Naureen Ghafoor

Improving thermal stability of hard coating films via a concept of multicomponent alloying

We propose a design route for the next generation of nitride alloys via a concept of multicomponent alloying based on self-organization on the nanoscale via a formation of metastable intermediate products during the spinodal decomposition. We predict theoretically and demonstrate experimentally that quasi-ternary (TiCrAl)N alloys decompose spinodally into (TiCr)N and (CrAl)N-rich nanometer sized regions. The spinodal decomposition results in age hardening, while the presence of Cr within the AlN phase delays the formation of a detrimental wurtzite phase leading to a substantial improvement of thermal stability compared to the quasi-binary (TiAl)N or (CrAl)N alloys.

Decomposition and phase transformation in TiCrAlN thin coatings

Phase transformations and mechanisms that yield enhanced high temperature mechanical properties of metastable solid solutions of cubic (c)-(TixCryAlz)N coatings are discussed in this paper. Coating ...

Atomic scale interface engineering by modulated ion-assisted deposition applied to soft x-ray multilayer optics

Cr/Sc and Ni/V multilayers, intended as normal incidence soft x-ray mirrors and Brewster angle polarizers, have been synthesized by employing a novel modulated low-energy and high-flux ion assistance as a means of engineering the interfaces between the subnanometer layers on an atomic scale during magnetron sputter deposition. To reduce both roughness and intermixing, the ion energy was modulated within each layer. The flat and abrupt interfaces yielded soft x-ray mirrors with near-normal incidence reflectances of R = 20.7% at the Sc 2p absorption edge and R = 2.7% at the V 2p absorption edge. Multilayers optimized for the Brewster angle showed a reflectance of R = 26.7% and an extinction ratio of R(s)/R(p)=5450 for Cr/Sc and R = 10% and R(s)/R(p)=4190 for Ni/V. Transmission electron microscopy investigations showed an amorphous Cr/Sc structure with an accumulating high spatial frequency roughness. For Ni/V the initial growth mode is amorphous and then turns crystalline after approximately 1/3 of the total thickness, with an accumulating low spatial frequency roughness as a consequence. Elastic recoil detection analyses showed that N was the major impurity in both Cr/Sc and Ni/V with concentrations of 15 at. % and 9 at. %, respectively, but also O (3 at. % and 1.3 at. %) and C (0.5 at. % and 1.9 at. %) were present. Simulations of the possible normal incidence reflective properties in the soft x-ray range of 100-600 eV are given, predicting that reflectivities of more than 31% for Cr/Sc and 5.8% for Ni/V can be achieved if better control of the impurities and the deposition process is employed. The simulations also show that Cr/Sc is a good candidate for mirrors for the photon energies between the absorption edges of B (E = 188 eV) and Sc (E = 398.8 eV).

Characterization of worn Ti-Si cathodes used for reactive cathodic arc evaporation

The microstructural evolution of Ti1-xSix cathode surfaces (x=0, 0.1, 0.2) used in reactive cathodic arc evaporation has been investigated by analytical electron microscopy and x-ray diffractometry ...

Influence of chemical composition and deposition conditions on microstructure evolution during annealing of arc evaporated ZrAlN thin films

The influence of substrate bias and chemical composition on the formed microstructure and resulting hardness of arc evaporated Zr1−xAlxN films in the compositional span 0.12≤ x ≤0.74 is investigated. A cubic ZrAlN phase is formed at low aluminum contents (x ≤ 0.38) whereas for a high Al-content, above x = 0.70, a single-phase hexagonal structure is obtained. For intermediate Al-contents, a two-phase structure is formed. The cubic structured films exhibit higher hardness than the hexagonal structured ones. A low bias results in N-rich films with a partly defect-rich microstructure while a higher substrate bias decreases the grain size and increases the residual stress in the cubic ZrAlN films. Recrystallization and out-diffusion of nitrogen from the lattice in the cubic ZrAlN films takes place during annealing at 800 °C, which results in an increased hardness. The cubic ZrAlN phase is stable to annealing temperatures of 1000 °C while annealing at higher temperature results in nucleation and growth of hexag...

Nanolabyrinthine ZrAlN thin films by self-organization of interwoven single-crystal cubic and hexagonal phases

Self-organization on the nanometer scale is a trend in materials research. Thermodynamic driving forces may, for example, yield chessboard patterns in metal alloys [Y. Ni and A. G. Khachaturyan, Nature Mater. 8, 410–414 (2009)]10.1038/nmat2431 or nitrides [P. H. Mayrhofer, A. Horling, L. Karlsson, J. Sjolen, T. Larsson, and C. Mitterer, Appl. Phys. Lett. 83, 2049 (2003)]10.1063/1.1608464 during spinodal decomposition. Here, we explore the ZrN-AlN system, which has one of the largest positive enthalpies of mixing among the transition metal aluminum nitrides [D. Holec, R. Rachbauer, L. Chen, L. Wang, D. Luefa, and P. H. Mayrhofer, Surf. Coat. Technol. 206, 1698–1704 (2011)10.1016/j.surfcoat.2011.09.019; B. Alling, A. Karimi, and I. Abrikosov, Surf. Coat. Technol. 203, 883–886 (2008)]10.1016/j.surfcoat.2008.08.027. Surprisingly, a highly regular superhard (36 GPa) two-dimensional nanolabyrinthine structure of two intergrown single crystal phases evolves during magnetron sputter thin film synthesis of Zr0.64A...

Microstructure evolution of Ti3SiC2 compound cathodes during reactive cathodic arc evaporation

The microstructure evolution and compositional variation of Ti3SiC2 cathode surfaces during reactive cathodic arc evaporation are presented for different process conditions. The results show that p ...

Reflectivity and structural evolution of Cr/Sc and nitrogen containing Cr/Sc multilayers during thermal annealing

It is shown that the thermal stability in vacuum of Cr/Sc multilayer thin films used as reflective optical components in soft x-ray instrumentation has substantial dependence on incorporation of N. ...

Coherency strain engineered decomposition of unstable multilayer alloys for improved thermal stability

A concept to improve hardness and thermal stability of unstable multilayer alloys is presented based on control of the coherency strain such that the driving force for decomposition is favorably al ...

Interface engineered ultrashort period Cr-Ti multilayers as high reflectance mirrors and polarizers for soft x rays of lambda = 2.74 nm wavelength.

Cr-Ti multilayers with ultrashort periods of 1.39-2.04 nm have been grown for the first time as highly reflective, soft-x-ray multilayer, near-normal incidence mirrors for transition radiation and Cherenkov radiation x-ray sources based on the Ti-2p absorption edge at E = 452 eV (lambda = 2.74 nm). Hard, as well as soft, x-ay reflectivity and transmission electron microscopy were used to characterize the nanostructure of the mirrors. To achieve minimal accumulated roughness, improved interface flatness, and to avoid intermixing at the interfaces, each individual layer was engineered by use of a two-stage ion assistance process during magnetron sputter deposition: The first 0.3 nm of each Ti and Cr layer was grown without ion assistance, and the remaining 0.39-0.72 nm of the layers were grown with high ion-neutral flux ratios phi (phiTi = 3.3, phiCr = 2.2) and a low energy Eion (ETi = 23.7 and ECr = 21.2), ion assistance. A maximum soft-x-ray reflectivity of R = 2.1% at near-normal incidence (approximately 78.8 degrees) was achieved for a multilayer mirror containing 100 bilayers with a modulation period of 1.379 nm and a layer thickness ratio of tau = 0.5. For a polarizing multilayer mirror with 150 bilayers designed for operation at the Brewster angle, 45 degrees, an extinction ratio, Rs/Rp, of 266 was achieved with an absolute reflectivity of R = 4.3%.