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Y.W. Gu
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Proceedings Papers
ITSC 2004, Thermal Spray 2004: Proceedings from the International Thermal Spray Conference, 843-845, May 10–12, 2004,
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A novel thermal plasma process, based on inductively coupled plasma torch is employed for producing nano-sized calcium phosphate powders from spray-dried hydroxyapatite (HA) feedstock. The phases during plasma process of HA feedstock under different working conditions have been studied. It is revealed that amorphous calcium phosphate is predominant in the nano-sized powders. HA, α-TCP and CaO are also detected in the nano-sized powders. After heat treatment at 800 °C in air, β-Ca 2 P 2 O 7 (β-DCP) and HA are found to dominate in the powders. The presence of β-DCP is attributed to the HA feedstock directly decomposed into DCP in the plasma flame, and this phase formed amorphous calcium phosphates region by the rapid quenching process. This region crystallized into β-DCP after heat treatment.
Proceedings Papers
ITSC2000, Thermal Spray 2000: Proceedings from the International Thermal Spray Conference, 1241-1248, May 8–11, 2000,
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Plasma sprayed thermal barrier coatings often face the problems of spallation and cracking in service owing to their poor bond strength and high residual stresses. Functionally graded thermal barrier coatings with a gradual compositional variation from heat resistant ceramics to fracture resistant metals are proposed to mitigate these problems. In this paper, functionally graded Y2O3 stabilized ZrO2 (YSZ) / NiCoCrAlY composite coatings were prepared using pre-alloyed and spheroidized composite powders. The mechanical and thermal properties of graded YSZ/NiCoCrAlY composite coatings, such as elastic modulus, bond strength, coefficient of thermal expansion, thermal cycling and oxidation resistance were investigated. Results showed that the bond strength and thermal cycling resistance of FGM coatings were much better than that of the duplex coatings. The coefficient of thermal expansion and elastic modulus changed gradually through the 5-layer functionally graded coating.
Proceedings Papers
ITSC2000, Thermal Spray 2000: Proceedings from the International Thermal Spray Conference, 325-331, May 8–11, 2000,
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In recent year, there has been an increasing trend to design the turbine coating together with the substrate. In this harsh environment, a failure in one quickly leads to a failure in the other. Materials that are used in structural applications are prone to mechanical vibration which when not attenuated, will lead to fatigue of components and shortening of life cycle. Therefore, it is not uncommon to find that it is important to understand the thermal stability and dynamic mechanical properties of coatings as well, which will be used in the dynamic conditions. In addition to these essentially noise reduction and vibration amplitude control motivated objectives, however, mechanical energy dissipation processes also find intrinsic applications in cases where a thorough understanding of the mechanisms responsible for the damping response of the material are required. This article describes the damping behavior and mechanisms that exist in plasma sprayed NiCoCrAlY coatings.
Proceedings Papers
ITSC2000, Thermal Spray 2000: Proceedings from the International Thermal Spray Conference, 625-633, May 8–11, 2000,
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This paper compares two types of hydroxyapatite (HA) composite coatings, HA/Ti-6Al-4V and HA/Y-ZrO2. The powders used in the study were prepared using a slurry process then deposited by plasma spraying. The resulting coatings were characterized based on their microstructure, mechanical properties, and biocompatibility. Both composite coatings performed better than pure HA coatings in tensile adhesion and indentation tests. Testing also revealed that the HA/Y-ZrO2 coatings had favorable strength and fracture toughness and that the HA/Ti-6Al-4V coatings had good affinity to living tissue and sufficient mechanical strength.
Proceedings Papers
ITSC1998, Thermal Spray 1998: Proceedings from the International Thermal Spray Conference, 1543-1548, May 25–29, 1998,
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Pre-alloyed and plasma spheroidized composite powders were used as the feed-stock in the plasma spraying of functionally graded ZrO 2 /NiCoCrAlY coatings. The ball milling parameters of the composite powders and the plasma spraying parameters for preparing FGM coatings were optimized in order to obtain the best performance for the thermal barrier coatings. Microstructure, physical, mechanical and thermal properties of ZrO 2 /NiCoCrAlY FGM coatings were investigated and compared with those of traditional duplex coatings. Results showed that the advantages of using prealloyed composite powders in plasma spraying were to ensure chemical homogeneity and promote uniform density along the graded layers. Microstructure observation showed the gradient distribution of ZrO 2 and NiCoCrAlY phases in the coating and no clear interface was found between two adjacent different layers. Oxidation of elements in Ni alloy occurred during plasma spray and the so-formed Al 2 O 3 and Cr 2 O 3 combined with ZrO 2 in a wide range of proportions. The bond strength of FGM coatings was about twice than that of the duplex coatings. Results also indicate that the thermal cycling resistance of FGM coating was much better than that of the duplex coating.
Proceedings Papers
ITSC1997, Thermal Spray 1997: Proceedings from the United Thermal Spray Conference, 259-266, September 15–18, 1997,
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Pre-alloyed, plasma spheroidized powders were used as feedstock in the plasma spraying of functionally graded ZrO 2 /NiCrAlY coatings. The advantage of using pre-alloyed powders was to ensure chemical homogeneity and promote uniform density along the graded layers, and these pre-alloyed powders could be successfully used to prepare the different inter-layers of functionally graded coatings. The microstructure, density and microhardness changed gradiently in the ZrO 2 /NiCrAlY coatings. The bond strength of ZrO 2 /NiCrAlY coatings with different graded layers was measured. Results showed that for as-sprayed coatings with the same thickness, the bond strength increased with the number of graded layers. The bond strength of the coatings with five graded layers was about twice as high as that of the duplex coatings because of the significant reduction of the residual stress in the coatings. Experimental results also showed that the bond strength of as-sprayed coating increased significantly after hot isostatic press (HIP) and vacuum heat treatments, and the reason can be attributed to the densification of the microstructure, the decrease of defects in the coatings, inter-diffusion between layers and further reduction in the residual thermal stress.