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Reactive magnetron sputtering of tungsten target in krypton/trimethylboron atmosphere
Abstract
W-B-C films were deposited on Si(100) substrates held at elevated temperature by reactive sputtering from a W target in Kr/trimethylboron (TMB) plasmas. Quantitative analysis by Xray
photoelectron spectroscopy (XPS) shows that the films are W-rich between ~ 73 and ~ 93 at.% W. The highest metal content is detected in the film deposited with 1 sccm TMB. The C and B concentrations increase with increasing TMB flow to a maximum of ~18 and ~7 at.%,
respectively, while the O content remains nearly constant at 2-3 at.%. Chemical bonding structure analysis performed after samples sputter-cleaning reveals C-W and B-W bonding and no detectable W-O bonds. During film growth with 5 sccm TMB and 500 oC or with 10 sccm
TMB and 300-600 oC thin film X-ray diffraction shows the formation of cubic 100-oriented WC1-x with a possible solid solution of B. Lower flows and lower growth temperatures favor growth of W and W2C, respectively. Depositions at 700 and 800 oC result in the formation of
WSi2 due to a reaction with the substrate. At 900 oC, XPS analysis shows ~96 at.% Si in the film due to Si interdiffusion. Scanning electron microscopy images reveal a fine-grained
microstructure for the deposited WC1-x films. Nanoindentation gives hardness values in the range from ~23 to ~31 GPa and reduced elastic moduli between ~220 and 280 GPa in the films deposited at temperatures lower than 600 oC. At higher growth temperatures the hardness decreases by a factor of 3 to 4 following the formation of WSi2 at 700-800 oC and Si-rich surface at 900 oC.
... After annealing, the W, WC, W 2 C and W 2 B phases are observed. The observed phases agree with those reported in the literature for the W-B-C system: W [11,39], WC 1−x [17,39], W 2 C [39] and WB 2 [11,40]. ...
... After annealing, the W, WC, W 2 C and W 2 B phases are observed. The observed phases agree with those reported in the literature for the W-B-C system: W [11,39], WC 1−x [17,39], W 2 C [39] and WB 2 [11,40]. ...
In this work, the phase formation and thermal stability of X-B-C powders are investigated, where X = {Nb, Ta, and W}. X-B-C powders are synthesized by pulsed-DC magnetron sputtering and compared to annealed mixtures of commercially available monocarbide and monoboride powders. The phase formation of all powders is investigated before and after annealing to 1500 °C in a protective atmosphere. Before annealing, powders prepared from coatings sputtered from X2BC targets exhibited a nanocomposite structure with small XC grains in the case of the Nb-B-C and Ta-B-C powders. No crystalline phases could be identified in the W-B-C powder. Annealing resulted in a significant increase in crystallite size and the formation of additional phases in all sputtered coating powders. Commercial powder mixtures showed XC and XB phases before and after annealing with no evidence of forming additional phases and minor exothermic reactions were observed by DSC measurements, which were linked to grain growth. No formation of the X2BC phases was observed. This can be explained by the energetically favorable binary XC and XB phases over the ternary X2BC phase in the case of Ta and W. The ternary Nb2BC phase was not observed despite being energetically favorable over binary phases, likely due to kinetic limitations.
... The main peak is located at 282.8 eV (FWHM = 0.8 eV) which can be attributed to the W-C bond. However, we can see a small second peak located at 283.7 eV this can be assigned to W 2 C (tungsten-semicarbide) 26,27 . The other peaks located at higher binding energies can be attributed to the C-C, C-H bond (shifted to 284.8 eV) and C-O species (FWHM = 1.25 eV). Figure 1b,c presents present typical C 1s XP spectra obtained from different depth of our sample which experienced medium energy noble gas ion irradiation (120 keV, 2.5 × 10 15 Xe + /cm 2 ). Figure 1b depicts the carbide region where we can see that the main peak is located at 283.6 eV confirming the presence of W 2 C, with no other peaks located at lower binding energies. ...
In spite of its superior chemical sensitivity, XPS depth profiling is rarely used because of the alteration introduced by the sputter removal process and the resulting inhomogeneous in-depth concentration distribution. Moreover, the application of XPS becomes increasingly challenging in the case of the analysis of thin layers, if the thickness is in the range of 2–3 inelastic mean free paths (IMFP) of the photoelectrons. In this paper we will show that even in these unfavorable cases the XPS depth profiling is applicable. Herein the XPS depth profiling of a model system tungsten-carbide-rich nano-layer of high hardness and corrosion resistance is presented. We will show that the problems arising because of the relatively high IMFP can be corrected by introducing a layer model for the calculation of the observed XPS intensities, while the alteration, e.g. ion mixing, compound formation and similar artefact, introduced by the sputter removal process can be handled by TRIDYN simulation. The method presented here overcomes the limitation of XPS depth profiling.
... Precursor development, however, is much less explored in the PVD community. It has been shown that reactive magnetron sputtering of tungsten target in krypton/trimethylboron (B(CH3)3) plasmas results in growth of W-rich 100-oriented WC1-x with a potential boron solid solution [160]. Moreover, there is an interesting process development in hybrid sputtering using typical CVD precursors such as B2H6, pentaborane(9) (B5H9) decaborane(14) (B10H14) as further discussed in section 7. We anticipate that a similar approach can be developed for TMB2 thin films. ...
We review the thin film growth, chemistry, and physical properties of Group 4-6 transition-metal diboride (TMB2) thin films with AlB2-type crystal structure (Strukturbericht designation C32). Industrial applications are growing rapidly as TMB2 begin competing with conventional refractory ceramics like carbides and nitrides, including pseudo-binaries such as Ti1-xAlxN. The TMB2 crystal structure comprises graphite-like honeycombed atomic sheets of B interleaved by hexagonal close-packed TM layers. From the C32 crystal structure stems unique properties including high melting point, hardness, and corrosion resistance, yet limited oxidation resistance, combined with high electrical conductivity. We correlate the underlying chemical bonding, orbital overlap, and electronic structure to the mechanical properties, resistivity, and high-temperature properties unique to this class of materials. The review highlights the importance of avoiding contamination elements (like oxygen) and boron segregation on both the target and substrate sides during sputter deposition, for better-defined properties, regardless of the boride system investigated. This is a consequence of the strong tendency for B to segregate to TMB2 grain boundaries for boron-rich compositions of the growth flux. It is judged that sputter deposition of TMB2 films is at a tipping point towards a multitude of applications for TMB2 not solely as bulk materials, but also as protective coatings and electrically conducting high-temperature stable thin films.
... Precursor development, however, is much less explored in the PVD community. It has been shown that reactive magnetron sputtering of tungsten target in krypton/trimethylboron (B(CH 3 ) 3 ) plasmas results in growth of W-rich 100-oriented WC 1-x with a potential boron solid solution [160]. Moreover, there is an interesting process development in hybrid sputtering using typical CVD precursors such as B 2 H 6 , pentaborane(9) (B 5 H 9 ) decaborane(14) (B 10 H 14 ) as further discussed in section 7. We anticipate that a similar approach can be developed for TMB 2 thin films. ...
We review the thin film growth, chemistry, and physical properties of Group 4–6 transition-metal diboride (TMB2) thin films with AlB2-type crystal structure (Strukturbericht designation C32). Industrial applications are growing rapidly as TMB2 begin competing with conventional refractory ceramics like carbides and nitrides, including pseudo-binaries such as Ti1-xAlxN. The TMB2 crystal structure comprises graphite-like honeycombed atomic sheets of B interleaved by hexagonal close-packed TM layers. From the C32 crystal structure stems unique properties including high melting point, hardness, and corrosion resistance, yet limited oxidation resistance, combined with high electrical conductivity. We correlate the underlying chemical bonding, orbital overlap, and electronic structure to the mechanical properties, resistivity, and high-temperature properties unique to this class of materials. The review highlights the importance of avoiding contamination elements (like oxygen) and boron segregation on both the target and substrate sides during sputter deposition, for better-defined properties, regardless of the boride system investigated. This is a consequence of the strong tendency for B to segregate to TMB2 grain boundaries for boron-rich compositions of the growth flux. It is judged that sputter deposition of TMB2 films is at a tipping point towards a multitude of applications for TMB2 not solely as bulk materials, but also as protective coatings and electrically conducting high-temperature stable thin films.
... This, in turn, could give a real opportunity to promote ternary binding configuration, which might be formed directly within the impulse of neon plasma flux [50] Table 2. Figure 4a and b shows the W 4f and C 1s core-level XPS spectra, while Fig. 4c and d depicts the B 1s and O 1s spectra of W-B-C coatings, which were synthesized from the surface-sintered cathodes and differ in their chemical compositions. The W 4f high-resolution spectra (Fig. 4a) exhibit a double-peak structure due to the spin-orbit splitting (~2.2 eV) of the W 4f7/2 and 4f5/2 peaks of W-B-C-1, which are located, respectively, at 31.4 eV and 33.6 eV, indicating metallic W bonds [51,52]. Following, if the expected boron content increases, the double peaks tend to shift toward lower binding energies that result from the formation of W-B bonds, as indicated by recent studies [53,54]. ...
The surface sintering approach was examined in the present manuscript to obtain single cathode material consisting of multiple elements. Design of electromagnetic discharge without the use of a mechanical press was utilized for this purpose at pulsed pressure oscillations, ranging from 25 to 125 Pa. Meanwhile, each pulsed plasma flux provided absolute energy varying from 87.8 to 279.5 J, resulting in a well-sintered tungsten boride crust within carbon matrix (WB x-C). Its application in sense of sintering degree was estimated by measuring thermal diffusivity, with a maximum value of 3.65 mm 2 /s achieved at the mid-range neon injection (75-100 Pa). Further, as-surface-sintered cathode material provided suitable contribution to the promising deposition rate (54 nm/min) of W-B-C films by employing gas injection magnetron sputtering in the single magnetron cycle. The quantitative analysis examined using photoelectron X-ray spectroscopy showed that W-B-C films are composed of four elements, of which the W/B ratio was differed prior to prepared cathode materials stoichiometry. The chemical bonding state study revealed the C-W, B-W, B-C and B-O bonds, of which W2BC nanocrystallites (~ 2 nm) were formed in the amorphous structure under the arrangement of neon pulsed glow discharge, and as indicated results of fast Fourier transform and Raman spectra. Nucleation of those ternary nanoclusters promoted a Vickers microhardness result of 26.3 GPa, emphasizing a sensitive response to the mechanical performance of deposited W-B-C films.
... It can be attributed to the presence of WC 1-x particles, which harden the surface layer. studies for thin films (nanoindentation hardness values in the range from~23 to~31 GPa and Young's modulus between~220 and 280 GPa [43]). The average electrical resistance values of the coatings on copper substrates were determined at the value of flowing current equal to 105 · 10 −3 A by the four-probe cell method. ...
The deposition of protective hardening coatings allows increasing the lifetime of important technical materials and pure metals. This work shows the possibility of depositing protective coatings based on cubic tungsten carbide (WC1-x) on metal substrates using a high-speed (~1 km/s) plasma jet generated by a coaxial magneto-plasma accelerator. In a plasma spraying process, the necessary conditions (high sputtering and crystallization rates) is created for synthesizing the high-temperature WC1-x phase and stabilizing it in the coating structure. The formation of WC1-x phase was confirmed by X-ray diffraction and energy dispersive X-ray spectroscopy. The substrate parameters such as linear dimensions and heat dissipation rate were found to directly affect the phase composition (WC1-x yield) and the structure of the deposited coating. The coatings with a higher WC1-x content are characterized by an increased hardness. The firstly made direct measurements of a Berkovich hardness (H = 33.0 ± 0.9 GPa) and Young's modulus (E = 401 ± 14 GPa) for bulk WC1-x grains, exceeding several micrometers in size, confirmed the suggestion about higher mechanical properties of the metastable WC1-x phase in comparison with hexagonal tungsten carbides WC and W2C.
This study focused on the influence of the CH4/Ar flow ratio on the microstructure, chemical composition, morphology, and mechanical properties of TaCx coatings deposited on (111) silicon substrates using magnetron sputtering. To characterize the coatings, X-ray diffraction for the microstructural analysis, auger electron spectroscopy (AES) for the stoichiometry, a mechanical profilometer for roughness and thickness, and a nanoindenter for mechanical measurements were used. Two experiments were designed: the first consisted of producing TaCx coatings at room temperature (RT) using various CH4/Ar ratios, from 0.1 to 0.3, to identify the phases, stoichiometry, and hardness. These samples did not present crystalline phases and showed a transition from sub-stoichiometric to over-stoichiometric materials. The over-stoichiometric coatings produced at CH4/Ar ratios greater than 0.2 exhibited a decrease in their hardness. For the second experiment, the substrate temperature was set at 500 °C to reach crystalline phases; the CH4/Ar ratio values were set to 0, 0.1, 0.125, 0.15, 0.175, and 0.2 to avoid over-stoichiometric coatings that exhibited lower hardness. At low CH4/Ar ratios, TaCx coatings were produced in an amorphous phase (α-Ta) and underwent a mixture of phases (α-Ta and cubic TaC). The materials tended to crystallize at CH4/Ar ratios greater than 0.175 (cubic TaC). All of the coatings were sub-stoichiometric, showing values from Ta to TaC0.56. Most of the samples exhibited a hardness of ∼30 GPa and a high Young’s modulus, except for the coatings with the highest carbon concentration (TaC0.56). The Ta coatings exhibited the lowest roughness, although they had the highest thickness. This behavior was attributed to Ta exhibiting a higher deposition rate than TaC.
In conclusion, the CH4/Ar ratio strongly influenced the crystallinity and stoichiometry of the coatings. Moreover, the materials exhibited a high hardness caused by their tendency to be amorphous.
The present study investigates the deposition condition of W–Ti–Co–C–N thin films made by magnetron co-sputtering. The central composite design of experiments was applied for analyzing the effect of substrate temperature (X1) and nitrogen gas flow rate (X2) on the mechanical properties. Nanoindentation, sliding wear test (ball-on-disk method), scanning electron microscopy, atomic force microscopy, and X-ray diffraction techniques were employed to evaluate the tribological properties and microstructure of the thin films. The optimum values of variables were found to be in the range of 500 °C < X1 and 6.5 < X2 < 9 sccm, giving a high hardness of 36.5 GPa, while the friction coefficient and wear rate have the lowest amount of 0.10 and 4 × 10⁻⁸ mm³/N.m, respectively.
The Brinell, Vickers, Meyer, Rockwell, Shore, IHRD, Knoop, Buchholz, and nanoindentation methods used to measure the indentation hardness of materials at different scales are compared, and main issues and misconceptions in the understanding of these methods are comprehensively reviewed and discussed. Basic equations and parameters employed to calculate hardness are clearly explained, and the different international standards for each method are summarized. The limits for each scale are explored, and the different forms to calculate hardness in each method are compared and established. The influence of elasticity and plasticity of the material in each measurement method is reviewed, and the impact of the surface deformation around the indenter on hardness values is examined. The difficulties for practical conversions of hardness values measured by different methods are explained. Finally, main issues in the hardness interpretation at different scales are carefully discussed, like the influence of grain size in polycrystalline materials, indentation size effects at micro- and nanoscale, and the effect of the substrate when calculating thin films hardness. The paper improves the understanding of what hardness means and what hardness measurements imply at different scales.
a Epitaxial growth of rhombohedral boron nitride (r-BN) on different polytypes of silicon carbide (SiC) is demonstrated using thermally activated hot-wall chemical vapour deposition and triethyl boron and ammonia as precursors. With respect to the crystalline quality of the r-BN films, we investigate the influence of the deposition temperature, the precursor ratio (N/B) and the addition of a minute amount of silicon to the gas mixture. From X-ray diffraction and transmission electron microscopy, we find that the optimal growth temperature for epitaxial r-BN on the Si-face of the SiC substrates is 1500 °C at a N/B ratio of 642 and silicon needs to be present not only in the gas mixture during deposition but also on the substrate surface. Such conditions result in the growth of films with a c-axis identical to that of the bulk material and a thickness of 200 nm, which is promising for the development of BN films for electronic applications.
The authors deposited N-doped tungsten carbide thin films on Si(100) substrates at 500 °C using direct-current reactive magnetron sputtering in a mixture of CH4/N2/Ar discharge and explored the effects of N doping on the preferred orientation, phase transition, and mechanical properties of the films by using x-ray diffraction, x-ray photoelectron spectroscopy, and nanoindentation measurements. They found that N doping significantly influenced the compressive stress, which led to a pronounced change in the preferred orientation, phase structure, and hardness for the tungsten carbide film. A phase transition from β-WC to α-WC occurred when N doping was in the range of 2.9 and 4.7 at. %, meaning that α-WC can be obtained at relatively low temperature (500 °C). To reveal the relationship between the stress and phase transition, as well as preferred orientation, the density-functional theory based on first principles was used to calculate the elastic constants and shear modulus for tungsten carbide with a structure of β-WC or α-WC. The calculated results showed that the preferred orientation depended on the competition between strain energy and surface energy, as well as the grains competitive growth, and the phase transition can be attributed to a decrease in the strain energy. The hardness of α-WC was harder than β-WC because the shear modulu- s for α-WC was larger than that of β-WC, whereas the bulk modulus for α-WC was almost equal to that of β-WC.
With contributions by Paul F. Fewster and Christoph Genzel. While X-ray diffraction investigation of powders and polycrystalline matter was at the forefront of materials science in the 1960s and 70s, high-tech applications at the beginning of the 21st century are driven by the materials science of thin films. Very much an interdisciplinary field, chemists, biochemists, materials scientists, physicists and engineers all have a common interest in thin films and their manifold uses and applications. Grain size, porosity, density, preferred orientation and other properties are important to know: whether thin films fulfill their intended function depends crucially on their structure and morphology once a chemical composition has been chosen. Although their backgrounds differ greatly, all the involved specialists a profound understanding of how structural properties may be determined in order to perform their respective tasks in search of new and modern materials, coatings and functions. The author undertakes this in-depth introduction to the field of thin film X-ray characterization in a clear and precise manner.
In this study mid-frequency pulsed-DC magnetron sputtering was used to deposit W-B-C coatings. The effect of coating composition and the deposition parameters on the structure, mechanical and tribological properties of the coatings was investigated. The broad diffraction peaks in the X-ray diffractogram indicated the amorphous to nanocrystalline nature of the coatings. The hardness of the coatings ranged from 22.6 to 26.2 GPa. The coatings prepared at the elevated temperature of 500 °C exhibited greater hardness compared to the coatings prepared at the ambient temperature. The amount of W in the coatings was the main parameter influencing the friction coefficient with a higher W content leading to a higher friction coefficient. The coatings prepared at 500 °C showed lower wear in tribological measurements due to their higher hardness compared to the coatings prepared at ambient temperature. The friction coefficient decreased as the testing temperature increased beyond 500 °C. XRD analyses after the high temperature tests have shown that this decrease was due to the formation of W-O Magnéli phases, which acted as a solid-state lubricant.
Preparation of coatings simultaneously exhibiting high hardness and enhanced fracture resistance is a hot topic, as nowadays used ceramic protective coatings show difficulties to cope with increased demands due to their inherent brittleness. Material exhibiting seemingly contradictory combination of mechanical properties - high hardness and moderate ductility enhancing the fracture resistance - was recently predicted by ab initio calculations in the crystalline X2BC system. The presented study is focused on the study of the influence of the C/W ratio on the microstructure, the content of different chemical bonds and the mechanical properties of W‐B‐C coatings prepared by magnetron sputtering at moderate temperature. It was shown that change of the deposition conditions to achieve different C/W ratios influences the energy flux and momentum transfer to the coating. The coating with the lowest C/W ratio experienced the highest energy flux and momentum transfer, which resulted in a dense coating microstructure. The microstructure progressively coarsened as the C/W ratio increased, i.e. as the energy flux and momentum transfer decreased. The level of the energy flux played no apparent role in determining the level of crystallinity of the coatings. Whereas coatings prepared at the lowest and the highest energy flux were amorphous, coatings with medium energy flux with the composition closer to that corresponding to W2BC formed small crystallites and were of nanocomposite nature. All the prepared coatings exhibited hardness >20 GPa. The presence of nanocrystalline domains played no apparent role in determining their mechanical properties. The densest yet amorphous coating exhibited the highest hardness, while coatings with columnar structure exhibited lower hardness. Therefore, the coating chemical composition was found out to be important for enhancing their crystallinity, while their microstructure and high relative content of W‐B bonds was identified as the key parameter for achieving high hardness.
We report x-ray photoelectron spectroscopy (XPS) core level binding energies (BE's) for the widely-applicable groups IVb-VIb transition metal carbides (TMCs) TiC, VC, CrC, ZrC, NbC, MoC, HfC, TaC, and WC. Thin film samples are grown in the same deposition system, by dc magnetron co-sputtering from graphite and respective elemental metal targets in Ar atmosphere. To remove surface contaminations resulting from exposure to air during sample transfer from the growth chamber into the XPS system, layers are either (i) Ar⁺ ion-etched or (ii) UHV-annealed in situ prior to XPS analyses. High resolution XPS spectra reveal that even gentle etching affects the shape of core level signals, as well as BE values, which are systematically offset by 0.2–0.5 eV towards lower BE. These destructive effects of Ar⁺ ion etch become more pronounced with increasing the metal atom mass due to an increasing carbon-to-metal sputter yield ratio. Systematic analysis reveals that for each row in the periodic table (3d, 4d, and 5d) C 1s BE increases from left to right indicative of a decreased charge transfer from TM to C atoms, hence bond weakening. Moreover, C 1s BE decreases linearly with increasing carbide/metal melting point ratio. Spectra reported here, acquired from a consistent set of samples in the same instrument, should serve as a reference for true deconvolution of complex XPS cases, including multinary carbides, nitrides, and carbonitrides.
WC/a-C:H tribological nanocomposite coatings were deposited by reactive magnetron sputtering of tungsten carbide targets in acetylene atmosphere on substrates of various materials – cemented carbide and three types of steel. All the substrates were pretreated by various magnetron plasma-based techniques to evaluate their efficiency in promotion of coating adhesion. All the studied processes took place at temperatures lower than 180 °C. The structure of the WC/a-C:H coatings and their mechanical properties did not depend on the pretreatment technique nor the type of substrate material. On the other hand, the surface roughness of the substrate and the deposited coating can be affected by the intensity and duration of the pretreatment process. The adhesion of the coatings is significantly dependent on the substrate material and the particular plasma pretreatment process, as well as their combination. HiPIMS pretreatment processes with lower duty cycle at higher substrate bias voltage result in excellent adhesion with scratch critical load values higher than 130 N. Further, the substrate-target distance influences the efficiency of the pretreatment process and also partly affects the performance of the coatings. It is concluded that magnetron plasma-based pretreatment and deposition processes might involve potential practical problems in industrial coating of various objects in the same process batch. This is particularly connected with their shape and their composition.
The study is mainly to optimize the process parameters (sputtering pressure and sputtering power) of the AlB2-type WB2 films deposited by DC magnetron sputtering by comparing their microstructure, elemental composition, and tribo-mechanical properties. As the argon pressure (PAr) increases (0.3–1.0 Pa), the particle density increases first and then decreases, with a corresponding decrease for particle energy. Under this condition, both the deposition rate and the B/W atomic ratio of the WB2 films increase first and then decrease under the competition effect between the sputtering and scattering process, the film orientation changes from (0 0 1) to (1 0 1), the microstructure changes from dense fine-fiber to porous column coupled with the stress evolution from compressive stress to increased tensile stress. Consequently, films deposited at PAr > 0.5 Pa have the poor hardness and wear-resistance. As the sputtering power increases (150–310 W), both the particle density and particle energy increase. Thus, the deposition rate increases greatly, the B/W atomic ratio declines slightly due to the resputtering process, the film structure becomes dense but rough by the particle bombardment causing the stress change from tensile stress to enhanced compressive stress, and the film orientation changes from the well-crystallized (0 0 1) to poor-crystallized (1 0 1). In conclusion, films with (0 0 1) orientation, high B/W atomic ratio, dense structure and proper compressive stress, which can be deposited at PAr = 0.5 Pa and sputtering current 0.5 A (corresponding to target power about 150 W), show the excellent tribo-mechanical properties with high hardness about 39.4 GPa and low wear rate of 2.2 × 10⁻⁷ mm³/mN.
The study is mainly to develop a new type of hard nanocomposite films based on the AlB2-type WB2 (h-WB2) system by studying the influence of carbon contents on the microstructures, mechanical and tribological properties, and thermal stability of the WBC films. By increasing the C2H2 partial pressure (PC2H2), carbon atoms present in forms of solid solution h-WBxCy and amorphous carbide a-WC, a-WBxCy, a-C:H (polymeric carbon) successively. Consequently, two- or three-phase nanocomposite films with microstructures comprising of nanocrystalline grains embedded in an amorphous matrix have been developed. Films with the h-WBxCy/a-WC and h-WBxCy/a-WC/a-C:H (or h-WBxCy/a-WBxCy/a-WC/a-C:H) nanocomposite structure deposited at PC2H2 ≤ 0.01 Pa (14.9–26.4 at.% C) exhibit highest hardness about 45 GPa and lowest wear rate about 10− 8 mm3/mN. And the transformation temperature of the h-WB2 phase has been enhanced greatly from 700 to 1000 °C by slight C-doping due to the formation of the strong interfaces between the nanocrystalline and amorphous phase in the films deposited at PC2H2 = 0.008 Pa. Moreover, proper C content (less than 26.4 at.%) can refine the film structure and reduce the oxygen impurity content and thereby improve the film properties. The residual stress of the films changes from tensile to compressive stress with the introduction of carbon, and the value of compressive stress is closely related to the film structures.
Thermodynamic equilibrium calculations were performed using solgasmix-pv at varying partial pressures of WCl6-CH4-H2 and WF6-CH4-H2. Chemical vapour deposition (CVD) phase diagrams, which display equilibrium condensed phases, were then constructed from the results of these calculations. These results indicated that for the WF6 and WCl6 systems the phase transformation from W to WC occurs at a lower partial pressure of CH4 with decreasing total pressure. However, the partial pressure of CH4 needed to produce WC is lower for WF6 than WCl6. Information from these CVD phase diagrams was used to determine process parameters for the deposition of monolithic hexagonal WC. It was observed that coatings grown at low temperatures contained phases other than those predicted in the CVD phase diagrams. Experimental results on the development of WC coatings based on CVD phase diagrams have been presented and discussed.
Thin epitaxial TiC and VC films and superlattices have been deposited on MgO(001) by simultaneous sputtering of the metals and evaporation of C60. It was found that epitaxial growth conditions for TiC could be maintained down to a temperature of 100 °C, while the epitaxial growth of VC required 200 °C. Epitaxial VC films were completely relaxed at all growth temperatures, while a change from a relaxed to a strained growth behavior was observed for TiC films. The structural quality of the TiC films was better than for the VC films. A general observation was that a plasma-assisted deposition process yields films with a higher quality and allows epitaxial growth at lower temperatures than for a pure coevaporation process.
Tungsten–carbon thin films have been reactively sputter deposited in various Ar–CH4 gas mixtures and the growth kinetics of the reactive deposition process have been elucidated. The films are amorphous as-deposited with partial crystallization of W2C and WC occurring following a 1100 °C–1 min rapid thermal anneal. Carbon incorporation within the W–C films is attributed to the flux of CH3 radicals impinging on the growth surface. Although they have a significantly lower concentration (∼0.1%) than the CH4 molecules contained within the plasma, their sticking coefficient is significantly larger than that of CH4. In addition, the change in the incorporation rate of carbon in the W–C films at higher CH4 (and subsequently CH3) concentrations has been shown to be due to the changes in the growth surface; as the CH3 flux increases, the growth surface becomes carbon terminated and decreases the incorporation of carbon because of the low CH3–C sticking coefficient. © 2001 American Vacuum Society.
Tungsten carbide films were prepared by rf magnetron sputtering of a tungsten carbide target onto high-speed steel (HSS), stainless steel, silicon and platinum. During deposition the substrate temperature remained below 100°C. The substrate potential was varied between 0 and - 120 V. From 0 to -40 V substrate bias we observed a decrease of the deposition rate whereas the intrinsic stress increased from 3 to 8 GPa. Simultaneously, the composition changed from WC1 to ca. WC0.5. The drastic variation of the chemical composition at low plating energies can be explained by a simple sputter model. With higher negative substrate potentials the films became harder (ca. 4500 HV 0.025) and denser. The adhesion to the substrates increased as well. SEM micrographs of such films deposited onto a non-biased substrate showed a less dense zone I structure. The crystal structure of the films, investigated by X-ray diffraction, was found as that of β-WC1-x and α-W2C. The stress can be reduced by adding approx 1% C2H2 to the working gas.
Tungsten carbide coatings were deposited onto molybdenum and cemented carbide substrates using d.c. and r.f. magnetron sputtering. Tungsten targets were used in reactive (argon plus acetylene) atmospheres and tungsten carbide targets were used in non-reactive (argon) atmospheres. Substrates were r.f. biased with d.c. potentials of up to -1000 V. Sputtering from WC targets produced carbon-deficient mixed-phase structures with β-WC1-x as the main component. Reactive deposition led to highly disordered W-C films which X-ray studies showed to be almost amorphous. Fractograms revealed very fine-grained to fractured amorphous film structures in all deposition modes. Hardness values higher than 3000 HV 0.05 were reached using non-reactive d.c. sputtering.
Instrument-size gas-bearing applications demanding hard wear-resistant coatings on lightweight thermally conductive substrates frequently require that properties such as electrical resistivity, magnetic susceptibility and adhesion integrity be evaluated. Titanium carbide and tungsten carbide coatings, approximately 1 to 2 μm thick, were sputter deposited on beryllium oxide substrates. In addition, titanium carbide coatings approximately 50 μm thick were prepared on beryllium oxide substrates by activated reactive evaporation. Electrical resistivity was measured at various locations on the coating surface with a square four-point probe array. Magnetic susceptibility was measured using the Gouy test method in which a specimen is partially suspended in a magnetic field and the resulting force of attraction is measured. Since adhesion integrity is essential to the success of any coating application, a direct tensile pull test was performed on each specimen. The resulting fracture surface showed coating thickness, and the structure and characterization was accomplished by scanning electron microscopy and X-ray analysis.
The feasibility of using organoboranes as precursors for the deposition of boron-carbon thin films in a hot-wall CVD furnace was investigated. The reagents studied include trimethylborane, triethylborane, and tributylborane. Triethylborane was the most suitable reagent in that higher boron to carbon ratios were obtained.
Tungsten carbide (WC) layers have been grown epitaxially on tungsten single crystals for the first time by using microwave plasma and electron cyclotron resonance plasma carburization of single-crystalline tungsten. WC on tungsten is grown epitaxially as (0001)WC|| (110)W in the alignment [1\bar 210]WC\| [\bar 111]W.
The method we introduced in 1992 for measuring hardness and elastic modulus by instrumented indentation techniques has widely been adopted and used in the characterization of small-scale mechanical behavior. Since its original development, the method has undergone numerous refinements and changes brought about by improvements to testing equipment and techniques as well as from advances in our understanding of the mechanics of elastic–plastic contact. Here, we review our current understanding of the mechanics governing elastic–plastic indentation as they pertain to load and depth-sensing indentation testing of monolithic materials and provide an update of how we now implement the method to make the most accurate mechanical property measurements. The limitations of the method are also discussed.
The synthesis of tungsten monocarbide (WC) thin films has been performed by physical vapor deposition on various substrates including glassy carbon, carbon fiber sheet, carbon foam, and carbon cloth. The WC and WâC phase contents of these films have been evaluated with bulk and surface analysis techniques such as x-ray diffraction, x-ray photoelectron spectroscopy, and scanning electron microscopy. These characterization techniques were also used to determine the effects of synthesis by nonreactive and reactive sputtering. The synthesis of WC particles supported on the carbon fiber substrate has also been accomplished using the temperature programmed reaction method. Overall, the results demonstrate that the phase purity of tungsten carbides can be controlled by the deposition environment and annealing temperatures.
In this paper we present the first results from processing and characterisation of epitaxial tungsten carbide films. The films were deposited in ultra-high vacuum by DC magnetron sputtering of tungsten with co-evaporated C60 as carbon source. It was found that the growth rate was crucial for the crystalline film quality. Epitaxial growth of β-WC1−x could be achieved at 400°C using a very low deposition rate of 6Å/min. Single-crystal films were thus formed on both MgO(100) and MgO(111). High-resolution transmission electron microscopy shows that the films grow with a very good match relative the substrate. Deposition rates of 10Å/min and higher resulted in renucleation and nanocrystalline growth with grain sizes of 15–80Å depending on substrate temperature. We have also found that a small addition of titanium stabilises the epitaxial growth of the cubic tungsten carbide films at higher growth rates. A titanium contribution of ∼2% give epitaxial growth at deposition rates of 40Å/min. X-ray diffraction study of the epitaxial films using reciprocal space mapping shows a cross-like broadening that is possibly related to ordering.
Tungsten carbide films have been deposited by CVD from a WF~/C~HS/H~ gas mixture on several different substrate materials (Ta, Si, Sic and C). Single-phase WC films could easily be obtained on Ta substrates at 900 "C using a total pressure of 100 mTorr and a high linear gas flow velocity (7 mls). It was found that the low pressures favoured the growth of carbon-rich films and made the deposition zone for WC longer. The phase composition of the films and deposition rates were also strongly affected by the substrate material. The substrate dependence was attributed to the chemical reactivity of WF6.
W1−xCx coatings (with x being in the range of 0.05–0.19) were deposited by reactive magnetron sputtering on to AISI 316 stainless-steel substrates in order to study the influence of the carbon content on the tribological properties of the coating–substrate composite. Knoop microhardness (HK), scratch adhesion, pin-on-disc sliding, ball-on-plate impact and abrasive wheel wear tests were performed to evaluate the mechanical and tribological properties of the coatings. X-ray diffraction (XRD) was used for phase- and peak shape parameter analysis, whilst the coating morphology was evaluated by scanning electron microscopy. It was found that all coatings consist mainly of the b.c.c. αW phase. With increasing carbon content from 5 to 19 at.%, an expansion of the αW lattice occurs progressively, associated with an increase in the peak full width at half maximum (FWHM) and a decrease in intensity. The film density and hardness increased with increasing carbon content up to 15 at.%, where hardness values of 4000HK0.025 were observed. Coatings comprising W1−xCx with x≤0.08 showed the best abrasive wear resistance and adhesion with no through-coating failure in the wear track for dry pin-on-disc sliding and no crack development around the indentation areas in impact tests after 50 000 impacts against both steel and cemented tungsten carbide balls.
Tungsten-carbon coatings have been deposited on stainless steel substrates by reactive magnetron sputtering from Ar–CH4 mixtures. The carbon concentration in the coatings measured by electron microprobe analyses was found to be proportional to the CH4 flow rate. Only the cubic α–W phase with a dilated lattice parameter was identified in W–C coatings having a carbon content lower than 25 at. %. Since the lattice parameter of the α–W phase in these W–C coatings increased with increasing carbon content, these coatings may be assumed to be W–C solid solutions. Only the nonstoichiometric β–WC1−x carbide (cubic phase) was detected in W–C coatings containing 30 to 70 at. % of carbon. The chemical state of the elements was investigated by x-ray photoelectron spectroscopy. The Vickers hardness of the W–C coatings was found to be considerably dependent on the carbon concentration. A maximum microhardness of 26 000 MPa was measured for W–C coatings containing either 14–15 at. % or 40–45 at. % of carbon. The correlation between crystallographic structure and microhardness is analyzed and discussed in this paper.
Epitaxial growth of sp(2)-hybridized boron nitride (sp(2) BN) films on sapphire substrates is demonstrated in a hot wall chemical vapor deposition reactor at the temperature of 1500 degrees C, using triethyl boron and ammonia as precursors. The influence of the main important process parameters, temperature, N/B ratio, B/H-2 ratio, and carrier gas composition on the quality of the grown layers is investigated in detail. X-ray diffraction shows that epitaxial rhombohedral BN (r-BN) film can be deposited only in a narrow process parameter window; outside this window either turbostratic-BN or amorphous BN is favored if BN is formed. In addition, a thin strained AlN buffer layer is needed to support epitaxial growth of r-BN film on sapphire since only turbostratic BN is formed on sapphire substrate. The quality of the grown film is also affected by the B/H-2 ratio as seen from a change of the spacing between the basal planes as revealed by X-ray diffraction. Time-of-flight elastic recoil detection analysis shows an enhancement of the C and O impurities incorporation at lower growth temperatures. The gas phase chemistry for the deposition is discussed as well as the impact of the growth rate on the quality of the BN film.
Epitaxial growth of sp2-hybridized boron nitride (BN) using chemical vapour deposition, with ammonia and triethyl boron as precursors, is enabled on sapphire by introducing an aluminium nitride (AlN) buffer layer. This buffer layer is formed by initial nitridation of the substrate. Epitaxial growth is verified by X-ray diffraction measurements in Bragg–Brentano configuration, pole figure measurements and transmission electron microscopy. The in-plane stretching vibration of sp2-hybridized BN is observed at 1366 cm–1 from Raman spectroscopy. Time-of-flight elastic recoil detection analysis confirms almost perfect stoichiometric BN with low concentration of carbon, oxygen and hydrogen contaminations. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Tungsten-carbon thin films have been deposited by reactive (Ar + C6H6) DC magnetron sputtering onto various substrates. Deposition onto glass, monocrystalline silicon, tantalum and stainless steel at room temperature yielded W–C films, having XRD patterns corresponding to the structure of heavily disordered W2C or WC1–x carbides. The samples deposited upon the Au or Cu foils were nanocrystalline cubic WC1–x with the grain size of 2.9 nm. Disordered tungsten-carbon films were stable up to 1200°C. Microhardness of the films with disordered W2C phase was about 5–6 GPa while that of the films with disordered WC1–x phase was about 17 GPa. The characteristics of films can be understood considering the effects of the incorporation of free carbon and/or carbon-hydrogen fragments into the tungsten carbide layer.
Tungsten carbide was obtained by chemical vapor deposition from a mixture of WCl6, C3H8, H2, and Ar on a graphite substrate in the temperature range of 1200 to 1500 °C. The influence of various factors on the crystal growth was investigated. The geometry in the furnace proved to be an important factor for the process of crystallization. The adherent coating of α-W2C could be obtained at relatively high flow rates of H2 and WCl6. The appropriate gaseous concentrations for the growth of WC were as follows: WCl6: 1.0–1.5 mole%; C3H8: 1.7–2.8 mole%; H2: 10–20 mole%; and Ar: 75–85 mole%; and the total flow rates was 1.78 ml/sec. Various crystal morphologies composed of uniform films, networks, dendrites and pillars with diameters of 2 to 40 μm were deposited on the WC-plated graphite. The dendritic or pillar crystals were obtained at temperatures between 1250 and 1400 °C, while the network-like crystals were formed at lower temperatures of 1200 to 1300 °C on the substrate surface.
Thin films in the W–C system were prepared by magnetron sputtering of W with coevaporated C60 as carbon source. Epitaxial deposition of different W–C phases is demonstrated. In addition, nanocrystalline tungsten carbide film growth is also observed. At low C60/W ratios, epitaxial growth of α-W with a solid solution of carbon was obtained on MgO(001) and Al2O3(001) at 400 °C. The carbon content in these films (10–20 at.%) was at least an order of magnitude higher than the maximum equilibrium solubility and gives rise to an extreme hardening effect. Nanoindentation measurements showed that the hardness of these films increased with the carbon content and values as high as 35 GPa were observed. At high C60/W ratios, films of the cubic β-WC1−x (x=0–0.6) phase were deposited with a nanocrystalline microstructure. Films with a grain size <30 Å were obtained and the hardness of these films varied from 14 to 24 GPa. At intermediate C60/W ratios, epitaxial films of hexagonal W2C were deposited on MgO(111) at 400 °C. Polycrystalline phase mixtures were obtained on other substrates and hexagonal WC could be deposited as minority phase at 800 °C.
It is shown that small variations of the deposition parameters during magnetron sputtering of tungsten carbide thin films may result in drastic changes of film properties. An increasing working gas pressure for example lowers stress and hardness values. Simultaneously, the texture of the WC1–x cristallites turns from 200 preferential orientation to 111, whereas the composition of the films does not change. In reactive sputtering with a tungsten target there is a narrow range from 2 to 3% C2H2 gas admixture to the working gas where the films are stochiometric (WC) and hard, and grain size and morphology are similar to that of non-reactively sputtered films. The generation of different crystallite structures and orientations in the range of 0–3% C2H2 admixtures are used to produce a multiphase thin film with extremely low crack propagation.
In this study, tungsten carbide, with its hardness, chemical inertness, thermal stability and low resistivity (25 μΩ cm)1 is shown as a reliable contact material to n- and p-type 6H-SiC for very high temperature applications. WC films with thicknesses
of 100–150 nm were deposited by chemical vapor deposition (CVD) from a WF6/C3H8/H2 mixture at 1173 K. A method to pattern CVD-tungsten carbide is suggested. TEM analysis of as deposited samples displayed
a clear and unreacted interface. The electrical investigations of the p-type 6H-SiC Schottky contacts revealed a high rectification
ratio and a low reverse current density (6.1 × 10−5 A cm−2, −10 V) up to 773 K. On n-type, a low barrier (ΦBn=0.79 eV) at room temperature was observed. The low ΦBn value suggests WC to be promising as an ohmic contact material on highly doped n-type epi-layers. We will show a temperature
dependence for the barrier height of tungsten carbide contacts that can be related to the simultaneous change in the energy
bandgap, which should be considered when designing SiC devices intended for high temperature operation.
Superhard nanocomposite coatings are currently of great interest for wear protection of tools. Within this work, after some consideration of the design and failure of nanocomposites, results on nanocomposite Ti–B–N and Ti–B–C films are presented and discussed. Coatings with different compositions in the quasi-binary systems TiN–TiB2 and TiC–TiB2 were deposited onto austenitic stainless steel and molybdenum sheets by means of unbalanced d.c. magnetron co-sputtering using segmented TiN/TiB2 and TiC/TiB2 targets. Coating chemical, structural and mechanical properties were investigated using electron-probe microanalysis (EPMA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), X-ray diffraction (XRD), and depth-sensing nanoindentation. Coatings with elemental compositions of about 35–45 at.% Ti, 18–44 at.% B and 20–36 at.% N or 22–40 at.% C consist of a nanocrystalline arrangement of TiB2 and TiN or TiC, respectively, with crystallite sizes of 1–5 nm. Coating hardness varies between 50 and 70 GPa and elastic moduli are close to 500 GPa.
The electrical characteristics of W‐Si(100) Schottky barrier junctions formed by sputter deposition of W on both n‐ and p‐type Si(100) have been measured in the temperature range 95–295 K using current‐voltage and capacitance‐voltage techniques. Auger electron and Rutherford backscattering spectroscopies were used to characterize the Si(100) surface prior to metal deposition, and to monitor the reaction between W and Si upon annealing. The results showed that initial silicide formation has very little or no effect on the barrier height. Annealing after initial silicide formation caused the junction characteristics to strongly deviate from the ideal thermionic emission behavior. For junctions with ideal thermionic emission behavior the barrier height was found to decrease with increasing temperature with a coefficient consistent with the predictions of recent models of barrier formation based on Fermi‐level pinning in the center of the semiconductor indirect band gap.
Tungsten carbide films have been deposited on stainless steel substrates held between 300 to 500 °C in a planar rf magnetron sputtering system at rates as high as that of pure tungsten (825 A°/min). The effect of substrate temperature on formation of WC x films has been investigated. A mixture of hexagonal WC, A‐15 W 3 C and carbon in graphitic and diamond form have been observed by AES and XRD techniques. The microhardness of these films has been found to be as high as 2365 kgf/mm2. The adhesion of these films as measured from indentation crack patterns has been found to depend on substrate temperature as well as on the amount of dispersed carbon in the film.
Tungsten and tungsten–carbon thin films have been produced from a W target sputtered in argon and argon–methane mixtures, respectively. The deposition rate of W films was measured as a function of the sputtering power and argon pressure varying in the range of 0.3–3 Pa. The crystallographic structure and composition of W films deposited on silicon and carbon substrates were investigated by x‐ray diffraction and Rutherford backscattering spectroscopy. The electrical resistivity of the W films was minimum (12 μΩ cm) when the internal stresses in the films were negligible. The carbon concentration in the W–C films determined by nuclear reaction analyses and Rutherford backscattering spectroscopy was varied from 10 to 95 at. % with increasing CH 4 content in the gas phase. The crystallographic structure of the W–C films was found to be dependent on the carbon concentration. Below 25 at. % of carbon, the structure of the W–C films was that of the cubic α‐W phase with a dilated lattice parameter. For higher carbon concentrations, the bcc α‐W phase disappeared and the structure was that of the nonstoichiometric cubic β‐WC 1-x phase. The structure of W–C films with a carbon content greater than 65 at. % was nearly amorphous. Internal stresses and electrical resistivity of W–C films were determined as functions of the carbon concentration. The experimental parameters suitable to produce W and W–C films with low resistivities and reduced internal stress level are reported in this article.
Tungsten carbide films have been deposited by low-pressure chemical vapour deposition from a WF6/C3H8/H2 mixture on Ta and Ni substrates. Single-phase WC films could be deposited on Ta in a broad vapour composition range at 900 °C. A mixture of WC and W2C was deposited in the temperature range 700–850 °C, while an amorphous film was obtained at 650 °C. The temperature behaviour suggests that deposition of carbon is a limiting factor in the growth process. The deposition process on Ta could be separated into two parts: a fast substrate reduction step of WF6 leading to the formation of metallic W followed by a slower formation and deposition of WC. The growth behaviour on Ta was also affected by tantalum carbides at the film-substrate interface. A different growth behaviour was observed on Ni. It was found that several η-carbides (i.e. Ni2W4C and Ni6W6C) were formed during a fast initial growth stage. Later on, the η-carbides reacted with carbon under the formation of WC and free Ni particles. It was also found that the carbon deposition rate on Ni substrates was higher than on Ta. This was explained by a catalytic process where Ni particles on the film surface favoured carbon deposition.
Tungsten-carbon coatings have been deposited on stainless steel substrates by reactive magnetron sputtering in ArCH4 mixtures. The composition of coatings was determined by electron microprobe analyses. Only the cubic α-W phase with an expanded lattice parameter was detected by X-ray diffraction in the W-C coatings containing less than 25 at% of carbon. Since the lattice dilatation of the α-W phase increased with increasing carbon content these coatings were assumed to be tungsten-carbon solid solutions. The dilatation of the W crystal lattice and Vickers hardness of the solid-solution-type W-C coatings were measured as functions of the carbon concentration. A lattice dilatation-microhardness relationship was established.
Instrument-size gas-bearing applications frequently require hard wear- resistant coatings on lightweight thermally conductive substrates. Titanium carbide and tungsten carbide coatings approximately 1–2 μm thick were prepared for this purpose on both beryllium and beryllium oxide substrates by sputter deposition. Titanium carbide coatings approximately 50 μm thick were also prepared using activated reactive evaporation techniques. Test fixturing was developed using a vertical-axis turbomolecular vacuum pump to provide both a vacuum atmosphere and a high speed spindle for specimen motion. Coated specimens having beryllium substrates were mounted directly on the pump spindle, while specimens with beryllium oxide substrates were brought into contact with the rotating member at a specified load for a given length of time. Specimen contact was made at a sliding velocity of approximately 7.0 × 103 cm s-1 in a vacuum pressure of less than 5.0 × 10-6 Torr. Specimen surfaces were completely characterized before and after testing using surface profilometery, light microscopy and scanning electron microscopy with X-ray analysis and X-ray diffraction techniques.
The refractory compounds TiC, ZrC, HfC, VC, NbC and TaC comprise a family of materials characterized by high melting point, great hardness and low chemical reactivity. These properties, which are exploited in tribological applications, are reviewed and discussed in terms of models taken from solid state physics. Particular attention is given to the important role of non-stoichiometry in these compounds and its influence on bonding and on transport properties. It is argued that the non-stoichiometry is a metastable condition, however, in view of the presece of ordered phases at certain values of carbon-to-metal ratio. The substantial impact of order-disorder transformations on transport properties is analyzed. The partially covalent bonding is discussed in terms of mixing of 2p and metal 3d or 4d atomic states, and the high Peierls stress resisting dislocation motion is thereby justified.
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The electronic structure of nanocrystalline (nc-) TiC/amorphous C nanocomposites has been investigated bysoft x-ray absorption and emission spectroscopy. The measured spectra at the Ti 2p and C 1s thresholds of thenanocomposites are compared to those of Ti metal and amorphous C. The corresponding intensities of theelectronic states for the valence and conduction bands in the nanocomposites are shown to strongly depend onthe TiC carbide grain size. An increased charge transfer between the Ti 3d-eg states and the C 2p states hasbeen identified as the grain size decreases, causing an increased ionicity of the TiC nanocrystallites. It issuggested that the charge transfer occurs at the interface between the nanocrystalline-TiC and the amorphous-Cmatrix and represents an interface bonding which may be essential for the understanding of the properties ofnc-TiC/amorphous C and similar nanocomposites.
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Comparion of WCl 6 -CH 4 -H 2 and WF 6 -CH 4 -H 2 systems for growth of WC coatings
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