Bonding Strength of DLC Film on Zirconia Coating Prepared by Gas Tunnel Type Plasma Spraying

Plasma sprayed zirconia coatings are now commonly used as the thermal barrier coatings (TBC) for high-temperature parts in many mechanical applications such as the engines of automobiles, planes and rockets. However, coating adhesion is frequently the cause for the failure of the coatings. As the same as the zirconia coatings, the deposition of diamond-like carbon (DLC) films on the metal substrate has some problem in the adhesion between DLC film and the substrate. But the adhesion property was improved by the interlayer of a hard ceramic coating on the metal substrate. In order to show the failure mechanism for coating adhesion, zirconia coatings with different thickness were prepared on stainless steel and/or aluminum substrates by a novel kind of gas tunnel type plasma spraying. The as-sprayed coatings were firstly polished and then subjected to deposition of diamond-like carbon (DLC) films using hydrocarbon gas as the carbon source. The adhesion of the DLC covered zirconi coatings was evaluated by the scratch test method. The evaluation showed that thicker as-sprayed zirconia coatings on aluminum alloy substrate showed much higher scratch durability, even with thinner zirconia coatings. The porosity and hardness of as-sprayed coatings were measured on the coating cross-sections using a micro-hardness tester and optical microscopy. The results showed that thicker coatings had higher hardness and lower porosity. The adhesion failure of DLC films on zirconia coatings is caused by two different mechanisms: one is the separation of the DLC films from the zirconia coatings and the other is the breaking of the DLC films under stress load because of the brittleness of the material. From the microscopic photos of the scratch tracks, the adhesion failure of DLC film on the thinner zirconia coatings is due to the flaking of DLC film by the distortion of aluminum alloy substrate, while the adhesion failure of DLC films on thicker zirconia coating is due to the chipping of the DLC film itself