Model Wear Tests Research Articles

Tribological properties of r.f. sputtered Ti-B-N coatings under various pin-on-disc wear test conditions

TiN, Ti(B,N) and TiB 2 hard coatings have been deposited on steel substrates by means of r.f. sputtering. The fundamental coating properties, such as the structure, adhesion, hardness, internal stresses and chemical composition, have been determined. The coatings have been tested with regard to their tribological behaviour by means of unlubricated pin-on-disc model wear tests under an ambient atmosphere as well as under vacuum conditions. Balls made of ball bearing steel and high speed steel have been used as pin materials. It was found that the dominant type of chemical bonding within the film is an important coating property with regard to the wear behaviour under pin-on-disc test conditions. It is shown that TiB 2 films are of limited benefit in a sliding contact against steel. Ti(B,N) and the well-known TiN are much more promising coating materials.

Tribological behaviour and corrosion performance of Ti-B-N hard coatings under plastic manufacturing conditions

During plastics processing, an increasing content of abrasive fillers and corrosive components in combination with high working temperatures up to 450°C produce extreme wear conditions. As shown in previous work, TiN coatings are wear resistant against hot molten plastic material. In this paper we deal with thin hard single-phase and multiphase coatings within the extended coating system Ti-B-N. The films were deposited onto steel substrates using an r.f. sputter process with r.f. substrate bian. In order to cover a broad spectrum of Ti-B-N coatings, the substrate bias, nitrogen gas flow and total deposition gas pressure were all varied systematically. The coatings were tested with regard to their fundamental mechanical properties such as hardness and adhesion. Additionally, the coating structure was investigated by scanning electron microscopy. Corrosion tests were also done to see how the coatings performed under this important condition. Following these tests the wear behaviour of the coating systems under plastics processing conditions was investigated in a model wear test. Corresponding to the melting zone of screw barrel systems, the test pieces were subjected to abrasive-corrosive wear under elevated temperatures. It is shown that the combination of high hardness, dense structure, good adhesion and—most important—excellent corrosion resistance leads to superior wear resistance of r.f. bias sputtered Ti-B-N coatings. The wear behaviour can be further improved if a dense covalent Al 2O 3 surface layer is deposited on top of the Ti-B-N film.

Comparison of abrasion model test results and machining tests with PVD-coated indexable inserts

The development of physical vapour deposition (PVD) coatings for improved wear resistance of cutting tools now is addressing more complex coating systems. For example, this is verified by the fact that most of the commercial PVD coating companies now are offering ternary coating systems, such as TiAlN or Ti(C, N), in addition to the standard TiN. Quaternary and quinternary systems are in development. While developing these new coating systems, more parameters have to be optimized which is resulting in larger series of experimental runs. Because it is impossible to consider all the coating variations in machining tests under real turning or milling conditions, it has to be proved how far model wear tests can help in preselecting the coatings with a good potential to perform successfully. The fact that model tests are able only to prove a single wear mechanism needs very careful verification. In this paper, we compare the results of turning or milling tests using TiN-, TiCN-, TiZrN-, ZrCN-and TiZrCN-coated tools with the results of abrasive wear tests. It is shown how far the model test result can give information about the potential of a coating in special cutting conditions, if the whole tribological system with the other affecting factors is to be evaluated.

Fundamental properties and tribological wear behaviour of sputtered CrCrN coatings on light metals

CrN hard coatings were deposited by sputtering onto pure titanium alloy TiA 16V4 in order to improve the wear protection and the oxidation resistance of the substrate materials. To obtain dense coatings with high hardness and good adhesion to the substrate, the bias voltages (r.f., d.c.) were varied systematically and different thin intermediate layers (chromium, NiCr) were deposited prior to sputtering of the CrN coatings. Furthermore, the influence of thermoshock treatment (fast cooling from 700 C to 10 C in water) on the film properties was analysed. The wear behaviour of the coatings was evaluated using a plate on cylinder model wear test apparatus.

Wear mechanisms of ultrahard, non-metallic cutting materials

Basic wear mechanisms of polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) composite cutting materials were examined. The investigation included the planning and realization of model wear test systems closely connected to and including the main tribological parameters of the real cutting process during turning of metals. Considerable attention has been given to the chemical composition and the microstructure of the cutting tool materials. A series of diffusion and abrasion model tests were carried out to reveal the effect of the diffusion mechanism on the abrasion resistance of PCD and PCBN during cutting processes. The forms of wear which occurred during the model tests were compared with the wear state produced by means of real turning tests and found to be useful in explaining the wear behaviour of PCD and PCBN during cutting operations. The diminution of the abrasion resistance of the cutting material owing to graphitization seems to be the dominant wear mechanism when turning a titanium alloy at elevated cutting speed with PCD composites. However, the wear rate obviously is dependent on the grain size of the diamond crystals. In the case of PCBN, the chemical composition of the binder and its percentage in the composite is important for thermal stability, as it is required for cutting tools. PCBN samples, exposed to diffusion tests with steel as the workpiece material at elevated temperatures up to 950 °C suffered significant changes in the structure of the binder. This seems to result in a reduction of the bulk material resistance to abrasive wear. Scanning electron microscopy and energy-dispersive spectroscopy investigation of inserts used in real cutting tests turning a bearing steel helped to ascertain the practical relevance of the results achieved with the model test series.

Simulation of the tribological behaviour of tools for cutting ductile materials

Cutting tests are very costly when expensive materials are to be cut. Therefore model tests are required which allow a preselection of materials or coatings to increase the lifetime of cutting tools. The wear of indexible inserts for milling a soft magnetic highly ductile nickel - iron alloy is simulated by means of a modified pin-on-disc system. For the simulation a detailed systems analysis of the tribological processes during cutting and model wear testing is necessary. Cutting tests revealed that flank wear is the wear-determining process. Therefore the flank of an original indexible insert was pressed against the flat surface of a rotating disc which was made of the nickel - iron alloy. By reversing the direction of rotation in comparison to the cutting process only sliding took place. The flank wear - distance curves of the model wear tests resembled the wear curves of the cutting tests. In both tests nickel - iron was transferred to the flank of the indexible inserts. By physical vapour deposition (PVD) of TiN, friction, adhesive transfer and wear were considerably reduced. These tests give valid information on the tribological behaviour of uncoated and coated indexible inserts for milling highly ductile nickel - iron alloys, but real lifetimes of the tools can only be determined by milling tests.

Tribological properties of r.f.-sputtered titanium-based hard coatings and their behaviour under plastics-processing conditions

Owing to the use of abrasive fillers and corrosive components in plastics, wear appears on plastics-processing equipment. As shown previously, TiN coatings offer high wear resistance in other tribological systems. Therefore it seems obvious to use TiN coatings for wear reduction in plastics processing. This paper deals with titanium-based hard coatings. The coatings were deposited on steel substrates by means of an r.f. sputter process with r.f. substrate bias. The coatings were tested with regard to their fundamental properties such as hardness, adhesion and coating structure. An increased substrate bias voltage leads to higher hardness and a denser coating structure. The optimum of adhesion and cohesion of the coatings is found in the range from −20 to −50 V. The wear behaviour of the coatings under plastics-processing conditions was investigated by two model wear tests corresponding to the tribological systems inside a screw-barrel system. For the melting zone a remarkable improvement in wear resistance was achieved.

Evaluation of some new titanium-based ceramic coatings in tribological model wear and metal-cutting tests

The results of tests on new multielement ceramic coatings based on (Ti,Al)N and Ti(B,N) deposited by plasma-assisted physical vapour deposition are reported. The work follows on from earlier studies based on standardized test procedures and now includes model cutting tests. The paper emphasizes the markedly different tribological behaviour of these coatings depending on the contact conditions—even on different faces of the same tool. The need to further develop such coatings for specific contact conditions is therefore highlighted.

Tribological properties and wear behaviour of sputtered titanium-based hard coatings under sheet-metal-forming conditions

In recent years, grey cast iron tools have been widely used for sheet metal forming because of their low friction coefficient and excellent emergency running properties. Low hardness and therefore reduced wear resistance are the main restrictions for this material. Tool steels are harder, but against steel workpiece material a tendency to adhesive wear might cause problems under insufficient lubrication. As shown in previous work, TiN coatings reduce the abrasive and adhesive wear of cast iron and steel samples under several tribological conditions. The present paper deals with thin hard coatings within the system TiBN, deposited on grey cast iron and tool steel by means of a d.c. or r.f. sputter process with r.f.-biased substrates. The coatings were analysed with regard to their fundamental properties such as hardness, adhesion and structure. The most promising titanium-based hard coatings were tested in a tribological model wear test and under metal-forming conditions. For the grey cast iron tools it is shown how the graphite lamellae disturb the coating structure. In this case the main wear mechanism is cracking of the coatings in the areas of graphite lamellae. The cracking leads to highly abrasive wear debris containing hard coating and substrate particles. The influence of the graphite on the coating growth can be suppressed by means of a thin pure titanium layer deposited prior to the hard coating. As the main result the wear behaviour of the coated grey cast iron tools is remarkably improved without a significant change in coating hardness or adhesion. On steel tools no influence of the base material on the coating was observed. For the coating a low tendency of adhesion to the workpiece material and high hardness values were obtained as well, which leads to a superior wear resistance of coated steel tools under sheet-metal-forming conditions.

Fundamental and tribological properties of R.F.-sputtered TiN coatings in plastic manufacturing model wear tests

In plastic manufacturing an increasing content of abrasive particles and corrosive components in combination with working temperatures up to 450 °C have led to a new dimension of wear. As shown in previous work, TiN coatings have been wear resistant under corrosive conditions and elevated temperatures. In this paper we deal with thin hard TiN coatings deposited onto steel substrates by means of an r.f. sputter process with r.f. substrate bias. The coatings were tested with regard to their fundamental properties such as hardness, adhesion and structure. An increased substrate bias voltage up to —100 V leads to higher hardness values and a more dense coating structure. Adhesion and cohesion of the coatings are best in the range from —30 V to —50 V. The wear behaviour of the coatings under plastic manufacturing conditions was investigated by two model wear tests. Corresponding to the conveying and the melting zones of screw barrel systems the test pieces were subject to abrasive and abrasive-corrosive wear under elevated temperatures. It is shown that the combination of high hardness, dense structure and good adhesion leads to superior wear behaviour of r.f. bias sputtered TiN coatings. For the melting zone model wear test a remarkable improvement in wear resistance by a factor of more than 10 was achieved.

Wear behaviour of r.f.-sputtered TiN, TiB 2 and Ti(B,N) coatings on metal forming tools in a model wear test

Within the last few years grey cast iron tools have been widely used for sheet metal forming owing to their low friction coefficient and excellent emergency running properties. Low hardness values and therefore reduced abrasive wear resistance are the main restrictions for this material. As shown in previous work, TiN coatings reduce the abrasive wear of cast iron test pieces under several tribological conditions. In this paper we deal with thin hard coatings of the system Ti-B-N deposited onto cast iron by means of an r.f. sputter process with r.f.-biased substrates. The coatings were tested with regard to their fundamental properties such as hardness, adhesion and structure. The most promising TiN, TiB 2 and Ti(B,N) coatings were tested under metal forming conditions. It is shown how the graphite lamellae of grey cast iron substrates disturb the coating structure. The main wear mechanism results from cracking of the coatings in the areas of graphite lamellae. The cracking leads to highly abrasive wear debris containing hard coating and substrate particles. Therefore a 1 μm thick pure titanium layer is deposited prior to the hard coating to reduce the influence of the graphite on the growth of the hard coating. As a main result the wear behaviour of the coated tools is remarkably improved without a significant change in coating hardness or adhesion.

Wear behaviour of r.f.-sputtered alumina coatings on cast iron substrates in a model wear test under dry conditions and the influence of residual stresses and adhesion

Alumina coatings were reactively r.f. sputtered onto cast iron substrates and their wear behaviour under unlubricated conditions was investigated using a model wear test (Timken test). The wear behaviour of the cast iron substrates was improved.The influence of the sputter cleaning process and the deposition pressure on the wear properties of the alumina coatings was investigated. The presence of oxygen in the sputter cleaning atmosphere and the use of a prolonged cleaning time can improve the wear behaviour. A reduction in the deposition pressure leads to a marked improvement in the wear properties of the coatings. This seems to be due to the better adhesion of the low pressure coatings which is caused by higher compressive residual stresses.

Properties and optimization of RF-bias-sputtered TiN for the wear reduction of plastic manufacturing tools and components

TiN was reactively sputtered using RF magnetron sputtering with RF-biased substrates for the purpose of wear protection of parts working under plastic manufacturing conditions. A complete optimization procedure is described starting with the investigation of fundamental coating properties such as structure, adhesion, hardness and their dependence on the deposition process and concluding with the evaluation of the tribological properties. These were investigated using the unlubricated Timken test (plate on cylinder) and a special model wear test for the simulation of the conditions of plastic manufacturing. The RF bias allows deposition with low bias voltages. Highest hardness values and best adhesion are obtained at -20 V and -50 V respectively. In both tribological model wear tests the optimized coatings showed excellent wear behaviour. For use under the corrosive and abrasive conditions of plastic manufacturing, the important role of coating structure could be demonstrated. Using these results in a practical application, the coating of a plastic forming mould with RF-bias-sputtered TiN resulted in a marked improvement in the lifetime under production conditions by a factor of more than four.

Modelling of two-body abrasive wear

A model of two-body abrasive wear, caused by abrasive particles harder than the wearing material, is presented. It shows the most important factors which influence wear loss. Properties such as work hardening, ductility and fracture toughness of a wearing material are considered in addition to hardness. The model includes the operating conditions of the wear system, e.g. size, shape (and/or attack angle) and type of abrasive particles and the applied surface pressure. Other parameters, e.g. sliding velocity, are implicitly included by their effect on the effective surface deformation or the capability of deformation or the work hardening of a wearing material. The predictions of the model were checked by using model wear tests at low sliding speeds. Good agreement was found between the experimental results and the theoretical model.

Rolling-sliding wear on precipitation-hardened structures of an austenitic steel

Microstructures of equal and different hardnesses were produced by the aging of a precipitation-hardened austenitic steel. This material was used as a model for studying the effects of microstructural elements on rollingsliding wear on underaged and overaged or peak-aged and thermomechanically treated structures of equal hardness. The effect of different hardness was evaluated by comparing the underaged and peak-aged structures. Model wear tests were run using a marginally lubricated contact by using an Amsler A 135 tribometer. The results can be interpreted in terms of the resistance to plastic deformation and the crack formation and crack growth behaviour of the different structures. The latter was studied using the usual fatigue and crack growth tests.