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In the present work, the friction and wear behaviour of the AM60 Mg-alloy (nominal chemical composition: Mg–6% Al–0.3% Mn; hardness: 52 HV) dry sliding against coated AISI M2 high-speed steel has been investigated. Different coatings were deposited on the high-speed steel by means of the PVD technique: three hard coatings (TiN, CrN and ZrN) and two self-lubricating coatings (MoS 2 and WC/C). The sliding tests were carried out using a block on disc testing rig. The testing conditions were selected with the aim of simulating the tribological loading present during dry drilling of Mg-alloys. The tests were carried out at a load of 200 N and a sliding speed of 0.8 m/s. In the second part of the work, tests at lower loads (150, 100 and 50 N) were also conducted. The analysis of the wear behaviour (in particular the formation of a Mg transfer layer on the coatings), along with the continuous monitoring of friction coefficient and contact temperature, allowed us to understand the acting wear mechanisms and therefore to propose specific guidelines for the selection of the drilling parameters.

The wear mechanism in disk-on-disk tests of a Cu–Be alloy against AISI D2 steel counterparts changes from metallic to oxidative for increasing loads. At low load the wear debris are made of the Cu–Be alloy, whereas above 50 N the main constituents are copper oxides, with a few residual particles of the copper-base alloy. Despite the lower oxidation state, cuprite (Cu2O) is the main fraction of the high-load debris, whereas tenorite (CuO) is less than 10%. An analysis of the X-ray diffraction line profile, supported by high-resolution transmission electron microscopy (TEM), shows that cuprite domains are nanocrystalline, with domain sizes distributed about a mean value of 12 ÷ 13 nm, and contain a high density of dislocations (∼5 × 1016 m−2). The small domain size is considered as a possible stabilization mechanism of cuprite against the higher oxidation state oxide tenorite, whereas the large dislocation content is a consequence of the heavy plastic deformation in the contact area. As a further support to the size-stabilization mechanism, a diffraction measurement repeated on the wear debris after a 6 month aging shows a marked increase in the tenorite fraction. According to line profile analysis, the remaining cuprite fraction is made up of nanocrystalline domains (∼6 nm) smaller than in the as-produced debris, thus supporting the hypothesis that small cuprite grains are more stable than larger ones, which more easily transform to tenorite.

Tensile stress-strain and dynamic acoustic resonance tests were performed on Fe-C-Ni- Cu-Mo high-strength steels, characterized by a heterogeneous matrix microstructure and the prevalence of open porosity. All materials display the first yielding phenomenon and, successively, a continuous yielding behavior. This flow behavior can be described by the Ludwigson equation and developes through three stages: the onset of localized plastic deformation at the pore edges; the evolution of plastic deformation at the pore necks (where the austenitic Ni-rich phase is predominant); the spreading of plastic deformation in the interior of the matrix. The analytical modeling of the strain hardening behavior made it possible to obtain the boundaries between the different deformation stages.

This critical review presents several aspects related to the use of copper as a main component in brake pads in road vehicles. The compositions of these materials are attracting increasing interest and concern due to the relative contribution of wear products to particulate matter emissions in the environment as a result of braking action even though there has been a reduction in exhaust products from internal combustion engines. We review the data on the main wear mechanisms in brake systems and highlight the positive role of copper. However, similar to other heavy metal emissions, even the release of copper into the atmosphere may have important environmental and health effects. Thus, several replacement strategies are being pursued, and the positive and negative features will be critically reviewed. Additionally, the future perspectives in materials development will be discussed.

The tribological behaviour of two Cu–Be alloys was investigated under dry sliding conditions against an AISI M2 steel counterface. The first alloy, Cu–Be1, was characterized by a hardness of 390 HV and a thermal conductivity of 106 W/mK, whereas the second one, Cu–Be2, had a lower hardness, 270 HV, and a higher thermal conductivity, 208 W/mK. At low loads, wear was metallic in nature. The Cu–Be1 alloy displayed the lowest wear rate, as expected. As the applied load was increased, however, the wear rate of the Cu–Be1 alloy increased, whereas the wear rate of the Cu–Be2 alloy was found to decrease and, in particular, to become lower than that of the Cu–Be1 alloy. A transition in the wear mechanism from metallic wear to tribo-oxidative wear was observed as the applied load was increased. The results were explained in terms of the characteristics of the tribological layer, which forms during sliding on the surface of the Cu–Be specimens.

Tensile properties of 3%Cr–0·5%Mo sintered steels vary greatly depending on the sintering temperature, compaction method and cooling rate. Given the high cooling rate and carbon content, the microstructure is either fully martensitic or bainitic–martensitic, with different amounts of bainite. Microhardnesss of the sintered samples is very high, ranging between 650 HV0·05 and 950 HV0·05, therefore, materials have a tendency to be brittle and their properties improve after a low temperature tempering. The best mechanical properties were obtained on the bainitic–martensitic materials sintered at high temperature.The present work demonstrates the possibility of obtaining excellent tensile properties using a 3%Cr–0·5%Mo prealloyed powder to form a dual phase microstructure, comprising of martensite and lower bainite in equal amounts.

A model to describe the strain hardening behaviour of porous alloys, produced by powder metallurgy, is presented. It accounts for the influence of the matrix strain hardening ability, the initial porosity content and the pore geometry, and its predictive ability is verified with reference to sintered iron and an Fe–0·3%C sintered alloy. The necessity to introduce in the model a parameter able to account for the internal notch strengthening effect exerted by pores is emphasised. This parameter is found to be dependent on the sintering degree of the materials.

Instrumented impact testing has been used to investigate the influence of the microstructure of the heat affected zone (HAZ) on the impact fracture behaviour of ASTM A105/AISI 304L friction weldments. The friction layer in the HAZ has been found to consist of two different parts. In the A105 side, a mechanically mixed layer made of bainite containing 304L 'protrusions' is present. The hardness of the bainite was found to increase as the friction pressure was decreased. In the 304L side, the friction layer was made of a thick shear band, formed by thermoplastic instability during welding. The impact fracture toughness was found to depend on both the crack nucleation and propagation stages, whose characteristics were related to the dynamic fracture toughness of the friction layers.

In the present investigation, the effect of load and external heating on the friction and wear behaviour of two Al-based metal-matrix-composites (SiC10 and SiC20, containing 10 and 20 vol.% of reinforcement) dry sliding against a semi-metallic friction material was studied. For loads lower than 200 N wear was by abrasion and adhesion, and friction coefficient was quite high, around 0.45. For loads higher than 200 N, friction decreased with load in both materials, whereas wear increased with load for SiC10 and became negative, because of transfer for SiC20. External heating induced a decrease in wear of both composites (and was negative in both cases) but also an unacceptable decrease in friction and increase in wear of the counterface friction material. The particular friction and wear behaviour displayed by the materials under study was explained making reference to the characteristics of the transfer layer, which forms on the surface of the rotating disc.

The lubricated rolling–sliding wear of an untreated and ion nitrided Ti–6Al–4V alloy has been investigated. All the tests were carried out under boundary lubrication using commercial SAE 14-W oil. The results show that nitriding improved wear resistance of the Ti–6Al–4V alloy. In particular the material nitrided for 24 h displayed the best wear resistance because the compound layer was effectively sustained by the hardened diffusion layer at the contact loads here investigated. Friction coefficient, on the other hand, was found to be higher in the nitrided materials, although the λ -factors were similar in all the tribological couplings. This was attributed to the lower wettability of the lubricant versus the nitrided layer than the passivated surface of the untreated alloy.

Wear debris from brake system materials : A multi-analytical characterization approach (vol 94, pg 249, 2016)

The aim of this work is the evaluation of corrosion behaviour of some industrial coatings on carbon steel, utilised to improve tribological behaviour. In particular, composite electroless nickel coatings with polytetrafluoroethylene (PTFE) particles, sintered bronze in PTFE matrix layers and zinc phosphates—with and without MoS2 coatings—were analysed. Salt spray fog exposure and electrochemical tests were carried out. Electroless nickel coatings showed the best protection properties with a good barrier protection, because of the coating structure: an external layer, which contains PTFE particles improving tribological properties, and an inner nickel layer, which protects against corrosion because of barrier effect. PTFE-coated sintered-bronze samples showed limited corrosion resistance due to the high number of defects present. The application of a MoS2 polymer-embedded layer improves the characteristics of phosphate conversion coatings, which, by themselves, cannot assure good protection.

The influence of gas and plasma nitriding on thermal fatigue resistance of a 41CrAlMo7 steel is considered. The role of compound and diffusion layers is discussed on the basis of the damage observed in service. The thick and porous compound layer, resulting from gas nitriding, is not very effective in preventing thermal crack nucleation and propagation up to the interface with the underlying diffusion layer. On the contrary, the thin and compact compound layer obtained by plasma nitriding is able to prevent crack nucleation. Overnitriding, i.e. excessive nitriding induced embrittlement, has to be avoided in order to obtain high crack arrest fracture toughness in the diffusion layer. This suggests the need to realize surface layers with optimized hardness. A simplified approach to predict the conditions for crack propagation in the diffusion layer as a function of the thermal loading and the properties of the nitride layer has been also proposed.

A simple model for the determination of the equivalent plastic strain for void growth and coalescence is proposed. It is based on the Rice and Tracey approach and on the concept of ‘intrinsic matrix ductility’, and considers the increase during plastic deformation of the void fraction and of the local plastic strain jointly. The model is tuned using data for pure iron with pre-existing voids, and is applied to different metals with ferritic matrix, showing that it is able to account for the influence on ductility of selected microstructural parameters and of the remote state of stress.

The functional modification of the surfaces is in many cases the best way to control the tribological damage of a component. Following a surface engineering approach, it is possible to choose the base material, or substrate, with tailored properties (for example, special mechanical or workability properties) and delegate to the modified surface the role of counteracting the tribological loadings. In most cases, the choice of a proper surface treatment results in cost saving, since it allows using cheaper substrate materials. The use of a low-cost substrate may compensate the additional costs of the surface treatment, required to improve system performances [1, 2, 3, 4].

Different engineering parts are subjected to tribological loading, such as sliding at different loads and speeds, abrasive interactions with hard particles or repeated contact stresses. Therefore, it is necessary to select materials according to specific engineering requirements, and then to verify their ability to perform adequately under the acting wear conditions.

Friction and wear depend on the characteristics of the mating surfaces. The difficulty to explain and to predict with high accuracy such phenomena reflects the complex nature of the surfaces, which is determined by the material properties (such as the microstructure), the geometrical irregularities, the presence of oxides due to the interaction with the surrounding atmosphere, and the presence of organic molecules, water vapour or other impurities adsorbed from the environment.

A reduction of friction between two sliding surfaces can be obtained by interposing a substance capable of reducing the shear stress, τm, necessary to allow the relative motion. This substance, which can be solid, liquid or gaseous, is called lubricant.

Duplex treatments are becoming increasingly interesting in several technological applications through which complex forms of stresses act during service. In pressure die casting, for example, corrosion and erosion by molten metal and oxidation contribute, together with thermal fatigue, to excessive degradation of the dies and inserts. While the benefits introduced by combining nitriding and PVD coating have been assessed in counteracting soldering and wear problems, the resistance of such duplex systems with respect to thermal fatigue has not yet been established.In this work, the role of plasma nitriding, PVD coating (CrN, ZrN), and duplex treatment in counteracting thermal cracking, i.e. heat checking, of hot work tool steel is considered. Thermal fatigue tests carried out on a laboratory scale, highlight the negative role of coatings, although this result has to be partially ascribed to an unsuitable prenitriding treatment and heavy oxidation.

Sintered steels in the density range 6.7-7 g cm- 3 and with a wide range of matrix hardnesses were produced with the main objective of understanding the interplay between porosity and matrix strength in determining their tensile and impact behaviour. The materials can be subdivided into two groups, characterised by different deformation and fracture mechanisms. The first group pertains to the as sintered materials (with a microhardness lower than 350HV0.1).They are characterised by a strain hardening stage before fracture, and both tensile strength and impact fracture load increase as density and matrix strength are increased. The second group pertain to the heat treated materials with a higher matrix hardness. These materials are characterised by a macroscopically brittle behaviour, since fracture takes place before general yielding. Because of the high matrix hardness, the peak stresses at the pore edges are not relieved by plastic deformation, and the pores behave as internal cracks. Fracture i...

Evolution of friction and wear of 42CrAlMo7 steels with different nitriding processes was investigated during boundary-lubricated rolling–sliding tests. The wear behaviour of nitrided steel with a thin compound layer (produced by plasma nitriding and by gas nitriding followed by oxidation) was characterised by the early removal of the compound layer, and the wear resistance was thus, given by the underlying diffusion layer. In the case of the material with a thick compound layer (produced by gas nitriding) wear was restricted to the compound layer. In this material, at low applied load (300 N, i.e. 485 MPa of Hertzian pressure, in this work), after the removal of the external porous layer wear tended to be negligible. At high applied load (1000 N, 890 MPa), on the other hand, the wear rate became higher than that of the diffusion layer. The friction behaviour was followed by determining the λ-factor evolution during each test. For a given λ-factor, the friction coefficients at 300 N were lower than at 1000 N.

The dry sliding behavior of a 304L austenitic stainless steel and a duplex 2205 austenitic-ferritic stainless was investigated. The evolution of wear was characterized by the existence of a sliding-distance transition. In particular, wear passed from delamination to tribo-oxidation, with a reduction in wear rate. The occurrence of such a transition was interpreted with reference to a theory of sliding wear based on the formation, by subsurface plastic deformation, of a tribological layer and its detachment during sliding. It has been found that the transition is controlled by the ability of the tribological system to form, on its outer part, a protective oxide-rich scale. This introduces a kinetic limitation, which is particularly important in the case of the duplex 2205, because of its lower ductility in comparison to the 304L steel. In this frame, the influence of sliding velocity, the particular frictional behavior, the role of chromium in the oxidative wear, and the surface temperature evolution during sliding could be explained.

The environmental and health concerns related to Particulate Matter (PM) released in the environment from different sources have attracted increasing attention in recent years. The present paper is focused on the PM pollution coming from the wear of automotive brake systems. Generally, brake pads have a complex composition containing even more than 30 different components, some of them more polluting than others. The extent of such emissions depends on their physical and chemical properties and on the tribological interactions with the counter face disc during the braking stages. The main topic of the ongoing research regards the potential impact of the emitted PM on the human health, depending on the mechanisms of formation and toxicity of the particles. Since specific epidemiological studies indicated a link between airborne PM and adverse health effects experienced by urban populations throughout the world, it is important to reduce emissions of pollutants at source, i.e. in the present context, brake systems. The European Union (EU) funded REBRAKE project is one of the many international research projects that are tackling the problem of wear debris emissions from brakes. The first results achieved by REBRAKE team are presented and discussed with regard to the updated situation as emerging from a detailed review of the present knowledge on this topic.

This study is an extension of continuing research on the wear behaviour of gamma-TiAl base alloys, which has demonstrated how a surface reaction layer, made of native oxides, can dramatically improve the dry sliding wear resistance of the Ti-48Al-2Cr-2Nb-1B at.- alloy. The results presented provide a further insight on the wear behaviour of the coated alloy under different loading conditions. Moreover, specific attention has been paid to the study of the tribological coupling and of the wear mechanisms affecting an M2 steel counterface disc. The steel counterface is abrasively worn by the oxide asperities present on the alloy surface. Wear debris remains trapped between the sliding surfaces and gets oxidised. Therefore, wear debris may contribute to further accelerate the wear rate before leaving the system or being compacted on the wear tracks to form protective glazes. Once the main wear mechanism was clearly identified, some tests were conducted on alloy specimens which had their surface oxide ...

The effect of deep cryogenic treatment (−196°C) on the properties of some tool steels was studied by means of both field tests on real tools and laboratory tests. The execution of the deep cryogenic treatment on quenched and tempered high speed steel tools increases hardness, reduces tool consumption and down time for the equipment set up, thus leading to cost reductions of about 50%. A laboratory investigation on an AISI M2 and an AISI H13 steel confirms the possibility of increasing the wear resistance and toughness by carrying out the treatment after the usual heat treatment.

In the present work, the oxidative sliding wear of a heat-treated steel at low-sliding velocities (less than 1 m/s) has been investigated. It has been shown that this type of wear can be described by a mechanism that considers the formation and agglomeration of oxide debris during sliding. The results have been modeled using available equations for two different types of oxidative wear and it has been shown that the model proposed by Sullivan and Hodgson can be used to predict the sliding wear rate of heat-treated steels in the low-sliding velocity wear regime. In this context, the important role of the surface bulk temperature is highlighted. In view of the above consideration, it is also pointed out that wear maps, developed from laboratory test results, have to be critically used for the design of tribological systems.

Thermal fatigue represents one of the most important stresses encountered in technical applications like aluminum pressure die casting and hot forging. The resulting damage is the well-known heat checking phenomenon, characterized by nucleation of a fine cracks network on die and insert surfaces. Usually, many other detrimental stresses, i.e. wear, erosion–corrosion by the molten aluminum and oxidation act during service, thus suggesting the introduction of protective surface treatments. The beneficial effect of nitriding, PVD-coating and, more recently, of their combination in duplex systems, in counteracting these stresses has been proved. On the contrary, the coating influence on thermal fatigue resistance is not yet clearly established. In this paper the influence of plasma nitriding, multi-layer PVD coating (i.e. CrN, ZrN) and duplex treatment (plasma nitriding and PVD) on the thermal fatigue resistance of AISI H11 hot work tool steel was studied. For this purpose a specific experimental apparatus, based on induction heating and water cooling, has been realized. Particular attention was given to plasma nitriding, used as pre-treatment in duplex plasma surface engineering. The role of compound layer, diffusion layer and coating(s) is discussed on the basis of the damage observed during the test.

The effect of three conditioning treatments (scratching, S; scratching and nitric acid etching, SE; scratching, Murakami etching and nitric acid etching, SM) on the properties and dry sliding wear behavior of diamond coatings on WC–Co substrates produced by hot filament CVD was investigated. In specimen S the critical strain energy G c for coating spallation was found to be similar to the available strain energy due to the residual stresses alone, G 0 . In the other two specimens, where cobalt was removed from the substrate surface before coating, G c was found to be 1.1 (specimen SE) and 1.8 (specimen SM) times G 0 . The dry sliding tests against an austenitic stainless steel counterface revealed, for all the coatings, a high run-in friction coefficient (about 0.6) followed by a tribological stage characterized by a very low friction coefficient (about 0.05). However, due to the high counterface ductility a transfer layer was soon formed, which increased friction coefficient and, thereby, the applied friction stress. In specimens S, this produced a rapid removal of the coating by buckling. In the other two specimens, only localized damage regions were detected, due to buckling for specimen SE and wedging for specimen SM. Additional sliding tests of specimen SM were carried out against an Al alloy counterface, a very soft material, and a granite, a very hard and brittle material, in order to highlight the influence of counterface on the dry sliding behavior of the coating. In the case of sliding against the Al alloy counterface, transfer phenomena started immediately after the run-in stage and a tribological stage with a low friction coefficient was not observed. In the case of sliding against granite, the friction coefficient continuously decreased during sliding tending to zero.

The influence of the chromium content and of the volume fraction of primary carbides on the thermal oxidation behaviour of spin cast high speed steels and semi-high speed steels used for the production of hot mill rolls was studied at 700°C. Oxidation nucleates at the carbide–matrix interface and carbides have a higher oxidation resistance than the matrix. Moreover carbides dissolve a higher amount of chromium than the matrix. As a consequence of these effects, the oxidation rate of these steels decreases by increasing the chromium content of the matrix and by decreasing the carbide volume fraction.

In the present paper, a simplified approach to the adhesive theory of friction, which encompasses both the concept of work of adhesion and the growth of the real area of contact, is presented. The starting point is to consider the average shear strength of the asperity junctions as dependent on the effective work of adhesion, which is influenced by the thermodynamic work of adhesion and irreversible local phenomena. Simple analytical equations for the average shear strength and friction coefficient are thus obtained, and their validity is checked with reference to experimental data relevant to copper and aluminum alloys dry sliding against high strength steel counterfaces. It is further shown that this approach is able to rationalize the presence and intensity of the oscillations on the friction trace (the so-called stick-slip phenomenon), and the influence of the initial roughness of the mated materials on friction evolution.

Lubricated rolling-sliding damage in a relatively soft Fe-0.6 pct C alloy and a relatively hard carbonitrided iron, both produced by powder metallurgy, has been investigated. Damage mechanisms were controlled by large-scale as well as small-scale plastic deformations. A large-scale, bulk plastic deformation process produced surface densification in the Fe-0.6 pct C alloy. Formation of surface cracks by asperity-scale plastic shearing was also observed in both materials. Small-scale plastic deformation processes, restricted to the pore edges, gave rise to the formation of fatigue microcracks at the boundary between the densified and undensified region in the Fe-0.6 pct C alloy. A similar effect was found at a depth of between 550 and 1000 µm in the carbonitrided material. Moreover, in the Fe-0.6 pct C alloy, these plastic deformations also triggered the formation and propagation of macrocracks, which produced macroscopic damage by spalling. The damage mechanisms due to small-scale plastic deformations were explained on the basis of a local approach model, able to account for the influence of pores on the mechanical behavior of the materials. However, this approach could not explain the microcracks, which were found at the surface pores in the carbonitrided material. Their formation was ascribed to the interplay between the surface tensile (friction) stresses and the low matrix toughness of the material near the surface.

The dry sliding wear of three Al-based metal–matrix composites containing a high amount of reinforcement (up to 65 vol.%) sliding against a steel counterface was found to be dominated by an oxidation mechanism. Wear, in fact, is given by the fragmentation of an oxidized scale, which forms during sliding. Although the wear mechanism is the same, the three materials display very different wear rates. This was found to be connected with the different roles of subsurface plastic deformation. The interplay between the subsurface plastic deformation and the microstructure of the composites is thus experimentally investigated and its influence on the stability of the surface scale clarified.

The effect of different boron concentrations on the sintering behavior of an Fe-1.5 pct Mo (wt pct) prealloyed powder was investigated. Sintering was carried out in a dilatometer so that all dimensional changes involved with the densification process could be followed. Several transformations were found to occur by heating powder compacts to 1200 °C and then cooling them to room temperature. At high temperatures, boron promoted the formation of liquid phases that, through a more-efficient sintering kinetics, promoted a satisfactory densification. Faster heating rates also had beneficial effects on the density of the final products. From a microstructural point of view, boron tended to destabilize the ferritic phase and to form iron and molybdenum borides. These borides can be found both in the intergranular regions, with a typical eutectic morphology, and dispersed in the ferritic matrix, in the form of nanometer-sized precipitates. This feature, having a significant effect on the hardness of the bulk material, has been ascribed to a bainite-like precipitation of borides from an undercooled austenitic phase.

The impact fracture toughness of sintered iron and high-strength sintered steels, with densities between 7.0 and 7.25 g/cm3, have been investigated by means of instrumented impact testing on fatigueprecracked as well as 0.17-mm-notched specimens. Experimental results show that the fracture behavior is controlled by the properties of the resisting necks at the crack/notch tip. The materials with impact yield strengths of up to 700 MPa display an increase in fracture toughness as the yield strength is increased. These materials undergo continuous yielding during loading, and ductile fracture takes place once the critical plastic strain is attained within a large process zone. A process-zone model, physically consistent with the fractographic observations, correctly rationalizes their impact fracture toughness. The materials with higher impact yield strengths display an impact curve which is linear up to fracture and are characterized by a fracture toughness which is independent of the yield strength. For these materials, the process zone reduces to the first necks at the crack/notch tip, and fracture takes place once the local applied stress-intensity factor reaches the fracture toughness of the matrix.

Immersion tests in a molten aluminium alloy of a surface-treated AISI H11 hot work tool steel were carried out in order to study the corrosion mechanisms and the effect of different surface treatments on the resistance of the material. The steel has a great reactivity with molten aluminium, which causes a generalised attack on the exposed surface. Plasma nitriding and a proprietary gas oxynitriding treatment (NIPRE®) do not change the attack morphology, but improve the corrosion resistance because they reduce wettability and require the dissolution of the surface compound layer formed by the diffusion treatment, before the alloy reacts with the steel. PVD coatings are able to greatly improve the corrosion resistance. They change the mechanism from a generalised attack to a localised one. The improvement is further enhanced by a preliminary nitriding of the base material. The coating density of through-coating defects and the resistance to the thermomechanical stresses arising during immersion are the key properties of the coated surfaces.

The oxidation behaviour of two high speed steels (HSS) employed for the production of hot rolls was studied. The steels have slightly different chromium contents and volume fractions of primary carbides. Because oxidation nucleates at the matrix–carbide interfaces and propagates in the matrix without involving the carbides, the oxide scale grows less uniform. All the primary carbides have a higher Cr content than the matrix; therefore they tend to reduce the oxidation resistance. The slight differences in chromium content and in carbide volume fraction are responsible for the different oxidation resistance at 600 and 700°C, whilst at 500°C the two steels have almost the same resistance.

The dry sliding wear behaviour of the Ti–6Al–4V alloy sliding against itself and AISI M2 steel was investigated at different sliding velocities (between 0.3 and 0.8 m/s) and applied loads (between 50 and 200 N). Two wear mechanisms were identified, irrespective of the counterface and applied load: oxidation wear at the lowest sliding velocities (0.3–0.5 m/s) and delamination wear at the highest (0.6–0.8 m/s). Wear rate was higher against the AISI M2 at the lowest sliding velocities, and it continuously decreased as sliding velocity was increased. On the other hand, as the sliding velocity was increased it first decreased, experienced a minimum and then became very severe in the case of sliding against the Ti–6Al–4V alloy. This behaviour was explained by making reference to the effect of the counterface. At the lowest sliding velocities, the AISI M2 counterface exerted an abrasive effect on the Ti–6Al–4V alloy, thus accelerating its oxidative wear. At the highest sliding velocities, metallic delamination (which developed through the formation of a mechanically mixed layer (MML) on the surface) was the controlling wear mechanism and the thermal effects connected with the frictional heating became of primary importance. Thus, as surface temperature increased (due to an increase in load or a decrease in the thermal conductivity of the counterface, i.e., in passing from the Ti–6Al–4V counterface to the AISI M2) the plastic strain rate at the contacting asperities also increased (by reversible dislocation motion) and wear rate also increased, in accordance with the theory of delamination.

Different Ni–P composite coatings containing SiC particles, to improve wear resistance, and PTFE, MoS 2 and BN particles, to reduce friction, were produced on a steel substrate by means of electroless deposition. A reference Ni–P deposit and a double Ni–P/SiC–PTFE co-deposit were also produced. The dry sliding behaviour against a steel counterface (made of AISI M2) was found to develop through two stages (indicated with stage I and II). Stage I was found to be connected with the initial wear damage of the co-deposit and is therefore representative of its surface durability. In this respect, the best performances were displayed by the Ni–P/PTFE and Ni–P/MoS 2 co-deposits, since their wear rates were negligible during stage I. For the Ni–P/PTFE co-deposit, friction coefficient also remained very low (0.07) during this stage, whereas in the case of the Ni–P/MoS 2 co-deposit, stage I, which also had a shorter duration, was characterized by a higher friction coefficient (0.18). The removal of the co-deposit (stage II) was, however, more difficult in the case of the Ni–P/MoS 2 co-deposit than for the Ni–P/PTFE co-deposit. The Ni–P/SiC–PTFE co-deposit displayed better properties than the reference Ni–P deposit but worse than the Ni–P/MoS 2 and Ni–P/PTFE co-deposits and, finally, the Ni–P/BN co-deposit showed the worst sliding behaviour, since its intense removal started from the beginning of the test. As far as the rolling–sliding behaviour of the co-deposits against the same steel counterface is concerned, the best properties were shown by the Ni–P/BN co-deposit and the worst performances were shown by the reference Ni–P deposit and the Ni–P/SiC co-deposit because of their brittleness.

Instrumented impact testing was used to investigate the effect of both notch severity and matrix strength on the impact fracture behaviour of different Fe–Mo–C high-strength steels produced by powder metallurgy, in the as-sintered as well as in the heat-treated condition. As the matrix strength and/or the notch severity are increased, two fracture mode transitions were observed: from a ductile to a very localized (but still ductile) type of fracture, and from a very localized to a mixed type of fracture. The fracture behaviour was found to be directly correlated with the shape of the impact curves. In the case of the ductile type of fracture, two types of load-deflection curves were encountered. The first type shows the presence of a distinct yield point and a strain hardening stage up to the fracture load. In the second type (encountered in specimens with increased matrix strength and/or notch severity) the distinct yield point disappears and the load–deflection curve shows a continuous yielding up to fracture. In the case of the ductile but very localized and the mixed type of fracture, the load–deflection curve is linear up to fracture. The deformation and fracture behaviour of the specimens which display this type of impact curve is explained using fracture mechanics arguments and it is concluded that the surface pores act as crack precursors.

A study has been made of the impact deformation and fracture behaviour between room temperature and −60°C of porous iron and a porous heterogeneous Fe-Ni-Cu-Mo-C alloy, both produced by powder metallurgy. Unnotched as well as sharp notched specimens (in order to obtain impact fracture toughness data) were tested. In the case of porous iron, the main contribution to the total absorbed energy for the unnotched specimens arose from the energy for plastic bending and the total absorbed energy was found to decrease as the testing temperature was decreased. On the other hand, the impact fracture toughness was found to increase as the testing temperature was decreased. The heterogeneous Fe-Ni-Cu-Mo-C alloys showed a decrease, as testing temperature was decreased, both in the total absorbed impact energy, in the case of unnotched specimens, and in the impact fracture toughness. All these results were interpreted on the basis of a process zone model, making reference to the particular deformation and fracture mechanisms.

This study deals with some aspects of the dry sliding behavior of a gamma TiAl-based alloy with the following composition: Ti-48Al-2Cr-2Nb-1B (at. pct). The tribological system consisted of an AISI M2 steel disk sliding against TiAl alloy sheets. Different alloy microstructures—duplex, equiaxed, and lamellar—were considered, in order to check for their possible effect on the wear behavior. At low loads, a mainly abrasive mechanism was inferred from the phases present in the debris collected at the end of each test. An additional contribution from oxidational phenomena was found at higher loads. In such conditions, the equiaxed samples displayed a slightly better wear resistance. A remarkable improvement in wear resistance was observed when the alloy specimens were tested without a preliminary removal of the surface layer which resulted from the heat treatments carried out to obtain different microstructures. Again, some differences were found in the wear behavior of alloys with different microstructures. In this case, though, the difference can rather be attributed to the different morphology of the surface layers. Better performances were observed in those samples, both duplex and equiaxed, in which a good mechanical matching between the alloy and the outer scale was accomplished, thanks to the presence of a hardened interdiffusion zone. An aspect which should be considered further is the tribological coupling of the as-treated alloys. Indeed, the elevated hardness and surface roughness of the coating yields a significant wear of the sliding counterface.

Residual porosity in ferrous powder metallurgical alloys induces the phenomenon of localized yielding, or first yielding, during tensile testing. This gives rise to the existence of a true (E1) and apparent (E2) Young’s modulus. The true Young’s modulus is similar to the dynamic modulus (Ed) determined by the acoustic resonance method, whereas the apparent Young’s modulus is lower than both E1 and Ed. For alloys with hard microstructures the apparent Young’s modulus turned out to be about 6% lower than the true Young’s modulus and a negligible influence of matrix hardness and pore morphology was highlighted. However, for ferritic or ferritic–pearlitic materials this difference was higher, ranging between 14 and 31% and it decreases as pore roundness is increased. For austenitic AISI 316L alloys both E1 and E2 are lower than Ed because of the presence of oxides on the powder surface, which favour early decohesion at the necks during tensile testing.

Sintered materials show a different stress-strain behavior when subjected to tensile or compressive loading, the response to compression being characterized by a higher elastic modulus, yield stress, and strain hardening rate. These differences tend to make the bending behavior somewhat more complex to analyze, particularly in the elasto-plastic field, as compared to conventional materials, having equal mechanical properties under tension and compression. As a consequence, the use of widely applied test techniques, such as the Three Point Bending (TPB), becomes more difficult for sintered materials, due to the lack of reliable analytical models capable of evaluating elasto-plastic stress-strain distribution as a function of applied load and deflection. In the present investigation, the results of uniaxial tensile-compressive and bending tests conducted on sintered ferrous alloys characterized by different microstructures and porosity are reported and briefly discussed. Then an analytical model, specifically aimed to analyze the elasto-plastic monotonic behavior of a TPB specimen made with a material having different tensile and compressive properties, is presented. Its predictions as regards load-deflection curves and elasto-plastic stress-strain distributions are compared with the results of TPB tests and of numerical (Finite Element) analysis, showing a fairly good agreement.

Instrumented impact testing and slow bend (Three-Point Bending, TPB) testing were used to study the influence of strain rate on the bending behaviour of four types of powder metallurgical materials (material A: pure iron; material B: Fe–0.4C; material C: Fe–0.4C in the heat treated state; material D: Fe–4Ni–1.5Cu–0.5Mo–0.5C) produced at two density levels. Whereas materials A and B are characterized by the presence of a distinct yield point and a pronounced plastic deformation before fracture, materials C show a reduced plastic deformation and materials D are characterized by a continuous yielding behaviour both under impact and TPB testing. The ratio between the impact and TPB yield loads was always larger than unity and increased as the materials hardness decreased. This ratio is mainly influenced by the matrix microstructure. On the other hand, both matrix microstructure and porosity influence the ratio between the impact and TPB maximum loads, which is in any case close to unity. Concerning the fracture energies, all the materials except materials A display a fracture energy which is larger under impact than under TPB. The highest ductility of materials A is responsible for the formation of an intense plastic deformation and a stable microcrack system on the tensile side of the specimen during TPB, which produces values for the TPB fracture energy similar to the impact fracture energy values.

The wear behaviour of two nitrided steels was studied in order to evaluate the tribological properties of the compound layer and of the diffusion layer. The specimens were treated in different conditions, in order to obtain different microstructural and compositional characteristics of the nitrided surfaces. Concerning the diffusion layer, microstructural homogeneity tends to prevail over the microhardness profile in affecting wear resistance, since grain boundary carbonitrides worsen the mechanical properties and oxidation behaviour of the subsurface layers and contribute to abrasion of the counterface. As far as the compound layer is concerned, the effect of thickness and compactness is at least as significant, if not more so, than that of composition. The results demonstrate the importance of designing the microstructure of nitrided layers to maximise the wear resistance of the treated components.

Some results of diverse experimental studies aimed at the optimisation of the process parameters of continuous steam treatment of sintered iron are presented in order to show how the compactness and constitution of the oxide layer strongly influence wear resistance. Since these parameters are not in direct correlation with others usually measured in industrial practice, the results presented clearly indicate that a steam treatment quality control procedure must involve careful oxide characterisation.