歯科用陶材エナメル層の表面応力測定 Measurement of Surface Stress in Superficial Layer of Dental Porcelain

陶材焼成後の焼成物の表面に局在する応力は, 冷却方法に大きく依存すると考え, 冷却条件を(a)炉冷, (b)コップ内での冷却, (c)空気中放冷, (d)強制空冷とした際の焼成物の冷却時における温度変化, 熱膨張曲線および表面応力値を測定したところ, 次のような結論が得られた.1)温度変化測定の結果, 炉冷では試料の中央部と表層部との温度降下速度の差はほとんど見られない.また, コップ内での冷却および空気中放冷ではやや差が見られ, 強制空冷の場合には差が見られ, 温度差も100℃以上におよんでいた.2)200〜700℃までの平均熱膨張率は, 炉冷1.21%, 空気中放冷1.08%, コップ内での冷却1.05%, 強制空冷0.95%と順に小さくなった.3)試料の温度降下速度および中央部と表層部との温度差が応力の発生に影響している.4)表面応力測定の結果, 強制空冷8.5kg/mm^2, 空気中放冷2.5kg/mm^2の値が得られたが, コップ内での冷却および炉冷の場合には測定できず, 応力の発生がごく小さかったものと思われる.

Considering that the appearance of the post-sintering stress localized in the surface layer of the sinter depends greatly on the methods of cooling, investigations were made on thermal changes in the sinter at cooling and thermal expansion curves by experiments with varying conditions:(a)cooling in the furnace, (b)cooling in the cup, (c)natural air cooling and(d)forced air cooling.Also, quantitative analysis of the surface stress induced by them was made, with results as follows:1)In the results of investigation on thermal changes, there was hardly any noticeable difference in the rates of depression between the interior and the surface layer of the sample by cooling in the furnace, a slight difference was observed in the cases by cooling in the cup and natural air cooling, and in the cases by forced air cooling, there was a great difference between the two parts, the thermal difference surpassing 100℃.2)Average coefficients of thermal expansion at 200〜720℃ showed a gradual diminution in the following order cooling in the furnace 1.21%, natural air cooling 1.08%, cooling in the cup 1.05% and forced air cooling 0.95%.3)The velocity of thermal depression and the thermal difference between the interior and the surface layer of the sample gave influence on the generation of stress.4)Whereas the surface stress measurement in the cases of forced air cooling and natural air cooling resulted in 8.5kg/mm^2 and 2.5kg/mm^2, no value was obtained from the cases by cooling in the cup and cooling in the furnace probably because of the generation of extremely little stress.