D.J. Schipper

Friction behaviour of solid oxide lubricants as second phase in alpha-Al2O3 and stabilised ZrO2 composites

The influence of metal oxide additives within alumina (?-Al2O3) and yttria-stabilised tetragonal zirconia (Y-TZP) matrices was studied with respect to the tribological behaviour of the composites. The solid lubricants CuO, ZnO, MgO, MnO2 and B2O3 were added in sufficiently small quantities (1 or 5 wt.%) to preserve the mechanical properties of the matrix phase without increasing the specific wear rate. The additives selected were an attempt to provide the materials with a self-lubricating mechanism and therefore decrease the coefficient of dry-friction. The ceramics were prepared by means of wet-ball milling powders, compaction by isostatic pressing and densification by sintering. Near-full densities (98?99%) were obtained for all ceramics except that containing CuO (92%). Tribological behaviour of the various composite compositions was tested on a pin-on-disk assembly using both ?-Al2O3 and ZrO2 as counter surface. Friction reductions of almost 40% for the CuO composite was obtained while the influence of the other oxide additives were less pronounced.

The effect of lubricant selection on galling in a model wear test

Galling is a known failure mechanism in sheet metal forming (SMF) processes. As a result of this wear process, the amount of waste increases, the production process becomes hard to control and eventually expensive maintenance is required in order to continue production. Delaying or avoiding galling mechanisms by optimising the contacting materials and lubricant is therefore of high industrial importance. The presented work focuses on the effect of lubricant selection on galling, using a model wear test: the TNO slider-on-sheet tribometer. With this tribometer, galling mechanisms are studied using tool steel in sliding contact with lubricated deep draw steel DC 06. The wear data obtained with the test method agrees well with a general framework of lump initiation, lump growth and scoring (galling). Experimental data clearly demonstrates that a low coefficient of friction at the start of the experiment is not a reliable indicator for galling prevention. Hence, lubricant evaluation should be done based upon long-term testing instead of on short-term testing. Comparative results with a lubricant set shows that the TNO slider-on-sheet model wear test is suited for ranking forming lubricants with respect to their ability to avoid material transfer.

Effects of a second phase on the tribological properties of Al2O3 and ZrO2 ceramics

The tribological properties of four different materials are investigated, tetragonal zirconia (Y-ZTP), Al2O3 dispersed in Y-TZP (ADZ), ZrO2 dispersed in Al2O3 (ZTA) and Al2O3 (with 300 ppm MgO). These materials are used as a cylinder sliding against a plate of Y-TZP (TZ-3Y)). Compared to Y-TZP, the wear resistance of ADZ composites is increased by a factor of 4?10. At a contact pressure of 230 MPa, a wear transition for Y-TZP is observed from plastic deformation to microchipping and microfracture due to the high interfacial temperature (450°C?550°C) generated by frictional heating. Because of the higher elastic modulus, hardness and fracture toughness at high temperature, ADZ composites show better wear resistance and a higher transition contact pressure (over 400 MPa) under the present conditions. For Al2O3, the transition from mild to severe wear occurs when the contact pressure is changed from 250 to 400 MPa. For ZTA ceramics, the wear behaviour does not change because of the presence of a compressive layer due to the zirconia phase transformation during sliding. In water the wear resistance for ADZ and ZY5 is almost two orders of magnitude higher than the results under dry conditions. Reduction of the interfacial temperature by using water and the formation of a hydroxide layer at the contact surface by the tribochemical reaction of water with the ceramic, as observed by XPS, gives a positive effect on wear resistance.

Wear of soft tool materials in sliding contact with zinc coated steel sheet

In order to reduce costs of tooling for press operations, efforts are made to use alternative tool materials like wood or plastic. Friction and wear characteristics in sliding contact with zinc-coated steel sheet could, however, limit the applicability of these tool materials for automotive applications. In this work the wear resistance of alternative tooling materials is studied by a combination of forming tests at a high speed stamping line and model wear tests using the TNO slider-on-sheet tribometer. With this tribometer, volume loss of alternative tooling materials can be determined as a function of the sliding distance, using sheet materials from automotive practise. Results show that the wear rate of a soft tool material can change two orders of magnitude as a result of the zinc layer type used. Furthermore, it is shown that the relative performance of alternative tool materials is strongly related to the hardness of the (tooling and sheet) materials. Industrial forming tests with a selection of alternative tooling materials confirmed the model wear test results. The same ranking of the tooling materials with respect to volume loss is obtained per sheet material.

The influence of laser line hardening of carbon steel AISI 1045 on the lubricated wear against steel AISI 52100

To diminish wear in tribological systems it is not always necessary to provide the entire surface with a wear resistant layer. Depending on the application it is sufficient to harden locally the load carrying areas which are subjected to wear. Such areas can be treated properly by a laser, either totally or partially. This paper describes the effects of laser line hardening on the wear behaviour of carbon steel AISI 1045 against ball bearing steel AISI 52100. It is shown that the wear resistance of carbon steel AISI 1045 can be improved considerably by line hardening the surface. The wear resistance of the laser line hardened surfaces is comparable with that of carburised steel AISI 1045. However, the properties of the laser line hardened areas determine the wear behaviour of the entire system. Furthermore, the experimental work indicates that the type of heat treatment, carried out prior to line hardening in order to improve the microstructure of the steel, has no significant effect on the wear behaviour of the tribological system.

On the Frictional Heating in Single Summit Contacts: Towards Failure at Asperity Level in Lubricated Systems

The influence of surface roughness and thermal conductivity on seizure in lubricated contacts is described in this work by quantifying the action of individual asperities in relation to local surface temperature rise. Application of the model to a contact situation in metal forming of stainless steel sheet material reveals the importance of a high quality finish in terms of the center line average roughness of the surface. Calculations show that seizure of single summit contacts can be avoided by surfaces with enhanced thermal conductivity.

A wear measurement method based on the comparison of local surface heights

A new wear measurement method capable of measuring changes of the microgeometry due to wear by image processing techniques is presented. An advantage of this method over other methods is that it gives information on local height changes on the surface caused by material removal by wear and material addition by material transfer. Using this method, it is possible to measure and characterise wear and material transfer at the surface microgeometry level. The method will be explained in detail and some examples illustrating the method will be shown.

Analysis of Material Transfer From a Soft Workpiece to a Hard Tool: Part II—Experimental Verification of the Proposed Lump Growth Model

In this study, the lump growth model, described in an accompanying paper (de Rooij and Schipper, 2000) is validated by means of experiments performed on a deepdrawing simulator. In the experiments, the influence of material and roughness properties of both sheet and tool on the galling behavior is determined. For these experiments, a deepdrawing simulator and a selection of aluminum and zinc coated sheets with several (coated) deepdrawing tools are used. Good agreement is found between results of the lump growth model and the sheet metal forming experiments.

Existence of a Tribo-Modified Surface Layer on SBR Elastomers: Balance between Formation and Wear of the Modified Layer

In most of the tribological contacts, the composition and tribological properties of the original interface will change during use. The tribo-films, with modified properties compared to the bulk, are dynamic structures that play a significant role in friction. The existence of a tribo-modified surface layer and its importance on the overall friction of elastomers has been shown both theoretically and experimentally before. The characteristics of the modified surface layer deserve specific attention since the tribological properties of elastomers in contact with a rough counter-surface are determined by these modified surfaces together with the properties of bulk of the material. Both the formation of the modified layer and the break down (wear) of it are of importance in determining the existence and thickness of the tribo-modified layer. In this study, the importance of the wear is emphasized by comparing two styrene butadiene rubber-based elastomers in contact with a granite sphere. A current status of perception of the removal and the stability of the modified surface layers on rubbers is introduced as well as experimental work related to this matter and discussion within literature. Pin-on-disk friction tests are performed on two SBR-based samples in contact with a granite sphere under controlled environmental conditions to form the modified surface layer. Although the hysteresis part of the friction force which has a minor contribution in the overall friction is not markedly different, the total measured friction coefficient differs significantly. Mechanical changes both inside and outside the wear track are determined by atomic force microscope nano-indentations at different timescales to examine the modified surface layer on the test samples. The specific wear rates of the two tribo-systems are compared, and the existence of the modified surface layer, the different measured friction coefficient and the running-in distances toward steady-state friction are explained considering different wear rates. A conceptual model is presented, correlating the energy input into the tribo-system and the existence of a modified surface layer.

New Material Development for Surface Layer and Surface Technology in Tribology Science to Improve Energy Efficiency

This paper reviews the development of new material and surface technology in tribology and its contribution to energy efficiency. Two examples of the economic benefits, resulted from the optimum tribology in the transportation sector and the manufacturing industry are discussed. The new materials are proposed to modify the surface property by laminating the bulk material with thin layer/coating. Under a suitable condition, the thin layer on a surface can provide a combination of good wear, a low friction and corrosion resistance for the mechanical components. The innovation in layer technology results molybdenum disulfide (MoS2), diamond like carbon (DLC), cubic boron nitride (CBN) and diamond which perform satisfactory outcome. The application of the metallic coatings to carbon fibre reinforced polymer matrix composites (CFRP) has the capacity to provide considerable weight and power savings for many engineering components. The green material for lubricant and additives such as the use of sunflower oil w...