In situ IR pyrometric analysis during thermal treatment in air of TiOxNy coatings

IR pyrometry is an original diagnostic tool for in situ analysis of surface transformations of coatings or bulk materials during the thermal treatment under reactive atmosphere such as high temperature oxidation. Significant oscillations of the pyrometric signal were observed during annealing in air of TiO_1.5N_0.5 coatings in the temperature range 673-823 K. This is due to interferences resulting from multi-reflections at the interfaces of a transparent growing film. This reveals the formation of a TiO₂ thin film on the top of the TiO_1.5N_0.5 coating. Modeling of the time dependence of the IR pyrometric signal allows the determination of the oxide layer thickness, transformation rate and optical properties of the films under the growth conditions. The progressive oxidation of a compact amorphous TiO_1.5N_0.5 coating from the external surface to the substrate interface was supported by SIMS, XRD and reflectance analyses.     

Effect of Oxidation and SiO<Subscript>2</Subscript> Coating on the Bonding Strength of Ti-Porcelain

Investigations on the effect of oxidation on titanium-ceramic adhesion were performed. Cast pure titanium was subjected to surface modification by preoxidation and introduction of an intermediate layer of SiO₂ by sol-gel process. Specimens were characterized by TG-DSC, XRD, and SEM/EDS. The adhesion between the titanium and porcelain was evaluated by three-point flexure bond test. Failure of the titanium-porcelain with preoxidation treatment predominantly occurred at the titanium-oxide interface. Preoxidation treatment did not affect the fracture mode of the titanium-ceramic system and did not increase the bonding strength of Ti-porcelain. The SEM results revealed the existence of microcracks on the SiO₂ coating surface oxidized at 800 °C in an air furnace. During the porcelain fusion, minute amounts of oxygen were able to penetrate the cracks and caused localized oxidation of the Ti-substrate. Failure of the titanium-porcelain with SiO₂ coating predominantly occurred at the SiO₂ layer. The SiO₂ coating served as an effective oxygen diffusion barrier and improved the mechanical and chemical bonding between porcelain and titanium.     

Oxidation resistance of refractory γ-TiAlW coatings

The resistance to oxidation of the W-alloyed magnetron sputtered γ-TiAl fine-crystalline 4 μm thick coatings have were investigated in this work. The oxidation tests were performed in an atmosphere of pure oxygen or in the air at a temperature of 1173 K for 120 h. The resistance to high-temperature oxidation was investigated by means of micro-thermogravimetric analysis with continuous or stepwise control of the substrate weight. Before and after the oxidation the morphology of the coatings as well as their chemical and phase composition were investigated by SEM, EDS and EBSD, respectively. The results have been compared with those obtained for the uncoated γ-TiAl substrate.It was found that: (1) the W-alloyed γ-TiAl coatings have a considerably higher oxidation resistance (about four orders of magnitude) than the uncoated γ-TiAl substrates and that their resistance to oxidation increases with the concentration of alloying element in the range of the concentrations investigated in the work; (2) the high resistance to oxidation of the coatings is a result of the thin α-Al₂O₃ layer formation on the surface of the substrate during oxidation; (3) the formation of dense and uniform α-Al₂O₃ layer on the coating surface is due to a fine-crystalline structure of the magnetron deposited γ-TiAlW coating.     

Si-W-Mo coating for SiC coated carbon/carbon composites against oxidation

In order to prevent carbon/carbon (C/C) composites from oxidation at 1773 K, a Si-W-Mo coating was prepared on the surface of SiC coated C/C composites by a simple pack cementation technique. The microstructures and phase composition of the as-received multi-coating were examined by SEM, XRD and EDS. It was seen that the compact multi-coating was composed of α-SiC, Si and (W_xMo_1 − x)Si_2. Oxidation behaviour of the SiC/Si-W-Mo coated C/C composites was also studied. After 315 h oxidation in air at 1773 K and thermal cycling between 1773 K and room temperature for 17times, no weight loss of the as-coated C/C composites was measured. The excellent anti-oxidation ability of the multi-coating is attributed to its dense structure and the formation of the stable glassy SiO₂ film on the coating surface during oxidation.     

Metastable alumina formation during oxidation of FeCrAl and its suppression by surface treatments

The influence of various surface treatments of industrial FeCrAl grades was investigated in order to reduce the formation of transition aluminas during thermal oxidation in the 800–950°C temperature range. High temperature gas phase annealing in H₂‐H₂O mixtures promoted the initial formation of an alpha thin film and no transition alumina formed during subsequent oxidation at lower temperatures, showing very low weight gains compared to non‐treated grades. Such a treatment was shown to be efficient for isothermal oxidation in oxygen of laboratory foil specimens but also for cyclic air exposure of fiber mats in near‐real operating conditions. Surface modification by application of a slurry TiO₂ coating before oxidation was also shown to greatly reduce the amount of transition alumina, observed by X‐Ray Diffraction and Laser Induced Optical Spectroscopy. For both treatments, the optimal conditions were determined and the influence on oxidation rate was assessed.     

The use of the 18O2‐exposure + SIMS‐based approach to investigate the spallation mechanisms of alumina scales

Spallation of the protective alumina scale accelerates the oxidation‐induced degradation of ?alumina formers’?. Despite of the vast evidence of this process its mechanism has not been elucidated, so far. This paper reports the results of the scale spallation mechanism investigation using a novel ¹⁸O₂‐exposure + SIMS‐based approach. The FeCrAl‐type alumina‐forming alloys, without and with Zr additions, were subjected to high temperature exposure at 1473 K in ¹⁸O₂‐rich atmosphere and, subsequently, cooled down in air. The surfaces exposed after spallation were observed using SEM, while high resolution SIMS was used to determine the elemental distributions. The obtained results indicate that: (i) in all cases the spallation occurs according to the adhesive‐mode; (ii) for reactive‐element‐free alloys the scales spalls away at temperatures high enough to enable the re‐oxidation of the exposed surface already during cooling; (iii) for the Zr‐containing alloys the spallation occurs at fairly low temperatures. Two practical implications of the obtained results should be noted: (i) oxide layers and not ‘bare substrate’ result from the scale spallation process during cooling; (ii) the formation of oxide layers during cooling may affect the behaviour of the system during subsequent heating to the reaction temperature.     

Al2O3微粉添加量对镁合金微弧氧化膜特性影响研究

为了研究添加Al₂O₃微粉对AZ31A镁合金微弧氧化膜特性影响,在不同浓度Al₂O₃微粉电解液中对其进行了微弧氧化处理。利用扫描电镜(SEM)观察微弧氧化膜形貌,能谱仪(EDS)分析了膜层表面Ca、Mg、O、Al元素分布,X射线衍射仪(XRD)分析了相组成,测定了膜厚、硬度和氧化液中Al₂O₃表面电荷,讨论了掺杂改性机理。结果表明,加入Al₂O₃微粉后,氧化电压随Al₂O₃添加量增加先增加后降低;氧化膜表面孔洞数量和尺寸减小,膜层表面Ca元素分布逐渐减少,成膜效率降低,膜层致密度和表面疏松层硬度提高,氧化膜主要由MgO和MgO₄等相组成。     

SiON/SiO2复合膜层对太阳能电池阵板间电缆的表面改性及其耐空间环境性能

在空间站工作的太阳电池阵板间电缆上下表面为聚酰亚胺薄膜,在低轨运行时会受到原子氧的强烈侵蚀,需要采取措施对其进行保护。采用射频磁控溅射法在电缆表面制备了颗粒尺寸均匀、排列致密的SiO₂膜层。通过表征空间环境试验前后样品发现由于电缆表面的凸起颗粒等缺陷无法完全被SiO₂膜层覆盖,导致原子氧会对缺陷位置产生侵蚀作用。采用全氢聚硅氮烷溶液对板间电缆基底进行表面改性处理,制备的聚硅氧氮烷涂层（SiON）可以有效地覆盖电缆基底表面的凸起颗粒等缺陷,使得其上溅射的SiO₂膜层表面光滑平整。经原子氧暴露试验,SiON/SiO₂层内部没有受到其侵蚀作用,可以防止原子氧对电缆基底的破坏。经多次冷热循环试验,SiON/SiO₂复合膜层仍然具备良好的结构特性与结合性能。     

Effect of (NaPO3)6 concentrations on corrosion resistance of plasma electrolytic oxidation coatings formed on AZ91D magnesium alloy

Different plasma electrolytic oxidation (PEO) coatings were prepared on AZ91D magnesium alloy in electrolytes containing various concentrations of (NaPO₃)₆. The morphologies, chemical compositions and corrosion resistance of the PEO coatings were characterized by environmental scanning electron microscopy (ESEM), X-ray diffractometer (XRD), energy dispersive analysis of X-rays (EDAX), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) test. The results showed that the PEO coatings were mainly composed of MgO, Mg₂SiO₄, MgAl₂O₄ and amorphous compounds. As the (NaPO₃)₆ concentrations increased from 0 to 10 g/l, the thickness and surface roughness of the coatings approximately linearly increased; the MgO and Mg₂SiO₄ phase increased within the concentration range of 0–3 and 0–5 g/l, and then decreased within the range of 3–10 and 5–10 g/l, respectively, while the MgAl₂O₄ phase gradually decreased. Moreover, the corrosion resistance of the coatings increased within the range of 0–5 g/l and then decreased within the range of 5–10 g/l. The best corrosion resistance coating was obtained in electrolyte containing 5 g/l (NaPO₃)₆, it had the most compact microstructure. Besides, a reasonable equivalent circuit was established, and the fitting results were consistent with the results of the EIS test.     

SiO_2含量对激光熔覆含La_2O_3生物陶瓷涂层性能的影响

采用激光熔覆技术,利用CaHPO_4·2H_2O,CaCO_3,La_2O_3,SiO_2粉末和Ti粉在医用钛合金Ti-6Al-4V表面制备掺杂的质量分数分别为0%,5%,10%,15%SiO2的含La2O3梯度生物陶瓷涂层,并研究了SiO_2含量对涂层性能的影响。利用金相显微镜、扫描电镜、能谱仪、X射线分析仪分别对涂层进行显微结构、表面元素、物相组成分析,利用MTT细胞活性检测法对涂层上种植的小鼠破骨前体细胞(RAW264.7)进行细胞活性分析。结果显示:掺杂10%~15%的SiO_2后,涂层的裂纹明显减少,且厚度变均匀。涂层在模拟体液(SBF)中浸泡2周后,羟基磷灰石(HA)在掺杂10%SiO_2涂层表面的沉积量最多,涂层表面的金属元素含量稀少,这表明含10%SiO_2涂层在SBF中具备更加优良的沉积HA的能力,同时有效的阻碍了基体中重金属离子的析出。MTT结果表明,掺杂SiO_2的涂层对破骨前体细胞的活性有更加明显的抑制作用。     

A surface study of the oxidation of type 304L stainless steel at 600 K in air

The oxidation of type 304L stainless steel at 600 K in air was studied using a number of surface-analytical techniques, including Auger electron spectroscopy (AES), scanning electron microscopy with energy-dispersive analysis of X-rays (SEM-EDAX), secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS). Spectral analysis showed that a duplex oxide was formed, the outer layer of which formed rapidly and was essentially iron (III) oxide. Beneath this was a mixed iron-chromium oxide. SIMS sputter-profile curves showed region of relatively low iron concentration in the oxide film at the metal-oxide interface. This resulted from the rapid diffusion of iron within the oxide film. The oxide grain boundaries were examined using SEMEDAX. Higher chromium and silicon levels were detected in these regions compared with the corresponding grain centers. AES indicated the presence of silicon as SiO₂.     

Analytical Electron-Microscopy Study of the Breakdown of α-Al2O3 Scales Formed on Oxide Dispersion-Strengthened Alloys

Alumina scales formed during cyclic oxidation at 1200°C on three Y₂O₃–Al₂O₃-dispersed alloys: Ni₃Al, -NiAl, and FeCrAl (Inco alloy MA956) were characterized. In each case, the Y₂O₃ dispersion improved the -Al₂O₃ scale adhesion, but in the case of Ni₃Al, an external Ni-rich oxide spalled and regrew, indicating a less-adherent scale. A scanning-transmission electron microscope (STEM) analysis of the scale near the metal–scale interface revealed that the scale formed an ODS FeCrAl showed no base metal-oxide formation. However, the scale formed on ODS Ni₃Al showed evidence of cracking and Ni-rich oxides were observed. The microstructures and mechanisms discussed may be relevant to a thermal-barrier coating with an Al-depleted aluminide bond coat nearing failure.     

Surface engineering analysis of detonation-gun sprayed Cr3C2–NiCr coating under high-temperature oxidation and oxidation–erosion environments

Detonation-gun (D-gun) spray technology is a novel coating deposition process which is capable of achieving very high gas and particle velocities approaching 4–5 times the speed of sound. This process provides the possibility of producing high hardness coatings with strong adherence. In the present study, this technique has been used to deposit Cr₃C₂–NiCr coating on T22 boiler steel. Investigations on the behaviour of this coating subjected to high-temperature oxidation in air and oxidation–erosion in actual boiler environment at 700 ± 10 °C under cyclic conditions have been carried out. The weight change technique was used to establish the kinetics of oxidation. X-ray diffraction (XRD), field emission-scanning electron microscopy/energy-dispersive spectroscopy (FE-SEM/EDS) and EDS elemental mapping techniques were used to analyse the oxidation/oxidation–erosion products. The uncoated boiler steel suffered from a catastrophic degradation in the form of intense spalling of the scale in both the environments. The Cr₃C₂–NiCr coating showed good adherence to the boiler steel during the exposures with no tendency for spallation of its oxide scale.     

Synthesis of silicate coating by layer-by-layer self-assembly of Yb2O3 and SiO2 particles

Multilayer particle assemblies that contained both Yb₂O₃ and SiO₂ particles were prepared on Si substrates by layer-by-layer self-assembly (LBLSA) as a new non-line-of-sight avenue of producing dense and uniform Yb₂O₃-SiO₂ coating microstructures. Morphological characteristics of the multilayer particle assemblies upon viscous sintering in the temperature range of 1300 to 1350 °C were evaluated in reference to those of multilayer assemblies that only contained SiO₂ or Yb₂SiO₅ particles. A 4-layer 0.5-μm SiO₂ particle assembly could be easily transformed to a dense and continuous coating structure after sintering. In contrast, a 4-layer 1.7-μm Yb₂SiO₅ particle assembly could not be consolidated and densified, as anticipated from the absence of the viscous sintering mechanism provided by the SiO₂ phase. Most of the coatings produced from sintering of Yb₂O₃-SiO₂ (0.5 μm) particles assemblies were dense and continuous with Yb₂Si₂O₇ and Yb₂SiO₅ as two predominant phases. The results suggest the feasibility of producing dense and uniform silicate coating structures by integrating the LBLSA concept with the viscous flow sintering mechanism provided by the SiO₂ particles.     

Influence of siliconizing on the oxidation behavior of plasma sprayed MoSi2 coating for niobium based alloy

Plasma spraying combined with halide activated pack cementation (HAPC) was used to deposit silicide coating on Nb-based alloy. X-ray diffraction (XRD) and energy disperse spectrum (EDS) indicate the formation of the siliconized NbSi₂ transition layer and the sprayed MoSi₂ outer layer. NbSi₂ layer prepared with HAPC exhibits relatively uneven surface which could promote the deposition of the sprayed MoSi₂. The coating specimen with 5 h siliconizing presented the best oxidation resistance with only 0.18% mass gain after 25 h oxidation at 1200 °C in air. The synergistic protection effect, depending on the continuous silica layer formed on the coating surface and the dispersal silica within the coating and interface, is responsible for the excellent oxidation resistance of the coating.     

An experimental study on synthesizing submicron MoSi2-based coatings using electrothermal explosion ultra-high speed spraying method

The MoSi₂-based coatings were synthesized on the surface of the low carbon steel substrate using electrothermal explosion ultra-high speed spraying method. Microstructure, phase structure, elements distribution and microhardness of the coatings were characterized by SEM, XRD, EDS and Vickers hardness tester, respectively. It is found that MoSi₂ coating and MoSi₂ + MoB₂ multiphase coating were in-situ formed. The coatings have compact microstructure, submicron-grain and high hardness. The bonding of coating-substrate is metallurgical one. The hardness and microstructure of the MoSi₂ coating were improved by boron alloying. The average and highest hardness of the MoSi₂ coating are 1340 HV_0.2 and 1390 HV_0.2, respectively, and that of MoSi₂ + MoB₂ multiphase coating are 1650 HV_0.2 and 1785 HV_0.2, respectively.     

Formation of WSi2 coating on tungsten and its short-term cyclic oxidation performance in air

Microstructural aspects of WSi₂ coating on pure W and its short-term oxidation performance under cyclic heating and cooling conditions in air at 1100 and 1300 °C have been studied. Cyclic oxidation performance of this coating has been compared with its performance under isothermal oxidation. The coating was applied by using a pack siliconization method. The as-formed coating consisted of an outer WSi₂ layer and an inner W₅Si₃ layer. The WSi₂ layer had a columnar structure and had several through-thickness cracks generated due to the mismatch of coefficient of thermal expansion between the coating and the substrate. Based on the coating microstructure, the mechanism of coating growth during siliconizing has been suggested. Weight change data obtained under cyclic oxidation in air at 1100 and 1300 °C suggested that the above coating can provide protection to W substrate against oxidation for about 2 h. The oxide scale that formed on the coating during oxidation exposure consisted of SiO₂ and WO₃ at 1100 °C and only SiO₂ at 1300 °C. The protective silica layer underwent spallation during thermal cycling, leading to a diminishing of the protective capability of the coating. More importantly, localized oxidation of the W substrate through discontinuities present in the coating at sharp corners caused severe damage to the coated samples. Isothermal oxidation exposure of the coating, in comparison, resulted in a much lower degree of damage and the coating provided protection for a much longer duration (up to 10 h) at the above temperatures. In this study, apart from reporting a hitherto unreported oxide scale morphology, the microstructural degradation of the coating during oxidation has been linked to the columnar structure of the WSi₂ layer.     

Oxidation behavior of MoSi2 and Mo(Si,Al)2 coated Mo–0.5Ti–0.1Zr–0.02C alloy

MoSi₂ and Mo(Si, Al)₂ coatings were prepared on Mo–0.5Ti–0.1Zr–0.02C alloy using pack cementation process. Oxidation studies revealed that Mo(Si, Al)₂ coating had a much superior oxidation resistance in the temperature range from 400 to 900 °C, where pest disintegration of MoSi₂ occurs due to internal oxidation. The growth kinetics of Al₂O₃ layer formed on Mo(Si, Al)₂ coating was much slower than that of SiO₂ layer formed on MoSi₂ coatings during oxidation.     

Electrochemical study on hot corrosion of Si-modified aluminide coated In-738LC in Na2SO4-20 wt.% NaCl melt at 750 °C

The corrosion performance of the slurry Si-modified aluminide coating on the nickel base superalloy In-738LC exposed to low temperature hot corrosion condition has been investigated in Na₂SO₄-20 wt.% NaCl melt at 750 °C by combined use of the anodic polarization and characterization techniques.The coated specimen showed a passive behavior up to −0.460 V vs. Ag/AgCl (0.1 mol fraction) reference electrode, followed by a rapid increase in anodic current due to localized attack in the higher potential region. In the passive region, the anodic dissolution of constituents of the coating occurred through the passive film, probably SiO₂, at slow rate of 20-30 μA/cm². The passive current for the Si-modified coating was two orders of magnitude smaller than that for bare In-738LC, which is known as Cr₂O₃ former in this melt. This indicates that the SiO₂ film is chemically more stable than Cr₂O₃ film under this condition. However, pitting-like corrosion commenced around −0.460 V and proceeded at the high rate of 100 mA/cm² in the higher potential region than +0.400 V. The corrosion products formed on the coating polarized in different anodic potentials were characterized by SEM, EDS and XRD. It was found from the characterization that oxidation was dominant attack mode and no considerable sulfidation occurred at 750 °C. The SiO₂ oxide was not characterized in the passive region because the thickness of the passive film was extremely thin, but was detected as the primary oxide in the localized corrosion region, where the selective oxidation of Al was observed by further progress of the corrosion attack front into the inner layer of coating.     

High temperature oxidation resistance of FeCrAl alloys covered with ceramic SiO2–Al2O3 coatings deposited by sol–gel method

One-, three- and five-layer SiO₂–Al₂O₃ coatings were deposited on a FeCrAl alloy basis by the sol–gel method. Sols in which the molar ratio of tetraethoxysilan to aluminium tri-sec-butoxide was 1:1 and 1:3 were used.As the samples were being soaked at T = 1200 °C for t = 700 h the mass of the samples increased. Thermal shock (T = 1200 °C, 10,000 cycles) causes greater degradation of the surface than soaking at a constant temperature. The XPS and EDS results show that the composition of the top layer of the coatings changes during high temperature oxidation and thermal shock. The outward movement of aluminium cations results in surface enrichment with aluminium, particularly for the single-layer coatings. The measured energies of bonds Si 2p and Al 2p in the multilayer coatings indicate that a structure of aluminosilicates with a composition between that of mullite and that of sillimanite forms during sintering.