Hydrothermal Crystallization with Microstructural Self-Healing Effect on Mechanical and Failure Behaviors of Plasma-Sprayed Hydroxyapatite Coatings - Biological and Biomedical Coatings: Processing and Characterization - page 272

From the mechanics of a flexural beam, the residual strain at the top surface and the HAC/
substrate interface can be calculated for a concave coating using Equation 6.25:

ε top , ε inter = -[( l - l 0 )/ l 0 ] ± ( t /2 r ) (6.25)

where t is the coating thickness and r is the radius of curvature measured at the neutral
surface of the HA coating. The first term -( l − l 0 )/ l 0 in Equation 6.25 is defined as the linear
strain and the second term ( t /2 r ) is the curvature strain. For convex coatings, the calculated
symbol “±” before the second term in Equation 6.25 should be reversed, and then Equation
6.25 will be changed into:

ε top , ε inter = -[( l - l 0 )/ l 0 ] ∓ ( t /2 r ) (6.26)

By measuring the variation of length and curvature of substrate-removed HA coating
layers, the residual strain ( ε ) of the 400°C to 900°C vacuum heat-treated HACs is obtained,
and the results are shown in Figure 6.25. The reason of performing vacuum heat treatments
is to prevent the significant oxidation of metallic substrates, especially for the Ti alloy
substrates. It can be seen that the plasma-sprayed HACs represents a compressive residual
strain. When applying heat treatments, the crystallized-HACs displays tensile residual
strains for heating temperatures performed at 400°C to 600°C. It is worth noting that the
crystallized-HACs produced compressive residual strains again at heating temperatures
higher than 600°C. Referring to the thermal dilatometry results represented in Figure
6.11 and Table 6.1, a significant crystallization-induced contraction is demonstrated after
high-temperature heat treatments. As shown in Figure 6.24, Young's moduli of heat-
treated HACs are advanced with increasing heating temperatures, and this is attributed
to the crystallization effect of HACs. However, the comparison of Figures 6.23 and 6.24
showed the bonding strength of vacuum and atmospheric heat-treated HACs did not
increase in accordance with increasing heating temperatures. They lowered with the
temperatures after attaining the maximum value at 600°C. This results from the impact of
the interfacial adhesive force between coating and substrate besides the cohesive strength
of the coating layers. Figure 6.26 shows the mechanism of residual stress on the debonding
of the coating [221,222]. Before debonding of a coating, interfacial and interlamellar splat
cracks are assumed to exist at the coating/substrate interface, as shown in Figure 6.26a.

0.4

0.3

Tensile strain

0.2

0.1

0

−0.1
−0.2
−0.3
−0.4

Compressive strain

25 400 500 600 700 800 900

Temperature of heat treament ( º C)

FIGURE 6.25
Measured residual strains of as-sprayed HACs and vacuum heat-treated HACs from 400°C to 900°C.