Tribological properties of rock-bit journal bearings tribo-pair with micro-dimples under contaminated grease lubrication

The failure of bit bearings is one of the main factors that restrict the life of a bit, and studies revealed that sealing and adhesive wear failure are common failure modes of bit bearings. To study the adaptability of the optimal textures to anti-wear and anti-friction performance of an 8 1/2” rock bit journal bearingunder sealing leakage conditions, the rheological properties of grease in the range of 0%–50% of the water-based drilling fluid volume in the compound lithium-based grease (rock bit grease) were tested. For the cylindrical dimples (diameter: 300 μm; depth: 40 μm, area ratio: 5%) and elliptical dimples (major and minor axis of 720 and 360 μm, respectively; depth: 40 μm, area ratio: 10%), the experimental study on the wear resistance of the optimized texture tribo-pairs was carried out as the grease invaded different drilling fluid contents based on the pin-disk pair. Results showed that the drilling fluid volume in the rock bit grease significantly affected the rheological properties of the grease. Furthermore, the cylindrical and elliptical dimples still had good anti-wear and anti-friction effects. Especially when the drilling fluid volume ratios in the grease reached 50%, the elliptical dimples can still reduce the friction coefficient and wear amount of the pair by 19.88% and 56.99%, respectively. With the increase of drilling fluid invasion into grease, the wear morphology of the un-textured tribo-pairs showed that the wear form changed from abrasive to adhesive wear, while that of the preferred textured tribo-pairs indicated slight abrasive wear.

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.

Effect of High−Speed Powder Feeding on Microstructure and Tribological Properties of Fe−Based Coatings by Laser Cladding

In order to improve the wear resistance of 27SiMn steel substrate, Fe−based alloy coatings were prepared by laser cladding technology in the present study. In comparison to the conventional gravity powder feeding (GF) process, high−speed powder feeding (HF) process was used to prepare Fe−based alloy coating on 27SiMn steel substrate. The effect of diversified energy composition of powder materials on the microstructure and properties of coatings were systematically studied. X−ray diffractometer (XRD), optical microscope (OM) and scanning electron microscope (SEM) were used to analyze the phase structure and microstructure of Fe−based alloy coatings, and the hardness and tribological properties were measured by the microhardness tester and ball on disc wear tester, respectively. The results show that the microstructure of conventional gravity feeding (GF) coatings was composed of coarse columnar crystals. In comparison, owing to the diversification of energy composition, the microstructure of the high−speed powder feeding (HF) coatings consists of uniform and small grains. The total energy of the HF process was 75.5% of that of the GF process, proving that high−efficiency cladding can be achieved at lower laser energy. The refinement of the microstructure is beneficial to improve the hardness and wear resistance of the coating, and the hardness of the HF coating increased by 9.4% and the wear loss decreased to 80.5%, compared with the GF coating. The wear surface of the HF coating suffered less damage, and the wear mechanism was slightly adhesive wear. In contrast, wear was more serious in the GF coating, and the wear mechanism was transformed into severe adhesive wear.

A Multiscale Accuracy Degradation Prediction Method of Planetary Roller Screw Mechanism Based on Fractal Theory Considering Thread Surface Roughness

The wear problems are vital to the planetary roller screw mechanism (PRSM) as they have a great influence on transmission accuracy, working efficiency, and service life. However, the wear characteristics of the PRSM have been rarely investigated in the past. In this paper, a multiscale adhesive wear model is established by incorporating the effective wear coefficient and considering the thread surface roughness. The variation of surface roughness is characterized by the two-dimension Majumdar–Bhushan (MB) function. The multi-asperity contact regimes are used to estimate microcontact mechanics of the rough interface. Moreover, the influences of surface roughness, material properties, and working conditions on the wear depth and precision loss of the PRSM are studied in detail. The results reveal that as the surface roughness increases, the total actual contact area, wear depth, and precision loss rate rise. In addition, the adhesive wear increases with the growth of the axial load, and decreases with the increase in the material hardness and material elastic modulus ratio to a certain extent. The investigation opens up a theoretical methodology to predict the wear volume and precision loss with regard to thread surface roughness, which lays the foundation for the design, manufacturing, and application of the PRSM.

Investigations on the Influence of Annealing on Microstructure and Mechanical Properties of Electrodeposited Ni-Mo and Ni-Mo-W Alloy Coatings

Ni-Mo and Ni-Mo-W coatings were electrodeposited on a stainless steel sheet, and then were annealed at 200, 400, and 600 °C. The effect of annealing heat treatment on the microstructure of Ni-Mo and Ni-Mo-W electrodepositions, their nano-hardness, and tribological properties were investigated. It was revealed that the average crystalline are refined and phase separation are promoted with formation of Mo-W related intermetallic precipitates at temperature exceed 400 °C on account of the co-existence of Mo-W elements within Ni-Mo-W coatings. Annealing heat treatment leads to hardening, and the hardness and elastic module increase significantly. The grain boundary (GB) relaxation and hard precipitated intermetallic particles are responsible for the annealing-induced hardening for ≤400 °C annealed and 600 °C annealed Ni-Mo-W coatings, respectively. In addition, both adhesive wear and abrasive wear are observed for coatings, and abrasive wear becomes predominant when annealing temperature up to 600 °C. The wear resistance of coatings is improved eventually by formation of a mixture of lubricated oxides upon annealing at 600 °C and the enhancement of H/E ratio for ≤400 °C annealed Ni-Mo-W coatings.

An Adhesive Wear Model Based on a Complete Contact Model for a Fractal Surface

An adhesive wear model based on a complete contact model for a fractal surface is presented in this work. A contact model which contains effect of adhesion is firstly presented based on ME model. A complete contact model is then proposed. Finally, an adhesive wear model based on this model is given. The results suggest that the maximum contact area increases firstly and then decreases as fractal dimension increases. The percentage of plastic contact area increases with increase of the fractal dimension. And the experimental results for wear volume have shown a good consistency with the results calculated by the wear model.