Oxide layers with segmentation cracks in thermal barrier coatings. Review and analysis

<jats:p>Currently, oxide layers in thermal barrier coatings are formed with a defective structure, porous or with segmentation cracks specially created perpendicular to the substrate, which reduce residual stresses during thermal cycling. To analyze the possibility of forming segmentation cracks in thermal barrier coatings by plasma spraying, three engineering criteria describing their production were used in this work: 1) the specific enthalpy of the plasma jet — Neff/Q — is the ratio of the effective power of the plasma torch (Neff) to the flow rate of plasma-forming gases (Q); 2) Neff /(Q d) is the ratio of the specific enthalpy of the plasma jet to the average particle size of the sprayed powder (d); and 3) linear energy — the ratio (Neff·K)/(V·B) — where K is the coefficient determining the residual effective power at the spraying distance, V is the speed of movement of the plasma torch relative to the substrate, B is the coating width in one pass. The second and third criteria determine the temperature of the sprayed particles and the temperature of the coating in the spray spot. These two temperatures determine the cohesive strength and the possibility of segmentation crack formation. By approximating literature data, equations were derived describing the dependence of the number of segmentation cracks on the values of the proposed criteria, the substrate temperature, and the average size of the sprayed particles. As a first approximation, the proposed ratios Neff/(Q·d) and (Neff·K)/(V·B) can be used when selecting spraying modes at the initial stage of experimental planning. For example, by increasing the value of the second criterion to 400 GJ/(kg·m) and the third criterion to 350 kJ/m2, the average number of segmentation cracks increases to 5 per 1 mm of coating length.</jats:p>