Pack Mixture Research Articles

Niobium Boronizing: Influence of the Treatment Temperature and Time

Abstract The application of niobium borides to components such as lamination cylinders, hightemperature devices, and medical equipment shows their importance and versatility in engineering. To improve niobium’s mechanical resistance and possible oxidation resistance at temperature, this research applied boronizing to pure niobium, carried out with double pack cementation. Boronizing at 950°C and 1,100°C was carried out for 1 and 4 h. Ekabor commercial pack mixture with a nominal chemical composition of 90 % silicon carbide, 5 % boron carbide, and 5 % potassium tetrafluoroborate was used with and without 10 percent by weight (wt%) silicon addition. Scanning electron microscope, energy dispersive spectroscopy, and X-ray diffraction analyses and microhardness tests were used to characterize the treated samples. A continuous high-hardness 2,394-HV0.1 (23.5 GPa) niobium diboride layer was formed at the surface of the niobium substrate. A maximum layer thickness of 53.6 ± 2.9 µm was measured after 4 h at 1,100°C, whereas after 1 h at 950°C, no visible layer was identified with the applied characterization techniques, suggesting a threshold in this temperature. Adding 10 wt% silicon to the pack mixture impacted the kinetics of the diffusion process, which resulted in an increase in layer thickness of 72.6 ± 10.1 µm after processing for 1 h at 1,100°C, but cracks formed in the processed surface.

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.

Assessing the influence of wildfire on leaf decomposition and macroinvertebrate communities in boreal streams using mixed‐species leaf packs

Abstract We investigated how compositional differences in riparian leaf litter derived from burned and undisturbed forests influenced leaf breakdown and macroinvertebrate communities using experimental mixed‐species leaf packs in boreal headwater streams. Leaf pack mixtures simulating leaf litter from dominant riparian woody‐stem species in burned and undisturbed riparian zones were incubated in two references and two fire‐disturbed streams for 5 weeks prior to measuring temperature‐corrected breakdown rates and macroinvertebrate community composition, richness, and functional metrics associated with decomposers such as shredder abundance and % shredders. Leaf litter breakdown rates were higher and had greater variability in streams bordered by reference riparian forests than in streams where riparian forests had been burned during a wildfire. Streams bordered by fire disturbance showed significant effects of litter mixture on decomposition rates, observed as significantly higher decomposition rates of a fire‐simulated leaf mixture compared to all other mixtures. Variation among sites was higher than variation among litter mixtures, especially for macroinvertebrate community composition. In general, fire‐simulated leaf mixtures had greater shredder abundances and proportions, but lower overall macroinvertebrate abundance; however, the shredder abundance trend was not consistent across all leaf mixtures at each stream. These results show that disturbance‐driven riparian forest condition and resulting composition of leaf subsidies to streams can influence aquatic invertebrate community composition and their function as decomposers. Therefore, if one of the primary goals of modern forest management is to emulate natural disturbance patterns, boreal forest managers should adapt silvicultural practices to promote leaf litter input that would arise post‐fire, thereby supporting stream invertebrate communities and their function.

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.

Effect of Y2O3 content in the pack mixtures on the cyclic-oxidation of Y2O3-modified low temperature aluminide coatings on 309 stainless steel

By using Y2O3 nanoparticles, instead of part or all of Al2O3, acting as filler, Y2O3-modified aluminide coatings were produced on 309 stainless steel using a conventional pack-cementation method at 700 °C for 6 h. Effect of Y2O3 content in the pack mixtures on the cyclic-oxidation resistance of different aluminide coatings was comparably investigated. The results show that the phase of aluminide coatings was Fe24Al76.8. With the increase of Y2O3 content in the pack mixtures, the thickness of aluminide coatings decreased but the cyclic-oxidation resistance at 900 °C in air increased, because of prevention of cavities at the alumina scale/coating interface, and because of mitigation of degradation of the aluminide coating due to decreased outward diffusion and inward interdiffusion between the aluminide and the 309 stainless base. The reasons for the results are discussed.

Effect of diffusion coatings on the high temperature properties of nickel-chromium-superalloys

The halide-activated pack cementation method was utilized to deposit aluminide or silicide coatings on Inconel 617 and Hastelloy X superalloys. Aluminide and silicide diffusion coatings were formed at 850[Formula: see text]C for 2 h in nitrogen atmosphere, using a pack mixture containing pure aluminum (Al) or silicon (Si) and aluminum oxide (Al2O3) powders with activators of NH4Cl and AlF3. Aluminide-coated alloys showed homogeneous and uniform microstructures. Al diffused into the alloy inwards and aluminide diffusion coatings of [Formula: see text]17 [Formula: see text]m thick were formed inside the alloy. It was shown that the Al coatings played a key role in blocking off the excessive corrosion products at a high temperature for the alloys. The enhanced thermal stability and improved wear resistance were achieved in the aluminide coatings. In contrast to the aluminide coating, the silicide coating played a negative role, unable to provide the protective layer. The microstructural evolution and thermal stability of the aluminide- and silicide-coated alloys have been elucidated.

A nonlinear packing model for multi-sized particle mixtures

Linear packing models are commonly used to predict the specific volume of a packing mixture composed of several size classes of particles. In a linear packing model, the specific volume of a packing mixture is a linear function of the volume fractions of each size class of particles. However, predictions from linear packing model have been observed to deviate from measured experimental results of both binary and ternary mixtures. For some cases, the deviation is significant. The concept of dominant size is an important element in the packing model. In the linear packing model, the dominant size is assumed to be one of the size class of the particle mixture. However, this assumption is not suitable for mixtures of particles with certain solid volume fractions. In the present model, we recognized that the dominant skeleton is composed of more than one class of particles. To represent the state of dominant skeleton, we hypothesize a dominant size, which is a continuous variable. Using this hypothesis, a non-linear packing model is developed. The proposed model is evaluated by experimental results obtained from binary mixtures of sand-silt and ternary mixtures of steel ball bearings, steel shots, glass beads, and washed sand. Comparison of the predicted and measured results indicates that this model is able to capture the non-linear behavior of density variation with respect to solid volume fractions.

Influence of Pt concentration on structure of aluminized coatings on a Ni base superalloy

Aluminum diffusion coatings have been processed by chemical vapor deposition techniques. Particularly, pack cementation is frequently used to process competitive oxidation resistance coatings on Ni based alloys. The superior oxidation resistance is associated with the presence of a NiAl layer that supplies Al to maintain a continuous and adherent α-Al2O3 film. Further enhancements on the performance are gained when a layer of Pt is deposited on the Ni based alloy before aluminization. Typically, the role of Pt on the performance of coatings has been characterized using aluminizing procedures by immersion of parts in the pack mixture. This work assessed the influence of Pt concentration on the structure of out-of-pack aluminized coatings aiming to clarify some contradictory results in the literature. Pt layers of 3 and 7μm thicknesses were electroplated on a Ni superalloy. Aluminization at 1100°C for 5h used out-of-the pack procedures avoiding direct contact between samples and the pack mixture. Analysis revealed that aluminized coatings are composed of two main regions: a layer of β-(Ni,Pt)Al and a interdiffusion zone (IDZ). Increasing Pt concentration increased the total thickness of coatings, smoothen the Al gradient in coatings and reduced the Kirkendall porosity. These effects are a consequence of Pt in reducing the activation energy for Al diffusion contributing to a higher atomic mobility of Al in the β-(Ni,Pt)Al layer. Richer Pt coatings enhanced oxidation performance due to the formation and maintenance of a continuous and adherent α-Al2O3 film and the lower density and dimensions of voids at the NiAl/Al2O3 interface.

Galvacoat – 2015

Zn–Al pack co-cementation was performed on medium carbon steel in a pack mixture containing Zn and Al powders with NH4Cl as activator. An Fe–Zn layer was formed first during pack co-cementation. Fe–Al phases then nucleated locally on the surface of the Fe–Zn layer. With continued reaction, a complete Fe–Al layer was formed and grew at the expense of the Fe–Zn layer. The Fe–Zn was compact, but the Fe–Al layer was porous. The Fe–Al layer had better high-temperature oxidation resistance, while the Fe–Zn layer had better corrosion resistance in chloride solution. Based on microstructural evolution and the chemical reactions associated with Zn–Al pack co-cementation, the growth behavior of Zn–Al pack coating on steel was discussed in detail.

Effects of Y2O3/Y on Si-B co-deposition coating prepared through HAPC method on pure molybdenum

In order to clarify the effects of reactive element Y on the properties of Si-B co-deposition coating on Mo substrate, the Si-B-Y2O3 and Si-B-Y co-deposition coatings were prepared at 1300 °C for 5 h by using the pack mixtures 16Si-4B-xY2O3/Y-4NaF-(76-x)Al2O3 (wt.%, x=0, 0.5, 1, 2, 4, 8). X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and wavelength dispersive spectroscopy (WDS) techniques were used to analyze the structure and oxidation behavior of these coatings. The results revealed that the Si-B-Y2O3 and Si-B-Y co-deposition coatings had the same structure with that of the Si-B co-deposition coating. However, Y was incorporated into these coatings and the thicknesses of these coatings were thicker than that of the Si-B co-deposition coating. In addition, the Si-B-Y co-deposition coating demonstrated better cyclic oxidation resistance than the Si-B co-deposition coating at 1100 °C. The modifying mechanism of Y on the Si-B co-deposition coating was discussed.

Sintering of SiC Coating Layer on Graphite Spheres Prepared by Pack Cementation

Pack cementation is an effective method in the manufacture of SiC coating on carbon material substrate surface, which is a controllable and simple process. Meanwhile, due to the reactive infiltration process of powder mixture into the substrate, a gradient transition structure layer is formed between the substrate and outer layer. In this paper, SiC was coated on the spherical substrates taken from the matrix graphite pebbles of high temperature gas-cooled reactor (HTR) fuel element. Relations between the Si/C content ratio of the pack mixture and the thickness of SiC layer were studied. Analysis found that Si/C content ratio, powder size and sintering time are factors influenced the thickness of the coating layers. When the Si/C content ratio was higher than 3:1, a uniform thickness coating bonded well with the substrate was obtained. The composition phases and thickness of coating layer etc. also had obvious changes along with the change of the Si/C content ratio in this research.Results also show that sintering atmosphere and particle size of powders are important factors affecting coating microstructure, while a vacuum atmosphere can smaller powder size can help to get a dense structure.

Growth behavior and properties of Zn–Al pack cementation coatings on carbon steels

Abstract Zn–Al pack co-cementation was performed on medium carbon steel in a pack mixture containing Zn and Al powders with NH4Cl as activator. An Fe–Zn layer was formed first during pack co-cementation. Fe–Al phases then nucleated locally on the surface of the Fe–Zn layer. With continued reaction, a complete Fe–Al layer was formed and grew at the expense of the Fe–Zn layer. The Fe–Zn was compact, but the Fe–Al layer was porous. The Fe–Al layer had better high-temperature oxidation resistance, while the Fe–Zn layer had better corrosion resistance in chloride solution. Based on microstructural evolution and the chemical reactions associated with Zn–Al pack co-cementation, the growth behavior of Zn–Al pack coating on steel was discussed in detail.

Effect of Y2O3 content in the pack mixtures on microstructure and oxidation resistance of B–Y modified silicide coating

B–Y modified silicide coatings were prepared on Nb–Si based alloy by pack cementation at 1300℃ for 10h. The effect of Y2O3 content in the pack mixtures on microstructure and oxidation resistance of the coatings was investigated. The results show that the four coatings have similar structures, which possess a (Nb,X)Si2 outer layer and a (Nb,X)5Si3 transitional layer. Y2O3 content in the pack mixtures has an obvious effect on the Si content in the coating. The mass gains of the coatings prepared with 0.5, 1, 2 and 3wt% Y2O3 in pack mixtures are 2.33, 1.96, 2.05 and 2.86mg/cm2 after oxidation at 1250℃ for 100h, respectively. The coating prepared with 1wt% Y2O3 exhibits the best oxidation resistance due to the formation of a dense glass-like borosilicate scale.

Duplex Surface Treatment of AISI 1045 Steel via Plasma Nitriding of Titanized Layer

In this Study, surface of medium carbon steel (AISI 1045) was modified by titanizing followed by plasma nitriding. Titanizing was performed using pack cementation method, in a pack mixture of ferrotitanium, ammonium chloride and alumina, at 1223 K for 45 and 180 min. Plasma-nitriding was conducted at two temperatures of 803 and 823 K for 5 h, in a gas mixture of 75%N2+25%H2. The samples were characterized by scanning electron microscope (SEM), x-ray diffractometre (XRD), Vickers microhardness and pin-on-disk wear tests. The thicknesses of titanized layers was 8-10μm, the thicknesses had no significant change after plasma nitriding. The duplex treated layers consisted of Fe4N, TiN and TiCN phases. Lower titanizing time and higher plasma nitriding temperature provided superior wear resistance and hardness. The highest wear resistance and the lowest friction coefficient were achieved in the sample which was titanized for 45 min and then plasma nitrided at 823 K, with 2130 HV microhardness.

Cyclic oxidation resistance of Ce/Co modified aluminide coatings on nickel base superalloys

Ce/Co modified aluminide coatings with different CeO2 content were prepared on nickel base superalloys by pack cementation method. The microstructure and cyclic oxidation behavior of the coatings were investigated. The results show that the Ce addition is beneficial to improving adhesion of oxide scales by forming the “oxide pegs” structure and reducing voids at the scale/coating interfaces. When the content of CeO2 addition in the pack mixture is 2wt.%, the Ce/Co modified aluminide coating has the highest content of Ce in the outer layer and exhibits the best cyclic oxidation resistance at 1323K.

Synthesis of clean aluminide coatings on Ni‐based superalloys via a modified pack cementation process

With the aim of seeking a modified pack cementation process as a potential low‐cost alternative to form clean and reproducible aluminide coatings for laboratory diffusion coating research, four non‐contact arrangements were employed to coat Ni‐based superalloy specimens. The advantages and disadvantages of each arrangement were discussed by comparing the cleanliness of coating surface, the homogeneity of surface Al content and coating thickness, as well as the easiness of preparation. By utilizing the arrangement with the best overall quality, an Ni‐rich β‐NiAl coating with a clean surface and a low S level (0.3 ppmw) was achieved, which was comparable to the coatings previously made by an ultra‐clean laboratory chemical vapor deposition (CVD) process. Reactive element (RE) additions such as Hf, Zr, or Y were codeposited by including a small amount of the respective RE oxide in the pack mixture, and approximately 500 ppmw Hf was detected in the Hf‐modified NiAl coating.

Structure and Oxidation Behavior of Zr–Y Modified Silicide Coatings Prepared on an Nb–Ti–Si–Cr Based Ultrahigh Temperature Alloy

Zr–Y modified silicide coatings have been prepared on an Nb–Ti–Si–Cr based ultrahigh temperature alloy by a pack cementation process. The effects of amount of Zr powder in the pack mixture and co-deposition temperature on the coating structure were assessed. The coatings prepared at 1,250 °C with different amounts of Zr powders in the pack mixture have similar structures, mainly consisting of a thick (Nb,X)Si2 (X represents Ti, Cr and Hf elements) outer layer, a thin (Ti,Nb)5Si4 middle layer and a 1–2 µm thick (Nb,X)5Si3 inner layer. The increased amount of Zr powders in the pack mixtures led to a significant decrease in the coating thickness. The coatings prepared at 1,150 and 1,200 °C have similar structures to that prepared at 1,250 °C. However, when the co-deposition temperature increases to 1,300 and 1,350 °C, a (Ti,Nb)5Si3 outermost layer formed in addition to the three layers mentioned above. The Zr–Y modified silicide coating can protect the base alloy from oxidation at least for 100 h at 1,250 °C in air. The good oxidation resistance of the Zr–Y modified silicide coating is attributed to the formation of a dense scale consisting of SiO2 and TiO2.

Effect of Y2O3 content in the pack on microstructure and hot corrosion resistance of Y–Co-modified aluminide coating

Y–Co-modified aluminide coatings on nickel base superalloys were prepared by pack cementation method. Effect of Y2O3 content in the pack mixture on microstructure and hot corrosion resistance of the coatings was investigated. The results show that with the increase in Y2O3 content, the content of Co in the coatings increases. The mass gain of the coatings with Y2O3 addition of 1, 2 and 3wt.% is 0.6, 0.55 and 0.42mg/cm2 after hot corrosion at 1173K for 100h, respectively. Y2O3 addition accelerates the diffusion of Co and thus increases the hot corrosion resistance of the coating.

Formation of B-modified MoSi2 coating on pure Mo prepared through HAPC process

B-modified MoSi2 coatings were prepared on pure Mo through pack Si–B co-deposition process. The effects of pack mixture, co-deposition temperature and time on the formation of the Si–B co-deposition coating were studied by preparing the coatings in the pack mixtures 16Si–xB–4NaF–Bal.Al2O3 (wt.%, x=0.5–4) at 1100–1400°C for 2.5–10h. XRD, EDS, and WDS techniques were used to examine the microstructures of the coatings. Thermodynamics calculation about the equilibrium partial pressures of the volatile fluoride vapors in the pack aided experimental observation was introduced to reveal the coating formation mechanism. The results showed that the Si–B co-deposition coating had four layers: the MoSi2 outer layer with MoB precipitates distributed within its lower part, the Mo5Si3 second layer, the MoB third layer and the Mo2B inner layer. Pack mixture, co-deposition temperature and time made negligible effect on coating structure. The growth of the coating was dominated by the inward diffusion of Si and B. The deposited Si atoms mainly formed MoSi2 phase; the deposited B atoms either diffused into the substrate to form the MoB and Mo2B layers or generated MoB precipitates within the MoSi2 outer layer. The effects of B atoms within the Si–B co-deposition coating on coating oxidation manifested in two aspects: firstly, B atoms diffused to coating surface and oxidized to B2O3, which promoted the formation of a dense scale and reduced the coating weight loss; and secondly, the MoB layer acted as a barrier to the inward diffusion of Si, which slowed the degradation of MoSi2 to poor oxidation-resistant Mo5Si3 and would increase the coating lifetime.

Characterization of Aluminum Coatings Formed on Porous Ferritic Stainless Steels by Pack Cementation

Pack cementation is an attractive method for the formation of aluminum coatings on metal substrates. When applied to porous stainless steel substrates, the aluminum vapour species can diffuse into the pores and coat the interior surfaces. In this study, aluminum coatings were grown on commercial porous UNS 430 stainless steel and freeze cast UNS 440 stainless steel substrates by the pack cementation method. The effects of pack mixture composition, temperature, time, and the morphology of the substrates were characterized by scanning electron microscopy. It was shown that the degree of coverage of the aluminum coatings increased with porosity of the substrate. In addition, increased reaction time and temperature of the pack led to a more homogeneous distribution of aluminum across the substrate.