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.     

Novel pretreatment of hard metal substrate for better performance of diamond coated cutting tools

A surface engineering approach for a novel pretreatment of hard metal tool substrate for optimum adhesion of diamond coatings is presented. Firstly, an alkaline solution was used to etch the WC grains to generate a rough surface for better mechanical interlocking. Subsequently, surface Co was removed by etching in acid solution. Then the hard metal substrate was boronized to form a compound interlayer which acted as an efficient diffusion barrier to prevent the outward diffusion of Co. Novel nano-microcrystalline composite diamond film coatings with a very smooth surface was deposited on the surface engineering pretreated hard metal surface. Promising results of measurement in adhesion strength as well as field cutting tests have been obtained.     

Comparative study of diamond films grown on silicon substrate using microwave plasma chemical vapor deposition and hot-filament chemical vapor deposition technique

Diamond films on the p-type Si(111) and p-type(100) substrates were prepared by microwave plasma chemical vapor deposition (MWCVD) and hot-filament chemical vapor deposition (HFCVD) by using a mixture of methane CH₄ and hydrogen H₂ as gas feed. The structure and composition of the films have been investigated by X-ray Diffraction, Raman Spectroscopy and Scanning Electron Microscopy methods. A high quality diamond crystalline structure of the obtained films by using HFCVD method was confirmed by clear XRD-pattern. SEM images show that the prepared films are poly crystalline diamond films consisting of diamond single crystallites (111)-orientation perpendicular to the substrate. Diamond films grown on silicon substrates by using HFCVD show good quality diamond and fewer non-diamond components.     

圆柱形衬底金刚石涂层的制备与表征研究

采用偏压增强热丝化学气相沉积法(BE-HFCVD),以WC-Co硬质合金圆柱体为衬底沉积金刚石薄膜.研究了提高金刚石薄膜形核密度和涂层附着力的新型复合衬底预处理方法,研究结果表明,采用新型复合衬底预处理工艺后,衬底表面凸凹不平,粗糙度达到366nm,相比未采用预处理工艺的表面粗糙度94.5nm,可以大大提高金刚石形核密度,并且处理后钻的成分含量从6%减低到0.4%,在很大程度上提高了金刚石涂层与村底之间的附着强度.研览结果还表明制备的金刚石涂层均匀且具有较好的表面质量.     

Direct nucleation of cubic boron nitride on silicon substrate

Cubic boron nitride thin films were deposited on silicon substrates by plasma chemical vapor deposition, and the nanostructure of the interface between the film and substrate was investigated by cross-sectional high-resolution transmission electron microscopy. Cubic boron nitride was found to nucleate directly on the properly pretreated silicon substrates in some local areas; in particular, cBN nuclei with a size of approximately 3 nm are nucleated on (111)–(100) step surfaces with an epitaxial relationship. This suggests the possibility of direct growth of cubic boron nitride in special surface environments of silicon substrates.     

The orientation effect of silicon grains on diamond deposition

Diamond deposition on mirror-polished polycrystalline silicon substrates which have grains in various orientations has been investigated using electron backscatter diffraction (EBSD) method with scanning electron microscopy (SEM). Diamond was deposited by microwave plasma chemical vapor deposition with application of a negative bias voltage on the substrate. The evidence from systematic SEM observations shows that silicon orientation determined by EBSD has a strong effect on diamond nucleation. In general, the diamond nucleation density on Si grains oriented close to <100> is the highest, while it is the lowest for those grains close to <111>, under the same experimental conditions for deposition. The same phenomena have been observed in the range of methane concentration from 2% to 4% in hydrogen.     

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.     

The synthesis of graphene nanowalls on a diamond film on a silicon substrate by direct-current plasma chemical vapor deposition

Graphene nanowalls have been synthesized on diamond by direct-current plasma enhanced chemical vapor deposition (CVD) on silicon substrates pre-seeded with diamond nanoparticles in gas mixtures of methane and hydrogen. Switching from diamond CVD to graphene CVD is done by increasing the methane concentration and decreasing the plasma power without breaking the vacuum. Graphene nanowalls stand on the CVD diamond film to form a 3-dimensional network. Scanning electron microscopy, high-resolution transmission electron microscopy, UV and visible Raman scattering and electrochemical cyclic voltammetry measurements are used to characterize the multi-layer turbostratic graphitic carbon nanostructure and demonstrate its electrochemical durability with a low background current in a wide electrochemical potential window.     

Influences of the substrate temperature on the films plasma-polymerized at the low system pressure

Summary Influences of the substrate temperature and the system pressure on the plasma polymerization were investigated from a viewpoint of the chemical composition of the deposited polymers. Both parameters influenced the plasma polymerization of tetrafluoroethylene (TFE). The deposition at the high substrate temperature as well as at the low system pressure led to the formation of plasma polymers being poor in fluorine moieties. This result indicates that the chemical composition of the deposited plasma polymers may be determined by interactions of activated molecules with substrate surfaces.     

Large area deposition of boron doped nano-crystalline diamond films at low temperatures using microwave plasma enhanced chemical vapour deposition with linear antenna delivery

We report on the preparation and characterisation of boron (B) doped nano-crystalline diamond (B-NCD) layers grown over large areas (up to 50 cm × 30 cm) and at low substrate temperatures (< 650 °C) using microwave plasma enhanced linear antenna chemical vapour deposition apparatus (MW-LA-PECVD). B-NCD layers were grown in H₂/CH₄/CO₂ and H₂/CH₄ gas mixtures with added trimethylboron (TMB). Layers with thicknesses of 150 nm to 1 μm have been prepared with B/C ratios up to 15000 ppm over a range of CO₂/CH₄ ratios to study the effect of oxygen (O) on the incorporation rate of B into the solid phase and the effect on the quality of the B-NCD with respect to sp³/sp² ratio. Experimental results show the reduction of boron acceptor concentration with increasing CO₂ concentration. Higher sp³/sp² ratios were measured by Raman spectroscopy with increasing TMB concentration in the gas phase without CO₂. Incorporation of high concentrations of B (up to 1.75 × 10²¹ cm³) in the solid is demonstrated as measured by neutron depth profiling, Hall effect and spectroscopic ellipsometry.     

Effect of substrate temperature on the selective deposition of diamond films

Selective diamond films on roughened Si(100) substrates with patternings have been achieved by microwave plasma chemical vapor deposition (MP-CVD). The films have been characterized by scanning electron microscopy (SEM) and Raman spectra. The influence of substrate temperature on the selective deposition of diamond films has been discussed in detail: the diamond nucleation density on the SiO₂ mask increased with substrate temperature while the effect of the selective deposition of diamond films deteriorated; the optimized deposition temperature conditions have been concluded.     

Roles of nitrogen and boron hydride adsorptions on silicon (001) substrate in boron nitride growth

The reactivities of nitrogen and boron hydrides with silicon (001) substrate were studied with first-principle theories in combination with a frontier orbital theory. Either positive or negative ionic species of the hydride were found to be particularly highly reactive on the silicon surface in our ab-initio calculations. The different characteristics of NH₂ and BH₂ species deposited on a Si(001) substrate were further revealed by calculating their potential energies on the surface using density functional theory. The hydronitrogen species were found to form a more oriented bond with the substrate in comparison with the hydroboron species. Accordingly, a nitride substrate is suggested to improve the interfacial bonding and therefore BN film adhesion in chemical vapor deposition for synthesis of high-quality cBN films.     

Effect of titanium metal in the prenucleation of ultrananocrystalline diamond film growth at low substrate temperature

Ultrananocrystalline diamond (UNCD) film is usually grown in methane–argon plasma unlike methane–hydrogen plasma conventionally used to deposit microcrystalline diamond film. The prenucleation and growth mechanism of these two types of diamond films are different as well. The present study introduces titanium metal powder during ultrasonication of silicon substrate to enhance the nucleation density of UNCD. A titanium thin film was also used at the interface to find the effect of metal on the growth of diamond film. The nucleation density of as-grown film was estimated from the FE-SEM images. After 20 min of growth, nucleation density reaches to 10¹¹/cm² on a surface pretreated by titanium mixed nanodiamond powder. Raman study was carried out for qualitative analysis of different carbon phase present in the UNCD films. X-ray photoelectron spectroscopy (XPS) was used to understand the growth mechanism by detecting the formation of carbon phase and metal carbide formation at the surface after stopping the growth at different time intervals.     

High-quality diamond film deposition on a titanium substrate using the hot-filament chemical vapor deposition method

Diamond film on titanium substrate has become extremely attractive because of the combined properties of these two unique materials. Diamond film can effectively improve the properties of Ti for applications as aerospace and biomedical materials, as well as electrodes. This study focuses on the effects of process parameters, including gas composition, substrate temperature, gas flow rate and reactor pressure on diamond growth on Ti substrates using the hot-filament chemical vapor deposition (HFCVD) method. The nucleation density, nuclei size as well as the diamond purity and growth tendency indices were used to quantify these effects. The crystal morphology of the material was examined with scanning electron microscopy (SEM). Micro-Raman spectroscopy provided information on the quality of the diamond films. The growth tendency of TiC and diamond film was determined by X-ray diffraction analysis. The optimal conditions were found to be: CH₄:H₂ = 1%, gas flow rate = 300 sccm, substrate temperature T_sub = 750 °C, reaction pressure = 40 mbar. Under these conditions, high-quality diamond film was deposited on Ti with a growth rate of 0.4 μm/h and sp² carbon impurity content of 1.6%.     

Electron microscopy of interfaces in chemical vapour deposition diamond films on silicon

The structure of interfaces in diamond films grown on Si(100) has been investigated by transmission electron microscopy for the early stages of microwave-assisted chemical vapour deposition. Using conditions optimized for achieving so-called highly-oriented diamond films the depositions were performed in two steps, a bias-enhanced nucleation step and a subsequent growth step. Characteristic for the early deposition stages is the self-organized formation of regular arrays of predominantly {111}-facetted Si substrate surface grooves and islands elongated along [1̄10] and [110] directions. Subsequently, an interlayer of nanocrystalline β-silicon carbide islands forms, followed by the formation of epitaxially oriented diamond nanocrystals with high fractions of {111} interfaces. High-resolution electron microscopy of the interface regions depicts arrays of terminating {111} diamond planes at an average ratio of five diamond to four SiC lattice planes which corresponds to a remaining lattice mismatch of 2.3%. The orientation relationships between the lattices may be described by a coincidence site lattice model if the elastic lattice distortions are taken into account. Only small fractions of amorphous inclusions are present near interfaces, essentially consisting of amorphous carbon as could be deduced from analyses of the C K edge fine structure in electron energy loss spectra. The observations are compared with cases for which diamond nucleation directly on silicon has been obtained.     

Characterization of silicon films grown by atomic layer deposition on nanocrystalline diamond

Ultrathin silicon films were deposited on nanocrystalline diamonds by means of atomic layer deposition (ALD) from gaseous monosilane. The silicon deposition was achieved through the sequential reaction of SiH₄ saturated adsorption and in-site pyrogenation. X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution electron microscopy (HREM) and Fourier transform infrared (FTIR) spectra were utilized to investigate the structure and the morphology of Si-coated nanocrystalline diamonds. The results confirmed that continuous silicon films were successfully deposited on both basal planes and edges of nanocrystalline diamond particles by this ALD method and the structure of the film was mainly determined by deposition temperature and deposition cycle.     

On the pressure limits of diamond chemical vapor deposition

A detailed examination of proposed diamond growth mechanisms was performed. Within the framework of existing models, a simple argument is presented to explain the experimentally observed limits to diamond growth by chemical vapor deposition (CVD) at pressures below 1 Torr. Based on data from a variety of conventional CVD reactors it is suggested that the CVD of diamond is not possible in systems where the partial pressure of atomic hydrogen is less than 20 mTorr. The addition of oxygen appears to depress the pressure limit by a factor of 10, which is attributed to the presence of hydroxyl radicals that remain active at lower growth temperatures.     

Modeling of substrate surface temperature distribution during hot-filament assisted diamond deposition

The importance of substrate temperature in determining the quality, uniformity and growth rate of diamond films is now well recognized. In the hot-filament assisted chemical vapor deposition of diamond, the mechanism of heat transfer is unique. In addition to conduction, convection and radiation, filament-to-substrate heat transfer takes place by dissociation of molecular hydrogen at or near the filament and recombination of atomic hydrogen at the substrate surface. In this paper, the role of atomic hydrogen recombination in heat transfer is examined. Furthermore, the effects of system geometry and process variables on the substrate temperature distribution are analyzed. The results indicate that atomic hydrogen recombination at the substrate plays a significant role in substrate heating. In hot-filament assisted diamond deposition, system geometry, filament temperature and pressure are the most important factors in determining the substrate temperature distribution.     

Microwave plasma enhanced chemical vapor deposition of diamond in silicon pores

In this work, the feasibility of growing boron-doped diamond coatings, approximately 0.3 μm thick, on thin silicon substrates that have 50-μm diameter pores etched 125 μm deep has been demonstrated using deep reactive ion etching (DRIE) in combination with chemical–mechanical polishing (CMP). Using a microwave plasma enhanced chemical vapor deposition (MPECVD) cyclic growth process consisting of carburization, bias-enhanced nucleation, diamond growth and boron-doped diamond growth, uniform diamond coatings throughout the pores have been obtained. The coatings were characterized by Raman spectroscopy and scanning electron microscopy and the secondary electron emission coefficients were found to increase from 4 to 10 between 200 and 1000 V, in agreement with reported values for thicker polycrystalline diamond films grown under similar conditions.     

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.