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 283

(a)

(b)

1

0.4

As-sprayed HACs
HT125
HT150

ln( σ i - σ o )

0

0.3

1.2 1.6 2.0

-1

0.2

R 2 = 0.95

-2

R 2 = 0.99

As-sprayed HACs
HT125
HT150

0.1

-3

R 2 = 0.98

0.0

28 32 36 40

Bonding strength (MPa)

FIGURE 6.33
(a) Failure probability density function f ( σ ) curves and (b) Weibull distribution plots of plasma-sprayed and
HT-HACs. F ( σ i ) is cumulative failure probability at corresponding bonding strength ( σ i ), and slope represents
Weibull modulus ( m ), which calculated by least-squares fitting method of Equation 6.36 at a maximum coef-
ficient of determination ( R 2 ).

( m > 1) of increasing failure rate (IFR) [249,251]. The HT-HACs also show a wearout failure
model with a higher Weibull modulus ( m = 5.0 and 6.4 for HT125 and HT150, respectively),
which represents the microstructural self-healing and homogeneity resulted from hydro- hydro-

thermal crystallization has evident benefits to effectively enhance the bonding strength of
plasma-sprayed HACs. A material with a higher Weibull modulus is selected as it may be
an indicator for clinical use or lesser technique sensitivity. The Weibull modulus can be
also a measure of the variability of the data, which gets larger as the degree of bonding
strength fluctuation decreases. Evident benefits of the failure probability density function
(Figure 6.33a) and failure rate (Figure 6.34a) curves shift to a higher bonding strength with
a concentrated data distribution are achieved for HT150 specimens. The hydrothermal
treatment not only effectively enhances the bonding strength of plasma-sprayed HACs,
but helps to acquire more stable HACs with less reliability decrease (Figure 6.34b) while
the loading exceeds the minimum strength.
The representative failure morphologies of these coatings are shown in Figure 6.35. It
can be seen that the failure morphology represents a combination of the cohesive failure
region (co) and the adhesive failure region (ad). The cohesive failure is dominated by micro-
structural features such as crystallinity, defects, lamellar texture, and a large area fraction
of cohesive failure can be commonly observed at high strength coatings [63,99]. Compared
with the failures of plasma-sprayed HACs shown in Figure 6.35a, since strengthening

self-healing and homogeneity resulted from hydro- homogeneity resulted from hydro- resulted from hydro-

TABLE 6.6
Results of Bonding Strength Measurements and Weibull Statistical Analysis of Plasma-Sprayed
HACs and HT-HACs

BondingStrength
(MPa), σ a

MinimumStrength
(MPa), σ o b

Characteristics
Strength(MPa), η b

WeibullModulus,
m b

Plasma-sprayed HACs 32.4 ± 1.5 26.8 33.1 3.8

HT125 (12 h) 38.1 ± 1.2 32.6 38.7 5.0

HT150 (6 h) 38.9 ± 1.0 33.3 39.4 6.4

a Each value is the average of 20 tests ( n = 20).
b Data obtained from lnln(1/1 − F ( σ i )) vs. ln( σ i − σ o ) plots (Figure 6.32b) using Equation 6.36.