Tribological and Thermal Properties of Mullite Coating Prepared by Atmospheric Plasma Spraying

The primary mullitized andalusite powders were spray-dried and heat-treated to improve sprayable capability. Then, mullite coating was deposited by atmospheric plasma spraying and heat treatment was contributed to recrystallization of the amorphous phase present in the as-sprayed mullite coating. Scanning electron microscopy and x-ray diffraction were used to characterize the microstructure and phase composition of mullite coating. Meanwhile, the phase transition temperature, enthalpy, and specific heat capacity of as-sprayed coatings as well as recrystallized mullite coatings were determined by means of differential scanning calorimetry (DSC). Moreover, tribological properties of as-sprayed coating were investigated by SRV-IV friction and wear tester from 200 to 800 °C. It has been found that the as-sprayed coating possesses good thermal stability. DSC analysis reveals that recrystallization of the glassy phase present in the mullite coating occurs at about 980 °C. The friction coefficient of mullite coating was gradually increased from 0.82 at 200 °C to the highest value of 1.12 at 800 °C, while wear rates of the coating were at the order of 10^?5 mm³/Nm. The as-sprayed coating suffered the most severe wear at 800 °C. The observed wear mechanisms were mainly abrasive wear, brittle fracture, and pulling-out of splats.     

Mechanical Behavior of Air Plasma-Sprayed YSZ Functionally Graded Mullite Coatings Investigated via Instrumented Indentation

Yttria-stabilized zirconia (YSZ)-mullite multilayer architectures with compositional grading between the bond coat and YSZ top coat are envisioned as solutions to ease their coefficient of thermal expansion mismatch induced stress. In this work, two different types of mullite powder (spray-dried and freeze-granulated) and a mullite-YSZ 75/25 vol.% mixture spray-dried powder were employed. Using instrumented indentation with loads between 10 and 500 mN, the role of the powder characteristics on the mechanical behavior of air plasma-sprayed mullite bond coats deposited on SiC substrates was investigated. Hardness (H) and elastic modulus (E) were measured for the as-sprayed coatings and for coatings heat-treated at 1300 °C, in water vapor environment, for periods up to 500 h. Both H and E values of the coatings are found to be highly dependent on the size distribution of the starting powders. It is aimed the fabrication of an efficient and cost-effective EBC prototype based on YSZ compositionally graded mullite.     

Plasma spraying of lanthanum silicate electrolytes for intermediate temperature solid oxide fuel cells (ITSOFCs)

A new challenge in the field of solid oxide fuel cells (SOFCs) concerns reducing their operating temperature to 973 K. Apatite ceramics are interesting candidates for SOFC electrolytes due to their high ionic conductivity at this temperature. The present work reports on the fabrication and characterization of La₉SrSi₆O_26.5 coatings obtained by atmospheric plasma spraying with two different plasma spray powers. The microstructure and the composition of the as-sprayed and heat-treated coatings were investigated by several techniques including X-Ray Diffraction, Inductively Coupled Plasma-Atomic Emission Spectroscopy and Scanning Electron Microscopy. The open porosity of the coatings was evaluated by the Archimedean method. It was found that the as-sprayed apatite coatings were composed of an amorphous phase as well as of a crystalline apatite phase, and that they contained chemical heterogeneities resulting from Si volatilization in the high-temperature plasma. Furthermore, a heat treatment rendered it possible to obtain denser, fully crystallized apatite coatings. Ionic conductivity measurements carried out with impedance spectroscopy demonstrated that the conductivity of the apatite coatings - depending on the spraying conditions - increased with sintering.     

Mullite-rich plasma electrolytic oxide coatings for thermal barrier applications

A study has been undertaken of the characteristics exhibited by mullite-rich plasma electrolytic oxide coatings grown on aluminium alloys by using silicate-rich electrolytes. It is found that they can be grown at a higher rate, and to a greater thickness, than alumina PEO coatings on aluminium. The thermal conductivity of these coatings has been measured using a steady-state method. It is shown to be of the order of 0.5 W m^− 1 K^− 1, which may be compared with ∼ 1.5 W m^− 1 K^− 1 for pure alumina PEO coatings and ∼ 10-15 W m^− 1 K^− 1 for dense polycrystalline mullite. Coupled with excellent substrate adhesion and good mechanical properties, this relatively low conductivity makes these coatings attractive for thermal barrier applications. Furthermore, they are shown to exhibit a relatively low global stiffness (∼ 40 GPa), which will reduce the magnitude of thermally-induced stresses and improve the resistance to spallation during temperature changes.     

Protective Al-rich mullite coatings on Si- based ceramics against hot corrosion at 1200 °C

The hot-corrosion behavior of uncoated SiC, bulk mullite and CVD grown mullite coatings in contact with Na₂SO₄ were investigated at 1200 °C. The range of thermodynamic activity of Na₂O (10^− 4 to 10^− 6) simulated in this study makes the oxide very basic leading to high reaction rates with SiO_2. Uncoated SiC corroded severely, forming various Na₂O·SiO₂ compounds with a significant weight gain. Even though the thermodynamic activity of silica is reduced in mullite, several compounds in Na₂O·SiO₂·Al₂O₃ system were formed in case of bulk mullite. CVD based mullite coatings with high alumina content at the top surface of the coating, and therefore reduced SiO₂ activity, offered protection to the underlying SiC in corrosive environments. As predicted correctly by thermodynamic calculations, the trend in weight gain with the coated SiC samples followed the theoretically calculated SiO₂ activity in Al-rich mullite.     

The Effect of Heat Treatment on the Oxidation Behavior of HVOF and VPS CoNiCrAlY Coatings

Free-standing VPS and HVOF CoNiCrAlY coatings were produced. The as-sprayed HVOF coating retained the γ/β microstructure of the feedstock powder, and the VPS coating consisted of a single (γ) phase. A 3-h, 1100 °C heat treatment in vacuum converted the single-phase VPS coating to a two-phase γ/β microstructure and coarsened the γ/β microstructure of the HVOF coating. Oxidation of free-standing as-sprayed and heat-treated coatings of each type was carried out in air at 1100 °C for a duration of 100 h. Parabolic rate constant(s), K _p, were determined for free-standing, as-sprayed VPS and HVOF coatings as well as for free-standing coatings that were heat treated prior to oxidation. The observed increase in K _p following heat treatment is attributed to a sintering effect eliminating porosity from the coating during heat treatment. The lower K _p values determined for both HVOF coatings compared to the VPS coatings is attributed to the presence of oxides in the HVOF coatings, which act as the barrier to diffusion. Oxidation of the as-sprayed coatings produced a dual-layer oxide consisting of an inner α-Al₂O₃ layer and outer spinel layer. Oxidation of the heat-treated samples resulted in a single-layer oxide, α-Al₂O₃. The formation of a thin α-Al₂O₃ layer during heat treatment appeared to prevent nucleation and growth of spinel oxides during subsequent oxidation.     

Mechanical properties, deformation behaviors and interface adhesion of (AlCrTaTiZr)Nx multi-component coatings

In this study, (AlCrTaTiZr)N_x multi-component coatings with quinary metallic elements were developed as protective hard coatings for tribological application. The mechanical properties, creep behaviors, deformation mechanisms and interface adhesion of the (AlCrTaTiZr)N_x coatings with different N contents were characterized. With increasing the N₂-to-total (N₂ + Ar) flow ratio, R_N, during sputtering deposition, the (AlCrTaTiZr)N_x coatings transformed from an amorphous metallic phase to a nanocomposite and finally a crystalline nitride structure. The hardness of the coatings accordingly increased from 13 GPa to a high value of about 30 GPa, but the creep strain rate also increased from 1.3 × 10^− 4 to 7.3 × 10^− 4 1/s. The plastic deformation of the amorphous metallic coating deposited with R_N = 0% proceeded through the formation and extension of shear bands, whereas dislocation activities dominated the deformation behavior of the crystalline nitride coatings deposited with R_N = 10% and 30%. With increasing R_N, the interface adhesion energy between the coatings and the substrates was also enhanced from 6.1 to 22.9 J/m².     

Investigation of interfacial properties of atmospheric plasma sprayed thermal barrier coatings with four-point bending and computed tomography technique

A modified four-point bending test has been employed to investigate the interfacial toughness of atmospheric plasma sprayed (APS) yttria stabilised zirconia (YSZ) thermal barrier coatings (TBCs) after isothermal heat treatments at 1150 °C. The delamination of the TBCs occurred mainly within the TBC, several to tens of microns above the interface between the TBC and bond coat. X-ray diffraction analysis revealed that the TBC was mainly tetragonal in structure with a small amount of the monoclinic phase. The calculated energy release rate increased from ~ 50 J/m^− 2 for as-sprayed TBCs to ~ 120 J/m^− 2 for the TBCs exposed at 1150 °C for 200 h with a loading phase angle about 42°. This may be attributed to the sintering of the TBC. X-ray micro-tomography was used to track in 3D the evolution of the TBC microstructure non-destructively at a single location as a function of thermal exposure time. This revealed how various types of imperfections develop near the interface after exposure. The 3D interface was reconstructed and showed no significant change in the interfacial roughness after thermal exposure.     

Effect of Cr7C3 on the mechanical, thermal, and electrical properties of Cr2AlC

In this work, phase pure Cr₂AlC and impure Cr₂AlC with Cr₇C₃ have been fabricated to investigate the mechanical, thermal, and electrical properties. The thermal expansion coefficient is determined as 1.25 × 10⁻⁵ K⁻¹ in the temperature range of 25-1200 °C. The thermal conductivity of the Cr₂AlC is 15.73 W/m K when it is measured at 200 °C. With increasing temperature from 25 °C to 900 °C, the electrical conductivity of Cr₂AlC decreases from 1.8 × 10⁶ Ω⁻¹ m⁻¹ to 5.6 × 10⁵ Ω⁻¹ m⁻¹. For the impure phase of Cr₇C₃, it has a strengthening and embrittlement effect on the bulk Cr₂AlC. And the Cr₂AlC with Cr₇C₃ would result in a lower high-temperature thermal expansion coefficient, thermal conductivity, specific heat capacity and electrical conductivity.     

Structure and properties of selected (Cr–Al–N,TiC–C,Cr–B–N) nanostructured tribological coatings

The paper will present the state-of-art in the process, structure and properties of nanostructured multifunctional tribological coatings used in different industrial applications that require high hardness, toughness, wear resistance and thermal stability. The optimization of these coating systems by means of tailoring the structure (graded, superlattice and nanocomposite systems), composition optimization, and energetic ion bombardment from substrate bias voltage control to provide improved mechanical and tribological properties will be assessed for a range of coating systems, including nanocrystalline graded Cr_1−xAl_xN coatings, superlattice CrN/AlN coatings and nanocomposite Cr–B–N and TiC/a-C coatings. The results showed that the superlattice CrN/AlN coating exhibited a super hardness of 45 GPa when the bilayer period Λ was about 3.0 nm. Improved toughness and wear resistance have been achieved in the CrN/AlN multilayer and graded CrAlN coatings as compared to the homogeneous CrAlN coating. For the TiC/a-C coatings, increasing the substrate bias increased the hardness of TiC/a-C coatings up to 34 GPa (at −150 V) but also led to a decrease in the coating toughness and wear resistance. The TiC/a-C coating deposited at a −50 V bias voltage exhibited an optimized high hardness of 28 GPa, a low coefficient of friction of 0.19 and a wear rate of 2.37 × 10⁻⁷ mm³ N⁻¹ m⁻¹. The Cr–B–N coating system consists of nanocrystalline CrB₂ embedded in an amorphous BN phase when the N content is low. With an increase in the N content, a decrease in the CrB₂ phase and an increase in the amorphous BN phase were identified. The resulting structure changes led to both decreases in the hardness and wear resistance of Cr–B–N coatings.     

X-ray diffraction measurement of residual stress in WC-Co thermally sprayed coatings onto metal substrates

Investigation of the residual stresses and microstructural properties associated with HVOF thermal spray coating of WC-17 wt% Co of same thickness on three substrates with coefficients of thermal expansion different to that of WC. The residual stresses were measured by X-ray diffraction sin²ψ techniques using CoKα radiation. The results indicated residual stresses that have different natures for the as-sprayed coatings despite using the same powder as feedstock. The magnitudes of the stresses in the as-sprayed condition are low.     

Electrochemical characterisation of the porosity and corrosion resistance of electrochemically deposited metal coatings

Electrochemical techniques for the assessment of porosity in electrodeposited metal coatings are reviewed. The determination of porosity and corrosion, resistance is illustrated by electrochemical data from three coating/substrate systems namely: electroless nickel on aluminium and steel and immersed gold coatings on an electroless copper-plated ABS polymer. Nickel coatings were up to 24 μm thick while gold deposits had thickness between 75 and 190 nm. Tafel extrapolation and linear polarisation resistance methods were used to determine the corrosion rate of the coated substrates. The aluminium samples were tested in 5% w/v (0.85 mol dm^− 3) NaCl, while coated steel and ABS samples were immersed in 0.125 mol dm^− 3 H₂SO₄ and 0.1 mol dm^− 3 NaBH₄, respectively, at 295 K. Current vs. time curves and anodic polarisation behaviour have also been considered.     

Preparation and Microstructure Characterization of Mullite Coatings Made of Mullitized Natural Andalusite Powders

Natural andalusite powders were heated at high temperature in air allowing mullitization. The mullitized powders were spray-dried and then air plasma sprayed to form mullite coatings. Microstructure and phase composition of the plasma-sprayed mullite coatings were investigated by means of scanning electron microscopy and x-ray diffraction. The effect of spraying power on the microstructure and phase composition of the mullite coatings was investigated. It has been found that spraying power has significant effect on the content of amorphous phase in the mullite coatings. At the same time, good adhesion is reached between the mullite coating and the bound coating (NiCrAlY) and between the bound coating and the substrate. The mullite coating deposited at a spraying power of 22.5 kW is free of cracks. The processes reported here are applicable to preparing mullite coatings used in high-temperature environment.     

The Effect of Heat Treatment on the Oxidation Mechanism of Blended Powder Cr3C2-NiCr Coatings

The advantageous oxidation and wear properties of Cr₃C₂-NiCr thermal spray coatings have resulted in them being extensively applied to combat erosion at high temperatures. Under these conditions, oxide layers take on an ever more significant role in determining the composite response. The response of blended powder-based carbide coatings for erosion applications has formed the basis for application of cermet-based coatings at elevated temperature. In this study, the oxidation mechanisms of as-sprayed and heat-treated Cr₃C₂-NiCr blended powder-based coatings are characterized. Interdiffusion between the coating phases with long-term exposure increased the Cr content of the matrix phase. This had a significant effect on the oxidation mechanism. The implications of the change in oxidation mechanism and oxide morphology on the coating response to high-temperature erosion are discussed.     

Temperature driven evolution of thermal, electrical, and optical properties of Ti-Al-N coatings

Monolithic single phase cubic (c) Ti_1−xAl_xN thin films are used in various industrial applications due to their high thermal stability, which beneficially effects lifetime and performance of cutting and milling tools, but also find increasing utilization in electronic and optical devices. The present study elucidates the temperature-driven evolution of heat conductivity, electrical resistivity and optical reflectance from room temperature up to 1400 °C and links them to structural and chemical changes in Ti_1−xAl_xN coatings. It is shown that various decomposition phenomena, involving recovery and spinodal decomposition (known to account for the age hardening phenomenon in c-Ti_1−xAl_xN), as well as the cubic to wurtzite phase transformation of spinodally formed AlN-enriched domains, effectively increase the thermal conductivity of the coatings from ∼3.8 W m⁻¹ K⁻¹ by a factor of three, while the electrical resistivity is reduced by one order of magnitude. A change in the coating color from metallic grey after deposition to reddish-golden after annealing to 1400 °C is related to the film structure and discussed in terms of film reflectivity.     

Annealing and oxidation study of Ta-Ru hard coatings

Refractory metal alloy coatings have been widely used as protective coatings on glass molding dies. The formation of intermetallic compounds in the coatings inhibits grain growth at high-temperature environment in the mass production of optical components. The current work presents Ta-Ru coatings with a Cr interlayer on cemented carbide substrates and silicon wafers deposited by direct current magnetron co-sputtering at 400 °C. The as-deposited Ta-Ru coatings possessed a hardness of 13-14 GPa and a surface roughness of 1.3-4.0 nm. The annealing treatments were carried out at 600 °C under two vacuum levels of 3 × 10^− 3 and 3 Pa, respectively. After annealing in vacuum at 3 × 10^− 3 Pa, the Ta-Ru coatings showed grain size, hardness, surface roughness and phase stability comparable to those of the as-deposited coatings. While annealing in vacuum at 3 Pa, preferential oxidation of Ta in the Ta-Ru coatings was verified by X-ray photoelectron spectroscopy, a variation of the chemical composition in depth was analyzed by Auger electron spectroscopy and the internal oxidation zone consisting of a laminated structure was observed by transmission electron microscopy.     

Hydrogen-sorption and thermodynamic characteristics of mechanically grinded TiH1.9 as studied using thermal desorption spectroscopy

Isobaric thermal desorption spectroscopy and X-ray diffraction analysis were used to study the influence of mechanical dispersion during high-energy milling γ-TiH_1.9 hydride upon its hydrogen-sorption characteristics, temperature and enthalpy of the γ → β (TiH_1.9 → Ti[H]_β) phase transition at isobaric heating the sample under hydrogen atmosphere with speed 3°/min. Isobars of hydrogen thermal desorption in the regions of the γ and β phases of the Ti-H₂ system at pressures of 0.1, 0.25, 0.315 and 0.45 MPa of hydrogen in the reactor have been derived. Experimental data obtained for initial titanium hydride and mechanically grinded for 20 min in a planetary ball mill have been used for construction of Van’t Hoff plots and for determination of enthalpy of formation of γ-hydride from solid solution of hydrogen in bcc titanium. Our experimental data reveal that 20 min high-energy influence on titanium hydride powder leads to increasing the specific surface of the samples from 0.13 to 8.58 m²/g and to significant (more than 250°) decreasing the temperature of the beginning of hydrogen release when heating the sample (i.e., to a decrease of thermal stability of mechanically activated TiH_1.9). However, mechanical dispersion does not change the temperature of the γ → β phase transition. It has been established that high-energy milling TiH_1.9 powder causes the effect of a decrease of enthalpy of the formation of γ-hydride from 248 kJ/mole H₂ to 175 kJ/mole H₂.     

Investigation on thermal and microstructural characterization of the TeO2-WO3 system

Thermal and microstructural characterization of the TeO₂-WO₃ binary system was accomplished by applying differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. Different compositions of the (1 − x)TeO₂-xWO₃ system, where x varies between 0.02 and 0.80 in molar ratio were studied. The samples were prepared by melting high purity powder mixtures of TeO₂ and WO₃ in a platinum crucible with a closed lid at 750 °C for 30 min and quenching in water bath. The glass forming range of the binary system was detected as 0.04 ≤ x ≤ 0.35 in molar ratio. As-cast samples were heat-treated above the crystallization peak temperatures at 550 °C for 24 h to obtain thermal stability and the phase stability of the binary system was investigated by performing systematical thermal, phase and microstructural characterizations with the heat-treated samples. The eutectic reaction of the binary system was detected at 617 ± 3 °C, the endothermic reaction indicating the phase transformation reaction of WO₃ from orthorhombic to tetragonal was determined at 743 ± 1 °C. α-TeO₂ and orthorhombic WO₃ crystalline phases were found to be present in the final structure when the total crystallization was achieved. Microstructural characterization of the TeO₂-WO₃ system was realized for a wide compositional range for the first time in the literature.     

Development and characterization of single wire-arc-plasma sprayed coatings of nickel on carbon blocks and alumina tube substrates

A single wire-arc-plasma spray torch has been used to develop metal coatings on carbon and alumina substrates under argon atmosphere for various applications. Nickel coatings of around 1 mm thickness have been deposited on selected area (60 mm × 200 mm on each side) of large size carbon blocks by intermittent arc spraying and cooling to reduce thermal stresses and possibility of coating de-lamination from the base substrate. The same process is also used for depositing about 3 mm thick nickel metal coatings (8 mm dia. × 40 mm long) on alumina tubes for developing electrical feed throughs. The nickel coated alumina tubes were tested for the vacuum compatibility of the coated material with the base tube. The coated assemblies could withstand vacuum of the order of 1 × 10^− 6 Torr and the leak rate was found to be less than 1 × 10^− 9 Std. cc/s for Helium gas, indicating excellent bonding of the coated metal with alumina ceramics and no connected open porosity in the coatings. X-ray diffraction studies were conducted for identifying the phases and the optical microscope with image analysis technique was used for studying the microstructure and porosity in the coatings.     

An investigation on the microstructure and oxidation behavior of laser remelted air plasma sprayed thermal barrier coatings

NiCrAlY bond-coat was coated on Inconel 718 substrate by air plasma spraying (APS) followed by APS ZrO₂-8 wt.%Y₂O₃ as top-coat. Using CO₂ laser of different energy densities, ceramic top-coat surface was remelted. Laser remelting with high energy density (4 J/mm²) produced a dense microstructure over the whole thickness of top-coat, while low energy density (0.67 J/mm²) laser remelting produced a ~ 50 μm thick dense layer on the top-coat surface. It was found that the volume fraction of monoclinic phase decreased from 9% in as-sprayed coating to 4% and 3% after laser remelting with high and low energy density respectively. After isothermal oxidation at 1200 °C for 200 h, the thickness of oxide layer (TGO) in the sample produced by low energy density laser remelting was ~ 5.6 μm, which was thinner than that of oxide layer in as-sprayed (~ 7.6 μm) and high energy density laser remelted (~ 7.5 μm) samples. A uniform and continuous oxide layer was found to develop on the bond-coat surface after low energy density laser remelting. Thicker oxide layer containing Cr₂O₃, NiO and spinel oxides was observed in both as-sprayed and high energy density laser remelted coatings. After cyclic oxidation at 1200 °C for 240 h, the weight gain per unit area of as-sprayed coating was similar to that of high energy density laser remelted coating while a significantly smaller weight gain was found in low energy density laser remelted coating.