Microstructure control and wear of Al2O3-SiC-(Al,Si) composites made by melt oxidation

Ceramic matrix composites of Al2O3-SiC-(Al,Si) have been fabricated by directed melt oxidation of aluminum alloys into SiC particulate preforms. The proportions of Al2O3, alloy, and porosity in the composite can be controlled by proper selection of SLC particle size and the processing temperature. The wear resistance of composites was evaluated in pin-on-disk experiments against a hard steel substrate. Minimum wear rate comparable to conventional ceramics such as ZTA is recorded for the composition containing the highest fraction of alloy, owing to the development of a thin and adherent tribofilm with a low coefficient of friction.

Al2O3---ZrO2 composite coatings for thermal-barrier applications

Graded alternate layers of Al2O3 and 8% Y2O3-ZrO2 and their admixtures were plasma sprayed onto bond-coated mild steel. They were evaluated for thermal-shock resistance, thermal-barrier characteristics, hot corrosion resistance (molten NaCl corrodant) and depth of attack, adhesion strength and the presence of phases. Although front-back temperature drops of 423-623 K were observed, some of the coatings showed good adherence even after 100 thermal shack cycles. In the sequence of the graded layers, the oxide which is directly in contact with the bond coat appears to influence the properties especially in coatings of 150 and 300 mu m thickness. Molten NaCl readily attacks the films at high hot-face temperatures (1273 K for 1 h) and the adhesive strength falls significantly by 50-60%. Diffusion of alkaline elements is also found to depend on the chemical composition of the outer coating directly facing the molten corrodant. (C) 1997 Elsevier Science Limited.

Microwave assisted preparation and sintering of Al2O3, ZrO2 and their composites from metalorganics

Controlled pyrolysis of Al(OBus)(3), Zr(OPrn)(4) and their mixtures in ethyl acetate induced using microwaves of 2.45 GHz frequency has been carried out. Microwave irradiation yields second-stage precursors for the preparation of respective oxides and their composites. It is observed that the microwave irradiation has a directive influence on the morphology of the ultimate oxide products. Al2O3, ZrO2 and the two composites 90% Al2O3-10% ZrO2 and 90% ZrO2-10% Al2O3 are also found to be sintered to very high densities within 35 min of microwave irradiation by the use of beta-SiC as a secondary susceptor.

Phase evolution and densification of spray pyrolysed ZrO2-Al2O3 powders

Devitrification of spray pyrolysed, amorphous ZrO2-Al2O3 solid solution produces nanocrystalline microstructures (grain sizes 10-20 nm). In this study, spray pyrolysed amorphous ZrO2-40 mol% Al2O3 powder displayed good sinterability during decomposition, after spraying, of the nitrate precursors up to 1023K. Hot pressing of fully pyrolysed, pre-sintered (more than 70% dense) pellets at 923K and 750 MPa produced an amorphous pellet with less than 2% porosity. The results indicate the possibility of producing dense, amorphous pellets that can be heat treated further to produce nanocrystalline microstructures conducive for superplasticity.

Sliding wear of Al2O3-SiC-(Al,Si) composites against a steel counterface

The sliding-wear behavior of Al2O3-SiC-Al composites prepared by melt oxidation against a steel counterface has been recorded in a pin-on-disk machine. At high speeds and pressures (10 m/s, 20 MPa), friction and wear appear to be principally controlled by the in-situ formation of an interfacial film that consists of a layer of Fe3O4. The formation of this him is examined as a function of sliding speed, lubrication, and composite microstructure. A model is proposed in which high surface temperatures cause the preferential extrusion of aluminum from the composite onto the pin/disk interface. This promotes the adhesive pickup of iron and its oxidation to form a stable tribologically beneficial layer of Fe3O4.

Wear of Al2O3-SiC-(Al,Si) melt oxidised ceramic composites

Al2O3-SiC-(Al,Si) cermets are fabricated using the melt oxidation route. The tribological properties of the composites are tested under adhesive sliding and two body abrasion conditions. Under adhesive conditions, the network of residual aluminium in the matrix plays a role in the formation of a thin tribofilm on the interface while in abrasion the hardness of the composite plays a prominent tribological role. The work suggests that microstructural control can make this low temperature composite competitive with the conventional high temperature monolithic ceramics. (C) 1999 Published by Elsevier Science S.A. All rights reserved.

In situ synthesis of Al2O3-ZrO2-SiCW ceramic matrix composites by carbothermal reduction of natural silicates

In situ formations of Al2O3 + ZrO2 + SiCW ternary composite powders have been obtained by carbothermal reduction of a mixture of Sillimanite. Kaolin and Zircon using two different carbon sources. Products formed were mixtures of alumina and zirconia along with silicon carbide in the form of whiskers. The effects of composition of the reactants, the role of fineness of the starting precursors and the nature of the carbon Source on the final product powder obtained are presented. XRD and SEM analyses indicate complete reaction of the precursors to yield Al2O3 + ZrO2 + SiCW as product powders, with the SiC having whisker morphology. It is also seen that zirconia could be stabilised to some extent in the tetragonal form without any stabilising agent by tailoring the starting materials and their composition. (C) 2002 Published by Elsevier Science B.V.

Low-pressure MOCVD of Al2O3 films using aluminium acetylacetonate as precursor: nucleation and growth

A study of the deposition of aluminium oxide films by low-pressure metalorganic chemical vapour deposition from the complex aluminium acetylacetonate, in the absence of an oxidant gas, has been carried out. Depositions on to Si(100), stainless steel, and TiN-coated cemented carbide are found to be smooth, shiny, and blackish. SIMS, XPS and TEM analyses reveal that films deposited at temperatures as low as 600 degreesC contain small crystallites Of kappa-Al2O3, embedded in an amorphous matrix rich in graphitic carbon. Optical and scanning electron microscopy reveal a surface morphology made up of spherulites that suggests that film growth might involve a melting process. A nucleation and growth mechanism, involving the congruent melting clusters of precursor molecules on the hot substrate surface, is therefore invoked to explain these observations. An effort has been made experimentally to verify this proposed mechanism. (C) 2002 Elsevier Science B.V. All rights reserved.

Oxidation and decomposition of NH3 over combustion synthesized Al2O3 and CeO2 supported Pt, Pd and Ag catalysts

The catalytic oxidation and decomposition of NH3 have been carried out over combustion synthesized Al2O3 and CeO2 supported Pt, Pd and Ag catalysts using temperature programmed reaction (TPR) technique in a packed bed tubular reactor. Metals are ionically dispersed over CeO2 and fine metal particles are found on Al2O3. NH3 oxidation occurs over 1% Pt/Al2O3, 1% Pd/Al2O3 and 1% Ag/Al2O3 at 175, 270 and 350 C respectively producing N-2, NO, N2O and H2O, whereas 1% Pt/CeO2, 1% Pd/CeO2 and 1% Ag/CeO2 give N-2 along with NO, N2O and H2O at 200, 225 and 250degreesC respectively. N-2 predominates over other nitrogen-containing products during the reaction on all catalysts. At less O-2 concentration, N-2 and H2O are the only products obtained during NH3 Oxidation. NH3 decomposition over all the catalysts occurs above 450degreesC.

Properties of Al2O3 films prepared by argon ion assisted deposition

Aluminum oxide films have been prepared by ion assisted deposition using argon ions with energy in the range 300 to 1000 eV and current density in the range 50 to 220 μA/cm2. The influence of ion energy and current density on the optical and structural properties has been investigated. The refractive index, packing density, and extinction coefficient are found to be very sensitive to the ion beam parameters and substrate temperatures. The as-deposited films were found to be amorphous and could be transformed into crystalline phase on annealing. However, the crystalline phases were different in films prepared at ambient and elevated substrate temperatures.

A kinetic study of the phase transformations in Al2O3 and Na2O · 6Al2O3 by a combined use of 27Al MAS NMR spectroscopy and x-ray diffraction: Importance of nucleation

Phase transformations of Al2O3 and Na2O · 6Al2O3 prepared by the gel route have been investigated for the first time by 27Al MAS NMR spectroscopy in combination with x-ray diffraction. Of particular interest in the study is the kinetics of the γ → α and γ → β transformations, respectively, in these two systems. Analysis of the kinetic data shows the important role of nucleation in both these transformations.

Influence of matrix characteristics on fracture toughness of high volume fraction Al2O3/Al–AlN composites

The role of matrix microstructure on the fracture of Al-alloy composites with 60 vol% alumina particulates was studied. The matrix composition and microstructure were systematically varied by changing the infiltration temperature and heat treatment. Characterization was carried out by a combination of metallography, hardness measurements, and fracture studies conducted on compact tension specimens to study the fracture toughness and crack growth in the composites. The composites showed a rise in crack resistance with crack extension (R curves) due to bridges of intact matrix ligaments formed in the crack wake. The steady-state or plateau toughness reached upon stable crack growth was observed to be more sensitive to the process temperature rather than to the heat treatment. Fracture in the composites was predominantly by particle fracture, extensive deformation, and void nucleation in the matrix. Void nucleation occurred in the matrix in the as-solutionized and peak-aged conditions and preferentially near the interface in the underaged and overaged conditions. Micromechanical models based on crack bridging by intact ductile ligaments were modified by a plastic constraint factor from estimates of the plastic zone formed under indentations, and are shown to be adequate in predicting the steady-state toughness of the composite.

A study of Al2O3:C films on Si(100) grown by low pressure MOCVD

We report the characterization of carbonaceous aluminium oxide, Al2O3:C, films grown on Si(100) by metalorganic chemical vapor deposition. The focus is on the study of the effects of carbon on the dielectric properties of aluminium oxide in a qualitative manner. The carbon present in the aluminium oxide film derives from aluminium acetylacetonate used as the source of aluminium. As-grown films comprise nanometer-sized grains of alumina (∼ 20–50 nm) in an amorphous carbonaceous matrix, as examined by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The films are shiny; they are smooth as observed by scanning electron microscopy (SEM). An attempt has been made to explore the defects (viz., oxide charge density) in the aluminium oxide films using room temperature high frequency capacitance – voltage (C-V) and current–voltage (I-V) measurements. The hysteresis and stretch-out in the high frequency C-V plots is indicative of charge trapping. The role of heteroatoms, as characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy, in the transport of charge in Al2O3:C films is discussed.

Fracture and R-curves in high volume fraction Al2O3/Al composites

Fracture toughness and fracture mechanisms in Al2O3/Al composites are described. The unique flexibility offered by pressureless infiltration of molten Al alloys into porous alumina preforms was utilized to investigate the effect of microstructural scale and matrix properties on the fracture toughness and the shape of the crack resistance curves (R-curves). The results indicate that the observed increment in toughness is due to crack bridging by intact matrix ligaments behind the crack tip. The deformation behavior of the matrix, which is shown to be dependent on the microstructural constraints, is the key parameter that influences both the steady-state toughness and the shape of the R-curves. Previously proposed models based on crack bridging by intact ductile particles in a ceramic matrix have been modified by the inclusion of an experimentally determined plastic constraint factor (P) that determines the deformation of the ductile phase and are shown to be adequate in predicting the toughness increment in the composites. Micromechanical models to predict the crack tip profile and the bridge lengths (L) correlate well with the observed behavior and indicate that the composites can be classified as (i) short-range toughened and (ii) long-range toughened on the basis of their microstructural characteristics.