Pack Cementation Research Articles

Dual coating layers to enhance oxidation resistance for refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs) possess excellent high-temperature mechanical properties, making them have the promising potential for applications at temperatures higher than Ni-base superalloys. However, their development has been limited by their poor oxidation resistance, which demands an urgent and effective improvement. In particular, the oxides of RHEAs generally suffer from severe pest phenomenon between 500 °C and 800 °C. In this study, the low-density, strong and ductile BCC Mo0.3Nb1.3TiZr RHEA was designed and prepared, and the isothermal oxidation mechanism was studied at mainly 800 °C.To effectively improve the oxidation resistance of the alloy, dual coating layers with inner-layer aluminide and outer-layer silica protection were designed and investigated. Firstly, pack cementation was used to prepare aluminide coating on RHEA. Secondly, the sol-gel method was used to deposit silica films over the aluminide coating. With single aluminide coating deposited on the base metal, the weight gain was reduced from 92.2 mg/cm2 of uncoated specimen to 4.5 mg/cm2 at 800 °C for 24 h. With the dual coating layers, no pest phenomenon was observed between 700 °C and 900 °C and the weight gain dropped sharply from 88.4 mg/cm2 of aluminized specimen to 2.8 mg/cm2 at 800 °C for 48 h. The aluminide coating and SiO2 film successfully adhered to the surface of the RHEA. In addition, the SiO2 film reduced the consumption of aluminide coating. Cracking, accelerated oxygen penetration, and pest phenomenon were not observed. This dual coating layers are very promising in solving the poor oxidation resistance of RHEAs.

Novel Cr/Si-Slurry Diffusion Coatings for High Temperatures

Surface enrichment in Al, Si, and Cr can greatly improve high temperature oxidation resistance of many alloys. Al, Si, and Cr coatings are commonly applied via simple slurries or more complex pack cementation processes. Due to the high melting point of Cr, the deposition of Cr-based diffusion coatings by the slurry technique has proved challenging, and to date, Cr has mostly been applied by pack cementation. Here, a novel Cr-Si coating process via the slurry technique is described which has been developed and then demonstrated on two Ni-based superalloys, Rene 80 and Inconel 740H. The addition of Si to the slurry lowers the melting point via a Cr-Si eutectic and enables the formation of a liquid phase during heat treatment. Through this Cr-Si slurry coating process diffusion layers enriched by Cr and Si of about 150 µm were achieved. Oxidation behavior was studied through isothermal exposures at 900 °C for 1000 h in lab air. Uncoated Rene 80 and IN740H both showed formation of a Ti-containing Cr2O3 scale below a thin TiO2 top layer. Underneath the external scale a zone of internally oxidized Al grew over the exposure time and reduced the load-bearing cross-section progressively. In comparison, the Cr/Si-coated samples did not show internal Al oxidation, but a slow-growing Si-rich oxide film underneath the external Cr2O3 scale. This subscale represents an additional oxygen diffusion barrier. Thus, the weight gain during exposure for the coated samples was significantly lower than for the uncoated materials.

Aluminide diffusion coatings for improving the pesting behavior of refractory metals

Refractory metal based alloys show great potential as structural materials for high-temperature applications due to their high melting points, but they are prone to catastrophic oxidation at around ▪. The effectiveness of aluminium diffusion coatings on the oxidation resistance of pure molybdenum, niobium, tantalum, and tungsten was investigated. Homogeneous diffusion layers of Al-rich intermetallic phases were formed on all four refractory metals after pack cementation. Thermodynamic calculations and experimental assessments (e.g., of the activator) determined the most suitable conditions for the pack cementations. Subsequent thermogravimetric analysis of the uncoated and coated samples at ▪ and ▪ for up to ▪ in dry synthetic air was used to determine the protective effect of the coatings and investigate the underlying oxidation mechanisms. Optical and electron microscopy, EPMA, XRD and Raman spectroscopy were used to investigate the diffusion layers and the oxide scales. For two refractory metals (Mo and Ta) the aluminization resulted in an improved oxidation behavior and a substantially decreased oxide growth rate compared to uncoated substrates.

Electrodepositing a Mo layer and composite siliconized coating on Nb-based alloy for high temperature protection

In order to protect Nb-based alloy from oxidation attack, a Mo layer was first electrodeposited under different current densities and then its siliconized coating was obtained via halide activated pack cementation (HAPC) process. Microstructure, chemical composition, hardness and adhesive strength of the Mo pre-layer on the Nb-based alloy were analyzed and oxidation behavior of the siliconized coating exposed at 1200 °C and 1300 °C was investigated. The results showed that the electrodeposited Mo layer was composed of amorphous Mo phase and MoO3. The oxygen content in the Mo layer was low as 38.2 at% when the current density during electrodeposition was 400 mA/cm2 with the microhardness as 238 ± 18 HV. The HAPC siliconized coating exhibited a multilayer structure of MoSi2/(Mo, Nb)5Si3/NbSi2, which showed an increase in the cohesive/adhesive strength compared to the Mo layer. Mainly Nb2O5 and SiO2 were formed on the exposed siliconized coating. An accelerated mass gain and a thicker oxidation layer were found as the exposure temperature increased with a thinning of NbSi2 inner layer. The siliconized coating provided an effective oxidation resistance for the Nb-based alloy and the increased exposure temperature made the protection of the coating deteriorate.

Multilayer synergistic design of NbSi2/Nb2O5-SiO2/MoSi2 ceramic coating on niobium alloys for multiple thermal protection properties

A novel NbSi2/Nb2O5-SiO2/MoSi2 multilayer thermal protection ceramic coating is designed and fabricated via halide-activated pack cementation and liquid-plasma-assisted particle deposition and sintering technology, endowing the coating with multiple high-temperature protective properties. The incorporation of MoSi2 nanoparticles not only gives superior high-temperature oxidation resistance/hot corrosion resistance of coating by forming a dense SiO2 barrier layer, but also promotes passive radiative heat dissipation. When NbSi2/Nb2O5-SiO2/MoSi2 coating was prolonged exposure to air at 1250 °C for 50 h of oxidation and at 900 °C for 100 h of corrosion, the mass gains are only 0.37 mg/cm2 and − 0.52 mg/cm2, respectively. The multilayer coating exhibits enhanced emissivity (>0.88) over the thermal infrared range of 3–20 µm, due to the lattice vibrational absorption-matching synergy of Nb2O5, SiO2, and MoSi2, as well as the mutual doping of SiO2/Nb2O5, mismatch of atoms at the interface, and a high degree of lattice distortion within the coating.

Synthesis, protection performance, and failure analysis of α-Al2O3/Mo(Si,Al)2 coatings

To prepare the advanced cooling structure of hollow turbine blades, molybdenum metal core (MMC) has been proposed as a substitute for ceramic core substrate, but the protective coating needs to be redesigned to resist ablation during the shell burning process and erosion during the casting process. To this end, in this study, an α-Al2O3/Mo(Si,Al)2 barrier coating is fabricated on a molybdenum surface utilizing a two-step pack cementation process, which includes first siliconizing and then aluminizing, combined with pre-oxidation treatments. The pre-oxidation behavior of the new Mo(Si,Al)2 coating under isothermal oxidation (at 1300 °C) and step oxidation and its protection performance during the superalloy casting process are comprehensively examined. The results indicate that the as-deposited coating consists of an outer Mo(Si,Al)2 layer and an inner Al8Mo3 layer. After pre-oxidation, an admixture (SiO2▪Al2O3) monolayer is first formed on the surface of MMC. Then, internal Al2O3 emerges in the lower part of (SiO2▪Al2O3) mixture. Finally, the SiO2 → Al2O3 displacement reaction encapsulates MMC with an exclusive α-Al2O3 layer. Besides, the step oxidation process can obviously enhance the formation of external α-Al2O3 compared to the isothermal oxidation process. Unfortunately, after pre-oxidation treatment, a bulge arises at the interface of Mo(Si,Al)2 and Al8Mo3. The buckled coating induces partially destruction of the MMC coating by the alloy melt, whereas the intact coating can protect MMC during the casting process. Furthermore, the interfacial gap of MMC varies with the addition of alumina.

A novel gradient CrSi2-ZrSi2-SiC-Si coating for long-term oxidation protection of C/C composites at 1773 K

A novel gradient CrSi2-ZrSi2-SiC-Si coating for carbon/carbon (C/C) composites was prepared by optimized pre-oxidation and two-step pack cementation (PC) process. The microstructure and oxidation behavior of the coating were investigated in comparison to SiC-Si coatings. The results showed that the composite coating is mainly composed of CrSi2, ZrSi2, SiC and Si phases with a certain composition gradient, but no sharp interface. The coated specimens had a 4.83 mg/cm2 weight gain at 1773 K oxidation for 604 h, providing better oxidation resistance than the SiC-Si coated specimens (9.3 mg/cm2, 225 h). Its excellent performance was mainly attributed to the formation of glass and crystalline phases such as SiO2, ZrO2, Cr2O3 and ZrSiO4, which provided better oxygen barrier ability and structural stability than the single SiO2 glass. In addition, the maximum weight gain and steady-state linear weight loss rate of the coating during the oxidation process were related to long-term anti-oxidation performance.

Chromium Diffusion Coatings for Mo-Based Silicides to Improve Their Oxidation Resistance

The oxidation behavior of two ternary Mo-Si-Ti alloys (eutectic Mo-20.0Si-52.8Ti and eutectoid Mo-21.0Si-34.0Ti) was investigated using thermogravimetric analysis at 700 °C, 900 °C, and 1300 °C for 100 h. The eutectic alloy formed a protective SiO2/TiO2 oxide scale, whereas the eutectoid alloy showed catastrophic oxidation. To improve the oxidation behavior of these alloys, chromium diffusion coatings deposited via pack cementation on the surface were investigated. Cr coatings were found to be suitable for improving oxidation resistance at 700 °C and 900 °C but failed at 1300 °C due to the evaporation of thin scales. At 700 °C and 900 °C, the formation of a Cr2O3 scale was proven on the Ti-rich/Mo-lean eutectic composition. After exposure, the Ti-lean/Mo-rich eutectoid composition, which is intrinsically more prone to oxidation, was found to form a continuous Cr2O3 scale only at 700 °C.

Effect of Si, Al2O3 additives on the ablation behavior of ZrB2-SiC coating with Al2O3-modified SiC bond coat for C/C composites

This work firstly evaluated the effect of temperature on the preparation of SiC coating using Al2O3 as catalyst by pack cementation (PC) method. SiC/ZrB2-SiC-Si (ZSS) and SiC/ZrB2-SiC-Al2O3 (ZSA) coating were prepared on the surface of C/C composites by PC combined with atmospheric plasma spraying (APS). The objective of this work is to study the influence of Si and Al2O3 additives for ablation performances of ZrB2-SiC coating. The coated samples were subjected to ablation test, and the phase composition and microstructure evolution were investigated. The results indicated that SiC/ZSA coating had better ablation resistance because phase transformation and growth of ZrO2 grain was restrained by Al2O3.

Refractory high-entropy aluminized coating with excellent oxidation resistance and lubricating property prepared by pack cementation method

Refractory high-entropy alloys (RHEAs) exhibit superior mechanical properties at high temperatures compared to that of conventional high-temperature alloys. However, some refractory metallic elements are highly susceptible to oxygen absorption at room temperature and the resulting oxide films are not protective, resulting in catastrophic oxidation before they can exhibit excellent mechanical properties. Therefore, it is essential for them to improve the high-temperature oxidation properties of RHEAs. The pack cementation method can be used to prepare the aluminized coatings on the surface of RHEAs to improve their high-temperature oxidation resistance. In the present work, the aluminized high-entropy coatings were prepared on the surface of as-cast Ta25Nb25W25V25 RHEAs. The diffusion activation energy required for aluminization was experimentally calculated to be 137.667 kJ/mol, thus the growth kinetics of the coating was constructed. Aluminized coatings with the thickness of ∼75 μm and Vickers hardness of ∼680 HV0.01 obtained by holding at 1100 °C for 9 h were selected for the study. The oxidation activation energy of the aluminized coating at 800–1000 °C was 96.981 kJ/mol, while that of as-cast RHEAs was only 48.816 kJ/mol, so the oxidation resistance of the specimen was greatly improved. At 20–600 °C, the aluminized coatings also exhibited excellent wear resistance and lubrication properties. In particular, the average coefficients of friction were reduced by 0.13–0.24 compared to that of as-cast RHEAs, which can be attributed to the AlW intermetallic compound particles formed within the aluminized coating acting as self-lubricating particles.

Niobium Silicide Diffusion Coatings: The Impact of Cu and of Ti on the Microstructure and High-Temperature Behavior

Abstract Silicide diffusion coatings on niobium offer attractive properties for applications at temperatures above 1,000°C because of the ability to form a protective oxide, silicon dioxide. The combination of a double layer of silicides, NbSi2 and Nb5Si3, provides a gradient of properties to the coating, mitigating the formation and propagation of cracks that otherwise allow oxygen to interact with the substrate. However, a temperature above 1,200°C is required to process double-layer silicide coatings, increasing the challenge of controlling the process and incrementing the cost of the process. The need to enhance silicide coatings without compromising their competitiveness motivates the study of the individual effects of copper and of titanium. This study processed silicide coatings by double pack cementation adding copper or titanium, respectively, to the pack mixture. Three pack mixture compositions were used: with 15 % wt. silicon, with 15 % wt. silicon + 7 % wt. copper, and 15 % wt. silicon + 7 % wt. titanium. Processing was carried out at 1,000°C for 6 hours. The coatings processed with silicon resulted on a single NbSi2 layer, but the pack mixture containing silicon and copper led to thick double-layer coatings of NbSi2 and a continuous layer of Nb5Si3, in spite of the low processing temperature. In contrast, the pack mixture containing silicon and titanium led to thinner coatings composed of a layer of NbSi2 and the discontinuous presence of Nb5Si3. Results suggest that copper and titanium impact the gaseous diffusion during processing and also the vacancy concentration in the ordered structure of NbSi2. Exposure to temperature reveals a change on the oxidation mechanisms after processing with titanium and a more significant mass gain of coatings processed with copper.

Microstructure and high-temperature oxidation behaviour of MoSi2 protective coatings on Mo-based combustion chamber in rocket engines

This paper presents a detailed characterization of the microstructure and high-temperature oxidation behaviour of MoSi2 coatings obtained by pack cementation methods on a Mo-based substrate. The author focused on the challenges of applying Mo-based combustion chambers in rocket engines. Due to these assumptions, the high-temperature oxidation resistance of MoSi2 coatings was analyzed in the temperature range of 1200–1700 °C. A particular area of interest was the surface condition of MoSi2 coatings after oxidation tests, characterized by their roughness. The presented studies showed very beneficial behaviour of oxidized MoSi2 coatings from the point of view of both protective coating's morphology and durability under these conditions and the surface roughness responsible for the gas flow stability in actual working conditions. The identified microstructural phenomena related to MoSi2-based coating formation, its thermal stability and oxidation interaction were described from the thermodynamic point of view. Those analyses revealed that thermodynamically the most stable silicide compound during the first stage of coating formation is Mo5Si3, not MoSi2. It suggests that the interaction between the Si-rich gas atmosphere and the Mo5Si3 layer determines the growth progress of silicide coating. The necessary condition for further growth is an excess of active silicon sources in the reactive atmosphere in the pack cementation process.

Modeling of Surface Modification of Stainless Steel by Halide Activated Pack Cementation Method

In high-temperature applications, ferrous-based materials are important due to their excellent combination of desirable mechanical properties, ease of production, corrosion resistance at room temperature and cost-effectiveness. However, mechanical properties must be optimized against environmental effects. Depending on the industrial applications, various corrosion types may also occur. An approach to preserve the mechanical properties of the structural alloy being protected against corrosion is the application of protective coatings to the surfaces. Diffusion coatings are an effective method to obtain corrosion, oxidation and abrasion resistance against detrimental conditions of high temperature. According to the literature, the halide activated pack cementation method has been widely used for ferrous-based materials for a long time. However, most studies concerned with developing coating applications are based on experimental investigations that include microscopic, chemical, and mechanical analyses. Limited studies have been conducted based on computational alloy approaches. In this study, Cr coating of the AISI 316L steel by halide activated pack cementation method was considered as a diffusional problem and the kinetics of the coating deposition process were examined. The effect of process variables such as temperature, time and the compositions of coating layers formed on the surfaces were investigated thermodynamically with Thermo-Calc software and kinetically with DICTRA module. This approach provides insight into the dependence of solid-state diffusions on the processing parameters, and a better understanding of the phases that form along the coating and substrate material.

Effect of pack characteristics and process parameters on the properties of aluminide-coated Inconel 625 alloy

Abstract In this study, Inconel 625 alloy was initially aluminide coated by halide-activated pack cementation at 700 °C for 4 h using fine (40–45 µm) globular and coarse (10–75 µm) ligament aluminum particles. Microstructural features and hardness variation of the coatings along with their oxidation behavior at 1000 °C during 50 h were investigated to reveal the effect of pack characteristics on the properties. Investigations revealed that (i) a homogeneous and continuous coating layer was formed on the surface without internal oxidation, (ii) a thicker coating (∼60 µm) was formed due to the pack consisting of coarse particles, (iii) a higher hardness value (1369 HV0.5) was measured for the coated alloy using fine particles in the pack, and (iv) the coating with coarse powder exhibited higher oxidation resistance during the first 50 h of oxidation test period. Secondly, superalloy was coated at different temperatures (700 and 1000 °C) and times (2 and 4 h) using coarse particles. In this stage, the findings showed that by increasing process temperature and time, the accumulation of aluminum on the superalloy surface increased and the oxidation tendency remained at a lower level in coatings containing thicker layers.

Structure and High-Temperature Oxidation Performance of Si-Co Diffusion Coatings Prepared on a TiAl-Nb Alloy

Si-Co diffusion coatings were prepared on a Y-modified TiAl-Nb alloy using the pack cementation process. The structures of the coatings prepared at different temperatures (1050, 1080, and 1120 °C) and pack Co contents (5, 10, and 20 wt.%) were comparatively studied. The coatings possessed the typical structure of a (Ti,X)Si2+Ti5Si4 (X represents Nb and Al elements) outer layer with a Co-rich superficial zone, a Ti5Si4+Ti5Si3 middle layer, and a TiAl2 inner layer. Increasing the co-deposition temperature in the range of 1050–1120 °C led to a larger coating thickness but a more-porous coating structure, while increasing the pack Co contents in the range of 5–20 wt.% caused a lower coating growth rate. The formation of the Si-Co diffusion coating followed an orderly process of depositing Si first and then Co. The Si-Co diffusion coating had much better anti-oxidation performance than both the TiAl-Nb substrate and pure silicide coating. After undergoing oxidation at 1000 °C for 100 h, the oxidation parabolic rate of the Si-Co diffusion coating was approximately 6.16 × 10−3 mg2/cm4h1, which was lower than those of the TiAl-Nb substrate by about two orders of magnitude and pure silicide coating by about one order of magnitude.

Antioxidant performance and oxidation mechanism of a liquid silicon infiltration (LSI) SiC–Si coating at an ultra-high temperature of 1873 K

A novel SiC–Si coating with a high amount of net-like distributed Si was fabricated on the graphite substrate by liquid silicon infiltration (LSI) method to improve the oxidation-resistant performance. The mass gain of this LSI coating after 300 h oxidation at 1873 K was only 3.018%, suggesting an excellent static antioxidant property at 1873 K. The detailed oxidation mechanism of this LSI coating was explored in this work. It was found that the high amount of Si with the net-like distribution in this LSI coating promoted the passivation growth of SiO2 glass layer and prevented the SiO2 glass from crystallizing into cristobalite. In addition, the Si in this coating retarded the active oxidation and enhanced the coating stability. In contrast, the infiltration (VSI) SiC–Si coating presented an apparent mass loss, and the pack cementation (PC) SiC coating showed the catastrophic failure. The oxidation mechanisms of these two coatings were also discussed against the backdrop of the LSI coating with high antioxidant performance.

ZrO2-nanoparticle assisted phase transformation and oxidation kinetics of thermally grown alumina on nickel aluminide coatings

In this study, a nanocomposite of Ni-ZrO2 was fabricated using composite electroplating. The fabricated nanocomposite was further aluminized by the process of pack cementation at 620 °C for 5 h. We examined the effect of ZrO2 nanoparticle dispersions in the Ni2Al3 on the oxidation kinetics and phase promotion of θ-Al2O3 to α-Al2O3 at high temperatures. The phase transformation, surface morphology, and oxidation behavior of ZrO2-dispersed Ni2Al3 were analyzed by Photo-stimulated luminescence (PSLS), TGA, XRD, and SEM, respectively. It was observed that as nano- ZrO2 content increased from 1 wt% to 5 wt% in the Ni2Al3 aluminide, the transformation of θ-Al2O3 into α-Al2O3 tends to became fast, resulting in an improvement in the low oxidation rate of the coating. In addition, the dispersion of ZrO2 nanoparticles in the Ni2Al3 matrix significantly mitigated the cavities and cracks at the interface between the aluminide and oxide scale, enhancing the interfacial strength.

Orthogonal Experimental Optimization of Preparation and Microstructural Properties of a Diffusion Barrier for Tantalum-Based Silicide Coatings.

To solve the problem of silicide coatings on tantalum substrates failing due to elemental diffusion under high-temperature oxidation environments and to find diffusion barrier materials with excellent effects of impeding Si elemental spreading, TaB2 and TaC coatings were prepared on tantalum substrates by the encapsulation and infiltration methods, respectively. Through orthogonal experimental analysis of the raw material powder ratio and pack cementation temperature, the best experimental parameters for the preparation of TaB2 coatings were selected: powder ratio (NaF:B:Al2O3 = 2.5:1:96.5 (wt.%)) and pack cementation temperature (1050 °C). After diffusion treatment at 1200 °C for 2 h, the thickness change rate of the Si diffusion layer prepared using this process was 30.48%, which is lower than that of non-diffusion coating (36.39%). In addition, the physical and tissue morphological changes of TaC and TaB2 coatings after siliconizing treatment and thermal diffusion treatment were compared. The results prove that TaB2 is a more suitable candidate material for the diffusion barrier layer of silicide coatings on tantalum substrates.

The high temperature oxidation and thermal shock behavior of a dense WSi2–TaSi2 coating on Ta substrate prepared by a novel two-step process

In order to improve the oxidation resistance of the Ta substrate, a novel two-step process including molten salt electrodeposition (Na2WO4-WO3 system) and halide activated pack cementation was adopted to prepare a WSi2–TaSi2 coating on tantalum substrate. During the electrodeposition process, dense tungsten coatings were fabricated at current densities of 30 mA/cm2, 40 mA/cm2 and 50 mA/cm2. It was observed that the grain size exhibited a log-normal distribution. When the current density was 40 mA/cm2, the grain size and flattest surface of the tungsten coating reached 9.50 ± 0.23 μm and 6.792 μm, respectively. When performing the static oxidation test, the WSi2–TaSi2 coating could effectively protect the Ta substrate oxidized at 1600 °C for 30 h. This is attributed to the presence of dense SiO2 and Ta2O5, which acted as a protective layer and suppressed the further penetration of oxygen. Furthermore, due to the matching thermal expansion coefficient between each layer and the sealing ability of semi-molten SiO2, the four-layer SiO2–W5Si3–WSi2–Ta5Si3 coating could successfully pass 721 thermal shock tests from 1600 °C to room temperature.

Development and Mechanical Characterization of Ni-Cr Alloy Foam Using Ultrasonic-Assisted Electroplating Coating Technique

Metal foams and alloy foams are a novel class of engineering materials and have numerous applications because of their properties such as high energy absorption, light weight and high compressive strength. In the present study, the methodology adopted to develop a Ni-Cr alloy foam is discussed. Polyurethane (PU) foam of 40PPI (parts per inch) pore density was used as the precursor and coating techniques such as electroless nickel plating (ELN), ultrasonic-assisted electroplating of nickel (UAEPN), and pack cementation or chromizing were used to develop the Ni-Cr alloy foam. The surface morphology, strut thickness and minimum weight gain after each coating stage were evaluated. It was observed from the results that the adopted coating techniques did not damage the original ligament cross-section of the PU precursor. The minimum weight gain and the coating thickness after the UAEPN process were observed to be 42 g and 40–60 μm, respectively. The properties such as porosity percentage, permeability and compressive strength were evaluated. Finally, the pressure drop through the developed foam was estimated and verified to determine whether the developed foam can be used for filtering applications.