FINITE ELEMENT MODELLING OF STRESS INDUCED CHEMICAL ATTACK

摘要

Physical stresses upon polymer chains reduce covalent bond strength, accelerating chemical attack. This paper describes a process used to model and predict stress induced chemical attack in semi-conductor processing equipment. The methodology used was to characterise material behaviours at elevated temperatures and then perform three dimensional finite element analysis of a seal trapped within a groove. The study predicted the location of failure and the stress levels required to induce chemical attack. This method has proven to increase the service life of elastomers in aggressive chemical environments by controlling stresses through seal and groove design. Stress induced chemical attack is a macroscopic material failure due to mechanically propagated cracks, initiated by chemical degradation of a material. By applying a stress to an elastomer we deform the molecular chains, which both reduces the energy needed to break covalent bonds and introduces steric twisting that exposes the more chemically susceptible cross linking. This mechanism is known as "Stress induced chemical attack". The chemical attack initiates the formation of microscopic fissures. Once formed a crack will continue to propagate, growing slowly until it reaches a critical length, at which point the fracture energy causes rapid propagation of the crack across the whole section of the seal, causing the seal to break in a brittle-like manner. The most common output derived from finite element analysis is a plot of the Von-Mises stresses. Although this form of analysis can prove useful in predicting contact forces, or wholly mechanical failure, both Von-Mises and Tresca fail to predict the location or occurrence of stress induced chemical attack. When predicting stress induced chemical attack it is not purely the magnitude of the principal stresses which governs failure, but the direction, i.e. whether they are in tension or compression. Testing, together with numerous analyses, has shown that plasma may initiate stress induced chemical attack in Perfluoroelastomer materials with as little as 0.6MPa tensile stress on the exposed surface. The magnitudes of the principal stresses causing failure may be significantly below the ultimate tensile strength of the material at the operating temperature. When installed into a groove perfluoroelastomer seals should not be stretched by more than 2%, rather than the 5% often quoted. The use of interference at the top of a dove-tail groove to retain an o-ring should be avoided where prolonged direct exposure to chemical attack may occur.