Preparation and characterization of multilayered ZrO<Subscript>2</Subscript> coatings on silicon carbide fibers for SiC/SiC composites

We have studied the surface morphology, phase composition, and oxidation resistance of multilayered tetragonal zirconia coatings produced on silicon carbide fibers by a sol-gel process and measured the tensile strength of individual fibers as a function of the number of layers in the coating. SiC-fiber-reinforced silicon carbide minicomposites have been prepared through pyrolysis of an organosilicon polymer, and their fracture surfaces have been examined. Using microindentation, we have determined the critical fiber-matrix debonding stress. The results demonstrate that the ZrO₂ coating on the fibers has the form of uniform, weakly bonded layers. The presence of a multilayered ZrO₂ interphase alters the fracture behavior of the SiC/SiC composites. The fiber debond stress in the composites markedly decreases with an increase in the number of layers in the interphase.     

Fabrication and luminescence properties of europium oxysulfide/(ZrO<Subscript>2</Subscript> + TiO<Subscript>2</Subscript>)/Ti composites

Combining plasma electrolytic oxidation and extract pyrolysis, we have produced composite oxide coatings on titanium, which exhibit bright luminescence in the red spectral region. The present results and data in the literature suggest that combining these approaches is potentially attractive for the ability to produce composite coatings with various properties on the surface of valve metals and alloys.     

Aqueous Sol-Gel Coating of Pitch-Based Graphite Fibers for Inclusion in Aluminum Matrix Composites

A coating on graphite is required to provide a wettable surface and prevent reaction between the graphite and the molten metal during the fabrication of high thermal conductivity metal matrix composites. An aqueous sol-gel procedure using zirconium acetate precursors to form continuous, crack-free, bridge-free zirconia coatings approximately 0.1 µm thick on graphite fibers 10 µm in diameter is described. Parameters were determined for dip-coating the fibers as a tow in an aqueous zirconium acetate solution and developing the coating structure by thermal desiccation, pyrolysis, and crystallization in a controlled environment.     

Effect of ZrO<Subscript>2</Subscript> on the alkali resistance and mechanical properties of basalt fibers

We have studied the effect of zirconia additions on the properties of basalt glasses and fibers. The solubility limit of ZrO₂ in basalt glasses is determined to be 7.1 wt %. Fibers produced from modified basalt glass contain both tetragonal and monoclinic zirconia. The highest ZrO₂ concentration in basalt fibers is 3.1 wt %. We have determined the fiber drawing temperature ranges and assessed the tensile strength and alkali resistance of the fibers. With increasing ZrO₂ content, the tensile strength of the fibers (d = 11−12 μm) decreases from 1.8 to 0.6 GPa. The addition of less than 3.1 wt % ZrO₂ increases the alkali resistance of the basalt fibers by 37%. The addition of more than 3.1 wt % ZrO₂ to the glass batch reduces the alkali resistance and tensilestrength of the basalt fibers.     

YSZ纤维增强等离子喷涂Al2O3/8YSZ涂层耐磨性能研究

将纤维增韧理念应用在等离子喷涂涂层设计中,可提升陶瓷涂层的断裂韧性,解决等离子喷涂陶瓷涂层韧性不足的问题。采用大气等离子喷涂技术制备了添加4%和8%(质量分数)氧化钇稳定氧化锆(YSZ)的YSZ纤维增强Al_2O_3/8YSZ涂层,对纤维增强涂层的断裂韧性及耐磨性能进行了研究。结果表明:等离子喷涂YSZ纤维增强Al_2O_3/8YSZ陶瓷涂层由α-Al_2O_3、γ-Al_2O_3和t′相组成;添加YSZ纤维后,涂层的断裂韧性明显改善,添加8%YSZ纤维复合涂层的KIC达2.924 MPa·m~(1/2),涂层的显微硬度变化较小;在相同磨损工况下,相比于未添加纤维的涂层,YSZ纤维增强涂层的耐磨性显著提高,其中,添加8%YSZ纤维后复合涂层的耐磨性是未添加涂层的2.5倍。     

Effect of impurities on characteristics of ZrO<Subscript>2</Subscript> and ZnO ceramic powders produced by spray pyrolysis

Previously it was observed that addition of impurities to a precursor solution may alter the size and morphology of the particles produced by spray pyrolysis. To investigate this further, the spray pyrolysis technique was used to prepare zirconia (ZrO₂) and zinc oxide (ZnO) ceramic powders, with addition of slight amounts of NaCl in various concentrations. The results show an increase in the percentage of nondisrupted particles which corresponds to an increase in the weight percentage of NaCl in the precursor in ZrO₂ powder produced at 400 °C. This effect is not repeated in ZnO powder produced at 400 °C, as the addition of NaCl to the precursor results in the disruption of individual particles into much smaller particles. As far as the morphology and strength of particles are concerned, it is concluded that the addition of NaCl to the precursor solution has a beneficiary effect on the morphology of ZrO₂ particles and an adverse effect on ZnO particles, both of which are negated at a higher reactor temperature of 600 °C.     

Hollow biomorphic Al<Subscript>2</Subscript>O<Subscript>3</Subscript> fibers produced from sisal

Al₂O₃ fibers with a hollow morphology were produced by Al-vapor infiltration-reaction and subsequent oxidation from pyrolysed fibers of natural sisal. Following pyrolysis, the bio-fiber template was reacted with gaseous Al at 1,400 °C–1,600 °C in vacuum to form Al₄C₃. After an oxidation/sintering process at 1,550 °C, the biomorphic Al₄C₃ fibers were fully converted into Al₂O₃, maintaining the microstructural features of the native sisal. Phase and microstructural characterization during processing were evaluated by high temperature X-ray diffractometry and scanning electron microscopy, respectively. Thermo-analyses were performed in the Al₄C₃ samples in order to estimate the reactions and the weight change during the oxidation step.     

Nanocoating of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> additive on ZrO<Subscript>2</Subscript> powder and its effect on the sintering behaviour in ZrO<Subscript>2</Subscript> ceramic

ZrO₂ powder was coated with Al₂O₃ precursor generated by a polymeric precursor method in aqueous solution. The system of nanocoated particles formed a core shell-like structure in which the particle is the core and the nanocoating (additive) is the shell. A new approach is reported in order to control the superficial mass transport and the exaggerated grain growth during the sintering of zirconia powder. Transmission electron microscopy (TEM) observations clearly showed the formation of an alumina layer on the surface of the zirconia particles. This layer modifies the sintering process and retards the maximum shrinkage temperature of the pure zirconia.     

Structure,thermal stability,and microhardness of ZrO<Subscript>2</Subscript> coatings produced by different techniques

Using ion-beam and rf magnetron sputtering of oxide targets, we have grown thin ZrO₂ coatings on 12Kh18N10T high-temperature steel substrates. Depending on the deposition technique, amorphous or crystalline (t + c) coatings have been obtained. The amorphous zirconia retains a disordered structure and high microhardness values up to 600°C. Its crystallization leads to a sharp drop in its microhardness and peeling from the substrate. The initially crystalline coating retains continuity and high microhardness values despite the t → с phase transition at 600 and 700°C. Only annealing at 800°C or higher temperatures reduces its microhardness. This is tentatively attributed to the influence of the substrate.     

Microstructural evaluation of thermally fatigued SiC-reinforced Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript> matrix composites

Using the water-quenching technique, the thermal fatigue behaviour of an alumina/zirconia and platelets- and particulates-SiC reinforced alumina/zirconia matrix composites hot-pressed at 1500 and 1700^∘C has been studied. The addition of 10 wt% SiC either in the form of platelets or particulates can obviously improve the thermal fatigue resistance of alumina/zirconia composites. The damages present after 40 thermal fatigue cycles in the composites was illustrated by the microstructure examinations and EDX.     

Protective coatings for carbon fibers

Protective refractory nanocoatings on carbon bundles and tapes have been investigated. Processes have been developed for growing thin layers of refractory oxides—alumina, zirconia, and silica—on continuous carbon fibers and tapes using sol-gel processing. ZrC/ZrO₂ bilayer coatings have been produced by chemical vapor transport. The surface morphology, phase composition, and elemental composition of the coatings have been studied by x-ray diffraction, high-resolution scanning electron microscopy, and qualitative energy-dispersive x-ray microanalysis. The results demonstrate that the refractory oxide coatings are homogeneous along the length and perimeter of individual fibers and adhere well to the fiber surface, with no peeling. Their thickness is within 200–300 nm. The effect of the nature of the coating on the oxidation resistance of carbon materials is analyzed.     

Nanostructured Pt-γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CeO<Subscript>2</Subscript> catalytic materials and coatings

A new process is described for the suspension synthesis of polycrystalline Pt-γ-Al₂O₃-CeO₂ catalytic materials and coatings, with maltose, C₁₂H₂₂O₁₁, as a reductant and structure former. The process parameters have been optimized in terms of the dispersion medium composition and the way in which Pt is introduced. The coatings are highly uniform in chemical and phase composition, as evidenced by quantitative analysis, optical microscopy, and x-ray microanalysis results. Characteristically, the coatings have a highly porous structure and good adhesive properties. Catalysts prepared by the proposed process show high activity for the oxygen oxidation of CO. The process can be used to fabricate Pt-Pd-Rh catalysts on block supports for the purification of vehicle exhaust gases and industrial off-gases. It has the advantage that high-porosity multicomponent catalytic coatings can be produced on cordierite block supports in a single step.     

Synthesis and characterization of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–ZrO<Subscript>2</Subscript> nanocomposite powder by sucrose process

Nanocrystalline alumina–zirconia powders were prepared by a modified chemical route using sucrose, polyvinyl alcohol (PVA) and metal nitrates followed by a post calcination process. The process involved dehydration of Al³⁺–Zr⁴⁺ ions-sucrose–PVA solution to a highly viscous liquid which on decomposition process produced a black precursor material. The obtained precursor were then calcined at various temperatures: 1,050, 1,100, 1,150, 1,200 and 1,250 °C for different soaking times (1, 2, 4 h) in air. The formation of a nanocomposite composed of α-alumina (~20 nm) and tetragonal (t) zirconia (~19 nm) crystallites were confirmed for the sample calcined at 1,200 °C for 2 h, based on our XRD and TEM results. However, for the samples calcined below 1,150 °C the composite formed were composed of metastable alumina (γ, δ, θ) as well as t-zirconia phases. Interestingly, the zirconia phase retained its tetragonal structure for all the samples calcined above 1,050 °C. This is possibly related to the “size effect” and reduction of surface enthalpy of the zirconia crystallites surrounded by Al³⁺ cations.     

Effect of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> stabilizer content and annealing on the structural transformations of ZrO<Subscript>2</Subscript>

The structure of partially stabilized zirconia crystals has been studied by transmission electron microscopy before and after annealing. Structural characterization of Y₂O₃-doped (2.8 to 4 mol %) zirconia before annealing showed that all of the samples consisted of twin domains whose size was dependent on the stabilizer content. Annealing at 2100°C increased the domain size in the composition range 2.8–3.7 mol % Y₂O₃ and reduced it at 4 mol % Y₂O₃. These structural changes allowed us to determine the position of the representative point relative to the phase boundary in the equilibrium phase diagram of the system.     

Effect of Growth Conditions on the Structural Properties of Calcium Phosphate Coatings Prepared in the System CaCl<Subscript>2</Subscript>-KH<Subscript>2</Subscript>PO<Subscript>4</Subscript>-KOH-HCl-H<Subscript>2</Subscript>O

The effect of growth conditions (supersaturation, relative concentrations of precursors, solution pH, and process duration) on the phase composition and structural properties of calcium phosphate coatings is studied. The results are used to determine the concentration boundary of metastable, supersaturated solutions at 37°C and a Ca/P molar ratio of 1.67 (hydroxyapatite stoichiometry). High supersaturations are shown to result in the growth of mixed-phase coatings: hydroxyapatite + octacalcium phosphate. At low supersaturations, single-phase hydroxyapatite coatings are obtained. The coatings consist of preferentially oriented nanocrystallites, independent of the growth conditions. The texture of the coatings and the size of the nanocrystallites are most sensitive to the growth duration.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 8, 2005, pp. 985–989.Original Russian Text Copyright © 2005 by Savvin, Kryzhanovskaya, Tolmachev.     

Mechanical behavior of SiCf/SiC composites with alternating PyC/SiC multilayer interphases

In order to tailor the fiber–matrix interface of continuous silicon carbide fiber reinforced silicon carbide (SiC_f/SiC) composites for improved fracture toughness, alternating pyrolytic carbon/silicon carbide (PyC/SiC) multilayer coatings were applied to the KD-I SiC fibers using chemical vapor deposition (CVD) method. Three dimensional (3D) KD-I SiC_f/SiC composites reinforced by these coated fibers were fabricated using a precursor infiltration and pyrolysis (PIP) process. The interfacial characteristics were determined by the fiber push-out test and microstructural examination using scanning electron microscopy (SEM). The effect of interface coatings on composite mechanical properties was evaluated by single-edge notched beam (SENB) test and three-point bending test. The results indicate that the PyC/SiC multilayer coatings led to an optimum interfacial bonding between fibers and matrix and greatly improved the fracture toughness of the composites.     

Fabrication of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite nanofibers by electrospinning

PVA(Polyvinyl alcohol)/chromium nitrate/aluminum nitrate composite nanofibers were prepared by using sol–gel processing and electrospinning technique. By high temperature calcinating the above precursor fibers, Cr₂O₃/Al₂O₃ composite nanofibers were successfully obtained. The fibers were characterized by XRD, IR, and SEM, respectively. The results showed that the crystalline phase and the morphology of the fibers depended on the calcination temperatures.     

Preparation of Y-TZP ceramic fibers by electrolysis-sol-gel method

Polycrystalline yttria stabilized tetragonal Zirconia (T-ZrO₂) fibers were obtained by pyrolysis of gel fibers using zirconium oxychloride octahydrate as raw material. The spinnable zirconia sol was prepared by electrolyzing the zirconium oxychloride octahydrate solution in the presence of acetic acid and sugar (sucrose, glucose or fructose), in which the molar ratio of CH₃COOH/ZrOCl₂ · 8H₂O and sugar/ZrOCl₂ · 8H₂O was in the range of 1.0–4.0 and 0.2–0.4, respectively. The relation of spinnability to the shape of colloidal particle was discussed. The as-prepared zirconia fibers sintered at different temperatures show smooth and crack-free surface with the diameter of 5–10 μm. Slow heating rate below 600 °C and then sintering at 1,400 °C for 30 min were necessary to obtain the dense tetragonal zirconia ceramic fibers, the particles composed the fibers had the size of ∼150 nm.     

Sol-gel synthesis of zirconia barrier coatings

A method for applying zirconia barrier coatings using a sol-gel method is described. The coatings of 8 wt % yttria-stabilized zirconia are applied by spin coating a solution containing zirconium alkoxides and yttrium acetate on to stainless steel substrates. Crystallization of the films was observed for thermal treatments in the range 750 to 1050° C. Excellent adhesion at the interface, due to significant coating-substrate interfacial reactions, indicates that this sol-gel route is a feasible method for applying zirconia coatings.