Mechanical, tribological and erosion behaviour of super-elastic hard Ti-Si-C coatings prepared by PECVD

Titaniun carbide (TiC) based coatings prepared by low temperature Plasma Enhanced Chemical Vapor Deposition (PECVD) are investigated as attractive candidates for wear resistance, and particularly for protection against solid particle erosion. In the present work, we incorporated silicon (Si) as an alloying element to TiC, to obtain ternary nanostructured Ti-Si-C films. The incorporation of Si in TiC resulted in significant microstructural, mechanical and tribological modifications. By controlling the Si content in the films, we observed a transition between films consisting of fine nano-sized TiC crystallites (nc-TiC) embedded in an amorphous C:H matrix (a-C:H) to a microstructure formed by nc-TiC encapsulated in a-SiC/a-C:H matrix. This allowed one to selectively control the main mechanical characteristics, namely the hardness (H), the Young's modulus (E), and the friction coefficient (μ), in the range of 14-32 GPa, 140-240 GPa, and 0.16-0.6, respectively. For films prepared under optimized conditions, high elastic strain to failure and high resistance to plastic deformation of the Ti-Si-C films, expressed by H/E and H³/E² ratios, resulted in an 8 fold increase of the erosion resistance at an impact angle of 90° compared to a bare steel substrate. Erosion resistance at 30° increased by a factor of 22 compared to bare substrate due to a simultaneous combination of high H and low μ. Taking into consideration the severe erosion test conditions and the Ti-Si-C film thickness of less than 5 μm in this work, further improvement is expected for thicker films.     

Effect of deposition temperature on structural, surface, optical and magnetic properties of pulsed laser deposited Al-doped CdO thin films

The objective of this work is to study the influence of deposition temperature on structural, surface, optical and magnetic properties of the Al doped CdO thin films prepared by pulsed laser deposition (PLD) technique. KrF excimer laser (λ = 248 nm, τ_l = 20 ns, ν = 10 Hz, ?_l = 2.5 J/cm²) was employed for the deposition of thin films. It is observed by XRD results that films grown at room temperature and 100 °C show preferential growth along (1 1 1) and (2 0 0) directions while high temperatures (200-400 °C) lead to preferential growth along the (2 0 0) direction only. The optical constants (n, k, α, and optical band gap energy) of films measured by spectroscopic ellipsometry show strong dependence upon deposition temperature. M-H loop of films, measured by vibrating sample magnetometer, deposited at 25 °C and 100 °C show paramagnetic nature while films deposited at temperatures (200-400 °C) exhibit ferromagnetic character. Scanning electron micrographs show degraded elongated grains at lower deposition temperatures, while smooth and compact surface is observed for films deposited at higher deposition temperatures.     

Structure and electrical conduction behavior of LiZnVO4 ceramic prepared by solution-based chemical route

In the present work, an evaluation of the structural and electrical properties of a compound (LiZnVO₄) has been undertaken. This compound was prepared by solution-based chemical route. The electrical properties were measured using a.c. impedance spectroscopy method in the frequency range of 10³-10⁶ Hz at various temperatures from 28 to 300 °C. X-ray diffraction study indicates a rhombohedral unit cell structure with lattice parameters a = 14.1934 Å, b = 14.1934 Å, c = 9.4926 Å, V = 1656.12 (Å)³, α = 90°, β = 90° and γ = 120°. A field emission scanning electron micrograph reveals a polycrystalline texture of the compound with grains of unequal sizes ∼0.2-2.0 μm. The electrical conduction in the material is a thermally activated process due to the bulk effect. Frequency dependence of a.c. conductivity obeys Jonscher's universal law (σ_ac = σ_dc + Aωⁿ).     

Fabrication of superhydrophobic Cu surfaces with tunable regular micro and random nano-scale structures by hybrid laser texture and chemical etching

Hydrophobic Cu surfaces with tunable regular microstructure and random nanostructures were fabricated by nanosecond pulsed laser texturing and chemical etching. The regular micro-scale roughness can be tuned from 5 μm to 30 μm Rz by nanosecond laser texturing while the ligament width and spacing of the random nano porosity about 10-50 nm can be realized by selective chemical etching. Wettability tests demonstrate that the Cu surface with this micro/nanostructural hierarchy, analogous to that of lotus leaves found in nature, is superhydrophobic with a water contact angle around 153°. The random nanostructure by selective etching plays an important role for hydrophobicity in addition to the regular microstructure by nanosecond laser texturing.     

Electrochemical and mechanical properties of superhydrophobic aluminum substrates modified with nano-silica and fluorosilane

Nano-silica particles were deposited on acid-etched hydrophilic aluminum (Al) substrates by immersion in well-dispersed nano-silica aqueous suspension and tetramethylamonium hydroxide, followed by a heat treatment. The surface was then further treated by a reaction with fluorosilane. The hydrophobicity, surface morphology, and mechanical properties of the coated Al substrates were investigated, along with their electrochemical properties over time of exposure to two NaCl solutions (0.3% and 3% by weight). All the coated Al surfaces exhibited a water contact angle of 155–158°, i.e., superhydrophobicity. The use of nano-silica suspension significantly enhanced the hydrophobicity of the coated Al. Artificial neural networks were used to provide quantitative understanding in how the microstructure of the treated Al surface contributed to its superhydrophobicity and electrochemical properties. When R_{a, total} (nano-roughness + micro-roughness) exceeds 450 nm, WCA is greater than 154°, independent of the nano/micro-roughness ratio (RR_NM). FESEM and AFM images of these surfaces suggest that a rough two-length-scale hierarchical structure coupled with the low surface energy of fluorosilane topcoat led to the superhydrophobicity of the formed coatings. The coating prepared with the 0.2% nano-silica suspension (vs. other concentrations) featured the highest Young's modulus and the best corrosion protection to the Al substrate in both NaCl solutions.     

Properties of hydrogenated amorphous carbon films deposited by PECVD and modified by SF6 plasma

Hydrogenated amorphous carbon (a-C:H) films were grown at room temperature on glass and polished silicon substrates using RF-PECVD (Radio-Frequency Plasma Enhanced Chemical Vapor Deposition). Plasmas composed by 30% of acetylene and 70% of argon were excited by the application of RF signal to the sample holder with power ranging from 5 to 125 W. After deposition, the films were submitted to SF₆-plasma treatment for 5 minutes. SF₆ plasmas were generated at a pressure of 13.3 Pa by a RF power supply operating at 13.56 MHz with the output fixed at 70 W. The resulting films were characterized in terms of their molecular structure, chemical composition, surface morphology, thickness, contact angle, and surface free energy. During the SF₆ plasma treatment, fluorine species were incorporated in the film structure causing chemical alterations. The interaction of chemical species generated in the SF₆ plasmas with surface species was responsible for the decrease of the film thickness and surface energy, and for the increase of the film roughness and hydrophobicity.     

Surface modification of silica glass by CHF3 plasma treatment and carbon negative-ion implantation for cell pattern adhesion

We have investigated a method for the patterning of cell adhesion on a silica glass by using two-steps of surface modification processes of CHF₃ plasma treatment and negative-ion pattern implantation. For the first step, exposure of CHF₃ plasma to silica glass (SG) was used to obtain hydrophobic surface, leading to eliminate cell-adhesion property. After treatment with RF power of 20 W and exposure time of 120 s, the hydrophobicity was occurred from the increase in contact angle of SG from 43° to 88° and its reason based on XPS analysis was due to formations of C―F, C―F₂, and C―F₃ bonds, so-called fluorocarbonated bonds. Culture of mesenchymal stem cells (MSC) and rat adrenal pheochromocytoma cells (PC12h) showed the degradation of cell adhesion property on the plasma-treated SG surface. For the second step, carbon negative-ion implantation into the hydrophobic fluorocarbonated-SG surface was used to pattern the hydrophilic region, leading to enhance cell adhesion property. The contact angle of C-modified surface decreased to 76° at conditions of 15 keV and 1 × 10¹⁵ ions/cm². XPS showed that the hydrophilicity was due to reduction of C―F_x bonds and formation of C―O and C═O bonds. After 3 days culture of MSC and PC12h on the C-implanted surface of the plasma-treated SG, a fairly good adhesion patterning of both cells was obtained on the ion-implanted regions.     

High-rate deposition of amorphous and nanocomposite Ti-Si-C multifunctional coatings

Amorphous (a) and nanocomposite Ti-Si-C coatings were deposited at rates up to 16 μm/h by direct current magnetron sputtering from a Ti₃SiC₂ compound target, using an industrial pilot-plant system, onto high-speed steel, Si, and SiO₂ substrates as well as Ni-plated Cu cylinders, kept at a temperature of 200 or 270 °C. Electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction analyses showed that TiC/a-C/a-SiC nanocomposites were formed consisting of textured TiC nanocrystallites (nc) embedded in a matrix of a-C and a-SiC. Elastic recoil detection analysis showed that coatings deposited at a target-to-substrate distance of 2 cm and an Ar pressure of 10 mTorr have a composition close to that of the Ti₃SiC₂ compound target, as explained by ballistic transport of the species. Increased target-to-substrate distance from 2 cm to 8 cm resulted in a higher carbon-to-titanium ratio in the coatings than for the Ti₃SiC₂ compound target, due to different gas-phase scattering properties between the sputtered species. The coating microstructure could be modified from nanocrystalline to predominantly amorphous by changing the pressure and target-to-substrate conditions to 4 mTorr and 2 cm, respectively. A decreased pressure from 10 mTorr to 4 or 2 mTorr at a target-to-substrate distance of 2 cm decreased the deposition rate up to a factor of ~ 7 as explained by resputtering and an increase in the plasma sheath thickness. The coatings exhibited electrical resistivity in the range 160-800 μΩ cm, contact resistance down to 0.8 mΩ at a contact force of 40 N, and nanoindentation hardness in the range of 6-38 GPa.     

Comparison testing of diamond-like a-C:H coatings prepared in plasma cathode-based gas discharge and ta-C coatings deposited by vacuum arc

The method of amorphous carbon coating deposition based on decomposition of acetylene in a non-self-sustained hollow cathode pulsed-DC discharge is investigated. The discharge is maintained by the electron emission of a grid-stabilized plasma cathode based on a DC glow discharge. The method allows the gas pressure in the discharge gap and the non-self-sustained discharge parameters to be varied in a wide range. It makes it possible to optimize the properties of the deposited coating and to perform in situ the preliminary ion cleaning of sample surface and the plasma immersion ion implantation to form an interface and to improve the coating's adhesion. The 0.1-10-μm-thick a-C:H films were deposited on tungsten carbide and stainless steel substrates at a deposition rate of 0.5-8 μm/h. The coatings were investigated using the methods of atomic-force microscopy (AFM), scanning electron microscopy (SEM) and Raman spectroscopy. The arithmetic average surface roughness (9-34 nm), the friction coefficients (0.01-0.3), the density (2.2-2.4 g/cm³), the microhardness (16-75 GPa) and the internal stresses in the films (3-7 GPa) were measured. Comparison was made between the properties of the resulted a-C:H coating with the properties of the ta-C coating obtained by cathodic vacuum arc deposition.     

Surface characterization of Ti-7.5Mo alloy modified by biomimetic method

Titanium and its alloys have been used in dentistry due to their excellent corrosion resistance and biocompatibility. It was shown that even a pure titanium metal and its alloys spontaneously form a bone-like apatite layer on their surfaces within a living body. The purpose of this work was to evaluate the growth of calcium phosphates at the surface of the experimental alloy Ti-7.5Mo. We produced ingots from pure titanium and molybdenum using an arc-melting furnace. We then submitted these ingots to heat treatment at 1100 °C for one hour, cooled the samples in water, and cold-worked the cooled material by swaging and machining. We measured the media roughness (Ra) with a roughness meter (1.3 and 2.6 μm) and cut discs (13 mm in diameter and 4 mm in thickness) from each sample group. The samples were treated by biomimetic methods for 7 or 14 days to form an apatite coating on the surface. We then characterized the surfaces with an optical profilometer, a scanning electron microscope and contact angle measurements. The results of this study indicate that apatite can form on the surface of a Ti-7.5Mo alloy, and that a more complete apatite layer formed on the Ra = 2.6 μm material. This increased apatite formation resulted in a lower contact angle.     

CrAlYCN/CrCN nanoscale multilayer PVD coatings deposited by the combined High Power Impulse Magnetron Sputtering/Unbalanced Magnetron Sputtering (HIPIMS/UBM) technology

CrAlYCN/CrCN coating combining high hardness (H_p = 36 GPa) and low friction coefficient (µ = 0.42 against Al₂O₃) has been developed for machining of Si containing Al-alloys. The coating was deposited by the combined High Power Impulse Magnetron Sputtering/Unbalanced Magnetron sputtering, (HIPIMS/UBM) technology. Macroparticle free Cr⁺ ion flux was generated by HIPIMS discharge to sputter clean the substrates prior to the coating deposition. The use of HIPIMS for surface pre treatment resulted in excellent adhesion, scratch test adhesion critical load value of Lc = 55 N on HSS and Lc = 68 N due to the local epitaxial growth and extremely smooth coating surface, Ra = 0.012 μm due to the elimination of growth defects.The coating crystallised in fcc structure with a preferred {220} orientation. XTEM analysis revealed a nanoscale multilayer structure of the coating with carbon segregated at the column boundaries but also vertically to form a lateral phase at the interfaces between the individual nanolayers.Addition of C to CrAlYN/CrN increased the chemical inertness between cutting tool and workpiece material without deteriorating the oxidation resistance of the coating. Thermo gravimetric analysis showed that the temperature for the onset of rapid oxidation was as high as 940 °C.In dry milling of AlSi9Cu1 alloy, CrAlYCN/CrCN coated 8 mm diameter cemented carbide end mills outperformed non coated end mills by factor of 2.5 with effective hindered built up edge formation mechanism.     

Effect of corrosion severity on fatigue evolution in Al-Zn-Mg-Cu

The effect of existing-localized corrosion on fatigue cracking of 7075-T6511 was established using crack surface marker-band analysis and a fracture mechanics model. The substantial reduction of fatigue life due to EXCO solution L-S surface pre-corrosion is nearly independent of exposure time after initial-sharp degradation, scaling with the evolution of pit-cluster size and initial stress intensity range with exposure time. Independent of exposure time, formation of a resolvable fatigue crack (∼10 μm) accounts for a similar-low (∼5%) fraction of total fatigue life at low stress range (σ_max = 150 MPa, R = 0.1). Crack formation occurs at microscopic protrusions into the corroded volume. A corrosion-modified-equivalent initial flaw size (CM-EIFS); predicted with the AFGROW tool using measured initial aspect ratio, initiation cycles, and total fatigue life inputs; accurately represents the corrosion damage effect on fatigue for a range of exposures. The similar deleterious effect of several corroding environments for various-exposed surfaces is described by a lower-bound CM-EIFS with a 300 μm depth and 1200 μm surface length suggesting fatigue is governed by a microscopic pit-based topography. Either an approximate lower-bound, or specific CM-EIFS calibrated by limited measurements of fatigue life for service-environment exposed specimens, can be used to assess the impact of corrosion in a damage tolerant framework. Complexities (e.g., local H embrittlement, 3D pit geometry, topography dependent initiation, and microstructure sensitive small-crack growth) do not compromise the CM-EIFS estimation, but must be better understood for refined modeling.     

Aluminum-rich TiAlCN coatings by Low Pressure CVD

Ti_1 − xAl_xN is a well established material for cutting tool applications exhibiting a high hardness and an excellent oxidation resistance. A main route for increasing the performance of Ti_1 − xAl_xN is the incorporation of further elements. Therefore the main objective of this work is to improve the properties and wear resistance of aluminum-rich CVD-TiAlN coatings by incorporating carbon. A new Low Pressure CVD process was employed for the deposition of a very aluminum-rich TiAlCN layers. The process works with a gas mixture of TiCl₄, AlCl₃, NH₃, H₂, N₂, Ar and ethylene as carbon source. In this work microstructure, composition, properties and cutting performance of CVD-TiAlCN coatings were investigated.Hard aluminum-rich TiAlCN coatings were obtained at 800 °C and 850 °C consisting of a composite of fcc-Ti_1 − xAl_xN and minor phases of TiN, h-AlN and amorphous carbon. WDX analysis indicates only a low carbon content < 2 at.%. Lattice constant calculations suggest that carbon atoms should not be incorporated in the Ti_1 − xAl_xN lattice. From TEM analysis and Raman spectroscopy it is evident that carbon is mainly located at the grain boundaries as a-C phase. Therefore these fcc-Ti_1 − xAl_xN(C) coatings with low carbon content are rather a composite of fcc-Ti_1 − xAl_xN and an amorphous carbon phase (a-C). At 900 °C the metastable fcc-Ti_1 − xAl_xN nearly disappears and co-deposition of TiN and h-AlN occurs. The layers deposited at 800 °C and 850 °C possess a high hardness around 3000 HV and compressive stress. CVD-TiAlCN coatings prepared at 850 °C shows also an amazing thermal stability under high vacuum conditions up to 1200 °C. Aluminum-rich composites fcc-Ti_1 − xAl_xN/a-C with x > 0.8 exhibit a superior cutting performance in different milling tests.     

Nanocarbon-dependent synthesis of ZrB2 in a binary ZrO2 and boron system

Thermodynamically, ZrO₂ may react with boron to form B₂O₃/B₂O₂ and ZrB₂ at room temperature. However, this reaction is incomplete at temperatures lower than 1550 °C, even with the use of metastable reactants, i.e., as-synthesized amorphous hydrous nano-ZrO₂ and amorphous boron powders. In this study, a complete disintegration of ZrO₂ was achieved by introducing nanocarbon to the binary system of ZrO₂ and boron at 1550 °C. The metastable reactants affected the temperature required for the solid-state reactions and also strongly affected the kinetics of the transformation. Single crystal and plate-like ZrB₂ particles with a uniform distribution and a size of ca. 1.0 μm in two-dimensions were obtained using 5 wt.% nanocarbon and a B/Zr molar ratio of 4.     

Coefficient of friction and wear of sputtered a-C thin coatings containing Mo

The paper reports on preparation of ~ 3000 nm thick a-C coatings containing Mo, interrelationships between their mechanical properties, a coefficient of friction μ and wear rate k and the effect of Mo content in the a-C coating on these interrelationships. The Mo-C coatings were prepared by sputtering using an unbalanced magnetron (UM) equipped with a graphite targets (∅ = 100 mm, 99.9% purity) fixed to the UM cathode with Mo ring of different inner diameter ∅_i. The content of Mo in the a-C coating was controlled by ∅_i. It is shown that μ and k of the coating strongly depend not only on its hardness H but also on its effective Young's modulus E^? = E/ (1 − ν²), the ratios H/E^?, H³/E^?2 and the elastic recovery W_e; here E is the Young's modulus and ν is the Poisson ratio. The ratio H³/E^?2 characterizes the resistance of coating to plastic deformation. Coatings with a low amount of Mo composed of nanograins of carbides dispersed in a-C matrix exhibit low values of μ ≈ 0.07 and k ≈ 10^− 7 mm³/Nm measured with WC ball at the rotation speed v = 0.05 m/s, total sliding length l = 1000 m and the load L = 2 N.     

Workpiece surface integrity when slot milling γ-TiAl intermetallic alloy

Slot milling is presented as a potential manufacturing route for aerospace component feature production when machining γ-TiAl intermetallic alloy Ti–45Al–2Mn–2Nb + 0.8 vol.% TiB₂XD using 2 mm diameter AlTiN coated WC ball nose end milling cutters. When operating with flood cutting fluid at v = 88 m/min, f = 0.05 mm/tooth, d = 0.2 mm, maximum flank wear was ∼65 μm after 25 min. SEM micrographs of slot surfaces show re-deposited/adhered and smeared workpiece material to a length of ∼50 μm. Brittle fracture of the slot edges was restricted to <10 μm with sporadic top burr formation observed up to ∼20 μm. Cross sectional micrographs of the slot sidewalls showed bending of the lamellae limited to within 5 μm.     

Grain boundaries between bismuth nanocrystals

Grain boundaries in thin Bi(1 1 0) films deposited on highly oriented pyrolytic graphite are investigated at atomic resolution using scanning tunnelling microscopy and high-resolution transmission electron microscopy. We find preferred misorientation angles Θ equal to 216°, 87°, 49°, 31°, 20°, 12° and 5°, the majority of which can be classified as large-angle boundaries. We find good agreement between the experimental results and a model of the tilt [1 1 0] grain boundary developed here. A method for estimating the surface unit cell based on measurement of dihedral angle in low-resolution images is also developed.     

An investigation on oxidation wear mechanisms of Ti-46Al-7Nb-0.7Cr-0.1Si-0.2Ni intermetallic-based alloys

The results of investigation on oxidation wear mechanism of Ti-46Al-7Nb-0.7Cr-0.1Si-0.2Ni-based intermetallic alloy are presented. Oxidation was carried out in air at temperatures: 900 °C, 925 °C and 975 °C taking into account the micro-geometry of surfaces being heated and oxidized. It was determined that the rise of surface roughness to Ra = 5.8 μm definitely reduces scale of chipping in higher temperature (975 °C). Investigation of the material structure of the specimen and chemical composition of oxidation products was performed. Possibilities of an increase in the heat resistance of the tested alloy by means of application of high roughness surfaces were shown.     

Selective single pulse femtosecond laser removal of alumina (Al2O3) from a bilayered Al2O3/TiAlN/steel coating

We studied surface modification of a double layer protective coating on steel induced by single fs laser pulse irradiation in ambient air. The outer alumina (Al₂O₃) layer, which protects against aggressive environments, was 1.7 μm thick and the titanium aluminum nitride (TiAlN) layer in contact with the steel surface had a thickness of 1.9 μm. The pulses (λ = 775 nm, τ = 200 fs) were generated by a Ti:sapphire laser source. The pulse energy was varied from 0.32 μJ to 50 μJ, corresponding to an incident laser fluence of 0.11 J cm^− 2 to 16.47 J cm^− 2. The surface damage threshold was found to be 0.20 J cm^− 2 and the alumina layer removal was initiated at 0.56 J cm^− 2. This selective ablation of alumina was possible in a wide range of fluences, up to the maximum applied, without ablating the TiAlN layer beneath.     

Erosion-oxidation behaviour of pack-aluminized 9% chromium steel under fluidized-bed conditions at elevated temperature

Pack-aluminized 9% chromium steel specimens were exposed to angular silica sand particles in a fluidized-bed erosion-oxidation rig for 200 h. The exposures were conducted in air at temperatures of 550 °C to 700 °C for particle impact angles of 30° and 90°, at speeds of 7.0-9.2 m s⁻¹. Subsequently, the mean thickness changes of the specimens were determined and the specimens were examined and analyzed by scanning electron microscopy and X-ray diffraction. The specimens experience only slight thickness changes for 30° angle impacts but significant material loss for 90° angle impacts, typical of a brittle erosion process. Under 30° angle impacts, the coatings were mostly retained on the substrate surface and slightly deformed. Thin oxide scales were detected on the surface at all test temperatures. Under 90° angle impacts, thickness losses increased with increase in speed and temperature up to 650 °C, resulting in complete loss of the coating in the test period. A porous, cracked, but continuous, oxide scale was observed on the surface of the exposed substrate. At 700 °C, the coating was partially retained on the substrate, with the residual coating thickness decreasing with increase in speed. Explanations for these observations are presented, the interactions between the erosion and oxidation processes for the specimens are discussed and the degradation mechanisms for the coatings under the test conditions are described in this paper.