A methodology,based on sintering-induced stiffening,for prediction of the spallation lifetime of plasma-sprayed coatings

This study concerns the effect of sintering-induced stiffening in promoting spallation of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings. Coatings with thickness in the range 350–800 μm were sprayed onto dense alumina substrates. In order to ensure a tough interface, the surface of the alumina substrates were first roughened by laser treatment. Specimens were heat treated at 1500 °C and periodically quenched to ～100 °C, using nitrogen jets. During cooling, specimens were monitored for spallation via a webcam. Spallation lifetimes were observed to be shorter for thicker coatings. Using a simple fracture mechanics approach, with the strain energy release rate obtained using measured coating stiffness values, the behaviour was found to be consistent with an approximately constant interfacial fracture energy value of the order of 300 J m^?2. If this interfacial toughness had been known beforehand, then the rationale presented here could have been used for prediction of coating lifetime. While the experiments are based on use of a ceramic substrate, the approach could be applied to conventional metallic substrate systems.     

Microstructure and oxidation resistance of Mo–Si–B coating on Nb based in situ composites

The development of robust high temperature oxidation resistant coatings for Nb–Si based alloy was evaluated for a Mo–Si–B coating system that was applied by a two step process. It is observed that the coating is composed of an outer layer of MoSi₂ containing boride dispersoids and an inner layer of unreacted Mo. The mass gain of substrate and Mo–Si–B coating is 190.08 and 1.28 mg cm⁻² after oxidation at 1250 °C in dry air for 100 h, respectively. The good oxidation resistance of the coating is attributed to the formation of a continuous borosilicate glass coverage.     

Novel thermal protection coating based on Zr0.75Ce0.25O2/phosphate duplex system for polyimide matrix composites fabricated via a combined sol–gel/sealing treatment process

Thermal protection coating based on Zr_0.75Ce_0.25O₂/phosphate system was fabricated on polymer–matrix composites via a combined sol–gel/sealing treatment process. Phosphates sealed the cracks and enhanced the adhesion property via chemical bonding and binding. The Zr_0.75Ce_0.25O₂/phosphate duplex coating exhibited good thermal shock resistance and improved thermal oxidation resistance of the substrate. Due to the protection of the duplex coating, the weight loss of the specimen reduced from (4.83 ± 0.12)% to (0.98 ± 0.08)% and the mass ablation rate decreased from 0.088 ± 0.002 mg cm⁻² s⁻¹ to 0.018 ± 0.002 mg cm⁻² s⁻¹ when testing at 810 °C. Coating failure was attributed to the formation of cracks and delamination.     

Damping capacity of nanotwinned copper

Copper with different twin densities was obtained by its deposition from the vapour phase in the form of coatings or separated condensates (foils) on substrates kept at different temperatures. True values of the logarithmic decrement (LD) for copper were calculated from LD amplitude dependences, measured for the coating–substrate system in the range of coating deformation amplitudes 5 × 10⁻⁵ < ε < 10⁻³ at bending vibrations of 140–150 Hz of cantilever specimens. It is shown that with the reduction of the twin domain thickness to the nanosize range, the copper damping characteristics undergo a qualitative change, which is expressed in an abrupt weakening of the amplitude dependence of LD and in a considerable increase in LD values at its heating in the entire range of deformation amplitudes. The observed changes of the dissipative properties of copper are associated with the influence of twin boundaries on its microplastic deformation.     

Ablation resistance of SiC–ZrC coating prepared by a simple two-step method on carbon fiber reinforced composites

SiC–ZrC ablation resistance coating was prepared on the surface of carbon fiber reinforced carbon (C/C) composites by simple pack cementation combined with low-cost slurry infiltration method. The results showed that SiC–ZrC coating could effectively protect C/C composites from ablation for 45 s at 3723 K under oxyacetylene torch. The mass and linear ablation rates (0.038 ± 0.01 mg/(s cm²) and 2.42 ± 0.15 μm/s) were largely reduced compared with that of uncoated C/C composites (0.530 ± 0.01 mg/(s cm²) and 1.75 ± 0.15 μm/s) after ablation for 20 s. The good ablation protective ability of SiC–ZrC coating is mainly attributed to the volatilization of SiO₂ and the formation of ZrO₂.     

Compositional pathways and capillary effects during isothermal precipitation in a nondilute Ni–Al–Cr alloy

For a Ni–5.2Al–14.2Cr at.% alloy with moderate solute supersaturations, the compositional pathways, as measured with atom-probe tomography, during early to later stage γ′(L1₂)-precipitation (R = 0.45–10 nm), aged at 873 K, are discussed in light of a multi-component coarsening model. Employing nondilute thermodynamics, detailed model analyses during quasi-stationary coarsening of the experimental data establish that the γ/γ′ interfacial free-energy is 22–23 ± 7 mJ m⁻². Additionally, solute diffusivities are significantly slower than model estimates. Strong quantitative evidence indicates that an observed γ′-supersaturation of Al results from the Gibbs–Thomson effect, providing the first experimental verification of this phenomenon. The Gibbs–Thomson relationship for a ternary system, as well as differences in measured phase equilibria with CALPHAD assessments, are considered in great detail.     

Femtosecond laser modification of multilayered TiAlN/TiN coating

A target of multilayered TiAlN/TiN coating deposited on steel was irradiated by 200 fs pulses of a Ti:Sapphire laser, operating at 775 nm. Laser fluences of 1.16 J cm^? 2 to 116 J cm^? 2 produced a range of modifications, from well defined parallel surface structures on the coating, to deep craters in the substrate. Parameters for coating ablation were determined in terms of laser fluence and pulse count. A single-pulse damage threshold was found to be 0.66 J cm^? 2 and the damage incubation factor 0.642. At lower laser fluences formation of laser induced parallel surface structures was evident, beginning from the first pulse, with a periodicity of about 580 nm. These structures remained and were pronounced at the rim of the damage spot even when the coating was completely ablated in the centre.     

Characterization studies of pulse magnetron sputtered hard ceramic titanium diboride coatings alloyed with silicon

The composition, structure and mechanical properties of pulsed-DC unbalanced magnetron sputtered Ti–Si–B thin films—hard coatings with the potential for excellent thermal stability and oxidation resistance—are investigated and reported in this paper. Fully dense, hard (19–37 GPa) Ti–Si–B coatings were deposited at substrate bias voltages (V_s) ranging from floating potential to −150 V which resulted in substrate temperatures of ∼90–135 °C. We found that variation of substrate biasing conditions critically affected film composition, structure and resultant mechanical properties. For instance, concentration of Si in films decreased from 18.4 at.% to 3.8 at.% as V_s was increased from floating potential to −150 V; composition profile analysis of the near-surface region of films (0–10 nm) revealed them to be rich in Si with significant differences among specimens produced at different substrate bias conditions. Variation of substrate biasing conditions provided coating structures that ranged from completely amorphous at floating substrate potential to nanocrystalline at V_s = −50 to −100 V and crystalline nanocolumnar at V_s = −150 V. We found that each of the structures obtained exhibited different specific values of hardness and elastic modulus, which is also in a good agreement with results reported for other coatings possessing similar micro- and nano-structures. Film structure was analyzed in detail by conventional and analytical transmission electron microscopy. Coatings that exhibited the highest values of hardness (37 GPa) were found to possess features such as crystalline nanocolumnar grains a few nanometres in diameter and disordered intergranular regions of different chemical composition, thus qualifying as nanocomposite films. Results of this work allowed relationships to be drawn between deposition parameters and Ti–Si–B coating composition, structure and mechanical properties. Qualitatively similar relationships are also expected for other biased plasma-assisted physical vapour deposited transition-metal-based ceramic coatings alloyed with Si (e.g. Ti–Si–N, Cr–Si–N, Cr–Al–Si–N).     

Oxidation resistance and mechanical characterization of silicide coatings on the Nb-18Ti-14Si-9Al alloy

The Nb-Si alloys are attractive candidate for more advanced aircraft engines, however their oxidation resistances are poor. In this work, silicide coatings were prepared on the Nb-18Ti-14Si-9Al substrate, and we present the concern of this Nb-Si alloy with high Al content, and focused the modification effect of Al on NbSi₂ coatings. It is found that composition of the substrate alloy have an essential effect on coatings, which is composed of (Nb,Ti)Si₂ outer layer and (Nb,Ti)Si₂ + (Nb,Ti)₃Si₅Al₂ inner layer. Underneath inner layer, NbAl₃ is formed and surrounded by Nb₅Si₃. Beyond fracture toughness test, the coating still preserved the integrity and tightly adhered to substrate, no cracks nucleated between substrate and the coating. After oxidation at 1250 °C for 50 h, the mass gain of substrate and silicide coating is 398.85 mg/cm² and 2.34 mg/cm² respectively. The excellent oxidation resistance of the coating is proved to benefit from modification effects of high Al in the substrate.     

Cracking investigation of W and W(C) films deposited by physical vapor deposition on steel substrates

This paper presents the investigation of the cracking of coatings deposited on steel substrates. The coating on substrate systems consisted on pure tungsten films (W) and films of solid solutions of carbon in tungsten [W(C)], which were deposited by direct current reactive magnetron sputtering on stainless steel substrates. The systems were strained uniaxially with a microtensile device adapted to a scanning electron microscope. The mechanical response was analyzed from the experimental results: the straining of the samples showed an evolution of the density of cracks in the coating, which was described trough an empirical equation based on the Weibull distribution function. The density of cracks, which corresponds to the crack saturation of the coating, appeared to vary inversely with coating thickness. Critical parameters relative to their mechanical stability were also determined from the experimental results: the strain energy release rate for crack extension through the film, G^f_c, and the fracture toughness, K^f_Ic, of the coatings. These values are included between 0.2 and 14 J m⁻², and between 0.1 and 2.5 MPa m^−1/2. The fracture resistance of W and W(C) coatings was found to be correlated to their thickness and microstructure.     

Effect of additives on the properties of plasma electrolytic oxidation coatings formed on AM50 magnesium alloy in electrolytes containing K2ZrF6

A plasma electrolytic oxidation (PEO) coating on AM50 magnesium alloy was obtained in a K₂ZrF₆–NH₄H₂PO₄–KF–C₆H₅O₇Na₃ electrolyte solution. The influence of the electrolytes on the properties of the PEO coating had been investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), roughness determination and electrochemical measurements. The experimental results show that 10 g L^? 1 K₂ZrF₆ and the presence of 8 g L^? 1 NH₄H₂PO₄ are beneficial for the passivation effect of the solution on AM50 Mg alloy as well as the compactness of the PEO coating. Many pores with large dimension on the surface of the coating are filled with coating compounds (mainly MgF₂), and the characteristics of these pores are influenced by the concentration of KF. The addition of KF and C₆H₅O₇Na₃ enhances the growth rate of the coating. The coating shows high corrosion resistance in the presence of 10 g L^? 1 K₂ZrF₆ combined with 8 g L^? 1 NH₄H₂PO₄, 3 g L^? 1 KF and 5 g L^? 1 C₆H₅O₇Na₃.     

Glass coatings on stainless steels for high-temperature oxidation protection: Mechanisms

The oxidation behavior of a martensitic stainless steel with or without glass coating was investigated at 600–800 °C. The glass coating provided effective protection for the stainless steel against high-temperature oxidation. However, it follows different protection mechanisms depending on oxidation temperature. At 800 °C, glass coating acts as a barrier for oxygen diffusion, and oxidation of the glass coated steel follows linear law. At 700 or 600 °C, glass coating induces the formation of a (Cr, Fe)₂O₃/glass composite interlayer, through which the diffusion of Cr³⁺ or Fe³⁺ is dramatically limited. Oxidation follows parabolic law.     

Microstructure and cavitation erosion behavior of WC–Co–Cr coating on 1Cr18Ni9Ti stainless steel by HVOF thermal spraying

A WC–Co–Cr coating was deposited by a high velocity oxy-fuel thermal spray (HVOF) onto a 1Cr18Ni9Ti stainless steel substrate to increase its cavitation erosion resistance. After the HVOF process, it was revealed that the amorphous phase, nanocrystalline grains (Co–Cr) and several kinds of carbides, including Co₃W₃C, Co₆W₆C, WC, Cr₂₃C₆, and Cr₃C₂ were present in the coating. The hardness of the coating was improved to be 11.3 GPa, about 6 times higher than that of the stainless steel substrate, 1.8 GPa. Due to the presence of those new phases in the as-sprayed coating and its higher hardness, the cavitation erosion mass loss eroded for 30 h was only 64% that of the stainless steel substrate. The microstructural analysis of the coating after the cavitation erosion tests indicated that most of the corruptions took place at the interface between the un-melted or half-melted particles and the matrix (Co–Cr), the edge of the pores in the coating, and the boundary of the twin and the grain in the stainless steel 1Cr18Ni9Ti.     

Magnetron-sputtered oxidation protection coatings for Mo–Si–B alloys

Oxidation protective Mo–70Al, Mo–37Si–15B and Mo–46Si–24B (at.%) coatings with 5–10 μm thickness were deposited on Mo–9Si–8B alloys by magnetron sputtering; and their oxidation behavior was studied at 800, 1000 and 1300 °C in air. On the Mo–70Al layer a dense aluminum borate scale grew at 800 °C; however, this coating rapidly degraded at 1000 °C linked to substrate oxidation at uncoated areas. The Mo–37Si–15B and Mo–46Si–24B layers provided oxidation protection to the Mo–Si–B alloy at 800 and 1000 °C for up to 100 h due to formation of a borosilicate scale. The latter coating was protective for short times even at 1300 °C.     

Microstructure and mechanical properties of hot pressed Mo–Cr–Si–Ti in-situ composite,and oxidation behavior with silicide coatings

The present study deals with the synthesis of Mo–16Cr–4Si–0.5Ti (wt.%) alloy by means of the reactive hot pressing method. The microstructure of the synthesized alloy consisted of (Mo, Cr, Ti)₃Si, and the discontinuous α-(Mo, Cr, Ti)_SS phases. The isothermal oxidation behavior of the alloy was investigated in air at 1273 K for 50 h. The alloy exhibited superior oxidation behavior in comparison with single phase molybdenum alloys, because of the formation of SiO₂ and Cr₂O₃ over the alloy surface. The flexural strength determined from three-point bend testing of single edge notch bend specimens was 615 ± 15 MPa. The dominant mechanism of fracture was identified as transgranular mode of crack propagation. To extend the life of the alloy under oxidizing atmosphere, silicide based oxidation resistant coatings were developed, using halide activated pack cementation process. The kinetic behavior of growth of the coating was established and the activation energy of the coating process was determined to be 52.5 kJ/mol. Isothermal oxidation tests of the coated alloy at 1273 K for 50 h, revealed a small weight gain at the initial stages of oxidation followed by no change of weight, indicating the protective nature of the coating.     

Heat transfer behavior of top side-pouring twin-roll casting

The interfacial heat transfer between the casting and the substrate from liquid/solid contact to solid/solid contact with pressure was investigated using a set of equipment designed according to the characteristics of the top side-pouring twin-roll casting process. The interfacial heat transfer behavior of this process consists of 4 stages: chilling, solidification shrinkage, compression and cooling. High values of the IHTC ranging from 50,000 to 90,000 W/m² °C were detected in the chilling stage, followed by a sharp decrease in solidification shrinkage stage (4000–8000 W/m² °C). Due to the pressure, which modeled the effect of rolling in twin-roll casting, the IHTC bounced back to 6000–20000 W/m² °C, according to different conditions. The influence of process variables such as pressure magnitude, compress speed, pouring temperature, surface roughness and alloy composition had been discussed. Because of the compress action, the influence of these variables performed in a different way, but it was concluded that the way to improve the contact conditions always accompanied with an increase in the IHTC.     

Microstructural characteristics of tungsten-base nanocomposites produced from micropowders by high-pressure torsion

Micropowder mixtures of W–50% Al, W–50% Ti and W–50% Ni were subjected to severe plastic deformation at 573 K using high-pressure torsion (HPT). The powder mixtures were consolidated and nanocomposites of W/Ti, W/Ti and W/Ni, with average grain sizes as small as ～9, ～15 and ～12 nm, respectively, were formed by imposing large shear strains. The nanocomposites exhibited Vickers microhardness as high as ～900 Hv, a level that has rarely been reported for metal–matrix composites. X-ray diffraction analyses together with high-resolution transmission electron microscopy showed that in addition to grain refinement, an increase in the fraction of grain boundaries up to 20%, the dissolution of elements in each other up to ～15 mol.%, an increase in the lattice strain up to 0.6%, and an increase in density of edge dislocations up to 10¹⁶ m^?2 occurred by HPT. The current study introduces the HPT process as an effective route for the production of ultrahigh-strength W-base nanocomposites, fabrication of which is not generally easy when processing at high temperatures because of interfacial reaction and formation of brittle intermetallics.     

Kinetics study on non-isothermal crystallization of the metallic Co43Fe20Ta5.5B31.5 glass

The crystallization kinetics of metallic Co₄₃Fe₂₀Ta_5.5B_31.5 glass has been studied by continuous heating differential scanning calorimetry. The DSC traces have been analyzed in terms of activation energy and kinetic model. It is found that all the DSC traces have a single exothermic peak which is asymmetrical, with a steeper leading edge and a long high temperature tail. The heating rate has a significant influence on the shape of the DSC curve, activation energy and transformation mechanism. The existence of a critical heating rate, β_crit = 20 K min⁻¹, is evident. The activation energy for crystallization are determined as 594.8 and 581.4 KJ mol⁻¹ for the heating rates β = 5–20 K min⁻¹, and 437.7 and 432 KJ mol⁻¹ for the heating rates β = 25–65 K min⁻¹, when using the Kissinger equation and the Ozawa equation, respectively. For the volume fraction crystallized, α, E_c dependence was obtained by the general Ozawa's isoconversional method. Using the Suriñach curve fitting procedure, the kinetics was specified. Namely, the crystallization begins with the Johnson–Mehl–Avrami nucleation-and-growth mode and the mode which has been well described by the normal grain growth kinetic law. These two modes are mutually independent. The proportion between the JMA-like and the NGG-like modes is related to the heating rate. The JMA kinetics is manifested as a rule in the early stages of the crystallization. The JMA exponent, n, initially being larger than 4 and continuously decreases to 1.5 along with the development of crystallization. The NGG-like mode dominates in the advanced stages of the transformation with the NGG exponent, m = 0.5 and is the major and principal kinetic characteristics for heating rate, β > 25 K min⁻¹.     

SiO2–Al2O3–glass composite coating on Ti–6Al–4V alloy: Oxidation and interfacial reaction behavior

A SiO₂–Al₂O₃–glass composite coating was prepared on Ti–6Al–4V alloy by air spraying and subsequent firing. The oxidation behavior of the specimens at 800 °C and 900 °C for 100 h was studied. The thermal shock resistance of the coating was tested by heating up to 900 °C and then quenching in water. The composite coating acted as an oxygen migration barrier and exhibited good resistance against high temperature oxidation, thermal shock, and oxygen permeation on the Ti–6Al–4V alloy. Coating/alloy interfacial reaction occurred, forming a Ti₅Si₃/Ti₃Al bilayer structure. A thin Al₂O₃ rich layer formed beneath the composite coating during oxidation at 900 °C.     

In-depth analysis of residual stress in an alumina coating on silicon nitride substrate using confocal Raman piezo-spectroscopy

Confocal Raman piezo-spectroscopy was applied to the non-destructive evaluation of residual stresses as they develop in a chemical-vapor-deposited Al₂O₃ coating on a Si₃N₄ ceramic substrate. According to a selected confocal configuration of the optical probe, with its focal plane set to in-depth scan the sample, the residual stress could be measured at various depths along the thickness of both coating and substrate. The residual stresses stored in the Al₂O₃ coating layer were measured using both the Cr³⁺ fluorescence band, located at 14,400 cm⁻¹ (R₁), and the Al₂O₃ Raman band at 417 cm⁻¹. When the R₁ fluorescence band was used, no variation could be resolved for the residual stress along the coating depth direction; in contrast, a clear in-depth stress distribution was observed when the 417 cm⁻¹ Raman band was used. The minimum stress magnitude was located at the coating external surface and the maximum at the coating/substrate interface. Given the high transparency of the Al₂O₃ coating, the residual stress field stored within the Si₃N₄ substrate could also be measured as a function of depth (according to the piezo-spectroscopic shift of the 206 cm⁻¹ Raman band of Si₃N₄). A deconvolution procedure of confocal spectra was proposed, which is based on the knowledge of the probe response functions of both coating and substrate materials. Laser probe deconvolution enabled us to retrieve the actual in-depth stress distribution from the stress distribution experimentally observed by defocusing experiments.