Volatile species in halide-activated diffusion coating packs

An atmospheric pressure sampling mass spectrometer was used to identify the vapor species generated in a halide-activated, cementation pack. Pack powder mixtures containing a Cr-Al binary masteralloy powder, an NH₄Cl activator salt, and either ZrO₂ or Y₂O₃ (or neither) were analyzed at 1000°C. Both equilibrium calculations for the pack and mass spectrometer results indicated that volatile AlCl_x and CrCl_y species were generated by the pack powder mixture; in packs containing the reactive element oxide, volatile ZrCl_z and YCl_w species were formed by the conversion of their oxide sources.     

Simultaneous chromizing-aluminizing coating of austenitic stainless steels

Chromium and aluminum were simultaneously co-deposited by diffusion into austenitic stainless steel substrates, by a single-step, pack-cementation process. The mechanism for the formation of diffusion-coated products on 304 and 316 stainless steels and on Incoloy 800 is discussed. The morphologies of the phases formed at the surface, i.e., an external beta layer and an underlying multiphase interdiffusion zone, are presented. The formation of the brittle, , outer layer was minimized by variations in the pack composition and activator. The coated 304 and 316 steels exhibited excellent scaling resistance upon oxidation in air at 1000°C.     

Codeposited chromium and silicon diffusion coatings for Fe-Base alloys via pack cementation

The simultaneous deposition of Cr and Si into plain carbon, low-alloy, and austenitic steels using a halide-activated pack-cementation process is described. Equilibrium partial pressures of gaseous species have been calculated using the STEPSOL computer program to aid in designing specific processes for codepositing the desired ratios of Cr and Si into a given alloy. The calculations indicate that NaCl-activated packs are chromizing, while NaF-activated packs deposit more Si with less Cr. The use of a dual activator (e.g., NaF+NaCl) allows for the deposition of both Cr and Si in the desired amounts. Single-phase ferritic coatings (150–250 microns thick) with a surface concentration of 20–35 wt.% Cr and 2–4% Si have been grown on AISI 1018, Fe-2.25 Cr-1.0Mo-0.15C, and Fe-0.5 Cr-0.5 Mo-0.2C steels using packs containing a 90 wt.% Cr-10Si binary source alloy, a NaF+NaCl activator, and a silica filler. Two-phase coatings (approximately 75 microns thick) containing 20–25 wt.% Cr and 2.0–2.4% Si have been obtained on 304 stainless steel using packs containing a 90 wt.% Cr-10Si binary source alloy, a NaF activator, and an alumina filler. The same pack chemistry allowed the diffusion of Cr and Si into the austenitic Incoloy 800 alloy without a phase change. A coated Fe-2.25 Cr-1.0 Mo-0.15 C coupon with a surface concentration of Fe-34 wt.% Cr-3Si was cyclically oxidized in air at 700°C for over four months and 47 cycles. The weight gain was very low (<0.2 mg/cm²) with no scale spalling detected. Coated coupons of AISI 1018 steel, and Fe-0.5 Cr-0.5 Mo-0.2C steel have shown excellent oxidation-sulfidation resistance in reducing, sulfur-containing atmospheres at temperatures from 400 to 700°C and in erosion and erosion-oxidation testing in air at 650 and 850°C.     

Simultaneous chromizing — Aluminizing coating of low-alloy steels by a halide-activated,pack-cementation process

The simultaneous chromizing — aluminizing of low-alloy steels has achieved Kanthal-like surface compositions of 16–21Cr and 5–8 wt.% Al by the use of cementation packs with a Cr-Al masteralloy and an NH₄Cl activator salt. An initial preferential deposition of Al into the alloy induces the phase transformation from austenite to ferrite at the 1150°C process temperature. The low solubility of carbon in ferrite results in the rejection of solute C into the austenitic core, thereby preventing the formation of an external Cr-carbide layer, which would otherwise block aluminizing and chromizing. The deposition and rapid diffusion of Cr and Al into the external bcc ferrite layer follows. Parabolic, cyclic-oxidation kinetics for alumina growth on the coated steels in air were observed over a wide range of relatively low temperatures (637–923°C).     

Morphology, structure and formation mechanism of silicide coating by pack cementation process

The MoSi2 coating on C 103 niobium based alloy was prepared by pack cementation method. The formative mechanism, morphology and structure of coating were investigated. The silicide coating was formed by reactive diffusion obeying parabolic rule during pack cementation process. It is found that the composite structural coating is composed of three inferior layers as follows. The main layer is composed of MoSi2, the two phases＇ transitional layer consists of NbSi2 and a few NbsSi3 and the diffuse layer is composed of NbsSi3. The dense amorphous glass layer formed on the surface at high temperature oxidation circumstance can effectively prevent the diffusion of oxygen into coating.     

铌基合金包埋渗法制备抗氧化硅化物涂层及其组织形成

利用包埋渗法在铌基合金上制备抗氧化硅化物涂层，渗硅温度分别为1050、1150和1250℃，渗硅时间分别为2、5、10、15和20h。利用SEM、EDS和XRD等检测手段分析涂层的结构、元素分布及相组成等，并对涂层形成的反应机理及反应动力学进行讨论。结果表明：涂层的相组成为（Nb，X）Si2（X表示Ti，Cr,Hf和Al等元素）；涂层具有双层结构，且上层较为致密；涂层与基体之间达到了冶金结合，通过扩散形成过渡层及互扩散区；在包埋渗温度分别为1050、1150和1250℃时，涂层生长的动力学曲线均符合抛物线规律。     

包渗温度对Ti-Al渗层组织和耐磨性能的影响(英文)

为了提高纯铜表面的耐磨性能,采用电镀/浆料包渗相结合的方法,以TiO2粉为渗Ti源,纯Al粉为还原剂,在Cu表面预镀Ni随后表面浆料包渗Ti-Al,制备Ti-Al共渗层。研究了包渗温度对Ti-Al渗层组织和耐磨性能的影响。采用SEM和XRD分析了渗层表面形貌和结构。结果表明:在800～950°C共渗12 h时,随着温度的升高,渗层组织变化过程为NiAl+Ni3(Ti,Al)→NiAl+Ni3(Ti,Al)+Ni4Ti3→Ni4Ti3+NiAl→NiAl+Ni3(Ti,Al)+NiTi;Ti-Al渗层的摩擦因数随着包渗温度的升高而降低,最小摩擦因数约为纯铜的1/3,最小硬度为纯铜的5倍。     

Microstructure and corrosion resistance of simultaneous Al-Fe coating on copper by pack cementation

Simultaneous Al-Fe coatings on copper were prepared by pack cementation to investigate the microstructure and corrosion resistance. The cross-section of prepared specimen was analyzed by scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). Then the cross-section of specimen corroded by solution of ferric chloride in ethanol was evaluated using SEM and X-ray diffraction (XRD). The coating consists of an intermetallic layer and an interdiffusion layer, and the thickness of coating layer increases parabolically with holding temperatures from 730 °C to 900 °C. The aluminum content in the coating varies between 7.65 and 4.0% from the surface to the inside layer while the iron content varies between 2 and 0.5%. The coating layer is composed of а-Cu/Al solid solution with a small amount of iron. Alumina formed on the surface of the coating layer during corrosion provides very good protection for the coating layer in the corrosive atmosphere and enhances the corrosion resistance of the coating.     

铌基超高温合金表面Si-Al包埋共渗抗氧化涂层的组织形成

通过1000～1150℃Si-Al包埋共渗8h的方法在铌基超高温合金表面制备Al改性硅化物抗氧化涂层,结果表明：实验温度下制备的涂层均具有多层复合结构;不同温度下共渗涂层外层的相组成不同,但最内层均由（Nb,Ti）Al3和（Cr,Al）2（Nb,Ti）组成。1050℃、8hSi-Al共渗在合金表面形成了Al改性的硅化物涂层,其最外层主要为Nb3Si5Al2,依次往内为（Nb,X）（Si,Al）2（X代表Ti,Cr和Hf元素）层、（Nb,X）5Si3层以及最内层;而在1000℃、8h条件下Si-Al共渗形成的涂层以（Nb,Ti）Al3为主,其中含有少量的（Nb,X）5Si3,没有形成以硅化物为主的涂层;在1100℃、8h和1150℃、8h条件下Si-Al共渗形成的涂层外层以（Nb,X）（Si,Al）2为主,但其中Al含量（摩尔分数）仅为2.35%,没有形成Nb-Si-Al三元化合物层。     

Development and growth of boron-modified and germanium-doped titanium-silicide diffusion coatings by the halide-activated,pack-cementation method

A halide-activated, cementation pack has been developed to codeposit either silicon and boron or else silicon and germanium in a single processing/reaction step to grow Ti-silicide diffusion coatings on commercially pure (CP) titanium, Ti-22Al-27Nb, and Ti-20Al-22Nb. Since boron is nearly insoluble in TiSi₂, a TiB₂ layer is localized at the surface of the B-modified silicide coatings. The thickness of the TiB₂ layer is controlled by the choice of boron activity and halide activator in the pack. Germanium is soluble in the Ti-silicide layers but inhomogeneously distributed in the Ge-doped silicide coating. The germanium content is controlled by choices of the Si-to-Ge ratio and the halide activator in the pack. The growth kinetics for the five-layered B-modified silicide coatings are generally similar to the undoped silicide coatings. The growth mechanism for the five-layered Ge-doped silicide coatings is generally different from the undoped silicides. The growth of dual-layer Ti-boride coatings was also studied.     

Formation of a chromium-carbide conversion coating on silicon carbide

The formation of a chromium-carbide conversion coating on SiC was achieved using the pack-cementation technique. The conversion coating is intended to improve the corrosion resistance of SiC and its derivatives, such as SiC-base continuous fibers and composites, by forming a protective Cr₂O₃ scale upon exposure to high-temperature corrosive environments. Different pack chemistry and processing parameters were evaluated in the laboratory. Results indicated that the coating morphologies and compositions achieved were significantly affected by variation of these processing factors. In this paper, the conversion coating obtained from one of the systems investigated is reported. The coating consists of a multilayered structure with each of the sublayers containing a high-Cr concentration. In addition, the coating surface is relatively dense and pore free compared to the underlying SiC substrate material. A dense and pore-free morphology is highly desirable for coating applications, especially on porous substrates. The multilayered coating structure consists of the following sublayers: Cr₂₃C₆/Cr₇C₃/Cr₇C₃+Cr₃Si/Cr₅Si₃C_x/SiC substrate.     

High-temperature coating for titanium aluminides using the pack-cementation technique

The practical application of titanium aluminide metal-matrix composites (MMCs) at high temperatures requires suitable surface coatings to provide the needed oxidation resistance. Without a coating, the titanium aluminide alloys suffered from rapid oxidation attack at elevated temperatures, particularly under thermal cyclic conditions. The pack-cementation coating process was utilized to aluminize the surface region of a Ti₃Al-base alloy to TiAl₃, the most oxidation-resistant phase. With the existence of an adherent conversion coating, a thin protective alumina scale formed on the outer surface, and a significant improvement in the corrosion resistance was observed. Excellent coating efficiency and geometric flexibility were demonstrated in this study by the pack-cementation technique. Further development of the cementation process will focus on the elimination of surface cracking in the coating.     

Formation and Characterization of Titanium Modified Aluminide Coatings on IN738LC

Up to now, the aluminide coatings used to protect industrial components at high temperature and corrosive environments have been modified by Pt, Cr, Si and Ni. In this investigation, aluminide coatings were modified by titanium and the microstructural feature and formation mechanism were evaluated. The coatings were formed on a Ni-based superalloy(IN738LC) by a two stage process including titanizing at first and aluminizing thereafter. Pack cementation titanizing performed at temperatures 950℃ and 1050℃ in several mixtures of Ti, Al2O3 and NH4Cl. At the second stage,aluminum diffused into surface of the specimens by an industrial aluminizing process known as Elcoatl01(4 hrs at 1050℃C). The modified coatings were characterized by means of standard optical microscopy, scanning electron microscopy,energy dispersive spectroscopy and X-Ray diffraction methods. The results show that Ti in the coatings is mainly present in the form of TiNi and Al67CrsTi25. Titanium modified coatings grew with a mechanism similar to simple aluminizing; this includes inward diffusion of Al from the pack to the substrate and then outward diffusion of Ni from the substrate to the coating. The advantages and characteristics of this two-stage modified coating is discussed and the process parameters are proposed to obtain a coating of optimum microstructure.     

Formation and Characterization of Titanium Modif Aluminide Coatings on IN738LC

HISTORICALLY superalloys have been used in gasturbine applications with a protective coating to combatoxidation problems in aero engines and hot corrosionproblems in industrial and marine engines.Theprotective coatings used are predominantly of thediffusion type produced by variety of techniques suchas pack cementation,chemical vapor deposition,slurryfusion and hot dipping[1,2].The purpose of this typeof coatings is to increase the amount of Al at thesurface of a component,so that a protec…     

Microstructural study on oxidation of aluminized coating on inconel 625

The high-temperature oxidation behavior of aluminized Inconel 625 has been examined using scanning electron microscopy (SEM) and fine-probe spot and linescan energy dispersive spectroscopy (EDS) microanalysis techniques. The formation of adherent slowly growing metallic coatings is essential for protection against severe environments. The coated (aluminized) and uncoated samples were subsequently oxidized in air at 1000 and 1100 °C to examine the performance of the aluminized coating. The micro-structural changes that occurred in both coated and uncoated samples were examined. It was found that coated samples showed superior performance against high-temperature oxidation, as compared to the uncoated samples. The coated samples revealed uniform and adherent oxidized layer, as compared to the uncoated samples. The oxidation behavior and multilayered microstructural morphology observed in the coated samples can be attributed to the interdiffusion and variation in kinetics of oxidation.     

MOSi2涂层的组织结构与高温抗氧化性能

采用包渗法在Nb-10W合金基体上制备MoSi2涂层,测试了涂层试样的1 600℃静态抗氧化和室温至1 600℃循环抗氧化性能,分析了硅化过程中涂层的形成机理、涂层表面和截面的形貌以及氧化后涂层成分与结构变化。采用包渗法制备的硅化物涂层是通过反应扩散形成的,该涂层为复合结构:MoSi2相主体层;以NbSi2相为主、并含少量Nb5Si3相的两相过渡区;Nb5Si3相扩散层;该涂层结构表现出良好的高温抗氧化和抗热震性能;Si的扩散是通过空位的反应流动实现的,空位在主体层与两相过渡区界面处聚集成微孔带,氧化后微孔带宽度增加;致密的低硅化物扩散层能有效地阻止裂纹向基体扩展。     

CeO₂改善低温渗铝涂层抗氧化性能的研究

在Ni基体上电沉积纯Ni镀层和Ni-CeO2复合镀层并对其进行620 °C低温渗铝,制备了无CeO2和CeO2改性的铝化物涂层。将以上两种涂层在1000 °C下氧化,研究CeO2颗粒的加入对氧化膜的生长速率和粘附性能的影响。结果表明,在δ-Ni2Al3涂层中加入纳米CeO2颗粒可以推迟一层完整α-Al2O3膜的形成时间,降低氧化膜的生长速率。此外,纳米CeO2颗粒的加入提高了氧化膜的粘附性,原因是与没有CeO2掺杂的涂层相比,CeO2改性铝化物涂层在氧化膜/涂层界面上形成的空洞尺寸较小。     

CeO2改性渗铝层的生长动力学研究

本文对钢试样进行复合电镀Ni＋CeO2并包埋渗铝。在粉末渗铝过程中保温不同的时间，获得了CeO2改性的渗层。利用SEM和EDS对渗层组织与成份进行分析，通过对渗层厚度的测量，得出合金层生长动力学曲线，建立了改性渗层的生长动力学方程。结果表明，在改性的渗层中，CeO2在富集区的含量达到了5．21wt％。通过对动力学曲线的研究，为控制和预测渗层厚度、制定工艺提供了理论依据。