K.C. Walter

Adherent diamond-like carbon coatings on metals via plasma source ion implantation

Various techniques are currently used to produce diamond-like carbon (DLC) coatings on various materials. Many of these techniques use metallic interlayers, such as Ti or Si, to improve the adhesion of a DLC coating to a ferrous substrate. An alternative processing route would be to use plasma source ion implantation (PSII) to create a carbon composition gradient in the surface of the ferrous material to serve as the interface for a DLC coating. The need for interlayer deposition is eliminated by using a such a graded interface. A PSII approach has been used to form adherent DLC coatings on magnesium, aluminum, silicon, titanium, chromium, brass, nickel and tungsten. A PSII process tailored to create a graded interface allows deposition of adherent DLC coatings even on metals that exhibit a positive heat of formation with carbon, such as magnesium, iron, brass and nickel.

Investigation of mechanical and tribological properties of amorphous diamond-like carbon coatings

We investigated the mechanical and tribological properties of amorphous diamond-like carbon (DLC) coatings deposited on Si(100) by a pulsed bias deposition technique. Tribological studies were performed using a pin-on-disc (POD) apparatus under a normal load of 6.25 N and at 10% relative humidity, with a ruby pin as a slider. Hardness measurements were performed using a nanoindenter and apparent fracture toughness using indentation techniques. We studied the influence of residual stresses on apparent fracture toughness. The data revealed that the thickness, hardness and compressive stress of the coating play different roles in the apparent fracture toughness. Crack initiation is influenced by the thickness and hardness of the coating, whereas crack propagation is influenced by the compressive stress in the film. The apparent fracture toughness of DLC coatings increased with coating hardness.

Increased wear resistance of electrodeposited chromium through applications of plasma source ion implantation techniques

Abstract Three plasma source ion implantation (PSII) processes, using either ammonia (NH 3 ), methane (CH 4 ), or oxygen (O 2 ) gases, are shown to increase the surface hardness and wear resistance of electrodeposited hard chromium. Characterization of the implanted surfaces indicates the formation of hard compounds, namely polycrystalline chromium-nitride (CrN), chromium-carbide (Cr 3 C 2 ), and chromium-oxide (Cr 2 O 3 ). For wear resistant applications of hard Cr coatings, PSII processes could extend the wear lifetime of coated components. By extending the lifetime and thus reducing the need for re-coating, PSII processes could also reduce the amounts of carcinogenic Cr +6 released into the environment.

Surface modification of AISI-4620 steel with intense pulsed ion beams

A 300 keV, 30 kA, 1 μs intense beam ofcarbon, oxygen, and hydrogen ions is used for the surface treatment of AISI-4620 steel coupons, a common material used in automotive gear applications. The beam is extracted from a magnetically-insulated vacuum diode and deposited into the top 1 μm of the target surface. The beam-solid interaction causes a rapid melt and resolidification with heating and cooling rates of up to 1010 K/s. Treated surfaces are smoothed over 1 μm-scale lengths, but are accompanied by 1 μm-diameter craters and larger-scale roughening over ≥ 10 μm, depending on beam fluence and number of pulses. Treated surfaces are up to 1.8 × harder with no discernible change in modulus over depths of 1 μm or more. Qualitative improvements in the wear morphology of treated surfaces are reported.

Multilayer hard carbon films with low wear rates

Abstract Vacuum arc deposition has been used to grow thin, ultrahard carbon films. Multilayer structures consisting of four alternating double layers of carbon with sp 3 contents of 80 and 40%, respectively, were grown using a pulse bias technique for ion energy control. The relative proportion of the soft layer (40% sp 3 content) in the multilayer structure was varied from 10 to 90%. The hardness of the multilayer structure is a weighted average of the hardnesses of the component layers. The overall compressive growth stress is less than would be expected from the proportions of the layer components. Pin-on-disk wear testing showed that multilayer films have a lower wear rate than monolithic films, especially at higher loads. The lowest wear rate was measured in a film with alternating layers of approximately equal thicknesses of the hard and soft layers.

Advances in PSII techniques for surface modification

Abstract Recent activities in plasma source ion implantation (PSII) technology include scale-up demonstrations for industry and development of variations on the original PSII concept for surface modification. This paper presents an overview of the continued growth of PSII research facilities world-wide and the industrial demonstrations within the USA. In order to expand the applicability of PSII, Los Alamos is actively researching a PSII-related technique called plasma immersion ion processing (PIIP). In one case, a pulsed-biased target can be combined with cathodic arc sources to perform ion implantation and coating deposition with metal plasmas. Erbium plasmas have been combined with oxygen to deposit erbia (Er 2 O 3 ) coatings that are useful for containment of molten metals. In a second case, hydrocarbon, inorganic and organometallic gases are utilized to create a graded interface between the substrate and the coating that is subsequently deposited by using pulsed-bias techniques. PIIP represents a significant advance since it allows coating deposition with all the strengths of the original PSII approach. Diamond-like carbon (DLC) and boron carbide are two such coatings that will be highlighted here for tribological applications.

FRACTURE TOUGHNESS OF DIAMONDLIKE CARBON COATINGS

The fracture behavior of diamondlike carbon (DLC) coatings on Si substrates has been examined using microindentation. The presence of DLC coatings reduces the radial crack length to less than one-half the crack length observed in uncoated Si at the same indenter load. A total work of fracture analysis of the radial cracks formed in the DLC-coating/Si-substrate system gives 10.1 MPa m 1/2 as the average fracture toughness for DLC alone. A bond-breaking calculation for DLC suggests that the elastic limit fracture toughness should be 1.5 MPa (m) 1/2 . The higher value obtained from experiment and total work analysis suggests that plastic work and/or a tortuous path crack evolution occurred during DLC fracture process.

DIAMONDLIKE CARBON DEPOSITION ON SILICON USING RADIO-FREQUENCY INDUCTIVE PLASMA OF AR AND C2H2 GAS MIXTURE IN PLASMA IMMERSION ION DEPOSITION

Diamondlike carbon (DLC) was deposited on silicon using a plasma immersion ion deposition (PIID) method. Inductive radio-frequency plasma sources were used to generate Ar and C2H2 plasmas at low gas pressures ranging from 0.04 to 0.93 Pa. The film stress and hardness were sharply dependent upon bias voltage at an operating pressure of 0.04 Pa. A maximum hardness of 30 GPa and compressive stress of 9 GPa was observed at a pulsed bias of −150 V bias (carbon energy of 80 eV). The mechanical properties of DLC films are correlated with UV Raman peak positions which infer sp3-bonded carbon contents.

Recent advances in plasma source ion implantation at Los Alamos National Laboratory

Abstract Plasma source ion implantation (PSII) is an environmentally benign, potentially cost-effective alternative to conventional lineof-sight, accelerator-based implantation and wet-chemical plating processes. PSII offers the potential of producing a high dose of ions in a relatively simple, fast and cost-effective manner, allowing the simultaneous implantation of large surface areas (many square meters), complex shapes and multiple components. The dynamics of the transient plasma sheath present during PSII have been modeled in both 1 1/2-D and 2 1/2-D (one or two spatial dimensions, plus time), and recent results from these efforts are compared with measurements of the uniformity of the implanted ion dose in complex configurations. Ammonia gas (NH 3 ) has been used as a nitrogen source for PSII processing of electroplated hard chromium. A retained dose of 2.2 × 10 17 N atoms cm −2 has been demonstrated to increase the surface hardness of the electroplated Cr by 24%, and decrease the wear rate by a factor of four, without any evidence of increased hydrogen concentration in the bulk material. By adjusting the repetition rate of the applied voltage pulses, and therefore the power input to the target, controlled, elevated temperature implantations have been performed, resulting in enhanced diffusion of the implanted species with a thicker modified surface layer. Experimental work has been performed utilizing cathodic arcs as sources of metallic ions for implantation, and preliminary results of this work are given. The area of ion-beam-assisted deposition (IBAD) has been explored utilizing PSII, with large surface area diamond-like carbon (DLC) layers being generated which can exhibit hardnesses in excess of 20 GPa.

Preparation of diamondlike carbon films by high‐intensity pulsed‐ion‐beam deposition

Diamondlike carbon (DLC) films were prepared by high‐intensity pulsed‐ion‐beam ablation of graphite targets. A 350 keV, 35 kA, 400 ns beam, consisting primarily of hydrogen, carbon, and oxygen ions was focused onto a graphite target at a fluence of 15–45 J/cm2. Amorphous carbon films were deposited at up to 30 nm per pulse, corresponding to an instantaneous deposition rate greater than 1 mm/s. Electrical resistivities were between 1 and 1000 Ω cm. Raman spectra indicate that diamondlike carbon is present in most of the films. Electron‐energy‐loss spectroscopy indicates significant amounts of sp3‐bonded carbon, consistent with the presence of DLC. Scanning electron microscopy showed most films contain 100 nm features, but micron size particles were deposited as well. Initial tests revealed favorable electron field‐emission behavior.