Surface coating for degradable magnesium and magnesium alloys and method for preparing the same

Abstract

A surface coating for degradable magnesium and magnesium alloys, including: an inner layer and an outer layer. The inner layer is a magnesium phosphate conversion layer, and the outer layer is a hydroxyapatite layer. A method for fabricating the surface coating is also provided herein, which includes: soaking the magnesium or magnesium alloy in an acidic solution containing magnesium salt and phosphate under heating to form the magnesium phosphate conversion layer on a surface of the magnesium or magnesium alloy; and transferring the magnesium or magnesium alloy to an alkaline solution containing calcium salt and phosphate followed by soaking under heating to form a hydroxyapatite layer on a surface of the magnesium phosphate conversion layer.

What is claimed is: 
     
         1 . A surface coating for degradable magnesium and magnesium alloy, comprising:
 an inner layer; and   an outer layer;   wherein the inner layer is a magnesium phosphate conversion layer, and the outer layer is a hydroxyapatite layer.   
     
     
         2 . The surface coating of  claim 1 , wherein the magnesium phosphate conversion layer has a thickness of 50 nm-10 μm; and the hydroxyapatite layer has a thickness of 0.1 μm-500 μm. 
     
     
         3 . The surface coating of  claim 1 , wherein a bonding strength between the surface coating and a magnesium substrate or a magnesium alloy substrate is equal to or larger than 50 MPa; and an atomic ratio of calcium to phosphorus in the hydroxyapatite layer is (1.5-1.67):1. 
     
     
         4 . A method for preparing the surface coating of  claim 1 , comprising:
 (S1) soaking a magnesium or magnesium alloy substrate in an acidic solution containing a magnesium salt and a first phosphate under heating to form a magnesium phosphate conversion layer on a surface of the magnesium or magnesium alloy substrate; and   (S2) transferring the magnesium or magnesium alloy substrate with the magnesium phosphate conversion layer to an alkaline solution containing a calcium salt and a second phosphate followed by soaking under heating to form a hydroxyapatite layer on a surface of the magnesium phosphate conversion layer.   
     
     
         5 . The method of  claim 4 , wherein in step (S1), the magnesium salt is selected from the group consisting of magnesium nitrate, magnesium chloride, magnesium sulfate, magnesium phosphate, magnesium perchlorate, magnesium acetate, magnesium hydroxide and a combination thereof; a concentration of the magnesium salt in the acidic solution is 0.001˜10 mol/L; the first phosphate is selected from the group consisting of sodium phosphate, potassium phosphate, magnesium phosphate, ammonium phosphate, calcium phosphate, and a combination thereof; and a concentration of the first phosphate in the acidic solution is 0.001˜10 mol/L. 
     
     
         6 . The method of  claim 5 , wherein a molar ratio of magnesium ions to phosphate ions in the acidic solution in step (S1) is (0.2˜5):1. 
     
     
         7 . The method of  claim 4 , wherein in step (S1), the acidic solution is adjusted to pH 2˜7 with nitric acid, sulfuric acid, hydrochloric acid, phosphoric acid, acetic acid or a combination thereof; and the soaking is performed at 5˜99° C. for 5 min˜12 h. 
     
     
         8 . The method of  claim 4 , wherein in step (S2), the second phosphate is selected from the group consisting of sodium phosphate, potassium phosphate, magnesium phosphate, ammonium phosphate, calcium phosphate, and a combination thereof; and
 a concentration of the second phosphate in the alkaline solution is 0.001˜10 mol/L; the calcium salt is selected from the group consisting of calcium phosphate, ethylenediaminetetraacetic acid calcium disodium salt (EDTA-Ca), calcium citrate, calcium acetate, calcium chloride, calcium nitrate, calcium maleate, calcium polyacrylate, calcium polymethacrylate and a combination thereof; a concentration of the calcium salt in the alkaline solution is 0.001˜10 mol/L; and a molar ratio of the second phosphate to the calcium salt is (0.2˜5):1.   
     
     
         9 . The method of  claim 4 , wherein in step (S2), the alkaline solution is adjusted to pH 7˜13 with sodium hydroxide, potassium hydroxide, aqueous ammonia or a combination thereof; and the soaking is performed at 5˜99° C. for 5 min˜48 h. 
     
     
         10 . The method of  claim 4 , wherein the magnesium alloy substrate is Mg—Zn alloy, Mg—Ca alloy, Mg—Li alloy, Mg—Mn alloy or Mg—Re alloy.