High-dose and multi-energy aluminum implantation of α-SiC (0001) was carried out to achieve a broad aluminum distribution extending from the sample surface to a depth of approximately 350 nm. Oxidation resistance of the implanted crystals was studied in 1 atm flowing oxygen at 1300°C. Aluminum implantation resulted in a 45% improvement in the oxidation resistance of α-SiC as compared with unimplanted crystals due to the formation of structurally dense mullite (3A12O3.2SiO2) in the oxidation scale.

This investigation outlines a process for the synthesis of preceramic/ceramic hard coatings on glass substrates. The precursor used was an organosilazane monomer [CH3SiHNH]n, with n=3 or 4. The glass substrates were dip coated in organosilazane solutions. The coated samples were then heat treated in different gaseous environments to produce the desired preceramic and ceramic coatings. The mechanical properties of the coatings were evaluated, such as rupture strength and microhardness. The former was measured using a four-point bend test (ASTM c150). Vicker's hardness rupture strength measurement were performed on both coated and uncoated specimens subjected to the heat treatment conditions. Microstructural examination was performed using a SEM. Chemical information, such as bonding was investigated via FTIR spectroscopy.

The reconstruction patterns of Si- and C-terminated surfaces of wurtzite SiC are studied by ab initio molecular dynamics. The (1×1) relaxed, (1×2) buckled, and (2×1) л-bonded chain geometries are considered. In the (l×1) geometry, inward relaxation of the C-terminated surface is much stronger than that of the Si-terminated surface. The C-terminated surface is stable with respect to (2×1) buckled reconstruction, but the Si-terminated surface is not. At both surfaces, the (2×1) л-bonded chain geometry is energetically the most favorable. The Si-C л-bonded chains are buckled and slightly dimerized. In both cases, the outermost atoms are carbons, due to the bond length constraints of the л-bonded chain reconstruction.

Nonstoichiometric defects in carbides of the Group IV and V transition metals and radiation-induced atomic defects in SiC were studied by positron lifetime measurements before and after low-temperature (80 K) electron irradiation and subsequent thermal annealing up to 1900 K. Agglomeration of radiation-induced atomic defects which strongly depends on the energy of the irradiation electrons and subsequent decay of the agglomerates in SiC is observed.

SiC thin films less than 1 μm thick are grown by thermal decomposition of CH3SiC13 in a hydrogen atmosphere in an open CVD system. The interface stress is shown to be depend on the growth conditions. The optimal conditions for the preparation of uniform SiC films have been determined. Thermal annealing of the films has also been studied, and the conditions that provide for relaxation of the interface stresses are determined.

Reduction of MCln, n = 3 for M = Cr, Mo, n = 4 for W with LiBEt3H in THF results in formation of nanometer-sized crystalline metal carbide, M2C particles whereas the same reduction reaction in toluene results in formation of the corresponding metal particles. As an example of this methodology, the reduction of MoC13(THF)3 and WC14 in THF by Li/C10H8 is described in detail. Reduction of MoC13(THF)3 in THF by Li/C10H8 resulted in formation of a black suspension from which black powder can be isolated. Electron diffraction and X-ray powder diffraction data showed the black powder is amorphous. Energy dispersive spectroscopy confirmed the presence of molybdenum, and no chlorine was detected. Ex-situ variable temperature X-ray powder diffraction experiments showed the presence of fec Mo2C which upon further heating underwent a phase transformation to hexagonal Mo2C. Crystalline molybdenum was also observed after heating to 750°C for 3 hours. Reduction of WC14 in THF by Li/C10H8 results in formation of a black powder which on heating underwent phase transitions from amorphous to fcc W2C to hexagonal W2C as determined by X-ray powder diffraction.

Surface and bulk interactions between a metal M (M = Au, Cu or Ni) and monocrystalline α-SiC are studied. Surface interactions are quantified by the work of adhesion measured by the sessile drop technique under high vacuum. Products of M-SiC bulk reactions are characterised by electron microscopy and microprobe analysis. Interpretation of experimental results is based on classical thermodynamics for equilibria of bulk phases and on interfacial thermodynamics for adsorption phenomena and wetting.

The gaseous alkali corrosion kinetics of α-SiC were systematically investigated from 950°C to 1100°C in an atmosphere containing 0.98 vol% sodium nitrate vapors. The reaction of the SiC in the alkali-containing atmosphere follows a linear rate law with an activation energy of 104 kJ/mol. The alkali corrosion is composed of dissolution of SiO2 in the sodium silicate liquid, oxidation of SiC, and adsorption of Na2O from the vapor phase. The overall reaction appears to be controlled by the oxidation at the SiC substrate/sodium silicate liquid interface.

Fully densified WC-A12O3 composites were successfully consolidated by both hot-pressing and pressureless sintering. The optimum hot-pressing condition for the composites was 1700°C for 2h under a pressure of 40MPa. A remarkable improvement in mechanical properties was achieved in the composite system, especially in WC-30 and -70vol%A12O3, compared to the monolithic WC and A12O3 ceramics. The addition of MgO as a sintering aid had a great effect on the properties of the composites. WC-30vol%A12O3 composite with 1.Owt% MgO addition exhibited flexural strength higher than 1000MPa up to 1200°C, fracture toughness; KIC≥7MPa√m, and hardness; HV ≥2450. In pressureless sintering with the addition of MgO as a sintering aid and subsequent HIP treatment, the WC-30vol%A12O3 composite exhibited the flexural strength higher than 900MPa up to 1200°C.

In this study, we employed continuous microscratch testing to determine the effects of interface roughness on adhesion and fracture toughness of thin Ta2N films. These films were sputterdeposited on single crystal sapphire and polycrystal alumina substrates to a thickness of 0.5 μm. Comparison of the results showed that the interfacial fracture energy increased from 0.4 J/m2 for films on the single crystals to 1.5 J/m2 for films on the polycrystals with a corresponding increase in fracture toughness values. These results are consistent with crack deflection and interface roughness models and are useful for understanding adhesion and toughness on the submicron scale. The results also show that the continuous microscratch technique is a viable approach to determining adhesion and toughness of bi-material systems.

The applications of thin film technologies are increasing. A variety of different ceramic and hard coating materials are deposited. Silicon nitride is one of the materials which seems to be quite interesting for the use as coating material. An use as a wear and corrosion protective coating seems to be conceivable. Silicon nitride is known to have a good thermalshock resistivity, thermal and chemical stability and good mechanical and wear resistant properties. If these properties are also given for deposited metastable PVD Si-N-coatings has to be evaluated yet.

This paper describes the influence of different coating process parameters of the MSIP process like DC and RF mode, reactive and non reactive sputter processes and target orientation on the properties of silicon nitride films. It is shown, that coatings deposited with a 100 oriented single crystal target have the highest deposition rate and the best mechanical properties. The thermal stability of these thin films is insufficient although it can be improved by a carbon incorporation.

The effects of heat – treatment on the mechanical properties of AIN – SiC whisker composites with Y2O3 and SiO2 additives has been studied. When the sample containing 10 wt% Y2O3 +SiO2 (Y2O3/SiO2= 1/0. 66) was treated at 1300°C in air for 140 hours the strength of compositqs was raised from 481 MPa to 784 MPa.

The phase composition, microstructure and grain boundary phase structure have been characterized by combining XRD, SEM, TEM/EDXA and HREM techniques. The reinforcement mechanism of the composites results from crystallization of the glass phase in the grain boundary at the high temperature oxidizing atmosphere to form the crossing structure of 2Hδ Sialon fibrous phase and SiC whisker.

Pure metals with nearly the same elastic properties as hard, covalent ceramics are often very soft, even at low temperatures. Why the difference? “Directed bonds” is the common answer. Or, “the Peierls-Nabarro stress is large”. However, the former is only qualitative, while the latter does not distinguish between covalent and metallic bonding, and is quantitatively incorrect. The rules of quantum chemistry, as developed to account for chemical reaction rates, indicate why dislocation mobilities are low in crystals bonded through hybrid sp and spd orbitals. Resistance to the motion of kinks on dislocation lines arises from the way in which the initial electronic states of these kinks are correlated with the final states (after a unit of motion has occurred). The initial bonding states correlate with the final antibonding states, and vice versa. Thus, the correlation lines cross, creating substantial barriers to the motion. The barrier magnitudes can be calculated from the energy gaps (e.g., for Sic, the HOMO-LUMO gap); together with the quadratic dependencies of bond energy on bond-bending. The calculated results agree with observations. A second prototype, TiC is also discussed.

B and N K-edge x-ray absorption spectroscopy measurements have been performed on three BN thin films grown on Si substrates using ion-assisted pulsed laser deposition. Comparison of the films’ spectra to those of several single-phase BN powder standards shows that the films consist primarily of sp2 bonds. Other features in the films’ spectra suggest the presence of secondary phases, possibly cubic or rhombohedral BN. Films grown at higher deposition rates and higher ion-beam voltages are found to be more disordered, in agreement with previous work.

Using first-principles electronic structure calculations in the local density approximation combined with lattice dynamics, we investigate the effect of III/V impurities on the ideal strength of covalent solids (C, Si, and Ge). Our results show that undoped crystalline solids are stronger in tension along [111] than n-type crystalline solids. P doping has a negligible effect on ideal tensile strength, while n-type doping causes a small reduction in strength of about 6%. The n-type impurity effect is due to the negative (repulsive) contribution from the bottom state of the distorted conduction band to the ideal strength of the solid.

Recently there has been very much interest in understanding the structure of amorphous carbon nitride thin films, it has been shown that the introduction of nitrogen in amorphous carbon leads to better properties. In this paper we use four different cluster to study the changes in the local density of states at the carbon and nitrogen sites introduced by different environments. The method used is a SCF Hartree-Fock using pseudo potentials. We have considered carbons with sp3 and nitrogens with sp2 and sp3 hybridization. The results show that when N occupies a tetrahedral position (sp3) it introduces a localized state in the gap, while in the sp2 configuration, it introduces states near the edges of the bands.

The paper suggests a criterion for selection of radiation stable semiconductors and dielectrics.

The search for materials with great radiation stability (RS) is one of the basic problems in materials science. The absolute RS of crystals could be provided only in case when all elementary radiation defects (Frenkel pairs) are unstable, and annihilated immediately at any low temperatures in the places whete they were born. The presence of an instability zone (IZ) of point defects vacancy (v) - interstitial atom (i) is a necessary condition for RS [1,2]. The characteristic radius r0 of IZ at v-i Coulomb interaction is on the order of 10-30 Å [3]. The important parameter is a mean distance x between stopping point of atom i, knocked out from its site, and the v born at this site. Without taking into account the periodic structure, the analytical expression for x was obtained[4]. The value of x appears to be not large. For example, if the knocked out atom has the energy of 10 keV, the mean distance between v and i is 10 Å in iron, and only 3 Å in copper (x < r0). So, the greatest part of i's seem to remain in IZ, and therefore must recombine with “their” own v's. This would result in RS being greater than what was really observed in majority of nonmetal crystals.