Search Results (22)

In sealing, sliding, and power transmission operations, surface quality and contact tolerances have high impacts on material system efficiency. Although the boriding process improves the wear resistance of metallic surfaces, it increases surface roughness, affecting the tribological efficiency of material systems. This study presents the tribological results of AISI 4140 boriding surfaces tested using a dehydrated paste pack boriding method with and without a finishing polish process to reduce the roughness. The duration of the boriding process was 1 h at 1123, 1173, 1223, and 1273 K using boron paste obtained from a commercial source and using a pot-polishing process with Al₂O₃ with a particle size of 0.5 μm for 25 min. The samples with and without the finishing polish process were structurally characterized using X-ray diffraction, and the boride coating adhesion was determined using Rockwell C indentation. The tribological properties of the boride surface with and without the finishing polish process were determined using a reciprocating sliding test, with a ZrO₂ ball as a counter body. The boride surfaces’ crystalline structure changed with polishing, which revealed the FeB phase and reduced the roughness value. These modifications in the surface characteristics altered the adhesion and tribological performance of the coating, resulting in a more stable tribological performance on the polished boride surfaces, with a reduction in the coefficient of friction (Cof) value from 0.75 ± 0.02 for the tribological test on the 1123 K-P sample to 0.59 ± 0.002 for the 1273 K-P sample surface at 20 N of applied load. Full article

The influence of alternating current (AC) field on the pack boriding process for medium carbon steel was investigated through characterization of microstructure, phase composition, microhardness, and corrosion resistance of the boride layer and its mechanism was revealed. Results showed that the boride layer obtained by AC field boriding was composed of the outer FeB and the inner Fe₂B phase, which was similar to that of conventional boriding. Meanwhile, the effective thickness of the boride layer and proportion of Fe₂B increased gradually with increasing current during AC field boriding. The introduction of an AC field during the boriding process served dual purposes. First, it facilitated the decomposition of the boriding medium, leading to an elevation in the concentration of active boron atoms. Second, it reduced the activation energy required for atomic diffusion, thereby accelerating the diffusion of both boron and iron atoms. These combined effects significantly enhanced the hardness distribution and corrosion resistance of the steel. Further insights into the process were gained by fitting the parabolic kinetics curves, which confirmed that the boriding process in an AC field was exclusively controlled by diffusion. This study also clarified the growth mechanism of the boride layer within an AC field. Full article

This study presents new results on the practical adhesion behavior of a boride layer formed on Monel 400 alloy, developed using the powder-pack boriding (PPBP) at 1223 K for 2, 4, and 6 h of exposure times, obtaining layer thicknesses from approximately 7.9 to 23.8 µm. The nickel boride layers were characterized using optical microscopy, Berkovich nanoindentation, X-ray diffraction (XRD), and scanning electron microscopy (SEM) to determine microstructure, hardness distribution, and failure mechanisms over the worn tracks. Scratch tests were conducted on the borided Monel 400 alloy according to the ASTM C-1624 standard, applying a progressively increasing normal load from 1 to 85 N using a Rockwell-C diamond indenter, revealing that critical loads (L_C1, L_C2, and L_C3) increased with layer thickness. The tests monitored the coefficient of friction and residual stress in real time. Critical loads were determined based on the correlation between the normal force and visual inspection of the worn surface, identifying cracks (cohesive failure) or detachment (adhesive failure). The results exposed those cohesive failures that appeared as Hertzian cracks, while adhesive failures were chipping and delamination, with critical loads reaching up to 49.0 N for the 6 h borided samples. Also, the results indicated that critical loads increased with greater layer thickness. The boride layer hardness was approximately 12 ± 0.3 GPa, ~4.0 times greater than the substrate, and Young’s modulus reached 268 ± 15 GPa. These findings underscore that PPBP significantly enhances surface mechanical properties, demonstrating the potential for applications demanding high wear resistance and strong layer adhesion. Full article

Pack boriding with CeO₂ was performed on the powder metallurgical (PM) near-α type titanium alloy at a temperature of 1273–1373 K for 5–15 h followed by air cooling. The microstructure analysis showed that the boride layer on the surface of the alloy was mainly composed of a monolithic TiB₂ outer layer, inner whisker TiB and sub-micron sized flake-like TiB layer. The growth kinetics of the TiB₂ and TiB layers obeyed the parabolic diffusion model. The diffusion coefficient of boron in the boride layers obtained in the present study was well within the ranges reported in the literature. The activation energies of boron in the TiB₂ and TiB layers during the pack boriding were estimated to be 166.4 kJ/mol and 122.8 kJ/mol, respectively. Friction tests showed that alloys borided at moderate temperatures and times had lower friction coefficients, which may have been due to the fine grain strengthening effect of TiB whiskers. The alloy borided at 1273 K for 10 h had a minimum friction coefficient of 0.73. Full article

DIN 16MnCr5 is commonly used in mechanical engineering contact applications such as gears, joint parts, shafts, gear wheels, camshafts, bolts, pins, and cardan joints, among others. This study examined the microstructural and mechanical properties and tribological behavior of different surface treatments applied to DIN 16MnCr5 steel. The samples were hardened at 870 °C for 15 min and then quenched in water. The surface conditions evaluated were as follows: quenched and tempered DIN 16MnCr5 steel samples without surface treatments (control group), quenched and tempered DIN 16MnCr5 steel samples with gas-nitriding at 560 °C for 6 h, quenched and tempered DIN 16MnCr5 steel samples with pack boriding at 950 °C for 4 h, and quenched and tempered DIN 16MnCr5 steel samples with duplex gas-nitriding and pack boriding. Microstructure characterization was carried out using metallographic techniques, optical microscopy, scanning electron microscopy with energy-dispersive spectroscopy, and X-ray diffraction. The mechanical properties were assessed through microhardness and elastic modulus tests using nanoindentation. The tribological behavior was evaluated using pin-on-disc tests following the ASTM G99-17 standard procedure under dry sliding conditions. The results indicated that the surface treated with duplex gas-nitriding and pack boriding exhibited the highest wear resistance and a reduced coefficient of friction due to improved mechanical properties, leading to increased hardness and elastic modulus. Full article

One of the primary challenges in the automotive industry is the wear of engine components, such as the crankshaft and camshaft, which is the most pronounced during the engine’s startup phase, when the amount of lubricant fluid is at its lowest. This study aims to enhance the surface wear resistance of automotive crankshaft steel by applying a boriding thermochemical process. This process forms a hard surface layer on the steel, improving its mechanical properties and bolstering its wear resistance, especially under dry conditions. Boride layers were achieved using the powder-pack boriding process in a conventional furnace, with meticulous treatment times of 2, 4, and 6 h at a constant temperature of 950 °C. The nature of the layers was analyzed using X-ray diffraction, and their tribological behavior was evaluated using the pin-on-disk test. The growth of the layers was directly proportional to the treatment time and was estimated at 145 µm and 48 µm for the 6 and 2 h of treatment, respectively. The surface hardness increased from 320 HV for the non-treated steel to 2034 HV for the sample exposed to 950 °C for 6 h. The results indicate a significant reduction in the coefficient of friction from 0.43 for the non-treated steel to 0.12 for the samples exposed to 950 °C for 6 h, suggesting potential wear protection during the engine starting period. Full article

A nano-dual-phase powder with ultra-fine grain size was synthesized by the liquid precursor method at 1200 °C. A series of single-phase high-entropy ceramic powders ((Ti, Zr, Hf, Nb)B₂, (Ti, Zr, Hf, Nb, Ta)B₂, (Ti, Zr, Hf, Nb, Mo)B₂, (Ti, Zr, Hf, Nb, Ta, Mo)B₂) with high purity (C content less than 0.9 wt% and O content less than 0.7 wt%) and ultrafine (average grain sizes of 340–570 nm) were successfully synthesized at 1800 °C. The sample of (TiZrHfNbTa)B₂ exhibited a hexagonal close-packed (HCP) structure, and the metal elements were uniformly distributed at the nanoscale, microscale, and macroscale. This method did not apply to the preparation of all high-entropy ceramic powders and was unfavorable for the formation of single-phase high-entropy borides when the size difference factor exceeded 3.9%. The present work provides a guide for the development of ceramic-based composites through precursor impregnation pyrolysis. Full article

AISI 5120 steel, which underwent 30 min single-surface nanocrystallization via supersonic fine particle bombarding (SFPB), was pre-boronized at 600 °C for 2 h, and then final-boronized at 800 °C or 900 °C for 2–6 h, i.e., two-step pack-boronizing. The specimens’ microstructure and mechanical characteristics before and after two-step boronizing were examined in detail. The results showed that the hardness of the SFPB surface reached 570 HV, which was 2.73 times higher than its original hardness (209 HV). The nanocrystallized surface exhibited the increased thickness and hardness of the boride layer, in comparison to that of the un-SFPB surface. Two-step pack-boronizing further improved the thickness and hardness of the SFPB surface of AISI 5120 steel. When final-boronizing at 900 °C for 6 h, the thickness and hardness of the boronizing layer in the SFPB surface was 88 µm and 2196 HV, respectively, which was 10.5 times higher than the original hardness. Additionally, the CeO₂ added in the boronizing agent was helpful in obtaining the boride layer with the ductile-serrated Fe₂B phase rather than the brittle phase of FeB in the boride layer, which was expected for industrial applications. Full article

In the present work, two mathematical diffusion models have been used to estimate the growth of the iron monoboride and diiron boride coating formed on AISI 420 steel. The boronizing of the steel was carried out with the solid diffusion packing method at a boronizing temperature of 1123 K–1273 K. Experimental results show the two-coating system consists of an outer monoboride and an inner diiron boride coating with a predominantly planar structure at the propagation front. The depth of the boride coating increases according to temperature and treatment time. A parabolic curve characterizes the propagation of the boride coatings. The two proposed mathematical models of mass transfer diffusion are founded on the solution corresponding to Fick’s second fundamental law. The first is based on a linear boron concentration–penetration profile without time dependence, and the second model with time dependence (exact solution). For both models, the theoretical law of parabolic propagation and the average flux of boron atoms (Fick’s first fundamental law) at the growth interfaces (monoboride/diiron boride and diiron boride/substrate) are considered to estimate the propagation of the boride coatings (monoboride and diiron boride). To validate the mathematical models, a programming code is written in the MATLAB program (adaptation 7.5) designed to simulate the growth of the boride coatings (monoboride and diiron boride). The following parameters are used as input data for this computer code: (the layer thicknesses of the FeB and Fe₂B phases, the operating temperature, the boronizing time, initial formation time of the boride coating, the surface boron concentration limits, FeB/Fe₂B and Fe₂B/Fe growth interfaces, and the mass transfer diffusion coefficient of boron in the iron monoboride and diiron boride phases). The outputs of the computer code are the constants ${\epsilon }_{FeB}$ and ${\epsilon }_{F{e}_{2}B}$. The assessment of activation energies of AISI 420 steel for the two mathematical models of mass transfer is coincident (${\mathrm{Q}}_{FeB}=$221.9 kJ∙mol⁻¹ and ${\mathrm{Q}}_{F{e}_{2}B}=$209.1 kJ∙mol⁻¹). A numerical analysis was performed using a standard Taylor series for clarification of the proximity between the two models. SEM micrographs exhibited a strong propensity toward a flat-fronted composition at expansion interfaces of the iron monoboride and diiron boride coating, confirmed by XRD analysis. Tribological characterizations included the Vickers hardness test method, pin-on-disc, and Daimler–Benz Rockwell-C indentation adhesion tests. After thorough analysis, the energies were compared to the existing literature to validate our experiment. We found that our models and experimental results agreed. The diffusion models we utilized were crucial in gaining a deeper understanding of the boronizing behavior of AISI 420 steel, and they also allowed us to predict the thicknesses of the iron monoboride and diiron boride coating. These models provide helpful approaches for predicting the behavior of these steels. Full article

This work attempts to model the powder-pack boronizing kinetics of 4Cr5MoSiV1 steel in the interval of 1133 and 1253 K in order to predict the layers’ thicknesses. The first approach is referred to as the bilayer model and relies on the conservation principle of mass balance equations at the two phase fronts accounting for the linearity of boron distribution across each boride phase. The second approach deals with the application of dimensional analysis to simulate the boronizing kinetics of 4Cr5MoSiV1 steel. Using the bilayer model and the classical parabolic law, the boron activation energies in FeB and Fe₂B were evaluated and discussed in light of the literature data. The estimated boron activation energies from the bilayer model were respectively equal to 164.92 and 153.39 kJ mol⁻¹. These values were very comparable to those calculated from the classical parabolic law. Finally, it was proven that the dimensional analysis was able to simulate the layers’ thicknesses for the selected processing parameters. Full article

The influence of prior pack boriding on the microstructure and properties of nanobainitised X37CrMoV5-1 hot-work tool steel was investigated in the present work. Pack boriding was conducted at 950 °C for 4 h. Nanobainitising consisted of two-step isothermal quenching at 320 °C for 1 h, followed by annealing at 260 °C for 18 h. A combination of boriding with nanobainitising constituted a new hybrid treatment. The obtained material exhibited a hard borided layer (up to 1822 ± 226 HV0.05) and a strong (rupture strength 1233 ± 41 MPa) nanobainitic core. However, the presence of a borided layer decreased mechanical properties under tensile and impact load conditions (total elongation decreased by 95% and impact toughness by 92%). Compared with borided and conventionally quenched and tempered steel, the hybrid–treated material retained higher plasticity (total elongation higher by 80%) and higher impact toughness (higher by 21%). It was found that the boriding led to the redistribution of carbon and silicon atoms between the borided layer and substrate, which could influence bainitic transformation in the transition zone. Furthermore, the thermal cycle in the boriding process also influenced the phase transformations during subsequent nanobainitising. Full article

The CoCrFeNiMn high-entropy alloys were treated by powder-pack boriding to improve their surface hardness and wear resistance. The variation of boriding layer thickness with time and temperature was studied. Then, the frequency factor D₀ and diffusion activation energy Q of element B in HEA are calculated to be 9.15 × 10⁻⁵ m²/s and 206.93 kJ/mol, respectively. The diffusion behavior of elements in the boronizing process was investigated and shows that the boride layer forms with the metal atoms diffusing outward and the diffusion layer forms with the B atoms diffusing inward by the Pt-labeling method. In addition, the surface microhardness of CoCrFeNiMn HEA was significantly improved to 23.8 ± 1.4 Gpa, and the friction coefficient was reduced from 0.86 to 0.48~0.61. Full article

The effect of a new hybrid heat treatment consisting of pack-boriding and nanobainitising on the microstructure and properties of EN 66SiMnCrMo6-6-4 bearing steel was investigated. The hybrid treatment produces a new high-strength (ca. 1480 MPa) material with a hard boride (ca. 2000 HV0.05) surface layer and a relatively ductile nanobainitic core. The formation of the boride layer significantly improves wear resistance. The boride layer, which is hard but susceptible to cracking, reduces the mechanical properties under tensile and impact loads. However, the borided and nanobainitised steel exhibits much higher tensile strength and ductility and slightly better impact toughness than steel after post-boriding quenching and tempering. Full article

(1) Background: Boriding is one of the most common methods of thermal-chemical treatment due to its excellent hardness and wear resistance of the produced diffusion layers. However, it has limited application compared to carburizing and nitriding because of fragility and chipping. Introducing another alloying element into the boron media helps avoid those drawbacks and improve other surface properties of the layer. The purpose of this work is to improve the surface mechanical properties of L6 and 5140 low alloy steels by two-component surface hardening with boron and copper. (2) Methods: The treatment was performed by means of a powder-pack method using boron, copper, and aluminum powders in the following proportions: 60% B₄C + 20% Al₂O₃ + 16% CuO + 4% NaF. The time–temperature parameters of the treatment were four hours exposure at 950 °C. Microstructure, elemental, and phase composition were investigated as well as microhardness and wear resistance of the obtained layers. (3) Results: Layers of up to 180–200 μm thick are formed on both steels as a result of treatment. Needle-like structures similar to pure boriding was obtained. The maximum microhardness was 2000 HV on L6 steel and 1800 HV on 5140 steel. These values correspond to iron borides and were confirmed by XRD analysis revealing FeB, Fe₂B, and Cr₅B₃. The wear resistance of both steels was about ten times higher after the treatment compared to non-treated samples. (4) Conclusions: Surface hardening with boron and copper significantly improves the mechanical properties of both alloy steels. The results obtained are beneficial for different tribo-pair systems or three-body wear with abrasion and minimum impact loads. Full article

The powder-pack boriding technique with an open retort was used to form borided layers on X165CrV12 tool steel. The process was carried out at 1123, 1173, and 1223 K for 3, 6, and 9 h. As a result of boriding the high-chromium substrate, the produced layers consisted of three zones: an outer FeB layer, an inner Fe₂B layer, and a transition zone, below which the substrate material was present. Depending on the applied parameters of boriding, the total thickness of the borided layers ranged from 12.45 to 78.76 µm. The increased temperature, as well as longer duration, was accompanied by an increase in the thickness of the FeB zone and the total layer thickness. The integral diffusion model was utilized to kinetically describe the time evolution of the thickness of the FeB and (FeB + Fe₂B) layers grown on the surface of powder-pack borided X165CrV12 steel. The activation energy of boron for the FeB phase was lower than that for the Fe₂B phase. This suggested that the FeB phase could be formed before the Fe₂B phase appeared in the microstructure. The high chromium concentration in X165CrV12 steel led to the formation of chromium borides in the borided layer, which increased the hardness (21.88 ± 1.35 GPa for FeB zone, 17.45 ± 1.20 GPa for Fe₂B zone) and Young’s modulus (386.27 ± 27.04 GPa for FeB zone, 339.75 ± 17.44 GPa for Fe₂B zone). The presence of the transition zone resulted from the accumulation of chromium and carbon atoms at the interface between the tips of Fe₂B needles and the substrate material. The presence of hard iron and chromium borides provided significant improvement in the wear resistance of X165CrV12 steel. The powder-pack borided steel was characterized by a four times lower mass wear intensity factor and nine times lower ratio of mass loss to the length or wear path compared to the non-borided material. Full article