Interdiffusion in SiC–AlN and AlN–Al2OC Systems

AlN, Al₂OC, and the 2 H form of SiC are isostructural. Both SiC–AlN and AlN–Al₂OC form homogeneous solid solutions above 2000° and 1950°C, respectively. The kinetics of phase separation in the two systems, however, are quite different. Interdiffusion in both SiC–AlN and AlN-Al₂OC systems was examined in the solid-solution regime in an attempt to elucidate differences in the kinetics of phase separation that occur in the two systems when annealed at lower temperatures. Diffusion couples of (SiC)_0.3(AlN)_0.7/(SiC)_0.7(AlN)_0.3 and (AlN)_0.7(Al₂OC)_0.3/(AlN)_0.3(Al₂OC)_0.7 were fabricated by hot pressing and were annealed at high temperatures by encapsulating them in sealed SiC crucibles to suppress loss due to evaporation. Interdiffusion coefficients in (SiC)_0.3-(AlN)_0.7/(SiC)_0.7(AlN)_0.3 diffusion couples were measured at 2373, 2473, and 2573 K, and the corresponding activation energy was determined to be 632 kJ/mol. (AlN)_0.7(Al₂OC)_0.3/ (AlN)_0.3(Al₂OC)_0.7 samples were annealed at 2273 K. The interdiffusion coefficient measured in the AlN–Al₂OC system was much larger than that in the SiC–AlN system.     

Tribological Behavior of SiC-Whisker/Al2O3 Composites against Carburized 8620 Steel in Lubricated Sliding

Mineral oil lubricated sliding tests of Sic-whisker (SiC_w,)/A1₂0₃ composites and monolithic alumina against carburized 8620 steel were conducted on a cylinder-on-cylinder machine. The wear rate of the composites was one or two orders of magnitude less than that of pure alumina. Hot-pressed 25 wt% SiC_w,/A1₂0₃ composite had a lower wear rate than sintered and HIPed 7.5 wt% SiC/Al₂O₃ composite under the same conditions. The weight loss of the steel mating ring against the 25 wt% SiC_w, composite was a factor of four lower than against the 7.5 wt% SiC_w, composite, but the former was a factor of 50 to 60 less than that against pure alumina. The composites showed lower friction coefficients than alumina during the run-in stage. The friction coefficients decreased with initial wear. The steady-state friction coefficient decreased with increasing load up to 500 N for hot-pressed 25 wt% SiC,/Al₂0₃ composite. Further, SEM observation showed much less microfracture in composites than in alumina. EDAX analysis revealed less Fe transfer from the steel ring to the composites than to pure alumina. Wear by microfracture and by adhesion in composites was suggested to be suppressed by SIC whiskers. This in turn reduced wear of the steel because of the generation of fewer hard particles.     

Effect of Carbon Precursors on the Microstructure and Bonding State of a Boron–Carbon Compound Grown by LPCVD

Methane (CH₄) and propylene (C₃H₆) were used to fabricate a boron–carbon coating by a low-pressure chemical vapor deposition (LPCVD) technique. The effects of carbon precursors on the phase, microstructure, and bonding state of the deposits have been investigated. X-ray diffraction results show that the 2θ value of the deposit from the C₃H₆ precursor shifts to 25.78° when the coating is deposited at 1223 K, and shifts to 26.1° when deposited at 1273 K, compared with the 2θ value of the pyrocarbon (PyC) peak deposited by LPCVD, which is 25.42°, and no boron–carbon (B–C) compound peak exists. However, the phases of coating deposited from CH₄ include B₂₅C, B₁₃C₂, elemental carbon, and boron. X-ray photoelectron spectroscopy (XPS) results show that the percent contents of boron atom in the coatings from the CH₄ precursor are 61.18% and 67.78% when deposited at 1223 and 1273 K, respectively, much higher than that from the C₃H₆ precursor, 10.85% and 15.30%, respectively. Scanning electron microscopy (SEM) results show that the coatings deposited from CH₄ have a coarse crystal-like morphology; however, the coatings deposited from the C₃H₆ precursor are smooth. The formation of PyC from C₃H₆ is more facile than that from CH₄, which leads to differences in the phase, atom content, and microstructure of coatings from CH₄ and C₃H₆ precursors.     

Spark Plasma Sintering of Nano-Sized TiN Prepared from TiO2 by Controlled Hydrolysis of TiCl4 and Ti(O-i-C3H7)4 Solution

Nano-sized TiO₂ powders were prepared by controlled hydrolysis of TiCl₄ and Ti(O-i-C₃H₇)₄ solutions and nitrided in flowing NH₃ gas at 700°–1000°C to form TiN. Nano-sized TiN was densified by spark plasma sintering at 1300°–1600°C to produce TiN ceramics with a relative density of 98% at 1600°C. The microstructure of the etched ceramic surface was observed by SEM, which revealed the formation of uniformly sized 1–2 μm grains in the TiCl₄-derived product and 10–20 μm in the Ti(O-i-C₃H₇)₄-derived TiN. The electric resisitivity and Vickers micro-hardness of the TiN ceramics was also measured.     

Effect of Na2SO4 on Structure and Corrosion Resistance of Ceramics Coatings Containing Zirconium Oxide on Ti–6Al–4V Alloy

A micro-plasma oxidation (MPO) technique has been developed in recent years, by which ceramic coatings are reported to possess improved properties and promising application prospects in many fields. The aim of this work was to study the effect of sodium sulfate, as an additive in the zirconate system, on the structure and corrosion resistance of ceramics coatings containing zirconium oxide grown on Ti–6Al–4V by MPO process. The phase composition, morphology, and element distribution in the coating were investigated by X-ray diffractometry, scanning electron microscopy, and energy distribution spectroscopy. Meanwhile, the corrosion resistance of the coated samples was examined by polarizing curves and potentiodynamic anodic curves in 3.5% NaCl solution. The results show that ceramic coatings were composed of m -ZrO₂, t -ZrO₂, and KZr₂(PO₄)₃. The Ti content in the coating near the substrate decreased sharply, and then remained at 5 wt% or so through the coating, while the Zr content near the substrate increased greatly, and then remained at about 55 wt% through the coating. The addition of sodium sulfate did not change the composition of the coatings, but increased the relative proportion of zirconium oxide to KZr₂(PO₄)₃ in the coating. Sodium sulfate decreased the thickness of the coating, while improving the density of the coatings. Moreover, the addition of the sodium sulfate improved the corrosion resistance of the coated samples in a 3.5% NaCl solution, whether considering localized pitting corrosion resistance or uniform corrosion resistance.     

Fabrication and Characterization of a (100) Three-Axis-Oriented Single-Crystal (Ba0.7Sr0.3)TiO3 Thin Film Prepared by the Chemical Solution Deposition Method

Epitaxially grown single-crystal perovskite (100) three-axis-oriented (Ba_0.7Sr_0.3)TiO₃ thin films were prepared on a (100) platinum-coated (100) magnesium oxide (MgO) single-crystal substrate by the chemical solution deposition method using a solution derived from Ba(CH₃COO)₂, Sr(CH₃COO)₂, and Ti(O- i -C₃H₇)₄.
The growth of the film was found to depend on the annealing condition. A (Ba,Sr)TiO₃ thin film annealed at 1073 K was found to be a single crystal by transmission electron microscope. The single-crystal film exhibited a (100) three-axis orientation that followed the (100) orientation of the Pt substrate, as observed from an X-ray pole figure measurement and selected area electron diffraction patterns.     

Fabrication of TiN/Si3N4 Ceramics by Spark Plasma Sintering of Si3N4 Particles Coated with Nanosized TiN Prepared by Controlled Hydrolysis of Ti(O-i-C3H7)4

TiN-coated Si₃N₄ particles were prepared by depositing TiO₂ on the Si₃N₄ surfaces from Ti(O- i -C₃H₇)₄ solution, the TiO₂ being formed by controlled hydrolysis, then subsequently nitrided with NH₃ gas. A homogeneous TiO₂ coating was achieved by heating a Si₃N₄ suspension containing 1.0 vol% H₂O with the precursor at 40°C. Nitridation successfully produced Si₃N₄ particles coated with 10–20 nm TiN particles. Spark plasma sintering of these TiN/Si₃N₄ particles at 1600°C yielded composite ceramics with a relative density of 96% at 25 vol% TiN and an electrical resistivity of 10⁻³Ω·cm in compositions of 17.5 and 25 vol% TiN/Si₃N₄, making these ceramics suitable for electric discharge machining.     

Flexible Ceramics in the System KZr2(PO4)3–KAlSi2O6 Prepared by Mimicking the Microstructure of Itacolumite

A ceramic composite mimicking the pervasively cracked microstructure of flexible sandstone (itacolumite) was successfully synthesized by sintering two ceramic materials with different thermal expansion coefficients. A combination of granular KZr₂(PO₄)₃ (high thermal expansion) and powdered KAlSi₂O₆ (low thermal expansion) resulted in a material with a jigsaw-like three-dimensional cracking microstructure similar to that of itacolumite. The synthesized composite was found to exhibit ductile deformation.     

Using Simultaneous Deposition and Rapid Growth to Produce Nanostructured Composite Films of AIN/TiN by Chemical Vapor Deposition

Brittleness has been the major obstacle in using ceramics. Previous research has shown, however, that ceramic materials that have small grain size show plasticity. We therefore propose two methods to produce nanostructured ceramic films by chemical vapor deposition (CVD): (1) high-speed deposition and (2) simultaneous deposition of insoluble materials (contained in a mixture of insoluble solids). These methods were successfully applied to aluminum nitride/ titanium nitride (AlN/TiN) films produced by CVD. The AlN/TiN nanostructured composite films were synthesized by atmospheric-pressure CVD (APCVD), using aluminum chloride (AlC1₃), titanium chloride (TiCl₄), and ammonia (NH₃) as reactant gases at temperatures ranging from 923 to 1123 K in a horizontal tubular reactor. For the highspeed deposition strategy, we obtained growth rates as high as 1.2 mm/h. Using either method, we were able to attain AlN/TiN composite films that had a grain size of 8 nm (AlN crystals) and 6 nm (TIN crystals), showing that these methods are effective in producing nanostructured composite films by CVD. Measurements of the fracture toughness of the prepared materials indicated that these strategies can be used to improve the ductility of ceramics.     

Novel Deposition of Columnar Y3Al5O12 Coatings by Electrostatic Spray-Assisted Vapor Deposition

A novel and cost-effective electrostatic spray-assisted vapor deposition (ESAVD) was used to deposit Y₃Al₅O₁₂ (YAG) coatings. Polycrystalline single-phase Y₃Al₅O₁₂ coatings were synthesized using the ESAVD method in an open atmosphere at 650°C, and then annealed at 700°–900°C for 1 h. The ESAVD process involves the decomposition and chemical reactions of charged aerosol in vapor phase. The low-temperature coating deposition characteristics of the ESAVD process using a suitable sol precursor decreases the reaction and crystallization temperatures for forming Y₃Al₅O₁₂ coatings. The microstructure of the Y₃Al₅O₁₂ coating prepared using the ESAVD method is columnar and such strain-resistance microstructure could be useful for thermal barrier coating applications.     

Anodic Spark Deposition from Aqueous Solutions of NaAlO2 and Na2SiO3

Coatings were deposited on Cu⁰, Ni⁰, and Al⁰ anodes from aqueous solutions of NaAlO₂ and Na₂SiO₃ by anodic spark deposition. Some coatings formed at constant current density (1.0 A/cm²) were characterized by scanning electron microscopy, electron beam microprobe analysis, X-ray diffraction, and emission spectroscopy. A relation for the current decay during a multiple-spark, constant-voltage process was determined. This relation, which describes the behavior of most of the systems studied, is

where φ_t, φ₀, and φ_∞ are the current densities at time t , at the start of the decay ( t =0), and at the steady state, respectively. K is a constant that is voltage and system dependent.     

Preparation of TiO2-Coated Al2O3 Particles by Chemical Vapor Deposition in a Rotary Reactor

A process of coating Al₂O₃ particles with TiO₂ by hydrolysis of Ti(OC₄H₉)₄ using chemical vapor deposition in a rotary reactor has been developed. The process resulted in (1) a coating film of TiO₂ which was compact and uniform with the fraction of TiO₂ being 0.1%–10.0% and (2) an amorphous TiO₂ coating at a low reaction temperature converted to anatase at a reaction temperature higher than 673 K.     

Preparation of Composite Particles by Pulsed Nd-YAG Laser Decomposition of [(CH3)2N]4Ti to TiN-Coat TiO2, Al2O3, or Si3N4 Powders

Irradiation of Ti[N(CH₃)₂]₄ by the 1.064-μm line of a pulsed Nd: YAG laser in the presence of TiO₂, Al₂O₃, or Si₃N₄ particles has been found to form amorphous deposits on the oxide particles. The resulting materials can be processed into TiN/TiO₂, TiN/Al₂O₃, or TiN/Si₃N₄ composites with the TiN component on the surface of the particles. The powders have been characterized by Raman spectroscopy and X-ray powder diffraction studies. The surface analysis of the composites by X-ray photoelectron spectroscopy and high-resolution electron microscopy is presented.     

AI2O3 Coatings as Diffusion Barriers Deposited from Particulate-Containing Sol-Gel Solutions

A procedure for the formation of A1₂O₃ coatings as diffusion barriers between ductile reinforcements (e.g., Nb and Ta) and intermetallic matrices (e.g., MoSi₂ and NiAl) is described. The coating technique involved sol-gel processing of alumina -forming sols with the addition of submicrometer-sized A1₂O₃ particles. Cracking in the coatings, a typical shortcoming of alumina sol-gel coating, was overcome by the addition of the fine particles into the sols. The surface charge of the A1₂O₃ particles was adjusted to be the same as the AIO(OH) colloids in the sols and electrophoresis was used to codeposit A1₂O₃ and AIO(OH) onto the surfaces of the reinforcements. The alumina gel derived from the sols acted as binder for the alumina particles, while the particles reduced the shrinkage of the sol-gel coatings and promoted the formation of dense coatings. The thickness of the coatings could be easily controlled without cracking and the effectiveness of the coatings as diffusion barriers was improved substantially.     

硼化钽增强聚醚醚酮复合涂层的制备及性能评价

采用阴极电泳沉积技术在纯钛基板表面沉积聚醚醚酮（PEEK）/硼化钽（TaB₂）复合涂层。采用透射电子显微镜、扫描电子显微镜、X射线衍射仪和摩擦磨损试验机等方式对电泳沉积液分散性、PEEK/TaB₂复合涂层表面形貌、微观结构、结晶行为、摩擦学性能和生物学性能进行表征。结果表明，通过调节电泳沉积参数可以制备形貌均匀、具有一定厚度的PEEK/TaB₂复合涂层，在390 ℃热处理后，涂层均匀致密无孔隙；热处理可以提高PEEK/TaB₂涂层的结晶性能，TaB₂颗粒的加入使PEEK涂层获得更高的结晶度；添加较低含量的TaB₂颗粒时，复合涂层在小牛血清（fetal bovine serum，FBS）介质中表现出良好的摩擦学性能，与纯PEEK涂层相比，磨损率分别下降了48.1 %，69.1 %；但过量TaB₂颗粒在PEEK基质中出现明显的团聚现象， 摩擦系数和磨损率呈现上升趋势；细胞实验表明，TaB₂良好的生物活性促进了样品表面细胞增殖。     

Fiber Strength Retention of Lanthanum- and Cerium Monazite-Coated Nextel™ 720

Nextel™ 720 fibers were coated with LaPO₄ and CePO₄ monazite. The coatings were applied using washed and unwashed rhabdophane sols derived from La(NO₃)₃/(NH₄)₂HPO₄ and a washed sol derived from Ce(NO₃)₃/H₃PO₄. The coatings were cured in-line at 900°–1300°C. Multiple coatings were also applied. Fiber strength was retained after coating with washed sols, but not with unwashed sols. These results are consistent with earlier work on LaPO₄ monazite fiber coatings derived from La(NO₃)₃/H₃PO₄.     

Auger Electron Spectroscopic Analysis of Hydroxylapatite Coatings on Titanium

Investigating the bioreactive behavior of hydroxylapatite [Ca₁₀(PO₄)₆ (OH)₂] coatings on titanium requires an exact knowledge of structure and composition of the coating. Auger electron spectroscopic analyses were performed in addition to other microstructural analyses. Studies of the outer surface and of the interface between hydroxylapatite and titanium showed that the energy and the signal sensitivity of the Auger phosphorus peak depend on the chemical environment. Argonion bombardment was found to affect the spectrum of the low-energy P peaks.     

Preparation and Aging Behavior of Chemical-Solution-Deposited (Pb(Mg1/3Nb2/3)O3)1-x–(PbTiO3)x Thin Films without Seeding Layers

The aging behavior of the solid-solution series (Pb(Mg_1/3Nb_2/3)O₃)_{1− x} –(PbTiO₃) _x (PMN_{1− x} −PT _x ) prepared by chemical-solution deposition without seeding layers was investigated. A strong influence of the rapid thermal annealing step on the film density was determined. The best nucleation and density of the thin films occurred when each deposited layer was separately pyrolyzed and crystallized. The thin-film microstructure was investigated using scanning electron microscopy. Conventional capacitance-voltage and hysteresis measurements were performed. For the first time, investigations on the fatigue performance and the leakage current for alternating-current and direct-current voltage were executed, which are important for the reliability in device applications.     

R-Curve Behavior of In-Situ-Toughened Al2O3:CaAl12O19 Ceramic Composites

The mechanical behavior of reaction-sintered alumina: 30 vol% calcium hexaluminate (Al₂O₃:CaAl₁₂O_19,or A1₂O₃: CA₆) composites was evaluated using the indentation strength in bending technique. A composite in which the hexaluminate (CA₆) phase possessed a platelike morphology showed more-pronounced R -curve behavior than a composite in which the CA₆ phase consisted of equiaxed grains. Toughness curves derived from the indentation-strength data exhibited a "crossover," such that the platelet composite exhibited the lower toughness at small flaw sizes, but the higher toughness at large flaw sizes. Incorporation of the platelet CA₆ resulted in enhanced toughening, compared to single-phase alumina of comparable grain size, thus demonstrating the viability of the in-situ -toughening approach. A simple grain-pullout model was used to estimate the toughening increment due to bridging by the platelet grains; the value obtained was in good agreement with toughness curves derived from indentation-strength measurements. Finally, fabrication of trilayer specimens, whereby outer layers of equiaxed A1₂O₃:CA₆ composite were strongly bonded to the platelet A1₂O₃:CA₆ composite, demonstrated high strength over the range of tested flaw sizes.     

Direct Synthesis and Sintering of Silicon Nitridenitanium Nitride Composite

Electrical discharge machining is an attractive machining technique which requires electrically conductive ceramic materials. Silicon nitride based composites with 35–40% of TiN possess electrical conductivity high enough to use this machining technique. The composite powder was prepared by combustion synthesis and densified by pressure sintering (sintering aids 6 wt% of Y₂O₃ and 4 wt% of A1,0₃). High fracture toughness, K _c 10.5 MPa^.m^1/2, is typical for the composite. It can be expected that this material can compete with pure silicon nitride in applications requiring complex machining.