The effect of substrate position on the microstructure and mechanical properties of SiC coatings on carbon/carbon composites

In this work, silicon carbide (SiC) coatings were produced on carbon/carbon composites using a chemical vapour deposition (CVD) method. During deposition, the temperature was fixed at 1200 °C and the coatings were produced by placing substrates at three different positions (340, 380 and 420 mm from the inlet) in the CVD reactor. The effect of substrate position on the microstructure and mechanical properties of the SiC coatings were experimentally investigated. The phase composition, surface morphology, defects and microstructure were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman Spectroscopy. The hardness and Young's modulus were measured using a nano-indentation method while fracture toughness was evaluated by micro-indentation. It was found that the deposition rate decreased linearly as substrate position moved far from the reactor inlet. In all coatings, only β-SiC phase was observed with a high density of stacking faults. It was found that the substrate position in the CVD reactor has a significant effect on the microstructure, grain size and crystallinity of the coating. At 340 and 420 mm substrate positions, a well-developed faceted microstructure with high crystallinity was observed while at 380 mm substrate position, the coating having lenticular-like fine grains with low crystallinity was obtained. The hardness values obtained from the top surface of the coatings are found to be higher than those from the cross-section, although the Young's modulus data (measured from the top surface and cross-section) were observed to be similar. At 380 mm substrate position, hardness, Young's modulus and fracture toughness were found to be the lowest compared to that of the coatings produced at 340 and 420 mm substrate positions. It is concluded that the SiC coatings with better mechanical properties can be produced by adjusting the substrate position in the CVD reactor.     

Mechanical properties of polyaniline

In spite of extensive electrical characterization of polyaniline, the information on its mechanical properties is missing in the literature. Complex Young's modulus of polyaniline compressed into pellets was measured at room temperature and an influence of preparation conditions of the polyaniline pellets on mechanical properties was studied. Young's modulus of PANI hydrochloride pellets was 0.9 ± 0.2 GPa and that of polyaniline base 1.3 ± 0.2 GPa. These values are comparable with common polymers, such as bulk polystyrene, 1.8 ± 0.1 GPa, or compressed polystyrene powder, 0.80 ± 0.02 GPa. Modulus of polyaniline is independent of the compression pressure above 300 MPa, the time of compression had no effect.     

Mechanical properties and oxidation resistance of CrAlN/BN nanocomposite coatings prepared by reactive dc and rf cosputtering

CrAlN/BN nanocomposite coatings were deposited through reactive cosputtering, i.e., pulsed dc and rf sputtering, of CrAl and h-BN targets, respectively. X-ray diffraction (XRD) and selected area electron-diffraction (SAED) analysis indicated that the CrAlN/BN coating consists of very fine grains of B1 structured CrAlN phase. With an increasing BN volume fraction of over 8 vol.%, the nanocrystalline nature of the grains is revealed through a dispersion of fine grains in the CrAlN/BN coating. A cross-sectional observation using a transmission electron microscope (TEM) clarified that the coating demonstrating the highest level of hardness has a fiber-like structure consisting of grains that are ~ 20 nm in width and ~ 50 nm in length. X-ray photoelectron spectroscopy (XPS) analysis revealed that the coating consists mainly of CrAlN and h-BN phase. The indentation hardness (H_IT) and effective Young's modulus (E*) of the coatings increased with the BN phase ratio, reaching a maximum value of ~ 46 and ~ 440 GPa at ~ 7 vol.% of BN phase; it then decreased moderately to ~ 40 and ~ 350 GPa at 18 vol.% of BN, respectively. Furthermore, CrAlN/BN coatings showed superior oxidation resistance compared with CrAlN coatings. After annealing at 800 °C in air for 1 h, the indentation hardness of CrAlN coatings decreased to 50% of the as-deposited hardness; in contrast, the hardness of CrAlN/BN nanocomposite coatings either stayed the same or increased, attaining a value of about 46 GPa. After annealing at 900 °C for 1 h, the hardness of all the coatings decreased to about 40%.     

Electro-stiffening in polypyrrole films: Dependence of Young's modulus on oxidation state,load and frequency

Electrochemically driven actuation of polypyrrole in aqueous sodium hexafluorophosphate (NaPF₆) solution has been shown to produce repeated large strains (>6%) at low voltages and with high conductivity, making it one of the most promising electroactive conducting polymers. Little is known about the voltage dependent stiffness of this version of the polymer. This information is important in determining the strain as a function of load. In this paper the complex Young's modulus (storage and loss components) of a hexafluorophosphate-doped polypyrrole film in aqueous NaPF₆ electrolyte at different oxidation states, under various loads and as a function of the frequency of the applied load, is investigated. Uniformity of doping was ensured by allowing enough time to reach steady state charge levels, and the creep during measurements was minimized by using preconditioning cycles. The results of this study show that storage modulus decreases (from 1 GPa to 0.80 GPa) as the polypyrrole oxidation potential increases (from ?0.4 V to +0.4 V versus Ag/AgCl reference electrode). The loss modulus, on the other hand, increases from 55 MPa to 80 MPa. An increasing trend in the Young's modulus is also observed with the applied load. The storage modulus increases from 0.65 GPa to 1 GPa by increasing the applied load from 0.2 MPa to 2.5 MPa. The modulus is found to increase with time through the experiment, which may be due to stretch alignment of the polymer. It is also observed that complex Young's modulus increases in proportion to the logarithm of frequency.     

Effect of powder feed rate on the mechanical properties of WC-5 wt%Ni coatings deposited using low pressure cold spray

WC-5 wt%Ni coatings were fabricated onto 3CR12 stainless steel plates using low pressure cold spray deposition. The powder feed rate of the volumetric feeder was varied at 50% (23.6 rpm, 6.8 ± 1 g min^− 1), 75% (35.7 rpm, 10.1 ± 0.9 g min^− 1), and 100% (58.9 rpm, 15 ± 1.1 g min^− 1), and the resulting influence on the mechanical properties of the coatings was investigated. The results were analyzed using a two-parameter Weibull distribution and linear regression analysis. A new parameter to quantify WC particle refinement is proposed. A powder feed rate of 75% produced the best coating properties, achieving a high hardness (2.56 GPa), low porosity (0.55%), high interfacial toughness (12.61 MPa m^0.5), high Young's modulus (69.23 GPa), and good fracture toughness (2.81 MPa m^0.5). This feed rate appeared to be the optimum deposition parameter to produce dense coatings, achieving a high level of Ni plastic deformation and a high amount of refined WC particles, which are too small to cause significant erosion during impact, and are therefore better able to be retained by the ductile binder.     

The effect of vacuum annealing on the microstructure,mechanical and electrical properties of tantalum films

The microstructure, mechanical and electrical properties of vacuum annealed tantalum films were studied. X-ray diffraction spectra confirmed the presence of mixed (α and β) phases in the as-deposited Ta films. After vacuum annealing (at 750 °C for 1 h), the metastable β-phase was completely transformed to stable α-phase. The grain size increased (from 35 ± 3 nm to 92 ± 3 nm) with the increase in annealing temperature. The mixed (α and β) phases resulted in higher hardness and higher Young's modulus. The film annealed at 750 °C for 1 h exhibited lower resistivity (52 ± 4 μΩ-cm), lower hardness (H = 10.4 ± 1.3 GPa) and lower Young's modulus (Y = 185 ± 5 GPa) as compared to the as-deposited and annealed (at temperature < 750 °C) films. This is attributed to the phase transformation from β to α at an annealing temperature of 750 °C.     

Mechanical and barrier properties of MOCVD processed alumina coatings on Ti6Al4V titanium alloy

This study focuses on the implementation of different aluminum oxide coatings processed by metal-organic chemical vapor deposition from aluminum tri-isopropoxide on commercial Ti6Al4V titanium alloy to improve its high temperature corrosion resistance. Films grown at 350 °C and at 480 °C are amorphous and correspond to formulas AlOOH, and Al₂O₃, respectively. Those deposited at 700 °C are composed of γ-Al₂O₃ nanocrystals dispersed in a matrix of amorphous alumina. Their mechanical properties and adhesion to the substrates were investigated by indentation, scratch and micro tensile tests. Hardness and rigidity of the films increase with increasing deposition temperature. The hardness of the coatings prepared at 350 °C and 480 °C is 5.8 ± 0.7 GPa and 10.8 ± 0.8 GPa respectively. Their Young's modulus is 92 ± 8 GPa (350 °C) and 155 ± 6 GPa (480 °C). Scratch tests cause adhesive failures of the films grown at 350 °C and 480 °C whereas cohesive failure is observed for the nanocrystalline one, grown at 700 °C. Micro tensile tests show a more progressive cracking of the latter films than on the amorphous ones. The films allow maintaining good mechanical properties after corrosion with NaCl deposit during 100 h at 450 °C. After corrosion test only the film deposited at 700 °C yields an elongation at break comparable to that of the as processed samples without corrosion. The as established processing–structure–properties relation paves the way to engineer MOCVD aluminum oxide complex coatings which meet the specifications of the high temperature corrosion protection of titanium alloys with regard to the targeted applications.     

Chromium carbide–CNT nanocomposites with enhanced mechanical properties

Chromium carbide is widely used as a tribological coating material in high-temperature applications requiring high wear resistance and hardness. Herein, an attempt has been made to further enhance the mechanical and wear properties of chromium carbide coatings by reinforcing carbon nanotubes (CNTs) as a potential replacement of soft binder matrix using plasma spraying. The microstructures of the sprayed CNT-reinforced Cr₃C₂ coatings were characterized using transmission electron microscopy and scanning electron microscopy. The mechanical properties were assessed using micro-Vickers hardness, nanoindentation and wear measurements. CNT reinforcement improved the hardness of the coating by 40% and decreased the wear rate of the coating by almost 45–50%. Cr₃C₂ reinforced with 2 wt.% CNT had an elastic modulus 304.5 ± 29.2 GPa, hardness of 1175 ± 60 VH_0.300 and a coefficient of friction of 0.654. It was concluded that the CNT reinforcement increased the wear resistance by forming intersplat bridges while the improvement in the hardness was attributed to the deformation resistance of CNTs under indentation.     

Structural,mechanical and electrical-contact properties of nanocrystalline-NbC/amorphous-C coatings deposited by magnetron sputtering

Niobium-carbide nanocomposite coatings with a carbon content varying from 43 to 64 at.% were deposited by dual DC magnetron sputtering. X-ray diffraction, x-ray photoelectron spectroscopy and electron microscopy showed that all coatings consisted of nanometer sized NbC grains embedded in a matrix of amorphous carbon. Mechanical properties and electrical resistivity showed a strong dependency on the amount of amorphous carbon (a-C) and NbC grain size in the coating. The highest hardness (23 GPa), elastic modulus (295 GPa) and the lowest resistivity (260 μΩ cm) were measured for the coating with about 15% of a-C phase. Contact resistance measurements using a crossed cylinder set-up showed lowest contact resistance for the coating containing 33% a-C (140 μΩ at a contact force of 100 N), which is comparable to a Ag reference (45 μΩ at a contact force of 100 N). Comparison with TiC-based nanocomposites studied under similar conditions showed that the NbC system has less tendency to form a-C and that lowest contact resistance is obtained at comparable amounts of a-C phase in both material systems (33% for NbC compared to 35% for TiC). With these good electrical contact properties, the NbC nanocomposites can be considered as a potential material for electrical contact applications.     

Effect of Ti content on the microstructure and mechanical properties of TiAlSiN nanocomposite coatings

TiAlSiN coatings has been proposed and studied because of their desirable properties in hardness and coating-substrate adhesion. Further improvement of their performance can be achieved by better understanding the effect of the concentration of each element on the microstructure and mechanical properties of the coatings. In this paper, the TiAlSiN coatings with different Ti content were deposited by reactive DC magnetron sputtering method. The microstructure and mechanical properties of the coatings were analyzed by energy dispersive spectroscopy, X-ray diffraction, transmission electron microscope, scanning electron microscope, nano-indentor and Rockwell indentation tester. The results reveal that TiAlSiN coatings consisted of amorphous phase and crystalline phase. With a Ti content of 63 at.%, as well as a Si content of 7 at.%, a super-hard TiAlSiN coating with a nanoindentation hardness of 66 GPa was achieved. What is more, in contrast to the well-described super-hard nanocomposite TiAlSiN coatings, another “nanocomposite” microstructure coating with a Ti content of 29 at.% in which the amorphous phase is wrapped in a crystalline phase was identified, with a comparatively low hardness value of 20 GPa. The highest adhesion strengths with a Rockwell indentation classes HF2 was achieved for a coating with a Ti content of 63 or 65 at.%.     

Thermal and mechanical properties of perovskite-type barium hafnate

Polycrystalline samples of the barium perovskite-type oxide, BaHfO₃ were prepared by solid-state reactions from HfO₂ and BaCO₃ powders. The thermal expansion coefficient, heat capacity, thermal diffusivity, thermal conductivity, elastic modulus, Debye temperature, and micro-Vickers hardness were measured. The crystal structure of BaHfO₃ is of the cubic perovskite type with the lattice parameter 0.4171 nm at room temperature. The sample bulk density is 91% of the theoretical density. The average linear thermal expansion coefficient is 6.93 × 10⁻⁶ K⁻¹ in the temperature range between 300 and 1500 K. The Young's modulus equals 194 GPa. The thermal conductivity at room temperature is 10.4 Wm⁻¹K⁻¹.     

Microstructure,hardness, toughness and oxidation resistance of Al2O3–ZrB2 composite with different Ti percentages prepared by in-situ SHS dynamic compaction

Effect on microstructure, hardness, modulus, fracture toughness and oxidation behaviour of Al₂O₃–ZrB₂ composite with different percentages of Titanium as diluents during the SHS reaction has been studied. It was observed that Ti addition leads to formation of different phases such as TiB₂, ZrO_2, TiB besides ZrB₂ and Al₂O₃. The hardness and modulus were found to be around 21 GPa and 350 GPa, respectively for 10 wt.% Ti addition in dynamic depth indentation. Oxidation study of the sample having 25 wt.% Titanium showed no significant change in phase and mechanical properties after 700 °C oxidation. The toughness of 20 wt.% Ti sample showed highest fracture toughness.     

Laser gas assisted nitriding and tin coating of Ti–6Al–4V alloy: Experimental and numerical investigation of mechanical properties

Three-point bending tests are carried out to measure the Young's modulus of laser gas assisting nitrided, laser gas assisting nitrided and coated, heat treated and coated, specimens. The testing conditions are incorporated to simulate the material response to the bending load using finite element modelling (FEM) and the ANSYS code is utilized in this regard. Laser gas assisted nitriding is carried out using a CO₂ laser system while TiN coating is realized using the PVD coating unit. It is found that crack-free laser gas assisting nitrided layer of about 80 μm is achieved while TiN coatings thickness is about 2 μm. Young's modulus of TiN coating is similar to that obtained for laser gas assisting nitrided layer.     

Adhesion and dynamic impact wear of nanocomposite TiC-based coatings prepared by DCMS and HiPIMS

Nanocomposite nc-TiC/a-C:H coatings exhibit a unique combination of mechanical properties such as high hardness, and low friction and wear. These physical and mechanical properties make those coatings attractive for application in industry. However, the properties of the whole coating/substrate system such as adhesion of the coating to substrate and its response on repeated impact loading known such as dynamic impact wear are also important for industrial applications. Thus, this paper is focused on the adhesion and the dynamic impact wear of nc-TiC/a-C:H coatings prepared by the hybrid PVD-PECVD process. Two series of nc-TiC/a-C:H coatings with a different amount of carbon were deposited onto commonly used industrial cemented tungsten carbide substrates using DC magnetron sputtering (DCMS) and the high power impulse magnetron sputtering (HiPIMS) of a titanium target in argon and acetylene mixture atmosphere. Both series of coatings were analysed using a scratch test and dynamic impact tester with an impact load of 600 N. The HiPIMS prepared coatings exhibited lower thickness and lower thicknesses of the Ti adhesive interlayers between the substrates and coatings than the DCMS prepared coatings. Thus, the adhesion and the impact wear of both series were discussed separately. These properties were discussed with respect to the coating microstructure, phase composition and mechanical properties such as the hardness H, the effective elastic modulus E, and the H/E and H³/E² ratios. The scratch adhesion of coatings depended on the H³/E² ratio and coating microstructure, hardness and surface roughness. The impact wear of the nc-TiC/a-C:H coatings depended on the H/E ratio and coating microstructure.     

The structure and mechanical and tribological properties of TiBCN nanocomposite coatings

TiBCN nanocomposite coatings were deposited in a closed field unbalanced magnetron sputtering system using pulsed magnetron sputtering of a TiBC compound target with various Ar/N₂ mixtures. TiBCN coatings were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, nanoindentation, Rockwell C indentation and ball-on-disk wear tests. The coatings with a nitrogen content of less than 8 at.% exhibited superhardness values in the range of 44–49 GPa, but also showed poor adhesion and low wear resistance. Improvements in the coating adhesion, H/E ratio and wear resistance were achieved together with a decrease in the coating hardness to 35–45 GPa as the N content in the coatings was increased from 8 to 15 at.%. The microstructure of the coatings changed from a nano-columnar to a nanocomposite structure in which 5–8 nm nanocrystalline Ti(B,C) and Ti(N,C) compounds were embedded in an amorphous matrix consisting of BN, free carbon and CN phases. With a further increase in the N content in the coatings to levels greater than 20 at.%, the inter-particle spacing of the nanocrystalline compounds increased significantly due to the formation of a large amount of the amorphous BN phase, which also led to low hardness and poor wear resistance of the TiBCN coatings.     

In-situ synthesis of Ti-Si-C fine grained composite with different amount of TiC: Microstructure and mechanical properties

The present work reports the in-situ synthesis of the Ti-Si-C composite so that TiC phase is increased in the composite by suitable adjustment of starting reactants during self-propagating high temperature synthesis. The detailed microstructure and mechanical behaviour of the composite has been studied. The microstructure, density, phase present, amount of melt phase, hardness and modulus were directly affected by adiabatic temperature of the reaction. The highest value of microhardness was found to be around 2200 HV_0.05. The nanoindentation revealed the hardness range from 22 to 12 GPa and modulus varied from 445 to 281 GPa for different percentages of TiC into matrix.     

Mechanical behavior of diamond matrix composites with ceramic Ti3(Si,Ge)C2 bonding phase

Polycrystalline diamond, PCD, compacts are usually produced by high pressure–high temperature (HP–HT) sintering. This technique always introduces strong internal stresses into the compacts, which may result in self-fragmentation or graphitization of diamond. This may be prevented by a bonding phase and Ti₃(Si,Ge)C₂ was so investigated. This layered ceramic was produced by Self Propagating High Temperature Synthesis and the product milled. The Ti₃(Si,Ge)C₂ milled powder was mechanically mixed, in the range 10 to 30 wt.%, with 3–6 μm diamond powder (MDA, De Beers) and compacted into disks 15 mm in diameter and 5 mm high. These were sintered at a pressure of 8.0 GPa and temperature of 2235 K in a Bridgman-type high pressure apparatus. The amount of the bonding phase affected the mechanical properties: Vickers hardness from 20.0 to 60.0 GPa and Young's modulus from 200 to 500 GPa, with their highest values recorded for 10 wt.% Ti₃(Si,Ge)C₂. For this composite fracture toughness was 7.0 MPa m^1/2, tensile strength 402 MPa and friction coefficient 0.08. Scanning and transmission electron microscopy, X-ray and electron diffraction phase analysis were used to examine the composites.     

Phase and microstructural evolution of Ir–Si binary alloys with fcc/silicide structure

To search hardening approach or new probable phases benefiting to high temperature behavior of Ir-based superalloys, Ir alloyed with Si was employed. Investigations on phase and microstructural evolution of a series of Ir–xSi (x=2.5, 5, 15, 20, 30, 36 and 45 mol%) binary alloys were carried out by XRD, EPMA and SEM analysis. A schematic plot of the Ir–Si binary diagram with the nominal Si content ranging from 0 to 50 mol% was primarily drafted. Room temperature mechanical properties, the Vicker hardness and Young's modulus, of bulk material or each kind of phases were also measured. Researches reveal that with Si addition up to 50 mol%, the microstructures are respectively composed of primary Ir solid solution fcc+peritectic Ir₃Si silicide (nominal Si content: 0–25 mol%), primary Ir₃Si+eutectoid silicide (Si: 25–33.3 mol%), Ir₃Si₂+eutectoid silicide (Si: 33.3–40 mol%) and primary IrSi+Ir₃Si₂ silicide (Si: 40–50 mol%). With plastic characteristic, the fcc phase has the low Vickers hardness and Young's modulus, while both of the silicides are high and the silicides behave brittle. For the high temperature applications over 1400 °C, Ir-based alloys with Si dropping must avoid the appearance of any kind of Ir/Si silicides in microstructure because the melting points of silicides (Ir₃Si, Ir₂Si and Ir₃Si₂) are close to 1400 °C; instead, solid solution hardening on Ir by Si is recommended.     

Characterization studies of pulse magnetron sputtered hard ceramic titanium diboride coatings alloyed with silicon

The composition, structure and mechanical properties of pulsed-DC unbalanced magnetron sputtered Ti–Si–B thin films—hard coatings with the potential for excellent thermal stability and oxidation resistance—are investigated and reported in this paper. Fully dense, hard (19–37 GPa) Ti–Si–B coatings were deposited at substrate bias voltages (V_s) ranging from floating potential to −150 V which resulted in substrate temperatures of ∼90–135 °C. We found that variation of substrate biasing conditions critically affected film composition, structure and resultant mechanical properties. For instance, concentration of Si in films decreased from 18.4 at.% to 3.8 at.% as V_s was increased from floating potential to −150 V; composition profile analysis of the near-surface region of films (0–10 nm) revealed them to be rich in Si with significant differences among specimens produced at different substrate bias conditions. Variation of substrate biasing conditions provided coating structures that ranged from completely amorphous at floating substrate potential to nanocrystalline at V_s = −50 to −100 V and crystalline nanocolumnar at V_s = −150 V. We found that each of the structures obtained exhibited different specific values of hardness and elastic modulus, which is also in a good agreement with results reported for other coatings possessing similar micro- and nano-structures. Film structure was analyzed in detail by conventional and analytical transmission electron microscopy. Coatings that exhibited the highest values of hardness (37 GPa) were found to possess features such as crystalline nanocolumnar grains a few nanometres in diameter and disordered intergranular regions of different chemical composition, thus qualifying as nanocomposite films. Results of this work allowed relationships to be drawn between deposition parameters and Ti–Si–B coating composition, structure and mechanical properties. Qualitatively similar relationships are also expected for other biased plasma-assisted physical vapour deposited transition-metal-based ceramic coatings alloyed with Si (e.g. Ti–Si–N, Cr–Si–N, Cr–Al–Si–N).     

Excellent mechanical properties of metastable c-WN fabricated at high pressure and high temperature

Tungsten nitrides (WNs) are promising functional materials with high hardness, but the greatest challenge is to synthesize stoichiometric and bulk materials. In this paper, bulk tungsten mononitride (c-WN) with sodium chloride structure, which is a metastable phase, has been successfully synthesized at high pressure and high temperature (HPHT) using W₃N₄ as precursor. It is found that synergistic effect of pressure and temperature was useful to control the complete decomposition of W₃N₄ and to suppress further decomposing of as-synthesized c-WN. The compression ability and Vickers hardness were investigated by in situ high pressure X-ray diffraction (XRD) and Vickers microhardness tests, respectively. It is worth noting that the bulk modulus of c-WN is 422.9 ± 6.7 GPa, which is comparable to diamond. The Vickers hardness, 29 GPa obtained under an applied load of 0.49 N, is nearly 45% higher than that of TiN which is widely used as hard wear protective coatings. The excellent mechanical properties of c-WN may be ascribed to strong pd hybridization which has been further proved by XPS.