Gábor Kalácska

The tribological behaviour of engineering plastics during sliding friction investigated with small-scale specimens

Abstract For economical, ecological and even technical reasons for some years there has been a tendency to introduce self-lubricating materials for bearing applications. In this way external lubricants such as oil or grease can be excluded, the design can be simplified and maintenance cost can be reduced. Among the self-lubricating materials the so-called engineering plastics have increasing importance. Unfortunately, data on their friction and wear characteristics are very disparate and often there is a lack of general understanding of the physical phenomena involved. In the present paper some basic types of engineering plastics (PA, oil-filled PA PTFE-filled PETP, POM-H) are experimentally investigated by means of small-scale reciprocating tribotesting. The tribological behaviour is explained in correlation with the chemical and mechanical properties of the materials. The basic failure processes are described for mild wear conditions as well as for overload conditions.

Experimental study by means of thin layer activation of the humidity influence on the fretting wear of steel surfaces

Wear processes in general are influenced by the surrounding atmosphere, humidity especially. For fretting wear several authors have examined the influence of humidity on the wear scar morphology, the wear volume and the composition of the wear debris. No agreement between the different results has been found, yet. In the present study, the influence of the humidity in the ambient air on the fretting process is examined. The friction, the wear volume and material transfer are measured for the fretting of a bearing steel ball against a flat steel specimen under gross-slip regime, in running-in as well as steady state conditions. The wear and material transfer are quantitatively measured with Thin Layer Activation method. It has been observed that high ambient humidity inhibits adhesion and accompanying material transfer. As a result, high humidity has a beneficial influence on fretting wear, in opposition to generally accepted hypothesis.

Design of a tribotester for evaluation of polymer components under static and dynamic sliding conditions

Abstract Coefficients of friction and wear lifetime for machine elements are usually estimated from pin-on-disc tests under fixed test conditions. The experimental design of a tribotester with variable sliding path, dynamically changing normal loads, and sliding velocities is presented in this paper together with a preliminary evaluation of sliding properties for engineering polymers. The design of a dynamic loading mechanism and control of the sliding motion are detailed. The importance of dynamic testing is illustrated for polyamides and polyacetals under pure, internally lubricated and glass fibre-reinforced conditions. Dynamic tests induce lower friction compared with static tests because of influences of visco-elastic deformation, inertia forces, and possibly molecular orientation on the sliding surface. Internal lubricants efficiently reduce friction and wear both under static and dynamic conditions. Glass fibres more effectively reduce wear rates on dynamic tests indicating different wear debris mobility in the interface.

Electrical Properties of Magnesium Catalyzed Cast PA6 Semi-Finished Products

Graphite and conducting carbon black have been added to magnesium catalyzed cast polyamide-6. Surface resistivity has been measured on samples containing various amounts of additives in wet, standard, and dry states. The samples exhibited antistatic properties at already 2.5 wt% graphite loading. To reach similar properties 1 wt% high conducting graphite and 0.5 wt% high conducting carbon black was needed. It is important to note, that in the case of the high conducting additives above a certain concentration (above 2 wt% for high conducting graphite and above 0.5 wt% for high conducting carbon black) the surface resistivity decreases rapidly to the order of 107 Ω, belonging to the electrostatic dissipative class.

Dynamic Mechanical Tests on Magnesium Catalyzed Cast Polyamide 6 Composites Having Different Additives

Due to the effect of graphite and montmorrilonit additives, the impact strenght slightly decreases, but by the influence of softening material, the value of impact strength can be three times more than the pure PA6. It is determined that graphite can be added until 3%, montmorrilonit can be added until 6% and softening material can be added until 15% to the base matrix. These samples give basis for special characteristics examinations.

The Influence of Humidity on the Fretting Wear of Steel Surfaces Using Thin Layer Activation

It is generally known that the humidity in the surrounding atmosphere influences wear processes in general. For fretting wear several authors examined the influence of humidity on the wear scar morphology and the composition of the wear debris. No agreement between the different results has been found, yet. In the present study, a distinction is made between running-in and steady-state fretting wear. Especially for the running-in stage, the influence of the humidity in ambient air on the friction and wear behavior and material transfer of steel surfaces subjected to contact vibrations has been investigated. The fretting experiments were conducted with a spherical against a flat specimen under gross-slip regime. The fretting wear and accompanying material transfer between the contacting specimens quantitatively are measured with Thin Layer Activation method. It has been observed that high ambient humidity has a beneficial influence on the running-in fretting wear of steel surfaces. It has also been found that the results of several authors concerning the influence of humidity on the fretting wear of steel have to be treated with great care, as classical wear measuring techniques can give erroneous wear results.

Global Approach of Tribomechanical Development of Hybrid Aluminium Matrix Syntactic Foams

Hybrid syntactic foams with AlSi12 aluminium matrix were produced by pressure infiltration. The volume ratio of iron to ceramic hollow sphere reinforcement (in the same size range) was varied, and hybrid syntactic foams were also produced with bimodal size ceramic reinforcement. Previously, a very detailed analysis of the mechanical properties of the composites was made with quasi-static compression tests, and their tribological properties were investigated by pin-on-disc method in dry and lubricated conditions. The present article establishes and clarifies the correlations between mechanical and tribological properties. The coefficient of friction, height loss of the specimens and specific wear showed good correlation with different mechanical parameters, e.g. density, structural stiffness and yield strength. The established trends and correlations between mechanical and tribological behaviour enable a better understanding of materials design and selection for further applications of mechanically loaded sliding machine parts.

Surface characterization of polytetrafluoroethylene treated by atmospheric plasma

Polytetrafluoroethylene surface was treated by atmospheric DBD plasma for 1 min in ambient conditions. The effect of DBD plasma introduces signifi cant increasing of the surface energy (wettability...

Extruded and Injection Moulded Virgin PA 6/6 as Abrasion Resistant Material

Polyamide (PA6/6) is often used as a tribological pair in abrasion prevalent applications such as hinges and sliders. PA6/6 is frequently processed by injection moulding and extrusion process. It is known that these processes influence the polymers mechanical behaviour, but their influence on the polymers wear response has not been studied. Hence the present research attempts to study the influence of different manufacturing processes on tribological behaviour for PA6/6. Wear tests were performed on a pin abrading tester (DIN 50322). Abrasion resistance of both extruded and injection moulded PA6/6 were tested at different loads (20 and 35 N). Single-pass (nonoverlapping mode) and multipass testing (overlapping mode) were used to understand the influence of clogging of wear debris. It is evidenced that with increasing load the specific wear rate decreases; moreover, fine abrasives tend to reduce the wear rate. In multipass testing a transfer layer clogged on the counterface that acted as a protective agent and lowers wear rate. Poor mechanical strength of injection moulded polymers is apparently compensated by microstructural response for having a similar wear behaviour between extruded and injection moulded PA 6/6. Hence a proper balance between microstructural and mechanical characteristics is an absolute must in PA 6/6 for better wear performance.

Investigation on the Flammability of Diverse Cast PA6 Semi-Finished Products

Cast PA6 (magnesium–polyamide 6 (MgPA6)) compounds produced using a magnesium catalysis are presently replacing other polyamides in semi-finished engineering products. Because of this increased use, the flammability of MgPA6 containing montmorillonite, graphite, carbon black, and a plasticizer is investigated. In these studies, we observed that the presence of montmorillonite deteriorates rather than improves the flame-retardant properties of MgPA6. We also noted that the presence of a plasticizer enhances the dripping tendency and consequently the burning rate of PA6 in the UL 94 Test for Flammability of Plastic Materials. In addition, the presence of carbon black reduces the fire performance of the resin while graphite improves the UL 94 classification of PA6 to a V-2.