Mechanical behavior of metaphosphate glasses used as coating on metals in relation to their structure and preparation method

Abstract

The purpose of the present work was to investigate the mechanical behavior of metaphosphate glass coatings of the Na2O-CaO-P2O5 system on steel substrates. These coatings may be used as thermal barriers or to give anti-corrosion protection. Coacervates of glasses under consideration were obtained by a coacervation process, from Graham's salt and deposited on aluminized steel substrate. Chemical composition of coacervate phases was improved by the choice of cations and addition of a filler to reduce the residual stresses in glass metal joints due to the thermal expansion coefficient mismatch. The chemical and mechanical properties of both the coating and the interface were studied by energy dispersive spectrometry (EDS) and hardness measurements. It was shown that an alumina layer is formed by the reaction between the phosphates compounds during curing. This layer acts as a barrier for the iron diffusion. The fracture toughness of the glass coating was investigated by Vickers indentations. Inasmuch the cracks are of the Palmqvist form, the Evans and Charles formula holds for calculating KIC. A value near 0.85 MPa m1/2 was found for all the glasses investigated. Residual stress distribution was calculated. The glass coating was in compression and residual stresses were reduced by the interlayer formation. A good adhesion was found since cracks produced by indentation near the glass steel interface propagated in the coating rather than along the interface.