Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint The effects of dry and wet grinding on the strength of dental zirconia C.M.B. Ho a , H. Ding b , X. Chen c , J.K.H. Tsoi b , M.G. Botelho a, ⁎ a Prosthodontics, Faculty of Dentistry, The University of Hong Kong, Hong Kong b Dental Materials Science, Applied Oral Sciences, Faculty of Dentistry, The University of Hong Kong, Hong Kong c Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Hong Kong ARTICLE INFO Keywords: Zirconia Abrasion Dental ceramics Biaxial flexural strength Surface roughness X-ray diffraction ABSTRACT The purpose of this study was to evaluate the effect of different dry and wet surface finishing on the mechanical strength and surface characters of a dental yttria-stablized zirconia ceramic (Y-TZP). Surface grinding treatments with a dental air turbine handpiece were performed with: coarse diamond (DC) and fine diamond (DF), tungsten carbide (Tc) and fine tungsten carbide (TcF) burs with or without water coolant. Air particle abrasion with 50 μm alumina (APA), combination of burs treatments or burs-abrasion, i.e. DC-TcF and DC-APA, were also performed with non-treatment group as control (C). Statistical analyses (α = 0.05) on results revealed that all surface treatments significantly increased the surface roughness (Ra) than control (p < 0.05), whilst decreased breaking force (BFN) and biaxial flexural strength (BFS). Tungsten carbide surface treatment could significantly lower (p < 0.05) BFS and BFN, but DC only significantly lowered BFN. DC and tungsten carbide treatments exhibited significantly lower BFS values in wet than dry. A positive correlation was found between the BFS and BFN with the number of fragments. Only tetragonal phase of ZrO2 was presented by XRD. Synchrotron XRD revealed the (101) peak exhibits a broadening effect in the tungsten carbide treated specimens (38 nm for Tc and 30 nm for TcF), i.e. grain sizes in these specimens were smaller than the control (60 nm for C). This study outcome suggested that tungsten carbide burs should be avoided for grinding Y-TZP because of significant re- duction in the BFS. Water cooling during grinding did not consistently reduce the potential heat damaging effects expected with dry grinding. 1. Introduction The popularity and clinical use of dental zirconia has increased due to its superior physical properties, high aesthetic potential, bio- compatibility and chemical stability. In dentistry, zirconia is primarily used as a substructure for fixed dental prostheses and has traditionally been veneered with porcelain to provide good aesthetics. However, the most commonly seen clinical complication of this type of restoration is chipping of the veneering layer of porcelain [1–3]. To overcome these problems it has been suggested to use monolithic zirconia for the full contour restoration which then requires staining, polishing or glazing thereby eliminating the problems associated with the veneering por- celain chipping during clinical function. “Smooth surfaces” are considered to be important for providing a hygienic surface by reducing plaque accumulation and bacteria reten- tion [4,5] as well as reducing the amount of wear of the opposing teeth [6]. However, chairside adjustment of indirect dental restorations is often necessary before and/or after cementation of the fitting surface to allow full seating of the restoration and of the “polished” surfaces on the interproximal contacts and occlusal surface prior to cementation. Such adjustments will result in rough surfaces that may affect the strength of the ceramic. In addition, polished zirconia is recommended because it produces minimal wear effects on opposing enamel [7,8]. Zirconia is a poly-crystalline ceramic that exists as three crystal- lographic forms depending on the temperature: monoclinic, tetragonal and cubic. Tetragonal state is stable between 1170 °C and 2370 °C and has better mechanical properties than its monoclinic state. Below 1170 °C, a volume increase of about 3–5% occurs during the transfor- mation from tetragonal to monoclinic phase [9,10]. Dental zirconia is stabilized by metal oxides such as yttria (Y 2 O 3 ) to allow the tetragonal form to exist at room temperature. Loads and stresses can cause micro- crack formation which will generate tensile stress and so induce a change from the tetragonal phase to the monoclinic phase, producing a localized volume increase. This associated volumetric increase has the effect of compressing crack defects which in turn prevents further propagation and results in improving the flexural strength [11,12]. The change in configuration helps to prevent crack propagation as com- pressive forces counter the tensile force at crack tips preventing its https://doi.org/10.1016/j.ceramint.2018.03.062 Received 16 January 2018; Received in revised form 6 March 2018; Accepted 8 March 2018 ⁎ Correspondence to: Faculty of Dentistry, The University of Hong Kong, The Prince Philip Dental Hospital, 34 Hospital Road, Rm 3B19 Prosthodontics, Hong Kong. E-mail address: botelho@hku.hk (M.G. Botelho). Ceramics International 44 (2018) 10451–10462 Available online 09 March 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved. T