Staff

Abstract:
In this work, (W,Ti)B2 films with different stoichiometric ratio Ti/W deposited on silicon and 304 stainless steel by radio frequency magnetron sputtering are presented. The coatings were deposited from plasma spark sintered targets obtained from the mixture of pure boron, tungsten and titanium powders. It is shown that during plasma spark sintering process using overstoichiometric boron and a low content of titanium change the WB2 to WB4 phase with almost no secondary phases. Subsequently, the impact of titanium content on the films properties is investigated systematically, including the chemical and phase composition, crystalline structure, surface and cross-section morphology. Simultaneously, nano-indentation test and ball-on-disk tribometery are performed to analyse the hardness and tribological properties of the films. It is shown that deposited films with titanium content of 3.6 and 5.5 at.% are formed in the zone T of the Thornton's Structural Zone Model. In opposite to α-WB2 magnetron sputtered coatings they are more flexible and hard nanocomposite coatings. The results show that the addition of titanium is apparently changing the film structure from nanocrystalline columnar to amorphous, very dense and compact structure with the addition of TiB2 phase. That films are simultaneously hard (H > 37.5 GPa), have high hardness to effective Young's modulus ratio values (H/E* > 0.1) and elastic recovery (We > 60%) appropriate for tough and resistant to cracking materials. The presented (W,Ti)B2 films exhibit also tribological and corrosion properties better than unalloyed coatings.

Keywords:
superhard films, ternary tungsten borides, RF magnetron sputtering, wear resistance, corrosion

Abstract:
The coatings of tungsten borides (WBx) and tungsten borides doped with titanium (WyTi1−yBx) were deposited by using combined magnetron sputtering - pulsed laser deposition technique. In the case of WBx coatings, pure tungsten target was evaporated by a laser pulse at 1064 nm wavelength and pure boron target was sputtered by a magnetron. In the case of WyTi1−yBx coatings, the W2B5 target was sputtered by the magnetron and titanium target was evaporated by the laser pulse at 1064 nm wavelength. The content of titanium dopant changed from 1.1 to 5.5 at.%. The microstructure, chemical and phase composition of deposited coatings were investigated by means of Scanning Electron Microscopy, Energy Dispersive X-Ray Spectroscopy, X-Ray Photoelectron Spectroscopy and X-Ray Diffractometry, respectively. The Vickers hardness and Young's modulus were determined by using the nanoindentation test. Nanocrystalline WB coatings with dominant WB2 phase were obtained at a substrate temperature of 520 °C. The coatings were superhard with a hardness of 47–50 GPa and the mean value of surface roughness was <6 nm. The WBx coatings doped with 5.5 at.% Ti had hardness similar to the coatings sputtered by magnetron from W2B5 target.

Keywords:
The superhard WyBx thin films was deposited with hybrid laser-magnetron technology, The MS-PLD method allows for the deposition of ternary borides such as WyTi1−yBx, The MS-PLD method enables to control the chemical and phase composition of films, The deposited films are smooth with hardness above 50 GPa, Crystalline films were deposited only on a substrate kept an elevated temperature

Abstract:
In this paper, the mechanical properties of composites consisting of electroplated Ni and co-electrodeposited SiC particles, coated with a thin protective layer of Ni, were studied. The protective layer was on the SiC particles prior to adding them to the electrolyte bath. It was demonstrated that due to the application of the protective layer it was possible to decrease sliding friction force, and improve the wear resistance, of the composite coatings in comparison with standard electroplated composite coatings made of Ni and co-electrodeposited pure SiC particles. Coating SiC particles with Ni was achieved by means of a PVD process. The main advantage of this step is avoiding oxidation of the surface of the particles during the contact with an electrolyte. Particles protected from oxidation lead to stronger interfacial bonding between the matrix and the reinforcement. Furthermore, better bonding protects the SiC particles from being extracted from the matrix by a counter-probe during friction and wear tests. The influence of the particle's concentration is studied. A smaller friction force and constant wear rate was observed.

Keywords:
wear, friction, interfacial bonding strength, metal matrix composites, silicon carbide, electrodeposited nickel

Abstract:
Tungsten and boron compounds belong to the group of superhard materials since their hardness could exceed 40 GPa. In this study, the properties of the tungsten boride WBx coatings deposited by radio frequency magnetron sputtering were investigated. The sputtering was performed from specially prepared targets that were composed of boron and tungsten mixed in a molar ratio of 2.5 and sintered in Spark Plasma Sintering (SPS) process. WB films were deposited on silicon (100) and stainless steel 304 substrates at temperatures of 23 ÷ 770 °C. Microstructure, chemical and phase composition were investigated by using Scanning Electron Microscope (SEM), X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Diffraction (XRD), respectively. The mechanical properties like Vickers hardness and Young's modulus were obtained by using nanoindentation test at a load of 5 ÷ 100 mN. The friction coefficient and wear resistance of αWB coatings were investigated in scratch test and reciprocal sliding wear instrumentation. Moreover, in order to investigate thermal properties, the αWB films were annealed at 1000 °C in argon/air for 1 h and at 250 °C for 2 h in air atmosphere. Results of our research confirm that αWB coatings can be considered as an alternative to superhard materials in the production of wear resistant, long-lasting tools.

Keywords:
Hard coatings, Hard materials, Magnetron sputtering, Mechanical properties, Thermal properties, Tungsten boride

Abstract:
In this paper, the study of the mechanical properties of composites consisting of electrodeposited Ni and co-electrodeposited SiC particles coated with a thin Cu layer was presented. It was demonstrated that the coating allowed to increase the concentration of ceramic particles in the composite. Although the plating parameters were the same for both types of composites, the concentration of SiC was 15% for the composite containing coated particles (Ni/SiC-Cu) and 10% for the composite containing uncoated particles (Ni/SiC). Furthermore, tensile tests showed that the Ni/SiC-Cu samples exhibited higher Young's modulus than the pure electrodeposited Ni samples or Ni/SiC samples. The measured Young's modulus of the Ni/SiC-Cu composite was 250 ± 10 GPa. However, the ultimate tensile strength of the Ni/SiC-Cu composite was lower than that of pure Ni. To explain the mechanical behaviour of the Ni/SiC-Cu composite, the microstructure of the interface of this composite and its bonding strength were studied. Microstructure studies conducted using a scanning electron microscope (SEM) revealed that the SiC/Cu interface was smooth and of good quality whereas the Cu/Ni interface was rough but also of good quality. The measured bonding, normal, and shear strength values demonstrated that the SiC/Cu interface was weak, and that was the main reason for the low ultimate tensile strength of the composite. The shear strength of the SiC/Cu interface was measured using a novel method: micropillars shearing including atomic force microscopy (AFM). Finally, a simple finite element model of the Ni/SiC-Cu composite, based on cohesive elements, was developed.

Keywords:
Interfacial bonding strength, Metal matrix composites, Tensile strength, Silicon carbide, Electrodeposited nickel

Abstract:
Tungsten borides belong to the group of potentially superhard materials which hardness could be compared to cubic boron nitride and diamond. However, difficulty in fabrication of single phase material using conventional methods is the main drawback of this group of ceramics. In order to overcome this problem material can be deposited as a thin layer e.g. in the pulsed laser deposition process. In this paper, the effect of laser wavelength and energy density of nanosecond Nd:YAG laser on the WBx-type layers were analyzed using wavelengths 355 and 1064 nm with the energy density of laser beam from 1.7 to 5 J/cm2 and from 1.7 to 9.3 J/cm2, respectively. The WB2.5 and WB4.5 targets synthesized in Spark Plasma Sintering process were used and the layers were deposited onto Si (100) substrate heated to a temperature of 570 °C. Layers' microstructure were analyzed using X-ray Diffraction and scanning electron microscope equipped with energy dispersive X-ray spectrometer. Change of laser wavelength and energy density resulted in variations of the chemical composition and morphology of deposited layers. Finally, W2B-βWB, αWB-WB-WB3 and WB3, and boron layers were deposited wherein WB3 structure is formed in a wide range of laser fluences and at both investigated wavelength. Next, WB3 layers were investigated in the indentation test at a load of 5–30 mN and its hardness was up to 50 ± 10 GPa

Keywords:
Pulsed laser deposition, Super-hard materials, Tungsten borides, Tungsten triboride

Abstract:
The results of synthesis of the fluorescent carbon dots (CDots) from graphite target and reduced graphene oxide (rGO) nanoparticles performed by the nanosecond laser ablation in polyethylene glycol 200 (PEG200) are shown. Two-step laser irradiation (first graphite target, next achieved suspension) revealed a very effective production of CDots. However, the ablation in PEG appeared to be effective with 1064 nm laser pulse in contrast to the ablation with 355 nm laser pulse. In the case of rGO nanoparticles similar laser irradiation procedure was less efficient. In both cases, received nanoparticles exhibited strong, broadband photoluminescence with a maximum dependent on the excitation wavelength. The size distribution for obtained CDots was evaluated using the DLS technique and HRTEM images. The results from both methods show quite good agreement in nanoparticle size estimation although the DLS method slightly overestimates nanoparticle's diameter

Abstract:
The plasma induced during underwater pulsed laser ablation of graphite is studied both experimentally and theoretically. The results of the experiment show that the maximum plasma temperature of 25000 K is reached 20 ns from the beginning of the laser pulse and decreases to 6500 K after 1000 ns. The observed OH absorption band shows that the plasma plume is surrounded by the thin layer of dissociated water vapour at a temperature around 5500 K. The hydrodynamic model applied shows similar maximum plasma temperature at delay times between 14 ns and 30 ns. The calculations show also that already at 14th ns, the plasma electron density reaches 0.97·1027 m−3, which is the critical density for 1064 nm radiation. At the same time the plasma pressure is 2 GPa, which is consisted with earlier measurements of the peak pressure exerted on a target in similar conditions.

Keywords:
underwater laser processing, pulsed laser ablation in liquid, laser induced plasma, numerical modelling

Abstract:
The most promising areas of research of new super-hard materials are transition metal borides. These materials are one of the candidates for future superhard layers that will be competitive to DLC and c-BN layers. In this paper MoB-type tungsten boride (WB) layers were examined. WB layers have been deposited by radio frequency magnetron sputtering on Silicon (100), 304 stainless steel (SS 304) and Inconel 601 substrates. Measured thickness of herein prepared layers was about 1 µm, and all studied samples were dense, uniform and smooth. Surface investigation was performed by using an optical profilometer, atomic force microscopy, and scanning electron microscopy. The structure analysis was examined by using X-ray diffractometer (XRD) and transmission electron microscopy (TEM) techniques. Results from the XRD and TEM analysis showed that WB layers were dominated by (101) reflection and indicated a fine grain structure with a grain size of 20–40 nm. The effect of target sputtering power and ambient gas pressure was investigated. The hardness of WB layers deposited on silicon substrate was compared under the load from 1 mN to 5 mN. The hardness of WB layers deposited on SS 304 and Inconel was measured up to 50 mN. All layers of WB revealed excellent hardness exceeding 40 GPa.

Keywords:
Borides, Nanohardness, RF magnetron sputtering, Superhard layers, Tungsten boride

Abstract:
Reactive spark plasma sintering (SPS) of WB2/WB3 ceramics from elements is studied; the sintering pressure dependence of the ratio of WB3 to WB2 in samples produced by SPS is discussed. Regardless of the sintering pressure, the obtained samples are very hard ~20 GPa. WB3 superhard films prepared by pulsed laser deposition (PLD) from selected SPS targets are presented.

WB3 coatings were prepared on Si (100) substrates using a nanosecond, Nd:YAG laser operating at a 355 nm wavelength. The phase analysis, crystallography, and orientations have been studied using X-ray diffraction (XRD). A WB2 to WB3 phase transformation from 8.2% WB3 in a sintered target to 93.3% WB3 in a deposited film was observed. Additionally, the surface of a SPS sintered WBx target after the ablation process was examined. XRD studies show that already during the laser ablation there is a significant WB2 to WB3 phase transformation. Vickers hardness of sintered samples was measured in macro- and micro-scale, and PLD films in the nanoscale.

Keywords:
Films, Tungsten borides, SPS – sintering, Pulsed laser ablation/deposition

Abstract:
Rhenium diboride (ReB2) thin films have been deposited by a nanosecond pulsed laser deposition method on Si (100) substrate heated to 570°C. The coatings were formed in the ablation process of SPS sintered ReB2 target. The effect of laser wavelength, energy density and postannealing on the films' properties was studied. Investigated wavelengths were 355 and 1064 nm of Nd:YAG nanosecond laser. Laser beam energy density varied from 2.1 to 6.1 J cm− 2 and from 4.1 to 9.4 J cm− 2 at 355 and 1064 nm, respectively. Layer thickness was of the order of several hundred nanometres. Deposition efficiency increases with an energy density in quasi linear way and strongly grows for shorter wavelength. The layers consist of two characteristic microstructures: a smooth basis and stick debris (typical diameters of several hundred nanometres). Lower energy density of laser beam and longer wavelength favour dominance of smooth basis and minimization of debris. The XRD analysis of all samples indicate the crystalline ReB2 with preferred (002) orientation and fine grain size of about 20 nm. Shorter wavelength and higher energy density foster stronger (002) orientation. Moreover, an annealing right after the deposition (25 min in 350°C) causes minimization of degree of orientation and decrease of hardness. The Vickers hardness of ReB2 films is at about 60 GPa and is reduced to about 40 GPa after the annealing process. Deposition efficiency and physical and chemical structures of layers produced under variety of conditions were studied and compared.

Keywords:
Ceramics ablation, Pulsed laser deposition, Rhenium diboride, ReB2 optical properties, Super-hard films, Wavelength influence

Abstract:
In this paper, the effect of absorption of the laser beam and subsequent radiation on the dynamics of a tungsten plasma plume during pulsed laser ablation is analyzed. Different laser wavelengths are taken into consideration. The absorption and emission coefficients of tungsten plasma in a pressure range of 0.1–100 MPa and temperature up to 70 000 K are presented. The shielding effects due to the absorption and radiation of plasma may have an impact on the course of ablation. The numerical model that describes the tungsten target heating and the formation of the plasma and its expansion were made for 355 nm and 1064 nm wavelengths of a Nd:YAG laser. The laser beam with a Gaussian profile was focused to a spot size of 0.055 mm2 with a power density of 1 × 109 W/cm2 (10 ns full width half maximum pulse duration). The plasma expands into air at ambient pressure of 1 mPa. The use of the shorter wavelength causes faster heating of the target, thus the higher ablation rate. The consequences of a higher ablation rate are slower expansion and smaller dimensions of the plasma plume. The higher plasma temperature in the case of 1064 nm is due to the lower density and lower plasma radiation. In the initial phase of propagation of the plasma plume, when both the temperature and pressure are very high, the dominant radiation is emission due to photo-recombination. However, for a 1064 nm laser wavelength after 100 ns of plasma expansion, the radiation of the spectral lines is up to 46.5% of the total plasma radiation and should not be neglected.

Keywords:
Plasma temperature, Tungsten, Laser ablation, Laser beams, Plasma pressure

Abstract:
The effect of laser wavelength on single-wall carbon nanotubes synthesis yield and their properties was studied. A double-pulse Nd:YAG laser, working at a wavelength of 355 or 1064 nm, was used for carbon nanotubes production. The synthesized carbon nanotubes (CNTs) were investigated using the SEM/STEM microscopy and Raman spectroscopy. The results show that the useful range of UV laser radiation fluence is narrower and the properties of synthesized CNTs depend much more on the laser fluence than in the case of infrared laser radiation.

Keywords:
carbon nanotubes, laser radiation fluence, pulsed laser vaporization, Raman spectra

Abstract:
The ablation of graphite is studied as a function of laser fluence for 355, 532 and 1,064 nm wavelength generated by a nanosecond Nd:YAG laser. It has been found that in the case of lower wavelengths, the transition from the thermal ablation to the phase explosion takes place at lower laser fluences. The change of crater shape due to the effect of deep drilling in the proximity of the phase explosion threshold was observed. The calculations of plasma radiation flux to the target surface were made, and the considerable increase of absorbed energy density was found in the case of 355 nm wavelength.

Keywords:
laser ablation, ablation rate, carbon

Abstract:
Materials containing suspended micro- or nanomaterials are used extensively in multiple fields of research and industry. In order to understand the behavior of nanomaterials suspended in a liquid, the knowledge of particle stability and mobility is fundamental. For this reason, it is necessary to know the nanoscale solid-solid interaction and the hydrodynamic properties of the particles. In the presented research we used a hybrid Atomic Force Microscope coupled with Optical Tweezers system to measure the femtonewton scale interaction forces acting between single particles and the walls of a microchannel at different separation distances and environmental conditions. We show an important improvement in a typical detection system that increases the signal to noise ratio for more accurate position detection at very low separation distances.

Keywords:
Optical Tweezers, Atomic Force Microscopy, particle-wall interaction, colloid stability

Keywords:
carbon nanoparticles, C-dots, laser ablation in liquid, photoluminescence

Keywords:
magnetron sputtering, pulsed laser deposition, superhard materials, tungsten borides

Keywords:
RF magnetron sputtering, hard materials, PLD, tungsten boride

Abstract:
Tungsten boride compounds are very promising new hard and super - hard materials. This kind of materials could be used in production of high-speed tools, durable bearings and nonabrasive surfaces. The hardness of tungsten borides strongly depends on boron content as well as material microstructure. For example hardness of W2B is about 12.4 GPa, WB hardness is 18 – 36 GPa, WB2 hardness is 28.5–39.7 GPa and WB3 hardness is 28.6 – 36.9 GPa. Moreover production of tungsten borides in the form of layers or nano -grained structured increase its hardness even above 40 GPa. Although the properties of tungsten boride structures are known from theoretical calculations, the data concerning structures produced experimentally are scare. Therefore, the properties of tungsten boride layers deposited by pulsed laser deposition (PLD) process are investigated in this paper. Layers were deposited using two types of targets: with boron to tungsten ratio of 2.5:1 and 4.5:1 and with the use of two laser wavelengths: 355 and 1064 nm.

Keywords:
pulsed laser deposition, tungsten borides

Abstract:
In this paper the interaction of Nd - YAG nanosecond laser pulse with tungsten or boron target and lasma induced during ablation are studied theoretically. Tungsten and boron were chosen because of the significant differences in their chemical and physical properties. Both materials are of a great practical importance. The model consists of equations of conservation of mass, momentum and energy and is solved with the use of commercially available program Ansys - Fluent. The calculations show the fundamental differences in ablation of both specie s. In case of tungsten the material evaporation is controlled by the plasma formation and consequently the absorption coefficient. The dense plasma plume can block laser radiation and limit energy transfer from the laser beam to the material. In case of bo ron the explosive ablation is observed. The calculations also show the sharp increase of the plume pressure after plasma formation and resulting significant increase of the velocity of plasma plume. The lume velocity obtained from the model is close to th at observed in the experiment carried out in similar conditions. Moreover, the effect of laser wavelength on the quality of the deposited boron and tungsten films has been discussed on the base of ablation model.

Keywords:
ablation mechanism, boron, critical temperature, plasma velocity, pulsed laser deposition, tungsten

Keywords:
pulsed laser deposition

Keywords:
pulsed laser deposition, RF magnsteron sputtering, tungsten borides

Keywords:
pulsed laser deposition, rhenium borides

Keywords:
pulsed laser deposition, rhenium borides, tungsten borides

Abstract:
In answer to research interest in production of super hard coatings prepared by pulsed laser deposition (PLD) method, Rhenium Diboride was taken under consideration. PLD is characteristic by necessity to define deposition parameters best for particular substance. ReB2 coatings were prepared with the use of 355 nm and 1064 nm wavelengths of Nd:YAG laser and the influence of laser wavelength on those films preparation was investigated. Deposition efficiency increased with shorter wavelength, however layer’s smoothness was better for longer wavelength. The XRD analysis shows crystalline ReB2 character of achieved samples.

Keywords:
pulsed laser deposition, rhenium diboride

Abstract:
Inexpensive Tungsten Tetraboride super hard coatings prepared by pulsed laser deposition (PLD) method are investigated. WB4 coatings were prepared on Si (100), by PLD technique using nanosecond, Nd:YAG laser operating at 355 nm wavelenght. Additionally the surface of SPS sintered WBx target before and after ablation process was examined. Phase change of WB2 to WB4 at the surface of target is observed. The phase analysis, crystallography and orientations have been studied using X-ray diffractometry. The surface layer of target after ablation and deposited films are found to be mostly WB4 phase with hardness HV>28 GPa.

Keywords:
laser mikro/nano engineering, thin films, pulsed laser ablation/deposition, tungsten tetraboride WB4

Keywords:
laser ablation, graphite, carbon nanotubes