Microstructure and mechanical properties of boron nitride nanosheets-reinforced fused silica composites

In order to overcome intrinsic brittleness and poor mechanical properties of fused silica (FS), boron nitride nanosheets (BNNSs) as a novel reinforcement were employed for fabrication of BNNSs/fused silica composites. BNNSs with micron lateral size were homogeneously dispersed with FS powder using a surfactant-free flocculation method and then consolidated by hot pressing. The flexural strength and fracture toughness of the composite with the addition of only 0.5 wt.% BNNSs increased by 53% and 32%, respectively, compared with those of pure FS. However, for higher BNNSs contents the improvement in mechanical properties was limited. Microstructural analyzes have shown that the toughening mechanisms are combinations of the pull-out, crack bridging, and crack deflection mechanisms.     

Microstructure and mechanical properties of alumina ceramics reinforced by boron nitride nanotubes

Boron nitride nanotubes (BNNTs)/alumina composites were fabricated by hot pressing. The mechanical properties of the composites are greatly dependent upon the content of BNNTs. In comparison with monolithic alumina, the incorporation of BNNTs results in the improvement of bending strength and fracture toughness owing to the effective inhibition of grain growth. A routine toughening mechanism, especially the bridging of BNNTs at grain boundaries and the sufficient physical bonding between BNNTs and alumina matrix, is dominantly responsible for the increase in mechanical properties.     

Improving the composition and multifunctional properties of amorphous boron nitride films prepared by post-annealing assisted femtosecond pulsed laser deposition method

Amorphous boron nitride (aBN) materials have similar density to crystalline phases and retain many unique electronic properties, valuable chemical inertness and high thermal stability characteristics. However, the current research on aBN materials has mainly focused on the synthesis and electrical properties of ultrathin aBN films. In this study, we developed a post-annealing assisted femtosecond laser deposition route towards stoichiometric, continuous, and multifunctional aBN films with thickness values of ∼1 μm. A series of boron nitride films were deposited on silicon wafers using a 1030 nm, 300 fs laser with a pulse energy of ∼1 mJ and a high repetition rate of 2 kHz to ablate a hexagonal boron nitride target. The deposited films were then annealed at 900 °C in a nitrogen atmosphere. The structures and chemical compositions of these obtained films were analysed by X-ray diffraction and X-ray photoelectron spectroscopy. Fourier transform infrared and nano-scratch tests were performed to measure the infrared optical and frictional properties of the adhered films. An infrared thermal imager was used to investigate the heat-dissipation performance of these films. The results indicate the components of the aBN film are further purified, the number of large heterogeneous particles is effectively reduced, and the surface becomes smooth after post-annealing treatment. This improvement promotes the transfer of heat flux and increases the transmittance in the mid-infrared light band. The significant effect mechanisms of the post-annealing treatment on the enhancement of the composition and multifunctional properties of aBN films prepared by the femtosecond pulsed laser were provided. The uniform coverage of the aBN films on the substrates, as well as the mid-infrared optical transparency and the protective performance are highly valuable and practical for infrared window protection applications.     

Electrical and optical properties of diamond-like carbon films deposited by pulsed laser ablation

Pulsed laser ablation of a graphite target was carried out by ArF excimer laser deposition at a laser wavelength of 193 nm and fluences of 10 and 20 J/cm² to produce diamond-like carbon (DLC) films. DLC films were deposited on silicon and quartz substrates under 1 × 10^? 6 Torr pressure at different temperatures from room temperature to 250 °C. The effect of temperature on the electrical and optical properties of the DLC films was studied. Laser Raman Spectroscopy (LRS) showed that the DLC band showed a slight increase to higher frequency with increasing film deposition temperature. Spectroscopic ellipsometry (SE) and ultraviolet–visible absorption spectroscopy showed that the optical band gap of the DLC films was 0.8–2 eV and decreased with increasing substrate temperature. These results were consistent with the electrical resistivity results, which gave values for the films in the range 1.0 × 10⁴–2.8 × 10⁵ Ω cm and which also decreased with deposition temperature. We conclude that at higher substrate deposition temperatures, DLC films show increasing graphitic characteristics yielding lower electrical resistivity and a smaller optical band gap.     

Optical and electrical properties of boron nitride oxide films

Polycrystalline boron nitride oxide (BNO) films are synthesized by RF magnetron sputtering. It is found that the bandgap of the BNO film increases with increasing oxygen composition. The bandgap energy as wide as 5.5 eV is obtained with oxygen composition of 17%. The electrical resistivity is estimated to be as high as 10¹³ Ωcm. Metal/BNO/metal structures are fabricated with various metals such as Ni, Cu and Al, and electrical characterization are performed for metal/BNO contacts. The true Schottky barrier heights are estimated for metal/BNO contacts. The true Schottky barrier height decreases with increasing metal work function. This behavior of the Schottky barrier height suggests that the BNO film has p-type electrical conduction.     

Fabrication and mechanical properties of boron nitride nanotube reinforced silicon nitride ceramics

In this study, silicon nitride (Si₃N₄) ceramics added with and without boron nitride nanotubes (BNNTs) were fabricated by hot-pressing method. The influence of sintering temperature and BNNTs content on the microstructures and mechanical properties of Si₃N₄ ceramics were investigated. It was found that both flexural strength and fracture toughness of Si₃N₄ were improved when sintering temperature increases. Moreover, α-Si₃N₄ phase could transform into β-Si₃N₄ phase completely when sintering temperature rises to 1800 °C and above. BNNTs can enhance the fracture toughness of Si₃N₄ dramatically, which increases from 7.2 MPa m^1/2 (no BNNTs) to 10.4 MPa m^1/2 (0.8 wt% BNNTs). However, excessive addition of BNNTs would reduce the fracture toughness of Si₃N₄. Meanwhile, the flexural strength and relative density of Si₃N₄ decreased slightly when BNNTs were added. The related toughening mechanism was also discussed.     

The microstructure and properties of energetically deposited carbon nitride films

The intrinsic stress, film density and nitrogen content of carbon nitride (CN_x) films deposited from a filtered cathodic vacuum arc were determined as a function of substrate bias, substrate temperature and nitrogen process pressure. Contour plots of the measurements show the deposition conditions required to produce the main structural forms of CN_x including N-doped tetrahedral amorphous carbon (ta-C:N) and a variety of nitrogen containing graphitic carbons. The film with maximum nitrogen content (~ 30%) was deposited at room temperature with 1.0 mTorr N₂ pressure and using an intermediate bias of − 400 V. Higher nitrogen pressure, higher bias and/or higher temperature promoted layering with substitutional nitrogen bonded into graphite-like sheets. As the deposition temperature exceeded 500 °C, the nitrogen content diminished regardless of nitrogen pressure, showing the meta-stability of the carbon–nitrogen bonding in the films. Hardness and ductility measurements revealed a diverse range of mechanical properties in the films, varying from hard ta-C:N (~ 50 GPa) to softer and highly ductile CN_x which contained tangled graphite-like sheets. Through-film current–voltage characteristics showed that the conductance of the carbon nitride films increased with nitrogen content and substrate bias, consistent with the transition to more graphite-like films.     

Enhanced mechanical and anisotropic thermal conductive properties of polyimide nanocomposite films reinforced with hexagonal boron nitride nanosheets

To attain thermally conductive but electrically insulating polymer films, in this study, polyimide (PI) nanocomposite films with 1–30 wt% functionalized hexagonal boron nitride nanosheets (BNNSs) were fabricated via solution casting and following imidization. The microstructures, mechanical and thermal conductive properties of PI/BNNS nanocomposite films were examined by taking account of the relative content, anisotropic orientation, and interfacial interaction of BNNS and PI matrix. The scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffractometry data revealed that BNNSs with hydroxy and amino functional groups have specific molecular interactions with PI matrix and they form stacked aggregates in the nanocomposite films with high BNNS loadings of 10–30 wt%. The tensile mechanical strength/modulus, thermal degradation temperatures, and thermal conductivity of the nanocomposite films were found to be significantly enhanced with increasing the BNNS loadings. For the nanocomposite films with 1–30 wt% BNNS loadings, the in-plane thermal conductivity was measured to be 1.82–2.38 W/mK, which were much higher than the out-of-plane values of 0.35–1.14 W/mK. The significant anisotropic thermal conductivity of the nanocomposite films was found to be owing to the synergistic anisotropic orientation effects of both BNNS and PI matrix. It is noticeable that the in-plane and out-of-plane thermal conductivity values of the nanocomposite film with 30 wt% BNNS were ~1.31 and ~3.35 times higher than those of neat PI film, respectively.     

Structure,electrochemical properties and functionalization of amorphous CN films deposited by femtosecond pulsed laser ablation

Amorphous carbon nitride (a-C:N) material has attracted much attention in research and development. Recently, it has become a more promising electrode material than conventional carbon based electrodes in electrochemical and biosensor applications. Nitrogen containing amorphous carbon (a-C:N) thin films have been synthesized by femtosecond pulsed laser deposition (fs-PLD) coupled with plasma assistance through Direct Current (DC) bias power supply. During the deposition process, various nitrogen pressures (0 to 10 Pa) and DC bias (0 to − 350 V) were used in order to explore a wide range of nitrogen content into the films. The structure and chemical composition of the films have been studied by using Raman spectroscopy, electron energy-loss spectroscopy (EELS) and high-resolution transmission electron microscopy (HRTEM). Increasing the nitrogen pressure or adding a DC bias induced an increase of the N content, up to 21 at.%. Nitrogen content increase induces a higher sp² character of the film. However DC bias has been found to increase the film structural disorder, which was detrimental to the electrochemical properties. Indeed the electrochemical measurements, investigated by cyclic voltammetry (CV), demonstrated that a-C:N film with moderate nitrogen content (10 at.%) exhibited the best behavior, in terms of reversibility and electron transfer kinetics. Electrochemical grafting from diazonium salts was successfully achieved on this film, with a surface coverage of covalently bonded molecules close to the dense packed monolayer of ferrocene molecules. Such a film may be a promising electrode material in electrochemical detection of electroactive pollutants on bare film, and of biopathogen molecules after surface grafting of the specific affinity receptor.     

Microstructure and mechanical properties of silicon nitride with tri-laminate structures

Silicon nitride ceramics with tri-laminate structures were prepared using two kinds of layers; layer with the aligned silicon nitride whisker seeds (named as “S” layer) and layer without the seed (“N” layer). The fracture toughness values on the casting surface of N layer of sample with a tri-laminate structure (N–S–N structure) showed an anisotropy, and this is contrary to the isotropic fracture toughness observed from the casting surface of sample consisting of only N layers. The fracture toughness anisotropy observed from N layer of the former sample is explained in terms of the microstructural anisotropy induced by the sintering shrinkage anisotropy within the casting plane.     

Microstructure control and mechanical properties of porous silicon nitride ceramics

Porous silicon nitride ceramics with a fibrous interlocking microstructure were synthesized by carbothermal nitridation of silicon dioxide. The influences of different starting powders on microstructure and mechanical properties of the samples were studied. The results showed that the microstructure and mechanical properties of porous silicon nitride ceramics depended mostly on the size of starting powders. The formation of single-phase β-Si₃N₄ and the microstructure of the samples were demonstrated by XRD and SEM, respectively. The resultant porous Si₃N₄ ceramics with a porosity of 71% showed a relative higher flexural strength of 24 MPa.     

Bioactivity of hydroxyapatite/wollastonite composite films deposited by pulsed laser

Hydroxyapatite/wollastonite (HA/WS) composite films on titanium alloy were prepared by pulsed laser deposition, and their bioactivity was studied. The dissolution and precipitation behaviors of the films were evaluated by soaking in simulated body fluid (SBF), and the osseointegration ability was evaluated by in vivo test. In the early soaking stage, the dissolution action will dominate, thus resulting in the gradual disappearance of the smooth spherical feature of the particles. After 7 days of soaking, new precipitates were observed which indicates that reprecipitation reaction dominates, and the surface was almost completely covered by new precipitates after the film was soaked for 28 days. The in vivo test showed that the composite films have excellent osseointegration ability. When the sample was embedded in the shin bone of rabbit for 3 weeks, a good combination of bone tissue and implant was achieved, and after embedding for 6 weeks, osteoblasts were observed between the bone tissue and implant.     

Tribological study of boron nitride films

Boron nitride (BN) films with different cubic and hexagonal phase compositions were deposited on silicon substrates via diamond interlayers by magnetron sputtering and electron cyclotron resonance microwave plasma chemical vapor deposition. The tribological behaviors of the BN films were investigated systematically using a ball-on-disc tribometer with silicon nitride as the counterpart. Comparison studies were also performed on sintered cubic and hexagonal BN compacts. The influence of phase compositions and surface roughness of BN coatings on their tribological characteristics was studied. The cubic BN (cBN) films showed excellent wear resistance against silicon nitride. The wear rate of the cBN films was estimated to be about 1.0 × 10^?7 mm³/N m by measuring the cross-sectional area of the wear track after the sliding test over a distance of 12 km.     

A comparative study of composition,structure and elastic properties of boron nitride films deposited by magnetron and ion beam sputtering

Sputtering from hexagonal BN targets, using a conventional magnetron (MS) or ion beam (IBS), produces films consisting of both sp²- and sp³-bonded BN and BN_x. We present here the dependence of BN film composition, structure, morphology and elastic properties on the bias voltage (V_b) applied on the substrate in the case of MS, compared with those of IBS-deposited BN films. The optical properties and the composition of the BN films were examined by spectroscopic ellipsometry (SE), whereas the chemical bonding was identified by Fourier Transform IR SE. The films' structure, morphology and density were also examined by X-ray diffraction and reflectivity and transmission electron microscopy. The elastic properties of the films were studied by nanoindentation techniques. Significant differences were found in composition, chemical bonding and structure of films grown by MS at various bias voltages.     

Microwave dielectric properties of bismuth zinc niobate thin films deposited on alumina by pulsed laser deposition

Bi₂Zn_2/3Nb_4/3O₇ thin films were prepared on Al₂O₃ substrates by pulsed laser deposition. The phase compositions and microstructures were characterized by X-ray diffraction and atomic force microscopy. The as-deposited films were all amorphous in nature. All films were crystallized after the post annealing at the temperature range of 700–900 °C for 30 min in air. The texture characteristics change with annealing temperature. A split post dielectric resonator method was used to measure the microwave dielectric performance at the resonant frequencies of 10, 15 and 19 GHz. For the films annealed at 900 °C, the preferential orientation is similar to the monoclinic BZN bulk. The microwave dielectric constants at 10, 15 and 19 GHz are 69.4, 58.9 and 47.9, respectively, which are closer to these of the monoclinic BZN bulk.     

The electrical and thermal properties of polyimide/boron nitride nanocomposite films

In the paper, the polyimide (PI)/boron nitride (BN) nanocomposites were prepared by in situ polymerization and exhibited enhanced electrical property and thermal stability. The structure of synthesized PI was confirmed by scanning electron microscopy, energy dispersive spectrometer, and Fourier transform infrared. The influence of doping concentrations on the relative permittivity, electrical conductivity, loss tangent, corona-resistant lifetime, and thermal stability of PI composites was investigated. Results showed that the relative permittivity of PI/BN composites increases after doping BN nanoparticles. It was noteworthy that both the electrical conductivity and loss tangent of PI composites were enhanced in low frequency (0–3000 Hz) and the situations were shifted in high frequency (>3000 Hz). It was observed that the corona-resistant lifetime of PI/BN composite with 20 wt% BN increases more than eight times. Moreover, significant improvements in the thermal stability of PI composites were achieved by addition of only a small amount of BN. The decomposition temperatures at 5 and 10% weight loss were 518.7 and 551.6 °C for 15 wt% doped PI/BN composite, respectively, which increases by 37.3 and 40.5 °C compared to those of pure PI. The resulting properties expand further the application range of polyimides.     

The effect of boron nitride nanosheets on the mechanical and thermal properties of aluminum nitride ceramics

The boron nitride nanosheets (BNNSs)/aluminum nitride (AlN) composites were prepared by hot press sintering at 1600°C. The microstructure, mechanical properties, and thermal conductivity of the samples were measured, and the effect of adding BNNSs to AlN ceramics on the properties was studied. It is found that the addition of BNNSs can effectively improve the mechanical properties of AlN. When the additional amount is 1 wt%, the bending strength of the sample reaches the maximum value of 456.6 MPa, which is 23.1% higher than that of the AlN sample without BNNSs. The fracture toughness of the sample is 4.47 MPa m^1/2, a 68.7% improvement over the sample without BNNSs. The composites obtained in the experiment have brilliant mechanical properties.     

Wear and corrosion properties of wollastonite and boron nitride doped,hydroxyapatite-based HAp-Wo-BN composite coatings prepared by pulsed laser deposition

In this study, hydroxyapatite-based hydroxyapatite-wollastonite-boron nitride (HAp-Wo-BN) composite film was formed on the surface of Ti6Al4V by pulsed laser deposition (PLD). Based on a survey in scientific literature, it is presumed that this is the first time such a process is being undertaken. The wear and corrosion resistance of this film were analyzed comparatively in simulated body fluid (SBF) to simulate the human body environment. In the coating, HAp was used to form a bone-like layer, wollastonite was to enhance bone-tissue regeneration and BN was used for its bone-tissue healing and anti-bacterial properties. The results showed that the wear as well as the corrosion resistance of all samples after PLD treatment increased. Relatively the best wear resistance was achieved from boron nitride and wollastonite doped hydroxyapatite layers, where the best corrosion resistance was from the ones that consisted of only hydroxyapatite.     

Microstructure and tribological properties of cubic boron nitride films on Si3N4 inserts via boron-doped diamond buffer layers

Cubic boron nitride (cBN) coatings were deposited on silicon nitride (Si₃N₄) cutting inserts through conductive boron-doped diamond (BDD) buffer layers in an electron cyclotron resonance microwave plasma chemical vapor deposition (ECR MPCVD) system. The adhesion and crystallinity of cBN coatings were systematically characterized, and the influence of doping level of BDD on the phase composition and microstructure of the cBN coatings were studied. The nano-indentation tests showed that the hardness and elastic modulus of the obtained cBN coatings were 78 GPa and 732 GPa, respectively. The tribological properties of the cBN coatings were evaluated by using a ball-on-disc tribometer with Si₃N₄ as the counterpart. The coefficient of the friction and the wear rate of the cBN coatings were estimated to be about 0.17 and 4.1 × 10^− 7 mm³/N m, respectively, which are remarkably lower than those of titanium aluminum nitride (TiAlN) coatings widely used in machining ferrous metal. The results suggest that cBN/BDD coated Si₃N₄ inserts may have great potentials for advanced materials machining.     

Microstructure and properties of the AlCrMoZrTi/(AlCrMoZrTi)N multilayer high-entropy nitride ceramics films deposited by reactive RF sputtering

The AlCrMoZrTi/(AlCrMoZrTi)N multilayer high-entropy nitride ceramic films (HENCFs) fabricated by reactive RF magnetron sputtering presented (200) preferentially oriented FCC crystal structures. With the increase in the modulation period, the nitrogen content and surface roughness of the multilayer films gradually increased, the template effect between the nanocrystalline and amorphous forms was weakened, and the multilayer interface structure decreased. The S4 film with a modulation period of 1500 nm had the highest hardness and modulus (16.6 and 225.7 GPa, respectively) and the highest H/E* and H³/E*² values. The results of friction experiments showed that the S1 film with the smallest modulation period had a stable friction coefficient and small wear rate on both Si and Cu substrates, and it exhibited the best friction and wear performance due to its low surface roughness, high toughness and compressive yield resistance, and dense multilayer structure. The friction mechanisms of the HECNFs on Si and Cu substrates were mainly adhesive wear, abrasive wear, and a small amount of oxidative wear.