Evaluation of oxidation behavior of laser clad CoWSi–WSi2 coating on pure Ni substrate at different temperatures

Microstructural aspects of CoWSi–WSi₂ coating on pure Ni and its oxidation performance under cyclic heating and cooling conditions in air at 900, 1100 and 1300 °C have been studied. The coating was applied by laser cladding process. The microstructure evaluations demonstrated that a uniform free cracks coating formed which was metallurgically bonded to the substrate. XRD and EDS analysis results showed that the coating consisted of CoWSi, WSi₂ and γ-Co. The mass gain/unit area versus oxidation time plot for all the samples was parabolic indicating that the oxidation process was diffusion-controlled. The structure of scales depended on oxidation temperatures; at 900 °C, a dense scale layer consisting of SiO₂ and small amounts of WO₃ and CoO, at 1100 °C, a characteristic scale structure, namely a mixed layer structure and at 1300 °C, the dense oxide scale enrichment of SiO₂ and porous interdiffusion layer with low oxygen concentration was formed.     

Modification of the oxidation behaviour of Si3N4–TiB2 composites by PECVD alumina coatings

A nearly fully dense (>98%) electroconductive silicon nitride—35 vol.% titanium diboride composite was obtained by hot isostatic pressing (HIP) in presence of a low content of sintering aids (0.5 wt.% Y₂O₃ + 0.25 wt.% Al₂O₃). To improve the oxidation behaviour of this composite material, a 3-μm thick protective coating of aluminium oxide was deposited on cubic samples (4 mm × 4 mm × 4 mm) by microwave plasma-enhanced chemical vapor deposition (PECVD) using an oxygen plasma and an organometallic precursor (trimethylaluminium). SEM images demonstrated that the coating was homogeneously distributed on the external surface of the specimens.Non-isothermal and isothermal oxidation tests were carried out with a Setaram Microbalance under pure flowing oxygen (10 L/h) on both uncoated and coated Si₃N₄–TiB₂ samples. In the case of non-isothermal oxidation of a substrate without coating, the reaction started at 600 °C. Between 1100 and 1350 °C, a plateau was observed and above 1350 °C the weight gain increased significantly. In presence of an Al₂O₃ coating, the composite started to oxidize at higher temperature (1200 °C). Isothermal kinetics recorded for 24 h, at 1350 and 1400 °C, revealed that the presence of the Al₂O₃ coating improved drastically the oxidation resistance and changed the shape of the curves from globally parabolic to almost logarithmic. An explanation of this protective behaviour, based on the characterization by XRD, SEM and EDS of the reaction products, is proposed.     

Porosity analysis and oxidation behavior of plasma sprayed YSZ and YSZ/LaPO4 abradable thermal barrier coatings

In this research, the high temperature oxidation behavior, porosity, and microstructure of four abradable thermal barrier coatings (ATBCs) consisting of micro- and nanostructured YSZ, YSZ-10%LaPO₄, and YSZ-20%LaPO₄ coatings produced by atmospheric (APS) method were evaluated. Results show that the volume percentage of porosity in the coatings containing LaPO₄ was higher than the monolithic YSZ sample. It was probably due to less thermal conductivity of LaPO₄ phases. Furthermore, the results showed that the amount of the remaining porosity in the composite coatings was higher than the monolithic YSZ at 1000 °C for 120 h. After 120 h isothermal oxidation, the thickness of thermally growth oxide (TGO) layer in composite coatings was higher than that of YSZ coating due to higher porosity and sintering resistance of composite coatings. Finally, the isothermal oxidation resistance of conventional YSZ and nanostructured YSZ coating was investigated.     

Oxidation behavior characteristics of an aluminized Ni-based single crystal superalloy CM186LC between 900 °C and 1100 °C in air

Oxidation behavior of an aluminized Ni-based single crystal superalloy CM186LC was performed between 900 °C and 1100 °C in air. The oxidation kinetics approximately followed a parabolic oxidation law at 900 °C and 1100 °C. The mass gains were significantly increased owing to the formation of θ-Al₂O₃ during initial oxidation stage. After 100 h oxidation, the mass gain rates were then decreased due to the transformation from θ-Al₂O₃ to α-Al₂O₃. The microstructures after 500 h oxidation at all temperatures generally consisted of scale, coating layer, interdiffusion zone (IDZ), substrate diffusion zone (SDZ) accompanied with the topologically close-packed (TCP) and substrate.     

Oxidation behavior of hot pressed ZrB2–SiC and HfB2–SiC composites

Non-isothermal, isothermal and cyclic oxidation behavior of hot pressed ZrB₂–20 (vol.%) SiC (ZS) and HfB₂–20 SiC (HS) composites have been compared. Studies involving heating in thermogravimetric analyzer have shown sharp mass increases at 740 and 1180 °C for ZS, and mass gain till 1100 °C followed by loss for HS. Isothermal oxidation tests for 1, 24 and 100 h durations at 1200 or 1300 °C have shown formation of partially and completely stable oxide scales after ～24 h exposure for ZS and HS, respectively. X-ray diffraction, scanning electron microscopy and energy or wavelength dispersive spectroscopy has confirmed presence of ZrO₂ or HfO₂ in oxide scales of ZS or HS, respectively, besides B₂O₃–SiO₂. Degradation appears more severe in isothermally oxidized ZS due to phase transformations in ZrO₂; and is worse in HS on cyclic oxidation at 1300 °C with air cooling, because of higher thermal residual stresses in its oxide scale.     

A study of damage evolution in high purity nano TBCs during thermal cycling: A fracture mechanics based modelling approach

This work concerns the study of damage evolution in a newly developed high purity nano 8YSZ thermal barrier coating during thermal cyclic fatigue tests (TCF). TCF tests were conducted between 100 °C–1100 °C with a hold time of 1 h at 1100 °C, first till failure and later for interrupted tests. Cross section analysis along the diameter of the interrupted test samples revealed a mixed-type failure and that the most of the damage occurred towards the end of the coating’s life. To understand the most likely crack growth mechanism leading to failure, different crack growth paths have been modelled using finite element analysis. Crack growing from an existing defect in the top coat towards the top coat/TGO interface has been identified as the most likely mechanism. Estimated damage by the model could predict the rapid increase in the damage towards the end of the coating’s life.     

Thermal stability of nanostructured 13 wt% Al2O3–8 wt% Y2O3–ZrO2 thermal barrier coatings

Nanostructured 13 wt% Al₂O₃–8 wt% Y₂O₃–ZrO₂ (13AlYSZ) coatings were developed by atmospheric plasma spraying (APS). The phase structure and the morphology of the 13AlYSZ coatings were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). It was found that the as-sprayed coatings mainly consisted of tetragonal zirconia, with the Al element solid solution in ZrO₂. Heat treatment at 1100 °C increased the average grain size of the ZrO₂ phase from 61 to 120 nm and decreased the porosity from 23.8 to 18%. The addition of the nano-Al₂O₃ can effectively inhibit the grain growth of the zirconia phase. The mechanism on inhibiting the grain growth of nanostructured 8 wt% Y₂O₃–ZrO₂ thermal barrier coatings has been discussed in detail.     

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.     

Protective properties of Al2O3 + TiO2 coating produced by the electrostatic spray deposition method

Mechanical resistance of Al₂O₃ + TiO₂ nanocomposite ceramic coating deposited by electrostatic spray deposition method onto X10CrAlSi18 steel to thermal and slurry tests was investigated. The coating was produced from colloidal suspension of TiO₂ nanoparticles dispersed in 3 wt% solution of Al₂(NO₃)₃, as Al₂O₃ precursor, in ethanol. TiO₂ nanoparticles of two sizes, 15 nm and 32 nm, were used in the experiments. After deposition, coatings were annealed at various temperatures, 300, 1000 and 1200 °C, and next exposed to cyclic thermal and slurry tests. Regardless of annealing temperature and the size of TiO₂ nanoparticles, the outer layer of all coatings was porous. The first five thermal cycles caused a rapid increase of aluminum content of the surface layer to 30–37 wt%, but further increase in the number of thermal cycles did not affect the aluminum content. The oxidation rate of coating-substrate system was lower during the thermal tests than during annealing. The oxidation rate was also lower for smaller TiO₂ particles (15 nm) forming the coating than for the larger ones (32 nm). The protective properties of Al₂O₃ + TiO₂ coating against intense oxidation of substrate were lost at 1200 °C. Slurry tests showed that coatings annealed at 1000 °C had the best slurry resistance, but thermal tests had weakened this slurry resistance, mainly due to decreasing adhesion of the coating.     

Thick thermal barrier coatings with different segmentation crack densities: Microstructure analysis and thermal oxidation behavior

Thick thermal barrier coatings (TTBCs) have been developed to increase the lifetime of hot section parts in gas turbines by increasing the thermal insulating function. The premeditated forming of segmentation cracks was found to be a valuable way for such an aim without adding a new layer. The TTBC introduced in the current study are coatings with nominal thickness ranging from 1 to 1.1 consisting of MCrAlY bond coat and 8YSZ top coat deposited by air plasma spray technique (APS). TTBCs with segmented crack densities of 0.65 mm^?1 (type-A) and 1 mm^?1 (type-B) were deposited on a superalloy substrate by adjusting the coating conditions. It was found that the substrate temperature has an influential role in creating the segmentation crack density. The crack density was found to increase with substrate temperature and liquid splat temperature. The two types of coatings (type-A and B) with different densities of segmentation crack were heat-treated at 1000 °C (up to 100 h) and 1100 °C (up to 500 h). The variation of hardness measured by indentation testing indicates a similar trend in both types of coatings after heat treatments at 1000 °C and 1100 °C. Weibull analysis of results demonstrates that higher preheating coating during the deposition results in a denser YSZ coating. The growth rate of TGO for TTBCs was evaluated for cyclic and isothermal oxidation routes at 1000 °C and 1100 °C. The TGO shows the parabolic trend for both two types of coatings. The Kps value for two oxidation types is between 5.84 × 10^?17 m²/s and 6.81 × 10^?17 m²/s. Besides, the type B coating endures a lifetime of more than 40 cycles at thermal cycling at 1000 °C.     

Short-term oxidation of a ternary composite in the system AlN–SiC–ZrB2

The present study investigates the oxidation behaviour in air of a structural ceramic composite with the following volumetric composition: 55% AlN–15% SiC–30% ZrB₂. This kind of ternary composite is electroconductive (3 × 10⁻⁴ Ω cm) and has significant strength (∼700 MPa) and toughness (4 MPa m^1/2) up to 1000 °C. Oxidation tests were carried out in a TG equipment from 700 to 1300 °C with exposition time of 30 h. Significant weight gain is observed at T > 1000 °C. In the range 700–900 °C, the process is dominated by the oxidation of ZrB₂ into zirconia and boria and the kinetic is nearly parabolic. At temperatures in the range 1000–1100 °C, boria reacts with alumina forming aluminium borate and borosilicate glass and the kinetic largely deviates from parabolic behaviour. In the samples oxidized at temperatures in the range 1200–1300 °C, aluminium borate and mullite crystallize on the surface. The kinetics is para-linear in this temperature range but at 1300 °C, the rupture of the outer layer, results in accelerated damage of the sample. The composite is recommended for applications up to 1100 °C.     

Influence of the deposition energy on the composition and thermal cycling behavior of La2(Zr0.7Ce0.3)2O7 coatings

Lanthanum–zirconium–cerium composite oxide (La₂(Zr_0.7Ce_0.3)₂O₇, LZ7C3) coatings were prepared under different conditions by electron beam-physical vapor deposition (EB-PVD). The composition, crystal structure, surface and cross-sectional morphologies, cyclic oxidation behavior of these coatings were studied. Elemental analysis indicates that the coating composition has partially deviated from the stoichiometry of the ingot, and the existence of excess La₂O₃ is also observed. The optimized composition of LZ7C3 coatings could be effectively achieved by the addition of excess CeO₂ into the ingot or by properly controlling the deposition energy. Meanwhile, when the deposition energy is 1.15 × 10⁴–1.30 × 10⁴ J/cm², the coating has a similar X-ray diffraction (XRD) pattern to the ingot, and the thermal cycling life of the coating is also superior to other coatings. The spallation of the coatings occurs either within the ceramic layer approximately 6–10.5 μm above its thermally grown oxide (TGO) layer or at the interface between ceramic layer and bond coat.     

Synthesis of BaCeO3 and BaCe0.9Y0.1O3−δ from mixed oxalate precursors

An oxalate precipitation route is proposed for the synthesis of BaCe_1−xY_xO₃ (x = 0 and 0.1) after calcination at 1100 °C. The precipitation temperature (70 °C) was a determinant parameter for producing a pure perovskite phase after calcination at 1100 °C for 1 h. TG/DTA measurements showed that the co-precipitated (Ba, Ce and Y) oxalate had a different thermal behaviour from single oxalates. Despite a simple grinding procedure, sintered BaCe_0.9Y_0.1O_3−δ pellets (1400 °C, 48 h) presented 90.7% of relative density and preliminary impedance measurements showed an overall conductivity of around 2 × 10⁻⁴ S cm⁻¹ at 320 °C.     

Oxidation protection of carbon/carbon composites with SiC/indialite coating for intermediate temperatures

In order to improve the oxidation resistance of carbon/carbon composites at intermediate temperatures, a novel double-layer SiC/indialite coating was prepared by a simple and low-cost method. The internal SiC transition layer was prepared by pack cementation and the external indialite glass–ceramic coating was produced by in situ crystallization of ternary MgO–Al₂O₃–SiO₂ glass. The microstructures and morphologies of coating were determined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). Oxidation resistance of the as-coated C/C composites was evaluated in ambient air at temperature from 800 °C to 1200 °C. Nearly neglectable mass loss was measured after 100 h isothermal oxidation test, indicating that SiC/indialite coating possesses excellent oxidation protection ability. The as-coated samples have a good thermal shock resistance and no obvious damage was found in the coating even after suffered more than 11 thermal cycles between test temperature and room temperature. The oxidation protection mechanism of this coating was also discussed.     

Increased high temperature strength and oxidation resistance of Al4SiC4 ceramics

Al₄SiC₄ bulk ceramics were synthesized by reaction hot-pressing using Al, graphite powders and polycarbosilane (PCS) as starting materials. The present work confirmed that this process was an effective method for the preparation of Al₄SiC₄ ceramics having high relative density and well-developed plate-like grains. The mechanical, thermal properties and oxidation behaviors of the Al₄SiC₄ ceramics were also investigated. The flexural strength, fracture toughness (K_IC) and Vickers hardness at room temperature were 297.1 ± 22 MPa, 3.98 ± 0.05 MPa m^1/2, 10.6 ± 1.8 GPa, respectively. The high-temperature bending strength showed an increasing trend with increasing test temperatures, with the value of 449.7 ± 26 MPa at 1300 °C. The thermal expansion coefficient was 6.2 × 10⁻⁶ °C⁻¹ in the temperature range from 200 °C to 1450 °C. The isothermal oxidation of Al₄SiC₄ ceramics at 1200–1600 °C for 10–20 h revealed that it had excellent oxidation resistance.     

Fabrication of a SiC/Si/MoSi2 multi-coating on graphite materials by a two-step technique

A SiC/Si/MoSi₂ multi-coating for graphite materials was prepared by a two-step technique. SiC whisker reinforcement coating was produced by pyrolysis of hydrogen silicone oil (H-PSO) at 1600 °C, and then the dense coating was formed by embedding with the powder mixture of Si, graphite and MoSi₂ at 1600 °C in argon atmosphere. The microstructure, thickness, phase and oxidation resistance of the coating were investigated. Research results showed that, the phase of multi-coating was composed of SiC, Si and MoSi₂. The thickness of the coating was about 300 μm. In addition, the coating combined with matrix well, and surface was continuous and dense. The oxidation pretreatment experiment was carried out in the static air at 1400 °C for 4 h before thermal failure tests and the specimens had 0.045% weight gain. Subsequent thermal failure tests showed that, the SiC/Si/MoSi₂ multi-coating had excellent anti-oxidation property, which could protect graphite materials from oxidation at 1000 °C in air for 12 h and the corresponding weight loss was below 1 wt%. Based on the surface morphology changes, oxidation pretreatment experiment and thermal failure tests enhanced densification of multi-coating and the coating had a certain self-healing ability.     

Design of an inlaid interface structure to improve the oxidation protective ability of SiC–MoSi2–ZrB2 coating for C/C composites

To improve the oxidation protective ability of SiC–MoSi₂–ZrB₂ coating for carbon/carbon (C/C) composites, pre-oxidation treatment and pack cementation were applied to construct a buffer interface layer between C/C substrate and SiC–MoSi₂–ZrB₂ coating. The tensile strength increased from 2.29 to 3.35 MPa after pre-oxidation treatment, and the mass loss was only 1.91% after oxidation at 1500 °C for 30 h. Compared with the coated C/C composites without pre-oxidation treatment, after 18 thermal cycles from 1500 °C and room temperature, the mass loss was decreased by 30.6%. The improvements of oxidation resistance and mechanical property are primarily attributed to the formation of inlaid interface between the C/C substrate and SiC–MoSi₂–ZrB₂ coating.     

Fabrication and properties of ZrB2–SiC–BN machinable ceramics

ZrB₂–SiC–BN ceramics were fabricated by hot-pressing under argon at 1800 °C and 23 MPa pressure. The microstructure, mechanical and oxidation resistance properties of the composite were investigated. The flexural strength and fracture toughness of ZrB₂–SiC–BN (40 vol%ZrB₂–25 vol%SiC–35 vol%BN) composite were 378 MPa and 4.1 MPa m^1/2, respectively. The former increased by 34% and the latter decreased by 15% compared to those of the conventional ZrB₂–SiC (80 vol%ZrB₂–20 vol%SiC). Noticeably, the hardness decreased tremendously by about 67% and the machinability improved noticeably compared to the relative property of the ZrB₂–SiC ceramic. The anisothermal and isothermal oxidation behaviors of ZrB₂–SiC–BN composites from 1100 to 1500 °C in air atmosphere showed that the weight gain of the 80 vol%ZrB₂–20 vol%SiC and 43.1 vol%ZrB₂–26.9 vol%SiC–30 vol%BN composites after oxidation at 1500 °C for 5 h were 0.0714 and 0.0268 g/cm², respectively, which indicates that the addition of the BN enhances oxidation resistance of ZrB₂–SiC composite. The improved oxidation resistance is attributed to the formation of ample liquid borosilicate film below 1300 °C and a compact film of zirconium silicate above 1300 °C. The formed borosilicate and zirconium silicate on the surface of ZrB₂–SiC–BN ceramics act as an effective barriers for further diffusion of oxygen into the fresh interface of ZrB₂–SiC–BN.     

Pressureless sintering of HfB2–SiC ceramics doped with WC

Using WC as sintering aid, nearly full dense (～99%) HfB₂–20 vol% SiC ceramics were sintered at 2200 °C for 2 h without external pressure. The densification mechanism, microstructure evolution, mechanical properties and oxidation resistance were investigated. The results indicated that complex chemical reactions of WC in HfB₂–SiC system strongly related to the densification, microstructure and properties. The Young's modulus, fracture toughness and 3-pt bending strength of HfB₂–20 vol% SiC with 10 wt% WC were 511 GPa, 4.85 Mpa m^1/2 and 563 MPa, respectively, which were comparable to some hot pressed HfB₂–SiC ceramics in literature. The oxidation of HfB₂–20 vol% SiC with 10 wt% WC at 1500 °C in air exhibited parabolic kinetics. After oxidation at 1500 °C for 10 h, its weight gain and SiC-depleted layer thickness were 3.7 mg/cm² and 43 μm, respectively, and its residual flexural strength was comparable to or even a little higher than the value before oxidation.     

The relationship between microstructure and flexural strength of pressureless liquid phase sintered SiC ceramics oxidized at elevated temperatures

The oxidation behavior of pressureless liquid phase sintered SiC ceramics with Al₂O₃ and Y₂O₃ as sintering additives was investigated in the temperature range from 1000 °C to 1600 °C at the interval of 100 °C for 5 h. The relationship between residual flexural strength and microstructure was analyzed in detail. It was found that the SiC specimens suffered from mild oxidation below 1300 °C. The flexural strength of SiC specimens after oxidation at 1100 °C was the highest (90% of the original strength) due to the formation of dendritic grains, which filled pores and healed cracks. And the flexural strength was almost above 80% of the original flexural strength when the oxidation temperature was below 1300 °C. Meanwhile, the weight of specimens underwent steady increase. However, when the oxidation temperature was elevated to above 1400 °C, the specimens began to suffer from severe oxidation, which resulted in a lot of through pores and cracks on the surface, bringing about the sharp decrease of flexural strength to 30% of original strength when the oxidation temperature of 1600 °C was reached. And the weight of the specimens after huge increase began to show downtrend when the oxidation temperature was elevated to 1600 °C due to the spalling of oxidation products.