Structural state and fracture strength upon friction of boron-containing alloys and coatings with an amorphous structure

The structural condition and wear resistance of electrochemical Co-B and Ni-B coatings with different amounts of boron, as well as Ni-Co-Fe-Cr-Mo-Si-B and Ni-Fe-Si-B amorphous alloys obtained by rapid solidification from the melt have been studied. It has been found that in Co-B and Ni-B coatings with a boron concentration less than 8 at %, there is formed a solid solution of boron in the matrix phase, and the coatings are characterized by a high fracture strength upon friction. At a boron concentration from 8 to 15 at %, there is formed in the coatings a metastable amorphous structure with a decreased wear resistance. An increase in the B concentration to 20 at % stabilizes the amorphous structure of coatings, which leads to an increase in their fracture strength. A conclusion is made that during friction interaction at the surface of amorphous layers there are formed crystalline layers, which are subject to tensile stresses and are characterized by a decreased fracture strength. The difference in the wear resistance of the alloys studied is explained by a different deformation and thermal stability of the amorphous structure of the material.