Richard N. Tarrant

Plasma-based ion implantation utilising a cathodic arc plasma

Plasma-based ion implantation (PBII) is usually carried out with isotropic gaseous plasmas, such as a discharge in nitrogen. More recently, it has been applied using drifting plasmas, such as those produced by cathodic arcs, in order to allow efficient implantation of metallic species. The condensable nature of a cathodic arc plasma allows for the deposition of ion-stitched thin film coatings, as well as surface modification by ion implantation. In this paper the promising results for biomaterial fabrication are discussed in light of current limitations of the technique. The use of PBII to control preferred orientation in titanium nitride films is also discussed, together with implications for the physical mechanisms involved in the development of preferred orientations in thin films.

Control of stress and microstructure in cathodic arc deposited films

The almost fully ionized cathodic arc plasma is a versatile source for the deposition of thin films. Ion energies impinging on the growth surface can easily be controlled by applying substrate bias. The natural energy of the depositing ions is moderate (tens of electron volts) and generates substantial compressive stress in most materials. In hard materials (such as tetrahedral-carbon and titanium nitride), the high-yield stress makes the problem particularly severe. Recent work has shown that stress relaxation can be achieved by pulses of high ion-energy bombardment (/spl sim/10 keV) applied to the substrate during growth. In this paper, we describe the variation of intrinsic stress as a function of applied pulsed bias voltage (V) and pulse frequency (f) for deposition of carbon and titanium nitride films. We found that stress relaxation depends on the parameter Vf, so it is possible to achieve the same level of stress relief for a range of voltages by selecting appropriate pulsing frequencies. With the right choice of parameters, it is possible to almost completely eliminate the intrinsic stress and deposit very thick coatings. Our experimental results showed correlations between intrinsic stress and film microstructures, such as the preferred orientation. This leads to the possibility of controlling microstructure with high energy ion pulsing during growth. Molecular dynamics computer simulations of isolated impacts provide insight into the atomic-scale processes at work. Using the results of such simulations, we describe a model for how stress relief might take place, based on relaxation in thermal spikes occurring around impact sites of the high-energy ions.

Combined deposition and implantation in the cathodic arc for thick film preparation

Delamination is a major mode of failure for many thin films. A film may delaminate spontaneously if the strain energy released exceeds the adhesion energy per unit area. In this work we explore the use of pulsed ion bombardments with energies up to 20 keV. At this energy, which is much higher than that normally used in ion-assisted deposition, implantation into the underlying substrate will occur. The process may have beneficial effects on the film adhesion. We report that we have successfully prepared carbon films in excess of 4.5 μm in thickness on silicon substrates. The thick films demonstrate indentation properties similar to bulk glassy carbon. In addition, we have been able to modify the stoichiometry of films by using a combination of implantation and deposition.

Multilayered carbon films for tribological applications

Recent work has shown that for nano-layered structures consisting of two materials, an effect may be shown in which the elastic modulus and fracture toughness depend on the period of the structure. In this paper we create carbon multilayer structures by two methods using cathodic arc deposition with pulsed bias applied to the substrate. The multilayers are created in one class of structures simply by interrupting the deposition to form relaxed layers. In the other class of structures, the pulse bias conditions are varied periodically. The in-service performance of the structures is assessed by impact-enhanced pin-on-disc wear testing and indentation hardness. The structures are examined using bright field and dark field transmission electron microscopy. A 10 nm non-convergent electron probe was used for the micro-diffraction studies. A model for the structure of the interfaces between the relaxed layers and the as-deposited material, including evidence of preferred orientation is presented.

Carbon coating of Ti-6Al-4V for reduced wear in combined impact and sliding applications

Abstract A range of carbon coatings with different hardness and modulus was compared for wear and frictional behaviours using one-side-carbon-coated Ti-6Al-4V alloy couples tested under conditions of combined impact and sliding contact. Carbon films with hardness over 10 GPa were found to cause far greater volume loss of the uncoated counterpart, and the volume loss was approximately proportional to the extent of hardness deviation above 10 GPa. The coefficient of friction was shown to correlate positively with coating hardness. The tendency of a softer coating to possess a greater sp2 or graphite-like content provides more effective solid lubrication in a wet environment, hence minimising both wear and friction. The corresponding low film modulus also provides an optimal structural integrity of the composite system by minimising the elastic modulus mismatch between the film and the underlying substrate.

Influence of gas flow rate and entry point on ion charge, ion counts and ion energy distribution in a filtered cathodic arc

Abstract We report the results of an investigation in which we measured ion charge, ion counts and ion energy distributions of a titanium cathodic arc operated in a nitrogen atmosphere, using a Hiden mass selected ion energy analyser. Measurements were taken under standardised conditions of arc current and magnetic confinement. Two different gas entry points (over the cathode and in the main chamber close to the Hiden port), three different background gas pressures (2, 4 and 7.5 mtorr) and two different flow rates (23 and 71 sccm) were investigated. Counts were taken at 14, 24, 28, 48 and 62 amu, representing N + or N 2 2+ , Ti 2+ , N 2 + , Ti + and TiN + , respectively. Ion energy was measured up to a maximum of 80 eV relative to vessel wall potential. Multiple data sets were collected for each combination of gas entry, gas pressure and flow rate. Ion counts, ion energy and mean charge all decrease as flow rate increases. For cathode entry, Ti ion counts increase greatly, N ion counts decrease slightly, mean ion energy increases and mean ion charge does not show a clear trend. Similarly, the responses of ion counts, ion energies and mean ionic charges to increases in pressure do not show a single, clear trend. The results of the study are reported and some implications for PBII processing are discussed.

A pulsed cathodic arc spacecraft propulsion system

We investigate the use of a centre-triggered cathodic arc as a spacecraft propulsion system that uses an inert solid as a source of plasma. The cathodic vacuum arc produces almost fully ionized plasma with a high exhaust velocity (>104 m s−1), giving a specific impulse competitive with other plasma or ion thrusters. A centre trigger design is employed that enables efficient use of cathode material and a high pulse-to-pulse repeatability. We compare three anode geometries, two pulse current profiles and two pulse durations for their effects on impulse generation, energy and cathode material usage efficiency. Impulse measurement is achieved through the use of a free-swinging pendulum target constructed from a polymer material. Measurements show that impulse is accurately controlled by varying cathode current. The cylindrical anode gave the highest energy efficiency. Cathode usage is optimized by choosing a sawtooth current profile. There is no requirement for an exhaust charge neutralization system.

A novel pin-on-apparatus

A novel pin-on-disk apparatus was developed that provides a repetitive impact loading between periods of sliding through alternate lifting and dropping of a spring-suspended spinning disk, away from, and onto, a spring-supported pin, respectively. The combination of the repetitive impact loading and sliding achieved in the apparatus was able to induce film adhesion failure of a thin film coated disk within 20 min, which, in the absence of the impact loading, would have survived the test due to the adequate sliding wear resistance. The impact/sliding pin-on-disk apparatus developed is therefore a useful means of predicting the sliding wear resistance and film adhesion of a coated system simultaneously.

Langmuir probe study of a titanium pulsed filtered cathodic arc discharge

A Langmuir probe has been used to make measurements of plasma parameters as a function of time at the substrate position in a magnetically-filtered pulsed cathodic arc discharge. Electron density, ne, and effective electron temperature, Teff, were calculated as a function of time from the I–V curves. The Druyvesteyn method was used to determine the electron energy distribution. Ion density was calculated using the assumption of plasma quasi-neutrality and an average ion charge state. Results show that over the plateau region (350–600 µs) of the pulse, the electron energy distribution is Maxwellian with Teff = Te = (10 ± 1) eV. During the rise and fall times of the pulse, the electron energy distribution is non-Maxwellian with an effective temperature of up to 15 to 20 eV during the rise time and ~7 eV during the fall time. The electron density during the plateau is ne = (3.0–6.0 ± 0.5) × 1017 m−3.

Voltage dependence of cluster size in carbon films using plasma immersion ion implantation

Carbon films were prepared using a cathodic arc with plasma immersion ion implantation (PIII). Using Raman spectroscopy to determine cluster size, a comparison is made between cluster sizes at high voltage and a low duty cycle of pulses with the cluster sizes produced at low voltage and a higher duty cycle. We find that for ion implantation in the range 2-20 kV, the cluster size depends more on implantation energy (E) than implantation frequency (f), unlike stress relief, which we have previously shown [M.M.M. Bilek, et al., IEEE Trans. in Plasma Sci., Proceedings 20th ISDEIV 1-5 July 2002, Tours, France, Cat. No. 02CH37331, IEEE, Piscataway, NJ, USA, p. 95] to be dependent on the product Ef. These differences are interpreted in terms of a model in which the ion impacts create thermal spikes.