C. H. Carter

The Occurrence and Behavior of Dislocations during Plastic Deformation of Selected Transition Metal and Silicon Carbides

The Group IV and Group V transition metal monocarbides (TMMC) possess a combination of some of the highest melting points, hardness values and mechanical strengths of any known materials. Because of these extremes in properties, interest in these materials continues, especially for structural and coating applications for utilization at temperatures where most materials would be molten or would seriously degrade and for employment as abrasives and cutting tools. However, to date, extreme notch-sensitivity, difficulty of fabrication and poor oxidation resistance have limited their acceptance. Several fundamental studies have provided significant information concerning the mechanisms of plastic flow induced by diamond indentation at reduced temperatures and by standard deformation equipment above the ductile-brittle transition temperature. A review of this research as it pertains to single crystals of these materials is presented elsewhere in these Proceedings.1 The principal thrust in this paper is a review of the results and a discussion of the possible mechanisms of high temperature deformation and steady state creep in both single and polycrystalline TMMC, including recent results obtained at NCSU on single crystals of NbCx. This will allow a comparison of the derived results and proposed deformation processes using different materials and applied stress parameters.

Kinetics and Mechanisms of Constant Stress Creep in the Non-Oxide Ceramics of SiC, SiC Whisker-Reinforced Si3N4 Composites and AIN

Constant stress, uniaxial compressive creep studies have been conducted within inert environments on polycrystalline SiC, SiC-whisker reinforced Si3N4 and AIN. Creep of reaction-bonded SiC within the ranges of temperature and stress of 1573–1923 K and 69–220 MPa, respectively, occurred by glide and the controlling process of climb of dislocations. Deformation in the preferentially oriented SiC produced by chemical vapor deposition (CVD) occurred solely by dislocation glide controlled via Peierls stress in the ranges of 1573–2023K and 69–220 MPa. The onset of creep of sintered α-SiC within the ranges of 1520–2073K and 69–414 MPa occurred by dislocation glide; however, subsequent formation of Si-containing B4C precipitates and their interaction with moving dislocations caused the controlling mechanism to become lattice and grain boundary self-diffusion above and below 1920±20K, respectively. All studies were conducted in one atm. of Ar. Silicon nitride composites containing 30 vol.% SiC whiskers and an initial vitreous phase composed of Al2O3, Y2O3, and SiO2 were investigated using the conditions of 1470–1670K, 50–350 MPa and purified N2 at 1 atm. Significant changes in the stress exponent and activation energy indicated a change in the controlling creep mechanism at 225 MPa and 1570K. Microscopy revealed these changes were caused by the removal of the amorphous material at the grain boundary and resultant contacts between Si3N4 grains. The composite creeps via grain boundary sliding accommodated by viscous flow at low stresses and temperatures and by diffusion at high stresses and temperatures. No cavitation was observed. The presence of the SiC whiskers had no observable effect on deformation. Phase pure and theoretically dense AIN was studied under the conditions of 1470–1670K, 100–370 MPa and 1 atm of N2. The controlling mechanism for creep under these conditions was diffusion-accommodated grain-boundary sliding. This mechanism was accompanied in parallel by relatively small amounts of unaccommodated grain-boundary sliding. Cavitation was not observed.

Structure and Chemistry of Interfaces in Silicon Carbide-Containing Materials

Transmission electron microscopy (TEM) and other techniques were employed to investigate the character of high and low angle boundaries and interfaces in several α-SiC-containing multiphase materials. In reaction-bonded SiC, the reaction of Si vapor with excess free C contained in an α-SiC matrix caused epitaxial growth of additional α-SiC and resultant sub-grain boundary formation. Climb-controlled deformation above 1773K caused the formation of additional, unique subboundaries in this material. In sintered α-SiC containing B and C, the occasionally predicted and reported amorphous boundary phase was not discerned in high resolution TEM or Auger analyses. The addition of MgO to an β-SiC whisker-containing Si3N4 composite resulted both in the formation of an amorphous phase and in the epitaxial crystallization of an MgO-containing silicate phase. Finally, ruby or eximer laser annealing of Ni-coated SiC caused the melting of the Ni and the diffusion of SiC into this molten phase. By comparison, the ion mixing of the Ni via the implantation of Si+ through this metal layer resulted in the sequential formation of numerous layers of varying chemistry. The following sections describe the several forms of these interphase regions and their effect on the structural properties of the various materials.

Interfaces Generated by Electronic Device Development in Beta SiC Thin Films

Beta silicon carbide (β-SiC) holds considerable contemporary interest as a candidate material for use in severe environments. It possesses high thermal conductivity, a high melting point, excellent resistance to radiation, a wide energy bandgap and a high saturated electron drift velocity. This unique combination of properties is ideal for the production of high temperature, high frequency or high power electronic devices and the next generation of very high speed microprocessors.

The Occurrence of Defects in Silicon Carbides as a Result of Processing Mode and Applied Stress

Silicon carbide (SiC) possesses extreme hardness, very good electrical, thermal and mechanical properties as well as excellent resistance to corrosion and thermal shock. As such, it is one of the primary candidate materials for use in systems for the production and conversion of energy at elevated temperatures. It is currently employed in or being considered for use in heat exchangers or waste heat recouperators in various prototype fossile fuel systems. Specific examples of its potential use include (1) indirectly fired turbine engines wherein the turbine section is separated and protected from the combustor by a SiC heat exchanger; (2) coal fired systems which heat air in a fluidized bed containing a SiC heat exchanger; (3) coal gasifiers wherein the outlet channels will be of SiC because of the particle erosion, high temperatures (1673K) and pressures on these systems; (4) critical parts for gas turbines, the Sterling engine and adiabatic diesel engines; (5) high temperature bearings and (6) first wall materials for fusion reactors.