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The stress corrosion cracking resistance of two API X65 microalloyed steels, with different microstructures has been evaluated both at room temperature and at 55 °C, by means of Linear Elastic Fracture Mechanics (LEFM) specimens known as MWOL (Modified WOL specimen), under constant displacement. The material with a ferritic–pearlitic microstructure showed crack tip dissolution, a transgranular crack growth mode and some evidence of Hydrogen Induced Cracking (HIC). At room temperature, besides crack propagation, decohesion between grain boundaries was also observed, as well as some cavitation at the front of the crack tip. The material with martensitic microstructure tested at room temperature showed a mixed transgranular-intergranular crack growth mode, without cavitation at the crack tip front. At 55 °C, the crack growth path observed was transgranular, with cracks longer than those observed at room temperature, indicating that some Hydrogen mechanisms, probably thermally activated, could be present in addition to the anodic dissolution mechanisms.

Resumen: Las propiedades de los recubrimientos a base de circonia estabilizada con ytria (YSZ) confieren a estos un excelente desempeño en una amplia gama de aplicaciones que van desde el uso odontológico hasta barreras térmicas, incluyendo también su empleo como electrolitos sólidos en celdas de combustible. En el presente trabajo, se depositaron recubrimientos nanoestructurados delgados de hasta 75 μm de 8YSZ sobre un sustrato de la superaleación Haynes 230, mediante el proceso de termorrociado por plasma por suspensión (SPS), con el fin de servir de enlace (subcapa) entre este sustrato liso, sin preparación previa, y un recubrimiento más grueso microestucturado de YSZ. Se estudió el efecto del espesor de la subcapa y la influencia de los tratamientos térmicos posteriores (TT) a 300 y 600 ̊C sobre su desempeño tribológico a temperatura ambiente (Tamb) y a 650 ̊C. Durante los ensayos de desgaste deslizante con una carga de 2 N contra alúmina, se determinó que la reducción del espesor del recubrimiento de 75 a 25 μm incrementa la resistencia al desgaste aproximadamente 5 veces. Asimismo, para el espesor de 25 μm el TT a la temperatura de 600 ̊C disminuye 6 veces su resistencia al desgaste con respecto a la condición original del recubrimiento. Similarmente, cuando el ensayo se realiza a 650 ̊C, el comportamiento tribológico desmejora significativamente. Los resultados han sido relacionados con los cambios morfológicos que tienen lugar durante el tratamiento térmico. Abstract: The outstanding properties of ytria stabilized zirconia (YSZ) coatings allow an excellent performance of these materials for many applications, ranging from dentistry to thermal barriers, including its use as solid electrolytes in fuel cells. In the present work, nanostructured coatings of 8YSZ with a maximum thickness of 75 μm were deposited on a Haynes 230 superalloy substrate by means of a suspension plasma spraying (SPS) process. The deposited coatings act as a sub-layer between the smooth and unprepared substrate and a thicker microstructured YSZ coating. The effect of the deposited sub-layer thickness and the influence of subsequent heat treatments (TT) at 300 ̊C and 600 ̊C, on its tribological performance at room temperature and 650 ̊C, were studied. During sliding wear tests with a load of 2 N versus alumina, it was determined that reducing the coating thickness from 75 μm to 25 μm increases the wear resistance by approximately 5 times. Also, it was found that the heat treatment at 600 ̊C applied to the coating of 25 μm thickness decreases its wear resistance 6 times with respect to the as-sprayed condition. Similarly, when the tests were performed at 650 ̊C, the tribological behavior was significantly deteriorated. These results have been related to the morphological changes that take place during the heat treatment process. Palabras clave: Termorrociado, Plasma por suspensión, Desgaste deslizante, Tratamientos térmicos, Circonia estabilizada por itria, Keywords: Thermal spray, Suspension plasma spraying, Sliding wear, Heat treatments, Ytria stabilized zirconia

Conventional electroless nickel–boron deposits are produced using solutions that contain lead or thallium, which must be eliminated due to their toxicity. In this research, electroless nickel–boron deposits were produced in a stabilizer-free bath that does not include any toxic heavy metal. During processing, the plating rate increased from 10 to 14.5 µm/h by decreasing the concentration of the reducing agent, leading to increased bath stability. The thickness, composition, roughness, morphology, hardness, wear, and corrosion resistance of the deposits were characterized. The new deposit presents an excellent hardness of 933 ± 56 hv50, 866 ± 30 hk50, and 12 GPa from the instrumented indentation test (IIT), respectively, which are similar to that of hexavalent hard chromium coating. Moreover, by using both potentiodynamic polarization and salt spray tests it was shown that the coating presents higher corrosion resistance as compared to standard nickel-boron coatings. The new deposit exhibits properties close to those of the conventional electroless nickel–boron deposits. Therefore, it could replace them in any industrial applications.

International audience; This investigation addresses the effect provided by industrial surface finishes on the tribocorrosion properties of 316L stainless steel exposed to NaCl solution. Three distinct surface treatments were evaluated: passivation (SSO), electropolishing-passivation (SSEP) and micro-undulation (SSM mechano-chemical + electropolishing + passivation). For the tribocorrosion tests, a potentiostatic approach was considered in order to highlight the alloy behavior under two opposite situations, where repassivation of the surface would be thermodynamically possible or not (anodic or cathodic polarization, respectively). The outcomes demonstrated that the surface treatments were either harmful (SSEP) or beneficial (SSM) in terms of resulting tribocorrosion resistance. The specific topography of the micro-undulated sample decreased the real contact area and improved the surface lubrication in aqueous medium. SSEP presented the highest chemical wear and several factors seemed to have contributed for it, including the chemical, mechanical and structural properties of the passive film. Regardless the surface treatment, the tribocorrosion response was modified by the applied potential and more severe damage was determined under anodic polarization. At this potential, calculations of the total surface degradation suggested that volume loss was mainly dominated by chemical wear.

The present work has been conducted in order to assess the mechanical and tribological performance of a ZrN/Mo2N coating deposited onto a H13 steel by means of a closed field unbalanced magnetron-s...

International audience; The results of the mechanical and tribological characterization of a prototype a-C:Cr, Si sputtered coating are presented. The hardness and the elastic modulus of the coated system have been determined by means of nanoindentation taking into account the actual architecture of the bi-layer coating. Both mechanical properties were recorded continuously versus the indentation depth, h, up to approximately 2000 nm, at a constant indentation rate and maximum applied loads of 700 mN. The results were analyzed by means of the Oliver and Pharr method and modeled on the basis of novel approach proposed recently by some of the authors. Wear tests were conducted at 25 °C, 400 °C and 450 °C against alumina, employing a contact pressure of 540 MPa. Characterization of the worn surfaces by SEM and elemental X-ray mapping has also been carried out. A wear rate as low as 1.2×10−18 m3/N m was determined for the coating tested at 25 °C, which is approximately one order of magnitude and three times less than those found from the tests performed at 400 °C and 450 °C, respectively. It has been determined that the a-C:Cr,Si coating exhibits a very good wear resistance even at temperatures up to 450 °C, as consequence of the Si and Cr oxides formed due to the oxidation process. Also, it has found that at this temperature, a continuous oxide film is formed, which reduces the wear rate of the coated system in comparison to that determined at 400 °C. However, the volume increase due to the oxidation process at 450 °C and the elimination of CO2, Ar and H2O vapors, induces a severe surface cracking of the coating.

International audience; In the present work, a Si/Zr based sol–gel (SG) coating was deposited on 316L stainless steel plates, previously treated by passivation (SSO) or electropolishing (SSEP) producing two different surface states. The SG coatings were compared for SSO and SSEP substrates in terms of morphology, topography and tribocorrosion response. The coating topography revealed a smoother surface for the Si/Zr-SSEP system. The coating deposited on the smoothest surface (Si/Zr-SSEP) presented half of the thickness of the one deposited on the roughest surface (Si/Zr-SSO). Tribocorrosion behavior was studied under potentiostatic control at anodic potential with a continuous recording of current (I) during sliding (pin-on-disc and alumina ball counterbody). Both SG systems showed an increase of current upon 100 sliding contact cycles indicating corrosion activity. After tribocorrosion tests, both systems revealed scratches, typical of abrasion, and coating removal in the wear tracks; the alumina counterparts presented accumulation of wear particles adhered to their surfaces. In conclusion, the initial surface state of the substrate modified the coating thickness, topography but did not significantly alter the tribocorrosion response of the studied SG systems.

International audience; Deux revêtements à base nickel obtenus par des bains réducteurs, l’un brut et l’autre ayant subi un traitement thermique sont étudiés. L’essai d’indentation a été mis en œuvre pour caractériser la dureté et le module de Young de ces dépôts. Les résultats montrent une bonne homogénéité des propriétés mécaniques dans l’épaisseur du revêtement, alors que le traitement thermique a eu pour effet d’augmenter sa dureté et son module de façon significative. Les contraintes résiduelles dans les revêtements sont étudiées par la méthode du micro anneau incrémental. Les contraintes calculées sont faibles, dispersées et ne semblent pas dépendre du traitement thermique. En l’absence de contraintes résiduelles avant et après traitement thermique, les variations de propriétés mécaniques ont été attribuées à la modification microstructurale du revêtement et à la précipitation de phases dures.

International audience; Thermal sprayed coatings based on WC-Co are widely used for providing wear resistance to engineering components. The High Velocity Oxygen Fuel (HVOF) thermal spraying technique is one of the most commonly employed for depositing wear resistant coatings on steel substrates and constitutes one of the coating processes that have been technically validated for the replacement of electrolytic hard chrome (EHC) coatings, especially for extreme operating conditions. The present work aims at studying the tribological behavior, under sliding wear conditions, of a coating based on WC-12Co, with and without the reinforcement of carbon nanotubes (CNTs). The coating has been deposited by HVOF thermal spraying on a SAE 1045 substrate steel. Wear tests were carried out under the ball-on-disk configuration, at a constant sliding velocity of ∼ 0.2 m.s-1 and an applied load of 10 N, employing WC-6Co balls as static counterparts. The results for the CNTs reinforced coating have shown a decrease of ∼ 58% and 86% in the values of the average friction coefficient and wear rate, respectively, as compared with the conventional coatings. The observed wear mechanism was mainly of an abrasive type.

This paper presents the friction and wear behaviour of TiB 2 –40vol%B 4 C ceramic composite disc in rotating sliding contact against SiC-ball and examines how the tribological characteristics of this system are affected by the interaction of wear particles, loading conditions and sliding speed. The transition from mild to severe wear is analytically modelled based on Linear Elastic Fracture Mechanics theory (LEFM). This model takes into account the mechanical and newly defined effective thermal induced stresses in the contact. The latter considers the Peclet number corresponding to the experimental parameters. Based hereon wear maps are constructed that show the transition in wear mechanisms, further corroborated by extensive SEM observation of the wear tracks.

This paper reports the friction and wear response of WC–10%Co(Cr/V) cemented carbide with different surface finishes, attained by grinding (G) and wire-EDM, respectively, during sliding experiments at 400 °C. For comparison, tests under the same conditions were carried out at 25 °C. The wear experiments were performed under a normal force of 14 N, which produced a Hertzian maximum pressure of 3.10 GPa, and a sliding speed of 0.3 m/s against WC–6%Co(Cr/V) balls of 6 mm diameter. At 25 °C the average values of the friction coefficients were 0.36 ± 0.04 and 0.39 ± 0.06 for the ground and wire-EDM surface finishes, respectively. The mechanical behavior of both systems at 25 °C was assessed by carrying out analytical calculations of the stress field created by a circular sliding contact under a spherical indenter, where the residual stresses were considered. The theoretical results are in agreement with the experimental data, indicating that the wire-EDM sample has a specific wear rate, which is approximately 3.1 times greater than that corresponding to the G sample at 25 °C. At 400 °C, an increase in the friction coefficients takes place up to values of 0.75 ± 0.1 and 0.71 ± 0.8, for the ground and wire-EDM surface finishes, respectively. The increase was associated to an adhesive mechanism, which is more pronounced for the G sample. However, for the wire-EDM sample this increase was more linked to a marked abrasive mechanism. The wear rates for both samples at 400 °C are similar to those obtained at 25 °C, which indicates that apparently the test temperature does not have an important effect on the wear rate. However, it is known that temperature influences considerably the residual stress nature. Therefore, these results were explained by taking into account the wear mechanisms between the tribopairs in view of the mechanical characteristics and the morphological features obtained from SEM coupled with EDS analysis.

International audience; For describing the indentation size effect (ISE), numerous models, which relate the load or hardness to the indent dimensions, have been proposed. Unfortunately, it is still difficult to associate the different parameters involved in such relationships with physical or mechanical properties of the material. This is an unsolved problem since the ISE can be associated with various causes such as workhardening, roughness, piling-up, sinking-in, indenter tip geometry, surface energy, varying composition and crystal anisotropy. For interpreting the change in hardness with indent size, an original approach is proposed on the basis of composite hardness modelling together with the use of a simple model, which allows the determination of the hardness‐depth profile. Applied to copper and copper alloys, it is shown that it is possible to determine the maximum hardness value reached at the outer surface of the material and the distance over which both the ISE and the workhardening take place.

International audience; Instrumented indentation is very useful to determine the elastic modulus and hardness of materials. To obtain reliable values of the mechanical properties, the measurement of the indenter displacement into the material must be very precise. In practice, the frame compliance of the instrument and the indenter tip defect are among the most influential parameters. In microindentation, the frame compliance term must be calculated for each analysis. On the other hand, the tip defect can lead to a wrong estimation of the contact area, which is involved in both the elastic modulus and the hardness calculations. The present work is mainly focused on the influence of the tip defect on the determination of the mechanical properties of a composite TiB2-60% B4C hard material using Berkovich, Vickers, Knoop and spherical indenters. This composite system was obtained by sintering in vacuum, employing a pulsed electric current, without any sintering additives. It has been shown that the tip defect greatly affects the results related to the mechanical properties of the system under study, even for low defect sizes, which are usually neglected in practice. After correction, the elastic modulus of the TiB2-60% B4C is found to be equal to 490 GPa whatever the indenter shape used. This result corroborates the value of 500 GPa found for this system by employing the resonance frequency method according to the ASTM Standard e1876-99. For analyzing the hardness-load dependence, the representative parameter of the Proportional Specimen Resistance model becomes null when the tip defect is introduced into the hardness calculation. Additionally, this parameter is found to be directly proportional to the tip defect size, suggesting a special attention when characterizing the indentation size effect of materials. Finally, the Meyer hardness found for the TiB2-60% B4C is close to 20 GPa.

The tensile stress–strain behaviour of AA 7075-T6 aluminium alloy deformed at temperatures in the range of 123–248 K, at strain rates between 0·0023 and 0·21 s−1 has been analysed in order to develop a rational constitutive description of the material under such deformation conditions. The constitutive formulation has been derived on the basis of the mechanical threshold stress model developed at Los Alamos National Laboratory. The constitutive equation thus proposed is able to separate the contribution of the different components that give rise to the mechanical strength of the alloy. It has been determined that the flow stress of the material arises mainly from the contribution of three different components: athermal barriers to dislocation motion, solid solution and precipitation hardening and work hardening (dislocation–dislocation interaction). The flow stress in each strain increment during the course of plastic deformation is computed recursively from its previous value, before the updating of the ...

International audience; A methodology for obtaining a nanocrystalline Al91-Mn6-Nd3 alloy and characterizing its mechanical properties is presented. The fabrication procedure, based on the melt spinning method, allowed the preparation of thin ribbons of not, vert, similar30 μm in thickness. The hardness and elastic modulus have been determined by classical and instrumented indentation tests considering the influence of the ribbon thickness on the indentation measurements. The Martens hardness has been calculated by means of instrumented indentation tests, for which the fixing support of the sample has no influence on the indentation measurement. The results have been subsequently compared with those obtained from classical Vickers hardness measurements, for which some composite hardness models have been applied in order to separate the contribution of the fixing support to the hardness measurement. It has been concluded that both methodologies lead to similar values for the mechanical properties of the alloy, i.e. not, vert, similar3.4 GPa for the macrohardness and not, vert, similar3.8 MPa m1/2 for the hardness length-scale factor of the material. The elastic modulus of the material, resulting from the analysis of the unloading indentation curves, has been found to be not, vert, similar131 GPa.

The present investigation has been conducted in order to develop a constitutive description for a low C-Mn steel employed in structural applications, when this material is deformed under hot-working conditions. The set of equations that encompass the constitutive description of the steel are based on a simplified form of the Mechanical Threshold Stress Model. Accordingly, it has been shown that the flow stress of the material can be considered to arise from two different contributions: solid solution and precipitation hardening, which do not evolve in the course of plastic deformation and work-hardening, the only evolving component of the flow stress. The evolution of work-hardening has been introduced in the model by means of the work-hardening law proposed by Estrin and Mecking, in differential form. Also, the temperature and strain rate dependence of the contributions of solid solution and precipitation hardening to the flow stress, as well as that of the saturation stress in the work-hardening law, are described by means of the model earlier advanced by Kocks. The constitutive formulation encompasses seven material parameters which have been determined employing the experimental data corresponding to the strain interval where the material only exhibits positive work-hardening. It is able to predict accurately both work-hardening and work-softening transients when changes in deformation conditions occur. Furthermore, it can be easily implemented in any advanced computational tool employed for the analysis of hot-working processes of this material.

The present investigation has been conducted in order to develop a rational approach able to describe the changes in flow stress of AA7075-T6 aluminum alloy with deformation temperature and strain rate, when this material is deformed at temperatures in the range of 123–298 K at strain rates in the range of 4 × 10 −4 to 5 × 10 −2 s −1 . The constitutive formulation that has been advanced to accomplish these objectives represents a simplified form of the mechanical threshold stress (flow stress at 0 K) model developed at Los Alamos National Laboratory (Los Alamos, New Mexico, USA). Thus, it is assumed that the current flow stress of the material arises from both athermal and thermal barriers to dislocation motion. In the present case, the effect of three thermal barriers has been considered: solid solution, precipitation hardening and work-hardening. The first two effects do not evolve during plastic deformation, whereas the last one is considered as an evolutionary component of the flow stress. Such an evolution is described by means of the hardening law earlier advanced by Estrin and Mecking (1984) [20] . The law is implemented in differential form and is integrated numerically in order to update the changes in strain rate that occur during tensile tests carried out both at constant and variable crosshead speed. The extrapolation of the hardening components from 0 K to finite temperatures is accomplished by means of the model earlier advanced by Kocks (1976) [19] . The results illustrate that the constitutive formulation developed in this way is able to describe quite accurately both the flow stress and work-hardening rate of the material, as well as temperature and strain rate history effects that are present when deformation conditions change in the course of plastic deformation. The evaluation of the ductility of the alloy indicates that the changes in this property are mainly determined by deformation temperature rather by strain rate. When deformation temperature decreases from 298 to 123 K, ductility also decreases from ∼35 to 24%. However, despite these relatively small variations, significant changes in the fracture morphology could be observed on the fracture surfaces of the examined specimens, with the predominance of a mixed ductile-brittle mechanism at lower temperatures.

The present work has been conducted in order to determine systematically the influence of the spraying distance on the microstructure and mechanical properties of a Colmonoy 88 alloy deposited by means of HVOF thermal spray onto a SAE 1045 steel substrate. The spray distance varied between 380–470 mm and the evaluation of the deposits characteristics and properties was carried out both on their surface and on cross section. Both hardness and elastic modulus of the coatings were determined according to the model of Oliver and Pharr. The yield strength of the coatings was also estimated following the methodology developed by Zeng and Chiu for the analysis of the loading and unloading curves obtained from nanoindentation experiments, as well as from classical static spherical indentation tests. The microstructural analysis indicated a significant increase in the unmelted particles volume fraction and the development of interlamellar microcracks as the spraying distance increases, leading to a decrease in the elastic modulus of the coatings. Both hardness and elastic modulus showed an anisotropic behavior and were found to be higher on the cross section of the coating than on the deposition plane. A satisfactory comparison between the predicted and experimental values of the coatings yield strength was observed for all the conditions investigated.

The present study has been carried out to evaluate the corrosion performance of three different (Ti1–xAlx)N/TiN multilayers coatings commercially deposited on WC–Co inserts. Two of these coatings were nanostructured, and all of them presented different Ti/Al atomic ratios varying from 2 : 3 up to 7 : 3 and were deposited by employing a commercial physical vapour deposition cathodic arc process. X-ray diffraction analysis indicated that the grain size for both the TiAlN and TiN layers in the coatings architecture, varied between approximately 9 to 14 nm. The corrosion performance was evaluated in a 3 wt-%NaCl solution using both open circuit potential and potentiodynamic polarisation measurements. Scanning electron microscopy and energy dispersive spectroscopy surface characterisation before and after corrosion testing allowed the determination that all the coatings under study show a corrosion current density of approximately one order of magnitude smaller than the uncoated substrate, indicating t...

The present work has been conducted in order to determine the influence of the spray distance, on the corrosion-fatigue behavior of a SAE 1045 steel substrate coated with a Ni base coating deposited by high velocity oxygen fuel (HVOF) thermal spray. The spray distances employed in the present investigation were of 380, 425 and 470 mm. The mechanical properties of the coated systems were evaluated by means of tensile and corrosion-fatigue tests conducted with cylindrical samples. Corrosion-fatigue tests were carried out under rotating bending conditions ( R = − 1), at a frequency of 50 Hz and maximum alternating stresses in the range of 250–420 MPa, employing a 3 wt.% NaCl solution. The results indicate that varying the spray distance in the range of 380–470 mm has apparently no significant influence on the corrosion-fatigue behavior of the coated systems. However, the presence of the Ni base coating gives rise to a significant increase in the corrosion-fatigue life of the coated substrate, in comparison with the uncoated steel. Such an increase varies between ∼ 90 and 440% in the interval of maximum alternating stresses investigated in the present work. Also, the coated systems exhibited a better corrosion-fatigue performance in comparison with the same steel substrate coated with an electrolytic hard chromium (EHC) deposit.

International audience; 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 not, vert, similar 2.6 μm in thickness and a hydrogenated, amorphous carbon (a-C:H) solid lubricant of not, vert, similar 2 μm.

An investigation has been conducted in order to determine the effect of both the metallic transfer mode (pulsed arc or short circuit) and the O2 content in the Ar/O2 gas mixture, of the gas-metal arc welding process (GMAW), on the fatigue life under uniaxial conditions of welded joints of 316L stainless steel. It has been concluded that the mixture of the protective gases employed in the process could have an important influence on the fatigue life of the welded joints of such steel in two different ways. Firstly, through the modification of the radius of curvature at the joint between the welding toe and the base metal and, secondly, through a more pronounced degree of oxidation of the alloying elements induced by a higher O2 content in the mixture. As far as the metallic transfer mode is concerned, it has been determined that the welded joints obtained under a pulsed arc mode show a greater fatigue life in comparison with the joints obtained under short circuit for both gas mixtures. Se ha llevado a cabo una investigación con la finalidad de determinar el efecto, tanto del modo de transferencia metálica (arco pulsado o cortocircuito) como del contenido de O2 en la mezcla de gases protectores Ar/O2, del proceso de soldadura a tope mediante arco metálico con protección gaseosa (GMAW), sobre la vida a la fatiga en condiciones uniaxiales de juntas soldadas del acero inoxidable AISI 316L. Dicho trabajo ha permitido concluir que la composición de la mezcla de gases protectores del proceso GMAW pudiera tener una influencia importante en la vida a la fatiga de las juntas soldadas de dicho material, a través de dos formas distintas: primero, mediante la modificación del radio de curvatura entre la raíz del cordón de soldadura y el metal base y, en segundo lugar, a través del mayor grado de oxidación de los elementos de aleación. En cuanto al modo de transferencia metálica, se determinó que las juntas soldadas mediante arco pulsado presentan una mayor vida a la fatiga en comparación con las juntas obtenidas por cortocircuito para ambas mezclas gaseosas.

In the present research, a nickel base coating was deposited on an AISI 1020 substrate using a high-velocity oxy-fuel technique. The coating was subsequently heat treated (HT) by means of an oxyacetylene flame. For the conditions evaluated in the present study, it was found that the HT coating has 1.15 times better wear resistance for the lower level of applied load and lower sliding speed (SS), and nearly 50 times better resistance for the highest level of the applied load and highest SS when compared with the non-HT coatings. These results have been attributed to a better distribution of the hardening phases, better cohesion between particles and an increase in hardness, as a consequence of the post-deposition heat treatment process. A severe wear regime was found for all the samples, since the wear rates presented values which were higher than 1 × 10− 5 mm3/m. The presence of the adhesion, oxidization, three-body abrasion, and cyclical deterioration phenomena, all competing with each other, was noted. ...

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).

In the present work, tribological performances of duplex coatings (Ti/TiN PVD and WC-Co HVOF) on Ti6Al4V substrates were investigated by analytical modelling and experimental assessment of mechanical/tribological properties. An analytical study of the contact stress distribution under a spherical indenter was performed, considering changes in stress distribution after adoption of the WC-Co intermediate coating as a load support layer. Experimental activities consisted of micro- and nanoindentation testing, HRC adhesion test and ball-on-disc sliding wear test (Alumina counterpart) under dry atmosphere. Morphology of the worn surface and layer failure mechanisms (i.e. wear mechanisms, coating surface and sub-surface delamination, crack propagation modes, role of interfaces) were then analysed by FlB-SEM techniques. Results showed that the use of a WC-Co interlayer with intermediate hardness and stiffness provides a more effective distribution of contact strain, with a significant increase of load carrying capacity and wear resistance of samples. A correlation between results of simulations, H/E and H(3)/E(2) ratios and wear resistance of coatings is finally proposed. (C) 2008 Elsevier B.V. All rights reserved.

Axial fatigue experiments have been conducted employing notched samples of a 316L stainless steel under both constant and variable amplitude loading. A number of samples were subjected to high cycle fatigue (HCF) in the range of ∼144,000–271,000 cycles and subsequent tensile tests, in order to evaluate the change in tensile mechanical properties due to prior fatigue damage. The results obtained have shown that HCF in this range of cycles gives rise to a significant increase in the yield stress of the material, while other mechanical properties determined in tension remain almost constant. It has been determined that the increase in yield stress with the cyclic ratio ( D ) is more significant as the maximum stress applied to the material increases. Such a behavior has been described by means of a simple parametric relationship, which could be employed for the practical determination of D in parts and components made of this material, by means of simple tensile testing. A plausible explanation of the increase in yield stress with D , based on the development of the dislocation substructure able to modify such a property, has been proposed. The fractographic analysis that has been conducted on the samples tested in tension after fatigue damage indicates that such a phenomenon also gives rise to the development of fatigue cracks, whose size increases with D . It has been shown that the number of cycles to fracture in two-stress step fatigue tests can be satisfactorily predicted by means of the Palmgren–Miner linear damage cumulative rule. A non-linear numerical procedure has been presented in order to re-compute the fatigue strength coefficient and fatigue exponent of the material, from data obtained under variable amplitude loading conditions, whose values give rise to a satisfactory description of the S–N experimental data. The use of more complex non-linear damage cumulative rules has not given rise to any significant improvement in the number of cycles recorded in the second stress step. It is believed that, due to the significant plastic deformation that occurs at the notch root during fatigue testing, cyclic creep (or ratcheting) damage in addition to fatigue damage could also be present.

The present investigation has been conducted in order to determine the microstructural features, hardness and elastic modulus of two different WC–Co and WC–Co–Cr high velocity oxy fuel sprayed coatings. Particular emphasis has been given to the evaluation of the elastic modulus on the surface and cross-section of both coatings, employing three different indenters and the influence of the applied load on such property. For this purpose, both conventional and instrumented indentation tests were carried out on the surface and cross-section of the coatings. The elastic modulus was computed by means of the Oliver and Pharr method (1992; 2004) and compared with the values determined by means of the Marshall et al. method (1982). The values of the elastic modulus found both on the surface and on the cross-section of the samples were ~250 GPa, which indicates a quasi-isotropic behaviour due to the uniform and dense microstructure observed along both directions.

Martensitic AISI 420 stainless steel was plasma nitrided using a DC N2 – H2 pulsed discharge, after thermal treatment at two different tempering temperatures. Sliding wear behavior was determined by means of a ball-on-disk tribometer at room temperature and without lubrication. Wear volume was calculated from the information provided by an optical profilometer and wear scars were analyzed by Scanning Electron Microscopy. The results indicated that the nitrided samples exhibited improved wear resistance when compared to the untreated specimens, but corrosion resistance was diminished due to the presence of the outer nitrided layer. However, when this layer was removed mechanically considerable improvement in the corrosion resistance was achieved without impairing the wear resistance of the whole system, which was 200 % higher compared to the wear resistance of the non-nitrided specimens.

The interfacial indentation test allows determining the interface toughness of a coating obtained by thermal spraying. During this test, a Vickers indentation is carried out on a cross-section of the sample. A crack is initiated and propagated along the interface. An analytical model allows defining an interface toughness representing the coating adhesion. The objective of this study is to compare this test with other tests (tensile adhesive test, shear test) and to specify its applicability. The residual stresses are also estimated by two different methods. Their influence on adherence is discussed in a third part. Those experiments were conducted on NiCr 80-20 VPS coatings with different thicknesses and roughnesses. In particular, it is shown that the interfacial indentation test is the most universal one and that compressive residual stresses improve coating adhesion.

Zirconium nitride was deposited by reactive unbalanced magnetron sputtering at different N 2 partial pressures, on an AISI 316L stainless steel substrate. The mechanical properties of the coatings were evaluated by means of nanoindentation tests employing a Berkovich indenter and loads which varied between 120–9000 µN. The sliding wear behavior of the substrate-coating systems was studied under a normal load of 2 N using a ball-on-disc tribometer, with an AISI 52100 ball (6 mm diameter) as counterpart. It has been found that N 2 partial pressure has a significant effect both on the hardness and corresponding Young's modulus of the coatings. As the N 2 partial pressure increases from 1 × 10 − 4 Torr to 10 × 10 − 4 Torr, the hardness and Young's modulus of the coatings decrease from 26 to 20 GPa and 360 to 280 GPa, respectively. The nanoindentation tests revealed the presence of a third oxide layer (10 nm thick, approximately) on the surface of the coating. Scanning electron microscopy (SEM) analysis performed on the worn triboelements indicated that both abrasive and adhesive wear mechanisms could take place in addition to the substrate plastic deformation. The deposition conditions and coating mechanical integrity determine the predominant wear mechanism.

The present investigation has been conducted in order to develop a rational approach, able to evaluate quite accurately the depth of a nitrided tool steel that should be removed previous to hard PVD coating deposition, and the dependence of such depth on the nitriding conditions, information that cannot be obtained easily by means of simple metallographic observations. The method thus proposed is based on the use of the hardness data corresponding to the hardness depth profile of the nitrided material, in conjunction with Fick's second law. The analytical procedure is described employing the experimental data obtained from a gas nitrided AISI H11 steel (X38CrMoV5.1), and has been validated by computing a rough estimate of the diffusivity parameters of nitrogen in the α-Fe matrix. In the particular case of the H11 steel, it has been determined that nitriding at a temperature of 580 °C for 8 h seems to have important advantages over the process conducted at 510 °C and 48 h, particularly in terms of the characteristics of the hardness profile obtained and the ability of the nitrided material to withstand indentation loads, a measure of its load-carrying capacity, as a feasible substrate for hard PVD coating deposition. The GDOS analysis that were conducted showed that within the first 8–25 μm of the sample depth, depending on the nitriding conditions, a layer of compounds with a high nitrogen content (7–19 wt.%) is formed, which is followed by a different layer with a lower nitrogen content, in the range of ∼ 2 wt.%, presumably constituted by a solid solution of nitrogen dissolved in the octahedral interstitial position of the α-Fe, together with a large volume fraction of sub-microscopic coherent and/or semi-coherent nitride precipitates, which explains the high hardness, in the range of ∼ 9–10 GPa observed. As expected, at a constant temperature, the extent of the compound layer was found to increase with the nitriding potential.

The aluminum alloy 7075-T6 is a structural alloy widely used for aeronautical applications due to its high relationship between mechanical resistance and weight. Depending upon the environmental conditions, many types of corrosion mechanisms such as intergranular, exfoliation, have been found to occur in aircraft structural aluminum alloys. A significant advance in order to improve the behavior of this alloy is related to the application of the autocatalytic Ni-P coating which confers an excellent corrosion resistance coupled with both reduced erosive wear and higher hardness. The purpose of this work was to investigate the effect of the application of a Ni-P coating on the corrosion resistance of an aluminum 7075-T6 alloy. The results obtained indicated that the application of the Ni-P coatings diminishes the susceptibility to pitting and makes the aluminum 7075 T6 alloy immune to the exfoliation corrosion attack.La aleación de aluminio 7075-T6 es una aleación estructural ampliamente utilizada para aplicaciones aeronáuticas, debido a su alta relación entre resistencia mecánica y peso. Dependiendo de las condiciones ambientales, algunos mecanismos de corrosion tales como intergranular, exfoliacion, picadura y crevice se ha encontrado que ocurren en estructuras de aviones de aleaciones de aluminio. Un avance siginificativo para mejorar el comportamiento de esta aleación es la aplicación de recubrimientos autocatalíticos de Ni-P, los cuales confieren una excelente resistencia a la corrosión acoplado con una reducción del desgaste erosivo y un aumento de la dureza. El propósito de este trabajo fue investigar el efecto de la aplicación de un recubrimiento de Ni-P sobre la resistencia a la corrosión de una aleación de aluminio 7075-T6. Los resultados obtenidos indican que la aplicación del recubrimiento de Ni-P disminuye la susceptibilidad a la picadura y hace a la aleación de aluminio 7075 T6, prácticamente inmune al ataque por corrosion expoliación.

The present investigation has been conducted in order to determine the influence of both the chemical composition of the bath and current density employed, on the adhesion, hardness and wear resistance of electrolytic Sn–Ni coatings deposited on a steel substrate. The micro-scratch test method was employed to evaluate the coating adhesion. Three scratches were performed at progressive loads. The subsequent observation of the surface damage by means of optical microscopy allowed the determination of the critical load, defined as the smallest load at which coating delamination will occur. The composite hardness of the system was determined by means of Vickers indentation and represented as a function of the relative indentation depth (R.I.D.). The absolute hardness of the coatings was computed by using a model published previously in the literature by one of the authors. The evaluation of the wear resistance was carried out through an accelerated wear test, without lubricant, under a ball-on-disc configuration, using a ball of AISI 52100 steel as a static counterpart. The wear scar morphology was studied by Scanning Electronic Microscopy (SEM) techniques coupled with the energy dispersive X-ray analysis (EDS). The results indicate that the Sn–Ni coatings obtained by means of the chloride–fluoride bath (CF) have apparently a higher adhesion to the steel substrate and wear resistance, in comparison with the coatings which were obtained by using the pyrophosphate bath (PF), irrespective to the current density value employed during deposition. However, the hardness results indicate that the coatings obtained from the pyrophosphate bath have a better endurance against indentation loads. Such a behavior has been partially explained in terms of the different microstructural features that characterized both coatings. The CF deposits were observed to present an elongated columnar grain structure free of cracks, whereas the PF coatings were observed to have an equiaxial structure with a high crack density. The present study has allowed the conclusion that the CF coatings have an overall better performance than the PF coatings.

International audience; In this work, the total and plastic hardness of a variety of materials were determined from the work-of-indentation. We show that by taking into account the influence of the tip defect, both the total and plastic hardness have the same values and that the hardness-load dependence is considerably reduced, thus explaining that part of the indentation size effect can be related to an incorrect estimation of the tip defect. To consider the influence of the indenter tip defect, we propose a calculation of the impression volume involving the length of the truncated indenter tip. For the residual impression volume, we have demonstrated that it can be simply calculated from the total volume using a proportionality constant equal to the residual to maximum depth ratio. Moreover, the hardness derived from work-of-indentation is found to be equal to the Meyer hardness. Finally, the ratios of Meyer hardness to the reduced modulus and elastic work to total work of indentation are found to be very close to the proportionality factor (π tanα) of Stilwell and Tabor.

In this work, CrAlN films with Al/Cr atomic ratios between 0.02 and 1.4 were deposited on tool inserts by AC magnetron sputtering at 5 kW for three different Ar/N2 flow rates. The unique configuration of the inverted cylindrical magnetron sputtering (ICM-10) system enables the deposition of compositionally graded Cr–Al–N coatings under a single deposition condition. The effect of aluminum on the structural, mechanical and tribological properties of (Cr,Al)N coatings are reported and compared with ALCRONA and CrN coatings from Balzers. Mechanical properties have been evaluated by the microhardness indentation technique, and the hardness is 18 and 15 GPa for CrN and CrAlN films, respectively. The tribological tests have been performed using the pin-on-disc method at room temperature and 700 °C. The results indicate that the coefficient of friction at room temperature and 700 °C for all the deposited coatings, Balzers-ALCRONA and CrN fall between 0.4–0.6. The wear volume at high temperature (700 °C) decreases with increase in Al incorporation. Auger Electron Spectroscopy reveals that all films have about 10 at.% of oxygen. Atomic Force Microscopy showed that our films had roughness ranging from 80 to 150 nm. X-ray mapping on (Cr, Al)N coatings for different Ar/N2 gas compositions showed transitions from cubic to hexagonal with decreasing nitrogen.

An investigation has been carried out in order to study the fatigue performance, both in air and in a 3 wt.% NaCl solution, of a quenched and tempered (Q&T) SAE 4340 steel substrate coated with a TiCN film of ∼4 μm in thickness deposited by plasma assisted physical vapor deposition (PAPVD). The results obtained show that the presence of the coating provides a significant increase, in the range of ∼140–180%, in fatigue life, in comparison with the uncoated substrate, when the material is tested in air under rotating bending conditions at maximum alternating stresses in the range of ∼550–700 MPa. It is also shown that when the material is tested in the NaCl solution only a marginal increase (25% at the most) in the number of cycles to failure is observed, particularly at maximum alternating stresses above ∼570 MPa, which suggests that the TiCN film is susceptible to the corrosive effect of the solution under the combined action of cyclic loading. The increase in the number of cycles to failure in air is explained in terms of the high mechanical strength of the film, its compressive residual stress state and excellent adhesion to the steel substrate. It is shown that fatigue cracks nucleate at the surface of the coating and propagate normal to the substrate–coating interface without bifurcation along the latter, after going through the entire coating thickness. Under the assumption of the validity of a simple linear law of mixtures, the tensile strength of the film is estimated in ∼34 GPa, very close to its absolute hardness of ∼33 GPa, whereas its fatigue limit is found to be in the range of ∼12.6 GPa.

The tensile and fatigue properties of a 6063-T6 aluminium alloy coated with an electroless Ni–P (EN) deposit have been investigated. The EN plating was in the as deposited condition, that is to say, without any post-heat treatment. The thickness of the coating was of about 40–50 μm and it had a P content of ∼9·5 wt-%. The results obtained indicate that such a plating gives rise to a decrease in the number of cycles to failure N f, when the substrate coating system is tested both in air and in a 3 wt-%NaCl solution. When the coated system is tested in air the decrease in N f varies from about 56–45%, whereas during testing in the NaCl solution such a decrease ranges between about 65–44%. In both cases, larger decrements in N f are observed to be associated with smaller maximum alternating stresses. It is shown that the EN deposit employed in the present investigation has a very good adhesion to the substrate, since no delamination is observed even when the coated system is subjected to tensile stre...

The wear resistance of a PVD (Ti 0.7 Al 0.3 )N coating deposited on an as-received and gas nitrided AISI H13 has been examined by using ballon-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 0.7 Al 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 0.7 Al 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 0.6 Al 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.

Vickers and Knoop hardness measurements performed on various ceramics (hard metals) and light alloy materials (soft metals) are compared. The results show that the Knoop hardness number is, in general, lower than the Vickers hardness number for the highest values of hardness, and this behaviour is reversed when the hardness values are low. This change in values, which occur at 8 GPa, has no real physical meaning and, therefore, it is difficult to interpret such behaviour in terms of the elasto-plastic deformation around the indent such as sinking-in, piling-up, and bulging of the indent faces, phenomena which take place during indentation or after the withdrawal of the indenter. Prior to interpreting the hardness difference, it is very important to consider the same area in the hardness calculations. That is why we have compared the available hardness data obtained from the literature and recalculated them by considering the projected and true areas of the contact. If the objective is to compare the two hardness numbers, it seems more suitable to consider the true area of contact, procedure which will provide a Vickers hardness number higher than the Knoop hardness number all over the range of the hardness values.

International audience; As observed from the current literature, the vast majority of the constitutive models that have been proposed for the description of the changes in the high-temperature flow stress of high-Mn austenitic steels, in the course of plastic deformation as a function of deformation conditions, have been expressed in terms of the total applied strain to the material (ε), which, as indicated by Follansbee and Kocks, is not a valid state parameter. In order to overcome this disadvantage, which hinders the prediction of the changes in flow stress under transient deformation conditions, commonly found during industrial hot-working processes, a different constitutive description is proposed. The advanced approach is based on the temperature and strain rate description of three important parameters: yield stress, hypothetical saturation stress and actual steady-state stress, which is considered equal to the critical stress for the onset of dynamic recrystallization, as proposed by Jonas et al. This description is accomplished by means of the well established Sellars–Tegart–Garofalo model, employing a unique value of the apparent activation energy for hot-working determined from the analysis of the peak stress values, as proposed earlier by Glover and Sellars. The model also requires a description of the time for achieving 50% dynamic recrystallization, which is expressed as a function of deformation conditions by means of the simple parametric relationship also proposed by Jonas et al. This information is subsequently input into two evolution equations, which describe the change in the work-hardening and work-softening rates of the material, as a function of deformation conditions, either when the work-hardening is counterbalanced only by dynamic recovery or when it is counterbalanced by both dynamic recovery and dynamic recrystallization. The results reported in the present communication indicate that the proposed constitutive description is able to provide an accurate reproduction of the experimental flow stress values determined under constant deformation conditions, obtained by Wietbrock et al. However, the analysis of a hypothetical deformation schedule conducted under transient deformation conditions suggests that a second internal state variable, other than dislocation density, might be required for the improvement in the prediction of the corresponding stress–strain curves.

The change in the composite hardness with penetration depth derived from nanoindentation tests conducted on coated systems, which involve the deposition of multilayer coatings, in general exhibits a complex shape, as a consequence of the sequential contribution of each coating layer to the composite hardness during indentation loading. In spite that there are a number of models, which have been proposed for describing the change of the composite hardness with penetration depth for monolayer coatings, as well as for determining the coating and substrate hardness, very few research works have addressed the problem of describing this kind of data for multilayer coatings. In the present communication, a rational approach is proposed for extending two models widely used for the analysis of monolayer coatings, in order to describe the composite hardness data of multilayer coatings, as well as for determining the hardness of each individual layer and that of the substrate. Thus, a modified form of the models earlier advanced by Korsunsky et al. and Puchi-Cabrera, as well as their computational instrumentation, are proposed. The extension of both models to deal with multilayer coatings is conducted on the basis of the model developed by Iost et al., in order to adapt the Jönsson–Hogmark model to the analysis of indentation data of multilayer coatings. Such a methodology provides a means of computing the volume fraction of each individual layer in the coating, which contributes to the composite hardness. According to the results obtained, this scheme seems to be general enough to be applicable to different hardness models other than the Jönsson–Hogmark model. The proposed modified models are validated employing nanoindentation results obtained from a 2024-T6 aluminum alloy coated with a diamond-like carbon film, employing electroless NiP as intermediate layer. The advantages and disadvantages of the different models employed in the analysis are thoroughly discussed.; International audience; The change in the composite hardness with penetration depth derived from nanoindentation tests conducted on coated systems, which involve the deposition of multilayer coatings, in general exhibits a complex shape, as a consequence of the sequential contribution of each coating layer to the composite hardness during indentation loading. In spite that there are a number of models, which have been proposed for describing the change of the composite hardness with penetration depth for monolayer coatings, as well as for determining the coating and substrate hardness, very few research works have addressed the problem of describing this kind of data for multilayer coatings. In the present communication, a rational approach is proposed for extending two models widely used for the analysis of monolayer coatings, in order to describe the composite hardness data of multilayer coatings, as well as for determining the hardness of each individual layer and that of the substrate. Thus, a modified form of the models earlier advanced by Korsunsky et al. and Puchi-Cabrera, as well as their computational instrumentation, are proposed. The extension of both models to deal with multilayer coatings is conducted on the basis of the model developed by Iost et al., in order to adapt the Jönsson–Hogmark model to the analysis of indentation data of multilayer coatings. Such a methodology provides a means of computing the volume fraction of each individual layer in the coating, which contributes to the composite hardness. According to the results obtained, this scheme seems to be general enough to be applicable to different hardness models other than the Jönsson–Hogmark model. The proposed modified models are validated employing nanoindentation results obtained from a 2024-T6 aluminum alloy coated with a diamond-like carbon film, employing electroless NiP as intermediate layer. The advantages and disadvantages of the different models employed in the analysis are thoroughly discussed.

Duplex treatments consisting of a plasma nitrided layer followed by a PAPVD hard coating have gained attention in recent years. The aim of this work was to determine the corrosion resistance of AISI H13 steel treated by using this duplex treatment. The samples were pre-nitrided using experimental equipment and, subsequently, TiAlN coating was deposited commercially by a PAPVD process. A series of electrochemical tests such as potentiodynamic polarization and linear polarization were carried out to investigate the corrosion resistance of the surface engineered steel. Surface characterization before and after the corrosion tests was performed by means of the scanning electron microscopy (SEM) technique coupled with the energy dispersive X-ray analysis (EDS). The experimental results indicate that the duplex systems TiAlN coating/nitrided steel protect the substrate against corrosion due to the passivation of the compound layer in the presence of the corrosive solution. Therefore, a better corrosion resistance is obtained as long as the extension of the nitrided layer underneath the hard coating is increased.

The AISI 316L stainless steel has been widely used in artificial knee or hip joints, as well as internal fixation devices. It is well known that this material has a good corrosion resistance and acceptable biocompatibility properties. Ion nitriding is a well established process for steel hardening that can also be applied to this kind of steels with the aim of enhancing its hardness but without reducing its corrosion resistance. In this work, the effects of ion nitriding on the corrosion performance of a 3l6L stainless steel was evaluated in a 0.9% sodium chloride solution by using electrochemical tests such as potentiodynamic polarization and linear polarization in both nitrided and untreated AISI 316L steel condition. Surface characterization before and after corrosion testing was performed using scanning electron microscopy (SEM) with an energy dispersive X-ray analysis (EDS). It was shown that an ion nitriding treatment in a 25% N2–75% H2 atmosphere performed at a temperature of 410 °C improves the surface hardness of the AISI 316L stainless steel. However, under the experimental conditions carried out in this research, the nitrided steel is as prone to localized corrosion as the untreated one. It is considered that this behavior is mainly due to the presence of CrN, which precipitates during processing, contributing to the depletion of chromium from the adjacent matrix and leading to a galvanic corrosion mechanism.

The present investigation has been conducted to study the fatigue behavior of a SAE 1045 steel both uncoated and coated with a Colmonoy 88 alloy (NiCrBSiW) of approximately 410 μm thick, deposited by HVOF thermal spraying. Previously to deposition the samples were grit-blasted with alumina particles of approximately 1 mm in equivalent diameter. Tensile and fatigue tests were carried out with the uncoated and coated specimens. Fatigue tests were conducted under rotating bending conditions (R = − 1) at a frequency of 50 Hz. The samples tested were in three different surface conditions, including polished, grit-blasted and coated. The fatigue limit was determined by means of the staircase method employing a stress step of 5 MPa. The results indicate that the presence of the coating gives rise to a reduction in the fatigue life of the coated samples tested in air in comparison with the uncoated specimens. On the contrary, when the coated samples were tested in a NaCl solution at alternating stresses less than 350 MPa, these showed an increase in fatigue life in comparison with the polished uncoated samples. The analysis of the fracture surfaces of the specimens tested in air revealed that alumina particles present on the surface of the grit-blasted samples acted as stress concentrators, inducing the nucleation of fatigue cracks at the substrate–coating interface, which explains the reduction in fatigue life. However, under corrosive conditions and low alternating stresses, the presence of the coating provides an effective protection against corrosion-fatigue failures, giving rise to an improvement of the corrosion-fatigue performance of the coated system. On the contrary, at elevated alternating stresses, the coating was observed to delaminate from the substrate, leading to an impairment of the corrosion-fatigue behavior of the coated samples.

In the present work, study on a systematic variation of the processing parameters such as gas mixture, time of nitriding and current density has been carried out in order to determine how these will influence the sliding wear behaviour of plasma nitrided layers on AISI H13. Optical profilometry and scanning electron microscopy were used to characterise the worn sample and to determine the wear mechanism. The results revealed a strong influence of the current density on the formation of different diffusion and nitrided layers, which explained the wear behaviour of the studied samples, especially for those tested at room temperature. It was found that the wear volume of the nitrided samples without the existence of a compound layer was ∼200 times smaller than the wear volume of the untreated samples and nearly seven times smaller than the wear volume of nitrided samples when the compound layer was present. When the tests were carried out at 300°C, the wear behaviour was worse for the nitrided sample...

Adhesion is one of the most important parameters which influences the development of thermal spray coatings. Therefore, the level of adhesion should be known for a given application. Apart from the standardized Tensile Adhesive Test (TAT), more than 80 methods are reported to measure the coating adhesion. Most of them are energy consuming in terms of time, cost and equipment. Moreover, they do not fulfil the necessary requirements of accuracy, confidence and representation of the real delamination process observed in service. To address this problem, the interfacial indentation test is used here to initiate and propagate a crack at the interface between the substrate and the coating. Studying the extension of the crack, an interfacial toughness is defined and deduced analytically from the experimental results. The new shear test, developed in the frame of the EU-CRAFT-project “Shear Test for Thermally Sprayed Coatings”, is also employed to assess the coating adhesion. Both tests are compared to the standardized TAT for various spraying systems, materials, substrate roughness and coating thickness. Advantages and disadvantages of the three tests are discussed. Correlations between the tests results obtained for different coating–substrate combinations are presented and general trends are described.

In the present paper, five different operating conditions of a plasma nitriding process on annealed AISI H13 were investigated. A systematic variation of processing parameters such as gas mixture, time of processing and current density has been carried out in order to study their effect on microstructure. X-ray diffraction, microhardness testing and scanning electron microscopy techniques coupled with semiquantitative energy dispersive X-ray analysis were used to characterise the nitrided samples. The results revealed that the current density during plasma processing has a considerable influence on both the compound layer formation and depth of the diffusion zone. Also, it has been shown that the geometrical configuration of the workload may affect the uniformity of the diffusion zone, giving rise to the presence of heterogeneous nitrogen distribution zones along it.

Interfacial indentation is used to create and propagate a crack in the interface plane between a coating and its substrate. It has been shown earlier that this methodology allows the measurement of an apparent interface toughness, which can represent the adhesion of the coating to its substrate. In service thermal spray coatings are subjected to various external influences, which could be detrimental to their adhesion, e.g. thermal shock, fatigue, wear, corrosion and combinations of these. The objective of the present work is to study some of these effects and their influence on the interface adhesion toughness of various substrate/coating systems based on new phenomenological approaches to the residual stress state existing in the coating: (a) annealing treatment – for as sprayed specimen, it was first shown that the interface toughness is directly proportional to the reciprocal of the squared coating thickness. Extrapolated to an infinite thickness, the toughness could be assumed then to repres...

The present study has been carried out in order to evaluate the tribological behavior of HfN-based multilayer-coated tools used for milling. The initial results of the wear behavior of multilayer HfN coatings, during the laboratory friction and wear tests conducted at room temperature against WC as tribological pair, are reported. As part of the characterization of the mechanical properties of the coatings, Vickers microindentation tests were also carried out employing loads in the range of 0.01–10 N, in order to evaluate the composite hardness of the coating-substrate systems. Information regarding the absolute hardness of the HfN and TiCN monolayer films has been obtained employing a model recently developed by one of the authors. This information is then used in order to determine the properties of the HfN/TiCN multilayer coating. It has been found that the wear volume of the single layer HfN coatings is approximately 15 times greater than the wear volume corresponding to the HfN/TiCN for the same amount of cycles.

This investigation has studied the effect of a WC–12Co alloy deposited by plasma spray on the fatigue and corrosion fatigue properties of a 7075-T6 aluminium alloy, as part of its validation as a viable replacement of electrolytic hard chromium plating for aircraft applications. It has been determined that coating this substrate alloy with a deposit of ∼50 μm gives rise to a significant decrease in the fatigue and corrosion fatigue properties of the substrate coating system in comparison with the uncoated substrate. Thus, if the substrate coating system is tested in air at maximum alternating stress in the range 219–377 MPa, the fatigue life debit varies between 86 and 73% in comparison with the uncoated substrate. On the other hand, if the substrate coating system is tested in a 3%NaCl solution in the same range of alternating stress, the fatigue life debit varies between 23 and 73%. However, at alternating stresses