The purpose of the present study is to develop a novel implant material which has a superior biocompatibilities and high strength by means of a coating technology.In this study,the cold spray technique was selected and used as surface modification method of a titanium alloy for the development of implant material;the porous pure titanium coating was sprayed on a titanium alloy,Ti-6Al-4V,by using the cold spray technique.The influences of spray conditions and post-sprayed heat treatments on the mechanical properties of the cold sprayed biomedical coatings were investigated.It was revealed from the experimental results that the developed biomedical titanium coatings had a low elastic modulus,high adhesion and enough tensile strength.Most important factor to get the superior biomedical coatings was the post-spray heat treatment because both the adhesion and cohesion strengths of the coatings could be improved significantly due to the post-spray heat treatment.It was also revealed that both the adhesion and cohesion strengths were improved by the recrystallization of nanoparticles generated by the high speed collision of particle during the cold spray process.The cold sprayed biomedical coatings developed in this work have enough fatigue properties.
Aerosol Deposition Method(ADM)is a technique to form films by impacting solid particles on substrates.Dense ceramic films can be formed even at room temperature.Principal characteristics of ADM are particle diameter of starting powder,which is around several μm,and the process temperature. In this study,to improve the deposition efficiency of the ADM,the effect of heating Cu substrates on the deposition efficiency of Al2O3 films was investigated.The deposition efficiency increased about 2 times by heating the substrate at around 80℃.It was revealed that the films contained H as an impurity and the H content tended to decrease by heating substrate.The volume resistivity of the film also increased by heating the substrate.Considering that the quantity of H2O detected on the starting powder was consistent with the H content in the films,it could be concluded that H2O on the starting powder deteriorated the deposition efficiency and contaminated the films during the deposition.These results suggest that developing a process which can prevent H2O from contaminating the films can improve the deposition efficiency of the ADM.
A new test method is proposed in this study to evaluate the adhesion strength of thermal spray coatings.The proposed method can evaluate the adhesion strength of coatings against to the shear loading acted at the interface by the compressive external loading.In order to verify the validity of the proposed method,the influence of the gage length on the evaluated shear adhesion strength was investigated.With the decreasing of the gage length of the specimen,the evaluated shear adhesion strength decreased and the dispersion of it increased.However,the dependence of gage length on the evaluated shear adhesion strength and the dispersion of it decreased with the increasing of the gage length.It was revealed from these experimental results that there are limits of the specimen gage length and the specimen width for the proposed test method.And to obtain the valid shear adhesion strength the specimen should have the larger gage section than a certain length and width which depend on the mechanical properties of coating and substrate.It was also discussed to expand the proposed test method to evaluate the interfacial fracture toughness and the resistance to the fatigue crack propagation at the interface.
A new method called“STAC-method”to measure adhesion shear stress of plasma sprayed coating was suggested by Dr.Yamazaki.In reference to this study,the author investigated whether this method can adapt to cold sprayed coating.Test pieces were fabricated by spraying copper powder on aluminum substrates and adhesion shear stress of them was measured by compressive loading so that gage length(Lg)was optimized.In addition the influence of gas temperature and coating thickness(tc)on the shear stress was investigated.As a result,the Lg was required to be more than 3 mm to suppress the dispersion of the shear strength,although the influence of tc was not significant.In investigation of gas temperature,the shear strength at 200,400,600 and 800℃ in the spray temperature were about 57,72,114 and 120 N/mm2 respectively.The test pieces at 200 and 400℃ exhibited brittle fracture and broke in the interface between the coating and substrates.On the other hand,those at 600 and 800℃ exhibited ductile fracture and broke in the aluminum substrates.These results suggest that STAC-method is useful as an evaluation for cold sprayed coating as well as plasma sprayed coating.