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Purpose Fretting wear exists widely in the field of matching mechanical parts whereas previous research studies mostly focus on the point contact through a ball-plate tribometer. This paper aims to study the influence of wear debris on the fretting wear characteristics of the nitrided medium carbon steel under line contact condition at elevated temperature. Design/methodology/approach Fretting wear behavior of the nitrided medium carbon steel was experimentally investigated under line contact condition at elevated temperature and different normal loads without lubrication. Wear loss, worn surface and wear debris were studied to analyze the wear mechanism of nitrided steel. Findings The results showed that surface hardness of the medium carbon steel was notably improved because of the generation of a 230 µm nitrided case. Wear loss increased with the normal load, which was associated with the damage of a thin solid film formed by the wear debris, consisting of iron oxides and chromium oxide rather than only iron or iron oxides. The wear debris became partially amorphous and spherical because it was trapped within the contact interface and was ground, rolled, oxidized under line contact conditions. The spherical wear debris acted as a third body and formed a lubricating film between the contact faces. This lubricating film helped to stabilize the friction coefficient and reduced the wear rate, which further caused the acceleration of wear volume to gradually decrease. The wear mechanisms of the nitrided steel were oxidation wear, abrasive wear and fatigue spalling of the oxide layer. Originality/value The findings are helpful to understand the fretting wear behavior of the friction pair under line contact and enrich the fretting tribology theory.

A 95W–3.5Ni–1.0Fe–0.5Co heavy alloy was surface modified by pack carburization. A new carburized layer consisting of outside surface layer and inner surface layer was formed. The double layered microstructure showed porous WC aggregates in the outside surface layer and W grains coated by shell-like (Ni, Co, Fe)6W6C carbides in the inner surface layer. There were two paths for C atoms diffusion and reaction through W grains and matrix phase. Co changed the formation orders of carbides and acted as a medium for promoting C atoms reaction with Ni, Fe, Co and W in the matrix to form shell-like (Ni, Co, Fe)6W6C carbides in the inner surface layer. Meanwhile, C atoms diffused in the W grains and reacted with W in an order of W→W2C→WC. WC nucleated and grew in carburized W grains and micropores produced in the growing WC crystals due to the lattice expansion from W→W2C→WC. As the carburization process continued, the shell-like (Ni, Co, Fe)6W6C carbide was also carburized to WC, and finally the whole W grain was carburized into porous WC aggregates in the outside surface layer.

The increasingly strict emission regulations in combustion engines are raising high requirements for the engine valve train system. In this paper, a novel multifunctional wear apparatus is designed to study the performance of engine valve train components. The apparatus employs a mechanical loading system, which consists of a special eccentric wheel and disc springs that apply the combustion loads, and the contact configurations and loading conditions of valve train components are simulated. It has three test functions for different components through specifically designed fixtures. The first function aims to evaluate the interaction between the valve seating face and the seat insert at high temperatures and loads. The second function is used to study the friction and wear properties of the valve stem and the valve guide. The third function is designed to evaluate the performance of the valve seals. At last, a verification test was carried out by the proposed experimental method. A pair of new exhaust valve and seat insert is tested for the performance evaluation of the first function. The wear mechanisms acting on the pairs interface are shown to be a combination of oxidative wear, adhesive wear, as well as fatigue flaking.

Rolling contact fatigue (RCF) behavior of carburized 30CrSiMoVM steel was experimentally investigated at elevated temperature. The results show that RCF crack was drive by asperity contact rather than the orthogonal shear stress or the maximum shear stress. The value of L 10 , L 50 , L a life at 100 °C decreased by 48.3%, 44.1%, 43.2%, respectively, compared with that at 40 °C, which was attributed to lubrication status changing from mixed lubrication regime to boundary lubrication regime. The formation of the synergistic tribofilm containing Fe 2 O 3 , FeS, ZnS, FePO 4 and Ca 3 (PO4) 2 around the center of the contact area effectively prevented initiation and propagation of RCF crack under boundary lubrication status, resulting in the obvious variation of the failure mode of the carburized layer.

The friction and wear properties of 30CrMnSiA steel were investigated at elevated temperature from 100 to 600 °C. Thereafter, the wear debris and worn surfaces were examined to understand the wear mechanisms. The remained debris with relatively high hardness created three-body abrasion at lower temperatures (100-300 °C). Abrasive wear prevailed at the conditions with high friction coefficients and wear rates. A significant change in friction and wear behavior occurred at 400 °C. At the temperature of 400 °C, oxidation induced mild wear was found because of the formation of load-bearing oxide film. Both the friction coefficients and wear rates of the steel were lowest at 400 °C. At the temperatures of 500-600 °C, a mild-to-severe wear transition occurred which resulted in an increase in the friction coefficients and wear rates of the steel. This is related to the decrease in the strength of matrix and hardness of worn surfaces and subsurfaces. The predominant wear mechanism is considered to be severe abrasive, adhesive wear and a fatigue delamination of the oxide film.

Rolling contact fatigue (RCF) characteristics of 0.1C–3Cr–2W–V nitrided and untreated steel have been investigated at ambient temperature. The results indicate that the friction coefficients of the nitrided specimens decreased from 6% to 29%, and the RCF life of the nitrided specimens have been significantly prolonged. The failure modes of the nitrided surface could be classified as spalling and delamination within the layer, and the inner defects generated by the plasma nitriding play an important role in the surface and subsurface initiated RCF. Meanwhile, decreased surface roughness and better wear-resistance are demonstrated to be useful to improving the RCF performance.

The effects of Zr and Si co-substitution on the magnetic properties, oxidation and corrosion resistances of TbCu 7 -type Sm–Co alloys have been investigated. Zr addition is very effective in promoting the formation of TbCu 7 -type structure and it contributes to the most of the coercivity enhancement. The largest coercivity of 1389 kA/m was obtained for SmCo 6.3 Zr 0.4 Si 0.3 alloy. Si addition can significantly improve the oxidation resistance. After exposure in air at 500 °C for 90 h, the thickness of the oxidation layer was just 106 μm for as-cast SmCo 6.4 Zr 0.3 Si 0.3 alloy compared to 230 μm for the SmCo 6.7 Zr 0.3 alloy. The room temperature coercivity of SmCo 6.4 Zr 0.3 Si 0.3 ribbon is as high as 884 kA/m after exposure at 300 °C in air for 10 min. The improved corrosion resistance for Si added alloys was also confirmed by polarization curve measurement. Zr and Si co-substitution provides a good approach for enhancing both the coercivity and resistances to oxidation and corrosion of the TbCu 7 -type Sm–Co alloy.