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.     

Influence of substrate material and ion bombardment on plasma-deposited fluorocarbon thin films

The influence of substrate material and ion bombardment on fluorocarbon thin films deposited using a C₂F₆ glow discharge in an rf, parallel plate reactor was investigated. Monitoring of the plasma process by optical emission spectroscopy indicated that the dominant species in the glow discharge was CF₂. Studies of bulk polytetrafluoroethylene (PTFE) and plasma-polymerized fluorocarbon thin-film samples in an XPS system demonstrated that the formation of non-CF₂ species can be induced by ion bombardment of CF₂ molecules. Characterization of the deposited fluorocarbon films by XPS found that the F/C ratio and CF_x distribution (0 < x < 3) in the films were dependent on processing conditions. Fluorocarbon films deposited simultaneously onto Al, glass, steel, and PTFE substrates using a C₂F₆ plasma and a graphite sputter target had measurably different F/C ratios, with the F/C ratio of the films deposited onto the Al substrates consistently lower than the F/C ratios of the films deposited onto the other substrates. When a C₂F₆ plasma was used without a graphite target, the F/C ratio in the film was constant, but the CF_x distribution was different for each of the substrate materials. Analysis of the plasma-polymerized films by TEM revealed that localized growth of fluorocarbon particles occurred during the initial stages of deposition, consistent with an activated growth mechanism. Differences in the F/C ratio for films deposited onto the various substrate materials were attributed to the interaction of the fluorocarbon plasma with the exposed surface of the substrate prior to complete coverage by the polymeric film. © 1994 John Wiley & Sons, Inc.     

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.     

Comparison of experimental and theoretical transverse elastic modulus of carbon fibers

The transverse elastic modulus of PAN-based carbon fibers as measured by experimental methods, calculated from theoretical equations and analyzed by the finite element method (FEM) is discussed. Raman spectroscopy was the primary method utilized to measure the transverse elastic modulus of carbon fibers in carbon-fiber reinforced plastics (CFRP). A lead oxide (PbO) thin film was deposited on the surface of a CFRP specimen using physical vapor deposition as the pretreatment in order to measure the strains of the carbon fibers and epoxy matrix phases by Raman spectroscopy. Since the relation between the Raman peak wave number of PbO thin films and tensile strain has already been developed, the transverse strain of the carbon fibers could be measured. The transverse strain of the carbon fibers was analyzed using a 2-D FEM model. The transverse modulus of the carbon fibers was determined by fitting the experimental result from Raman spectroscopy to the FEM model. The determined transverse modulus (10.4 GPa) is compared with those experimentally measured by nanoindentation (13.4 GPa), numerically analyzed using 2-D and 3-D FEM models (5.25 GPa and 28.7 GPa, respectively), and theoretically calculated from the Mori-Tanaka, Halpin-Tsai, and Uemura equations (24.8 GPa, 17.4 GPa, and 28.4 GPa, respectively).     

Mechanical properties of MWPECVD diamond coatings on Si substrate via nanoindentation

The mechanical properties of polycrystalline diamond coatings with thickness varying from 0.92 to 44.65 μm have been analysed. The tested samples have been grown on silicon substrates via microwave plasma enhanced chemical vapour deposition from highly diluted gas mixtures CH₄-H₂ (1% CH₄ in H₂). Reliable hardness and elastic modulus values have been assessed on lightly polished surface of polycrystalline diamond films.The effect of the coating thickness on mechanical, morphological and chemical-structural properties is presented and discussed. In particular, the hardness increases from a value of about 52 to 95 GPa and the elastic modulus from 438 to 768 GPa by varying the coating thickness from 0.92 to 4.85 μm, while the values closer to those of natural diamond (H = 103 GPa and E = 1200 GPa) are reached for thicker films (> 5 μm). Additionally, the different thickness of the diamond coatings permits to select the significance of results and to highlight when the soft silicon substrate may affect the measured mechanical data. Thus, the nanoindentation experiments were made within the range from 0.65% to 10% of the film thickness by varying the maximum load from 3 to 80 mN.     

Substrate effect on wear resistant transition metal nitride hard coatings: Microstructure and tribo-mechanical properties

Four types of different hard transition metal nitrides (TMN:ZrN, CrN, WN and TiN) coatings were deposited on Si (100) and 316LN stainless steel substrates using DC magnetron sputtering. A comprehensive study of microstructure and substrate dependent tribo-mechanical properties of TMN coatings was carried out. Higher hardness (H) and elastic modulus (E) were obtained for WN (H=40 GPa and E=440 GPa) and TiN (H=30 GPa and E=399 GPa) coatings. This is related to the formation of (100) and (111) preferred orientations in WN and TiN coatings, respectively. However, the less hardness and elastic modulus were obtained for ZrN and CrN coatings where (200) orientation is preferred. Remarkably, low friction coefficient (0.06–0.57) and higher wear resistance in the coatings deposited on steel substrates are directly associated with the higher resistance to plastic deformation (H³/E²) and the presence of intrinsic compressive stress. Three body wear modes enhanced the friction coefficient (0.15–0.62) and the wear rate in the coatings deposited on Si substrates. This is primarily associated with low fracture toughness of brittle single crystalline Si (100) substrates. Steel-on-steel contact was dominated in ZrN/steel sliding system. This occurs due to the severe adhesive wear mode of steel ball, whereas, the abrasive wear modes were attained for the CrN, WN and TiN coatings sliding against steel balls.     

Mechanical and tribological properties of silicon carbide coating on Inconel alloy from liquid pre-ceramic precursor

Liquid polycarbosilane (LPCS) derived hard coatings of silicon carbide (SiC) were deposited on Inconel alloy at three different moderately high temperatures by chemical vapour deposition. The deposited films were characterized by X-ray diffractometry and Field emission scanning electron microscopy. Liquid PCS yielded a mixture of α-SiC and β-SiC during decomposition having uniform round-shaped particles of dimension around 200–300 nm without extensive cracking and few discrete shaped particles were also found to form at higher temperature (i.e. 1100 °C and 1200 °C) deposited films. The coated samples showed substantial increment in hardness and fracture toughness as compared to the uncoated sample. The fracture toughness (K_IC) values of the deposited films were in the range of 6.7–10.7 MPa(m)^1/2. The tribological properties and hardness of the films were also found to vary with deposition temperature. The scratch tracks of the films revealed that brittle failures occurred in all SiC coated substrates.     

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.     

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.     

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.     

Influence of Si-addition on wear and oxidation resistance of TiWSixN thin films

TiWSi_xN films were deposited using a magnetron co-sputtering system on silicon (111), 316L stainless steel, and M2 high-speed steel substrates. The silicon target current density was varied from 0 mA/cm² to 4.32 mA/cm² in order to modify the Si content in the films. The microstructure and chemical composition were determined by means of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. The surface of the films was explored via scanning electron microscopy (SEM) and atomic force microscopy (AFM). Mechanical, tribological, and thermal properties were investigated by means of the nanoindentation, ball-on-disc, and cyclic oxidation tests, respectively. Our results indicated that as the silicon target current density was increased, the microstructure changed from crystalline to amorphous, and the hardness and elastic modulus improved from initial values of 7.5 ± 0.3 GPa and 181 ± 8 GPa to 15 ± 1 GPa and 229 ± 9 GPa, respectively. Furthermore, films deposited at high silicon target current exhibited better resistance to thermal oxidation. The failure mechanism of the WTiSi_xN thin films under cyclic oxidation was attributed to the microstructure of the films, WO₃ sublimation, and the thermal coefficient mismatch between the film and the substrate.     

Nitrogenated carbon films deposited using filtered cathodic arc

Nitrogenated carbon films were deposited on various substrates using filtered cathodic arc. Non-uniformity of the film thickness was less than 5% over a 15 cm diameter area. Mechanical, optical (refraction index, extinction coefficient versus wavelength) and electrical properties were investigated as a function of nitrogen flow rate. Deposited coatings demonstrated high hardness of 40–65 GPa, Young's modulus 200–285 GPa, excellent elastic recovery, high critical pressure for scratch formation, and surface smoothness. While the hardness showed a relatively small decrease with nitrogen flow increase, the stress decrease was more significant (8–3.8 GPa). Extremely low wear rates were observed, even at high contact pressures, and no substantial debris was detected indicating that carbon is oxidized during wear. Clear correlation was found between transparency, electrical resistivity and stress of the films. Transparency and resistivity showed a significant rise with an increase of stress. An explanation of the film properties is based on the assumption that the basic characteristics of the deposited films were determined by the relative proportion of two three dimensional complementary type of bonds; the tetrahedral sp³ bonds leading to stiff networks, and the trigonal sp² arrangments close to fullerene-like, or nanotube-like, structures.     

Mechanical Properties and Microstructural Characterization of Amorphous SiCxNy Thin Films After Annealing Beyond 1100°C

The effect of thermal annealing on structure and mechanical properties of amorphous SiC_xN_y (y ≥ 0) thin films was investigated up to 1500°C in air and Ar. The SiC_xN_y films (2.2–3.4 μm) were deposited by reactive DC magnetron sputtering on Si, Al₂O₃ and α‐SiC substrates without intentional heating and at 600°C. The SiC target with small excess of carbon was sputtered at various N₂/Ar gas flow ratios (0–0.48). The nitrogen content in the films changes in the range 0–43 at.%. Hardness and elastic modulus (nanoindentation), change in film thickness, film composition, and structure (Raman spectroscopy, XRD) were investigated in dependence on annealing temperature and nitrogen content. All SiC_xN_y films preserve their amorphous structure up to 1500°C. The hardness of all as‐deposited and both air‐ and Ar‐annealed SiC_xN_y films decreases with growth of nitrogen content. The annealing in Ar at temperatures of 1100°C–1300°C results in noticeable hardness growth despite the ordering of graphite‐like structure in carbon clusters in nitrogen free films. Unlike the SiC, this graphitization leads to hardness saturation of SiCN films starting above 900°C, especially for films with higher nitrogen content (deposited at higher N₂/Ar). This indicates the practical hardness limit achievable by thermal treatment for SiC_xN_y films deposited on unheated substrates. The ordering in carbon phase is facilitated by the presence of nitrogen in the films and its extent is controlled by the N/C atomic ratio. The suppression of graphitization was observed for N/C ranging between 0.5–0.7. Films deposited at 600°C show higher hardness and oxidation resistance after annealing in comparison with those deposited on unheated substrates. Hardness reaches 40 GPa for SiC and ~28 GPa for SiC_xN_y (35 at.% of nitrogen). Such a high hardness of SiC film stems from its partial crystallization. Annealing of SiC_xN_y film (35 at.% of N) in Ar at 1400°C is accompanied by formation of numerous hillocks (indicating heterogeneous structure of amorphous films) and redistribution of film material.     

Nanoindentation and nanoscratching studies of amorphous carbon films

Hydrogen-free amorphous carbon (a-C) films prepared by RF magnetron sputtering were deposited on Si substrates in thin films, at various negative bias voltages V_b (i.e. Ar-ion energies), and in thick layered-structure films with alternative values of V_b. The main purposes of this work are to present preliminary results concerning the effect of Ar-ion bombardment during deposition on the elastic properties of thin a-C films with Ar⁺ energies in the range 30–200 eV, and the adhesion failure which limits their thickness and usefulness for practical applications, and the enhancement of hardness and scratch resistance of sputtered a-C films developed in a layered structure. The results show a significant improvement in the elastic properties of layered structure films and their stability. The combination of high hardness and relative low elastic modulus which the layered films exhibit make them more resistant to plastic deformation during contact, as confirmed by scratch testing.     

Structure and mechanical properties of W incorporated diamond-like carbon films prepared by a hybrid ion beam deposition technique

W incorporated diamond-like carbon films were prepared on silicon(1 0 0) wafers using a hybrid deposition system composed of an end-Hall-type hydrocarbon ion gun and a tungsten DC magnetron sputter source. The W concentration in the films was controlled by changing the fraction of Ar in the Ar and C₆H₆ reaction gas. The chemical composition, atomic bond structure, and mechanical properties were investigated for W concentrations ranging from 0 to 8.6 at.%. When the W concentration was <2.8 at.%, the W atoms were dissolved in the amorphous carbon matrix without forming a WC_1−x phase. Amorphous and crystalline WC_1−x nano-particles appeared when the W concentration was >2.8 and >3.6 at.%, respectively. It was found that the hardness and elastic modulus were not sensitive to the W concentration in this concentration range. On the other hand, the residual compressive stress was strongly dependent on the chemical state of the incorporated W atoms. The change in mechanical properties is discussed in terms of the microstructural changes induced by W incorporation.     

Thermal chemical vapor deposition of fluorocarbon polymer thin films in a hot filament reactor

Formation of fluorocarbon polymer films with a linear (CF₂-CF₂)_n molecular structure similar to polytetrafluoroethylene, PTFE is described by a hot filament chemical vapor deposition method. Growth process is analyzed by infrared absorption and C(1s), O(1s) and F(1s) core level electron spectroscopy of films deposited at −5 and +70 °C. Absorption doublet at 1220 and 1160 cm⁻¹ assigned to C-F₂ asymmetric and symmetric stretches, rock at 518 cm⁻¹ and wag at 637 cm⁻¹ indicate formation of linearly organized CF₂ groups with minimum hindrance to molecular vibration modes in CVD grown films. Absorption bands at 1660 and 3389 cm⁻¹ show O and OH groups in the films which diminish on annealing. The C(1s) components, CF₃, CF and C-CF bonding show branching, cross-liking and defects sites which increase as substrate temperature is increased. The O(1s) line analysis shows O₂ in fluorocarbon films is chemically bonded as C-O and F₂CO with relative ratio depending on the film growth temperature. Both O₂ and OH are the result of additional reaction pathways involving the species generated from fragmentation of CF₃C(O)F. Molecular structure of fluorocarbon polymer films involving these species are discussed which are in conformity with the XPS and IR absorption data.     

Studies on characterization and cryogenic mechanical properties of polyimide-layered silicate nanocomposite films

Polyimide/layered silicate nanocomposites were prepared via in situ polymerization process from PMDA-ODA and organo-MMT in a solution of N,N-dimethylacetamide. XRD, FTIR, UV-vis analyses showed that at the content of 1 wt% MMT, MMT were well intercalated, exfoliated and dispersed in polyimide matrix. As the MMT content is more than 3 wt%, MMT agglomerates became severe as shown by the transparency and transmittance of the hybrid films. The cryogenic mechanical properties of the films at 77 K were studied and compared with those at room temperature. The cryogenic tensile strength showed the highest value at 1 wt% MMT content, and its strength, modulus and elongation at break were simultaneously increased than the pure PI film. The cryogenic elastic modulus exhibited an increasing trend until the MMT content reached 10 wt%. The cryogenic failure strain of hybrid films with 1-3 wt% MMT contents was greater than 10%, showing good ductility at 77 K. The tensile strength and modulus of the hybrid films at 77 K were generally higher than those at room temperature except at 20 wt% MMT for the strength.     

Structural and mechanical properties of NbN and Nb-Si-N films: Experiment and molecular dynamics simulations

The structural and mechanical properties of NbN and Nb-Si-N films have been investigated both experimentally and theoretically, in their as-deposited and annealed states. The films were deposited using magnetron sputtering at substrate bias (U_B) between 0 and −70 V. While NbN films were found to crystallize in the cubic δ-NbN structure, Nb-Si-N films with Si content of 11–13 at% consisted of a two-phases nanocomposite structure where δ-NbN nanocrystals were embedded in SiN_x amorphous matrix. Films deposited at U_B=0 V were highly (001)-textured. Application of substrate bias potential led to a depletion of light atoms, and caused a grain size refinement concomitantly with the increase of (111) preferred orientations in both films. The maximum hardness was 28 GPa and 32 GPa for NbN and Nb-Si-N films, respectively. NbN and Nb-Si-N films deposited at U_B=−70 V exhibited compressive stress of −3 and −4 GPa, respectively. After vacuum annealing, a decrease in the stress-free lattice parameter was observed for both films, and attributed to alteration of film composition. To obtain insights on interface properties and related mechanical and thermal stability of Nb-Si-N nanocomposite films, first principles molecular dynamics simulations of NbN/SiN_x heterostructures with different structures (cubic and hexagonal) and atomic configurations were carried out. All the hexagonal heterostructures were found to be dynamically stable and weakly dependent on temperature. Calculation of the tensile strain-stress curves showed that the values of ideal tensile strength for the δ-NbN(111)- and ε-NbN(001)-based heterostructures with coherent interfaces and Si₃N₄–like Si₂N₃ interfaces were the highest with values in the range 36–65 GPa, but lower than corresponding values of bulk NbN compound. This suggests that hardness enhancement is likely due to inhibition of dislocation glide at the grain boundary rather than interfacial strengthening due to Si-N chemical bonding.     

Local structural analysis of a-SiCx:H films formed by decomposition of tetramethylsilane in microwave discharge flow of Ar

Hydrogenated amorphous silicon carbide (a-SiC_x:H) films were prepared by the decomposition of tetramethylsilane (TMS) with microwave discharge flow of Ar. When radio-frequency (RF) bias voltage (− V_RF) was applied to the substrate, the film hardness increased as (2.39 ± 1.12)-(9.15 ± 0.55) GPa for − V_RF = 0-100 V. The a-SiC_x:H films prepared under various − V_RF conditions were analyzed by the carbon-K near edge X-ray absorption fine structure (NEXAFS), by the elastic recoil detection analysis (ERDA), and by the X-ray photoelectron spectroscopy (XPS). From a quantitative analysis of NEXAFS, the sp²/(sp²+ sp³) ratios of C atoms were evaluated as 67.9 ± 2.0, 55.4 ± 2.7, and 51.7 ± 0.7% for − V_RF = 0, 60, and 100 V, respectively. From ERDA, hydrogen content of the film prepared under the condition of − V_RF = 100 V was found to decrease 28% comparing with that under − V_RF = 0 V. It is suggested that the cause of the increase of the film hardness when applying − V_RF is predominantly the growth of the sp³-hybridized structure of C atoms accompanied by the decrease of hydrogen terminations.     

Determination of hardness, Young's modulus and fracture toughness of lanthanum tungstates as novel proton conductors

Lanthanum tungstate is a promising material to be used as electrolyte in proton conducting fuel cells, or as a mixed proton-electron conducting membrane for hydrogen separation, and its mechanical properties are crucial for these applications. Lanthanum tungstates with a La/W atomic ratio between 4.8 and 6.0 have been investigated at room temperature at micro/nanoindentation range. Lanthanum tungstates exhibit a strain gradient plasticity at the vicinity of the imprints, which implies that the hardness presents an indentation size effect that was corrected using the Nix and Gao approach. The hardness and Young's modulus have therefore been determined to be 8-9 GPa and 130 ± 15 GPa, respectively. The fracture toughness was estimated to be ∼2 MPa m^1/2 for LWO56 using the Palqmvist equation. Both hardness and Young's modulus did not present a significant dependence with neither the sintering temperature nor the composition. The different imprints were visualized by Atomic Force Microscopy.