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A biodegradable Mg/MgO composite coating was deposited onto AZ31B magnesium alloy by high-velocity suspension flame spraying technique using nano-sized MgO and micron-sized Mg powders. Phase constitution, microstructure, mechanical property and corrosion behavior of Mg/MgO composite coatings with different Mg/MgO ratios were investigated. Mg/MgO composite coatings with different Mg/MgO ratios presented rough and porous surfaces with typical features of solid-fluid co-deposition and exhibited well bonding coating/substrate interfaces with some pores within the coatings. Mg/MgO composite coatings prepared by co-deposition of Mg/MgO mixed powder were mainly composed of Mg rather than MgO indicating superior deposition of Mg. Microhardness, elastic modulus and tensile strength of Mg/MgO composite coatings were influenced by the Mg/MgO ratio. Mg/MgO composite coatings presented a limited corrosion protection for Mg alloy substrate after immersion in Hanks' simulated body fluid for different times. Corrosion behavior of Mg/MgO composite coatings was also discussed. This study proposed a strategy to prepare biodegradable Mg/MgO composite coating on metal substrates for potential applications in the implant field. [ABSTRACT FROM AUTHOR]

To slow down the corrosion rate and increase the bioactivity of magnesium alloys, biodegradable hydroxyapatite/magnesium (HA/Mg) composite coating was deposited onto AZ91D alloy substrate by high-velocity suspension flame spray technique using nano-sized HA and micron-sized Mg powders. Microstructures and phase constitutions of HA/Mg composite coating were analyzed by scanning electron microscopy and x-ray diffraction. Corrosion resistance of composite coating was also evaluated by potentiodynamic polarization test and electrochemical impedance spectroscopy in Hanks' balanced salt solution (HBSS). The result shows that the HA/Mg composite coating mainly consisted of Mg and HA in addition to a few of MgO phases. The coating showed a rough surface morphology with homogeneous elemental distribution and a lamellar structure in cross section as well as well-bonded coating/substrate interface. Mg particles presented superior deposition during spraying due to their higher melting degree compared to HA particles. AZ91D alloy substrate was significantly protected from corrosion in HBSS by the HA/Mg composite coating with Mg dissolution on surfaces of the composite coating. This strategy can offer a new route to fabricate HA/Mg composite coatings on other metal substrates for use in biodegradable implant fields. [ABSTRACT FROM AUTHOR]

In order to improve the corrosion resistance and bioactivity of biodegradable Mg alloy substrate, novel Zn-HA/Zn double-layer coatings with different HA/Zn ratios in weight were deposited on AZ91D substrates by cold spraying. Phase compositions and microstructures of as-sprayed coatings and coatings after corrosion tests were analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Electrochemical corrosion behaviors of both Zn-HA/Zn double-layer coatings were investigated in Hanks' simulated body fluid using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Results showed that both the pure Zn coating and HA/Zn composite coatings presented the similar phase compositions with their primary powders in addition to Zn oxidizations. Zn powders were plastically deformed and partially oxidized due to its low melting point, while HA powders were mainly crashed into fragments and hill-like splats. Both Zn under layer and HA/Zn upper layer were well bonded and presented dense structures, differences in HA/Zn upper layers were related to the HA/Zn ratios. Potentiodynamic polarization and EIS measurements illustrated that the cold-sprayed Zn-HA/Zn double-layer coatings not only improve the corrosion resistance of Mg alloy substrates, but also enhance its bioactivity due to the HA existed in composite upper layer. [ABSTRACT FROM AUTHOR]

In this paper, a new cold spraying shot peening (CS-SP) technology was used to produce surface nanocrystallization (SNC) of AZ91D magnesium alloy for the first time. The structure characteristics of SNC layer on AZ91D alloy were investigated by means of X-ray diffractometer (XRD), optical microscopy (OM), scanning electronic microscopy (SEM) and transmission electron microscopy (TEM), respectively. The microhardness distribution of AZ91D specimen after CS-SP treatment was tested by a microhardness tester. The wear property and the coefficient of friction (COF) of AZ91D alloy before and after CS-SP treatment were investigated using the sliding wear tester. Experimental results showed that SNC layer with thickness about 40 μm can be prepared on AZ91D alloy by the CS-SP process. The average grain size of SNC layer was in range of 50–80 nm and a large number of deformation twin bands existed beneath the nanocrystalline layer. The microhardness of AZ91D specimen after CS-SP treatment exhibited a gradient distribution with the depth and the microhardness value of the outermost layer was up to 160 Hv 0.025 about twice as that of the matrix. In comparison with the untreated one, the AZ91D alloy after CS-SP treatment showed the lower COF and the enhanced wear resistance. • Surface nanocrystallization of AZ91D alloy was induced by cold spray shot peening. • The hardened layer consisted of nanograin region and deformation twin bands region. • The average grain size of nanograin region was in a range of 50–80 nm. • The microhardness variation of harden layer in the depth showed a gradient decrease. • The treated AZ91D alloy exhibited the improved mechanical properties. [ABSTRACT FROM AUTHOR]