Long-term oxidation kinetics of aluminide coatings on alloy steels by low temperature pack cementation process

The long-term oxidation kinetics of the P92 steel and iron aluminide diffusion coating formed on its surface by the pack cementation process have been investigated at 650 °C over a period of more than 7000 h both in 100% steam and in air under normal one atmospheric pressure by intermittent weight measurement at room temperature. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques were used to analyse the oxidised surfaces. For the P92 steel substrate, the scale formed by oxidation is largely magnetite (Fe₃O₄) in steam and haematite (Fe₂O₃) in air. Despite this difference in the type of oxide scales formed, it was found that the long-term oxidation kinetics of the P92 steel substrate in both steam and air can be described by a logarithmic time relationship: Δm _t = k _lln(t/t° + 1); the constants k _l and t° were subsequently determined using a closest fit process for oxidations in steam and air. For the coating, the oxide scale formed in both steam and air was Al₂O₃, which provided the long-term oxidation resistance. It was observed that the long-term oxidation kinetics of the coating in both steam and air can be best described by Δm _t = Δm ₀ + k _c t ^1/3; the rate constant k _c of oxidation in steam and air was then determined by the least squares method. For both the P92 steel substrate and coating, the rate of oxidation is faster in steam than in air at 650 °C particularly in the case of the P92 steel substrate.     

Low temperature aluminisation of alloy steels by pack cementation process

Abstract

The pack aluminisation process is normally applied at temperatures >973 K at which the mechanical properties of alloy steels would degrade. Thus, the present study was undertaken to apply this process to aluminising the alloy steels at temperatures <973 K in order to increase their high temperature oxidation resistance while maintaining their microstructure and hence mechanical strength and creep resistance. A type of commercial alloy steel P92 (9Cr–1Mo) was used for the present study. Pack powder mixtures consisting of Al, AlCl₃ (anhydrous) or NH₄Cl and Al₂O₃ were used to carry out the process. The aluminising temperature was varied from 773 to 973 K, pack Al content from 1 to 30 wt-% and aluminising time from 1 to 16 h to investigate their effects on the coating growth kinetics in the AlCl₃ activated packs. It was observed that all the coatings formed in the AlCl₃ activated packs were of a single layer structure with Fe₂Al₅ as the main coating phase. It was established that the interrelationship between the thickness h (in μm) of this coating layer and aluminising temperature T (in K), time t (in h) and pack Al content W (in wt-%) can be described by h=83005·9W^1/2t^1/2e^?73330/(RT). In the NH₄Cl activated packs, it was found that coating formation and dissolution took place simultaneously at 923 K and stable growth of a coating layer was only possible when the pack Al content was sufficiently high. However, the coatings formed in these packs had highly uneven regions.     

活化剂对渗铬层结构和成分的影响

采用粉末包埋法,分别以氯化铵、氯化钠、氯化镍为活化剂在镍基高温合金上于950℃进行了渗铬处理。扫描电镜和能谱分析结果表明,以氯化铵或氯化钠为活化剂得到的渗铬层以向外生长模式为主,但前者在与基体界面处夹杂氮/氧化物颗粒较多,同时含有高达约11.4%(质量分数)的Fe杂质,而后者在与基体界面处夹杂较少,渗铬层Fe含量仅为约1.4%(质量分数),远低于前者。以氯化镍为活化剂得到的渗铬层则以向内扩散为主,与基体无明显界面,但Ni及杂质Fe(19.7%(质量分数))含量最高。氯化钠表现出净化渗铬层成分的作用,而氯化镍具有增强与基体互扩散的作用,因此,活化剂种类对渗铬层成分及结构具有重要影响。     

Preparation of CNFs surface to coat with copper by electroless process

In the present work, the effects of oxidative pretreatments of carbon nanofibers in different mineral acids were studied. It has been found that the pretreatment of carbon nanofibers was very important to obtain the different grain size copper nanoparticles on the CNF. This material was pretreated with three different oxidative solutions, in order to increase their specific surface: HNO₃, KMnO₄ + HNO₃ and K₂Cr₂O₇ + H₂SO₄. The highest oxidative coarsening conditions imply the highest active surface, increasing possible copper nucleation points. Sensitized solution has been NiCl₂ instead of SnCl₂, which was used in conventional electroless process. In all cases, electroless process was carried out at room temperature and pH of the plating solution was maintained at the value of 12.5.     

Oxidation behavior of aluminide coatings on carbon steel with and without electrodeposited Ni-CeO2 film by low-temperature pack cementation

A CeO₂-dispersed aluminide coating was fabricated through aluminizing the electrodeposited Ni-CeO₂ nanocomposite film on carbon steel using pack cementation method at 700 °C for 4 h. The isothermal and cyclic oxidation behavior of the CeO₂-dispersed aluminide coating at 900 °C, including the growth of oxide scale and the microstructure of the coatings, have been investigated comparing with the aluminide coating on carbon steel. The results show enhanced oxidation performance of the CeO₂-dispersed aluminide coating, which is concerned with not only CeO₂ effect on the microstructure and oxidation, but also decreased interdiffusion between the aluminide and the Ni film. The CeO₂ benefit effects and interdiffusion are discussed in detail.     

包埋法制备TiC内涂层的影响因素分析

用包埋法制备了碳/碳复合材料TiC内涂层.对制备涂层的影响因素进行了分析.最终确定的制备条件为制备温度2173K、保温时间为2h、Ti和C的配比为3:1.     

Deposition of silicon modified aluminide coatings on nickel base superalloys by pack cementation process

Abstract

The equilibrium partial pressures of vapour species generated in halide activated pack powder mixtures at high temperatures were calculated for a series of compositions using thermochemical analysis tools. The results obtained were applied to identify suitable activators and pack compositions for codepositing Al and Si to form diffusion coatings on nickel base superalloys by the pack cementation process. The calculation results suggested that compositions of the packpowder mixtures activated by CrCl₃.6H₂O may be adjusted to create deposition conditions favourable for codepositing Al and Si, but, those activated byAlF₃or AlCl₃ may only deposit Al.A series of coating deposition experiments were also carried out at 1000 ° C and 1100 ° C and the results obtained confirmed that, with adequate control of pack compositions and deposition conditions, codeposition of Al and Si can be achieved with CrCl₃.6H₂O activated pack powder mixtures. A mixture of elemental Al and Si powders may be used as a depositing source instead of using Al-Si master alloy powders as conventionally recommended. The coatings could be formed either through the inward diffusions of Al and Si or through the outward diffusion of Ni together with other substrate elements such as Cr and Co, depending on the deposition temperature used. Prolonged deposition at 1100 ° C ledtothe formationofa coatingwith amultilayeredstructure consistingofanouter nickelsilicide layerand a middle Simodified NiAl layer followed by a diffusion zone. The pack compositions and deposition conditions may be adjusted to control the microstructure of the coatings formed by the codeposition process.     

Microstructure and tribological behaviour of super-hard Ti-Si-C-N nanocomposite coatings deposited by plasma enhanced chemical vapour deposition

In this study, a series of new quaternary Ti-Si-C-N nanocomposite coatings have been deposited on HSS substrate at 550 °C using an industrial set-up of pulsed direct circuit plasma enhanced chemical vapour deposition (PECVD) equipment with a gas mixture of TiCl₄/SiCl₄/H₂/N₂/CH₄/Ar. The composition of the coatings can be controlled through the adjustment of CH₄ flow rate and the mixing ratio of the chlorides. Detailed structural and chemical characterisations using transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) suggest the formation of a Ti (C, N)/a-C/a-Si₃N₄ nanocomposite structure. Ti (C, N) films show a (200) texture, which change to random orientation of the crystallites when the silicon content reaches about 9 at.%. The tribological behaviour of these coatings was investigated at room and elevated temperature. The results show that the nanocomposite Ti-Si-C-N coatings with low Si and high C contents have a lower friction coefficient of 0.17-0.35 at room temperature. The Ti-Si-C-N nanocomposite coating containing 12 at.% Si and 30 at.% C shows excellent tribological properties with a low friction coefficient of 0.30 and a low wear rate of 4.5 × 10⁵ mm³ N^− 1m^− 1 at 550 °C.     

Formation of nickel aluminide/nickel hybrid coatings on alloy steels by two step process of nickel plating and low temperature pack aluminisation

Abstract

The present study investigates the conditions required for forming a hybrid coating consisting of an outer nickel aluminide layer and an inner nickel layer on alloy steels. A commercial alloy steel of 9Cr–1Mo was used as a substrate. Electroless and electronickel plating processes were used to form an initial nickel layer on the steel. The AlCl₃ activated packs containing pure Al as a depositing source were then used to aluminise the nickel deposit at temperatures ≤650°C. The effect of phosphorus or boron content in the initial nickel layer deposited with the electroless nickel plating solutions using hypophosphite or boron–hydrogen compound as reducing agent was investigated in relation to the spallation tendency of the coating either immediately after the aluminising process or during the thermal annealing post-aluminising process. Under the aluminising conditions used, the outer nickel aluminide layer formed was Ni₂Al₃. For the electroplated nickel deposit, the growth kinetics of the outer Ni₂Al₃ layer during the pack aluminising process was found to obey the parabolic rate law with a parabolic rate constant being 12·67 μm at 650°C for 2 wt-%AlCl₃ activated pack containing 4 wt-% pure Al as a deposition source.     

Effects of substrate bias voltage and target sputtering power on the structural and tribological properties of carbon nitride coatings

Effects of substrate bias voltage and target sputtering power on the structural and tribological properties of carbon nitride (CN_x) coatings are investigated. CN_x coatings are fabricated by a hybrid coating process with the combination of radio frequency plasma enhanced chemical vapor deposition (RF PECVD) and DC magnetron sputtering at various substrate bias voltage and target sputtering power in the order of −400 V 200 W, −400 V 100 W, −800 V 200 W, and −800 V 100 W. The deposition rate, N/C atomic ratio, and hardness of CN_x coatings as well as friction coefficient of CN_x coating sliding against AISI 52100 pin in N₂ gas stream decrease, while the residual stress of CN_x coatings increases with the increase of substrate bias voltage and the decrease of target sputtering power. The highest hardness measured under single stiffness mode of 15.0 GPa and lowest residual stress of 3.7 GPa of CN_x coatings are obtained at −400 V 200 W, whereas the lowest friction coefficient of 0.12 of CN_x coatings is achieved at −800 V 100 W. Raman and XPS analysis suggest that sp³ carbon bonding decreases and sp² carbon bonding increases with the variations in substrate bias voltage and target sputtering power. Optical images and Raman characterization of worn surfaces confirm that the friction behavior of CN_x coatings is controlled by the directly sliding between CN_x coating and steel pin. Therefore, the reduction of friction coefficient is attributed to the decrease of sp³ carbon bonding in the CN_x coating. It is concluded that substrate bias voltage and target sputtering power are effective parameters for tailoring the structural and tribological properties of CN_x coatings.     

原位生成SiC-MoSi_2-TiSi_2涂层的工艺条件研究

用一次包埋法制备了C/C复合材料SiC-MoSi2-TiSi2复相陶瓷内涂层,对制备涂层的影响因素进行了分析。最终确定的制备条件为制备温度2573K,保温时间为2h,MoSi2和TiC的配比为5∶2。结果表明C/C复合材料表面SiC-MoSi2-TiSi2复相陶瓷涂层在1500℃有氧环境下氧化49h,失重仅仅2.18%,失重率为1.17×10-4g/（cm2.h）,具有良好的抗氧化性能。     

The combined effect of nickel and bismuth on the structure of hot-dip zinc coatings

The combined effect of nickel and bismuth on the structure and the morphology of zinc hot-dip galvanized coatings were studied with optical microscopy, scanning electron microscopy and X-ray diffraction. From this investigation it turned out that the coating produced from a zinc melt containing 1% Ni and 1% Bi is composed of three layers corresponding to δ- and η-phase of the Fe-Zn system and T-phase of the Fe-Zn-Ni system. Bi was not detected in these phases. However inclusions of this element were identified between the grains of the T-phase. Their presence could negatively affect the corrosion performance of the coating because bismuth is cathodic to zinc.     

Microstructural studies and wear assessments of Ti/TiC surface composite coatings on commercial pure Ti produced by titanium cored wires and TIG process

Tungsten Inert Gas (TIG) process and titanium cored wires filled with micro size TiC particles were employed to produce surface composite coatings on commercial pure Ti substrate for wear resistance improvement. Wire drawing process was utilized to produce several cored wires from titanium strips and titanium carbide powders. Subsequently, these cored wires were melted and coated on commercial pure Ti using TIG process. This procedure was repeated at different current intensities and welding travel speeds. Composite coating tracks were found to be affected by TIG heat input. The microstructural studies using optical and scanning electron microscopy supported by X-ray diffraction showed that the surface composite coatings consisted of α′-Ti, spherical and dendritic TiC particles. Also, greater volume fractions of TiC particles in the coatings were found at lower heat input. A maximum microhardness value of about 1100 HV was measured which is more than 7 times higher than the substrate material. Pin-on-disk wear tests exhibited a better performance of the surface composite coatings than the untreated material which was attributed to the presence of TiC particles in the microstructure.     

NbSi基原位复合材料表面包埋渗制备SiCrY共渗涂层

通过热力学计算分析了分别使用NH₄Cl和CrCl₃·6H₂O两种不同激活剂时, 各个共渗元素的卤化物蒸汽压分压的变化, 得到了实现Nb-Si基原位复合材料上Si-Cr-Y三元包埋共渗的最佳条件。采用包埋共渗法在Nb-Si基原位复合材料表面制备了Si-Cr-Y共渗涂层, 研究了涂层的组织形貌、 成分及其相组成。结果表明: 使用NH₄Cl做激活剂, 通过调整包埋渗料的成分, 可以在适当的温度下实现Si-Cr-Y的三元共渗。当渗料成分为12Cr-6Si-0.75Y₂O₃-5NH₄Cl-76.25Al₂O₃(质量分数)时, 在1350℃可以实现Si-Cr-Y三元共渗。制备的涂层具有多层结构, 分为外层、 内层和明显的互扩散层。互扩散层的存在, 表明涂层的形成是一个连续生长过程, 伴随着Cr、 Si、 Y元素向基体内的扩散。涂层的主要成分由Cr₂(Nb,Ti)、 (Nb,Ti)₅Si₃和HfSi₂组成, Y元素的添加起到了细化涂层的作用。     

Influence of deposition temperature on the hardness and dimensions of steels with hard coatings produced by chemical vapour deposition

CVD hard coatings are applied to steel parts at high and at medium reaction temperatures. The paper presents an introduction to the effect of CVD temperature on hardness and on the dimensions of the CVD-coated steel parts.In combined high temperature CVD and hardening processes, the coatings are deposited at temperatures which are equal to the austenitizing temperature of the steel base; the coated pieces are then quenched in the CVD reactor. Hence the hardness and dimensions of the steel bases depend on the CVD reaction temperature. The required hardness values of the base may be adjusted by tempering according to the hardness-austenitizing temperature-tempering temperature diagram. If the dimensions of the steel parts exceed about 20 mm, the dimensional changes due to heat treatment are larger than those involved in coatings of the usual thickness of 5–10 μm. Dimensional corrections may be made by means of cold treatment and tempering.In medium temperature CVD processes good dimensional stability of the treated steel parts may be achieved by pretempering the prehardened steel parts at temperatures that are equal to or very near to those which are applied in CVD, and by working the pretempered parts to dimensions which will change to within the allowed tolerances during deposition of the coatings. In these processes, high hardness values in D-type steel bases may be reached by the application of hardening temperatures higher than the usual ones.The anisotropic behavior of D-type tool steels may cause some problems in dimensional stability and in the distribution of internal stresses in the hard coatings; the stresses will be compressive in most kinds of CVD hard coatings.     

The effect of carbon content on the microstructure of hydrogen-free physical vapour deposited titanium carbide films

Titanium carbide (TiC) coatings for tribological applications were deposited on high speed steel. Several coatings with different titanium to carbon ratio were deposited by means of physical vapour deposition in which titanium was evaporated and carbon was sputtered. The coatings were characterised using analytical electron microscopy. It was observed that the change in titanium to carbon ratio significantly changed the microstructure of the coatings. The low carbon containing coatings consisted of columnar grains exhibiting a preferred crystallographic orientation whereas the coating with highest carbon content consisted of randomly ordered TiC grains in an amorphous carbon matrix. Energy filtered transmission electron microscopy revealed a change in Ti/C ratio as the distance from the substrate increased. The titanium to carbon ratio was observed to increase with distance from the substrate until a stable level was reached. This is due to a variation in the titanium evaporation during the early stages of film growth. This change of the titanium to carbon ratio affected the columnar growth in the initial stage of coating growth for the coatings with low carbon content.     

The effect of alloying elements on the structure and mechanical properties of ultra low carbon bainitic steels

Microalloying with Nb and B leads to a granular bainite microstructure which is composed of a bainitic ferrite matrix and a uniformly distributed martensite/austenite-constituent in the as-rolled condition. Due to this transformation strengthening mechanism, high strength and toughness could be achieved even though the C content was extremely low. It was found that both dissolved Nb in austenite and free B are prerequisites for granular bainite formation. Furthermore, there is a critical B content to achieve the complete bainitic transformation strengthening effect. The critical B content increases with C content. C thus diminishes the effect of B in promoting bainite transformation, due to the formation of boron carbides or the depletion of dissolved Nb in austenite. The effect of Mn, Mo and Ni on the decomposition of austenite is similar. A parameterMneq which relates the effect of these alloying elements on the Bs temperature was derived. It was confirmed that the strength of bainitic steels is inversely proportional to theBs temperature.     

Effect of carbon steel microstructure and molecular structure of two new Schiff base compounds on inhibition performance in 1 M HCl solution by DC,SEM and XRD studies

In this investigation, attempts have been made to study the inhibitive effect of N,N′-ortho-phenylen acetyle acetone imine (S1) and 4-[(3-{[1-(2-hydroxy phenyl) methylidene] amino} propyl) ethanemidol]-1,3-benzenediol (S2) in the concentration range of 50–400 ppm for mild steel with two different microstructures resulted from two different heat treatments (annealed (A) and quenched and tempered (Q&T)) in 1 M hydrochloric acid solution. Both Schiff bases acted as a mixed type inhibitors. The S1 inhibitor for both microstructures showed better inhibition efficiency than S2. The A samples indicated slightly less corrosion than Q&T samples in 1 M HCl solution in absence of inhibitor due to the formation of duplex γ-Fe₂O₃/Fe₃O₄.     

The process of electrochemical deposited hydroxyapatite coatings on biomedical titanium at room temperature

Calcium phosphate (CaP) ceramics, especially hydroxyapatite (HA), have received much attention and have been clinically applied in orthopaedics and dentistry due to their excellent biocompatibility. Among several methods for preparing HA coating, electrochemical deposition is a relatively new and possible process. However, documented electrochemical processes were conducted at elevated temperature. In this study, uniform HA coatings have been directly deposited on titanium at room temperature. X-ray diffractometry (XRD) results demonstrated that dicalcium phosphate dihydrate (CaHPO₄·2H₂O, DCPD) was the main component of the coating deposited at lower current densities (1 and 5 mA/cm²). HA structure was obtained at current density above 10 mA/cm² and remained stable after heat treatment at 100–600 °C for 1 h. Part of HA phase was transformed into β-TCP and became a biphasic calcium phosphate coating after annealing at 700 °C. Scratch tests showed that HA coating was not scraped off until a shear stress of 106.3 MPa. Coatings deposited at room temperature exhibited stronger adhesion than those at elevated temperature. HA coating revealed a dense inner layer and rough surface morphology which could fulfill the requisition of implant materials and be adequate to the attachment of bone tissue.     

化学气相沉积低温热解炭的微观组织结构与沉积模型

利用扫描电子显微镜(SEM),透射电子显微镜(TEM)结合选取电子衍射(SAED)研究了化学气相沉积低温热解炭的微观组织结构。结果表明:低温热解炭是由直径小于2μm的球形颗粒状炭组成,该球形颗粒状炭的核心是炭黑颗粒,外层为中织构热解炭。其沉积过程主要经历了:微小炭黑颗粒的萌生,炭黑颗粒外层的生长,炭颗粒表面热解炭的沉积和炭颗粒的聚集长大四个过程。