Influence of metal surface finishing on porcelain porosity and beam failure loads at the metal-ceramic interface

Statement of the problem. Investigators suggest that metals should be finished in 1 direction before porcelain application to minimize metal irregularities and trapped contaminants. These irregularities are thought to be focal points for porosity and crack propagation. Purpose. This study investigated the influence of metal finishing and sandblasting on (1) porosity production at the porcelain-metal interface, and (2) porcelain-metal beam failure load. Material and methods. Eighty cast metal samples were divided into 4 test groups: (A) bidirectional finish/sandblasting; (B) unidirectional finish/sandblasting (C) bidirectional finish only and(D) unidirectional finish only. The porcelain applied was 1.5 mm thick. Samples were sectioned longitudinally. Half of the samples were subjected to 3 3-point flexural test. The remaining samples were sectioned into 4 slices and were examined with a light microscope (x500). Number and diameter of porosities at the metal-porcelain interface were recorded. Results. Mean loads at failure (Ibs) were as follows: A, 11.1 +/- 1.3 (5.03 +/- 0.58 Kg); B, 11.2 +/- 1.7 15.08 +/- 0.77 Kg); C, 4.0 +/- 1.8 (1.81 + 0.81 Kg); and D, 5.0 +/- 2.1 (2.26 +/- 0.95 Kg). Groups A and B were significantly different from groups C and D (P<.0001). Nonsandblasted samples (C and D) exhibited a separation at thtr ceramometal interface, which prevented quantification of porosity size and number. Average interface porosity sizes (mu m) (A, 8.99 +/- 1.92; 13, 10.03 +/- 1.86) showed no significant difference. The mean interface porosity number (A, 62.3 +/- 16.02; B, 67.4 +/- 10.01) showed no significant difference. Conclusion. Direction of metal finishing did not affect the porosity number and size at the ceramometal interface or the beam failure lends. Sandblasting increased the beam failure loads. Nonsandblasted samples showed detachment of the porcelain from the metal.