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This work investigates the tribocorrosion properties of the nickel boride layer on an Inconel 718 superalloy immersed in a neutral solution of sodium sulfate and sodium chloride. The solution simulates the corrosive environment created by the combustion reaction of sodium, sulfur, and oxygen when Ni-based superalloys are used in turbine blades. A powder-pack boriding process was conducted at 950 °C for 6 h, obtaining a nickel boride layer thickness of around 50 μm on the surface of the superalloy; microstructural analysis revealed that the boride layer was composed of Ni 4 B 3 –Ni 2 B–Ni 3 B. Further, depth-sensing Vickers microindentation tests were performed to assess the ratios of H/E and H³/E2 on the nickel boride layer. Additionally, tribocorrosion tests were conducted with a reciprocating tribometer and a typical three-electrode electrochemical cell. The counterpart was a 4.8 mm diameter alumina ball that applied 20 N for a sliding distance of 100 m. According to the ASTM G119-09 procedure, the total material loss rate due to tribocorrosion (T), which includes mass loss rate due to wear (W c) and corrosion (C w), was estimated. The results revealed a wear-dominant regime for the Reference material (Inconel 718 superalloy), attributed to passive film removal and high contact pressure at the tribopair, which increases the volume loss rate. On the other hand, the nickel boride layer on the Inconel 718 superalloy displayed a wear-corrosion regime due to boride layer debris that increased the corroded area. Under these experimental conditions, the presence of the nickel boride layer on the Inconel 718 superalloy improved tribocorrosion resistance by approximately three times. [ABSTRACT FROM AUTHOR]

New results about the scratch adhesion resistance of nickel boride layer on Inconel 718 superalloy were estimated in the present study. The nickel boride layer was developed on the surface of Inconel 718 superalloy by means of the powder-pack boriding process conducted at 1173 K with 2, 4, and 6 h of exposure. The microstructure of the nickel boride layer was analyzed from optical microscopy, X-ray diffraction and energy dispersive spectroscopy (EDS). Furthermore, and before the scratch tests, indentation properties of the nickel boride layers such as hardness, Young's modulus, and the distribution of residual stresses were evaluated using Berkovich nanoidentation tests applying a constant load (50 mN) across the diffusion layers. The scratch tests were performed over the surface of the nickel boride layer-substrate systems using a Rockwell-C diamond indenter with a continuously increasing normal force from 1 to 80 N, whereas the behavior of the coefficient of friction and the residual depth as a function of the scratch length were monitored during the tests. For the determination of the critical loads, the combination of acoustic emission signal with microscopic observations of the worn tracks were used; the critical loads were estimated at which the layer cracks (cohesive failure) or is detached (adhesive failure) and they explained according to the mechanical properties of the nickel boride layer-substrate system. For all the set of experimental conditions, the presence of three types of failure mechanisms over the worn tracks were detected, while the results showed that the critical loads increase with enhancing nickel boride layer thickness. [ABSTRACT FROM AUTHOR]

New results about the growth kinetics of nickel boride layers on an Inconel 718 superalloy were obtained. The nickel boride layers were developed by means of the powder-pack boriding process at 1173–1223 K with different exposure times for each temperature. According to optical and scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and nanoindentation techniques, the microstructure of the boride layer consisted in three zones: a high content of Ni 4 B 3 on the outer-zone of the borided surface, while Ni 2 B and Ni 3 B were revealed at the inner zone of the layer and diffusion zone, respectively. Moreover, the boriding growth kinetics of nickel boride layers was estimated using the Heat Balance Integral Method (HBIM) based on a numerical resolution of a system formed by differential algebraic equations, and considering that the evolution of the layers is governed by the parabolic growth law. For the range of boriding temperatures, the boron activation energies in the Ni 4 B 3 , Ni 2 B and Ni 3 B resulted in 233, 206 and 218 kJ mol−1, respectively. • New results of growth kinetics of boride layer on Inconel 718 alloy were obtained. • The HBIM was used to evaluate the growth kinetics of Ni 4 B 3 , Ni 2 B and Ni 3 B phases. • Boron diffusion in Ni borides was obtained by a particular solution of DAE system. • The Q values were 233, 206 and 218 kJ mol −1 for Ni 4 B 3 , Ni 2 B and Ni 3 B phases. [ABSTRACT FROM AUTHOR]

New results about the wear resistance of nickel boride layers under dry sliding conditions were obtained in this work. For this purpose, an Inconel 718 superalloy was borided under different conditions: 1173 K with 2 h of exposure and 1223 K with 6 h of exposure, using the powder-pack process. Before the wear tests, the nickel boride layer was characterized by the depth-sensing Vickers microindentation technique to determine indentation properties such as hardness, Young's modulus, residual stresses and fracture toughness. The dry sliding wear tests were performed on the borided Inconel 718 superalloy obtained for the two boriding conditions, and on the surface of the reference material (Inconel 718 superalloy), using the ball-on-flat technique, under a constant load of 20 N and considering relative wear distances between 50 and 200 m. In addition, the development of the failure mechanisms over the surface of the wear tracks was analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) techniques. For the overall set of experimental conditions, the results showed that the presence of nickel boride layers on the surface of the Inconel 718 superalloy increased the wear resistance compared with the reference material. However, the presence of a deeper zone of compressive residual stresses at the inner-zone of the nickel boride layer formed at 1223 K with 6 h of exposure, enhanced the wear resistance by approximately two- and three-fold compared with the values estimated for the nickel boride layer obtained at 1173 K with 2 h of exposure. • New results about the sliding wear resistance of nickel boride layer were obtained • The wear resistance of the nickel boride layer is influenced by the layer thickness • Indentation properties of the boride layer are correlated with wear resistance • A thick nickel boride layer developed a deeper zone of compressive residual stresses • A deeper zone of compressive stresses increased the wear resistance of the layer [ABSTRACT FROM AUTHOR]

Recently, electric arc-based welding processes have gained significant attention in the manufacturing sector due to their usefulness for repairing or remanufacturing damaged mechanical components. The changes in microstructure and mechanical properties of materials and components repaired by electric arc-based welding techniques have been the focus of most of current research. Since most of components are damaged by wear during operation, the improvement of such repaired/welded materials or components in terms of tribological properties for each specific application is wanted. In this work, the effects of applying a thermochemical treatment (boriding) as a tribological booster on a low carbon steel repaired by arc welding were studied. The tribological behavior (coefficient of friction (CoF) and wear volume) of the repaired steel before and after being borided was evaluated by means of micro-abrasion tests to replicate three-body abrasion under muddy environments. In addition, the physical and mechanical characteristics of the boride coating in both the original material and the repaired zone were evaluated by optical microscopy, X-ray diffraction, instrumented hardness tests and the VDI adhesion test. The wear scars were analyzed by scanning electron microscopy (SEM) and non-contact profilometry to identify the resulting wear mechanisms and to measure wear scars, respectively. The results showed that boriding treatment was effective to increase the hardness and wear resistance of the repaired low carbon steel without creating gradients of hardness, CoF, wear resistance or adhesion strength between the surface of the original material and the repaired zone. • Boriding was applied on steel repaired by wire and arc additive manufacture (WAAM). • Boriding was useful to homogenize mechanical properties of welded surfaces. • Tribological properties were evaluated by micro-abrasion tests. • Mechanical and tribological properties of WAAM repaired region were improved. • Boriding increases the hardness and wear resistance of the WAAM repaired region. [ABSTRACT FROM AUTHOR]