Search

In this study, novel findings were obtained regarding the influence of a 10 A current intensity on the growth of an FeB–Fe2B layer during pulsed-DC powder-pack boriding. Boride layer formation was carried out on AISI 316 L steel at 1123–1223 K for different exposure times at each temperature, considering 10 s polarity inversion cycles. The boride layer was characterized by x-ray diffraction and high-speed Berkovich nanoindentation, the latter being used to determine the hardness and reduced Young's modulus mappings along the depth of the layer-substrate system. Moreover, the growth kinetics of the FeB–Fe2B layer on the steel's surface was modeled using the heat balance integral method (HBIM). This involved transforming Fick's second law into ordinary differential equations over time, assuming a quadratic boron concentration profile in space to determine the B diffusion coefficients in FeB and Fe2B, respectively. From the Arrhenius relationship, the B activation energies in the boride layer were estimated considering the contribution of the electromigration effect; the results showed an approximately 30% reduction compared to the values obtained in the conventional powder-pack boriding for AISI 316 L steel. Finally, the theoretical layer thickness obtained by the HBIM demonstrated an error of no more than 5% against the experimental FeB and FeB + Fe2B layer thickness values. [ABSTRACT FROM AUTHOR]

In this study, novel findings were obtained regarding the influence of current intensity and electrode distance on the growth of the FeB-Fe2B layer during pulsed-DC powder pack boriding (PDCPB). Boride layer formation was carried out on AISI 1018 and AISI 4140 steels at 900 °C for 2700 s, considering current intensities ranging from 2.5 to 7.5 A and electrode distances of 10, 15, and 20 mm for each current intensity. The growth of the FeB-Fe2B layer was enhanced as the current intensity/electrode distance increased. This was related to the contribution of electromigration, the amount of powder mixture (used during the PDCPB) between electrodes, and Joule heating. Analysis of variance was performed on borided steels to assess the impact of the current intensity/electrode distance on the boride layer growth. The results revealed weight coefficients of approximately 50% for current intensity, around 40% for electrode distance, and a combined contribution of both variables of no more than 3.5%. Finally, multiple regression analyses were conducted to estimate boride layer thickness expressions as a function of the independent variables. The model results demonstrated a 5% error when compared to the experimental boride layer thickness. [ABSTRACT FROM AUTHOR]

New results about the influence of a small iron borided layer (~ 36 µm) obtained by the powder-pack boriding process (PPBP) and diffusion annealing post-process (DAP) were evaluated. Dry sliding wear tests on PPBP, PPBP + DAP and untreated AISI 1018 steel were performed under the pin-on-disc configuration, and balls of Al2O3 and WC were used as a counterpart at different sliding distances. Also, the contact pressures of the layer/substrate system were evaluated using the finite element model with the aid of the ABAQUS software. The results showed that the Al2O3 ball presented the major coupling (ball-disc), and the PPBP + DAP reduces the specific wear rate around ~ 304 times related to the untreated AISI 1018 steel to 500 m under severe sliding conditions. [ABSTRACT FROM AUTHOR]

New results about the tribocorrosion resistance of borided ASTM F1537 alloy immersed in Hank's solution were obtained in this study. A CoB-Co 2 B layer, with around 30 μm of thickness, was obtained at the surface of the cobalt-based alloy using the powder-pack boriding process at 1273 K with 6 h of exposure. The tribocorrosion tests were carried out in the borided ASTM F1537 alloy and the non-borided ASTM F1537 alloy (reference material) using a linear reciprocating tribometer coupled with a standard three-electrode electrochemical cell. A counterpart of alumina (ball of 4.8 mm-diameter) was used, with a constant applied load of 20 N, and a total sliding distance of 100 m. The total material loss rate due to tribocorrosion (T), which included the mass loss rate due to wear (W) and that due to corrosion (C), was estimated according to the ASTM G119 procedure. The results showed that the presence of CoB-Co 2 B layer at the surface of the ASTM F1537 alloy increased the tribocorrosion resistance around 1.2 times compared with the reference material. In addition, for the reference material, 55% of the material loss rate was attributed to the wear-corrosion synergism in comparison with 47% estimated for the borided ASTM F1537 alloy. Finally, the influence of wear affected in greater extent than corrosion in the reference material, whilst the corrosion-wear regime was obtained for the borided ASTM F1537 alloy. • New results about the tribocorrosion behavior of the CoB-Co 2 B layer were obtained. • The tribocorrosion resistance was enhanced by the presence of the CoB-Co 2 B layer. • 47% of the total material loss rate was attributed to the corrosion-wear synergism. • A corrosion-wear regime was obtained for the borided ASTM F1537 alloy. [ABSTRACT FROM AUTHOR]

New results about the growth kinetics of CoB–Co2B layers developed at the surface of CoCrMo alloy using the powder-pack boriding process assisted by a direct current field (PBDCF) were estimated in this work. The PBDCF was conducted at temperatures of 1048 – 1148 K with different exposure times for each temperature, whereas the growth kinetics of the cobalt boride layers was modelled using a system of two differential equations. In addition, indentation properties such as hardness, Young's modulus and residual stresses were estimated along the depth of the borided CoCrMo surface. The growth kinetics of the cobalt boride layers developed by PBDCF indicated that thicker boride layers were formed on the material's surface which was in contact to the current field at the anode, in contrast to the surface exposed at the cathode. The kinetics of cobalt boride layers were compared with those obtained by conventional powder-pack boriding process. [ABSTRACT FROM AUTHOR]

New results about the tribocorrosion resistance and cytotoxicity of FeB-Fe 2 B layer developed by the powder-pack boriding process on the surface of AISI 316 L steel are presented in this work. Initially, the tribocorrosion tests in the borided AISI 316 L steel and the untreated material (AISI 316 L steel) were performed in Hank's solution, using a ball-on-flat configuration, which was connected with a three electrode-chemical cell. The sliding tests, in the presence or absence of corrosion, were performed under 20 N normal force using an alumina ball as a counter body. For the overall experimental conditions, the material loss for each material was estimated according to the ASTM G119 procedure. Further, the in vitro cytocompatibility of FeB-Fe 2 B layer on AISI 316 L steel and the untreated material was evaluated by the indirect contact method, in which cell lines of immortalized human fibroblast (CHON-002) and normal renal epithelium of Cercopithecus aethiops (Vero) were used. The results showed that the presence of FeB-Fe 2 B layer on the AISI 316 L steel improves the tribocorrosion resistance 1.5 times than that of the untreated material, with a presence of a wear-corrosion degradation mechanism in contrast with a wear-dominated regime for the untreated material. Finally, the cytotoxicity tests revealed satisfactory properties in terms of effects on survival and proliferative activity of human fibroblasts and Vero cells on the surface of the borided AISI 316 L steel. [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 corrosion resistance of borided CoCrMo alloy exposed to the Hanks' solution during different days were estimated by means of the electrochemical impedance spectroscopy technique. The CoB-CoB coating was developed on the surface of the borided alloy using the powder-pack boriding process at 1223 K during 6 h of exposure. The corrosion resistance of the borided cobalt alloy was evaluated by the fitting of suitable equivalent electrical circuits using Nyquist and Bode plots to obtain the electrochemical parameters; the results were compared with the CoCrMo (non-borided) alloy. The samples (borided and non-borided) were characterized by the scanning electron microscopy and by the energy-dispersive x-ray spectrometry techniques to determine the elemental chemical composition developed on the surface of the materials. In addition, the reaction products formed on the surface of the borided CoCrMo alloy exposed to the Hanks' solution after the tenth day of immersion were analyzed by the x-ray photoelectron spectroscopy (XPS) technique. The results showed that the corrosion resistance of the borided cobalt alloy was affected (or reduced) by the presence of BS and CrPO clusters formed on the material's surface. Finally, the electrochemical reactions developed during the immersion of the borided cobalt alloy on the tenth day of exposure were proposed according to the XPS results. [ABSTRACT FROM AUTHOR]

New results about the evolution of the FeB-Fe 2 B layers during a diffusion annealing process (DAP) are presented in this work. First, the growth of the boride layers over the surface of an AISI 1045 steel was developed by means of the powder-pack boriding process (PPBP) at temperatures of 1173–1273 K with different exposure times for each temperature. The boron diffusion coefficients in the FeB and Fe 2 B were estimated according to the mass balance equations on the growth interphases, and expressed as a function of the boriding temperatures by the Arrhenius equation. Moreover, the DAP was conducted on borided samples obtained at 1273 K with 4–8 h of exposure using a SiC atmosphere, and considering the theoretical values of annealing times proposed by the extended model. The evolution of the boride layer microstructure was represented by the interphase velocities of the FeB/Fe 2 B and Fe 2 B/substrate, and the relationships between the growth of the Fe 2 B at the expense of the FeB layer for the applied range of annealing times. [ABSTRACT FROM AUTHOR]

Different kinds of materials are broadly employed to manufacture bioimplants. Metals are commonly used for the fabrication of lower body components such as knee prostheses and hip replacements because of their good mechanical properties. Among the available metallic biomaterials, cobalt alloys are commonly used. However, the hostile environment of the human body promotes the corrosion of the metallic bioimplant, and the wear due to the sliding contact could also increase the corrosion. There is, therefore, a constant attempt to study the surface of metallic biomaterials to enhance their functionality. In the present work, two kinds of surface modification techniques were employed on ASTM F-1537 cobalt alloy samples: laser surface texturing and boriding. Tribocorrosion tests were carried out using a linear reciprocating tribometer coupled with a standard three-electrode electrochemical cell. Tafel polarization plots were used to calculate the material loss due to corrosion. Also, non-contact profilometry was employed to estimate the tribocorrosion volume loss. In addition, the worn tracks were analyzed by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The textured + borided samples increased 1.5 times the tribocorrosion resistance compared to the untreated samples. The differences in the tribocorrosion performance among the modified surfaces were discussed in this paper. • The texturing and boriding increased the tribocorrosion resistance of the surface. • The material loss due to corrosion was lower than the material loss due to tribocorrosion. • The texturing decreased the real contact area and the boriding retarded wear. [ABSTRACT FROM AUTHOR]

In this study, new results about the practical adhesion resistance of boride coating/substrate system formed at the surface of AISI 316 L steel and improved by means of a diffusion annealing process are presented. First, the boriding of AISI 316 L steel was performed by the powder-pack method at 1173 K with different exposure times (4-8 h). The diffusion annealing process was conducted on the borided steels at 1273 K with 2 h of exposure using a diluent atmosphere of boron powder mixture. The mechanical behavior of the boride coating/substrate system developed by both treatments was established using Vickers and Berkovich tests along the depth of the boride coatings, respectively. Finally, for the entire set of experimental conditions, the scratch tests were performed with a continuously increasing normal force, in which the practical adhesion resistance of the boride coating/substrate system was represented by the critical load. The failure mechanisms developed over the surface of the scratch tracks were analyzed; the FeB-FeB/substrate system exhibited an adhesive mode, while the FeB/substrate system obtained by the diffusion annealing process showed predominantly a cohesive failure mode. [ABSTRACT FROM AUTHOR]

New fracture toughness ( K C ) data for cobalt boride (CoB and Co 2 B) coatings were obtained using the Vickers depth-sensing microindentation technique. The coatings were developed on the surface of a CoCrMo alloy using the powder-pack boriding process at temperatures between 1223 and 1273 K using various exposure times for each temperature. The mechanical characterization of the boride coatings was divided into two procedures: first, Vickers indentations were conducted at constant distances from the surface using loads ranging from 15 to 450 mN. For the entire set of experimental conditions, the behavior of the indentation load as a function of the diagonal length ( d m ) was examined on the CoB and Co 2 B coatings. Second, the crack lengths emanated on the corners of the indentations marks (with applied loads above 250 mN) were measured on both coatings using a scanning electron microscope (SEM). Based on the indentation results, the K C values of the cobalt boride coatings were estimated using the universal crack equation, which is applicable independent of the cracking mode. The results indicated that the CoB and Co 2 B coatings exhibited two types of cracking modes (intermediate and radial-median, respectively), and that the fracture toughness of the Co 2 B coating was ten-fold greater than that of the CoB coating. [ABSTRACT FROM AUTHOR]

Abstract: In this study, new data about the growth kinetics and indentation properties of cobalt boride layers formed on the surface of the ASTM F-75 biomedical alloy were estimated. The boron diffusion at the surface of the biomedical alloy was conducted using a powder-pack boriding process at temperatures of 1223–1273K with different exposure times for each temperature. Two mathematical approaches were proposed to determine the boron diffusion coefficients of the CoB and Co2B layers in the range of boriding temperatures, in which the experimental results of the kinetics of the cobalt boride layers were compared with those estimated using diffusion models. Finally, the mechanical characterization of the cobalt boride layers was evaluated by indentation techniques with applied loads of 0.98N and 50mN, respectively. According to the nanoindentation tests, a maximum hardness value (30GPa) and a maximum Young's modulus (380GPa) were obtained near the surface region (5μm) of the borided cobalt alloy. [Copyright &y& Elsevier]

Abstract: In this study, the fracture toughness of boride coatings formed at the surface of AISI 1045 steel was improved by means of a diffusion annealing process. First, the boriding of AISI 1045 steel was performed by the powder-pack method at a temperature of 1223K and a range of exposure times (8–12h). The diffusion annealing process was conducted on the borided steels at a temperature of 1273K with 8h of exposure using a diluent atmosphere of SiC powder and bentonite. To establish the mechanical behavior of the boride coatings developed by both treatments, properties such as the real hardness and the Young's modulus were estimated at 50μm from the surface using Vickers and Knoop testing, respectively. The fracture toughness of the boride coatings was estimated using a universal crack equation applicable independently of the cracking mode. The boride coating obtained by the boriding process exhibited an intermediate cracking mode, while the coatings obtained by the diffusion annealing process showed a radial-median mode. The effect of the diffusion annealing process on the fracture toughness of the boride coatings revealed an increase of approximately 50% in comparison with the coatings developed by the powder-pack boriding process. [Copyright &y& Elsevier]

Abstract: In this study, the indentation properties of the Fe2B layer were evaluated. The powder-pack boriding process on low-carbon steels was carried out at a temperature of 1273K for 4, 6 and 8h of exposure. First, Vickers indentation testing was performed on the boride layer at a distance of 50μm from the surface of the borided steel with applied loads in the range of 0.0981 to 9.81N for the set of experimental conditions of the boriding process. Furthermore, the hardness–depth profile, the behavior of the Young's modulus, and the elastic and plastic work of indentation were estimated by the instrumented (Berkovich) indentation testing using a constant load of 250 mN along the depth of the Fe2B layer. The results of the Vickers hardness testing on 50 μm from the surface indicated the presence of the indentation size effect (ISE) in the boride layer, in which the apparent or real hardness was estimated according to the proportional specimen resistance (PSR) model, and the dislocation band (DB) model. Finally, using the results of the instrumented indentation tests along the depth of the boride layer, the thermal residual stresses and the hardness dissipation parameter (HDP) were evaluated as a function of the boriding temperature and exposure time. [Copyright &y& Elsevier]

Abstract: In this study the mechanical behavior of FeB and Fe2B layers formed at the surface of AISI D2 steels was estimated by the Berkovich nanoindentation technique. The boriding of AISI D2 steels was developed by the powder-pack method at temperatures of 1223, 1273 and 1323K with exposure times of 3–7h for each temperature. The mechanical characterization was performed to determine the effect of the set of experimental parameters of the boriding process with three experimental procedures: first, nanoindentation was performed along the depth of the surface layers at a constant load of 250mN to determine the hardness gradient and the state of the thermal residual stresses in the boride layers; then, loads in the range of 10 to 300mN were applied to the “pure” zone of the FeB layer at a distance of 10μm from the surface, and in the “pure” zone of the Fe2B layer (40μm), respectively. Finally, the fracture toughness, compressive residual stresses, and brittleness of the boride layers were evaluated using the length of the cracks that originated from the corners of the Berkovich nanoindentation site after the application of loads ranging from 300 to 500mN. For a constant load of 250mN, the results showed that the state of thermal residual stresses and the hardness of both the FeB and Fe2B layers were a function of the temperature and exposure time of the process. Moreover, the presence of the indentation size effect (ISE) in the FeB and Fe2B layers was verified in the range of applied loads (10–300mN), in which the apparent or real hardness was estimated by the elastic recovery (ER) model according to the boriding experimental parameters. Finally, the estimated values of the fracture resistance and brittleness of the boride layers fell within the range of 1.48–3.02 and 2.01–4.65 , with the presence of compressive stresses in the range of 428 to 1604MPa for the FeB and Fe2B layers, respectively. [Copyright &y& Elsevier]

Abstract: A reverse analysis of the Vickers indentation fracture toughness was carried out to derive a numerical expression for estimating the fracture resistance of the Fe2B layer. The boride layers were created at the surface of AISI 1018 borided steels by the paste boriding process, in which a 4-mm-thick layer of boron carbide paste was applied to the material surface. The treatment was carried out at temperatures of 1123–1273K with exposure times of 4, 6 and 8h at each temperature. From the set of experimental conditions used in the boriding process, Vickers indentations were performed with an applied load of 1.96N at 25μm from the surface of the borided steels. The crack lengths created from the corners of the Vickers indentation prints were analyzed in the Palmqvist crack regime. The numerical expression of the stress intensity factor (K) of the Fe2B layer was assisted by both a dimensional analysis and the superposition principle. The analysis considered the residual stress field generated by the indentation load, which was superimposed upon different crack lengths; the stress intensity factor was expressed as a function of the elastic–plastic properties of the layer, the dimensionless crack length, and the location on the Palmqvist crack front. From an extensive numerical analysis, the results of the fracture toughness (K C) values of the Fe2B layer were estimated with the finite element method, and then compared with the values obtained by traditional Palmqvist crack models proposed by other researchers. [Copyright &y& Elsevier]

Boriding is a thermochemical surface treatment in which boron diffuses into and combines with a substrate material to form a single- or double-phase metal boride layer at the substrate's surface. This contribution discusses the importance of boriding for ferrous materials in which the formation of boride layers, such as FeB and Fe2B, improves hardness, wear resistance, temperature resistance and corrosion resistance at the surface of steels. The article includes a discussion of the growth kinetics of boride layers associated with a diffusion model. To understand the intended industrial applications, the article then reviews the mechanical characterisation by induced-fracture Vickers indentation and Berkovich nanoindentation of the boride layers. [ABSTRACT FROM AUTHOR]

Abstract: This study evaluated the indentation size effect on the Fe2B/substrate interface using the Berkovich nanoindentation technique. First, the Fe2B layers were obtained at the surface of AISI 1018 borided steels by the powder-pack boriding method. The treatment was conducted at temperatures of 1193, 1243 and 1273K for 4, 6 and 8h at each temperature. The boriding of AISI 1018 steel resulted in the formation of saw-toothed Fe2B surface layers. The formation of a jagged boride coating interface can be attributed to the enhanced growth at the tips of the coating fingers, due to locally high stress fields and lattice distortions. Thus, the mechanical properties achieved at the tips of the boride layer are of great importance in the behavior of borided steel. Applied loads in the range of 10 to 500mN were employed to characterize the hardness in the tips of the Fe2B/substrate interface for the different conditions of the boriding process. The results showed that the measured hardness depended critically on the applied load, which indicated the influence of the indentation size effect (ISE). The load-dependence of the hardness was analyzed with the classical power-law approach and the elastic recovery model. The true hardness in the tips of the Fe2B/substrate interface was obtained and compared with the boriding parameters. Finally, the nanoindentation technique was used to estimate the state of residual stresses in this critical zone of the Fe2B/substrate interface. [Copyright &y& Elsevier]

Abstract: The present study uses interfacial indentation testing to estimate the adhesion of the FeB/Fe2B coating formed on the surface of borided AISI 316 steel. This technique creates and propagates a crack along the FeB/Fe2B interface and defines the apparent fracture toughness, which can then be related to the adhesion and mechanical support of the aforementioned interface. The boriding process was performed on the surface of AISI 316 steel by means of the powder-pack method at temperatures of 1123, 1173, 1223 and 1273K with 2, 4, 6, 8 and 10h for each temperature. The Young''s modulus for each surface layer was obtained by Knoop microindentation at a constant indentation load. Vickers microindentation fracture technique was used to generate microcracks at the FeB/Fe2B interface with varying indentation loads. The applied load, Young''s modulus, hardness, and lateral crack lengths generated from the corners of the indentations, along with the depth of the FeB layer, were used to determine the apparent fracture toughness and adhesion of the FeB/Fe2B interface. The apparent fracture toughness of the FeB/Fe2B interface varied between 3.56 and 4.45MPa . Finally, the intensity of residual stress at the FeB/Fe2B interface was estimated as a function of the FeB layer thickness. [Copyright &y& Elsevier]

Abstract: The kinetics of the FeB/Fe2B layers and diffusion zone at the surface of AISI 316 steels exposed to the powder-pack boriding process were studied in this work. FeB/Fe2B layers and diffusion zone measurements were taken at different temperatures and exposure times to validate diffusion-controlled growth during the boriding process. In order to obtain the boron diffusion coefficients at the FeB/Fe2B layers and diffusion zone, a mathematical model based on the mass balance at the growing interfaces was proposed. The activation energy values estimated for the FeB and Fe2B layers were 204 and 198kJmol−1 respectively. In addition, the activation energy value obtained for the diffusion zone was 116kJmol−1. The diffusion model was extended to estimate the FeB/Fe2B layer thicknesses, and the depth of the diffusion zone at the temperature of 1243K with 3 and 5h of exposure, based on the experimental parameters ascribed to the boriding process. Finally, the effects of the FeB/Fe2B growth and diffusion zone, on the weight gain of borided steels and on the instantaneous velocity of the interfaces were incorporated in the model. [Copyright &y& Elsevier]

Abstract: The present study characterizes the surface of AISI 4140 steels exposed to the paste-boriding process. The formation of Fe2B hard coatings was obtained in the temperature range 1123–1273K with different exposure times, using a 4mm thick layer of boron carbide paste over the material surface. First, the growth kinetics of boride layers at the surface of AISI 4140 steels was evaluated. Second, the presence and distribution of alloying elements on the Fe2B phase was measured using the Glow Discharge Optical Emission Spectrometry (GDOES) technique. Further, thermal residual stresses produced on the borided phase were evaluated by X-ray diffraction (XRD) analysis. The fracture toughness of the iron boride layer of the AISI 4140 borided steels was estimated using a Vickers microindentation induced-fracture testing at a constant distance of 25μm from the surface. The force criterion of fracture toughness was determined from the extent of brittle cracks, both parallel and perpendicular to the surface, originating at the tips of an indenter impression. The fracture toughness values obtained by the Palmqvist crack model are expressed in the form K C (π/2)> K C > K C (0) for the different applied loads and experimental parameters of the boriding process. [Copyright &y& Elsevier]

Abstract: The growth kinetics of Fe2B layers formed at the surface of gray cast iron were evaluated in this study. The pack-boriding process was applied to produce the Fe2B phase at the material surface, and the variables included three temperatures (1173, 1223 and 1273K) and four exposure times (2, 4, 6 and 8h). Taking into account the growth fronts obtained at the surface of the material and the mass balance equation at the Fe2B/substrate interface, the boron diffusion coefficient on the borided phase was estimated for the range of treatment temperatures. Likewise the parabolic growth constant, the instantaneous velocity of the Fe2B/substrate interface, and the weight gain in the borided samples were established as a function of the parameters τ(t) and α(C), which are related to the boride incubation time (t 0(T)) and boron concentration at the Fe2B phase, respectively. Observation of the growth kinetics of the Fe2B layers in gray cast irons suggest an optimum value of boron concentration that is in good agreement with the set of boriding experimental conditions used in this work. [Copyright &y& Elsevier]

Abstract: This study evaluates the morphology of borided interfaces by means of the fractal theory. The boride layers were formed in the AISI M2 steel by applying the paste boriding treatment at temperatures of 1253 and 1273K and treatment times of 2 and 6h, while a boron carbide paste thickness of 4 or 5mm covered the samples surface in order to produce the boron diffusion. The morphology of interfaces formed between FeB and Fe2B layers and between Fe2B layer and steel substrate was analyzed by the rescaled-range (R/S), root-mean-square (RMS), and Fourier power spectrum (FPS) methods. Moreover, the multi-affine spectra of roughness exponent were obtained by calculating the q-order height–height correlation functions. We found that both interfaces are multi-affine, rather than self-affine. The multi-affine spectra of roughness exponents are found to be different for FeB/Fe2B and Fe2B/substrate interfaces, but independent on the treatment parameters (boron carbide paste thickness, temperature, and boriding time). Furthermore, we found that the multi-affine spectra of both interfaces behave as it is expected for “universal multi-fractals” with the Lévy index γ =1, associated with the multiplicative cascades with a log-Cauchy distribution. Furthermore, our data suggest a great homogeneity of the boron diffusion field, characterized by universal fractal dimension D diff =2.90±0.01. These findings provide a novel insight into the nature of phase formation during the boriding treatment. [Copyright &y& Elsevier]

An alternative method called pulsed-DC powder-pack boriding process (PDCPB) is presented in this study. The main components of the PDCPB consisted of a metal box containing the specimen embedded in a powder mixture, and placed between two electrodes, which were connected to a DC power supply, and a programmable electronic control device producing the polarity changes during the process. A set of boriding conditions were carried out on the surfaces of AISI 316 L stainless steel and Inconel 718 superalloy using a constant current input of 5 A with polarity inversion cycles of 10 s. After the PDCPB, the boride layers were characterized by optical microscopy, X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The growth kinetics of the boride layers was established using a diffusion model that considered the mass balance equations at the growth interfaces, in which the boron diffusion coefficients in the layers were expressed as a function of the boriding temperatures to estimate the boron activation energies in the borided materials. The change of polarity in the electrodes allowed a uniform flux of boron during the process, obtaining similar layer thicknesses on the surfaces of the material exposed to the pulsed-DC field. Finally, the results showed that the growth rate of the layers was increased by the effect of the pulsed-DC field, whilst the boron activation energies, in the borided materials, decreased drastically compared to those obtained for the conventional powder-pack boriding process. • A novel method denominated pulsed-DC powder-pack boriding is presented in this work. • Uniform boron fluxes were produced on the surfaces exposed to pulsed-DC field. • Pulsed-DC field developed similar layer thicknesses at the material surface. • Pulsed-DC field increased the growth kinetics of boride layer in treated materials. [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]

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]

This study presents novel insights into the Pulsed-DC Powder-pack Boriding (PDCPB) process applied to AISI 1018 steel at low (700 °C) and medium (750–850 °C) temperatures for 1, 2, and 3 h of exposure. The electric field, induced with periodic polarity inversion half-cycles of 10 s under 5–10 A, promoted a constant B+ flux, ensuring a uniform boride layer thickness. The growth of the boride layers was estimated using Dybkov's Chemical Interaction Model (DCIM) and analysis of variance (ANOVA). DCIM established the B activation energies in FeB and Fe 2 B phases, while ANOVA determined the influence of treatment parameters on the total layer thickness. The results demonstrated a reduction of B activation energies in FeB and Fe 2 B by up to ∼ 23 % and ∼ 15 %, respectively, compared to conventional powder-pack boriding. These reductions were attributed to electromigration and Joule heating phenomena. ANOVA results indicated that temperature was the most significant parameter (77 %) for total (FeB + Fe 2 B) layer thickness. At the lowest treatment temperature (700 °C), the highest average boriding media resistance (∼ 3.06 Ω) was measured, and the maximal electric field magnitude (3098 V m−1) was determined through computational analysis. • Joule heating depends on the boriding media's electrical resistance. • The electric field magnitude is inversely proportional to treatment temperatures. • The B activation energies reduced as the current intensity increases. • ANOVA results demonstrated that PDCPB highly depends on temperature. [ABSTRACT FROM AUTHOR]

• Boriding reduced friction by up to 30% compared to heat-treatment in dry tests. • In dry tests, the borided steel showed mild wear and the heat-treated steel showed severe wear. • Grease lubrication reduced friction similarly in the borided and heat-treated steel. • The wear resistance of the borided steel improved by about 10 times using grease. This work presents new results about the dry and grease-lubricated wear resistance of borided AISI 52100 steel. Boriding was conducted at 850 °C for 1 h. The reciprocating wear tests were conducted with an alumina ball of 6 mm of diameter, with normal load of 20 N and sliding distances of 100, 150 and 200 m; a lithium grease was employed for lubrication. In comparison with heat-treated AISI 52100 steel, boriding reduced the coefficient of friction by 15–30% in dry conditions, although both stabilized similarly in grease-lubricated tests. In addition, the results showed that, for the longer sliding distance, boriding increased the wear resistance up to 8 times in dry conditions and up to 10 times using grease for lubrication. The implementation of boriding significantly improved the tribological performance of AISI 52100 steel in both tests conditions. [ABSTRACT FROM AUTHOR]

• CoB-Co 2 B layer was changed to Co 2 B by diffusion annealing on borided CoCrMo alloy. • The scratch resistance notably improved after a diffusion annealing process. • The accomplished layer avoids severe scratch failure mechanisms. • The scratch residual depth decreased around 60% with this procedure. • The enhanced layer reduced around 50% the volume loss in multipass scratch. This work studies the effect of a diffusion annealing process in cobalt boride layers by progressive load scratch (PLST) and multipass scratch (MPST) tests. First, a double-phase CoB/Co 2 B layer was formed on CoCrMo alloy at 1000 °C for 6 h. Next, a single-phase Co 2 B layer was obtained through a diffusion annealing process. The scratch tests were conducted with a progressive load from 5 to 150 N in PLST, whereas in MPST, up to 400 passes were applied with a subcritical load of 38 N. Conducting the diffusion annealing process significantly improved the scratch resistance; in PLST, the achieved layer avoided severe failure mechanisms and decreased residual depth by 60%. In MPST, friction was reduced and the volume loss decreased around 50%. [ABSTRACT FROM AUTHOR]

Deep drawing of stainless steel and aluminum is a widely adopted method for manufacturing various industrial goods. However, the wear damage produced to die tools is a recurring problem in these processes. While lubricants are often employed to lubricate and reduce die wear, dry deep drawing is the current best alternative for meeting green manufacturing demands. Hence, enhanced solid lubricants or surface treatments for dies are highly required. This study aims to evaluate and compare the tribological behavior of a heat-treated AISI D2 steel and a boriding AISI D2 tool steel against stainless steel (AISI 304) and aluminum alloy (AA 6061 T6) in the absence of lubricant. The boride coating was formed on the AISI D2 steel by the closed-pack boriding technique, resulting in a tooth-saw morphology with an overall thickness of 33.6 μm. X-ray diffraction analysis confirmed the presence of FeB, Fe2B, and Cr2B3 phases on the surface of the borided AISI D2 tool steel. Notably, the boriding treatment led to a significant increase in the surface hardness of the tool steel, reaching a value of 20.7 GPa. The coefficient of friction (CoF) behavior and wear were evaluated by pin-on-disk tests, simulating conditions similar to those found in cold deep drawing operations. The wear surfaces were analyzed by optical profilometry and scanning electron microscopy (SEM). The results suggest boriding as a suitable treatment to reduce wear damage on the surface of deep drawing dies used for stainless steel and aluminum forming processes in the absence of lubricant. Two-body abrasion and adhesion were identified as the main wear mechanisms in AISI D2 steel against stainless steel and aluminum, with a significant amount of oxidation observed when aluminum was tested. Additionally, a reduction of approximately 20 % in the CoF was observed when employing borided tool steel against stainless steel. However, there was an observed increase of around 75 % in the CoF when the borided tool steel was tested against aluminum. • Boriding reduced wear in dry sliding of tool steel against Inox steel and aluminum. • CoF decreased for Inox steel vs. tool steel with borided samples. • CoF increased for aluminum vs. tool steel with borided samples. • Stainless steel mass loss decreased significantly with borided tool steel. • Abrasion and adhesion were identified as the primary wear mechanisms in all tests. [ABSTRACT FROM AUTHOR]

• Borided AISI 316L steel wear maps provide a general view of its wear performance. • Wear maps in terms of load and sliding speed for ball-on-flat tests are reported. • Mild regime is characterized by grooving, material agglomeration and a tribofilm. • Debris, pitting and severe grooving predominate in the transition zone. New results on wear maps of borided AISI 316L steel, obtained by powder-pack boriding process, are presented to provide a general visualization of its wear performance under different dry sliding conditions. Wear maps showed wear behavior in terms of specific wear rate, volume of material removed and coefficient of friction as a function of load (5–20 N) and sliding speed (5–30 mm/s). Specific wear rates between 5 × 10−6 to 9.7 × 10−6 mm3/Nm were found, corresponding to mild wear and the transition region to severe wear. Grooving, material agglomeration and the formation of a tribofilm characterize the mild zone as observed by SEM-EDS, while debris, pitting and severe grooving predominate in the transition zone. [ABSTRACT FROM AUTHOR]

• Erosion resistance of the X12CrNiMoV12-3 steel was improved by boriding process. • Material loss on boride layer decreased at 400 °C under the impact of SiO 2 particles. • At 400 °C and low incidence angle, the erosion on borided surface was null. • Cracking was the failure mechanism observed on the borided surface after erosion. The X12CrNiMoV12-3 stainless steel, used in aeronautic industry, was borided and characterized for its resistance to particle erosion. The performance of boride layer, developed at 950 °C with 6 h of exposure, was evaluated at two temperatures and three impact angles using SiO 2 particles. The results indicated, for all the experimental conditions, that the erosion resistance was improved for the presence of the boride layer on the surface, where the mass loss was null at 400 °C for the impact angle of 30°. Erosion mechanism of cracking was revealed on the borided surface compared with cutting and pitting on the untreated stainless steel. [ABSTRACT FROM AUTHOR]

In pursuit of innovating the conventional powder-pack boriding (CPB) process by introducing an electrical driving force, novel findings regarding the kinetics growth of boride layers produced on an AISI 8620 steel through the pulsed-DC powder-pack boriding process (PDCPB) were obtained. The PDCPB accelerates B diffusion on the surface of the specimen, resulting in an augmentation of the growth kinetics around 12 % and 15 % for FeB and Fe 2 B, respectively, compared to those observed in the CPB process. Additionally, the effect of polarity inversion half-cycles on the growth of the FeB-Fe 2 B layer during the PDCPB was studied. The experiments were conducted at temperatures of 850 °C, 900 °C, and 950 °C, with different exposure times, and polarity inversion half-cycles of 30 s and 50 s, maintaining a constant current density of around 177 mA cm−2. An estimation of the electric charge (q) using a transfer function considering minimal and maximum temperatures was achieved. It was confirmed that, with a symmetrical inversion, q was similar for both 30 s and 50 s polarity inversion half-cycles, indicating no influence on the growth kinetics of the boride layers. • Each component of boriding media influenced its electrical behavior during PDCPB. • A holistic explanation of the boriding media electrical behavior was developed. • Transfer function was used for the estimation of electric charge during PDCPB. • Similar electric charge was obtained for half-cycles >7 ms. • Electric charge did not influence the boride layer growth kinetics. [ABSTRACT FROM AUTHOR]

Thermochemical treatments are used for increasing the surface mechanical properties of iron and steels. The wear on mechanical components caused by sliding contact is an important parameter in the field of tribology. In this study, the wear resistance of ARMCO pure iron hardened by the powder-pack boriding process was evaluated using unidirectional dry sliding. The thermochemical treatment of boriding was carried out at temperatures of 850 °C for 1, 2 and 3 h of exposure time. Surface properties such as hardness and Young's modulus were obtained using the nanoindentation technique. The wear testing was performed by multi-pass scratch using an Al 2 O 3 sphere as counterpart, constant loads of 20 and 30 N and 50, 100 and 150 passes (with a sliding distance of 5 mm for each pass). The Archard's model was used to obtain the wear coefficient for each treatment condition. The finite element method applying mesh nonlinear adaptivity was used to evaluate the stress field in the borided samples under sliding contact. Results showed that borided ARMCO pure iron under unidirectional sliding loading caused both adhesive and abrasive wear. Furthermore, for the boriding conditions established in this work, the wear behavior at loads ≥30 N was governed by both the mechanical properties of the boride layer and its thickness, whereas at lower loads only the mechanical properties of the boride layer influenced the wear resistance. • Hard iron borides were formed on ARMCO pure iron by pack boriding process • Thicker boride layers exhibited higher sliding wear resistance in multi-pass scratch • Stress field in the layer/substrate systems are correlated with the wear resistance. [ABSTRACT FROM AUTHOR]

The implementation of post-treatments for the enhancement of the mechanical behavior of boride layers has been conducted in recent years. For this reason, the borided 8 % Cr steel underwent austempering-interrupted treatment and austempering-interrupted + cryogenic treatments. Resulting microstructures have been characterized using light optical microscopy, scanning electron microscopy and energy dispersive X-ray spectrometry, and X-ray diffraction. The brittleness grade and dry sliding wear resistance of the borided steel with and without post-treatments were achieved through a Vickers microindentation approach (at various loads of 2 N, 3 N, and 5 N) and the ball-on-flat wear test (at a constant load of 20 N for 100 m). In addition, a non-borided material (hardening + tempering) was tested employing the ball-on-flat wear test to demonstrate the improvement of the boride layers. As expected, the wear resistance of the 8 % Cr steel subjected to conventional boriding was 8 times greater than that of the non-borided material due to the surface hardening achieved through the thermochemical treatment. On the other hand, the austempering-interrupted treatment conducted after conventional boriding resulted in a 1.2 times improvement in wear resistance. This enhancement was attributed to the transformation of the FeB–Fe 2 B dual-phase layer into a Fe 2 B single-phase layer, which reduced the brittleness grade of the boride layer from 4 to 3.1. Nonetheless, the Fe 2 B single-phase layer achieved the highest wear resistance, which was 1.3 times greater, after undergoing austempering-interrupted + cryogenic treatments (resulting in a slight decrease in the material's brittleness grade from 3.1 to 2.9) compared to the borided steel treated with austempering-interrupted alone. Grooving and debris were observed on the entire wear tracks for both non-borided material (ductile) and boride layers (brittle). Additionally, brittle layers exhibited the development of cracks and spalling, while ductile non-borided steel showed plastic deformation. • Austempering-interrupted treatment caused the dissolution of the FeB layer. • Austempering-interrupted + cryogenic treatments reduced boride layer brittleness. • Both post-treatments enhanced the wear performance of boride layer. • Wear behavior of boride layers were not influenced by substrate microstructures. [ABSTRACT FROM AUTHOR]

Abstract This work presents an experimental-numerical study on the resistance of cobalt borides (CoB/Co 2 B) to contact fatigue. The boride layers were formed at the surface of a CoCrMo alloy using the powder-pack boriding process at temperatures of 1123 K for 3 h and 1223 K for 1 and 5 h of exposure times, in order to obtain three different thicknesses. The contact fatigue tests were performed with a servo-hydraulic universal testing machine by cyclic loading of a sphere on the layer-substrate system. The methodology of the contact fatigue test consisted of two main stages. Firstly, a critical load under monotonic loading was determined, where circular cracks were considered as the failure criterion. Secondly, fatigue conditions were conducted in low-cycle using sub-critical monotonic loads with a frequency of 6 Hz. A comparison of the results showed differences in the characteristics of the hard coating degradation in each case. The stress affiliated to the contact damage modes in the coating was obtained by the finite element method. The results of this study showed that contact damage modes in cyclic loading with sphere in cobalt boride coatings were a function of the coating thickness, number of cycles and contact load. Highlights • Cobalt boride layers were evaluated through standing contact fatigue. • Stresses affiliated to contact damage modes in the coating were obtained by the finite element method. • Contact damage modes in cyclic loading were function of layer thicknesses. [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]

Novel results about the growth kinetics of Fe 2 B layers produced by pulsed-DC powder-pack boriding (PDCPB) on AISI 1018 and 4140 steels were obtained. In the process, a DC-field is applied to ionize the B atoms and implements a programmable electronic control device (PECD) to maintain a constant B flux on both surfaces of the material. During PDCPB, the electrical resistivity (ER) was reduced due to the percolation of KBF 4 in the boriding media. Nevertheless, the increase of ER was caused by the depletion of KBF 4 after 1000s of the current induction. The PDCPB enhanced the growth kinetics of the Fe 2 B layer around of 9% for both borided steels compared to those estimated in the conventional powder-pack process. • A low potential boriding media allows the formation of Fe 2 B layer. • The pulsed-DC field enhances growth kinetics of the Fe 2 B layer. • The electromigration energy contribution decreases boron activation energy in Fe 2 B. • Boron activation energy in Fe 2 B is 9% lower than traditional powder-pack boriding. • DC field modifies the electrical behavior of boriding media due to Joule effect. [ABSTRACT FROM AUTHOR]

An experimental and numerical study of the scratch test on FeB/Fe 2 B bilayers is presented. The boride layers were formed at the surface of AISI 304 steels by developing the powder-pack boriding process at temperatures of 1223 K for 2, 6 and 10 h of exposure times. From the set of experimental conditions of the boriding process, scratch tests were performed with a linearly-increasing load mode of 1 to 90 N on 7 mm in length to determinate the most effective and informative testing conditions and to estimate the critical load (L c ) at which the boride layer fails. The damage in the boride layer was examined by high resolution SEM. Experiments tests indicated that at a critical load the boride layer fails through brittle fracture. Numerical calculations considering the residual stress field generated by the scratch load showed that at this load the tensile stresses inside the boride layer become large enough to cause brittle failure. The residual stress fields generated by the scratch load were analyzed and related with the failure mechanisms observed by the experimental tests. [ABSTRACT FROM AUTHOR]