Brittle Compound Layer Research Articles

High dislocation-density silver particle interlayer for high-quality YSZ and Al2O3 joints

An Ag particle interlayer was adopted to join YSZ to Al2O3 ceramics in air directly. In comparison with the traditional Ag-CuO braze used in our previous work, the Ag particle interlayer prevented the formation of the brittle compound (CuAl2O4) layer along the ceramic interface. Sintering effect of particulate Ag and high-density dislocations in the interlayer are the major contributors to recrystallization of Ag and elemental interdiffusion. High Resolution Transmission Electron Microscope images confirmed that atomic bonding was achieved between the interlayer and Al2O3. Joining parameters, including temperatures and pressures, were optimized, resulting in the maximum shear strength of the joint (∼80 MPa) obtained at 950 °C/30 min with the external pressure of 0.9 MPa, which is 78 % higher than that of the joint brazed by Ag-CuO.

Effect of surface characteristics on the axial fatigue properties and wear resistance of Ti-6Al-4V alloy nitrided by gas blow induction-heating

The effects of surface characteristics on the axial fatigue properties and wear resistance of the GBIH-nitrided titanium alloy were investigated to improve the fatigue strength and wear resistance of Ti-6Al-4V alloy rapidly nitrided by induction-heating in a nitrogen atmosphere with blowing nitrogen gas (GBIH-nitriding). The fatigue strength of the GBIH-nitrided Ti-6Al-4V alloy was lower than that of the mirror-polished specimen owing to the formation of a brittle nitrogen compound layer and grain coarsening. Fatigue tests using specimens prepared by combining GBIH-nitriding and particle collision treatments revealed that the formation of nitrogen diffusion layers at the outermost surface and the generation of compressive residual stress without forming surface asperities increased the fatigue strength of the GBIH-nitrided Ti-6Al-4V alloy. Moreover, the formation of a nitrided layer and suppression of surface asperities effectively improved the wear resistance of the Ti-6Al-4V alloy. Therefore, formation of a nitrogen diffusion layer and the introduction of compressive residual stress without grain coarsening, a compound layer, and surface asperities increased the fatigue strength and wear resistance of the GBIH-nitrided alloy.

Achieving balanced enhancement of mechanical performance and heat resistance in Cf/SiC-superalloy joints by regulating active element Ti in brazing filler

The application of Cf/SiC-superalloy joints in aerospace typically demands high strength and good heat resistance. However, the simultaneous improvement of the two properties is a pressing problem that needs to be addressed. This paper proposes the addition of diamond (C) powder to the Cu-Ni-Ti brazing filler to regulate the interface reaction between the filler and Cf/SiC composite and the initial liquefaction temperature of the interlayer, thereby achieving a balanced enhancement in both the mechanical performance and high-temperature resistance of the Cf/SiC-GH3044 joints. The results indicate that during the joining process, a brittle compound layer composed of TiC, Ti5Si3, and Ni16Ti6Si7 is formed at the Cf/SiC/interlayer interface. The thickness of the compound layer increases with elevated joining temperatures or prolonged holding times, which is detrimental to the strength of the joint. The added C particles react with the active element Ti to synthesize in situ some TiC reinforcing particles with the low thermal expansion coefficient in the interlayer. This not only suppresses the excessive growth of the brittle compound layer but also alleviates the residual thermal stress in the joint. Additionally, since Ti is a lowering-melting element in the Cu-Ni-Ti filler, the consumption of Ti by the C/Ti reaction raises the initial liquefaction temperature of the interlayer, thus improving the heat resistance of the joint. When the joining temperature and holding time were kept constant, the shear strength of the joint reached a maximum value as the C particle content increased. If the joining temperature was elevated or the holding time was extended, the C content associated with the peak strength also increased. By optimizing the C particle content, both the shear strength and heat resistance of the joint were improved. The maximum shear strength of the joint at room temperature and 900 °C were measured to be 245 MPa and 138 MPa, respectively. Moreover, the joint exhibited a maximum temperature capability of 1064 °C.

Titanium-modified plasma nitriding layer and enhanced properties for 42CrMo steel

In order to obtain a nitriding layer with thicker ductile diffusion layer and thinner brittle compound layer, plasma nitriding with titanium addition was carried out at 540 °C for 4 h for 42CrMo steel, the characteristic of titanium modified nitriding layer and the enhanced properties were systematically investigated by optical microscope (OM), scanning electron microscope (SEM), XRD, hardness tester and wear tester etc. The results show that titanium addition can effectively modify the characteristics of nitrided layer, and thus a plasma nitrided layer owning thicker ductile diffusion layer and thinner brittle compound layer and higher surface hardness can be obtained. Titanium addition can significantly decrease the compound layer and increase the effective hardening layer, and also greatly enhance the surface hardness, especially with the optimal amount of 0.8 g, the compound layer can be reduced from 19 μm to 10 μm, while the effective hardening layer can be increased from 225 μm to 370 μm. And the surface hardness can be enhanced from 703HV0.05 to 889HV0.05, more importantly, both wear resistance and toughness can be greatly promoted by the novel titanium modified plasma nitrided layer, attributed to the formation of dispersedly distributed TiN particles with very high hardness in the nitriding layer.

Influence of the compound layer on the rolling contact fatigue properties of nitrided medium-carbon steel at the elevated temperature

Influence of the compound layer (CL) on the rolling contact fatigue (RCF) properties of plasma nitrided steel was experimentally investigated at the elevated temperature. The results show that with the removal of its CL, the rated life (L10), the median life (L50) and characteristic life (L63.2) of the nitrided steel increased by 157.9%, 57.9%, and 54.0%, respectively. Spalling and delamination are two main RCF failure modes of the nitrided steel independent on the CL. Spalling was caused by the mutual collection of two ring-type cracks or a ring-type crack and the main crack perpendicular to the rolling direction, while delamination was associated with the convergence of the ring-type crack and the subsurface main crack which was induced by not only the maximum shear stress but also the vein-like structure. Early spalling of CL made more severe stress concentration, which accelerated the initiation and propagation of the ring-type crack and then significantly reduced the RCF life of the nitrided steel. Consequently, a brittle CL is suggested to be removed to improve the service life of the industrial parts made of the nitrided steel and operating under the alternating stress.

Microstructure and mechanical properties of a laser welded Ti–6Al–4V titanium alloy/FeCoNiCrMn high entropy alloy with a Cu filler layer

A high-power solid-state laser was used to weld the Ti–6Al–4V titanium alloy and the FeCoNiCrMn high-entropy alloy. By adding a pure Cu filler layer for the laser welding, a strong welded joint is obtained and the average tensile strength of the laser welded Ti–6Al–4V/FeCoNiCrMn joint exceeds 140 MPa. Composition and mechanical properties of phases in the laser welded Ti–6Al–4V/FeCoNiCrMn joint were investigated by the optical microscope, the electron probe microanalysis technique, the scanning electron microscope and the nanoindentation technique. The fusion zone is mainly composed of the Cu-rich and the FeCoNiCrMn-rich regions. The Cu-rich phases can disperse brittle intermetallic compounds such as Ti–Fe, to prevent the formation of a continuous brittle compound layer, thereby improving the plasticity of the welded joint and promoting the formation of the joint. The temperature distribution model in the fusion zone was established, and combined with the element distribution and phase composition analysis results in the fusion zone, microstructure mechanism for formation of the fusion zone was proposed.

The Controlled Compound Layer of Ni-Coated Nitrided Pure Iron.

In order not to sacrifice nitrided layer thickness and reduce brittle compound layer thickness, Ni-coated pretreatment was carried out with electrodeposition on a pure iron surface, followed by gas nitriding. The brittle compound layer thickness of duplex surface treated samples was reduced, and the nitrided layer thickness increased to 320 μm. The microhardness was 4 times harder, and the wear loss was reduced by 68% compared with the original material. The results indicate that Ni-coated pretreatment could effectively improve microhardness and wear resistance and realize the controlled microstructure of a brittle compound layer of pure iron without compromising nitrided layer thickness. Ni coating plays an important role in ammonia adsorption and decomposition, and in the transfer of active nitrogen atoms during nitriding.

Microstructural changes and mechanical properties of AerMet100 steel surface-treated by plasma nitriding

In this work, quenched AerMet100 steel is plasma nitrided at 460 °C for 8 h in a gas mixture of 50% N2 + 50% H2. The microstructures and mechanical properties are investigated. Results show that the current nitriding conditions can effectively inhibit the formation of brittle white compound layer in the surface of AerMet100 steel. Further investigations on orientations of martensite laths and prior austenite confirm that all 24 variants of martensite laths produced by one prior austenite grain follow the KS orientation relationship and this relationship holds during nitriding. Besides, a memory phenomenon that the orientation of precipitated γ′-Fe4N is exactly the same as that of prior austenite is observed, due to the strong variant selection. Compared with the un-nitrided specimen, the microhardness of AerMet100 steel after nitriding treatment is remarkably improved, and volume wear rate is reduced by an order of magnitude. Such a large improvement is primarily attributed to a desirable combination of high surface hardness, considerable nano-grains and secondary hardening of the substrate.

A novel titanium enhanced plasma nitriding for 42CrMo steel

Titanium enhanced plasma nitriding was primarily developed for 42CrMo steel, excellent characteristics of nitriding layer with thinner brittle compound layer, thicker effective hardening layer and higher hardness was obtained by the novel titanium enhanced plasma nitriding. Under the same nitriding parameters of 540 °C × 4 h, the thickness of effective hardening layer was increased from 225 μm to 380 μm; the thickness of brittle compound layer was decreased from 19 μm to 10 μm. More importantly, the surface hardness was significantly increased from 703HV0.05 to 895HV0.05 due to the formation of TiN nitrides. The research can provide new insights into plasma nitriding technology.

The effect of plasma nitriding treatment time on the tribological properties of the AISI H13 tool steel

In a conventional plasma nitriding mechanism and actual processing, the sample is primarily treated with nitrogen ions. Hence, nitriding is not mainly performed using neutral nitrogen species, such as nitrogen atoms. In this study, a novel method to control nitrogen ions was implemented, and nitriding of the AISI H13 tool steel was performed in electron-beam-excited plasma using neutral nitrogen species and nitrogen ions. The effect of various treatment times on the formation of a hard but brittle compound layer were investigated, along with subsequent tribological change to the surface of the material. The results show that tool steel nitrided by neutral nitrogen species for 6 h produced a diffusion-based nitrided layer (up to 60 μm) without altering the pretreatment surface finish (Ra = 14 nm), and there was a specific wear rate of less than one hundredth (4.3 × 10−7 mm3/N·m) compared with the untreated sample (2.9 × 10−5 mm3/N·m). Conversely, nitriding by ions produced a rough, brittle compound layer on the surface of the tool steel. The samples with a compound layer exhibited a lower coefficient of friction than those with no compound layer.

Modification of Low-Alloy Steel Surface by High-Temperature Gas Nitriding Plus Tempering

The low-alloy steel was nitrided in a pure NH3 gas atmosphere at 640 ~ 660 °C for 2 h, i.e., high-temperature gas nitriding (HTGN), followed by tempering at 225 °C, which can produce a high property surface coating without brittle compound (white) layer. The steel was also plasma nitriding for comparison. The composition, microstructure and microhardness of the nitrided and tempered specimens were examined, and their tribological behavior investigated. The results showed that the as-gas-nitrided layer consisted of a white layer composed of FeN0.095 phase (nitrided austenite) and a diffusional zone underneath the white layer. After tempering, the white layer was decomposed to a nano-sized (α-Fe + γ′-Fe4N + retained austenite) bainitic microstructure with a high hardness of 1150HV/25 g. Wear test results showed that the wear resistance and wear coefficient yielded by the complex HTGN plus tempering were considerably higher and lower, respectively, than those produced by the conventional plasma nitriding.

Plasma nitriding without formation of compound layer for 38CrMoAl hydraulic plunger

In this study, low temperature plasma nitriding (LPN) was primarily used in 38CrMoAl hydraulic plunger to overcome the peeling problem of the compound layer in real application. The cross-sectional microstructures of the treated samples were observed by optical metallography, phase constituents were determined by X-ray diffraction (XRD), the microhardness profile and brittleness of surface were measured by microhardness tester. The results showed that no compound layer was formed while plasma nitriding at 450 °Cfor 6 h, and XRD also confirmed that there existed no γ′-Fe4N. Meanwhile, the brittleness was decreased at lower temperature plasma nitriding comparing with normal plasma nitriding (NPN), due to no brittle compound layer formed on the outmost layer.

DLC Duplex Coating on High-Speed Tool-Steel Substrates Using Plasma Nitrocarburizing After Radical Nitriding

Abstract Diamond-like carbon (DLC) is applied to a wide variety of fields in the industry because of its tribological properties and hardness. However, one of the major issues is the adhesion strength between DLC films and the metal substrate. Many efforts have been made to obtain a higher adhesion strength, such as the formation of an interlayer and the modification of the surface layer. In contrast, radical nitriding is one of the most suitable methods for the pretreatment of duplex coatings, because it makes it possible to increase the surface hardness while maintaining the roughness of the original surface. According to this process, a diffusion layer is formed in the surface region using state plasma control. In the present investigation, we used existing hot-wall plasma nitriding equipment to treat high-speed tool steel using both plasma nitrocarburizing and radical nitriding to form a DLC film. Our results indicated that radical nitriding formed a nitrogen-diffusion layer, followed by the formation of a carbon layer by nitrocarburizing without the formation of a brittle compound layer. It maintained surface roughness compared with the untreated substrate. Although the hardness of the steel did not improve, the resulting DLC films exhibited better adhesion to the treated-steel substrates than to the untreated ones.

Formation of nitrided layer using atmospheric-controlled IH-FPP and its effect on the fatigue properties of Ti-6Al-4V alloy under four-point bending

The purpose of this study is to develop a rapid nitriding using a surface modification which is a combination of atmospheric-controlled induction-heating fine particle peening (AIH-FPP) and nitrogen gas blow. AIH-FPP was performed for titanium alloy (Ti-6Al-4V ELI grade) at 1173 K for 180 s in a controlled nitrogen atmosphere with an oxygen concentration less than 10 ppm without supplying particles. A nitrogen compound (TiN) was formed on the AIH-FPP treated surface, which results in increasing the surface hardness of Ti-6Al-4V alloy. To examine the effect of AIH-FPP on the fatigue properties of Ti-6Al-4V alloy, four-point bending fatigue tests were performed at a stress ratio of 0.1 in air at room temperature. It was found that the AIH-FPP treated specimens showed low fatigue limit due to the formation of a brittle compound layer and coarse microstructure.

TRIBOLOGICAL BEHAVIOR OF 42CrMo4 STEEL NITRIDED BY PLASMA

This paper presents wear behavior of the plasma nitrided 42CrMo4 steel. This steel is used in mechanical industry; it has been assessed by evaluating tribological properties and surface hardness by using a pin-on-disk wear machine and microhardness tester. Experimental results showed that the nitrides ε-Fe2-3N and γ’-Fe4N present in the compound layer increase the microhardness. It is found that plasma nitriding improves the wear rate and the presence of a hard and brittle compound layer on the surface causes an increase in wear of the specimen surface.

Ti-6Al-4V 合金の機能性および強度に及ぼす複合処理の効果

This study was conducted to comprehensively investigate the effect of combination treatment on the wear resistance, corrosion resistance, mechanical properties and fatigue strength of Ti-6Al-4V alloy. The combination treatment was composed of plasma nitriding, short-time heat treatment and fine-particle bombarding (hereafter, FPB). Plasma nitriding was performed at 1123 K. The obtained results were compared with those of the previous study concerning the combination treatment in which plasma nitriding was performed at 1023 K. A thick hardened layer (thickness: 200 μm) was formed by plasma nitriding. The substrate was strengthened by short-time heat treatment. The surface brittle compound layer was eliminated by FPB, and high compressive residual stress was introduced at the same time. The combination treatment improved wear resistance without the deterioration of corrosion resistance. Moreover, the combination treatment improved tensile strength and fatigue strength although it markedly reduced ductility. The comparison with the results obtained in the previous study showed that the combination treatment in which plasma nitriding was performed at 1023 K was more effective to improve the functionalities and strength of Ti-6Al-4V alloy.

Effect of combination treatment on wear resistance and strength of Ti–6Al–4V alloy

Abstract This study was conducted to comprehensively investigate the effect of combination treatment on the wear resistance, mechanical properties, and fatigue strength of Ti–6Al–4V alloy. The combination treatment was composed of plasma nitriding, short-time duplex heat treatment (hereafter, duplex heat treatment), and fine-particle bombarding (FPB). A hardened layer was formed by plasma nitriding. The substrate was strengthened by duplex heat treatment. The brittle compound layer, formed by plasma nitriding, was eliminated by FPB, and also high compressive residual stress was introduced. Wear resistance was improved by combination treatment; however, hybrid surface treatment, composed of plasma nitriding and FPB, was more effective to improve wear resistance than combination treatment. The tensile strength was improved 30% by combination treatment. Fatigue crack initiation from the surface was strongly suppressed by the formed layer and introduced high compressive residual stress. Moreover, subsurface crack initiation was suppressed by the strengthened substrate. As a result, fatigue strength was greatly improved, by 59%, by the combination treatment.

Microstructure and wear properties of AISI M2 tool steel on RF plasma nitriding at different N2–H2 gas compositions

Wear behavior of quenched-tempered AISI M2 tool steel samples has been studied after plasma nitriding at different N2–H2 plasma gas flows containing 25, 50 and 75sccm N2. Plasma nitriding was performed at 450°C for 8h under floating potential using a plasma reactor equipped with a radio frequency power generator. Microstructure, phase composition, nitrided layer thickness, hardness and surface roughness of the samples were studied using optical microscopy, X-ray diffraction, microhardness and surface profilometry measurements. Dry sliding wear resistance of samples was determined by performing ball-on-disc wear testes. The results revealed formation of mainly a diffusion zone at the 25sccm N2–75sccm H2 gas flow and mono-phase ε-Fe2–3N compound layer at higher N2 concentrations. Plasma nitriding increases near surface hardness up to 50% (about 1600HV0.025) irrespective of the N2:H2 ratio, where nitrided layer depth and surface roughness increase with increasing the N2 flow rate in the plasma gas. Depending on the nitrogen content, sliding wear resistance may be improved between 20 and 90% with respect to the un-nitrided substrate. Among the nitrided samples the maximum and minimum wear resistance was obtained at plasma gases containing higher and lower H2 fractions, respectively. Decreasing wear resistance with increasing N2 flow rate in the plasma gas attributed to formation of the hard and brittle compound (white) layer on the sample surface and development of residual stress profiles.

Improvement of Fatigue Strength of Ti–6Al–4V Alloy by Hybrid Surface Treatment Composed of Plasma Nitriding and Fine-Particle Bombarding

This study was conducted to improve the fatigue strength of Ti­6Al­4V alloy by hybrid surface treatment composed of plasma nitriding and fine-particle bombarding (FPB). Plasma nitriding to form a hardened layer was performed at relatively low temperature to suppress the growth of the microstructure of the substrate as far as possible. Subsequent FPB eliminated the outermost brittle compound layer leading to a reduction in fatigue strength, and also introduced compressive residual stress at the same time. Hybrid surface treatment formed a hardened layer that can improve wear resistance without marked deterioration of the mechanical properties. Since the formed layer possessed high compressive residual stress and relatively high hardness, initiation of fatigue cracks from the surface was strongly suppressed and the crack initiation site was located in the substrate under the formed layer. As a result, the fatigue strength improved by 47% and the fatigue ratio (fatigue strength/tensile strength) reached a very high level of 91%. [doi:10.2320/matertrans.M2013228]

Effect of Hybrid Surface Treatment Composed of Plasma Nitriding and Fine Particle Bombarding on Fatigue Strength of Ti–6Al–4V Alloy

This study investigated the effect of hybrid surface treatment on the fatigue strength of Ti­6Al­4Valloy. The hybrid surface treatment was composed of plasma nitriding and fine particle bombarding (FPB). Plasma nitriding generated a hardened layer with no marked growth of the microstructure of the substrate, and subsequent FPB eliminated a compound layer formed outermost through plasma nitriding. Tensile strength was maintained at the initial level, although ductility was somewhat reduced by fracture of the hardened layer. Plasma nitriding decreased the fatigue strength because the brittle compound layer fractured at relatively low stress amplitude and hastened the propagation of cracks. On the other hand, the effect of the hardened layer appeared through the elimination of the compound layer, so that hybrid surface treatment greatly improved the fatigue strength. Correspondingly, a crack initiation site was found beneath the hardened layer and suggested that the formed layer strongly suppressed crack initiation from the surface. [doi:10.2320/matertrans.M2012262]