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Abstract : Residual stresses in chemically vapor deposited monolithic and graded coatings in the Ti-C-N system were investigated as a function of substrate material and coating composition by X-ray diffraction Sin2 ψ method. The thermal expansion coefficients CTEs of the graphite substrates ranged from 2.5×10-6 K-1 to 8.6×10-6 K-1. Titanium nitride CTE 9.35×10-6 K-1 and titanium carbide CTE 7.5×10-6 K-1 coasings deposited on the low-expansion substrates CTEs 2.5-3.5×10-6 K-1 exhibited crack networks which accounted for low stress levels measured in the coatings. A phenomenalogical explanation of the crack patterns was given. The coatings grown on the substrates with high coefficients of thermal expansion CTEs 7.8-8.6×10-6 K-1 had no cracks. Residual stresses in the TiN coatings on these substrates were measured to be tensile. Whereas TiC coatings always exhibited compressive stresses ranging from -54±10 MPa to -288±18 MPa. The TiCxNy coatings deposited on the substrate with a thermal expansion coefficient of 8.6×10-6 K-1 also had compressive stresses increased with increasing TiC mole fraction in the TiCxNy up to about 0.9 above which stresses decreased. The residual stresses in the top TiC layers of the graded TiN-TiC coatings with linear, parabolic and exponentional composition profiles grown on the the same substrate CTE 8.6×10-6 K-1 were measured to be compressive and about 475 MPa. Stresses in the coatings were calculated and attributed to the thermal expansion mismatch between the coating and the substrate. It was shown that the measured stresses were, in general, found to be in good agreement with the calculated ones.





Autor: S. Eroglu B. Gallois

Fuente: https://hal.archives-ouvertes.fr/



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