A novel approach for protecting the surface of TZM alloy against high-temperature oxidation is proposed, integrating pack cementation with the deposition of a preceramic polymer coating. Pack cementation was performed via a contact method in a powder mixture. The borosiliconizing process resulted in the formation of a two-layer coating: an outer layer composed of MoSi2 and Mo5Si3 with inclusions of (Ti, Zr)B2, TiC, MoB2, Mo2B5, and SiO2, and an inner layer consisting of a boride phase MoB. A mechanism for protective layer formation was established. To seal microcracks, reduce porosity, and enhance oxidation and thermal resistance, a second treatment stage was applied-deposition of a polyhydridosilazane-based preceramic coating (PHPS) loaded with silicon and boron particles. The chemical and phase compositions of the coatings were characterized, and the oxidation kinetics of coated TZM alloy were investigated. The combined borosiliconizing-PHPS treatment was found to promote the formation of a thermally stable oxide-nitride film that seals defects, reduces oxygen diffusivity, and provides an effective barrier against high-temperature oxidation. The resulting multilayer coating system exhibits excellent thermal stability and is suitable for prolonged service under extreme temperature conditions.

The use of molybdenum-based alloys as materials for components operating under high temperatures and significant mechanical loads is widely recognized due to their excellent mechanical properties. However, their low high-temperature resistance remains a critical limitation, which can be effectively mitigated by applying protective coatings. In this study, we investigate the influence of a two-step coating process on the properties and performance of the TZM molybdenum alloy. In the first step, pack cementation was performed. Simultaneous surface saturation with aluminum and silicon, a process known as aluminosiliconizing, was conducted at 1000 °C for 6 h. The saturating mixture comprised powders of aluminum, silicon, aluminum oxide, and ammonium chloride. The second step involved the application of a pre-ceramic coating based on polyhydrosiloxane modified with silicon and boron. This treatment effectively eliminated pores and cracks within the coating. Thermodynamic calculations were carried out to evaluate the likelihood of aluminizing and siliconizing reactions under the applied conditions. Aluminosiliconizing of the TZM alloy resulted in the formation of a protective layer 20–30 µm thick. The multiphase structure of this layer included intermetallics (Al63Mo37, MoAl3), nitrides (Mo2N, AlN, Si3N4), oxide (Al2O3), and a solid solution α-Mo(Al). Subsequent treatment with silicon- and boron-modified polyhydrosiloxane led to the development of a thicker surface layer, 130–160 µm in thickness, composed of crystalline Si, amorphous SiO2, and likely amorphous boron. A transitional oxide layer ((Al,Si)2O3) 5–7 µm thick was also observed. The resulting coating demonstrated excellent structural integrity and chemical inertness in an argon atmosphere at temperatures up to 1100 °C. High-temperature stability at 800 °C was observed for both coating types: aluminosiliconizing, and aluminosiliconizing followed by the pre-ceramic coating. Moreover, additional oxide layers of SiO2 and B2O3 formed on the two-step coated TZM alloy during heating at 800 °C for 24 h. These layers acted as an effective barrier, preventing the evaporation of the substrate material.

Theoretical calculations of the physicochemical conditions of molybdenum-based alloys' siliconizing and boron–siliconizing processes were carried out. Based on the obtained data, the phase composition and its quantitative ratio in the coatings were predicted. The theoretical calculations were experimentally confirmed. The coatings' microstructure, phase, and chemical composition obtained by siliconizing and boron–siliconizing TZM molybdenum alloy in a chlorine medium were studied. The thickness and microhardness of protective coatings are determined. The heat resistance of the obtained coatings in an air atmosphere at 800⁰C for 48 hours was investigated. A recommendation for further use is provided.

The physicochemical conditions of the siliconizing and boron–siliconizing processes of molybdenum-based alloys in a closed reaction space in an environment of chlorine and fluorine at reduced pressure were studied. Theoretical calculations of the equilibrium composition of systems with the participation of silicon, boron, molybdenum, nitrogen, oxygen, chlorine, and fluorine were carried out, which made it possible to determine the influence of process parameters (temperature, composition of the reaction medium) on the probable phase composition of the obtained coatings. Based on thermodynamic calculations, the composition and rational consumption of the initial powders and the temperature intervals of the chemical heat treatment (CHT) during the complex saturation of molybdenum-based alloys with silicon and boron were modeled. It was established that it is advisable to use chlorine as an activator, which leads to the formation of molybdenum chlorides MoCl4 and MoCl3 in the composition of the gas phase and can indicate the flow of exchange reactions between chlorides and the matrix of the processed material in the reaction space. The rational saturation temperature of alloys based on molybdenum with silicon and boron is determined—1100–1250 °C. The possibility of the existence of condensed phases MoSi2, MoB2.15, B6Si, MoB1.65, and MoB is shown.

Improvement of wear, corrosion, and heat-resistant properties of coatings to expand the operational capabilities of metals and alloys is an urgent problem for modern enterprises. Diffusion titanium, chromium, and aluminum-based coatings are widely used to solve this challenge. The article aims to obtain the corrosion-electrochemical properties and increase the microhardness of the obtained coatings compared with the initial Ti-6Al-4V alloy. For this purpose, corrosion resistance, massometric tests, and microstructural analysis were applied, considering various aggressive environments (acids, sodium carbonate, and hydrogen peroxide) at different concentrations, treatment temperatures, and saturation times. As a result, corrosion rates, polarization curves, and X-ray microstructures of the uncoated and coated Ti-6Al-4V titanium alloy samples were obtained. Histograms of corrosion inhibition ratio for the chromium-aluminum coatings in various environments were discussed. Overall, the microhardness of the obtained coatings was increased 2.3 times compared with the initial Ti-6Al-4V alloy. The corrosion-resistant chromaluminizing alloy in aqueous solutions of organic acids and hydrogen peroxide was recommended for practical application in conditions of exposure to titanium products.

Copper, aluminum, and alumina foams are prepared by a combination of reticulated foaming and freezing processing. This results in all of these foams in an additional strut porosity consisting of lamellar pores and material lamellae; the lamellar pores enable designed access to the inner volume of the struts. To increase the functionality in terms of enlarging the total surface area and increasing the adsorption activity, the foams are loaded with microporous materials by direct crystallization: the copper foams are combined with the metal organic framework HKUST‐1, the aluminum foams are combined with the zeolite SAPO‐34, and the alumina foams are combined with the zeolite silicalite‐1. All microporous materials are shown to access the inner strut pore surface area. Because of different crystal sizes of the different microporous materials, the load of the inner strut surface area varies significantly.

Reticulated alumina ceramic foams manufactured by the Schwartzwalder technique are coated with a sodium aluminosilicate glass using an aqueous precursor containing alumina particles and sodium silicate. After a heat treatment, a well‐adhering glass layer with a thickness between 10 and 120 μm is obtained. Besides the strut surface, the glass phase also penetrates the hollow strut cavities as well as strut cracks. The total porosity of the glass‐coated foams is between 88% and 90%, and the open cellular structure is not negatively affected by the glass coating. The compressive strength increases by a factor of two under consideration of the decrease in total porosity; a compressive strength in the order of magnitude of 2 MPa is feasible. An ion exchange of Na+ by K+ into the glass coating layer (chemical strengthening) is successfully demonstrated, but does not result in an additional improvement of the compressive strength.

Open-celled aluminum nitride ceramic foams were prepared by the polymer sponge replication technique involving aqueous dispersions of passivated AlN. The amount of the Y2O3 and Dy2O3 as sintering aid was varied, and the effects on the densification, microstructure formation, phase composition, and finally, the thermal conductivity were investigated. A typical thermal conductivity of 1.1 W m(-1) K-1 was determined for foams at a porosity level of 94.3 vol.%, on average. This measured foam thermal conductivity was subsequently modeled using different porosity <-> thermal conductivity relations considering the different hierarchical levels of porosity in these foams. From these models, the thermal conductivity of the bulk AlN strut material was determined, correlated with the strut microstructure and the phase composition, and compared to literature data.

Ceramic foams are made of zinc oxide using different amounts of Sb 2 O 3 and Bi 2 O 3 as sintering aids. The effect of a ball milling processing of the starting powders and the sintering temperature on the microstructure and the properties of the ZnO foams is investigated. The focus is set on the evolution of the secondary phases formed within the microstructure of ZnO. A determining effect is identified in the amount of an Al 2 O 3 impurity which is introduced by abrasion of the milling vessels during ball milling. Alumina is partially dissolved in a spinel α–Zn 7 Sb 2 O 12 secondary phase which is stabilized by a reduction of the unit cell volume. Remaining Al 2 O 3 is incorporated into zinc oxide under formation of a defect wurtzite phase. The phase evolution is a complex function of the content of sintering aids, the Al 2 O 3 impurity level and the sintering temperature. The shrinkage during sintering and the porosity evolution are correlated to the phase composition within the ZnO material. The thermal conductivity and the compressive strength of the foams are determined, normalized with respect to their porosity, and correlated to the microstructure and phase composition of the ZnO strut material.

Poly(silsesquioxane)‐based polymer‐derived ceramics (PDCs), may be manufactured by a great variety of processes and process combinations leading to ceramic parts seven with high functionality. Advantage of the use of poly(silsesquioxane)s as starting materials with a defined amount of oxygen is that low‐temperature‐processing steps such as shaping and cross‐linking may be carried out in air, and a disadvantage often discussed is a maximum service temperature below 1200 °C. In this article, the focus is set to PDC tapes, coatings, tailored surfaces, and tailored porosity. And, a rough distinction is made between materials pyrolyzed below 1200 °C and far above 1200 °C. The later provides porosity formed by high‐temperature reactions between particulate fillers and the ceramic residue from pyrolysis.

The manufacturing of aluminium foams with a total porosity of 87% using the sponge replication method and a combination of the sponge replication and freezing technique is presented. Foams with different cell counts were prepared from polyurethane (PU) templates with a pore count per inch (ppi) of 10, 20 and 30; consolidation of the foams was performed in an argon atmosphere at 650 °C. The additional freezing steps resulted in lamellar pores in the foam struts. The formation of lamellar pores increased the specific surface area by a factor of 1.9 compared to foams prepared by the sponge replication method without freezing steps. The formation of additional lamellar pores improved the mechanical properties but reduced the thermal conductivity of the foams. Varying the pore cell sizes of the PU template showed that-compared to foams with dense struts-the highest increase (~7 times) in the specific surface area was observed in foams made from 10 ppi PU templates. The effect of the cell size on the mechanical and thermal properties of aluminium foams was also investigated.

Open cellular ceramic foams were manufactured from plain and chromia-doped alumina, with a chromium concentration ranging between 1.25 mol% and 5.0 mol%. The (AlCr)(2)O-3 starting powders were prepared by precipitation of a chromia precursor onto the surface of an alumina powder and subsequent calcination. Characterization of the starting powders as well as the foam samples made therefrom were carried out with respect to the chromium concentration in the alumina phase and the influence of the dopant on the cellular structure and sintering behavior of the doped material. While no positive effect on the compressive strength of the ceramic foams was found, the dopant influences the sintering behavior resulting in an increased shrinkage and in a reduction of total porosity.

Highly porous SiOC ceramic foams with gradient or uniform macrostructures were obtained through polymer derived ceramic routes. Precuring of preceramic polymers and introduction of SiO2 powders were used to tailor precursor viscosity and hence SiOC foam macrostructure. Effects of polymer viscosity on porosity, pore size, pore distribution were investigated by light microscopy and micro-computed tomography techniques. SiOC ceramic foams. Foams from one unmodified precursor, showed pore size gradient with small pores located at bottom and large pores at the top. To address this non-uniformity, the viscosity of the precursor was increased by pre-curing the preceramic polymer, which resulted in decrease of the average pore size and improvement in pore size uniformity. For a different system with a self-foaming preceramic polymer, because of the simultaneous release of foaming gases and rapid increase in viscosity during crosslinking, the foam had non-uniform macrostructure with large pores and thick struts at the bottom. By addition of SiO2 fillers, the crosslinking reaction rate was reduced leading to homogeneous pore nucleation and uniform small pore size foams.

Copper foams are manufactured from polyurethane template foams with a cell size of 10 and 30 ppi using two techniques: Sponge replication and a combination of sponge replication with additional freezing steps to obtain lamellar strut pores. The total porosity of foams is ≈93% with and ≈87% without additional freezing, respectively. The foams possess randomly oriented lamellar pores with a thickness of 40–56 μm. The thickness of the material lamellae is 72–80 μm. The specific surface area is increased by the additional lamellar pores; this value ranges from 24 to 42 cm 2  g −1 for the 10 ppi templated foams. The influence of the cell size of the template foams and the lamellar pores within the struts on the (strut) porosity, the mechanical properties and the thermal conductivity is investigated.

To get a better insight into the coating behavior of a polymer-derived ceramic material, we model and simulate the diffusion, oxidation and reaction-induced volume expansion of a specimen without outer mechanical loads. In this macroscale approach, we use an oxidation state variable which determines the composition of the starting material and the oxide material. The model contains a reaction rate which is based on the change of the free energy due to a change of the concentrations of the starting material, the oxide material and a diffusing gaseous material. Using this, we model a growing oxide layer in a perhydropolysilazane (PHPS)-based polymer-derived ceramic (PDC), containing silicon filler particles. Within the mechanical part of the modeling, we use the Neo-Hookean material law which allows for the consideration of volume expansion and the diffusion kinematics in terms of finite deformations. We derive this continuum formulation in 3D and reduce it later to 1D, as we show that a 1D formulation is sufficient for thin oxide layers in our consideration. In such a case, the reaction-induced volume expansion is mostly limited to strains orthogonal to the oxide layer, as the bulk material hinders transversal deformation. Both formulations, i.e., 1D and 3D, are implemented in the finite element software FEAP. We perform a parameter study and fit the results with experimental data. We investigate the diffusion kinematics in the presence of volume expansion. Additionally, we discuss the influence of the elastic energy on the reaction rate.

Open‐porous copper foams with additional strut porosity are manufactured by two different manufacturing routes. The first is based on the Schwarzwalder sponge replication technique. The second method is a combination of Schwartzwalder sponge replication and freezing technique in which an additional strut porosity is generated inside the struts of the sponge‐replicated foams by freezing at −20 °C for 24 h and subsequent sublimation. Thermal processing of both types of foams is conducted at 500 and 900 °C for 6 h in a hydrogen‐containing atmosphere to reduce copper oxides and to facilitate the sintering process of the copper powder particles. Despite significant shrinkage of both foam series after thermal processing, hollow struts and lamellar pores keep their shape and do not collapse. The influences of the additional lamellar pores and thermal processing temperature on the cellular structure, porosity, specific surface area, yield strength, absorbed energy, and thermal conductivity are studied. The additional strut porosity generated by the freezing step significantly increases the specific surface area of the copper foams by a factor of 2 in comparison to the sponge replicated foams.

Ceramics are widely used as implant materials; however, they are brittle and may emit particles when used in these applications. To overcome this disadvantage, alumina foams, which represent a 3D cellular structure comparable to that of human trabecular bone structures, were sputter coated with platinum, tantalum or titanium and modified with fibronectin or collagen type I, components of the extracellular matrix (ECM). To proof the cell material interaction, the unmodified and modified materials were cultured with (a) mesenchymal stem cells being a perfect indicator for biocompatibility and releasing important cytokines of the stem cell niche and (b) with fibroblasts characterized as mediators of inflammation and therefore an important cellular component of the foreign body reaction and inflammation after implantation. To optimize and compare the influence of metal surfaces on cellular behavior, planar glass substrates have been used. Identified biocompatible metal surface of platinum, titanium and tantalum were sputtered on ceramic foams modified with the above-mentioned ECM components to investigate cellular behavior in a 3D environment. The cellular alumina support was characterized with respect to its cellular/porous structure and niche accessibility and coating thickness of the refractory metals; the average cell size was 2.3 mm, the average size of the cell windows was 1.8 mm, and the total foam porosity was 91.4%. The Pt, Ti and Ta coatings were completely dense covering the entire alumina foam surface. The metals titanium and tantalum were colonized very well by the stem cells without a coating of ECM components, whereas the fibroblasts preferred components of the ECM on the alumina foam surface.

Molybdenum‐based materials offer high melting temperatures and promising mechanical properties; therefore, they are potential candidates for high‐temperature components, such as turbine blades. However, at temperatures above 700 °C, molybdenum suffers from severe pesting phenomenon, leading to decomposition of the component. Therefore, oxidation‐resistant environmental barrier coatings are crucial to prevent the material from degradation and to maintain its excellent mechanical properties at high temperatures. This review provides a detailed overview on the different coating concepts for Mo and Mo‐based alloys.

Alumina replica foams were manufactured by the Schwartzwalder sponge replication technique and were provided with an additional strut porosity by a freeze-drying/ice-templating step prior to thermal processing. A variety of thickeners in combination with different alumina solid loads in the dispersion used for polyurethane foam template coating were studied. An additional strut porosity as generated by freeze-drying was found to be in the order of ~20%, and the spacings between the strut pores generated by ice-templating were in the range between 20 µm and 32 µm. In spite of the lamellar strut pore structure and a total porosity exceeding 90%, the compressive strength was found to be up to 1.3 MPa. Combining the replica process with freeze-drying proves to be a suitable method to enhance foams with respect to their surface area accessible for active coatings while preserving the advantageous flow properties of the cellular structure. A two-to-threefold object surface-to-object volume ratio of 55 to 77 mm−1 was achieved for samples with 30 vol% solid load compared to 26 mm−1 for non-freeze-dried samples. The freeze-drying technique allows the control of the proportion and properties of the introduced pores in an uncomplicated and predictable way by adjusting the process parameters. Nevertheless, the present article demonstrates that a suitable thickener in the dispersion used for the Schwartzwalder process is inevitable to obtain ceramic foams with sufficient mechanical strength due to the necessarily increased water content of the ceramic dispersion used for foam manufacturing.