Alan M. Russell

Superhard self-lubricating AlMgB14 films for microelectromechanical devices

Performance and reliability of microelectromechanical system (MEMS) components can be enhanced dramatically through the incorporation of protective thin-film coatings. Current-generation MEMS devices prepared by the lithographie-galvanoformung-abformung (LIGA) technique employ transition metals such as Ni, Cu, Fe, or alloys thereof, and hence lack stability in oxidizing, corrosive, and/or high-temperature environments. Fabrication of a superhard self-lubricating coating based on a ternary boride compound AlMgB14 described in this letter has great potential in protective coating technology for LIGA microdevices. Nanoindentation tests show that the hardness of AlMgB14 films prepared by pulsed laser deposition ranges from 45 GPa to 51 GPa, when deposited at room temperature and 573 K, respectively. Extremely low friction coefficients of 0.04–0.05, which are thought to result from a self-lubricating effect, have also been confirmed by nanoscratch tests on the AlMgB14 films. Transmission electron microscopy st...

Pulsed laser deposition of AlMgB14 on carbide inserts for metal cutting

Abstract Nanocrystalline AlMgB 14 containing 0–30 mol% additives are a family of new superhard materials with hardness comparable to that of TiB 2 on the lower end and to that of cubic BN on the higher end. Compared with diamond and cubic BN, AlMgB 14 is an equilibrium material with excellent electrical conductivity, high chemical stability, and lower density. The projected cost of manufacture of the boride is 10% of the cost of diamond and cubic BN. AlMgB 14 materials appear to be congruently melting/evaporating, which would allow them to be processed with techniques such as pulsed laser deposition (PLD). In this work, the feasibility of PLD for synthesizing thin films of baseline AlMgB 14 (0% additive) is demonstrated and compared with TiB 2 . A 248-nm, 23-ns KrF excimer laser was used to prepare baseline boride thin films on cemented carbide (ANSI C-5 and C-2) tool inserts. The films were dark blue, continuous and fairly uniform with few particulates. An impact fracture test showed that adhesion of the films to the substrate was excellent. The deposition rate was 0.08 nm per pulse at an energy density of 7 J/cm 2 . Nanoindentation hardness tests revealed that the films exhibited hardness 60% higher than the carbide substrate. Lathe turning tests with cold-drawn 1045 steel bars indicated that C-5 tools coated with 0.5 μm baseline AlMgB 14 have an average flank wear reduction of 12% compared to uncoated C-5 tools. Further machining tests on C-2 tools showed that the tools coated with baseline boride have much better flank (23% reduction) and nose wear resistance (26% reduction) compared with TiB 2 coated tools. In addition, multilayer composite coating of AlMgB 14 and TiB 2 outperformed single layer boride coating in minimizing the tool wear. This pioneering work sets the stage and serves as a catalyst for rapid and innovative advances in the development of new boride materials for numerous tool and hard coating applications, including bulk cutting tools, hard and erosion-resistant coatings, wear-resistant electrical switch contacts, and conductive thin films for MEMS.

Déformation processed métal-métal composites

Deformation processed metal–metal composites (DMMCs) are composites in which both the matrix and the reinforcing phase are ductile metals. The ductility of the metals permits the composite to be heavily deformed by drawing or rolling to reduce the thickness and spacing of the two phases to as small as 10 nm. Their nano-scale microstructures are exceptionally strong, with strengths approaching whisker strength in some cases. And since the materials are comprised of two essentially pure metals, their electrical conductivities are quite high for current flow in directions parallel to the composite’s filaments or lamellae, leading to numerous potential applications. During the 1990s, continuing research has produced DMMCs in a wide range of matrix metals, including Al, Au, Mg, Sc, and Ti. Methods are also being developed to produce DMMCs with much larger dimensions in rod and sheet forms to permit their use in a wider variety of engineering applications.

Coefficient of thermal expansion of AlMgB14

Abstract The coefficient of thermal expansion (COTE) of AlMgB14 was measured by dilatometry and by high temperature X-ray diffraction using synchroton radiation. The COTE over the temperature range 298 K to 1373 K was determined to be 9×10−6 K−1, which is relatively close to the value of 8.3×10−6 K−1 for pure B. The anisotropy of the COTE appears to be relatively small.

Microstructure evolution of Al–Mg–B thin films by thermal annealing

The growth of Al–Mg–B thin films on SiO2/Si(100) substrates was performed by nanosecond pulsed laser deposition at three different substrate temperatures (300 K, 573 K, and 873 K). The as-deposited films were then annealed at 1173 K or 1273 K for 2 h. X-ray photoelectron spectroscopy, x-ray diffraction (XRD), and atomic force microscope were employed to investigate the effects of processing conditions on the composition, microstructure evolution, and surface morphology of the Al–Mg–B films. The substrate temperatures were found to affect the composition of as-deposited films in that the Mg content decreases and C content increases at higher substrate temperatures, in particular for the 873 K-deposited film. XRD results show that the as-deposited films were amorphous, and this structure may be stable up to 1173 K. Annealing at 1273 K was found to fully crystallize the room temperature and 573 K-deposited Al–Mg–B films with the formation of the polycrystalline orthorhombic AlMgB14 phase, accompanied by the ...

Al2MgO4, Fe3O4, and FeB impurities in AlMgB14

Abstract AlMgB 14 materials with Si and Ti additions produced by mechanical alloying/hot uniaxial pressing have recently been found to display hardness greater than 40 GPa. Al 2 MgO 4 and Fe 3 O 4 , and FeB are common impurities in AlMgB 14 produced in this manner. The characterization of these impurities and their effects on the hardness and electrical properties of AlMgB 14 without Si and Ti additions are presented in this study.

Electrical transport in amorphous semiconducting AlMgB14 films

The electrical transport properties of semiconducting AlMgB14 films deposited at room temperature and 573K are reported in this letter. The as-deposited films are amorphous, and they exhibit high n-type electrical conductivity, which is believed to stem from the conduction electrons donated by Al, Mg, and/or Fe impurities in these films. The film deposited at 573K is less conductive than the room-temperature-deposited film. This is attributed to the nature of donor or trap states in the band gap related to the different deposition temperatures.

Anomalously high impact fracture toughness in B.C.C. Mg-Li between 4.2K and 77K

Mg-Li alloys` unique combination of low density and extraordinary ductility have interested engineers and scientists since the 1940`s. As a preliminary phase of a larger program to investigate the potential for strengthening Mg-Li with dispersed metallic reinforcing phases, this study was designed to measure the grain size dependence of the ductile-brittle transition temperature of a single-phase, body-centered cubic alloy of Mg-12 weight % Li. Most body-centered cubic (BCC) metals undergo a transition from ductile to brittle fracture as the temperature is lowered. These transition temperatures vary widely for different BCC metals; for example, plain carbon steel transition temperatures range from 200K for very low C content to 450K for high C content. Hexagonal close-packed metals often experience embrittlement at low temperatures, although the transition is usually not as abrupt. The ductile to brittle transition has been studied by tensile test in several HCP Mg alloys (8-9), but not in binary BCC Mg-Li. No impact test data for binary BCC Mg-Li alloys were found in the literature.

Characterization of strength and microstructure in deformation processed Al-Mg composites

The microstructures, mechanical properties and electrical resistivity have been evaluated for deformation processed Al-20 vol % Mg and Al-13 vol % Mg composites. The Mg second phase adopts a convoluted, ribbon shape filamentary morphology after deformation. Both the size and spacing of these filaments decreases with deformation. The strength of these composites increases exponentially with reduced spacing of Mg filaments. The electrical resistivity of these Al-Mg composites is slightly higher than that of pure Al.

Mechanical properties of magnetostrictive iron-gallium alloys

Single crystal specimens of Fe-17 at. % Ga were tested in tension at room temperature. Specimens with a tensile axis orientation of [110] displayed slip lines on the specimen faces corresponding to slip on the {110}<111> with a critical resolved shear stress of 220 MPa. Yielding began at 0.3% elongation and 450 MPa. An ultimate tensile strength of 580 MPa was observed with no fracture occurring through 1.6% elongation. The Young’s modulus was 160 GPa in the loading direction with a Poisson’s ratio of -0.37 on the (100) major face. A specimen with a tensile axis orientation of [100] showed slip lines corresponding to slip on the {211}<111> with critical resolved shear stress of 240 MPa. Discontinuous yielding began at 0.8% elongation, which was thought to result from twinning, kink band formation, or stress-induced transformation. The Youngs modulus was 65 GPa in the loading direction with a Poisson’s ratio of 0.45 on the (001) major face. A maximum tensile strength of 515 MPa was observed with fracture occurring after 2% elongation. A sizeable elastic anisotropy of 19.9 was identified for Fe-27.2 at. % Ga accompanied by a Poissons ratio of -0.75 to produce a large in-plane auxetic behavior.