The significance of brittle reaction layers in fusing of dental ceramics to titanium

Mikko Tapani Saloniemi
Abstract & Cover

This thesis comprises four intercomplementary parts that introduce new approaches to brittle reaction layers and mechanical compatibility of metalloceramic joints created when fusing dental ceramics to titanium. The first part investigates the effects of TiO2 layer structure and thickness on the joint strength of the titanium-metalloceramic system. Three groups of standard metalloceramic samples with different TiO2 layer thickness and crystal structure were tested. The TiO2 layers were produced using atomic layer deposition (ALD). Scanning acoustic microscopy (SAM), three-point bending (TPB), cross-section microscopy, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) were employed. Samples with all TiO2 thicknesses displayed good ceramics adhesion to Ti, and uniform TPB results. The fracture mode was independent of oxide layer thickness and structure. Cracking occurred deeper inside titanium, in the oxygen-rich Ti[O]x solid solution surface layer. During dental ceramics firing TiO2 layers dissociate and joints become brittle with increased dissolution of oxygen into metallic Ti and consequent reduction in the metal plasticity. To accomplish an ideal metalloceramic joint this needs to be resolved. The second part introduces photoinduced superhydrophilicity of TiO2.  Test samples with ALD deposited anatase TiO2 films were produced. Band gap energy (EBG) for the TiO2 layers was estimated from transmittance measurements. Samples were irradiated with UV light (> EBG) to induce superhydrophilicity of the surfaces through a cascade leading to increased amount of surface hydroxyl groups. Samples were divided into two groups D and E to study the required irradiation time. Hydrophilicity of the TiO2 surfaces was assessed by sessile drop contact angle measurements. The reference contact angle prior to UV radiation was ~55˚. Superhydrophilicity (contact angle ~0˚) was achieved within 2 minutes of UV radiation. After initial partial recovery during the first 10 minutes, the contact angle remained below 20˚ for 1h. Total recovery was not observed within 24h storage. Photoinduced ultrahydrophilicity can be used to enhance wettability of titanium surfaces, an important factor in dental ceramics veneering processes. The third part addresses interlayers designed to restrain oxygen dissolution into Ti during dental ceramics fusing. The main requirements for an ideal interlayer material are proposed. Based on these criteria and systematic exclusion of possible interlayer materials silver (Ag) interlayers were chosen. Six groups of standard metalloceramic samples were studied. Groups F, J, K and L were Al2O3-blasted, Groups G and H were left polished. Thin silver interlayers were produced on Groups G, H, J and K by using the DIARC®  plasma coating method and thicker interlayers on Group J by electrochemical baths. Analysis methods were as in the first part. Good ceramics adhesion to titanium was observed in all test groups save for G and H, which both exhibited several areas of poor contact. SEM/EDS analyses revealed attachment of alumina particles on the Al2O3-blasted titanium. Ag covered this contamination in Group L. TPB results were significantly better in Group L samples compared to Group F. Generally, cracking occurred inside titanium in oxygen-rich Ti[O]x solid solution (F, G, H, J, K), locally also between Ti and dental ceramics (K, L). In Group L multiple cracks occurred inside dental ceramics, none inside Ti structure. Ag interlayers of 5 μm on Al2O3-blasted samples can be efficiently used to retard formation of the brittle oxygen-rich Ti[O]x layer, thus enhancing metalloceramic joint integrity. Based on the literature, isolation of alumina blasting particle contamination was also considered beneficial. The most brittle component in metalloceramic joints with 5 μm Ag interlayers was bulk dental ceramics instead of Ti[O]x. The fourth part investigates the importance of mechanical interlocking and presents a new approach to overcome mechanical problems of brittle reaction layers. Mechanically polished, Al2O3-blasted, and photolithographically etched standard metalloceramic samples, Groups M, N and P, showed no significant TPB test differences. Cracking occurred through Ti[O]x, but in photolithographically etched samples also locally through dental ceramics. Hence, the significance of mechanical interlocking achieved by conventional surface treatments can be questioned as long as the formation of the brittle layers (mainly oxygen-rich Ti[O]x) cannot be sufficiently controlled. Photolithographically etched pits can be used to cause cracking of dental ceramics instead of the more brittle reaction layers. The current depth and steepness of the pits, however, were insufficient for extensive stress redistribution. In summary – in contrast to former impressions of thick titanium oxide layers – this thesis clearly demonstrates diffusion of oxygen from sintering atmosphere and SiO2 to Ti structures during dental ceramics firing and the following formation of brittle Ti[O]x solid solution as the most important factors predisposing joints between Ti and SiO2-based dental ceramics to low strength. This among other predisposing factors such as residual stresses created by the coefficient of thermal expansion mismatch between dental ceramics and Ti frameworks can be avoided with Ag interlayers. 

Source of Information
FinALD40 exhibition material,
Institute of Dentistry; University of Helsinki, Finland
(Helsinki/Espoo, Finland)
External Link
Read Thesis
linkedin invite