Improvement of oxidation resistance of metals and alloys by high temperature coating and laser treatment

An overview is presented of the phenomenon of high temperature oxidation and its deleterious role in the performance of high temperature components. A brief description of some surface modification techniques (ion implantation, plasma coating, and laser treatment) and their relevance for improving high temperature corrosion properties is also included. An example is discussed where it is described in detail how a suitable combination of plasma coating and laser treatment techniques could be used to confer best protection to metallic substrates for a particular service condition. Mild steel was plasma coated with stainless steel. By giving different laser treatments (by changing the travel speed of the laser beam) a variety of modifications were introduced in the coating (such as the removal of only the inherent deficiencies, e.g. porosity, and alloying of the substrate material). The plasma coated and the laser treated materials were characterised and oxidation tests were carried out on them in air both in isothermal (973 and 1073 K) and under thermal cycling (between 1073 and 298 K) conditions, along with the uncoated mild steel.As expected, the plasma coating and laser treatments caused marked improvements in the oxidation resistance of the mild steel. A laser treatment that could only remove the irregularities of the coating had improved the oxidation resistance of the coating during the isothermal tests, whereas the treatment causing alloying of the substrate and consequent dilution of the coating materials was found to reduce the resistance. However, interestingly, this trend was found to be reversed during the long term thermal cycling test. This study has led to an understanding of the applicability of a given laser treatment for a particular service condition, and further, may help in designing coatings and treatments for high temperature components.