G. Straffelini

Effect of deep cryogenic treatment on the mechanical properties of tool steels

Abstract 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.

Dry sliding wear mechanisms of the Ti6Al4V alloy

Abstract The dry sliding behaviour of the Ti6Al4V alloy was studied in order to highlight the mechanisms responsible for the poor wear resistance in different load and sliding speed conditions. By increasing the sliding speed, a transition from oxidative wear to delamination occurs with a corresponding minimum in the wear rate. This minimum shifts towards lower speeds by increasing the load. The results were supported by the analysis of the wear debris and of the worn specimens, and discussed on the basis of the main mechanical and chemical characteristics of Ti alloys which influence the wear resistance.

Dry sliding wear of Ti–6Al–4V alloy as influenced by the counterface and sliding conditions

Abstract 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.

Surface durability of electroless Ni–P composite deposits

Abstract 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.

Oxidative wear of heat-treated steels

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.

Influence of matrix hardness on the dry sliding behaviour of 20 vol.% Al2O3-particulate-reinforced 6061 Al metal matrix composite

Abstract In the present investigation the wear behaviour of the 6061 Al alloy reinforced with 20 vol. % Al 2 O 3 particles dry sliding against a tool steel counterface was studied as a function of load and with reference to different values of the matrix hardness, obtained by submitting the extruded composite to thermal and forging treatments. The obtained wear rates were interpreted on the basis of the analysis of the surface and subsurface damage to the composite due to sliding. If the hardness of the materials is increased by a treatment, their wear rate also increases, because of the occurrence of subsurface softening and the formation of surface mixed scales prone to leaving the tribological system. On the other hand, the as-extruded composite is characterized by a lower matrix hardness (and thus a higher ductility) which prevents the conditions for subsurface softening and formation of the mixed scale from being reached. In this case a transfer layer mainly consisting of iron oxides then forms and protects the composite, thus reducing its wear rate.

Influence of load and temperature on the dry sliding behaviour of Al-based metal-matrix-composites against friction material

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.

Corrosion protection properties of electroless Nickel/PTFE, Phosphate/MoS2 and Bronze/PTFE coatings applied to improve the wear resistance of carbon steel

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.

Effects of load and sliding speed on the tribological behaviour of Ti6Al4V plasma nitrided different temperatures

Abstract Dry sliding tests were carried out under different load and sliding speed conditions on Ti6Al4V alloy plasma nitrided at three temperatures 973, 1073 and 1173 K. The results were interpreted on the basis of the evolution of the frictioc basis of the evolution of the friction coefficient and characterizing the wear debris and worn surfaces in order to understand the acting wear mechanisms. A comparison was also made with the results obtained with untreated specimens, tested under the same conditions. Plasma nitriding can noticeably improve the dry sliding resistance of the Ti6Al4V alloy. The nitriding temperature must be chosen according to the main wear mechanism observed under specific load and sliding speed conditions. When wear is determined by the resistance of the compound layer (low loads and low sliding speeds), the nitriding treatment has to be carried out at 1073 K. In this case, the compound layer has optimal properties with respect to resistance to adhesion and fragmentation. When the material is exposed to delamination (high loads and high sliding speeds), the strength of the diffusion layer has to be maximized. In this case, the nitriding temperature should be as high as possible (1173 K in the present investigation) in order to enhance the hardening of the diffusion layer.

Thermal fatigue resistance of gas and plasma nitrided 41CrAlMo7 steel

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.