Which of the following type of failure may occur, if the coefficient of thermal expansion of ceramic is much lower than that of metal?
(A) Cohesive failure of ceramic (B) Adhesive failure of metal ceramic bond (C) Cohesive failure of metal (D) Cohesive failure of metal ceramic bond
(B) A thermal contraction mismatch that results from a higher coeﬃcient of contraction of porcelain (αP is greater than αM) will introduce residual tangential and hoop tensile stresses. The additive eﬀect of tangential tensile stress induced in the porcelain by the intraoral force can exceed the tensile strength of the porcelain and causes crack propagation in the porcelain veneer. When the contraction coeﬃcient of the porcelain is much lower than that of the metal (αP much lower than αM), porcelain cracking or metal-ceramic bond failure can occur near the metal-porcelain interface. This incompatibility failure is likely caused by the development of radial tensile stresses that exceed the tensile strength of porcelain.
Contraction coeﬃcient of the porcelain - αP  
Contraction coeﬃcient of the metal - αM
 
Phillip’s 12th edition page 423