Hydroxyapatite and Other Biomedical Coatings by Electrophoretic Deposition - Biological and Biomedical Coatings: Processing and Characterization - page 119

and Ti6Al4V [104,160]. In particular, Fecralloy has good high-temperature stability, which
allows it to be heated to high temperatures without sacrificing its mechanical properties.
Interfacial Stresses and Adhesion
Despite the achievement of chemical bonding formed at the HAp/metal interface, the
adhesion of the HAp layer to the substrate tends to be extremely weak [163]. This outcome
is largely a result of stresses at the HAp/metal interface [164]:
• Drying shrinkage (HAp). When the HAp is removed from the suspension, it dries
and undergoes shrinkage while the Ti does not.
• Heating expansion (Ti). When the coated substrate is heated initially, the Ti expands
significantly while the deposit, which expands to a lower degree, is unfired and
very weak.
• Thermal expansion mismatch. Upon cooling, the brittle ceramic, which is bonded to
the metal, is subjected to tensile stress owing to the different coefficients of ther-
mal expansion ( α ) [165]:
α HAp = 11-14 × 10 −6 °C −1
α Ti = 8.7-10.1 × 10 −6 °C −1
α Ti6Al4V = 8.7-9.8 × 10 −6 °C −1
Owing to the stresses arising from these mismatches, slow heating and cooling rates are
required.
Metals with coefficients of thermal expansion similar to that of HAp have attracted con-
siderable attention. Fecralloy, with a coefficient of thermal expansion of 11.1 × 10 −6 °C −1 ,
has been used as a substrate for HAp coatings, resulting in dense and crack-free HAp
coatings [104]. Another advantage of using Fecralloy is that a dense, alumina, passivating
layer forms in situ on the metal surface during the sintering process and this can protect
the metal from corrosion in the body environment. Metals with much higher coefficients
of thermal expansion than that of HAp also have been used. The coefficient of thermal
expansion of 316L stainless steel is 16.0 to 19.0 × 10 −6 °C −1 , which is advantageous because
it places the HAp in compression during and after cooling [117,166]. It is well known that
ceramics are brittle and hence weak in tension but they are very strong in compression.
There have been considerable efforts made to improve the interfacial strength between
the HAp coating and the substrate. One method involved the fabrication of a dual-layer
electrophoretic deposit strategy, which required applying a HAp coating and heat treat-
ing twice [117]. The purpose of this was to use the bottom layer as a diffusion barrier for
metallic ions, thereby reducing the decomposition of the HAp coating in the top layer.
Since the top coating layer was applied seamlessly to the bottom coating layer, it filled the
cracks created in the bottom layer, thus improving the adhesive strength of the coating to
the substrate. Shear strengths up to 23 MPa were achieved.
Another method is to obtain densified HAp coatings without significantly increasing the
heat treatment temperature. This has been approached by electrophoretically depositing a
well-packed green ceramic coating. The two main methods of doing this are careful adjust-
ment of the coating parameters [167] or use of fine HAp particles, especially nanoHAp