Y.W. Gu

In vitro studies of plasma-sprayed hydroxyapatite/Ti-6Al-4V composite coatings in simulated body fluid (SBF)

The bioactivity of plasma-sprayed hydroxyapatite (HA)/Ti-6Al-4V composite coatings was studied by soaking the coatings in simulated body fluid (SBF) for up to 8 weeks. This investigation was aimed at elucidating the biological behaviour of plasma-sprayed HA/Ti-6Al-4V composite coatings by analyzing the changes in chemistry, and crystallinity of the composite coating in a body-analogous solution. Phase composition, microstructure and calcium ion concentration were analyzed before, and after immersion. The mechanical properties, such as tensile bond strength, microhardness and Youngs modulus were appropriately measured. Results demonstrated that the tensile bond strength of the composite coating was significantly higher than that of pure HA coatings even after soaking in the SBF solution over an 8-weeks period. Dissolution of Ca-P phases in SBF was evident after 24h of soaking, and, a layer of carbonate-apatite covered the coating surface after 2 weeks of immersion. The mechanical properties were found to diminish with soaking duration. However, slight variation in mechanical properties was found after supersaturation of the calcium ions was attained with the precipitation of the calcium phosphate layers.

Spark plasma sintering of hydroxyapatite powders

Dense hydroxyapatite (HA) compacts have been successfully fabricated by a spark plasma sintering (SPS). The sintering behavior of HA powders at different temperatures ranging from 850 degrees C to 1100 degrees C was studied. Results showed that spark plasma sintering resulted in rapid densification to near theoretical density. The HA compact was homogeneously sintered at 950 degrees C in a short sintering duration of 5 min, while maintaining high quality and high relative density (>99.5%). The density, microhardness and Youngs modulus of HA sintered compact initially increased with the sintering temperature, reached a maximum value at around 950-1000 degrees C, then decreased with further increase in the temperature due to the decomposition of HA into beta-tricalcium phosphates. Fracture toughness results showed no significant difference with increasing temperature due to the combined influences of density and grain size. Microstructure analysis showed no noticeable grain growth under different sintering temperatures due to the short time exposure at high temperatures.

Effects of residual stress on the performance of plasma sprayed functionally graded ZrO2/NiCoCrAlY coatings

Abstract Functionally graded ZrO2/NiCoCrAlY coatings were produced by plasma spraying using pre-mixed and spheroidized powders as the feedstock. The microstructure, density, elastic modulus, thermal conductivity/diffusivity, microhardness and coefficient of thermal expansion were found to change gradually through the five-layer functionally graded coatings which was beneficial for the improvement of mechanical and thermal properties of the coatings. The residual stresses of the as-sprayed coatings with different graded layers and different thicknesses, as well as the changes of residual stresses during thermal cycling were simulated by finite element analysis (FEA). Results showed that residual stress was the lowest for the five-layer functionally graded coating compared to that of the duplex coating and three-layer coating with the same thickness, and the residual stresses increased with a decrease in coating thickness. For the coatings with the same thickness, the bond strength and thermal cycling resistance were found to increase with an increase in the number of graded layers which is due to the decrease in the residual thermal stresses. The bond strength of the five-layer functionally graded coating was about twice as high as that of the duplex coating and the number of thermal cycles of functionally graded coating was five times higher than that of the duplex coating. Results also showed that the bond strength decreased with an increase in the coating thickness.

Thermal properties of plasma-sprayed functionally graded thermal barrier coatings

Abstract Plasma-sprayed thermal barrier coatings often have 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 yttria stabilized ZrO2/NiCoCrAlY coatings were prepared using pre-alloyed and spheroidized composite powders as the feedstock and the thermal properties of the coatings were studied. Results show that the coefficient of thermal expansion and thermal conductivity/diffusivity changed gradually through the five-layer functionally graded coating. The thermal diffusivity and conductivity were found to increase with an increase in NiCoCrAlY content and heating temperature. The resistance of functionally graded coatings to thermal fatigue was approximately five times better than that of the duplex coatings. Results also show that the graded coating was superior to the duplex coating in long-term oxidation resistance.

Plasma spraying of functionally graded hydroxyapatite/Ti–6Al–4V coatings

Abstract Functionally graded hydroxyapatite (HA)/Ti–6Al–4V coatings were produced by plasma spray process using specially developed HA-coated Ti–6Al–4V composite powders as feedstock. The microstructure, density, porosity, microhardness, and Youngs modulus ( E ) were found to change progressively through the three-layered functionally graded coating that composed of the layers 50 wt.% HA/50 wt.% Ti–6Al–4V; 80 wt.% HA/20 wt.% Ti–6Al–4V, and HA. No distinct interface between adjacent layers of different compositions was evident from scanning electron microscope observation. X-ray diffractometry showed that the coatings composed of HA and Ti phases. Microhardness, as measured through the indentation technique, and tensile adhesion strength decreased correspondingly with increasing HA content in the single-layered composite coatings. The application of HA/Ti–6Al–4V composite powders improved the tensile adhesion strength of the coatings significantly. The Youngs modulus and fracture toughness results showed highly anisotropic elastic behavior with relatively higher E and K IC (fracture toughness) values parallel to the coating surface due to the intrinsic lamellar structure of the plasma sprayed coatings.

Plasma sprayed functionally graded thermal barrier coatings

Functionally graded thermal barrier coatings of the system yttria stabilised zirconia/NiCoCrAlY were fabricated through plasma spraying using pre-alloyed composite powders as feedstock. Composite powders with different compositions (75% NiCoCrAlY:25% YSZ; 50% NiCoCrAly:50% YSZ and 25% NiCoCrAlY:75% YSZ) were prepared by mechanical alloying and plasma powder spheroidisation, and are subsequently sprayed successively in a single plasma torch to form the functionally graded coating. This method has ensured a homogeneous coating with satisfactory inter-layer formation, and a flexibility for depositing different numbers of inter-layers by changing the composition of the composite powders. In the present study, five-layered functionally graded coatings have been plasma sprayed. These coatings demonstrated attractive properties compared to the conventional duplex thermal barrier coatings. This is because of the unique microstructure formed in the functionally graded coating with this new approach which also results in reduced residual thermal stresses in the coatings.

Mechanical behavior of plasma sprayed functionally graded YSZ/NiCoCrAlY composite coatings

Abstract Functionally graded Y2O3 stabilized ZrO2 (YSZ)/NiCoCrAlY composite coatings were prepared using pre-alloyed and spheroidized composite powders. During the preparation of pre-alloyed powders and composite coatings, aluminum and chromium were oxidized and the so-formed metal oxides combined with zirconia forming unique mixtures of oxides. In this study, the mechanical properties of graded YSZ/NiCoCrAlY composite coatings, such as the elastic modulus, were investigated using a four-point bending test. The results showed that the elastic moduli and bending strength of individual layers of FGC decreased with an increase in the content of YSZ in the layer because of the increase in porosity of the coatings. The elastic moduli of the coatings also decreased with an increase in temperature. The elastic modulus for a five-layer functionally graded coating with a ceramic layer in tension during the bending test was lower than that of the coating with the ceramic layer in compression. This can be explained by the differences of the deformation behavior between the ceramic layer and metal layer during the four-point bending test. A laminated composite beam theory was used to explain these results.

Microstructure formation in plasma-sprayed functionally graded NiCoCrAlY/yttria-stabilized zirconia coatings

Functionally graded NiCoCrAlY/yttria-stabilized zirconia coatings were fabricated by plasma spraying using mechanically alloyed, plasma-spheroidized NiCoCrAlY/yttria-stabilized zirconia (YSZ) composite powders. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) analysis revealed that oxidation of aluminum, chromium and yttrium in the NiCoCrAlY alloy occurred in the high-temperature plasma-spray stream during powder preparation and coating deposition. The oxidized products, predominantly aluminum oxide, subsequently mixed with zirconia at elevated temperatures in a wide composition range. These mixtures are observed as fine lamellae structures with different gray levels in the back-scattered electron (BSE) mode. Among the lamellae structures, the regions containing high aluminum oxide concentration appeared darker, compared with the regions with high zirconia concentration. It was observed that aluminum oxide formed a preferential combination with zirconia in the coating. Dendritic structures were frequently observed in the coating interlayers and were considered to have formed during the solidification process of the molten splats. The observations on polished cross-sections also showed that the resultant coatings were dense and had no clear interface between adjacent layers. It is believed that the graded distribution of the coating compositions helped to decrease the high level of residual thermal stresses experienced during thermal cycling, and hence contribute positively to the bond strength of the coating. These functionally graded coatings have demonstrated superior tensile adhesive bond strength to duplex coatings.

Functionally graded ZrO2-NiCrAlY coatings prepared by plasma spraying using pre-mixed, spheroidized powders

Functionally graded ZrO2-NiCrAlY coatings were prepared by plasma spraying. Pre-mixed, plasma spheroidized powders were used as the feedstock. The advantage of using pre-mixed, spheroidized powders was to ensure chemical homogeneity and promote uniform density along the graded layers, and these pre-mixed powders are used to form the different interlayers of functionally graded coatings in the present study. The microstructure, density and microhardness changed gradiently in the ZrO2-NiCrAlY FGM (functionally graded materials) coatings. The bond strength of ZrO2-NiCrAlY FGM 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 FGM 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 FGM coating increased significantly after hot isostatic press (HIP) treatment and the reason can be attributed to the densification of the microstructure, the decrease of defects in the coatings, interdiffusion between layers and reduction in the residual thermal stresses.

SiC whisker toughened Al2O3-(Ti, W)C ceramic matrix composites

Nowadays, more and more researchers begin to utilize more than one type of reinforcing agent to fabricate ceramic matrix composites in order to improve the mechanical properties of ceramic materials to a greater degree 1 and 11. The aim of this research work is to use SiC whiskers to improve the fracture toughness and flexural strength of (Ti, W)C reinforced Al2O3 composite material. The effects of SiC whisker content and sintering temperature on the relative density, flexural strength and fracture toughness of Al2O3-(Ti, W) composites were investigated.