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.     

The role of ion-induced substrate damage in thin film adhesion strength

Variable-energy positrons are combined with Auger electron spectroscopy, scanning electron microscopy, and scratch test critical load measurements to study interfacial properties in thin film adhesion. This work complements an earlier investigation of the adhesion strength enhancement produced by ion bombardment of the substrate surface before deposition. In this study, we have investigated SiO₂ films deposited by radio-frequency (RF) sputtering onto stainless steel substrates. Extended ion bombardment etching leads to three related effects: the scratch test critical load is increased significantly, Auger electron spectroscopy shows a greater penetration of the film material into the substrate; and the interfacial positron annihilation signal is dominated by large, open-volume defects. These results are interpreted as confirmation that ion bombardment leads to the formation of microvoids in the interface layer and, consequently, to an increased adhesion strength by allowing mechanical interlocking between the coating and the substrate.     

Low-temperature deposition of optically transparent diamond using a low-pressure flat flame

The radial uniformity and scaleable nature of flat flames make them an attractive technique for diamond deposition. Due to the high temperatures involved in combustion synthesis, typically molybdenum and silicon have been used as substrates. Here we report low-temperature diamond deposition on glass substrates. Diamond deposition was achieved on ordinary sodium silicate glass at substrate temperatures of 500°C; however, film delamination occurred during cooling after deposition. Vycor™ and Pyrex™ are two glasses that have thermal expansion coefficients that are similar to diamond. Continuous, optically transparent films were successfully deposited on both glasses. The diamond films have been characterized by scanning electron microscopy, Raman spectroscopy and secondary ion mass spectroscopy (SIMS). The dependence of hydrogen and sp²-bonded carbon incorporation in the films on reactant composition was quantified. These films were optically transparent and showed good adhesion as measured by a simple tape test.     

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.     

Effect of Adhesion, Film Thickness, and Substrate Hardness on the Scratch Behavior of Poly(carbonate) Films

The effect of adhesion, film thickness, and substrate hardness on the scratch behavior of poly(carbonate) (PC) films was investigated. Films of various thickness were prepared by spin-coating solutions of PC in chloroform onto glass, ferroplate, Al 1100, Al 6022, and Al 6111 substrates. Adhesion between the films and the substrates was controlled by pretreatment of the substrates and the thickness of the films was controlled by the concentration of the PC solutions. Adhesion of the films to the glass substrates was measured by a blister test. Scratch tests were performed using a custom-built, progressive-load scratch tester with interchangeable diamond indenters; the resulting scratches were observed by optical microscopy, atomic force microscopy (AFM), and environmental scanning electron microscopy (ESEM). The critical normal load (i.e., the smallest applied normal load for which delamination of the film from the substrate was observed) was used as a criterion to determine the scratch resistance of the films. It was found that better film/substrate adhesion resulted in a higher critical load for delamination. As film thickness increased, the critical load and, thus, scratch resistance also increased. Substrate hardness had a strong influence on the scratch behavior of the PC films. For a low-hardness substrate (i.e., Al 1100), the work from scratching was mainly consumed by deforming the substrate. In the case of substrates with intermediate hardness (i.e., Al 6022, Al 6111, and ferroplate), the substrates were more resistant to the stresses that were generated in the films; hence, the deformation of the substrates was less severe. A high-hardness substrate (i.e., glass) resisted the applied load and resulted in higher stress concentrations in the films and at the interface. Consequently, a rougher surface inside the scratch track was observed.     

Effect of Adhesion,Film Thickness,and Substrate Hardness on the Scratch Behavior of Poly(carbonate) Films

The effect of adhesion, film thickness, and substrate hardness on the scratch behavior of poly(carbonate) (PC) films was investigated. Films of various thickness were prepared by spin-coating solutions of PC in chloroform onto glass, ferroplate, Al 1100, Al 6022, and Al 6111 substrates. Adhesion between the films and the substrates was controlled by pretreatment of the substrates and the thickness of the films was controlled by the concentration of the PC solutions. Adhesion of the films to the glass substrates was measured by a blister test. Scratch tests were performed using a custom-built, progressive-load scratch tester with interchangeable diamond indenters; the resulting scratches were observed by optical microscopy, atomic force microscopy (AFM), and environmental scanning electron microscopy (ESEM). The critical normal load (i.e., the smallest applied normal load for which delamination of the film from the substrate was observed) was used as a criterion to determine the scratch resistance of the films. It was found that better film/substrate adhesion resulted in a higher critical load for delamination. As film thickness increased, the critical load and, thus, scratch resistance also increased. Substrate hardness had a strong influence on the scratch behavior of the PC films. For a low-hardness substrate (i.e., Al 1100), the work from scratching was mainly consumed by deforming the substrate. In the case of substrates with intermediate hardness (i.e., Al 6022, Al 6111, and ferroplate), the substrates were more resistant to the stresses that were generated in the films; hence, the deformation of the substrates was less severe. A high-hardness substrate (i.e., glass) resisted the applied load and resulted in higher stress concentrations in the films and at the interface. Consequently, a rougher surface inside the scratch track was observed.     

Diamond deposition on copper treated hardmetal substrates

The adhesion of diamond coatings onto hardmetal substrates is improved by a copper deposition produced by a cementation from aqueous CuSO₄ solutions. During this reaction Co is dissolved from the substrate surface and copper is deposited. To obtain homogeneous Cu deposits, the influence of CuSO₄ concentration and reaction time on cementation were investigated.During diamond deposition, Cu reduces the surface mobility of Co, which is necessary to decrease deposition of non-diamond carbon and therefore increase adhesion. Indentation tests showed the good adhesion of the diamond coatings qualitatively.Cu precipitation and diamond deposition were examined by scanning electron microscopy (SEM). The diamond quality was detected by Raman spectroscopy. Using secondary ion mass spectroscopy (SIMS) depth profiles the interface and the Cu distribution were characterized indicating that during diamond deposition Cu is dissolved and forms an intermetallic Co–Cu mixed crystal.     

Processing–structure–adhesion relationship in CVD diamond films on titanium substrates

The hardness and adhesion properties of diamond films deposited on pure Ti and Ti–6Al–4V alloy are related to the structural characteristics of the films and of the intermediate layers formed at the film/substrate interface. Deposition experiments were performed by hot-filament CVD systematically varying the deposition temperature (650–850 °C) and time (60–360 min). The morphology and structure of the coatings and interfaces were investigated by optical and electron microscopy (SEM) and by the combined use of reflection diffraction (RHEED) and X-ray powder diffraction (XRPD), used also in the GID (Grazing Incidence Diffraction) mode. Hardness measurements (HV) of the deposits were made at a constant loading of 25 g on specifically prepared transversal sections of the specimen. Only small differences concerning the mechanical properties have been found for films grown on pure Ti and Ti–6Al–4V substrates. Adhesion of diamond on the substrate has been determined by the scratch test. The analysis of the micrographs of the indenter scratches and of the fracture-correlated acoustic emissions clearly indicates the occurrence of a multi-step detachment process, leading progressively to effective delamination of the diamond coatings.     

Chemical vapour deposition of diamond on nitrided chromium using an oxyacetylene flame

In this paper we present our first preliminary results on chemical vapour deposition (CVD) of diamond onto nitrided chromium using an oxyacetylene flame. Polycrystalline diamond films were obtained after deposition at very low substrate temperatures (<400°C). At these low temperatures there was extremely weak bonding, or no bonding at all, between the deposited layer and the substrate. To obtain stronger bonding, four growth experiments were carried out at initially higher substrate temperatures (700–1000°C). Whilst growth continued, the substrate temperatures were lowered step by step to 250°C. It was observed that on lowering the substrate temperature by more than about 500°C from the initial temperature, delamination occurred, suggesting that the thermal stresses exceeded the bonding strength. Subsequently, adherent diamond coatings were grown on the freshly exposed substrate surface whilst further lowering the substrate temperature. These diamond coatings were characterized using scanning electron microscopy and the adhesion of the diamond coatings to the substrates was assessed by means of the scotch tape test.     

Bilayered coatings of BN/diamond grown on Si3N4 ceramic substrates

Cubic boron nitride (c-BN) is a well known material to be used in machining of ferrous metallic alloys, namely as a coating. However, in most practical cases, there is a lack of adhesion to the substrate material. In this work, BN coatings were deposited by magnetron sputtering on silicon nitride (Si₃N₄) ceramic substrates using an intermediate layer of CVD microcrystalline (MCD) or nanocrystalline diamond (NCD). The goal was to improve the c-BN content by using diamond interlayers, and to optimize film adhesion to the substrate by employing such ceramic, which is known to provide very high adhesion strength to CVD diamond. The BN/NCD/Si₃N₄ combination demonstrated to be unique regarding the absence of delamination at both the BN/diamond and diamond/substrate interfaces, also providing the highest c-BN phase content.     

Adhesion Improvement of Poly(Phenylene-Vinylene) Substrates Induced by Argon-Oxygen Plasma Treatment

Copper films evaporated on argon-oxygen plasma-treated poly(phenylene-vinylene) films have been studied by scratch test, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The adhesion of the metallic film to the polymer substrate was greatly enhanced after treatment and found to increase with the treatment time. SEM observation of the treated samples revealed that the morphology of the polymer surface was gradually changed with the treatment time as compared with that of the bare polymer film. On the other hand, XPS analysis of the polymer-metal interface showed that the bonding between carbon, oxygen and copper were subsequently modified as compared with those obtained in untreated samples. The high adhesion strength observed on these substrates was related to the modification in the surface morphology on the one hand and to the formation of new compounds at the polymer-metal interface on the other. The nature of the interfacial layer and its influence on the adhesion of the copper layer was discussed by comparing the results with those obtained in poly(phenylene-vinylene) (PPV)-Al systems.     

Adhesion Improvement of Poly(Phenylene-Vinylene) Substrates Induced by Argon-Oxygen Plasma Treatment

Copper films evaporated on argon-oxygen plasma-treated poly(phenylene-vinylene) films have been studied by scratch test, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The adhesion of the metallic film to the polymer substrate was greatly enhanced after treatment and found to increase with the treatment time. SEM observation of the treated samples revealed that the morphology of the polymer surface was gradually changed with the treatment time as compared with that of the bare polymer film. On the other hand, XPS analysis of the polymer-metal interface showed that the bonding between carbon, oxygen and copper were subsequently modified as compared with those obtained in untreated samples. The high adhesion strength observed on these substrates was related to the modification in the surface morphology on the one hand and to the formation of new compounds at the polymer-metal interface on the other. The nature of the interfacial layer and its influence on the adhesion of the copper layer was discussed by comparing the results with those obtained in poly(phenylene-vinylene) (PPV)-Al systems.     

Study of interface properties between Cu2ZnSnS4 thin films and metal substrates

In this study, the effects of various metal substrates on the interface properties of Cu₂ZnSnS₄ (CZTS)/metal structures were investigated. The crystal phases, morphologies, and element distributions at the interfaces between CZTS thin films and various metal substrates (Mo, W, Ti, and Al foils) were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and secondary ion mass spectroscopy. MoS₂ and WS₂ phases were formed at the CZTS/Mo and CZTS/W interfaces, respectively. No Ti-S and Al-S compounds were formed at the CZTS/Ti and CZTS/Al interfaces. The formation of these phases was dependent on the lowest reaction temperature between the metal foil and S vapor. The size of particles at the back surface of the CZTS thin film on the Ti substrate was larger than that on the other substrates, because the Ti element improved the crystallinity of CZTS. The presence of a thick WS₂ layer at the CZTS/W interface was attributed to the fact that the (211) plane of the W foil caused exposure of a greater number of W atoms in the sulfurization process because of the body-centered cubic crystal structure of W. The diffusion of Ti atoms into the CZTS thin film was due to the large average hop distance of defects in the CZTS thin film and the relatively low activation energy of Ti atoms. Current–voltage curves and energy band diagrams revealed that the ohmic contacts formed at the CZTS/Mo and CZTS/Ti interfaces were better than those formed at the CZTS/W interface and that a Schottky contact was formed at the CZTS/Al interface.     

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.     

Electrochemical behaviour of graphite- and molybdenum electrodes modified with thin-film diamond

Films of undoped polycrystalline diamond were deposited on molybdenum or graphite substrates by hot filament chemical vapour deposition (HFCVD), using a precursor gas mixture of methane in excess hydrogen. The morphology and quality of the as-deposited films were monitored by scanning electron microscopy, Raman spectroscopy, and X-ray diffraction, and the electrochemical activity monitored with cyclic voltammetry. The results suggest a direct correlation between electrochemical activity, film thickness, and quality. Analysis of the ferrocyanide–ferricyanide couple at a diamond-modified molybdenum or graphite electrode suggest some extent of electrochemical reversibility, but the rates of charge transfer across the diamond–substrate interface varied with the methane concentration and substrate type. The ratio of the anodic and cathodic peak currents was always close to unity. Ex situ studies with energy dispersive spectroscopy show that some Prussian Blue was deposited on the graphite-modified electrode, grown using 0.5% CH₄ in H₂.     

Characterization of diamond fluorinated by glow discharge plasma treatment

The surface fluorination of diamond by treatment in glow discharge plasmas of CF₄ for different times has been investigated. High quality diamond films were deposited onto silicon substrates using hot filament chemical vapor deposition (HFCVD). Subsequently, the films were exposed to a radiofrequency glow discharge plasma of CF₄ for times ranging from 5 min to 1 h. The effects of the plasma treatment on the surface morphology, diamond quality and elemental composition were investigated using atomic force microscopy (AFM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. Differences in film roughness caused by the plasma treatment were detected by AFM and confirmed by scanning electron microscopy (SEM). Raman spectroscopic analyses showed that the original diamond was of high quality and that the bulk of each film was unchanged by the plasma treatment. Analyses using XPS revealed increased surface fluorination of the films at longer treatment times. In addition, the density of free radicals in the films was probed using electron paramagnetic resonance spectroscopy (EPRS), revealing that untreated diamond possesses an appreciable density of free radicals (6×10¹² g⁻¹) which initially falls with treatment time in the CF₄ plasma but increases for long treatment times.     

Nucleation and growth of diamond films on single crystal and polycrystalline tungsten substrates

Silicon has been the most widely studied substrate for the nucleation and growth of CVD diamond films. However, other substrates are of interest, and in this paper, we present the results of a study of the biased nucleation and growth of diamond films on bulk single and polycrystalline tungsten. Diamond films were nucleated and grown, using a range of bias and reactor conditions, and characterized by Raman spectroscopy and scanning electron microscopy (SEM). High-quality (100) textured films (Raman FWHM<4 cm⁻¹) could be grown on both single and polycrystalline forms of the tungsten substrate. On carefully prepared substrates, by varying the bias treatment, it was possible to determine the nucleation density over a 4–5 order range, up to ∼10⁹ cm⁻². Raman measurements indicated that the diamond films grown on bulk tungsten exhibited considerable thermal stress (∼1.1 GPa), which, together with a thin carbide layer, resulted in film delamination on cooling. The results of the study show that nucleation and growth conditions can be used to control the grain size, nucleation density, morphology and quality of CVD diamond films grown on tungsten.     

Study on metal-doped diamond-like carbon films synthesized by cathodic arc evaporation

In this study, we developed a novel method of synthesizing metal-doped diamond-like carbon films (DLC) using the cathodic arc evaporation (CAE) process. Intense Cr plasma energy activated the decomposition of hydrocarbon source gas C₂H₂ to form a metal-doped amorphous carbon film on steel substrates. We deposited a Cr interlayer to prevent interdiffusion between DLC and the steel substrates. When the C₂H₂ partial pressure is higher than 1.3 Pa, the deposition reaction switched from Cr₃C₂ to DLC formation. The result is a hydrogenated DLC thin film possessing excellent microhardness as high as 3824 Hv(25g), and for which the incorporation of a Cr interface and Cr doping in the DLC matrix ensure film ductility and sufficient film adhesion. We employed Raman spectroscopy to evaluate the influences of reactive gas flow and substrate bias on the DLC composition; we carried out the microstructure and mechanical property measurements by scanning electron microscopy (SEM), X-ray diffraction (XRD), glow discharge optical spectroscopy (GDS) and wear tests.     

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.     

Analysis of diamond nucleation on molybdenum by biased hot filament chemical vapor deposition

The nucleation and initial growth of diamond on molybdenum using biased hot filament chemical vapor deposition were investigated by scanning electron microscopy, Raman spectroscopy and adhesion force tests. The studies showed that the negative biased pre-treatment greatly enhanced the nucleation density and adhesion force of diamond films on molybdenum. The experimental evidence was confirmed that there is large stress near the interface between the diamond and the Mo substrate, which were originated from the disordered graphite phases and molybdenum carbide near the interface. This may play an important role during nucleation stage. However, larger stress can cause the degradation of the adhesion force of diamond films on Mo substrate. However, the adhesion force was enhanced with increasing bias voltage. The theoretical relationship between the adhesion force and the bias voltage is given by theoretical calculation.