Pack Cementation Research Articles

Study on Microstructure and Oxidation Resistance Mechanism of Y-Modified NiCrAlY Coating Prepared by Pack Cementation

The existence mode of Y is an important factor affecting the oxidation resistance of NiCrAlY coating. The traditional thermal sprayed NiCrAlY coating had some problems such as internal-stress-induced coating peeling, when it was used at high temperature for a long time. In this paper, Y-modified NiCrAlY coatings were prepared on the surface of thermal-sprayed NiCrAlY coatings by halide-activated pack-cementation technology. The microstructure and oxidation resistance of the coatings after thermal insulation at 1100 °C for 100 h were analyzed. The results show that the oxidation resistance of Y-modified NiCrAlY coatings was improved significantly. When the oxidation temperature and time were 1100 °C and 100 h, the oxidation weight gain of thermal sprayed NiCrAl0.5Y and NiCrAl1.0Y coatings was 3.8 and 2.2 times that of Y-modified coatings, respectively. In addition, the NiCrAlY coatings prepared by pack cementation had better oxidation resistance than thermal sprayed coatings when the content of Y was equal. The oxidation resistance of the pack-cemented NiCrAlY coatings were increased with the increase in Y content when it was in a range of 0~1 wt.%. When oxidized at 1100 °C for 100 h, the oxidation weight gain of the pack-cemented NiCrAl0.5Y and NiCrAl1.0Y coatings was about 1.8 and 0.9 mg/cm2, respectively. The formation of highly stable intermetallic compounds Ni5Y in the pack-cemented Y-modified coatings is concluded to be the reason for improving the oxidation resistance of the coatings. This work can provide a solution to improve the oxidation resistance of traditional MCrAlY coatings.

Mechanism of accelerating creep rupture under high stress level of pack cementation aluminide coatings deposited onto T92 steel tube

An aluminide coating was deposited on commercial T92 steel by pack cementation. The coating exhibits a typical bilayer structure composed of an outer FeAl layer and an inner interdiffusion layer. The creep rupture life of the coated and uncoated T92 was investigated. Results show that the rupture life of the coated T92 decreases significantly, i.e., decreases by 46 % at the creep stress of 110 MPa compared with the uncoated T92. By examining the microstructure and fractography of the coated specimen, the mechanism of aluminide coating deteriorating creep rupture life is clarified. The main fracture mode of T92 substrate with and without coating is microvoid accumulation fracture. The characteristic microstructure of the aluminide coating is responsible for the premature creep rupture. The formation of columnar grains and brittle nature of FeAl phase make it tend to crack during creep. The phase transformation and needle-like AlN precipitates in the inner interdiffusion layer contribute to the inward propagation of cracks and accumulation of creep damage.

Improvement of High-Temperature Oxidation Resistance of Iron-Base Heat Storage Materials by Aluminizing Using Pack Cementation Method

Improvement of High-Temperature Oxidation Resistance of Iron-Base Heat Storage Materials by Aluminizing Using Pack Cementation Method

Synthesis and thermoelectric properties of Cr-substituted higher manganese silicides prepared by pack cementation

Higher manganese silicides are attractive low-cost thermoelectric materials finding application in middle to high-temperature power generation. However, for practical use their performance needs to be enhanced. Pack cementation is a simple, green and economic method applied so far on the preparation of pure thermoelectric metal silicide powders. In this study, pack cementation is employed for the synthesis of doped thermoelectric powder for the first time as a promising single-step process. Compounds Mn1-xCrxSi1.80 (0.02≤x ≤ 0.15) at which Cr substitutes Mn site in the matrix are prepared and examined regarding their composition, microstructure and performance. Considerable amounts of CrSi2 impurity are present for x ≥ 0.07. The highest thermoelectric efficiency is achieved for Cr 4% substitution of Mn leading to the peak figure of merit ZTmax=0.59 at 814 K (∼35% enhancement). The measured oxidation resistance in this temperature region is high. Pack cementation has been successfully utilized to synthesize high-performance Cr-substituted higher manganese silicide powder.

The Diffusion Coatings for Industrial Tool Application

The diffusion boriding process allows to increase the abrasion and corrosion resistance of the majority of steel grades. The aim of this study was to determine the influence of the chemical composition of the substrate of various steel grades on the microstructure of boride coatings produced using two diffusion methods: pack cementation boriding using EKABOR-2 and paste-pack boriding using EKABOR-PASTE on the substrate of tool and structural steels:. The boriding processes were carried out at 1000oC in an argon atmosphere for 4h. Scanning microstructure investigations of the obtained coatings indicate that a high content of alloying elements increasing the FeB (Cr, Mo, W) phase, results in the formation of an external, continuous layer of FeB borides. It was found that with increasing alloying element and carbon concentration, the total thickness of the boride coatings decreases. In particular, a chromium content below about 1%, with a carbon content below about 0.4%, significantly limits or prevents the formation of the FeB phase. Increasing the content of alloying elements and carbon, results in a change in the morphology of the iron borides.

Formation and oxidation behavior of SiO2/NbSi2 multilayer coating fabricated by one-step method

SiO2/NbSi2 multilayer coatings were prepared on Nb alloy to enhance its high-temperature oxidation resistance by utilising nano-Al2O3 as the inert filler in halide-activated pack cementation (HAPC). In this study, the effect of nano-Al2O3 on the morphology and formation of the silicide coating was studied. Moreover, the oxidation resistance of the SiO2/NbSi2 multilayer coating and single NbSi2 coating were investigated. The results revealed that nano-Al2O3 promotes the formation of the SiO2 layer during HAPC. After oxidation at 1250 °C for 100 h, the weight gain of the SiO2/NbSi2 multilayer coating was only 7.20 mg/cm2, which was significantly lower than that of the single NbSi2 coating. The SiO2 outer layer effectively prevents the inward diffusion of oxygen at 1250 °C; however, it quickly transformed into a porous layer of mixed oxides at 1350 °C owing to the outward diffusion of Nb and crystallisation of SiO2.

Effects of the La2O3 addition contents on the ablation performance of in-situ La-doped ZrC-SiC-ZrSi2 coating for C/C-ZrC-SiC composites

The C/C-ZrC-SiC composites coated by the La-doped ZrC-SiC-ZrSi2 coatings containing different La contents were fabricated by reactive melt infiltration (RMI) combined with gradient pack cementation in a one-step process. The composites' ablation behaviors under the oxyacetylene flame were comparatively studied. The effects of the La2O3 addition contents on the microstructure and phase composition evolution on the surface of the composites caused by ablation were investigated. The results indicated that the C/C-ZrC-SiC composites with an in-situ La-doped ZrC-SiC-ZrSi2 coating prepared by the mixed powders with 15 wt% La2O3 (15LZS) showed a better ablation performance. After ablation for 120 s, their mass and linear ablation rates were − 0.62 ± 0.15 mg/s and − 2.42 ± 0.24 μm/s, respectively. Such good ablation resistance should owe to appropriate La contents on the surface of the 15LZS. During ablation, the La elements could promote the formation of a dense oxide layer, which was composed of the m-ZrO2, t-ZrO2, and Zr0.9La0.1O1.95 phases. It could efficiently inhibit oxygen diffusion and endure the mechanical denudation of the oxyacetylene flame.

Oxidation Behavior of Si-Modified Aluminide Coatings on K438 Superalloy Prepared Using a Hybrid Slurry/Pack Cementation Process

A Si-modified aluminide coating was prepared on nickel-based K438 superalloy using a hybrid slurry/pack cementation process. The microstructures indicated that the coatings exhibited a two-layer structure: the outer Al-rich Ni0.9Al1.1 layer with the dispersion of minor Ni2Al3 and Cr-rich phases of Al13Cr2 and Cr5Si3 and the inner interdiffusion layer. For comparison, simple aluminide coatings were also produced using normal pack cementation. Compared with the K438 superalloy and simple aluminide counterpart, the Si-modified aluminide coatings exhibited a lower oxidation rate and improved alumina scale adhesion in air at 1,100°C. Furthermore, the Si-modified aluminide coatings showed longer service life compared to the simple aluminide coatings because the addition of Si mitigated the degradation of coatings. Therefore, the simple aluminide lost protection while the Si-modified aluminide coatings were still protective after 300 h.

Microstructure and oxidation resistance of in-situ silicide composite coating on Mo-2.5Zr-2.5ZrO2-xW (x=0, 20, 30) substrate

In-situ silicide composite coatings were prepared on Mo-2.5Zr-2.5ZrO2-xW (x=0, 20, 30) substrates by pack cementation technique. The microstructure evolution, oxidation behavior and oxidation resistance mechanism of the composite coatings at 1300 °C were studied. The experimental results revealed that the coating thickness increases from 89 μm to 144 μm when the W content in the substrate increases from 0 at.% to 30 at.%. During the oxidation process, due to the presence of many cracks in the MoSi2 coating on the Mo-2.5Zr-2.5ZrO2 substrate, it was rapidly oxidized to form Mo5Si3 and volatile MoO3 in turn. When the oxidation time reached 150 h, the mass of MoSi2 coating increased to −937.19 mg/cm2, and the sample was completely destroyed. The mass gain of the coating on Mo-2.5Zr-2.5ZrO2-30W substrate (mainly (Mo,W)Si2) was only 2.86 mg/cm2 after 210 h cyclic oxidation. In the process of high temperature oxidation, the molten SiO2 oxide film with flowability formed on the surface can well isolate oxygen, while the presence of high melting point ZrO2 particles can form pinning effect on the surface of the oxide film, which limits the irregular flow of molten SiO2, and helps to improve the high temperature oxidation resistance of the coating.

Investigation of the Preparation, Corrosion Inhibition, and Wear Resistance of the Chromized Layer on the Surfaces of T9 and SPCC Steels.

To improve the corrosion inhibition and wear resistance of materials, the pack cementation method was used to prepare chromized coatings on the surfaces of high-carbon T9 steel and low-carbon SPCC steel. The results showed the formation of a uniform and dense double-layer structure with a thickness of ~10 μm on the surfaces of two different types of steel. The coating layer for T9 steel was mainly composed of Cr23C6 and Cr7C3, while that for SPCC steel was mainly composed of Cr23C6 and Fe-Cr solid solution. Additionally, both of the steels showed different hardness distributions. The hardness measurements of the outer layers of the T9 steel and SPCC steel were ~1737.72 HV and 1771.91 HV, while the hardness values of the secondary layers were 1378.31 HV and 448.52 HV, respectively. The polarization curves in 3.5 wt.% NaCl solution demonstrated the better corrosion resistance of the chromized coating. Chromizing increased the corrosion potential by ~0.2 V and reduced the corrosion current density by one order of magnitude. Under the presence of an 8 N load, the friction factor before and after the chromizing of T9 steel was about 0.69, and the mass wears were 2 mg and 0.6 mg, respectively. Meanwhile, the friction factor of the SPCC steel before and after chromizing was about 0.73, with respective mass wears of 2 mg and 2.9 mg. The wear resistance of T9 steel after chromizing was superior, but it became worse after chromizing for the SPCC steel.

Influence of oxidation protective SiAl coatings on the tensile and fatigue behavior of Ti–48Al–2Cr–2Nb at elevated temperatures

This study considers the influence of novel oxidation protective SiAl-coatings on the tensile and fatigue behavior of the γ-Titanium Aluminide alloy Ti–48Al–2Nb–2Cr. The SiAl-coatings were applied by pack cementation (PC) and magnetron sputtering (MS) processes. After coating, the specimens were pre-oxidized for 300 h at 850 °C. As reference, bare untreated and bare pre-oxidized specimens were tested. In ambient temperature tensile tests, pre-oxidation reduced both ultimate tensile strength and elongation. In comparison, PC-coated specimens showed higher ultimate tensile strength and elongation indicating that the coatings can effectively prevent embrittlement due to surface oxidation. The fatigue lifetimes of PC-coated, MS-coated and pre-oxidized specimens reduced drastically at higher stress amplitudes. Damage observations indicate that pre-oxidizing facilitates early crack initiation in the embrittled surface layer above a certain transition stress amplitude. Both coating types can attenuate embrittlement of the surface layer. However, the coatings themselves tend to initiate early cracks above a transition stress amplitude leading to a lifetime behavior similar to pre-oxidized specimens. Below the respective transition stress amplitude, the lifetimes of both coated specimen types reach almost those of bare untreated specimens. The results suggest that the benefits of the coatings in the form of reduced oxidation and embrittlement outweigh the slight fatigue life penalty at service relevant low stress amplitudes.

Design and preparation of MoSi2/SiO2-Nb2O5/NbSi2 multilayer coating on Nb alloy: Microstructure and hot corrosion behavior

A MoSi2/SiO2-Nb2O5/NbSi2 multilayer coating was designed on Nb alloys using halide activated pack cementation (HAPC) and an innovative liquid plasma-assisted particle deposition and sintering (LPDS) method to improve its hot corrosion resistant. The weight gain of MoSi2/SiO2-Nb2O5/NbSi2 coating was 3.204 mg/cm2 after 200 h, which was 85.7% lower than that of NbSi2/Nb2O5-SiO2 coating, while the life of NbSi2 coating was restricted only 100 h (20.539 mg/cm2) at 900 °C in Na2SO4 +NaCl salt. This improved performance is attributed to the prefabricated SiO2-based barrier layer and MoSi2 as a Si source to supplement the SiO2 content, thus effectively alleviating the penetration of oxygen and molten salts.

Microstructure Investigation and Cyclic Oxidation Resistance of Ce-Si-Modified Aluminide Coating Deposited by Pack Cementation on Inconel 738LC

The synergistic effect of Si and Ce addition on the oxidation resistance of a pack cementation aluminide coating applied on a Ni-based IN738LC superalloy substrate was investigated in this study. The structural and thermal influences of both Si and Ce, focusing on morphology, oxidation behavior, and scale spallation tendency, are accordingly discussed based on the experimental results using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray analyses (EDX). For this purpose, the oxidation resistance of the modified coatings was evaluated by measuring the weight gain of the coated samples after 16 h for each cycle at 1100 °C for a total of 50 cycles of the oxidation process. The investigations indicated that Si addition to the modified aluminide coating improves the oxidation resistance through the formation of β-NiAl and δ-Ni2Al3 phases, and also δ-Ni2Si phases. Furthermore, the addition of 1% Ce to the modified aluminide coating enhances the formation of the fine-grained microstructure of the β-NiAl and δ-Ni2Al3 and reduces the outward/inward diffusion of elements (so-called blocking effect), which significantly modifies the cyclic oxidation resistance. The oxidation enhancement also may be attributed to synergistic effects of Ce and Si addition during the deposition process that reduce the inward oxygen diffusion and reduce the growth rate of α-Al2O3 during oxidation tests.

Enhanced High Temperature Oxidation Resistance of the Conventional Ti-48Al-2Nb-2Cr Alloy through Carbon Pack Cementation and Analysis of the Carbon Diffusion Path

γ-TiAl alloys have attracted interest as lightweight materials for structural applications at temperatures below 800 o C. With recent improvements in microstrutural control of the γ-TiAl alloys, resulting in excellent creep properties and high temperature strength, their use can be extended to temperatures higher than 800 o C. To increase their application temperature range, the oxidation resistance of γ-TiAl alloys must be improved through alloy design or coating techniques. To improve the high temperature oxidation resistance of γ-TiAl alloys, it is important to suppress the growth of TiO2 oxide by forming a dense coating layer to block oxygen atoms. In this study, we tried to improve the high temperature oxidation resistance of a conventional γ-TiAl alloy, Ti-48Al-2Nb-2Cr (Ti 4822) using a pack cementation process, with carbon sources and activating materials, XRD and EDS analysis results for the surface products indicated the formation of coating layers comprising oxides and carbides including TiC, Al4C3, Ti2AlC, and Ti3AlC. Based upon the sequential formation of coating layers and phase diagrams, the diffusion path of the carbon during the pack cementation process is proposed. Also, detailed carburization and oxidation behaviors of the γ-TiAl alloys in the temperature range of 800-1000 o C are discussed in terms of microstructural observations. Enhanced high temperature oxidation resistance was obtained in Ti-48Al2Nb-2Cr (Ti 4822) through a proper carbon pack cementation process.

Improved hot corrosion resistance of TiBw/TA15 composite via pack cementation aluminide coating

An aluminide coating was prepared on the TiBw/TA15 composite by pack cementation to improve the hot corrosion resistance. After 100 h test, the mass gain for uncoated and coated samples was 39.9 and 2.1 mg cm−2, respectively, which showed that the aluminide coating effectively improved the hot corrosion resistance. For the uncoated sample, the circulation of sulfurization/chlorination–oxidation and the basic dissolution with basicity/acidity change in molten salt caused serious hot corrosion. For the coated sample, the continuous and dense Al2O3 film effectively weakened the basicity/acidity change by preventing the diffusion of S and Cl elements. The high Al content in the TiAl3 phase could maintain the continuous Al2O3 film, although parts of it could be dissolved by molten Na2SO4.

ZrSi2-SiC/SiC Gradient Coating of Micro-Structure and Anti-Oxidation Property on C/C Composites Prepared by SAPS

A ZrSi2-SiC/SiC gradient coating system was designed to reduce the thermal stress of anti-oxidation coatings for C/C composites and prolong their anti-oxidation time at a high temperature. The SiC transition layer was prepared by pack cementation and the gradient ZrSi2-SiC outer coatings with different ZrSi2 contents added were deposited by supersonic air plasma spraying (SAPS). The micro-morphologies and phase compositions of the coatings were studied by SEM, EDS, XRD, and TG/DSC, and their anti-oxidation performances were tested by a static oxidation experiment. The findings suggested that the gradient coating with 30 wt.% ZrSi2 content displayed the optimum and dense microstructure without obvious pores and microcracks, compared with the other three proportional coatings. During the oxidation test, because of the oxidation reaction of ZrSi2 and SiC phases, a large amount of silica was formed in the coating to fill the pores and microcracks and densify the coating further. Oxidation products ZrO2 and ZrSiO4, having a high melting point and outstanding anti-oxidation property, were embedded in the SiO2 glass layer to reduce the layer volatilization rate and improve the ability to block oxygen. Therefore, the specimen with 30 wt.% ZrSi2 still kept mass gain after 188 h oxidation time at 1500 °C. However, when the oxidation time was increased to 198 h, it had a mass loss of 0.1%, because the coating compactness was destroyed by the escape of the oxidation gases.

Development of Cr and Al Pack Cementation Coatings on Co-Based γ/γ′ Superalloys

Co-based superalloys have been developed as candidate materials to replace Ni-based superalloys in hot sections of turbine engines, however, their oxidation resistance is limited. Therefore, in this work chromium and aluminum diffusion coatings were developed via the pack cementation process for novel γ/γ′-strengthened superalloys of the Co–Ni–Al–W system. This alloy system also offers the opportunity to vary the cobalt-to-nickel ratio and the tungsten content systematically to investigate their influence on the coating process. The coating process was shown to successfully enrich the surfaces of such alloys either in aluminum or chromium. For Al, the result was the formation of (Co, Ni)Al on top of an interdiffusion zone, while for chromium a Cr-rich layer in solid solution with the substrate along with the formation of σ-phase were observed. The coating formation mechanisms are discussed for both coatings in the light of activities and phase formation.

Development and oxidation behavior of high entropy silicide (NbMoTaWV)Si2 coatings on NbMoTaWV alloy

A novel high entropy silicide (NbMoTaWV)Si2 coating was prepared on a NbMoTaWV high entropy alloy by silicon pack cementation. The single-phase structure of (NbMoTaWV)Si2 was obtained due to the high entropy effect and sluggish effect of NbMoTaWV refractory high entropy alloy. The processing parameters could affect the coating structure and thickness. Low temperature favored the formation of single-layer (NbMoTaWV)Si2 layer while high temperatures (1200 ℃ and 1300 ℃) produced two layers coating, (NbMoTaWV)Si2 layer and (NbMoTaWV)Si2/(NbMoTaWV)5Si3 mixed interdiffusion layer. Coating with reasonable thickness and minimized cracks could be obtained under proper processing parameters. The oxidation test of coating specimens was conducted at 1200 ℃ for up to 96 h. The high entropy of (NbMoTaWV)Si2 suppressed the generation of metallic oxides and promoted the formation of dense and intact SiO2 layer, which largely improved the oxidation resistance of NbMoTaWV. Failure in corners was observed in most coating specimens after exposed for 48 h, and the failure mechanism was also discussed.

Aluminide Diffusion Coatings on IN 718 by Pack Cementation.

This paper addressed the issues of both direct and indirect synthesis of Ni aluminides by pack cementation (pure Ni and IN 718 superalloy). On the Al-Ni diffusion twosome under pressure, at temperatures below and above the Al melting temperature, the appearance and evolution of diffusion porosity because of the Kirkendall–Frenkel effect manifestation was highlighted. It has been confirmed that, as the temperature rises above the Al melting temperature, the porosity decreases. Nickel-based superalloys, and in particular IN 718, significantly increase their performance by increasing the aluminides proportion in the top diffusion coating. This is made possible by changing the value of the Al and Ni weight percentage ratio in this area (noted as Al/Ni). In the case of the diffusion twosome between IN 718 and pack aluminizing mixtures, having in their composition as active components Al powder, Ferroaluminum (FeAl40) or mixtures of Al and Fe powders, at processing temperatures above the Al melting temperature, by modifying the active component of the mixture, substantial changes in the Al/Ni values were observed, as well as in the maximum %Al in the diffusion coating and of its thickness. It was found that, when switching from Al to FeAl40 or powder mixture (Al + Fe), the Al/Ni value changes between 3.43 and 1.01, from initial subunit values. The experiments confirmed that the highest %Al in the top aluminized diffusion coating, for IN 718, was obtained if the powder mixture contained 66.34 wt.% Al.

Oxidation behavior of pack-cemented, thermal etched SiC coating with Al–B–C additions

Silicon carbide (SiC) coating with Al–B–C additions (designated ABSC2) was fabricated by pack cementation and subsequent thermal etching (argon and nitrogen atmosphere). In addition, the roles of thermal etching on the microstructures and oxidation behaviors for ABSC2 coating were examined. The ABSC2 coatings subjected to thermal etching show superior oxidation resistance to that without thermal etching. Further, the ABSC2 coating subjected to thermal etching in a nitrogen atmosphere (designated ABSC2N) shows the best oxidation resistance of −0.3% (weight gain) after 12 h at 1500 °C. This may be attributed to the elimination of bulk secondary phases and the extra oxidation of the numerous SiC whiskers.