Coating—substrate interdiffusion diffusion coatings

The interdiffusion of various elements among the substrate and plasma sprayed coatings was observed to be marginal as has been indicated by EPMA. The diffusion between the bond coat and the top coat was relatively high. Aluminium was found to be the most vulnerable element to the diffusion phenomenon. 

As the TBCs are applied onto advanced single crystal superalloy substrates, interdiffusion between the bond coat and the substrate has become a serious problem. The local concentration change caused by the interdiffusion would promote the precipitation of detrimental phases in the substrate. Outward diffusion of elements from the substrate, such as W, Mo and Cr, is detrimental to the interfacial bonding between TOO and bond coat, accompanied by volatilization of oxides of those elements at elevated temperatures. In addition, inward diffusion of A1 into the substrate could result in A1 depletion in the bond coat and finally degrade the oxidation resistance of the bond coat in long-term thermal exposure [7,65]. 

Silver coatings may blister above 200°C because of oxygen diffusion. A nickel undercoat stops interdiffusion with a copper substrate above 150°C. Alloying with antimony, selenium, sulphur or rhenium increases hardness—the coefficient of friction is also much reduced in the last case. ... 

Interdiflusion between the components of catalyst coatings and substrates can also lead to catalyst deactivation. If Nb, Ta, Ti, V or Zr diffuses through palladium or other noble metal protective layers and reacts on the outer surface to form stable oxides, carbides or nitrides, the catalytic dissociation of molecular hydrogen can be poisoned. Interdiffusion, linked to loss of hydrogen flux, has been reported by Edlund and McCarthy [47] and Pagheri et al. [56]. Membrane surfaces can also become depleted of palladium if palladium diffuses into the substrates. Apparent complete loss of palladium has been observed by Rothenberger et al. for 40 nm thick Pd films on Ta foils after 48 h use at 1173 K (900 °C) [41]. 

Depending on the curing conditions and material properties, atoms will diffuse across the interface to varying extents when two phases of coating and a substrate achieve molecular contact by wetting. The phenomenon is a two-stage process Wetting is followed by interdiffusion of an element across the interface to establish a chemical bond (Lee, 1991 Kendall, 2001). 

---