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      Hardness

      ?? First of all DLC is a form of a diamond and diamond... well it is Hard and Strong, there no doubt about it.

      ??What is DLC coating? hardness serves many purposes in all of its applications from cutting tools for obvious purposes to luxury watches for scratch protection.

      Diamond-Like Carbon Coatings Hardness Chemical Formula Topics Covered Properties of Diamond-Like Coatings Methods for Producing DLC Films Plasma Assisted CVD Ion Beam Deposition The Closed Field Unbalanced Magnetron Sputter Ion Plating Process Deposition of Stress-Free Films Applications Extrusion Dies Engine Applications Bone Saws Summary Properties of Diamond-Like Coatings Amorphous (a-C) and hydrogenated amorphous carbon (a-C:H) films have high hardness, low friction, electrical insulation, chemical inertness, optical transparency, biological compatibility, ability to absorb photons selectively, smoothness, and resistance to wear. For a number of years, these economically and technologically attractive properties have drawn almost unparalleled interest towards these coatings. Carbon films with very high hardness, high resistivity, and dielectric optical properties, are now described as diamond-like carbon or DLC, table 1. Table 1. Properties of diamond and DLC materials. Thin Film Bulk Property CVD Dia. a-C a-C:H Diamond Graphite Crystal Structure Cubic ao=3.561? Amorphous Mixed sp2 and sp3 bonds Amorphous sp3/sp2 Cubic ao=3.567? Hexagonal a=2.47 Form Faceted crystals Smooth or rough Smooth Faceted crystals Diamond-like carbon coatings enhance the hardness and resilience of bearing surfaces for use in joint arthroplasty. The purpose of this study was to evaluate the potential of a hard diamond-like carbon (DLC) coating to enhance the hardness and resilience of a bearing surface in joint replacement. The greater hardness of a magnesium-stabilized zirconium (Mg-PSZ) substrate was expected to provide a harder coating-substrate composite microhardness than the cobalt-chromium alloy (CoCr) also used in arthroplasty. Three femoral heads of each type (CoCr, Mg-PSZ, DLC-CoCr and DLC-Mg-PSZ) were examined. Baseline (non-coated) and composite coating/substrate hardness was measured by Vickers microhardness tests, while nanoindentation tests measured the hardness and elastic modulus of the DLC coating independent of the Mg-PSZ and CoCr substrates. Non-coated Mg-PSZ heads were considerably harder than non-coated CoCr heads, while DLC coating greatly increased the microhardness of the CoCr and Mg-PSZ substrates. On the nanoscale the non-coated heads were much harder than on the microscale, with CoCr exhibiting twice as much plastic deformation as Mg-PSZ. The mechanical properties of the DLC coatings were not significantly different for both the CoCr and Mg-PSZ substrates, producing similar moduli of resilience and plastic resistance ratios. DLC coatings greatly increased hardness on both the micro and nano levels and significantly improved resilience and resistance to plastic deformation compared with non-coated heads. Because Mg-PSZ allows less plastic deformation than CoCr and provides a greater composite microhardness, DLC-Mg-PSZ will likely be more durable for use as a bearing surface in vivo. Hardness (Hv) 3000-12000 1200-3000 900-3000 7000-10000 Density (g/cm3) 2.8-3.5 1.6-2.2 1.2-2.6 3.51 2.26 Refractive Index - 1.5-3.1 1.6-3.1 2.42 2.15 Electrical Resistivity (Ω/cm) >1013 >1010 106-1014 >1016 0.4 Thermal Conductivity (W/m.K) 1100 - - 2000 3500 Chemical Stability Inert Inert Inert Inert Inert Hydrogen Content (H/C) - - 0.25-1 - - Growth Rate (μm/hr) ~1 2 5 1000 (synthetic) - Methods for Producing DLC Films Several methods have been developed for producing diamond-like carbon films: · primary ion beam deposition of carbon ions (IBD) · sputter deposition of carbon with or without bombardment by an intense flux of ions (physical vapour deposition or PVD) · deposition from an RF plasma, sustained in hydrocarbon gases, onto substrates negatively biased (plasma assisted chemical vapour deposition or PACVD). Until recently, the work on DLC worldwide has not yielded the expected benefits in the field of wear resistance and general mechanical performance. Most of the success has been in applications for magnetic storage media and optical coatings. The reasons for this are: · only thin coatings (< 1μm) have been used · the 2D aspect of most of the deposition routes · the difficulty in gaining good adhesion to metallic substrates. Plasma Assisted CVD Plasma assisted CVD techniques employing RF and DC glow discharges in hydrocarbon gas mixtures produce smooth amorphous carbon and hydrocarbon films, which have mixed sp2 and sp3 bonds. These exhibit hardness values of 900-3000Hv. The CVD processes will generally require deposition temperatures of at least 600°C to give the required combination of properties, however, low temperature deposition is possible. The CVD technique gives good deposition rates and very uniform coatings, and is suited to very large-scale production. Ion Beam Deposition Another technique for DLC deposition is based on ion beam deposition. This has the advantage of being able to deposit high quality coatings at very low temperatures (near room temperature). The disadvantages are that the deposition rate is very low (1μm/hr maximum) and that even substrates of simple geometry need complex manipulation to ensure uniform deposition. The Closed Field Unbalanced Magnetron Sputter Ion Plating Process A technique has now been developed that can readily apply a-C:H films (>4μm) to substrates of any shape. The process is based on closed field unbalanced magnetron sputter ion plating (CFUBMS), figure 1, combined with plasma assisted chemical vapour deposition. The commercial importance of such a development is already being seen and the potential range of applications and possibilities are enormous. The technique is highly innovative and it provides the flexibility required to ensure excellent adhesion to any substrate, and the coating of any component shape or material, in a high productivity industrial process. Figure 1. Schematic of a four magnetron closed field unbalanced magnetron sputtering system (DG Teer, UK Patent 2258343A) The new technique combines the benefits of both plasma CVD and ion beam deposition. The deposition is carried out at 200°C in a closed field unbalanced magnetron sputter ion plating system (Teer Coatings UDP 400 or 800 series). The system was originally designed for reactive deposition of metal nitrides, carbides and oxides. The inherent versatility of the process has enabled the deposition of DLC in the system by combining two established techniques, PVD and CVD. Low pressure RF plasma CVD is adopted for high rate deposition (>5μm/hr), in combination with simultaneous ion assistance and physical vapour deposition from unbalanced magnetron sputtering sources, to give very high quality films. As with beam techniques, the low pressure of the process means that deposition is to some extent line-of-sight, which means that substrate manipulation is necessary to ensure uniform deposition. However, because the substrates are surrounded by four long magnetrons (>1m in length if necessary) the coating flux impinges on the substrates from all directions and, usually, only simple single axis rotation during deposition is necessary. Deposition of Stress-Free Films One of the main problems with DLC deposition at low temperature, is the creation of very high internal stress levels in the films. This, combined with the ensuing lattice mismatch when DLC is applied to a wide range of substrates, commonly leads to poor adhesion. In high mechanical stress applications, the adhesion of the films is of paramount importance. This problem has now been overcome by ensuring that there are no stress concentrations near the coating/substrate interface. The magnetron sources are used to reactively deposit a series of multilayer compounds prior to deposition of the DLC. The layers have graded interfaces. This ensures that there are no abrupt changes in composition, and that the stress is introduced into the film gradually. The optimum multilayer structure series is: titanium, titanium nitride, titanium carbonitride, titanium carbide, and then the DLC. It has also been subsequently found that the mechanical properties of the hard carbon films can be improved by incorporating a small percentage of metal dopant (usually ~5% titanium) in the final carbon structure. The resulting films have excellent friction and wear properties: · microhardness of up to 4000Hv · coefficient of friction during dry running in air against cemented WC always

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