Preparation of TiC-Ti3AlC composite coated graphite flakes and their improved oxidation resistance

The weak oxidation resistance has severely hindered graphite from various high temperature applications, therefore in this paper a molten salt technique was proposed to prepare titanium aluminium carbide based coatings on graphite flakes to overcome this problem. The resultant TiC-Ti₃AlC coated graphite showed higher peak oxidation temperature (~?900?°C) than the uncoated one (700?°C), suggesting that the coatings will afford graphite with superior oxidation resistance. Such improvements can be largely ascribed to not only the homogenous and crack-free TiC-Ti₃AlC coatings and their resultant relicts of TiO₂ and Al₂O₃, but also the molten salt technique for preparing the coatings. Especially the molten salts offer fast dissolution/melting/dispersion of Ti/Al powders, rapid reaction with graphite in the salt melts, and homogenous growth of the carbide coatings on the surface of graphite flakes at as low temperatures as 950–1150?°C.     

Synthesis of fine and high purity Cr2AlC powders by novel method

Cr₂AlC powders using Cr/Al/C and Cr/Al/Cr₃C₂ systems as raw materials were successfully synthesised by a microwave hybrid heating method for the first time. The mixtures of Cr, Al and graphite or Cr₃C₂ with different molar ratios were used to investigate the formation of Cr₂AlC phase. For Cr/Al/C with the molar ratios of 2:(1.0–1.2):1 system, Cr₂AlC with a small amount of Cr₇C₃ and Cr₂Al was synthesised at 1000°C for 3 min, and the average particle size was ?1?μm. For Cr/Al/Cr₃C₂ with the molar ratio of 1:2:1 system, high purity Cr₂AlC powders was synthesised at 1000°C for 3 min, and the average particle size was ?1?μm. The synthesis of high purity Cr₂AlC powders for short time was attributed to the combination of the hybrid heating effect and the introduction of Cr₃C₂ as carbon source. Microwave hybrid heating is a promising method for the preparation of various other MAX phases.     

Molten salt synthesis of orthorhombic CrB and Cr2AlB2 ceramics

High-purity and fine CrB powders are firstly synthesized from Cr and B powders in (Na, K)Cl molten salt at 850 °C for 2 h. Then the as-achieved CrB powders are used as precursors to synthesize homogeneous and size-controlled Cr₂AlB₂ powders in (Na, K)Cl molten salt at 900 °C for 2 h. Additionally, the formation mechanisms of CrB and Cr₂AlB₂ in (Na, K)Cl molten salt are also investigated. Results show that the Cr atom, formed by disproportionation of Cr²⁺ ions in molten salt, reacts in-situ with B powders to form CrB. For the formation of Cr₂AlB₂, Al and CrB migrate mutually and assemble firstly in the molten salt, and then Al intercalates into CrB to generate Cr₂AlB₂.     

Preparation of in situ grown silicon carbide whiskers onto graphite for application in Al2O3-C refractories

C-SiC composite powders were prepared by salt-assisted synthesis from Si powders, graphite, and a molten salt medium (NaCl and NaF) with the molar ratio of Si/C =?1/2 at 1300?°C for 3?h. After the C-SiC composite powders part and complete replacement of the graphite, the mechanical properties, oxidation resistance and slag-corrosion resistance of the Al₂O₃-C materials were studied by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), as well as with dedicated equipment. The results indicated that SiC whiskers, with lengths of 10–50?nm, formed on the surface of the flake graphite, and the activation energy of oxidation of the C-SiC composite powder increased by 45.72?kJ?mol^?1 as compared to that of flake graphite. Furthermore, the decarburization area and slag erosion area of the Al₂O₃-C material decreased after 3?wt% of C-SiC composite powder was substituted for the flake graphite. Meanwhile, the cold modulus of rupture was maintained when 3?wt% of C-SiC composite powder was added. This improved both the oxidation and slag resistance of the Al₂O₃-C materials.     

Phase formation and microstructure evolution of plasma sprayed Cr2AlC MAX phase coatings under post annealing

In this study, thick Cr₂AlC coatings were first synthesized via plasma spraying of Cr₃C₂–Al–Cr agglomerated powders and post annealing. The microstructure evolution and mechanical properties of the Cr₂AlC coatings annealed at 500–1000 °C were investigated. The as-sprayed coatings exhibited a lamellar structure, primarily consisting of Cr₂AlC, Cr₇C₃, Cr₂₃C₆, and (Cr, Al)C_x solid solutions. The short residence time during spraying led to incomplete reactions in the Cr₃C₂@Al–Cr agglomerates, resulting in the formation of (Cr, Al)C_x. Post annealing provided sufficient energy for the transition of (Cr, Al)C_x → Cr₂AlC. With an increase in the annealing temperature (<900 °C), gradual transition of the (Cr, Al)C_x phase led to a slight increase in the Cr₂AlC content, and thus, the as-annealed coatings maintained high hardness (>1000 HV_0.2) with improved fracture toughness. Higher annealing temperatures (>900 °C) promoted clear enhancement of the Cr₂AlC content, thus reducing the coating hardness. The transition phase (Cr, Al)C_x and high temperature annealing were the primary factors to promoting the formation of the Cr₂AlC phase in sprayed coatings. This study indicates that the Cr₃C₂@Al–Cr agglomerates can be effective alternatives to expensive MAX phase powders as feedstock for plasma spraying of Cr₂AlC coatings.     

Hot corrosion behavior of BaLa2Ti3O10 exposed to calcium-magnesium-alumina-silicate at elevated temperatures

Calcium-magnesium-alumina-silicate (CMAS) attack has been regarded as one of the significant failure mechanisms for thermal barrier coatings (TBCs). In this study, CMAS corrosion behavior of BaLa₂Ti₃O₁₀, a novel TBC material, is investigated at 1300?°C and 1350?°C for 0.5?h, 4?h, 12?h and 24?h. Results reveal that BaLa₂Ti₃O₁₀ has high resistance to molten CMAS infiltration, attributable to the formation of a dense reaction layer. X-ray diffraction, scanning electron microscope, energy dispersive spectroscope, transmission electron microscope confirm that the layer consists of apatite, celsian and perovskite phases. With increased corrosion duration, the layer retains good phase stability and the thickness increases. The formation of corrosion products and the reaction layer are discussed according to a dissolution-reprecipitation mechanism and the optical basicity theory.     

Influence of Al content and post-annealing on synthesis and mechanical properties of plasma sprayed Cr–Al–C composite coatings

Due to ultra-high temperature and short reaction time, it was very challenging to produce high purity MAX phase by plasma spraying. In this study, Cr–Al-graphite agglomerated powders with different Al additions (x = 0.2–1.5) was used to prepare Cr–Al–C composite coatings by atmospheric plasma spraying followed with annealing. Results showed that the as-sprayed coatings displayed typical lamellar structure, mainly composed of Cr–C binary carbides (Cr₇C₃ and Cr₂₃C₆) and residual Al. After annealing at 700 °C, the newly formed Cr₂AlC phase increased significantly in the coatings. The higher addition of Al, the more Cr₂AlC phase formed after annealing. The enhanced atomic diffusion, sufficient Al source and existence of (Cr, Al)C_x contributed to the formation of Cr₂AlC under annealing. Annealing treatment improved the hardness of the coating, but with the increase of Cr₂AlC phase content, the hardness decreased slightly. The Al content and post-annealing had a synergistic effect on the formation of Cr₂AlC phase in the sprayed coatings. This provided an effective route to control the Cr₂AlC content in sprayed Cr–Al–C composite coatings.     

Application of Cr3C2/C composite powders synthesized via molten-salt method in low-carbon MgO–C refractories

High-performance and low-carbon MgO–C refractories are important refractories for smelting ultra-low carbon steel and clean steel. Based on this, Cr₃C₂/C composite powders were synthesized by the molten-salt method, and used as an additive to prepare low-carbon MgO–C refractories under nitrogen atmosphere. The phase, morphology and oxidation kinetics of Cr₃C₂/C composite powders were studied. In addition, the effect of Cr₃C₂/C composite powders on the morphology, mechanical properties, thermal shock resistance, and corrosion resistance of MgO–C refractories was investigated. The results indicated that the Cr₃C₂/C composite powders exhibited superior oxidation resistance than flake graphite. Moreover, the Cr₃C₂/C composite powders were introduced into the MgO–C refractories. Compared with the sample without Cr₃C₂/C composite powders, the introduction of 1 wt% Cr₃C₂/C composite powders significantly improved the thermomechanical properties and corrosion resistance of the material, its CMOR, CCS before and CCS after thermal shock were 9.06 MPa, 50.40 MPa and 32.60 MPa, respectively, and the corrosion index was significantly reduced from 44.6% to 26.5%.     

The effect of carbon and nickel additions on the precursor synthesis of Cr3C2-Ni nanopowder

Decreasing crystal size to nanoscale is a proven method to enhance material properties. In this study, nanosize Cr₃C₂ and Cr₃C₂-Ni were synthetized and the reaction sequence was studied. Aqueous precursors using only water-soluble raw materials with varying carbon contents and a nickel addition were spray-dried. Glycine was used as a carbon source and chromium acetate hydroxide as a chromium source in the precursor solutions. Nickel nitrate hexahydrate was introduced as a nickel source to yield a metallic binder into the carbide nanopowder.Resulting powders were heat-treating to identify an applicable precursor composition producing the targeted Cr₃C₂ phase with crystal size of tens of nanometers. Thermal synthesis tests of the precursor powders to yield Cr₃C₂ took place at a temperature between 900 and 1300?°C under an Argon atmosphere. The synthesis of nanosize Cr₃C₂-Ni powder was successful at 1000?°C in 30?min, in a case of the best precursor. In order to produce the carbide phase with no residual oxide traces, relative carbon load has to be 48?wt%, while the stoichiometric amount of carbon in Cr₃C₂ is 13?wt%. When also introducing the nickel source into the precursor, an even higher carbon load was required. The carbon surplus needed to enable the Cr₃C₂ synthesis attributes to the non-homogeneity of the precursor composition.The chemical synthesis starting from water-soluble raw materials is a promising way of preparing nanosize Cr₃C₂-Ni with the targeted phase configuration.     

Molten salt synthesis of MAX phases in the Ti-Al-C system

The molten salt method was used to synthesise the MAX phase compounds Ti₂AlC and Ti₃AlC₂ from elemental powders. Between 900–1000?°C, Ti₂AlC was formed alongside ancillary phases TiC and TiAl, which decreased in abundance with increasing synthesis temperature. Changing the stoichiometry and increasing the synthesis temperature to 1300?°C resulted in formation of Ti₃AlC₂ alongside Ti₂AlC and TiC. The type of salt flux used had little effect on the product formation. The reaction pathway for Ti₂AlC was determined to be the initial formation of TiC_1-x templating on the graphite and titanium aluminides.     

Microstructure and mechanical properties of Nb4AlC3 MAX phase synthesized by reactive hot pressing

The present study aims at synthesizing the Nb₄AlC₃ MAX phase by reactive hot pressing using Nb:Al:NbC as starting materials. In order to identify the reaction path, interrupted tests at intermediate temperatures were performed as well as differential thermal analyses (DTA) of powders. Coupling between DTA and XRD data and SEM/EDS analyses of the samples allows a better understanding of the reaction mechanisms. Pure and fully dense Nb₄AlC₃ samples were obtained and characterized for the first time by EBSD and SEM to assess, using an original method, grain size and microstructure. For instance, in the present study, an average grain length of 5–7?µm was obtained.Standard mechanical characterizations showed interesting properties: K_Ic≈?6?MPa?m^1/2, E?≈?350?GPa and α?≈?7.10^?6 °C^?1. Oxidation performance of Nb₄AlC₃ was evaluated at 1100?°C under cyclic conditions. A breakaway regime was instantaneously established for this condition, thus demonstrating the impossibility of using such an unprotected material for structural applications at high temperature in air environment.     

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.     

High velocity oxygen fuel sprayed Cr3C2-NiCr coatings against Na2SO4 hot corrosion at different temperatures

Cr₃C₂-NiCr/NiCrAlY coating was prepared by high velocity oxygen fuel (HVOF) spraying. The microstructure and the Na₂SO₄ hot corrosion behavior at different temperatures of the coating were investigated. The Na₂SO₄ hot corrosion mechanism of Cr₃C₂-NiCr coating was also discussed. The results showed that HVOF Cr₃C₂-NiCr coating was relatively dense and mainly composed of Cr₃C₂, NiCr and a small amount of Cr₇C₃ three phases. The dense Cr₂O₃ layer was formed on the surface of Cr₃C₂-NiCr coating after Na₂SO₄ corrosion at 750 °C to further prevent corrosion. The coating had produced the obvious longitudinal crack with the increase of hot corrosion temperature up to 900 °C. The corrosion mechanism of Cr₃C₂-NiCr coating against the Na₂SO₄ salt at high-temperature was as follows: firstly, the protective oxidizing film was formed at 750 °C, then the protective oxide film dissolved at the interface between the coating and Na₂SO₄ salt with the hot corrosion temperature increasing up to 900 °C, and subsequently the dissolved anions and cations could migrate to the interface between molten salt and air and the loose and unprotected oxides were regenerated, thereby exacerbating the failure of the coating.     

Large-scale fabrication of TiC@C powders and its effect on the properties of Al2O3-MgO-C castables

High performance carbon containing castables have always been pursued by researchers and steelmaking producers, unfortunately, poor water-wettability of graphite flakes was greatly limited their application in castables. To respond this, we proposed a large-scale and low-cost modified molten salt shielding synthesis technique for fabricating TiC coated graphite in air atmosphere using graphite flake and Ti powder as raw materials. Microstructure, wettability and oxidation resistance of TiC@C powders, and effect of TiC@C powders on the properties of Al₂O₃-MgO-C castables were investigated. The results demonstrate that TiC coated graphite was synthesized via modified molten salt shielding synthesis route in air atmosphere. A uniform and continuous TiC layer was formed on the surface of graphite, thereby significantly improving the water-wettability and oxidation resistance of graphite flakes. The castables with 5% TiC@C powders possessed lower apparent porosity, higher cold strength, good oxidation resistance, and slag resistance in comparison with the castables with graphite flakes, and slag resistance were also better than Al₂O₃-MgO castables. The as-fabricated TiC@C powders have good water-wettability and oxidation resistance, making them as a prime carbon source for producing carbon containing castables for steel ladle linings.     

Tribological properties of duplex coatings of chromium-vanadium carbide produced by thermo-reactive diffusion (TRD)

Carbide coatings are very important in molding industries due to their good anti-wear properties increasing the life of molds of hot and cold forging, extrusion and powder metallurgy exposing to abrasive forces. Diverse processes are applied to produce carbide coatings. One of them is the thermo-reactive diffusion method (TRD) using a molten salt bath. This process has many advantages including cost-effectiveness over other similar surface coating methods. The aim of this study is the formation of carbide-composite coatings using molten salt baths containing oxides of carbide forming elements individually and in a mixed form on SKD-11 cold work tool steel at 1000 °C. For this purpose, two sets of experiments were considered in this study. In experiment A, a chromium oxide bath and then a mixed chromium oxide and vanadium oxide bath with a molar ratio of Cr to V equal to 0.66 were used. In experiment B, at first a mixed chromium oxide and vanadium oxide bath (Cr/V ratio = 0.66) and next a single bath of vanadium oxide were utilized. To evaluate and compare the produced coatings, FE-SEM, EPMA (point, line and map), XRD analysis, nano-indentation and wear tests were performed. The results showed that the coatings include chromium carbides (Cr₃C₂, Cr₇C₃ and Cr₂₃C₆) and vanadium carbides (V₂C, V₆C₅ and V₈C₇) as well as a carbon-chromium-vanadium triple phase with the composition of Cr₂C₂V. Moreover, the best hardness and abrasion resistance were gained for the coating produced in the molten bath containing chromium oxide and vanadium oxide after experiencing a chromium oxide bath. For the sample, the hardness was between 17.2 and 19.6 GPa and the lowest amount of COF was 0.32.     

Microstructure of HVOF-sprayed Ag–BaF2⋅CaF2–Cr3C2–NiCr coating and its tribological behavior in a wide temperature range (25 °C to 800 °C)

Ag–BaF₂?CaF₂–Cr₃C₂–NiCr composite powders were prepared by physically blending commercial BaF₂?CaF₂–Cr₃C₂–NiCr and Ag powders. Ag–BaF₂?CaF₂–Cr₃C₂–NiCr composite coatings were deposited on Inconel 718 alloy substrate by high velocity oxy-fuel (HVOF) spraying. The friction and wear behavior of the coatings under dry sliding against Si₃N₄ balls from 25 °C to 800 °C was evaluated with a ball-on-disk high temperature tribometer. The microstructure and composition of the samples were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectrometer. Results showed that the composite coatings were mainly composed of hard phase of Cr₃C₂, binder phase of NiCr, high-temperature lubrication phase of fluorides and low-temperature lubrication phase of Ag. The fluorides existed in the forms of both crystal particles and amorphous state, while the silver featured as typical thermally sprayed splats. Due to the high flame temperature, some fluorides have been oxidized to chromates and around 30 wt% of Ag was lost during spraying. In addition, it was found that Ag content had an important influence on the composite coating, and an appropriate dosage of metallic silver could effectively improve the tribological performance of the coating. The generation of AgCrO₂ at moderate (500 °C and 650 °C) temperature and BaCrO₄ at high temperature (800 °C) could contribute to the decline in friction coefficients and wear rates of Ag–BaF₂?CaF₂–Cr₃C₂–NiCr coatings.     

Molten salt synthesis of continuous tungsten carbide coatings on graphite flakes

Continuous tungsten carbide (WC) coatings were prepared on graphite flakes (G_f) by molten salt synthesis (MSS) technique using NaCl and KCl as reaction medium. The effect of reaction temperature, dwelling times and WO₃/G_f molar ratio on the compositions and morphologies of resultant samples was investigated, and the related formation mechanism for the coatings was also discussed. The results show that continuous WC coatings can be prepared at 1100 °C for 60 min with a WO₃/G_f molar ratio ranging from 1/15 to 1/5. These coatings exhibit homogeneous and crack free features, and their thickness increases with the increase of molar ratio. With the WO₃/G_f molar ratio below or beyond the above range, discontinuous WC coatings or W/W₂C/WC flake-like particles without graphite are obtained, respectively. The varied compositions of the samples obtained with different WO₃/G_f molar ratio can be related to the detailed chemical reaction process between WO₃ and G_f in the molten salts. A “template-growth” mechanism is proposed to explain the formation of WC coatings in the MSS process.     

Thermal cycling and hot corrosion behavior of a novel LaPO4/YSZ double-ceramic-layer thermal barrier coating

LaPO₄ powders were produced by a chemical co-precipitation and calcination method. The ceramic exhibited a monazite structure, kept phase stability at 1400?°C for 100?h, and had low thermal conductivity (~ 1.41?W/m?K, 1000?°C). LaPO₄/Y₂O₃ partially stabilized ZrO₂ (LaPO₄/YSZ) double-ceramic-layer (DCL) thermal barrier coatings (TBCs) were fabricated by air plasma spray. The LaPO₄ coating contained many nanozones. Thermal cycling tests indicated that the spallation of LaPO₄/YSZ DCL TBCs initially occurred in the LaPO₄ coating. The failure mode was similar to those of many newly developed TBCs, probably due to the low toughness of the ceramics. LaPO₄/YSZ DCL TBCs were highly resistant to V₂O₅ corrosion. Exposed to V₂O₅ at 700–900?°C for 4?h, La(P,V)O₄ formed as the corrosion product, which had little detrimental effect on the coating microstructure. At 1000?°C for 4?h, a minor amount of LaVO₄ was generated.     

Synthesis and growth behavior of micron-sized rod-like ZrB2 powders

Using ZrOCl₂·8H₂O, Na₂B₄O₇·10H₂O and C₁₂H₂₂O₁₁ as raw materials, micron-sized rod-like ZrB₂ powders were prepared via a molten-salt-mediated carbothermal reduction from chemically homogenous precursors obtained by sol-gel method. The effects of Zr/B/C molar ratio, firing temperature, holding time and molten salt on the composition of products have been investigated, respectively. Pure micron-sized ZrB₂ powders with controllable rod-like morphology were obtained at 1400 °C for 4 h holding with Zr/B/C of 1/5/5 in presence of molten salt, which has a diameter of 1–2 µm and aspect ratios of 3–10. The investigation of growth behavior showed that at the first stage, nano-size ZrB₂ columns grew along the c-axis with ZrC_x thin film on their top as “active-site”. Then, with consuming ‘active sites’, ZrB₂ columns started to grow in diameter direction, and finally small columns merged into a larger rod.     

Effects of the CaO/SiO2 ratio and Cr2O3 on the hydrothermal synthesis of xonotlite

Formation of xonotlite was attempted by hydrothermal reactions of Ca(OH)₂, silica gel and coprecipitated SiO₂Cr₂O₃ gel at C/S molar ratios of 0.65, 0.83, 1.0 and 1.2 at a saturated water vapor pressure with addition of 0–10% of Cr₂O₃. Cr₂O₃ enters 10% or more into CS(I) to form a solid solution, interferes with the formation of tobermorite and expands the range of formation of CSH(I) towards the higher temperature side. The temperature of formation of xonotlite rises above that in the absence of Cr₂O₃: from 210 to 450°C at C/S = 0.83 from 160 to 280°C at C/S = 1.0 and from 250 to 270°C at C/S=1.2.