The effect of changing coating length on the cohesive and adhesive failure of the coating on metallic flat substrates is analyzed by uniaxial loading. The criterion for determining the fragmentation length of the coating with mixed (adhesive-cohesive) failure was developed. Calculating the critical length as the intersection point of the two lines (interfacial failure shear stress-length curve and the normal failure stress-length curve) does not always give correct value. The critical length of coating should be evaluated as maximum value within the annular trapezoid. It is established that the kind of probability density function both the adhesion strength and the cohesion one should be considered when determining the critical length of the coating. Verification of the developed criterion was performed on the example of plasma-sprayed coatings. It was noted, the formation of multiple cracks in the plasma-sprayed coating has random character due to microstructural defects, such as pores and microcracks.
Author: Dolgov N. A.
ADHESION STRENGTH EVALUATION OF CERAMIC COATINGS ON CAST AND SELECTIVE LASER MELTED Co-Cr DENTAL ALLOYS USING TENSILE SPECIMENS
Studying the fracture characteristics of porcelain coatings plays a main role in selection of materials for metal-ceramic restorations. The aim of this work is to study the effect of the substrate manufacturing process on the adherence of the porcelain. The coatings of porcelain IPS.Inline (Ivoclar Vivadent) are fused onto dental Co-Cr alloys fabricated via casting (Biosil F) and Selective Laser Melting (SLM) (Co212-f). The adhesion strength of the ceramic coatings is studied under tensile load of flat specimens. The interfacial shear strength is determined using experimental results. The shear stress distributions in the metal-ceramic interface at the critical load are evaluated by analytical approach. It is established that the interfacial shear strength values of ceramic coating are 67.5 MPa for cast Biosil F alloy and 83.8 MPa for SLM Co212-f alloy. The higher shear strength of the porcelain to the SLM samples is due to the nearly two times higher surface roughness, which is reason for increasing both the mechanical and the chemical adhesion. The nature of the fracture of the ceramic coating on the Co-Cr alloys, produced by casting and SLM, is similar and is mixed adhesive–cohesive mode. The higher adhesion strength of the porcelain coating to the SLM dental alloy is a good precondition for the SLM application in production of metal-ceramic fixed partial dentures for areas with heavy loads.
The aim of the present paper is to investigate the mechanical properties (hardness and tensile strength) of dental Co-Cr alloys fabricated via casting and selective laser melting (SLM). Two groups of metallic specimens (four-part dental bridges and standard tensile test specimens) made of Co–Cr dental alloys were produced by lost-wax casting and SLM processes. Vickers hardness distribution along the depth of the dental bridges as well as the Rockwell hardness and tensile strength of the samples were studied out. The hardness of Co–Cr dental alloys are dependent on the manufacturing technique employed. It was established that the average Vickers hardness of the samples, produced by SLM, was higher than that of the cast samples 382 HV and 335 HV respectively. The nearly even hardness distribution in the bridges, produced by SLM, and fluctuations of the hardness values along the depth of the cast bridges were observed. The Rockwell measurements confirmed the higher hardness of the SLM samples – 39 HRC in comparison with that of the cast ones – 33 HRC. The tensile strength is in good agreement with the hardness values. Due to the unique microstructure, the yield strength and tensile strength for the SLM samples were higher than those of the as-cast alloy.