Thermal Sprayed Bioceramic Coatings: Nanostructured Hydroxyapatite (HA) and HA-Based Composites - Biological and Biomedical Coatings: Processing and Characterization - page 169

The major problems pertaining to the HA-coated implants are their poor long-term
functional service due to the failure of the coatings. Extensive studies revealed that the
long-term functional performance of the CP coated implants was influenced significantly
by the microstructure of the coatings (Suominen et al. 1996; Szivek et al. 1996). In addition,
HA coating has a potential problem of introducing an additional interface into the implant
system. The interface between HA coating and metallic alloy is critical in determining the
reliability of the implant (Shaw et al. 1998; Piveteau et al. 1999; Gross et al. 1998c; Bonfield
1987; Krauser et al. 1991). Studies have reported that fracture often occurred at the HA/
substrate interface rather than at the bone/HA interface from a direct shear loading (Wang
et al. 1993b; Inadome et al. 1995; Hayashi et al. 1993). The analysis of malfunctional dental
implants evidenced the failure to be primarily located at HA/metal interface (Krauser et
al. 1991). Consequently, estimation of the interface properties is essential for evaluating
the HA-coated implants. When used as dental and orthopedic implants, due to identi-
fied clinical environment, HA coatings experience compound stresses, which can include
shear, bending, tensile, and compressive forces. Among all the mechanical performances
required, adhesion and fracture toughness are the most critical variables. Besides the adhe-
sive strength, strain energy release rate is also capable of evaluating the interface (Chung
et al. 1997). The strain energy density theory has been extended to study the growth char-
acteristics of three-dimensional cracks within coatings (Sih 1991). Four-point bend test was
usually used to determine the strain energy release rate of the coatings at the coating/sub-
strate interface upon the considerations that the interface between the two different mate-
rials is usually the weakest part and failure initiated from an interfacial defect was often
observed (Howard et al. 1991, 1993, 1994). The four-point bend test to evaluate the bonding
between coating and substrate has the advantages of providing stresses more like those
experienced in vivo of HA coatings (Haman et al. 1997). As discussed, the strain energy
release rate is an appropriate variable for evaluating the energy of the inception of cleavage
and dislocation emission along the interface. It is essential to relate the resistance of the
material to crack propagation to a parameter that can characterize the fracture toughness of
the material. With regard to the complex stresses at the coating/substrate interface, it is not
easy to get the fracture toughness to represent interface property. Generally used variable
as an alternative of the fracture toughness is based on the determination of strain energy
release rate, G . For thermal-sprayed coatings, the four-point bend test was believed to be
an effective technique for the determination of critical strain energy release rate ( G ss ) at the
coating/substrate interface (Howard et al. 1994; Clyne et al. 1996). The specimen loaded by
four-point bend for the G ss determination is schematically depicted in Figure 4.25. G can be

P /2 b

P /2 b

h 1

b

h

h 2

2 a

1

1

2 c

FIGURE 4.25
Schematic depiction of the coating/substrate bimaterial specimen for 4-point bend test (h1, coating thickness;
h2, substrate thickness). (From Li et al., Eng. Fract. Mech. , 74, 1894-1903, 2007a. With permission.)