To manufacture a protective coating with low thermal conductivity and good frictional wear performance, a Fe 59 Cr 12 Nb 5 B 20 Si 4 coating was designed and produced by high velocity oxygen fuel (HVOF) spraying; the properties and performance of this coating where then compared with those of a commercially available AISI 316L stainless steel coating. In the as-deposited state, both coatings exhibit dense layered structures with porosity below 1% and slight oxidation. The microstructure of the Fe-based coating has an amorphous matrix and some precipitated nanocrystals. The result is that the designed Fe-based coating has a thermal conductivity (2.66 W/m·K) that is significantly lower than that of the 316L stainless steel coating (5.87 W/m·K). Based on its advantageous structure, the Fe-based coating exhibits higher microhardness, reaching 1258±92 HV. The friction coefficient and wear rate of the Fe-based coating show an increase at 200°C followed by a decrease at 400°C, due to the evolution of the wear mechanism at different temperatures. The dominant wear mechanism of the Fe-based coating at room temperature is fatigue wear accompanied by oxidative wear. At 200°C, due to the existence of “third body” abrasive wear, the wear process was accelerated. The large-area oxide layer is likely responsible for the decrease of friction of the coating at 400°C.

This paper considers the deposition of a commercial steel powder with a chemical composition that allows the coating to obtain an amorphous structure using thermal spray techniques. The processes used are characterized by high cooling speeds of the particles after the impact upon the substrate. The powders were sprayed with two different processes: cold gas spray (CGS) and high velocity oxyfuel (HVOF). A comparison between the samples obtained reveals that only the CGS coatings are completely amorphous; the HVOF samples exhibit nanocrystalline phases, detected with XRD analysis and SEM micrographs. Furthermore, the CGS coatings are more compact and show lower hardness with a comparable Young’s modulus. A hypothesis is that the formation of the amorphous structure is related to plastic deformation at impact (due to the high energy of the particles), rather than to the temperature; the mechanism could resemble that of a severe plastic deformation process. Additional thermal treatments and mechanical tests are in progress to investigate the toughness and other mechanical properties of the coatings.

In this study, the spray-dried Al 2 O 3 /Y 2 O 3 composite powder was prepared using commercially available nanosized Al 2 O 3 and Y 2 O 3 feedstock. The mass ratio of Al 2 O 3 to Y 2 O 3 was 2:1. Atmospheric plasma spraying possesses high thermal enthalpy, large temperature gradient and rapid cooling rate. The microstructure and wear resistant performance of plasma-sprayed Al 2 O 3 .Y 3 Al 5 O 12 (YAG) coating were investigated. As-sprayed Al 2 O 3 .YAG coating was chiefly composed of amorphous phase, which may reveal superplastic feature in supercooled liquid region (from 503.0°C to 906.5°C). The as-sprayed Al 2 O 3 .YAG coating exhibits fine plasticity and toughness. Friction and wear tests of the coatings were executed on a MMU-5GL tribological tester using a ring-on-disk arrangement. The coatings were deposited on end flat surfaces of the wear rings. The graphite disks were prepared. The wear tests were conducted at following conditions: relatively high load of 2000N; a rotational speed of 500rpm (equivalent to a sliding velocity of 0.68m/s). Friction coefficients could be obtained real-timely. The thermocouple was applied to measure the worn surface temperatures. The Al 2 O 3 /YAG amorphous coating/graphite pair possessed lower friction coefficient and worn surface temperature, compared with Al 2 O 3 coating/graphite and Al 2 O 3 -Cr 2 O 3 coating/graphite pairs. After wear tests, many network cracks visible to naked eye appeared the surfaces of Al 2 O 3 and Al 2 O 3 -Cr 2 O 3 coatings. However, no cracks were observed on the worn surface of the Al 2 O 3 /YAG amorphous coating. Therefore, plasma-sprayed Al 2 O 3 .YAG coating possesses excellent wear resistance under severe conditions with high pv values (p: contact pressure; v: friction velocity).

The lower wear and poor impact resistance of the amorphous coatings has been a great problem in the past few years for their use in industrial applications. Several research methods have been reported recently to overcome this issue. The present paper addresses the main work done recently on iron based amorphous composite coatings by the addition of 0-20% Alumina particles. These particles were homogeneously distributed in the amorphous matrix of the coatings which improved wear and impact resistance as compared to the monolithic coatings without any decrease in corrosion resistance. The hard alumina particles enhanced wear resistance to several times not only in air but also in salt water solution with a decrease in friction coefficient. The combined effect of wear and corrosion were also observed to become better by the alumina addition. Furthermore, the impact resistance was also improved three times by the addition of alumina particles. The hard second phase particles present in the amorphous coating matrix disperses the residual stresses generated during the impact loading. The brittle alumina particles absorb the impact energy by breaking itself which stop the initiation of cracks and also play a vital role in the crack arresting and blocking of the crack propagation.

In this study, the in-situ corrosion behavior of an Fe-based amorphous coating is investigated in a simulated deep sea environment (80 atm). FeMoCrYCB powder produced by gas atomization was deposited on 316L stainless steel substrates by HVOF spraying. The amorphous iron coatings exhibited greater pitting resistance than stainless steel under high hydrostatic pressures, evidenced by higher pitting potential, longer pitting incubation time, and reduced pitting growth. Passive films that formed on the amorphous coatings were also analyzed and found to be thicker, more uniform, and harder than those that developed on 316L stainless steel, indicating that the former are more difficult to break down and more resistant to Cl- ion penetration.

Amorphous coatings, despite their high strength and hardness and outstanding corrosion and wear properties, have been limited in application due to poor bonding strength and low impact resistance. This paper reviews the progress that has been made in that regard through the addition of ductile metals, ceramic particles, and polymer phases and through laminar structure design consisting of alternating amorphous and NiCrAl layers. Test results show that the composite amorphous coatings realized by the various methods exhibit significantly improved bonding strength and impact resistance along with their other superior properties.

This study assesses the influence of alumina particle additions on the impact and wear behavior of iron based amorphous coatings. Test results show that the presence of Al 2 O 3 particles improved the impact and wear resistance of the coatings by a factor of three. Deformation and fracture mechanisms under impact loading were also investigated. It was revealed via SEM analysis and finite element simulations that hard second phase particles in the amorphous coating matrix disperse residual stresses generated during impact loading and that brittle particles absorb impact energy by fracturing, which plays a vital role in crack prevention and arresting. Abstract only; no full-text paper available.