High-Temperature Healing of Cracklike Flaws in Mg- and Ca-Ion-Implanted Sapphire

Controlled-geometry voids were introduced into Mg-implanted and Ca-implanted sapphire substrates using microfabrication techniques and ion beam etching, and were subsequently transferred to an internal interface by hot-pressing. The morphological evolution of cracklike and channel-like defects in response to anneals at 1700°C was studied. The healing behavior of defects in the Ca- and Mg-ion-implanted samples differs significantly. Mg additions appear to reduce the directional dependence of the healing characteristics and thus homogenize the evolution. Destabilization of the basal plane by Ca may contribute to the rapid healing of cracks oriented parallel to the basal plane. The healing characteristics of pore channels in Ca-implanted samples indicate a strong residual energetic barrier to healing stemming from surface energy anisotropy. Despite this, defects in Ca-implanted sapphire healed more rapidly than crystallographically and geometrically identical defects in Mg-implanted sapphire. Thus, the results suggest that Ca additions increase transport rates sufficiently to more than compensate for the relatively higher energetic barriers to pore channel breakup.     

High-Temperature Healing of Lithographically Introduced Cracks in Sapphire

A new method for producing controlled-geometry, controlled-crystallography, cracklike defects using photolithography has been developed. The method has been applied to sapphire, and used to study crack healing behavior at 1800°C. Effects of crack face and crack perimeter crystallography, crack face microstructure, and impurities on healing behavior have been identified.     

High-Temperature Fracture Toughness of Sapphire

The fracture toughness of sapphire with crack propagation parallel to the basal plane was measured from 1200° to 1500°C. Fracture surfaces of near c -axis fibers tested in tension were used to determine the values. The toughness was constant and equal to 1.4 ± 0.1 Mpa^.m^1/2 over the entire temperature range.     

High-Temperature Wetting of Sapphire by Aluminum

Sessile drop studies of molten aluminum on single-crystal sapphire substrates were conducted to investigate the effects of atmosphere on contact angle, substrate reactions, and interfacial crystal growth. Unlike previous investigations performed briefly in a vacuum environment in a temperature range within 600°C of the aluminum melting point, these experiments were conducted at higher temperatures (1200° to 1600°C) and at 1-atm total pressure over longer experimental times to more closely approach equilibrium conditions. A continuously flowing buffered gas system utilizing high-purity metered mixtures of hydrogen and helium in combination with a thoria ceramic electrolyte sensor were employed to achieve variations of the oxygen partial pressure from 10⁻¹⁹ to 10⁻¹⁵ atm while continuously maintaining the total pressure at 1 atm. At constant temperature, it was found that neither the oxygen partial pressure nor the crystallographic orientation of the sapphire substrate had a significant effect on the observed contact angles. A continuous decrease of acute contact angles and a single reaction ring characterized the 8-h experiments without the alternating spreading and contracting behavior repeatedly reported in the literature. This phenomenon can be attributed to the lower rate of metal evaporation and interfacial reaction at the higher total gas pressure and yet extremely low oxygen partial pressure of these experiments. Profilometric analysis of sapphire substrates subsequent to the removal of the quenched sessile drops indicates a reduction in metal–solid interaction due to the closer approach to equilibrium than in previous studies. An epitaxial orientation with respect to the substrate was observed in α-alumina crystallite formation at the metal–ceramic interface. Experimental evidence suggests that it was formed by a nucleation and growth process during the cooling period.     

Strengthening of Sapphire by Precipitates Containing Titanium

Sapphire single crystals alloyed with 0.5 cation % Ti and aged at 1500° C to form a precipitate showed considerable increase in hardness and fracture strength over unalloyed sapphire. It is suggested that propagation of a crack requires local plastic deformation and that precipitate particles interfere with local plastic deformation.     

Orientation Effects on the High-Temperature Morphological Evolution of Pore Channels in Sapphire

Arrays of semi-infinite and of controlled-aspect-ratio pore channels, both of controlled orientation, were introduced into undoped basal-plane sapphire substrates, using microfabrication techniques, ion-beam etching, and hot pressing. The breakup of these channels via Rayleigh instabilities during a series of annealings at a temperature of 1700°C was monitored. In all cases, the channels broke up with a characteristic wavelength (λ) that was much larger than that expected for a material with isotropic surface energy, which reflected stabilization effects that were due to anisotropy of the surface energy. The break-up wavelength also was very dependent on orientation: channels that were oriented along the [11¯00] and [112¯0] directions yielded the smallest and largest pore spacings (λ-values), respectively, which is in qualitative agreement with prior observations. The critical (minimum) aspect ratio for the breakup of finite-length channels into multiple pores also is dependent upon channel orientation, and the trend mirrors that observed for semi-infinite channels. The pattern of channel evolution suggested two-fold rotational symmetry within the basal plane, where, because of the nature of the experiment, apparent six-fold symmetry is expected. Several factors that may contribute to or cause an apparent or real loss of symmetry have been discussed.     

高温钛乳浊生料釉的研究

本文对钛釉用于卫生瓷生产进行了研究．讨论了ＴｉＯ２外加量以及长石，石灰石含量对釉面色泽，白度的影响．利用Ｘ射线衍射和偏光显微镜，对析出晶相的种类和分布进行了分析，在此基础上对钛釉的乳浊机理进行了探讨．     

Resistance of Titanium Diboride to High-Temperature Plastic Yielding

Polycrystalline TiB₂ specimens of 3 μm average grain size but free of lamellar precipitates were compression-tested to 1900°C and 500 MPa. No plastic yielding was detected. Under such conditions, most other structural ceramics do exhibit yield behavior. The results suggest the existence of a high Peierls stress, presumably related to Ti—B bonding. This finding contrasts with a report on ZrB₂ in which resistance to yielding was attributed to lamellar precipitates.     

Ambipolar Diffusion Contribution to High-Temperature Thermal Conductivity of Titanium Carbide

The odd thermal conductivity behavior of transition metal carbides– K increasing with increasing T –has been interpreted as resulting from the strong scattering of electrons by carbon vacancies and polar optical phonons and from the strong scattering of phonons by vacancies and conduction electrons. These scattering processes have been used in a Callaway analysis to fit the thermal conductivity of TiC from the liquid He temperature to 1000 K; however, at the highest temperatures a residual contribution to K was noted which increased with increasing T. An analysis of the Lorenz function has indicated that this contribution is electronic. The present work indicates, using a theory modified for semimetals, that the additional conductivity may be ambipolar diffusion (electron-hole migration and recombination).     

High-Temperature Stability of Titanium Boride Reinforced Alumina-Silicon Carbide Based Composite

Silicon - Alumina and Silicon Carbide based composites are routinely used in mechanical, automobile, ceramic and aerospace industries with numerous reinforcements. Reinforcements increase the...     

Lattice Modification in Ion-Implanted Ceramics

The effect of ion implantation on alumina (Al₂O₃) and silicon carbide (SiC) was investigated by Rutherford backscattering (RBS), indentation hardness, fracture toughness, transmission electron microscopy (TEM), and linear scratching with a diamond stylus. The implanted (10^l6 to 10¹⁷ Cr.cm⁻² at 280 to300 keV, 1 to 4 |MX 10^l6Ti.cm ⁻²at 150 keV,2 |MX 10¹⁶Zr.cm⁻² at 150 keV) AI₂O₃ lattice is significantly damaged but remains crystalline; the lattice hardness increases, and the scratched surface is less sensitive to fracture. The implanted (10¹³ to 10^l6 N.cm⁻² at 62 keV, 10¹⁴ to 10^l6 Cr.cm ⁻² at 280 keV) Sic lattice becomes amorphous to a depth of 250 nm, becomes less hard, and deforms without fracture when scratched. The variation in Rutherford backscattered spectra and in surface hardness with annealing are reported and interpreted in terms of lattice defects for the Al₂O₃ specimens.     

Determination of Residual Stresses in Titanium Carbide-Base Cermets by High-Temperature X-Ray Diffraction

The crystal structure and thermal expansion of titanium carbide, nickel, and two titanium carbide-base cermets were determined between room temperature and 1100°C. (2012°F.). No structural changes were observed. An abnormal rate of expansion was observed for pure nickel near the Curie temperature, 353°C. (665°F.), and for the nickel and carbide phases in the cermets at about 816°C. (1500°F.). The expansion coefficient of pure nickel and the nickel phase in the cermets was found to be approximately twice that of pure titanium carbide and the carbide phase in the cermets. The brittleness and poor impact strength of the cermets was attributed to the large residual stresses present in these materials as a result of this difference in thermal expansion. The stress-strain relations were interpreted on the basis of a mechanical interaction between the phases in the cermets. The carbide phase was found to be essentially under triaxial compression and the nickel phase under a triaxial tension of 158,000 lb. per sq. in. At elevated temperatures, increased solid solubility of carbide in the nickel phase and plastic deformation of this phase was believed to influence the stress-strain relations and the thermal-expansion behavior of the phases. It was concluded that replacement of the nickel phase in the cermets with a metal or alloy, such as a nickel-chromium-molybdenum alloy, which has a coefficient of thermal expansion similar to the carbide phase in the cermets, would improve the impact strength of these bodies. Equations were developed for the thermal expansion of titanium carbide and nickel. Values of the expansion coefficient were computed for each of the materials by differentiation of these equations.     

Synthesis of Ternary Titanium Aluminum Carbides Using Self-Propagating High-Temperature Synthesis Technique

Different ternary carbide phases, namely Ti₃AlC₂, Ti₃AlC, and Ti₂AlC, were successfully synthesized in a self-sustaining regime. Direct reactions among elemental powders of titanium, aluminum, and carbon are strongly exothermic, and the resulting reaction products consist of binary carbides and they are partially molten. The use of TiAl, instead of elemental titanium and aluminum, significantly reduces the combustion temperature. As a result, ternary titanium aluminum carbide phases are formed. In addition, the combustion-synthesized products are not sintered and easy to deagglomerate. Reaction conditions and X-ray diffraction patterns of different ternary phases formed in a self-sustaining regime are presented.     

Ambient- and High-Temperature Properties of Titanium Carbide–Titanium Boride Composites Fabricated by Transient Plastic Phase Processing

Ambient- and high-temperature properties of a class of titanium carbide-titanium boride composites that have been produced by transient plastic phase processing are presented. The composites produced are comprised of Ti₃B₄, TiB₂, and TiC_0.65 at their equilibrium composition (34.5, 30.5, and 34.9 vol%, respectively), and the Ti₃B₄ phase in these composites occurs either as equiaxed grains or as platelets, depending on the starting mixture composition. Measurements of the ambient- and high-temperature flexure strength and fracture toughness, thermal shock susceptibility, oxidation resistance, and wear resistance of this class of composites are presented. The role of various microstructural parameters—such as the morphology of the Ti₃B₄ phase, the length scale of the microstructure, and the volume fraction of borides—on these properties has been identified.     

Comparative Studies on Sintering Behavior of Self-Propagating High-Temperature Synthesized Ultra-Fine Titanium Diboride Powder

Sintering behavior of nano-sized titanium diboride (TiB₂) powder prepared by a self-propagating high-temperature synthesis (SHS) technique was compared with that of the commercially available powder. The SHS-made powder showed excellent sinterability at low temperatures and a maximum of 97% densification was achieved at 2223 K, whereas the identically sintered commercial powder could only be densified to ∼86% at 2223 K. The estimated activation energies for sintering of 35±0.07 kJ/mole and 46±0.7 kJ/mole for the SHS and commercial powder, respectively, indicated the possibility of a different mechanism during sintering.     

海南高岭土制备蓝宝石级高纯Al2O3除钛工艺

以海南文昌高钛高岭土为原料,采用盐酸酸浸法制取铝盐,进行铝盐中杂质钛的除杂方法研究,优选出PAM除杂法,并采用均匀设计试验法进行PAM除杂的工艺优化研究.结果 表明:通过对絮凝体系加热时间、温度、pH值、PAM加入量进行均匀设计试验并对其实验数模进行建立与优化,可使铝盐中钛杂质的除去率达到98.895％;采用该优化工艺所制备的低钛高纯球形α-Al2O3,其Ti元素含量低至0.34ppm,杂质总量低至39ppm,金属Ti杂质及总量杂质均优于AO-B级蓝宝石Al2O3质量要求.     

Interfacial Reactions between Sapphire and Silver—Copper—Titanium Thin Film Filler Metal

Wetting and brazing studies of sputtering-deposited, submicrometer thin film filler metal in an Ag—Cu—Ti/Al₂O₃ system were performed. The interfacial reaction layer between the filler metal and Al₂O₃ was investigated. It is possible to make a brazing joint even with a reaction layer of less than 100 nm thickness. Different types of interfacial reaction layers were observed when the Ti content in the filler metal was varied. The Cu—Ti—O system compounds were observed in the samples with high wetting capabilities, but not in the sample with low wetting characteristics. It was found that these compounds are substances that promote effective brazing.     

Liquid Corrosion and High-Temperature Oxidation Effects on Silicon Carbide/Titanium Diboride Composites

A silicon carbide composite containing titanium diboride particulate reinforcement was subjected to room-temperature acidic corrosion in aqua regia solutions for 200 h, and 50% NaOH+50% H₂O and 10% HF+57% HNO₃+33% H₂O solutions for up to 500 h. A small increase in room-temperature strength was observed after the aqua regia exposure. Separately, this ceramic was oxidized at 1400°C for 24 h and then tested for strength retention at room temperature. Contrary to the observations reported in the literature on similar material, there was no significant room-temperature strength decrease in this particular composite.     

Metalorganic Chemical Vapor Deposition by Pulsed Liquid Injection Using an Ultrasonic Nozzle: Titanium Dioxide on Sapphire from Titanium(IV) Isopropoxide

A new method for metalorganic chemical vapor deposition has been developed in which a precursor solution is directly injected into a reaction chamber. The vaporization of short pulses of liquid is facilitated by atomization with a piezo-electrically operated nozzle. The system has been used successfully to grow oriented, thin TiO₂ (rutile) films on sapphire substrates with the crystalline relationship (101)[010]_R || (1120)[0001]_s. A film growth rate of approximately 2.5 monolayers of TiO₂ (rutile) per pulse was achieved at 650°C for a reactant impingement rate of about 250 monolayers of Ti(OPrⁱ)₄ per pulse.