Synthesis and Friction Behavior of Chemically Vapor Deposited Composite Coatings Containing Discrete TiN and MoS2 Phases

Composite coatings containing discrete crystalline phases of TiN and MoS₂ were deposited on Si, graphite, and Ti-6A-14V alloy substrates by simultaneous chemical vapor deposition. MoF₆ and H₂S were utilized as the precursors for MoS₂ formation, while Ti((CH₃)₂N)₄ and NH₃ were used for TiN deposition. The composition and microstructure of the coatings were investigated by X-ray diffraction, Auger electron spectroscopy, and transmission electron microscopy. The molar ratios of TiN and MoS₂ in the composite microstructure could be controlled by adjusting the MoF₆concentration in the reagent mixture. Encouraging friction and wear characteristics against silicon nitride were obtained for a composite coating which was rich in MoS₂ at the coating surface, with TiN as the major phase near the substrate interface. Friction coefficients at room temperature in air were typically in the range of 0.07 to 0.3. The friction coefficients remained comparable at 573 K, but increased to 0.7 to 1.0 at 673 K. A friction coefficient value of −0.3 was, however, obtained from a composite coating tested at 973 K.     

Effect of H2O Vapor on the Internal Friction of Binary Alkali Silicate Glasses

Internal friction and shear modulus of Na, K, and Li disiticate glasses were measured by a torsional pendulum as a function of H₂O vapor treatment at T=300°C. H₂O produced changes in the acoustic spectra (between—100° and 400°C and f=0.5 Hz) similar to the mixed-alkali effect which appears upon introduction of a second alkali to a single-alkali glass. An Na₂O·2SiO₂ glass free from the common nonbridging oxygen relaxation was produced .     

Tribological Characteristics of Si3N4 Ceramic in High-Temperature and High-Pressure Water

The effects of sliding speed and dissolved oxygen on the tribological behavior of Si₃N₄ sliding on itself in water were investigated at room temperature and at 120°C saturated vapor pressure. The friction coefficients and specific wear rates at 120°C were much larger than those at room temperature and had a minimum at about 0.4 m/s, whereats -the specific wear rate of the disk increased with increasing the sliding speed. The wear rate at lower sliding speeds in water at 120°C is considered to be primarily controlled by the increase of the contact stress on the asperities which are formed by the dissolution of grain boundaries of the Si₃N₄ ceramic and the subsequent dissolution of the silica layer of the reaction product However, the wear rate at higher sliding speeds is governed by the direct oxidation and microfracture of the Si₃N₄ substrate. The tribochemical reaction to produce NH₃ mainly occurred at all sliding conditions in water at room temperature and 120°C, and the reaction to produce H₂ gas appeared slightly only at the sliding speeds above 0.4 m/s at 120°C. The tribological behavior was independent of dissolved oxygen concentration for all sliding conditions in water at room temperature and 120°C.     

Mechanochemical Changes of Weinschenkite-Type RPO4·2H2O (R = Dy, Y, Er, or Yb) by Grinding and Thermal Reactions of the Ground Specimens

It is reported that, on mechanochemical treatment, weinschenkite-type RPO₄·2H₂O (R = Dy, Y, or Er) gradually transforms into rhabdophane-type RPO₄· nH₂O (n = 0.5 to 1) and weinschenkite-type YbPO₄·2H₂O into xenotime-type YbPO₄, at room temperature in air. Rhabdophane-type YPO₄·0.8H₂O and ErPO₄·0.9H₂O obtained by grinding weinschenkite-type RPO₄·2H₂O (R=Y or Er) are new. The new rhabdophane-type YPO₄·0.8H₂O and ErPO₄·0.9H₂O gradually transform to xenotime-type YPO₄ and ErPO₄ when heated above 900°C (R = Y) and 700°C (R = Er) in air.     

Effect of Heat-Treatment on the Constitution and Mechanical Properties of Some Hydrated Aluminous Cements

The compound compositions of four aluminous cements were determined on anhydrous as well as hydrated specimens which had been heat-treated at temperatures between room temperature and 1400° C. Phases were identified by X-ray diffraction and differential thermal analysis. Specimens were also tested for transverse strength, dynamic modulus of elasticity, and thermal length change. A study of the dehydration characteristics of CaO - Al₂O₈ - 10H₂O₃ 3CaO.Al₂O₃. 6H₂O, and Al₂O₃. 3H₂O was included. The data indicated that CaO. Al₂O₃ 10H₂O was the primary crystalline hydrate formed in the cements at room temperature. At 50° C., 3 CaO Al₂O₃-6H₂O and Al₂O₃. 3H₂O were formed as by-products of the dehydration of CaO.Al₂O₃.10H₂O. When heated alone in an open system, CaO.Al₂O₃.10H₂O did not convert to 3CaO. Al₂O₃. 6H₂O and A1₂O₃. 3H₂O. A correlation between the mechanical properties and compound compositions was noted.     

High-Temperature Stability of Alumina in Argon and Argon/Water-Vapor Environments

The high-temperature stability of alumina (Al₂O₃) in argon and argon/water-vapor (Ar/H₂O) environments has been investigated. Samples were exposed at temperatures of 1300°C–1700°C for 10 h. The microstructure, flexural strength, and volume all showed significant changes in the Ar/H₂O environment at 1700°C. Samples also became whiter, because of the oxidation of graphite impurities that had diffused from the hot-processing dies. In the Ar/H₂O environment at 1700°C, grain-boundary etching occurred and was much more severe than in the pure-argon environment, which was very likely caused by the enhanced formation of gaseous Al(OH)₃ and Al(OH)₂ along grain boundaries. In addition, in the Ar/H₂O environment, substantial grain growth occurred in the surface vicinity. This grain growth, together with grain-boundary etching, led to a decrease in flexural strength.     

Thermal Decomposition of Some Substituted Barium Titanyl Oxalates and Its Effect on the Semiconducting Properties of the Doped Materials

Thermal analysis has been performed on BaTiO(C₂O₄)₂.4H₂O, Ba_0.6Sr_0.4TiO(C₂O₄)₂.4H₂O, Sr(TiO(C₂O₄)₂.4H₂O, Ba_0.9Pb_0.1TiO(C₂O₄)₂.4H₂O, and BaTi_0.9Zr_0.1O(C₂O₄)₂.4H₂O. It was observed that the strontium compound decomposes differently than the others. Previous investigators have proposed conflicting mechanisms for the pyrolysis of the barium salt and these results are discussed in comparison with this work. The electrical resistivity and temperature coefficient of fired lanthanum-doped materials were found to vary with the calcination temperature. Maximum conductivity was observed in samples calcined at 900°C whereas maximum positive temperature coefficient was observed for materials calcined at 1050°C. Particle sizes of the calcined material were compared with grain sizes in the fired pieces and correlated with the electrical properties. A cursory examination was made on the effects of fabrication pressure, 1.25 to 15 tsi, on the electrical conductivity. Both the conductivity and positive temperature coefficient were found to increase with decreasing fabrication pressure.     

Infrared Absorption Due to H2O and D2O in a Fluorozirconate Glass

A comparative study of infrared absorption due to H₂O and D₂O impurities in a fluorozirconate glass (53ZrF₄·20BaF₂·4LaF₃·3AlF₃·20NaF) was carried out. The H₂O and D₂O were introduced into the glass by reaction of the surface at 260°C with H₂O and D₂O vapor entrained in a stream of N₂. Reaction with H₂O produced IR absorption bands at 2.9 μm (O–H stretch) and 6.1 μm (H₂O bend). Reaction with D₂O produced bands at 3.9 μm (O–D stretch) and 8.3 μm (D₂O bend). The ratios of the corresponding D₂O/H₂O peak frequencies are 0.74 for both the stretching and bending vibrations, in good agreement with the value of 0.727 predicted from the difference in the OH and OD reduced masses.     

Formation and Transformation of WO3 Prepared from Alkoxide

Amorphous WO₃ or WO₃ or H₂O is formed by hydrolysis of tungsten ethoxide. The temperature of hydrolysis influences the crystallization of WO₃·H₂O. Tungsten hydrate (WO₃·H₂O) has an orthorhombic unit cell with a=0.5235 nm, b = 1.0688 nm, and c=0.5123 nm. Orthorhombic WO₃ crystallizes at 350° to 500°Cfrom amorphous WO₃. Cubic WO₃ is formed at 200° to 310°C with dehydration of WO₃·H₂O. WO₃ transformations are examined by high-temperature X-ray diffraction. The kinetics of formation of the cubic modification have been studied by measuring the weight decrease with a thermobalcnce. Formation isotherms can be interpreted in terms of the first-order equation –In (1–f)=kt; activation energies are 110 and 80 kJ mol⁻¹ for initial and final stages, respectively.     

Stability of SrFeO3-Based Materials in H2O/CO2-Containing Atmospheres at High Temperatures and Pressures

The chemical stability of SrFeO₃-based perovskites in H₂O- and CO₂-containing atmospheres at high temperatures and pressures has been examined. The extent of reaction as a function of p CO₂, p H₂O, temperature, and time has been determined. Either strontium carbonate or Sr(OH)₂·H₂O was observed on sample surfaces after exposure. Observation of two different reaction-rate behaviors could be explained by the formation of different products. The stability of the perovskite has been found to increase when the activity of Sr is decreased. Chemical stability in H₂O/CO₂ is important to understand in order to use these membrane materials for syngas production.     

Kinetics of Kaolinite Dehydroxylation

When kaolinite is heated under pressures of self-generated H₂O vapor from 0.4 to 3.2 atm, the dehydroxylation is best described by a nucleation-and-growth equation with m varying from 1.10 to 2.16 as the pressure is increased. The temperature coefficient of the rate constant, expressed as an apparent activation energy, varies from 62.4 to 260 kcal/mol. The H₂O vapor evolved in a self-generated-atmosphere sample holder was automatically swept into a gas chromatograph at fixed intervals. The measurement is specific for H₂O.     

Alternative Route for Synthesis of Barium Titanyl Oxalate: Molecular Precursor for Macrocrystalline Barium Titanate Powders

An important molecular precursor to barium titanate, namely, barium titanyl oxalate [BaTiO(C₂O₄)₂.4H₂O], has been synthesized by an alternative route. An alcoholic solution containing 1 mol of butyl titanate monomer [(C₄H₉O)₄Ti] is reacted with alcoholic solution containing 2 mol of oxalic acid (H₂C₂O₄:2H₂O) to form an intermediate soluble oxalotitanic acid [H₂TiO(C₂O₄)₂.nH₂O]. The oxalotitanic acid in alcoholic medium is subjected to cation exchange reaction with aqueous solution containing equimolar barium acetate to form an insoluble barium titanyl oxalate (BTO) in yields of 80–85% at room temperature. The pyrolysis of BTO in air at T .750°C/5 h produced barium titanate (BT) powders.     

Suggested Chemistry of Zinc Oxychloride Cements

Solutions of ZnCl₂ were prepared from ZnCl₂· n H₂O and by reaction of zinc metal with HCl. Specific gravities and pH values were determined as a function of composition. A ternary phase, 4ZnO·ZnCl₂·5H₂O, was precipitated when the ZnCl₂ solutions were diluted to a pH of 5.48 ± 0.05. Mixtures of ZnO with ZnCl, and HCI solutions were equilibrated in sealed containers and analyzed by X-ray diffraction. The resulting phase diagram shows 2 ternary phases, 4ZnO·ZnCl₂·5H₂O (4·1·5) and ZnO·ZnCl₂·2H₂O (1.1.2), both of which are soluble to the extent of < 1 wt% in ZnCl₂ solutions. Thermogravimetric data indicate that the 1.1.2 phase loses half the constituent H₂O at ∼230°C and the remainder, with ZnCl₂, at higher temperatures. The 4.1.5 phase dissociates to ZnO and 1.1.2 at ∼160°C. The system ZnCl₂-H₂O is not binary, but is a section through the ternary system ZnO-HCl-H₂O, with the solubility curve of the 4.1.5 phase intersecting the ZnCl₂-H₂O section in dilute solutions.     

The System MgO─P2O5─H2O at 25°C

The phase diagram for the ternary system MgO─P₂O₅─H₂O at 25°C has been constructed. The magnesium phosphates represented are Mg(H₂PO₄)₂· n H₂O ( n = 4, 2, 0), MgHPO₄·3H₂O, and Mg₃(PO₄)₂· m H₂O ( m = 8, 22). Because of the large differences in the solubilities of these compounds, the technique which involves plotting the mole fractions of MgO and P₂O₅ as their 10th roots has been employed. With the exception of MgHPO₄·3H₂O, the magnesium phosphates are incongruently soluble. Because incongruency is associated with a peritectic-like reaction, the phase Mg₂(PO₄)₃· 8H₂O persists metastably for an extended period.     

Effect of Water Content on the Electrical Conductivity of Na2O≅3SiO2 Glass

Na₂O–3SiO₂ glasses with up to ≅12 wt% water were prepared under high-pressure, hydrothermal conditions and their electrical conductivities were measured. The conductivity (σ) was found to depend on H₂O content in a manner similar to the "mixed-alkali" effect. At constant temperature, σ decreased initially with increasing H₂O content to a minimum at 3≅4 wt% H₂O and increased with further increase in water content. Infrared spectroanalysis of these glasses was made to determine the type of water present in the glass and the results were interpreted in the context of the measured activation energy and preexponential factor of dc conductivity. The sodium ion diffusion coefficient for a glass containing 0.76 wt% H₂O was less than that of water-free glass and in agreement with the electrical conductivity results.     

Partition of Hydrogen in the Modified Chemical Vapor Deposition Process

In the modified chemical vapor deposition (MCVD) process for making glass fibers, most of the hydrogen coming into the reaction from hydrogen-bearing impurities in the starting materials is not incorporated in the glass. It is instead mostly converted to HCl, which is not absorbed by the newly formed silica particles, and passes out the exhaust stack. The residual partial pressure of H₂O formed in equilibrium with HCl in the presence of O₂ and Cl₂ accounts quantitatively for the OH appearing in the glass when the known solubility of H₂O in silica is considered. The H₂O/HCl equilibrium is quenched at a temperature below that of the reaction zone at a value for which the rate of reaction approximates the transit time across the deposition zone. Quantitative agreement is obtained for published OH concentrations produced by SiHCl₃ doping.     

The System Alumina-Gallia-Water

A study of the system Al₂O₃–Ga₂O₃–H₂O has resulted in the determination of equilibrium diagrams for the systems Al₂O₃–Ga₂O₃ and Al₂O₃.-H₂O–Ga₂O₃.H₂O. Extensive solid solution characterizes the α -Al₂O₃ and β -Ga₂O₃ structures at high temperatures, but it is shown that below 810°C. a compound, GaAlO₃, and a new series of (Al, Ga)₂O₃ structures are stable. Among the hydrates, a complete series of diaspore solid solutions extends from Al₂O₃.H₂O to Ga₂O₃.H₂O. Boehmite solid solutions extend to approximately the composition 70Al₂O₃.H₂O, 30Ga₂O₃.H₂O.     

Solubility of Water Vapor in Alkali Borate Melts

The solubility of water vapor at 750° to 1050°C was determined for alkali borate melts containing 0 to 40 mole % Li₂O, Na₂O, or K₂O. In all cases the solubility in these melts is linearly proportional to the square root of the H₂O partial pressure. At p H₂O = 1 atm and T = 900°C, o.5 to 2.2 mole % H₂O are dissolved in the melts in equilibrium. In the potassium borate melts a minimum of solubility was observed at about 25 mole % K₂O; in the sodium borate melts the minimum was at 35 to 40 mole % Na₂O. In the lithium borate melts a minimum of solubility was not reached in the range of compositions investigated. These results are discussed in terms of a concept for the acid-base properties of melts and glasses in which the position of the solubility minimum corresponds to the "neutral point" of an acidity-basicity scale. Some corroborating qualitative observations concerning the evaporation of components from the glass melts and the chemical resistivity of the corresponding solid glasses are discussed.     

Oxidation of HIPed TiC Ceramics in Dry O2, Wet O2, and H2O Atmospheres

Isothermal oxidation of dense TiC ceramics, fabricated by hot-isostatic pressing at 1630°C and 195 MPa, was performed in Ar/O₂ (dry oxidation), Ar/O₂/H₂O (wet oxidation), and Ar/H₂O (H₂O oxidation) at 900°–1200°C. The weight change measurements of the TiC specimen showed that the dry, wet, and H₂O oxidation at 850°–1000°C is represented by a one-dimensional parabolic rate equation, while the oxidation in the three atmospheres at 1100° and 1200°C proceeds linearly. Cross-sectional observation showed that the dry oxidation produces a lamellar TiO₂ scale consisting of many thin layers, about 5 μm thick, containing many pores and large cracks, while H₂O-containing oxidation decreases pores in number and diminishes cracks in scales. Gas evolution of CO₂ and H₂ with weight change measurement was simultaneously followed by heating the TiC to 1400°C in the three atmospheres. Cracking in the TiO₂ scale accompanied CO₂ evolution, and the H₂O-containing oxidation produced a small amount of H₂. A piece of single crystal TiC was oxidized in ¹⁶O₂/H₂¹⁸O to reveal the contribution of O from H₂O to the oxidation of TiC by secondary ion mass spectrometry.     

Low-Temperature Formation of Aluminum Orthophosphate

Factors influencing the low-temperature formation of AIPO₄ and its precursor phases, AIPO₄· x H₂O (1 x 2), were investigated. AIPO₄ formed by reaction between 33.3 wt% H₃PO₄ solution and alumina. Five aluminas (three anhydrous and two hydrated) were utilized. Each differed in particle size, surface area, and crystallinity. The reaction temperatures investigated were 113°, 123°, and 133°C. The high-surface-area aluminas were sufficiently reactive in the phosphoric acid solution at these temperatures to produce crystalline reaction products. However, only hydrated forms of AIPO₄, AIPO₄· x H₂O (1 x 2), crystallized directly out of solution. x generally decreased as the curing temperature was increased. Upon dehydration of these hydrated reaction products, anhydrous AIPO₄ was formed, primarily in the berlinite and/or cristobalite modifications. Both the temperature of reaction and the alumina used influence the hydrates that form. In turn, the hydrates which form, the macroscopic assemblages into which they may crystallize, and the morphologies of the crystallites all affect the polymorphic form and the crystallinity of the anhydrous AIPO₄ phase ultimately produced on dehydration. Phase-pure and highly crystalline AIPO₄-cristobalite (the high-temperature modification) was formed by the dehydration of AIPO₄·H₂O at a temperature as low as 113°C.