On the evaluation of adhesion of coatings by automatic scratch testing

Automatic scratch testing is an expedient technique for comparatively evaluating the cohesive failure load and adhesion failure load of thin coatings on various substrates. In combination with SEM examination of the scratch track, this technique has been used herein to detect and evaluate various effects on coating strength and adhesion. For soft Triballoy T-800 and Stellite SF-6 cobalt-base coatings on 4340 low alloy steel, adhesion was found to be strong and failure was found to be cohesive in the coating. In the presence of a plated chromium interlayer, pre-existing cracks lowered substantially the cohesive failure load, which was also lowered by an increase in the coating deposition pressure. The spacing of transverse cracks within the coating was found in all cases to decrease with increasing applied normal load. In soft aluminum coatings on depleted uranium (DU)-0.75% Ti alloy specimens, alloying aluminum with magnesium or zinc enhanced the coating strength and adhesion. In (Al-Mg) coatings on this substrate, a smoother surface led to a lower friction coefficient and a higher adhesion failure load. In hard, thin TiN coatings on 17-4 PH steel, a lower bias voltage applied to the substrate yielded higher cohesive and adhesion failure loads. In hydrogenated amorphous SiC thin coatings on 4340 steel, loss of hydrogen by annealing converted the residual compressive stresses into tensile stresses and lowered both the cohesive and the adhesion failure loads. Finally, automatic scratch testing proved helpful in determining delamination loads in multilayer TiN/Ti/TiN coatings on DU-0.75% Ti alloy.     

Deposition and mechanical property evaluation of amorphous silicon-containing diamond-like carbon (Si-DLC) coatings on steel

Amorphous silicon-containing diamond-like carbon (Si-DLC) coatings were deposited by Ar⁺ ion beam-assisted physical vapor deposition of tetraphenyl-tetramethyl-trisiloxane (704 Dow Corning diffusion pump oil). The steel substrates studied included AISI 4130, 17-7 PH, 440-C, and 4340 (bare and nitride-precoated) specimens. DLC coating thicknesses ranged from 1.8 to 4.31 μm. Deposition rates increased with increasing beam current density and varied with the steel substrate composition. Nanoindentation measurements of the hardness and elastic modulus at two different depths yielded values of 9-10 GPa and 99-128 GPa, respectively. Film cohesion and adhesion failure loads increased with increasing underlying layer hardness, chromium content in the substrate, or the presence of a titanium nitride precoat. The friction coefficient of a diamond stylus against the coating surface decreased and wear resistance increased with nitride precoating.     

Effect of substrate rotational speed during deposition on the microstructure,mechanical and tribological properties of a-C:Ta coatings

Rotational speed has an important influence on the performance of coating materials. The a-C:Ta composite coatings were prepared by controlling the substrate rotational speed during deposition process using PVD technique. The results showed that the coating transformed from dense structure to columnar structure. Due to the changes of deposition time and the vapor incident angle of the sputtering ions, the sp² hybrid structure increased and the C–Ta bonds contents decreased as a function of the rotational speed, which led to the improvement of adhesion force. The average friction coefficient of the composite coatings did not fluctuate significantly for the amorphous carbon matrix and the transfer films formed during friction, while the wear rates were gradually increased. The sample at 0.5 rpm possessed the lowest wear rate, which was mainly associated with the cooperative behavior of the dense structure and the formation of TaC nanoclusters in the composite coating.     

Effect of wax on the adhesion strength between acrylic urethane film and chromated steel sheet,and the friction coefficient of the film

The effect of wax in an acrylic urethane (AU) film on the adhesion strength between AU film and steel sheet treated with Zn-Ni and chromium was investigated by varying the amount of wax using a pull-off test. The adhesion strength decreased as the amount of wax increased. Since the wax in the AU film can react with the reactive functional groups of imine ester (hardener), the cohesive strength of the AU film can be reduced. The distribution of wax in the AU film on a polar and a nonpolar substrate, whose contact angles with water were 30° and 75°, respectively, was examined using contact angle measurements and X-ray photoelectron spectroscopy (XPS). The distribution of wax in the AU film was greatly affected by the degree of polarity of the substrate. In the case of the nonpolar substrate, the wax in the AU film migrated towards the film/nonpolar substrate interface to form a weak boundary layer, where the failure occurred. In the case of the polar substrate, the amount of wax at the film/polar substrate interface was much smaller than that at the film/nonpolar substrate interface and increased linearly with increasing wax content in the AU film. The friction coefficient of the AU film against chromated steel sheet was also measured with varying amounts of imine-ester and additives such as wax and colloidal silica. The friction coefficient appears to be influenced only by the amount of wax.