Junhu Meng

Tribological Behavior of MoSi2 and Its Composites in Sliding Against Ni-Based Alloys

In this paper, MoSi2 and its composites were fabricated by hot pressing method. The roles of the second phase as mechanical and tribological components were studied. The results showed that B4C and ZrO2 particulates could be used for the strengthening and toughening of MoSi2 respectively. The tribological behaviors of MoSi2 and its composites sliding against Ni–Cr alloy and Ni–Cr–S alloy at room temperature and 600 °C were investigated on a tribometer with a pin-on-disk configuration. MoSi2–SiC exhibited the best tribological performance. The wear behaviors of MoSi2 and its composites were well correlated with their toughening mechanisms in sliding against Ni–Cr alloy at room temperature. Due to the introduction of chromium sulfides, the formation and removal process of the transferred layers at 600 °C controlled the wear mechanism of MoSi2 and its composites. The tribo-oxidation was an important factor favorable to wear reduction.

Soft lithography of ceramic microparts using wettability-tunable poly(dimethylsiloxane) (PDMS) molds

Green alumina microparts were fabricated from a high solid content aqueous suspension by microtransfer molding using air plasma-treated poly(dimethylsiloxane) (PDMS) molds. The wettability of the air plasma-treated PDMS molds spontaneously changed between the hydrophilic and hydrophobic states during the process. Initial hydrophilicity of the air plasma-treated PDMS molds significantly improved the flowability of the concentrated suspension. Subsequent hydrophobic recovery of the air plasma-treated PDMS molds enabled a perfect demolding of the green microparts. Consequently, defect-free microchannel parts of 60 μm and a micromixer with an area of several square centimeters were successfully fabricated. In soft lithography, tuning the wetting behavior of PDMS molds has a great effect on the quality of ceramic microparts. Using wettability-tunable PDMS molds has great potential in producing complex-shaped and large-area ceramic microparts and micropatterns.

High Temperature Wear of Ti3AlC2 Sliding against Al2O3

Bulk Ti3AlC2 was prepared via hot pressing TiC, Ti and Al powders in vacuum. The sliding wear of Ti3AlC2 against Al2O3 at room temperature up to 800°C was investigated on a SRV-IV oscillating friction and wear tester. Reduced wear and wear transition of Ti3AlC2/Al2O3 tribo-couple were found by increasing temperature. The worn surfaces of both Ti3AlC2 and Al2O3 were analyzed by scanning electron microscope and X-ray photoelectron spectrometer. It was found that the severe wear of Ti3AlC2 and Al2O3 from room temperature to 200°C was related to mechanical wear, i.e. grain fracture and pullout of Ti3AlC2. At temperature higher than 400°C, the tribo-oxidation layer on the worn surface of Ti3AlC2 containing TiO2 and Al2O3 was beneficial for reducing wear of both Ti3AlC2 and Al2O3.

Structural and crystal-chemical characteristics of the apatite deposits from human aortic walls

Thermal behavior of biological apatite is the object of several studies. Crystal size, carbonate content, phase composition, and other parameters change during annealing up to 900 °C in biological minerals with apatite structure. The way these parameters change reflects the specific properties of the initial bioapatite. This work presents data on thermal transformations of pathological bioapatite from the human cardiovascular system, namely aortic wall deposits. Some minor elements, foreign to calcium hydroxyapatite (e.g., Na and Mg), can be both incorporated in the apatite structure and localized in the surface layers of crystals, modifying functions of the mineral. A new approach was proposed to determine the predominant location of minor elements, such as Mg, Na, and K, in the mineral of pathological deposits. Mg and Na in pathological apatite can be in both structurally bound (substituting calcium in lattice) and labile (localized on the crystal surface) states, while K is not able to join the apatite structure in significant amount or be chemically bound to it. This approach, based on atomic spectrometry, can be used effectively in combination with a set of traditional techniques, such as like EDS, IRS, and XRD.

Variation of Surface Roughness of Ti3SiC2 Disk during Polishing in Water and Alcohols

The surface polishing of Ti3SiC2 disk in fluids (water, ethanol, propanol, glycol, and glycerol) is conducted on a Buehler grinder/polisher and evaluated using surface roughness. Using Buehler automatic grinder/polisher, the Ti3SiC2 disks are grinded and polished in the as-mentioned lubricants by grinding disk of diamond with sizes of 45 μm to 3 μm. The surface roughnesses of Ti3SiC2 disks at each stage are measured by 3D surface profiler. The results show that the lowest surface roughness (Ra) of Ti3SiC2 disk obtained by mechanical polishing is 0.04 μm. The optimum polishing process of Ti3SiC2 disk is as follows: using water as lubricant, at a load of 5 N, for steps 1 to 4, the Ti3SiC2 and grinding disk rotates comparatively and the sizes of diamond particles on the abrasive disk are 45, 15, 9, and 3 μm, respectively. For step 5, the abrasive disk is woven cloth with no diamond particles. The duration of each step is 5 min. Using the same polishing process, the surface roughness of Ti3SiC2 disk by direct hot pressing is lower than that by in situ reactive hot pressing. Using the same polishing process but different lubricants, the surface roughness of the Ti3SiC2 disks increases in the order of water, ethanol, propanol, glycol, and glycerol. In water, the surface roughness of Ti3SiC2 disk decreases with the increasing quantity of water and polishing duration.

Friction and Wear of Ti3SiC2-Ag/Inconel 718 Tribo-Pair under a Hemisphere-on-Disk Contact

Room temperature friction and wear of Ti3SiC2-Ag sliding against Inconel 718 with a hemisphere-on-disc configuration were investigated in air. The effects of Ag content and TiAlN coating on Inconel 718 substrate were also included. Ti3SiC2/Inconel 718 tribo-pair showed high friction coefficient (0.6) and severe wear due to pullout of Ti3SiC2 grains was observed at a sliding speed of 1 m/s. Ti3SiC2-Ag composites had better tribological behavior than that of monolithic Ti3SiC2 in sliding against Inconel 718. At a sliding speed of 0.01 m/s, Ti3SiC2-Ag/Inconel 718 tribo-pairs exhibited moderate friction coefficient (0.32-0.4). At a sliding speed of 1 m/s, severe wear was not observed for Ti3SiC2-15vol.%Ag and Ti3SiC2-20vol.%Ag composites although the tribo-layer was not rich in Ag. When Ti3SiC2-Ag composites mated with TiAlN coating on Inconel 718 substrate, moderate friction coefficient (0.29-0.36) and low wear rate (10-6 mm3N-1m-1) were obtained at 0.01 m/s. A transition from mild wear to severe wear of Ti3SiC2-Ag composites at 0.1 m/s can be attributed to the ploughing effect by hard asperities on TiAlN coating.

Evolution of Indentation Induced Cracks on WC Ceramic under Cavitation Loading in Water

For engineering ceramics, cracks produced in preparation process, i.e. sintering and surface finishing, have a profound impact on the mechanical strength and wear. Under cavitation loading, the flaws on the surface and subsurface are preferentially eroded. In this paper, artificial cracks are introduced on WC ceramic by indentation of a Vickers indenter at a load of 10 kgf. Five positions on the cavitation eroded area are selected. The average length of the cracks is 71.6 mm. The cavitation erosion tests are terminated at 1, 3, and 5 min to reveal the evolution of cracks. A single pattern for the evolution of cracks is recognized. Two new cracks at the tip of indentation cracks first nucleate and then propagate (accompanied by pullout of grains). The angle between the two new cracks ranges from 30° to 45°. The evolution of adjunct two groups of cracks gives birth to local breakout finally. Steps which are provided limited resistance to breakout are observed. The nucleation and propagation of the four indentation cracks do not always occur simultaneously.

Friction and Wear of Al2O3-Based Composites with Dispersed and Agglomerated Nanoparticles

In recent years, Al2O3-based nanocomposite which is composed of micro-size Al2O3 matrix and a second phase nanoparticles (e.g. SiC, Ni) as the reinforcement phase have received considerable attention and come a long way mainly because of their good mechanical property and resistance to abrasive wear and erosive wear. Up to now, practice on tribologcial design and testing of high temperature self-lubricating Al2O3-based nanocomposites is an exciting field to be explored. In this chapter, the principle for fabrication of Al2O3-based nanocomposite for better mechanical property than that of monolithic Al2O3 ceramic is briefly introduced on basis of literature review. In other words, the microstructure, and mechanical property of Al2O3-SiC, Al2O3-Ni nanocomposites are briefly introduced and discussed. This chapter focuses on the design and tribological property of high temperature self-lubricating Al2O3-based nanocomposites. As the first step, the tribological consideration of Al2O3-SiC nanocomposite is discussed according to Todd’s work which gives useful information on the pullouts of grains during grinding and polishing. In addition, a concept for designing high temperature self-lubricating Al2O3-based nanocomposite with dispersed and agglomerated ceramic nanoparticles is proposed and discussed using Al2O3-TiCN composite as an example. Al2O3-5 wt % TiCN and Al2O3-10 wt % TiCN composites had high hardness and good tribological property at room temperature in air. Al2O3-TiCN composites containing 10 and 20 wt % TiCN nanoparticles in sliding against Ni-Cr alloy are self-lubricating at 500 °C.

Formation and Evolution of Tribo-Layer on Al2O3 and Al2O3-Based Ceramics in Sliding against Si3N4 under Dry Sliding

Alumina (Al2O3) and Al2O3-based ceramics are candidate wear-resistant materials for many applications. In the past decades, the friction and wear behaviors of Al2O3 and Al2O3-based ceramics in dry sliding and lubricated conditions have been extensively investigated. In dry sliding, surface modification of Al2O3 and Al2O3-based ceramics at tribo-interface can be observed. In other words, the original surfaces of Al2O3 and Al2O3-based ceramics can be modified by friction and friction-induced material interaction (fracture, mechanical mixing, tribo-sintering) at the tribo-interface, and hence give birth to a tribo-layer. The formation and evolution of the tribo-layer on the worn surfaces of Al2O3 and Al2O3-based ceramics are the keys to understand the friction and wear behaviors of Al2O3 and Al2O3-based ceramics. This is conducted by scanning electron microscopy (SEM) observation and classification of the morphology and composition of the worn surfaces of Al2O3 and Al2O3-based ceramics at different sliding stages. The typical characteristics of the tribo-layers on the worn surfaces of Al2O3 and Al2O3-based ceramics in sliding against Si3N4 in a reciprocating motion are smooth surface with many interlinked cracks in morphology, mechanical mixing in chemical composition. The condition allowing the formation of the tribo-layer is briefly discussed.

Tribological Performance of Translucent Dy-α-Sialon Ceramics

A considerable test was made to figure out the effects of temperature and sliding conditions on the wear properties of the translucent Dy--Sialon. The friction coefficient was 0.54 at RT, 0.26 at 100 oC, and 0.81 at 600 oC respectively under an applied load of 5N. The wear rate was 6.91×10-15 at RT and 1.0×10-15 at 100 oC for the same Dy--Sialon sample. Obviously Dy-Sialon shows an excellent wear resistance under a suitable sliding condition, a load of 5N and at 100 oC. This appears attractive and important for Dy-Sialon ceramics to be used as a type of special wear resistant materials, with an optical translucence.