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.     

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.     

Oxidation resistance coatings of Mo−Si−B alloys via a pack cementation process

Oxidation behavior of Si pack cementation coatings on a three-phase Mo−Si−B alloy was examined. During Si pack cementation of a three-phase Mo−Si−B alloy, a MoSi₂ outer layer was synthesized on the exterior layer. Following oxidation in air of the pack-coated alloys at 1200°C, the MoSi₂ phase layer was consumed and replaced by the Mo₅Si₃ (or T₁) phase. The synthesized T₁ phase provided excellent oxidation resistance when exposed to high temperatures. The silicide-coated alloys exhibit higher oxidation resistance compared with uncoated Mo−Si−B alloys. The enhanced oxidation behavior and its mechanism are discussed in terms of the thicknesses of the oxide layers under exposure to high temperatures.     

粉末包埋法制备镍基高温合金Y-Cr-Al涂层高温氧化行为

本研究在不同的Al含量的条件下应用粉末包埋法制备了Y-Cr-Al涂层,并对其高温氧化行为进行了研究。结果显示：Al含量为2%所生成的涂层为三层结构,Al含量为2.5%,3%所形成的涂层为两层结构。Al含量为2%,3%的条件下涂层主要以α-Cr ,NiAl为主,Al含量为2.5%时生成的涂层富含Cr的NiAl相,许多Cr原子固溶于NiAl相中。Al含量为2%所生成的涂层氧化性能最差,Al含量为2.5%所生成的涂层展现了较为优异的氧化性能。     

MoSi2 oxidation resistance coatings for Mo5Si3/MoSi2 composites

In order to improve the oxidation resistance properties of 30 at.% Mo5Si3/MoSi2 composite at high temperature in air, a molybdenum disili-tide coating was prepared on its surface by a molten salt technology. XRD and SEM analysis showed that only tetragonal MoSi2 phase ex-isted in the coating after being siliconized for 5 h at 900℃. The oxidation film formed on the uncoated sample was not dense, so that oxygen diffused easily through it. The volatilization of MoO3 resulted in the oxidation film separating from the substrate. The MoSi2coating was proved to be an effective method to prevent 30 at.% MosSi3/MoSi2 composites from being oxidized at 1200℃. A dense glassy SiO2 film was formed on the MoSi2 coating surface, which acted as a barrier layer for the diffusion of oxygen atoms to the substrate. The 30at.% Mo5Si3/MoSi2 composites with a MoSi2 coating showed much better oxidation resistance at high temperature.     

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。     

Preventing the accelerated low-temperature oxidation of MoSi2 (pesting) by the application of superficial alkali-salt layers

Previous work showed that MoSi₂ diffusion coatings formed by a NaF-activated pack cementation process did not pest. A Na–Al-oxide by-product layer resulting from the NaF activator formed a Na-silicate layer to passivate MoSi₂. Superficial NaF layers were then used to prevent the pesting of MoSi₂ diffusion coating that were otherwise susceptible to pest disintegration. In this study, the use of superficial alkali-salt layers to prevent the accelerated oxidation of bulk MoSi₂ at 500°C is investigated more broadly. The application of Na-halide, KF, LiF, Na₂B₄O₇, or Na-silicate layers prior to oxidation prevented accelerated oxidation and pesting for at least 2000 hr at 500°C in air. The formation of a fast-growing, Na-silicate layer passivates MoSi₂. The MoO₃ that forms during oxidation absorbs sodium by intercolation to form stable Na-molybdate precipitates. Na₂B₄O₇, Na-silicate, LiF, and KF prevented accelerated oxidation at 500°C by a similar mechanism. The application of alkali-halide salts is a simple, effective solution to prevent the accelerated oxidation and pesting of MoSi₂.     

Erosion-corrosion in a laboratory-scale coal-firing FBC of various aluminized coatings prepared by low-temperature pack cementation

Using a halide-activated pack-cementation method but at a temperature (600 °C) noticeably lower than normal, an η-Fe₂Al₅ coating and two δ-Ni₂Al₃ coatings with and without dispersions of CeO₂ nanoparticles were developed respectively on a low-carbon steel and the steel pretreated with an electrodeposited film of Ni or Ni-CeO₂. The erosion-corrosion (E-C) performance of the three aluminide coatings during 100 h exposure at ~ 600 °C in a coal-firing laboratory-scale fluidized-bed combustor (FBC) was investigated, by mounting the aluminized samples onto a rig which maintained rotation for accelerating the relative impacting speed of flying solid particles (mainly SiO₂ bed materials). The η-Fe₂Al₅ and the CeO₂-free δ-Ni₂Al₃ coatings experienced an unacceptable recession rate. Compared to the two CeO₂-free aluminide coatings, the CeO₂-dispersed δ-Ni₂Al₃ coating offered profoundly improved E-C resistance, because the latter coating was not only strengthened by the CeO₂ dispersion and grain refinement, it also could grow a more adherent alumina scale.     

Steam oxidation resistance of Ni-aluminide/Fe-aluminide duplex coatings formed on creep resistant ferritic steels by low temperature pack cementation process

Steam oxidation resistance and thermal stability were studied at 650 °C for a coating with an outer Ni₂Al₃ layer and an inner Fe₂Al₅ layer formed on P92 steel surface. The parabolic rate law of oxidation was obeyed only in less than 2000 h with positive deviations occurring at longer oxidation times. The outer layer of the coating was transformed to NiAl during oxidation, but it remained stable once it was formed. The mechanisms for the enhanced thermal stability were discussed and a simple approach to enhancing the lifetime of the coating was proposed.     

Structure of Al-modified silicide coatings on an Nb-based ultrahigh temperature alloy prepared by pack cementation techniques

In order to prepare Al-modified silicide coatings on an Nb-based ultrahigh temperature alloy, both a two-stage pack cementation technique and a co-deposition pack cementation technique were employed. The two-stage process included siliconizing a specimen at 1150 °C for 4 h followed by aluminizing it at 800-1000 °C for 4 h. The coating prepared by pack siliconization was composed of a thick (Nb,X)Si₂ (X represents Ti, Cr and Hf elements) outer layer and a thin (Nb,X)₅Si₃ transitional layer; after the siliconized specimens were aluminized at or above 860 °C, a (Nb,Ti)₃Si₅Al₂ phase developed at the surface of the coating, and furthermore, when aluminizing was carried out at 860 °C, a new (Nb,Ti)₂Al layer formed in the coating between the (Nb,X)₅Si₃ layer and the substrate, but when aluminizing was performed at 900-1000 °C, the new layer formed was (Nb,Ti)Al₃. The co-deposition process was carried out by co-depositing Si and Al on specimens at 1000-1150 °C for 8 h under different pack compositions, and it was found that the structure of co-deposition coatings was more evidently affected by co-deposition temperature than pack composition. An Al-modified silicide coating with an outer layer composed of (Nb,Ti)₃Si₅Al₂, (Nb,X)Si₂ and (Nb,Ti)Al₃ was obtained by co-depositing Si and Al at 1050 °C.     

Ti-Nb-Si基高温合金表面包埋Si-Cr共渗涂层的组织

采用Si-Cr包埋共渗法在Ti-Nb-Si基高温合金表面制备了Cr改性的硅化物涂层,共渗温度为1250和1300℃,时间为10h。利用SEM,EDS和XRD等检测手段分析了涂层的结构、元素分布及相组成等,并对涂层的形成机理进行了讨论。结果表明:Si-Cr共渗温度为1250℃时,降低渗剂中的催化剂NaF含量会降低Si和Cr的反应扩散速度并且改变了涂层的结构和相组成。催化剂NaF含量为8wt%,涂层外层由(Nb,Ti)Si2及少量(Ti,X)5Si3(X代表Nb,Cr和Hf等元素)组成,中间层由(Ti,X)5Si4组成,过渡层由(Nb,Ti)5Si3组成;降低NaF含量至5wt%,Si-Cr共渗温度仍为1250℃时,涂层外层由(Ti,X)5Si3组成,且有较多孔洞出现,中间层为(Ti,X)5Si4,而过渡层很薄。与渗Si涂层相比,Si-Cr共渗涂层中的裂纹明显减少,但在涂层外层存在较多孔洞且涂层厚度明显减小。提高包埋共渗温度至1300℃时,Cr的反应扩散速度得到提高,且在涂层外层出现了(Nb1.95Cr1.05)Cr2Si3三元相。     

Isothermal and cyclic oxidation resistance of boron-modified and germanium-doped silicide coatings for titanium alloys

Since titanium alloys with an adequate balance of mechanical properties and high-temperature oxidation resistance have not been developed, protective coatings are required. In our previous paper, B-modified and Ge-doped silicide diffusion coatings grown on CP Ti, Ti–24Al–11Nb, Ti–22Al–27Nb, and Ti–20Al–22Nb by the halide-activated, pack-cementation method were described. In this study, isothermal and cyclic oxidation were used to evaluate the oxidation performance of these coatings in comparison to uncoated substrates. The rate-controlling mechanism for isothermal oxidation at high temperature was solid-state diffusion through a SiO₂ scale, while the mechanism for low-temperature oxidation involved grain-boundary diffusion through TiO₂. Both isothermal and cyclic oxidation rates for the B-modified and Ge-doped silicide coatings were much slower than for pure TiSi₂. Oxygen contamination was not detected by microhardness measurements in the coated substrates after 200 oxidation cycles at 500–1000°C for the Ti–Al–Nb alloys, or at 500–875°C for CP Ti. The excellent oxidation resistance for the optimum coating compositions is discussed.     

固体渗铬层组织结构及性能稳定性研究

采用固体粉末法在GCr15钢表面进行渗铬处理,研究了不同渗铬时间下渗层的结构和性能,并研究了自然时效对渗层结构、力学性能的影响.采用扫描电子显微镜及EDS能谱仪、X射线衍射仪研究了时效前后渗层组织结构的变化,采用显微硬度仪、纳米压痕仪、洛氏硬度计分析了时效前后渗层表面硬度、截面微区硬度、与基体的结合强度.结果表明:渗层...     

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。     

Boride coatings obtained by pack cementation deposited on powder metallurgy and wrought Ti and Ti-6Al-4V

Wear resistance of Ti alloys needs to be improved, and an effective way to achieve this is through surface treatment. Boronizing is a surface treatment in which boron diffuses into the surface of Ti leading to the formation of hard and wear-resistant Ti borides. Boronizing of wrought and/or cast Ti alloys by pack cementation has been studied, while similar coatings on Ti alloys produced by powder metallurgy (PM) have not been reported. Also critical process parameters for boronizing Ti alloys, such as pack cementation powder composition and the process temperature have not been systematically studied and analysed. The present work reports on the surface modification of PM Ti and PM Ti-6Al-4V by boronizing, and presents some important thermodynamic aspects of the process comparing it with similar coatings applied to wrought Ti-6Al-4V. The coatings were characterised using scanning electron microscopy and X-ray diffraction. For both Ti and Ti-6Al-4V alloys the use of amorphous B as a B element supplier in the boriding powder pack led to the formation of a uniform external boride layer, while the use of B₄C as a B element supplier in the pack and under the same boronizing conditions, led to the formation of an external TiN layer and an internal layer containing B. The thermodynamic calculations performed proved successful in determining the appropriate conditions for boride coating deposition and estimating the phases likely to be formed. Finally the effect of surface roughness on the coating quality is discussed.     

电热爆炸喷涂法原位合成MoSi2涂层的试验研究

利用电热爆炸喷涂设备,在低碳钢基体上制备了MoSi:涂层.对涂层进行了显微组织、相结构、元素分布及显微硬度分析.结果表明,涂层中存在树枝晶、柱状晶和等轴晶,晶粒细小致密均匀.涂层主要由MoSi2相组成,并含有微量的Mo5si3相.涂层与基体间存在着原子扩散现象,为冶金结合.涂层显微硬度为1000～1400 HV0.1.