TiAlN) deposited industrially at Balzers (Amherst, NY, USA), by means of PAPVD. The results indicate that coating the nitrided D2 tool steel substrate with these two films gives rise to an improvement of ∼97% (TiN) and 99% (TiAlN) in the wear behavior at the test temperature of 300°C, in comparison with the uncoated substrate. However, at a temperature of 600°C, besides oxidation of the coatings, the mechanical strength of the substrate decreases giving rise to fracture and delamination of the films. At this temperature the uncoated substrate exhibited the highest resistance to sliding wear, presumably due to the formation of a well bonded surface glazed layer which gives rise to a significant reduction in the friction coefficient. The indentation experiments that were conducted with the nitrided steel substrate and the coated systems indicates that the nitriding process applied to the D2 steel prior to PAPVD coating provides a satisfactory load support which contributes to the improvement of the coated systems capability to withstand indentation loads at room temperature. In this regard, the coated system with a TiAlN coating displayed a better behavior than that shown by the system with a TiN coating. An experimental procedure is proposed in order to predict the hardness profile of the nitrided tool steel, along the cross section of the material, just from hardness measurements taken on the surface of the sample, employing different indentation loads.
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