Nanoscale Surface Modification of Biodegradable Materials by Severe Shot Peening

Author:  Sara Bagherifard
Source:  ICSP-13
Doc ID:  2017039
Year of Publication:  2017
Introduction: Metallic materials commonly used as bone implants have a notable mismatch of mechanical properties with those of natural bone. This incongruity induces stress shielding by not transferring the applied load to the surrounding tissue [1]. Another downside for these materials is their redundancy when used for temporary fixation purposes. Biodegradable metallic materials with mechanical characteristics comparable to those of mammal hard tissue that can stabilize damaged segments under relatively large amplitudes of static and dynamic loading over time, and then resorb without adverse tissue reactions, can be of significant importance to address the existing challenges in hard tissue engineering. They have high potential to overcome the aforementioned issues and eliminate the need for costly and complicated retrieval surgeries. However, regardless the long list of favorable characteristics of these materials with bioresorbable non-toxic degradation products, their low fatigue strength, uncontrolled corrosion rate and undesirable hydrogen gas production in physiological environment have considerably limited their application in biomedical field [2, 3]. Mg based alloys can be of high potential to be used for fixation plates in orthopedic, trauma and maxillofacial surgery, if their general mechanical and corrosion properties are improved. Many solutions have been suggested to enhance Mg based materials’ corrosion resistance, including alloying [4] and surface coatings [5, 6]; however, some of these approaches can cause further complications for example compromising biocompatibility and mechanical properties. New Mg alloys have been recently developed for orthopedic implants [7], but their application particularly for load bearing cases still remains a challenge. Nanocrystallization has the potential to promote general mechanical characteristics. It also gives the prospect of decreasing the risk of prompt localized failure of biodegradable metallic biomaterials that is an issue particularly under cyclic loading. There are some studies on the effect of grain refinement on Mg based material obtained via severe plastic deformation (SPD) methods. SPD results in significant grain refinement by applying large plastic deformations at high strain rates [8-10]. Enhanced mechanical properties are reported through application of SPD methods to different Mg alloys [11-14]. Nonetheless, many opposing results have been reported about corrosion and degradation resistance of Mg alloys after SPD [15-18]. The electrochemical data are reported to be sensitive to a wide range of parameters including applied plastic strain, reduced grain size, crystallographic orientation and basal texturing [19, 20]. The broad scatter of the applied SPD treatments (equal-channel angular pressing (ECAP), surface mechanical attrition treatment (SMAT), high pressure torsion (HPT), etc.), various studied Mg alloys and the wide range of resultant grain size and orientation, in some way justify the contradicting results available in the literature.

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