Erosive wear resistance of NCD coatings produced by pulsed microwave discharges

Erosion tests on nanocrystalline diamond (NCD) films are relevant not only for the evaluation of the erosive wear resistance, anticipating applications where coated materials are exposed to particle impacts, but also as a way to evaluate their adhesion to the substrates. NCD films were grown on Si₃N₄ ceramic by microwave plasma assisted deposition in continuous (CW) and pulsed (PW-50 Hz and PW-500 Hz) discharge modes in argon-rich gas mixture. The films grown in PW modes presented lower crystallite size and lower surface roughness than those grown in CW one, while the use of CF₄ plasma pre-treatment of the substrate lead to further film homogeneity. The erosive wear resistance of NCD was evaluated by solid particle impact using SiC (45–250 μm size) as erodent material, with selected parameters accordingly to Hertzian stress field calculations. Film weight loss was undetectable until delamination took place. When tested with 150 μm SiC particles, the CF₄ plasma pre-treated substrates yield a three-fold increase (15 min) in delamination time comparing to untreated specimens, while samples coated under PW-50 Hz conditions presented a six times lower erosion rate compared to CW ones. It is believed that the improved nucleation behaviour by the use of PW mode and its higher homogeneity on the CF₄ plasma pre-treated samples decrease the flaw population on the diamond/substrate interface, leading to improved adhesion levels.     

Pretreatment of substrate surface for improved adhesion of diamond films on hard metal cutting tools

The d.c. plasma jet CVD process is one of the most promising coating processes used for the production of polycrystalline diamond films. In comparison with other CVD processes, its obtainable linear growth rates, in the range of 100 μm/h, are much higher than growth rates of microwave or hot filament CVD (1–10 μm/h).

One interesting application is the diamond coating of cutting tools. The main problem here is the poor adhesion of the films. Therefore, a mechanical or chemical pretreatment or intermediate layers are used to improve the adhesion.

In these investigations the influence of mechanical pretreatment by grinding and polishing with diamond powder of different grain sizes as well as chemical etching are examined on WC-Co hardmetals. Sputtered metallic interlayers of different thicknesses and arc-ion plated amorphous carbon films are deposited on these substrates, and diamond films were synthesized on these pretreated cutting tools by d.c. plasma jet CVD.

Adhesion and wear resistance of the diamond films have been examined by dry turning tests on very abrasive metal-matrix composites. Distinct improvement in adhesion of diamond coatings on hard metal substrates was achieved by two methods of substrate surface pretreatment: etching with Murakami's solution and following ultrasonically seeding with diamond particles or using an amorphous carbon film as intermediate layer.     

Growth and Adhesion Enhancement of Diamond Films Deposited on Steel Substrates by a Cr–N Interlayer

Diamond film deposition onto iron-based substrates by chemical vapor deposition methods is complicated by the formation of black carbon or graphitic soot on the substrate surface prior to diamond nucleation and growth, by fast diffusion of carbon into the iron substrate, and by poor adhesion of the deposited film. These complications suggested the use of a buffer layer between the deposited diamond film and the iron-based substrate. We review different methods used to improve the adhesion of diamond film to steel substrates. In particular we describe in detail our own studies which involve the use of a Cr-N interlayer. The use of a chromium nitride interlayer has been found to improve significantly the adhesion of diamond films deposited on ferrous substrates. This is achieved by hindering diffusion processes of carbon and iron, very stable mechanical and chemical bonding between the interlayer and the diamond film, and good adhesion of the interlayer to the steel substrate. We also report on our studies related to residual stress present in the films, as well as a correlation between the interlayer properties and adhesion strength of deposited films.     

Hot filament chemical vapour deposition and wear resistance of diamond films on WC-Co substrates coated using PVD-arc deposition technique

Different Cr- and Ti-base films were deposited using PVD-arc deposition onto WC-Co substrates, and multilayered coatings were obtained from the superimposition of diamond coatings, deposited on the PVD interlayer using hot filament chemical vapour deposition (HFCVD). The behaviour of PVD-arc deposited CrN and CrC interlayers between diamond and WC-Co substrates was studied and compared to TiN, TiC, and Ti(C,N) interlayers. Tribological tests with alternative sliding motion were carried out to check the multilayer (PVD + diamond) film adhesion on WC-Co substrate. Multilayer films obtained using PVD arc, characterised by large surface droplets, demonstrated good wear resistance, while diamond deposited on smooth PVD TiN films was not adherent. Multilayered Ti(C,N) + diamond film samples generally showed poor wear resistance.Diamond adhesion on Cr-based PVD coatings deposited on WC-Co substrate was good. In particular, CrN interlayers improved diamond film properties and 6 μm-thick diamond films deposited on CrN showed excellent wear behaviour characterised by the absence of measurable wear volume after sling tests. Good diamond adhesion on Cr-based PVD films has been attributed to chromium carbide formation on PVD film surfaces during the CVD process.     

Partially stabilized zirconia substrate for chemical vapor deposition of free-standing diamond films

In this work we investigated the use of partially stabilized zirconia (PSZ) as the substrate for deposition of CVD diamond films. The polycrystalline PSZ substrates were sintered at high temperatures and the results showed that this material has unique properties which are very appropriated for the growth of free-standing diamond films. The diamond nucleation density on PSZ is high, even without seeding, and the CVD diamond film was totally released from the substrate after the deposition process, without cracking. Micro-Raman analysis revealed that the free-standing diamond film had a good crystallinity on both surfaces with practically no stress in the structure. The same PSZ substrate can be reutilized for the deposition of a large number of diamond films. The average growth rate is about 5–6 μm/h in a microwave plasma reactor at 2.5 kW. The deposition process causes the reduction of ZrO₂, producing ZrC. The high mobility of oxygen in the zirconia matrix at high temperature would probably help to etch the interface region between the substrate surface and the diamond film, decreasing the adhesion strength and eliminating some defects in the film structure related to non-diamond carbon phases.     

A study of the initial stages of diamond deposition on ferrous substrates coated by a nitrided chromium interlayer and on nitrided polycrystalline chromium substrates

The use of a nitrided chromium interlayer has been found to improve the interfacial properties of diamond films deposited on ferrous substrates. This is achieved by hindering diffusion process of carbon and iron, good adhesion of the interlayer to the steel substrate, and very stable mechanical and chemical bonding between the interlayer and the diamond film. In the present study the initial stages of diamond deposition on steel substrates coated by a nitrided chromium interlayer and on nitrided polycrystalline chromium substrates are reported. Nitridation of chromium films deposited by electrochemical methods and polycrystalline chromium substrates resulted in the formation of two chromium nitrides phases, CrN and Cr₂N, and a rough surface morphology. The initial stages of diamond deposition were found to be accompanied by carburization of the substrates surface resulting in chromium carbide formation. The incubation time, diamond particle density and growth rate at the very initial stages of the deposition process were found to differ for these two substrates. It is suggested that these differences originate from different carburization rates of the two substrates. Phase transformation, recrystallization and diffusion processes in the near surface regions of both substrates resulted in very stable chemical bonding and good adhesion of the diamond film to the substrates. Raman spectra of the deposited films, on both substrates, show shift of the diamond peak position to higher wave numbers and split of the peak. These effects are associated with compressive stresses in the diamond film. Residual stresses in the deposited films were calculated from the shift of the diamond Raman peak. The residual stresses, as calculated from the Raman spectra, were found to increase with deposition time reaching values of 8.4 and 6.9 GPa for continuous diamond films on steel substrate coated with the nitrided chromium film and on nitrided chromium substrates, respectively. Based on a simple model it was estimated that thermal stress, arising from mismatch between the thermal expansion coefficient of diamond and the underlying substrates, is the major component of the compressive stress in the diamond films.     

Fretting wear and electrochemical corrosion of well-adhered CVD diamond films deposited on steel substrates with a WC–Co interlayer

Diamond films have been grown on carbon steel substrates by hot-filament chemical vapour deposition methods. A Co-containing tungsten-carbide (WC–Co) coating prepared by high velocity oxy-fuel spraying was used as an intermediate layer on the steel substrates to minimize the early formation of graphite (and thus growth of low quality diamond films) and to enhance the diamond film adhesion. The effects of the WC–Co interlayer on nucleation, quality, adhesion, tribological behaviour and electrochemical corrosion of the diamond film were investigated. The diamond films exhibit excellent adhesion under Rockwell indentation testing (1500 N load) and when subjected to high-speed, high-load, long-time reciprocating dry sliding ball-on-flat wear tests against a Si₃N₄ counterface in ambient air (500 rpm, 200 N, 300,000 cycles). A WC–Co interlayer with appropriate chemical pretreatment is shown to play an important role in improving the nucleation, quality and adhesion of the diamond film, relative to that shown by substrates without such pretreatment.     

Influence of plasma ionization on the elastic modulus and tribology behavior of carbon films deposited by the HiPIMS technique

This work reveals the influence of discharge current on carbon-ion energies of plasma, elastic modulus, and friction coefficient at the nano- and macroscale of carbon films deposited via high-power impulse magnetron sputtering. Three applied discharge current conditions in deposition processes were employed to obtain three-carbon films of interest. The number of carbon ions with their energies was obtained via mimic tests of the deposition process using three similar discharge currents through a quadrupole mass spectrometer detector based on the time-averaged ion energy distribution function. The bonding structure of the films was evaluated using Raman spectroscopy, fitting the Diamond and Graphite peaks to obtain a semiquantitative analysis. The elastic modulus of the carbon films was determined from atomic force acoustic microscopy measurements avoiding the influence of the substrates. The friction coefficient was analyzed at the nanoscale via atomic force microscopy and at the macroscale via tribometry. Significant alterations were observed in the number and energy level of the carbon ions with the variation of discharge current. These alterations significantly influenced the bonding properties, elastic modulus, and tribology behavior. A higher elastic modulus and higher sp³ bond content were observed for the film with a lower number of carbon ions and less energy.     

Selective growth of diamond and carbon nanostructures by hot filament chemical vapor deposition

Diamond and carbon nanostructures have been synthesized selectively on differently pretreated silicon substrates by hot filament chemical vapor deposition in a CH₄/H₂ gas mixture. Under typical conditions for CVD diamond deposition, carbon nanotube and diamond films have been selectively grown on nickel coated and diamond powder scratched silicon surface, respectively. By initiating a DC glow discharge between the filament and the substrate holder (cathode), well aligned carbon nanotube and nanocone films have been selectively synthesized on nickel coated and uncoated silicon substrates, respectively. By patterning the nickel film on silicon substrate, pattern growth of diamond and nanotubes has been successfully achieved.     

Selective synthesis of diamond and CNT nanostructures directly on stainless steel substrates

Without surface pretreatment or applying additional interlayer, diamond films have been directly synthesized on an Fe-25Cr-5Al steel substrate by a hot filament chemical vapor deposition method from an H₂-1vol.% CH₄ gas mixture. Due to an effective removal of intermediate graphite phase from the diamond-steel interface, the coated diamond films were continuous and adherent well to the steel substrate. Aligned conical diamond structures were also achieved on this steel substrate by negatively biasing the substrate holder and inducing a glow discharge. The deposition behavior of carbon on Fe-Cr-Ni steel substrate was different. A graphite-rich carbon film incorporated with diamond particles grew in the absence of biasing, then aligned carbon nanotube bundles were formed in the presence of negative biasing and glow discharge. The different deposition behavior of carbon on the two kinds of steel substrates was addressed in terms of the effect of their chemical compositions.     

Deposition of DLC film from adamantane by using pulsed discharge plasma CVD

Diamond like carbon films are deposited on silicon and quartz substrates using adamantane as a sole source of carbon by pulsed discharge plasma chemical vapor deposition. Tauc band gap of such films has been successfully tuned from 1.7eV to 2.9eV. Iodine incorporation is observed to favor the growth of such films and induces disorder in the films. It also brings down in energy the on-set of photon absorption. Such iodine incorporated diamond like carbon films may be interesting candidates for the new coming applications such as for heterojunction photovoltaic devices.     

In situ surface oxide reduction with pulsed arc discharge for maximum adhesion of diamond-like carbon coatings

With filtered pulsed arc discharge (FPAD) method it is possible to achieve very high adhesion of high quality diamond-like carbon (DLC). Here we explain this high adhesion with the oxide reduction and consequent carbide formation and ion mixing of the substrate when exposed to high temperature carbon plasma ions. The use of intensive high energy (> 2 keV) carbon plasma is the only practical method to achieve ultimate adhesion of DLC. With this unique method presented, the adhesion properties and the substrate interface electron spectroscopy for chemical analysis (ESCA) spectra of DLC coatings are independent of the pre-treatment of silicon substrates. High adhesion and proper selection of substrate enables to deposit thick DLC coatings (> 10 μm). We also show how the DLC deposition system can be improved and simplified.     

Evolution and properties of adherent diamond films with ultra high nucleation density deposited onto alumina

In this work, we report on adherent diamond films with thickness of up to 4.5 μm grown on polycrystalline alumina substrates. Prior to deposition, alumina substrates were ultrasonically abraded with mixed poly-disperse slurry that allows high nucleation density of values up to ∼5×10¹⁰ particles/cm². It was estimated that the minimal film thickness achieved for continuous films was ∼320 nm, obtained after a deposition time of 15 min with diamond particles density (DPD) of ∼4×10⁹ particles/cm². Continuous adherent diamond films with high DPD (∼10⁹ particles/cm²) were obtained also on sapphire surface after abrasion with mixed slurry and 15 min of deposition. However, after longer deposition time, diamond films peeled off from the substrates during cooling.The poor adhesion between the diamond and sapphire is attributed to the weak interface interaction between the film and the substrate and to difference in coefficient of thermal expansion. On the other hand, it is suggested that the reason for good adhesion between diamond film and alumina substrate is that high carbon diffusivity onto alumina grain boundaries allows strong touch-points at the grooves of alumina grains, and this prevents the delamination of diamond film. This adhesion mechanism, promoted by sub-micron diamond grain-size, is allowed by initial high nucleation density.The surface properties, phase composition and microstructure of the diamond films deposited onto alumina were examined by electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and high-resolution scanning electron microscopy (HR-SEM). The residual stress in the diamond films was evaluated by diamond Raman peak position and compared to a theoretical model with good agreement. Due to the sub-micron grain-size, the intrinsic tensile stress is high enough to partially compensate the thermal compressive stress, especially in diamond films with thickness lower than 1 μm.     

Comparison of the adhesion of diamond films deposited on different materials

Indentation tests combined with acoustic emission spectra were used to compare the adhesion of diamond films deposited on various substrates, including Ti, Cr, Si and Ti coated Cu. We show that indentation in the diamond coatings may cause the following failure modes: (a) the substrate cracking; (b) the film cracking and localised detachment; and (c) the film delamination and the delamination propagation. Acoustic emission during indentation loading provided essential information in predicting what mode of failure occurs. Combined with the acoustic emission spectra, the indentation tests are reliable in comparing the adhesion of diamond films deposited on the same or similar substrate materials. However, the comparison of the film adhesion on very different substrates, like Cu and Ti, is not so straightforward. Acoustic emission spectra also revealed that indentation caused substrate cracking prior to the failure of the film/substrate interface for diamond coatings on Si. In this case, the indentation tests are not valid to compare the coating adhesion.     

Influence of substrate nature on the d.c.-glow discharge induced nucleation of diamond

The nature of the nucleation centers, formed during the so called bias enhanced nucleation (BEN) of chemical vapor deposition (CVD) diamond is still an open question. We address this question by investigating the chemical composition and structure of the material deposited during the “nucleation” stage on various substrates by near edge X-ray absorption fine structure technique (NEXAFS) and Raman spectroscopy.The key step of the BEN of diamond in hot filament CVD systems is the generation of a stable d.c.-glow discharge between the grounded substrate and a positively biased electrode. This process results in the deposition of a carbon based film which contains the diamond nucleation and growth centers. Different materials, such as Si(100), CVD diamond films, and Si(100) onto which thin films of Ni were evaporated were used as substrates.It was found that the structure of the material deposited during the d.c.-glow discharge process is affected by the nature of the substrate. The d.c.-glow discharge process applied to the Si substrate resulted in the formation of a graphite-like film in the earlier stages (5 min), which after prolonged treatment time (30 min) was predominantly composed of nanosized diamond. The CVD diamond film, used as a substrate, promoted the formation of nanosized diamond particles even after 5 min of the d.c.-glow discharge process. However, C-13 labeling experiments have shown that microcrystalline diamond does not grow on the pre-existing CVD diamond substrate under the d.c.-glow discharge conditions. In the case of the Ni modified Si, the deposited film was graphitic in nature both after short and prolonged d.c.-glow discharge treatment times.     

Investigation of diamond deposition uniformity and quality for freestanding film and substrate applications

In this paper, we report on microwave CVD deposition of high quality polycrystalline diamond and on related post-processing steps to produce smooth, flat and uniformly thick films or diamond substrates. The deposition reactor is a 2.45 GHz microwave cavity applicator with the plasma confined inside a 12 cm diameter fused silica bell jar. The deposition substrates utilized are up to 75 mm diameter silicon wafers. The substrate holder is actively cooled with a water-cooled substrate holder to achieve a substrate surface temperature of 600–1150 C. The pressure utilized is 60–180 Torr and the microwave incident power is 2–4.5 kW. Important parameters for the deposition of thick films with uniform quality and thickness include substrate temperature uniformity as well as plasma discharge size and shape. As deposited thickness uniformities of ± 5% across 75 mm diameters are achieved with simultaneous growth rates of 1.9 μm/h. The addition of argon to the deposition gases improves film deposition uniformity without decreasing growth rate or film quality, over the range of parameters investigated. Post-processing includes laser cutting of the diamond to a desired shape, etching, lapping and polishing steps.     

Coating of DLC film by pulsed discharge plasma CVD

Diamond-like carbon (DLC) films were deposited onto Ti plate substrate by means of pulsed discharge (PD) plasma chemical vapor deposition (CVD) from gas mixture of methane and hydrogen, and their structures were investigated with transmission electron microscope (TEM). When the polarity of the substrate was negative, the DLC film was grown on the substrate. The transmission electron diffraction (TED) pattern of the deposited film, which was shaved with knife from the surface of the substrate, showed that both TiC and diamond structures were formed, showing that the DLC film can be coated with good adhesion by means of the formation of TiC interlayer. The coatings of DLC films onto a stainless steel plate and a drill of WC, on which Ti film were deposited previously, was also succeeded by the PD plasma CVD method with good adhesion.     

Growth of microcrystalline and nanocrystalline diamond films by microwave plasmas in a gas mixture of 1% methane/5% hydrogen/94% argon

Well-faceted microcrystalline diamond (MCD) films were deposited along with nanocrystalline diamond (NCD) films on the same substrate by a microwave plasma in the gas mixture of 1% CH₄+5% H₂+94% Ar. This was achieved by forcing a microwave plasma ball generated at 170 torr gas pressure to touch a silicon substrate that was pre-seeded by nanocrystalline diamond powder resulting in a high concentration of atomic hydrogen on the surface of growing diamond. Previously reported compositional mapping of the argon–methane–hydrogen system for MCD and NCD growth was not valid in this process parameter space. The non-uniform concentrations of atomic hydrogen and carbon containing radicals such as C₂ as well as varied local substrate temperature resulted in the simultaneous deposition of well-faceted MCD films in some areas with nanograined NCD films in others. Dilution of methane/hydrogen microwave plasmas by as much as 94% of argon alone could not suppress the growth of MCD.     

Friction force microscopy study of diamond films modified by a glow discharge treatment

A glow discharge treatment technique has been developed which enables control of the surface roughness and morphology of diamond films for applications in optical and electrical components. A conventional hot filament chemical vapour deposition (CVD) system was used to deposit the diamond films onto silicon substrates via a three-step sequential process: (i) deposition under normal conditions; (ii) exposure to either a pure hydrogen plasma or 3% methane in an excess of hydrogen using DC-bias; and (iii) diamond deposition for a further 2 h under standard conditions. The frictional characteristics and roughness of the film surfaces were investigated by atomic force microscopy (AFM) and the morphology and the growth rates determined from scanning electron microscope images. Lateral force microscopy (LFM) has revealed significant differences in frictional behaviour between the high quality diamond films and those modified by a glow discharge treatment. Friction forces on the diamond films were very low, with coefficients ∼0.01 against silicon nitride probe tips in air. However, friction forces and coefficients were significantly greater on the DC-biased films indicating the presence of a mechanically weaker material such as an amorphous carbon layer. A combination of growth rate and frictional data indicated that the exposure to the H₂ plasma etched the diamond surface whereas exposure to CH₄/H₂ plasma resulted in film growth. Re-Nucleation of diamond was possible (stage iii) after exposure to either plasma treatment. The resultant friction forces on these films were as low as on the standard diamond film.     

Improvement in adhesion of diamond films on cemented WC substrate with Ti–Si interlayers

Diamond films were deposited on the cemented WC+(3–5)% Co substrates by a microwave plasma chemical vapor deposition system. The substrates were pretreated with various processing steps before diamond deposition, including: polishing, etching for Co removal, Ti coating by DC sputter, and amorphous Si coating by E-gun. The residual stress of the films was determined by both Raman shift and low incident beam angle X-ray diffraction (LIBAD) methods. The adhesion of the films was evaluated by indentation adhesion testing. The film morphology and film–substrate interface structure were examined by SEM and Auger electron spectroscopy, respectively. The results show that Ti–Si can be a good interlayer to improve film adhesion and inhibit diffusion of Co to the substrate surface on diamond nucleation. This is due to the formation of strong TiC and SiC bonding to enhance film adhesion; Si acts as a promoter for diamond nucleation, and the residual stress with application of interlayer is much lower than that interlayer-free. The results also show the existence of an optimum Ti thickness for the best film adhesion.