2.1

Materials

A feldspathic porcelain with well-dispersed crystallites was used for this study. ¹

1 Mark II for the CEREC ^® system, Vita Zahnfabrik, Bad Säckingen, Germany.

^{, 2}

2 Commercial products and equipment are identified only to specify adequately experimental procedures and does not imply endorsement by the authors, institutions or organizations supporting this work, nor does it imply that they are necessarily the best for the purpose.

It was a relatively strong pressed porcelain, making the fracture surfaces conducive to fractographic analysis. The material is described by the manufacturer as consisting of natural feldspar materials with very fine crystalline portions homogeneously embedded in the surrounding glass matrix and fired at high temperature (1170–1200 °C). An average flexural strength of 154 MPa ± 15 MPa was reported by the manufacturer . ³

3 The uncertainty type was not reported, but may be assumed to be one standard deviation.

The data is quoted as having been from another study where the values were listed slightly differently as 154 MPa ± 12 MPa. The tests were done in three-point flexure with 1.5 mm × 3 mm × 20 mm specimens on 15 mm outer spans. The fracture toughness of this material by the single edged precracked beam method was 1.19 MPa m ^1/2 ± 0.05 MPa m ^1/2 (one standard deviation) .

This material was identified as “Porcelain 2” in our earlier study on the applicability of the Weibull statistics to dental materials strength analysis . The bend bars were furnished by the manufacturer with no information provided about the grinding or polishing steps used. The bars appeared to have been polished since there were no machining striations on the ground surfaces. The edges were not rounded or beveled.

2.2

Methods

Flexural strength of 26 test pieces was measured in 1/4-point, 4-point flexure with 3 mm × 4 mm × 28 mm specimens on a semi-articulating fixture with 10 mm and 20 mm spans. Bars of this short length were necessary since the bend bars were cut from CAD/CAM blanks. The crosshead speed was 0.2 mm/min and all testing was done in laboratory ambient conditions. This crosshead rate produces stress or strain rates similar to those achieved with longer 3 mm × 4 mm × 40+ mm bend bars tested on standardized 20 mm × 40 mm bend fixtures. Every fracture surface of every test piece was examined with a stereoptical microscope at up to 300 magnification and also a scanning electron microscope (SEM) using procedures outlined in . We did this since we initially did had difficulty characterizing some of the fracture origins. One specimen was selected for further intensive work with a field emission scanning electron microscope. In a few instances, fracture surfaces were etched with hydrofluoric acid (1% for 20 s) to ascertain whether the origin tended to dissolve differently than the matrix. A compound optical microscope was used on a polished broken half of a specimen to examine the overall microstructure as well as to measure the approximate concentration of certain flaws per volume using quantitative microscopy techniques. A rough single estimate of the number of flaws per unit volume was made by counting the number of large flaws that were exposed per unit surface area on a 50 cm diagonal digital computer monitor image of the piece which had an exposed area of 4 mm × 17 mm. Only flaws larger than a few tens of micrometers were counted since these are the most likely fracture origins. The number of flaws of a certain size per unit volume <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='N¯V’>N¯¯¯VN¯V
N ¯ V
was estimated from Eq. (5.5) of Ref. :

where <SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='N¯A’>N¯¯¯AN¯A
N ¯ A
is the number per unit area and <SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax style="POSITION: relative" data-mathml='D¯’>D¯¯¯D¯
D ¯
is the average flaw diameter. Flaws of size 30 μm or greater were easily discernable when the exposed surface was magnified onto the computer monitor screen. <SPAN role=presentation tabIndex=0 id=MathJax-Element-5-Frame class=MathJax style="POSITION: relative" data-mathml='N¯A’>N¯¯¯AN¯A
N ¯ A
was estimated by counting the number of flaws exposed on the polished section and dividing by 4 mm × 17 mm. The microstructure of the polished specimens was also examined with the scanning electron microscope, with both unetched and etched specimens (1% hydrofluoric acid for 20 s).

The crystalline phase assemblage was evaluated by X-ray diffraction analysis with copper Kα radiation with two theta scans from 5° to 65°. An aluminum reference was scanned over the same range for comparison.