Abstract: Inconel 625, a nickel-based superalloy, is used in a wide range of applications including the marine and petroleum industries under where it is subjected to harsh conditions such as high temperatures and highly corrosive environments. However, its wear resistance is limited and can be often considered unsatisfactory in some applications. If this alloy were to be used under abrasive wear conditions, its surface would have to be protected by a wear resistant coating. In this study, a two-step thermo-chemical borotitanizing treatment (including an initial boriding step followed by titanium diffusion) is proposed. Microstructural characterization (optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction) and mechanical properties (including micro-hardness and micro-abrasion wear) of the coated samples were conducted. Microstructural studies revealed a compact, homogenous, silicide-free coating, consisting of four distinct regions: a TiB 2 layer, a multi-phase boride layer, a diffusion zone and the substrate. Hardness values were significantly higher than those obtained by standard boriding treatments. Due to the nano-sized boriding agents used, the coatings formed on the surface were thicker than coatings obtained by methods such as nitriding, paste boriding and pack-boriding, and comparable to that of laser boriding. The wear resistance was improved by up to ten times in comparison with untreated Inconel 625. Grooving was the effective wear mechanism in untreated Inconel 625. However the increase in surface hardness achieved by the borotitanizing treatment changed the wear mechanism in the coated samples from grooving to rolling.

Abstract: New results about the evolution of the FeB-Fe2B 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 Fe2B 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/Fe2B and Fe2B/substrate, and the relationships between the growth of the Fe2B at the expense of the FeB layer for the applied range of annealing times.

Abstract: The Ni- based superalloy Inconel 625 is extensively used owing to its high strength, excellent fabricability, good weldability and outstanding resistance to high temperature corrosion against aggressive environments. However, despite its unique properties and extensive use, its wear resistance is in some cases unsatisfactory. In this study, Inconel 625 was subjected to boriding treatments at different temperature and different durations. Microstructural characterization was made by conventional methods (scanning electron microscopy, optic microscopy, X-ray diffraction analysis) and mechanical characterization was made by microhardness and micro-abrasion wear test. Micro-abrasion wear tests were conducted against a AISI 52100 steel under 5, 7.5 and 10 N load in a 25 wt.% SiC slurry at room temperature. Microstructural results revealed that multi-phase boride layer (nickel boride, chrome boride and iron boride) and silicide layer were formed. The hardness and thickness of the boride layers were found to be 1175--2432 HV0.1 and, 6.61 -- 92.03 I¼m, respectively. Depending on the boriding temperature and time. In spite of silicide layer formation the wear resistance of borided Inconel 625 alloy increased up to 8 times thanks to the increase in the surface hardness which was caused by boriding process. The boriding treatment caused a transition in the wear mechanism. While grooving was observed in the untreated Inconel 625 samples, mixed (grooving and rolling) and rolling type wear was predominant in the borided samples.

Abstract: In this study, the surface of steel tooth drill bits (Ni-Cr-Mo based) was subjected to the solid-state boriding treatment with 10- to 50-nm nanoboron powder. Boriding processes were carried out at a constant temperature of 1273 K for 30, 45, 60, 75, 90, and 105 min using a solid-state box boriding technique. Borided drill bit samples were characterized by conventional methods (microstructure, microhardness, X-ray diffraction, and chemical analysis). The wear behavior of borided samples was tested at different loads and sliding speeds by a microabrasion experimental setup. Metallographic studies showed that the boride layers have a sawtooth morphology and consist of FeB and Fe2B. The thickness and hardness of the boride layer were 35.29–202.56 μm and 1300–2333 HV0.1, respectively, depending on the duration. The wear resistance of borided samples increased significantly due to the increase in surface hardness and lubricating effect, both of which were caused by the boriding process. A groove wear me...

Abstract: To modify the surface properties of pure titanium, boride layers had been fabricated by the boron molten-salt diffusion on pure titanium surfaces in the temperature range of 900-1100 °C for 5- to 30-h treatments. The results demonstrated that the boride layers were mainly composed of TiB whiskers and TiB2 layers without the rutile titanium oxide TiO2. Two diffusion models were introduced to model the growth kinetics of boride layers. The parabolic growth constants and the boron diffusion coefficients were obtained. The boron activation energies for TiB2 and TiB were 225.617 and 165.266 kJ mol−1, respectively. The surface microhardness of the borided titanium decreased with the increase in distance from the surface. The results of wear tests indicated that the wear properties had been improved significantly compared to the pure titanium under dry sliding conditions.

Abstract: In this work, the EN-GJS-400-15 cast iron was pack-borided in a powder mixture composed of 5% B4C, 5% NaBF4 and 90% SiC at the three temperatures: 900, 950 and 1000°C for 2, 4 and 6 h, respectively. The pack-borided EN-GJS-400-15 cast iron was characterized by the following experimental techniques: optical microscopy, XRD analysis and Microhardness Vickers tester. The growth kinetics of boride layers was also investigated. As a consequence, the boron activation energy was found to be 212.28 kJ mol–1 for the EN-GJS-400-15 cast iron. Based on a regression model, a useful equation was derived to estimate the boride layer thickness as a function of the boriding parameters (time and temperature). A good agreement was then obtained between the predicted values of boride layers thicknesses and those measured experimentally. In addition, an iso-thickness diagram was proposed to be used as a simple tool to select the boride layers thicknesses according to the potential applications of EN-GJS-400-15 cast iron in industry.

Abstract: 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-Co2B 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 B2S3 and CrPO4 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: Plasma electrolytic boriding (PEB) is a method of combination surface strengthening and surface texturing on metal. In this study, the kinetics and the lubrication friction of borided layers in the plasma electrolytic boriding on the Q235 were investigated in an aqueous solution for 5–15 min. The cross-section and surface morphologies of the boriding layers were confirmed using scanning electron microscope (SEM). The presence of phases on the surface was determined using the X-ray diffraction. The hardness and the lubrication friction were evaluated using a micro-hardness tester and pin-on-disk friction tester in an oil sliding condition, respectively. The PEB layer contains phases in FeB, Fe2B, Ni3B4, NiB, and Ni2B. It is indicated that the value of activation energy in the PEB treatment is approximately 186.17 kJ/mol. The random micro-pores in surface texturing are unevenly distributed on the surface of the Q235. The micro-hardness of the boriding layer is up to 900 HV, whereas that of the substrate is approximately 181 HV. The weight loss of PEB sample in 10 min is 0.0017 mg in the lubrication friction, whereas that of untreated sample is 0.0047 mg in the same condition. The formation of boriding strengthening surface texturing in PEB improves lubrication friction greatly.

Abstract: Surface of low carbon austenitic stainless steel (AISI 316L) was modified by duplex surface treatment to improve its wear behavior, without compromising corrosion resistance of the steel. The duplex surface treating process composed of pack boriding (B) and pack chromizing (Cr) stages. Some specimens were duplex surface treated by boriding followed by chromizing (B–Cr), while some others were treated via chromizing followed by boriding (Cr–B), to evaluate the effect of duplex treatment sequence on wear and corrosion resistances. Microstructural studies, phase identification, wear and corrosion behaviors of the specimens were evaluated by scanning electron microscope (SEM), X-ray diffractometer (XRD), pin-on-disc wear test, and potentiostat electrochemical test. The results show that the precedence of boriding stage to chromizing stage makes significant improvement in wear and corrosion resistances of the duplex coating, whereas B-Cr coating provided superior wear resistances (up to 49%) and reduced corrosion rate (up to 4.5 times) in comparison to Cr-B and boriding coatings. Corrosion resistance of B-Cr coating was comparable to that of the stainless steel.

Abstract: In this study, the Response Surface Methodology (RSM) and Central Composite Design (CCD) were used to optimize the hardness of boride diffusion layer on ASTM F-75 alloy (also called Haynes alloy). A boronizing thermochemical treatment was carried out at different temperatures and for different time periods. Hardness tests were conducted. The boride diffusion layer was verified by the X-ray diffraction (XRD) analysis indicating the formation of CoB, Co$_2$B, CrB and Mo$_2$B phases. An optimal hardness of 3139.7 HV was obtained for the samples subjected to the boriding process for a duration of 6.86 h at 802.4$^{\circ}$C.

Abstract: The gas boriding in N2–H2–BCl3 atmosphere was applied in order to produce a wear resistant surface layer on Nimonic 80A-alloy samples. The microstructure, microhardness and corrosion resistance of the boride layer were investigated. The produced layer consisted mainly of the compact boride zone (with average thickness 71 μm), including the mixture of nickel and chromium borides of high hardness (up to 1861 HV). In order to evaluate the corrosion behavior, the two methods of corrosion tests were used: potentiodynamic corrosion test in 5% NaCl solution and immersion corrosion test in a boiling solution of H2O, H2SO4 and Fe2(SO4)3. The results showed that gas boriding could provide the excellent corrosion resistance if the whole surface of a Nimonic 80A-alloy sample was covered by the continuous boride layer. Otherwise, as a consequence of selective boriding, the significant difference in electrochemical potentials caused an accelerated uniform corrosion of the base material.

Abstract: In the present study, the AISI P20 steel was pack-borided in the temperature range of 1,123-1,223 K for treatments times between 2 and 8 h. A new kinetic model based on the integral method was used to estimate the boron diffusion coefficients in the Fe2B layers. As a result, the activation energy for boron diffusion in the AISI P20 steel was estimated as 194.3 kJ mol−1. This value of energy was compared with the literature results. Furthermore, to validate experimentally the present model, two additional boriding conditions were used. The morphology of cross-sections of borided samples was examined by scanning electron microscopy. The formation of a single boride layer was confirmed by x-ray diffraction analysis. The Daimler-Benz Rockwell-C indentation technique was used to evaluate the cohesion of boride layers on the surface of AISI P20 steel. Finally, the scratch and pin-on-disc tests for wear resistance were carried out by means of a LG Motion Ltd and a CSM tribometer respectively under dry sliding conditions.

Abstract: Kinetics of boride layer growth and tensile behaviour in borided commercial-purity nickel was investigated. Boriding was carried out in a solid medium consisting of Ekabor-II powders at 1173, 1223 and 1273 K for periods of 3, 5 and 8 h. Scanning electron microscopy (SEM) and optical microscopy showed column morphology in the boride layer. X-ray diffraction (XRD) analyses indicated that the boride layer formed on the surface consisted mainly of Ni2B, with precipitates of Ni6Si2B. A parabolic relationship between layer thickness and processing temperature was observed. The obtained results showed that although the boride layer thickness increased with increasing boriding temperature and time, boriding parameters had no significant effect on the hardness of the boride layer or the matrix. Tensile properties were negatively influenced by the boriding treatment; both yield and tensile strength values decreased due to the presence of the hard yet brittle surface coating. In addition, the growth kinetics of boride layers was also analysed. The results showed a nearly parabolic relationship between the layer thickness and the process temperature, with activation energy of 47.3 kJ mol.

Abstract: The possibility of application of boriding media based on boron carbide—which additionally contain chromium, titanium, and silicon—for the diffusion hardening of titanium alloys is considered. Boriding in amorphous boron is performed for comparison. The microstructure, elemental composition, and phase composition of diffusion coatings on the OT4 titanium alloy formed by saturation in powder media are investigated. Hardening boride layers are formed on the titanium alloy form saturating media based on amorphous boron and multicomponent mixtures based on boron carbide. In all cases, the phase composition of the coating corresponds to phases TiB, Ti2B5, and Fe2Ti. It is revealed that coatings from 30 to 150 μm thick are formed in conditions of the solid-phase saturation of titanium from powder mixtures due to the diffusion. Temperature-temporal conditions of formation of boride layers on OT4 titanium from powder saturating media are investigated and optimal modes for the formation of operable boride coatings are established. The optimal temperature range for processes of chemical-thermal boriding of titanium (900–1150°C) and saturation time (from 2.5 to 5 h) are determined. The maximal thickness of the operable boride coating on the OT4 titanium alloy is established, being from 180 μm in the case of saturation from Bamorph and up to 240 μm for the 50% B4C + 20% SiC + 25% CrB2 + 5% NaCl mixture at 950°C and saturation time of 4 h. Herewith, it should be noted that it was considered that the largest coating thickness is that retaining on the hardened sample surface.

Abstract: Austenitic stainless steels form the largest family of alloys in terms of number and applications. They are characterized by having good toughness, weldability, cold formability, and corrosion resistance in various situations. However, because they cannot harden by heat treatment, they exhibit low wear resistance. Suitable coatings can increase their wear resistance and expand their usability range. Boride coatings, with their high hardness and wear resistance are a good candidate for this purpose. In this work, samples of stainless steels, AISI 304 and UNS S31254, were subjected to liquid boriding at 950°C for 2 and 4 h. Optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Vickers hardness tests were performed, along with tests for micro-adhesive wear (fixed-ball type). The boriding treatment resulted in the formation of layers with high hardness, in the range of 1800 to 2000 HV, which is typical for boride layers. Micro-adhesive wear tests demonstrated the great increase in wear resistance obtained through this treatment.

Abstract: In the present work, the EN-GJL-250 gray cast iron was pack-borided in a powder mixture consisting of 5 % B4C, 5 % NaBF4, and 90 % SiC. The boriding treatment was carried out in the temperature range 900°C–1000°C during 2, 4, and 6 h. The morphology of the generated boride layers was observed by an optical microscope and a scanning electron microscope. The phases in the borided layer were identified by X-ray diffraction (XRD) analysis, and Vickers microhardness testing was used to determine the microhardness profiles along the boride layers. The growth kinetics of boride layers was studied and the mass gain was estimated for the given boriding conditions. The regression model was also used to predict the total boride-layer thickness. As a result, the boron activation energy for EN-GJL-250 gray cast iron was estimated as 134.21 kJ mol−1 and compared with the literature data.

Abstract: In this study, an alternative surface hardening method called as “CRTD-Bor” (Cathodic Reduction and Thermal Diffusion based Boriding) was introduced for the heavy-duty applications of medium carbon, chromium-silicon martensitic steel, also known commercially Silchrome 1. The influences of process parameters (e.g. electrolyte temperatures and both electrolysis and phase homogenization durations) on the chemistry, thickness and hardness of boride structures were investigated to yield a modified surface within the industrially desired compositions, namely single Fe 2 B or the layer containing max. 10% of FeB in vol. Furthermore, adhesion as well as thermal oxidation behaviors of borided substrates were examined. Cross sectional SEM investigations revealed that it was possible to grow 35 μm or 45 μm thick boride layers within the preferred compositions after 40 min and 55 min of CRTD-bor, respectively. Thin film XRD analyses confirmed single Fe 2 B formation with minor Cr 2 B peaks. The hardness of boride layers varied in the range of 1400 ± 200 HV. The grown boride layers exhibited the ideal adhesions to the steel matrix with either perfect HF1 or acceptable HF3 qualities according to their constitutions. CRTD-Bor boriding process improved the oxidation resistance of the steel at 650 °C remarkably by forming thin ~ 7 μm thick protective layer composing of mixed iron-, chromium - oxides and borates as well as boron oxide (B 2 O 3 ) which was firstly identified in the hexagonal crystalline structure after oxidizing borided steel samples at 450 °C and 650 °C.

Abstract: In this study, the structural characterization and boriding kinetics of the molybdenum borides formed on the surface of borided pure molybdenum (Mo) have been investigated Boronizing was carried out in solid medium with boron component forming Ekabor ® 2 (90% SiC, 5% KBF4, 5%B4C) powders at 1273 K, 1373 K for 2, 4, 6, 8 hours under a controlled atmosphere containing argon gas flow The boride layer was characterized by the scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), Energy dispersive spectroscopy (EDS) and Vickers microhardness tester X-ray diffraction analysis showed that the boride layers on molybdenum consisted of MoB and Mo2B phases However, the MoB phase was observed at certain boriding temperature and boriding times The thickness of boronized layers almost ranged from 12 to 425 μm with boriding time A parabolic relationship was observed between boride layer thickness and boriding time The growth rate constant and activation energy for the boride layer were calculated The hardness of borides compounds formed on the surface of molybdenum ranged from 925 to 1150 HV005, whereas the hardness of the untreated molybdenum sample was 258 HV005

Abstract: Austempering heat treatment of gray cast iron can significantly improve its mechanical properties. In addition, increasing its surface hardness by means of thermochemical treatments such as boriding can further extend its range of use. In this work, gray cast iron samples with the composition 3.6 % C - 2.1 % Si - 0.43 % Mn - 0.26 % Cr - bal Fe were subjected to austempering treatment using austenitizing at 900°C, with subsequent cooling using austempering salt baths at temperatures of 240°C, 300°C, and 360°C for 1, 2, 3, and 4 hours with subsequent cooling in air. Another set of samples was subjected to boro-austempering treatments, which consisted of boriding at 950°C for 2 to 4 hours in molten borax and followed by cooling in salt baths at temperatures of 240°C, 300°C, and 360°C for 1, 2, 3, and 4 hours, with subsequent air cooling. This approach avoids the need for further heating the work piece to perform the austempering treatment. Subsequently, the samples were characterized for micro-hardness and adhesive wear behavior. The austempering treatment significantly increased material performance, and the boro-austempering treatment further improved its properties.

Abstract: The purpose of the investigation was to examine the possibility of improving tool life by reducing the wear effect and improving the adhesion of a thin film through a compound configuration that consists in boriding and PVD deposition. Single layer coatings of TiN were deposited by PVD (cathodic arc) on quenched and tempered and on borided powder metallurgy (P/M) AISI M2 steel. Adhesion test was performed according VDI 3198. Microhardness measurements were performed on Vickers scale and the tribological behavior evaluated through dry sliding wear test, using a ball-on-disk apparatus. The wear tracks were analyzed through scanning electron microscopy (SEM) and confocal microscopy. After the wear test the samples were transversally cut, coating and substrate were investigated using scanning electron microscopy (SEM). The results showed a better adhesion of the coating for the borided sample comparing to the quenched and tempered sample. The wear mechanisms of quenched/tempered-TiN (Q/T-TiN) samples against Al2O3 ball were different from the wear mechanisms of borided-TiN (B-TiN) sample against Al2O3 ball.

Abstract: Boronizing/boriding is a thermo mechanical process which produced protective surface layers to enhance the performance of engineering components utilized in mechanical, wear and corrosion. The present study investigate the microstructure and the hardness of boride layers formed on 0.28% Vanadium and 0.87% Nickel alloyed ductile iron after boronizing process. Specimens were boronized at 950° C for 6, 8 and 10 hours holding time before being cooled in the furnace. The microstructure and boride layer formed on the surface of substrates were observed under Olympus BX60 Optical Microscope. Vickers Micro Hardness Tester was also performed to determine the hardness of boride layers. Boride layer was formed by diffusion of the boron into the metal lattice at the surface which composed double phase of FeB and Fe2B with saw-tooth morphology. The results of this study indicated that the thickness of boride layers increased from 109.8μm at 6 hours to 195.4μm at 8 hours holding time before they crack at 10 hours. The hardness of the material surface also increased from 1535 HV to 1623 HV at 6 and 8 hours respectively. In conclusion, the microstructure, borides thickness and hardness of borides layer were depending on boronizing time while temperature kept constant.

Abstract: The article provides a detailed analysis of formation features for surface phases in technical iron and Cr20-Ni80 alloy samples that undergo case-hardening at a temperature of 850°C for 2, 4 and 6 hours of saturation in two different environments: acetylene, and molten salt consisting of sodium tetraborate and amorphous boron. We carried out an X-ray phase analysis to determine the phase structure of surface material layers that formed as a result of the case-hardening process. We discovered that after carburising it was possible to detect Fe3C and Fe-α phases on the surface of technical iron samples, and after boriding we found FeB, Fe2B and Fe3B phases; we noted a lack of characteristic Fe-α and Fe-γ peaks on the X-ray diffraction pattern. We detected many different phases in the Cr20-Ni80 alloy after the same type of case-hardening. Titanium oxides appeared after case-hardening of titanium in air at 800°C. We provide data on surface structure of samples subjected to vacuum carburising: over a 2 to 6 hour interval, the layer thickness is a parabolic function of time. When carrying out electrolysis-free liquid boriding, increasing exposure time from 2 to 6 hours alters the thickness of the strengthened layer only slightly, so, when carrying out case-hardening, it is less efficient to increase saturation time in molten salt containing sodium tetraborate and amorphous boron.