K.M. Chen

Characteristics of Extrusive Wear and Transition of Wear Mechanisms in Elevated-Temperature Wear of a Carbon Steel

The dry sliding wear of a medium carbon steel with different microstructures was measured under the normal load range of 50–150 N at 400°C by a pin-on-disc high-temperature wear setup. The wear behavior and wear mechanism were systematically studied; in particular, the characteristics of extrusive wear and the transition of wear mechanisms were investigated. Under low normal loads, the wear is oxidative type wear. Once the normal load reached a critical value, a mild-to-severe wear transition occurred, and subsequently an extrusive wear prevailed. The mild-to-severe wear transition depended on the microstructure of matrix; the critical normal load of the transition was 112.5 N for tempered sorbite, 125 N for lamellar pearlite, and 137.5 N for tempered martensite and tempered troostite. As oxidative wear prevailed, a thick oxide layer about 20–30 μ m and a plate-like wear debris with regular outline were recognized. However, as the extrusive wear occurred, the wear rate abruptly increased but the friction coefficient was reduced. The extrusive wear predominated due to thermal softening of the matrix and presented a superthin oxide layer (less than 0.5 μ m) and low oxide content on worn surfaces, accompanied by the appearance of ribbon-like wear debris.

Comparative Study of Wear Behaviors of a Selected Titanium Alloy and AISI H13 Steel as a Function of Temperature and Load

A comparative study of the wear behaviors of a selected titanium alloy and AISI H13 steel as a function of temperature and load was performed on a high-temperature wear tester. The titanium alloy and H13 steel presented totally different wear behaviors with the variation in temperature and load. Their behaviors are suggested to be attributed to the protective ability of tribo-oxides and the thermal softening resistance of the matrix. Compared to H13 steel, the titanium alloy presented poor room-temperature wear resistance, excellent high-temperature wear resistance, and an extremely protective function of tribo-oxides.

Effect of Microstructures on Elevated-Temperature Wear Resistance of a Hot Working Die Steel

Elevated-temperature wear tests under atmospheric conditions at 400 °C were performed for a hot working die steel H21 on a pin-on-disk wear tester. The phase and morphology of worn surfaces were examined using XRD and SEM, and the relation of wear resistance to tempered microstructures was studied for H21 steel. XRD patterns exhibit that oxidative wear is a predominated wear mechanism with Fe3O4 and Fe2O3 on worn surfaces. It is found that with increasing normal load, obvious plastic deformation of substrate appears on worn surfaces. Microstructures start to affect apparently wear resistance of the steel with an increase of load. Under loads of 50–100 N, wear losses of steel retain low values and relatively approach for steels with various microstructures. As loads are increased to 150–200 N, wear losses of steel start to increase obviously and present apparent difference for steel with various microstructures. Wear resistance is found to increase in the sequence as follows: tempered sorbite, tempered martensite, tempered troostite without secondary hardening and tempered troostite with secondary hardening or upcoming one. Higher strength and microstructural stability are required for steels with excellent wear resistance.

Selection of Heat Treatment Process and Wear Mechanism of High Wear Resistant Cast Hot-Forging Die Steel

Dry sliding wear tests of a Cr-Mo-V cast hot-forging die steel was carried out within a load range of 50–300 N at 400 °C by a pin-on-disc high-temperature wear machine. The effect of heat treatment process on wear resistance was systematically studied in order to select heat treatment processes of the steel with high wear resistance. The morphology, structure and composition were analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS); wear mechanism was also discussed. Tribo-oxide layer was found to form on worn surfaces to reduce wear under low loads, but appear inside the matrix to increase wear under high loads. The tribo-oxides were mainly consisted of Fe3O4 and Fe2O3, FeO only appeared under a high load. Oxidative mild wear, transition of mild-severe wear in oxidative wear and extrusive wear took turns to operate with increasing the load. The wear resistance strongly depended on the selection of heat treatment processes or microstructures. It was found that bainite presented a better wear resistance than martensite plus bainite duplex structure, martensite structure was of the poorest wear resistance. The wear resistance increased with increasing austenizing temperature in the range of 920 to 1120 °C, then decreased at up to 1 220 °C. As for tempering temperature and microstructure, the wear resistance increased in following order: 700 °C (tempered sorbite), 200 °C (tempered martensite), 440 to 650 °C (tempered troostite). An appropriate combination of hardness, toughness, microstructural thermal stability was required for a good wear resistance in high-temperature wear. The optimized heat treatment process was suggested for the cast hot-forging steel to be austenized at 1020 to 1120 °C, quenched in oil, then tempered at 440 to 650 °C for 2 h.

Identification and characterization of an atypical 2-cys peroxiredoxin from the silkworm, Bombyx mori.

Peroxiredoxins (Prxs) play an important role in the protection of insects against the toxicity of reactive oxygen species. Here, we identified and characterized a novel, atypical 2‐cysteine (Cys) peroxiredoxin (BmPrx3) from an expressed sequence tag database in a lepidopteran insect, Bombyx mori. The BmPrx3 cDNA contained an open reading frame of 684 bp that encodes a 228‐amino‐acid protein with a calculated molecular mass of 25 kDa. Sequence comparison revealed that BmPrx3 belongs to the atypical 2‐Cys Prxs. Quantitative real‐time PCR revealed that BmPrx3 can be detected in all tissues and developmental stages. Recombinant BmPrx3 purified from Escherichia coli exhibited antioxidant activity that removed hydrogen peroxide and protected DNA from oxidative damage. Disc diffusion and viability assays revealed that recombinant BmPrx3 increased bacterial survival under H2O2‐mediated oxidative stress. In addition, quantitative real‐time PCR analysis indicated that BmPrx3 transcription levels were significantly increased in response to various oxidative stresses. Furthermore, BmPrx3 transcription levels in the midgut were regulated by bacterial infection. Taken together, these results suggest that BmPrx3 acts as an antioxidant enzyme to protect the silkworm from various oxidative stresses.

Effect of Various Nanoparticles on Tribo-Layers and Wear Behavior of TC11 Alloy

Multilayer graphene (MLG), Fe2O3, and their nanocomposites with various proportions and amounts were applied as additives and directly participated in the formation of tribo-layers during sliding wear of TC11 alloy against AISI 52100 steel. Their ingredients and amounts were found to exert substantial effects on the additive-containing tribo-layers and wear behavior. Irrespective of the added amount of MLG or Fe2O3, the formed tribo-layers, because of the lack of load-bearing or lubricant capacity, readily lost stability and protection function, causing a high wear rate. However, a small quantity of MLG/Fe2O3 nanocomposites could result in a remarkable decrease in the wear rate. This was attributed to the stable existence and continuous protection of a friction-reduced and wear-resistant double-layer tribo-layer. In particular, Fe2O3-rich nanocomposite additives produced more protective tribo-layers to markedly improve the wear resistance of TC11 alloy.

Dusky works upstream of Four‐jointed and Forked in wing morphogenesis in Tribolium castaneum

Dusky (dy) is required for cytoskeletal reorganization during wing morphogenesis in Drosophila melanogaster, but which genes participate together with dy for wing morphogenesis has remained unclear. In Tribolium castaneum, dy is highly expressed at the late embryonic stage. Tissue‐specific expression analysis indicated high expression levels of dy in the epidermis, head and fat body of late‐stage larvae. RNA interference (RNAi) targeting dy significantly decreased adult wing size and caused improper folding of the elytra. Meanwhile, dy knockdown reduced the transcription of four‐jointed (fj) and forked (f). Our results show that fj RNAi reduces adult wing size and that silencing f results in abnormal wing folding in T. castaneum. Interestingly, knocking down fj and f simultaneously phenocopies dy RNAi, suggesting that dy probably acts upstream of fj and f to regulate wing morphogenesis in T. castaneum.

Effect of nanoparticles on the tribo-layers and the tribology of a steel-on-steel couple:

The tribological behavior and tribo-layers of AISI 1045 steel sliding against 52100 steel were investigated in the case of supplying MoS2, Fe2O3, and their mixtures onto the sliding interface. When nanoparticles were supplied, tribo-layers were formed on the worn surfaces. The tribological behavior of the sliding pair depended on the characteristics of tribo-layers, which were decided by different nanoparticles. As the additives—especially the ones containing MoS2—were supplied onto the sliding interface, the wear rates and friction coefficients of both 1045 steel and 52100 steel were markedly decreased to extremely low values, approaching zero and marginally undulated with the increase in load. Single-component Fe2O3 nanoparticles markedly reduced the wear rate of 1045 steel with slightly increased friction coefficient, but its decreased extent was merely half of that of the additives containing MoS2. The improvement of the tribological performance of steels was attributed to the formation of protective tribo-layers. The addition of pure Fe2O3 resulted in the formation of insert-type tribo-layers, while cover-type tribo-layers were formed by the addition of the mixture additives of Fe2O3+MoS2 and pure MoS2. The cover-type tribo-layers provided more protective and lubricative functions than that of the insert-types.

Comparative research on the effect of an oxide coating and a tribo-oxide layer on dry sliding wear of Ti–6Al–4V alloy

The effect of an oxide coating and a tribo-oxide layer on dry sliding wear of Ti–6Al–4V alloy was comparatively studied. The oxide coating was prepared on Ti–6Al–4V alloy by a thermal oxidation/diffusion process; the tribo-layer was an in situ produced mechanically mixed layer during dry sliding. The oxide coating markedly improved the wear performance of Ti–6Al–4V alloy at room and elevated temperatures. Tribo-layers were classified into three types: no-oxide tribo-layer, porous, and dense tribo-oxide layers. Both porous and dense tribo-oxide layer presented protective function, thus significantly improving wear performance of Ti–6Al–4V alloy. However, no other than dense tribo-oxide layer was qualified to be comparable to the oxide coating, which almost possessed the same wear-reduced function as the oxide coating. Even when the oxide coating was severely delaminated, the tribo-oxide layer would replace the oxide coating and took effect to protect from wear.