Nanocrystalline diamond coatings for mechanical seals applications

A mechanical seal is a type of seal used in rotating equipment, such as pumps and compressors. It consists of a mechanism that assists the connection of the rotating shaft to the housings of the equipments, preventing leakage or avoiding contamination. A common cause of failure of these devices is end face wear out, thus the use of a hard, smooth and wear resistant coating such as nanocrystalline diamond would be of great importance to improve their working performance and increase their lifetime. In this paper, different diamond coatings were deposited by the HFCVD process, using different deposition conditions. Additionally, the as-grown films were characterized for, quality, morphology and microstructure using scanning electron microscopy (SEM) and Raman spectroscopy. The topography and the roughness of the films were characterized by atomic force microscopy (AFM).     

Residual stress, Young''s modulus and fracture stress of hot flame deposited diamond

Chemical vapour deposition (CVD) diamond coatings deposited on various substrates usually contain residual stresses. Since the residual stress affects the adhesion of the coating to the substrate, as well as the performance of the coating/substrate composite in many technical applications it is of importance to study the magnitude of these stresses.

In the present study the hot flame method was used to deposit diamond coatings on cemented carbide inserts by scanning the surface with a nine flame nozzle. By varying the oxygen to acetylene flow ratio and the deposition time coatings of different qualities and thicknesses were obtained. The residual strain/stress of the coatings was measured by three different methods: X-ray diffraction using the sin² (Ψ) method, Raman spectroscopy and disc deflection measurements. To extract the residual stress from the strain data the Young's modulus was obtained from bending tests of diamond cantilever beams manufactured from free standing diamond films. The latter technique was also used to determine the fracture stress of the diamond films.

All deposited coatings displayed a residual compressive strain/stress state. The residual strain in the diamond coatings did not vary with coating thickness (1.5 μm to 20 μm) but was found to increase from −1.8 × 10⁻³ to −2.2 × 10⁻³ with decreasing diamond quality. The compressive residual stress was found to decrease from −2 GPa to −1.3 GPa with decreasing diamond quality. This is mainly due to a decrease in Young's modulus (from 1.1 TPa to 0.6 TPa) with decreasing diamond quality. Also the fracture stress was found to decrease (from 1.8 GPa to 0.8 GPa) with decreasing diamond quality. The three methods used for measuring the stress state in the coatings, X-ray diffraction, Raman spectroscopy and deflection measurement, all give the same result. The deflection technique has the advantage that no information about the elastic properties of the coating is needed, whereas Raman spectroscopy has the best lateral resolution (≈5 μm) and is the fastest method (≈5 min).     

Deposition processes for films and coatings

Deposition of films and coatings on different substrates is carried out mainly by using physical and chemical vapour deposition (CVD). Various films and coatings of metals, oxides, carbides, nitrides, suicides, borides, etc., are successfully deposited by the CVD process. An update on the deposition process and its applications is discussed in this paper.     

Growth of nanocrystalline diamond films in CCl4/H2 ambient

We investigated the growth characteristics of the nanocrystalline diamond films using CCl₄/H₂ as gas sources in a hot-filament chemical vapor deposition (CVD) reactor. Successful growth of nanocrystalline diamond at typical growth condition of 1.5-2.5% CCl₄ and 550-730 °C substrate temperature has been demonstrated. Glancing angle X-ray diffraction (XRD) clearly indicated the formation of diamond in the films. Typical root-mean-square surface roughness of 10-15 nm and an optimal root-mean-square surface roughness of 6 nm have been achieved. Transmission electron microscopy (TEM) analyses indicated that nanocrystalline diamond film with an average grain size in the range of 10-20 nm was deposited from 2.5% CCl₄/H₂ at 610 °C. Effects of different source gas composition and substrate temperature on the grain nucleation and grain growth processes, whereby the grain size of the nanocrystalline film could be controlled, were discussed.     

用氢气/甲醇混合气体在微波等离子体CVD中合成纳米晶粒金刚石膜

用微波等离子体增强化学气相沉积方法(MPECVD),利用氢气和甲醇的混合气体,在硅片上沉积出纳米晶粒的金刚石薄膜.用扫描电子显微镜(SEM)、拉曼光谱(Raman)、原子力显微镜(AFM)及扫描隧道显微镜(STM)对薄膜的晶粒平面平整性及纯度进行了表征.通过SEM发现,提高甲醇浓度或降低沉积温度可以减小金刚石膜的晶粒尺寸.拉曼光谱显示薄膜中确实存在纳米晶粒的金刚石,并且薄膜的主要成分为金刚石.用AFM测得薄膜表面的粗糙度Rms＜80m,STM观测晶粒的平均尺寸在10～20m之间.研究结果表明,用MPECVD方法,利用氢气和甲醇的混合气体是制备纳米晶粒金刚石膜的一种理想方法.     

Effect of slurry composition on the chemical mechanical polishing of thin diamond films

Nanocrystalline diamond (NCD) thin films grown by chemical vapour deposition have an intrinsic surface roughness, which hinders the development and performance of the films’ various applications. Traditional methods of diamond polishing are not effective on NCD thin films. Films either shatter due to the combination of wafer bow and high mechanical pressures or produce uneven surfaces, which has led to the adaptation of the chemical mechanical polishing (CMP) technique for NCD films. This process is poorly understood and in need of optimisation. To compare the effect of slurry composition and pH upon polishing rates, a series of NCD thin films have been polished for three hours using a Logitech Ltd. Tribo CMP System in conjunction with a polyester/polyurethane polishing cloth and six different slurries. The reduction in surface roughness was measured hourly using an atomic force microscope. The final surface chemistry was examined using X-ray photoelectron spectroscopy and a scanning electron microscope. It was found that of all the various properties of the slurries, including pH and composition, the particle size was the determining factor for the polishing rate. The smaller particles polishing at a greater rate than the larger ones.     

Characteristics of a well-adherent nanocrystalline diamond thin film grown on titanium in Ar/CH4 microwave CVD plasma

The fabrication of a well-adherent diamond film on titanium and its alloys is always problematical due to the different thermal expansion coefficients of the two materials, the complex nature of the interlayer formed during diamond deposition, and the difficulty in achieving very high nucleation density. In this work, well-adherent and smooth nanocrystalline diamond (NCD) thin film is successfully deposited on pure titanium substrate by microwave plasma-assisted chemical vapor deposition (MWPCVD) method in Ar/CH₄ environment. It is found that the average grain size is less than 20 nm with a surface roughness value as low as 12 nm. Morphology, surface roughness, diamond crystal orientation and quality are obtained by characterizing the sample with field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy, respectively. Detailed experimental results and mechanisms for NCD film deposition are discussed.     

Studies of nanocrystalline Pd alloy films coated by electroless deposition

The microstructures of thin coating films of pure palladium and palladium alloys deposited from organic electrolytes onto different metallic substrates by electroless plating method have been investigated. The coatings are dense, pore-free 0.005–1 μm thick films with high adhesive strength to the substrate surface. X-ray spectral analysis, X-ray phase analysis, transmission and scanning electron microscopy were used to determine the composition and structure of alloy coatings of binary systems: Pd–Au, Pd–Ag, Pd–Ni, Pd–Pb, and ternary system Pd–Au–Ni. The coatings of Pd–Au, Pd–Ag, and Pd–Ni have a solid solution structure, whereas Pd–Pb is intermetallic compound. It has been found that the deposited films consist of nanocrystalline grains with sizes in the range of 11–35 nm. Scanning and transmission electron microscopy investigations reveal the existence of clusters formed by nanocrystalline grains. The origin for the formation of nanocrystalline structures of coating films is discussed.     

The versatility of hot-filament activated chemical vapor deposition

In the field of activated chemical vapor deposition (CVD) of polycrystalline diamond films, hot-filament activation (HF-CVD) is widely used for applications where large deposition areas are needed or three-dimensional substrates have to be coated. We have developed processes for the deposition of conductive, boron-doped diamond films as well as for tribological crystalline diamond coatings on deposition areas up to 50 cm × 100 cm. Such multi-filament processes are used to produce diamond electrodes for advanced electrochemical processes or large batches of diamond-coated tools and parts, respectively. These processes demonstrate the high degree of uniformity and reproducibility of hot-filament CVD. The usability of hot-filament CVD for diamond deposition on three-dimensional substrates is well known for CVD diamond shaft tools. We also develop interior diamond coatings for drawing dies, nozzles, and thread guides.Hot-filament CVD also enables the deposition of diamond film modifications with tailored properties. In order to adjust the surface topography to specific applications, we apply processes for smooth, fine-grained or textured diamond films for cutting tools and tribological applications. Rough diamond is employed for grinding applications. Multilayers of fine-grained and coarse-grained diamond have been developed, showing increased shock resistance due to reduced crack propagation.Hot-filament CVD is also used for in situ deposition of carbide coatings and diamond-carbide composites, and the deposition of non-diamond, silicon-based films. These coatings are suitable as diffusion barriers and are also applied for adhesion and stress engineering and for semiconductor applications, respectively.     

A study of polyaniline deposited nanocrystalline diamond films for glucose detection

Nitrogen-doped nanocrystalline diamond (NCD) films have been deposited on Si substrates in CH4/Ar/N2 gas mixtures by the microwave plasma enhanced chemical vapor deposition (MPECVD) technique. Such films contain very small diamond grains (10 to 30 nm) with high electrical conductivity (126 ohms(-1) cm(-1)) compared to un-doped ones. The films were characterized by scanning electron microscopy (SEM) and Raman spectroscopy. Near edge X-ray fine structure studies showed that the nitrogen-doped NCD had slightly higher sp2/sp3 bonding ratio compared to the un-doped sample. A nitrogen-doped NCD electrode was functionalized by conducting polymer films (polyaniline) to work as an interface for biomedical applications. Glucose sensing has been demonstrated based on this functionalized electrode. Linear response of the sensor has been observed for glucose concentration up to 9 mM.     

Mechanical properties of diamond thin films

Abstract

The mechanical properties of diamond films deposited via hot filament chemical vapour deposition have been determined using a range of techniques, and related to the composition and morphology of the diamond films as determined by laser Raman spectroscopy. As the quality of the film increases, its hardness (as determined by the volume law of mixtures hardness model) also increases until it is larger than values often reported for polycrystalline bulk material, a consequence of the very small grain size in the films. Coating adhesion, as determined from indentation adhesion tests, also appears to improve with coating quality. Variations in the behaviour of the friction coefficient between diamond films and diamond and steel counterfaces are less well defined, but it appears that the surface morphology of the film is important in dictating the behaviour rather than the quality of the diamond. These results are discussed in the context of the potential use of diamond coatings in tribological applications.

MST/1695     

Synthesis of ultrasmooth nanostructured diamond films by microwave plasma chemical vapor deposition using a He/H(2)/CH(4)/N(2) gas mixture

Ultrasmooth nanostructured diamond (USND) films were synthesized on Ti-6Al-4V medical grade substrates by adding helium in H(2)/CH(4)/N(2) plasma and changing the N(2)/CH(4) gas flow from 0 to 0.6. We were able to deposit diamond films as smooth as 6 nm (root-mean-square), as measured by an atomic force microscopy (AFM) scan area of 2 μm(2). Grain size was 4-5 nm at 71% He in (H(2) + He) and N(2)/CH(4) gas flow ratio of 0.4 without deteriorating the hardness (~50-60 GPa). The characterization of the films was performed with AFM, scanning electron microscopy, x-ray diffraction (XRD), Raman spectroscopy, and nanoindentation techniques. XRD and Raman results showed the nanocrystalline nature of the diamond films. The plasma species during deposition were monitored by optical emission spectroscopy. With increasing N(2)/CH(4) feedgas ratio (CH(4) was fixed) in He/H(2)/CH(4)/N(2) plasma, a substantial increase of CN radical (normalized by Balmer H(α) line) was observed along with a drop in surface roughness up to a critical N(2)/CH(4) ratio of 0.4. The CN radical concentration in the plasma was thus correlated to the formation of ultrasmooth nanostructured diamond films.     

Annealing of sputter-deposited nanocrystalline Cr-Ta coatings in a low-oxygen-containing atmosphere

As a protective hard coating on glass molding dies, Cr-Ta coatings were fabricated on binderless tungsten carbide substrates with a Ti interlayer by RF magnetron sputtering. The nanocrystalline Cr-Ta coatings were deposited at 550 °C, which revealed one nanocrystalline phase for the Ta-rich coating and two nanocrystalline phases for the Cr-rich coating. Annealing treatment was conducted at 600 °C in a 12 ppm O₂-N₂ atmosphere to evaluate the coating performance in a realistic glass molding environment. Both Auger electron spectroscopy and X-ray photoelectron spectroscopy depth profiles verified the outward diffusion of Cr, which formed a protective coating for the Cr-rich coatings. A scale of Cr₂O₃ and a Cr-depleted transition zone near the surface were identified by conducting a transmission electron microscopy investigation on the annealed Cr_0.71Ta_0.29 coating. The Cr-rich coating absorbed a smaller amount of oxygen, exhibited greater hardness, and maintained nanoscale surface roughness after annealing in the glass molding atmosphere, thus making it an appropriate protective coating for the die material.     

Characterization of diamond films deposited on titanium and its alloys

Titanium and its alloys have important applications for example in aerospace or as bioimplants. Some of these applications would be improved by diamond coatings. However the large thermal expansion mismatch between diamond and titanium or its alloys creates high residual stresses, up to about 7 GPa at 800 °C, which represent an important drawback. In this study, polycrystalline diamond films were deposited on pure titanium and Ti-6Al-4V in a classical tubular microwave plasma reactor from C-H(-O)-containing gas mixtures, at a temperature in the range 600–900 °C. Raman spectroscopy provided information about the diamond grain stress, which is obviously related to the deposition temperature. X-ray diffraction indicates the presence of titanium carbide or oxycarbide. Some other characterizations by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) are reported. It is shown that XPS coupled to argon ionic etching allows us to study the first steps of the deposition process. The structure and the chemical composition at the interface of a thicker deposit are obtained by TEM and EELS.     

Chemical vapor deposition of highly adherent diamond coatings onto co-cemented tungsten carbides irradiated by high power diode laser

The present investigation deals with the definition of a new eco-friendly alternative to pretreat Co-cemented tungsten carbide (WC-Co) substrates before diamond deposition by hot filament chemical vapor deposition (HFCVD). In particular, WC-5.8 wt %Co substrates were submitted to a thermal treatment by a continuous wave-high power diode laser to reduce surface Co concentration and promote the reconstruction of the WC grains. Laser pretreatments were performed both in N(2) and Ar atmosphere to prevent substrate oxidation. Diamond coatings were deposited onto the laser pretreated substrates by HFCVD. For comparative purpose, diamond coatings were also deposited on WC-5.8 wt %Co substrates chemically etched by the well-known two-step pretreatment employing Murakami's reagent and Caro's acid. Surface morphology, microstructure, and chemical composition of the WC-5.8 wt %Co substrates after the different pretreatments and the deposition of diamond coatings were assessed by surface profiler, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction analyses. Wear performance of the diamond coatings was checked by dry sliding linear reciprocating tribological tests. The worn volume of the diamond coatings deposited on the laser pretreated substrates was always found lower than the one measured on the chemically etched substrates, with the N(2) atmosphere being particularly promising.     

A comparative machining study of diamond-coated tools made by plasma torch,microwave, and hot filament techniques

An effective metal-cutting tool is usually a combination of a hard coating and a tough substrate. The successful deposition of diamond outside its thermodynamic stability range has stimulated the development of a new class of cutting tools: those with diamond-coated inserts of any desired style and edge geometry. The successful implementation of diamond coatings also expedited similar research in the deposition of cubic boron nitride. This paper presents superhard coating tools, with emphasis on diamond-coated WC-Co tools, the corresponding deposition of technologies and the foreseen metal-cutting applications.     

Chemical vapour deposition of microdrill cutting edges for micro- and nanotechnology applications

Conventional cemented tungsten carbide-cobalt (WC-Co) microdrills generally have a low cutting efficiency and short lifetime mainly because they operate at very high cutting speeds. Since it is relatively expensive to make microtools it is highly desirable to improve their lifetime and in-service performance. Microtools used to make microelectronic and mechanical systems (M.E.M.S) devices with sharp cutting edges, such as milling or drilling tools need protective coating in order to extend life and improve performance. One method of achieving this objective is to use a suitable surface engineering technology to deposit a hard wear resistant coating, such as diamond. Diamond has excellent mechanical properties, such as ultra-high hardness and a low friction coefficient. One of the most promising surface treatment technologies for depositing diamond onto complex shaped components is chemical vapour deposition (CVD). However, CVD of diamond coatings onto the cemented WC-Co tool has proved to be problematic. Binder materials such as cobalt can suppress diamond nucleation resulting in poor adhesion between the coating and substrate. In this paper the effects of pre-treated substrate material on the coating structure are reported. The morphology and the crystallinity of the as-grown films was characterised by using scanning electron microscopy (SEM). Raman spectroscopy was used to assess the carbon-phase purity and give an indication of the stress levels in the as-grown polycrystalline diamond films. The diamond coated tools have potential applications in micro- and nanomachining of micro- and nano-sized components used in M.E.MS.     

Cutting Performance and Wear Mechanism of Multilayer Diamond-Coated Tools

Monolayer and multilayer diamond films are deposited on WC-Co cemented carbide by hot-filament chemical vapor deposition. The growth characteristics of diamond coatings are analyzed. Cutting performance characteristics such as tool life and the stability of machining process in the machining of presintered ZrO₂ are compared based on the variation of cutting speed and resultant cutting force, and workpiece surface roughness. For the monolayer diamond coatings, as the concentration of CH₄ increases from 1% to 5%, the diamond crystal is transformed from micron columnar crystal to nanocluster crystal. The multilayer diamond coatings combine the advantages of micron- and nanocrystalline structures. The multilayer diamond-coated tool exhibits longer service life and better machining quality. Because of the appearance of the brittle–plastic conversion mechanism, the surface integrity of ZrO₂ processed by multilayer diamond-coated tool is relatively high. As for the uncoated tool, the workpiece is mainly machined by brittle spalling. The interfacial stratified fracture system between the interlayers is proposed to be the toughening mechanism of the multilayer structure.