Polymer derived oxidation barrier coatings for Mo-Si-B alloys

Thermodynamic analysis was carried out to predict the phase composition of a perhydridopolysilazane-type polymer derived ceramic coating on Mo₅SiB₂ matrix particles after heat treatment. The most probable chemical reactions between these constituents and resulting phases were calculated. The feasibility of PHPS/Mo₅SiB₂ chemical reactions was proved experimentally. An amorphous SiO_xN_y phase and free Si were found by X-ray diffraction analysis and Raman spectroscopy. The presence of these two phases explain the improvement in oxidation resistance of the Mo₅SiB₂ particles, which was found to be as twice as high at 800 °C and 1100 °C in air, compared to the unprotected plain Mo₅SiB₂ phase. The oxidation of the free silicon provided by the PHPS conversion was addressed as an oxygen trap.     

TaSi2–SiC high-emissivity coating modified by SiB6 on flexible alumina fibre fabric with enhanced oxidation resistance and high interfacial bonding strength

To improve the oxidation inhibition of TaSi₂-based high-emissivity coatings at high temperatures, TaSi₂–SiC coating modified by SiB₆ was prepared on the surface of alumina fibre fabrics. The effects of the SiB₆ content on the surface appearance and emissivity of the coating were investigated, and the mechanical properties of the coated fabrics were compared. When the SiB₆ content in the coating was 2.5%, the borosilicate glass liquid phase generated by SiB₆ oxidation effectively prevented the oxidation of TaSi₂. The bond strength between the coatings and fibre fabric was 207 kPa after calcination at 1200 °C, which was 39% higher than that of the coated fabric without SiB₆. The emissivity of the TaSi₂–SiC coating, modified by a SiB₆ content of 2.5%, reached above 0.92 after calcination at 1200 °C for 5 h. Therefore, the TaSi₂–SiC high-emissivity coating modified by SiB₆ has good application prospects in the field of thermal protection.     

The improvement of the self-healing ability of MoSi2 coatings at 900–1200 °C by introducing SiB6

The brittleness of MoSi₂ ceramic and the thermal mismatch between MoSi₂ coating and C / C composite lead to brittle cracking of the coating at 900−1200 °C. This problem has been overcome in this studyby introducing submicron-SiB₆ into the coating. The pre-fabricated cracks and a kinetics model of hot-pressed SiB₆-MoSi₂ ceramic could quantitatively predict the glass growth and crack healing. As expected, enhancing temperature and SiB₆ content increased the growth rate of the borosilicate glass and the crack healing ability of MoSi₂ ceramic, which was ascribed to the lower oxidation activation energy and larger specific surface area of submicron-SiB₆. For the plasma sprayed coating, SiB₆ with submicron structure was benefit for cracking inhibition and formation of borosilicate glass during oxidation, reducing the oxygen permeability and the consumption of inner coating. Hence, the 15 % SiB₆-MoSi₂ coatings raised the protection times to 84 and 120 h at 900 and 1200 °C respectively, presenting favorable oxidation protective performance.     

Microstructure and oxidation resistance of a multiphase Mo-Si-B ceramic coating on Mo substrates deposited by a plasma transferred arc process

To improve the oxidation resistance of a Mo substrate, a multiphase Mo-Si-B ceramic coating was deposited using a plasma transferred arc (PTA) process. The phase constituents, microstructure, and oxidation resistance of the coating were investigated. The results show that the microstructure of the Mo-Si-B coating is characterised by rod-like Mo₅SiB₂ dendrites, a lamellar structure of Mo₃Si/Mo₅SiB₂ binary eutectics, and cellular Mo₅Si₃ dendrites. Compared with the Mo substrate, the Mo-Si-B coating exhibits a significant improvement in the oxidation resistance at 1300?°C in an air atmosphere. This is mainly owing to a dense and continuous borosilicate layer formed on the surface of the Mo-Si-B coating, which acts as an oxygen diffusion barrier. Moreover, it is observed that the Mo₅SiB₂ dendrites exhibit a higher oxidation resistance compared to Mo₃Si/Mo₅SiB₂ eutectics during high-temperature oxidation exposure. The oxidation behaviour is discussed based on the oxidation kinetics and a cross-sectional microstructure analysis of the oxidised Mo-Si-B specimen.     

Mechanical,Thermal, and Oxidation Properties of Refractory Hafnium and zirconium Compounds

The thermal conductivity, thermal expansion, Youngs Modulus, flexural strength, and brittle–plastic deformation transition temperature were determined for HfB₂, HfC_0·98, HfC_0·67, and HfN_0·92 ceramics. The oxidation resistance of ceramics in the ZrB₂–ZrC–SiC system was characterized as a function of composition and processing technique. The thermal conductivity of HfB₂ exceeded that of the other materials by a factor of 5 at room temperature and by a factor of 2·5 at 820°C. The transition temperature of HfC exhibited a strong stoichiometry dependence, decreasing from 2200°C for HfC_0·98 to 1100°C for HfC_0·67 ceramics. The transition temperature of HfB₂ was 1100°C. The ZrB₂/ZrC/SiC ceramics were prepared from mixtures of Zr (or ZrC), SiB₄, and C using displacement reactions. The ceramics with ZrB₂ as a predominant phase had high oxidation resistance up to 1500°C compared to pure ZrB₂ and ZrC ceramics. The ceramics with ZrB₂/SiC molar ratio of 2 (25 vol% SiC), containing little or no ZrC, were the most oxidation resistant.     

A new route to fabricate SiB4 modified C/SiC composites

In order to improve the oxidation and thermal shock resistance of 2D C/SiC composites, dense SiB₄–SiC matrix was in situ formed in 2D C/SiC composites by a joint process of slurry infiltration and liquid silicon infiltration. The synthesis mechanism of SiB₄ was investigated by analyzing the reaction products of B₄C–Si system. Compared with the porous C/SiC composites, the density of C/SiC–SiB₄ composites increased from 1.63 to 2.23 g/cm³ and the flexural strength increased from 135 to 330 MPa. The thermal shock behaviors of C/SiC and C/SiC–SiB₄ composites protected with SiC coating were studied using the method of air quenching. C/SiC–SiB₄ composites displayed good resistance to thermal shock, and retained 95% of the original strength after being quenched in air from 1300 °C to room temperature for 60 cycles, which showed less weight loss than C/SiC composite.     

Improved mechanical properties of SiB6 reinforced silica-based ceramic cores fabricated by 3D stereolithography printing

Silica-based ceramic core is an extremely critical component in the manufacture of hollow blades during investment casting. However, the traditional preparation methods rely more on the molds, and the manufacturing costs are relatively high. In this study, silica-based ceramics with silicon hexaboride (SiB₆) addition were prepared via 3D stereolithography printing. And the effects of the SiB₆ content on mechanical properties of the obtained ceramic samples were explored. As the SiB₆ content increased to 2.0 wt%, the linear shrinkage gradually decreased, while the room temperature and high temperature flexural strength were enhanced at the SiB₆ content from 0 to 1.0 wt% and reduced as the SiB₆ content further rose. As the SiB₆ content increased to 1.0 wt%, the linear shrinkage was reduced to 1.86% resulting from the oxidation reaction of SiB₆. Furthermore, with 1.0 wt% SiB₆ addition, the flexural strength of the samples at room temperature was enhanced from 6.75 MPa to 14.63 MPa due to the sintering promotion of oxidation product B₂O₃, and the flexural strength at 1550 °C was improved from 7.68 MPa to 13.08 MPa because of the enhanced β-cristobalite content, which is suitable for high temperature casting of ceramic cores. Therefore, it demonstrates the capability of fabricating SiB₆ reinforced silica-based ceramic cores with high performance via stereolithography.     

Passive filler loaded polysilazane-derived glass/ceramic coating system applied to AISI 441 stainless steel,part 1: Processing and characterization

This study describes an oxidation and corrosion resistant environmental barrier coating (EBC) applied to an AISI 441 stainless steel substrate. For this purpose, four polymer-derived ceramic (PDC) coating systems were developed. These coating systems consisted of a bond coat applied by dip coating, and a top-coat that was loaded with passive fillers and deposited by spray coating. The microstructures of the coatings were investigated using optical microscopy and scanning electron microscopy, including energy dispersive spectroscopy (EDS). X-ray powder diffraction (XRD) was used to investigate the phase composition of the coatings. The optimized composite top coatings were prepared from the preceramic polymer HTT1800, filled with yttria-stabilized zirconia and a specially tailored Al₂O₃–Y₂O₃–ZrO₂ (AYZ) passive filler, and commercial barium silicate glasses were used as sealing agents. After thermal treatment in air at 750°C, uniform and crack-free composite coatings on stainless steel substrates were developed, with thicknesses of up to 93 μm. Oxidation tests, which were performed at 850°C in synthetic air, showed that every tested coating system remained undamaged by oxidation and showed good bonding to the metal substrate.     

Boron-modified phenol formaldehyde resin-based self-healing matrix for Cf/SiBOC composites

C_f/SiBOC was fabricated from 2D carbon fabric as reinforcement and slurry-containing boron-modified phenol formaldehyde (BPF) resin with silicon as matrix resin using reaction-bonded silicon carbide method. The processing involves synthesis of (BPF) resin by reacting various amount of boric acid with phenol formaldehyde resin, polymer to ceramic transformation at 1450°C under argon atmosphere, with and without silicon, thermal transformation of the polymer matrix composite into a ceramic matrix composite and evaluation of isothermal oxidation for ceramics and its composites at 1000, 1250 and 1500°C. The ceramic studies, confirmed the formation of B₄C, SiC and SiB₄ (SiBOC) mixed phase and the role of boron as a catalyst for graphitisation of free carbon present in the ceramic. Oxidation of C_f/SiBOC composite at various temperatures leads to the formation of borosilicate glass which heals the cracks, hindering the inwards diffusion of oxygen.     

Effect of silicon/graphite ratio and temperature on oxidation protective properties of SiC/ZrB2–SiC coatings prepared by pack cementation

To investigate the silicon/graphite ratio and temperature on preparation and properties of ZrB₂–SiC coatings, ZrB₂, silicon, and graphite powders were used as pack powders to prepare ZrB₂–SiC coatings on SiC coated graphite samples at different temperatures by pack cementation method. The composition, microstructure, thermal shock, and oxidation resistance of these coatings were characterized and assessed. High silicon/graphite ratio (in this case, 2) did not guarantee higher coating density, instead could be harmful to coating formation and led to the lump of pack powders, especially at temperatures of 2100 and 2200 °C. But residual silicon in the coating is beneficial for high density and oxidation protection ability. The SiC/ZrB₂–SiC (ZS50-2) coating prepared at 2000 °C showed excellent oxidation protective ability, owing to the residual silicon in the coating and dense coating structure. The weight loss of ZS50-2 after 15 thermal shocks between 1500 °C and room temperature, and oxidation for 19 h at 1500 °C are 6.5% and 2.9%, respectively.     

Experimental and first-principles simulation study on oxidation behavior at 1700 °C of Lu2O3–SiC-HfB2 ternary coating for SiC coated carbon/carbon composites

The oxidation behavior and microstructure evolution of Lu₂O₃–SiC-HfB₂ ceramic coating specimen at 1700 °C were investigated systematically by experimental study and first-principles simulation. The prepared ternary coating possesses a compact morphology, which effectively defends C/C substrate against oxidation at 1700 °C for 130 h, showing a good antioxidant property. The formed HfSiO₄, Lu₂Si₂O₇, and HfO₂ with high melting points play an active role in developing the thermal stability of the oxidized scale. Besides, Lu and Hf atoms incline to diffuse into SiO₂, which enhances its structural stability. The improved thermal property of the oxidized scale for the Lu₂O₃–SiC-HfB₂/SiC ceramic coating can delay the effective delivery of oxygen inwardly and thus prolong its oxidation protection time. The quick volatilization of SiO₂ at 1700 °C induces that some glass phase evaporates with being not completely stabilized, which causes the formation of holes and the consumption of the inner coating.     

Oxidation behavior of Al2O3 reinforced MoSi2 composite coatings fabricated by vacuum plasma spraying

MoSi₂, MoSi₂–10 vol.% Al₂O₃, MoSi₂–30 vol.% Al₂O₃ (denoted as MA0, MA1, MA3, respectively) coatings were fabricated by vacuum plasma spraying (VPS), and their oxidation behavior was examined at low temperature (500 °C) and high temperature (1500 °C). The test at 500 °C showed that the addition of Al₂O₃ effectively restrained the pest oxidation of MoSi₂. The MA1 coating had satisfactory fluid surface and presented good oxidation resistance at 1500 °C. However, the MA3 coating showed worse oxidation resistant behavior compared with the MA0 coating because of mullite formation.     

Oxidation protective MoSi2-SiC-Si coating for graphite materials prepared by slurry dipping and vapor silicon infiltration

A novel kind of dense MoSi₂-SiC-Si coating was prepared on the surface of graphite substrate by slurry dipping and vapor silicon infiltration process. Mo-SiC-C precoating was fabricated via slurry dipping method, and then MoSi₂-SiC-Si coating with dense structure consisting of Si, MoSi₂ and SiC was obtained by vapor silicon infiltration process. The isothermal oxidation tests at temperatures from 800 to 1600 °C and TGA test from room temperature to 1500 °C were used to evaluate the oxidation resistance ability of the MoSi₂-SiC-Si coating. The experimental results indicate that the prepared coating has good oxidation protection ability at a wide temperature range from room temperature to 1600 °C. Meanwhile, the oxidation of the coated samples is a weight gain process at temperatures from 800 to 1500 °C due to the formed SiO₂ layer on the surface of coating. After oxidation for 220 h at 1600 °C, the weight loss of the coated sample was only 0.96%, which is considered to be the excessive consumption of the outer coating and the appearance of defects in the coating. Two layers can be observed in the coating after oxidation, namely, SiO₂ layer and MoSi₂-SiC-Si layer.     

Effect of SiC-mullite coatings on oxidation resistance of graphite

Abstract

Two layers oxidation protecting coatings of SiC and mullite were successfully created on the graphite substrate by pack cementation and plasma spray methods, respectively. Phase synthesis was studied by X-ray diffraction. Microstructure and morphology were investigated by optical and scanning electron microscopes (SEM). X-ray diffraction results of the first coating showed that the secondary SiC was completely synthesised by the heat treatment of Si, Al₂O₃, C and SiC at 1500°C. Alumina and kaolin reaction at 1400°C led to the formation of mullite as the second layer. The oxidation resistance of two layers coating was considerably improved in comparison with mono layer coating and raw graphite. Oxidation resistance was decreased at higher temperatures in all samples. Formation of SiO₂ glassy phase improved the graphite oxidation resistance which was confirmed by SEM.     

Comparative study of failure mechanisms of MoSi2 coating on Mo1 wire mesh under isothermal oxidation and hot-fire test using a hydroxylammonium nitrate based monopropellant

A MoSi₂ coating was prepared on the Mo1 wire mesh via pack cementation method, and its failure mechanisms under isothermal oxidation and hot-fire test using a hydroxylammonium nitrate based monopropellant were comparatively studied. Under isothermal oxidation at 1300 °C and 1400 °C, degradation of MoSi₂ into Mo₅Si₃ caused failure of the coating, and interdiffusion made a much larger effect relative to oxidation. However, the MoSi₂ coating failed because of the synergy of oxidation, ablation, and interdiffusion under hot-fire test. Besides, dissolution of mullite into SiO₂ and ablation of high velocity flame contributed to the failure of the coating as well.     

Enhanced mechanical properties of carbon fibre/lithium aluminosilicate composites modified by SiB6 addition

ABSTRACT

Carbon fibre-reinforced lithium aluminosilicate matrix composites (C_f/LAS) with different SiB₆ contents were prepared by the hot pressing method to assess their mechanical properties and oxidation resistance. Composite that was incorporated with 2 wt-% SiB₆ exhibited the highest flexural strength of 500 ± 22.3 MPa. Weight loss and residual strength of C_f/LAS modified by SiB₆ were analysed. The results indicated that the addition of SiB₆ had a remarkable effect on improving the oxidation resistance for C_f/LAS. To establish a direct relationship among interfacial microstructure, mechanical and oxidation behaviour of the studied composites, their connection was examined and discussed.     

Dynamic oxidation and protection of the PAN pre-oxidized fiber C/C composites

Pre-oxidized fibers as reinforcement are candidates for reducing the overall cost of C/C composites with superior properties. This study investigated the dynamic oxidation and protection of the pre-oxidized fiber C/C composites (Pr-Ox-C-C). According to the Arrhenius equation, the oxidation kinetics of the Pr-Ox-C-C consisted of two different oxidation mechanism with the transition point was at about 700 °C. Scanning electron microscopy investigation showed that oxidation initiated from the fiber/matrix interface of composites, whereas the matrix carbon was easily oxidized. To improve the anti-oxidant properties of Pr-Ox-C-C, a ceramic powder-modified organic silicone resin/ZrB₂-SiC coating was prepared by the slurry method. The coated samples were subjected to isothermal oxidation for 320 h at 700 °C, 800 °C, 900 °C, 1000 °C and 1100 °C with incurred weight losses of ? 1.6%, 0.77%, ? 1.28%, 0.68% and 1.19%, respectively. After 110 cycles of thermal shock between 1100 °C and room temperature, a weight loss of 1.30% was obtained. The Arrhenius curve presented four different phases and mechanisms for coating oxidation kinetics. The excellent oxidation resistance properties of the prepared coating could be attributed to the inner layer which was able to form B₂O₃-Cr₂O₃-SiO₂ glass to cure cracks, and the ZrB₂-SiC outer layer that could provide protective oxides to reduce oxygen infiltration and to seal bubbles.     

Passive filler loaded polysilazane-derived glass/ceramic coating system applied to AISI 441 stainless steel,part 2: Oxidation behavior in synthetic air

This article features the oxidation behavior of ferritic stainless steel grade AISI 441 coated with protective polymer-derived ceramics (PDC). Two PDC compositions are studied with respect to their oxidation resistance in a flow-through atmosphere of synthetic air at temperatures of up to 1000°C. The coatings contain a combination of six passive fillers: Y-containing ZrO₂, glass microspheres, alumina-yttria-zirconia (AYZ) powder, and three commercial glasses. They are pyrolyzed in air for 1 hour at 800°C with heating and cooling rates of 3 K/min. Detailed microstructural examination of the oxide products formed at the surface of samples after exposure to air at 900°C, 950°C, and 1000°C for 1-48 hours is analyzed. Both uncoated steel and steel coated with two of the protective systems described in part 1 of this article are investigated. Fe, Cr₂O₃, TiO₂, and a spinel of the composition (Mn,Cr)₃O₄ are identified at the oxidized surface of the steel substrate using X-ray diffraction. A significant weight gain of the unprotected steel is measured after all experiments, while oxidation tests of the coated steel show a negligible weight gain after 900°C and 950°C. During the early stages of coating oxidation, the monoclinic-to-tetragonal ratio in the zirconia filler is shifted toward the monoclinic modification. Longer exposures and higher temperatures lead to the formation of yttrium aluminum garnet (YAG) due to glass microsphere crystallization and solid state reactions in the AYZ powder. The crystallization of the three commercial glasses functioning as sealants leads to the formation of Ba(AlSiO₄)₂ also known as hexacelsian, which subsequently transforms to celsian. YZr₈O₁₄ is also formed. The protective effect of the PDC coatings applied to the stainless steel is demonstrated up to 950°C.     

Effect of HfO2 framework on steam oxidation behavior of HfO2 doped Si coating at high temperatures

HfO₂ doped Si is designed as bond coat material in thermal/environmental barrier coatings (TEBCs). In this work, the HfO₂-Si composite coatings with different HfO₂ contents were prepared by atmospheric plasma spraying (APS). The steam oxidation behavior of the coatings was comparatively studied at 1300 °C and 1400 °C. Volatilization of Si occurred during spraying, leading to the deviation of coating compositions. The sprayed coatings contained different HfO₂ structures. During steam oxidation, HfSiO₄ phase was formed at the SiO₂/HfO₂ interface by solid-state reaction between them. The HfSiO₄ or HfO₂/HfSiO₄ mixture particles worked to deflect or pin micro-cracks, thus improving the resistance of the coating to cracking. At 1300 °C, a protective oxide scale was formed on the traditional Si coating or the HfO₂-Si coating with isolated HfO₂ particles. However, the HfO₂-Si coating with inter-connected HfO₂ framework revealed poor oxidation-resistance. At 1400 °C, accelerated oxidation degradation, steam corrosion volatilization, interface reaction and sintering occurred. The HfO₂ framework structure played a dominating role in determining the steam oxidation resistance of the HfO₂-Si coating, and the connected HfO₂ framework and TGO network provided a rapid diffusion path for oxidants (H₂O, O^2? and OH^?) and deteriorated the oxidation resistance.     

Influence of annealing temperature on microstructure evolution of TiAlSiN coating and its tribological behavior against Ti6Al4V alloys

TiAlSiN multicomponent coating, owing to its high hardness and excellent high temperature resistance, was widely used in the cutting field of difficult-to-cut materials such as titanium alloys. For machining titanium alloys, high temperature is easy to gather on the tool chips and deteriorate the cutting tools. Moreover, high temperature will also promote the microstructure evolution and make the wear mechanism more complex. In this paper, TiAlSiN coatings were deposited on cemented carbides and annealed at 400 °C, 600 °C and 800 °C respectively for 60 min in air, followed by reciprocating friction tests against Ti6Al4V counterparts. AFM, SEM, EDS and XPS were applied to investigate the microstructure evolution and tribological behavior of TiAlSiN coating after high temperature annealing. The results demonstrated that the oxidation resistance of TiN phase in TiAlSiN coating was worse than Si₃N₄ and AlN phases. These nitrides can be oxidized to TiO₂, SiO_x and AlO_x under 600 °C, and the depth of oxide layer was increased with the rising annealing temperature, resulting in the coarsened microstructure. The wear mechanisms of as-deposited TiAlSiN coating were oxidation wear and adhesion wear. With the rising annealing temperature, abrasive wear was gradually enhanced. For the TiAlSiN coating annealed at 800 °C, abrasive wear became the dominant wear mechanism.