Superhard self-lubricating AlMgB14 films for microelectromechanical devices

Abstract

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 studies show that the as-deposited films are amorphous, regardless of substrate temperature; however, analysis of Fourier transform infrared spectra suggests that the higher substrate temperature facilitates the formation of the B12 icosahedral framework, therefore leading to the higher hardness.

Bonding and elastic properties of amorphous AlYB

“…Many compounds are known, such as AlMgB 14 , AlNaB 14 , AlLiB 14 , AlYB 14 , AlTbB 14 , AlDyB 14 , AlHoB 14 , AlErB 14 , AlYbB 14 and AlLuB 14 [1][2][3][4][5]. Bulk modulus of AlMgB 14 was measured to be in the range of 196-264 GPa [6]. Kölpin et al performed ab initio calculations on XMgB 14 (X¼ Al, C, Si, Ge, Mg, Sc, Ti, V, Zr, Hf, Nb, Ta) [7] as well as Al x Y y B 14 (x, y¼0.25, 0.5, 0.75, 1.00) [8] and reported bulk moduli in the range from 196 to 228 GPa.…”

“…AlYB 14 (space group Imma) belongs to a group of boron-rich solids with fascinating properties. Many compounds are known, such as AlMgB 14 , AlNaB 14 , AlLiB 14 , AlYB 14 , AlTbB 14 , AlDyB 14 , AlHoB 14 , AlErB 14 , AlYbB 14 and AlLuB 14 [1][2][3][4][5].…”

Electrical transport in amorphous semiconducting AlMgB14 films

“…13 XPS also indicated that there is no appreciable difference in composition between room-temperature-and 573 K-deposited AlMgB 14 films. The microstructure of AlMgB 14 films, which was examined by transmission electron microscopy, 12 remains amorphous regardless of substrate temperature, and no evidence of conducting inhomogeneities or crystalline domains was observed.…”

“…Figure 3 shows the Tauc plots of AlMgB 14 films, from which the optical band gaps are determined to be ϳ0.5 eV, and it appears that the deposition temperature does not have a strong effect on the optical gaps. Therefore, the different carrier concentrations in AlMgB 14 films can only be attributed to the trap states of different characters, i.e., an increase in deposition temperature leads to a higher density of well-formed B 12 icosahedra, 12 which, in turn, gives rise to more electron trap states in the gap. As the density of these trap states increases, a decrease in carrier concentration is expected, which is indeed observed in this study.…”

“…8 Considerable research has focused on the electrical transport mechanism, 6 dielectric properties, 7 doping behavior, 9,10 and device performance 3,5 of boron compounds with C, N, and O, such as B 4 C/B 5 C, B x N ͑x Ͼ 1͒ and boron suboxide ͑B x O,x Ͼ 1͒. Recently, the ternary boride compound AlMgB 14 has attracted attention due to its interesting mechanical properties; 11,12 however, no studies have been reported on the electrical properties of AlMgB 14 thin films. This letter discusses the effect of deposition temperature on the electrical properties of AlMgB 14 films.…”