Surface Compound Layer Research Articles

Effects of rare earth on nitriding microstructure and properties of a La-Ce micro-alloyed Q235RE steel

Accelerating nitriding is an interesting topic in chemical heat treatment of ferrous parts. In order to have a deeper understanding of the catalysis principles of rare earth in nitriding, a Q235 carbon steel and a Q235RE steel micro-alloyed with rare earth elements La and Ce were nitrided respectively by plasma nitriding and gas nitriding. The microstructure, phase constitution, and composition of the nitrided specimens were analyzed. The hardness profile of nitriding layer and the brittleness of surface compound layer were measured. The results demonstrate that the Q235RE specimens obtained a higher hardness and deeper effective hardening layer than the Q235 specimens without rare earth microalloying. The rare earth catalysis effects were presented as the acceleration of nitrogen diffusion and the inhibition of nitride precipitation in the diffusion layer of the Q235RE specimens. In addition, the surface compound layer of the Q235RE specimens showed higher toughness and better adhesion to the matrix than that of the Q235 specimens, which is considered to be strongly related to the rare earth elements diffused into the compound and the enhanced residual compressive stress in the layer.

On the role of chemisorption in the formation of the target surface compound layer during reactive magnetron sputtering

Modeling of the reactive sputter processes requires a description of the target processes which results in the formation of a compound layer on the target. The sputter removal of the formed compound is balanced by compound formation mechanisms. Chemisorption of reactive gas atoms and molecules was initially identified as one of these processes. Further improvement of these chemisorption based models was achieved by the inclusion of processes such direct reactive ion implantation, knock-on implantation of chemisorbed species, and the redeposition of sputtered material on the target. In a recent study, the importance of ion implantation was questioned and an alternative model proposed. The results of this model are confronted with existing literature to investigate its validity.

Tribocorrosion Behavior of γ′-Fe4N Nitride Layer Formed on Mild Steel by Plasma Nitriding in Chloride-Containing Solution

Nitriding has long been used to engineer the surfaces of engineering steels to improve their surface and subsurface properties. The role of the surface compound layer (γ′-Fe4N and/or ε-Fe2-3N) in improving the tribological and corrosion-resistant properties of nitrided steels has been established. However, there have been very few studies on the response of the compound layer to tribocorrosion in corrosive environments. In this work, the tribocorrosion behavior of a 5 μm thick γ′-Fe4N nitride layer produced on mild steel (MS) by plasma nitriding has been studied in a NaCl-containing solution under various electrochemical conditions. The results show that at a cathodic potential of −700 mV (saturated calomel electrode, SCE), where mechanical wear is predominant, the total material removal (TMR) from the γ′-Fe4N layer is 37% smaller than that from the untreated MS, and at open circuit potential, TMR from the layer is 34% smaller than that from the untreated MS, while at an anodic potential of −200 (SCE), the γ′-Fe4N layer can reduce TMR from mild steel by 87%. The beneficial effect of the γ′-Fe4N nitride layer in improving the tribocorrosion behavior of mild steel is derived from its high hardness and good corrosion resistance in the test solution and its ability to resist both mechanical wear and corrosion and to reduce wear–corrosion synergism.

Martensitic Induction Hardening of Nitrided Layers*

Abstract In this research a combination of nitriding and induction hardening is investigated, as this is expected not only to result in significant savings in process time and energy, but also to produce surface layer properties that cannot be set with one of the individual processes. The focus of the current investigations was on the dissolution of the compound layer during inductive heating and the resulting microstructure formation and the hardness profile. Furthermore, it was investigated how the absence of a compound layer affects the subsequent martensitic transformation. For this purpose, differently nitrided surface layers were martensitically hardened and the microstructure was investigated metallographically and physically. After the martensitic transformation of the nitrided layer porosity and retained austenite were observed due to the decomposition of the nitrides of the compound layer. The retained austenite could be reduced by higher temperatures during surface hardening and compound layer removal. The investigations showed, that the optimum initial condition for induction hardening is nitriding with compound layer and a mechanical removal of the latter prior to induction heat treatment.

Improvement on pitting wear resistance of gears by controlled forging and plasma nitriding

In the present investigation, the use of a bainitic steel for forged gears applications and the suitability of different plasma nitriding treatments is discussed. The gears were forged, machined, ground, polished, and nitrided at 500 °C with three sets of N2–H2 gas mixtures, containing 5, 24, and 76 vol.% N2, to develop a nitrided case of approximately 300 μm depth. Gears were characterized before and after the nitriding concerning the phase composition, microstructure, microhardness, fracture toughness, and residual stresses states. Pitting wear tests were performed on a standard Forschungsstelle für Zahnräder und Getriebebau (FZG) test rig. The nitrided gears with 24 and 76 vol.% N2 formed a biphasic compound layer of ε-Fe2-3(C)N and γ′-Fe4N. As the volume fraction of nitrogen in the gas mixture was decreased, the detected content of γ′-Fe4N in the compound layer increased, but a monophasic compound layer was only reached with 5 vol.% N2. The nitrogen rich gas composition increased the surface hardness and decreased the fracture toughness of the compound layer, as they have more ε-Fe2-3(C)N. The diffusion zone of the different nitrided surfaces showed residual compressive stresses. The best performance was obtained in the nitrided gears with 24 vol.% N2, due to the better combination between the surface hardness, fracture toughness, residual stresses, and compound layer thickness. The nitrided gears with 24 vol.% N2 have ten times improvement over the non-nitrided gears, while the nitrided gears with 5 and 76 vol.% N2 have an improvement of three point seven and five point four times.

Surface degradation of nitrided hot work tool steels under repeated impact-sliding contacts: Effect of compound layer

Wear behaviour of quenched and tempered (QT) hot work tool steels (Uddeholm QRO90) was investigated against SAE 52100 grade bearing steel balls after gas nitriding using a dedicated laboratory scale impact-sliding wear test rig. Gas nitriding was employed in a fluidised bed reactor under two alternative regimes: i. “High Temperature Nitriding (HTN)” carried out at 510 °C and ii. “Low Temperature Nitriding (LTN)” carried out at ≤ 400 °C. The HTN process resulted in the formation of ∼2 μm thick external compound layer, whereas the LTN processed steels were free of any surface compound layer formation. After the impact-sliding wear tests employed at room temperature (RT), the prevailing wear mechanisms of the examined steels were assessed as tribo-oxidation and fatigue wear. The testing at 600 °C induced different wear mechanisms for the HTN and the LTN steels. While tribo-oxidation and fatigue wear were preserved for the HTN steel, plastic deformation dominated the wear that progressed on the compound layer free surface of the LTN steel. Impact-sliding wear testing at 600 °C showed that the wear rate of LTN > HTN steels, as opposed to the wear rate at RT where wear rate of HTN > LTN.

The Influences of Non-Volatile Surface Compound Layer on the Dry Etching of Borosilicate Glass

In the dry etching of borosilicate glass (BSG), the process gas reacts with the impurities of borosilicate glass and forms a non-volatile compound layer on the surface. This surface compound layer (SCL) makes it difficult to control the surface morphology and device structure dimension with precision and significantly influences the etch rate. We examined the influence of SCL on the etch rate of quartz and BSG and discussed its etching mechanism depending on the thickness of the SCL. The formation of SCL was dominated by both the plasma gas composition and substrate material, and the etching regime can be defined by the thickness of the SCL. A thin SCL was formed on the BSG by CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /Ar gas plasma etching, and the etch rate was suppressed by the SCL. A significantly thin SCL was formed on the quartz specimens under in same etching conditions, and a direct etching reaction was occurred. As the ratio of physical etching gas (Ar) increased, both the ion energy flux and the physical ion bombardment increased and consequently improved the etch rate. The SCL plays the role of an F radical supplier, while the thick SCL reduces the etch rate because it takes time for the etchants to transit from the surface to the interface. The SCL acts as a barrier in the etching reaction, which could be adjusted by the regulation of the etching gas composition or the plasma parameters, such as the inductive power and the bias power of the RIE reactor. [2021-0071]

On the kinetics of nitride and diffusion layer growth in niobium plasma nitriding

This work reports results of a study on niobium plasma nitriding kinetics, conducted at relatively low temperatures, regarding the N-Nb system (1523 K) eutectoid temperature, and short times (on the 1253–1453 K and 1–4 h ranges). Results confirm that the niobium plasma nitriding is diffusion-controlled. The obtained nitride (compound) layer is multiphase, mainly constituted of ε-NbN, γ-Nb4N3 and β-Nb2N nitrides. The estimated compound and diffusion layer growth activation energy was 97.11 and 120.11 kJ mol−1, respectively. Considering the hardness distribution all over the nitrided layer, the transition between the compound and diffusion layers is very sharp. As example, it can vary from 18.5 GPa (⁓1850 HV) at the surface compound layer to decreasing values from ⁓480 to 100 HV, along the diffusion layer, depending on the studied condition. All results obtained from the Sa and Sz roughness characterization indicated increasing measured values for longer treatment times and higher temperatures, for atom clusters islands-like surface morphology. Finally, apparently crack-free compound layers were yielded, highlighting the importance of the use of relatively low treatment temperatures, below the N-Nb system (1523 K) eutectoid temperature, and relatively short treatment times for the plasma nitriding treatment studied here.

Effects of Surface Compound Layer on Bending Fatigue Strength of Nitrided Chromium-Molybdenum Steel

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;Carburized and quenched materials with high fatigue strength are often used for motorcycle engine parts. Nitrided materials exhibit less deformation during heat treatment than carburized and quenched materials, so if the same or higher fatigue strength can be achieved with nitrided materials as with carburized and quenched materials, the geometric precision of parts can be increased and we can reduce engine noise as well as power loss. When the fatigue strengths of a nitrided material with its compound layer surface put into γ’ phase through nitriding potential control (hereafter, G), and a nitrided material put into ε phase (hereafter, E) were measured, the results showed the fatigue strength of the G to be about 11% higher than that of carburized and quenched materials. It was inferred that the strength of the compound layer determines fatigue strength. The reason the fatigue strength of the G is higher is that initial cracks do not readily form, and it can be inferred that when cracks do form, they progress readily and lead to final fracture. In the case of the E, it is thought that when the stress intensity factor, ΔK, due to initial cracks exceeds the threshold of the stress intensity factor range, ΔK&lt;sub&gt;th&lt;/sub&gt; (&lt;span class="formula inline"&gt;&lt;math id="M1" display="inline"&gt;&lt;mo&gt;≃&lt;/mo&gt;&lt;mn&gt;5.9&lt;/mn&gt;&lt;mi mathvariant="normal"&gt;M&lt;/mi&gt;&lt;mi mathvariant="normal"&gt;P&lt;/mi&gt;&lt;mi mathvariant="normal"&gt;a&lt;/mi&gt;&lt;msqrt&gt;&lt;mi&gt;m&lt;/mi&gt;&lt;/msqrt&gt;&lt;/math&gt;&lt;/span&gt;), it leads to fatigue fractures. While the G has higher fatigue strength than carburized and quenched materials, it is likely to have a big effect on microcrack fatigue strength. This is a factor we believe requires consideration when designing engine parts for strength and in planning part manufacturing.&lt;/div&gt;&lt;/div&gt;

Surface engineering of titanium by multi-interstitial diffusion using plasma processing

Titanium and its alloys possess a range of highly interesting properties such as excellent corrosion resistance, high specific strength and biocompatibility, but suffers from poor wear resistance. The present work addresses plasma assisted surface treatment of CP 2 titanium using various combinations of oxygen and nitrogen, i.e. mixed interstitials. The sequence of controlled plasma nitriding and oxidizing treatments plays a significant role for the evolution of the hardness depth profiles and the development of the surface compound layer and the underlying diffusion/transition zone. Composition profiles of oxygen and nitrogen are obtained by GDOES; Mixed interstitial solubility of nitrogen and oxygen is found in both h.c.p. α titanium and in the compound layer. The combination of interstitials leads to larger case depth, in particular for the diffusion zone (expanded h.c.p. α titanium). Therefore, it highlights the advantages of combined nitriding and oxidizing compared to single nitriding treatments on the mechanical properties.

Gaseous surface hardening of Ti-6Al-4V fabricated by selective laser melting

The present work investigates the response of different gaseous thermochemical treatments on selective laser melted (SLM) Ti-6Al-4V. The resulting microstructures after thermochemical treatment were investigated with X-ray diffraction, light optical microscopy, scanning electron microscopy and Vickers-microhardness indentation. Nitriding, performed at 1000–1050 °C resulted in a diffusion zone of nitrogen in solid solution and surface compound layers consisting of TiN (and Ti2N at 1000 °C). Below the compound layer Al-enrichment of the α-zone was observed. Carbo-oxidising in a CO atmosphere at 1000–1050 °C resulted in deep diffusion zones and thick compound layers of the ternary compound TiC1-xOx. Both surface hardness and layer depth were found to increase with temperature and treatment time. Chemically controlled carbo-oxidising, applying the gas redox system CO-CO2, was performed at temperatures in the range 850–1050 °C, resulting in carbo-oxides and formation of oxides with increasing Ti:O ratio with increasing temperatures (rutile and Magnéli phases). Nitriding followed by (carbo-)oxidising treatment resulted in higher surface hardness owing to the formation of mixed interstitial compounds TiC1-x-yNxOy in the compound layer. The compound layer grew into the Al-rich zone as elongated structures.The improvement of wear by nitriding, carbo-oxidising and duplex nitriding/(carbo-)oxidising on SLM Ti-6Al-4V was evaluated by dry sliding wear testing. Lowering of the wear volume by up to a factor of 450 compared to an annealed reference sample was realised. Carbo-oxidising in CO at 1000 °C offered the best wear resistance and resulted in a lowering of the friction coefficient, averaging μ = 0.22, compared to μ = 0.45 for an annealed reference sample.

Ultrasonic bonding of 2024 Al alloy using Ni-foam/Sn composite solder at ambient temperature

Ultrasonic bonding of 2024 Al alloy was successfully performed at ambient temperature by using Ni-foam reinforced Sn-based composite solder foils. Ni-foam sheets with porosities of 60% and 80% were used to strengthen pure Sn solder. The Ni skeletons in the solder seam changed from three-dimensional continuous network structure into strip type with increasing ultrasonic bonding time, accompanied with the formation of Ni3Sn4 phase on Ni skeleton surface and Al3Ni intermetallic compound (IMC) layer on Al substrate surface. While, further prolonging the ultrasonic bonding time could largely consume Sn solder and break the Ni skeletons. The Sn-based solder added with 80% porosity Ni-foam exhibited better soldering performance than that added with 60% porosity Ni-foam. The Al/Ni80–Sn/Al joint ultrasonically bonded for 4 s exhibited the highest shear strength of 52.34 MPa, the shearing failure mainly happened in the composite solder layers.

Evaluation of growth behaviour of vanadium carbides-reinforced iron-based surface compound layer by in-situ reaction

The growth of vanadium carbide reinforced iron substrate surface compound layers are investigated, where the compound coatings were fabricated by in situ synthesis process at 950 °C, 1000 °C and 1050 °C in 1–5 h, respectively. A detailed characterization about the microstructural change of compound layer is given by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The obtained vanadium carbide compound layers are composed of V2C and V8C7. V2C and V8C7 have different microstructures and crystal orientations. The thickness of the vanadium carbide layer ranges from 16.62 ± 0.16 μm to 81.47 ± 0.63 μm in different process conditions. Growth kinetics of the layer follows the parabolic rules and the calculated activation energy value is Q=415.7KJ/mol. Both the heating treatment temperature and the holding time constantly influence on the microstructure and growth behaviour of the vanadium carbide compund layer.

Phase composition and stress state in the surface layers of burnished and gas nitrided Sverker 21 and Vanadis 6 tool steels

The effects of plastic deformation, by slide diamond burnishing as a pre-nitriding treatment, of the surface layers of a conventional, Sverker 21, and a powder metallurgy, Vanadis 6, tool steel were examined by metallography and X-ray diffraction for phase and internal stress analyses. Two sequential processes: turning-nitriding and turning-burnishing-nitriding were compared in the affected nitrided regions: surface (compound layer) and sub-surface (diffusion zone). Compared to nitriding only, prior burnishing leads to an increase in the thickness of the affected region, containing Fe3N, Fe4N, CrN nitrides, by 60% from ~36 μm for Sverker 21 and by more than 300% from ~3 μm for Vanadis 6 steels. A homogenization of the stress state to isotropic compression was found in the surface layers of both prior burnished tool steels.

Microstructural characterization of a V2C and V8C7 ceramic-reinforced Fe substrate surface compound layer by EBSD and TEM

Microstructural characterization of a vanadium carbide (V2C and V8C7) surface compound layer was performed by electron backscatter diffraction (EBSD), scanning electron microscopy and transmission electron microscopy. The phase composition and phase distribution of the compound layer and the morphologies, grain sizes and crystallographic orientations of the vanadium carbides were investigated through elemental distribution and crystallographic analyses. The results indicated that the compound layer was composed of four distinct zones consisting of a compact and uniform V2C layer (thickness of 3.2 μm) over the topmost surface followed by a nonuniform V + V2C (2.4 μm) zone, a V8C7 layer (10.7 μm) and a relatively deep V2C + V8C7+α-Fe + Fe3C zone (23.2 μm). The V2C layer was found in the V-rich region, and the EBSD results revealed that this phase grew mainly along the {0001} and {11–20} orientations. However, the V8C7 phase appeared to be nearly random, which indicates that the growth of this phase had no specific orientation. The V2C and V8C7 phases exhibited low-angle grain boundaries, and their misorientation angle distributions were mainly concentrated within the range of 0–5°. Furthermore, nanoindentation tests demonstrated that the hardnesses of the V2C and V8C7 layers were 22.16 ± 0.52 and 32.42 ± 0.61 GPa, respectively. Examination of the vanadium carbide powders obtained by chemical extraction indicated that the V2C grains with a hexagonal structure exhibited equiaxial and short rod-like morphologies, and the V8C7 particles with a cubic structure possessed short rod-like and spherical morphologies.

Effect of Crystal Structure of Surface Compound Layer on Fatigue Strength of Nitrided SCM 435 Steel

Effect of Crystal Structure of Surface Compound Layer on Fatigue Strength of Nitrided SCM 435 Steel

Effect of Phase in Surface Compound Layer on Rotating Bending Fatigue Strength for Gas-nitrided SCM435 Steel

Effect of Phase in Surface Compound Layer on Rotating Bending Fatigue Strength for Gas-nitrided SCM435 Steel

Mechanical Properties and Tribological Behavior of In Situ NbC/Fe Surface Composites

The mechanical properties and tribological behavior of the niobium carbide (NbC)-reinforced gray cast iron surface composites prepared by in situ synthesis have been investigated. Composites are comprised of a thin compound layer and followed by a deep diffusion zone on the surface of gray cast iron. The graded distributions of the hardness and elastic modulus along the depth direction of the cross section of composites form in the ranges of 6.5-20.1 and 159.3-411.2 GPa, respectively. Meanwhile, dry wear tests for composites were implemented on pin-on-disk equipment at sliding speed of 14.7 × 10−2 m/s and under 5 or 20 N, respectively. The result indicates that tribological performances of composites are considerably dependent on the volume fraction and the grain size of the NbC as well as the mechanical properties of the matrices in different areas. The surface compound layer presents the lowest coefficient of friction and wear rate, and exhibits the highest wear resistance, in comparison with diffusion zone and substrate. Furthermore, the worn morphologies observed reveal the dominant wear mechanism is abrasive wear feature in compound layer and diffusion zone.

Fabrication of TiB&lt;sub&gt;2&lt;/sub&gt;/TiC Duplex Particles Reinforced Carbon Steel Matrix Surface Composite

A process that combines a vacuum expandable pattern casting V-EPC with self-propagation high-temperature synthesis SHS of TiB2/TiC particles for the fabrication of the TiB2+TiC duplex particulates reinforced carbon steel matrix surface composite are introduced in this paper. The effect of ferrotitanium addition in the SHS reactants on the macro and micro structures as well as hardness is investigated. It has been found that with the increase of ferrotitanium addition, the concentration of synthesized particles in the layer gradually increases to maximum, then decreases. The hardness gradually decreases from the surface compound layer to core and the maximum hardness of 1682HV is obtained on the composite surface.

The Effect of Different Tungsten-Containing Materials Addition on the Reinforcements in the Surface Compound Layer with Steel Substrate

The effect of different tungsten-bearing materials addition in a surface preform, such as Ni-base WC particles, ferrotungsten powders and casting WC particles, on the reinforcement phases at the surface compound layer with a carbon steel substrate is investigated under the condition of vacuum expandable pattern casting V-EPC. The microstructures and reinforcement phases are characterized by optical microscopy, SEM and EDS. Experimental results show that it is impossible to synthesize the independent WC particles in each condition. The tungsten-containing materials are all inclined to decompose during steel infiltration and the released tungsten elements tend to combine with carbon to form fish born-like or strip-like WC or W2C carbides and dissolve in other type of carbides and matrix.