The processes of passive oxidation, deposit-induced corrosion, active oxidation, scale/substance interactions, and scale volatility are presently studied in the case of high-purity SiC and Si3N4 in pure oxygen, giving attention to such secondary elements in the ceramics as water and CO2 oxidants, combustion environment impurities, and thermal cycling. Deposit-induced corrosion is discussed for the cases of NaSO4 as well as vanadate and oxide-slag deposits; issues associated with the active-to-passive oxidation transition are noted.

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Corrosion of Silicon-Based Ceramics in Combustion Environments

A detailed microstructural investigation of reaction-bonded silicon carbide has been performed using both fully-bonded and quenched samples and other specially prepared specimens containing large original single crystals of known crystallographic habit. The development of the epitaxial SiC overgrowth on the original SiC particles has been followed and found to proceed by the progressive growth and coalescence of identically-oriented nuclei. This epitaxial layer grows with a habit characteristic ofβ (cubic) SiC and then transforms toα-SiC in the high temperature region behind the reaction front. The formation of faceted grain boundaries is explained by this growth morphology. Furthermore, SiC:SiC grain boundaries, SiC:SiC epitaxial boundaries and SiC:Si interfaces have all been characterized by TEM techniques. The grain boundaries are of particular interest since they usually comprise a thin (∼1 nm) layer of amorphous SiC with occasional suicide and graphite inclusions. The general cleanliness of the vast majority of interfacial area is a result of the removal of the impurities insoluble in SiC by liquid silicon moving through the sample. The overall distribution of impurities is discussed. Other microstructural features have been characterized and texturing due to original particle alignment during fabrication of the green compact investigated. The control of the mechanical properties of reaction-bonded silicon carbide by these various microstructural features is discussed. A basis for an explanation of the interesting trace-impurity-controlled contrast seen in secondary electron images of these materials is also established.

Microstructural evolution in reaction-bonded silicon carbide

“In situ toughened” silicon carbides, containing Al, B, and C additives, were prepared by hot pressing. Densification, phase transformations, and microstructural development were described. The microstructures, secondary phases, and grain boundaries were characterized using a range of analytical techniques including TEM, SEM, AES, and XRD. The modulus of rupture was determined from fourpoint bend tests, while the fracture toughness was derived either from bend tests of beam-shaped samples with a controlled surface flaw, or from standard disk-shaped compact-tension specimens precracked in cyclic fatigue. The R-curve behavior of an in situ toughened SiC was also examined. A steady-state toughness over 9 MPa·m1/2 was recorded for the silicon carbide prepared with minimal additives under optimum processing conditions. This increase in fracture toughness, more than a factor of three compared to that of a commercial SiC, was achieved while maintaining a bend strength of 650 MPa. The mechanical properties were found to be related to a microstructure in which platelike grain development had been promoted and where crack bridging by intact grains was a principal source of toughening.

In Situ Toughened Silicon Carbide with Al‐B‐C Additions

A study of the distribution of Si in rice hulls was carried out to aid in understanding the observed formation of SIC whiskers by the thermal decomposition and reaction of these natural materials. Scanning electron microscopy, Auger electron spectroscopy, and energy-dispersive X-ray analysis confirm the presence of a major amount of Si in the inner and outer epidermal regions of the rice hull. The Si is present as a hydrated amorphous form of SiO2 which is evidently bound in the inner layer to organic constituents, placing it adjacent to a source of carbon but interfering with its solubility in HF. The SIC produced by pyrolysis is in the form of ultrafine particles with cubic crystal structure and whiskers exhibiting stacking faults and twins perpendicular to the whisker axis, easily visible by transmission electron microscopy. These whiskers are very similar to ones obtained by chemical vapor deposition with SiO and CO as the primary reactants.

Formation and Structure of Silicon Carbide Whiskers from Rice Hulls

The oxidation kinetics of sintered α- and hot-pressed SiC were investigated over the temperature range 1200° to 1500°C. Oxide thicknesses were measured by ellipsometry, interferometry, and profilometry. The two materials behaved similarly, exhibiting parabolic behavior for short times (up to 100 min) followed by decreasing rates for longer times. The oxidation rates for the sintered material were lower than for the hot-pressed material at all temperatures studied. Apparent activation energies varied with temperature, from 134 to 389 kJ/mol for the sintered alpha material and from 155 to 498 kJ/mol for the hot-pressed variety. Platinum-marker experiments indicated that new oxide is formed at the silicon carbide/ oxide interface. It is proposed that the diffusion of oxidant through the growing oxide film is the rate-controlling process. The details of the oxidation kinetics were complicated by the partial crystallization of the oxide film at higher temperatures, leading to complex paths for transport of oxidant through the film so that the apparent activation energies are difficult to interpret.

Oxidation Kinetics of Hot‐Pressed and Sintered α‐SiC

The effects of annealing conventionally sintered, hot-pressed, and reaction-sintered Sic have been investigated by optical and transmission electron microscopy. In conventionally sintered and hot-pressed bodies, “composite” grains consisting of α-SiC plates sandwiched between recrystallized β-SiC envelopes grow rapidly into the fie-grained β matrix; thickening of these α plates within the p envelopes is relatively sluggish. The existence of the composite plates can be explained by the extreme anisot-ropy of the interfacial energy between β- and α-Sic: {111)β(000l)α interfaces have energies several orders of magnitude lower than random β/α interfaces. In reaction-sintered Sic, this anisotropy is manifested by rapid growth of α seeds along their basal planes into the epitaxial β reaction product; slow growth occurs perpendicular to the basal planes.

β→α Transformation in Polycrystalline SiC: I, Microstructural Aspects

The final stages of the β→α polymorphic phase transformation in conventionally sintered and hot-pressed SiC were studied by optical and transmission electron microscopy. The interface between the α and β regions of partially transformed grains invariably contains thin α lamellae adjacent and parallel to coherent β twins (or β stacking faults). These planar defects serve as nucleation sites for the transformation, with growth of α occurring by the motion of partial dislocations nucleated at the intersection of coherent and incoherent twin boundaries. Various structural phenomena in SiC, including faulting, mi-crosyntaxy, and long-period polytypism, are discussed in terms of this transformation mechanism.

The β→αTransformation in Poly crystalline SiC: III, The Thickening of α Plates

Microstructural characterization of conventionally sintered, hot-pressed, reaction-sintered, and chemically vapor-deposited polycrystalline SiC shows that the initial stages of the βα transformation in the first three materials are similar, the process being dominated by the elimination of high-energy βα interfaces. In the final stage of transformation, all four materials transform by nucleation and growth of α lamellae at {111}β twin boundaries or at β stacking faults.

The β→α Transformation in Polycrystalline SiC: IV, A Comparison of Conventionally Sintered, Hot‐Pressed, Reaction‐Sintered, and Chemically Vapor‐Deposited Samples

The β→α transformation in polycrystalline Sic occurs by the rapid growth of composite grains consisting of α-SiC plates “sandwiched” between grains of recrystallizedp-SiC. Growth of these composite grains into the fine-grained β matrix occurs much more rapidly than thickening of the α plates into their β“envelopes.” A phenomenological analysis of the energetics of these several growth processes is presented and it is shown that the observations can be explained by the extreme anisotropy of the interfacial energy between β− and α−SiC; {lll}β(0001)α interfaces have much lower energies than random, β/α interfaces. In reaction-sintered Sic, this anisotropy is manifested by rapid growth of a seeds along their basal planes into the epitaxial β reaction product; slow growth occurs perpendicular to the basal planes.

L. U. Ogbuji

Papers

β→α Transformation in Polycrystalline SiC: I, Microstructural Aspects

The β→αTransformation in Poly crystalline SiC: III, The Thickening of α Plates

The β→α Transformation in Polycrystalline SiC: IV, A Comparison of Conventionally Sintered, Hot‐Pressed, Reaction‐Sintered, and Chemically Vapor‐Deposited Samples

β→α Transformation in Polycrystalline Sic: II, Interfacial Energetics

The Microstructure of Oxide Scales on Oxidized Si and SiC Single Crystals