Low-Temperature Carburization of AL-6XN Enabled by Provisional Passivation

Employing AISI-AL-6XN as example, we introduce a new method of surface activation for low-temperature carburization. This method consists of two steps: (i) removing the passivating surface oxide and a potentially existing severely plastically deformed surface layer (Beilby layer) by aqueous (liquid) hydrochloric acid, and (ii) immersion in ethanol and subsequent drying in nitrogen. Upon carburization with a gas mixture of acetylene, hydrogen, and nitrogen, this new method of surface activation enables the formation of a fully developed “case”, a uniform solid solution of interstitial carbon in austenite with carbon fractions up to 0.20 near the alloy surface. The underlying mechanism of surface activation is shown to involve the formation of a provisional passivating layer. It consists of chlorides or ethoxides that are insoluble in ethanol. It prevents the reformation of the regular Cr-rich passivating oxide layer and is readily removed upon heating and exposure to the carburizing gas. As the new activation method is quicker, more effective, and less destructive to furnace hardware than activation with hot gaseous hydrochloric acid that is currently applied in industrial manufacturing, it may have considerable technological impact.

Titanium Bulk Micromachining for BioMEMS Applications: A DEP Device as a Demonstration

Titanium has been widely used as a biomedical material in orthopedics, dentistry, cardiology, and cardiovascular surgery due to the excellent biostability and biocompatibility that results from its spontaneous formation of a highly passivating oxide layer in air and blood. However, little research has been done on the development of titanium for bioMEMS applications. This is likely due to the immaturity of titanium bulk micromachining technology to date. Here we report the application of new high-aspect-ratio bulk titanium micromachining techniques recently developed within our group towards the fabrication of a titanium-based multi-frequency traveling wave dielectrophoresis (DEP) device targeted for the separation of bioparticles. The device serves to illustrate the potential of these techniques for enabling the realization of novel bioMEMS devices with enhanced functionality and capability.

Nucleation and Pattern Formation of Coherent Islands During Epitaxial Growth of Ge on Si(110) Surfaces

Molecular beam epitaxial growth of Ge on Si(110) surfaces reveals interesting aspects of the heterogeneous nucleation of coherent Ge islands. Cleaning of the Si substrate by desorption of a passivating oxide layer at high temperature creates surface pits. Two sets of experiments, including deposition of Ge on as-cleaned substrates, and surfaces with a thin Si buffer layer are compared to illustrate the nucleation behavior of Ge. Typical Ge deposition temperatures range from 600°C to 725°C.For Ge deposition on as-cleaned surfaces, the faceted edges of pits serve as preferential sites for the heterogeneous nucleation of coherent Ge islands. Experiments were also performed on surfaces with thin (˜20nm) Si buffer layers grown on the as-cleaned surface. Though the faceted pits have not been completely covered by the Si buffer layer, they have decreased in lateral size. In addition, the Si(110) surface shows ledges that are formed along specific crystallographic directions. Ge deposited on the Si buffer nucleates first at the corners of the pits, in an interesting dipole orientation, as well as along the ledges on the surface.