The results of XRD analysis of plasma sprayed HA/YSZ/Ti–6Al–4V coatings are shown in Fig. 1. The XRD pattern of as-sprayed composite coatings retains the characteristics of powders in Fig. 2 except a small amount of CaO and a-TCP. For plasma sprayed pure HA coatings, the formation of additional calcium phosphate phases such as a-TCP, b-TCP, TTCP and CaO are likely to be induced by high-temperature plasma flame. However, in the composite coatings produced using the composite powders, there are no indications of b-TCP and TTCP formation except a small amount of CaO and a-TCP, as shown in Fig. 1. The absence of TTCP and b-TCP in the composite coatings is probably due to different degrees of decomposition of HA and the interdiffusion of HA and YSZ. The thermal conductivity of YSZ (0.007 J s-1 cm K) is lower than that of HA (0.013 J s-1 cm K), which can affect the thermal diffusivity and molten state of the materials during plasma spraying. Moreover, there is no oxidation of Ti compound in the as-sprayed composite coating. This proves the effectiveness of the design of the composite powder in preventing of the oxidation of Ti alloy, in which Ti–6Al–4V powders are fully covered by HA and YSZ.
Fig. 1. XRD patterns of as-sprayed and heat treated coatings (spraying parameters: 8.5 cm, 12 kW).
Fig. 2. XRD pattern of HA/YSZ/Ti–6Al–4V composite powder.
In comparison between Figs. 1 and 2, the as-sprayed coating exhibits much lower apatite crystallinity than the initial powders. YSZ and Ti alloys remain stable during plasma spraying. The plasma spraying process results in the formation of amorphous coating that is indicated by the broad peaks with low intensities.
XRD revealed t-ZrO2/c-ZrO2 and Ti as the major phases in the as-sprayed coating (Fig. 1). Heat treatment at 600-C and 3 h resulted in a prominent re-emergence of HA peaks in the XRD pattern followed by traces of the TiO2 (rutile) peaks. The TiO2 peaks became increasing prominent as the heat treatment duration increased (maximum 12 h), and as the temperature rose to 700-C. Concurrently, the intensity of the Ti peaks decreases. A distinguishing feature in the XRD pattern is the absence of supplementary calcium phosphate phases that often accompany plasma sprayed HA.
The influences of plasma net energy and standoff distance on HA relative crystallinity of heat-treated HA/ YSZ/Ti–6Al–4V composite coatings are listed in Table 1. The crystallinity of HA firstly increases with an increase in power net energy, reaching a maximum value at a net energy of 12 kW, then decreases with further increase in net energy. The influences of power net energy and stand off distance on the relative crystallinity are the combined effects of amorphorization and recrystallization during plasma spraying. At higherpower net energy of 12–14 kW and a shorter standoff distance of 7–8.5 cm, the high temperature of the plasma flame and the high-temperature gradient among substrate, coating and the surrounding accelerate the transformation of crystalline HA to amorphous calcium phosphate, thus decreasing the relative crystallinity of HA. However, at lower-power net energy of 8–12kW, recrystallization of the amorphous calcium phosphate may be more significant than the amorphorization of HA, resulting in an increase in the crystallinity of HA.
Table 1 Relative crystallinity of HA in composite coatings
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