Mechanical And Biocompatibility Properties Of Boron Nitride Nanoplatelets Reinforced Plasma Sprayed Hydroxyapatite Coatings

Boron nitride nanoplatelet(BNNP)exhibits excellent mechanical properties,biocompatibility properties and high temperature stability,and therefore BNNP was introduced into HA coating fabricated through plasma spray.Meanwhile,careful analysis of phase constituents and microstructure,assessment of mechanical properties and biocompatibilities of the as-sprayed coatings were also investigated?SEM observation showed that BNNPs homogeneously distributed in the as-sprayed coatings.Also,the typical microstructure of the as-sprayed coatings diaplayed biomodal characteristic with microsized grains and sintered nanostructureed grains.XRD results showed HA,amorphous phase,tricalcium phosphate(TCP),Ca O phases detected in coatings.SEM observations and Raman results revealed the added BNNPs retained their original structure even after harsh process.Micro-hardness,fracture toughness,micro indentation and micro scratch were carried out for mechanical properties.Results showed that the hardness,elastic modulus and fracture toughness were improved with an addition of BNNP at 1% weight.Synergetic strengthening and toughening mechanisms which are operative through splat boundaries and individual splats were proposed.At splat boundaries,these embedded BNNPs induced stronger adhesion between the adjacent splats to resist splat sliding.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.Instrumented micro-scratch experiments showed that the addition of BNNPs ascribes to improve the resistance of composite coating.Osteoblast adhesion,ALP activity and mineralizaion were evaluated for BNNP/HA in-vitro biocompatibility.The results showed the wettability and large stiffness of composite coatings significant affect the attachment.The nanostructureed grains in BNNP/HA increased the ALP activity of the osteoblast.After immersion in SBF for 7 days,the mineralized layer was formed on the surface of the BNNP/HA coating,indicating that the addition of BNNP did not affect the mineralization ability of HA.After heat treatment at 600? for 2 hours,the above-mentioned amorphous phase and decomposed phase of as-sprayed coating were substantially changed to crystalline HA,and the crystallinity of the HA coating increased from 60% to 93%.The experimental results showed that the HA hardness,elastic modulus,and fracture toughness of the HA coating after heat treatment were improved by 2.3%?32.8%? 21%.Owing to the synthesized nano-grain HA during heat treatment,the ALP activity was increased by 23%.However,immersed in simulated body fluid for 7 days,the high crystallinity reduces the dissolution of calcium ions,and the mineralized deposition rate of bio-apatite decreases.Abovementioned results illustrate that BNNP is an excellent reinforcement for HA coating and the BNNP/HA composite coating is expected to be a promising material for load-bearing orthopedic implant.