Mild Wear Conditions Research Articles

The cohesive zone crack analogue for incomplete contacts under mild wear conditions

Fretting fatigue may appear in mechanical components that are in contact under vibrational loading, inducing oscillatory frictional stresses, often combined with alternating bulk stresses. Fretting fatigue cracking is important, with its initiation being difficult to estimate. This work extends a well-known fretting fatigue model, the crack analogue model that examines fretting fatigue induced by incomplete contacts of metallic surfaces. This is done by inserting a mode II type cohesive zone at the contact edges. In our previous work on fretting fatigue of complete contacts, we proposed mixed mode I and II cohesive zones because of the nature of the stress fields around the contact edges. The proposed cohesive zones address several issues related to the mechanics of friction and wear, by combining them in the case of mild wear where asperity plastic shakedown is the dominant inelastic mechanism. Thus, the micromechanical behaviour of the surface topology and its evolution are naturally introduced into the analysis. The surface roughness becomes an internal variable that evolves with the cyclic loading, establishing a steady state friction coefficient. Alternatively, friction evolution can be established pointwise by monitoring the accumulated micro-slip. The work of friction dissipates the mechanical work and at the same time becomes the precursor of large scale wear. A predictive methodology for fretting crack nucleation is proposed through the fatigue threshold stress intensity factor used in ordinary fatigue.

Tribological behaviour and wear mechanism of ADI with different hardness values

The tribological behaviour and wear mechanism of austempered ductile iron (ADI) with different hardness values were investigated. Results showed that the wear resistance of ADI was sensitive to hardness and service conditions. Under mild wear conditions, the low-hardness ADI sample exhibited high wear resistance because graphite was easy to transfer to the tribosurface and acted as a lubricant to reduce the friction coefficient. However, the high-hardness ADI sample was hard to deform plastically, which restricted the transfer of graphite from the matrix to the tribosurface. Under severe wear conditions, the plastic deformation of the low-hardness ADI sample gave rise to abrasive wear, which resulted in poor wear resistance. With the deterioration of wear conditions, the wear mechanism of ADI was transformed from micro-cutting wear to oxidative wear.

Wear and aging behavior of vulcanized natural rubber nanocomposites under high-speed and high-load sliding wear conditions

The wear damage and aging behavior of natural rubber (NR) nanocomposites are very complex under severe conditions (high load, high-speed wear, or ablation). Little is known about the influence of strong interfacial friction on the high-temperature aging mechanism in the internal of NR nanocomposites. In this study, a set of UMT-TriboLab wear tester was used to study the wear and aging of NR nanocomposites, and to reveal their influence on the internal aging of specimens under severe conditions. Sliding wear tests were carried out with rubber-metal tribopair using a Block-on-Ring configuration. Different from mild wear conditions, the wear mechanism of NR nanocomposites under severe conditions experienced a transition from abrasive wear to adhesive wear. The viscous film formed by the degradation products on the wear surface was found to strongly affect the subsequent wear and aging behavior (wear weight loss, friction coefficient, temperature rise, etc.) of specimens. Layer-by-layer analysis showed that the heat diffusion caused by surface wear resulted in noticeable post-crosslinking in the internal layer of specimens while the oxidation reaction was limited to occur only on the specimen surface.

Properties of tribofilm formed on self‐mated stainless steel lubricated by palm methyl ester mixed petroleum diesel fuel

AbstractThis paper discussed the tribofilm properties formed on the contact interface of self‐mated austenitic stainless steel lubricated by petrol diesel fuel mixed with 7.5% wt palm methyl ester. To simulate a severe contact condition between the material, a ball‐on‐disc tribometer was employed. Mild wear condition was found in the case of methyl ester mixed petrol diesel fuel lubrication whereas catastrophic wear was found in the case of petrol diesel fuel lubrication. EDX images revealed significant traces of manganese metallic film on worn scar of the ball in the case of petrol diesel fuel condition. On the contrary, manganese metallic film was found scarcely localised in the case of palm methyl ester mixed petroleum fuel. This indicates that the methyl ester component in the petrol diesel fuel has acted as an agent to prevent the adhesion of manganese metallic film on the contact interface resulting in mild wear of the material.

Dry wear behavior of rheo-casting Al–16Si–4Cu–0.5Mg alloy

Dry wear behavior of the rheo-casting Al–16Si–4Cu–0.5Mg alloy was investigated by micro-scratch and dry sliding wear tests. Analyses of the microstructure, scratch grooves, wear tracks, worn surfaces and wear debris of the alloy were carried out by optical microscope and scanning electron microscope. The microstructural analysis showed that via rheo-processing, the primary Si was refined and rounded, eutectics dispersed more homogenously, and even the skeleton AlFeMnSi phase was fragmented into facet shape. Micro-scratch test showed that the microstructural refinement resulted in better wear resistance. Dry sliding wear test revealed that the rheo-processed sample exhibit obviously superior wear resistance because of the microstructure improvement. The dominant mechanism in mild wear condition is abrasion, but it turned to adhesion and oxidation in high applied load and fast sliding velocity conditions.

Wear Characteristics of Mo-W-Type Hot-Work Steel at High Temperature

The friction and wear characteristics of new Mo-W-type hot-work die steel, known as SDCM-S, were studied at high temperature. The results showed that the new Mo-W-type steel had greater wear resistance compared with H13 steel, which was due to its high oxidizability and high-temperature property. The high oxidizability and high temper stability features facilitate the generation, growth, and maintenance of a tribo-oxide layer at high temperature under relatively stable conditions. The thick tribo-oxide layer and high temper stability postpone the transition from mild to severe wear and ensure that conditions of mild oxidative wear are maintained in SDCM-S steel. The M2C and M6C carbides in Mo-W-type steel increase the temper stability and provide sufficient additional support for the formation and growth of a single thick tribo-oxide layer at high temperature. Mild oxidative wear is the dominant wear mechanism for SDCM-S steel between 400 and 700 °C.

Wear Resistance of H13 and a New Hot-Work Die Steel at High temperature

The friction and wear behaviors of a new hot-work die steel, SDCM-SS, were studied at high temperature under dry air conditions. The wear mechanism and microstructural characteristics of the SDCM-SS steel were also investigated. The results showed that the SDCM-SS steel had greater wear resistance compared with H13 steel; this was owed to its high oxidizability and temper stability. These features facilitate the generation, growth, and maintenance of a tribo-oxide layer at high temperature under relatively stable conditions. The high oxidizability and thermal stability of the SDCM-SS steel originate from its particular alloy design. No chromium is added to the steel; this ensures that the material has high oxidizability, and facilitates the generation of tribo-oxides during the sliding process. Molybdenum, tungsten, and vanadium additions promote the high temper resistance and stability of the steel. Many fine Mo2C and VC carbides precipitate during the tempering of SDCM-SS steel. During sliding, these carbides can delay the recovery process and postpone martensitic softening. The high temper stability postpones the transition from mild to severe wear and ensures that conditions of mild oxidative wear are maintained. Mild oxidative wear is the dominant wear mechanism for SDCM-SS steel between 400 and 700 °C.

Micro/Nanostructure and Tribological Characteristics of Pressureless Sintered Carbon Nanotubes Reinforced Aluminium Matrix Composites

This study reports the manufacture, microstructure, and tribological behaviour of carbon nanotube reinforced aluminium composites against pure aluminium. The specimens were fabricated using powder metallurgy method. The nanotubes in weight percentages of 0.5, 1.0, 1.5, and 2.0 were homogeneously dispersed and mechanically alloyed using a high energy ball milling. The milled powders were cold compacted and then isothermally sintered in air. The density of all samples was measured using Archimedes method and all had a relative density between 92.22% and 97.74%. Vickers hardness increased with increasing CNT fraction up to 1.5 wt% and then reduced. The microstructures and surfaces were investigated using high resolution scanning electron microscope (SEM). The tribological tests showed that the CNT reinforced composites displayed lower wear rate and friction coefficient compared to the pure aluminium under mild wear conditions. However, for severe wear conditions, the CNT reinforced composites exhibited higher friction coefficient and wear rate compared to the pure aluminium. It was also found that the friction and wear behaviour of CNT reinforced composites is significantly dependent on the applied load and there is a critical load beyond which CNTs could have adverse impact on the wear resistance of aluminium.

Dry Sliding Wear Behavior of EB Surface-Alloyed and Dispersed AZ91 Magnesium Alloy under Mild Wear Conditions

This paper reports on investigations of the beneficial effects of electron beam alloying (EBA) and electron beam dispersion alloying (EBDA) on the wear behavior of AZ91D Mg alloy under mild wear conditions with applied normal loads of 1…10 N. The layers generated had a thickness of 1.5 mm with Al contents of 30 wt.%. For dispersion alloyed layers, TiC was added with particle sizes of 20…100 µm. At a sliding distance of 20 m, the wear rates of alloyed layers (150 HB) and dispersion alloyed layers (180 HB) were almost the same and could be reduced by half compared to the untreated AZ91D (60 HB). Due to their large size and the large spaces between them, TiC particles were pressed into the layer matrix, or were torn out and acted as additional abrasives. Therefore, at a sliding distance of 50 m, the wear rate of dispersion alloyed layers increased to the level of the base material.

Braking pad-disc system: Wear mechanisms and formation of wear fragments

Abstract The tribological phenomena that occur in brake systems are interesting even in other respects than just the braking action. One important issue, that is gaining increasing importance over the last years, concerns the environmental impact of wear debris produced by the braking action. In this context, the present study is focussed on the tribological behaviour of a commercial friction pad material dry sliding against a cast iron disc. Pin-on-disc tests were conducted at room temperature under mild wear conditions, as concerns load and rotating speed. The effect of some components of pad material, in particular of copper, on the dynamic of formation of tribological layer and wear debris is presented. The results obtained so, although referring to quite a simpler system that real brake systems, still may provide interesting indications, for instance in view of the development of novel brake pad materials and braking control systems.

Tribological Behaviour of Aluminium Hybrid Metal Matrix Composite

Aluminum based metal matrix composites (MMCs) are appropriate materials for structural applications in the aircraft and automotive industries because they are ductile, highly conductive, lightweight and have a high strength to weight ratio. Wear and Friction behaviour of Al6061 with various percentage volumes of Multiwall carbon nanotube and Silicon Carbide reinforcement through stir casting method followed by die-casting was investigated. Wear test was made by using Pin on Disc Apparatus on the prepared Specimen. It was found that, under mild wear conditions, the composite displayed lower wear rate and friction coefficient compared to Aluminium. However, for severe wear conditions, the composite displayed higher wear rate and friction coefficient and it was clarified that the friction and wear behaviour of Al–SiC-MWCNT composite is largely influenced by the applied load and there exists a critical load beyond which CNTs could have a negative impact on the wear resistance of aluminium alloy. Also the hardness of the composite was increased by increasing the percent volume of reinforcement.

Improved resistance of alumina to mild wear by aluminium titanate additions

Abstract The wear behaviour of an alumina (Al 2 O 3 )–aluminium titanate (Al 2 TiO 5 ) composite containing 10 vol.% of second phase is studied and compared to that of single phase alumina. A careful control of the microstructure has been done in order to compare materials with similar alumina grain sizes. Wear tests have been performed on a pin on disk tribometer with an alumina ball as pin, at room temperature, under a normal force of 10 N and at sliding speeds from 0.06 to 0.15 m/s. Extensive analyses of the microstructural modifications due to wear have been done by a combination of field emission scanning microscopy and confocal microscopy. Mild wear conditions were attained for both materials. The main wear mechanism identified in both materials involves the formation of a hydroxide film and its cracking and delamination. The composite specimens presented increased wear resistance compared to the single phase ones.

Influence of mild and severe wear condition in the formation and stability of friction film in clutch system

Pin-on-disc tests frequently shows a transition from mild to severe wear when steel slides against steel in unlubricated condition. Studies have shown that the transition is influenced by hardness, normal load, roughness, sliding velocity, temperature and time. This phenomenon was also observed when a brake friction material was tested against perlitic cast iron. In this work the same regime transition of standard clutch friction material was evaluated for varying loads and sliding velocity. For this purpose tribometric tests were carried out varying the normal load from 200N to 450N in two different sliding speeds: 750rpm (2.05m/s) and 1200rpm (3.27m/s). Each test was run continuously for 3h at room temperature. The transitions from mild to severe wear have been observed only at 1200rpm and loads ranging from 380N to 450N. Characterization of sliding surface and cross-section of the disc by scanning electron microscopy and dispersive energy X-ray has shown that material transferred from friction material to sliding surface of disc. The characterization has shown that the quantity of friction material found on the surface of disc after the tests decreased when the transition from mild to severe wear occurred. In the mild wear the friction film is relatively thick and continuous. However, in the severe wear regime the film is thinner and less continuous and the severity may influence the formation and stability of friction film. The friction film may act as a solid lubricant reducing the wear rate.

Wear and friction behavior of Al6061 alloy reinforced with carbon nanotubes

Friction and wear behavior of Al6061 monolithic alloy and 1wt% CNTs reinforced Al6061 composite prepared through ball milling and spark plasma sintering was investigated. It was found that, under mild wear conditions, the composite displayed lower wear rate and friction coefficient compared to the monolithic alloy. However, for severe wear conditions, the composite displayed higher wear rate and friction coefficient compared to the monolithic alloy. Analysis of worn surfaces revealed that, at lower loads, abrasion was the dominant wear mechanism for both materials. At higher loads, adhesion was found to be dominant for the monolithic alloy while excessive sub-surface fracturing and delamination were mainly observed for the composite. Also, it was clarified that the friction and wear behavior of Al–CNT composites is largely influenced by the applied load and there exists a critical load beyond which CNTs could have a negative impact on the wear resistance of aluminum alloy.

J111035 TiCN複層膜を被覆した工具鋼のトライボロジー性能の評価

Tribological performances of TiCN/TiN coating on tool steels were evaluated by two dry sliding tests, i.e., a ring-on-block test under severe wear condition and a ball-on-block test under mild wear condition. From the ring-on-block test against an aluminum alloy ring, it was revealed that the main failure mode of TiCN/TiN coating was only micro-chipping. And by TiCN/TiN coating a tendency of elongation of the film life of TiN coating, whose failure modes are both micro-chipping and scratching, was observed. From the ball-on-block test against an alumina ball, it was revealed that TiCN/TiN coating, which has a low surface energy and a property of easy to oxidize, shows markedly small wear rate. Possible mechanisms of these improvements are discussed.

Calcium sulphonate and its interactions with ZDDP on both aluminium–silicon and model silicon surfaces

The boundary lubrication of an aluminium–silicon cylinder liner operating under mild wear conditions has been investigated. The interface was lubricated with synthetic six centistokes poly alpha olefin base oil, which contained either calcium sulphonate or calcium sulphonate and zinc dialkyldithiophosphate. Effective replication of silicon grains within an aluminium–silicon alloy using a silicon crystal substrate affords valuable reproduction of tribofilms. The physical and chemical characteristics of films generated on either substrate were comparable. A marked difference in structure was exhibited between the tribofilms formed from either lubricant. Detailed tribological and tribochemical analysis has been conducted on either substrate.

Effect of surface temperature rise on friction characteristics for sliding speed under unlubricated condition

This article presents a theoretical model of the kinetic friction characteristics for sliding velocity under mild wear condition, assuming a simple effective hardness at asperity contacts. The proposed model depends on the surface temperature distribution obtained by using Jaeger's estimation for a moving homogeneous heat source with an infinitely long band on a semi-infinite solid and a relation between steel hardness and temperature measured by Tominaga. Further, the model takes into consideration the stochastic model of a rough surface contact that was proposed by Greenwood and Williamson. The calculation results show that the surface temperature increases and the surface hardness and the friction coefficient decrease as the sliding velocity increases. However, the variation in the friction coefficient is less than that in the surface hardness. The initial value of the friction coefficient and the contact pressure has a great influence on the relation between the friction coefficient and the sliding velocity. In contrast, the standard deviation of the asperity summit heights has little influence on the relation.

Wear Behavior of Laser-Hardened GCr15 Steel under Lubricated Sliding Conditions

Microstructure, microhardness and tribological properties of laser hardened GCr15 steel were investigated in this paper. The wear resistance under lubricated sliding conditions was compared between specimens treated with laser and those of conventionally hardened. The tribological properties of laser surface-quenched GCr15 steel specimens were slightly better due to the effects of the microstructure hardening, high hardness and toughness, with the wear rate (in the order of 10-6mg/Nm) lower than that of the conventionally treated specimens. At the steady state, the frictional coefficient of laser-treated samples had no obvious difference from that of the conventionally treated samples. The wear mechanism for both cases was similar, generally involved surface fatigue wear and slight abrasion wear. LeiQ.K. Liu S. Lei Introduction H. Li In recent years, among the various surface modification methods, laser-induced surface modification has gained much attention for achieving the desired properties for applications[1]. This method is mainly used for ferrous alloys which undergo martensitic transformation and thus form a very hard surface layer with negligible surface roughness and distortion[2]. Some ideas demonstrated that the wear rate at a particular contact pressure can be strongly influenced by the microstructure of the steel, but there is also contrary idea that under the conditions of mild wear, the microstructural constituents of steels have no significant influence on the wear rate, although they affect the rate of severe wear. Previous studies of the authors demonstrated that under the dry sliding wear conditions, laser surface-hardened specimens of ferrous alloys exhibited enhanced wear resistance than conventionally hardened specimens. The aim of the study is to investigate the lubricated sliding wear behaviors of laser surface hardened GCr15 steel specimens and to compare the effect of the different microstructure compositions for laser transformation hardening with those of conventionally hardened and quenched. Moreover, the wear properties of the GCr15 steel and its corresponding wear mechanism under the lubricated wear conditions will also be studied.

Microstructural alterations within thermal spray coatings during highly loaded diesel engine tests

The good wear characteristics of thermal spray coatings are related to structural, productional, and topographical properties. Finally, the alteration of the microstructure in different contact zones is essential for mild wear conditions that will guarantee a long-life cycle because it reveals the ability of the material to adjust to the current load situation. Previous studies have shown that in diverse tribosystems an in situ formation of a nanocrystalline layer together with mechanical mixing maintains long-lasting, wear resistant surfaces with low wear rates. This investigation exemplifies results from a cylinder that was run in a road test motor. The relevant piston ring positions that fully describe the tribosystem have to be defined at the combustion chamber (CC), top dead centre (TDC), stroke and bottom dead centre (BDC). Transmission electron microscopy (TEM) and energy-filtered transmission electron microscopy (EFTEM) are used to analyse these contact zones in terms of microstructure and chemistry. Nanocrystalline surface layers up to a thickness of 100 nm occurred in all areas of contact whereas subsurface zones revealed differing microstructural changes. Thus the influence of thermal impact and different mechanical load conditions in a motor cylinder are evident. EFTEM elemental mappings verify the incorporation of elements that stem from lubrication or combustion residues.

Effects of a stepwise change in load on the subsequent friction and wear characteristics of carbon steel under dry sliding

Machines and components in operation are usually subjected to varying loads. However, wear tests of metals have usually been conducted under constant loads since early times in wear study. In this study, in order to investigate the effects of a change in load on the transition from mild to severe or quasi-mild wear, pin-on-disk type wear tests of carbon steel in contact with itself were conducted in moist air under dry sliding. During wear testing, the load was changed in a step-wise manner just one time from low to high levels. A longer sliding distance under mild wear conditions at low load in the first stage produces a more flattened and oxidized sliding surface having high wear resistance. In other words, a quasi-mild wear regime similar to mild wear with low rate occurred even at high loads in the second stage. The critical sliding distance in the first stage necessary for the transition to quasi-mild wear in the second stage is given by an empirical formula. The proposed formula indicates that the critical sliding distance decreases with increasing load in the first stage and decreasing load in the second stage. In conclusion, the quasi-mild wear can be attained under shorter sliding distance at higher loads in the first stage.