Corrosion resistance properties of plasma nitrided Ti-6Al-4V alloy in nitric acid solutions

The corrosion resistance of plasma nitrided Ti-6Al-4V titanium alloy in nitric acid solutions was investigated using polarisation curves, cyclic voltammetry, XRD analysis and surface microhardness. For comparison, untreated alloy samples were tested under the same conditions. While the untreated alloy shows a passive behaviour, the compound layer obtained by glow-discharge nitriding treatment, which is composed of an outer TiN and an inner Ti₂N layer, shows a high electrochemical inertia; however, if the polarisation potential is higher than a threshold or the period of immersion in high oxidising acid solution is sufficiently long, then the compound surface layer will be attacked and removed with corrosion rate higher than in the case of the untreated Ti-6Al-4V alloy. The corrosion resistance of Ti₂N layer results higher in comparison with that of TiN layer.     

The effects of gas nitriding on fatigue behavior in titanium and titanium alloys

Fatigue behavior has been studied on gas-nitrided smooth specimens of commercial pure titanium, an alpha/beta Ti-6Al-4V alloy, and a beta Ti-15Mo-5Zr-3Al alloy under rotating bending, and the obtained results were compared with the fatigue behavior of annealed or untreated specimens. It was found that the role of the nitrided layer on fatigue behavior depended on the strength of the materials. Fatigue strength was increased by nitriding in pure titanium, while it was decreased in the Ti-6Al-4V and Ti-15Mo-5Zr-3Al alloys. Based on detailed observations of fatigue crack initiation, growth, and fracture surfaces, the improvement and the reduction in fatigue strength by nitriding in pure titanium and both alloys were primarily attributed to enhanced crack initiation resistance and to premature crack initiation of the nitrided layer, respectively.     

Titanium alloys after surface gas nitriding

Experimental studies using differential scanning calorimetry (DSC) for nitriding of four titanium-alloys near α Ti-8Al-1Mo-1V, near α Ti-6Al-2Sn-4Zr-2Mo, α + β Ti-6Al-4V and near β Ti-10V-2Fe-3Al at different temperatures and for different periods of time are presented. The X-ray diffraction (XRD) technique was used in order to study the phase transformations that occur during gas nitriding. As a result of the nitrogen interaction, a nitrided layer was formed that consists of titanium nitrides, followed by an interstitial solution of nitrogen in the hcp α titanium phase. The microstructural changes of these alloys in relation to the alloy composition and processing parameters were studied. It was found that the microstructure of alloys nitrided at temperatures below their β transus temperatures for various periods of time is uniform and homogeneous. With the increase of the temperature above their β transus temperatures the microstructure changes to irregular. Microindentation hardness testing using a Knoop indenter was conducted on the nitrided titanium alloys to analyse their hardness evolution in relation to the nitriding processing parameters and alloy composition. It was found that the microhardness increases with the increase of the temperature and time of nitriding. The surface morphology of the Ti-6Al-2Sn-4Zr-2Mo alloy in relation to the nitriding processing parameters was analysed.     

Ti-6Al-4V合金渗氮层的生物腐蚀行为

采用真空感应快速渗氮技术对Ti-6Al-4V合金进行表面渗氮,将渗氮前后的试样置于SBF溶液中探究其耐蚀性。利用维氏硬度计、光学显微镜、电化学工作站、光学3D表面轮廓仪、扫描电镜(SEM)和X射线衍射(XRD)等技术对渗氮层进行测量和观察。试验结果表明:820 ℃下对Ti-6Al-4V钛合金进行真空感应渗氮处理后,渗氮层厚度达20 μm,其氮化物层硬度得到明显提高。同时,经过感应渗氮处理后样品的表面粗糙度增加。电化学试验表明经感应渗氮后Ti-6Al-4V钛合金的耐蚀性明显增强。     

Fretting contact study of Ti-6Al-4 V/graphite couples in a dry shaft/bearing contact with thrust: Influence of plasma nitriding of the titanium alloy

Fretting wear or wear by small displacements is defined when two contacting surfaces (first-bodies) are subjected to small amplitude reciprocating motion of micron order. This phenomenon is observed in many mechanical assemblies and it can significantly reduce the contact mechanisms life.This paper reports on the influence of nitriding treatment of the titanium alloy on the fretting tribological behaviour of graphite/Ti-6Al-4 V couple in a dry shaft/bearing contact with thrust. Two contact geometries are investigated: cylinder-in-cylinder and flat/flat. These fretting contacts are subjected to low-amplitude oscillatory movements, with temperature reaching 270 °C.The nitriding treatment for the Ti-6Al-4 V was carried out with a gas mixture N₂/H₂ at moderate temperature (700 °C) for 12 h. In these conditions, the maximum surface hardness was improved by a factor three.In this study, the mechanism of transfer and wear of graphite against Ti-6Al-4 V nitrided or not, have been studied with a scanning electron microscope, an optical microscope and an interferential microscope.The morphological and profile analysis performed on rubbing surfaces showed various aspects of wear: prints, craters, transfer ... and allowed to explain the location, development and origin of the degradation.The friction couples have showed differences between the tests realised with the shafts with or without nitriding and especially at an elevated temperature.We discuss the experimental results and we suggest several possibilities in order to understand some specific tribological behaviour: impact of the third body, of the abrasion, etc.     

Improving tribological properties of Ti6Al4V alloy with duplex surface treatment

Ti-doped diamond like carbon films were deposited on both untreated and plasma nitrided Ti6Al4V alloy using Closed Field Unbalanced Magnetron Sputtering (CFUMBS) method and their tribological properties were evaluated by conducting sliding wear conditions. The influence of the nitrided layer on tribological behavior of Ti-DLC films was studied by means of XRD, SEM, scratch tester, microhardness tester and pin-on-disc tribotester. The microhardness results pointed out that the duplex treatment dramatically increased the surface hardness and reduced the plastic deformation of the alloy. Wear tests showed that Ti-DLC coatings on both untreated and nitrided surfaces caused a reduction in the coefficient of friction. The reason of the reduction in the coefficient of friction was found to be the formation of transfer film between the sliding surfaces. Wear rates demonstrated that wear resistance of duplex treated (Ti-DLC coating after nitriding) Ti6Al4V alloy was significantly improved.     

A study into effects of CO2 laser melting of nitrided Ti-6Al-4V alloy

Multiple treatment of engineering surfaces can provide improved surface properties that cannot be obtained by a single surface treatment. Consequently, this study investigates the effects of laser melting on the microstructures of plasma nitrided Ti-6Al-4V alloy. The study consists of two parts. In the first part, governing equations pertinent to the laser melting process are developed, and temperature variation across the melted zone is predicted. In the second, an experiment is conducted to nitride the surface of the alloy through plasma nitriding process and to melt the plasma nitrided and the untreated alloy surfaces with a CO₂ laser beam. The resulting metallurgical changes are examined using x-ray diffraction (XRD), bdenergy-dispersive spectrometry (EDS), and scanning electron microscopy (SEM) techniques. It is shown that three distinct nitride layers are formed in the vicinity of the alloy surface prior to the laser melting process, and that after the melting process nitrided species are depleted while cellular and dendritic structures are formed. In addition, the structure consisting of transformed β containing coarse and fine acicular α is observed in the melted regions.     

Improvement of corrosion and wear resistances of AISI 420 martensitic stainless steel using plasma nitriding at low temperature

The influence of low temperature plasma nitriding on the wear and corrosion resistance of AISI 420 martensitic stainless steel was investigated. Plasma nitriding experiments were carried out with DC-pulsed plasma in 25% N₂ + 75% H₂ atmosphere at 350 °C, 450 °C and 550 °C for 15 h. The composition, microstructure and hardness of the nitrided samples were examined. The wear resistances of plasma nitrided samples were determined with a ball-on-disc wear tester. The corrosion behaviors of plasma nitrided AISI420 stainless steel were evaluated using anodic polarization tests and salt fog spray tests in the simulated industrial environment.The results show that plasma nitriding produces a relatively thick nitrided layer consisting of a compound layer and an adjacent nitrogen diffusion layer on the AISI 420 stainless steel surface. Plasma nitriding not only increases the surface hardness but also improves the wear resistance of the martensitic stainless steel. Furthermore, the anti-wear property of the steel nitrided at 350 °C is much more excellent than that at 550 °C. In addition, the corrosion resistance of AISI420 martensitic stainless steel is considerably improved by 350 °C low temperature plasma nitriding. The improved corrosion resistance is considered to be related to the combined effect of the solid solution of Cr and the high chemical stable phases of ?-Fe₃N and α_N formed on the martensitic stainless steel surface during 350 °C low temperature plasma nitriding. However, plasma nitriding carried out at 450 °C or 550 °C reduces the corrosion resistance of samples, because of the formation of CrN and leading to the depletion of Cr in the solid solution phase of the nitrided layer.     

Triode plasma nitriding and PVD coating: A successful pre-treatment combination to improve the wear resistance of DLC coatings on Ti6Al4V alloy

Diamond-like carbon (DLC) coatings have found great applicability in the automotive industry because of their low friction coefficient and high wear resistance. Nevertheless, their tribological performance can be greatly reduced on soft substrates such as titanium alloys. The hard DLC coating cannot usually follow elastic and plastic deformation of the substrate without failing. In order to overcome this property mismatch between hard coating and soft substrate, triode plasma nitriding was applied as a pre-treatment to improve the mechanical properties of the Ti6Al4V alloy and further enhance the load support for the DLC coating. DLC and multilayered TiN/DLC, CrN/DLC CrAlN/DLC coatings were deposited onto “standard” and plasma nitrided Ti6Al4V substrates. Triode plasma nitriding increased the load-bearing capacity of the coating/substrate system, as higher critical adhesion loads were recorded for DLC coatings on plasma nitrided Ti6Al4V substrates. This treatment also reduced the wear rate of the DLC coating/substrate. Further load support and lower wear rates were achieved by using TiN, CrN and CrAlN as intermediate layers on plasma nitrided Ti6Al4V substrates.     

Corrosion behaviour of glow discharge nitrided titanium alloys

The aim of this work is the study of the corrosive behaviour of glow discharge nitrided Ti-6Al-4V alloy, using electrochemical techniques.Potentiodynamic polarisation and electrochemical impedance measurement show the excellent corrosive resistance of titanium alloy after thermochemical treatment, as well as different behaviour between the alloy before and after the nitriding treatment.Also, in very hostile environments, such as 5 wt.% HCl, where the titanium alloy is heavily corroded, the nitrided samples show good resistance. Nevertheless, 750 °C nitrided samples have worse behaviour than those treated at 900 °C. This is probably due to the lower corrosion resistance of the nitride ε present in the nitrided layer created at the lower temperature than the nitride δ, and the limited thickness of the modified layer obtained at this temperature.     

TC4钛合金表面沉积CrAlSiN涂层的组织与性能

为了提高TC4钛合金表面硬度和耐磨性能,通过等离子渗氮技术和多弧离子镀技术相结合的方法对TC4钛合金进行表面改性处理。通过扫描电镜、维氏显微硬度计、三维轮廓仪、高速往复摩擦磨损试验仪和电化学工作站,对比研究了TC4钛合金、渗氮层和CrAlSiN涂层的显微组织、硬度、耐磨性能和耐腐蚀性能。结果表明,经渗氮处理后,TC4合金表面渗氮层硬度提高了约2倍,在此基础上制备的CrAlSiN涂层的平均硬度高达3222 HV0.025,涂层表面存在少许大颗粒和凹坑;CrAlSiN涂层平均摩擦因数为0.22,磨损机理主要为粘着磨损,对磨副的材料粘着到涂层表面,而涂层几乎无磨损,耐磨性能显著提高。CrAlSiN涂层的自腐蚀电位为-0.542 V,比TC4钛合金基体的自腐蚀电位-0.747 V正移了0.205 V,表明在渗氮层基础上沉积CrAlSiN涂层显著提高了合金的耐电化学腐蚀性能。     

等离子渗氮与喷丸强化复合改进钛合金抗微动损伤性能

利用直流脉冲等离子电源装置对Ti6A14V钛合金表面渗氮处理,研究了渗氮层的相组成、硬度分布、韧度及摩擦学性能,采用喷丸形变强化(SP)对渗氮层进行后处理,以达到联合提高钛合金微动疲劳(FF)抗力的目的.研究结果表明:脉冲电源等离子技术可在钛合金表面获得由TiN、Ti2N、Ti2A1N等相组成的渗氮层,该改性层能够显著地提高钛合金常规磨损和微动磨损(FW)抗力,但降低了基材的FF抗力.渗氮层的减摩和抗磨性能与SP引入的表面残余压应力协同作用,使钛合金FF抗力超过了SP单独作用.提高渗氮层韧度对改善钛合金FF和FW性能均十分重要.     

喷丸预处理对Ti6Al4V合金低温等离子体氮化的影响

为改善钛合金表面耐磨性能,同时达到防止薄壁零部件变形和节约能源的目的,以Ti6Al4V钛合金为对象,研究了喷丸强化预处理对钛合金低温渗氮层及耐磨性的改善作用。结果表明,喷丸强化预处理能够有效促进钛合金表面低温离子渗氮过程,在500 ℃低温渗氮试验条件下,随着喷丸预处理强度的增大,钛合金渗氮效率逐步提高,渗氮层的表面硬度、承载能力和表观韧性逐步增加,使得渗氮层的耐磨性能逐步提高。当喷丸预处理强度增加到0.25 mmA时,Ti6Al4V钛合金渗氮层的表面硬度比单纯渗氮处理试样提高32.7%,磨损率降低42.3%,使钛合金基体的磨损率降低70.5%,较好地实现了喷丸预处理促进钛合金低温离子渗氮的目标。     

快速多重旋转碾压诱导Ti-6Al-4V表面纳米晶及性能研究

目的 使Ti-6Al-4V能更好地应用于海洋领域.方法 采用快速多重旋转碾压技术(FMRR)对Ti-6Al-4V表面进行冷变形处理,研究其力学性能.然后,对其进行低温等离子渗氮,渗氮温度为550℃,保温时间4 h.利用高分辨透射电子显微镜(HRTEM)、透射电子显微镜(TEM)、扫描电子显微镜(SEM)、X射线衍射仪(XRD)和电子显微硬度计等测试设备,对FMRR处理后的Ti-6Al-4V表层组织结构和性能以及等离子渗氮处理后表层组织结构进行表征.结果 经15、30、45 min的FMRR处理后,Ti-6Al-4V表层晶粒细化并获得了纳米结构,对应的平均晶粒尺寸为65～90、45～70和20～40 nm.此时,晶界明显增多,局部存在孪晶、小角度晶界和高密度位错等结构缺陷,电子衍射环连续.同时,Ti-6Al-4V表层结构未出现新相,晶粒细化导致衍射峰略有变宽,也提高了Ti-6Al-4V表面的显微硬度,显微硬度为325～453HV,比处理前提高了约41％.FMRR预处理的试样经过低温等离子渗氮后,基体表面的渗氮层主要包括白色化合物层和过渡层两个区域.结论 FMRR处理为后续低温渗氮提供了能量条件和结构条件,提高了渗氮的速度,最终的渗氮层厚度约为100μm.     

Effect of Microstructural Variation on Dry Sliding Wear Behavior of Ti-6Al-4V Alloy

The present article evaluates the influence of independent control factors such as microstructural variation, normal load, sliding velocity, and test duration on dry sliding wear behavior of Ti-6Al-4V alloy at room temperature using a statistical approach. Ti-6Al-4V alloy has been heat treated in a controlled manner in order to produce different microstructural features (i.e., lamellar, bimodal, and equiaxed). Lamellar microstructure is found to be harder than bimodal microstructure followed by equiaxed microstructure in Ti-6Al-4V alloy. Dry sliding wear tests have been carried out using a multiple Tribo tester following a well planned experimental schedule based on Taguchi’s L₉ orthogonal array design. Dry sliding wear behavior of Ti-6Al-4V alloy consisting of various microstructural features is related to their hardness values. Results indicated that lamellar microstructure has the lowest sliding wear resistance followed by bimodal and equiaxed microstructure. With the help of signal-to-noise ratios, optimal combination of control factors to minimize the dry sliding wear in Ti-6Al-4V alloy has been determined. Normal load is the most significant control factor influencing the dry sliding wear behavior of investigated Ti-6Al-4V alloy followed by sliding velocity, test duration, and microstructural variation. Normal load has greater static influence of 27.02%, sliding velocity has an influence of 18.07%, test duration has an influence of 12.71%, and microstructural variation has an influence of 10.55% on weight loss of Ti-6Al-4V alloy due to wear having R ² = 0.89. Two wear mechanisms have been identified: oxidative wear occurs at the lowest sliding velocity and delamination wear occurs at the highest sliding velocity. Optical microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, and Rockwell hardness measurements have been used to characterize the microstructures in order to correlate the results obtained.     

Tribology and cyclic oxidation behavior of plasma nitrided valve steel

In this study, the tribology and cyclic oxidation behavior of plasma nitrided DIN 1.4871 austenitic valve steel were investigated. For this purpose plasma nitriding treatments were carried out in nitrogen and hydrogen with ratio N₂/H₂: 1/3 at 10 Torr pressure. Nitriding cycles of 400, 450, 500 and 550 °C for 7 h were selected. To remove oxide layer and to enhance diffusion, an effective sputter cleaning procedure was applied in argon and hydrogen gases. The pin-on-disc sliding wear experiments were performed at a load of 6 N and sliding velocity of 0.1 m/s in normal atmosphere under dry condition. Cyclic oxidation tests used to evaluate the oxidation characteristics of the samples consisted of 50 cycles each 30 min at 750 °C. The structure and properties of the samples were examined by optical and scanning electron microscopy (SEM), microhardness measurements and X-ray diffraction. The results indicated plasma nitriding at all temperatures increased the wear resistance of valve steel when sliding against bearing steel. The 550 °C nitrided layer, with CrN, Fe₄N and Fe_2-3N on the surface, was most effective in improving wear resistance. In the case of cyclic oxidation, the results showed that oxidation resistance depends strongly on nitriding temperature. Nitriding at 450 °C produced a layer of predominantly “S” phase which was more effective in improving the oxidation resistance of valve steel.     

Effect of Gas Composition on Nitriding and Wear Behavior of Nitrided Titanium Alloy Ti-15V-3Cr-3Al-3Sn

Titanium alloy, Ti-15V-3Cr-3Al-3Sn was nitrided at different temperatures with low pressure plasma with 100% nitrogen, and nitrogen diluted with hydrogen and argon. The nitrided layers were characterized for hardness, structure, and composition. Nitrided samples show weight gain that depended on temperature and duration of nitriding. EDS results show that intake of nitrogen is significant at temperatures above 750 °C. Hydrogen dilution increases intake of nitrogen. Samples nitrided with hydrogen dilution have lower surface roughness and higher nitrogen concentration. Depth profiling by XPS shows the formation of nitride in the near-surface region and also that nitrogen concentration in the interior of the nitrided layers is higher at higher temperatures. Micro Raman shows that formation of nitride takes place at higher temperatures. XRD shows that the nitrided layers consist predominantly of alpha Ti and Ti₂N. This is reflected in the hardness increase and hardness profile in the nitrided samples. The low intake of nitrogen by the alloy is attributed to the low solubility of nitrogen in beta alloy and low diffusion coefficient of nitrogen. Reciprocating wear studies showed a lower coefficient of friction and lower wear loss for nitrided samples compared to that of substrate.     

Plasma nitriding behavior of Ti6Al4V orthopedic alloy

The influence of plasma nitriding on mechanical, corrosion and tribological properties of Ti6Al4V has been investigated using X-ray diffraction, microhardness tester, scanning electron microscopy, pin-on-disc tribotester, electrochemical polarization and impedance spectroscopy. Plasma nitriding treatment of Ti6Al4V has been performed in 25%Ar-75%N₂ gas mixture, for treatment times of 1-4 h at the temperatures of 650-750 °C. The corrosion tests were carried out in Ringer solution at 37 °C, and the wear tests were performed in dry sliding conditions. XRD analyses confirm the formation of δ-TiN and tetragonal ?-Ti₂N phases in the modified layer. It was observed that the surface hardness and wear resistance increase as the treatment time and temperature increase. The electrochemical impedance measurements indicate a decrease in double layer capacitance value and increase in charge transfer resistance for the nitrided specimens compared to the untreated ones.     

Plasma nitriding of plastic mold steel to increase wear- and corrosion properties

In this study, the wear- and corrosion resistance of the layers formed on the surface of a precipitation hardenable plastic mold steel (NAK55) by plasma nitriding were investigated. Plasma nitriding experiments were carried out at an industrial nitriding facility in an atmosphere of 25% N₂ + 75% H₂ at 475 °C, 500 °C, and 525 °C for 10 h. The microstructures of the nitrided layers were examined, and various phases present were determined by X-ray diffraction. Wear tests were carried out on a block-on-ring wear tester under unlubricated conditions. The corrosion behaviors were evaluated using anodic polarization tests in 3.5% NaCl solution.The findings had shown that plasma nitriding does not cause the core to soften by overaging. Nitriding and aging could be achieved simultaneously in the same treatment cycle. Plasma nitriding of NAK55 mold steel produced a nitrided layer consisted of a compound layer rich in ε-nitride and an adjacent nitrogen diffusion layer on the steel surface. Increasing the nitriding temperature could bring about increase in the thickness of the nitrided layer and the nitride volume fraction. Plasma nitriding improved not only surface hardness but also wear resistance. The anti-wear property of the steel was found to relate to the increase in the thickness of the diffusion layer. Corrosion study revealed that plasma nitriding significantly improved corrosion resistance in terms of corrosion potential and corrosion rate. Improvement in corrosion resistance was found to be directly related to the increase in the nitride volume fraction at the steel surface.     

Effect of PostNitride Annealing on Wear and Corrosion Behavior of Titanium Alloy Ti-6Al-4V

Journal of Materials Engineering and Performance - Titanium alloy, Ti-6Al-4V, was plasma nitrided using RF plasma with 100% N at 800 °C and annealed at 850 °C in vacuum....