D. Klaffke

Fretting wear of ceramics

Abstract The tribological behaviour of engineering ceramic materials can be influenced by several tribosystem parameters. Friction and wear properties of different types of ceramics (silicon nitride, silicon carbide, alumina and zirconia) are evaluated for the case of oscillating sliding motion at room temperature. The influence of such parameters as: counterbody material; contact geometry; stroke; normal force; running time; frequency; and humidity of the sorrounding air are studied. For many situations the formation of reaction layers governs the tribological behaviour, leading to a two-phase behaviour of friction and wear. The transition from phase I (severe wear) to phase II (mild wear) depends on several parameters of the tribosystem and operational conditions.

Advances in tribology: the materials point of view

The application of advanced materials in various areas of contemporary technology can lead to improvements in the function, quality and performance of engineering components and systems. In this paper, an overview of the developments in high performance materials, both organic and inorganic based, is given. This includes thin hard coatings because of their increasing importance in tribological improvements. For these types of materials the requirements for tribo-engineering applications are analysed. Research results from BAM concerning ceramics and ceramic composites, polymers and polymer composites as well as hard coatings illustrate the friction and wear behaviour of these materials and their potential for tribo-engineering applications.

Are thin hard coatings (TiN, DLC, diamond) beneficial in tribologically stressed vibrational contacts?—Effects of operational parameters and relative humidity

Abstract Vibrating contacts often fail by triboinduced damage. In the frame of a BRITE/EURAM project thin hard coatings have been considered as a solution to overcome such contact problems. TiN, Me-C:H (metal-containing DLC) and diamond coatings were investigated under oscillating sliding conditions in a ball-on-disk configuration. The tribological behaviour of flat coated specimens was investigated in comparison to uncoated steel specimens against steel and alumina balls in the gross-slip regime in a broad range of strokes (25 µmm to 10 mm), frequencies (1–20 Hz) and loads (1–20 N). Tests were performed at room temperature in dry air, normal air and moist air, respectively. Friction force, total linear wear and electrical contact resistance were measured continuously. In the present study, for TiN, Me-C:H and diamond coatings, friction coefficients of 0.15–0.9, 0.03–0.20 and 0.05–0.8, respectively, were determined and wear coefficients varying by orders of magnitude were found in dependence of test conditions. Sharp friction and wear transitions were observed in dependence of stroke, frequency, load, test duration and relative humidity. Preselection and development of coatings for vibrating contacts, development of testing strategies, standardisation activities, as well as valuation of tribotest results of thin hard coatings have to consider effects of test parameters. For comparison and preselection of coatings under conditions of oscillating sliding a data base was developed. The results reveal that thin hard coatings, especially carbon-based coatings can reduce friction and wear in vibrating contacts for a broad range of test conditions. Investigations on wear scars with different methods of surface analysis indicate that the tribological behaviour is determined in many cases by tribochemical formation of surface layers and debris.

Comprehensive tribological characterization of thin TiN-based coatings

Innumerable papers have been published so far describing tribological investigations of thin hard coatings based on TiN. Analysis of the presented results demonstrates a large dispersion of measured friction and wear numbers, whereas TiN-coated pieces and tools have proved their benefits in a broad area of application. Therefore an attempt was made to clarify the influences on friction and wear test results by varying the coating process, the tribological stresses due to sliding, fretting and rolling motion and by changing the surrounding medium. The results reveal that machining of substrate surfaces and type of tribological stresses due to sliding, fretting and rolling have an important influence. The formation of reaction layers is dominating the tribological behaviour in most cases.

Tribological optimization of CVD diamond coated Ti-6Al-4V

Abstract CVD diamond coatings on Ti-6Al-4V substrates were found to be highly effective in reducing fretting fatigue which, for example, can occur in the root section of aerospace compressor blades. Optimization of a mechanical polishing process leads to significant improvements in the tribological properties of these diamond coatings. This is shown by extremely low wear rates and low coefficients of friction against bearing steel, alumina and diamond. The wear and friction behavior was investigated in a ball-on-disk tribometer and characterized by profilometry, scanning electron microscopy (SEM), atomic force microscopy (AFM) and micro-Raman spectroscopy.

Ceramic particulate composites in the system SiC-TiC-TiB2 sliding against SiC and Al2O3 under water

Abstract The tribological behaviour of SiC, SiC–TiC and SiC–TiC–TiB 2 was determined in oscillating sliding against SiC and α-Al 2 O 3 in water at room temperature. The tribo-systems with the composite materials containing TiC and TiB 2 differ significantly from the systems with the single phase SiC: The wear is reduced and the friction is increased. The wear reduction up to a factor of 10 is mainly due to the formation of an oxide film containing titanium oxides which is soft, stable in water and well adhering to the bulk material. This oxide film is transferred to the alumina ball but not to the silicon carbide ball.

Wear of multiphase SiC based ceramic composites containing free carbon

Abstract Ceramics are of increasing interest for application in many branches of technique. In order to improve the tribological performance of ceramic materials, composites on the basis of silicon carbide with different amounts of titanium carbide, boron carbide, titanium diboride and free carbon in the form of graphite were manufactured by a reaction sintering process. The friction and wear behaviour of these composites was investigated with oscillating sliding motion in unlubricated tests at room temperature on different levels of relative humidity. Additional tests were performed under water lubrication, revealing the superior behaviour of composites for special test conditions. A major influence of the free carbon on friction or wear behaviour was not observed. The tribological behaviour in air as well as under water is mainly dependant on the amount of titanium phases.

Tribological characterization of hard carbon films prepared by Laser-Arc evaporation☆

Abstract Hard diamond-like carbon films were deposited onto silicon wafers and hardened steel substrates using laser-induced vacuum arc evaporation (Laser-Arc). The deposition was carried out in a vacuum of approximately 10−4 Pa with an arc repetition frequency of 100 Hz, a peak current of 500 A and a pulse duration of approximately 37 μs. After substrate cleaning by Ar+ ion bombardment films were prepared at different deposition temperatures up to a thickness of approximately 0.5 μm. The tribological behaviour of as-deposited carbon films was studied using a vibration wear technique with a steel ball (AISI 52100) as counterbody. To check the effect of the water content of the surrounding air, tests were performed in air at ambient temperature but of varying relative humidities. From in situ measurements of friction and wear, the tribological lifetime of the films was determined. From a wear scan at the end of testing and additional profilograms, the wear of the ball and that of the disk were calculated separately. The results demonstrate that very good wear resistance and a low coefficient of friction is obtained for specific conditions of the Laser-Arc coating process. However, the friction and wear of all coatings increased strongly in dry air. These results are the basis of further investigations to modify the coating process by Laser-Arc.

Tribological performance of SiC and TiB2 against SiC and Al2O3 at low sliding speeds

Abstract The friction and wear behaviour of a new developed, pressureless sintered TiB 2 ceramic was studied in comparison to SiC against SiC and Al 2 O 3 balls under unlubricated conditions at room temperature. Special attention was paid to the influence of relative humidity on friction and wear results. For both ceramics the coefficient of friction against both ball materials decreases with increasing humidity and is higher in tests against alumina than against SiC. The wear rate is affected significantly by humidity and decreases by one order of magnitude for Al 2 O 3 /SiC system and by 2 orders of magnitude for SiC/SiC system. For TiB 2 wear rates are nearly not affected by humidity against both ball materials and are much smaller than for SiC.

Wear behaviour of i-carbon coatings

Abstract The wear behaviour of i-carbon coatings on steel and ceramic specimens was investigated under conditions of continuous and oscillating sliding. It was found that both the counterbody material and the operating conditions (sliding velocity and temperature) as well as the environmental humidity have a pronounced influence on the tribological behaviour. The carbon layers show excellent tribological behaviour with low values of friction and wear provided that the frictional specific power input is below a certain level (approximately P F W mm −2 ). With respect to the environmental humidity, the carbon layers behaved best under conditions of high humidity. Surface studies by electron spectroscopy for chemical analysis indicated that the carbon bonding states of the coatings remain stable during the tribological loading below the critical transition conditions. Studies of the transferred carbon layers indicated that selective tribo-oxidative processes have occurred during the tribological loading.