Factors affecting the progressive development of wear-protective oxides on iron-base alloys during sliding at elevated temperatures

The friction behaviour of two commercial Fe-12%Cr alloys during reciprocating sliding in air at 100–400 °C has been studied and a model is proposed to account for the changes observed. After relatively high initial values, associated with metal-metal contact, the friction decreases progressively with sliding time, reaching a minimum value that is very reproducible for a given set of conditions after a short period, the length of which decreases with increasing temperature. Subsequently the friction increases somewhat and attains a steady value which is maintained throughout the remainder of the sliding run. This value can be correlated with oxide-oxide contact only. The decrease in friction in the early stages is associated with the progressive development of adherent compacted oxide regions. The model proposed to account for these changes assumes that these regions are uniform in thickness, that the volume rate of oxide production is proportional to the remaining area of bare metal surface and hence that the area growth rate of compacted oxide follows an exponential decay law. The model relates the changes in coefficient of friction with time to several interfacial metal, oxide and metal-oxide parameters. There is very close correlation between the model and practice at 400 and 300 °C. However, the correlation is less exact at the lower temperatures. These results are considered in the light of two possible mechanisms of oxide generation during sliding wear.     

The transition from severe to mild sliding wear for Fe-12%Cr-base alloys at low temperatures

A study of the friction and wear behaviour of two commercial Fe-12%Cr-base alloys Jethete M152 and Rex 535 during like-on-like reciprocating sliding in air at ambient temperatures up to 200 °C has been carried out. As expected from practical experience, the overall wear resistance of Rex 535 is superior to that of Jethete M152. In all cases, the wear processes are characterized by an initial period of primary severe wear with associated high, but irregular, coefficients of friction, followed by a transition to a steady state period of secondary mild wear with associated reduced and steady friction values. The time of this transition is load independent and decreases with increasing ambient temperature but occurs more rapidly for Rex 535 than for Jethete M152, which accounts entirely for the former's superior overall wear performance. The wear rate during sliding in the primary severe wear period is independent of alloy, of applied load and, possibly, of temperature while the secondary wear rate is independent of alloy but is dependent on temperature, although not in a regular manner. The transition from severe to mild wear can be correlated with the generation and comminution of metal wear debris particles during the severe wear period until the particles are small enough for substantial oxidation of the exposed surfaces to take place at the ambient temperature of sliding. The subsequent temperature dependence of the secondary mild wear rate is probably related to changes in the adhesive properties of this tribogenerated wear debris. The faster transition from severe to mild wear for Rex 535 compared with that for Jethete M152 is associated with easier comminution of the metal wear particles of Rex 535 owing to their lower ductility. Hence the significance of oxidation of the debris surfaces becomes important at an earlier stage in the sliding process.     

The effectiveness of oxides in reducing sliding wear of alloys

During like-on-like reciprocating sliding in air (amplitude 2.5 mm, load 1.5 kg, speed 500 double traversais per minute), the formation of oxides can have considerable influence on the friction and wear characteristics of high-temperature alloys, such as Jethete M152 and Rex 535. In particular, above a certain transition temperature, between 200 and 300°C for these alloys under these conditions, an adherent, smooth wear-protective oxide layer is developed on the load-bearing surfaces. At lower temperatures, oxide debris reduces the extent of metal-metal contact, thereby reducing the friction and wear rate, but does not eliminate it completely. The oxide debris is produced by two processes; one involves transient oxidation of the metal surfaces, removal of such oxide during each transversal, and reoxidation of the exposed metal; the other involves the formation, fracture, comminution, and oxidation of metal debris particles. At temperatures above the transition temperature, the oxide debris is compacted and comminuted between the sliding surfaces to develop the wear-protective oxide layer. This paper considers the reasons for the effectiveness of such oxides in terms of the influence of the hydrostatic pressures generated on plastic deformation of the very fine oxide particles or asperities in the surface. The resulting friction during sliding is less than during metal-metal contact because only limited asperity junction growth occurs before the asperities become sufficiently large and the hydrostatic pressures sufficiently reduced to allow fracture within the oxide-oxide junctions. The oxide-wear debris produced is recompacted into the surface, resulting in only very low wear rates. It has been shown that the number of asperity-asperity contacts during sliding of wear-protective oxide layers is relatively high, typically 5×10³/mm² of apparent contact area, while the mean surface flash temperature rise is low, typically 2°C. Consideration is given to some of the conditions that favor development of wear-protective oxide layers.