Coating—substrate interdiffusion

In general, aqueous electroplating has minimal effect on substrate properties (apart from hydrogen embrittlement). Coated substrates can also be heat treated to promote interdiffusion, although this may result in concentration of elements at grain boundaries, causing embrittlement. Specific elemental electrodeposition processes and properties are reviewed in Ref 25 some examples are given here. 

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]. 

With tin coatings on brass, the interdiffusion of coating and substrate brings zinc to the surface of the tin the action can be rapid even with electrodeposited coatings. The effect of zinc in the surface layers is to reduce the resistance of the coating to dulling in humid atmospheres, and the layer of zinc corrosion product formed makes soldering more difficult. An intermediate layer of copper or nickel between brass and tin restrains this interdiffusion . 

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. ... 

We have used one-dimensional model [3] to calculate the residual stresses in the system involving a ceramic-like (TiC) coating connected to a metal-like (Ti) substrate by an inter-mediate FGM region, consisting (ultimately after the interdiffusion anneal) of titanium carbide with a carbon concentration that varies with distance. The FGM plate has a thickness 2c, unit dimension in depth (z-direction) and is infinitely long in the x direction, as shown in Fig. 2. 

Optimised LPPS parameters were plasma power of 30 kW, argon gas flow rate of 30 slpm, hydrogen flow rate of 4 simp, powder feed rate of 12 g min-1, chamber pressure of 60 mbar and stand-off distance of 220 mm. Immediately adjacent to the interface substrate-coating an approximately 0.1 pm thick interdiffusion zone developed in the Ti6Al4V substrate with elevated Ca, P and O contents followed by a thin layer of ACP and then by a layer of oxyhydroxyapatite (Ca/P 1.67) as ascertained by Raman spectroscopy (absence of the stretching vibration of OH-... 

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]. 

The degradation modes of coatings are essentially the same as those described for cyclic oxidation of alloys in Chapter 5, mixed-oxidant corrosion of alloys in Chapter 7, and hot corrosion of alloys in Chapter 8. However, additional factors arise with coatings since they are relatively thin, they contain a finite reservoir of the scale-forming elements (Al, Cr, Si), and interdiffusion with the substrate can both deplete the scale-forming element and introduce other elements into the coating. Additional mechanical affects also arise, which can lead to deformation of the coating. Mechanical effects are also critical to the durability of TBCs. 

---