Microstructure, Microhardness, Fracture Toughness, and Abrasive Wear of In-Situ Synthesized TiC/Ti-Al Composite Coatings by Cold Spraying Combined with Heat Treatment

TiAl intermetallic compounds, as a new kind of high-performance light-weight structural material, are widely applied in many fields. Titanium carbide (TiC) as the reinforcing phase could improve the mechanical properties, wear resistance, and heat-resistance stability of TiAl intermetallic compounds. Ti(Al, C) mixture powders were deposited by cold spraying at gas temperature of 250 °C, 450 °C, and 550 °C. Then, Ti(Al, C) coatings were annealed at temperatures of 650 °C for different times and following holding at 1100 °C for 3 h. The microstructure, microhardness, fracture toughness, and abrasive wear of Ti-Al composite coatings were investigated. The research results were that the particle size of mixture powders decreased as the ball milling time prolonging. Ti(Al) solid solution appeared in the mixture powders as the milling time increased to 30 h. The average porosity of the coating sprayed at 550 °C was the lowest (0.85%). The as-sprayed coatings exhibited the same phase compositions with the mixture powders. The coating sprayed at gas temperature of 550 °C has the highest microhardness and the lowest weight loss. Ti-Al intermetallic was in-situ synthesized after annealing at 650 °C. The average porosity of the annealed coating (sprayed at 450 °C) was the lowest. The content of Ti-Al intermetallic compounds of the annealed coating sprayed at 450 °C is the highest. The X-ray diffraction (XRD) analysis results are consistent with the EDS analysis of the annealed coatings after annealing at 650 °C. Ti-Al intermetallic compounds were almost completely formed in the three kinds of the coatings after annealing at 650 °C for 20 h and following holding at 1100 °C for 3 h. TiAl and TiAl3 intermetallic phases were in-situ synthesized in the coatings based on the energy dispersive spectroscopy (EDS) and XRD analysis. TiC was also in situ synthesized in the coatings as the annealing temperature increased to 1100 °C. The annealed coating (sprayed at 450 °C) has the highest microhardness, fracture toughness, and wear resistance properties after annealing at 1100 °C for 3 h.

Effect of Vacuum Annealing on Microstructure and Hot-Salt Corrosion Behavior of CoNiCrAlY/YSZ/LaMgAl11O19 Double-Ceramic Coating

This study investigated the potential effect of vacuum annealing on the microstructure and hot salt corrosion behavior of CoNiCrAlY/YSZ/LaMgAl11O19 (LMA) double-ceramic coatings. A hot-salt corrosion test revealed that sprayed coatings exhibited an unsatisfactory anti-corrosion performance, and the LMA layer underwent severe fracture and corrosion degradation. Vacuum annealing induced a prominent recrystallization of the amorphous phase in LMA layer, triggering severe volume shrinkage and microcrack initiation. The recrystallization and volume shrinkage of the LMA layer were aggravated by an increase in the annealing temperature. The annealed coating with a higher fraction of the LaMA phase showed superior resistance to hot-salt corrosion. However, the salt mixture diffused simultaneously along the microcracks and eventually eroded into the YSZ layer. These results confirmed that vacuum annealing significantly enhanced the hot-salt corrosion resistance of the LMA layer. However, it deteriorated the barrier effect of the salt mixture through microcrack formation.

A Novel Oxide Layer Formed on the 800 °C-Annealed CoMoCrSi Coating Significantly Reduced Friction and Wear at Room Temperature

In the present work, we investigated the microstructures and properties of as-sprayed and annealed CoMoCrSi coatings. Specifically, the annealed treatment at 800 °C resulted in good recrystallization, improved microstructure, and enhanced properties of CoMoCrSi coatings. An oxide layer formed on the annealed coating surfaces; it was mainly composed of nano-sized Cr2O3 and micro-sized CoMoO4, and could account for the increased surface microhardness and enhanced anti-wear performance of annealed coatings. In particular, the very hard Cr2O3 played a critical role of resisting press-in and wear during the tests, and the CoMoO4 had a lubricating effect during the friction process. Finally, the annealed coatings exhibited low coefficients of friction (COFs) of 0.4 and wear rates of 0.7–0.8 × 10−6 mm3·N−1·m−1 after a long sliding distance of 1000 m at RT. Consequently, the wear mechanism transferred from brittle fracture coupled with abrasive wear for the as-sprayed coating to slight abrasive wear for annealed coatings.

Room-Temperature and High-Temperature Wear Behaviors of As-Sprayed and Annealed Cr3C2-25NiCr Coatings Prepared by High Velocity Air-Fuel Spraying

Cr3C2-25NiCr coatings were deposited on stainless steel substrates by high velocity air-fuel (HVAF) spraying. Friction and wear behaviors of as-sprayed and annealed coatings were investigated both at room-temperature (RT) and 600 °C (high-temperature, HT). The results show that annealing at 600 °C in air is effective to enhance the wear performances of the coating both at RT and HT. The enhanced wear resistance of annealed coatings is discussed from the oxide growth and the microstructural evolution of the coatings. The wear behavior of the annealed coating follows the abrasive mechanism at RT and changes to an oxidative wear at HT, in which formation of a tribo-oxide layer plays a critical role to reduce the friction coefficient and to protect the underlying coatings from abrasive damage. The findings of this work reveal the significance of oxide-scale growth and microstructural evolution on the HT wear behaviors of the Cr3C2-25NiCr coating, which provides strategies for enhancing the wear properties of such coatings for HT applications.

Thermal Stability of CrWN Glass Molding Coatings after Vacuum Annealing

CrWN glass molding coatings were deposited by plasma enhanced magnetron sputtering (PEMS). The microstructure and thermal stability of these coatings were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscope, transmission electron microscope, atomic force microscope and nanoindentation tests. The as-deposited coating exhibited an aggravated lattice expansion resulting in a constant hardness enhancement. The vacuum annealing induced surface coarsening and the spinodal decomposition of the coating accompanied by the formation of nm-sized c-CrN, c-W2N, and h-WN domains. The annealed coating with low W content had mainly a face-centered cubic (f.c.c) matrix, strain fields caused by lattice mismatch caused hardness enhancement. Following an increase in W content, the annealed coating showed a mixed face-centered cubic (f.c.c) and hexagonal close-packed (h.c.p) matrix. The large volume fraction of h-WN phases seriously weakened the coating strengthening effect and caused an obvious drop in hardness.

Effect of Sputtered AlY Coating on High-Temperature Oxidation Behavior of Stainless Steel

AlY coating on 1Cr18Ni9Ti stainless steel was prepared by magnetron sputtering method and some of them were pre-oxidized or vacuum diffusion annealed at 600°C, and the effect of the coating with different treatments on the oxidation behavior of the stainless steel was studied at 1,100°C in air. Results show that the order of the 24-h oxidation mass gain for the specimens is the stainless steel without coating > the stainless steel with coating but without any pre-treatment > the stainless steel with AlY coating after pre-oxidation treatment > the stainless steel with AlY coating after vacuum diffusion annealing. After oxidation, a thick and loose Fe2O3/Cr2O3 film is formed on the stainless steel without coating, while thinner Fe2O3/Cr2O3 film is formed on the stainless steel with AlY coating. Compared to the oxidation film formed on the steel with pre-oxidized coating, the one formed on the steel with vacuum diffusion annealed coating is thinner and denser. The rare earth Y and its oxides Y2O3 in the coating produce reactive element effect and improve the ductility/adhesion of the oxide film, which enhances the oxidation resistance of the stainless steel, especially in the vacuum diffusion annealed AlY coating.

Microstructure and Electrical Characteristics of Plasma Sprayed Thick Film Mn-Co-Ni Oxide Thermistor

ABSTRACTMn-Co-Ni-O spinel, owing to its large temperature dependant resistivity, is an attractive ternary system for negative temperature coefficient (NTC) thermistor applications, particularly in oxygen rich environments. This research explores the potential of plasma spray as a method for the fabrication of thick film Mn-Co-Ni-O thermistors. Nanostructured Mn-Co-Ni-O spinel powders were synthesized by gel combustion method and were air-plasma-sprayed on to alumina substrates. The as-sprayed deposit formed a rocksalt structure rather than a cubic spinel structure. Annealing in air at 600°C for 3 hours resulted in transformation to the spinel structure while higher temperature annealing resulted in phase separation of rocksalt phase at 1250°C and recombination into spinel monophase at 1350°C. Temperature dependent measurement of resistivity showed -4 to -1% temperature coefficient of resistivity (TCR) from room temperature to 250oC with high reproducibility, making the low temperature annealed coating suitable in thermistor applications.