Showing papers by "Jari Koskinen published in 1996"

Effect of tribofilm formation on the tribological performance of hydrogenated carbon coatings

Abstract: Amorphous hydrogenated carbon (a-C:H) films were deposited by the r.f. plasma technique on stainless steel substrates. Pin-on-disc experiments were carried out over a wide range of normal loads (5–40 N) and sliding velocities (0.1–3.0 m s−1) in order to study the friction and wear performance of the coating against steel and alumina. The friction coefficient of a-C:H films against both steel and alumina pins decreased with increasing load and sliding velocity. On the pin wear surface, tribolayer formation was detected. The wear of the alumina pins increased with increasing load and sliding velocity when they contacted the coating. However, the thick tribolayer formed on the wear surface of the steel pin protected it from excessive wear when high loads and sliding velocities were applied. The wear surfaces were analysed by secondary ion mass spectroscopy and Auger electron spectroscopy. The analyses revealed that the thick tribolayer formed on the pin wear surface mainly consisted of the oxides of the pin material. However, evidence of carbon was found in the sliding deposit formed in front of the contact area of the pin and also in some cases on the pin wear surface. Carbon played an important role in the low friction behaviour although the amount of carbon was low. It is assumed that a thin tribolayer with low shear strength, consisting of carbon species, is formed on the disc wear surface. The coating wear increased when the normal load was increased. Some transfer of pin material was observed on the coating wear surface.

Present progress in the development of low friction coatings

Abstract: Low friction coatings are being developed for a variety of reasons. Practical conditions in which machines and machine elements have to operate are ever expanding and more often involve features which preclude liquid or grease lubrication. An example of such conditions very common in everyday life is the food-processing industry, where any kind of lubrication is impossible owing to the risk of contamination of a product. Some other conditions include hostile environments such as high temperature or vacuum. High temperatures are common, for example, in the chemical industry and power generation, whereas vacuum is often associated with space technology. A new motivation for development of low friction coatings is a general trend to reduce lubrication for reasons of environmental compliance. All these together challenge materials technology and surface engineering especially. Accordingly, activity in the area of the development of coating materials, sophisticated deposition methods as well as post treatments of coatings has been vigorous in recent years. In this paper we first review typical conditions in which low friction coatings are sought as well as the demands for properties which coatings should fulfil in order to function in an appropriate way. This is followed by a more detailed presentation of three different coatings for three different conditions. Amorphous hard carbon films deposited by arc discharge deposition are hydrogen free, have high hardness and are best characterized as an amorphous diamond. The films have been shown to possess a friction coefficient below 0.2 or even below 0.1 in dry-sliding conditions associated with good wear resistance. Potential conditions in which these films could be used are dry sliding at room temperature and in a normal atmosphere with adequate humidity. The second coating family which will be presented is based on molybdenum disulphide (MoS2). These are typical coatings for use in a vacuum. Many attempts have been made to overcome the many limitations of these coatings with post-treatments. In this presentation, ion beam and laser processing of MoS2 coatings with or without alloying are dealt with. The third set of conditions includes high temperatures. This is one of the most difficult areas of low friction coatings and no final solutions for practical purposes have been developed.

Arc discharge deposition of stainless steel coatings at different nitrogen pressures

Abstract: A filtered arc discharge process was employed to deposit stainless steel films using an AISI316 cathode. In this procedure, macroparticles and droplets, which are the most serious drawback of arc deposition processes especially in corrosion applications, are mostly filtered out. Films were deposited in vacuum or in the presence of a nitrogen plasma at different partial pressures. Low carbon steel and silicon single crystals were employed as substrates. Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA) and X-ray diffraction (XRD) were used to characterize the films. The corrosion properties were examined using electrochemical polarization measurements. The corrosion current density was clearly lower than that of bulk steel, but higher than that of bulk AISI316. Increasing the film thickness and nitrogen content lowered the corrosion current density.

Ion beam analysis of diamond-like carbon films

Abstract: Diamond-like carbon films are deposited with a number of different methods. This results in a wide variety of different microstructures and even compositions of the films, which challenges characterization capabilities. Ion beam analysis can be used in many ways to probe the properties of the films. A classic application is the determination of hydrogen concentrations in hydrogenated amorphous carbon films, a-H : C. This is performed by using either a resonance of the nuclear reaction 1 H( 15 N, αγ) 12 C or a forward recoiling technique. Diamond-like films have been doped with metals, Si, F, etc. in order to modify mechanical, tribological, or optical properties. Dopand concentrations can be qualitatively measured using either a backscattering or nuclear reaction analysis (NRA) method. A fairly trivial application is the utilization of either Rutherford or non-Rutherford backscattering to obtain the number of carbon atoms per square centimetre. Combined with other information on the chemical composition of the film and with the thickness obtained with a profilometer, for example, the density of the very thin films can be calculated. The density is an interesting parameter of the films and may vary considerably, depending on the hydrogen concentration and the structure of chemical bonding. If the thickness of the films is not constant or if a diamond-like material is just a deposition with an irregular shape, the above method cannot be utilized. The ion beam technique may still be useful and the Doppler shift attenuation of the resonance can be used. Resonance techniques are useful in analysing some other carbon related films, for example in analysing nitrogen in CN x films. In this paper, the most useful ion beam techniques for analysing diamond-like materials are given with examples.

A barometer of plasma and ion-assisted methods of surface modification

Abstract: Plasma and ion-assisted methods of surface modification have been studied for over three decades in the field of metals, semiconductors, ceramics, polymers, etc. A large number of different methods and approaches exist. A typical factor is the exploitation of the beneficial effects of ion bombardment on material surfaces. The methods have been developed into industrial processes for the manufacture of electronic materials and of protective coatings and surface treatments for metals. New methods, such as plasma-based ion implantation, have recently been developed, indicating the speed at which techniques are currently advancing. A survey has been carried out of plasma and ion-assisted methods of surface modification. A total of 44 people working in industry or research answered a series of questions on the present situation and future prospects in the field. For example, comments were requested on the following aspects relating to the particular methods used. (1) An evaluation of the current state of the art and the future of the method. (2) The major force driving the development process. (3) The major difficulties in commercialization. (4) The major theoretical challenges associated with the method. The answers received and comments given were analysed to form a picture of the current situation in the field of plasma and ionassisted methods of surface modification, and a view of the future for the community. This paper provides a summary of the results.

Tribological characterization of ion implanted PMMA films

Abstract: PMMA films spun on steel samples were irradiated by 230 keV 15 N + -ions at a fluence of 1·10 16 at/cm 2 . The polymer films were characterised by Nuclear Resononce Analysis (NRA) and scanning electron microscopy. The tribological properties of the films were tested by using a pin-on-disk test. Low wear rates comparable to values generally reported for diamond-like films were measured. Some tribological tests were carried out also by using pins which were coated with hard carbon coating resulting the coefficient of friction of 0.15.