Boron nitride nanoplatelets induced synergetic strengthening and toughening effects on splats and their boundaries of plasma sprayed hydroxyapatite coatings

Significance 

The field of biomaterials is a rapidly emerging one owing to its direct relation with healthcare and impact on human health related issues. Among them, titanium alloys have been widely used as metallic implants for repair and replacement of damaged bones due to their unique properties. Unfortunately, they have not been able to achieve the expected long-term performance owing to the inferior adhesion bonding with the bone tissues and low wear resistance.

As such, hydroxyapatite coating has been widely to be deposited onn the surfaces of these metallic implants. However, the low fracture toughness limits its functionality, particularly in load-bearing metallic implants, and therefore toughening HA coating has been one of the most concerned topics in the materials community. More importantly, an ideal reinforcement in HA composites and/or coatings should possess significant toughening effect even with smaller amount addition to avoid negative influence on the bioactivity/osteointegration of HA.

Recent advancement in nanomaterials has resulted in the use and development of carbonaceous nanomaterials such as graphene and carbon nanotubes as ceramic fillers to enhance their fracture toughness. However, lower oxidation temperature (~400-450℃) of carbonaceous nanomaterials [18] makes them not suitable as nanofillers in composites synthesized through the high-temperature processes. In very recent years, boron nitride nanoplatelets (BNNPs) with two-dimensional structure have been reported to possess comparable mechanical properties to those of carbonaceous nanomaterials. Furthermore, BNNPs are chemically inert up to 950℃ and white color of BNNPs matches well with that of HA. Hence, BNNPs are expected to be an promising reinforcing nanofiller in HA matrix for orthopedic applications.

Lab of Plasma Spray led by Dr. & Prof. Yao Chen at Soochow University, China, innovatively proposed to introduce boron nitride nanoplatelets into hydroxyapatite composite coating fabricated through plasma spray, and the synergetic strengthening and toughening mechanisms which are operative through splat boundaries and individual splats were highlighted. The work was published Ceramics International.

The authors observed that the boron nitride nanoplatelets were homogenously distributed in the as-sprayed coating, and significant enhancement in the fracture toughness and moderate improvement in yield strength were achieved as compared to its monolithic hydroxyapatite coating counterpart. At splat boundaries, these embedded BNNPs induced stronger adhesion between the adjacent splats to resist splat sliding, leading to the fact that the inter-splat friction force of the resultant hydroxyapatite coating with the addition of 2% boron nitride nanoplatelets weight fraction was considerably increased by 7.3%. Within splats, toughening mechanisms such as BNNP pullout, crack bridging by both anchored BNNPs and nanosized HA grains, crack deflection and crack propagation arrested by the embedded BNNPs were observed to improve toughness. Moreover, thermal mismatch between HA matrix and BNNPs during cooling process after plasma spray would induce the pre-existing dislocations formed around these BNNP nanofillers, which was assumed to hold out the effect of Orowan-type strengthening within splats.

In a nutshell, Dr. Chen’s group successfully fabricated boron nitride nanoplatelets reinforced hydroxyapatite composite coating using plasma spray, and various mechanical properties of the hydroxyapatite composite were significantly improved. With the commercialization of this new composite coating developed by Chen and his research group we can hope that the problems related to biocompatible coatings on the metallic implants may have found promising solution.

Reference

Zhu, J., Chen, Y., Ren, J., Zhao, D., & Liu, W. (2018). Boron nitride nanoplatelets induced synergetic strengthening and toughening effects on splats and their boundaries of plasma sprayed hydroxyapatite coatings. Ceramics International, 44(9), 10604-10610.

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