Twin disc micropitting tests

Gears are important power transmission components and their efficiency and durability strongly depend on their tribological performance. The demand for higher efficiency has further increased the requirements for controlled tribological performance of gear contacts. One of the typical tribological failures that gears experience is micropitting, which is a fatigue failure of the surface strongly associated with low lubricant film thickness. Micropitting is claimed to be the dominant life limitation for ground hardened gears operating under modest conditions. In this work the factors affecting micropitting performance of gears were studied. Twin disc experiments were carried out to investigate the effect of surface roughness, lubricant type and surface treatment on micropitting performance of two case hardening steels. The test results showed that the roughness of the gear has a major impact on the micropitting performance. Also the use of DLC coating or carbonitriding surface treatment and the use of ester type lubricant had a beneficial effect compared to untreated or mineral oil lubricated surfaces.     

Load-carrying capacity evaluation of coating/substrate systems for hydrogen-free and hydrogenated diamond-like carbon films

Diamond-like carbon (DLC) coatings have shown excellent tribological properties in laboratory tests. The coatings have also been introduced to several practical applications. However, the functional reliability of the coatings is often weakened by adhesion and load-carrying capacity related problems. In this study the load-carrying capacity of the coating/substrate system has been evaluated. The DLC coatings were deposited on stainless steel, alumina and cemented carbide with two different deposition techniques: the tetrahedral amorphous carbon (ta-C) coatings were deposited by a pulsed vacuum arc discharge deposition method and the hydrogenated carbon (a-C:H) films by radio frequency (r.f.) plasma deposition method. The load-carrying capacity of the coated systems was evaluated using a scratch test, Rockwell C-indentation test and ball-on-disc test. The effect of substrate material, substrate hardness, coating type and coating thickness was studied. An increase in substrate hardness increased the load-carrying capacity for the coated systems, as expected. The two coating types exhibited different performance under load due to their different physical and mechanical properties. For the load-carrying capacity evaluations the ball-on-disc configuration was found to be most suitable. This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterization of wear surfaces in dry sliding of steel and alumina on hydrogenated and hydrogen-free carbon films

Hydrogenated amorphous carbon coatings were deposited by r.f. plasma and hydrogen-free carbon films in pulsed arc discharge on stainless steel substrates. The coatings were characterized and evaluated in tribological tests. Pin-on-disc tests were used over a wide range of test parameters: normal load, 5–40 N; sliding velocity, 0.1–3.0 m s⁻¹. The wear of both coatings was of the same order of magnitude (0.7 × 10⁻³−5.1 × 10⁻³ mm³). However, the wear of the counterface was one order of magnitude higher for the hydrogenfree carbon coatings. Increasing the normal load generally caused an increase in coating wear and in most cases also an increase in counterface wear. When the steel pin was sliding against the hydrogenated carbon coating with a high sliding velocity and load, a rather thick tribofilm was formed on the pin wear surface, lowering the coefficient of friction and reducing the pin wear. The tribofilm formed on the alumina pin sliding against the hydrogenated carbon film also seemed to reduce the friction coefficient but could not prevent the pin wear. A tribofilm was also formed on the pin wear surface when the hydrogen-free carbon coating was sliding against the steel and alumina pins, but the layer was not able to protect the pins. The tribofilm did, however, lower the coefficient of friction, which was rather insensitive to the different test parameters used. According to secondary ion mass spectroscopy analyses, material transfer of the pin was detected on the disc (coated) wear surfaces. The tribofilms formed on the pin wear surfaces consisted of pin material, hydrogen, oxygen, and carbon.     

Improvement of a-C:H film adhesion by intermediate layers and sputter cleaning procedures on stainless steel, alumina and cemented carbide

The adhesion of amorphous hydrogenated carbon (a-C:H) films deposited in a radio frequency (r.f.) plasma discharge on stainless steel, alumina and cemented carbide with different intermediate layers (Ni, Ti and TiC) and sputter cleaning procedures was studied. The composition of the carbon films and the intermediate layers as well as the interface between the coating and the substrate was determined by secondary ion mass spectroscopy (SIMS). The adhesion experiments were carried out using a scratch tester. Tested specimens were also studied by scanning electron microscopy (SEM) to reveal the morphology of the coatings and the scratches.

Without any intermediate layer, the a-C:H coatings generally had insufficient adhesion to the substrate materials studied. For stainless steel and cemented carbide substrates, the TiC intermediate layer and, for alumina substrates, the titanium intermediate layer gave the best adhesion values evaluated by the scratch test. Also, the sputter cleaning of the substrates prior to deposition was necessary for sufficient adhesion of the coating. The intermediate layers also change the failure mode of the coating in the scratch test in some cases.     

Fibre Optic Sensors for Long-Term Monitoring of Oil Film Pressure in Diesel Engine Main Bearing

The oil film pressure is one of the key parameters in journal bearings influencing the performance of the bearings. A fibre optic sensor was developed for online determination of actual oil film pressure under load without disturbing the actual tribological contact. Four optical sensors were integrated in a hydrodynamic journal bearing of a Wärtsilä Vasa 4R32 LN E medium-speed diesel engine with four cylinders, maximum power of 1,640 kW and rotating speed on 750 rpm. Online engine tests were carried out with different loads to study the sensor operation in real operating conditions. The engine tests were repeated six times over 4 years of operation reaching up to thousand hours of the engine use. The results showed differences in bearing pressure depending on the position of the sensor and on the operating cycle of the cylinders. The pressure peaks of all four cylinder work cycles could be identified in the measured pressure curves, and the pressure variations within the pressure curves fit well to the diesel engine’s work cycle and mass forces. The sensors had good repeatability over the whole test period.     

Microbial growth control in water hydraulic systems by conventional filtration

As a service to readers who understand German, French or Spanish better than English, the abstract of the research article in this issue follows in these languages.     

Friction reduction by texturing of DLC coatings sliding against steel under oil lubrication

It has been demonstrated that tetrahedral amorphous carbon (ta-C) films provide excellent wear and friction properties in dry sliding. Recently the applications of ta-C coatings in lubricated conditions have become more important. The use of carbon coatings aims at reducing the wear and coefficient of friction under minimum lubrication and without hazardous lubricant additives. For optimum tribological performance, a modification of the ta-C coated surfaces is required. The present paper describes an innovative method of coated surface texturing, by which nanometer and micrometer size pores are processed by various methods. Particle masking was used for processing micrometer size pores and for controlling the coating growth conditions in order to produce nanometer size pores in the ta-C surface. The masking by particles yielded a pore geometry which varied from complex shaped channels to small individual pores. The texturing was performed by distributing metallic powder particles on the surface or by direct chemical deposition of metal particles on the substrate in prior to pulsed vacuum arc deposition. The tribological characterization was carried out by applying reciprocating friction tests with controlled lubricant replenishment, in order to simulate metal forming processes. The friction reducing effect, which was observed in the tribological tests, indicated a microlubrication effect of the textured coating surfaces.