Objectives. Recent advances in subtractive and additive computer-aided design/computer-aided manufacturing (CAD/CAM) technologies have provided alternative routes for metal processing, resulting in microstructural diversity of the frameworks and great differences in mechanical properties and surface characteristics between the alloys. The different routes used for metal processing could affect the porcelain bonding and the overall performance of the porcelain-fused-to-metal (PFM) prostheses. The aim of the first part of this doctoral thesis was therefore to determine the effect of thermo-mechanical cycling on the shear bond strength (SBS) of dental porcelain to Co-Cr and Ti-based alloys fabricated by casting, milling, or by selective laser melting (SLM). The second part of this research was designed to investigate the effects of surface airborne-particle abrasion and bonding agent application on the porcelain bonds to titanium dental alloys fabricated by milling and by selective laser melting. In the third part, the relation between the strength of a porcelain bond to SLM titanium and the temperature of titanium preoxidation was described.
Methods. In the first part of the study seven groups of metal cylinders (n = 22 per group) were fabricated by casting (Co-Cr and commercially pure-cpTi), by milling (Co-Cr, cpTi, Ti-6Al-4V) or by SLM (Co-Cr and Ti-6Al-4V). Dental porcelain was applied onto the metal substrates and each lot of metal-ceramic combinations was divided into two subgroups – one stored in deionized water (24 hours, 37 ℃), the other subjected to thermal (6,000 cycles, between 5 and 60 ℃) and to mechanical cycling (105 cycles, 60 N load). SBS test-values and failure modes were recorded. The thickness of oxide layers on intact Ti-based substrates was measured using Auger electron spectroscopy (AES).
In order to study the effects of surface airborne-particle abrasion and subsequent application of bonding agent on the strength of porcelain bonds to Ti-alloys, eight groups of Ti-6Al-4V substrates (n = 11 per group) were fabricated – half of them by SLM, and half by CNC milling. The groups represented a fully crossed experimental protocol of abrasion with airborne Al2O3 particles under a pressure of 2 bars (intact – controls or airborne-particle abraded) and bonding agent application (with or without bonder) for the CNC milled and SLM titanium substrates.
The effects of titanium preoxidation at elevated temperatures were investigated on five groups of SLM Ti-6Al-4V substrates (n = 11 per group). Each group received a different preoxidation treatment (25 ℃ – control, 300 ℃, 600 ℃, 750 ℃ or 800 ℃) before porcelain firing. The thickness of surface oxide after preoxidation was determined (AES). A bonding agent and dental porcelain were applied onto the metal substrates and the metal-ceramic bond strength determined by a three-point bend test according to ISO 9693-1:2012.
Further, the average profile roughness (Ra) of the metal substrates was determined in each group. Representative metal-ceramic interfaces were analysed using a focused ion beam/scanning electron microscope (FIB/SEM) and energy dispersive spectroscopy (EDS). Metal-ceramic bond strength data and roughness data were analysed statistically with ANOVA, Tukey’s HSD and t-tests or by linear modelling (α = 0.05).
Results. The mean SBS values differed according to the metal-ceramic combination (p < 0.0005) and to the fatigue conditions (p < 0.0005). The failure modes and interface analyses suggest stronger adhesion of porcelain to Co-Cr than to Ti-based alloys. Values of Ra differed according to the metal substrate (p < 0.0005). Ti-based substrates were not covered with thick oxide layers following digital fabrication. Both airborne-particle abrasion (p < 0.0005) and bonding agent (p < 0.0005) enhanced the titanium-ceramic bond strength significantly. The former increased the surface roughness of CNC milled titanium but decreased the roughness of the SLM substrates. The elevated preoxidation temperatures led to thicker surface oxide layers (AES) and to increased roughness of the SLM titanium substrates. The titanium-ceramic bond strength, however, correlated negatively with the preoxidation temperature. Air passivation at room temperature following airborne-particle abrasion of SLM titanium, together with bonding agent application, resulted in a titanium-ceramic bond (36.73 ± 4.90 MPa) stronger than that of alternatively prepared titanium-ceramic systems.
Conclusions. Ti-based alloys are more susceptible than Co-Cr to reduction of the porcelain bond strength following thermo-mechanical cycling. The strength of the porcelain bond to Ti-based alloys is affected by the metal processing technology applied, the cast titanium-ceramic system being inferior to digitally fabricated systems and the CNC milled titanium-ceramic systems being less affected by the thermo-mechanical cycling. Air/room temperature passivation following surface airborne-particle abrasion of SLM titanium and application of a bonding agent resulted in a titanium-ceramic bond well above the minimal ISO 9693-1:2012 recommended value of 25 MPa for metal-ceramic systems.