The effects of ultrasonic nanocrystal surface modification (UNSM) on pack aluminizing for the fabrication of Pt-modified aluminide coatings at low temperatures

An 8–9 μm thick Pt layer was coated on a superalloy and transformed to a Ni–Pt alloy layer by the interdiffusion of Ni and Pt at 1050 °C for 3 h. The surface of the Ni–Pt alloy layer was pack aluminized to form a Pt-modified aluminide coating. Ultrasonic nanocrystal surface modification (UNSM) was applied to the alloy layer prior to pack aluminizing. The effects of UNSM on Pt-modified aluminide coatings fabricated at 750, 850, 950, and 1050 °C were studied. The treated Ni–Pt alloy layers had finer grain sizes than the untreated specimens. In addition, UNSM made the grain size of the Ni–Pt alloy finer and reduced the surface roughness. During pack aluminizing, the Pt-modified aluminide coatings fabricated following UNSM uptook more Al and were thicker than the untreated Pt-modified aluminide coatings at the various temperatures (750, 850, 950, and 1050 °C). The untreated Pt-modified aluminide coatings with pack aluminizing performed at 750 and 850 °C were composed of only a two-phase (NiAl + PtAl₂) layer, due to insufficient diffusion of Pt at the lower temperatures. However, two-phase and one-phase (NiAl) layers were obtained in the treated Pt-modified aluminide coatings which were pack-aluminized at 750, 850, 950, and 1050 °C, due to the diffusion of Pt through the greater amount of grain boundaries and increased volume generated by UNSM before the pack aluminizing. Additionally, the treated coatings had smoother surfaces even after the pack aluminizing. During cyclic oxidation at 1150 °C for 1000 h, the treated Pt-modified aluminide coatings aluminized at relatively low temperatures (750 and 850 °C) showed better cyclic oxidation resistance than the untreated Pt-modified aluminide coating aluminized at 1050 °C.     

Cyclic oxidation of Pt/Pd-modified aluminide coating on a nickel-based superalloy at 1150 °C

Pt-, Pd-, and Pt/Pd-modified aluminide coatings were prepared on Inconel 738LC by pack aluminizing at 1034 °C. During pack aluminizing, Pt-modified aluminide coating formed a two-phase β-NiAl + PtAl₂ layer and a β-NiAl layer on an interdiffusion zone, whereas Pd- and Pt/Pd-modified aluminide coatings formed only the thicker β-NiAl layer. However, Pd-modified aluminide coating had many pores. During cyclic oxidation, Pt/Pd-modified aluminide coating had a surface that was less rumpled than that of Pt-modified aluminide coating due to its thicker thickness. Pt/Pd-modified aluminide coating had a 22% greater Al-uptake than Pt-modified aluminide coating. Cyclic oxidation tests at 1150 °C showed that Pt/Pd-modified aluminide coating had the best cyclic oxidation resistance. After the cyclic oxidation, an additional γ-Ni phase was seen beneath the outermost alumina scale on the the γ′-Ni₃Al phase in Pt/Pd-modified aluminide coating. The γ-Ni phase, which had a higher Cr content, increased the adhesion and stability of the alumina.     

A study on the microstructure and cyclic oxidation behavior of the pack aluminized Hastelloy X at 1100 °C

A pack aluminizing process at 950 °C for 9 h has been employed on the nickel-base superalloy Hastelloy X to deposit a 75 μm thick β-NiAl aluminide layer on the surface. A nanoscale dendritic structure is observed on the surface of the aluminide coating. A finger-like interdiffusion zone is found between the aluminide layer and the substrate. Fine precipitates with complex phases are distributed in the NiAl layer. The cyclic oxidation tests of aluminized alloys and untreated substrates were conducted at 1100 °C for 196 h. It was observed that the aluminizing process greatly enhances the cyclic oxidation resistance of Hastelloy X at 1100 °C due to a dense and protective alumina layer formed on the surface. Complex phase transformation reactions occurred in the aluminide layer. Owing to the oxidation and interdiffusion reactions at high temperature, the Al content of the NiAl layer was depleted to form some low Al containing γ-substrate grains on the surface and a continuous γ layer between the aluminide layer and substrate. Thermal stress induced, transverse cracks in the interdiffusion zone, were observed possibly due to the difference of thermal expansion coefficients among the substrate, aluminide layer and interdiffusion zone.     

Microstructural aspects of plain aluminide and Pt-aluminide coatings on Ti-base alloy IMI-834

One of the major limitations of the near-α titanium alloys such as IMI-834 and IMI-829 that is currently restricting their use at high temperatures is their poor oxidation resistance. Several protective coatings including diffusion aluminide coatings are currently being examined to enhance the oxidation performance of these alloys in the temperature range of 600–750 °C. In the present study, various microstructural aspects of plain aluminide and Pt-aluminide coatings on Ti-base alloy IMI-834 have been studied. Plain aluminide coating shows a single layer consisting of only Al₃Ti phase in the as-aluminized state. Pt-aluminide coating consists of three layers, namely an outer platinum-rich layer, an intermediate Al₃Ti layer, and the inner interdiffusion layer. The above structure of Pt-aluminide coating on IMI-834 alloy is very similar to that reported on Ni-base superalloys. It has further been found in the present study that the structure of Pt-aluminide coating depends on its Pt and Al contents. The nature of such dependence is similar to that reported for Pt-aluminide coatings on Ni-base superalloy substrates.     

分散剂Y2O3对渗铝层的影响

通过改变传统渗剂来研究分散剂（Al2O3，Y2O3）在渗铝过程中的作用和对渗铝层的影响。利用XRD和SEM对渗层相的组成、氧化前后表面形貌进行分析。结果表明：分散剂Y2O3在最初的渗铝过程中参加了渗铝过程，不仅防止铝粉粘结，而且影响渗层相的组成。分散剂为Al2O3时，渗层主要为Al3Ti＋AlTi＋AlTi3相，而分散剂中含有Y2O3时，渗层土要为AlTi3相，这与传统渗铝机理认为分散剂在渗铝过程中不参加渗铝反应相矛盾。同时，分散剂也影响着渗铝层氧化前后的表面形貌及Al2O3的晶格类型。     

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.     

The Effect of NiAl Coatings on Oxidation Behavior of AISI 403 Stainless Steel

STAINLESS STEEL TYPE AISI403possess a highdegree of resistance to atmospheric corrosion becauseof its ability to form a dense adherent oxide film,whichprotects the material from further attack[1].Thiscomposition was developed to meet the requirementsfor some gas turbine components.The alloy is notparticularly recommended for use in hot corrosion andoxidation environments.Therefore,a protective coatinglayer is essential for such applications as hot sectioncomponents in gas turbine[2].A…     

Corrosion-erosion resistance of Zn-Al co-cementation coatings on carbon steels in aqueous media

A novel Zn-Al co-cementation coating was obtained by a pack cementation method,This coating possesses a two-layered structure,The outer layer is mainly composed of Fe2Al3 and FeAl intermetallics with a small amount of Zn,and the inner layer consists of Zn,Fe and a small amount of Al.The corrosion-erosion resistance of Zn-Al con-cementation coatings on carbon steel was studied by a rotary corrosion method in various NaCl and H2S containing solutions and relevant SiO2 containing media,The experimental results are compared with those of carbon steels and the sherardizing and aluminizing coating ,showing that the Zn-Al co-cementation coating have excellent corrosion-erosion resistance in various aqueous media.     

A study on composite coatings of yttrium ion-plating and pack aluminizing for high-temperature applications

Yttrium (Y) was incorporated by an ion-plating method either before or after pack aluminizing to maximize the corrosion resistance of IN 713C. Various combinations of pack aluminizing and yttrium-ion plating were examined with respect to coating sequence, aluminum activity, and corrosive environment. Of all the various coating combinations examined, the best corrosion resistance was obtained from H/A + Y (high-activity aluminizing + Y-ion plating) type composite coatings. Uniformity of the Y deposition was greatly dependent upon the surface condition of the aluminide-coating layer. The high-activity aluminide coating gave better uniformity of Y deposition than did the low-activity-aluminide coating.Improvement of corrosion resistance by the Y-modified-aluminide composite coatings of H/A + Y type occur because the presence of Y between the Al₂O₃ columns improves Al₂O₃ scale adherence and substantially prevents depletion of Al in the aluminide-coating layer.     

梯度NiCrAlY涂层的1000和1100℃氧化行为研究

采用电弧离子镀技术及后续热处理工艺在镍基高温合金上制备了均匀NiCrAlY涂层和梯度NiCrAlY涂层,分析了2种涂层的组织结构,对比研究了2种涂层静态空气下1000和1100℃恒温氧化行为以及1100℃的循环氧化行为.结果表明:均匀NiCrAlY涂层由γ′/γ相和少量β-NiAl相、α-Cr相组成,成分分布均匀;梯度NiCrAlY涂层具有外层富Al和内层富Cr的结构,其中外层由β-NiAl相和少量γ′/γ相、α-Cr相组成.一方面,梯度涂层的初始Al含量较高;另一方面,氧化过程中其富Cr区两侧出现了对富Al区的Al向基体扩散起阻碍作用的Cr(W)析出带.这两方面使梯度涂层长时间维持更多的Al存储相,提升了氧化膜的迅速生成及再生成能力,从而使涂层具有较好的抗氧化性能.     

Y对Nb基体表面包埋渗铝涂层微观组织和生长机制的影响

通过向包埋渗铝剂中添加Y₂O₃粉末在纯Nb基体表面制备了Y改性的渗铝涂层,研究了Y对涂层微观组织和生长机制的影响。结果表明,Y对涂层的相组成和NbAl₃相的晶粒形态均无明显影响。随包埋剂中Y₂O₃添加量的增加,涂层表面的Y含量升高。Y改性后,涂层的生长机制由受Al原子的沿晶扩散控制转变为受Al原子的晶内扩散控制,降低了涂层的生长速率,并使NbAl₃相的柱状晶区形成了<010>//ND和<110>//ND的2种丝织构。     

Synthesis and oxidation behavior of platinum-enriched γ + γ′ bond coatings on Ni-based superalloys

Simple Pt-enriched γ + γ′ coatings were synthesized on René 142 and René N5 Ni-based superalloys by electroplating a thin layer of Pt followed by a diffusion treatment at 1150-1175 °C. The Al content in the resulting γ + γ′ coating was in the range of 16-19 at.% on superalloys with 13-14 at.% Al. After oxidation testing, alumina scale adherence to these γ + γ′ coatings was not as uniform as to the β-(Ni,Pt)Al coatings on the same superalloy substrates. To better understand the effect of Al, Pt and Hf concentrations on coating oxidation resistance, a number of Ni-Pt-Al cast alloys with γ + γ′ or β phase were cyclically oxidized at 1100 °C. The Hf-containing γ + γ′ alloys with 22 at.% Al and 10-30 at.% Pt exhibited similar oxidation resistance to the β alloys with 50 at.% Al. An initial effort was made to increase the Al content in the Pt-enriched γ + γ′ coatings by introducing a short-term aluminizing process via chemical vapor deposition or pack cementation. However, too much Al was deposited, leading to the formation of β or martensitic phase on the coating surface.     

Influence of Manufacturing Route on the Oxidation Resistance of Platinum-Modified Aluminide Bond Coatings and Their Performance in Thermal Barrier Coatings Deposited on a Ni-Based Superalloy

The importance of manufacturing route of Pt-aluminide bond coatings in determining the life of thermal barrier coatings on Ni-based superalloys is demonstrated. It is shown that bond coatings aluminized by chemical vapor deposition exhibit higher resistance to oxidation in comparison with the pack cementation route, which results in about twofold increase in the life of the thermal barrier coating as determined under cycling oxidation conditions. This difference in behavior is correlated with the initial microstructures of the bond coatings and their effect on thermal stability and oxidation resistance. However, thermal barrier coatings utilizing the two types of bond coatings are found to fail by the same mechanism involving spallation of the top coating due to loss of adhesion between the thermally grown oxide and underlying bond coating. It is concluded that manufacturing by the CVD route with low Al activity decelerates the kinetics of the processes leading to degradation of the coating system in comparison with the pack cementation route with high Al activity.     

Si-Al COATING ON PURE MOLYBDENUM SUBSTRATE AND ITS CYCLIC OXIDATION BEHAVIOR

The halide-activated pack cementation method is utilized to codeposit aluminum and silicon on Mo substrate. Emphasis is placed on the microstructure and elevated-temperature oxidation resistance of coatings. The results show that hexagonal Mo (Si, Al)2 as a main phase and a little amount of the lower disilicide Mo3Si3 was formed on Mo substrate through the halide-activated pack cementation method. The resultant Si-Al coating on Mo substrate exhibits excellent cyclic oxidation resistance. The excellent cyclic oxidation resistance of the coatings is attributed to the formation of alumina on the coatings during the oxidation.     

DZ40M钴基合金铝化物涂层的循环氧化

以新型定向凝固钴基高温合金ＤＺ４０Ｍ为基体,研究其低压化学气相沉积铝化物涂层的循环氧化行为,发现该涂层具有较高的抗循环氧化性能,涂层与基体结合良好。涂层退化主要是由外表面氧化膜的愈合消耗Ａｌ源所造成,沉积渗剂中加入Ｔｉ可加速涂层的退化。     

OXIDATION RESISTANCE OF NANOCRYSTAL ODS ALUMINIDE COATINGS PRODUCED BY PACK ALUMINIZING PROCESS ASSISTED BY BALL PEENING

Nanocrystal ODS （oxide dispersion strengthening） aluminide coatings were produced on a stainless steel and nickel-based superalloy by the pock aluminizing process assisted by ball peening, Pure Al powders and 1% of ultra-fine Y2O3 powders were mixed by ball milling. The ultra-fine Y2O3 powders were dispersed in Al particles. Ball peening welded the Al particles onto the substrate and accelerated the formation of aluminide coating. Nanocrystal ODS aluminide coatings were produced by the outward growth at a much low temperature （below 600℃） in a short treatment time. The effects of the operation temperature and treatment time on the formation of the coatings were analyzed. SEM （scanning electron microscope）, AFM （atomic force microscope）, EDS （energy dispersive X-ray spectroscopy）, XRF （X-ray fluorescence spectrometer） and XRD （X-ray diffraction） methods were applied to investigate the microstructure of the coatings. High-temperature oxidation tests were carried out to evaluate the oxidation resistance of the ODS aluminide coatings.     

Pd—Ni—Al涂层的循环氧化和在Na2SO4熔盐中的热腐蚀行为

研究了底温高活性（ＨＴＨＡ）Ｐｄ－Ｎｉ－Ａｌ涂层在１０５０℃人的循环氧化和Ｎａ２ＡＳＯ４熔盐中的热腐蚀结果表明,Ｐｄ－Ｎｉ－Ａｌ涂层表面生成一层致密的氧化膜,抗热腐蚀性能接近Ｐｔ－Ａｌ涂层,优于单渗铝涂层,初步分析了Ｐｄ－Ｎｉ－Ａｌ涂层的高温腐蚀机理。     

加速固体粉末渗铝的两段法新工艺

采用金相显微镜、扫描电子显微镜、X射线衍射仪、显微硬度计等仪器，研究了经两段法固体粉末渗铝工艺处理后，HK40钢渗铝层的组织和性能。试验结果表明，该渗铝层是由NiAl相和Ni3Al相组成，与传统固体粉末法渗铝工艺相比，具有渗速快、渗层显微硬度过渡平缓、渗层表面质量好等特点，并对加速渗铝过程的原因进行了初步探讨。     

A Controlled New Process of Pack Aluminization

IRON ALUMINIDES exhibit excellent corrosionresistance in aggressive atmospheres at elevatedtemperature'1"41,which enables steels or alloys to beprotected by Fe-Al intermetallic coatings.However,itshould be borne in mind that aluminides with a highaluminum content—FeAl3,Fe2Al5—tend to embrittlethe coated layer and reduce its oxidation resistance.Bycontract,iron-rich iron aluminides possess excellentmechanical properties and oxidation resistance15"61.Aluminum diffusion coatings are oft…     

Microstructures and cyclic oxidation behaviour of Pt-free and low-Pt NiAl coatings on the Ni-base superalloy Rene-80

Aluminizing of bare and 3 μm-Pt-electroplated specimens has been utilised to prepare NiAl and low platinum (Ni,Pt)Al coatings. Cyclic oxidation of the coatings was investigated by exposing samples to 1 h cyclic oxidation at 1100 °C. The modified coating exhibited an external layer of NiAl-25 vol.% PtAl₂ above a three-zone structure. This structure endured over the whole testing time, while the NiAl coating failed after 77 cycles. The (Ni,Pt)Al coating did not reduce the scale growth rate, but it improved scale adhesion. In addition, Pt limited the outward diffusion of Ti from substrate and hence prohibited formation of undesirable TiO₂.