Wear of Dry Sliding Al 6061-T6 Alloy Under Different Loading Conditions

In the present work, wear of Al 6061-T6 alloy under different normal loads, sliding speeds and temperatures was investigated. Pin on disk type tribometer was used to conduct dry sliding experiments. Different load combinations comprising of normal loads (1 kg, 1.5 kg and 2 kg), sliding speeds (1.25 m/s, 2 m/s and 3 m/s) and temperatures (room temperature (31 ± 1 °C), 60 °C, 100 °C and 150 °C) were applied during dry sliding experiments. Adhesive and abrasive wear mechanisms were observed in dry sliding of Al 6061-T6 alloy contacts from the microscopic analysis of worn contact surfaces. The wear rate was more influenced by increase in normal load than increase in sliding speed and temperature. Under normal loads of 1 kg and 1.5 kg, Al 6061-T6 alloy showed better wear resistance at higher temperatures when compared to that at room temperature.

Atomistic insights into anti-wear mechanisms and protective tribofilm formation in polytetrafluoroethylene composites

Polytetrafluoroethylene (PTFE) has a low friction coefficient but poor wear resistance (k ~ 10−3 mm3/Nm) against various surfaces. Mechanical modeling suggests that the enhanced anti-wear performance of PTFE composites (k ~ 10−5 mm3/Nm) relies on load support by filler in the matrix. Recent studies found that tribochemical polarization of PTFE polymers triggered the formation of highly protective transfer film, thus resulting in an exceptionally low wear (k ~ 10−7 mm3/Nm) in certain composites. However, atomistic interaction was believed to play an important role in the known anti-wear mechanisms, which has yet to be fully described. Here, environmental and computational experiments allowed detailed mechanistic studies for representative PTFE composites, including metal-, ceramic-, carbon-, and polymer-filled composites. Experimental results found that the protective and polarized transfer film formed only in environmental water/oxygen, which could also reduce the composite wear by 10-fold or more. Density-functional-theory (DFT) calculations revealed that the electrophilic atom at solid surface tends to defluorinate PTFE molecule, which enables the tribochemical products of polarized PTFE accumulated near the sliding surfaces. Molecular dynamics simulations suggested that the strengthening of nonbonding interactions resulted from polar polymers improved polymer composites' adhesion and cohesion strengths against steel counterface, which was responsible for the achievement of macro-scale ultralow wear in PTFE composites. The relation between the atomistic interactions and the macroscopic wear behavior of composites was systematically discussed.

Corrosive-wear behavior of LSP/MAO treated magnesium alloys in physiological environment with three pH values

A laser shock peening (LSP) layer, a micro-arc oxidation (MAO) coating, and an LSP/MAO composite coating were fabricated on the surface of AZ80 magnesium alloy by laser shock and micro-arc oxidation process. The ball-disc grinding method was used to perform wear test on the three treated specimens in simulated body fluids (SBF) with pH values of 4, 7.4 and 9. The morphology and element content of worn surface were investigated by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The results indicated that the wear rates of the three treated specimens in three pH environment in numerical order were pH 4 > pH 7.4 > pH 9, respectively. The wear rates of the three treated specimens in the same pH environment were arranged in the order of MAO > LSP > LSP/MAO, respectively. The main wear mechanisms of the LSP specimen in pH 4 environment were fatigue wear and corrosion wear, while it were corrosion wear and adhesive wear in pH 7.4 and pH 9 environments. Abrasive wear, fatigue wear and corrosion wear were the main wear mechanisms of the MAO specimen in pH 4 environment, while abrasive wear, adhesive wear and corrosion wear were the main wear mechanisms of that in pH 7.4 and pH 9 environments. The corrosion wear resistance of the LSP/MAO specimen in SBF solution with three pH values was improved due to the synergism of LSP fine crystal layer and MAO coating.

The Friction and Wear Behaviours of Inconel 718 Superalloys at Elevated Temperature

Machine parts made of nickel-based alloys usually work in high-temperature service environments such as aircraft turbines. The mechanical properties and antioxidant properties of materials tend to be reduced at high temperatures. Therefore, it is of great practical significance to reveal the wear mechanisms of materials at different temperatures. In the present investigation, the tribological behaviour of an Inconel 718 superalloy at different temperatures was investigated. First, the coefficient of friction curves obtained under different test conditions were analysed in detail to illustrate the dynamic change process of friction at high temperature. Next, the morphology of the wear surface, surface morphology of friction pairs and material transfer during friction were analysed via scanning electron microscopy 3D morphology and energy dispersive spectroscopy measurements to reveal the wear mechanisms of materials in a high-temperature environment. Finally, the microstructure of the cross section of the wear tracks was analysed by using optical microscopy electron back-scattered diffraction etc., to clarify the mechanisms of crack initiation and material removal. The results show that the friction properties of the Inconel 718 superalloy have differences at different test temperatures. Although increasing the test temperature does not necessarily aggravate the wear of the material, the oxidation of the wear surface during the friction process significantly increases. In addition, when the contact load increases, the thickness of the oxide layer and wear of the material simultaneously increase.

Effect of Cryogenic Treatment on Microstructure, Mechanical Properties and Distortion of Carburized Gear Steels

The effects of cryogenic treatment and low temperature tempering on the microstructure, mechanical properties and distortion of the 20Cr2Ni4A and 17Cr2Ni2MoVNb carburized gear steels were investigated. The results showed that the case hardness of the experimental steels was increased after the cryogenic treatment, due to the decrease of the retained austenite content and the precipitation of the tiny carbides. The wear resistance of the two steels after cryogenic treatment was improved, although the wear mechanisms were different for 17Cr2Ni2MoVNb and 20Cr2Ni4A steels. The distortion of the Navy C-ring specimens underwent shrinkage before expansion during the cryogenic process, and the distortion of 17Cr2Ni2MoVNb steel was smaller than that of 20Cr2Ni4A steel.