Nanoindentation response of high performance fullerene-like CNx

Amorphous carbon nitride (CN_x) coatings are now being developed for a range of applications, e.g. as a protective top layer for hard disks or as a coating to reduce the friction between synthetic joints in the human body. The purpose of this work is to assess the mechanical properties of the latest generation of fullerene-like CN_x deposited on different substrates in order to expand the number of potential applications. Samples of CN_x on four different substrates have been studied using quasistatic nanoindentation with a wide range of peak loads, from 500 μN to 500 mN and dynamic nanoindentation for peak loads from 100 μN to up to 10 mN. Improved deposition techniques generate samples with extremely high values of hardness/Young's modulus; in some cases greater than 0.4 which is not achieved by any other hard material. Adhesion and fracture resistance are comparable to or better than that of traditional high hardness coatings, such as SiC and TiN, on similar substrates. The sample of CN_x on titanium showed differences in hardness and Young's modulus at low loads, where the influence of the substrate is negligible, compared to coatings deposited on other substrates. This arises due to the fact that Ti from the substrate may have diffused into the coating in the deposition process creating a sort of C–N–Ti high hardness layer which would have some advantages of both the fullerene-like and traditional hard coating systems.     

Processing and Characterization of Plasma Spray Coatings of Glass Microspheres Premixed with Al2O3 Particles

Various ceramics and metals are being deposited as functional, protective, and near-homogenous coatings on engineering components by exploiting the characteristic properties of plasma medium. Such coatings are known to exhibit improved wear, thermal, and corrosion resistance. Although a lot of studies have been reported on coatings made up of a large number of metals and ceramic particles, hardly any effort is made to coat glass microspheres on metals despite their high hardness. In view of this, the present work was undertaken to study the preparation and characterization of a new class of coatings made up of borosilicate glass microspheres (BGM) premixed with micro-sized aluminum oxide (Al₂O₃) in different proportions. Deposition of these BGM and BGM/Al₂O₃ coatings is carried out at five different levels of torch input power. Coatings are characterized in terms of their thickness, hardness, adhesion strength, and porosity. The coatability of BGM and the BGM/Al₂O₃ mixture on metallic substrates is assessed by evaluating the coating deposition efficiency. This work reveals that the torch input power and the Al₂O₃ content in the feedstock affect the major coating characteristics, and premixing of Al₂O₃ with BGM results in better coating properties.     

Hard transparent dielectric coatings

Hard transparent corrosion-resistant dielectric coatings were investigated (a) for application to the direct protection of glass surfaces, (b) for application to the protection of thin film metallic window coatings used for reflection or transmission of solar radiation and (c) for use in codeposited metal-ceramic coatings. The dielectric coatings were Al₂O₃, SiO₂ and SiC in the thickness range 100–100 000 Å. The coatings were deposited under varying conditions of substrate temperature, gas composition, r.f. power and substrate preparation. The results of the following measurements on the dielectric coatings are presented: diamond pyramid hardness tests, optical reflectance and transmittance in the region 0.6–4.0 eV and corrosion resistance tests under cyclic conditions. The results showed a considerable improvement in the scratch resistance and hardness of glass by layering of hard dielectric coatings 2–5 μm thick.     

Mechanical properties and scratch resistance of filtered-arc-deposited titanium oxide thin films on glass

The mechanical properties and the scratch resistance of titanium oxide (TiO₂) thin films on a glass substrate have been investigated. Three films, with crystalline (rutile and anatase) and amorphous structures, were deposited by the filtered cathodic vacuum arc deposition technique on glass, and characterized by means of nanoindentation and scratch tests. The different damage modes (arc-like, longitudinal and channel cracks in the crystalline films; Hertzian cracks in the amorphous film) were assessed by means of optical and focused ion beam microscopy. In all cases, the deposition of the TiO₂ film improved the contact-mechanical properties of uncoated glass. Crystalline films were found to possess a better combination of mechanical properties (i.e. elastic modulus up to 221 GPa, hardness up to 21 GPa, and fracture strength up to 3.6 GPa) than the amorphous film. However, under cyclic sliding contact above the critical fracture load, the amorphous film was found to withstand a higher number of cycles. The results are expected to provide useful insight for the design of optical coatings with improved contact-damage resistance.     

Impact of Al and Cr alloying in TiN-based PVD coatings on cutting performance during machining of hard to cut materials

The microstructure and electronic structure of nanodispersed (Al₆₇Ti₃₃N) and (Ti₁₀Al₇₀Cr₂₀N) PVD coatings were investigated by high-resolution transmission electron microscopy (HRTEM) and electron spectroscopy techniques: X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS). The grain size measured by HRTEM was 5-20 nm for Al₆₇Ti₃₃N and TiAlCrN type coatings, correspondingly. The spinodal decomposition of Al supersaturated nitrides under its deposition was found. Chromium increases the metastable solubility of h-AlN in c-Ti_1 − xAl_xN. The lifetime of the cutting tools with Al-rich coatings has been evaluated under ball nose end milling of hardened tool steel H13 (HRC 50-52) and aerospace materials. TiAlCrN coatings are preferable for steel machining, and AlTiN coatings are better for aerospace material processing. It was found that AlTiN coating has lower hardness but higher plasticity and improved impact fatigue fracture resistance. The TiAlCrN coating has much better hot hardness and oxidation stability at high temperatures, but it is stiffer than AlTiN.     

A new high temperature resistant glass-ceramic coating for gas turbine engine components

A new high temperature and abrasion resistant glass-ceramic coating system (based on MgO-Al₂O₃-TiO₂ and ZnO-Al₂O₃-SiO₂ based glass systems) for gas turbine engine components has been developed. Thermal shock resistance, adherence at 90°-bend test and static oxidation resistance at the required working temperature (1000°C) for continuous service and abrasion resistance are evaluated using suitable standard methods. The coating materials and the resultant coatings are characterized using differential thermal analysis, differential thermogravimetric analysis, X-ray diffraction analysis, optical microscopy and scanning electron microscopy. The properties evaluated clearly showed the suitability of these coatings for protection of different hot zone components in different types of engines. XRD analysis of the coating materials and the resultant coatings showed presence of a number of microcrystalline phases. SEM micrographs indicate strong chemical bonding at the metal-ceramic interface. Optical micrographs showed smooth glossy impervious defect free surface finish.     

Influence of composition and processing parameters on mechanical properties and erosion response of Ni–TiB2 coatings

Abstract

Sputtered Ni–TiB₂ coatings have been shown to protect Ti–6Al–4V and Inconel 718 substrates from solid particle erosion. However, before new erosion resistant coatings can be efficiently designed, it is essential that the role of mechanical properties in determining erosion resistance be fully understood. In this investigation, nanoindentation techniques were used to quantify the effects of substrate preparation, coating composition, and sputtering process parameters on the elastic moduli and indentation hardness of thin coatings deposited on Ti–6Al–4V and Inconel 718 substrates. The influence of these parameters on coating adhesion was determined using a conventional scratch test. Elastic moduli, indentation hardnesses, and coating adhesion were correlated with erosion behaviour. The erosion resistance of the coatings that exhibited microscopic ductility is dependent on the nodule diameter and coating properties such as hardness, elastic modulus, and fracture toughness.

MST/1697     

Combined Langmuir-Blodgett and sol-gel coatings

Optical properties and in-depth structure of double-layer coatings on glass substrates were investigated. One of the layers was prepared by dip coating either from silica or titania sol, the other layer was made from ca. 130 nm Stöber silica particles by the Langmuir-Blodgett (LB) technique. Two different types of combined coatings were prepared: (1) nanoparticulate LB films coated with sol-gel (SG) films and (2) nanoparticulate LB films drawn onto SG films.Scanning electron microscopy and optical methods, i.e. UV-vis spectroscopy and scanning angle reflectometry were applied for analyzing the structure and thickness of coatings. These measurements revealed that the precursor sols could not penetrate into the particulate LB film completely in case of coating type (1). For coating type (2) very little overlap between the SG and LB layers was found resulting in significant improvement of light transmittance of combined coatings compared to single SG films.To show some possible advantages of the combination of these techniques additional studies were carried out. Surface morphology of combined coatings (1) was studied by atomic force microscopy. Surfaces with different roughness were developed depending on the thickness of the sol-gel film (titania: ca. 70 nm; silica: ca. 210 nm). The adhesive peel off test revealed improved mechanical stability of combined coatings (2), in comparison to LB films which makes them good candidates for further applications.     

Chemomechanical control of sliding friction behaviour in non-metals

Recent work has shown that surface active environments can be used to significantly and predictably influence the near-surface flow behaviour (i.e. hardness) of such solids as magnesium oxide, calcite, alumina, quartz and soda-lime (s.l.) glass. Specifically, these solids are hardest in environments in which theirζ-potential is approximately zero. The results of the present study demonstrate that such chemomechanical effects can, under certain conditions, also be used to affect and control sliding friction behaviour. In particular, it is shown that for magnesium oxide and s.l. glass in various environments the coefficient of frictionμ _f is a minimum when ζ ? 0. This and other results are described, and some mechanistic and practical implications discussed.     

Carbon,carbide and silicide coatings

The elements carbon and silicon are the significant constituents of the refractory coating materials discussed in this paper, namely B₄C, pyrocarbon, TiC, SiC, Si₃N₄ and MoSi₂. Because of their hardness and their chemical and oxidation resistance, these materials are of increasing interest as coatings on various substrates. However, their physical and chemical properties limit the number of suitable plating methods. Whilst the CVD technique is the most versatile, the sintering technique is a good alternative, especially if thicker coatings are required.The different coating materials are discussed in detail. The ways in which different coatings can be modified for special applications, e.g. isotropic pyrocarbon coatings for bioengineering purposes, are discussed. Pure silicon carbide and silicon nitride coatings become important as protection against internal oxidation and as barriers in carbon fibre reinforced composites.The problems arising from the simultaneous deposition of two elements to form a specific compound are demonstrated with the examples of SiC, Si₃N₄ and B₄C. The necessity of avoiding chemical reactions with the substrate during deposition is exemplified by the case of TiC coatings on carbon fibres.Finally the technique of preparing metal silicide coatings on refractory metals and carbon by plasma spraying and isostatic hot pressing as protection against oxidation is explained, and the modification of the coatings, in particular to improve thermal cycling behaviour, is discussed.     

Electrical and mechanical properties of nano-structured TiN coatings deposited by vacuum cold spray

Titanium nitride (TiN) coatings were fabricated by vacuum cold spray (VCS) process at room temperature with nano-sized starting powder (about 20 nm in size). The microstructure of the powder and coating was examined by scanning electron microscope and X-ray diffraction. The porosity and pore distribution of the VCS TiN coatings were measured by the N₂ adsorption-desorption method. The microhardness and fracture toughness of the coatings were evaluated by using the micro-indentation technique. The sheet resistance and electrical resistivity of the coatings were characterized by the four-point probe method. The results show that the sheet resistance of coatings is significantly reduced from 13565 to 127 Ω with increasing the coating thickness. A minimum electrical resistivity of 1.8 × 10⁻³ Ω m is achieved. The VCS TiN coatings with high porosity ranging from 58.3 to 67.6% exhibit low hardness of 279-490 HV and relatively good fracture toughness of about 3.12 MPa m^1/2.     

Properties of Al2O3 nano-particle reinforced copper matrix composite coatings prepared by pulse and direct current electroplating

Cu–Al₂O₃ nano-composite coatings have high potential for use in applications in which high mechanical properties together with high corrosion resistance are required. In the present study it is intended to produce copper nano-alumina composite coatings with various nano-alumina contents in order to investigate the effect of alumina reinforcement particles on corrosion resistance and mechanical properties such as hardness and wear resistance. The composite coatings were deposited using direct current (DC) and pulse current (PC) plating. The microstructures of the coatings produced from both methods were examined via scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The wear behaviors, micro hardness, coating thickness, corrosion rate and coating porosity were examined using appropriate methods. Compared to DC deposition, PC plating facilitated higher amounts of particle incorporation with more uniform distribution. The results indicated that the mechanical properties of the applied coatings with incorporated nano-alumina reinforcement were far more superior as compared to its own matrix as well as non-composite copper coatings. It was also found out that increasing the amount of nano-alumina content in the coating, led to enhanced general properties of the coatings.     

Investigating the wear characteristics of engineered surfaces: low-temperature plasma nitriding and TiN + MoS x hard-solid lubricant coating

Significant progress has been made in the past decade in plasma nitriding with a majority of the research work focusing on improving hardness and wear resistance of the nitrided surface through the reduction of nitriding temperature, pressure or time. Hard-solid lubricating coatings have also been extensively studied for lowering the wear rate and coefficient of friction of traditional hard coatings such as TiN by the combined effect of hardness and solid lubrication. In this study, the wear characteristics of low-temperature plasma-nitrided steel substrate performed using a Saddle-field fast atom beam source and TiN + MoS _x hard-solid lubricant coating deposited by a closed-field magnetron-sputtering technique have been investigated. The thin hard layer in plasma-nitrided substrates exhibited much higher hardness and lower wear compared to the untreated substrate in pin-on-disc wear testing. In addition, the study of the wear track morphology of the nitrided samples evidenced significant reduction of deeper ploughing and plastic deformation due to higher hardness and load supporting of the nitrided layer. On the other hand, due to the incorporation of MoS₂ in TiN coating, the wear resistance and coefficient of friction were greatly improved in TiN + MoS _x coating compared to pure TiN coating. In contrast to TiN coating, a relatively smoother wear track with less abrasive wear also supported the beneficial effects of adding MoS₂ in TiN coating.     

Scratch resistant optical coatings on polymers by magnetron-plasma-enhanced chemical vapor deposition

The magnetron-plasma-enhanced chemical vapor deposition (magPECVD) provides silicon-organic thin films for optical, electrically insulating or diffusion barrier coating applications. With process pressures of ≤ 1 Pa this technology is well adapted to the sputtering process of optical interference coatings and also facilitates an inline-process implementation. This paper describes the deposition process for scratch resistant coatings on polycarbonate (PC) and allyl diglycol carbonate substrates. Based on the optical, chemical and mechanical characterization of single magPECVD thin films of varied chemical composition, several types of layer stacks (e.g. of gradient or alternating hardness distribution) were deposited with varied total thickness on PC substrates. Abrasion test results indicate two main effects: the resistance against scratches of high load abrasion (50 N) mainly depends on the total coating thickness. The durability against scratches of low load abrasion (5 N) shows a clear advantage for the multilayer design in contrast to homogeneous single layers even of higher thickness. Finally a 5-layer antireflective system was reactively sputtered onto the magPECVD coating and successfully passed adhesion and environmental tests.     

Interfacial study of magnesium-containing fluoridated hydroxyapatite coatings

Magnesium-containing fluoridated hydroxyapatite (Mg_xFHA) coatings have been developed to improve the biological performances of fluoridated hydroxyapatite (FHA) coatings. The coatings are deposited on Ti6Al4V substrates via a sol-gel process. The interface between the coating and substrate is characterized by scanning electron microscopy, glow discharge optical emission spectroscopy and X-ray photoelectron spectroscopy for coating thickness, elemental distribution and chemical states. Pull-off test is used to evaluate the adhesion strength. The results show that the interdiffusion of elements happens at the coating/substrate interface. The incorporation of Mg ions into FHA coatings enhances the pull-off adhesion strength between the coating and the substrate, but no significant difference is observed with different Mg concentrations.     

Optical properties of hydrogenated hard carbon thin films

Hydrogenated amorphous carbon (a-C:H) films were deposited onto glass, silicon and germanium substrates. The films are transparent in the IR and are extremely hard (Mohs' hardness of about 8). The a-C:H coatings were prepared in an r.f.-excited discharge sustained by various hydrocarbon gases.The thickness, density, refractive index (at 0.3 μm and 2–10 μm) and relative hydrogen content were determined. Variations in the IR refractive index and the relative hydrogen content could be correlated with the deposition conditions. With a refractive index of approximately 2 a-C:H is an ideal antireflection coating for germanium (n = 4).Laser calorimetric measurements of optical absorption at 10.6 μm give a loss as low as 3% for a coating 1.3 μm thick on germanium (λ/4 for n = 2 at 10.6 μm).     

Mechanical,tribological and anti-corrosive properties of polyaniline/graphene coated Mg-9Li-7Al-1Sn and Mg-9Li-5Al-3Sn-1Zn alloys

The mechanical, tribological and corrosion protection offered to Mg-9Li-7Al-1Sn and Mg-9Li-5Al-3Sn-1Zn alloys by the epoxy coating containing polyaniline/graphene (PANI/Gr) pigments is undertaken in the current work. PANI/Gr containing coatings were observed to be strongly adherent with a higher scratch hardness (H_s) and plowing hardness (H_p), i.e. H_s of 0.43 GPa, and H_p of 0.61 GPa, respectively when compared to that of neat epoxy coating (H_s of 0.17 GPa, and H_p of 0.40 GPa, respectively). Due to their higher H_s and H_p values, PANI/Gr based coatings displayed an enhanced wear resistance (Wear volume, W_v = 4.53 × 10^-3 m³) than that of neat epoxy coating (W_v = 5.15 × 10⁻³ m³). The corrosion protection efficiency in corrosive environment of 3.5 wt% NaCl solution was obtained to be >99% for PANI/Gr containing coatings when compared to that of neat epoxy coating. The charge-transfer resistance (R_ct) of the PANI/Gr containing coatings were estimated to be >10⁶ Ω cm², which indicates their highly protective nature when compared to that of neat epoxy coating (R_ct ˜10⁵ Ω cm²). Hence, PANI/Gr containing coatings can be potentially used for wear resistance and corrosion protection applications in marine environments.     

Optimising deposition parameters of W-DLC coatings for tool materials of high speed steel and cemented carbide

The paper presents the results of investigations on the selection of optimal deposition parameters for W-DLC coatings produced by pulsed reactive magnetron sputtering in order to obtain the coatings with high adhesion to the high speed steel (HSS) and cemented carbide (HM) substrates. To optimise the deposition parameters for W-DLC coatings Taguchi's method was used. An acetylene flow rate, the substrate bias voltage, the thickness of W-DLC coatings and the thickness of chromium sublayer were selected as deposition parameters and for each of them three levels of values were determined. Adhesion, wear resistance and hardness of coatings were chosen as the criteria for selecting the optimum deposition parameters. The test results showed that all the selected parameters have a significant effect on the adhesion of coatings. The thickness of W-DLC coatings has a very significant effect on the adhesion to HSS substrates and in a case of HM substrates the same effect has the thickness of a chromium sublayer. The wear resistance of coatings increases with an increasing acetylene flow rate and decreases with the increasing substrate bias voltage. High correlation was found between the H/E ratio of coatings and their wear resistance as well as toughness. The W-DLC coating showing the best properties (L_C3 ≈ 90 N, k_V = 3.8 × 10⁻⁷ mm³/Nm) was marked by the nanocomposite structure containing about 40% of nanocrystalline tungsten carbide phase and 60% of amorphous carbon matrix.     

Hardness and indentation-induced damage of SiC-coated graphite

The hardness and indentation damage of chemical vapour-deposited SiC coating/graphite substrate composites have been studied. Experimental results indicate that hardness is a non-linear function of coating thickness, and is not significantly affected by the changes in the magnitude of the residual stresses in SiC coating. The size of indentation lateral crack, observed using optical microscopy, varies with coating thickness. Acoustic emission spectra show that thinner coatings suffer more extensive fracture as compared with thicker coatings.     

Effect of ZrB2 and Polyvinyl Butyral Content on the Oxidation Resistance of ZrB2–SiC Coatings Produced by Slurry Brushing Method

ZrB₂–SiC coatings are prepared on the surface of graphite by slurry brushing method to improve the oxidation resistance. Effects of ZrB₂ content and polyvinyl butyral (PVB)–ethanol solution concentration on microstructure and static oxidation behavior of the ZrB₂–SiC coatings are investigated at 1200 °C in air. The results indicate that increasing ZrB₂ content improves the oxidation resistance of the coatings. When ZrB₂ content increases from 30 to 45 wt%, weight loss rates of the coated samples after oxidation at 1200 °C for 120 min decrease from −0.92% to −1.67%. Increasing binder solution concentration raises component content in the coatings. As PVB–ethanol binder concentration increases from 0.025 to 0.075 g mL⁻¹, weight loss rates of the coated samples after oxidation at 1200 °C for 120 min decrease from 0.32% to −0.38%. Excellent oxidation resistance of ZrB₂–SiC coating is attributed to self-sealing ability of B₂O₃ and borosilicate glass. The composite glass can inhibit oxygen diffusion by filling defects in the coating promptly. The borosilicate glass phase can enhance the fluidity of the composite glass. ZrO₂ and ZrSiO₄ particles restrict the growth of the microcrack, which improves the oxidation resistance of ZrB₂–SiC coating.