Two-step Pack Cementation Process Research Articles

Deposition of a low-activity type cobalt-modified aluminide coating by slurry aluminizing of a pre-Co-electroplated Ni-based superalloy (IN738LC)

Incorporating the Co element into the β-NiAl phase as a third element enhances the hot corrosion and oxidation properties of aluminide coatings. The deposition of Co-modified aluminide coatings is limited to co-deposition by the pack cementation method or a two-step pack cementation process of Co and Al, so far. This study investigates the possibility of depositing a low-activity type of Co-modified aluminide coating by combining the electroplating process with the slurry aluminizing method. To accomplish this objective, different thicknesses of Co-electroplated layers, deposited by a Watts bath, ranging from 5 to 20 μm are slurry aluminized using a slurry consisting of Al particles as the Al source and a PVA aqueous solution as the binder at 1100 °C for 2 h. Formation mechanisms are discussed through slurry aluminizing of 5, 10, and 20 μm thick Co-electroplated layers at the temperature range of 700–1100 °C with 100 °C intervals at zero time. FE-SEM surface morphology, BSE-SEM cross-sectional observation, surface XRD characterization, and EDS elemental analysis are utilized to characterize the coatings. Results show a critical thickness of the Co layer is required to deposit a defect-free low-activity type of Co-modified aluminide coating, which is determined to be ∼10 μm in this study. The final aluminide coating thickness decreases from ∼140 to ∼65 μm by increasing the Co-electroplated layer thickness from 5 to 20 μm. The identified phases on the surface differ from β-NiAl to β-CoAl by increasing the Co-electroplated layer thickness up to critical value. The Void formation occurs by the Kirkendall effect in coatings with more Co-electroplated layer thickness than the critical value, segregating the coatings within the β-CoAl phase.

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.

Evolution of the Microstructure and Properties of Pre-Boronized Coatings During Pack-Cementation Chromizing

The effect of chromizing time on the microstructure and properties of B–Cr duplex-alloyed coating prepared by a two-step pack-cementation process was investigated. The phases, microstructure, and element distribution of three coatings obtained were characterized by X-ray diffraction (XRD), secondary electron imaging (SEI), backscattering electron imaging (BSEI), and energy dispersive spectroscopy (EDS), respectively. The results show that as the chromizing time increases, the net-like Fe2B and rod-like CrFeB phases in the coating gradually disappear, and finally completely transform into the block-like Cr2B and CrxCy (Cr7C3 and Cr23C6) phases. The growth kinetics analysis shows that interface reaction dominates the coating growth during the early stage of chromizing, while atomic diffusion gradually controls the coating growth at the later stage. The evolution mechanism of the B-Cr duplex-alloyed coating was also discussed.

Microstructures and Wear Resistance of Boron-Chromium Duplex-Alloyed Coatings Prepared by a Two-Step Pack Cementation Process

In this study, a two-step pack cementation process (preboronizing and then chromizing) was employed to prepare the B-Cr duplex-alloyed coating on the steel. After the first step of preboronizing (PB sample), box-type furnace chromizing (BC-1 sample) and induction heating chromizing (BC-2 sample) were carried out, respectively. The phases and microstructure of the coatings were characterized by X-ray diffraction (XRD), backscattering electron imaging (BSEI), and energy dispersive spectroscopy (EDS). The results reveal that the heating mode of the second step of chromizing has a significant effect on the phase composition and microstructure of the B-Cr coating. The efficiency of induction heating is higher than that of the box furnace heating, resulting in a thicker, denser, flatter surface, and B-Cr coating with fully reacted B and Cr elements. The wear and corrosion resistance of the steel is found to be significantly improved by the formation of effective B-Cr coating. The formation mechanisms and properties of the two duplex-alloyed coatings are investigated and discussed.

Deposition and oxidation behavior of Mo(Si,Al)2/MoB layered coatings on TZM alloy

A two-step pack cementation process including first boronizing and then co-depositing of Si-Al-Y with different Al contents (0, 5 and 10wt%) in the packs was used to deposit oxidation-resistant coatings on TZM alloy. The as-formed coatings are MoSi2/MoB, Mo(Si,Al)2/MoB, and Mo(Si,Al)2/Mo-Al-B/MoB layered coatings respectively. Compared with the MoSi2/MoB coating, the improved oxidation-resistant performance is presented in the Mo(Si,Al)2/MoB and Mo(Si,Al)2/Mo-Al-B/MoB coatings upon oxidation at 1250°C, which should be attributed to the sluggish inward diffusion of Si into substrate and forming dense scales composed of SiO2 and Al2O3. The microstructure change and oxidation behavior of these coatings are investigated.

ZrSi<sub>2</sub>-SiC/SiC Anti-Oxidant Coatings Prepared on Graphite Spheres by Two-Step Pack Cementation Process

A ZrSi2-SiC/SiC double layer was fabricated on the surface of graphite spheres by two-step pack cementation with zirconium, silicon, and graphite powders as mainly raw materials. The microstructure, phase, oxidation behavior, and thermal shock resistance of the ZrSi2-SiC/SiC composite coatings were studied. The results show that the outer coating is composed of ZrSi2 and SiC and the inner coating is composed of SiC, of which the thickness is 450 μm. Isothermal oxidation test was conducted in air at 1773 K for 20 h and a weight increase of 1.69% were recorded. After 10 times of thermal shock tests between 1473 K and room temperature, the weight of the coated specimens decreased by only 1.14%. The excellent oxidation resistance is due to the production of SiO2 and ZrSiO4 in the process of the oxidation of SiC and ZrSi2, forming a glass phase that can effectively prevent further oxidation.

Characterization of a multiphase coating formed by a vapor pack cementation process to protect Nb-base alloys against oxidation

Abstract In order to improve the oxidation resistance of Nb–Nb 5 Si 3 composites, a multilayer coating with a top coating consisting of mixed phases of oxides and chromium borides was made. The substrates were firstly coated with a pure Cr layer by cathodic triode sputtering and then borosiliconized according to a two-step pack cementation process. The as-obtained coatings were characterized using scanning electron microscopy (SEM), energy and wavelength dispersive X-ray spectroscopies (EDS, WDS), X-ray diffraction (XRD), infrared (IRRAS) and Raman spectroscopies. The results showed that the multiphase layer is composed of mixed phases of oxides (silica and borosilicate) and chromium borides (outer layer) and chromium silicide (inner layer). The coating formation mechanism involves a eutectic liquid (43 mol%SiO 2 –57 mol%CrO) created by the reaction between Cr and SiO gas. Oxidation of the chromium borides leads to the formation of boron oxide, which enhances the fluidity of borosilicate phases with a healing effect, the efficiency of which depends on the mass coating.

Preparation and oxidation resistance of silicide/aluminide composite coatings on an Nb–Ti–Si based alloy

Silicide/aluminide composite coatings were prepared on an Nb–Ti–Si based alloy by a two-step pack cementation process including siliconizing firstly and then aluminizing. The siliconized coating is composed of a thick (Nb,X)Si2 outer layer and a thin (Ti,Nb)5Si4 inner layer. During the aluminizing process, the original (Nb,X)Si2 outer layer retains and Al moves through it to form an Al3(Nb,X) layer beneath it. Besides, a thin Al3(Nb,X) outermost layer also forms above the (Nb,X)Si2 layer. This composite coating possesses good oxidation resistance at 1250°C within 50h, and a double-layered scale with an Al2O3 outer layer and a SiO2-matrix inner layer forms.

Improvement in oxidation resistance of silicide coating on an Nb–Ti–Si based ultrahigh temperature alloy by second aluminizing treatment

Modified silicide coating was prepared on an Nb–Ti–Si based alloy by a two-step pack cementation process including first siliconizing and then aluminizing. The two-step coating is composed of an Al-rich outermost layer, a (Nb,X)Si2 outer layer and a (Ti,Nb)5Si4 inner layer. The coating possesses an enhanced oxidation resistance at 1250°C due to the presence of the Al-rich outermost layer. The mass gain of the coating after oxidation at 1250°C for 100h is 3.7mg/cm2, and a duplex scale consisting of an Al2O3 outer layer and a SiO2-matrix inner layer forms.

A dual-layer Ce–Cr/Al oxidation resistant coating for 3D open-cell nickel based foams by a two-step pack cementation

The dual-layer Ce–Cr/Al coating was deposited onto the open-cell nickel-based foam by a two-step pack cementation process. The first layer was composed of Ce-modified chromium; the second (external) layer was made of Al. The research aimed to highlight the influence of Ce modified on the microstructure and oxidation resistance of the Ce–Cr/Al coating. The phase composition and microstructure of the inner layer Ce–Cr coating and the dual-layer Ce–Cr/Al coating before and after oxidation were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrum (EDS), respectively. Simultaneously, in order to distinguish the effect of Ce, the microstructure characteristics of the Cr/Al coatings without Ce were investigated. The oxidation resistance of Ce–Cr/Al coating was also compared with the Ce–Cr coating, Cr/Al coating and bare nickel-based foams under isothermal conditions for 120h at 1173 and 1273K, respectively. The results show that the dual-layer Ce–Cr/Al coatings are compact and uniform with no microcracks. The Ce added can effectively restrain the interdiffusion between the Ce–Cr coating and Al coating during the oxidation process. Therefore, a continuous Al2O3 layer is formed on the surface of the Ce–Cr/Al coating, which has a slower oxidation rate than the Cr2O3 layer formed on the surface of the Ce–Cr and Cr/Al coatings. In addition, the Ce–Cr/Al coated foams have a relatively good mechanical performance as compared to the Ce–Cr coated foams, the Cr/Al coated foams and bare nickel-based foams before and after oxidation.

Preparation of a SiC/Cristobalite-AlPO 4 Multi-layer Protective Coating on Carbon/Carbon Composites and Resultant Oxidation Kinetics and Mechanism

In order to improve the oxidation resistance of carbon/carbon (C/C) composites, a SiC/C-AlPO 4 multi-layer coating was fabricated on the C/C composites by a simple and low-cost method. The internal SiC bonding layer was prepared by a two-step pack cementation process and the external C-AlPO 4 coating was deposited by hydrothermal electrophoretic deposition process. Phase compositions and microstructures of the as-prepared multi-layer coating were characterized by X-ray diffraction (XRD), scaning electron microspocy (SEM) and energy dispersive spectrometer (EDS). Anti-oxidation properties, oxidation behavior and the failure behavior of the coated composites were investigated. The results indicate that the multi-layer coating exhibits obviously two-layer structure. The inner layer is composed of β-SiC, α-SiC phase with a scale of silicon phase. The outer layer is composed of cristobalite aluminum phosphate (C-AlPO 4 ) crystallites. The SEM observation shows the good bonding between the inner and outer layers. The multi-layer coating displays an excellent oxidation resistance in air in the temperature range from 1573 to 1773 K, and the corresponding oxidation activation energy of the coated C/C composites is calculated to be 117.2 kJ/mol. The oxidation process is predominantly controlled by the diffusion of O 2 through the C-AlPO 4 coating. The failure of the multi-layer coating results from the generation of the microholes that may be left by the escape of the oxidation gases.

High Performance Si-SiC Composite Coating for C/C Composites Prepared by a Two-Step Pack Cementation Process

High performance oxidation resistant Si-SiC coating for C/C composites was prepared by a two-step pack cementation method using silicon, graphite powders as source materials and Al2O3, MgO, B2O3 as additives. XRD analyses revealed that the obtained coating was composed by β-SiC, Si and α-SiC; SEM analyses displayed that the as-prepared coating exhibited dense surface and cross-section microstructures. Isothermal oxidation test in air at 1773 K showed that the weight loss of the as-obtained Si-SiC coating coated sample after 200h was only 4.42 × 10–4 g.cm–2. The main cause of the weight loss of the coated sample after long time oxidation is due to the vaporization and exhausting of SiO2 glass film which could not provide sufficient glass phase to seal the holes.

Influence of preparation technology on the microstructure and anti-oxidation property of SiC–Al2O3–mullite multi-coatings for carbon/carbon composites

Abstract Oxidation protective SiC–Al2O3–mullite multi-coatings for carbon/carbon (C/C) composites were prepared with a two-step pack cementation process. The influence of preparation temperature and SiO2/Al2O3 ratio of the pack powder on the phase, microstructure and oxidation resistance of the multi-coatings were investigated. It showed that the multi-coatings that contained mullite could be produced at 1700–1800 °C. A denser coating surface was acquired with the decrease of SiO2/Al2O3 ratio in the pack chemistries while a little damnification to the interface of the coating and C/C substrate. The as-prepared coating could effectively protect C/C composites from oxidation at 1600 °C for 81 h.