The growth and characteristics of nanocrystalline diamond thin films with thicknesses from 20 nm to less than 5 µm are reviewed. These materials contain between 95% and >99.9% diamond crystallites, the balance being made up from other forms of carbon. Within this class of materials there is a continuous range of composition, characteristics, and properties which depend on the nucleation and growth conditions. It is convenient to classify these films as either ultra-nanocrystalline-diamond (UNCD) or nanocrystalline-diamond (NCD) based on their microstructure, properties, and growth environment. In general, UNCD materials are composed of small particles of diamond ca. 2–5 nm in size with sp2-carbon bonding between the particles. UNCD is usually grown in argon-rich, hydrogen-poor CVD environments, and may contain up to 95–98% sp3-bonded carbon. NCD materials start with high density nucleation, initiating nanometer-sized diamond domains which grow in a columnar manner with the grain size coarsening with thickness. NCD is generally grown in carbon-lean and hydrogen-rich environments. NCD and UNCD exhibit an interesting range of physical properties which find use in X-ray windows and lithography, micro- and nanomechanical and optical resonators, tribological shaft seals and atomic force microscopy (AFM) probes, electron field emitters, platforms for chemical and DNA sensing, and many other applications.

The CVD of Nanodiamond Materials

Today the development of clean technologies in all spheres of industrial manufacturing is an essential task, not only for material and metal finishing but also for plasma surface engineering. Among the most critical group of technologies which needs to be replaced by alternative technologies are processes used to produce functional galvanic and decorative coatings. The electroplating of finishes, such as hard chromium, cadmium and nickel in metal finishing is today recognized as a major source of environmental pollution in every country. Therefore wet bath technologies have started to lose favour compared with high performance dry coating methods such as physical vapour deposition (PVD), plasma-assisted chemical vapour deposition, chemical vapour deposition and thermal spraying. Among these techniques, the results obtained with PVD coatings in metal cutting and forming in the last 15 years show the most promising solution of the complicated situation in which galvanic coatings seemed to be technologically and economically irreplaceable. In this paper the general situation in this field is shown. Already today it is possible to replace efficiently some of the galvanic processes in specific cases (e.g. Cr, Ni, Cd, Zn, Au). It is important to point out that PVD is considered to be a technique which can provide not only metallic, but also alloyed and ceramic coatings with a virtually unlimited range of chemical composition and therefore controlled protective, mechanical and wear-resistant properties. Entering into competition with galvanic coatings the manufacturers of PVD coaters were confronted with new requirements: a huge quantity of substrates of the same size, to be chemically and plasma cleaned and then coated at the highest possible deposition rate. For industrial mass production one can therefore use only large PVD batch systems or in-line coaters. The alternative for today's low price galvanic coatings is therefore dry and clean PVD technologies, fully supported by legislation on environmental protection. The economics depend directly on the substrate type and the quantity. The first positive results on the replacement of electrodeposited nickel on aluminium substrates and hard chrome on soft iron are also reported here. A soldering test was made on a sputtered nickel layer. Wear and corrosion tests were performed with iron cores, coated with PVD CrN coating. All tests were made in the Slovenian automotive industry. Results show that for a large number of substrates PVD clean technology is already economically competitive with galvanic coatings.

PVD coatings as an environmentally clean alternative to electroplating and electroless processes

This paper reviews the growth of diamond by chemical vapour deposition (CVD). It includes the following seven parts: (1) Properties of diamond: this part briefly introduces the unique properties of diamond and their origin and lists some of the most common diamond applications. (2) Growth of diamond by CVD: this part reviews the history and the methods of growing CVD diamond. (3) Mechanisms of CVD diamond growth: this part discusses the current understanding on the growth of metastable diamond from the vapour phase. (4) Characterization of CVD diamond: we discuss the two most common techniques, Raman and XRD, which have been intensively employed for characterizing CVD diamond. (5) CVD diamond growth characteristics: this part demonstrates the characteristics of diamond nucleation and growth on various types of substrate materials. (6) Nanocrystalline diamond: in this section, we present an introduction to the growth mechanisms of nanocrystalline diamond and discuss their Raman features.This paper provides necessary information for those who are starting to work in the field of CVD diamond, as well as for those who need a relatively complete picture of the growth of CVD diamond.

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Diamond growth by chemical vapour deposition

Cr1−xNx coatings were deposited by magnetron sputtering at a substrate temperature of 200°C onto AISI 316 stainless-steel substrates immersed in an Ar/N2 plasma. The goal of this investigation was to study the influence of nitrogen content on the structural, mechanical and tribological properties of the Cr1−xNx coatings (with x being in the range of 0–0.4). The coating composition and microstructure were evaluated utilizing glow discharge optical emission spectroscopy and glancing angle X-ray diffraction, whereas the morphology was evaluated by scanning electron microscopy. Knoop microhardness, scratch adhesion, pin-on-disc sliding, ball-on-plate impact and abrasive wheel wear tests were performed to evaluate the mechanical and tribological properties. The presence of Cr, Cr2N, CrN (and mixtures of these phases) has been identified and related to the film composition. For Cr1−xNx coatings with x≤0.16, only the α-Cr phase could be detected. A progression towards a denser microstructure was found with increasing nitrogen content up to x=0.29, associated with an increase in hardness from 700 up to 2400 HK0.025. Cr1−xNx coatings with x=0.10–0.16 showed good adhesion and the best abrasive and pin-on-disk sliding wear resistance, together with less crack development around the indentation area (and thus improved toughness) in impact tests after 50 000 impacts, against both steel and cemented tungsten carbide balls. The hardest coating (Cr0.71N0.29) performed best in terms of reducing the resulting impact indentation volume.

Structure, mechanical and tribological properties of nitrogen-containing chromium coatings prepared by reactive magnetron sputtering

Effect of grinding on the erosion behaviour of a WC–Co–Cr coating deposited by HVOF and detonation gun spray processes

With the increasing concern over toxic wastes produced by conventional metal finishing operations there is a strong drive in the USA to replace “dirty” electroplating processes (especially chrome and cadmium) with “clean” technologies. While many companies and military agencies continue to think in terms of less polluting electroplates, others are turning away from bath technologies completely, in favor of the modern high performance dry coating methods such as physical vapor deposition (PVD), chemical vapor deposition (CVD) and thermal spraying. Under funding from the US Defense Department's Advanced Research Projects Agency, various dry alternatives to electrolytic hard chrome, namely PVD, plasma nitriding, high velocity oxyfuel (HVOF), laser CVD and laser cladding, have been examined for use both in original equipment manufacture and in rebuilding worn components. In this paper hard chrome replacement is used to illustrate the requirements for replacement of electrolytic coatings in general. Hard chrome alternatives are being evaluated to ensure not only that their performance is at least comparable with the chrome they replace, but also that they fit with the way in which the coated components are produced, used, and refurbished. They must be reliable and cost-effective, and must fit the needs of the end user over the entire life cycle of the coated component. HVOF, PVD and duplex plasma-nitride PVD coatings show great promise as replacements in a variety of applications, from bearing surfaces and hydraulics to decorative finishes. The combination of performance data with cost analyses shows that these alternatives can be cost-effective chrome replacements. For example, HVOF coatings can provide more than twice the life in sliding wear at half the cost.

The replacement of electroplating

The corrosion behavior of several coating/substrate combinations was determined using the ASTM B117 Salt Fog Test. The coatings were electrodeposited hard chromium (EHC) and two high-velocity oxygen-fuel (HVOF) thermal-sprayed coatings, tungsten-carbide/cobalt (WC/Co) and Tribaloy 400 (T400). The substrates were 4340 steel, 7075 aluminum alloy, and PH13-8 stainless steel. On the 7075 Al alloy, a sulfamate nickel layer was deposited prior to the deposition of hard chromium. The results indicated that on the 4340 steel none of the coatings provided significant protection, with equivalent performance between the EHC and WC/Co coatings and slightly poorer performance for the T400. On the 7075 Al alloy, the EHC with sulfamate nickel exhibited excellent performance as no pits or blisters were noted on the faces or edges of the samples. The WC/Co showed no pitting or blistering on the faces but had a significant amount of pitting along the edges. The EHC and WC/Co coatings performed well on the 13-8 stainless steel as no pits or blisters were noted on the faces or edges. The T400 coatings had rust stains on the faces and edges but defects could not be seen with the unaided eye or at a 7× magnification.

Salt fog corrosion behavior of high-velocity oxygen-fuel thermal spray coatings compared to electrodeposited hard chromium

The possibility of low temperature growth of diamond films on silicon substrates was investigated using an electron cyclotron resonance microwave plasma source to provide activated precursor species from CH 4 -H 2 and CO-H 2 gas mixtures at low pressures (1.33 Pa (10 mTorr) to 5.32 Pa (40 mTorr)). Diamond crystallites and continuous diamond films were nucleated and grown on (100) and (111) oriented silicon substrates over the temperature range 500–600 °C. Above 600 °C, microcrystalline graphite was formed. The morphology of the films was investigated using scanning electron microscopy (SEM) and the structure was investigated using laser micro-Raman spectroscopy and X-ray diffraction (XRD) as a function of the deposition parameters. The Raman and XRD results indicated that the films were nanocrystalline with a high defect concentration.

Diamond thin film growth on silicon at temperatures between 500 and 600 °C using an electron cyclotron resonance microwave plasma source

Optical emission spectroscopy, Langmuir probes, and B‐field probes have been used to characterize electron cyclotron resonance plasmas used in the deposition of polycrystalline diamond films. These plasmas were generated at 1.33 Pa using selected ratios of CO:H2 and CH4:O2:H2 that produced the highest quality films (i.e., high degree of faceting and an intense Raman peak at 1332 cm−1). Electron temperature, ion density, plasma potential and B‐field profiles obtained at optimum growth conditions are presented. The best diamond films were produced on silicon (100) substrates biased to +40 V dc and maintained at a constant temperature of 500 °C. These substrates were placed downstream of an electron cyclotron resonance (ECR) layer (the region of high plasma density created by resonant microwave absorption at the 87.5 mT B‐field contour), characterized by a Te near 1 eV and ion densities near 4×1012 cm−3. Actinometry was used to quantify the optical emission spectra. In the CO:H2 system, quality films were ob...

Characterization of electron cyclotron resonance plasmas optimized for the deposition of polycrystalline diamond films

Diamond crystallites and thin films have been deposited onto polycrystalline aluminum substrates utilizing an electron cyclotron resonance microwave plasma-assisted chemical vapor deposition (ECR-PACVD) method. For all depositions, the substrates were biased to +40 V dc with respect to ground and their temperature was maintained at 500 °C. Similar deposits were obtained from two different feedgas systems at a total pressure of 1.33 Pa (10 mTorr). The first system consisted of a carbon monoxide (CO) and hydrogen (H2) mixture (CO:H2 = 20:80), and the second was a methane (CH4), oxygen (O2), and hydrogen (H2) mixture (CH4:O2:H2 = 21:10:69). The deposits were subsequently characterized by scanning electron microscopy, micro-Raman spectroscopy, and x-ray diffraction. The results of these analyses indicate that polycrystalline diamond was deposited onto aluminum substrates, as both individual crystallites and continuous films.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400019117

B.D. Sartwell

Papers

The replacement of electroplating

Salt fog corrosion behavior of high-velocity oxygen-fuel thermal spray coatings compared to electrodeposited hard chromium

Diamond thin film growth on silicon at temperatures between 500 and 600 °C using an electron cyclotron resonance microwave plasma source

Characterization of electron cyclotron resonance plasmas optimized for the deposition of polycrystalline diamond films

Deposition of diamond onto aluminum by electron cyclotron resonance microwave plasma-assisted CVD