Halide Activated Pack Cementation Process Research Articles

Formation and oxidation behavior of refractory high-entropy silicide (NbMoTaW)Si2 coating

A single high-entropy silicide (NbMoTaW)Si2 coating was prepared on NbMoTaW refractory alloy by a halide-activated pack cementation process. The formation mechanism and growth kinetics of the (NbMoTaW)Si2 coatings were analysed. The oxidation behavior of the (NbMoTaW)Si2 coatings was investigated at 1300 ℃ for 24 h in air. The results showed that the high entropy effect and sluggish effect of the high-entropy NbMoTaW alloy makes it form a single-phase silicide (NbMoTaW)Si2 during the Si diffusion growth process. The lattice distortion effect of substrate provides a channel for the diffusion of Si to accelerate its diffusion rate. The (NbMoTaW)Si2 coating effectively protects the NbMoTaW alloy from oxidation at 1300 ℃. The oxidation resistance of the coating is attributed to the preferential oxidation of Si to produce a dense continuous SiO2 scale and the precipitation of WSi2 to suppress the interdiffusion of the silicide layer and the alloy substrate.

Application of the pack cementation process on SiC/SiC ceramic matrix composites

The potentials and limitations of a halide-activated pack cementation process on SiC/SiC Ceramic Matrix Composites for the development of bond coats as part of environmental barrier coating (EBCs) systems were investigated. Different pack compositions using chromium, aluminum and alloys of these elements were tested and the kinetics of coating formation were examined in addition to their microstructure. The results and their analogy to diffusion couples were discussed and it was shown that coating elements which form silicides and carbides are promising candidates for coatings deposited on SiC/SiC via pack cementation. Based on such considerations a two-step pack cementation was proposed, which used chromium, one of the suitable elements, in a first step, to finally achieve an alumina-forming coating. The oxidation resistance of the developed coating was tested via thermogravimetric analysis and compared to the uncoated material. The coating protected the fiber-matrix interface of the SiC/SiC Ceramic Matrix Composites from oxidation.

Microstructure and Oxidation Behavior of Anti-Oxidation Coatings on Mo-Based Alloys through HAPC Process: A Review

Mo and Mo-based alloys are important aerospace materials with excellent high temperature mechanical properties. However, their oxidation resistance is very poor at high temperature, and the formation of volatile MoO3 will lead to catastrophic oxidation failure of molybdenum alloy components. Extensive research on the poor oxidation problem has indicated that the halide activated pack cementation (HAPC) technology is an ideal method to solve the problem. In this work, the microstructure, oxide growth mechanism, oxidation characteristics, and oxidation mechanism of the HAPC coatings were summarized and analyzed. In addition, the merits and demerits of HPAC techniques are critically examined and the future scope of research in the domain is outlined.

Aluminide coating on Mar-M246 nickel superalloy by halide activated pack cementation (HAPC)

Nickel based superalloys are widely applied in high temperatures systems because they have excellent mechanical properties. However their use over long period can lead to excessive degradation oxidation and corrosion. Protective aluminum-based coatings increase the oxidation resistance of these alloys which may be achieved using the Halide Activated Pack Cementation (HAPC) technique. In this study, the Mar-M246 nickel superalloy was coated with aluminum rich intermetallic layers, obtained by the HAPC process at 700 °C, 800 °C, 900 °C and 1000 °C, for treatment duration of 9 h. Thermodynamic calculations have been carried out previously to determine process parameters, such as the choice of the activator and the suitable temperature. The microstructural characterization of the coatings was performed by SEM and EDS analysis. The coatings obtained were uniform and homogeneous in terms composition, and free of failures and cracks containing Ni2Al3 and/or NiAl3 phases. The efficiency of the coatings to enhance the oxidation resistance of the substrate was proved through the oxidation tests carried out at 1000 °C for about 240 h in which there was the formation of protective Al2O3 scale.

Microstructures and Oxidation Behaviors of Silicide Coated Nb Alloys by Halide Activated Pack Cementation Process

In this study, we tried to improve the oxidation resistance of Nb-12Si (wt%) alloys at 1200 °C or higher through pack cementation coatings. Nb-12Si (wt%) alloys were prepared by arc-melting under Ar atmosphere. When the alloys were coated using pack powder mixtures composed of Si, Al2O3 and NaF, two silicide layers composed of NbSi2 and Nb5Si3 phases were successfully produced on the substrate. The Si-pack coatings were performed with various heat treatment temperatures and time conditions. The microstructures and thickness changes of the coating layers were analyzed to determine the growth behaviors of the coating layer. The growth constant of 8.4 10–9 cm2/sec was obtained with a diffusion growth mode. In addition, in order to examine the resistance of the Si-pack coated alloys, isothermal static oxidation tests were performed at 1200 °C and higher temperatures. As a result, the oxidation resistance of the alloys was determined by protecting the surface of the alloys with silicide oxide layers formed by the silicide coatings. The uncoated specimens exhibited an abnormal weight increase due to the formation of Nb oxide. The coated specimen showed excellent oxidation resistance at 1200 °C for up to 12 hrs, while the previous reports on the same alloy verified oxidation resistance only up to 1100 °C. It appears that the excellent oxidation resistance is closely related to the NbSi2 coating layer thickness. The oxidation behaviors of the coating layers after the oxidation tests were discussed in terms of microstructural and phase analyses.

Studies on growth mechanism of intermediate layer of (Mo,W)5Si3 and interdiffusion in the (Mo,W)-(Mo,W)Si2 system prepared by pack cementation coating

Silicide based oxidation resistant coatings were developed over Mo-30 W wt.% alloy using halide activated pack cementation process. Formation of single-layer [only (Mo,W)Si2] and double-layer [consisting of (Mo,W)Si2 outer and (Mo,W)5Si3 intermediate] coatings were developed below and above a critical coating temperature of 1100 °C using a pack mixture having a composition of 20Si-2.5NH4F-77.5Al2O3 (wt.%). The activation energy for the growth of (Mo,W)Si2 coating layer was found to be 51 kJ/mole. The diffusion mechanism of the species in (Mo,W)-(Mo,W)Si2 system (using single layer coated samples) was studied by conducting isothermal annealing experiments at different temperatures of 700, 800 and 1000 oC for a period of 1000 h. The growth rate of the (Mo,W)5Si3 interlayer during the isothermal annealing treatment was found to be parabolic. The mechanism of the formation of (Mo,W)5Si3 intermediate layer, its microstructure and growth kinetics in (Mo,W)-(Mo,W)Si2 system were investigated by cross-sectional electron microscopy and EBSD analysis. The growth mechanism of the intermediate layer was further discussed using a Physico-chemical approach.

The addition of zirconium to aluminide coatings: The effect of the aluminide growth mode

Alumina-former coatings such as nickel aluminides have been the essential approach to combating the aggressive service environment of gas turbine hot section components. The positive effects of zirconium on the cyclic oxidation lifetime of aluminide coatings have been reported. The current paper has reported the effect of the growth mode of the aluminide coating on the microstructure of the Zr-modified aluminide coating. The aluminization process was carried out using the halide activated pack cementation process in the powder mixtures of aluminum, ammonium chloride, zirconium oxide, and alumina at 760 and 1040 °C for the inward and outward growth, respectively. The microstructures of the coatings were studied using FESEM, EDS and XRD. The results showed that the outward coatings contain a high level of Zr mainly incorporating it in the second phases such as AlNi2Zr in a matrix of NiAl. In contrast, the Zr concentration in the inwardly grown aluminide coatings did not exceed the 1 atomic percentage and the typical microstructure of the high activity aluminide coatings was observed. The chemical reactions involved in the pack cementation process and the mechanism of the coating formation were discussed.

Microstructure and tribological properties of Si-Y/Al two-step deposition coating prepared on Ti2AlNb based alloy by halide activated pack cementation technique

Abstract To enhance the tribological properties of the Ti2AlNb based alloy, a Si-Y/Al coating was prepared by a two-step halide activated pack cementation process including Si-Y co-deposition first and then aluminization. The Si-Y coating and the Si-Y/Al coating both had multilayer structures. After the aluminization, the microhardness of the Si-Y/Al coating decreased a little, but was still much higher than that of the substrate. Its toughness was obviously improved. The tribological properties of the bare alloy, the Si-Y coating and the Si-Y/Al coating were comparatively studied at room temperature and 873 K using SiC balls as the counterparts. The results showed that the Si-Y/Al coating could provide better protection for the Ti2AlNb based alloy than the Si-Y coating.

Effect of Austenite Stability on Pack Aluminizing of Austenitic Stainless Steels

Aluminide coatings were applied to the surfaces of several austenitic stainless steels—UNS S30300, S30400, S30900, S31000, and S31600 (Type 303, 304, 309, 310, and 316)—by the halide activated pack cementation process. The coating compositions, microstructures, and hardness were determined for the different steels coated at 850°C for 25 h. The stability of the austenite phase for each type of steel was calculated by determining the ratio of the nickel to the chromium equivalents based on their nominal compositions. The thickness of the inner diffusion zone in the coating was shown to be inversely related to the austenite stability of the steels. Microhardness measurements were obtained across the coating thickness and into the substrate. The hardness values followed the same trends as the aluminum composition profile into the substrate.

Microstructure and mechanical properties of hot pressed Mo–Cr–Si–Ti in-situ composite, and oxidation behavior with silicide coatings

The present study deals with the synthesis of Mo–16Cr–4Si–0.5Ti (wt.%) alloy by means of the reactive hot pressing method. The microstructure of the synthesized alloy consisted of (Mo, Cr, Ti)3Si, and the discontinuous α-(Mo, Cr, Ti)SS phases. The isothermal oxidation behavior of the alloy was investigated in air at 1273K for 50h. The alloy exhibited superior oxidation behavior in comparison with single phase molybdenum alloys, because of the formation of SiO2 and Cr2O3 over the alloy surface. The flexural strength determined from three-point bend testing of single edge notch bend specimens was 615±15MPa. The dominant mechanism of fracture was identified as transgranular mode of crack propagation. To extend the life of the alloy under oxidizing atmosphere, silicide based oxidation resistant coatings were developed, using halide activated pack cementation process. The kinetic behavior of growth of the coating was established and the activation energy of the coating process was determined to be 52.5kJ/mol. Isothermal oxidation tests of the coated alloy at 1273K for 50h, revealed a small weight gain at the initial stages of oxidation followed by no change of weight, indicating the protective nature of the coating.

Microstructural analysis and growth mechanism of single-step aluminum–titanium diffusion coatings on a nickel-based substrate

Aluminum–titanium coatings have shown promising results for some aggressive high temperature environments in which protective oxides can hardly form. In this study, a series of aluminum–titanium diffusion coatings were developed on a nickel-based substrate via a single-step halide activated pack cementation process. Powder mixtures of aluminum, titanium, aluminum oxide and ammonium chloride were prepared with different mass ratios of aluminum to titanium. Coating experiments were carried out at 850, 900, 950, 1000 and 1050°C for at least four hours. Coated specimens were characterized using conventional metallography, glow discharge optical emission spectroscopy and X-ray diffraction analysis. The results show that a zone of NiAl phase is formed in all the coatings right above the coating–substrate interface. Other zones towards the surface including τ2-Al2NiTi and τ1-(Al,Ni)3Ti in some of the specimens and τ4-AlNi2Ti and Ti2Ni in most of the others were found. The growth mechanism of the coatings was discussed based on the microstructural features.

Formation of MoSi2 and Al doped MoSi2 coatings on molybdenum base TZM (Mo–0.5Ti–0.1Zr–0.02C) alloy

Formation of aluminium (Al) doped molybdenum di-silicide (MoSi2) coatings was studied to improve the high temperature oxidation behavior of TZM (Mo–0.5Ti–0.1Zr–0.02C) alloy. The pack composition of the halide activated pack cementation process was successfully optimized to form silicide and Al doped silicide coatings on the TZM alloy substrates. Mo(Si, Al)2 phase was found to form at the outer layer of the coating prepared by doping Al in MoSi2. A change in composition of the phases with increase in coating temperature was detected with Al doping, whereas un-doped silicide coating process was dominated by the formation and growth of MoSi2 phase. Oxidation test and the characterization studies using SEM, EDS, XRD, and micro-hardness measurements indicated the improved performance of Al doped silicide coating during high temperature oxidation in dry air due to the formation of the protective alumina scale.

Surface molybdenizing on titanium by halide-activated pack cementation

To improve the surface hardness of titanium, molybdenized layer was fabricated on titanium surface by halide-activated pack cementation process. Coupons were analyzed using optical microscopy (OM), scanning electron microscopy (SEM) with X-ray energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). Microhardness values were obtained by Vickers hardness test. It was found that molybdenized layer consists of diffusion layer and deposition layer formed above 883 °C. Attributed to the difference of Mo content, the phase transformation of Mo → β → α″ → α′ occurred from outside to inside in the diffusion layer, which led to the gradual decrease of microhardness values from the deposition layer to the substrate for the different hardness levels of β, α″ and α′ phases. Molybdenized layer could obviously improve the surface microhardness of the titanium substrate. The highest microhardness value of deposition layer and diffusion layer is about 1400 HV and 1200 HV respectively, which is approximately four times higher than that of the titanium substrate. Based on Fick's second law, the relation between the thickness of diffusion layer, process temperature and time is discussed.

Formation and Oxidation Resistance of NbSi<sub>2</sub> Coatings on Niobium by Pack Cementation

NbSi2 coatings were formed on niobium by halide-activated pack cementation process. The as-coated niobium samples were oxidized in air up to 1723 K by thermogravimetry method. The surface and cross-sectional morphology, phase composition and element distribution of the NbSi2 coatings before and after oxidation were characterized by SEM, XRD and EPMA. The results show that the as-formed coatings consist of single phase of hexagonal NbSi2 and the oxidation resistance of pure niobium can be greatly improved by pack siliconizing.

Effect of temperature on microstructure of molybdenum diffusion coating on titanium substrate

The halide-activated pack cementation process was used to form molybdenum diffusion coating on titanium substrate. The morphology, structure, elements diffusion distribution and microhardness of the coatings formed at different diffusion temperatures were studied. The results indicate that the coating is made up of deposition layer and diffusion layer, and the surface roughness of specimens is increased after diffusion. In the diffusion layer, the major phases are Mo and β-Ti phase with addition of α′-Ti phase and α″-Ti phase. And the phase composition of Mo →β→β″→β′ is formed for different Mo contents in the diffusion layer from outside to inside. The diffusion of Ti element is very obvious as well as Mo element. With increasing the diffusion temperature, the thickness of diffusion layer is increased rapidly, and the microhardness is changed more smoothly with diffusion depth, which shows the same distribution rules as the Mo content.

Modifying Cr-Al Coatings by Reactive Element Additions: Eiperimental and Theoretical Considerations

Cr-Al diffusion coatings have been modified by Hf addition using a one step halide activated pack cementation process. The substrate was a low alloy steel 2.25Cr-1 Mo. Cyclic Oxidation properties of the coatings were evaluated, and a Hf modified coating was the most resistant coating. Codeposition pack thermodynamics were computed and analyzed using available thermodynamic tools in order to compare experimentally obtained results with theoretical predictions.

Oxidation-resistant Ge-doped silicide coating on Cr-Cr 2Nb alloys by pack cementation

The halide-activated pack cementation process was modified to produce a Ge-doped silicide diffusion coating on Cr-Cr 2Nb alloys in a single processing step. The morphology and composition of the coating depended both on the pack composition and processing schedule and also on the composition and microstructure of the substrate. Higher Ge content in the pack suppressed the formation of CrSi 2 and reduced the growth kinetics of the coating. Ge was not homogeneously distributed in the coatings. Under cyclic and isothermal oxidation conditions, the Ge-doped silicide coating protected the Cr-Nb alloys from significant oxidation and from pesting by the formation of a Ge-doped silica film.

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.