Super-hard coatings with materials based on ternary AlMgB matrices

Abstract

Mechanical engineering, automotive, aeronautic and mining industries, as well as military and space industry require hard and wear resistance materials. However military and space industry often also needs light-weight materials in combination with the above wearable and hardness properties. Diamond is the hardest material known but some properties of the second hardest material, cubic boron nitride (cBN), surpass those of diamond. Although diamond is superior to cBN in hardness, its chemical stability is lower than that of cBN. Diamond is dissolved in molten steel and ferrous materials, whereas cBN is inert. The two parameters as well as friction coefficients are the reasons for using diamond and cBN as materials of choice in commercial applications. Since diamond is unsuitable for machining steels and ferrous materials and cBN coatings have not been mastered on commercial levels, new superhard coatings are needed. The single elements and binary composite films have been investigated thoroughly, but less effort has been given to ternary and quaternary composites that might reach the hardness of superhard materials. In consistence with the properties for advanced technological applications novel superhard materials in forms of films have been synthesized and studied. In this project, new materials based on aluminum, magnesium and boron were prepared and their mechanical properties were investigated in correlation with their structural composition. It is noted that composite films mentioned are related to the nano-scale since there are nanocrystallites inside the film with amorphous structure, so it means nanocomposite films. Composite films of aluminum magnesium boride (AlMgB) with ultra-hard properties could be suitable alternatives to existing lower performance materials that are currently used in mechanical applications such as cutting tools, or even they could be used as more advanced materials in military and space applications. With suitable structures and chemical compositions, these novel materials based on ternary compounds of Al, Mg and B are remarkable light and counted in the group of superhard materials. Possibly by adding optimum amount and some additives (TiB2) into the AlMgB composite, the mechanical properties of the composite films are enhanced. The hardness can be effectively increased to 45 - 50 GPa, approaching the hardness of the cBN. However, the addition of AlN and TiC would reduce film hardness. In addition, the AlMgB films are highly resistive to abrasion and have low coefficients of sliding friction. Therefore, the AlMgB materials approach the mechanical properties of cBN and diamond. The ternary AlMgB thin films were coated on silicon substrates by a sputter deposition technique using three unbalanced rectangular magnetrons (one AlMg and two boron targets). The magnetrons were installed on a deposition chamber in a closed magnetic field configuration. The deposition parameters and power supplied to individual targets are adjusted to optimize the structure and chemical composition of the film and achieve the extreme level of mechanical properties of the AlMgB films. The surface morphology and roughness were studied by atomic force microscopy and scanning electron microscopy, structural properties were analysed using x-ray diffraction (XRD) and fourier transform infrared (FTIR) spectroscopy, while compositional analysis and chemical states were investigated by energy dispersive x-ray (EDX) spectroscopy and x-ray photoelectron spectroscopy (XPS), respectively. Hardness and elastic modulus were evaluated using a nanoindentation technique. Analysis shows that the root mean square (rms) roughness varies between 1.0 and 18 nm when the AlMg target at power density alters from 0.2 to 1.0 W/cm2 and the boron target is maintained at 2 W/cm2. The chemical compositional analysis presents that aluminum, magnesium and boron are the dominant elements in the film. The coating also comprises some trace elements of oxygen, carbon and argon. The chemical composition is changed when different power densities to the AlMg targets are applied. By changing the power densities, the metal rich film Al1.38Mg0.64B with only about 33 at.% boron content was prepared. The films demonstrate the hardness value of about 30 GPa. The obtained hardness is already impressive when it is compared to the hard hydrogenated diamond-like carbon (DLC) films synthesized by plasma enhanced chemical vapor deposition (CVD) showing hardness of 24 – 28 GPa. On the other hand, boron rich films exhibit similar hardness (~30 GPa). The XRD pattern suggests that the prepared AlMgB films confine nanocrystals in amorphous matrices. The nanocrystals are identified to be B12 icosahedra forming network in amorphous AlMgB matrices that considerably contributes to the hardness enhancement of the films.