Mariusz Bielawski

Development of unbalanced magnetron sputtered Al–Mo coatings for cadmium replacement

Abstract Aluminum is emerging as a key material system to replace environmentally unacceptable cadmium, especially for application on high strength steels. This is a particularly challenging application, which requires deposition of highly corrosion resistant, sacrificial coatings having excellent adhesion properties and in many applications, high lubricity. Al–Mo coatings, deposited using the unbalanced magnetron sputtering (UMS) deposition process, can meet these requirements. Depending on the level of alloying, coatings with enhanced corrosion properties or coatings with a low coefficient of friction can be obtained. Results of microstructural examination and tribological and electrochemical tests are presented for a series of UMS Al–Mo coatings produced over the full binary range. The coatings have uniform thickness, a fully dense, featureless or columnar structure and a good adhesion to steel. The passivating effect of molybdenum results in a gradual increase of the corrosion potential (from −0.750 V for pure Al to −0.220 V for Al-90Mo) and pitting potentials (respectively, from −0.650 to +0.200 V). Coatings with the Mo content below 14 wt.% are sacrificial to AISI 4340 steel. The coefficient of friction of Al–Mo coatings was tested to be between 0.2 and 0.9, with the lowest value obtained for Al-95Mo composition. At this level, the coefficient of friction of UMS Al-95Mo is comparable to that reported for cadmium.

Residual Stress Evaluation in TiN Coatings Used for Erosion Protection of Aerospace Components

A laser beam profilometry technique was used to investigate residual stress accumulation during TiN deposition and stress relaxation during post-deposition heat treatment. The test coatings were reactively sputtered on silicon and steel substrates using a UMS technique. TiN coatings, deposited at different bias and pressure levels, were evaluated for residual stress and microhardness. It was found that both the residual stress and the hardness were strongly affected by the coating deposition conditions. In addition, stress-temperature correlations were obtained by subjecting the coatings to temperature cycles up to 450°C. Stress-temperature plots revealed that the level of residual stress relaxation depended on deposition conditions and only coatings deposited at low ion bombardment could be fully annealed. The role of intrinsic and thermal stresses in the total residual stress in the coating/substrate system was also discussed.

Computational Evaluation of Adhesion and Mechanical Properties of Nanolayered Erosion-Resistant Coatings for Gas Turbines

A computational method to evaluate fracture toughness of prospective erosion-resistant coatings using a combination of first-principles density functional theory (DFT) calculations and fracture mechanics is proposed. Elastic coefficients C11 , C12 and C44 , the ideal work of adhesion Wad , bulk moduls B, shear moduls G, and Young’s moduls E of transition metal nitrides with a cubic structure such as TiN, CrN, ZrN, VN and HfN are calculated. Both the G/B ratio and Cauchy pressure C12 -C44 indicate brittle behaviour for TiN, ZrN and HfN and more metallic behaviour for CrN and VN. The fracture toughness KIC and interfacial fracture toughness KICInt for bi-layer combinations of these five nitrides is calculated along the [100] and [110] directions. The largest KIC value is obtained for HfN (2.14 MPa·m1/2 ) in (100) orientation and for TiN (2.16 MPa·m1/2 ) in (110) orientation. The lowest fracture toughness, in both orientations, is found for CrN. Among ten coherent interfaces of the five investigated nitrides the largest value of interfacial fracture toughness, KICInt = 3.24 MPa·m1/2 , is recorded for the HfN/TiN interface in the (110) orientation.Copyright