Transition of elevated-temperature wear mechanisms and the oxidative delamination wear in hot-working die steels

Dry sliding wear tests of H13 and H21 steels were performed at 400 °C. The wear mechanisms and their transitions were studied, and an oxidative delamination wear was suggested. A mild oxidative wear prevailed with oxide fatigue delamination under less than 3.54 MPa. Under 3.54-5.31 MPa, the oxidative wear prevailed with oxide fatigue delamination and the oxidative delamination wear. As the pressure surpassed 5.31 MPa, a severe wear prevailed with the oxidative delamination wear and the plastic extrusion. The oxidative delamination wear meant that the delamination occurred inside the matrix underneath tribo-oxides with long-ditch delamination and belt-like wear debris.     

Relations between oxidative wear and Cr content of steels

Dry sliding wear tests at 25-400 °C were performed for 45, 4Cr5MoSiV1 and 3Cr13 steels; the relations between oxidative wear and Cr content of steels were explored. The low and medium-Cr steels had a substantially lower wear rate and increasing tendency than the high-Cr steel at 25-200 °C, but the contrary case occurred at 400 °C. With an increase of ambient temperature, the wear rate of the low and medium-Cr steels first decreased, then increased and reached the lowest value at 200 °C, while the wear rate of the high-Cr steel decreased monotonously with the lowest value at 400 °C. At 25 °C, trace tribo-oxides reduced wear to some extent in adhesive-dominated wear for the low and medium-Cr steels. At 200 °C, a small amount of tribo-oxides formed and reached a thickness of 10 μm on contacting asperities in the low and medium-Cr steels, thus oxidative mild wear prevailed. At 400 °C, a great amount of tribo-oxides appeared in the low and medium-Cr steels; unexpectedly, the high-Cr steel had more tribo-oxides than the low or medium-Cr steels in some cases. Its high wear resistance may be attributed to Cr-strengthened adhesion power of tribo-oxides and matrix.     

A simplified theory for the oxidative wear of steels

A simple model, based on linear oxidation, is proposed to represent oxidative wear by flaking. The theory developed predicts that log (N/ .w) varies linearly with θ⁻¹, where N is the normal applied force, .w is the wear rate per time and θ the absolute temperature of the metal substrate. The results of tests using En1A pins on N75 and En1A tracks show reasonable agreement with the theoretical predictions for θ values between 300°C and 500°C for both the N75 track and the En1A track. In these regions oxidation may reasonably be expected to constitute the majority of the wear.     

Characteristics of wear debris in impact sliding

The characteristics of debris arising from repetitive compound (sliding) impact are presented. The debris was collected from a variety of tests. Microscopy and X-ray diffraction analysis establish (1) that debris may arise from specimen and counterface, (2) that metallic particles consist of very small and/or distorted primary crystals, (3) that oxide formation occurs and (4) that phase transformations of metallic constituents may occur. Further, particular debris characteristics are shown to be dependent on the nominal contact stress and the relative sliding velocity.     

Model of cutting-tool wear based on the analysis of oxidative frictional processes

The ambient impact to structural transformations and operation ability of cutting tools has been examined. On the base of the experimental data the model of tribo—oxidation wear of cutting tools has been developed.     

Characteristics of wear results tested by pin-on-disc at moderate to high speeds

The use of a pin-on-disc configuration for investigating the wear mechanism or behaviour of solid materials is examined carefully since the results of such configurations differ from published data and some existing theories cannot be applied to such a configuration directly. The obvious contradictions include the following. The results between the arrangements of the rotating pin and the stationary pin under the same load and speed are different. The bulk temperatures of the rubbing specimens increase with the duration of testing, which may eventually arrive at a steady state. However, before the wear condition reaches a steady state, it will have continuously varied. Moreover, the friction coefficient increases with sliding speed when the applied load on the rubbing specimens is over certain levels. All these contradictions can be reasonably explained with the accurate prediction of bulk and flash temperatures at the contact area. To this end, this paper provides a more reasonable method for the calculation of temperatures and the real and apparent contact areas.     

Characteristics of fracture during the approach process and wear mechanism of a silicon AFM tip

The wear of an atomic force microscope (AFM) tip is one of the crucial issues in AFM as well as in other probe-based applications. In this work, wear tests under extremely low normal load using an AFM were conducted. Also, in order to understand the nature of silicon tip wear, the wear characteristics of crystal silicon and amorphous silicon oxide layer were investigated by a high-resolution transmission electron microscope (HRTEM). It was found that fracture of the tip readily occurred due to impact during the approach process. Experimental results showed that the impact should be below 0.1 nNs to avoid significant fracture of the tip. Also, it was observed that wear of the amorphous layer, formed at the end of the tip, occurred at the initial stage of the silicon tip damage process. Based on Archard's wear law, the wear coefficient of the amorphous layer was in the range of 0.009-0.014. As for the wear characteristics of the silicon tip, it was shown that wear occurred gradually under light normal load and the wear rate decreased with increase in the sliding distance. As for the wear mechanism of the silicon tip, oxidation wear was identified to be the most significant. It was shown that the degree of oxidation was higher under high normal load and in a nitrogen environment, oxidation of the silicon tip was reduced.     

Investigation of abrasive wear resistance and wear mechanism of composite alloys

The abrasive wear resistance of composite alloys comprising hard tungsten carbide and soft CuNiMn matrix under different wear conditions has been investigated and compared with CrMo cast iron. It was found that Yz-composite alloy with hard cast angular tungsten carbide has greater wear resistance than CrMo cast iron under two-body wear conditions, but lower resistance than Cr-Mo cast iron under three-body wear conditions. It was found that under three-body wear conditions selective wear of the matrix and digging or fragmentation of tungsten carbide particles dominate in Yz-composite alloy, and microcutting and deformed ploughing is dominant under two-body wear conditions. The abrasive wear resistance of composite alloys under two-body wear condition is independent of bulk hardness, but is closely related to the microhardness of tungsten carbide.     

基于磨耗稳定态的高性能摩擦磨损实验机研制

电气化铁路中电力机车靠受电弓滑板与接触网导线的滑动接触来获取动力,它们之间的摩擦接触状态直接影响到机车的运行速度和牵引力,同时其摩擦磨损性能决定了机车连续运行的时间和接触网导线的使用寿命.实际应用中二者构成了一对机械与电气耦合的特殊摩擦副,它们之间的磨耗过程是1个非常复杂的物理化学过程.通过研究发现,机车在正常运行过程中,滑板与接触网导线在运行一段时间以后会出现1个磨耗量较低、磨耗率相对稳定的阶段,在该阶段中摩擦副表面有一层光亮的膜层,称此时为磨耗稳定态,磨耗稳定态对磨擦副的性能及运行寿命具有重要的影响.为了对受电弓滑板与接触导线的载流磨耗特性进行实验研究,分析磨耗稳定态出现的规律,制作了1台高性能摩擦磨损实验机.实验机实验参数调节范围宽,测量精度高,功能强,也可用于电刷和电机滑环等滑动电接触实验.     

Friction and wear behavior of human teeth under various wear conditions

Previous reports have described the differences in the friction and wear behavior between different zones of human teeth. The objective of this research was to study the friction and wear behavior of human teeth under different wear conditions to extend the understanding of the tooth wear process, as well as to provide a more rational explanation for wear mechanism of teeth. Two typical wear tests, namely two- and three-body wear, were conducted on human tooth enamel using a reciprocating apparatus. The effect of food particles was of particular interest. Three loads, 10, 20 and 40 N, were used. Wear was assessed by sample wear volume. The results show that human tooth enamel exhibits lower friction and smaller wear volume under three-body wear conditions than under two-body wear conditions. Under three-body wear conditions, although increasing normal load results in a progressive increase in the wear volume of enamel, the increasing rate is lower at high load than that under two-body wear conditions. Further analysis of wear surfaces indicates that human tooth enamel experiences different wear mechanisms under different wear conditions.     

The delamination theory of wear and the wear of a composite surface

The delamination theory of wear postulates that there is a “nonworkhardening” soft surface layer which deforms continuously due to the instability of dislocations, and that the low speed sliding wear of metals is caused by the subsurface crack nucleation and propagation nearly parallel to the surface. A corollary of the theory is that when hard metal surfaces are plated with a soft metal to reduce the coefficient of friction and the wear rate, the soft metal layer must be thinner than a critical thickness so as to prevent the accumulation of dislocations in the plated layer and the formation of the delaminated layer. This corollary was investigated by plating annealed AISI 1018 steel with cadmium. The wear rate of the steel specimen plated with 0.1 μm cadmium on both of the contacting surfaces was three orders of magnitude smaller than the unplated specimen when they were tested in argon. In the case of thicker coatings, the cadmium layer wears by the delamination process which occurs within the plated layer. The very thin cadmium plate is also effective in reducing wear in inert oil, but not effective in an oxidizing atmosphere. The coefficient of friction of the 0.1 μm Cd plated steel was less than the unplated steel under all test conditions.     

On the identification of wear modes and transitions using airborne wear particles

A novel test method was used to identify how contact conditions influence the wear modes and transitions for sliding steel-on-steel contacts. The test equipment was a pin-on-disc tribometer equipped with instruments for counting airborne particles. The results show that the dominant wear mode significantly influences the number of airborne particles generated from the contact. During mild wear few or no measurable airborne particles were generated. The transitional running-in process could also be identified by airborne particle measurement analysis.     

Impact wear of MgO single crystals II. Impact and wear damage

The impact wear damage of MgO single crystals was investigated under an impact load of 60 kgf. Two types of impact damage, a zero wear process and a measurable wear process, are distinguished. The impact scar shows three types of cracks: subsurface cracks parallel to the surface, and tangential and radial cracks on the surface. In the early stages of the wear process the impact scar is surrounded by surface and subsurface cracks and grows as a unit square cell; wear occurs at the scar bottom and on the scar wall. Consequently the profile of the wear scar changes to a hemispherical or parabolic shape. The zero wear limit under each impact load is determined from the static contact stress and the number of impacts.     

Friction and wear of ceramics

The adhesion, friction, wear and lubricated behaviors of both oxide and non-oxide ceramics are reviewed. Ceramics are examined in contact with themselves, other harder materials and metals. Elastic, plastic and fracture behavior of ceramics in solid state contact is discussed. The contact load necessary to initiate fracture in ceramics is shown to be appreciably reduced with tangential motion. Both friction and wear of ceramics are anisotropic and relate to crystal structure as with metals. Grit size effects in two- and three-body abrasive wear are observed for ceramics. Both free energy of oxide formation and the d valence bond character of metals are related to the friction and wear characteristics for metals in contact with ceramics. Surface contaminants affect friction and adhesive wear. For example, carbon on silicon carbide and chlorine on aluminum oxide reduce friction while oxygen on metal surfaces in contact with ceramics increases friction. Lubrication increases the critical load necessary to initiate fracture of ceramics both in indentation and with sliding or rubbing.     

Rebinder effect and wear

The relation between the Rebinder effect and wear through the general form of the chemomechanical interaction is discussed from a fundamental viewpoint. An attempt has been made to merge the theories of the Russian school and that of Westwood et al. into one unified theory which can be made quantitative and used to explain many facets of the wear of metallic and non-metallic materials.