Enhancement of surface mechanical properties by using TiN[BCN/BN]n/c-BN multilayer system

The aim of this work is to improve the mechanical properties of AISI 4140 steel substrates by using a TiN[BCN/BN]_n/c-BN multilayer system as a protective coating. TiN[BCN/BN]_n/c-BN multilayered coatings via reactive r.f. magnetron sputtering technique were grown, systematically varying the length period (Λ) and the number of bilayers (n) because one bilayer (n = 1) represents two different layers (t_BCN + t_BN), thus the total thickness of the coating and all other growth parameters were maintained constant. The coatings were characterized by Fourier transform infrared spectroscopy showing bands associated with h-BN bonds and c-BN stretching vibrations centered at 1400 cm⁻¹ and 1100 cm⁻¹, respectively. Coating composition and multilayer modulation were studied via secondary ion mass spectroscopy. Atomic force microscopy analysis revealed a reduction in grain size and roughness when the bilayer number (n) increased and the bilayer period decreased. Finally, enhancement of mechanical properties was determined via nanoindentation measurements. The best behavior was obtained when the bilayer period (Λ) was 80 nm (n = 25), yielding the relative highest hardness (∼30 GPa) and elastic modulus (230 GPa). The values for the hardness and elastic modulus are 1.5 and 1.7 times greater than the coating with n = 1, respectively. The enhancement effects in multilayered coatings could be attributed to different mechanisms for layer formation with nanometric thickness due to the Hall-Petch effect; because this effect, originally used to explain increased hardness with decreasing grain size in bulk polycrystalline metals, has also been used to explain hardness enhancements in multilayered coatings taking into account the thickness reduction at individual single layers that make up the multilayered system. The Hall-Petch model based on dislocation motion within layered and across layer interfaces has been successfully applied to multilayered coatings to explain this hardness enhancement.     

Influence of bilayer periods on structural and mechanical properties of ZrC/ZrB2 superlattice coatings

ZrC/ZrB₂ multilayered coatings with bilayer periods ranging from 4.4 to 35.5 nm were synthesized by r.f. magnetron sputtering. X-ray diffraction, scanning electron microscopy and nanoindention were employed to investigate the microstructure and mechanical properties of the nanoscale multilayers. The results indicated that all coatings had the clear multilayered structure with mixed ZrB₂(0 0 1), ZrB₂(0 0 2) and ZrC(1 1 1) preferred orientations. The maximum hardness (41.7 GPa) was observed in the multilayer with 27.5-nm thick period, which is about 25% higher than the rule-of-mixture value of the monolithic ZrC and ZrB₂ coatings. It also exhibited the best adhesion. Its critical load was over 70 mN. While through insert ZrB₂ into ZrC layer periodically, higher residual stress built in ZrC layer can be released.     

Microwave-absorbing and mechanical properties of carbonyl-iron/epoxy-silicone resin coatings

Coatings with flake carbonyl-iron particles as absorber and epoxy-silicone resins as matrix were prepared. The complex permittivity, complex permeability and microwave-absorbing properties were investigated in the frequency range of 2-18 GHz. Both the real part of permittivity and permeability were increased with carbonyl-iron weight concentration. The minimum reflection loss shifts to the low-frequency region with increase in carbonyl-iron weight concentrations. The minimum reflection loss value of −42.5 dB was obtained at 10.6 GHz for the coatings with 55 wt% carbonyl-iron. The values of adhesive power and impact strength are up to 24 MPa and 50 kg cm, respectively. These results show that the coatings possess good microwave-absorbing and mechanical properties.     

Structure and mechanical properties of reactive sputtering CrSiN films

CrSiN films with various Si contents were deposited by reactive magnetron sputtering using the co-deposition of Cr and Si targets in the presence of the reactive gas mixture. Comparative studies on microstructure and mechanical properties between CrN and CrSiN films with various Si contents were carried out. The structure of the CrSiN films was found to change from crystalline to amorphous structure as the Si contents increase. Amorphous phase of Si₃N₄ compound was suggested to exist in the CrSiN film. The growth of films has been observed from continuous columnar structure, granular structure to glassy-like appearance morphology with the increase of silicon content. The film fracture changed from continuous columnar structure, granular structure to glassy-like appearance morphology with the increase of silicon content. Two hardness peaks of the films as function of Si contents have been discussed.     

Mechanical properties of InAs/InP semiconductor alloys

The microindentation studies have been reported for undoped and doped InAs/InP semiconductor alloys grown by metal organic vapor phase epitaxy (MOVPE). It was found that the microhardness value increases with increase of applied load and attains a constant value for further increase in the load. The mechanical properties like, fracture toughness, brittleness index, fracture surface energy and indentation size effect coefficient were determined using the microhardness value. The indented samples were etched in H₂SO₄:H₂O₂:H₂O of the ratio of (1:1:1) for 30 s. This reveals the dislocation rosette patterns generated around the edges of the indentation on subsequent etching process.     

Mechanical properties of metallic ribbons investigated by depth sensing indentation technique

The paper presents mechanical properties of two kinds of Co-based and one Fe-based metallic ribbons by the depth sensing indentation (DSI) technique. Investigations were carried out on two kinds ternary alloy Co₇₇Si_11,5B_11,5 and Fe₇₈Si₁₃B₉ and multicomponent Co₆₈Fe₄Mo₁Si_13,5B_13,5, which are so-called “zero-magnetostriction” materials. Metallic ribbons were investigated in amorphous state and partially crystallized state after annealing in 400°C in argon atmosphere. Heating of ribbons obtained by melt spinning technique was performed to check its effect on changes of mechanical properties.     

Effect of strain relaxation of oxidation-treated SiGe epitaxial thin films and its nanomechanical characteristics

In this study, we examined the effect of high-temperature oxidation treatment on the SiGe epitaxial thin films deposited on Si substrates. The X-ray diffraction (XRD), atomic force microscopy (AFM), and nanoindentation techniques were employed to investigate the crystallographic structure, surface roughness, and hardness (H) of the SiGe thin films, respectively. The high-temperature oxidation treatment led to Ge pileup at the surface of the SiGe thin films. In addition, strain relaxation occurred through the propagation of misfit dislocations and could be observed through the cross-hatch pattern (800-900 °C) and SiGe islands (1000 °C) at the surface of the SiGe thin films. Subsequent hardness (H) measurement on the SiGe thin films by continuous penetration depth method indicated that the phenomenon of Ge pileup caused a slightly reduced H (below 50 nm penetration depth), while relaxation-induced defects caused an enhanced H (above 50 nm penetration depth). This reveals the influence of composition and defects on the structure strength of high-temperature oxidation-treated SiGe thin films.     

Crystal Structure and High Hardness Mechanism of Orthorhomic Si2N2O

A quasi-single-phase orthorombic Si2N20 compound is obtained by hot-pressing sintering using homogeneous precursors as raw materials under nitrogen atmosphere. The bulk hardness of orthorombie Si2N20 （o-Si2N2 O） is investigated by a nanoindenter experiment; the results show that o-Si2N20 with maximal value about 19 GPa has a high hardness covalent crystal besides β-Si3N4. It is discovered that the high hardness is mainly attributed to the unique crystal structure. The bridging O atoms in the o-Si2N20 are responsible for decreasing hardness. It is found that the Si-O bonds in the open tetrahedral crystal structure are more easily broken and tilted than other bonds.     

Nanoindentation and nanoscratch behaviors of Ag/Ni multilayers

The hardness, elastic modulus and scratch behaviors of Ag/Ni mulitlayers deposited by evaporation have been carried out by nanoindentation and nanoscratch. It has been found that the hardness (H) increases, while the modulus (E) decreases, that is to say an increase of H/E as the periodicity decreases. Many mechanisms are included in nanoscratch, including initial elastic contact, plowing and fracture stage, in each multilayer. Coefficient of friction during plowing decreases with the decrease of the periodicity, which can be ascribed to decreasing material pile-up due to the increase of H/E. Elastic recovery after scratching also increases as the periodicity decreases because of the increase of H/E, which leads to improved wear resistance. The fracture stage will be postponed with decreasing periodicity, which also leads to better wear behavior.     

Surface modification of exchange-coupled Co/NiOx magnetic bilayer by bias sputtering

We have investigated the effect of bias voltage on sheet resistance, surface roughness and surface coverage of Co/NiO_x magnetic bilayer. In addition, interface topography and corrosion resistance of the Ta/Co/Cu/Co/NiO_x/Si(1 0 0) system have been studied for Co layers deposited at an optimum bias voltage. Atomic force microscopy (AFM) and four point probe sheet resistance (R_s) measurement have been used to determine surface and electrical properties of the sputtered Co layer at different bias voltages ranging from 0 to −80 V. The Co/NiO_x bilayer exhibits a minimum surface roughness and low sheet resistance value with a maximum surface coverage at V_b=−60 V resulted in a slight increase of magnetic resistance and its sensitivity for the Co/Cu/Co/NiO_x/Si(1 0 0) magnetic multilayers, as compared with the same magnetic multilayers containing unbiased Co layers. The presence of Ta protection layer improves the corrosion resistance of the multilayers by three orders of magnitude in a humid environment.     

Microstructure, mechanical properties, and oxidation resistance of nanocomposite Ti-Si-N coatings

Ti-Si-N coatings with different silicon contents (0-12 at.%) were deposited onto Si(1 0 0) wafer, AISI M42 high speed steel, and stainless steel plate, respectively. These coatings were characterized and analyzed by using a variety of analytical techniques, such as XRD, AES, SEM, XPS, nanoindentation measurements, Rockwell C-type indentation tester, and scratch tester. The results revealed that the hardness was strongly correlated to the amount of silicon addition into a growing TiN film. The maximum hardness of 47.1 GPa was achieved as the Si content was 8.6 at.%. In the mechanical and oxidation resistance measurements, the Ti-Si-N coatings showed three distinct behaviors. (i) The coatings with Si contents of no more than 8.6 at.% performed good adhesion strength quality onto the HSS substrates. (ii) The fracture toughness of the coatings decreased with the increase in Si content. (iii) The Ti-Si-N coating with 8.6 at.% Si showed the excellent oxidation resistance behavior. The cutting performance under using coolant conditions was also evaluated by a conventional drilling machine. The drills with Ti-Si-N coatings performed much better than the drills with TiN coating and the uncoated drills.     

Indentation Load Effect on Young＇s Modulus and Hardness of Porous Sialon Ceramic by Depth Sensing Indentation Tests

Depth sensing indentation （DSI） tests at the range of 200-1800mN are performed on porous sialon ceramic to determine the indentation load on Young＇s modulus and hardness values. The Young modulus and hardness （Dynamic and Martens） values are deduced by analysing the unloading segments of the DSI test load-displacement curves using the Oliver-Pharr method. It is found that Young＇s modulus ET, the dynamic hardness HD and the Martens hardness HM exhibit significant indentation load dependences. The values of Young＇s modulus and hardness decrease with the increasing indentation load, as a result of indentation load effect. The experimental hf /hm ratios lower than the critical value 0.7, with hm being the maximum penetration depth during loading and hf the final unloading depth, indicate that our sample shows the work hardening behaviour.     

Ion beam sputter deposited W/Si multilayers: Influence of re-sputtering on the interface structure and structure modification at ultra short periods

X-ray multilayer mirrors of period ranging from 9.6 to 1.7 nm, deposited using ion beam sputtering, have been examined using grazing incidence X-ray reflectivity (GIXRR) and grazing incidence X-ray diffraction. Detailed analysis of GIXRR data revealed that significant amount of re-sputtering of Si layer takes place while W deposition is underway. Re-sputtering is mainly due to bombardment of high-energy neutrals getting reflected from the W target. Due to re-sputtering interface of the multilayer becomes asymmetric. This puts a major hindrance in avoiding the intermixing and achieving sharp interfaces at shorter periods. Maximum thickness of Si which gets lost due to re-sputtering during deposition is ∼0.8 nm. The shortest period multilayer estimated, that could be deposited without intermixing, was 2.7 nm. These results are of significance for developing low period W/Si multilayers.     

Low-temperature ordering and enhanced coercivity of L10-FePt thin films with Al underlayer

FePt multilayer films with and without Al underlayer were prepared by magnetron sputtering on SiO₂ substrate and subsequently annealed in vacuum. Experimental results suggest that the existence of Al underlayer can effectively reduce the ordering temperature and increase the coercivity of FePt films. Due to the slight larger lattice constant of Al underlayer than that of FePt films, [Fe (0.66 nm)/Pt (0.84 nm)]₃₀ films begin to order at 350 °C and the coercivity of them reach to 5.7 kOe after annealing at 400 °C for half an hour.     

A new superhard material: Osmium diboride OsB2

Superhard materials have many industrial applications, wherever resistance to abrasion and wear are important. The synthesis of new superhard materials is one of the great challenges to scientists. We re-examined the phase diagram of the binary osmium-boron system and confirmed the existence of two hexagonal phases, OsB_1.1, Os₂B₃, and an orthorhombic phase, OsB₂. Almost nothing is known about the physical properties of osmium borides. Microhardness measurements show that OsB₂ is extremely hard. Ab initio calculations show that this is due to formation of covalent bonds between boron atoms. OsB₂ is also a low compressibility material. It can be used as hard coating.     

The bulk modulus of SmFeAs(O0.93F0.07)

The crystal structure of SmFeAs(O_0.93F_0.07) has been investigated under high pressure (up to ∼9 GPa) by means of synchrotron powder diffraction analysis followed by Rietveld refinement. The bulk modulus was calculated (K₀ = 103 GPa) using a 3rd order Birch–Murnaghan equation of state and resulted in quite good agreement with theoretical calculations reported for LaFeAsO. The linear compressibilities β_a and β_c are 2.11(4) and 4.56(7) × 10⁻³ GPa⁻¹, respectively.     

Composition dependence of thermal stability, micro-hardness and compactness in glassy Se90In10−xGex alloys

In the present work, the effect of Ge incorporation is studied on the thermal stability, micro-hardness and compactness of glassy Se₉₀In₁₀ alloy.Thermal stability of glassy Se₉₀In_10−xGe_x alloys has been studied using differential scanning calorimetric technique. Micro-hardness of glassy Se₉₀In_10−xGe_x alloys is measured at room temperature. The compactness of the structure of Se₉₀In_10−xGe_x alloys is also determined from the measured densities. The variation in micro-hardness and compactness with composition has been discussed.     

Ge/Sb2Te3 nanocomposite multilayer films for high data retention phase-change random access memory application

The amorphous-to-crystalline transition of Ge/Sb₂Te₃ nanocomposite multilayer films with various thickness ratios of Ge to Sb₂Te₃ were investigated by utilizing in situ temperature-dependent film resistance measurements. The crystallization temperature and activation energy for the crystallization of the multilayer films increased with the increase in thickness ratio of Ge to Sb₂Te₃. The difference in sheet resistance between amorphous and crystalline states could reach as high as 10⁴ Ω/□. The crystallization temperature and activation energy for the crystallization of Ge/Sb₂Te₃ nanocomposite multilayer films was proved to be larger than that of conventional Ge₂Sb₂Te₅ film, which ensures a better data retention for phase-change random access memory (PCRAM) use. A data retention temperature for 10 years of the amorphous state [Ge (2 nm)/Sb₂Te₃ (3 nm)]₄₀ film was estimated to be 165 °C. Transmission electron microscopy (TEM) images revealed that Ge/Sb₂Te₃ nanocomposite multilayer films had layered structures with clear interfaces.     

Sub-ps laser microstructuring of soft X-ray Mo/Si multilayer gratings

The sub-picosecond laser microstructuring of multilayer gratings is presented in this paper. A micromachining system operating with a 0.5 ps KrF laser at 248 nm was used to etch grating structures with a groove width of 1–2 μm in Mo/Si and Si/Mo multilayers. Atomic force microscopy, scanning electron microscopy and X-ray reflectivity were used to characterize the microetched patterns. The ω-scans around the 1st Bragg maximum show symmetric satellites up to 3rd order, with positions corresponding to the grating period. The use of sub-picosecond laser pulses minimizes the thermally affected zone and enhances the quality of the etched features. Short pulse laser processing is advantageous for the fabrication of high spatial resolution microstructures required in X-ray optics. Received: 21 May 2002 / Accepted: 19 August 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Email: dpapa@iesl.forth.gr