Tribological studies on PVD/HVOF duplex coatings on Ti6Al4V substrate

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

Friction and wear response of pulsed electric current sintered TiB2-B4C ceramic composite

This paper presents the friction and wear behaviour of TiB2-40vol%B4C 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 Péclet 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. © 2014 Elsevier B.V.status: publishe

Influence of wire-EDM on high temperature sliding wear behavior of WC10Co(Cr/V) cemented carbide

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.status: publishe

Influence of tip defect and indenter shape on the mechanical properties determination by indentation of a TiB₂-60%B₄C ceramic composite

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.status: publishe

Measurement of residual stress in thermal spray coatings by the incremental hole drilling method

The experimental measurement of residual stresses originating within thick coatings deposited by thermal spray processes onto solid substrates plays a fundamental role in the preliminary stages of coating design and process parameters optimization. The main objective of the present investigation was to determine the residual stresses by means of the incremental hole drilling method in order to perform the measurement of the stress field through the thickness of two different HVOF Nickel-based coatings. The holes through the coatings were carried out by means of a high velocity drilling machine (Restan). A finite element calculation procedure was used to identify the calibration coefficients necessary to evaluate the stress field. The Integral method was used for the analysis of non-uniform through-thickness stresses. The results for both coatings indicate that the nature of the residual stresses is tensile and their values are between 150-300 MPa. (c) 2006 Elsevier B.V. All rights reserved

Characterization and residual stresses of WC-Co thermally sprayed coatings

The present investigation has been conducted in order to determine the residual stresses of an as-ground WC-12Co coating of two different thicknesses, by means of two different methods. Firstly, X-ray diffraction techniques, which allowed the determination of the surface residual stresses of the coating by means of the method called "sin(2) psi" method. Secondly, an incremental hole drilling technique together with the integral method, which allowed the analysis of the non-uniform through-thickness residual stresses present in the coatings. It has been determined that the surface residual stresses are of a Compressive nature, which could be due to the grinding that was applied to the coatings in order to achieve the desired thicknesses. On the contrary, the results of the incremental hole drilling tests indicated that the through-thickness residual stress distributions are not uniform and are characterized by the presence of tensile peak stresses, at depths in the range of similar to 50-125 mu m. Such stresses were observed to decrease towards the coating-substrate interface where the compressive component of the stress state becomes greater than the tensile component. It has been found that the mean residual von Mises stress is higher in the thinner coating than in the thicker one, of approximately 180 and 107 MPa, respectively. (C) 2008 Elsevier B.V. All rights reserved