The strength of the HAp coating is affected by complex factors. One of them is residual stresses that lead to cracking due to its positive value. The peak value of Raman shift is moved in the case of plasma-sprayed one compared with bulk sintered one. Raman micro spectroscopy can then visualize the microscopic distribution through the thickness of coating. However, during the plasma-spraying process or immersion process in SBF, the phases of HAp coating are easily decomposed into calcium phosphates that can also change the raman characteristics. The authors observed the raman shift data of HAp coating with various immersion period from 1 day to 30 days. Raman spectroscopy can successfully distinguish HAp peak from combined TCP peaks, which is stable during immersion. Calculated residual stress from HAp peak changed from compressive to tensile during immersion. These changes indicates that residual stress value is affected by the crystal structure of HAp or TCP.

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