Evaluation of functional coatings on ceramic dental crowns

Ceramic dental implants combine advanced materials science, esthetics and manual skill. Every single dental crown must be individually modified in terms of size, shape and color to ensure successful reconstructive dentistry.

The production process is complex: from the manufacturing of blanks, to precise milling in reproducing the original tooth from the mold, to sintering, and then varnishing. Dental technicians apply several layers of ceramic glaze to adjust the color of the implant, for the most natural look. The coatings must be functional, namely resistant to wear and pressure, but also esthetical with a smooth surface and just the right color. 

Such a manufacturing process is expensive. This is why efforts have been made to simplify and accelerate it by using special spray coatings. Two Lithium silicate sprays, LiSiPURE and LiSi LOW FUSE, have been developed in a joint effort by Fraunhofer IKTS in Hermsdorf and the company elaboro®. To demonstrate the high quality and functionality of the new coatings in terms of form and uniformity, optical coherence tomography (OCT) was used.

OCT is a fast, non-destructive imaging method that delivers volumetric information. It uses short coherent light in the near infrared spectrum. Materials that are at least partially transparent at wavelengths ranging from 800 to 1300 nm can be inspected with commercially available instruments at depths ranging from a few hundreds of microns to a few millimeters. As the laser scans the sample, photons are backscattered at the surface and at inhomogeneities within the sample, both regions of discontinuous optical properties. The intensity of the backscattered light is recorded as a function of depth. As the focused light moves along a line, a virtual cross-section of the sample is recorded (B-scans). 

Figures 1 and 2 show B-scans of zirconia dental crowns with different coatings – in the same location on the vesitibular tooth cusp, in occlusal view. The exact positioning is made possible by a motorized sample stage and a special sample holder. Samples 4 and 14 were coated with a combination of commercially available ceramic glazes and LiSi PURE (sample 4) and LiSi LOW FUSE with color particles (sample 14). The OCT images show that the coatings are uniform and coat the raw ceramic substrate with a uniform and smooth layer. The surface quality is comparable to that of a commercially available glaze layer (sample 13). Figure 2 shows two additional glaze sprays where the coating distribution was problematic: due to the rough surfaces, the uniformity of coating was not ensured.

The images show that the OCT technology can provide information on the quality of coatings for dental crowns. Additionally, a specialized software was developed which delivers information on the layer thickness, surface roughness and presence of defects. Special systems for characterizing ceramic coatings and substrates can be adapted in cooperation with industrial partners.