The influence of surface roughness on porcelain strength N. de Jager, A.J. Feilzer * , C.L. Davidson ACTA, Department of Dental Materials Science, Louwesweg 1, 1066 EA Amsterdam, The Netherlands Received 15 July 1999; received in revised form 2 December 1999; accepted 29 February 2000 Abstract Objectives: In order to adjust occlusion, the functional surfaces of porcelain restorations are often ground and mechanical machining is even an essential part of the CAD–CAM process for these restorations. The aim of this study was to investigate the influence of the finishing procedures on the biaxial flexure strength of four commercial porcelains. Methods: Four commercial porcelains of which two are used for metal –ceramic restorations (Flexo Ceram Dentine and Vita VM K68) and two for veneers and inlays (Duceram LFC Dentine and Cerinate BODY) are used in this study. For each porcelain, sixty discs (  22 mm, h  ^2.0 mm) were produced using twelve different finishing procedures. Twenty discs were left untreated, twenty discs were milled, using a high-speed diamond disc, and twenty discs were machined in a high-speed grinding/polishing device. Half of the samples were glazed. In each of these six groups, half of the samples were stored for 16 h at 80°C in a 4% acetic acid solution. The biaxial flexure strength was determined using the ball-on-ring method. In each group the roughness of the surface was determined and examined via SEM. Results: With the exception of Flexo Ceram Dentine, a significant correlation was found between the roughness of the surface and the biaxial strength: the smoother the surface, the stronger the sample. The differences in biaxial strength may be attributed to the stress concentration of an applied load due to the roughness of the surface caused by mechanical finishing or chemical action. The fact that the strength of Flexo Ceram Dentine was not affected by the different surface treatments is probably due to the size of the leucite particles, which apparently induce more stress concentration than the surface flaws and the roughness of the surface. Significance: It was concluded that surface roughness determines the strength of a porcelain material, except where the inner structure of the material causes greater stress concentration than that caused by the combination of surface roughness and surface flaws.  2000 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Ceramics; Porcelain; Roughness; Strength; CAD–CAM 1. Introduction The essential step in the CAD–CAM production of porcelain restorations is the mechanical machining of cera- mic materials. Machining reshapes the surface and intro- duces surface flaws. The roughness produced by the finishing procedure and the introduction of surface flaws may be accompanied by a reduction in strength. It is indeed customary to expect that, within certain ranges, an increase in surface roughness will lead to a decrease in strength. According to the literature, strength is determined by the shape, sharpness, size and depth of the surface flaws as well as by internal defects [1]. Various techniques such as glaz- ing have been proposed to strengthen the material after the introduction of surface flaws. The effectiveness of such strengthening mechanisms is disputed. Studies on dental porcelains both support and refute the strengthening effect of a glaze layer on porcelains [2–4]. The objective of the present study was to establish criteria for the influence of surface finishing on the strength of dental porcelain. It was hypothesized that the surface rough- ness obtained by the different finishing procedures will concentrate an applied stress. As a consequence of the stress concentration, failure will occur at a lower stress. The flaws which cause failure may not occur randomly, but rather may be concentrated around points with higher stress, as a result of surface roughness. For this reason, the influence of finishing procedures on the roughness and biaxial flexure strength of four commer- cial porcelains were investigated. 2. Material and methods Four commercial porcelains were investigated. Two of these are used for metal–ceramic restorations (Flexo Dental Materials 16 (2000) 381–388 dental materials 0109-5641/00/$20.00 + 0.00  2000 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. PII: S0109-5641(00)00030-0 www.elsevier.com/locate/dental * Corresponding author. Tel.: +31-20-5188335; fax: +31-20-6692726. E-mail address: a.feilzer@acta.nl (A.J. Feilzer).