Jari Koskinen

Tribological characteristics of diamond-like films deposited with an arc-discharge method

Using an are-discharge method, we deposited a diamond-like carbon film 600 nm thick on hardened steel. Characterization of the film was carried out with Raman spectroscopy. In dry sliding wear and friction tests, with a hardened steel pin as a counterpart, we obtained a friction coefficient between 10000 and 20000 cycles, with the maximum value of 0.18. The value decreased to 0.12 after about 100000 cycles. We obtained a wear coefficient of 7 × 10−17 m3/mN. A transfer layer formed on the pin during sliding and probably had the dominating effect on the tribological behavior. We observed in nanoindentation measurements that the film softened in a wear track during the first 20000 cycles. Although fracture pits on the wear track occurred, fracture is not the dominant failure mechanism of these films. Degradation of good tribological properties was caused mainly by partial wear-through of the film after 370000 cycles and by a subsequent redeposition of the transfer film on the wear track during prolonged sliding.

Comparison of diamondlike coatings deposited with C+ and various hydrocarbon ion beams

The mechanical properties of the diamondlike coatings deposited with mass‐separated C+, CH+3, CH+4, and C2H+2 ion beams have been compared. The hardness, abrasive wear resistance, and adhesion of the coatings prepared with the C+ ion beam were superior to those of the coatings prepared with other ions. The most serious drawback of the films prepared with hydrocarbon beams was their brittleness and weak adhesion.The mechanical properties of the diamondlike coatings deposited with mass‐separated C+, CH+3, CH+4, and C2H+2 ion beams have been compared. The hardness, abrasive wear resistance, and adhesion of the coatings prepared with the C+ ion beam were superior to those of the coatings prepared with other ions. The most serious drawback of the films prepared with hydrocarbon beams was their brittleness and weak adhesion.

Abrasive wear resistance of ion‐deposited hard‐carbon films as a function of deposition energy

Hard‐carbon layers were deposited on WC‐Co hard‐metal substrates using mass‐separated 12C+‐ion beams with ion energies varying between 10 eV and 2 keV. The abrasive wear resistance, hardness, density, and dc resistance of these films were measured as a function of ion energy. Coatings deposited at ion energies between 100 eV and 2 keV possessed the highest hardness, best wear resistance, and good adhesion to the substrate, whereas coatings prepared at ion energies Ei ≤50 eV suffered from poor adhesion. The resistance of the hard‐carbon coating against the abrasive wear was found to be about 300 times better than that of a typical ceramic Al2O3. In addition, Vickers hardness 120±20 GPa, density 3.3±0.3 g/cm3, and dc resistance 2×109 Ω cm were obtained. The hardness was found to remain unchanged after the thermal annealing up to 600 °C.

Diamond-like carbon coatings by arc-discharge methods

Abstract Pulsed arc-discharge deposition has been used to prepare diamond-like carbon (DLC) films on various substrates. Both direct and curved plasma methods have been used. Coatings with excellent tribological properties and good adhesion have been demonstrated. The coefficient of friction of diamond-like films against a Si3N4 ball was between 0.045 and 0.084 which is an order of magnitude smaller than for most other materials. The corresponding wear coefficient was between 0.5 × 10-16 and 1.7 × 10-16m3N m-1. The diamond-like films produced using mass-separated ion beams seem to be similar in structure and properties except that the arc-discharge deposited films suffer from graphite particles on the depositions, even when using a curved solenoid.

A model for particle growth in arc deposited armophous carbon films

Abstract Arc evaporation processes are connected with the emission of small particles which are incorporated in the growing film on the substrate. SEM, AFM and optical microscopy indicate a size increase of such objects during film growth. Because of their cone-like shape we call them cone-like nodules. Using conventional measurement methods that can determine the structure, it is hard to explain the behaviour and properties of those nodules, as they do not provide clear information about their nature. Square resolved Raman measurements show a mixture of graphitic and diamond-like structures, that allows to conclude that the nodule consists of a graphitic particle originated at the cathode, covered by an amorphous carbon film. We propose a new model that describes the growth of cone-like nodules in the homogeneous carbon film very well. It is based on the dependence of the DLC film properties on the incidence angle of ions. The cone-like shape and the growth behaviour at perpendicular and grazed ion incidence was simulated using a simple computer program. A comparison of the simulated data and the SEM-images show a good qualitative and quantitative agreement.

Diamond-like carbon (DLC) thin film bioelectrodes: effect of thermal post-treatments and the use of Ti adhesion layer.

The effect of thermal post-treatments and the use of Ti adhesion layer on the performance of thin film diamond like carbon bioelectrodes (DLC) have been investigated in this work. The following results were obtained: (i) The microstructure of the DLC layer after the deposition was amorphous and thermal annealing had no marked effect on the structure, (ii) formation of oxygen containing SiOx and Ti[O,C] layers were detected at the Si/Ti and Ti/DLC interfaces with the help of transmission electron microscope (TEM), (iii) thermal post-treatments increased the polar fraction of the surface energy, (iv) cyclic voltammetry (CV) measurements showed that the DLC films had wide water windows and were stable in contact with dilute sulphuric acid and phosphate buffered saline (PBS) solutions, (v) use of Ti interlayer between Pt(Ir) microwire and DLC layer was crucial for the electrodes to survive the electrochemical measurements without the loss of adhesion of the DLC layer, (vi) DLC electrodes with small exposed Pt areas were an order of magnitude more sensitive towards dopamine than Pt electrodes and (vii) thermal post-treatments did not markedly change the electrochemical behavior of the electrodes despite the significant increase in the polar nature of the surfaces. It can be concluded that thin DLC bioelectrodes are stable under physiological conditions and can detect dopamine in micro molar range, but their sensitivity must be further improved.

Tribological characterisation of hard carbon films produced by the pulsed vacuum arc discharge method

Abstract Hard diamond-like carbon (DLC) films were deposited on silicon and high-speed steel substrates using a pulsed vacuum arc discharge method. The plasma plume was focused on the substrate using a direct electromagnetic coil. Several methods were used for coating characterisation. The film composition was analysed using Rutherford backscattering spectroscopy and forward recoil spectroscopy. About 0.5 at.% oxygen and about 1 at.% hydrogen was detected in the film. The tribological properties of the carbon films were studied using pin-on-disc tests. The counterface materials employed were alumina and hardened steel (AISI 52100 and M50) pins, which were slid against the coated substrates. The friction coefficient was measured and the wear surfaces were studied. The sliding speed was in the range 0.02–0.6 m/s and the load in the range 5–20 N. The tests were carried out in air with a relative humidity of 50±2% and at a temperature of 24±3 °C. The test results show that the DLC coatings produced for this study generally had a coefficient of friction (μ) of about 0.2. The lowest value measured was μ =0.14. The wear resistance of the coatings was good, provided that the adhesion to the substrate was sufficient. The comparative tests with titanium nitride and titanium aluminium nitride coatings showed that DLC films are considerably more wear resistant than titanium-based coatings.

Wear and hardness of diamondlike coatings prepared by ion beam deposition

The wear rate of 10‐μm‐thick 12C coatings prepared by mass separated ion beam deposition has been measured to be 60 times lower than WC+Co hard metal and 10 times lower than the conventional hard coating material TiN. The hardness of the 12C coatings was measured to be at least the same as that of the natural diamonds.

TRIBOLOGICAL PROPERTIES OF HARD CARBON-FILMS PRODUCED BY THE PULSED VACUUM-ARC DISCHARGE METHOD

Abstract Hard diamond-like carbon (DLC) films were deposited on silicon substrates using a pulsed vacuum arc discharge method. Several methods were used for coating characterization. The film composition was analysed using secondary ion mass spectrometry (SIMS), Rutherford backscattering spectroscopy and forward recoil spectroscopy. The film contained about 0.5 at.% oxygen, 0.3 at.% chlorine, 0.3 at.% copper and 1 at.% hydrogen. The tribological properties of the carbon films were studied using pin-on-disc tests. The counterpart materials employed were alumina and hardened steel (AISI 52100 and M50) pins, which were slid against the coated substrates. The friction coefficient was measured and the wear surfaces were studied. The sliding speeds were varied in the range 0.02–0.6 m s −1 and the load in the range 5–20 N. The tests were generally carried out in air with a relative humidity of 50±3%. The effect of relative humidity on wear and friction behaviour was also studied in the range 10–75% relative humidity. The test results show that the DLC coatings produced for this study generally had a coefficient of friction (μ) of about 0.2. The wear resistance of the coatings was good, provided that the adhesion to the substrate was sufficient. The sliding distance influenced the friction and wear behaviour of the pin-disc system. When the sliding distance was longer, the coefficient of friction increased. When the steel counterpart materials were used, the wear rates showed a decreasing trend. This phenomenon was probably caused by formation of a thick and rough reaction layer on the pin wear surface. With the alumina pins, no visible reaction layer formation was detected and the wear rate of the DLC coating rose when the sliding distance was increased. The wear surfaces were studied in detail using SIMS depth profiling and elemental imaging techniques. The relative humidity had a minor effect on the wear of these coatings in the range studied. The coefficient of friction, however, was higher at lower humidity levels and decreased when the relative humidity was increased. The comparative tests with TiN coatings showed that DLC films have a lower coefficient of friction and are considerably more wear resistant than TiN coatings.

Density measurements of diamond-like coatings using a low energy accelerator

Abstract With the aid of nuclear physical methods based on the use of a low energy accelerator it is shown that the density of even a small, uneven and rough diamond-like coating prepared by mass-separated ion beam deposition can be determined reliably. The density of the coating corresponds to that of diamond. Outlines of the used methods, i.e. Rutherford backscattering and/or nuclear resonance shift combined with the use of a profilometer and Doppler shift attenuation, are presented.