Dale Niesz

The effect of abrasive hardness on the chemical-assisted polishing of (0001) plane sapphire

A series of abrasives with various hardness values including monocrystalline and polycrystalline diamond, α- and γ-alumina, zirconia, ceria, and silica were used to examine the concept of chemical-assisted polishing for finishing the (0001), c-plane (basal plane), of sapphire. Diaspore, a monohydrate of alumina, was also evaluated. Atomic force microscopy suggested that the hydrated layer of the c-plane surface was about 1 nm thick. Polishing experiments were designed to determine whether the chemically modified surface hydration layer forms on the basal plane in water. The results indicate that harder abrasives do not necessarily cause faster material removal and better surface finish for similar abrasive particle size. Abrasives with hardness equal to or less than sapphire such as α-Al2O3 and γ-Al2O3 achieved the best surface finish and greatest efficiency of material removal. It is proposed that the (0001) c-plane sapphire surface was modified by water to form a thin hydration layer with structure and hardness close to diaspore. This reaction layer can be removed by an abrasive that is softer than sapphire but harder than the reaction layer. α-Al2O3 was particularly effective. This result is attributed to adhesion between identical reaction layers on the basal planes of the alumina abrasive and the sapphire. This demonstrates that high removal rates and good surface finish can be achieved without costly diamond polishing.

Automated generation and analysis of powder compaction diagrams

Abstract The compaction diagram is a useful characterization tool for measuring the strength of powder agglomerates (e.g. granules) and characterizing the microstructural development of powder compacts in dry forming operations. The current work describes a new method for the rapid measurement and analysis of compaction diagrams. A computer-controlled testing machine was used to collect crosshead versus load data during powder compaction in a uniaxial die. The net crosshead translation during loading included the permanent compaction of the powder plus the combined elastic deflections of the powder compact and the testing apparatus. In order to compensate for the elastic deformation in the testing apparatus, baseline data were collected by compressing an empty die and were subtracted from the net crosshead position. At the high pressure endpoint, the elastic deformation in the powder compact (i.e. springback) is the difference between the adjusted crosshead position and the thickness of the pellet after ejection from the die. At intermediate pressures, mathematical models were used to estimate the springback. This approach resulted in improved accuracy, significant time savings and an expanded scope of analytical capability compared to other methods of generating compaction diagrams.

The Role of Multiple Polytypes in Determining the Catastrophic Failure of Boron Carbide at High Shock Velocities

The absence of a plastic phase in boron carbide and its failure at shock impact velocities just above the Hugoniot elastic limit (HEL) has been a puzzle for a long time. In the present work, using self-consistent field density functional simulations we are able to account for many experimental observations by noticing that several boron carbide polytypes [(B 11 C)C 2 B, (B 12 )C 3 , etc …] coexist without significant lattice distortions. Our analysis also indicates that above a threshold pressure all the candidate microstructures are less stable than a phase involving segregated boron (B 12 ) and amorphous carbon (a-C) but the energetic barrier between boron carbide and B 12 + 3C, is by far lower for the B 12 (CCC) microstructure, requiring the lowest atomic displacement for a transformation B 4 C→3B+a-C, occurring at pressures of 6 GPa = P(H EL ). For such a configuration, segregation of free carbon occurs in layers orthogonal to the (113) lattice directions, in excellent agreement with recent transmission electron microscopy (TEM) analysis