Da-Yung Wang

Catalysis effect of metal doping on wear properties of diamond-like carbon films deposited by a cathodic-arc activated deposition process

Abstract Diamond-like carbon (DLC) films containing various metal dopings were synthesized by using a cathodic-arc activated deposition process. Metal plasma with intensive ion energies catalyzes the decomposition of hydrocarbon gases (C 2 H 2 ), and induces the formation of hydrogenated DLC films with a mixture of sp 2 and sp 3 carbon bonds. The composite film structure consists of a metal-doped DLC film on top of a graded metal nitride interlayer, which provides enhanced mechanical and tribological properties. The catalysis effect of three common transition metal plasmas, including Cr, Ti, and Zr was investigated. This experiment depicts the advantage of the catalysis effect of Cr plasma in synthesizing DLC films with a higher sp 3 carbon bond ratio comparing with that of Ti and Zr plasma. The wear properties were correlated with the metal doping. The optimized Cr-C:H films with Cr and CrN interlayers give satisfactory wear performance in a ball-on-disk test with a lower wear rate of 2×10 −17 m 3 /Nm and a lower friction coefficient of 0.1 sliding against WC counterparts. Wear life of Cr plasma activated DLC (Cr-C:H) outperforms that of Ti and Zr by nine- and twofold, respectively. The Cr-C:H film exhibits a dense microstructure and a higher sp 3 bond ratio, showing great potential for wear applications.

Microstructural and tribological characterization of MoS2–Ti composite solid lubricating films

Abstract Sputtered MoS 2 thin films provide lubrication and wear improvements for vacuum and space applications. When exposed to humid environments, however, the MoS 2 films are prone to rapid deterioration by oxidation. In this study, we synthesized a composite coating, which consists of titanium metal and solid lubricating MoS 2 layers by the unbalanced magnetron sputtering process. Experimental results indicate that the Ti interlayers among the MoS 2 –Ti composite films effectively enhance the density and stability of the film structure. Transmission electron microscopy, X-ray diffractometry, and Raman analyses reveal the amorphous nature of the MoS 2 –Ti coating, which contains only a short-range order within the Ti interlayers. Ball-on-disc tribotests of the MoS 2 –Ti composite films show a higher friction coefficient, increased wear life and microhardness, and reduced wear debris as Ti interlayers increased in thickness. In addition to its densification and strengthening effect, the Ti interlayer also reacts preferentially with oxygen to form TiO 2 and thus effectively suppresses adverse oxidation of MoS 2 . Thus, in humid environments, the MoS 2 –Ti composite film is highly promising as a solid lubricant, as evidenced by its prolonged wear life and resistance to oxidation.

Microstructure and adhesion characteristics of diamond-like carbon films deposited on steel substrates

Abstract For tribological applications, the low friction coefficient and high microhardness of diamond-like carbon (DLC) films give significant advantages in cutting and forming non-ferrous materials. The inherently large residual stress of DLC films, however, prevents the depositing of thicker films. This study designed and implemented a compound interface, comprising a series of metal, metal nitride, and metal carbonitride interlayers deposited in a graded structure, between the DLC (a metal-doped a-C:H) film and M2 steel substrates. The tribological performance of the interface was evaluated using a scratch tester and ball-on-disk tribometer. Meanwhile, the failure mechanism of DLC deposited on M2 steel substrates was examined using SEM/EDS and TEM microscopy. Experimental results demonstrate an improved DLC hard coating with superior adhesion strength on the steel substrates.

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.

Characterization of Cr2O3/CrN duplex coatings for injection molding applications

Abstract In this study, we deposited Cr 2 O 3 /CrN duplex coatings, consisting of a thin Cr 2 O 3 oxide film on top of a CrN layer, with an unbalanced magnetron (UBM) sputtering technique. Microstructure characterization was conducted with X-ray diffraction (XRD) and scanning electron microscopy (SEM). Tribological evaluations were also performed with a scratch tester and ball-on-disc tribometer. Experimental results showed a well-adherent Cr 2 O 3 /CrN duplex coating morphology. The Cr 2 O 3 top-layer possesses an amorphous lattice structure. Gas flow rates and bias voltages had an impact upon mechanical and chemical properties of the Cr 2 O 3 /CrN coatings. To compensate the fluctuation of pumping efficiency, target poisoning and sputtering parameters, the flow rate of the reactive gas was rapidly and accurately regulated via a set of piezo valves and an optical emission monitor (OEM). OEM readings revealed the exact proportions of reactive gas consumed by metal plasma (US Patent 4 525 (1985) 417; Appl. Phys. Lett. 50 (1987) 1056; Surf. Coat. Technol. 49 (1991) 543). Gas flow rates were thus controlled, dynamically, with variations of processing conditions. At an OEM setting of 30–50% and a pulsed bias voltage of −55 V, the Cr 2 O 3 /CrN duplex coating demonstrated excellent micro-hardness (2533 Hv 25g ) and adhesion strength ( L c =75 N). The highest contact angle between water and Cr 2 O 3 was measured at 103° at an OEM setting of 30%. An improved mold releasing capability is anticipated, especially compared with the traditional hard chrome plating. Finally, an outstanding wear life of 4250 m was achieved on the Cr 2 O 3 /CrN duplex coating deposited at 50% of OEM setting. In summary, the enhanced properties of micro-hardness, film adhesion, mold releasing and wear life of the Cr 2 O 3 /CrN duplex coating demonstrated its potential for injection molding applications.

Microstructure analyses of CrN coating synthesized by a hybrid PVD and metal-plasma ion implantation process

Abstract Chromium nitride (CrN) coating is promising for precision forming and molding applications due to its superior tribological properties and environmentally friendly nature. In this study, enhancement of the surface properties of CrN, such as the density and wetting characteristics, were conducted with energetic ion implantation treatment. CrN coatings were deposited using a hybrid physical vapor deposition (PVD) and metal-plasma ion implantation (MPII) technique. MPII is a plasma-based ion implantation process based on an accelerated (10–80 keV) vacuum-arc metal plasma source with multiple charge states. At the initial coating stage, low dosage of MPII ion flux helps in surface activation and ion mixing. Subsequently, surface treatments of the as-deposited CrN coating with implantation of metal and/or carbon ions result in densification and phase transformation at a near-surface regime. The wear resistance, corrosion resistance, fatigue strength and mold-releasing mechanism are significantly improved. The surface study was followed by a series of microstructural and tribological analyses.

Deposition of CrN coatings by current-modulating cathodic arc evaporation ☆

Abstract Conventional cathodic arc evaporation (CAE) suffers from macroparticle contamination and excessive heat load on the substrates, resulting from the energetic ion flux emitted from hot cathode spots. In this study, a low frequency current-modulating controller was used to modulate the arc currents in high-low cycles. The deposition is conducted primarily during the high-current cycles, while a minimum background-current is maintained to sustain the arc discharge. Cathodes can be operated at currents lower than in the continuous CAE process. Macroparticle ejection is reduced substantially due to the effective cathode cooling. The preferred orientation of CrN changes from Cr 2 N (211) to CrN (111) by reducing heat load on substrates. The residual stress of CrN coatings is reduced from 3.8 GPa to 3.0 GPa in a current-modulating CAE process. The critical load of CrN deposited by the current-modulating CAE process is 78 N, which is comparable with that of CrN deposited by the continuous CAE process. Fracture toughness and film adhesion were significantly improved by the current-modulating mechanism, leading to a prolonged wear life of CrN coatings by three times. Results of this study provide a practical and effective way to operate CAE process with reduced substrate temperature, reduced coating roughness, and improved film adhesion.

Corrosion and tribological studies of chromium nitride coated on steel with an interlayer of electroplated chromium

Abstract The electrochemical and tribological behavior of CrN coatings on steel are investigated. A single layer of chromium nitride coating is compared with a double layer of the coating (CrN/Cr/steel) in which the interlayer chromium was produced by electroplating with the aim of improving the corrosion and tribological performance of the steel. The CrN coatings are deposited by using a reactive cathodic arc plasma deposition technology in an industrial scale, while the interlayer of chromium produced by electroplating. The coating assemblies have been compared in terms of hardness, wear and corrosion resistance. The composition and structure of the chromium nitride have been studied by X-ray diffraction (XRD), using both θ/2θ diffraction mode and the glancing-incidence X-ray diffraction mode. The surface morphology was examined by using SEM. The improvement in wear and corrosion resistance after cathodic arc plasma deposition with and without a hard chrome as an interlayer are discussed in considering the microstructure changes.

Synthesis of (Ti, Zr)N hard coatings by unbalanced magnetron sputtering

Abstract In this study, ternary (Ti,Zr)N thin films were synthesized using unbalanced magnetron sputtering with pulsed substrate bias. The pulsed bias effectively eliminated the arcing damage caused by surface contaminants and oxides on the substrate. The results show that a solid solution of (Ti,Zr)N (evidence from XRD and TEM analysis) was formed at all deposit parameters in which the multicomponent Ti–Zr–N coatings were deposited. The (Ti,Zr)N grains are columnar and grow in the (111) orientation. The ternary (Ti,Zr)N coating demonstrates an enhanced microhardness compared with the binary TiN and ZrN coatings deposited under equivalent conditions. A better combination of condition of the values of microhardness and adhesion was obtained at OEM of 60% and bias of –70 V.

Microstructure analysis of MoS2 deposited on diamond-like carbon films for wear improvement

A compound solid lubricating film containing a MoS2 top layer deposited on DLC interlayer by UBM sputtering technique was investigated for its tribological applications in humid environments. TEM, Raman and XRD analysis revealed the amorphous nature of the MoS2 film which contains only short-range order in its lattice structure. The as-deposited MoS2 compound films showed well-bonded interfaces. The MoS2, however, is very susceptible to humidity and oxidation, which resulted in higher friction coefficients and lower wear life. A friction coefficient of 0.05 was measured between steel balls and MoS2 in atmosphere of 90% RH. Excessive abrasive wear was identified, as a result of the wear debris and the oxidized transfer layers between MoS2 and its counterpart. The inclusion of a supportive DLC interlayer has effectively improved the wear behavior of MoS2 films under various loading conditions. The overall wear mechanism of MoS2 was complicated due to its oxidation problem which needs to be resolved for successful usage of MoS2 in humid environments.