Mechanical properties of nanocrystalline diamond/amorphous carbon composite films prepared by microwave plasma chemical vapour deposition

Nanocrystalline diamond films (NCD) have been deposited by microwave plasma chemical vapour deposition from CH₄/N₂ mixtures with varying methane content. They consist of diamond nanocrystallites with sizes of 3–5 nm embedded in an amorphous matrix with grain boundary widths of 1–1.5 nm. The CH₄ content in the gas phase has almost no influence on the microscopic structure but a strong effect on the macroscopic structure and morphology. The mechanical and tribological properties of these films have been investigated by nanoindentation, nano tribo tests, and nano scratch tests. The hardness of a 4-μm-thick film deposited with 17% methane was about 40 GPa, the indentation modulus 387 GPa, and the elastic recovery 75%. Ball-on-disk tests against an Al₂O₃ ball revealed, after initially higher values, a friction coefficient of ≤0.1. Tribo tests and scratch tests proved a strong adhesion and a protective effect on silicon substrates. Finally, the correlations between the macroscopic structure of the films and their mechanical and tribological properties are discussed.     

Influence of microstructure and phase composition on the nanoindentation characterization of bioceramic materials based on hydroxyapatite

The aim of the study was to investigate the influence of microstructure and phase composition on the mechanical behaviour of hydroxyapatite (HAp) and biphasic HAp/β-tricalcium phosphate (β-TCP) bioceramic materials using nanoindentation. The formation of β-TCP phase in the HAp ceramic had the predominant influence on the nanomechanical properties of compact ceramics. For investigated microstructures there appear to be a slight decrease in the elastic modulus with increasing load and a higher decrease in hardness, which are in agreement with upper bounds of the results reported in literature. Maximal value of reduced modulus and hardness is yielded with pure HAp, and is measured to be 133.76 GPa for average grain size of 3 μm and 12.18 GPa for average grain size of 140 nm, respectively. The average modulus and hardness results for HAp/β-TCP ceramics with higher (101.61 GPa, 6.76 GPa) and lower grain size (115.72 GPa, 8.76 GPa) show sufficient mechanical properties in order to serve as hard tissue replacement material.     

Study of an on-line precision microgroove generating process on silicon wafer using a developed ultra-thin diamond wheel-tool

This study presents a novel and economical method for precisely developing an ultra-thin diamond grinding wheel-tool and using the finished wheel-tool to on-line fabricate crisscross microgrooves on silicon wafer. The wheel-tool blank is made of diamond grain of 0-2 μm grade via a designed micro co-deposition. A non-continuous cathode design, in which current crowding effect can be suppressed, is used to obtain a diamond wheel-tool with good surface characteristics. With abrasive content of 8 g/l, a suitable interval chip-pocket of 2-3 μm can be generated. The grinding wheel blank is thinned and dressed simultaneously down to a thickness of 15 μm using micro wire Electro Discharge Dressing (w-EDD). The finished wheel-tool is directly utilized to grind the crisscross microgrooves on the silicon wafer using ‘high-speed and fast-shallow grinding’ technique. A grinding depth of 0.5 μm per stroke is exactly controlled to ensure that the removal mechanism transfers to a ductile grinding mode. The width, depth and surface roughness R_a of the microgrooves are 15 μm, 9 μm and 0.087 μm, respectively.     

Mechanical characterization of ultra-thin fluorocarbon films deposited by R.F. magnetron sputtering

Fluorocarbon films were deposited on silicon substrate by R.F. magnetron sputtering using a polytetrafluoroethylene (PTFE) target. Structure of the deposited films was studied by X-ray photoelectron spectroscopy (XPS). Hardness, elastic modulus and scratch resistance were measured using a nanoindenter with scratch capability. -CF_x (x = 1, 2, 3) and C-C units were found in the deposited fluorocarbon films. The hardness and elastic modulus of the films are strongly dependent on the R.F. power and deposition pressure. The film hardness is in the range from 0.8 GPa to 1.3 GPa while the film elastic modulus is in the range from 8 GPa to 18 GPa. Harder films exhibit higher scratch resistance. Differences in nanoindentation behavior between the deposited fluorocarbon films, diamond-like carbon (DLC) films and PTFE were discussed. The fluorocarbon films should find more applications in the magnetic storage and micro/nanoelectromechanical systems.     

Deposition behavior and characteristics of hydroxyapatite coatings on Al2O3, Ti,and Ti6Al4V formed by a chemical bath method

A simple chemical bath method was used to deposit hydroxyapatite (HA) coatings on Al₂O₃, Ti, and Ti6Al4V substrates at ambient pressure by heating to 65–95 °C in an aqueous solution prepared with Ca(NO₃)₂·4H₂O, KH₂PO₄, KOH, and EDTA. The deposition behavior, morphology, thickness, and phase of the coatings were investigated using scanning electron microscopy and X-ray diffractometry. The bonding strength of the coatings was measured using an epoxy resin method. The HA coatings deposited on the three kinds of substrates were fairly dense and uniform and exhibited good crystallinity without any additional heat treatment. A coating thickness of 1–1.8 μm was obtained for the samples coated once. By repeating the coating process three times, the thickness could be increased to 4.5 μm on the Al₂O₃ substrate. The bonding strength of these coatings was 18 MPa.     

Improved mechanical properties of SnO2:F thin film by structural modification

This study reports the improvement in the mechanical properties of SnO₂:F (FTO) thin films through the modification of the structure and surface morphology. The FTO thin films are deposited on glass substrates by the atmospheric pressure chemical vapor deposition method on an industrial production line. Both the average grain size and the surface roughness were progressively increased by increasing the flow rate of metal organic monobutyltin trichloride (MBTC). The hardness and Young's modulus of the FTO films increased from 9.01 GPa to 15.08 GPa, and from 125.24 GPa to 206.93 GPa, respectively, according to the nanoindenter results. Post-heat treatment at 650 °C for 10 min resulted in a further increase in the hardness and Young's modulus, reaching maximum values of ~15.89 GPa and ~235.9 GPa, respectively. The enhancement in mechanical properties can be attributed to the reduced grain boundaries and the improved structural densification.     

Effective thermal conductivity of a diamond coated heat spreader

Thin diamond coatings are often suggested to enhance thermal conductivity of some substrate. We measured the effective thermal conductivity of varying thicknesses of diamond on tape cast, polycrystalline silicon carbide. The effective thermal conductivity of 30 μm diamond on tape cast silicon carbide is 1.7 W/(cm K). The effective thermal conductivity can be increased to 2.2 W/(cm K) by increasing the diamond thickness to approximately 70 μm. With the measured effective thermal conductivity, the thicknesses of the diamond film and substrate, and knowledge of the thermal conductivity of the substrate material, the thermal conductivity of the diamond layer can be calculated from a simple formula. The thermal conductivity of the 30-μm and 69-μm diamond layers were found to be 3.9 W/(cm K) and 5.8 W/(cm K), respectively.     

Tribological properties of partly polished diamond coatings

Extremely low friction coefficient was achieved with “partly polished diamond coatings”. Diamond coatings were deposited onto Si substrates by MWCVD with the mixture of CH₄ and H₂. Deposited films were characterized by X-ray diffraction (XRD), Raman spectroscopy and Electron Spectroscopy for Chemical Analysis (ESCA). Sharp peak derived from polycrystalline diamond was observed by XRD. Whereas Raman profile of partly polished diamond coatings was close to that of ta-C. This result suggests that small diamond grains were surrounded by amorphous carbon structure in the diamond coatings. Deposited diamond coating was polished with each other. Surface roughness R_a was reduced to 0.3, 0.2 and 0.08 μm, respectively. The hardness of the polished diamond coatings investigated by Nanoindentation technique was approximately 40.8 GPa, which was relatively lower value compared with conventional as-deposited CVD diamond coatings. For the tribological properties, we examined the effect of surface roughness using flat-ended pin-on-disk apparatus and ball-on-disk apparatus with bearing ball (SUJ2) and stainless steel (SUS304). Diamond coatings were deposited onto flat-ended pin and disk, and they were polished to R_a = 0.3, 0.2 and 0.08 μm. After the 6000 cycle process extremely low friction coefficient, μ = 0.05, was achieved with the pair of R_a (flat-ended pin, disk) = R_a (0.08, 0.3) in flat-ended pin-on-disk apparatus. In order to clarify the effect of surface roughness, ball-on-disk was carried out with different surface roughness, R_a = 1.7, 0.3, 0.2 and 0.08 μm. Here as-deposited diamond coating, R_a = 1.7 μm, was used as a reference point. Friction coefficient of μ = 0.09 was obtained for both balls. After the tribological tests balls were analyzed by scanning electron microscope (SEM) and energy dispersed X-ray spectrometer (EDX).     

Influence of porosity on the mechanical properties of microporous β-TCP bioceramics by usual and instrumented Vickers microindentation

Beta-tricalcium phosphate [Ca₃(PO₄)₂, β-TCP] is a bioresorbable material showing an excellent biocompatibility. However, sintering of β-TCP is difficult and the material presents poor mechanical strength and a low resistance to crack-growth propagation. In this study, influence of the porosity on the hardness and the elastic modulus is studied by means of usual and instrumented microindentation tests. Nevertheless, indentation diagonals measurement by optical observations is not accurate due to the crack formation around the residual indent. That is why instrumented indentation test which allows deducing the hardness and the bulk modulus from the load-depth curve analysis is used as an alternative method. The corresponding hardness number can be calculated by using the maximum indentation depth (Martens Hardness) or the contact depth determined by Oliver and Pharr's method (Contact Hardness). But in order to give representative values when comparing classical and instrumented hardness measurements, Martens hardness is preferred because its value can be directly related to the value of the Vickers hardness number by simple geometrical considerations.In this work, bioceramics were produced by conventional sintering of β-TCP powders synthesized by aqueous precipitation. Different process conditions were chosen to obtain microporous ceramics with a porosity rate between 0 and 14% in volume. As main results, the elastic modulus is found decreasing between 166 GPa and 108 GPa and the hardness number from 4.4 GPa to 2.2 GPa when increasing the porosity rate. A model connecting mechanical properties to porosity rate and grain arrangement is validated for the elastic modulus whereas deviation is observed for the hardness number. However, we propose an original approach where the relative variation of the two mechanical properties can be expressed with a unique relation as a function of the porosity volume fraction.     

Mechanical,biocorrosion, and antibacterial properties of nanocrystalline TiN coating for orthopedic applications

The current study analyzes the surface, mechanical, biocorrosion, and antibacterial performances of a nanocrystalline TiN ceramic coating synthesized using cathodic arc-physical vapor deposition (PVD) on biomedical Ti6Al4V substrates. The surface hardness and modulus of elasticity were assessed using the microindentation method. The adhesion, friction coefficient, and antibacterial properties of the coating were evaluated. The in vitro corrosion of the prepared coated Ti alloy substrate was analyzed in simulated body fluid (SBF) via cyclic potentiodynamic polarization (CPP), dynamic electrochemical impedance spectroscopy (DEIS), and scanning vibrating electrochemical technique (SVET). The results demonstrated that a nanocrystalline TiN coating with a crystallite size of 10.33 nm and a thickness of 5 μm was formed with good adhesion on the alloy surface. The coating had an enhanced surface hardness of 38.63 GPa and a modulus elasticity of 358 GPa, and exhibited enhanced resistance to plastic deformation compared with the substrate – features that can enhance the service life of an implant. The antibacterial experiments indicated an upgraded antibacterial performance of the TiN coating compared to the bare alloy. The in vitro corrosion-resistance analyses confirmed the enhanced surface protective performance of TiN ceramic coatings against biocorrosion in SBF. The results showed higher impedance values in DEIS, a higher passive region in the CPP analysis, and a lower anodic current density in the SVET analysis compared with the bare substrate.     

Electrochemical corrosion and materials properties of reactively sputtered TiN/TiAlN multilayer coatings

TiN/TiAlN multilayers of 2 μm thickness were successfully prepared by reactive DC magnetron sputtering method. XRD pattern showed the (1 1 1) preferential orientation for both TiN and TiAlN layers. XPS characterization showed the presence of different phases like TiN, TiO₂, TiON, AlN and Al₂O₃. Cross sectional TEM indicated the columnar growth of the coatings. The average RMS roughness value of 4.8 nm was observed from AFM analysis. TiN/TiAlN coating showed lower friction coefficient and lower wear rate than single layer coatings. The results of electrochemical experiments indicated that a TiN/TiAlN multilayer coating has superior corrosion resistance in 3.5% NaCl solution.     

Thermotropic liquid crystalline polymers as protective coatings for aerospace

Liquid crystalline polymers (LCPs) have potential as multifunctional, environmentally friendly coatings for aerospace, overcoming the disadvantages of current materials. Their use, however, has been hindered mainly by their poor adhesion strength. The present work studies novel liquid crystalline thermosetting polymers (LCTs), which can overcome the disadvantages of commercial LCPs for protective coatings in aerospace applications. Phenylethynyl terminated liquid crystalline oligomers based on 4-hydroxybenzoic acid (HBA) and 6-hydroxy-2-naphthoic acid (HNA) were synthesized and melt-pressed on grit-blasted aluminum to obtain 25 μm and 80 μm thick coatings. The presence of coating defects and curing kinetics were investigated, and the adhesion, mechanical properties and environmental resistance were compared with a commercial LCP reference material (Vectra^®). The LCTs showed highly improved adhesion; moreover, fully cured LCTs are harder and stiffer than commercial LCPs, which are expected to increase their wear and impact resistance. The coatings showed no swelling, peeling, or blistering after 500 h of full immersion in fluids such as jet fuel and turbine oil; furthermore, LCTs resisted 1000 h in corrosive fog (salt-spray) and hot moisture. Exposed samples retained their hardness, modulus, and pull-off strength, evidencing the outstanding chemical resistance of these LCTs. Our findings showed the potential of LCTs as protective and wear resistant coatings, particularly in the aggressive environments of aerospace applications. However, results suggest that exposed coating/substrate interfaces constitute paths for environmental attack. Further research aims at elucidating the possible mechanisms.     

Processing and room temperature mechanical properties of a zirconium carbide ceramic

Zirconium carbide (ZrC) powder, batched to a ratio of 0.98 C/Zr, was prepared by carbothermal reduction of ZrO₂ with carbon black. Nominally phase-pure ZrC powder had a mean particle size of 2.4 μm. The synthesized powder was hot-pressed at 2150°C to a relative density of > 95%. The mean grain size was 2.7 ± 1.4 μm with a maximum observed grain size of 17.5 μm. The final hot-pressed billets had a C/Zr ratio of 0.92, and oxygen content of 0.5 wt%, as determined by gas fusion analysis. The mechanical properties of ZrC_0.92O_0.03 were measured at room temperature. Vickers’ hardness decreased from 19.5 GPa at a load of 0.5 kgf to 17.0 GPa at a load of 1 kgf. Flexural strength was 362.3 ± 46 MPa, Young's modulus was 397 ± 13 MPa, and fracture toughness was 2.9 ± 0.1 MPa•m^1/2. Analysis of mechanical behavior revealed that the largest ZrC grains were the strength-limiting flaw in these ceramics.     

Boron doped diamond deposited by microwave plasma-assisted CVD at low and high pressures

Boron doped diamond is deposited over a range of pressures and chemistries including pressures from 35–120 Torr and gas chemistries including hydrogen–methane–diborane and argon–methane–hydrogen–diborane mixtures. The diamond deposition system is a 2.45 GHz microwave resonant cavity system. Diborane (B₂H₆) gas chemistry has been utilized with flow rates of 2.5–100 ppm. At low pressures of 35 Torr polycrystalline films are deposited using a feed gas mixture of hydrogen and 0.5% methane. At moderate pressures of 95 Torr, diamond films are grown using 60% Ar, 39% H₂ and 1% CH₄. For the high pressure experiments of 120 Torr, polycrystalline films are deposited using 98% H₂ and 2% CH₄. The deposition rate ranges from 0.3 to 1.6 μm/h. This investigation describes the relationship of the diborane flow rate and pressure versus the resulting film morphology, electrical properties, and morphology of the deposited films. The deposition of boron-doped polycrystalline diamond is done on 5 cm diameter silicon and silicon dioxide coated substrates. The resistivity spatial variation across the wafer was ± 5% indicating a good uniformity.     

Microstructure and performance of block copolymer modified epoxy coatings

The effects of two diblock copolymers, poly(ethylene-alt-propylene)-b-poly(ethylene oxide) (PEP–PEO) and poly(1,2-butadiene)-b-poly(2-vinyl pyridine) (PB–P2VP) on the mechanical properties of epoxy coatings were studied. Both modifiers self-assembled into spherical micelles of 10–20 nm diameter in cured bulk epoxy. This morphology was preserved in 15 μm thick coatings; however, micelle segregation to the coating/substrate interface was also observed. The critical strain energy release rate, G_1c, of bulk thermosets was enhanced by up to fivefold with the addition of block copolymers. Likewise, the abrasive wear resistance of thin coatings increased with modifier inclusion. The results showed that at 5 wt.% of loading, block copolymers were able to impart a 40% increase in abrasive wear resistance to modified coatings over neat ones. Block copolymer modifiers did not sacrifice the modulus and glass transition temperature of bulk thermosets and coatings, or the hardness and transparency of coatings.     

Mechanical behaviour of the constituents inside carbon-fibre/carbon-silicon carbide composites characterised by nano-indentation

The Young's modulus, hardness, fracture toughness and ductility of the key constituents were characterised using nano-indentation for three types of carbon-fibre/carbon silicon carbide composite manufactured through different routes and/or using different carbonaceous raw materials. Under indentation, all of the carbon constituents demonstrated much less ductile deformation than the silicon carbide and silicon did in these composites. Between two types of PAN-based carbon fibre, as well as of pyrolytic carbon, a difference of around a factor of two was evident in the Young's modulus and hardness. For the silicon carbide, a difference of around 100 GPa and 5 GPa was recorded for the mean Young's modulus and hardness respectively; for silicon, only a small variation was evident. The estimated mean fracture toughness of the silicon carbide ranged between 0.7 and 1.2 MPa.m^1/2, whilst the silicon was approximately 0.6 MPa.m^1/2. Results for the constituents were discussed in terms of their elastic/plastic behaviour.     

Superconductive superhard phase of BC7: Predicted via ab initio calculations

We present a first-principles study on a phase of BC₇ with Amm2 space-group symmetry (we call o-BC₇), which is selected from more than a dozen of candidates via energetic, mechanical and dynamical stabilities within a wide pressure scale from 0 to at least 100 GPa. Our calculated results show that this structure is highly incompressible with large bulk and shear modulus of 388 GPa and 430 GPa, respectively, which is even higher than cubic BN. Further investigations revealed that the Vickers hardness (H_v ≈ 49.5 GPa) is higher than t-BC₃ reported recently. And the superconducting critical temperature is calculated to be 38 K, which is comparable to that of MgB₂.     

An elevated temperature infrared emissivity ceramic coating formed on 2024 aluminium alloy by microarc oxidation

An infrared emissivity coating material containing γ-Al₂O₃ was prepared on 2024 aluminium alloy surface by the microarc oxidation (MAO) method. The microstructure of the coatings was analysed by SEM, XRD and EDS techniques. The infrared emissivity properties tested at 500 °C were investigated by an infrared radiometer based on a Fourier transform infrared spectrometer. The results show that the infrared emissivity values of coated Al samples depend on the phase composition and surface roughness of the coatings. Corresponding to increasing coatings thickness, the gradually increasing γ-Al₂O₃ content and some oxide compounds containing Si and P contribute to the higher infrared emissivity value (about 0.85) in the wavelength range of 8–20 μm. The increasing surface roughness leads to an obvious increase in emissivity from 0.2 to 0.4 at wavelength 3–5 μm.     

Growth of high-quality carbon nanotubes on free-standing diamond substrates

Carbon nanotubes (CNTs) were grown on diamond-coated Si substrates and free-standing diamond wafers to develop efficient thermal interface materials for thermal management applications. High-quality, translucent, free-standing diamond substrates were processed in a 5 kW microwave plasma chemical vapor deposition (CVD) system using CH₄ as precursor. Ni and Ni-9%W-1.5%Fe catalyst islands were deposited to nucleate CNTs directly onto the diamond substrates. Randomly-oriented multi-walled CNTs forming a mat of ∼5 μm thickness and consisting of ∼20 nm diameter tubes were observed to grow in a thermal CVD system using C₂H₂ as precursor. Transmission electron microscopy and Raman analyses confirmed the presence of high-quality CNTs on diamond showing a D/G peak ratio of 0.2-0.3 in Raman spectra.