PFM Bonding Mechanism — MFDS Part 1 MCQ
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Correct answer: A — A thin adherent oxide enables chemical bonding, surface roughness adds micromechanical retention, and a slightly higher alloy coefficient of thermal expansion leaves the porcelain under residual compression
Explanation lettering: D = shown as B · E = shown as C · C = shown as D · B = shown as E
A is correct. The principal interfacial mechanism is chemical interaction between the porcelain and a thin, adherent metal-oxide layer. Air abrasion increases surface area and provides supplementary micromechanical interlocking. Thermal compatibility strengthens the complex rather than creating the chemical bond: the alloy is normally selected to contract slightly more than the porcelain on cooling, placing the brittle porcelain under beneficial residual compression. An excessively thick or poorly adherent oxide can weaken the interface, so B is incorrect; a lower alloy contraction would tend to produce harmful tensile stress. C and E incorrectly exclude the essential oxide-mediated chemical contribution. D describes resin bonding or repair protocols: silane is not the principal mechanism by which porcelain bonds to a PFM alloy during firing. Van der Waals forces may contribute secondarily but should not be presented as equivalent to oxide-mediated bonding.
Reference: Mehulić K, Laus-Sosić M. Metal-ceramic bond: how to improve? Minerva Stomatologica. 2009;58:367-373. https://pubmed.ncbi.nlm.nih.gov/8463354/