Fine-grained SiC Research Articles

Thermodynamic guidance for implantation treatments to improve coating–substrate adhesion

A novel ‘‘seeding’’ method is disclosed, in which ion implanted Al atoms promote bonding between SiC and a subsequently deposited Al2O3 coating. Al+ ions were implanted in SiC substrates near room temperature to a fluence of 4×1017 Al/cm2 at an energy of 110 keV. The substrates were coated with 100–200 nm of aluminum oxide and then heated in air (i.e., oxidized) for 4 hours at temperatures from 500 to 1000 °C. Scratch tests demonstrated that coatings spalled from nonimplanted substrates but not from Al+-implanted substrates. Auger sputter depth profiles of substrates oxidized at 850 °C showed that Al diffused out of the implanted layer and an Al-Si-O layer formed at the coating-substrate interface. Cross-section and plan TEM of a substrate oxidized at 1000 °C identified the interface phase as mullite and found fine-grained SiC and mullite (Al6Si2O13) in the implanted layer. The composition and phase changes were explained in terms of the thermochemical reactions expected of the Al-Al2O3-SiC-O2 system.

Preparation of Fine-Grained SiC Ceramics from Pyrolyzed Polycarbosilane

Dense, (> 99% of theoretical) fine-grained SiC ceramics were fabricated by a hot-pressing technique using very fine, crystalline SiC powders made from polycarbosilane. The grain sizes were {approx} 0.3 or 0.7 {mu}m when they were hot-pressed at 1,800{degree}C with Al or at 2,050{degree}C with AlN, respectively. Fabrication processes, the characteristics of the synthesized starting SiC powders, and the microstructures and properties of the SiC ceramics were investigated.

Microstructure and abrasive wear resistance of cast Ni-Cr-C alloys

The abrasive wear behaviour of directionally solidified Ni-Cr-C alloys was investigated using a pin-type test. M 7C 3 carbide volume fractions (CVF) were varied from 0 to 40%. Two sets of alloys with different carbide and dendrite spacings were abraded with bonded SiC and corundum particles, varying the grit size and applied load. M 7C 3 carbides greatly improved the abrasive wear resistance against fine-grained SiC particles within the whole range of compositions. By refining the primary carbide structure in hypereutectic alloys, the wear resistance against coarse-grained SiC particles was also improved with increasing CVF although SiC is known to be much harder than M 7C 3. Coarse SiC abrasive particles had a detrimental effect on the wear resistance of all hypoeutectic alloys and, even more, of hypereutectic alloys if the primary carbides were coarse. In testing with corundum, the wear resistance always improved with increasing carbide volume fraction. Wear damage was arranged in three classes. First, SiC and corundum abrasives were partially broken from the substrate at the entrance edge of the specimen. The edges of SiC grains stayed sharp during the wear process whereas the edges of corundum particles were rounded or the corundum was crushed by M 7C 3 carbides. Secondly, damage in the wear surface occurred by fracturing of the edges of carbides facing the wear surface. In addition, SiC abrasives were able to groove carbides. Thirdly, coarse SiC grains transmitted shear stresses causing severe subsurface damage leading to microstructure disintegration and spalling of primary carbides. SiC transmitted larger shear stresses than corundum because the latter was separated by a thin layer of wear debris from the unworn material. The microstructural parameters influencing wear were CVF, size, morphology and distribution of carbides. Optimum wear resistance depended on the abrasive mineral. Alloys with high CVF and coarse primary carbides were best suited for wear with corundum whereas fine primary carbides were required to resist wear by SiC.

Improvements in Mechanical Properties in SiC by the Addition of TiC Particles

Silicon carbide ceramics containing up to 24.6 vol% dispersed TiC particles yielded fully dense composites by hot‐pressing at 2000°C with 1 wt% Al and 1 wt% C added. The micro‐structure consists of fine TiC particles in a fine‐grained SiC matrix. Addition of TiC particles increases the critical fracture toughness of SiC (to ∼6 MPa·m1/2 at 24.6 vol% TiC) and yields high flexure strength (≥680 MPa), with both properties increasing with increasing volume fraction of TiC. The strengths at high temperatures are also improved by the TiC additions. Observations of the fracture path indicate that the improved toughness and strength are a result of crack deflection by the TiC particles.